DEPARTMENT OF THE INTERIOR

OF THE

UNITED STATES

GEOLOGICAL SURVEY

. 136

WASHINGTON

GOVERNMENT PRINTING- OFFICE 

1896

UNITED STATES GEOLOGICAL SUEVEY

CHARLES B. WALCOTT, BIKECTOK

THE

ANCIENT VOLCANIC ROCKS

SOUTH MOUNTAIN, PENNSYLVANIA

FLORENCE BASCOM

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1896

CONTENTS.

Page.

Letter of transmittal................................ ........................ 11

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

CHAPTER I. An account of geological surveys in the South Mountain below

the Susquehanna.......................................................... '14

 Historical review........................................................ 14

Bibliography ....................................................... 19

CHAPTER II. Geological relationship of the rocks of the Monteroy district.... 20

General description of the area studied.................................. 20

Extent and character of the three rock types ............................ 20

Sedimentary rocks....................................................... 21

Areal distribution............................................... 21

Structural features.............................................. 21

Thickness ...................................................... 21

Age and superposition .......................................... 21

Acid eruptives.......................................................... 23

Areal distribution..................................'.....^....... 23

Character....................................................... 23

Previous descriptions ........................................... 23

Economic value..i.............i................................. 24

Basic eruptives ......................................................... 24

Areal distribution............................................... 24

Character ...................................................... 24

Previous descriptions ........................................... 25

Ore deposits of the Monterey district .................................... 25

Comparative age of sedimentary and igneous rocks...................... 27

Contacts described.................................................. 27

Conclusions ........................................................ 28

Comparative age of acid and basic eruptives............................ 29

Field relations....................................................... 29

Probable explanation............................................... 29

Summary............................................................... 30

CHAPTER III. Petrographical description of the Cambrian rocks ............ 31

Macroscopical description............................................... 31

Microscopical description ............................................... 31

Quartzite........................................................... 31

Slates .............................................................. 33

Chemical analysis........................................................ 33

Summary............................................................... 34

CHAPTER IV. Petrographical description of the acid eruptives .............. 35

Nomenclature ............................................................ 35

Quartz-porphyries...................................................... 39

Distribution........................................................ 39

Macroscopical description........................................... 39

Microscopical description ........................................... 39

Phenocrysts .................................................... 39

Feldspar ................................................... 39

Quartz ..................................................... 40

5
6 CONTENTS.

CHAPTER IV Continued. Page. 

Quartz-porphyries Continued.

Microscopical description Continued.

Groundmass .................................................... 41

Accessory constituents .......................................... 41

Aporhyolites............................................................ ' 42

Distribution........................................................ 42

Macroscopical description.....;..................................... 42

Microscopical description ........................................... 44

Phenocrysts..................................................... 44

Feldspar and quartz ........................................ 44

Other porphyritic constituents .............................. 45

Groundmass .................................................... 46

Fluidal structure ........................................... -46

Micropoikilitic structure.................................... 47

Spherulitic structure. ................ i... .................,: 51

Chain spherulites........................................... 53

Axiolitic structure.......................................... 54

Rhyolitic structure.......................................... 54

Lithophysal structure............. .......................... 55

Micropegmatitic structure................................... 55

Perlitic structure ........................................... 55

Amygdaloidal structure..................................... 55

Taxitic structure............................................ 57

Summary of proof of devitrification ............................. 57

Opinions of petrographers................................... 59

Chemical composition of the acid eruptives............................... 61

Acid volcanic breccia ......................... .......................... 63

Distribution ...................... .................................. 63

Tuff................................................................ 63

Flow breccias....................................................... 63

Tuffaceous breccias ................................................. 64

Metamorphosed acid eruptives: sericite-schists and slates ............... 64

Summary ............................................................... 66

CHAPTER V. Petrographical description of the basic eruptives ............... 68

Nomenclature........................................................... 68

Melaphyres and augite-porphyrites...................................... 69

Distribution................................'........................ 69

Macroscopical description........................................... 70

Microscopical description ........................................... 72

Original structures.............................................. 72

Secondary structures............................................ 73

Original constituents.........................^.................. 73

Secondary constituents.......................................... 74

Accessory minerals.............................................. 77

Discussion of chemical analyses......................................... 78

Basic slates......................................................"....... 79

Distribution and description ........................................ 79

Basic pyroclastics................... .................................... 80

Distribution and description ........................................ 80

Crushed porphyrites ............................................ 80

Tuffaceous breccia .............................................. SO

Ash............................................................. 80

Summary............................................................... 81
CONTENTS. 7

Pago.

CHAPTER VL Summary of conclusions.......;........... ; .. ; ..;... i.... >. ... ; 82

Evidence of the eruptive character of the two rock types........ i. ... -. -. -.:' 82

Field evidence .......... ^ .. .... ^ i.............. -. ......... ; ;. ^... : .-. 82

Schistosity.^ ^ ^.. ^.....;.......;.;..m.i....... i...^.;...;...... 82

Lamination .....i..^.^^. ....i.......-.a. .....i.^..;;..!.-^.. 82

Theslates .........;................i....................... ; .,-. 82

Absence of gradation between igneous and clastic rocks ...i.. -.;-.. 83

Surfaces-flow features.i. . i..... ... . ;.........i.... i......;. -. .;... 83

Pettographical evidence.... ^..:;.;..... i.... ^... i;.... -. i.... i;.... ^ - 83

Structural.........;,........: : ............ -.:..-.,. .....^...-.-.:-. 83

Mineralogical. i. i....... i. i;......; i.. i...; . .. i..... -.. -.-. i.. ^: i ^. 84

Chemical........ -.. ... ^.. -........ .^-. -... -.-. . . .;... i ; .. ^...:-. ; .. -... 84

Original rock types ....:;;. . ...:;..:.;.... ^; . ;. .. 11.... . .... i i i -. ... ^.... 84

ACid ighCOUS rO^ks .........::. i ......... i .;.........; : ..... i ; : .:.:. : . 84

Basic igneous rocks .... ; : ; :......;...... -.. . ....... . ...... ^:... i -..;-. 85

Similar rocks in other regions .....: ^;................:: .......... .^. -. .. . 86

Literature..:.;.....;::..... ^ij:... ;i , i.......!,.. t ..;..-. .L.i!.-.,..tli 87

ILLUSTRATIONS.

Page. 

PLATE I. Map showing location of Monterey district......................... 13

II. Panoramic view of mountains of the Monterey district............. 16

III. Geologic and topographic map of the Monterey district............. 20

IV. Crumpling of sandstone at the east end of the tunnel through Jacks

Mountain on the Gettysburg Railroad ........................... 22

V. Sections through the Monterey district............................. 24

VI. Junction of felsite and sandstone on the old Tapeworm Railroad.... 28

VII. Flow structure in aporhyolite, Monterey district ................... 42

VIII. Flow structure in aporhyolite, Raccoon Creek...................... 44

IX. Spherulitic aporhyolite, Mouterey district.......................... 46

X. Aporhyolite with spherulites in layers.............................. 48

XI. LithophysjB, Raccoon Creek ....................................... 54

XII. Flow hreccia ....................................................... 62

XIII. Acid hreccia, Raccoon Creek ....................................... 64

XIV. Sericite-schist, Gettysburg Railroad................................ 66

XV. Thin sections: a, quartzite; 1), feldspar crystal..................... 96

XVI. Thin sections: a, perthitic structure in feldspar; b, stretched feldspar crystal ..................................................... 98

XVII. Thin sections: a, quartz-albite mosaic rilling crack in feldspar crystal ; b, micropoikilitic structure.................................. 100

XVIII. Thin sections: a, flow structure in an aporhyolite; b, chain spherulites

in an aporhyolite ................................................ 102

XIX. Thin sections: a, b, augite-porphyrite in ordinary and in polarized

light............................................................ 104

XX. Thin sections: a, b, perlitic parting in an aporhyolite in ordinary

and in polarized light............................................ 106

XXI. Thin sections: a, perlitic parting in an aporhyolite; I, axiolites in

an aporhyolite................................................... 108

XXII. Thin sections: a, b, altered and unaltered spherulites in aporhyolites. 110

XXIII. Thin sections: a, altered spherulites in ordinary and in polarized

light; b, chain spherulites with phenocryst ...................... 112

XXIV. Thin sections: a, b, rhyolitic structures in aporhyolites ............. 114

XXV. Thin sections: a, rhyolitic structure in an aporhyolite; b, piedmontite.............................................................. 116

XXVI. Thin sections: a, b, amygdaloidal aporhyolites..................... 118

XXVII. Thin sections: a, b, tridymite spherulites in an aporhyolite......... 120

XXVIII. Thin sections: a, augite-porphyrite; b, melaphyre.................. 122

9

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY,

Baltimore, Md., March 17, 1894.

SIR: I have the honor of transmitting herewith, for publication as a 

bulletin of the Survey, a paper by Miss Florence Basconi on the 

Ancient Volcanic Kocks of South Mountain, Pennsylvania.

This work was done in the field during the summer of 1892 and in 

my laboratory during the winter of 1892-93. That part of it relating 

to the acid eruptive rocks was accepted in June, 1893, as a thesis for 

the degree of doctor of philosophy at the Johns Hopkins University.

Since that time the paper has been amplified and improved, and I 

believe that the detailed investigations which it describes will prove to 

be of permanent value in- elucidating the character and history of 

ancient volcanic rocks, not merely in South Mountain, but in many 

other regions of the Eastern United States. 

Very respectfully,

GEORGE H. WILLIAMS,

Geologist.

Hon. J. W. POWELL,

Director United States Geological Survey.

11

U.S. GEOLOGICAL SURVEY. BULLETIN NO. 136 PL.I.

MAP SHOA\^HSTG LOCATION OF MONTE RE Y DISTRICT.
THE ANCIENT YOLCANIC ROCKS OF SOUTH 

MOUNTAIN, PENNSYLVANIA.

By FLORENCE BASCOM.

INTRODUCTION.

The mountain range known in Vermont as the Green Mountains, in 

Massachusetts as the Taconic Mountains, and in New York and New 

Jersey as the Highlands, is the South Mountain in Pennsylvania and 

Maryland, the Blue Eidge in Virginia, and the Smoky Mountains in

North Carolina. The South Mountain in Pennsylvania lies just east 

of the middle of the State, and stretches from Maryland north and 

east in a sickle-shaped curve toward the Susquehaima. While in the 

New England States the mountains of this range rise to heights from 

3,000 to 4,000 feet above sea level, and in its southern extension from 

4,000 to 7,000 feet, in Pennsylvania its summits rarely exceed 1,500 feet.

South Mountain is 50 miles in length, and 10 miles wide at its broadest point. It covers from 150 to 175 square miles, and abundantly 

exposes distinct and interesting rock types. These rocks, prevailing 

throughout the entire South Mountain range, have long been known 

to geologists, although their true character was not recognized until 

recently.

In December, 1892, as the result of field work on the part of Dr. G. 

H. Williams in the northern and of the writer in the southern portion 

of South Mountain, there appeared a preliminary description 1 of two of 

the rock types, in which their identification as ancient volcamcs was 

announced. In this bulletin it is proposed to substantiate that identification with more detailed proof. It is further proposed to show that 

these ancient igneous rocks were, at the time of their consolidation, 

identical in character with their recent volcanic analogues, and that 

their present differences are due to subsequent changes, chief among 

which has been devitrification. It is also proposed to recognize these 

facts in the nomenclature.

The petrographical features of the third and only remaining rock 

type of the South Mountain will also be described in some detail.

A brief report upon the previous geological work accomplished in 

the South Mountain, and some account of the structural features of 

the mountain and the age of its rocks, will precede the petrographical 

discussion.

'The volcanic rocks of the South Mountain in Pennsylvania and Maryland: Am. Jour. Sci., 3d 

series, Vol. XLIV, Dec., 1892, pp. 482-490, PI. I. Reprinted in the Scientific American for Jan. 14,1893.

13
CHAPTER I.

AN ACCOUNT OF GEOLOGICAL SURVEYS IN THE SOUTH 

MOUNTAIN BELOW THE SUSQUEHANNA.

H1STOEICAL REVIEW.

The first topographical description of South Mountain appeared as 

early as 1755. 1 It was made by Lewis Evans, of Philadelphia, who 

describes the South Mountain with a fair degree of accuracy, as "not 

in ridges like the Endless Mountains, but in small, broken, steep, stony 

hills; nor does it run with so much regularity." He continues: "In 

some places it gradually degenerates to nothing, not to appear again 

for some miles, and in others it spreads several miles in breadth."

In a publication 2 which appeared in Germany in 1787 several pages 

are devoted to a general description of South Mountain. Two of the 

type rocks (the sandstone and the porphyry) were noted and aptly 

described, as the following quotation shows:

One finds here and there gray laminated sandstones with quartz veins; fragments 

of coarse ferruginous quartz. At one spot on the road [from Sharpsburg to Frederick town] I found blocks of gray-reddish porphyry with little transparent quartz 

grains intermixed, and milk-white opaqne feldspars. * * * The South Mountain 

in its entire extent contains rich crevices, gangnes, and nests of ore, especially of 

iron and copper. * * * I have still to add, from the observations made upon 

this journey, that the eastern slope is gentler and more gradual than the western.

The most important publications on the South Mountain have 

appeared under the auspices of the various surveys of Pennsylvania

and Maryland. The First Geological Survey of Pennsylvania was 

organized in 1836 under the distinguished geologist, Henry D. Eogers. 

The difficulties encountered, however, were so great that it was not 

until 1858 that the two quarto volumes of the survey were issued. 

Professor Eogers deals somewhat cursorily with the South Mountain 

region. He says: 3

In its geological constitution, this tract is without much variety, for it contains 

scarcely any roclcs except those of the Primal series. It is doubtful if the true 

gneissic rocks anywhere reach the surface within its borders, and only in one or two 

localities have even the lowest members of the Auroral limestones been met with 

covering the upper Primal slates. Even of intrusive igneous rocks, it embraces a 

singularly small amount, those met with being chiefly greenstones and trap rocks.

. 'Analysis of a Map of the British Colonies in America, 1755.

5Schopf's Beytrago zur mineralogisehen Kenntniss des ostlicheu Theils von Nordamerika urid

seine Gebiirge, chapter 30, pp. 96-101, Erlangen, 1787. A translation of this work has been made by

Prof. John M. Clark, of the New York State Museum of Natural History, to whose kindness the

writer its indebted for a copy of the unpublished manuscript of the pages relating to South Mountain.

3 Geology of Pennsylvania, Vol. I, Part II, pp. 203-209, 1858.

14
BASCOM.] HISTORICAL REVIEW. 15

The geological structure or mode of stratification of this belt is equally simple. 

The "whole tract consists of two or three groups of high, narrow, nearly parallel 

anticlinal ridges, expanding and subdividing toward the southwest. These are 

composed of the Primal white sandstone. Between them are high parallel valleys 

and plateaus of the Primal upper slates, which, from being softer and more fissile, 

have been worn and trenched by the plowing force of waters to somewhat lower 

levels than the more resisting, better cemented sandstones. The crests of the ridges 

are therefore stony and rugged, their flanks usually smoother, being formed chiefly 

of the slate, 1

That Professor Rogers did not include in the "singularly small 

amount" of igneous rocks the rocks forming the valleys, the mountain 

flanks, and even the summits of the mountains, is further indicated by 

the following:

Another section across the mountain more to the southwest extends from south to 

north along the Baltimore and Carlyle turnpike. The first important stratum of the 

hills is the usual gray siliceous altered rock, so common along their southern side."

North of this, about 3 miles from Petersburg, occurs the dark green slate, with its 

epidote and Avhite intrusive quartz. Succeeding this is an extremely compact siliceous altered slate, and beyond this a reddish gray rock, of the same series, containing 

specks of reddish feldspar aud small veins of epidote, and near this the fissile talcosc rock, several times mentioned before. * * * The summit of the ridge exhibits 

a dark blue and greenish blue indurated rock, weathering a dark brown, and evidently very ferruginous. It appears to be a band of the Primal slate in a highly metamorphic condition, approaching jasper. * * * These lower Primal slates are 

highly indurated, and even decidedly crystalline, containing in some of their layers 

segregated specks, and even half-formed geodes of epidote and other minerals.

On page 206 of the same report Professor Eogers has figured a section across South Mountain, along the Gettysburg Railroad, from Fairfield to Mouterey Springs. This section shows stratified rocks lying 

in a series of anticlinal flexures, which accord rather with Professor 

Eogers's conception of "rock waves" than with his observed dips. 

These dips are, with a single exception, to the southeast. Accompanying the section is the following description, which bears directly upon 

the portion of South Mountain that is particularly discussed in this 

bulletin:

Passing now to the eastward'of the Green Ridge axis we cross a high slope of 

slate, apparently the upper Primal, in a synclinal fold, and then traverse a succession of outcrops of the Primal white sandstones and slates to the eastern base of the 

high land called Jack's Mountain, at the foot of which the older rocks disappear 

under the Meso/oic red sandstone of the plain of Adams County.

The exposures in the sandstone near the tunnel opposite Jack's Mountain indicate 

 a probable thickness of 1.000 feet. Near the tunnel at the northwest side of the 

mountain there is a hard epidotic rock; and not far from it a highly altered greenish 

slate, a rock found in several other localities farther west and containing layers of 

gray slate, spotted luith epidote. Farther west occurs epidote with asbestus. Near 

Miuie Branch search was made many years ago for copper ore, but nothing was found 

to justify the expectation of finding a productive vein of that mineral. A small 

quantity of copper ore was once obtained and a furnace built for smelting it in a 

small ridge north of Jack's Mountain, but the exploration was abandoned. The 

metal occurs in the form of a green and blue carbonate, with a little native copper. 

Evidently the ore is not abundant.

1 The it.alics in this ami the following quotatioiis are the writer's.
16 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

Many of the characteristic features of the South Mountain rocks were 

aptly described by Professor Rogers, yet, plainly, their nature was not 

fully understood nor their importance appreciated at the close of the 

First Geological Survey of Pennsylvania. In 1860 the Primal series 

of Pennsylvania, as ifc occurs in Maryland, was thus characterized by 

Tyson:1

1. A hard sandstone, made up of grains of quartz, with occasionally grains of feldspar and kaolin.

2. A slate, varying in color from gray to brownish and greenish. It is ranked as 

an argillite, but portions of it assume a marked talcose appearance, especially in the 

Catoctin Mountain, and in parts of Middletown Valley, Avhere it has been much disturbed and altered by proximity to intrusive rocks. These last consist of amphibolites (trap), porphyries, amygdaloid, serpentine, and epidote. This last-named rock 

is extensively developed, both in large masses and intercalated between the slates.

 The Second Geological Survey of Pennsylvania was organized in 1874 

under Prof. J. P. Lesley, who had been an assistant to Professor Eogers 

in the earlier survey.

With improved facilities for scientific work, and with more accurate 

methods of mapping, a new survey of South Mountain was undertaken 

by Dr. Persifor Frazer, with A. E. Lehman as topographical assistant. 

Five sections (Nos. 7, 8, 9, 10,11), more or less incomplete, were made 

through the mountain, and as the result of his investigations Dr. Frazer 

says: 2

It is apparent that the great South Mountain is composed essentially of two 

groups of rocks, the lower (and along this line, the northwestern) consisting of 

various modifications of the quartz conglomerate above spoken of, and in which 

quartz occurs in various forms.

Th&upper and southeasterly group is felsitic in character, but contains, also, large 

beds of hydromica and chlorite schists intersected by veins of milk quartz, while 

the orthofelsite presents every variety of appearance, from a sandy and earthy slate, 

in which the crystals of orthoclase are very much decomposed, indeed, are sometimes almost clay, through the jasper-like variety to the massive and coarsely porphyritic Structure in which it is suited to be used as an ornamental building stone.

In the same year (1877) Dr. Frazer published a further account 3 of 

the nature and origin of these "orthofelsites." This account was in 

substance twice repeated by him, in 1879 4 and in 1880: 5

The rocks of this region [South Mountain] may be divided into two great series 

a western (underlying), of which the characteristic strata are composed of quartzite 

and of arenaceous schist containing quartz pebbles (Mountain Creek rock), and the 

eastern (overlying) of hydromica and chlorite schists, and orthofelsite, both porphyritic and unporphyritic. Both these series show indications of having been penetrated by dikes of plutonic character within this area.

The porphyry, which carries the copper in this region, shoivs no character of igneous 

action, but occurs in coarse and thin beds, more or less disintegrated, and in certain

'Keporton the Geology of Maryland, Jan., 1860, pp. 34, 35.

2Keport of Progress in the Counties of York, Adams, Cumberland, and Franklin, 1877, CC, pp 285-295. "' 

3Tho copper ores of Pennsylvania: Polytechnic Keview, Nos. 16-and 17, Vol. Ill, April, 1877, 

"Trails. Am. Inst. Min. Eng., Vol. VII, 1879, p. 338. 

6 Second Geol. Survey, Pa., CCC, Appendix, 1880, pp. 312-313,
U. S. GEOLOGICAL SURVEY 

PANORAMIC VIEW OF THE MOUNTAINS OF THE MONT
BASCOM.] HISTORICAL REVIEW. ; 17

localities reduced almost to the state of kaolin, Nothing which might correspond 

to the term sandstone was observed, though all tho above sediments were free of 

grit and sandy particles. * * * It seems fair to conclude that the region of the 

copper-bearing rocka belongs to the Huronian cycle.

With these views Dr. T. Sterry Hunt expressed entire concurrence. 1 

Dr. Hunt had previously made some study of the South Mountain

rocks, and published, at various times, his observations concerning

them. In 1876 he said: 2

In the southern part of Pennsylvania, to the west of Gettysburg, this mountainous belt, rising between the Mesozoic on the east and the great limestone valley 

on the west, presents an immense development of a peculiar type of crystalline 

rocks which I detected there last year, and which has a considerable geological 

importance. It is a tedded petrosilex, grayish, reddish, or purplish in color, sometimes granular but more often jasper-like in texture, and frequently porphyritic from 

the presence of small crystals of orthoclase-feldspar or of glassy quartz. There is 

found here a great brecidtli of this rock distinctly bedded, presenting different varieties, and alternating with dioritic, or diabasic, epiclotic, and chloritic rocks, with 

argillites, in which are sometimes included thin beds of the petrosilex, the strata 

generally dipping at high angles to the east.

These rocks Dr. Hunt provisionally referred to a position near the 

base of the Huronian division, adding:

This petrosilex is identical in its lithological character with the hillleflinta, or 

stratified flint rock of the Swedish geologists, which is by them assigned a similar 

position, i. e., above the most ancient gneisses.

In 1878 Dr. Hunt expressed essentially the same views,3 and in 18794 

he opposed the correlation of the South Mountain rocks with the copperbearing rocks of Lake Superior (Keweenawau series), although at an 

earlier date he notes a resemblance. He says: 5

I may also note that I have observed bedded petrosilex rocks like those just 

noticed [South Mountain porphyries] to the north of Lake Superior, both in an 

island south of St. Ignace and on the adjacent mainland. Tho conglomerates or 

breccias, which, in the rocks of tho Keweenawan series on tho south shore of the 

lake, include the native copper of the Calumet and Hecla and the Boston and Albany 

mines, are also made up of tho ruins of a precisely similar petrosilex porphyry.

In October, 1879, Mr. J. F. Blandy made a brief reconnoissance of 

the lower portion of the South Mountain with not unfruitful results. 

He makes a suggestive correlation of the copper-bearing rocks of 

southern Pennsylvania with the Lake Superior copper formation," 

thus recognizing the volcanic nature of the greenstones, at least, 

which he called " amygdaloidal trap." The acid rocks still remain 

"slates."

In the final report of the. Pennsylvania survey,7 Professor Lesley

1 Trans. Am. Inst. Min. Eng., Vol. VII, 1879, p. 339.

2 Proc. Am. Ass. Adv. Sci., 1876, pp. 211-212.

3 Second Geol. Surv. Pa., Vol. E, p. 193.

4 Trans. Am. Inst. Min. Eng., Vol. VII, p. 331.

8 Proc. Am. Ass. Adv. Sci., 1876, p. 211.

"Trana, Am. Inst. Min. Eng., Vol. VII, pp. 331-333.

7 Final Report of the Pa. Geol. Surv., Lesley, Summary, Vol. 1,1892,

. Bull. 136 2
18 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

gives in substance the views of Dr. Frazer, which have already been 

quoted. He refers to Dr. Frazer's "sectixm S" as representative of 

the South Mountain rocks and structure. In this section, as in the 

others in which the orthofelsite appears, notably section 9, the bedded 

orthofelsite is represented as overlying the quartzose conglomerate. 

Professor Frazer concludes his summary with the statement that "it 

is hard to avoid the inference that our South Mountain rocks represent the Huronian section of Murray and Logan."

The geological map of Adams, Cumberland, and Franklin counties 

made by this final Pennsylvania survey refers all the rocks of the 

South Mountain region by the use of a single color to the "Azoic 

system."

The Second Geological Survey of Pennsylvania, while recognizing 

the extent and crystalline character of the South Mountain rocks and 

emphasizing the absence of the Primal of Kogers, still failed to solve 

the problems of the structure of the region and the age and origin of 

its rocks.

Professor Lesley has been very ready to acknowledge the incompleteness of the survey in the South Mountain, and since Mr. Walcott's 

determination of the age of its sedimentary rocks has appeared, he has 

expressed himself as desiring a new investigation of that region. 

While he claims that the survey of Pennsylvania "has been so minute 

and complete that comparatively little remains to be desired in the 

future," he adds: "The geology of the South Mountain is therefore 

[because of the new aspect given to it by the investigation of Mr. 

Walcott] in a very unsatisfactory condition arid requires, in fact, a 

special and protracted investigation in the field. It is the most unsatisfactory part of the work of the Geological Survey of the State." 1

Subsequent workers in the South Mountain are indebted to the 

Survey for the superior topographical maps of the mountain made on 

a scale of 1 inch to 1,600 feet (about 3 inches to the mile) by A. E. 

Lehniau. These maps are invaluable to the field geologist and render 

possible accurate areal mapping. Owing to a confusion of cleavage 

planes and bedding the dips recorded on these maps are not reliable.

While the geological explorations of the South Mountain have been 

careful and minute and conducted by able geologists, the petrography 

of its rocks has never been thoroughly investigated. The microscope 

has not been used to assist in determining the nature and origin of the 

rocks, and to correct impressions colored by preconceived ideas or by 

an experience more or less limited to sedimentary structures. Under 

microscopic scrutiny and the comparative study of recent lavas, an 

increasing number of the so-called sedimentary rocks are proving to 

be igneous in origin. That such has proved to be the case in the South 

Mountain, while not surprising, is a fact of considerable significance, 

both because of characteristics which the rocks themselves possess and

1 Keport of the Board of Couuu., pp. 5-6, 1893.
BASCOM.] HISTORICAL EEVIEW. 19

because they form au important part of a belt of similar rocks extending northward aud southward along the Atlantic Coast.

Following is a list of publications on the rocks of South Mountain, 

Pennsylvania:

BIBLIOGRAPHY.

1755. Lewis Evans. Analysis of a map of the British colonies.

1787. Schopf. Beytriige zur miueralogischen Kentuiss des ostlichen Theila YOU Nordaruerika und seiner Gebtirge, chap. 30, pp. 66-101. 

1858. Rogers. Geology of Pennsylvania, Vol. I, pp. 203-209. 

1860. Tyson. Report on the Geology of Maryland, pp. 34-35.. 

1875-1877. Frazer. Second Geological Survey of Pennsylvania; Report of progress

in the counties of York, Adams, and Franklin, Vol. CC, p. 285.

1876. Hunt. Proc. Am. Ass. Adv. Sci., pp. 211-212.

1877. Frazer. Copper ores of Pennsylvania; Polytechnic Review, Vol. Ill, p. 170.

1878. Hunt. Second Geological Survey of Pennsylvania, Vol. E, p. 193.

1879. Frazer and Hunt. Trans. Am. Inst. Min. Eng., Vol. VII, pp, 331, 332, 338.

1879. Blandy. The Lake Superior copper rocks in Pennsylvania; Trans. Am. Inst. 

Min. Eng., Vol. VII, p. 331.

1880. Frazer. Second Geological Survey of Pennsylvania, Vol. CCC. Appendix. 

1883. Frazer. Iron ores of the middle James River.; Trans. Am. Jour. Miu. Eng.,

Vol. II, p. 203. 

1883-1884. Frazer. An hypothesis of the structure of the copper belt of South

Mountain; Trans. Am. Inst. Min. Eug., Vol. XII, p. 82. 

1883-1884. Heuderson. The copper deposits of South Mountain; Trans. Am. lust.

Min. Eug., Vol. XII, p. 90.

1891. H. R. Geiger and A. Keith. The structure of the Blue Ridge near Harpers 

Ferry; Bull. Geol. Soc. Am., Vol. II, pp. 155-164.

1892. Lesley. A Summary Description of the Geology of Pennsylvania, Vol. I, p. 146. 

1892. G. H. Williams. The volcanic rocks of South Mountain, in Pennsylvania

and Maryland; Am. Jour. Sci., Vol. XLIV, pp. 482-496, PI. X. Reprinted

in the Scientific American, Jan. 14, 1893. 

1892. C. D. Walcott. Notes ou the Cambrian rocks of Pennsylvania and Maryland,

from the Susquehauna to the Potomac; Am. Jour. Sci., Vol. XLIV, pp.

469-481.

1892. A. Keith. The geologic structure of the Blue Ridge in Maryland and Virginia; The American Geologist, Vol. X, No. 6, pp. 362-369.

1893. G. H. Williams. Piedrnoiitite and scheelite from the ancient rhyolite of 

South Mountain, Pennsylvania; Am. Jour. Sci., Vol. XLVI, pp. 50-57.

1896. C. D. Walcott. The Cambrian rocks of Pennsylvania; Bull. U. S. Geol. Survey No. 134.
CHAPTER II.

GEOLOGICAL RELATIONSHIP OF THE ROCKS OF THE MONTEREY

DISTRICT.

GENERAL DESCRIPTION OF THE AREA STUDIED.

This bulletin will treat chiefly of the geology of that portion of South 

Mountain which lies in Franklin and Adams counties and is embraced 

between the lower Cambrian quartettes of the Green Eidge on the 

west and the Triassic red sandstones of the plains on the east. The 

southern limit is the State line, while a parallel line passing through 

the Russel copper mine, some 6 miles to the north, furnishes a northern 

boundary. 1 (See PI. I.) Well-wooded mountains (Green Ridge, Mon 

terey Peak, Pine Mountain, Jacks Mountain, Raven Rock Mountain, 

Haycock Mountain, etc.), varying in altitude from 1,500 or 1,850 feet, 

with a general trend from northeast to southwest, inclosing high 

plateau-like valleys and sloping toward the eastern plains, constitute 

a region of exceptional natural beauty and geological interest. (See 

PI. II.)

