DEPARTMENT OF THE INTEBIOB

BULLETIN

OF THE

UNITED STATES

GEOLOGICAL SURVEY

. ISO

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1898
IJO

-. r
UNITED STATES GEOLOGICAL STJEVEY

CHARLES D. WALCOTT, DIRECTOR

THE

EDUCiTIONIL SERIES OF ROCK SPECIMENS

COLLECTED AND DISTRIBUTED

BY THE

UNITED STATES GEOLOGICAL SURVEY

BY

JOSEPH SILAS DILLER,

» 

WASHINGTON

v

GOVERNMENT PRINTING OFFICE 

 1898

CONTENTS.

Paga

Letter of tranamittal.......-.......................................'........... 9

Preface..................................................................... 11

The study of rocks.......................................................... 13

Introduction............................................................ 13

Structural features of rocks............................................. . 13

Methods of physical analysis............................................ 18

Specific-gravity method............................................. 19

Electro-magnetic method............................................ 22

Microscopic method................................................. 23

The principal rock-making minerals..................................... 27

Minerals illustrated by this series of rock specimens.................. 29

Minerals of the isometric system ................................ 30

Minerals of the tetragonal system............................... 33

Minerals of the hexagonal system................................ 34

Minerals of the orthorhombic system ............................ 37

Minerals of the monoclinic system............................... 40

Minerals of the triclinic system ................................. 45

Homogeneous aggregates ...................... ................. 46

Classification of rocks................................................... 48

Tabular view and classification of the rocks of this scries............ 52

Descriptions of specimens................................................... 56

Unaltered sedimentary rocks of mechanic.il origin (Nos. 1-22)............ 56

Unaltered sedimentary rocks of chemical origin (Nos. 23-38) ............. 91

Unaltered sedimentary rocks of organic origin (Nos. 39-56)............... 115

Unaltered igneous rocks (Nos. 57-114) ................................... 145

Granite-rhyolite family (Nos. 58-69).................................. 146

Syenite-trachyte family (Nos. 70-71)................................. 181

Nephelite-leucite rocks (Nos. 72-78).................................. 186

Diorite-andesite family (Nos. 79-94).................................. 211

Gabbro-basalt family (Nos. 95-109)................................... 245

Peridotite family (Nos. 110-114)...................................... 286

Metamorphic sedimentary rocks (Nos. 115-135)........................... 298

Metamorphic igneous rocks (Nos. 136-146) ............................... 343

Residual rocks (Nos. 147-150)............................................ 376

Illustrations of surface modifications (Nos. 151-156)...................... 385

Index ...................................................................... 393

5

ILLUSTRATIONS.

Page. 

PLATE I. BeJ'^d structure, bedding, and cross bedding, OakCatiyon, Arizona. 14

II. Massive structure in diorite, Sliasta County, California.........:. 16

III. Rhombohedral jointing in gneiss, Potoinac River near Washington,

District of Columbia........................................... 18

IV. Columnar jointing, Watchung Mountain, Orange, New Jersey..... 20

V. Petrographic microscope ......................................... 24

VI. High part of South Manitou Island, Lake Michigan, showing forest 

formerly huried beneath drifting sand and now exposed by eolian 

erosion ........................................................

VII. A typical exposure of southern loess, Muscatine, Iowa............

VIII. Clay bank at Brick Haven, Virginia,, opposite Washington, District of Columbia .............................................. 68

IX. Thin sections of sandstone: A, pebbly sandstone from Barron, Ore- 

 gon, as seen under a microscope in ordinary light; B, brown 

sandstone from Huinmelstowu, Pennsylvania, as seen under a 

microscope in ordinary light.................................... 76

X. Thin section of Potsdam sandstone from Ablemans, Wisconsin, as 

seen under a microscope: A, in ordinary light; B, between crossed 

iiicols.......................................................... 80

XI. Ripple marks on sand dunes near Golden Gate Park, San Francisco,

California ..................................................... 82

XII. Ripple marks cut by mud cracks filled with sand, from the Grand

Canyon of the Colorado, Arizona................................ 84

XIII. Thin section of graywacke from Hurley, Wisconsin, as seen under

a microscope in ordinary light.................................. 86

XIV. Old Faithful geyser in action, Yellowstone National Park......... 92

XV. Vein of quartz in gneiss near Chain Bridge, District of Columbia.. 94

XVI. Thin sections of siliceous oolite from Center County, Pennsylvania, 

as seen under a microscope: A, in ordinary light; J5, between 

crossed nicols.................................................. 96

XVII. Deposits in Marengo Cave, Indiana............................... 98

XVIII. Deposits in Luray Cave, Virginia................................. 100

XIX. Terraces of Mammoth Hot Springs, Yellowstone National Park.... 102

XX. Concretions of marcasite from Thatcher, Colorado................ 110

XXI. Fossil trees, Yellowstone National Park .......................... 114

XXII. Section of a flint nodule from Austin, Texas....................... 120

XXIII. Beds of fossiliferons iron ore between beds of limestone, Rochester,

New York ..................................................... 138

XXIV. Dike of audesite cutting andesitic breccia, Absaroka Range, Wyoming ........ ............................................:^..... 146

XXV. Lithophysaj in lithoidite of Obsidian Cliff, Yellowstone National

Park; natural size ............................................. 154

XXVI. Sections of spherulites, from Yellowstone National Park; figs. 1-7. 156

XXVII. Branching crystals of orthoclase, Manebacher twins, from Yellowstone National Park............................................ 158

XXVIII. Sections of metarhyolite from Utley, Wisconsin: A, secondary 

growth of microperthite about plagioclase in polarized light; 

B, pheuocryst of microperthite in polarized light............... 168

7
8 ILLUSTRATIONS.

Paga 

PL. XXIX. Thin section of nephelite-syenite from Litchfield, Maine, as seen

under a microscope between crossed nicols.................... 204

XXX. Thin section of dacite from Bear Creek, California, as seen under

a microscope ................................................. 214

XXXI. Nodules in dacite, Lassen Peak, California ..................... 218

XXXII. Thin section of hypersthene-andesite from Mount Shasta. California, as seen under a microscope............................ 228

XXXIII. Vesuvius in eruption, April 26, 1872 ............................ 246

XXXIV. Cinder Cone, 10 miles northeast of Lassen Peak, California..... 248

XXXV. Volcanic bombs at base of Cinder Cone, 10 miles northeast of

Lassen Peak, California...................................... 250

XXXVI Surface of lava field 10 miles northeast of Lassen Peak; latest

lava, not covered by sand ..................................... 252

XXXVII. Sections of quartz grains and surrounding shells of glass and

augite, from same locality.................................... 254

XXXVIII. Thin sectiou s as seen under a microscope: A, olivine-diabase from 

Pigeon Point, Minnesota; B, garnetiferous gabbro from Granite 

Falls, Minnesota............................................. 278

XXXIX. Thin sections.of kiinberlitefrom Elliott County, Kentucky, as

seen under a microscope...................................... 292

XL. Thiu.sectiou of marble from Cockeysville, Maryland, as seen under

a microscope: A, in ordinary light; H, bet ween crossed nicols.. 300 

XLI. Syncline and anticline ......................................... 316

XLII. Thin section of tourinaline-biotite schist from the Black Hills, 

South Dakota, as it appears under a microscope in polarized 

light ........................................................ 328

XLIII. Thin section of aporhyolite from South Mountain, Pennsylvania,

showing altered spherulites, as seen under a microscope....... 346

XLIV. Thin sections of aporhyolite: A, from Lake Superior, Minnesota, 

showing perlitic structure; B, from South Mountain, Pennsylvania, showing perlitic, spherulitic, and lithophysal structures ......................................................... 348

XLV. Thin sections: J,quartz-uorite-gneissfromOdes8a,Minuesota; B,

hornblende-gabbro-gneiss from Minnesota Falls, Minnesota.... 360

XLVI. Spheroidal weathering in granite near Woodstock, Maryland.... 386

XLVII. Irregular weathered surface of Cambrian limestone at York,

Pennsylvania .. ...................... ........................ 388

FIG. 1. Pegmatitic structure .......................... ...................... 17

2. Smeeth separating apparatus........................................ 20

3. Westphal balance ................................................... 22

4. Balsam bottle ....................................................... 25

5. Wire saw, United States Geological Survey........................... 26

6. Grinding apparatus, United States Geological Survey ................. 27

7. Section of coast, showing beach and sediments ...................... 57

8. Shells in marine sand at Carter's Grove, on James River, Virginia..... 65

9. Brecciated Devonian limestone, Fayette, Iowa ....................... 73

10. Section of vein...................................................... 94

11. Travertine fan, Mammoth Hot Springs, Yellowstone National Park... 100

12. Cells of silicified Avood, seen under a microscope ..................... 114

13. Globigerina and Textularia from the chalk of White Cliff, Texas... 116

14. Globigerina ooze, from Indian Ocean, at a depth of 1,800 fathoms .... 117

15. Fragments of volcanic glass in tuff, as seen under a microscope....... 212

16. Pyrope, showing border of biotite and magnetite.................... 293

17. Cross section of stanrolite from staurolitic-mica-schist............... 334

18. Glaciated rock surface showing frontal and lateral grooves caused by

a projecting nodule of harder rock ................................ 389
LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL STJUYEY,

Washington, D. 6\, November 5, 1897.

SIR,: I have the honor to submit herewith the manuscript'and illustrations for a bulletin to accompany the Educational Series of Bock 

Specimens.

Very respectfully,

J. S. DILLER,

Geologist. 

Hon. CHARLES D. WALCOTT,

Director United States Geological Survey.

PREFACE.

It was early perceived that the field parties of the Geological Survey 

occasionally had favorable opportunities for the collection of important 

rock types not readily accessible to others, and that, by systematic 

cooperation, these parties, without much additional expense, could in 

the course of years gather a large collection of duplicate type specimens of rocks which would be valuable to educational institutions for 

the purposes of teaching.

This collection was started in 1882, under the directorship of Maj. 

J. W. Powell, and at first contemplated 200 suites containing only 100 

specimens each of typical rocks belonging about equally to the two 

great groups, sedimentary and igneous.

To Messrs. Becker, Hague, and Emrnons was assigned the collecting 

of the crystalline rocks, while the others were to be obtained by Messrs. 

Gilbert, Powell, Chamberlin, Hague, and Kerr. The work progressed 

steadily for several years, but, in order that important types not met 

by any of the field parties in their regular work might be obtained, it 

was found necessary to assign more definitely the task of completing 

tiie series and preparing it for distribution.

Although the responsibility for the selection, numbering, and arrangement of the series rests almost wholly Avith the present writer, these 

were determined after much correspondence and conference with his 

colleagues and were controlled largely by circumstances. Although 

most of the specimens in this collection are intended to illustrate rock 

types, there are a number such as concretions (33-36), dike(57), jointing 

(103,123), and others intended to illustrate special features, and it lias 

been thought best to treat each with its kind among the classified rocks 

rather than under a separate heading.

Acknowledgments are due to nearly all the geologists of the Survey 

and to many persons outside who have generously aided in collecting 

specimens within their reach. Special acknowledgment should be 

made to Prof. C. H. Hitchcock, who collected all the specimens from 

New Hampshire.

In the preparation of this bulletin, 29 specimens have been described 

by Prof. J. P. Iddings, of the University of Chicago; 16 by Prof. W. 

S. Bayley, of Colby University, Waterville, Maine; 9 by Mr. Whitman 

Cross, of the United States Geological Survey; 5 by Mr. George P.. 

Merrill,of the United States National Museum; 5 by Prof. J. E. Wolff, of

11
12 PREFACE.

Harvard University, Cambridge, Massachusetts; 4 by Mr. George Otis 

Smith, of the United States Geological Survey; 3 by Mr. G. K. Gilbert, 

of the United States Geological Survey; 3 by Mr. Waldemar Lindgren, 

of the United States Geological Survey; 3 by Prof. 0. E. Yan Hise, of 

the State University, Madison, Wisconsin; 2 by Mr. Bailey Willis, of 

the United States Geological Survey; 2 by Mr. H. W. Turner, of the 

United States Geological Survey; 2 by Mr. Walter Harvey Weed, of 

the United States Geological Survey; 1 by Prof. E. B. Matthews, of 

the Johns Hopkins University, Baltimore, Maryland; 1 by Prof. Florence Bascom, of Bryn Mawr College, Bryn Mawr, Pennsylvania; 1 by 

Prof. L. V. Pirsson, of the the Sheffield Scientific School in Yale University, New Haven, Connecticut; and 1 by Mr. S. Weidman, of the 

University of Chicago; in all of which cases the descriptions are duly 

accredited in the text. All of the descriptions not thus accredited and 

the other portions of the bulletin have been prepared by the present 

writer, who desires to acknowledge the aid he has received from 

Microscopical Physiography of Bock-making Minerals, by J. P. Iddings 

(translation of Eosenbusch); the excellent Handbook of Eocks, by J. F. 

Kemp; Petrology for Students, by Alfred Harker; Stones for Building 

and Decoration, by G. P. Merrill; Eocks, Eock-weathering, and Soils, 

by the same author, as well as to the more complete works on petrography, especially those of Eosenbusch and Zirkel.

J. S. DILLER. 

JUNE 9, 1897.
THE EDUCATIONAL SERIES OF ROCK SPECIMENS.

By J. S. DILLER.

THE STUDY OF KOCKS.

INTRODUCTION.

A rock is a large mass of mineral matter forming an essential part of 

the earth. Granite and sandstone are rocks made up of a number of 

minerals, but in other rocks, as, for example, limestone and serpentine 

when pure, the mass is composed wholly of one mineral. In still other 

exceptional cases, like obsidian, the mineral matter may be in a wholly 

amorphous condition, like glass not made up of any definite mineral or minerals. The material of which the rock is composed may 

be loose, as sand and gravel, or cemented (lithified), as sandstone and 

conglomerate.

Eocks may be studied (1) in the field, as large masses, where their 

greater structural features and relations to other rocks are discovered; 

and (2) in the-laboratory, where only hand specimens (stones) are 

available for investigation. The science of rocks is petrology. It is the 

branch of geology that treats of the origin, mode of occurrence, struc1 - 

ture, composition, and alteration of rocks. The branch of petrology 

that embraces a knowledge of the structural (megascopic and microscopic), mineralogic, and chemical characters of rocks is petrography, 

The knowledge of rocks especially as masses of minerals, acquired 

largely by microscopic and other laboratory methods, is called by some 

litliology.

It is hoped that the lithology and petrography to be learned in 

studying this educational series of rock specimens may arouse an 

interest in the study of petrology, and also in that of geology, of which 

the former is a part.

STRUCTURAL FEATURES OF ROCKS.

Among the first features of rocks noted by an observer are those 

arising from structure. They uiay be megascopic that is, large enough 

to be seen by the unaided eye or microscopic, visible only with the aid 

of a microscope.

Fragmental or clastic, brecciated, agglomerated, conglomerated, pebbly, 

granular, cryptoclastic or compact, horny. Specimens 10,11, and 12 are 

all made of fragments and illustrate a kind of structure characteristic

13
14 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150,

of such rocks, viz, clastic. Clastic rocks composed of fragments of 

igneous rocks are said to be pyroclastlc. Specimens 79 and 99 are 

pyroclastic. Clastic or fragmented structure, as it is sometimes called, 

may be coarse or fine, according to the size of the component parts, 

and other designations may be applied to it on account of variation in 

the shape of the fragments. When the fragments are angular the 

structure is brecciated (11). If they are coarse, with occasional large 

blocks of many shapes and sizes irregularly intermingled with finer 

material, the structure is agglomerated. When the fragments are distinctly rounded pebbles the structure is conglomerated (10). Such 

pebbles intermingled with sand produce pebbly structure (12). Eock 

composed of sand, either angular or well rounded, has psammitic or 

sandstone-like structure (12-19). It is also called granular (fragmental 

granular); but this term has a wider application than the others, for 

granites (67), diorites (94), gabbros (109), and similar igneous rocks, 

whose mineral constituents form distinct, angular, often interlocking 

grains, are also granular (crystalline granular), but their fundamental 

structure is crystalline and not fragmental, as is the case with sandstone.

In specimens 21 and 22, whose fragments, although visible in thin 

sections under the microscope, are so small that they can not be seen 

by the unaided eye, the structure is cryptoclastic or compact. The 

structure of specimen 134 is more than compact; it is especially fine 

and close, with the great hardness and conchoidalfracture of flint, due 

to the induration it has experienced in contact with an igneous rock. 

Its structure is horny or flinty.

Stratified, banded, cross bedded, laminated. Most fragmental rocks 

are formed in water, and the material of which they are composed is 

stratified, i. e., arranged in layers and beds (strata), as illustrated in the 

accompanying PI. I, which represents a series of parallel horizontal 

strata consisting chiefly of sandstones. A stratum may be thick or 

thin, and may contain one or more layers. In specimen 16 an indistinct 

banded structure is due to the presence of differently colored layers in a 

larger stratum of sandstone. A similar structure is produced in igneous rocks by flowing (see below). The layers are usually parallel to 

the stratification, but in some cases, as in the lower beds of PI. I, the 

layers run diagonally across the stratum and produce cross bedding. 

The bedding may be thick and without layers or it may be thin, as in 

PI. I, and also in specimen 17, where the fine material is arranged in 

sheets so thin that it is called laminated. This structure is often well 

illustrated in deposits of sedimentary clay.

Unstratified or massive. Successive flows of lava, where they spread 

over a flat country, arrange themselves in sheets so as to show a bedded 

structure similar to that of stratified rocks, but igneous rocks generally, such as gabbros, diorites (Pi. II), granites, etc., show no such 

arrangement. They are unstratified that is, massive.

Flow structure, streaked. Rhjolites and other acid lavas are usually
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. 1

BEDDED STRUCTURE, BEDDING. AND CROSS BEDDING, OAK CANYON, ARIZONA.
DILLEB.] STRUCTURAL FEATURES OF ROCKS, 15

viscous at the time of their eruption, and occasionally preserve lines 

produced by flowing. Specimen 80 shows a streaked arrangement of its 

black glass, due to this cause. The elongated parts may be drawn out 

so as to form bands and produce banded structure. Alt structures of 

this kind are included tinder the general synonyms fluxion structure, 

fluidal structure, andjflow structure. When not visible to the unaided 

eye it may frequently be seen under the microscope in the stream-like 

arrangement of the small crystals and other material variously colored.

Porous, cavernous, cellular, pumiceous, vesicular Specimens 23 and 29 

are full of small irregular cavities, and their structure is porous. Both 

are hot-spring deposits, and the porous structure originated at the time 

the rock was formed. In specimen 153 the cavities are larger. It is 

cavernous. Similar structures occur in acid volcanic rocks. The special 

form illustrated by specimen 100 occurs most frequently in basic lavas. 

The cavities are nearly round cells or vesicles, produced by expanding 

vapor contained in the molten rock material at the time of its eruption. 

This structure is cellular. Sometimes the lava is so inflated by expanding 

vapors as to be froth-like, pumiceous, as in specimen 59. Pumice is 

usually a product of highly explosive volcanic eruptions. Yast quantities of it were thrown into the air by the great eruption of Krakatoa 

in 1883. When there are but few cells, so that they appear as separate 

vesicles, the rock is vesicular. The cells and vesicles of lava are formed 

while it is yet viscous, and if the lava afterward moves, the cells will 

be elongated in the direction of motion.

Amygdaloidal. After vesicular lavas solidify, percolating waters 

frequently deposit mineral matter of various kinds in their cavities. 

Quartz, calcite, zeolite, etc., are deposited in the cells, producing, as iu 

specimen 139, an amygdaloidal structure. The almond-shaped kernels 

filling the cells are known as amygdules.

Vitreous or glassy, perlitic, denitrified. Specimen 60 illustrates vitreous 

or glassy structure, so named on account of similarity to artificial glass. 

It is limited to igneous rocks, and results from the chilling of molten 

rock material (magma) so suddenly as not to permit an appreciable 

amount of crystallization before it becomes solid. Specimen Gl is almost 

wholly glass, and is divided by sets of more or less concentric fissures, so 

as to give it the globular (perlitic) structure characteristic of perlite. 

Some very ancient acid volcanic rocks which were originally glassy have 

by slow crystallization gradually lost their glassy character and become 

devitrijied. Specimen 336 illustrates this feature. The minerals developed are chiefly quartz and feldspar.

Crystalline,porphyritic, liolocrystalline, hypocrystalline, amorphous, Uthoidal, even-crystalline granular.-^-Iu specimen 90 the prominent white 

portions are feldspar crystals, the dark blade-shaped crystals are hornblende, and the hexagonal ones are mica. These forms result from crystallization. The rock structure produced by crystallization is crystalline, 

a feature which is common to many igneous and metamorphic rocks.
16 THE EDUCATIONAL SERIES OP EOCK SPECIMENS. [BULL. 150.

Specimen 90 is a good example of one type of crystalline rock. The feldspar, hornblende, and mica each has its own peculiar form, i. e., is idiomorphic. The gray portion of the rock appears uniform to the unaided 

eye. It is the groundmass, in which the prominent crystals (phenocrysts) 

are embedded, producing the porphyritic structure. In a thin section of 

90, it may be seen under the microscope that all the gray matter of the 

groundmass is crystallized. There is no amorphous matter (base) present; the rock is completely crystalline, i. e., liolocrystalline. Granites, 

syenites, diorites, and similar igneous rocks, as well as gneiss, micaschist, and others among the metamorphics, are holocrystalline. The 

other extreme is represented by obsidian (60), in which there may be 

no visible crystallization, its structure being amorphous. Many lavas, 

such as 62, 63, 82, 86,101, etc., contain various proportions of both crystalline and amorphous material, and their structure is said to be hypocrystalline. Specimen 62 is an acid lava having a low degree of crystallization. On account of its compact, stony character the structure is 

called lithoidal. In granite the minerals crystallized under circumstances that made them interfere with one another and prevented the 

development of perfect crystals, such as produce porphyritic structure. 

On the other hand, the mineral grains are angular, interlocking, and of 

approximately equal dimensions, so that the rock is even-crystalline 

granular. Even-crystalline granular structure is so well illustrated by 

granite (66-69) that it is often called granitic or granitoid. The various 

minerals, instead of being bounded each by its own peculiar form (idio. 

morphic), as are the phenocrysts in porphyritic rocks, have irregular 

angular forms determined by interfering crystallization; each takes the 

form imposed on it by its growing neighbors, and is allotriomorpliic. In 

specimen 90, among the phenocrysts of feldspar, hornblende, and mica 

there are occasional prominent round grains of quartz embedded in the 

gray groundmass. These grains of quartz differ froin the other phenocrysts in not being idiomorphic. They lack the crystalline form peculiar 

to quartz and are said to be anhedral.

Phanerocrystalline, cryptocrystalline, compact, aplianitic. The series 

of terms already noted designate structures arising from varying 

amounts of crystalline and amorphous material. There is still another 

series (besides coarse and fine), to designate the size of the crystallized 

grains. When the particles are sufficiently large to be visible to the 

naked eye the structure is phanerocrystalline. When the particles are 

so fine as to be visible only with the aid of the microscope the structure 

is microcrystalline or cryptocrystalline. Very fine-grained igneous rocks, 

like fraginental ones, are said to be compact. In diabases and diorites 

compact structure has been called aplianitic.

Pegmatitic, granophyric, graphic. In granite, quartz-porphyry, and 

rhyolite, quartz and feldspar sometimes intercrystallize in parallel positions so as to produce, in certain sections, a graphic appearance. The 

structure is pegmatitic, granophyric, or graphic, and is well illustrated
i. i

U. S. GEOLOGICAL SURVEY 

MASSIVE STRUCTURE IN DIORITE, SHASTA COUNTY
DILLER.] STRUCTURAL FEATURES OP ROCKS. 17

in graphic granite (fig. 1), of which there is no specimen in the collection. When the structure is so fine as to be visible only under the 

microscope, it is micropegmatitic, microgranophyric, or micrographic.

Spherulites and lithophysce. In some acid lavas there is a low grade 

of crystallization, illustrated by specimen 62, in which there is a more 

or less distinct radial arrangement of the crystalline particles and 

development of spheroidal bodies (spherulites). These are more distinct under the microscope between crossed uicols, when each is marked 

by a black cross. The hollow spherules (litliophysce) contained in specimen 62 form its most conspicuous feature. The lithophysse often show 

concentric shells, and are lined by a multitude of minute crystals. Like 

spherulites, they are found chiefly in acid lavas, especially in the 

lithoidal form of rhyolite.

Microlites or crystallites, and glol>ulites. Crystals vary in size, and may 

be so small as to be visible only under the microscope. Extremely minute 

crystals are called miorolites or crystallites. Feldspar and augite occur as 

microlites. Generally 

microlites are acicular, or rod-shaped, 

and, although crystallized and doubly 

refracting, are not 

determinable mineralogically. Besides 

these there are other 

extremely minute 

bodies which do not

pOSSeSS Crystallo- FIG. I. regmatitic structure of graphic granite. The dark areas

, . ,. -, , are quartz; the light, feldspar. The tiguro was prepared from a

graphic form ana dO specimen loauedby Mr. G. P. Merrill, of the National Museum.

not sensibly affect

polarized light. The simplest of these are round and are termed globulites. They are common in Lypocrystalline lavas, especially basalt and 

rhyolite. Branching hair-like forms are called triohites and straight 

ones belonites.

Concretionary, oolitic. The spheroidal forms lately noted are found 

in igneous rocks. Forms somewhat similar, but of different origin, are 

found in stratified rocks. Specimen 34 shows a piece of fern embedded 

in sand, which has been indurated in nodular form about the fern by 

the deposition of carbonate of iron collected from the surrounding rock. 

Such a structure, formed about a center by additions to the outside, is 

concretionary. Specimens 33 and 35 also are concretions; so is each 

of the many little round grains in specimen 30, as well as in 26 and 31. 

When, as in 20, they are abundant and so small that the mass resembles 

the roe of fish, the structure is oolitic.

Secretionary, geode. A nodule that is hollow and lined with crystals, 

as specimen 36, is a geode. It is a secretion, and is formed in a cavity 

Bull. 150 2
18 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

l)y coating its walls and then gradually making additions on the inside. 

Amygdules, as illustrated in specimen 139, are secretions and are of the 

same general character as the geode.

Shaly structure, slaty cleavage, foliated or schistose, schist, gneiss. 

Some fine-grained fragmental rocks, whether laminated (17) or not 

(21 and 22), may be much more easily split into irregular chips or plates 

parallel to the stratification than perpendicular to it, and the structure 

that renders them thus fissile, being characteristic of shales, is called 

shaly structure. Specimen 122 may be split readily into thin sheets, 

like roofing slate (122). The character that renders such splitting 

possible is called slaty cleavage, since it is a common feature of slate. 

Shaly structure is parallel to the stratification and is determined in the 

deposition of the material, but slaty cleavage may be parallel or inclined 

to the plane of stratification, and is induced by movements in the mass 

due to pressure in the process of mountain building. (See description 

of specimen 122.) Specimen 129 splits readily in one direction and yet 

it possesses neither shaly structure nor slaty cleavage, but has what 

is usually denominated foliated or schistose structure. This fissility is 

due to the parallel, more or less lenticular arrangement of foliated and 

other minerals. The plane of easiest splitting is through the sheets or 

lenses of the foliated mineral. Foliated structure is a common feature 

of metamorphic rocks, both sedimentary and igneous. When the 

foliated structure is coarse the rock is gneiss, and when fine the rock is 

schist. Formerly the term gneiss had a mineralogic significance, but 

here, following Gordon 1 and Kemp,2 the writer uses it in -a structural 

sense only. Gneiss is well illustrated in the collection by specimen 

128, which may be called conglomerate-gneiss as well as rnetamorphic 

conglomerate. Specimens 137 and 138 also illustrate gneiss, and 129, 

131, and 133 are good examples of schist.

Jointing, rhombohedral, columnar. Eocks are frequently intersected 

by divisional planes called joints, which divide them into more or less 

regular bodies. PI. Ill represents jointing in metamorphic rocks, in 

which the several systems of fissures divide the rock into rhombohedral 

bodies, illustrated by specimen 123. In igneous rocks the jointing gives 

rise to columns (PI. IV), like those of the Giant's Causeway and Fingal's 

Cave. Specimen 103 is a small column. .

METHODS OF PHYSICAL ANALYSIS.

Within the last thirty years great advances have been made in the 

study of rocks, especially in the appliances for their detailed chemical 

and physical investigation in the laboratory. Chemical analysis gives 

the sum total of the chemical elements they contain, but in most cases 

this does not disclose the mineral composition of the rocks analyzed,

1 C. H. Gordon, Bull. Geol. Soc. America, Vol. VII, p. 122.

2 J. F. Kemp, Handbook of Eocks, p. 95.
S. GEOLOGICAL SURVEY 1ULLETIN NO. 150 PL. Ill

RHOMBOhEDRAL JOINTING IN GNEISS, POTOMAC RIVER NEAR WASHINGTON, 

DISTRICT OF COLUMBIA.
DILLEK.] METHODS OF PHYSICAL ANALYSIS. 19

nor does it throw any light on their structure, which is closely related 

to their origin and history. For these we must depend on physical 

analysis.

There are three important methods of partial or complete physical 

analysis of rocks: (1) By means of heavy solutions, (2) by means of an 

electro-magnet, the rock in both cases being used in the form of small 

particles, and (3) by means of a polarizing microscope, thin sections of 

rock being used.

The first method depends for its efficiency upon a difference in the 

specific gravity of the minerals composing the rocks, and is quantitative as well as qualitative; the second depends upon the relative magnetic susceptibility of the associated minerals; and the third depends 

upon their optical properties. The last is by far the most important and 

complete method, and when practicable should be employed first, as it 

will certainly furnish information, of value in the application of the 

other methods. It is chieliy qualitative, determining the minerals and 

structural elements of rocks, but affords also a means of estimating 

approximately the quantitative relations of the minerals.

SPECIFIC-GRAVITY METHOD.

If a mixture of dry sawdust and iron filings be thrown into water 

the sawdust will float and the iron filings will sink, the two being separated by means of a liquid whose specific gravity is between those of the 

mixed substances. Minerals of which rocks are composed differ in 

specific gravity, and by employing heavy liquids of intermediate specific gravity the minerals may be separated. The best liquids for this 

purpose readily mix with water, so that their specific gravity can be 

conveniently changed by the addition or evaporation of water.

Preparation of the material. The rock to be investigated should be 

pulverized by stamping (not grinding) in a mortar. To separate the 

grains and dust a series of sieves or bolting cloths may be used. Those 

found most generally useful are bolting cloths Nos. 10 and 16. The 

grains which go through No. 10 and not through No. 16 are usually of 

the best size for separation in the heavy liquid, and, having been rubbed, 

are clean and free from dust.

Solutions most commonly used. Thoulet brought into general use a 

solution of potassium iodide and mercuric iodide mixed in the proportions of KI to HgI 2 as 1 to 1.24. With this solution a specific gravity 

of 3.196 may be obtained. It is readily miscible with water, and as 

easily evaporated, so that its specific gravity may be conveniently 

changed as desired.

Cadmium borotungstate, barium mercuric iodide, and methyl iodide 

have been used, but the separation of minerals having high specific 

gravity is most easily attained by means of a solution prepared by 

mixing nitrates of silver and thallium in the proportion of 1 to 1. 

For the methods of preparing the solutions and descriptions of the
20 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

apparatus used, as well as its manipulation in making the separation, 

reference may be made to the following works:

Mikroskopische Physiographie der Mineralien und Gesteine, I. Band 

(die petrographisch wichtigen Mineralien), by EL Eosenbusch, third edition, pp. 226-250.

Translation of the above, Vol. I, by J. P. Iddings (third edition, 1893), 

pp. 98-107.

Lehrbuch der Petrographie, by F. Zirkel, second edition, Vol. I, pp. 

94-108.

American Journal of Science, December, 1895, third series, Vol. L, 

p;446.

To convey a more definite idea of this method of analysis, a description may be here inserted of the Smeeth separating apparatus, which 

is illustrated in fig. 2.

This consists of a wineglass-shaped base (a), with a hollow standard, 

a tube (b) to contain the heavy liquid in which the separation takes 

place, and stoppers (c and d) to close the upper and 

lower ends of the tube.

All of these separate parts have ground fittings, 

so as to be water-tight. To make a separation, 

place the tube b in a, as in the figure, but with 

the stoppers c and d removed. Fill the tube b 

about three quarters full of the heavy solution; 

add the rock powder, then the stopper c, and after 

the tube has been shaken so as to thoroughly wet 

the powder allow it to stand for separation. The 

particles heavier than the liquid will sink to the 

bottom and those lighter will rise to the surface. 

On account of adhesion among the particles some 

of the heavier ones will be lifted to the top, while 

lighter ones will be pulled down to the bottom. 

These may be liberated by stirring both portions 

at the same time with a small glass rod inserted 

from above. By repeating the stirring several times 

it is possible to secure a complete separation. 

It will be readily seen that by inserting the stopper d the tube 6, 

with its contents of heavy liquid and light material floating on top, 

may be removed and the heavy material washed out of a for study.

The tube & may be again inserted in a and the stoppers removed, 

allowing the heavy liquid to fill the standard of a. Distilled water 

may be added to the liquid to lower its specific gravity, so that the 

next heavier particles will sink and separate as before. Thus by successive additions of small quantities of water a powdered rock composed of many minerals differing in specific gravity may be separated 

iuto its constituents. 

In making mineral separations it is especially desirable to discover

FIG. 2. Smeeth separating apparatus.

,- 
U. S. GEOLOGICAL SURVE

COLUMNAR JOINTING, WATCHUNG MOUNTAIN, ORANGE, NEW JERSEY.
DILLER.) METHODS OF PHYSICAL ANALYSIS. 21

as closely as possible the specific gravity of the mineral separated, and 

this may be accomplished approximately by determining the specific 

gravity.of the solution at the time of separation. There are several 

ways of doing this: (1) By means of indicators of known specific 

gravity; (2) by using the Westphal balance; or (3) by using Solomon's 

apparatus for determining the specific gravity of liquids. Only the 

first two of these need be noticed more fully here.

With a series of indicators, the specific gravity of which has been 

accurately determined, the specific gravity of the liquid may be found 

by placing the indicators in the liquid. These indicators may be suitable fragments of minerals, or other specially prepared material. Those 

heavier than the liquid will sink; the others will rise; if one has the 

same specific gravity as the liquid it will remain suspended and thus 

indicate the specific gravity of the liquid. If several such indicators 

are placed in the tube with the powder to be separated, the specific 

gravity of the material may be approximately determined by comparison at the time of separation.

A simple but efficient series of indicators has been devised in the 

petrographic laboratory of Harvard University, under the direction of 

Prof. J.E.Wolff. To prepare them, seal up by melting at one end a number of short lengths of small glass tubes; put in each a certain amount 

of mercury or lead,, so that they will form a regular series increasing in 

weight from the lightest to the heaviest; seal up the other end of each, 

leaving a short piece of platinum wire projecting to form a loop or hook 

by which the indicator can easily be removed from the heavy liquid. It 

is best to make a large number of these at once, and then by successive 

dilutions of the heavy solution to determine the specific gravity of the 

series by using the Westphal balance. Thus solutions are obtained in 

which the minerals remain exactly suspended, and by measuring with 

the balance the specific gravity of the solution, that of the minerals is 

determined.

The Westphal balance, made by G. Westphal, of Cella, province of 

Hanover, Germany, is illustrated in fig. 3. It consists of a base with 

an upright hollow cylinder a, in which slides the rod fo, controlled by 

the screw c. On I is supported the frame d, which at e forms the fulcrum of the beam /. The base may be leveled by the screw g, and when 

& is vertical and the two points at li are together the beam/is horizontal. From the hook k hangs the sinker I. The liquid whose specific 

gravity is to be determined is put in a glass vessel m, and the specific 

gravity is weighed directly by a decimal series of weights and riders hung 

at Tc or upon the dividing beam/between e and If. The specific gravity 

indicated by the riders in the figure, that at fc having the value 2, is 

2.915.

As the particles of different minerals in the same rock vary greatly 

in size, structure, and hardness, it is not to be expected that all of them 

will be present in the powder prepared for separation in the same pro-
22 THE EDUCATIONAL SERIES OF .ROCK SPECIMENS. [BULL. 150.

portion as in the original rock. In fact, some of them may disappear 

entirely in the dust while the powder is preparing for separation. 

Upon examination of the powder it is generally found also that many 

of the grains, instead of being homogeneous, are composed of more 

than one mineral. Furthermore, different minerals may have the same 

specific gravity. For these reasons it is not possible by using heavy 

liquids to make accurate mineral analyses of rocks, but only a rough 

approximation of their mineral composition can be attained. This 

method, however, affords a most convenient means of obtaining separately considerable quantities of particular rock-forming minerals for 

study. 

Thin laminated minerals may be separated from others by allowing

"l\l " l " " * *O

a.

1

FIG. 3. Westphal balance.

the rock powder to glide gently over paper, or to fall upon the moistened 

sides of a glass funnel. On account of their shape they sink slowly in 

water, and may be separated to some extent by vigorously stirring the 

powder in water and, after allowing it to stand a few moments, decanting quickly.

ELECTRO-MAGNETIC METHOD.

Magnetite may readily be removed from rock powder prepared as 

already indicated, by passing an ordinary magnet closely over the 

powder spread out upon paper. With a much stronger magnet an 

electro-magnet a number of other minerals, such as amphibole, pyroxene, olivine, epidote, garnet, etc., having considerable iron in their
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. V

PETROGRAPHIC MICROSCOPE.
DILLKB.] METHODS OF PHYSICAL ANALYSIS. 23

composition, may be lifted from the paper and thus separated from 

feldspar, quartz, and other nonmagnetic minerals. By varying the 

magnetic moment of the instrument the iron-bearing minerals may be 

more or less completely separated from one another.

MICROSCOPIC METHOD.

As minerals differ among themselves in the forms of their crystals, 

so also they differ in their effect on light which passes through them. 

The optical properties of minerals are directly dependent on crystallographic relations, and afford especially valuable data for their determination in rocks.

Petrographic microscopes are arranged with reference to studying 

rocks by transmitted ordinary and polarized light so that the optical 

properties of their component minerals may be discovered.

In the accompanying cut (PL V) is shown such a microscope, 1 a 

description of which is here given.

The screw supporting the arm between the pillars allows the instrument to bo 

inclined at any angle. The main tube is provided with a cloth lining, into which 

the drawtubo carrying the ocular is fitted. There is a coarse adjustment by rack 

and pinion, and a fine adjustment by a micrometer screw. The mirror is both flat 

and concave and the mirror-bar is adjustable.

Coining now to the peculiarly petrographical features, we have the lower nicolprism, or polarizer, inclosed in a cylindrical metal box, both ends of which are protected by glass. This box is capable of a complete revolution, and is provided with 

a graduated silvered circle and index. It is held by a cylindrical frame, in which 

it may be raised or depressed at will by a rack-aud-pinion movement. This frame 

is attached to the under side of the stage by a swinging #rm, so that the whole polarizing apparatus may be thrown to one side if desired. A strong compound lens may 

be screwed upon the upper end of the polarizer whenever strong illumination or 

converged polarized light is needed.

The circular stage (9.5 cm. in diameter) is provided with a beveled silvered edge, 

graduated to degrees. Upon this is mounted, for smooth and concentric revolution, 

the admirable mechanical stage known in the manufacturers' catalogue as No. 1052. 

This carries an index for reading the graduated circle, and is also provided with 

silvered graduations for its two rectangnlar movements, whereby any point within 

a section can be readily located. The upper sliding bar, which carries the object, 

has been shortened, so as to be flush with the revolving stage only when pushed to 

its extreme limit on either side. With this, square or short rectangnlar glasses must 

be usod for mounting, which will avoid any interference with the revolution of the 

stage.

Into the nose piece, just above the objective, is an opening, intended to receive 

the four following accessories, each mounted in a separate brass frame: (1) A Bertram! lens for magnifying the interference figures; (2) a quarter undulation mica 

plate; (3) a quartz wedge; (4) a Klein quartz plate or a gypsum plate, with red of 

the first order.

The centering of the various objectives is secured by two screws, having motions 

at right angles to each other.

The upper nicol-prism, or analyzer, is inserted in tho tube, in order to avoid the 

diminishing of the size of the field, which is unavoidable when the prism is placed 

over the ocular as a cap. To accomplish this, and at the same time to keep the

1 Manufactured l>y the Bausch &. Loinb Optical Company, Rochester, New York.
24 THE EDUCATIONAL SEKIES OF ROCK SPECIMENS. [BULL. 150.

tube dust-tight, the nicol is inclosed in one side of a double-chambered box. The 

other side is left vacant, and the box may be slid to and fro as ordinary or polarized 

light is desired. A metal sheath protects this box from above. 1

For the microscopic study of rocks the material is best prepared in 

the form of thin sections.

Preparation of thin sections. For the preparation of thin sections of 

rock, a thin but strong chip about as large as a 25-cent piece is selected. 

The chip is held in the hand and ground smooth on one side. The 

grinding may be done on a fine sandstone, emery stone, whetstone, or 

iron plate on which emery, first coarse (No. 70) and finally fine (flour), 

is used to make the surface perfectly smooth. It is much more convenient, however, and less laborious to grind on a revolving iron plate 

mounted on the top of a vertical axis to which power is applied by the 

foot or from some other source.

After making a smooth plane surface as large as the chip will permit, 

cement it with Canada balsam to a piece of heavy glass about 1 inch 

square in the following manner. Place the square glasses and ground 

chips on a small, gently heated brass table. When the chips are 

thoroughly dry, place on the glasses some balsam, hardened by evaporating part of the turpentine, and press the flat side of the chips on 

the glasses, so as to remove all bubbles of air. When cooled, the chips 

should adhere firmly to the glass.

A cement which is preferred by some to Canada balsam is obtained 

by slowly melting together a mixture of 16 parts by weight of viscous 

Canada balsam and 50 parts of shellac and keeping them heated for 

some time. The mass, as it completely cools, may be drawn out in 

strings and rolled between the hands into convenient rods about 1 cm. 

thick and 20 to 30 cm. long.

Holding the chip by means of the thick glass, after mounting as 

above described, grind down its other side carefully until the rock shows 

signs of breaking away. The glass should be thick, so as not to bend, 

and should be pressed evenly, so that the section may be ground, 

uniformly throughout. Finer emery (No. 120) should be used toward 

the last, on a perfectly flat iron plate, and for the final grinding the 

finest washed emery (No. FFFF), on a smooth glass plate, until the 

section is completely transparent.

The extra balsam around the section should be removed with a warm 

knife blade and the section thoroughly cleaned by washing it first 

in alcohol with a soft camel's hair brush, and finally with clean water. 

It is then put, section upward, in a shallow bath of turpentine for some 

hours, until the balsam is dissolved and the thin section completely 

loosened from the thick glass. After removal from the bath it is carefully washed again, to prepare it for transfer to a thinner glass. The 

thoroughly cleaned object-glass on which the section is to be finally

"Copied from a paper on a new petrographic microscope of American manufacture, by the late 

Prof. G, H. Williams: Am. Jour. Sci., Feb., 1888, 3d series, vol. XXXV, pp. 114-117. 
DILLKB.] METHODS OF PHYSICAL ANALYSIS. 25

mounted is placed on a small table as before, and a drop of balsam is 

placed on it from the bottle (fig. 4) by pressing the rubber bulb. The 

sections are then carefully transferred by sliding them with mounted 

needles from the thick glass to the drop of balsam. The balsam, heated 

by a lamp beneath the table, becomes more liquid, and the section is 

manipulated by the mounted needles until all parts of it above and 

below are covered with balsam and all bubbles are removed. Bubbles 

are easily removed by pricking them with a heated needle. When the 

balsam has been evaporated sufficiently, a bit of it removed by a mounting needle will not adhere to the thumb-nail if applied while yet fluent. 

It must be heated gently and not too long, else it will become too 

brittle. The thin cover glasses, having been thoroughly cleaned with 

alcohol and water, are then x>ut on the section and pressed down gently, 

so as to squeeze out the 

superfluous balsam, 

which, after cooling, is re 

moved and the cover see 

tion cleaned as before.

Oriented and other sections are often desired of 

rocks which may not be 

chipped. In such cases, 

to obtain a slice for grinding, it is necessary to have 

a rock saw. An apparatus combining both saw 

and grinding disk with 

motive power was devised 

by the late Prof. George H. Williams. It contains a storage battery 

and an electric motor, a grinding disk, an emery wheel, and a diamond 

saw.' Diamond saws may be made by taking a flat circular sheet-tin 

(tinned iron) disk 6 or 8 inches in diameter and placing it in a vise 

between flat, wooden disks, allowing the tin. to project one-fourth of an 

inch all around. Place a knife-edge across that of the tin disk at an 

angle of about 15 to 20 degrees and strike it a blow, so as to cut a 

nick an eighth of an inch deep into the edge of the disk. Eepeat this 

operation until the entire periphery of the tin disk has been notched 

or nicked, the nicks to be about one-tenth of an inch apart, cut in 

directions which may vary in different quadrants of the disk. While 

nicks in the same quadrant of the disk's edge should be parallel, those 

of -adjacent quadrants should be nearly perpendicular to each other. 

Mix diamond dust or bort with a little oil to make it adhere, and fill 

the nicks, using a small pointed stick. Lay the edge of the tin disk on 

a flat iron and hammer gently on the sides along the edge, so as to

Fio. 4. Balsam bottle.

Tor a complete description of this machine by Professor Williams, see Am. Jour. Sci., February, 

1893,3d series, Vol. XLV, p. 102.
26 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

close the nicks and hold the diamond dust. . If the dust is deeply 

buried in the nick the saw will not cut at first.

A very serviceable saw may be made of annealed iron wire, as illustrated in fig. 5. c is an endless wire (gauge 16) running over two 

pulleys, a and 6, 18 inches in diameter. To the lower pulley, &, the 

power is applied by a shaft. These pulleys at work should make about 

250 revolutions a minute. The upper one is pulled up on a sliding 

frame by the weight and cord #, to keep the wire stretched. The ends of 

the wire must be carefully soldered together and filed smooth. Water

should be dropped by the tube i 

where the specimen to be cut is 

pressed against the wire, which may 

be supplied with emery by the fingers of the operator. The wire 

may be used as a scroll saw and a 

slice may be thus cut out of the 

middle of the specimen. This saw 

has been successfully used in the 

petrographic laboratory of the 

United States Geological Survey 

for the last seven years.

In the same laboratory the grinding apparatus represented in fig. 6 

has been found equally successful. 

The cast-iron frame a carries the 

steel spindle fc, to which is screwed 

on top the cast-iron plate c, 12£ 

inches in diameter. The plate revolves when at work about 1,200 

times a minute, in a box, d d, and is 

underlain by a block of same size, 

e, to prevent emery from getting to 

the spindle. Separate plates may 

be kept for coarse and fine grinding, 

and a buff wheel may be fitted to 

the same spindle for polishing. For 

grinding and polishing large specimens for exhibition a larger plate may be used.

Soft, porous material may be hardened by heating it in Canada 

balsam, and when cooled a section may be made as already indicated.

Bock powder may in many cases be most conveniently studied by 

mounting it between a glass slide and cover in a drop of water. To 

study the internal structure and optical properties, one must avoid thetotal reflection from the surface of the particles, and it is better to 

mount the powder, which should be of nearly uniform size, in a substance whose index of refraction is nearly the same as that of the 

powdered mineral. For this purpose glycerin (n 1.46), Canada

FIQ. 5. Wire saw. United States Geological 

Survey, a, b, pulleys with V groove; c, endless 

iron wire; d, frame; e, guide; /, bolts, threefourths inch; g, tension; ft, clamp; i, water 

tube.
DILLEK.] METHODS OF PHYSICAL ANALYSIS. 27

balsam (n = 1.54), and a concentrated .solution of iodide of potassium 

and mercury (n = 1.73) have been used, but for preservation Canada 

balsam is preferred. To secure a uniform distribution of the powder 

in the balsam, put a thin film of solid balsam on the central portion of 

the slide and spread the powder properly on it, then upon heating it 

slightly the powder \v\\l adhere. It can then be covered with balsam 

dissolved in ether or chloroform and furnished with a glass cover.

Three of the methods of physical analysis by means of which rocks 

may be separated more or less completely into their mineral components have been given in the preceding pages. In applying the first 

and second of these methods, the observer discovers some of the 

important properties of the minerals by which they are distinguished 

from one another, but their final 

determination generally depends

largely upon the microscopic and

optical study of the separate 

grains.

The third method, the microscopic method of rock analysis, is 

by far the most comprehensive, and 

affords the most valuable means of 

discovering not only the mineral 

composition but also the structure 

of rocks, and throws much light 

upon their genesis and history. To 

apply it successfully it is necessary to know something of minerals, 

especially of their optical proper 

ties. In the succeeding chapter are 

given brief descriptions of the minerals which play a part, more or less 

important, in any of the rocks of the series. Only brief mention is 

made of their optical properties.

Full aud systematic accounts of rock-making minerals may be found 

in the works of Dana,1 Eosenbusch,2 Zirkel,3 and Fouqu<§ and Michel 

Levy.4

For the convenience of English and American students an excellent 

translation of Roseubusch has been made by Prof. J. P. Iddings/'

THE PRINCIPAL ROCK-MAKING MINERALS.

Although there are over 800 species of minerals, there are only about 

100 which enter, to any considerable extent, into the composition of

'System of Mineralogy of J. D. Dana, 6th ed., by Edward S. Dana; 2Tew York, 1892. 

2 Mikroskopische Physiographic der Mineralien uud Gestoine, von H. Rosenbnsch; Band I: Die 

petrographischSvichtigen Miueralieii; 1892. 

"Lehrbuch der Petrographio, voii Dr. Ferdinand Zirkel; Vol. I; Leipzig, 1893.

*Min6ralogie inicrographique, Koches eruptives francaises, par Fouqu6 ot A. Michel L6vy; 1879.

 Microscopical Physiography of the Rock-making Minerals; An Aid in tho Study of Rocks, by 

H. Rosenhusch; translated aud abridged for Use ill Schools and Colleges by Joaeph P. Iddiugs; Third 

Edition. 1893.

FIG. C. Grinding apparatus, TJiiited States Geological Survey.
28 THE EDUCATIONAL SERIES OF ROCK SPECIMENS.

rocks, and scarcely a score of them are of prime importance as rock 

constituents. Bock-making minerals have been variously classified 

according to their conditions and the part they play in the constitution 

and history of rocks.

'those minerals whose presence is necessarily implied in the definition of the rock are essential minerals, while the others present are 

nonessential or accessory. This is well illustrated by granite, of which 

the essential minerals are quartz, orthoclase, and a ferromagnesian silicate. They are always present, otherwise the rock would not be 

granite. Apatite, magnetite, titariite, and a number of other minerals 

are accessory in granite, because they may be present or absent without affecting the kind of rock.

Minerals present in the rock at the time of its origin are original 

or primary minerals. The others now present are secondary minerals, 

in that they originated in the rock after it was formed. Secondary 

minerals may originate within a rock either from the alteration of 

primary minerals or from the accession of new material from without. 

Orthoclase, which is an original constituent of granite, is frequently 

altered to muscovite, quartz, and kaolin, all of which, when so derived, 

are secondary minerals. A granite may contain at the same time not 

only the essential aud original quartz and muscovite, but also some 

secondary quartz and muscovite which have resulted from the alteration of the feldspar.

The mineral matter of rocks is in one of two conditions, either amorplious or crystalline. The amorphous condition arises in the solidification of matter from a molten or gelatinous state under such circumstances as prevent its crystallization. In this way glasses and opals 

are produced.

The degree of crystallization in solidification from a molten condition

depends chiefly oil the rate of cooling. If very sudden, practically all

the material becomes glass, but in other cases, where the cooling is 

slower, some of the matter crystallizes, and the amount that becomes 

glass grows less and less as the rate of cooling decreases. It follows 

that, if allowed to cool slowly enough, all the mineral matter will crystallize and the rock will become holocrystalline. While many lavas, 

like pumice (59), obsidfen (60), and perlite (61), are glassy, and others, 

like nevadite (64) and dolerite (105), are holocrystalliue, a large number of volcanic rocks, like many basalts, andesites, and rhyolites, contain more or less glassy material. The chemical composition of the 

glassy material found in rocks varies greatly, not only in its original 

constitution, but in proportion to the degree of crystallization in the 

solidification of the lava. Under the microscope between crossed nicols 

glass is seen to be isotropic and generally to contain minute but more or 

less distinct crystals.

Opal is hydrated silica; composition, SiO2+aq. Like glass, it is 

wholly isotropic, unless affected by internal strains which give it anom-
DILLBB.] THE PRINCIPAL ROCK-MAKING MINERALS. 29

alous double refraction. Hardness, 5.5 to 6; specific gravity, 2.2 infusible, and so distinguishable from glass, which is fusible. It forms irregular patches, strings, and veins, as well as pseudomorphs after various 

minerals, in altered acid eruptive rocks. Its most important occurrence 

is in siliceous sinter deposited by many hot springs and geysers (23), and 

in silicified wood (37).

In the folio wing list are given the minerals commonly found in rocks, 

but it must not be supposed that all are equally important and abundant. The most important mineral of unaltered sedimentary rocks of 

mechanical origin is quartz, on account of its ability to withstand attrition, as well as its resistance to solvents, both chemical and organic. 

Calcite and dolomite, although of no importance among sedimentary 

rocks of mechanical origin, are of greater importance than quartz in the 

unaltered sedimentary rocks of chemical and organic origin. Among 

the igneous and metainorphic rocks, quart/,, the feldspars, nephelite, 

leucito, the ainphiboles, the micas, the pyroxeues, and oliviue are the 

most important rock-making minerals, and are generally regarded as 

of prime significance in their classification. Magnetite, zircon, and 

titanite are of somewhat less importance, while the scapolites, garnet, 

tourmaline, topaz, epidote, andalusite, cyanite, sillimanite, staurolite, 

chlorite, talc, and serpentine are of nearly equal importance.

MINERALS ILLUSTRATED BY THIS SERIES OF ROCK SPECIMENS.

The minerals of the succeeding list are arranged according to their 

systems of crystallization, which bring together generally those having 

similar optical and other properties.

Isometric system.

Pyrite. Fluorite. Hailynite.

Magnetite. Garnet. Noselite.

Chromite. Leucite. Analcite.

Spinel. Socialite. Perofskite.

Tetragonal system. 

Rutile. Zircon.

Hexagonal system.

Graphite. Quartz. Apatite.

Pyrrhotite. Tridymite. Nephelite.

Hematite. Calcite. Cancrinite.

Ilmenite-leucoxene. Dolomite. Tourmaline.

OrtliorhomMc system.

Marcasite. Pyroxenes (orthorhombic). Phrenite,. 

Andalusite and Cliiastolite. Olivine. Talc. 

Topaz. Fayalite. 

Staurolite. Zoisite.
30 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Gypsum.

Pyroxenes (mcmoclinic).

Amphiboles.

Micas.

Feldspars (triclinic).

Serpentine. 

Kaolin.

Monoclinic system.

Chlorite. 

Epidote. 

Allanite. 

Titanite.

Zeolites.

Feldspars (inonoclinic).

Triclinic system. 

Microcline. Plagioclase.

Homogeneous aggregates. 

Glauconite. Limonite.

MINERALS OF THE ISOMETRIC SYSTEM.

Pyrite. Bisulphide of iron. Composition, FeS2 . Isometric. When 

well crystallized, usually appears in the form of cubes or pentagonal 

dodecahedrons. Fine-granular, sometimes radially fibrous, forming 

nodules. Brittle ; hardness, 6 to. 6.5. 1 Specific gravity, 4.8 to 5.2. 

Luster, metallic; color, pale brass-yellow. Streak, grayish black. 

Opaque. Before the blowpipe on charcoal or in a closed tube gives 

off sulphurous odors and becomes magnetic. Often occurs as crystals and grains, rarely in large masses. In many rocks, both sedimentary and igneous, but is always an accessory constituent. It is readily distinguished by its color, hardness, and chemical reaction. It 

occurs as small grains, generally microscopic, in minette (91), camptonite (92), diorite (93), garnetiferous.gabbro (109), and hornblende-schist 

(131). Marcasite (page 37) is essentially like pyrite in all respects 

excepting crystallization and its easier alterability. Instead of being 

isometric, it is orthorhombic.

Magnetite. Magnetic oxide of iron. Composition, Fe3O4 . Isometric, 

octahedrons common, dodecahedrons less common, and generally occurs 

in irregular grains. When massive, often granular. Hardness, 5.5 to 

6.5. Specific gravity, 4.9 to 5.2. Color, iron black. Streak, black. 

Opaque. Strongly magnetic. Magnetite occurs occasionally in large 

deposits, and is mined as a valuable ore of iron. It is one of the most 

widely distributed accessory constituents of igneous and inetarnorphic 

rocks, in which it occurs as microscopic crystals and grains. Being 

strongly magnetic it is readily recognized. It forms the whole of 

specimen 146, but in most of the other metamorphic and igneous rocks 

it can not be seen without the aid of the microscope unless the rock is 

powdered and brought under the influence of a magnet, when the little

'The scale of hardness by Mohs and that of fusibility by Von Kobell, used in these descriptions, are 

as follows:

Hardness. 1. Talc. 2. Gypsum. 3. Calcite. 4. Fluorite. 5. Apatite. 6. Feldspar. 7. Quartz. 

8. Topaz. 9. Corundum. 10. Diamond.

Fusibility. 1. Stibuite: fusible in the flame of a candle in large fragments; 2. .Natrolite: fusible 

in the flame of a candle in small fragments; 3. Almandite garnet: infusible in the flame of a candle, 

but easily fusible before a blowpipe, even in somewhat large pieces; 4. Green actinolite: fusible before 

the blowpipe in rather fine splinters; 5. Orthoclase: fusible before the blowpipe in finer splinters; 

6. Bronzite: before the blowpipe becomes rounded only on the finest points and thinnest edges.
DILLEH.] THE PRINCIPAL ROCK-MAKING MINERALS. 31

grains of magnetite will leap forth. Grains of magnetite resemble 

chromite and ihnenite, but may be distinguished from them by the 

strong magnetism and the lack of definite blowpipe reaction showing 

the presence of chromium or titanium.

Chromite. Isometric, generally in regular octahedrons. Composition, FeCr2O4 . Color, black, but in thin sections by transmitted light, 

brown. Hardness, 5.5. Specific gravity, 4.3 to 4.5. It commonly 

occurs iu serpentine and olivine and is closely related to picotite, from 

which it differs chiefly iu the amount of chromium it contains. It 

occurs in the olivine of saxonite (114) and in serpentine (145).

Spinels. The spinels are aluminous minerals whose crystals are 

regular octahedrons. The common varieties are given in the table:

The common varieties of spinel.

Spinel ...........

Formula.

MgAlA-- 

(MgFeMAlFe)A- - 

FoAlA .............--.....-.:.

(FoMg)0( AlCr) A ---....--....

Color.

Kod...... .............

Specific 

gravity.

3. 5-4. 1

3.9

4.0

H.

8 

8

7.5-8

8

The first two occur most commonly in gneiss, while the last, occurring 

in eruptive rocks, is generally enveloped by olivine. All the spinels 

are accessory minerals, usually small, but unchangeable, for they 

remain perfectly fresh in rocks whose other minerals are decomposed. 

Although the spinels occur in many of the rocks of the collection, they 

are most common in the oliviue of the basalts and peridotites, but are 

visible only iu thin sections with the aid of the microscrope. Picotite 

occurs in the oliviue of quartz-basalt (101) and olivine nodules (104), 

and pleonaste has been observed in cortlandite (113).

Fluorite. Calcium fluoride. Composition, CaE2. Isometric crystals, 

usually cubical. Hardness, 4. Specific gravity, 3.1 to 3.25. Luster, 

vitreous. Color, various, usually bright, making pretty cabinet specimens. Occurs generally in veins, and rarely in small particles as an 

accessory constituent of rocks, as in the biotite-granite (67) of Platte 

Canyon, Colorado.

Garnets. Silicates of aluminuniyiron, manganese,chromium, calcium, 

and magnesium. The principal varieties are given in.the table:

The principal varieties of garnet.

Groasularito ....... 

Almandite.........

Spesaartite ........ 

Pyrope ............

Formula.

Ca3Al2Si3012 .. ........................ 

Fe3Al2Si3O l2 .......................... 

Ca3Fe2Si30| 2 .. ........................

Mn3Al2Si30|2 . . ........................ 

Mg3Al2Si30,j... .......................

Color.

Pale green or yellow. . . 

Hod...... .............. 

Black..................

lowish red. 

Red or yellow.......... 

Blood-red..............

Specific 

gravity.

3. 4-3. 75 

3. 5-4. 3 

3. 6-4. 3

3. 7-4. 3 

3. 7-3. 8
32 THE EDUCATIONAL SERIES OF ROCK SPECIMENS.

Isometric crystals, usually rhombic dodecahedrons. Hardness, 6.5 to 

7.5. Luster, vitreous-resinous. Transparent. Sometimes massive, forming garnet rock. As an accessory constituent it occurs usually in crystals in altered rocks, both eruptive and sedimentary. In the collection, 

crystals of common garnet appear in garnetiferous gabbro (109), micaschist (130), and garnetiferous hornblende-schist (141). Pyrope appears 

in kimberlite (112).

Leucite. Silicate of aluminum and potassium. Composition, KA1- 

Si2O6 . Probably isometric, with the characteristic roundish crystals 

icositetrahedral, generally in grains, rarely massive, granular. Hardness, 5.5 to 6. Specific gravity, 2.4 to 2.5. Luster, vitreous. Color, white 

or gray. Translucent to opaque. Infusible, but decomposed without 

gelatin ization by hydrochloric acid. Resembles analcite, but distinguished by infusibility and greater hardness.

In thin sections under the microscope leucite is polyhedral or round. 

Small crystals are practically isotropic, but larger ones show complicated twinning lamellae between crossed nicols, and frequently contain 

numerous inclusions arranged in zones. Leucite is known only in 

igneous rocks, chiefly in volcanics. It occurs in orendite (72), where,, 

although abundant, it can be seen only in the thin section with the 

microscope.

Socialite. Chlorosilicate of aluminum and sodium. Composition, 

Na4Al3ClSi3OJ2 . Isometric. Crystals dodecahedral or octahedral. 

Hardness, 5.5 to 6. Specific gravity, 2.1 to 2.3. Luster, vitreous. Color, 

blue, greenish, or gray. Easily and completely soluble in hydrochloric 

acid. On standing, gelatinous silica separates out, and when the solution is evaporated crystals of common salt appear. Heated in a closed 

tube, blue varieties become white. Before the blowpipe, at 3.5 they 

fuse, with intumescence, to a colorless glass.

Under the microscope, cross sections of its crystals are quadratic or 

hexagonal, and appear isotropic, with a low index of refraction. Sodalite occurs chiefly in granular-crystalline igneous rocks of the nepheliteleucite series. It occurs in some parts of pulaskite (74), and also in the 

uephelite-syenite (77) of Litchfield, Maine, where its small blue grains 

may be seen occasionally in a hand specimen.

Haiiynite and noselite. Sulphure to-silicate of aluminum, sodium, calcium. Composition, an isomorphous mixture of NasAl^SO^SiaO^ 

and Na3CaAl3 (S04 )Si3O12 . The members rich in sodium are called 

noselite, while those rich in calcium are called haiiynite. Isometric 

crystals as in sodalite. Hardness, 5.5 to 6. Specific gravity, 2..3 to 

2.5. Color, blue, gray, red, or various. Heated in a closed tube, unlike 

sodalite it retains its color, but before the blowpipe at 4.5 it fuses to a 

white glass.

Both haiiynite and noselite gelatinize easily in hydrochloric acid, and 

on evaporation abundant needles of gypsum are found in the case of 

haiiynite, but few or none in the case of noselite. JSToselite in some
DILLBR.] . THE PRINCIPAL ROCK-MAKING MINERALS. 33

rocks has a characteristic dark border. Haiiynite and uoselite are frequently full of inclusions. Haiiynite occurs in the theralite (75) of 

Gordons Butte, Montana.

Analcite. Hydrous silicate of aluminum and sodium. Composition, 

NaAlSi2O(iH2O. It is one of the zeolites, and, although sometimes a 

primary constituent, in this series of rocks it is always a secondary mineral, frequently resulting from the alteration of uephelite or leucite. 

It resembles leucite, and may be distinguished by fusing at 2.5 and 

gelatinizing in hydrochloric acid. It occurs in the grouudmass of porphyritic theralite (76), and possibly also in camptonite (92).

Perofslcite. Calcium titanate. Composition, CaTiO3 . Isometric. 

Crystals generally very small octahedrons or rounded grains. Hardness, 5.5. Specific gravity, 4.1 to 4.4. Luster, metallic. Grayish 

yellow or brown. Has high index of refraction and anomalous double 

refraction. It frequently occurs in melilite, nephelite, and leucite 

rocks, and also in some peridotites. It occurs abundantly in kimberlite 

(112)j but can be seen only with the aid of a microscope.

MINERALS OF THE TETKAGONAL SYSTEM.

Rutile. Titanic oxide. Composition, TiO2. Tetragonal crystals, 

generally prismatic but sometimes in grains, and also extremely 

minute microscopic crystals in certain schists and slates. Hardness, 

G to G.5. Specific gravity, 4.19 to 4.25. Luster, metallic, adamantine. 

Color, yellow, brownish, or red. High index of refraction. Not affected 

by acids. It is often an original accessory mineral, but in other cases 

it appears to be secondary from titauite or ilmeuite. Kutile is common 

in gneisses, mica-schist, and rocks rich in hornblende and augite. The 

hairlike crystals often seen in quartz are supposed to be rutile. It 

occurs in the quartzite of Eureka, Nevada (118), the phyllite of Ladiesbnrg, Maryland (126), and the schistose biotite-gneiss of Manhattan 

Island (132).

Zircon. Silicate of zirconium. Composition, ZrSiO4 . Tetragonal. 

Crystals usually small, square prisms, ending in pyramids. Hardness, 

7.5. Specific gravity, 4.05 to 4.75. Colorless to yellow, pink, violet, or 

brownish. Luster, adamantine, translucent. Double refraction strong, 

with high index of refraction. Infusible, not affected by acids excepting by concentrated sulphuric acid when the mineral is in powdered 

form.

Zircon is one of the most widely distributed original accessory minerals in many igneous rocks as well as metamorphosed sediments. 

Like spinel, it does not decompose readily, but remains unaltered in 

the residual material of the crystalline rocks in which it was contained. 

It may sometimes be obtained by washing such material. It is contained in hornblende-biotite-granite (68), nephelite-syenite (77), schistose biotite-gneiss (132), and many other specimens of this series. 

Bull. 150 3
34 THE EDUCATIONAL SERIES OF KOCK SPECIMENS. [BULL. 150.

MINERALS OF THE HEXAGONAL SYSTEM.

Graphite. Nearly pure carbon. Hexagonal crystals, flat six-sided 

tables, and foliated grains. Hardness, I to 2. Specific gravity, 2.09 

to 2.23. Luster, metallic. Color, iron-black. Opaque. Thin laniell;er 

flexible, but not elastic. Feel, greasy. Infusible and not affected by 

acids. It is widely disseminated as a pigment in the older metamorphic rocks and occasionally occurs in beds. It occurs in metamorphic 

conglomerate (128?).

Pyrrhotite. Sulphide of iron, often containing nickel. Composition, 

chiefly Fe7S8 to FenSl2. Irregular opaque grains of a bronze-yellow to 

copper-red color and metallic luster. Magnetic, and thus readily distinguished from pyrite. Occurs in cortlandite (113), but is usually 

microscopic; occasionally it may be seen among the scales of talc in 

steatite (142).

Hematite. Ferric oxide. Composition, Fe2O> Hexagonal. Crystals 

usually thin hexagonal plates. In transmitted light, blood-red. Luster 

of crystals splendent, hence called specular iron. Sometimes irregular 

scales. Hardness, 5.5 to 6.5. Specific gravity, 4.5 to 5.3. Color, ironblack to red. Streak, red. Usually opaque, but when thin enough to 

be transparent is blood-red in transmitted light. Is infusible, but 

becomes magnetic when heated.

Micaceous hematite and red hematite of themselves form rock masses, 

often of large size, occurring in beds among the metamorphic rocks, 

chiefly Archeau. Specular iron is widely distributed as an accessory 

constituent of acid eruptive rocks. It is one of the first minerals to 

crystallize, and in the form of minute red scales is usually included by 

the minerals of later crystallization. Occurs forming almost the whole 

of specimen 121, and also in 52 and 120, as well as in many others.

Ilmenite. Mainly ferrous titanate, but variable in composition. 

Formula commonly FeTi03 . Hexagonal. Crystals rhombohedral, but 

generally in irregular grains. Hardness, 5 to 6. Specific gravity, 4.5 

to 5.2. Color, black and opaque. Luster, submetallic. Streak, nearly 

black. Not magnetic, or feebly magnetic; less so than magnetite. It 

frequently alters to a white or yellowish-brown, strongly refracting 

substance, called leucoxene, which, although it contains titanium, is of 

variable chemical composition. Ilrneuite is a widely distributed ingredient of eruptive rocks, especially diorite, diabase, gabbro, and peridotite, and is found less frequently in metamorphic sedimentary rocks. 

It occurs in kimberlite (112). Leucoxene occurs in aporhyolite (136) 

and in the residual sand of diabase (148).

Quartz. Pure or nearly pure silica. Composition, SiO2 . Hexagonal. 

Six-eided prisms terminated by six-sided pyramids. In eruptive rocks 

the prismatic faces are very short or altogether absent, so that the faces 

of the two terminal pyramids come together, forming bi-pyramidal 

crystals. Hardness, 7. Specific gravity, 2.6. Generally colorless in 

rocks, but sometimes smoky or bluish.
DILI.BB.] THE PRINCIPAL ROCK-MAKING MINERALS. 35

Under the microscope it is clear and transparent. It is nniaxially 

positive, and these features usually distinguish it from other minerals 

with which it is associated. It often contains numerous extremely 

minute cavities filled with water or liquid carbon dioxide in which there 

is a bubble in continuous rapid motion.

Quartz is one of the most important and widely distributed of all 

rock-forming minerals, in both altered and unaltered sedimentary and 

igneous rocks. It is an essential constituent of granite, quartz-diorite, 

rhyolite, and dacite, as well as of quartzite, many gneisses, schists, and 

sandstones.

Bock-making quartz occurs in three phases: granitic phase, porphyritic phase, and clastic phase.

Granitic quartz is such as occurs in granite. It is the youngest original constituent of the rock, and fills the irregular spaces left between 

the other minerals. Its form is therefore determined by its surrouudings that is, it is allotriomorpMc. It is usually filled with fluid inclusions. The quartz in gneiss, mica-schist, and other similar crystalline 

rocks is irregular,'granular, and closely related to granitic quartz. It 

forms veins and sometimes contains gold. In California the auriferous 

gravels are derived from rocks containing such veins. Granitic quartz 

is well illustrated by that in the granites (66 to 69); and that in many 

of the gneisses (132 and 137), schists (119, 129 to 131), and veins (25) 

is closely related to it. Jaspilite (120) is composed largely of'quartz 

of this character.

Porphyritic quartz usually has well-developed crystal form that is, 

it is idiomorpliic. Fluid inclusions are common, but not so abundant as 

in granitic quartz, and, furthermore, porphyritic quartz often contains 

glass inclusions. In many cases crystals of porphyritic quartz have 

been corroded by the magma in which they originated and reduced to 

round grains; that is, they have become anhedral (90) or are completely 

destroyed. Porphyritic quartz is illustrated by nevadite (64) and daciteporphyry (90). It belongs wholly to igneous rocks.

Clastic quartz is such, as is found in sandstones and conglomerates 

of sedimentary origin. It is illustrated in specimens 3 to 5, 10, 12 

to 19. Some of the quartz found in rocks is occasionally in the form of 

chalcedony, which is optically negative and usually fibrous. Of such 

material agates are composed. Silica in the amorphous state usually 

contains water and is called opal.

Tridymite. Pure silica. Composition, SiO2 , like quartz. Hexagonal 

crystals, usually minute, thin tabular forms. Imbricated, like tiles 

on a roof. Within rocks tridymite is always of microscopic dimensions. 

It is found chiefly, almost exclusively, in acid volcanic rocks. It occurs 

in lithoidite (62) and aporhyolite (136).

Calcite. Calcium carbonate. Composition, CaCO3. Hexagonal. 

Crystals rhombohedral, but massive in limestone. Hardness, 2.5 to 3.5. 

Specific gravity, 2.5 to 2.7. Khombohedral cleavage perfect, so that it
36 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

may generally be split with ease into rhombohedrons. Sometimes colorless and transparent (Iceland spar) and used to illustrate double 

refraction. By placing it on a dot on white paper two dots appear. 

Often yellowish gray or white. Infusible, but glows and becomes lime. 

Effervesces readily in cold hydrochloric acid. When powdered, will 

effervesce in strong vinegar.

Calcifce is one of the most important and widely distributed of minerals in rocks. It forms essentially the Avhole of many limestones (28 

to 31, 39, 40, 42, etc.), is the cementing material in calcareous fragmental rocks (12), and is generally abundant in the altered basic 

eruptives.

Under the microscope it is usually clear and may. be distinguished 

by its cleavage two sets of lines crossing at an acute angle and its 

peculiar pinkish-yellow gray, often irridescent, colors between crossed 

nicols.

Dolomite. Carbonate of calcium and magnesium. Composition, 

(CaMg)CQ3 , in which varying amounts of magnesia may be replaced by 

iron or manganese. Hexagonal. Crystals, cleavage, color, and optical 

and other properties very like those of calcite, but it may usually be 

distinguished from the latter by means of the characteristic that, unless 

powdered, it does not effervesce readily in acetic or cold dilute hydrochloric acid. Hardness, 3.5 to 4. Specific gravity, 2.8 to 2.9.

Dolomite, like calcite, is widely distributed, often in crystals whose 

cross sections under the microscope appear triangular, six-sided, or 

rhombic. It is an essential constituent of limestone and massive dolomites. Much of the limestone in the United States is dolomite. It 

occurs in specimens 116 and 117 of the collection, also as a secondary 

product in the kimberlite (112) of Kentucky.

Apatite. Calcium phosphate -f- calcium chloride or uuoride. Compoposition, 3(Ca3P208 )+Ca(ClFl)2. Hexagonal, crystals generally microscopic, six-sided prisms. Granular in crystalline schists. Hardness, 5. 

Specific gravity, 3.16 to 3.22. Usually colorless or whitish under the 

microscope. Rather high index of refraction, so as to appear to stand up 

in surrounding minerals of later crystallization by which it is included. 

Soluble in nitric acid without gelatinization, and the solution gives, 

with ammonium rnolybdate, a yellow precipitate. Moistened with sulphuric acid and heated, it colors the name pale bluish green. This distinguishes it from nephelite, with which it is often associated. Widely 

distributed as an original accessory constituent of granular igneous 

rocks; common, but less abundant, in basic lavas. Eeddish-brown 

grains of apatite may be seen in the magnetite (146) from Port Henry, 

New York. It is a microscopic constituent of granite (68), pulaskite 

(74), nephelite-syenite (78), dacite-porphyry (90), and many other rocks 

of this series.

Nephelite. Silicate of potassium, sodium, and aluminum. Composition probably NaAlSiO4 , in which one-fourth of the Na is generally
THE PRINCIPAL ROCK-MAKING MINERALS. 37

replaced by K. Hexagonal. Crystals six-sided prisms, or massive. 

Hardness, 5.5 to 6. Specific gravity, 2.6. Colorless. Luster vitreous 

in crystals, but greasy when massive. Fuses at 3.5 into colorless glass. 

Gelatinizes in acid. The glassy colorless variety in younger volcanic 

rocks is nephelite; the massive form, usually with greasy luster and 

often colored, in intrusive rocks is sometimes called eleolite. They are 

identical in chemical composition. Nephelite readily alters to zeolites. 

They are essential constituents of a whole series of extrusive and intrusive igneous rocks. Nephelite occurs in phonolite (73), pulaskite (74), 

theralite (75 and 76), and, in the phase commonly called eleolite, in 

nephelite-syenite (77 and 78).

Gancrinite. Silicate and carbonate of aluminum, sodium, and hydrogen. Composition, Na4HAl3(CO3)Si:iOi2. Hexagonal. Crystals columnar 

and massive. Hardness, 5 to 6. Specific gravity, 2.42 to 2.5. Colorless 

and transparent when fresh, but frequently yellow and of other colors. 

Fuses easily, effervesces in hydrochloric acid, and when heated gelatinizes. Like nephelite, cancriuite alters to zeolites, and as yet lias been 

found only in nephelite-syenite. It occurs in that of Litchfield, Maine 

(77), usually as conspicuous yellowish grains.

Tourmaline. Silicate and borate of aluminum, magnesium, and iron; 

of complicated composition. Hexagonal. Crystals usually three-sided. 

Hardness, 7 to 7.5. Specific gravity, 2.94 to 3.24. Color, usually black; 

luster, vitreous. Eeadily distinguished under the microscope by its 

strong absorption when the longer axis of the section is perpendicular 

to the short diagonal of the polarizer. Tourmaline occurs in granitic 

igneous rocks, especially near the borders of regions of contact inetaniorphism and in fissure veins. It occurs commonly also in the members 

of the crystalline schists; for example, mica-schist (130) and quartzschist (119). It occurs also in phyllite (126) and other altered rocks.

MINERALS OF THE ORTHO.KHOMBIC SYSTEM.

Marcasite. Iron disulphide. Composition, FeS2 . Orthorhombic. 

Before the blowpipe, on charcoal or in a closed tube, gives off sulphurous 

fumes and becomes magnetic. Color, pale bronze-yellow. Like pyrite 

.in all essential respects, excepting form of crystallization, by which 

means alone are the two distinguishable. Marcasite alters much more 

easily than pyrite. Marcasite is illustrated by specimen 35. The general form of its crystals is suggested by the points projecting from 

the surface of the nodule. Both pyrite and inarcaslte readily change 

to limonite. In specimen 35 the marcasite is almost wholly changed to 

liinonite.

Andalusite and chiastolite. Aluminum silicate. Composition, Al2 SiO5 . 

Orthorhombic. Crystals generally in square thick-set prisms, rarely 

in rounded grains. Hardness, 7 to 7.5. Specific gravity, 3.1 to 3.2. 

Color, various, often reddish brown or dark; under the microscope 

usuallv colorless or reddish; when colored, is pleochroic. Transparent.
38 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

replaced Cleavage quite perfect, parallel to prismatic faces. Infusible. 

Saturated with cobalt solution after ignition and then again ignited, 

becomes blue. Occurs in argillaceous rocks affected by contact metamorphism, less commonly in mica-schists and gneisses of the Archean, 

and rarely in granites.

Chiastolite is audalusite containing carbonaceous matter so arranged 

as to form light and dark squares or a cross on a cross section of a 

crystal. It is especially abundant in some zones of contact metamorphism, and is well illustrated by the chiastolite-schist (135) near 

Mariposa, California.

Topaz. A fluosilicate of aluminum. Composition, Al2SiO4F2. Orthorhombic. Crystals usually short, flattish prisms, rarely in grains. 

Hardness, 8. Specific gravity, 3.52 to 3.56. Color, light yellow, blue, 

or colorless. Transparent, infusible, and only partially attacked by 

sulphuric acid. - Often contains inclusions of hematite and ilmenite 

and minute cavities filled with water or carbonic acid in a liquid state.

Crystals of topaz occasionally occur in cavities of the rhyolite 

(nevadite, 64) of Chalk Mountain, Colorado.

8taurolite. Silicate of iron and aluminum. Composition, HAl5FeSi2<>| 3 . Orthorhombic. Generally crystallized in short prisms, often 

twinned, forming a cross. Hardness, 7 to 7.5. Specific gravity, 

3.3 to 3.8. Color, usually brownish; under the microscope pleochroic. 

High index of refraction, and yields brilliant colors between crossed 

nicols.

Staurolite does not occur in igneous rocks, but appears in crystalline 

rocks, especially of the Archean, in gneiss, mica-schist, and others poor 

in ainphibol'e. It is found also in rocks highly altered by contact inetaniorphism, and is well illustrated by prominent crystals in the staurolite-mica schist (133) of Charlestown, New Hampshire.

Pyroxenes.^ -The pyroxenes are silicates chiefly of iron, magnesium, 

and calcium, and form a series with considerable range of chemical 

composition. With the amphiboles, micas, and olivine they are the 

most important rock-forming ferromagnesian silicates in igneous masses. 

Pyroxenes may be divided according to their crystallization into three 

groups rhombic, mouoclinic, and triclinic. At this place only the 

rhombic pyroxenes, enstatite, bronzite, and hypersthene, will be considered. Their crystals are prismatic. Cleavage parallel to the pinacoidal 

planes is sometimes better developed than that of the prismatic planes. 

Enstatite contains less than 5 per cent of iron, bronzite from 5 to II 

per cent, and hypersthene over 14 per cent. While enstatite in thin 

sections is colorless and not pleochroic, hypersthene is usually greenish, 

light red, or brownish, and strongly pleochroic. The most important 

distinguishing feature under the microscope is the parallel extinction 

in prismatic sections and distinct rectangular cleavage in cross sections, which are usually octagonal, with four of the sides larger than 

the others.
' THE PRINCIPAL ROCK-MAKING MINERALS. 39

The orthorhoinbic pyroxenes, especially hyperstheue, are important 

rock-forming minerals, occurring in many andesites and basalts, as well 

as gabbros and peridotites, but are of little consequence in rocks of 

sedimentary origin, either unaltered or metamorphic.

Enstatite (MgSiOa ) occurs as an essential constituent in saxonite 

(114), also in some of the olivine nodules (104), and occasionally in 

basalt. It occurs also in the pyroxenite (110) from near Baltimore. 

Bronzite occurs in the feldspathic peridotite (111) of the same region. 

Hypersthene occurs in hypersthene audesites (SO and 87), and forms an 

important part of the gabbro (108) of Mount Hope.

A fibrous substance having the composition of serpentine, but usually 

forming pseudomorphs, resulting from the alteration of orthorhombic 

pyroxene, is lastite. It occurs in peridotite from near Baltimore (111).

Olivine. Silicate of magnesium and iron. -Composition, (MgFe) aSi04 . Orthorhombic. Crystals short, prismatic. Sections usually four, 

six, or eight sided. Often granular. Hardness, 6.5 to 7. Specific gravity, 3.3 to 3.45. Colorless or yellowish. Decomposed, with separation of 

gelatinous silica, by heated hydrochloric acid. Under the microscope, 

in thin section, olivine is seen to be without distinct cleavage and to 

have a high index of refraction. Its rough surface apparently rises 

above the adjacent minerals, and becomes brilliantly colored between 

crossed nicols.

Three phases of olivine have been distinguished: (1) That of granular igneous rocks, such as olivine-gabbros and peridotites, where it is 

granular; (2) that of porphyritic igneous rocks, where it forms prominent crystals, as in many basalts; ('3) that of crystalline schists, where 

it occurs as an accessory constituent. In the first two cases it is one 

of the .minerals which crystallize early in the solidifying molten mat 

ter, and includes only such minerals as magnetite, ilmenite, picotite, 

etc., of earlier crystallization. In igneous rocks it is always an original, 

and generally an essential, constituent.

Olivine alters to calcium carbonate, iron oxide, silica, and serpentine, 

and a large proportion of the masses of serpentine exposed among the 

metamorphic rocks have resulted from the alteration of rocks rich in 

olivine. The process will be described under rocks of the peridotite 

series (110 to 114) and serpentine (145). Specimens 104 and 114 illustrate granular oliviue. The ijorphyritic form may be seen in porphyritic theralite (70); also in kimbeiiite, as illustrated in PI. XXXIX.

Fayalite. Silicate of iron. Composition, Fe2SiO4 . Orthorhombic, 

usually in minute tabular crystals. Hardness, 6.5. Specific gravity, 

about 4. Color, light yellow, but upon exposure generally becomes 

opaque, dark brown, or black. Fuses readily to magnetic globule and 

gelatinizes in acids. The minute nonmagnetic crystals in the lithophysre 

of specimen (62) are fayalite.

Zoiftite. Silicate of calcium and aluminum. Composition, HCa^AlaSiuO13 . Orthorhombic. Usually in columnar aggregates. Hardness, 6.
40 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 15J.

Specific gravity, 3.25 to 3.36. Has two cleavages, at right angles to 

each other, one perfect and the other imperfect. Usually colorless or 

gray, index of refraction high, but double refraction low, and between 

crossed nicols its color is a feeble blue. Epidote, which it resembles, 

gives brilliant colors between crossed nicols.

It occurs in some altered gabbros (143), and more commonly in hornblende-schists (131 1?), but is much less common than epidote.

Prehnite. A silicate of lime and aluminum. Composition, H2Ca2Al2 - 

Si3OJ2 . Orthorhombic, but usually in irregular plates. Hardness^ 0 to G.5. 

Specific gravity, 2.8 to 2.95. Colorless or greenish. Fuses at 2 with 

intumescence, and then readily gelatinizes in acid. Occurs only as a 

secondary mineral in basic eruptive rocks (102), but occasionally may 

be an original constituent of calcareous rocks and crystalline schists.

Talc. Hydrous silicate of magnesium. Composition, Mg3H2Si4O12 . 

Orthorhombic. Usually occurs in plates or rods. Basal cleavage 

perfect, giving it a foliation like mica, but soft (hardness, 1), and not 

elastic. Specific gravity, 2.8. Colorless in thin scales, but in larger 

bodies usually greenish or gray, with pearly luster and greasy feel. 

Like muscovite, gives brilliant colors between crossed nicols, but may 

be distinguished by the fact that after ignition, if it is moistened with 

a solution of nitrate of cobalt and heated again, it becomes when 

cooled pale red.

Talc is an essential constituent of talc-schist and steatite (142), and is 

not uncommon among metamorphic rocks. In basic eruptive roci<s it 

sometimes appears as a secondary mineral, resulting from the alteration of hornblende.

MINERALS OF THE MOMOCLINIC SYSTEM.

Gypsum. Hydrous calcium sulphate. Composition, CaSO4+2aq. 

Monoclinic, crystals tabular, but in rocks generally irregular, granular, 

lamellar, or fibrous aggregates. -Hardness, 2. Specific gravity, 2.2 to 

2.4. Colorless to gray and yellowish. Yields water in a closed tube, 

is slowly soluble in hydrochloric acid without gelatinization or effervescence, and fuses rather easily. Occurs in beds as gypsum rock. 

It is of chemical origin and is interstratified with other sedimentary 

rocks.

Pyroxenes (monoclinic). The pyroxenes are silicates, chiefly of iron, 

magnesium, and calcium, with sufficient variation in chemical composition and physical properties to give rise to a number of species. The 

Orthorhombic pyroxenes are treated on page 38. The only mouoclinic 

forms which appear in this series of rocks are augite, diallage, diopsidc, 

and cuf/irite. Crystals, short prismatic. Also in irregular grains. 

Hardness, 5 to 6. Specific gravity, 3.23 to 3.5. Color, grayish white, 

ranging through green and black. By transmitted light it is usually 

greenish, yellowish, or pale brownish. Sections parallel to the prism 

show one cleavage, and between crossed nicols the extinction angle
DIU.EIL] THE PRINCIPAL ROCK-MAKING MINERALS. 41

may be as large as 45°. Corresponding sections of orthorliombic pyroxenes have parallel extinction, and those of amphibole only a small 

extinction angle. Cross sections are often nearly square, with slightly 

truncated corners, or octagonal, with sides of nearly equal length. 

They show two directions of cleavage nearly at right angles. Diallage 

differs from augite in having, besides prismatic cleavage, a distinct 

parting, parallel to the orthopinacoid, giving diallage a more or less 

lamellar structure. Like bronzite, diallage often contains miuute 

inclusions which give it a metallic sheen.

Augite is one of the most common essential primary minerals in igneous rocks. The phenocrysts of dolerite (105) and theralite (76) are 

augite. In granular form augite constitutes a large part of diabase 

(106), basalt (95 to 102), and quartz-norite-gneiss (140). It occurs as an 

accessory constituent in many eruptive rocks. Among metamorpine 

rocks it is not common, for in the process of dynamo-metamorphism 

augite is usually changed to hornblende.

Diallage lias rather limited distribution, occurring with olivine forming nodules in basalt (104), in olivine-diabase (107), and as an essential 

constituent in gabbros (108 and 109). Diopside (malacolite) is a monoclinic pyroxene, poor in alumina and without lamination parallel to the 

orthopinacoid. It occurs in pulaskite (74), basalts (102), and pyroxenite 

(110), as well as in some olivine nodules (104) and some of the Archean 

granites and limestones. Acinite and regirite are forms rich in sodium 

and iron. ^Egirite occurs in the theralite (75 and 76) of Crazy Mountains, Montana.

Amphibole. The amphiboles are silicates, mainly of magnesium and 

calcium, in which the former predominates over the latter. The orthorhombic forms do not occur in any of the rocks of this series. Only 

the monoclinic forms will be considered. Crystals prismatic, with 

cleavage angle 120°, well marked and. characteristic. Hardness, 5 to 6. 

Specific gravity, 2.9 to 3.55. Not affected by acids. Color, various. 

The colorless or white variety is tremolite. The green is actinolite, arfvedsonite, or uralite. The black and brown are hornblende or yriinerite, 

and the blue is generally fjlaucophane.

Cross sections of amphibole prisms are four or six sided, with promi 

nent cleavage lines parallel to the principal sides and crossing each 

other at an angle of 324°. In prismatic sections the extinction angle 

is small, which distinguishes it from augite. Pleochroism usually 

strong in green, brown, and blue amx>hiboles. The cleavage, angle of 

extinction, and pleochroisin distinguish the amphiboles from the 

pyroxenes, which they so closely resemble chemically.

Tremolite (CaMg3Si4O] 2 ) occurs as lamellar masses or crystals in 

Archean limestones, silicate hornfels, and some serpentines. Actinolite occurs in many green schists of the Archean. It occurs also as 

tufts of radiating needles in the cortlandito (113) of Stony Point, New 

York. Common black hornblende is an essential constituent of horn-
42 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

blende-granite (69), many dacites (81 and 82), hornblende-an desites 

(83 to 85), camptonite (92), and cortlaudite (113) among igneous rocks. 

In hornblende-schist (131) and garnetiferous hornblende-schist (141) the 

hornblende is green. Arfvedsonite occurs in leacite and nephelite rocks 

(pulaskite 74), griiuerite in rocks rich in iron (hematite 121), and glaucophane in glaucophane-schists, not represented in this series. Uralite 

and fibrous green hornblende occur- in metamorphic rocks, and are 

usually paramorphic after pyroxene. As such, uralite occurs in hornblende-gabbro-gneiss (143 and 144).

The micas. The micas are silicates, chiefly of aluminum, magnesium, 

iron, and the alkaline metals. In composition and other properties 

there is considerable variation, and the group is divided into a number 

of'species. The most common, for the sake of brevity, may be included 

under the names of Motite and muscovite. The former, containing considerable iron, is dark colored, strongly pleochroic, and approximately 

uniaxial, while the latter, rich in potassium, is colorless or gray and 

biaxial. The micas are all characterized by crystallizing in thin hexagonal plates with eminent basal cleavage, producing foliated structure 

and easily affording very thin elastic laminae which show a black cross 

or bisectrix apparently at right angles to the cleavage surface. They 

are generally regarded as belonging to the monocliriic system, although 

their habit is hexagonal.

In porphyritic igneous rocks the mica is often in well-developed hexagonal phites, but in granitic rocks and schists it is in irregular scales. 

Under the microscope sections parallel to the cleavage are frequently 

isotropic, yield an optic-axial figure, and show no cleavage lines, but 

sections perpendicular to the cleavage show conspicuous cleavage lines, 

brilliant polarization colors, and parallel extinction.

The colorless transparent micas, included for petrographic convenience under muscovite, are lepidolitc, zinmcaldite, sericite, damourite, 

paragonite, and margarite. The micas generally black by reflected 

light but by transmitted light colorless, deep brown, yellowish, reddish, 

or green, with strong pleochroism, included under biotite, are anomite, 

rubellan, and 'phlogopite.

Muscovite (KELjAlsSisO^) is not a volcanic .mineral. As an original 

constituent in igneous rocks it is limited to the granitic series (66), 

but is a widely distributed and common constituent of metamorphic 

sedimentary rocks, gneisses, and schists (119, 129, and 130). In the 

form of sericite it is often an early product of metamorphism in phyllite 

(126), staurolite-niica-schist (133), and aporhyolite (136).

Eiotite has a wide distribution in igneous rocks. In hornblendemica-andesite (83), dacite (81 and 82), and dacite-porphyry (90) it 

occurs iii regular crystals among phenocrysts. It is an essential constituent also of biotite-granite (67), minette (91), and cortlandite (113), 

and an accessory constituent in many others. It is widely distributed 

as an essential constituent of many gneisses and schists, especially in
DILUSR.] THE PRINCIPAL ROCK-MAKING MINERALS. 43

specimens 66, 129, 130, and 132. Lepidomelane occurs in nephelitesyenite (77).

Chlorites. The chlorites are hydrated silicates of magnesium, iron, 

and aluminum. On account of differences in chemical composition 

and also to a limited extent in physical properties, the chlorite group 

has been divided into a number of species, but for ordinary petro 

graphic purposes these need not be distinguished. Monocliuic. Crys 

tals often hexagonal, but occurring generally, like mica, in flat or 

bent plates of irregular outline. Hardness, 1 to 3. Specific gravity, 

2.0 to 2.96. Color, green. Basal cleavage perfect, producing foliated 

structure. Laminre flexible. Basal sections show a distinct black cross 

or bisectrix. Sections perpendicular to the cleavage are lath-shaped. 

Pleochroic, yielding greenish or bluish to yellowish or red colors. 

Interference colors between crossed nicols much lower than those of 

mica.

Ohlorites are widely distributed among igneous and metamorphic 

rocks, but are always secondary, resulting from the alteration of silicates rich in magnesium and iron, such as mica, pyroxene, and arnpliibole. They occur in hornbleude-pyroxene-andesite (85) and many other 

igneous rocks, but can be seen only in the thin section under the 

microscope. They occur also in chlorite-phyllite (127), which is composed chiefly of chlorites, and in steatite (142), of which chlorite sometimes forms a considerable portion, although steatite is usually talc.

Epidote. A hydrous silicate of aluminum, iron, and calcium. Composition, H(CaFe)2(AlFe)3Si;)O]3. Monoclinic. Crystals prismatic and, 

in rocks, generally granular. Hardness, 6.5. Specific gravity, 3.3 to 3.5. 

Basal cleavage perfect. Color, generally yellowish greeu, sometimes 

brownish. Under the microscope, in thin sections, usually colorless, 

but yields between crossed nicols especially brilliant colors, which 

distinguish it from zoisite.

Epidote is never a primary or essential constituent of igneous rocks. 

It frequently occurs as a secondary mineral resulting from the alteration of igneous and sedimentary rocks. It occurs in some gneisses (1.38), 

schists, and phyllites (127), and in aporhyolite (136), and is one of the 

commonest of all silicates resulting from weathering.

Allanite. Silicate of aluminum, iron, cerium, and other rare elements. Occurs in crystals and irregular grains, like epidote. Hardness, 5.5 to 6. Specific gravity, 3 to 4.2. Color, generally pitch, black, 

brown, or gray; in thin sections reddish brown or greenish, usually 

with strong pleochroism. Cleavage indistinct or absent. Optical 

properties vary; many allanites are isotropic, others highly doublerefracting.

Allanite occurs in gneisses (138) as well as in diorite (93), dioriteporphyry (89), and other igneous rocks.

Titanite. Silico-titanate of calcium. Composition, CaSiTio5. Monoclinic. Crystals frequently double-wedge shaped. Occurs also in
44 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL. 150.

irregular grains. Hardness, 5 to 5.5. Specific gravity, 3.3 to 3.7. 

Color, brown to black and reddish yellow. Feebly transparent. Index 

of refraction very high and surface appears rough, but double-refraction low, giving between crossed nicols a brownish-gray color. Not 

affected by acids. Light-yellow titanite usually in wedge-shaped crystals is commonly called spliene. It is one of the primary accessory 

minerals in many granites, syenites, diorites, and other igneous rocks. 

Granular titanite is of secondary origin, frequently resulting from the 

alteration of titaniferous magnetite, ilmenite, or rutile in igneous and 

metamorphic rocks. Titanite occurs in pulaskite (74), diorite (93), and 

in the form of sphene iu ininette (91), and in many other igneous and 

metamorphic rocks, but rarely in particles sufficiently large to be seen 

without the aid of a lens.

Zeolites (monoclinic). Zeolites are hydrous silicates of aluminum, 

calcium, potassium, and sodium, with other bases, and, excepting rare 

cases of aualcite, all are secondary minerals. They have low index of 

refraction and low double refraction. They are transparent, usually 

colorless or white, readily attacked by hydrochloric acid, and easily 

fusible. The most common forms besides analcite, already mentioned 

(page 33), are lieulandite, stilbite, SGolecite, epistilbite, and laumontitc, 

which may be distinguished chiefly on optical grounds. Laumontite 

is well illustrated in the ainygdules of the diabase amygdaloid (139); 

scolecite, in the feldspathic peridotite (111) near Baltimore.

Feldspars (monoclinic). The feldspars are silicates, chiefly of aluminum, with more or less potassium, sodium, or calcium. According to 

crystallization they may be divided into two groups, monoclinic and 

triclinic, but all are very closely related in chemical and physical 

properties.

Their principal common feature is the possession of two cleavages, 

one parallel to the base, perfect, and the other parallel to a lateral pinacoid, less perfect and variable. The angle between the two cleavage 

planes in monoclinic feldspar is 90 degrees, and the basal cleavage 

shows no twinning striae. In the triclinic feldspars, however, the two 

cleavage planes are slightly inclined to each other, and the basal plane 

is striated. These stri;e are well illustrated by the large feldspar crystals in specimen 90. The striation is due to oft-repeated lamellar 

twinning, which under the microscope and between crossed nicols pro 

duces numerous distinct parallel alternating color bands, and affords an 

easy means of distingnishingthe triclinic from the monoclinic feldspars.

Monoclinic feldspar, usually called orthoclase or potash feldspar, is a 

silicate of alumina and potash. Composition, KAlSijOij. Crystals 

short tables and square prisms, but in rocks it usually occurs in irregular grains. Crystals may be simple, but are often twinned parallel to 

the orthopinacoid (Carlsbad twins). Color generally white or gray, but 

often reddish. Frequently more or less turbid and sernitransparent. 

Not acted ou by ordinary acids, and before the blowpipe fuses at 5.
DiLLBB.1 THE PRINCIPAL ROCK-MAKING MINERALS. 45

Sections of simple crystals of orthoclase between crossed nicols are 

seen to be optically the same throughout, but Carlsbad twins show two 

bands of interference colors, less bright than those of quartz.

Orthoclase is an essential constituent of granites (66-69), syenite (71), 

inetarhyolite (quartz-porphyry, 65), phonolite (73), and pnlaskite (74), 

and traces of it have been found in dolerite (305).

In the recent acid volcanic rocks orthoclase is clear and glassy, and 

has been designated sanidine. This form of orthoclase is an essential 

constituent of nevadite (64) and trachyte (70). It occurs also in the 

cavities of lithoidite (62).

MINERALS OF THE TKICLINIC SYSTEM.

Feldspars (triclinic). As already stated, feldspars are silicates, clue fly 

of aluminum, with more or less potash, soda, or liine. There are three 

essentially distinct chemical compounds which exist as separate varieties 

of feldspar, often designated, respectively, potassium feldspar (orthoclase)) sodium feldspar (albite), and calcium feldspar (anortkite). The 

second of these compounds combines in definite proportions with the 

first and third, forming the intermediate members of several series of 

feldspars, designated, respectively, the potash-soda series (anorthoclase) 

and the soda-lime series (plagioclase}. Orthoclase is monoclinic, but 

anorthoclase and plagioclase are triclinic. Although the cleavage 

angle of plagioclase is clearly oblique (86° to 87°), that of anorthoclase 

varies scarcely any from a right angle.

The characteristic feature of triclinic feldspar is the lamellar or polysynthetic twinning, producing striations on the basal cleavage plane. 

The twinning parallel to the brachypinacoid (least perfect cleavage 

plane) is the albite type, while that at right angles to it is the pericline 

type. The fine parallel banding such twinning produces is evident 

under the microscope between crossed nicols, and is highly characteristic 

of the triclinic feldspars.

The potassium-feldspar compound crystallizes in the monoclinic system 

as orthoclase, and in the triclinic system it is called microcline. It occurs 

in irregular grains, and in the thin section under the microscope between 

crossed iiicols may be readily distinguished by its colored rectangular 

grating, produced by the two sets of fine parallel lamellar twins, crossing each other at right angles. Basal cleavage plates between crossed 

nicols extinguish at an angle of 15° 30' to the line of the piuacoidal 

cleavage, and a piuacoidal cleavage plate extinguishes at an angle of 

5° to the line of basal cleavage. Eeddish microcline is abundant in 

the biotite-gnimte of Platte Canyon, Colorado (67). It is present also 

in the granite of Fox Island, Maine (68), granitoid gneiss (137), and 

epidote-rnica-gneiss (138). Microcliue often contains parallel intergrowths of orthoclase and albite.

Plagioclase feldspars are those of the lime-soda series, including 

albite and auorthite and their isoinorphic mixtures, oligoclase, andesine,
46 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

labradorite, and bytownite. The composition of albite is NaAlSi3O8 ; 

of anorthite, CaAl2Si2O8 . The average intermediate mixtures are:

Intermediate mixtures of alMte and anorthile. 

Albite ........................................................ Ab

OligocLiso .................................................... Ab8An3

Andosinc ..................................................... Ab7 An5

Labradorite .................................................. Ab2An:)

Bytownite .................................................... Ab^Aiir

Anorthite..................................................... An

These feldspars are much alike, and are most readily distinguished by 

their specific gravity and extinction angles, which are given in the following table and in the diagram prepared by Michel Levy: J

Extinction angles and specific gravities of plagioclase feldspars.

Albite ............................................

Extinction 

antrle on base, 

OP,

0 0

2 to 3

17 to 27

28 to 39

Extinction 

angle on 

pinacoids, il.

o o 

-, 19 to +13

Specific 

gravity.

2.62

2.64

2.65

2 69

2.71

2

7C

Plagioclase is an important and essential constituent of diorite (93 

and 94), basalt (100 to 102), dolerite (105), diabase (100 and 107), and 

gabbro (108 and 109). It occurs as an accessory mineral in many 

metamorphic rocks; for example, gneiss (132) and schists (129 and 131). 

It is especially prominent, showing fine striations, in specimen 90.

Albite occurs in albite-schist (129), oligoclase in trachyte (70), audesine in dacite (81 and 82) aud uornblende-mica-andesite (S3)rlabradorite 

in most of the andesites (83 and 88), basalt (102), and diabase (10C and 

107); bytownite in gabbro (108), peridotite (111), and gneiss (144); and 

auortliite in the hornbleiide-gabbro-gueiss near Baltimore (143).

Auorthoclase, as far as is yet definitely known, is very rare in rocks 

as compared with orthoclase and plagioclase. It often shows intersecting systems of extremely fine, twinning lamellae, which pass into 

areas free from, them without there being any visible boundary between. 

Like orthoclase. it occurs in the acid series (granitic and syenitic) of 

rocks. It possibly occurs in theralite (76).

HOMOGENEOUS AGGREGATES.

Serpentine. Serpentine is a hydrous silicate of magnesium. Chemical composition, H4Mg3Si2Og. Serpentine is always a secondary mineral, 

and when in the form of crystals is generally pseudomorphous. It is 

usually massive and sometimes fibrous (chrysotile). Hardness, 2.5 to

'Etude sur la determination des feldspaths, 1894, p. 32. See, also, Becker's PI. XI in Eighteenth 

Ann. Kept., Part III, 1898, p. 36.
DILLBR.] THE PRINCIPAL ROCK-MAKING MINERALS. 47

 4. Specific gravity, 2.5 to 2.6. Color, generally green, sometimes red 

or mottled. Feel, often greasy. Structure, splintery. Chrysotile, a 

silky, fibrous form of serpentine, belongs to the orthorhombic system 

and has parallel extinction. Massive serpentine has low double refraction with aggregate polarization, as if composed of minute scales, fibers, 

or grains.

Serpentine results chiefly from the alteration of olivine. The change 

begins along the cracks in the oliviue, and often gives rise to a peculiar reticulated structure, in which the net is serpentine.-* If the alteration is not complete, remnants of olivine occur in the meshes.

Serpentine results also from the alteration of augite and hornblende, 

and iu. the latter case it is generally characterized by a grating structure 

with rhombic interspaces, due to the arrangement of the fibers normal 

to the cleavage of the hornblende. In that derived from augite the 

meslies tare rectangular,

Serpentine lias a wide distribution among metamorphic ro.cks, and 

has been derived from intrusive rocks rich in olivine, such as peridotite; 

also sometimes from gabbro-diabase and pyroxenite.

Specimen 145 illustrates serpentine derived from the alteration of 

; pyroxenite.

Kaolin. Kaolin forms earthy aggregates which under the microscope 

are seen to be composed of minute scales resembling inuscovite, but 

they have a weak double refraction. It is usually Avhite and results 

from the alteration of feldspar, nephelite, scapolite, and similar minerals 

in the process of weathering. It occurs in specimen 149, also in the 

white portions of 147 and 148.

Glauconite. Glauconiteis essentially a hydrous silicate of potassium 

** and iron, and is amorphous. It occurs in small grains, which are generally spherical, but also elliptical or irregular. Hardness, 2. Specific 

gravity, 2.2 to 2.4. Refraction and double refraction weak. It constitutes the essential part of greeusand (5).

Limonitc. Limonite is a hydrous ferric oxide having the following 

composition: 2Fe/): j3H2O. It is not crystallized,, but often fibrous, 

>, stalactitic, botryoidal, concretionary, or earthy. Its color is various 

shades of yellow or brown, whence its other name, broicn hematite. 

When heated before a blowpipe it becomes magnetic, and in a closed 

tube yields water. Its streak is yellowish brown and differs from the 

cherry-red streak of red hematite. Hardness, 5 to 5.5. Specific gravity, 

3.G to 4. Iron rust and most of the yellowish-brown stains resulting 

^ from the alteration of iron-bearing minerals are limonite. It accompanies siderite in specimen 32, and results from the alteration of marcasite in specimen 35. The yellow tint of phyllite (126) is due to the 

presence of limonite.
48 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

CLASSIFICATION OF ROCKS.

Eocks may be classified according to any of their many features, but 

the most fundamental one upon which the primary divisions are based 

is genesis. From this point of view rocks are of two great groups, 

igneous and sedimentary. Igneous rocks are those which, like basalt 

and granite, have solidified from a molten state. The material of which 

they are formed, like that erupted from volcanoes, has come up from the 

interior of the earth in a highly heated igneous condition. Sedimentary 

rocks, on the other hand, are formed by the accumulation of sediments 

brought together by some transporting agent, generally water, and are 

well illustrated by sandstones, conglomerates, shales, and limestones. 

The material of which they are made up is derived from the degradation 

of the land of the earth's surface. Sedimentary rocks are sometimes 

called aqueous, fragmental, or stratified, and igneous rocks are correspondingly designated eruptive, massive, or unstratified, but the various 

terms in the two series are not strictly synonymous.

Before proceeding further with the classification, attention should be 

given to the various stages through which a rock may pass in the cycle 

of its existence.

Under the influence of the weather, rocks exposed upon the earth's 

surface gradually decompose and disintegrate, and the loose material 

thus formed is carried away by the rills, brooks, and rivers as sediment 

to the sea, where it is deposited to form new rocks. By the long-continned accumulation of sediments and outpouring of igneous material, 

sedimentary and igneous rocks once upon the surface may become 

deeply buried and affected by the internal heat of the earth. Furthermore, many of them become involved in mountain-building processes 

and subjected to such high degrees of heat and pressure that the material of which they are composed is rearranged more or less completely, 

in accordance with crystallographic and other forces. By this process, 

which is largely dynamic, the unaltered sedimentary and igneous rocks 

become altered or metamorphosed into slates, schists, gneisses, and 

other forms of metamorphic rocks. In specimen 128, inetainorphic 

conglomerate, the original fragmental structure may yet be seen. In 

Hoosac Mountain this conglomerate passes into gneiss, in which the 

original structure has entirely disappeared. It should be borne in mind 

that this change, called metamoryliism, does not produce new rocks; it 

simply modifies old ones.

The earth movements by which rocks are metamorphosed may raise 

them in mountain masses above the sea. After erosion has removed tbe 

surface material the once deeply buried metamorphic rocks become 

exposed, weathering takes hold of them, and they disintegrate, forming 

residual sands and clays, which are ultimately washed awayanddeposited in the sea. Old rocks are thus destroyed in providing material for 

new rocks. The history of a rock embraces three stages: (1) the stage
DILLEB.] CLASSIFICATION OF ROCKS. 49

of its origin, including the time it remained unaltered5 (2) the stage of 

its alteration; and (3) the stage of its disintegration, which is usually 

very local and brief.

In accordance with these stages rocks may be arranged under three 

heads: (1) unaltered rocks; (2) altered or metamorphic rocks; (3) 

residual rocks.

It is evident that the classification of rocks should refer primarily to 

those which are unaltered. Metamorphic rocks should, as far as possible, 

be traced back to their original condition, and be arranged accordingly. 

As already indicated, the two fundamental groups of unaltered rocks, 

based on origin, are sedimentary and igneous, and to the subdivision of 

these two great groups attention will now be given, beginning with the 

sedimentary.

Sandstone (13), vein quartz (25), and diatom earth (51) are all sedimentary rocks, and being composed chiefly of silica have essentially 

the same chemical composition. In each case water was the agent 

which bore the material to the point of accumulation, but the modes of 

deposition were unlike. To make the sandstone, the sediment was precipitated from mechanical suspension in the water, while for both vein 

quartz and diatom earth the material was precipitated from solution, 

the former by a chemical agent and the latter by an organism. On 

account of these differences sedimentary rock may be divided into 

mechanical, chemical, and organic, as indicated in the table, page 53.

The ultimate classification of igneous rocks is a matter concerning 

which there is much difference of opinion. The chief divisions are often 

based in large measure upon mode of geologic occurrence and chemical 

composition, for these two factors are the principal, ones iu a determination of the structural and inineralogic features of igneous rocks.

Igneous material ("magma") rising from the earth's interior may 

break entirely through the overlying rocks and reach the surface, where, 

exposed to the atmosphere, it cools rapidly; or it may stop beneath the 

surface on the way up and cool slowly to solidification under circumstances very unlike those which obtain upon the surface. Igneous 

rocks have thus been divided by some geologists into two groups, volcanic and p.lutonic, sometimes called extrusive and intmsive rocks. 

Although the igneous rocks of this collection are not divided into 

these two groups, some of the structural features of these, as usually 

given, will now be considered, for convenience in other subdivisions.

Volcanic or extrusive rocks are delivered upon or near the surface 

by volcanic action and spread out in streams or layers, either as continuous flows or as accumulated fragments of lava. On account of their 

sudden cooling many volcanic rocks are either wholly glassy or only 

partially crystalline. Others are holocrystalline, but the mineral particles forming the principal mass in such cases are usually, but not 

always, very minute.

Plutonic rocks, on the other hand, cooling at some distance beneath 

Bull. 150 4
50 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

the surface, solidify more slowly than volcanic rocks and attain a rather 

coarsely crystalline granular structure, excepting along their borders, 

where they come in contact with cooler rocks. They occur in masses 

of various shapes and sizes,- ranging from dikes to bosses and irregular 

batholiths miles in extent.

Eising through stratified rocks, the molten material often spreads 

out between the strata, forming low dome-shaped masses (laccoliths) or 

sheets of nearly uniform thickness. Intrusive sheets lying conformably 

between strata resemble contemporaneous surface Hows which were 

buried in the sediments as they were deposited. Generally the two 

cases may be easily distinguished by studying the contact. Intrusive 

rocks highly heat the rocks with which they come in contact, and frequently metamorphose them, and are themselves finer grained near the 

contact, owing to the cooling influence of neighboring masses.

Although plutonic rocks generally do not extend to the surface, it is 

evident that volcanic rocks always connect, at the orifice from which 

they issue, with plutonic masses extending into the earth. Plutonic 

and volcanic rocks grade into each other, forming more or less continuous series. All these, coming from essentially the same magma, may be 

regarded as belonging to the same series.

As to chemical composition, igneous rocks have a wide range. For 

example, their content of silica may range from 30 per cent to 80 

per cent, and on this basis alone igneous rocks may be conveniently 

spoken of as acid, that is, containing over 65 per cent of silica; intermediate, 55 per cent to 65 per cent; and basic, below 55 per cent.

The chemical nature of the magma determines almost wholly the 

mineral composition of the resulting igneous rocks. For this reason, 

the classification of igneous rocks on a chemical basis, which is really 

fundamental, may be expressed in large measure also in terms of mineral 

association, and its application may be thus rendered much more convenient.

On a cheinical-mineralogic basis igneous rocks may be divided into a 

number of families, each embracing all rocks, both volcanic and plutonic, which have essentially the same composition, and each series ranging in structure from the glassy and porphyritic forms of lava to the 

coarsely crystalline, even-granular forms, like granite and gabbro.

Silica is one of the most abundant and widely distributed components 

of rocks. For this reason the principal rock-forming silicates become 

the most important basis of classification, especially the silicates of the 

alkalies feldspar, nephelite, and leucite and the ferromagnesian silicates amphibole, mica, pyroxene, and olivine. Only those families that 

;are represented .by. specimens in the collection will be here considered, 

and some dfithese families, are not limited as they might be if the collection were larger and of greater variety. The following classification 

corresponds closely to that reported May 27,1897, by: a committee con- 

.sisting of 0. JR. Van .Hise, W.. H, Weed, H, W, Turner^ Whitman .Cross,
DILLKR.] CLASSIFICATION OF ROCKS. 51

and J. S. Diller, appointed by the Director of the United States Geological Survey to consider the nomenclature of the igneous rocks and to 

adopt a system for use in the geologic folios published by the Survey.

Granite-rkyolite family. Granite among plutonic rocks and rhyolite 

among volcanic rocks are the types of this family. They are rich in 

silica, usually containing about 70 per cent (65 to SO), with approximately 8 per cent of the alkalies, K2O. generally but not always being 

a little in excess of the Na2O. When holocrystalline, their essential 

minerals are quart/ and alkali feldspars. Oligoclase is present, sometimes in considerable but subordinate quantities; also one or more of 

the ferroinagnesiau silicates hornblende, mica, or augite, with smaller 

amounts of accessory minerals.

Syenite-trachyte family. Syenite among plutonic rocks and trachyte 

among volcanic rocks are the types of this, family. They are not so 

rich in silica as the members of the granite-rliyolite family, usually 

averaging about 59 per cent (55 to 68), with approximately 9 per cent 

of the alkalies, K^O being in general slightly in excess of Na^O. Mineralogically they are characterized by the general absence of quartz 

and the predominance of alkali feldspars. As in the granite-rhyolite 

family, there is often a large but always subordinate amount of oligoclase present. Hornblende, mica, or angite may be rather abundant, 

but the whole amount of the ferromagnesian silicates present is less 

than that of the feldspar. The rocks of this series are rare.

Nephelite-leuciteroclcs. Nephelite-syenite and phonolite are respectively the abyssal and surface igneous rocks of this family. They contain so low a percentage of silica (about 54 per cent) and so high a 

percentage of the alkalies (about 14 per cent) that nephelite and leucite 

are formed and, although generally associated with feldspar, are considered the characterizing minerals. In the phonolite and nephelitesyenites the associated feldspar is largely orthoclase, but in the theralites it is chietly plagioclase, and on account of such differences the 

nephelite-leucite rocks, although rare, may be divided into several 

families. The hornblende and augite which these rocks contain are 

usually rich in alkalies.

Diorite-andesite family. Diorite and andesite are the plutonic and 

volcanic type rocks of this family. They average about 60 per cent 

(48 to 70) of silica and 7 per cent of the alkalies, Na^O being in excess, 

with about 6 per cent of the alkaline earths, of which the greater part 

is generally lime. Consequently the characterizing minerals are sodalime feldspars. Quartz is often present, and also hornblende or mica, 

but not in predominating quantities. Augite is common in. andesites, 

but less so in diorites, where its relative abundance indicates a degree 

of approach to the gabbros.

Gabbro-basalt family. Gabbro and basalt are respectively the plutonic and volcanic types of this family. They average about 50 per 

cent of silica, 9 per cent of iron oxides, 9 per cent of CaO, 6 per cent
52 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

of MgO, and 4 per cent of the alkalies. Feldspar is usually less ^ 

abundant in this family than in the preceding ones, and on account of 

increase in the CaO and decrease in the alkalies, it is prevailingly 

of the lime-soda variety, as distinguished from the soda-lime variety 

which prevails in the diorite family, although in some cases the alkali 

feldspars become important. As the CaO and MgO increase, pyroxene 

becomes more abundant, so that the holocrystalline members of this 

family in general are essentially labradorite-pyroxene rocks. Theniem 

bers of this family and of the diorite-andesite family are the most 

abundant of igneous rocks.

Peridotite family. Pyroxenite and peridotite are the plutonic types 

of this family and are composed, essentially, the first of pyroxene and 

the last of olivine, by which they are characterized. In chemical composition they are especially rich in MgO, containing an average of about 

29 per cent, with rather low silica (about 44 per cent) and a very small 

percentage 'of th.e alkalies and alumina, so that they are essentially 

feldspar-free rocks. Volcanic rocks of this series are rare, and as yet, 

perhaps, wholly unknown in the United States.

TABULAR VIEW AND CLASSIFICATION OF THE ROCKS OF THIS SERIES. * 

STAGES OP A KOCK CYCLE.

Origination. Alteration. Disintegration.

CLASSIFICATION OF ROCKS.

Unaltered. Metamorphic. } Residual. 

Sedimentary:

Mechanical. Mica-schist. <-

Chemical.

Organic. Crystalline limestone. Clay of limestone. 

Igneous:

. Granite-rhyolite family. Granite-gneiss. Sand of granite.

Syenite-trachyte family.

Ncpheliue-leucite rocks.

Diorite-audesite family. ' « Gahbro-basalt family. Gabbro-gneiss. Sand of diabase.

Peridotite family. Serpentine.

1 Only a few examples of metamorphic and residual rocks are given, for the purpose of showing their 

relations to the unaltered forms.
DILLEK.] CLASSIFICATION OF KOCKS.

Unaltered sedimentary rocks.

53

'

Mechanical :

Clay...............................................

Shale ..............................................

Chemical:

LimestoneOolite..........................................

Concretions 

Silioifled shell .................................

Organic:

Number of specimen in the series.

1,2.

3, 4, 5, 6.

7.

8,9.

10.

11.

19 1 Q 11 T". 1 ft 17 1 fi 1 0

20.

2;(.

24, 25.

20.

27.

28.

29.

::o.

81.

32.

33.

34.

35.

36.

37.

38.

39, 40, (41),42, 43, 44, (45),40, 47,48,40,50.

52.

11

54, 55, 00.
54 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL. 150.

a

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1DILLEK.] CLASSIFICATION OF ROCKS. 55

Melaniorjiliio sedimentary roclcs.

No. of 

specimen,

Crystalline limestone................................................ 115

Marble............................................................... 110

Marble (dolomite).................................................... 117

Quartzite............................................................ 118

Quartz-schist........................................................ 119.

Jaspilite............................................................. .120

Magnetic specular hematite.......................................... 1.21

Slate (clay slate, roofing slate)....................................... 122

Indurated .jointed shale.......................................... 123

Crumpled shale .................................................. .124

Faulted pebble................................................... 125

Phyllito (sericitc-achist) ............................................. 120

Phyliite (chlorito-phyllite)........................................... 127

Metamorphic conglomerate (conglomerate gneiss)................... 128

Albite-schist......................................................... 129

Mica-schist .......................................................... 130

Hornblende-schist................................................... 131

Schistose biotito-gneiss.............................................. 132

St aurolitc-mica-schist................................................ 133

Horufels.............................................................. I'M

CliiastoHte-schist.................................................... 135

Metamorphic igneous rod's.

No. of 

specimen

Aporhyolite ......................................................... 130

Granitoid gneiss..................................................... 137

Epidote-mico-gneiss ................................................. 138

Diabase-amygdaloid ................................................. 139

Qnartz-jiorite-gueiss................................................. 140

Garuotiferous hornblende-schist..................................... 141

Steatite............,...................!............................. 142

Hornblendio gabbro-gnoiss (gabbro-diorito).......................... 143

Do............................................................... 144

Serpentine........................................................... 145

Magnetite ........................................................... 140

Residual roclcs.

No. of 

specimen.

Residual sand of gneiss ..................................... ....... 147

Residual sand of diabase ............................................ 148

Residual clay of I'ehlspathic rock (kaolin)........................... 149

Residual clay of limestone........................................... 150
56 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Illustrations of surface modifications.

No. of 

specimen.

151

152

153

154

155

156

DESCRIPTIONS OF SPECIMENS.

UNALTERED SEDIMENTARY ROCKS OF MECHANICAL ORIGIN.

No. 1. BEACH GRAVEL.

(FROM NAHANT, MASSACHUSETTS. DESCRIBED BY J. S. DILLER.)

Specimen No. 1 consists of smooth pebbles of various sizes. These 

illustrate the character of the pebbles accumulated to form the beach 

gravel of Natoant. They range in size from that of a pea to that of an 

egg. When the fragments are as large as a man's head, the accumulation of pebbles and cobblestones is called shingle. The round form and 

smooth surface of the pebbles in specimen No. 1 show that they have 

been subject to much attrition on the beach. The breaking waves 

(breakers) are continually dashing against the shore, often with great 

violence, knocking the pebbles together, breaking off their corners, and 

grinding all the material within their grip to finer gravel, sand, and 

clay. The water of the broken wave returning to the sea flows down 

the beach, carrying with it some of the gravel, sand, and clay. As the 

water at the surface of the ocean is carried landward by the wind 

waves,1 that on the beach flows seaward as an undercurrent (undertow) 

and distributes the fragmental material, according to size, over the 

bottom. This process is illustrated in the accompanying fig. 7.

The beach is composed largely of pebbles, with some sand. The 

undertow is generally not strong enough to carry pebbles far from 

shore, but off shallow and stormy coasts sand may be carried to a considerable distance (nearly 100 miles off the Atlantic coast), and the silt 

and clay will be deposited still farther beyond, where the undertow 

meets the deoper water. In this manner the waste of the coast land is 

spread over the bottom of the sea to form new rocks. As the transporting power of the undertow varies with the force of the waves 

in changing tide and winds, so its deposits will vary and become 

arranged in layers i. e., stratified.

The cliff cut by the sea in forming the beach is a sea cliff. It is the 

source of the beach gravel. Pebbles may already exist in the loose

'G. K. Gilbert, Lake Bonneville: Mon. U. S. Geol. Survey, Vol. I, 1890, pp. 29 and 30.
DESCRIPTIONS: NO. i, BEACH GKAVEL. 57

material of the cliff, as along the New England coast at many points, 

bnt this is not generally the case. The gravel of the beach is usually 

made from the solid rocks which form the cliff. The force of the waves 

is so great on stormy coasts that fragments, small and large, sometimes 

weighing tons, are broketf off from the fissured cliff and tumbled about 

by the dashing waves, pounding one another until ground to small pebbles and clay. The rocks of the sea cliff are weakened for the attack 

of the waves by weathering, but a large part of the destructive effect 

of the waves is due to the fact that they are armed with fragments of 

rock, which are hurled with great force against the cliff, as in a bombardment. The noise of such rock-pounding by the waves on a stormy 

coast may sometimes be heard a number of miles.

Pebbles of soft material are readily ground to pieces and disappear. 

It is only those, like specimen No. 1, of hard, relatively tough, homogeneous material that stand the continuous battering and become well 

rounded. Of all minerals, quartz is the most common one which is 

both physically and chemically hard and can well resist the wear of 

wave action. For this reason many of the beach pebbles are composed

J=Gra\/e/. ^=Sanct. 3°C/ay.

FIG. 7. Section of coast, ahowiug beach aud scdinieuts.

largely of quartz. When the waves are not normal to the beach the 

pebbles are thrown up the beach in the direction of the moving wave 

and made to travel along the beach. Such pebbles are well rounded, 

and their si/e is usually, but not always, proportional to the distance 

they have traveled. The original shape and structure of the fragments 

are the chief factors in determining the ultimate form of the pebbles 

subject to the attrition brought about by the waves on a beach. If the 

original fragments are flattish, and especially if their material splits 

readily in one direction, the pebbles will be flat with rounded edges, 

but if of nearly cubical blocks of practically homogeneous material, 

such as 'granite, syenite, aud many other igneous rocks, they will be 

spheroidal, as specimen No. 1.

There is another natural process of gravel formation which is of 

much importance and should be mentioned in this place, although 

not represented by specimens. Streams of water flowing over the 

surface of the land carry with them rock fragments which have been 

detached in various ways, rolling them along the bottom of the channel and sometimes picking them up and dropping them again. The
58 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

stones thus carried give and receive many blows, not only striking 

the bed rock of the channel, but striking one another, and by these 

blows they are rounded, just as are the stones rolled upon the beach. 

The resulting forms are the same, and if a typical sample of stream 

gravel were added to this collection it would merely duplicate the 

sample of beach gravel.

No. 2. GLACIAL GRAVEL (STRIATED PEBBLES).

(FROM EL-KHORN, WALWOKTH COUNTY, WISCONSIN. DESCRIBED HY G. K. GILBERT.)

A glacier is a large mass of ice moving slowly under the influence of 

gravity. Glaciers on mountain slopes descend valleys and have the 

form of broad, deep rivers. They are fed at the upper end by annual 

accumulations of snow, and waste away by melting at the lower end. 

Glaciers on plains are called ice sheets. They are built up by accumulations of. snow until their own weight forces them to spread 

 outward, and they waste away by melting at their margins. A mountain glacier carries a load of rock. Some stones fall upon its back and 

are borne along without modification; others are picked up from 

beneath and embedded in the ice. As one part of the ice moves faster 

than another, the embedded pebbles are rubbed against one another 

and are thereby scratched. Some of them are also rubbed against the 

rocky bed over which the glacier moves and are still more vigorously 

abraded. Often they are rubbed until quite flat on one side, the flat 

surface being polished and marked by parallel scratches. Sometimes 

after such flattening they are turned and similarly rubbed upon another 

side, and in this way a large number of facets may be ground upon the 

same pebble. Ice sheets, similarly, have stones embedded in their 

lower portions, but usually cany none upon their backs. The parts of 

glaciers and ice sheets which waste by melting are traversed by 

streams, which may flow across the upper surface, beneath the under 

surface, or in tunnels midway. These streams wash along the pebbles 

that come in their way, rolling them as they go and giving them all 

the ordinary characters of stream gravel or beach gravel (see specimen 

No. 1). The stones carried by glaciers are thus of three types: (I) The 

unmodified material of the back load, which usually consists of fragments 

fallen from cliffs and is angular; (2) the pebbles and bowlders embedded 

in the ice, which are usually subangular in general form, and are characterized by scratches, or by scratches and ground facets; (3) the pebbles 

finally acted upon by glacial streams, which are well rounded and have 

smooth surfaces. Those of the first type resemble the stones of talus 

slopes; those of the third are not distinguishable from other streain- 

\vorn pebbles, nor from beach-worn pebbles (specimen No. 1); those of 

the second type are peculiar to glaciers and are distinctively said to 

be (jlaciatecL They are illustrated by the specimen No. 2.

The corners of glaciated pebbles are usually rounded, so that there
DILLER.] DESCRIPTIONS; NO. 2, GLACIAL GRAVEL. 59

-" ' are DO sharp angles, but the rounding does not approach in perfection 

that illustrated by stream pebbles and beach pebbles. On each flat facet 

the strise are parallel, but the direction is often different on contiguous 

facets. Usually the facets show also irregular scratches, and such 

scratches, with little parallelism, characterize the unfaceted portions 

of the surface.

y A glacier moves continuously, but slowly, from the region of accumulating snow to the region of ablation or melting, so that the stones it 

receives are all carried in the same direction, and where the ice stream 

ends by melting, the stones stop and are accumulated. Where pebbles 

are deposited without other material they constitute glacial gravel, 

but more frequently the glaciated pebbles are deposited in association

^ with clay, and sand, constituting a material called loiolder clay or till. 

(See specimen No. 9.)

