Bulletin No. 275 Series A, Economic Geology,

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

SLATE DEPOSITS'AND SLATE INDUSTRY 

OF THE UNITED STATES

T. NELSON DALE

WITH SECTIONS HY

E. C. ECKEL, W. E. HILLEBIUND, AND A. T.

WASHINGTON

GOVERNMENT PRINTING OFFICE 

190G

CONTENTS.

Page.

PART I. ORIGIN, COMPOSITION, AND STRUCTURE OF SLATE .................... 5

Definition and classification ............................................ 5..

Origin............................................................... 6

Petrographic characters............................................... 9

History of research............................................... 9

Spotted slates .................................................... 15

Igneous slates..................................................... 18

Structure ............................................................ 18

Bedding......................................................... 18

Slaty cleavage.................................................... 22

Slip cleavage (" false cleavage")................................... 24

Grain ........................................................... 25

Joints............................................................ 27

Faults........................................................... 29

Shear zones (hogbacks)........................................... 29

Cleavage bands .................................................. 31

Veins............................................................ 32

Nodules ......................................................... 33

Dikes..................._....................-....-......,....... 33

Geological relations................................................... 34

Chemistry of slate................................... ................. 34

Chemical composition ............................................ 34

Chemical changes in weathering................................... 37

Chemical composition of New York and Vermont slates............. 39

Economic geology of slate............................................. 42

Difficulties in slate quarrying...................................... 42

Bedding and cleavage how distinguished.......................... 42

"Flints" their nature and cause ................................. 43

Relations of joints, dikes, and shear zones.......................... 44

Use of geological map and compass in prospecting for slate........... 44

Methods of testing slate........................................... 45

Technical description of a slate quarry............................. 50

PART II. SLATE DEPOSITS OF THE UNITED STATES ........................... 51

Arizona.............................................................. 51

Arkansas ............................................................ 51

California............................................................ 56

Georgia....'....................,..................................... 59

Maine............................................................... 60

Maryland....:......................-................................. 67

Minnesota....................................-....'...'................ 70

New Mersey .......................................................... 70

New York ........................................................... 70

Pennsylvania ........................................................ 75

Pennsylvania and Maryland (Peach Bottom)........................... 85

Tennessee............................................................ 88

Utah ................................................................ 89

Vermont....... ; ..................................................... 89

Virginia ............................................................. Ill

West Virginia......................................................... 119

3
4 CONTP:NTS AND ILLUSTRATIONS.

PART II. SLATE DEPOSITS OF THE UNITED STATES Continued. Page.

Merriman's recent tests of slates....................................... 122

Comparative characteristics of slates ............'....................... 124

Statistics of production, imports, and exports........................... 125

BIBLIOGRAPHY ........................................................... 138

GLOSSARY OF GEOLOGICAL AND SLATE-QUARRY TERMS ....................... 146

INDEX ........................:.......................................... 149

ILLUSTRATIONS.

	Page. 

PLATE I. Map showing location of slate deposits.................... 1....... 5

II. A, Eed slate with bed of limestone and green slate; B, Bed of sandstone resting on slate ........................................... 18

III. Slate syncline at West Castleton, Vt.............................. 18

IV. Plicated and faulted beds in slate................................. 20

V. A, Quartzite beds in overturned folds in slate; B, Quartz veins in

slate ......................................................... 22

VI. Slip cleavage after slaty cleavage ................................. 26

VII. Shear zones..................................................... 30

VIII. Map showing Arkansas slate area ................................ 52

I.. Sketch map showing location of slate deposits of Montgomery-Polk

County slate district, Arkansas................................. 54

X. Geologic map of the Placerville slate district, California............ 56

XI. A, Thin section of Brownville, Me., slate; B, Cleavage banding.... . 64

XII. Merrill quarry at Brownville, Me................................. 66

XIII. Old Bangor quarry (north-northeast end), Pennsylvania........... 76

XIV. Old Bangor quarry (south-southwest end), Pennsylvania .......... 76

XV. Derricks and carriers, Old Bangor quarry, Pennsylvania ........... 78

XVI. Map of Slatington quarries, Pennsylvania...............;......... 80

XVII. Blue Mountain quarry, near Slatington, Pa........................ 82

XVIII. Eureka quarry, near Slatington, Pa................................ 82

XIX. Eemoving the "ribbon," Slatington, Pa.......................... 84

XX. Map of quarries in New York and Vermont slate belt, south part.... 88

XXI. Map of quarries in New York and Vermont slate belt, north part... 88

XXII. Sections across New York and Vermont slate belt................. 90

XXIII. Quarry diagrams, New York and Vermont slate belt............... 92

XXIV. Quarry diagrams, New York and Vermont slate belt............... 96

XXV. Slate syncline at West Pawlet, Vt................................ 98

FIG. 1. Diagram, after Gosselet, showing relations of cleavage and bedding... 23

2. Diagram of cleavage banding ...................................... 32

3. Diagram showing divergent strike of bedding and cleavage........... 43

4. Map of slate region in Maine....................................... 61

5. Map of slate prospects in Frederick and Montgomery counties, Md ... 68

6. Sketch of anticline in Provident Slate Company's quarry, near Slatington, Pa......................................................... 82

7. Sketch of anticline in old quarry north of Slatington, Pa............. 82

8. Sketch of syncline at William Hughes's quarry, Slatington, Pa....... 83

9. Sketch of folds at Eureka quarry, near Slatington, Pa................ . 83

10. Microscopic drawing of Peach Bottom slate ........ ^................ 87

11. Diagram showing structure at Vermont Black Slate Company's quarry,

near Northfield,Vt.............................................. 91

12. Map of slate region in Buckingham and Fluvanna counties, Va...... ^ 112

13. Map showing location of Snowden quarries, Arnherst County, Va....-. 116

14. Map of slate prospects in Fauquier County, Va...................... 117

15. Map of slate region about Martinsburg, W. Va...................... 120
U. S. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. I

' 189

MAP OF THE UNITED STATES 

SHOWING LOCATION OF SLATE DEPOSITS

BY T.. NELSON DALTE

 Productive districts

 Prospective/deposits

Scale

0 100 200 300 400 500 600 miles
SLATE DEPOSITS AND SLATE INDUSTRY OF THE UNITED STATES.

By T. NELSON DALE AND OTHERS.

PART L-ORIGIN, COMPOSITION, AND STRUCTURE OF SLATE,

By T. NELSON. DALE. 

DEFINITION AND CLASSIFICATION OF SLATE.

Definition. The term slate, in ordinary usage, denotes a rock which has more or 

less perfect cleavage, adapting it to various commercial uses, and in which the constituent particles, with very few exceptions, can not be distinguished except in thin 

section under a microscope. In contradistinction a schist is a rock -of sometimes 

identical chemical and mineralogical composition, but is either made up of coarser 

particles or possesses a wavy structure, or else is marked by both of these features. 

Both slates and schists may have originated in deposits of identical character, but 

they have undergone different processes.

Slates as above denned vary greatly in color from black through various shades 

of gray to greens, reds, and purples of different hues. They vary also in luster 

from having none to being almost as bright as mica itself. They vary greatly not 

only in grade of fissility, but in surface texture, as seen by the unaided eye or determined by touch, and still more in microscopic texture, as seen in thin section. They 

differ also in mineralogical and chemical composition and in physical properties.

On account of their peculiar properties slates are adapted to a great variety of uses 

roofing, flooring, electric switch boards, blackboards, hand slates, billiard and laboratory table tops, vats, mantels, grave linings, .wainscoting,, hearths, chimney and 

well caps, memorial tablets, bread boards, refrigerator shelves, etc. This stone has 

thus become linked with some of the principal necessaries of life and death.

Slates divide themselves naturally into (1) those derived from aqueous sediments 

and (2) those of igneous origin. The latter, however, are very exceptional. For 

commercial purposes the basis of classification of the aqueous sedimentary slates must 

be, first, structural, for the cost of their production and the degree of their strength 

depend primarily upon that; secondly, it must be mineralogical, for their durability 

depends upon their content of certain mineral constituents.

Those slates in which the particles have been merely compressed by weight or pressure and cemented by carbonates of lime and magnesia, by kaolin or different combinations of iron, and whose grade of fissility, strength, and elasticity are therefore low, 

must be distinguished from those in which, under metamorphic processes, the kaolin 

and feldspar have passed into mica, forming a more or less dense and regular crystalline fabric of overlapping scales and fibers inclosing any remaining sedimentary particles. Such slates necessarily possess a high grade of fissility and considerable ' 

strength and elasticity. The first group includes clay slates, the second mica slates 

or phyllite slates. Those in which the micaceous matrix is but partially formed,

5
6 . SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

and which represent a transition from clay slate to mica slate, are more conveniently 

put into the clay-slate group. Thus the dark-purple ("red") roofing slate of Penrhyn, 

in Wales, is a clay slate, and so also is the black slate of Martinsburg, W. Va.; but 

the French Ardennes slate, the Welsh Festiniog, and the Peach Bottom of Pennsylvania and Maryland, are all mica slate.

But mica slate includes slates of very different composition and structure, and 

therefore of properties and value. The first distinction to be made is based upon 

the amount of ferrous carbonate whether or not it is sufficient to produce discoloration on continued exposure to the weather for this is an important factor in the 

value of slates used for roofing. This distinction gives a group of fading and one of 

unfading slates, each of which can be further subdivided as to color. Bwt each of 

these colors of fading and unfading mica slate embraces a wide range of texture. 

Some, like the Arkansas black and light-greenish slates, have a matrix of extremely 

fine sericite fibers, while the black slates of Arvonia, Va., and the Peach Bottom 

slates have a texture approaching that of a schist. Such differences in texture can 

not but result in difference of physical properties. The grades of texture are sufficiently designated by the words very fine, fine, medium, and coarse. There are also 

distinctions in the grade of fissility which do not always correspond to the grade of 

texture. These are conveniently numbered from 1 to 4, beginning with those of the 

best fissility. Finally there are differences in luster and amount of magnetite 

differences which are purely specific. The general distinctions are formulated in 

the following table:

CLASSIFICATION OF SLATE.

(I) Aqueous sedimentary.

(A) Clay slates: Matrix without any or with but very faint aggregate polarization.

(B) Mica slates: Matrix with marked aggregate polarization.

(1) -Fading: With sufficient FeC03 to discolor considerably on prolonged 

exposure.

(a) Carbonaceous or graphitic.

(b) Chloritic (greenish).

(c) Hematitic and chloritic (purplish).

(2) Unfading: Without sufficient FeC03 to produce any but very slight

discoloration on prolonged exposure, 

(a) Graphitic. 

(6) Hematitic (reddish), 

(c) Chloritic (greenish). 

 (d) Hematitic and chloritic (purplish).

(II) Igneous.

(A) Ash slates. 

(B) Dike slates.

The scientific basis for these subdivisions will be seen by consulting the table 

facing page 124 and the microscopic and chemical analyses of the slates there 

named. The application of this scheme to 13 kinds of American slates will be found 

on page 125.

ORIGIN OF SLATE.

With the exception of the rare slates of igneous origin, referred to in the previous 

section, slates originate in marine deposits of clay and sand. The common occurrence of angular grains of feldspar and of quartz in slate implies the nearness of 

shores or land masses of granitic rocks to such deposits. The alternation of beds of 

slate with beds of quartzite or grit ("ribbons," "hards," altered sandstone) corresponds to the alternation of extremely fine clayey sediments, derived from the waste 

of granitic land masses, with sandy sediments consisting of coarser material from
ORIGIN OF SLATE. 7

the same source. The repeated alternation of such fine and coarse sediments is 

attributed to the alternation of calm water, favorable to the deposition of fine material, with strong currents that bring coarse sediments more rapidly from the shore. 

These materials consisted largely of quartz, feldspar, and mica, but included also 

zircon and other silicates, various compounds of iron, lime, and magnesia, as well as 

kaolin arising from the decomposition of the feldspar. In such a slate region as that 

of western Vermont and eastern New York, where the slate is interbedded with fossiliferous limestone, it is evident that alternating with such changing conditions in 

the water there were also periods when marine life abounded and the sediments 

were entirely calcareous. Black slates owe their blackness to carbonaceous matter, 

probably derived from the decomposition of marine organisms on the sea floor. Eed 

slates owe their color to the access of ferruginous matter from the land, and purplish 

ones to an admixture of this and a green magnesian mineral (chlorite) of secondary 

origin. In both the reddish and purplish slates the iron is supposed to have been 

originally precipitated in the form of the rust-colored limonite (2FeaO;,, 3H2 0) from 

iron-bearing solutions, and afterwards altered by loss of water (H2 0) to the reddish 

hematite (Fe203 ). «

An accumulation of several hundred feet of such clayey and sandy sediments 

when buried under several thousand more of other sediments of like origin on a 

gradually subsiding sea bottom must have been subjected to sufficient vertical pressure, in connection with a small amount of moisture, to have been cemented together 

and hardened the clay into shale and the sand into sandstone. During this process 

the particles in these sediments retained the general horizontal and parallel arrangement which they had received from their distribution by sea water, but became 

firmly compacted and thus acquired a bedding foliation.

The next stage in the formation of slate is attributed ultimately to the radiation 

of heat from the earth's interior into space, resulting in a contraction of the interior, 

and consequently in a corrugation of the upper portion. This corrugation, for reasons not yet perfectly evident, took place, as far as observation extends, chiefly 

within certain belts along the axes of our mountain systems, producing lateral compression in a great mass of parallel strata. The first effect of this compression was 

to bend at least the lower portion of the strata into wave-like folds, and thus to 

shorten its horizontal area in one direction and increase its vertical thickness. The 

character of this folding is well shown in the Pennsylvania and western Vermont 

slate regions (see Pis. Ill, XIII, XIV, XVII, XVIII/XXII, XXV). But another 

effect of this compression was to' metamorphose the shale into slate. This metamorphism probably did not take place until the folding was well initiated. The 

transformation included two processes, and it is uncertain which preceded. There 

was a rotation of each individual sedimentary particle from its original horizontal 

position in the bedding foliation into one forming a considerable angle to the direction of pressure. There was also, under the combined presence of moisture and the 

effect of pressure and heat, both that which must have been generated by the pressure and that which pervaded the strata at the depth at which they were buried, such 

a chemical recombination of the silica, alumina, potash, iron, and water of the feldspar, kaolin, and iron of the shale as to generate new potash-mica in amount sufficient to constitute, in the case of the mica-slates, over 33 per cent of the resulting 

slate. This muscovite was formed in scales of infinitesimal thinness and other dimensions, and generally of longish, tapering, or ribbon-like outline. Most of these scales 

arranged themselves with their flat sides parallel to or overlapping one another, but 

facing the direction from which the pressure came, and also with an angle of inclination governed by that pressure. A small, but variable proportion, however, of these

"See on these changes Van Hise, C, R A treatise on ineUunorphism: Mon. U. S. Geol. Survey, vol. 

47, pp. 225,232.
8 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

scales took such a position that their flat sides became parallel to the direction of 

the pressure, and the longer axes of the other scales also lay in that direction. As 

mica crystallizes in columnar crystals, and as the plates or scales due to its molecular 

structure are transverse to the crystal column, and as a slab of slate consists largely 

of parallel scales of mica it may be said to correspond when held horizontally to 

such a crystal held vertically. When arnica-slate is cut in thin section across the 

cleavage its optical behavior under polarized light is like that of a mica crystal cut 

across its crystal cleavage. Yet as not only a considerable number of the mica 

scales in slate lie across the cleavage, but as some scales of chlorite and crystals of 

other minerals do also, the texture of a mica-slate combines some of the features of a 

crystal with some of those of a tissue. Extremely thin sections of slate transverse 

to the cleavage sometimes show this interlacing of the two sets of scales on their 

attenuated edges. It is to this microscopic texture that slate largely owes its peculiar 

properties.

This crystalline fabric may inclose in its meshes any sedimentary particles of quartz, 

zircon, feldspar, kaolin, or other minerals which were not or could not be made over 

into mica or secondary quartz, but whose alignment became more or less parallel to 

that of the major part of the new mica. During this metamorphism other chemical 

combinations arose from the constituents of the shale. These crystallized in isolated 

scales or crystals of chlorite, biotite (magnesia mica), various carbonates, pyrite, magnetite, graphite, tourmaline, andalusite, etc. These arranged themselves variously 

some in the cleavage direction, some in the grain direction. Various lenses, consisting of one mineral surrounded by one or two others, were also formed and arranged. 

The limonite became hematite. And during these changes in the fine sediments 

the intercalated beds of sandstone passed into quartzite and metamprphic grit by the 

formation of siliceous and micaceous cement between their particles.

The essential part of the process of the transformation of a granitic rock into clay 

and of that into mica may be made generally intelligible by recalling two well-known 

experiments of Daubr^e, the French experimental geologist. In the first, angular 

fragments of granite and quartz were rapidly rotated in a horizontal iron, cylinder 

filled with water and moving on an axis. The trituration resulted in producing a 

very fine clay. The second consisted in inclosing clay and water in a closed wroughtiron tube, which was then placed in a gas furnace and exposed for several weeks to 

a very high temperature. The result was the production of scales of mica. In the 

first experiment the factors of stream and wave erosion were duplicated, and in the 

second the elements of moisture, pressure, and heat involved in the crystallization 

of new minerals from clay were reproduced.

Two other experiments illustrating the arrangement of the mica thus formed, as 

well as the sedimentary mica and other minerals, may well be cited here, also. The 

familiar one of Tyndall, which consisted in subjecting wax to pressure between glass 

plates and, after cooling it in ice and salt, breaking it edgewise with a hammer. The 

fractures showed that it had acquired slaty cleavage. The other experiment was 

made by Jannetaz, and consisted in subjecting a mass of clay in a wrought-iron box, 

open at one side, to hydraulic pressure, which resulted in the formation not only of 

slaty cleavage but also of the grain structure.

The general microscopic structure of slate is such as to warrant the assumption 

that the compression which produced it operated not only with great uniformity but 

also very gradually.

The microscopic features of slate will be found more fully considered in the following section on the petrographic characters of slate. The above will suffice, however, 

to convey, with as few technical terms as possible, a general idea of the principal steps 

in the formation of a mica-slate of aqueous-sedimentary origin. The history of clayslate is similar, but metamorphism has been insufficient to produce the micaceous 

texture in it. After the slate was formed the mass was subjected to various stresses
PETROGRAPHIC CHARACTERS. . 9

(tensions, shears, contractions), which resulted in several systems of joints, in faults, 

slip cleavage ("false cleavage"), shear /ones ("hogbacks"), and in irregular openings in which veins of quartz and calcite were formed by infiltration; and the mass, 

in some cases, was traversed by fissures penetrating to the molten zone and thus 

permitting the exudation of lava-like material which formed 'dikes.

The folds into which the slate beds were bent are rarely complete, for the reason 

that their upper portions have generally been removed through various erosive processes the decomposition due to atmospheric agencies, the action of streams, and, 

in places, of the continental glacier. Pis. Ill, XIII, XIV, XVII, XVIII, and 

XXV all illustrate this feature. In the Appalachian region generally the thickness 

of this eroded material Avas very considerable.

PETROGRAPHIC CHARACTERS^ 

HISTORY OF RESEARCH.

However uncertain the structural, mineralogical, and chemical features of roofing 

slates may once have seemed, these are all now well understood. Sedgwick, Sorby, 

Phillips, Tyndall, Daubree, Gosselet, Jannetaz, and Becker have 'studied their 

structure either in the field or the laboratory, or in both, and Sorby, Zirkel, Eenard, 

and others have investigated their mineralogical composition. Summaries on the 

cleavage of slate were published by Loretz in 1880, and Harker in 1886, and on its 

petrography by Kalkowsky in 1886, Zirkel in 1894, and Kosenbusch in 1898. Chemical analyses are given in Roth's Chemical Geology (1890), and in Zirkel's Petrography 

(1894). Hillebrand, Reade, and Holland made a number of complete analyses 

between 1899 and 1901. Van Hise, in his recent Treatise on Metamorphism & 

(1904), discusses the theory of metamorphism in schists and slates, andLeith, in his 

paper on rock cleavage 6 (1905), treats of the"theory of bedding and cleavage, basing 

his conclusions more largely-on the microscopic study of schists than of roofing 

slates. The bibliography on pages 138-145 shows how much has been written on the 

different aspects of the subject. It is not proposed to give here a summary of all this 

literature, but an attempt will be made to set forth, in a series of statements, quotations, and questions, the state of scientific opinion on the more important and interesting features of slate.

Slaty cleavage may be defined simply as a rearrangement of the particles of a deposit 

by lateral pressure, together with the arrangement of any new crystalline particles 

formed during and in consequence of that pressure. This arrangement of old and 

new particles is related to the directions of pressure and resistance. The older 

authorities on slaty cleavage usually define the direction of pressure as being at 90° 

to the cleavage. This definition of the direction has, however, been under discussion 

in recent years. The latest phase of opinion appears in the writings of Van Hise, 

Becker, and Leith. Van Hise d expresses his views in these words:

During the process of recrystallization at any given moment there is maximum shortening in the 

direction of greatest stress, maximum elongation in the direction of least stress, and shortening or 

elongation in the direction of mean stress. Consequently the shape of the modified particle may be 

that which would be produced if a plastic grain were rolled out, the sides being confined in one 

direction, but with liberty to elongate in another direction in the same plane; oritrnay be that which 

would be produced if a roundish cake of dough were flattened between two boards, and consequently 

elongated in all directions at right angles to the direction of greatest pressure. . . . The pa'rticles 

are arranged with their greater, mean, and minor diameters, corresponding to least, mean, and greatest 

pressures. In proportion as the movement involves shearing motion or scission the mineral particles 

are rotated from a position in which the direction of greatest elongation is at right angles to the direction of greatest pressure, although at any given time the mineral particles tend to develop with their 

longer axes at right angles to the maximum pressure.

 'This chapter in its original form appeared in part 8 of Nineteenth Ann. Kept. U. S. Geol. Survey, 

in 1899, but its matter is here revised and brought up to date. 

&Mon. U. S. Geol. Survey, vol. 47. 

cBull. U. S. Geol. Surve'y No. '239. 

^Treatise on metamorphism: Mon. U. S. Geol. Survey, vol. 47, 1904, pp. 752, 753.
10 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Becker's experimental studies were directed to determining the mathematical 

character of the deformation undergone by a plastic but crystalline mass under pressure. For this purpose he pierced a block of ceresiu with needle holes at regular 

intervals in both a vertical and a horizontal direction, drawing a thread smeared with 

coloring matter after the needle, and subjected the block to powerful vertical pressure. The curves then assumed by the colored lines enabled him to determine the 

position of the axes of the strain ellipsoid with relation to the direction of pressure. 

The result of this and of other experiments described in the same paper is that 

cleavage does not coincide even approximately with the direction of the major axes of the strain 

ellipsoid. Neither does the cleavage correspond to the position of the major axes of the strain ellipsoids at any previous stage of the strain.a

In other words, slaty cleavage is not perpendicular to the smallest axis of the strain 

ellipsoid, but makes with "that axis an acute angle equal or greater than 45° and 

increasing.as the strain grows greater." Both the experiments and the reasoning 

seem simple and clear.

Leith, following Van Hise, states that 

Cleavage is always tending to develop normal to the greatest principal stress, but its final position 

may or may not*be inclined to the greater stress, depending upon the nature of the strain.6

This question of the relation of the dip of slaty cleavage to the direction of pressure is not a purely speculative one, but bears directly upon problems in field geology. Thus where cleavage is nearly horizontal it is probable that a secondary crustal 

movement must have diminished the original inclination of the cleavage, and consequently the whole attitude of the folds. Were the limiting angle at which slaty 

cleavage forms established, then, in the case stated, a maximum figure for the angular displacement of the cleavage could be obtained.

There is some vagueness in.scientific literature, as well as in popular conception, as 

to what constitutes a slate. Sorby drew this distinction:

When a section [of a fine-grained slate] cut at right angles to the cleavage is rotated in polarized 

light it becomes, over nearly the whole surface, very bright, and much darker at different azimuths, 

like a doubly refracting crystal, whereas there is little or no such change in the case of true clay 

slates of the normal granular type containing much kaolin and very little mica.o

Micaceous luster is not a satisfactory criterion, for some slates, like those from 

Monson, in Maine, are almost lusterless and yet possess a brilliant aggregate polarization and are very sonorous. They are mica-slates (phyllite-slates), as are the Brownville slates, from the same State, which have a micaceous luster.

An interesting transition from a shale to a mica-slate, described on page 81, occurs 

at Lehigh Gap, Pennsylvania. The new muscovite (sericite) has begun to form across 

the bedding of the shale without as yet any change of orientation in the clastic particles. In a specimen of shale from Rensselaer County, N. Y.,<^ the incipient alignment of clastic muscovite across the bedding is well shown in thin section, and slip 

cleavage seems about to be set up. Some specimens of clay-slate from Martinsburg, 

W. Va., show a slaty cleavage Avithout any matrix of muscovite whatever, but others 

show the beginning of one by a faint aggregate polarization. These cases suffice to 

show that all transitions from a shale to a clay-slate and to a mica-slate occur.

While it is supposed that mica-slate is the product of a less intense metamorphism 

than schist 6 and it is also assumed that the process by which slaty cleavage was 

produced was marked by great uniformity, and also took place slowly there are as 

yet neither observations nor experiments for a perfectly satisfactory reply to the fola Experiments on schistosity and slaty cleavage: Bull. U. S. Geol. Survey No. 241, 1904, p. 21.

bllock cleavage: Bull. U. S. Geol. Survey No. 239, 1905, p. 138.

cQn the structure and origin of noncalcareous stratified rocks: Quart. Jour. Geol. Soc. London, vol. 

36, p. 76.

dSee Bull. U. S. Geol. Survey No. 242, PI. II, B.

« Van Hise, op. cit., p. 894: " When the depth is not great and the mass-mechanical action is not 

very severe, slates are likely to form. When the depth is greater and the mass-mechanical action is 

severe, schists or gneisses are likely to develop."
. PETEOGEAPHIC CHAEACTEES. 11

lowing questions: Why should one part of a sedimentary .formation have been 

altered into roofing slate and an adjacent part of the same into schist? Would continued metamorphism transform a roofing slate into a schist? Were schists of sedimentary origin at one stage roofing elate? Why should metamorphism begin in some 

shales by the formation of slip cleavage leading to schist, and in others by slaty 

cleavage?

As to the mineral constituents of slate, Sorby gives the average size of the scales of 

sericite « in the best Welsh slates as one two-thousandths of an inch in breadth by one 

six-thousandth in thickness. 'J Scattered about among the meshes of the muscovite are 

minerals which extinguish irregularly. Chief among these is quartz, mostly in clastic 

grains, sometimes formed in situ or in lenses of chalcedony. This secondary quartz 

may attach itself to the quartz fragments or to the various other minerals to be named. 

Next in importance is a chloritic mineral, dichroic (light yellow, dark green), polarizing from a Prussian or plum blue, to a violet or olive when the section is traasverse 

to the scale, but remaining dark between crossed nichols during a complete revolution when the section is parallel to it. These scales are usually intergrown with 

lamelke of muscovite, sometimes of biotite, and lie some transverse to the cleavage 

and parallel to the grain, others parallel to the cleavage. c Chlorite and quartz somettimes make up minute lenses with their long axes parallel'to the cleavage, or chlorite 

surrounded by sericite makes lenses with their long axes in the grain direction. 

These scales of chlorite sometimes contain needle-like crystals, probably rutile, crossing one another at angles of 60° and 120° (see p. 80). This chloritic mineral is 

regarded as of secondary origin. Eutile needles are generally, but not always, present, and in great abundance/' This mineral occurs also in irregular masses up to 

0.05 mm. in diameter, consisting of a network of prisms forming angles of '60° and 

120° (sagenite twins). Thus at Bremo/N^a., as stated on page.115, hematite occurs in 

very minute scales and dots and very abundantly in the reddish and purplish slates. e

There are also cubes, lenses, and spherules of pyrite. The spherules abound in 

black slates. Carbonaceous matter and graphite abound in the black and dark-gray 

slates. Clastic grains of feldspar and of zircon are characteristic. Hemimorphic 

prisms of tourmaline are not very unusual.

Calcite and other carbonates, particularly one of lime, iron, and magnesia, are apt 

to be evenly disseminated in rhombs and plates. Ehodochrosite (MnC03 ) probably 

also occurs (see p. 71). The following have also been identified: Ottrelite, staurolite, 

garnet, andalusite, sphene, anatase, biotite, hornblende, epidote, apatite, pyrrhotite, 

gypsum, magnetite, limonite, pyrophyllite, talc.

Slates are often speckled with minute protuberances which under the microscope 

resolve themselves into the lenses just referred to so-called "eyes" or "knots." 

Instead of consisting of chlorite and quartz, they may consist of an octahedron of 

magnetite partially surrounded by quartz and that entirely by chlorite./ This quart/ 

is then regarded as a later infiltration into a cavity formed between the magnetite

«See Laspeyres, Sericit: Zeitschr. flir Kryst., vol. 4, 1880, p. 224.

<?Sorby, iDid. ^

cSee Renard, Recherches sur la composition et la structure des phyllades ardennais: Bull. Mus. roy. 

d'hist. nab.'deBelgique; vol. 3, p. 235, pi. 12; Zirkel, Lehrb., 1894, p. 298; and G. Rose, Uber die regelmassige Verwachsung.der versciiiederien Glimrnerarten mil einander sowie rnit I'ennin und Eisenglanz: Monatsber. K.'Akad. zu Berlin, 1869.

it On these see-Zjrkel, Ueber die mikroskopische Zusammensetzung von Thonschiefern und 

Dachschiefern: Poggenclorft Annalen, vol. 144,1891, p. 319; Van Werweke, N. Jahrb. Min. Geol., 1881, 

vol. 1, p. 178; 'Saner,'N. Jahrb. Min. Geol., 1881, vol. 1, p. 227-238; Cathrein, Ueber das Vorkommen 

mikroskopischer Zirkone u. Titan-Mineralien in den Gesteinen, WOrzburg, 1884; Kalkowsky, Elemente der Lithologie, 188C, p. 257. Sorby (loc. cit., p. 68) gives the diameter of the slate needles as less 

than &, inch.

e Renard gives the size of the granules of Fe2 03 as 0.020-0.005 mm.; op. cit., vol., 3, p. 234. See also 

his PI. XII, fig. 2, Of purple and green slate, in same volume. See also Gosselet (Etudes sur 1'origine 

de 1'ottrelite: Ann. Soc. Geol. du Nord,.vol. 15,1887-1888, Lille, 1888, pp. 188-189) who describes hematite 

us occurring in three forms in the reddish slates: (1) In irregular grains 0.01 to 0.02 mm. or less; (2) 

in scales with a bluish steel-like luster under a mixture of reflected and transmitted light; (3)' in 

minute granules which are always red, a brick-red under reflected light.

/See Geinitz, Der Phyllit von Rimogne in der Ardennen: Tschermaks Min. Mittheil. (new folge), 

vol. 3; also Renard, op. cit., vol. 2,1883, p. 133 et seq., and pi. 6.
12 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

and the chlorite by pressure. Others consist of chlorite surrounded by calcite and 

that by quartz.«

Others have a central crystal of pyrite instead of magnetite, 6 or the pyrite may 

have been changed to lirnonite. c Still others consist of chalcedony surrounded by 

chlorite scales,^ or of quartz surrounded by radial plates of muscovite, e or of biotite 

surrounded by quartz (see p. 65). These lenses may, as in the Peach Bottom slates 

(see p. 87), be so minute as not to result in any visible speckling of the surface, and 

may consist of crystals of andalusite surrounded by chlorite, quartz, or muscovite, 

the orientation of the lens being different from that of the crystal. These crystals of 

andalusite may have quartz on one side and muscovite on the other.

Finally, the discoloration once attributed by Bischof to the hydration and oxydation of a ferrous oxide has been shown, in some slates at least, to 'be due to the alteration of a ferrous carbonate to limonite, and is thought to be probably due to this in 

most slates (see pp. 36,39).

The presence of kaolin is, of course, to be assumed in all clay-slates and also in all 

mica-slates in which the micatization of the matrix is incomplete. Its presence in 

minute quantities in those mica-slates which have little or no luster, like the Monson, 

Me., slates, and possibly the Lehigb and Northampton County slates of Pennsylvania, 

may be suspected, but not'demonstrated.

Leaving out the rarer and less significant constituents and basing his estimates on 

the chemical and microscopic analyses of the principal varieties of slate from the 

French Ardennes, Kenard computes the mineral percentages as follows:

Percentages of minerals in slates of Ardennes, France.

Mineral.

Muscovite (sericite)

Rutile. ........................................................

Purple.

40.69

7.75

40.41

6.23

i ^

Green.

39.54

6.81

45.78

2.90

1.04

Bluish 

black.

37.75

12. 55

40.58

4. SI

Grayish 

green.

37.97

17.99

30.97

1 <M

3.09

In round numbers this amounts to 

Muscovite ..................................................................................... 38 to 40

Chlorite ....................................................................................... 6 to 18

Quartz......................................................................................... 31 to 45

Hematite ...................................................................................... 3 to 6

Kutile ......................................................................................... 1 to li

Renard/ calls attention to the fact that the green slates of Fumay in the Ardennes 

contain 4 per cent more Si02 than the purple ones and about 3£ per cent less Fe2 03 .

Eosenbusch,? in comparing IS.analyses of clay slates from different parts of Europe, 

calls attention to the strikingly characteristic preponderance of the MgO as compared 

with the CaO, along with a uniformly high percentage of iron oxides and of A12 03 , 

and also to the like preponderance of the K2 0'over the Na2 0, which he explains thus:

Clays and clay slates constitute the finest mechanical detritus from quartz feldspar rocks; whatever 

silicates of lime they contain was removed as a soluble bicarbonate and for this reason very little 

lime-soda feldspar can occur in such a detritus.