Though but a limited portion of an extended South Mountain area, 

this district is in many respects quite typical and furnishes ample material for some general conclusions in regard to the geology of the entire 

area. Although, because of the short time at the disposal of the writer, 

detailed field work was limited to the Monterey district, the petrographical study included material collected throughout the South 

Mountain.

EXTENT AND CHARACTER OF THE THREE ROCK TYPES.

Three distinct rock types are readily recognized. (PI. III.)

1. A siliceous sedimentary rock, represented by aquartzose conglomerate, a sandstone, and a quartzite. This is rarely accompanied by an 

interbedded argillaceous slate.

2. An acid volcanic rock, which shows all phases of crystallization, 

from a spherulitic rhyolite to a true quartz-porphyry, is amygdaloid a I 

or compact, is accompanied by pyroclastics and breccias, and is" sometimes sheared into a perfectly fissile slate or sericite-schist.

3. A basic, holocrystalline, volcanic rock, which is usually amygdaloidal, massive, or more frequently schistose, and is also accompanied 

by pyroclastics and breccias and sheared to a slate.

1 As a matter of convenience this area will be called the Monterey district. 

20
U.S.QEOLOGICAL SURVEY BULLETIN NO. 136 PL.III.

LEGEND

AciclVolcani.cs (Pre-Cambrian.)

Basic Volcanios (Pre-Carahrian.)

JULIUS BIEN & CO. LITH. N.Y.-

GEOLOGIC AND TOPOGRAPHIC MAP OF THE MONTEREY DISTRICT

S c ale i
BASCOM.] THE SEDIMENTARIES. 21

The slates of the region thus prove to be both sedimentary and 

igneous. The former are argillaceous. The latter are either acid or 

basic, and are far more abundant than the former.

SEDIMENTARY ROCKS.

Areal distribution. The first-mentioned of the rock types occupies 

the high altitudes only, Green Ridge, Monterey Peak, Pine Mountain, 

and Jacks Mountain being largely formed of it, while it caps Haycock 

Mountain and the foothill to the east of Jacks Mountain.

Structural features. The strike of the sediments is uniformly northwest and southeast, with a mean dip of about 45 degrees .southeast. 

There are some exceptions to this southeasterly dip, notably at Monterey 

Peak, where the dip is northwest, forming, with the Green Ridge sandstone, a gentle syncline. (See PI. V.)

In the Jacks Mountain quartzite the bedding is exceedingly obscure 

and the cleavage very marked. There is an opportunity for error in 

determining the dip by the confusion of the two.

At the northeast end of the tunnel which is cut through a spur of 

Jacks Mountain, on the Gettysburg Railroad, stratification is distinctly visible, and shows that minor crumpling of the sediments as 

well as folding on a large scale resulted from their uplift. The rock 

is here folded in a series of small anticlines and synclines. (See PI. IV.)

Thickness. Professor Rogers estimated the thickness of the sandstone at 1,000 feet. Professor Lesley, on the other hand, considers it 

"immensely thick,"and states that Frazer's "section 11" shows 32.000 

feet of quartzite and 64,000 feet of schistose conglomerate. 1

Walcott 2 and Keith,3 in their examination of a section of the same 

quartzite, sandstone, and conglomerate, displayed to the west of the 

Monterey district, agree essentially with Professor Rogers's estimate 

(1,000-1,200 feet). The writer did not find sufficient data within the 

Monterey district for a reasonably correct estimate of the thickness of 

the quartzite. There are undoubtedly a very great number of minor 

crumplings and foldings resulting in compressed synclines, anticlines, 

and thrust planes in Jacks Mountain. Since there are no means of 

determining the number of these folds, no estimate can be made of the 

thickness of the formation. If the minor folds are ignored, the dip and 

horizontal extent of the sediments indicate a thickness which is enormous, an indication uusustained by sections only 2 miles to the west. 

Probably the estimate made by Mr. Walcott (1,200 feet) in that more 

favorable locality will be approximately correct for the Jacks Mountain 

quartzite.

Age and superposition. These siliceous elastics are the "Primal white

1 Summary, Final Report Geol. Pa., Vol. 1,1892, pp. 145-146.

2 Walcott, Notes ou the Cambrian rocks of Pennsylvania and Maryland from the Susquohanna to the 

Potomac: Am. Jour. Sci., Vol. XLIV, De'c., 1892, p. 481.

3Keith, The geologic structure of the Blue Ridge in Maryland and Virginia: Am. Geologist, Dec., 

1892, Vol. X, No. C, p. 365.
22 VOLCANIC EOCKS OF SOUTH MOUNTAIN. [BULL. 136.

sandstone" of Rogers, and are represented by him as interbedded with, 

slates and occurring in a series of narrow anticlines. They form the 

"Mountain Creek rock" of Frazer and Lesley, mapped by them as 

Azoic and represented both in sections and text as underlying the 

"chlorite schists" and "bedded orthofelsites." Messrs. Geiger and 

Keith, 1 in their earlier investigation 'of the southward extension of 

this sandstone in the Blue Ridge, placed it above the limestone of the 

Cumberland Valley, and, upon structural grounds, determined its age 

as Upper Silurian (Medina). The true age of these sediments has 

recently been determined beyond question, through the discovery of 

fossils, by Mr. Walcott.2 No fossils were found in the quartzose conglomerates and quartzites of the Mouterey district (No. 2 of Mr. Walcott's section), but the interbedded argillaceous shales, of .which mention has been made, are found to the west of the Monterey district passing .beneath quartzite and other shales in which were discovered the 

remains of Scolitlms linearis, Olenellus, and Camerella minor. These 

discoveries undoubtedly refer the conformably underlying Monterey 

quartzite to a Lower Cambrian age.

In a recent section made in the light of Mr. Walcott's discoveries and 

of his own subsequent discovery of fossils in the Blue Ridge, Mr. Keith * 

gives the Monterey sediments a similar position (No. 4), describing them 

as a "massive white sandstone with bluish black bands, feldspathic, and 

in places conglomeratic." Mr. Walcott calls them "a coarse-grained 

quartzite, sometimes conglomeratic."

The sections (PI. V) through the Monterey district show the relative 

distribution and position of the three type rocks. It will be observed 

that the sandstone lies in a gentle syncline, or almost flat, and wholly 

above the eruptives. These observations made by the writer in the 

Monterey district and elsewhere in the South Mountain, accord with the 

observations of Mr. Keith 4 in Maryland. Owing to the entire absence 

of exposure, save at the tunnel, the minor folding, which is undoubtedly 

present at many other points, is not indicated in the sections.

Mr. Keith 5 finds that in Maryland, as a result of faulting, the igneous 

rocks occasionally overlie the sandstone. This is nowhere the case 

in the Monterey district. The facts that no inclusions of sandstone 

were found in the volcanic breccia, and that fragments of the acid eruptive occur in the quartzose conglomerate, suggest the superficial character of the latter.

Further indications of the same nature will be discussed in considering the comparative age of the sedimentary and igneous rocks.

'H. E. Geigev and Arthur Keith, The structure of the Blue Kidge near Harpers Perry: Bull. Geol. 

Soc. Am., Vol. II, p. 1G3, pis. 4 and 5.

2 C. D. Walcott, Notes on Cambrian rocks of Pennsylvania and Maryland: Am. Jour. Sci., Vol. 

XLIV, 1892, p. 481.

3 A. Keith, Geologic structure of Blue Kidge in Maryland and Virginia: Am. Geologist, Vol. X, 1892, 

p. 365.

4 Geiger and Keith, loc. cit., p. 155, pis. 4 and 5. 

6 Loc. cit.,p.365.
CRUMPLING OF SANDSTONE AT THE EAST END OF TUNNEL THROUGH JACKS MOUNTAIN, ON 

THE GETTYSBURG RAILROAD.
BASCOM.] ERUPTIVE ACID ROCKS. 23

ACID EBUPTIVES.

Areal distribution. The flanks of the mountains, the hills, and the 

valleys are all formed of volcanic material ("Primal upper slate"), 

which, as Professor Bogers suggests, furnishes less resistance to erosion 

than the hard sandstone. Of these rocks the acid lavas are much less 

abundant and tend to occupy the lower altitudes in this portion of the 

South Mountain. Farther north the reverse is true.

With the exception of a large district northwest of Jacks Mountain, 

the acid rocks occur in insignificant areas, such as might be exposed 

by the erosion of the surrounding greenstone. There are nineteen such 

limited and isolated occurrences.

Character. The acid volcanics are always readily distinguished from 

the basic rocks by their bright colors, which range from a brick-red, 

through pink, purple, blue, or blue-gray, to a grayish-green, light-green, 

or buff. Opaque white or red phenocrysts. may be conspicuous or 

almost absent. Bands of spherulites, simulating bedding lines, are a 

prominent feature, while spherutaxites composed of spherulites ranging 

in size from a pea to a butternut are by no means uncommon. Butaxites with a marked yet delicate flow structure are abundant.

Only faint suggestions of the lithophysal structure were found in the 

Monterey district. To the northwest, at Eaccoon Creek, it was found 

in great perfection.

An amygdaloidal character is less general than with the basic rocks. 

In a few localities, however, notably the Bigham copper mine region, 

it is very pronounced. Both flow breccias and tuff breccias occur. 

The rocks split readily into slabs, and in general are cleaved parallel 

to the structure planes of the sedimentary rocks. In some instances 

the porphyries have been sheared into spotted slates, still preserving 

the crystalline outlines of the feldspar phenocrysts, or into fissile sericite-schists. Such metamorphism is displayed in an exposure of a 

few feet, or even in a single hand specimen.

There are not sufficient data for an estimate of the thickness of these 

rocks; In the porphyry area north of the Olerniont House, Monterey 

Springs, several borings have been made for water. One well was 

bored to the depth of 6G feet in 1878, and carried 48 feet lower in 1886, 

when it was reported that a " soft rock" was reached. A few rods west 

of this other attempts were made,.in 1892, to bore through the porphyry. The greatest depth reached was 62 feet, when, the character of 

the rock not altering, the undertaking was abandoned because of the 

refractory nature of the porphyry.

Previous descriptions. These are the rocks characterized by Professor Rogers as " a gray siliceous altered rock," " a reddish gray rock 

of the same series" (Primalupper slate), "ahighly metamorphicPrimal 

slate." They are the "orthofelsites" of Frazer, "showing no character 

of igneous action," and the "bedded petrosilex" of Hunt. Blandy 

covers them by the term " slates."
24 VOLCANIC ROCKS OF SOUTH MOUNTAIN.

Economic value. The susceptibility of these porphyries to a tine 

polish and their suitability for ornamental purposes have been remarked 

by many of those who have studied them.

The earliest mention of this sort was made in 1822 by Dr. Hayden, 1 

who notes " handsome porphyry, Nicholson's Gap, Blue Ridge, Pennsylvania, crystals red and distinct."

Speaking of Catoctin Mountain, the southern extremity of South 

Mountain in Maryland, Tyson says: 2

Its porphyries and amygdaloids are deserving of the attention that I propose hereafter to bestow upon them. Some of them will receive a beautiful polish, but their 

hardness renders the process expensive. This can, however, be overcome by appropriate machinery.

Dr. Frazer 3 has, as already quoted, described the porphyries as 

" suitable for an ornamental building stone."

Dr. Hunt 4 lias also called attention to the fact that "these peculiar 

rocks, which make such a conspicuous figure in the South Mountain of 

Pennsylvania south of the Susquehanna, are of interest economically 

from the fact that they are in other regions the repositories of rich 

iron ores, and also because they afford ornamental porphyries of rare 

beauty, similar to those wrought in Elfdalen, in Sweden."

The Tenth Census 5 reports that "it [the South Mountain porphyry] 

is well adapted to'ornamental work, as it is rich in color, durable, and 

susceptible of a good polish, and in many cases could be obtained in 

abundant quantities. It has not yet been quarried for purposes of 

construction."

BASIC ERUPTIVES.

Areal distribution. The basic eruptives occupy, in this district, an 

area fully twice as large as that covered by the acid eruptives constituting the major part of the valleys, foothills, and mountain flanks.

Character. The rocks are massive, schistose, or slaty. They are 

usually conspicuously amygdaloidal, and associated with these amygdaloids are banded fine-grained schists, which have been considered alteredaccumulations of volcanic ash. In a section exposed on the Gettysburg 

Eailroad there are alternating bands, from 2 to 3 feet wide, of a compact, 

fine-grained, epidotic rock which may also represent a basic volcanic 

ash. Bombs were found embedded in this epidotic rock. As with the 

acid rocks, there are accompanying.basic breccias, though the latter are 

not abundant. The cementing material in every case was epidotic. 

An agglomerate formed of rounded fragments from an inch to 6 inches 

in diameter was also found. There was no opportunity for estimating 

the thickness of the accumulated basic flows. Wells have been bored 

in the basic rock to the depths of 55, 85, and 110 feet.

1 H. H. Hayden, Mineralogical notes: Am. Jour. Sci. and Arts, Vol. V. 1822, p. 255. 

 * Tyson, First Annual lleport, 1860, Appendix, p. 3. 

3 Frazer, Vol. CO, p. 285. 

4 Hunt, Proc. Am. Ass. Adv. Sci., 1876, p. 212. 

' Teiitli Census lleport on the Uuilding Stones of tbe United States, p. 168.
U.S.GEOLOGICAL SURVEY BULLETIN NO. 136 PL.V.

Green Ridge

i Toms Creek

Old Furnace Road Pine Mi

Tributary to Toms Cr. Gladhills Road

I

Jacks Mt.

Tape-worm R.R.

Gettysburg R.R. Tributary to Miney Branch 

Cove Hollow

-Tributaries to Friends CreekFriends Creek

 n-jj Old Furnace Road Green Ridge

Devils Race Course Monterey Peak

Gettysburg R.R. 

and Turnpike. Miney Branch 

Wood Road

r _.._.._- ... -Tributaries to Miney Branch-

!

| Hay Cock Mt.

Tributaries to Friends CreekRoad

Clines Hill SECTIONS THROUGH THE MONTEREY DISTRICT.

Cambrian Sandstone. 

Volc anic s (Pre - C ambrian.)

Triassic Formation.

B a si <*"Vbl c ani c s (Pre -Cam bri an.)
luscoM.] ORES. 25

/ 

The basic rock, by reason of its softer character, is more subject to

alteration under dynamic action than are the hard acid rocks. The 

effect of this is seen in the almost universal schistosity of the basic 

rock. This metaniorphisin is accompanied by a correspondingly greater 

chemical alteration than is shown by the acid rocks. The alteration 

consists largely in the abundant development of epidote and chlorite, 

which gives to the rock its uniform green color and its popular name of 

"greenstone."

Previous descriptions. Dr. Hayden 1 notes the remarkable development of epidote as follows:

Most beautiful epidote, with green and other shades of copper scattered in quartz. 

The blue is prevalent. Abundant in Blue Ridge. * * *

Quartz aud epidote. with green carbonate and rod oxid of copper and native 

copper. Blue Ridge; abundant.

The greenstones are locally known as the "copper rock," because of 

the copper ore which they carry. They are Rogers's "lower Primal 

slates," which "are highly indurated, and even decidedly crystalline, 

containing in some of their layers segregated specks, and even halfformed geodes [amygdules] of epidote and other minerals." "A highly 

altered, greenish slate." "Dark-green slate, with its epidote and 

white, intrusive quartz." Tyson describes them as " slates." Dr. Frazer 

calls them "chlorite schists," and Dr. Hunt terms them epidotic or 

chloritic rocks. Blandy approaches most nearly the present idea of 

their nature in describing them as " amygdaloidal trap."

ORE DEPOSITS OF THE MONTEREY DISTRICT.

The workable deposits of ore in the South Mountain fall into two 

classes: The limonite ores, which are deposited in Paleozoic sedimentary rocks, and the copper ores, which are associated with pre-Paleozoic 

igneous rocks.

Profitable liniouite iron mines are being worked at various localities 

in the South Mountain, in the Cambrian sandstone, and along the contacts between it and the Silurian limestone. They are fully discussed 

in the Final-Report of the Second Geological Survey of Pennsylvania, 

Vol. I.

One such ore bank occurs within the Monterey district, on its northeastern boundary, at the contact between Triassic limestone and the 

Cambrian sandstone, near Old Maria furnace. This bank is exhausted 

and has not been worked for many years.

An interesting occurrence of copper ore and native copper 2 is to be 

found within the South Mountain chain, in a belt extending from a 

point some 6 miles north of the State line, in a southwesterly direction,

"Loc.cit.,1822, p. 255.

"Frazer, Copper ores of Pennsylvania: Polytechnic Review, Vol. Ill, 1887, No. 16, p. 159; Au hypothesis of the structure of the copper holt of the South Mountain, pp. 82-85. Hendersou, The copper 

deposits ot the South Mountain: Traus. Am. lust. Mill., Eng., Vol XII, pp. 85-90.
26 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

to and beyond the State line into Maryland. This belt lies along the 

contact between the acid and basic eruptives. 1

The principal localities where shafts have been sunk within this belt 

are the Snively copper mines, the Eussel farm (the Bechtel shaft), the 

Eeed copper pits, the Bigharn copper mine, and the Headlight mine. 

(See map, PI. III.) To the east of this belt, on the road between Fairfield and Fountaindale, a fifth old copper shaft is located.

The Snively copper mine, which is a mile north of the Mouterey district, was not visited by the writer.

Frazer reports that the copper is associated with quartz, epidotic 

rock, and azurite.2

At the Eussel copper mine, which is located on the Eussel farm, at the 

fork of Copper Eun, native copper occurs in quartz veins, associated 

with calcite, in a very siliceous epidotic rock, an epidosite. The material thrown out at the lower shaft was amygdaloidal greenstone; at the 

upper or Bechtel shaft, an acid slate, which indicates that the copper 

belt is along the contact between the two igneous rocks.

The Eeed copper pits lie on the north side of Toms Creek, at the 

forking of the highroad near Spring Eun. It has not been worked for 

forty years, and there is nothing visible to indicate the character of the 

copper occurrence. The dump pile shows scoriaceous greenstones 

with malachite stains. Dr. Suively reports 'that native copper occurs 

here in quartz veins.

The excavation for copper ore on the Bighani property, which lies 

just northwest of the forking of Gladhills road, furnishes interesting 

material for the petrographer. It is here that acid amygdaloids have 

been exposed in great perfection. They are abundantly stained with 

malachite, azurite, or cuprite. Metallic copper occurs in quartz veins 

traversing the amygdaloids, and in submacroscopic quantities in the 

amygdules. In the latter case the copper is frequently surrounded by 

zones of the oxide and the carbonate. An analysis of these rocks gives 

4 per cent of copper. The Headlight mine is tunneled beneath the turnpike half a mile (eastward) below the Clermont House. Here the dump 

pile shows only greenstones, more or less stained with copper carbonates. 

This is also the case at the old copper shaft on the Fairfield road. 

Asbestos in quartz is quite generally found at these copper mines.

Throughout this belt the copper is evidently of secondary origin, 

occurring in seams and amygdules, where it has been deposited from 

solution. This occurrence of metallic copper in the igneous rocks of 

South Mountain is interesting because of its similarity to the Lake 

Superior copper-ore deposits. Its associations are quite analogous. 

In the Lake Superior region it occurs in veins, quartzose and epidotic, 

in the open-textured amygdaloids (basic), and in interbedded coni Frazev says: " The ore belt lies in the orthofelsite which forms this portion of the chain." 

2P, Frazer, Copper ores of Pennsylvania: Polytechnic Review, Vol. Ill, 1877, No. 16, p. 170; also, 

Appendix, Vol. CCC, Second Geol. Surv. Pa., p. 310.
BASCOM.] AGE. OF THE ROCKS. 27

glomerates. In the South Mountain, in the absence of contemporaneous 

interbeddecl elastics, the occurrence is limited to the igneous rocks.

The replacement of the feldspar by chlorite and epidote, and their 

replacement in turn by copper, characterizes the amygdaloids of the 

South Mountain also.

The explanation of the precipitation of the copper is doubtless the 

same in both regions.1

A correlation of the South Mountain formations and the Keweenawan copper-bearing rocks was suggested by Mr. Blandy in the article 

before quoted. There is great petrographical similarity between the 

porphyrites and felsites and diabase porphyrites of the Keweeuawan 

series and their equivalents in the South Mountain, but it seems to the 

writer unwise to parallelize on petrographical evidence the South 

Mountain igneous rocks and the Keweenawan of Lake Superior. All 

correlation upon such grounds, when applied to rocks widely separated, 

is well known to be untrustworthy. There has not as yet been found 

sufficient evidence to show to what horizon in the Algorikian series the 

igneous rocks of South Mountain should be referred.

COMPARATIVE AGE OF SEDIMENTARY AND IG-NEOUS ROCKS.

CONTACTS DESCRIBED.

That the Cambrian rocks do not underlie the "slates" and "orthofelsites," as stated in the reports of the Pennsylvania surveys, is quite 

plain, but whether the sediments are entirely subsequent to the 

igneous, rocks or are in part, at least, contemporaneous, it is not so 

easy to decide.

Contacts between the sedimentary and igneous rocks are finely 

exposed at two localities. About halfway through the tunnel on the 

Gettysburg Eailroad the basic igneous rocks and the Cambrian rocks - 

are in contact. Both formations dip gently to the southeast (±20°). 

The sandstone has become an indurated quartzite. Close to the greenstone it has acquired a green color, due to the abundant development 

of chlorite. It has also become very schistose, and in fact might 

readily be confused with the greenstone itself. Granulation, undulatory extinction, and the obliteration of all direct evidence of clastic 

origin, as revealed by the microscope, indicate the action of dynamic 

force on the quartzite. The greenstone is slaty and too decayed at the 

contact for microscopic study.

Southwest of Old Maria Furnace, in an abandoned cut on the Tapeworm Eailroad, a great surface of red felsite 2 is partially exposed and 

partially overlain by the clastic formation. (PI. V.I.) Here again 

there is a gentle dip of both rocks to the southeast (±45°). The sand-

'Pumpelly, Geology of Michigan, Vol.1, Part III, p. 43.

2 TTelsite is used here as a field term for a compact, stony, nonporphyritic or inconspicuously porphyritic volcanic rock. See pp. 37-38.
28 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BOLL. 136.

stone lias become a vitreous quartzite. The hard felsite does not showalteration. That the igneous and sedimentary rocks have been subjected to the same forces of folding is shown by the uniformity of their 

cleavage dips at both these localities.

This conformity of structure planes, the absence of contact metamorphisni, and the evidences of dynamic action indicate planes of 

thrust rather than planes of original deposition or of subsequent 

igneous intrusion.

CONCLUSIONS.

These, two contacts do not decisively prove the younger age of the 

overlying sediments, although they very strongly indicate it. If the 

sandstone has been thrust over the lava from beneath, it would be necessary to suppose an enormous amount of erosion to account for the 

entire absence of volcanic material above the sandstone. On the other 

hand, it is very easy to suppose that overlying sediments, on being 

subjected to pressure, were thrust up over the igneous rocks from the 

east. This explanation of the facts coincides with other evidence relative to the comparative age of the sedimentary and volcanic rocks.

(1) Nowhere in the South Mountain has there been found a dike of 

the volcanic material in the sedimentary rock. Were the lavas more 

recent than-the sediments, the former could hardly fail of filling cracks 

in the latter.

(2) No sedimentary beds have been found intercalated with the lava 

flows.

(3) There is no evidence of the alteration of the sedimentary rock 

by igneous contact.

(4) The igneous rocks, in their fluxion and arnygdaloidal structures, 

and in their accompanying pyroclastics and ash deposits, bear every 

evidence of being subaerial lava flows.

(5) The presence of igneous fragments in the basal conglomerate of 

the sedimentary formation shows that erosion of the igneous rocks was 

taking place while the sediments were accumulating along their edges.

While it is true that some erosion of the eruptives must have taken 

place before and during the deposition of the Cambrian sediments, 

there is no evidence of extended erosion; indeed, the character of surface flows, which the lavas retain so conspicuously, would incline one to 

suppose that no great load of material has been removed. Mr. Keith's 

conception that the granite which in Maryland is intruded into the 

"diabase" (greenstone) is younger than the volcanics necessitates 

an immense original thickness of the diabase flows and an extended 

subsequent erosion to expose the plutonic eruptive. In this respect 

his argument for the comparative age of the deep-seated and surface 

igneous rocks seems at fault 1 . Such unlimited erosion is unsupported 

by field observations in the South Mountain.

1 Geologic structure of Blue llidgo in Maryland and Virginia: Am. Geologist, Vol. X, 1892, p. 368.
U. S. GEOLOGICAL SURVEV BULLETIN NO. 136 PL. '

Felsite. Sandstone. 

JUNCTION OF FELSITE AND SANDSTONE ON THE OLD TAPEWORM RAILROAD.
BABCOM.]. AGE OF THE ROCKS. . 29 

COMPARATIVE AGE OF ACID AND BASIC ERUPTIVES.

FIELD RELATIONS.

The relative age of the acid and basic volcamcs is an interesting 

question, a question to which, it is not possible with our present knowledge to give an entirely satisfactory answer.

On the one hand we find the acid rocks occupying, as a rule, the 

lower altitudes, and even overlain by the basic rock. Occurrences of 

the latter nature are located as follows:

(1) At the junction of Gladhill's road and the Fountaindale turnpike, in the northeast angle of the roads, a slight excavation exposes 

an outcrop of gray, slaty felsite overlain by greenstone.

(2) Not far from the source of Minie Branch there is exposed by the 

cutting of the stream a gray felsite, overlain by greenstone.

(3) On the road from Fountaindale to Fail-field, southeast of Jacks 

Mountain, a much-altered felsite is overlain by greenstone.

(4) Lastly, on the Fountaindale turnpike, about half a mile above the 

Eminetsburg tollgate, the same superposition is exposed by the roadside.

The only published statement of opinion as to the relative age of the 

volcanics has been made by Mr. Keith. 1 From his observations of the 

relative positions of the acid and basic rocks in Maryland, Mr. Keith 

expresses himself as confident that the ll quartz-porphyry underlies the 

diabase."

On the other hand, over against this somewhat meager proof must 

be considered a few facts of another sort. Professor Williams reports 

that a position of the acid and basic rocks the reverse of that which has 

been described in the Monterey district occurs north of that district. 

Throughout the northern portion of the range the acid eruptives, which 

abound almost to the exclusion of the basic eruptives, form the mountains, while the latter occupy the valleys.

We must, moreover, bear in mind that the principles of stratigraphy 

employed in determining the age of sedimentary rocks may prove very 

misleading if applied to igneous rocks. The younger lava may be 

found overlain by the materials of an earlier eruption where the 

former, in coming to the surface, breaks through the older formation. 

The latest eruption may fill the depressions.

Where two different lavas occur on the same level, as is the case 

south of the Clermont House, the lava (in this case the acid lava) occurring as a narrow strip inclosed by the other might be au intrusive 

eruptive, and thus younger than the inclosing basic lava.

PROBABLE EXPLANATION.

With these principles in mind, with conflicting field evidence, and in 

the absence of genuine dikes of either acid or basic character, the data 

for the determination of the comparative age of the acid and basic

> Loc. cit., p. 367,
30 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

volcanics are tiot sufficient. In all probability there were several 

sources of lava flow.

The southern vents furnished the great areas of basic lava (greenstones) in Maryland and southern Pennsylvania, while the northern 

vents poured out the enormous acid flows.

In the Monterey district the two lavas are mingled, and apparently 

the basic flow was preceded by the acid flow.

Thus the facts observed by Professor Williams in the north, Mr. 

Keith in the south, and the writer in the Monterey district, may be 

brought into accord.

SUMMARY.

Three types of rocks are present in the South Mountain: (1) Siliceous 

sedimentary rocks of Lower Cambrian age, overlying with structural 

conformity and strati graphical unconformity igneous rocks. These 

igneous rocks are surface flows of (2) an acid and (3) a basic constitution. They are probably pre-Cambrian, and lithologically resemble the 

Keweenawau copper-bearing rocks of Lake Superior.

There is not sufficient evidence to decide their comparative age, 

though in the Monterey district field observations, on the whole, indicate that the acid rocks are the older.

The subaerial flows were subjected to a limited erosion before the 

sediments were deposited and while they were being deposited, so that 

the region must have possessed some elevation at that time.

The intense dynamic action shown by the igneous rocks occurred 

after the deposition of the sediments. Since the sediments were laid 

down the whole region has been subjected'to lateral pressure (at the 

time of the Appalachian uplift), whereby the igneous rocks were cleaved 

and sheared and the sedimentary formation was thrust up over them 

from the east where it has been largely eroded, occurring now only 

sporadically and the whole region was elevated. That erosion has 

removed a great thickness of material since this elevation is indicated 

by the cleavage dips of the igneous rocks.
CHAPTER III.

PETROGRAPHICAL DESCRIPTION OF THE CAMBRIAN ROCKS. 

MACROSCOPICAL DESCRIPTION.

The sedimentary formation exhibits two marked phases, the conglomeratic and the quartzose. Its lowest member is conglomeratic.

These conglomerates are frequently slaty, through the development 

of more or less sericite. They contain quartz pebbles from an inch to 

an inch and a half in length, and sometimes show included fragments 

of quartz-porphyry and of a green slate.

From a conglomeratic character the sediments pass through a coarse 

sandstone into a compact quartzite exhibiting under the microscope the 

characteristics of a re-crystallized clastic.

The quartzose sandstone is exposed in great masses on the flanks and 

summit of Monterey Peak, on Haycock Summit, at the Gladhills Switch 

on the Gettysburg Railroad and at many points along the railroad 

as it skirts the east side of Jacks Mountain, and in massive pinnacles 

along the crest of that mountain. That the sandstone has been greatly 

fissured and broken is indicated by the conspicuous and frequent quartz 

veiuiug, but metamorphism, save in a few cases, has been limited to 

the formation of a vitreous quartzite by the deposition of a siliceous 

cement. The original stratification planes are usually preserved, and 

cross bedding is frequently conspicuous. A secondary cleavage is also 

very marked, so much so as, in the absence of well-defined stratification planes, to be mistaken for the bedding.

MICROSCOPICAL DESCRIPTION. 

QUARTZITE.

Four specimens of -typical quartzite were studied in the thin section. 