In the Pleistocene period there were many glaciers in the high 

mountains of Colorado, Utah, Nevada, California, and the area lying 

north of these States, and there were ice sheets in the northern part of 

the continent. One of the ice sheets, having its center of accumulation 

in Canada, spread over New England, covered most of the area bordering the Great Lakes, and occupied the northern part of the Great 

Plains. During its existence it moved an immense quantity of clay, 

sand, and stones southward, producing a deposit which in many 

districts deeply buries the bed rock. The pebbles constituting 

specimen No. 2 are from a deposit-at Elkhorn, Walworth County, Wisconsin, where they were embedded in clay.

The reader will find further information in the descriptions of speci- 

^ mens Nos. 9 and 155. He is also referred to the second chapter of The 

Great Ice Age, by James Geikie; to The Drift: Its Characteristics 

and Kelationships, by B. D. Salisbury, in Vol. II of the Journal of 

Geology, and to The Bock Scorings of the Great Ice Invasion, by 

T. C. Chambeiiin, in the Seventh Annual lieport of the United States 

Geological Survey.

No. 3. BEACH SAND.

(FROM SULLIVANS ISLAND, NBA a CHARLESTON! SOUTH CAROLINA. DESCRIBED BY

J. S. DILLEU.) .

The mineral fragments next smaller than pebbles are grains, and an 

accumulation of them is sand. Fragmental material grades in size from 

k . gravel through sandy gravel and pebbly sand to pure sand. Sand is 

variously designated, according to its mode of occurrence and origin, 

as well as with reference to its color, shape, and miueralogic composition. Beach sand, river sand, residual sand, black sand, sharp sand, 

quartz sand, etc., are among the names thus given it.

Specimen No. 3, from the ocean beach on Sullivaus Island, at the entrance of Charleston Harbor, is a uniformly fine-grained sand, composed 

chiefly of angular grains of colorless transparent quartz. Some of the
60 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL. 150.

grains of quartz are rounded and a few are stained yellowish by oxide 

of iron. If the sand is stirred with a small horseshoe magnet, only a few 

small grains of magnetite may be found adhering to the poles. Some 

of the black grains are relatively earthy material, such as results from 

the decomposition of ferromagnesian silicates like augite. Other black 

grains are yellowish green by transmitted light, and slightly pleochroic, 

with a large cleavage angle, but a small angle of extinction, and are ^ 

believed to be hornblende. There are a few thin, light-gray, pearly 

scales of a cloudy, transparent, biaxial mineral like somewhat-altered 

muscovite. Clear, angular, glassy grains of uiicrocliiie and plagioelase 

were seen, without traces of kaolinization. Numerous small white fragments of molluscan shells are present.

The sand is so largely composed of quartz that it affords a good ^ 

example of quartz sand. Many of the grains are sharp-angular, showing shallow-curved pittings, resulting from couchoidal fracture. A 

larger proportion of the grains show more or less rounded corners with 

dull surfaces like ground glass, due to the attrition of the grains among 

themselves when moved. Grains of sand are so light in water that the 

feeble knocks they give one another when hustled about by the waves 

and currents make little impression as compared with that made and 

received by pebbles. For this reason grains of beach sand will remain 

angular where pebbles become well rounded. Sand of well-rounded 

grains worn only by water indicates long-continued exposure to the 

action of the waves. Waves and currents inclined to the coast cause 

the sand to travel along the coast; The farther the sand travels the 

larger the proportion of its rounded grains.

The character of the sand in specimen No. 3 is such as to indicate that 

it is derived from two sources: (1) From the decomposed and disintegrated crystalline rocks, like gneiss and granite, rich in quartz; and 

(2) from the breaking up of quartz-feldspar rocks without decomposition. The iron-stained grains of quartz and the complete absence 

of decomposed feldspar indicate that it was derived from residual 

materials; the feldspar, having completely decomposed, forming kaolin 

and muscovite, was entirely separated from the quartz by the sorting 

action of the waves. The glassy quartz with conchoidal-fracture surfaces and the clear fresh feldspar indicate rock breaking, for such 

unaltered material could not be of residual origin. The feldspar would 

show clouding, due to alteration.

The beach sand of the Atlantic coast is part of the formation now 

developing in that region from the waste of the land. Other minerals r< 

besides feldspar are reduced to finer particles by decomposition, and 

removed in the process of sorting, thus concentrating the quartz to 

form sand. The bulk of nearly all sand is quartz, a condition which 

naturally results from the chemical and i>hysical durability of that 

mineral. A considerable portion of it has been derived from the erosion 

of the Cretaceous, Tertiary, and Pleistocene sandstones of the Coastal
DESCRIPTIONS: NO. 4, DUNE SAND. 61

Plain. Tims it happens that the same grain of sand in the course of 

its history may have belonged successively to a series of different sandstones, and be much older than the formation in which it occurs.

Closely related to beach sand is river sand, which is deposited along 

the courses of streams through the land. The grains of river sand 

are usually less rounded than those of beach sand; When composed 

wholly of angular grains, it is often called sharp sand by builders, who 

use it in making mortar. Varieties of both river and beach sand are 

sometimes based on mineralogic composition. Magnetic sand or iron 

sand is rich in magnetite or other ore of iron. Micaceous sand contains 

much mica. Feldspathic sand is rich in feldspar. Gold sand or auriferous sand contains particles of gold, aud at some places along the 

coast of Oregon and elsewhere has been washed for gold. Auriferous 

sand is usually black aud is sometimes called blade sand, because of 

the magnetite and other heavy black minerals it contains.

No. 4. DUNE SAND (EOLIAN SAND).

("FROM SAN FKANCISCO, CALIFORNIA. DKSCIUMED MY J. S. DITJLISK.)

This sand was collected from the dunes near Golden Gate Park, 

between San Francisco and the Cliff House. PL XI illustrates a portion of the wind rippled surface of one of these dunes. The twigs in 

the foreground show the direction of the prevailing wind from, the 

coast. The sand from the adjacent seabeach, like that on many other 

coasts, is blown inland by the storms from the sea to form, the knolls 

and ridges of the dunes. They are well developed occasionally in arid 

regions, and also along the shores of lakes; for example, Lake Michigan 

(see PL VI), where the evidence of strong winds is seen in the inclination of the trees.

A glance at specimen No. 3 with a lens discloses the fact that the grains 

are all well rounded. In this respect dune sand differs from ordinary 

beach and river sand, but the difference is only one of gradation. Sand 

is relatively much heavier in air than in water, so that when hurled by 

wind the grains strike much harder blows than when borne by water, 

and this fact fnlly accounts for the greater roundness of dune sand.

In mineralogic composition dune sand is generally as variable as 

river and beach sand. Specimen No. 4 is composed chiefly of quartz, 

although the proportion of quartz is not so large as in specimen No. 3. 

In No. 4 there are so many grains of hornblende and other black 

minerals as to give the sand a decidedly dark color. Most of the 

coarse grains are clear and colorless or have a slight rusty tinge clue 

to oxide of iron. Considerable sand was examined, but no feldspar 

was found in it. The magnet picks up numerous small black grains of 

magnetite and grains of other colors inclosing small particles of magnetite. Most of the larger black grains in the sand, when crushed, are 

found to be a transparent, pleochroic, greenish-yellow mineral with the
62 THE EDUCATIONAL SERIES OF KOCK SPECIMENS. [BULL. 150.

extinction and cleavage angle of hornblende. Other clear-green vitreous grains without pleochroism and with a large angle of extinction 

are probably pyroxene. The grains with a dull or waxy luster, ranging 

in color between red, light yellow, gray, green, and brown, when 

crushed and exposed under a lens appear to be fine, powdery, siliceous 

material like that obtained by crushing small pieces of variously colored 

siliceous slates and jasper, such as are abundant in the Coast "Range. 

Sand dunes border nearly all lacustral and marine coasts that are not 

rocky, and are usually in the form of irregular rounded ridges or 

mounds parallel to the beach and with the steeper slope to the laudward. They occur not only along the coasts, where the sand is derived 

from the beaches, but also in arid regions of the interior. They are 

abundant in tbe great desert region of North Africa and Arabia, as 

well as at a number of points in the western part of our own country. 

They sometimes cover many square miles, and the accumulated sand 

may be hundreds of feet in thickness. San Francisco is largely built 

on sand dunes, which cover most of the northern end of the peninsula. 

The prevailing winds from the southwest blow the sand inland in a 

rhythmical way and beautifully ripple-mark tbe surface of the dunes. 

Owing to the local eddies and frequent shifting of the winds, the structure of the dunes is very irregular. It may be compared with the 

cross bedding occasionally seen in water deposits. (See PI. I.)

With variation in the direction and force of the winds and in tlie 

supply of sand, the dunes change their form and sometimes travel 

slowly inland. On the western border of Europe, where blowing sand 

occupies nearly half the coast from the Pyrenees to the Baltic, their 

advance varies from place to place, and ranges between 3 and 24 feet 

per annum. Fertile fields and houses of once populous districts have 

been buried by advancing dunes. Streams have been turned from 

their courses, and tbe whole region has been converted into sand wastes.

Such devastation can be averted to a great extent by pi an ting grass to 

hold the sand, and trees to break the wind. In Gascouy, on the southwest coast of France, where dunes are very large, extending along the 

sea for 150 miles, with a breadth of belt at times as much as 6 miles, 

and rising frequently to the height of 300 feet, the devastation by the 

advancing dunes was checked and averted to a considerable extent by 

planting and growing pines.

Sands shifted by the winds are not confined to the seacoast, but 

occur on the borders of many lakes, as well as in the arid regions of 

the interior. They may be seen on the western shore of Lake Michigan (see PI. VI). where the dunes reach a height of 100 to 200 feet. 

Forests once entombed beneath them are being again exposed by eolian 

erosion.

In desert regions the absence of vegetation exposes the rocks directly 

to the Aveather, and the dryness is especially favorable to transportation 

.of sand by the winds, which are often vigorous. The blown sand in
U. S. GEOLOGICAL SURVEY 

HIGH PART OF SOUTH MANITOU ISLAND, LAKE MICHIGAN, SHOWING FOREST FORM

EXPOSED BY EOLIAN EROSION.
DIIABK.] DESCRIPTIONS: NO. 5, GREENSAND. 63

such regions carves the exposed stones and ledges in a peculiar manner. 

The discovery of this geologic phenomenon led to the invention-of the 

sand blast, which is now a mechanical process of considerable importance in the arts.

The eolian sands of the arid regions of North America have, been 

described by Prof. I. C. Eussell: Geological Magazine, July 1889, p. 289. 

Like those of the coast, the dunes of the interior regions are composed 

chiefly of quartz, but exceptions to this rule occur occasionally. Professor Eussell reports small dunes in Carson Desert composed of casts 

of small crustaceans (Gypris), and others near Fillniore, Utah, composed of crystals of gypsum. On parts of the coast of Cornwall, 

England, the dune sand consists mostly of fragments of land and sea 

shells. The material has been used as a fertilizer. In places it is so 

firmly lithitled by calcareous cement that the rock can be used for 

building. References to the most important papers on dime sand are 

given by Geikie in his Text-Book of Geology, third edition, pp. 335, 336.

No. 5. GREENSAND.

(FiiOM FAKMINGDALE, MONJVIOUTH COUNTY, NKW ,'JTCUSKY. DKSCKHVED UY

J. S. DILLEK. )

Greensand is a sand characterized by the presence of the greenish 

mineral glaucouite, which is essentially a hydrous silicate of aluminum, 

iron, and potash. It is abundant in the Cretaceous formation along 

the Atlantic coast, especially in New Jersey, where the series which it 

characterizes has a thickness of nearly 550 feet. The greensand series 

is well stratified, and contains in places an abundance of marine fossils.

Typical greensand, of which specimen No. 5 is an example, is composed, 

besides the glaucouite, of some clay, and generally, also some calcareous 

matter, with minerals like those derived from, the disintegration of 

crystalline rocks in the waste of the laud. Quartz'is most abundant, 

but feldspar, hornblende, magnetite, augite, zircon, epidote, tourmaline, 

garnet, and other minerals are present in small amounts. It contains 

also a few small fragments of gneiss and other rocks.

The principal component, glaucouite, is usually in more or less 

rounded, sometimes mammillated grains about I 11 "" in diameter. By 

means of a phosphatic cement the smaller grains are occasionally bound 

together, forming nodules. The color of the glauconite grains is black 

or dark green when fresh, and brownish when altered. The mineral is 

soft and easily crushed, yielding in thin section light-green colors by 

transmitted light and fine aggregate polarization between crossed 

nicols. Fine punctures may occasionally be seen upon the surface of 

the grains; at other times they are smooth and shining; but generally 

they are dull and the surface is rather irregular. Some of the grains are 

distinct internal casts of foraininiferal calcareous shells, but generally 

the form of the shell chambers is not sharply preserved. Dr. C. E. Lord, 

who examined in the laboratory the upper marl of Farmingdale, reports
64 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

that, besides containing glauconite, the marl is characterized niiueralogically by the presence of raicrocline, "biotite, and muscovite, a great 

scarcity of chlorite and amphibole, and an increase in the size of the 

quartz grains, which are round and often attain 3 nim in diameter, 

although generally in the greensands the average diameter is about l'"m . 

Gypsum and phosphorite are present. These minerals are rarely separated, and occur in large quantities in the argillaceous gray marl near 

Farmingdale. The phosphorite occurs in the form of an earthy, amorphous, colorless substance, frequently inclosing grains of glauconite or 

fragments of other minerals, and occasionally the spines and teeth of 

fishes. It is easily separated and subjected to qualitative analysis to 

prove that it is phosphate of lime.

Much light has been thrown upon the formation of green sand by the 

work of the Challenger expedition. Approximately 1,000,000 square 

miles of the sea floor is now covered by glauconitic deposits, and they 

are limited in their development to certain areas adjacent to the land 

where Foraminifera are present and the amount of sediment is small. 

The depth generally is between 100 and 200 fathoms, although glauconite is sometimes found at much greater depths.

In the glauconitic material dredged up from the present sea floor by 

the Challenger and other expeditions, the grains of glauconite are commonly found in the cavities of Foraminifera and other organisms, and 

in many other cases the grains show clearly the interior casts of such 

shells, the shells having disappeared. This association is so general 

that all the probabilities appear to favor the opinion that the glauconite 

was formed originally in the cavities of organisms. Green sand has been 

found in greater or less amounts in nearly every geologic horizon from 

the Cambrian down to the present time, but is especially abundant, as 

already indicated, in portions of the Cretaceous. Greensand is extensively used in New Jersey as a fertilizer.

The student will find much additional information concerning the 

formation of the glauconitic deposits of the present oceans in the Challenger Eeport of Deep Sea Deposits, especially on pp. 378 to 391. Concerning the greensands of the Atlantic coast, reference may be made 

to A Preliminary Keport on the Cretaceous and Tertiary formations 

of New Jersey, by W. B. Clark, in the Annual Eeport of the State 

Geologist of New Jersey for 1892, pp. 169 to 245; also to the Journal of 

Geology (February and March, 1894), Vol. II, p. 161.

No. 6. FOSSILS FROM MARINE SAND.

(FROM GROVE WHARF, JAMES CITY COUNTY, VIRGINIA. DESCRIBED BY

J. S. DILLER.)

In the beach sand of specimen No. 3 there are fragments of shells 

broken tip by the waves. At many places along the coast the shells are 

buried in the sand before they are broken. This takes place especially
DILLKR.]' DESCRIPTIONS: NO. 7, LOESS. 65

beneath the water a little offshore, or in a bay, where the force of the 

waves is not so violent as on an exposed beach and animal life is 

abundant to supply the shells. When buried so as to become a part of 

the earth, and thus to form a record of the kind of animals that lived 

while the sand was being deposited, the shells are fossils. The two 

shells of specimen.No. 6 are fossils whose specific names are Venus 

mercenaries and Pectenjeffersonius. They were found buried in the sand 

as illustrated in fig. 8. The clams and scollops which bore these shells 

lived and died in the sea during the geologic epoch known as the Miocene. The bodies disappeared, but the shells remained unaltered, 

buried in the sand, to tell of the life of that ancient time.

The presence of the shells in the bluffs on the land far from the sea 

shows that during the Miocene epoch the.ocean covered much of what 

is now dry land, and that since then the sea bottom at that particular 

place, and iu fact all ____ ,___ 

along the Atlantic coast 

of the United States from 

Massachusetts to Florida, 

has been raised and the 

coast line moved farther 

eastward to its present 

position.

Specimen No. 6 furnishes an example of the 

simplest type of fossilization. The organism is 

buried and a large part 

of it, the shell, remains 

unaltered. Specimen No. 

38 illustrates a type of 

fossilization iu which the 

shell is completely 

changed, and only the 

outer form is preserved, while in specimen No. 37, which is vi third type, 

although the organic matter is completely replaced, the outer form and 

delicate internal structure of the wood are completely preserved.

No. 7. LOESS.

(FROM MUSCATJNK, ]OWA. .DESCRIBED «Y J. S. DlLIJSK.)

The pale-yellowish, homogeneous, fine-granular, somewhat calcareous earthy material forming the bluffs at Muscatine, Iowa, and elsewhere along the Mississippi is called loess. The name was first given 

to material of the same sort occupying the valley of the Khine.

The loess of Muscatine is intermediate in fineness of grain between 

sand and clay, and, although it appears remarkably uniform, there is 

considerable range in the size of the particles from about 1 mm. to 

Bull. 150 5

Fio. 8. Shells in marine sand at Carters Grove, on Jauies 

lliver, Virginia.
66 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL. 150.

0.0001 mm. in diameter the finer material forming by far the greater 

portion of the mass.1 Under the microscope it is seen that most of the 

grains are angular or subangular, but many are distinctly rounded, 

showing that at some stage of its existence they have been subjected 

to considerable attrition. Quartz is the most abundant mineral in 

both rounded and angular grains. Clear, colorless grains of orthoclase 

and plagioclase feldspar and green pleochroic fragments of hornblende 

are common but not abundant. Yellowish, strongly pleochroic, foliated 

scales of biotite occur occasionally, and rarely fragments of a palereddish, strongly absorbing mineral, supposed to be tourmaline. There 

are a small number of black, opaque grains, some of which are probably ores of iron, although none were found to be magnetic, even after 

heating. The flocculent yellowish or rusty-gray particles are clay, colored chiefly by oxide of iron. They become decidedly reddish by ignition, just as in the manufacture of red bricks the yellowish hydrous 

oxide of iron (limouite), by heating and driving off the water, is 

changed to red hematite.

In dilute hydrochloric acid the loess effervesces vigorously, but only 

for a few moments, showing the presence of small amounts of carbonate of lime. It generally occurs in the form of small nodules or tubes, 

and by studying the loess in the field it has been found that the 

minute tubes represent the rootlets of vegetation penetrating from the 

surface. They ramify the deposit, but as their general direction is vertical they give to the loess a vertical structure, which tends to make 

it form cliffs where sufficiently thick and properly exposed. This feature can be seen in PI. VII, which shows a typical exposure of the 

loess at Muscatine. At this exposure the loess appears to be massive, 

i.e., without evident stratification. Such is the general character of 

the loess, but at other exposures in the same region there are distinct 

traces of stratification due to aqueous deposition.

Besides the mineral constituents already noted, the loess from Muscatine contains a few minute coiled shells of land snails. According 

to W J McGee,2 the loess of the southern part of the State contains 

also the fragments of other land animals, a few water snails, and still 

fewer mussels, the latter of subarctic and arctic facies. The land animals are most closely related to arctic and subarctic forms, indicating 

that the loess originated during a cold epoch.

The distribution of the loess in the Mississippi Valley, as recently 

pointed out by Chamberlin, may be summed up in two great features:. 

It is distributed (1) along the leading valleys, and (2) along the border 

of the former ice sheet at the stage now kuown as the lowan. The 

loess occurs principally in the valleys of the Missouri, Mississippi, 

Illinois, and Wabash rivers. Along the Missouri Eiver it is found 

from southern Dakota to its mouth, and along the Mississippi River

1 Report of the Illinois World's Fair Commission, by Milton Wliitney, pp. 93-114. 

2Eleventh Ann. Rept. U. S. Geol. Survey, Part I, 1891, pp. 300 and 471.
U. S. GEOLOGICAL SURVEY

A TYPICAL EXPOSURE OF SOUTHERN LOESS, MU
DILLER.] DESCRIPTIONS: NO. 7, LOESS. 67

from Minnesota to southern Mississippi. Along the Illinois and the 

Wabash it occurs from the points of their emergence from the territory 

of the later glacial sheet to their mouths. In all these valleys the 

loess is thickest, coarsest, and most typical in the bluffs bordering the 

rivers, and becomes finer, thinner, and nontypical as the distance from 

the rivers increases.

The distribution of the loess along the front of the ice sheet in glacial 

times was elaborately worked out for Iowa by Mr. McGee, who showed 

that the loess material was the fine stuff ground up by the glacier 

which, during the loess epoch, terminated in that region. Many others 

have so greatly extended the evidence of this relation that it is now 

generally accepted as one of prime importance in considering the origin 

of the loess. While it appears to be clear that the material of the 

loess in the Mississippi Yalley was furnished by-the ice sheet, and that 

in some way it was distributed chiefly by water, there are many puzzling features of its distribution that are not easily accounted for; as, 

for example, the wide range in altitude of the loess deposits. The 

extent of the vertical range, as pointed out by Chambeiiin, is about 

1,000 feet, and frequently the range within a score of miles is from. 

500 to 700 feet. The fluctuating floods of the loess epoch must have 

exposed portions of the loess-covered flats to the action of the winds, 

and the fact that evidences of wind action in the original deposition of 

the loess have been observed has led Chamberlin to suggest that eolian 

as well as aqueous agencies may have played an important part in the 

deposition of the loess of the Mississippi Valley.

Loess similar to that of the Mississippi Yalley has been recognized 

in many parts of the earth, especially in the valley of the Rhine, along 

the Danube, and in various portions of southeastern Asia. Notwithstanding the similarity of its general features in many regions, its 

origin may be very diverse. In China, for example, its large masses 

are supposed by Richthofen to be chiefly wind-blown material derived 

from the disintegration of the rocks in the adjacent hills. Prof. I. C. 

Russell 1 has shown that the "adobe" in the arid regions of the West 

is practically identical with the loess of China, and that its subaerial 

accumulation is to be mainly ascribed to the action of ephemeral 

streams. Wherever it occurs the loess or adobe is a fertile soil and, 

excepting in arid regions, sustains a large agricultural population.

The student will find further information on this subject in the following papers: The Driftless Area of the Upper Mississippi Yalley, by 

T. C. Chamberlin and R. D. Salisbury, Sixth Annual Report of the 

United States Geological Survey (1884-'85), pp. 278-307; The Pleistocene History of Northeastern Iowa, by W J McGee, Eleventh Annual 

Report of the United States Geological Survey (18S9-790), Part I, pp. 

291-303; Supplementary Hypothesis Respecting the Origin of the Loess 

of the Mississippi Yalley, by T. C. Chamberlin, Journal of Geology, 

Yol. Y (November-December, 1897), pp. 795-802.

1 Geological Magazine, 1889, p. 349.
68 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

No. 8. BRICK CLAY.

(FROM BRICK HAVEN, ALEXANDRIA COUNTY, VIRGINIA. DESCRIBED BY J. S.

DILLER. )

Pine earthy material like specimen No. 8, which is somewhat firm and 

brittle when dry. but plastic and tenacious when wet, is clay. When 

pure, it is composed essentially of silicate of aluminum. Generally, 

however, it is impure from the presence of fine grains of quartz and 

oxide of jron, as well as other minerals.

. Specimen No. 8 is clay from a bank, represented in PI. VIII, on the 

Virginia side of the Potomac, nearly opposite Washington. Its paleyellowish color is due to the presence of hydrous oxide of iron (litnonite), 

which when the brick is burned loses water and becomes hematite, 

coloring the brick red.

.When stirred in water the clay readily goes to pieces; the coarse 

particles subside rapidly, the finer ones slowly. By decanting, the 

material can be separated into portions differing in size of the particles. 

Under the microscope the coarser portion is seen to contain numerous 

grains of quartz, with a few of clear banded feldspar. There are occasional minute round nodules of limonite; arid nearly all the grains of 

sand, as well as of clay, are stained yellowish by oxide of iron. The 

coarser portion is made up chiefly of quartz grains and oxide of iron, 

arid many of the grains of quartz are well rounded. In the finer portion clay particles are more abundant, but they are much smaller than 

the quartz and other mineral fragments. The clay is in very fine scales 

and is stained reddish yellow by oxide of iron. The scales look like 

minute flakes of mica, but may be distinguished by their very weak 

double refraction. The basis of clay is silicate of aluminum, and when 

pure in raiimte scales it is colorless. The aggregation of such scales 

looks white, like snow, on account of its porous structure.

Clay is the insoluble residue left from the decomposition and disintegration of aluminous silicates, especially feldspar, but it is derived also 

from nephelite, scapolite, and other minerals containing much silica and 

alumina. The white clay (kaolin) derived from feldspar is illustrated in 

specimens Nos. 147,148, and 149, which are the residual material of felclspathic rocks. Specimen No. 150 is clay derived from the decomposition 

of an argillaceous limestone. The clay was originally deposited with 

the carbonate of lime in the limestone as it formed. Later, when the 

limestone was raised above the sea and exposed to the weather, the 

rain dissolved the soluble carbonate of lime and left the insoluble 

residual clay.

The clay represented by specimen No. 8 forms a deposit of considerable extent along the Potomac near Washington,, and differs from the 

clays mentioned above in that it has been transported by water from the 

place where it originated to its present position. The deposit rises less 

than 150 feet above the river. It is extensively used in the manufacture
CK HAVEN, VIRGINIA, OPPOSITE WASHINGTON, DISTRICT OF COLUMBIA.
DILLER.] DESCRIPTIONS: NO. 9, BOWLDER CLAY. 69

of brick, and a view in one of the clay pits is given in PI. VIII. According to W J McGee, 1 the clay belongs to the Columbia formation, 

and was deposited in an estuary on the borders of a river delta of the 

ancient Potoinac. At the time this clay was deposited (i. e., during 

the later Columbia epoch) the Atlantic slope in the vicinity of Washington stood about 150 feet below its present level. The Potomac entered the estuary at Washington and dropped its load of sediment, of 

which the clays were the finer portion. They were derived, at least 

in large part, from the loose mantle of residual material resulting 

from the decomposition and disintegration of the rocks in the Potoinac 

basin.

From a study of the relation of the Columbia formation to bowlderbearing strata and to the great terminal moraine which stretches 

across the country from Cape Cod in Massachusetts to Dakota, Mr. 

M'cGee concludes that the clays of the Potomac river near Washington 

were deposited during the first and second glacial periods, in the early 

part of the Pleistocene.

No. 9. BOWLDER CLAY.

(FROM ROCHESTER, NEW YORK. DESCRIBED MY G. K. GILBERT.)

Certain clays produced by glaciers contain pebbles and bowlders, 

and are therefore called borrlder clays. In the description of specimen 

No. 2 some account is given of glaciers and ice sheets, and mention is 

made of the ice sheet which in Pleistocene time covered a large portion 

of northeastern North America. As the stones held in an ice sheet 

rub one on another and are gradually reduced in size, the particles 

ground off remain embedded in the ice. Where the stones rub against 

the bed rock, not only are they ground away, but the bed rock itself is 

worn, aud the product of all this abrasion is received by the ice and 

carried forward. Where the ice melts, its load comes to rest, forming a 

deposit, and in this deposit the coarser and finer fragments may be 

indiscriminately mixed. Such a mixed deposit is called till. Sometimes its finer part is sand, but usually there is enough clay to make it 

coherent, and the name bowlder-clay is then commonly used. Bowlderclays are heterogeneous not only in mechanical structure but in composition. The ice in its journey abrades many rocks, and the particles 

ground from all these are mingled together in the resulting till. Tho, 

ratio in which the different rocks traversed are represented in the till 

depends partly on the extent of their outcrops and partly on the readiness with which they are ground away; and it is also found that the 

nearest* rocks are relatively better represented than the more remote. 

In the northern States the motion of the ice sheet was southward, and 

the till at each locality is largely composed of pebbles and rock flour 

from the formation lying immediately north of it. Specimen No. 9 is.

1 Prou. Am. Assoo. Adv. Sci., Vol. XXXVI, ]> 221. Also Am. Jour. Sci., ad series, Vol. XXXV, p. 331,
70 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

from a till deposit underlain by the Niagara limestone, but a short distance to the north begins a broad outcrop of Medina shale, and this 

formation furnished the chief material of the clay, giving its reddish 

color.

The specimen appears, on first examination, to contain a few fine 

pebbles and very little sand; but when the coarser portion is separated 

from the finer by successive washings, sand is found to form a considerable proportion of the whole mass. The sand is chiefly quartz, with a 

small amount of fe'ldspar and green hornblende and much oxide of iron. 

Some of the larger grains are angular, but the smaller ones are often 

rounded. The very fine material is not a normal clay, such as results 

from the decomposition of aluminous silicates, but is a rock flour, due 

to the trituration of a variety of materials, including various unaltered 

as well as decomposed silicates and the carbonates of lime and magnesia. The sand grains were probably derived chiefly from the sandstone of the Medina formation, and had been rounded before they were 

received by the ice.

Further information will be found in the descriptions of specimens 

Nos. 2 and 155. The reader is also referred to The Terminal Moraine of 

the Second Glacial Epoch, by T. 0. Chainberlin, in the Third Annual 

Eeport of the United States Geological Survey; and to The Surface 

Geology of New Jersey, by E. D. Salisbury, in the Annual Eeport of 

the State Geologist of New Jersey for 1891.

No. 10. CONGLOMERATE.

(FROM UTFCA, ONEIDA COUNTY, NEW YORK. DESCRIBED BY J. S. DILLER. )

When gravel is cemented so that the pebbles and sand of which it is 

composed are bound together, the rock formed is conglomerate. It differs from gravel'only in containing a cementing substance which converts the loose material into a solid mass.

In specimen No. 10 the pebbles are quartz. They are small and 

generally well rounded. Some, indeed, are subangular, but most of 

them have the corners completely reduced. The sand, and also the 

cement which holds the fragments together, are chiefly quartz. 

Although there are a few brass-yellowish grains of pyrite, and some 

black ones of other minerals, almost the whole mass of the rock is 

silica. It is a deposit in which quartz is so abundant as to indicate 

that at the time the conglomerate was formed the conditions were 

especially favorable for the accumulation of quartz.

The bed of conglomerate from which specimen No. 10 was taken is 

well exposed in Oneida County, New York, and on this account is called . Oneida conglomerate. It is associated with a thick sandstone the 

Medina which varies in color from red to white, and which may be 

traced almost continuously from western New York eastward to near 

the Hudson, where its upturned edge swings to the southwest, stretching
DILLEB.] DESCRIPTIONS: NO. 10, CONGLOMERATE. 71

away through Few Jersey, Pennsylvania, Maryland, Virginia, and Tennessee into Alabama. From this long line the outcrops of tlie Medina 

sandstone and conglomerate extend westward to the Mississippi Valley, 

bntiu that direction the material gradually becomes finer.

The shells found in the sandstone, as illustrated in specimen No. 19, 

show that it was deposited in the ocean. The reduction in the size of 

the particles of the sediment toward the Mississippi Valley, in accordance with the general relations of littoral deposits illustrated in fig. 7 

(p. 57), indicates that the shore of the ocean lay to the eastward, in the 

Appalachian region, and that at that time the sea occupied the Mississippi Valley.

Most conglomerates contain a considerable proportion of quartz pebbles, or pebbles of siliceous rock, but besides these they usually contain 

pebbles of many other kinds. The most abundant pebbles are those of 

the most durable rocks, such as vein quartz, quartzite, graywacke, 

granite, and various kinds of volcanic and plutonic rocks, especially 

those which contain a high percentage of silica. When pebbles of any 

one kind predominate, special names, such as quartz conglomerate, limestone, conglomerate, and volcanic conglomerate, may be given to the rock, 

according to the prevailing constituent.

Conglomerates may be coarse or fine, according to the size of the 

pebbles of which they are composed. Fragments larger than pebbles 

are often called bowlders, and conglomerates containing them have 

been designated boivlder conglomerates. The size of the fragments and 

their surface features are indexes to the character and strength of the 

transporting power by which they were deposited.

As indicated in the descriptions of specimens Nos. 1 and 2, gravel 

originates wholly in the process of erosion, either by glaciers or streams 

of water on the land, or by waves of the sea beating on the coast. On 

stormy coasts, where much gravel is produced and the undertow is 

strong enough to carry it seaward, it is spread over the bottom near 

shore to form a bed of conglomerate, as illustrated in fig. 7. Being 

beneath the sea, the deposit is below the level of erosion and is preserved so long as it remains in that position. On the land, however, 

the conditions are different. The gravel deposits of glaciers and water-, 

courses are ever exposed to erosion. By shifting floods they are gradually washed down to lower levels toward the sea, and finally the material 

is carried into the sea, where it finds a resting place. For this reason 

gravel deposits of the land are ephemeral. In the process of erosion 

they are carried into the sea, where, like those formed by the waves, 

they are added to the more permanent deposits which contribute to the 

upbuilding of new formations. The coarsest material is deposited 

nearest shore and marks approximately the direction of the shore line, 

so that by tracing out the coarse conglomerate among the ancient sedimentary rocks we can to a considerable extent make out the geography 

of the land aud sea. The distribution of the sandstone and conglomerate
72 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

of which specimen No. 10 is a sample shows clearly that the shore line 

of the Medina epoch was in the Appalachian region and that the laud 

furnishing the sediment lay to the eastward.

The cement which binds the loose material together and converts 

gravel into conglomerate varies from place to place, and sometimes holds 

a more or less definite relation to the composition of the rock. In conglomerates where the sand and pebbles are chiefly quartz the cement 

is often siliceous; in those containing fragments rich in iron it is 

generally ferruginous. Silica and oxide of iron are the most common 

cementing substances, but carbonate of lime also occurs in some 

regions. These three cements are illustrated in sandstones by specimens Nos. 12, 14, and 15. In some cases there is no visible cement, the 

pebbles and matrix beiug so closely pressed together that they adhere.

No. 11. BRECCIA.

(FROM VIRGINIA, OPPOSITE POINT OF ROCKS, MARYLAND. DESCRIBED BY

J. S. DILLER.)

Breccia differs from conglomerate in the shape of the fragments of 

which it is composed. In conglomerate most of the pebbles are rounded, 

but in breccia the fragments are angular. Intermediate stages between 

the two rocks have been called brecciated conglomerate.

Breccias are much less common than conglomerates, and are produced in various ways. Those of sedimentary origin are of little 

importance and grade into conglomerate. Specimen No. 11 is of this 

type. It was selected on account of its distinctly fragmental structure, 

its availability, and its architectural application. Although many of 

the fragments are angular, others are well rounded; in fact, at most 

places where this rock crops out the round pebbles predominate, so 

that generally the rock is a conglomerate. The fragments are nearly

all limestone, and at the time the breccia originated the fragments 

were transported only a short distance from their source. This fact is 

readily determined by studying the rock in the field, where it occurs 

near the limestone from which the fragments were derived.

The color of the limestone fragments in specimen No. 11 varies according to that of the parent rock, but the interstitial material of sand, carbonate of lime, and oxide of iron in which the pebbles are embedded is 

uniformly red, like much of the Triassic sandstone belonging to the 

same formation.

As in conglomerate, there are three substances which act as cement 

in the breccia carbonate of lime, oxide of iron, and silica and all are 

of nearly equal importance. If a piece of the red material between the 

pebbles is placed in hot hydrochloric acid until the carbonate of lime 

and oxide of iron are dissolved away, the fragment usually retains its 

form, owing to the siliceous cement present.

. The rock of which specimen No. 11 is a sample has been used for ornamental building purposes. A series of large columns of this material
DILLER.] DESCRIPTIONS: NO. 11, BRECCIA. 73

adorn the old Hall of Eepresentatives, now called Statuary Hall, in 

the Capitol at Washington. But it is so difficult to dress and polish 

evenly that it is not extensively used for such purposes.

For further information concerning this rock, reference should be 

made to a paper by Arthur Keith on The Geology of the Oatoctin Belt, 

in the Fourteenth Annual Keport of the United States Geological

' FIG. 9. Brecciated. Devonian limestone, Fayette, Iowa.

Survey, Part II, page 346, and to Stones, for Building and Decoration, 

by G. P. Merrill, page 93.

Besides the breccias of sedimentary origin, illustrated by specimen 

No 11, there are tahis breccias, fault or friction breccias, and volcanic or 

eruptive breccias. Under the influence of the weather, on steep slopes 

rocks break up into angular fragments, and beneath cliffs such fragments generally accumulate and form a talus, which in some places 

becomes cemented so as to form talus breccia. In breccias of this sort
74 THE EDUCATIONAL SEEIES OP ROCK SPECIMENS. [BULL. 150.

none of the fragments are rounded. They occur perhaps most frequently in regions of extensive limestones, especially such as are 

cavernous, limestones furnishing both the fragments and the cement.

Along lines of fracture, where rocks have been faulted and crushed, 

the angular fragments thus formed may be cemented by substances held 

in solution by water circulating in the fissures, and thus form breccia. 

Such breccia is called fault breccia or friction breccia. Breccias of a 

smilar sort, whose origin is in some cases at least not yet clearly understood, are illustrated by fig. 9, which represents a limestone breccia 

described by W J McGee in the Eleventh Annual Keport of the 

United States Geological Survey, Part I, pages 319-321.

Fragments ejected from volcanoes are often angular, and a consolidated accumulation of them forms volcanic breccia. Acid lavas are 

usually viscous, and during their eruption are sometimes so crushed 

'and broken as to become a mass of angular fragments. When cemented 

together by material of the same kind as the fragments, as is often the 

case, such lavas are breccias, and to distinguish them from others are 

called lava breccias or brecciated lavas. Lava flowing over angular fragments on the surface may pick them up, and the mass may thus become 

brecciated.

In the various kinds of breccias there may be a wide range not only 

in the size of the fragments but also in their chemical composition. 

Usually, however, there is less variety in composition than among the 

pebbles of a conglomerate, but a greater range in size.

No. 12. PEBBLY SANDSTONE.

(FROM BARKON, NEAR ASHLAND, JACKSON COUNTY, OREGON. DESCRIBED BY J. S.

DILLER.)

Conglomerates are composed of pebbles, and sandstones are composed 

of sand. They are often found intermingled as alternating layers of the 

same mass and pass into each other gradually or abruptly, recording the 

gradual or sudden change in the currents by which the material was 

deposited. Between conglomerates and sandstone there are many 

intermediate grades. These may be represented by specimen No. 12.

Pebbly sandstone is composed chiefly of sand, but contains so large 

a proportion of conspicuous pebbles that these deserve mention in the 

name. This specimen was collected from a pebbly bed in a mass of 

Cretaceous sandstone resting unconformably on the older rocks of the 

Klamath Mountains, from which the material to make the sandstone 

was derived. The dark-colored pebbles are chiefly slates, while the 

lighter-colored ones are from masses of serpentine and other eruptive 

rocks. The fragments are all metamorphic rocks, and some are full of 

small veins.

Gray sand, which constitutes the greater portion of the rock, is composed chiefly of quartz and feldspar, with some mica and. other minerals 

derived from the diorities, granites, and similar eruptive rocks, as well 

as from the slates with which they are associated in the Klarn'ath
DILLER.] DESCRIPTIONS: NO. 13, GRAY SANDSTONE. 75

Mountains. Some of the small, dark grains of sand are, like the pebbles, intersected by microscopic veins. Although the pebbles are well 

rounded, the grains of quartz and feldspar are angular. This feature 

is well illustrated in PI. IX, A. Some of the grains are of plagioclase 

feldspar, but their banding can not be seen in ordinary light. The 

other grains are of quartz, with sharp, angular outlines, strongly 

contrasting with the rounded forms of the pebbles with which they are 

associated. This association shows clearly that the pebbles are rounded 

more easily than the grains of sand, and the reason for this is to be 

found in the fact that the grains of sand, being so light in the buoyant 

water, strike such tiny blows when they collide with one another 

during transportation that but little effect is produced. On the other 

hand, the pebbles, on account of their greater weight, strike much 

more effective blows and soon get their corners knocked oft'.'

A drop of acid on specimen No. 12 causes brisk effervescence, showing the presence of carbonate of lime as a cementing substance between 

the grains of sand and pebbles.

At the time the pebbly sandstone was formed the Klarnath Mountains 

were an island in the Cretaceous seas and received the beat of the 

waves, which, to a large extent at least, produced the fragments and 

deposited them to make the .pebbly sandstone. Although all the 

fragments in the pebbly sandstone are of metamorphic rocks, the 

pebbly sandstone itself is entirely unaltered. So it is evident that the 

rocks of the Klamath Mountains were metamorphosed before the 

Cretaceous sandstone was deposited.

No. 13. GRAY SANDSTONE.

(FROM BEREA, CUYAHOQA COUNTY, OHIO. DESCRIBED BY J. S. DILLER.)

Sandstone is consolidated sand. As long as the material is loose and 

incoherent it is sand, but whenever by any process the particles are 

made to cohere so as to form a solid rock, they become sandstone. All 

sandstones were originally sarjd and show similar variations in composition and texture. The range of variation, however, is greater 

than in sands, on account of the differences in the composition, color, 

and other properties of the cement.

The grains of some porous sandstones are angular and so loosely 

cemented that when broken the surface of the rock is rough and gritty 

to the feel. Such sandstones are commonly called grits, and of these 

specimen No. 13 is a good example. It is well exposed at Berea, Ohio, 

where it is extensively quarried for building stones and for grindstones 

and is generally known as the Berea grit.

The Berea grit is a tine-grained, homogeneous sandstone, composed 

almost wholly of quartz and orthoclase feldspar. The latter is greatly 

altered and in most cases completely changed to kaolin, which is readily 

distinguished by its whiteness. With the gray quartz, it gives color to 

the rock. Some grains of kaolin contain fresh cores of feldspar, and a few 

comparatively fresh grains of microcline and plagioclase may be seen.
76 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Muscovite and pyrite.are rather rare. The cementing substance is 

argillaceous and is impregnated by oxide of iron, but there is not a 

sufficient quantity present to modify the color of the rock. The cement 

is weak and does not fill all the interstices between the grains of sand. 

On this account the rock is soft and .porous, so that it can be easily 

carved and readily absorbs a large amount of water.

Kaolin occurs in distinct grains of essentially the same shape as the 

quartz, although somewhat more rounded. These grains, as well as 

the thick coating of soft kaolin which envelops some of the rounded 

grains of feldspar, had not yet been formed at the time the sandstone 

was deposited, else it would have been removed by the attrition the 

grains have experienced during their transportation.

The original freshness of the material and the presence of such a 

considerable portion of feldspar suggest that the surface of the land 

at the time the sandstone was formed must have been one of considerable relief. It is only when streams have considerable fall that 

the currents are swift and strong enough to carry grains of sand and 

pebbles. In a rapid stream pebbles and bowlders are rolled along on 

its bed. They frequently knock together, break to pieces, and by 

long-continued attrition are reduced, to sand and finer sediment, but 

the sand formed in such cases is composed of fresh minerals. The 

feldspar, at least when the sand originates, is chiefly unaltered, but 

on subsequent exposure to weathering it may readily become changed 

to kaolin, as in the Berea grit. Had the land been one of gentle relief, 

.worn down to almost a plain (peneplain), the streams would have been 

sluggish and able to remove only the material resulting from the 

'decomposition and disintegration of the rocks. The.fine particles of 

kaolin derived from the alteration of the feldspar are readily separated 

from the quartz during transportation, leaving the sand composed 

almost exclusively of quartz. Thus it appears that the original presence of a considerable proportion of unaltered feldspar indicates that 

the land from which it was derived at the time the Berea grit was 

formed was one of considerable relief. Chemical analysis of the rock, 

according to Mr. G. P. Merrill, 1 shows that the rock contains about 

95 per cent of silica, with a small amount of lime, magnesia, oxide 

of iron, alumina,.and alkalies. When freshly quarried it contains 

from 5.83 to 7.75 per cent of water, but when dry only 3.39 to 4.28 per 

cent.

The Berea grit has a wide distribution in Ohio, having an extent of 

about 15,000 square miles above and below ground. This wide extent 

is remarkable considering its thickness, as it seldom reaches 50 feet. 

In the northern part of the State it is medium grained and contains 

some pebbles, but in the middle and southern portion of the State it 

is fine grained. Its surface is often ripple-marked, and worm burrows 

abound as on the sands of modern beaches, indicating that the Berea 

grit was formed along an ancient shore line.

1 Stones for Building and Decorafciou, p. 77.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. IX

THIN SECTIONS OF SANDSTONE.

A. Pebbly sandstone from Barren, Oregon, as seen under a microscope in ordinary light, X 50. 

B. Brown sandstone from Hummelstown. Pennsylvania, as seen under a microscope in ordinary light, X 50.
DILLEK.] DESCRIPTIONS: NO. u, BROWN SANDSTONE. 77

On account of its agreeable color, its durability, and the ease with 

which it is worked, it is a.valuable building stone. Its grit makes it 

valuable for grindstones, and its porosity makes it a reservoir for 

petroleum and gas. According to Prof. Edward Ortou, 1 it is "the most 

important single stratum in the entire geological column of Ohio. Its 

economic value above ground is great, but it is greater below. In its 

outcrops it is a source of the finest building stone and the best grindstone grit of the country, and when it dips beneath the surface it 

becomes the repository of valuable supplies of petroleum, gas, and 

salt water."

No. 14. BROWN SANDSTONE.

(F.ROM HUMMKLSTOWN, DAUPHIN COUNTY, PENNSYIA'ANIA. DESCRIBED HY

J. S. DlLLEU.)

The sandstone of Hummelstown, Pennsylvania, is a typical sandstone 

with ferruginous cement. In color it is usually purplish brown with 

minute white specks. The uniformity in the size of its rather small grains 

gives the rock an even texture. Its feel is decidedly gritty, owing to 

the angular form of the grains. It is composed chiefly of angular grains 

of quartz with some clear, fresh microcliiie and plagioclase, showing distinct twinning. Occasionally fragments of a mineral with very strong 

absorption perpendicular to the prismatic axis, and parallel extinction 

like that of tourmaline, may be found. The kaolin present is sometimes 

in distinct grains of about the same size as those of quartz. It was 

deposited chiefly as finer silt between the grains of sand, and is much 

less abundant than the quartz. The largest grain shown in the lower 

left-hand quarter of PI IX, J3, is kaolin. The others are nearly all 

quartz. The brownish cement which is the chief interstitial substance 

and coats many of the grains of quartz is ferric oxide. On account of 

its abundance it gives color to the whole mass. As its color varies 

through shades and tints of brown and red, so also the rock varies in 

color.

The following analysis, by E. A. Schneider, shows the chemical composition of the rock:

Analysis of broicn sandstone from Hmnmelstown, f'unnsyluania.

SiO, .............................................

A1203 ... .........................................

Tfe.jQ.t.................. ..........................

FeO. .............................................

CaO................................... ...........

MgO.. ...........................................

K20 ..............................................

Na,O- ............................................

H.jO (105°).. ......................................

Total .......................................

Per cent. 

88.13

5.81

1.77

.31

.20'

.53

U.63

.06

90

.26

99.93

1 Geological Survey .of Ohio, Economic Geology, vol. G, p. 28.
78 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

At the quarry from which specimen No. 14 was obtained the rock near 

the surface is reddish brown, and the greater body of the rock deeper 

in the earth is purplish brown. The bedded arrangement of the rock 

is a prominent feature of the quarries. The even layers are usually less 

than 10 feet in thickness and are cut by joints, which greatly facilitate 

quarrying.

The formation represented by the brownstone of Hummelstown, 

Pennsylvania, has a wide and irregular distribution along a belt 

stretching from New England to South Carolina. Throughout the 

whole belt, although it varies in texture considerably, ranging all the 

way from a coarse conglomerate and angular breccia to shale, it is 

everywhere deeply colored by oxide of iron and frequently associated 

with compact, dark, heavy, igneous rock, such as the basalt of Orange, 

New Jersey, illustrated by specimen No. 102. Specimen No. 11 is 

breccia from the border of the same formation near the Potomac.

Much has been written on this formation. Its bibliography is given 

by Prof. I. C. Russell,1 who illustrates its distribution by maps and 

fully describes its character and the hypotheses concerning its history. 

He calls it the Newark system. Fossil plants and fishes, and also the 

footprints and bones of huge reptiles and batrachians, have been found 

at a number of places, and their evidence fixes the age of the Newark 

as Juratrias. Whether it was deposited rn a series of local basins, 

corresponding to the present disconnected distribution of the rocks, or 

as a broad terrane in one irregular and continuous arm of the sea 

stretching from New England to South Carolina, is as yet a matter of 

discussion.

The uniformly red and brown color of the formation throughout its 

whole extent indicates uniform conditions over the whole area. As 

shown by Mr. Russell,* it suggests a mild, moist climate. Gneiss, 

schists, and similar rocks containing much pyroxene, hornblende, and 

mica or other ferromagnesian silicates are not usually red when undecoinposed. Under the influence of the weather, however, these ironbearing minerals may be altered and much ferric oxide developed, coating the grains of quartz and other unchanged minerals red or brown. 

If these products of subaerial decay are washed away and deposited to 

form sandstones and shales without wearing off the ferric oxide coating 

the grains, the new rock will be red or brown; and thus, it is thought, 

the red color of the Newark system may be explained.

The great length of the formation, taken in connection with its small 

breadth and great thickness, and its intimate association with basaltic 

igneous rocks having the same lineal arrangement and being, at least 

in part, of contemporaneous origin, are among its most important 

features. The rock is extensively used as a building stone in cities of 

the Atlantic States, and at several points coal beds of importance have 

been discovered.

1 Bull. U. S. Geol. Survey No. 85,1892:

2 Bull. U. S. Geol. Survey No. 52,1889, p. 56.
DILLEB.] DESCRIPTIONS: NO. 15, POTSDAM SANDSTONE. 79

No. 15. POTSDAM SANDSTONE.

(.FHOM AKLEMANS, SAUK COUNTY, WISCONSIN. DESCRIBED HY J. S. DILLKJI.)

The Potsdam sandstone is so named from its occurrence at Potsdam, 

hi northern New York. It has been traced through a wide stretch of 

country southwest and west of New York; and at many points, as in 

Wisconsin, where it has been positively identified, the same name is 

applied.

It is a typical quartz sandstone in which the sand is largely siliceous. 

It is light, almost colorless, on account of the transparency of the quartz 

of which it is composed, although there is here and there a suggestion 

of pale rusty yellow, due to the trace of ferric oxide present in the 

cement.

Its structure is decidedly granular and somewhat porous. The interstices between the grains are in many cases not completely filled. The 

feel of specimen No. 15 is less gritty than that of specimen No. 14, and 

ii the surfaces of the two specimens be examined with a lens it will be 

observed that when specimen No. 15 is fractured many of the grains 

break, but in specimen No. 14 the cement breaks and the grains pull 

apart, leaving the surface with more angular projections.

The grains are nearly all quartz, colorless and transparent, excepting 

the faint gray clouding due to the occasional presence of the large number of liquid inclusions. The dark material forming part of the cement 

between the grains in Fig. A, PI. X, is ferric oxide. The outlines of 

the original well-rounded grains are generally indicated by a clouded 

border which marks off the siliceous cement between them. In Fig. 

jB, PI. X, it may be seen that the cement is sometimes oriented, so as to 

be optically continuous with the adjoining grain and extinguished at 

the same time. In the lower right-hand portion of Fig. J5, PI. X, is a 

banded grain of feldspar. At the upper left hand of the feldspar,, as 

seen in Fig. A, PI. X, is an elongated triangular grain of ferric oxide, 

and at its right a well-defined area of interstitial quartz which belongs to 

the adjoining grain. The fragment of quartz crystal represented by the 

grain grew by additions to the outside until the intervening space was 

completely occupied. In some places complete crystal faces have been 

developed. The growth of quartz grains and of other minerals in this 

manner, and the consequent induration of the rocks containing them, 

is a nietamorpbic process. It has been illustrated and discussed by a 

number of authors, especially by Irving and Van Hise.1

This rock is much used for building purposes, and on account of its 

siliceous cement is especially durable. In some localities, however, 

where the formation crops out, the cement is not siliceous, or the rock 

may contain small accumulations of clay. Both of these features, wherever they occur, lessen the value of the stone for building purposes.

The sand of which the Potsdam sandstone was formed is much more

'Bull. U. S. Geol. Survey No. 8, ami Am. Jour. Sci., Vol. XXX, p. 231, and Vol. XXXIII, p. 285.
80 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BUU..ISO.

distinctly rounded than most of the beach saud.along the Atlantic coast 

from New England to Florida. It has been so much worn that the feldspar and other minerals softer than the quartz have been almost completely ground to silt and removed. It suggests, also, that the material 

from which the sand was derived may have been in a greatly decomposed and disintegrated condition, and the landscape one in which 

gentle slopes prevailed; for under such circumstances the altered minerals are unusually soft, so that they are easily ground fine during transportation, and separated from the quartz.

The following chemical analysis, by E. A. Schneider, shows the 

highly siliceous character of this sandstone:

Analysis of Potsdam sandstone from Ablemans, Wisconsin.

SiO,

HjO (ignition) 

Total

Per cent.

99.42

.31

.18

99.91

No. 16. BANDED SANDSTONE.

(FKOM PEOA, SUMMIT COUNTY, UTAH. DESCKIBED BY J. S. DIIXEH.)

This sandstone is arranged in distinct layers, beds, or bands, which 

in the field, where large exposures may be seen, are clearly expressed, 

chiefly in differences of color. The banding may be seen in the hand 

specimen, but is not conspicuous. It is parellel to the stratification, 

and was determined -when the material was deposited, although the 

peculiarities of color may not all have been developed at that time.

The sand of which this stone is composed is almost wholly quartz. 

Here and there are traces of unaltered feldspar. Grains of kaolin are 

more abundant than those of fresh feldspar, and they may in some 

cases be seen as minute white specks in the hand specimen.

Under the microscope the granular structure is much more distinct. 

The grains of quartz are well rounded. Both silica and ferric oxide 

appear in the cement. The former is perhaps the more abundant, and 

is in places optically continuous with the adjoining grain, showing that 

the crystallographic force in the grain controlled its deposition, and 

the matter was so arranged as to form a growth in the crystal like that 

in the Potsdam and many other sandstones. The ferric oxide is sufficiently abundant to give a decidedly reddish color to the rock, and its 

arrangement has given rise to the banding of the rock. Some bands 

contain much oxide of iron and others but little. The grains of sand 

in the various bands do not differ among themselves essentially in size 

or material. This is not generally the case in beds of stratified rocks, 

for in such rocks the sediments are ordinarily arranged according to the
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. X

THIN SECTIONS OF POTSDAM SANDSTONE. FROM ABLEMAN, WISCONSIN, AS SEEN

UNDER A MICROSCOPE, X 50. 

A. In ordinary light. B. Between crossed nicols.
DILLER.] DESCRIPTIONS: NO. 17, LAMINATED SANDSTONE. 81

size and weight of the fragments, thus producing stratification, as 

illustrated in the laminated sandstone, specimen No. 17.

Following is a chemical analysis of the sandstone, by E. A. Schneider, 

which shows the very siliceous character of the rock:

Analysis of bunded sandstone from Peoa, Utah.

Si04 .............................................

ALA )

Fe2 03 * 

EoO.. ............................................

CaO..............................................

MgO....... ......................................

K20.. ............................................

Na2 0... .........................................

H2 0 (105°) .......................................

Total ......................................

T:'er cent.

00.60

2.02

.04

.08

.11

.29

No. 17. LAMINATED SANDSTONE.

(FKOM HOI-YOKE, HAMPDKN COUNTY, MASSACHUSETTS. DESCRIBED UY J. S.

DILLER.)

The sand of which this stone is composed is very fine and is arranged 

in such thin sheets as to produce laminated structure. The material is 

much finer than that of other sandstones in the series,' and approaches 

mud or clay in character. Upon the broad surfaces of the hand specimen may be seen many glistening scales of mica, which lie parallel to 

the stratification. On account of their extreme thinness the scales of 

mica readily float and are carried away to be deposited with finer sand 

and mud.

The composition varies greatly perpendicular to the stratification or 

lamination, but parallel to it within the same layer the composition 

is comparatively uniform. The lighter-colored layers are composed 

chiefly of quartz grains with mica, some grains of feldspar, tourmaline, 

and other minerals. The quartz is often well rounded and coated with 

oxide of iron. The darker-colored, red layers contain finer material and 

more angular particles. Mica, oxide of iron, and argillaceous material 

are much more abundant, and represent a quieter stage of the water 

than the coarser films.' Systematic variation of these layers indicates 

a corresponding variation in the conditions of deposition. Where 

the coarser and thicker portions were laid down the water was more 

vigorously in motion than where the fine sediment was deposited. 

The coarser may represent times of heavy rains, melting snow, or floodtide, and the finer, periods of low, quiet water, carrying a much smaller 

amount of sediment. 

Bull. 150 6
82 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULUISO.

Specimen No. 17: illustrates the laminated sandstone of the Triassic 

rocks in the Connecticut Valley, and belongs to the .same formation as 

the brownstone of Hummelstown, Pennsylvania, the distribution of 

which is indicated under specimeni No. 14. Throughout this large area 

the conditions vary greatly; and while the laminated sandstone \vas 

forming in some places, coarser sandstone and conglomerate were form 

ing in other places, so that the same stratum may show lateral transitions 

from the finer to the coarser sediments.

NO. 18. RlPPLE-MAHKED SANDSTONE.

(FROM HOLYOKB, HAMPDEN COUNTY, MASSACHUSETTS. DESCRIBED BY J. S.

DILLEK.)

The wavelike marks upon the upper surface of specimen No. 18 are 

ripple marks, and were produced by corresponding movements of the 

water at the time the sediments were deposited. In the water they are 

formed only where it is shallow, and they do not extend beyond the 

depths to which the water is agitated by the wind. At low water they 

are well exposed along the sandy shores of the ocean, but they generally attain their most regular development upon the land in regions of 

wind-blown sand.

PI. XI illustrates the ripple-marked surface of one of the sand dunes 

near Golden Gate Park, San Francisco, California. The bent twigs in 

the foreground show the direction of the prevailing strong winds to be 

from-the left; that is, from the ocean. The axes of the ripple marks 

are perpendicular to the course of the wind, and the slopes of the 

small ridges are not equal. Upon the windward side the slope is long 

and gentle; to the leeward it is short and steep. The sand blown by 

the wind moves up the long slope and falls over the shorter one, causing the ridge to move forward with the wind, but at a much sloAver 

rate. Under the influence of strong winds from the Pacific, the ripple 

marks illustrated in PI. XI gradually advance from, left to right. The 

whole surface is in motion and the dunes travel landward.

The development of ripple marks under water is not so simple a matter as their subaerial development, where they are due wholly to the 

influence of the wind. The ripple marks formed by water are rarely 

so regular as those illustrated in PI. XI. In specimen No. 18 the upper 

surface is of finer material than that of which the ripple marks are 

chiefly composed, and in the deposition of this sediment the irregularities of the rippled surface were rendered less conspicuous. Originally 

the material now exposed upon the rippled surface was mud. It was 

soft and easily impressed.. At low water the surface was uncovered 

and exposed to the weather. Insects, birds, and other animals crossing 

the mud flat afoot left tracks. The footprints were covered up and preserved by later deposits so as to remain in the rocks, and to-day afford 

evidence of the character of the animals that lived when the rock was
U. S. GEOLOGICAL SURVEY 

jl^^^^^^^^^Epf w£&Si^^S3i£!j^^^^^fau^^t>ri&^*ai!i 

^lipllklppli^

RIPPLE MARKS ON SAND DUNES NEAR GOLDEN GATE PARK S
DESCRIPTIONS: NO. 19, FOSSILIFEROUS SANDSTONE. 83

formed. . The sandstone of the Connecticut Valley, from which specimens Nos. 17 and 18 were collected, have long been celebrated for the 

large fossil footprints it contains in certain localities. During the early 

portion of the Juratrias period amphibians and reptiles of large size 

traversed the muddy flats of the Connecticut Valley and left tracks in 

some cases nearly 2 feet in length. In places where the mud was 

exposed long enough to dry, reticulated cracks were developed. The 

succeeding flood filled the mud cracks with sand, and when the rock is 

split apart the filled mud cracks intersecting the ripple marks appear 

as in PI. XII.

NO. 19. FOSSILIFEROUS SANDSTONE. 

(PuoM MEDINA, ORLEANS COUNTY, NEW YORK. DESCRIBED BY J. S. DILLEH.)

Fossiliferous sandstone differs from other sandstone only in that it 

contains fossils. In this specimen from Medina, New York, commonly 

called the Medina sandstone, the sand, like that of the Potsdam sand-. 

stone, is almost exclusively quartz. The fossils it contains are either 

casts or shells shaped somewhat like a tongue, on which account the 

little mollusk was named Linyula. Lingula is one of the most ancient 

genera, ranging from the lower Cambrian to the present day. In the 

course of evolution it has been remarkably persistent, and, unlike most 

forms, has suffered comparatively little modification under the influence 

of a long series of geologic changes.

The fossils found in the sandstone are generally of animals which 

lived upon, or directly above, the sandy bottom of the sea. Barely the 

fossils are of land animals or plants which have been brought to the 

sea by rivers and buried in the sand.

The fossiliferous sandstone at Medina is composed chiefly of quartz, 

the grains of which are rather angular. The preservation of the delicate shells indicates that the sand, at the time of its deposition, was 

not subjected to great attrition, else the fragile shells would have been 

ground to pieces. ° The cementing substance between the grains is 

chiefly carbonate of lime, probably derived in large part from the 

fossils.

This sandstone has a wide distribution through the eastern part of 

the United States, especially along the Appalachian Mountains, where 

it is evidently a shore deposit, passing into conglomerate, and containing ripple marks, rill marks, and other evidences of littoral origin. 

The Medina sandstone is associated with the Oneida conglomerate, 

and its distribution is more fully given on pages 70 and 71. Being a 

hard, siliceous rock, iuterstratified with shales, limestones, and other 

softer rocks, it more effectually resists the general degradation of the 

laud than its associates, and in the course of long continued exposure has come to be the principal mountain-forming rock in the immediate vicinity of its outcrop. The North Mountain, of Pennsylvania, 

the Massanutteu, of Virginia, and the Clinch Mountain, of Tennessee,
84 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

are good examples of mountains formed by the Medina sandstone. 

They are remarkable for their smooth, even crests, and give evidence 

of an earlier topographic cycle, when the land of that region stood at a 

much lower level with reference to the sea, having been, in fact, .worn 

down by the streams during the long-continued period of degradation 

almost to a plain (peneplain). The even-crested mountains of hard 

rocks are the only remnants of this ancient peneplain.

NO. 20. GrRAYWACKE.

(FROM HURLEY, IRON COUNTY, WISCONSIN. DESCRIBED BY W. S. BAYLEY.)

The graywackes differ from the sandstones in composition. Whereas 

the latter consist essentially of quartz grains (or of quartz and feldspar, as in the case of the arkoses) cemented by quartzitic, calcareous,, 

or other cement simple in composition, the graywackes contain grains 

of many different minerals and small fragments of rocks, united by a 

cement of the composition of many slates. In the formation of the 

sandstones the rocks from which the sands were derived were broken 

down into their constituent mineral components, and these were sorted 

by the waters in which they were deposited. On tbe other hand, the 

rocks from whose detritus the graywackes were made were not so completely disintegrated. The sands contained not only quartz and other 

mineral grains, but also little particles of rock, all so intermingled that 

we can not believe that much sorting took place. When rock particles 

are not to be found in the graywackes, the distinction between these 

rocks and the sandstones must rest upon the cementing material, which 

in the former is dark in color and contains much chlorite and some mica. 1

The specimen in the collection was taken from a low ledge on the . 

south side of the Milwaukee, Lake Shore and Western Eailroad, about 

three miles west of Hurley, .Wisconsin. According to Irving and Van 

Hise, the rock belongs among the upper beds of the Peuokee iron 

formation,2 which is Huronian. It is not foliated that is, it is not a 

schist but it is heavily bedded, the different beds appearing as finer 

or coarser grained bands, in the latter of which rock fragments are 

quite conspicuous, while in the former the grains are so fine that the 

rocks seem to grade upward into black slates. All the beds have a low 

dip, a little east of south, and a strike north of east. The low dip of 

the rock over large areas indicates that it has not been subjected to 

severe orographic forces a fact explaining its lack of foliation, and 

thus serving to distinguish it, together with all the other PenokeeGogebic rocks, from the much squeezed, highly foliated rocks of preHuronian age in this region.

The specimen is an excellent representative of the typical graywacke, although but few rock fragments can be discovered in it. It

1 Cf. Geikie: Text-Book of Geology, 2d ed., 1885, p. 162; and J. Roth: Allgemeine und Chemische 

Geologic, 33.11, p. 022.

2 Tenth Ann. Kept. U. S. Geol. Survey, Part I, pp. 420 el ne»[.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XII

RIPPLE MARKS CUT BY MUD CRACKS FILLED WITH SAND, FROM THE GRAND CANYON 

OF THE COLORADO, ARIZONA.
DILLER.] DESCRIPTIONS: NO. 20, GRAYWACKE. 85

is a fine-grained gray rock of nearly uniform texture. In it in ay be 

detected a few grains of quartz that often appear black, small dull 

white grains of feldspar, and occasionally tiny black streaks that look 

like slate fragments. It splits quite easily parallel to its bedding, and 

breaks with a more or less conchoidal fracture in other directions. Its 

density is 2.687.

Under the microscope its coarser components are easily distinguished 

from the matrix or cement, which is not in very large quantity. The 

most numerous grains are clear fragments of quartz, some of which 

present the rounded outlines of waterworn grains, while others are subangular. They often contain rows of tiny vacuoles filled Avith liquid, 

and sometimes a few little specks of dust. Many of them have a wavy 

extinction, which is usually ascribed to deformation in the internal 

structure of the mineral showing it, as a consequence of pressure. A 

few grains are seen to be composed of several portions of different crystals, since their various parts extinguish, in different positions. All 

these features are those belonging to the quartz of crystalline schists. 

Hence, we may conclude that some of the material of the graywacke 

was derived from rocks belonging to the crystalline-schist series.

Next to quartz in quantity are feldspar grains. Some of these are 

quite fresh, and consequently clear, grams of an unstriated variety 

that is probably orthoclase. Others show the bars of plagioclase, while 

a few are marked by the gridiron crossbarring of microcliue. A few 

are as well rounded as those of the quartz, but most are more or less 

angular. The greater portion of the feldspar is highly altered. In 

natural light such grains appear cloudy, and often they possess a reddish tiuge. Under crossed nicols they break up into a sort of mosaic 

of tiny particles of quartz and small needles and shreds of a brightly 

polarizing micaceous mineral that may be kaolin. In the most highly 

colored grains the pigment is discovered to be a red or reddish-brown 

iron compound intermingled with the other components of the mosaic. 

The shapes, of the altered feldspar grains no longer resemble those of 

waterworn fragments. Decomposition has destroyed their original 

outlines, for now the kaolin needles extend into the interstitial cement, 

so that the margin formed by the replacing mosaic is rough and ragged.

The only other substances that are clearly seen to be grains are a 

few hornbleudic pieces and fragments of dark rock. The pieces of 

hornblende are very few in number. They are irregular in their outline, as if broken, unworn pieces, and are strongly pleochroic, being 

yellowish green in a direction perpendicular to their cleavage, and 

dark green parallel to this. The rock fragments are dark, and sometimes nearly black. Under high powers they are found to consist of 

fine grains of quartz and of a dark substance, probably some mineral 

colored black by magnetite.

As a rule, all the fragments lie with their longer axes in one direction, which a little examination of the hand specimen shows to be in 

the plane of bedding.
86 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The grains make up the larger part of the rock mass. Its most characteristic portion, however, is the material cementing these. This is 

present in but small quantity. It consists largely of chlorite, which 

gives it its greenish hue and the rock its dark color. The minerals that 

can be detected in it are chlorite, quartz, biotite, magnetite, muscovite 

or kaolin, and a very little hematite. The first two are the most common. Under high powers much of the quartz is found to be iii small 

grains, while a good deal of the same mineral is intermingled with 

chlorite, etc., as a very fine aggregate of secondary origin. Professor 

Van Hise, who has studied the graywackes of the Penokee district1 

very carefully, believes that this quartz has come principally from, 

feldspar, whose other product of decomposition is chlorite. In a few 

instances the origin of the quartz and chlorite may be told from the 

general shape possessed by their aggregates, which is that of feldspar 

grains, but in most cases the aggregates possess such indefinite outlines that nothing can be learned from them.

The true cement is the interstitial substance between the smallest 

recognizable grains. This represents what was originally clay. At 

present it is composed largely of tiny flakes of green chlorite a little 

secondary quartz between the chlorite and, embedded in this matrix, 

little particles of magnetite and pyrite, both of which are opaque, 

small round aggregates of a dark-brown translucent rutile, and occasionally a little flake of green biotite and tiny shreds of kaolin or rauscovite. Since the same muscovite or kaolin shreds are observed surrounding some of the larger grains of feldspar, it is probable that this 

mineral in the cement is derived, like the larger pieces, from, substances 

of the composition of feldspar in the original clay.

The entire cement is thus found to be crystalline. None of the original clay remains. The alteration is regarded by Van Hise. in the article 

alluded to above, as due entirely to the influence of circulating waters 

holding certain substances in solution, the most important being some 

magnesium salt. By the action of this salt on the material of the 

plagioclase and of the iron-bearing minerals in the original deposit an 

abundance of chlorite was developed, and at the same time the excess 

of silica in the feldspar was separated as quartz in the interstices 

between the chlorite flakes in the matrix. 2

Such a change as this, produced by the partial solution of substances 

in a rock and their chemical reaction, upon one another, is known as 

a rnetasomatic change. By it a clastic rock sometimes loses nearly 

all evidences of its original fragmeutal nature and becomes crystal 

line. If the metasomatic change is attended by pressure, the rock may 

have developed in it a foliation, and may thus give rise to a new rock, 

which, if its origin were not known, would undoubtedly be called a 

crystalline schist. Eocks of this nature have been described by Van

'Am. Jour. Sci., 3d series, Vol. XXXI, 1886, p. 453,

2 The percentage of SiOj in oligoclase is about 62 per cent, while the proportion in chlorite is only 

about 28 percent.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL, XIII

THIN SECTION OF GRAYWACKE FROM HURLEY, WISCONSIN, AS SEEN UNDER A MICROSCOPE

IN ORDINARY LIGHT, X 33.
DILLER.] DESCRIPTIONS: NO. 21, SHALE.

Hise, 1 and the chemical processes that have changed them from graywackes to mica-schists have been very carefully worked out.

PI. XIII shows the fraginental character of the rock quite plainly. 

The lighter grains, both the rounded ones and the sharply angular 

ones, are quartz. The large clouded grains are altered feldspar. In 

the lower portion of the figure are two grains that have retained their 

waterworn outlines, while to the right of the center and a little above 

it is one whose original outlines have nearly disappeared through 

decomposition. The muscovite, biotite, and the fine components of the 

groundmass are not visible in the photograph.

The chemical composition of the graywacke, as reported by H. N. 

Stokes, is as follows:

Analysis of graywacke from Hurley, Wisconsin. '

Si02 .............................................

A1A ............................................

FeA............................................

FoO. .............................................

MnO.............................................

CaO...... .........................................

MgO............ .................................

K.,0. .............................................

Na,0 ............................................

Total......................................

Per cent.

7G.84

11.70

tt

2.88

.70

1 39

1.62

9 H7

1.87

100. 18

This is not very different from the composition of a feldspathic quartzite. Nor should we expect it to be different, for the original rock of 

which the graywacke is an altered phase was a plagioclase-quartz-clay 

rock, as we learn from its microscopic study.2

No. 21. SHALE.

(FROM CASHAQUA CREEK, LIVINGSTON COUNTY, NEW YORK. DESCRIBED HY 3. S.

DILLER.)

Shale is composed of sediment finer than sand. It usually splits or 

breaks most easily parallel to its stratification, showing that this line 

of weakness originated at the time of deposition. The deposit is chiefly 

clay with very fine sand, and the rock became shale by induration.

The rock from which specimen No. 21 was collected is best exposed on 

Cashaqua Creek, Liviugston County, New York. On this account it 

was locally called the Cashaqua shale. It is light greenish gray, and 

rather solid for shale. It was obtained at a fresh exposure, where the

'Cf. C. K. Van Hise, upon the Origin of the mica-schists anil black mica-slates of the PenokeeGogebic iron-bearing series: Am. Jour. Sci., 3d series, Vol. XXXI, 1880, p. 453.

!For ot.lier descriptions of gray.vackes see P. D. Adams: Appendix to Ann. Kept. Canadian Geol. 

Survey for 1880-81-82, Montreal, 1883, pp. 20-23; and Irviug ami Van Hise: Tenth Ann. llopt. 'U. S. 

Geol. Survey, pp. 420 et seq.
88 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL. 150.

corrasion by the stream is rapid, so that the weathered portion of the 

sha.le has been removed and brought the solid rock to the surface. 

When exposed for a considerable time to the weather, shale becomes 

fissile, crumbles to small pieces, and is ultimately reduced to a tenacious 

clay, sometimes more or less sandy, ready to be carried away by the 

rains, rills, and rivers into the sea, and again deposited to initiate a new 

rock cycle. In the field the Cashaqua shale contains a few fossil shells 

and some flattened calcareous concretions, with traces here and there, 

but no continuous beds, of sandstone. On the Genesee River this shale 

is 110 feet thick. It gradually thins toward the west, appearing on Lake 

Erie with a thickness of 33 feet. To the east of Cashaqua Creek sandy 

layers become more and more abundant and the shale is gradually 

replaced by sandstone, indicating that the source from which the sediment of the shale was derived lay in that direction.

Under the microscope the specimens of shale from Cashaqua Creek 

are seen to vary considerably in the size of the particles of which they 

are composed. The component material may be conveniently divided 

into argillaceous and sandy. Both occur in the same specimen, and 

may be present in nearly equal amounts, or either may predominate, 

forming almost the whole of the mass.

The sharp, angular sand grains are chiefly quartz. It is generally 

clear and colorless, but occasionally contains liquid and other inclusions. Greenish grains of hornblende and chlorite are rather common. 

'No fresh feldspar appears among the grains of sand; it has all disappeared in the process of decomposition and disintegration of the rock 

from, which the sediment was derived. The space between the grains 

is occupied chiefly by argillaceous matter. It incloses scales of muscovite, numerous crystals of ferruginous carbonate of lime, and a multitude of minute prismatic crystals, the exact nature of which is not 

well understood. The muscovite is usually in very small scales and 

stripes intermingled with argillaceous material sach as one sees ordinarily resulting from the alteration of feldspar. The mica in large 

part, and perhaps wholly, and also the clay, originated in the decomposing feldspar, although it is possible that the former may have been 

derived in part from a rock containing primary muscovite.

The small colorless or reddish and yellowish brown crystals are some-. 

times perfect rhornbohedra, but more commonly they are round grains 

or groups of crystals. When sufficiently transparent the colors between 

crossed nicols are high. In acetic acid they effervesce like calcite, 

and when dissolved in hydrochloric acid the solution, upon the addition 

of ammonia, yields a precipitate of ferric oxide, showing that the carbonate contains iron. Its easy solubility suggests that it is not doloraitic. 

The mineral is apparently ferrocalcite. In weathered portions of the 

shale the ferrocalcite is reddish and yellowish brown, owing to the 

oxidation of the iron, and occasionally the iron oxide is in sufficient 

abundance to give a rusty tinge to the rock.

The sharp, angular cystals and grains of ferrocalcite are quite uiii-
UILLER.] DESCRIPTIONS: NO. 21, SHALE. 89

formly distributed through the whole mass of the rock, and were 

apparently deposited with the other sediment. It is evident, however, 

that they differ widely in their origin from the fragments of quartz 

and the greater portions of the sediments in which they occur. The 

grains of quartz are minute fragments of the rocks from which they 

were derived. They are not crystals, but their boundaries are not crystallographic lines. The fact that many of the ferrocalcite grains are 

perfect crystals indicates that they were deposited directly from solution in water. Whether their precipitation took place with the deposition of the sediment from suspension, or at a later date, may be learned 

from their relation to the associated material in the shale. In altered 

rocks carbonates frequently originate in the decomposition of other 

minerals, but in such cases there are usually more or less distinct traces 

of the decomposed mineral. In the Cashaqua shale there are no traces 

of decomposed minerals to indicate that the ferrocalcite originated that 

way. The complete absence of veins and other irregular accumulations 

of the material suggests that it was not deposited by waters circulating 

tlirough the mass after the rock was formed. The regularity of its 

distribution and its idiomorphic character indicate that it was precipitated from solution in the ocean waters at .the same time that the suspended sediments were, so that the chemical and mechanical sediments 

intermingled upon the bottom. It is known that mechanical precipitation tends to promote chemical precipitation, and it is possible that the 

association of the two sediments may be in some measure accounted 

for iii this way.

The occurrence of ferrocalcite in this shale is interesting on account 

of its bearing upon the origin of extensive deposits of iron ore among 

the crystalline schists. 1

The minute, brownish, prismatic crystals occurring in irregular 

abundance, frequently in small swarms, throughout the .argillaceous 

material range in length from 0.01mm to 0.005mm . Generally they are 

so small that their effect upon transmitted light cau not be observed, 

but the larger ones have a high index of refraction, giving strong color 

between crossed nicols, and have parallel extinction. They sometimes 

join in such a way as to suggest twinning, and occasionally are bent. 

These minute crystals, to which the Germans give the name "Thouschiefernadelchen," occur not only in shale, but more frequently in clay 

slate. They are referred to in the description of specimen No. 122, and 

are less abundant than in the Cashaqua shale. The exact nature of 

these minute crystals has been the subject of much investigation. On 

account of their small size it is difficult to isolate them for chemical 

determination, but Sauer, Oathrein, and Werveke have shown conclusively that at least in many cases the minute crystals are rutile.

The Cashaqua shale in $!ew York is a member of the Portage group 

of the Devonian system of rocks, which has a wide distribution over 

the whole country between the Appalachian and Rocky mountains.

i B. D. Irving: Am. Jour. Scl., third aeries, Vol. XXXII, 1886; pp. 255-272.
90 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BOLL. 150.

No. 22. CARBONACEOUS SHALE.

(FROM DUG GAP, WALKEK COUNTY, GEOKGIA. DESCRIBED BY J. S. DILLEH.)

This shale was collected at Dug Gap, near Lafayette, in the northwestern corner of Georgia. On account of its color it is sometimes 

called the black shale, or Devonian black shale, to indicate also its 

geologic age.

The upper portion of the shale, 3 or 4 feet in thickness, is usually 

dark gray in color, and often carries a layer of round concretions about 

an inch in diameter. The remainder of the formation, the portion from 

which the specimens were taken, is jet black from an abundance of carbonaceous matter, and when freshly broken it emits a strong odor like 

petroleum. 1

To the unaided eye it is homogeneous and compact, but under the 

microscope it is minutely granular. The grains are chiefly angular 

particles of quartz, with minute globules and crystals of pyrite embedded 

in dark-colored material which is largely carbonaceous, whence the 

name carbonaceous shale. That this material is for the most part carbonaceous is indicated by the fact that when heated red-hot for a short 

time the dark color disappears, owing to the combustion of the carbonaceous matter. When highly heated for a longer time before the blowpipe it turns black again and becomes magnetic, due to the presence 

of iron.

The highly carbonaceous character of the shale is most clearly indicated by the following chemical analysis, made by L. G. Eakins in the 

chemical laboratory of the United States Geological Survey:

Analysis of shale from Day Gap, Georyia.

SiO-j.. ............................................

Al,03 ------ ......................................

Fe.,03 ............................................

PaO

MgO..................... ........................

K20.. ............................................

N.a2O .............................................

T> f\

s ................................................

H2 0....... .......................................

0. for S ..........................................

Total ......................................

Per cent.

51. 03

8.00

CO

1.15

3.16

.41

91

7.29

TQ 11

Q QO

.81

102. 90 

2.74

100. 16

1 In some States this black shale has been distilled for oil, yielding 30 to 40 gallons yier ton.
^ DILLER.] DESCRIPTIONS: NO. 23, SILICEOUS SINTER. 91

The iron pyrites in the shale oxidizes and, according to Hayes, 1 frequently stains the weathered surface of the rock with iron oxide and 

sulphate and mineralizes many springs. The shaly structure is not 

prominent on fresh fractures, but is brought out somewhat by weathering.

Associated with the black shale locally in Tennessee, are small nodules and beds of phosphate of lime, which is of economic importance 

as a fertilizer. Although only a few feet in thickness, this shale is 

perhaps the most persistent and uniform of all0 the Paleozoic formations 

of the South. It extends over the whole of middle Tennessee from the 

Tennessee River to the eastern edge of the Cumberland Plateau and 

^ southward across northwestern Georgia and northern Alabama, its 

outcrops indicating an original extent over at least 38,000 to 40,000 

square miles.

This shale, on account of its distinctive and striking appearance, has 

attracted much attention from miners and has been prospected in many 

places for coal and various ores. Such exploitation, however, has 

always been attended by failure, as the shale contains nothing of present 

economic importance excepting the local deposits of phosphate of lime. 

Although it carries a large proportion of carbonaceous matter Avhich 

burns when it is placed in a hot fire, the amount is not sufficient to 

constitute it a fuel, and no true coal is ever found associated with it. 

Besides the fertilizer it affords, this formation is of economic importance 

chiefly as a starting point in prospecting for the red fossil ore, like No. 

52 of this series, which belongs below it at a uniform depth over coux siderable areas.

UNALTERED SEDIMENTARY ROCKS OF CHEMICAL ORIGIN.

JSTo. 23. SILICEOUS SiNTEB.2 

(FKOM YELLOWSTONK NATIONAL PARK. DESCRIBED HY WALTER HARVEY WEED.)

The hot springs and geysers of the Yellowstoue are surrounded by 

large areas of siliceous sinter that often entirely cover the floor of the 

geyser basins. About the spouting vents this material has been built 

up into mounds and cones of unique forms and great beauty. The 

more quiet pools have built up more or less regular mounds of white 

sinter, which are in places as much as 20 feet in height above the sur- 

+ rouuding level. Besides these deposits, the alkaline waters of the 

geyser regions have left deposits of silica wherever they have flowed, 

and many square miles within the park are covered by white and 

glistening deposits of this material.

Until the Yellowstone deposits were studied it was the generally 

accepted theory that the geyser waters reached the surface heavily

1 . 'Geologic Atlas TT. S., folio 2, Ringgold, Georgia-Tennessee. Also, Sixteenth Ann. Kept. U. S. Geol. 

Survey, Part IV, pp. 6U-C23.

"See Formation of hot-spring deposits, by W. H. Weed: Ninth Ann. Bept. U. S. Geol. Survey, 1890, 

pp. (528-663.

>
92 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL. 150.. ^

charged with silica, which by relief of pressure, by cooling, and by 

evaporation was precipitated out and deposited by the waters. Observation of the natural conditions un'der which the Yellowstone deposits . r 

are forming, together with experiments and a study of the chemical 

analyses of-the geyser waters, sliowed that the silica brought to the 

surface by the geyser waters was rarely separated out and deposited > 

by the first two causes, but that deposits are formed about the geysers 

and the margins of springs by evaporation, producing a true geyser lie. 

A new mode of deposition was then recognized, namely, the separation t 

of silica by plant life,"by the algse that are abundant in the hot waters 

of the region. It is by this agency that much the largest part of the 

sinter deposits of the region have been formed. . . ,y,

This algous vegetation is sure to be observed, by every visitor to 

the region. Its varied tones of pink, yellow, orange, red, brown, and 

green adorn the slopes of geyser cones, flush the Avhite silica of the 

little basins with their tints, and mark the waterways with their brilliant colors. It is ever present where the temperature does not exceed 

185° F., often lining the great bowls of the cooler springs and pools 

with leathery sheets of brown or green. Where a constant overflow 

prevails, the channel is often filled by a vigorous growth in which an 

algie mat is formed having the consistency of a firm jelly, and most 

beautifully colored. In whatever form it is found, and no matter how 

brilliantly tinted, this algous material, if removed from the water and 

dried in the hot sun of the region, rapidly loses its color, shrinks in 

size, and becomes an opaque white mass of silica, whose weight is not 

one per cent of its former state. Chemical analysis, made in the Sur: -rt 

vey laboratory by J. E. Whitfield, shows this dried material to be silica 

and water, viz:

Analysis of dried algous vegetation from geyser margin, Yellowstone National Park.

&i02 . .............................................

H20... ............................................

Per cent.

93 37

4.17

1.50

Experiments showed the writer that the growing algte form a jelly 

of hydrous silica.. It is of this material that the alga? filaments are 

formed, and the algse slime of other waters is here a hydrous silica 

binding the threads together. The nature of this separation may be 

seen under the microscope, though the fresh hydrous silica is difficult 

to study, and the dried, material becomes opaque. In most cases the 

glassy rods can be readily distinguished, and the inclosing paste 

usually shows globules and pellets of the dehydrated silica.

The specimen No. 23 is a sinter formed by algse. It is light and 

porous, showing in part a fibrous structure, is soft, and resembles chalk 

in appearance because it soils the fingers with a white impalpable
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XIV

OLD FAITHFUL GEYSER IN ACTION, YELLOWSTONE NATIONAL PARK.
DESCRIPTIONS: NO. 24, VEIN. 93

powder. The specimens represent the lighter, truly algous form of 

siliceous sinter, such as is commonly formed by the gelatinous mats 

about the hot springs. *

In thin section, when under low powers, the siliceous sinter usually 

appears as an indistinct aggregate of glassy silica. If, however, a 

power of 200 diameters be used, an.d the section is thin enough, the 

sinter is seen to be composed of a delicate network of minute fibers or 

threads which pass in and out, and are often composed of extremely 

minute globules of silica glass. In most cases the algous origin of the 

sinter is not apparent except under high powers. Generally the 

organic nature of the sinter can not be satisfactorily determined in thin 

section, owing to the fact that the plant rods are composed of hyaline 

silica. It is only in the minutest details that the structure - may be 

recognized. In some specimens of the sinter we see extremely minute, 

thread-like particles of the silica held in a mat of small Spheres of 

silica closely crowded together. In such cases it is not reasonable to 

assume that the former organic jelly made by the living plant has, 

upon drying, separated into such globular particles. It is rather more 

probable that the silica was itself in the globular form during the life 

of the plant.

The specimen represents the most common form of freshly formed 

sinter about the the geyser basins of the Yellowstone. The illustration, PI. XIV, represents Old Faithful Geyser in action. The eruptions are of four or five minutes' duration, and take place every hour 

from an opening a couple of feet across in the summit of a mound built 

up of white siliceous sinter. The sinter forms a series of shallow 

terraced basins, in the cooler of which the alg;c colors are noticed.

No. 24. VEIN.

(FROM PASKE.NTA, TKHAMA COUNTY", CALIFORNIA. DESCRIBED MY J. S. Diuvim.)

Much of the rain that falls upon the earth sinks into it and circulates 

through the pores and fissures of the rocks beneath the surface. Soluble portions of the rocks are dissolved, and the water becomes charged 

with mineral matter. When this water again reaches the surface in 

springs it deposits the mineral matter held in solution, and thus siliceous 

sinter (No. 23), travertine (No. 29), etc., are formed. It may deposit its 

dissolved material beneath the surface, filling fissures in rocks, or form 

icicle shaped masses, as stalactites (No. 28), in caverns.

The fissures in rocks filled in this way with mineral matter are veins. 

They are illustrated by specimen No. 24, which was collected at Paskenta, 

in Tehama County, California, where they occur abundantly as stream 

pebbles brought down from the Coast Kange. The vein in this case is 

quite uniformly white, and is composed of quartz and calcite. When 

placed in dilute hydrochloric acid it effervesces briskly, dissolving the 

calcite and leaving the quartz. The calcite is most abundant in the 

middle portion of the vein, while the quartz usually predominates upon
94 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL. 150.

the sides; showing that much of the quartz was deposited before the 

calcite. This relation is shown in fig. 10, illustrating a vein from which 

the calcite has been removed by acid. The quartz is often toothshaped, projecting into the vein.

Some veins are composed almost wholly of calcite, with but a trace 

of quartz here and there along the edges, while others are chiefly 

quartz. Generally, however, the two minerals are present in nearly 

equal amounts.

The rock fissures or openings which, when filled with mineral matter, become veins are produced- in various ways. Some originate in 

the contraction of the rock while drying or cooling, others are due to 

solution, but the greater number that become veins are produced in 

connection with movements of the laud in making mountains and continents. It is for this reason that veins are most abundant in mountain 

regions among rocks which have been upturned and metamorphosed. 

Veins of quartz are perhaps more abundant than those of any other 

mineral, and those of calcite are next in number. Feldspar, barite, 

fluorite, hornblende, epidote, and pyroxene are other 

vein minerals, besides precious metals, such as gold and 

silver, and a large number of ores, which are sometimes 

of great economic value. Where native gold occurs it is 

usually found in quartz veins.

PI. XV illustrates a vein of quartz 2 feet in thickness 

cutting jointed gneissoid rocks on the Potomac Eiver 

near Washington, District of Columbia.

" . A vein containing ore is usually called a lode by miners, 

of vein composed and the mineral matter associated with the ore in the vein 

ofqi.iartz(i)and js the ganfjue, while the rock containing the vein is the

calcito (2). from *-_T . « . / T 11 j_ winch the calcite country TOOK, Veins when first formed are usually not 

lias boon re- far from vertical, but they may be displaced by subseinovedbyacici. queut movements and intersected by several series of 

later veins. They vary in breadth from a mere film to over a,hundred 

feet, and occasionally have banded structure produced by the deposition 

of corresponding layers on opposite sides of the fissure until it is completely filled. When the filling is not complete the cavities are frequently lined with beautiful crystals similar to those found in geodes. 

Fibrous minerals are usually found in veins, and the fibers are perpendicular to the walls.

No. 25. VEIN QUAKTZ.

(FROM CASTLETON, HARFOKD COUNTY, MARYLAND. DESCRIBED BY J. S. DILLER.)

Of the various kinds of vein-forming minerals, quartz is the most 

common and important. Veins of quartz in altered crystalline rocks 

are sometimes 100 feet or more in width and may be traced across the 

country for miles. It occurs in sufficiently large masses to be considered as a rock. Although usually associated with metamorphic rocks, 

it originated, like siliceous sinter, by deposition from aqueous solution.

.si.
U. S. GEOLOGICAL SURVEY 

VIEW OF QUARTZ IN GNEISS NEAR CHAIN BRIDGE, DIST
DIU.BU.] DESCRIPTIONS: NO. 26, SILICEOUS OOLITE. 95

Specimen No. 25 is from a large vein at Castleton, Maryland. The 

rock is light colored, vitreous lustered, hard enough to readily scratchglass, infusible, insoluble in hydrochloric acid, and in fact has all the 

characteristic properties of the mineral quart/,. Its structure is granular, but the grains being angular and interlocked, as in granite, they 

occupy all the space, and as all the particles are of the same material it 

does not appear granular to the naked eye. This structure is, however, 

well displayed under the microscope between crossed nicols, owing to 

the unlike optical orientation of the grains. The grains are transparent, although somewhat clouded by a multitude of minute liquid inclusions, usually less than 0.002""" in diameter. These minute inclusions 

can be seen only with a high magnifying power, but are known to be 

liquid ou account of the moving bubble which each one contains. If 

the section be gently heated, the liquid expands so as to occupy the 

whole cavity and the bubble disappears. The cause of the rapid motion 

of the bubble in these inclusions is not well understood.

Similar inclusions occur in the quartz associated with the calcite in 

the vein of specimen No. 24, as well as in the quartz of granite, gneiss, 

and schistose rocks generally.

The included liquid is usually water, but in some cases has been 

shown ' to be carbon dioxide in a liquid state, and the two may occur 

together in the same cavity. Yein quartz is a water deposit from solution, and the included water was usually imprisoned in the cavities at 

the time the quartz was formed. Occasionally minute cubes of salt are 

found in these liquid inclusions.

The quartz at Castleton is ruined and finely pulverized for use in the 

manufacture of pottery. In order to render it more pulverable it is 

highly heated in a kiln. It comes out snow-white and brittle, ready for 

grinding.

No. 26. SILICEOUS- OOLITE.

(FROM CENTER COUNTY, PENNSYLVANIA. DESCIMUEI> HY J. S. DILLTCK.)

The occurrence of silica in the form of sinter deposited by geysers is 

illustrated by specimen No. 23; that deposited in fissures as veins is represented by specimens Nos. 24 and 25. We now come to a form in which 

"the silica appears in small spherules like fish roe, whence the name 

oolite. In this case the chief component of the oolite is silica, and it is 

designated siliceous oolite to distinguish it from the more common form 

of oolite, such as specimen No. 31, which is composed of carbonate of 

lime. The material was collected near State College, in Center County, 

Pennsylvania, where it occurs as bowlders associated with flint over an 

area of several square miles.

This oolite is remarkable for the uniformity in shape, size, and distribution of the spherules. On a fresh fracture their centers are usually 

dark, while their rims and the matrix which fills the interspaces and

1 Gc. W. Hawes: Am. Jour. Sci., 3d series, Vol. XXI, i>. 20!i.
96 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL.150.

incloses the spherules are light colored. Both spherules and matrix 

are firmly coherent, so that a fracture passes directly through the 

spherules instead of around them. In some places the hand specimens 

show cavities which are lined with minute crystals of quartz. It is 

evident that if conditions had continued favorable for the develojimeut 

of quartz these drusy cavities would all have been filled.

The structure of the spherules may be readily seen in the hand specimen with a pocket lens. Their centers are usually dark, granular, and 

vitreous, and surrounded by a dense waxy envelope, which is in many 

cases beautifully banded, like agate. This relation is illustrated by the 

spherule in the center of the figures in PI. XVI. The concentric structure shows that the spherules result from growth and not from attrition. 

At the center of many of them rounded grains of sand are found as a 

nucleus, about which the silica was deposited in successive layers. 

Some of the layers are granular quartz; others are fine, granular, or 

fibrous chalcedony. The black bands and clouding in the figures are 

brown oxide of iron.

The two large spherules cut through the middle by the left edge of 

the figures in PI. XVI have each a round grain of quartz at the center. 

These grains are full of minute needles and liquid inclusions, like those 

of granitic quartz. The minute needles observed in these nucleal 

quartzes do not occur in the granular quartz of the concentric layers. 

As pointed out by O. E. Hovey, this nucleal quartz is occasionally 

surrounded by secondary growths of quartz optically continuous with 

the original grain. 1 In each of the two spherules alluded to the quartz 

is encircled by a rather coarsely granular band in which many of the 

longer diameters of the grains radiate from the center of the spherule. 

Some of the. spherules are composed wholly of such grains, and the 

radial structure is plain. Others appear to be made chiefly of chalcedony. All the sectious showing a nucleal .grain are corresponding 

sections, and supposed to be through the middle of the spherule. 

When compared it is evident that the composition and structure of 

the spherules vary considerably. Occasionally the nucleal quartz is 

enveloped in a thick, dense layer of oxide of iron, and more rarely 

there are finely fibrous layers nearly midway between the center and 

circumference of the spherule. The outermost layers of the spherules, 

which form the borders of the interstitial spaces, are generally fibrous. 

This structure is well illustrated about the clear, triangular area toward 

the lower left hand portion of Fig. B of PI. XVI.

Oolite composed of carbonate of lime is of common occurrence, sometimes in large masses containing marine shells, and must be of marine 

origin. Its formation has been observed in lakes and springs, also. 

On the other hand, siliceous oolite is rare, and from the fact that, as 

in specimen No. 28, silica is known to replace carbonate of lime, it has 

been suggested that the siliceous oolite Avas originally calcareous.

1 Bull. Geol. Soc. America, Vol. V, p. 628.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XVI

THIN SECTIONS OF SILICEOUS OOLITE FROM CENTER COUNTY, PENNSYLVANIA,

AS SEEN UNDER A MICROSCOPE, X 25. 

A. In ordinary light B. Between crossed mcols.
DIFXER.] DESCRIPTIONS: NO. 27, GYPSUM. 97

Pertinent to this view are the following chemical determinations taken 

from complete analyses published by Edwin H. Barbour and Joseph 

Torrey, jr.: l

Composition of oolite from near State College, Pennsylvania.

Lime-silica 

oolite.

3.7

88.71

Silioa-limo 

oolite.

56.50

36.84

Siliceous 

oolite.

95.83

J.93

The lime-silica oolite and silica-lime oolite occur in the same hand 

specimen, but are said to be separated by a sharp line. Siliceous 

oolite and flint occur at the same locality. The same observers report 

organic remains in the siliceous oolite they describe, but none were 

observed in the specimens of this series nor in the material described 

by Hovey.

It does not admit of doubt that the nuclear quartzes filled with 

minute needles were originally grains of sand about which the concentric layers of silica were formed, but whether or not the concentric 

layers when first formed were calcareous or siliceous may yet be questioned. Dr. W. Bergt,2 who has studied the material most carefully in 

the laboratory, and G. R. Wielaud,3 who has investigated it in the field, 

have advocated its direct deposition from hot siliceous springs and have 

described associated phenomena indicating such-origin. Furthermore, 

similar deposits have been observed by W. H. Weed now forming about 

the siliceous hot springs of the Yellowstone National Park. They 

will be described by him in Monograph XXXII of the Geological 

Survey series, under the head of "Hot-spring deposits."

No. 27. GYPSUM. 

(FROM GYPSUM, OTTAWA COUNTY, Onio. DESCRIBED BY J. S. DILLER.)

Gypsum, like quartz and calcite, occurs at times in such large masses 

as to be properly considered a rock. At Gypsum, Ohio, it is found 

near the middle of a great sheet of limestone of upper Silurian age. 

The specimens for this series were collected by Messrs. Marsh & Co.

Gypsum is generally fine-granular, or compact, and so soft as to be 

quite easily scratched with the finger nail, but, unlike limestone, does 

not effervesce in acid. Its normal color is white, though by admixture 

of clay, carbonaceous matter, and iron oxide, it becomes gray, brown, 

or reddish. The clear, transparent form of gypsum is seleriite, and the 

compact, white form, frequently used for statuary and ornaments, is 

alabaster. Its composition is OaSO4 2H2O. When heated, the water

1 Am. Jonr. Sci., 3d series, Vol. XL, pp. 248, 249. See also Petrology for Students, l>y Alfred Harkcr, 

p. 232.

2 Gesell8cliaftIsis, in Dresden, 1892, p. 115. 

8 Am. Jour. Sci., Oct., 1897, 4th serieu, Arol. IV, pp. 262-264.

Bull. 150 7
98 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

is readily driven off, and the resulting powder is the common plaster of 

paris of commerce.

Gypsum is usually found in beds or lenticular masses associated with 

clay and rock salt, or anhydrite, and it has been found locally in rocks 

of all ages later than the Silurian. This intimate association with rock 

salt clearly indicates its marine origin in many cases at least; for it is 

well known that'in the manufacture of salt by evaporation from sea 

water gypsum is precipitated before the more soluble salt. Gypsum 

occurs also in irregular'masses in limestone and is not associated with 

salt. It then appears to have resulted directly from the action of sulphuric'acid upon the limestone, converting the carbonate into the 

hydrous sulphate of lime. Gypsum is rarely found forming incrustations in caves. 1

Gypsum is used chiefly in the manufacture of plaster of paris and 

land plaster. The former has many applications in the arts, and the 

latter is a valuable fertilizer. Thin plates of selenite are used to secure 

certain optical effects in microscopic investigations, and from alabaster 

statuary, vases, and other ornaments are carved.

No. 28. STALACTITES. 

(FROM LURAY CAVE, PAGE COUNTY, VIRGINIA. DESCRIBED BY J. S. DILLER.)

Limestone is one of the most soluble among the common rocks, and in 

certain regions, as for example Virginia and Kentucky, where circumstances are favorable, large caves are made in it by solution. The underground water in such regions contains much dissolved limestone, and 

when it seeps through the roof of a cave and evaporates the dissolved 

limestone is deposited in various shapes, depending chiefly upon the rate 

of supply. Tlje most common and characteristic feature is the stalactite 

illustrated by specimen No..28. A number of stalactites in position are 

shown in the accompanying PI. XVII, a view of Marengo Cave, Indiana, 

taken by Mr. Ben Haines.

A drop of water with carbon ate of lime in solution, upon reaching the 

roof of the cave where it is exposed, begins to evaporate and lose its 

carbonic acid by virtue of which it holds the limestone in solution. As 

a result the calcareous matter is deposited around the edge of the drop, 

and if the proper supply continues a tubular pendant will develop. 

While some of the small stalactites are hollow, they do not long remain 

so, for the hollow soon fills and the growth of the stalactite continues 

by the addition of successive layers upon the outside. A cross section, 

of a stalactite shows it to be composed of one or more concentric bands, 

which are usually radial fibrous. Their color is generally yellowish, 

owing to the presence of iron oxide, but occasionally they are snowwhite and translucent or beautifully colored by copper or other substances. Their chemical composition is CaCO3 , which generally crys1 See paper l>y G. P. Merrill, Proc. U. S. 2sT at. Mus., Vol. XVII, pp. 80-81.
U. S. GEOLOGICAL SURVEY

DEPOSITS IN MARENGO CAVE, INDIA
DIUJIK.] DESCRIPTIONS: NO. 29, TRAVERTINE. ' 99

tallizes in the hexagonal system as calcite, but occasionally in the 

orthorhombic system as aragonite.

In PI. XVII stalactites are seen hanging like icicles from the roof of 

a cave. When the supply of water is greater than can be evaporated 

from the stalactites, some of it drops to the floor and forms a stalagmite 

which grows upward as the stalactite grows downward, and if the 

process be continued long enough the two will meet and form a column, 

as shown in the plate. Variations in the rate and distribution of the 

supply give rise to many beautiful as well as grotesque forms. Luray 

Cave, of Virginia, from which specimen No. 28 was collected, is remarkable for the abundance and varied character of its deposits. Part of 

these are illustrated in PI. XVIII, from a photograph taken by Mr. 0. H. 

James, of Philadelphia. All caverns are not so beautifully ornamented. 

Mammoth Cave, of Kentucky, although remarkable for its size, contains 

a very small amount of cave deposits such as are shown by the two 

illustrations.

The formation of stalactites has been described and illustrated by 

G. P. Merrill in Proo. U. S. Nat. Mus., Vol. XVII, pp. 77-80.

No. 29. TRAVERTINE.

(FROM MAMMOTH HOT SPRINGS, YELI,OWSTONE NATIONAL PARK. DESCRIBED «v 

WALTEU HARVEY WEED.)

Among the many interesting natural phenomena that claim the 

attention of the visitor to the Yellowstone National Park, the geysers 

and hot springs rank .first in general interest. Their novelty and 

beauty are sure to attract universal admiration, while the vast quantities of hot water that flow from the ground are convincing evidence 

of the nearness of internal heat. These steaming fountains and boiling 

pools are usually surrounded by snowy white borders of mineral matter 

deposited by the hot waters.

At the Mammoth Hot Springs the deposit consists of carbonate of 

lime; this forms the unique marble terraces and pulpit basins of those 

springs. The total area covered by travertine at this place is over a 

thousand acres, and the greatest thickness is about 250 feet. When 

first seen the main mass of the deposit is striking from its whiteness, 

resembling an immense snow bank, filling a narrow valley environed by 

dark-green, pine-clad slopes. This deposit of travertine forms benches 

or terraces, the largest one, on which the hotel stands, being 83 acres 

in extent. While the deposit is white, or if old and weathered a light 

gray, the hot-spring bowls and overflow basins, and especially the slopes 

wet by the streams of hot water, are brilliantly tinted with red, yellow, 

orange, green, and brown. These colors are due to a low form of plant 

life alg;e whose presence is not easily recognizable, owing to the 

covering of travertine. The illustration shows the general character 

of the terraced basins that are built up on the overflow slopes.
100 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The illustration, PI. XIX, shows the hot-water basins that cover the 

slopes, like irregular steps. They are formed by the more rapid deposition about the edges of pools of hot water; and the creuulated border 

produced by the rippling of the water, together with the presence of 

alga', causes an overflow that builds up stalactitic masses that hang 

like a fringe down the sides of the basins.

The occurrence of plant life in these hot waters has a special significance, as the travertine deposits are built up chiefly by tlie activity of 

these little plants, which abstract carbon dioxide from the hot waters, 

thus causing the precipitation of the carbonate of lime held in solution. 

The specimen shows the fibrous variety of travertine formed by the 

encrusting of the yellow, silky skeins of alg;e filaments growing in the 

hottest waters near the orifices of the springs. This deposit forms 

fan-shaped masses (fig. 11), which eventually choke up the spring.

The specimen shows a beautiful fibrous structure, and is apparently 

compact, but the transverse section shown by the ends of the specimen

. 11. Travertine fan, Mammoth Hot Spriugs, Yellowatono National Park.

prove the rock to be cellular. The freshly formed rock is soft and 

friable, but becomes indurated and compact with age, the change 

being unaccompanied by any variation in chemical composition. While 

freshly formed and wet, the travertine retains the brilliant colors of 

the algie, but these gradually fade upon drying- and the death of the 

plant life, and the deposit becomes chalky white. The older deposits 

have a gray surface, due to the organic matter from the algse present 

in the rock.

Though the specimen of travertine described is a hot-spring deposit, 

this rock is also formed by cold-spring waters which are highly charged 

with carbonate of lime. The beautiful cave onyx and other cave 

deposits are similar in composition, but they are formed more slowly 

and are therefore of different physical character.

In thin section under the microscope the organic origin is not disclosed, but fresh specimens may be dissolved in dilute acid, which will 

reveal the algae sheaths and threads. The following analysis, made in
DEPOSITS IN LURAY CAVE, VIRGINIA.
DILLEH.] DESCRIPTIONS: NO. 29, TRAVERTINE. 101

the laboratory of the United States Geological Survey by J. E. Whitfield, shows the material to be a nearly pure carbonate of lime:

Analysis of travertine from vent near Blue Springs, main lerrace, Mammoth Hot Springs,

Yelloivstone National Park.

A1203 + FO,OH . . - . ...............................

Per cent.

0.70

55.02

ft(9C.02)

O.OC

0.20

60.08

 42.25

' 0.24

99. 81

alt all the C02 be supposed to l.o combined with lime. 

6 Potassio chloride.

The striking feature of the analysis is the remarkably low percentage 

of carbon when it is remembered that the travertine is formed of limeincrusted algre threads.

The waters of the Mammoth Hot Springs are so heavily charged 

with carbonate of lime in solution that this substance is deposited by 

other agencies than plant life, but they are insignificant in comparison 

with it. This is taken advantage of in the preparation of coated 

articles of metal and glass often sold to tourists the articles being 

prepared by exposing them to hot-water spray.

In thin section under the microscope the travertine shows tangled 

fibers, which are surrounded by a furry growth of calcite spicules, which 

are in general at right angles to the fibers. Under high powers with 

crossed uicols these rods appear brownish gray in color, and the spaces 

between the fibers are filled with a finely cryptocrystalline aggregate 

of calcite. In many of the sections, where the rods are thickly crowded, 

the lace-like arrangement is not so apparent and the calcite aggregate is 

wanting. The calcite spicules which surround the rods are but rudely 

radial in appearance, and sometimes appear felted, but generally 

resemble delicate frostwork. This microscopic structure quite accords 

with what we know of the origin of the travertine. The rods are the 

coatings of the algae threads, which in the section are probably merely 

shells and contain no recognizable organic matter.
102 THE EDUCATIONAL SEEIES OF KOCK SPECIMENS. [BULL. 150.

No. 30. OOLITIC SAND.

(FROM PYRAMID LAKE, WASTIOE COUNTY, NEVADA. DESCRIBED BY J. S. DILLER.)

Among the Needles on the shore of Pyramid Lake in Nevada, accord- *" 

ing to Prof. I. 0. Bussell, the sbort beaches are made up of oolitic 

sand, represented by specimen No. 30. Although most of the grains are 

well rounded, there are many subangular ones. In size they are very ^ 

irregular, ranging from l min to 6 mm in diameter. The smaller, when 

.cemented, give rise to oolite, but the larger ones produce pisolite. 

Among the well-rounded grains are occasional shells of a small snail. v

The middle portion of each spherule is minutely granular and enveloped by series of concentric fibrous granular layers. Each spherule is 

a concretion. Some have grains of sand or minute shells at the center 

as a nucleus, about which the carbonate of lime accumulated. Occasionally the nucleus is large and complex, including one or more 

concretionary grains, each having its own concentric banding. Most .4 

of the spherules, however, are without such foreign nucleus. They 

appear to have started by the segregation of the more or less flocculent particles of the precipitate itelf, forming globular accumulations 

upon which the further precipitation continued with variations, giving 

rise to successive rings of growth. This subject will be considered 

more at length under " Concretions" (Nos. 33 to 30).

The spherical form of the grains and the uniform thickness of the 

concentric layers indicate that the grains were in motion while the sue- * 

cessive coats were formed. Professor Eussell 1 remarks that "this 

material is evidently now forming, and in places has been cemented 

into a compact oolite by the deposition of a paste of calcium carbonate r 

between the grains, and forms irregular layers several inches in thickness that slope lakeward at a low angle," The formation of oolitic 

sand on the shore of Pyramid Lake is limited to the immediate vicinity 

of the Needles, where warm calcareous springs rise near the shallow 

margin of the lake.2

Similar sand, but much finer, is now forming locally upon the shores . 

of Great Salt Lake. The waters of Pyramid Lake and Great Salt Lake 

differ widely in chemical composition, and it is evident that the conditions under which oolitic and pisolitic sand may be formed vary ± 

considerably.

Dr. A. Eothpletz has shown 3 that the oolitic sand of Great Salt Lake 

is of vegetal origin. It is due to certain algae having the power to 

secrete carbonate of lime. When the grains of sand are dissolved in 

weak acid, there remain the dead and shriveled bodies of such algae, 

and in the lake these granules are usually covered, at least in part, with

' MOD. U. S. Geol. Survey, Vol. XI, p. 168. 

"Ibid., p. 61.

3BotaEische8 Ceutralblatt, ST. 35, 1892; translated by F. W. Cragiu iu Am. Geologist, Nov. 1892, 

Vol. X, p. 279.
TERRACES OF MAMMOTH HOT SPRINGS, YELLOWSTONE 
DILLKR.] DESCRIPTIONS: NO. 31, OOLITIC LIMESTONE. 103

a bluish-green algra mass. In Pyramid Lake, however, no such masses 

have been reported in connection with the oolitic sand, and the evidence does not appear to indicate that it is of vegetal origin. To settle 

this matter, if possible, specimens of this oolitic sand from Pyramid 

Lake were referred to Prof. W. G. Farlow, of Harvard College, who 

reports that they contain " nothing like Glosocapsa and Gloeothece." 

He adds: "I find no trace of vegetable structure except the rare occurrence of Leptothrix-like threads broken up into fragments. These 

are so rare and so imperfectly shown that I can only regard them as 

accidental impurities, coming, perhaps, from the outside. There is 

nothing like the mass of algoe found in the Salt Lake material."

No. 31. OOLITIC LIMESTONE. 

(FROM BEDFORD, LAWRENCE COUNTY, INDIANA. DESCRIBED BY J. S. DULLER.)

The oolitic limestone, or oolite, from Bedford, Indiana, represented by 

specimen No. 31, is composed chiefly of oolitic grains, among which there 

is great variety in form, but a smaller range in actual size. They are 

generally less than I11 "" in diameter, and can not be studied to advantage without a microscope. They do not have the usual concentric 

banding, like the grains of siliceous oolite (No. 20) and oolitic sand (No. 

30), and are much more irregular. Discoidal and elongated forms predominate, possibly for the reason that under their conditions of growth 

the environment was not symmetrical in all sections, and development 

along different radii proceeded unequally. Much of the variation in 

shape, however, is due to the fact that the grains are fragments of coral, 

inolluscau shells, worm tubes, and numerous other more or less rounded 

forms.

Each rounded oolitic grain is enveloped by a coat stained more or 

less deeply by oxide of iron. Many of these grains are completely 

crystalline, and, all parts being in parallel position, they extinguish at 

the same time when rotated between crossed nicols. Some of these 

grains, as shown by the distribution of their iron oxide, were evidently 

once organic, and, besides being optically continuous within, they are 

in some cases continuous upon the outside with the clear granular 

calcite of the matrix. It is evident that such grains have grown by 

additions of calcite upon the outside, in just the same manner as the 

quartz has grown in many slightly altered sedimentary rocks.1 The 

matrix is usually more crystalline and clearer than the grains which 

it incloses, and is often optically continuous about several grains.

Mr. George H. Girty has examined this limestone microscopically, 

and furnishes the following remarks concerning the fossils it contains:

Protozoa are very abundantly represented by the forarainiferal genus Endothyra. 

Most of the forms observed can probably be referred to Endothyra baileyi Hall, a common fossil in the St. Louis limestone of the Mississippian series. A trace of sponges

1 See descriptions of Nos. 15 and 118.
104 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.150.

has been noticed, and corals, belonging chiefly to the genus Zaphrentis, occur sparingly. Fragments of echiuoderm plates and spines constitute, perhaps, the greater 

portion of the organic remains of this rock. Brachiopods and bryozoa are not well 

represented, and the articulates are represented by ostracods and trilobifces.

In the oolitic sand (specimen No. 30) of Pyramid Lake there are 

usually a few shells of a small fresh-water gastropod; but in the oolite 

from Bedford, Indiana, the organisms of various kinds are marine and 

so abundant as to indicate a rich fauna. This commingling of fossils 

and oolitic grains is illustrated in the formation of oolites upon the 

seashore to-day. Oolitic sand and oolite, as shown by Dana,1 are of 

common occurrence on the beaches of coral islands. Coral rock is 

broken off and ground up to calcareous sand by the force of the waves 

that break against the coast. These sands, worm shells, etc., are 

thrown by the tides and waves upon the beach. The water of such 

calcareous shores gets much carbonate of lime in solution, and as the 

sand dries, after the tide recedes, the dissolved material is deposited, 

thus forming successive coats about the grains, filling the spaces 

between them, and, if the grains are not frequently moved, ultimately 

cementing them together to form oolite or pisolite, according to the size 

of the grains. Dana observed that such "beach deposits consist of 

regular layers, commonly from an inch to a foot in thickness, and are 

generally consolidated up to a line a little above high-tide mark. In 

all instances observed the layers dip at an angle of from 6° to 8° down 

the beach. The dip is nothing but the slope of the beach itself, and 

arises from the circumstance that the sands are deposited by the 

incoming waves or tides on such a sloping surface."

Such deposits are forming on the Florida Keys. Much of the drift 

sand of that region is consolidated into true oolite, containing numerous traces of organic remains, wholly unlike those of the oolite at Bedford, Indiana.

As shown by Biothpletz 2 and Wethered, 3 it is probable that-algae 

and other low forms of life which secrete carbonate of lime may have 

contributed much in the formation of oolite. The fragments of coral 

and other grains of calcareous sand in water containing much carbonate 

of lime in solution would, under especially favorable conditions, afford 

nuclei for the growth of algas and girvanella3, forms which are known 

to have played an important role in the formation of certain oolitic 

granules. The more we learn of sedimentary rocks precipitated from 

solution the more clearly we perceive the importance of the work of 

such organisms in the process.

The Bedford oolitic limestone is extensively quarried at many points 

in a belt extending southeast from Gosport by Bloomiugtou, Bedford, 

and Salem to near the Ohio River. The belt has a length of about 90 

miles and an irregular breadth averaging nearly 5 miles. The rock

'Coral and Coral Islands, 3d edition, pp. 153,156,206, and 392.

2 Botanisches Ceutralblatt, Nr. 35, 1892; translated by F. W. Cragin iii American Geologist, Nov., 

1892, Vol. X, p. 280. 

3 Quart. Jour. Geol. Soc. London, May, 1895, vol. 51, p. 196.
j BILLER.] DESCRIPTIONS: NO. 32, LIMONITE. 105

**-. varies in thickness from 25 to 100 feet, and is massive, showing only 

traces here and there of stratification. Upon a weathered surface these 

lines are more distinct and cross bedding is occasionally visible. The

^ dip is gentle toward the Mississippi Yalley. Joints are common, but 

not abundant, and the water thus gaining access to the rock mass has 

in places given rise to great caverns. Where weathered, the limestone

^ is buff colored, but in the deeper parts of the quarries it is blue. The 

difference in color is thought to be due to a change in the oxidation of 

the iron and the loss of carbonaceous matter upon exposure. When 

first taken from the quarry the rock is soft and easily carved, but 

hardens upon exposure.

* Oolitic limestones of great extent are rare, although a local development of oolitic structure in extensive limestones is not uncommon. Its 

.formation depends, apparently, upon the large local supply of carbonate of lime along the shore, but this must be combined with mobility 

of deposit in-order to produce spherical deposition.

* On account of the abundance of oolite at a certain horizon among 

the rocks of the Jurassic system in Europe, this portion of the system 

is called the Oolite. It must be understood, however, that as a name

* for a group of rocks in the Jurassic system it includes many other 

rocks besides oolite properly so called.

Oolite is generally carbonate of lime, but occasionally it is siliceous, 

as in specimen No. 26, or ferruginous, as in specimen No. 52. In the 

second case, however, the oolitic structure was originally calcareous; 

and it is possible, but not probable, that the same Avas true in the first

^ - case. Karely oolite is made of pyrite. Mr. G. P. Merrill has an excellent 

example from the Black Hills. The student will find an excellent paper 

on the Bedford oolitic limestone in the Twenty-first Annual Eeport of 

the Department of Geology and Natural History of the State of Indiana, 

for 1896, pp. 291-427. The paper includes a bibliography of oolites.

No. 32. LIMONITE.

(FROM LOWMOOR, ALLEGHANY COUNTY, VIRGINIA. DESCRIBED BY J. S. DILLER.)

Of the various substances accumulated by deposition from solution

in water, limonite is among the most common. When heated before

a blowpipe in the reducing flame it becomes magnetic on account of the

iron present. In composition it is a hydrated oxide of iron, expressed

\ by the formula 2Fe2O33H2O. Its streak is usually yellowish brown,

^, like iron rust, which is practically the same substance. Another name

for it is brown hematite, to distinguish it from red hematite (No. 121),

which has a red streak and contains no water.

The simplest form of limonite is represented by specimen No. 32, in 

which it occurs as a stalactite. Carbonate of lime frequently occurs in 

the form, of well-developed stalactites, but such forms of limonite are 

rare, although it is frequently seen in small shapes which are somewhat 

stalactitic or botryoidal. The mode of occurrence, origin, and structure of the stalactite of which specimen No. 32 is a fragment are in a
106 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULLED.

general way exactly analogous to those of specimen No. 28, which is ,.*. 

composed of carbonate of lime.

The stalactites of limonite were collected by Mr. G. L. Welch from the 

roof of a limestone cavern within a mile of Lowmoor. That there were w 

some stalactites of carbonate of lime in the same cavern is indicated 

by the fact that in a few of the specimens the limonite is completely 

inclosed by calcite, and the limonite core is discovered only when a . 

cross section of the stalactite is examined.

It is not quite correct to say that this stalactite is limonite, for it is 

in some cases largely carbonate of iron (siderite), and a few of the 

stalactites in the series are coated by carbonate of lime, like specimen 

No. 28, although the middle portions of such are limonite or siderite.

The structure of specimen No. 32 is well shown at the ends. The brown 

concentric layers differ in texture; some are very compact and more or 

less waxy or dull, while others, upon a fresh fracture showing cleavage 

faces, have a somewhat vitreous luster when seen in the proper position; still other bands are spongy or pulverulent. Upon applying a * 

drop of .hydrochloric acid it is found that the lustrous layers effervesce 

vigorously, while the others are not affected. The lustrous layers are 

siderite, the others are limonite. By a study of the soft ocherous " 

layers it is found that the limonite, in many cases, grades into the 

siderite in the same layer, the oxide being the exposed portion, while 

the carbonate is within, where not so exposed to the influence of 

atmospheric agents. This relative position at least suggests, if in fact 

it does not demonstrate, that the limonite is derived from the carbonate by oxidation. As shown by T. Sterry Hunt 1 this change is -( 

accompanied by a considerable loss of volume, and we are enabled to 

understand why liinouite derived from the carbonate is so porous.

The compact layers of limouite which usually form the outer coating 

of the Stalactite, and occur also within, sometimes alternating with the 

layers of siderite, do not appear to be derived from the carbonate. 

Their structure is not porous, and precludes the idea that they were 

derived directly from the carbonate, but indicates rather that the 

change took place before the 'deposition. It is possible that in all cases "* 

the iron may have been in solution as a carbonate, but was oxidized 

before actual deposition, so as to show no loss of volume.

At Lowmoor there are large deposits of limonite, and it occurs in the 

form of an extensive bed, averaging 15 feet in thickness. It is of 

Devonian age. This is but one of many similar examples throughout ' 

the Appalachian region.

The conditions necessary to the accumulation of iron upon the earth's 

surface are well illustrated in many places to-day, and deserve special 

consideration. By the decomposition of rocks containing ferriferous 

minerals the iron is usually liberated in the form of ferric oxide, which, 

being insoluble, remains behind, coloring the residual material red-

_ : y

1 Mineral Physiology and Physiography, p. 202.
DESCRIPTIONS: NO. 33, CLAYSTONE. 107

*. dish. To render it soluble it must be reduced to a ferrous state, and 

this is accomplished by acids, chiefly carbonic acid, resulting from the 

decomposition of vegetal matter. Ferrous oxide readily combines

^ with carbonic acid to form carbonate of iron, which is soluble in water, 

containing an excess of carbonic acid, and it is thus that the accumulation of large bodies of iron is rendered possible.

, Ferrous carbonate when exposed to pure air loses carbonic acid and 

takes oxygen, thus returning to the insoluble ferric oxide. This reaction forms a precipitate and deposits limonite. The process is illustrated about many chalybeate springs where the reddish ferric oxide 

accumulates.

} The precipitation and deposition of carbonate of iron, such as that 

which is associated with the limonite in specimen No. 32, could not take 

place with full exposure to pure air. There must be present some 

reducing agent as, for example, carbonic acid to prevent its oxidation. As shown by Le Conte, 1 the condition most favorable for its 

deposition is the presence of a large amount of decomposing organic 

matter for example, a peat bog, where there is an excess of carbonic 

acid. This subject will be reverted to in considering fossiliferous iron

-*' ore, specimen No. 52.

Limonite is a valuable ore of iron, and when pulverized is used also as 

a polish and for paint.

No. 33. CONCRETION (CALCAREOUS). (CLAYSTONE.) 

\- (FROM HARTFORD, CONNECTICUT. DESCRIBED BY J. S. DILLER.)

', In studying the oolitic sand (p. 102) it was noted that each grain is 

essentially a concretion, i. e., it is nodular in shape and composed of 

material which accumulated by systematic additions upon the outside. 

It began to form at the center, and gradually grew outward.

Specimen No. 33 is a concretion, and, like the grains of oolitic sand, 

is composed of carbonate of lime, but it was formed under very different 

conditions. On account of its composition it is sometimes called a 

calcareous concretion, and in consideration of the fact that it occurs in 

clays, it is frequently designated claystone.

It was obtained from the bank of the river at Hartford, Connecticut,

where many of them have been washed out of the adjoining clay bank.

1 Their mode of occurrence at other localities was described and their

great variety of form illustrated as early as 1841, by President Edward

Hitchcock.2

In form there is great variety, including spherical, discoidal, anuulated, and cylindrical. Most of the forms represented in this collection 

are discoidal, and show traces of the planes of stratification in the clay 

where they were found.

'Elements of Geology, p. 137.

Tiiial report on the Geology of Massachusetts, Vol. JJ, pp. 407-417, pis. 15, 16, 17, ami 18.
108 THE EDUCATIONAL SERIES OF ROCK SPECIMENS.

- . tj

The stratification planes are marked directly through the claystones, A 

and they have their greatest extension parallel to the stratification, 

which is horizontal. That the concretions did not exist at the time the 

clay was deposited, but originated in place later, is clearly indicated by ^ 

their mode of occurrence and structure. The lines of stratification in 

both clay and claystone were marked out by differences of size, color, 

or composition of the particles at the time the clay was deposited. At . 

a later period the carbonate of lime, of which the greater portion of the 

concretion is composed, was deposited by waters circulating through 

the porous layers of the sandy clays.

Why the circulating waters bearing calcic carbonate should deposit 

it in such shapes is not always clear, but it appears from the general 

form that the precipitating influence must have emanated from the center. Many calcareous concretions contain fossils, and the decomposing 

organic matter furnished the surrounding atmosphere with the precipitant that developed the concretion from the center outward. The 

form of growth, however, is determined largely by the relative porosity 

of the various layers. The greatest development takes place along the 

most porous layers, where precipitation is most free. As the concretion 

grows, it loses organic matter at the center, and this process may be ^ 

carried on until the latter has entirely disappeared in such a way as 

not to preserve its original form. No record of the fossil will remain. 

Many concretions have no visible traces of organic matter, and its 

absence may be explained in this manner.

Excepting the conditions which mark the planes of stratification, 

claystones are often homogeneous throughout, but in other cases they -i 

possess concentric banding like that so well shown in the siliceous oolite, 

specimen No. 26.

For further information on the subject of claystones, the student is 

referred to Monograph XXIX, United States Geological Survey, by 

Prof. B. K. Emerson, where he will find the matter discussed under the 

heading "-Concretions." .

No. 34. CONCRETION (FERRUGINOUS, CONTAINING A FOSSIL).

(FROM MAZON CREEK, GRUNDY COUNTY, ILLINOIS. DESCRIBED BY J. S. DILLER.)

The rocks along Mazon Creek, Grundy County, Illinois, from which "* 

specimen No. 34 was collected, according to Mr. Frank H. Bradley, 1 consist of very sandy clay shales and sandstones, in some places becoming 

nearly pure clay shales with many fossiliferous nodules of carbonate of 

iron. These nodules contain many species of ferns, as well as numerous 

fossil insects, occasional crustaceans, fish scales, and shells of mollusks. 

It appears that in this case most, if not all, the concretions have .. 

organic nuclei, and in this respect they are strongly contrasted with 

the claystones of Hartford. It should be remembered, however, that

1 Geol. Survey of Illinois, Vol. IV, p. 196.
DIUJHL] DESCRIPTIONS: NO. 34, CONCRETION. 109

& in the shales of Mazon Creek there are numerous fossils, but in the 

clays of Hartfoid fossils are rare or entirely absent.

The ferruginous sandy shale, which contains the concretions (speci- 

, men No. 34) irregularly scattered through it, is 25 feet in thickness and 

overlies the main coal bed of that region.

In form and size the concretions vary according to the inclosed

i, organism. A small fern leaflet may be contained in. a concretion less

than an inch in diameter, while another concretion, inclosing a piece of

calaniite, may be a foot in diameter. Generally, however, the concretions are flattened in one direction and elongated oval in another.

As with the calcareous concretions, the plane of greatest extension 

4 is the plane of stratification. Traces of stratification may be seen in 

the banding or shaly structure of the concretion.

The fossil vegetal fragments contained in these concretions have been 

studied, described, and figured by Leo Lesquereux,1 who remarks:*

Peculiar species of plants and animals or their fragments seem to bave been 

selected as tbe nuclei of tbe nodules. They contain, for example, an abundance of 

leaflets of various species of Neuropteris . . . which are either rare or bave not yet 

been found in the shale at Morris, while these shales are rich in [other] remains 

scarcely or not at all preserved in the concretions.

He states further: :i

The piunic or leaflets of terns are always found in them in a flattened position, 

their axis or rachis extending through the center of the elongated nodule, with the 

divisions on both sides; the surface of the pinnules, slightly swollen, as when in 

their living state, is marked by recognizable hairs or fruit dots, with distinct veins 

and veiulets, and their appendages, like the scales, are seen in the various modifications which they present in living specimens; for example, long, straight, flat, 

diArerging, or primary rachis, and becoming shorter, ruffled and curled on their

-j upper divisions. The small organs of plants appear, therefore, in a better state of 

preservation than in the shales. ' With small animals like crustaceans, scorpions, 

insects of a fleshy and very delicate texture, the preservation oj; form is still more 

remarkable. They are found entombed in the middle of the nodules just as if they 

were in life, or as if they had been transformed into stone while still living. The 

fruits or nutlets are not flattened. By the section of the nodules, which generally 

break into two equal halves by hard strokes on their edges, the middle and internal 

part of the fruit is exposed to view, while the outside surface is immersed in the 

stone. The numerous cones also of Lepidodendron found in these concretions are 

equally well preserved, either in whole or in part, by horizontal cross sections. Some 

specimens not only show distinctly the pedicels of the sporangea and the blades in 

their natural position, but even sporauges with their seeds have been found in 

them, without'perceptible alteration. In the cross section of these Lepidostrobi

 the sporange cells form a central row, which is surrounded by the blades in the 

form of a star.