«Renard, op. cit., p. 248.

fiSeeHarker, on "eyes" of pyrite and other minerals in slate; Geol. Mag., Dec. Ill, vol. 6, No. 103, 

p. 396. London, Sept., 1889.

cLoretz, Ueber Transversalschieferung, etc., 1882, pp. 283-289. (For full titles of papers and works 

cited see Bibliography, pp. 138-145.)

d Mallard, Sur 1'examen microscopique de quelques schistes ardoisiers: Bull. Soc. mineralogique de 

France, vol. 3, No. 4, 1880, p.'101.

«Zirkel, Lehrb., p. 744, Thonschiefer

/Op. cit, vol. 2, 1883, pi 147.

o Eleinente der Gesteinslehre, p. 424.
PETEOGEAPHIC C.EAEACTEES. 13

While there has been substantial agreement as to the general microscopic and 

chemical character of roofing slates, there have been questions as to the origin of 

some of their constituents. Is the muscovite (sericite) which makes up from a third 

to a half of the mica-slates the product of the metamorphism of argillaceous material, 

or are these shred-like scales the result of the disintegration of some micaceous rock, 

the effect of pressure having been simply to bring the shreds into parallelism and to 

mat them together? A shale under the microscope shows a considerable amount of 

apparantly clastic muscovite in scales of various sizes with more or less arrangement 

in the direction of the bedding. Sorby regarded the micaceous mineral of slates as 

formed in situ by an alteration of partially decomposed feldspar, yet admitted that 

the structure was just such as would result from the deposition of material sorted by 

gentle currents and subsequently compressed, but the chlorite he considered as undoubtedly secondary.«

Rosenbusch wrote:

Of course the material of clay slates was mechanically brought together, but the mineral constituent of that part which is mainly micaceous and without feldspar was certainly the result of metamorphic processes which were intimately connected with dynamic-geologic processes. &

Hutchins expressed his conclusions thus:

This flue mixture of biotite, muscovite, kaolin, the minutest waste of feldspar, and in less degree 

of quartz, and probably other substances, under the joint action of pressure, warmth, and mineral 

solutions, gives rise to various decompositions and recombinations which result, among other things, 

in the formation of new mica, with the separation of titanic acid in the form of rutile. Into these 

reactions, whatever may be their exact course, even the muscovite in very fine state of division appears 

to enter: and there is good reason to conclude that in fine-grained sediments of suitable composition, 

exposed long enough to the necessary conditions as to pressure, temperature, and percolation of solutions, an almost complete regeneration of the " paste'' to mica can and'does take place, and that this 

regenerated material? under intenser dynamo-metamorphic action, is converted into some of the forms 

of micaceous slates known to us. The mica so formed is probably what in its more advanced stages 

of development is often known as sericite. c

As slates are evidently derived from the waste of granitic rocks this material must 

have contained clastic mica, as do the shales, and wherever the feldspar of the granite 

or gneiss had previously been sericitized such sericite must have found its way into the 

sediments and into the slate without very great loss by decomposition. As a matter 

of observation, fine-grained mica-slates do contain occasional scales of muscovite apart 

from the matrix, which may be of clastic origin, but the micaceous matrix of micaslate is regarded as mainly of metamorphic origin.

A question has also been raised as to the origin of the rutile needles. Roth a is 

decidedly of the opinion that they belong to the original sediment. Thiirach, e Pfaff,/ 

and Credner!/ find an abundance of them in clay. Rosenbusch A states that while 

zircon and apatite bear traces of their clastic origin, such traces are entirely absent 

in both the rutile and the tourmaline. It will be noticed in looking over the 40 microscopic analyses of roofing slates made for this volume by the writer that the abundance 

of these needles varies greatly. In some very crystalline slates (Peach Bottom, 

Maine) they are hard to find, and in some less crystalline ones (Arkansas, Vermont) 

they are very abundant.

The sizes of the mineral particles and the numerical abundance of some of them, as 

determined during the microscopic study of the roofing slates of Arkansas, California,

a See On the structure and origin of noncalcareous stratified rocks: Quart. Jour. Geol. Soc. London, 

vol. 36, pp. 67-77.

bNeues Jahrb. fur Min., etc., 1881, vol. 1, p. 399.

cHutchins, W. M., Clays, shales, and slates: Geol. Mag., vol. 7, p. 317.

^Doubtless some of the constituents are of secondary origin (as quartz, pyrite, and the products of 

its oxidation, calcite, hematite, and limonite, gypsum), but the evidence of the secondary origin of 

the rutile needles does not seem to me convincing. Unstratified sands also contain a series of minerals. Allgem.u. Chem. Geologic, vol. 2, p. 586.

e Thiirach, H.,Ueber das VorkommenmikroskopischerZirkoneu. Titan Mineralien in den Gesteinen: 

Verb, phys.-math. Gesellsch., Wiirzburg, N. F., vol. 18, No. 10, 1884.

/Pfaff, E., PetrographischeUntersuchungen iiber die eocenen Thonschiefer der Glarner Alpen: Sitzungsb. math.-phys. Classe k. k. Akad. der Wissenschaft., Munich, vol. 10, 1880, p. 479.

a Credner, G. R., Die Krystallinischen Gernengtheile gewisser Schieferthone und Thone: Zeitschr. 

ges. Naturwiss., Halle, 1874.

AElemente der Gesteinslehre, p. 424.
14 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Maine, Maryland, New York, Pennsylvania, Virginia, Vermont, and West Virginia, 

are here summarized:

Quartz, grains: 0.013 to 0.1 millimeter by 0.004 to 0.03 millimeter (exceptionally 

0.347 long).

Feldspar (plagioclase), grains: up to 0.047 by 0.052.

Muscovite (sericite) of matrix: thickness down to 0.00017, length from 0.006 to 

0.06 and over.

Biotite, scales: up to 0.085 by 0.047 (exceptionally 0.2 by 0.09) and numbering up 

to 22 per square millimeter.*

Chlorite, scales: 0.047 to 0.17 by 0.006 to 0.1 (exceptionally 0.38 by 0.25) and numbering up to 100 per square millimeter.

Carbonate, rhombs: 0.002 to 0.065.

Rutile-"needles:" 0.0017 to 0.095 by 0.0006 to 0.006 millimeter and numbering 

from 1,000 to 1,850 per square millimeter, or 900,000 per square inch. Those in 

Arkansas slates number up to about 2,400 per square millimeter.

Pyrite, lenses and distorted cubes: 0.002 to 0.094 by 0.47 (rarely 0.6 by 0.1) and 

numbering up to 300 per square millimeter.

Pyrite, spherules: 0.0017 to 0.027 millimeter and numbering up to 600 per square 

millimeter.

Magnetite, distorted octahedra: 0.009 to 0.14 by 0.02 (rarely 0.17 by 0.04) and 

numbering up to 43 per square millimeter.

Hematite, dots: 0.0004 to 0.01 millimeter.

Tourmaline, prisms: 0.005 to 0.076 by 0.001 to 0.009 millimeter. "

Andalusite, prisms: 0.008 by 0.001 to 0.11 by 0.03 (exceptionally 0.2 by 0.02) and 

numbering up to 80 per square millimeter. *

Lenses-of chalcedonic quartz and rhodochrosite: 0.32 by 0.15 millimeter (sometimes 

1 millimeter long).

Lenses of biotite and quartz: up to 0.565 by 0.14 (rarely 1 by 0.075).

Lenses of pyrite and quartz: up to 0.75 by 0.12 millimeter.

Lenses of chlorite or quartz or muscovite or combinations of them, containing each 

an andalusite prism and measuring 0.04 to 0.25 by 0.04 to 0.1 millimeter.

As the mineral constituents of aqueous-sedimentary slates were either (1) derived 

from older rocks and deposited either as mechanical sediments or chemical precipitates, or (2) formed during metamorphism, or (3) derived from marine organisms, 

they are to be classified as follows:

Classification of mineral constituents of slates.

Clastic. Clastic or authigenous. Authigenous. Organic.

Quartz grains. 

Feldspar grains. 

Zircon grains. 

Muscovite scales. 

Kaolin. 

Apatite. 

Magnetite? 

Carbonates, granular.

Kutile needles. 

Tourmaline.

Quartz, chalcedonic 

Quartz, vein. 

Muscovite (serieite). 

Biotite.

Chlorite, interleaved with 

rnuscovite or biotite.

Pyrite.

Magnetite.

Hematite.

Carbonates of lime, iron, 

magnesia.

Carbonate of manganese.

Andalusite.

Barite.

Gypsum.

Talc.

Carbonaceous matter. 

Graphite.

« The square millimeter given comprises the thickness of the thin section.
PETROGRAPHIC CHARACTERS. 15

Of course these secondary or authigenous minerals were merely new combinations, 

sometimes in crystalline form, of clastic ones. Thus the hematite originated in sedimentary limonite. The andalusite (A] 2 0:i ,Si02 ) probably originated in kaolin.

The principal elements in roofing slates, as determined by the chemical analyses 

given in the section on the chemistry of slate (p. 34), are to be attributed to the mineral constituents in the following way:

Si02 to quartz, muscovite, biotite, feldspar, chlorite, tourmaline, andalusite, zircon, 

kaolin.

Ti02 to rutile.

A1 2 0S to muscovite, biotite, feldspar, chlorite, tourmaline, andalusite, kaolin.

Fe2 03 to hematite, magnetite, muscovite, biotite.

FeO to chlorite, magnetite, carbonate, biotite.

MnO to rhodochrosite.

CaO to plagioclase, calcite, carbonate of lime, iron and magnesia, apatite.

BaO to barite and some silicate.

MgO to chlorite. biotite, carbonate.

K.jO to muscovite, orthoclase.

Na.2Q to muscovite, plagioclase.

Li 2 0 to tourmaline. 

 H2 0 to muscovite, chlorite, limonite, kaolin.

PL,05 to apatite.

C02 to calcite, rhodochrosite, and carbonate of lime, iron, and magnesia.

FeS2 to pyrite.

SO,, to barite and gypsum.

C to graphite, coaly matter.

Zr2 0 to zircon.

The mineral sources of the nickel, cobalt, chromium, vanadium, and ammonia 

detected by Doctor Hillebrand in the slates of Vermont and New York have not 

been determined.

SPOTTED SLATES.

The spots in roofing slates have long attracted attention.« In the eastern New 

York and western Vermont slate region the purple slates often have green spots of 

circular or oval, but frequently of irregular outline. These spots sometimes occur 

only along lines of bedding and correspond to or pass into green "ribbons." In 

places, however, an entire bed of purple slate several feet thick is irregularly spotted 

throughout. The red slates are also often spotted. The spots are frequently circular 

or oval and measure from a fraction of an. inch to several inches in diameter and of 

pale-green color with or without a purple border. Some of the spots, however, have 

no symmetry whatever. In order, if possible, to throw some new light on this subject a few thin sections were prepared across small spots in directions parallel to and 

across the cleavage, and in the case of the spotted red slates chemical analyses were 

made by Doctor Hillebrand of the green center of the spot, of its purple rim, and of 

the outer red slate.

MICROSCOPIC. ANALYSES.

An elliptical green spot, 1 by f inch, in purple Cambrian slate from the Lake 

Bonioseen' Slate Company's quarry, at Cedar Point, Castleton, Vt., in a section cut 

parallel to cleavage, shows, in the green part, muscovite scales lying in all directions, 

large chlorite scales, quartz fragments, carbonate rhombs, and a few irregular spherules of pyrite. In the center is some opaque, noncalcareous matter, partly sura Comparative view of the cleavage of crystals and slate rocks, by John Tyndall: Phil. Mag., vol. 12, 

July, 1856. On the disposition of iron in variegated strata, by George Maw: Quart. Jour. Geol. Soc., 

vol. 24, p. 379; also On variegated Cambrian slates, by the same author, PI. XIV, figs. 29, 31, 32, London, 18C8. Les schistes de Fumay, by Gosselet: Ann. Soc. Geol. du Nord, vol. 10. pp. 63-80, Lille., 1884; 

same author, L'Ardenne, 1888, p. 35. Text-book of Geology, by Archibald Geikie, 4th ed., vol. 1, p. 

451 (2), 1893. Lehrbuch der Petrographie, by F. Zirkel, 2d ed., vol. 3, pp. 296-297,1894.
16 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

rounded by an aggregation of spherules of pyrite in a cloud of rutile needles. There 

are also cracks filled with secondary sericite. In the surrounding purple the same 

elements recur, but the pyrite is much more abundant, measuring up to 0.021 mm. 

There are also many dots of Fe2 03 from less than 0.003 to 0.009 mm., and rutile needles 

up to 0.012 mm. in length.

An elliptical green spot, 3 inches long, with a purple rim, in Ordovician red slate 

from the National Ked Slate Company's quarry north-northwest of Kaceville (Specimen D. XIV, '95, 397a), when cut transversely to the cleavage, measures a half inch 

in thickness and shows a black streak 1 inch long in the center. The central streak 

consists of strings of minute irregular lenses of cryptocrystalline quart/ and possibly 

carbonate of manganese (rhodochrosite) containing spherules of pyrite. The green 

part consists of a mass of fibers of muscovite, which polarize as one mineral with 

much carbonate and many lenses, and also quartz grains. In the purple rim there 

is a decrease of carbonate and the hematite fragments begin to appear and become 

still more abundant in the surrounding red slate itself.

A green spot in Ordovician red slate (D. XIV, '95, 20lc), from the Empire Red 

Slate Company's quarry, a mile north of Granville, cut parallel to the cleavage, shows 

slate needles (Ti02 ) up to 0.043 mm. long, carbonate rhombs up to 0.030 mm., chlorite scales up to 0.030 mm., angular quartz grains up to 0.039 mm., and prisms of 

tourmaline up to 0.021 by 0.002 mm. The surrounding red slate, that of Analysis 

K, p. 73, has been described in the general description on p. 72.

Another spot, almost circular, 0.44 inch in diameter, from a piece of red slate (D. 

XIV, '95, 201 1) from the same quarry, cut parallel to the cleavage, shows a central 

dot 0.03 inch in diameter, consisting mainly of carbonate and of a dense brown material. About this is a zone about 0.1 inch wide, of elliptical shape, of carbonate, with 

some fibrous quartz along the margin. Then comes a zone 0.08 inch wide, of green 

slaty material, containing angular quartz grains, muscovite scales, rutile needles, 

nodules of pyrite, and thinly disseminated areas and rhombs of carbonate; then a 

very narrow zone, made up entirely of carbonate and pyrite. Outside of this, another 

green slate zone, 0.08 inch wide, like the first, but with very little carbonate. The 

angular quartz grains measure up to 0.030 mm. There are also slender tourmaline 

prisms. Outside of all comes the red slate, full of Fe2 03 pigment. Chlorite was 

not detected in the green zones, but it may be present in minute scales.

CHEMICAL ANALYSES OF SPOTS IN RED SLATE.

The specimen (Q, R) analyzed by Doctor Hillebrand came from the same quarry 

as the large spot described above. It was a green spot with purple rim, in red slate. 

The analysis of the red slate M, on page 73, is repeated for comparison.

Chemical analyses of spotted red slate.

Ti02 (titanium dioxide, 

rutile) .................

A12 03 (alumina) .........

Fe2 03 (ferric oxide) ...... 

FeO (ferrous oxide) ...... 

MnO (manganousoxide) . 

NiO (nickelous oxide) . . . 

CoO (cobaltous oxide) ... 

CaO (lime)...............

Specimen, a

M.

63.88

.47 

9.77 

3.86 

1.44 

.21 

Trace. 

Trace. 

3.53 

.05

Q.

 64.59

. .51 

10.23 

1.79 

1.19 

.26 

Trace. 

Trace. 

4.07 

.05

R.

65.44

.52 

9.38 

1.09 

1.06 

.32 

Trace. 

Trace. 

4.53 

.06

Li,0 (lithia)............. 

H2 0 (water below 110° C.) 

H20 (water above 110° C.) 

Po05 (phosphoric oxide). 

C02 (carbon dioxide) .... 

FeSo (pyrite) .............

Total ..............

Specimen, a

M.

5.37 

3.45 

.20 

Str. tr. 

.27 

2.48 

.08 

5.08 

Trace.

100. 14

Q.

5.12 

3.70 

.23 

Str. tr. 

.28 

2.29 

.08 

5.84 

Trace.

100. 23

R.

4.92 

3.57 

.22 

Str. tr. 

.25 

2.10 

.08 

6.55 

.04

100. 13

«M (=D. XIV, '95, 397 o), red slate, 1 mile north-northwest of Raceville, in Granville, Washington 

County, N. Y., about a spot; Q, purple rim of the spot; R, green portion of the spot.
PETROGRAPHIC CHARACTERS. 17

Doctor Hillebrand adds this observation:

Calculation shows that with no CO;, there would be only enough CaO for the PoOf,, and, further, 

that the result would be no MnO. How much FeO, if any, exists as carbonate is not indicated. If, 

after allowing for apatite, for MnCOg, and CaCOa, the remainder-of the CO^ is charged to MgO, we 

find the proportions shown in the columns below, a

MgC03 (in part FeC03 ) .........................................................

M.

6.14

4.22

.38

Q.

7.11

4.77

.47

R.

7.93

1 qfi

.57

«See Doctor Hillebrand's chemical note, p. 39. 

DISCUSSION OF THE SPOTS.

From Doctor Hillebrand's analyses it would appear that there is a decrease of the 

carbonates of lime and manganese and magnesia and of silica and rutile from the 

center of the spot outward and an increase of Fe2 03 in the same direction.

The main results of the microscopic and chemical analyses agree even as to the 

relative amount of pyrite. The1 difference in color from the green to purple and red is 

manifestly due to the differences in the amount of hematite. Pyrite, rutile, carbonate, and tourmaline are more abundant within the spots than without them.

Certain green fossil impressions in purple slate at Middle Granville, N. Y., may 

throw some light on the origin of these spots. In this case the effect of organic 

matter, whether the carbonaceous matter of the lining of an annelid boring or from 

a marine alga, has been to diminish the quantity of Fe2 03 in the slate, and possibly 

to increase the amount of chlorite.« Gosselet regards the spots as the result of the 

reduction of the hematite (Fe2 03 ) by decaying organisms to the ferrous oxide (FeO) 

and its removal as an organic salt or as a carbonate. He observes that the green 

spots in purple tiles wear less readily than the rest of the tile, because they contain 

more quartz, and this Si02 ho attributes to infiltration.'1

in the spots examined from the New York and Vermont slates the marked 

decrease of Fe2 03 is accompanied by a marked increase of carbonate of lime, iron, 

and manganese/ and of Si02 , also by a slight increase, in some of the thin sections 

at least, of FeS2 . Carbonates are also characteristic of the spots in some European, 

slates. c The increase of the carbonates may be directly connected with the produc- 

. tion of C02 by decaying organisms and the consequent decrease of the Fe2 03; Not 

impossibly the organism may have had a calcareous exoskeleton which was dissolved 

and then redeposited as crystalline CaC03 . The infiltration of Si02 and the formW 

tion of chalcedony may be purely secondary, and likewise the deposit of FeS2 , or 

there may have been some precipitation of FeS2 about the decaying organism, as 

seems to have been the case in some fossiliferous sediments. At any rate, the rim 

of intermediate" cornpositon would be the zone'in which chemical reaction was less 

.effective.

In view of all these facts and indications, the spots may be safely regarded as 

probably produced by chemical changes in the sediments consequent upon the 

decay of organisms.

If this be the correct view, the green ribbons, which traverse both purple and red 

slate, would correspond to small deposits of decomposing organic material that

"See Tyndall, Maw, Gosselet, Geikie, and Zirkel, as indicated by titles given in footnote on p. 15.

6 Maw (loc. cit.) had analyses made of dark greenish ribbons in the Welsh blue slates, and found 

that the ribbons contained 6 per < ent more SiO.>, 7 per cent more ALOa , 4£ per cent more MgO (=7 

times as much), but 4 per cent less Fe..0;1 .1 per cent less FeO, and 3i per cent less K2 0 than the adjacent 

blue beds. Under the microscope thegreen ribbons showed more feldspar and chlorite. He attributes 

these differences to change in sedimentation. 

Zirkel, loc. cit.

Bull. 275 06 2 
18 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

effected similar changes in the Fe203 of the argillaceous sediments. Where a bed of 

quartzite forms the center of such a ribbon quartzose sedimentation must have taken 

place also, and possibly may have been the very condition which proved favorable 

to marine life.

IGNEOUS SLATES.

Most remarkable are the green slates from the English Lake district (Buttermere, 

Tilberthwaite, etc.), which consist of volcanic ash and which have long been known 

in England as excellent roofing material. These have recently been chemically 

and microscopically analyzed and described. a These slates are found to consist 

chiefly of chlorite, calcite, quartz (mostly secondary), and muscovite, but contain 

also anclesitic lapilli, feldspar, garnets, sphene, and anatase. Slate needles and tourmaline are conspicuously absent. The chemical analyses show the following important constituents:

Si02 ................................................................ 50.16-54. 02

A1 2 03 .............................................................. 11. 94-17. 85

CaO ...........................................I........'............ 3. 67- 6. 46

FeO:............:.....................................'............ 5.97- 7.06

COz ...................................................:........... 2. 45- 5. 41

C02 if calculated to CaC03 would give from 5.56 to 12.29 per cent of CaC03 . Specific gravity ranges from 2.775 to 2.788. The percentage of Si02 is low, and that of 

FeO is near that of the "unfading green" slate of Vermont (p. 107).

Still more remarkable are the slates first described by Mr. E. C. Eckel in 1903, 6 

which seem to have been formed directly from igneous rock by shearing. They 

show from 3 to 9 per cent less Si02 than the English ash slates and more than double 

the per cent of MgO of the European and American roofing slates of aqueous-sedimentary origin, analyses of which are given under the heading Chemistry of slate, 

(p. 34). These slates are fully described by Mr. Eckel on page 57.

The state of science on the remaining structural, geological, and chemical features 

of slate will be given under the three headings which follow.

STRUCTURE OF SLATE, c 

BEDDING.

Ordinary planes of bedding may be defined as those which are approximately 

parallel to the surface of the water in which the sediment is formed. If the sediment changes in character, then a horizontal bed of different material results. If, 

deposition being interrupted, annelids creep over the bottom or algse decompose 

there, and the same kind of sedimentation be afterwards resumed, then two horizontal beds of the same material will result, separated by a plane covered with trails 

and impressions. Some bedding planes are due to the changing size'or arrangement 

of the particles, some may be the result of contraction in drying, others are the 

effect of vertical compression. Whatever explanation may be offered'for bedding, 

the bed is the starting point in a slate quarry, for the direction of the bed indicates 

(cleavage, etc., being equal) where the same quality of slate is likely to recur.

In those places where folding has brought the bedding plane to the angle of inclination required by the cleavage, the unaltered sedimentary particles of the slate still 

retain their parallelism to the bedding, i. e., cleavage and bedding are parallel, but 

in all other places cleavage is necessarily more or less transverse to bedding. This,

a Hutchins, Maynard, The ash slates of the Lake District: Geol. Mag. London, 1892. Reade, T. Mellard, and Holland, Philip, The green slates of the Lake District, with a theory of slate structure and 

slaty cleavage: Proc. Liverpool Geol. Soc. (1900-1901), 1901.

bBull. U. S. Geol. Survey No. 225, p. 419; also " On a California roofing slate of igneous origin," in. 

Jour. Geol., vol. 12, p. 15, Jan.-Feb., 1904. '

eThis chapter in its original form appeared in part 3 of Nineteenth Ann. Rept. U. S. Geol. Survey, 

in 1899, but its matter ib here revised and brought up to date.
A. RED SLATE WITH BED OF LIMESTONE AND GREEN SLATE,

Block of reddish slate from quarries U miles south of Hatch Hill, in Whitehall, N. Y., with bed of limestone 

bordered by green and purple slate, making a "ribbon," an inch in vertical thickness, crossing the cleavage 

diagonally, Scale on rule is 4 inches long.

B. BED OF SANDSTONE RESTING ON SLATE.

Horizontal bed of sandstone with calcareous concretions having their longer axes parallel to the bedding, overlying greenish slate with cleavage dipping 20° and joints 45°. An irregular cleavage continues in the 

sandstone. Eddy Hill, Fair Haven, Vt One foot of sledge handle is in sight.
U. S. GEOLOGICAL SURVEY 

SYNCLINE OF BLACK AND GRAY BANDED SLATE AT WEST 

Showing the effect of erosion upon the more calcareous beds. Cleavage dips 3
STRUCTURE. 19

of course, assumes that the slate beds still retain the position they had when slaty 

cleavage was set up in them.

PL II, B, shows a ledge of green roofing slate with a cleavage dipping 20° east 

(strike N. 15° W. ); joints striking N. 30° E. and dipping 45° east. The upper part 

is a quartz sandstone or grit, with calcareous concretions containing Lower Cambrian 

trilobites. The direction of the axial planes of the calcereous bodies and the direction of the line of contact between the slate and sandstone show the direction of the 

bedding to be horizontal.

The "ribbons" of the Pennsylvania quarries shown in figs. 6-9 and Pis. XIII, XIV, 

XVIII-XX, consist of small beds of quartz sandstone or grit in which the particles 

are held together by a calcareous and sometimes a sericitic cement, the original clay 

having gone into Muscovite. Sometimes the slate beds are separated, as in the Vermont and New York quarries (Pis. XXIII, XXIV), by beds of calcareous quartzite 

reaching several inches in thickness and consisting of grains of quartz sand cemented 

together by secondary quartz and calcite and other carbonates. Such beds may be 

but a fraction of an inch in thickness, and consist mainly.of carbonate, as in the 

syncline at West Pawlet, Vt., shown in PI. XXV. In some cases bedding is indicated simply by a variation in the amount of lime in successive beds, as in the 

syncline at West Castleton, Vt., shown in PI. III. Here the solution of the lime by 

the acids of the atmosphere has etched the joint face, as it were, and the more 

calcareous beds thus stand back from the less calcareous ones.

Planes of bedding may be indicated by the position of fossils, as the brachiopods 

and trilobites at the Arvonia quarries in Virginia, or by bifurcating impressions, 

possibly made by seaweeds,- and serpentine ones by annelids, or by the small black 

beds in green slate due to the decomposition of various marine organisms, all of which 

may be seen in the slate quarries of western Vermont and eastern New York.

PI. II, j\, from a photograph of a piece of red slate from the old quarries of the 

Fair Haven .Red Slate Company, in the southeast corner of Whitehall, N. Y., shows 

a bed an inch thick crossing the cleavage diagonally, and therefore spreading out 

' to double that width on the cleavage surface. In the center of the "ribbon" is a 

bed one-fourth inch thick of greenish limestone, and on either side of the ribbon is 

a very thin rim of green slate; the rest of it is purple. Under the microscope the 

composition of these little beds is this: The central green consists of calcite and siderite rhombs; some of the siderite is altered to limonite. There are large quartz grains, 

muscovite scales without parallel orientation, and occasional plagioclase grains. The 

purple consists chiefly of scales of muscovite and chlorite, lying in two directions, at 

right angles to each other, irregular dots of hematite, spine carbonate rhombs, quartz 

grains, and rarely a grain of plagioclase. The thin green strips on the sides contain 

less hematite than the purple, and a large number of the muscovite and chlorite scales 

lie parallel to the bedding and transverse to the cleavage. The red slate itself is like 

the purple, but contains far more hematite and probably less chlorite. The iron 

obscures the other minerals. In this specimen the central bed of quartzose limestone 

is probably due to change of sediment. The varying amount of ferric oxide in the 

purple and green parts of the ribbon as compared with the red of the slate beds may 

be due to a chemical change brought about by the decomposition of organisms on the 

sea floor, as has been shown on pages 15-18 to be probably true of the green and 

purple spots in the same slate. Sometimes the quartzose ribbons of the red slate are 

parallel to the cleavage. This parallelism between bedding and cleavage characterizes some of the Maine and Vermont quarries (see PI.- XII).

The Vermont purple slate frequently has small beds of green slate, with or without a central band, crossing the cleavage; and the village sidewalks in that region 

are sometimes flagged with such purplish green-banded slate. When, owing to folding and pitch, the grain of such slates is not at right angles to the bedding the roofing slates have diagonal green ribbons. These green ribbons, both in the red slate
PLATE IV.

PLICATED AND FAULTED BEDS IN SLATE.

(A) Extreme!}' plicated bed of quartzose limestone or calcareous quartzite in Cambrian slate at Fair Haven, Vermont. Natural size. The slate on'both sides of 

the bed for a half inch is green; beyond that, purplish.

(7:?) Extremely plicated bed of quartzose limestone or calcareous quartzite in Cambrian slate at Fair Haven, Vermont. Eeduced one-half.

(C) Faulted qnart/ite bed in slate at Meadow Slate Company's quarry, Fair Haven, 

Vermont. Normal fault.

(D) Faulted quartzite bed in slate at Eureka quarry, Poultney, Vermont. Reversed 

fault.

(E) Thin section of faulted calcareous bed at Pawlet, Vermont, given in PI. XXII!, 

7.), showing adjustment of cleavage to faulting and production of secondary cleavage. Enlargement, 05 diameters,

(F) Thin section cf plicated and faulted beds of calcite and quartz separating beds 

of purple and green Cambrian slate at Blissville, Castleton, Vermont. Enlargement, 2 diameters. Both calcite and quartz beds are bordered on both sides 

with chlorite scales.

(( ») Dislocated beds of quartzite in purple Cambrian slate at old quarry, about one 

mile south of West Castleton, Vermont. A fragment of a bed of quartzite has 

been turned about into the cleavage foliation and across smaller beds of quartz- 

. ite. By L. M. Prindle.

(//) Plicated and folded quartzite in Cambrian slate at Fair Haven, Vermont. 

Height, 40 feet. Minor plications somewhat enlarged. 

20 .
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. IV

PLICATED AND FAULTED BEDS IN SLATE.
STRUCTURE. 21

of New York and the purple of Vermont, run into planes or rows of spots of various 

sizes, usually more or less oval in outline. When a series of such spots are in line it 

indicates the course of the bedding.

The small beds or ribbons are often plicated, as in figs. Q, R, PL XXIV. PI. IV, 

E, is a microscopic drawing of a thin section of the plicated bed of PI. XXIII, D. 

The bed here consists of calcite and vein quartz, the original calcareous sediment 

having been crystallized and vein quartz deposited. The diagram shows the faulting of the bed, the bending of the cleavage foliation, and the flip cleavage caused by 

the dislocation. The plication of such small beds of quartzose limestone and calcareous quartzite is sometimes extreme, as shown in PI. IV, A, B, H. 'The folding in 

such cases seems to have been preserved only in the hard beds, while in the more 

plastic material of the slate on either side slaty cleavage has obscured or effaced the 

bedding."

Fig. F, PI. IV, represents a plicated bed of quartz and one of calcite separating beds 

of purple and green slate. Under the microscope both quartz and calcite beds are 

bordered with chlorite scales on the outside, and separated by such scales; there is 

also some pyrite along the edges. The cleavage of the slate is at right angles to the 

course of the bedding, but is slightly deflected near the plicated beds. In several 

places the slaty material has been drawn partly into the bed.

A thin section of a small plicated bed of quartzite in the purple slate of the Cedar 

Point slate quarry, in Castleton, Vt., shows the following: The bed consists mainly 

of quartzite, but this contains grains of plagioclase, rhombs of carbonate, probably 

calcite, and scales of muscovite. Toward the slate there are coarse fibers of muscovite. 

The slate merges into the quartzite, sending out long streamers of Sericite, which 

penetrate between the grains of quartz and calcite. The slate contains large scales of 

chlorite within the meshes of sericite, which scales lie at right angles to the cleavage, 

i. e., parallel to the course of the bed. It also contains grains of quartz. The fibrous 

character of the slate is apparent at the border of the quartzite bed. " The significance of such a bed is that sandy material was deposited for a brief interval during 

the deposition of the finer material which produced the slate; there were grains of 

quartz and of feldspar, and probably scales of mica, together with calcareous mud. 

Under the compression and the chemical changes which accompanied it the quartz 

grains were cemented into quartzite, the calcareous mud was crystallized, and the bed 

was plicated and became entangled with the slaty material. Theslaty material itself was 

also somewhat plicated, and a secondary cleavage (slip cleavage) was produced in it.

PI. V, A, from the Ordovician syncline at West Castleton, also illustrates this plication of quartzite beds and the overturning of the folds. Fig. T, on PI. XXIV, shows 

these features on a larger scale.

Exceptionally the quartzite beds seem to have been pushed out of their normal 

parallelism, even without folding or faulting.

A case of brecciation on a-somevvhat large scale is shown in fig. G, PI. IV, taken 

near fig. T, PL XXIV.

Frequently, however, the bed surface is simply a parting whose meanderings must 

be carefully followed in order to distinguish it from fractures of various kinds, as at the 

West Pawlet syncline, PI. XXV; or the bedding may be indicated by the weathering out of calcareous matter from the slate itself, some beds containing more of it 

than others, as at the syncline at West Castleton, PL III. The rock at West 

Castleton is a shaly slate, consisting of alternating light and dark gray bands, 

i. e., beds of muscovite and chlorite scales, grains of quartz, spherules of pyrite, and 

some carbonate, but there is more carbonate in the gray bands than in the black ones, 

which contain more carbonaceous matter. Now and then there is a minute bed coiiaSee C. H. Hitchcock, Second Ann. Kept. Geol. Maine, 1862, p. 285, fig. 45, where a bed of strongly 

plicated limestone lies between implicated beds of slate. See also the classic figure of H. C. Sorby given 

in his paper,- On the origin of slaty cleavage: Edinb. New Phil. Jour.; vol. 55, July, 1853, pp. 139,-140, 

which was reproduced by Tyndall in his Roy. Inst. Lecture, and also by Phillips in his British Assoc. 