They were obtained from widely separated localities, and range in color 

from gray to a light green. Two of them illustrate the enlargement of 

quartz fragments and the genesis of a quartzite as perfectly as do any 

of the Lake Superior sandstones. 1 (PI. XV, a.}

'Quartz enlargements were first described by Tiirnobohui, subsequently by Sorby, Young, Irving 

and Vau Hiso, Bonney, Phillips, and Iddiugs:

A. E. Torneboiun, Ein Beitrag Kiir Frage der Quartzbildung: Geol. Foreu. StockL. 1876, Vol. Ill, 

p. 35. Kef. Neues Jahrbucli fur Mineral., 1877, p. 210.

H. Clifton Sorby: Anniversary Address, Quart. Jour. Geol. Soc. London, Vol. XXXVI, 1880, p. 62.

A. A. Young: Am. Jour. Sci., Vol. XXII, July, 1881.

11. D. Irving: Am. Jour. Sci., Vol. XXV, June, 1883.

E. D. Irving and C. E. Van Hise: Bull. TJ. S. Geol. Survey No. 8, 1884.

T. G. Bouncy and J. A. Phillips: Quart. Jour. Geol. Soc. London, Vol. XXXIX, 1883, p. 19.

E. D. Irviug and C. E. Van Hise: The Penokee iron-bearing series of Jjlichigitu and Wisconsin: 

Tenth Ann. Eept. U. S. Geol. Survey (1888-89), p. 375, PI. XXVII, fig. 2,

J. P. Iddings: Mon. U. S. Gcol, Survey, Vol. XX. 1892, pp. 346-347.

31
32 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [u0LL,i36.

The original grains are conspicuously outlined by a rim of iron oxide 

and surrounded by interlocking areas which are optically continuous 

with the inclosed grain. Barely the grain itself is composed of two 

differently oriented areas, in which case the enlargement is also composed of two different areas oriented with the two portions of the grain.

These same specimens show feldspar grains, usually with the " gridiron" structure of microcline. Occasionally a hornblende fragment 

is also present. The rock is evidently derived from, granitic debris 

and might more accurately be called arkose. The fresh character of 

the fragments is in general noteworthy and suggests a source not far 

distant.

The best examples of the vitreous quartzite, the extreme phase of 

metamorphism in the sediments, are to be found in the tunnel at and 

in the neighborhood of the contact with the greenstone, and on the 

unfinished Tapeworm Eailroad southwest of the Old Maria Furnace at 

the similar contact of the sandstone and -felsite. At both of these 

localities the rock shows microscopically and microscopically the effect 

of dynamic action. Of two specimens from the first locality, which 

has already been described in sufficient detail on page 27, one is a pure 

quartzite with prominent cleavage, on the surface of which iron rust is 

deposited. The thin section shows a quartzite from which every vestige of the original waterworn grains has been obliterated. The rock 

consists of interlocking areas of quartz and some interstitial material. 

This is a tine aggregate* of quartz grains, and may be a remnant of the 

granulation which must have occurred as a result of the movement of 

the grains against one another.

A similar phenomenon in the sandstone of Sugar Loaf Mountain .has 

been figured by Dr. Keyes. 1

Another specimen from the same locality, but nearer the greenstone, 

shows a rock which, because of the shearing and chloritization that it 

has undergone, very closely resembles a chlorite schist. Its clastic 

character, however, becomes quite evident under microscopic examination. This shows it to be a siliceous rock, greatly sheared and altered 

in character by the formation of sericite and chlorite. It is the only 

section of. quartzite in which chlorite has been observed. Its close 

proximity to the greenstone, in which that mineral is so abundantly 

developed, suggests a probable source of the magnesia, iron, and alumina. Tourmaline, zircon, magnetite, and feldspar are also present in 

this rock.

The quartzite at the felsite contact is similar, both in the hand specimen and in the thin section, to the first of the two specimens just described. No trace of the original grain remains. Undulatory-extinction 

is pronounced and recrystallization complete. Specimens obtained close 

to the acid igneous rock differ only in the presence of a large amount of

1 C. ~R. Keyes, A geological section across the Piedmont Plateau in Maryland: Bull. Geol. Soc. America, 

Vol. H, 1891,p.321. ' ' "" ' " ""
SLATES. ' 33

red iron oxide. The quartz crystals are frequently cracked, testifying 

to more intense dynamic action. The extreme induration at both these 

contacts is confined to a selvage of the sandstone.

SLATES.

In a cut on the G-ettysburg Railroad, southwest of the Old Maria Furnace, the sedimentary rock is locally of a different character from that 

which has been described. It resembles the second specimen described 

from the contact in the tunnel more closely than any other of the elastics. Shearing has been accompanied by the development of sericite 

and chlorite, producing a soft, green, slaty rock. The chloritic areas 

have usually clearly denned boundaries, and possibly represent former 

hornblende grains. Zircon is present. The major part of the rock consists of quartz grains, with nndulatory extinction.

Another local alteration of the clastic rock is to be found on the hill, 

tops southeast of Jacks Mountain. Here it appears as a yellowish 

schistose rock. The thin section is characterized by the development of 

a large amount of sericite and granular epidote. The" sericitic scales 

show a parallel arrangement, which is the product of shearing. The 

color of the rock is due largely to an iron hydroxide.

Among the sedimentary rocks within the Monterey district there 

rarely occurs, associated with the sandstone, an argillaceous slate. 

This is exposed just north of Jacks Mountain Station. It is the only 

representative in the Monterey district of the interbcdded slates occurring west of that district. It is silky, pearl-gray, crinkled, and cleaves 

readily into slabs.

The thin section shows that the rock has been somewhat recrystallized, though its clastic origin is still apparent in the angular shape of 

its quartz fragments. Quartz and sericite are the principal constituents of the rock. The other constituents are magnetite, hematite, and 

a gray-cloudy aggregate which, under the highest power, obscurely 

suggests leucoxene and granular epidote.

CHEMICAL ANALYSIS.

The analysis of the quartzite given below shows proportions which 

would be possible only in a derivative rock where the exact chemical 

composition is a matter of accident and subject to no fixed laws.

The silica percentage, is disproportionately high for any but a clastic 

rock. The alkalies indicate the presence of a considerable amount of 

either orthoclase feldspar or sericite, probably the latter. The alumina 

and lime percentages denote the presence of epidote. 

Bull. 136 3
34 VOLCANIC ROCKS OF SOUTH MOUNTAIN. 

Anfllysis of quartzite. 1

[BULL. 130.

Ti02 ... ..........

P.A .............

AlsO,- -.. ..

Fe2 03 .... ........

Per cent.

7.870

.145

.205

4.210

1.800

fnO

K2O. ....... .......

Na,0 .............

Total .......

Per cent.

.160

.960

100. C80

SUMMARY.

The Cambrian sediments of the Monterey district may be classified 

as conglomerate, sandstone, quartzite, and slate. < 

The first two of these phases represent original sedimentary deposits, 

of which the latter has been locally metamorphosed by induration into 

a quartzite. The slates are in some cases from deposits of a more 

finely divided material, and in other cases they are the product of shearing.

There has been a limited and local introduction of new material, 

which shows itself in the production of clilorite and epidote. In every 

case the clastic character of the rock remains indubitable.

1 Specimen No. 1157, Ifc miles S. of "W. of the burned sawmill ou tlie Conococheagno. This analysis 

was made by Dr. IT. A. Gentli, formerly of Pennsylvania University, for the Second Geological Survey of Pennsylvania, Vol. CCC, Second Geol. Surv. Pa.
CHAPTER IV.

PETROGRAPHICAL DESCRIPTION OF THE ACID ERUPTIVES.

NOMENCLATURE.

The question of the nomenclature of the acid volcauics has proved 

to be one of considerable interest and importance. In the discussion 

of suitable terms for the description of these rocks, it will he necessary 

to anticipate, in some degree, the results of their microscopic study.

A variety of names have been applied by petrographers to the acid 

type of the oldei1 volcanic rocks. Under the general group of quartzporphyries, Rosenbusch classifies them as microgranites, with a microgranitic groundinass; granophyres, with a rnicropegmatic groundinass; 

felsopliyrcs, with a microfelsitic base; and mtropliyres (including pitchstones and pitcbstone-porphyries), with a vitreous base. Fouque" and 

L6vy employ microgranitite, micropegmatite, and porpliyre petrosiliceux 

as corresponding terms. British petrographers have described these 

acid rocks under the terms kornstones, clay stones and claystone-porphyries, felsites, quartz-felsites, and felsite-poryhyries, agreeing in this 

respect with the older German usage, when they have not followed 

Rosenbusch. In America both German and English usages have been 

followed, with more or less confusing results. In the nomenclature of 

the South Mountain rocks an effort has been made to avoid such confusion and to use such a term or terms as shall accurately characterize 

them and all similar rocks.

Among the acid eruptives of South Mountain are typical representations of Rosenbusch's quartz-porphyries. Closely associated with 

them arid impossible of separation by any sharp line of demarcation are 

acid rocks wifch every structural characteristic of modern lavas.

Although possessing some characteristics in common with the/e/sopJiyres, these ancient prototypes of the rhyolite can not be included 

under that term, since they have a holocrystalline grouudmass. Insomuch as many of the English felsites have been shown by Rutley, 

Allport, Cole, and Boniiey to be devitrified obsidians and pitchstones, and thus, like the American rocks, representatives of the glassy 

lavas of pre-Tertiary times, these South Mountain lavas might, with 

some propriety, be termed felsites. Strict consistency would then compel the replacement of the term, quartz-porphyry by the more limited 

and less well-established term quartz-felsite, which was felt to be a disadvantage. Moreover, the distinction between the rhyolitic lavas of

35
36 VOLCANIC KOCKS OF SOUTH MOUNTAIN. [BULL. 136.

South Mountain and the typical quartz-pophyries is not one of the 

absence or presence of phenocrysts. Thus the term felsite would be 

forced to cover not only nonporphyritic acid rocks, but also conspicuously porphyritic rocks. Finally, felsite, though useful as a field name, 

may well be objected .to as an inaccurate petrographical term.

A brief history of the word felsite and its synonyms in different countries (petrosilex, eurite, and halleflinta,) will serve to illustrate the 

unfitness of it and these allied terms for exact petrographical usage.

"Felsit" was introduced into German petrographical nomenclature 

by Gerhard 1 in 1814, who applied the term to a matrix which he claimed 

was common to all feldspar, claystone, and hornstone porphyries. This 

matrix was a compact homogeneous aggregate of feldspar and "quartz, 

supposed by Gerhard to be compact feldspar.

In 1858 Naumann 2 adopted Gerhard's word for the feldspar-quartz 

groundmass, and called all porphyries with such a base felsite-porphyries. This term has since been applied by Tschermak to porphyries 

without quartz phenocrysts (orthofelsites, orthophyres). 

. The more accurate German usage of the present day discards felsite 

save as a macroscopic term for an unresolvable porphyry base,3 Avhile 

"felsitfels" is used if, in the absence of pheuocrysts, the rock is composed of this base only.4

This same quartz-feldspar mosaic, confused with compact feldspar, 

was distinguished as petrosilex by Wallerius 5 as early as 1747, and 

subsequently by Dolomieu. .This term was also early used by Brongniart. In 1819 the same groundmass was called eurite by Daubisson" 

because of its fusibility. Michel-Levy 7 uses the term petrosilex, but 

with a more limited meaning. He defines it as essentially synonymous 

with Eosenbusch's microfelsite, making " porphyre petrosiliceux" equivalent to felsophyre. Petrosilex is, however, generally used by Levy and 

other French petrographers in a more or less loose and vagne way, to

cover a crystalline or cryptocrystalline quartz-feldspar aggregate or a 

partially amorphous siliceous feldspathic magma.

In Sweden, the fine-grained, apparently homogeneous acid rocks 

consisting essentially of quartz and feldspar and rarely porphyritic, 

are called hallefliuta.8 .They may be of aqueous or of igneous origin.

The early meaning of felsite in England was quite similar to that 

given it on the Continent. According to Pinkerton 9 the term was

'Bcitrage zur Goschichto des Weissteins des Felsit und anderer verwaiidten Arten: Abhandl. 

K. Akad. Wiss. zu Berlin, 1814, 1815, pp. 18-26. 

2 Lelirbucli dor Geognosie, 2<1 ed., Vol. I, 1858, p. 597., 

3Kosenbusch, Mikro. Pliys., etc., 2d ed., Vol. II, p. 373. 

4Rosenbusch, loc. cit., p. 354.

8 Systematica Mineralogicum, 1847, French translation, 2 vols., Paris, 1853. 

«Trait6 de g6ognosie, 1st ed., Vol. 1,1819, p. 112.

7 Structures et classification des roches eruptives, p. 17.

8 Justus Eotb, Allgemeine und cbemiscbe Geologie, Vol. II, p. 494. F. Zirkel, Lehrbucli der Petrographie, Vol. I, p. 564. 

9 T. Pinkerton, Petralogy, A Treatise on EockSj Vol. I, 1811, p. 161.
BASCOM.] NOMENCLATURE. 37

introduced in 1794 by Kirwan : for compact feldspar. Current usage as 

defined by Teall 2 applies the term to " compact stony rocks, the mineral composition of which can not be ascertained by examination with 

the naked eye or with the lens. * * * These rocks are anhydrous 

(or nearly so), and except in this respect agree in composition with the 

acid glassy lavas."

Dana 3 (as an exponent of American usage) defines "felsyte," a rock, 

as "compact orthoclase with often some quartz intimately mixed, fine 

granular to flint-like in fracture, * * * both metamorphic and 

eruptive." The mineral he defines as follows: "Felsite is compact,' 

uncleavable orthoclase, having the texture of jasper or flint, which it 

much resembles. It often contains some disseminated silica. It is 

distinguished from flint or jasper by its fusibility. * * * It is the 

base of much red porphyry." This is substantially the " felsitfels " and 

 " felsit" of Rosenbnscli.

In short, "felsite" has been used to describe an acid base. unresolva,ble 

by the naked eye, and once supposed to be a single mineral. With the 

introduction of the microscope this macro-"felsitic" base was resolved 

into the microgranitic, micropegmatitic, and microfelsitic groundmass, 

the point of ignorance having been shifted from the felsitic base, 

macroscopically unresolvable, to the microfelsitic base, which is microscopically unresolvable.4

On the Continent "felsite" has practically been replaced by these 

terms. British and American petrographers retain it as a useful field 

name for rocks formed of this macroscopically unresolvable base without phenocrysts or with inconspicuous phenocrysts. In this sense the 

word will be used, when used at all, in this paper.

It is very generally recognized that structural features are not conditioned by the geological age of rocks, but are a function of the conditions of consolidation. That these conditions, while very varied and 

complex in any geological period, have not essentially altered since 

Paleozoic times, has been shown to be the case by some of the most 

able observers.5

With this recognition has come the growing conviction among petrographers that mere age should be eliminated as a factor in geological nomenclature. While this is true, it is felt on the other hand 

that the rock name should show some recognition of the alteration

1 Richard Kirwan, Elements of Mineralogy.

2 J. J. Harris Teall, British Petrography, p. 291.

3J. D. Dana, Manual of Mineralogy and Lithology, 3d ed.. 1883, pp. 280, 442. Manual of Geology, 

3ded., 1879, pp. 73, 77.

"Rosenbusch does not assent to this interpretation of microfelsite, but regards it as a definite 

chemical compound allied to feldspar, just as folsite was once regarded.

6 Teall, Address of the Pres. of the Geol. Soc. of the British Ass, Adv. Sci., 1893.

Judd, On the gabbros, dolerites, and basalts of Tertiary age in Scotland and Ireland: Quart. Jour. 

Geol. Soc., London, Vol. XLH, 1880, pp. 49-07.

Allport, Tertiary and Paleozoic trap rocks: Gool. Mag., London, 1873, p. 196. British Carboniferous 

Dolerites: Quart. Jour. Geol. Soc., London, Vol. XXX, 1874, pp. 529-567.

Iddings and Hague, On the development of crystallisation in the igneous rocks of Washoo, Nov., 

with notes on the geology of the district: Bull. U. S. Geol. Survey No. 17,1885
38 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

which the rock has undergone subsequent to its solidification. If at 

the time of its solidification the rock presented the features of a 

rhyolite, as is believed to have been the case with much of the South 

Mountain acid lava, but since that time has become holocrystalliue, 

both these facts its original character and its present character 

should be recognized in the name. It is believed that this result may 

be secured by the retention of such well-established names as rhyolite, 

obsidian, trachyte, etc., preceded by a prefix which shall have such a 

signification as will indicate the altered character of the rock. The 

'prepositions meta, epi, and apo all indicate, as prefixes, some sort of an 

alteration. Their exact force has been thus defined by Professor Gildersleeve. Meta indicates change of any sort, the nature of the 

change not specified. This accords Avifch the use of the prefix by Dana 

in such terms as "metadiorite" and " metadiabase." These terms have 

been recently revived to designate "rocks now similar in mineralogicalcomposition and structure to certain igneous rocks, but. derived by 

metamorphism from something else." 1

jEpi signifies the production of one mineral out of and upon another. 

This prefix has not been much used. We find it in such terms as epidiorite, epigenetic hornblende, and epistilbite. Apo may properly be 

used to indicate the derivation of one rock from another by some specific alteration.

If, therefore, we decide to employ this prefix to indicate the specific 

alteration known as devitrification (Entglasung) we may obtain, by compounding it with the names of the corresponding glassy rocks, a set of 

useful and thoroughly descriptive terms like aporhyolite, apoperlite, 

apobsidian, etc., as to the exact meaning of which there can be no doubt.

In accordance with this usage, it is proposed in this paper to call all 

the acid volcanic rocks the structures of which prove them to have once 

been glassy, aporhyolites, while such as were originally holocrystalliue, 

or whose original character is in doubt, will be termed quartz-porphyries.

The writer feels that the introduction of a new name into petrographical literature is to be deplored unless it can be shown that the name is 

formulated in accordance with certain well-defined principles. A good 

rock name should express composition, origi nal structure, and as far as 

possible the process of alteration, if alteration has occurred. It is 

thought that aporhyolite and the suggested series of similarly formed 

terms meet these requirements. They are therefore adopted as preferable to any in present use.2

1 Whitman Cross, "On a series of peculiar schists near Salida, Colo.: " Proc. Colo. Sci. Soc., Jan., 

1893, p. G.

: Since the above was written tho term eorhyolite has been proposed by,Dr. Nordenskjold to cover 

ancient acid volcanics identical with those of the South Mountain. In a review of Dr. Nordenskjold's 

able paper, "Ueber arcluoische Ergussgesteiuo aus Smaland " (Bull. Gool. Instit. Upsala No. 2, vol.], 

1893), tho writer has discussed the disadvantages that attend the use of any term or series of terms 

which carry with them tho idea of age. The reader is referred to this review, which appeared in the 

American Geologist for March, 189C, pp. 179-184, for n fuller statement of the writer's idea of the relation existing between devitrification and ago of volcanics.
BASCOM.] QUARTZ-PORPHYRIES. 39

It is a question whether it is always possible to distinguish between 

a primary and a secondary crystalline groundinass, and no attempt has 

beeii made to draw a sharp line between the quartz-porphyries and tlie 

aporhyohtes. In the absence of some of the more marked structures 

of a glass the presence of a secondary crystalline structure has not 

been considered sufficient evidence for the completely secondary character of the crystallization. It is very probable that while a large 

portion of the lava flow consolidated as a glass, much of the lava solidified at a sufficient depth to have secured a holocry stall me groundinass.

QUARTZ-PORPHYRIES. 

DISTRIBUTION.

A deep-red porphyry outcrops along the turnpike leading from the 

west to Monterey Station, and covers a small area north of the highroad, 1 Quo mile north of this, in the neighborhood of u Gum Spring,"

there are several limited areas of blue, purple, and brick-red porphyries. 

About 4 miles northeast, on the "Old Furnace Koad," by the unfinished 

viaduct, is a large area of dark-blue porphyry, passing toward the 

southwest by insensible gradations into typical aporhyolites.

These are the more important areas of porphyries. There are other 

porphyries so closely associated with the devitrified glassy lava that 

they will not be separately located.

MACROSCOPICAI, DESCRIPTION.

The beauty and variety of ^olor of these porphyries have already 

been noted. The pheuocrysts are not large (5 to 11 Inm long), but are 

conspicuous against the dark or brilliantly colored matrix. The orthoclase phenocrysts are opaque white, pink, or brick-red. They usually 

possess a well-defined crystalline outline and show twinning in. the hand 

specimen. The quartzes are colorless or a rich wine-red. . The fracture 

is conchoidal and there is a less-marked tendency to cleavage than in 

the aporhyolites.

The porphyries show a reddish-brown color on the weathered surface, 

and on decomposing form a red earth. The presence of manganese is 

abundantly exhibited in. dendritic markings on the cleavage surfaces. 

This is especially true in the case of the slates developed from the porphyries. (PL XIV.)

MICROSCOPICAL DESCRIPTION.

PHENOCRYSTS.

Feldspar. The porphyritical feldspars are more abundant than the 

quartz phenocrysts, and are remarkably fresh and unaltered. That 

they belong to the group of alkali feldspars, and that both monoclinic 

and triclinic varieties are undoubtedly present, are indicated by chemical analysis (given on page 61), by their specific gravity, and by their

1 See tho geological map of the Monterey district accompanying this bulletin, PI. III.
40 VOLCANIC EOCKS OF SOUTH MOUNTAIN. [BULL. 136.

optical properties. Feldspar crystals were separated from specimens 

of the porphyry taken from different localities and their specific gravity 

was determined." "The range was from 2.6 to 2.62. On account of the 

presence in the heavier feldspars of minute inclusions of piedniontite, 

the lowest specific gravity was considered to represent the purest 

feldspar.

Twinning is very common, according to either the Carlsbad or Manebacher (pericline) laws. PI. XV, b, shows a Manebacher penetration twin 

of feldspar. The section is nearly parallel to the clinopinacoid, as the 

following observations indicate: The axis of least elasticity makes an 

angle with the twinning line (rhombic section) of about 15 degrees on 

one side and 21 degrees on the other; the obtuse positive bisectrix 

emerges. There is a slight trace of a basal cleavage and a microperthitic intergrowth parallel to c. The position of the plane of the optic 

axes, the distribution of the axes of elasticity, and the specific gravity 

all indicate that the crystal is the triclinic soda feldspar anorfchoclase.

The resemblance to this of the other feldspar phenocrysts makes it 

 probable that this is the prevailing feldspar. Other sections show the 

albitic intergrowth (microperthite) developed in a pronounced manner 

(PI. XVI, a). Every gradation of this structure is present, from the 

microperthitic to the cryptoperthitic. Sometimes the perthitic growth 

does not persist throughout the crystal, but is present in an incipient 

stage along its edge.

The feldspars are often cracked and drawn apart in the direction of 

the schistosity .of the rock, and the cracks cemented with sericite scales 

whose parallel arrangement conditions the schistosity (PL XVI, b). 

This is most striking in porphyry obtained 40 feet below the surface, 

which shows an abrupt passage into a sericite-schist. The phenocrysts 

are crushed and pulled apart. This action had been accompanied by

an abundant development of sericite, Some of the phenocrysts possess in the hand specimen a red color, which is due to a flue admixture 

of red iron oxide. These pheuocrysts frequently prove, from a test of 

their optical properties, to be orthoclase.

The alteration of the feldspars to epidote may indicate the presence of lime in the feldspar. Undoubtedly, however, much of the lime 

is of secondary origin. -This alteration of feldspar to epidote seems 

to be a direct one. There is no intermediate kaolin stage, such as 

observed byEutley. 1 Accompanying the epidote is granular quartz. 

These two alteration products usually occupy the center of the crystal 

and are surrounded by a rim of unaltered feldspar.

Brilliantly colored piedmoritite frequently fills cavities in the feldspar 

crystals. This mineral is often surrounded by a rim of epidote.

Quartz. The bipyramidal or rounded quartz phenocrysts are remarkable only for their undulatory extinction and the cracks by which strain

'Rutley, On some perJitic felsites and on the possible origin of some epidosites: Quart. Jour. 

Geol. Soc. London, Vol. XLIV, 1888, pji. 741-742.
BASCOM.] QUARTZ-PORPHYRIES. 41

lias been relieved. They are fresh and show characteristic embaymeuts 

and inclusions. Like the orthoclase, they are sometimes reddened by 

inclusions of hematite, or are more rarely given a rich wine-red color 

by inclusions of piedmontite.

GROONDMASS.

The holocrystalline groundmass of the porphyries consists essentially 

of quartz and feldspar. These minerals form, either a finely microgranitic mosaic or a structure which becomes of considerable interest 

in connection with the question of the original character of the crystallization.

° In the first case there is no reason per se for supposing the crystallization to be other than primary. It is only as the crystallization 

is associated with secondary structures, or structures testifying to a 

primitive glassy condition, that the presumption favors its secondary 

character. When the crystallization is microgranitic and presumably 

primary, the rock is a genuine quartz-porphyry.

The structure alluded to, which occasionally replaces the quartz-feldspar mosaic in the porphyries, is the micropoikilitic, 1 where all the quartz 

of the groundmass has crystallized in irregular areas inclosing the feldspar as lath-shaped microliths. These feldspar niicroliths are oriented 

quite independently of one another. This structure characterizes the 

aporhyolites, and the question of its primary or secondary character 

will be fully discussed in connection with these rocks.

The alteration of the groundmass to sericite and the formation of a 

sericite schist will be discussed in connection with the acid slates.

ACCESSORY CONSTITUENTS.

Piedmontite (manganese epidote) is the most remarkable of the accessory constituents. It is not only disseminated in microscopic quantities through the deep-red porphyry, but it also occurs in macroscopic 

masses, as a radiating aggregate, filling veins and cavities in the red 

felsite or replacing spherulitic crystallization.

Two localities in the Monterey district furnish a great abundance of 

the piedmoutite: the southwest flank of Pine Mountain, 1 mile northeast of Monterey station, and the hillside south of the Clermont House, 

between the turnpike and Miuie Branch. Microscopic sections of the 

aggregate show brilliantly colored needles of piedmontite intergrown 

with clear quartz. (PI. XXV, &.) This intergrowth with quartz is also 

marked in the hand specimen. The radiating needles have the appearance of being broken, stretched apart, and the spaces filled with secondary quartz. The only other locality where piedmontite was found 

in macroscopic quantities is in the Buchanan Valley, 2 miles north of

'G-. H. Williams, On the nso of the terms poikilitic and uiicropoikilitic in. petrography: Jour, of 

Geol., Vol. I (No.2, Peb.-March, 1803), pp. 170-179,
42 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BOLL. 136.

the Chambersburg turnpike, and one-eighth mile west of Musser's store. 

Piedinontite occurs here in cavities associated with scheelite. 1

Epitiote is also abundantly present in the porphyries, passing by 

gradations of rose-colored epidote to the deep carmine of the piedtnontite. Both of these silicates are undoubtedly secondary products.

Innumerable black globnlitesof manganese oxide and black and red 

iron oxide crowd the groundinass. In their arrangement they show 

fluxion, structure, or form meshes inclosing the quartz areas which 

constitute a part of the micropoikilitic structure. This arrangement is 

sometimes so marked as to be conspicuous in the hand specimen, giving 

the rock a mottled appearance. Crystals of zircon are not infrequently 

present.

A study of the quartz-porphyries of the South Mountain shows us 

that they are present there in characteristic development, and offer no 

marked variations from the normal type described from other localities. 

For this reason a brief space only has been devoted to themv

APORHYOLITES. 

DISTRIBUTION.

There remains to be described a large and important portion of the 

acid rocks of the Monterey district. These will be termed aporhyolites, 

for reasons already suggested and to be more fully discussed in the 

sequel.

The localities colored red upon the geological map of the Monterey 

district (PI. Ill) are aporhyolitic areas associated, in the cases already 

mentioned, with quartz-porphyries. The largest area begins just south 

of the Bigham copper mine, and, widening toward the north, extends 

far beyond the Monterey district. This area furnishes most of the 

splierulitic structures. A small detached area, south of the Glermont 

House furnishes slaty and'Spherulitic aporhyolites. These aporhyolites 

and those of the four detached areas near the Maryland State line, 

separated from each other by a distance of a mile to a mile and a half, 

show some marked points of resemblance, to which attention will be 

called later.

The areas covering the foothills southeast of Jacks Mountain show 

an altered aporhyolite. There are several sin all areas of red aporhyol ite 

on the Gladhill road leading to the Bigham property. At the forking 

of this road with the Fountaindale turnpike occurs the aporhyolite slate 

mentioned on page 29.

MACROSCOPICAI, DKSCRIPTION.

The aporhyolites have about the same range in color as the porphyries, varying from light bluish-gray, or buff, to many shades of red 

and purple. While usually compact and always fine-grained, they are

1 G. H. Williams Pieclraontito and sctioolito from the ancient rhyolito of South Mountain, Pennsylvania. Am. Jour. Sci., Vol. XLVI, 3d series, July, 1893, pp. 50-57.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. VII

FLOW STRUCTURE IN APORHYOLITE, MONTEREY DISTRICT. 

Four-fifths natural siza> The specimen is deep red, with lines of flow in pink.
BABCOM.] . APORHYOLITES. 4d

also sometimes very vesicular. In the latter case the amygdules of 

dark-green epidote and clear quartz, elongated by fluxion and conspicuous against a pale-pink background, render the rock strikingly 

handsome. Phenocrysts are usually present, but generally inconspicuous.

Delicate lines of flow structure, which are brought out in great 

detail by weathering or are painted in rich colors on the material 

washed by the mountain brooks, are another marked feature of the 

aporhyolites. These flow lines are frequently very sinuous, showing contortion and crumpling of the lava. (Pis. VII and VIII.) The flowage 

is often emphasized by the mingling of two contrasting magmas, forming taxites of either a eutaxitic or an ataxitic character. A fuller 

description of these taxites is given on page 57.

Another characteristic which these aporhyolites possess in common 

with their modem analogues is the spherulitic structure. Spherulites

are rarely, if ever, altogether absent, and in some localities they are 

crowded so close together as to constitute the major part of the rock 

mass. They range in size from microscopic dimensions to those of a 

butternut. Where there is no regularity of arrangement and they are 

brought out in relief by weathering, the rock has a superficial resemblance to a conglomerate composed of rounded pebbles of remarkably 

uniform size (BE shot) and shape. (PL IX.) The rich grays and blues 

and purples of the spherulites and matrix render, this a conspicuous 

rock. Other specimens from, the same locality (the south flank of the 

mountain northeast of the junction of Copper Run and Toms Creek) 

show spherulites elongated by flow. They are thus drawn out into 

solid cylinders with a diameter of I"11 " arid a length of some 2C1 ".

Specimens of aporhyolites composed of spherulites about the size 

and shape of almonds (18min by 9""") were.also found. The rock had 

been greatly sheared and the spherulites flattened.