Aside from the inclosed organism, part of whose carbon usually 

remains in the fossil, the concretion is composed chiefly of carbonate of 

iron. When dissolved in dilute hydrochloric acid, upon the addition of 

4. ammonia the solution becomes at first greenish, then reddish, precipitating an abundance of ferric oxide. There is but little calcium and

1 Geol. Survey of Illinois, Vol. IV, pp. 375-508, and uuiueroiis plates.

2 Op. cit., pp. 482-483. 

a Ibid., p. 482.
110 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

magnesium in the solution, and it is evident that the iron,.being in the 

ferrous state, as shown by the greenish color of the solution, is present in the nodule in the form of a carbonate. The presence of the iron 

is most simply shown by the fact that a small fragment of the concretion heated in the reducing flame becomes magnetic. It is interesting to 

note this occurrence of carbonate of iron in connection with organisms 

where carbonic acid was probably present to prevent the oxidation of 

the iron at the time of deposition.1

No. 35. CONCRETION (MARCASITE). 

(FROM THATCHER, LAS ANIMAS COUNTY, COLORADO. DESCRIBED BY J. S. DILLER.)

These concretions of marcasite were obtained, according to Mr. G. K. 

Gilbert,2 from the lower layers of the limestone at the base of the Tiinpas 

formation. 3 The limestone has a light-gray color, which becomes 

creamy white on weathered surfaces. It is compact and fine grained, 

and in the eastern portion of the district its texture becomes somewhat 

coarser, its color paler, and the fractured surface resembles chalk.

As the limestone is broken up and removed by the action of the weather, the more 

resistant nodules ;ire freed from the matrix, so as to lie loose on the surface. They 

are of a dark-brown color, and of oval or cylindrical form, with a diameter of about 

half an inch. Their surfaces are not even, but are set Avith angular projections, the 

ends of crystals.'

In PI. XX, taken from Gilbert's report, the variation in form and 

size of the nodules is illustrated upon the natural scale.

That the crystals referred to above are marcasite is known by their 

form in connection with the fact that, when pulverized and heated in a 

closed tube, they yield a sulphurous odor and become magnetic, thus 

indicating that they are composed of sulphur and iron.- From the shape 

of the projecting crystals it appears that their axes, although at right 

angles, are of unequal length, thus showing that the mineral is marcasite and not pyrite, which has nearly the same color and composition 

but differs in form of crystallization.

The structure of the concretion is radial and connected directly with 

the arrangement of the pointed crystals upon its surface. Although 

all the concretions are rusty brown outside, when broken open some of 

them are found to be brass-yellow within, and the radial structure distinct. In most of the nodules, however, the marcasite has completely 

changed to limonite, and the structure is not well marked. No nuclei 

were observed.

The chemical composition of marcasite is sulphur 53.4, iron 46.6,

j represented by the formula FeS2. When exposed to the air, it usually

decomposes, forming limonite, which at first only coats the nodules but

1 See also description of Limonite, specimen No. 32, and Fossiliferous Iron Ore, specimen No. 52.

2 The underground water of the Arkansas Valley in eastern Colorado: Seventeenth Ann. Eept. 

. TJ. S. Geol. Survey, Part II, 1896, p. 566.

3 The Tiinpas formation is the upper portion of the Niobrara (Cretaceous) group of that region. 

* Gilbert, op. cit., p. 566.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XX

CONCRETIONS OF MARCASITE FROM THATCHER, COLORADO. 

Natural size.
DII.USB.] DESCRIPTIONS: NO. 36; GEODE. Ill

in time completely replaces the marcasite. When the replacement is 

complete the crystals are pseudomorphs of limonite after marcasite. 

In the process of decomposition sulphuric acid is set free and may 

attack the surrounding minerals, thus contributing to the effects of 

weathering. Sulphate of iron is frequently formed, and the nodules 

fall to pieces. This is especially likely to be the case if the air is damp. 

The nodules found loose upon the weathered surface of the limestone 

are chiefly limonite, but according to Mr. G. W. Stose, who collected 

some of the specimens, those found firmly embedded beneath the surface of the limestone are marcasite, and when broken exhibit the radial 

structure much better than those changed to limonite.

In the case of the concretions Nos. 33 and 34 of this series, it is evident that they were formed after the deposition of the argillaceous 

strata in which they are contained; but the concretions of marcasite 

from the Timpas limestone do not indicate so clearly their origin. In

some other cases, however, fossils which were originally calcareous are 

fonud completely replaced by marcasite or pyrite. Such replacements 

were not found in the strata from which specimen No. 35 was collected. 

Nodules of marcasite or pyrite are perhaps most frequently found 

in limestone, but they are of common occurrence also in shales and 

sandstones. 1

No. 36. GEODE.

(FKOM WARSAW, HANCOCK COUNTY, ILLINOIS. DESCRIBED BY J. S. DILLER.)

Geodes, like concretions, are nodules, but differ widely in their mode 

of growth. A concretion begins at the center and increases in size by 

additions upon the outside. On the other hand, a geode begins upon 

the outside as the lining of a cavity, and grows by successive additions 

upon the inside of the cavity. As in specimen No. 36, the cavity is 

usually well studded with crystals.

The geodes at-Warsaw are contained in a bed of the Keokuk group, 

of lower Carboniferous age, and it has been said that there is no other 

locality known in the West where a few hours of labor of a good collector will be rewarded by so large a variety of finely crystallized specimens. The occurrence of these geodes in the field has been studied by 

Mr. A. H. Worthen,2 and their structure and mineralogic composition 

have been described by Prof. George H. Brush.3

Mr. Worthen says:

The geodes occur disseminated through the shale and shaly limestone, sometimes 

so thickly dispersed through it that the individuals press against each other as they 

lie embedded in the matrix; and again, are so sparsely disseminated that several 

cubic feet of the shale Avill afford not more than a single specimen. They are most 

abundant at Warsaw, in the lower part of the bed, which also affords nearly all the 

largo-sized individuals. The general form of those filled with siliceous minerals is

1 The formation of sandstone concretions, by G. P. Morrill: Proc. Nat. Mus., Vol. XVII, p. 87.

2 Geol. Survey of Illinois, Vol. I, pp. 89-98. 

. 3 Op. cit., pp. 00-96.
112 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

globular, and many of them are solid spheres of quartz, the interior of which is 

generally crystalline, with a thin crust of chalcedony coating the exterior surface. 

Through the middle of the geode beds there is a band, of shale, which, at Warsaw, 

is. from 8 to 10 feet thick, iu which nearly all the geodes are lined with calcareous 

minerals, and these present less regularity of form than those lined with quartz. 

Many of them are flat disks, nearly or quite solid, but always containing calcite, 

and frequently fine crystals of blende.

Mr. Worthen states also that in Hancock County a siliceous geode 

was found filled with liquid bitumen, and at St. Francisville, Missouri, 

others were observed to be partially filled with clear water.

The collection of thirty-three geodes studied by Professor Brush was 

specially selected, and his report describes the variations. The geodes 

of this series of rock specimens, although generally quite uniform, are 

somewhat varied, as were those described by Professor Brush, from 

whose report the following is quoted:

You will observe that, in every case I have examined, the outer layer of the geode 

is siliceous and is of that form of silica which is called chalcedony, although sometimes this outer siliceous rim is extremely thin. The next in the order of superposition is crystalline quartz. In every geode which contains crystalline quartz this rests 

directly on the chalcedony. In some instances a second layer of chalcedony rests 

on the quartz crystals (Nos. 11, 14, and 22), and in one instance a second series of 

quartz crystals rests on the second layer of chalcedony. Calcite occurs in great 

beauty and variety of form, sometimes resting directly on the chalcedonic crust, 

and sometimes resting on the lining of quartz. In no instance where calcite and 

quartz occur in the same geode have I found the quartz resting on calcite; they all 

indicate that the calcite is subsequent in formation to the quartz. The calcite 

crystals are worthy of special crystallographic study.

The occurrence of pyrites shows that in some cases its formation was simultaneous 

with that of calcite, while in other instances it was apparently subsequent to it. 

The elongated crystals of tarnished pyrites are quite remarkable and might easily 

be confounded with rutile; but they show a yellow color and a cross fracture, and 

a blowpipe examination reveals their real character.

Blende seems to have been simultaneous in formation with the calcareous layer of 

the geodes in which it occurs, for in two instances I have observed it embedded in 

the calcareous layer without resting on the chalcedonic base.

Gypsum, observed in minute crystals in only two instances, is subsequent in formation to the second layer of chalcedony in the geodes in which it occurs. Pearlspar, dolomite, or brown-spar, as it might very appropriately be called, occurs in 

several geodes, and is almost always of subsequent formation to the calcite. Iu a 

few instances, however, calcite crystals appear resting on a dolomite base, and this 

leads me to call attention to the occurrence of calcite of at least two distinct periods 

of formation, as shown by the form and color of the crystals (Nos. 20 and 21). The 

dolomite in the geodes seems to be peculiarly liable to decomposition by the 

oxidation of the iron. An analysis of it shows it to contain a large percentage of 

carbonate of iron with the carbonates of lime and magnesia.

Arayonite was found in but one instance, and then resting on dolomite. Geode No. 

32 contained a considerable amount of a loose white powder, which, on chemical 

examination, proved to be a-hydrous silicate of alumina; and it is exceedingly 

curious that the crystals of calcite in what must have been the lower part of the 

cavity, contain, disseminated through them, this same silicate, as, upon solution in 

acid, they leave behind an insoluble white powder, similar in character to that 

 found loose in the geode. Moreover, the crystals differ in form from those lining the 

upper portion of the cavity.
DILLER.] DESCRIPTIONS: NO. 37, SILICIFIED WOOD. 113

GeocTe No. 4 is one of the most interesting of the snite, being almost filled -with, 

asphaltum, and having isolated quartz crystals embedded in the asphaltum.

The geodes in this series are composed of a thin crust of chalcedony, 

here and there showing indistinct banding parallel to the surface of the 

geode. Inside of the chalcedony is a thick layer of crystalline quartz 

with granular or radial structure about the inclosed cavity, which is 

bristling with the pyramidal points of quartz crystals. Superimposed 

upon the faces of the quartz crystals, here and there in some of the 

geodes, are a few crystals of dolomite, calcite, pyrite, or other minerals, 

but they form a very subordinate portion of the whole mass.

That geodes originate in cavities there can be no question, but the 

way in which the cavity was produced is often a matter of doubt. The 

form of the gepcle is in some cases conclusive evidence that the cavity 

was produced by dissolving and removing a shell or other fossil. The 

shape of the geodes in this series does not clearly indicate any organism, 

but it has been suggested that the geodes of the Keokuk limestone of 

Iowa and Illinois (including those of Warsaw) u occupy the centers of 

sponges that were at some time hollowed out by siliceous solutions, like 

the hollowed corals of Florida, and then lined with crystals by depositions from the same or some other mineral solution." !

Among the most interesting geodes are those occasionally found in 

fossiliferous shales, as at Yaquina Bay, on the coast of Oregon, where, 

after the calcareous shells are removed, leaving a cavity, the mold is 

tilled with translucent quartz by successive additions upon the walls 

of the cavity. In most cases the cavities are completely filled, but in 

others the cavity is only partially filled with quartz, the remaining 

portion containing water with a movable bubble, whose motion may be 

observed through the translucent quartz.

Similar small geodes are occasionally found in amygdaloids, where 

the original vesicles in the lava are not completely filled. The same 

term is also applied to crystal-lined cavities in veins, without reference 

to their form.

No. 37. SILICIFIED WOOD.

(FROM GALLATIN BASIN, GALLATIN COUNTY, MONTANA. DESCRIBED BY J. S.

DILLER.)

The occurrence of silicified wood is illustrated in PI. XXI, prepared 

from a photograph taken by Prof. J. P. Iddings upon the slopes of 

the Lamar Eiver. The trunks of the two trees are shown standing 

erect where they grew. They were buried by the accumulation of sand 

and gravel of an ancient geologic flood, which gave birth to the Tertiary 

conglomerate. At the time of their burial they were not silicified, but, 

while under the ground, the circulating siliceous waters effected a complete change iu their composition. The wood was removed particle by 

particle, and in the position of each was placed a particle of silica, so

1 Manual of Geology, 4th edition, by J. D. Dana, pp. 97,98.

Bull. 150 8
114 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

tliat ultimately the wood was all removed and the whole trunk was 

completely changed to silica; yet the replacement occurred in such a 

way as to preserve with wonderful detail, not only the remarkable cellular structure of the original wood, but also the peculiar markings upon 

the walls of the cells. .Thin sections have been made of specimen No. 37, 

and fig. 12 shows its cellular structure, as seen under a microscope. The 

cells of the wood are filled with silica, slightly different in appearance 

from that which replaces the vegetal matter, and it is this difference 

that marks the cellular structure so distinctly. It has been studied by 

Mr. 1<\ H. Knowltou, who describes it as follows:

Pilyoxylonpealei. Annual ring very pronounced, 2.10 mm broad; cells of summer 

wood large, thin-walled; cells of fall wood thick, much compressed; cells of summer wood with a single series of large, scattered punctations; medullary rays in a 

single series of two to about twenty long cells, marked radially with one to three 

small-bordered pits in the width of each wood cell; resin tubes rather numerous, of 

large size.

The fossil wood contains in places much amorphous silica, but at 

other places is generally cryptocrystalline, with here and there small 

radial fibrous groups with negative optical properties, 

indicating that it is chalcedony.

The special lesson to be learned from this specimen 

is the excellent example it affords of the replacement 

by substitution of one substance for another, in connection with fossilization by casting.

The double character of the process may be illustrated by an experiment. If wood be repeatedly 

FicKi2^ceiisofsiiici- soaked in a solution of sulphate of iron until its cells 

fied-wood as seen un- are filled, and then burned, its structure will be preder a microscope. serve(i in the remaining ferric oxide in the form of a 

cast. The place formerly occupied by the wood will be vacant. In 

silicified wood, however, there is the cast of silica as beforehand also 

the replacing silica which occupies the position of the organic matter. 

Such perfect replacement by silica may occur also in the'animal kingdom. The internal structure of a spirifer, as well as the delicate parts 

of other mollusks, may be replaced and preserved in the greatest detail 

by silica.

Silicified wood is of common occurrence in many of the sandstones 

and conglomerates of all ages, especially in those later than the Jurassic. These rocks usually contain much quartz, and, being porous, 

afford special facilities for the circulation of underground waters, 

through whose agency the silicification of the wood is effected.

No. 38. SILICIFIED SHELL. 

(FROM CHARLESTOWN, CLARK COUNTY, INDIANA. DESCRIBED BY J. S. DILLER.)

This shell, Spirifer oiveni, is one of the Devonian forms found in the 

hydraulic limestone, or cement stone, which extends northeastward from 

Louisville, Kentucky, to Charlestown, Indiana. The shell was origi-
U. S. GEOLO3ICAL SURVEY tULLETIN NO. 150 PL. XXI

. ;__ .. . _NE NATIONAL PARK.
DILLBR.] DESCRIPTIONS: NO. 39, CHALK. 115

nally carbonate of lime, but is now quartz, the carbonate of lime having 

been removed and the silica deposited in its place. This has been done 

in such a way as completely to preserve the external form of the shell, 

but the internal structure is lost. It appears that a perfect impression 

of this shell was made in the material by which it was inclosed at the 

time the rock was formed. The shell was then removed by solution in 

the water circulating in the rocks, thus leaving a mold which was 

afterwards filled with silica, producing a cast. The method of fossilization by molds and casts is common and of much importance in the 

geological record. Molds of shells are perhaps more frequently seen in 

the Oriskany sandstone than in any other formation.

Casts may be of other substances than silica, and when of a comparatively stable substance, like oxide of iron, the fossil may be preserved 

although the rock inclosing it may undergo great change. One of the 

most striking examples of this kind is in a mica-schist of Scandinavia, 

where the original sediment was so inetamorphosed as to have been 

completely crystallized, and yet the forms of large trilobites it contains 

are perfectly preserved in the casts of oxide of iron.

There are three principal types of fossilization illustrated by specimens in this series. In the first type, represented by specimen No. 6, 

the organism is simply buried and remains wholly or partly unchanged; 

in the second type, represented by specimen No. 38, the organism has 

been completely removed and its mold or cast preserves its external form 

only; in the third type, represented by specimen No. 37, the organism 

is completely removed and replaced by mineral matter in such a way as 

to preserve not only the external form but also its delicate internal 

structure.

UNALTERED SEDIMENTARY ROCKS OF ORGANIC ORIGIN.

No. 39. CHALK.

(FROM WHITE CLIFFS OF LITTLE RIVER, SEVIER COUNTY, ARKANSAS. DESCRIBED

BY J. S. DILLER.)

Of the various rocks deposited by water, limestone is among the 

most important. It is composed essentially of either carbonate of lime 

or carbonate of magnesia, or both. Its various forms are quite fully 

illustrated in this series by the following specimens: Chalk (No. 39), 

Patellina limestone (No. 40), coquina (No. 42), shell limestone (No. 43), 

cherty limestone (No. 44), compact limestone (No. 46), lithographic limestone (Nq, 47), hydraulic limestone or cement rock (No. 48), amorphous 

marl (No. 49), and shell marl (No. 50) among the unaltered rocks, and 

crystalline limestone (No. 115), marble (No. 116), and dolomite (No. 117) 

among the metamorphic rocks.

Chalk is a white earthy limestone which is so soft as to be easily 

marked by the finger nail, and is composed of fine calcareous sediment 

derived chiefly from the shells of foraminifera. When pure, its chemical composition is almost wholly carbonate of lime.
116 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Although well known for centuries in England, its occurrence among 

the rocks of the United States was not appreciated fully until 1887,1 

when it was described by Mr. R. T. Hill, who has shown 2 that there 

are several distinct beds of this material, having wide vertical and 

areal distribution, in the Cretaceous rocks of Texas and neighboring 

States. The specimens for this series, which are a little harder and 

less porous than typical chalk, were collected from the uppermost of 

the various chalk beds of the Texan region at White Cliffs, on Little 

River, in Arkansas, where, according to Mr. Hill, a great bed of the 

purest white chalk occurs in section 35, T. 11 S., E. 29 W. He says:

These cliffs have long been a landmark of the region, are about 150 feet in height, 

perpendicular, and as white and almost as pure as the celebrated chalk cliffs of 

Dover, England. Their remoteness from the lines of travel is the probable explanation of their having so long been overlooked by American geologists.

The chalk of these cliffs scales off rapidly in great conchoidal flakes, and owing to the irregularity of this 

process, its face, instead of being a continuous plane, is 

composed of many acute and reentrant angles, resembling the bastions of a fortress. The summit of the cliff 

is covered with gravel, but, measuring from the top of the 

hill a short distance from the margin, the present thickness of this chalk is found to be about 135 feet from the 

summit to the bed underlying it. This chalk has a low 

southeastern dip.

The regularity of this bed throughout its exposure 

about one-fourth of a mile and its reappearance a few

miles to the east and across the Saline watershed shows that it is not a local bed, 

but the remnant of a great and extensive horizon, worn away by the denudation 

through Tertiary and Quaternary times of the deposits of the Red River embayment.

In the large fragments from which specimen No. 39 was prepared 

fossils were rather common and conspicuous, although but few traces of 

them cau be seen in the hand specimen. Caniptonectes, Inocerainus, 

Baculites, and Ananchytes ovalis are the fossils reported by Hill from 

this bed, but it appears that generally "the chalk is almost free from 

fossils." This statement, however, refers only to fossils which can be 

seen by the unaided eye, for if properly prepared and examined under 

the microscope the chalk is found to be composed almost wholly of 

material derived from the shells of minute organisms. Some of the 

shells are complete, more are broken, but most are reduced to a fine 

powder. In fig. 13 are represented two of the most common forms 

which occur in specimen ISTo. 39. These have been studied by Mr. 

George H. Girty, who reports as follows:

The chalk sections which I have examined seem to be scantily supplied with 

recognizable organic fragments. The latter consist of foraminifera, with an occasional coccolith. 'The foraminifera can be referred to the genera Globigerina and 

Textularia, of which the former seems much more common.

FIG. 13. &lobigerina(a, XlOO) 

and Textularia (6,' X140) from 

the chalk of White Clifl's, 

Texas.

1 Am. Jour. Sci., 3d series, Vol. XXXIV, 1887, p. 308.

2 Aim. Eept. Geol. Survey Arkansas for 1888, vol. 2, pp. 87 and 88.
DILLER.] DESCRIPTIONS: NO. 39, CHALK. 117

The chemical analysis 1 of the chalk from White Cliffs is as follows

Analysis of chalk from White Cliffs, Scvier County, Arkansas.

Alumina. .. .\

Ferric oxkleJ

Total......................................

Per cent.

3.49

' 1.41

94.18 

1.37

101. 00

PiIt contains some iron, which here and there tinges it yellowish. The 

insoluble matter and silica are readily accounted for by the presence of 

spiculse of sponges and other siliceous organisms, which may be seen 

under the microscope. Mr. Hill reports that in the field it does not 

contain nodules of flint. In this 

respect it is like the " chalk without flints" of Europe, and differs 

from the "nodular chalk" or 

"chalk with flints," so well exposed in the, cliff's at Dover, 

England. Although flints do 

not occur in the chalk at White 

Cliff's, they do occur in the 

Caprina limestone near Austin, 

Texas, from which specimen No. 

41 was collected.

Much light has been thrown 

upon the origin of chalk by the 

deep-sea soundings of the Challenger expedition.2 From the 

sea floor, at depths of between 

2,500 and 17,000 feet, where not 

too cold, the dredge brought up a white ooze, consisting largely of the 

shells of foramiuifera and other organisms having calcareous tests, intermingled with a small amount of radiolarian and other siliceous shells.

In fig. 14 is shown the appearance of globigerina ooze as seen 

under the microscope. It is composed chiefly of the calcareous shells of 

members of the family Globigerinidae, but with these are a few lozengeshaped and other siliceous shells/1 This is so strikingly similar to 

chalk in its structure and composition that its deposition practically 

illustrates the origin of chalk.

1 Ann. Kept. Geol. Survey Arkansas for 1888, vol. 2, p. 220.

2 Report of the scientific results of the exploring voyage of H. M. S. Challenger, 1873 to 1870; DeepSea Deposits, 213-223. 

"Ibid., upper one-third of fig. 4, PI. XII.

FIG. 14. Globigeriiia oo/c from Indian Ocean, at J 

depth of 1,800 fathoms.
118 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The organisms whose remains now form the chalk lived in the sea 

under various circumstances. The larger animals lived upon the bottom, 

where they died and their shells contributed to the accumulating 

chalk.

The minute foraminifera, of whose tests the chalk is chiefly composed, 

lived not upon the bottom, but far above it, near the surface of the ocean. 

The species of foraminifera caught in the surface nets of the Challenger, 1 

are the ones whose dead shells have sunk to the bottom to make up a 

large part of the globigerina ooze.

It is evident that these minute organisms must have drawn from the 

ocean water the carbonate of lime of which their shells are composed. 

The same is true of corals, mollusks, and all other marine organisms 

whose skeletons or shells are calcareous and whose remains play such 

an important part in the formation of limestones.

Notwithstanding the fact that chalk is one of the rarest of sedimentary 

rocks, globlgeriha ooze is one of the most widely distributed of the 

marine deposits forming at the present day.

Chalk is very porous, so that it will absorb in some cases an amount 

of water equal to about one-third its own bulk. On this account beds 

of chalk are great reservoirs of underground water, and in some places, 

as in the vicinity of London, such beds yield a large supply of water by 

means of artesian wells.

Chalk has wide application in the arts, industries, and agriculture. 

It is calcined to make lime, producing a superior quality of that material for chemical and structural uses. In the semihumid portion of its 

extent, where the material indurates through a process of interstitial 

hydraulic setting, it is sawn and extensively used as a building material. 

By saturating chalk with siliceous solutions to give it hardness, and 

mineral stains to give it color, ornamental marble of great variety and 

beauty is manufactured iii Europe. Its most remunerative application, 

however, is its use in the process of making hydraulic and Portland 

cements. For this purpose it is mixed in definite proportions with clay 

and silica. It is the use of chalk that has enabled Europe to control 

the supply of superior Portland cement in the United States, and Hill 

has pointed out the fact that the neglect to utilize the extensive deposits 

of chalk in our own country amounts to a serious commercial loss. When 

pulverized, washed, and elutriated it is known as creta preparata, and 

is extensively used for toilet and fine abrasive purposes, as well as for 

medicinal purposes. It is also used in the manufacture of carbonate of 

soda and carbonic acid. On wet clay soil it is a valuable fertilizer, and 

for such purpose it is extensively used in parts of England, and could 

be most profitably so employed upon the noncalcareous lands of the 

southern coastal region of the United States.

The white crayons used for marking purposes were formerly made of 

chalk, but now they are composed chiefly of artificially produced sul1 Challenger Keports; Deep Sea Deposits, p. 213.
. DESCRIPTIONS: NO. 41, FLINT.

phate of lime, with a small admixture of lime carbonate. The lump 

chalk used by carpenters and other tradesmen is the natural chalk as 

removed from the ground.

No. 40. PATELLINA LIMESTONE.

^ . (FROM AUSTIN., TEXAS. DESCRIBED BY J. S. DILLER.)

The specimens of this series were collected on Bull Creek, 5 miles 

west of Austin, Texas. It is a light-colored, earthy limestone, which 

to a considerable degree resembles the chalk of Arkansas, although it 

belongs to a much older horizon. According to Hill, 1 it forms a stratum 

10 feet or more in thickness near the middle of the Glen Eose beds, in 

A the basal portion of the lower Cretaceous.

The typical Patellina limestone differs from the chalk (No. 39) chiefly 

in hardness and in the macroscopic fossils present) but the material 

also occurs in nature as beds of pulverulent chalk, or marl. As in the 

case of chalk (No. 39), there are a number of mollusks present in this 

limestone, but few, if any, of them appear in the hand specimens. 

> It is composed largely of a small, flat-conical foraminiferal shell, 

Patellina texana Eoemer,2 and on this account, at the suggestion of 

Mr. Hill, has been called the Patellina.limestone. While Patellina texana is the only foraminifer in the limestone visible to the naked eye, 

in a thin section of the rock under the microscope the finer material is 

seen to be composed chiefly of foraminifera similar to those of chalk, 

illustrated in figs. 13 and 14, and might well be called cliaUcy limestone. 

Its conditions of formation must have been in general quite like those 

of chalk.

No. 41. FLINT.

(FROM AUSTIN, TEXAS. DESCRIBED BY J. S. DIT/LER.)

Flint nodnles are of common occurrence in the upper chalk beds of 

England, but, as noted under the description of chalk (No. 39), they do 

not occur in the equivalent of that horizon in Texas. The specimens of 

the series were collected from the lower-lying Caprina limestone (Shumard) of Hill's section, 1 ' 2 miles west of Austin, Texas.

In this chalky limestone are well-defined layers of exquisite (lint nodnles, occupying, apparently, persistent horizons iu localities. These flint nodnles aire oval and 

kidney shaped, ranging in size from that of a walnut to about 2 feet in diameter. 

Exteriorly they are chalky white, resembling in general character the Hint nodules 

of the English chalk cliffs. Interiorly they .are of various shades of color, from 

light opalescent to black, sometimes showing a handed structure. These Hint 

nodnles arc beautifully displayed in situ in the Deep Eddy Canyon of the Colorado, 

above Austin, where they can he seen occupying three distinct belts in the white 

1 chalky limestones. ...

The fact that these are the only flint horizons, so far at least as is known to the 

writer, in the whole of the immense Cretaceous deposits of the United States is very

l Paleontology of the Cretaceous formation of Texas: Proc. Biol. Soc: Washington, Vol. VTJT, 1893, 

pp. 14, 20, and 21.

2 Illustrated in Dana's Manual of Geology, 4th eel., 1895, p. 834. 

3 Geol. Survey Texas, Bull. No. 4, p. xix.
120 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULUISO.

interesting, and especially since they occur about the middle of the Lower Cretaceous series instead of at the top of the upper series, as in England. It was from 

them that the Indians made their flint implements, and the ease of their lithologic 

identity "will be of value to the anthropologist in tracing the extent of the intercourse and depredations of former Indian tribes inhabiting this region. 1

These flints have been distributed in later geologic epochs over a 

wide area coastward of the present outcrops of the Capriua limestone.

The specimens illustrate the light-colored exterior of the flints as 

well as the extremely compact texture and the perfect conchoidal 

splintery fracture of the darker interior. Pi. XXII, taken from a photograph, shows a section of one of these nodules in which the concentric 

banding, as well as the planes of original stratification, are distinctly 

marked. When highly heated in a flame, the dark-colored flint becomes 

white, indicating that the color is due to the presence of organic matter. 

Mr. Hill reports a nodule containing a small cavity filled with liquid; 

others are found with fossils (Eequieuia and Monopleura) as nuclei, but 

these occurrences are exceptions. The nodules are generally without 

nuclei. Nevertheless, they contain a large number of fossil fragments, 

which are visible only with the aid of a microscope.

Mr. J. A. Merrill 2 has made a special study of the fossils in the flint 

nodules of the Lower Cretaceous of Texas, and not only described the 

forms, but considered the conditions of their preservation and the origin 

of the nodules. He examined a number of slides of specimens in this 

series, and reports three species of monactiuellid and three tetractinellid forms of sponge spicules, besides the remains of foraminifera, 

echinoderms, and shell fragments.

The large number of the siliceous organisms found in the flint, both 

of this country and of Europe, leave scarcely any doubt as to the 

source of the silica of which they are formed. It was origin ally taken, 

at least in large part, directly from the sea water by siliceous organisms, especially Osponges, for their skeletons and shells, in much the 

same way as the carbonate of lime is secured from the same water by 

organisms having calcareous parts. In globigerina ooze calcareous 

and siliceous organisms are found intermingled, and the Texas material 

contains a similar association of forms. The organisms found by Mr. 

Merrill in the flint of Texas are foraminifera, sponges, mollusks, and 

fish scales. The foraminifera were principally globigeriua whose 

shells are well known to have been originally calcareous. In the flints, 

however, the calcareous matter is completely replaced by silica, and it 

is evident that the flint is not made wholly by an accumulation of siliceous organisms, but in part, at least, by the replacement of calcareous 

organisms by silica brought in solution. Spicules of sponges and other 

organisms are found in all stages of preservation. A few are well preserved; more are partially destroyed, while the greater number have 

either almost or entirely disappeared under the attack of mechanical 

attrition and solvents.

1 Hill, Geol. Survey Texas, Bull. No. 4, p. xlx.

2 Bull. Mus. Comp. Zool. Harvard College, Vol. XXVIII, No. 1, pp. 1-26. See alao a Review of 

the general work by Mr. Wayland Vaughan, in Jour. Geol., Vol. IV, p. 112.
U. S. GEOLOGICAL SURVEY 

SECTION OF A FLINT NODULE FROM 
WI.LER.] DESCRIPTIONS: NO. -12, COQUINA. 121

Although the forms of some of the sponge spicules are well preserved, 

the siliceous material of which they are now composed is not amorphous, 

as it was originally, but so arranged, or perhaps we may say rearranged, 

as to be crystalline. The silica of the flint is in two forms, crystalline 

and amorphous. The first is practically insoluble, but the second is 

soluble in caustic potash. By far the larger portion of the silica in the 

flint is in a crystalline condit ion. Mr. George Steiger, by treating specimen No. 41 with a 10 per cent solution of caustic potash for one hour 

over a water bath, found that 15.39 per cent of the flint was dissolved. 

It is generally very fine microgranular, but occasionally it is radial 

fibrous like chalcedony. However, it is optically positive, while chalcedony is negative. Even that of the sponge spicules and other fossils, 

which was originally amorphous, is now crystalline. Perhaps this 

structural change may have resulted directly from the removal of the 

spiculin originally associated with the silica in the sponge spicules. 

However this may be, it is certain, as already noted, that there is 

much actual replacement by silica in the fossils of flint nodules.

It has generally been supposed that originally the silica was rather 

uniformly distributed throughout the bed, and that the flint nodules 

were formed in much the same manner as claystoues by concretional 

action. A somewhat different view is suggested by Sollas,1 advocated 

by Merrill,2 and commented upon by Vaughan 3 that each nodule represents a separate sponge bed, in which many generations of sponges 

have lived and died in all stages of development.

In the local accumulations thus produced Sollas and Merrill see the 

origin of the nodules, but in support of a somewhat different view 

Vaughan refers to a fact noted by Murray that sponge spicules collect around shells. That flint nodules, sometimes at least, have nuclei 

has been shown by Hill, and the weight of present opinion appears to 

favor the view that flint nodules are largely concretionary.

Flintnodules are extensively imported from England into the United 

States, where they are ground and mixed with kaolin for the purpose 

of making potter's clay, such as is used in the manufacture of porcelain 

and other finer grades of china. The material is also valuable, especially 

in Texas, for road making, and is extensively used for track ballast.

No. 42. COQUINA.

(FROM ST. AUGUSTINE, FLOKIDA. DESCRIBED BY J. S. DILLER.)

Ooquina is a very porous limestone, composed almost exclusively of 

shell fragments cemented together by carbonate of lime. It is a shell 

limestone, but on account of its peculiarities is generally known throughout this country by the local Spanish name of coquina.

According to Mr. R. Dietz,4 it forms a considerable portion of Anas1 Annals Mag. Nat. Hist., 5th series, Vol. VI, pp. 441-443.

2 Bull. Mus. Comp. Zool. Harvard College, Vol. XXVIII, p. 22.

s Jour. Geol., Vol. IV, p. 114.

«Jour. Acad. Nat. Sci. Philadelphia, Vol. IV, 1824, p. 73.
122 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. ISO.

tasia Island, and occurs in horizontal layers, which easily separate, 

forming slabs. The layers are from 1 inch to 18 inches in thickness. 

The fragments of shells composing them vary in size, and occasionally 

entire shells are found. In general, the material is finest near the surface. When first removed from the ground the rock is soft and may 

easily be cut into any desired shape, but upon exposure to the air it 

becomes indurated. On this account it is a good building material, 

and has been extensively used in the construction of the fort, the quays, 

and other structures at St. Augustine.

The shells are chiefly, if not wholly, of species now living along the 

adjoining coast. In the coquina studied by Dietz they belong principally to the genus Area, but Dr. W. H. Ball says that they vary greatly 

from place to place, according to the locally dominant species.

The shell fragments are all arranged with their largest surfaces parallel to the plane of stratification. The space between them is partially 

filled with clear quartz sand, and the whole is cemented by calcite, and 

in such a way as to give the rock, when examined under-a small lens, a 

crystalline appearance. The quartz is easily loosened and isolated by 

dissolving a small piece of the coquina in acid. Some of the grains 

thus liberated are well rounded, but generally they are sharp, angular, 

and clear, as if near their original source. Numerous minute rutile 

needles occur in some of them, as in the quartz of granitic rocks. The 

sand drifts southward along our Atlantic coast, and it is probable that 

the sand in the coquina has been carried from far northward, for the 

beach of Florida exposes no rocks from which it could have been 

derived.

Dr. Dall tells me that coquina is now forming at many points along 

the coast of Florida. The shallow-water shells washed up by the 

waves to the beach, when placed about high-tide level, are alternately 

wet and dry. The water laving the shell beach gets a large amount of 

carbonate of lime in solution, and as it dries, after the waves recede, 

the lime carbonate is deposited upon the fragments, gradually binding 

them together and forming a more or less solid shell rock coquina.

The loose shell-beach material is mixed with cement to make an artificial building stone, quite extensively used in St. Augustine. The 

trimmings of buildings made of this material are of coquina.

No. 43. SHELL LIMESTONE. 

(FPQM ROCHESTER, NEW YORK. DESCRIBED BY J. S. DILLE'R. )

This limestone, like coquina, is composed of shells, and is therefore a 

shell limestone. It differs from coquina, however, in being compact 

and containing almost exclusively the shell of one species, formerly 

called Atrypa hemispherical but now known as Anoplotheca hemispherical 

As the limestone is composed almost entirely of Anoplotheca, it is 

sometimes called Anoplotheca limestone by paleontologists.

i Geology of New York, by James Hall, 1843, pp. 04 and 73.

2 Paleontology, of New York, Vol. VIII, by James Hall, 1894, p. 136.
DILLER.] DESCRIPTIONS: NO. 44, CHERTY LIMESTONE. 123

This bed of limestone is only 3 or 4 inches in thickness along the 

Genesee Eiver near Kochester, and lies within the upper green shale 

of the Clinton group. The sediments associated with it are all fine, 

and, although of littoral origin, evidently do not belong to beach deposits. The shells are so well preserved as to retain their pearly appearance, and but few of them are broken. They accumulated upon the 

sea floor at a. favorable spot, where not disturbed by the influx of ordinary sediments or the beat of waves.

The relation of the strata in that region is illustrated in PI. XXIII. 

At the base of the exposure shown in the figure is a mass of shale, 

which is overlain by a thin-bedded limestone, near the bottom of which 

is a bed of iron ore (No. 52). The limestone above the ore, having a 

thickness of 14 feet, forms the middle falls of the Genesee. As its most 

abundant shell is a Pentamerus, it is referred to as the Peutaraerus 

limestone. Immediately above the Peutaraerus limestone, which is well 

shown in the figure, is the mass of green shale containing the thin bed 

of Anoplotheca limestone from which specimen No. 43 was obtained.

No. 44. CHEBTY LIMESTONE.

(FROM BUFFALO, NEW YORK. DESCRIBED BY ,T. S. DILLER.)

The bed of limestone from which specimen No. 44 was taken extends 

from near the Hudson westward through the State of New York into 

Canada, Ohio, Indiana, and other States of that region. At many 

places in New York it contains a large amount of siliceous material 

called chert or hornstone. The limestone containing it is cherty limestone. It was formed during the Corniferous 1 period of the Devonian 

era. Fossils, especially corals, are so abundant in some places that 

the limestone looks like the reef-rock of modern coral reefs. The fossils 

are often silicified, forming chert.

The chert occurs irregularly distributed throughout the mass of limestone, as in specimen No. 44, or it may be arranged in layers, nodular 

sheets, or series of separate nodules in the same plane, alternating with 

layers of limestone. In such cases the limestone layers are generally 

thicker than those of chert, although the relative proportions of chert 

and limestone vary greatly from place to place. In general, the occurrence of chert in limestone is analogous to that of flint and chalk.

The limestone, being soluble under conditions of Aveathering, is gradually carried away, leaving the exposed surface of the limestone rough 

with chert. As weathering progresses the surface in places becomes 

paved with angular fragments of chert.

In hand specimen No. 44 the bluish-gray compact chert and the 

dark limestone are distinctly separable. In some specimens the chert 

predominates, in others the limestone. Their boundaries are almost 

always sharply denned, although in some places there is gradation

iJFroiu the Latiii "coriiu," horn.
124 THE EDUCATIONAL SERIES Otf ROCK SPECIMENS. [BULL. 150.

from one to the other. Both effervesce in acid, but the soft, dark portion, which will hereafter be referred to as the limestone, effervesces 

much more freely than the hard, flinty chert.

1 That the limestone is crystalline, at least in part, is indicated by the 

minute glistening grains, visible under a sjnall hand lens. When 

viewed in a thin section under a microscope it is found to be irregularly granular, containing numerous perfect crystals of calcite, ranging from 0.005mm to 0.03mm in diameter. Most of the calcareous material is dark or brownish, owing to the presence of carbonaceous matter, 

which disappears upon ignition, leaving the limestone white. Clear, 

colorless calcite occurs in veins, but more commonly in single crystals 

or variously shaped patches, irregularly intermingled with the clouded 

material.

Embedded in the limestone are occasional angular grains of clear 

quartz. They occur in chert also, but are much less common.

The chert effervesces rather feebly in acid, owing to the presence in 

it of some carbonate of lime, but when that is dissolved away nothing 

but the hard, horny siliceous material is left. Under the microscope 

it appears cryptocrystalline, with here and there more coarsely crystalline, clear areas, having confused radial fibrous structure, some of which 

appears to be optically negative, as chalcedony.

Scattered throughout the chert is much carbonate of lime, often in 

sharp rhombohedral crystals. These crystals are completely enveloped 

by the chert, as if they were formed before the chert was deposited, or 

at least before it had hardened so as to prevent the development of 

crystals of calcite. This matter will be more fully considered under 

"Chert," as illustrated by specimen No. 45, which was taken from the 

same bed of cherty limestone as that which occurs at Buffalo, but at a 

different locality.

No. 45. CHERT. 

(FROM-LEROY, GENESEE COUNTY, NEW YORK. DESCRIBED BY J. S. DILLER.)

The chert at Leroy, New York, according to Hall, 1 occurs in the Corniferous limestone of the Devonian group, where it is arranged in irregular bands between layers of compact gray or blue limestone. In a 

thickness of about 50 feet of the Corniferous limestone there are at 

least twelve horizons of chert, ranging from a few inches to several feet 

in thickness. Westward the proportion of chert in the limestone 

diminishes, and in some places the chert nearly disappears.

Chert, illustrated by specimen No. 45, is a highly siliceous material of 

light-gray color. When freshly broken some of it effervesces for a little 

while, showing the presence of a small amount of carbonate or lime; 

but a fragment long exposed to the weather does not effervesce, the 

carbonate of lime having been removed in solution. Upon exposure it

1 Geology of New York, part 4, 1843, p. 161?.
DILLER.] DESCRIPTIONS: NO. 45, CHERT. 125

breaks up into small angular pieces, which accumulate in the soil at 

the surface.

The chert contains shells, corals, and other fossils which were originally composed of calcareous matter, but are now completely silicified. 

In addition to these fossils, it contains numerous organisms which were 

once siliceous. Mr. George H. Girty, who examined the thin sections of 

this chert, states that "sponge spicules and fragments of the spicular 

skeleton of sponges of both the hexactinellid and lithistid orders are 

not uncommon, although they frequently are much broken. The hexactinellid elements are chiefly flesh spicules ornamented with numerous 

sharp nodes."

The structure, as revealed by the microscope, is cryptocrystalline, 

and ou the whole considerably finer than that of the chert in specimen 

No. 44. The minute grains are rarely greater than 0.001 mm in diameter,

and there is comparatively little variation in size. Radial fibrous chal- 

 ceclony, such as occurs in specimen Noi 44, was not seen in the body of 

the chert, but does occur in a few remarkably well-developed veins. 

These veins are made up of several bands of fibrous .chalcedony, with a 

final filling of granular quartz in the middle. Here and there throughout the chert are small areas of amorphous silica, and there may be 

much in the cryptocrystalline mass, where it can not be so readily distinguished.

The chert, excepting that which replaces calcareous organisms, is 

remarkable for the abundance of sharp rhoinbohedral crystals, like 

those of calcite, it contains. There is some variation in the forms of 

the crystals, but in general they are rhombohedral and average about 

0.02 "1m in diameter. A few minute crystals of other substances besides 

carbonates, as well as irregular grains of quartz, are present.

The siliceous material of the Corniferous limestone, illustrated by 

the specimens Nos. 44 and 45, is sometimes called hornstone. There 

appears to be no distinct line of division between flint, hornstone, and 

chert. The term "flint," although used in a comprehensive sense to 

include chert, so that chert may be defined as impure flint, is applied 

chiefly to the more purely siliceous rock which occurs in chalk,. Flint 

is often, but not always, rather dark colored. A special characteristic 

of flint, according to Griswold, seems to be that a considerable part of 

the silica is in the amorphous soluble form of opal.

In New York the siliceous material of the Oorniferous limestone 

was formerly called "hornstone," 1 and the name is still used to some

'Geikio applies the term "hornstono" (Text-Book of Geology, 3d ed., p. 154) to "an exceedingly 

compact siliceous rock, usually of some dull tint, occurring in nodular masses or irregular bands or 

veins. The name has sometimes been applied to more flinty forms of felsite." In the United States, certain more or less flinty rocks which result from the alteration of sediments in contact with igneous 

rocks have been called hornstone. It is what the Germans call "hornfels." Hawes described such 

in New Hampshire, Am. Jour. Sci., 3d series, 1881, Vol. XXI, p. 27; Emerson in New Jersey, AMI. 

Jour. Sci., 3d series, 1882, Vol. XXIII, p. 303; and Komp in Trans. New York Acad. Sci., Vol. XI, pp70, 128. It thus appears that the term "horustone' 1 has been used to designate rocks of widely different origin, and the needs of science would be better subserved by dropping it altogether, and using 

tlio terms Jlint and chert for rocks like specimens Nos. 41 and 45, and hornfels for those like specimen 

No. 130.
126 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

extent,1 but with growing infrequency,2 while the term "chert" is coming into more general use to designate impure flint, especially when 

it is calcareous.

Much has been written concerning the origin of flint and chert, and 

it is evident that all have not been produced in the same way. Some 

observers consider that the silica of chert is^derived from sponges and 

other siliceous organisms, while others consider that by some chemical 

reaction the silica was precipitated directly from sea water to make the 

chert.

The presence of siliceous organisms 3 in many cherts leaves no doubt 

that their silica, at least in large part, was derived from siliceous 

organisms.

Oolitic structure (specimen No. 26) occurs in some cherts, and has 

been regarded as indicating the replacement of carbonate of lime by 

silica. The silicified corals and mollusks (specimen No. 38), so common 

in the chert of the Ooruiferous limestone, afford positive evidence of this 

replacement, and since we often find in the same specimen of Corniferous 

chert both sponge spicules and replaced calcareous fossils, the traces of 

its history are essentially the same as those noted under flint (specimen 

No. 41). In fact, the chert (specimen No. 45) differs from flint chiefly in 

containing numerous crystals of carbonate of lime. These crystals can 

not be regarded as remnants of the calcareous organisms. They crystallized in place before they were enveloped in hard chert to interfere 

with their development. Irving and Van Hise,4 after an extensive 

study of the cherty limestone and the cherty carbonates of the Penokee 

iron-bearing series of Michigan and Wisconsin, conclude: "First, that 

the chert was mainly deposited simultaneously with the iron carbonate 

with which it was so closely associated; and, second, that it is probable 

that the chert is of organic origin, although we have no positive proof 

that it is not an original chemical sediment, while it may in part be 

from both sources."

The acids resulting from the decomposition of organisms affect the 

solubility of silica, and, as suggested by Julien,5 may cause its precipitation. This would account for the silicification of organisms, both 

vegetal and animal, so common in the various formations.

The evidence of the formation of chert by direct precipitation from 

sea water without the intervention of life is negative. Although there 

are cherts in which no trace of life has been found, it is possible that such 

traces have been obliterated by more pronounced activity of the same 

agents which in other cases only partially destroy them.

i See Dana, Manual of Geology, 4th ed., 1895, p. 583. 

2 See " Hornstone " in Century Dictionary.

3W. J. Sollas, Annals Mag. Nat. Hist., 5th series, Vol. VII, 1881, p. 141; G. J. Hinde, Geol. Mag., 

'1887, p. 435; J. A. Merrill, Bull. Mus. Comp. Zool. Harvard College, Vol. XXVIII, pp. 1-26. 

''Tenth Ann. Kept. U. S. Geol. Survey, Part I, p. 397. 

«Proc. Am. Assoc. Adv. Sci., 1879, p. 396.
DESCRIPTIONS: UNALTERED SEDIMENTARIES. 127

No. 46. COMPACT LIMESTONE.

(FROM GREASON, CUMBERLAND COUNTY, PENNSYLVANIA. DESCRIBED «Y J. S.

DILLER.)

This limestone is one of the most extensive and most ancient, as well 

as the most important, economically, in the United States. It occurs 

in the great limestone belt extending from western. New England through 

the corner of New York, New Jersey, Pennsylvania, Maryland, and 

'Virginia into Tennessee, and represents widespread, long-continued, 

comparatively uniform conditions of the sea in Cambro- Silurian time. 

On account of its softness and solubility by long exposure to atmospheric agents it has wasted away more than the harder rocks adjacent 

to it. The latter rocks form mountains, while the limestone appears 

in the intervening valley. Lebanon and Cumberland valleys of Pennsylvania and the Great Valley of Virginia have this limestone as their 

fundamental rock. The soil in these valleys is rich, and furnishes the 

basis of one of the greatest agricultural regions of the country.

The limestone is compact, with numerous minute glistening particles, 

visible with a hand lens. Its dark color is due to impurities, probably 

carbonaceous, at least in part, for when highly heated the dark color 

disappears. In dilute hydrochloric acid it effervesces freely, but not so 

vigorously as pure calcite, and after the carbonate of liine is completely 

dissolved there remains a very fine, dark sediment. Under the microscope the structure of this limestone is found to be what would be 

termed microporpliyritic. It contains a multitude of minute rhombohedral crystals of calcite, about 0.05 to 0.06mm in diameter, embedded in a 

very fine-granular matrix, which is chiefly carbonate of lime, but contains in addition nearly all the various impurities found in the limestone.

Many of the crystals are very sharply denned and contain traces of. 

the impurities in the matrix. Occasionally the material is arranged in 

bands, of which the darker and more carbonaceous usually contain the 

largest number of well-developed crystals. In places over very small 

areas they have so grown as to mutually interfere and interlock in a 

way suggesting the crystalline structure of marble. In the lightercolored bands the micropheuocrysts are less abundant, and occasionally the line-granular groundmass prevails.

After the carbonate of lime is removed by acid, much of the fine 

residual material is doubly refracting, but in general it is too fine for 

a definite iniueralbgic determination. The chemical analysis of the 

whole rock, however, shows that the residual material must be composed chiefly of quartz, with silicates of alumina, magnesia, and iron.
128 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL 150.

The chemical analysis, made by,E. A. Schneider, is as follows:

Analysis of limestone from Greason, Pennsylvania.

Si02 .............................................

A1203 )

Fe203 5

CaO.............................. ................

MgO................... ..........................

co2 .... ..........................................

H20 (105°).......................................

Total ................................. ....

Per cent.

7.00

3.62

.45

39. 26

9.00

38.82

.75

.18

99.08

Specimen No. 46 contains no fossils, although there are beds of the 

same belt in that region containing an abundance of marine shells, and 

there can be no doubt in such cases that a considerable part of the 

limestone is of organic origin. For this reason, in the classification of 

the limestones of this series it was placed among those of organic 

origin. There is reason to believe, however, that it may be of chemical 

origin, and occasion will be taken at this point to consider the evidence 

concerning such a view.

The principal evidence furnished by the limestone itself is to be 

found in its porphyritic structure. The relative age of the groundmass and the minute crystals so conspicuous under the microscope 

(microphenocrysts, which produce the porphyritic structure) may be 

best understood by considering a porphyritic igneous rock, such as 

dacite-porphyry (specimen No. 90), where the phenocrysts of quartz 

and feldspar are clearly older than the groundmass by which they are 

enveloped; that is, these crystals were formed before the groundmass 

solidified. This is shown especially by the corrosive action of the 

magma upon the quartz crystals. At the time the large feldspar crystals developed the inclosing material was soft, so as not to interfere 

with their symmetrical growth and structure. The same must have 

been true of the minute crystals of calcite in the limestone. They 

must be the oldest solid portion of the mass. Although it may have 

accumulated about the same time, it was not lithified until the crystals 

of calcite were fully developed.

This microporphyritic structure of the limestone is not a local modification in the rock; it belongs to the AY hole mass, and may not be 

attributed to rnetamorphism, either local or regional, for both are absent. 

The associated layers of shale and fossiliferous limestone are unaltered. 

It appears as if the crystals of calcite developed directly in the solution 

from which they drew their carbonate of lime, and that they are the 

fundamental portion of the original mass.

The conditions under which large masses of limestone originate by
DILLBR.] DESCRIPTIONS: NO. 46, COMPACT LIMESTONE. 129

chemical deposition are not well understood, and there is much difference of opinion concerning the early history of such rocks. This subject has been recently discussed by Mr. Bailey Willis, 1 and from his 

paper the following quotations are taken. The conditions .favorable to 

chemical deposition are 

(ft) Evaporation from an inclosed sea.

(/>) Precipitation of lime and magnesia from ocean waters, charged by solution 

from the laud, through evaporation, through reaction of salt -water on fresh, and 

through varying atmospheric conditions at the surface of the sea.

(a) Evaporation from an inclosed sea. When a limited body of water, suchasalake, 

is subjected to a change of climate, so that evaporation exceeds precipitation of rain, 

the volume will shrink, outflow will cease, and the solution of salt will be concentrated. If the process is sufficiently continued, the solution will become saturated, 

first for one salt, then another, and they will he-deposited in the order of their insolubility. This process is important as an indication of climatic variation in the past. 

It has been fully described by Gilbert, Russell, and Chatard for Pleistocene lakes 

and the chemical relations, and these studies suggest the conditions to which appeal 

must be made to explain the less exact facts known in ancient formations of the kind.

(b) Precipitation from brackish-waters. The chemical precipitation of lime and magnesia from sea water is a much mooted question. There are two lines of evidence 

relating to it which are apparently opposed. On the one hand, the scientists who 

have described material obtained by soundings on modern limestone deposits have 

recognized only organic remains. The Challenger in the open oceans, remote from 

great rivers; the Coast Survey vessels in the Caribbean, the Gulf of Mexico, and off 

the Atlantic coast; the Norwegian expedition in the North Atlantic, and English 

vessels in the Indian Ocean have found calcareous oozes of various kinds and rocky 

limestone formations, but in every case the calcareous matter is described as composed wholly of the tests of pelagic organisms, many of them of microscopic size. 

It is known that carbonates of lime and magnesia are to a greater or less extent soluble in waters containing carbonic acid, and that the proportion of these carbonates 

dissolved in ocean waters is small. According to Dittmar, the salts in solution in 

ocean waters contain 0.345 per cent of carbonate of lime and 3.600 per cent of sulphate of liiue,2 and the ocean is capable of dissolving all tbe lime poured into it by 

rivers. 11 This view being accepted, it follows that pelagic organisms, which possess 

the power of secreting solid carbonate of lime from solution, alone can cause liine 

deposits. Chemical precipitation is, according to this view, impossible, or, if it 

occurs, is followed by speedy re-solution, and all. limestones deposited under conditions of the existing oceans are of organic origin.

On the other hand, there are many limestones, deposited at different periods of 

geologic time, from Algoukian to the present, including some now forming, which 

consist of more or less clearly crystalline calcite, devoid of organic structure. If 

this calcite was originally built into organic forms they have been entirely obliterated. Such limestones do indeed contain fossils which sometimes exhibit more 

or less crystalline texture, but the occurrence of these organic forms in the holocrystalliue matrix ouly raises the question, If the mass was originally all organic 

and has undergone secondary crystallization after lithifaction, why was the process 

so complete in the matrix and relatively so ineffective in structures whose delicate 

anatomy can still be traced even to microscopic details? Thin sections of limestone which show a mass of interferant crystals suggest that this wa» the primary

1 Jour. Geol., July-August, 1893, Vol. I, No. 5, pp. 500-517.

2 Report on tlio Scientific Results of the Voyage of H. M. S. Ghullvityer; Pliyttiuu tind Chemistry, 

Vol. I, p. 204. 

S0p.cit..p.221.

Bull. 150- 9
130 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BDLL.ISO.

structure of the rock, and organic remains appear to bo foreign bodies which are 

accidentally of the same substance as the matrix. If this view be correct, then only 

the alteration of the organic carbonate is the measure of the alteration of the rockmass, and it is a fair inference from the original crystalline structure that the limestone may liave been produced by chemical precipitation.

Li explanation of this contradiction it may be suggested that broad 

shallow seas resemble inclosed bodies of water rather than the open 

ocean, so far as concentration of their dissolved salts is concerned; and 

that observations on organic deposits have been made in the ocean, 

whereas it is not improbable that many limestones were deposited in 

relatively shallow seas. If oceanic waters enter a broad basin which 

is so nearly inclosed as to impede their outflow, they may circulate 

until more or less concentrated by evaporation, much as they would be 

in a completely inclosed water body. Under such conditions limestones may have a chemical origin.

It is not proposed here to argue that limestones are prevailingly of one origin or 

the other, but only to show that the assumption of organic origin for all the calcareous deposits of the stratified series is too sweeping. To this end it is desirable 

to consider the chemical and mechanical conditions A\rhich affect the precipitation 

of carbonate of lime, to estimate the solubility of the carbonate in salt water, to 

review the conditions under which lime is contributed to, and distributed in, the sea, 

and to describe several cases of modern limestone formation by precipitation. . . .

As to the chemical and mechanical conditions which affect the precipitation of 

carbonate of lime, chemists describe two under which bicarbonate of lime held in 

solution may be decomposed, liberating carbonic acid and precipitating the neutral 

carbonate: First, by diminution of the tension 'of the carbonic acid in the atmosphere; second, by agitation of the solution.

Theoretically, either one of three things may occur to the neutral carbonate of 

lime if it be thrown out of solution by either one of these processes, which we may 

admit are active on some portions of the salt-water surface. The carbonate may be 

redissolved, or deposited as a calcareous mud, or built into organic structures. We 

may discuss these alternates in turn.

The solvent action of sea Avater has been the subject of direct observation in the 

ocean and of experimental determination. ...

The pelagic pteropods and foraininifera, living at the surface, sink on dying and 

are slowly dissolved. If the water be too deep, the carbonate of lime never reaches 

the bottom; only the insoluble residue gets there. The limits below Avhich the calcareous remnants are not found are about 1,500 fathoms for pteropods, thin shells 

exposing large surfaces to solution, and 2,800 for globigerina, smaller shells, relatively more massive. . . .

The solvent power of sea Avater is very moderate and may be satisfied, so far as 

carbonate of lime is concerned, by two sources by organic tests in suspension, and 

by chemical precipitate. The lime used by organisms is derived from the solution 

to which it is partly returned by re-solution, but another part is deposited, and the 

sea thus suffers constant loss. This loss is supplied by the streams from the land. 

If this terrigenous supply is less than the amount of organic deposit, the sea will 

become less alkaline and \vill more efficiently dissolve calcareous tests, until the 

solvent is satisfied. If the land contribution is continuously equal to the amount 

organically subtracted, there will be equilibrium. If the land yields more carbonate 

of lime than that which is being locked up in organic limestones, the alkalinity of 

the sea Avill gradually increase until there is chemical precipitation. This condition 

is favored by the entrance of lime-bearing fresh Avater into a sea free from active 

currents and exposed to evaporation Avhich balances the inflow.
UILLKR.] DESCRIPTIONS: NO. 46, COMPACT LIMESTONE. 131

After reviewing the conditions under which lime is carried from the 

land and distributed in the sea, Willis finds that 

The lime brought down by rivers, though measurable by hundreds of thousands 

of tons per annum, is so widely diffused in the vast volume of the ocean, that it 

escapes recognition.

There are several instances of modern limestone formation which, 

though local, illustrate the processes of chemical deposition on a large 

scale. A reference to this may close the suggestion concerning limestone deposition by other than organic means.

Chemically deposited limestone is forming in the southern part of 

Florida, probably over extensive areas. It occurs in the Everglades, 

and the precipitation is in two forms:

First, from the mass of the water as a flocculent mud; second, from the lower 

layers of the water in. contact with limestone as crystals forming an integral part 

of the solid rock.

The limestones formed upon the shores of the Pleistocene lakes 

Bouueville 1 and Lahontan,2 of Utah, and Nevada, as well as the one now 

developing at the mouth of the Rhone, are referred to as examples of 

limestones formed under the conditions considered in the preceding 

discussion.

These conditions are favored at the mouth of the Rhone by the salinity of the 

Mediterranean and the absence of strong currents.

The examination of a few thin sections of limestone of different ages, from Cambrian to the present, shows that they have three principal types of structure. There 

are those which resemble the Everglades limestone in that they consist of more or 

less coarsely crystalline calcite, yet include unaltered organic remains. Of these 

the Trenton limestone and the marbles of corresponding age in Tennessee, which 

occur interstratified with unaltered calcareous shales, are the most striking examples examined. Cambrian limestones and the Kuox dolomite show similar crystalline 

structure. The second type, the precipitated sediment which forms, the muds of the 

Everglades and which was deposited in. Lake Bonneville, is represented by specimens composed of exceedingly fine grained, apparently pulverulent, material; the 

best of these are from the Knox dolomite and the Solenhofen lithographic stone. 

The third variety of limestone consists of the thoroughly crystalline marbles, which 

contain no unaltered material, and which occur in such field relations that they are 

known to be completely metamorphosed. Extended study is required to determine 

the nature of deposition of the first and second types. They may have been organic 

and have suffered moderate alteration only, but there is a reasonable presumption 

that they did to some extent crystallize in place from sea water, and were, to a still 

greater extent, precipitated from the outspread fans of fresh water radiating from 

rivers' mouths, whence they spread as fine silt over the bottom of the sea. . . .

In discussing the solubility of shells in sea water it has been pointed out that the 

layer of organic matter which accumulates at the sea bottom contains a solvent 

formed by the evolution of carbonic acid in the process of decay. Through this 

layer all substances must pass before they can become part of a lithified stratum. If 

they are plant tissue or flesh they will become more or less oxidized; if they are calcareous tests they will be more or less completely dissolved, and if there beany 

chemically precipitated lime arriving on the sea bottom it, too, would be dissolved 

in this menstruum. The earlier forms of dredge which scooped into the sea bottom

i Mon. TJ. S. Gcol. Survey, Vol. I, by G. K. Gilbert 

* Mou. TJ. S. Geol. Survey, Vol. XI, by I. C. Kuasell.
132 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

brought up a mass of ooze, formed of fine particles, burying organic forms. The 

later forms of dredge, arranged to skim the .surface of the bottom, bring up shells 

and organisms remarkably free from mud. Now, it may be conceived that the layer 

of mud on "which the creatures live, die, and with sunken organic remains decay, 

grades from the fresh surface of recent accumulations downward into a much more 

completely decayed and dissolved mass, and that this rests upon a surface of limestone. In the upper part of this unconsolidated stratum carbonic acid may most 

abundantly be evolved; in its lowest part the more concentrated solution of lime 

may accumulate. Then it is conceivable that lithiflcation by crystallization of the 

carbonate of lime from the more concentrated solution is constantly proceeding on 

the limestone surface. If this conception be correct, the formation of limestone by 

organic means involves the re-solution and crystallization, of more or less of the calcite in the primary formation, and only those organic forms can remain unchanged 

which resist the solvent action. If they are delicate, as the trilobites' brauchia 

from the Trenton limestones, described by Walcott, they give evidence that they 

were rapidly buried and protected.

It is thought by some that limestones are evidences of organic life at whatever 

period of sedimentary history they were deposited, but it has here been shown 

that the source of all lime in the sea is the land, and that under conditions existing in certain localities both crystalline limestone and calcareous mud are now 

forming chemically. It has also been shown that lime converted into organic forms 

is subtracted from that which would otherwise go to saturate the sea water. If, 

then, in any early age of the earth's history, lime-using organisms were not present 

to subtract and deposit lime from sea Avater, and if the atmospheric agencies 

worked then as now, the contributions from the land must have continually added 

to the alkalinity of the sea until chemical precipitation occurred. Such a process 

must have been limited to seas rather than extended to oceans, because the conditions of delivery of lime from the land were then, as now, localized. With the 

development of marine life and the increased demand for lime for organic use, and 

with the corresponding deposition of organic limestone, the sea water must have 

become less alkaline, and the conditions of chemical precipitation must have been 

still more restricted. In time it might occur that pelagic organisms should demand 

so much lime for circulation from the water to calcareous algte, to herbivorous, and 

then to carnivorous forms, and so back into solution, that lime could escape from 

solution by precipitation only under exceptional conditions. If it be true that the 

oceanic oozes, the muds of the Caribbean, the mud flats of Florida, and similar 

calcareous deposits in different seas the world over, be wholly organic, then marine 

life has locked up more lime than the continents could concurrently supply, and the 

balance is now turned against chemical precipitation. But it has not always been so.

No. 47. LITHOGRAPHIC STONE. 

(FROM FLINT RIDGE, GREENWOOD COUNTY, KANSAS. DESCRIBED BY J. S. DILLER.)

Lithographic stone is a limestone characterized by its very fine, uniform texture, structure, and composition so fine and compact, indeed, 

that it will receive very delicate markings by the engraver's tools, as 

well as by etching with acids in lithography.

The best lithographic stone comes from the neighborhood of Solenhofen, near Munich, in Germany, where the rock is extensively quarried. 

Besides the uniformity of texture and composition which makes it 

equally resistant throughout to the engraver's tool, it is soft enough to 

be easily engraved and possesses a degree of porosity which renders it 

properly absorbent, so that it will receive and retain the greasy preparations used by the lithographer in transferring and printing.
DESCRIPTIONS: NO. 48, HYDRAULIC CEMENT ROCK. 133

Specimen No. 47 is a poor example of lithographic stone, although it 

is one of the best that could be readily obtained in this country. It 

lacks uniformity of texture and composition, and for this reason is not 

good for lithographic purposes." Under the microscope, in a thin section, 

it is seen to contain not only very fine-granular carbonate of lime, but 

also numerous angular particles of quartz, which, although usually less 

than 0.02 mm in diameter, render it worthless for lithography. The 

minute particles would turn the engraver's tools aside, and in etching 

would not be affected by acid, like the surrounding carbonate of lime. 

It contains also numerous small patches of clear calcite, which modify 

the absorption of the stone.

^Vhen a bit of this stone is dissolved in acid, a large amount of 

residual material is obtained that is composed chiefly of quartz. 

When similarly dissolved, the Solenhofen stone, used in printing the 

United States Geological Survey maps, leaves a considerable insoluble 

residue of dark argillaceous matter with some minute grains of quartz, 

although chemical analysis has shown that in many cases it is composed 

almost wholly of carbonate of lime. All the other substances it contains put together rarely make as much as 4 per cent of the whole 

mass. 1 A simple means of testing the lithographic qualities of a stone 

which to the naked eye appears so fine grained and homogeneous as k> 

promise to be of use for lithographic purposes, is to examine a thin 

section under the microscope. If the stone is of value,- it will appear 

homogeneous in composition and have a very fine texture.

Good lithographic stones have been much sought for in this country, 

but thus far with but little success. Stones have been found in several 

of the States within the Mississippi Yalley region, but so far as known 

none of them have proved very satisfactory. Some rocks have been 

discovered yielding small stones, but none of these have come into 

extensive use. It is possible, however, that good lithographic stone 

may yet be found in this country. A paper by Mr. G. P. Merrill 2 on 

lithographic limestone will be found useful to students and others 

interested in this subject.

No. 48. HYDRAULIC CEMENT BOCK.

(FROM AKRON, ERIE COUNTY, NEW YORK. DESCRIBED BY J. S. DILLER.)

Hydraulic cement rock is a limestone containing nearly half as much 

clay as carbonate of lime. It affords a quicklime, the cement from 

which, when properly prepared, will harden under water to a stonelike mass. On this account the rock is often called by geologi.sts:i 

hydraulic limestone. It occurs interstratified with other limestones of

"Paper and Press (Philadelphia, January, 189C), Vol. XXII, p. 90.

2The Mineral Industry, by E. K. Rothwell, 1893, vol. 2, p. 453.

3Mr. TJ. Cummings, general manager of the Standard Cememt Company, who kindly obtained the 

specimens of hydraulic Cement rock for this series, informed me that the term hydraulic limestone is 

in the trade applied to a limestone that contains only about half as much 'slay as the cement rock, and 

that the lime derived from it will not make a cement that will harden under water.
134 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

various compositions, and, in places, contains fossils of marine origin. 

The cement made from it is of great importance for building purposes.

Hydraulic cement rock is usually of a gray color, and has a more or 

less fine-granular crystalline structure. ' Under the miscroscope it is 

seen to contain a large number of angular grains of quartz, and here 

and there a grain of fresh feldspar. Some of the feldspar is microcline, 

and so fresh as to be clear and show distinctly the characteristic crossed 

striations. The microscope reveals numerous circular spots or pellets, 

which are fine-granular, and contain much of the argillaceous material.

When dissolved in acid the rock leaves a large amount of gray 

residual material, which, under the microscope, is found to be chiefly 

argillaceous, with much quartz, some feldspar, and a trace of a few 

other minerals. These represent the sediment deposited in the limestone while forming.

The chemical analysis given below, by George Steiger, shows the 

large amount of impurity present; and that the greater portion of it 

is quartz, with much clay, is evident. It seems hardly proper to call 

these materials impurities, for the value of the rock, as a source of 

cement, depends upon their presence in the limestone.

Analysis of cement rock from Akron, New York.

Tio2 . ............................................

ALA ..................................... .......

FeA ............................................

FeO..............................................

MuO.............................................

CaO.. ............................................

MgO........................ .....................

K2O .............................................

Na2 0 ............................................

Water 100° ....................................

Water 100° +....................................

P2o6 - .........................................

CO.,....'..........................................

Total ........................................

1 

Per cent.

9.03

.16

2.25

.85

.52

20.84

18.37

.85

.21

.77

.03

40.33

100. 21

The property of hardening under water, possessed by hydraulic 

cement, is attributed to a chemical union of the clay (silica and alumina) 

with the lime and water. Mortar made of ordinary quicklime will 

harden only upon evaporating to dryness, and is therefore of no value 

for many of the most important structures.

The hydraulic cement rock represented by specimen No. 48 belongs 

to what is called the Water lime group in the upper Silurian system of 

New York, where it is extensively used for making cement, especially 

in Ulster County. Large quantities of cement rock' are quarried also 

in Indiana and Kentucky, and to a less extent at various points in
DESCRIPTIONS: NO. 49, AMORPHOUS MARL. 135

Georgia, Illinois, Kansas, Maryland, Missouri, New Mexico, Ohio, 

Pennsylvania, Texas, Utah, Virginia, West Virginia, and Wisconsin. 

The total product of hydraulic cement in the United States in 1895 

was nearly eight million barrels, and the output is rapidly increasing.

Portland cement, which is of the same character as the hydraulic 

cement already referred to, is made in England by mixing 70 per cent 

of chalk with 30 per cent of flue mud from the Thames. It is now 

being made quite extensively in this -country from a nonmagnesian 

argillaceous limestone.

Information concerning the production of cement in this country is 

published annually by the United States Geological Survey in the 

report on the Mineral Kesources of the United States, and special 

mention may be made to the report for 1891, pp. 529 to 538, the report 

for 1894 (Part III of the Sixteenth Annual Eeport), pp. 576 to 585, and 

the report for 1896 (Part V of the Eighteenth Annual Eeport), pp, 

1179-1182.

No. 49. AMORPHOUS MARL.

(FROM CORTLANJ), CORTLAND COUNTY, NEW YORK. DESCRIBED BY .1. S. DlIVLER.)

A belt of limestones and highly calcareous rocks extends across the 

State of New York from the Niagara to near the Hudson. During the 

Glacial period much material from this belt was carried in the drift 

southward, so that there is a broad belt in which the springs and other 

waters rising in the drift carry much carbonate of lime in solution.

Upon the loss of the carbonic acid by means of which the carbonate 

is held in solution, as well as by means of plants and animals, much of 

the carbonate is precipitated, forming calcareous tufa or marl. The 

calcareous tufa generally contains traces of the vegetation which grows 

upon the bottoms of the lakes and streams or upon the adjacent slopes. 

On the other hand, the marl generally contains shells. Marl is an 

earthy calcareous rock in which the carbonate of lime is intermingled 

with much clay, sand, or other earthy material. The proportions may 

vary from a small percentage to over one-half of the whole mass. 

James Hall 1 says that " in nearly all situations the muck swamps are 

underlain by a deposit of calcareous marl. This is usually very finely 

pulverulent, and, though cohering when wet, is very friable when dry."

Specimen No. 49 is of this character. A few small shells of both 

univalve and bivalve rnollusks occur in it where the specimens were 

collected, but as they are not in large numbers the marl appears 

amorphous, although in other portions of the mass shells are locally 

abundant. It contains 94.32 per cent of carbonate of lime, and is 

almost completely soluble in dilute hydrochloric acid, leaving a whitish 

residue composed chiefly of more or less rounded grains of quartz.

The calcareous material is very fine-granular, the particles averaging 

apparently less than 0.01"1 '" in diameter, while the associated quartz

1 Geology of New York, Part IV, 1843, p. 360.
13G THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

grains are generally about six times as large. Both are crystalline, 

although neither shows crystallographic outlines. The carbonate of 

lime closely resembles that obtained by pulverizing the contained shells, 

and suggests that it may have originated by their disintegration. Professor Hall remarks: !

In the greater .number of the marl beds the remains of fluviatile testacea are very 

abundant, though it is only in a few situations where they have formed any large 

proportion of the deposit. The shells appear to have flourished in immense numbers, probably from the facility with which they obtained calcareous matter, and 

other favorable circumstances; but still it is plain that the formations of this kind 

are generally due to calcareous springs or to the percolation of rain water through 

the surrounding rocks, which, from its excess of carbonic acid, dissolves the calcareous particles in the soil or the harder strata.

No. 50. SHELL MARL. 

(FROM NEAR ROCHESTER, NEW YORK. DESCRIBED BY J. S. DILLER.) 

A few miles east of Rochester, New York, is a small place which until 

recently was a swamp, but is now dry land and cultivated. Originally 

it was a small lake, which was gradually filled by sediments washed 

from the adjacent slopes, and became first a swamp and then arable 

land. In the lake lived numerous mollusks, whose remains were 

buried in the mud of the lake, converting it into marl. As the shells 

may still be plainly seen, the material is shell marl. In some cases the 

shells are so abundant as to form the greater portion of the mass, but 

in specimen No. 50, although numerous, they form but a small portion 

of the whole. When the marl is placed in dilute acid it effervesces 

vigorously for a short time only, and the greater portion of the material, 

which under the microscope is seen to be composed of sand and argillaceous matter, with traces of fibrous vegetal remains, is left as a 

residue.

No. 61. DIATOM EARTH. (INFUSORIAL EARTH, OR TRIPOLITE.)

(FROM WHITE PLAINS, CHURCHILL COUNTY, NEVADA. DESCRIBED BY J. S.. DILLER.)

Diatom earth, sometimes called tripolite (or tripolyte 2 ), and more 

frequently infusorial earth, is a soft earthy material like chalk (No* 39), 

volcanic dust (No. 58), or kaolin (No. 149), but differs from all of these 

in being composed chiefly of extremely minute siliceous plants, or diatoms. The diatoms were once included under the general term "infusoria," hence the name infusorial earth. Tripolite, or tripoli, takes its 

name from the country in Africa, where similar material occurs. In 

that case, however, instead of being composed of diatoms, it appears to 

be derived from the leaching of a siliceous limestone.

The shells of organisms iu lacustrine waters of limestone regions, as 

we have already seen, are calcareous, for there large masses of calcareous rocks give carbonate of lime to the waters of the lakes. In

1 Geology of New York, Part IV. 1843, p. 361'. 

2 Dana's Manual of Geology, 4th edition, p. 81.
Diu.un.1 DESCRIPTIONS: NO. HI, DIATOM EARTH. 137

regions where the prevailing rocl.cs are rich in silica the shells of 

organisms which flourish in the waters are siliceous. The most common of such siliceous organisms are diatoms, a very minute species of 

plant, of which there are many forms. A few of these are illustrated 

in Dana's Manual of Geology, fourth edition, pages 164 and 894. Occasionally they are so abundant that their dead shells, falling to the 

bottom of the water in which they lived, accumulate and form large 

deposits. Such deposits are common in the volcanic regions of the 

Northwest, where the streams carry much silica from decomposing 

lavas and are occasionally interrupted and ponded by outflows of new 

coulees. Several excellent examples of such ponding, produced by the 

recent outflows of lava forming dams in the bed of the stream, occur 

along Pit Eiver and Klamath Eiver, in northern California. In such 

cases there was developed above the dam a temporary lake in which

diatoms flourished and gave rise to small local deposits, now exposed

on the banks of the river, which by long-continued corrasion has cut a 

canyon through the lava and drained the lake. The diatom earth of 

White Plains, Nevada, forms a bed of larger extent. It is of Miocene 

age, and has been tilted with the associated volcanic rocks. 1 Its mode 

of accumulation is illustrated by Shaler.2 Although diatom earth is 

often of lucustral origin, it is produced a.lso in the warm waters of the 

siliceous springs of the Yellowstone National Park, where, as Weed 

has shown, beds 3 to 6 feet in thickness cover many square miles.3

Diatoms flourish in the surface water of parts of the ocean, especially 

in the South Atlantic, where they are so abundant as to becloud it and 

where they serve as food for whales. Their remains sink to the bottom 

and form great accumulations of diatom ooze.4 Their tests, unlike the 

calcareous ones of foraminifera, are insoluble and may sink to the 

bottom of the deepest ocean.

Diatom earth is found in many parts of the world, and is extensively 

used for polishing. It has been used also as an absorbent in the manufacture of explosives, and as a packing about steam boilers. The 

"silver white" of commerce is diatom earth.

In the United States it occurs at many localities, of which two may 

be mentioned. Near Eichmond, Virginia, it forms a bed 30 feet thick 

and 100 miles in extent; and near Monterey, California, there is a bed 

of it 50 feet in thickness, but of unknown extent. There are many 

other localities. The output for 1896 in this country was 2,796 tons, 

valued at $16,042.

'U. S. Geol. Expl. Fortieth Par., Vol. I, Systematic Geology, p. 421; ATol. II, Descriptive Geology, 

p.770.

2 The origin and nature of soils: Twelfth Ann. Kept. IT. S. Geol. Survey, Part 1,1891, p. 310, fig. 22. 

8Botanical Gazette, Vol. XIV, No. 5, p. 117,1889. 

4 lioports of the Challenger Expedition, Deep Sea Deposits, p. 208.
138 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

NO. 52. FOSSILIFEROTJS IRON ORE. 

(FROM ROCHESTER, NEW YORK. DESCRIBED BY J. S. DILLER.)

The rocks of the Clinton series, exposed along the Genesee Eiver, 

near Kochester, are illustrated in PL XXIII. The main mass is a thinbedded limestone of the Clinton series in the upper Silurian system, but 

near its base, plainly visible in the figure, is a bed of iron ore red 

hematite illustrated by specimen No. 52. At this point the ore bed 

has a thickness of 14 inches. Twenty miles to the east it attains its 

greatest thickness, 24 inches. Westward from Eochester the ore bed 

extends only a short distance, for it does not reach the Niagara Eiver. 

To the south, however, in the Appalachian region, it has a remarkable 

distribution. Sometimes there is only one bed, as at Eochester; then 

again there may be three beds, ranging from 1 foot to 10 feet in thickness. They can be traced from New York through Pennsylvania, 

Virginia, Kentucky, and Tennessee into Alabama. They occur also in 

Wisconsin.

The ore is usually fossiliferous, as is specimen No. 52, and is sometimes 

called "red fossil ore." At other times it is oolitic, and is referred to 

as the oolitic iron ore; also as the Clinton ore, on account of its age 

and place of best exposure. The fossils are chiefly broken crinoids and 

bryozoa.

The rock is made up of flattish or elongated grains, many of which 

are fragments of shells, but when seen in the hand specimen all appear 

to be oxide of iron. Under the microscope, however, these fossil fragments are, in most cases, found to be only partially made up of iron 

ore. In some cases there is a fine coating of the oxide, such as may be 

seen about the grains of beach sand, as well as about the grains of 

many sandstones and quartzites, but generally it is thicker than 'a mere 

' coating, and in many cases it completely replaces the carbonate of lime 

of the original fossil. Some of the grains look oolitic, but in a thin 

section no concentric or radial arrangement, suggesting concretionary 

structure, was observed. At many other places, however, as shown by 

Smyth,1 the oolitic structure is well developed. In specimen No. 52 the 

cement binding the ferruginous grains together is calcite and silica. 

They may be intermingled or may occur separately, but in either case 

the cement contains but little oxide of iron. The silica is occasionally 

radial-fibrous and optically negative, like chalcedony.

By dissolving a fragment of this ore in hydrochloric acid, after the 

carbonate of lime has completely disappeared it is found that the ore 

is intimately associated with much silica, which is not easily recognized 

before the carbonates are removed. Upon close examination of a thin 

section a considerable part of the carbonate of lime in. the fossil fragments is seen to be replaced by silica.

'Am. Jour. Sci., June, 1891,3d series, Arol. XLIII, p. 487.
U. S. GEOLOGICAL SURVEY 

BED OF FOSSILIFEROUS IRON ORE BETWEEN BEDS OF LIMEST
15ILLEB.] DESCRIPTIONS: NO. 52, FOSSILTFEROUS IRON ORE. 139

When this rock is long exposed, at or nenr the surface, to atmospheric influences and drainage, it is softened by the removal of much 

of the carbonate of lime cement, and thus the proportion of oxide 

of iron is greatly increased, making the rock a valuable ore of iron. 

Below drainage the carbonate of lime is not removed, and the rock is 

harder and of less value for the iron it contains; but as it affords the 

necessary flux for more siliceous ores, it may still be used for its iron. 

It is important, however, to note that the ore in both cases is the anhydrous oxide of iron that is, red hematite and not the carbonate, or 

limonite, as might be expected from its association with carbonate of 

lime.

This ore has been extensively used of late years in the Southern 

States, especially at Birmingham, Alabama, and at various points in 

Tennessee. Originally it was considerably used in Tennessee, not only 

as a source of iron, but also in dyeing, and it'has been known in that 

region for many years as the "dyestone ore."

The fossiliferous and bedded character of this ore, and its extensive 

distribution, are altogether exceptional, so that its origin is a matter 

of much interest. James Hall 1 regarded it as derived from pyrite, in 

part at least, through the action of thermal waters, but this view has 

long since given place to other hypotheses.

The facts that the oxide of iron in the bed replaces bryozoa and other 

calcareous fossils, and that below the drainage level the rock is largely 

carbonate of lime, have led to the view that the ore originated entirely 

by the replacement of limestone after the strata were tilted up into 

their present position. This view was advocated by A. F. Foerste,2 

and especially by James P. Kimball.3

The type locality of the rocks of the series to which the iron ore 

belongs is at Clinton, New York, where, instead of one bed, there are 

three beds of ore, and, as pointed out by 0. H. Smyth, they are 

associated with shales, sandstones, and conglomerates, whose ripple 

marks and mud cracks clearly indicate the littoral condition under 

which they were deposited along the borders of an interior sea that 

once filled the Mississippi Yalley, and at an earlier stage of its history 

deposited the great mass of limestone noted in the description of 

specimen No. 46 (p. 127). The abundance of marine life furnished the 

material for the coquina-like accumulation of shells, and the streams 

of the adjacent lands brought the oxide of iron to tide-washed, saltwater flats, where, in the association of the two, the deposit of iron 

originated.

According to Dana,4 "The beds were evidently made over tidewashed, salt-water flats, where trituration is gentle. They indicate a 

wonderful degree of uniformity in continental level over a wide area."

1 Geology of Now York, part 4, p. 61.

2 Am. Join-. Sci., Jan., .1801, 3d series, Vol. XLI, p. 28.

3 Am. Geologist, Dec,., 1891, Vol. VIIJ, pp. 356-357. 

4 Dana's Manual of Geology, 4th eel., p. 539.
140 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The oolitic "structure of the ore, as shown by Newberry, 1 favors the 

view which regards the ore as an original deposit instead of a subse-" 

quent replacement of the limestone.

0. Willard Hay'es has recently examined many of the Clinton ironore mines of Tennessee,2 Georgia, and Alabama with special reference 

to data concerning the origin of .the ore, and he reports 3 that he has 

never found it passing into a nonferruginous limestone. ''While it is 

true that the proportion of iron to lime in the unweathered ore varies 

in some cases rather rapidly, it is quite as apt to vary along the strike 

as upon the dip, showing that the variation is original and not connected with depth below the surface." His observations lead him to 

conclude, as did dewberry and Smyth, that the iron ore is an original 

 constituent of the bed, and is not due to later replacement. ,

Xo. 53. PEAT.

(FROM NORTH CAMBRIDGE, MIDDLESEX COUNTY, MASSACHUSETTS. DESCRIBED BY

GEORGE OTIS SMITH.)

Peat is of recent origin, and the processes of its formation can be 

observed by anyone who has opportunity to visit a swamp. It is generally formed by the accumulation of moss-like plant matter that has 

been preserved from the usual decay by moisture which prevented the 

free access of air. Classified by origin, peat may be termed phytogenous and sedimentary; with respect to composition, it is essentially 

carbonaceous. The specimen of this collection well exhibits the character of peat in its dry, compact form, in which it is used as a fuel. Jn 

color, it is dark brown or black; in texture, somewhat earthy, but 

with almost all of the characteristics of matted plant fibers. In its 

natural condition peat varies much in degree of compactness and the 

amount of water it contains.

Peat is a sedimentary deposit in stagnant or almost stagnant water. 

Thus the plant remains are usually deposited in close proximity to the 

place of growth, and often the growing plants are in contact with the 

underlying peat, which simply represents past generations of the same 

species. Peat-forming conditions obtain in countries of humid temperate or subarctic climate. The places of peat accumulation are small 

lakes, marshes, or bogs, where the moisture favors both the growth of 

mosses and water plants and the preservation of their remains.4 The 

drainage modifications which have resulted from the glacial invasion 

have favored the existence of such swamps and bogs in the northern 

parts of this country; but in Ireland peat bogs attain the greatest 

extent, one having an area of over 100 square miles, with the deposit 

of peat almost 50 feet thick.

'The genesis of our iron ores, by J. S. Newberry: School of Mines Quart., Nov., 1880, Vol. II, 

pp. 13-14.

2 Geologic Atlas 17. S., folio 8, Sewanee, Tenn.

8 Letter, June 10, 1896.

4 Professor Sbaler gives the best description of these marsh conditions, in his paper on Fresh-water 

morasses of the United States: Tenth Ann. Jl^pt. IT. S. Geol. Survey, 1890, pp. 255-339.
DILLEK.] DESCRIPTIONS: NO. 54, CANNEL COAL. 141

Specimen No. 53 was collected in North Cambridge, Massachusetts, 

where it occurs in a small basin eroded in brick clay similar to No. 8 of 

this collection, and of inter-Glacial age. 1 The shallow pond has been 

filled during post-Glacial time by this deposit of plant remains, which 

is from 3 to 4 feet thick. In the lower part of this deposit the peat is 

compact and composed of finely comminuted material, which grades 

upward into a vegetal mat, on the surface of which the peat-making 

plants are growing in some places. The origin of the compact peat is 

thus clearly shown.

Many varieties of peat have been distinguished, classification being 

based upon the kind of peat-forming plants, or upon the physical and 

chemical characters of the peat itself. The amount of carbon present 

varies from 50 to 62 per cent, but usually there is little increase over the 

amount contained in peat moss, Sphagnum. The principal use of peat 

is as a fuel, but it is far inferior to other fuels; and it is also used as an 

absorbent and as a fertilizer.

No. 54. CANNEL COAL.

(FROM WALLINS CREEK, HARLAN COUNTY, KENTUCKY. DESCRIBED BY GEORGE

OTIS SMITH.)

Coal, like peat, is composed essentially of carbon derived from vegetal matter accumulated under conditions in which water played an 

important part. It may therefore be regarded as a phytogenous sedimentary rock, but it is unlike peat in all its physical and chemical 

characters, and far superior to it in economic importance. Coal is not 

believed to have been deposited as coal, hence its origin needs to be 

discussed under two heads the conditions of accumulation and the 

character of subsequent change. Peat has be'en defined as simply i>reserved plant de"bris, and is essentially an unaltered rock; but in the case 

of coal the changes subsequent to deposition have been considerable. 

This alteration has consisted of changes in structure or texture and in 

chemical composition. Thus, the coal specimens might be described 

later as altered rocks were it not for their natural relation to the peat 

just described.

That coal represents old deposits of vegetal matter is well proved by 

both its megascopic and its microscopic structure. Distinct plant impressions are often seen on coal. Logs are found changed to coal, and 

its inner structure in many cases is fibrous or cellular. The microscope 

reveals even delicate spores as constituents of coal. The fossil leaves 

and other plant remains, which are so abundant in the associated shales 

and sandstones, strengthen the proof of its organic origin. Concerning 

the conditions of accumulation of this mass of plant d<§bris, two rival 

hypotheses may be mentioned, but a complete discussion of them Avould 

involve the consideration of the most complex of geologic phenomena.

'The glacial brick clays of Rhode Island and southeastern Massachusetts; Chap. Ill, The clays 

about Boston. Ly C. F. Marbut and J. B. AVoodworth: Seventeenth Anu. Rept. "U. S. Geol. Survey, 

Part 1,1890, pp. 989-998.
142 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The one hypothesis gives to coal a growth-iii-place origin, the other a 

drift or transport origin.

The growth-in-place origin involves a comparison^ of Paleozoic coal 

beds and recent peat deposits. The hypothesis extends peat-forming 

conditions over vast areas, and pictures a luxuriance of vegetation 

hardly to be comprehended from our knowledge of present conditions. 

Such a view is supported by the presence of the underclay, supposed 

to represent the soil which supported this growth, and also by the 

freque'nt occurrence of roots (Stigniaria) in this clay and of tree trunks 

in the sandstone above the coal. An accumulation as great as that 

represented in the coal beds seems'to imply tropical vegetation ; yet the 

process was one of preservation as well as of production of the organic 

matter. The climate that would promote the latter might prevent the 

former, since, as has been stated in the description of peat, accumulation of vegetal matter takes place to-day only in humid temperate and 

subarctic climates.

The drift origin of coal deposits involves sedimentation processes 

differing somewhat from those which have produced other sedimentary 

rocks. Sedimentation of this character would take place in the quiet 

waters of marginal lagoons, into which abundant vegetal matter drifts. 

Along the margins of these lagoons marsh vegetation would flourish, 

and here subaerial decay and subaqueous preservation might proceed 

side by side. The invasion of currents bearing mud or sand into portions 

of the lagoons would occasion the deposition of shale or sandstone; and 

thus the splitting of a coal bed may be explained as a simple phenomenon of sedimentation.

The two hypotheses have much in common. Both involve base-level 

conditions, luxuriance of vegetation, and slow subsidence of the area 

of accumulation. They differ in that growth-iu-place necessitates 

subsidence of an oscillatory character to explain the alternation of 

coal beds and subaqueous sediments, while the sedimentation of 

drift material needs only slow subsidence, possibly somewhat intermittent. Both hypotheses are ably supported, the latter receiving more 

attention from later workers on the subject. It is reasonable to believe, 

moreover, that the coal deposits, with so great a distribution both geologically and geographically, have not all had exactly the same origin, 

and that even in the same coal field both hypotheses may be necessary 

to explain all the phenomena.

The subsequent alteration of these deposits of carbonaceous debris 

has resulted in the destruction of much of the organic structure and 

in quite marked chemical changes. Peaty fermentation doubtless 

initiated such changes, while later geological processes contributed to 

the metamorphism. It must be noted that the metamorphism has been 

selective, the associated shales and sandstones remaining unaltered. 

This is explicable from the readiness with which hydrocarbon compounds respond to changes in their physical environment. Increase of
DIIJJIB.] DESCRIPTIONS: NO. 55, BITUMINOUS COAL. 143

temperature and of pressure attending the folding and faulting of the 

rocks of the.coal formation has caused the chemical changes which 

constitute the difference between woody fiber and coal. The alteration, 

in short, consists of a decrease in volatile or gaseous constituents and 

a corresponding relative increase in carbon. The principal physical 

change has been a corresponding loss of volume, and from compression 

there have resulted various structures more or less characteristic of the 

different varieties of coal. These changes have been slow, but the 

degree of alteration attained in some of the Tertiary coals shows that 

time is not the most important factor. Coal beds are found in formations of the Paleozoic, Mesozoic, and Tertiary ages, but the Carboniferous coals are the most important, and the coal specimens described 

below are all of this age.

Caunel coal is black in color, with a rather dull luster, compact and 

homogeneous in texture, and with a flat conchoidal fracture, although 

often with a fissility approaching that of shale. Its specific gravity 

is lower than that of most bituminous coal, but higher than that of 

lignite or brown coal. Chemically, cannel coal is characterized by its 

richness in'volatile hydrocarbons a quality Avhich makes it preeminently a gas coal. Its name, canuel, or caudle, was given because of 

the readiness with which it burns with a bright flame, so that pieces 

were used as candles by the poor people in England.

Cannel coal is more clastic in character than other coals, and is composed of carbonaceous material thoroughly macerated. Fossil fishes 

and remains of other aquatic animals are often found associated with 

caunel coal, showing its sedimentary origin. The proportion of impurities or ash is larger than in other coals, and cannel coal thus grades 

into carbonaceous shale by increase in amount of clayey material.

The specimens of this collection came from the Carboniferous area of 

southeastern Kentucky. Here the measures lie in a broad syncliue, 

Avith a gentle fold as its southeastern limit, and an overthrust fault 

bounding it on the northwest. Analyses of cannel coal from the same 

part of the basin show a rather low percentage of carbon, with ash 

amounting to over 25 per cent.

No. 55. BITUMINOUS COAL.

(FROM GEORGES CREEK, ALLEGANY COUNTY, MARYLAND. DESCRIBED BY GEORGE

OTIS SMITH.)

This coal is intensely black, with a velvet or pitch luster and a 

cubical or uneven fracture. A block of bituminous coal usually shows 

a banded structure, with some of the bauds dull, others bright and glistening. The hardness of this coal is less than that of anthracite, hence 

its common name, soft coal; and it soils the hand, thus differing from 

cannel coal. Its specific gravity varies from 1.2 to 1.5. Chemically, it 

varies considerably in the proportion of carbon and volatile constituentsjbut is intermediate in composition between anthracite and lignite;
144 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Bituminous coal ignites easily, burns with a yellow flame, and gives off 

much smoke. The rapidity of its combustion fits it for general use in 

the industries, while certain varieties are especially adapted for the 

production of gas and coke.

The Georges Creek Basin, from which locality this specimen was collected, is situated in western Maryland. It is a shallow basin of Coal 

Measures, containing several valuable beds of coal.. The " Big Vein," 

which occurs near the top of the 1,200 feet of Carboniferous rocks hereexposed, is correlated with the Pittsburg coal bed, the most valuable 

of the Appalachian field. It forms the lowest member of the upper 

Coal Measures, and its structure persists over large areas, showing wide 

prevalence of nearly uniform conditions at the time of its deposition. 1 

The coal represented by specimen No. 55 is a high-grade coal, standing 

well in the market.

No. 56. ANTHRACITE COAL.

(FROM GILBERTON MINE, SCHUYLKILL COUNTY, PENNSYLVANIA. DESCRIBED BY

GEORGE OTIS SMITH.)

As will be observed from an examination of this specimen, anthracite coal is black, but with a bluish or brownish tinge. Its luster is 

adamantine to submetallic and its fracture is plainly conchoidal. It is 

dense and brittle, and no megascopic structure can be distinguished, 

yet the microscope sometimes reveals traces of plant remains. Anthracite is the hardest and heaviest of all the coals, with a hardness of 2 

to 2.5 and a specific gravity of 1.4 to 1.7. Its chemical characteristic 

is the high proportion of carbon, which ranges from 85 to 92 per cent 

in the Pennsylvania anthracite and is known to reach even 95 per 

cent in the anthracite of Wales. Because of the decrease in volatile 

constituents, anthracite ignites with more difficulty than does bituminous coal, and burns with a small bluish flame and no smoke, but atfords 

more heat, pound for pound, than any other coal.

The locality from which this anthracite was collected is in the Western 

Middle Anthracite Field of Pennsylvania.2 The eastern basin of this 

field is a long and narrow spoon-shaped syncline, subdivided by anticlines into many smaller basins. Overturned strata are quite common, 

and the coal beds of this area are steeply inclined, the average dip 

being not less than 35° to 40°. Such close folding has tended to crush 

the coal, thus increasing'the proportion of waste.

The chemical and physical characteristics of anthracite are such as 

seem due to a greater degree of alteration tha^n that attained in the 

other coals. It has been suggested that there were differences in 

the original vegetal deposits, since beds of anthracite have been found 

between or even above beds of bituminous coal. However, these rela-

'The stratigraphy of the bituminous-coal fields is described by I. C. White iu Bull. U. S. Geol. 

Survey No. 65. 

2 Described iu the Filial Keport of the Pennsylvania Geol. Survey, vol. 3, Part I, Chap. GXXI.
DESCRIPTIONS: NO. 57, DIKE. 145

tions are exceptional, and it is well known that in the same bed 

bituminous coal may grade into anthracite, and in many cases this 

change is seen to be the result of contact metamorphisin. Perhaps 

the best instances of alteration are found in the Cretaceous coals of the 

Anthracite-Crested Butte area in Colorado. 1 The noncoking bituminous coals are found in regions of least metamorphism, the coking coals 

in the localities of more advanced alteration, and the anthracite only 

in areas of great regional metamorphism or in the neighborhood of 

large bodies of porphyrite.2

If the metamorphism of coal beds is more profound, the coal becomes 

graphitic, as is the case in the anthracite of Rhode Island.

In short, there is a complete gradation in chemical composition from 

wood which contains about 50 per cent of carbon to graphite, which is 

pure carbon. Intermediate members of this series are the peat and the 

coals which have already been described. The group is a natural one, 

the members having an origin essentially the same, but with present 

characters dependent upon the degree of subsequent alteration.

UNALTERED IGNEOUS ROCKS.

No. 57. DIKE.

(FROM WlLLIAMSONS POINT, LANCASTER COUNTY, PENNSYLVANIA. DESCRIBED

BY J. S. DlLLEK.)

By the activity of forces not fully understood, fissures (great and 

small) are produced in the rocks at and near the earth's surface and 

filled from below with molten rock material. Such rock-filled fissures 

are dikes. They are common in volcanic regions, and illustrate one of 

the forms of connection between the volcanic effusions upon the surface and the highly heated interior of the earth in which the lavas 

originate. They are generally nearly vertical, as in PI. XXIV, breaking through sedimentary as well as igneous rocks, and occasionally 

show a marked columnar structure perpendicular to the walls of the 

dike, like that of specimen No. 103. Specimen No. 57, which includes 

the whole thickness of the dike, shows the cross fractures distinctly. 

This jointing was determined by the cooling influence of the adjacent 

rocks.

Dikes range in size from a few inches to several hundred feet in 

thickness and from a few rods to miles in length. Igneous rocks such 

as iorm dikes are generally harder than the adjoining rocks, so that 

they are able longer to withstand the destructive influence of weathering, and give rise to ridges which may be traced for miles. The same 

dike may not continue for many miles, but the group to which it belongs 

may be more or less continuous for long distances. In connection with

'Geologic Atlas U. S., folio 9, Anthracite-Crested Butte. 

"Eldridgo, descriptive text of folio 9.

Bull. 150 10
146 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

the Triassic formation of the Atlantic coast, they extend with interruptions from Massachusetts to North Carolina.

The material of large dikes cools more slowly than chat of small dikes, 

and becomes more coarsely crystalline. Small dikes, .like specimen No. 

57, are usually fine grained. Large dikes are in general more coarsely 

crystalline in the middle than upon the borders, where they come in 

contact with the country rock and are sometimes glassy. A number 

of volcanoes may be in line on the same fissure, affording escape at 

certain points for the rising magma. In other places the outflow upon 

the surface appears to have taken place all aloug the fissures in sheets, 

as observed by Russell, 1 Le Conte,2 and others.

Dikes may occur in radial groups about a volcano. Volcanoes sometimes become plugged up, and the great pressure of the molten material 

within bursts the mountain asunder. The fissures afford an escape 

for the magma, and dikes are produced. This arrangement is well 

marked in the Highwood and Crazy mountains, as well as other mountains in Montana 3 and about the Spanish Peaks of Colorado.4 Mount 

Etna affords an illustration of such a group in connection with an active 

volcano. Upon its. slopes are numerous minor vents arranged in lines 

radiating from the main crater.

Like joints, dikes may occur in parallel or intersecting systems. 

Where they intersect, their relative age can be determined; the newer 

cut across the older.

Many kinds of igneous rocks, both plutonic and volcanic, occur in 

dikes. Some forms are known only in dikes, and for this reason have 

been placed in a separate group by some authors.5

The most common of all dike-forming rocks is probably diabase, the 

group to which the small dike illustrated by specimen No. 57 belongs.

FAMILY.

No. 58. VOLCANIC DUST (RHYOLITICI*).

(FROM GALLATIN VALLEY, GALLATIN COUNTY, MONTANA. DESCRIBED MY J. P.

IDDINGS.)

This fine dust forms a deposit about 20 feet thick within Neocene 

lake beds of the Gallatin Valley, Montana,6 where it has been studied 

by A. C. Peale. The major part of these lake beds consist of volcanic 

dust, similar but less pure, and presumably brought into the lake basins 

by waters from the neighboring slopes, where it has been deposited by 

the wind. The purer material occurring in these beds is considered to

' Bull. U. S. Geol. Survey No. 108, pp. 11 and 22. 

* Am. Jour. Sci., 3d series, Vol. VII, p. 167.

8 Highwood Mountains of Montana, by Walter H. Weed and Louis V. Pirsson.- Bull. Geol. Soc. 

America, vol. 6, p. 392. See, also, Geologic Atlas U. S., Little Belt folio, by W. H. Weed. 

4 Geologic Atlas U. S., Spanish Peaks folio, by R. C. Hills.

6 " Ganggesteine " of Rosenbusch: Mikroskopische Physiographie der Massigen Gestciuo, 1887, p. (i. 

0 Peale, A. C.: Geologic Atlas U. S., folio 24, Three Forks, Montana, 1896,
U. S. GEOLOGICAL SURVEY IULLETIN NO. 150 PL. XXIV

DIKE OF ANDESITE CUTTING ANDESITIC BRECCIA, ABSAROKA RANGE, WYOMING
DILLEK.] DESCRIPTIONS: NO. 58, VOLCANIC DUST. 147

have been deposited directly from the air. It occurs in beds 2 to 5 feet 

thick, separated by thin calcareous layers, the thickness of the whole 

being 20 feet.

When e'xamined with a microscope, it is seen to be made up of minute 

fragments of colorless glass, whose angular shapes in some instances, 

and thread-like form in others, together with the presence of air pores, 

which are spherical, elliptical, and tubular, indicate plainly that the 

fragments are broken pumice: The size of the fragments is very small, 

the largest being about 0.4""" in diameter, the average diameter being 

about 0.2mi".

A very small percentage of the fragments are pieces of crystals, and 

these appear to be feldspar, hornblende, and pyroxene, and possibly 

some quartz. But quartz and unstriated feldspar may be easily confused when in angular fragments, unless cleavage is pronounced. This 

small percentage of crystals as compared with glass Diay be due to the 

original paucity of crystals in the magma exploded into dust, or it may 

be the result of a partial separation of the material during its transportation through the air, by. which means the denser and more compact 

particles settled nearer the vent from Avhich the eruption took place 

than the lighter' and .more attenuated ones. Hence it can not be 

assumed that the material found in this deposit necessarily represents 

the composition of the lava before explosion. The glass itself is absolutely free from microlites and is perfectly colorless in the thin bits 

forming the dust.

The chemical composition of the dust is shown by the following 

analyses:

Analyses of volcanic dust and of rhyolile.

A1203 . .......................

Fe.,0:,. .......................

FeO .........................

MnO............ .............

MgO. ...... ..................

CaO ......:..........-.......

K.,0 .........................

Li.,0 .......... ...............

so3 - .........................

I.

Per cmt. 

08. 68

12. 69

1.14

1.17

1.14

1.23

5.58

7.99

100. 73

II.

Per cent. 

74. 59

13. 78

1.23

].27

1.23

1 91

1.34

6.06

100. 71

in.

Percent. 

75.19

13.77

.61

1.37

.09

.68

' { fl'-i

.29

«r

Analysis II shows the percentage when the loss upon ignition is left 

out. This loss is unusually large and indicates a highly hydrated glass. 

When compared with the rhyolitic rocks of the Yellowstone Park 

region, Avhich is 50 miles to the southeast, the volcanic dust of the
148 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULUISO.

Gallatiu Yalley is found to be much richer in K2O and poorer in Na^O, 

and to contain more MgO and a little more OaO and iron oxide. Analysis III, of a rhyolite from the North Madison Plateau, Yellowstoue 

Park, is placed by the side of that of the volcanic dust for comparison. 

.It is a fair representative of the composition of most of the rhyolite 

of the region. The relatively high percentage of MgO and CaO, and 

the presence of fragments of hornblende and pyroxene and feldspar, 

suggests that the volcanic dust may be the glassy portion of a lava 

having the composition of a dacite, in which potash was concentrated 

in the groundinass by the crystallization of the soda in lime-soda feldspars. Since this is purely speculative and the actual history of this 

particular deposit of dust is unknown, it will be better to refer it to the 

rhyolites on the basis of its high content of silica, remembering that 

its chemical composition is not normal when compared with the great 

bodies of rhyolite that were erupted in the region to the southeast, and 

noting the fact that it maybe related to the explosive eruptions of 

andesite which also took place in earlier Neocene times in the same 

region.

For accounts of the transportation of volcanic dust, sometimes to 

distances of 500 and 700 miles, the student is referred to the text-books

of Dana, Geikie, and others.

No. 59. EHYOLITIC PUMICE.

(FROM MONO LAKE, MONO COUNTY, CALIFORNIA. DESCRIBED BY WALDEMAR

LINDGREN.)

At most volcanic eruptions the effused or ejected masses are partly 

of a porous or scoriaceous structure, due to the expansion of gases and 

vapors contained in the magma. The extreme result of such a dilation 

of the magma by expanding gases is a light, spongy, froth-like substance, usually more or less drawn out in threads and sometimes, 

indeed, forming fibrous masses with silky luster. It is called pumice.

Pumice may be formed during the eruption of any magma, and we 

may thus speak of rhyolitic pumice, of andesitic pumice, etc. No doubt, 

on account of the more viscous character of the magma the kind first 

mentioned is most common, and such is the rock here described.

The late Tertiary or Pleistocene volcanoes south of Mono Lake, at 

the eastern base of the Sierra Nevada, when active poured out heavy 

masses of rhyolitic glass or obsidian.

Yiolent explosions accompanied the eruptions, and fine volcanic dust, 

together with "volcanic bombs" of pumice, were scattered all over the 

surrounding country for miles beyond the craters.

The pumice is light gray, usually somewhat fibrous or silky on 

account of the long drawn-out pores which it contains. As a rule the 

pieces will float on water. Under the microscope it is found to consist 

of a colorless glass, perfectly isotropic, containing a few large por-
DILLER.] DESCRIPTIONS: NO. 59, RHYOLIT1C PUMICE. 14!)

phyritic crystals or pheuoerysts of plagioclase withnarroAv striatious; 

one or two larger greenish pyroxene grains were also noted. More 

abundant are foils of a brown biotite, sometimes lmm long. The glass 

is much purer than that of the obsidian from the same locality. It 

contains no or very few trichites, and only a moderate quantity of 

microlites. Among these, biotite foils, sometimes hexagonal and occasionally bent and twisted, together with minute feldspar microlites, are 

the most abundant. Local accumulation of opacite grains and obscure 

microlites are frequent. Very characteristic is the stringy or drawnout structure of the glass, caused by long, winding, thread-like gas 

inclusions.

The macroscopic pores of the pumice of course appear prominently in 

the sections. They are usually drawn out in an elongated shape parallel to the thin, microscopic gas inclusions in the glass, and are sometimes so abundant that only a slender network of glass remains.

The appended analysis by W. H. Melville shows by the high percentage of alkali and small amount of lime that the rock belongs in the 

group characterized by the prevalence of alkali feldspars, albite, and 

orthoclase, though probably a little oligoclase is also present. The 

silica is rather low for a rhyolitic pumice, and in this respect the rock 

approaches the trachytes.

Analysis of rhyolUic pumice from Mono Lake, California.

sio., .............................................

Al,03 . ......... ..................................

FoA...... ......................................

FeO .............................................

CaO.. ............................................

MgO.... ..........................................

K,0. .............................................

Na.,0 ............................................

Total......................................

'Per cent.

2.00

07. !)!)

15.99

0.56

1.99

1.03

0.77

4.80

4.74

99.93

. 00. KHYOLITIC OBSIDIAN.

(FROM MONO LAKE, MONO COUNTY, CALIFORNIA.

LINDGREN.)

DESCRIBED WATVDEMAH

Just south of Mono Lake, in the Great Basin, and a few miles east 

of the great fault scarp of the Sierra Nevada, there rises a long row of 

volcanic cones with a maximum height of 2,700 feet above the level 

of the lake. Probably in Pliocene, and certainly in Pleistocene times, 

these craters were in full activity, pouring out acid viscous lavas, which 

consolidated as rhyolite and obsidian, and scattering volcanic bombs, 

lapilli, and ashes all over the surrounding country. The volcanoes are 

no longer active, but their cones are still excellently preserved.
150 THE EDUCATIONAL SERIES' OF ROCK SPECIMENS. |BULL. 150.

The lava flows are principally composed of a rhyolitic obsidian, which 

is a rapidly cooled and consequently glassy lava. The obsidian, which 

was collected at the foot of one of these ridges, is of deep-black color, 

has a glassy luster, and beautiful conchoidal fracture; in thin fragments it is translucent with a grayish or smoky color.

Under the microscope the principal constituent is a colorless, isotropic 

glass, filled with a great variety of minute inclusions and crystals, very 

frequently arranged so as to give the rock a banded or fluidal appearance. Once an inclusion of a glassy andesitic rock with feldspar and 

augite microlites was noted, showing that on its way up the magma 

broke through more basic eruptives. Although there are no large 

phenocrysts, the glass abounds with minute crystals and inclusions, to 

a degree not to be suspected by the appearance-of the rock.

While the mineralogical character of some of these may be distinctly 

recognized, others are so small and obscure that we can only infer from 

their form that they possess the physical characteristics of crystals. 

These are designated microlitic forms.

Among the crystals which may be identified we note biotite, a few 

distinct augite prisms, small magnetite grains, pretty generally distributed, and minute colorless prisms, with feeble refraction, which 

almost certainly are feldspars.

The most abundant mineral is brown biotite in long-drawn foils with 

a maximum length of 0.066min , although generally much smaller. 

Sometimes these biotite foils are bent or broken. Further we note a 

few yellowish-green, strongly refracting grains of augite, and some 

colorless rounded masses (maximum diameter O.I1 ""1 ) of a strongly 

refracting doubtful substance. Between crossed nicols these latter 

show undulous extinction and gray colors of interference. Besides 

these the glass swarms with microlites, which are usually of slender 

prismatic forms and act on the polarized light, but which can not be 

positively identified.

Most prominent, however, are other microlitic forms which are designated as trichites. They are defined as capillary crystals of exceedingly small thickness compared with their length. They are nearly 

always beut, broken, or curved in complicated forms. The trichites 

have a great tendency to combine in groups, radiating spider-like from 

a common center, usually a small grain of magnetite. On account of 

their minute thickness, rounded form, and the total reflection caused 

thereby, they usually appear opaque, but occasionally larger ones may 

be found which transmit the light with a dark-brown color. Sometimes 

the trichites appear isolated, as curved or straight hairs with a maximum length of 0.15mm, and it seems, indeed, as if there were a transition between the recognizable narrow biotite foils and the trichites, 

although such a connection has not been conclusively proved. At any 

rate, the trichites seem to be composed of some silicate rich in iron.

Besides these microlitic forms the glass contains a great many gas 

inclusions, generally more or less drawn out in one direction.
DILLER.] DESCRIPTIONS: NO. 61, RHYOLITIC PERLT.TE. 151

The structure and microscopic appearance of the obsidian containing 

all these minute forms are by no means constant and similar throughout the mass. The principal cause of this is the microfluidal structure 

prevailing in the rock and showing that the magma has been in motion 

after the separation of the crystals and the microlitic forms. Most 

slides have a more or less distinct banded appearance. In some bands 

biotite crystals abound, in parallel arrangment; in others, trichites predominate; in still other streaks, minute feldspar crystals may prevail. 

The bands are often curved in a complex way.

Sometimes the glass may be comparatively free from microlitic forms, 

while in other places they accumulate in dense masses.

A faint double refraction of the glass is often noticed; in most cases, 

however, this is due to extremely small microlites, discernible with difficulty even with strong objectives, but in a few places it really appears 

as if the glass itself were faintly double refracting.

The appended analysis by W. H. Melville shows that the rock has 

the composition of a typical rhyolite, indicated by the high percentage 

of silica, small amount of lime, and relatively large quantities of soda 

and potassa.

Analysis of rhyolitic obsidian from Mono Lake, California.

Si02 ..............................................

M,03 .... ........... .............................

Ee203 ............................................

FeO .............................................

CaO..--...........^...................... ........

MgO...... .......................................

K2 0. .............................................

Na,O ............................................

Total ......................................

Per <;ei it.

75.78

12.39

0.22

1.25

0.81

0.31

4.64

No. 61. EHTOLITIC PEKLITE.

(FROM YKLLOWSTONE NATIONAL PARK. DESCRIBED BY J. P. IDOINGS.)

A dark-gray rhyolite glass forms a cliff on the east side of the Firehole River, opposite Excelsior geyser and the Midway geyser basin. 

It is part of the great body of rhyolite which, as a surficial flow of lava, 

constitutes the plateau of the Yellowstone Park.

The rock varies in character in bands, which are alternately glassy 

and lithoidal. The glassy portions are distinctly perlitic, and also 

finely porous and vesicular, the cavities containing minute white pellets 

of tridymite. In places the centers of perlitic shells are black glass, 

the surrounding thin shells being light gray. This is a common feature 

of perlites in many localities in the Yellowstone Park. The less glassy 

or lithoidal layers lack the vitreous luster, and are dull gray; they are
152 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

also porous, the vesicles or cavities in the rock being unequally distributed in different layers. The banding produced by the variously 

constituted layers of the rock, which express what is called the flowstructure, becomes more apparent in large masses of rock. The whole 

is filled with small phenocrysts of quartz and feldspar. In some parts 

of the rock there are small spherulites.

In thin sections the microscopic character of the rock is quite varied. 

The grouridmass consists, in places, of colorless glass, with numerous 

trichites and microlites with marked fluidal arrangement. Such places 

exhibit perlitic cracks. Through this glass are scattered spherulites 

that appear brown in transmitted light and show the optical characteristics of those in the lithoidite from Obsidian Cliff. They are close 

together in the lithoidal portion of the rock and have attained various 

degrees of crystallization. Occasionally they have formed around the 

phenocrysts as a spherulitic border. Their microstructure, as well as 

that of the clusters of tridyrnite, is the same as in the lithoidite of 

Obsidian Cliff, and needs no special description. The phenocrysts are 

quartz, sanidine, plagioclase, and augite, with smaller crystals of magnetite. Quartz occurs in idiomorphic crystals, whose sections are 

those of double hexagonal pyramids; some are rounded. They frequently carry a number of brown glass inclusions in negative crystal 

cavities,, and less frequently bays of groundmass.

Sanidine forms idiomorphic crystals, which sometimes have rounded 

corners. These crystals often carry numerous inclusions of groundmass, and also of colorless glass. Sanidine in some instances surrounds 

plagioclase more or less completely, the two feldspars having parallel 

orientation.

The plagioclase exhibits polysynthetic twinning, in thin lamellae with 

low extinction angles in symmetrical sections. It is probably oligoclase. Frequently it is crowded with inclusions of groundmass and 

glass, so as to be fairly honeycombed. Augite occurs in comparatively 

large phenocrysts, but in small amount. It is light green and frequently incloses magnetite and glass. Magnetite forms crystals and 

grains of considerable size, 0.4mm in diameter. With it are generally 

associated colorless crystals of zircon and some of apatite. On the 

edges or walls of cavities and in places where tridymite and sanidine 

have crystallized in microscopic crystals there is a small amount of 

brown biotite in six-sided plates. 1

The chemical composition is shown by the following analysis, made 

by H. N". Stokes in the chemical laboratory of the United States Geological Survey.

 ' For fuller description of the rhyolites of the Tellowstone National Park see the forthcoming monograph on that region: Mon. TJ. S. Geol. Survey, Vol. XXXII.
J)IM,ER.] DESCRIPTIONS: NO. G2, LITHOIDITE. 

AualysiH of rliyolitic perlito from Yelloiostone National Park.

153

Si02 .............................................

A12 03 .. ..........................................

PeA ............................................

FeO..............................................

MnQ. ............................................

CaO... ........ ...................................

MgO. ............................................

K20. .............................................

Na.2 0 ............................................

Percent.

73.84

12.47

.32

.90

trace

1.08

.25

5.38

2.88

2.76

09.88

NO. 02. LITHOIDITE.

(FBOM OBSIDIAN CLIFF, YELLOWSTONE NATIONAL PARK.

IDDINGS.)

DESCRIBED BY J. P.

The lithoidite iu this series of rocks was collected from the northern 

end of Obsidian Cliff in the Yellowstoue National Park. It represents 

the laminated and more or less crystalline portion of a great flow of 

rhyolitic glass which forms the picturesque cliff of obsidian, well known 

to the tourist by its glossy black columns that rise high above the road.

The jet-black obsidian is filled with spherulites and lithophysre, which 

are scattered through it in layers. In places the spherulites are minute 

and crowded together, so as to form light-gray layers having a stony or 

lithoidal appearance. As these lithoidal bands become more numerous 

the rock is composed of alternating layers of black glass and gray lithoidal ones. At the northern end of the cliff the lower portion of the 

whole mass is lithoidal, as in the specimen.

The composition and structure of the sphesulites and lithophysaj and 

the character and probable origin of the lamination of the lithoidite 

have been described in detail by the writer in an article on Obsidian 

Cliff, 1 from which the following extracts are taken:

This lithoidite [loc. cit., p. 264] is a light purplish-gray rock which shows, on 

cross-fractures, delicate bands of light and dark colored layers. The former are 

crystalline, with small cavities scattered along them, which form planes of weakness and permit the rock to split into thin plates, often one-sixteenth of an inch in 

thickness. The dark layers are microsphern]itic and dense [and are sometimes 

glassy]. . . .-. The lithoidal rock is as full of spherulitic forms as the obsidian, 

but it appears more porous and contains a multitude of hollow spherules of the 

utmost delicacy and beauty. An idea of their great abundance is given by PI. XIII, 

fig. 2 [PI. XXV of this bulletin, p. 154], which was drawn from a slab of lithoidite 

and is the natural size. Most of them are hemispherical and consist of a group of 

concentric shells which curve one over another like the petals of a rose. The shal1 J. P. Iddings, Obsidian Cliff, Yellowstone National Park: Seventh Ann. Eept. TJ. S. Geol. Survey, 

1888. A part of this paper was previously published under tlio title, The nature and origin of litliophysio and the lamination of acid lavas: Am. Jour. Sci., Jan., 1887,3d series, Vol. XXXIII, pp. 30-45.
154 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

lower ones present small, rose-like centers surrounded by thin, circular shells. The 

disks are sometimes oval and sometimes composed of several sets of shells which 

have started from centers near together and developed in sectors, giving a scalloped 

form to the curves. Others are eccentric or send out long, curving arms, cross-walled 

like a chambered ammonite.

The partition walls are generally very thin and often close together, in one instance 

50 occurring within a radius of 2 inches. They are very fragile and crumble under 

the touch, being made up of small and slightly adhering crystals with brilliant, 

glistening faces.

The minerals occurring in the lithophysae and hollow spherulites and 

other cavities in the Htboidite are quartz, tridyiuite, sanidine, fayalite, 

and magnetite, the detailed descriptions of which will be found in the 

the paper referred to and in a more recent one- in which the quartz is 

especially described. 1 . The microscopical character of this lithoidal rhyolite is extremely 

varied, and exemplifies the structure or phases of crystallization common to many rhyolitic lavas in most parts of the world. It has been 

made the subject of a special paper on spherulitic crystallisation 2 from 

which the following description has been taken:

In thin-sections the rock is irregularly banded or mottled according as the sections have been ground across or parallel to the layers of lamination. The most, 

crystalline parts are colorless^ the dark-gray portions are mottled with minute spots, 

and with a low magnifying power these dark portions are seen to be minutely spherulitic, exhibiting characteristic black crosses. The transparent parts are quite 

crystalline aggregations of tridymite or quartz and feldspar, often with numerous 

irregular cavities between the crystals. Occasionally these places contain irregular 

grains of fayalite, and still more rarely brown mica, besides scattered spherulites, 

which also border these more crystalline portions.

The thin sections show a few of the larger spherulites which are porous, and in 

some places branching, arborescent or feather-like growths of feldspar in fact, all 

of the modifications of spherulitic crystallization described in the paper on Obsidian 

Cliff.

Studied with higher magnifying power, it is seen that the finely sphemlitic portions are crowded Avith trichites, which are more perfect as the spherulitic structure 

is more minute, but which lose their form and uniformly fluidal arrangement when 

the spherulites are more developed, sometimes being crowded out toward the margin 

of the spherulites and aggregated in opaque lines between the spherulite individuals. The finely spherulitic parts of the rock section also exhibit an extremely 

minute granulation by transmitted light and appear brown; but by incident light 

this granulated portion is white, evidently in consequence of the reflection of .the 

light from innumerable small surfaces or cracks. In the small spherulites that lie 

isolated in and also bordering on the more crystalline portions of the rock the centers of the spherulites are granulated and brown while the margins are often colorless and transparent. In some cases the centers of the spherulites are colorless, and 

the brown color is confined to an outer zone. In such spherulites the fibers of the 

outer zone are more delicate than those of the central portion, showing a lower 

degree of crystallization.

These small spherulites when investigated with the quartz plate prove to be optically negative that is, the axis of greatest elasticity, a, lies approximately parallel 

to the direction of the radial fibers. This is also true of the most minute colorless

1 J. P. Iddings and S. L. Penfleld, The minerals in hollow sphernlites of rhyolite from Glade Creek, 

Wyoming: Am. Jour. Sci., July, 1891, 3d series, Vol. XLII, pp. 39-46.

2 J. P.Iddings, Spherulitic crystallization: Bull. Philos. Soc. Washington, Vol. XI, 1891, pp. 445- 

464, pie. 7,8.
U. S. GEOLOGICAL SURVEY 

LITHOPHYS/E IN LITHOIDITE OF OBSICIAN CLIFF, YELL

Natural size.
DIU.EB-.] DESCRIPTIONS: NO. 62, LITHOIDITE. 155

spherulites which occur in the glass of the obsidian, and are represented by fig..l, 

PL VII'-[fig. 1, PL XXVI of tins bulletin, p. 156]. In the lithoidite the spherulites 

have formed in juxtaposition, so that they adjoin one another Avith more or less 

polygonal boundaries. Occasionally there is a small space between several sphernlites where the magma has crystallized differently. These spaces may attain a considerable size, relatively, or may constitute layers of the rock. The sphernlites 

bordering such spaces frequently continue their crystallization a short distance into 

them, and exhibit distinct prismatic rays that project beyond the apparent periphery 

of the spherule, and resemble the teeth of a cogwheel. Sometimes the projecting 

rays assume a comparatively great size. These forms are illustrated by figs. 3 and 

5, PI. VII [PL XXVI of this bulletin, p. 156]. In such cases the mineral character of 

the rays is clearly determinable. The projecting rays are prisms with parallel sides 

and crystallographic terminations. They extend with uniform optical, orientation 

toward the center of the spherulite. They exhibit in a few instances distinct, 

cleavage parallel to the sides of the prism. The angle of extinction ranges from 

0° to 10° or 12°, being usually low. The prisms are invariably optically negative, 

and are therefore orthoclase crystals elongated in the direction of the clino axis. 

The high limit of the extinction angles, as well as the chemical composition of the 

roclc and the spherulites, since there is no evidence of the presence of more than 

one species of feldspar, indicates that the orthoclase is rich in soda, the molecular 

ratio of the potash to soda in the rock being 1 to 1.

There is a difference between the end of the projecting prisms of feldspar and the 

part of the ray within the sphernlite. The former is transparent and clear, without 

inclusions; the part within the spherulite proper is clouded and granulated, as 

already stated. In some instances the granulation assumes a more definite character and has a radiating feather-like structure, which at once suggests the granophyric arrangement of quartz in feldspar. This is unquestionably its true character, 

although the quartz does not appear to affect the optical behavior of the feldspar 

rays.

An examination of the microscopic granophyre groups of feldspar and quartz which 

occur in the same thin sections, and which have been described in the article on 

Obsidian Cliff (p. 274), shows the same optical characters and feather-like structure. 

Such an intergrowth is represented in fig. 2, PL VII [PL XXVI of this bulletin.] 

It is made up of feldspar crystals, which cross one another at a common point, or 

which radiate from a common center.. These feldspars invariably have the axis of 

greatest elasticity, a; approximately parallel to the direction of radiation. They 

have the same crystallographic orientation as the feldspar rays of the spherulites. 

The intergrown quartz does not alter perceptibly the optical orientation; therefore 

it must be either so oriented as to have its axis of greatest elasticity more or less 

coincident with that of the feldspar, or it is not present in sufficient amount to 

influence the interference phenomena appreciably. The latter is most probably the 

case, for in a large granophyric group with the same structure, which was studied 

for comparison, it was observed that the quartz, though appearing to be present in 

considerable amount, was not sufficient to change the character of the double refraction of the feldspar, which was the predominant mineral. It modified it, however, 

to a variable extent; and in places where quartz was more abundant its optical character was predominant.

The small spherulites of this rock are unquestionably composed of orthoclase 

prisms or needles elongated in the direction of the clinoaxis, which radiate from a 

center and are intergrown with quartz, after the manner of granophyre or micropegmatite; and it is this microscopic intergrowth which gives them the granulated 

or feather-like structure.

In the case of the spherulites with projecting rays of pure feldspar, it is evident 

that the free silica ceased to crystallize as quartz in intimate connection with the 

orthoclase and allowed the latter to continue alone and project into a highly siliceous 

residual paste, which finally crystallized as tridymite in most instances.
EXPLANATION OF PLATE XXVI.

SECTIONS OF SPHERULITES.

FIG. 1. Colorless microscopic spherulite, showing irregular dark cross between 

crossed uicols, enlarged 153 diameters.

FiO. 2. Simple form of granophyre group of quartz and feldspar between crossed 

nicols, enlarged 235 diameters.

FIG. 3. Microscopic spherulite with projecting rays of orthoclase, enlarged 120 

diameters^

FIG. 4. Like fig. 3, with crescent-shaped area of pure feldspar substance, enlarged 

130 diameters. 

FIG. 5. The same as fig. 3, enlarged 120 diameters.

FIG. 6. Portion of large spherulite, showing different forms of feldspar needles.

FIG. 7. Branching group of orthoclase needles occurring in the outer portion of 

the spherulite of fig. 6. 

156
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXVI

SECTIONS OF SPHERULITES FROM YELLOWSTONE NATIONAL PARK.
DESCRIPTIONS: NO. 62, LITHOIDITE. 157

In certain cases the zone of clear feldspar does not occur on the margin of the 

spherulite, but forms a crescent-shaped transparent belt within it, as shown iuiig. 4, 

PI. VII [PI. XXVI of this bulletin]. In ordinary light this belt appears to be a gaping, circular crack, though its definition is lost at one end. Between crossed nicols it 

is found to consist of pure feldspar, oriented in accord with the radiating prisms, and 

producing no disturbance of the dark cross which passes regularly through it. At 

the lower end, fig. 4, it is seen to be part of the same crystallization as the purer feldspar rays of that part of the spherulite; and in the upper part it differs from the rest 

of the spherulite simply by being free from the granulated or micropegmatitic structure. There can be no doubt that it is a part of the original crystallization of the 

spherulite, and that from some cause .the free silica ceased to crystallize for a short 

space and then continued as in other portions of the spherulite. This is in harmony 

with the observation that the micropegmatitic structure and the other phases of 

crystallization in this part of the rock are irregularly scattered in. patches, so that 

adjoining parts of adjacent spherulites are micropegmatitic, while other portions of 

them are free from this structure. Such crescent-shaped spaces in the spherulites of 

this rock have undoubtedly been produced at the time of the crystallization of the 

feldspar rays, by conditions which affected the quantity or distribution of tlie free 

silica or of the mineralizing agencies engaged in its crystallization.

1-n parts of the rock the small feathery spherulites bordering an area of tridymite 

do not terminate in well-defined feldspar prisms, but pass out into irregularly shaped 

feldspars and send out acicular rays of extreme delicacy. These transparent needles 

also lie in various directions in the tridymite area. They have apparently the same 

double refraction as orthoclase, and have the axis of greatest elasticity, a, parallel 

to their length. A transverse parting is slightly developed. Their mineralogical 

character can not be made out Avith any degree of satisfaction from the smallest 

needles, but they can be traced to stouter ones which are undoubtedly orthoclase; 

so that there would seem to be no question that the delicate acicular rays of these 

spherulites are needles of orthoclase elongated parallel to the clinoaxis. They are 

then the same as the stouter prismatic rays, but have probably developed an acicular 

form because of some slight difference in the conditions under which they crystallized.