Report on cleavage. ' :
22 SLATE DEPOSITS AND INDUSTRY OF "UNITED STATES.

sisting largely of calcite. This explains why the beds are so clearly and yet so delicately brought out on the joint face. The original sediments had varying amounts of 

calcareous material in them. The carbon dioxide brought down from the atmosphere 

by the rain has, as it were, carried away the more calcareous parts, leaving the less 

calcareous ones in relief.

In many of the eastern New York and western Vermont quarries change of color 

alone is an indication of the passage from one bed to another. This change may be 

gradual or abrupt. But color is not an infallible guide, as the red slate sometimes 

passes into the green along the same bed, and there is no reason why the Cambrian 

purple should not likewise pass into the green of the same formation.

In cases where there are no fossil impressions or intervening beds of very different 

material or partings or slight changes in the composition of the slate itself, producing changes of color or different degrees of erodibility, the course of the bedding can 

sometimes be made out in a thin section cut transverse to the cleavage when examined under the microscope. There maybe an occasional arrangement of the particles parallel to the original bedding or an extremely minute bed of other material, 

or lines of different particles may cross the cleavage foliation. Fig. C on PI. VI 

illustrates cases of this kind.

Gosselet« gives some remarkable instances of intense and complex folding of beds 

of slate on a large scale. These great folds are very acute and overturned. In some 

places shafts have been dug through other rocks in order to reach underground portions of synclines and anticlines and quarry the slate. Slate quarrying in the Ardennes thus resembles coal mining in a region of intense folding.

SLATY CLEAVAGE.

The causes and the structure of slaty cleavage have already been discussed under 

the headings "Origin of slate" and "Petrographic characters."

In most slate regions cleavage is not coincident with bedding. Its relations to 

bedding are illustrated in figs. 6-9 and Pis. Ill, V, A; XII-XIV, XVII-XIX, 

XXIII-XXV. The lowest inclination of slaty cleavage in slate districts visited by 

the writer is 5°-10° and occurs in Pennsylvania (see Pis. XIII, XIV). & The lowest 

in the eastern New York-;and western Vermont belt is 20°. Where the cleavage 

so nearly approaches horizontally as it does in the first instance its position is probably due in part to a secondary crustal movement and the occasional curvature of 

both joints and cleavage there also point to such a movement.

Amount of compression in the formation of slaty cleavage. Sorby calculates on a small 

bed of intensely plicated sandy slate, inclosed in ordinary slate, that the amount of 

shortening by plication was about 75 per cent, and reasons that the clayey material 

of the slate itself must therefore have-been compressed to the same extent. This is 

the only way in which the amount of compression actually suffered by a mass of slate 

could be computed. But this calculation does not take into consideration the elongation of the slate in the shearing.

Relation of cleavage dip'to dip of inclosing hard beds. Gosselet brings out the fact 

that where a bed of slate lies between beds of a hard rock like quartzite there is a 

constant geometrical relation between the degree of the cleavage dip of the slate and 

of the dip of the beds of quartzite. The same thing is true, as he shows, in the horizontal relations between the strike of the cleavage, that of the quartzite bed, and the 

direction of movement:

Unless the ancient shore yielded to the pressure from the south the lower beds must have had a 

tendency to rise again&t that obstacle, to slide as a wedge between the obstacles and the overlying 

beds. The slaty material inclosed between the two beds of quartzite is thus pushed upward in a 

direction which is the component between a vertical line (i.'e., vertical to the horizon) and the 

oblique movement of the wall (i. e., along the surface of the slate bed). Cleavage will be developed 

along that component, and we actually find that it dips 40° while the bed dips 27°.

aL'Ardenne, p. 41, fig. 7, Les rapports de Ste. Marie avec les Tr6sfoss6s. Les schistes de Fumay, 

PI. Ill, fig. 1: Bond dans les schistes de Fumay.

bSee also Thirteenth Ann. Kept. U. S. Geol. Survey, PI. XC, showing a slate quarry near Lebanon, 

N. Y., with cleavage dipping 10°. - .
A. QUARTZITE BEDS IN OVERTURNED FOLDS IN SLATE.

Area (3£ by <H feet) of joint face of slate syncline in Plate 111, showing 

4 small beds of quartzite in overturned folds, with axes dipping in the 

direction of the cleavage. At left is a quartz vein in the cleavage. Scale, 4 inches.

Quartz vein of 
SLATY CLEAVAGE. 23

His figure is here repeated (see fig. 1) with the construction added.« 

The gist of this is that if \ve knew the dip of the hard beds on either side 'of a bed 

of slate we could foretell the cleavage dip of the slate. This, however, would be 

applicable only where no secondary disturbance of sufficient force to disturb the 

relations had occurred.

Relations of cleavage to axes of folds. Some pre-Cambrian and Paleozoic schist 

masses have two transverse systems of folding which within limited areas interfere 

with one another; both systems are also intersected by a cleavage with a constant 

strike different from that of each.'' Where only one system of folding occurs the 

strike of the cleavage is not necessarily parallel to that of the bedding. In such a. 

case the cleavage is attributed to a change in the direction of the pressure. c

The cleavage planes of the slate rocks of North Wales are always parallel to the main direction of 

the great anticlinal axes, but are not affected by the small undulations or contortions of these lines. 

The strike of the cleavage in a district is far more constant and regular than the strike of the beds.d

surface

Quartz/te

Slate

FIG. ]. Diagram from Gosselet showing relations of cleavage of slate to dip of inclosing hard beds.

To these facts should be added this that in the case of a pitching fold the cleavage, 

although parallel to the axis of the fold, must necessarily intersect the strikes of the 

sides of the fold. Phillips gives a section .from Sedgwick, e which he calls a local 

exception, in which the cleavage planes, while coinciding in strike with that of the 

anticline which they traverse, incline on either side of it toward its axis. Rogers/ 

describes a case of this fan-like cleavage in an anticline. Such a structure could be 

produced by secondary movement creating an anticline in horizontal beds already 

possessing a vertical cleavage, and in the synclinal part of tlxe fold the fan structure 

would radiate downward.

Sorby figures from Ilfracombe, North Devon// a small, highly plicated bed of coarsegrained light-colored sandy slate traversing a mass of vertically cleft shaly slate. 

The gritty beds show a coarse and imperfect fan-like cleavage which curves slightly 

around the anticlines into the synclines. Here the fan structure seems to be due in 

part, at least, to the deflection of the clea.vage by the coarser material, and there is 

no need of supposing a secondary movement. The fine and more plastic material 

has developed a vertical cleavage which in the coarser has become rudely fan like.

a Gosselet, Les schistes de Fumay, pp. 68, 69, fig. 5. 

bLoretz, Ueber Schieferung, pp. 69-70. 

oloid., pp. 83,84.

dSharpe and Phillips, Contrib. to Geology of North Wales: Quart. Jour. Geol. Soc. London, vol.2 

1846.

e Report on cleavage, p. 374. 

/Geol. Surv. Pa., vol. 2, part 2, p. 903, fig. 715. 

00n the origin of slaty cleavage, p. 138.
24 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Effect of frost on cleavage. As all slate quarry men know, repeated freezing and 

thawing, is disastrous to the cleavability of roofing slates. The material must be 

split fresh from the quarry. In order to ascertain, if possible, what difference in 

microscopic structure freezing and thawing produced this experiment was tried: A 

specimen was obtained from the quarry early in the winter fresh and unfrozen, and 

was kept moist in a moderate temperature until severe weather set in. It was then 

broken into two equal parts, one of which was kept moist indoors, the other exposed 

on the sill of a north window for a week, during which the temperature went down 

to 10° below zero F. This part was then thawed out over a furnace register. Both 

frozen and unfrozen pieces were after some accidental delay sliced and examined 

microscopically. The. whole texture of the frozen slate was found to be perceptibly 

closer than that of the unfrozen. The test would have been more satisfactory had 

the thin sections been made at once. The loss from freezing and thawing is so considerable that means to recover it have been sought, and a process has recently been 

patented for restoring the fissility of slate by the use of liquid air.«

Curvature of the cleavaqe. As far back as 1839 De la Beche called attention to the 

curvature of cleavage planes when approaching a bedding plane. & Baur in 1846 

observed S-like cleavage foliation in Germany, and describes certain slates which 

were so much curved as to be fit for use only on the roofs of towers, but he does not 

explain whether this curvature was parallel or transverse to the bedding. c John 

Phillips, in his British Association Report, ascribed these curvatures to the differing 

density of the beds.^ Barker in some cases attributes it to a gradual change in the 

texture of the beds.« In other cases a secondary motion is called in to explain it./ 

Curvature of the cleavage is not uncommon in the Lehigh and Northampton County 

quarries in Pennsylvania, where it is plainly not dependent upon change of texture 

(see pp: 76, 83). Slates from the beds so affected are also used for roofing towers.

Phillips ff gives a figure, the original authorship of which is not mentioned, representing the cleavage surface of a piece of slate in which gently plicated ribbons are 

shown. A normal fault crosses the piece diagonally, displacing the beds. The 

cleavage surface also shows the "flexuous" lines of a third foliation oblique to the 

cleavage. Finally two small calcite veins cross the primary cleavage, the plicated 

bedding, the plicated secondary cleavage, and also the fault plane. The specimen 

thus bears traces of at least five, if not six, motions.

SLIP CLEAVAGE ("FALSE CLEAVAGE.")

Several writers Sedgwick,^ Phillips, * De la Beche,.? Zirkel,* Loretz^ describe 

a striation or an extremely fine plication that appears on the cleavage surfaces 

of some slates. This is the "bate"or "false cleavage" of quarrymen. Two sys-

"By W. A. McLaughlin, Delta, Pa.

6 Report on the Geology of Cornwall, Devon, and West Somerset, p. 620, fig. 31, London, 1839.

eUeber die Lagerung der Dachschiefer, etc., pp. 392, 393, fig. 9. (For full titles of works cited see 

Bibliography, pp. 138-145.) 

(*0p. cit., p. 384, fig. 23. See also Jukes: Quart. Jour. Geol. Soc., vol. 22, p. 359, 1866. That the angle 

and amount of cleavage change with the density of the rock was shown by Phillips in 1828. See also 

Harkness, op. cit., 1855.

eOn. slaty cleavage, etc.

/Hughes, T. M., quoted in Lyell's Students' Elements, 7th ed., pp. 53, 573, fig. 625, 1871.

ff Report on the Geology of Cornwall, etc., p. 372, fig. 2. '

7i" While my first observations on cleavage planes were made during long bygone years in Cumberland, I had hardly noticed the phenomenon of a second cleavage plane; but on many occasions I 

have subsequently collected, from various parts of England, a considerable and unpublished mass of 

materials in illustration of this second plane. The second cleavage plane is generally inclined at a 

great angle to the first plane. Most beautiful examples of this double structure were seen in 1839 by 

Sir R. Murchison and myself in the quarries of the Ardennes, where the fine, glossy surfaces of the 

slates are frequently marked by the parallel striae of second cleavage, and the economical value of 

the slates is sometimes much deteriorated by the second plane. By a powerful reflected sunlight I 

have frequently been able to trace these strife of a second cleavage on the surface of the Bangor slates 

which have been brought to Cambridge." Synopsis Classif. British Paleozoic Rocks, p. xxxv; London 1855. Also, by same author, Trans. Geol. Soc. 1840, p. 655.

i On a group of slate rocks, 1829, p. 1.

/Geol. Observer, second ed., 1853, p. 588, fig. 239.

fcLehrbuch, 1894, pp. 307, 308.

ZUeber Transversalschiefernng, pp. 263, 264. Spurr, J. E.. describes slates in Minnesota with two 

and three cleavages and bedding:'Am. Jour. Sci., 3d ser., vol. 48, p. 159,1894.
SLIP CLEAVAGE. 25

teins of such lines or plications may occur in the same slate. These are due to a 

secondary and tertiary cleavage, a slip cleavage developed upon the primary slaty 

cleavage. This consists simply of very minute plications which usually result in 

microscopic slippages or faults along which the slate easily breaks. This structure 

may show itself where slaty cleavage alone is visible or in the bedding also (see 

Pis. VI and XI, A). There is a readjustment of the slaty cleavage and the bedding 

foliation with reference to the new pressure instead of a rearrangement of all the 

particles as in slaty cleavage. The presence of "false cleavage" can be detected 

microscopically in a piece a half-inch 'square as certainly as from tests applied to 

pieces of commercial size. The specimen shown in PI. VI came from a quarry which 

proved a failure on account of this structure, but it was not detected until after the 

expenditure of much money. The Arizona slate, described on page 51, shows two slip 

cleavages crossing one another, as well as both bedding and slaty cleavage.

False cleavage has received in recent years a variety of technical names: Close-joint 

cleavage, strain-slip cleavage, Ausweichungs-clivagp, fissility, fracture cleavage. Science is 

not advanced by the mere multiplication of technical terms. The term "fissility," 

being a synonym of Latin derivation for cleavability of Anglo-Saxon origin, is likely 

to be misleading. "Close-joint" and "fracture cleavage" are objectionable because 

they may be applied to jointing. The term "slip cleavage" has brevity and definiteness in its favor, as well as priority over the last two. Slip cleavage is a common feature in schist and is particularly characteristic of theTaconic region in western Massachusetts, Vermont, and eastern New York, where it seems to be due to a continued, 

if not, in some instances at least, a secondary, crustal movement consequent upon the 

first metamorphism. Good illustrations of slip cleavage will be found in the following publications of the United States Geological Survey: Monograph XXIII(1894), 

figs. 44, 45, 46, 53, 56; Thirteenth Annual Report (1894), p. 319, fig. 25; Fourteenth 

Annual Report (1895), p. 537, fig. 57; Sixteenth Annual Report, parti (1896), figs. 89, 

96, 97. A careful study of these microscopic drawings and photographs will show 

that in all cases these minute faults along which more or less slippage has occurred 

are the necessary result of the lateral shortening of the rock mass by plication. This 

plication Avas as much due to a lateral compression as was the major folding of the 

Appalachian system. Slip cleavage should therefore not be confounded \yith two 

intersecting systems of fractures (jointing on a small scale), the strike of neither of 

which would be at right angles to the direction of strain but diagonal to it or to a single system of such fractures which might-be the result of stretching. The same pressure which produced slip cleavage in buried masses of micaceous matter (slate and 

schist) may have produced fractures at the surface; but where both schist and a rigid, 

vitreous rock like quartzite have (in contact) been subjected to the same pressure, in 

the Taconic region, both have usually been folded, the schist in smaller folds, the 

quartzite in larger ones, but slip cleavage has been confined to the schist, as shown 

in fig. 25 of Thirteenth Annual Report, already referred to.

The faults or fractures in slip cleavage belong strictly to the process of "rock 

flowage" and not to that of "rock fracture." AsHeim put it: " By real cleavage is 

always to be understood a cleavability of the material pervading the entire mass and 

visible in each little fragment. . . ."« .

THE GKAnST.

Sharpe's explanation for slates splitting more readily along the "grain" than 

across it is that the mineral particles lie with their flat surfaces parallel to the cleavage 

and their longer axes in the direction of the cleavage dip. A fracture across the 

cleavage and parallel to the dip is parallel to the longer sides of the particles, whereas 

one parallel to the strike of the cleavage is across both longer axes and sides. 6

"Mechanismus der Gebirgsbildung, vol. 2, p. 59. 

6 Op. cit., p. 114.
PLATE VI.

THIN SECTIONS OF ROOFING SLATE WITH SLIP CLEAVAGE ("FALSE

CLEAVAGE ").

(^4) Microscopic view of a thin section of purple Cambrian roofing slate from an old 

quarry three-fourths of a mile south of Fair Haven, Vt., showing slip or "false" 

cleavage originating in minute wrinkles. Section transverse to both cleavages. 

Enlargement, 55 diameters.

(B) Microscopic view of a thin section of green Cambrian roofing slate from the 

Huckleberry Hilj quarry, 2 miles southeast of West Pawlet, Vt., showing slip 

or "false" cleavage. Section transverse to both cleavages. Enlargement, 55 

diameters. The black spots are pyrite. This figure represents a small part of 

the dark band shown in Fig. C.

(C) Microscopic view of the same thin section, entire, showing alternation of fine 

and coarse beds, some of which are pyritiferous, and only in one of which is the 

false cleavage pronounced. Enlargement, 4 diameters. 

26
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. VI

SLIP CLEAVAGE AFTER SLATY CLEAVAGE.
THE GEAIN. 27

Renard states that the scales of chlorite lie perpendicular to the cleavage, i. e., 

about in the direction of the grain." Jannetaz's experiments in reproducing grain 

have already been referred to (p. 8),& arid his experiments showing that the direction of the grain is that of the greatest elasticity are given on page 45.

Daubree, in one of his experiments, produces cleavage in the direction of pressure 

and motion which is the relation of grain to pressure. c It seems, therefore, that 

besides the cause assigned to grain by Sharpe there is the formation of exceedingly 

obscure vertical divisional planes in the direction of the pressure and the crystallization of secondary minerals along these planes. rf The variation qf the strike of the 

grain from the direction of the cleavage dip at Rimogne (Ardennes) is from 1 to 20°. 

At Fumay, in the same region, that variation is 6°. « At Rimogne the strike of the 

grain bisects the acute angle formed by two sets of joints. In some of the Ardennes 

slates plates of hematite lie in the "grain" and indicate its direction./ Watrin 

states that as the longer axes of distorted octahedra of magnetite all lie in the direction of the grain in some of the Ardennes slates, their combined magnetism gives a 

polarity to the slate in the direction of the grain and enables the quarry men to ascertain its direction by the magnetic needle. A like arrangement of lenses and dis-' 

torted cubes of pyrite characterizes the slate at Northfield, Vt. (seep. 91), and of 

folia of inuscovite in certain .slates near New Canton, Va.

The same feature is occasionally seen in schists. Thus scales of biotite in a schist 

in Beekman and Pawling, Dutch ess County, N. Y., and actinolite crystals in schist in 

Hubbardton, Vt., have their axes in any direction, but their flat sides transverse to 

bedding and cleavage. . The "grain" shows itself in a more or less obscure striation of the cleavage surface 

in a direction nearly parallel to the cleavage dip and sometimes to the dip joints.

In several of the Maine quarries the grain is abnormally nearly horizontal, while 

the.bedding is steeply inclined. As the strike of the grain is about the same as the 

direction of the pressure which produced the cleavage the logical inference seems to 

be that the folds at these quarries, if reconstructed, would be found to have a nearly 

vertical pitch, and this would have to be attributed to a secondary movement. The 

workmen at these quarries speak of the slate as being "on end." The slate blocks 

are broken along the grain to reduce them to workable proportions, and as the grain 

is the direction of weakness roofing slates are always cut with their long sides parallel 

to the grain, but in some localities there is hardly any grain.

In making thin sections for the microscopic studies of the writer for this bulletin 

fresh specimens were obtained from the quarries with the grain direction marked on 

them by the foremen. Thin sections were then prepared transverse to the cleavage 

and parallel to the grain and also at right angles to it. Where any difference is 

noticeable between the two sets it consists in the presence of large flakes of chlorite 

with their flat sides lying in the grain direction or in the longer axes of lenses and 

crystals lying also in that direction, and, where the sections are very thin indeed, in 

many small scales of muscovite being similarly oriented. Where the matrix is very 

micaceous and not obscured by carbonate or other minerals, sections parallel to the 

grain polarize more brilliantly than those across it.

JOINTS.

Nature of joints. Joints are simply ruptures of continuity due to various strains. 

Exceptionally, later movements may cause slippage along joint planes and result in 

polishing the joint faces (slickensides). The usual character of joint planes, how-

« Renard, op. cit., vol. 3, p. 235.

bSee Jannetaz, M&noire sur les clivages dea rouhes (schistosit6, longrain), et sur leur reproduction: 

Bull. Soc. Gepl. France, 3d Ser., vol. 12, p. 211, 1883-84.

eDaubre'e, Etudes synthgtiques, etc., p. 422. (For full titles to works cited see Bibliography, p. 138.) 

rfRosenbusch figures biotite scales transverse to cleavage: Elemente der Gesteinslehre. fig 73 

(p. 432), p. 437.

Renard, op. cit., vol. 3, p. 3. 

e, op. cit., p. 336.
28 . SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

ever, points to a sudden rupture of large masses of rock affected in all its parts by 

one and the same mechanical expression of energy.«

Sedgwick termed the three commoner kinds of joints occurring in slate regions 

strike joints (joints parallel to the strike), dip joints (joints parallel to the direction of 

the dip), and diagonal joints (joints diagonal to strike and dip), terms which ought 

to be ever kept in use. ' 

To these should be added horizontal joints (" bottom " or "flat joints ") which in 

some slate regions are of much economic importance.

Rogers called attention to the parallelism of joints to dikes in Pennsylvania, a 

parallelism previously observed in England. c

This is so true in western Vermont that the proximity and course of a dike can be 

foretold by the prevalence of certain diagonal joints. Joints are so-nef inies open and 

filled with vein matter and more frequently, when scarcely parted, have their sides 

coated with crystals of chlorite, calcite, gypsum, pyrite,. barite, anatase, etc.

Carved joints. In exceptional instances joint planes undulate like bedding planes 

across both bedding and cleavage (see PI. XVIII). This is attributed, in the Pennsylvania slate region, at least, to a secondary crustal movement. The flat joint near 

the surface of the Peach Bottom slate belt, with its mass of crushed slate, is probably 

also of secondary origin.^

At Foulk Jones & Sons' quarry at Slate Hill, 2 miles northeast of Delta, Pa., such 

a curvature in jointing occurs as to have produced a conical structure'20 to 30 feet 

high and 15 feet in diameter at the base. In view of the peculiar curvature of fractures across the cleavage of slate resulting from the use of explosives such conical 

forms may also be the result of shock.

Plicated joints. In the quarry just referred to a vertical dip joint filled with quartz 

three-twentieths of an inch thick is in plications from one-fourth to one-half inch 

wide. These plications may have been due to the zigzag course of the original fracture. Some finely plicated jointing has, however, a different history. Near the 

extreme north end of the western Vermont slate belt (more exactly, one-fourth mile 

north of the northwest end of Hincum Pond, in Sudbury, Rutland County) there is 

a gray slate with a micaceous matrix in which bedding is clearly indicated by the 

alternation of small carbonaceous beds with noncarbonaceous ones, and also by 

minute beds with quartz grains up to one-tenth mm. in diameter alternating with 

beds without any. All of these beds are crossed at an angle of 40° by slaty cleavage, 

and both bedding and cleavage are crossed by joints which in places are plicated, 

measuring an inch from crest to crest, but they are plicated in two directions. In 

some parts of the rock these joints are only one-fourth inch apart and are parted from 

one-tenth to one-fifth inch, the openings being filled with banded veins of quartz 

and fibrous calcite. The solution of these veins at the surface has left a series of 

plicated gaping joints crossing both bedding foliation and slaty cleavage. Under the 

microscope these joint veins are seen to consist on either side of a border of vein 

quartz, followed similarly by two bands of calcite fibers, then two bands of quartz, 

followed by a central band of gently curving calcite fibers 'with a central parting 

containing a little quartz at intervals. The slate between these joints also shows a 

slip cleavage crossing bedding and slaty cleavage in the general direction of the joints, 

but not plicated. The history of this slate thus appears to be: (1) The folding of a 

mass of marine sediments of alternating composition and the formation of slaty 

cleavage at an angle of 40° to the folded beds; (2) compression along the strike

oLoretz: Uber Schieferung, pp. 98-100.

*>A Synopsis of Classification of the British Paleozoic Hocks, by Adam Sedgwick, London. 1885, 

p.,36.

cH. D. Rogers, Geology of Pennsylvania, vol. 2, pt. 2, p. 912, fig. 718, joints in red shale parallel to 

dike; also fig. 719, joints in argillaceous sandstone; 1858.

<*See on curved joint planes in the Peach Bottom district: Edward B. Mathews, Maryland Geol. 

Survey, vol. 2, 1898, p. 223, PI. XXII, rig..l.
FAULTS AND SHEAE ZONES. 29

resulting in slip cleavage; (3) the elongation of the mass alo.ng the strike producing 

open joints; (4) compression, both in the direction of the dip of the slaty cleavage 

and in a direction at right angles to that, thus plicating the joints in a twofold way; 

(5) the infiltration of Si02 and CaC03 in alternation into the open joints, resulting 

in banded veins; (6) the solution of the CaC03 by carbonic and organic acids and 

trig crumbling away of the Si02 -in consequence of exposure by erosion. Such a 

slate shows what complex structures successive crustal movements may produce, in 

ordinary sediments, as well as how fatal such movements may be to the commercial 

value of slate. These joints were probably first formed by elongation, and thus are 

different from ordinary joints.

FAULTS.

Faults of no great magnitude are .common in slate regions. The fault plane is 

frequently a cleavage or a joint plane. From the shear involved in slaty cleavage 

reversed faults'are more frequent in slate than normal ones. a Minor faults from the 

western Vermont region are shown in PI. IV, C, D. One of these is a normal fault, 

the part overlying the fault plane having slidden down; the other is a reverse fault, 

the similar part having been forced up. A microscopic section across the reverse fault 

shows the sharp bending of the beds at the fault plane, and the deposition of a thickness of one-sixteenth of an inch of vein matter in bands along that plane. This 

matter consists of chlorite, calcite, and quartz.

Some of the slate flagstones in the village of Granville, N. Y., the exact source of 

which could not be ascertained, are full of small faults, which come out finely in the 

rain, and show how much secondary compression there must have been at that 

locality.

Faulting may occur within a slate mass in two directions at right angels to one 

another. & ...

The dropping of a block of some superjacent valueless strata into a mass of commercial slate between two fault planes is a possibility that should not be overlooked 

in slate quarrying.

SHEAR ZONES (HOGBACKS).

These terms, as well as Knickungsebenen, Querknicke, zones of shearing, apply to 

one and the same feature. Rosenbusch, c Brogger/z Reusch, e and Turner/ describe 

it from Norway, Saxony, Alsatia, and California. Its occurrence in the slate belt of 

eastern New York and western Vermont is fully described on pages 31, 44. These 

writers explain it as an angular plication, or a series of such plications, due to shearing pressure on somewhat rigid material. In places the pressure was great enough 

to produce a slight faulting on either side of the deflected portion or zone. Very 

rarely a cleavage foliation is produced within the zones. Mr. Turner's term abbreviated to shear zone affords a convenient designation, with the understanding, however, that in this sense it applies only to sedimentary rocks.

The term "hogback" is used by coal miners to describe a sharp rise in the floor 

of a coal seam. The propriety of its application in-slate quarries is not so obvious. 

It is used there to designate peculiar bends or fractures, Avhich consist of two angular 

bends in opposite directions and near each other, traversing a mass of slate. These

oSee Herbert, E. J., Reversed faults in bedded slates. Geol. Mag. N. S. Dec. II, vol. 4, p. 441,1877.

&See a ease of this in the Hudson slates, of Rensselaer County, Thirteenth Ann. Kept. U. S. Geol. 

Survey, 1894, PI. CL, p. 320, fig. 26« Also, in this connection, A faulted slate slab, by J. J. H. Teall: 

Geol. Mag., Dec. Ill, vol. 1, PI. I, London, 1884.

cDie Steiger Schiefer, 1877, p. 95.

dBrogger, Die silurischen Etagen 2 u. 3 im Kristiania Gebiet auf Eker, p. 216, fig. 31, Kristiania, 1882.

cReusch, H., Die fossilieHfuhrenden krystallinischen Schiefer von Bergen in Norwegen; German 

translation by R. Baldauf, pp. 52, 53, fig. 35; and p. 38, fig. 23, Leipzig, 1883.

/Turner, H. W., Further contributions to the geology of the Sierra Nevada: Seventeenth Ann. Rept. 

U. S. Geol. Survey, p. 662, fig. 22,1896.

Bull. 275 06 3
PLATE VII.

SHEAR-ZONES ("HOGBACKS.").

(A*) Diagram of thin section of "sea-green" Cambrian slate from Williams and 

Edwards' quarry in Wells, Vermont. Enlarged 2 diameters. Section across the 

cleavage and an incipient "hogback," showing the two bends in opposite directions and developed in but two places into fractures. Secondary fractures at 

15° to cleavage cross the zon,e. A calcareous strip, possibly a trace of bedding, 

crosses the cleavage at 45°. All the fractures filled with calcite.

(J5), (C), (D}' Microscopic drawings from above thin section, enlarged 40 diameters, showing in reversed position the outlines of several of the vertical and diagonal fractures within and at edge of zone.

(£') Diagram from specimen of "hogback," from E. E. Lloyd's sea-green slate 

quarry, Poultney, Vermont.

(F) Slate from within a fully developed "hogback," showing outline of fracture. 

Same location as E.

(G) Same location as above slate, showing two "hogbacks."

(H) Diagram from specimen of Cambrian slate from Eddy Hill, Fair Haven, Vermont, showing development of "hogbacks." Secondary quartz occurs along the 

fractures. Keduced one-half.

(I) Shear-zone in Silurian schist. 1£ miles east of Rupert, Vermont.

(K) Diagram from specimen from top of the Pattern, in Pawlet, Vermont.

(X) Diagram from thin section of Cambrian slate from Eddy Hill, Fair Haven, Vermont, enlarged two diameters, showingoneof the main fractures of a "hogback" 

with diagonal fractures which are filled with quartz, and beside it slip-cleavage. 

' 30
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. VII

SHEAR ZON'ES.
CLEAVAGE BANDS. 31

flexures may and often do merge into fractures, and the slate between the two planes 

of fracture is broken into small fragments. The two bends or fractures may be 

anywhere from one-sixteenth of an inch to 4 feet apart. At one of the old Middle 

Granville, N. Y., quarries a slate surface shows within a space of 4 inches four " hogbacks " varying from one-sixteenth to one-fourth inch in width. Another has six in 

a space of 6 inches. At one of the Arvonia, Va., quarries a mass 5 feet thick is traversed by numerous shear zones. As they generally traverse the cleavage diagonally, 

the blocks of slate adjacent to them come out in triangular form, and thus occasion 

much waste. The strike of some of the shear zones in western Vermont is parallel 

to that of several of the dikes, and both may have been formed under the'same stress. 

Figs. E, G, I, on PL XXIII, and figs. L, U, and V, on PL XXIV, show the relations of 

"hogbacks" at several quarries. Fig. A, on PL VII, shows the microscopic structure 

of the "hogback" of Fig.V, PL XXIV. The section was made where the bends had 

not as yet developed into complete fractures. The entire width of the "hogback" is 

nine-tenths of an inch. Between the two sides is a system of cracks crossing the 

cleavage at an angle of 15°; another system crosses the cleavage at 25° and extends 

beyond the side. This may be the original bedding. Both of these systems of cracks 

are filled with secondary calcite. Another set, likewise filled with it, crosses the 

cleavage here and there about at right angles, but in zigzag (see figs. B, C, D, PL 

VII). These, farther on in the quarry, become continuous fractures. The secondary fractures within the zone probably result eventually in breaking up the slate, as 

it occurs usually in fragments within a fully developed shear zone. The observations 

at this quarry were: Strike of bed N.±, dip 30° E.; strike of cleavage N.±, dip 42° 

to 45° E.; strike of shear zone N. 35° to 40° E., dip 65° NW. In this case the strike 

of the shear zone corresponds to that of the diagonal joints of the region and of 

a number of the dikes. Figs. E to L, on PL VII, illustrate the development of a 

shear zone still further. In fig. L the diagonal fractures are filled with quartz, and 

a slip cleavage (false cleavage) also occurs. Other observations of "hogbacks" are 

given on the quarry tables, pp. 102, 103.

CLEAVAGE BAISTDS.

While characteristic of slate regions, cleavage bands do not occur or have not yet 

been reported in commercial slate. They would be quite as objectionable as are 

shear zones.

PL XI, B, is from a photograph taken at a locality in Rupert, Vt., found by Mr. 

L. M. Prindle. Cleavage banding is of frequent occurence in the Vermont and New 

York slate belt and in the schist mass east of it, although not always as well shown 

as'at this point. It resembles the shear-zone structure just described, but presents 

further stages. In PL XI, B, the rock is divided into alternate bands of hard, uncleft 

quartzose shale and of bands of very finely cleft shale. The bedding zigzags across 

both bands. The material of both bands was originally identical. The present differences are the result of a difference in the amount of motion i. e., of slip cleavage 

along alternating strips of rock and of the consequent difference in resistance to erosion. There is also a difference in color, some infiltration of limonite having taken 

place along the more highly cleft bands.

A similar structure in purple slate is shown in fig. 2, but here the bands themr 

selves have slipped. a Small beds of green slate indicate the course of the stratification and show the amount of slippage suffered by the bands.

In the Poultney River, about a mile east of East Poultney, certain hard, bright 

green and purple slates near the schist mass show the cleavage banding well. In 

breaking up such a rock the denser uncleft parts come out in slab-like blocks, the 

larger surfaces of which lie transverse to the bedding.

"See also Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1,1896, pp. 561-564, figs. 88-91.
32 SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

Doctor JBecker« explains this structure by the alternate interference and coincidence of waves of vibration produced by shock. When the waves of vibration coming from opposite directions coincide, cleavage fractures i. e., planes of slip cleavage will be numerous.

Professor Van Hise regards such structures simply as the result of the concentration or sparseness of slip cleavage. &

In the ledge figured in PL XI, B, there are between 360 and 530 such planes to 

the inch in the cleft bands. The hard bands in the same ledge shoAV as many fractures, but they are ..discontinuous or merely incipient. Both hard and soft bands 

consist of stratified shale made up of quartz fragments, muscovite and chlorite 

scales, etc., but in the soft ones the pressure has in places brought about sufficient 

alignment of the muscovite to produce aggregate polarization, .not in the direction of 

the slip cleavage but in that of the bedding. There has also been a formation of 

sericite along the slip-cleavage planes, and thus the rock is becoming a schist.