Spherulites become a still more striking feature of the aporhyolites 

when arranged in layers which traverse the face of the rock in long, 

parallel, dotted bauds. This arrangement has been described by 

Iddings in the obsidian of the Yellowstone National Park. 1

While sometimes these bands are 4mm wide, at a nearly uniform distance apart, and of an indefinite length, in other cases they are very 

narrow, dwindling into mere lines and dying out, to be succeeded by 

other lenticular bauds. (PI. X.) The planes of these spherulites have 

become planes of weakness and solution. The rock cleaves readily 

parallel to them and shows a coating of secondary silica on the cleavage surfaces. This deposition of silica causes these planes to become 

the hardest part of the rock, and hence they often stand out as 

parallel ridges on. the weathered surfaces.

At the locality mentioned on pages 41-42, piedmontite, in radiating 

needles, associated rarely with scheelite, has, together with quartz, been

'Obsidian Cliff: Seventh Ann. Kept. TJ.'S. Geol. Survey, 1888, p. 276, PI. XVIII.
44 VOLCANIC ROCKS OP SOUTH MOUNTAIN.. [BULL. 136.

deposited along these planes of former spherulitic crystallization. The 

silica occupies the center of the deposit and gives rise to ridged surfaces. 

These layers of spherulites do not always lie in a single plane. Cross 

sections of the layers show irregular and minute sinuosities, which were 

doubtless caused by movement in the lava during consolidation.

The bands consist always, when not composed of piedmontite, of a 

central dark line (impurities) with a lighter band (opaque quartz) on 

either side. These in turn are bordered by dark lines (iron oxide) 

(PI. X). This parallel banding, conspicuous even at a considerable distance, though recognizable as a true igneous structure by one familiar 

with the acid lavas of the Yellowstoue National Park or of the Lipari 

Islands, counterfeits a sedimentary structure so closely that it is not 

surprising that the rocks should be described as "bedded orthofelsites."

Still other specimens show spherulites with a tendency to an arrangement in rows and chains which are in turn collected in bands some 

2 inches in width traversing the rock and alternating with bands 

approximately free from spherulites. This arrangement differs from the 

layer spherulites in the fact that the outline of individual spherulites is 

quite distinct and the grouping irregular.

Some fine examples of the lithophysal structure were found in the 

Raccoon Creek region (PI. XI). Nothing that could be definitely recognized as lithophysae. was observed in the Monterey district.

In the areas southeast of Jacks Mountain the aporhyolites have 

been more or less silicifie'd or epidotized, or both, and are sometimes 

very difficult to distinguish, in the hand specimen, from the neighboring sandstone. A good example of this r^ck is seen in the exposures 

on the turnpike about one-fourth of a mile above the Emmitsburg tollgate, where silicification has produced a close resemblance to a quartzite. The fresh surface is white, Feldspar phenocrysts are sparsely

distributed and scarcely discernable. Quartz blebs are more numerous. Pyrite is finely disseminated, and the exposed surfaces of the 

rock are tinged with yellow and red iron oxide. The rock cleaves in 

two directions oblique to each other. In this respect it resembles 

many of the felsites.

The microscopic evidence of igneous character is supported by the 

field evidence, which shows a continuity between this altered rock and 

that which would be at once recognized as a felsite.

MICROSCOPICAL DESCRIPTION.

PHENOCRYSTS.

Feldspar and quartz. The feldspar phenocrysts are very like the 

feldspars of the quartz-porphyries, and little needs to be added to the 

description of them already given. They are fresh, contain inclusions 

of a once glassy magma, show perthitic intergrowth, and are twinned in 

accordance with the Carlsbad (albite) and Manebacher (pericline) laws.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. VIM

'f-''~' ''*.- - J»"''. , "' ': ' jOS^.

.«'. > < * ":: -i"1 '- ' - '- :>:.' ' *ss 

*-' ' ' ":: -'' i^-~'...--^. 4'-- -jfr^

I <.f .« «:><. ^"

FLOW STRUCTURE IN APORHYOLITE, RACCOON CREEK. NATURAL SIZE.

Tne specimen is hght pink, showing on a smooth, weathered surface delicate and sinuous lines of an original flow 

structure, in shades of red and purple.
BASCOM] APORHYOL1TES. 45

In the former case the twinning is sometimes repeated, and furnishes 

another indication of the triclinic character of the feldspar.

The feldspars are frequently bent or broken, causing the twinning 

striatious to show curvature and faulting. Occasionally a phenocryst 

has been completely broken, the fragments pulled apart, and the space 

filled with a quartz-albite mosaic of much coarser grain than the groundmass, and evidently of subsequent formation. (PI. XVII, a.) A complete replacement of the feldspar crystal by a quartz mosaic sometimes 

occurs. 1

The quartz phenocrysts have rounded outlines and characteristic 

embayments and inclusions; undulatory extinction is almost universal; 

frequently the quartzes are cracked, and sometimes completely granulated. Both the porphyritical constituents of the aporhyolites give 

rise, in connection with curving flow lines, to the "augen" structure 

which has often been described,2

These flow lines leave clear, triangular spaces on one side of the 

phenocrysts, such as Futterer describes.3

Other porphyritic constituents. The only ferromaguesian silicate 

unmistakably present in the aporhyolites is biotite. This was found 

in sections representing three different areas of acid volcanics which lie 

along the southern limit of the Monterey district. The first area is a 

small one at the head of Minie Branch. The second area is larger and 

caps the mountain southeast of Raven Eock Mountain. The third 

area is directly to the east of this, on Friends Creek. The lavas of 

these three localities show a marked similarity, both in the hand specimens and in the thin sections. They are compact, dirty-gray rocks, 

weathering a yellowish-gray. Feldspar phenocrysts are somewhat 

sparsely and inconspicuously present. On a fresh surface biotite can 

b.e detected with the naked eye. Magnetite is present, and in one of 

two specimens from the mountain locality pyrite is abundant in minute 

crystals.

All of the thin sections of these rocks are distinguished by finely 

striated feldspars. The carlsbad and pericline twinning are sometimes 

both present in a single crystal. Quartz phenocrysts are not numerous.

The presence of titanic iron oxide in abundance is attested by the 

formation of leucoxene. The groundmass is holocrystalliue and finely 

microgranitic, with a tendency toward the micropoikilitic structure. 

This tendency is fully developed in. one of the sections. The hand specimen, from which this section was cut, shows a rock more completely 

silicified than the other specimens indicate, and also contains pyrite, 

which is indicative of secondary crystallization.

'Similar replacements have been described by Dr. Milch, Beitrage zur Kenntnia dea Verrucano, 

1892,, p. 126,

?J. Lehmann, Untersuchnngon iiber die Entstehimg der altkrystalliniacheu Schiefergesteine.Bonn, 

1884. G. H. Williams, Bull. U. S. Geol. Survey No. 62, pp. 85, 118, 207, PI. XV, fig. 1.

3Karl I'utterer, Der '' Ganggranit'' von Grossachseu und der Quartz-porphyr von Thai im Thiiringer 

Wald, luaug. disser., 1890, p. 32.
46 VOLCANIC EOCKS OF SOUTH MOUNTAIN. [BULL. 136.

There is some epidotization of the groundmass, resulting hi the production of finely granular epidote concentrated at certain centers of 

development.

The mica by which these lavas are characterized has a fibrous- character, and is the andesitic type described by Kosenbusch. 1 It is in 

idiomorphic clusters of reddish-yellow fibers. The absorption is characteristically strong and the pleochroism marked. Parallel to the 

cleavage the fibers show a deep reddish-brown color, and at right 

angles a light yellowish-red or straw-yellow color. The marked resemblance of the lava from these localities suggests a continuity in the 

lava flow a continuity probably maintained underneath the greenstone.

If this is" the case these areas are exposed by erosion of the overlying basic lava, if it ever completely overlay the aporhyolite. That the 

continuity was maintained above the greenstone is a scarcely tenable 

hypothesis, because of the extended erosion Avhich such a superposition 

would necessitate.

If any other ferroinagnesian constituents were present in the aporhyolites and there are indications that there were they have been 

totally decomposed and removed.

Harker 2 found it to be true of the pre-Cambrian acid volcanics of 

Wales, that clusters of biotite flakes were preserved in a comparatively fresh condition, while only slight trace of augite or hornblende 

remained.

In the South Mountain aporhyolites these minerals must have been 

rare originally, and are now completely replaced by epidote. Occasionally the outline of a perfect crystal section parallel to 010 is preserved', while the substance of the crystal is entirely replaced by epidote individuals. In the absence of basal sections there is no clue to 

the specific character of the original crystal.

GROUNDMASS.

The groundmass is always a quartz-feldspar- aggregate, of varying 

structure and grain (about -8-V of a millimeter in diameter), more .often 

finely than coarsely inicrogranitic. The structures of the groundmass 

are of the highest interest and importance in their disclosure of the 

original character of the rocks which possess them.

The fluidal, inicropoikilitic, spherulitic, axiolitic, lithophysal, rhyolitic, micropegmatitic, perlitic, taxi tic, amygdaloidal, and trichitic structures are characteristically developed and merit detailed description.

Fluidal structure. The fluidal structure, which is so familiar to all 

students of rhyolitic lavas, is a conspicuous feature of the aporhyolites, 

both macroscopically, as has been described, and microscopically. 

Globulites of magnetite and hematite and indefinable opaque crystallites follow sinuous lines of flow, twisting around the pheuocrysts and

'Rosenbusch, Massige Gesteino, 2d ed., Vol. II, p. 658, 

lcanic Series of Rocks, p. 18,
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. IX

SPHERULITIC APORHYOLiTE, MONTEREY DISTRICT. 

Seven-tenths natural size.
BASCOM.] APORHYOLITES. 47

imparting to them the appearance of eyes, previously mentioned. PI. 

XVIII, a, is the reproduction of a photomicrograph of a section showing this structure.

Micropoikilitic structure. The micropoikilitic structure has been 

defined in connection with the quartz-porphyries, where it was not 

infrequently a significant feature of the groundinass. It is still more 

characteristic of the aporhyolites, and is occasionally present in the 

basic eruptives.

These irregular quartz patches, inclosing inicrolites of lath-shaped 

feldspars or other minerals of independent orientation, give a pronounced mottled or "patchy" appearance to the groundrnass, an 

appearance which has been noted in volcanics of all ages. It has been 

observed and variously described, usually without being named, in 

quartz-porphyries, felsites, porphyrites, rhyolites, and peridotites, by 

numerous writers Irving, 1 Williams,2 Haworth, 3 Cross,4 Iddings,5 

Diller,6' Liudgren. 7 Teall, 8 Barker," Brogger, 10 and Nordenskjold. 1 ' Pelsite from the Archean rocks of Georgia shows the same structure. 12 This 

structure is likewise present in the felsites from the neighborhood of 

Boston, 13 as is the case also with the quartz-porphyries and felsites of 

Marblehead Neck, Massachusetts.

An acid lava of the Keweenawau series from Minnesota, recently 

examined by the writer, shows with a high' power and polarized light 

the micropoikilitic structure in perfection. 14

While the term micropoikilitic is not restricted to a quartz-feldspar

1 R. D. Irving, Copper bearing rocks of Lake Superior: Mou. IT. S. Gool. Survey, Vol. V, 1883, pp. 

99-100, I'l. XIII, figs. 13 and 14.

2 G. H. Williams, Neues Jabrbucli fur Min. u Pet., Supp. Vol. II, 1882, p. G07, PI. XII, fig. 3. The 

peridotites of the Cortlaudt aeries: Am. Jour. Sci., Vol. XXX, p. 30; Vol. XXXIII, p. 139.

3 E. Hawortli, A contribution to the Arehtuaii geology of Missouri: Am. Geologist, 1888, Vol. I, p. 

368, PI. 1, figs. 1 and 2; also, Crystalline rocks of Missouri: Ann. Kept. Mo. Geol. Survey, Vol..VIII, 

1894, p. 195, PI. XXII.

4 Whitman Cross, On some eruptive rocks from Ouster County, Colo.: Proc. Colo. Sci. Soc., Vol. II, 

1888, pp.232, 242. On a series of peculiar schists near Salida, Colo.: Proc. Colo. Sci. Soc., Jan., 1893, p.8. 

 ° J. P. Iddings, The eruptive rocks of Electric Peak and Sepulchre Mountain, Tellowstone National 

Park: Twelfth Ann. Kept. U. S. Geol. Survey, pp. 589, 646.

°J. S. Diller, Mica-peridotite from Kentucky: Am. Jour. Sci., 3d series, Vol. XLIV, Oct., 1892, p. 

287.

7 Waldemar Lindgren, On sodalite syenite and other rocks from Montana: Am. Jour. Sci., 3d aeries, 

Vol. XLV, Apr , 1893, p. 287.

8 J. J. Harris Teal], British Petrography, 1888, p. 337. 

"Alfred Barker, Bala Volcanic Scries of Hocks, pp. 23, 53, 54.

10 W. C. Brogger, Der Miueralieu dor Syouitpcgmatitgiinge tier stiduorwegischen Augit und Nephelinsyenit: Groth's Zeitschrift fiir ICrystallographie, Vol. XVI, p. 46.

11 Otto Nordenskjold, Zur Kenntniss dor sogon. Hiilleflinta des nordb'stlichen Smalands: Bull. Geol. 

Inst. Upsala, Vol. I, No. 1, 1893.

12A section of this felsite was loaned by Prof. L. V. Pirsson. It is of especial interest in its 

great similarity to the South Mountain felsite, thereby showing the southward persistence of this 

rock type.

13 J. S. Diller, 1'elsites and assoc. rocks north of Boston: Proc. Boston Soc. Nat. Hist., Vol. XX 

Jan. 21, 1880, pp. 355-368; also Bull. Mus. Comp. Zool. Harvard Coll., whole series, Vol. VII; geol., 

series, Vol. I. Thin sections of these felsites were kindly loaned by Mr. Diller to the Johns Hopkius 

laboratory. They have many microscopic features iu common, with the South Mountain rocks, and 

like them were first supposed to be of sedimentary origin.

IJ The writer's attention was called to the presence of this structure in the Minnesota acid volcauics 

by Dr. U. S. Grant, of the Minnesota State Geological Survey.
48 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136

intergrowth, in most of the occurrences described these have been the 

component minerals. In the South Mountain rocks the feldspathic 

material is usually so abundant as not to admit of the determination 

of the mineral character of the host. In such cases a clue to the 

nature of the cementing material is found in its optical continuity with 

the porphyritic quartz. These phenocrysts are severally included 

within a single micropoikilitic area, with which they are always similarly oriented (PI. XVII, 6). The cementing material acts as a sort of 

secondary enlargement of the quartz phenocrysts. The feldspar phenocrysts, on the other hand, have no effect upon the orientation of the 

cement.

In the basic rocks, which are coarser: grained, the character of the 

host can be directly tested and proved to be quartz. The mottled 

appearance, previously alluded to, is usually emphasized by the 

arrangement of globulites, longulites, and trichites of iron oxide. This 

is such as both to accord with the flow structure and to outline the 

quartz areas, which in these cases have a somewhat oval shape.

When shearing has led to the production of sericite, this mineral is 

formed around the micropoikilitic areas, rarely traversing a single 

area, when it seems to be cementing material filling a crack (PI. XVII, 

&). These areas are persistent, and slowly disappear in the development of a slate.

While in some cases this structure is undoubtedly of primary character, as Professor Iddings considers it to be in many porphyrites, in a 

large class of rocks its secondary origin seems equally certain. Irving, 

who gives one of the best as well as the earliest (1881) descriptions of 

this structure, considered it of a secondary character. His statements 

as to its nature and origin are so applicable to the South Mountain 

aporhyolites that they are quoted here in full: 1

Although wholly absent from some sections, a very highly characteristic feature of 

the sections of many of these rocks, and more particularly of the felsites without 

porphyritic quartz, is a network quartz which can only he regarded as of secondary 

origin. I find no mention of such a feature in any of the descriptions of the felsites 

of other regions which I have examined. Only occasionally * * * is this network quartz coarse enough to he readily seen with a low power in the ordinary 

light. Usually both a high power and the use of the polarized light are required 

for its detection, when it appears in its most characteristic development as a delicate aborescent tracery or frost-work saturating the ground mass in all directions.

In the polarized light all of the quartz network within each of these numberless 

irregularly round areas, whose existence would not be suspected in the ordinary 

light, is found to be similarly oriented.

From these more pronounced deyelopments the secondary quartz is found through 

many degrees of lessening amount and less plainly marked character until it disappears altogether. It is plainly of the same nature as the secondary quartz of the 

already described orthoclase-gabbro, diabase-porphyrite, and quartzless porphyry, 

and of the augite-syenite described below. It never, however, reaches in the rocks 

now under description the coarseness nor presents the graphic forms with which, it 

appears in the augite-syenites, its characteristic development here being the deli-

^oc. cit., pp. 99-100.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. X

 - 

 * ' . **vH: "' '"* - -v - s*5 "^"-''. '*'

APORHYOLITE WITH SPHERULITES IN LAYERS.

From Jim Saunder's farm, Bigham Copper mine district. Five-sevenths natuial size. The deta-'s of the 

spherulitic chains are lost in the photograph.
BASCOM.} . APORHYOLITES. 49

cate arborescent clusters above mentioned. Whether this secondary quartz may ever 

bo rather a result of devitrification than a truly secondary or alteration product^ I 

have no means of deciding, though it is certainly the latter often, and I should suppose always. It surely can have no connection with the original solidification of the 

rock.

Observations made on the South Mountain aporhyolites lead to 

essentially the same conclusions as those reached by Irving. As the 

nature of the structure is of both interest and importance in its bearing upon the question of the primary or secondary character of the 

groundmass, attention will be called to some of the observations which 

prove suggestive.

It has been stated that a few sections of the basic lavas of South 

Mountain exhibited this structure. In these sections the nature of the 

structure could be more readily detected. (PI. XIX, a and I.)

The outline of the lath-shaped feldspars, forming an original ophitic 

structure, is completely preserved, though none of the original constituents of the rock remain, unless some of the titaniferous iron oxide is 

original. . At present the rock consists entirely of quartz, epidote, magnetite 

(or ilmenite), and leucoxene. It is amygdaloidal, and the vesicles are 

filled with secondary quartz. Quartz is also a cement for the minerals 

of the groundmass, and forms irregular interlocking areas, which are 

quite similar to the micropoikihtic areas of the finer-grained acid rocks, 

and produce in polarized light the familiar patchy effect. Fine cracks 

traversing the sections are still preserved in outline by the ferrite, but 

are prior to the quartz areas, which have obliterated all trace of cementing material. The epidote. which replaces crystals of some former ferro 

maguesian constituent, is often pierced by these cracks, which become 

invisible in the quartz areas save for the outlining ferrrte. There can 

be no question as to the secondary character of the micropoikihtic 

structure in these cases.

Some structures described by Zirkel J in the rhyolites of the Washoe 

district are. suggestive in this connection. He notes (slide 350, fig. 1, 

PI. VI) perlitic parting in certain rhyolites (southeast from Wadsworth), 

where the cracks, semicircular and oval, traverse a glassy groundmass 

and are bordered by a narrow zone of nricrofelsite. "giving [to the section] the appearance of a network." The same general effect is produced 

in other rhyolites of the district (sees. 351, 352, figs. 2, 3, PI. VI) by 

faint granular lines "which, by their fluidal running, form a net with 

a multitude of meshes of oval shape." These lines seemed to be the 

vestiges of perlitic cracks, though they could not be certainly determined as such. A widespread and characteristic type of rhyolite (sees. 

333, 407, fig. 1, PI. VIII) shows the same network of dark granular lines, 

but in this case the meshes are filled with a spherulitic crystallization. 

i There are, then, two types of crystallization which may occupy the

1 Geol. Expl., 40th parallel, Vol. VI, Microscopic Petrography.

Bull. 136 4
50 VOLCANIC EOCKS OF SOUTH MOUNTAIN. |.uuu,136.

oval spaces, the spherulitic and the microfelsitic. The former does so to 

the exclusion of a glassy grouudinass, and must have been formed prior 

to the consolidation of the rock. The latter shares the space with the 

glass into which it gradually passes and of which it may be a molecular 

alteration. Turning to the South Mountain aporhyolites (particularly 

the spherutaxites), Ave find a similarity in the development of faint granular lines forming irregular oval meshes and giving to the section the 

network appearance figured by Zirkel. These lines maybe traces of a 

former perlitic parting or of a microfluidal structure. The meshes are 

now filled either by the quartz areas which condition the micropoikilitic structure or by the vestiges of a spherulitic crystallization.

The latter represents a primary structure, as in the rhyolites; the 

former may represent the molecular rearrangement of a spherulitic 

crystallization or of a glassy groundmass. In the last case it is the 

direct result ot devitrification and infiltration, processes more readily 

initiated along the borders of the perlitic cracks, as in the rhyolites of 

the Washoe district.

A comparative study of some sections' of the rhyolites of the Eosita 

Hills, Colorado, and of Obsidian Cliff, Yellowstone National Park, 

side by side with the aporhyolites under discussion, also suggests the 

secondary character of the micropoikilitic structure with reference to 

the spherulitic crystallization. In the trichitic spherulites of the modern rhyolites there is an appearance analogous to the micropoikilitic 

mottling^ caused by the breaking up of the radiating spherulitic fibers 

into irregular areas which extinguish differently. An altogether similar phenomenon occurs in some of the spherulites of the ancient rhyo 

lites. It is indicative of an intermediate stage between the spherulitic 

and a completely micropoikilitic crystallization. This change from the 

spherulitic to the micropoikilitic structure is carried still further in 

some sections, notably in the case of a specimen crowded with minute 

red spherulites. Bach spherulite extinguishes as an individual filled 

with inclusions of feldspar and iron oxide (hematite). The host can be 

determined by optical tests to be quartz. The shape of the spherical 

bodies in the hand specimen and in thin section, their tendency to form 

bands and chains, and their uniform color put their original spherulitic 

character and the secondary nature of the micropoikilitic structure 

beyond doubt. It is not supposed that a prior spherulitic crystallization always existed where now the aporhyolites show a micropoilitic 

structure, but these evidences of the derivation of the structure from 

spherulites establish a presumption for its secondary origin in other 

aporhyolites, where it may be the direct result of devitrification or 

may be due to the subsequent alteration, by infiltration, of a granular 

crystallization.

On the whole,-the plainly secondary character of the micropoikilitic

1 Sections of material from those localities wore kindly loaned the writer by Dr. Cross and Professor 

Iddings. 
BASCOM.] APORHYOLITES. , 51

structure in the basic volcanic, the evidence in the aporhyolites of its 

being subsequent to fiuidal lines or to perlitic parting, the indications 

that in many cases it is subsequent to a spherulitic crystallization, all 

denote a secondary origin for this structure in the South Mountain rocks. 

Spherulitw structure. There are two sorts of spherulitic crystallization in the aporhyolites. They differ in no essential respect, but are 

unlike in appearance. The most numerous spherulites are also the 

simplest and smallest. They are colorless, microscopic spheres, scarcely 

or not at all perceptible in ordinary light, but between crossed nicols 

showing a distinct dark cross. Spherulites in every respect similar 

have been described and figured by Professor Iddings from the rhyolites of the Yellowstone National Park. 1 Similar spherulites also occur 

in the rhyolites of Hungary and in the felsites of the Boston Basin. 

These radial growths are grouped in bunches and along lines, and are 

composed of positive fibers. Further optical determination of the fibers 

could not be made. Their positive character indicates either a prismatic

section of quartz elongated in the direction of the vertical axis or a 

clinopinocoidal (010) section of orthoclase elongated in the same direction. In the latter case the extinction would be slightly inclined. 

This is impossible, however, of determination.

While it is not impossible that some of these spherulites are secondary, in some cases there is evidence of their primary character. One 

such case of spherulites whose formation was coincident with the consolidation of the rock occurs in an aporhyolite from a cut made by the 

Gettysburg Eailroad north of Toms Creek trestle. Minute colorless 

spherulites are embedded in a base which suggests in every way its 

former glassy condition. In ordinary light there is no evidence of 

crystallization except the porphyritical.

The groundmass is traversed by irregular, angular cracks, evidently 

the result of crushing. These cracks, which are cemented by epidote, 

pass through the spherulitic aggregates (seen with crossed nicols), sometimes cutting directly across a spherulite, portions of which appear on 

either side of the crack. Between crossed nicols the field breaks up 

into a mosaic of quartz and feldspar. The granular crystallization 

disregards the cracks, filling the spaces left by the cementing epidote. 

It seems fair to conclude from these observations that the spherulitic 

crystallization is prior to the cracking, that the granular crystallization 

is subsequent, and that the cracking occurred in an already solidified 

glass.

The secondary character of the granular crystallization admitted, it 

is easy to suppose that it is due to devitrification continued through 

lapse of time. The spherulites, on the other hand, being prior to 

the crystallization of the groundmass, and prior to the cracking, are 

doubtless primary and contemporaneous with the consolidation of the 

glass.

i Obsidian Cliff: Seventh Anil. Kept. TJ. S. Geol. Survey, PI. XVII, p. 276.
52 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [BULL. 136.

The other class of spherulites correspond to those figured by Professor 

Iddings. 1 They are much larger than those that have just been 

described. The smallest is easily discernible by the naked eye, while 

the largest is 12£ cin. in diameter. They are spherical, hemispherical, cylindrical, fan-shaped, oval, or irregular in form. While they 

all possess a clear-cut, conspicuous outline in ordinary light, in many 

sections they completely disappear between crossed nicols. The fresh 

spherulites (PI. XXII, 6), which still show in polarized light a radiating structure, are usually colored red by a finely disseminated iron 

oxide. The globulites of hematite are distributed homogeneously 

throughout the spherulite, or they are grouped in radial and concentric 

lines. These lines are most dense near the margin of the spherulite, 

and may be separated from the central portion by a narrow clear zone, or 

it is the outer rim of the spherulite which is clear. In the central portion of the spherulite the coloring matter shows a tendency to collect 

in bunches that correspond with areas which extinguish as individuals. 

Between crossed nicols the field is broken up into the minute areas 

which were referred to on page 50 as forming a structure approaching 

the micropoikilitic. Feldspar phenocrysts usually occupy the center of 

the radiating crystallization, and two or more spherulitic centers may 

be included within a single outer zone. In specimens from Kaccoou 

Creek 2 these radial growths were remarkably well preserved and occur 

in a groundmass which retains the characteristics of a glass in great 

perfection (PI. XX, a; PI. XXI, a). It bears a close resemblance to 

the grouudmass of some of the Colorado rhyolites, and in ordinary light 

would certainly be mistaken for the base of a fresh glassy lava. Delicate perlitic parting, which because of its delicacy is easily obliterated, 

is here preserved in wonderful detail. It is evidently subsequent to 

the radial crystallization to which it accommodates itself. The presence of innumerable globulites accentuates the perlitic and rhyolitic 

structures. With crossed iiicols the grouudmass at once betrays its 

holocrystalline character (PI. XX, &). All glassy structures disappear, 

to be replaced by granular quartz and feldspar, a crystallization which 

closes the perlitic parting and thereby completely obliterates it.

The porphyritic feldspars still show inclusions of a glassy base. It is 

impossible by any description to produce that definiteness of conviction 

as to the original glassy nature of the groundmass which the character 

of the sections justifies. To one who has studied these sections in both 

ordinary and polarized light there can be no question as to the secondary 

character of the holocrystalline groundmass. One can not escape the 

conviction that the rock originally consolidated as a spherulitic perlite 

and has become holocrystalline by a process of devitrification.

The sequence of events, as revealed by microscopic study, is as folows: There was first the intratelluric development of the porphyritic

1 Op. cit., p. 277, PI. XVII.

2 In Franklin County, Pa., east of Kooky Ridge and south of Graeffeuburg.
BASCOM.] APORHYOLITES. 53

crystals, followed by the emergence of the magma, the development of 

globulites and fluxion structure, the commencement of radial crystallization, and finally the consolidation of the magma as a glass and the 

development of perlitic parting. .Subsequent to all this, and extending 

over a much longer period of time, the process of devitrification took 

place.

Associated with a holocrystalline groundmass, which bears less 

marked evidence of an original glassy character, are the more altered 

spherulites. Their circular outline in the thin section and their spherical shape in the hand specimen testify to their former presence.

In both hand specimen and thin section a threefold zonal arrangement 

is often clearly defined by the distribution of red or black iron oxide. 

"With crossed nicols these boundaries become inconspicuous, and the 

field of the microscope shows only a uniform quartz-feldspar mosaic, 

or the former radial growth is indicated by zones of a finer-grained 

crystallization than the groundmass (PI. XXIII, a), or a niicropoikilitic 

structure is present within the spherulitic boundaries when absent in 

the groundmass. Occasionally, vestiges of a radiate structure still 

remain.

In the specimen already referred to on page 50 and figured in PI. 

XXII, a, the spherulites are colored red by a uniform dissemination of 

hematite particles, and are not more than one-half mm. in diameter. o 

They traverse the rock in rows and chains, which are in turn grouped in 

bands about 2 inches wide. The rock has been sheared, a ready cleavage has been produced, and sericite has developed around each spherulite. As has already been mentioned, the spherulitic individuals have 

become niicropoikilitic individuals. Another specimen shows spherulites which have been rendered almond-shaped through shearing or, 

less probably, by the fluidal movement of the magma during this 

consolidation. That the former was probably the case is indicated by 

the gradual passage of the rock into a slate and the development of 

biotite.

Spherulites which, like these, have been replaced by a fine-grained 

mosaic of quartz and feldspar have been described by Klockmanu. 1

Chain spherulites. The arrangement of spherulites in layers and along 

planes so that a cross-section shows a chain of spherulites has been 

described on page 43 and figured on PI. XVIII, &, as it appears in the 

hand specimen. In ordinary light the microscope discloses somewhat 

sinuous or straight dark bands, with scalloped borders sharply outlined, 

inclosing an irregular clear chain which .also has scalloped edges.

Frequently there are detached clear spots with circular outline. 

These bands sometimes spread out so as to include phenocrysts and 

sometimes curve around them. (PI. XVIII, &.) A comparative study of 

these spherulitic chains and the chains of fresh spherulite of the Yel-

' Die Porpbyre; Dergeologische Aufbau des sogen. Magdeburger Dferrandes mit besonderer Beriicksicbtignng der nuftretenden Eruptivgesteiue: Jabrbuch K. prcnss. geol. Landesanstalt, Vol. XI, 

3890, p. 170.
54 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136-

lowstone obsidians discloses.a striking similarity in ordinary light 

the same irregularly scalloped outline, the same central chain of clear 

spherules. With crossed nicols the close similarity vanishes, for in the 

ancient rocks the radial growth has utterly disappeared. The central 

clear chain consists now of a fine quartz mosaic. The dark borders, 

except for the crowded magnetite globulites, can not be distinguished 

from the holocrystalline quartz-feldspar groundmass. This clear central zone where the spherulites converged evidently furnished the plane 

of weakness and easy solution, along which silica was infiltrated and 

parallel to which the rock cleaves.