The spaces between the spherulites already mentioned are in most instances occupied by tridymite in comparatively large crystals, often twinned and carrying 

numerous gas cavities. These patches may be completely filled Avith tri.dym.ite or 

may be but partially filled, there being open spaces between the crystals which 

cross one another in all directions, and in extreme cases the tridymite may simply 

coat the walls of a hollow cavity. In most instances the area is completely occupied, when the free silica is sometimes in the form of quartz. There are always other 

minerals present in variable amounts, including orthoclase and magnetite, with less 

regularly tourmaline, mica, and fayalite. The fayalite occurs in relatively large 

irregular individuals, with an opaque border, which at times entirely replaces the 

original mineral.

The tourmaline and mica occur in minute crystals about 0.025 inm long and 0.01 nim 

thick. They are abundant in places and lie scattered through the tridymite or 

quartz, and also in the margin of the bordering spherulites. They occur sometimes 

together, but usually one is present to the exclusion of the other. The tourmaline 

is recognized by its decided pleochroism, the strong absorption being across the 

prisms, O > E. Its color is brownish-green; colorless parallel to E. The double 

refraction is strong and negative. Transverse sections exhibit a uniaxial cross, and 

are bounded by six sides, alternately three short and three long. It also occurs in 

the tridymite coating the walls of the hollow cavities in some cases.

The mica is green and also yellowish brown to reddish. It forms stout tablets 

with six sides, and exhibits strong absorption, from colorless to almost opaque, 

which is of course in the opposite direction from that of the tourmaline, the axes 

of elasticity also in the long and narrow sections being reversed in the two minerals. 

The dark-green mica may be easily mistaken for the tourmaline.
158 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The tourmaline and mica are idiomorphic and must have crystallized just before 

the outer portion of the small spherulites and the tridymite and quartz in which 

they lie. They are confined to the region of these interspherulitic spaces, and are 

not found scattered indiscriminately through the compactly spherulitic portion of 

the rock. Their period of crystallization is therefore later than that in which the 

small spherulites began to crystallize and earlier than the final crystallization of 

the residual magma or paste. Their separation from the magma was preceded by 

that of quartz and orthoclase, and was also followed by the same. Their crystallization in such a siliceous lava is abnormal, for they are locally abundant in very 

small spaces within the body of the rock, and not along a contact face of it. The 

crystallization of the tourmaline at least involves the presence of a small amount 

of boron and fluorine within the magma before its final solidification; but they 

were probably present in extremely small amounts and only locally.

AVhile the occurrence of the tourmaline, like that of the fayalite, may be referred 

to the category of "fumarole action," still this is only correct when the term is so 

defined as to include any mineralizing influence which heated vapors may have upon 

crystallization. It would thus include their primary action within fused magmas, as 

well as their secondary action on solidified ones. The effect of heated vapors which 

permeate the rocks in many places in the Yellowstoue National Park is distinctly a 

destructive or metamorphosing process; and all such fumarole action is plainly 

secondary in the sense that it effects changes in the crystalline character of rocks 

already solidified. It would seem advisable, therefore, in order to avoid confusion, 

to use some other terms for the primary mineralizing influence of absorbed Arapors 

upon the crystallization of molten magmas.

The second kind of spherulites occurring in this rock, which were described in 

the paper on Obsidian Cliff under the head of porous spkenditcs (p. 278), are distinguished by being composed of more or less distinct rays of feldspar, which are generally branched, and a cementing material of tridymite, with numerous hollow 

spaces or gas cavities. The branching feldspars may also lie in an isotropic base, 

which appears to be glass. The arborescent feldspar growth may form a complete 

sphere or only a plume-shaped growth, or it may even resemble the stem and 

branches o± a shrub. The optical investigation of these feldspar rays shows them 

in some cases to consist of many small stout prisms of feldspar grown together with 

their longer axes parallel and forming long crooked rods in the direction of this 

axis.

In thiii section these rods or ray's are partly positive and partly negative that is, 

the axis of elasticity, which is approximately parallel to their length, is sometimes 

less and sometimes greater than that at right angles to this direction. The prisms 

have a small extinction angle of variable size, and it is observed that the negative 

rays exhibit less double refraction than the positive ones and have a lower extinction angle. From these characters it is evident that the rays are prisms of orthoclase elongated parallel to the vertical axis c, and that the plane of the optic axes is 

normal to the plane of symmetry. In spherulites of this sort the positive and negative rays are generally tiniformly mixed throughout the whole. Such a spherulite 

sometimes has an outer zone or border of compact, finely fibrous structure, which is 

negative and is the same growth as the small grauophyric spherulites. Tridymite 

is scattered through these spherulites, besides small grains of magnetite, and sometimes a few grains of fayalite and a little mica.

In other cases the branching rays are all positive. This indicates that the feldspar prisms have the same development that is, parallel to the axis c but that the 

plane of the optic axes is in the plane of symmetry, which is frequently observed to 

be the case in prisms of orthoclase which have crystallized independently in the 

tridymite in other parts of the rock.

The distinctly arborescent growths of feldspar in which the long slender rays 

branch off from a stouter stalk is shown in the figure on PI. VIII [PI. XXVII of this 

bulletin]. The prisms become thinner and more crowded together as they grow
U, 3. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXVII

BRANCHING CRYSTALS OF ORTHOCLASE, MANEBACHER TWINS, FROM 

YELLOWSTONE NATIONAL PARK, X 43.
DESCRIPTIONS: NO. 62, LITHOIDITE. 159

outward, and terminate in broad fronds like leaves. The stems are usually twinned 

throughout their length, as are also the fronds, which are sometimes found as 

isolated growths. These are twinned in the same manner, the composition plane 

dividing the crystal in two in the direction of its length. These twinned prisms of 

orthoclaseare always negative, and the inclination to the twinning plane of the axis 

of greatest elasticity is about 7° or 8°. These characteristics could only be found in 

orthoclase prisms elongated parallel to the cliuoaxis a, and twinned according to 

the Mnnebach law, which is the orientation already given for this form of feldspar 

in the article on Obsidian Cliff (p. 278).

In the large porous spherulites several forms of feldspar growths occur together. 

In one instance the center consists of an aggregation of partial spherulites of small 

size and positive character. This is surrounded by a narrow zone of cloudy material, 

but slightly doubly refracting, and with a positive character. Outside of this zone 

the porous portion of the spherulite begins. It is made tip of nearly straight radiating fibers of feldspar, with weak double refraction, which are all positive. From 

points at various distances from the center of the \vholo spherulite, within these 

positive libers, there start stouter fibers of feldspar with stronger double refraction 

and negative character. These branch out into radiating bunches, which unite to 

form the outer zone of the spherulite, where the fibers are partly negative and partly 

positive. In this outer /one it is observed, on closer inspection, that the negative 

feldspars form stems from which thinner positive feldspar fibers branch like the 

needles of a pine twig. These needles curve to a position parallel to the stem and 

to the radii of the sphere. All of the porous portion of the spherulite is thickly 

spotted Avith pellets of tridymite. The structure is very crudely represented by 

tigs. 6 and 7 of PL VII [PI. XXVI of this bulletin], the actual structure being 

extremely complex, formed as it is by innumerable delicate crystals. The first porous 

zone of weakly refracting rays of feldspar, all of which are positive, must be composed of prisms elongated in the direction of the vertical axis, c, with the plane of 

the optic axes in the plane of symmetry. The branching groups of strongly refracting feldspars, which are all negative, must be prisms parallel to the clinoaxis a; 

they are twinned according to the Manebach law. In the outer zone these latter 

prisms send out thinner ones in the direction of the vertical axis, c, which are positive. These thinner needles branch forward from both sides of the twinned stem; 

consequently the crystallization, of the twinned prism must have advanced out from 

the angle 2 /? made by the c axes of the twinned halves-of the crystal.

The synchronous growth of crystals of the same mineral with two distinct habits 

is a natural consequence of crystallographic branching as distinguished from that 

due to the splitting or cracking of microlites. 1 Its occurrence in this rock indicates 

how slight may be the difference in the conditions under which either form of crystal 

is induced. This accords with the fact that we find no fixed order in which positive 

and negative spherulitic growths havo been developed. In the rock of Obsidian Cliff' 

they alternate Avith each other, sometimes one having started first and sometimes 

the other.

1 0. Lehman, Molekular Physik, Leipzig, 1888, p. 378.
160 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL.ISO.

The chemical composition of the lithoidite at Obsidian Cliff is shown 

by the following analysis, made by J. E. Whitfield:

Analysis of lithoiditcfrom Obsidian Cliff, Yellotostono National Park.

Si02 .............................................

Ti02 .............................................

ALA ......................................... ...

FeA ............................................

PeO

MnO.............................................

CaO-. ............................................

MgO...... .......................................

Li.,0... ..........................................

Na,0.............................................

K20... ...........................................

p2 os ---- .........................................

S03 -- ............................................

H20.......... .............. .....................

Total ......................................

Per cent.

75.50

13.25

1.02

.91

.90

.07

.06

4.76

2.85

.32

100. 05

No. 63. LIPARITE. 

(FROM PINTO PEAK, EUREKA COUNTY, NEVADA. DESCRIBED BY J. P. IDDINGS.)

The rhyolite of Pinto Peak is part of a large body of rhyolitic lava 

that has been poured out over a limestone country, and that is accompanied by breccia, tuff, and pumice of the same magma. It is mostly 

white, or light gray, and lithoidal, with abundant minute cavities, producing a rough fracture, and sometimes an earthy appearance. The 

rock is filled with phenocrysts of dark-colored quartz and colorless 

and white feldspars, besides minute crystals of biotite. In some cases

minute red garnets may be seen with a pocket lens. The most conspicuous constituent is quartz, whose dark-colored substance is strongly 

contrasted with the white groundmass. The quartzes are cracked, 

and seldom exhibit crystallographic forms. Feldspar, though, equally 

abundant, is less noticeable because of its color. It is brilliant and 

glassy, and is partly sanidine and partly plagioclase. Biotite occurs 

in distinct six-sided crystals.

In thin section the rock consists of a holocrystalline groundmass and 

porphyritic crystals and angular fragments. The groundmass exhibits 

flow-structure, which is expressed by streaks of semiopaque, faintly 

yellow material with no definite characteristics. The arrangement of 

the crystal particles presents no special connection with, the flowstructure, except in some instances. The form of the grains in the 

groundmass is indefinite and allotriomorphic; their size varies from 

microscopic to submicroscopic proportions, so that their existence is 

inferred from their action on polarized light. The structure is then 

known as rnicrocryptocrystalline.
DESCRIPTIONS: NO. 63, LIPARITE. 161

A study of favorable spots shows the groundmass to consist of microscopic interpenetrating grains of quartz and feldspar, through Avhich 

are scattered larger grains, about 0.06 1Iim in diameter, for the most 

part quartz; a portion is orthoclase and plagioclase. There are also 

minute crystals of biotite in some cases, and a small amount of light 

red garnet in irregularly shaped grains. The latter is quite uniformly 

distributed through the mass of the rock.

The phenocrysts of quartz occur in AV ell-developed dihexahedrons, 

and in angular fragments; rarely as rounded grains. They are irregularly cracked and free from impurities, but sometimes contain bays 

of groundmass and a few colorless glass inclusions. Around the inclusion may sometimes be seen double-refraction phenomena, Avhich have 

been produced by internal strain. This in some cases has caused small 

cracks that pass through the center of the dihexahedral glass inclusion 

and extend short distances into the quartz crystal aloiig three planes

corresponding to three planes of symmetry in the crystal. Occasionally there are fluid inclusions with moving bubbles.

The feldspar phenocrysts are sanidine, and also plagioclase in variable proportions. The substance of the feldspars is fresh and very free 

from impurities. Their forms are sometimes crystallographic, often 

angular and fragmental. The tAvinning of the feldspars is characteristic. Many of the sanidines are Carlsbad twins; the plagioclase 

exhibits polysynthetic twinning according to the albite law, and sometimes according to the pericliue law. Zonal structure is rarely observed. 

Cleavage is frequently perfect, though often wanting; conchoidal fractures occur, and the resemblance to quartz is so strong thatpptical«tests 

are necessary to distinguish them. The sanidine often exhibits a very 

small angle betAveen the optic axes, and the plane of the optic axes is 

sometimes normal to the plane of symmetry and sometimes parallel 

to it.

The plagioclase feldspar is probably oligoclase, its substance is like 

that of sanidine, and the polysynthetic twins are also twinned according to the Carlsbad law. Gas cavities sometimes occur, but none of 

glass. Biotite occurs in small crystals, dark brown, Avith small optical 

angle. Zircon occurs sparingly. Magnetite is almost entirely absent; 

occasionally a minute crystal of it may be seen. 1

1 For further description see Monograph XX, TJ. S. Geological Survey, Geology of (ho Eureka District, by Arnold Hague, pp. 237, 264, 374.

Bull. 150 11
162 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The chemical composition. is given in the following analysis by Dr. 

Edward Hart:

Analysis of liparile from Pin to Peak, Nevada.

SiOy...... ........................................

TiO.,. .......... ..................................

Al O

1-6,03...... .....................................

PeO........... ...................................

MnO. ............................................

MgO- ............................................

CaO................. -..-.....--.-.-.....-..-.....

NasO..... ........................................

K20.. ............................................

PA---- :.....................-..................

Total ......................................

Per cent.

73. !)1

15. 29

0.89

0.77

3. 62

0.07

1.19

No. 64. NEVADITE.

(FROM CHALK MOUNTAIN, EAGLE COUNTY, COLORADO. DESCRIBED BY WHITMAN

CROSS.)

Geologic occurrence. The nevadite of which No. 64 is a specimen 

forms a low mountain mass between the headwaters of the Arkansas, 

Eagle, and Tenmile rivers. The mountain lies on the continental 

divide, and has steep slopes with a cliff at the top on the south, east, 

and north, while cut into by stream branches on the west. The nearly 

level summit lies at an elevation of a little less than 12,000 feet on the 

eastern side of the mass. The mountain derives its name from the 

white color of the cliffs.

All about the mass, which, is 2 miles in longest diameter, Carboniferous sandstones and conglomerates of the Weber formation are 

exposed, but the contacts with the nevadite are so much obscured by 

talus slopes that the relation of the. sedimentary to the igneous rock 

can not be fully made out. From the structure of the uevadite it is 

inferred, that the mass seen muskhave consolidated very near the surface. The body is interpreted as an irregular intrusive sheet cutting 

obliquely across the Carboniferous strata, and it is assumed that parts 

of the magma may have reached the surface and formed lava streams.

Definition. The term nevadite is here applied, as denned by Hague 

and Iddings, 1 to a rhyolite " characterized by an abundance of porphyritic crystals embedded in a relatively small amount of groundinass."

General description. This rock has many pheuocrysts of sanidine, 

oligoclase, smoky quartz, and a few of biotite, embedded in a lightgray groundinass which a hand lens shows to be evenly granular and 

specked by numerous small biotite leaves and a dust of magnetite.

1 Am. Jour. Sci, 3d series, Vol. XXVII, 1884, p. 461.
DILUBR.] DESCRIPTIONS: NO. 64, NEVADITE. 163

Two somewhat different modifications are represented in the collection. 

One has large sauidiiie crystals., reaching a maximum length of 3cni, 

which usually show a delicate satiny luster parallel to a steep ortho1 K __

dome, apparently -^ P 56, (15.0.2). In the other form of the rock the

sanidines are smaller and clearer with faint luster, if any. Both varieties have numerous smoky quartz crystals, generally much fractured, 

and showing prismatic and pyramidal faces. Oligoclase is present in 

many small phenocrysts, and these are often included in the larger 

sauidine crystals. The mica is a very subordinate element of the rock.

The plienocrysts. The especially noteworthy mineral of the rock is 

sauidiue. Its crystals are fresh and glassy, usually developed as thick 

tablets parallel to the cliuopinacoicl, with' prism and two-or three 

orthodoines as the terminal planes. Carlsbad twinning is common. 

The delicate satiny luster referred to is very beautifully developed in 

many crystals. The luster is due to films of air between the thin 

plates of a very perfect parting which is not exactly like a cleavage. 

When actual separation of these plates has not occurred there is sometimes an exquisite blue color to be seen parallel to the steep orthodome. 

This color is most brilliant in the small sanidines of certain rhyolites, 

where no cleavage parallel to the dome in question can be detected.

The numerous small crystals of oligoclase in this nevadite are quantitatively much less than the sanidines. They are frequently included 

in the latter.

Quartz occurs very abundantly in more or less rounded or resorbed 

crystals. The material is very pure, free from inclusions of other minerals or of glass, and from penetrating arms of the groundmass. Gas 

pores of very minute size are numerous, and also fluid intrusions with 

relatively little fluid in them. .

Biotite is sparingly developed in rudely hexagonal leaves, containing 

many included grains of magnetite.

The groundmass. Quartz and feldspar in very even-grained mixture, 

with flakes of biotite, constitute the groundmass. Quartz is very often 

developed in double pyramids, leaving the feldspar to occur in irregular 

grains. But little triclinic feldspar can be detected. There are sometimes small gas pores between groundmass grains, and in some sections 

there appe.ars to be a very small amount of clear amorphous substance 

in angular spaces between the quartz and feldspar particles of the 

groundmass. This is never sufficiently developed to form a basis, and 

while it seems to be residual glass its nature is not clearly determinable. 

It is identical in appearance and development with what has been 

determined as glass in the rhyolite of the Hohenburg, near Berkum on 

the Bhine.

Occasionally there are drusy cavities in this nevadite which contain 

many thin sanidine tablets and, rarely, a few stout crystals of quartz 

and topaz. This occurrence of topaz was the first to be described as
164 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL. 150.

primary in igneous rocks. Topaz is now known in lithophysal cavities 

of many rhyolites, and the development of these drusy cavities in the 

Chalk Mountain nevadite, with the occurrence of topaz in them, is one 

of the principal indications that the rock consolidated near the earth's 

surface.

Chemical composition. A specimen of the finer-grained rock of Chalk 

Mountain was subjected to quantitative analysis by W. F. Hillebraud 

with the following result, under I. The clear sanidine pheuocrysts 

have the composition given under II.

Analyses of nevadite and sanidine from Chalk Mountain, Colorado.

Si02 ...................................

A12 03 ............ ......................

Fe2 03 ...... ............................

reo...:.. ..............................

Mn02 . .................................

CaO ...................................

MgO.... ...............................

K2 6 ...................................

Na/> ..................................

Li20. ..................................

11,0 ...................................

 p r\

'Total ............................

I.

Per cent. 

74.45

14.72

0.56

0.28

0.83

0.37

4.53

3.97*

0.66

0.01

II.

Per cent. 

65.04

20.40

0 79

9.74

4.11

.29

100. 37

Specific gravity of rock, 2.61 at 32° C

There probably should have been a very small amount of Fe2O3 

found, but the rock analyzed was nearly free from magnetite, and 

contained little biotite. Mn02 is present as a stain of pyrolusite in 

patches through the rock. It is clear that the rock has a large excess 

of silica as quartz and that much, of the soda is present in the sanidine.

The following publications by the writer refer to the nevadite of 

Chalk Mountain:

Bull. U. S. Geol. Survey, No. 20, Contributions to the Mineralogy of the Rocky 

Mountains, pp. 75-80. On the luster exhibited by sanidine in certain rhyolites.

Ibid., pp. 81-82. An unusual occurrence of topaz.

Mon. U. S. Geol. Survey, Vol. XII, Geology and Mining Industry of Leadville, 

Colorado, by S. F. Emmons. In Appendix I, Petrography, pp. 345-349.

No. 65. METARHYOLITE. 

(FROM UTLEY, GREEN LAKE COUNTY, WISCONSIN. DESCRIBED BY S. WEIDMANJ)

The Utley metarhyolite (quartz-keratophyre) is of pre-Cambrian age, 

probably Keweenawan, and occurs as a single knob in unconformable

1 Adapted from an article on the TJtley metarhyolite: Bull. No. 3, Geol. and Nat. Hist. Survey of 

Wisconsin.
DIUJUI.] DESCRIPTIONS: NO. 65, METARHYOLITE. 165

contact with lower Ordovician sandstone and limestone. It is intruded 

by a few small dikes of diabase of pre-Cainbrian age.

The normal rock of this area has a dark almost black groundmass, 

in which are embedded numerous phenocrysts of feldspar and quartz. 

The rock is very compact and brittle, and breaks easily, with a conchoidal fracture.

The phenocrysts of feldspar, which are more numerous than those 

of quartz, are grayish white in the fresh specimens, but are somewhat 

reddish on the weathered surface, owing to alteration. They vary in 

size from linm to 5inm in diameter, and sometimes in certain coarse spots 

they attain a thickness of 8mm and even 10nmi . These feldspar crystals 

are triclinic, as is indicated by their well-develop v cl cleavage, the angle 

between the two most important cleavage faces Ueing about 93° 32".

The quartz crystals, as seen in the hand specimen, have the characteristic limpid appearance, and usually possess good crystal forms, 

but are sometimes rounded. They vary in size and are approximately 

equidiinensional with the feldspar phenocrysts with which they are 

associated.

.The dark-colored groundmass is megascopically very dense and 

aphanitic and always breaks along curved planes. Thick masses of 

nodules or spheroids, produced by spheroidal parting, which phenomenon is brought out clearly on the weathered surface, form an interesting feature of this rhyolite outcrop. The spheroids vary in size 

from £ inch to 1£ inches in diameter, and. do not differ in composition 

or texture from the normal rock in which they occur. They have separated out as nodules by weathering or altering along the spheroidal 

partings. These partings are produced by contraction during the 

original cooling of the magma.

Under the crossed nicols of the microscope and A\7 ith low power the 

feldspar crystals of thin sections of the rock always have a mottled 

appearance. With a higher power, this mottled appearance is seen to 

be due to a close intergrowth of two varieties of feldspar, producing 

what is termed a microperthitic feldspar or microperthite. 1 Besides this 

microperthite there area few phenocrysts of polysynthetically twinned 

plagioclase, probably near the albite end of the series. There is a definite relation between this twinned plagioclase and the microperthite 

which will be referred to again.

1 The term microperthite, which is visually meant to include an iutcrgrowtli of orthoclaso with albite 

or other acid plagioclase, is here extended and meant to include iiitergrowths within the plagioclase 

series as well.
166 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [Buix.150.

The chemical composition of this feldspar, as determined by 0, F. 

Tolman, is as follows:

Analysis of feldspar of the Utley metarhyolite.

KiO^.... ............................... .........

Al^.... .............................. ..........

FeO .............................................

CaO .............................................

K,,0 ...'..........................................

Na20 ...............'.............................

Total ......................................

Per cent.

05. 57

18. 12

1.32

5.52

5.11

7.20

99. 84

The microperthite is distinctly tricliuic, as shown by its cleavage 

system. Its chemical composition and optical properties are such as to 

place it entirely within the plagioclase series, the intergrown plagioclase 

species ranging from albite to andesiue.

The microperthite phenocrysts are usually composed of intergrowths 

of two varieties or species of feldspar. One species that is always 

present is albite, having an extinction angle of + 19° on the brachypinacoid (oo P&). Another species that is usually if not always present 

is a variety of oligoclase-andesine, having an extinction angle of from 

-\- 5° to 7° on the same face (co P ob). In one case the microperthite 

proved to be albite and andesine, the latter having an extinction angle 

of - 3° on oo P&.

The nature of the microperthite is that of a lamellar intergrowth of 

albite with a more basic plagioclase, each species tending to have a definite crystal form with vertical axes in a common direction. It has a 

regular and invariable arrangement of its component feldspars, unless, 

as in a few cases, it has been interfered with by excessive pressure. 

Tbe appearance of the microperthite in a section cut parallel to the 

brachypinacoid (oo P 66) is shown in Fig. B of PI. XXVIII.

The microperthite is wholly secondary that is, its development by 

processes of metamorphism has been long subsequent to the original 

crystallization and solidification of the magma. This process of change 

in the upbuilding of the microperthite may be said to have been accomplished by three somewhat different, yet related, methods.

First, by outgrowths on older feldspar.. Growth of this kind is shown 

in Fig. A, PI. XXVIII, photographed in polarized light. The outer 

shell of microperthite is seen to inclose a twinned plagioclase of perfect 

crystal form. The outer boundary of the enlargement is somewhat 

irregular. The inner crystal contains fractures which do not extend 

into the microperthite. The outer zone of microperthite is fresh, while 

the inner core shows much decomposition., The relation of the microperthite to the twinned plagioclase, as seen in this photograph, shows 

that the latter was fractured and considerably altered before the formation of the rim of microperthite.
DESCRIPTIONS: NO. 65, METARHYOLITE. 167

Second, by regeneration of older plagioclase. In this process the 

perthitic intergrowths of albite and oligoclase-andesiue are products 

of the recrystallization of the older twinned plagioclase. This process 

begins along the outer boundaries and fractures of the older crystal 

and very probably takes place along the easy solution planes of the 

latter. The c axes of the perthitic growths are, except in a few cases, 

parallel to the c axes of the original crystal. This relation is shown in 

Fig. B, PI. XXVIII. This process, to a small extent, has gone on in 

the crystal shown in the photograph Fig. A, PI. XXVIII. In the 

lower left-hand part of the inner crystal core, above the dark line separating the original from the secondary feldspar, is a small area of perthite forming out of the older crystal. Numerous other cases could be 

cited 1 where the alteration of the twinned plagioclase to microperthite 

has been brought about. The unperthitized feldspar is always in an 

advanced stage of alteration, while the inicroperthite is always fresh.

Third, by new growth from the groundmass. This process has developed small irregular crystals. These irregular crystals of microperthite 

have a medial suture from which extend normally radiating fibers of 

microperthite.

These processes of change have very probably been aided by pressure. In some cases cross twinning after the pericline law has been 

induced in the original crystal by pressure. In such cases the later 

growth of perthite seems to follow along the composition faces of the 

pericline twins. When the latter phenomenon occurs, the laths of 

perthite have their longer direction almost normal to the c axis of the 

older crystal.

The quartz phenocrysts have a fairly perfect crystal form, but are 

sometimes rounded. They are often corroded and have einbayinents, 

and, like some of the feldspar crystals, show the effect of a moving 

magma by being cracked and broken, the dismembered parts being 

separated by films of groundmass. They often contain rounded and 

rhombic-shaped areas of included groundmass. Bhoinbic cleavage is 

developed in some of the crystals.

Biotite, although not apparent in the hand specimen, is seen to be 

rather abundant in the thin section. It occurs as minute, short, tabular crystals, to some extent scattered throughout the groundmass, but 

also seems to be congregated in little patches and in zones about the 

feldspar crystals, and in the fractures as vein material. It is probably 

wholly secondary.

Hornblende was once present in this rock, but has been completely 

altered. The fact that it was once present is indicated by angular 

areas having the typical outlines of basal sections of this mineral. The 

hornblende is replaced by other minerals, such as magnetite, calcite, 

epidote, biotite, sphene, apatite, garnet, and quartz. These minerals 

also occur scattered throughout the groundmass. Minute flakes of 

brown hornblende, of secondary origin, occur to some extent in the 

rock.

1 A nnoro comprehensive discussion of tlio origin and nature of the niicropcrtliito is given in tho 

paper cited on page 165.
EXPLANATION OF PLATE XXVIII.

SECTIONS OF METARHYOLITE.

Fig. A. Metarhyolite, Utley, Wisconsin; slide 4379, University of Wisconsin Collection : Secondary growth of microperthite about plagioclase. In polarized light, 

X 30. Twinned plagioclase core with crystal form, fractured and containing many 

secondary inclusions of biotite in the fractures, and what is probably sericite distributed through the plagioclase. The fractures and decomposition products are confined to the core. The outer rim is made up of short laths and Avedges of feldspar 

lying in a parallel direction, giving a distinct difference in appearance to the 

ordinary twinned feldspar of the core. The secondary enlargement has been interfered with in its development by the small phenocryst above and to the right. In 

the lower left-hand corner of the older plagioclaso crystal, above the boundary 

line of core and rim, is a patch of microperthite/which is similar to that of the rim. 

The inner patch is a product of the regeneration of the twinned plagioclase; the 

surrounding rim is a development from the grouudmass. The position of the fractures and decomposition products within the plagioclase core establishes the earlier 

origin and advanced metamorphism of the plagioclase previous to the development 

of the microperthite.

Fig. B. Metarhyolite, Utley, Wisconsin; slide 3846, University of Wisconsin Collection: Phenocryst of the microperthite in polarized light, x 30. Section cut 

parallel to oo P ob ou the M face, showing basal cleavage. A bisectrix nearly normal. The angle of extinction of dark part is from + 3° to + 6°; of the light part 

about +19°. The large amount of lime in the feldspar coupled with the lower 

angle of extinction of the dark part indicates that the latter is oligoclase with a 

composition of Ab.jAni. The light part is albite. The microperthite is secondary, 

and the two varieties of feldspar are developed with their c axes parallel to the c 

axis of the original feldspar. 

168
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXVIII

SECTIONS OF METARHYOLJTE FROM UTLEY, WISCONSIN.

A. Secondary growth of microperthite about plagioclase; in polarized light, X 30. 

B. Phenocryst of microperthite; in polarized light, X'30.
niLLER.] DESCRIPTIONS: NO. 65, METARHYOLITE. 169

The groundmass is composed of microscopic grains of interlocked 

quartz and feldspar, intermingled with which are minute opaque specks 

and grains of iron oxide. To this iron oxide and other dark-colored 

microscopic minerals is due the blackish color of the groundmass as 

seen in the hand specimen. These small dark-colored minerals are 

arranged in sinuous, lines and curve about the porphyritic crystals, 

producing the common fluxion texture characteristic of lava flows. 

There is also developed in some specimens a radial spherulitic texture 

of the groundmassj and in other specimens the mottled pcecilitic texture. The groundmass was once partly and probably wholly glassy, 

and is now devitrified. This devitrification has been accomplished, in 

great part at least, by secondary processes since the original solidification of the magma.

In the thin sections of the rock from the nodular or spheroidal beds 

occur abundant examples of perlitic parting in the groundmass. The 

perlitic parting assumes the shape of curved fractures sometimes 

approaching circles, but more often only parts of circles or of other 

curvilinear forms. From these minute curved cracks there extend, with 

their longer axes perpendicular to the parting, small crystals of feldspar 

or of feldspar and quartz, giving the appearance of a radial or spherulitic crystallization along the fracture. In the circular and curvilinear 

forms of perlitic parting, the radii of feldspar appear as zones surrounding an inner cryptocrystalline groundmass which is like that outside of the zone. This phenomenon is in close similarity to the larger 

structure of the spheroids of the hand specimens and indicates a common 

origin of the two structures. Throughout the area, as shown in both 

the hand specimen and thin sections, are numerous fractures that are 

filled with secondary quartz and feldspar. The vein material in the 

fracture of the feldspar is very often feldspar, while that in the groundmass and in the quartz crystals is usually quartz.

The chemical composition of the rock is as follows:

Analysis of metarhyolite from Utley, Wisconsin.

Si0.j.. ............................................

AlA ................................

Fe,03 ............................................

IfoO..... .........................................

MnO.... .........................................

CaO.. ............................................

MgO...... .......................................

TSa^O. ............... .............................

K2 0. .............................................

IEL.,0.. ............................................

Percent.

7!!. 09

13.43

} 2.57

2.20

1.03

3.85

1.58

.76
170 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Besides the constituents enumerated in the analysis, there are also 

present in the rock very small amounts of CO2, ZrO2, TiO2, and P2O5 , 

as indicated by the microscopic examination. The relatively large 

amount of lime and soda present explains the plagioclase composition 

of the microperthite. The proportionately large amount of soda would 

class this rock as a soda-quartz-porphyry or quartz - keratophyre. 

Applying the nomenclature of the more recent volcanic rocks to those 

of pre-Cainbrian age, the Utley rock is in composition and texture a 

typical rhyolite, and in consideration of its altered condition may be 

called a metarhyolite.

No. 66. GRANITE. 

(FROM ROCKLIN, PLACER COUNTY, CALIFORNIA. DESCRIBED BY WALDEMAR

LlNDGREN.)

The granitic area of Rocklin forms an irregular rounded mass about 

30 miles long and 20 miles wide, inclosed towards the north, east, and 

south by the auriferous slates of the Sierra Nevada. Westward it is 

bordered by the alluvial plains of the Sacramento Valley. 

 As a rule, the rock in this area contains only black mica and hornblende, sometimes, indeed, only hornblende together with quartz and 

feldspar, and is more closely allied to a diorite than to a granite. 

At Eockliu, which is a railroad station on the line of the Central 

Pacific, with an elevation of about 200 feet, the rock assumes a more 

granitic character and contains both black and white mica. It is somewhat finer grained than in other parts of the area and makes an excellent building stone. The extensive quarries at Eockliu supply a large 

proportion of the granite used for building purposes in San Francisco.

The rock is of light-gray color and of medium fine grain. Grayish 

quartz grains, white feldspars sometimes with striation on the cleavage 

planes, black or dark-brown biotite, and silvery muscovite in small 

scales may readily be distinguished by the naked eye.

Under the microscope the following primary constituents may be 

discerned: Magnetite, apatite, and zircon are the oldest minerals 

occurring as minute inclusions in the later consolidated grains; then 

come, in order of succession, biotite, muscovite, plagioclase, orthoclase, 

and quartz.

The quartz, being the last consolidated mineral, forms irregular 

grains, the outlines of which are determined by the older constituents, 

although there are indications that the consolidation of orthoclase and 

quartz in part went on simultaneously. It is clear and fresh, being, of ( 

course, much less subject to decomposition than the other constituents. 

It often shows notable uudulous extinction. Fluid inclusions with / 

moving bubbles are common, as usual, in the quartz of the granites. 

Small needles and foils of biotite and mnscovite occasionally abound; 

while there is a little apatite in more or less slender prisms and small
DESCRIPTIONS: NO. 66, GRANITE. 171

colorless crystals of zircon, recognizable by their strong double 

refraction.

The orthoclase occurs, as a rule, in clear, irregular grains, together 

with the quartz, between the more or less idiomorphic plagioclase crystals. More rarely it shows rough crystallographic outlines. Its period 

of consolidation is nearly identical with that of the quartz, and grains 

showing a micropeginatitic intergrowth of the two minerals are not 

uncommon.

Many orthoclase grains are filled, with minute quartz grains, all 

extinguishing together. Undulous and zonal extinctions occur frequently. Fluid inclusions are not common. The orthoclase is less 

subject to decomposition than the plagoclase, to be described presently, 

but the alteration to white, fine, micaceous aggregates may often be 

observed, proceeding along the cleavage lines. As infiltrations from 

neighboring minerals chloritic aggregates and calcite may be noted.

'The plagioclase is found to be more abundant than in a perfectly 

normal granite. In many cases there is considerably more plagioclase 

than orthoclase. The plagioclase is a very acid soda-lime feldspar, 

belonging to the series of the oligoclases; in only one instance was a 

grain of microcline noticed. No albite appears to be present. Twin 

striation narrow, not infrequently absent, or shown by very few polysynthetic lamella}. Double refraction somewhat stronger than the 

orthoclase. Zonal structure exceedingly common. The plagioclaee ig, 

in contrast to the orthoclase, more or less idiomorphic, showing slfort 

prismatic or lathlike forms, without well defined terminal faces. Decomposition has, on the whole, made greater headway than in the orthoclase, 

and proceeds along the cleavage line and also sometimes concentrically 

in crystals with zonal structure. As it begins the crystals grow cloudy 

and milky, by reason of minute, irregular air, more seldom fluid, inclusions. Small scales of hematite and other indeterminable inicrolites 

are noted in the clouded mass. The final products of decomposition 

are fine aggregates of white mica, calcite, and, where a migration of 

magnesian solutions from adjoining minerals has taken place, single 

foils, and aggregates of chlorite and epidote.

The muscovite occurs in large and small straight foils, showing the 

usual characteristics of the mineral strong double refraction, white 

color with notable absorption, etc. Idiomorphic foils of biotite may 

sometimes be noticed embedded in a larger foil of muscovite. The 

muscovite does not decompose readily.

The biotite is rather more abundant than the muscovite; it is from 

yellowish to greenish brown, has very strong absorption, and occurs as 

irregularly bordered straight foils. Inclusions are not abundant; apatite, magetite, and zircon are occasionally noted.

The biotite is decomposed to a considerable extent, the resulting 

minerals being a green chlorite and a yellowish-green pleochroic epidote. In a brown biotite foil green chloritic lamella} are sometimes
172 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

intercalated, showing the way in which a complete pseudomorph of 

chlorite after biotite may take place.

The chlorite is deep green, pleochroic, and shows the usual deep blue 

or purple colors of interference.

The structure of the rock is characterized as hypidiomorphic granular, in contrast to the structure designated as allotriomorphic granular. 

In the latter all of the constituents are irregular grains, while in the 

former one or more of them show crystallographic outlines. In this 

case the more or less lathlike oligoclase crystals, together with the 

micas, lie embedded in the clear, irregular grains of the orthoclase and 

quartz.

The chemical composition is shown by the appended analysis, made 

by W. H. Melville in the chemical laboratory of the United States 

Geological Survey:

Analysis of granite from Eocklin, Placer County, California.

Si02 .............................................

Al.Oa----- .......................................

FejOs .. .......... ................................

FeO

CaO .............................................

Mgo!-...... .....................................

K2o.. ................................:...........

No f)

Total ......................................

Per cent.

73

16.38

.90

2.42

.48

1.87

4.53

.52

100. 19

From this and from a study of the thin section it is apparent that 

the rock has the composition of a normal granite, except that sodium 

is present in unusually large quantity and that a considerable part of 

the orthoclase is replaced by oligoclase.

No. 67. BIOTITE-GRANITE.

(FROM PLATTE CANYON, JEFFERSON COUNTY, COLORADO. LESCRIBED BY

E. B. MATHEWS.)

The granites from the Platte Canyon are but a portion of the large 

granitic area which extends along the whole length of the Colorado or 

Front Eange. In the area described in the Pikes Peak folio of the 

United States Geologic Atlas a large portion of the district is occupied 

by a granitic complex which has been found to consist of several varieties of granites that can be grouped under four heads,1 of which the 

Pikes Peak type is the most prominent. To this type belong the rocks 

of this series. The fresh unaltered granites are coarse grained aggre-

'Tlie granites of Pikes Peak, Colorado, by Edward B. Matbews: Bull. Geol. Soci America, Vol. 

VI, 1894-95, pp. 471-473.
DESCRIPTIONS: NO. 67, BIOTITE-GRAN1TE. 173

gates of quartz, perthitic feldspar, and biotite with an occasional fluorite inegascop.ically developed. The microscope also reveals apatite, 

zircon, magnetite, rutile (?), hematite, limouite, and epidote.

The rocks are generally of a pinkish tone, which becomes more accentuated during incipient alteration and fades away when the rocks 

have undergone considerable metamorphism. The grain of the granite 

varies widely from a case in which the feldspar phenocrysts are 6 

inches in length to the more normal grain in which the length of the 

feldspar individuals is not more than one-fourth of an inch. In the 

specimens belonging to this series the usual dimensions are one-half 

an inch for the feldspar and one-fourth to one-eighth of an inch for the 

quartz. The biotite areas are generally smaller than the quartz individuals, although sometimes they may be one-half an inch in width. 

The texture of the Pikes Peak type varies from an aggregate of minerals in which the feldspar is only slightly larger than the quartz to one 

in which the feldspar stands out in large imperfectly formed porphyritic crystals. The mass of the rock is composed of three or four mineral 

species, of which microcline is by far the most important.1

The quartz is present in large, irregularly rounded grains, which are 

distinctly outlined against the feldspathic and micaceous constituents. 

Toward the feldspars the quartz appears normally younger, although its 

contact with the larger porphyritic microcline sometimes indicates that 

the quartz had already consolidated before the formation of the phenocrysts. In thin sections the quartz is seen to occur either as primary 

individual grains, or in pegmatitic intergrowths with the feldspar, or 

as secondary grains filling the interstices and cracks which were formed 

subsequent to the consolidation of the rock. The largest fragments of 

the quartz may be one-fourth of an inch in diameter or the individual 

grains may scarcely be distinguished with the highest power of the 

microscope. Within the quartz are included numerous fluid and 

individualized interpositions. They are arranged in long bands or are 

disseminated irregularly throughout the host. Sometimes there are 

long hair-like inclusions whose mineral characteristics can not be specifically determined. These are the well-known " quartz needles," 

which have been considered by various authors as rutile, apatite, tourmaline, hornblende, or silliniauite. In a few instances their nature has 

been determined, but in the granites of this series the filaments are too 

fine to permit such specific determination.

1 A. mechanical separation of .20 grams of the powered rock shows the constituents to be present in 

the following proportion by weight:

Per cent. 

Microcline........................................................................... 53.33

Quartz............................................................................... 33. 41

Biotite...................... ^. ....................................................... 10. 71

Oligoclase ........................................................................... 2.55

Tinder the term "quartz" are included those fine filaments of albito which wore separated from 

the microcline in tho crushing, while in "biotite" are included all of those minerals which wore not 

floated in a Thoulet solution when at its maximum density of 8.1.
174 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Microcline constitutes more than one-half of the rock mass, and in 

all instances shows the characteristic microcline twinning. The color 

of this mineral is the principal source of the pink tone noticed in the 

rock specimens, and is usually due to the presence of minute hematite 

flakes evenly disseminated through the feldspar, or to iron staining 

along the cracks. The size of the feldspar individuals varies widely, 

but in the rocks of this collection it may be large or small, since the 

microcline usually occurs both in phenocrysts and as .a constituent of 

the groundinass. The phenocrysts may occur in rndely automorphic 

crystals, showing poorly defined pinacoidal and prismatic faces, which 

give to the mineral a prismatic habit in which the elongation is parallel 

to the clino-axis. In the groundmass the feldspar individuals are small, 

somewhat larger than the quartz and without crystal form, occurring 

only as irregularly outlined grains or anhedrons.

The microcline of these rocks may show twinning in addition to that 

according to the albite law which produces the so-called microcline 

structure. Such twinning may be according to the Carlsbad law, in 

which the twinning plane is the orthopinaciod, or more commonly the 

twinning plane may be the basal pinacoid, according to the Manebach 

law. Throughout these potassium feldspars are numerous fine disks, 

lenses, or stringers of a plagioclastic feldspar, probably albite, in perthitic intergrowths with the host. These fine albite stringers occur in 

the basal piuacoid normal to the clinopinacoidal cleavage and on the 

clinopinacoid they show an inclination of about 73° in obtuse /5; accordingly these albite lamellae lie approximately parallel to a steep positive 

orthodome of the value of 8 P<X. In certain instances the albite occurs 

in small rounded disks, which oriented sections show a-re not cross 

sections of spindles, but of more or less spherical bodies.

Micropegmatitic intergrowths between the quartz and the feldspars 

are not common iu these rocks, although they are sometimes present in

minute branching or arborescent areas. A more extensive study of 

the rocks of the surrounding country shows that rnicropegmatite is not 

common in the granites of this class (Pikes Peak type), but when present is probably the result of alteration of the original constituents.

Oligoclase occurs in a few instances, but never constitutes any considerable portion of the rock mass. It is usually easily separated from 

the other constituents by the fine lamellae twinned according to the 

albite law. The fineness of the lamellae and the low extinction of one 

or two degrees clearly indicates the oligoclase nature. Within the 

mic-roclines are also minute plagioclase disks which may or may not be 

twinned. These disks show the effect of marked alteration and are 

clouded by the alteration products. The altered disks are separated 

from the microcline host by a rim of clear feldspar which is in physical 

continuity with the inner core. At times the twinning of the inner 

core is continued in the rim either directly or alternately, as in the Eoc 

Tourne twins described by Gustav Eose. 1

'Pogg. Ami., 1865.
DILLER.] DESCRIPTIONS: NO. 67, BIOTITE-GRANITE. 175

The cause of the altered plagioclase with surrounding clear rims is 

not definitely known. The difference between the two parts may be 

the result of original 'differences of crystallization, the inclusion being 

more basic, more porous, or filled with interpositions; or the rim may 

be entirely secondary, a product of the alteration which the granites 

have undergone.

Biotite is the third in importance of the constituents of the granites, 

and is present either in fine shreds closely aggregated or in single, 

somewhat larger, individual flakes. This mica shows a deep-brown 

color, metallic luster, and a strong pleochroism. The absorption is 

very marked for the ray vibrating parallel to the basal cleavage where 

the plate becomes practically opaque; the ray vibrating at right angles 

to the cleavage is yellow sometimes even straw-yellow. The plane 

of the optic axes apparently lies normal to the leading ray of the percussion figure, and is therefore of the first sort anomite, This determination seems to be corroborated by the peculiar brown color, the 

small optic angle, and the brittleness of the perfectly fresh cleavage 

flakes.

Fluorite is present in many sections of this series where it occurs in 

small irregular grains, and rarely in well-defined crystals, which suggest either cubes or octahedrons, The mineral is specially characterized by a highly perfect octahedral cleavage, which is well developed 

in the larger areas, but is lacking in the minute crystals. The little 

anhedrons are clear, and may be colorless, purple, faintly pink, or 

green, and when the grains are colored the pigment is unevenly disseminated through them, and seems to be more intense about inclusions 

than in the clearer portions of the mineral. Between crossed uicols the 

areas show complete isotropism and no anomalous double refraction. 

The index of refraction of the fluorite in these rocks appears exceptionally high, even as high as that for garnets. The cause of the 

anomaly is not apparent, but that the mineral is fluorite, and is not a 

garnet, is shown by the fluorine content in. the bulk analysis, where the 

fluorine sometimes reaches 1 per cent.

Zircon, apatite, and epidote are sometimes present in the sections. 

They are usually in minute, more or less prismatic crystals, colorless 

or faintly yellow, that may be distinguished by the low double refraction of the apatite, the high index, double refraction, and lack of 

cleavage of the zircon, and the irregularity of outline and cracked or 

shagreened upper surface of the epidote.

The order of crystallization of the minerals in the rocks seems to have 

been as follows: The magnetite when present formed first, and was 

immediately followed by the zircon and the apatite. Biotite formed 

somewhat later, including all of the minerals then existing in the 

magma. Probably the separation of the biotite was followed by the 

crystallization of a small amount of plagioclase and a much larger 

amount of microcline. The microcline of the groundmass is clearly 

older than the quartz, but in the larger pheuocrysts of microcline the
176 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150..

relative age of the quartz and feldspar is different, and it is very probable that the large phenocrysts of microcline succeeded the separation 

of the quartz and were the last of the original minerals to crystallize 

from the magma. The age relationships of. the epidote and fluorite are 

unknown. The facts indicate that the epidote is a product of moderate 

alteration of the feldspar, while the fluorite is closely associated with 

the biotite or irregularly disseminated throughout the rock mass. 

When the former is associated with the biotite, the mica is clear and 

unaltered, and appears younger than the fluorite, and does not suggest 

that the fluorite is in anywise formed at the expense of a fluorinebearing mica. On the other hand, the fluorite is not clearly a miarolitic 

filling, but appears instead to be one of the older constituents of the 

rock, perhaps of the same age as the apatite and zircon.

The granites from the Platte Canyon very often show evidences of 

alteration as the result of dynamic metambrphism. The extinction of 

the quartz becomes undulatory or mottled, and very frequently the 

quartz has been crushed into a mosaic in which individual fragments 

are drawn out in a common direction, producing the well-known 

"strieferiquarz."

The feldspars have been deformed by a rounding of their angles and 

a drawing out or stretching of their material into long bands or flat 

ellipsoids. This deformation of the feldspars changes the well-formed 

phenocrysts into large lenses or eyes, so that the granite passes from a 

massive rock into a schistose augen-gneiss. Few, if any, new minerals 

are formed by the deformation of the original constituents which differ 

in specific character from the original minerals. The rocks when 

crushed show secondary quartz, microcline, and biotite, with rarely 

any other additional minerals, unless occasionally muscovite, which is 

formed through the bleaching of the biotite and the decomposition of 

the feldspars,

The granite under discussion is one relatively rich in silica, sodium, 

and fluorine, and poor in calcium, iron, and magnesium. The following 

analysis, by W. F. Hillebraud, is that of a granite from the upper slope 

of Pikes Peak, closely related to the granite of the Platte Canyon in 

structure, and in mineralogical and chemical composition.
DILLEH.] DESCRIPTIONS: NOS. 67, 68, BIOTITE-GRANITE.

Analysis of Uotite-granite from Pikes Peak, Colorado.

177

CiH

Tin

A1203 . ...... ..................................

POjOg ................................... .........

FeO. .............................................

MnO........... ..................................

CaO.. ............... .............................

BaO .............................................

MgO.............................................

K,0................ I.............................

N'o n

H2 0 below 110° C.... ............................

H20 above 110° C.... ............................

P2 o6 ... ..........................................

Tf ................................................

PO

Total......................................

Per cent.

i<?

12.00

.76

.86

.80

.04

4.92

3.21

14.

.30

.36

100. 55

.15

100. 40

The practical absence of phosphoric acid indicates that only a small 

proportion of the fluorine is needed for the apatite seen in the rocks, 

even though the apatite is a pure fluor-apatite. The remaining fluorine 

is sufficient to combine with most of the calcium and still be present in 

the biotite. The .calcium occurring as a calcium-fluoride leaves little 

which may enter into the composition of the feldspar, and consequently 

the oligoclase must be very near the albite limit. The strong pleochroism and the deep color of the mica suggest that almost all of the iron 

is used up in the relatively slight development of biotite. The percentage of alumina indicates that much of this is in the feldspar, and that 

only a small amount is in combination in the mica. After all of the 

elements are satisfied the silica is considerably in excess, as is evident 

from the abundance of quartz.

From the texture and from the mineralogical and chemical composition it is evident that the granite from the Platte Canyon is a somewhat porphyritic example of granitite (Rosenbusch), biotite-granite 

(Zirkel), and true granite (Michel L6vy).

jSTo. 68. HOENBLENDE-BEARING BlOTITE-GKANITE. 

(FROM Fox ISLAND, MAINE. DESCRIBED BY J. P. IDDINGS.)

This rock occurs on Vinal Haven Island, one of the Fox Islands, in 

Penobscot Bay, Maine, where, according to George O. Smith, 1 it occupies a roughly circular area nearly 5 miles in diameter.

1 See Geological Map of the Pox Islands, PL II of the Geology of the Pox Islands, Maine, by George 

Otis Smith, 1890.

Bull. 150 12
178 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

It is medium grained and light colored, and consists of pinkish-gray 

feldspars and about an equal amount of brownish quartz, besides a 

smaller quantity of black ferromaguesian minerals, mostly biotite, but 

partly hornblende. These dark-colored constituents are in much 

smaller crystals than the feldspars, which are partly idiomorphic.

In thin section the rock is hypidioinorphic-granular in texture, each 

of the constituent minerals possessing some degree of idiomorphism.

The large feldspars are for the most part microcline, with very 

delicate double polysynthetic twinning that is, lamellar twinning 

according to the albite and the pericline laws. There is also pronounced 

microperthitic intergrowth of lime-soda feldspar, whose index of refraction relative to that of microcline, and whose low angles of extinction 

indicate that it is oligoclase. Some of the large feldspars are orthoclase, and exhibit no polysynthetic twinning, while oligoclase also 

occurs in small partially idomorphic crystals.

In some microclines the twinned lamellae are curved and crowded in 

certain spots, and since the quartz crystals exhibit a strain phenomenon 

of an analogous kind, it is probable that the microcliue twinning is, in 

part at least, due to mechanical strains undergone by the rock.

In places the feldspar is clouded with minute dots which have a lower 

index of refraction than the feldspar and are white by incident light. 

Their exact nature can not be determined, but they are probably 

in part cavities, in part kaolin. In some cases very small flakes of 

colorless mica or muscovite are present. The same form of alteration 

occurs in the orthoclase. In the perthitic microcliue the intergrown 

lime-soda-feldspar is more altered than the microcline. A zonal structure is present in some of the microclines, but is more noticeable in the 

oligoclase.

Quartz exhibits the characters commonly found in the quartz in 

granite, it is aiiotriomorphic, and was probably the last mineral 

crystallized, though its crystallization undoubtedly began while the 

feldspars were being formed, and its growth was contemporaneous 

with that of the latter part of the feldspar. Some crystals exhibit 

irregular double refraction when examined between crossed nicols, 

showing that different parts of one crystal have different optical orientation. In some cases the portions are very minute, and appear like 

indistinct lamination in two directions, as in microcline.

Fluid inclusions are abundant and very small. They sometimes 

occur in planes which occasionally traverse several quartz crystals in 

one direction, proving that they are of secondary origin that is, they 

were formed subsequent to. the crystallization of the quartz and consequently of the rock as a whole.

Biotite occurs in irregularly shaped plates. Its color is brown, with 

strong absorption for rays vibrating at right angles to a that is, for . 

rays vibrating nearly in the plane of cleavage. It is light brown for 

rays vibrating parallel to a or at right angles to the plane of cleavage.
DESCRIPTIONS: NO. 69, HORNBLENDE-GRANITE. 179

It incloses numerous crystals of colorless apatite and fewer of zircon 

and of magnetite. Hornblende is in smaller amount than biotite, and 

occurs in irregular anhedrons. Its color is brownish green, with the 

usual pleochroism from greenish brown for rays vibrating parallel to 

a, the direction of vibration of the fastest traveling ray, to dark green 

parallel to IT, and .c, £ >'Xt > a. Biotite and hornblende are sometimes 

grown together. They are both quite fresh and unaltered.

Sphene occurs in yellowish anhedrons, associated with the hornblende. Zircon is comparatively abundant in small, short prisms, but 

apatite is much more abundant in small, stout crystals, which are colorless. These are readily distinguished from one another by their optical, 

properties. Spheue, having very high index of refraction and comparatively low double refraction, stands out prominently in the section, 

has a roughened or shadowed surface, and is not brilliant between 

crossed nicols. Zircon, having both high refraction and high double 

refraction, but being in very small crystals, exhibits strongly marked 

outlines, has a strong relief, and is brilliant between crossed nicolsj 

while apatite, with strong index of refraction and very low double 

refraction, exhibits strong relief, and very low color between crossed 

nicols.

Iron oxide is sparingly present, presumably as magnetite. There 

are also small microscopic crystals, which are brown to opaque, whose 

character is indeterminable.

NO. 69. BlOTITE-BEARING HORNBLENDE-GRANITE. 

(FROM CAPE ANN, MASSACHUSETTS. DESCRIBED BY J. P. IDDINGS.)

This rock occurs along the coast of Massachusetts north of Boston, 

where, in the vicinity of Gloucester, it forms the greater portion of Cape 

Ann, and is extensively quarried for building purposes. Its texture is 

medium grained and its color light greenish gray. It is composed of 

greenish-gray feldspar, with somewhat pearly cleavage surfaces, about 

an equal amount of brownish quartz, and a much smaller amount of 

black mineral, which is mostly hornblende. In thin section the texture 

is hypidiomorphic granular, the quartzes and feldspars having very 

irregular outlines and an arrangement suggestive of that in gneisses. 

The auhedrons of quartz have jagged outlines and interlock with one 

another, a number of quartzes being clustered together in places. 

They exhibit irregular double refraction, and are often separated into 

patches having slightly diiferent optical orientation (feldererscheinungen), which is the result of mechanical strain. There are numerous 

minute fluid inclusions, and, rarely, inclosed crystals of zircon.

The feldspars are microcline and lime-soda feldspar, which is mostly 

oligoclase. These are intimately intergrown, as in perthite. The microcliue appears to form disconnected pieces, while oligoclase constitutes 

a continuous intersecting web between the pieces of microcline. The 

microcline looks as though a large crystal had been cracked and
180 THE EDUCATIONAL SERIES OF ROCK SPECIMENS.

had been pulled apart, the interstices being filled with oligoclase; but 

all parts of the microcline have one orientation. The microcline is 

clouded with minute particles, indicating partial alteration, while the 

substance of the oligoclase is perfectly fresh and pure. These large 

complex feldspars are usually twinned after the Carlsbad law.

In some cases there appears to be an intergrowth of two large feldspars iu much the same manner as in the graphic intergrowth of quartz 

and orthoclase that is, the crystals cross one another at oblique angles.

Between the large crystals of feldspar is a border of smaller feldspars which are mostly lime-soda-feldspar, aud probably oligoclase. 

.With these small crystals are also^ small crystals of hornblende and 

biotite. These minerals occur in the same manner adjacent to large 

anhedrons of quartz.

The hornblende is brownish green with the usual pleochroism. The 

shape of its anhedrons is very irregular against both feldspar.and quartz. 

Its outline is quite jagged, with minute projections of hornblende and 

of biotite, which have grown in the margin of the anhedron. It carries 

inclusions of biotite, magnetite, and zircon j besides larger ones of monoclinic pyroxene, probably malacolite. These have irregular outlines and 

various orientations. Hornblende also occurs in clusters of minute 

anhedrons, together with similar anhedrons of biotite, inclosed in feldspar and quartz.

The biotite is brown with the usual absorption and other optical properties. Its shape is quite irregular. Its aiihedrons are not so large as 

those of the hornblende. It appears to have crystallized at the same 

time as the last part of the hornblende, being inclosed in the margins 

of crystals of the latter mineral.

Monoclinic pyroxene is quite subordinate in amount, and occurs with 

irregular outlines, both as inclosures in feldspar and in hornblende. 

Its color in thin section is pale green, and its double refraction is fairly 

high. It is probably nia.lacolite.

Zircon occurs in comparatively numerous, minute, short crystals. 

Apatite is not abundant. It is in small stout prisms, and also in needlelike forms. It sometimes contains rounded isotropic inclusions resembling glass. Iron oxide, which is apparently magnetite, occurs sparingly, 

and is generally associated iu clusters or in juxtaposition with crystals 

of the ferromagnesian minerals.

Allanite is present in occasional anhedrons, irregularly shaped. Its 

color is chestnut-brown with strong pleochroism.

The microstructure of the rock is that of a rnetamorphic granite or 

of a metamorphosed granite rather than that of a purely igneous or of 

an unaltered eruptive granite.

The quarries at Cape Ann are said to be next to the largest in the 

United States, aud have furnished stone for many of the buildings in 

the cities of the Atlantic coast. For further information on the geology 

of that region see paper by N. S. Shaler on The Geology of Cape Ann,
DILLEH.] DESCRIPTIONS: NO. 70, TRACHYTE. 181

Massachusetts, in the Ninth. Annual Eeport of the United States Geological Survey, pp. 535 to 611.

SYENITE TRACHYTE FAMILY.

No. 70. TRA.CHYTE.

(FROM GAME RIDGE, NEAR ROSITA, CUSTER COUNTY, COLORADO. DESCRIBED BY

WHITMAN CROSS.)

Occurrence. This trachyte forms a surface mass resting on andesitic 

tuff and breccia. The rock was the last one in the sequence of seven 

. distinct eruptions from the Kosita volcano, the others having been 

andesites of various types, dacite, and rhyolite. Game Ridge is a small 

mass of trachyte preserved from erosion by faulting, and none of the 

rock now remaining exhibits the outer structural features of lava 

streams. Dikes of the same rock seen near by show the channels 

through which the magma came to the surface.

Description. The rock is light-gray, porphyritic, its most conspicuous constituent being sanidine in characteristic glassy crystals, much 

fissured and occurring both in tabular and stout prismatic forms, with 

a diameter rarely exceeding 1 centimeter. These crystals are not very 

abundant, the greater part of this feldspar being in the gray groundmass. There are many small phenocrysts of oligoclase, but they are 

not prominent, as a rule; while the less numerous leaves of biotite, the 

only dark silicate of the rock, are naturally distinct.

Microscopical study shows the predominant gray groundmass to consist chiefly of orthoclase feldspar in short prisms, which, by their more 

or less distinct parallel arrangement and the curving of the bands 

about the phenocrysts, cause a fluidal structure. This case illustrates 

very well that groundmass structure commonly called "trachytic," 

which is due to the predominance of feldspar and its tendency to crystalize in these microlitic prisms.

The groundmass contains a small amount of quartz, usually found 

in little, elongated clusters of clear grains, or as matrix for the orthoclase staves. This latter association is rarely so developed as to 

become truly a poikilitic structure. The amount of quartz is not 

enough to invalidate the reference of the rock to the trachytes. Oligoclase appears in a very few minute crystal* in the groundmass, 

and a scanty magnetite dust is found in the fresh rock. Ordinarily, 

there is much liinonite in flakes all through the rock, often giving it a 

pinkish tinge.

Apatite and zircon are very sparingly present, in characteristic form.

The sanidine crystals are very free from included mineral grains 

except in the outer zone, where they contain groundmass particles, this 

zone being an added growth of the groundmass period. The plagioclase of the rock is oligoclase, as shown by the extinction very near to 

the albitic twinning line in all sections normal to the twinning plane.
182 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BOLL. 150.

Biotite is developed only in thin brown leaves which seldom have regular outline and are often very ragged, with magnetite grains included 

in the border zone or lying adjacent to the leaf, as though resulting 

from recrystallization after partial resorption of the mica.

Chemical composition. The freshest available specimen of this trachyte has the composition shown in the following analysis, by L. G. 

Eakins:

Analysis of trachyte from Rosita, Colorado.

Si02 ............................"................

A1A ................................... .........

Fe^Oa---. ........................................

FeO... ........... ................................

MnO......... ....................................

CaO.. ............................................

MgO... ..........................................

K2 0... ...........................................

TS&.&. ............................................

H2 0. .............................................

"P O

co2 ..............................................

Total......................................

Sp. gr. at 29° C...... .............................

Per cent.

66.03

18. 49

2.18

.22

.96

.39

5.86

 5.22

.85

.01

100. 24

2.59

This composition confirms the deductions from mineral constitution 

that the rock is nearly a typical trachyte. Silica is higher than is 

normal, but not sufficient to place the rock with the rhyolites. The 

high percentage of alkalies and the low amount of lime makes it clear 

that alkali feldspar must make up the great bulk of the rock and that 

the oligoclase can not play a prominent r61e in the groundiuass. The 

percentages of the iron oxides and of magnesia correspond to the small 

amount of biotite and the change of magnetite into limonite.

Relationship of the rode. The Game Eidge trachyte is to be compared 

especially with the well-known occurrences of the Siebengebirge, on 

the Khine, near Bonn. Many of the rocks of this classic locality are 

very similar to that of Game Eidge in structure, in relative importance 

of the oligoclase, and in general chemical composition. The similarity 

with the trachyte of the Drachenfels, as represented in specimens 

accessible to the writer, is particularly striking, especially in regard to 

amount and development of quartz and biotite and the structure of 

the groundmass.

Literature. -On some Eruptive Eocks from Ouster County, Colorado, 

by Whitman Cross, Proceedings Colorado Scientific Society, Vol. Ill, 

pp. 233-237; Geology of Silver Cliff and the Eosita Hills, Colorado, by 

Whitman Cross, Seventeenth Annual Eeport, United States Geological 

Survey, Part II, 1896, pp. 263-403.
DILLBR.] DESCRIPTIONS: NO. 71, SYENITE. 183

No. 71. SYENITE.

(FROM OUSTER COUNTY, COLORADO, TWO AND ONE-THIRD MILES A LITTLE EAST OE 

NORTH FROM SlLYER CLIP]?, AND ONE-HALF MILE SOUTHEAST 01? THK BULL 

DOMINGO MINE. DESCRIBED BY WHITMAN CROSS.)

Geological occurrence and distribution. This syenite occurs in narrow 

dikes cutting gneiss or granite in a considerable area of mountainous 

country lying south of the lower portion of the Grand Canyon of the 

Arkansas Eiver. The dikes have been specially observed in the vicinity 

of Silver Cliff, but they occur all through the area-drained by Grape 

Creek, which enters the Arkansas near the mouth of the Royal Gorge, 

and they probably have a still wider distribution in the Wet Mountain 

Range, to which the district mentioned is adjacent.

These dikes, standing in nearly vertical position, cut in all directions 

across the strike of the steeply upturned gneisses and schists. They 

are therefore younger than the folding of those gneisses, but are older 

than the great erosion which took place probably in very early Tertiary 

times and produced surface features not far different from those of the 

present day. The dikes are certainly pre-Tertiary, but their age can 

not be more closely fixed from known evidence. Associated with the 

syenite dikes through this area are numerous narrow dikes of normal 

diabase.

The dikes of syenite vary in width from an observed maximum of 

about 20 feet down to 1 foot or less. They are continuous for distances 

of more than a mile, in several instances, and their courses are often 

nearly straight. The dike from which the specimens of this series 

came was traced for more than a mile, and varies in width from 2 to 12 

feet. At the point where the specimens were obtained it was about 6 

feet wide and had been penetrated by a prospect shaft, furnishing the 

material for collection.

Description. The field habit of this syenite is very characteristic. 

The'rock has commonly a strong brick-red color, sometimes with a 

yellowish shade. This causes outcrops of dikes to be distinguishable 

often at considerable distances as red lines, contrasting with the darker 

shades of the ordinary gneiss.

The predominant mineral of the rock is red feldspar, developed in 

thin tablets, and by the prevailing arrangement of these tablets 

approximately parallel to the dike walls a schistose structure is produced. There is commonly a dense contact zone of three or four inches 

in width which grades off rapidly into the average coarser rock of the 

dike. These contact bands are usually of darker color, and have a 

dense porphyritic structure.

On macroscopical examination this rock is seen to be chiefly made 

up of red feldspar tablets, with a subordinate amount of dark material 

in fine particles, among which hornblende, mica, and epidote can here 

and there be recognized. The feldspar tablets are like rounded disks, 

seldom more than 2mm across. The schistose structure arising from
184 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

their rudely parallel arrangement is clearly illustrated by most of the 

specimens collected, for the rock fractures easily in the direction of this 

lamination and with difficulty across it. Occasionally larger crystals 

of orthoclase are developed, and, more rarely, long thin prisms of 

hornblende.

A microscopical study shows the primary constituents of this syenite 

to be orthoclase, plagioclase, amphibole, biotite, and apatite, while epidote, chlorite, calcite, and a ferritic pigment are secondary products.

The coloring pigment of the rock occurs in extremely small particles, 

which are doubtless limonite, and which impregnate nearly all of the 

feldspar crystals as a dense cloud. A large number of the feldspar 

tablets are plagioclase, as the common polysynthetic twin structure 

clearly demonstrates, but the pigment often so obscures the optical 

action that the angles of extinction can not be closely measured. In 

the zone normal to the twinning plane the extinction angle is always 

small, however, and it is probable that oligoclase is the only member of 

the soda-line series here developed. Orthoclase certainly predominates, 

over plagioclase.

The feldspar tablets are never perfectly idioinorphic. The crystallization of the oligoclase individuals was finished before that of orthoclase, and the predominant pinacoidal plane was then sharply defined, 

while the prismatic and terminal planes seem to have been irregular. 

The orthoclase tablets are throughout of irregular outline, owing to 

their mutual interference in the last stages of growth.

The schistose structure referred to was doubtless produced by a 

movement of the magma during crystallization. By this means the 

feldspar tablets, which were already large, were forced into a position 

approximately parallel to the dike walls. Many angular spaces were 

naturally left, into which the free amphibole prisms and biotite leaves 

were crowded. The biotite, however, seems to have continued its 

growth after this movement, for it often fills the irregular spaces completely, and has the usual hexagonal form only when included by some 

feldspar individual. Some of the angular spaces between clouded feldspars are filled by a colorless mineral, and in polarized light this matter 

almost invariably resolves into segments having the optical orientations of the adjoining feldspar individuals. The material seems in all 

cases to be orthoclase, and the ferritic pigment never penetrates into 

substance of this origin. Epidote and calcite also occur as fillings of 

these angular spaces, without their proper crystal form. Epidote occasionally appears as a direct decomposition product of biotite, but does 

not replace feldspar substance except in the more decomposed rocks.

From the facts above given it seems probable that upon final solidification of the syenitic magma there remained some angular spaces 

between crystals, which were unoccupied by any mineral substance, and 

that epidote and calcite were deposited from solutions in these cavities 

at a later period. Whether the clear feldspar substance also belongs
DILLER.] DESCBIPTIONS; NO. 71, SYENITE. 185

to this later period, or to the final stage of crystallization of the magma, 

is not fully evident. Porous structure originating in this way is known 

as the miarolitic structure.

The amphibole of this syenite is unlike any of the common varieties 

of the group. The prisms are seldom of perfect form in the coarsergrained parts of dikes, and are usually made up in part of a brown stnd 

in part of a dark-blue amphibole, the brown forming the inner portion 

and being surrounded in very irregular manner by the blue variety. 

The relations of the two are such as to indicate that the blue is merely 

the later part of the crystal, and grounds for considering it as a paramorphic product from the brown have not been observed. In other 

dikes of this syenite the brown and the blue varieties have been found 

in separate crystals. The blue amphibole has the following pleochroisiH: % = strong yellowish-brown, lr = dark blue; £ = dark greenishblue; CAJC = about 10°, absorption strong, jc>ft>a. The brown 

amphibole has dark chestnut-brown color parallel to ft and £, and a 

lighter shade parallel to a. Extinction 12° or more, not closely determinable. The brown variety also has a strong absorption parallel to fa 

and £. Both varieties seem to have the optical orientation of normal 

hornblende, and the blue is, therefore, unlike glaucophane on the one 

hand, and arfvedsonite or riebeckite on the other, while the brown is 

apparently related to barkevicite.

The biotite is almost uniaxial, and has ft + £ = dark green, and a == 

yellow-brown. Absorption is very strong.

Allanite was observed in a few small characteristic prisms in the 

contact zone of one of the narrower syenite dikes. 

Chemical composition.- The freshest specimen of the average rock 

that could be obtained was analyzed by L. G. Bakins, with the following result:

Analysis of syenite from near Silver Cliff, Colorado.

Si02 .............................................

ALA, ............................................

!?« (\

FcO .............................................

HnO... ..........................................

CaO.. ............................................

MgO.............................................

K20... ...........................................

Na-jO ............................................

H.2 0. ................ .............................

coa . ................................... ..........

Total ......................................

Sp. gr. at 30° C., 2.C89.

Per cent.

59.78

10.86

a. 08

3.72

.14

.69

5.01

5.39

1.58

.75

99.90

The rock analyzed contains calcite and epidote as secondary minerals, developed chiefly as fillings of the miarolitic cavities. The feld-
186 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BuLi,i50.

spar is not visibly decomposed, and it seems more likely that the lime 

of the epidote and calcite was brought in by solutions than that it was 

derived from the adjacent feldspars. The character of the feldspar 

contents of the rock analyzed must in any case be calculated after 

deducting lime for calcite, epidote, and amphibole. Nearly one- third 

of the lime is required for the carbonic acid to form calcite, and at 

least another third may be deducted for the other two minerals. This 

leaves one-third as belonging to an oligoclase.

If one-third of the lime belongs to oligoclase of the composition 

AbsAii], and the remainder of the soda is referred with the potash to 

a pure alkali feldspar, the amount of the oligoclase in the rock will be 

nearly 30 per cent and of the alkali feldspar nearly 50 per cent. The 

percentage of oligoclase will rise rapidly with an increased amount of 

lime introduced into the calculation, but it is quite probable that the 

oligoclase molecule is richer in lime than has been assumed, and this 

would reduce the percentage of possible oligoclase in the rock.

Literature. This syenite was first described in an article entitled, 

On some Eruptive Eocks from Ouster County, Colorado, by Whitman 

Cross, published in Proceedings Colorado Scientific Society, Vol. Ill, 

1887, pp. 237-240. It is further discussed, with details of geological 

relations, in a report on the Geology of Silver Cliff' and 'the Eosita 

Hills, Colorado, by Whitman Cross, in Seventeenth Annual Eeport 

United States Geological Survey, Part II, 1896, pp. 263-403.

BOCKS.

 3

No. 72. ORENDITE.

(FROM LEUCITE HILLS, SWEETWATER COUNTY, WYOMING. DESCRIBED BY WHITMAN CROSS.)

Introductory. When the project of gathering the rocks of this collection was first considered, the only leucite-bearing rock known in the 

United States was that occurring in the Leucite Hills, discovered by 

S. F. Emmons, and described by Prof. F. Zirkel.1 It was decided to 

collect that rock for the Educational Series. When the material was 

brought in and subjected to microscopical study, it was found that the 

greater part was not like that described by Zirkel, but contained both 

sanidine and leucite. The rock collected has, however, been described 

under the name of orendite,2 being made the type of the kind of volcanic rock rich in leucite and sanidine with diopside and maguesian 

mica as other essentials. The rock is still representative of the types 

especially characterized by leucite.

< Megascopical description. The orendite of this collection has a dull 

reddish-brown color and is quite vesicular, the pores being small and

1 Keports of tho Fortieth Parallel Survey, Vol. VII, Microscopical Petrography, 1876, p. 259.

2 Igneous rocks of the Leucite Hills and Pilot Butte, Wyoming, by Whitman Cross: Am. Jour. 

Sci., 4th series, Vol. IV, 18S7, pp. 123-120.
DILLBB.] DESCRIPTIONS: NO. 72, ORENDITE. 187

irregular in shape, with divergent smooth-walled arms. The vesicles 

are usually much less than 1C1 " in size, but vary considerably in different specimens. On examination with a hand lens the walls of these 

cavities may, in most specimens, be seen to be coated with a network 

of very pale yellowish needles of a peculiar amphibole, which will be 

described below. Hyaline opal, in clear globular droplike forms, is 

not uncommon in the pores. A dull white indistinctly crystalline substance of undetermined character is also often present.

The only distinct megascopic constituent of the rock mass is a 

reddish-brown mica, found by chemical analysis to be phlogopite, 

occurring in hexagonal leaves only 3 or 4mm broad at most, and sinking to microscopic dimensions. The general color tone of the rock is 

due to this mica.

In some specimens are occasional dull grains of orthoclase, which 

are corroded and belong to foreign rocks found in larger inclusions in 

many places throughout the mass.

Microscopical description. Microscopical study shows the rock under 

discussion to consist of leucite and sanidine in slightly predominant 

amount as compared with the ferromagnesian silicates, phlogopite, 

amphibole, and diopside. Apatite is unusually abundant and a few 

much-resorbed flakes of brown biotite may commonly be found in each 

section. Needles of a yellow mineral, rutile (*?), are present in minute 

quantities, but magnetite, ilmenite, or pyrite have not been found.

In quantitative development leucite and sanidine vary considerably; 

now the one, now the other, seeming to predominate. Of the darker 

silicates phlogopite is the most important, while diopside varies with 

leucite, and amphibole with sauidine.

Leucite occurs in this rock in a multitude of minute rounded grains, 

between 0.01lllin and 0.05mm in diameter. Some of them have the common icositetrahedral form, with a zone of minute inclusions, but more 

of the grains are auhedral. All are perfectly isotropic. In every way 

these leucite crystals and grains are identical in habit with those of the 

sanidiue-free rock.

The sanidine of orendite occurs in stout, square prisms, of rough 

outline and seldom exceeding lmm in length. The prismatic axis is a, 

as shown by the optical orientation. It is always the axis a of elasticity which lies near the prismatic axis, and the maximum observed 

angle of extinction is 10°. Rude dome and prism faces terminate the 

crystals as a rule. Twinning after the Carlsbad law is not uncommon.

The reddish-brown mica appears in thin sections to have a very 

weak pleochroism, varying only from a pale salmon-pink to pale yellow. 

Sections normal to the cleavage often reveal a polysynthetic basal twinning, which is made evident through the perceptible angle between the 

axis, a, of elasticity and the crystallographic axis, c. This reaches 3° or 

4°. Thin basal sections, not twinned, when examined in convergent, 

polarized light show a negative bisectrix and an unusually large optic
188 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

angle. The leaves are usually of hexagonal form, and they are fresh 

and free from inclusions except rare ones of glass and still rarer yellowish needles, most probably of rutile.

The mica of this rock was isolated and analyzed by W. F. Hillebrand 

with the result given below, which shows it to have the composition of 

phlogopite, a species of mica not before identified as an original constituent of igneous rocks. The observed physical properties agree with 

this determination.

Analysis of phlogopite of orendite from Wyoming.

Si02 .............................................

Ti02 .. ...........................................

AljOs . ...........................................

fr n

Fe,03 ...........................................:

FeO .............................................

CaO .............................................

 BaO .............................................

MgO ............................................

KaO .............................................

Na20 ............................................

H,0 .............................................

P c\

V

Total ......................................

Less 0 for F.. ...................................

Total ......................................

Per cent.

42.56

2.09

12.18

.73

, 2.73

.90

.20

1.00

22.40

10. 70

.44

2.35

.00

2. 46

100. 80

1. 03

99.77

Diopside occurs in small prisms; pale green or colorless, which sink 

to microlitic needles not easily determinable. In some of the other 

rocks of the region this mineral is developed iii somewhat larger prisms,

and W. F. Hillebrand has isolated and analyzed it, showing it to be an 

entirely normal diopside.

Analysis of diopside of orendite from Wyoming.

Si02 .............................................

TiO-n

P-A/ 

A12 03 - ...........................................

Fe2 03 - .......... :...............................-

FeO .............................................

MnO...... .......................................

CaO..............................................

MgO.............................................

K2 0. ................. ............................

Na20 ............................................

Hy.0.. .............................................

Sp. gr. at 20° C. 3.29.

Per cent.

50.36

3.03

1.19

1.82

.03

23. 32

17.42

.42

.76

.31

99.16
DIU.EB.] DESCRIPTIONS: NO. 72, ORENDITE. 189

The amphibole of this rock is a very unusual one in its optical characters, and its determination as a member of this group depends largely 

on the strongly marked cleavage parallel to a prism \vhose angles 

measure about 124° and 56°, together with the general habit of the 

mineral. It occurs in rudely prismatic individuals, between the sanidiue and leucite grains, producing structures soon to be described. 

Occasionally it has nearly regular crystal form with prism and pinacoid 

as in amphibole, and a termination apparently made up of pyramid and 

dome faces. Refraction and double refraction are of about the strength 

of actinolitic amphibole. Extinction is parallel to c, as far as has been 

ascertained, a = a is pale yellow, ft = & is red, £ = c is bright yellow. 

The reddish tone is similar to that of hypersthene, and all colors increase 

rapidly in intensity with increasing thickness of the sections. Absorption, Ii>,c>a.

The association of minerals in orendite leads to several interesting 

microstructures. Phlogopite appears to have formed first, and it is 

almost free from inclusions. Leucite and sanidine are as a rule developed in separate patches or areas, the former in swarms of minute 

round grains, the latter in aggregates of a few irregular prisms seldom 

exceeding linm in length. Diopside is developed mainly in minute 

needles and microlites, a large share of which are iucluded quite irregularly in the sanidiues, producing a certain inicropceeilitic structure. 

The remainder of the diopside occurs between other larger mineral 

grains.

The amphibole seems one of the latest crystallizations of the rock, 

and varies in development. In the angular spaces between the sanidiues the yellowish amphibole occurs exactly as does augite in ophitic 

diabase, acting as an oriented cement for several sanidine crystals. In 

the leucitic areas the same amphibole appears in stout prisms inclosing numerous leucites, just as aDgirine holds the nephelines in many 

phonolites. In this manner another form of the micropoecilitic structure is produced. It also occurs in very minute needles in many of the 

vesicles.

In occasional spots and adjacent to the pores of the rock the minerals are less intimately intergrown. Leucite is sometimes found 

inclosed in sauidiue, but frequently the separation as described is very 

sharp. There are thus in this orendite two kinds of micropoecilitic 

structures, a curious separation of the analogous silicates, leucite and 

sanidine, ophitic structure, and through the prominence of phlogopite 

leaves a porphyritic structure.

Chemical composition. Two analyses of orendite have been made by 

W. F. Hillebrand, which are given below. Analysis I is of the rock 

from Fifteen Mile Spring, on the eastern edge of the Leucite Hills, the 

locality from, which all the specimens of the educational collection were 

obtained. Analysis II is of a very similar rock from North Table 

Butte, in the northern part of the Leucite Hills.
190 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150. 

Analyses of orendite from Leucite Hills, Wyoming.

SiO, ...................................

Ti02 ................... ................

ZrO,. I..... ............................

A12 03 - .................................

Cr203 ..................................

Fe2 03 ----------------------------------

FeO. ...................................

MnO.............. .......:.......-.....

CaO. ........ ...........................

StrO

EaO. ......... ..........................

MgO..... ..............................

ir n

Na.,0 ..................................

Li20 ............ .......................

H2 0 below no°C. ......... ............

H/) above 110° C ......................

P

r\

so3 ............ .................. ... ...

a

.

p ......................................

CO*...... .............................

Total .................,........:.

Total ............................

I.

Per cent. 

54.08

2.08

9.49

.07

3.19

1.03

.05

3.55

.20

.67

0.74

11.76

1.39

.79

2.71

1.35

.29

.04

.49

99. 97

.21

99.76

H.

Per cent. 

54.17

2.07

.22

10. 16

.05

3.34

.05

.06

4 19

.18

.59

6.02

11.91

1.21

.52

1.01

1.59

.16

.06

.36

.49

100. 21

.17

100. 04

The rocks are remarkable for the large number of rare elements contained in them in determinable quantities. ZrO2 was not tested for in 

the first analysis. It does not belong in the mineral zircon, as that can

not be identified in thin sections, and it seems most probable that the

peculiar amphibole is allied to certain silicates which may be classed 

with the pyroxenes, in having TiO2 and ZrO2 replace apart of the SiO2. 

Cr2O3 and most of the P belong in phlogopite, together with a large 

part of the BaO. SO3 represents some mineral easily soluble in weak 

acids, probably noselite, which may be indistinguishable from leucite 

in thin sections. Both the mica and the pyroxene contain TiO2 , but 

the rare yellow needles occasionally seen may be rutile.

As regards the commoner rock constituents, the predominance of potash in orendite causes characteristic minerals contrasting with those 

of the corresponding soda-rich rock, phoiiolite. Leucite is.developed 

in place of nepheliue. A potash-bearing mica appears instead of regirine 

and arfvedsonite, which are common in phouolite. The pyroxene of 

orendite is almost a pure lime-magnesia mineral^ and soda is prevented 

from entering into plagioclase.

Occurrence and relationships. The rock now under discussion occurs 

in lava flows of Tertiary age. In the same flows a part of the magma has 

crystallized free from sanidine, but with leucite, diopside, and phlogo-
DILLKB.] DESCRIPTIONS: NO. 72, PHONOLITE. 191

pite of the same development observed in orendite. This rock, the oue 

originally described by Zirkel, has the same bulk chemical composition 

as orendite. Since leucite contains less silica than sanidine, there is of 

necessity an excess of this acid radical in the pure leucite rock, apparently in the form of an obscure glassy base. This rock has been called 

"wyomiugite 77 in the cited paper on the rocks of the Leucite Hills.

At Pilot Butte, west of the Leucite Hills, occurs a rock rich in diopside, phlogopite, and perofskite, with a glassy base which the analysis 

of the rock shows must have nearly the composition of leucite. The 

name " madupite " has been proposed for this type.

Orendite belongs in the group called leucite-trachyte by Zirkel, and 

leucite-phonolite by Eoseubusch. A special name has been proposed in 

the belief that both the compound terms above mentioned are objectionable, and that it is appropriate and desirable to have a distinctive' 

name for the leucite-sanidme rocks corresponding to phonolite.

Ko./73. PHONOLITE.

(FROM BLACK HILLS, SOUTH DAKOTA, DESCRIBED mr WHITMAN CROSS.)

Megascopical description. This rock has a dense aphanitic texture 

except for a few small sanidine tablets. Its general color is a dull 

brownish green, mottled by numerous dark bluish-green spots of 

indistinct sheaf-like form. These dark spots are due to the bundles 

of segirine needles. A peculiar greasy or semivitreous luster is produced by the reflections from the multitude of minute faces of nepheline or sanidine grains forming the mass of the rock.

Microscopical description. The principal constituents of this phonolite 

are nepheline, sanidine, and cegiriue, with a characteristic accompaniment of noselite and sodalite. Nepheline is very distinctly developed 

in short prisms causing the hexagonal or nearly square outlines seen 

in thin sections. The average diameter of these nepheline crystals is 

a,bout O.lnim and their length less than 0.2mm . * In this development the 

nephelines lie in a fine-grained mass consisting chiefly of sanidine 

scales and tablets, which overlap so much that very thin sections and 

high powers of the microscope are necessary clearly to resolve the 

mass into its elements.

To a subordinate- degree in the sections examined the sanidine is 

developed in more elongated staves and needles, and these are commonly arranged in nearly parallel position, producing an apparent 

fluidal structure. It is noticeable that in these portions of the rock 

nepheliue is developed in much smaller crystals than elsewhere.

.ZEgirine is the mineral causing the dark spots of the rock. It occurs 

in bundles or sheaves of minute needles which in the central zone seem 

more or less of a common orientation or are united practically into one 

crystal which feathers out at each end. Owing to the great numbers 

of included nepheline or sanidine grains, even in the most massive part
192 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BDLL.ISO.

of these bundles, the thin sections rarely show the actual existing continuity of the aegirine substance. Where the latter is most massive 

the abundant inclusioDS of other minerals produce a typical microposcilitic structure. The optical characters of this segirine are entirely 

normal. Its pleochroism is strong, as follows: a = pure green; -Ir = 

olive green; £ = yellowish green, a lies very near the prismatic axis. 

No considerable angle of extinction was observed, so that segiriueaugite is not developed in this rock.

Scattered through the rock in amount greatly subordinate to nepheline, but still an important constituent, is a mineral of the regular system characterized by a cloud of dark, dusty interpositions. While this 

substance is apt to be decomposed, it has the characters often found 

in noselite as occurring in phonolites. Its rudely irregular crystals are 

somewhat larger than the nephelines and may reach lmm in diameter. 

From the chemical analysis, which shows both sulphuric acid and 

chlorine in the rock, it is to be inferred that sodalite is a companion of 

noselite here as in many other phonolites, but no means of distinguishing the two have been found in this case.

In almost every section of this phonolite may be found small areas 

of clear, colorless, isotropic substance in angular spaces between welldefined crystals of other minerals. It is thought probable that this 

isotropic substance is analcite, through analogy with the occurrence of 

that mineral in the very similar phonolites of the Cripple Creek region 

in Colorado.

Other minerals occur in this rock'only in minute traces. There are 

occasional specks of magnetite and possibly of pyrite. The titanic and 

zirconic acids shown by analysis are most probably contained in a mineral occurring sporadically in a manner much like the aegirine; that is, 

in irregular particles in residual spaces or inclosing minute grains of 

nepheline or feldspar. This mineral is almost colorless, of high single 

and double refraction, with no or very slight pleochroism and apparent parallel extinction. It resembles a mineral observed, but not 

positively identified, in the Cripple Creek phonolites, though there the 

association with laveuite led to the supposition that it was to be referred 

to that species or to some allied complex silico-titanate with zirconic 

acid also present.

Chemical composition. In the subjoined table of analyses, Column I 

represents the composition of this phonolite as analyzed by W. F. Hillebrand; II, an analysis by Pirsson of phouolite from the Devils Tower, 

near the Black Hills; 1 III, analysis by Hillebrand of a phonolite from 

Miter Peak near Cripple Creek, Colorado; 2 IV, analysis by vom Hath 

of a typical German phonolite from Zittau, Saxony.3

1 Phonolitic rocks from the Black Hills: Am. Jour. Sci. 3d series, Vol. XLVII, 1894, p. 341.

2 General geology of the Cripple Creek district, Colorado: Sixteenth Ann. JRept. TJ. S. Geol. Survey, 

Part II, p. 39.

3 Quoted by Zirkel, Lehrbuch der Petrographie, 2d ed., p. 446.
D1LLKR.] DESCRIPTIONS: NO. 72, PHONOLITE. 

Analyses of phonolite from various localities.

193

SiO*. .............

Zr02 . .............

TiOz..... .........

A12 03 . ............

Tfo n

EoO ..............

MnO... ..........

CaO ..............

SrO-. .............

BaO ..............

MgO. .'............

K20 ..............

NaaO .............

Li,O ..............

H20 110° C..... 

HS0 + 110° C .....

P

f\

S03 - ..............

Cl ................

F. ................

CO*.. .............

S .................

I.

Per cent. 

57.86

.15

.22

90 9fl

9 ^

.39

.21

.89

.04

no

.04

5.19

9.47

.21 

2.40

.03

.06

.08

(?)

.03

99.97

II.

Per cent. 

58.98

.20

.24

20.54

i fii

.48

.26

.67

.11

5.31

9.95

.19 

.97

.20

.28

100. 07

III.

Per cent. 

61.08

.18

18. 71

1.91

.63

1.58

.05

.08

4.63

8.68

a 2. 21

.12

99.86

IV.

Per cent. 

59.17

19.74

3.39

.92

.15

6.45

8.88

a 1.18

99.88

a, total water.

The analysis shows the phonolite of this collection to be quite typical, and very closely related to the rock so abundant in the Cripple 

Creek district of Colorado. It was ascertained that the Cripple Creek 

 ' phonolites contained from 35 to 40 per cent of nepheline, and it is 

evident that there must be about the same amount in this rock. Sanidine probably constitutes at least 40 per cent of the rock, leaving 

about 20 per cent for segirine, noselite, sodalite, and the accessory 

minerals.

The presence of zirconic acid in determinable amount without discernible zircon crystals, and the similar amount of titanic acid, seem 

to indicate the presence of some one or more of the rare minerals con- 

» taining these acid radicals, such as lavenite, which have been noted of 

recent years in eleolite-syenite or phonolite.

Literature. The first phonolite to be discovered in the United States

^ was found at Black Butte in the Black Hills, and was described by

. J. H. Caswell.1 In 1894, L. Y. Pirsson described the rock of Devils

Tower, near the Black Hills, as a phonolite and gave an analysis

of the same which has been reproduced in the above table. It is

known that there are many other occurrences of phonolite in and near

1 Report on the Geology and Resources of the Black Hills of Dakota, by Henry Newton and Waiter 

V P. Jenney, 1880, p. 492.

Bull. 150 13
194 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL.ISO.

the Black Hills, but no systematic examination of their occurrence r 

and relationships has been made. L

NO. 74. PULASKITE (ELEOL1TE-HORNBLENDE-SYENITE). 

(FROM LITTLK ROCK, ARKANSAS. DESCRIBED BY J. P. IDDINGS.)

This rock is the so-called "blue granite" of Arkansas, which has been 

studied and fully described by J. F. Williams, 1 from whose report the 

following has been extracted.

The rock is an intrusive body occurring in a wide, dike-like mass, 

whose eruption took place about the close of the Cretaceous period, and 

which forms the main ridges of the Fourche Mountain region. It is a 

bluish-gray, crystalline rock, in some parts dark in others light colored. , 

It has a peculiar semi-porphyritic appearance when viewed megascopically. The feldspar phenocrysts are conspicuous on account of their , 

size and their highly perfect cleavage-planes and the light reflected 

from them. They appear to be crudely tabular parallel to the clinopiuacoid (010), and give a trachytoidal texture to the rock. The crystals 

are not sharply defined, since they interlock with the smaller crystals 

of the groundmass. The groundinass is subordinate to the feldspar 

phenocrysts, is phanerocrystalline, but dubiodiagnostic, consisting of , 

whitish, gray, and dark-colored grains. Occasionally small plates of 

mica and crystals of dark amphibole or pyroxene are recognizable. The 

greater portion, however, is feldspathic.

The texture of the rock varies throughout the body from granitic 

porphyritic to hypidiornorphic granular. The following minerals are 

usually found in every specimen of the rock, but are present in variable 

quantities. Especially among the dark-colored minerals is this noticeable, since in many cases one of them predominates to the almost complete exclusion of the others. The minerals are orthoclase (cryptoperthite), hornblende (arfvedsouite), augite (diopside), biotite, eleolite, 

sodalite (rare), titanite, apatite.

Orthoclase is by far the most important mineral in the rock. It 

appears usually in two forms as the result of two distinct periods of 

crystallization. The crystals belonging to the first are the phenocrysts, from 10 to 30mm in length. They often show an idiomorpliic 

form, although this is frequently impaired by the juxtaposition and < 

mutual penetration of the smaller crystals of the second period of 

crystallization. Where crystal faces have been recognized they are 

OP (001), o>P o> (010), and ooP (110). Some crystals are twinned -' 

according to the Manebach law, others according to the Carlsbad. ^ 

In one instance both kinds of twinning were observed in one crystal. 

The feldspar phenocrysts are impellucid as a result of more or less 

advanced kaolinization. ,

'J.P.Williams, The igneous ropks of Arkansas: Ann. Kept. Geol. Survey Arkansas, Vol. II, 1890, 

pp. 39-71; Little Kock, 1891. >
DILLEE.] DESCRIPTIONS: NO. 74, PULASKITE. 195

The angle between the two cleavage planes is nearly 90°. Some 

crystals exhibit delicate microcline twinning which, taken in connection 

with the chemical composition, indicate soda-microcline. In some crystals there is a microscopic intergrowth with another feldspar showing 

albite twinning and extinction angles of .oligoclase. The feldspar corresponds to Brogger's kryptopertbite. Some of the feldspars exhibit 

no polysynthetic twinning. Some contain minute cavities elongated so 

as to appear like needles. These are generally in parallel groups, 

either perpendicular to the cleavage planes or parallel to them. A few 

larger cavities appear to contain fluid with a gas bubble. There are 

also inclusions of the ferr.omagnesian minerals with apatite and magnetite, besides more numerous grains of eleolite, irregularly shaped, 

and sometimes with hexagonal outline. Less frequently there are 

colorless isotropic inclusions with lower refraction than feldspar, which

are probably sodalite. 

The chemical analysis of the feldspar is as follows:

Analysis of feldspar of pulaskite from Little Rock, Arkansas.

Si02 .............................................

A\,03 . ...........................................

FeA ................................. ..........

CaO .............................................

MgO. ...... ......................................

K.,0. .................... ^-. .......................

'N&tO................ .............................

Total ......................................

Per cent.

66.95

0.90

0.52

0.24

7.82

0.30

99.80

There is an excess of silica not accounted for. No quartz has been 

observed in the rock, and no individual crystals of lime-soda feldspar. 

The small feldspars constituting the groundrnass have the same characters as the large ones. The greater part of the rock is orthoclastic 

feldspar. The ferromagnesian minerals are generally very small, and 

are monoclinic pyroxene, amphibole, and biotite. The monoclinic 

pyroxene is pale green in thin section, without pleochroism. Its form 

is irregular, and it is generally surrounded more or less completely by 

greenish-brown hornblende. In some parts of the rock the margin has 

a decided green color, probably segirite, the principal portion of the 

pyroxene being diopside or inalacolite. The green margin shows an 

increase in the amount of sodium taken up from the magma toward the 

end of the crystallization of the pyroxene. The cleavage isthat common 

in pyroxene; and the ordinary twinning parallel to the orthopinacoid 

(100) is present in some crystals. Among the inclusions noted are 

apatite, magnetite, titanite, biotite, and irregularly distributed gaspores.
196 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

The amphibole when seen in thin section is rich chestnut brown in 

some cases and greenish brown or dark green in others. When idiomorphic it is bounded by the faces o> P (110), o> PO> (100), <x> P do (010), 

and OP (001) and some orthodome (mOl) has also been observed. 

Amphibole cleavage is well developed.

The pleochroism in some cases is as follows :

fo = reddish brown; a = light yellowish brown; .c = dark reddish 

brown; the absorption being c > It > a. In other cases the pleochroism 

is: ft = deep bluish green; a = brownish yellow; c = yellowish green; 

the absorption being ft > c > a. A dark green edge, or border, surrounds many of the brown crystals. Some of the brown crystals are 

completely free from such a border, but it almost always appears about 

the greenish ones.

It is probable that these amphiboles belong to the arfvedsouite group, 

but no chemical tests have been attempted in proof of this supposition. 

Inclusions are similar to those contained in the pyroxene.

The biotite is reddish-brown in thin section, with fairly strong pleochroism from yellow to reddish brown. It forms plates parallel to the 

basal plane, which are idipmorphic in some cases and irregularly outlined in others. It carries inclusions of magnetite, sphene, apatite, and 

zircon. It often surrounds comparatively large crystals of magnetite, 

sphene, and apatite. Sometimes it is on the outside of the pyroxene 

and amphibole, and sometimes it is inclosed by hornblende and pyroxene. Its crystallization appears to have begun before that of horoblende. But in general the pyroxene appears to be older than the 

hornblende, and the biotite continued its growth to the end of the 

series of ferromagnesian minerals. 1

Eleolite or nephelite occurs in variable quantities. In some parts of 

the rock it is quite insignificant, while in other parts it is an important 

constituent. It occurs in allotriomorphic anhedrons, often occupying 

spaces between large feldspars or forming irregularly shaped inclusions 

within them. It is colorless, with a higher refraction than the feldspar, 

and is sometimes more or less altered to analcite. Eleolite is nearly 

free from inclusions, but contains a few needles of apatite and occasionally slender crystals of segirite.

Sodalite is found in some parts of the rock. It is generally filled 

with dust-like inclusions, but more often is altered to indeterminable 

decomposition products. The sodalite crystals are about half a millimeter in diameter.

Sphene is the most important of the subordinate minerals. It forms 

light to dark yellow idiomorphic crystals, some of which are 1.5mm in 

length. In thin section the crystals exhibit sharp outlines and a roughlooking surface, and the mineral appears to have been the earliest to 

crystallize.

1 This is not the order stated by Professor Williams, but appears to bo that shown by the sections of 

specimens collected for the Educational Series. J. P. I.
DILLKR.] DESCRIPTIONS: NO. 75, THERALITEr .197

Apatite is quite abundant in crystals, sometimes 2mm long. The 

larger crystals are comparatively short and stout, the smaller ones 

being slender prisms, occasionally very thin and hair-like. In most 

cases colorless, some of the crystals are dusted and contain rod-like 

inclusions. Magnetite occurs sparingly in small anhedrons. Fluorite 

has been observed in some varieties of the rock; also minute crystals 

of ;egirite, which appear to be of secondary origin.

According to Williams there is no zircon present. But certain 

minute crystals with high refraction and double refraction occur in the 

thin sections from specimens in the educational series, which appear to 

be zircon. They may be easily confused with sphene.

Williams points out the resemblance between pulaskite and the laurvikite of Norway, described by Brogger. The two rocks resemble one 

another mineralogically and chemically, but differ somewhat in texture.

Laurvikite is coarser grained.

No. 75. THERALITE.

(FROM GORDONS BUTTE, CRA/Y MOUNTAINS, MEAGHER COUNTY, MONTANA. 

DESCRIBED BY J. E. WOLFF.)

The Crazy Mountains, from the borders of which specimens 75 and 

76 were collected, constitute an outlying range situated a few miles east 

of the main mass of the Rocky Mountains in Montana and near the 

boundary between the great eastern plains and the mountains. The 

Yellowstone Eiver flows around the southern end of the range and the 

Musselshell, a branch of the Missouri, bounds it on the north.

The mountains are composed of strata of Cretaceo-Tertiary age, 

lying for the most part nearly flat or at low angles, through which 

many kinds of eruptive rocks have been intruded as stocks, laccoliths, 

sheets, or dikes. Among these eruptive rocks certain peculiar dark 

basaltic rocks are found which occur generally near the periphery of 

the range, the center of which is formed of masses of dioritic rocks.

These basaltic rocks are coarsely crystalline when they occur in thick 

sheets or laccoliths, but fine grained and porphyritic when occurring 

in thin sheets or dikes, where the conditions of quicker cooling have 

been effective. They are characterized chemically by a low per cent 

of silica and high per cents of the alkalies and of lime, magnesia, and 

iron, together with exceptional amounts of barium and strontium, as 

will be seen in the accompanying analyses. Mineral ogically they 

are peculiar in the combination of augite, nephelite, a mineral of the 

sodalite group, and a feldspar which is chemically a potassium feldspar 

with barium and some sodium, calcium, and strontium. In the original 

descriptions of the rocks this feldspar was determined as in part ordinary sanidine, in part a soda-lime feldspar, and the rocks were therefore 

made the types of a plutonic rock characterized by the mineral combination nephelite, soda-lime feldspar, and named theralite by Professor 

Eosenbusch. The feldspar appears to be, strictly speaking, neither
198 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

ordinary sanidine nor soda-lime feldspar, and hence for classificatory 

purposes the weight should be put on the chemical composition and 

the presence of nephelite and feldspar.

Specimen 75 is from Gordons Butte, an outstanding hill on the northwestern side of the range, which is oval in outline, with a diameter of 

2£ miles. The base of the butte is formed by sandstones, shales, 

and limestones, which lie nearly flat but dip gently in toward the 

center. Above these and toward the base, frequently interlaininated 

with the sedimentary rocks, is a great intrusive sheet of theralite which 

is nearly 600 feet in total thickness. The original cover of shale has 

been worn off and only little patches of loose, baked shale can be 

found. The great laccolith of eruptive rock has very perfect vertical 

prismatic structure or jointing, the columns of which lean gently 

inward, corresponding to the position of the contacts which conform 

to the dip of the containing strata. A great line of cliffs, nearly 

circular in outline, forms the outside edge of the summit, which in the 

interior is a basin with gently sloping sides. The specimens were gathered from the northwest base of the cliffs and represent the coarsest 

variety of theralite found in the range. They come from the center of 

the mass, where the crystallization was coarsest.

Description of the specimen (hand specimen). The prisms of augite 

are the most striking characteristic. They have a roughly parallel 

arrangement, caused by the motion of the rock as it flowed parallel 

to the bounding country rock. The light green border of segirite can 

be occasionally noticed. Large brown plates of biotite and occasional 

clear yellow grains of titanite are visible. The nephelite and feldspar 

make up the colorless part. The former is recognized by'its yellowishgray color and somewhat greasy luster, while the feldspar is white, 

with distinct glassy luster on the cleavage surfaces, In other places in

this same laccolith the feldspar contains little crystals of haiiynite and 

nephelite distributed through it, and this causes a luster mottling or 

pcecilitic structure on the cleavage surfaces. In the variety represented 

here this character is obscure.

In the thin section the augite is seen in stout prisms with both pinacoids developed and rarely terminal planes. It is pale green in color, 

passing into a deeper green toward the periphery, owing to mixture 

of the segirite molecule, which often results in a border of deep-green 

tegirite in parallel orientation to the augite. The segirite also occurs 

independently in radiating prisms. The augite incloses crystals of 

apatite and magnetite, and is often touched by plates of biotite, which 

may be oriented with the basal cleavage parallel to the augite prism.

The biotite plates contain apatite, magnetite, and rarely augite. Their 

rounded outline and occasional embayment by the later-formed minerals shows that they have been subjected to resorptive action. Small 

serpeutinized grains of olivine inclosed in the biotite are uncommon in 

this occurrence, although found at other localities (see description of 

specimen No. 76).
DILLBE.] DESCRIPTIONS: NO. 76, THERALITE. 199

, The feldspar occurs in large, irregular areas, which can be recognized 

as such by their clear, transparent look and polarization tint, bluish 

white of the lower order. The two cleavages show distinctly only in 

very thin sections or near the edge, where the strain of grinding has 

developed it. It incloses little prisms of nephelite and rarely in this 

occurrence square or hexagonal sections of haiiynite crystals, although 

this mineral is abundant in other parts of the same rock mass.

The nephelite appears also in large prismatic masses which have an 

imperfect crystal outline; the two cleavages, prismatic and basal, are 

well developed, giving generally two rectangular cleavages of which 

the prismatic is best developed. It polarizes with almost the same 

tint as the feldspar, extinguishes parallel to the cleavages when both 

show, and basal sections are isotropic and give an indistinct cross in 

converging light. The nephelite is in places fresh, but generally more 

or less filled with decomposition products; some of these with bright 

polarizing tints and slightly yellow color (which is distinct in the hand 

specimen) are probably cancriuite, the rest zeolites, which give an 

aggregate fibrous appearance.

The feldspar in this rock has generally the optical properties of sauidine, with the plane of the optic axes 5° from the base, small angle of 

the optic axes, and the axis of greatest elasticity an acute bisectrix. 

Its chemical composition has, however, already been mentioned; the 

specific gravity seems to vary from 2.63 to 2.57.

The order of crystallization is evidently first the apatite, magnetite, 

and olivine (since they are inclosed in the biotite and augite); then 

the augite and biotite; then the nephelite, and lastly the feldspar. 

The.formation of the segirite at the close of the augite period and its 

occurrence as the periphery of augite crystals, suggests a caustic action 

of the magma which impregnated the forming or already formed augite 

with the cegirite material. The chemical analysis of the theralite from 

Gordon's Butte is given with that of the specimen from Alabaugh. 

Creek, on page 197.

No. 76. PORPHYRITIC THERALITE.

(FROM MOUTII OF ALABAUGH CREEK, CRAZY MOUNTAINS, MEAQHER COUNTY, 

MONTANA. DESCRIBED BY J. E. WOLFF.)

Specimen 76 is from a sheet or bedded dike from Alabaugh Creek at 

the north end of the mountains. The structure and inineralogical composition are different from 75, although they represent the same rock.

Description of the specimen (hand specimen). A grayish-black rock 

containing long slender prisms of augite with octagonal outline, hexagonal plates of biotite and grains of olivine, which have a yellow core 

and rusty red exterior. The groundmass is seen to glitter with little 

flakes of biotite, and small augite prisms can be detected in it. The 

augite crystals have a rough parallel arrangement due to the flowing 

of the rock.
200 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

In the thin section the augite phenocrysts have the same pale-green 

color and rcgirite border as in 75, but the latter is much less marked. 

In addition to apatite and magnetite, they are honeycombed with inclusions of the residual magma, which is generally individualized to nephelite or feldspar. Sometimes the groundmass has pushed into the augite 

in little bays which may contain biotite. The biotite is in distinct 

hexagonal plates inclosing apatite, magnetite, and olivme; it has been 

corroded by the caustic action of the magma, which has produced the 

rounded forms sometimes seen. This biotite has a very marked obliquity of extinction to the cleavage. The olivine is in rounded grains 

touched or even surrounded by the biotite; between the two minerals 

there is a deep brown zone (iddingsite) which seems to grade into the 

biotite. The olivine incloses apatite, which occurs also independently 

in quite large crystals.

The groundmass contains augite, biotite, magnetite, apatite, olivine, 

a mineral of the sodalite group in deep blue crystals with square or 

hexagonal outline (due to the fact that they are sections of the rhombic dodecahedron), and isotropic character. The colorless part of the 

groundmass is threefold; there are long colorless laths of feldspar, 

sometimes in Carlsbad twins, which optically correspond to vsanidine 

(in other occurrences of this rock they may be anorthoclase); between 

these is a fibrous substance, often in prismatic aggregates, which is the 

zeolitized nephelite; and lastly clear glassy areas, which are nearly or 

quite isotropic, show traces of cubic cleavage and are analcite.

It will be seen on careful study that there are two forms of feldspar in the groundmass, one occurring in slender crystals, the other in 

larger irregular grains which seem to have formed later; there is probably a chemical difference in these two forms corresponding to the 

different habit and period; whether one be sanidine and the other 

anorthoclase is not apparent,

The distinction between the phenocrysts (crystals of the first generation) and the same minerals in the groundmass is not very sharp, 

but the relations between the feldspar and nephelite are different from 

those iii the other specimen, for here the nephelite is later than the 

prismatic feldspar, although perhaps contemporaneous with the other. 

We see that the olivine followed the apatite and magnetite and preceded 

the biotite.

The following two analyses represent the chemical composition of the 

coarse and porphyritic theralite. No. 1, coarse theralite from Gordon's 

Butte, was made by W. F. Hillebrand in the laboratory of the United 

States Geological Survey, at Washington, and Nos. 2 and 3 by E. A. 

Schneider, in the same laboratory. Analysis No. 2 is of specimen No. 

75, and No. 3 of specimen No. 76.
DILLER.] DESCRIPTIONS: NO. 77, NEPHELITE-SYENITE.

Analyses of theralite from Crazy Mountains, Montana.

201

SiOa .........................

Tin

Al aOs -"-" ..................

ITeO .........................

MnO. ........................

CaO .........................

SrOa. .......................

BaO .........................

MgO. .........................

KaO. .........................

^0 ........................

Li.,0.. .......................

H.,0(alKM'0110°C.)..........

PA-- ................... ....

CO*. .........................

Cl ...........................

S03 - .........................

Total.................

1.

Per cent. 

44.65

.95

13.87

C.OG

2.94

.17

9.57

.37

.76

5.15

4.49

5. 67

trace

.95

2.10

1.50

.11

.61

99.92

2.

Per cent. 

44.31

(b)

17. 20

4.64

3.73

.10

10.40

W

(b)

6.57

3.64

4.45

(&)

.77

3.30

(b)

(b)

(b)

(b)

99.11

S.

Per cent. 

47.07

(b)

18.22

3.05

3.85

.28

8.03

(b)

(b)

6.35

3. 82

4. 93

W

.38

2.97

(b)

(b)

(b)

(b)

100.15

ft Contains some CaO. b Undetermined.

No. 77. NEPIIELITE-SYENITE (ELEOLITE-SYENITE). 

(FROM LITCHFIELD, KENNEBEC COUNTY, MAINE. DESCRIBED BY W. S. BAYLEY.)

It is not certain that the rock represented by this specimen has been 

found in place. It is known to occur in five or six localities in the 

vicinity of Litchfield, Kennebec County, Maine, usually in the form of 

bowlders lying on the surface, but sometimes in low ledge-like exposures nearly covered by glacial sands. The specimen's in the collection 

were obtained from a pile of loose material lying on both sides of the 

road running from South Litchfield post-office, in the town of Litchfield, 

in Kennebec County, Maine, to the city of Gardiner, on the Maine Central Kailroad, about 6 miles south of Augusta. The distance of the 

locality from South Litchfield is about three-quarters of a mile, and 

from Gardiner about 8 miles.1

The rock is a moderately coarse-grained crystalline aggregate of 

three principal substances, none of which are porphyritically developed. Its texture is thus hypidiomorphically granular, or granitic. 

Occasionally a general parallel arrangement of the. constituents may 

be detected, when the structure becomes schistose.

The most abundant of the components is a white mineral, with brilliant cleavage faces, often characterized by a pearly luster. This occurs 

both in large columnar grains from a quarter to a half inch in length.

1 1'or description of other localities see article by W. S. Bayley in Bull. Geol. Soc. America, Vol. Ill, 

pp. 231-252.
202 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

and in small irregular ones, so arranged in certain areas as to cause 

their aggregation to resemble a fine-grained marble. The larger grains, 

which are albite, have a density varying between 2.600 and 2.608 and 

a composition 1 as follows:

Analysis of albite of neplielite-syenite from Litchfield, Maine.

tiin

Al20a - - ' ........

K2 0 .............................................

Na2 0...... ......................................

H2 0 ....................................'.........

Per cent.

66.39

19.69

.99

10.17

.52

2.24

Another of the prominent constituents that may be seen in the hand 

specimen is in irregularly shaped masses of a grayish color, and with an 

oily luster. It .possesses a well-marked cleavage, which is emphasized 

by the interposition of long black needles between the cleavage plates. 

A fragment of this substance dissolves quite readily in hydrochloric 

acid, leaving a residue of gelatinous silica. Its composition, as determined by Dr. Clarke,2 is that of nephelite (eleolite):

Analysis of nephelite of nephelite-syenite from, Litchfield, Maine.

Si02 .............................................

A1A ............................................

CaO ............................................

MgO...... ..............:........................

Krf> .............................................

NajO ............................................

H20 .............................................

Total ................................. i....

Per cent.

43.74

34.48

4.55

16.62

.86

100. 25

The only dark-colored mineral present in the rock is a lustrous black 

lamellar one, which cleaves with such ease that large plates can be split 

from it. These plates are elastic, and they show a nearly uniaxial interference figure in converged light between crossed nicols. The mineral 

is a biotite of the variety known as lepidomelane, as shown by Dr. 

Clarke. 3

1 E. W. Clarke, Am. Jour. Sci., 3d series, Vol. XXXI, p. 268.

2 Ibid., p. 262.

3 Ibid., Vol. XXXIV, p. 133.
DILLEH.] DESCRIPTIONS: NO. 77, NEPHELITE-SYEN1TE. 203

Analysis of lepidomelane of nephelite-eyenite from Lilcltfield, Maine.

- - .

A12 0S ... ......................... ................

Fes03 ......................................

PeO .............................................

'M.-uQ. ............................................

c&o .............................................

K20 .............................................

 NotQ........... .................................

HSO ...............:.............................

Total ......................................

Per cent.

32.35

17. 47

24, 22

13. 11

1.02

.89

.70

6.40

4.67

100. 83

The three substances, albite, eleolite, and lepidomelane, occur in all

specimens of the rock wherever found, and are thus to be termed 

essential constituents. The other components, though very common, 

are not found in all specimens, and hence are accessory. They comprise can crinite, sodalite, and zircon, and are the minerals that have 

made the rock famous all over the world. The most common and at 

the same time the most striking of the accessory constituents is cancrinite, which is in very irregular lemon-yellow and orange grains, that 

are scattered indiscriminately among the other components, but appear 

to prefer the neighborhood of the nephelite. The orange-yellow variety 

yielded Dr. Clarke 1 the following analysis:

Analysis of cancrinite of neiihelite-syenite from Ecnnelec County, Maine.

Si02 .............................................

A12 03 . ...........................................

FejjOg... .........................................

CaO........ ......................................

yfa^Q. ............................................

K20.- ............................................

H,,0.. ............................................

C02 .... ..........................................

Total......................................

Per cent.

36.29

30.12

4.27

19.56

.18

2.98

6.96

100. 36

Upon comparing this analysis with that of the nephelite, Dr. Clarke 

reaches the conclusion that the former is an alteration product of the 

latter. The sodalite is much less common than the cancriuite. It is 

disseminated in small blue grains throughout the rock mass, and occurs

1 Am. Jour. Sci., 3d series, Vol. XXXI, p. 263.
204 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL/ISO.

as coatings on its joint cracks. The composition of the sodalite has 

also been determined by Dr. Clarke. It is as follows:

Analysis of sodalite of nephelite-syenite from Litchfield, Maine.

SiO* .............................................

AI n

JSajO ............................................

K2 0 .............................................

C1-. ........................... ..................

H20.. ..........................................:.

0 01..... ......................................

Total......................................

Per cent.

37.33

31.87

24.56

.10

6.83

1.07

1.54

300.22

The zircon, the only remaining mineral that can be recognized in the 

hand specimen, is the single component of the rock that possesses a crystal form. It is in hard, pinkish-brown grains, with an octahedral habit, 

that may be found here and there among the older constituents. Its 

crystals are bounded by the tetragonal prism and pyramid of the same 

order. An analysis by Gibbs 1 gave:

Analysis of zircon of nephelite-syenitc from Litclificld, Maine.

SiOjj .............................................

Zr02 .. ...........................................

JFeA ......................................... .:.

Total......................................

Percent.

35.26

63.33

.79

.36

119. 74

The texture of the rock as seen under the microscope is thoroughly 

granitic, in that none of its essential components possess crystal outlines, although many of the nephelite grains and the larger albites have 

quite well defined rectangular cross sections. In ordinary light it 

appears to consist of plates of green mica embedded in a nearly homogenous, colorless, transparent groundmass, clouded here and there 

with opaque white and yellowish substances, that seem to be decomposition products of some constituent. Under crossed nicols, however, 

this groundmass is resolved into large and small grains of the minerals 

detected in the hand specimen, besides orthoclase and microcliue. (See 

PL XXIX.)

The mica, which is the oldest mineral present, with the exception 

of the zircon, occurs not only in the large plates already mentioned, but 

also as inclusions in the other components, more particularly the eleolite. In basal sections it is so dark as to be almost opaque, except in 

extremely thin pieces, when it is of a clear, deep-green color. The 

optical figure when tested with the mica-plate is found to be negative,

1 Pogg. Anu., LXXI, p. 559.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXIX

THIN SECTION OF NEPHELITE-SYENITE AS SEEN UNDER A MICROSCOPE

BETWEEN CROSSED NICOLS, X 33.

The black mineral in the upper right-hand quadrant is lepidomelane, whose arms include several grains of nephelite. 

In the lower left-hand quadrant are two other grains of the same mineral. !n the extreme lower right-hand corner is 

part of an albite grain. The rest of the figure shows the fine-grained feldspathic mosaic. The twinning lainf>llae are 

very plain in several of the grains, and in one, in the upper left-hand quadrant, the lamellae disappear to the left of the 

grain. Several pieces of cancrinite are also discernible in this portion of the section, but in the photograph they 

can not be distinguished from the untwinned feldspar.
DILLBK.] .DESCRIPTIONS: NO. 77, NEPHELITE-SYENITE. 205

consequently the acute bisectrix, which, in the biotites, is nearly coinciclent Avith the crystallographic axis c, is the axis of greatest elasticity, 

a. In cross sections the pleochroisin is very pronounced. The ray 

vibrating at right angles to the cleavage is a bright yellowish green, 

while that vibrating parallel to it is nearly all absorbed. The scheme 

for the absorption is therefore a < b = c. The extinction in these sections, as measured against the cleavage, varies from 0° to 1°. In its 

optical properties, as well as in its chemical nature, the mica corresponds 

to biotite.

The largest of the colorless components is that clouded with opaque 

substance. This is present with rudely rectangular cross-sections, in 

which a single cleavage may be detected, parallel to which is the extinction. The relief is slight, consequently the index of refraction of the 

substance is low. Its opacity in places is due to the crowding together 

of little glass and fluid inclusions, in some of the latter of which are 

tiny, movable bubbles, and of small flakes of a brilliantly polarizing 

micaceous mineral that are visible only between crossed nicols, and are 

probably secondary in origin. The only other inclusions noticed are 

narrow flakes of lepidomelane. These are arranged with their long axes 

in parallel directions, that are likewise parallel to the extinction plane 

of their host. A few of the rectangular sections when revolved between 

crossed nicols remain dark during the entire revolution. These in converged light show a imiaxial interference figure. All other sections 

polarize in gray or bluish-gray tints, and these tested by the mica plate 

are discovered to be negative. If the slide be uncovered and treated 

with hydrochloric acid, and then washed and dipped in a dilute solution 

of aniline purple, the areas occupied by this mineral will be found to 

be stained purple, indicating the presence of gelatinous silica. All 

these properties are those of nephelite.

The most abundant of the feldspars is the albite occurring in the 

columnar forms already mentioned. In the thin section it possesses 

long quadrangular outlines. Its grains are characterized by a series 

of remarkably fine twinning lamelhe that bend and curve, disappear 

suddenly at cleavage cracks, and reappear again in other parts of the 

grains phenomena indicating that the mineral exhibiting them has at 

some time been subjected to great pressure. A close inspection of the 

grains will disclose the fact that some of them are made up of very fine 

lamelhe of different feldspars, in which the extinction is different. 

Others consist partly of laineike in which the gridiron structure of 

rnicrocline is plainly apparent. The resemblance of such sections to 

the pictures of cryptoperthite and mlcrocline-inicroperthite l is so strong 

as to suggest the probability of their being, like the latter, intergrowths of two feldspars. A feature well worthy of notice in connection with the singly striated grains is the fact that they are penetrated 

in all directions by jagged einbayinents of a pellucid plagioclase with 

broader twinning lamelhe than those of the rest of the grain. Small

1 W. C. Brugger, Zoitschr. fur Krystallograpbie, Vol. XVI, JL'l. XXII, Ug. 3, ami PL XXIII, flg. 4.
206 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [BULL. 150.

areas of this glassy feldspar occur all through the albites, so that the 

latter appear to be completely saturated with it. The clear feldspar 

polarizes in gray and blue tints and always has ragged outlines against 

the inclosing albite. From the fact that the saturating material is so 

much fresher than the material of the large grains, and because of its 

peculiar saturating character, it must be regarded as probably younger 

than the albite which it penetrates and as having been formed after 

the rock had consolidated.

By revolution of the slide between crossed nicols the transparent, 

colorless groundrnass in which the mica and the cloudy albite and 

nephelite are embedded is found to break up into a mosaic of very 

brilliantly polarizing grains all of about the same size. This mosaic 

occupies all the space between the large grains and is so distributed as 

to appear to fill what were at some time fissures in the rock. (Of. fig. 

A, PI. XXVIIf. 1 ) The greater number of tbe grains in the mosaic are 

feldspar. Some are marked by a single set of twinning bars, others by 

two sets crossing each other at nearly right angles, and still others are 

untwinned. The large number of the latter noticed is an indication of 

the presence of orthoclase, though this can not be proved on account 

of the lack of cleavage lines, and of crystallographic contours in the 

grains. Those showing two sets of twinning lamellae are probably 

microcline. The larger proportion of grains are of the first kind, but 

their lamellae are so bent and bowed that their extinction angles can not 

be read with any degree of accuracy. The only method by which a 

knowledge of the nature of the various feldspars may be obtained is by 

their separation and analysis, and a comparison of the figures thus 

obtained with those indicating the composition of the rock as a whole. 

By use of the Thoulet solution, it will be found that two lots of feldspar 

fall when the density of the solution reaches 2.622 and 2.56, respectively. 

That which falls at 2.622 consists of grains usually striated in a single 

.direction, and others in which no striations are visible. The latter 

extinguish at 19° from the cleavage, and show in converged light the 

bar of an axial figure. An analysis of some of this powder made by 

Mr. W. H. Melville gave:

Analysis of albite of nephelitc-syenite.

SiOs.. ...........................................

AltOt- ...........................................

FeO..... .........................................

PiO

MgO.. ...... .....................................

K20.... ..........................................

"NTa n

TT f\

Total ......................................

Per cent.

68.28

19.62

.23

01

.09

.39

10.81

no .09

99.82

1 The structure produced by the embedding of the larger components of a rock in a finer grained 

aggregate, has been termed the "mortar " structure by Tornebohm. It is thought to be the result of 

pressure. .
DESCRIPTIONS: NO. 77, NEPHELITE-SYENITE. 207

This is the composition of a very pure albite. In a separation made 

by the writer, the powder whose density was 2.56 comprised some 

untwinned grains, arid many with the cross twinning of microcline. 

Its analysis gave the figures:

Analysis of niici'ocline and orthoclane from nepheltte-si/vuUe.

Percent.

65.14

18.19

SiO, ..........................................

A1A .........................................

FeO...........................................

CaO...........................................

MgO............................................. .16

K2o.............:................................ 14.14

1.08 

.17

99.82

H,,0.

Total

This powder was thus a mixture of microcline with a little orthoclase.

It is very evident that the feldspar grains of the mosaic are younger 

than the eleolite and the large grains of albite. Their smaller size, 

perfect transparency, lack of cleavage lines, and their method of occurrence, in narrow stringers and small areas between the undoubted 

primary constituents, point to a secondary origin for them. The cause 

of the production of this new feldspar in the rock, which had already 

consolidated and been fissured before the formation of the mosaic, was 

probably the pressure to which it was subjected at some time in its 

history, and of the action of which we have ample proof in the bending 

of the twinning lamella? of the feldspars and in the "mortar" structure 

of the rock itself.J

In addition to the feldspars in the mosaic there is also present in it 

another brilliantly polarizing substance forming prismatic and lenticular grains. In natural light it is indistinguishable from the newer 

feldspar, except in very thick sections, where it has a slightly yellowish 

tinge. It is transparent and free from inclusions of all kinds, save 

little liquid ones, inclosing movable bubbles. Two cleavages cross 

nearly all its grains at right angles to each other, and the extinction is 

parallel to these. Sections that remain dark between crossed nicols 

show a uniaxial negative interference figure. The index of refraction 

is so low that grains have no relief. These properties sufficiently characterize the mineral as cancrinite. It is older than the other constituents of the mosaic, but is younger than the eleolite and the albite of 

the larger grains.

A few other grains in the mosaic and in the neighborhood of the 

nephelite remain completely dark in all positions between crossed

1 For a discussion of the origin of secondary minerals in dynamically metamorphosed rocks, see 

C. Callaway, Quart. Jour. Geol. Soc. London, Aug., 1889, p. 475, and G. H. Williams, Bull. IT. S. <!eol. 

Survey No. 62.
208 THE EDUCATIONAL SEKIES OF ROCK SPECIMENS.

nicols, and when examined in converged light show no axial figures. 

These are sodalite. In ordinary light the mineral has a very pale-blue 

tint that can be recognized only by the contrast afforded by the colorless minerals in the field of view, -which appear to be tinged with 

yellow.

The inclusions most abundant in it are grains of plagioclase, small 

plates of lepidomelane, cancrinite, and a few flakes of a brightly polarizing micaceous substance. ISTephelite is often intergrowu with it in 

such a way that a large number of apparently isolated areas of the 

former mineral polarize together. Since the sodalite includes all the 

minerals of the rock except nephelite, even those that are younger than 

this,, and since it is intergrowu with the latter, it must be an alteration 

product of it. It is the youngest of all the constituents. Not only is 

it present in irregular grains that include small particles of the other 

components, but it is found also as a cement binding together the 

grains of the mosaic.

The only remaining mineral to be spoken of is zircon, but this is so 

rarely seen in thin section that its discussion may be dismissed with a 

very few words. It occurs as small irregular grains, of a brownishyellow color, with a very high index of refraction, and strong double 

refraction. No cleavage cracks cross them, nor are crystal contours 

sufficiently sharply marked to aid in the determination of the extinction. Occasionally a fragment remains dark during its revolution 

between crossed nicols. This may be made to show a uniaxial figure, 

which, when tested, is found to have a positive character.

The composition of the rock as a whole as determined by Mr. L. Cr. 

Eakiiis, is as follows:

Analysis of nephelite-syenite from Kennebev County Maine.

SiO* .............................................

A.ltO3 ...... ............. .........................

FeA ............................................

FoO

MnO. ............................................

CaO. .............................................

MgO....... ......................................

K2 0....... .......................................

ITazO. ............................................

H20.-.. ..........................................

C02 .. ............................................

Per cent.

CO. 3D

22.5!

.42 

2.20

.08

.32

.13

4.77

8.44

.57

The small quantity of K2O as compared with the large amount of 

Na2O present would indicate a scarcity of orthoclase and an abun- 

,dance of albite among the feldspars. A calculation of the proportions 

of the different constituents based upon the analyses given leads to the 

figures: 7 per cent of lepidomelane, 2 per cent of caucrinite, 17 per
DILLER.] DESCRIPTIONS: NO. 78, NEPHELITE-SYENITE. 209

cent of nephelite, 27 per cent of orthoclase and microcline, and 47 

per cent of albite. No other feldspars are present than albite, orthoclase, and microcline.

The rock from Litchfield is thus a granitic aggregate of the essential 

constituents, lepidomelane, uephelite, and albite, and the accessories 

zircon, cancrinite, sodalite, albite, orthoclase, and microcline. It has 

been shattered and fissured as the result of the action upon it of great 

pressure, and the crevices thus formed have been filled with new feldspar and other minerals. As its original structure was granitic, and 

its original components are nephelite, biotite, and an acid feldspar, the 

rock must be classed with the nephelite-syenites. But as the acid 

feldspar is largely albite, whereas in normal nephelite-syenites it is 

principally orthoclase, the Maine rock represents a well-marked variety 

in the nephelite-syenite group, a variety that has been given the 

descriptive name litchfieldite. 1

Most of the eleolite-syeuites that have been described from North 

America are normal phases, containing large quantities of orthoclase, 

and, in addition, pyroxene, amphibole, and sphene. For descriptions 

of these the reader is referred to the original articles.2

No. 78. NEPHELITE-SYENITE (ELEOLITE-SYENITE).

(FROM BEEMERVILLE, SUSSEX COUNTY, NEW JERSEY. DESCRIBED BY J. P.

IDDINGS.)

The nephelite-syeuite from Beemerville, New Jersey, is a medium- to 

coarse-grained, dark-gray rock, with a somewhat greasy luster. Its 

texture varies considerably from a fine-graiiu d, evenly-granular mass 

to a coarse-grained one, with prominent feldspars, which present long, 

narrow sections on the surface of the rock. Tlie rock forms a dike cutting Hudson Eiver shales, and has been described in detail by Prof. 

B. K. Emerson. 11 In places the rock contains 90 per cent of nephelite. 

Under tbe microscope it is seen to consist of nephelite, orthoclase, 

aegirite, biotite, with melanite, spheue, apatite, and zircon in smaller 

amounts and in quite variable proportions. In fact the mineral composition differs considerably in different parts of the mass.

In general nephelite (eleolite) appears to have crystallized before the 

feldspar, when the nephelite is idioinorphic or is inclosed within the 

feldspar in irregularly-shaped crystals. Its substance is quite fresh, 

with little or no indication of decomposition. In some places there

> Bull. Geol. Soc. America, Vol. Ill, p. 243.