In a piece of sericite schist from the mass east of Rupert the rock consists of alternating strips with and without slip cleavage, those without being wider. The slipcleavage planes meander about and run into each other, though having a general 

parallelism. The plication is much more intense and irregular in the cleft strips 

and also appears to be more sericitic there.

FIG. 2.2-Cleavage banding showing slippage of bands; brook in Gorham, Poultney, Vt.

In all these cases the fractures are the result of the plications.

It seems probable that the shear zones represent but a variation of the process 

involved in the cleavage bands. The force bent and crushed the slate in the bands 

instead of producing very numerous planes of slip cleavage within them.

VEINS.

Quartz veins or "flints," as the quarrymen call them, are a striking feature of 

slate as well as a cause of perplexity in quarrying. They appear at the most unexpected points, frequently ramify very irregularly, and disappear as suddenly. PI. V, 

B, from the old quarries at Jamesville, N. Y., shows a good type of such veins of

a Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. Am., Vol. IV, Jan., 1895. 

On page 16 he says: " Thus there seems sufficient reason to believe that a pressure very rapidly 

applied, producing primary ruptures attended by shock, will be immediately followed by secondary 

ruptures in the same direction as the others at intervals dependent upon the wave length of the 

impulse. In much the same way a high explosive shatters a rock far more than black powder. A 

phenomenon of which no explanation has been offered in this paper is that of thick slates and of 

those flags which are to be considered as very thick slates. These, though cleavable in a certain 

thinness, are not capable of further splitting. Such rocks indicate a flow which is not uniformly distributed through the mass, but on the contrary passes through maxima at intervals corresponding to 

the thickness of a slate or flag. It is possible that at the inception of strain such masses were in a 

state of tremor so intense that the interference of waves determined surfaces along which flow began. 

These surfaces would be weakened by the flow, and further strain would be distributed among them 

rather than over the intervening solid sheets. Effects of a similar kind are produced on a pile of 

sheets of paper, such as ' library slips,' resting on an inclined cloth-covered table which is jarred by 

rapid blows."

& Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1, Principles of North Am. pre-Cambriau Geology, 

1896, pp. 662-664.
VEINS NODULES DIKES. 33

segregation. In some cases there is a rough parallelism in them. They may occur 

partly in the bedding and partly in the cleavage, or may cross both bedding and 

cleavage, their general dip being roughly at right angles to that of the cleavage, and 

their strike probably not very different from that of cleavage and bedding. Such 

veins may be several hundred feet long and several feet thick, or may form.' lenses 

several feet in length. At Benson, Vt., and Ijamsville, .Md., small joint veins of 

fibrous calcite form series of gashes "en Echelon." The fibers or prisms lie transverse to the course of the vein. Paubree regards such veins as due to stretching." 

That the material of the veins described, of whatever sort, must have come from the 

adjacent slate itself is evident, and also that it was deposited in solution.

In the cases adduced there is some relation between the foliations of the slate and 

the course of the veins, but veins do occur which appear to be quite lawless, crossing 

in every direction, anastomosing, and intersecting one another, and sometimes 

inclosing fragments of the adjacent slate arid constituting the cement of a brecciated 

area. That the veins are the result of various secondary stresses, producing openings 

of more or less irregularity, is manifest. Where the stress ceased to operate the vein 

tapers out; where the stresses were complex the veins are so.

The material which filled the openings thus made is chiefly quartz, frequently 

"milky " in color, and often,finely crystallized in small cavities. With the quartz 

are often associated chlorite, biotite, calcite, and possibly dolomite. The chlorite 

sometimes occurs in hexagonal scales, in vermicular aggregations, or in tortuous 

columns. Some of the smaller veins are banded, presenting alternations of quartz 

and fibrous calcite, or of quartz and rhombs of calcite, as in the case of those in plicated 

joints described on page 28. Sometimes the quartz contains cavities measuring from 

0.002 to 0.005 millimeter in diameter, partly filled with fluid. Galenite in small 

particles was found in veins at the Jamesville, N. Y., quarries.

NODTTLES.

In some of the western Vermont quarries very hard nodules, a few inches in diameter and of lenticular form, occur along the bedding planes. They consist of a 

quartzite nucleus containing much calcite and large scales of inuscovite surrounded 

by slaty layers with calcite, quartz, and inuscovite scales. Pyrite is disseminated 

throughout both nucleus and outer zone. Such nodules are evidently of sedimentary 

origin.

Miiller describes concretions of pyrite and quartz in the slate quarries of Thiiringen 

and observes that the slate in their vicinity is of superior quality, containing less 

pyrite than it does at a distance from the nodules. The explanation is that all the 

pyrite has congregated in the large nodules instead of being more widely disseminated 

in small crystals. The nodules are thus of economic advantage. &

DIKES.

Volcanic dikes are not infrequent in slate deposits. Leaving out the very exceptional one described by Mr. Eckel on page 57, dikes are almost as detrimental as 

veins. They are usually more regular than veins of segregation and, as stated under 

"Joints," are apt to give warning of their proximity by an abundance of joints parallel to them. For several feet on each side of the dike these joints become very 

close and are sometimes crossed by an equally numerous set of horizontal ones, thus 

breaking up the slate into small blocks. At Arvonia, Va., it appears that the quality 

of the slate a little beyond the dike is better than it is farther away, so that a dike 

traversing a slate deposit is not necessarily without economic value.

«Daubr6e, Geologic exp6rimentale, p. 144, fig. 166.

bMiiller, F. E., Die normalen Schiefer des Hennbergs,p.218. (See Bibliography, pp. 138-145.)
34 SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

GEOLOGICAL RELATIONS OF SLATE.

In view of the origin and structure of the marine sedimentary slates it is evident 

that such slate deposits are to be looked for along the borders of or in proximity to 

granitic land-masses where the dominant pressure has come mainly from one direction and where it'has not been sufficiently intense to result in extreme nietamorphism. The Maine slate belt lies not far south of a granitic area. Two of the 

Vermont slate belts lie but a few miles west of the Green Mountain granitic axis 

and two others east of it. In eastern Pennsylvania, West Virginia, and Maryland 

the slates lie not far northwest of a granitic area, and the Virginia slate lies not 

many miles southeast of one.

From the facts brought out in considering the structure of slate it is also-evident 

that slate belts generally bear evidence of more than one crustal movement. Along 

the entire Appalachians, where at least two major mountain-making movements 

occurred the Ordovician and the Carboniferous and where the slates are mostly 

of Cambrian or Ordovician age, they must needs show the effect of at least one 

disturbance of the slaty cleavage.

Where, as in western Vermont and eastern New York, slates of two geological 

periods occur in proximity, and where one is even superposed on another, the question of their mutual relations becomes of economic and scientific importance. a 

There was here a crustal movement at the close of Lower Cambrian time which 

resulted in the cleavage of the Lower Cambrian sediments and their emergence. 

They were afterwards, in places submerged and received Ordovician deposits, which, 

at the close of Ordovician time, were folded and also received a slaty cleavage. 

Moreover, the folds of both periods are overturned.

CHEMISTRY OF SLATE. 

CHEMICAL COMPOSITION.

Comparatively few complete chemical analyses of roofing'slates have found their 

way into scientific literature. Several of the rare elements are usually omitted. FeO 

and Fe2 03 are not distinguished, nor CaO and C02 , so that several of the percentages 

are more or less misleading. Fourteen selected analyses, all but one from European 

sources, are here given for reference:

Selected analyses of roofing.slates.

Si02 (silica) ........

Ti02 (titanium dioxide) ...........

Alo03 (alumina)....

Fe203 (ferric oxide)

FeO (ferrous oxide)

CaO (lime). ........

MgO (magnesia) . . .

K20 (potassa) ......

Na20 (soda) ...:....

COa (carbon dioxide) ..............

C (carbon)

I.

58 30

.23

21.89

7.05

'2.57

39

1.09

2.45

1.18

II.

61 57

1.31

19.22

6.63

1.20 ' m

2.00

3.63

.93

III.

65 63

.94

20.20

2.72

.85

19

1,54

3.81

.71

IV.

61 43

.73

19.10

4.81

3.12

<*1

 2.29

3.24

.83

V.

67.56

......

12.23

2.87

6.99

.27

3.03

1.76

1.28

3.11

VI.

59 35

1.00

13.56

1.10

4.75

5 20

3.60

1.77

1.48

4 45

VII.

55 880

1.270

21. 849

9.033

.155

1.495

3.640

.460

1.794

VIII.

55. 06

22. 55

1.97

5.96

2.92

3.82

2.17

IX.

60.68

.59

21. 20

5. 68

.46

1.71

.88

. 3.64

2.09

.07

X.

60.17

1.15

18.89

6.17

.95

1 7^

04

1.85

2.76

' 1.39

1.04

XI.

56.71

.70

14.43

1.98

3.65

3.83

.04

3.47

2. 61

2.59

3.71

.77

XII.

56 9?

.90

16. 41

.53

3.52

? 94

03

3.14

3.27

1.47

91,68

1.54

XIII.

57.23

.89

20.43

1.33

5.64

1.54

.02

2.49

2.39

3.97

Trar.fi

XIV.

63.06

.73

18.03

2.24

4.07

 .81

2.21

3.07

1.51

"See Am. Jour. Sci., 4th ser., vol. 17, 1904, p. 185, and Bull. U. S. Geol. Survey, No. 242, 1905.
CHEMICAL COMPOSITION.

Selected analyses of roofing slates Continued.

35

MnO (manganous

P205 (phosphoric

S03 (sulphuric oxide) ..............

HoO. ................

FeSo. ...............

CoO (cobaltous oxide) ..............

Total......... 

Specific gravity b. . .

I.

4.61

99.76 

2.81

II.

3.25

99.96

III.

3.17

99. 76

IV.

3.52

99. 38

V.

0.10 

1.00

100.20

......

VI.

0.31 

3.41

99.98

- 

VII.

0.586

.022 

3.385 

051

99. 801

C2.89

VIII.

4.35

100. 10

2.78

IX.

0. 16

2.88

100. 04

X.

0.20 

.11

a 3. 70

100. 17

......

XL

0.06 

.05

.12 

02.74 

2.64

100. 10

XII.

0.16 

.09

.10 

a2. 21 

3 07

99.88

XIII.

0.05 

.54

.03 

3.94

100. 09

XIV.

0.30 

.06

.09 

a 3. 62

99.80 

2.83

a Combined water.

&The specific gravity of the Festiniog black slates of Wales was determined as 2.751 at the physical 

laboratorvof Williams College in 1899. Since the publication of Doctor Hillebrand's note on New 

York and Vermont slates, Keade and Holland find as a mean of 13 Welsh slates CroOt , 0.003; VoOf,, 

0.032; ZrOo, 0.022. See Proc. Liv. Geol. Soc. (1900-1901), p. 101, and table.

cSpec. grav. from Merriman.

I. Gray roofing slate, best quality, Delabole, Camelford, Cornwall; two analyses by J. A. Phillips, 

London, Edinb. and Dublin Phil. Mag., 4th ser., No. 27, pp. 95-96, Feb., 1871.

II. Purple roofing slate, Fumay, Ardennes, northwest France; by A. Ren'ard, Recherches sur la 

composition et la'structure des phyllades ardennais; Bull. Mus. Roy. d'Hist. Nat. de Belgique.Vol. I, 

p. 239,1882.

III. Green roofing slate beds from purple, Fumay, Ardennes, as above.

IV. Blue-gray roofing slate, La Richolle quarry, Rimogne, Ardennes, northwest France; by Klement, pub. by A. Renard, op. cit. supra, p. 233.

V. Roofing slate (probably black, Devonian), Westphalia; by H. von Dechen; Roth. Allgem. und 

Chem. Geol., II, pp. 586, 587,1884. (107.)

VI. Roofing slate (color not given, Devonian), Fran ken berg, near Goslar, in Prussia; by A. von 

Groddeck; Jahrb. pr. Geol. Landesanst, 1885-86; quoted in Roth. Allgemeine Chem. Geol., II, pp. 

586, 587.

VII. Black roofing slates ("Peach Bottom") from J. Humphreys Co.'s quarry, half a mile east of 

Delta, York County, Pennsylvania; by Andrew S. McCreath, in 2d Geol. Surv.'Pa., Report of Progress, 1877, Vol. CCC, pp. 269, 270,1880.

VIII. Bluish roofing slate of Carboniferous age, Mohradorf, near Wigstadl, Austrian Silesia; by 

Nikolic, in Tschermaks Min. Mitth., 1871, p. 207; quoted by Roth, op. cit. supra, pp. 588-589.

I.. Blue slate, Glyn quarries, Llanberis, Wales; analysis made at Museum of Practical Geology, 

London, for George Maw, Geol. Mag., London, 1868, vol. 5, p. 123.

.. Reddish rooting slate (best), Alexandra quarry, Moel Tryfaen, North Wales. Anal, by Reacle 

and Holland, Proc. Liverpool Geol. Soc. (1899-1900), 1900. (Anal. No. 1.)

.I. Very dark banded roofing slate, Llansantfraid, Glyn Ceiriog, near Llangollen, North Wales. 

Anal, by Reacle and Holland, Proc. Liverpool Geol. Soc. (1899-1900), 1900. (Anal. No. 2.)

.II. Very dark roofing slate, Moel Ferna, North Wales. Anal, by Reade and Holland, Proc. 

Liverpool Geol. Soc. (1899-1900), 1900. (Anal. No. 3.)

.III. Green roofing slate from quarry between Camlyn and Cemmaes, Anglesey. Anal, by Read* 

and Holland, Proc. Liverpool Geol. Soc. (1899-1900), 1900. (Anal. No. 4.)

.IV. Green roofing slate, Velenhelli, Wales. Anal, by Reade and Holland, Proc. Liverpool Geol. 

Soc. (1900-1901), p. 112. (Anal. No. 6.)

.The fifteen complete analyses of New York, Vermont, and Pennsylvania roofing 

slates made by the chemists of the United States Geological Survey and given under 

the descriptions of those slates are summarized in the following table. Wherever 

several analyses were made of one kind of slate the average is given. The rarer 

elements and water below 110° C. have here been thrown together.
36 SLATE DEPOSITS AND 1NDUSTKY OF UNITED STATES.

Summarized analyses of roofing slates from New York, Vermont, and Pennsylvania.

Ti02 (titanium dioxide) ....

CaO (lime).................

H2 0 (water above 110° C.) . .

Sundry and water below 

110°C.... .................

Total .................

Sea 

green.

(^

63.33 

.73 

14.86 

1.12 

4.93 

1.20 

2.98 

4.06 

1.22 

1.41 

.11 

3.37 

Trace

.69

100. 01

2.776

Unfading 

green. 

(2) Vt.

59.37 

1.00 

18.51 

1.18 

6.69 

.49 

2.36 

3.78 

1.71 

.30 

.14 

4.01

.51

100. 05

2.795

Bright 

green.

A

67.89 

.49 

'11.03 

1.47 

3.81 

1.43 

4.57 

2.82 

.77 

1.89 

.04 

3.21

.66

100. 08

2. 717

Variegated (EurekaMl)

60.24 

.92 

18.46 

2.56 

5.18 

.33 

2.33 

4.09 

1.57 

.08 

.16 

3.81

.39

100. 12

2.805

Purple.

(2) 

Vt.

61.29 

.77 

16.24 

4.63 

2.62 

.60 

2.99 

5.27 

1.38 

.54 

.04 

3.16

.56

100. 09

2.806

Red. (4) 

N.Y.

63.89 

.52 

11.80 

4.56 

1.33 

2.25 

4.57 

3.95 

.50 

3.15 

.02 

2.82

.77

100. 13

2.796

Black.

(1) 

Vt.

59.70 

.79 

16.98 

.52 

4.88 

1.27 

3.23 

3.77 

1.35 

1.40 

1.18 

3.82 

.46

.70

100.05

2.774

Black.

Lenigh 

Co., Pa.

56.38 

.78 

15.27 

61.67 

63.23 

4.23 

2.84 

3.51 

1.30 

3.67 

1.72 

4.09 

.59

1.11

100.39

2.783

General average.

61.51 

.75 

15.39 

2.21 

4.08 

1.47 

3.23 

3.90 

1.22 

1.55 

.42 

3.53

.67

2.783

a Figures in parentheses indicate the number of analyses averaged. 6 Approximate.

If analysis K2 of red slate on page 73 be included with the four others, the per cent 

of Fe2 03 in the red slates would range from 3.48 to 7.10 per cent and would average 

5.08. Comparing, then, the amount of Fe2 03 in the colored slates we shall find that 

it steadily increases from the variegated to the purple and to the red, as the microscopic sections show.

On the other hand, there is a decrease of FeO in passing from the unfading green 

to the variegated, sea-green, -bright green, purple, and red. This decrease corresponds to and is probably consequent on the decrease of chlorite, a hydrous silicate 

of MgO and FeO.

There is more lime (CaO) and carbon dioxide (C02 ) in the Lehigh slate than in 

any of the others. There is less CaO and C02 in the unfading green and in the variegated analyzed than in any of the slates.

There is less pyrite (FeS2 ) in the red and most in the black.

Doctor Hlllebrand's discussion of the analyses of New York and Vermont slate and 

of the causes of the discoloration of the "seagreen" is given on pages 39-42. While 

that investigation was undertaken primarily to determine the cause of discoloration 

in one kind of slate, microscopic analyses show that discoloration in the black slates 

of Pennsylvania is also greatest where carbonate is most abundant. It is safe to 

assume that ferrous carbonate plays an important part in the discoloration, not only 

in these slates, but in all slates. The red slates of New York State contain much 

carbonate, but do not discolor, the carbonate probably not being ferrous.

These 29 analyses are sufficient to give within certain limits the essential chemical 

composition of commercial slate of aqueous sedimentary origin.

Range of composition of slate.

Per cent. 

.......... 55-67

.......... 11-23

.......... 0.52-7

.......... . 46-9

Potash...................... 1.76-5.27

Silica.........

Alumina......

Ferric oxide... 

Ferrous oxide.

Per cent. 

Soda ....................... 0.50-3.97

Magnesia ................... . 88-4. 57

Lime ....................... . 33-5. 20

Water above 110° C.......... 2. 82-4. 09
CHEMICAL CHANGES IN WEATHERING. 37

For comparison a composite analysis of 51 American Paleozoic shales made by 

H. N. Stokes of the United States Geological Survey« is added, the unessential elements being also omitted.

Composite analysis of 51 Paleozoic shales.

Per cent. 

Silica........................... 60.15

Alumina........................ 16. 45

Ferric oxide..................... 4.04

Ferrous oxide................... 2. 90

Potash.......................... 3.60

Per cent. 

Soda............................ 1. 01

Magnesia ....................... 2.32

Lime ........................... 1.41

Waterabove 110° C.............. 3.82

As these figures come between the extremes of the slate analyses, and as numerous 

clays and some schists do also, & it is evident that a chemical analysis of a slate is not 

sufficiently characteristic to prove that it is not that of a shale, a clay, or a schist. 

Indeed, but for the low maxima for soda (2.17, 2.59, and 3.97 per cent) in 3 out 

of the 29 analyses, and the high minima for combined water, many granites, syenites, porphyries, diorites, and basalts could pass chemically as roofing slate.

That there should be such chemical similarity between slate, shale, and clay results 

primarily from the fact that the slate here considered is simply metamorphosed shale 

and that shale is compressed clay. Whatever miueralogical changes metamorphism 

brought about, the same elements persisted. In some mica-slates the grains of quartz, 

feldspar, zircon, and other minerals are the identical ones of the original clay sediment. The economic bearing of the chemical analyses of slates will be brought out 

under "Methods of testing slate," in the section on economic geology.

CHEMICAL, CHANGES LIST WEATHERING.

The changes in substance and color which slates undergo in weathering are striking. 

The black slate of the Peach Bottom district (Pennsylvania and Maryland) and of 

Buckingham County, Va., passes into a bright reddish-brown clay, while the black 

slate of Lehigh and Northampton counties, Pa., passes into a yellowish ocher and 

finally into a white clay; and the black clay slate of West Virginia passes through 

similar changes. These alterations will be referred to in describing each district. 

These changes in weathering should not, however, be confounded with the relatively 

slight discoloration which slates on the roof undergo during a fraction of a century 

or even several centuries. The former changes probably required tens of centuries.

Dr. George P. Merrill has investigated the weathering of Peach Bottom slates c 

and shown that in the passage from the black slate to the red clay the following 

losses occur: 57.57 per cent of the Si02 ; 8.78'per cent of the iron oxides; all of the 

lime; 28.16 per cent of the MgO; 77.95 per cent of the K2 0, and 99.64 per cent of the 

Na2CT; while all of the A1 2 03 remains, as well as the water. A'piece of Peach Bottom 

slate in the first stages of weathering, examined microscopically by the writer, shows 

the magnetite crystals and lenses passing into hematite and the andalusite crystals 

becoming limonitic from the oxidation of its ferruginous inclusions.

In connection with some chemical analyses of slates from Lehigh County, Pa., Doctor Hillebrand made some partial analyses of a piece of black slate from a ledge near 

the Berks and Lehigh County line, H mile north-northwest of Rothrocksville, on the 

road from that place to Hynemansville, in which the stages of weathering were finely 

shown. Analysis A. is of the black but slightly altered part of the slate; B is of the 

greatly altered white part of the same specimen. The alteration has proceeded first 

along the bedding planes, which there undulate in a vertical direction and from

nBull. U. S. Geol. Survey No. 168, p. 17 (B), 1900.

foFor clays see H. Ries, Technology of the clay industry, Sixteenth Ann. Rept. U. S. Geol. Survey, 

pt. -I, pp. 523-576, 1895, pp. 554-575; also Prof. Paper No. 11,1903. For schist, see Bull. 148, Analyses of 

Rocks, etc., Clarke and Hillebrand, p. 221 (L) and p. 99 (K), 1897.

c Treatise on rocks, rock weathering, and soils, N. Y., 1897.
38 SLATE DEPOSITS AND 1NDUSTKY OF UNITED STATES.

these along the cleavage, which dips only about 12°. These analyses, when compared with analysis A of fresh black slate from the Slatington quarries (see p. 84), 

14 miles farther north-northeast, show a probable loss of 4 per cent of CaO during 

a still earlier stage of weathering.

Analyses of weathered slates from Pennsylvania.

Alo03 (alumina) .......................................................... 1

A.

Per cent. 

65.56

17.06

4.19

1.31

.20

.26

3.81

.68 .

.G2-.04FeS.,

100. 18

B.

Per cent. 

68.81

16.44

3.14

1 fift

.20

.27

4.32

C 00

100.28

"Including carbonaceous matter.

A thin section of another specimen from, the same ledge, made across both dark 

and light parts, shows under the microscope:

1. Absence of carbonate in both, this having been previously dissolved out.

2. Many spherules of pyrite in the dark part, some of which, altered to limonite, 

still remain in the white part.

3. Quartz grains, equally numerous in both.

4. Muscovite (sericite) much more conspicuous in the light than in the dark part.

To the above data should be added two analyses of white "shale clays," by Doctor 

Hillebrand, which are regarded as the residual product of the weathering of the 

black slate. . Analyses of shale clays from Lehigh County, Pa..

Si0.2 (siiicii). .......................................................................

TiOo (titanium dioxide) ...........................................................

C.

Per cent. 

64.50

"21.67

1.83

1.09

.18

.20

4.26

c5.65

.85

100. 23

D.

Per cent.

75.77

&15.30

.81

.20

2.85

4.69

.36

99.98

a Including Po05 . MVith trace of Fe2O3 and (perhaps) P2O5 . Ignition loss.

C. White "shale clay" quarried for fire clay a half mile south of Fogelsville, Upper Macungie, 

Lehigh County, Pa.

D. White "shale clay" associated with limonite one-fourth mile northwest of Guth Station, 

South Whitehall, Lehigh County, Pa.
ROOFING SLATES OF NEW YOKE AND VERMONT. 39

The significance of these analyses and microscopic examinations is that this slate

in weathering first loses its carbonate, then its carbonaceous matter, then its pyrite,

and has its feldspathic material, if any, changed to kaolin, while its muscovite

. remains partly unaltered and its quartz largely so. The bleaching process in the

Slates of Lehigh and Northampton counties, Pa., apparently proceeds by these steps:

1. Removal of carbonates of lime and magnesia, accompanied by oxidation of the 

ferrous carbonate and deposition of limonite. 

. 2. Oxidation and removal of carbon; also oxidation and removal of pyrite.

3. Removal of almost all limonite.

In general, therefore, while slate in weathering returns to a clay, this clay, in some 

cases, differs from that in which the slate must have originated by the absence of 

carbon and the small amounts of silica, potash, and soda, and in others by the 

absence or extremely low percentages of carbon, lime, and iron.

. NOTES ON THE CHEMICAL COMPOSITION OF THE HOOFING SLATES 

OF EASTERN NEW YORK AND WESTERN VERMONT."

BY W. F. HlLLEBRAND.

Before the analyses of any of the slates were undertaken, it was well understood 

that the chief question of economic importance to be solved was the cause of fading 

observed in some, but not all, of the green slates after longer or shorter exposure to 

atmospheric influences. Attention was therefore primarily directed toward its elucidation, and the evidence accumulated will be herein set forth. Naturally, in the 

course of the work, opportunity was afforded for drawing conclusions as to the combination of certain chemical elements concerning which the microscope could furnish 

no satisfaction. . A few remarks in this connection may therefore well be in place.

It soon became clear that there was no apparent connection between the percentage of iron sulphide (FeS2 ) in any green slate and its liability to fade; that, furthermore, the visible sulphide, was of an extremely resistant type, and often after years 

of exposure retained its original luster undinuned; that it was probably true pyrite. 

The possibility remained that there might be finely divided FeS2 in the much more 

readily decomposable form of marcasite. Microscopical evidence made it clear that 

if present at all it was in amount altogether too trifling to produce the fading effect 

often observed.

It was found that the ferrous iron, other than that in the pyrite, was almost entirely 

in a very soluble condition. Even when carbonates were absent, a large proportion 

of it dissolved in moderately dilute acids after very few minutes boiling. But the ferrous iron thus shown to exist in the silicate minerals could not be accepted as the 

cause of fading, because it exists in all slates; and some of the most ferruginous, as 

314/, are "unfading." Microscopical evidence is also opposed to oxidation of ferrous 

silicates as the cause of fading.

There remained as its sole probable cause the carbonate found in nearly all of the 

slates. Its chemical behavior toward acids proves beyond question that this carbonate can be in no observed case calcite. Were it even in small part so, an instantaneous disengagement of gas would be apparent on adding acids, whereas their action 

is tardy and becomes pronounced only on application of heat. To those familiar with 

the behavior of different carbonates this evidence alone is decisive.

The carbonate is, therefore, an isomorphous mixture, and indeed the C02 found, as 

compared with the CaO, shows at once a great excess of the former. That CaC03 is 

one of the chief isomorphous constituents appears from the fact that absence of C02 

uniformly accompanies the lowest trifling percentages of CaO, and that increase of 

CaO carries with it an approximately corresponding one of C02 . The C02 is usually 

in molecular excess of the CaO, and sometimes of the MnO and FeO as well (compare

"Reprinted from Nineteenth Ann. Kept.,part 3. The analyses aiid microscopic descriptions he;c 

referred to will be found under New York and Vermont slates.
40 SLATE DEPOSITS AND INDUSTEY OF UNITED STATES.

analyses L, M, Q, R);« hence MgC03 is indicated as the next most important isomorphous element of the carbonate. Analyses M, Q, and R, taken together in the order 

named, afford positive evidence that MnC03 is present either as an independent mineral or as a component of the complex carbonate, for as C02 decreases from the green 

center through the purple rim to the outer red, so does MnO, and at such a rate that 

with no carbon dioxide MnO would likewise disappear. In general, too, those slates 

high in C02 show a relatively high MnO content. Analysis 2Qld (p. 71) shows also 

that manganese carbonate is found as a vein formation in the slates, and Professor 

Dale has found with the microscope some indications of its presence in the lenses of 

the red slate, at least. In order to be able to credit the fading to the carbonate, it is 

necessary still to show that it contains FeC03 as a constituent, for, while the manganese might produce discolorization by oxidizing-, its amount is slight, and the color 

due to its oxidation products alone would be black rather than brown. Because of 

the above-mentioned ready solubility of the ferrous silicate or silicates in acids, 

chemical proof of the presence of ferrous carbonate is not obtainable, but from our 

general.knowledge of its relations to those of magnesium and manganese it is in the 

highest degree probable that it does exist either as a distinct mineral or as a component of the dolomitic carbonate, and, in fact, it would be somewhat surprising if, with 

magnesium and manganese present as carbonates, iron should be absent. The needed 

proof, however, is supplied by the microscope, as detailed in the text of Professor 

Dale's report (p. 105).

If it is objected that some green slates comparatively rich in carbonate fade less than 

others which are much poorer, it may be reasonably urged that the relative proportions of the several carbonates are doubtless subject to change in different slates, as 

in the natural order of things they must be, since the original composition of the 

elate-forming materials must have differed in each case. This being so, it follows that 

a mere test as to the relative amount of C02 can furnish no guide in advance as to the 

fading qualities of a slate, as might be the case were the composition of the mixed 

carbonates always constant.

While pyrite is never a visible constituent of the red slates, there seems no other 

way to account for the sulphur found, since careful tests failed by far to show an 

equivalent amount of S03 . Hence in the analysis pyrite is reported in all cases.

The'phenomenal percentage of barium in 201e (K), as compared with other slates, 

has been of service as showing that, in this case at least, it is mainly, if not altogether, a constituent of some silicate, or silicates, and not of barite only, which latter 

mineral, according to Professor Dale, has been-observed on joint planes and in crystalline concretions, for the sulphur is totally inadequate to form barite in this case.

The condition of the nickel and cobalt has not been definitely ascertained. From 

the analysis of the manganese carbonate 2QId it might appear as if they were constituents of the carbonate in the slates, but it is not impossible that they may be in 

combination with arsenic, or arsenic and sulphur, as arsenides and sulpharsenides, 

even if the presence of arsenic has not been revealed, no test having been made for 

that element. In all the tests made for nickel and cobalt, both elements were found, 

and in one case, probably a maximum, their combined percentage as oxides was 

0.025.

Heating of the slates in tubes closed at one end gave without exception alkaline 

vapors and whitish sublimates, which latter reacted for S03 , Cl, and strongly for NH3 . 

In eight cases the amount of ammonia thus given off in both conditions was ascertained by nesslerization and found to range from 0.0075 per cent in the case of 760a 

(H) to 0.04 per cent for 305d (P). This last is a black slate carrying nearly 0.5 per 

cent of carbon, but the ammonia obtained was not sensibly higher than in some of 

the green slates, as 645a (F) with 0.035 per cent NH'3 , or 314 of '95 (I), with 0.03 

per cent NH3 .

a See pp. 16, 73.
ROOFING SLATES OF NEW TOEK AND VERMONT. 41

Neither -the condition nor the source of the nitrogen thus revealed can be stated 

with any degree of positiveness. It is found in the interior of fresh and unbroken 

masses, and hence is not to be regarded as derived from infiltration of nitrogenous 

matter since the opening of the quarries. Whatever may be its manner of combination, nitrogen is coming to be recognized as a primary constituent of many rocks and 

minerals. It seems probable that in the present instance its presence is to be attributed to. that of the organic matter which was doubtless not wanting when the materials now composing the slates were originally laid down.

Chromium "and vanadium are probably minor constituents of all the slates. They 

were not looked for in making the foregoing analyses, but in connection with some 

more recent work relating to the distribution of vanadium in the rocks of the earth's 

crust," the-sea-green slate 230a (A) and the red slates 201e (K) and 397e (L) were 

examined. In 201e there was found 0.017 per centof vanadium as V2 05 and 0.007 per 

cent of chromium as O2 03 . In a mixture of equal parts of the two red slates there 

was found of V205 0.008 per cent and O203 about as in the first case.

To the novice a cursory examination of the tabulated analyses might seem to indicate a greater diversity in essential composition than comports with the facts. Thus, 

there seems to be, perhaps, little relation in composition, except qualitatively, between 

the red slates 358ci! (J) and 397'e (L). Yet, after deduction of all CaO as phosphate 

and carbonate, of all MnO as carbonate, and of enough MgO as carbonate to satisfy 

the remainder of the C02 , also of the trace of pyrite, amounting to 16.58 per cent in 

all, and calculating the remaining 83.80 per cent to 100, there result the figures 

given in the first column below, for comparison with which analyses 358d (J) and 

201e (K) of slates naturally altogether or nearly free from carbonates are partially 

reproduced:

Si02 . ...............

TiOo...... ..........

A1203 . ..............

FeaOs ..............

BaO. ...............

MgO ...............

397e(L).

67.40

.57

13.84

4.15

1.70

.07

3.34.

358d (J) .

67.61

.56

13.20

5.36

1.20

.04

3.20

201e(K).

67.55

.58

12.59

5.61

1.24

.31

3.27

K2 0 ...............

NajO ..............

HoO below 110° C..

Total ........

397e (L).

4.50

.62

.44

3.37

100. 00

358d (J).

4.45

.67

.45

0 Q7

99.71

201e(K).

4.13

.61

.40

3.03

99.32

When it is borne in mind that a portion of the FeO in 397e should doubtless be 

credited to the carbonate, whereby an equivalent of MgO would be released for the 

silicate, it will be seen that the agreement as to FeO in the above comparison would 

be much closer, and the three slates may be said to have almost identically the same 

composition when compared in this manner.

Applying a.similar correction to the three analyses of 397a R, Q, and M, in the 

order named, we obtain the-following corrected figures for the green spot, the purple 

rim, and the outer red, which bring into special prominence their relatively high 

silica contents as compared with all the other slates analyzed:

SiOa .... ............

Tio2 - ...............

A12 03 - ...... ........

Fe203 ..............

FeO. ...............

BaO................