The impurities which have entered the rock along this cleavage plane 

give rise to the central dark line mentioned in the macroscopic description, while the silica forms the opaque white band on either side. The 

central zone is sometimes more than a millimeter wide. Where a feldspar crystal lies across this plane of weakness with its longest axis at 

right angles to the latter, the strain has proved too great for the crystal, 

which has been broken apart and the break cemented by infiltrated 

silica (PI. XXIII, b).

The chain spherulite structure is of more common occurrence in the 

aporhyolites of the Monterey district than any other form of spherulitic growth. The acid rocks east of the Bigham copper mine show 

otbern in great perfection. Eutley 1 has figured some similar chain 

spherulites in the felsitic lavas of England and Wales. In felsite of 

the Keweenaw series from the Minnesota shore of Lake Superior the 

writer has recently observed fine bands of silica so similar to the altered 

chain spherulites as to suggest a like explanation for them.

Axiolitic structure. Closely related genetically to the chain spherulites, but unlike them in being radial linearly rather than centrally, is 

the axiolitic growth.2

Axiolites are not particularly characteristic of the South Mountain 

aporphyolites. Curving, linearly radiating growths do occur, however, 

in specimens from more than one locality. PI. XXI, 6, shows this 

structure.

Rliyolitic structure. The rocks in which the axiolites were observed 

are holocrystalline, yet they exhibit most strikingly the characteristics 

of a glass. Flow and vesicular structures, stringers and shreds, and 

curved patches of a brownish-red color, forming what has been called 

the rhyolitic structure, abound. (PI. XXIV, a and &; PI. XXV, a.) 

This latter structure has been figured and described by Eutley, 3 Nordenskjold, 4 and de la Vallee-Poussiu,5 and on a macroscopic scale by

'Felsitic lavas of England and Wales: Mem. Geol. Survey, Gt. Brit., 3885, PI. VII, figs. 11 and 12.

'Zirkel, Microscopic Petrography: Geol. Expl. 40th parallel, p. 1C7.

3Kutley, On the microscopic structure of devitrified rocks from Beddgelert .and Snowden: Quart. 

Jour. Geol. Soc. London, Vol. XXXVII, p. 406, figs. 1 and 2.

^Nordenskjold, Zur Kenntuiss des sogen. Hsilleflinta des uordostlichen Smalands: Bull. Geol. Inst. 

Upsaln, No. 1, Vol. I, p. 5, 1893. v

f Do la Vallue-Ponssin, Les ancieuuea rhyolites, dites eurites, deG-rand-Manil: Bull. Acad. Roy. Belgique, Ud series, Vol. X, 1885, p. 271.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XI

LITHOPHYS/£, RACCOON CREEK.

The reck is a pa's pink, while the originally hollow spaces between the concentric layers of the lithophysse are 

. . filled with a deeper pink siliceous deposit.
BASCOM.] APORHYOLITES. .55

Irviiig. 1 A perlite from Deer Creek Meadows, 25 miles south of Lassen 

Peak, displays a similar rhyolitic structure. This structure is essentially a phase of the fluidal structure.2

JLithophysal structure. Very often the macroscopic features of the 

aporhyolites disclose their original character more convincingly than do 

the microscopic. Lithophysse are best revealed in the hand specimen? 

where they are brought out in delicate relief by weathering. In such 

specimens from the Raccoon Creek locality, the rose-pink petals of the 

lithophysne in a pale-pink base produce quite as beautiful examples of 

this glassy structure as any obsidian or rhyolite offers. (PI. XI.) No 

undoubted lithophysaj were found within the Monterey district. The 

microscope discloses some vesicular structures which bear slight trace 

of a lithophysal character, but the alteration has been too great to 

allow of their identification as hollow spherulites.

Micropeymatitic structure. Themicropegmatitic structure shows itself 

in microscopic pegmatoid groups of phenocrysts, such as have been 

frequently described in porphyries and rhyolites.3 It does not play an 

important part in the aporhyolites.

Perlitic structure. That this structure is present in the South Mountain rocks, and in great perfection, has already been noted. (PI. XX, a, 

PI. XXI, a.) While its presence is a most reliable proof of the former 

character of the rock, its absence furnishes no evidence against the previous glassy condition of the rock, both because many recent rhyolites 

showed no trace of that structure and because it is most readily effaced 

by devitrification.

Amygdaloidal structure. At Raccoon Creek, at the Bigham copper 

mine and its near vicinity, are light-colored (pink and yellow), extremely 

vesicular aporhyolites. The vesicles are oval or elongated by How movement. (PI. XXVI, a and b.) They are uniformly filled with epidote or 

quartz, with both, or with either to the exclusion of the other. When 

both are present the quartz forms a rim around the epidote. The epidote 

has often a radial arrangement, while crystal boundaries are absent. Its 

color varies from a deep yellowish green to light yellow, and pleochroism is marked. In some of the larger amygdules the radiating needles 

of epidote have been broken and pulled apart at right angles to their 

longer axis and the spaces filled with silica. Piedmoutite and quartz 

show the same relation, as described on page 41. The groundwork of 

these amygdaloids is the usual holocrystalline quartz-feldspar aggregate. Incipient alteration to granular epidote is more frequent in these 

open-textured amygdaloids than in the compact aporhyolites. These

'Irving, Copper-bearing rocks, etc.: Mon. U. S. Geol. Survey, Vol. V, pp. 312-313, fi^. 22.

2 This is the " Aschenstructur" of Miigge (Untersuchungen iibcr dio "Lenneporphyre" in Westfalen 

und don angrenzenden Gobieten: Neues Jahrb. fur Min., Gool. 11. Pal., B. B. VIII, 3893, pp. 048, 649, 

713), who considers it dne to the original fragmental character of the lava. Whether it is always to 

be so interpreted is a question for further investigation.

>Iddings, op. cit., p. 275, PI. XV, fig. 5.
56 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

rocks are pierced with coarse quartz veins which bear native copper. 

Copper in microscopic quantities and copper oxide also occur in the 

amygdules. Hand specimens are frequently coated with the copper 

carbonates, malachite and azurite. There were picked up on the roadside some specimens of auiygdaloidal aporhyolites that are quite 

diverse from the amygdaloids of the Bigham copper mine which have 

just been described. They are similar to specimens found north of the 

Monterey district at Eaccoou Creek. The amygdules, which are black 

against a yellowish-white background, are finely attenuated and elongated in long parallel hair lines, lending to the rock the appearance of 

an eutaxite. The black color is due to magnetite, which either is finely 

disseminated in quartz (the other infiltrated mineral), or is present in 

masses, or simply forms a heavy rim around the amygdules.

Some of the amygdaloids from. Eaccoon Creek merit a detailed description. In these rocks the vesicles are usually bordered by a broad riin 

like the groundmass, in its present crystallization, but separated from, 

it by a narrow, clear zone of quartz, and characterized by a greater 

abundance of magnetite (or ilmenite). On the inner edge of this 

border are'spherulitic growths, while the rest of the vesicle is filled 

with quartz (PI. XXVII, b) or with quartz and an opaque black oxide 

(PI. XXVII, a). In the latter case the black oxide occupies the center 

of the vesicle, leaving a clear zone of silica around the spherulites. 

Crossed nicols show that the spherulites are optically continuous with 

the quartz, and that the radial appearance which has been retained is, 

due to the arrangement of the impurities. The appearance of these 

vesicles is very suggestive of those figured by Professor Cole. 1 Professor Cole explains this type of spherulite by a dual mode of growth 

a radial growth outward from the grouudmass, as well as inward, originating in the glass aiid converging toward a spherulitic center! He 

does not give the mineral character of the spherulite. Whatever may 

be the facts with reference to the Eocche Eosse obsidian, it is not 

necessary to postulate an abnormal method of crystallization to explain 

the phenomena observed in the South Mountain aporhyolites.

The spherulites projecting into the vesicle, with their bases sunk into 

its walls, were recognized by Professor Iddings, who kindly examined 

the section, as tridymite spherulites, such as form on the walls of vesicular cavities in all kinds of modern lavas. Tangential sections of such 

spherulites are represented by granular aggregations. The form of 

the tridymite has been preserved by impurities, while its molecular 

arrangement has been altered to that of quartz. The presence of a 

border between the groundmass and the cavity suggests that crystallization, starting from the walls of the cavity, took place within the 

magma, initated, perhaps, by the gaseous content of the vesicle.

'Greuvillo A. J. Cole and Gerard W.Butler, On-the lithopliyses iu the obsidian of the Kocche 

Rosse, Lipari: Quart. Jour. Geol. Soc. London, Vol. XLVIII, 1892, p. 438.
BASCOM.J . APORHYOL1TES. 57

Somewhat similar radial growths within vesicles in ancient rhyolites 

have been described and figured by de la Yallee-Poussin. 1

Taxitic structure. Another structure which the South Mountain rocks 

possess in common with rhyolites is what has been called the taxitic. 2 

This consists in the intimate mingling of two portions of the magma 

which from some cause (liquation) are slightly differentiated. The 

iron constituent, which evidently separated out in the original glass, 

has been still further crowded into bands and curved lines by the 

secondary crystallization. The result is the production, in some cases, 

of an irregular mottling, when the rock is called an ataxite; and in 

other cases of a more or less complex network of interlacing bands 

following lines of flow and forming a cutaxite. This mottling and 

banding is made the more striking by a marked contrast in color. The 

body of the rock is light-gray or pink, and the lines are dark blue-gray 

or red, according to the varying degrees of oxidation of the iron. 

Where the dark lines outline oval and spherical spaces and contain 

porphyritical crystals in or near their centers, the crystallization is 

regarded as having once been spherulitic and the rock is termed a 

spherutaxite. These have been described on page 50.

The eutaxites are frequently so sheared as to give a hair-like tenuity 

to the bands in cross section, while the microscopic slide shows the 

effect of pressure on the rock in the parallel arrangement of the globulites of black oxide. The universal presence of globulites, trichites, 

and microlites of black and red iron oxide in flow bands, or indifferently distributed, or in concentric zones around spherulites and vesicles, is worthy of mention as a further point of resemblance to the modern rhyolite. Such a trichitic structure in similar rocks has been 

described by various petrographers.3

SUMMARY OF PROOF OF DEVITRIFICATION.

It is not easy to present the evidence for the secondary nature 

of the holocrystalline groundmass so that it shall have the weight 

which properly belongs to it. Very much depends upon effects which 

it is impossible to convey by description, but which carry conviction 

to the student of these rocks. The contrasting appearance of many of 

the sections in ordinary and polarized light can not be adequately reproduced. The disappearance under crossed nicols of rhyolitic, perlitic,

'Les anciennes-rhyolites, dites eurites, de Graud-Manil: Bull. Acad. roy. Belgifjue, iid series, Vol. X, 

1885, p. 292.

'Fritsch and Eeiss, Tenerirte, 1808, p. 414.

Kosenbusch, Mic. Phys. der Massigen Gesteine, 2d ed., p. 025.

F. Loewinson-Lessiug, Zur Bildungsweise rind Classification der klastiscben Gesteine, 1888, pp. 228- 

235. Note sur les taxitos et sur les roches clastiquo vulcaniqiie: Bull. Soc. Belg. geol., etc., Vol. V., 

1891. Loewinson-Lessing's division of the taxites into ataxites, cutaxitcs, and splierutaxites has 

been followed in this bulletin.

"11. D. Irviug, Copper bearing Rocks, etc.: Mon. U. S. Geol. Survey, Vol. V, p. 312.

S. Allport, On certain ancient devitrifled pitchstones and perlitca from the Lower Silurian district 

of Shropshire; Quart. Jour. Qeol. Soc. London, Vol. XXXIII, p. 449.

Nordeuskjold, op. cit.
58 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BOLL. 136.

spherulitic, and fluxion structures, so clearly indicated in ordinary 

light, in a homogeneous holocrystalline mosaic is one of the strongest 

evidences for the secondary character of the crystallization. 

There are also instances where the nature of the crystallization is 

distinctly proved. On page 51 it was shown to be subsequent to the 

cracking -which must have occurred in a solid rock. Pages 52-53 

describe the replacement of the radiating crystallization of the spherulites and chain spherulites by a granular crystallization which is homogeneous with a granular grouudmass. Finally, on pages 49-50, the 

secondary character of the inicropoikilitic crystallization has been 

indicated. One or another of these indications of secondary crystallization is almost invariably present in the rocks which have been 

included under the name aporhyolite.

The exceptional occurrences, where these structures are absent, show 

genetic relationship in the field to typical aporhyolites. The determination of the character of the grouudmass in the cases described 

thus practically determines it for all the aporhyolites.

The secondary character of the holocrystalline groundinass once 

admitted, and the indications of an original glassy base recognized as 

sucbj one is forced to conclude that the former was developed from the 

latter by a process of devitrification.

That the processes of crystallization do not necessarily cease with 

the solidification of a magma is well known, for experiment has proved 

that crystallizing forces are active in a glass as well as in a molten 

magma. 1 This action is exceedingly sluggish, and requires, unless 

accelerated by heat and moisture, an immense amount of time. Devitrification has been considered the result of dynamic action only; 2 but 

while dynamic action undoubtedly accelerates the process, if it does not 

initiate it, devitrification may also take place independently of dynamic 

action, as was the case in the famous example of the old cathedral window glass 3 and the ancient devitrified glass from Nineveh investigated 

by Sir David Brewster.4 The nature of the process is in no way different from the process of crystallization in a fluid magma save in the 

rapidity of the action, and is of both a physical -and a chemical character.

The devitrification which has occurred in the South Mountain aporhyolites is not attributed to dynamic action, of which there are many 

evidences of another nature in the South Mountain, but to statical 

metamorphisni. The former would, by shearing, obliterate the original 

structures of a glassy rock and produce a slate, while the latter might 

be an important initiatory and accelerating factor in the process of 

devitrification of the glassy rocks.

Jl)aubr6e, G6ologie experimentale, 1879, p. 158.

: Do la bailee Poussin, Les curitos quartzeuscs (rhyolites ancienues) de Nivelles ct dea environs: 

Bull. Acad. roy. sci. lett. ct des beaux arts do Belgique, 57 auntie, 3d series, Vol. XIII, Xo. 5, 1887, 

pp. 521-522. T. G. JJornemann, Der Quartzporpliyr von Heiligeuateiu nud seine Fluidalstructur: 

Zeitschr. Deutscli. geol. Gosell., Vol. XXXIX, 1887, p. 793.

3British Ass. Eept., ]840.

'Trans. Royal Soc. Edinburgh, Vols. XXXII, XXXIII.
KASCOM.] AtORttYOLlTES. 59

Opinions of petrogr ciphers. Paleozoic and pre-Paleozoic 'acid volcanics have long been studied on the European continent. Although 

tlieir variation from the modern type of acid volcanics, rather than their 

resemblance to that type, has, for the most part, been emphasized by 

German and French petrographers, there have not been wanting able 

advocates of devitrification and of an original glassy base for the 

ancient lavas.

R. Ludwig 1 (1861) and Vogelsaug 2 (1867) inclined to-the opinion that 

the grouudmass of certain quartz-porphyries is the result of the devitrification of a glassy lava.

The late Dr. K. S. Lossen 3 (1869), on comparing the spherulitic porphyries of the Harz Mountains with the obsidians of Lipari, Mexico, and 

Java, found the resemblance sufficiently striking to lead him to declare 

that the porphyry groundmass was originally crystallized as glass, and

became cryptocrystalline through molecular rearrangement. Later, 

Kalkowsky 4 (1874) suggested that devitrification through the chemical activity of water was the process by which the microfelsitic base of 

certain pitchstones and felsites was developed; and still later, H. Otto 

Lan'g 5 (1877) described a macroscopically unindividuahzed base which 

is similar macroscopically to the devitrified base described by Kalkowsky. Sauer (i (1889) considered the Dobritz porphyries as the final alteration product of a pitchstone. More recently Klockniann 7 (1890) has 

described the replacement of the spherulitic crystallization in quartz 

porphyries, through secondary processesrby a fine-grained aggregate 

of quartz and feldspar.

Osann 8 (1891) described incipient devitrification in perlite and other 

glassy rocks from Cabo de Gata. Finally, Link (1892) considered that 

the fine-grained groundmass of some American rocks closely related 

to mica-syeuite-porphyries was once glassy, or at least partially glassy, 

and C. Vogel 9 (1892) reached the same conclusion, as to the Umstadt 

porphyries in Hesseu. Many no less capable observers still hold to 

an original difference between ancient and recent acid volcanics, and 

the possibility of devitrification and an original similarity is yet an 

open question in Germany.

In France, La Croix 10 describes andesites from Martinique in which 

the glass has altered into quartz spherulites and a granular quartz 

aggregate.

It is interesting to note that many of the halleflinta of Sweden,

'Erl. zur geol. Karte Hessens, Bl. Dieburg, 1861, p. 56. 

2 Phil, do g6ologie, pp. 144, 153, 194.

3 Beitra'ge zur Petrograpbio tier plutonischen Gesteine: Abhamll. tier Berliner Acatl., 1869, p. 85. 

''Tscherruaks mineral. Mittheil, pp. 3J, 58. 

'Grundriss dor Gesteiuskundo, p. 43.

G Erl. zur geol. Specialkarto Saehsens. Bl. Meissen, pp. 81-91.

'Die Porphyro; Der geol. Auf'bau sogen. Magdeburger Uferrandos init besoiulerer Beriicksichtigung dor anftretenden Eiliptivgesteiue: Jahrbucb K., prenss. geol. Laudesaiistalt.Vol. XI.

8 Zeitschr. Deutsch. geol. Gesell., Berlin, pp. 691,716.

9 Abhandl. geol. Landeaanstalt vou Hessen, Vol. II, p. 38.

10 Coniptes-rendus, CXI, p. 71.
60 . VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

 which, like the South Mountain, volcanics, were once described as sedimentary, are proving to be acid volcanics, preserving the features of 

their modern equivalents. Quite recently glassy and rhyolitic structures in these rocks have been observed and described by Otto Nordenskjold. 1

In Belgium, de la Vallee-Poussin seems to be the only writer who has 

brought out the resemblance between the eurites of that country and 

modern rhyolites. He describes at some length structures similar to 

those possessed by the aporhyolites of South Mountain. A vacillating 

state of mind as to the matter of nomenclature is indicated in the 

titles of his successive papers. 2

In England the rhyolitic character of ancient acid volcanics has been 

recognized and emphasized, and the idea of devitrification is widely 

accepted. Allport, Cole, Bonney, Eutley, Judd, and Harker have 

accomplished most valuable work along this line.

Dr. Wadsworth 3 was the first American petrographer to advocate 

the abandonment of age as a factor in rock classification, while at the 

same time he recognized devitrification as the process which was forming felsites out of rhyolites. What he says is of interest in its anticipation of ideas now more*generally accepted:

This devitrification gives rise in the older and more altered rhyolites to the feldspar, quartz, and microfelsitic (so called) base that has so puzzled lithologists in the 

study of the felsites. The rhyolites of all volcanic rocks preeminently show lamination produced by flowing, a fact which is doubtless due to their being so siliceous. 

This structure and their devitrification enable us to trace a direct connection 

between the rhyolites and felsites, which are simply the older and more altered rhyolites. * * * One of the best illustrations of this is to bo found on Marblehead 

Neck, Massachusetts, where at least two distinct flows of felsite occur, one cutting 

the other. They show the fluidal structure so characteristic of rhyolites a character that has been mistaken for lines of sedimentation by geologists, while the 

inclosed crystals of orthocl.ise have been taken for pebbles. * * * While to the 

naked eye and under the microscope this rock shows the fluid al structure of a rhyolite, in polarized light it is seen that the base has been completely devitrified, a 

process that is carried to a great extent in many known modern rhyolites.

No other American petrographer has so distinctly advocated the 

identity of felsites and ancient rhyolites, in spite of the fact that many 

of our felsites illustrate it as unmistakably as do the English felsites.

Dr. Irving,4 in his description of the Beaver Bay group of the Keweenaw series, repeatedly calls attention to the resemblance between the 

ancient felsites and quartz-porphyries and the modern rhyolites, though 

he does not express an opinion as to their equivalence.

'Op. cit. Also TJober archfcische Erguasgesteine aus Smaland: Bull. Geol. Inst. Upsala, No. 2, 

Vol 1,1893, pp. 1-127.

: Les anciennes rhyolites, diteseurites, du Grand-Manil: Bull. Acad. roy. Belgiqne, 3d series Vol. X, 

1885,pp. 253-315. Lea eurites quartzeuses (rhyolites anciennos) do Nivellea et des environs: Bull. 

Acad. roy. des sci. et des beaux-arts de Belgique, 57 aunee, 3d series, Vol. XIII, No. 5,1887.

;M. E. Wadsworth, Notes on the mineralogy and petrography of Boston and vicinity: Proc .Boston 

Soc.Nat. Hist., Vol. XIX (May, 1877), p. 236. On the classification of rocks1 : Bull. Mass. Comp. Zool. 

Harvard Coll., Vol. V, No. 13, June, 1879, p. 277.

'Op. cit., pp. 312, 313, note 5, p. 436.
BASCOM.] CHEMICAL COMPOSITION OF THE ACID ERUPTIVES. 61

Messrs. Hague and Iddings l make the statement "that the degree of 

crystallization developed in igneous rocks is mainly dependent upon 

the conditions of heat and pressure under which the mass has cooled, 

and is independent of geological time."

On the question of devitrification the writer finds no more direct 

expression of opinion, but the fact of devitrification is recognized by 

Iddings, Williams, Cross, and Diller. In none of the felsites elsewhere 

described have the varied structures of the modern rhyolites been more 

perfectly and conspicuously preserved than in the aporhyolites of the 

South Mountain.

CHEMICAL COMPOSITION OF THE ACID ERUPTIVES.

The chemical study of the South Mountain rocks has been rendered 

easy by a number of analyses of these rocks made by the late Dr. Genth, 

under the auspices of the Second Geological Survey of Pennsylvania. 

Analyses of the sedimentary, acid, and basic igneous rocks of South 

Mountain are distributed through pages 252 to 282 of Eeport CCC.

The analyses of the acid volcanics have here been brought together 

and tabulated.

Table of analyses.

Si02 ..............

PsOs ..... ........

AJA- . .... 

FeA-- - ---- 

FeO. ...... ....... 

Ti02 .... .........

MffO.............

CaO .............

K,0 .............

Na.?0 ............

H2 0 .............

Total ......

I.

79. 970

13. 860

2. 700

1.230

.220

95. 038

II.

79. 920

.190

12.400

2.950

0.490

2.320

.440

100.71

111.

70. 70

.11

2.63

' ."08

.11

6.15

1.53

100. 77

IV.

70.02

12.26

2.08

4.03

.26

.34

2.57

.40

99.09

V.

75. 570

.190

11.940

3.830

5.250

3.010

.270

100. 00

VI.

75. 31

.05

12.90

3.05

.........

3.15

5.11

.61

100. 65

I. Fissile green schist. Between Piuo Grove Furnace and Laurel forge.*

II. " Orthofelsite." " Oue-fonrtli of a mile north of Lerew's store." 3

III. "Orthofelsite." "Cut 011 turnpike 5 miles northwest of Petersburg, Cumberlaud County." 3

IV. "Laminated felsite." "East of Bigham Copper Mine." 4

V. "Laminated orthofelsite." "One-fourth of a mile southeast of Caledonia Furnace." 3

VI. " Finely laminated orthofelsite." "One-fourth of a mile west'of Cole's sa\ymill, on the Shippenburgroad." 3

Their general uniformity and their close agreement with the analyses 

of typical rhyolitic lavas is the most striking feature of these analy-

'On the development of crystallization in the igneous rocks of Waahoo, Nov., with notes ou the 

geology of the district: Bull. U. S. Geol. Survey No. 17,1885, p. 40:

"Analysis made by A. S. HcCreath for commercial purposes; alkalies undetermined.

3Aualyses made by Dr. Gentli and Henry Trimble, Second Geo). Surv. Pa., Vol. CCC, pp. 263-269.

4 Analysis made by C. Hanford Hendorson, of Philadelphia, The copper deposits of tho South 

Mountain; Traus.-Am. lust. Mm. Eug., Vol. XII, 1884, p. 90. . _ . . .. _ 
62 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

ses, and is a convincing proof of the igneous origin of the rocks which 

they represent. In the absence of samples of the rocks analyzed or of 

exact descriptions of their character, special points can be brought out 

only in ferenti ally.

The high percentage of the alkalies and the slight trace of lime 

plainly denote the character of the feldspathic constituents. This 

indication of their chemical character coincides with the optical and 

physical properties enumerated on page 40. The rock from which 

Analysis IV was made is reported to be from the same locality from 

Avhich many of the aporhyolites were* obtained, the optical character of 

whose feldspars were tested.

Microscopic study of these rocks leads us to expect a percentage of 

titanium oxide. In two instances the analyses show it. It is not 

unlikely that in the other cases the titanium oxide was not determined. 

The absence of manganese oxide from these analyses is surprising, as 

the Monterey porphyries and aporhyolites show a high percentage of 

it. The lime and magnesia present are doubtless due to the presence 

of epidote in the rock. The silica, alumina, and iron percentages are 

exactly normal and call for no remark.

Analysis I, of a fissile schist, illustrates the slight change of chemical 

constitution which accompanies dynamic action in the acid rocks0

Among the analyses made by Dr. Genth for the Second Geological 

Survey of Pennsylvania are the following, which, because of their 

anomalous character, have not been tabulated with the others:

~t> n

A12 03 ......... .............

Fe-iOs.-.. ..................

FeO .......................

TiO,.... ...................

MnO....... ................

MgO. ........ ..............

CaO .......................

KjO .......................

Na3 0 ......................

Total ................

I.

53.86

99 -}Q

7.11

2.79

1 OQ

4.56

4.12

. 2.55

100.42

II.

r,o 09

.21 *>n 73

1.92

.52

2.03

6.68

4.28

99.62

I. "Slaty rock." "Nine miles southwest of Dillerslmrg."

II. "Purplish slaty orthofelsite." "One and one-ball'miles southeast of Mount 

Alto."

These rocks plainly do not represent the normal type of the South 

Mountain acid rock. In the absence of specimens or means of determining the character of the rocks from which these analyses were made, 

it is impossible to explain altogether satisfactorily the abnormally low 

percentage of silica and.the high percentages of alumina and iron.

If these slates \vere once normal aporhyolites, the shearing which 

produced the subsequent slaty character must have been accompanied 

by an abundant development of sericite from the feldspar. If the silica
U. £. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XII

FLOW BRECCIA.

From crest of hill east of the Ciermont House. One-half natural size. The details of the brecciated structure 

are greatly obscured in the photograph. Some of the fragments are spherulitic ; many of them fit togethcrf 

forming what v, as once a large fragment. The colors are shades of pink, red, and purple.
t 

BASCOM.T' ACID VOLCANIC BRECCIA. 63

thus set free was carried off by percolating' water, a low silica percentage and a correspondingly high alumina percentage would result, while 

the alkalies would remain about the same. The increase in iron may be 

due to infiltration.

ACID VOLCANIC BRECCIA.

DISTRIBUTION.

The presence of acid pyroclastics in the Monterey district has already 

been mentioned. Although a conspicuous feature of a portion of South 

Mountain,, notably of the Buchanan Valley north of the Chainbersburg 

turnpike, where they cover about 2 square miles, they play an insignificant rdle among the rocks of the Monterey district. Their character, however, is unmistakable. They may be classified as tuffs, flow 

breccia, and true breccia or tuffaceous breccia.

This is a dark-purple banded rock, the clastic character of which is 

hardly evident in the hand specimen. Microscopic examination discloses its tuffaceous nature. Minute angular fragments, not exceeding 

a millimeter in length, are thickly distributed through a crystalline 

groundmass.

The fragments are usually sphernlitic, with the replacement of the 

spherulitic crystallization, as in the massive aporhyolites described on. 

page 52, by a fine mosaic, so that in ordinary light the spherulites are 

traceable only from their outline. In the same way, under crossed nicols 

a uniform granular crystallization obscures the fragmental character 

of the rock a character which, in ordinary light, is sharply brought 

out by an outlining pigment of red iron oxide. Fragments of quartz 

and feldspar are among the inclusions. The grouudmass, which is of 

the same chemical and mineralogical constitution as the included fragments, doubtless represents an ash recrystallized.

FLOW BRECCIAS.

These occur at widely separated localities northwest of Old Maria 

furnace, near the source of Toms Creek, on the Gladhills road, and on 

the brow of the hill east of the Clermont House. These breccias are 

composed of fragments of considerable size, which plainly were caught 

in a viscous acid magma, as is evidenced by their linear arrangement 

in flow lines and by the way in which different fragments fit together, 

forming what was once a larger fragment. On the weathered surface 

of the rock its brecciated character is rendered very manifest in the 

varying tints of pink, red, purple, and blue (PI. XII). The fragments 

range in size from the submacroscopic to those that are 2£ inches in 

diameter. Their spherulitic character is discernible by the naked eye. 

Under the microscope, in ordinary light, the fragments frequently show 

either perlitic parting, a spherulitic character, or a regular arrangement
64 VOLCANIC ROCKS Otf SOUTH MOUNTAIN. [BOLL. 136.

of the coloring matter parallel to the boundaries of the fragments, due 

to water deposition. Crossed nicols again show uniform crystallization 

or a micropoikilitic structure. In one specimen of breccia this was not 

the case, however. A coarsely crystalline siliceous cement is quite 

.distinct in grain from the uniformly finely crystalline fragments. This 

may be a lava flow crushed and recemented.

TUFKACEOUS BKECCIA.

In some instances the grouudmass doubtless represents an altered 

ash, when the rock becomes a true breccia. A breccia of this sort from 

the Monterey district has epidote largely developed in the matrix. 

The granulated quartzes and the perthitic feldspars of the included 

fragments show in a marked way the effect of dynamic action.

At Raccoon Creek a fine specimen of breccia was found (PI. XIII), 

and in the Buchanan Valley breccia is extensively exposed. Some of the 

fragments contain chain spherulites. At Coles Corner '(Buchanan Valley), some G miles northeast of Graeffenburg, the breccia is sheared, 

and with the development of sericite the rock has become more or less 

slaty, while still conspicuously"retaining its brecciated character.

The presence of these tuff's, flow breccias, and breccias properj which 

are the natural accompaniments of surface lava flows, is inexplicable 

under any other hypothesis of the origin of. the acid rocks.

METAMORPHOSED ACID ERUPTIVES: SERICITE-SCHISTS AND SLATES.