"JR. K. Emerson, On a great dike of foyaite or eleolite-syeuite, cutting the Hudson Eiver shales in 

northwestern New Jersey: Am. Jour. Sci.,3d series, Vol. XXIII, 1882, p. 302. Lacroix, Sur la sy6nite 

eleolithique de Montreal (Canada): Comptes Rendus, ex. 1890, p. 1152. J. F. Williams, The igneous 

rocks of Arkansas: Ann. Kept. Geol. Survey Arkansas, 1890, Vol. II, p. 129 et seq. W. S. Bayley, 

The eleolite-syenite of Litchfield, Maine, and Hawes' hornblende-syenite from Eed Hill, New Hampshire: Bull. Geol. Soc. America, Vol. Ill, 1892, pp. 231-252.

3B. TL Emerson, On a great dike of foyaite or eleolito-syenite, cutting Hudson Kiver shales in 

northwestern Kew Jersey: Am. Jour. Sci., 3d series, Vol. XXIII, 1882, pp. 302-308.

Bull. 150 H
210 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

are minute irregular crystals of a transparent mineral, with strong 

double refraction and lower single refraction than nephelite; this is 

probably cancrinite, and may be secondary. The prismatic cleavage is 

somewhat developed, but not marked. Its index of refraction being 

higher than that of orthoclase, the nephelite stands out in distinct 

relief when in this mineral. The double refraction for both minerals 

ranges from zero to grayish white of the first order, and is not a ineaus 

of distinction between thorn.

The feldspar appears to be wholly orthoclase, in some cases exhibiting microperthite intergrowth with plagioclase, presumably albite. 

The outline of the feldspar crystals is generally allotriomorphic. 

Twinning according to the Carlsbad law is common, that according to 

Baveuo law also occurs, in simple twins, and also in crossed twins, 

furnishing cross sections with triangular quadrants, of which the opposite pairs have like orientation. The ordinary cleavage is often distinct. A slight decomposition has produced a cloudy in deter mi Liable 

alteration product in some cases. Jn general the feldspars are less 

fresh than .the nephelite. Both of these minerals are traversed by 

veins filled with an isotropic medium, probably sodalite. JEgirite 

occurs in crystals with irregular outlines, sometimes better defined iii 

the prismatic zone. The color is dark green with pleochroism to 

brown. Cleavage is pronounced. The large crystals of aegirite inclose 

most all of the other rock constituents in small crystals or grains; that 

is, orthoclase and nephelite, and more often sphene, biotite, inelanite, 

and magnetite. In some places all of these minerals are so intermingled as to appear to be contemporaneous in growth.

Biotite forms irregular crystals, with dark reddish-brown color and 

strong absorption. It may inclose spheue, tegirite, magnetite, apatite, 

and zircon. Melanite forms irregular grains, with a dark-brown color, 

high refraction, and isotropic character. It is not uniformly disseminated through the rock. It incloses aegirite, magnetite, and sphene, 

and appears intergrown with feldspar and nephelite to some extent.

Sphene is generally idiomorphic, and is abundant. It occurs in most 

all the other constituents in well-defined crystals, but is sometimes 

allotriomorphic with respect to tegirite and biotite, and occasionally 

incloses small crystals of cegirite. Twinning is frequently observed.

Apatite is abundant in more or less rounded, short, stout crystals, 

associated with the ferromagnesian minerals. It is sometimes in sharp 

crystals. Magnetite forms irregularly shaped grains intimately connected with the dark colored minerals. Zircon occurs in minute crystals, sometimes short and stout and rounded, sometimes long and 

narrow.

On account of the variability of this rock in texture and in mineral 

composition, it will be found that the specimens in the collection differ 

considerably in both these respects. The largest of the feldspars, 

according to Professor Emerson, are 30mm in length. In some parts 

of the rock he estimated the percentage of nephelite present at 90.
DESCRIPTIONS: NO. 79, ANDESITIC TUFF. 211

The chemical composition of the average rock is given in the accompanying analysis, which was made by L. Or. Ealdus.

Analysis of 11 epli elite-syenite.

Si02 .............................................

AI n

j<v>3 - - - 

TeO .............................................

MnO........ .....................................

MO....:.......................................-.

MgO.... .........................................

'KfO. .............................................

Na-jO. ............................................

HaO... ...........................................

rirv

Total ....................................

Per cent.

53.56

2.19

1.22

.10

'l. 24

.31

9.50

6.48

.93

DIORITE-ANDESITE FAMILY.

No. 79. ANDESITIC TUFF.*

(FROM STILLWATER CREEK (NEAR RINEHART'S), EIGHT MILES NORTHEAST ov REDDING, CALIFORNIA. DESCRIBED BY J. S. DILLER.)

This gray, earthy rock iu the hand specimen is not conspicuously 

fragmeutal, but if examined closely it will be found to contain lighter 

and darker colored pebbles embedded in a gray grotmdinass. In the 

cliff from which these specimens were collected it is readily seen that 

the white portions are fragments of pumice and the darker ones are 

amiesitic lava, so that the material of which the rock is composed is 

evidently of volcanic origin.

On Stillwater Creek there is a small mass of this tuff now exposed, 

and it is many miles from the nearest volcanic center. Formerly it 

was connected both eastward and westward with larger masses of tuff, 

whic.h have a wide distribution upon the borders of the Sacramento 

Valley. Upon the western side of the Sacramento Valley the material 

is very fine. Along the Stillwater it is intermediate in size, and on 

the eastern side of the valley, where the mass is very thick, it is coarse. 

Since sediments of volcanic origin are coarsest and their accumulation 

is thickest very close to the point of eruption, it is evident that the 

source of the tuff on Stillwater Creek is to be found to the eastward in 

the Lassen Peak district. Where best exposed the tuff is distinctly 

stratified, and was evidently deposited in a body of water which filled 

the Sacramento Valley.

In the hand specimen, besides the small fragments of andesite and 

pumice already mentioned, there are small, dark specks, which, when

'Instead of tuff the word tu/ais sometimes written. The former shonkl be unod for fragmental 

volcanic rocks only, and the latter for certain forms of carbonate of lime, as " calcareous tufa," deuosted from solution in water.
212 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

removed from the specimen with the point of a knife blade, crushed 

aud examined in polarized light, are found to have the cleavage, strong 

pleochroism, and inclined extension belonging to hornblende.

Grains of feldspar, too, are quite common in the hand specimen, but 

on account of the fragile nature of the matter in which they are 

embedded many of the crystals break out of the thin sections when 

the material is being ground. In the thin section the remaining feldspars are rarely idioinorphie. They are usually found to be very irregular, corroded or broken crystals, containing many glass and liquid 

inclusions. Their tabular form, zonal structure, and angle of extinction agree very closely with those of andesine from the andesites of 

the Lassen Peak region. Fragments of magnetite and of hornblende 

crystals are not common, and those of hypersthene are rare.

The gray groundmass, which constitutes at least 75 per cent of the 

tuff, is made up of minute fragments of volcanic glass. This is easily 

discovered when examined with a higher magnifying power. It is then 

seen to be composed of such curiously formed particles as are represented in fig. 15. a has the same tubulo-vesicular structure as the

Y

Fig. 15. Fragments of volcanic glass in tuff as seen under the microscope, X 50.

larger fragments, and in fact is a minute particle of pumice. Where 

thin, the glass is clear and transparent, but where thick it is slightly 

dark colored, b represents the curved wall of a broken bubble, c is a 

small vesicle, still complete aud surrounded by tubular glass, aud d is 

interstitial glass between several bubbles and is not tubular. All are 

sharp, angular particles of glass, exactly analogous to volcanic dust. 

Although the macroscopic evidence indicating that the rock is tuffaceous is strong, the microscopic evidence is still stronger, and demonstrates completely its volcanic origin.

The analysis given below shows its chemical composition, as determined by W. H. Melville:

Analysis of andesitic tuff from Slillwater Creek, California.

SKV............ .................................

A1203 ................... ..........................

POiOs--. .........................................

FeO.- ............................................

CaO........................ ......................

MgO- ............................................

K20..... .........................................

NajO..... .........................................

Total ......................................

Per cent.

69.51

15. 01

0.56

1 97

2.80

0.61

2. 81

3.43

100. 23
DILLEB.] DESCRIPTIONS: NO. 80, DACITE. 213

Tuff, in its widest sense, is applied to nil fragmental rocks composed 

of volcanic material, but it is more frequently used to designate only 

those which are composed of fine volcanic detritus, such as lapilli, 

sand, or dust. Those made of c6arse fragments, somewhat assorted 

according to size, are called volcanic conglomerates; while others, 

which are composed of large, angular blocks as well as fine material 

all intermingled, as they frequently are in the immediate vicinity of 

the volcanic vent are designated volcanic agglomerate.

Although much of .the fragmental volcanic material about volcanic 

vents is more or less rounded, forming conglomerates, there are also 

many cases in which it is sharp and angular, forming breccias. These 

result not only from accumulations of angular ejected material, but 

also, and perhaps frequently, from the breaking up or complete brecciation of brittle viscous lava at the time of its eruption,

Uses. The so-called pozzolana, of Italy, and trass, of the Eifel, in 

Germany, are tuffs, and are extensively used in the manufacture of 

hydraulic cement.

Near Paskenta, California, a tuff of the same stratum as that which 

occurs on the Stillwater is used for making water coolers. As it is 

porous it allows the water to evaporate rapidly through the sides of the 

vessel and thus cools the contents of the jar. Tuff, on account of its 

composition, is a poor conductor of heat, and does not readily crack 

when exposed to fire. On this account it is used quite extensively in 

volcanic regions for constructing chimneys. It is soft and easily cut 

into any desired shape, but, as it is easily crushed, it can not be used for 

large structures where it will be subjected to great pressure.

No. 80. DACITE.

(FttOM BEAR CREEK FALLS, SHASTA COUNTY, CALIFORNIA. DESCRIBED RY J. S.

DILLER.)

In the field this dacite is of comparatively small extent, covering 

only a few square miles. The forks of Bear Creek, in cutting their 

canyons, have severed the original mass into five smaller masses. They 

all rest upon audesitic tuff, which at the time the dacite was erupted 

was unconsolidated, so that the overflowing dacite picked up and 

included a multitude of small fragments of andesite. The flow has a 

maximum thickness of about 80 feet. Its upper portion is comparatively free from included fragments, but below it is full of them, and 

so closely resembles the tuff by which it is underlain that the line 

between the two rocks is generally indistinct. Some of the cliffs of 

this dacite along Bear Creek exhibit a remarkably well-developed 

columnar structure.

It is 'a rough, gray rock, containing short streaks of black glass, 

approximately parallel, giving to the rock a decided fluidal structure. 

This feature is usually much more conspicuous in the rhyolites than in 

the dacites, In the field this structure is parallel to the surface, and
214 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BUM.ISO.

is plainly seen to have been produced by the flowing of the lava at the 

time of its eruption.

In the hand specimen it may be observed that the larger streaks of 

black glass are distinctly perlitic, and sprinkled with phenocrysts of 

feldspar. The gray portion is somewhat mottled, and envelops occasionally small fragments of andesite picked up at the time the lava was 

effused.

Under the microscope, as seen in PI. XXX, the rock becomes plainly 

porphyritic, with angular fragments of feldspar, hypersthene, hornblende, and quartz embedded in a light-colored groundmass of glass, in 

which the fluidal structure is well displayed.

The feldspar occurs in irregular tabular crystals and fragments of 

crystals, which generally show the twinning lamella? and angle of symmetric extinction belonging to andesine and labradorite. Polysynthetic twinning occurs according to both albite and pericline laws, but 

the former is much more frequent than the latter, and they are generally combined. The angle of extinction of cleavage plates on M and P 

ranges from zero to 20°, and lies chiefly about 13°, indicating labradorite.

There is a small amount of sanidine present, indicated in part by the 

absence of twinning lamellre, but chiefly by the low specific gravity, 

2.50, and the presence of considerable potassium, as shown by the chemical analysis of the rock. Only a portion of the potassium is to be relegated to sanidine, for the greater portion, as has been shown by 

chemical tests, is within the glassy groundmass.

The feldspar contains numerous liquid and glass inclusions, besides 

a smaller amount of magnetite, quartz, pyroxene, and hornblende, which 

crystallized at an earlier stage than the feldspar in the solidification of 

the rock. A striking feature of the feldspar is the rarity of perfect

crystals. In form it is irregular and fragmental, affording evidence 

that the crystals were broken by the flowing viscous lava during its 

eruptions. Feldspar is more abundant than all the other minerals 

combined.

In ferromagnesian silicates this rock is especially poor, and of these 

hypersthene and hornblende are the only ones present. Hypersthene 

is most abundant, and, like the hornblende, it occurs in irregular phenocrysts and small grains.

Quartz is rare in angular fragments and round grains, containing 

dihexahedral glass inclusions.

The glassy groundmass constitutes nearly two-thirds of the rock, 

and has a marked fluidal .structure due to irregular streaks of clear, 

colorless glass, alternating with others that are clouded. The clear 

streaks are frequently full of elongated glass cavities, as in pumice, and 

have perlitic structure, as shown in PI. XXX. The clouded ones are 

composed chiefly of glass containing a multitude of amorphous, dustlike particles, and a few irregular grains of feldspar and other minerals. 

Occasionally they contain spherulitic portions.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 F=L. XXX

THIN SECTION OF DACITE FROM BEAR CREEK, CALIFORNIA, AS SEEN UNDER A MICROSCOPE 

BETWEEN CROSSED NlCOLS, X 35.
D1I.LBK.1 DESCRIPTIONS: NO. 81, DACITE. 215

The chemical composition of the dacite, determined by E. B. Biggs, 

is as follows:

Analysis of dacite from Bear Creek Falls, Sliasta County, California.

sioa .. .................................:.........

Ti 0 a .............................................

A1203 .............................................

 Fet03 . ...........................................

FeO... ...........................................

1W-.... .......................................

MnO................................... ..........

BaO .............................................

SrO ..............................................

CaO ................................ ............

MgO..... ........................................

Li,O .............................................

5sTasO ............................................

IL20...... .............................. .........

 H£>............ ..................................

Total ......................................

Dried at, 105° C.

Percent.

68.10

0.15

15.50

3.20

A

0.03

O.OC

3.02

0.10

4.20

3.13

2.72

100.21

The small amount of iron oxide and magnesia present is due to the 

small proportion of ferromaguesian silicates. The rather large amount 

of lime present is found in the labradorite, while the soda and potassa 

are found, as in the Lasseit Peak dacite, chiefly in the groundmass.

No. 81. DACITE.

"(FROM SPRING VALLEY ROAD, EUREKA COUNTY, NEVADA. DESCRIBED BY J. P.

IDDINGS.)

This rock occurs as part of a surficial body of andesitic perlite, 

which varies in mineral composition and character, the most siliceous 

modification being dacite.

The dacite is compact, with an earthy texture and rough fracture. 

It is buff, with lumps of pink, yellow, or white tuff, and is crowded 

with phenocrysts of black biotite, small amounts of hornblende and 

pyroxene, abundant feldspars, and numerous dark-colored quartzes. 

The quartzes resemble those in the rhyolite of Pinto Peak in this collection. The character of the rock differs somewhat in different specimens. In thin section the groundmass of the rock is seen to have been 

glass, which is more or less altered and devitrified.. It exhibits characteristics of glasses that appear as welded fragments and bits of tuff, 

having a marked flow structure. It is seldom isotropic, but is faintly 

doubly refracting and in places consists of minute colorless globules. 

There is much semiopaque cloudy material of an indeterminable 

nature. Occasionally«there is distinct spherulitic crystallization, and 

in places the groundmass is microcrystalline in irregular and ill-defined
216 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

grains. The microstructure of the rock varies considerably in different 

parts of it.

The phenocrysts of feldspar are plagioclase; four-fifths of the individuals are striated, and the symmetrical extinction angles range from ; 

a few degrees to about 30°. They belong to the andesine-labradorite 

varieties. The double refraction corresponds to that of feldspars richer y 

in lime. For the most part they are in angular fragments, but some 

exhibit the customary crystal outlines. Inclusions of colorless glass in 

rectangular shapes, and with an inclosed gas bubble, are common, and ^ 

in some individuals are abundant. Clouds and streaks of dust-like 

particles prove to be made up of minute glass inclusions, 0.002mni in 

diameter, together with grains of iron oxide, magnetite, and opaque * 

rods. The streaks are often in parallel lines, whose orientation in the 

feldspar crystal is not determinable. They suggest the dust-like inclusions characteristic of the plagioclase of many gabbros. Minute crys- ) 

tals of apatite and zircon, and of the accompanying ferromagnesian 

minerals are also included. The substance of the feldspar is fresh and 

unaltered.

Quartz is much less abundant and occurs in rounded and irregular 

grains. Its substance is colorless in thin section. Glass inclusions in 

negative crystal cavities are common, and also those of groundmass. 

Other inclusions are seldom observed.

Hornblende is quite abundant in crystals and fragments. The crystals are well developed in the prismatic zone, with unit prism and 

clinopinacoid, less often the orthopinacoid. Terminal planes are rare. 

Its prismatic cleavage is characteristic. The color is brown, with a 

tinge of green; the absorption is strong, and, as usual, ,c>tY>a. It is 

free from dark border, and has no characteristic inclusions, being ingeneral quite free from them. Its substance is unaltered.

Of the pyroxenes, the orthorhombie form is more abundant than the 

moriocliriic. Hypersthene in irregularly shaped crystals is more or less 

altered to a fibrous decomposition product. The unaltered hypersthene 

exhibits a slight pleochroism in thin sections, becoming more marked 

in thicker sections. It is green parallel to the axis £, light reddish 

brown parallel to a and ft. The substance is generally free from inclusions, but sometimes bears numerous magnetite grains and glass 

inclusions, besides apatite needles and grains of hornblende. When 

altered the hypersthene passes into a fibrous green mineral, whose 

fibers are about parallel to the crystallographic c axis. The alteration 

product has the optical properties of actinolite. 1 Augite occurs sparingly in pale-green grains and crystals, and is distinguished from the 

hypersthene by its lack of pleochroism, monoclinic characters, and by 

its generally unaltered condition when near hypersthene which is more 

or less decomposed.

Biotite is the most prominent ferromagnesian mineral in the more

_ : - -Q 

1 See figs. 5 and 9, PI. Ill, Monograph XX, Geol. of the Eureka District, Washington, 1892.
DILLER.] DESCRIPTIONS: NO. 82, DACITE. 217

quartzose varieties of this rock. It forms six-sided plates, is dark 

brown in thin sections, with strong absorption. It possesses .a small 

optic angle, behaving almost as a uuiaxial mineral. It sometimes 

exhibits the common twinning. Inclusions are rare.

Magnetite, apatite, and zircon occur as subordinate minerals. The 

zircon crystals often have sharp outlines and well-developed forms, 

including the two unit prisms and pyramids, and the ditetragonal 

pyramid 3 P 3 (311). The more common forms are shown in figs. 15-20, 

PI. Ill, Monograph XX, United States Geological Survey.

The chemical composition is .probably similar to that of the dacite 

fr.om northeast of South Hill in the same district, given on page -64, 

Monograph XX. (Consult Monograph XX, U. S. Geol. Surv., pp. 236 

and 308 et seq.)

No. 82. DACITE.

(FROM LASSEN PKAK, CALIFORNIA. DESCRIBED BY J. S. DILLER.)

Lava which is characterized by lime-soda feldspars and quartz is 

dacite. It is sometimes called quartz-audesite.

Lassen Peak, like Mount Shasta, Mount Hood, and many other 

prominent peaks of the Cascade range, was once an active volcano, 

and of its newer lavas dacite forms by far the greater portion. It is 

widely distributed about the base of the peak, and at one place, known 

as Chaos, it is of so recent eruption that it still remains in its original 

extremely rough, broken, rocky condition.

It is a porphyritic gray lock, which has a rough fracture and porous 

structure. At first sight it looks somewhat like granite,1 from which, 

however, it differs essentially in containing a large amount of glassy 

base.

Among the phenocrysts embedded in the light-gray groundmass, 

hornblende and biotite are the most conspicuous. Plagioclase is most 

abundant. Qua tz is common and pyroxene scarce. Idiomorphic 

phenocrysts are rare; the crystals are nearly all rounded or broken by 

the movements of the lava at the time of its eruption.

The hornblende is black where fresh, becoming brownish by altera 

tiou. It appears occasionally in well-defined crystals, but usually its 

form is broken or irregular. Pyroxene is occasionally found grown 

upon hornblende in parallel position. Grains of magnetite are frequently included in hornblende, but the black border so common about 

the hornblende and mica in many andesites is entirely absent her*e. 

Apatite, almost black, as seen in the hand specimen, by transmitted 

light appears deep brown and strongly pleochroic. It sometimes contains inclusions of both magnetite and apatite.

The feldspar phenocrysts, as shown by Hague and Iddings, are all 

plagioclase, and belong chiefly if not wholly to audesiue. Well defined

'Baron von Richthofen, The Natural System of Volcanic Rocks, p. 10. Hague and Iddings, Am. 

Jour. Sci., 3d series, Vol. XXVI, Sept., 1883, p. 231.
218 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL. 150.

crystals are rare. The usual form is rounded or broken, and frequently 

contains numerous glass inclusions.

The quartz, when seen in thin sections, is found to be well rounded, 

and generally contains the characteristic dihexahedral glass inclusions.

The pyroxene is chiefly hypersthene, although augite is sometimes 

present. The crystals are usually so small that they belong rather 

to the groundmass than to the phenocrysts.

The gray groundmass is porous, sometimes fibrous and pumiceous, a 

leature which can be best observed in the hand specimen with a small 

lens. In the thin section it is seen to be composed chiefly of clear 

glass swarming with acicular colorless crystallites. It contains comparatively small crystals and grains of feldspar with a smaller proportion of hornblende, hypersthene, biotite, and magnetite, besides round 

or irregular dark gray felty spherulitic patches.

The chemical composition of the lava at Lassen Peak shows that it 

is a typical dacite. Analyses have been made of different flows and 

are given in the accompanying table.

Analyses of lava from Lassen Peak, California.

SiOa ...............

Ti04 .. ............

A12 03 -. .......... --

CrA ........

FeA------ - -

PeO ..........:....

MnO-... - 

CaO ...............

SrO................

EaO. ..............

MgO. ..............

K20 ...............

Na^.0 ..............

LiaO. ..............

H20 .............

PA 

Total........

1.

Per cent. 

69.51

15.75

3.34

1.71

2 OQ

3.34

3 89

0.56

100. 19

2.

Per cent. 

68.20

16.98

3.95

4.33

2 ft7

1.52

2 gg

0.44

100. 27

3.

Per cent. 

68.72

0.31

15.15

1.16

1.76

0.11

3.30

0.03

0.07

2.78

4.26

0.74

0.09

99.76

4.

Per cent. 

68.32

0.31

15.26

1.66

1 26

0.04

3.26

i Q9

9 81

4.27

1.37

0.12

100. 07

5.

Per cent. 

55.14

0.52

6.16

0.54

0.11

8

qc

0.07

4 99

1.04

3 71

/Y A1

0.18

100. 07

6.

Per cent. 

65.77

91 51

5 79

O fiQ

5 92

0 34

100 09

7.

Per cent. 

76.75

12.32

1.36

i ia

3 08

0 54

QQ fia

No. 1. Gray dacite at southeast base of Lassen Peak. Analyst, T. M. Chatard.

No. 2. Reddish dacite at northeast base of Lasseu Peak. Analyst, T. M. Chatard.

No. 3. Dacite of latest eruption at Chaos, north base of Lassen Peak. Analyst, W. F. Hillebrand.

No. 4. Gray dacite, west base of Lassen Peak, envelopes 5.

No. 5. Nodule contained in 4.

No. 6. Feldspar (Am. Jour. Sci., 3d series, Vol. XXVI, p. 232, Sept., 1883). Analyst, P. W. Shimer.

No. 7. Glass base (Am. Jour. Sci., 3d series, Vol. XXVI, p. 232, Sept., 1883). Analyst, P. W. Shimer.

In the first column is given an analysis of the material in this series. 

The composition of the feldspar, determined by Messrs. Hague and 

Iddings, is given in column 6, and that of the glass base in column 7, 

where it appears that the latter contains the chief portion of the potas-
U. S. GEOLOGICAL

NODULES IN DACITE, LASSEN PEAK, CALIFO
DILLBB.] DESCRIPTIONS: NO. 83, HORNBLENDE-MICA-ANDESITE. 219

siuni and that none of the feldspar is sanidine. It is evident also that 

the glass base contains a larger percentage of silica than the crystallized portion of the dacite. It is almost always true that in the process 

of crystallization the basic elements become mineralized with greater 

rapidity, relatively, than the silica, so that the amorphous residue in 

hypocrystalline eruptive rocks is generally richer in silica than the 

average of that portion which is crystallized.

Although the great mass of dacite at Lassen Peak contains a larger 

proportion of glass base and has attained a lower degree of crystallization than most of the other lavas in the same region, there are within the 

mass isolated nodules, as seen in PI. XXXI, that are holocrystalline, 

or contain a relatively small amount of glass base. The complete crystallization of the mass at the time of its eruption was doubtless prevented by the condition of the magma taken in connection with its

environment. The irregular holocrystalline nodules, on account of their 

higher degree of crystallization, may be regarded as the first portion of 

the mass to solidify.

In these nodules hornblende is much more abundant than in the 

dacite which envelops the nodules. It is frequently allotriomorphic 

against the feldspar, as the augite in diabase; 1 biotite and pyroxene 

are usually less abundant, and olivine is occasionally present, and some 

tridymite. The feldspar are generally lath-shaped. A chemical analysis of one of these nodules, with that of the enveloping dacite, is given 

in the table.

Similar nodules are found in many eruptive rocks, especially such as 

granite, diorite, etc., which have crystallized at a considerable depth 

beneath the earth's surface. They are generally more basic than the 

rock inclosing them, and are regarded by many petrographers, but not 

all, as the part of a once homogeneous magma, separated out in the 

process of differentiation and first to crystallize.

Dacites are sometimes holocrystalline, but frequently contain more or 

less amorphous matter. In a general way they are intermediate in the 

rock series between rhyolite and andesite. The structure and habit of 

many dacites appears most closely related to those of rhyolite, but in 

the field they are commonly found associated with andesites. In the 

Lassen Peak region the dacites and rhyolites are of approximately the 

same age, and both are younger than the andesites which they have 

overflowed.

No. 83. HORNBLENDE-MICA-ANDESITE.

(FROM HOOSAC MOUNTAIN, EUREKA COUNTY, NEVADA. DESCRIBED BY ,T. P.

IDDINGS. )

This rock occurs in a small exposure on the road northeast of Hoosac 

Mountain, Eureka County, Nevada. It appears to be a lava flow or

1 See also Hague and Iddiuga, Am. Jour. Sci., Sept., 1883, 3d aeries, Vol. XXVI, p. 234, 235j andRosenbuech, Mikroskopische Physiogr -,phie der inassigen Gesteine, 2d edition, 1887, p. G75.
220 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL-ISO.

possibly an intrusive body, its contact with the limestone not having 

been discovered. It is a dense, compact rock having a reddish-purple, 

to purplish-gray groundmass, rich in megascopic crystals of feldspar, 

hornblende, and biotite, the feldspar predominating. Pyroxene is 

wholly absent.

In thiu sections the groundmass is seen to be holoerystalliue, composed of microlites of plagioclase, largely oligoclase, in an aggregate of 

feldspar and quartz grains. These grains are nearly free from inicrolites 

at their centers. There are, besides, minute crystals of magnetite and 

opaque microlites which correspond to shreds of brown inica in some 

instances. The lath-shaped feldspars show fluidal arrangement. The 

feldspar pheuocrysts are wholly triclinic. They are finely developed 

crystals, for the most part equidimensional, yielding sharply outlined 

sections having the usual forms. Zonal structure is pronounced and is 

well shown in fig. 1? PI. V, MonograpliXX, United States Geological Survey. They sometimes exhibit evidence of successive periods of growth 

and resorption. The customary cleavage is not well developed and is 

often wanting, there being, instead, irregular cracks, as in sanidine. 

The polysyuthetic twinning after albite and pericliue is very unevenly 

developed. (This is shown by figs. 3 and 4, PI. V, and fig. 2, PL VI, 

Monograph XX.) The largest feldspars have quite irregular outlines 

and abundant narrow stria3; the medium-s:zed ones have sharp crystal 

outlines and fewer broader strife. Most individuals are also twinned 

in two parts alter the Carlsbad law (fig. 7, PI, III, Monograph XX), 

and frequently several have grown together in parallel orientation (fig. 

2, PI. VI, Monograph XX). Most of the feldspar pheuocrysts are 

labradorite or audesine; the, microlites are probably oligoclase. The 

precise character of the. grains in the groundmass is indeterminable; 

they may be in part orthoclase. The phenocrysts bear colorless glass 

inclusions and sometimes clouds and streaks of dust-like particles, 

besides occasional minute apatites and zircons.

The hornblende pheuocrysts, which are recognized by their crystal 

form and characteristic six-sided cross section, are seen in thin section 

to be wholly decomposed. The opaque black border remains, but the 

interior of the crystal, when not lost during the process of grinding the 

section, is altered to a finely fibrous, yellowish-green mineral having 

the optical properties of amphibole (actinolite). Its fibers start from 

transverse fissures and run parallel to the c axis of the original crystal 

for a short distance. They form a network, the meshes.of which are 

filled with a colorless subslance, probably opal. There are sometimes 

opaque and also transparent globulites, besides ferrite and hematite. 

In some cases the decomposed hornblende is replaced by calcite, and 

in others by chlorite.

Biotite occurs in fewer and larger crystals, tabular and six-sided in 

form, and often quite thick. The crystals are frequently twinned 

parallel to ooP (110), with O P (001) as the composition plane, the com-
DIU.BB.J DESCRIPTIONS: NO. 83, HORNBLENDE-MICA-ANDESITE. 221

mon law. Absorption strong; pleochroism marked, sections parallel 

to the base being brownish red, those inclined to it, orange, yellow, or 

green. Optic angle small. There are sometimes opaque, needle-like, 

possibly tabular inclusions arranged parallel to the six sides of the 

mica plates; also apatite and zircon iuclosures. In some cases crystals 

of plagioclase are inclosed in the pheuocrysts of biotite, and occasional 

iutergrowths of these two minerals prove their crystallization to have 

been synchronous. It should be noted that the biotite is almost perfectly fresh, while the hornblende is completely altered.

Quartz occurs in sporadic phenocrysts, rounded and often greatly 

corroded and cracked. Its substance is very pure and free from inclusions. Microscopic quartz grains form a constituent of the groundmass. Magnetite occurs in grains among the phenocrysts and as a 

microscopic constituent of the grouudniass. Apatite is well developed 

in small crystals, dusted red and orange, with noticeable absorption 

parallel to the c axis. It contains in some instances glass inclusions 

in negative crystal cavities (figs. 1, 4, 6, PI. Ill, Monograph XX). 

Zircon is a constant ingredient in small quantities.

The chemical composition, determined by K. W. Mahon, is as follows:

Analysis of hornblende-mica-andesite from Hoosa,c Mountain, Nevada.

SiOa. ............................................

Ti02 ........... ...... ................... . ff . ......

Al,0j-.. .........................................

l^03 . ...........................................

PeO.. ......... ...................................

MiiO.............. ...............................

MgO...... .......................................

CaO ........ ...................................

NnjO ............................................

K.,0.. .......................... ^.. ...............

P206 . ........ . ........... .............. ..........

Ign.. ............................................

Per cent.

07. 83

1.04

15.02

5. 10

.29

a. o?

2.40

a. 20

.26

1.11

99.38

For further details, see Monographs United States Geological Survey, 

Vol. XX, Geology of Eureka District, Nevada, pp. 233, 1464, 364, et seq.

NO. 84. HORNBLENDE-ANDESITE.

(FROM BLACK BUTT.K AT WESTERN BASE OF MOUNT SHASTA, CALIFORNIA. 

DESCRIBED BY J. S. DILLER.)

Andesites are lavas of intermediate chemical composition and are 

distinguished from the rhyolites and trachytes on the one hand and 

basalts on the other by the predominance of the soda-lime feldspar.

They are commonly divided into horublende-audesites, mica-andesites, 

augite-andesites, and hyperstheue-andesites, according to the predouii- 
222 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

nating ferromagnesian silicate. Hornblende-andesite is one in which 

the characterizing ferromagnesian silicate is hornblende.

Mount Shasta, 1 California, during, the latter part of the Tertiary 

period was an active volcano, and among the earlier lavas of its western 

slope hornblende-andesite plays an important role. The later lavas are 

hypersthene-andesites (specimen No. 87) and basalts. Where unaltered 

the hornblende-andesite is composed chiefly of a compact, light-gray 

groundmass, in which are sprinkled prominent crystals of black hornblende. Upon the surface the rock is usually altered and becomes reddish, owing to the liberation of oxide of iron in the process of alteration. 

It is a uniformly dense lava without any of the vesicular structure 

common to basaltic rocks of the same region.

Under the microscope it is seen that the gray groundmass contains 

small phenocrysts of feldspar which can not be readily detected in a 

hand specimen. They are all banded, and their angles of extinction 

indicate that they are anorthite and labradorite. In form they are 

commonly irregular but sometimes idiomorphic and nearly as broad as 

long. Their well-marked zonal structure affords an opportunity to 

observe the wide difference in angle of extinction between the outer 

and inner zones, a difference which is usually assumed to indicate that 

the central portion of the crystal is more basic than the outer zones. 

It may be sometimes observed, however, that the center and outer 

zones have the same angle of extinction, while the intermediate ones 

have a much larger angle.

The hornblende phenocrysts are larger than those of feldspar. They 

are usually deep brown, strongly dichroic, and surrounded by dark gray 

or black borders. Where «the border can be resolved it is found to be 

composed chiefly of augite and magnetite, which originated in the 

caustic action of the magma upon the hornblende at the time of the 

eruption.

The groundmass is a mat of microlites. It is composed chiefly of 

lath-shaped crystals or irregular grains of feldspar, with a smaller proportion of hypersthene, magnetite, and amorphous substance. The 

feldspars, many of which are very minute, have the small angle of 

extinction which belongs to andesine. They are occasionally arranged 

in streams about the older crystals of feldspar and hornblende. The 

minute slender crystals of hypersthene in the fresh rock are inconspicuous. They are scarcely pleochroic, between yellowish and greenish. When the rock begins to alter they become yellowish red and 

impart color to the whole mass.

Hornblende-andesite from Black Butte, at the western base of Mount 

Shasta, contains among its phenocrysts neither mica nor pyroxene, and 

is a good type. Such forms, however, are rather exceptional, for the 

most common phase of hornblende-andesite, even in that region, coni Mount Shasta a Typical Volcano, by J. S. Diller: Nat. Geog. Monograph ]ST o. 8. Published 

under the auspices of the National Geographic Society by the American Book Company.
DILLER.J NO. 83, HORNBLENDE-PYROXENE-ANDESITE. 223

tains phenocrystic hypersthene, but it is always less conspicuous than 

the hornblende. Augite, which is common in hornbleude-aridesites in 

many regions, is rarely if ever present in those of Mount Shasta. The 

same is true of mica. The groundmass, too, varies greatly; on the one 

hand it may be holocrystalline and on the other may contain an abundance of brown, glassy base, forming the principal constituent of the 

groundmass. Generally, however, there is only a trace of clear, vitreous base present. In general habit, as well as in chemical composition, the hornblende-andesite of Mount Shasta is related to .the trachytes, and on this account 1 Eosenbusch classifies them as trachytoid 

hornblende-andesites.

The following chemical analysis of specimen No. 84 was made by 

W.H.Melville:

Analysis of hornbleiide-andcsite, from western base of Mount Shasta, California,

AIA ............................... .............

Fe2O3 ....... .....................................

FeO .............................................

CaO... ...........................................

MgO.......... ...................................

KaO............................ '-........-.......

Na20 ................................... ........

Per cent.

0.06

04.48

19 28

1.40

1.78

5.06

1 64

1.12

4.41

on 90

No. 85. HORNBLENDE-PYROXENE-ANDESITE.

(FROM NEAK THE COMSTOCK LODE, A7IRGINIA ClTY, NEVADA. DESCRIBED BY

J. P. IDDINGS.)

This rock, like the pyroxene-andesite from Virginia City, Nevada 

(specimen No. 88), forms massive volcanic lavas whose occurrence has 

been described by the authors mentioned in connection with the 

description of that rock (q. v.). It is dense and dark gray, with a 

rough fracture, an aphanitic groundmass carrying numerous small 

crystals of feldspar with brilliant cleavage planes. In thin sections it 

.is seen to be partly altered, the ferroinagnesian minerals, hornblende 

and pyroxene, being almost wholly changed.

It consists of a holocrystalliue groundmass with abundant phenocrysts of plagioclase and few of altered ferromagnesian minerals. The 

plagioclase phenocrysts are fresh and unaltered, and exhibit zonal 

structure and polysynthetic twining according to the albite law, besides 

Carlsbad twinning. The symmetrical extinction angles are those of 

labradorite. The inclusions are minute rectangular glass ones, in some

IMikroskopische Physiographic, 3d edition, 1896, Vol. II, p. 888. See also paper by Hague and 

Iddiugs, Am. Jour. Sci., Sept., 1883, Vol. XXVI, pp. 222-235.
224 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

cases -devitrified, and opaque grains and crystals, besides "colorless 

apatite prisms. There is also included in some cases a highly doubly 

refracting mineral in flakes and films, which is possibly caloite. There' 

are occasional rounded grains which are probably augite, and opaque 

magnetite grains.

Hornblende, which can be recognized by its forms in cross section, 

is altered to an aggregation of opaque grains (magnetite), and colorless mica (muscovite), and yellow, strongly refracting grains (epidote), 

green chlorite, and calcite. In some cases the hornblende is largely 

replaced by calcite. These pseudomorphous aggregations sometimes 

inclose small plagioclase feldspars and apatites, which were undoubtedly 

originally inclosed in the hornblende.. Pyroxene, which is recognized 

by the shapes of its sections, is altered to chlorite with seams of calcite 

and some quartz and numerous grains of magnetite, which latter mineral may have been originally inclosed in the pyroxene. Epidote also 

occurs in these pseudoraorphs. The chlorite is quite uniformly oriented 

and sometimes exhibits marked fibration and cleavage, like that of 

bastite. It is somewhat pleochroic, being green parallel to the fibers, 

and yellow for light vibrating at right angles to them. A few small 

phenocrysts of unaltered pale-green augite were found in the thin 

sections of this rock. One showed the customary twinning parallel to 

the orthopinacoid (100).

Microscopic phenocrysts of magnetite and apatite occur associated 

together in groups. The apatite is in the stout, colorless prisms usually 

found in andesites.

The groundmass is holocrystalline, both microcrystalline granular 

and microlitic. It consists of micropoikilitic quartz, feldspar microlites, 

and magnetite grains, besides chlorite and some other secondary minerals. The micropoikilitic quartz is primary, having formed as the last 

act of crystallization of the molten magma. The microscopic anhedrOMS 

of quartz inclose microlites of feldspar and other minerals. The larger 

microscopic feldspars are lath-shaped plagioclases. The magnetite 

grains and crystals are abundant. There are scattered through the 

groundmass small patches of calcite and aggregations of secondary 

quartz, in which the calcite occurs as minute rhombohedrous. Chlorite 

forms pseudomorphs after microscopic pyroxenes, and also occurs in 

spherical aggregates.1

No. 86. HYPERSTHENE-ANDESITE.

(FROM NORTHEAST SHOULDER OF BUFFALO PEAK, PARK COUNTY, COLORADO. 

DESCRIBED BY WHITMAN CROSS.)

Occurrence. Buffalo Peak (elevation, 13,541 feet) lies between South 

Park and the Arkansas Eiver, a little southeast of the south end of 

the Mosquito Eange.

'Further information concerning this rock may be found in Mou. TJ. S. Gool. Survey, Vol. Ill, 

pp. 53-62, and Bull. U. S. Geol. Survey No. 17, pp. 22 and 23.
PILLBB.] DESCRIPTIONS: NO. 86, HYPERSTHENE-ANDESITE. 225

It is a double pointed mountain whose base consists of easterly dipping Carboniferous strata, while the upper part is made up of horizontal 

beds of audesitic tuff', capped by audesitic lava flows. The summit 

sheet is .a hornblende-andesite containing some hypersthene, and the 

northeastern shoulder is made up chiefly of the dark hyperstheneandesite of this collection. The geology of the region has not been 

worked out in detail, but the mountain was visited in the summer of 

1880 by members of the Eocky Mountain Division of the Survey, then 

engaged in the study of the adjacent Mosquito Range.

General description. This hypersthene-aiidesite is a dark, almost 

black, rock, exhibiting to the eye many very small white feldspar crystals embedded in a black groundmass having a dull vitreous luster. 

On careful examination a number of dull green grains and small prisms 

may be distinguished, which the microscope shows to be hypersthene 

or augite. Glistening particles of magnetite may also be detected.

The rock varies somewhat in texture, but the specimens collected 

show a large number of the white or clear glassy feldspar phenocrysts 

which the microscope proves to be labradorite. In other parts of the 

mass the groundmass is more prominent and more clearly vitreous.

The phenocrysts. Microscopical examination reveals a lime-soda feldspar, hypersthene, augite, and magnetite as very distinct phenocrysts, 

in a groundmass which has a glassy base holding numerous augite and 

plagioclase microlites and magnetite particles. The pheuocrysts equal 

or exceed the groundmass in quantity. All are very small, few surpassing 2im" in diameter.

Plagioclase is probably developed in several varieties, but the chief 

one is certainly labradorite. The crystals vary much in size and form, 

in number and character of inclusions, and in twinning. The larger 

crystals are usually clouded by many original brown glass inclusions, 

now often devitrified, and obscured by an opaque ferritic dust. These 

inclusions are often connected and occupy more than half the space of 

the crystal. Zonal extinction is almost universal in the plagioclase, 

but does not always represent great difference in optical character. 

Various twinning laws have been noted, the common albitic law being 

accompanied by the Carlsbad, pericline, and other laws not determined.

A gradation in size occurs between the largest phenocrysts and the 

grouudmass microlites, and these intermediate crystals usually carry 

few inclusions.

Both hypersthene and augite are present, as phenocrysts of very 

similar development, but hypersthene greatly predominates. The 

pyroxenes occur in prisms usually two or three times as long as they 

are thick, but of variable size. The largest are, however, only 3 or 

4inm in length.

The hypersthene of this rock has been determined chemically and 

optically, and it is typical of the hypersthene now known to be a very 

frequent constituent of basic andesites the world over. The pleochroism 

Bull. 150 15
226 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL. 150.

of the hypersthene, while not very strong, is sufficient to distinguish it 

from augite, and the parallel extinction of the orthorhombic prisms is 

also a very commonly applicable test. The augite is pale green in thin 

sections and not Visibly pleochroic. The hypersthene gives & = a reddish brown, b = & reddish yellow, .c = c green, almost identical with 

that of the augite. In the thicker prisms the colors are quite strong. 

Cleavage is ordinarily not very markedly developed parallel to either 

piuacoid of the hypersthene. Sections parallel to the macropinacoid 

show that a is the acute bisectrix, but the optic angle is not very small. 

The hypersthene of the first specimen of this andesite collected at 

Buffalo Peak was isolated from the augite by Dr. Hillebraud through 

continued treatment of the rock powder with strong hydrofluoric acid, 

which attacks augite much easier than hyperstheue. The purest material isolated was found on microscopical examination to be almost free 

from augite, and the analysis yielded Dr. Hillebrand the following 

result:

Analysis of liyperstkene of hypersthene-andcsite from Buffalo Peak, Colorado.

Si02 ................................................

AljOs ............................................

Te.,0, .................... ..........i. ............

FeO ..............................................

MnO.. ...........................................

P'if»

MgO...... .......................................

Total ......................................

Per cent.

. 51.70

1.72

17.99

0.36

2 87

25.09

100. 03

This analysis, made in 1882, is very similar to many that have been 

made since that time of hypersthene from other rocks. It appears that 

andesitic hypersthene commonly contains magnesia and ferrous oxide 

in nearly equal amounts, but varieties richer in magnesia certainly 

occur in some cases.

The larger magnetite grains are to be considered as phenocrysts. 

They are frequently iucluded in the pyroxenes, which are free from the 

more minute specks of iron ore characterizing the groundmass.

The groundmass consists of an almost colorless glass base, appearing 

brownish by low powers through globulitic specks, holding numerous 

short rnicrolites of plagioclase and augite, and very minute magnetite 

grains. No hypersthene could be found among the minute prisms of 

the groundmass.
DESCRIPTIONS: NO. 87, HYPERSTHENE-ANDESITE. 227

Chemical composition. The following analysis of the fresh rock was 

made by W. F. Hillebrand:

Analysis of hypcratlivne-aiidesitefrom Buffalo Peak, .Colorado.

SiOi .............................................

Al,08 ................... .........................

FoA...... ........ ..............................

FeO .............................................

MnO. ............................................

CaO .............................................

J3aO .............................................

SrO.. ............................................

MgO..................... ........................

NajO ...... ......................................

K20. .............................................

HjO. .............................................

PA------ .......................................

Cl ...............................................

Sp. gr. at 16° C., 2.742.

Per cent.

56.19

1C. 12

4 no

4.43

7.00

4 60

9 on

1.03

.27

.02

Q'l 01

The percentages of lime and magnesia explain the development of 

hypersthene. The potash is so high as to indicate that if the rock had 

completely crystallized there must have been a considerable amount of 

orthoclase in the grouudtnass, as this alkali does not normally enter 

into any of the phenocrystic minerals.

Literature. An outline of the geology of Buffalo Peaks and descriptions of the hypersthene-audesite were given in Bulletin No. 1, United 

States Geological Survey, On Hypersthene-andesite and on Triclinic 

Pyroxene in Augitic Rocks, by Whitman Cross, with a geological sketch 

of Buffalo Peaks, Colorado, by S. F. Emmons, 1883. The " triclinic" 

pyroxene referred to was augite, cut in sections nearly normal to the 

prism, and the erroneous determination was retracted in a note in the 

American Journal of Science, 3d ser., Vol. XXVI, 1883, p. 76.

No. 87. HYPBESTHENE-ANDESITE.

(FROM WEST SLOPE OF MOUNT SHASTA, CALIFORNIA. DESCRIBED BY J. S. DILLER.)

The later lavas of Mount Shasta are chiefly hypersthene-audesite, 

and this specimen, No. 87, collected at Horse Camp, near the timber line, 

upon the western slope of Mount Shasta, represents one of the earlier 

of the late flows. The final flows upon the same slope are much darker 

colored and more basaltic in appearance.

The lava illustrated by specimen 87 is a compact, even-grained nonporphyritic rock, of a light-gray color, and contains only a few small 

crystals of pyroxene, which are visible to the naked eye.

In thin section, however, it becomes conspicuously porphyritic, as
228 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

illustrated in PLXXXH, whore the numerous square and brick-shaped 

crystals of feldspar and grains of hyperstheue are seen distributed 

through a light-gray ground mass.

The feldspar is clear and colorless, showing polysynthetic twinning 

and zones of growth. It is all plagioclase, apparently, and the angle of 

extinction, as well as its composition, judging from the chemical analyses 

below, indicates that it is labradorite.

The chemical analyses were made by W. H. Melville. For the purpose of comparison, a chemical analysis of the later, dark-colored, more 

basnltic flow is given. In column I is an analysis of specimen 87, and 

in II is an analysis of a specimen taken from the latest flow. 1

Analyses of hypersthenc-andesite from Mount Sha8ta, California.

Si02 .............. .....................

Al 0-

Ye,03 . .................................

FAO

CaO...... ..............................

MgO.. .................................

K,0.. ...J.. ...................... ......

' Total............................

I.

0.20

04.52

ia 01

0.90

2.51

5.11

2.35

1.25

99.79

II.

0.44

63.03

17 79

2.27

1 09

3.63

1.06

3; 92

99.96

Hypersthene occurs in irregular grains and oblong crystals, like 

those in the hypersthene-audesite on Buffalo Peak, Colorado. As it is 

sometimes included in the feldspar, some of the hypersthene must have 

crystallized before the feldspar. Occasionally dark spots are found 

to be composed chiefly of magnetite and pyroxene, and suggest the 

former presence of hornblende.

The groundmass contains much glass, clouded by a multitude of 

feldspar microlites and minute grains of pyroxene and magnetite.

No. 88. OLIVINE-BEARING PYROXENE-ANDESITE.

(FKOM NEAK THE COMSTOCK LODE, VIRGINIA ClTY, NEVADA. DESCRIBED BY

 J. P. IDDINGS.)

This rock forms a massive lava in the neighborhood of the Comstock 

Lode, Virginia City, Nevada. The rocks of this region have been 

described by Mr. George F. Becker,2 Prof. F. Zirkel,3 and Messrs. 

Hague and Iddings. 4 They form a great series of volcanic eruptions, 

the history of which has been variously given by the authors quoted. 

The pyroxeue-audesite in this educational series is one of these volcanic 

lavas.

1 See also Hague.and Iddings paper in Am. Jour. Sci., 3d ser., Vol. XXVI, Sept., 1883, pp. 222-235. 

. 2 G. -F. Becker, Geology of the Comstock Lode, etc.: Mon. U. S. Geol. Survey, Vol. Ill,-1882. 

»F. Zirkel, Microscopical Petrography: U. S. Geol. Expl. Fortieth Parallel, Vol. VI, 1876. 

4 A. Hague and J. P. Iddings, The Development of Crystallization in the Igneous Rocks of Washoe, 

Nevada, etc.: Bull. U. S. Geol. Survey No. 17,1885.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXXII

THIN SECTION OF HYPERSTHENE-ANDESITE FROM MOUNT SHASTA, CALIFORNIA, 

AS SEEN UNDER A MICROSCOPE, X 40.
DILLBE.] DESCRIPTIONS: NO. 88, PYROXENE-ANDE8ITE. 229

The rock is dense and grayish black^ with, an uneven fracture. It 

sparkles with minute crystals that crowd a dark aphaiiitic groundmass. 

The megascopic crystals are feldspars with brilliant cleavage faces, 

arid dark-colored minerals, more or less brilliant, which are pyroxenes. 

The mineral composition of the rock, however, can be discovered only 

in thin sections with the aid of the microscope. The rock is then seen 

to consist of a brown, glassy, and microlitic grouudmass, bearing innumorable'small phenocrysts of plagioclase feldspar, hypersthene, augite, 

altered divine, and a few anhedrons of quartz.

'J he phenocrysts of feldspar exhibit marked zonal structure and 

polysynthetic twinning according to albite, .pericline, and Carlsbad 

laws. Cross sections-are rectangular, and also show evidences of prismatic and domnl faces. The symmetrical extinction angles read from 

Carlsbad and albite twins, and, interpreted according to the method 

employed by Michel Levy, 1 show that the feldspars are labradorite of 

about the composition Ab2Ann . The feldspar is perfectly fresh, and 

carries fine glass inclusions, besides others of magnetite, rounded grains 

of pyroxene (both augite and hypcrsthene), apatite, and portions of 

the gronndmass. There are also minute rectangular inclusions, with 

clear, colorless margins and dusted centers. They have one orientation 

in one feldspar, and are apparently parallel to the crystallographic 

axes. Some are comparatively large, others very minute. In some 

cases they appear isotropic, in others doubly refracting, and resemble 

feldspar.

Jn one feldspar crystal there was observed a large inclusion of brown 

globulitic glass, containing curved lines of opaque grains and rods 

that stand at right angles to the lines of opaque grains. In some cases 

these cross rods appear to be transparent prisms, probably pyroxene. 

These appear to be connected with hair-like needles of pyroxene, and 

larger needles of augite are present, studded with similar opaque rods 

and grains, and also with crystals of magnetite. The glass base immediately surrounding these black grains is colorless for a short distar.ee, 

the brown coloring matter having been concentrated in the augite 

needles and opaque grains. Along the margin of the feldspar surrounding the brown glass inclusion are projecting crystals of feldspar, 

like the teeth of a saw. They have lower refraction than the main 

crystal of feldspar, and also lower angle of extinction, so that they are 

undoubtedly more alkaline, and may be of the same substance as the 

minute rectangular inclusions which have just been mentioned, which 

are sometimes doubly refracting.

The pyroxene is hypersthene and augite in nearly equal proportions, 

with possibly more augite than hypersthene. The two minerals appear 

almost identical in thin sections, and are easily confused with one 

another. The hypersthene has slight pleochroism between reddish and 

greenish, while the augite is not pleochroic and is pale green. The 

characteristic prismatic and piuacoidal cleavage is less well developed

1 A. Michel L6vy, fitude sur la deterniiuation cles fcldspatha dans lea plaques mincea, Paris, 1890.
230 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL..ISO.'

in the hypersthene than in the augite. General form, twinning, and 

inclusions are similar in both pyroxenes. The optical properties in one 

case are those of orthorhombic crystals, in the other those of monoclinic crystals.

The crystals are more or less idioinorphic, stout prisms, with the two 

piiiacoids and unit prism in the prismatic zone, and terminal faces not 

readily recognizable. Cross sections show the characteristic 8-sided 

outline of pyroxene crystals. Twinning, in which the orthopinacoid or 

the macropinacoid is the twinning plane, is common in both pyroxenes.

Inclusions of magnetite are often observed, and also those of feldspar, 

apatite, and of glass in negative crystal cavities or in irregularly 

shaped cavities. They frequently carry a gas bubble. In some crystals they are numerous, and often they are very minute.

In certain spots in the rock are clusters of crystals of labradorite 

and pyroxene with magnetite and some interstitial brown, globulitic, 

and trichitic glass.

Serpentine derived from the alteration of olivine occurs in pseudomorphs having the shape of sections of olivine, usually 4 or 6 sided, 

with orthorhombic symmetry. The pseudomorph is surrounded by a 

narrow border of augite grains. The serpentine is green in some cases 

and brown in others. The amount of olivine originally present was 

not great, so that the rock may be classed with the andesites rather 

than with the basalts.

Magnetite occurs in comparatively large grains or anhedrons, somewhat irregularly shaped. In some cases the magnetite partly incloses 

pyroxene. It is frequently inclosed by the pyroxene.

There are relatively few crystals of apatite in stout prisms, which 

are comparatively large! They have the usual hexagonal forms, and 

are often dusted and slightly pleochroic, between brown and dark sepia, 

E>O. In some crystals there are minute opaque needles lying parallel 

to the prismatic axis c.

In one thin section studied, there is a rounded anhedron of quartz 

containing colorless glass inclusions with gas bubbles, and in one case 

what appears to be a colorless crystal with rhombic outline. The 

quartz is surrounded by brown glass and a shell of pyroxene crystals.

The groundmass is a fine example of glassy andesitic groundmasses. It consists of brown glass full of stout microlites of pyroxene, probably monoclinic for the most part, and those of lath-shaped 

feldspar, together with grains of magnetite. These are not mingled 

with complete uniformity throughout the rock sections examined, 

there being spots which are mostly glass and others mostly crystals. 

The lath-shaped feldspars are plagioclase, whose exact position in the 

albite-anorthite scale has not been determined. Besides these microlites, there are small rectangular and lath-shaped feldspars with large 

extinction angle, probably labradorite, and small anhedrons and prisms 

of pyroxene and crystals of magnetite. These are slightly larger than
DII.LER.] NO. 89, HORNBLENDE-DIORITE-PORPHYRY. 231

the smallest microlites of the groundmass, but are still microscopic 

and may be classed as microlites.

NO. 89. HORNBLENDE-DIORITE-PORPHYRY.

(FROM SOUTH SIDE OK BUCKSKIN GULCH, 2| MILES ABOVE ALMA, MOSQUITO 

RANGE, PARK COUNTY, COLORADO. DESCRIBED BY WHITMAN CROSS.)

Occurrence. This rock occurs as an intrusive sheet in Silurian strata. 

Near the point.of collection the sheet is 20 feet or less in thickness and 

it has been traced for several miles with no great increase at any point, 

although often less than 20 feet thick on account of splitting into two 

or more parallel sheets. While following the same horizon for long 

distances, the rock locally cuts obliquely to slightly higher or lower 

planes of stratification, and in the cliff sections of the gulches it is not 

always continuous.

The rock maintains the structure and texture of the specimens in 

this collection except in narrow contact zones, where it is finer grained.

General description. The freshest rock obtainable has a general 

greenish-gray tone, and consists of numerous phenocrysts of plagioclase 

and hornblende lying in a somewhat subordinate tine-grained, greenish 

groundmass. The phenocrysts vary in size, but seldom if ever reach 

a diameter of 1 centimeter. Pinkish crystals of orthoclase are usually 

developed in very small numbers, and biotite may occasionally be distinguished, though commonly altered to chlorite or epidote.

Microscopical examination shows the groundmass to consist of orthoclase, hornblende, and quartz, with a small quantity of plagioclase and 

magnetite. The accessory constituents, apatite and zircon, are present 

as usual, with a little titanite, and, sporadically, minute prisms of 

allanite, which can not be found in all the thin.sections. Allanite was 

not observed raegascopically.

The phenocrysts. The abundant plagioclase phenocrysts are developed in various forms, but chiefly in tabular crystals parallel to 

the brachypiuacoid, often elongated somewhat parallel to the brachyaxis a. Twinning is common according to the albite, Carlsbad, and 

pericline laws, and all three are often combined. Zonal structure indicating change in composition during crystallization is frequently very 

plain,-but the variation in extinction between center and periphery is 

not great. Primary inclusions are few, apatite and magnetite being 

most common.

Optical examination snows that labradorite is the predominant 

member of the series here developed. The maximum of extinction 

noted is about 30°, and in crystals combining albite and Carlsbad laws 

of twinning the extinctions in the zone normal to the lamellae indicate 

labradorite of the composition 1 albite + 1 anorthite to be most common. Some andesine is thought to be present in crystals characterized 

by very fine albitic twinning.

The hornblende is the common green variety, with normarpleochroism 

and extinction. Its crystals are usually well terminated by the com-
232 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL 150.

inon pyramid and dome faces. There is no resorptiori, and alteration 

to chloiite lias begun externally and on many cleavage fissures. The 

phenocrysts vary in size and seem to grade rapidly downward to the 

needles' of the ground mass, which have the same characteristics of 

color and form.

Biotite has not been seen in fresh condition, but stout-prisms 

believed to represent this mineral, now entirely altered to chlorite and 

epidote, may be found in nearly all sections.

The few orthoclase crystals are clinopinacoidal tablets, usually of 

irregular boundary, including small plagioclase crystals. This is 

regarded as an exceptional development of orthoclase during the general groundmass period.

Groundmass. The groundmass is a.distinctly granular mixture of 

predominant orthoclase with abundant hornblende needles, and some 

quartz and plagioclase. Orthoclase is developed iu irregular grains, 

plagioclase in small, stout crystals, and quartz either in clear grains or 

most commonly in a cement for the other constituents, and then usually 

oriented over small areas so that it practically includes several grains 

and causes a rudimentary micrppoikilitic structure.

The hornblende needles are not very acicular, but they, are long 

enough and numerous enough to express a fluidal structure had there 

been movement after their formation. As such a structure is not 

found, these hornblendes are considered as crystallizing during the 

groundmass period.

Plagioclase is very subordinate, and while its small crystals probably belong to oligoclase or andesine, this has not been proved. Magnetite seems to have formed iu two periods, a scanty dust through the 

groundmass belonging to the later period.

Chlorite, epidote, calcite, and muscovite are secondary products of

variable development in the different specimens collected.

Chemical composition. The average rock has the composition of the 

following analysis by W. F. Hillebrand:

Analysis of h .rnblende-diorite-porphyry from Colorado.

SiOj.... .........................................

A\2 03 .................... ........................

Fe.O,.... ........................................

FeO .............................................

MnO... ..........................................

CaO .............................................

MgO.- ...........................................

K.O.......... ....................................

Na20. ............................................

H2 0 .............................................

pn

P r»

Total......................................

Sp. gr., at 16° C, 2.77.

Per cent.

56.62

16.74

4.94

3.27

.15

7 QQ

4.08

1.07

y.so

.02

1.15
DILLER.] DESCRIPTIONS: NO. 90, DACITE-PORPHYRY. 233

The analysis agrees very well Avith the mineral constitution as described. It is clear that the plagioclase must be one containing much 

lime, and the contents in potash is sufficient to represent a considerable 

amount of orthoclase in the grouudmass. Calcite and chlorite must 

have been well developed in the material analyzed. It would seem necessary to have some free silica in the rock in view of the large proportion of hornblende and labradorite.

Literature. This rock and its occurrence, together with the associated eruptives of the Mosquito Range, were described in Monograph. 

XII, United States Geological Survey, Geology and Mining Industry 

of Leadville, Colorado, by S. F. Emrnons, with Petrographical Appendix by Whitman Cross; 1887, pp.228-240,334-340. The rock was there 

called hornblende-porphyrite.

No. 90. DACITE-POUPHYRY.

(FROM CLEAR CREEK, SHASTA COUNTY, CALIFORNIA. DESCRIBED BY J. P.

IDDINGS.)

This rock occurs in Smiths Gulch, Shasta County, California. It consists of a dense, greenish-gray, aphanitic groundmass, filled with phenocrysts of white plagioclase feldspar, 15mm long and smaller, and 

numerous rounded quartzes, the largest of which have a diameter of 7mm , 

besides much biotite in small crystals, 2-"m across, and numerous hornblende prisms, 4 or 5mi" long. There are also many very small crystals 

of these minerals in the groundmass. The specimens collected are 

probably not all of the same degree of freshness, for in the specimen 

examined the hornblende crystals are unaltered, while in the thin sections prepared from chips of the same rock the hornblende is decomposed. The rock presents a fine example of porphyritic texture, which 

is emphasized by the contrasted colors of the feldspar phenocrysts and 

the grouudmass. By porphyritic texture is understood that appearance 

which is produced when the mass of a rock, having any texture whatever, is sprinkled with crystals larger than those constituting the main 

mass of the rock. They usually stand out prominently in contrast to 

the groundmass.

In thin sections of this dacite-porphyry the large feldspars are seen 

to be twinned according to the albite law, with thin lamellne, whose 

symmetrical extinction angles range from 0° to 4°, and are undoubtedly 

oligoclase. Other modes of twinning, after pericline and Carlsbad laws, 

are rarely seen in these feldspars. A zonal structure, which shows 

itself faintly between crossed nicols, is sometimes observed. The crystals are idiomorphic, and exhibit outlines in cross section due to the 

faces commonly developed in the feldspars of similar rocks, i. e. (001), 

(010), (HO), (110), (101), (201), and sometimes other subordinate faces.

In a few cases the outlines of the sections of feldspar are irregular 

and are interrupted by "bays" of the groundmass.

The feldspar is partly altered, and as a consequence is clouded white
234 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

in places by innumerable, minute, irregularly shaped inclusions, which 

appear colorless in thin section and have a lower refraction than the 

feldspar substance. Their exact mineral nature is indeterminable. 

These minute inclusions are sometimes confined to alternate lamellae 

of the polysyuthetic twins, the intermedite lamella} being free from 

them. In other cases they occur in irregularly shaped clouds across 

all the twin lainellne of one crystal.

Some of the feldspar phenocrysts inclose green hornblende and brown 

biotite with sphene, zircon, apati te, and possibly epidote. In one instance 

the biotite is the same size as that in the adjacent groundmass and 

lies near the margin of the feldspar. The inclosed mica is parallel to 

that in the groundmass, which exhibits a fluidal arrangement. This 

indicates that the feldspar pheuocrysts continued to grow aftei1 the 

motion in the groundmass ceased without showing signs of any break 

or interruption in the act of crystallization. Hence, the growth of these 

phenocrysts probably took place in the molten magma at a period immediately preceding the solidification (crystallization) of the groundmass 

of the rock.

The quartz pheuocrysts are almost as numerous as those of feldspar. 

Their outlines are partly idiomorpbic, partly rounded, and sometimes 

exhibit "bays" of groundmass. In some cases the outline is quite 

sharp against the surrounding grouudmass, in others it is jagged and 

uneven in consequence of the merging of the quartz of the phenocrysts 

into that of the small grains in the groundmass, proving that the quartz 

phenocrysts continued to grow in the period of crystallization of the 

groundmass. Sometimes several quartz phenocrysts have grown in 

juxtaposition without there being any evidence of definite relation 

between their orientations. Most of the quartzes exhibit uniform extinction between crossed uicols and optical homogeneity. Others, however, 

show an irregularly banded extinction, as though they were twinned in 

lamella}. These ill-defined, fairly broad bands appear to be parallel to 

a principal optic section, that is, they may lie in the prismatic zone. 

In one instance there are also extremely thin, poorly defined bauds 

nearly at right angles to the broad bands. In another case these thin 

bands cross the broad ones at an inclination of 32°. They may be 

parallel to a rhombohedral face. A rhombohedral cleavage is marked 

in some cases and in others there are minute inclusions parallel to the 

broader bands. The exact nature of these striations has not been 

determined. They may be the result of molecular readjustment produced by dynamical strains.

The quartz crystals carry inclusions of groundmass, zircon, sphene, 

monoclinic pyroxene, and very abundant, minute particles, irregularly 

shaped, apparently fluid inclusions with gas bubbles; in some cases 

simply gas inclusions.

Biotite occurs in bent flakes, which are crudely hexagonal plates with 

irregular outline. In thin sections they show an arrangement in lines
DIUJSR.] DESCRIPTIONS: NO. 90, DACITE-PORPHYRY. 235

according to the flow of the magma before solidification. They are 

strongly pleochroic, being pale yellow for light vibrating parallel to the 

a axis, i. e.,nearly at right angles to the cleavage; and dark brown to 

almost black for light vibrating parallel to the cleavage and at right 

angles to the a axis. The angle between the optic axes is apparently 

zero, and the mineral behaves as an optically liniaxial crystal.

The hornblendes in the rock sections studied are completely replaced 

by calcite and chlorite; with numerous colorless prisms of apatite, formerly exisling as inclusions in the hornblende. In some cases calcite 

preponderates over chlorite, or alone constitutes the pseudomorph after 

hornblende. In other cases chlorite occurs alone.

There is some sphene present in small, sharply idiomorphic crystals, 

easily confused with those of zircon, which also occur. It possesses 

the usual optical properties. Zircon is sparingly present in small 

4-sided prisms with pyramids of the other order. There is considerable 

apatite in stout, colorless crystals of small size, having prismatic, basal, 

and subordinate pyramidal faces.

Epidote occurs in comparatively large, irregularly shaped individuals, and also in sharply defined prisms. Kepeated twinning is present 

in some crystals. There is faint pleochroism in thin sections, from 

colorless to yellow. From the arrangement of the epidote prisms it is 

evident that they existed in the magma before the crystallization of 

the groundmass, for they take part in the flow structure, and the mica 

plates bend around them in some instances. Either they are primary 

crystals of epidote that crystallized from the molten magma before its 

final solidification, or they are pseudomorphs after hornblende prisms, 

having the b axis of the epidote parallel to the c axis of the hornblende. No cross sections of the prisms that might have shown the 

characteristic angles of either mineral were observed. While epidote 

is a frequent product of the alteration of hornblende, it does -not 

appear probable that these epidote crystals have been formed in that 

manner. They are simple or twinned crystals in each case, and not 

aggregations of minute anhedrons, as is usual when forming pseudomorphs. Moreover, no epidote was noticed in association with the 

chlorite and calcite pseudomorphs, which is its most frequent mode of 

occurrence when secondary after hornblende. The study of rock sections from specimens in which hornblende is not altered might settle 

this question.

Calcite occurs in comparatively large, but microscopic, anhedrons; 

sometimes alone, with numerous apatites, and probably replacing a 

ferromagnesian mineral; sometimes with chlorite,replacing hornblende.

The groundmass of the rock is a niicrogranular aggregate of feldspaT and quartz in allotriomorphic anhedrons of various sizes from 

.0009 mm to .0025 mm in diameter. The larger anhedrons are mostly 

striated feldspar, a few only being uustriated and idiomorphic. With 

these are mingled many microscopic crystals of magnetite, mostly
236 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL. 160.

4-sided, and probably octahedrons, bits of chlorite, and curved plates 

of biotite, besides many minute, rounded anhedrons having high refraction, and probably epidote.

The very fine grain of the groundtnass, which is raicrocrystalline 

and megascopically aphanitic, together with its composition, places 

the rock among the dacite porphyries.

The chemical composition of this rock is shown in the accompanying 

analysis by J. Edward Whitfield. The absence of CO2 and the low 

content of H2O indicate that the material analyzed was free from 

calcite and very slightly altered. The marked preponderance of soda 

over potash is to be noted.

Analysis of (ladle-porphyry from Clear Creek, California.

Si02 . .............................................

Ti02 ....... ......................................

A1A ............................................

Fe203 ............................................

FoO............................... ................

MuO... ..........................................

MgO. ............................................

CnO..... .........................................

Na2 0 ............................................

K2 0..................... .........................

Li.O .............................................

P.A .............................................

S03 .... ..........................................

01................................................

C02 ....... .......................................

H2O .............................................

Total ......................................

Less 0 for Cl .....................................

Per cent.

04.24

.76

18. 67

1.40

1.96

1.48

4.11

'4. 14

1.71

.08

.22

.25

none

1.18

100. 20

.05

100. 15

NO. 91. MlNETTE.

(FROM FRANKLIN FURNACE, SUSSEX COUNTY, NEW JERSEY. DESCRIBED BY J. P.

IDDINGS.)

This rock was first described as micaceous diabase by B. K. Emerson.1 

in 1832, and has been referred to subsequently as mica-diabase and 

kersantite. Tt is, however, miuette that is, a more or less porphyritic 

holocrystalline rock, poor in feldspar which is some variety of alkali 

feldspar, orthoclase or albite, or both, and rich in biotite and mouocliuic 

pyroxene. It is a lamprophyre in Eoseubusch's sense.

The rock is dark greenish gray and dense, with a small hackly fracture. Abundant small micas are the only mineral constituent that is 

recognizable megascopically. There are besides occasional lumps of a 

white mineral which appears to be either feldspathic or zeolitic. Accord1 B. K. Emerson, On the dikes of micaceous diabase penetrating the bed of zinc ore at Franklin 

Furnace, Sussex County, New Jersey: Am. Jour. Sci., 3d series, Vol. XXIII, 1882, pp. 376-379.
DILLEH.] DESCRIPTIONS: NO. 91, M1NETTE. 237

ing to Emerson the rock effervesces freely with acid, owing to the presHI> e of calcite. In thin secti n the rock is seen to be holocrystalline, 

with a hypidiomorphic granular texture. The ferromagnesian minerals 

and iron oxide are idiomorphic with respect to the feldspar. Together 

they also preponderate over the feldspar, which is more abundant, 

however, than any other one kind of mineral. Taken in the order of 

rlieir relative abundance the constituents are feldspar, inonoclinic 

pyroxene, mica, magnetite, epidote, calcite, apatite, pyrite: but this 

order of abundance varies with the degree of alteration, and is different 

in different parts of the rock.

The feldspar is in part twinned with polysynthetic lamellae. In some 

cases twinning is not recognizable. The index of refraction of the 

feldspar in both cases being lower than that of Canada balsam indicates that the feldspar is albite or orthoclase, or both. It is quite possible that the unstriated crystals, more or less clouded by alteration 

products, are orthoclase. They sometimes yield long, rectangular 

sections arranged in fan-like groups, an arrangement often assumed by 

alkali feldspars. The less clouded, polysynthetically twinned crystals 

are undoubtedly albite, since their maximum symmetrical angles of 

extinction are about 15° to 17°, which corresponds to those of albite or 

andesine, but the lower index of refraction as compared with that of 

balsam proves it to be albite. The minute flakes and anhedrons 

scattered through the feldspar appear to be epidote, calcite, and some 

little chlorite. There are also microscopic crystals of apatite.

Pyroxene is more abundant- in some sections than in others, having 

become more or less altered to epidote and chlorite. The pyroxene 

crystals are idiomorphic to a high degree, and are bounded by the 

prism and two pinacoids, the terminal planes not being determinate. 

The crystals are short, stout prisms. The outlines are sometimes 

indistinct, owing to the partial alteration to epidote. The prismatic 

cleavage is distinct, and a zonal structure is occasionally observed. 

The color in thin section is pale brown, which renders it difficult to 

discriminate between the pyroxene and epidote. It is monoclinic, with 

a high extinction angle, and is probably augite. Pleochroism is not 

noticeable. Inclusions of magnetite and pyrite are numerous. The 

pyroxene has been replaced, in some cases almost completely, by epidote, and to a less extent by chlorite, quartz, and calcite.

Biotite occurs in abundant crystals, which are six-sided plates, comparatively thick. They are idiomorphic with respect to the feldspar, 

having crystallized before it; but they are allotriomorphic with respect 

to the pyroxene, sphene, and magnetite, having crystallized later than 

these minerals. It partly incloses these minerals, besides numerous 

small crystals of apatite. Some of the mica crystals are bent and distorted. Twinning is present in some cases. The color in thin sectfou 

is reddish brown, with strong pleochroism and absorption, between 

light brown or yellow and dark reddish browii. In convergent polarized light the mineral appears uniaxial, the angle between the optic
238 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

axes is so small. Alteration has not affected the biotite to the same 

extent as the pyroxene. Many biotites exhibit no alteration. In some 

crystals there are lenticular inclusions, placed between the lamina}, 

which appear to be aggregations of minute grains of epidote, exhibiting 

aggregate polarization. Chloritization has set in at the margin of some 

mica crystals.

Spheue is very abundant in comparatively large idiomorphic crystals, 

yielding the characteristic sharply rhombic cross sections. Cleavage 

is pronounced. .The color in thin section is pale brown, Avith slight 

absorption. The index of refraction is somewhat higher than that ofpyroxene, and the double refraction is somewhat lower. Otherwise its 

resemblance to this mineral is very close, and the two are easily confused. It is rather free from inclusions of other minerals, even magnetite, which occur rarely.

Magnetite is present in numerous small crystals and in some auhedrous, Pyrite is quite abundant in crystals apparently bounded by 

faces of the cube and rhombic dodecahedron in combination. Apatite 

is also abundant in small, stout prisms, often sharply outlined and 

bounded by the prism, a pyramid and the basal pinacoid. It occurs 

scattered through tbe other minerals feldspars, micas, and pyroxenes.

Epidote, a secondary mineral in this rock, occurs in aggregates of 

crystals usually having allotriomorphic outlines, sometimes occupying 

the place of former pyroxene, sometimes scattered irregularly through 

the rock in larger or smaller anhedrons. It has nearly the same color 

as the pyroxene and sphene, but is noticeably pleochroic, with yellow 

color. Its index of refraction is almost the same as the minerals just 

named, but its double refraction is higher.

Calcite is present in irregularly outlined anhedrous and to a variable 

extent. It was not abundant in the thin sections studied. Tbere is 

also a little secondary quartz and chlorite.

The chemical composition of the rock, as determined by L. G. Eakins, 

is .given in the accompanying analysis, except for the fact that titanium 

has not been determined.

Analysiy of minette from -Franklin Furnace, New Jersey.

SiO....... ........................................

AU>, ............................................

li^O............. .................... ..............

1A;O.. ............................................

MnO ............................................

JMilO........... ..................................

CaO ..............................................

Xa^O. . ...........................................

K20. .............................................

CO^....... .......................................

H.,0. ................................ ............

1

Per cent.

19. 40

7. 46

..18

0.21

11. gy

].80

3.26

.74

J.53

100.01
DILLBH.]. DESCRIPTIONS: NO. 92, CAMPTONITE. . 239

The rock is very low in silica and high in alumimi, with comparatively low maguesi.a, high lime, and relatively high alkalies, chiefly potash. The alumina must have entered largely into the augite together 

with liine and iron, which accounts for the abundant production of 

epidote. The formation of alkali feldspars in a magma so rich in lime 

and alumina and so low in silica is noteworthy. It followed the crystallization of these elements into pyroxene, the alumina having entered 

largely in the hypothetical subsilicate moleculeNo. 92. CAMPTONITE.

(.FKOM CAMPION FALLS, GHAFTON COUNTY, NEW HAMI>SUIKE. DESCIUBKD BY

J. P. IDDJNGS.)

"At Campton Falls there are several dikes which furnish handsome 

Specimens for those who admire dark, porphyritic rocks. The black 

crystals of hornblende are not large enough to determine with the 

unaided eye, but they are very brilliant and numerous."' These rocks, 

originally called diorite by Hawes, who recognized their extremely 

.basic character, have become the type of Koseubusch's camptonite 

group. The rock in the educational collection is compact and bluish 

black, with crystalline luster, reflecting light from minute needles. It* 

carries numerous small pheuocrysts of hornblende.

It occurs in an 8-foot dike in mica-schist, according to a subsequent 

description by Hawes.2 In this description he also furnishes a more 

complete analysis of the rock, which will be cited later on. In thin 

section the rock is seen to be holocrystalline and to consist of abundant 

hornblende crystals and a subordinate amount of feldspar, with some 

augite and iron oxide, a little biotite and apatite and pyrite, and in 

some cases an isotropic mineral, which appears to be aualcite or sodalite. There is also a variable amount of calcite, serpentine, or clilorite. 

The preponderant constituent is hornblende, which forms long prisms 

that are icliomorphic in the prismatic zone, having the prism faces (110) 

and clinopiuacoid (010) well developed. In some cases the clinopinacoid is more developed than the prism faces. Twinning parallel to the 

orthopinacoid (100) is sometimes present. The color in thin section is 

chestnut-brown, with marked pleochroism .c and fo strong brown, a 

light brown jc;>ft>a. The inclination of jc to o appears to be about 

12°. A study of the chemical analysis of the rock in connection with 

its mineral composition iudicates that the hornblende is rich in alumina 

and contains considerable soda. Its exact chemical composition has 

not been determined.

The next mineral in abundance is feldspar, which appears to be of

1 G. W. Hawes, Mineralogy and Lithology: Part IV of the Geology of Now Hampshire, Vol. Ill, 

p. 161, Hitchcock, Concord, 1878.

2 G. W. Hawes, On a group of dissimilar eruptive rocks in Cainptou, New Hampshire: Am. Jour. 

Sci., 3d series, Vol. XVII, 1879, p. 147 et. seq.
240 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL. 150.

several varieties. The greater number of crystals exhibit little or no 

polysynthetic twinning, aud have very low angles of extinction. These 

crystals are probably oligoclase and orthoclase. They occur in long, 

slender prisms, sometimes with a tendency to divergent arrangement. 

They have a lower refraction than those feldspars in the rock which 

exhibit more pronounced twinning in thin lamellae, and yield symmetrical extinction angles as high as 20°. These feldspars may be andesine. They are subordinate to the more alkaline ones in amount. So 

that it appears that the feldspars of this rock are decidedly alkaline 

and are not basic, as they were supposed to be by Hawes.1

Some of the rock sections contain a "colorless, isotropio mineral in 

comparatively large individuals. Its outline is irregular, and it is traversed by irregular cracks, aud no distinct cleavage cracks. It is probably analcite, but no determination of its exact nature has yet been 

made. Its quantity varies in different rock sections; from some it is 

absent.

There is a variable amount of inonoclinic pyroxene present. It is 

quite abundant in some sections and scarce in others. It is pale yellow to violet in thin section, and is slightly pleochroic. The violet 

color may be .due to titanium, which, according to Hawes's analyses, 1 is 

present in the rock in considerable amount. The pyroxene is more or 

less altered to serpentine. In some sections the change has been complete. When unaltered it has the usual form aud cleavage, and sometimes the twinning parallel to the orthopinacoid. Some crystals of 

pyroxene inclose brown hornblende. The pyroxene is probably augite.

A brown inica, biotite, is also present in comparatively few large 

crystals; its outline is ill-defined, and it incloses numerous crystals 

of other minerals. The rock contains many small crystals of iron 

ores, mostly in well-defined crystal forms, which yield sections with 3 

and 4 sides. They are white by incident light, and appear to be partly 

altered to leucoxene. They may be titaniferous magnetite, or ilmeuite. 

Some of the opaque auhedrons of iron ore are pyrite, having a brassy 

color in incident light. There are numerous microscopic needles of 

colorless apatite. There is considerable calcite and serpentine which 

do not occupy spaces that may be certainly referred to former pyroxene crystals. These are secondary minerals.

1 Loc. cit.
DILLEK.] DESCRIPTIONS: NO. 93, DIORITE. 241

The chemical composition of the rock is given by the following 

analyses, I by G. W. Hawes, and II by L. G. Eakins:

Analyses of camptonite from Campton Falls, New Hampshire.

Si02 ...................................

Ti02 . .............. .............:......

A1203 .... ..............................

Fe.,03 .- ................................

FeO............ ........................

MnO. ..................................

MgO- ...... ............................

CaO... .................................

Na^O- ..................................

K,O...... ..............................

PA

HS0....... .............................

I.

Per cent. 

41.94

4..15

15.36

3.27

9.89

.25

5.01

9.47

5.15

.19

2.47

3.29

II.

Per cent.

19.68

11.15

6.65

9.37

2.77

1.72

4.82

1.49

100. 11

These analyses differ from one another considerably, chiefly in the 

alumina and alkalies and in the determination of titanium oxide. The 

rock varies in the content of analdte, but it is doubtful whether the 

high alumina percentage given in II is correct. Titanium is undoubtedly present in considerable quantity. The difference in carbon dioxide 

may well be due to variability in the amount of calcite present.

No. 93. DIORITE.

(FKOM MIDDLE BUUSH CREEK, AT BASE OF THE TEOCALLI MOUNTAIN, GUNNISON 

COUNTY, COLORADO. DESCRIBED BY WHITMAN Cnoss.)

Occurrence. This diorite occurs as a very irregular stock with ramifying dikes cutting through the Carboniferous in the southern Elk 

Mountains. The part of the stock included within the Crested Butte 

quadrangle, is more than 10 miles in length and it extends northward 

into the Aspen quadrangle for several miles more. Large stocks of 

tho same rock occur elsewhere in the Elk Mountains and in other parts 

ol Colorado. The specimens collected show the average grain and 

composition of tho diorite mass, but variations in grain are occasionally 

met with, and a subordinate porphyritic structure is lot-ally developed.

Description. The rock is light gray, fine grained, composed chiefly 

of plagioclase, orthoclase, quartz, biotite, and hornblende. The lightcolored minerals predominate, with but slight variation in the ratio of 

biotite to hornblende, over large areas; but, locally, quartz decreases in 

amount and then augite often appears as the associate of the other 

dark silicates. Magnetite is quite subordinate. The rarer accessory 

constituents, seen only under the microscope, are. apatite, titanite, 

zircon, pyrite, allanite, and an unknown dark-brown mineral. 

Bull. 150 16
242 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Plagioclase is the most important element of this diorite. It occurs 

almost entirely in idiomorphic crystals, varying in size between 0.5"im 

and 1.5""". These are developed in tabular form parallel to the brachypinacoid. Twinning according to the albite laws is usual, united with 

the Carlsbad and pericline laws in many cases. Zonal extinctions are 

very marked, indicating a change in composition of the feldspar molecule as crystallization progressed. The main part of the crystals seem 

to be labradorite, as the symmetrical extinctions in the zone normal to 

the albitic twinning reach nearly to 40°. Tests on crystals showing both 

Carlsbad and albitic twinning point to labradorite of the composition 

3 albite + 4 anorthite as most common. Doubtless andesiue and oligoclase are present as outer zones in some cases.

Orthoclase is much subordinate to plagioclase, but is still a very 

important constituent. It occurs only in irregular grains, or added to 

the plagioclase crystals in oriented position, especially in the zone of the 

ortho '(macro) axis. Quartz appears in irregular grains exactly analogous to the orthoclase, these two minerals being the last elements to 

form, and mutually interfering with each other.

Biotite is reddish brown in color, in thin, irregular leaves. Horiiblewde occurs in irregular prismatic grains, green in color. Now one 

of these dark minerals predominates, now the other. They are of 

similar size, seldom reaching 2mm diameter, and are both more or less 

altered, chlorite and epidote being common products.

Of the accessory constituents magnetite, apatite, and zircon occur in 

common forms. Titauite appears in irregular grains, seldom in characteristic crystals. Allanite Vas seen in two slides, out of a large number 

examined. It was there developed in irregular grains.

An unknown mineral is present in some sections in minute, darkbrown, strongly pleochroic, and absorptive prisms, included in feldspar 

or quartz.

Chemical composition. An analysis of the fresh diorite by L, G. 

Eakins yielded the following:

Analysis of diorite from .Elk Mountains, Colorado.

Si02 .............................................

TiOi.. ...........................................

A1A ...................................... ......

FeA ............................................

FcO

MnO... ..........................................

CaO... ......... ......^... ........................

MgO... ..........................................

TT C\

NajO ............................................

H.O..............................................

" Sp.gr., at 30° C., 2.79.

Per cent.

62.71

17.06

3.79

2.74

5.51

1.78

2 Oft

3.54

.24

100, 33
DILLBH.] DESCRIPTIONS: NO. 94, DIORITE. 243

^ These figures show silica to be so high as to explain the quartz content, and the relative strength of potash explains the abundance of 

orthoclase. The lime is strong enough to produce a plagioclase rather

^ - rich in this element. 

literature. The geological occurrence of this rock is illustrated in

 ,-. the Anthracite-Crested Butte Folio, No. 12, Geologic Atlas of the

United States. *

No. 94. DIORITE.

A

^ (F.KOM E.LKCTK1C PEAK, Y.KI/LOWSTONK NATIONAL PARK. DESCR1HKI) BY J. P.

IDDINOS.')

This diorite occurs as a part of a stock of igneous rock which was

 } forced through Cretaceous strata in early Tertiary times. The surrounding sandstone and shale have been metamorphosed by the contact, 

with the intruded lavas. The stock and connecting dikes were 

once part of a volcano which built its cone upon the surface of the 

Cretaceous rocks. The interior of the volcano has been laid bare by 

faulting and erosion.

The diorite mass has a variable grain, but is for the most .part 

coarsely crystalline, the clusters of feldspars and of ferromagnesian 

minerals ranging from 5mm to 2""" in diameter., These clusters, which 

give the apparent texture to the rock, are composed of from two to a 

dozen crystals of feldspar and an intermingled aggregate of ferromag- 

\ nesian minerals. The constituent minerals are hypersthene, augite, 

hornblende, biotite, lime-soda feldspar, orthoclase, quartz, magnetite, 

with apatite. Of these, augite, hornblende, biotite, and plagioclase are

V the most abundant. The structure is hypidiomorphic granular. The

plagioclases are more nearly idioinorphic than the other constituents,

but are not strictly so. They are rectangular to lath-shaped. They

n were formed in large part early in the course of crystallization. Some

are found penetrating brown hornblende. Orthoclase is present in some

cases iu small amount, aud either surrounds crystals of plagioclase as a

f partial border or forms irregular grains as cement between them. Quartz

also forms irregular grains'between the other minerals, as though the

last mineral to crystallize. It carries fluid inclusions. Hornblende,

^ pyroxene, and biotite rarely.exhibit crystal boundaries. Their outlines 

are usually very irregular, and they penetrate one another in a most 

complex manner. Magnetite is scattered through the rock in crystals 

or grains, being most commonly found within the ferromaguesian minerals. Apatite occurs iu short, stout crystals, not very well formed, 

aud is colorless. Zircon is rare.

The plagioclase feldspars exhibit the characteristic polysynthetic 

twinning and sometimes zonal structure. Some carry abundant minute

\ inclusions. In some cases these are colorless rectangular bodies of an

A ' Tlio eruptive rocks of Electric Peak and Sepulchre Mountain, etc.: Twelfth Ann. Kept. U. S. 

Geol. Survey, 1892, pp. 569-664.
THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

indeterminable nature, but suggesting glass. Others are clots and 

minute rods or needles in swarms. Small grains of the ferromagnesian 

.mint'ials may also be included.

Hypersthene is not always present, but occurs in parts of the rock. 

It exhibits a faiut pleochroism in thin sections, green || jc, yellow || a, 

and light red 11 IY. Its outline is irregular, and it is generally surrounded by hornblende, occasionally by augite or biotite. The augite 

is pale green in thin sections and is not pleochroic. It seldom exhibits 

crystal!*(graphic outline, but forms irregular grains, and is generally 

surrounded by hornblende. Biotite is dark brown to yellow in thin 

sections, with strong absorption. Its outline is irregular. It occurs 

outside of the pyroxenes in most instances, as though a younger mineral, but it may be partly inclosed by them, as if to some extent 

contemporaneous. . Hornblende is in part green, in part brown, the latter kind being 

usually within the former. The green hornblende often surrounds 

pyroxene and biotite wholly or partially. Generally they form an intricate intergrowth, as though they had crystallized at about the same 

time. The problem of .this iutergrowth is discussed at length in the 

article on The Eruptive Eocks of Electric Peak and Sepulchre Mountain, Yellowstone National Park, by J. P. Iddiugs, in the Twelfth 

Annual Keport of the Director of the United States Geological Survey, 

for 1890-91, page 006 et seq.

The chemical composition of the diorite as determined by J. E. Whitfield is given in the analysis:

Analysis of diorite from Electric Peak, Yelloivstone National Park.

SiO.............. .................................

i^n

A1A---- - ------------ - - 

Fe2 03 .. :......... .................................

FeO

MnO.............................................

CaO.. ............................................

MgO....... ......................................

Li20 .............................................

NajO. ............................................

K,0.... ..........................................

 p s\

<sn

el................................................

H2O.. ............................................

LesaO for Cl ....................................

Total .....'..........................-......

Per cent.

.84

14.23

4.69

4.05

.16

7.94

6.37

.01

2.98

1.23

.40

.17

.93

100. 28 

.04

100. 24
DILLEB.] DESCRIPTIONS: NO. 95, VOLCANIC SAND. 245

* GABBBO-BASAI/T FAMILY.

No. 95. VOLCANIC SAND.

(FROM SNAG LAKE CINDER CONE, LASSEN COUNTY, CALIFORNIA. DESCRIBED BY

. J. S. DlLLER.)

Volcanic sand is sand that has been produced from molten rock

** material by a volcanic explosion. Taken with volcanic dust, it is often 

referred to as volcanic ash, but is not ash in the same sense as is that 

resulting from the burning of wood or coal. In order that its formation

A . may be more clearly understood, it is necessary to consider some of the 

conditions and surface features of volcanic eruptions.

Deep borings, mines, and wells hare been sunk at many points on the 

^ earth's surface, and wherever observations have been made the temperature has been found to increase downward toward the earth's interior. 

The increase is by no means regular, and yet the rate generally does 

not vary greatly from an average of about 1° F. for every increase of 

57 feet in depth.

If the temperature increased at this rate regularly to a depth of 20 

miles, the temperature would be over 1,850° F., and at 50 miles over 

4,600° F., or far higher than the fusing point of all rocks under ordinary 

conditions.

We need not stop here to inquire further into the condition of the 

earth's interior, whether it be solid or liquid, or as to the source of heat,

^ whether it is a residue of the original incandescent earth, or is clue to

. chemical action, or is produced by the mechanical crushing of rocks.

Let it be sufficient for our purpose to know that the interior portion of

^ the earth, below a depth of a few miles from the surface, is very hot.

Bain falls on the mountain slopes. Some of it gathers into rills, runs 

into brooks, creeks, and rivers, and, finally finds its way back into the 

sea whence it came. Another portion enters the soil, and, under the 

influence of gravity, passes through the pores, cracks, and fissures of 

the rocks to various depths within the earth. On the lower slopes of 

f the mountains and in the valleys much of the water which entered 

above reappears from springs, most of which are cool and refreshing. 

In some cases, however, the water penetrates so far into the earth

'* before reappearing in springs that it is warmed by the internal heat. 

Thus warm springs, hot springs, and boiling springs are produced.

t' In those boiling springs in which the outlet is large enough to allow 

v< the heat to escape, the movements of the water are comparatively uniform; but in certain cases the outlet is narrow in proportion to the 

length of the more or less vertical tube in the ground, and there is not

> sufficient opportunity for the heat to escape. The heat increases until 

the expansive force of the highly heated water and steam is sufficient

": to produce an explosion. The overlying water and steam, as illustrated

^ in PI. XIV (p. 92), are thrown into the air by the eruption. Such 

springs are geysers, and steam is the motive power in their eruption.
246 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL.IM.

Next more important than eruptions of water in geysers are eruptions of mud. A notable one occurred in 1888 at Bandai-sau in Japan. 

Large quantities of mud, saturated with steam or highly heated water 

under pressure, were developed a short distance beneath the surface. 

A great explosion occurred, removing the'whole side of the mountain. 

A vast quantity of steam escaped, and streams of mud flowed dawn the 

valley, damming water courses to form lakes, and destroying a number 

of villages.

In true volcanic action the material transferred from the interior of 

the earth to the surface is neither simple water, as in the geyser, nor 

mud, as in the semivolcanic eruption at Bandai-san, but melted rock. 

It may come from greater depths than either of the others, where the 

temperature is higher, and the rocks may be either in a molten condition 

Or so hot that when the pressure upon them is relieved they fuse and 

become eruptible.

A fine sample of a truly volcanic eruption is afforded by the stupendous explosion of Krakatoa, in the Straits of Sunda, in August; 1883. 

The explosion was heard for a distance of more than 150 miles, and a 

mass of matter 1£ cubic miles in bulk was blown to pieces and hurled 

high into the air in the form of pumice, ashes, and. fine dust. The 

dust, very like that of specimen No. 58, was thrown to a height of 17 

miles in the air and spread far and wide by the winds. Some of it 

fell hundreds of miles from its source. The air wave set in motion by 

this great explosion traveled around the earth three times, and the sea 

in the neighborhood was thrown into waves, one of which was computed to have risen more than 100 feet above tide level, destroying villages and 38,380 people.

The eruption at Krakatoa differed from that at Bandai-san chiefly in 

that the material was, at least in large part, not in the form of mud, 

but in a state of aqueo-igneous fusion, i. e., actually molten. In the 

geyser and the eruption of mud the material is impelled to the surface 

by steam. The same is true also in the final delivery of molten rock, 

or magma, in such eruptions as that of Krakatoa, where the explosion 

must have been due chiefly to the expansive force arising from water 

or its component elements. In that case, however, all the molten 

material was blown out by the explosion. None of it flowed out. . The large quantities of steam given off' by volcanoes in eruption, as 

shown in PI. XXXIII, is illustrated by the great clouds of vapor which 

rose from Vesuvius during its eruption in April, 1872. The steam 

clouds were given off not only near the summit of the mountain Ayhere 

the eruption took place, but also from the streams of molten rock 

coursing down its sides. These streams are marked by lines of vapor. 

On the left the lava reaches the very base of the mountain, while on 

the right are two separate streams reaching nearly to its base. This 

eruption differed from those already noted in that some of the material 

was blown out by explosion at the mountain's summit, while the larger
VESUVIUS IN ERUPTION, APRIL 26, 18
DILIER.] DESCRIPTIONS: NO. 95, VOLCANIC SAND 247

part of the rnolteu material flowed out and formed lava streams or 

coulees.

It must not be supposed from the fact that attention has been called 

to the action of steam in volcanic eruptions that it is the principal 

agent involved; such a conclusion would be incorrect; steam has 

probably little to do with the raising up of great masses of molten 

material from the earth's interior; the cause of such upraising is not 

well understood, but when the molten material has reached the earth's 

surface, steam has much to do with the form of its delivery.

Volcanic eruptions are of two forms explosive and effusive. In the 

first form the material is blown to fragments and violently hurled into 

the air; in the second, the magma that is, the molten rock material 

wells up within the volcano and flows out upon the surface, forming 

coulees. In some cases the volcanic eruption is wholly explosive, as at 

Krakatoa; in others it maybe wholly effusive, as at Mauna Loa, in 

Hawaii, but generally both forms occur together, as at Vesuvius.

The same was true also at the small volcano at Snag Lake, 10 miles 

northeast of Lassen Peak, California, where the products of the explosive and effusive eruptions are quite distinct, one forming the cinder 

cone, and the other a lava field, both of which are illustrated in part 

by PI. XXXIV.

Volcanic sand, specimen No. 95, was obtained from the sand field 

shown in the foreground in PI. XXXIV. Lapilli, specimen No. 96, 

came from the cinder cone in the distance, and quartz basalt, specimen 

No. 101, was collected from the lava field to the right, where the lava 

was. not covered by sand as in the view.

The finest material blown from this volcano, volcanic .sand, spread in 

all directions from the vent, covering" the ground as a sheet for a distance of about 8 miles. The coarser fragments fell close to the vent, 

and, piling up, formed the cinder cone, surmounted by a cup-shaped 

crater from which its material was blown.

The maximum thickness of the sand at the base of the cinder cone 

could not be distinctly ascertained. It is loose and slides easily, so thafc 

a great deal of excavating is required to get through it where the layer 

has considerable magnitude. One-fourth of a mile from the base of the 

cone, where specimen No. 95 was collected, the sheet of fragmental 

material is 4 feet 4 inches thick. It consists of two portions. The upper 

part is properly called volcanic sand; the lower part, however, is composed of small, light brown pumiceous fragments, ranging in size from 

that of a pea to an inch in diameter. The fragments are very vesicular 

but still not sufficiently light to float on water like ordinary pumice (No. 

59). They are very rough and jagged, with surfaces torn by the bursting cells once filled with steam or other eruptive gases. Some of them 

were produced by concussion. The pieces hurled violently into the air 

frequently struck others, and all were partially pulverized as in a great 

mill. This sort of sediment is more abundant in the volcanic sand which
248 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [HULL. 150.

o.verlies the puiniceons material, and was therefore ejected at a subsequent stage of the eruption. Many of the sand grains are vesicular, but 

less so than the pumieeous fragments, and are generally rounded. 

Other grains are angular and composed of compact lava with few vcsiculse. They are bounded by fracture surfaces, and were evidently produced by the violent concussion of larger fragments. A few grains of 

quartz, feldspar or olivine of early crystallization, are present.

The sand exhibits irregular stratification, due to the sorting action 

of the atmosphere upon the subsiding particles during the eruption. 

.The thin layers are lenticular in shape and continuous for short distances only, not as sharply denned as beds laid down in water, but indicating a decided tendency to form beds under favorable conditions. 

This tendency is most clearly manifested a short distance away from 

the volcanic vent, where the air was not so much disturbed by the violent current expelled from the crater.

The selective influence of the atmosphere maybe seen not only in the 

imperfect stratification of the sand, as exhibited in a vertical section, 

but also in its distribution upon the surface. Near the cinder cone the 

sand is coarse and the bed thick, but the bed becomes thinner and the 

sand becomes finer as the distance from the cone increases.

On the borders of the large circular field covered by the sediment 

thrown out from the crater the fine material is sand, and there is no 

evidence to show that any considerable amount of volcanic dust or material still finer than the finest sand was formed.

The almost complete absence of volcanic dust in this case is surprising when we consider the highly explosive character of the eruption, 

and it may be attributed in part, perhaps, to the viscous condition of 

the magma at the time of the outburst. Had the pumiceous fragments 

in the layer already referred toj/contained sufficient eruptible gases at 

the time of the ejection to blow them to atoms, considerable dust would 

doubtless have resulted.

For an illustrated description of the volcanic phenomena exhibited 

at this recently active center, reference maybe made to Bulletin No. 79, 

United States Geological Survey, and to the Lassen Peak Folio.

No. 96. LAPILLI:

(FROM SNAG LAKE CINDER CONE, LASSEN COUNTY, CALIFORNIA. DESCRIBED BY

 J. S. .DILLER.)

. The volcanic fragments next larger than grains of sand are lapilli. 

They are,usually.very porous and differ from the sand particles chiefly 

in size, ranging from that of a pea to several inches in diameter. Most 

of them are black, but shades of red and yellow prevail locally, and 

when abundant may give color to the whole cone. The lapilli of the 

cinder cone illustrated in PI. XXXIV, from which specimens 96 were 

obtained, are generally black, giving it a somber aspect. A cinder 

cone in.Arizoha, northeast of Flagstone, noted for the brilliant colors
U. S. GEOLOGICAL SURVEY 

CINDER CONE, 10 MILES NORTHEAST OF LASSEN 
PIIXER,] DESCRIPTIONS: NO. 96, LAPILLI. 249

of its lapilli, is called Sunset Peak. Lapilli are also called "volcanic 

.cinders;" hence the mime cinder cone, applied to all cones of loose volcanic material about the vent from which it issued.

The lapilli of the Snag Lake cinder cone remained long enough in 

the air to become completely solidified before striking the ground, for 

they show no signs of flattening by impact, nor do they show upon 

their surfaces any lines of flowage, such as are so plainly marked in 

the next specimen (No. 97), from the cinder cone of another locality.

The following chemical analysis of lapilli from Snag Lake cinder 

cone, made by W. F. Hillebrand, shows that its composition is essentially the same as that of the lava in the lava field (No. 101). Both are 

quartz basalt.

Analysis of lapilli from Snag Lake Cinder'Cone, Lassen County, California.

Si62 ............................................. Tio2 . ..;.........................................

A1A------ ...-.........-.........:..............

Cr203 ............................................

Fe2 03 ............................................

FeO

MnO................... ..........................

CaO .............................................

SrO. .............................................

BaO .............................................

MS0.~. .........................................

K.,0..............................................

Na,0 ............................................

LiaO.............................................

H.,0. ............................................. PA ............:::..............................

' Total ......................................

Percent.

50.53

.54

17.50

1.35

5.03

.12

8.07

trace (?)

5.94

1.55

3.51

.27

.15

100. 50

No. 97. SCORIA.

(FROM ICE SPRING CRATERS, MILLARD COUNTY, UTAH. DESCRIBED BY J. S. DILLER.)

The volcanic fragments of some cinder cones have surfaces like slag, 

showing lines of flowage. Such fragments, especially when cellular, 

have been called scoria. The cinder cone from which specimen No. 97 

was taken shows many traces of the original molten condition of its 

material. Upon the surface of the lava which escaped from this cone 

there are distinct flow lines, and also upon the inner slopes of the 

crater where the/ subsiding magma left trickling remnants.

The fragments of scoria in many cases show by flattening that they 

were yet soft when they'fell, and other well preserved flow'forms indicate that they were ;;ot hurled violently from "the crater. If that were 

the case they would have been broken by collision or rounded by impact, 

an specimen No. 96. Instead they were ejected gently that is, sputtered
250 THE EDUCATIONAL SERIES OF ROCK SPECIMENS.

out and were so soft and sticky as to mold themselves and adhere to 

that upon which they fell. Mr. Gilbert graphically describes them as 

" vclcanic bombs whose aerial flight was too short to permit them to 

harden." For a description of the volcanic features of Ice Spring 

craters reference should be- made to Gilbert's monograph on Lake 

Bonueville, Monographs United States Geological Survey, Vol. I, pp. 

320 to 325.

No. 98. VOLCANIC BOMB.

(FROM NEAR MOUNT TRUMBULL, YAVAPAI COUNTY, ARIZONA. DESCRIBED BY

J. S. DILLER.)

Besides the sand and various'forms of lapilli ejected from an active 

volcano there are other fragments, generally larger, designated volcanic 

blocks or bombs. The angular ones are blocks and the round ones are 

bombs. The latter are illustrated in PL XXXV, which is a photograph 

of those at the base of the Snag Lake cinder cone. They were hurled 

into the air, fell upon the steep slopes of the cinder cone, and rolled 

to its base. They are compact throughout, excepting upon the surface, where they are more or less vesicular. This vesicular covering 

shows that their form is not due to the impact of the neighboring 

particles during the eruption, but to some earlier cause. Some of these 

bombs are over 8 feet in diameter, and it is difficult to believe that they 

became round and cool while flying through the air, for that would 

postulate an unreasonable rapidity of cooling. That they were solid 

when they struck the ground is evident, because they did not flatten 

as did those ejected from the Ice Spring craters of Utah, the Mono 

craters of California, and many others. It appears probable that these 

large bombs were the first part of the lava to solidify and were suspended in the magma before the eruption, when they were hurled out 

of the crater in their present form. This view of their origin explains 

also their compact structure as compared with the vesicular character 

of the other ejected material.

The volcanic bombs collected for this series were obtained from one 

of the cinder cones on the Uinkaret platform, a short distance southeast of Mount Trumbull, Arizona. On this platform are a number of 

symmetrical cinder cones, the result of geologically recent volcanic 

activity, the eruptions from which cascaded over the cliffs for thousands 

of feet to reach the bottom of Grand Canyon. Among the fragmental 

material of which these cones are made up are found great numbers of 

these "lava balls," ranging in size from that of a pea to 5 feet or more 

in diameter. The average size is nearly that of the specimen in the 

collection. Many of them are red from the oxidation of the iron, but 

others are dark, with surfaces ramified by a series of cracks.

The origin of these bombs is believed to be similar to that of those at 

Snag Lake cinder cone, where they were certainly ejected as independ-
VOLCANIC BOMBS AT BASE OF CINDER CONE, 10 MILES NORTHEAST 
'- DILLBR.] DESCRIPTIONS: NO. 99, BASALT TUFF. 251

**^ ent fragments. Their spherical form is generally attributed 1 to rotation while flying through the air. Professor Dana 2 says that at Hawaii 

such forms are produced " by the rolling movement of the front of the 

stream due to friction at the bottom." They sometimes have a center 

of olivine or more scoriaceous lava.

Some of the bombs near Mount Truinbull were found containing oliviue nodules, such as specimen No. 104, which were evidently solid at the

time of the eruption, and may have been shot into the air like a bullet.

. The same mineral nodules occur also in the lava stream, especially in

 Ji its vesicular portions, and when they are released by weathering closely 

resemble volcanic bombs.

"^ The volcanic phenomena of that region are described by 0. E.

A Button in Vol. II of the Monographs of the United States Geological 

Survey, p'. 101 to 112, to which the student is referred for further information. A fuller discussion, On the Fragmentary Ejectameilta of Vol-

,. canoes, is given by H. J. Johnston-Lavis in the Proceedings of the 

Geologists' Association of London (1885-86) Vol. IX, pp. 421 to 432.

No. 99. BASALT TUFF.

,.u

(FROM BATTLE CREEK MEADOWS, TEHAMA COUNTY, CAU.FORNIA. DESCRIBED 

 BY J. S. DILLER.)

Fine fragmental, volcanic material, such as dust, sand, and small 

lapilli, when cemented so as to be more or less firmly coherent is tuff. 

The term,-as explained under andesite tuff (No. 79), is made to cover all 

pyroclastic rocks of which the component fragments are finer than 

coarse volcanic conglomerate. It varies greatly, owing to the character 

and size of the component parts, from a very fine granular, light-colored 

rock,.such'as would result from lithifying volcanic dust like specimen 

No. 58, to a conglomerate made up of lapilli, such as specimen No. 96, 

and is designated according to the kind of lava fragments it contains, as 

basalt tuff, andesite tuff, rhyolite tuff, etc.

Specimen No. 99 was obtained near the summit of a cone from whose 

base issued a stream of basalt that flowed down the canyon of Battle 

Creek for miles. The cone was made up by the accumulation of volcanic sand and lapilli about the orifice from which they issued, but

', instead of being loose material, as at the Snag Lake cinder cone, it is 

cemented and forms a tuff cone. The cementation probably occurred

) in connection with the volcanic outbreak. Eruptions are generally

-T-. accompanied by rains from condensed steam. When the rain is sufficiently copious the saturated mass of fragmental material may flow 

as mud and become consolidated, forming tuff. Mud flows are sometimes disastrous! Pompeii was buried by such a stream from the slopes 

of Vesuvius. Cones made of tuff are tuff cones. They are generally 

less steep than cinder cones, and distinctly stratified.

 ^ ' Geikie, Text Book of Geology, 3d edition, pp. 200 mid 201. Also Judd's Volcanoes, p. 70. 

* Dana, Manual oi' Geology, 4th edition, p. 280.
252 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BOLL. 160.

The basalt tuff, specimen No. 09, is strongly contrasted with the andesite tuff (No. 79), not only in the size and composition of its fragments but 

especially in its distribution with reference to the source of material. 

The sand and dust of which the andesite tuff was made spread far and 

wide over the country, and may have been derived from numerous 

distant volcanoes about Lassen Peak. They were deposited in a body 

of water and formed part of an extensive bed. On the other hand, the 

basalt tuff presented by specimen No. 99 is a land-made deposit, limited 

iu its distribution to one small cone. It is somewhat stratified parallel 

to the slope of the cone. This arrangement is due to assorting done 

by the air and the sliding of the material down the steep slopes.

No. 100. CELLULAR BASALT. 

(FROM ICE SPRING CRATERS, MILLARD COUNTY, UTAH. DESCRIBED BY J. S. DILLER.)

By the expansion of gases contained in lava cavities are developed. 

If the cavities are small the lava is porous. If they are large and rather 

regular, either spherical or elongated, the lava is vesicular. When the 

vesicles are so abundant, as in specimen No. 100, that the space between 

them is reduced to a fine wall, the structure is cellular.

Specimen No. 100 was obtained from the interior portion of a large 

ejected fragment or bomb near the eastern base of the Mitre, the same 

crater upon whose slope the specimen of scoria (No. 97) was collected. 

Much of the lava of this region is vesicular, but it is rarely cellular.

The lava having the cellular structure (No. 100) is basalt. Vesicular 

structure is commou iu basalts, but is rare or entirely wanting in the 

more.siliceous lavas. The vesicles in many cases are elongated by, 

movements of the mass, and show the direction of the flow. When 

the vesicles are filled with; secondary minerals, such as occur in specimen No. 139, the structure becomes amygdaloidal,

No. 101. QUARTZ-BASALT.

(FROM SNAG LAKE CINDER CONE, LASSEN COUNTY, CALIFORNIA. DESCRIBED BY

J. S. DILLER.)

. Quartz-basalt is basalt which is characterized by the presence of 

primary quartz. '

One of the latest volcanic eruptions in .this country, so far as is yet 

definitely known, occurred a little more than 200 years ago near Snag 

La'ke, 10 miles northeast of Lassen Peak, California. The cinder cone, 

ash field, and lava field, seen in PI. XXXIV, look as fresh as though 

the eruption took place only yesterday.

. There were two lava flows from -tho base of the cinder cone. The 

older is partially covered by volcanic sand, as shown in PI. XXXIV. 

The surface of the younger flow is shown iu PI. XXXVI to be without 

any covering of volcanic sand. Specimen No. 101 was collected from 

the younger flow.

The lava is quartz-basalt. It is a dark, compact, more or less porphyritic rock in which crystals and grains of quartz are distinct, but
U. S. GEOLOGICAL SURVEY BULLFTIN NO. 150 PL. XXXVt

 

SURFACE OF LAVA FIELD 10 MILES NORTHEAST OF LASSEN PEAK, CALIFORNIA. 

Latest lava, not covered by sand.
DILLEB.] DESCRIPTIONS: NO. 101, QUARTZ-BASALT. 253

usually not conspicuous. Under a microscope it is seen <o be composed 

of quartz, plagioclase feldspar, pyroxene, and olivine, together with 

accessory magnetite and a Lirge proportion of uuindividualized material 

which is generally glolmlitic. The feldspar is most abundant; then 

follows pyroxene in nearly equal amount, witli less olivine and quartz. 

The globulitic base makes up nearly 25 per cent of the whole mass.

The olivine occurs in grains or well defined crystals, which often 

include small coffee brown isotropic crystals, supposed to be picotite. 

It is one of the first minerals to crystallize in the magma, and is rarely 

surrounded by a dark border. The pyroxene is chiefly hypersthene, 

but augite is sparingly present, especially in the shells surrounding the 

quartz.

The feldspar occurs in two sets of crystals. The largest have their 

interiors clouded by a multitude of glass aud globulitic inclusions, and 

their irregular outlines indicate that their initial forms were modified

by the corrosive action of the magma. The other feldspar crystals are 

lath-shaped, poly synthetic twins, such as are common in rocks of this 

class.

Grains and crystals of quartz in considerable numbers occur uniformly distributed throughout the lava, and are especially remarkable 

on account of the shell of augite and glass by which they are enveloped. This feature is illustrated in PI. XXXVII, where it is seen that 

the quartz is occasionally wholly resorbed by the magma, and its place 

marked by a group of granular augite.

The chemical composition of the quartz-basalt from. Snag Lake is 

given below, as determined by W. F. Hillebrand. It shows a large 

percentage of silica, but this feature can not be considered as necessarily the one which determined the presence of quartz; its origin may 

be referred to other causes. 1

Analyses of quartz-basalt from Snag Lake lava field, Lassen County, California.

SiOj.. ............................................

Ti02 ... ..........................................

A1203 .......................................... ..

Fe203 ............................................

FeO- ....................... ...1..................

MnO.............................................

CaO..............................................

SrO. ............................................

BaO....... .......................................

MgO...... .......................................

K.,0.... ..........................................

Nn2 0.. ...........................................

. Li.O..............................................

rr f\

 p,o, ................. ............................

Total ......................................

Per cent.

VI 9K

0.60

Ifl At\

I

on

4.72

0.10

7.65

0.03

. 6.74

1.57

3.00

0.40

0.20

100. 38

1 See Bulletin U. S. Geol. Survey No. 06, by J. P. Iddings.
254 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Quartz-basalts are relatively rare. They hold essentially the same 

relation to basalts that the dacites do to the andesites, or the rhyolites 

to the trachytes. A fuller illustrated description of the quartz basalt 

(No. 101) and its associated volcanic phenomena may be found in Bulletin No. 79, United States Geological Survey, by J. S. Diller.

No. 102. BASALT.

(FROM WATCHUNG MOUNTAIN, ORANGE, NEW JERSEY. DESCRIBED BY J. P.

IDDINGS.)

The basalt which forms a sheet intercalated in the red sandstone 

(Juratrias) of New Jersey and constitutes a capping to the ridge known 

as the First Mountain of the Watchuug Eange was undoubtedly a 

sheet of lava that flowed over the surface of the country in Juratrias 

times. The rock in the collection is from the large columns exposed in 

John O'Eourke's quarry, near those illustrated in PI. IY (p. UO), in 

West Orange Township, and is, therefore, from the lower portion of the 

lava sheet.

The rock is dark bluish-gray when freshly fractured, usually turning 

greenish upon exposure. It is compact and breaks with an evengrained texture. Megascopically it is finely crystalline to aphauitic, 

sometimes slightly porphyritic, with small phenocrysts llum or 2m111 

long. Owing to its durability and to the readiness with which it is 

broken into regular blocks, it is extensively quarried for paving stones, 

and is also crushed for road metal and used for macadamizing many 

miles of roads in this region. It is frequently columnar, or exhibits 

less regular prismatic jointing, and is the well-known "trap" rock 

forming the crests of the Watchung Eange, Bergen Hill, and the 

Palisades on the Hudson Eiver.

In thin sections, under a microscope,, the rock is seen to consist of 

abundant monoclinic pyroxene and much plagioclase feldspar, with 

magnetite and scattered patches of microlitic and globulitic glass 

base, and a variable amount of serpentine or chlorite. The pyroxene, 

which is in excess of the feldspar, is mostly malacolite, being pale 

green to colorless in thin sections, with high double refraction and 

poorly developed cleavage. It may easily be confounded with olivine. 

However, the occurrence of completely altered areas inclosed in perfectly fresh pyroxene indicates that the serpentine represents a much 

more easily altered mineral, such as olivine. The pyroxene of similar 

basalts and diabases occurring in Connecticut was analyzed by G-. W. 

Hawes,1 and shown to be an iron-lime-magnesia pyroxene, low in 

alumina, corresponding to the composition of malacolite. In the basalt 

of Orange Mountain it does not exhibit the basal parting, or twinning, 

or the idiomorphism that characterize salite. It is probable that olivine 

was present in the rock before decomposition set in. A few partly

1 G-. W. Hawes, On the mineralogical composition of the normal Mesozoic diabase upon the Atlantic 

border; Proc. U, S. Nat, Museum, 1881, pp. 129-134.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXXVII

SECTIONS OF QUARTZ GRAINS AND SURROUNDING SHELLS OF GLASS AND AUGITE' 

FROM LAVA FIELD 10 MILES NORTHEAST OF LASSEN PEAK, CALIFORNIA.
DILLER.] DESCRIPTIONS: NO. 102, BASALT 255

altered crystals of this mineral have been observed in some tliiu. sections. In others there are brown serpentine pseucloniorphs which are 

unquestionably decomposed olivines. It is possible that the scattered 

patches of serpentine which have been deposited in irregularly shaped 

spaces have resulted from the alteration of olivine. But serpentine 

may also be derived from the decomposition of the inalacolite.

The plagioclase feldspar forms lath-shaped crystals with polysynthetic twinning, often with only 3 or 4 stripes. The high extinction 

angles and relatively strong double refraction show it to belong to the 

more calcic species, probably labradorite. Hawes has shown that two 

species of feldspar often occur together in these rocks, and has demonstrated the presence of labradorite and anorthite. 1

The feldspar is in part altered to an almost colorless, brilliantly polarizing mineral, without definite crystallographic boundaries, probably

prehnite.

Remnants of a glass base are occasionally observed. They form 

angular patches, the glass being colorless with globulites and inicrolites, mostly of augite with attached grains of magnetite. The magnetite is sometimes present in small aggregations. In places this residual 

base is holocrystalline, possibly through alteration. A study of the 

whole rock-mass showed that glass was more abundant in the upper 

portion of the lava sheet.

The chemical composition of this rock is shown in the analysis made 

by L. (1. Eakins:

Analysis of basalt from Watchuny Mountain, New'Jersey.

'

.SiOi.-.-.. ..........................................

A\,0d ...... ......................................

Y^O....... ......................................

FvO............................... ...............

MnO .............................................

*no. .............................................

CaO...... ........................................

MgO............. ................................

lv,0..-. ..........................................

Na^O. ............................................

H,0.. ............................................

co2 .......... .....................................

Total......................................

Per cent.

51. 30

10. 25

2.H

8.24

.00

.(M

10. 27

7.97

1.00

1.54

l.st:;

100. 28

For a description of the columnar structure of this rock, see The 

columnar structure in the igneous rock on Orange Mountain, New Jersey, by J. P. Iddings: Am. Jour. Sci., 3d series, Vol. XXXI, May, 

1886, pp. 321-331.

1 G. W. Hawes, loc. cit., p. 131.
256 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

No. 103. COLUMNAR JOINTING IN LAVA. 1 /--.

(FROM KARNAK EIDGK, MONTEZUMA RVNGK,'CHURCHILL COUNTY, NEVADA.

DESCRIBED BY J. P. IDDINGS.) . ^

The occurrence of this rock, under the name of rhyolite, is described '< 

by Messrs..Hague 2 and King 3 in the Reports of the U. S. Exploration i 

of the Fortieth Parallel, where illustrations of its columnar structure 

are given.

Along the crest of this [KarnakJ ridge the rhyolite forms a series of clusters of 

prismatic columns of all sizes from 3 feet down to an inch in diameter. They show / 

from 3 to 7 sides, most frequently 5, but in many cases the fifth side will be much 

longer than the other four, with a slightly curving outline and«a-^fendency to develop t 

a sixth side. The four-sided figure would seem to be the least common. Usually 

they stand in an approximately vertical position, that is, above an angle of 60°. .4 

The tendency to columnar structure shows itself in various degrees of perfection, 

from the symmetrical prism to a single set of parallel planes diagonal to the bedding 

of (he rock. The most perfect prismatic forms are found near the summit, becoming 

less and less sharply developed farther down the slopes. The exterior of the columns, generally of a dark, almost chocolate brown color, fades in many instances 

into a reddish gray. The interior is an exceedingly brilliant, pure gray.

The microscopical characters of the rock have been described by ;' 

Zirkel 4 and referred to those of rhyolite, but they are those of hornblende-mica audesite or possibly of some form of dacite.

The rock has a light-gray, aphauitic groundmass, with many small 

phenocrysts of feldspar, biotite, and hornblende. The feldspar phenocrysts are all lime-soda feldspar in idiomorphic forms, with marked 

zonal structure and numerous glass inclusions. The values of the syin- v 

metrical extinction angles are such as to indicate that the feldspars are 

labradorite in part. The glass inclusions are colorless in some cases 

and dark brown in others. Other inclusions in the feldspars are thin 

prisms of apatite, magnetite, zircon, and occasionally hornblende and 

biotite.

The hornblende phenocrysts are very small and have poorly developed forms. Cross sections show the presence of the unit prism (110) 

and clinopinacoid (010). The crystals are sometimes irregularly shaped 

anhedrons. Pleochroism is pronounced from greenish brown and 

brown to light brown: £ greenish brown, ft = brown, and a = light 

brown. The absorption is £>b>it. Some crystals in thin section 

exhibit a narrow border of magnetite grains and augite microlites; 

others are free from it.

Biotite occurs in thin plates and also in comparatively thick crystals 

with more or less irregular outline. It has a brown color similar to 

that of hornblende, with strong absorption. It carries inclusions of

'This rock; although n quartz-bearing raica-andesite or dacite. is given this place in the series 

because columnar 'jointing is most common in basalts. J. S. D.

2 A. Hague, U. S. Exploration of the Fortieth Parallel, Vol. II. Washington, 1877, p. 761.

3 C. King, ibid, Vol. I. Washington, 1878, p. 644.

4 F. Zirkel, Microscopical Petrography: Vol. VI, U. S. Geol. Expl. Fortieth Parallel. Washington, . 

1876, p. 177.
DILLEB.] NO. 103, COLUMNAR JOINTING IN LAVA. 257

magnetite, sometimes iu the central part, sometimes in the margin; 

besides few opaque needles.

Magnetite occurs in minute crystals. Apatite forms numerous long, 

slender prisms with dust-like inclusions. Zircons are very few and 

small. There are a few phenocrysts of quartz with rounded and irregular outlines, carrying glass inclusions.

The groundmass is holocrystalline, consisting of irregularly shaped 

auhedrons of feldspar and quartz in micropoikilitic growth, the whole 

being clouded by minute particles, probably gas cavities in most cases. 

There are, besides, microscopic idiomorphic feldspars with rectangular 

outlines, sometimes forked at the corners. These exhibit polysynthetic 

twinning in some cases, but not in others. The smaller ones have a low 

extinction angle and are probably oligoclase. It is questionable 

whether any may be orthoclase. In places there are idiomorphic crystals of quartz surrounded by a clouded zone of quartz and feldspar in

micropoecilitic intergrowth. The groundmass also contains minute 

crystals of magnetite and scales of mica, besides microscopic prisms 

of apatite.

Columnar structure, or the separation of a rock into prisms more or 

less straight and parallel to one another, is not confined to any one 

kind of rock, although it is most frequently developed in igneous rocks 

and especially in basalts. The most familiar examples of columnar 

rocks are basalts, notably the Giant's Causeway and Fingal's Cave iu 

Ireland, the columnar lavas in the Auvergne in central France, and 

those of the Snake Eiver Canyon in Idaho and of the valley of the 

Columbia in Oregon. Excellent columnar structure occurs in the basalt 

or trap of the Palisades on the Hudson and of Watchung Mountain, 

west of Orange, as shown in PI. IV (p. 20), and at Paterson and Little 

Falls, iu New Jersey. But equally good columns are found in the rhyolites and obsidian in numerous localities in western America, notably 

in the Yellowstone National Park and in Nevada. They also occur in 

andesites and other kinds of igneous rocks, and even in granite in rare 

instances. Less perfect columns are sometimes observed in limestone, 

baked clays, and coal. In all cases the prismatic cracking is the result 

of a contraction of the rock mass, either through cooling, as in the case 

of volcanic lavas, or through loss of substance upon drying or baking, 

when water or vaporizable portions of the mass have been driven off 

by the heat of adjacent intruded rocks.

In igneous rocks the prismatic cracks start at right angles to the 

plane or surface of cooling, and if the rate of cooling is uniform over 

the surface the cracks continue in a straight line, producing straight 

prisms. If the cooling is not uniform at the surface, the columns or 

prisms become curved, often in diverging groups. When the rock mass 

is perfectly homogeneous and the cooling is uniform, the columns are 

hexagonal and of uniform thickness; but these conditions are seldom 

realized in nature, and the number of sides may vary, as may also the 

Bull. 150 17
258 THE EDUCATIONAL SERIES OF BOOK SPECIMENS. [BULL. 150.

thickness of different prisms. The slower and more gradual the shrinkage of the mass, the larger the columns. And since the rates of cooling 

at the upper and lower surfaces of a lava sheet resting; upon the surface 

of the earth are usually quite different, it often happens that the lower 

portion of a lava sbeet will be separated into larger columns than the 

upper portion. Moreover, since the cooling at the bottom is generally 

uniform, while tbat from the top may be irregular, the bottom columns 

are usually straight and normal to the bottom plane, and the upper 

columns are curved in diverging groups. Small prisms in volcanic 

lavas sometimes correspond exactly in shape to the well-known starch 

prisms which have been produced by the drying of a mass of starch. 

For a discussion of the production of columnar structure, consult the 

following articles: ' . On the origin and mechanism of production of the prismatic (or columnar) structure of basalt, by R. Mallet: Philosophical Magazine (4), 

vol. 1,122,201.

On columnar fissile arid spheroidal structure, by T. G. Bonney: 

Quart. Jour. Geological Society, 1876, p. 140. 

The columnar structure in the igneous rock on Orange Mountain, 

New Jersey, by J. P. Iddings: Am. Jour. Sci., 3d series, vol. 31, 1886, 

p. 321. ' - - 

No. 104. OLIVINE NODULE FROM BASALT.

(FROM NEAR MOUNT TKUMBULL, YAVAPAI COUNTY, ARIZONA. DESCRIBED BY

J. P. IDDINGS.)

The rock which carries these nodules is a recent lava flow whose 

place of occurrence is illustrated and described in Monograph II, 

United States Geological Survey, pp. Ill and 112. These nodules are 

found not only in the lava flow, but also as bombs among the ejected 

material of the cinder cone from whose base the coulde issued.

The rock is a basalt, in part highly vesicular and scoriaceous, having 

a dark gray to black color, and in part solid and compact. In the vesicular portion the nodules are several inches long and thick; in the 

compact rock they are small, nearly the size of lima beans or almonds 

or walnuts. The shape of the nodules is quite irregular, some being 

rounded, others angular with smooth surfaces. A megascopic examination shows that in some cases the nodules are composed of several 

kinds of minerals, though chiefly olivine. Some appear to consist of 

diallage or a mixture of diallage and olivine. The small ones in the 

compact basalt seem to be nearly pure olivine in nearly all cases. The 

basalt also carries comparatively large crystals of black hornblende 

and feldspar, which are less noticeable than the nodular inclusions, 1 

These nodules are granular crystalline aggregates of pale-green, olivine,

'In the field, when the nodules of olivine and diallage were collected, crystals of hornblende and 

feldspar, occasionally an inch in diameter, were seen. The outer surfaces in nearly all cases were 

irregular, showing embayments due to the corrosive action of the magma. J. S. D.
DILLEK.] NO. 104, OLIVINE NODULE FROM BASALT. 259

with a variable amount of green pyroxene. The olivine crystals have 

a vitreous luster and subconchoidal fracture.

The vesicular basalt containing the nodules when seen in thin section 

is found to have an almost opaque groundmass, filled with small cavities, abundant small olivines and occasional augite crystals, besides 

relatively few lath-shaped microlites of lime-soda feldspar. In places 

these microlites are more abundant and the grouudinass is partly 

transparent and is crowded with minute opaque dots. The opaque 

matter is undoubtedly magnetite in very minute grains, or crystals so 

numerous as to prevent the transmission of light through rock sections 

of the ordinary thinness. In the very thinnest edges it is possible to 

make out the presence of other constituents, whose character, however, 

is not determinate directly.

The almost microscopic phenocrysts of olivine scattered through this 

groundmass are-generally idiomor.ph.ic, with the outlines usual to such 

crystals that is, the sections are rhombs or six-sided figures; but some 

of the crystals are imperfectly developed and have irregular outlines, 

or are skeleton forms of growth, with jagged outline and numerous 

pockets or bays of groundmass, and also inclosures of groundmass. 

Such forms result from rapid crystallization in a quickly cooling magma. 

The olivine is nearly colorless, with strong double refraction, yielding brilliant colors between crossed nicols. Some of the crystals are 

twinned, so that two crystals cross one another, or are in juxtaposition, 

Avith their vertical axes inclined to one another at an angle of about 

60°. This arrangement may be brought about when the twinning and 

composition planes are the unit brachydome (Oil), an observed mode 

of twinning in olivine. Augite crystals about the size of the olivines 

are scarce. They form somewhat rounded and irregularly shaped 

crystals, which are pale green in thin section and are not pleochroic. 

They are also strongly doubly refracting, and are brilliant between 

crossed nicols, being distinguished from olivine by their color and 

.more marked cleavage, which is the usual pyroxene cleavage in the 

prismatic zone. The inclined extinction angles measured against these 

cleavage cracks reach as high as 42°. The augites are full of irregularly shaped inclusions of glass, and of multitudes of minute dot-like 

inclusions, which also may possibly be glass.

There are still less frequently irregularly shaped crystals of dark 

brown hornblende with strong pleochroisrn, which contain inclusions 

of groundmass. They are the same as the large megascopic crystals 

of black hornblende already mentioned.