MgO...............

397a (R).

7fi (VI

.60

10.90

1.27

. £&

.06

2.75

397a (Q).

73.64

.58

11.66

2.04

JLt Ov>

.05

3.25

397a(M).

71.59

.53

10.95

4.33

.05

3.77

K2 0 ...............

NaoO ..............

tt,0 below 110° C . .

HoO above 110° C..

Total........

397a(R).

4.15

.26

.29

2.45

100. 00

397a (Q).

4.22

.26

.32

2.61

99 99

397a(M).

3.87

.22

.30

2.78

a Distribution and quantitative occurrence of vanadium and molybdenum of rocks in the United 

States, by W. F. Hillebrand: Am. Jour. Sci., 4th ser., vol. 6,1898, p. 209.
42 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

The foregoing calculations for 397 e and 397a (L and M) show at the same time the 

general dolomitic character of the carbonate, as appears from the following table, 

wherein the percentages of carbonates are given as calculated on the above-predicated 

assumptions:

CaC03 . ..............

M^CO. ..............

397e (L).

8.95

6.93

.48

16.38

Molecular. ratio.

'1.03

| 1.00

397a(R).

7.93

5.36

.57

13.86

Molecular ratio.

1.15

| 1.00

397a(Q).

7.11

4.77

.47

12.35

Molecular ratio.

1.16

1 1.00

397a(M).

6.14

4.22

.38

10.74

Molecular ratio.

1.15

| 1.00

If, as is quite probable, a small fraction of the CaO should be charged to the silicates, the true dolomitic ratio would be more closely approached. It is of no consequence for the above calculations that an undeterminable portion of FeC03 should 

appear as MgC03 . The ratios are not thereby affected.

ECONOMIC GEOLOGY OF SLATE. 

DIFFICULTIES IN SLATE QUARRYING.

The difficulties in slate quarrying are many. Assuming that the commercial value 

and the grade of fissility of the slate itself have been fully.determined by scientific 

and practical tests, the opening of a quarry offers serious problems. These concern 

the thickness of the deposit and of the weathered "top," the character of the jointing, the presence of faults, shear-zones, and dikes. There are also the practical 

matters of drainage, of the location of dumps, of transportation facilities, and of fuel. 

The cost of slate at some quarries is increased by the necessity of removing the 

dumps of former workings, which, for lack of capital or of good judgment, were 

placed on good slate. Sometimes the .only way to remove these old dumps is to 

throw them into the quarry on one side and hoist them up at the other. In places 

where the beds are steeply inclined or vertical as the quarry deepens one of the 

walls falls in and the removal of this material entails great expense. The occasional 

employment of a consulting engineer would obviate mechanical accidents of this 

nature by the mathematical determination of the points at which supporting walls 

should be left. There are also uncertainties of a stratigraphic character which the 

use of the core drill would eliminate from many a slate-quarrying venture.

The percentage of waste is generally high in slate quarrying. Watrin (see Bibliography, p. 145), referring to the Ardennes region, gives the total waste at from 70 to 

75 per cent in weight, of which 20 to 25 occur in the quarry and 50 in splitting. 

Merrill estimates the waste in the Peach Bottom region as 88 per cent. In the Maine 

quarries it is also large, owing to the frequent interbedding of the slate with quartzite.

BEDDING AND CLEAVAGE-HOW DISTINGUISHED.

Wherever the slates are traversed by "ribbons," or marked changes in color 

occur and persist through a thickness of several feet, or wherever strips of quartzite 

or limestone occur at intervals and continue longitudinally for several hundred feet, 

quarrymen of any experience know that they have to do with beds, and that the 

quality of the slate of any one bed may be expected to continue along that bed, 

unless some change should occur in the character of the cleavage. The quality of 

the slate is primarily dependent upon the character of the sediment. This changes 

less frequently in a horizontal than in a vertical direction. The changes in the character of the materials brought into the sea and deposited at one time throughout a
N. N.I t;

ECONOMIC GEOLOGY OF SLATE. 43

moderately large area were fewer than between those brought in at different times at 

any one spot. Cleavage being the result of a later compression may traverse sediments 

of slightly different composition with Itttle change in direction, but will be very much 

affected by great changes in the material or the grain of the sediment (see PI. II, B}. 

The prime factor is, then, the bed, the second one the cleavage.

In the soxithern part of the slate belt of western Vermont, where beds of quartzite 

or limestone are few and inconspicuous and the difference of color is slight, the distinction between bedding and cleavage is not easily made. Quarrymen and prospectors sometimes regard them as identical when they differ. Where the strikes of 

the bedding and cleavage are divergent, if that of the cleavage be mistaken for that 

of the bedding, a new opening may easily be made at

the wrong point and the looked-for bed may be missed 

(see fig. 3). In such places the readiest means of distinguishing cleavage and bedding are:

(1) The fossil impressions (trails or algse, sometimes 

called "wavers") are always on a bed surface.

(2) Minute plicated beds of calcite and quartz indi- | $ 

cate bedding. s ^

(3) A microscopic section transverse to the cleavage, 

if other means fail, may indicate the amount of divergence between the bedding and the cleavage (see fig. 

C, PL VI). 

In some places, however, bedding and cleavage are ^1 " ' 

identical in both strike and dip.

"FLINTS" THEIR NATUHE A1STD CAUSE. pBeds of quartzite, often calcareous, micaceous, pyri- / c/eaxsympof, 

tiferous (see PI. V, A), should never be confounded symbo/r^ « 

with veins of quartz (see PL V, B). In some regions / 

they, are both indiscriminately designated by the FlG 3 ._Diagraui illustrating the 

quarry men as "flints!" The former are sediments effect of divergence in strike 

mainly of quartz sand, and, although varying consid- of slftte bedding and cleavage, 

erably in thickness, are generally more persistent than

the veins which, as has been already shown, are chemical infiltrations into fractures 

produced at a much later tirne in consequence of various stresses. Ordinarily the 

quartzite has a more granular and less glassy surface than the vein quartz. A microscopic section under polarized light will almost always show the difference when 

ordinary means fail. The importance of not confounding the quartzite beds and 

the quartz veins lies in this that while the quartzite beds indicate the direction and 

thickness of adjacent beds of slate, and thus prove helpful, the quartz veins constitute perhaps the most fortuitous and the most pernicious element in slate quarrying. 

The strains which the slate masses have suffered have been so various that it is almost 

impossible to foretell the probable presence, course, extent, or thickness 1 of a quartz 

vein. A few things should, however, be noted. While the fractures.which occasioned the veins are to be looked upon somewhat as accidental, they are the result of 

stresses affecting large areas or of the complex interactions of pressure in a few definite directions. The course of a vein which is tapering out should be taken with a 

compass, and another should be somewhat expected in the same line or in directions 

parallel to it, or at nearly a right angle to it.

Bull. 275 06 4
44 SLATE DEPOSITS AND 1NDUSTBY OF UNITED STATES.

RELATIONS OF JOINTS, DIKES, A15TD SHEAR ZONES.

From the similarity of the courses of the diagonal joints and of many of the dikes 

and "hogbacks" in western Vermont and eastern New York there seems to be a 

close relationship between them.

They may all have been produced by the same stress at the same time, in some 

cases the strain resulting in a hogback, in others in a diagonal joint; and these joints, 

when very deep, may have given rise to dikes. The practical application of this is 

that the possibility of such a relationship should lead the quarryman, whenever he 

finds diagonal jointing, to suspect the proximity of hogbacks and dikes with a similar 

course, and so with either a hogback or a diagonal dike, and this suspicion may 

sometimes save expenditure of time and labor.

USE OF A GEOLOGICAL MAP AND COMPASS IN PROSPECTING FOR SLATE.

Although the following paragraphs refer particularly to the western Vermont and 

eastern New York slate belt and necessarily so because this is the only slate region 

which has been mapped in this way the general principles and method Deprocedure 

are applicable to -any slate region where the beds consist of close and more or less 

overturned folds.

The quarry maps, Pis. XX and XXI, are designed to be of practical utility. The 

coloring shows where the Cambrian green and purple and the Ordovician red slates 

may be looked for or not looked for. The dovetailing of the Cambrian and Ordovician areas represents to a certain extent structural relations and not mere 

"accidents" of erosion. Thus, the Jamesville Cambrian belt is closely related to the 

Cambrian belt which lies east of South Granville.

On the quarry maps the course of bedding and cleavage has been shown at several 

quarries by special symbols. The scale of these maps is sufficiently large to admit 

the entry of many more .quarries and symbols. By using a small geological compass 

to determine the strike of any bed of good slate at any of the located quarries, and 

transferring it to the quarry map by means of a protractor, the probable direction of 

the recurrence of the bed can be ascertained, and so with joints, hogbacks, or dikes. 

Such a compass should be provided with sights, spirit levels, movable ring to set off 

magnetic variation, and have a clinometer attachment to indicate angle of dip.

Where, as at West Pawlet (see section VII, PI. XXII, and figs. A, B, C, PI. XXIII), 

the slate is closely folded, a succession of repetitions of the same series of beds may 

be looked for in an east-and-west direction at varying intervals. The possibility of 

the pitching of the axis of a fold in a northerly or southerly direction should be 

looked out for. In such cases older or newer beds are traversed in following the 

direction of the pitch. Where an Ordovician belt abruptly cuts off a Cambrian 

one on the north or south, the Cambrian one must ordinarily be supposed to plunge 

under the Ordovician one.

From the relations already explained, quarrymen need not be surprised, here and. 

there, as the excavation proceeds, to come upon the Ordovician red and bright green 

slates at the bottom of a sea-green or unfading green quarry, or to come upon these 

Cambrian slates at the bottom of a red slate quarry (sections I, II, V, VI, VII, PI. 

XXII).

 Quarrymen are very skilled in detecting the presence of good slate from the peculiar 

appearance of the weathered edge surface, and that skill appears to have been their 

only guide in prospecting in this region. It would be well if this skill were reenforced by the use of the following method in exploration:

First. Make reference to a geological map for the areas in which the various slates 

may occur.

Second. Determine on quarry map or general map the good slate beds already 

exploited.
METHODS OF TESTING SLATE. 45

Third. Make compass determination of the strike of such beds.

Fourth. Explore along that strike.

Fifth. Explore at right angles to that strike to see if the series is repeated by folding. (Note order of horizons in table facing p. 94.)

Sixth. Trench at promising localities across the strike to expose as large a series 

as possible.

Seventh. When surface indications are favorable, make an opening large enough 

to determine angle of dip of both bed and cleavage and to obtain specimens sufficient 

for tests given on pages 45-49.

Eighth. Bore with diamond or steel-shot drill at 45° to cleavage dip, so that the core 

will split up into elliptical pieces sufficiently larger than diameter of core to be conveniently tested.

Ninth. Measure thickness at right angles to bedding planes on the core.

The map of the Slatington, Pa., slate district (PI. XVI), upon which the quarries 

are carefully located and the strike and pitch shown, is also designed for practical use.

METHODS OF TESTING SLATE.

Methods of testing the elasticity, absorption, fissility, and resistance of roofing 

slates have been in use for many years, and many more or less complete chemical 

analyses of slate have been published. In recent years, however, more exact methods 

of reaching these results have been devised. All suchmethods have here been brought 

together. If parts of one specimen, fairly representing the average quality of the 

product of any quarry or prospect, or if parts of each of a series of specimens, fairly 

representing all the different varieties and qualities there obtained, were to be subjected respectively to the tests described, such a slate or slates may be said to have 

been for all economic purposes exhaustively investigated. Several of these tests are 

of so simple a character as to be very easily applied. This list of methods is largely 

compiled from Bottinger, Fresenius, Hutchins, Jannetaz, Merriman, Reverdin and 

de la Harpe, Sorby, Umlauft, and J. F. Williams/' Although they all offer valuable 

suggestions, the most useful papers on the subject are those of Fresenius, Umlauft, 

and Merriman.

Sonorousness. One of the first and most time-honored tests of roofing slate is to 

suspend a good-sized piece of the usual thinness and tap it with some hard object. 

If it possesses the molecular structure of a slate it will yield a sernivitreous ring. A 

clay slate will be less sonorous than a mica slate, but mica slates with a large percentage of chlorite and possibly a little talc will be deficient in sonorousness. It is 

because of this property that when at the quarries refuse slates are thrown upon the 

dumps the sound produced is not unlike that made by the smashing of a large 

quantity of crockery.

Cleavabiliiy. This test should be applied by an experienced workman. The block 

should be freshly quarried, unfrozen, and moist. The chisel should be very thin and 

about 2 inches wide. The cost of slate is closely related to the degree of its cleavability.

Cross fracture ("sculping"). This is to determine the character of the "grain." 

This test should also be applied by an experienced hand to a large block several 

inches thick, with a stout chisel and a long-handled heavy mallet. Jannetax'' published a method for determining with scientific precision the direction of the grain 

in slate when it is but obscurely shown on the cleavage surface. The slate is sawn 

in a direction parallel to its cleavage and one of the sawn surfaces is made exceedingly smooth and covered with an even and very thin coat of grease. The point of 

a red-hot platinum wire is applied to the slate opposite the center of the greased

«Full titles are given in the economic Bibliography, pp. 138-145.

b Relations entre la propagation de la chaleur et I'61asticit6 sonore dans les roches, 1877, p. 417.
46 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

surface. The greased area reached by the heat will, in cooling, leave an oval outline, the long axis of which will show the direction of the grain, the heat having 

traveled more rapidly within the slate in the direction of the grain than in any other. 

He also made a disk of slate 5 inches in diameter of ordinary thickness, with a 

central perforation. This disk was fastened by the extremities of the diameter 

parallel to the grain and afterwards by that at right angles to the grain, and was 

made to vibrate by tapping the side of the perforation. The sound produced when 

the disk was fastened by the diameter at right angles to the grain was louder than 

when fastened by that parallel to it. In other words, the direction of the grain was 

that in which elasticity and vibration were greatest.

Character of cleavage surface. The cleavage surface should be examined with an 

ordinary magnifying glass. A superior slate should scale along the cleavage surface 

into very thin chips with translucent edges. If the grain is pronounced it will appear 

in fine transverse lines. If false cleavage, which is fatal, be present, it can usually 

be detected on the cleavage surface. Ribbons, which are sometimes lines of weakness, should be noted. .There is great-difference in the smoothness of the surface in 

slates of different regions. Ordinarily the constituent minerals ought not to be 

visible. Minute lenses or crystals are not necessarily detrimental, but they retain 

dust and thus afford a foothold for mosses and other cryptogams, which gather 

moisture and thus aid the decomposition of the slate.

Presence of lime. This can be determined by the application of cold dilute hydrochloric acid to the edges of a freshly-quarried slate. Eapid effervescence implies 

presence of carbonate of lime; slow, that of a lesser quantity of it or of dolomite . 

carbonate of lime and magnesia. Reverdin and de la Harpe call attention to the 

fact that good slate may have a high percentage of calcium carbonate, while others 

free from it may be poor.« But it is nevertheless true that slates containing such a 

percentage of lime must, other things being equal, in time be acted upon more 

powerfully by the acids of the atmosphere than those without any or with an infinitesimal one.

Color and discoloration. The color of the freshly-quarried slate should be noted 

and compared with that of pieces exposed for several years to the weather, either 

on a roof or on the quarry dumps, or with that at the top of the quarry close to the 

gravel, although this last comparison may not always be perfectly conclusive. The

value of slates is somewhat affected by the extent of their discoloration.

Presence of clay. This should be tested by breathing upon a fresh and clean piece 

of slate and observing whether there is any argillaceous odor. The very best slate 

will not emit any such odor.

Presence of marcasite. A slate containing lenses or crystals of a pale-yellowish 

metallic mineral which, on exposure, decomposes, forming a yellowish white film 

and rusty spots, is poor;'' but pyrite in small quantity is not necessarily detrimental, 

for it does not decompose readily. .It is present in nearly all slates, and particularly 

in black slate.

Presence of magnetite. Since slate has come to be used so largely in electric appliances the determination of the relative abundance of this mineral is of practical 

importance. A cubic inch of the slate should be weighed, broken up, and reduced 

to a fine powder in an agate mortar or else between sheets of. zinc or copper, and 

then spread on a flat surface and thoroughly traversed by a strong magnet and the 

extracted magnetite weighed. The percentage, by weight, per cubic inch of slate can 

thus be determined.

a See Bibliography, p. 145.

bSee Alexis A. Julien, On the variation of decomposition in the iron pyrites; its cause, and its 

relation to density: Annals N. Y. Acad. Sci., vol. 3, pp. 365-403; vol. 4, pp. 125-221 and Pis. VIII, IX; 

1886,1887. Also, A. P. Brown, On marcasite and pyrite; a comparative study of the chemical behavior 

of pyrite and marcasite: Proc. Am. Phil. Soc., vol. 33, pp. 225-243,1894. Also, H. N. Stokes, On pyrite 

and marcasite: Bull. U. S. Geol. Survey No. 186,1901.
METHODS OF TESTING SLATE. 47

Strength. Merrirnan's apparatus for testing the strength of slate is described in 

his paper:«

The pieces were supported in a horizontal position upon wooden knife-edges 22 inches apart, and 

these loads were placed upon another knife-edge halfway between the supports. This load was 

applied b>; means of sand running put of an orifice in a box at the uniform rate of 70 pounds per 

minute, and by the help of an electric attachment the flow was stopped at the instant of rupture.

He computes the modulus of rupture by the use of this formula, S=<>r- , in which

w is the load in pounds which causes the rupture, 1 the distance between the supports in inches, b the breadth and d the thickness of the plate in inches. S is the 

number of pounds per square inch. According to these tests the modulus of rupture 

in the best slates should range from 7,000 to 10,000 pounds.

J. F. Williams b made tests of compression and elastic limit on purple, red, and 

green slates from Rutland and Washington counties. His results give a modulus of 

rupture ranging from 7,310 to 10,800 pounds per square inch. Campbell & Donald c 

give 20,000 pounds as the crushing weight for 1 cubic inch of slate. Wilkinson, in 

his Practical Geology of Ireland, gives 30,730 pounds as the crushing weight of the 

Killaloe slates/* Watrin«gives the maximum crushing weight of some French slates 

as 2,000 kilograms per square centimeter, but 1,700 as the average.

Merriman has also devised an impact test for determining the strength of slates. 

He lets a wooden ball weighing 15.7 ounces fall 9 inches upon a piece of slate measuring 6 by 7f inches by from 0.20 to 0.28 inches thick, and repeats the blows until 

the slate breaks. The foot pounds of work per pound of slate are then calculated 

from the weight and thickness of the slate and the number of blows. The resulting 

figures range from. 3.50 to 126.66.

Toughness or elasticity. Merriman finds the ultimate deflection in certain Pennsylvania slates, when placed on supports 22 inches apart, to range from 0.270 to 0.313 

inch. Certain blue-black "slates in Eldorado County, Cal., when split seven to 

the inch arid 18 inches square and fastened solidly at the two ends are said to bend 

3 inches in the center without any sign of fracture./ J. F. Williams!/ tested beams 

of slate from Rutland and Washington counties, 1 inch square and 10 inches long, 

with supports 6 inches apart. Bending without breaking was effected by from 770 

to 1,200 pounds, and when the supports were placed. 3 inches apart by from 1,710 to 

2,400 pounds. The great elasticity of the slates of western Vermont and of Northampton and Lehigh counties, Pa., is apparent to any one visiting the shanties where 

the splitting is done.

Density or specific gravity. This is determined'in the usual way, by weighing a 

piece of the slate iii and out of water and dividing its weight out by the difference 

between its weight in and out. All air should first be removed by boiling the piece 

of slate in distilled water. The specific gravity will be considerably affected by the 

amount of magnetite or pyrite.

Porosity. This is best determined by drying, then weighing, then immersing for 

twenty-four hours and weighing again, in order to ascertain the percentage of water 

absorbed. Merriman takes a piece 3 by 4 inches, with rough edges, dries it in an 

oven at 135° F. for twenty-four hours, cools to the normal temperature of room, 

weighs, and immerses it for twenty-four hours, and weighs again. His tests of PennnSee Bibliog., p. 144. An extended abstract of Merriruan's paper is given by Wm. C. Day in part C, 

continued, of Nineteenth Annual Report (pp. 257-203), and on p. 122 of this bulletin the results of his 

recent tests of 33 specimens of slate are given.

bSee Bibliog., p. 145.

cEncycl. Brit., ninth ed., 1887.

^Quoted by Hull; see Bibliography, p. 144.

'» Les ardosieres des Ardennes, pp. 192, 193.

/California State Mining Bureau, twelfth report of State Mineralogist J. J. Crawford, Sept. 15,1894, 

p. 400.

ffSee Bibliography, p. 145.
48 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

sylvania slates showed from 0.099 to 0.303 per cent of absorption. Porosity is sometimes roughly indicated by immersing a roofing slate edgewise one-half in water and 

observing how far the^water ascends by capillary attraction. In good slates it ought 

to rise but very little.

Eeverdin and de la Harpe a state that slates are liable to deterioration from the 

chemical action of gases arising from woodwork beneath the slate, as w6ll as from 

the action of the atmosphere, and that they are also liable to an increase of porosity 

by the physical action of changes of temperature and by the unequal conductivity of 

heat in the direction of cleavage and of grain. They state that the porosity in a fresh 

slate should be below 0.1 per cent, and after treatment less than 0.2 per cent. Their 

somewhat elaborate method is this: For determining porosity as produced by acids 

the slate is treated with 10 per cent cold acetic acid and the flask is made vacuous 

from time to time. The piece is then washed, dried, weighed, and 'immersed in 

diphenylamine in a thick-walled tube 12 by 3£ centimeters. The tube is exhausted, 

heated two hours in oil bath at 170° C., air pressure is restored, and heating continued for four to five hours at 150° C., after which the test pieces are removed, the 

diphenylamine wiped off with ether, and the increase in weight taken.

For determining porosity as produced by changes of temperature, the slate is heated 

in a wrought-iron tube for half an hour to 300° C., and the tube is then suddenly 

cooled by a stream of water for half an hour. This process is repeated twenty-four 

times, and the slate is then impregnated with diphenylamine and the procedure is as 

in previous test.

Fresenius is accredited with a method of testing the effect of heat and cold on slate 

by saturating it with water and putting it for twenty-four hours in a freezing mixture 

and heating another from 250° to 350° for five or six hours and then immersing it in 

water. The porosity, strength, and elasticity of the pieces so treated should then 

be tested. Bottinger points out that the greater the porosity of a slate the more 

damaging is the action of frost likely to, be. The effect of frost on the microscopic 

structure has already been referred to (p. 24).

Hardness, or abrasion. Merriman has also devised a method of testing the relative 

hardness of slates by subjecting a piece of determined weight to the action of a grindstone revolved 50 times, the .slate being held against it by a lever with a constant 

 pressure of 10 pounds. The loss of weight in the process was then determined. As 

the softness depends largely upon the amount of quartz and mica a microscopic 

analysis would also throw light upon this quality.

Corrodibility. An important quality in roofing slates is their resistance to the acids 

of the atmosphere, particularly in "cities, where gases increase its destructive power. 

Fresenius in 1868 suggested testing the weathering qualities of a 'slate by immersing 

it for three days in dilute sulphurous acid in a closed vessel. At the end of that time 

poor slates are softened or broken up into thin laminae or easily fractured, while good 

ones preserve both their density and hardness.

Merriman for the same purpose prepared a solution consisting of 98 parts of water, 

1 part of hydrochloric acid, and 1 part of sulphuric acid. Pieces of slate 3 by 4 inches 

were carefully weighed, then immersed in the solution for one hundred and twenty 

hours, then dried for forty hours, weighed, the solution strengthened, and specimen 

reimmersed for another one hundred and twenty hours, and weighed again. This 

was repeated once more. The losses in weight ranged from 0 to 2.76 per cent.

Microscopic analysis. One of the most satisfactory tests of slate is the examination 

of a thin section of it under the microscope. A cubic inch thus tested will suffice to 

show the character of the cleavage, the presence of false cleavage, if any, the probable durability, or indurability, of the color, as well as the presence of any mineral 

constituents likely to determine its other qualities. The specimen should be carefully

a Op. cit.
METHODS OF TESTING SLATE. 49

selected, so as to fairly represent the general quality of the bed. It should be fresh, 

unfrozen, and a half inch thick across the cleavage. At least two sections should he 

prepared although the more the better one parallel to the cleavage and another at 

right angles to it, never diagonal to it. The sections should be exceedingly thin, 

much more so than ordinary sections of eruptive rocks, and the slide cover should be of 

the very thinnest kind and should.be fastened on with a minimum of balsam to admit 

the use of the highest objectives. Both slides should be examined first in ordinary 

light then in polarized light, with powers ranging from 140 to 700 and in some cases 

even 1,100 diameters. The transverse section will show the quality of the cleavage, 

the false cleavage, if any, and under polarized light whether the specimen is a mica 

slate or a clay slate by the entire matrix becoming in a mica slate four times dark 

and four times light in complete rotation. The observer should, however, not be 

misled by the obscuration of aggregate polarization caused by the abundance of carbonaceous matter or carbonate or hematite. The carbonate, which extinguishes 

irregularly, is very troublesome. This could be eliminated by the application of 

HC1 to the bare section and then recovering it. The aggregate polarization may also 

be obscured by the thickness of the section and the whole analysis thus vitiated. 

Sections parallel to the cleavage reveal the amount of carbonate and indicate the 

probable amount of discoloration by exposure. Both sections, under incident light, 

will show pyrite, if any exists.

Chemical analysis. This, in order to give a correct idea of the composition of the 

slate, should not be partial, but complete.« Such an analysis should then be compared with complete analyses of the best slates of like color, and before a final conclusion is reached as to the value of the slate its microscopic analysis and the results 

of the tests of its strength, elasticity, porosity, and corroclibility should be considered 

in connection with its chemical analysis. Merriman concludes from six different 

kinds of tests applied to each of 24 specimens of Old Bangor and Albion (Pennsylvania) slates, as well as from the results of several general chemical analyses, that 

The strongest slate stands highest in weathering qualities, so that a flexural test affords an excellent index of all its properties, particularly if the ultimate deflection and the manner of rupture be 

noted. The strongest and best slate has the highest percentage of silicates of iron and alumina, but 

is not necessarily the lowest in carbonates of lime and magnesia. Chemical analyses give only 

imperfect conclusions regarding the weathering qualities of slates and do not satisfactorily explain 

their physical properties. *>

As the discoloration in slate has been shown to be probably due to the oxidation 

of the ferrous carbonate (FeC03 ), one of the chief objects of a chemical analysis 

should be to show its amount or absence, but this is just what a chemical analysis 

can not do with so complex a rock.

Hutchins finds that the presence of chlorite minerals can be detected by heating 

the slate to dull redness, thus dehydrating and discoloring those minerals, then preparing a thin section of the slate so treated and comparing it with sections of the 

normal rock. c

The most decisive of all these tests are probably those for strength and toughness, 

as applied by Merriman, and the microscopic analysis.

It should be added that shearing, compression, and expansion tests of slate have 

been made at the United States Arsenal at Watertown, Mass.

"See, on the advantage of complete analysis, Principles and methods of analysis applied to silicate 

rocks, by VV. F. Hillebrand, Bull. U. S. Geo'l. Survey No. 148; Analyses of rocks and-analytical methods, Clarke and Hillebrand, pp. 1-64,1897.

l> Merriman. See Bibliography, p. 144.

o Hutchins, Clays, shales, and slates. See Bibliography, p. 140.
50 8LATE DEPOSITS AND INDUSTRY OF UNITED STATES.

TECHNICAL, DESCRIPTION OF A SLATE QUARRY.

The following outline of a description of a slate quarry for economic purposes is 

proposed as covering the features of practical geological and economic importance, 

aside from the ordinary statistical matters as to amount and value of productj number of employees, etc.:

Quarry name or number.

Location (exact).

Geological formation as given on the geological map.

Distance from railroad and means of transporting product.

Diagram of plan of quarry to scale.

Depth of quarry. >

Dimensions of working face.

Distance and direction of dumps from working face.

Means of drainage.

Number of inclines or of horse derricks.

Number of slate-trimming machines.

Kinds of slate produced.

Thickness of good and bad beds in natural order.

Diagram showing course of beds.- " 

Strike and dip of beds.

Strike and dip of cleavage.

Strike and dip of strike joints.

Strike and dip of dip joints.

Strike and dip of diagonal joints.

Strike and dip of grain.

Strike and dip of shear zones.

Location and diameter ot posts.

Proximity of dikes.

Course and thickness of veins.
PART TL-SLATE DEPOSITS OF THE UNITED STATES,

ARIZONA.

By T. NELSON DALE.

A specimen of roofing slate recently received from Arizona by Dr. David T. Day, of 

the division of mineral resources of the United States Geological Survey, came from 

a deposit about 6£ miles north of Phoenix, which is said to measure from 800 to 1,000 

feet in width and about 5,000 feet in length. Its examination has yielded the following results: N

It is bluish gray with a lustrous surface, marked by two sets of minute wrinkles 

that lie at right angles.to each other, a coarser set numbering from 25 to 40 per inch 

and a finer one numbering many more. The slate breaks along the finer set much 

more readily than along the coarser set. It has a marked argillaceous odor, shows 

considerable pyrite on the sawn edge, contains but little magnetite, does not effervesce in cold dilute hydrochloric acid, is sonorous, and has a fair grade of n'ssility.

Under the microscope this slate shows a matrix « consisting of rnuscovite (sericite) 

with a brilliant aggregate polarization.« Cleavage and bedding foliations are parallel, but are crossed by two incipient slip-cleavages (see p. 24 and Pis. VI, XI, ./I), 

the plications of which have not resulted in fractures, but which give the slate a 

somewhat schistose structure. The finer of these slip-cleavages has from 100 to 150 

plications to the inch. There is considerable quart/, which appears to be mostly secondary. The conspicuous feature, however, is th°e great abundance of lenses and 

particles of pyrite and hematite. In the cross sections these measure from 0.0043 to 

0.043 millimeters in length by from 0.002 to 0.008 millimeters in width and number 

about 800 per square millimeter. & In parallel section they have irregular angularroundish outlines.

There are also large plates of muscovite, a few crystals of tourmaline, a few lenses 

or decomposed crystals of (?) up to 0.0425 by 0.025 to 0.075 millimeters and considerable rutile in very minute prisms. No carbonate detected.

The constituents of this slate, arranged in descending order of abundance, appear 

to be muscovite (sericite), .quartz, pyrit?, hematite, kaolin, magnetite, rutile, and 

tourmaline.

If properly cut with reference to the direction of weakness this may prove a serviceable slate, but false cleavage is generally objectionable. In this case the false 

cleavages are, however, only incipient.

ARKANSAS. 

. By EDWIN C. ECKEL.

LOCATION AND CHARACTER OF DEPOSITS.

Location The slate deposits of Arkansas are included in an area of Ordovician 

rocks in the west-central portion of the State. This area is about 100 miles in its 

greatest dimension from east to west, and varies from 12 to 20 miles in breadth from, 

north to south. The towns of Little Rock, Benton, Malvern, Hot Springs, and Mena

"For definitions of technical terms see Glossary, pp. 146-147.

6 The square millimeter referred to comprises the thickness of the thin section.

51
52 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

are all located just outside the boundaries of this Ordovician area, as will be seen 

from an examination of PI. VIII, which gives the outlines of the district. Within 

this district deposits of roofing slate occur at numerous localities, but outside of its 

limits no slate can reasonably be expected to occur in Arkansas. Prospecting should 

be confined', therefore, to the area shaded in PL VIII.

This should not be construed as meaning that all this area is underlain by deposits 

of roofing slate, for such is by no means the case. On the contrary, workable roofing slate deposits make up only a small portion of the shaded area, the remainder 

being occupied by sandstones and shales.

Description of deposits. Within the district noted roofing slates have been worked 

in a small way at several points near Hot Springs, Garland County, for a number of 

years. None of the workings, however, has been'extensive enough to give a definite 

idea of the commercial possibilities of the deposits.

During the last few years a new slate area has been exploited in western Arkansas, 

the principal deposits here being in the counties of Polk and Montgomery. Here 

black, gray, red, and greenish slates occur in abundance, and a number of companies 

have been organized to work various properties. In the spring of 1905 I spent a few 

days in this area, visiting most of the deposits which had been opened. This area 

is shown in PL IX, where a number of quarry locations are indicated. , The only 

quarries in operation at that time were those of the Southwestern Slate Company, at 

Slatington, about 20 miles southwest of Mount Ida, Montgomery County. Both red 

and black slates occur on the property of this company.

In a report made recently Buckley gives the following general section:

Feet. 

Quartzite and sandstone..................................................... 200

Red slate .................................................................. 300

Quartzite and sandstone...................................................... 500

Black slate................................................................. 400

The black slate quarry of this company is located on Crooked Creek, in section 34, 

T. 3 S., R. 27 W. The slate here strikes N. 70° E. and dips 25° N. It is a finegrained, deep black slate, and seems excellent for either roofing or mill slate. A few 

beds of rock (sandstone) are included in the section, but altogether a thickness of 

about 400 feet of black slate is exposed in the quarry along Crooked Creek.

Nearer to the company's office, at Slatington, red slates are quarried in section 33, 

T. 3 S., R. 27 W. The slate here strikes N. 30° W. and dips 20° NE. The mill contains one saw, taking an 8 by 5 foot slab; one planer, taking an 8 by 4 foot slab, and 

one 12-foot rubbing bed.

In 1904 the Kansas City Slate Quarrying Company opened a quarry in the northwest quarter of section 12, T. 3 S., R. 28 W. The cleavage here strikes N. 80° VV. 

and dips 45° N., and the bedding is apparently parallel to the cleavage. The slate 

is badly jointed near the surface. Most of that exposed is green, with occasional red 

streaks and much pyrite. The slate on the dump has disintegrated very badly on 

weathering, and is really hardly more than a clay shale.