The alteration of the feldspathic constituent of quartz-porphyries to 

sericite or some other micaceous mineral, under the action of dynamic 

forces, has been frequently described. 1

The production, in this way, from massive acid eruptives, of schists 

and slates resembling true porphyroids,2 is finely illustrated in the 

South Mountain. In a single exposure (west end of Long Mountain, 

west of Gettysburg) felsites with distinct phenocrysts grade in sensibly 

into a crinkled sericite slate. The shear zone is of limited width (10 

feet), and bounded on each side by massive felsites. The phenocrysts 

are only slowly obliterated and can be distinguished until the last stage 

in the alteration has been reached.

Thin sections of five successive stages were studied. They show a 

development of sericite first around the feldspar phenocrysts and in a 

plane of dislocation. It is only sparingly developed in the ground-

'J. Lehmann, TJntersuchung,'iviber dieEntstehuug der altkrystallinischen Schiefergesteine, Bonii, 

J884, Cap. IX, Druclcschieferung uud Glimtnerbildung, p. 130.

A. Ton Groddeck, Zur Kenutniss einigcr Sericitgesteine, Avelche neben und in Erzlangerstatten 

auftreten : Neues Jahrbuch fur Mineral., etc. Supp. Vol. IV, 1886, p. 428.

G. H. Williams, Bull. TJ. S. Geol. Survey No. 62, pp. 61, 121, 212.

Bonnoy, On some nodular felsites in the Bala group of North Wales: Quart. Jour. Geol. Soc. London, Vol. XXXVIII, p. 289.

Callaway, On tho genesis of the crystalline schists of the Malvern Hills: Quart. Jour. Geol. Soc. 

London, Vol. XLI1I, pp. 530, 531.

P. L. Milch, Beitriigo zur Konntnia des Verrucano, 1892, pp. 128, 129.

sKoseubusch, Pet. Massigen Gesteine, Vol. II, 2d ed., p. 411.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XIII

ACID BRECCIA, RACCOON CREEK.

Natural size. The photograph does not bring out clearly the brecciated character of the work, which is 

very pronounced in the specimen. The matrix is apparently an ash containing angular fragments of a larger 

size (average diameter 1 cm.). The colors are blue, gray, and buff.
BASCOM.] SERICITE-SCHISTS AND SLATES. 65

mass. In the next stage the groundmass shows a decided tendency 

to a parallel arrangement, and sericite is more abundantly developed. 

Eventually the phenocrysts are obliterated and there is much sericite 

in the groundmass. It is not, however, developed to the exclusion of 

the feldspar, while the quartz remains unaltered.

In general, the development of sericite stands in direct relation to the 

shearing, and increases up to an almost complete, if not quite complete, replacement of the feldspathic constituents of the groundmass. 

Silica remains as a constituent of the groundmass.

Material obtained from an artesian well at a point 40 feet below the 

surface furnished a similar shear zone, which displayed an even more 

abrupt transition from a porphyry to a sericite-schist. A single microscopic section included both porphyry and schist in typical development. The former showed an early stage of the alteration which was 

complete in the latter, where only sericite and a very schistose siliceous 

microgranitic groundmass remained. In this case, and in the extreme 

stage of the transition previously described, it would be impossible, 

with the microscope alone, to decide whether the schists were of clastic 

or uonclastic origin. This is one of the instances where field evidence 

is quite essential to the authoritative determination of the origin of the 

rock.

At the Bechtel shaft there has been thrown out a mottled red and 

white schist which has been produced by the shearing of a massive 

felsite. Here the phenocrysts have been replaced by a quartz mosaic 

and some sericite, which is also largely developed in the groundmass. 

The red mottling is due to a more or less parallel arrangement of red 

iron oxide globulites.

A light green sericite-schist found on the railroad near Blue Eidge 

Summit station, and closely resembling some schists in situ exposed 

on the Gettysburg Eailroad below the Clermont House, shows under 

the microscope phenocrysts of feldspar containing inclusions of a 

former glassy magma, still well preserved and showing twinning striations. These phenocrysts occur in a groundmass of quartz, a little 

feldspar presumably, much sericite, epidote, ilmenite, or magnetite, and 

leucoxeue.

The color of the schist is due largely to the epidote.

At the exposure just now mentioned on the'Gettysburg Eailroad, 

east of the Olermont House, there occurs a handsome, light, silverygreen, crinkled sericite-schist. Several rods to the north of this 

exposure the railroad cuts through quartz-porphyry, but the contact 

between the porphyry and the schist is not exposed. The schist is similar to those already described, whose gradual passage into a massive 

porphyry could be followed in the field, and shows traces of phenocrysts under the microscope, and in the hand specimen on the surface 

at right angles to the cleavage. The cleavage surfaces often display 

exquisitely delicate and manifold dendritic tracery. (PL XIV.) 

Bull. 136 5
66 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BDLL. 136.

On the highroad from Fountaindale to Fairfield, not far from the 

"Old Copper Shaft/' occurs a dark purple-gray spotted slate. The 

light-green spots are sometimes irregular, but more frequently possess 

crystalline outlines and prove under the microscope to be a sericitic 

alteration of feldspar phenocrysts. Much of the feldspathic material 

still remains. The groundmass consists largely of iron oxide, which by 

its prevalence obscures the other constituents leucoxene and quartz.

Microscopic evidence is sufficient in this instance to determine the 

origin of the rock. It is plainly a sheared eruptive, and probably a porphyritic aporhyolite, although the irregular outline of some of the 

sericitic areas suggests a brecciated aporhyolite.

The occurrence of these slates is an interesting feature of the geology 

of the South Mountain. In the hand specimen they might readily be 

confused with a porphyroid, that is, a metamorphosed clastic rock, and 

have been so confused by geologists. They did not escape the attention of Professor Eogers, who alludes to them as "the fissile talcose 

rock" near the "reddish gray rock, containing specks of reddish feldspar," and includes them among the primal slates whoso highly altered 

condition he repeatedly contrasts with the other slightly altered sediments (sandstone). If these slates were of clastic origin, a high degree 

of metamorphism was necessary to produce their present crystalline 

condition, and Professor Eogers was quite right in drawing a contrast 

between their extreme metaraorphism and the comparatively unaltered 

condition of all the other sediments. The very fact that their development from a sediment calls for such a high degree of metamorphism confined to limited and isolated zones, and for which no adequate cause can 

be assigned, renders such an origin as improbable as it is unnecessary. 

Such a "selective metamorphism" is not demanded by the facts.

As a matter of fact, these slates are scarcely more altered than the 

sandstone. Dynamic action in the latter has developed a quartzite. 

Dynamic action in the less resistant porphyry and aporhyolite has 

produced a sericite slate. That the chemical character of the acid 

rock remains essentially unaltered is evinced by analysis I, given on 

page 61. This shows exactly the composition of a rhyolite, and is 

totally unlike that of the sediments of the region. The sedimentary 

argillaceous slates of South Mountain are very little altered, and 

exhibit no tendency toward the development of porphyroids.

All evidences field relationship, successive stages shown in the 

hand specimen and under the microscope, chemical character, inherent 

improbability of clastic origin combine to reveal the igneous character 

of these acid slates.

SUMMARY.

The acid igneous rocks of the South Mountain have proved to be 

quartz-porphyries, devitrified rhyolites (aporhyolites) with accompanying pyroclastics, and sericite-schists.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 130 PL. XIV

SERICITE SCHIST.

xposure on the Gettysburg Railroad just below (east of ) the Clermont House. One-half natural size. It 

shows the dendritic arkings which are frequently to be seen upon the cleavage surfaces of the acid slate.
SUMMAKY. 67

The first are typical holocrystalline -porphyries, characterized by a 

soda-feldspar and by the presence in many cases of accessory piedmontite. The second group are the prototypes of the modern rhyelites, 

differing from them only in the loss of a vitreous base through devitrification. They are without phenocrysts, with inconspicuous pheuocrysts, and with abundant and conspicuous pheuocrysts. Like the 

porphyries, they are characterized by a soda-feldspar that is, they are 

of the pantellerite type. The evidence for devitrification lies in the 

abundant presence of structures peculiar to glassy lavas, in the present 

holocrystalline character of the rocks, and in the empirical knowledge 

that glass may become crystalline through lapse of time.

The sericite schists are a metamorphic product of the first two 

classes by means of dynamic action.

The alteration which the original types have undergone subsequent 

to consolidation is, in the case of the aporhyolites, devitrification 

(statical metamorphism); in the case of the schists, sericitization 

(dynamical metamorphism); and in the case of all three groups, including the quartz-porphyries, an epidotization (weathering).
CHAPTER V.

PETROGRAPHICAL DESCRIPTION OF THE BASIC ERUPTIVES.

NOMENCLATURE.

Many of the petrographical. data upon which uniformitarian arguments have been based have been drawn from the comparative study 

of basic eruptives, and there is a marked disposition to disregard age 

in the nomenclature of these rocks. Among German petrographers, 

Eeyer, 1 Tietze,2 Reiser,3 Reusch (H. H.),4 and Suess 5 have supported the 

view that age is not a just ground of distinction between eruptive 

rocks, and Roseubusch 6 predicts that in no very distant future the 

separation of effusive rocks into an older and a younger series "will 

prove untenable." English and American petrographers are practically disregarding age in their nomenclature of the basic igneous 

rocks. AniOLig the former, Judd,7 Teall, 8 Allport,9 Bonuey, 10 Phillips, 11 

and Hobson 12 are notable. Among American petrographers, the 

Daiias 13 and Iddings 14 have disregarded age in their usage of basic 

rock names.

In plagioclase-augite rocks, the distinction, between the -gabbro and 

the diabase groups has been finally recognized as structural and 

not inineralogical, and the distinction between the diabase and the

'E. Reyer, BeitriigezurFisikderErup., 18"77, pp. 142-171; ref. Hussak: Neues JahrbuchfiirMinoia-1., 

etc., 1892, Vol. II, p. 147. Beitriigo zur Fisik der Erup. und der Eruptivgesteino. 1887, p. 135.

?E. Tietze, Das Altersprincip bei der Nomenclatur dor Eruptivgesteiue: Verhandl. k. k. geol. 

Keichsanstalt, Wien, 1888, p. 166; ref. F. Becke: Noues Jahrbuch fiir Mineral.. Vol. II, 1884,p. 303.

3 J£arl A. Keiser, Ueber die Eruptivgesteine des Algaii: Tschermaks mineral. Mittkeil., Vol. X, 

1889, pp. 500-550.

4 H. H. Reuscb, TJeber Vulkanismus, Berlin, 1883, 

6 Suess, Das Autlitz der Erde, Vol. I, pp. 204-206, 1883.

°H. Rosenbusch, Ueber die cliemiscbe Beziehungen der Eruptivgesteine: Tschermaks mineral. 

Mittheil., Vol. XI. 1890, p. 146.

'Judd, Oh the g.ibbros, dolorites, and basalts of Tertiary ago in Scotland and Ireland: Quart. Jour. 

Geol. Soc. London, Vol. XLII, 1886, pp.49-97. The secondary rocks of Scotland: Quart. Jour. Geol. 

Soc. London, Vol. XXX, 1874, pp. 220-303.

8 Tcall, Address of the president Geol. Sec. (e) of the British Assn. Adv. Sci., 1893. 

9 Allport, On the basaltic rocks of the Midland coal fields: Geol. Mag., Vol. VII, No. 70, 1870, pp. 

159-162 Tertiary and Palaiozoic trap rocks: Geol. Mag. Vol. X, 1873, p. 196. 

'"Bonney, Quart. Jour. Geol. Soc. London, Vol. XXX, p. 529.

"Phillips, On the so-called greenstones of central and eastern Cornwall: Quart. Jour. Geol. Soc., 

London, Vol. XXXIV, p. 471.

I2 Hobson,On the basalts and andesites of Devonshire: Quart. Jour. Geol. Soc..London, Vol. XL VIII, 

1892, pp. 496-507.

]3 J. D. Dana, On some points in lithology: Am. Jour. Sci., 3d series, Vol. XXXVIII, 1878, pp. 

336, 438. E. S. Dana, Trap rocks of the Connecticut Valley: Proc. Am. Assn. Adv. Sci., 1884.

14 Iddings, The columnar structure in the igneous rock on Orange Mountain, New Jersey: Am. 

Jour. Sci., 3d series, Vol. XXXI, May, 1886, p. 331.

68
BASCOM.] ' BASIC ERUiTIVfcS. $9

basalt (dolerite) as one of the degree and granosity of the crystallization. It is easy to see that these features are determined by the 

geological conditions of consolidation. It follows from this that the 

essential characteristics of the rock groups are independent of age.

A history of the classification of the gabbro and its allied groups 

and an exact statement of the final definition of these groups have been 

concisely given by Dr. Bayley. 1 He suggests that the group of melaphyres and augite-porphyrites will eventually be dispensed with, when 

the olivine diabases and the diabase "will take the position thus left 

vacant, and the plagioclase-augite rocks will be found to occupy these 

places with respect to each other5 the gabbros, the position of a deepseated rock; the diabases, that of the corresponding holocrystallino 

effusive; and the basalt, that of the hypocrystalline equivalent." 

With this understanding of the use of the terms diabase and basalt, 

the South Mountain basic rocks fall into the diabase group.

They are holocrystalline, effusive, plagioclase-augite rocks, with or 

without olivine. They thus possess the characteristics of the augiteporphyrites and melaphyres (diabase group). They are so fine-grained 

as to appear homogeneous in the hand specimen, yet show no evidence 

in the thin section of an originally hypocrystalline character. There is 

no proof for or against devitrification. In the absence of such proof 

their present holocrystalline character will be recognized in their nomenclature as primary.

MELAPHYRES AND AUGITE-PORPHYRITES. 

DISTRIBUTION.

The quartz-porphyries and aporhyolites in the Monterey district are 

limited to numerous small detached areas. The melaphyres and augiteporphyrites, on the other hand, occupy a large, irregular area, covering 

the valleys, the foothills, and the mountain flanks. Besides this area, 

which constitutes about one-half of the entire district, there are two 

small areas north of the old Maria Furnace, which are surrounded by 

the acid rocks, thus reversing the usual relation of the basic and acid 

eruptives. Along the State line and to the south of that line the diabases are massive or schistose, and inconspicuously amygdaloidal. In 

the Monterey district the amygdaloidal character of the diabases is their 

most marked feature. In the exposures on the Gettysburg Railroad 

narrow zones of inconspicuously amygdaloidal or nonamygdaloidal melaphyres grade above and below into conspicuously scoriaceous rocks.

Basic igneous slates occur at the west end of the Gettysburg tunnel, 

where they grade into massive diabases. They also occur on Colonel 

Benchoffs place, at a locality just north of Gum Spring, on a line 

northeast of the Blue Ridge Summit station, and form, a knoll south of 

the Fountaindale post-office. (See map, PI. I.)

1 The basic massive rocks of the Lake Superior region: Jour, of Geology, July-August, 1893, Vol. 

I, No. 5, pp. 433-456.
70 VOLCANIC SOCKS 0V SOUTH MOUNTAIN. [BOLL. 136.

There are a few other localities where slates occur. Where the slates 

were not studied in thin section their igneous origin has not been 

considered as proved.

Ash beds, diabases crushed and recemented with epidote and quartz, 

are exposed along the Gettysburg Railroad in the cuts west of the 

tunnel.

A tuffaceous breccia, composed of fragments so rounded as to appear 

water worn, was found at the head of Minie Branch. At the Russel copper mine and a few other localities epidosites are abundant. At the 

former place they are evidently vein material, and carry the native 

copper. In other localities they undoubtedly represent the last stages 

of decomposition and alteration of the massive or more often of the 

tuffaceous diabases.

MACROSCOPICAL DESCRIPTION.

The augite-porphyrites and raelaphyres vary in color from a slateblue or purple to all shades and tones of green. Where epidote is the 

predominating alteration mineral the prevailing color is light yellowishgreen; with chlorite or actinolite as the alteration products the color is 

a dark green.

The most persistent and striking feature of the augite-porphyrites 

and melaphyres is their amygdaloidal character, to which allusion has 

already been made.

Bowlders on the roadside and in the fields show a curiously rough 

and pitted surface, due to epidote or quartz ainygdules brought out in 

relief by weathering. Sometimes the bowlders closely resemble conglomerates composed of green or white oval pebbles; or the quartz 

ainygdules, when perfectly spherical, mimic the spherulites of the 

acid rocks.

The diabases (augite-porphyrites and melaphyres) are rarely massive, usually schistose, sometimes slaty, and almost universally amygdaloidal.

As the amygdules quickly respond to pressure, they furnish a delicate test of the degree of schistosity present in the rock. Macroscopically the schistosity is otherwise more or less obliterated by subsequent 

epidotization or chloritization.

Genuinely massive diabases are exceptional in occurrence and limited in extent. Occasionally a mass of rock has moved as a whole, 

under pressure, and thus close to schistose or even slaty diabase the 

rock may retain its massive character. In these cases, since there has 

been no shearing, the vesicles are often perfect spheres, showing no 

elongation from magmatic movement. This is notably the case just 

north of Gum Spring, on the Old Furnace Road. Quartz amygdules 

show conspicuously as round white spots on the fresh surface of the 

rock, which is a dark blue-gray, or are brought out in relief on the 

weathered surface, giving the rock the appearance of being riddled
BASCOM.) MELAPHYKES AND AUGITE-PORPIIY RITES. 71

with shot. In other localities there is a greater diversity both in the 

shape and in the composition of the amygdules.

There is frequently a zonal arrangement of three minerals epidote, 

chlorite, and quartz the light-green epidote being developed on the 

edge of the vesicle and surrounding the chlorite and quartz, which are 

successively developed in the interior: Where the vesicles are large, 

epidote sometimes occurs in beautiful radiating crystals, not completely filling the vesicle, and occasionally associated with crystallized 

malachite.

It is doubtless to these amygdules that Professor liogers refers when 

he describes tbe " decidedly crystalline" primal slates as containing 

"segregated specks and even half-formed geodes of epidote and other 

minerals," or " as gray slate spotted Avith epidote." Wherever there 

has been sufficient shearing to form a slate a micaceous mineral is formed 

in the vesicles. This micaceous mineral is either sericite, when the 

slate is conspicuously ornamented with greenish-white oval spots, or it 

is chlorite, when the slate is marked with brilliant dark-green oval 

spots.

The uonamygdaloidal diabase is always more or less schistose and 

frequently slaty. At the second railroad cut beyond Gladhill's switch 

it hasa banded appearance, due to an alternation in color, purplish green, 

dark and light green rapidly succeeding one another. The rock is finegrained. The form of the banding and the structure of the rocks as 

disclosed by the microscope suggest that the bands represent ash beds.

At the west end of the tunnel there is a curious differentiation of 

the diabase in color and sensitivity to pressure. This differentiation is 

limited to an irregular band which suggests in many ways an intrusive 

dike. The apparent dike traverses the nouainygdaloidal diabase in a 

direction oblique to the schistosity of the latter. At one end it diverges 

and sends out a branch which pursues a course vertically downward 

and disappears beneath the surface.

Fragments of the schistose diabase are included within the dike. 

Its upper surface is somewhat amygdaloidal, the-interior compact, and 

the lower surface bordered by a band of light-yellow epidote. This 

band is more irregular in outline than the upper surface.

The dike is intersected by two systems of fine parallel, or approximately parallel, quartz veins. Parallel to one of these systems is an 

easy cleavage.

The diabase just above and below the dike is finely schistose. This 

schistosity is parallel to the course of the dike, and is particularly 

remarkable above, the dike, where it follows every curve of the latter. 

The color of the dike is purplish, and contrasts with the surrounding 

dark-green diabase.

The obliquity of this seeming dike to the general schistosity of the 

diabase, its inclusions of fragments of the surrounding schist, its 

divergent branches, and the foliation of the diabase parallel to the
72 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

band are all more readily explained on the supposition that we are dealing with a genuine igneous"dike than in any other way. The structure 

of the rock under the microscope and its analysis (see Analysis III, 

p. 78) show that there is no essential difference in these characters 

between the band and the schistose diabase which it traverses. Its 

chemical composition differs only in the high percentage of iron which 

it carries. It is possible that for some cause there has been a local concentration of iron (to which the color is due) within the limits of this 

band, which renders it harder than the surrounding diabase and enables 

it to resist pressure more successfully. Hence, while not yielding itself 

to the pressure which produced the schistosity of the diabase, it has 

also been the means of in-educing a foliation in the diabase parallel to 

itself. The only other tenable hypothesis is that it represents a later 

intrusive Java flow of the same general composition as that of the rock 

into which it was intruded. The manner in which it grades into a 

finely vesicular rock on its upper surface, and the inclusions of fragments of a green diabase, would be explained by this hypothesis. Its 

resistance to pressure would be due to the same cause in either case.

In only a few instances do the diabases show a porphyritical structure apparent to the naked eye. The diabases have not infreqently 

suffered crushing, and are recemented with quartz, epidote, and hematite, the former minerals predominating. Veins of asbestos with quartz 

occur in the more epidotic diabase.

MICROSCOPICAL DESCRIPTION.

Original structures. There is a marked uniformity of structure and 

of mineral constituents in the South Mountain diabases.

Unlike the aporhyolites, the porphyrites and rnelaphyres do not show 

the effects of maginatic movement. Their structure is universally the 

ophitic, which is produced only in a magma in a state of equilibrium. 

The vesicles also, as has already been noted, do not betray any fluidal 

movement. (PI. XXVIII, a.)

Crystallization is fine-grained (see p. 69), corresponding to what has 

been called the " microophitic." That originally this microophitic structure was associated with and passed insensibly into the hyalopilitic is 

not impossible, although subsequent processes of alteration, chief among 

which is silicificatiou, have destroyed all trace of an uuindividualized 

base.

Shearing has obscured and sometimes.obliterated the delicate ophitic 

structure through processes detailed later. Where dynamic action 

found relief in the crushing of the rock rather than in the production 

of a schist, the ophitic structure remains perfectly preserved in the rock 

fragments. The porphyritic structure is inconspicuous. Among the 

iionolivinitic porphyrites iutratelluric crystallization is nearly absent. 

Feldspar and augite phenocrysts are rare. This characteristic, together 

with the widespread development of the amygdaloidal structure, allies 

these rocks to Eosenburch's spilite type.
BASCOM.] MELAPHYRES AND AUGITE-PORPHYEITES. 73

The olivinitic porphyrites, or true melaphyres, contain olivine as a 

constituent of the groundmass as well as in the very plentiful porphyritical crystals.

The distribution of the melaphyres is quite similar to that of the 

spilites. The history of the two types since consolidation has been the 

same, and they will be discussed together.

The vesicular structure is a conspicuous feature of the melaphyres 

and spilites. They range from rocks almost as vesicular as a sponge to 

a compact rock containing only a few scattered vesicles. These vesicles 

are filled with material furnished.by percolating waters, and a solid 

amygdaloid is formed.

The mineral nature of the amygdules will be described under the 

secondary constituents. The vesicles vary in size from microscopic 

dimensions to 5 centimeters in length and 3 in breadth. They are very 

significant, both of the amount of shearing and of alteration present 

in the rocks which they characterize, and they have undoubtedly been 

a factor in determining the character of both processes (pp. 74-75).

Secondary structures. The micropoikilitic structure, as has been noted 

on page 49, is occasionally present. It is found in those melaphyres 

and spilites Avhich have been thoroughly silicified by infiltration. The 

secondary nature of the structure is very plain. The original structure 

(the ophitic) is so well preserved, in spite of the replacement of the 

mineral constituents, that in ordinary light the altered character of 

the rock is scarcely apparent (PI. XIX, a and &). Polarized light at 

once betrays the extent of the alteration.

Where the schistose character of the rock is pronounced in the hand 

specimen, it is also a marked feature of the thin section. The constituents, which in these cases are for the most part secondary, are 

arranged with their longest axes at right angles to the pressure.

There has been so complete, a recrystallization of the rock as to 

obscure its original character. It has been repeatedly pointed out that 

under conditions of pressure igneous rocks acquire a degree of schistosity which renders it almost impossible to determine their true character and to distinguish authoritatively between foliated traps and 

metamorphosed slates (elastics). In the schists under discussion their 

relation to undoubted porphyrites leaves no room for doubt as to their 

origin, nor is their alteration so extended as has been described in 

other localities. The structure of the South Mountain porphyrites is 

usually far less altered than is the case with the greenstone schists of 

the Menoininee and Marquette regions.

Original constituents. It is to be expected that these ancient rocks, 

comparatively soft and extremely vesicular, exposed as they have been 

to pressure, percolating waters, and weathering, should exhibit alteration. It is surprising that the alteration has not been so complete as 

to obscure altogether the original structures and constituents. The 

original constituents of the rock plagioclase feldspar, augite, olivine, 

fcitaniferous magnetite are either present in a comparatively fresh con-
74 VOLCANIC ROCKS OF SOUTH MOUNTAIN.

dition or are represented by characteristic alteration products, Avhich are 

often paramorphs of the original mineral. The former is only rarely 

the case, while the latter is the rule.

The feldspar occurs both as porphyritic crystals and as a constituent 

of the groundinass. The crystals of the first generation are from 0.6 to 

0.8 millimeters in length to 0.2 millimeters in breadth. Those of the 

second generation are lath-shaped, 0.4 millimeters in length to 0.4 

millimeters in breadth, and condition the microophitic structure. They 

are both striated, and show undulatory extinction when any of the 

original substance remains. Usually the feldspars are altered to epidote and quartz, or they have been completely replaced by quartz, while 

their crystal outline is preserved by the iron constituent.

It is doubtful whether augite is present otherwise than as a constituent of the grouudmass where it is allotriomorphic. It is universally 

replaced by the more stable amphibole minerals or by epidote or 

chlorite, and some porphyritic crystals of the latter minerals may 

represent augite phenocrysts. The chief alteration product of augite 

in the schistose porphyrites is actinolite. This mineral is not limited 

in its development to the augite outlines, and it thus obscures the 

ophitic structure. Olivine crystals of two generations are readily 

recognized by means of their characteristic form, their irregular fracturing, and sometimes by their alteration products. Usually the olivine 

is altered to epidote. In a few cases (PI. XXVIII, b) the crystals are 

still sufficiently unaltered to respond to the optical tests for olivine.

There is a large amount of an opaque black oxide in the porphyrites. 

Where this was tested the powder was found to be magnetic. This 

fact, the crystal form of some of the oxide, and the analyses of these 

rocks point to the conclusion that much of it is magnetite, though 

undoubtedly very titaniferous. Occasional rhombohedral forms and 

cleavages indicate that ilmenite is also present. Both these minerals 

have given rise to an abundant development of leucoxene (PI. 

XXVIII, 6).

In order of abundance the original constituents rank as follows: 

feldspar, augite, magnetite and ilmeuite, olivine.

Secondary constituents. The processes of alteration have been greatly 

assisted by the opeu-textured, vesicular nature of the rocks, and the 

mineral character of the amygdules is indicative of the character of 

the alteration of the rock mass. The vesicles are almost universally 

filled with one or two or all of'three minerals: epidote, quartz, and 

chlorite. Whatever is the ainygdaloidal filling is also the prevailing 

alteration mineral. If quartz fills the amygdules the rock mass is 

more or less completely silicified. The ophitic structure, while preserved in outline, is replaced by the inicropoikilitic. Quartz, titaniferous 

magnetite, leucoxene, and some epidote constitute the rock, which is 

distinguished in the hand specimen by its blue-gray color and white 

amygdules.

One of the most common ainygdaloidal fillings is epidote with a
BABCOM.I MELAPHYUES AND AUGITE-PORPHYRITES. 75

little quartz. In this case epidote is the predominating' alteration 

product. These rocks are recognized in the hand specimen by a light-, 

green color and green amygdulcs. Feldspar, augite, and olivine have 

all been replaced by epidote. The material for this mineral has 

undoubtedly been furnished liy the interaction of feldspar and augite, 

and also has been brought to the rock by percolating water from overlying rocks. In the extreme phase of this alteration these rocks are 

trueepidosites.

In the -presence of a larger amount of iron, actinolite is abundantly 

developed. There is also much free iron oxide, and the rock becomes 

dark green in color. Where there has been shearing movement, as in 

the case of the acid rocks, a micaceous mineral is developed/ In the 

aporhyolites the mineral is sericite; in the porphyrites it is chlorite. 

In the incipient stages of the schistose structure chlorite occupies the 

center of the amygdules, with quartz and epidote filling the rest of the 

space. When the rock is so schistose as to be fairly called a slate the 

amygdules are represented by brilliant dark-green spots and consist of 

chlorite only.

Chlorite in turn becomes the prevaling alteration product. Often 

none of the original constituents remain. Actinolite, chlorite, epidote, 

and secondary silica are the invariable constituents of the "spotted 

greenstone schists." The actinolite and chlorite both blur the outlines 

of the original constituents and obliterate the original structure. These 

rocks are a medium green in color.

The important secondary constituents of the porphyrites are quartz, 

epidote, actinolite, chlorite, and leucoxeue. The prevalence of one or 

the other of these alteration products can be determined in the hand 

specimen by means of the color of the rock and the character of the 

amygdules. With reference to the character of the alteration which 

they have undergone, the rnelaphyres and spilites thus fall into the following groups:

1. A blue-gray rock with quartz amygdules which do not show 

shearing. Under the microscope it shows an ophitic structure well 

preserved, and a silicified ground mass. Localities: Near Gum Spring, 

on the Old Furnace road, on Minie Branch, and along the Gettysburg 

Kailroad.

2. A light yellowish-green rock with epidote-quartz amygdules, and 

epidote as a prevailing constituent. The original structure is obscured. 

Localities: South of the Clermont House on the Gettysburg Railroad 

and at the Russel copper mine.

3. A medium-green spotted schist. Chlorite is the prevailing mineral. The original structure is more or less completely obscured. 

Localities: Along the State line at the west end of the tunnel and at 

many other places. This is a prevailing type of the porphyrite.

4. A dark-green rock, more or less schistose. Epidote, quartz, and 

chlorite form the amygdules. Actinolite is abundant as aii alteration
76 VOLCANIC EOCKS OF SOUTH MOUNTAIN. [BOLL. 136.

product. Feldspar is often fresh and unaltered and the structure 

.preserved.

In the first three types actinolite may also be present, but not so 

abundantly as in the last. The first type passes into the second by 

increase in epidote, and the second type grades readily into the third 

by increase in chlorite. As this increase accompanies the development 

of schistosity, most of the schists belong in the third group.

Types 3 and 4 are not sharply separated. Chlorite and actinolite are 

present in both. In the former chlorite predominates, and hrthe latter 

actinolite.