The feldspar microlites are exceedingly small, are in many cases 

forked at the ends, and are twinned in two strips parallel to the long 

axis of the microlite. The symmetrical extinction angles, reaching 32° 

in some instances, indicate that the feldspar is labradorite.

The nodules in thin section are seen to consist of anhedrons of olivine, with a subordinate amount of pyroxene, which is in part orthorhombic, in part monoclinic.
260 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BCLL.ISO.

The olivine is colorless and has wholly allotriomorphic outlines. In 

some cases it is without cleavage, in others there is a more or less distinct pinacoidal cleavage. In general the substance is quite pure, but 

inclusions are numerous in some individuals. They are in part negative cavities having the form of hollow crystals of olivine; in part they 

are rounded, or form an irregular network of cavities. These seem to 

be filled with gas in some cases and with liquid in others, there being 

a moving gas bubble within the liquid inclusion. Occasionally the 

cavities are cut in making the thin section, and balsam enters and 

partly fills them. There are also rod like inclusions of a brown mineral, which lie in parallel lines. They appear to be the s>ame as stouter 

inclusions of similar shape, which have the same direction and the same 

color. They also occur in irregular shapes and as rounded grains 

like the larger brown grains which have a higher index of refraction 

than olivine, are isotropic, and are either picotite or chromite. Some 

individuals of olivine, when seen between crossed nicols, exhibit a 

parallel banding, as though composed of comparatively broad lamellae 

with slightly different angles of extinction, which might be due to 

poly synthetic twinning.

The pyroxene resembles the olivine very closely, but has a grayishgreen tinge, with no pleochroism, but with marked cleavage as in 

pyroxenes. Some of it has the parallel extinction and lower double 

refraction of orthorhombic pyroxene, and is probably enstatite. Other 

individuals have higher double refraction and inclined extinction, and 

are probably diopside. Both pyroxenes in some cases carry numerous 

gas and fluid inclusions with moving bubbles, which have the shape of 

crystals of the matrix, or may be irregularly shaped. Occasionally the 

pyroxene is striated as though by poly synthetic twinning.

In. one section examined there is a little violet-brown glass between 

crystals of olivine within the nodule. When olivine lies in contact

with this glass it has its proper crystal form, which indicates that the 

glass is a remnant of the matrix out of which the olivine crystallized. 

This kind of glass was not observed around the margin of the olivine 

nodule.

The outline of the margin of these nodules in thin section is irregularly jagged, with minute bays and pockets of the groundmass of the 

surrounding basalt penetrating into the crystals of olivine and pyroxene. There is no sign of crushing of the olivine along the margin of 

the nodule.

In mineral composition these nodules resemble certain varieties of 

peridotite, since they consist of olivine with picotite, or oliviue with 

diopside and enstatite and picotite, and might possibly be considered 

inclusions of fragmentsyof such rocks. But the olivine is the same as 

that in the basalt, and in some cases there is brown glass within the 

nodule. It is probable that these nodules are segregations that have 

formed within the molten basalt magma, either in lumps the size of
DILLBB.] DESCRIPTIONS: NO. 105, DOLERITE. 261

the present nodules or in larger masses which have been cracked upon 

the eruption of the basalt magma. Analogous segregations of hornblende often occur in audesites. Similar nodules of olivine are found 

in basalt in various localities, some of them being very large masses. 

For a review of the different hypotheses that have been advanced to 

explain the formation of oliviue nodules, the student is referred to Prof. 

Zirkel's Lehrbuch der Petrographie, Second Edition, Vol. II, p. 931.

3*o. 105. DOLERITE.

(FROM VALMONT, BOULDER COUNTRY, COLORADO, THREE MILES EAST OF THE TOWN 

OF BOULDER. DESCRIBED BY WHITMAN CKOSS.)

Geological occurrence. The rock to be described occurs as a vertical 

dike, running east and west, 20 to 40 feet wide, and nearly 2 miles in 

length. The formation cut by the dike is the Fox Hills, or upper division of the Montana group of the Cretaceous, a series of sandy shales 

and friable sandstones. The locality is on the plains, 4 miles from the 

foothills, and the shales are nearly horizontal. The specimens were 

obtained from the west end of the dike, just above the village of Valmont. Here the dike rises about 200 feet, with vertical walls near the 

top and d6bris-covered slopes below.

It is supposed that this rock was erupted during the Denver period 

of the post-Laramie, and that at the time of eruption there may have 

. been a thickness of 2,000 feet, more or less, of sedimentary rocks above 

tlie horizon at which the dike is now exposed. Basaltic magma of 

practically identical composition was erupted at this time at several 

places, some 14 to 1C miles nearly south from Yalniont, and the product 

is now seen in dikes and in the surface flows of Table Mountain at 

Golden. It is considered probable that the Yalmont dike also represents a channel through which the basaltic magma rose to the surface. 

Aside from analogy with the occurrence near Golden, a further support 

of this supposition is afforded by the uniform texture and structure of 

the Valmont rock from, wall to wall and from end to end of the dike. 

This is interpreted to mean that the magma cooled at a very uniform 

rate, such as could be experienced only where the adjacent shales had 

become highly heated by the long-continued passage of lava through 

the fissure conduit.

General description. The Valraont dolerite is a very dark porphyritic 

rock, whose most distinct phenocrysts are dark augite prisms, with a 

maximum length of lcm, while most of them are less than 0.5cm long, 

and there is a regular gradation down to those less than 1""" in length. 

By careful search a few glassy olivine crystals may be seen, especially 

where serpentiuization has begun. Eeflection from the curved fissure 

planes of the fresh olivine pheuocrysts often gives them a metallic 

luster. Some olivine crystals reach a length of 3mm , but the average is 

about 1""".

Feldspar phenocrysts are rarely discernible with the naked eye, but
262 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BUJ.L.ISO.

examination -with, a lens shows the dark-gray mass in which olivine and 

augite crystals are embedded to consist very largely of plagioclase 

tablets, whose narrow cross sections exhibit a very fine striation on the 

basal cleavage plane. These tablets lie irregularly, and their boundaries seem indistinct. The lens also shows the dark color of the mass 

to be largely due to many minute black and green particles regularly 

disseminated through the whole.

Microscopical examination of thin sections shows the rock to consist 

of the minerals already mentioned, with the addition of orthoclase and 

biotite and the accessories magnetite and apatite. The dark specks 

giving color to the mass are magnetite, augite, and biotite. The angular spaces left between the plagioclase tablets are for the most part 

filled by orthoclase. No glassy or globulitic base exists in this rock.

The constituents are fresh, excepting some of the olivines, which are 

partly changed into golden-yellow serpentine, and patches of feldspar, 

which are cloudy. . Characteristics of constituents. The augite is of a dull, greenish-gray 

color, occasionally exhibiting a yellowish tinge or becoming nearly colorless in certain zones. The phenocrysts are stout prisms of the usual 

forms, but the outlines as seen in thin sections are almost always somewhat irregular lines. Zonal structure is rarely seen, but a few crystals 

show a colorless zone near the outer border. This is free from inclusions, while the green portion commonly contains numerous round or 

irregular sack-shaped brown glass inclusions, together with grains of 

magnetite, oliviue, and prisms of apatite. Very rarely these are 

arranged in zones, though an irregular kernel of the crystal is frequently free from inclusions. Biotite leaflets are sometimes included 

in augite.

There are many small phenocrysts of augite, but there is a decided 

gap between these and the irregular greenish grains of a second generation, which average 0.05 to 0.20mm in diameter.

Olivine is developed in a manner closely corresponding to that of 

augite. Its most distinct phenocrysts show prisms and domes. Inclusions of magnetite, apatite, and glass are numerous. The latter are 

usually small and round and show black globulitic devitrification. 

They are sometimes zonally arranged. Biotite leaves are rarely found 

in the olivine.

All stages of serpentinization can be seen in almost every thin section, but the majority of the crystals are very fresh. The fibrous 

golden-yellow product contrasts markedly with the colorless olivine. 

It develops in fibers normal to the various fissure planes in the crystal.

Plagioclase is so developed in this rock that it is difficult to determine what varieties are present. The larger crystals are tablets parallel to the brachypiuacoid, but their other boundaries are either 

imperfect or are obscured by the later growths. There are probably at 

least two varieties of the lime-soda series present. The older of these
DILLEB.] DESCRIPTIONS: NO. 105, DOLERITE. 263

forms the larger tablets, 1 to 3""" long, and quite narrow. These usually 

present very thin lamina;, twinned according to the albite law, and 

such polysynthetic parts are often further twinned by the Carlsbad 

law. In addition, twinning by the pericline law is often found. The 

maximum observed extinction in the zone normal to the predominant 

pinacoid is nearly 40°, which indicates that such crystals must be at 

least as rich in lime as labradorite.

Probably different from these larger crystals are the small staves of 

plagioclase, which are abundant. It is also difficult to make out the 

form of these crystals.

Orthoclase is probably an element of considerable importance in the 

Valmont dolerite, but it is practically impossible to prove its identity 

in most thin sections. The cause of this difficulty lies in the manner 

in which it is developed in irregular grains, as the last crystallized 

mineral, between other constituents. But there are some cases where 

plagioclase crystals are more or less completely surrounded by an 

irregular border of apparent feldspathic substance which extinguishes 

uniformly parallel to the pinacoidal line of sections normal to the lamina;. That this substance and many of the simply polarizing grains are 

orthoclase is strongly indicated by the chemical analysis of the rock 

and by analogy with the basalts of similar composition near Golden, 

where the development is much more distinct.

Biotite occurs abundantly and very regularly distributed throughout 

the rock in little greenish-brown flakes having a tendency to attach 

themselves to olivine and magnetite. They are apparently included 

both in angite and olivine. No large leaves have been noticed. Only 

rarely do the flakes have a hexagonal form.

Magnetite is scattered through the rock, but is less abundant than in 

most basalts. It is the most abundant inclusion in the olivine and 

augite crystals. 

Apatite is present in characteristic prisms, .but is not so prominent 

as in andesite. No zircon crystals have been observed, and no titanium, 

minerals.

Structure. The large augite and oliviue prisms distributed through 

the dense feldspathic mass of the rock give it a porphyritic structure, 

but microscopical study shows that there is not quite that relation 

between pheuocrysts and groundmass which is commonly found in 

porphyritic rocks. The distinct crystals of augite and olivine lie in a 

mass which also contains crystals of labradorite, but these grade downward in size to correspond with a probably distinct feldspar occurring 

in staves, while orthoclase with some oligoclase fills the interstices 

between the older tabular crystals.

It is true that augite occurs in what is apparently a second generation in small irregular grains, which are embedded in or lie between 

the feldspar crystals. This rock has therefore a peculiar structure 

not commonly met with, and it serves to illustrate one way in which
264 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [nuLL.150.

porphyritic structure may be producer! without a second generation of 

the chief minerals; for while the augite and possibly other minerals 

may have had a second period of formation, the porphyritic structure 

seen is not due to that cause.

Chemical composition. Below is given the analysis of the typical 

rock (I), and that of augite isolated from the same (II). Both analyses 

are by L. G. Eakins.

Analyses of dolerite from Valmont, Colorado, and of anr/ite therefrom.

Ti02 ... ..........................................

A12 03 . ...........................................

Fe203 . .........:..................................

FeO .............................................

MnO............. ................................

CaO '.............................................

BaO .............................................

MgO-. ...........................................

K20.- ............................................

Na20..... .......................................

H20 .............................................

T> A

sos ...... ........................................

I.

Per cent. 

48.25

.89

16.73

3.99

G. 28

a v>

.013

5.77

4.08

3.24

1.72

.68

no

.12

II.

Per cent. 

49.10

7.95

8.30

99 <\i

12.37

The greater part of the titanic acid of the rock is undoubtedly contained in the augite, but it was not determined in the analysis of this 

latter.

The analysis is that of a typical basalt, except as to the alkalies. 

The high amount of potash found confirms the belief that tbe last feldspar crystallizing in this basalt is orthoclase. In the closely allied 

basalts of Table Mountain at Golden there is likewise a larger amount 

of potash than soda. Perhaps the presence of biotite is also due to 

this abundance of potash. This mineral is also developed in the Golden 

basalt.

Literature. The basaltic occurrences of this region are fully described in Monograph XXVII of the United States Geological Survey, 

Geology of the Denver Basin, Colorado, by S. F. Emmons, Whitman 

Cross, and G. H. Eldridge.

No. 106. DIABASE.

(FROM WEST ROCK, NEW HAVEN, CONNECTICUT. DESCRIBED BY L. V. PIRSSON.)

Northward from the city of New Haven there extends through Connecticut and into Massachusetts a long, narrow area of sandstones 

which, from their fossil contents, are known to be of Triassic age. The
DIU.KR.] DESCRIPTIONS: NO. 106, DIABASE. 265

length of this belt is about 110 miles and its width about 20. The 

rocks composing it are chiefly red sandstones which pass, on the one 

hand, into coarse conglomerates and, on the other, into sandy shales. 

The belt itself lies in a great trough of upturned crystalline metamorphosed rocks of uncertain, but probably Paleozoic, age. The thickness of the Triassic strata in this trough is not known, but artesian 

borings down to 4,000 feet made in New Haven have not passed 

through it. The belt is one of a system occurring in independent 

areas along the Atlantic border, which represent deposits made in 

Mesozoic time and to which the name of the Newark group has been 

given. The belt extending northward through Connecticut and Massachusetts is known as the Connecticut Valley area.

The general structure of this area has produced the belief that the 

trough was the former valley of the ancient Connecticut Eiver, which, 

through submergence, passed into an estuarine condition, and was 

then filled with the sediments. The character of these beds, their 

frequent cross bedding, and the sudden changes from sandstone into 

conglomerates show rapid shifting of currents and powerful stream 

action.

A small outlying area of these sandstones about 15 miles west of 

the main one indicates that the formation had once a wider area than 

at present, but denudation since its emergence has carried away an 

unknown amount, leaving, however, the thicker mass lying in the 

former trough.

The beds in this area are not in their original horizontal position. 

After their emergence, by the action of orogeuic forces, they were 

faulted into a series of great blocks, which dip eastward and have 

their upthrow on the western side. They thus form a series of monoclines which give rise to north and south ridges.

The type of topography to which this structure has given rise is 

strongly accentuated by the fact that the upturned blocks of sandstone contain numerous intrusions of diabase, or " trap rock," as it has 

been commonly called. These intrusions are of all sizes and of thicknesses up to 250 feet. The sandstone which formerly covered the 

intrusions has been largely carried away, especially along the crests 

of the faulted blocks, by denudation and glacial erosion, thus exposing 

the igneous rock. It therefore produces a series of curved north-andsouth ridges which, on account of the upthrow on the westward side, 

face toward the west and southwest with bold precipitous cliffs with 

columnar faces. These projecting masses of diabase dominate the 

topographic character of the region and form the most striking element in its scenic features.

It is admitted by all geologists who have studied the region that the 

diabase occurs in intrusive masses, but in regard to the period at which 

the intrusions took place there has been much discussion. Concerning 

this, two views have been held. One of these supposes that the main
266 THE EDUCATIONAL SERIES OP ROCK SPECIMENS [BULL.ISO.

portion, at least, of the intrusions took place after the upturning- of the 

beds. The magma rose through fissures, often passing along between 

the bedding until, nearing the top, it lifted the upper layers and, sometimes abrading the upturned beds on the lower side, formed large 

intrusive masses. This view has been chiefly upheld by the late Prof. 

J. D. Dana.

The other view supposes that the intrusions of diabase took place 

before the upturning of the strata, and that it was injected between 

them in immense horizontal intrusive sheets conformable with the 

bedding, or poured out in contemporary lava flows as the sediments 

were deposited. Later, when the sandstones were faulted into blocks 

with upthrow to the west, the trap sheets were brought to light. This 

idea has been chiefly urged and developed by Prof. W. M. Davis. It 

would be out of place here to enter into a discussion of these two conflicting views; suffice it to say that some features of the area seem to 

be best explained by the former, others by the latter, and that it is by 

no means certain that either view is everywhere correct. The main 

point is that the igneous rock described here is intrusive in origin, and 

that the peculiarities of structure and texture it exhibits are due to an 

igneous mass cooling and crystallizing under at least a moderately 

beavy cover of sediments. Those who desire more information in 

regard to the structure and occurrence of these intrusive masses of 

diabase will find it in the works given -in the list of literature at the 

end of this article.

West rock is the name given locally to the extreme southern end of 

the most.western of the "trap" ranges mentioned above. The ridge, 

which has here a height of about 400 feet above tide, breaks off along 

the western front and southern end in a bold cliff from which the mass 

slopes back toward the east at a moderate angle until it merges into 

the lower country.

Along the crest the rock exposures are those of the diabase itself, 

the sedimentary covering having been carried away; but down the 

slopes toward the north and east the covering sandstones are met 

with. Along the AYestern side and southern front the cliff gives an 

excellent section, and it may be seen here that the diabase is in part 

unconformable to the strata, and then dips and passes in between the 

planes of bedding of the upturned sandstones. The thickness of 

the mass above the sandstones on the west front is about 200 feet. It 

is cut by a series of joint planes which divide it into rude columns, 

so that the cliff front, viewed from below, has a pronounced columnar 

structure. .

The rock has long been quarried and used as a building stone and 

for road metal in the city of New Haven; For this latter purpose it is 

especially well adapted by its hardness and tough, resistant qualities. 

For building purposes material having smooth planes of the natural 

joint faces is especially sought, as these, from the oxidation of the ironbearing minerals, are covered by a thin skin of varying shades of
DILLBB.] DESCEIPTIONS: NO. 106, DIABASE. 267

brown which produce a very pleasing effect in surfaces of masonry. 

The quarry is at the extreme southern end of the cliff, and is in the 

village of Westville, one of the suburbs of New Haven, and about 2 miles 

from the center of the city. It is from this quarry that the specimen 

in the collection has been taken.

The diabase of the New Haven region possesses a special interest 

from the petrographic standpoint in that it was, so far as known, the 

first rock in America to be investigated by modern petrographical 

methods. This investigation was made in 1874 by Hawes and E. S. 

Dana, who showed by analyses and by examination of thin sections 

by the polarizing microscope, that the "trap" rocks of the Connecticut 

valley area were composed of augite, iron ore, and a feldspar to which 

the composition of labradorite was assigned, with at times the addition 

of some chlorite. Later, by analyses and separations, Hawes showed 

that two varieties of feldspar were commonly present in these rocks.

The specimen shows a rock of a very dark stone-gray color, heavy, 

and rather dense in texture. On a close examination with the eye, this 

dark color is seen to be due to the spotting of shapeless masses of a 

blackish mineral (mostly augite) with tiny flecks of white (feldspar). 

It may be noticed that at times the white flecks take the form of minute 

rods. The clean fractured surface of the rock is nearly devoid of luster. 

The lens only serves to bring out these features more strongly, and to 

show that the light and dark minerals are mingled in shapeless masses. 

It does not bring out any pronounced feature which would serve to 

characterize the rock. The rock, indeed, does not megascopically show 

any pronounced features which would serve to classify it, beyond its 

weight and the dark color due to a large proportion of aferromagnesian 

mineral, characters which would at once place it among the basic rocks 

of basaltic habit.

On examining a thin section of the rock under the microscope, however, its structure and mineral composition are at once clearly seen, and 

it also proves a very interesting one for petrographic study.

The minerals composing it are found to be apatite, iron ore, biotite, 

pyroxene, piagioclase feldspar, orthoclase, and quartz, while chlorite 

and serpentine-like minerals occur as secondary products produced by 

weathering. The minerals are given in the order of their formation, 

which is told by the younger including the older, or by the latter projecting into the former with crystal boundaries. The pyroxene and 

plagioclase feldspar are by far the most abundant and constitute the 

chief mass of the rock.

Apatite is not common. It occurs in small stout prisms, which are 

apt to be associated with the colored components, and long slender 

needles, which occur almost wholly in the feldspar. It is colorless and 

easily told by its high single refraction, low double refraction, and 

optically negative extension. It may occur in any of the other components, and is therefore the oldest.

Iron ore, though well distributed throughout the section in black
268 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

opaque masses, is by no means abundant. It is undoubtedly in part 

ilmenite, since the analysis of the rock shows a considerable amount of 

titanic oxide present, and there is no other mineral to which it can be 

referred, except that pyroxene sometimes carries a small amount. 

When it enters into their composition, however, they are almost invariably of violet color and have a marked dispersion of the optic axes, 

which is not the case in the pyroxene'of this diabase. Both magnetite 

and ilmenite are present, as shown later.

There are two varieties of pyroxene present. The one is the usual 

brownish kind characteristic of this class of rocks; the other is a white 

or colorless one, which has a tendency to be more idiornorphic than the 

brown, in columnar shapes. The difference between them, since the 

brown one is light in tone, is not extremely marked, but it can be easily 

seen by studying the section with a rather low power, so that a considerable number of the augites are brought into the field at once. The 

exact nature of. the white augite is not known. It occiirs very commonly throughout the diabase of the Connecticut valley area. It is 

also found in occurrences of diabase in Sweden, in northern England, 

at Rio Janeiro, Brazil, and in Nova Scotia. It was formerly supposed 

that the mineral was the same as that variety of pyroxene found at 

Sala, iu. Sweden, and hence called salite. From this the rock has often 

been called salite diabase.1 Recently, however, E. O. Hovey 2 has investigated the salite from Sala and has found that the angle of the optic 

axes in air 2Ea = 112° 30', while the mineral under discussion is characterized by a remarkably small angle for a pyroxene 2Ea = 32° 39' 

(Brazil), 34° 47' (Halleberg, in Sweden). Hence it can not be salite, and 

this name should no longer be given to this variety of diabase. The 

mineral has a good prismatic cleavage and also a pronounced cross 

parting parallel to the base. It is sometimes twinned with a (100) as 

the twinning plane. It is more idiomorphic than the brown variety, 

and therefore older. It suffers from alteration much more easily than 

the brown, and while the latter is very fresh the colorless one is everywhere beginning to be attacked by processes which are converting it 

into a fibrous serpentine-like substance. This fact also helps to distinguish it from the brown. In this respect the section is a very instructive one, as it shows very clearly how such processes of alteration by 

weathering go on, whereby minerals rich in magnesia are converted 

into serpentine-like substances.

The process begins on the outside and works inward, or it starts from 

cleavage cracks, which have enabled the capillary moisture to creep 

into the mineral, and works from both sides. The first stage consists 

in the production of a number of fine, parallel, colorless fibers which 

pass from the edge or crack into the mineral. They look like a series 

of fine parallel scratches on the surface of the otherwise unaltered

] See Rosenbusch, Mass. Gest. 1887, p. 202. 

2 Tscliermak's Min. Mitt., vol. 13, p. 218.

.LOO* , \). 6V6.

13, p. 218. 1893. Also p. 213.
DESCRIPTIONS: NO. 106, DIABASE. 269

mineral, and are to be seen best with very high powers. From this 

stage they grow more and more numerous, until finally that portion of 

the mineral attacked is converted into a cloudy opaque substance of a 

brownish or yellowish color.

The brown pyroxene is of the usual aluminous variety of augite found 

in diabases. It is generally much fresher than the white. It has a 

much larger optic angle. Both varieties have a large angle of extinction in sections parallel to 6(010) or nearly so, which sections are easily 

told by the high polarization colors they exhibit. The brown variety 

has an excellent prismatic cleavage, a very poor parting parallel to 

6(010), which may be occasionally observed, and a much better one 

parallel to a(100), on which face it is also frequently twinned.

The pyroxenes in this rock have been chemically investigated by 

Hawes. He did not know, however, that there were two species present, 

and the analysis probably represents a mixture of both of them. The 

material probably contained also a little feldspar. It is of value, however, for it shows the nature of the average augitic component of the 

rock and enables us to determine the average composition of the feldspar 

from the mass analysis.

Analysis of pyroxene of West Rock, Neio Haven, Connecticut; "by G. W. Hawes.

JxaO ' 

Total ......................................

Per cent.

50.71 

 3.55 

15.30 

.81 

13.35 

13.63 

1.17

1.48 

100. 00

The analysis does not, of course, yield ratios which can be construed 

into satisfactory formulas. It must stand, however, until means for 

the separation of the two pyroxenes have been devised, which has not 

yet been successfully done.

The plagioclase feldspar occurs in the shape of intergrowu laths, 

sometimes long and slender, sometimes short and broad, which may 

attain a length of lmm . It is clear, colorless, and very fresh. Twinning according to the albite law invariable; according to the Carlsbad 

law very common; pericline twinning also occurs, but is much less 

common.

The chemical and microscopic work of Hawes and Dana had early 

shown that the general composition of the feldspar was that of labradorite. Later, Hawes, by means of separations made by heavy liquids, 

showed that the feldspars in a similar diabase from Jersey City con-
270 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [auLL.150.

sisted of labradorite and andesite. Since in a dike cutting West Eock 

there occurred phenocrysts which analysis proved to be auorthite, 

Hawes seems to have concluded that there was also more than one 

feldspar present in the West Eock diabase, and that therefore it was 

made up of a mixture of anorthite and. albite feldspars, not of mixtures 

of these constituting intermediate species, as in the Jersey City diabase. 

This singular idea was justly and promptly combated by the late Prof. 

J. D. Dana.

As a matter of fact, the feldspars do belong to intermediate species, 

and range from a rather basic labradorite to andesite, or say from 

Ab2An3 to Ab3An2 . This would give them as an average the composition of an acid labradorite, which is exactly what the chemical work 

calls for.

The section is very well adapted for the study of the plagioclases 

according to the excellent methods elaborated by Michel Le'vy 1 and 

Fouque',2 based on optical properties.

Thus, if sections oriented in the zone of a (100) on c (001), which are 

easily told by their equal illumination of the albite twins when the 

twinning plane coincides with the cross hairs or is 45° from it, are 

studied, the angles of extinction are found to be large, usually above 

20°, and therefore indicative of a basic feldspar. Such sections may 

be easily recognized by the disappearance of the albite twinning in the 

45° position, while in parallel position to the cross hairs the twinning 

lamellae, though equally illumined, are separated by fine, black hair 

lines of the sutures. The 45° position offers the best method of discrimination between the albite and Carlsbad twinning, the former 

disappearing and the latter becoming very evident, especially in the 

basic feldspar.

A number of such sections were selected and measured, of which the 

following will serve as examples:

Angles of extinction of labradorites.

No. 1.

o 

35

35

22

No. 2.

o 

15

15

40

No. 3.

o 

20

18

6

No. 4.

o 

25

27

No. 5.

0

30

30

........

Eeferring now to the tables given by Michel Le'vy, it will be seen 

that No. 1 is that of a labradorite of about the composition Ab2An:i , 

and that the section is inclined about 10° from a (TOO) on c (001). No. 2 

is that of a labradorite a little more basic, the section cut inclined 40° 

from a (100) on c (001). ^No. 3 is a labradorite, Ab2An3 , the section cut 

about 50° from c (001) toward a (100). E"o. 3 has the composition of

1 Determination des feldspaths, Paris, 1894.

2 L'fitude des feldspaths des roches volcaniques, Paris, 1894.
DILI.BE.] DESCRIPTIONS: NO. 106, DIABASE. 271

about Ab2An3 , the section being cut about 65° from a (100), sloping 

toward c (001). No. 4 is AbiAn3 , the section inclined 50° from a (100) 

on G (001). No. 5, which does not show the Carlsbad twinning, indicates a probable labradorite.

It will be noticed that the feldspars are frequently zonally built. 

Often in such sections as those mentioned above this is shown by a 

decreasing angle of extinction from center to periphery. If sections 

are chosen for study parallel to b (010), this is still more marked, and 

extinction angles referred to the trace of the cleavage parallel to 

c (001) will be found to vary from 20° to 10° (negative direction), or 

from basic labradorites to andesites. It must be said, however, that 

everything indicates that labradorite is by far the predominant type of 

feldspar in this rock.

The small amount of potash in the rock, shown by the analysis, indicates a little orthoclase to be present. The chance of recognizing a

scattered grain or two in the section is naturally very small, but as 

the mineral mostly occurs associated with quartz in micropegmatite 

intergrowths it is easily seen. These intergrowths will be found in 

little angular interspaces between the laths of labradorite'. They can 

be studied only by the use of moderately high powers, and though well 

distributed, are not very common. Sometimes a little quartz, alone 

will be found in the interspaces. This micropegmatitic intergrowth 

of quartz and feldspar, or of quartz alone, is the very last product of 

crystallization in the rock.

Biotite will only very rarely be found in the section, and may be 

wanting. It may sometimes occur as an irregular small leaflet of the 

usual brown color and strong pleochroisrn. Sometimes it is bleached 

to a greenish color, but more often it is changed into chlorite.

The distinct idiomorphic lath-shaped form of theplagioclase is characteristic for diabases, and it conditions angular interspaces which are 

sometimes filled with augite, sometimes with glass, and sometimes with 

greenish fibrous material held to be chlorite. The occurrences of these 

masses has been called mesostasis. The exact nature of the greenish 

substance is not known. It has received the name of viridite and 

chloropite, which are, of course, only a cover for ignorance and under 

which various substances are thrown together.

In the West Bock diabase it will be found, on studying the section 

with a high power, that the angular interspaces between the feldspars 

are .frequently filled with material of this kind. It occurs iri fibers, 

sometimes packed in bundles which extinguish simultaneously, sometimes in divergent or radial masses, often in intertwined irregular little 

clumps. It will be noted to have a pale-greenish to yellowish color. It 

extinguishes, so nearly as can be told, parallel to.the fibers, and they 

are extended in an optically positive direction. The maximum double 

refraction of the quartz in one section reached a tone of very pale 

yellow, which shows that the section has a thickness of .03mm (the
272 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

average thickness of the sections). In this, the fibers under discussion 

attain a double refraction color of a brilliant yellow inclining to orange. 

This shows the maximum double refraction of the mineral to be about 

0.01:2, and therefore that it is not the ordinary variety of chlorite commonly found in eruptive rocks and frequently an alteration product of 

mica, as the latter has an extremely low double refraction. With the 

means at present available it can not be said what the precise nature 

of this substance is; it resembles members of the chlorite group of 

minerals in many respects, and, as suggested by Eosenbusch, 1 is best 

designated as the " chlorite-like substance.?' Its derivation from augite 

as an alteration product seems clear from study of the section. It is 

found around it, and in many interspaces the ratio of the amount of 

the mineral is inversely proportional to that of the augite.

The structure of the rock is conditioned by the amount and order of 

formation of its chief components and the conditions under which they 

crystallized. Pyroxene and labradorite are the chief constituents all 

others in amount compared with them are insignificant and they give 

the rock its character. Of them, the white pyroxene crystallized first, 

followed by labradorite, and finally the brown augite. The latter began 

crystallizing, however, before the labradorite had finished. It is this 

crystallization of the labradorite in idiomorphic lath-shaped forms, filled 

in and surrounded by the brown pyroxene, that gives to diabase its 

characteristic structure, termed "ophitic" by the French and wellnamed "intersertal structure" by Zirkel. The West Rock diabase 

shows this very well, but not in so marked a degree as may at times be 

found.

The chemical composition of the rock has been carefully studied and 

discussed by Hawes, to whom we owe the following analysis of the 

West Eock diabase :

Analysis of West Hock diabase, New Haven, Connecticut.

SiOt............................... ...............

TiO,t...... .......................................

A12 03 ................................... .........

FeA--. -- - ..................................

FeO .............................................

MnO... ..........................................

CaO .............................................

MgO.. ...........................................

Ua-jO. ...........................................

K2 0.............................................

T> f\

Total........................................

Per cent.

51.78

1.41

12.79

3.59

8.25

.44

10.70

7.63

2.14

.39

.63

.14

99.89

 Massige Gesteine, 1887, p. 183.
UILLEK.] 'DESCRIPTIONS: NO. 106, DIABASE. 273

This analysis expresses very clearly what the study of the section 

has already indicated the large amount of ferromagnesian minerals, 

shown by the high iron, lime, and magnesia, and that the feldspar 

must at least be a plagioclase of medium basicity, shown by the great 

preponderance of lime over soda. The small amount of potash shows 

that orthoclase can be present only in very limited amount, while the 

titanic oxide shows that part of the iron ore must be present as 

ilmenite.

Hawes l on the basis of his analyses has calculated the proportion 

by weight in which the various minerals enter into the composition of 

the rock. With some rearrangement, this is shown in the following 

table:

Minerals composing the West Rock diabase.

-

Total. .......................................

Per cent.

2.7

1.8

54.5

37.7

2.3

.3

99 3

The above expresses the average composition very well; a minute 

amount of quartz is neglected, as are also the products of alteration.

The rock from West Eock is a normal diabase, consisting chiefly of 

labradorite and augite; the minerals in one generation arranged in 

intersertal structure. It is of rather fine grain and quite fresh. It is 

distinguished by containing an augite of a light color in addition to the 

ordinary augite of such rocks.

Literature of the diabase of the New Haven region. 

DANA, J. D.:

Trans. Conn. Acad. Sci., vol. 2, p. 45, 1870. Amer. Jour. Sci., 3d eer., vol. 6, 

p. 104, 1873; vol. 22, p.. 230,1881; vol. 42, pp. 79 and 439, 1891; vol. 44, p. 165, 

1892. Manual of Geology, 4th edition. 

DAN-A, E. S.:

Proc. Am. Assoc. Adv. Sci., 23d meeting, Aug., 1874; Sec. B., p. 45. Abs. of same,

Am. Jour. Sci., vol. 8, p. 390, 1874. 

DAVIS, W. M.:

Seventh Ann. Eept. U. S. Geol. Survey, p. 455,1885-86. Am. Jour. Sci., vol. 32, 

p. 342,1886; vol. 37, p. 423,188'J. Bull. Mus. Comp. Zool. Harvard Coll., vol. 

XVI, No. 6, Dec., 1889. 

FRAZER, P.:

Second Geol. Surv. Pa., Vol. C., p. 118, 1874. 

HAWES, G. W.:

Am. Jour. Sci., 3d ser., vol. 9, p. 185,1875. Proc. U. S. Nat. Mus., 1881, p. 129. 

HOVEY, E. 0.:

Am. Jour. Sci., vol. 38, p. 361, 1889.

Bull. 150- -18

1 Proc. U. S. Kat,. Mus., 1881, p. 132.
274 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 130.

« 

No. 107. OLIVINE-DIABAS;E.

(FKOM PIGEON POINT, COOK COUNTY, MINNESOTA. DESCRIBED BY W. S. BAYJ.EY.) ' '

No. 107 is a representative of the rock that has always been called 

an olivine-gabbro in the writings of students of Lake Superior geology. 1 

The specimen was obtained from the north shore of Pigeon Point, 

the northeastern extremity of Minnesota. Here the rock occurs as a 

large-dike-like mass, cutting Huronian slates and quartzites. Its best 

exposures are on the north shore of the point at the water's edge, 

where the rock forms high cliffs whose face is always kept perpendicular 

through the action of frost, which penetrates joint cracks, and by the 

force of its expansion loosens large blocks that fall to the base of the cliff 

and there form a talus of fresh material. It is from one of the freshest 

of these blocks that the specimen in the collection was taken.

The rock is a medium-grained, gray, crystalline aggregate, in which 

two components may easily be extinguished. One of these is a white or 

nearly colorless mineral with glistening cleavage faces, often occurring 

in long, narrow crystals, upon whose surfaces may usually be detected ! 

longitudinal striations. Its color, cleavage, and structure indicate a 

plagioclase. The other component inacroscopically distinguishable is a 

jet-black substance also possessing glistening cleavages. Under favorable circumstances this substance, which is a pyroxene, may be seen to 

surround the feldspar crystals in such a way that the latter appear 

embedded in the former. Close inspection of the specimen will also -~ 

discover small areas of a finely granular texture and of a yellowish 

tinge. These areas consist chiefly of olivine. From a macroscopic 

examination, then, we learn that the rock is a uonporphyritic, crystalline 

aggregate of augite and plagioclase; in other words, that it is either a 

diabase or a gabbro. The long, narrow development of the plagioclase 

suggests a diabase.2

The density of the rock varies between 2.927 and 2.970, according as 

the specimen investigated contains a larger or smaller proportion of * 

feldspar. Its chemical composition, as found by Mr. W. F. Hillebrand,

is as follows:

______________ __________________________ 

'Aug. Strong, Nones Jahrb. f. Min., etc.. 1877, pp. 113-138; R. D. Irving, Geol. of Wisconsin, Vol. 

III. pp. 168-183; Alex. Julien : Geology of Wisconsin, Vol. Ill (1880), pp. 233-238; R. D. Irving, The 

copper-bearing rocks of Lake Superior, Mon. TJ. S. Geol. Survey, Vol. V, pp. 37-50.

2 Although the specimen in the collection is not porphyritic, in certain areas on Pigeon Point the ;. 

same rock is so developed that porphyritic crystals even 6 or 7 inches in length are not uncommon. 

The crystals are exactly similar to the crystals of plagioclase in the groundmass of the rock, except 

in point of size.
DILLElt.] DESCRIPTIONS: NO.-107, OLI VINE-DIABASE.

Analysis of olivine-diabase from Pigeon Point, Minnesota.

275

SiO,.. ...........................................

Ti02 .................................. ...........

ZrOj.............................................

AIA- . . - --- --- .- 

TiV A 

FeO.. ............................................

MnO. ............................................

CaO........................ ......................

SrO................ ..............................

BaO .............................................

MgO. ............................................

K,0.. ............................................

Li/).. ...........................................

H2 0 .............................................

 p t\

ci. ..................'.............................

Total ...................'...................

Per cent.

49.88

1 19

i o r,r.

2.06

8.37

.09

9.72

.02

5. 77

.08

2.59

1.04

100. 12

Its thin section, when examined under the microscope, is seen to be 

composed essentially of the three minerals above mentioned, viz, 

plagioclase, olivine, and augite. The plagioclase comprises about GO 

per cent of the sections. In ordinary light it appears as a colorless 

gi'oundmass in which everything else lies embedded. Under crossed 

nicols this apparently homogeneous matrix breaks up into numerous 

long, narrow crystals, ranging in length from 1 to 20""". Each of these 

is composed of a number of smaller individuals which, by their union, 

often give rise to broad lath-shaped forms, like those so characteristic 

of some of the European gabbros, notably those from Volpersdorf, in 

Silesia, and from the Harz. 1 Each of the smaller crystals is polysynthetically twinned according to the albite law, in which tha brachypiuacoid (cc P 06) is the twinning plane, and often two of them unite to form 

a Carlsbad twin with the inacropinacoid (ao P o>) as the twinning plane 

and the brachypinacoid (ao P'o>) as the composition face. The material 

of the crystals is very fresh, and is often almost as glassy as the feldspar of modern volcanic rocks. It contains as inclusions only a few 

particles of dust and minute flakes of brightly polarizing fibrous 

kaolin, except in those rare cases where slight alteration has developed 

a larger quantity of the latter mineral, when the feldspar substance 

becomes cloudy, and under crossed nicols is observed to be full of 

little needles, showing brilliant polarizing colors.

The polysynthetic twinning lamella} noticed in nearly every piece of 

the feldspar would lead to the suspicion that this is a plagioclase, but 

their occurrence is not positive proof of this, as a repeated growth of 

Carlsbad twins would give rise to the same phenomenon.. The angles 

made by its two systems of cleavage lines, however, are 85° and 95° on

1 BosenbusclV, Mikroskopische Physiographic, II, 1887, p. 155.
276 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [uuLL.150.

the macropiuacoid, and the cleavages are never at right angles to each 

other in any sections. Moreover, the maximum symmetrical extinction 

of lamella on each side of a twinning line is 24°. These results indicate a triclinic feldspar belonging in the lower portion of the labradorite group. 1 If a fragment of the rock is powdered and sifted, and its 

constituents are separated by means of one of the heavy solutions used 

for this purpose, it will be found that most of the plagioclase will fall 

when the density of the solution is between 2.700 and 2.716. If a 

portion of the material that falls at 2.700 be analyzed, it will give 

the figures in column I. In column II is given for comparison the 

composition of a labradorite 2 of the formula Ab3Au^, and in III that 

of a plagioclase :! with a specific gravity 2.700, from a European locality.

Analyses ofplagiocJases of olivine-diabane.

SiO-j .........................

AljOs---. ....................

Fe^Os...... ..................

CaO .........................

Na.,0 ........................

Total..................

I.

53.75'

30. 39

1.26

10.84

100. 00

2.700

II.

53.60

29. 80

11.70

100. 00

2.703

III.

54.55

28.68

1 fiQ

11.28

2.700

It will thus be seen that the density and the chemical composition of 

the mineral, as well as its optical and physical properties, agree very 

well with those of a basic labradorite consisting of a mixture of three 

nlbite molecules and four of auorthite.

The olivine constitutes about a tenth of the rock. It is undoubtedly 

older than the augite, since it is surrounded by this mineral (see PI. 

XXXVIII, A), and is probably older than most of the labradorite, 

although, in a few instances it may be seen to include portions of crystals of the feldspar. Its period of crystallization must have overlapped 

that of the plagioclase, i. e., while the chief portion of the olivine 

separated from the magma before the feldspar had begun to form, a 

small portion of it solidified after the labradorite began to crystallizeThe mineral is in rounded grains of a light .yellowish-green color. It 

has a high index of refraction and consequently a rough surface, and is 

traversed by irregular cracks filled with green decomposition products. 

Excepting the feldspar crystals included in the oliviue above referred 

to, no inclusions other than dust-like particles, like those in the feldspar, and certain green acicular secondary substances are met with 

in it. In but very few sections does the olivine remain unaltered. 

In even the freshest varieties a little chloritization has taken place, 

and this shows itself as a green rim around the edge of the grains 

and in the cracks. The small, brown, strongly pleochroic flakes, with 

a well-marked cleavage, sometimes intermingled with the chlorite,

1 Iddings's Eosenbiisch's Microscopical Physiography, p. 300.

* Sclmster, Tschermak's Min. u. Petrog. Mitth., Ill, 1880, p. 153.

3 Tscliennab, Sitzungsber. d. K. Akad. Wias. (July 3,1869), Band LX, AM. 1,1870, p. 145.
DIT.LER.] DESCRIPTIONS: NO. 107, OLIVINE-DIABASE. 277

are biotite plates, produced, like the chlorite, by decomposition of 

the olivine, and the bunches of dark-green fibers that penetrate the 

fresh mineral are probably some form of hornblende.

The youngest of the essential components, augite, is entirely allotriomorphic. It fills the interstices between the other constituents, and 

thus has its contours molded by these. (See Fig. A, PI. XXXVIII.) 

Often several feldspar laths are included in a single augite plate, which, 

when broken, reflects the light uniformly from its surface, leaving dull 

areas in those places occupied by the feldspar. This kind of intergrowth gives rise to what is known macroscopically as luster mottling. 1 

The microscopic structure produced by it has been denominated pceci 

litic.2 The gn-ater part of the augite is perfectly fresh, and, like the 

feldspar and the olivine, it contains no true inclusions other than 

the dust-like particles scattered throughout its mass. Its color is 

purplish-pink, with a faint pleochroisin; the a and .c niys being & 

deep purplish-pink, in highly-colored pieces, and the It ray yellowishgray. The two series of prismatic cleavage lines are very distinct in 

pieces ciit parallel to the basal plane, where they make the usual 

pyroxene angles of 87° and 93°. In other sections they form a series 

of parallel lines that are sometimes gently curved as the result of 

pressure. (See a, PL Y, Bull. U. S. Geol. Survey, No. 109.) The 

diallagic parting parallel to the orthopinacoid is only rarely seen. 

The extinction on plates cut parallel to the cliuopinacoid is 44° against 

the single cleavage.

The augite of nearly all Lake Superior rocks is uniformly tinged with 

purple, whereas that of foreign rocks is generally greenish, so that 

some explanation must be sought for the difference. Knop,3 who made 

an exhaustive study of the augite of the Kaiserstuhl in Baden, found 

that all the varieties of this mineral rich in titanium and poor in iron 

have a purple color in thin section. Although investigations in other 

regions do not fully substantiate Kuop's conclusion, it is interesting to 

note that the purple augite in the Pigeon Point rock contains a large 

percentage of titanium and a comparatively small proportion of iron. 

A partial analysis of the augite powder separated from the specimen 

was made in the laboratory of the United States Geological Survey by 

K. B. Biggs. His figures, indicating a very pure diallage, are :

Analysis of diallage of olimne-diabase from Pigeon Point, Minnesota.

Si02 .............................................

ALA..... ........................................

Fe,03 .......................................... ..

FeO.. ............................................

CaO........ ......................................

MgO....... ......................................

Ti 0* .............................................

Per cent.

2.00

4.C8

14.15

15.10

.11.34

1 K. Punipelly, Proc. Amer. A cad. Adv. Sci., XIII, p. 260; and It. D. Irviug, Copper-bearing rooks, 

p. 42.

*G. H. Williams: Am. Jour. Sci., 3d series, ^7"ol. XXXT, 1880, \>: 30. 

"Groth's Zeitsch. f. Kryst, X, p. 58.
278 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [BULL. 150.

In addition to the essential components, the rock, con tains only apatite and titaniferons magnetite as primary constituents. The apatite 

is only sparingly present in long, narrow, colorless crystals, with a 

parting parallel to the base. The magnetite appears both in idiomorphic and in allotriomorphic forms, the latter predominating. A large 

number of the grains, especially in slightly altered specimens, are 

surrounded by rims of reddish brown mica, and not a few show the 

beginning of an alteration into cloudy-white or gray leucoxene. A 

little quartz and a few flakes of brown and red biotite, strongly pleochroic masses of green chlorite, and some undeterminate substances, 

filling cracks in the essential components and occupying corners 

between them, may be found in some sections. The quartz is in micropegmatitic intergrowths with feldspar; the biotite is an alteration 

product of olivine, augite, and magnetite, and the chlorite a product of 

the decomposition of the first two of these.

As has already been stated, the rock has been pronounced a gabbro by all geologists who have studied it. Its pyroxene has the 

composition of diallage, and it sometimes possesses the orthopinacoidal parting characteristic of this mineral. 1 Its structure, however, is more nearly that of ithe diabases than of the gabbros (see PI. 

XXXVIII, A). Its plagioclase is in lath-shaped crystals, embedded 

in angite, so that its texture is not granitic, but ophitic. The texture, 

then, if alone considered, would place the rock with the diabases, while 

its mineralogic composition would place it among the gabbros. Recent 

investigations have shown that many holocrystaliine basic rocks, which 

no one would for an instant regard as gabbros, contain typical diallage. 

Therefore the possession of this constituent is not characteristic of 

gabbros; so there remains only the structure as a means of distinguishing between these rocks and the diabases. Professor Judd 2 has suggested that we limit the term gabbro to those rocks composed of 

plagioclase and pyroxene, that possess the granitic structure, i. e., 

that have their constituents developed in approximately equal sized 

grains, without crystal contours, and that we designate as diabases all 

rocks of this composition in which the structure is ophitic. According 

to this view, the Pigeon Point rock is a coarse-grained olivine-diabase.3

No. 108. GABBRO.

(FROM MOUNT HOPE, BALTIMORE COUNTY, MARYLAND. ABSTKACT BY .T. P. TDDINGS

FROM THE DESCRIPTION BY G. H. WILLIAMS.)

The occurrence and petrographical characters of this gabbro have 

been fully described by the late Prof. George H. Williams, and it will

'For a resum6 of the opinions of different investigators on the value of the orthopinacoidal part 

ing as a characteristic of diallage, as distinguishing it from augite, see AVadsworth, Bull. No. 2, Geol. 

and Nat. Hist. Survey, Minnesota, pp. 55-57.

2Quart. Jour. Geol. Soc. London, 1885, pp. 354-418.

3Por a fuller description of the rock, both in its fresh and altered condition, see W. S. Bayley, The 

eruptive and sedimentary rocks of Pigeon Point, Minnesota, and their contact phenomena: Bull. U. S. 

Geol. Survey No. 109, 1893.
U. 8. GEOLOGICAL SURVEY

THIN SECTIONS AS SEEN UNDER A MICROSCOPE.

A. Ol'vine-oiabase from Pigeon Point, Minnesota, X 40.

B. Garnetiferous gabbro from Granite Falls, Minnesota, X 33.

The light-colored, long, narrow crystals of A are plagioclase, in which dust inclusions and twinning striations may 

easily be seen. Surrounding these are large plates of diallage, with a series of parallel cleavage lines well developed- The rounded grains at the right of the section and elsewhere are olivine. They are traversed by irregular 

cracks and have apparently a fibrous texture. This appearance is due to the strong refraction of the mineral, and 

is purely a light phenomenon. No indication of fibrous texture is noticed in the thin section. The olivine and the 

plagioclase are clearly older than the pyroxene, which fills in the spaces left between these minerals, giving rise to 

the ophitic texture so characteristic of diabase,
nnxER.] . DESCRIPTIONS: NO. 108, GABBRO. 279

only be necessary to quote what he has published ou the subject in 

Bulletin 28 of the United States Geological Survey, which is entitled. 

The Gabbros and Associated Hornblende Epcks Occurring in the 

Neighborhood of Baltimore, Maryland. To this paper the student is 

referred for fuller details thaD can be given in this place.

The gabbro is well exposed in a railroad cut at Mount Hope Station 

on the Western Maryland Railroad, where it grades into hornblendegabbro-gneiss (diorite).

As a rule the two rocks alternate with each other in every direction horizontally 

aud vertically in the most perplexing manner, and it is frequently impossible to 

tell on the spot whether a given specimen is gabbro or diorite (hornbende-gabbrogueiss). . . . Just west of the bridge which crosses this cutting the gabbro 

shows a decided tendency to spheroidal weathering so often characteristic of basic 

massive rocks. [The hyperstherie gabbro aud the hornblende-gabbro-gneiss at this 

place are clearly one geological mass.]

Excellent exposures of the hypersthene-gabbro are very abundant witlliu the 

Baltimore area, especially in the northern portion of it. . . . Perhaps the best 

place to obtain fresh and typical specimens is at Mount Hope Statiou, although they 

may be secured from the huge bowlders which strew the surface at almost any spot 

within the area.

The rock is always very massive in appearance, rarely exhibiting the bunded and 

nowhere the schistose structure, which is frequently seen in the associated hornblendic rocks. Its irregular polygonal blocks are often covered with a thin coating 

of a deep red color due to decomposition, beneath Avhich, however, the rock is surprisingly fresh. . . .

The color of the hypersthene-gabbro is on the whole uniform, although it may 

vary from a purplish black to gray.

The most striking feature in the texture of the unaltered gabbro is the repeated 

and abrupt change in the coarseness of the grain which is seen at some localities. 

This phenomenon, as is well known, is one frequently observed in very ancient massivo rocks Avhich cover considerable areas. . . . Irregular patches of the coarsest 

kinds lie embedded in those of the finest grain without regard to any order. In 

other cases a more or less pronounced banded structure is produced by an alternation of layers of different grains or by such as have one constituent developed more 

abundantly than the other. Such bands are not, however, parallel, but vary considerably in direction, and show a tendency to merge into one another, as though they 

had been produced by motion in a liquid or plastic mass. . . .

The mineral constituents of the gabbro which are discernible with the unaided 

eye are plagioclase, diallage, and hypersthene. A black hornblende, which is brown 

in transmitted light, is also sometimes seen in good-sized crystals, and has every 

appearance of being a primary component. Magnetite and apatite are shown by 

the microscope to be universally present, although in varying quantities. Olivine 

was observed in only one specimen, collected near Orange Grove. With the exception of certain indeterminable inclusions, no other minerals were discovered in the 

wholly unaltered rock.

The grain of the hypersthene gabbro is, as a rule, uniform and fine, the component 

minerals averaging from 1 to 2mm in diameter. Exceptionally, however, the grain 

becomes coarser; in a few specimens the individuals of pyroxene and feldspar measured from 25 to 35mm in length. The coarsest varieties are rarely altogether fresh.

The feldspathic constituent of the hypersthene-gabbro is bytownite, corresponding to a mixture of six molecules of auorthite with one of albite. Stauroscopic 

measurements on the feldspar extracted from a Mount Hope specimen of the gabbro 

gave extinction angles (measured against the cleavage lines) of 16° to 19° on OP 

(001) and of 28° to 30° on ^ Pob (010).
280 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

A chemical analysis of this feldspar, made by W. S. Bayley, gave the following 

percentages:

Analysis of feldspar in gabbro from Mount Hope, Maryland.

Si02 .............................................

A1A-- --- --. ...................... ........

CaO.................................. ............

Total......................................

Specific gravity, 2.74.

Per cent.

' 46.17

35.23

16.29

2.31

100. 00

a Difference.

In the above-mentioned specimen of gabbro from Mount Hope, the feldspar constitutes somewhat over one-half of the entire mass. This may perhaps be regarded 

as a fair average, although great variations in the proportions of the component 

minerals caused the analyses of specimens of the gabbro from different localities to 

differ widely.

The feldspar individuals are generally quite irregular in shape, giving rise to the 

granular structure which is characteristic of gabbro in contrast to diabase. Rarely, 

ho wever, a lath-shaped cry stal in dicates a slight tenden cy to ward th e oph itic structure.

When seen between crossed nicols, the feldspar appears, as a rule, finely striated. 

Sometimes two systems of strife are seen to intersect at an angle of nearly 90°. In 

other cases the striations are very coarse or altogether wanting, in which case the 

extinction is more or less undulatory and irregular. [In some instances the striations are slightly curved or bent, as though by dynamic stress. J. P. I.]

The minute dust-like inclusions, which are so common in the feldspars of the 

older basic rocks, are admirably developed in the Baltimore gabbros. They answer 

very well to the descriptions given of them in rocks from other localities by Zirkel, 

Hagge, Hawes, and other investigators. When viewed with a IOAY magnifying 

power the plagioclase appears to be covered with a fine black or brown dust, which, 

under the highest magnifying power, is resolved into a mass of very minute opaque 

dots and lines. The arrangement of these inclusions is such that the needles occupy 

the center of the crystal; this is surrounded by a zone where only the dots or 

globulites are to be seen. The exterior of the crystal is generally free from inclusions of any kind. Frequently the acicular bodies are altogether absent, and thns 

the minute dots are arranged in lines which indicate the position of the twinning 

lamellfB. An occasional inclusion of another kind, like a fluid cavity, an apatite 

crystal, or a grain of magnetite, is surrounded by a narrow zone which is quite 

free from the dust-like particles. Neither a microscopical nor a chemical examination served to give any clue to the mineralogical nature of these minute bodies.

The diallage is the constituent of the gabbro next in order of importance to the 

feldspar, although in some specimens it is not so abundant as the hypersthene. The 

chemical composition of this mineral from a Gwynns Falls specimen is as follows:

Analysis of'diallat/e of yabbro from Gwynns Falls, Maryland,

SiO*........ ......................................

AT n

FeO. .............................................

MnO.. ...........................................

CaO..............................................

MgO...... ............................ ..........

Specific gravity, 3.26.

Per cent.

51.41

4.32

9.31

.04

20.60

15.14

100. 82
D1T.T.KTI.] DESCRIPTIONS: NO. 108, GABBRO. 281

]n hand specimens the dUllage appears black, but; when seen by tnnisuiit.tcd light 

in thin sections it has a light green color and exhibits no appreciable pleochroisni. 

This constituent, like all the others, shows no indication of a crystal form; it is 

present only in rounded or irregularly shaped grains, and appears to have crystallized 

somewhat later than the feldspar. The prismatic cleavage and the parting parallel 

to the orthopinacoid are both well developed; frequently a second parting parallel 

to the clinopiuacoid is also present. In sections approximately parallel to either 

OP (001) or oo P6b (100) an optical axis not a bisectrix appears, when the examination is made -with convergent polarized light. Clinopinacoiclal sections exhibit an 

extinction angle as large us 40°.

Twins, according to the ordinary law for augite, where the orthopiuacoid is both 

twinning plane and composition face, are quite common. Instances where the 

twinning trace is inclined from 25° to 35° to the cleavage lines, as described by 

Coheu, 1 Rosenbusch, 2 and others, have been also not infrequently observed.

Other systems of fine striations, visible only between crossed nicols, are very often 

seen traversing the individuals of diallage in several directions. They are by no 

means always straight in their coarse nor continuous across the crystal, but appear 

to be clue to some molecular disturbance produced by pressure, to Avhich, as is well 

known, this mineral is peculiarly sensitive. 3

In its freedom from inclusions the diallage, as a rule, presents a contrast to the 

hypersthene, although when such inclusions are exceptionally present they do not 

appear materially to differ from those of the rhombic pyroxene.

Hypersthene is a constant component of the Baltimore gabbros, although its 

amount varies widely in different specimens. It is readily detected in the rock by 

its bronzy luster. . . . This effect is, however, altogether due to inclusions in 

the hypersthene, and, since these may be wanting, the absence of the metallic rellection is of itself no proof that this mineral is not present. Its chemical composition 

is as follows:

Analysis of hypersthene of Maryland gabbros.

AlsO, ............................................

FeO.-.. ..........................................

CaO.. ........ ....................................

MgO.... .........................................

Total ......................................

Specific gravity, 3.35.

Per ceut.

52. 12

1.C9

20.94

3.20

21.56

99.51

Under the microscope this mineral exhibits no better crystalline form that the 

diallage, from which, however, it is easily distinguished by its marked trichroisin. 

The ray Avhich vibrates parallel to the brachydiagonal aixis (a = a) is brownish red, 

the one vibrating parallel to the macrodiagonal (& = b) is light greenish yellow, 

while that one which vibrates parallel to the vertical axis (c=C) is green. The 

absorption is a < C <C t>» C, being very nearly equal to a- ...

The cleavage parallel to the unit prism ooP (110) is well developed, but even more 

pronounced is a parting parallel to both pinacokls oo Poo (100) oo Poo (010). . . . 

The fact that the brachydiagonal axis is the acute bisectrix, and the optical angle 

comparatively small, is sufficient indication that it is hypersthene and not onstatite 

or bronzite. . . .

'Geognostische Bescbreibung tier Umgegeud von Heidelberg, p. 69, 1881. Sarnmlung von MikrophotograpbieD, etc., Taf. XXVIII, fig. 4, 1881.

2Mikro8kopische Physiographic, Vol. II: Die raassigen Gesteine, p. 410. 

8Van "Worveke: Nones Jabrbuch fur Miueralogie, Geologie, und Paliiontologio, II, p. 97,1883.
282 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOM. 150.

The peculiar inclusions so generally characteristic of Jhypersthene are, as a, rule, 

present in great perfection. . . . They are composed of fine needles arranged 

parallel to all three crystallographic axes and of little plates of a reddish brown 

color lying parallel to the brachypinacoid. To these latter the peculiar metallic 

reflection seen in many sections is due. They vary considerably in thickness, as may 

be seen by the different depths of color which they possess. These plates all extinguish simultaneously Avith the hypersthene, which may be due to their extreme 

thinness or to the fact that they have their axes parallel to those of their host. . . . 

Although generally abundant, they are sometimes entirely absent from the hypersthene.

A yellowish brown hornblende, undoubtedly of primary character, is sometimes 

met with in the hyperstliene-gabbro. . . . This primary hornblende is, in the 

Baltimore gabbros, strongly pleochrpic. The a ray, approximately parallel to the 

clinodiagonal axis, is light yellow; the b ray, coinciding with the axis of symmetry, 

is brownish yellow, while the c ray, inclined about 12° to the vertical axis, is yellowish brown. The absorption is, as usual in hornblende, C>b>florC b>8.

This hornblende is sometimes so full of minute black inclusions as to be nearly 

opaque, even when in very thin sections. These interpositions are, however, not 

evenly distributed, but are massed in irregular patches. No trace of external crystalline form is discernible. This mineral appears to have been the last to crystallize 

from the magma, a single individual often covering a considerable space and inclosing smaller grains of both pyroxene and feldspar. It is not infrequently intergrown 

with pyroxene in such a manner that the orthopinacoids of both minerals are coincident. ...

The occurrence of this hornblende in the least altered specimens of the gabbro, 

its compact structure, and the other marked contrasts which it presents to the green, 

fibrous amphibole, . . . are all a sufficient warrant for its original character.

Green, more or less fibrous amphibole occurs in variable amounts in 

some sections of the gabbro from Mount Hope. It seldom exhibits 

itliomorphic outlines, but is in confused aggregates of imperfectly 

developed individuals. The pleochroism is pronounced, and is that common in hornblende, i. e., the a ray, nearly parallel to the clinodiagonal 

axis, is light yellow; the IK ray, coincident with the orthodiagonal axis, 

is yellowish green, and the .c ray is dark bluish green. The absorption 

is JC >ft>&. It appears to be of secondary origin.

Of the other constituents occurring in the hypersthene-gabbro, the magnetite 

[pyrite] and the apatite present no peculiarities worthy of mention. The former is 

never very abundant; the latter rarely so, although in one specimen from Mount 

Hope it was found to compose 12 per cent of the entire mass of the rock.

NO. 109. GARNETIFEROUS GABBRO.

(FROM GRANITK FALLS, YELLOW MEDICINE COUNTY, MINNESOTA. DESCRIBED BY

W. S. BAYLEY.)

No. 109, like Nos. 140 and 144, is from a rock found interbedded with 

gneisses and schists in the valley of the Minnesota Eiver, in .Minnesota. 

The specimens in the collection were obtained from a pit some 20 feet in 

depth that had been blasted in the rock near a quartz vein which had 

been explored for gold. In the neighborhood of the vein the rock is 

quite massive, but at a distance of a few feet from the contact it has a
DILLBB.] DESCRIPTIONS: N.O. 109, GARNETIFEROUS GABBRO. 283

distinctly schistose character. The location of the pit is 1,500 paces 

north of the SE. corner of sec. A, T. 115 K, B. 30 W., Minnesota. 1

In the hand specimen the rock has a massive aspect. Its prevailing 

color is dark green, speckled .with large patches of dark red and small 

areas of greenish yellow or white. Upon close inspection the yellow 

and white areas are discovered to be the glistening cleavage surfaces of 

a striated plagioclase. The red areas are the surfaces of a .dark-red, 

transparent, hard mineral. It has no distinct cleavage, and is insoluble 

in acids. The properties are those of garnet. The nature of the darkgreen matrix in which the garnet and plagioclase are embedded can not 

be determined, though from its dark color it may be assumed to be very 

basic. The specific gravity of the rock is 3.105.

Under the microscope the thin section shows a granular aggregate of 

plagioclase, green augite, garnet, and magnetite, with small quantities 

of green hornblende, a few large grains of quartz, and tiny crystals of 

apatite.

The plagioclase is in irregular allotrioniorphic grains crossed by welldefined cleavage cracks and traversed by irregular fissures filled with 

decomposition products or stained by iron oxides. The material of the 

feldspar is clear and colorless except where rendered cloudy by inclusions. The most abundant of these are tiny flakes of chlorite and 

irregular masses of opaque, earthy substance, besides dust-like particles of magnetite and little nests of a brightly polarizing calcite. All 

these are quite abundant in the neighborhood of the fissures, but are 

rare in other parts of the grains. Other frequent inclusions are long, 

narrow crystals of a strongly refractive, colorless mineral, with a parallel extinction and weak double refraction. In cross sections they 

possess the hexagonal outline and the isotropic character of apatite. 

Their average length is about 1""" and their thickness about 0.05""", 

though a few have cross sections measuring 0.2111 " 1 in diameter. Augite, 

hornblende, and large masses of magnetite are also included in the 

plagioclase.

Under crossed uicols the plagioclase twinning becomes very apparent, 

as fairly wide bands, usually running entirely across the grains. Some 

of the bands curve slightly, others wedge out as they pass toward the 

interiors of the grains, and still others spring from the sides of cracks, 

etc. These phenomena, as explained in the description of iTo. 77, indicate that the rock has been subjected to pressure since it solidified. In 

certain restricted areas in the section, notably in the neighborhood of 

large garnets, there is often a second series of twinning lamellae cutting 

the first series at some acute angle. This second series comprises 

numerous lamellae, not as distinct as those of the first set, and not as 

sharply marked off from one another. They cross the bars belonging 

to the first set and have a more or less undulous extinction. The 

specific gravity of the greater portion of the plagioclase is somewhere

1 Seo map, pi. 28, Geology of Minnesota, vol. 1, of ITinal Buport, p. 58'J.
284 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL-ISO.

about 2.68, or near audesine, though probably a more basic member is 

also present, with a density of about 2.72. '

The next most abundant component of the rock is a pyroxene. This 

is in green, slightly pleochroic, allotriomorphic grains which are older 

than the plagioclase, since the latter conform to the former in shape, 

and occasionally include them. This augite when fresh is marked by 

two series of cleavage lines crossing each other at angles of about 90° 

on basal sections, and on other sections by a single series of parallel 

lines. The pyroxene may easily be distinguished in these sections 

from hornblende by the large angles of extinction against the parallel 

cleavage lines, which often reach as high as 43°, whereas those of hornblende rarely approach 24°, and by the lack of strong pleochroism.

In addition to the two cleavages mentioned as prominent on basal 

sections, there is often another, presenting itself as a series of closely 

crowded parallel lines, bisecting the larger of the two nearly rectangular intersections of the more prominent lines. Since the shorter of the 

two lateral axes in pyroxene is the orthoaxis, this cleavage must be 

parallel either to the orthopinacoid or to some orthodome. But since 

sections in the zone of the orthopinacoid and the clinopiuacoid show 

only a single series of parallel lines, it is evident that the cleavage 

under discussion must be orthopiuacoidal. This cleavage is characteristic of diallage. For some time it was regarded as original and the 

mineral exhibiting it was looked upon as a distinct species of pyroxene. 

Now, thanks to the investigations of Professor Judd, 1 it is known that 

the cleavage is secondary and that its origin is often the direct consequence of pressure,2 to which the rock containing it was subjected.

Though much of the diallage is fresh, a still larger proportion is 

more or less altered. The interior is often stained brown or yellowish 

brown, and at the same time is deprived of its power of polarizing 

brilliantly. A little magnetite in rounded grains separates around the 

edges ®f the altered portion, and a more or less fibrous cleavage is 

developed in it. In other places the yellow substance occurs in little 

plates or needles arranged in parallel lines inclined to the cleavage, 

thus giving rise to the appearance of a third set of fine cleavage lines. 

In some cases, the lines are so straight and narrow that they are with 

great difficulty distinguishable from true cleavage lines, even under a 

high power. The origin of somewhat similar inclusions, so common a 

feature iu diallage and hypersthene, has given rise to much discussion, 

Judd thinking them secondary infiltration products along planes of 

easy solution,3 and others regarding them as original inclusions taken 

up by the mineral during its growth.4

Around the outside edges of the diallage another alteration is

1 Quart. Jour. Geol. Soc. London, 1885, pp. 378-379.

2 M. E. Wadsworth: Bull. No. 2, Geol. and Nat. Hist. Survey Minnesota, p. 55 et seq. 

3 Quart. Jour. Geol. Soc. London, 1885, p. 354.

4 G. H. Williams: Am. Jour. Sci., 3d series, Vol. XXXI, Jan. 1886, p. 33; and Vol. XXXII r, Pob.1887, 

p. 143.
DILLEB.] DESCRIPTIONS: NO. 109, GARNETIFEROUS GABHRO. 285

observed. Here the pyroxene is surrounded by large plates or small 

granules of a bright-green substance with strong pleochroism, in yellowish-brown and dark bluish-green tints, and an extinction inclined to 

the cleavage. This substance, which is a hornblende, surrounds the 

augite as a narrow rim, which sometimes extends into the pyroxene 

grain, and at other times is formed of granules that seem to have been 

added 1 to the grain after its formation. In either case the substance 

is to be regarded as secondary in origin. In a few cases, when the 

pyroxene grain was small, the entire substance has been changed, and 

in its place is now an area of hornblende, that might be regarded as 

original, were it not for the fact that so much, of the hornblende of the 

rock is undoubtedly secondary.

Another product sometimes formed by the alteration of the pyroxene 

is biotite. This is in small reddish-brown flakes, mingled with the 

hornblende on the periphery of the diallage.

The magnetite and pyrite appear as large irregular grains that are 

present more frequently near the pyroxene and garnet than elsewhere 

in the rock. Both have resisted alteration, and both are equally 

opaque, so that the only method of distinguishing between them is by 

their lustre in reflected light. The magnetite is black and the pyrite 

brassy yellow.

The garnet is the characteristic mineral of the rock. It is in large 

cellular masses, sometimes measuring half an inch in diameter. In 

the section it appears as a highly refractive, isotropic, deep pink substance, filled with inclusions and crossed by many irregular cracks, 

along the sides of which are stains of yellow iron oxides. So large 

and so numerous are the inclusions that the garnet substance in its 

arrangement reminds one of the section of a coarse sponge saturated 

with various colorless products. The largest and most striking of the 

inclusions are quartz grains. These are colorless and without cleavage 

traces. They inclose little mica plates, apatite crystals, dust particles, 

and thousands of little liquid-filled pores, arranged in lines. Under 

crossed uicols most of the larger grains break up into an aggregate 

with the lines of inclusions passing from one grain into another without interruption. Occasionally some of the clear inclusions in the 

garnet are discovered to be plagioclase, but these are comparatively 

rare. The other substances inclosed by it are small pieces of biotite, 

crystals nnd particles of magnetite, crystals of apatite, and thousands 

of tiny cavities filled with liquid. As in the quartz, these are arranged 

in lines, and these lines are sometimes continuous in both substances. 

This latter phenomenon would indicate that the inclusions are of secondary origin, and that they were formed after the quartz and garnet 

had assumed their present positions. The age of the garnet is shown 

by its associations to be younger than that of the other constituents.

1 .For descriptions of added growths to pyroxene and hornblende, see C. R. Van Hiso: Am. Jour. Sci., 

M series, Vol. XXXIII, May 1887, p. 385, and G. P. Merrill, Ib, Vol. XXXV, June 1888, p. 488.
286 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Another form in which the garnet exists is in small granules surrounding magnetite. Here the mineral has the same properties as 

when in large pieces, except that it contains no large inclusions of 

quartz and feldspar.

The quartz is in colorless masses; surrounded by garnet, as already 

mentioned, and also in larger pieces associated with the garnet, but 

not included in it. It usually occurs most abundantly near the garnet, 

in those sections containing a great deal of this mineral, and is almost 

if not totally absent from those sections in which there is none.

Since the garnet is probably secondary, i.'e., since it was probably 

formed after the main portion of the rock solidified, it is also probable that the quartz is of secondary origin,'and that the original 

components of the rock were essentially diallage and plagioclase, with 

magnetite, pyrite, and apatite as accessory components.

A rock of this composition is a gabbro, if its structure is not schistose. The principal mass of the rock under study is schistose, but it 

is very probable that this is an imposed structure, since, even in the 

apparently massive variety represented by the hand specimen, abundant indications of pressure are apparent. The massive phase, since it 

is characterized by garnet, must be denominated a garnetiferous 

gabbro, while the schistose'variety is a squeezed gabbro or a zobtenite, midway in character between Nos-. 135 and 137.

The photograph (PI. XXXVIII, B] shows a large grain of plagio 

clase, surrounded by garnets, with the interstices between them filled 

with quartz. The black grains to the right are magnetite arid pyrite.

The composition of the rock, as found by H. N. Stokes, is:

Analysis of garnetiferous gabbro from Granite Falls, Minnesota.

AlgO,... .........................................

FeA...... ......................................

FeO.. ............................................

CaO.. ............................................

MeO .............................................

K20.... ..........................................

NagO....... ......................................

Per cent.

. 52. 31

18.35

5.90

11.06

7.33

1.00

.49

2.90

. ;!5

99.60

PEKIDOTITE FAM1Z/Y.

No. 110. PYEOXENITB.

(FROM PlKESVILLE, BALTIMORE COUNTY; MARYLAND. ABSTRACT BY J. P. IDDINGS 

FROM THE DESCRIPTION BY GEORGE H. WlLTJAMS.)

Together with gabbros and peridotites, the pyroxenites break

1 The noufeldspathic intrusive rocks of Maryland and the course of their alteration: Am. Geologist, 

July, 1890. Also, Guide to Baltimore, with nil account of the geology of its environs, and three maps: 

Am. lust. Hin. Eng., Baltimore meeting, February, 1892.
DIUJSB.] DESCRIPTIONS: NO. 109, GARNETIFErtOUS GABBRO. 287

through the gneiss and marbles of the eastern part of the Piedmont 

area of Maryland. The pyroxenites and peridotites are younger than 

the gabbros, but are connected with them by intermediate varieties, 

and may be regarded as having originated from a gabbro-magma by a 

diminution in the alumina and silica.

The pyroxenites of Maryland are of two types. One, having darker 

color and being heavier, consists of bronzite or hypersthene and diallage; the other, having lighter color, consists of bronzite or hypersthene 

and diopside. The first is the more common. It is generally evenly 

granular in texture, with allotriomorphic crystals 1 or 2""" in diameter, 

and sometimes S""11 in length. Its fracture is hackly and very uneven. 

Its color is a mixture of dark brown and greenish black.

In thin section the rock consists almost wholly of anhedrons of reddish and greenish pyroxenes, both orthorhoinbic and monoclinic. The 

mineral composition is monotonous and simple, there being in addition 

to pyroxene only occasional specks of iron oxide or magnetite.

The pyroxenes are diallage and enstatite or bronzite. The diallage 

is pale green in thin sections, with the customary optical characters; 

high refraction and double refraction and inclined extinction in all 

'sections except those parallel to the orthopinacoid co PO> (100). The 

cleavage is prismatic and orthopinacoiclal. Some sections of diallage 

exhibit between crossed uicols fine lines which are parallel to the prismatic axis. These are due either to multiple twinning, probably parallel 

to the orthopinacoid, or to lamellar inclusions of orthorhoinbic pyroxene. 

Sometimes these fine striations are crinkled or crumpled as though by 

mechanical strain. In some sections of diallage there are inclusions 

of orthorhoinbic pyroxene in parallel lines crossing the diallage at an 

inclination to "the prismatic axis. These are surrounded by minute 

inclusions having relatively high refraction. In some places these 

inclusions take the form of parallel lamellae of pyroxene, which appears 

to be orthorhoinbic, in others the two kinds of pyroxene are intergrown, with quite irregular outlines. The orthorhoinbic pyroxene in 

some cases incloses irregularly shaped pieces of diopside or diallage, 

and also thin lamellre of this mineral in parallel position. The diallage 

is diopside or malacolite.

The orthorhoinbic pyroxene is pale reddish, with slight pleochroism, 

greenish to colorless parallel to ,c and the prismatic axis, and reddish 

parallel to a and .b at right angles to this axis. Sometimes there 

are rodlike inclusions giving the mineral a bronzy luster. In the 

absence of a chemical analysis it is not possible to determine whether 

the mineral is more properly enstatite or hypersthene. Professor 

Williams has called it bronzite or hypersthene.

Both kinds of pyroxenes are more or less dusted by minute inclusions. 

In some rock sections the orthorhombic pyroxene appears to be in 

excess of the monoclinic variety, but the following chemical analysis, 

made by J. B. Whitfield, of the pyroxenite from Johnny Cake road, 

shows that in this rock diallage is slightly in excess.
288 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL. 150. 

Analysis of pyroxenite from Johnny Cake road, Maryland.

Si02 .. ............................................

A12 03 . ............................................

Cr203 ........................................ ....

FeA......... ................................ ...

FeO.. ............................................

MnO.- ...........................................

CaO..............................................

MgO......................... ....................

Nil/) ............................................

K20.- ............................................

H20 (red heat) ...................................

Cl ...............................................

.Total........................................

Sp. gr., 3. 318.

Per cent.

50.80

3.40

.32

1.39

8.11

.17

' 12.31

22.77

.52

.24

100. 03

Magnetite occurs sparingly in rounded anhedrons. There are also 

small brown grains of chromite or picotite. A characteristic form of 

alteration of this rock is brought about by the change of the pyroxene 

into secondary hornblende, and subsequently the change of this into, 

talc, giving rise to extensive beds of steatite, in which the talc is mixed 

with more or less pale fibrous tremolite and chlorite.

In the vicinity of Pikesville, Maryland, the pyroxeuite often has an 

extremely coarse grain, and not infrequently contains porphyritical 

crystals of orthorhombic pyroxene an inch or two in length.

This type of nonfeldspathic rock, free from olivine and consisting of 

enstatite or hypersthene and diallage, has been called websterite by 

Professor Williams.1

No. ill. PEKIDOTITE (FELDSPATHIC PEBIDOTITE).

(FROM SUDBROOK PARK, BALTIMORE COUNTY, MARYLAND. ABSTRACT BY J. P. 

IDDINGS FROM THE DESCRIPTION BY GEORGE H. WILLIAMS.)

The peridotites of the Baltimore district have been fully described 

by the late Prof. George H. Williams in Bulletin 28, United States 

Geological Survey, from which the following has been taken : 2

The rock [pevidotite] sometimes occurs in comparatively narrow, well characterized dikes; sometimes in small oval or lenticular patches. In the latter case it is 

nearly always associated with serpentine, which has originated from its alteration. 

. . . All the rocks of this class occuring near Baltimore have a strong resemblance 

to each other. All are dark brown or greenish brown in color, and have a peculiar 

rough appearance on the weathered surface by which they may be recognized at a 

considerable distance. This is produced by the decay of the olivine, which leaves the 

crystals of the more stable bronzite stand ing out like knobs upon the surface. . . .

A macroscopical examination discloses a compact greenish-black groundmass, in 

which are embedded large crystals of a glistening yellow bronzite. In some cases 

no other constituent can be recognized with the unaided eye, but generally much

MJp. cit., page 47.
DILLER.] DESCRIPTIONS: NO. Ill, PERIDOTITE. 289

smaller crystals of black diallage can be detected. Feldspar is frequently thinly 

sprinkled in small white grains throughout the rock. . . .

The bronzite varies considerably in both the size and the number of its individuals. 

These range, even in the same hand specimen, from 1 to 8mm in diameter. In some 

cases this mineral seems to compose nearly the entire mass of the roctt, while in. 

others it is so sparingly present as to appear quite accessory.

Cleavage plates of this bronzite are readily obtained parallel to ooP o> (010) and 

ooP (110). They show, when examined in parallel polarized light, an extinction 

parallel to the vertical (c) axis. In thin sections of the rock this mineral is nearly 

colorless, and hence can exhibit no pleochroism. In thick plates, however, this 

phenomenon is plainly visible, viz: a = a = reddish-brown; 1) = b = reddish-yellow; 

c = C = green; absorption C^>fl]>b- . In thin sections this bronzite appears 

as large yellow or colorless plates. It has often a fibrous structure parallel to the 

vertical axis. Characteristic inclusions like those found in the hypersthene do not 

occur, but brightly polarizing spots with a highly inclined extinction angle are 

very prominent between crossed nicols when the bronzite itself is dark, although in 

ordinary light they are quite invisible. They seem to be due to an intergrowth of 

small portions of diallage substance. As alteration of the bronzite commences fine 

bands of serpentine are seen to be developed along cracks which traverse it. Minute 

fibers, resembling bastite, are also of common occurrence.

The diallage of the olivine rocks yields in isolated splinters cleavage plates parallel to oo P o> (100), and ooP (110). . . . [The latter] show an extinction angle 

of from 10° to 13° when measured against the vertical axis. Ill thin rock sections 

an extinction angle as high as 35° was observed in this mineral.

The diallage shows no pleochroism. Thin sections of it are nearly colorless, while 

such as are quite thick, but thin enough to transmit light, remain green in all positions, with no perceptible change in the intensity of the color when the transmitted 

ray is made to vibrate in different directions through tlie crystal.

The diallage lias a much more perfect cleavage than the bronzite. It likewise presents a marked contrast to this mineral in the frequency with which twinning 

lamelho occur in it. These seem to be of secondary origin, produced probably by 

pressure, as was also observed in the case of the diallage of the hypersthene-gabbro 

[from the Baltimore district]. Such secondary lamella) are often present in great 

numbers, and are especially distinct when the diallage crystal has been broken or 

bent.

The feldspar is shown by all its physical properties to be bytownite. , Its specific 

gravity ... is 2.722. The average of a large number of stauroscopic measurements on cleavage fragments gave extinction angles on OP (001) of 19°, and on QO Poo 

(010) of 32°, values Avhich agree well with the specific gravity. This feldspar is 

soluble with difficulty in concentrated hydrochloric acid and fuses readily in the 

flame of a Bunsen burner. This mineral shows a marked freedom from inclusions, 

differing in this respect from the feldspar of the hypersthene-gabbro. AVherever it 

comes in contact with the olivine the peculiar reactionary rims of amphibole are 

finely developed. 1

The interior zone of these rims next to the olivine is narrow, granular, and almost 

colorless. Outside of this is a darker-greenish portion, having a feathery appearance. This exterior zone projects in rounded tufts into the feldspar substance, at the 

expense of which it has apparently been formed.

The main mass of the feldspar is not always fresh. The change which it has 

undergone is the interesting and unusual one to a zeolite mineral (probably scolecite), instead of the more common one, to saussurite or calcite. While the larger 

portion of a feldspar crystal remains quite unchanged, radiating tufts or continuous 

bands of stout, colorless needles may frequently be seen. These show the most

' Bull. U. S. Geol. Survey No. 28, p. 43.

Bull. 150 19
290 THE EDUCA.TIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

brilliant interference colors in polarized light, even when the section is exceedingly 

thin, an effect well known to be characteristic of the zeolites. ...

The olivine, the most characteristic constituent of this class of rocks, exhibits no 

very striking peculiarities. It is present in small, rounded grains, forming the dark, 

compact groundmass in which crystals of the other minerals are porphyritically 

embedded. The structure of these rocks is therefore quite an exceptional one for the 

family of peridotites: First, on account of the porphyritic character, Avhich is rare in 

members of this class, and, second, because of the unusual position of the olivine in 

the groundmass, indicating that it is here the youngest instead of the oldest constituent, as is generally the case. The grains of olivine are always more or less completely changed to serpentine, although in many specimens cores of the original 

mineral remain intact. The serpentine is of a bright yellow color when seen in a thin 

section under the microscope, and shows in an admirable manner the well-known 

process of its formation out of the olivine substance. The fine serpentine fibers are 

developed along cracks and fissures in the olivine, working their way from these 

inward until finally none of the oliviue remains. A considerable quantity of magnetite in fine shreds and lines is always disseminated through the serpentine, having 

been separated from the olivine during the process of its alteration.

No other constituents were observed in the oliArine rocks of the Baltimore region, 

except in rare cases small quantities of a compact, brown, strongly pleochroic hornblende, which is filled with minute dustlike inclusions. This mineral is most intimately associated with the diallage, the two being completely intergrown in the 

same crystal. . . .

Feldspathic peridotite is the commonest type of peridotites in the 

Baltimore region. The feldspar iii it does not compose over 5 per cent 

of the rock. The following chemical analysis, prepared by .Dr. Leroy 

McCay, was made from a specimen from a dike on the Western Maryland Railroad, north of Howardsville.

Analysis of peridotite from Maryland.

Si02 .............. ................................

A12O3 ............................................

Fe2 03 ......................................... ...

FeO.... ..........................................

MiiO... .......... ................................

CaO.- ............................................

Mgo....'...:............. . .............. ......

Na.,0..... ............................ ............

 02 ...... ........................................

H20.... .........................................

Total. .......................................

Specific gravity, 2.989.

Per cent.

41.00

7.58

5.99

4.63

10.08

23.59

.52

3.62

4.73

101. 74

NO. 112. KlMBEKLITB. 

(FROM ELLIOTT COUNTY, KENTUCKY. DESCRIBED BY J. S. DILLER.)

This rock occurs in the midst of the coal fields of eastern Kentucky, 

where the sandstones and shales of the coal measures are nearly horizontal. It is one of a series of three peridotites found under somewhat 

similar conditions west of the axis of the Allegheny Mountains among
DILLEH.] DESCRIPTIONS: NO. 112, KIMBERLITE. 291

the comparatively little disturbed sedimentary rocks. The other localities referred to are the serpentine of Syracuse, New York, described 

by Dr. Williams, 1 and the niica peridotite of western Kentucky.2

The kimberlite of Elliott County, Kentucky, outcrops at only three 

points in the same neighborhood, but the form suggested by the residual material of the soil is that of dikes, and the large number of 

inclusions it contains in places shows that it has been erupted through 

Carboniferous strata.

It is a compact, dark-greenish rock, with a specific gravity of 2.781. 

In it are embedded many grains of yellowish olivine, uniformly distributed throughout the mass. Rarely it is fine granular and dense, 

like many darker colored basalts, but generally the grains of which it 

is composed are medium sized. Occasionally the olivine grains disappear and the deep-green serpentine pervades the whole mass. 

Besides the oliviue and serpentine, which together form nearly 75 per 

cent of the rock, there are other minerals which appear in the hand 

specimen. Most important among these are pyrope and ilmenite, the 

latter appearing in the form of irregular grains which sometimes attain 

a diameter of nearly 2cm . A few scales of biotite may be observed. 

Near the exposed surface the rock becomes yellowish, due to the oxidation of the iron, and softens so that it readily disintegrates. The 

garnet and much of the ilmenite withstand the atmospheric influences 

and are found quite fresh and abundant in -the residual sand resulting 

from the disintegration of the peridotite.

The following table is based directly upon estimates made under the 

microscope of the areal distribution of the various minerals in the 

freshest portions of the sections from the locality where the peridotite 

is least altered:

Primary minerals. Per cent.

1

1

8

2.2

Secondary minerals. Percent.

 JA 7

1.1

It is not claimed, of course, that this table represents with a high 

degree of accuracy the mineralogical composition of the rock, yet it 

closely approximates the real proportions in the sections studied. The 

table clearly indicates that originally at least 80 per cent of the rock 

was olivine and that ultimately it will be nearly all serpentine or, 

perhaps, in some places dolomite with a small proportion of magnetite, 

ilmeuite, garnet, and perofskite.

' Am. Jour Sci., 3d series, Vol. XXXIV, August, 1887, p. 137. 

2 Am. Jour. Sci., 3d series, Vol. XLIV, October, 1892, p. 286.
292 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The general structure of the rock is illustrated in PI. XXXIX, figs. 

A and B, which show the remaining more or less regular phenocrysts 

of olivine inclosed in a network of serpentine with other products of 

alteration. The high proportion of olivine in the rock places it among 

those peridotites which are generally designated dunites, but the presence of some enstatite shows its relationship to saxonite. The more or 

less distinct porphyritic structure was regarded by Carville Lewis 1 as 

so important a feature that he designated a similar rock of the Kimberly mines in the South African diamond field as "kimberlite." The 

original structure of the rock was not wholly porphyritic. In some 

places, at least, it has a holocrystalline even-granular structure like 

granite, the irregular grains of oliviue interlocking like those of quartz 

and feldspar in granite.

The olivine grains are generally irregular in form, varying from 0.1 

to 1.51""1 in diameter, and are penetrated by many fissures. Occasionally, however, as shown in the upper figure of PL XXXIX, they are 

bounded by sharply defined crystallographic planes, a feature which is 

unusual for the olivine in peridotites. It occurs in the form, which is 

common in basaltic lavas, of a short prism terminated by brachydomes 

without the base.

The alteration of the olivine to serpentine takes place rapidly in the 

cross fractures approximately parallel to the base, but very slowly along 

the numerous minute fissures in the prism zone. Cleavage parallel to 

the brachypiuacoid is scarcely discernible.

In the process of alteration the olivine is transformed into serpentine 

with the secretion of magnetite. Among the secondary products 

there is much dolomite, which appears to result from the transformation of the olivine. The abundance of the carbonate present suggests 

that the olivine might contain a considerable percentage of lime. This 

suggestion was proved true by a chemical analysis of the olivine.

Pyroxene plays so small a part among the minerals of this rock that 

it can not be considered an essential constituent. In the form of irregularly corroded grains, it is distributed throughout the mass with 

approximate uniformity, but it constitutes not more than 1 per cent of 

the whole. The cleavage is nearly rectangular, and the extinction in 

prismatic sections is parallel, indicating with a high degree of probability that the pyroxene is orthorhombic. It is generally transparent, 

with a sprinkling of fine, dark grains, and is surrounded by a clouded 

border conforming to the embayed contour.

The embayments of the irregular enstatite sometimes contain olivine, 

demonstrating that the pyroxene is an earlier product of crystallization 

than the olivine and owes its border, at least in part, to the subsequent 

corrosive action of the magma.

The mica is dark colored, strongly dichroic, with a very small optic 

axial angle in the plane of the principal ray of the radial figure 

(schlagfigur) produced by puncturing a thin plate of the mica with a

'Proc. Brit. Assoe. Adv. Soi., 1887, p. 720.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XXXIX

THIN SECTIONS OF KIMBERLITE FROM ELLIOTT COUNTY KENTUCKY, AS SEEN UNDER A

MICROSCOPE, X 30.
DILLBH.] DESCRIPTIONS: NO. 112, KIMBERLITE. 293

sharp needle. This biotite is sparingly distributed throughout the 

rock and is surrounded by a prominent border composed of colorless 

mica and oxide of iron. A

Pyrope can not be considered one of the essential minerals in this 

rock, yet it is among the most prominent. It occurs in spherical and 

ellipsoidal grains varying from 1""" to more than a do/en millimeters in 

diameter. They are found abundantly along the line of the dike in the 

soil resulting from its disintegration. The small, clear, deep-red grains 

have a specific gravity of 3.673 and are locally regarded as rubies of 

problematical value, but the paler red, much-fractured fragments of 

larger size have attracted little attention.

The most interesting feature of the pyrope is prominent under the 

microscope, where it is seen to be surrounded by a border of radial 

fibers analogous to that described by Fr. Becke 1 and A. Schrauf,2 and 

later critically examined by A. v. Lasaulx.3 The general character of 

the border is represented in fig. 16. It is composed of two essentially

FIG. 16. Pyrope showing border of biotito and magnetite.

different substances, both of which are always present, although varying much in proportions. First of these may be mentioned a dark 

powder, which is frequently so abundant as to render the border 

opaque. It occurs most abundantly in the outer portion of the border,' 

and is chiefly, if not wholly, magnetite, for when carefully detached by 

a sharp needle from an uncovered section it is found to be strongly 

magnetic. The second, usually inner, substance of the ring is of a 

grayish or reddish brown color, and is generally fibrous in structure. 

Schrauf studied the fibrous substance enveloping the garnets in the 

serpentine of Krem^e, Bohemia, and named it kelyphite. The investigations of Lasaulx have shown that in some cases the border, instead 

of being a single mineral, is a mixture of several minerals, chiefly of 

the pyroxene and amphibole groups. In the example under consideration its composition appears to be exceptional. Although it is commonly made up of closely compacted, very fine parallel fibers, perpendicular to the outer surface of the garnet, it frequently appears as an 

irregular uonfibrous fringe upon'the inner side of the border, or even

1 Tschermak's Mittheihmgen, IV, 1881, pp. 189, 285.

"Beitrage zur ICcnntniss des Associations-kreises der Magnesia-Silikato: Zeitsclirift f'iir Jvry*)tii.llographie, 1882, VI, pp. 321-388; also Ueber Kelyphite: Neuos Jahrbuch, 1884, Bd. II, p. 21.

3 TJeberdieTJinrindungeii voii Granat. Sitzungsberichto der niederrhein. Gesellschaf't in Bonn, 1882, 

Jnli 3; Verhandlungen des naturhistorisclien Voreines der preussischon. ELoinlaudo uud Westf'aleiis, 

Neuminddreissigster Jahrgaug, zweito Hall'to, Bonn, 3882, p. 114.
294 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. IBULL.ISO.

completely incloses within the garnet, where it is usually of a deepbrown color. Generally it is distinctly doubly refracting, and when 

finely fibrous is sometimes strongly colored red and green between 

crossed ntcols. The nonfibrous form of the substance, although deeply 

colored, is isotropic and consequently not dichroic, but when .fibrous 

the absorption parallel to the fibers is occasionally almost complete, 

like that of biotite, Avhich in one section, by its uniaxial, negative, 

and strongly dichroic character it was proved to be. \ Ilrnenite and 

magnetite are common and uniformly distributed constituents of the 

Kentucky peridotite. '

Abundantly scattered among the other secondary products in the 

serpentinous network enveloping the remnants of olivine are yellowish 

clouded grains ranging in size from .004 to .06""" in diameter. The 

intensity of the yellowish color varies considerably, with a strong 

inclination toward brown. Its index of refraction is very high, causing it to rise above the neighboring minerals, but its low grade of 

transluceucy scarcely more than allows the observer to discover that 

the mineral is distinctly doubly refracting without determining certainly its degree. With a very sharp steel point a number of these 

grains were removed-from an uncovered section and dissolved in fused 

KHSO4 . When the product was moistened with a solution of H2O2 it 

turned distinctly yellow, indicating the presence of titanium. At first 

these grains were supposed to be octahedrite, but Dr. G. H. Williams's 

discovery of similar grains of perofskite in the serpentine of Syracuse 

led to the separation and chemical examination of those in specimen 

112, and they also were found to be perofskite.

For a fuller description of this rock and its mode of occurrence the 

student is referred to United States Geological Survey Bulletin No. 38, 

Peridotite of Elliott County, Kentucky.

NO. 113. CORTLANDITE (HORNBLENDE-PERIDOTITE). 

(FROM STONY POINT, ON THE HUDSON, NEW YORK. ABSTRACT BY. J. P. IDDINGS

FROM THE DESCRIPTION BY GEORGE H. WILLIAMS.)

This rock was first described by Prof. George H. Williams, under the 

head of hornblende-peridotite,1 at which time it was sh®wn to belong 

to Bonney's class of hornblende-picrites, and to Oohen's class of hudsonites. The latter term, however, Williams showed had been given to 

a variety of diallage, and so he proposed the name cortlandtite for the 

rocks ordinarily classed as hornblende-peridofcites. The following 

description is taken from his paper just cited:

The best locality for specimens of this type of peridotite is at Kings Ferry on 

Stony Point, a small prominence on the west side of the Hudson River, somewhat 

southwest of Yerplanck. . . . [Many varieties of peridodite may be found among 

the blocks thrown out of the railroad cutting near this place.] The most. remark1 Peridotites of the " Cortlandt Series " on the Hudson River near Peekskill, 2Tew York: Am. Jour. 

Sci., 3d series, Vol. XXXI, 1886, pp. 26-41.
DILLEB.] DESCRIPTIONS: NO. 113, CORTLANDITE. 295

able among these varieties is a dark green, at.flrst sight apparently fine-grained, rock, 

which, however, when held in a proper light, exhibits glistening, bronze-colored 

cleavage surfaces, often measuring 3 by 4 inches. The reflection from these surfaces 

is not altogether continuous, being interrupted by small rounded grains of a dull 

green mineral whose nature can not be determined with the unaided eye, but which 

the microscope shows to be oliviue or serpentine.

This "luster-mottling," as a similar phenomena has been called by 

Pumpelly, 1 has been termed pcecilitic structure by Professor Williams.2

The glistening surfaces fin this rock], as the microscope shows, are those of a 

brown hornblende. The individuals of this mineral are very large, being often 4 

inches in diameter; but, notwithstanding that they are so abundant as to be everywhere in contact with each other, so full are they of inclusions of tho other constituents that they do not of themselves make up one-half of the entire mass of the 

rock. [The only other minerals recognizable without a microscope] are frequent 

particles of magnetic pyrites (pyrrhotite) and glistening flakes of alight green mica.

The hornblende is recognized by its characteristic prismatic cleavage

with an angle of 124° 30'.

In thin section it is rich brown, which is the color of basaltic hornblende. Its pleochroism is pronounced. The color of rays vibrating 

parallel to £, nearly parallel to the prismatic axis, is dark chesjbnut; 

that of the rays vibrating parallel to It, parallel to the orthodiagonal 

axis, is a slight tinge lighter, while the color for rays vibrating parallel 

to a is a light yellow. The absorption is .c = b > a. Hence some sections exhibit marked pleochroism, and others almost none. The extinction angle in clinopinacoidal sections is 9° to 10° from the prismatic 

axis, indicated by the direction of the prismatic cleavage cracks in 

such sections.

The inclusions in this hornblende are both numerous and characteristic. The 

most common are opaque black needles, ranging in size from the finest dust to about 

0.03""" in length. The majority are arranged either parallel to tho vertical axis, or 

else so as to make an angle of about 45° with this. Others appear quite irregular 

in their position. More rarely small transparent crystals, the largest of which are 

0.05mm long and 0.0211 "" broad, c ocur with the opaque needles. The nature of these 

could not be determined. . . . Still more rarely than these transparent crystals, 

the hornblende contains inclusions of thin brown plates similar to those which are 

so characteristic of hyperstheue. All of these interpositions, of which the opaque 

needles as a rule occur alone, show a tendency to concentration toward the center 

of tho hornblende, leaving a border near the edge comparatively free from foreign 

substances. Often they form irregular patches scattered like little clouds over the 

brown background.

The hornblende itself never shows any trace of crystalline form. It fills the 

irregular spaces between the other constituents, a single individual often covering 

a space some inches square. From its relations to tho other minerals in the rock it 

is evident that it was the last to solidify, while the great size of the crystals would 

seem to indicate that the process of their formation went on very slowly. . . .

The hornblende seems particularly subject to alteration, which is often far 

advanced before the oliviue or the pyroxene are materially affected. The first 

change which the hornblende undergoes is bleaching, accompanying which is the

'Metasomatic development of the copper-bearing rocks of Lake Superior. Proc. Amer. Acad., Vol. 

XIII, p. 260,1878.

2 Loc. cit.,p. 30; also, On the use of the terms poikilitic and micropoikilitic in petrology: Jour. 

G«ol., Vol. I, p. 176,1898.
296 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

almost total disappearance of the characteristic inclusions. The mineral becomes 

nearly colorless and consequently nonpleochroic, while retaining the compact structure and optical behavior of the unaltered portion. Later there is developed, particularly around the edge of the hornblende, a bright emerald-green substance, 

which, on account of its lack of dichroisni and very feeble action on polarized light, 

may be regarded as chlorite.

Next to the hornblende, the most important constituent of this rock is the oliviue, 

which is remarkable both for its freshness and for its beautiful inclusions. It is 

present in rounded grains or in well-defined crystals, upon which the usual combination of domes, prisms, and pinacoids may be seen. These crystals vary from onehalf to 2 inui in diameter. The mineral is quite colorless, with a high index of 

refraction, and is traversed by irregular cracks, along which serpentinizatiou may 

be frequently seen to have commenced, although in many sections there is hardly a 

trace of this alteration. . . . They [the inclusions] are black and opaque, 

having generally the form of minute, rounded grains, or long rods arranged parallel 

to one or more of the crystallographic axes of the oliviue, although they are sometimes more irregular in their distribution. Frequently these rods, instead of being 

straight, are variously bent and t\visted, exhibiting the form of trichites in obsidians. In such cases they show a tendency to form elliptical groups resembling a fine 

arabesque, as figured by Zirkel. 1 The same author has observed that while these 

inclusions are very characteristic of the olivine of the older rocks [i. e., the coarsergrained rocks J. P. I.] they are never found in that of the younger basalts. There 

seems little doubt that they are composed of magnetite, since they are readily 

decomposed by acid, and since such grains of oliviue as contain them in abundance 

are attracted by the magnet. ...

[In the exceptional cases, in which feldspar is present, and is in contact with 

olivine, there is a zone between the two minerals consisting of two parts.] The 

inner portion, nearest the olivine, is composed of square grains of nearly colorless 

pyroxene; the outer one [adjacent to the feldspar, consists] of tufts of radiating 

actinolite needles of a beautiful bluish-green color and strongly pleochroic. [The 

oliviue is sometimes partly altered to serpentine.]

The pyroxene constituent of the peridotite from Kings Ferry appears to be for 

the most part hyperstheue. It sometimes forms small irregular grains not larger than 

those of the olivine, but in other cases it is present in individuals which are over a 

centimeter iu length, inclosing the smaller grains of both oliviue and hypersthene 

like the hornblende. In all forms it possesses all the ordinary characteristics of hyperstheue, except that it is singularly free from the usual inclusions. Its pleochroism 

is very strong: a = a ray is red; & = b ray is yellow; c=Cray.is green. Its cleavage is well developed parallel to the prism (QO P) and also still better parallel to the 

brachypinacoid (oo Po6). A common, nonpleochroic augite, in which a diallage 

habit is frequently developed by the presence of a parting parallel to the orthopiuacoid [100] is also often to be observed in this rock, although iu many specimens it is 

altogether lacking. As this constituent increases in amount the rocks grade into 

those of the next group [augite-peridotite or picrite].

Biotite is also present in small amounts. This mineral rarely retains its brown 

color. It is generally so completely bleached as strongly to resemble muscovite in 

thin sections. It is much bent and twisted, often having small lenses of calcite iuterposed between its lamella}. . . . Aside from mere bleaching, the formation of 

the bright green, chloritic mineral, noticed as an alteration product of the hornblende, is also frequent. The true character of this mica is revealed by its very 

small axial angle ... 'as well as by the fact that rarely sections may be found 

which have escaped the bleaching. These have the characteristic color, and pleochroism of biotite, and sometimes contain acicular inclusions resembling needles of 

rutile. ...

1 ZeitscUrii't cler deutschen geologischeii Gesellscliaft, XXIII, 1871, p. 59, PI. IV, Fig, II. Mikrosk. 

Besckatieuheit, p. 214.
WLLKK.] DESCRIPTIONS: NO. 114, SA.XONITE. 297

Feldspar, though frequently accessory, is never an important constituent of these 

rocks. [It is a basic lime-soda feldspar, probably bytownite. J. P. I.]

Magnetite, aside from composing the inclusions in the olivine above referred to, 

forms small grains which line the cracks in this mineral, and are especially abundant 

about its edge, where it is in contact with the brown horn blende. The large opaque 

grains scattered through the rock are almost all pyrrhotite (magnetic pyrites (Fe7S«)); 

chromite or picotite were not observed; another form of spinel, however, pleonast, 

recognized by its dark-green color and isotropic character, is not uncommon. This 

mineral is filled with thin opaque plates almost exactly like the inclusions in the 

Avell-known hercynite from Rousperg, in Bohemia. Apatite was hardly ever observed.

NO. 114. SAXONITE. 1 

(FROM NEAR RIDDLES, DOUGLAS COUNTY, OREGON. DESCRIBED BY J. S. DILLER.)

The saxonite, of which specimen 114 is a sample, occupies an irregular 

area of about 2 miles in extent a short distance directly west of Riddles, Douglas County, Oregon. It is almost completely surrounded by 

rocks which are considerably metamorphosed, although upon one side 

it touches for a short distance the Cretaceous shales.

The rock has a dark yellowish-green color and high specific gravity, 

suggesting at once that it is rich in ferromaguesian silicates. It 

is holocrystalliue granular, and composed essentially of olivine and 

eustatite, with a small percentage of accessory chromite and traces of 

magnetite.

The olivine predominates, so that the enstatite forms less than onethird of the mass. Nevertheless, it plays a much more important role 

than in the kimberlite of Kentucky. Here it is of sufficient importance 

to rank as an essential constituent of the rock, and occasionally shows 

fine lamella? twinning. Both the olivine and the eustatite are clear 

and colorless, but may be readily distinguished by their cleavage and 

optical properties. They are allotriomorphic and do not contain inclusions, excepting the coffee-brown grains of chromite with a small 

amount of magnetite and fine ferritic dust.

Notwithstanding the comparatively fresh condition of the rock, it is 

in places completely permeated by a multitude of cracks filled with 

serpentine resulting from its alteration. The combination of enstatite 

and olivine would appear to be particularly favorable for the production of serpentine, as it supplies to tbe other the material needed 

besides the water to make serpentine.

Thus saxonite, including the serpentine derived from, it, is of particular interest in being the seat of deposit of genthite, a nickel silicate 

of economic importance. It was discovered by Will Q. Brown, and is 

associated with quartz in more or less distinct veins, Avhich, according 

to Mr. W. L. Austin, who has studied the deposits in tbe field, and 

mapped them, extend across the entire area in a northeast and southwest direction.

1 Saxonite was named l>y Dr. M. E. Wadsworth in hia Lithological Studies in 1884, Cambridge, 

Massachusetts. It is the same rock that UoBenbusch designates " hamburgito."
298 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150,

The genthite, like the vein quartz, is of secondary origin, and from 

the fact that the oliviue in the saxonite contains nickel, it is regarded 

as the source of that in the genthite.

The following analyses of tbe saxonite (I) and of the olivine (II) it 

contains were made by F. W. Clarke, of the United States Geological Survey:

Analyses of saxonite and its contained olir/inc.

Si02 . ...................................

A1A..................... .............

GrtQv. .................................

FeA------ ............................

FeO ...................................

MO......... ..........................

MnO............ .......................

MgO. .......................... ........

I.

Per cent. 

441

41. 43

.04

.76

2.52

C.25

.10

.55

43. 74

90.80

II.

Per cent. 

0.57

42.81

.79

2.61

7.20

.26

54.12

99.36

For fuller description of the mode of occurrence of this rock and ifs 

nickel ore the student is referred to United States Geological Survey, Bulletin No. 00, pages 20 to 26; also to a paper by W. L. Austin 

in tbe Proceedings of the Colorado Scientific Society of Denver for 

January 6, 1896, and to a communication by Mr. H. B. vou Foullon, 

Jahrbuch d. k. k. geol. Eeichsanstalt, Vienna, 1892, XLII, p. 223.

METAMORPHIC SEDIMENTARY ROCKS.

No. 115. CRYSTALLINE LIMESTONE.

EAST BASE OF MODOC PEAK, EUREKA COUNTY, NEVADA. DESCRIBED BY

J. P. IDDINGS.)

The crystalline limestone from the Devonian strata was collected at 

the east base of Modoc Peak, Eureka, Nevada, and is called the Nevada 

limestone (Monographs U. S. Geol. Survey, Vol. XX, p. 63 et seq.). It 

is light-gray, quite crystalline and saccharoid, and contains 40.62 per 

cent of magnesium carbonate. It is not distinctly bedded, being massive. In thin section it is evenly granular and wholly crystalline, with 

almost no admixture of other minerals than dolomite. A small amount 

of quartz of subsequent crystallization occurs as scattered patches, 

forming a filling in minute cavities between the dolomite crystals. No 

other minerals were observed. The dolomite exhibits the characteristic cleavage, but little or no twinning. There are numerous minute 

inclusions of an indeterminate nature, probably gas or fluid.
D1LLER.] DESCRIPTIONS: NO. 116, MARBLE. 299

No. 116. MARBLE.

(FROM LEE, BERKSHIRE COUNTY, MASSACHUSETTS. DESCRIBED BY.!. S. DILLEK.)

Unaltered limestones are Avell represented in the series by chalk 

(No. 39), patellina limestone (No. 40), coquina (No. 42), shell limestone 

(No. 43), cherty limestone (No. 44), compact limestone (No. 46), lithographic limestone (No. 47), and hydraulic limestone (No. 48). Specimen No. 115 is an example of a limestone that has been completely 

crystallized since its deposition. This process is metamorphic in its 

nature. Owing to the fact that calcite is readily soluble and much 

more easily changed than the mass of other rock-making minerals, 

limestones may become completely crystalline (metamorphosed), while 

the adjoining rocks remain but little altered.

Metamorphic limestone is sometimes called marble, but the name is 

usually restricted, as iu the trade, to limestone which will take a polish.

The marble of Lee, Massachusetts, is of Cambro-Silurian age, and is 

a part of the great limestone and marble belt extending from Georgia, 

through a number of the Southern, Middle, and New England States, 

into Canada. Specimens Nos. 46 and 150, and probably, also No. 117 

are from this belt. In the Middle States the rock is limestone; but in 

New England, as well as in Georgia and Tennessee, where it has been 

subjected to much greater disturbance and consequent metamorphism, 

it is converted into marble.

The Lee marble is a white, uniformly fine-grained rock, which looks 

to be a remarkably pure limestone; but if so, it should be completely 

soluble. In Mr. E. A. Schneider's chemical analysis, given below, it is 

seen to contain only 0.19 per cent of insoluble matter, of whose composition the analysis gives us but little information. The analysis shows 

that the limestone is rich in carbonate of magnesia, containing over 4.0 

per cent; so rich, indeed, that it may be classed among the dolomites. 

In this respect it is closely related to the Cockeysville marble (No. 

117); also that of Eureka, Nevada (No. 115).

Analysis of Lee marble.

Iu soluble ... 

ALOa, Pe^Oa 

CaO.........

MgO........

Organic matter. 

H.,0, 105°......

Total....................................... 9<). 45

I

Per cent.

0.19

.24

30.88

21.42

40.72

Under the microscope the Lee marble is seen to be completely crystalline granular, with small grains, comparatively uniform in size.
300 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

Many of these show twinning bands, but are generally quite free from 

visible inclusions. The presence of 0.19 per cent of insoluble material 

would be expected to reveal itself under the microscope. However, as 

minerals other than the carbonates, were not readily detected in the 

section, a fragment of the limestone was dissolved, and a residue 

obtained of clear colorless cleavage plates of a mineral which, has a 

very small optic axial angle and is negative. The interference colors 

of its cleavage plates are very low, in thin pieces, not even attaining 

yellow of the first order. V

According to Mr. Merrill,1 much of the marble of Lee and adjoining 

localities contains crystals of tremolite, which weather out upon exposure, leaving the rock with a rough pitted surface. They are doubtless p 

due to rnetamorphism. Mr. Hobbs 2 notes their abundant development 

in the Canaan limestone of the same region along the line of a great 

fault.

The Lee marble was used in the( structure of the Capitol extension, 

but Mr. Merrill reports that the weathering out of the tremolite crystals 

in the exterior walls is very noticeable. In England dolomite is con- ^ 

sidered much more durable than pure limestone, but that view does

not appear to be shared in this country.3

».

]ST o. 117. MARBLE (DOLOMITE). v

(FROM COCKEYSVILLE, BALTIMORE COUNTY, MARYLAND. DESCRIBED BY

J. S. DILLER.)

This rock is known throughout the Baltimore region as the " Cockeysville marble" because it is extensively quarried at that place. It is \. 

completely crystalline, as shown in PI. XL, which represents it as seen 

under a microscope of small amplification. The crystalline grains are Vmuch larger than those of the Lee marble. The rhomboheclral cleavage 

is especially distinct in the thin section, although not well shown in the 

illustration, which was taken between crossed nicols in order to bring * 

out the twinning lamellae. Grains of quartz, several of which are 

shown in the figure, are common, and account for the high percentage 

of silica present. Farther westward in Maryland, according to 

Williams, the limestones are much less crystalline and retain traces of 

their original sedimentary character. All of these features are obliterated in the Coekeysville marble. The impurities may originally have 

been distinct, but now, according to Williams, have crystallized and 

appear as accessory minerals, the original sedimentary character 

having been obliterated in the metamorphism. ,

There is little variation visible in the hand specimens, but in the 

large mass, where quarried, there are considerable variations in the 

size of the grains, as well as chemical composition and accessory con1 Stones for Building and Decoration, p. 94.

* Jour. Geol., Vol. I, p. 794. >' ~

3 Stones for Building and Decoration, by G-. P. Merrill, p. 381.
U. S. GEOLOGICAL SURVEY

BULLETIN NO. 150 PL. XL

THIN SECTION OF MARBLE, COCKEYSVILLE, MARYLAND, AS SEEN UNDER A MICROSCOPE, X 30. 

A. In ordinary light, B. Between crossed nicols.
DIMMER.] DESCRIPTIONS: NO. 118, QUARTZITE. 301

stituents. The coarse-grained variety, which is much coarser than 

specimen 117, is locally called "alum stone," and is burnt for lime. 

 Two analyses of this marble are given below. The first was made 

by J. E. Wbitfield, 1 and th.e second by E. A. Schneider. The second 

shows the composition of the specimen in this series.

Analyses of Cotfcey smile marble.

Per cont.

Si02 ...... .................

AW*. ..................... 

FeO ....... .........I.....

CaO .......................

MgO...... .................

C'f\

Ignition ...................

0.44 

1.22 

trace 

30.73 

20. 87 

45. 85 

.1.22 

100. 33

Al A, F° A ................ 

CaO.........................

MgO........................ 

CO., .........................

H2 0 at 103° .................

Per cent.

5.57 

.40 

29.08

20. !.<0 

44.20

99. 01

The amount of magnesian carbonate present, as shown by these two 

analyses, is variable, and averages a little over 30 per cent.

For information concerning the field relations of this rock, the student 

is referred to the "Guide to Baltimore, with an account of the Geology 

of its Environs," pp. 97 to 102, by George H. Williams. Its distribution 

is shown upon an accompanying- geological map.

NO. 118. QUAliTZITE. 

(FROM CAKIBOTJ HILL, EUKKKA COUNTY, NEVADA. DESCRIBED BY.!. P. IDDINGS.)

The quartzite'from Caribou Hill, Eureka, Nevada, is of Silurian age, 

and is designated on the atlas sheets accompanying Monograph XX, 

United States Geological Survey, as the Eureka quartzite.2 It is dense, 

white, and vitreous, in massive beds, with no distinct evidence of its 

fra-gmcntal character visible to the unaided eye.

In thin section it is seen to be a very pure quartz rock, consisting of 

grains of quartz about 0.25""" in diameter. In general they are very 

free from inclusions of foreign mineral matter, but usually exhibit 

minute fluid inclusions. Some grains contain hair-like needles which 

are probably rutile. The fluid inclusions are often arranged in lines 

or planes traversing the quartz grains in one or more directions. The 

shape and outline of the quartz grains in some cases suggest those of 

granitic quartz, in which each grain has crystallized until interrupted 

by the neighboring grains; but in many cases it can be seen that the 

outer portion.of the grain is an addition of quartz substance upon a 

central nucleus of .quartz, which itself is a well rounded water-worn 

grain. The inner and outer portions have the same optical orientation

'Bull. 00, TJ. S. Geol. Survey, p. .109. 

2 See Monograph XX. pp. 54 and 212.
302 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

and appear to be one continuous individual crystal. The line of 

demarcation between the two portions consists of minute inclusions 

surrounding the original rounded grain. This secondary enlargement 

of quartz grains has been described by a number of petrographers, 1 

and is due to the crystallization of quartz from aqueous solutions 

permeating the sandstone, the secondary quartz assuming the same 

crystallographic orientation as that of the nucleus. This feature may 

be observed also in the Potsdam sandstone (PI. X, p. 80). By this 

process a sandstone may be converted into a dense vitreous quartzite 

by the action of aqueous solutions without the accompaniment of 

abnormal pressure or of any increase of temperature.

In the quartzite from Caribou Hill there are veins of chalcedony that 

traverse the rock in all directions and sink to microscopic proportions. 

The chalcedony is developed in spherulitic growths, radiating from the 

surface walls of the veins inward, usually as narrow bands in thin 

section. In some of the broader veins the chalcedony in the central 

portion exhibits definite spherulites and in some cases microcryptocrystalline aggregation. It is distinguished from the quartz by lower 

index of refraction, and by the fact that the axis of greatest elasticity 

is parallel to the long axis of the chalcedony fibers, which are in a 

sense optically negative. Minute prisms of quartz would be optically 

positive, the axis of least elasticity being parallel to the length of the 

prisms.

It is evident that the sandstone, after being cemented by the secondary enlargement of the quartz grains, and having been converted to 

quartzite, was subjected to dynamical strain which fractured it into 

small pieces and minute particles. It was then permeated by solutions 

which deposited silica in the form of chalcedony in all £he fissures and 

interstices and recemented it into a dense quartzite.

The fractures, which are now indicated by the veins of chalcedony, 

traversed the grains of quartz and their quartz cement as across a 

compact mass, and incomplete parallel fractures, which accompany the 

more perfect ones, are shown by lines and planes of fluid inclusions. 

These are therefore of secondary origin, are subsequent to the first 

cementation and contemporaneous with the fracturing, and result from 

dynamic action.

No. 119. QUARTZ-SCHIST.

(FROM STEVENSON STATION, GRKEN SPRING VALLEY, BALTIMORE COUNTY, MARYLAND. DESCRIBED BY W. S. BAYLEY.)

The specimen numbered 119 was obtained from the Shoemaker 

Quarry, near Stevenson, a station on the Green Spring Valley Branch

'A. E. Tornebohm, Eiu Beitrag zur Frage der Quartzit.bildung: Geol. Eoren. Stocldu, 1876, Vol. 

Ill, p. 35, reviewed in Neues Jiihrbucli i'iir Miu., etc. 1877, p. 210. H. C. Sorby: Quart. Jour. Geol. 

Soc. London, 1880, Vol. XXXVI, p. 62. A. A. Young: Am. Jour. Sci., July, 1882. E. D. Irviug: Id., 

June, 1883. E. D. Irving, and C. E. Van Hise: Bull. 8, U. S. Geol. Survey, 1884. T. G. Bonuey and 

J. A. I'liillips: Quart. Jour. Geol. Soc. London , Vol. XXXIX, p. 19. J.P. Iddiuga: Monograph XX, 

Appendix B, U. S. Geol. Survey, 1892, p. 346.
DILLKR.] DESCRIPTIONS: NO. 119, QUARTZ-SCHIST. 303

of the Northern Central Railroad, in Baltimore County, Maryland. 

The rock is used for flagging, and for the foundations and abutments 

of bridges.

Geologically the rock is one of the members of the series underlying 

the eastern portion of the Piedmont Plateau. This is an elevated baseleveled area, which is divided by Dr. Williams 1 into an eastern and a 

western portion. In the western portion the rocks are unquestionably 

metamorphosed sediments (see description of IsTo. 126), while in the 

eastern portion they are nearly all holocrystalline, though they may 

originally have been clastic. Some of them, possess an obscure conglomeratic habit. They are beneath the metamorphosed sediments of 

the western plateau region, which are probably Cambrian and Lower 

Silurian in age, and hence they are regarded as probably Algonkian, 

or, at any rate, as pre-Cambrian.2 The rock represented by the specimen occurs along the contact between a dynamically metamorphosed 

hornbleudic and micaceous gneiss and a crystalline dolomitic limestone 11 known as theCockeysville marble 4 (No. 117). It passes insensib'y 

into the gneiss, of which it may be considered a phase. In the latest 

map 5 of the district no distinction is made between the two rocks, 

both being represented by the same color and described as metamorphosed sediments.

Different specimens of the rock present different appearances. All 

are more or less foliated, and some are platy. The least foliated varieties are light-gray rocks with a sugary texture. They consist of an 

aggregate of small quartz grains and tiny flakes of a light-yellow 

glistening mica. The more schistose phases contain much more mica, 

and so have a little darker color than the less schistose forms. All 

varieties of the rock are crossed by parallel joint planes whose surfaces 

are covered with mica scales. In the massive varieties the joints are 

less numerous than they are in the more schistose ones, in which they 

are often only a small fraction of an inch apart. The surface of a crossfracture through a specimen of this kind resembles very closely that of 

a micaceous gneiss.

No traces of bedding planes can be detected in any of the specimens. 

Their platy character is determined by the joint planes passing 

through them.

'G-. H. Williams, The Petrography and Structure of the Piedmont Plateau in Maryland: Bull. 

Geoh Soc. America, Vol. 2, 1891, pp. 301-322; and Guide to Baltimore, with an account of the Geology 

of its Environs, and three maps, Am. Inst. Miu. Engineers, Baltimore Meeting, February, 1892. Prepared by local committee for the use of the institute, pp. 77-124, with maps.

2 Cf. C. R. Van Hise, Correlation Papers Archean and Algonldan: Bull. TL S. Geol. Survey No. 86; 

especially pages 411 and 415. »

3 Good specimens of this marble are seeu in the columns and heavy platforms of the Capitol extension at Washington. A1 large portion of this building, the Washington Monument, and the PostOflieo building in the same city are constructed of it, as well as the spires of St. Patrick's Cathedral 

in New York.

4 See map in Guide to Baltimore; and Geological Map of Baltimore and Vicinity, G. H. Williams, 

editor, published by Johns Jlopkius University, 1892; also Baltimore sheet, Atlas IT. S. Geol. Survey, 

edilion August, 1892.

5 A. Preliminary Geological Map of Maryland, G. H. Williams, editor, 1893: Maryland, its Resources, 

Industries, and Institutions, prepared for the Board of World's Pair Managers of Maryland, Baltimore, 1893,
304 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150. "^

A close inspection of the joint surfaces of most specimens will show 

the presence of small grains of coal-black tourmaline in the midst of 

the mica scales. Dr. Williams, 1 in his account of the petrography 1 

of this schist, states that "its most characteristic feature is imparted 

to this rock by long crystals of black tourmaline, which have been 

developed in these inuscovite layers. These crystals are invariably 2 

broken and their fragments separated along one line, showing that 

the rock was compressed in one direction and elongated or stretched 

in another at right angles." sr

The rock is described by Williams 2 in the following words:

The least important of the rocks of probably sedimentary origin in the Baltimore 

region is a peculiar schist composed mostly of quartz and divided into beds of vary- ~r~ 

ing thickness by parallel layers of muscovite. ... Its quartz grains are of 

different sizes, but are so completely recrystallized that they form an interlocking * 

mosaic. Besides the flakes of muscovite, the only other constituents are iron stains and 

occasional crystals of tourmaline, microclines, and zircon. Sharply defined areas 

showing a minute spherulitic polarization are also common. They are identical with 

those occurring in the Saxon "greisen," and probably represent altered feldspar. 

The rock shows the effect of pressure in the undulatory extinction of its quartz / 

grains. The cleavage planes of the quartz-schist are due to thin layers of muscovite 

in good-sized scales, with their basal planes all parallel to the foliation.

With respect to its origin he declares that the rock is probably a 

u facies of the gneiss, produced by some dynamic agency, for it always 

shows the effect of internal mechanical action and motion. Moreover, 

the abundance of tourmaline points to the agency of fumaroles, which 

are always important factors in the recrystallization of deeply buried 

rocks."

The platy character of the rock is plainly seen in the hand specimen; 

its schistosity, however, is not so apparent. Nevertheless, on breaking 

the rock the fracture is much more easily produced in a direction parallel to that of the cleavage planes than in any other direction, even 

when the separation is not along the cleavage plane. This is due to 

the arrangement of the mica scales throughout the body of the rock, 

with their flattened sides parallel to the cleavage planes.

In the thin section the irregular grains with brilliant polarization 

colors are quartz; the long, narrow grains with a longitudinal cleavage, 

a very slight pleochroism, an extinction parallel to the cleavage lines 

and brilliant mottled polarization colors are muscovite; the opaque 

crystals and rounded grains are magnetite and pyrite, altered in many 

cases to limonite or ocher, and the few clear, colorless grains with palegreen polarization colors are plagioclase. Some of these latter are 

crossed by indistinct twinning bars, and many of them, particularly 

those near the edges of the section, are traversed by cleavage cracks. 

In some sections the plagioclase is altered to a cloudy aggregate of * 

scaly minerals and little dust particles. Grains of zircon are rare. 

They occur as small rounded or elliptical particles, with a very high /-

1 Guide to Baltimore, p. 103. ~

2 Op. cit:,pp. 102,105.
DIUBB.] DESCRIPTIONS: NO. 120, JASPILITE. 305

refractive index and strong double refraction. The latter property is 

shown in the bright polarization colors and the former by the dark 

lines bounding the grains.

The quartz is the most characteristic component. It is in irregularly 

interlocking grains. These are crossed by continuous lines of tiny dust 

inclusions and larger liquid inclosures with movable bubbles. The 

majority of the grains exhibit undulatory extinction, though this property is best seen in the thinnest grains near the edges of the section.

Along certain bands the larger quartzes are embedded in a -rubble 

of smaller grains, and in these bauds the inuscovite is most common. 

The mica, the quartz grains, and the feldspars are all elongated in the 

direction of the banding, producing a. well-marked schistosity. In 

other portions of the section the muscovite'also exhibits a tendency to 

a parallel arrangement, but this tendency is more noticeable when 

the section is viewed under a low-power hand lens than when examined under the microscope. The mica 1 and the feldspar occur between 

the quartz grains, the former lying along the boundaries between two 

grains and the latter occupying angular spaces between, several 

quartzes. Both the muscovite and the plagioclase show pressure 

effects the mica in the bending of occasional flakes and the feldspar 

in the occurrence of secondary twinning lamellae.

From, the microscopic study of the sections we are led to the same 

conclusion as that reached by the field study of the rock, namely, that 

it is a dynamically metamorphosed acid rock, which from its composition 

appears more likely to have been a sandstone than an eruptive. The 

rock is now a mica-schist in its most micaceous phases, or a micaceous 

quartz-schist in its less micaceous forms.

No. 120. JASPILITE.

(FROM ISHPEMING, MAHQUETTE COUNTY, MICHIGAN. DESCRIBED BY C. R.

VAN HISE.)

Macroscopical. The jaspilites of the Ishpeming area of the Marquette district in the Lake Superior region occur in the Eegauuee or 

iron-bearing formation of the Lower Marquette series. They are banded 

rocks, the alternate bands consisting mainly of small iron-stained particles of quartz, or jasper, and of iron oxide. The exposures present a 

brilliant appearance, due to the interlamination of the bright red jaspel 

and the dark red or black iron oxides. The iron oxide is mainly hematite, and includes both red and specular varieties; but magnetite is 

usually present.

The jasper bands frequently have oval terminations or die out in 

an irregular manner. The rocks have been folded in a complicated 

fashion, as a result of which the layers present an extremely contorted

1 In u few sections there are present, in addition to the inuscovite, a few flakes of a brownish-green, 

strongly ploochroic biotite. These, when present, are always closely associated with the muscovite. 

The mineral never occurs in quantity large enough to affect the general character of the rocfc.

Bull. 150 20
306 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

appearance. The folded layers frequently show minor faulting. Because of their brittle character, at many places the bands have become 

broken through and through, and sometimes they pass into reibungsbreccias. In some cases the movement of the fragments over one 

another has been so severe as to produce a conglomeratic aspect.

In the folding of the rock the readjustment has occurred mainly in 

the iron oxide between the jasper bauds. As a result of this, the iron 

oxide has been sheared, and when a specimen is cleaved aloog a layer 

it shows brilliant micaceous hematite. This sheared lustrous hematite, present before the last dynamic movement, is discriminated with 

the naked eye or with the lens from crystal-outlined hematite and magnetite which have filled the cracks in the jasper bands and the spaces 

between the sheared lamina} of hematite. The jaspilite differs mainly 

from the ferruginous chert of the iron-bearing formation, with which it 

is closely associated, in that the siliceous bands of the former are stained 

a bright red by hematite and the bands of ore between them are mainly 

specular hematite, while in the cherts the iron oxide is earthy hematite. 

The jaspilite in its typical form, whenever found, always occupies one 

horizon, the present top of the iron-bearing formation just below the 

Goodrich quartzite. In different parts of the area the jaspilite has a 

varying thickness. With this jaspilite, or just above it, are the hard 

iron ores of the district; hence it has been called by the miners, to 

discriminate it from the ferruginous chert, u hard ore jasper."

Microscopical. In thin section the jaspilites are seen to have a 

minutely laminated character, each of the coarser bands, as seen in 

hand specimen, being composed of many lamina, due to the irregular 

concentration of the iron oxide. These laminae are of greatly varying 

width. They unite and part in a most irregular fashion, producing 

a mesh-like appearance, and frequently laminae disappear, as do the 

coarser bands.

The complex, bright-red jasper bands are composed mainly of finely 

crystalline cherty quartz, but they are everywhere stained with minute 

particles of blood-red hematite. The grains of quartz average rather 

less than 0.01""" in diameter, and each of these minute grains contains 

one or more particles of hematite. These are concentrated in laminae, 

or are separate flecks included in the quartz grains. In some cases 

the hematite appears to be somewhat concentrated between the grains, 

but in general it is arranged in entire independence of them, as though 

it were present before the quartz had crystallized. The ferruginous 

bauds contain a predominating amount of iron oxide, but in them is 

included much quartz, exactly similar to that of the jasper bands. The 

original, translucent, red, sheared hematite is easily discriminated from 

the secondary crystal-outlined hematite and magnetite.

The folding, faulting, fracturing, and brecciation spoken of in hand 

specimen are beautifully shown under the microscope. The resultant 

cracks and crevices are filled with secondary quartz and crystal-outlined
DILLEK.] NO. 121, MAGNETITIC SPECULAR HEMATITE.

hematite and magnetite. This quartz is much more coarsely crystalline 

than the older quartz, the grains in many slides averaging from 0.05""" 

to O.lmm in diameter. While much of this secondary quartz occurs in 

veins that cut across the original lamination of the rock, a good deal of 

it has entered parallel to the lamination. Its coarseness and its likeness to the vein quartz readily distinguish it from the earlier quartz. 

The crystal-outlined hematite and magnetite also help to fill the veins 

and spaces between the micaceous hematite laminae. These secondary 

materials usually have entirely filled the spaces, thus completely cementing the rock, and because so much material has entered parallel to the 

original lamination this structure is emphasized by the secondary 

impregnations.