The Gulf Slate Company has an opening about 20 feet square and 30 feet deep on 

the southwest quarter of section 9, T. 3 S., R. 28 W. The cleavage here .strikes 

N. 65° E. and dips 35° N. The slate exposed in this shallow pit is mostly red. It is 

better than that last noted, but is rather soft.

A red slate of good grade is exposed in S. 17, T. 3 S., R. 28 W., with cleavage striking N. 10° E. and dipping 40° W.

Black slate of good quality is shown just north of Little Missouri Creek, in S. 3, 

T. 4 S., R. 28 W. The cleavage strikes N. 60° E. and dips 10 N.

On Danville's property in S. 28, T. 3 S., R. 28 W., a gray slate is exposed. This 

is a hard, durable slate, but ig badly jointed near the surface. The bedding strikes 

N. 30° W. and dips 30° SW.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 276 PL. VIII

| JOHNSON.J

FRANKLIN: ciarksvilie !

 ff Maivern , \ 

j_HOT SPRING [ (J RANT IJE

i Scale of miles

35 50 75

MAP SHOWING ARKANSAS SLATE AREA.
ARKANSAS. 53

Stratton and Kelley's property in S. 29, T. 3 S., R. 28 W., shows a red slate striking N. 70° and dipping 43° N. In the northeast quarter of the same section the 

National Slate Company has an opening.

Black and gray slates in large smooth slabs are exposed on theBrennan and McElwen property in the northeast qnarter of S. 30, T. 3 S., R. 28 W. The cleavage in 

these outcrops strikes N. 70° W. and dips 13° .N.

The Barden quarry in S. 25, T. 3 S., R. 28 W., shows good black slate; striking. 

N. 65° E. and dipping 20° N.

Chemical composition of Arkansas slates. The following analyses of. Arkansas roofing slates are fairly representative of the composition of these materials: 

Analyses of slates, Montgomery County, Ark. 

[Analyst, W. G. Waring.]

Silica (SiCv)...........--....................

Ferrous oxiclc fFcO)

Lime(CaO)..................................

Potash (K2 0) ................................

1.

66.10

9.04

3 44

1.77

 > .78

4.96

.64'

08

no

2.10

.09

.18

2.

68.79

14. 26

5.90

1.16

1.40

1.43

3.09

.09

.44

.01

2.01

.11

.47

3.

69.04

12.66

8.55

i 3fl

1.75

1.87

2.98

09

.06

.01

Tr.

.72

. 84-

4.

69.07

11. 40

7.66

1.04

1.56

3.14

3.88

.96

.02

.01

.11

.24

5.

67.90

10. 42

(i. 22

3.17

4. 34

4.11

.69

.06

.01

1. 78'

.01

.23

6.

64.00

11. 59

13. 71

1.56

2.03

1.36

 .64

.05

. .04

4.03

.01

.57

7.

63. 22

16. 76

9. 54

1.76

1.52

1. 43

.70

.05

.03

3.70

.01

1.01

1. Green slate, quarry No. 2.

2. Red slate, quarry No. 2.

3. Red slate from State House Cove.

4. Hard red slate from State House Cove.

5. Green slate from State House. Cove.

6. Black slate from west end of Crooked Creek vein.

7. Black slate from Crooked Creek Falls.

MICROSCOPIC ANALYSES OF ARKANSAS SL,ATE.

By T. NELSON DALE.

The microscopic analyses of Arkansas slates published in 1904, Bull. TJ. S. Geol. 

Survey No. 225, p. 414, have been revised and are here repeated and supplemented 

by several new ones of specimens collected by Mr. Eckel during his recent visit to 

the quarries of that State.

Mack slate from Mena, near Big Forks. This is a pure black slate, which to the 

unaided eye has an exceedingly fine texture and a remarkably smooth cleavage surface with a slight luster. It is both carbonaceous (or graphitic) and magnetitic; does 

not effervesce with cold dilute hydrochloric acid, is very sonorous, and very fissile. 

Under the microscope this slate shows a matrix of muscovite (sericite), with a very 

brilliant aggregate polarization and an unusually fine texture and great homogeneity. 

Quartz grains are few and not over 0.01 mm. in diameter. Rutile needles unusually 

minute. Many opaque particles of irregular shape, some of which are pyrite, others 

magnetite, and some coaly or graphitic matter. No carbonate.

The constituents, arranged in descending order of abundance, appear to be muscovite (sericite), carbonaceous or graphitic matter, quartz, pyrite, magnetite, and 

rutile.
54 SLATE DEPOSITS AND INDUSTEY OF UNITED STATES.

This is a very superior roofing slate, with a remarkably fine cleavage and not liable 

to discolor on exposure, but its strength and its behavior under frost should be tested.

Dark-reddish slate from Mena, near Big Forks. In color this slate is somewhat 

darker than the "red" slate of New York. To the unaided eye it has a minutely 

granular texture and a roughish, speckled, almost lusterless surface. Contains very 

little magnetite; does not effervesce with cold dilute hydrochloric acid, is sonorous, 

splits readily, and has some argillaceous odor. Under the microscope it shows a 

matrix of muscovite (sericite), with brilliant aggregate polarization, quartz grains 

ranging up to 0. 025 mm. muscovite and chlorite scales, and abundant hematite in 

minute dots. There are also rhombs, from 0.1 to 0.2 mm., of chlorite and rhodochrosite, probably pseudomorphs after ? No other carbonate.

The constituents, arranged in descending order of abundance, appear to be muscovite (sericite), hematite, kaolin, quartz, chlorite, rhodochrosite ?, and magnetite.

This slate compares favorably in texture with the New York "red" slate.

Reddish slate from Mammoth red and Lost Hannah beds (exact locality not given). 

Color lighter than above, but not quite so red as the New York slate. To the unaided 

eye has a fine texture and a fine cleavage surface, but no luster. Contains very little 

magnetite; does not effervesce with cold dilute hydrochloric acid, is sonorous and 

fissile, has some argillaceous odor.

Under the microscope it shows a matrix of muscovite (sericite), with brilliant 

aggregate polarization, quartz grains up to 0.03 mm., muscovite and chlorite scales, 

abundant hematite pigment, and no carbonate.

The constituents, arranged in descending order of abundance, appear to be muscovite (sericite), hematite, kaolin, quartz; chlorite, and magnetite.

This is a finer and softer slate than the dark red, and should be tested for strength 

and frost resistance.

Greenish-gray slate from Mena. In color this resembles the "sea-green" slate of 

Vermont. To the unaided eye it has a fine texture, a roughish cleavage surface, and 

a waxy luster; does not show pyrite on sawn edge, contains very little magnetite; 

does not effervesce with cold dilute hydrochloric acid, and is somewhat sonorous.

Under the microscope it shows a matrix of muscovite (sericite), with a brilliant 

aggregate polarization and is of very fine texture and homogeneity, but the cleavage 

is crossed at an angle of ]3° by a very close bedding foliation and also by an obscure 

slip or "false" cleavage at about 40°. Contains very few and very minute quartz 

grains, no carbonate, several pseudomorphic rhombs of chlorite 0.08, mm., and has 

a slight argillaceous odor.

The constituents, arranged in descending order of abundance, appear to be muscovite (sericite), quartz, kaolin, chlorite, and magnetite.

The two extra foliations are likely to prove directions of weakness.

Light-greenish slate from Mammoth red bed, locality not designated. This is more 

greenish than the above, to the unaided eye has an exceedingly fine texture and a 

very fine, almost lusterless cleavage surface; shows pyrite on sawn edge, contains a 

little magnetite; does not effervesce with cold dilute hydrochloric acid, is sonorous, 

very fissile, and has a slight argillaceous odor.

Under the microscope it shows a matrix of muscovite (sericite), with a brilliant 

aggregate polarization and great evenness of texture. A very minute bed of quartz 

grains, chlorite, and muscovite lies in the cleavage, which is, therefore, the bedding 

also. The grain is indicated by the transverse position of some of the muscovite 

scales. Quartz not very abundant, but occurs in grains up to 0.037 mm. Eutile 

needles abound from 0.0026 by 0.0009 up to 0.014 by 0.0014 mm. Muscovite and 

chlorite scales occur, the latter producing the green color. There are some opaque 

granules (limonite? and pyrite), occasional lenses, 0.14 mm. long, of a central mass 

(probably rhodochrosite), with secondary muscovite at both ends. jNo other carbonate. Showfl a number of tourmaline prisms up to 0.025 by 0.008 mm.
U. S. GEOLOGICAL SURVEY

R 30 W

BULLETIN NO. 275 PL. IX

R 29 W R 28 W

SKETCH MAP SHOWING LOCATION OF SLATE DEPOSITS OF MONTGOMERY AND POLK COUNTIES, ARKANSAS.
ARKANSAS. 55

The chief constituents, arranged in the descending order of abundance, appear 

to be mtiscovite (sericite), quartz, kaolin, chlorite; rutile, pyrite, magnetite, and 

tourmaline.

This is a very superior siate on account of its fissility, color, and freedom from 

carbonate, but its strength and resistance to frost should be tested.

Very dark bluish gray slate from sec. 25, T. 3 S., R. 29 W. (Specimen collected by 

Mr. E. C. Eckel.)

To the unaided eye this slate has a line texture and a smooth cleavage surface with 

a little luster, and shows a litt'e pyrite on the sawn edge. It contains very little 

magnetite, considerable carbonaceous or graphitic matter, do.es not effervesce with 

cold dilute hydrochloric acid, is sonorous, and has a high 'grade of fissility. Under 

the microscope it shows a fine-textured matrix of muscovite (sericite) with good 

aggregate polarization, but somewhat obscured by carbonaceous matter. Quartz is 

not abundant, the grains measuring up to 0.047 by 0.02 mm. Pyrite spherules, measuring up to 0.008 mm. in diameter, number about 120 per square millimeter. There 

are rutile needles but no carbonate.

The chief constituents, arranged in order of decreasing abundance, appear to be 

muscovite (sericite), quartz, pyrite, carbonaceous or graphitic matter, and rutile.

This slate has absence of carbonate in its favor. It is not so fine textured or. so fissile as that described on page 53, but may prove more durable.

Light-gray slate with a slightly greenish tinge from sec. 30, T. 3 S., E. 28 W. 

(Specimen collected by Mr. E. C. Eckel.) To the unaided eye this has a fine texture, but a lusterless, roughish surface, and shows a little pyrite o.n the sawn edges. 

It contains an insignificant amount of magnetite but no carbonaceous or graphitic 

matter, does not effervesce with cold dilute hydrochloric acid, and has an argillaceous odor and a fair degree of fissility and sonorousness.

Under the microscope it shows a fine-textured matrix of muscovite (sericite) with 

brilliant aggregate polarization, containing not abundant quartz grains, measuring 

up to 0.03 mm., scales of chlorite and of muscovite transverse to cleavage, some 

pyrite cubes up to 0.063 mm. (generally with a rim of secondary quartz) passing into 

limonite and staining the matrix. Rutile needles abound.

The chief constituents, arranged in decreasing order of abundance, appear to be 

muscovite (sericite), quartz, chlorite, kaolin, pyrite, rutile, and limonite, with accessory magnetite.

Whether the limonitic staining from the decomposition of the pyrite will produce 

any appreciable discoloration in the course of years can be determined only by 

experiment.

Very dark-gray spangled slate from quarry operated by the Southwestern Slate Manufacturing Company. (Specimen collected by Mr. E. C. Eckel.) To the unaided eye 

this has a coarsish texture and a roughish, almost lusterless surface, spangled with 

minute scales of mica, shows pyrite on sawn edges, contains very little magnetite and 

little carbonaceous matter, does not effervesce \vith cold dilute hydrochloric acid, is 

somewhat sonorous, tolerably fissile, and has a slight argillaceous odor.

Under the microscope it shows a fine-textured matrix of muscovite (sericite) with 

brilliant aggregate polarization, containing roundish and angular grains of quartz of 

variable and large size, even up to 0.27 by 0.17 mm.; also a few of plagioclase feldspar, scales of muscovite and biotite up to 0.2 by 0.1 mm., some lenses of carbonate 

up to 0.4'long, a few grains of tourmaline, and lenses of secondary quartz. There 

is.a faint incipient false cleavage, not apparent, however, in the hand specimen.

The chief constituents of this slate, arranged in order of decreasing abundance, 

appear to be muscovite (including sericite), quartz, carbonate, pyrite, carbonaceous 

matter, and kaolin, with accessory biotite, plagioclase, tourmaline, and magnetite.

This slate combines some of the features of a metamorphic graywacke or grit with 

those of a mica-slate. It is inferior to the dark bluish-gray slate described above. 

Bull. 275 06 5
56 SLATE DEPOSITS AND INDUSTRY .OF UNITED STATES.

CALIFORNIA.

By EDWIN C. ECKEL.

Location and general relations. Though roofing slate has at different times been 

quarried on a small scale in other parts of California, the only important slateproducing area in the State is located in Eldorado County. The quarries which have 

been opened in this district are located along a line running about N. 15° W. from 

Placerville, at distances of 1 to 6 miles from that town. The location and geographic 

and geologic relations^of the slate deposits and quarries can best be understood by 

reference to the maps included in the Placerville folio of the United States Geological Survey. The workable roofing-slate deposits of this district occur in a belt of 

the Mariposa slates, of late Jurassic or early Cretaceous age. The quarries which 

have been opened are all situated near the western boundary of this belt of Mariposa 

slates, where it is bordered, by a large area of diabase. This diabase, according to 

Lindgren and Turner, is "of the age of the Mariposa slates, or older." A number 

of linear areas of amphibolite occur in the Mariposa slates. These amphibolites are 

described as being derived from diabase or gabbro. They are in part altered to 

serpentine.

Previous work on the slate deposits. The Placerville folio, No. 3, United States Geological Survey, published in 1894, contains the results of detailed geologic work by 

Lindgren and Turner in the area in which the roofing-slate deposits occur. At that 

date the roofing-slate industry had not assumed its present importance, though all 

the quarries now worked had then been opened. The existence of roofing-slate 

deposits is noted in the text of the folio, and the locations of the quarries are indicated on the map showing the economic geology of the area. No reference is made 

to the "green slates," or to the dikes cutting the Eureka quarry.

Excellent though brief descriptions of the different quarries and of the condition 

of the slate industry at various dates are to be found in the reports of the State mineralogist of California, particularly in the eighth and twelfth reports. The geologic 

relations of the slate-producing areas are shown in PI. X.

At present the most important quarry is that of the Eureka Slate Company, and 

this is now being worked on a large scale.. This quarry is at Slatington, about onehalf mile southwest of Kelsey.

Structural relations in Eureka quarry. The cleavage planes of the slates in the Eureka 

quarry strike N. 25° W. The dip of the cleavage is practically vertical, with slight 

local variations to 80° E. or 80° W. The upper weathered beds in the quarry are 

overturned by local pressure so as to give dips of 40° to 60° to the east or west, 

according to local conditions. This overturning is evidently due merely to the weight 

of the overlying soil and decomposed slate, and the effects are shown only for a 

depth of from 3 to 15 feet. It is of interest, however, as a warning against accepting 

dip readings taken from surface beds of the slate.

The slate body shows rather frequent, but narrow, "ribbons." These ribbons are 

bands (from one-sixteenth to one-half inch thick usually, but occasionally as thick 

as 2 inches) of -material differing in composition from the mass of the slate. They 

are in general more siliceous than the normal slate, and do not furnish merchantable 

material. Their geologic interest arises from the fact that they represent differences 

of original sedimentation. The plane of the ribbons in a slate quarry is, therefore, 

the. plane of original bedding. In the Eureka quarry, and, indeed, throughout the 

roofing-slate belt, the plane of original bedding seerns to be usually within 10° of the 

plane of slaty cleavage.

The slate mass is cut by a series of joints parallel to the "grain" of the slates, 

striking N. 55° E. and dipping from 70° to 80° NW. Joints across the "grain" of
U. S. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. X

120 50

120 50

GEOLOGIC MAP OF PLACEEVJLLE SLATE DISTRICT, CALIFORNIA.

Scale . 

0______1_______2______3______4. miles

Mariposa slates Basic igneous rocks Slate quarries Line of cable
OALIFOENIA. . 57

the-slate, which«would be practically horizontal, do not occur in this quarry, but 

many of the thin quartz seams occupy this position.

Quartz and calcite occur in thin layers, filling joint spaces and occasionally cleavage spaces. Pyrite also occurs in very much flattened nodules, which were apparently parallel to the original bedding.

Character of the normal slate. The mass of the Eureka quarry product is a dense, 

deep-black slate, splitting very finely and regularly, with a smooth glistening surface, 

much like that of the Bangor and Lehigh slates of Pennsylvania. The frequency-of 

the ribbons and of the pyrite nodules prevents the slate from being serviceable as 

mill stock, but as a roofing material it is excellent.

A specimen of the black slate, free from ribbon, was selected for analysis in the 

laboratory of the United. States Geological Survey. The results of this analysis, by 

Mr. W. T. Schaller, follow:

Analysis of black slate, Eureka quarry, Slatington, Cal,

Silica(SiOj)..................................^........................... 63. 52

Alumina (A12 03 ) and titanic oxide (Ti02 ).................................. 16.34

Iron oxides(FeO, Fe2 03 )......^...............J........................... 6. 79

Lime(CaO).............................................................. .98

Magnesia (MgO) ......-.-.-.---.-.-.................1..................... 2. 50

Carbon dioxide (C02 ).................................................

Water .............................................................. c 4' 86

Prof. T. Nelson Dale has reported as follows on a sample of this slate:

This slate is very dark gray. To the unaided eye it has a fine texture and a smooth and lustrous 

cleavage surface. It contains considerable carbonaceous or graphitic matter but extremely little magnetite. The sawn edges show pyrite and lenses, and streaks of lighter gray material. There is no 

effervescence in cold dilute hydrochloric acid. It is sonorous and has a high grade of fissility.

Under the microscope it shows a matrix of muscovite (sericite) with a brilliant aggregate polarization and a general evenness of texture, interrupted, however, by lenses up to about 8 mm. long by 

about one-half mm. wide, consisting chiefly of quartz fragments (surrounded by a rim. of radiating 

secondary quartz) with muscovite scales, plates, and rhombs of carbonate, and rarely a grain of 

plagioclase. These lenses have their long axes parallel to the cleavage, but in sections parallel to it 

some of them have such irregular outline and are so large as to appear like minute beds. The matrix 

contains many quartz fragments, measuring up to 0.09 by 0.03 mm., much less carbonate than the 

lenses, muscovite scales, chlorite scales, a little pyrite in spherules and crystals, carbonaceous or 

graphitic matter, rutile needles, a few grains of tourmaline, and rarely one of zircon.

The main constituents of this slate, arranged in order of decreasing abundance, appear to be muscovite (sericite), quartz, chlorite, carbonate, pyrite, carbonaceous matter or graphite, rutile, and 

magnetite, besides accessory tourmaline and zircon.

This ought to prove a serviceable slate. It will be noticed that carbonate, although present, is 

insufficient to produce effervescence.

Eureka quarry green slate.® A band of green slate several feet wide crosses the 

Eureka quarry. On examination it is found that the borders of. this band are not 

parallel to the "ribbon" of the black slate. The green band can not, therefore, be 

interbedded with the black slates. The probability that it represents a dike of massive igneous rock which has been changed to a slate by pressure subsequent to its 

intrusion is strengthened when the chemical composition of the green slate is conr 

sidered. Two analyses of the green slates are presented on the next page. The first 

is of a sample selected by the-writer and analyzed in the laboratory of the United 

States Geological Survey by Mr. W. T. Schaller; the second was given by Mr. C. H. 

Dunton, manager of the Eureka quarry, but the name of the analyst is unknown. 

As the analyses show a close agreement in essential features, it is probable that they 

are fairly representative of the composition of the green slates, and that their avern For a more detailed discussion of this interesting "igneous slate" the reader is referred to the paper 

by the writer in the Journal of Geology, vol. 12,1904, pp. 15-29.
58 SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

age, which is given in the third column of the table, may be regarded as typical of 

this interesting and apparently unique type of roofing slate.

Analyses of iynenus green slate, Eureka quarry, Slatincjton, Cal.

Silica (SiOo) ...................................................................

Lime (CaO) .....................................................................

Sulphur (S) ....................................................................

Alkalies (KoO.NaoO) ..........................................................

Carbon dioxide (CO.) .........................................................

Water..........................................................................

(1)

45.15

10.33

8.42

6.42

8.72

(a)

(«)

I 11. 28

(2)

47.30

35.53

8.00

7.83

7.86

.12

3.17

9.92

Aver- 

* age.

40. 22

15.93

8.21

7.12

8.29

.12

3.17

10.60

a Not determined.

These analyses have been compared a with a series of 36 analyses of American 

roofing slates derived from clays by pressure, and remarkable differences in composition are apparent. The green slate, on the other hand, approximates closely in 

composition to certain "basic" igneous rocks of the district, and it is probable that 

it was derived from a gabbro or similar rock.

The "green slate" is in'reality grayish green in color. It splits readily, though 

with not so smooth a surface as the black slate.- It stands punching and trimming 

well, and is sufficiently strong for roofing use. Considering its origin and composition it is probable that it will be a highly durable slate, holding its color well. At 

present it is sold entirely for trimming and lettering on black slate roofs, for which 

purpose it is particularly well adapted, giving a strong but pleasant color contrast. 

. Chili Bar Slate Company quarry. This quarry is located about 3 miles north of 

Placerville, in-sec. 36, T. 11 N., R. 10 E., on the south side of the South Fork of 

American River, a few hundred yards east of the Placerville-Kelsey stage road.

This is the oldest quarry in the district, having been opened about twenty years 

ago. 'It has been shutdown since 1897.

Several openings were made in a bluff forming the river bank at this point. In 

the easternmost of these openings, which is about 40 feet high and 30 feet wide, a 

rather poor slate with irregular joints is shown. The cleavage strikes N. 20° W., 

and dips 75° E. The westernmost opening is small, with a tunnel which was apparently run in on a band of better slate. The slate piled in the yard has kept its color 

fairly well.

It seems possible that this quarry may be flooded at high water. Both it and the 

one next mentioned (San Francisco quarry) are badly located, having no large dumping area available near the quarries. Neither quarry has gone deep enough to get 

really good slate, which might have been found at a greater depth.

San Francisco Slate Company quarry. This quarry is located in T. 11 N., R. 10 E., 

within a quarter mile of that of the Chili Bar Slate Company, but on the north side 

of the river and west of the Placerville-Kelsey road. The principal opening was 

located about 600 feet north of the river, at an elevation of 150 feet above its bank. 

A tramway led down to the dressing yards, which were situated at the river bank.

The cleavage of the slates in the large opening strikes about N. 30° W., and has 

an almost vertical dip. No slate has been quarried here since 1897. A large stock 

of trimmed slates is still piled in the dressing yard, and many of these have already 

discolored badly.

aJour. Geol., vol. 12, 1904, p. 26.
CALIFORNIA GEORGIA.

Transportation and market. The Eldorado County slates have practically no competition on or near the Pacific coast, while the Eureka quarry lias recently placed 

large shipments in Hawaii and Guam. Until recently the principal problem has 

been the transportation of the slates from the quarry to the railroad. This was 

formerly done by-wagon hauling over a 6-rnile stretch of very hilly road. During 

the last season, however, the Eureka Slate Company has installed an aerial tramway 

system from its quarry to a point near Placerville. This tramway is an engineering 

feat of no mean order, the crossing of the South Fork of American River being the 

principal difficulty encountered.

Production of roofing slate in California. The following table, compiled from figures 

given in various volumes of Mineral Resources of the United States, shows the amount 

and value of California slate production for a number of years:

. California production of roofing slate, 1890-1901.

Year.

1890...........................

1891...........................

1892...........................

1893...........................

1894...........................

1895...........................

Quantity.

Squares. 

 3, 104

4,000

3 Kfin

.900

1,500

Value.

118, 089

24, 000

5,850

10, 500

Year.

1896 ..........................

1897 ..........................

1898 ..........................

1899 ..........................

1900 ..........................

1901 ..........................

Quantity.

Squares. 

4, 597

1, 000

dnn

928

3,500

2,500

Value.

820, 388

7,000

2,700

G, C42

20, 500

18,608

GEORGIA.

By EDWIN C. ECKEL.

The workable roofing slate deposits of Georgia occur in theRockmart (Ordovician) 

formation, and are developed only near the town of Rockmart, Polk County.

Geology of the slates. In a recent publication^ of this Survey, Dr. C. W. Hayes has 

described the geologic relations of the Georgia slates as follows:

The llockmart formation is confined to the Cedartown and Rockmart areas, in the southern portion 

of the Rome quadrangle. As stated previously, these rocks are contemporaneous with the upper 

portion of the Chickamauga limestone north of the Coosa Valley. The formation consists chiefly of 

Mack slates, originally calcareous shales, but sufficiently altered for the development of slaty 

cleavage. In addition to the slate, which occupies the lower portion of the formation, it contains 

beds of highly ferruginous sandstone and some cherty limestone. It also contains coarse conglomerates, made up of limestone pebbles embedded in an earthy matrix. This upper portion of the formation was evidently deposited near the margin of the sea, where the supply of sediment was abundant 

and variable in character.

The most extensive slate quarries in the United States south of Pennsylvania are located at llockmart. The formation in which the quarries are located, the Rockmart slate, extends across the' 

border into the Rome quadrangle, but it is not certain that any workable slate will be found in this 

area. The formation is variable in composition, and to the north of Rockmart consists largely of 

unaltered clay shales with beds of ferruginous limestone and sandstone,

The portion of the formation which now produces roofing slate was originally a fine-grained homogeneous clay shale. Under the influence of metamorphism, connected probably with the extensive 

faulting which the region has undergone, a very perfect slaty cleavage was developed, which generally obscures, and in some cases entirely obliterates, the original bedding. East of the quadrangle, 

along the Cartersville fault, the slate is generally wrinkled near the fault, so that it does not cleave 

readily, and at a considerable distance from the fault the cleavage is only imperfectly developed. 

Hence the best slate will be found within a comparatively narrow belt, from 1 to 5 miles from the 

fault.

a Hayes C. W. Description of the Rome quadrangle: Geologic Atlas U. S., folio 78, U. S. Geol. 

Survey, 1902.
60 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Use of slate in Portland cement manufacture. Slate is merely a form of shale in which 

a fine, even, and parallel cleavage has been developed by pressure. In composition, 

therefore, it will vary exactly as do shales, and, so far as composition alone is concerned, slate would not be worthy of more attention, as a Portland cement material, 

 than any other shale.

Commercial considerations in connection with the slate industry, however, make 

slate a very important possible source of cement material. Good roofing slate is 

relatively scarce and commands a good price when found. In the preparation of 

roofing slate for the market so much material is lost during sawing, splitting, etc., 

that only about 10 to 25 per cent of the amount quarried is salable as slate. The 

remaining 75 to 90 per cent is of no service to the slate miner. It is sent to the dump 

heap, and is a continual source of trouble and expense. This very material, however, as can be seen from the analyses quoted below, is often admirably adapted for 

use in connection with limestone in a Portland cement mixture. As it is a waste 

product, it could be obtained very cheaply by the cement manufacturer.

Only one American Portland cement plant is at present using roofing slate as one 

of its raw materials, and this plant is of recent construction. It is that of the Southern States Portland Cement Company, and is located about half a mile east of Bockrnart, Ga. The Portland cement manufactured here is made from a mixture of pure 

limestone and slate, both of which materials occur in the immediate vicinity of the 

plant.

East of the town the surface rock is the "Chickamauga limestone," which here 

contains beds of pure nonmagnesian limestone, which have been quarried at several 

points in the vicinity and burned into lime.

The cement company purchased the property of the old Georgia Slate Company, 

about half a mile southwest of Rockmart, and carried on extensive operations with 

the diamond drill. The intention was to quarry the slate, sell as slate the portions 

best suited for that use, and utilize the scrap and waste in the manufacture of cement. 

The quarries from which the limestone is obtained are located half a mile east of 

town, near the mill.

Analyses of slate used for Portland cement, Rockmart, Ga.

Silica (SiOs).. ....................

1.

57.40

23. 65

4.45

3.23

3.23

2.

58. 20

18.83

5.78

4.35

3.51

Alkalis (K.,0, NaaO) .............

Water............................

'1.

(«)

(«)'

(")

1 6.80

-

2.

3 on

.49

.82

{

fid

4.07

a Not determined.

1. J. F. Davis, analyst. Privately communicated.

2. Slocum and Vandeventer, analysts, Eighteenth Ann. Kept. U. S. Geol. Survey, pt. 5.

MAINE. 

By T. NELSON DALE.

The slate region of Maine lies about in the center of the State, in the southern 

part of Piscataquis County, south and southeast of Moosehead Lake and east and 

west of Sebec Lake, in the towns of Monson, Blanchard, and Brownville. (See map, 

fig. 4.) Commercial slate occurs also in the town of Forks, Somerset County.

Geological relations. The slate occurs in a belt consisting largely of slaty rocks, 

represented by Prof. Charles H. Hitchcock as from 15 to 20 miles wide, and extend-
MAINE. 61

ing from the Kennebec River, between Bingham and Dead rivers, northeastward to 

the sources of the Mattawamkeag River. The rocks are probably of early Paleozoic 

age.« The portion of this belt now yielding commercial slate lies south of the central granitic area of the State. The general structure of this belt is unknown. At 

North Blanchard on the west the strike of the bedding is N. 25°-39° E., dip 80° ESE. 

Near Blanchard and the Piscataquis River the strike is N. 55°-60° E., and the dip, 

40 feet below the surface, is south-southeast at about 80°; but at the top, owing 

either to the glacier, which moved here S. 20°-40° E., or else to the beginning of an 

anticline, the dip curves over to the north-northwest. Within l£ miles southwest of 

Monson the strike is N. 60° E., dip 90°. At Monson the strike is N. 47°-54° E., dip 

80° SE; but at Brownville, 20 miles east of Monson, the strike is N. 78° E., dip 75° 

NNW. As the grain is horizontal at Brownville and at points 3£ miles west-southwest and 1 £ miles southwest of Monson, a nearly vertical pitch may be assumed for 

the folds, but it is singular that the jointing in the quartzite beds should not furnish 

any clue to this pitch.

FIG. 4. Map of slate region in Maine. From Post-route map. The chief quarrying centers are shown

by crossed hammers.

Monson. In 1904 three quarries were in full operation in the town of Monson. The 

Monson Pond quarry of the Monson Maine Slate Company; the newly opened one 

of the Maine Slate Company, of Monson, about 3£ miles west-southwest of the village; 

and the West Monson quarry, about 1J miles southwest of it, operated by the Monson Consolidated Slate Company.

At the Monson Pond quarry the following series is exposed: 15 beds of slate, 

measuring altogether from 79 feet to 93 feet 6 inches, alternating with 15 beds of 

dark gray or black quartzite ("hards"), measuring altogether from 48 feet 5 inches 

to 49 feet 5 inches, both slate and quartzite amounting to from about 127 to about 

142 feet. The .deposit has been prospected for 200 feet farther southeast, but the

«See Hitchcock, C. H., The geology of northern New England, 1886; also his Preliminary Kept. 

Nat. Hist, and Geol., Maine, including geological map of northern Maine, pt. 1, 1861, pp. 316, 319, and 

Second Ann. Kept., 1862, pt. 2, pp. 280, 282,360.
62 SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

slate beds range only from 4 inches to 2 feet in thickness and the quartzite beds vary 

considerably. The entire thickness explored here thus measures from 327 to 342 

feet, and in that thickness there are no indications of duplication. This quartzite is 

usually very fine grained, and under the microscope proves to be biotitic and pyritiferous, with a little magnetite and muscovite and a few grains of zircon. In order to 

convey an idea of the great irregularity of the interbedding which marks the entire 

belt the following measurements of the Pond quarry series at the north edge of 

quarry are here given:

Section at, Monson Pond quarry, Monson, Me.

[Furnished by the courtesy of Mr. F. H. Crane, superintendent.]

Feet. Inches.

Slate ............................................................. 7-18 0

Quartzite .......................................................... 0 6

Slate ..................'..................... ............... ...... 1 8

Quartzite .......................................................... 0 2- 8

Slate ................................................ ............. 1 10

Quartzite.......................................................... 0 8

Slate ............................................................... 2 0

Quartzite .......................................................... 0 3

Slate .............................................................. 4 0

Quartzite ........................................................... 1 6

Slate .............................................................. 4 0

Quartzite .......................................................... 0 8

Slate ....--.-.---..---...-..-.-.-.-..---...------.---..-..--..--- 4 0

Quartzite ........................................................... 0 6

Slate ....:.............-.........--..--...-.....-......-.....---.... 15-18 0

Quartzite .......................................................... 4 0

Slate .............................................................. 5 -0

Quartzite .......................................................... 1 4

Slate .....................:......................-................. 7- 8 0

Quartzite .......................................................... 4 0

Slate ............................................................... 3 0

Quartzite ........................................................... 1 2- 4

Slate ...........'................................................... 3 0

Quartzite ........................................................... 0 8-12

Slate .............................................................. 8 0

Quartzite .......................................................... 20 0

Slate .............................................................. 7 0

Quartzite.......................................................... 2 0

Slate .............................................................. 6 0

Quartzite ..............................:........................... 11 0

Total slate, 79-93 feet 6 inches.

Total quartzite, 48 feet 5 inches to 49 feet 5 inches.

There is sometimes a transition from the quartzite to the slate, a quartzitic slate 

intervening. In the above lists such beds are classified as quartzite.