Olivine may be present in any of the four groups. The crystal outlines are best preserved in the first group; hence the ophitic structure 

is here best preserved; and it is most obscured in group 3, where original crystal outlines are lost.

Group 1 contains the rocks which have been the least sheared, a fact 

which is perhaps accounted for by the silicified character of the rock.

The peculiar dike-like band which traverses the basic eruptives at 

the Avest end of the tunnel is not unlike type 1 in color and compactness 

of texture. The color is several shades darker, and the specific gravity 

of the rock is greater. Under the microscope a further remseinblance 

is seen. The compact rock mass consists largely of titaniferous magnetite (arranged in layers, or outlining obscurely an ophitic structure), 

chlorite, epidote, and quartz.

The araygdaloidal selvage of the band shows the same constituents 

in an inverse proportion, and the ophitic structure is strongly marked. 

This was true in all the numerous thin sections made of the band. In 

the vesicular portion of the band the ophitic structure is well preserved, and even olivine crystals with unaltered outline are present. 

The amygdules are filled with quartz, granular and crystalline epidote, 

arid chlorite. It is probable that the yielding of the vesicles, which 

would offer the least resistance to pressure, saved the rock. The 

tendency to a parallel arrangement of the feldspars on either side of 

the amygdules accords with this supposition. The passage of this 

baud, which is sometimes only from an inch to two inches wide, into a 

green chlorite-schist or slate is very abrupt. The difference seems to be 

due to the presence in the band of a large amount of iron. Its chemical analysis, given on page 78, coincides with this view.

Epidote and quartz are by far the most abundant and widely distributed of the secondary minerals. This is true not only of the basic 

eruptives, but also of the acid eruptives when they are the prevalent 

alteration products and amygdaloidal filling, and where piedmontite, 

a member of the epidote group, occurs in macroscopic quantities. 

There seem to have been conditions favoring an extensive epidotization 

and silicificatiou. Undoubtedly there has been within the rocks themselves a mutual reaction between the decomposition products of feldspar and augite resulting in the production of epidote. The fact that
BASCOM.] MELAPHYEES AND AUGITE-PORPHYRITES. 77

rarely some fresh feldspar still remains, while the augite is always 

decomposed, indicates that the decomposition products of augite have 

acted upon fresh feldspar, thus also developing epidote. But the feldspars and augite of the porphyrites under discussion will not account 

for all the epidote and quartz present in them, composing, as they do, 

the amygdules, and in many cases the entire rock. The dip of the 

foliation planes of the basic eruptives indicates a thickness formerly 

much greater. A large amount of erosion of the igneous rocks has 

occurred since they were elevated to their present position. The water 

which percolated through this great thickness of igneous material 

brought with it the lime and alumina.

It is plain that these processes of epidotization and siliciiication took, 

place not only while the porphyrites were being elevated, but have continued since the cessation of all dynamic action. The filling of vents 

aiid cracks by these materials, the fresh, uuschistose character of the 

epidote and quartz in vesicles which themselves show the effect of 

squeezing, the presence of granular epidote in the schists and slates, 

all lead to this conclusion. While this is true, there are, on the other 

hand, amygdules of epidote where the fan-shaped radiating crystals 

of epidote have been broken and pulled apart in consonance with the 

alteration in the shape of the vesicle, and the spaces thus formed filled 

with silica.

The nonvesicular character of the acid eruptives has saved them 

from so extended an epidotization as characterizes the basic eruptives. 

In the case of the amygdaloidal aporhyolites of the Bigham copper 

mine, the same conditions which obtained with the porphyrites have 

operated to effect with them an extended development of epidote. 

While there is so complete an alteration in mineral constituents, there 

is surprisingly little change in structure.

In this respect the South Mountain basic eruptives are a contrast to 

similar greenstones of the Menoniiuee and Marquette regions. A comparative study of the greenstones of the two regions shows that the 

Lake Superior rocks, while more altered in structure, possess feldspars less altered than do the South Mountain greenstones. Calcite is 

much more abundant in the former rocks, and epidote in the latter, as a 

secondary product.

Acessory minerals. Copper occurs in microscopic quantities in the 

amygdules of the basic eruptives, just as it did in the amygdaloidal 

aporhyolites. This is true only of the arnygdaloids from the various 

copper-mine localities described on pages 25-27. At these localities the 

carbonates of copper, malachite and azurite, occur as thin stains. The 

former sometimes forms crystals of considerable size in the vesicular 

cavities (one-fourth inch). A silvery-green asbestos occurs in some 

abundance in quartz veins penetrating the basic eruptives. It is 

plainly a secondary product. A finely divided red hematite is 'sometimes quite conspicuous in the amygdules as a cementing material for
78 VOLCANIC KOCKS OF SOUTH MOUNTAIN. I BULL. 136.

the crushed porphyrites. It also occurs in crystalline form, and at a 

single locality (south of the Fountaiudale turnpike, on the,north flank 

of Haycock Mountain) micaceous hematite occurs in veins. Calcite is 

rare. It is occasionally present in the ainygdules or as vein material; 

and in the case of a single specimen, broken from a roadside bowlder, it 

almost completely replaces the substance of the rock.

The range of minerals, original, secondary, and accessory, found in 

the South Mountain rocks is a very limited one.

DISCUSSION OF CHEMICAL ANALYSES.

The analyses tabulated beloAv, with the exception of No. IV, are collected from analyses scattered through the publications of the Second 

Geological Survey of Pennsylvania. For Analysis IV the writer is 

indebted to the courtesy of Professor Daniells, of "Wisconsin University:

Table of analyses.

Si02 .......................

2,05.. ..................

A12 03 ....... ............

Fe2 03 -- -- -- - - - ----

FeO .......................

TiOj. ......................

CaO. .......................

MgO. ......................

K20 .......................

Na.,0 ......................

Total................

I.

48.93

15.42 

14. 71

18.10 

2.78

100. 54

II.

48.02 

1.45

17.84 

11.61 

.98

18. 25

1.50

99.65

III.

41. 280

18. 480 

9.440 

8.200

7 040 

7.486 

2.208

a. 523 

2.740

100. 397

IV.

37. 225

( 44.82

15.79 

1.14

(')

98. 975

V.

37.03 

0.51 

( 24. 13

) 19. 03 

4. 05 

0.21 

1.44 

8.93 

Trace. 

3.54

99.67

1 Not determined.

I. " Orthofelsite, containing epidote, llf miles west of Gettysburg." J

II. "Epidotic rock, 2^ miles from Mount Alto furnace." :

III. " Chloritic schist from Bechtel shaft." 3

IV. Differentiated baud at the west eud of the tunnel. 3

V. "Variegated chlorite-schisfc with chlorite (?), one-half mile northeast of Pine 

Grove." l

The characterization of these rocks by the Second Geological Survey 

is somewhat vague, and in the discussion of the analyses the writer 

is again hampered by the lack of hand specimens of the rock analyzed.

The percentages are about those of the normal augite-porphyrites 

and melaphyres as given in Eoth's tables. They scarcely show as 

much variation as the altered augite-plagioclase rocks of his tables.

Although the analyses show some phosphorous pentoxide, no apatite 

was noted in the microscopic study. The iron percentage is high in all 

of the analyses, though not abnormally so not higher than the microscopic study would lead us to expect.

'Second Geological Survey of Pennsylvania, Vol. OCC, pp. 255-275. Analyses made by the late Dr< 

P. A. Genth.

2 3Trazer: Hypothesis of the structure of the copper belt of the South Mountain, p. 82: Traps, Am., 

Inst. Mm. Bug..Vol. XII, pp. 85-90. Analysis made by C. Hanford Honderson.

3 Analysis made by Prof. W, W, Papiella, of Wisconsin University.
BASCOM.] BASIC SLATES. 79

The "epidotic rock" (I aiid II) shows, as would be expected, an 

abnormally high lime percentage.

The chief variation from the normal type lies in the addition of lime 

and iron oxide and the abstraction of the alkalies and magnesia.

BASIC SLATES.

DISTRIBUTION AND DESCRIPTION.

The localities where the basic slates occur in any considerable extent 

are colored a light yellow-green on the map of the Monterey district. 

The slight rounded eminence opposite the Fountaindale post-office is 

composed of a dark-gray crinkled and finely laminated slate. It is 

darker colored than, the clastic slate of the region and is surrounded 

by the basic eruptives. The thin section shows very distinct traces of an 

ophitic structure. Iron oxide, chlorite, and sericite are the only constituents that can be determined. About three-fourths of a mile north 

of Monterey station, on the road leading from the turnpike to the Old 

Furnace road, there is an exposure of a lighter-colored slate. The same 

constituents are found in this slate, with a larger proportion of sericite 

and the addition of leucoxene. The ophitic structure is barely discernible, and it is with considerable hesitancy that the slate is referred to 

the group of igneous rocks. Half a mile farther northeast, on the Old 

Furnace road, just beyond Gum Spring, occurs a light-gray spotted 

slate of undoubted igneous origin. It is not so finely foliated as the 

slates that have just been described, and much of the original structure 

remains. The constituents are the same as those of the last-mentioned 

slate. Leucoxene is more abundant and the ophitic structure is pronounced.

The amygclules, which give the slates a spotted appearance, are composed of quartz, sericite, and some chlorite. These slates are related 

to the porphyrites of Group I, some typical examples of which occur 

near by.

At the west-end of the Gettysburg tunnel, just above the iron-bearing band previously described, another spotted slate has been developed from the basic igneous rock. In this case the slate is green, and 

the spots are a brilliant dark shade of the same color. Iron oxide, 

chlorite, epidote,-and some silica are the constituents. The original 

structure is entirely obliterated. Some of the epidote grains faintly 

suggest olivinitic forms. Chlorite is the prevailing mineral. The spots, 

which as in the other slate are sheared amygdules, are-formed of chlorite only. This slaty zone is only a few inches wide and passes somewhat abruptly into a slightly schistose porphyrite.

The first tunnel on the old Tapeworm Eailroad, which was abandoned 

before an excavation was made, exposes a green slate which differs from 

the one just described only in its nonamygdaloidat character and in the 

greater abundance of epidote. The knoll northeast of Blue Eidge
80 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

Summit station, the roadway crossing the GettysburgBailroad in front 

of the Clermont House, and the hill to the east of the Fairfield-Fountaindale road and north of the Fountaindale turnpike, are the other localities where slates occur. While their association and their appearance in 

the hand specimen, which resembles that of the first two slates discussed, 

indicate an igneous origin, in the absence of thin sections that origin 

can not be considered beyond question.

BASIC PYROCLASTIOS. 

DISTRIBUTION AND DESCRIPTION.

The basic breccias of South Mountain may be classified as follows: 

(1) Crushed porphyrites which have been recemented with epidote and 

quartz and sometimes brilliantly colored with red hematite; (2) Tufface'ous breccia; (3) Ash.

 (1) Crushed porpkyrites. The crushed and broken porphyrites, while 

perhaps not in a strict sense breccias, present a strikingly brecciated 

appearance. Fragments of all sizes, of a blue or purple-gray rock, are 

embedded in a bright-green and white or rose-colored matrix. The 

fragments have undergone either silicification or epidotization without 

affecting materially their structure (microophitic), which is outlined 

by iron oxide. 0

(2) Tuffaceous breccia. Some large bowlders found near the source 

of Minie Branch furnish the only unmistakable tuffaceous breccia. 

The fragments show a considerable range in size (see page 24) and are 

thickly crowded in a basic cement. As there has been no shearing, 

the structure of the fragments is perfectly preserved. (PL XXVIII, a.) 

Epidote, quartz, and iron oxide are their present constituents.

(3)- Ash. Above the Headlight copper mine on the Fouutaindale 

turnpike and in the fourth cut beyond Mouterey station (northeast), on 

the Gettysburg Eailroad, are intercalated bands of a light-green rock 

which, for the following reasons, have been considered altered ash: 

At the west end of this cut a fine-grained homogeneous rock is striped 

with alternating bands of light green and reddish green. Under the 

microscope these bands show no trace of any structure save a slight 

schistosity. They are composed almost wholly of angular grains of 

epidote, magnetite and leucoxene, actinolite, chlorite> and quartz. The 

difference in color is due to the presence in the reddish bands of red 

iron oxide. Toward the eastern end of the same cut the whole face of 

the rock is banded with light-green epidotic layers from 1 foot to 2 

feet wide, running approximately parallel to one another.

A microscopic slide of one of these bands consists wholly of granular 

epidote and quartz, with a little iron oxide, usually the red oxide.

These rocks overlie and are in close proximity to scoriaceous basic 

lava. This fact, together with their variation in color and their structureless and fragmental character, is very suggestive of an altered ash. 

At the first locality mentioned, the Headlight copper mine, the so-
BASCOM.] . SUMMARY. - 81

called ash occurs as a light-green schist. It is not banded, but presents under the microscope the same structureless character as the 

rocks above described. It consists of actinolite blades and needles, 

epidote granules, and some-chlorite, magnetite, and leucoxene.

SUMMARY.

The basic igneous rocks display but little variety of structure or 

mineral constitution. The former is that common to porphyrites and 

melaphyres the microophitic and in spite of great alteration in the 

mineral constituents of the rocks it still remains a marked structure. 

Shearing obscures it, but in the extreme form of the sheared porphyrite the slate it is still discernible.

The formation of chlorite and actinolite tends to confuse outlines; 

hence some of the slates in which there is not much of these minerals 

preserve their original structure better than the chlorite-schists. The 

original mineral constituents, plagioclase, feldspar, augite, and olivine, 

have almost completely disappeared. No augite remains; olivine crystals are well preserved in outline, and sometimes a core of the original 

mineral remains. There is considerable feldspar still unaltered. It is 

always striated, but the crystals are too small to allow of an accurate 

determination of their character. That they belong to the basic end 

of the series is shown by their extended alteration to epidote, and by 

the chemical analyses of the rocks.

The vesicular character of these rocks has aided in the extended 

replacement of their original minerals, and the amygdules are an index 

of the character of that replacement. The presence of vesicles has also, 

doubtless, been a factor in preserving the internal structure of the rock 

in spite of dynamic action. Silicincation, epidotization, and chloritization are the processes of alteration which have been most active.

The source of material is twofold from the rocks in which these processes have been described, and from the overlying rocks which have 

been removed by erosion. 

Bull. 136 6
CHAPTEK VI.

SUMMARY OF CONCLUSIONS.

EVIDENCE OF THE ERUPTIVE CHARACTER OF THE TWO ROCKS.

The preceding chapters have treated in detail structural and chemical 

features possessed by the South Mountain rocks and characteristic of 

igneous rocks only. These are regarded as sufficient evidence of the 

igneous origin of the rocks which they characterize, without further 

proof on that point, .ft only remains to show on what grounds a sedimentary origin has been attributed to them, and to sum up the evidence 

against such an origin.

' FIELD EVIDENCE.

ScMstosity. The conformity of the foliation planes of the porphyries, 

the aporhyolites, and the porphyrites, with the foliation planes of the 

Cambrian sediments is a prominent and persistent feature, and one 

which, among others, has undoubtedly led to the ascription of a sedimentary origin to the former rocks. The confusion of foliation planes 

and bedding planes in the quartzite, owing to the obscurity of the 

latter, added force to this argument.

This conformable schistosity is not, of course, inconsistent with the 

igneous origin of the underlying rocks. The schistosity is a secondary 

feature, produced by forces which affected the igneous and aqueous 

rocks alike.

' The cleavage is quite as plainly secondary in the clastic rocks as in 

the nonclastic, and sometimes conforms to the bedding and sometimes 

does not.

Lamination. An. original lamination, conspicuous in the aporhyolites, the nature of which was described on pages 43-44, characterized 

as bedding by Hunt and others, doubtless furnished another reason for 

attributing stratification and an aqueous origin to the rocks possessing 

it. The real nature of the lamination has proved to be such that it 

becomes an evidence of the igneous character of the rocks in which it 

occurs.

That the lamination is due to bands of spherulites has been pointed 

.out. True spherulitic crystallization, such as has been described in 

these aporhyolites, has thus far been known only as the product of crystallization from a molten magma.

The slates. The slaty character of the rocks has been another reason 

for assigning a sedimentary origin to them.

The slates, both acid and basic, have been considered clastic slates 

by Professors Rogers and Lesley, by Frazer, Tyson, Blandy, and Dr. 

82
BASCOM.] SUMMARY OF CONCLUSIONS. 83

Hunt, as the quotations from these writers, given in Chapter I, show. 

While the igneous slates of South Mountain do not resemble any of 

the Cambrian sediments of that region, their resemblance to porphyroids from regions where there has been extended metamorphism is 

very great. This resemblance is internal as well as external, and furnishes an instance of the production of essentially similar results by 

either of two different methods.

The true nature of these slates and the manner of their production 

are conclusively revealed through field evidence. Where a single exposure shows a shear zone of not more than 20 feet in which every gradation from a porphyry to a fissile slate is displayed, or where a single 

hand specimen shows such a m.etainorphism, the evidence of such a 

genetic relationship is irrefutable.

Absence of gradation between igneous and clastic rocks. There is, on 

the other hand, no such gradation between the igneous rocks and 

undoubted elastics as might be expected if the former were metamorphosed elastics. We have holocrystalline rocks sharply separated 

from noncrystalline elastics, with an entire absence of intermediate 

stages.

Professor Kogers was impressed with the high degree of metamorphism which these rocks must have undergone in order to attain their 

present holocrystalline character. "A gray siliceous altered rock," "a 

compact siliceous altered slate," are the terms he uses to describe the 

porphyries and aporhyolites, while he speaks of the porphyrites as 

"primal slate in a highly metamorphic condition" and " highly altered 

greenish slate." The sediments and the igneous rocks have been subjected to the same dynamic forces, and, as a matter of fact, we find 

one no more highly metamorphosed than the other relatively to their 

respective powers of resisting alteration.

Surface-floio features. Positive field evidence for the nonsedimentary origin of these rocks is found in the features which they possess in 

common with surface flows. Their vesicular, scoriaceous, and puiniceous character, the accompanying pyroclastics, their flow structures, 

even grain, conchoidal fracture, and other characteristics of a glassy 

lava all testify to an eruptive origin.

PETROGRAPHICAL EVIDENCE.

Structural. The petrographical evidence of the origin of the porphyries, aporhyolites, and porphyrites is of an even more unmistakable 

character.

Their porphyritic structure is indicative of their origin. Olivine and 

feldspar pheuocrysts with crystalline outlines, idiomorphic quartz with 

embayments and edges rounded by niagmatic corrosion, are possible 

only in rocks which have once been molten.

The ophitic structure, preserved in great perfection in the porphyrites, is peculiar to rocks which have consolidated from a molten magma.
84 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BOLL. 136.

Other structures which furnish additional and convincing proof of an 

igneous origin need only be mentioned. A detailed description of their 

appearance in the aporhyolites and porphyrites has been given in the 

previous chapters. Such structures are the spherulitic, axiolitic, lithophysal, perlitic, rhyolitic, fluidal, and ainygdaloidal.

Mineraloyical. Metamorphosed sedimentary rocks are always accompanied by certain characteristic minerals. The absence of such minerals 

in these South Mountain rocks is conspicuous. Epidote and sericite 

are the only prominent alteration minerals. Of these, the former is the 

product of weathering rather than a true inetainorphic mineral; the 

latter is more or less limited to shear zones, where its development is 

directly related to the dynamic force acting upon massive porphyries 

and aporhyolites. The absence of all evidence of contact action indicates their effusive character.

Chemical. The close conformity of the composition of these rocks in 

the one case with that of the rhyolites and in the other with that of the 

diabases and inelaphyres from all parts of the world, as tabulated by 

Roth, indicates an igneous origin. Their uniform composition is a contrast to the composition of a series of clastic rocks, where the chemical 

proportions are largely a matter of accident. A similar test has been 

applied by Eoseubusch 1 to the determination of the origin of Archean 

gneisses. The association of these types of acid and basic lava accord 

with the laws of petrographical consanguinity.

ORIGINAL ROCK TYPES. 

ACID IGNEOUS KOCKS.

The acid-lava flows in South Mountain are regarded by the writer as 

quite comparable, at the time of their consolidation, to similar flows in 

post-Tertiary time, such, for instance, as those which have been recently 

studied in the Yellowstone National Park. Certain portions of the flow, 

as in the case of the Obsidian Cliff, were completely vitreous save for 

spherulitic and lithophysal crystallization. In other localities the lava 

was lithoidal, and in the central portion of thick flows holocrystalline. 

In this way three types of acid volcanics would be developed 

rhyolites, lithoidal rhyolites, and quartz-porphyries. Every gradation 

between these types would accompany these.

Thus, while there are certain areas in the South Mountain, notably 

the Bigham Copper Mine and Kaccoon Creek localities, which exhibit 

typical ancient rhyolites, other regions display genuine quartz-porphyries. While in the latter rocks, which constitute a not inconsiderable portion of the acid flows, the groundmass may have been, and probably was, originally holocrystalline, as in some modern lavas, in the case 

of the former rocks it is supposed that the groundmass was, at the time 

of consolidation, wholly or partly glassy.

'Zur Auffassung tier chemischen Natup des Grundgebirges.- Tsglieraiaks mineral. Mittheil., Vol, 

XII, 1891, pp. 49-61.
BASCOM.] SUMMARY OF CONCLUSIONS. 85

Chief among the processes of alteration which have been going on 

since that time is devitrification. Out of glassy and lithoidal rhyolites 

devitrification has been developing aporhyolites. This process consists 

in the replacement of whatever glassy base was present in the original 

rock by a uniform quartz-feldspar mosaic: Sometimes the alteration is 

carried still further, and the original spherulitic crystallization is also 

replaced by this secondary granular crystallization. Barely, if ever, 

does the secondary crystallization completely obscure the former character of the rock, while often all of the structures peculiar to a fresh 

glassy lava are retained.

The other processes of alteration, which all of the original rock 

types have undergone in some slight degree, and some of them in an 

extreme degree, are the processes of sericitization and epidotizatiou. 

The former process has been a chief factor in the development of slates 

from, massive porphyrites and aporhyolites, '

These three processes of alteration devitrification, sericitization, 

and epidotization represent statical inetainorphism, dynamic metamorphism, and weathering, respectively.

BASIC IGNEOUS KOCKS.

In contrast with the acid rocks, the basic rocks have their original 

constituents so completely replaced that it is not easy to determine the 

original type or types. The original constituents were plagioclase, pyroxene, olivine, ilmenite, and magnetite. The original structures were 

the microophitic, the porphyritic (inconspicuous and mostly confined to 

the olivine-bearing type), and the amygdaloidal, a universal structure.

In view of these constituents and structures, these rocks have been 

regarded as members of the diabase or augite-porphyrite group and of 

the olivine-diabase or melaphyre group. The augite-porphyrites resemble the spilites in their scanty porphyritic crystals, their ever-present 

inclination to the amygdaloidal structure, and their susceptibility to 

weathering.

The character of the alteration which has taken place in these melaphyres and spilites varies with the amount of shearing which has 

occurred. Where shearing has been a factor in the alteration, chlorite, 

actinolite, and quartz replace the pyroxene and plagioclase.

In the absence of shearing, epidote has resulted from the interaction 

of plagioclase and pyroxene. Olivine has altered to epidote, serpentine, and iron oxide; ilmenite has altered to leucoxene, and magnetite to 

hematite, when altered at all. In the absence of shearing the ophitic 

structure is preserved in outline, although sometimes the micropoikilitic is added to it through the infiltration of silica; with the presence 

of shearing the development of chlorite and actinolite has obliterated 

the original structure and produced the schistosity characteristic of 

chlorite-actinolite rocks.
86 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136. 

SIMILAR ROCKS IN OTHER REGIONS.

These South Mountain volcanics form a part of a belt of similar 

rocks which have been recently recognized along the Eastern Coast of 

the United States and Canada-.

Such volcanics have been described in New Brunswick by the Canadian geologists Bailey, 1 Matthew,2 and Ells.;i

More recently, in the Sndbury district, similar rocks have been observed by Bell.4 They have been described in Maine by Shaler, 5 and are 

recognizable in Canada, Maine, and New Hampshire through the 

writings of Hunt, Jackson, and Hitchcock, although they were otherwise interpreted by these observers.

Spherulitic volcanics have recently been definitely recognized by W. 

S. Bay ley 6 at Yinal Haven, Maine, and have been studied in detail by 

Mr. G-. O. Smith, of Johns Hopkins University.

They have been identified in the Boston Basin by Wads worth 7 and by 

Diller, 8 who has studied them in some detail. The thin sections loaned 

by the latter for comparative study have already been mentioned as 

showing a marked similarity to the South Mountain acid volcanics.

The continuation of the South Mountain volcanics in Maryland and 

Virginia has been studied by Keith.9 Similar volcanics have been found 

in North Carolina by Professor Williams, 10 in South Carolina by Lieber, 11 

and in Georgia by Professor Pirsson. 12

In Canada, Maine, in the neighborhood of Boston, and in Missouri n 

the felsites were, like the South Mountain rocks, first regarded as sedimentary in origin, and have only recently been identified as volcanic.

With continued petrographic investigation of the pre-Cambrian rocks 

of North America volcanics may yet be recognized at other points where 

the rocks have been interpreted as sedimentary.

In the Lake Superior region they have long been known through the 

writings of Irving and others, and their extent in that region has 

recently been still further enlarged. 14

'Bailey, Report on the pre-Silurian rocks of South New Brunswick: Kept. Can. Geol. Survey, 1877-78 

D. D.

5Bailey, Matthew, and Ells, Report on Southern New Brunswick: Rept. Can. Geol. Survey, 1878-79. 

3Ells, Volcanic rocks of Northern New Brunswick: Rept. Can. Geol. Survey, 1879-80, D. 

'Ibid., 1889-90, F, 1891.

'Shaler, Cobscook Bay, Maine: Am. Jour. Sci. (3), Voh XXXII, 1880, p. 40. Mount Desert: Eighth 

Ann! Rept. U. S. Geol. Survey, 1886-87, pp. 1043,1054. 

«Bull. Geol. Soc. Am., vol. 6,1894, pp. 474-476.

'Wadsworth: Bull. Mus. Comp. Zool. Harvard Coll., Vol. V, No. 13, p. 282. 

*Diller, op. cit. 

9 Keith, op. cit.

10 For full statement of distribution of volcanic rocks on Atlantic Coast, see paper by Professor Williams in Jour, of Geology, Vol. II, No. 1, pp. 1-31.

"Lieber, Report ou the survey of North Carolina, 185C, 2d ed., 1858, p. 31. 

12 A section of a Georgian felsite, loaned by Professor Pirsson, has already been alluded to. 

13Haworth: Am. Geologist, Vol. I, 1888, p. 280; Bull. Missouri Geol. Survey, No. 5. 

14 U. S. Grant, Volcanic rocks in the Keewatin of Minnesota: Science, Vol. XXIII, Jan. 12, 1884, 

p. 17.
BASCOM.] LITERATURE. . 87

The reported rarity of volcanic action.1 in America in pre-Cambrian 

times is perhaps more apparent than real, and is due rather to the failure 

to recognize the results of such action than to the actual absence of volcanic action. '

LITEBATURE.

A list of papers in which points of resemblance between ancient and 

modern acid volcanics have been emphasized, or in which devitrification has been described, and a list of articles on the nature and origin 

of sphernlites, are appended.

PArKUS ON ACID VOLCANICS AND DEVITKIFICATION.

Allport, S. On the microscopic study of the pitchstones and felsites of Arran. 

Geol. Mag., Vol. IX, 1872, pp. 536-545.

 . On ancient devitrified pitchstones and perlites from the Lower Silurian district of Shropshire. Quart. Jour. Geol. Soc., London, Vol. XXXIII, 1877, p. 449.

Bayley, W. S. Spherulitic volcauics at North Haven, Maine. Bull. Geol. Soc., 

Am., Vol. VI, 1894, pp. 474-476.

Blake, J. F. On the felsites and conglomerates between Bethesda and Llanllyfni, 

North Wales. Quart. Jour. Geol. Soc., London, Vol. XLIX, 1893, pp. 441-465.

Bonney, T. G. On certain rock structures as illustrated by pitchstones and felsites in Arran. Geol. Mag., Vol. IV, 1877, pp. 499-511.

 . Note on the felsite of Brittadon, North Devonshire. Geol. Mag., Vol. V, 1878, 

pp. 207-209;

 . On some nodular felsites in the Bala group of North Wales. Quart. Jour. 

Geol. Soc., London, Vol. XXXVIII, 1882, p. 289. .

Bornemaiin, T. G. Die Quartzporphyro von Heiligenstein und seine Fluidalstructur. Zeitschr. Deutsch. geol. Gesell., Berliu,Vol. XXXIX, 1887, p. 793.

Chapman, P. On a method of producing perlitic and pumaceous structures in 

Canada balsam. Geol. Mag., Vol. VII, 1880, p. 79.

Clements, J. Morgan. The volcanics of the Michigamme District of Michigan. 

Jour. Geol., Vol. Ill, No. 7, Oct.-Nov., 1895, pp. 801-822.

Cole, Grenville A. J. On the artificial production of the perlitic structure. 

Geol. Mag., Vol. VIII, 1880, p. 115.

 . On hollow spherulites and their occurrence in ancient British lavas. Quart. 

Jour. Geol. Soc., London, Vol. XLI, 1885, p. 162.

 . On the alteration of coarsely spherulitic rocks. Quart. Jour. Geol. Soc., 

London, Vol. XL1I, 1886, p. 186.

 . On the devitrification of cracked and brecciated obsidian. Min. Mag., 

Vol. IX, No. 44, 1891, pp. 272-274,

Cole, Grenville A. J., and Butler, G. W. On lithophysai in the obsidian of the 

Rocche Rosse, Lipari. Geol. Mag., Vol. IX, 1892, p. 488.

 . On the lithophysto in the obsidian of the Rocche Rosse, Lipari. Quart. 

Jour. Geol. Soc., London, Vol. XLVIII, 1892, p. 438.

Davies, Thos. Preliminary note on old rhyolites from Bouley Bay, Jersey. Min. 

Mag., Vol. Ill, 1880, pp. 118-119.

Diller, J. S. Felsites and their associated rocks north of Boston. Proc. Boston 

Soc. Nat. Hist., Vol. XX, 1880, pp. 355-368; Bull. Mus. Cornp. Zool. Harvard Coll., 

Vol. VII, 1881, pp. 165-178.

Futterer, Karl. Der Ganggranit vou Gros-Sachsen und der Quartzporphyr von 

Thai im ThiAringer Wald. Inang. Disser., 1890.

'Dana, J. D., Manual of Geology, 4th od., 1895, p. 938.
88 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BULL. 136.

Hague, Arnold, and Iddings, J. P. The development of crystallization in the 

igneous rocks of Washoe, Nov., with notes on the geology of the district. Bull. 