It has been noted that the jaspilite is characteristic of the uppermost horizon of the iron-bearing formation j that is, it is immediately 

below the next overlying series. The contact zone has been one of the 

great planes of accommodation, and thus the dynamic effects upon the 

jaspilite are explained. Between the jaispilite horizon and that of the 

ferruginous cherts is a transition zone. In this the layers of siliceous 

material sometimes have borders of red iron-stained quartz.

It has been explained that the chief differences between the jaspilites 

and the ferruginous cherts are the blood-red character of the minute 

hematite particles and the micaceous character of the ferruginous layers 

of the former. It appears highly probable, therefore, that dynamic 

movements have transformed the ferruginous chert into jaspilite, the 

layers of brown hematite being sheared into micaceous hematite and the 

inclusions of brown hematite being changed into the blood-red variety. 1

No. 121. MAGNETITIO SPECULAR HEMATITE.

(FROM THE REPUBLIC MINE, MARQUETTE COUNTY, MICHIGAN. DESCRIBED BY

C. R. VAN HISE.)

The specular hematites of the Marquette district occur at or near the 

contact of the Upper Marquette and Lower Marquette series, being 

either at the top of the Negaunee iron-bearing formation of the Lower 

Marquette series, or at or near the base of the Goodrich, quartzite of 

the Upper Marquette series. The greater quantities of the hard ores 

are probably in the latter position, and from this horizon the particular 

specimens described below are derived. These hard ores are all associated with jaspilite, No. 120 of the Educational Series.

The original source of the iron for the iron ores is a silicious siderite. 

The ore is a secondary concentration produced at favorable places by 

downward percolating waters. In many cases, after a first concen-

'li'or relations of the Negaunee formation containing jaspilite to other formations, and a description 

of the Marquotte series, to which the Negauneo formation" belongs, see Preliminary Report on tlio 

Marquette Iron-bearing District of Michigan, by C. R. Van Hise and AV". S. Bayley, with a chapter on 

the Republic Trough, by H. L. Smytli: Fifteenth Ann. Rept. U. S. Geol. Survey; also Mon. TJ. S. Geol. 

Survey, Vol. XXVIII.
308 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

£v

tratiou, there was a mashing of the deposits, and later a farther enrichment of the iron ore by infiltration. ' ^

The specimens consist mainly of brilliant flakes of micaceous hematite, which are arranged to a considerable degree with their greater / 

dimensions parallel, thus giving the ore a distinct schistosity or rift. 

Between the flakes of hematite are miuute granules of magnetite. 

Under the microscope the slides are best studied in reflected light. 

They seem to be made up mainly of numerous closely fitting grains of 

hematite, the majority of which take an imperfect polish," and have r 

therefore a gray, sheeny, spotted appearance. The grains, which are 

parted along the cleavage, reflect the light like a mirror. The large 

number of these reflecting surfaces is appreciated only by rotating the * * 

section, which brings successively different ones into favorable positions to reflect the light into the microscope tube. As both the mag- . 

netite and the hematite are opaque, the two minerals in general can 

not be discriminated, although in some cases the crystal forms of magnetite are seen, and a small part of the hematite, much of it in little 

crystals, shows the characteristic blood-red color. u

The accessory minerals are quartz, feldspar, muscovite, and griiuer- ^ 

ite. Some of the small areas of quartz and feldspar appear to be 

fraginental. The muscovite occurs mainly in small independent flakes, sbut some of it is apparently secondary to the feldspar. The griinerite 

is very sparse. The translucent, red hematite is closely associated with 

the feldspar, muscovite, and griinerite.

The iron ores and associated rocks of ther Marquette iron-bearing 

district are fully described in the Fifteenth Annual lieport of the United 

States Geological Survey, pages 561 to 589, and in Monograph XXVII, \ 

pages 328 to 407.

No. 122. SLATE. >"

(FROM MONSON, PISCATAQUIS COUNTY, MAINE. DESCRIBED BY W. S. BAYLEY.)

Specimen 'No. 122 is an excellent sample of the rocks that are known "* 

as clay slates. It is a very fine-grained, bluish-gray variety, with a 

close crystalline texture. It is so soft that it may easily be scratched 

with a knife blade, but at the same time it is so dense and elastic that 

it rings clearly when hit with a hammer. If struck on its side with a 

chisel whose cutting edge is parallel to the long edges of the specimen, 

it will split into plates whose minimum thickness is limited only by the 

skill of the workman. This peculiarity of splitting into thin slabs is 

the most characteristic property of slates, and is that which gives 

them their great economic value. The evenness with which they may >- 

be cleft and the size of the plates obtainable from them are important 

elements in determining the suitability of the material for roofing and 

manufacturing purposes. In a good slate the grain is fine, and the i 

cleavage planes run for long distances without interruption.

The principal localities in this country from which good slates come ~~
DESCRIPTIONS: NO. 122, SLATE. 309

are the Peach Bottom and neighboring regions in southeastern Pennsylvania and northern Maryland, the Arvon quarries and mines in the 

Avestern part of the upper peninsula of Michigan, various places in 

Vermont and Virginia, and the quarries of the Monsou Slate Company, 

at Monson, Piscataquis County, Maine.1 The specimens 2 in the collection are from the last-named locality.

Though not of as much importance as in the case of stone used in 

the foundation of large buildings, the strength 3 of various slates serves 

as a rough measure of their comparative merits. In massive rocks the 

strength is nearly equal in every direction. In schists and bedded 

rocks less pressure is required to produce crushing, when applied in a 

direction parallel to their schistosity or cleavage, than is necessary 

when applied perpendicularly to these structural planes. A cube of 

the Monsou slate measuring an inch on an edge, breaks across the 

cleavage, pressure being applied at right angles to the cleavage, when

the pressure reaches 30,425 pounds. A slab 12 by 6 by 1 inch, supported on knife edges 10 inches apart, breaks under a stress of 4,000 

pounds when the pressure is applied along a line midway between the 

supports.

The rock occurs in beds 4 of various thicknesses, ranging between. 20 

feet and 4 inches, interlaminated with hard, fine-grained, dark quartzites. The series strikes about 70° east of north and dips at an angle 

of about 78° to 83° in a direction north of west. The cleavage is 

nearly vertical, perhaps inclining about a degree therefrom, and strikes 

nearly in the same direction as the beds. The difference in the inclination of the cleavage and bedding causes the course of the former to 

cross that of the latter at a small angle, but since the quartzite is so 

much harder than the slate, the cleavage planes that are so marked in 

the softer rock stop abruptly when the contact of the quartzite is reached, 

or cross it as a few fractures or slight faults. Since the .cleavage and 

the bedding are not coincident, and not even parallel, it is evident that 

the former is not in any way dependent upon sedimentation. A shale 

splits easiest along its bedding planes. The Monson slate can not be 

split along its bedding, but it is easily cleft along its cleavage planes. 

In other words, the cleavage of shales is an original characteristic, 

while that of slates must be of secondary origin.

The cause of cleavage in slates has been carefully studied by many 

geologists. Sharpe,5 as long ago as 1846, held that the cleavage of

Tor amounts quarried in different districts see Mineral Resources of the "United States for .1888 

and later years, and for descriptions of tlie districts, consult G. 1\ Merrill, The Collection of Building 

and Ornamental Stones in theTJ. S. Kat. Mus.: Kept. Smithson. lust. 1885-86, 3:>t. II, p. 404, et seq.

2 The specimens in the collection and the thin slabs were kindly furnished by the Monson Company, 

as were also tlio results of the strength tests and the analysis.

3 For statements regarding tests of building stones, see Merrill, loc. cit., p. 489.

4 Thought by Prof. C. IT. Hitchcock to be Cambrian or Lower Silurian. See geological map of Maine 

in. Colbv's Atlas of Maine. Houlton, 1884.

8 Daniel Sharpe, On Slaty Cleavage; Quart. Jour. Geol. Soc. London, 1846, Vol. Ill, pp. 74-105; Vol. V, 

1849, pp. 369-396.
310 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL 150.

slates is due to the flattening of the constituent grains. Sorby' believed 

that slaty cleavage is caused mainly by the rotation of the mineral particles, and especially mica, until the flat grains assume a position where 

they may best resist further rotation, namely, a position at or near 

right angles to the pressure. As favoring this view, he subjected a 

mixture of clay and iron oxide to pressure, and obtained a cleavage 

structure at right angles to the pressure. Tyndall 2 and DaubreV have 

also produced a cleavage in substances by simple pressure. The former 

used beeswax, clay, etc., and the latter clay mixed with scales of mica. 

In each case in the resultant cleavable product the flat particles were 

always found with their broad sides in the plane of cleavage; but 

Tyndall held, with Sharpe, that in the case of beeswax, as in all cases 

of nature, the cleavage is produced mainly by the flattening of the constituent particles.

In nature the pressure that produces the cleavage is the same as that 

which bows and bends the rocks of the earth's crust. Since this pressure is rarely perpendicular to the bedding, it follows that the superinduced cleavage is rarely parallel to the bedding. In many cases it is 

probable that the force that upturned the rocks at the same time produced in soft beds a cleavage; in other cases the cleavage was produced 

subsequent to the upturning of the rocks; and in rare cases two cleavages were produced by pressure acting at different times along two 

different directions.4 In addition to the cleavage produced in a rock by 

pressure, it usually happens that this agency causes such a change in 

the conditions under which the rock exists that crystallization is set up 

in its material, so that its original nature is largely obscured. Slates 

are usually interbedded with undoubted fragmental rocks like quartzites, grading into these and into conglomerates. Consequently, it is 

assumed that its original condition was fragmental, though perhaps no

traces of this condition are now discernible. If the original bed was

fragmental, the deposit must have been of a very fine grain, like mud 

or silt. Mr. Hutchings 5 has recently examined such beds in which 

schistosity has not been produced, and has obtained from them a great 

deal of information with respect to the course of crystallization set up 

in them. He concludes that nearly all. the mica of slates is a secondary product, derived by the alteration of constituents of the original 

deposit. Other components of the slate are also new products, formed 

subsequent to the deposition, and hence the slate in its present condition is composed largely.of crystallized secondary material. In this 

sense the slate is crystalline, and should be classed with the crystalline 

schists. Since, however, we know that it was originally a sediment, 

we usually class it among the fragmeutal rocks, thus placing more

1 Edinburgh New Phil. Jour., vol. LV., 1853, p. 137.

2 Philos. Mag., IV, XII, p. 129.

3 G6ologie Exp6rimeijfcale, p. 391.

4 Cf. C. E. Van Hise: Bull. Geol. Soc. America, vol. 2, p. 209.

6 Geological Magazine, Vol. VII, June and J uly, 1890, and Vol. VIII, 1891, p. 164.
^_ DILLBB.] DESCRIPTIONS: NO. 122, SLATE. 311

emphasis upon the genetic relations of the rock than upon its present 

condition.

Yan Hise 1 has recently rediscussed slaty cleavage. He concludes 

% that the structure is due to the arrangement of the mineral particles 

with their longer diameters or readiest cleavage, or both, in a common 

direction, and that the cause of this arrangement is, first and most 

important, the parallel development of new minerals; second, the flattening and parallel rotation of old and new mineral particles, and third, 

and of least importance, the rotation into approximately parallel positions of random original particles. He further concludes that this 

structure is developed in rocks when they are so deeply buried as to be 

^ in the zone of plastic flow, and that the structure develops in the planes 

normal to the greatest pressure.

Under the microscope the principal mineral constituents of the Monsou slate in the collection are discovered to be quartz, chlorite, musco-

 vite, biotite, magnetite, and a few little black organic particles. These

x are nearly all in the form of small lenses, with their long axes in the same

direction. The largest grains are those of chlorite. This mineral is in

 light-green masses, with a dark-green pleochroism. Its double refrac-

'A tion is weak, so that between crossed nicols it polarizes in gray or bine 

tints. Around its grains bend the flakes of .muscovite, etc. a proof 

that the former must have existed when the cleavage was superinduced 

in tlie rock. It is probable that the grains were then plagioclase and 

that the chlorite has since been produced by its alteration. Chlorite 

also exists in little shreds between the other components, where it is 

always elongated in the direction of the foliation.

The next most abundant components are the micas. These occur as 

tiny shreds and flakes of muscovite that can be detected only between

-i crossed nicols, when it appears with bright polarization colors and as 

larger, ill-defined masses of brown biotite. The latter sometimes occurs 

also as small flakes lying parallel to the muscovite plates. It is by no

^ means so common as the muscovite, nor is it always arranged with its 

flat sides in the plane of cleavage. Since the flakes of muscovite are 

not crowded around the large masses of biotite as they are around the 

chlorite grains, it would seem that the biotite must have "been, formed 

after the potash mica. It includes shreds of the muscovite, and in its

> arrangement it by no means follows the rule that the long axes are 

parallel to the cleavage. Its genesis, consequently, was probably subsequent to the origin of the foliation.

The quartz is not in large quantity. Only occasionally can little 

grains be detected, when they have the usual elliptical cross section. 

The magnetite and the organic substance are both in comparatively

w large-sized grains. The former have badly defined crystallographic 

outlines, while those of the latter are irregular and ragged. If a small

"N 

'Principles of North American Pre-Cambrian Geology, by C. R. Van Hiso: Sixteenth Ann.Kept. 

TJ. S. Geol. Survey, Part I, pp. 633-668. See also Deformation of Kocks, Part III, Cleavage and Fissility: 

Jour. Geol., Vol. 1V, 1896rpp. 449-483.
312 THE EDUCATIONAL SERIES OF KOCK SPECIMENS. [BULL. 150.

portion of tlie section is heated for some time on a piece of platinum 

before the blowpipe the organic particles are burned out, while the 

magnetite remains unchanged. This is about the only method of distinguishing between the two substances when present in such small 

particles. It is impossible to determine, except by careful chemical 

tests, whether the organic material is carbon or a compound of this 

element. Besides these opaque constituents there are in the muscovite 

shreds, between the lamellae, numerous little black particles, most of 

which are in tiny short needles, though a few are in larger round or 

irregularly shaped masses. The latter have a brownish color, but their 

dimensions are so small that the nature of the material composing them 

can not be determined. From analogy, however, we may conclude that 

they are tiny grains of rutile. This mineral in small needles is so 

characteristic of slates that they were long known by the German 

petrographers under the indefinite name of thonschiefernadeln, until 

in ] 881 Cathrein 1 proved them to consist of rutile.

No other components are important enough to merit mention as 

essential to the rock, though perhaps in an occasional section a single 

grain of pyrite or of some other widely occurring mineral may be 

detected. ^

From the microscopic examination of the slate we easily discover 

why it splits so readily in one direction only and in such very thin 

plates. The different constituents, with the exception of the biotite, 

are arranged in layers composed of flat pieces lying in parallel po*itions and overlapping and dovetailing into one another. Across the 

laminae fracture is difficult, and the fracture surface is rough, for the 

breaks, following the paths of least resistance, refuse to pass across 

the grains in their courses when they can so easily turn aside and pass 

around them. The crystalline structure noticed on the cross-fracture 

surface is caused by the projection of the grains. The cleavage surface, 

on the contrary, is smooth, because so few grains extend from one layer 

into the other that the easiest path for the fracture is between successive 

layers.

In the following table the composition of the rock, as determined by 

L. M. Norton, is given in column I; in column II is given the composition of a specimen of the Peach Bottom slates, Pennsylvania.

____________________________________________________________:________. __ _ __

1 Neues Jahrb. f. Min., etc., 1881,1, p. 169.
DILLBR.] DESCRIPTIONS: NO. 123, INDURATED JOINTED SHALE. 313

Analyses of slates.

SiO2 ........ ............................

Tin

or\

A.l.,03 ...... ............................

FeO............ ........................

MnO............ ..................:....

CaO..... ..........^. ...................

MgO....... ............................

K20...... ..............................

Na.,0 ..................................

H20... ...... ...........................

Total ............................

I.

Per cent. 

56.42

24.14

4.4G

.52

2.28

5.53

3.15

} 3. 88

100. 38

II.

Per cent.. 

55. 880

1.270

.022

21. 849

9.034

.586

.155

1.495

3.640

.400

r .051

\ 1. 974

.051

99. 800

The specific gravity of the Mouson slate is 2.851.

The analysis accords well with the results of the microscopic study 

of the thin section. The small amount of magnesia indicates a small 

proportion of biotite. The absence of plagioclase is revealed by the 

small percentage of lime. The large proportion of potash points to an 

abundance of muscovite, while the water and ferrous iron show the 

presence of chlorite. The percentage of silica is so low that there can 

not be much quartz in the rock, and the alumina is just about sufficient 

to combine with the potash, magnesia, and iron protoxide to fogtn 

muscovite, biotite, and chlorite. The soda may indicate the presence 

of a very alkaline feldspar, or it may be present in the light-colored 

mica, some of which may be paragonite instead of muscovite. 1

No. 123. INDURATED JOINTED SHALE.

(FROM SOMEKVILLE, MIDDLESEX COUNTY, MASSACHUSETTS. DESCRIBED BY C. R.

VAN HISE.)

Various causes have been assigned for joints, of which the more 

important are tension, torsion, and compression. It is believed that 

joints may be classified into tension joints and compression joints, torsion joints being but a variety of tension joints. The first are ordinarily 

in the normal planes, or at right angles to the stretchiug force; the 

second are in shearing planes, or are inclined to the crushing force.2

Joints may be produced in rocks in the outer zone of the crust of the 

earth, where fracturing results from deformation. Bocks which are 

buried to a great depth are under such a load that it is impossible 

that crevices and cracks can exist; therefore jointing does not occur.

1 3Tor other descriptions of slates see Irviug and Van Hise, Tenth Ann. Kept. TJ. S. Geol. Survey, 

pp. 372, 428; Geology of AVisconsin, Vol. Ill; and for general account of slate regions,.etc., Report 

on Building Stones of the United States, Census of 1880, Washington, 1884.

-'For a fuller discussion of joints, see Principles of North American pre-Cainbrian Geology, by 

C. It. Van Hise: Sixteenth Ann. Kept, U. S. Geol. Survey, Part I, pp. 668-672.
314 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Jointing is almost certainly confined to the outer 10,000 meters of the 

crust of the earth, and is perhaps confined to the outer 5,000 meters.

Tension joints. Tension is often due to the contraction caused by 

cooling or by desiccation. It is well known that the peculiar columnar 

jointing of igneous rocks is due to the contraction and consequent tension caused by cooling, and the mud cracks of sedimentary rocks are 

due to the contraction and consequent tension caused by desiccation. 

However, it is probable that neither cooling nor desiccation is important 

in the production of systematic, parallel sets of joints.

When rocks are simply folded in the outer zone of fracture, the convex halves of the folds may be subjected to simple tension, and if this 

goes beyond the ultimate tensile strength of the rocks, radial cracks 

will form, which extend through the strata and strike parallel with the 

rocks. In rocks which are complexly folded or cross folded, there may 

be two sets of tensile joints which intersect each other nearly at right 

angles. Tensile joints in homogeneous rocks are exactly or nearly at 

right angles to the stretching force.

Daubree 1 has shown that if a brittle plate breaks when it is subjected 

to torsion, a double set of parallel fractures nearly at right angles to 

each other are produced. The forces which produce complex folding 

stretch the convex parts of the strata, where not too deeply buried, in 

two rectangular directions; or, in other words, they are subject to 

torsion. It therefore appears that Daubre"e's explanation of joints by 

torsion is but another statement of the production of joints by complex 

folding in the two principal planes of tensile stress.

Compression joints. Daubree 2 and Becker 3 have shown that joints 

may be produced by compression. Then there may be jointing in two 

sets of shearing planes when the rocks are subjected to a single stress, 

and, according to Becker, there may be joints in three or four planes 

when they are subjected to unequal stresses in different directions, in 

the latter case one of these sets of joints being normal to tensile stress 

and the other in shearing planes. In all cases in which jointing occurs 

in shearing planes the joints are inclined to the pressure. In a simple 

ideal case the joints should be at an angle of 45° to the pressure, but 

this rarely occurs in nature. Fracturing along shearing planes is illustrated when stones are crushed in a testing machine. The fractures 

do not form in the lines of pressure, but in general at any place in a 

homogeneous rock in two intersecting directions at angles somewhat 

less than 90° to each other. The direction of pressure bisects the acute 

angle.

Becker has explained that minor faulting is a common phenomenon 

of compression joints.

1 G6ologie Exp6rimentale, by A. Daubrec, pp. 30G-;tU, Pnria, 1879.

2 Ibid., pp.315-322, Paris, 1879.

3 George F. Becker: Finite homogeneous strain, flow, and rupture of rooks: Bull. Geol. Soc. 

America, Vrfl. IV, 1893, pp. 41-75. The torsioiial theory of joiiits: Trans. Am. Inst. Min. Engineers, 

Vol. XXIV, 1894, pp. 130-138.
DTLLER.] DESCRIPTIONS: NO. 124, CRUMPLED SHALE. 315

lu many cases when the rocks are jointed in more than one direction 

the different sets of joints may have been produced by successive 

orogenic movements, rather than at a single period of deformation.

The specimen (No. 121) from Somerville, Massachusetts, belongs to the 

so-called "Cambridge slates" whose geologic age is not yet positively 

known. Crosby has referred them to the Carboniferous, but others 

have considered them to be Cambrian.

The specimen is bounded by three sets of joints. One set is parallel 

to the bedding; the other two sets intersect the bidding at acute angles 

and are at acute angles to each other. As a result of the three sets of 

joints^ the rock is broken into rhomboidal blocks (or more accurately 

oblique parallelepipeds) bounded by the three sets of joint planes, each 

piece having as boundaries two parallel fractures along each of the three 

sets of joints. Probably each of the sets of joints is produced by compression rather than by tension, and. therefore is formed along shearingplanes. The joints parallel to the bedding are undoubtedly controlled 

in direction by planes of weakness along the bedding. As shown by the 

smoothed surfaces, there has been slipping along many of these joints 

during the folding process. In some cases the evidence of readjustment in slickensided faces is also seen along the sets of joints intersecting the bedding.

No. 124. CRUMPLED SHALE.

(FROM HOT SPRINGS, MADISON COUNTY, NORTH CAROLINA. DESCRIBED BY BAILEY

WILLIS.)

The specimen of crumpled calcareous shale illustrates the folding of. 

laminated rocks. When this shale was deposited as a sediment its 

successive layers of red and buff mud were level and parallel. In that 

position they hardened into firm rock. The shale was gradually buried 

beneath later sediments and sank into the earth's mass. At a depth 

of a few thousand feet the superincumbent pressure was such as to hold 

this shale in a condition in which it could not break apart. When 

compressed edgewise by a sufficient lateral force the layers were 

obliged to bend like paper. In thus bending, they slid one over 

another, they were squeezed thinner in certain parts of the fold, on the 

limbs, and they thickened where the combined pressure of load and 

edgewise thrust were less intense.

Folds of rock masses are not limited to small crumples such as can 

be shown in a specimen. The beds of strata folded may be 5,000 feet, 

or five times 5,000 feet thick; then the folds are large. For example, 

in central Pennsylvania there is a fold, known as the Nittany Arch, 

which is 25 miles across and about 150 miles long. Examples of still 

larger folds might be cited.

The accompanying illustrations 1 show two forms of single folds. 

One of these (Fig. A, PI. XLI), which is open upward like a trough, is

1 Plate XLVIII, Thirteenth Ann. Kept., U. S. Geol. Survey.
316 THE EDUCATIONAL SERIES OF BOOK SPECIMENS. [BOLL. 160.

called a syncline. The other (Fig. _Z?, PI. XLI), which is arched upward, 

is called an anticline.

Synclines and anticlines usually occur 4n association, side by side 

and overlapping end on end. They then form systems of folds, which 

may be of great extent. The Appalachian system of folds extends 

from Nova Scotia to Alabama, about 1,600 miles, and varies from 50 to 

125 miles in width. Along the eastern base of the Eocky Mountains 

there is a similar system of folds.

Folds develop many different forms, both in cross section and in longitudinal section.. They are usually classified according to the degree 

of compression, which varies from gentle undulations to structures in 

which the strata are pressed closely in vertical positions, or to those in 

which the beds are even overturned and squeezed out thin. Folds are 

also classified according to magnitude and complexity. Any large fold 

includes smaller folds, each such smaller fold includes still smaller 

folds, and so on down to microscopic crinkles.

Strictly speaking, in folds the relative order of strata is not changed; 

beds a, &, c, d, e, /, etc., after folding succeed one another in their 

original order. But there is another class of structures resulting 

from compression, called faults, overthrusts, or thrusts, which are characterized by a change in the order of strata. When rocks break, the 

separate parts may be pushed one over another. When folding, rocks 

are so intensely compressed that some strata, softer than others, are 

squeezed out, then that portion of the fold above the squeeze may be 

pushed over the portion beneath. When rocks flow under tremendous 

.pressure the movement of flow may be concentrated in a plane or in a 

number of planes in a narrow zone. Then the rock is divided at each 

plane, and the parts may be moved past one another. In all these 

cases the changes of form in the rock mass may result in placing a 

lower formation a, over a higher stratum /. Such a structure is an 

overthrust, and the rock is said to be faulted. 1

Overthrusts vary in magnitude from those which may be visible in 

the specimens of crumpled shale to those of the southern Appalachians, 

which exceed 300 miles in length and traverse strata 10,000 feet thick.

The deformation of rock masses is more fully illustrated and discussed 

by the writer in the Thirteenth Annual Report of the United States 

Geological Survey, pp. 211 to 281, and by 0. E. Van Hise in the Sixteenth Annual Eeport of the United States Geological Survey, pp. 581 

to 872; also in the Journal of Geology. Vol. IV, 1896, pp. 195 to 213 

and 313 to 353.

No. 125. FAULTED PEBBLE OF CEETACEOTJS CONGLOMERATE. 

(FROM SISKIYOU MOUNTAIN, CALIFORNIA. DESCRIBED BY BAILEY WILLIS.)

There is movement in the solid rock masses of the earth. Earthquakes are a violent but temporary effect of forces which act constantly 

to modify at least the surface of the globe. The nature of these forces

1 Plate LII, Fig. 1, Thirteenth Ann. Kept., U. S. Geol. Survey.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 150 PL XLI

^^frS^^^fcHi?^^

SYNCLINE AND ANTICLINE. 

A. Synclme. B. Anticline.
"**>

DESCRIPTIONS: NO. 120, PHYLLITE. 317

is but vaguely understood. They originate in gravity, in the earth's 

internal heat, and in physical and chemical reactions at temperatures 

and pressures which far within the earth greatly exceed human experi-

 ' * ence.

To these forces are due the differences of elevation of sea bottoms

^ and continents; they raise mountain ranges, from which the atmos-

. J pheric agents, heat, frost, wind and rain, carve individual mountains;

they are active now, and have been active throughout all the earth's

past.

v These forces act upon rock masses whose size may be measured by 

cubic miles, and whose Aveight may be expressed in millions of tons.

 V The power of these forces is beyond comprehension.

Against such forces the resistance of firm rocks is but as the plasticity of wax. In the movements within the earth's mass rocks break, 

bend, or even flow. Whether they break, or bend, or flow depends 

upon the amount of superincumbent load under which they are forced 

to change form. The softening effect of heat is for rocks of the outer

-^ crust insignificant as compared with the influence of pressure.

Consider any horizontal layer of the earth's crust. It rests with all 

its weight on the next layer below. The weight of the two layers is 

borne by a third below them ; and so on into the depths. The pressure 

due to this weight of the rocks increases rapidly downward. At 5 

miles below the surface it is sufficient to crush most rocks; at 10 miles

-v below the surface the pressure of load exceeds the strength of the 

hardest rocks at the surface.

Rock masses which are lightly loaded ascompared with their strength, 

that is, rocks near the surface, break when forced to move. Bock 

masses which are more heavily loaded as compared with their strength, 

that is, rocks lying deeper in the earth, bend when obliged to change 

form. Eock masses which are loaded in excess of their strength, that 

is, rocks at depths of 6 to 10 miles below the surface, flow from regions 

r of overwhelming force to regions of less resistance.

The specimen of a faulted pebble, No. 125, from a Cretaceous conglomerate in northern California, illustrates the breaking of rock 

masses. The pebble is composed of the very hard and brittle mineral, 

quartz. It has not been cracked by a blow, it was not suddenly

* shattered, but it has been broken across by pressure. Afterwards the 

separated parts have been cemented by the deposition of silica from 

solution. Many pebbles over a large area in this particular conglomerate are thus broken, showing that the effect of the force was distributed throughout the mass of the rock.

v No. 126. PHYLLITE (SEBICITM-SOHIST).

(FROM LADIKSBUUG, FREDERICK COUNTY, MARYLAND. DESCRIBED KY 

^ W. S. BAYJVEY.)

The western portion of the Piedmont Plateau in Maryland is underlain by rocks that are less. thoroughly crystalline than are those of the
318 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

eastern portion.1 While they have been subjected to a certain amount 

of metamorphism and alteration, they still plainly show that they were 

once sediments of an ordinary type, such as limestones, shales, etc. 

The line of demarcation between these rocks and the more crystalline * i 

schists to the east is sharp. Where occasionally the less crystalline 

rocks are infolded with the more crystalline ones, the former appear to /"' 

be much younger than the latter. Moreover, they are interbedded in l 

places with a lower Silurian limestone, and are always much less folded 

and distorted than are the eastern schists. The eastern schists are 

regarded by Williams 2 as Algonkian in age, and the western ones as ^ 

Cambrian and Lower Silurian.

The principal rock of the western area is a phyllite 3 of which, several A 

types occur. That represented by the specimen is the most characteristic. All the phyllites were originally argillaceous sediments. Since 

their deposition they have been subjected to dynamic and chemical 

metamorphism.

Dr. Williams describes these phyllites as constituted principally of 

a silky white mica (kaolin or sericite), whose individual scales vary ^ 

greatly in size in different specimens. This is sometimes wholly or in 

 part replaced by chlorite, forming a chlorite-schist. Quartz grains of 

varying size and outline are generally present, while feldspar is 

extremely rare. It is probably the alteration of this mineral in the 

process of weathering that has yielded the micaceous component. Iron 

oxide is also present in very small grains. Tourmaline in small crystals 

is very common, as are also microscopic needles of rutile (thonschiefernadeln). The phyllites have always a perfect cleavage and a satiny -; 

luster which increases with the crystallization of new mica. Their color 

ranges from black through every shade of purple, blue, and green to a 

pale gray. The darker varieties are largely worked as roofing slates.4 

Evidences of clastic structure are not.infrequently preserved in the 

shape of rounded grains and small pebbles of varying appearance and 

composition. Where least disturbed these slates are jointed and cut by , 

cross seams of chlorite or quartz. When they are more disturbed they 

become greatly puckered and filled with eyes of quartz.5

The specimen in the collection represents one of the lightest colored 

of the phyllites. It came from near the town of Ladiesburg, in Frederick County, on the western side of the area occupied by these rocks, "* 

near the contact of the phyllites with the overlying Kewark sandstones.0' The rock is so closely folded and so perfectly cleaved that its

'Cf. description of No. 119.

2 G. H. Williams, Guide to Baltimore; prepared for Am. Inst. Min, Engineers, Baltimore, 1892, 

.p. 80-87.

3 G. H. Williams, The petrography and structure of the Piedmont Plateau in Maryland: Bull. 

Geol. Soc. America, vol. 2, 1891, pp. 305-307. ^

4 The value of the slate quarried from these rocks in the State of Maryland amounted to $153,068 iu 

the year 1894.

8 Op. cit., p. 306.

"See Preliminary Geological Map of Maryland, G-. H. Williams, editor, 1893: Maryland, its 

Resources, Industries, and Institutions; prepared for the Board of World's Fair Managers of Maryland, Baltimore, 1893. Ladiesburg is at about latitude 59° 35' and longitude 77° 15'. \
DESCRIPTIONS: NO. 126, PHYLLITE. 319

bedding has been much obscured. From the differences in the geological structure of the regions in which the phyllites and the niica-schists 

(No. 119) occur, it is argued that the former rocks have been squeezed 

in one direction only, by a single earth throe, while the latter have 

been subjected to different squeezings at different times.

The specimens are soft, argillaceous, slaty rocks of a light-green 

color when fresh and a pinkish-yellow color when weathered. Along 

the cleavage surfaces and along the sides of joint cracks the material 

is yellow. All specimens are very fissile, their cleavage planes being 

very numerous and very close together, and nearly all are crossed 

either by joint planes perpendicular to the cleavage or by little puckerings, which appear in cleavage surfaces as series of tiny waves. 

These are little contortions that serve as evidence that the rock has 

been at some time subjected to great pressure. When breathed upon 

the specimens emit the argillaceous odor so characteristic of clays and 

slates.

When viewed under low powers of the microscope the thin sections 

occasionally reveal the j)reseuce of rounded areas that appear to be 

cross sections of sand grains. Between crossed nicols they often break 

up into matted fibrous aggregates of a strongly refracting mineral 

colored by yellow ocher.

Under high powers in natural light the rounded and irregularly 

shaped colorless areas are seen to be embedded in a brownish-yellow or 

light-green cloudy mass, through which are scattered fibers of alightgreen mineral, tiny grains of a very highly refractive one probably 

zircon small plates of red hematite, and little opaque particles of 

magnetite. Between crossed nicols the nature of the various components is not difficult to determine. The most prominent ones are quartz, 

sericite, chlorite, and masses of a finely fibrous substance, believed to 

be kaolin.

In most sections quartz and chlorite are the most abundant. Some 

of the former is in little rounded or irregular grains. To these as a 

nucleus has been added new quartz material, Avhich often extends in 

the form of long, narrow spicules into the surrounding mass of quartz 

and chlorite. In this the quartz forms a groundmass that is so thickly 

strewn with chlorite fibers and with opaque grains of different kinds 

that its polarization can scarcely be distinguished. It is evidently a 

secondary product which has resulted from the decomposition of some 

of the rock's original components or is a substance that has been introduced from Avithout. The chlorite, which is a ferruginous variety, is 

mainly in bunches o'f light-green fibers, with a very weak double refraction. In many instances where the chlorite is thick it appears like an. 

isotropic substance. Occasionally the chlorite is intergrown with a 

brightly polarizing, light-colored sericite. The two minerals together 

occupy areas that resemble the shapes of sharp-edged fragments. 

Through this quartz-chlorite groundmass are scattered large plates of 

sericite, crystals of zircon, opaque reddish-brown grains of some iron
320 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

oxide that lias been superficially changed to limonite, small, irregular 

masses of ocher, innumerable particles of various opaque bodies that 

may collectively be denominated dust, and thousands of very small, 

slender, apparently opaque needles. These are the rutile needles characteristic of slates. They are most abundant in those portions of the 

slides in which chlorite predominates, being especially numerous around 

the opaque granules of limonite.

The yellow tint of most of the specimens is due to the presence of 

ocher. This occurs not only in the grains and irregular masses already 

referred to, but it also occurs as a very finely divided pigment which 

saturates the chlorite fibers and colors them.

The structure of the rock is that of fine-grained slates which have 

suffered a large amount of alteration and which have been made fissile 

by movement under pressure. All of its present constituents are secondary, except the larger grains of quartz, which probably represent 

original sand grains. Its composition (I) as determined by George 

Steiger, of the United States Geological Survey, shows that however 

much it may differ from its original condition, it still possesses the 

composition of a clay slate. The analysis (II) of a typical clay slate 

from near Clausthal : in the Harz is given-for comparison.

Analyses of phyllite and clay slate.

Si02 . ...:.. ............................

Ti02 .. .................................

A1203 ..................................

Fe-A------------ - ----"--- --- - 

FeO...... ..............................

MnO.. .................................

CaO.. ..................................

BaO- ...... .............................

MgO... .....'...........................

K,0.. ..................................

N%0. - -- ...........................

TLUO+GT al"»ATr« 1 OftO

s...... .................................

c ......................................

PA--------------- -- ----------------

C02 ..... ...............................

Total............................

I.

Per cent. 

57.24

.08

23. 48

3.19

 4.87

.09

.93

3.55

1.18

4.C5/

.09

99.68

II.

Per cent. 

57.99

.32

23.42

.49

5.06

1.65

1.20

3.50

1.32

3.39

.17

.74

1.12

J J". Kotli: Allgemoino uiid Choinische Geologic, II, p. 588.
DILLEK.] . DESCRIPTIONS: NO. 127, PHYLL1TE. 321

No. 127. PHYLLITE (CHLORITE-PIIYLLITE).

( FROM LISBON, GRAFTON COUNTY, NEW HAMPSHIRE. DESCRIBED BY W. S. BAYLEY.)

y' Specimen No. 127 is from a characteristic member of the Lisbon 

^ group of green schists, which 0. H. Hitchcock' places in the lower 

division of the Huronian. The group embraces greenish schists, conglomerates, quartzites, jaspers, and dolomites. Van Hise 2 doubts the 

advisability of continuing to call these rocks Huron ian, in view of the 

fact that so many of the New England crystalline schists are being 

*" found to be Cambrian or later in age. The term as used by Hitchcock 

^ seems to have been chosen largely on lithological grounds. Whatever 

the age of the rocks, they have been squeezed and altered until the 

evidence of their origin has been more or less obscured.

The hand specimen shows a uniformly fine-grained, crystalline rock

of a grayish-green color. It has a typical though not very distinct

> schistose structure. A study of the hand specimen reveals little

respecting its composition. On surfaces at right angles to the

schistosity the rock has the appearance of a fine-grained graywacke;

t surfaces that are parallel to the schistosity have a more or less silky

y luster which is due to the presence of numerous small flakes of a green

^ micaceous mineral with the appearance of chlorite. Here and there

streaks of denser green occur. In these the green mineral is plainly

chlorite.

v If the thin sections of the rock are examined under a hand lens the 

cause of the rock's schistosity. is seen to be the elongation of its constituents in a common direction. The elongation is, however, not very 

strongly marked, and so the schistosity is not very prominent.

The constituents of the sections, as seen under the microscope, are 

quartz, calcite, chlorite, epidote, feldspar, magnetite, pyrite, and herna- 

>- tite, in the order of their abundance,.and in a few sections, in addition, 

a little biotite.

The quartz is in irregular grains aggregated in long, lenticular areas, 

and in very small grains that, together with a little feldspar, make up 

the matrix by which all the other components are surrounded. The 

larger, lens like areas resemble large, flattened sand grains that have 

» been squeezed until they were shattered internally, and the flue grains 

appear to be portions of the finer matrix of a sandstone in which the 

larger grains lay. Under high powers quartz is also seen forming a 

cementing mass uniting the small quartz grains and all the other components of the matrix. This form of the mineral may be a secondary 

product resulting from the decomposition of some original constituent

*r "C. H. Hitchcoolc, Geology of the Connecticut Valley .District: Geol. of New Hampshire, Vol. II, 

Chap. IV, pp. 277-29B, Concord, 1877.

-^ 'J Cf. C. Jl. "\rivn .Hise, Correlation Papers Archean and A.lgonkian: Mull. U. S. Geol. Survey No. 

8G, p. 382.

-Bull. 150 21
322 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

r

of the rock. All of the quartz, except the larger grains, contains ; 

numerous inclusions of chlorite and epidote.

The calcite occurs-in two forms. It is present as large, irregular 

grains, including within its mass grains of all the other, constituents, .<. 

and as the filling of little veins and nests that were at once tiny crevices and cavities in the rock mass. A third form exists in some sec- /"" 

tions, where it appears to replace an original component whose nature 

can not be determined. The calcite is easily recognized by its silvery 

white color in certain positions between crossed nicols, by the Newton's 

rings of color around the edges of many of its grains, by the two series ^ 

of cleavage lines present in most grains, and by the parallel twinning 

bars in the larger ones. The mineral is probably in all cases secondary. ^

Chlorite is present as flakes of a light-green color and with a faint 

pleochroism. Its double refraction is so weak that many flakes behave 

almost like isotropic substances. In a few instances portions of some 

of the plates have a strong pleochroism in green and brown tints. 

These portions resemble biotite in some of their characteristics, and 

they suggest the thought that some of the chlorite is an alteration 

product of this mineral. ^

The epidote is in small, prismatic crystals, small, rounded grains, and ' 

larger, irregular masses that appear to be aggregates of grains, -7 

imbedded in the quartz and chlorite. The color of the mineral is light 

greenish yellow, the tint being deeper in the large masses than in the 

smaller grains and crystals. Pleochroism is noticeable in all but the 

smallest particles. It is always slight, the mineral appearing in different tints of the same color in different directions. The epidote is 

easily distinguished from the other components by its color, its high 

refractive index, and its strong double refraction, producing bright 

interference colors. Although the mineral is scattered everywhere 

throughout the rock, it is accumulated more thickly in some areas than 

in others. Together with chlorite and quartz, it forms aggregates 

whose outlines suggest that they once belonged to sand grains that - 

have been replaced by the aggregate.

Feldspar is rare. It is present as small, irregular grains scattered 

among the quartz. In all cases it is altered, and in most cases so much 

so that the traces of its twinning bars are very obscure. Like the other 

components, it is filled with inclusions of epidote and chlorite, and in *r 

addition it contains included grains of quartz. As a rule, the more 

altered the feldspar, the more abundant the inclusions; hence it would 

appear probable that this mineral has in many cases given rise by its 

alteration to the aggregates of chlorite, quartz, and epidote mentioned 

above.as possessing the outlines of sand grains. Whether all of the 

epidote, chlorite, and secondary quartz has resulted in this manner or i* 

not is unknown, but it is probable that much of it was thus formed.

Biotite is rare. It occurs in only a few sections, where it appears as 

large, reddish-brown flakes and plates, with a distinct cleavage, and a 

strong pleochroism in reddish-brown and yellow^tints.
DILLBB.] NO. 128, METAMORPHIC CONGLOMERATE.. 323

The magnetite and pyrite are both opaque. The former is. in very 

irregular masses with a blue black luster, and the latter usually in welldefined crystals with an octahedral habit and a brassy yellow luster. 

The magnetite is more abundant than the pyrite. In some sections it 

is very plentiful. Like many of the other constituents, by its very 

irregular outlines it.presents the appearance of a secondary substance.

Hematite is only occasionally met with. It occurs as small bloodred plates included in quartz, chlorite, and feldspar, more particularly 

around the borders of magnetite grains.

From the general appearance and the composition of the rock, it is 

apparent that it is a product of the alteration of some preexisting 

rock of an entirely different character. At present it is completely 

crystalline. The structure, however, suggests that it was originally 

fragmeutal. The shattered quartz grains and some of the feldspa.rs 

may represent original sand grains, but all the other components are 

probably secondary. The rock as it now exists is a chlontic phyllite 

(Of. No. 126) or a chlorite-schist. It possesses ha.rdly sufficient chlorite 

to be typical of the chlorite-schists, consequently it seems preferable 

to call it a chlorite-phyllite. Were its grains fine and its cleavage surfaces even, it would be a chloritic slate.

No. 128. METAMORPHIC CONGLOMERATE.

(FuoM HOOSAC MOUNTAIN (TUNNEL), MASSACHUSETTS. DKSCIUHKD IVY J. E.

WOLFF.)

In the hand specimen the rock varies in character according to the 

distinctness of the.pebbles. These are of three kinds: First, quartz, 

either white or of a beautiful opaline blue; second, feldspar; third, 

a fine-grained quartz-feldspar rock, either granite or gneiss. These 

pebbles are separated from each other by the cement, a crystalline 

aggregate of mica, quartz, and little glistening glassy feldspars (albite). 

The cement winds about the pebbles, sometimes cutting across them 

along little fault lines; at other times, little tongues of the cement, 

parallel to the general foliation or structure, cuts off thin strips from 

the pebbles, giving them a sharpened, ragged look. In this case the 

cement appears to have followed*planes of break and slipping. Thus 

a quite banded phase of the rock is produced, in which the dark mica 

bands, by their obliquity to each other, may simulate cross-bedding. 

However, in the large blocks at the dumps, bands filled with pebbles 

(original conglomerate bands) are plainly to be seen alternating with 

fine-grained bands (original sandstone bands) in which little fragments 

of the same blue quartz found in the larger pebbles can still be 

recognized.

In the thin sections the quartz pebbles are distinguished by the 

same properties as those of the original quartz masses of the granitoid 

gneiss from which the former were evidently in part derived. The core 

of blue quartz, Avhich can be recognized by its iridescence on holding
324 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULL. 150.

the section up to the light, is stained and cracked, passing at the edges 

into a mosaic of fine quartz, evidently separated from the parent mass 

by pressure and motion. As soon as we pass from the core to the separate grains we find that the blue color disappears, so that \?e might 

imagine this color was due to some peculiar effect on the light produced 

by the strained but not granulated quartz. This granulated quart/ 

becomes gradually mingled with mica and metamorphic feldspar, and 

so merges into the cement.

The feldspar, pebbles are also generally much strained and invaded 

by secondary quartz, mica, epidote, etc., producing sometimes the 

appearance of an aggregate of individual grains of feldspar separated 

from each other by grains of quartz, but such aggregates are really a 

sort of feldspar breccia belonging originally to one jriece, as is evident 

by the fact that the cleavage cracks run about parallel through all the 

grains which have been but slightly moved. The feldspar of the 

pebbles is microcline, orthoclase, or plagioclase. It is often difficult to 

distinguish such aggregates, or those formed by the metamorphic feldspar of the cement, from pebbles of original quartz-feldspar rock. 

The latter contain the feldspar and quartz in grains of more uniform 

size, with sometimes a tendency to imperfect crystal form on the part 

of the feldspars (a characteristic of eruptive granites); moreover, the 

feldspar may be of several kinds in the same pebble, in different crystallographic positions, and free from the inclusions which abound in 

the metamorphic feldspar.

The cement is composed of occasional small pieces of clastic quartz 

and feldspar, but principally of metamorphic mica, feldspar, quartz, 

etc. The mica, both muscovite and biotite, and sometimes green chlorite, occurs in plates, sometimes in clumps or stringers. The muscovite and biotite are often iutergrown parallel to the base. The quartz 

is in isolated grains or interlocking aggregates. The metamorphic

feldspar is in grains, often elongated, but without crystal outline; the 

grains are either simple, or in simple twins (albite law). These feldspars are similar to those developed on a larger scale in the albite 

schist, and are probably albite. 1 They contain as inclusions grains of 

quartz, round flakes of biotite and niuscovite, dark grains of magnetite, 

reddish-brown hexagonal plates of ilrnenite (?), fluid inclusions, and 

dark aggregates of iron ore and graphite (?), among which occasional 

prisms of rutile may be recognized. Grains of calcite are found 

among the biotite or in the feldspar, and occasional large prisms of 

zircon, with high double refraction and positive character. Aggregates of dirty-white titanite grains are found near masses of black 

iron ore. Some of the little grains of microcline found in the cement 

are probably formed in place, i. e., metamorphic. The contemporaneous crystallization of these minerals of the cement.is shown by their

'Compare The metamorphism of clastic feldspar in conglomerate schist, by J. E. WolflT: Bull. 

Mus. Coiup. Zoology Harvard Coll., Vol. XVI, No. 10.
DILLEK.] DESCRIPTIONS: NO. 129, ALBITE-fcCHIST. 325

intimate association, and the honeycombing of the feldspars by the 

quartz, mica, etc.

For farther information concerning this rock, the student is referred 

to Monograph XXI1.T, United States Geological Survey, pages 49-59.

No. 129. ALBITE-SCHIST.

(FnoM HOOSAC MOUNTAIN (TUNNEL), MASSACHUSETTS. DESCRIBED BY

J. E. WOLFF.)

In the hand specimen the rock has a perfect schistose structure, with 

the mica in continuous layers separated by flat lenticular masses of 

quartz, while the crystals of albite are scattered irregularly through 

the rock. There is a dark, almost metallic luster on the foliation plane, 

due to the greenish color of the muscovite and its mixture with biotite 

and chlorite.

The crystals of albite are of about the same size and generally rounded 

or subangular, their longer dimension not necessarily parallel to the 

schistosity of the rock. Sometimes the basal cleavage sections (P) are 

seen to be bounded by crystallographic planes corresponding to the 

second cleavage (M) and the prisms T and I.

The following analysis of this feldspar was made by E. B. Riggs for 

Monograph XXIII, before cited:

Analysis of'feldspar of albite-schist from Hoosac Mountain, Massachusetts.

Si02 ... ...........................................

A12 03 ---. ........................................

CaO. .............................................

MgO... ..........................................

NaO .............................................

K20..............................................

C02 (combustion) 0.77 = 0.44 C. (graphitic material).

Percent.

69.09

13.00

.20

10. 28

.40

.42

99. 59

It is common to find the basal cleavage of a crystal reflecting the 

light in two parts, and this is due to the fact that the crystal is a simple 

twin parallel to (M) albite law.

Basal cleavage pieces, showing a simple twin, give an. extinction 4° 

oblique to the twinning plane and second cleavage (M). Twins measured on the goniometer give angles of 172° 46' to 172° 50' between the 

basal cleavages of the two twins. The chemical and physical properties are therefore those of albite.

In the thin section the albite grains are recognized by their lai'ge 

size, by the development of cleavage when the slide is thin, and by 

the characteristic inclusions. The latter consist of quartz in droplets
326 THE EDUCATIONAL SERIES OF KOCK SPECIMENS. [BULL. 150. y

or lenticular grains, muscovite or biotite in round or long flakes, crystals of zircon, round grains of apatite, magnetite in crystals or masses, 

rhombs of carbonate (calcite), grains of titanite, fluid inclusions, and 

irregular black masses or specks of iron oxides and graphitic sub- <*~ 

stances, among which little six-sided brown plates of ilmenite (H) can 

be observed with a high power. These inclusions may be few, or so i 

abundant as to honeycomb the crystal, and are'characteristic of many lxpseudoporphyritic crystals of the schdsts in general, such as ottrelite, 

staurolite, mica, etc., differing from true porphyritic crystals (pheiiocrysts) of eruptive rocks by the fact that they are of contemporaneous ,*.-- 

or even later origin than the constituents of the rock which.lie outside 

them, but which they inclose so abundantly. "^

It is a peculiarity of the dynamic metamorpliism of both sediments 

and eruptive rocks that the new feldspar is sometimes a pure albite 

or a soda-lime feldspar near to albite in the series, and that the habitus 

is' unlike that of similar feldspars in eruptive rocks, inasmuch as the 

crystal form is imperfect and the crystals uutwinued, even under the 

microscope, or in simple twins, unlike the multiple-twinned albite of 

eruptive rocks. An exception should, perhaps, be noted for the alkaline 

eruptive rocks.(e. g., theralite).

The mica, muscovite, biotite, and chlori-te occurs outside the albite x" 

in thick plates or irregular aggregates of plates, or clumps. The muscovite and biotite may be intergrown parallel to the base. The quartz 

grains are generally in the interlocking aggregates, which are crossed y 

by fluid inclusions arranged in lines (really in planes). The quartz is 

otherwise generally free from inclusions near the center of the lenses, "~ 

but at their edges, where individual grains lie in the meshes of the mica, 

they may contain flakes of mica.

Magnetite, calcite, titanite, apatite, and zircon are found in the masses 

of mica. The clumps of mica and quartz have sometimes a rough con- v 

centric arrangement.' The muscovite has a distinctly greenish color, 

even in the thin section, and in part the fibrous appearance due to the 

interweaving of irregular plates of the mineral, which is characteristic 

of the variety called sericite.

Neither in the hand specimen nor in the thin sections of the schist 

has any clastic material been observed, although such a derivation is 

certain both from the field relations and from the rocks of this class 

observed elsewhere in this field which are transitional to clastic rocks. 

Neither can we state from what source were derived the elements which 

produced the large amount of albite (soda-alumina silicate) in this and 

in (probably) the two other rocks. The granitoid gneiss may have been 

originally an eruptive granite; 1 the naetainorphic conglomerate was 

originally a mass of granitic d6bris at the base, becoming a feldspatuic ^" 

sandstone or arkose in succeeding stages until succeeded by the schist,

'Its similarity should be noted to the coarse eruptive granite called "Kapakiwi," which covers 

extensive areas iu Finland.
DESCRIPTIONS: NO. 130, MICA-SCHIST. 327

which must have been a shale or slate, perhaps at the base an impure 

limestone. Metamorphism has given all three rocks certain characters 

in common (regardless of diversity in origin and composition), such as 

schistosity, albitic feldspars, mica, etc., while the differences seem due 

to the remnants of the original material or the influence this exerted 

on the process of the new crystallization.

No. 130. MICA-SCHIST (TOUKMALINE-BIOTITE-SCHIST). 

(FROM BLACK. HILLS, SOUTH DAKOTA. DESCRIBED BY W. S. BAYLEY.)

'' This specimen is particularly interesting because it has every chauacteristic of a typical mica-schist, while at the same time its mode 

of origin is well understood. The rock of which it is a sample occurs 

in the Black Hills, South Dakota. In the southern part of the district 

is a large core of pie-Cambrian eruptive granite, forming Harney

Peak.1 Immediately surrounding this on all sides are crystalline 

schists, striking everywhere parallel to the boundary between the granite and themselves and dipping away from the eruptive mass. Close 

to the granite the schists are completely crystalline; at some little distance from it they are less so. A few miles away from the contact 

they become very much less crystalline, and possess, in addition to 

their foliation parallel to the granite boundary, a slaty cleavage which 

is parallel to the cleavage of slates, existing at a greater distance from 

the granite mass, and into which the schists pass gradually. Since no 

unconformity exists between the schists and the slates, and since the 

microscopical examination of both shows that the mineralogical gradation between them is as complete as the gradation noticed in the field, 

it is concluded that the schists are derived from the slates, and consequently belong to the same geologic horizon as do these, which are 

Algonkian, and probably Huronian.2

The position of the schists with respect to the granite, their more 

complete crystallization as this is approached, and their passage into 

true fragmeutal slates at a distance from the eruptive are all indications that not only is their crystallization due to the proximity of the 

granite, but their schistosity as well, since this structure disappears as 

the distance from the intrusive increases.

The action by which an eruptive mass produces crystallization in 

clastic rocks through which it breaks is known as contact action, and 

the change effected in thefragmentals is known as contact-metainorphism. This kind of metarnorphisin, while it may obliterate the proofs 

of the original fragmeutal nature of the material acted upon, never of 

itself produces in the resulting rock a schistose structure. This is 

always a result of movement of the rock particles, a consequence, in 

turn, of mechanical action, hence known as dynamical metamorphism.

1 The specimen came from a poiut north of Harney Peak, about I or 1£ miles west of the Ettn Mine. 

2 For farther information concerning the relations existing between the slates and schists, see C. 11. 

Van Hise, Bull. Geol. Sou. America, vol. I, p. 203.
328 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 160. _v

The force producing the schistosity in the case under discussion was 

that generated by the intrusion of the granitic mass, as is plainly shown 

by the strike of the foliation of the schists and its dip away from the 

core of granite. The mica-schist represented by the specimen may ^_ 

therefore be regarded as a dynamically metamorphosed contact-rock, 

whose original character was fragmeutal. Its present condition is due /"" 

entirely to the recrystallizatiou of its originally clastic grains through 

processes carefully described by Professor Van Hise in the article 

referred to in the footnote. It is now a typical crystalline schist.

It may be described inacroscopically as a light gray, fine-grained rock < _ 

with a well-marked schistose structure which is best seen on the two 

long sides of the specimen. Here thin parallel bands of light and dark -4- 

colors alternate. The grains comprising them are so small that the 

nature of the minerals forming the lighter bands can not be determined 

without the aid of the microscope. The dark color of the other bands 

is plainly seen to be due to what look like fine black needles. On the 

upper and lower flat surfaces of the specimen the needles are observed * 

to have more the appearance of narrow plates, with a brilliant luster. 

With a sharp knife thin flakes may be sprung from them. The mineral 

therefore is very easily cleavable, and may be regarded presumably as 

mica. The only other features of the hand specimen worthy of notice 

are the little black flakes or bunches that dot the planes of cleavage 

and the small rusty spots scattered throughout the rock. The latter 

are garnets, and the former are accumulations of mica flakes, as may 

be determined under a hand lens. They resemble very strongly the 

" knoten" described by Bosenbusch J and other German petrographers -- 

as characteristic of sedimentary rocks that have been made crystalline 

by the intrusion of eruptives through them. On some specimens there 

are also little circular areas with diameters of about 6 nim , glistening 

brilliantly from many tiny facets. These consist of aggregates of a 

light-colored mica, probably muscovite. The rock is quite soft, and is 

apparently much decomposed. The softness is due to the slight ->. 

amount of coherence existing between the -component grains, in consequence of which they fall apart very easily when rubbed.

The microscopic investigation shows the rock to be remarkably fresh. 

Its specific gravity is 2.806. Under the microscope the predominant 

light-colored mineral is discovered to be quartz, while some of the >x~ 

black plates and needles are biotite and others are tourmaline. Garnets 

and muscovite are also observed, but in very small quantity. The 

schistose structure is the result of the arrangement of the constituents 

with their longer axes approximately in the same plane, which is the 

plane of schistosity. (See PI. XLII.)

The lenticular forms of the light-colored quartz grains are so appar- --r 

ent in the photograph that they need merely be referred to. Between 

these grains is a matrix composed principally of large flakes of biotite

1 Eosenbusch: Mikroskopische Physiographie cler Hassigen Gesteine, Stuttgart, 1887, p. 47.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XLII

THIN SECTION OF TOURMALINE-BIOTiTE-SCHIST FROM BLACK HILLS, SOUTH DAKOTA, 

AS IT APPEARS UNDER A MICROSCOPE IN POLARIZED LIGHT.
D.LLE.R.) DESCRIPTIONS: NO. 130, MICA-SCHIST. 329

that appear dark in the picture, and slender needles of tourmaline that 

have a light color. Not all of the lighter colored portions between the 

larger .quartz grains are composed of tourmaline exclusively. Small 

grains of quartz, tiny flakes of biotite, some muscovite, and little 

masses of limonite are all found in it, but they are too minute to be 

distinguishable in the photograph.

The quartz is in transparent, colorless grains with lenticular cross 

sections, measuring from 0.15 to 0.351 " 1 " in length and about O.I to 

0.15"11" in width. It holds as inclusions many little liquid-filled cavities, 

and some filled with glass and devitrification products. Tiny specks 

of magnetite and other iron compounds and small crystals of apatite, 

besides many tourmaline crystals, are also inclosed by it.

The dark constituents, as has been stated, are biotite and tourmaline. 

In sections cut parallel to the plane of schistosity the biotite appears 

ill irregularly outlined plates, most of which are basal sections of the 

mineral. These are dark brown in color and show no pleochroism. In 

other sections the mica is cut at some inclination to the vertical axis, 

so that its pieces are marked by parallel cleavage lines running in the 

direction of the long axes of the flakes, which are, in the main, parallel 

to the long axes of the quartz lenses. The biotite in these sections 

extinguishes parallel to the cleavage. It is very dark brown, almost 

opaque, when the cleavages and the longitudinal axes are parallel to 

the vibration plane of the nicol, and is light yellow in directions at 

right angles to this. It includes tourmaline and contains round dark 

areas (halos) that are more or less pleochroic.

The tourmaline is in small, well-developed crystals of a dark-brown 

color, and with, nearly the same pleochroism as the biotite. These 

measure 0.2 to 0.5mm in length and from 0,05 to O.I"11" in thickness. 

They are often doubly terminated in such a way as to show clearly 

their hemimorphism. They may be distinguished from the biotite by 

the i'act that their absorption is much less in the direction of their long 

axes than at right angles thereto; consequently, they appear dark 

when in that position in which the biotite appears light, i. e., when 

their long axes are perpendicular to the vibration plane of the lower 

nicol. They may also be recognized by their basal parting running 

across the crystals at right angles to their long axes. The tourmaline 

extinguishes parallel. Its double refraction is negative. It is commonly arranged parallel'to the schistosity of the rock, though many 

exceptions to this general rule are noted.

The only other component that need be mentioned at any length is 

the garnet. This is present in almost colorless isotropic grains, with 

diameters of about 0.5 to 0.8""", except in rare instances, when they reach 

as much as 2mn3 . The mineral is not very abundant, and consequently 

is not present in all sections. From the fact that the mica plates are 

sometimes bent around the garnets, it follows that the latter mineral 

existed at the time when movement of the rock particles was in prog-
330 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL, 150.

ress. Around the borders of the grains is often a yellowish-green 

stain, resulting from the decomposition of magnetite or other iron compounds. It is this that causes the rusty color around the garnets in the 

hand specimen.

Another yellowish-green substance sometimes stains the constituents 

over small areas, and in some places forms little lenticular masses 

between the components. This substance possesses no definite structure and shows no double refraction, hence its true nature is difficult 

to determine. It is, however, neither essential to the rock nor characteristic of it, and therefore is of little importance for purposes of classification.

Muscovite flakes are very rare. Occasionally a tiny one may be seen 

lying between the quartz grains when the section is examined under 

crossed nicols. In ordinary light they are invisible. When the thin 

section includes one of the little groups of plates noticed in the hand 

specimen, the number of the flakes seen naturally becomes quite large, 

though the area covered by them is limited entirely by the outline of 

the group.

Though all the mineral components of the schist have been described, 

there remain a few structural features to be noted. The schistosity has 

already been stated to be the result of the arrangement of the minerals 

with, their long axes approximately parallel, and the difference in color 

of alternate bauds has been ascribed to the varying proportions of 

biotite and tourmaline in them. In some sections the difference in color 

is found to depend largely upon the accumulation of black iron compounds between the quartz grains in certain bands and its absence from 

others, in which case it is probable that there were variations in the 

composition of successive laminae in the original sedimentary beds.

Another structural feature of interest is the occurrence here and 

there of lenticular areas in which the quartz grains are larger tliail 

elsewhere in the rock, and the amounts of biotite and tourmaline are 

much less. These -areas may represent pebbles that were originally 

scattered through the slates and were afterwards flattened by the 

pressure that produced the schistosity,1 while at the same time chemical 

changes set up in them gave rise to the quartz, mica, and tourmaline 

that now usurp their places.

The biotite, quartz, magnetite, and muscovite in the schist may all be 

looked upon as resulting from the decomposition of theorigiual materials 

of the slate under the influence "of contact action. The garnets may 

likewise be ascribed to this action. The-tourmaline, on the contrary, 

contains a large amount of boron a substance not occurring in any 

large quantity in slates. Its formation is probably due to a reaction 

beteen the constituents of the slate and the boron gases given off by 

the granite before it finally solidified.2

1 Of. Geol. Soc. of America, Vol. I, pi. 4.

sFor account of gases exhaled from eruptive masses see Geikie, Text Book of Geology, 1885, 

pp. 180-184.
DII.LEK.] DESCRIPTIONS: NO. 131, HORNBLENDE-SCHIST. 331

The composition of the schist, which should be called a tourmalinebiotite-schist on account of the large amount of tourmaline in it, has 

been found by H. N. Stokes to be as follows:

Analysis of schist from the Blade .Hills.

OJ/-\

A12 03 ................................... .........

FeA ...................................... ......

FeO............. .................................

CaO........ ......................................

MgO. ...... ......................................

ic2 o..............................................

Na,0 ............................................

Ign....................................... .......

Total ......................................

Percent.

60.77

1.7. 65

1.55

3.29

.50

2. 13

4.49

.99

1.89

99.32

The boron was not determined, but it is evident that it is present in 

considerable quantity in the rock, since very little of the loss of weight 

upon ignition is due to the escape of water.J

No. 131. HORNBLENDE-SCHIST.

(FROM MANHATTAN ISLAND, NEW YORK. DESCRIBED BY J. P. IDDINGS.)

Bather massive crystalline schist, greenish black, with tendency to 

cleave in parallel plates. It occurs with mica-schist and gneiss, exposed 

in the northwestern portion of Manhattan Island, in the neighborhood 

of Eiverside drive and One hundred and twenty-fifth street, New York 

city. 2

In thin section it consists of dark-green hornblende, with subordinate 

amounts of feldspar, quartz, magnetite, biotite, apatite, and a little 

zircon and pyrite, besides some secondary minerals in places muscovite, epidote, or zoisite.

The lamination of the rock is produced by a somewhat parallel 

arrangement of the stout crystals of hornblende and by streaks of the 

other constituents in smaller grains. It is most pronounced in sections 

cut across the lamination, but also appears in those cut parallel to the 

schistosity.

The hornblende is strongly green in thin section, with tinge of brown 

and with pronounced pleochroism, the variation being from green 

through brownish green to light brown. Owing to the fact of easy 

cleavage of the rock due to the .parallel arrangement of the hornblende, thin sections are mostly prepared parallel to the prismatic 

axis of the crystals; consequently cross sections are scarce. Many

'For other descriptions of mica-schists see It. D. Irving, Geology of AVisconsin, "Vol. Ill, 1880, pp. 

145,148. PlateXV, Fig. 1 and 3; A. A. Julieu : Il>. p. 232. A. Wichinann, Ib. p. 034.

'Compare J". F. ICemp, The Geology of Manhattan Island: Trails. New York Acad.Sci., Vol. VII, 

pp. 49-65
332 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

individuals of hornblende exhibit no cleavage. The extinction angle 

is low. Twinning is seldom observed. The substance of the hornblende is very pure. There are inclusions of occasional rounded 

crystals of zircon surrounded by' brown halos, slightly pleochroic, 

considerable magnetite in places, besides feldspar, quartz, and biotite.

Feldspar occurs in irregular grains without crystallographic boundaries, and of much smaller size than the crystals of hornblende. It is 

mostly plagioclase with polysynthetic twinning according to albite 

and periclme laws. Some of the extinction angles suggest andesinelabradorite. The lamellae are in places curved and may have been 

produced by dynamic processes. There is some undulatory extinction 

in places, but not much. Orthoclase may be present in small amounts, 

as there are a few unstriated feldspars.

Quartz occurs in irregular grains which are sometimes rounded, 

especially when inclosed in hornblende. The substance is very pure, 

with almost no undnlatory extinction. Biotite is brown, with strong 

absorption, and occurs in irregularly shaped pieces, generally inclosed 

in the hornblende. Magnetite occurs in clusters and streaks of irregular grains, and is relatively abundant. Apatite forms colorless grains 

or stout crystals with irregular or rounded outline.

The rock in places is traversed by microscopic veins. When these 

cross crystals of hornblende they consist of aggregations of quartz 

and other minerals; or quartz may exist alone, having the same 

crystallographic orientation as that of adjacent grains of quartz; but 

where the minute vein should appear crossing quartz crystals, it is 

represented only by rows of dots included in otherwise very pure 

quartz. The hornblende sometimes exhibits a slight fibration on both 

sides of the vein, the direction of the fibers being parallel to that of 

the G axis of the hornblende.

No. 132. SCHISTOSE BTOTITE-GNEISS. 

(FROM MANHATTAN ISLAND, NEW YORK. DESCRIBED BY J. P. IDDINGS.)

Schistose,, fine-grained, dark-colored rock associated with amphibolite-schist and gneiss at the same locality as that of the hornblendeschist on Manhattan Island, near .Riverside drive and One hundred 

and twenty-fifth street, New York City. It consists of biotite, feldspar, and quartz, with a small amount of muscovite, apatite, and 

zircon. The thin sections are most easily made parallel to the schistosity of the rock and the foliae of the mica; hence they seldom show 

cross sections of the mica plates. There is somewhat of a flaserstructure, caused by flakes of mica grouped in lines curving about 

larger crystals of feldspar and quartz.

In thin section the biotite is dark brown, with strong absorption. 

Its form is irregular and seldom exhibits crystal boundaries. It 

includes grains of quartz and feldspar and other minerals, and in 

turn occurs as microscopic crystals included in quartz and feldspar.
DILLBB.] DESCRIPTIONS: NO. 133, STAUROLTIIC MICA-SCHIST. 333

Muscovite occurs sparingly as relatively large colorless plates, sometimes surrounded by biotite.

Quartz appears to be more abundant than feldspar. Its outline is 

quite irregular, the smaller grains often rounded. It exhibits slight 

undulatory extinction and in places minute rutile needles, and some 

inclusions of the other mineral constituents of the rock. Fluid inclusions are quite scarce.

Feldspar forms allotriomorphic grains like those of quartz, but often 

attains larger dimensions. It is mostly plagioclase, with poly synthetic 

twinning, in places curved, accompanied by uudulatory extinction. The 

extinction angles are low, indicating oligoclase. Orthoclase appears 

to be present in smaller amounts. No niicrocline was noted.

Apatite forms comparatively large, irregular grains, colorless and 

quite pure. Some grains are surrounded by a shell of brown, doubly 

refracting mineral. A' similar mineral, with paler color and low double 

refraction, occurs in spots through the rock. Its character has not 

been determined. Zircon occurs in rounded microscopic crystals. 

Magnetite is almost entirely absent.

As the proportion of feldspar grows less the rock approaches a 

gneissic mica-schist.

No. 133. STAUROLITIC MICA-SCHIST.

(FROM CHARLESTOWN, SULLIVAN COUNTY, NEW HAMPSHIRE. DESCRIBED BY W. S.

BAYLEY.)

The rock stratum from which specimen No: 133 was obtained constitutes one of the members of C. H. Hitchcock's Coos group, 1 which 

comprises (1) a quartzite, (2) a staurolitic mica-schist, and (3) a staurolitic and garnetiferous clay slate. In the Connecticut Valley the 

mica-schists gradually lose their micaceous character and become 

argillitic; hence there can be no question that the schists are rocks of 

sedimentary origin. In the New Hampshire reports the u Coos group" 

is included in the Paleozoic system. In a later paper 2 it is more definitely assigned to some horizon above the Cambrian, possibly the 

Devonian.

As represented in the hand specimen, the rock is dark gray in color 

aiLcl crystalline in texture. In structure it is typically schistose, but 

the schistosity is not developed in parallel directions. In all specimens 

it is more or less contorted in short waves, which show themselves on 

cleavage surfaces as a series of approximately parallel crumplings. 

The luster of these surfaces is silky, suggesting the presence in the 

rock of some micaceous mineral with its broader faces parallel to 

the cleavage surfaces. On cross fractures the specimens are characterized by the x>resence of rods of a very glistening, easily cleavable 

mineral in a dull, light-gray matrix, resembling in appearance a very

1 (J. H. Hitchcock: Geology of Now Hampshire, Vol. 11, Chap. IV, pp. 316-328. 

* C. H. Hitchcock: Jour. Geol., Vol. IV7 , 1890, p. 59.
334 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

fine-grained gneiss or graywacke. The rods are all elongated in the 

direction of the rock's schistosity. Cleavage plates- of this mineral are 

of a reddish-brown color. They are not pleochroic to the slightest 

degree. Between crossed nicols they remain dark through an entire 

revolution, and in converged light they exhibit the uniaxial figure. 

These characteristics belong to biotite.

The most noticeable feature of the specimens is the presence in them 

of irregular lumps whose cross sections are oval in outline. Their 

longer directions are approximately parallel to the schistosity of the 

rock. An inspection of a freshly fractured cross section of one of these 

rounded crystals for such they are reveals the presence of a core of 

a garnet-colored substance surrounded by a mass of lusterless greenishgray material. The nucleus consists of fresh staurolite arid the surrounding material of an aggregate of alteration products of this mineral. 

Hawes 1 describes these crystals as consisting of a core surrounded by 

a zone, the core and zone being separated from each 

other by a line of impurities. Sections made from 

some of the largest crystals in the specimens collected for this series leave no doubt that these crystals are staurolite. The appearance of a cross 

section of one of the best crystals is shown in the 

accompanying figure (fig. 17). The nucleus is an 

oval area of fresh staurolite, which is pleochroic in 

light-yellow and orange-yellow tints. Absorption 

c>a. Plane of its optical axes is <*> P oo. ThesurFIG. 17. Cross section rouuding material is a fine-grained aggregate of a

of staurolite in stauroliti£ , , i mi T -j j j. j- mica-scMst. white micaceous mineral.. This is divided into distinct areas, as indicated, by lines composed largely 

of chlorite flakes and smal? grains of an opaque substance that may 

be some carbonaceous material. The same opaque substance borders 

the periphery of ti:e section, which in outline is a rhomb with rounded 

corners. The micaceous aggregate is evidently the product of the 

decomposition of a staurolite crystal, the nucleus which it surrounds 

being a remnant of the original substance which has not yet suffered 

decomposition. The chlorite lines were in all probability produced 

from inclusions that so commonly exist'in both andalusite and staurolite crystals.2 A close inspection of the hand specimen will show that 

the micaceous minerals in the grouudinass wrap around the staurolite 

crystals in a manner suggesting the existence of the latter in their 

present positions at the time when the rock was attaining its schistose 

structure.

Under low powers of the microscope the grouudinass in which the 

staurolite crystals lie is seen to be composed of large plates of brown 

biotite or of green chlorite and comparatively large masses of a light1 G. W. Hawes: Geol. ISTew Hampshire, Vol. Ill, Pt. 4, pp. 109-111.

2 Of. J. D. Dana: System of Mineralogy, 6th Ed., pp. 497, 500; also G. W. Hawes: Geol. New Hampshire, Vol. Ill, PI. II, figs. 8 and 8a.
DILLEE.] DESCRIPTIONS: NO. 133, STAUROLITIC MICA-SCHIST. 335

green fibrous aggregate with crystal contours, embedded in a foliated 

matrix consisting'of quartz, sericite or muscovite, clilorite, ail occasional grain of zircon, rutile, aud tourmaline, and innumerable small, 

black particles scattered everywhere throughout the slide, except 

through the light-green fibrous masses.

Biotite is the most noticeable constituent. It is in large plates, usually elongated in the direction of the c axis, aud hence possessing 

a cleavage transverse to their longer directions. Its color is dark 

reddish-brown parallel to the cleavage and a light greenish-yellow perpendicular thereto. On the edges of many plates the mica has been 

changed to chlorite, and often the alteration has proceeded from the 

edge inward, so that lamellae of the green chlorite are intergrown with 

lamellae of brown biotite. In the smaller grains all the mica has often, 

been replaced by the chlorite, and in sections cut from weathered portions of the rock not a trace of biotite remains in even the largest 

grains. Both the biotite and the chlorite developed from it contain 

many inclusions of quartz, magnetite, rutile, zircon, and tiny opaque 

grains that may be some form of carbon or of a carbonaceous substance. 

A few of the inclusions, more particularly those of zircon, are surrounded by dark circles that are more or less pleochroic. These are 

known as "pleochroic halos." The biotite flakes were evidently formed 

in the rock after the constituents of the matrix, but probably before 

the cessation of the motion that rendered this schistose.

The light-green masses.with crystal outlines are composed of an 

intermixture of little bundles of a green fibrous chlorite aud spicules 

of a colorless micaceous mineral. The chlorite is recognized by its 

slight pleochroism and its dull interference colors. The colorless mineral is brilliantly doubly refracting. Its extinction is parallel to a welldefined cleavage that may be seen under high powers. It is probably 

muscovite or talc. From the shapes and composition of these masses 

it is clear that they are pseudomorphs after some mineral that originally was present in well-defined crystals. In all probability this 

mineral was garnet. The alteration of garnet into chlorite is not 

uncommon. Perfect pseudomorphs of this nature have been described 

by Pu.mpelly 1 from the Spurr mine in Michigan.

The relation of the garnets to the matrix is the same as that of the 

biotite and of the large staurolite crystals. Evidently they were already 

present in the rock prior to the final production of its schistosity. They 

appear to be the oldest of its present constituents, since they only are 

devoid of the black particles so thickly strewn, through all the other 

components.

The matrix surrounding the biotite and garnet pseudomorphs appears, in ordinary light, as a colorless aggregate of fibers and grains 

besprinkled more or less thickly with numberless small black or brown 

particles, and containing here and there an irregular flake of green

1 Aiu. Jour. Sci., 3d series, Vol. X, 1875, p. 17.
336 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. -[BULL. 150.

chlorite, a rod or a rounded grain of magnetite, a ragged grain of ilnienite 

or of titaniferous magnetite, a minute crystal of brownish-yellow rutile, 

and occasionally a small plate of greenish-brown tourmaline. The 

tourmaline is recognized by its pleochroisrn and its absorption G?>£, 

the titaniferous magnetite by its gray decomposition products, and the 

rutile by its color, its lack of distinct pleochroism and its high refractive index. Between crossed nicols the colorless or very light-green 

component of this matrix is the platy variety of muscovite known as 

sericite. It is identified by its lack of color, its lack of pleochroism, 

its brilliant polarization colors,- and by the fact that its extinction is 

parallel to its cleavage. The sericite occurs in long, acicular and columnar crystals or scales, often grouped in little bundles. Between 

them are colorless grains of quartz, usually elongated in a direction 

parallel to the long axes of the sericite individuals. No pressure effects 

are noted in the quartz grains. Their elongation appears to be an 

original condition. Both the quartz and the sericite have the aspect of 

minerals that have crystallized in situ, and the entire rock seems to 

have undergone a complete recrystallization from its original state as 

a sediment.

The structure of the matrix is plainly schistose, the schistosity beingdue to the elongation of the quartz grains, and the arrangement of the 

sericite flakes parallel to this elongation. The foliation, however, is 

not evenly parallel, as it is in specimens of some of the other rocks in the 

collection, but is contorted, little folds being crowded closely together 

in some portions of the sections and opening out into very gently curved 

folds in others. The quartz grains on the limbs of the folds are always 

much more elongated than those in their crests and along their axes, 

thus causing these microscopic folds in the thinning of their sides to 

resemble the larger folds noted in disturbed rock strata.

Nothing in the microscopic structure of this rock suggests its original 

fragmental character. As at present constituted, it is a thoroughly 

crystalline rock. No indications of a fragmental grain may be detected 

in it. Since, however, it has been traced into slates, there can be no 

question as to its derivation from a sediment composed of the same 

constituents as those composing an ordinary shale. The rock is a typical crystalline schist that has been derived from a sedimentary fragmental rock. In composition it is a mica-schist containing altered 

staurolite and garnet crystals.

The rock's foliation is unquestionably the result of pressure. The 

elongation of the quartz grains and the bending of the mica flakes 

point to this origin. There is, however, no fracturing of grains and no 

distortion in their optical properties, as in the case of schists derived 

by dynamic metamorphism from igneous rocks (see No. 140). The 

foliation of the rock under discussion was not produced by the flattening of grains of minerals already existing in it, nor by their rotation 

into positions of least resistance. It was caused by crystallization of
DILLBB.] DESCRIPTIONS: NO. 134', HORNFELS. 337

its components while under pressure, attended by motion in the plastic 

crystallizing mass.

The rook affords a good example of a schist formed from a fragrnental 

rock through metasoiuatic agencies accompanying dynamic action. In 

dynamically formed schists like No. 140 the present constituents are 

practically the same as those existing in the original rocks; in metasomatic schists the present components have been formed de uovo. 1

No. 134. HOBNPELS.

(FROM GENESEE VALLEY, PLUMAS COUNTY, CALIFORNIA. DESCIMUED BY H. VV.

TURNER.)

By contact inetamorphism is meant those alterations in a rock .mass 

which take place along the border of an intrusive mass in consequence 

of the lieat and mineralizing solutions whose origin is due to the intrusive igneous rock.

In the Educational Series collection this phenomenon is illustrated 

by two specimens from California one a hornfels from Plumas County 

and the other a chiastolite-schist from Mariposa County.

At both these localities areas of granitoid rocks are in contact with 

sedimentary masses, and along the border of the granite there is a 

zone of metamorphic rock which grades almost imperceptibly into less 

altered sediments at about the same distance from the granite contact 

at all points.

In the metamorphic zone new minerals have been formed, chiefly by 

the recrystall!zation of the material of the sediments. This phenomenon has been noted in many parts of the world, and careful investigations have shown beyond all reasouable doubt that the formation of 

these minerals took place after the intrusion of the igneous magma 

and as a result of it.

The investigations of Hawes 3 have, indeed, made certain that in one 

instance there has been an addition of boric acid to the schists from 

the mineralizing solutions, resulting in the formation of abundant 

tourmaline iu the zone nearest the .granite. Rosenbusch, 3 in a very 

thorough investigation of contact phenomena, however, found that, at 

the locality which he studied, the new minerals formed were almost 

Avholly due to the recrystallization of the material of the sediments 

which had been metamorphosed.

Hornfels is the term applied by Eosenbusch to contact metamorphic 

rocks which are massive and not schistose. Such rocks usually break 

with a conchoidal fracture, especially when tine grained. Often, as in - 

the specimens in the Educational Series collection, the hornfels is a

'For a discussion of (he processes concerned in the metaiuorphisin of sedimentary rocks see C. K. 

Vim Hise, The principles of North American pre-Cambrian geology : Sixteenth Ann. Kept. U. S. Geol. 

Survey, pp. C80-69J, 705. For a discussion of inotainorphisni in general see .Barker's Petrology for 

Stiulenls, University 1'ress, Cambridge, 1895, pp. 257-290.

"Am. .lour. Sci., 3d series, 1881, Vol. XXI, p. 21.

3 Pie Htei£?erschicfor. Strns.sburg, 1.877.

Bull. 150 22
338 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

fine-grained flinty rock, but the name may also be applied to coarser 

varieties which were originally argillaceous sandstones. The hornfels zone from which specimen 'No. 134 was obtained varies in width, 

according to Mr. Diller, who collected the specimens, from 160 to 1,300 

feet. This great variation in width is thought by Mr. Diller to be due 

.to the fact that the contact of the granite and the sedimentary mass 

is probably not a vertical one; that is to say, he supposes the granite 

to pitch in under the hornfels, so that when the zone is wide the distance of the metamorphic rock from the underlying granite surface 

may not. in reality be any greater than when the zone is narrow. The 

igneous rock which has caused the original shale to be metamorphosed 

into a hornfels has been called above a "granite." In reality it should 

be called a quartz-gabbro or a quartz-pyroxene-diorite, for it contains 

little or no alkali feldspar. The intrusive rock is a gray, medium to 

rather coarse-grained granitoid rock, of granitic or hypidiotnorphic 

structure, and is composed of plagioclase, hornblende, monoclinic 

pyroxene, quartz, iron oxide, biotite, and hypersthene, the relative 

abundance of the components being roughly indicated by the order in 

which they are named, plagioclase being the most abundant. Apatite 

is present in some specimens, and in one thin section no pyroxene was 

noted. The plagioclase is chiefly labradorite, and if th.e kind of feldspar be taken as the basis of classification, following Brb'gger and 

others, then the above rock must be called a quartz-gabbro.

Microscopically the rock is dark, very fine- and even-grained, with 

conchoidal fracture. None of the individual constituents are recognizable with the unaided eye.

Microscopically the rock presents, without the analyzer, a minutely 

mottled appearance, due to the presence of very abundant spots, lighter 

in color than the intervening spaces. These spots are often close 

together. With the analyzer they are seen to be made up of aggregates 

of translucent grains, with low, gray, interference colors. The grains 

of these aggregates often extinguish together; that is to say, they 

show aggregate polarization, indicating a like optical orientation of all 

of the grains of any one aggregate. No interference figure could be 

obtained in convergent light. There are very minute polarizing fibers 

scattered through these aggregates, which are presumably biotite. 

The aggregates" appear to represent the incipient formation of some 

mineral.

The spaces between the aggregates are dark colored and contain very 

abundant minute foils of a reddish-brown biotite and dull black grains, 

which are probably carbonaceous.

Scattered through the section, both in the aggregates and in the 

intervening spaces, are clear grains of 'clastic (?) quartz and large foils 

of biotite, some of them 0.4"1 "1 long.
DILLKB.] DESCRIPTIONS: NO. 135, CHIASTOLITE-SCHIST. 339

No, 135. CHIASTOLITE-SCHIST.

(FROM NEAR MARirosA, MARIPOSA COUNTY, CALIFORNIA. DESCRIBED BY H. W.

TURNER.)

The locality from which specimen No. 135 was obtained is in a zone of 

contact inetamorphie rocks at the south end of a long belt of the Mariposa slates which extends from Colfax, in Placer County, to the point 

at which the specimens were collected, a distance, following the curvature of the slate belt, of about 130 miles. To the west of Placerville 

this belt of Mariposa slates is in contact with an area of granite-porphyry and does not seem to be greatly altered, but the contact was 

only cursorily examined. Only at its south end is this slate belt in 

contact with any large body of a coarsely granular rock of a granitoid 

nature, and at no other point in its entire extent does ib show great 

alteration. The granitoid rock, which has caused the inetamorphism 

of the slates in Yaqui Gulch, extends at least as far south as the San 

Joaquin River, a distance of 35 miles, with an average width of 12 or 

more miles. This granitic rock is not near the contact with the schists 

a true granite, but a subsiliceous quartz-mica-diorite, as may be seen 

from the analysis in the table below, and indeed this is probably true 

of most of the area.

A fine-grained granite, No. 372 in the table below, from Eaymond, 

near the center of the area, is nearly a normal granite in composition, 

but analyses Nos. 369 and 851, from near the edge of the area, show 

the rock there to be much less siliceous. It has not, however, been 

determined whether or not No. 372 represents the average rock of the 

center of the area, or whether it is not a portion of an older or younger 

granite mass.

Analyses of yranile from the Raymond f/ranitoid area.

No. 369, 

Chowchilla 

Hiver.

Per cent. 

62.62

5.49

1.7C

3 49

No. 851, 

Yaqui 

Gulch.

Per cent. 

58.09

6.24

2.02

2.94

No. 372, 

Raymond 

Quarry.

Per cent. 

73.54

2.55

1.89

4. CO

In Yaqui Gulch, near the intrusive rock, the Mariposa schists have 

been irregularly displaced and their relation to the quartz-diorite is 

not a perfectly simple one, so that the study of the details of the metainorphism is not so satisfactory as it would otherwise be. The rocks 

near the granitoid rock (a quartz-diorite) are much obscured by soil.

The specimen collected nearest the quartz-diorite is a rather mediumgrained andalusite-hornfels, and was obtained at a point about 2,500
340 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. IBULL.ISO.

feet north of the main diorite contact. This hornfels is presumed to 

have been originally a sandstone. The expression " main diorite contact" is used for the reason that a small mass of quartz-diorite, doubtless an apophysis of the main area, occurs near the andalusite-hornfels 

noted above. After crossing the zone of andalusite-hornfels, the exact 

extent of which was not determined, we find in going away (or north) 

from the main diorite mass a considerable zone of knotted mica-schists 

and of chiastolite-schist.

In Yaqui Gulch, about 3,800 feet from the main diorite contact, these 

schists are well exposed in the bed of the stream. The normal strike 

of the main belt of Mariposa slates is about N. 30° W., with a dip of 

from 50° to 90° to the east. In the vicinity of the diorite these slates 

are much displaced and to some extent contorted. Along that portion 

of Yaqui Gulch, where the specimens were collected, the beds strike 

approximately north and south, and dip both east and west at an angle 

of 60° or more. The schists thus lie nearly at right angles to the course 

of the main diorite contact, which is nearly east and west.

The specimens of chiastolite-schist were not obtained all at one point, 

but at various points along the gulch within a distance of about 1,200 

feet, or in a zone of from 3,800 to 5,000 feet from the main diorite area. 

The chiastolite-schist occurs in layers interbedded with mica-schists 

and knotted schists. Often these layers are but 2 to 6 inches in thickness. It is evident that their formation depends largely upon the 

.original composition of the individual layers. Those containing a large 

percentage of argillaceous matter appear to develop into chiastoliteschists, and the more sandy layers into micaceous schists without 

chiastolite, or with only imperfectly developed crystals. This is to be 

expected from the composition of chiastolite, which is a silicate of 

alumina, the latter being a prominent constituent of all argillaceous

rocks.

Some of the specimens of chiastolite-schist collected in 'Yaqui Gulch 

contain distinct impressions of Aucella erringtoni, or one of its closely 

allied varieties described by Professor Hyatt. This fossil is said by 

Professor Hyatt, Professor Smith, of Stanford University, and others, 

to be of Jurassic age. It is thus evident that the granitoid rocks 

which have affected the inetamorphisra of the clay-slates are of late 

Jurassic or post-Jurassic age. °

More than 5,000 feet away from the diorite the Mariposa beds are 

not greatly metamorphosed. They are no longer schists, but clayslates. However, they still show some effects of the metainorphic 

action of the diorite in the presence of very abundant minute prisms, 

the distribution of which appears to be somewhat capricious, as some 

layers of the clay-slate show them and some do not, at exactly the 

same locality. At a distance of about 6,500 feet from the main diorite 

contact the inetamorpbic action of the igneous mass appears to have 

ceased entirely, but the contact of the diorite and the schists is perhaps
DILLER.J DESCRIPTIONS: NO. 135, CHIASTOLITE-SCHIST. 341

not a vertical one, and the distance of the schists from the underlying 

diorite mass in a vertical or inclined direction may be much less than 

the distances here given. Moreover, it should be .stated that these distances are all approximate. Some of them were measured by pacing, 

others only estimated. A contour map has not as yet been made of 

this region, which lies directly south of the Sonora quadrangle.

The specimen is from Yaqui Gulch, 2 miles southwest of the town of 

Mariposa.

Megascopically, when fresh, it is a hard, black, fine-grained schistose rock, with very abundant minute points with a silvery reflection 

and slender prisms which are square in cross section. These prisms 

are sometimes an inch in length, but usually shorter. In weathered 

specimens a dark center can be seen in some of the cross sections of 

the prisms.

Microscopically, when seen without the analyzer, it shows a finegrained, dark groundmass, composed of minute, clear .grains, many of 

them rounded; abundant rniimte, black particles, and reddish brown 

biotite scales, arranged in more or less nearly parallel lines, giving the 

schistose structure to the rock. In this groundmass are long, clear 

prisms which are square in cross section, and minute, clear prisms 

which are nearly of a size, having a width of 0.02mm and a length of 

about 0.2mi". Many of these lie at an angle to the plane of schistosity, 

suggesting their formation after the rock had been rendered schistose. 

The large prisms with square cross sections are chiastolite, and 

are from 1 to 2mm in diameter. The dark cross which distinguishes 

chiastolite from andalusite is feebly developed in many of the prisms, 

and consists in lines of minute granules extending from the center to 

the prism edges, bisecting the prism angles. The minute, black particles show on some surfaces a metallic luster by reflected light. Some 

of the rock was powdered and washed. A fine, black dust, which collected on the surface of the water and which presumably represents 

the black particles seen under the microscope, was consumed when 

placed on a platinum spatula in a flame at high temperature. This 

powder is therefore assumed to be carbon, and the metallic luster indicates that it may be in the form of graphite. The minute, clear prisms 

seen in natural light. probably represent the silvery points observed 

megascopically. They are muscovite. With the analyzer these prisms 

extinguish parallel to the direction of elongation and show bright interference colors.

The clear cross sections of the chiastolite crystals show the two 

prismatic cleavages intersecting nearly at right angles. In the cross 

sections the extinction is diagonal, bisecting the prism angles and the 

intersections of the cleavage lines. In longitudinal sections the two 

cleavages are indicated by parallel lines, and the extinction is parallel 

to the cleavage. In favorable light a slight pleochroism may be detected 

in some of the chiastolite prisms, ,c being faintly reddish. The outer
342 THE EDUCATIONAL' SERIES OF ROCK SPECIMENS. [BULL. 150.

edge of even the freshest chiastolite crystals is altered to a fibrous, 

colorless aggregate, which, according to Eosenbusch, may be a mixture 

.of sericite and kaolin, and occasionally irregular cracks extending into 

the crystals are filled with this same decomposition product. The fibers 

of the decomposition rim usually stand approximately normal to the 

prism planes. The particles of the grouudinass, to a certain extent, 

exhibit a tendency to flow around the chiastolite crystals that is to 

say, they are arranged in lines roughly parallel to the sides of the 

prism. This is best seen in the cross sections, and may be taken to indicate that the schists were in a plastic condition after the crystals were 

formed. A few minute ^7 einlets cut the section, filled with a clear mineral in little grains, which is apparently quartz. The powdered rock 

was tested for magnetite, but merely a few grains were found.

Analyses of contact-metamorphiv rocks. ' 

[Analyst, Steiger.]

Si02 .................................

Ti02 . ................................

A1203 ...... ....................... ...

T?6iOa ................ ................

FeO .................................

MnO. ................................

CaO .................................

SrO..... .............................

BaO .................................

MgO........... ......................

K2 0 .................................

Na^O ................................

Li,0...... ...........................

Water below 100" C ..................

p n

so3 ...... ............................

CO2 ............ ......................

Cl ...................................

F...... ..............................

BA-- . ............................

Total..........................

No. 851.

58.09

.95

17.46

1.12

5.08

0.24

.04

.07

4.06

2.02

2.94

.29

1.45

.]7

.05

 .21 '

.02

.11

inn 37

.01

100. 36

Xo. 432.

65.10

.72

17.77

1.05

3^29

1.38

1.43

2.45

.47

2.49

.03

fstrong\

\ trace. / 

.12

1.21

100. 80

.06

100. 74

No. 431A.

62.15

.89

19. 34

4.23

2.25

1.50

.04

1.88

3.07

1.60

.19

1.79

.15

.13

.22

1.12

100. 55

.10

100.45

2vro. 855.

7K

17.62

5.64

2.20

.12

3.10

1.02

4.36

.17

.05

.01

99 76

.01

99.75

No. 851 is a basic quartz-mica-diorite, collected near Yaqui Creek, 

about 300 feet from the border of the zone of contact-metamorphic rocks. 

It is the igneous rock which has caused the formation of the contact 

minerals.

No. 432 is a knotted mica-schist collected in Yaqui Gulch, about 

3,800 feet from the main diorite contact.
DILLEH.] DESCRIPTIONS: NO. 136, APORHYOLITE. 343

No. 431A (Educational Series Collection, No. 135), is a chiastolite- 

. schist collected in Yaqui Gulch, a few feet .to the north of No. 432, or 

about 3,800 feet from the main diorite contact.

No. 855 is a clay slate collected near the head of Yaqui Gulch, somewhat more than a mile from the main diorite contact.

The above analyses were made with the object of showing, as nearly 

as practicable, whether the new minerals in the zone of contact nietamorphism had formed as a result of certain elements being added to 

the schists from the quartz-diorite magma or from the mineralizing 

solutions and gases accompanying its intrusion, or had resulted merely 

from the recrystallization of the material of the clay slates. The specimens Nos. 432, 431A, and 855 may be regarded as having been all 

originally clay slates. Koseubusch found that the amount of the water 

of crystallization in a contact metaraorphic schist varied inversely with 

the distance from the igneous rock that caused the metamorphism. 

This same law appears to apply to the above series. In Nos. 432 and 

431A the amount of this water is 1.79 per cent and 2.79 per cent, respectively, while in No. 855, which is farthest from the igneous mass, the 

water of crystallization (100° C.-f) is 4.36 per cent. It will be noted 

that the amount of fluorine is greater in those schists nearest the 

diorite. This element probably exists in the authigenic micas, and 

may be regarded as having .been added to the schists from gases that 

ascended along the contact at the time of the intrusion of the diorite. 

The carbon content (0.11 per cent) in the quartz-diorite may have been 

derived from carbonates of lime and magnesia formed by the action of 

surface waters containing carbon dioxide. This is more than probable, 

since carbonates were noted in small amount in the thin sections of 

the rock. It should be borne in mind, however, that carbon dioxide 

occurs in a liquid form in minute cavities in the quartzes of diorite 

and granite, and when great masses of granitic rocks have undergone 

crushing the amount of carbon dioxide liberated would be considerable. The quartz-diorite No. 851, however, shows no evidence of crushing. The large majority of the newly formed minerals in the contact 

schists probably represents merely a molecular rearrangement of the 

original components.

METAMORPHIC IGNEOUS ROCKS.

No. 136. APORHYOLITE.

(FROM SOUTH MOUNTAIN, ADAMS COUNTY, PENNSYLVANIA. DKSCUIBKB BY Miss

F. BASCOM.)

The term aporhyolite, which has been recently introduced into petrographical nomenclature, 1 is designed to cover those acid volcanic rocks 

which are similar in chemical and mineralogical composition and in

'Bull. U. S. Geol. Survey, No. 130. The structure, origin, and nomenclature of the acid volcanic 

rocks of South Mountain: Jour. Geol., Vol. I, No. 8, pp. 813-832.
344 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [HUL...IOO.

structure to the rhyolite, but differ from that rock type in possessing a 

holocrystalline groundmass, presumably secondary.

The presence of those microscopic structures peculiar to glassy rocks, 

associated with evidence of the secondary character of the crystallization of the groundmass, is considered indicative of the original glassy 

character of the rock, and hence of a former identity with the rhyolite. 

It is maintained that the present difference is due to changes which 

have taken place subsequent to the solidification of the rock, chief 

among which has been devitrification.

By devitrification (entglasung) is meant the conversion of a glassy or 

partially glassy groundmass into a holocrystalline groundmass. That 

molecular motion does not cease with the solidification of a rock mass 

is an acknowledged fact. Daubree1 has shown experimentally that 

crystallization may take place in glass just as in. a molten magma. The 

action differs only in the amount of time required. In the former case 

it is exceedingly sluggish. Heat and moisture, which are not likely to 

have been altogether absent in the history of metamorphic rocks of 

any age, might be important factors in initiating and accelerating the 

process.

Such then are the facts the original character of the rock and the 

subsequent alteration indicated by the name aporhyolite.2

The structures pointing to a glassy origin and the secondary character of the crystallization of the groundmass will be indicated in the 

description of the specimen included in this collection.

The aporhyolite of the collection comes from a spur of South Mountain, about 2 miles west of the old Maria Furnace, in Adams County, 

Pennsylvania, and about 3£ miles northeast of Monterey Station, in 

Franklin County, Pennsylvania. The best specimens are found north 

of the junction of Toms Creek and Copper liun, upon the mountain side. 

They are of pre-Canibriau age, and have been subjected to pressure, 

rendering them more or less schistose.

In color the aporhyolites of this locality show considerable range j a 

blue-gray, bluish purple, and a reddish purple are the predominating 

colors. Both shades of purple are frequently present in a single hand 

specimen, when there is a tendency to alternating bands of these colors. 

Some specimens show a yellowish-green tone due to the presence in 

some abundance of a secondary micaceous mineral which will be characterized later. Phenocrysts are absent or inconspicuous. An amygdaloidal structure, so characteristic of aporhyolites from other localities 

in the South Mountain, is not observable in these specimens.

The conspicuous feature of the specimens is the globular or ellipsoidal 

spherulites which crowd the rock so thickly as almost to exclude the 

matrix in which they are embedded. These spherulites are from 3mnl 

to 5!nm in diameter. On the weathered surface of the rock they stand

1 Daubr6e: G6ologie Experimentale. 1879, p. 158.

2 TLe prefix apo may be used to indicate the derivation of one typo from another by some specific 

alteration.
DIU.ER.] DESCRIPTIONS: NO. 136, APORHYOLITE. 345 -* '

 > out in relief, giving it a superficial resemblance to a conglomerate.

They are irregularly distributed or are arranged in bands, and are 

'** often elongated by the movement of the magma during their formation. 

^ When seen in cross section the spherulites show a threefold zonal

banding of the blue and purple pigment. There is a dark center sur-

-^ rounded by light and dark zones, or this arrangement of zones is 

^ reversed.

A flow structure, while not conspicuous as in many aporhyolites, is

* obscurely indicated by the banding of the purple and blue shades, by 

the elongation of the spherulites, and by their arrangement in chains. 

The bright cleavage surfaces of minute feldspar crystals occasionally

*s reflect the light. These feldspars, magnetite, and the secondary mica- 

 ceous mineral before alluded to are the only constituents readily determined with the naked eye.

* Both, the matrix and the spherulites are cryptocrystalline.

The hardness of the rock is between G and 7. Its specific gravity is

* 2.678. The specific gravity of specimens of spherulitic glass ranges 

from 2.385 to 2.394. These figures show the increase in density which 

accompanies devitrification. The presence of manganese oxide is 

denoted in a brown stain on the weathered surface of the rock.

_ Under the microscope in ordinary light the spherulites appear as circular, elliptical, or irregularly oval areas outlined by minute particles 

of red iron oxide (hematite) and dusted by the same pigment. Sue-

^ cessive zones are faintly indicated and arise from the crowding in 

bands of innumerable particles of black and red iron oxide. Sometimes 

these circular areas merge one into the other, forming a chain of spherulites suggestive of those described by Professor Id clings 1 as characteristic of the Yellowstoue National Park volcanics.

-* The groundmass in which the spherulites lie, and which constitutes 

only a small portion of the field, is distinguished in ordinary light from 

the spherulitic areas by its comparative freedom from the iron oxide

^ particles. PL XLIII shows the altered spherulites of an aporhyolite 

from the type locality in the South Mountain, Pennsylvania. 

With the analyser in place the field has a much more homogeneous

u aspect. The spherulites unexpectedly disappear. Instead of the radiating fibers of quartz and feldspar, which constitute the well-known 

spherulitic structure, there is a finely granular quartz-feldspar mosaic, 

quite similar to the quartz-feldspar mosaic of the groundmass. 

A cloudiness due to the abundant iron oxide particles in the spheru-

 * litic areas, the presence of brightly polarizing scales of a secondary 

mineral, or the finer grain of the quartz-feldspar crystallization alone 

serve to distinguish the spherulites from the groundmass.

Karely the aporhyolites of this locality show traces of a radial growth 

of quartz and feldspar, not yet entirely obliterated by alteration to a 

granular crystallization, while sometimes the groundmass still preserves

1 Obsidian Cliff: Seventh Ann. Kept. U. S. Gcol. Survey, 1888, p. 277, PI. XVII.
346 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150. ^

faint indications of perlitic parting. This is a structure peculiar to a ^ 

glass and consists in. a concentric cracking due to the contraction on 

cooling. Subsequent crystallization might readily obliterate- such a ^ 

structure. In this case iron oxide particles are so arranged as to pre- < 

serve the outline of the cracks. This structure is illustrated in Fig. A, 

PI. XLIV, prepared from a thin section 1 of aporhyolite from the Lower & 

Keweenawan of the Lake Superior region. ^

Aporhyolites from Raccoon Creek in the South Mountain, about 

10 miles west of Copper Run, show spherulitic and perlitic structures * 

still perfectly preserved, associated with lithophysal flow and rhyolitic 

structures in great perfection. Fig. B, PI. XLIY, is aporhyolite from 

Raccoon Creek, showing spherulitic and perlitic structures. <4-

The association of any two of these structures is considered sufficient ^ 

proof that the rock which they characterize consolidated as a glass. 

Such is the nature of the evidence that the finely granular crystalliza- ^rtion of the groundmass and the spherulites alike is secondary.

The rock first consolidated as a glass (or in large part a glass), 

crowded with spherulites possessing the true radiating structure char- ^ 

acteristic of spherulites. Subsequent to this consolidation, devitrifica- . ' 

tion, the nature of which has already been explained, has brought 

about the uniform granular holocrystallirie character which the rock ^- 

now possesses.

The mineral constituents of these rocks are few in number. Chief 

among the crystals of the first consolidation is a clear, well-preserved 

feldspar. Crystals of this mineral are inconsiderable in size. They are 

frequently grouped, and are distributed without reference to the spheru- x»-~ 

lites. They may be twinned by the Carlsbad or Manebacher law and 

often show microperthic structure as result of pressure. They may 

contain inclusions of an original glassy magma. That they belong to * 

the alkali end of the series of feldspars is indicated by the chemical 

analysis of the rock, while their specific gravity (2.6) and the emergence 

on the M face of a positive bisectrix very slightly inclined, as may be v~ 

observed in converging light on the Mauebacher twins, suggest anorthoclase.

Occasional clear oval areas of granular quartz may sometimes rep- Wresent granulated quartz crystals of the first consolidation, or the secondary replacement of spherulitic centers, or they are, in many'places, 

the filling of minute vesicles elongated by the movement of the magma. 

In the last case there are frequently forms along the walls of the vesicles, outlined by the iron oxide, which suggest minute tridymite crys- w 

tals or spherulites, such as are found lining the walls of vesicles in 

modern lavas. They disappear in polarized light. Fig. J5, PI. XLIY, 

prepared from an aporhyolite from Raccoon Creek, shows quartz-filled - 

vesicles bearing tridyrnite spherulites on their walls. Rarely transparent colorless crystals of zircon, characterized by high index of

1 Thin section furnished by N. H. Winchell.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XLIII

THIN SECTION OF APORHYOL1TE FROM SOUTH MOUNTAIN, PENNSYLVANIA, SHOWING 

ALTERED SPHERUL1TES, AS SEEN UNDER A MICROSCOPE.
DiLLKu.] . DESCRIPTIONS: NO. 136, APORHYOLITE. 347

Tefraction and brilliant polarization colors, may be recognized, also 

minute grains of pleochroic hornblende. The only other original constituents are the iron oxides, magnetite, and hematite. They are both 

in part secondary. The latter is recognized by its nonmetallic luster 

and reddish color in incident light, the former by its rough metallic 

black surface. Neither show crystalline form. The former is magnetic 

in the powder. Hematite is a characteristic pigment for rhyolites.

The conspicuous secondary constituent is the micaceous mineral of 

which mention has already been made and to whose presence the rock 

owes a light greenish tinge. Under the microscope, in ordinary light, 

it appears as transparent pale greenish yellow, irregular plates showing lines of cleavage, with a low index of refraction and an oily 

luster. In parallel polarized light these plates show brilliant interference colors, and in converging polarized light a small axial angle. 

These plates are developed most abundantly around the feldspar crystals, filling the cracks in the feldspars that have suffered crushing, 

and around and in the spherulitic areas.

It is undoubtedly an alteration product of feldspar under pressure 

and that species of mica known as sericite. This mineral can be formed 

from the acid feldspars by the replacement by hydrogen of a portion of 

the alkali constituent and the setting free of silica. This kind of metamorphisin is of common occurrence in the development of schistose 

rocks from the massive acid eruptives, and has been carried so far in 

some of the acid volcanics of South Mountain as to'form a sericite-schist.

Another secondary constituent of these aporhyolites is epidote. This 

mineral is of a deeper, yellowish green than the sericite, and is easily 

distinguished from the latter by its high index of refraction. It occurs 

in irregular granular aggregates. Its high relief and bright iriterference colors serve to distinguish it. It is a product of the weathering 

of the feldspars in the presence of solutions carrying alumina.

That the magnetite of these rocks is titaniferous is plainly shown by 

its alteration products, of which there are two. The most abundant 

one is a cloudy, white (in incident light), or yellowish substance called 

leucoxene. Associated with this mineral there are, rarely, brown semitransparent grains of titanite, showing a higher relief than epidote, and 

without brilliant interference colors.

The evidence that these rocks have undergone pressure and some 

shearing lies in the development of a sericite and the resulting foliation, the cracking and pulling apart of the feldspars, the development 

of the perthitic structure, and the granulation of the quartzes.
348 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

An analysis of aporhyolite from Monterey, Franklin Oonaty,'Pennsylvania, as reported by H. N. Stokes, is as follows:

Analysis of aporhyolite.

Si02 .............................................

TiOz ..... ........................................

PA--- ................................ ..........

ALA .........:................. .................

Fe2 03 ... ......... .................................

EeO ....................................'..........

MnO............. ................................

BaO .............................................

SrO............... ................... .............

CaO.... ..........................................

MgO.............................................

JC,0.--.- .........................................

Li.,0 .............................................

H2 0 below 110° C ................................

Total............................-.....'....

Per cent.

76. 34

.26

11. 60

2.41

.30

.09

.55

.06

 2. 75

5.50

.10

.39

100. 35

The analysis is essentially that of a typical rhyolite. The relation of 

soda percentage to the potash accords, in its indication of the character 

of the feldspar, with the optical determinations. The lime may be 

referred to the epidote, which doubtless tilso explains the trace of manganese oxide. A manganese epidote has been found very abundantly 

in some of the aporhyolite near Monterey. The presence of titanium 

in the magnetite is also substantiated by the analysis.

Beside the locality from which these specimens were obtained typical 

aporhyolites also occur in the South Mountain along Eaccoon Creek, 

south of Caledonia Furnace, Franklin County, Pennsylvania. Elsewhere similar acid volcanics have recently been recognized, forming, 

with the South Mountain volcauics, a belt extending along the.eastern 

border of the United States and Canada. 1

In Newfoundland, Nova Scotia, New Brunswick, and on the Gaspe* 

peninsula old volcanic rocks, both acid and basic, are extensively 

developed and have been described by the Canadian surveys. Hitchcock and Shaler report their presence in Maine about Eastport and 

Mount Desert, along the coast,2 and ou Moosehead Lake, in the interior.1 '

Dr. Wadswprth 4 and Mr. Diller 5 have made the felsites (aporhyolites) 

of the Boston basin famous.

'Fora full account of these localities see a paper by G. H. Williams, The distribution of ancient 

volcanic rocks along the eastern border of North America: Jour. Geol., Vol. II, No. 1, pp. 1-31, pi. 1, 

1894.

2 Am. Jour. Sci., 3d series, Vol. XXXII, pp. 40-43,1886.

3 Geol. Maine, 1861, p. 190 and 432; also id. 1863, p. 330.

* The classification of rocks: Bull. Mus. Com. Zool. Harvard College, Vol. V, p. 282,1879.

0 The felsites and their associated rocks north of Boston: Bull. Mus. Comp. Zool., Vol. VII, p. 165,1881.
U. S. GEOLOGICAL SURVE 1 IULLETIN NO. 150 PL. XUV

THIN SECTIONS OF APORHYOLITE.

A. From Lake Superior, Minnesota, showing perlitic structure 

B. From South Mountain, Pennsylvania, showing perlitic, spherulitic, and lithophysal structures.
DESCRIPTIONS: NO. 137, GRANITOID GNEISS. 349

In Chatham and Orange counties, North Carolina, ancient acid volcanics showing spherulitic and flow structures have been collected by 

Dr. G. H. Williams, and at Lancaster, South Carolina, devitrified glasses 

have been found by Prof. S. L. Powell. With continued petrographic 

study of the pre-Cambrian rocks of America aporhyolites may be 

recognized at many other points. The similarity of the pre-Cambrian 

volcanics of the Lake Superior region to their modern equivalents was 

long ago remarked by Dr. Irviiig.J

In the Kewatin (Lower Huronian) of Minnesota holocrystalliue volcauics, showing perlitic parting, spherulitic, and other structures common to aporhyolites, have lately been described by Dr. Grant.2

While aporhyolites have only recently been recognized in America, 

in England such writers as Allport, Cole, Bonney, Eutley, and Harker 

have long investigated similar rocks as they occur in Scotland, the 

Lake District, and northern Wales. In Sweden and Belgium they 

have been recognized and described by Nordenskjold 3 and De La 

Vallee Poussin. 4

For a list of English, French, and German papers on this class of 

rocks see Bulletin 136, United States Geological Survey, pp. 87-91.

No. 137. GRANITOID GNEISS.

(FKOM HOOSAC MOUNTAIN (TUNNEL), MASSACHUSETTS. DESCRIBED BY J. E.

WOLPF.)

The three rocks, granitoid gneiss (137), raetainorphic conglomerate 

(128), and albite-schist (129), form an ascending series, with the granitoid gneiss at the base, and in their geologic relations give the key to 

the structure of the main axis of the Green Mountains in northwestern 

Massachusetts/'

The granitoid gneiss occurs in two areas. The first (from which the 

specimens are taken) is on Hoosac Mountain, where the rock occupies 

a long oval area on top of the mountain south of the tunnel line, and 

is again cut by the tunnel for a distance of several thousand feet 

at a considerable depth below the surface of the mountain. The second area lies a few miles northwest from the first, forming the crest 

and slopes of Stamford Mountain, in Vermont, and a part of its 

southern continuation in Massachusetts, called Clarksburg Mountain. 

Therock.has hence been called the "Stamford granite," 0 and occupies

1 K.D. Irviiig, Copper-bearing Rocks of the Lake Superior Region : Mon.U. S. Geol.'Survey, pp. 

312,313, note 5, p. 436. . ...

2 U. S. Grant, Volcanic Rocks in the Kewatiu of Minnesota: Science, V ol. XXIII, Jan. 12, 1894, 

p. 17.

3 Otto Nordeuskjold, TJeber archaeischo Ergussgestoiuo aus Snialand: Hull. Gool. Instit. TJpsala, 

No. 2, Vol. 1,1893.

 "DeLa Vall6e Poussin, Lea anciennes rhyolites tlites eurites de Grand Manil: Bull. Acad. R. do 

Uelg., 3d series, Tome 10,1885.

6 Consult Mon. XXIII, TJ. S. Geol. Survey, Part II, The Geology of Hoosac Mountain aud 

Adjacent Territory, by J. E. Wolff.

i; Geology of Vermont, p. 601.
350 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150..

a rounded area several miles in diameter, within which the rock displays considerable lithologic variation. On the outside of the core of 

this Stamford granitoid gneiss there is a mantle of younger sedimentary rocks, composed principally of quartzite and schist, in which, on 

the west side, near the summit of Clarksburg Mountain, in Massachusetts, Mr. 0. D. Walcott found the remains of Lower Cambrian trilobites. 1 This quartzite mantles around the southern end of the granite 

and is found again on the east side, Avhere it is partly quartzite, partly 

mica schist, so that the mass as a whole has a dome structure, with the 

granitoid gneiss as a central core. On the east side of this dome, west 

of the village of Stamford, Vermont, a remarkable contact between 

the two rocks is found. The quartzite is conglomeratic, and in the 

granitoid gneiss there is a curious cleft due to the weathering of what 

was originally a trap dike, some of the material from which is found in 

the overlying sandstone, whose layers thicken over the cleft and sag 

down into it, the whole showing plainly the uuconformable deposition 

of the quartzite arid the pre-Cambrian age of the granitoid gneiss. 

The latter rock is finely banded, almost schistose at the contact, but 

away from this it becomes more and more massive and coarse, with the 

development of large phenocrysts of feldspar in places. The schistose 

structure is evidently an effect of crushing (stretching), combined with 

mineralogical change, which decreases toward the solid core of massive 

rock, but this change was probably assisted by the previous disintegration of the granitic rock on the old pre-Cambrian land surface.2

On Hoosac Mountain the rock occurs in smaller mass and is somewhat more metamorphosed than the rock of the Stamford area. It 

occurs, however, in the same relation, forming the original Cambrian 

shore line and overlain by the representative of the Lower Cambrian, 

here a conglomerate, itself extremely metamorphosed (the metamorphic conglomerate of the collection). It is noticeable here, too, 

that the gneissic structure is often better developed near the contact, 

and is then roughly parallel to the banding of the conglomerate.

The second rock, the nietamorphic conglomerate (No. 128), typically 

developed, is found resting on the granitoid gneiss of Hoosac Mountain at the place where the latter rock leaves the surface of the mountain and plunges downward to the tunnel level. The rock forms a 

bed 600 to 700 feet in actual thickness, very coarse at the base and 

becoming gradually finer-grained toward the top, where ijb is succeeded by the albite-schist. The conglomerate outcrops run east and 

west from the central point in the mountain crest, curving around on 

each side from, an east-and-west trend to a norfch-and-south one, so 

that they bound the granitoid gneiss area, and, in fact, completely 

encircle it. While the conglomerate dips from the granitoid gneiss on 

the north and east sides, it dips toward or under it on the west and

1 Am. Jour. Sci., 3d series, Vol. XXXV, 1888, p. 236.

*K. Pumpelly, The relation of secular rock disintegration to certain transitional crystalline 

schists: Bull. Geol. Soc. America, Vol. II, pp. 209-224.
DILLER.] DESCRIPTIONS: NO. 137, GRANITOID GNEISS. 351

southwest sides, so that the structure is that of an anticlinal fold or 

dome overturned to the west, the axis of which inclines or pitches 

north, and therefore the formations successively leave the surface 

along the crest of the mountain and plunge northward and downward 

to the tunnel level. The conglomerate, therefore, having been folded 

over the granitoid gneiss with overturning on the west side, we find 

that the rock has lost nearly all traces of its original character on the 

sides of the fold where the motion or stretching action was greatest, 

but that this is better preserved on the axis or center of the arch, 

where the northerly pitch of the axis gives us the series in normal 

position.

The conglomerate specimens are also taken from the dumps of the 

central shaft of the tunnel, which cuts the rock 1,000 feet below the 

surface. They show conglomeratic character plainly, while the same 

rock found at the surface on the sides of the fold loses its character as 

a conglomerate and becomes a gneiss.

The third rock, the albite-schist (No. 129), overlies the conglomerate 

conformably on the crest of the mountain, exposing a thickness of at 

least 1,000 feet, and mantles over the conglomerate on the east and west 

sides, as the upper layer of the fold. The basal layer, 50 feet or more 

thick, is very, rich in garnet crystals, suggesting an original calcareous 

rock, and as the conglomerate represents the Cambrian quartzite, this 

overlying schist must represent the Stockbridge limestone and associated schists (Cambro-Silurian) which lie in the Hoosac valley and 

mountain mass of Greylock to the west of Hoosac Mountain, so that 

there is some basis for this explanation of the presence of the limesilicate (garnet) in such abundance.

The albite-schist is cut by the tunnel on both sides of the anticlinal 

axis, and the specimens are therefore taken from the dumps of the 

central shaft, which is a great storehouse for specimens of the various 

modifications of the three rocks, in an ideally fresh condition.

With this brief description of the geological occurrence of these 

specimens, we may x>roceed to their individual ^description. 

: In the hand specimen the granitoid-gneiss contains large elongated 

masses of pale-reddish feldpar, separated by thinner bands of white 

or bluish granular quartz, in the middle of which there are often 

larger grains of homogeneous blue quart/,. Thin, dark-green, branching bands of mica and epidote traverse the specimen in rough parallelism to the quartz-feldspar bands, while within the quartz or feldspar 

masses little patches of the same green color are found. A broad, flat 

cleavage face of feldspar is often distinctly curved, or may be broken 

up into several pieces which do not reflect the light simultaneously, 

and have therefore been moved from their original position, in one 

plane. When we remember that the more massive rock, as found in 

the center of the great Stamford core, has the same large feldspars 

in well-formed crystals, the blue quartz in large grains, and but little
352 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.IOO.

banding or development of thin mica bands, we can understand that 

the present structure is due to pressure, which strained and broke up 

the minerals, the granulated minerals moving a little over each other, 

and in the interstices thus formed, or along the larger planes of 

motion, the mica, epidote, etc., developing to form the little secondary 

bands. In this way, by dynamic metamorphism, a coarse banded rock 

has been produced from a coarse massive rock.

In the thin section these mechanical and mineralogical changes can 

be studied more in detail. Many of the large feldspar areas are recognized as microcline by the fine double twinning (albite and pericline 

laws), while others Avhich show no twinning are partly orthoclase, partly 

microcline, Avhich the section cuts parallel to the second cleavage 

(co Poo). In some cases the large feldspars contain narrow, spindleshaped masses of another feldspar, in parallel growth (microperthite). 

Other feldspar areas with but a single polysynthetic twinning are* 

probably albite (primary). These large crystals are crossed by little 

vein-like aggregates composed of rounded grains of quartz, little irregular grains of fresh, clean microcline, other grains of feldspar with a 

clear, even, polarizing tint, with often two straight parallel sides and 

a twinning line parallel to them which divides the crystal into two 

halves, all these properties being identical with those of the albites of 

the conglomerate and schist, so that it is probably the same feldspar 

in the case of the granitoid gneiss. In addition to these, the veinlets 

contain little grains of epidote and plates of biotite and niuscovite. 

They may run irregularly or be roughly parallel in direction to the 

general banding of the rock. It often happens that the large feldspar 

is broken into several minor pieces, which are separated along the lines 

of break, or of micro-faulting by an aggregate of these minerals, whose 

later origin, after pressure and motion had acted, is thus evident.

In the same way the larger masses of original quartz are identified 

by their large size and by the fact that they are surrounded by a zone 

of aggregate quartz broken off from the parent mass, which, in polarized light, is seen to be itself strained or cracked. This original quartz 

is moreover free from inclusions of any of the secondary minerals, 

unless in cracks, while the metaniorphic quartz of the little veinlets 

which penetrate the feldspar, and of the banded aggregates between 

the feldspar, frequently inclose biotite, showing the contemporaneous 

origin of the two minerals, and that the larger masses of original quartz 

existed before the period of the biotite formation.

The rest of the slide is composed of an aggregate of the followingminerals:

(i) Feldspar, in little imperfect crystals, which are often simple twins 

according to the albite law and are probably albite. 1 These are either 

clear or are filled with little plates of mica, grains of epidote, and droplets of quartz. There are also irregular grains of microcline, in part

1 Their refractive index i.s usually lower thnn that, of I ho balsam of the section, or barely equal to it. 

Obliquity of extinction in tlitj /-one of the twim.iiu.tr ii.xi.s is too high for oligoclase, i. e., albite.
DILLKH.] DESCRIPTIONS: NO. 138, EPIDOTE-MICA-GNEISS. 353

broken off from the large primary crystals, in part probably inetamorphic. 

(2) Quartz in interlocking aggregates or isolated grains. (3) Biotite 

and greenish muscovite in irregular stringers, inclosing in their meshes

I little prisms or grains of yellow epidote with high polarizing colors, 

, occasional grains of black opaque magnetite, white grains or aggregate

-V masses of titanite, and some areas of white calcite, recognized by its

ij rhombohedral cleavage and white polarization. There are occasional 

small prisms of zircon with terminal planes and with distinct uniaxial 

positive character.

This aggregate of minerals often invades the large original masses of 

feldspar, but it will be noticed that none of these minerals show signs

^ of straining or breaking, and that the little albitic feldspars inclose 

epidote, quartz, and mica. Eeasoning from this, we infer that they 

formed after or during the action of the forces which strained and

> broke the original feldspars and quart/,, and under conditions of contemporaneous crystallization, unlike those of eruptive rocks, where 

there is generally a certain succession in the formation of the different 

minerals some earlier, some later.

T The original rock was a coarse, massive, granitic rock, perhaps of

eruptive origin, and resembling the coarse granite of Finland, called

rapakiwi, while dynamic metamorphism has produced the present

^ gneissic condition, the mineralogical change consisting essentially

in the production of new feldspar, mica, quartz, and epidote.

No. 138. EPIDOTE-MICA-GNEISS.

(FROM LEBANON, GRAFTON COUNTY, NEW HAMPSHIRE. DESCRIBED BY J. P.

IDDINGS.)

This gneiss from Lebanon, New Hampshire, was supposed to be 

so-called protogene-gneiss, the characteristic minerals of which are 

"chlorite, talc, rotten mica, or other decomposition products," according to Hawes.1

It is in fact an epidote-mica-gneiss, according to present classification. It consists of relatively large, irregular crystals of microcliue, 

with small grains of quartz in aggregates equaling the feldspar in bulk, 

besides brown biotite in aggregates of small plates, variable amounts 

of colorless muscovite, and much epidote in aggregations of microscopic 

crystals. Subordinate minerals, occurring in relatively small amounts, 

are apatite, tourmaline, allanite, zircon, possibly sphene, and occasionally green mica or chlorite and, rarely, calcite. The rock is quite fresh 

and undecomposed, judging from the condition of the brown biotite. 

The epidote is grouped in aggregations with, quartz and sometimes 

with muscovite, or is scattered in various-sized crystals and grains 

through the microcline, but there are no remains or other evidence

1 G. W.Hawes: Mineralogy and Lithology of New Hampshire, Part IV, of Geology of Now Hampshire. Concord, 1878, p. 201.

Bull, 150 23
354 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.IBO.

of any ferromagnesian mineral, more or less altered, from which it 

could have been in part derived. It appears as a primary constituent.

The microcline exhibits microscopic multiple twinning with grating 

structure, and the n'bration due to an intergrowth of plagioclase, or 

microperthite structure. In certain sections the microcline twinning 

is scarcely recognizable. There is, besides, a simple Carlsbad twinning. Smaller feldspars occur mixed with the quartz. The feldspars 

are clouded to a greater or less extent by minute crystals of epidote, 

and sometimes of muscovite. The minerals are in some cases arranged 

in several directions in a feldspar, the substance of the feldspar being 

otherwise quite pure. Micropegmatitic intergrowth of quartz and 

feldspar is occasionally noticed. In some feldspars there are innumerable minute rectangular inclusions with a bubble of gas in each, 

presumably fluid inclusions of secondary origin. Microscopic grains of 

quartz with rounded outline are abundant in certain feldspars, the 

quartz being more numerous in the marginal part of the feldspar.

Quartz forms irregular grains, somewhat rounded in outline, often 

very free from inclusions or impurities, sometimes bearing numerous 

minute crystals of epidote, and less often those of biotite and muscovite. Fluid inclusions are not common.

Biotite forms small stout plates, set at all angles, without approach 

to parallelism. Its color is dark brovvu with a tinge of green. The 

absorption is strong, and the substance of the mineral quite pure. In 

a few cases it is altered to a green mica, and in some cases chlorite. 

Muscovite is almost absolutely colorless in thin section, with slight 

absorption of light for rays vibrating parallel to the cleavage of the 

mica. It is sometimes inclosed by biotite. Both micas are in places 

intergrown, or include one another, together with grains of quartz 

and epidote. Inclusions of apatite and zircon are scarce.

Epidote forms yellowish crystals or grains with faint pleochroism. 

Its high index of refraction and strong double refraction are characteristic. Occasionally the center of a crystal is chestnut-brown and 

pleochroic, and is probably allanite. It is closely associated with muscovite in some places, in others is independent of it. Its size sinks to 

the minutest microscopic dimensions. It might be assumed to be an 

alteration product, as it often is, were it not for the fact that no partially decomposed calcium-iron mineral is to be found in the rock.

Apatite is scarce, in colorless, irregular grains. Zircon is also scarce, 

and forms very small grains.

Tourmaline occurs sparingly in short prisms, with a purplish:brown 

color in thin section, and exhibits complete absorption of the ordinary 

ray. Several grains of calcite were noted in one section, and in another 

an irregular grain supposed to be sphene. Chlorite is present in only 

a few instances, when it exhibits a strong green color and marked 

pleochroism. It is evidently an alteration product derived directly 

from biotite. Magnetite is entirely absent.
DILLER.] DESCRIPTIONS: NO. 139, DIABASE AMYGDALOID. 355

It is possible that this rock may be the same as, or similar to, that 

mentioned by Hawes, in the report already referred to, as occurring at 

Walling's quarry, which contains epidote.

No. 139. DIABASE AMYGDALOID.

(FROM GRAND MAUAIS, COOK COUNTY, MINNESOTA. DESCRIBED BY W. S. BAYLEY.)

This rock forms one of the many flat-lying sheets that, together with 

conglomerates and other fragmental layers, make up the Keweenawan 1 

or copper-bearing series of the Lake Superior region. On the north 

shore of the lake the eruptive beds of the series come down to the. 

water's edge, forming cliffs, in whose sides several distinct layers may 

often be seeo. Some of the beds are dense throughout; others, and 

among them the bed from which the specimen was taken, are completely crystalline and moderately coarse grained toward their centers, 

and finer grained and vesicular at both the upper and the lower contacts.

The vesicles in modern lavas are known to be due to the expansion of 

water contained in the molten rock magma. As long as this remains 

in the interior of the earth the pressure there existing retains the water 

in a liquid condition. When the magma escapes to the surface, where 

the pressure is much less, the superheated water flashes into steam, 

which in attempting to escape produces bubbles in the liquid mass, just 

as bubbles are produced in boiling water. If the rock solidifies quickly 

at top and bottom some of the bubbles are caught, the steam escapes 

from them, and a vacuole results. If the vacuole is afterwards filled 

by material deposited from waters circulating through the rock, amygdules are formed, and the rock becomes an amygdaloid. The vesicles 

in old lavas are undoubtedly due to the same causes as give rise to the 

vacuoles of modern volcanic rocks. Hence, when a sheet of eruptive 

material is found with vesicles near its upper surface, and especially 

when these are found near its lower surface as well, we conclude that 

the sheet was a surface flow, like the lava sheets of modern volcanoes.

The specimen was taken from near the upper surface of a flow or 

sheet that reaches the water's edge 1£ miles east of Grand Marais, on 

the north shore of Lake Superior.

An inspection of the hand specimen shows it to be a very fine-grained 

purple rock, without any evidences of stratification or foliation. In it 

are a large number of vacuoles. In a few of the specimens these are 

arranged with their long axes approximately parallel, but in most no 

such regular arrangement is apparent. In the former case the regularity is due to the fact that movement continued in the pasty rock magma 

even after it had become viscous enough to prevent the escape of the 

bubbles. These were consequently drawn out in the direction of 

the motion.

'For characterization of the Keweenawan, see Irving and Chamberlin; Bull. U. S. Geol. Survey 

JJo. 23, and K. P. Irving, Copper-bearing Eocks of Lake Superior, Mon. TJ. S. Geol. Survey, Vol. V.
356 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

Some of the vacuoles are entirely empty. Others have their walls 

covered with a thin coating of a fibrous pink mineral, while others 

again are almost filled with larger needles of the same substance. A 

few are lined with a pistachio-green material resembling epidote, while 

still others have their sides lined with a layer of the green mineral 

under a coating of the pink substance. It is evident that the green 

mineral is older than the pink one, since the latter could not have been 

deposited upon the former until this had already existed. In a few of the 

vesicles a third substance maybe seen. It is a soft, colorless, or white, 

easily cleavable material that effervesces strongly when moistened 

.with cold hydrochloric acid. This mineral, which is evidently calcite, 

is younger than the pink mineral, since it occupies the central portions 

of the vesicles and is surrounded by the latter, which is present in 

little bundles of radiating needles, whose hardness is low. The needles 

easily dissolve in hydrochloric acid, yielding gelatinous silica, and give 

up water in the closed tube. Though no complete chemical analysis of 

them has been made, there is but little doubt that they are laumontite.