The Pond quarry measures about 500 feet along the strike and nearly 100 feet 

across it at the top, and from 250 to 300 feet in depth. The beds strike N. 47° E., 

dip 78°-80° SE., without any indication of turning, and the cleavage strikes N. 45° E., 

dips 90°, thus intersecting the bedding at a very acute angle. The grain strikes 

N. 45°-50° W. and dips 90°. The slate is traversed at intervals by horizontal joints, 

which are more frequent in the quartzite in places from 1 to 4 feet apart. The 

quartzite also has joints, striking N. 65° W. and dipping 25° N. 65° E., which often 

are veined with quartz. There are also vertical diagonal joints striking about north-
MAINE. . 63

west and thus parallel to the grain. The northeast half of the quarry is much 

broken up by diagonal jointing and faulting, but in the southwest half conditions are 

more normal, although veining is there more frequent. The difference between the 

jointing of the quartzite and the slate results from the difference in their rigidity. 

Their behavior under the same stress must needs have been very dissimilar. The 

quart/ veins traversing the slate sometimes contain biotite, chlorite, and a little calcite. 'The surface of the formation is glaciated and covered with from 5 to 10 feet of 

glacial clay and pebbles.

The slate itself is very dark gray, but at the glaciated surface some of the beds 

have in bright sunlight a very slightly purplish hue. The fifth bed from the north 

edge is slightly brownish. To the unaided eye both texture and surface are fine, but 

the latter is almost lusterless. It is slightly carbonaceous or graphitic, and has very 

little magnetite. The sawn edge shows a little pyrite. No effervescence in cold 

dilute hydrochloric acid. It is very sonorous.

Under the microscope this slate shows .a matrix of muscovite- (sericitej with a 

brilliant aggregate polarization, but there is considerable irregularity hr the size of 

 the particles. .Quartz fragments measure up to 0.017 .by 0.008 mm. Occasional 

quartz lenses measure 0.094 by 0.047 mm. There are to each square millimeter from 

30 to 40 scales of chlorite (interleaved with muscovite, rarely with biotite) measuring 

up to 0.047 by 0.03 mm. and lying transverse to the cleavage; also about ten scales 

of biotite to each square millimeter measuring up to 0.086 by 0.02 mm., lying both 

across and with the cleavage, and longish crystals and lenses of pyrite with their 

long axes parallel to the grain, numbering about fifty to the square millimeter and 

measuring up to 0.075 by 0.028 mm. These crystals are mostly distorted cubes, but 

mingled with them are probably some distorted octahedra of magnetite. .Scattered 

throughout is dark-gray carbonaceous or graphitic matter in extremely minute particles, to which and to the biotite the slate owes its blackness. Finally, a few delicate rutile prisms, 0.001 mm. long, some specks of hematite, and a few tourmaline 

.prisms up to 0.036 by 0.004 mm. No carbonate detected.

The constituents of this slate, arranged in the order of their decreasing abundance, 

appear to be muscovite (sericite), quartz, chlorite, biotite, pyrite, carbonaceous or 

graphitic matter, magnetite, rutile, and apatite.

The only available chemical analysis of this slate is that by L. M. Norton/' which 

shows 56.42 per cent of Si02 , 24.14 per cent of A1 2 03 , and 0.52 per cent of CaO. This 

small percentage of lime, taken in connection with the occurrence of a little calcite 

in the quartz veins, points to the presence of an insignificant amount of carbonate, 

which the microscope fails to detect. But a little of this lime belongs to the apatite. 

The specific gravity is given by Bailey as 2.851. Tests of the crushing weight and 

strength made at the Massachusetts Institute of Technology show that a cubic inch 

of this slate yields to the pressure of 30,425 pounds when applied at right angles to 

the cleavage, and that a slab, 12 by 6 by 1 inches, supported on knife edges 10 inches 

apart, breaks under a stress of from 3,950 to 4,000 pounds applied at the center with 

a steel rod, five-sixteenths inch in diameter, placed between the slate and the pressure block. This gives a modulus of rupture of 9,937 pounds per square inch.

This slate is split to seven thirty-seconds of an inch for roofing. It is also used for 

electric purposes, register borders, blackboards, refrigerator shelves, etc.

At the quarry of the Maine Slate Company of Monson, 3£ miles west-southwest 

of the village, opened in 1903, there are about 30 feet of slate and interbedded 

quartzite exposed. The thickest bed of slate measures about 8 feet. In 1904, the 

quarry measured about 100 feet along the strike, 30.feet across it, and 40 feet in 

depth. The bedding strikes N. 55°-60° E., and dips about 80° SSE., but at the surface curves over steeply north-northwest. The cleavage strikes about the same, but

a See Bailey, W. S., Bull. U. S. Geol. Survey No. 150, 1898, p. 313, and Twentieth Ann. Kept., 1't. VJ. 

(Cont.), 1899, p. 394.
64 SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

dips 90°. There are vertical dip joints, horizontal joints, and one diagonal one. 

The grain is parallel to the horizontal jointing, so that the slate has to be sawn in a 

northwest and southeast vertical direction and split ('.' sculped ") in a horizontal one.

The slate is very dark gray; to the unaided eye has a finer texture and finer cleavage surface than that of the Monson Pond quarry, and also more luster. It is slightly 

graphitic, has very little magnetite, but the sawn edges show considerable pyrite. It 

does not effervesce with cold dilute hydrochloric acid, and is very sonorous.

Under the microscope this slate shows a matrix of muscovite (sericite), with a 

very brilliant aggregate polarization. There are lenses of biotite and quartz, or of 

quartz with a nucleus of pyrite, measuring up to 0.565 by 0.14 mm., rarely 1 by 0.075. 

Quartz fragments, unusually abundant in sections parallel to cleavage, measure up 

to 0.064 mm.; biotite scales, about 18 to each square millimeter, measuring up to 

0.13 by 0.028 mm., lie across as well as in the cleavage. Little less abundant than 

these are scales of chlorite interleaved with muscovite, lying across the cleavage and 

measuring up to 0.13 by 2 mm. There are also about 300 lenses of pyrite to each 

square millimeter, with their long axes in the cleavage, and measuring from 0.002 

up to 0.094mm. in length, and up to 0.047 in width and breadth, This number 

probably includes a few crystals of magnetite. These lenses are sometimes surrounded by secondary muscovite. Generally distributed is a dark grayish or black 

material, probably graphitic, to which the slate owes its blackness. Tourmaline 

prisms up to. 0.047 by 0.009 mm. No carbonate was detected.

The relative abundance of these minerals appears to be, in descending order, muscovite (sericite), quartz, biotite, chlorite, pyrite, graphite, and magnetite.

The product is at present being prepared exclusively for roofing.

West Monson. At the quarry of the Monson Consolidated Slate Company a bed of 

black slate 9 feet thick, with a bed of quartzite 15 feet thick on its north side, and 

small alternating beds of quartzite and slate on its south side are exposed, the whole 

series measuring perhaps 50 feet. Th'e quarry in 1904 .measured 300 feet along the 

strike, 15 across it, and 160 in depth. The walls are supported by three pillars of 

slate. Bedding and cleavage both strike N. 60° E., and dip 90° E. There are vertical 

dip joints striking N. 15° W.; also horizontal joints to which the grain is parallel. 

There are about 15 feet of till on the edges of the glaciated slate. As only one bed 

of slate is worked, the percentage of waste at this quarry is very small.

The slate is a very dark gray. To the unaided eye texture and cleavage surface 

are very fine. It'has more luster than the Pond quarry slate, but not as much as 

that of the Maine Slate Company of Monson. It is slightly graphitic, has no magnetite, but shows pyrite on sawn edges; does not effervesce with cold dilute hydrochloric 

acid, and is very sonorous.

Under, the -microscope this slate shows a matrix of muscovite (sericite) with a 

brilliant aggregate polarization. There are a few lenses of quartz and biotite, measuring from 0.107 to 0.13 by 0.034 mm., some lying in the cleavage, others in the 

direction of the grain. The quartz fragments measure up to 0.02 by 0.012. There 

are about nine biotite scales to each square millimeter, -measuring up to 0.08 by 

0.02 mrn.; also about fourteen chlorite scales, measuring 0.047 by 0.02 mm., but 

sometimes 0.085 by 0.03, with their longer axes and laminae usually parallel to the 

cleavage and across the grain; and finally, twenty to fifty lenses and crystals of 

pyrite to each square millimeter, measuring, in sections across the cleavage, up to 

0.066 by 0.02, with their longer axes parallel to the cleavage, and the usual finely 

disseminated carbonaceous matter; also tourmaline prisms up to 0.007 by 0.008 mm.. 

No carbonate was detected.

The probable, relative abundance of these constituents, in descending order, is muscovite (sericite), quartz, chlorite, pyrite, biotite, and carbonaceous matter or graphite. 

Professor Merriman's tests of this slate are given on page 123.

This slate is used both for roofing and mill stock, particularly for electric purposes.
A. THIN SECTION OF BLACK ROOFING SLATE FROM THE 

MERRILL QUARRY AT BROWNVILLE, ME.

Showing a fine matrix of muscovite (sericite) with distorted octahedra of magnetite and (exceptionally for this quarry) a secondary plication resulting in slip 

cleavage ("false cleavage"). Ordinary light. Enlargement about 50 diameters. Bedding foliation and slaty cleavage here parallel. Lenses of chlorite and muscovite or quartz and muscovite or muscovite about some of the magnetite crystals.

Shales partly altered to 

finely cleft rock altefoliation, which is vi

cleavage. Lower bhandle is 30 inches 
MAINE. 65

North Blanchard. There are two quarries at North Blanchard, both operated by 

the Lowell Slate Company. At the State of Maine or Blanchard quarry 50 feet of slate 

and quartzite, ten beds of each, in alternation, are exposed, and 200 or 300 feet more 

have been prospected east of the quarry. The quarry measures between 250 and 300 

feet along the strike, 40 to 50 feet across it, and 200 feet in depth. Both bedding and 

cleavage strike N. 25° E. and dip east-southeast at 80°. The slate has vertical dip 

joints striking N. 70° W. and diagonal ones striking N. 40° W., dipping 32° SSW. 

There are also joints confined to the quartzite, dipping 65° to 70° SSW., and also 65° 

to 70° NNE. The grain strikes N. 65° W. and dips 90°, almost like the dip joints. . 

The surface of the deposit is glaciated and covered with 10 feet of till. Some of the 

quartzite surfaces show faint traces of marine life. At the Moosehead quarry, which 

lies one-half mile southwest or south-southwest of the last, more than 65' feet of slate 

and quartzite are exposed. The thickest beds of slate measure 4 and 7 feet. The 

quarry measures about 500 feet along the strike, 50 feet across it, and 125 feet in 

depth. Bedding and cleavage both strike about N. 37° E. and dip east-southeast at 

80°. Dip joints strike N. 55° W. and dip 90°. The quartzite on the west side of the 

quarry is broken up by undulating horizontal joints from 1 to 4 feet apart. The 

grain corresponds to the dip joints.

The slate from these quarries is a very dark gray. To the unaided eye the texture 

and cleavage surface are fine, but the latter is only slightly lustrous. The slate contains a little carbonaceous or graphitic matter and no magnetite, but the sawn edges 

show pyrite. No effervescence in cold dilute hydrochloric acid. It is very sonorous 

and very fissile.

Under the microscope this slate shows a matrix of muscovite'(sericite), with, brilliant aggregate polarization. A thin section parallel to the cleavage shows muscovite 

scales sufficiently numerous and parallel to produce a slight aggregate polarization. 

This may be due to an unusually pronounced grain. The quartz fragments occasionally measure 0.028 mm., and are not abundant. There are about one hundred 

scales of chlorite, interleaved with muscovite or sometimes biotite, to each square 

millimeter, with their lamina; across the cleavage and measuring up to 0.066 by 

0.028; also, about seven- biotite scales to each square millimeter, measuring up to 

0.085 by 0.047 mm., often bordered by secondary quartz or muscovite in the direction of the slaty cleavage, but with their lamina; transverse to it. There are about 

two hundred lenses of pyrite to each square millimeter, measuring from 0.004 to 

0.03 mm. in length and up to 0.01 in width; much dark-gray carbonaceous or graphitic matter in exceedingly fine particles; tourmaline prisms up to 0.07 by 0.009 mm. 

are plentiful. No carbonate or slate needles found. The chief constituents, arranged 

in descending order of abundance, appear to be muscovite, chlorite, quartz, pyrite, 

carbonaceous matter or graphite, and biotite.

This slate is used for roofing and mill stock, including electric appliances.

Brownville. Only one quarry is now in operation at Brownville, the " Old Merrill," 

operated by the Merrill Brownville Slate Company. This quarry lies less than a mile 

about northeast of the station (see PI. XII). Here are exposed 42 beds of slate 

alternating with as many of quartzite, and measuring altogether 165 feet in thickness. The slate beds range from 6 inches to 6 feet, and the quartzite beds from 6 

inches to 5 feet 6 inches. Any quartzitic slate is considered quartzite in these calculations. a . . - At the Hughes quarry, owned by the same company, but now idle, and situated 

a mile northwest of the Merrill, there are 28 beds of slate alternating with 28 of 

quartzite, measuring in all 161 feet 6 inches. The slate beds range from 1 to 9 feet 

and the quartzite from 4 inches to 20 feet in thickness. There is no evidence of 

duplication in these series of beds. Some of the quartzite is grayish and medium

. a Measurements obtained through the courtesy of C. H. Dunning, Supt.
66 SLATE DEPOSITS AND INDUSTEY OF UNITED STATES.

grained. Under the microscope it proves to be chloritic, pyritiferous, and slightly 

biotitic, with rare grains of zircon.

The Merrill quarry measures about 450 feet along the strike, between 165 and 200 

feet across it, and 250 in depth. The bedding strikes N. 78° E., dips 75° NNW.; 

the cleavage strikes N. 68° E., and dips 70° NNW. Dip joints strike N. 20° E., dip 

82° WNW.; diagonal joints strike N. 60° W., dip 90°. There are also horizontal 

joints, to which the grain is parallel. Quartz veins are not conspicuous, but there 

are some qiiartz lenses from 2 to 3 feet in diameter. These veins contain a little 

biotite. The south wall of the quarry (see PL XII), which is formed by a quartzite 

bed, is divided into rhombic blocks about 10 feet in their longer diameter, owing to 

the intersection of joints dipping 25° W. and 30° E. respectively. The surface, of 

this bed has also what resembles a coarse ripple marking, but is probably a minor 

effect of the rhombic jointing.

The slate is a very dark gray. To the unaided eye it has a very fine texture and 

a very smooth cleavage surface, with a very bright luster. It is slightly carbonaceous 

or graphitic. When powdered, it yields considerable magnetite to the magnet. The 

sawn edges show lenses of pyrite a millimeter and less in length. Some of the cleavage and other surfaces on the dumps show a very dark purplish coating. There is 

no effervescence in cold dilute hydrochloric acid nor any discoloration whatever. It 

is very sonorous.

Under the microscope it shows a very fine grained matrix of muscovite (sericite), 

with a very brilliant aggregate polarization. It contains much quartz in fragments 

up to 0.076 by 0.02 mm.; about 5 biotite plates to each square millimeter, measuring 

up to 0.076 by 0.03 mm., lying across the slaty cleavage. These often form the 

nuclei of quartz lenses which measure up to 0.4 by 0.03 mm. But the most conspicuous feature, next to the brilliant matrix, is the abundance of magnetite in tabular 

crystals, probably distorted octahedra, lying parallel to the cleavage, about 43 to each 

square millimeter, and measuring from 0.009 to 0.141 mm. in length and up to 0.02 

in width, rarely 0.17 by 0.04. These crystals are sometimes bordered by secondary 

quartz and muscovite or chlorite, 011 one or both sides, particularly whenever they 

happen to diverge from the cleavage direction. These secondary minerals occupy 

spaces resulting from a movement of the crystals after the commencement of slaty 

cleavage. There are also, but in less abundance, lenses of pyrite, up to 0.62 long, but 

sometimes 0.75 and 0.12 wide, and consisting of a nucleus of pyrite surrounded by 

secondary quartz or by this and biotite, these minerals forming the tapering part of 

the lens. There is also the usual abundance of dark gray graphitic? material in 

extremely fine particles. Not a few prisms of tourmaline occur, up to 0.043 by 0.008 

mm. No carbonate.

PI. XI, A will give some idea of the distribution of the magnetite crystals in this 

slate, but the "false cleavage" of the specimen is not typical of the product of the 

Merrill quarry indeed it is quite exceptional. The specimen was selected to illustrate "false cleavage" as well the fineness of slaty cleavage. The principal constituents, arranged in descending order of abundance, appear to be muscovite 

(sericite), quartz, magnetite, pyrite, carbonaceous matter or graphite, biotite, 

chlorite, tourmaline. Prof. W. 0. Crosby found that the slate of the East Brownville Slate Company had an average crushing strength of 29.270 pounds to the square 

inch, the weight being applied perpendicular to the cleavage, and that it required 

3,550 pounds to break a slab 6 inches wide, 1 inch thick, and 11 inches long between 

supports, the load being applied at the middle. This would give a modulus of rupture of 9,762 pounds per square inch. The results of Professor Merriman's recent 

tests of Brownville slate will be found on page 123.

The product of the Merrill quarry is now used exclusively for roofing purposes; 

its magnetite, it is thought, prevents its use for electric appliances. However, a piece 

6 by 4 by one-half inches makes no impression whatever on the magnetic needle,
U. S. GEOLOGICAL SURVEY 

THE MERRILL SLATE QUARRY AT BROWNVILLE, 

Seen looking S. 60° W. The end wall, working face, has 42 beds of slate alternating with as many of qua

The left wall consists of a quartzite bed with diagonal jointi
MAINE MAEYLAND. 67

and the section photographed in PI. XI, A came from that piece. The Brownville 

slate is highly crystalline.

Forks, Somerset County. A slate prospect opened in this town in 1890 was visited 

by the writer in 1905.«

This prospect is about 18 miles west of the North Blanchard quarries, in the southwest corner of the town of Forks, about 3 miles northeast of Caratunk, and about a 

mile northwest of Pleasant Pond. It is on Holly Brook, on land owned by Lawrence 

Hill. The nearest railroad is the Somerset Eailway extension at Mosquito Narrows, 

6 miles distant.

The cleavage strikes N. 55° E. and dips from 90° to steep northwest and southeast, 

owing to minor folding. The bedding is probably not far different.

The slate is bluish black, of fine texture and cleavage surface, with a luster not so 

great as that of the Brownville slate. It is graphitic, contains a very small amount 

of magnetite, has no argillaceous odor, does not effervesce in cold dilute hydrochloric 

acid, is sonorous, splits, and can be perforated readily.. Neither the ledge nor the 

fragments, said to have been exposed fifteen years, show discoloration.

Under the microscope the section shows a matrix of muscovite (sericite), with a 

brilliant aggregate polarization, proving it to be a mica slate. The cleavage is fine 

and regular. There are about 52 lenses of pyrite to each square millimeter, measuring (in transverse section) from 0.02 to 0.06 rnm. in length by from 0.004 to 0.016 

mm. in width. In sections parallel to the cleavage these lenses have a very irregular outline and are often as broad as long. These lenses account for the limonitic 

staining on cleavage surfaces of water-soaked specimens. Quartz is abundant but 

minute. No carbonate was detected. A few tourmaline prisms up to 0.11 mm. in 

length. Some scales of chlorite with interleaved muscovite measure up to 0.09 mm. 

There are rare zircon fragments and aggregations of rutile crystals.

The constituents of this slate, arranged in descending order of abundance, appear 

to be muscovite, quartz, chlorite, pyrite, and graphite, with accessory tourmaline, 

zircon, and rutile.

This Pleasant Pond slate differs from the Monson slates in having a lustrous and 

smooth surface, and from the Brownville slate in having much less magnetite and a 

little less luster. It would prove suitable for roofing or mill stock purposes.

Whether, like the other slates of this State, it is interbedded with quartzite at frequent intervals could not be determined.

The more important features of Maine slates as brought out in the above descriptions will be found set forth in tabular form opposite page 124.

MARYLAND.

' By T. NELSON DALE.

The slate of Cardiff, in Harford County, about 30 miles northeast of Baltimore, 

extends up into Pennsylvania, and will therefore be considered under a heading of 

its own (see pp. 85-88). Slate occurs also in Montgomery and Frederick counties, 

about 40 miles west of Baltimore and 33 miles northwest of Washington, where ill 

has been prospected and quarried to a small extent (see map forming fig. 5).

Geological relations. For the geology of the region the reader is referred to the 

writings of Williamsl Keyes, and Mathews. & Keyes's section passes through Sugarloaf Mountain and along the Baltimore and Ohio (Metropolitan Branch) Railroad to

«See note on a new variety of Maine slate in Bull. U. S. Geol. Survey No. 285.

l> Williams, G. H., The petrography and structure of the Piedmont Plateau in Maryland: Bull. Geol. 

Soc. Am., vol. 2, 1891, pp. 301-318, and map, PI. XII. Keyes, C. R., A geological section across the 

Piedmont Plateau in Maryland: Bull. Geol. Soc. Am., vol. 2, 1891, pp. 319, 322, and section fig. 3. 

Mathews, E. B., On liamsville slate: Maryland Geol. Survev vol. 2, 1898, pp. 231-232, and map, 

PI. XXX.
68 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

the northern corner of the District of Columbia. Its important features are these: 

Sugarloaf Mountain is a mass of eastward-dipping Cambrian sandstone passing 

toward the east into and under a belt of slaty or schistose rocks (phyllites), which

77°25' 77*20

FIG. 5'. Map of parts of Frederick and Montgomery counties, Maryland, showing (by dots) slate prospects and abandoned quarries.

Williams described as containing sericite, chlorite, quartz, hematite, and tourmaline. 

The phyllite area is traversed in several places by Mesozoic diabase dikes, and east 

of it are contorted gneisses.
MARYLAND. 69

^

This slate belt, whatever may be its exact bounds, is well exposed atljamsville, on 

Bush Creek, and the Baltimore and Ohio Railroad in Frederick County, where it has 

a well-defined strike of N. 10° E. Between Ijamsville and a point 2% miles southsouthwest of that place it is at least 1J miles wide, and reappears west of Hyattstown, 

3£ miles farther south in Montgomery County; and from that pointits exposures and 

cleavage strike range from S 20° to 37° W. It thus passes between Sugarloaf Mountain and the village of Mount Ephrairn on the west and Hyattstown and Barnesville 

on the east, its minimum length and width being about 12 and 1^ miles respectively. 

Immediately east of this belt are chloritic schists and altered diabases.'

Ijamsville. One-fourth mile west of Ijamsville station is an abandoned quarry in 

dark, slightly reddish purple slate. « The bedding, shown by light green ribbons, 

strikes N. 10°-15° E., and dips 30°-40° W., but the cleavage strikes N. 40° E. and 

dips 50° SE. There are intermittent joints striking N. 15°-20° E., dipping 75° ESE., 

which have been parted a fourth of an inch, lined with quartz and filled with calcite. Similar slate, but interbedded with light green, occurs also several hundred 

feet east and north of the station.

Thurston. On Little Bennett Creek, about a half mile east-southeast of Thurston, 

in Frederick County, is another quarry (opening 70 by 50 feet) operated twenty years 

ago but now abandoned. The slate is also dark purplish with light-green passages 

containing here and there a scaly bright-green mineral, pyrophyllite, which carries 

from 1 to 2 per cent of copper oxide. The bedding, indicated by coarsely plicated 

quartzose passages, appears to be about horizontal. A piece of this slate that is said 

to have been on a roof many years shows some lightening of the color, owing chiefly 

to the growth of lichens, but the change is only superficial. The cleavage strikes N. 

20° E. and.dips eastward at an angle of 60°. There is a 2-foot quartz vein in the "top."

About 600 feet south-southwest of this and one-half mile southeast ,of Thurston is the 

quarry recently opened or reopened by the Bennett Creek Slate Company, measuring 75 feet along the cleavage by 65 feet across it, which, in May, 1904. had a depth 

of 40 feet. The upper 25 feet are "top." The cleavage strikes N. 37° E. and dips 40° 

E., with green spots aligned in the same direction. Joints strike N. 30° E. and 

dip 40 ° W. The grain is almost at right angles to cleavage. The slate is dark 

purplish but has a bluer tinge than that of Ijamsville. To the unaided eye it has a 

slightly granular texture, a rather fine cleavage surface with distinct grain and a little luster. It contains no magnetite, does not effervesce with cold dilute hydrochloric 

acid, and is somewhat greasy to the touch. The sawn edges show light-green particles and more minute pyrite ones. The sonorousness is medium. Some of the slates 

on exposure develop dark spots, due to the oxidation of some mineral. The amount 

of quartz is so small that slabs a half inch thick may be easily sawn across the cleavage and the grain with a handsaw.

Under the microscope it shows brilliant aggregate polarization, but much irregularity in size of particles and very slight plications in the direction of the grain. 

The matrix of muscovite (sericite) contains abundant quartz fragments, measuring 

up to 0.047 and sometimes 0.094 mm. There are conspicuous scales of chlorite, 

often interleaved with muscovite lying transverse to the cleavage, measuring up to 

0.38 by 0.25 mm., exceptionally 0.66 by 0.14 mm., and forming the centers of lenses 

of sericite or talc? & measuring up to 1.5 mm. in length. There are also lenses of 

sericite or talc? & measuring up to 0.56 by 0.09 mm. with their long axes in the grain 

direction.

aSee Maryland Geol. Survey, vol. 2, pp. 231, 232,-pl. XXIV, fig. 2.

&Mathews, following some determination by G. H. Williams, regarded these slates as containing 

talc. (Maryland Geol. Survey, Vol. 2, p. 232.) A chemical test just made by George Steiger at the 

U. S. Geol. Survey laboratory shows 0.27 per cent of Si02 , soluble in 1-20 solution of NooSO^ before 

ignition, and 1.09 per cent after ignition. F. W. Clarke, chief chemist, notes that the difference, 0.82 

per cent, corresponds to a splitting off of SiO;. from the talc if talc is present. It would be one-fourth 

of the SiOo in the talc and thus represent a percentage of 5.17 of talc. As the only other silicates in 

the slate are crystalline quartz, chlorite, muscovite (sericite), and tourmaline, this result is reliable. 

The combination of chlorite and sericite alone would be sufficient to account for the "talcose" 

touch of the slate.

Bull. 275 06 0
70 SLATE DEPOSITS AND 1NDUSTKY OF UNITED STATES. 

f

These scales and lenses number from 10 to 14 per square millimeter, of which about 

half are chlorite. Multitudinous minute dots of bright red hematite occur throughout the slate, to which and the chlorite it owes its purplish color. Among these 

dots are many particles of pyrite, some of which measure up to 0.038 mm. A few of 

these have passed' into limonite. Slate needles (Ti02 ) are somewhat plentiful. No 

carbonate. Tourmaline 0.076 by 0.009 mm. The important constituents of this 

slate, arranged in order of abundance, appear to be muscovite, chlorite, quartz, 

talc, hematite, pyrite.

Its deficient sonorousness is not due to the absence of a micaceous matrix, but to 

the presence of a large amount of chlorite and to about 5 per cent of talc. It is a 

mica slate.

Mount Ephraim. About 4 miles southwest of the Bennett Creek quarries and onefourth mile south-southeast of the village of Mount Ephraim, in Montgomery 

County, east of road to Barnesville and north of Little Monocacy River, on the 

property of Abraham Harris, a similar slate has been prospected. The cleavage 

strikes K 30°-35° E. and dips 70° E. The bedding probably Strikes N. 30° E., dip 

uncertain. 

The characteristics of Maryland slates are shown in the table facing page 124.

MINNESOTA.

Location of deposits. Deposits of roofing slate occur in northern Minnesota, a few 

miles west of Duluth. At present, however, all the quarries formerly opened here 

are abandoned, and the quality of the slate, as seen in specimens on the old dumps, 

is hardly such as to justify reopening.

References on Minnesota slates. The following reports and papers, while not economic in intention, contain data of interest on the distribution and character of the 

Minnesota slates:

HALL, C. W. Keewatin area of eastern and central Minnesota. Bulletin Geol. Soc.

America, vol. 12, pp. 345-376. 1901. 

SPURR, J. E. The stratigraphic position of the Thomson slates. American Journal of

Science, 3d series, vol. 48, pp. 159-166. 1894. . WINCHELL, N. H. The geology of Carlton County: Vol. 4, Final Reports Minnesota

Geol. Survey, pp. 1-24. 189.9.

 The geology of the southern portion of St. Louis County: Ibid., pp. 212-221.

 The geology of the Carlton plate: Ibid., pp. 550-565.

 The geology of the Duluth plate: Ibid., pp. 566-580.

NEW JERSEY.

The Bangor-Slatington slate belt of Pennsylvania is prolonged eastward into New 

Jersey, and roofing-slate quarries have been opened at several points, notably near 

Newton and Lafayette.

NEW YORK.«

By T. NELSON DALE.

GEOLOGICAL, RELATIONS.

The slates of Washington County, N. Y., are a continuation of those of Rutland 

County, Vt. The Lower Cambrian affords greenish and purplish slates, like those 

in Vermont described on page 104, which were once extensively quarried at Middle

"The slate of Washington County, N. Y., was described by the writer in part 3 of Nineteenth 

Ann. Kept. U. S. Geol. Survey in 1899, but the matter appears here in revised form.
NEW YOKE. 71

Granville and Jamesville. Overlying the Lower Cambrian, in isolated lenticular 

synclinal areas or in long, ramifying masses of complex structure, are Ordovician 

(Hudson) grits and black, red, and green shales; and these red and green shales 

pass here and there into roofing slates.«

While the Lower Cambrian greenish and purplish slates are at present quarried 

almost exclusively 011 the Vermont side of the boundary, the red and green slates 

attain their best development on the New York side, particularly in the towns of 

Granville, Whitehall, and Hampton. The structural relations of the two slatebearing formations on the New York side are shown in sections I, II, VII of PI. 

XXII, and are explained in detail on page 100.

As to mode of occurrence, these red and green slates seem at several points to 

replace the Hudson grit along the strike. Certainly these slates occur in as close 

proximity to the Cambrian slates as does the grit. Black graptolite shale sometimes 

crops out very near to and probably underlies the red slate. Beds of red and green 

slate alternate vertically, and replace one another along the strike, and also pass 

into shales of the same colors. The thickness exposed at the quarries reaches 50 

and 75 feet, mostly red, with about 25 feet of green overlying, but, subtracting that 

which is too hard or too soft or badly veined, there are sometimes but 10 feet, rarely 

more than 25 feet, of good red slate exposed at any one quarry, although it sometimes reaches 42 feet. Owing to the character of 'the folds and their pitch, as well 

as the merging of the colors along the strike, it is not easy to ascertain the total 

thickness of the red and green. A few feet or inches of dark red or purple sometimes occur in the red. Beds of greenish quartzite, sometimes calcareous, and bordered by a purple slate, the whole "ribbon" measuring an inch or two in thickness, are not uncommon (see PI. II, A). Also beds one-half inch thick of rhodochrosite (manganese carbonate) with crystalline calcite.

An analysis of this (Specimen D. XIV, '95, 201d), made by Mr. George Steiger, 

yielded the following:

Analysis of rhodochrosite bed.

AljOj............................ 0. 68

Fe2 08 ........................... .14

FeO ............................ 1.13

MnO ........................... 3.2.22

NiO and CoO ................... .10

CaO ............................ 3.81

MgO ........................... 2.61

C02 ............................ 25.06

Insoluble matter, including all silica from dissolved silicates..... 32. 75

Total..................... 98.50

Under the microscope thin sections of this bed show, with polarized light, a finegrained bluish-brown matrix identical -in color and texture with that of the small 

lenses in the red slate and with some of the lenses in the green slate; also large areas 

of calcite and some quartz.

There are also quartz veins in both red and green, sometimes crystallized, and in 

the red light-green spots with or without a purple rim. Both red and green slates 

are frequently speckled with lenses (see p. 11). The bedding planes are often covered with glistening annelid trails and with possible impressions of algse. Thin films 

of barite and calcite occur-on the joint planes.

The location of the more important red slate quarries in the towns of Granville and 

Hampton are shown on PI. XX. The structure at two of the quarries is shown diagrammatically in figs. J/of PI. XXIII and t/of PI. XXIV, and the compass courses 

at the more important quarries, with reference numbers to their location on the map, 

PI. XX, are given below, but Nos. 33, 39, 41 lie west of the area mapped; 33 is 3J 

miles north-northeast of Middle Granville, in Granville township; 41 is 2f miles

"See map forming PI. XIII in Nineteenth Ann. Kept. U. S. Geol. Survey, pt. 3; also the Economic 

goology and structure section sheets of the forthcoming Mettawee and Fort Ticonderoga folios.
72 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

northeast of Middle Granville, also in Granville; and 39 is 2| miles west of Janesville, in Whitehall.

Quarry number.

33..................................

34 ..................................

35..................................

36.................................. 

37..................................

38..................................

39..................................

40 ..................................

41..................................

64..................................

66..................................

Quarry number.

33..................................

34..................................

35..................................

36..................................

37..................................

38..................................

39..................................

40..................................

41..................................

64..................................

66..................................

Bedding.

Strike.

N. 

N. 

N.+ 

N. 20° W. 

N. 12° W. 

N. 10° W. 

N. 5° E. 

N. 

N. 

N. 15° W. 

N. 30° W.

Dip.

30± E. 

45° E. 

15° E. 

E.-W. 

35° E, 

25° E. 

50° E. 

' 35° E. 

30° E. 

22° E. 

20° E.

Dip joints.

 Strike.

N. 65° W.

E.-W.

N. 75° W.

N. 77° E.

N. 80° W.

Dip.

90°

90°

90°

90°

90°

Cleavage.

Strike.

1

N. 15° E. 

N. 5° E. 

N. 12° W. 

N. 5° W. 

N. 5° W. 

N. 

N. 

N. 

N. 15° W.

Dip.

20 E. 

30° E.

25° E. 

25° E.

Diagonal joints.

Strike.

N.30°E.

Dip.

70° W.

I

Strike joints.