IT. S. Geol. Survey, No. 17, 1885.'

Harker. Bala volcanic series of Caernarvonshire aud associated rocks. Sedgwick prize essay for 1888. Cambridge, 1889.»

Hatch, P. H. On the Lower Silurian felsites of the southeast of Ireland. Geol. 

Mag., Vol. VI., 1889, pp. 545-549.

Irving, R. D. The copper-bearing rocks of Lake Superior. Mon. U. S. Geol. Survey, Vol. V, 1883.

Judd, J. W. On composite dikes in Arrau. Quart. Jour. Geol. S,oc. London, Vol. 

XLIX, 1893, pp. 536-565, pp. 546, 551, 560.

Kalkowsky, Ernst. Mikroskopische Uutersuchuugen von Felsiten uud Pechateiuen Sachsens. Tscherinaks mineral. Mittheil., 1874, pp. 31, 58.

Klockmann, F. Der Geologische Aufbau des sogen. Magdeburger Uferrandes 

mit besonderer Beriicksichtigung der auftretendeu Eruptivgesteine. Jahrbuch K. 

preuss. geol. Laudesanstalt, Vol. XI, 1890, pp. 171-203. -

La Croix, Alf. Comptes-rendus, CXI, p. 71.

Lang, Heinrich Otto. Grundriss der Gesteinekunde, 1877, p. 43.

Levy, A. Michel. Caracteres microscopiques des roches anciermes acides, consid<5rds dans leurs relations avec 1'age des e"ruptives. Bull. G6ol.'Soc., France, Feb., 

1875.

Lossen, K. A. Beitriige zur Petrographie der plutonischen Gesteine. Abhandl. 

K. Akad. Wiss. zu Berlin, 1869, p. 85..

McMahon, Lieut. Gen. C. A. Notes on some trachytes, metamorphosed tuffs and 

other rocks of igneous origin on the western flank of Dartmoor. Quart. Jour. Geol. 

Soc., London, Vol. L, 1894, p. 338.

Milch, L. Beitriige zur Kenntnis des Verrucano, Leipzig, 1892; Part 2,1896.

Miigge, O. Untersuchungen liber die."Leuneporphyre" in Westfalen und den 

angrenzenden Gebieten. Neaes Jahrb, fur Mm. Geol. u. Pal., Vol. VIII, 1893, pp. 

534-719.

Nordenskjold, Otto. Zur Kenntniss der sogen. Halleflinta des uordostlichen 

Smalande. Bull. Geol. Inst., Upsala, No. 1, Vol. I, 1893.

 . Ueber archteische Ergussgesteine aus Sinaland. Bull. Geol. Instit. Upsala, 

No. 2, Vol. 1,1893, pp. 1-127.

Osann, A. Zeitechr. Deutsch. geol. Gesell., Berlin, Vol. XLIII, 1891, pp. 691, 716. 

Rosiwal, A. Petrograpkisclie Notizen iiber Ernptivgesteine aus dem Tejrovieer

Cambrium. Verhandl. K. k. geol. Reichsanstalt, 1894, p. 210.

Rutley, F. Perlitic and sperulitic structures in the lavas of Glyder Fawn. Quart 

Jour. Geol. Soc., London, Vol. XXXV, 1879, p. 508.

 . On the microscopic structure of devitrified rocks from Beddgelert and Snowdon. Quart. Jour. Geol. Soc., London, Vol. XXXVII, 1881, p. 403.

 . The microscopic character of the vitreous rocks of Montana. Quart. Jour. 

Geol. Soc., London, Vol. XXXVII, 1881, pp. 391-402.

 . On strain in connection with crystallization and perlitic structure. Quart. 

Jour. Geol. Soc., London, Vol. XL, 1884, pp. 340-346.

 . Felsitic lavas of England and Wales. Geol. Survey of England and Wales, 

1885.

 . Notes on alteration induced by heat in certain vitreous rocks. Proc. Royal 

Soc., London, No. 245,1886.

 . On some eruptive rocks from the neighborhood of St. Minver, Cornwall. 

Quart. Jour. Geol. Soc., London, Aug., 1886, pp. 392-401.

 . On the rocks of the Malvern Hills. Quart. Jour. Geol. Soc., London, Vol. 

XLIII, 1887, p. 499.

 . On perlitic felsites and on the origin of some epidosites. Quart. Jour. Geol. 

Soc., London, Vol. XLIV, 1888, pp. 740-744.
BASCOM.] LITERATURE. , 89

Rutley, F. On tachylite from Victoria Park, Whiteinch, near Glasgow. Quart. 

Jour. Geol. Soc., London, Vol. XLV, 1889, pp. 623-632.

 . On composite Bpherulites in obsidian from Hot Springs, near Little Lake, 

Colorado. Quart. Jour. Geol.Soc., London, Vol. XLVI, 1890, pp. 423-428.

 . On some of the melaphyres and felsitea of Caradoc. Quart. Jour. Geol. Soc., 

London, Vol. XLVII, 1891, pp. 534-544.

 . On a spherulitie and perlitic obsidian from Pilas, Mexico. Quart. Jour. 

Geol. Soc., London, Vol. XXVII, 1891,-pp. 530-533.

 . On the sequence of perlitic and spherulitic structures: A rejoinder to a criticism. Quart. Jour. Geol. Soc., London, Vol. L, 1894, p. 10.

Rutley, F., and Herman, D. On the microscopic character of some specimens 

of devitrified glass, with notes on certain analogous structures in rocks. Proc. Royal 

Soc., London, No. 239, 1885, pp. 87-107.

Sederholm, J. J. Studien tiber archteische Eruptivgesteine aus dem siidwestlichen 

Finland. Tsch. Miu. Mith., Vol. XII, 1891, pp. 98-141.

Smith, G. O. The volcanic series of the Fox Islands, Maine. Johns Hopkins 

University Circulars No. 121, Oct., 1S95.

De la Vallee-Poussin, Chas. Les ancienues rhyolites, dites eurites, tie GrandManil. Bull. Acad. roy. Belgique, Vol. X, 1885, pp. 253-315.

 . Les eurites quartzeuses (rhyolites aucienues) de Nivelles et des environs. 

Bull. Acad, roy. des Sci. et des Lettr. et des Beaux-Arts de Belgique, Vol. XIII, 1887, 

pp. 498-535.

Watts, W. W. Note on the occurrences of perlitic cracks in quartz. Quart. Jour. 

Geol. Soc., London, Vol. L., 1894, p. 367.

 . On perlitic structure. Geol. Mag., Dec. IV, Vol. Ill, 1896, pp. 15-20.

Woods, Henry. The igneous rocks of the neighborhood of Biulth. Quart. Jour. 

Geol. Soc., London, Vol. L, 1894, p. 566.

Vogel, Christoph. Die Quartzporphyre der Umgegeud Von Gross-Umstadt. 

Abhandl. der grossherzlich. hessichen geol. Landesanstalt zu Darmstadt, Vol. II, 

Part 1,1891, pp. 1-52.

Vogelsang, H. Philos. d. Geologie, 1867, pp. 144, 153, 194.

Wadsworth, M. E. Notes on the mineralogy and petrography of Boston and 

vicinity. Proc. Boston. Soc. Nat. Hist., Vol. XIX, 1877, p. 236.

 . On the classification of rocks. Bull. Mus. Comp. Zool. Havard Coll., Vol. 

V, No. 13, 1879, p. 282.

Zirkcl, Ferdinand. Mikroskopische Uutersuchungen der glasigen uud halbglasigen Gesteiue. Zeitschr. Deutsch. geol. Gesell., Berlin, Vol. XIX, 1867, p. 784.

PAPERS RELATING TO SPHERULITES.

Brogger, W. C. Die Mineralien der Syenitpegmatitgange, etc. Zeitschr. fiir 

Kryst. u. Min., Vol. XVI, 1890, pp. 552-553.

Cohen, E. Gottingsche gelehrten Anzeigen, 1886, p. 915.

Cole, Grenville A. J. On hollow spherulites and their occurrence in ancient 

British lavas. Quart. Jour. Geol. Soc., London, Vol. XLI, 1885, pp. 162-169.

Cross, Whitman. On the occurrence of topaz and garnet in the lithophysse of 

rhyolite. Am. Jour. Sci. (3), Vol. XXXI, 1886, p. 432.

 . The constitution and origin of spherulites in acid eruptive rocks. Bull. 

Philos. Soc,, Washington, Vol. XI, 1891, pp. 411-444,

Dana, E. S. Contributions to the petrography of the Sandwich Islands. Am. Jour. 

Sci. (3), Vol. XXXVII, 1889, p. 441-467.

 . Characteristics of volcanoes, 1891, p. 318.

Delesse, E. Recherches sur les roches globuleuses. M6m. de la Soc. ge~ol., France, 

Vol. IV, 1852, pp. 301-364.

Hoist, N. O., and Eichstadt, F Klot Diorit Sliittmassa. Geol. Foreuingens 

Stockholm Forhandl, Vol. VII, 1884, p. 134.
90 VOLCANIC ROOKS OF SOUTH MOUNTAIN. [BOLL. 136.

Iddings, J. P. On the occurrence of fayalite in the lithophyste of obsidian and. 

rhyolite in the Yellowstone National Park. Am. Jour. Sci. (3), Vol. XXX, 1885) 

pp. 58-60.

 . Obsidian Cliff, Yellowstone National Park. Seventh Ann. Kept. U. S. Geol. 

Survey, 1885-86, pp. 249-295.

 . The nature and origin of lithophysae, and the lamination of acid rock. Am. 

.Jour. Sci. (3), Vol. XXXIII. 1887, pp. 36-45.

 . Spherulitic crystallization. Bull. Philos. Soc., Washington, Vol. XI, 1891, 

pp. 445-464,

Iddings, J. P., and Penfield, S. L. The occurrence of fayalite in the lithophysie of the obsidian from the Lipari Islands. Am. Jour. Sci. (3), 1890, pp. 75-78.

Johnston-Lavis, A. J. Note on lithophysse in obsidian of the Rocche Rosse, 

Lipari. Geol. Mag., Vol. IX, 1892, p. 488.

Judd, J. W-. Contributions to the study of volcanoes. Geol. Mag., Decade II, 

Vol. II, 1875, p. 65.

Langorin, A. Ueber die Natur der Glasbasis, sowie der Krystallizationvorgiinge 

in Eruptiven Magma. Tschermaks mineral. Mittheil., Vol. VIII, 1827, p. 440.

Lehmann, O. Molecular Physik, Vol. I, 1888, pp. 378-390.

L6vy, Michel, A. Stir divers e"tats fabulaires de la silica. Bull. Soc. g6ol. 

France, Vol. Ill, 1875, p. 140. Comptes-rendus Acad. Sci., Paris, Vol. XXII, 1876

 . Sur uue nouvelle e"tat fabulaire du quartz suivant une seule orientation 

crystallographique. Comptes-rendus Acad. Sci., Paris, Vol. XXVII, 1876.

 . Mdmoire sur les divers modes de structure des roches eruptives e'tuclie'es au 

microscope au moyen de plaque minces, Ann. Min., Vol. VIII, 1876, p. 337. Comptesreudus Acad. Sci., Paris, Vol. VIII, 1875. Ref. Science, No. 21,1871, p. 502.

. . Des differentes forms de spherulites dans les roches Eruptives. Bull. Soc. 

geol. France, Vol. V, 1877, p. 257.

 . Sur la nature des spherulites paisant partie integrale des roches e"ruptives. 

Comptes-rendus Acad. Sci., Paris, Vol. XCIV, 1882, p. 464. Ref. Neues Jahrbuch fur 

Mineral., Vol. IV, 1882, p. 381.

 . Structure et classification des roches Eruptives, 1889, p. 20.

McMahon, Lieut. Gen. C. A., and Hutchings, W. Maynard. Note on pseudospherulites. Geol. Mag., Dec. IV, Vol. II, 1895, pp. 257-259.

Raisin, Miss C. A. On some nodular felstones of the Lleyn. Quart. Jour. Geol. 

Soc. London, Vol. XLV, 1889, pp. 247-369.

 . Variolite of the Lleyn and associated volcanic rocks. Quart. Jour. Geol. Soc., 

London, XLIX, 1893, p. 145.

Reinhold. Proc. Phil. Acad. Sci., 1882, p. 59.

Richthofen, Baron von. Studien aus dem ungarischsiebenbiirgischeu Trachytgebergen. Jahrbuch, k. k. geol. Reichsanstalt, Vol. II, 1860, p. 180.

 . Natural system of volcanic rocks. San Francisco, 1868, p. 14. 

Rose, Gustav. Ueber die sogeuannte krystallisirten Obsidian. Poggendorff 

Aunalen, Vol. X, 1827, p. 323.

Rosenbusch, H. Mik. Phys. der Massigeu Gesteiue, 2d ed., 1887, pp. 392-393. 

Roth, Justus. Beitriige zur Petrographio der plutonis'chen Gesteine, 1869, p. 168.'

 . Allgemeine und Chemische Geologie, 1883, Vol. II, p. 216.

Scrope, G. P. Considerations on volcanoes. Trans. Geol. Soc., London, Vol. II, 

1825, p. 202. Repr. 1873, p. 118.

Szabo, Dr. Joseph. Die Trachyte und Rhyolite der Ungebung von Tokay. Jahrbuchs, k. k. geol. Reichsanstalt, Vol. XVI, 1866, p. 89.

Sztireniji, Hugo. Kugele und spherulitische Trachyte von Schomnitz und dem 

Matia Gebirge. Ref. Neues Jahrbuch fiir Mineral, Vol. II, 1883, p. 222.

Teall, J. J. Harris. British Petrography, 1880, p. 402.

Tenne, L. A. Ueber Gesteiue des Ceno do las Navajos (Wesserberg) Mexico. 

Zeitschr. Deutsch. geol. Gesell., Vol. XL, 1888, p. 610.
BABCIOM.] LITERATURE. .91

Vogelsang, H. Spherulitie structure iu dio.rites. Niederrh. Geol. Nutnr and 

Heilkuude, 1862.

 . Philosophic der Geologic und mikroskopische Gesteiustudien, 1867.

 . Die Krystalliten, 1875.

Von Hauer, Kail Ritter. Die Gesteine niit Lithophysenbiidnugen vonTclki Hau 

in Ungeru. Verhandl. k. k. geol. Reichsanstalt, 1866, p, 98.

Weiss, Charles E. Quartz Porpliyr aus Thuringen. Zeitschr. Deiitscli. geol. 

Gesell., Berlin, Vol. XXIX, 1877, p. 418.

Zirkel, Ferdinand. Lehrbucli der Petrographie, Boiiii, Vol. II, 1866, p. 253.

 . Microscopical Petrography, Geol. Expl. Fortieth Parallel, Vol. VI, 1876, p. 212.

PLATES.

93

PLATE XY.
PLATE XV.

Fig. a. Quartzite. Specimen I. Slide 1 D. Between Waterloo and the Blue 

Mountain House. In polarized light, X 32. Illustrates the induration of a sandstone hy enlargment of the original grains.

Fig. 1). Penetration Manebacher twin of anorthoclase in quartz-porphyry. Specimen 23. Slide 23 D. From well, at depth of 40 feet near Clermont House. In 

polarized light, X 10 (about).

Piedmoutite fills the cavities in the crystal. 

96
U. S. GEOLOGICAL SURVEY BULLETIN NO. 138 PL. XV

THIN SECTIONS. 

(i, Quartzite j 1>, feldspai crystal.
PLATE XVI.

Bull. 136- 7
PLATE XVI. -

Tig. a. Microperthic structure in a Carlsbad, twin of anorthoclase iu quartz-porphyry. Specimen 181. Slide 181 D. Near the old viaduct on the Tapeworm Railroad. In polarized light, X 85.

Fig. 1). Broken feldspar crystal in quartz-porphyry. Specimen 166. Slide 166 D. 

NearGrum Spring on the Old Furnace road. In polarized light, X28. Thephenocryst 

has been broken and pulled apart and the cracks cemented with sericito scales. Tbe 

ground mass shows the micropoikilitic structure and the niicropoildlitic a.reas are 

surrounded Avith sericite scales. 

98
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XVI

THIN SECTIONS. 

a, Microperthitic structure in feldspar; b, stretched feldspar crystal.
PLATE XVII.
PLATE XVII.

Fig. a. Broken feldspar crystal in quartz-porphyry, Specimen 177, Slide 6b. 

Old Furnace road north of the junction with the Gladhill's road. In polarized 

light, X 115.

Fig. 1). Quartz-porphyry. Specimen 25. Slide 64. Artesian well near Clerinont 

House. In polarized light, X 28.

It is attempted in this figure to show the continuity of orientation of a quartz 

phenocryst with the micropoikilitic area surrounding it, and to show (in a very 

crude way) the patchy effect given to the groundmass by the micropoikilitic structure. The quartz areas are bordered by sericite scales. 

100
U. 8. GEOLOGICAL SURVEV BULLETIN NO. 136 PL. XVII

THIN SECTIONS. 

U, Quartz -a I bite mosaic filling crack m feldspar crystal ; 6, micropoikilitic structure.
PLATE XVIII.

101
PLATE XVIII.

Fig. a. Flow structure in an aporhyolite. Specimen and slide loaned by Prof. S. 

L. Powcll, of Newbury, S. C. From the South Mountain. In ordinary light, X 24.

Fig. 1). Cbain sj.herulites in an aporhyolite. Specimen 34. Slide 34 D. From the 

neighborhood of the Bighani copper mine. In ordinary light, X 24. Quartz phenocryst (now granulated) inclosed by chain spherulites. Clear spherules in center of 

dark bauds are still preserved. 

102
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 138 PL. XVIII

THIN SECTIONS, 

a, Flow structure in an aporhyolite ; b, chain spherulites in an aporhyolite
PLATE XIX.
PLATE XIX.

Fig. a. Augite-porphyrite. Specimen 69. Slide 69 D. Gettysburg Railroad, southeast of the Clermont House. In ordinary light, X 24. Ophitic structure outlined by 

iron oxide. Colorless areas are quartz, and the considerable irregular areas are 

epidote.

Fig. 1). The same. In polarized light, X 24. The quartz breaks up into irregular 

areas, producing the mottled effect of the micropoikilitic structure. 

104
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XIX

THIN SECTIONS. 

a, 6, Augite-porphyrite in ordinary and in polarized light.
PLATE XX,

105
PLATE XX.

Fig. a. Perlitic parting in an aporhyolite. Specimen 279. Slide G. H. W. (unnumbered). Raccoon Creek, Franklin County. In ordinary light, X 88.

Fig. b. The same. In polarized light, X 225. A quartz-feldspar mosaic obscures 

all trace of glassy structures. 

106
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XX

THIN SECTIONS. 

ft, b, Perlitic parting in an aporhyoiite in ordinary and m polarized light
PLATE XXI.

107
PLATE XXI.

Fig. a. Perlitic parting in an aporbyolite. Specimen G. H. W. -Slide G. H. W. 

(unnumbered). Raccoon Creek, Franklin County. Ill ordinary light, X 40. 

. Fig. 1), Axiolites in an aporbyolite. Specimen 77. Slide 77. South Mountain. 

In ordinary light, X 40. 

108
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XXI

THIN SECTIONS, 

a, Perlitic patting in an aporhyolite ; b. axiolltes m an aporhyolite.
PLATE *XXII.

109
PLATE XXII.

Fig. a. Altered sphernlite in au aporhyolite. Specimen 280. Slide 280 D. Peach 

Orchard, southwest flank of Pine Mo ill) tain. In ordinary light, X 5. In polarized 

light the slide shows an even-grained quartz-feldspar mosaic, with the micropoikilitic structure and with no trace of spherulitic crystallization.

Fig. b. An unaltered sphernlite in an aporhyolite. Specimen 279. Slide G. H. W. 

(unnumbered). Raccoon Creek, Franklin County. In ordinary light, X 30.' Groundmass devitrified, but spherulitic crystallization still in a large part preserved. 

110
U.S.GEOLOGICAL SURVEY. BULLETIN N9. 136 PL.XXII.

a,~ ALTERED. SPHERL'LITK .IN" AN" APORHYOLITE. 

b - UNALTERED SPHERULITE IN APORHYOLITE.
PLATE XXIII.

111
PLATE XXIII.

Fig. a. Splierulitic uporhyolite.' Specimen 226. Slide 226 D. South flank of 

mountain northeast of the junction of Copper Eun and Toms Creek. In polarized 

light, X 28.

The left-hand side of the figure shows the spherulites in ordinary light; the 

right-hand shows the same spherulites in polarized light.

Fig. &. Aporhyolite. Specimen 121. Slide 121 D.° One-half mile beyond th.. 

Bigham copper mine, Old Furnace.road. In polarized light, X 80. A crystal of feldspar broken by the cleavage along the plane of former spherulitic crystallization. 

The figure shows the crystalline silica, which has replaced the spherulitic crystallisation and which is much coarser in grain than that of the groundruass. 

112
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XXIII

THIN SECTIONS. 

a, Altered spherulites in ordinary and in polarized light; 6, chain spherulites with phenocryst.
PLATE XXIY

Bull. 136 8 113
PLATE XXIV.

Fig. a. Rhyolitic structure in an aporhyolite. Specimen 153. Slide 153. South 

Mountain. In ordinary light, X 120.

Fig. Z>. The same. In ordinary light, X 40. The structure is not satisfactorily 

shown in either figure. 

114
U. S. GEOLOGICAL SURVEY BULLETIN NO. 136 PL. XXIV

THIN SECTIONS. 

ci y 6, Rhyolitic structures in aporhyolites.
PLATE XXV.

115
 ;- PLATE XXVv- - " _. - -- .--

Fig. a. Rhyolitic structure in an aporhyolite. Specimen 153. Slide'153. South 

Mountain. In ordinary light, X 120. Aschen-structur of Miigge.

Fig. b. Piedrnontite in an aporhyolite. Specimen 162. Slide 482. Southeast 

flank of Pine Mountain. In ordinary light, X 120. 

116 .
U.S.GEOLOGICAL SURVEY. BULLETIN N?. 136 PL. XXV.

a, - RHYOLITE STRUCTURE IX AN APORHYOLITE. 

d - PIEDMONTITE.
PLATE XXVI.

117
PLATE XXYI.

1 Fig. a. Amygdaloidal aporhyolite. Specimen 48. Slide 48 D. Bigham copper 

mine. In ordinary light, X 30. Epidote fills the center of the arnygdule and quartz 

surrounds the epidote. v

Fig. b. Amygdaloidal aporhyolite. Specimen G. H. W. Slide G.H.W. (unnumbered). Raccoon Creek, Franklin County. In ordinary light, X 30.

118
U.S.GEOLOGICAL SURVEY. BULLETIN N?. 136 PL.XXVI.

a,b - AMYGDALO1DAL APORETYO.LITE.
PLATE XXYII.

119
PLATE XXVII. - .

Fig. a. Amygdaloidal aporhyolite with tridymite sphenilites. Specimen 276. 

Slide G. H. W. Eaccoon Creek, Franklin County. In ordinary light, X 30.

Fig. &. The same. Specimen G. H. AV. Slide G. H. W. (unnumbered). Raccoon 

Creek, Franklin County. In ordinary light, X 30. 

120
U.S.GEOLOGICAL SURVEY. BULLETIN N9. 136 PL.XXVII.

a b - TRIDYMITE SPHERUUTES IN AN APORHYOLITE.
PLATE XXVIII.

121
PLATE XXVIII.

Fig. a. Augite-porphyrite. Specimen £37. Slide 237 D. Head of Minie Branch. 

In ordinary light, X 120.

Fig. 6. Melaphyre. Specimen 87. Slide 87 D. On Gettysburg Railroad, at fourth 

cut northeast of Monterey Station. In ordinary light, X 120. 

122
U.S.GEOLOGICAL SURVEY. BULLETIN N?. 136 PL.XXVIII.

a - AUGITE-PORPHYRJTE. 

b - MELAPHYRE.
INDEX.

Allport, S., cited......................... 37,57,68

Amygdaloidal structure in aporhyolites,

characters of....................... 55

Analyses, chemical...................... 33-34, 78

Aporhyolites, character and distribution of. 42-61 

Augite-porphyrites, character and distribution of..............'..............-. 69-78

Axiolitic structure in aporhyolites, characters of.............................. 54

Bailey, L. W., cited......................... 86

Bayley.W.S., cited ........................ 69,86

Bell, Robert, cited .......................... 86

Blandy, J. F., cited...................... 17,27,82

Bonney, T. G., cited ..................... 31,64,68

Bornemann, T.G., cited .................... 58

Bre Wster, David, cited...................... 58

Brogger, W. C., cited .......;............... 47

Brongniart, cited............:.............. 36

Butler, G.W., cited......................... 56

Callaway, cited............................. 64

Cambrian rocks, description of............. 31-34

Chemical analyses ...................... 33-34, 78

Clark, John M., acknowledgments to....... 14

Cole, G. A. J., cited......................... 56

Copper ore, occurrence of.................. 25-27

Cross, AVhitman, cited .'.................... 38,47

acknowledgments to ................... 50

Dana, J. D., cited........................... 37, 68

Daua.E.H.,cited .......................... 68

Daniells, analysis by....................... 78

Daubr6e, A.,cited.......................... 58

Devitrification, proofs of................... 57-58

Diller.J.S., cited............................ 47,86

Dolomieu, cited ............................ 36

Ells.E.W., cited........................... 86

Evans, Lewis, cited ........................ 14

Feldspar in quartz-porphyries, characters of 39-40 

Feldspar in aporhjolites, characters of.... 44-45

Frazer, Persifor, cited...... 16-17,24,25,26,78,82

Fritsch and Iteiss, cited.................... 57

Futterer, Earl, cited....................... 45

Geiger, H. B., cited......................... 22

Genth, F. A., chemical analyses by... 34, 61, 62, 78 

Gerhard, cited ............................. 36

Gildereleeve, cited......................... 38

Grant, U. S., acknowledgments to .......... 47

cited ..................... 1............. 86

Groddeck, A. von, cited..................... 64

Hague, Arnold,cited ....................... 37,61

Harker,Alfred,cited....................... 46,47

Haworth, E., cited.......................... 47, 86

Hayden,H.H.,cited........................ 24,25

Henderson, C. H., cited. 

analyses by ........

Hitchcock, cited........

Hobson, cited ..........

Page.' 

......I....... 25

.............. 61

.............. 86

.............. 68

Hunt, T. Sterry, cited .......... 17, 24, 25,82,83, 86

Iddiugs, J. P., cited.................. . ...... 31,

37,43,47, 48, 51, 52, 55, 56, 61, 68 

acknowledgments to................... 50

Irviug, E. D., cited............. 31.47,55,57,60,86 

Jackson, cited. .^............ . .............. 86

Jacks Mountain, geology of................ 21,42

Judd.J.W., cited........................... 37,68

Kalkowsky, E., cited....................... 59

Keith, Arthur, cited............... 21, 22,28, 29, 86

Keyes, C. E., cited.......................... 32

Kirwan, Eichard, cited ..................... 37

Klockmanii,F., cited ....................... 53, 59

LaCroix, A., cited.......................... 59

Lang, H. Otto, cited ........................ 59

La Vallee-Poussin, Ch. de, cited ......... 55, 57,60

Lehman, A. E., superior topographic maps

of South Mountain prepared by.... 18

Lehman, J., cited........................... 45, 64

Lesley, J. P..cited..................... 17-18,21,82

L6vy,Michel, cited...............'.......... 36

Lieber, cited ........ 1...................... 86

Lindgren, "Waldemar, cited................. 47

Link, cited................................. 59

Lithophysal structure in aporhyolites, characters of............................ 55

Loewinson-Lessing,F., cited ............... 57

Lessen, K. S., cited ......................... 59

Ludwig,E., cited........................... 59

Matthew, cited............................. 86

McCreath, A. S., analyses "by ............... 61

Melaphyres, character and distribution of.. 69-78 

Mica of aporhyolites, characters of......... 46

Micropegmatitic structure in aporhyolites,

characters of....................... 55

Micropoikilitic structure in aporhyolites,

description of...................... 47-51

Milch, P. L., cited ...............1......... 45,64

Monterey district named and defined ...... 20

ore deposits of.......................... 25-27

Naumann, cited ............................ 36

Nordenskjold, Otto, cited............. 38,47,57, 60

Osann, A., cited ............................. 59

Perlitic structure in aporhyolites, characters of.............................. 55

Phillips,J.A.,cited........................ 31,68

Piedmontiie in quartz-porphyries, character

and distribution of................. 41-42

123
124 INDEX.

Page. 

Piukerton, T., cited ........................ 36

Pirsson, L. V., cited ........................ 47,86

Pumpelly, R., cited......................... 27

Quartz in quartz-porphyries, character

of ...............................'40-41, 45

Quartz-porphyries, characters and distribution of.............................. 39-42

description of'.. ......................... 39-42

Eaccoon Creek, description of amygclaloids

from............................... 56

Reiser, cited................................ 68

Reusch.H. H., cited........................ 68

Reyer, E., cited............................. 68

RhyoHtic structure in aporhyolites, characters of.............................. 54-55

Rogers, IT. D., cited.... 14-16,21,23,25, C6,71,82,83

Rosenbusch, H., cited .... 35, 36, 37,46, 57, 64, 68, 84

Roth, Justus, cited ...... '. .................. 36

Rutley.F., cited............................ 40,54

Sauer, cited ................................ 59

Schopf, J.D.,cited ......................... 14

Sedimentary rocks, description of.......... 21-22

 Sericite schist, localities of................. 64-66

Shaler, K". S., cited.......................... S6

Slates, localities of................ 43, 64-66,79-80

Smith, G. 0., volcanic rocks in Maine

studied by ......................... 86

Sorby, H. C., cited .......................... 31

South Mountain, area of.................... 13

account of surveys in.................. 14-19

Spherulitic structure in aporhyolites, character of ...... i............... 43-44, 51-54

Suess, Eduard, cited........................ 68

Taxitic structure of rocks, description of... 57 

Teall, J. J. Harris, cited ................. 37,47,68

Tietze,cited................................ 68

Tornebohm, A. E., cited .................... 31

Trimble, Henry, analyses by ............... 61

Tyson, P. T., cited......................... 16, 24, 25

ATaii Hise, C. E., cited ............'.......... 31

Vogel, C., cited............................. 59

Vogelsang, H., cited........................ 59

Wadsworth.M.E.,cited ................... 60,86

Walcott, C. D., cited. ..............*......... 21, 22

"VVallerius, cited............................ 36

Williams, G. H., letter of transmittal by... 11

cited.................... 13, 29,41,42, 47, 64. 86

Young. A. A., cited......................... 31

Zirkcl.F., cited ......................... 36,49,54

L