When placed under the microscope the mass of the rock is found to 

consist largely of small microlites of a twinned feldspar and a darkbrown interstitial substance or groundmass. The feldspar is often 

clouded with decomposition products and is reddened by stains of iron 

salts. It occurs usually in skeleton crystals with forked ends in place 

of crystal terminations, and in cross section it frequently appears as a 

hollow shell including portions of the brown groundmass. The microlites extinguish at low angles as measured against their long axes, and 

consequently are composed of a substance near oligoclase in composition. The highest extinction to be noted is about 16°, while many of 

the tiny crystals extinguish nearly parallel: The microlites are scattered irregularly through the section, except in those parts near the 

vacuoles, where they are absent.

The interstitial substance or groundmass is opaque in most parts of 

the section and particularly so immediately around the vesicles. On 

the edges of the section, where it is thinnest, it is seen to be composed 

of many very slender feldspar needles aggregated into little radiating 

bunches or into sheaf-like forms. Between these is a dark matrix 

consisting of a noupolarizing substance originally glass darkened 

by little round grains and irregular masses of magnetite, hematite, 

or brown iron oxides. In other places this matrix is comparatively 

clear, when it exerts a feeble influence on polarized light, as if the 

glass of which it was composed had begun to differentiate into mineral 

species, whose nature, however, can not be determined because of 

the abundance of the iron compounds mingled with them. In the 

vicinity of the walls of the amygdaloidal cavities there is an absence of 

the larger feldspar microlites, as has already been stated. Under the 

higher powers small feldspathic microlites may be detected in these 

portions of the groundmass, but otherwise no special features are to be 

noticed in it.
DiLLEK.1 DESCRIPTIONS: NO. 139, DIABASE AMYGDALOID. 357

The general characteristics of the amygdalpid are those of a much 

decomposed glassy rock with feklspathic microlites. Its structure 

resembles that of a glassy diabase or basalt. Upon comparison with 

sections made from various "portions of similar vesicular flows from 

other horizons in the Keweenawan, it is found that the rock under 

investigation resembles very strikingly the fine-grained portions of 

those beds which in their interior have the composition and structure. 

of typical diabase. Its fine grain and the inicrolitic character of its 

feldspars are due to the rapid cooling of the rock mass. The lack of 

the larger microlites in the neighborhood of the vacuoles is due to the 

fact that in those places comparatively large surfaces of the rock mass 

were exposed to cooling which there went on more rapidly than elsewhere, " so that the time before the final solidification of the rock was

not sufficiently long to enable the crystals to grow to the size reached 

by those in other portions of its mass.

Not much can be learned from the section regarding the filling of the 

cavities. In most cases the softer materials contained in them are 

removed in the grinding. Along the walls may sometimes be seen 

adhering a small amount of a colorless mineral with a low double 

refraction. It occurs in little aggregates resembling very closely the 

quartz mosaics of certain quartz porphyries.

Other smaller cavities may be distinguished from the true vesicles by. 

the fact that their sides are ragged and not smooth. The filling, a 

dirty green substance having very little effect on polarized light, 

extends far back between the microlites of the rock; and that portion 

of the rock mass immediately contiguous to them is not more compact 

than the portions away from them. These two facts taken together 

indicate that the cavities have an origin different from that of the 

larger vesicles. The irregular penetration of the filling mass into 

the rock material indicates that the boundary between the two has 

encroached on the material of the rock. Since the feldspar microlites 

project into the greenish substance filling the cavities, it is evident 

that the interstitial groundmass yielded more rapidly than the feldspathic material. These cavities are consequently not true amygdaloidal ones, but they resemble these in that both are often filled with 

the same minerals. They are supposed to be secondary in origin, i. e., 

to have been formed by the decomposition of the groundmass of the 

rock containing them, and are distinguished from true aniygdules by 

the designation pseudamygdules.

No analysis of the rock is given, because it is so much decomposed 

and so full of secondary products that very little regarding its original 

composition would be learned from it. 1

1 For descriptions of similar aiuygdaloida from the Lake Superior region see 11. Pnmpolly: Froc. 

Am. Acad. Adv. Sci., 1878, XII, p. 282; and K. D. Irviug, Copper-bearing rocks of Lake Superior, 

Moil. U. S. Geological Survey, Vol, V, Washington. 1884, pp. 87-91.
358 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150. *f

NO. 140. QUARTZ-NOBITE-GNEISS. ^

(F«OM ODESSA, BIGSTONE COUNTY, MINNESOTA. DESCRIBED BY W. S. BAYLEY.)

. \ 

In the valley of the Minnesota River, in Minnesota, there exists an (

area of crystalline schistose rocks a few miles wide and extending with £ 

very many interruptions from New Ulm in a northwesterly direction 

to .the southeastern end of Big Stone Lake. Among the rocks occurring in this area are interbedded gneisses, crystalline schists, and 

other foliated rocks that may be regarded as squeezed eruptives. They 

are all thought to be pre-Algonkian in age, though so far as known no 

proofs of the correctness of this view have as yet been recorded.

The specimen described in this article was ta.ken from a small outcrop 1,200 paces north and 1,500 west of southeast corner, sec. 9, T. ^ 

120 N., E. 45 W., near Odessa, Minnesota. 1

In the field the rock appears to be quite schistose, with a strike ^" 

about northeast and a dip of about 45° to the northwest. According 

to Prof. C. W. Hall, of the U-niversity of Minnesota, who collected the 

specimen, the outcrop is isolated, being separated from all others by a 

distance of some two or three hundred paces. Consequently its relations to the other rocks in the vicinity can not be made out.

Very little can be learned from an inspection of the hand specimen. -/ 

From the ease with which thin slabs may be broken from it we are 

justified in supposing it to be schistose, and this supposition is rendered 

certain by an examination of the specimen with a lens, especially on 

slightly weathered surfaces, where black acicular particles maybe seen 

with their long axes lying in the plane along which the rock breaks 

most easily. . . .^

We may, then, describe the rock as a very dark-gray crystalline aggregate with an obscure schistose structure and a sugary texture. Its "*. 

grain is SO tine that but one of its constituents may be recognized with 

any degree of certainty. This is a black mineral with a tinge of brown. 

It is easily cleavable, and it glistens brilliantly from its cleavage surfaces. A flake of this mineral sprung from the specimen with the point ^ 

of a penknife and placed under the microscope is found to have a nearly 

uniaxial interference figure, which, with the other properties above -f* 

mentioned, indicates a biotite. In addition to this there can also be { 

detected a granular white mineral, whose particles are so small that 

their nature can not be determined. The specific gravity of the rock is A 

2.770. .

In thin section, the character of the rock becomes very apparent. It ' 

consists essentially of quartz, plagioclase, and a pyroxene, with biotite, % 

garnet, pyrite, and magnetite as accessory constituents.

In natural light there appears an apparently homogeneous colorless 

groundmass, in which lie large, irregular grains of a very highly refractive, almost colorless, mineral with well-marked cleavages, smaller dark- T" 

 V

' See Map, PI. 29, in The Geology of Minnesota, Vol. I of final report, Minneapolis, 1884, p. 614.
DILLER.] DESCRIPTIONS: NO. 140, QUARTZ-NORITE-GNEISS. 359

brown flakes, small irregular or rounded grains of an opaque substance, 

> and occasionally isolated highly refractive grains of a very light pinkish.

tiiige, or aggregates of these. When examined with a low power, a 

i certain parallelism may be detected in the arrangement of the various 

( minerals (see PI. XLV, JL), though this is not so well marked as in the

case of the mica-schist from the Black Hills (PI. XLII). The schistosity 

V of this rock is beautifully shown in the photograph, where the pyroxene

and biotite grains, and to a less marked degree the plagioclase grains as

 * well, have their long axes running in the same direction. The lightcolored substance without cleavage cracks is quartz, that containing 

dark inclusions, is plagioclase, and the dark inclusions are its decomposition products; the dark-gray grains with heavy cleavage lines are 

pyroxene, and the darker fibrous masses on the edges of these are biotite. In the upper right-hand corner is a large flake of the last-named

»- mineral, and on the right hand side of the center of the picture is a 

grain of magnetite. The supposition grounded upon the macroscopic 

examination of the hand specimen, viz, that the rock is schistose, is 

found to be correct, as is also the implied conclusion that the schistosity is not well marked. 

When examined uuder crossed nicols the apparently homogeneous

 ^ groundmass breaks up into an aggregate composed of very irregularly 

shaped interlocking grains of quartz and feldspar. Both are perfectly 

pellucid in natural light and both contain the same kinds of inclusions. 

They may be distinguished from each other by the brighter polarization colors of the quartz and the twinning striations of the feldspar. 

From the intricate manner in which they interlock it is impossible to

i determine which is the older. Both include small particles of all the 

other components, and therefore they must be younger than these.

r^ They contain, also, tiny liquid inclusions, a few with movable bubbles, 

large quantities of black, red, and green dust, and irregularly outlined 

green and reddish inclosures that appear to be either decomposition 

products of some substance whose nature can no longer be determined 

or secondary infiltration products. The latter supposition seems to be 

more1 plausible, since the character of the inclusions is so varied, Avhile

 V- at the same time they are undoubtedly such as are produced by secondary agencies. 

The twinning bars of the plagioclase are usually parallel, but somei times they appear in two series, crossing each other at nearly right 

angles. The maximum symmetrical extinction of two contiguous

 lamelhB, measured against their line of contact, is about 26°, indij eating a plagioclase somewhere in the labradorite series. The specific 

gravity of the mineral is 2.673. In any given position of the section 

between the nicols in most grains only one portion of the lamella} 

of any series is dark, the other ends of the same lamelkc remaining 

bright. As the section is revolved the dark portions become bright,

 ' the dark zone gradually moving toward what were the bright ends in
366 THE EDUCATIONAL fcEftlES OF feOCK SPECIMENS. [BULL. 150.

the first position. This gradual movement of the dark portion of the 

field from one part of a 'mineral to another is known as undulatory 

extinction. It is supposed in most cases to be due to a strain under 

which the mineral exhibiting it exists. Its occurrence in the present 

instance is probably evidence that the rock was produced by pressure 

from some previously existing rock of a somewhat similar character.

The irregular, highly refractive grains have a slight pleochroism in 

pale greenish and pinkish tints, the latter, when the most prominent 

of the two cleavages, being perpendicular to the vibration plane of the 

lower nicol. The extinction is parallel to the cleavage where only 

one set of cleavage lines is observed, and on sections on which two 

cleavages are seen it bisects the angle between these. The axis of 

elasticity parallel to the single cleavage is smaller than the one at 

right angles to this, and the double refraction is not great. The polarization colors are consequently not brilliant. Since the long axes of 

nearly all the grains run'in about the same direction, which is parallel 

to the plane of schistosity of the rock, and the sections are made 

parallel to this plane, it is not surprising that so few basal sections of 

the mineral can be found. In the one or two that may be discovered 

in each section there are two series of cleavage lines, which make with 

each other angles of about 88° and 92°. No inclined extinctions are 

to be seen in any sections where only one set of cleavage lines appears.

The mineral, which is an orthorhoinbic pyroxene, is usually quite fresh 

and free from inclusions. In sections cut from weathered portions of 

the rock, however, an interesting alteration is observed to be in prog 

ress. The cleavage cracks of the pyroxene are filled with a brown, 

pleochroic substance without definite morphological characteristics. 

This brown substance gradually spreads into the surrounding augite, 

replacing it in part, so that often what at one end is a perfectly fresh, 

almost colorless, pyroxene is at the other end a mass of brown, pleochroic substance. As the amount of the brown substance increases, its 

characteristics become more pronounced, until finally it takes the form 

of biotite. This biotite often lies embedded in augite, often it borders 

large grains of this mineral, and sometimes it occurs in the spaces 

between neighboring grains. There can be no doubt that this biotite 

is an alteration product of the pyroxene.

The greater portion of the biotite in the section is in isolated grains 

between the colorless constituents, quartz and plagioclase.. The long 

axes of its grains are nearly always approximately parallel to the 

long axes of the pyroxene. The mineral is dark brown, almost opaque, 

 in the direction of its single cleavage, and is a brownish yellow at right 

angles to this. The pleochroic halos observed in the micas of other 

rocks are found also in these, but their pleochroism is not very marked. 

It is, of course, impossible to state positively that this biotite has the 

same origin as that found in and near the pyroxene, but since it has
U. S. GEOLOGICAL SURVEY BULLETIN NO. 150 PL. XLV

THIN SECTIONS.

A. Quartz-norite-gneiss from Odessa, Minnesota. 

. Hornblende-gabbro-gneissfrom Minresota Falls, Minnesota
DILLKH]. DESCRIPTIONS: NO. 140, QTJARTZ-NORITE-GNEI8S. 361

exactly the same properties as the secondary mica, we may infer that 

it is probably of the same origin.1

J' The only constituents remaining to be described are the garnets, 

j ^ pyrite, and magnetite. The first mentioned are in almost colorless, very 

( highly refractive grains, without cleavage and without inclusions. 

They are irregular in cross section, but approximate more or less 

closely to circles. They may be easily distinguished from all other constituents, since they are perfectly isotropic between crossed nicols. The

 * large grains are traversed by irregular fracture lines, distinguishable 

from cleavage cracks by their lack of parallelism. The garnets are found 

frequently as inclusions in quartz and feldspar, or in the interstices

*V between these. They are evidently among the earliest formed of the 

rock's components, as the contours of all the others, except pyroxene, 

where they touch the peripheries of the garnet are molded by them. 

The pyr oxene is regarded as older than the garnet, for the reason that 

the contours of the garnet depend somewhat upon those of the

- ' pyroxene for their shape.

The pyrite and magnetite are both irregular, opaque grains that may 

be distinguished from one another by the brassy luster of the former 

k in reflected light and the bluish-black luster of the latter.

-. In an attempt to name the rock we are met with the difficulty always 

experienced in discussing the classification of schistose rocks. No 

systematic scheme for their classification has yet been proposed. It 

would seem best to confine the term schists to foliated aggregates of 

quartz and some bisilicate without feldspar, and the term gneisses to 

^ schistose aggregates of feldspar and some bisilicate with or without 

quartz. According to this distinction our rock is a gneiss, since it consists essentially of plagioclase and pyroxene; i. e., it is a schistose rock 

with the composition of quartz-norite. Roth 2 calls such rocks "zobtenites," while Dathe calls them "flaser-gabbros." 3 Gordon 4 suggests 

that when the origin of a gneiss is known it receive a compound name,

^ the first term of which shall designate the original rock from which the 

gneiss was derived, so that as much knowledge as possible concerning

" x the rock may be comprehended in its name. If we accept this sugges-

<_ tion the name of our rock should be quartz-norite-gneiss. Professor

Hall, who has made a careful study of the rock in the field, thinks

f^- there can be no doubt that it was once a typical member of the gabbro 

family. The microscopic examination confirms his opinion. It shows 

the rock to be a very slightly altered quartz-norite that has undergone

 pressure sufficient to render it schistose.

Norite-gneisses are exceedingly rare, as are also gabbro-gueisses in

, ' Cf. M. E. Wadsworlh, Bull. No. 2, Geol. Survey Minn., p. 65. For literature on alteration of augite 

'** to biotite, see W. S. Bayley, Bull. U. S. Geol. Survey, No. 109, p. 41.

2 Allgeraeine und Chemische Geologic, Bd. II, p. 484. 

' a Zeits d. deutsch geol. Gesell, 1877, p. 327.

«Bull. Geol. Soc. America, vol. 7,1895, p. 122.
362 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

which the pyroxene and plagioclase are perfectly fresh, 1 for the pressure 

that so squeezes a massive rock as to flatten its constituent grains 

produces a condition of affairs very favorable to the alteration of 

pyroxene into horublende. In the present case alteration has been 

effected in the rock, but the change has been from pyroxene into 

biotite.2 Schistose gabbros in which hornblende exists in large quantities have been described in a number of foreign publications, and 

in several published in this country.3 

An analysis of the rock, as reported by H. K Stokes, is here given:

Analysis of quartz-noritc-yneiss from Odessa, Minnesota.

Q\Q., .............................................

A1A ................................ ............

FeA....... .....................................

FoO... .................. '.........................

CaO..............................................

MgO. ............................................

K2 0. .............................................

Na.,0...... ...................... '.................

FeS, .............................................

Total ......................................

Per cent.

01. 04

16.97

5:58

5.99

3.62

.55

1.9G

.43

3.73

99.87

The 1.96 per cent of Na2O corresponds to about 34 per cent of labradorite among the rock's constituents. After making allowance for the 

amount of SiO2 required by the pyroxene, biotite, and garnet, there 

remains over about 20 per cent of the oxide, which must be regarded 

as existing in the rock as quartz.4

No. 141. GARNETIFEROUS HORNBLENDE SCHIST.

(FROM HANOVER, GRAFTON COUNTY, NEW HAMPSHIRE.

BA YLEY. )

DESCRIBED BY W. S.

The specimens of this rock were collected at Hanover, G-rafton 

County, New Hampshire, where it was originally thought by Professor 

Hitchcock 5 to belong with the Lisbon group of schists. In a later 

paper, however, the same author is inclined to regard it as igneous,

'For descriptions of hornblende free gabbroitic gneisses, see accounts of augite-schists, by Wichmann, Geology of Wisconsin, Vol. Ill, 1880, p. 045; and E. D. Irving, ib., Vol. IV, pp. 609 and 694-696; 

and Fifth Annual Eeport U. S. Geol. Survey, 18«5, p. 211.

'See G. H. Williams, Bull. U. S. Geol. Survey N"o. 62, for discussion of effect of pressure upon the 

minerals of the massive rocks.

s Cf. G. H. Williams, Bull. TJ. S. Geol. Survey ]ST o. 28, and description of specimens Nos. 109'and 143 

in this bulletin.

4 This rock will be further described in a paper now in preparation by C. W. Hall, designed for 

publication as a bulletin of the Ifnited States Geological Survey.

6 C.H. Hitchcock, Geology of the Connecticut Valley District: Geol. of New Hampshire, Vol.11, 

p. 363. Concord, 1877.
NO. 141, GARNETIFEROUS HORNBLENDE-SCHIST. 363

since it occurs in "bunches, varying in size from a peck measure to a 

mass 10 miles long. ... On the northwest side of the principal 

range, the schist comes successively in contact with mica-schist, hydro- 

- 4 mica-schist, argillite, and chlorite-schist, all of which have been altered 

\ through heat into vitrified and indurated rocks, usually richer in silica 

than when unaltered. On the southeast side the adjacent rock is 

... . invariably mica-schist, somewhat indurated. . . . The present attitude of the igneous hornblende is like that of the modern laccolite 

where the cap has been worn away." 1

All specimens of the rock.are schistose, and some are banded with 

alternating, narrow, dark and light bands. The rock as a whole has 

a dark greenish-gray color, which is produced by large numbers of 

slender black needles of hornblende embedded in a fine-grained white 

or gray matrix. The bands of the banded specimens are portions of 

the rock in which these two substances alternately predominate. In 

the thicker of the lighter layers the white material is quartz, that 

seems to have been introduced along joint cracks in the rock.2 a

Besides hornblende and quartz, two other minerals are observable 

in the hand specimens garnet and biotite. The former appears as 

little dark-red dodecahedral crystals, and the latter as dark bronzy 

plates with a very brilliant luster. The mica plates and the hornblende 

needles are arranged with their longer axes approximately parallel to 

the banding of the rock, thus accounting for the schistosity in this 

direction.

Under the microscope the rock is discovered to be composed of 

numerous green hornblende prisms and large plates of brown biotite, 

crystals of garnet, and grains of magnetite, in a colorless matrix which 

between crossed nicols is resolved into an aggregate of quartz and 

plagioclase.

The hornblende is usually elongated parallel to the c axis. Occasionally a cross section with the characteristic prismatic cleavage is 

observed, but not frequently. While the majority of the prisms are 

arranged with their longer directions approximately parallel, many of 

the larger crystals lie in a direction inclined to this, thus interfering 

with the perfection of the rock's schistosity. The hornblende is strongly 

pleochroic, as follows: .c, bluish green; fr, yellowish green; a, yellow. 

Its cleavage is fairly well marked parallel to the longer axes of the 

prisms. In addition to this there is a parting, nearly at right angles 

to the cleavage.

The biotite is not found in all sections, though it occurs in greater or 

less quantity in most of them. It is in large flakes with a distinct 

cleavage and a reddish-brown and yellow pleochroism. The extinction 

is nearly zero. The biotite is evidently one of the latest formed of the 

rock's components. It not only incloses needles of hornblende and

 Bull. Geol. Soc. America, V ol. VII, p. 511.

2 Cf. C. E. Van Hise. Principles of North American Pre-Cambrian Geology: Sixteenth Aim. Kept. 

IT. S. Geol. Survey, pp, 662-664,666,667.
364 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

grains of plagioclase and quartz, but its plates occur mainly in the 

spaces between neighboring hornblendes. . (

The garnets appear in thin section as light pink, perfectly isotropic 

bodies, that include a few grains of quartz and some magnetite. Though .^~ 

completely idioinorphic, they were plainly formed after the hornblende / 

and the biotite, as may be seen by the examination of their contacts 

with these minerals. ^.

The magnetite occurs as small irregular masses and grains scattered 

through the colorless groundmass and as comparatively large crystals ' /Lwith an octahedral habit. A noticeable feature in connection with the 4 

crystals is the presence around them of a zone free from hornblende 

and biotite. The iron that is necessary to the production of these * 

latter minerals appears to have been appropriated by the magnetite 

in the manufacture of its crystals.

The matrix in which these constituents lie is a hypidioraorphic gran- rular aggregate of triclinic feldspar and quartz. The latter mineral is 

the more abundant. It occurs in irregular grains of a larger size than 

the feldspar grains. The quartz grains are usually simple Occasionally *- 

they are complex, a single one sometimes breaking up between crossed 

nicols into a number of differently oriented parts. Very rarely a grain 

exhibits the undulatory extinction. Usually the quartz is water clear 4 

and is free from inclusions. In some sections, however, a few grains 

include small groups of colorless prismatic crystals that resemble a zeolite. The strong polarization of the quartz in which they are embedded 

has prevented the accurate determination of their nature.

The feldspar of the matrix is mainly plagioclase; a few grains resem- * 

ble microcline. The feldspars, like the quartz, are in irregular grains 

which show little or no effects of pressure. The majority of the grains 

are crossed by parallel twinning bars with a symmetrical extinction of * 

from 18° to 22° against their line of junction. These are probably 

andesine or labradorite. In the darker bands, where hornblende is 

abundant, the plagioclase contains few if any inclusions; but in some 

portions of the lighter bands, where the plagioclase is most abundant, 

the grains tire full of inclusions of hornblende, biotite, and magnetite. . *" 

The microcline is represented by a few grains only. They are distin- ,_$« 

guished from the plagioclase grains by the two series of twinning bars 

crossing each other at nearly right angles. <

The study of the thin sections throws but little light on the problem 

of the origin of the schist. The garnets, the biotite, and the hornblende are probably secondary, i. e., they are not original secretions * 

from a molten magma. Whether the components of the matrix are ^ 

also secondary or not can not be told. It is impossible to discover, 

therefore, from a study of the specimens in the collection, whether the «.-, 

rock from which the schist was derived was originally a quartz-diorite 

a diabase or a gabbro, From its miueralogical composition it may quite ^ 

safely be inferred that it was originally one of these three types. A
DESCRIPTIONS: NO. 142, STEATITE. 365

chemical analysis of a series of carefully selected specimens of the 

rock would probably show that it was originally igneous, and it might 

determine the type of igneous rock to which it belonged, for Adams 1 

has shown that in all probability sedimentary and igneous schists may 

by this means be distinguished from each other, and Koseubusch 2 has 

proved that a massive rock in its change to a schist suffers little chemical change. Whatever may have been the original nature of the rock it 

is now a hornblende-schist, since it consists largely of hornblende and 

quartz. The structure of the rock from which it was derived has been 

completely lost in the changes which it has suffered iu passing to its 

present condition.

No. 142. STEATITE, OR SOAPSTONE.

(FROM FRANCESTOWN, HILLSBORO COUNTY, NEW HAMPSHIRE. DESCRIBED BY

W. S. BAYLEY.)

Specimens No. 142 is from the quarries of the Francestown Soapstone Company, at Praucestown, New Hampshire. The rock has beeu 

quarried at this place since 1802. It is used largely for the manufacture of stoves, the refuse and waste being sold for packing material. The soapstone is said to occur as a bed in ferruginous and micaceous schists belonging to tho Merrimac group of sediments, which is 

placed by Hitchcock 3 in the Paleozoic.

"The peculiarity of the stone," writes Hitchcock,4 "consists iu the 

uniform distribution through it of spherical radiated aggregations of 

crystalline plates of talc. These make the stone uniformly strong in 

all directions." The rock in thin sections appears to be a fibrous mass, 

and where the fibers do not overlie one another so as to interfere they 

are dark when parallel to the plane of vibration of the light.5

The specimens in the collection are sawn slabs, consequently their 

surfaces do not present the characteristic appearance of the rock as 

seen in nature. On a fresh fracture, such as may be obtained by breaking Off an edge of the block, the rock exhibits an irregular aggregate 

of silvery gray plates, which here and there are grouped into the little 

spherical masses referred to by Hitchcock. Occasionally a small grain 

of a dark metallic mineral, pyrrhotite, may be detected among the talc 

plates, but no other constituents are observable. So far as can be seen 

the rock is massive. There is no evidence of foliation in it.

In thin sections the following components are noticed in addition to 

those mentioned above: chlorite, calcite, biotite, magnetite, quartz,

1 F. D. Adams: Am. Jour. Sci., 3d series, Vol. L, 1895, pp. 58-69.

2 H. lloseulmsch: Zur Auffasung der chemischen Natur des Grundgebirges, Tschermak's Min. und 

Petrog. Mitth., XII, p. 49.

3C. H.Hitchcock, Geology of the Merrimac District, East Part: Geology of New Hampshire, Vol. 

II, Chap. VI, pp. 588-591, Concord, 1877.

4 C. H. Hitchcock, Economic Geology, Pt. V: Geology of New Hampshire, Vol. Ill, p. 87.

5 G. W. Hawes, Mineralogy and Lithology, Pt. IV: Geology of New Hampshire, Vol. Ill, p. 113. 

Concord, 1878.
366 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

limonite, and rutile. The rutile and liinonite are present in very small 

quantity, the former as tiny irregular grains and the latter as little 

crystals disseminated throughout the chlorite and talc. The magnetite 

is also in irregular grains, but they are not evenly scattered over the 

section. They are much more abundant in the vicinity of calcite, some 

grains of this mineral being well filled with them. This fact suggests 

that the iron and the calcite are secondary substances derived from 

some original component by alteration. The quartz is not found in all 

sections. When it is present it occurs as hypidiomorphic grains associated usually with chlorite. From its general appearance and the 

shape of its grains, it appears to be a secondary infiltration. Biotite 

is more abundant than any of the other accessories. Under low powers 

its little plates look like brown.stains on the almost colorless talc and 

chlorite fibers. When examined under high powers the u stains" are 

discovered to be very thin scales of a reddish-brown biotite, with the 

usual properties of this mineral.

The calcite, chlorite, and talc are the essential components. All are 

practically colorless. The calcite is granular and the other two minerals are fibrous. The former appears often as large irregular granules, 

with a higher refractive index than any other of the colorless minerals 

in the rock. It is distinguished by its strongly emphasized rhombohedral cleavage and by its brilliant polarization colors. The most 

usual occurrence of the mineral, however, is iu the form of little veins 

interposed between the folia of the micaceous components and as nests 

filling the spaces between neighboring grains of talc and chlorite.

The two micaceous constituents, talc and chlorite, constitute the 

greater portion of the rock, and the talc is the more abundant. Both 

minerals are found in plates, fibers, and folia. They are both colorless, 

or nearly so, both possess a parallel series of well-marked cleavage 

lines,-and both extinguish parallel to these cleavages. The chlorite is 

slightly pleochroic in very light-green tints, and has a very low double 

refraction. Its interference colors between crossed nicols are gray, 

blue, or white. The talc is nonpleochroic. Its double refraction is 

strong, and its interference colors are in brilliant red and green tints. 

There is no difficulty in distinguishing between the two minerals.

In structure the rock is radially fibrous. The folia of talc and chlorite are rarely straight. They are usually curved and bent to such an 

extent that every plate shows the undulatory extinction. Large bent 

and crumpled folia lie in a felted mass of smaller fibers, which in turn 

are curved and crumpled in the same manner. It is this interlacing of 

fibers rather than the occurrence of spherical aggregates of talc that 

gives strength to the rock.

Nothing can be learned from thin sections of the rock concerning 

its origin, except that, since its components are all secondary, the rock 

itself is probably secondary. There is nothing in the nature of these 

components, however, and nothing in the structure of the rock that
DESCRIPTIONS: NO. 143, GABBRO-DIORITE. 367

suggests the nature of the original. Professor Hitchcock 1 in a recent 

paper regards steatite as an alteration product of material originally 

igneous, and Both 2 declares that generally steatite is a secondary substance produced from minerals that contain but little alumina, especially 

from augite, hornblende, and olivine. The chlorite associated with the 

talc originated from aluminous hornblende and augites. If the original 

rock contained olivine this may have yielded magnetite in its decomposition. If Roth's statement expresses a fact the soapstone of Francestown may be an altered diabase or diorite, containing some olivine. 

The only phenomenon observed in the thin section that may shed some 

light on the question is the accumulation of magnetite and biotite in 

certain areas, which may possibly have once been occupied by olivine.

NO. 143. HORNBLENDE-GAJBBRO-frNEISS (GABBRO-DIORITE). 

(FROM FRANKLIN, BALTIMORE COUNTY, MARYLAND. ABSTRACT BY J. P. IDDLNGS

FROM THE DESCRIPTION HY GEORGE H. WILLIAMS.)

This rock has been described by George H. Williams 3 as a product of 

alteration of gabbros by processes of dynamic metamorphism. The 

rock varies in character from place to place, grading from massive 

gabbro into massive hornbleudic rock in some places. In other places 

the hornblendic rock possesses a well-marked schistose structure.

When thus foliated the rock cleaves with greater or less readiness along one plane, 

which, so far as observed, appears to be parallel to the foliation of the adjacent 

gneisses. Where this structure is most eminently developed, all traces of the gabbro 

disappear, and the diorite has much more the appearance of one of the so-called 

crystalline schists than of a massive rock.-"

The specimens in the Educational Series represent the more schistose 

phase of the gabbro-diorite, and might be described as a somewhat 

feldspathic arnphibolite. The rock is greenish black and distinctly 

crystalline, and consists of hornblende and a subordinate amount of 

feldspar. These are arranged in parallel layers not clearly distinguished 

except in thin section.

The hornblende is the usual green variety common in massive diorite and crystalline schists. It almost never occurs in well-defined crystals, but in confused aggregates of imperfectly developed individuals. In the rock from Franklin, the hornblende is in compact crystals which are perfectly idiomorphic. The extinction 

angle, measured on splinters parallel to the prismatic cleavage plane, gave an average 

 value of about 13°. The pleochroism is very pronounced and of the character usual 

in hornblende, i. e.,' the a ray, nearly parallel to the clinodiagonal axis, is light 

yellow; the b ray, coincident Avith the orthodiagonal, yellowish green; and the c 

ray, dark bluish green. The absorption is here, as in all hornblende, C ^> b ]> ft.

The ordinary hornblende of the diorite is quite free from inclusions, but in certain 

specimens crystals much larger and more compact than the average occur, in which

i Jour. Geol. Vol. Ill, p. 58.

"J. Roth: Allgemeine nnil Cbemisclie Geologie, 'Vol. II, p. 541.

3George H. Williams, The gabbros and associated hornblende rocks occurring in the neighborhood 

of Baltimore, Maryland: Bull. U. S. Geol. Survey No. 28, Washington, 1886, p. 27 et sect4 Loo. cit.,p. 32.
368 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

minute black needles are so abundant as to render the mineral quite opaque, no 

matter how thin the section may be. These needles are short and stout, being often 

so rounded as to present an elliptical outline. They vary considerably in size, but 

even the largest are no*t more than .04 ram in length and .002 min in width. In prismatic 

sections of the hornblende, they are seen to be arranged parallel to the vertical axis, 

and also in two directions, making an angle of about 45° with this. la basal sections 

they appear to follow, for the rnqst part, the prismatic cleavage lines, though they 

are also less frequently disposed in other directions, especially in that parallel to the 

clinodiagoual.

These inclusions furthermore are always central in their arrangement, the periphery 

of the hornblende crystal being free from them. When particularly abundant the 

smaller needles appear to run together, forming irregular, opaque masses, much 

resembling magnetite. That these inclusions are in reality magnetite is indicated 

by the fact that such crystals of hornblende as contain them most abundantly are 

attracted by the magnet, while others are not.

The feldspar of this rock is anorthite, yielding extinction angles on 

the basal plane (001) as high as 22° to 28°, and on the brachypinacoid 

(010) as high as 36°. The chemical composition of the feldspar, as 

determined by W. S. Bayley, in the variety of this rock from Pikesville, 

is as follows:

Analysis of feldspar from hornblende-gabbro-gneiss of Pikesville, Maryland.

<2if\

A1203 . ...........................................

CaO... ...........................................

Na-jO. ...........................................

Total......................................

Specific gravity, 2.74.

Per cent.

45.06

35.69

18.30

100. 00

a By difference.

This corresponds closely to the composition of a feldspar composed

of eight molecules of anorthite to one of albite, An8 Abi. The crystals 

exhibit polysynthetic twinning, and occasionally dust-like inclusions, 

similar to those found in the feldspars in the gabbro from this region 

and elsewhere. The amount of feldspar in this rock is very small, more 

so than in the gabbro specimens.

The subordinate or accessory minerals are very few in number and 

of very small amount. They are colorless apatite, pyrite, sphene, and 

minute crystals of epidpte, and larger ones of irregularly shaped zoisite.
DILLKB.] DESCRIPTIONS: NO. 144, GABBRO-DIORITE. 369

The chemical composition of the gabbro-diorite from near Pikesville, 

Maryland, as determined by Leroy McCay, is given in the following 

analysis:

Analysis of yabbro-diorite from Pikesville, Maryland,

Cif\

Tio2 ............. ............................:...

A12 03 ------ ......................................

FeA ................... .........................

FeO.-... .........................................

MnO.--... .......................................

MgO..--. .............'......-.................-..

PnO

NajO ............................................

K,0.. ............................................

p,os . ....... .....................................

H20........ ......................................

Total ......................................

Sp. gr., 2.996.

Per cent.

46.85

.30

20.02

2.30

10.16

13.84

f.VIPft

.88

100. 27

Its similarity to those of the gabbro of the region has been pointed 

out by Williams.

NO. 144. HOBNBLENDIC GABBRO-GNEISS (GrABBRO-DIORITE).

(FROM MINNESOTA FALLS, YELLOW MEDICINE COUNTY, MINNESOTA. DESCRIBED

BY W. S. BAYLEY.)

Specimen No. 144, like No. 140, was found among the crystalline 

schists of the Minnesota Valley. It occurs at Minnesota Falls of the 

Minnesota River, 600 paces north and 100 west of southeast corner, 

sec. 11, T. 115 N., It. 39 W., Minnesota. 1 In the field it appears as a 

distinctly foliated rock, striking about east and west and dipping north 

at 30°.

The rock is a medium-grained, distinctly crystalline aggregate of a 

white striated plagioclase and a lustrous black mineral with a quite 

perfect cleavage. The cleavage surface is less even than that of mica 

and the cleavage plates that can be sprung from the mineral are less 

thin. The microscope investigation of thin sections shows it to be 

hornblende. In the hand specimen but little evidence of schistosity is 

at first sight apparent; but upon close examination it may be seen that 

the long axes of a majority of the grains lie approximately in the same 

plane, though not necessarily in the same direction in this plane.

A single glance at the thin section reveals a general parallelism in 

the arrangement of the rock's darker constituents (see PI. XLV, B). 

When placed under the microscope most sections show only a coarsegrained aggregate of plagioclase and light green hornblende. A few 

show in addition a very light-colored, highly refractive augite.

1 Sco pi. 28, Geology of Minnesota, vol. I of Finnl Report, 1884, p. 589.

Bull. 130 24 .
370 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

The plagioclase is in very irregular grains that are clear and colorless 

except for certain tiny dust-like inclusions scattered through them. In 

addition to these the plagioclase contains also tiny specks of magnetite 

and small flakes of green hornblende. Under crossed nicols it is seen 

to be crossed by series of twinning lamellae. The majority of these form 

a single series of parallel lines: others form two series, cutting each 

other at angles of about 78°. Some of the lamelUe run entirely across 

the grains; others spring from their edges and wedge out toward their 

centers, while still others spring from the sides of cleavage cracks and 

extend only a short distance into the feldspathic substance. In many 

grains the lamellae have an undulous extinction, different portions of 

the same lamellae extinguishing in different positions of the stage as it 

is revolved. All these phenomena indicate that the feldspars have 

been subjected to pressure. The maximum symmetrical extinction of 

contiguous lamellae is somewhere in the neighborhood of 33°, and the 

density of the mineral is 2.731. It is thus a basic bytownite. Though 

the main mass of the feldspar is fresh, as has been said, the corners of 

grains and the small areas between neighboring ones are often filled 

with a micaceous, brightly polarizing substance in tiuy flakes, forming 

by their accumulation little cloudy masses in and between the fresh 

plagioclase grains. This substance has the peculiarities of kaolin, a 

common alteration of all feldspars, and is undoubtedly this material.

The hornblende is in large masses with a green color and a moderately strong pleochroism. Many of the grains have but one set of 

cleavage lines. The extinction against these varies between parallelism and an inclination of 38°, according as the section is parallel to 

the orthopinacoid or to the clinopinacoid, and the pleochroism is in 

clear-green and yellowish-green tints, the former when the cleavage is 

nearly parallel to the vibration plane of the nicol. Other grains of the 

mineral have two series of cleavage lines making .angles with each 

other that the extinction bisects. Where the series are most distinct 

and most sharply cut the angles made by the two sets are 124° and 56°. 

The pleochroism in these sections is dark green and light greenish 

yellow, the latter when the shorter diagonals of the rhombs made by 

the cleavage are parallel to the principal plane of the nicol. The pleochroism is thus a = greenish yellow, ft and ,c = green. Absorption 

ft- = .c>a. The different individuals of the mineral are very irregular 

in shape and usually four or five aggregate into groups. Most of these 

are simple individuals, but others have two or more different parts in 

the twinned relation to each other, as may be attested by examining 

the section between crossed nicols, when the different parts of the same 

grain extinguish in opposite directions, and the polarization colors are 

different. The only inclusions noted in the hornblende are little grains 

of magnetite, small dust particles, and in certain sections masses of 

a light-colored, highly refractive mineral.

The latter substance is found only in the interior of hornblende
DILLER.J- DESCRIPTIONS: NO. 144, GABBRO-DIORITE. 371

masses, surrounded on all sides by the green hornbleudic material. It 

lias a very high index of refraction and is not pleochroic. It is either 

colorless or some very light shade of green. Under favorable conditions 

it may be seen to be crossed by two series of cleavage cracks making 

angles of nearly 90° with each other. The extinction bisects these 

angles, and the polarization colors are very brilliant. This augite, for 

such the mineral is, is not always sharply separated from the surrounding hornblende. It gradually assumes a greenish tinge on its 

edges. This color deepens as the distance from the interior of the 

augite increases, and the substance acquires more or less pleochroisin, 

until it finally becomes indistinguishable from the surrounding hornblende. This relation of the two minerals indicates that the hornblende 

has been derived in some way from the augite. This change of augite 

to green hornblende is known as uralitization, and the variety of green 

hornblende so produced is called uralite. Uralitization is especially 

characteristic of rocks that have undergone dynamic metamorphism.1

Occasionally on the edges of hornblende grains, and especially where 

these are in contact with feldspar, may be found small reddish brown 

flakes of strongly pleochroic biotite, sometimes extending with frayed 

ends far into the hornblende. It is present in small quantity, either 

between plagioclase and hornblende, as described, or between neighboring hornblende grains. Its origin is probably secondary, and its 

cause a reaction between the hornblendic and plagioclastic materials.2

The essential components of the rock as it at present exists are 

plagioclase and hornblende. The latter is evidently-an alteration 

product of augite, and the plagioclase is bytownite. Both minerals are 

allotriomorphically developed, but the rock has been given a gneissoid 

structure by pressure. If we reconstruct the predecessor of the gneiss 

from the material at hand, remembering that the hornblende represents 

an original augite, we shall have a granular aggregate of plagioclase 

and augite, i. e., a gabbro. Since the rock is a gneiss which has been 

derived from a gabbro, we must call it a gabbro-gneiss, if we adopt the 

nomenclature suggested by Dr. Gordon.3 Moreover, since its characteristic ferromagnesian component is hornblende, its more distinctive 

name is hornblendic gabbro-gneiss.4 Dr. Williams 5 has called the 

rocks of this character that occur so widely in the Baltimore region 

gabbro-diorites, but since these rocks at present are neither gabbros 

nor diorites the name is open to grave objections.

The hornblendic gabbro-gneiss illustrates the usual form taken

 ' : 

'See G. H. AVilliaius, Bull. U. S. Geol. Survey No. 62,p. 52, for general discussion of uralitization 

and the literature of subject.

2 The alteration of hornblende into biotite is not a common phenomenon, though by no means 

unknown; cf. G. H. Williams: Bull. U. S. Geol. Survey No. 62, p. 182.

3 See description of No. 140, p. 358.

4 The name hornblende-gabbro-gneiss describes a gneiss derived from hornblende-gabbro, i. o., a 

gabbro-gueiss in which the hornblende is original. In the rock under discussion the hornblende is 

regarded as secondary, hence the uso of the adjective hornblendic, which signifies merely that the 

gneiss contains hornblende.

6 Bull. U. S. Geol. Survey No. 28, pp. 17,27-itt.
372 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

by a gabbro when subjected to dynamical rnetainorphism, in which case 

the augite usually changes to hornblende. In the less common case 

the augite changes to biotite, and since the latter mineral contains but 

35 to 40 per cent of SiO2 , while the former contains about 53 to 54 per 

cent, there is left over a residue of silica, which may crystallize as 

quartz, yielding a quartz-biotite-augite phase of gabbro-gneiss. 1

The density of the rock is 2.935, and its composition, as found by 

Dr. Stokes, is given below in column I. The figures iu column II 

represent the composition of the-hypersthene-gabbro of the Baltimore 

area.2 Upon comparison it will be found that the schistose rock from 

Minnesota is almost identical, chemically, with the massive eruptive 

from Maryland.'

Analysis of gabbro-gneiss and hypersthene-gabbro.

SiOz ...................................

AlgO....... ...... .......................

FeA...... ............................

FeO..... ...............................

CaO ....................................

HgO....... ..................... .......

K20 ...................................

Na2O ..................................

Total............................

I.

48.29

20.87

i 13

4.93

14.32

7.54

.38

1.77

.89

100.12

II.

46.85

19. 72

% 99

7.99

13.10

7.75

.09

1.56

.56

100. 84

The hypidiomorphic texture and the schistose structure of the 

rock are both well brought out in Fig. J5, PI. XLV. The plagioclase 

appears as large white areas, and the hornblende as darker ones, crossed 

by parallel lines marking the cleavages of the mineral. In the center 

to the left is a core of augite, whose dark color and broad, irregular 

cracks are indications of the great amount of alteration suffered by 

the mineral.

No. 145. SERPENTINE.

(FROM GREENVILLE, PLUMAS COUNTY, CALIFORNIA. DESCRIBED BY J. S. DILLER.)

Serpentine is a green or greenish, hydrous silicate of magnesia. It 

is of common occurrence, forming large masses, chiefly among more or 

less altered rocks.

The serpentine of Greenville, California, forms an irregular mass 

bounded on the one hand by slates and on the other by quartz-porphyry 

and granite, through which it appears to have erupted. It is a typical, 

yellowish-green serpentine, and shows well the slickeusides due to 

internal movement of the mass. Adjoining parts are made smooth by 

rubbing against each other. The slickensides give the serpentine a 

schistose appearance.

1 See description of No. 140 in this bulletin, p. 358.

2 GK H. Williams: Bull. U. S. Geol. Survey No. 28, p. 39.
DILLBR.] DESCRIPTIONS: NO. 145, SERPENTINE. ' 373

Small fragments of the serpentine are readily picked up by a magnet, showing the presence of magnetite. In a thin section under the 

microscope the serpentine is seen to be irregularly fibrous. The fibers 

have parallel extinction in conformity to their orthorhoinbic character. 

Most of them lie in the same general direction as the slickensiding. 

In places the fiber's are so closely compacted in parallel position as to 

be optically continuous, giving bright yellow colors between crossed 

nicols. Generally, however, the interference colors do not rise above 

yellowish gray. There are no distinct indications of the mesh or grate 

structures, characterizing respectively serpentine derived from olivine 

and from hornblende, but rather the structure resembles the bar 

structure, indicating derivation from pyroxene. The movement of the 

mass has been so great as to obliterate much of the original secondary 

structure.

Magnetite is abundant in small grains distributed throughout the 

mass, but often arranged in streams and elongated patches parallel to 

the fibrous structure of the serpentine.

Besides magnetite, there are numerous yellowish-brown irregular 

isotropic grains, without definite crystallographic outlines. They are 

always less abundant than the magnetite, and although quite generally 

distributed throughout the section, they are not equally abundant in 

all portions. Considering the associations, and their optical properties, as well as their chemical reactions observed by Mr. Melville, these 

particles are regarded as chromite.

This serpentine, where deeply cut by the creek descending from 

Bound Valley reservoir, contains nodular masses ranging from 1£ feet 

to 8 feet in diameter. They are enveloped by the schistose serpentine. 

Upon the outside these nodules have a dark border of serpentine and 

magnetite. The interior of the nodules vary in color; the smaller ones 

are gray and appear to be composed of pyroxene altered to serpentine 

and chlorite. The larger nodules have a yellowish, very heavy interior, 

 which appears to be pyroxene, as in the other case, but is much less 

altered. It has a rather feebly pronounced fibrous structure, with a 

l.irge angle of extinction. Its outlines are entirely irregular and show 

no characteristic cleavage. In acid it shows no trace of gelatinization.

These nodules give a clue to the original composition of the rock 

from which the serpentine was derived, for the serpentine in the nodules, 

associated with the remnants of pyroxene, from which it was derived, 

has essentially the same structure as that in the slickensided serpentine, the difference being that in the slickensided serpentine the bars 

have been made more generally parallel by motion within the mass. 

This view is corroborated by the fact that upon the borders of the serpentine mass it passes into a rock, which in its outward appearance 

looks like gabbro with the pyroxene and feldspar distinctly marked. 

In the thin section, however, it is found that both are highly altered, 

the feldspar to a saussuritic mass and the pyroxene to serpentine with
374 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

the bar structure well marked. Considerable of the original fibrous 

pyroxene is still present.

The analysis of the serpentine (No. 145) was made by W. H. Melville, 

in the chemical laboratory of the United States Geological Survey, and 

is as follows:

Analysis of serpentine from Greenville, California.

Si02 .............................................

A1203 - -------- ............. ......................

Fe-jOs-. ........... ......................:........

FeO .............................................

' CaO .............................................

MgO... ..........................................

K20 .............................................

Na2 0 ............................................

Total ......................................

Per cent.

12.70

39.14

2 08

4.27

2; 04

39.84

0.11

100. 18

Serpentine when compact may be easily carved and will usually 

receive a high polish. When of good color it is used for ornaments and 

interior decorations. This is especially true when the serpentine occurs 

mixed with limestone, forming verde antique, which is frequently used 

as. a decorative marble.

No. 146. MAGNETITE.

(FROM PORT HENKY, ESSEX COUNTY, NEW YORK. DESCRIBED BY W. S. BAYI.KY.)

No. 146 is a specimen of one of the magnetite ores from the Mineville 

group of mines in Moriah Township, Essex County, New York. The

rock is mainly a coarsely granular aggregate of heavy, black, lustrous 

magnetite, which constitutes the larger portion of the specimen, a 

smaller proportion of reddish-brown grains of apatite, and a very few 

grains of a white substance that effervesces with acid. The latter i.s 

evidently calcite. When the thin section is viewed against a background of white paper, a fourth component is detected in the shape of 

irregular grains of a dark green color. These when examined under 

the microscope are discovered to be pyroxene.

The specimen is a good example of the magnetite ores formerly so 

extensively worked in New York. According to the Report of the 

Eleventh Census, the quantity of this ore mined in 1889 was 927,26!) 

tons. 1 Since this time the quantity has materially decreased, in consequence of the increased production of hematite in the Lake Superior 

region, until in 1894 it amounted to only 196,436 tons. 2 Samples of

1 Report on the Mineral Industries of the United States at the Eleventh Census, 1890. Washington, 

1892, p. 7. Cf. also plate opp., p. 22. 

8 Sixteenth Ann. Kept., TJ. S. Geol. Survey, Pt. Ill, p. 200.
^ DILLEB.] DESCRIPTIONS: NO. 146, MAGNETITE. 375

-f* the ore taken from the stock piles at the shipping docks of the Port 

, Henry Ore and Iron Company, at Port Henry, New York, upon analysis 

yielded: Fe,.61.39 per cent; P, 1.496 per cent; TiO2 , small percentage. 1 

^ The ore is non-Bessemer. According to Kemp 2 it occurs as irregular 

I flattened lenses that are usually associated with gneisses. In general

*) the long axes of the lenses run parallel to the foliation in the gnei'sses. 

These latter rocks 3 are aggregates of quartz, orthoclase, microline, 

pyroxene, biotite, hornblende, and microperthite. Near the ore bodies 

the rock is darker than elsewhere. It here consists essentially of

; K plagioclase, green hornblende, green augite, and a little magnetite and 

sphene. Above the gneisses and younger than them is a series of crys-

 v talliue limestones together with gneissoid and schistose rocks, among

t which black garnetiferous and horublendic varieties are most prominent. There is no marked stratigraphic break between the gneisses 

and the overlying series of limestones and schists, both of which may 

correspond to the Grenville series of Canada, regarded by Adams 4 and 

the other Canadian geologists as upper Laurentian and by Van Hise 5 

as Algonkian.

Ore occurs in both the gneissic and the limestone series, that represented by the specimen, as already stated, being in the former. The

,5 gneisses are so much contorted that no structure has yet been made out 

for them and their contained ore bodies. As for the origin of the ores, 

this also is obscure. Kemp 6 states that "no segregative agency, strictly 

so called, could have occasioned them, and some other method of origin 

must be invoked." But what this method is he does not state.

^ Under the microscope the thin sections of the ore show the presence 

of magnetite, green pyroxene, apatite, calcite, and pyrite. Tbe magnetite and pyrite are both opaque. They may easily be distinguished

1 by the blue-black color of the former and the brassy color of the latter 

in incident light. The magnetite is in large irregular grains in the 

spaces between which are triangular areas of pyrite.

The apatite is in large, colorless grains with rounded outlines. It 

possesses an indistinct cleavage parallel to which is the extinction. 

The brown color of the mineral as seen in the hand specimen is due to

.^ the dissemination through it of thousands of small inclusions, among 

which are numerous plates of hematite that may be recognized by

* . their red color. Among the other inclusions may be seen grains of 

magnetite, liquid inclosures, and hundreds of tiny specks whose nature 

can not be determined. They maybe included together as "dust."

1 Iron Ores of the United States: Report of tlie Tenth Census, Vc.1. XV, p. 112.

^i 2 J. F. Itenip, Crystalline limestones, ophicalcites and associated schists of the Eastern Adirondacks: Bull. Gool. Soc. America, VoL VI, 1895, p. 241.

3 J. F. Kemp, The Geology of Moriah and Westport Townships, Essex County, New York: Bull. 

^ N. Y. State Museum, Vol. Ill, No. 14, September, 1895, p. 325.

" F. D. Adams, On the typical Laureutlan area of Canada: Jour. Geol., Vol. 1,1893, p. 325. 

 ^ r> C. R. Van Hise, Principles of North American Pre-Cambrian Geology : Sixteenth Ann. Kept. U. S.

Geol. Survey. Washington, 1894-90, Pt. I, p. 707. 

> «Bull. N.Y. State Museum, Vol. Ill, No. 4, p. 342. "
376 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

From their arrangement parallel to the cleavage it may be assumed 

that some of them are of secondary origin.

The pyroxene is a very compact, green, variety, which in thick section 

is almost opaque. In thin section it exhibits a well-marked pleochroism 

in emerald-green and grass-green tints. Because of its pleochroism 

this augite at first sight resembles hornblende. It may, however, easily 

be distinguished from this mineral by the rectangular cleavage in cross 

sections and by the high extinction angles (over 40°) observed in longitudinal sections. As a rule, the mineral is in.irregular grains, which 

may surround grains of apatite, but occasionally it is in partially idiomorphic crystals, when it is apparently surrounded by magnetite. The 

order of crystallization in the rock appears to be apatite, augite, magnetite, pyrite. All the irregular grains of augite are crossed by ramifying veins of white calcite, whose directions appear to be quite independent of the direction of the mineral cleavages. They were probably 

formed by infiltration after the remainder of the rock had assumed its 

present condition.

The composition of the ore suggests the possibility of its origin by 

secretion from some eruptive magma, like the titaniferous ores associated with the gabbros of the Lake Champlain district in New York 1 

and those on the periphery of the great gabbro mass in northeastern 

Minnesota.2 No positive evidence on this question is yet available. 

The New York magnetites are similar in many respects to the magnetites that have been so long mined in New Jersey.

RESIDUAL ROCKS.

' No. 147. EESIDUAL SAND OP GKANITE.

(FROM DISTRICT OF COLUMBIA, ABOUT 3 MILES NORTHWEST OF WASHINGTON. 

DESCRIBED BY G. P. MERRILL.)

Rock-weathering is a purely superficial phenomenon, brought about 

through agencies that are in part physical and in part chemical in their 

nature. The chief physical agencies are expansion and contraction 

among the mineral particles owing to temperature changes, and the 

expansion and consequent rending action of absorbed water passing 

into the condition of ice. The chemical agencies are those comprised in 

the processes of oxidation, hydration, and solution, though many minor 

reactions doubtless take place. Hydration without loss of materials 

involves expansion, and hence an incidental physical agency is brought 

in play.

Kock-weathering manifests itself in a variety of ways, much depending on the character of the rock masses and the prevailing climatic 

conditions.

J J. F. Keinp: Bull. N. T. State Mus., "Vol. Ill, No. U, p. 351.

2W. S. Bay.ley, The peripheral phases of the.great gabbro mass of northeastern Minnesota: Jour. 

Geol., Vol. II, pp. 816-825.
DILLEB.] NO. 147, RESIDUAL SAND OF GRANITE. 377

The ultimate product of extreme weathering of any but purely quartzose rocks is almost invariably a highly ferruginous clay. This for the 

simple reason that of all substances forming any appreciable portion of 

rocks, the aluminous silicates, free silica, and iron oxides are the more 

refractory, and hence are left to accumulate as residuary materials, 

while the more soluble constituents are removed in solution. The proportional amount of material actually lost by leaching in the reduction 

of a rock mass to the condition of a soil, or residual clay, is necessarily 

widely variable. Among calcareous rocks it may amount to even 

upward of 99 per cent. Among siliceous rocks, however, the amount 

of soluble matter is vastly less, and the amount lost by purely chemical 

agencies rarely exceeds 50 per cent.1

The series of weathered products here described have been selected 

with a view of illustrating not merely the characteristic form of weathering, but also the fact that among granular crystalline rocks, as the 

granite from the District of Columbia and the diabase from Medford, 

Massachusetts, the beginnings of weathering are more in the nature of 

disintegration than decomposition. These two cases are also of further 

interest in that we are apparently enabled to set a time limit to the 

weathering, the first mentioned being judged as largely post-Cretaceous 

and the last post-Glacial.

Materials of the nature here described are subject to considerable 

variation, and the descriptions given must be accepted as descriptive 

of the general 'results produced rather than as definitely applicable to 

any particular sample.

The fresh rock, illustrated by the small fragment in the box with the 

residual sand, No. 147, is a strongly foliated micaceous granite, the 

foliation being secondary and due to dynamic causes. In its fresh 

state, it will be observed, it consists of a finely granular aggregate of 

quartz and feldspars arranged in imperfect lenticular masses from 2 to 

5mm jn diameter, about and through which are distributed the abundant 

folia of black mica. In the thin section the structure is cataclastic; 

that is to say, the various minerals of which it is composed have, since 

their formation, been more or less shattered and distorted through 

dynamic agencies acting upon the rock mass of which they form a part. 

Quartz and black mica are the most prominent constituents, though 

there are abundant feldspars of both potash and soda lime varieties. 

In addition to these are abundant accessory epidotes, a few small 

apatites, flakes of white mica (sericite), and widely scattering black 

tourmalines, with the usual sprinkling of iron ores.

Within a space of 15 vertical feet, as'displayed in the road cuttings, 

this rock passes from its fresh and unchanged condition into that of a 

sandy soil, upon which trees, shrubs, and a variety of flowering plants 

are growing. The change is quite gradual, the rock becoming first

Tor a more detniled discussion of the subject, see Rocks, Book-weathering, and Soils, by George P. 

Merrill, The Macmillau Company, New York, 1897.
378 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

browned through the oxidation of the mica and ultimately losing its 

rock-like character altogether on the immediate surface. The zone of 

disintegrated material is rarely more than 2 or 3 feet in depth at the 

point where these samples were collected, though where the material 

has not been subject to erosion it has accumulated to depths of upward 

of 50 feet.

The residuary material fi$o. 147), when submitted to mechanical 

separation, yielded results as below:

Mechanical analysis of residual sand of granite.

silt ...... ...... . .

Sand...................................

Sand...................................

Sand...................................

Sand...................................

Total ............................

Percent.

4.25

6.50

11. 25

3.75

11.00

23.50

29.50

10. 25

100. 00

Diameter 

of largest 

grain..

mm . 

0.10

.18

.25

.65

1.00

i. 50

8.00

The coarser of these particles, like the gravel and the coarse sand, 

are of a compound nature, being aggregates of quartz and feldspar, 

with small amounts of mica and other minerals. In the finer material, 

on the other hand, the process of disaggregation has gone so far as to 

largely free each individual from its associates, excepting of cour.-e in 

the case of microscopic inclusions, which could be liberated only by a 

complete.disintegration of the host itself. These particles, as seen 

under the microscope, are all sharply angular and in many cases quite 

fresh and uudecomposed. The mica shows the greatest amount of 

alteration, the change consisting mainly in an oxidation of its ferruginous constituent, whereby the folia become stained and reduced to 

yellowish-brown shreds. The feldspars are in some cases opaque 

through kaolinization, but in others are still fresh and unchanged even 

in the smallest particles. The finest silt, when treated with a dilute 

acid to remove the iron stains, shows the remaining granules of quartz, 

feldspar, and epidote beautifully fresh, and with sharp angular borders, 

the mica being, however, almost, completely decolorized, and resembling 

sericite more than biotite.

The chemical changes which have taken place in the process of 

breaking down, or weathering, are shown in the following table, in 

which the original analyses have been recalculated on a basis of 100. 

In columns 3, 4, and 5 are given .calculations showing the percentage

r

V-
NO. 118, RESIDUAL SAND OP DIABASE. 379

V

amounts of the entire rock and of each of the original constituents 

removed by the leaching action of water during the transformation.

Analyses of fresh and disintegrated granite from the District of Columb'u.

Constituents.

Si02 .......................

Al,0....... ................

Ti02 ... ....................

FeA- .................

FoO .......................

CaO .......................

MgO...... .................

Na-jO ......................

K,0 .......................

P,0«...... .................

Total................

Percentage of 

t'resli 

granite.

69.61

14.39

3.61

3.22

2.45

2.71

2.68

0.10

1.23

100. 00

Percentage of 

residual 

sand.

65.84

15. 20

0.31

4.40

2.64

2.65

2.12

2.00

0.06

4.72

Percentage loss 

for entire 

rock.

10.' 50

0.46

0.00

0.81

0.036

0.77

0.85

0.04

13. 4C6

Percentage of 

each constituent 

saved.

85.11

96.77

". 100.00

74.79

98.51

71.38

68.02

60.00

a 100. 00

Percentage of 

eacli constituent 

lost.

14.89

3.23

0.00

25.21

1.49

28.62

31.98

40.00

0.00

a Gain.

Detailed information, together with an illustration showing the 

locality from which, these samples were selected, may be found in 

Bulletin of the Geological Society of America, Vol. VI, 1895, pp. 

321-332, and in Bocks, Eock-weathering and Soils, The Macmillau Co., 

New York, 1897.

No. 148. KESIDUAL SAND OF DIABASE.

(FROM MEDFORD, MIDDLESEX COUNTY, MASSACHUSETTS. DESCRIBED IJY G. P.

MERRILL.)

The fresh rock from which this sand was derived, and which is illustrated by a small fragment in the box, is a coarse, almost granular, 

dark-gray aggregate of plagioclase feldspar, augite, and black mica, 

with orthoclase, apatite, magnetite, and ilmeuitein smaller proportions, 

and, as a rule, visible only with the microscope. As secondary products occur hornblende, chlorite, quartz, calcite, leucoxene, pyrite, and 

a soda-bearing zeolite.

The rock breaks down, giving rise to the brown gravel shown in the 

box, the change in color being due to the oxidation of the iron of the 

ferruginous silicates. In the natural outcrops all stages of weathering 

are to be seen, sound bowlder-like masses with only a slight amount of 

superficial disintegration lying still in place embedded in the brown 

residual sand. The material analyzed as fresh rock, and tabulated 

below, was obtained from, the interior of one of these bowlders. The
380 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

residuary sand when submitted to mechanical analysis yielded results 

as below:

Mechanical analysis of residual sand of diabase.

3

4 

5

6

7 

8 

9

in

11

Name.

Medium sand ......:..............

Very fine sand .................... 

Silt............................... 

Fine silt .......................... 

Clay..............................

Total........................

Diameter of particles.

0.5 to 0.25 '"...... ......... 

0.25 to 0.1"""......... .......

0.1 to0.05mm . .............. 

0.05 to 0.01" ............... 

0.01 to0.005""» ............. 

0.005 to 0.0001""" ............

Per cent.

42.3

20.66

12.72

9. 37 

4.97

4.18 

1.13 

0.37 

1.67

0.66

1.73

99.76

Concerning these separations, it should be stated, those represented 

by Nos. 1 and 2 are plainly, even to the unaided eye, of a compound 

nature, and easily recognizable as diabase derivatives, though somewhat discolored by iron oxides.

No. 3 shows particles of feldspar, augite, and mica fairly well disaggregated, though even here many of the granules are compound. No. 

4 differs mainly in being finer and of a lighter color, while No. 5, of a 

rich mahogany brown color, appears to the-unaided eye to be composed 

mainly of mica scales. The microscope, however, shows it to contain 

numerous badly stained but quite fresh feldspathic particles and cleavage flakes of augite. No. G, the particles of which lie between 0.1 and 

u.05mm in diameter, shows also only minute flecks of inica recognizable 

macroscopically, but contains both feldspathic and augitic particles 

like No. 5, while Nos. 7 and 8 are deep ocherous-browu silts, offering no 

distinctive features to the unaided eye, and No. 9 would p.iss for a lightbrown ocher. The material analyzed as silt (columns 5, 6, and 7 below) 

is the equivalent of numbers 7, 8, and 9 of this series.
DILLEH.] NO. 148, RESIDUAL SAND OF DIABASE. 381

The chemical changes which have taken place, together with cnlculations made as before, are shown in the accompanying tables:

Analyses of fresh, and disintegrated diabase from Medford.

Constituents.

_ rin HC1 ^

Sl °4»._Co>"- 

A103 ....... .......

Fe,0s 

T?t*f\

CaO ...............

MgO. ...... ........

MnO...... .........

KjO ...............

PA---- ...........

Fresh diabase.

1. 

Bulk 

analysis.

47.28

20.22 

3.66 

8.89 

7.09 

3.17 

0.77 

2.16 

3.94 

0.68 

2.73

100. 59

2. 

Analysis 

of portion 

soluble in 

HC1 and 

Na,Co3 .

i 1.19 

i 9.66 

4.74

} 10. 91

3.09 

2.20 

not (let. 

1.21 

0.50 

not det. 

2.73

36.23

Disintegrated diabase.

3. 

Bulk 

analysis.

| 44. 44 

23.19 

12.70

6.03 

2.82 

0.52 

1.75 

3.93 

0.70 

3.73

99.81

4. 

Analysis 

of portion 

soluble in 

HC1 and 

Na.fCo3.

, 0.85 

i 8.65 

4.86

10.00

1.50 

1.84 

not det. 

0.68 

0.17 

not det. 

3.73

32. 28

Silt from disintegrated diabiiae, numbers 7, 8, and 9 of 

table, on p. 380.

5. 

Per cent 

soluble in 

HC1 and 

^a-iCo^.

0.47 

22.63 

21.98

12.83

3.32 

3.23 

not det. 

1.30 

0.90 

not det. 

10.86

77.52

6. 

Per cent 

insoluble

HCl'and 

NogG'Os.

} 13. 51

1 5.88

0.12 

0.79 

not det. 

0.52 

1.24 

not dut. 

0.11

22. 17

7. 

Total.

36.61

40. 08

3.44 

4.02 

not det. 

1.82 

2.14

10.97

99.08

Calculated loss of material.

Constituents.

Si02 ...................................

A1A - - - - 

Fe,o3 - .................................

FeO ...................................

CaO ...................................

MgO.............................:.....

MnO....................................

KoO...... ..............................

2sTa20 ..................................

P20fi ...... .............................

Recalculated on 

basis of 100.

8. 

Fresh diabase.

47.01 

20.11 

3.63 

8.83 

7.06 

3.15 

0.77 

2.14 

3.91 

0.68 

2.71

100. 00

9. 

Decomposed 

diabase.

44.51 

23. 24

} 12. 71

6.04 

2.85 

0.52 

1.75 

3.94 

0.70 

3.74

100. 00

10. 

Percentage loss 

foriMitire 

rock.

8.48 

0.00

2.42

1.83 

0.68 

0.32 

0.62 

0.50 

0.08 

a 0. 53

14.93

11. 

Percentage of 

each constituent 

saved.

81.97 

100. 00

81.90

74.11 

78.30 

58.43 

70.85 

87.17 

88.61 

6 100. 00

12. 

Pe.rcentago of 

each constituent 

lost.

18.03 

0.00

18.10

25.89 

21.70 

41.57 

29.15 

12.83 

11.39 

0.00

a Gain.

b The calculation gives 119.49 per cent, showing that a gain in volatile matter is to be expected.

From these tables it appears that the process of decomposition has 

gone on more extensively than was the case with the granite described 

above, though the mechanical disintegration is not as great. This is
382 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BOLL. 150.

due to the fact that soda-lime feldspars are more readily attacked 'than 

the potash variety, and also to the abundance of iron-rich silicates, as 

black mica, augite, and hornblendes, which are also susceptible.

It is evident that the decomposition has been accompanied by a leaching process, since, as shown in column 10, some 14.93 per cent of the 

entire rock mass has been thus carried away.

Of the material classed as silt in columns 5, 6, and 7, or as silt, and 

clay in the table on.p. 380, and which constitutes only some 3.17 per cent 

of the entire residue, 77.87 per cent is soluble in dilute hydrochloric 

acid and sodium carbonate solutions. The insoluble portion, constituting 22.13 per cent of the silt, consists of unaltered feldspar, and iron, 

lime, and inagnesiau silicates, which are easily recognizable under the 

microscope in minute, sharply angular particles.

More detailed information concerning this rock and the residual 

products of its weathering may be found in the following publications: 

On the petrographic characters of a dike of diabase in the Boston 

Basin, by W. H. Hobbs: Bull. Museum of Comparative Zoology, Vol. 

XVI, No. 1, 1888; and Disintegration and decomposition of diabase at 

Medford, Massachusetts, by George P. Merrill: Bull. Geological Society 

of America, Vol. VII, 1896, pp. 349-362.

No. 149. BESIDTJAL CLAY OF FELDSPATBIC EOCK. KAOLIN.

(FROM HOCKESSIN, NEWCASTLE COUNTY, DELAWARE. DESCRIBED BY G. P.

MERRILL.)

The kaolin deposits of northern Delaware, like those of Delaware 

County, Pennsylvania, are the results of the decomposition of highly 

feldspathic gneisses, and to a less extent of felclspathic conglomerates, 

themselves derived from the gneisses. As both gneisses and conglomerates are here standing practically on edge, it follows that from the 

same pit may be mined materials derived from both sources. The 

materials sent out in this series are not intended to contain any derived 

from the conglomerate.

The gneisses carry in addition to quartz and feldspar both white 

and black mica. As the feldspars decompose the black mica also 

yields to the decomposing agencies, and the resultant clay, from both 

minerals, is variously stained and colored by the oxidation of the iron. 

Where the mica has been very abundant the staining has gone on so 

extensively as to ruin the clay for high-grade ware. Sample 149 shows 

the material as taken from the pits. Before it was broken the gneissoid 

structure was still in some cases discernible. 

The kaolin used in the manufacture of pottery is obtained from this 

crude material by a process of washing, whereby the larger fragments 

of quartz and still undecomposed feldspars and a portion of the mica are 

removed. The purest kaolin, as obtained by this process, still carries 

particles of feldspar in all stages of decomposition and many minute
 DILLBB.] NO. 149, RESIDUAL CLAY OF FELDSPATHIC ROCK. 383

shreds of white mica and a little quartz. The individual particles of 

this kaolin are extremely irregular in outline, and so far as observed 

never show the rhombic and rhomboidal crystal outlines characteristic 

of true kaolinlte. The crude material, when washed in water, yields 

many fragments of quartz, which are badly shattered and extremely 

irregular in outline, but which rarely show ttie clear glass-like fracture 

and brilliant polarization colors so characteristic of either vein quartz 

or the quartzes of granites and gneisses. On the contrary, the surfaces are dull, and by reflected light whitish, or glassy only in spots, the 

appearance being that of quartzes etched by alkaline solutions, and 

which in this case was doubtless produced by the action of the carbonates of potash, formed during the process of feldspathic decomposition. 

A mechanical analysis of this washed kaolin, made under the direction of Milton Whitney, of the Department of Agriculture, yielded 

results as below:

Mechanical analysis of washed kaolin.

Name.

Silt..........................

Fine silt. ....................

Diameter of 

particles.

mm. 

2 tol

1 to 0. 5

0. 5 to 0. 25

0.25 to 0.1

0. 1 to 0. 05

0.05 to 0.01

0.05 to 0.005

0. 005 to 0. 0001

Per cent.

0.00

0.00

0.00

0.00

0.00

30.86

7.31

47.78

0.40

11.40

A chemical analysis of this same washed kaolin yielded results as 

below. Although these results are approximately like those of kaolins 

as given by Dana (System of Mineralogy, Gth ed., 1803), the microscope 

shows the material to be far from uniform in composition but to still 

carry abundant crystalline particles, as quartz granules and shreds of 

still undecomposed feldspars. It is to the presence of this free quartz 

and the undecomposed silicates that is due the high total percentage of 

silica and the small amounts of lime, magnesia, and the alkalies shown.

The more striking features brought out by the analysis are the 

almost entire disappearance of the alkalies and the assumption of over 

12 per cent of water.
384 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULLJSO.

Analysis of kaolin.

Si02 . .............................................

Ti02 .... .........................................

A12 03 .. ..........................................

Fe>.2 03 . ........................ ...................

MnO.. ...........................................

CaO. .............................................

MgO.............................................

 KtO. .............................................

Na20. ............................................

H2 0 at 100°. .....................................

H2 0 at 100° +....................................

T> n

Total .......................................

Per cent.

48.73

.17

37.02

.79

.16

.11

.41

.04

.52

12.83

.03

100. 81

No. 150. EESIDUAL CLAY OF LIMESTONE. 

(FROM STAUNTON, AUGUSTA COUNTY, VIRGINIA. DESCRIBED BY G. P. MERRILL.)

The fresh rock is a dense, dark-gray, highly siliceous magnesian 

limestone, showing under the microscope a very fine granular aggregate of calcite granules and rhombs, but which are so muddied by 

included impurities as to be scarcely transparent, and of a lead-gray 

color.

The granules are not in all cases contiguous, but at times separated 

by opaque films of argillaceous matter. They never show the twin 

structure so characteristic of the metamorphic limestones and marbles.

The decomposition of the stone is almost purely chemical and consists in a removal, by leaching, of the calcium carbonate, leaving the 

less soluble silica., silicates, and iron oxides to accumulate in the form, 

of a deep-red, highly plastic clay, which on drying becomes so indurated as to be broken only with a hammer.

So abundant is the iron oxide that the residual material is stained 

almost beyond recognition, and it is only when it is first boiled in 

dilute hydrochloric acid to remove the iron that it can be studied at 

all satisfactorily. When thus treated and submitted to microscopic 

examinations, it is found to consist mainly of very irregularly rounded 

and angular quartz fragments, which are more or less corroded and 

unmistakably of clastic origin; i. e., they existed in the limestone, not 

in the form of particles crystallized in place, but as mechanically 

included detritus formed from the breaking clown of preexisting siliceous rocks. Particles of feldspars, some of which show twin bandings, are also present, and more rarely are found shreds of white 

and black mica, chlorite, epidote (?), and very rarely a minute but 

very perfectly preserved, doubly terminated, colorless crystal with 

forms characteristic of rutile. Both quartzes and feldspars are rough
DILLEH.] NO. 151, SPHEROIDAL WEATHERING IN IGNEOUS ROCKS. 385

and corroded, though even the plagioclase feldspars are still in many 

cases sufficiently fresh to show twin stride.

Analyses of the fresh rock and its residual clay, together with the 

portions soluble and insoluble in hydrochloric acid and sodium carbonate solutions, made by George Steiger, of the Geological Survey, 

gave results as tabulated below :

Analyses of fresh siliceous limestone and its residual clay, from near Staunton, Virginia.

Si02 .............

TiO». ............

A12O3 ...... ......

FeA------------

FeO .............

MnO- ............

CaO..............

MgO....... ......

K20...... ........

Na/) ............

Waterloo ......

Water 100+ ..... 

Pi\

coa .... ..........

Total......

Fresh limestone.

Iiisoluble.

Per cent. 

6.98 

.09 

1.39 

.25 

none, 

none. 

.04 

.15 

.91 

.04 

unclct. 

.15 

none, 

none, 

undet.

10.00

No. 1. 

Soluble.

Per cent. 

0.39 

none. 

.53 

.04 

.63 

none. 

28.39 

18.15 

.18 

.05 

undet. 

.34 

.03 

41.85 

undet.

90.58

Total.

Per cent. 

7.37 

.09 

1 92 

.29 

.63 

none. 

28.39 

18.30 

1.09 

.09 

.09 

.49 

.03 

41.85 

undet.

100. 63

Residual clay.

Insoluble.

Per cent. 

52.81 

.1C 

15. 9C 

1.05 

.09 

none. 

.20 

.75 

4.51 

.03 

vindet. 

4.42 

.06 

none.

80.04

No. 2. 

Soluble.

Per cent. 

3.09 

.04 

3.96 

6.25 

.30 

none. 

.30 

.43 

.28 

.20 

undet. 

2.10 

.04 

.38

Total.

Per cent. 

55. 90 

.20 

19.92 

7.30 

.39 

none. 

.50 

1.18 

4.79 

.23 

2.54 

6.52 

.10 

.38

i

17.37 99.95

SiOi dissolved by first treating with HC1 (1-10) on water bath for one hour, then with a 5 per cent 

solution of NajCOa; No, 1, 1.09 per cent; No. 2, 4.7,'i per cent.

1 ILLUSTRATIONS OF SURFACE MODIFICATIONS.

No. 151. SPHEROIDAL WEATHERING IN IGNEOUS BOCKS.

(FROM ROSEBUKG, DOUGLAS COUNTY, OREGON. DESCRIBED BY J. S. DILLER.)

All rocks exposed upon the surface of the earth are subject to 

ineteorologic changes which tend to their destruction. The change 

of material under these conditions has been fully considered by Mr. 

Merrill in connection with the residual rocks, Nos. 147 to 150. It 

remains to notice some of the morphological features developed in 

the course of this rock destruction.

The development of the larger topographic features incident to the 

degradation of the land, the carving of canyons, plateaus, and mountains, and, finally, the reduction of the whole to featureless plains, 

are subjects wholly within a department of geology which takes little 

cognizance of the kinds of rocks, and may not be discussed here; but 

there .are certain minor surface features developed in the larger process 

Bull. 150 25
386 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL.ISO.

that can be illustrated by hand specimens. The first of these to be 

noted is the spheroidal weathering of igneous rocks.

This peculiar feature is illustrated in PI. XL VI. 1 The ledge is coinposed of an igneous rock which, when it cooled and solidified, cracked 

and became permeated by numerous fissures. The rain water circulates through these fissures, and all portions of the rock above drainage 

level are exposed essentially to the weathering conditions of the surface. Fissures in rocks may originate in other ways than by contraction on the loss of heat. They may be developed by earth movements, 

and to this class probably belong the fissures in the figure above. 

They were produced long after the rock solidified.

Owing more especially to the solvent action of the water circulating 

in the fissures, each block bounded by them is exposed to external 

attack of meteorologic agents from all sides. The attack of the decomposing and disintegrating influences, since they act upon or through 

the exposed surface, must be proportional to the extent of that surface. The sphere has a smaller extent of surface in proportion to its 

volume than any other form, and is therefore the most stable form 

under external attack. The angular fragments of an exposed igneous 

rock are subject to severer attack than the same volume would receive 

if spherical. The angles afford a special opportunity for attack, and 

as a result are weathered off more rapidly than the other portions, 

thus reducing the angular blocks to spheroidal ones. As the weathering proceeds from without and depends upon the oscillating surface 

conditions, the effect is concentric, and the mass is divided into a series 

of shells which, under conditions affording transportation, break away, 

exposing the spherical surface, as seen in Plate XLYI.

Specimen 15 L is a nodule.frorn an exposure of weathered igneous rock 

near Eoseburg, Oregon. It is diabase, whose eruption occurred probably during the Eocene. Large masses of it are exposed in the Roseburg region, and most of them, where deeply weathered, show the 

development of spheroidal forms to a greater or less extent. While 

such forms in the Eoseburg diabase are generally developed by weathering along fissures, as already explained, it is possible that in some 

localities of the same region they may have resulted from the manner 

of its eruption,2 as explained by F. Leslie Eansome for spheroidal 

structure in similar rocks near San Francisco.

This sort of weathering of igneous rocks, although most commonly 

exhibited by basic eruptives, basalts, and diabases, occurs also in 

felsites, ns noted by Geikie,3 and in granites, good examples of which 

have recently been pointed out by Mr. C. E. Keyes.4

These spheroids are sometimes of large size, and by the gradual 

erosion of the surrounding rock they are left upon the surface as

1 Plate copied from Fifteenth Annual Report U. S. Geol. Survey, PI. XL III.

2 Eruptive Rocks of Point Bonita: Bull. University of California, Vol. I, pp. 110-113,

3 Text Book of Geology, 3d pd.'tiou, p. H91.

4 Fifteenth Ann. Kept. U. S. Geol. Survey, p. 725.
SPHEROIDAL WEATHERING IN GRANITE NEAR WOODSTO
DILLER.] DIFFERENTIAL WEATHERING OF IMPURE LIMESTONE. 387

bowlders of disintegration. Such bowlders are common in many 

regions, especially of granular igneous rocks.

The problem presented by nodules resulting from external attack 

has been mathematically solved in all its relations by Dr. George F. 

Becker 1 and applied not only to the spheroidal weathering of rocks, 

but also to the formation of pebbles.

No. 152. SPHEROIDAL WEATHERING IN SHALE.

(FROM DRY CREEK, 8 HAST A COUNTY, CALIFORNIA. DESCRIBED BY J. S. DILLEK.)

This shale is of lower Cretaceous age, and is exposed along the shallow 

canyon of a stream flowing from the Coast Range to the Sacramento. It 

dips eastward at a considerable angle, and is penetrated by numerous 

fissures, which give the water ready access to its mass. The spheroids 

are generally indistinct in the bluff, but when the shale IS dug out, its 

spheroidal fragments are often a conspicuous feature, and specimens 

such as No. 35'J are abundant. Their origin is essentially the same as 

that of specimen No. 151.

The concentric shell or "onion" structure is well marked, although not 

perfectly developed. It is not equally evident everywhere in the large 

mass of Cretaceous shale in Shasta and Teh am a counties. Some of the 

layers of these nodules contain a trace of carbonate of lime, and in 

pi noes calcareous concretionary masses are abundant. They occasionally contain fossils.

No. L~>3. DIFFERENTIAL WEATHERING OF IMPURE LIMESTONE.

(FKOM GALENA, Jo DAVIESS COUNTY, ILLINOIS. DESCRIBED BY J. S. DILLER.)

The two cases preceding illustrate the comparatively uniform effect 

of weathering upon essentially homogeneous material, where it gives 

rise to a nearly even surface; but in the case now to be noted the 

weathering develops surface differences, and may therefore be called 

differential.

The weathered surface of this limestone is very irregular. Thi s irregularity is apparently due, in part at least, to the impurity of the limestone itself, which contains, according to Mr. Steiger, 1.07 per cent of 

insoluble matter, irregularly distributed throughout the mass, rendering it unequally sensitive to the attack of circulating waters, and thus 

giving rise to the inequalities of surface. The surface, however, is not 

smooth, and has no evident direct relation to drainage.

A surface of this kind, on a large scale, is illustrated in PI. XLVII,2 

which shows a massive bed of lower Cambrian limestone, eroded irregularly by weathering beneath the soil. This irregular erosion by solution may be due to the unequal attack of the agents, rather than irregularity in the composition of the limestone. That the latter had little

1 M on. TJ. S. Geol. Survey, Vol. XIII, p. 68.

2 Plate copied from Bull. U. S. Geol. Suivey No. 134 (PI. IV).
388 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL. 150.

influence is indicated by the fact that the limestones generally, whether 

pure or not, have an irregular weathered surface beneath the soil. This 

is a conspicuous feature of some of the limestones laid bare by hydraulic 

mining upon the western slope of tbe Sierra Nevada.

No. 154. DIFFERENTIAL WEATHERING, FLUTED LIMESTONE.

(FROM BAIRD, SHASTA COUNTY, CALIFORNIA. DESCRIBED BY J. S. DILLER.)

This limestone is exposed in large masses, making a mountain ridge 

along the McOloud Eiver. At many points its surface bears peculiar 

small ridges and intervening depressions." In the field it is evident 

that the little valleys are lines of drainage, and it may readily be concluded that they are a form of rain erosion due to the solvent action 

of the water running over the Surface. An examination of many ridges 

and valleys discloses no differences in composition or structure to which 

tlie variations in surface form may be due. From the fact that the 

flutings are drainage lines, it appears that they may be referred to the 

rilling of the water on the surface of the limestone. Where the water 

flows the solvent actiou is most intense, and a minute valley is formed 

by solution.^ This fprm of differential weathering is common upon 

limestone exposures, but is rarely so well developed as in the examples 

from which specimen 154 was collected.

No. 155. GLACIATED BOCK.

(FROM ROCHESTER, NEW YORK. DESCRIBED BY G. K. GILBERT.)

Rock surfaces which have been crossed by a glacier and thereby 

polished, striated, or furrowed, are said to be glaciated.

In the description of specimens Nos. 2 and 9 some account is given of 

the action of glaciers and of the formations they deposit, and mention is 

also made of the great ice sheet which in Pleistocene time invaded the 

United States. The rock fragments, coarse and tine, embedded in a 

glacier are included chiefly in its lower part, so that as the ice moves 

over the rock there is friction between the bowlders, pebbles, sand 

grains, and clay particles above and the bed rock beneath. Although 

the motion is exceedingly slow, amounting ordinarily to only a few feet 

in twenty-four hours, the pressure exerted by the thick ice bed is so 

great that this friction has an appreciable effect, and the rocks are 

worn away. The fine mud tends to smooth the rock, and the result of 

its action is a beautifully polished surface marked by fine parallel 

striaj. The sand grains scratch the surface and must be efficient in 

wearing it away. Pebbles and bowlders of hard material sometimes 

act like engravers' tools, plowing furrows in the rock beneath. When 

the ice finally melts away, as it has melted from the North American 

region, there remains a rock surface which is in general beautifully 

polished, but is also scratched, furrowed, and otherwise sculptured.
U. S. GEOLOGICAL SURVEY 

IRREGULAR WEATHERED SURFACE OF CAMBRIAN LIMESTONE 
DILLEE.] DESCRIPTIONS: NO. 156. DESERT VARNISH. 389

The character of the sculpture depends also in part on the nature of 

the bed rock. If that is homogeneous it may be reduced to an even 

surface or carved in smooth forms outlined by sweeping curves; if it 

contains limited masses which are very hard, these may be left prominent, and in such cases a ridge of the softer rock is apt to be preserved on the lee side of the hard rock, and a U-shaped furrow carved 

about the front and sides. This feature is illustrated by fig. 18, photographed from a slab of limestone, containing a lump of chert.

When glaciated rock surfaces are exposed to the weather they gradually lose their polish and other characters. In the tract once covered 

by the great American ice sheet the best illustrations are found where 

the bowlder clay, which protects the rock surface from the weather, 

has been freshly removed, as, for example, in preparing rock for quarrying. Specimen 155 was obtained in this way. The rock is limestone 

belonging to the Niagara formation, and had been planed by the ice so

FIG. 18. Glaciated rock surface showing frontal and lateral grooves caused by a projecting nodule of

harder rock.

as to constitute an even floor on which the bowlder clay rested. The 

ice seems to have changed its direction of motion from time to time, 

for besides the principal set of parallel striiB there are others crossing 

them obliquely. The direction of the principal strife is in this case 

south 50° west.

The subject of rock glaciation is fully discussed in Rock Scorin-gs 

of the Great Ice Invasions, by T. C. Ohamberliu, Seventh Annual 

Report, United States Geological Survey.

No. 156. DESEKT VARNISH.

(FROM TOOKLE VALLEY, TOOELE COUNTY, UTAH, DESCRIBED BY G. P. MERRILL.)

The tendency of weathering is not always toward immediate disintegration. In many instances, and particularly among siliceous fragmental rocks, exposure to weather results in the formation on the imrne-
390 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BULL 150.

diate surface of a thin coating of a more refractory nature, which for , 

a time protects the material from further disintegration. This induration, it is assumed, is brought about through the deposition of matter 

held in solution by ground water, which is brought to the surface by 

capillarity and there evaporated. The same action, even when not 

attended by induration, may result in a superficial discoloration, as in 

the pebbles under discussion. These pebbles are of quartzite, rounded 

by wave action, and now, by the drying up of the lake, left exposed to 

the atmosphere. Mr. Gilbert describes their origin and occurrence as 

follows: In Tooele Valley the Provo shore-line contours alluvial cones 

descending from the mountains at the east and west and extended so 

as to meet one another at the south. The valley is open at the north 

and northerly winds must have brought powerful waves. These drifted 

the alluvial material southward along both sides of the valley and built 

curved bars about the head. As there was no lateral escape for the 

water driven up the bay by the wind, the undertow was powerful, carrying off not only mud and sand, but gravel, and leaving in the bars 

stones ranging in diameter from 3 or 4 inches to more than 1 foot. 

This coarse shingle is in places so devoid of finer material that it is 

barren of vegetation, and these barren tracts best exhibit the dark > 

coating on the bowlders.

Examined in thin sections under the microscope, the bowlders are 

found to consist essentially of quartz granules, but they carry in addition occasional grains of inicrocline, a soda lime feldspar and a considerable amount of interstitial calcite and disseminated earthy and ferru- J 

ginous matter. There is also an occasional fleck of colorless mica. 

The zone of discoloration is quite thin, rarely extending inward for a 

distance of more than 2""", the color being much more intense on the 

immediate surface.

When thin sections for microscopic study are cut so as to show portions of both the desert varnish and interior portions, the variation in 

color is found to be so slight as to be almost imperceptible, and on 

casual inspection there seems to be no essential difference in composition such as should account for the darker color shown in the hand y, 

specimen. Closer inspection shows, however, that the interstitial calcite of the interior of the pebbles is quite lacking in the discolored outer 

portions, and, further, that the cavities left by its removal in solution are 

often lined with dark, opaque amorphous matter. The discolored portion, when tested chemically, reacts for iron and slightly for manganese. Heated in a closed tube, it yields water and gives off a very faint y 

empyreumatic odor. Material from the interior portion of the pebbles, 

when treated Avith dilute acid, effervesces briskly, the solution reacting 

also for iron and manganese, but much more faintly than does the discolored portion. It is evident that the exterior coloring of the "desert 

varnish" is due mainly to a local segregation of oxide of iron with a 

little manganese and organic matter. The cause of such segregation
DILLKB.] DESCRIPTIONS: NO. 156, DESERT VARNISH. 391

is not, however, altogether apparent. In such cases it is often difficult 

to decide what, among certain phenomena, may be merely incidental 

and what maybe causative. All things considered, it seems safe to 

assume that this local discoloration is due to a superficial segregation 

of the metallic contents of the quartzite in a state of higher oxidation, 

the iron, originally in form of a ca'rbonate, becoming converted into a 

hydrated oxide, while the lime carbonate itself was removed in solution. The small amount of organic matter may have been added from 

external sources from the waters of the original lake.

The formation of the bars from which these pebbles were collected 

is described by Mr. Gilbert in his report on Lake Bonueville (Monographs United States Geological Survey, Vol. I).

INDEX.

Page. 

Acid rocks, definition of .................. 50

Acmite. characters of..................... 41

Actinolite, characters of........ ....... 41

JEgirite, characters of .................... 41

Agglomerated structure, nature of ...... 14

Albite, analyses of .................. 202,206,325

character and occurrence of.......... 45,46

Albite-schist, description of ............ 325-327

Algous vegetation (dried) analysis of.... 92

Allanite, characters of.................... 43

Allotriomorphic minerals, nature of..... l(i

Allotriomorphic quartz, nature of....... 35

A morphous marl, description of........ 135-130

Amorphousminerals, nature of.......... 28

Amorphous structure, nature of......... 10

Ainphibole, characters of................'. 41-43

Amygdaloid, .description of............. 355-357

Amygdaloidalstructure, nature of....... 15

Amygdules, nature of .................... 15

Analcite, characters of ................... 33

Analyses, albite ..................... 202.2U6,325

algous vegetation (dried)............. 92

andesine................................ 218.

andesites..................... 221,223,227,228

andesitic tuff.......................... 212

anorthite ............................... 368

aporhy elite............................ 348

augite.................................. 204

basalt (dolerite)....................... 204

basalt (quartz-)........................ 253

bytownite ............................. 280

camptonite............................ 241

cancrinite............. ............... 203

cementrock........................... 134

chiastolite-schist...................... 342

clay .............................. 383,384,385

clay slate.......:..................... 320,342

cryptoperthite ...'................. ... 195

dacite................................ 215,218

dacite-pqrphyry........................ 230

diabase.^.......-.................. 272,273,275

diallage.--..-........................ 277,280

diopside................ ................ 188

diorite ........................... 242,244,342

dolerite................................ 204

dolomite............................... 301

feldspars............................... 164,

166,195,202,206,207,218,270,280,325,308 

garnetiferous gabbro ................. 280

gneiss.................................. 302

granite.......................^......... 166

graywacke............................. 87

hornblende-andesito.................. 223

Page.

Analyses, hornblende-diorite-porphyry.. 232

hornblende-mica-andesite ............. 221

Hummelstown sandstone ............. 77

hydraulic cement rock................ 134

hypersthene-andesite ............... 227,228

hypersthene........................... 281

kaolin.................................. 384

kryptoperthite........................ 195

labradorite ...........................: 270

lapilli.................................. 249

lepidomelane .........."............... 203

limestone.............................. 128

liparite................................. 102

marble............................... 299,301

metarhyolite .......................... 169

mica-schist ........*................... 342

microcline.............................. 207

microperthite......................... 100

minette................................. 238

nephelite............................... 202

nephelite-syenite.................... 208,211

nevadite ............................... 104

obsidian.................. ............. 15L

olivine ....................:.........._. 298

olivine-diabase ........................ 275

oolite................................... 97

orendite................................ 190

orthoclase .............................. 207

peridotite.............................. 290

.perlite(rhyolitic)...................... 153

phlogopite............................. 188

phonolite .........^.................... 193

phyllite................................ 320

plagioclases............................ 276

porphyritic theralite.................. 201

porphyry (dacite-) .................. 232,230

porphyry (hornblende-diorite-)...... 232

Potsdam sandstone........ ............ 80

pumice(rhyolitic)..................... 149

pyroxene .............................. 209

pyroxenite............................. 288

quartz-basalt.......................... 253

quartz-mica-diorite ................... 342

quartz-norite-gneiss .................. 362

residual clay of f ejdspathic rock.... 383,384

residual clay of limestone . .......... 385

residual sand of diabase .............. 381

residual sand of granite .. ........... 379

rhyolite----..--........................ 147

rhyolitic obsidian...................... 151

rhyoliticperlite....................... 153

rhyolitic pumice....................... 149

sandstone.............................. 77,80

393
394 INDEX.

Page. 

Analyses, sanidine ........................ 164

saxonite ............................... 298

schists ........................... 320,331,342

serpentine............................. 374

shale................................... 90

slates.....:....................... 313,320,343

sodalite ................................ 204

syenite............. ............. 185,208,211

theralite............................... 201

trachyte............................... 182

travertine.............................. 101

tuff (andesitic)........................ 212

volcanic dust.......................... 147

zircon.-.........-..--.................. 204

Analysis of rocks, methods of ..'.......... 18-27

Andalusite, characters of................. 37-38

Andesite, plate showing dike of--........ 146

(hornblende-), description of....... 221-223

(hornblende-mica-), description of. 219-221 

(hornblende-pyroxene-), description

of................................. 223-224

(hypersthene-), description of...... 224-228

(pyroxene), description of------.--. 228-231

Andesitic tuff, description and analysis

of................................. 211-213

Anhedral crystals of porphyritic quartz,

characters of ...................... 35

Anhedral grains of quartz, features of... 16 

Anomite, characters ot.......^............ 42

Anorthite, analysis of.--.---.-.-..-....... 368

characters of........................... 45

Anorthoclase, character and occurrence

of................................... 45,46

Anthracite coal, description of......... 144-145 '

Anticline and syncline, features of .... 315-316

plate showing.......................... 316

Apatite, characters of.--....--.--.-..-...- 36

Aphanitic structure, nature of........... 16

Aporhyolite, description and analyses

of......'............ ............ 343-349

plates showing thin sections of.... 346,348

Arf vedsonite, characters of............... 41

Augite, analysis of........................ 264

characters of.......................... 40-41

Banded sandstone, description and analysis of...........-..........----..--- 80-81

Banded structure,nature of.............. 14,15

Basalt, characters of...................... 51-52

description and analysis-of. 254-255,261-264 

(cellular), description of.............. 252

olivine nodule from ................. 258-261

(quartz-), description of............ 252-254

Basalt tuff, description of.........:..... 251-252

Bascom, F., descriptions of rocks by... 343-349 

Basic rocks, definition of.................. 50

Bastite, characters of....................-- 39

Bayley, W. S., analyses by.......... 206,280,368

descriptions of rocks by _........_...- 84-87,

201-209,274-278,282-286,302-305, 

308-313,317-323,327-331,333-337, 

355-367,369-372,374-376. 

Beach gravel, description of... ........ 56-58

Beach sand, description of..............-. 59-61

Bedded structure, plate showing -..-....- 14

Berea grit, description of.................. 75-77

Biotite, character and distribution of.... 42-43

Biotite-bearing hornblende-granite, description of...................... 179-181

Pajra 

Biotite-gneiss (schistose) description

of. ............................... 332-333

Biotite-granite, description and analyses of....'...............-......... 172-177

(hornblende-bearing), descri p t i o n

of................................. 177-179

Bituminous coal, description of........ 143-144

Bowl der clay, description of.............. 69-70

Bowlder conglomerate, characters of.... 71

Breccia, description of-------------------- 72-74

Brecciated Devonian limestone, figure of. 73 

Brecciated structure, nature of-.......... 14

Brick clay, description of................. 68-69

Brown sandstone, description of......... 77-78

Brush, G. H., quoted on geodes......... 112-113

Bytownite of hypersthene gabbro, analysis of............................... 280

Calcareous concretion, description of.. 107-108 

Calcite, characters of .............. ...... 35-36

Calcite and quartz vein, description of .. 93-94 

Camptonite, description and analysis of. 239-241 

Canada-balsam-shellac mixture for cementing thin sections to glass 

slides, formula for................. 24

Cancrinite, analysis of .................... 203

characters of .......................... 37

Cannel coal, description of 4 .............. 141-143

Cashaqua shale, description of ........... 87-89

Cave deposits, plates showing............ 98,100

Cavernous structure, nature of .......... 15

Cellular basalt, description of............ 252

Cellular structure, character of.......... 252

Cement rock (hydraulic,), description and

analysis of....................... 133-135

Chalk,description and analysis of...... 115-119

Chatard, T. M., chemical analyses by.... 218

Chemical analyses. See Analyses.

Chert, d escription of.................... 124-126

Cherty limestone, description of....... 123-124

Chiastolite, characters of................. 37-38

Chiastolite-schist, description and analysis of ............................. 339-343'

Chlorite, characters of... .............. 43

Chlorite-phyllite, description of........ 321-323

Chromite, characters of............... _. 31

Chrysotile, characters of.........'......... 46^7

Cinder Cone, California, plate showing... 248 

Clarke; F. W.,chemical analyses by...... 202,

203,204,298 

Clastic structure,.nature of .............. 13-14

Clay, plate showing deposit of............ 68

(bowlder),description of.....--....... 69-70

(brick),description of................. 63-69

(residual, of feldspathic rock), description and analyses of....... 382-384

(residual, of limestone), description

and analyses of.................. 384-385

Clay slate, analyses of................... 320,343

Claystone,description of................. 107-108

Cleavage (slaty), nature of................ 18

Coal (anthracite), description of....... 144-145

(bituminous), description of......... 143-144

(cannel), description of............. 141-143

Cockeysville marble, description and

analysis of ...................... 300-301

Columnar jointing, description of-.-... 256-258 

nature of .............................. 18
INDEX. 395

Page. 

Columnar jointing,plate showing........ 20

Compact limestone, description and analysis of..........--...... . - 127-132

Compact structure, nature of............ 14,16

Concretion (calcareous), description of. 107-108 

(ferruginous), description of....... 108-110

(marcasite), description and figures

of................................. 110-111

Concretionary structure, nature of...... 17

Conglomerate, description of..----------- 70-72

description of faulted pebble of...- 316-317 

(metamorphic), description of------ 323-325

Conglomerated structure, nature of-...- 14 

Contact metamorphic rocks, analysis of. - 342 

Coquina, description of.--...-.........- 121-122

Cortlandite, description of ............. 294-297

Country rock, definition of............... 94

Cretaceous conglomerate, description of

faulted pebble of................ 311-317

Cross, Whitman, descriptions of rocks

by.................................162-164,

181-194,224-227,231-233,241-243,201-264 

Cross bedding,nature of.................. 14

plate showing ......................... 14

Crumpled shale, description of ........ 315-316

Cryptoclastic structure, nature ?f...... 14

Cryptocrystalline structure, uatuic. of .1 16

Crystalline limestone, description of .... 298

Crystalline minerals, nature of........... . 28

Crystalline structure, nature of ...... 14,15-16

Crystallites, nature of .................... 17

Crystallization, degrees of................ 28

Dacite, descriptions and analyses of .... 213-219

plate showing nodules in.............. 218

plate showing thin section of......... 214

Dacite-porphyry, description and analysis of ............... ............. 233-236

Damourite, characters of ................. 42

Desert varnish, description of.......... 389-390

Devitrification, nature of................. 15

Diabase, description and analysis of ... 264-273

description of residual sand of ..... 379-382

(oliyine-) description and analysis 

of..................... ........... 274-278

Diabase amygdaloid, description of .... 355-357

Diallage, analysis of...................... 277,280

characters of ......................'.... 40-41

Diatom earth, description of ........... 136-137

Differential weathering, impure limestone............................. 387-388

fluted limestone....................... 388

Dike, description of..................... 145-146

plate showing ......................... 146

Diller, J. S., descriptions of rocks by.... 56-58,

59-69,70-84,87-91, 93-99,102-140,145-146,

211-215,217-219, 221-223,227-228,245-254,

290-294, 297-298, 299-301,372-374,385-388

Diopside, characters of.................... 40-41

analyses of............................. 188

Diorite, descriptions and analyses of .... 241-

244,342

Diorite-andesite family of rocks, characters of.............................. - 51

descriptions of specimens of........ 211-244

Dolerite, description and analysis of... 261-264 

Dolomite, characters of................... 36

description and analysis of.......... 300-301

Page; 

Dune sand, description of............ ^..... 61-63

Dust (volcanic), description and analysis

of.-.. ........................... 146-148

Eakins, L. G., analyses by................. 90,

182,185,208,238,241,242,255,264 

Electro-magnetic method of analysis of

rocks, description of .............. 22-23

Eleolite-hornblende-syenite, description

and analyses of................... 194-197

Eleolite-syenite, descriptions and analyses of.....'... .................... 201-211

Enstatite, characters of................... 38-39

Eolia'n erosion, plate showing......... 62

Eolian sand, description of...... .......... 61453

Epidote, characters of..................... 43

Epidol e-mica-gneiss, description of..... 353-355

Extrusive rocks, definition of............. 49

Fault breccia, characters of............... 73-74

Faulted pebble of Cretaceous conglomerate, description of............. 316-317

Faults, features of......................... 316

Fayalite, characters of __......'._.......... 39

Feldspar, figure showing banded grain of. 79 

figure showing arborescent growth

of.............................. ..' 158

Feldspars, analyses of..................... 166,

195,202,206,207,218,280,325,368 

(monoclinic) character and occurrence of ........................ 44-45

(triclinic) character and occurrence

of............ ..........J......... 45-46

Feldspathic rock, residual clay of...... 382-384

Ferruginous (fossiliferous) concretion,

description of.................... 108-liO

Flint, description .of..................... 119-121

plate showing nodule of .............. 120

Flinty structure, nature of.........-.---. 14

Flow structure, nature of -.._.......... 14-15

Fluidal structure, nature of.............. 14-15

Fluorite, characters of.................... 31

Fluted limestone, differential weathering

of......--.-..............;....... 388

Fluxion structure, nature of.............. 15

Folds, featuresof ......................... 316

Foliated structure, nature of............. 18

Fossil trees, plate showing............... 114

Fossils from marine sand, description of. 64-65

Fossiliferous iron ore, description of .. 138-140

plate showing.......................... 138

Fossiliferous sandstone, description of.. 83-84 

Fragmental structure, nature of....--... 13-14

Friction breccia, characters of............ 73-74

Fusibility, scale of........................ 30

Gabbro, characters of...................... 51-52

description .......................... 278-282

(garnetiferous), description and analysis of ............................ 282-286

Gabbro-basalt family of rocks, characters of.................. ............ 51-52

descriptions of specimens of........ 245-286

Gabbro-diorite, descriptions and analyses of............................. 367-372

Gangue, definition of............._...... 94

Garnetiferous gabbro, description and

analysis of ....................... 282-286

plate showing thin section of......... 278

Garnetiferous hornblende-schist, description of............................ 362-365
396 INDEX.

Garnets, characters of...----..-..--.-.--. 31-32

table showing varieties of...----.-..-. 31

Geode, character of ........._........... 17-18

description of........................ 111-113

Geyser.plate showing..................... 92

Gilbert, G. K.. descriptions of rocks by.. 58-59,

69-70,388-389 

quoted on occurence of concretions of

marcasite.......................... 110

Girty, G. H., quoted on fossils contained

in chert .... ....^................... 125

quoted on fossils of chal k............. 116

quoted on fossils of oolitic limestone ................-..-----.---. 103-104

Glacial gravel, description of. ........... 58-59

Glaciated rock, description of.......... 388-389

figure showing......................... 389

Glass (volcanic), figure showing fragments of........................... 212

Glassy structure, nature of............... 15

Glauconite, characters of................. 47,63

Glaucophane, characters of.......--.-..-. 41

Globigerina, figure of..................... 116

Globigerina ooze, figure of................ 117

Globulites, nature of...................... 17

Gneiss, features of ........................ 18

(biotite), description of............. 332-333

(granitoid), description of .......... 349-353

(quartz-norite). description and analyses of ................... 1;...... 358-362

Granite, description and analyses of

specimens of................. 170-181,339

(biotite), description of............. 172-179

(graphic), figure showing pegmatitic

structure of........................ 17

(hornblende), description of........ 179-181

residual sand of................--.... 376-379

Granite-rhyolite family ot rocks, characters of.............................. 51

descriptions Of specimens of ....... 146-181

Granitic structure, nature of............. 16

Granitoid gneiss, description of ........ 349-353

Granitoid structure, nature of........... 16

Granophyrio structure, nature of........ 16-17

Granular structure, nature of............ 14

Graphic granite, figure showing pegmatitic structure of .......--..--..... 17

Graphic structure, nature of ............. 16-17

Graphite, characters of................... 34

Gravel (beach), description of............ 56-58

Gravel (glacial), description of----------- 58-59

Gray sandstone, description of.......--.- 75-77

Graywacke, description and analysis of.. 84-87 

plate showing thin section of......... 86

Greensand, description of.. _............ 63-64

Grit, Berea, description of.....-.......:.. 75-77

Groundmass, definition of................ 16

Grunerite, characters of.................. 41

Gypsum, characters of.................... 40

description of.......................... 97-98

Hall, James, quoted on occurrence and

nature of amorphous marl beds. 135,136 

Hardness, scale of......................... 30

Hart, Edward, analyses by............... 162

Hatiynite, characters of................... 32-33

Hawes G. W., chemical analyses by. 241,269.272 

Hematite, characters of................... 34

Paga 

Hematite (brown), characters of......... 47

(specular), description of........... 307-308

Hexagonal system, description of minerals of.-....................:..-.-.-. 34-37

listof minerals of............._...... 29

Hitchcock, C. H., acknowledgments to... 11 

cited on New Hampshire schists.... 362-363

cited on- New Hampshire steatite..... 365

Hill, R. T.. quoted on occurrence of chalk

inTexas.-.-.-..----.------------.- 116

quoted on occurrence of flint nodules

inTexas.......................... 119-120

Hillebrand, W. F., chemical analyses by. 164,

177,188,190,193,201,218,

226,227,232,249,253,275

Holocrystalline structure, nature of..... 16

Homogeneous aggregates, list of......... 30

Hornblende, characters of................ 14

Hornblende-andesite, description and

analysis of....................... 221-223

Hornblende-bearing biotite-granite, description of...................... 177-179

Hornblende-diorite-porphyry, d e s c r i ption and analysis of.............. 231-233

Hornblende-gabbro-gneiss, descr i p t i o 11

and analysis of.................. 367-369

plate showing thin section of --------- 360

Hornblende-granite (biotite bearing) description of ...................... 179-181

F ornblende-mica-andesite, desc r i p t i o n

and analysis of................... 219-221

Hornblende-peridotite, description of . 294-297 

Hornblende-pyroxene-andesite, descrip-,

tion of---.-.-...................... 223-224

Hornblende-schist, description of ...... 331-332

(garnetiferous) description of...... 362-365

Hornblendic gabbro-gneiss, description

and analysis of...... ............ 369-372

Hornfels, description of.................. 337-338

definition of............... ............ 125

Hornstone, description s of.............. 123-126

definition of............................ 125

Horny structure, nature of .............. 14

Hummelstown sandstone, description and

analysis of ......................... 77-78

Hydraulic cement rock, description and

analysis of ....................... 133-135

Hypersthene, characters of............... 38-39

analysis of ........................... 226,281

Hypersthene-andesite, description and

analysis of....................... 224-228

plate showing thin section of......... 228

Hypocrystalline structure, nature of.... 16

Iddings, J. P., descriptions of rocks by. 146- 

148,151-162,177-181,194-197,209-211,215- 

217,219-221,223-224.228-231.233-241,243- 

244, 254-261, 278-282, 286-290, 294-297, 

298, 301-302, 331-333, 353-355, 367-369 

Idiomorphic minerals, nature of ......... 16

Idiomorphic quartz, nature of............ 35

Igneous rocks, characters of.............. 49-f,0

definition of ............!.............. 48

(metamorphic) descriptions of specimens of .--------.-..-............ 343-376

(metamorphic) table of ............... 55

(unaltered) descriptions of specimens of .......................... 145-298
INDEX. 397

Igneous rocks (unaltered), table of....... 54

Ilmenite, characters of.................... 34

Indurated jointed shale, description of. 313-315 

Infusorial earth, description of......... 136-137

Intrusive roek, definition of.............. 49

Iron ore (fossiliferous), description of. 138-140

plate showing.......................... 138

Isometric system, description of minerals

of..............-----....--.---..-..- 30-33

list of minerals of...................... 29

Jaspilite, description of ................ 305-307

Jointed shale, description of............ 313-315

Jointing, nature of........................ 18

(columnar), description of.......... 256-258

(columnar), plate showing..... .... 20

(rhombohedral), plate showing ...... 18

Kaolin, characters of................... 47

d escription and analyses of --------- 382-384

Kimberlite, description of.............. 290-294

plate showing thin sections of........ 292

Knowlton, F. H., quoted on silicifled wood 114 

Kryptoperthite, analysis of............... 195

Labradorites, angles of extinction of..... 270

Laminated sandstone, description of.... 81-82

Laminated structure, nature of.......... 14

Lapilli, description and analysis of.... 248-249

Lava, analysis of.---........-...-.-........ 218

columnar jointing in................ 256-258

Lava breccia, characters of............... 74

Lava field, plate showing................. 252

Lee marble, description and analysis of. 299-300 

Lepidolite, characters of.................. 42

Lepidomelane, occurrence of............. 43

analysis of............................. 203

Lesquereux, L., quoted on fossils of ferru ginous concretion............... 109

Leucite, characters of..................... 32

Limestone, residual clay of..........'. .. 384-385

differential weathering of.......... 3S7-388

(cherty), description of............. 123-124

(compact), description and analysis

of.....-...........----. -------- 127-132

(coquina), description of............ 121-123

(crystalline), description of........... 298

(fluted), differential weathering of... 388 

(hydraulic), description of.......... 133-135

(oolitic), description of.............. 103-105

(Patellina), description of............ 119

(shell), description of............... 122-123

Limonite, characters of................... 47

description of............ ........ 105-107

Lindgren, Waldemar, descriptions of

rocks by............ .... 148-151,170-172

Liparite, description and analysis of... 160-162 

Lithographic stone, description of.. .. 132-133

Lithoidal structure, nature of............ 16

Lithoidite description, analyses, and

figures of ........................ 153-160

plate showing lithophysse in.......... 154

Lithology, definition of ................... 13

Lithophysae, description and figures of. 153-159 

nature of............................... 17

Lithophysal structure, plate showing.... 348

Lode, definition of......................... 94

Loess, description of...................... 65-67

plate showing deposit of.............. 66

Luray Cave, Virginia, plate showing deposits in............................ 100

Page. 

Luster-mottling, example of..--....-..... 295

Macroscopic (.see Megascopic).

Madupite, characters of .................. 191

Magma, definitions of..................... 15,49

Magnetite, characters of.................. 30-31

description of........................ 374-376

Magnetitic specular hematite, description of............................ 307-308

Mahon, R. W.', chemical analyses by...... 221

Marble, description and analysis of.... 299-301

plate showing thin sections of........ 300

Marcasite, characters of.................. 37

description of........................ 110-111

plate showing concretions of......... 110

Marengo Cave, Indiana, plate showing

deposits in......................... 98

Margarite, characters of.................. 42

Marine sand, description of fossils from . 64-65 

Marl (amorphous), description of...... 135-136

(shell), description of................. 136

Massive rocks, nature of.................. 14

Massive structure, plate showing ........ 16

Mathews, E. B., descriptions of rocks

by................................ 172-177

McCay, Leroy, chemical analyses by... 290,369 

Megascopic features of rocks, definition

of................................... 13

Melville, W. H., chemical analyses by.... 149,

166,206,212,223,374

Merrill, G. P., descriptions of rocks by. 376-385,

389-391 

Mesostasis, example of................._ 271

Metamorphic conglomerate, description

of................................. 323-325

Metamorphic igneous rocks, descriptions

of specimens of.................. 343-376

Metamorphic sedimentary rocKs, descriptions of specimens of ........... 298-343

Metamorphism of rocks, processes of.... 48-49

Metarhyolite. descriptions, figures, and

analyses of.........._.......... 164-170

plate showing.......................... 168

Micas, characters of....................... 42-43

analyses of.._........................ 188

Mica-schist, description and analyses

of............................. 327-331,342

(staurolitic) description of ......... 333-337

Microcline, characters of.................. 45

analysis of............................. 207

Microcrystalline structure, nature of.... 16

Microgranophyric structure, nature of.. 17 

Micrographic structure, nature of....... 17

Microlites, nature of ...................... 17

Micropegmatitic structure, nature of.... 17

Microperthite, analysis of................. 166

plate showing.......................... 168

Microscopic features of rocks, definition

of.....---.----....--....--.....-... 13

Microscopic method of analysis of rocks. 23-27 

Minerals forming rocks, enumeration and

description of...................... 27-47

essentialand accessory.......--------- 28

listof................................... 29-30

original (primary) and secondary.... 28

Minette, description and analysis of... 230-239 

Mohs scale of hardness.................... 30

Monoclinic system, list of minerals of.... 30

description of minerals of............. 40-45
398 INDEX.

Page

Muscovite, character and distribution of. 43. 

Nephelite, characters of..-.-........----.' 36-37

analyses of............................... 202

Nephelite-leucite rocks, characters of... 51 

descriptions of specimens of........ 186-211

Nephelite-syenite, descriptions and analyses of-............... ............ 201-211

plate showing thin section of......... 204

Nevadite, description and analysis of.. 162-164 

Nodiile of flint, plate showing............ 120

Nodule of olivine from, basalt.-description

of... ..-----.- -. -----. 258-261

Norton, L. M., chemical analysis by...... 312

Noselite, characters of... -....-.... -... 32-33

Obsidian (rhyolitic) description and analysis of.-....--......---..--........ 149-151

Old Faithful geyser, plate showing....... 92

Olivine, charactersof...................... 39

analyses of............................. 298

Olivine-bearing pyroxene-andesite, description of....................... 228-231

Olivine-diabase, description and analysis

of... -.--. --.-. .. 274-278 

plate showing thin section of......... 278

Olivine nodule from basalt, description

of............ .................... 258-261

Oolite (siliceous), description and analysis of............................... 95-97

plate showing................ ........ 96

Oolitic limestone, description of........ 103-105

Oolitic sand, description of............. 102-103

Oolitic structure, nature of.-.i........... 17

Opal, charactersof........................ 28-29

Orendite, description and analyses of.. 186-191 

Original or primary minerals, illustrations of.-.-.i.................... .. 28

Orthoclase, characters of................. 44-45

plate showing..----.................... 158

Orthoclase of nephelite-syeuite, analysis

of.............. .................. 207

Orthorhombic system, description of minerals of............................. 37-40

list of minerals of...................... 29

Overthrusts, features of.................. 316

Paragonite, characters of................. 42

Patellina limestone, description of..._. 119 

Peat, description of......;.............. 140-141

Pebble (faulted) of Cretaceous conglomerate, description of ............ 316-317

Pebbles, striated, description of ......... 58-59

Pebbly sandstone, description of.......... 74-75

plate showing thin section of......... 76

Pebbly structure, nature of .............. 14

Pegmatitic structure, character of....... 16-17

figure showing.......................... 17

Peridotite, description and analysis of. 288-291 

Peridotite family of. rocks, characters of. 52

descriptions of specimens of ....... 286-298

Perlite (rhyolitic) description and analysis of.. .. ... .. . ....-151-153

Perlitic structure, nature of...-..--...... 15

plate showing-....................:.... 348

Perofskite, characters of............---... 33

Petrographic microscope, plate showing. 24 

Petrography, definition of... .......... 13

Petrology, definition of . _.........._:._... 13

Phanerocrystalline structure, nature of. 16

Page. 

Phenocryst, definition of................... 16

Phlogopite, characters of................ 42

analysis of ..--.............--....--. . 188

Phonolite, description and analyses of.. 191-194 

Phyllite, descriptions and analyses of.. 317-323 

Physical analysis of rocks, methods of... 18-27 

Pirsson, L. V., chemical analyses by...... 193

descriptions of rocks by............. 264-273

Plagioclase, analyses of ................... 276

character and occurrence of.......... 45,46

Plutonic rocks, characters of... _......... 49-50

Poecilitic structure, example of.......... 295

Porous structure, character of.......... 15,253

Porphyritic quartz, anhedral crystals of. 35 

Porphyritic structure, nature of......... 16

Porphyritic theralite, description and

-analysis of^....................... 199-201

Potsdam sandstone, description and analysis 6f--. -.-- .-...- -... 79-80 

plate showing thin sections of........ 80

Prehnite, charactersof ................... 40

Primary or original minerals, illustrationsof----.........--..:...-.-.. 28

Psammitic structure, nature of.......... 14

Pulaskite, description of................ 194-197

Pumice (rhyolitic), description and analysis of............................ 148-149

Pumiceous structure, nature of.......... 15

Pyrite, characters of...:................'.. 30

Pyroclastic structure, nature of.......... 14

Pyrope of kimberlite, description and

figure of ........................... 293

Pyrope with border of biotite and magnetite, figure show ing............. 293

Pyroxene, analysis of.........'......____._ 269

Pyroxenes (monocliiiic), characters of... 40-41 

(orthorhomic), characters of......... 38-39

Pyroxene-andesite (olivine-bearing), description of...................... 228-231

Pyroxenite, characters of --....__.._.._.. 52

description and analysis of......... 286-288

Pyrrhotite, characters of................. 34

Quartz, characters of ..................... 34-35

(porphyritic). anhedral crystals of .. 35 

Quartz and calcite vein, description of.. 93-94 

Quartz-basalt, description and analyses

of.... ............................ 252-254

Quartz conglomerate, definition of.'...... 71

Quartz grains, plate showing thin sections' of---....--..-................. 254

Quartzite, description of................ 301-302

Quartz-mica-diorite, analyses of.......... 342

Quartz - norite - gneiss, description and

analysisof........................ 358-362

plate showing thin section of......... 360

Quartz sand, description of .............. 59-61

Quartz-schist, description of............ 302-305

Quartz vein, plate showing............... 94

Residual clay of feldspathic rock, description and analyses of..... _ 382-384

Residual clay of limestone, description

and analyses of.................. 384-385

Residual rocks, table of. .................. 55

descriptions of specimens of........ 376-385

Residual sand of diabase, description and

analyses of....................... 379-382

Residual sand of granite, description and

analyses of....................... 376-379
INDEX. 399

Page 

Bhombohedral jointing, nature of........ '18

plate showing........................... 18

Rhyolite, analyses of...................... 147

Rhyolitic dust, description and analysis

of..................... ............ 146-148

Rhyolitic obsidian, description and analysis of ............................. 149-151

Rhyolitic perlite, description and analysis of............. ................ 151-153

Rhyolitic pumice, description and analysis of............................. 148-149

Riggs, R. B., chemical analyses by.. 215,275,325 

Ripple-marked sandstone, description of. 82-83 

Ripple marks, plates showing ............ 82,84

Rock, definition of.-........................ 13

mode of preparing thin suctions of... 24-27 

Rock-making minerals, enumeration and

descriptions of .................... 27-47

essential and accessory ............... 28

original (primary) and secondary.... 28

titles of works containing accounts of. 27 

Rock powder, method of microscopic

study of............................ 26-27

Rock sections (thin), mode of preparation

of................................... 24-27

Rocks, methods of physical analysis of... 18-27 

methods of study of................... .13

minerals forming...................... 27-37

structural features of................. 13-18

tabular view and classification of.... 48-56

Rubellan, characters of................... 42

Rutile, characters of...................... 33

Sand, figure showing ripple marks on ... 82 

(beach), description of................ 59-01

(dune, or eolian), description of...... 61-63

(marine), description of fossils from. 64-65 

. (oolitic),description of.............. 102-103

(residual of diabase), description and

analyses of..-....-----......-.... 379-382

(residual of granite), description and

analyses of....................... 376-37!)

(volcanic), description of............ 245-248

Sandstone (banded), description and

analysis of ........................... 80-81

(brown), description and analysis of. 77-78 

(brown), plate showing thin sections 

of ......_ -.... ........................ 76

(fossiliferous), description of..--..... 83-84

(gray), description of ................. 75-77

(laminated), description of ........... 81-82

(pebbly), description of........... _ 74-75

(pebbly), plate showing thin sections 

of..................................... 76

(PotsdamV, description and analysis of 79-80 

(Potsdam), plate showing thin sections of...... ......................... 80

(ripple-marked), description of...... 82-83

Sandstone-like structure, nature of.._ 14 

Sanidine, analysis of...................... 164

character and occurrence of.......... 45

Saxonite, description and analysis of... 297-298 

Schist, features of ......................... 18

(albite-), description of ............. 325-327

(chiastolito-), description and analysis 

of................................... 339-343

(hornblenbe-), description of....... 362-365

(q uartz-), description of ............ 302-305

Page. 

Schist (sericite-), description and analysis

of................................... 317-320

(staurolitic mica-), description of.. 333-337 

(tourmaline-biotite), description and 

analysis of.-.--.....-.-.--.-.--.... 327-331

Schistose biotite-gneiss, description of 332-333 

Schistose structu re, nature of............ 18

Schneider, E. A., chemical analyses by 77,

81,201,29!) 

Scoria, description of ................... 249-250

Secondary minerals, illustrations of..... 28

Secretion, nature of ...................... 17-18

Sections (thin) of rocks, mode of preparation of................................. 24-27

Sedimentary rocks, definition of ......... 48

(me tamorphic), table of............... 55

of chemical origin (unaltered), descriptions of specimens of ....... 91-115

of mechanical origin (unaltered), descriptions of specimens of......... 56-91

of organic origin (unaltered), descriptions of specimens of............ 115-145

(unaltered), table of ................... 53

Sericite, characters of .................... 42

Sericite-schist, description and analysis

of...................-.--.......:.. 317-320

Serpentine, characters of................. 46-47

description and analysis of......... 372-374

Shale, description and analysis of........ 87-89

(carbonaceous), description and analysis of................................ 90-91

(crumpled), description of.......... 315-316

(jointed), description of............. 313-315

spheroidal weathering in ............. 387

Shaly structure, nature of................ 18

Shell (silicified), description of......... 114-115

Shell limestone, description of.......... 122-123

Shell marl, description of................. 136

Shimer, P. W., chemical analyses by..... 218

Shingle, definition of...................... 56

Siliceous oolite, description and analysis

of.................... ................ 95-97

plate showing.......................... 96

Siliceous sinter, description of........... 91-93

Silicified shell, description of........... 114-115

Silicifled wood, description of .......... 113-114

Sinter (siliceous), description of.......... 91-93

Slate, description and analyses of ..... 308-313,

320,342 

Slaty cleavage, nature of................. 18

Srneeth separating apparatus, description and figure of.................. 20-21

Smith, G. O., descriptions of rocks by. 140-145 

Soapstone, description of............... 365-367

Sodalite, characters of.................... 32

analyses of............................. 204

Specific-gravity method of analysis of

 rocks, description of. ........... 19-22

Specular hematite, description of...... 307-308

Sphene, character and distribution of.... 44

Spheroidal weathering, description and

figures of......................... 385-387

Spherulites, nature of..................... 17

description and figures of........... 153-159

plate showing sections of... ........ 156

Spherulitic structure, plate showing..... 348

Spinel, characters of...................... 31
400 INDEX.

Page. 

Spinel, table showing varietiesof......... 31

Stalactites and .stalagmites, description

of...............--... ......--.... 98-99

plates showing........................ 98,100

Staurolite, characters of.................. 38

figure showing cross section of crystal of............................... 334

Staurolitic mica-schist, description of.. 333-337 

Steatite, description of.................. 365-367

Steiger, Geo., chemical analyses by. 134,320,342 

Stokes, H. N., chemical analyses by.... 87.153,

286,348,362 

Stratified rocks, nature of................ 14

Streaked structure, nature of............ 15

Striated pebbles, description of.......... 58-59

Surface modification, table showing

classes of........................... 56

illustrations of....................... 385-391

Syenite, description and analyses of... 183-186, 

195-197,201-211

Syenite-trachyte family of rocks, characters of-------.--.--~-----------.--. 51

descriptions of specimens of........ 181-186

Syncline and anticline, features of..... 315-316

plate showing............................. 316

Talc, characters of ........................ 40

Talus breccia, characters of.............. 73

Terraces of hot springs, plate showing.. 102 

Tetragonal system, description of minerals of....... ..................... 33

listof minerals of............. ...... 29

Textularia, figure showing ............... 116

Theralite, analysis of...................... 201

description of........................ 197-199

(porphyritic) description and analysis of..._......_......_........... 199-201

Thin sections of rocks, mode of preparation of........---...---..........-. 24-27

Thoulet solution for analysis of rocks,

ingredients of...................... 19

Thrusts, features of....................... 316

Till, definition of .......................... 69

Titanite, character and occurrence of.... 43-44

Tolman, analysis by...-.....-.....--.--.-. 166 

Topaz, characters of ............. ...... 38

Tourmaline, characters of............._ 37

Tourmaline-biotite-schist, description

and analysis of .................. 327-331

plate showing thin section of......... 328

Trachyte, description and analysis of.. 181-182 

Trap rock, descriptions of...... 254-255, 264-273

Travertine, description, analysis, and figure of............................. 99-101

Trees (fossil), plate showing ............. 114

Tremolite, characters of.................. 41

Triclinic system, list of minerals of... 30

minerals of ............................ 45-46

Tridymite, characters of ................. 35

Tripolite, description of ................ 136-137

Tuff (basalt), description of............ 251-252.

Page.

Turner, H. W.,descriptions of rocks by 337-313 

Unstratified rocks, nature of............. 14

Uralite, characters of....._....._._.__..._ 14

Van Hise, C. E., descriptions of rocks

by.. ..................... 305-308,313-315

Vein of quartz, description of _..._ _...... 93-94

plate showing ......................... 94

Vein quartz, description of............... 94-95

Vesicular structure, nature of.......... 15, 252

Vesuvius in eruption, plate showing..... 246

Vitreous structure, nature of............ 15

Volcanic bombs, description of......... 250-251

plate showing..............._...._ 250

Volcanic breccia, characters 'of........... 73-74

Volcanic cone, plate sh owing............. 248

Volcanic dust, description and analysis

of................................. 146-148

Volcanic glass, figure showing fragments

of................................... 212

Volcanic rocks, characters of............. 49

Volcanic sand, description of........... 245-248

Volcano in eruption, plate showing...... 246

Von Kobell's scale of fusibility .......... 30

Weathering (differential) in impure limestone, description of.............. 387-388

plate showing.......................... 388

Weathering (spheroidal) in shale, description of ........................ 387

Weathering (spheroidal) in igneous rocks,

description of.................... 385-387

plate showing-.............!.........-.. 386

Weed, W. H., descriptions of rocks by... 91-93

99-101

Weidman, S., descriptions of rocks by. 164-170 

Westphal balance, description and figure

of.--.....-..........--....--..-.... 21-22

Whitney, Milton, chemical analyses by.- 383 

Whitfield, J. E., chemical analyses by.... 92,

101, 236, 287 

Williams, G. H., combined saw and grindingdisk devised by ................ 25

descriptions of rocks by............ 278-282,

286-290, 294-297, 367-369 

quoted on gabbro-diorite of Maryland .......................... 367,367-368

quoted on quartz-schist of Maryland- 304 

Willis, Bailey, descriptions of rocks by. 315-317 

quoted on conditions favorable to

chemical deposition .............. 129-132

Wolff, J. E., descriptions of rocks by.. 197-201, 

323-327,349-353

indicators for specific-gravity analysis of rocks devised by........... 21

Wood (silicified), description of........ 113-114

Worthen, A. H., quoted on geodes...... 111-1J2

Zeolites, characters of.............._.... 44

Zinn waldite, characters of... _.......... 42

Zircon, characters of...................... 33

analysisof............................. 204

Zoisite, characters of...................... 39-40