Strike,

N.+

N. 10° W.

N. 15° W.

Grain 

strike.

Dip.

55° E.

25° E.

50° E.

Shear zones. 

" Hogbacks."

Strike.

N. 7° E.

N. 55°W.

Dip.

77° E.

70° NE.

DESCRIPTIONS OF THE SLATES.

Red slate. The "red" slate is a decidedly reddish brown, becoming brighter on 

exposure. To the unaided eye its texture is fine, and its cleavage surface varies 

from slightly roughish to speckled with minute protuberances ('' eyes " or " knots "), 

in either case without luster. It is rnagnetitic, sometimes effervesces with cold dilute 

HC1, is sonorous, and has an argillaceous odor. Under the microscope shows a matrix 

of muscovite (sericite) with somewhat faint aggregate polarization, but this faintness 

is partly due to obscuration by carbonate and hematite, many of the plates of the 

former remaining light when all the mica is dark and the hematite darkening the mica 

when it is light. Distributed throughout the matrix are multitudinous bright-red 

hematite dots of circular or irregular oval outline, measuring from 0.0004 to 0.009 mm. 

There is much irregularity in the size of the particles, indicating but a fair grade of 

flssility. The larger particles consist of quartz fragments up to 0.06 by 0.03, rare 

grains of plagioclase feldspar and of zircon, considerable carbonate in rhombs or plates 

up to 0.047 mm., scales of chlorite interleaved with muscovite, up to 0.075 by 0.036 

mm., a very variable number of lenses up to 0.34 by 0.15, rarely 0.5 mm. long, consisting probably of rhodochrosite (carb. manganese) and chalcedonic quartz (containing carbonate rhombs and musco^dte scales, sometimea partial pseudomorphs of 

chlorite after carbonate), and finally some tourmaline prisms up to 0.005 by 0.001 mm.

The chief constituents of the red slate, arranged in descending order of abundance, appear to be muscovite (sericite), quartz, hematite,kaolin, carbonate, chlorite, 

magnetite, rhodochrosite. For colored lithographs of magnified thin sections of this
YOEK. 73

slate, as seen under both ordinary and polarized light, see Nineteenth Annual Report 

U. S. Geological Survey, part 3, PI. XXVIII, A, B, C. For a discussion of the 

causes of the green and purplish spots, which occasionally occur in this slate, seepage 

15 of this bulletin. Associated with the red slate is generally a little purplish slate, 

sometimes speckled, but of no commercial importance. Under the microscope it 

shows less hematite, and possibly more chlorite. Analysis N, below, shows that it 

contains from 2£ to over 4 per cent less hematite and about one-third of 1 per cent 

more ferrous oxide than the red.

The following analyses were made in the chemical laboratory of the United States 

Geological Survey, the complete analyses by Dr. W. F. Hillebrand, the partial ones 

by Mr. George Steiger:

Analyses of red slates from New York.

Li20 (lithia) .............................

H20 (water below 110° C) ................

S03 (sulphuric oxide) ....................

Total...............................

Specific gravity ..........................

Specimens. «

J.

67.61 

.56 

13.20 

5.36 

1,20 

.10 

Trace? 

Trace? 

.11 

.04 

3.20 

' 4.45 

.67 

Trace. 

.45 

2.97 

.05 

None. 

.03 

Trace. 

None.

100. 00 

.016

K.

67.55 

.58 

12.59 

5.61 

1.24 

.19 

Trace. 

Trace. 

.26 

.31 

3.27 

4.13 

 .61 

Trace. 

.40 

3.03 

.10 

.11 

.04 

Trace. 

None.

100.02 

.02

L.

56.49 

.48 

1.1.59 

3.48 

1.42 

.30 

Trace. 

Trace. 

5.11 

.06 

6.43 

3.77 

.52 

Str. tr. 

.37 

2.82 

.09 

7.42 

.03 

Trace. 

None.

100. 38 

.016 

2. 7839

M.

63.88 

.47 

9.77 

3.86 

1.44 

.21 

Trace.- 

Trace. 

3.53 

.05 

5.37 

3.45 

.20 

Str. tr. 

.27 

2.48 

.08 

5.08 

Trace.

None.

100. 14

..........

K.-

7.10 

1.00

2.8085

N.

1.02 

1.67

..........

a J (=D. XIV, '95, 358d), red slate, H. H. Matthews's quarry, 1 mile west of Poultney, in Hampton, 

Washington County, New York; K (=D. XIV, '95, 201e), red slate, Empire Red Slate Company's 

quarry, 1 mile north of Granville, in Granville, Washington County, N. Y.: L(=D. XIV, '95, 397e), 

red slate, National Red Slate Company's quarry, 1 mile north-northwest of Raceville, in Granville, 

Washington County, N. Y.; M(=D. XIV, '95, 397a),red slate, same locality as L, but near a green and 

purple spot; K2 (=D. XIV, '95, 201/t), red slate, same as K, but finer grained; N (=D. XIV, '95, 284a), 

purple bed in red slate at Fair Haven Red Slate Company's quarry (not worked), two miles'north of 

Truthville, in East Whitehall, Washington County, N. Y. For presence of chromium and vanadium 

in these see Doctor Hillebrand's note, p. 41.

Some points in these analyses are discussed by Doctor Hillebrand on pages 39-42.

As these red slates do not discolor and yet show a considerable amount of carbonate 

under the microscope, and as the discoloration of the Vermont "sea-green" slates is 

due to the oxidation of a ferrous carbonate, it may be inferred that the carbonate 

here is chiefly one of lime and magnesia, but the effect of a very small percentage of 

ferrous carbonate would probably be masked by the brightness of the hematite.

J. F. Williams reported for this slate a modulus of rupture of 7,310 pounds per square 

inch, and Merriman 126.66 foot-pounds of work per pound of slate as a result of impact 

tests. Th.3 results of Merriman's recent other tests of this slate are given on page 123.
74 SLATE DEPOSITS AND INDUSTRY OE UNITED STATES.

Bright-greenish slate. The reddish slate is usually interbedded and exceptionally 

passes along the strike into a light bluish-green slate, brighter in color than the 

"unfading green" Cambrian slates of Vermont. Its color is peculiarly bright by 

lamplight; its texture and surface are similar to jthose of the "red," and the surface 

is also sometimes speckled with minute lenses. It is magnetitic, effervesces very 

slightly with cold dilute hydrochloric acid, is sonorous and is said to be unfading.

Under the microscope shows a matrix of muscovite (sericite) with a more brilliant 

aggregate polarization than that of the red slates, which may be due to the absence 

of the hematite, but the coarseness of the particles does not indicate a superior fissility. There are quartz grains up to 0.084 by 0.056 mm., chlorite scales up to 0.043 

mm., some carbonate rhombs from 0.002 to 0.056 mm., and lenses measuring up to 

0.385 by 0.128 mm., that consist either of rhodochrosite or chalcedonic quartz inclosing 

very minute carbonate rhombs and chlorite scales. Sections parallel to the cleavage 

show these lenses with a more roundish outline. There are also particles of pyrite, 

tourmaline prisms, rutile needles, and grains of zircon.

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite (sericite), quartz, chlorite, carbonate, magnetite, rutile.

Its color is due to the abundance of chlorite.

Bright-greenish roofing slate. The following analysis (specimen 0=D. XIV, '95, 

397c), by Dr. W. F. Hillebrand, is of a bright-greenish speckled slate from the 

National Red Slate Company's quarry, 1 mile north of Raceville, in Granville, Washington County, N. Y.

Analysis of bright-greenish slate from New York.

Si02 (silica).................... 67. 89

Ti02 (titanium dioxide)......... .49

A1203 (alumina)................. 11.03

Fe203 (ferric oxide)............. 1:47

FeO (ferrousoxide)............. 3.81

MnO (manganous oxide)........ .16

MO (nickelous oxide).......... Trace?

CoO (cobaltous oxide).......... Trace?

CaO (lime)..................... 1. 43

BaO(baryta)................... .04

MgO (magnesia)................ 4.57

K2O (potassa).................. 2. 82

Na2O. (soda)................... .77

Li20(lithia)................... Trace.

H2 0 (water below 110° C.) ..... .36

H2 0 (water above 110° C.)..... 3.21

P2 05 (phosphoric oxide) ....... .10

C02 (carbon dioxide).......... 1. 89

FeS2 (pyrite),.................. .04

S03 (sulphuric oxide).......... Trace.

C (carbon).................... None.

Total................... 100.08

S (sulphur, total).............. .022

Specific gravity=2.7171.

The relatively high percentage of magnesia is attributed to the abundance of both 

dolomite and chlorite.

The results of Professor Merriman's recent tests of the bright-greenish slates 

quarried by the Mathews Consolidated Slate Company are given on page 123.

Black slate. Black roofing slate of Ordovician age was quarried many years ago, in 

a small way, 3 miles south of Hoosick Fails, near Hoosick, and also, at a later time, 

2 miles south of Stephentown, near Lebanon Springs, in Rensselaer County, N. Y., 

but did not prove to be of economic importance at either locality. a

The principal features of New York slates as brought out in the above descriptions 

will be found in tabular form opposite page 124.

a PI. CI and fig. 26, Thirteenth Ann. Kept. U. S. Geol. Survey, show the structure at the Lebanon 

Springs quarry.
SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 75

PENNSYLVANIA.

By T. NELSON DALE.

GENERAL, FEATURES.

The slates of Pennsylvania, aside from those of Lancaster County and the southeastern part of York County, which are described under the heading " Peach Bottom " 

slate (pp. 85-88), occur in Northampton and Lehigh counties in a strip from 2 to 4 

miles wide on the southern side of the Blue Mountain, extending from Delaware 

Water Gap in a west-southwest direction to a point 4 miles west of Lehigh Gap, a 

distance of about 32 miles. The chief centers of the slate industry here are at Bangor 

and Slatington. The geographical relations of these places and of the other groups 

of slate quarries in these counties are shown on the maps of the Second Pennsylvania 

Geological Survey of Merrill's Stones for Building and Decoration (third edition) 

and of Merrirnan's article in Stone for July, 1898.

The general geological relations of this slate are these: On the southeast, forming 

an east-northeast to west-southwest belt between Easton and Reading and beyond, 

are the pre-Cambrian gneisses, etc., of South Mountain, flanked and dotted over with 

strips of Lower Cambrian quartzite and sandstone. Northwest of this and parallel 

to it is a great Cambro-Ordovician dolomite and limestone plain from 3 to 6 miles 

wide, under which the quartzite dips. Still farther northwest is a slightly hilly belt 

of Ordovician shales, grits, and roofing slates of Hudson age, from 6 to 8 miles wide. 

At the southeast these shales and slates overlie the limestone, and at the northwest 

they dip under the Silurian conglomerate and sandstone of the Blue Mountain. The 

boundary between the shale and slate formation and the limestone is roughly parallel 

to the general course of the Oneida and Medina boundary, but passes a little north of 

Nazareth.

The shale and slate formation measures from a minimum exposure of 1,600 feet to 

an estimated maximum of 6,000 feet, of which the commercial slate comprises but a 

few hundred feet. In structure this formation consists of a succession of minor close 

folds, generally overturned to the northern horizon, so that their axial planes have 

a southerly dip but usually so that the synclines have a steeply inclined southern and 

a gently inclined northern limb. Sometimes the fold is extremely close and the 

overturn so complete that its axial plane has a very low southeasterly dip. The 

cleavage dips to the southern horizon at various angles, sometimes at one of 5°, pointing, like the curvature of cleavage and jointing, to a secondary movement. These 

folds vary greatly in width and their axes also pitch alternately east-southeast and 

west-northwest at angles ranging from 5° to 10°. They also bend laterally from 

north to south. These folds have been more or less truncated by surface erosion, and 

in places the cleavage foliation has heen crushed and bent over to the south by the 

friction of the southward-moving ice sheet.

"Ribbons," or small beds of grit, measuring from a fraction of an inch to two feet 

in thickness, characterize the slate belt throughout. This grit consists mostly of 

more or less angular grains of quartz and feldspar, rarely of shale and quartzite, 

together with scales of muscovite and lenses of chlorite, spherules of pyrite, carbonaceous matter, all in a cement of calcite and sericite with rhombs of carbonate more 

or less altered to limonite. The ribbons, as explained on page 6, represent coarse 

sediments, brought in, probably, by local marine currents.

Commercial slate is obtained along two belts. The upper and northerly one, known 

as the "soft vein," which is separated from the overlying Silurian conglomerate by 

an uncertain thickness of shale and slate, consists of beds of relatively soft slate of 

sufficient thickness between the ribbons to furnish large slabs suitable for mill stock 

or roofing purposes. Tne lower and southerly belt, the "hard vein," near the base
76 v SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

of the Hudson formation, consists of small beds of harder slate separated by small ribbons which are not coarse enough to interfere with their use either as mill stock or 

roofing slate.

The Bangor, East Bangor, Pen Argyl, Danielsville, Slatington, and Slatedale quarries are in the '' soft vein " belt and the Belfast and Chapman are in the '' hard vein.'' 

Judging from the map of the Second Geological Survey, Pennsylvania, which represents a limestone area, presumably anticlinal in structure, as extending from the 

Delaware near Portland to the vicinity of East Bangor, the "hard vein" ought to 

recur there near the limestone.

The literature of the slate of Northampton and Lehigh counties will be found in 

the Bibliography on pages 138-145, under the names Chance, Lesley, Merrill, Merrinian, Eogers (H. D.), Sanders.

A few typical quarries and slates from these belts will now be described.

NORTHAMPTON COUNTY.

Bangor. The "Old Bangor" quarry at Bangor, now operated by J. S. Moyer & Co., 

is the largest slate quarry in the United States and the oldest one in this region. It is 

regarded as being near the top of the "soft vein" belt. It measures over 1,000 

feet along the strike, 500 across it, and from 200 to 300 feet in depth. (See Pis. 

XIII-XV.) The general structure is a close overturned syncline, striking about 

N. 25° E., with an axial plane dipping east-southeast at a low angle and pitching 

5°-10° SSW., crossed by cleavage dipping from 5° to 10° S. 32° E. There is a gritty 

bed both at the top and bottom of the quarry, which is regarded as the limit of tne 

slate i. e., the lower limit, drill cores from a depth of 200 feet below the quarry 

having failed to show any good slate. The thickest bed of good slate is 9 feet thick. 

Vertical diagonal joints striking N. 52°-57° E. are conspicuous. There are also 

vertical dip joints and horizontal ones. Curvature of the cleavage occurs here and 

there. Certain diagonal or longitudinal joints intersecting the bedding planes at the 

northern corner of-the quarry produce an optical illusion, for the beds there seem to 

curve over to the west-northwest instead of the east-southeast, and thus, in connection with the syncline on the east-southeast wall, to form a complete ellipsoid. 

The ribbons are markedly more calcareous than the slate, and in weathering become 

white from incrustation of lime. Veins of white calcite and quartz, 4 inches thick, 

occur in the strike joints. Crystalline films of gypsum were collected on the horizontal joint faces.

The slate from ,the "Old Bangor" quarry is very dark gray, and to the unaided 

eye has a fine texture and a fine cleavage surface, almost without any luster. The 

sawn edge shows pyrite. It contains very little magnetite, is carbonaceous or graphitic; effervesces with cold dilute hydrochloric acid, is sonorous, and has an argillaceous odor.

Under the microscope this slate shows a matrix of muscovite (sericite), with a 

brilliant aggregate polarization, somewhat obscured by abundant carbonate in plates 

and rhombs. The cleavage is regular, but the particles are coarse. There are occasional chlorite scales up to 0.124 by 0.066 mm.; quartz grains not very abundant up 

to 0.047 mm. Pyrite spherules number about two hundred per square millimeter, 

measuring up to 1.01 mm. in diameter. Eutile needles are very plentiful. Carbonaceous or graphitic matter occurs in fine particles throughout.

A tranverse section of a coarse ribbon from this quarry showed, under the microscope, bedding crossed by cleavage at an angle of 27°, a matrix of sericite in irregular 

stringers, without any aggregate polarization, and carbonate and carbonaceous matter, containing angular quartz grains up to 0.17 by 0.12 mm., feldspar grains (plagioclase, orthoclase) up to 0.14 by 0.09 mm., the quartz and feldspar together making 

up no more than 25 per cent of the ribbon; also spherules of pyrite up to 0.07 mm.,
U. S. GEOLOGICAL SURVEY

OLD BANGOR SLATE QUARRY, BANGOR, PA., NORTH-NORT

Showing the close overturned syncline crossed by almost horizontal cleavage. Co
U. S. GEOLOGICAL SURVEY 

OLD BANGOR SLATE QUARRY, BANGOR, PA., SOUTH-SOU

Showing the eroded overturned close syncline crossed by almost horizontal cleavage
PENNSYLVANIA.' 77

sometimes in a lens of chlorite; some large plates of rauscovite and of chlorite interleaved with muscovite, up to 0.06 mm., and rarely a zircon, fragment, 0.03 mm.

The constituents of this slate, arranged in descending order of abundance, appear 

to be muscovite (sericite), carbonate, quartz, kaolin, pyrite, chlorite, rutile, magnetite, and carbonaceous matter or graphite.

The final results of Professor Merrirnan's physical and chemical tests of "Old Bangor '' slate are given here for convenience of reference: Strength (modulus of rupture), 

9,810 pounds per square inch; toughness (ultimate deflection on supports 22 inches 

apart), 0.312 inch; density (specific gravity), 2.780; softness (amount in grains 

abraded by 50 turns of a small grindstone), 128; porosity (per cent of water absorbed 

in twenty-four hours), 0.145; corrodibility (per cent of weight lost in sixty-three 

hours in acid solution composed of 98 per cent H2 0,1 per cent HC1, 1 per cent 803 ), 

0.446. Per cent of lime, 4.38; computed carbonate of lime, 7.82 per cent; computed 

carbonate of magnesia, 5.65 per cent."

The product from the large beds is used for roofing, but that from the ribboned 

beds goes into mill stock.

Quarries of the North Bangor Slate Company. The large quarry is about 200 feet 

square and 350 feet deep. The folded beds strike about N. 60° E. and the cleavage 

dips southwestvvard, at an angle of 15°. The total thickness of good slate here, 

measured across the cleavage, is 300 feet. In the smaller quarry the thickest bed 

measures 9 feet, and is reserved for blackboards.

In a quarry adjacent to the first of these two quarries, operated by Auld & Conger, 

a completely overturned syncline, several hundred feet from northwest to southeast, 

is exposed on the east-northeast wall, with an almost horizontal cleavage.

The slate from these quarries is very dark gray, and to the unaided eye has a fine 

texture and a very smooth cleavage surface, with but slight luster. It is somewhat 

carbonaceous or graphitic, and contains some magnetite. The sawn edge shows 

pyrite. It effervesces somewhat freely with cold dilute hydrochloric acid; is sonorous, 

and has an argillaceous odor.

Specimens from the North Bangor Slate Company's quarry show, under the microscope, a matrix of muscovite (sericite) with a somewhat brilliant aggregate polarization and a general-uniformity in size of particles. The aggregate polarization is 

partly obscured by the very abundant carbonate. Quartz fragments are not very 

abundant and measure up to 0.04 mm.. There are some chlorite scales up to 0.09 mm., 

and about three hundred and sixty spherules of pyrite, measuring up to 0.01 mm. to 

each square millimeter. Also carbonaceous matter or graphite and very abundant 

rutile needles. 

The constituents, arranged in descending order of abundance, appear to be muscovite (sericite), carbonate, quartz, kaolin, pyrite, chlorite, magnetite, rutile, carbonaceous matter or graphite. In a black-ribboned piece from the same quarries the 

half-inch ribbon shows much carbonaceous'matter and many spherules of pyrite. 

The quartz.particles arid the plates of carbonate are larger in the ribbon than in the 

adjacent slate. The product of this company's quarries is used for blackboards, billiard tables, fireboards, stationary tubs, well covers, tiles, lathe work, etc.

East Bangor. The large quarry of the East Bangor Consolidated Slate Company 

measures 350 feet along the strike and 700 feet across by 130 feet in depth. The beds 

dip 5° NNE. and the cleavage 15° to 20° SSW. The main joints strike north and 

south and undulate in a general vertical direction. The grain strikes N. 50°-60° W. 

"Ribbons are frequent. The slate is very dark-bluish gray, and to the unaided eye has 

a fine texture and a somewhat fine but almost lusterless cleavage surface. It is both 

carbonaceous, or graphitic, and magnetitic. The sawn edge shows a little pyrite. It

aTrans. Ain. Soc. Civ. Eng., vol. 27, 1892 (3), p. 342-347.
78 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

effervesces slightly with cold dilute hydrochloric acid; is sonorous, and has*a scarcely 

perceptible argillaceous odor.

Under the microscope a specimen taken between "ribbons" shows a matrix of 

muscovite (sericite) with a brilliant aggregate polarization, somewhat obscured by . 

carbonate, a texture a little finer than that of "Old Bangor" slate, but carbonate as 

abundant as in that. Not very much quartz in grains measuring up to 0.04 by 0.02 

mm.; about seventeen scales of chlorite, with interleaved muscovite, to each square 

millimeter, measuring up to 0.07 by 0.03 mm., exceptionally 0.08 by 0.04 mm.; about 

six hundred spherules of pyrite per square millimeter, measuring up to 0.008 mm.; 

rutile needles very abundant; carbonaceous or graphitic matter in fine particles. 

. The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite, carbonate, quartz, pyrite, chlorite, kaolin, rutile, carbonaceous 

matter or graphite, magnetite.

The product of this quarry is used both for roofing and for mill stock, but not for 

blackboards, as the 9-foot bed is crossed by ribbons. The roofing slates are said to 

show comparatively little discoloration after thirty years' exposure.

Heimbach. The Heimbach quarry about 1$ miles northeast of Slatington, but in 

Northampton County, is now operated by Jay S. Moyer & Co. There is a thick bed 

here known as the " Heimbach big bed," which is very dark gray and less bluish 

than " Old Bangor." To the unaided eye it has a fine texture and a somewhat fine 

but almost lusterless cleavage surface. It contains an exceedingly small amount of 

magnetite; effervesces with cold dilute hydrochloric acid, and is sonorous.

Under the microscope it shows a matrix of muscovite (sericite),with but faint aggregate polarization owing to the abundance of carbonate, which much exceeds that in 

the "Old Bangor" slate. The cleavage is considerably finer and there is greater 

regularity in the size of particles than in that slate. There are a few chlorite scales 

up to 0.06 by 0.04. Very little quartz is seen, in grains reaching only 0.036 mm. 

Pyrite spherules measure up to 0.-012 mm. and number about 116 per square millimeter. Kutile needles are very abundant. The usual carbonaceous matter is seen.

The constituents of the " Heimbach big bed," arranged-in descending, order of 

abundance, appear to be muscovite (sericite), carbonate, quartz, pyrite, chlorite, 

rutile, carbonaceous matter, magnetite, but the carbonate is larger in amount than in 

some slates in which its relative position is the same.

There is also in the same quarry a 6-foot bed, which is quite black, more so than 

any of the Northampton or Lehigh County slates. It has, to the unaided eye, a 

somewhat fine texture and cleavage surface with a slight luster. It shows pyrite 

on the sawn edge, is quite carbonaceous or graphitic, very slighly magnetitic, effervesces in cold dilute hydrochloric acid, is sonorous, and has a slight argillaceous odor.

Under the microscope the "Heimbach black bed" shows a fine matrix of muscovite (sericite), with but faint aggregate polarization owing in part to the very large 

amount of carbonate and carbonaceous matter. The number of quartz fragments is 

relatively small and they measure up to 0.028 mm. There are chlorite scales up to 

0.06 mm. Spherules of pyrite measure up to 0.008 mm., exceptionally 0.017 mm. 

and number about 135 per square millimeter. Rutile needles abound. Karely a 

scale of hematite is seen.

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite (sericite), carbonate, quartz, carbonaceous matter and graphite(?), pyrite, chlorite, rutile, magnetite, the percentage of carbonate being, however, 

large.

The principal difference between the Heimbach and "Old Bangor" slate is in the 

larger amount of carbonate, lesser amount of pyrite, and the finer structure of the 

former, judging from the microscopic evidence.

Pen Argyl. The Albion quarry at Pen Argyl is regarded as being near the bottom 

of the "soft vein" belt. It measures 650 feet along the strike, 600 feet across it,
U. S. GEOLOGICAL SURVEY 

DERRICKS AND CARRIERS, OLD BANGOR SLATE QUARRY, 

In background at right the slate splitters and trimmers' shanties on the dumps. Co
PENNSYLVANIA. 79

and 390 feet in depth. In 1904 it was operated by two companies, Wm. Lobb & 

Sons at the western end and Stephen Jackson & Co. at the eastern end. The 

structure is an S-shaped fold overturned to the north-north west. The bedding 

strikes N. 58°-60° E.; cleavage at the north-northwestern edge dips 10° S., halfway 

down about 10° N., and at the bottom about 40° S. The grain is at right angles to the 

ribbon. At the north-northwestern edge of the quarry is a series of hard beds 40 

feet thick. Under the microscope this rock shows no aggregate polarization and consists of the following minerals, named in order of diminishing abundance: Carbonate, 

quartz, chlorite, carbonaceous matter, muscovite, pyrite, and rarely plagioclase feldspar. It is a clay slate. Some of the beds are said to measure 20 feet between 

ribbons. There is an exceptional bed, 44 inches thick, of a dark, slightly greenish 

gray slate of superior quality, described beyond. Vein matter on the dumps consists 

of vein quartz and calcite, the former predominating. Associated with these veins 

are plicated very quartzose ribbons traversed by a coarse slip cleavage and also 

brecciated. Black slaty material has been pressed into the faults of the slip cleavage. 

One of these quartz masses was 20 feet thick and extended the entire length of the 

quarry.

The chief product of the Albion quarry is a very dark-gray slate which, to the unaided 

eye, has a fine texture and a roughish almost lusterless cleavage surface. It is somewhat carbonaceous or graphitic and magnetitic, shows pyrite on the sawn edge, effervesces slightly with cold dilute hydrochloric acid, considerably in the ribbon, and 

is sonorous.

Under the microscope this slate shows a matrix of muscovite (sericite), with brilliant 

aggregate polarization, somewhat obscured by abundant carbonate in plates and 

rhombs. The amount of carbonate is less than in the specimens from the Old Bangor 

and North Bangor quarries. Quartz is not very abundant, in grains measuring up to 

0.04, rarely 0.09 mm. Scales of chlorite, interleaved with muscovite, are conspicuous, 

lying transverse to the cleavage, sometimes forming lenses with their long axes parallel to the cleavage, measuring up to 0.14 by 0.07 mm. and numbering about 20 per 

square millimeter. Pyrite spherules occur up to 0.01 mm. and average about 150 

per square millimeter. There is finely disseminated carbonaceous (or graphitic) 

matter and a great abundance of rutile needles.

The mineral constituents, arranged in descending order of abundance, appear to be 

muscovite (sericite), carbonate, quartz, chlorite, pyrite, rutile, carbonaceous matter, 

magnetite. The carbonate, although second, is not as abundant as in specimen from 

"Old Bangor" quarry.

The results of Professor Merriman's physical and chemical tests of the dark-gray 

("black") slate from this quarry are here given for convenience of reference: 

Strength (modulus of rupture), 7,150 pounds per square inch; toughness (ultimate 

deflection on supports 22 incties apart), 0.270 inches; density (specific gravity), 

2.775; softness (amount in grains abraded by 50 turns of a small grindstone), 80; porosity (per cent of water absorbed in 24 hours), 0.238; corrodibility (per-cent of weight 

lost in 63 hours in acid solution), 0.547; per cent of lime, 4.09; computed carbonate 

of lime, 7.40 per cent; computed carbonate of magnesia, 4.41 per cent.«

The dark greenish-gray slate of the Albion quarry has to the unaided eye a somewhat granular texture and a roughish and almost lusterless cleavage surface. It is 

not perceptibly carbonaceous or graphitic," but is somewhat magnetitic. The sawn 

edges show pyrite. It effervesces slightly with cold dilute hydrochloric acid, and 

is sonorous.

Under the microscope it shows a matrix of muscovite (sericite), with brilliant 

aggregate polarization. Quartz fragments measure up to 0.09 mm. There is considerable carbonate. There are chlorite scales, interleaved with muscovite, lying transaMerriman, op. cit. .
80 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

verse to the cleavage and parallel to the grain; also chlorite scales transverse both 

to grain and cleavage, with inclusions of needle-like crystals, probably rutile, crossing one another, usually at 60° or 120°. All these chlorite scales average about 18 

per square millimeter, and measure up to 0.17 by 0.1 mm. Tbfe' pyrite spherules 

measure up to 0.01 mm., and number about 150 per square milliriieter.- Rutile needles 

also occur. ' '"'

These constituents, arranged in descending order of abundance", appear to be niuscovite (sericite), carbonate, quartz, chlorite, pyrite, rutile, and magnetite.

The product of the Albion quarry, at least that of the western end, is used for 

roofing.

Weathering at Pen Argyl. Some of the other quarries at Pen Argyl show the passage 

of the black slate by weathering into a yellowish light brown, and even white shaly 

substance. The dumps afford specimens showing all gradations from black slate to 

white rock. The weathered /one in these quarries measures up to 40 feet in thickness. At one of them the beds of the truncated southern limb of an overturned 

syncline dip about 55° south, while the cleavage dips lees than 10° in the same 

direction. The weathered top measures between 30 and 40 feet, and is covered with 

from 5 to 10 feet of subsoil. The weathering acids, taking advantage of the position 

of the bedding, have followed one of the ribbons and penetrated the workable slates 

to a depth of 60 feet below the weathered zone, and have also commenced to work 

horizontally along the cleavage. This has resulted in the formation along the ribbon 

of a central bed of reddish-yellow ocher about 1 foot thick, with a 6-inch band of 

white on either side of it. The white portions here probably represent less ferruginous.parts of the original sediment, and not a later stage of weathering, for similar 

alternations of yellowish and white belts of weathered material occur in hand 

specimens crossing the cleavage. The workable black slate is thus traversed here by 

a 2-foot bed of yellow and white ochraceous material at an angle of 65° to the cleavage as the effect of weathering upon material and structure. The process here is 

mainly, decarbonization. (See further, on the subject of weathering, p. 37.)

In this connection it should be stated that in the " soft vein " quarries generally, 

judging from the dumps, the amount of iron and lime in the ribbon varies greatly. 

In some cases pyrite or siderite predominates and limonitic staining ensues, in others 

lime and a white calcareous crust forms. The slate flags on the sidewalks in the 

village of Bangor wear along the ribbons more rapidly than along the slaty portions.

Chapman. This is in the "hard vein" belt/ The principal quarry is operated by 

the Chapman Slate Company. It measures from 700 to 800 feet along a longitudinal 

joint striking N. 63° E. across it, and is about 200 feet wide and 300 feet deep. In 

the center of the quarry is a completely overturned close fold, syncline, or anticline, still visible on the south-southeast wall of the quarry, striking N. 75°-78° E., 

pitching gently eastward; but the lower limb of this fold opens out on the eastnortheast and west-southwest walls so as to dip steeply northwest. This fold is 

crossed by cleavage striking N. 65° E. and dipping 14°-15° SSE. The principal 

joints are longitudinal, striking N. 63° E. and dipping 90°, forming two of the sides 

of the quarry. The grain strikes N. 37°-53° W. and dips 90°. The largest bed, 

which, although containing very small ribbons, can be used for roofing slate, measures-16 feet. The number of ribbons exposed on the east-northeast wall, where the 

fold opens out, is 112, averaging a little over 2 per foot of slate. a

Some quartz veins lying in the cleavage show a columnar structure parallel to the 

side of the vein. The "top" in an adjacent quarry of the same company measures 

fully 50 feet. .

The slate is very dark gray, but is crossed by frequent ribbons a trifle darker and 

measuring from one-eighth to one-half inch, which, however, scarcely deflect the

"Figures through the courtesy of Mr. Richard Chapman, superintendent.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. XVI

SECTIONS ALONG LINES (A), (B), AND (C)

DOTTED LINES I.N SECTIONS SHOW PROBABLE COURSE OF BEDS

MAP OF 

THE SLATI1STGTON SLATE QUARRIES

PENNSYLVANIA 

Contours from U. S. Geological Survey, Slatington quadrangle,

with additions by T. Nelson Dale.

Quarry locations by Fred H. Moffit, assistant geologist. 

Scale

H % . I mile

Contour interval 20 feet. 

1905.

 Direction of strike of bed. 

P- 10 and amount of pitch of axis of fold.

70
PENNSYLVANIA.. 81

cleavage. To the unaided eye it has-a fine texture and a slightly roughish and slightly 

lustrous cleavage surface. It is both graphitic (or carbonaceous) and magnetitic. The 

sawn edge shows pyrite. Under cold dilute hydrochloric acid there is a slight 

effervescence, but more in the ribbon. It is sonorous, and discolors but slightly in 

thirty years exposure. E. H. S. Bailey gives the specific gravity as 2.79. .For 

Professor Merriman's recent tests of this slate see page 122.

Under the microscope the Chapman slate shows a matrix of muscovite (sericite) 

with a brilliant aggregate polarization not obscured by carbonate. A ribbon of 

coarse materials, 0.62 mm. wide, crossing the cleavage at an angle of 14°, possesses 

aggregate polarization. There are some large lenses of coarse quartz and carbonate 

in the cleavage. There is considerable carbonate throughout, in plates and rhombs, 

but less than in "Old Bangor" slate. Quartz up to 0.06 mm., pyrite spherules up 

to 0.02 mm. and numbering from 200 to 600 per square millimeter, about 55 scales 

of chlorite, with interleaved muscovite, per square millimeter, lying across the cleavage and measuring generally up to 0.08 by 0.04 mm., graphitic or carbonaceous 

matter in fine particles, abundant rutile needles, rare grains of plagioclase feldspar. 

The ribbon contains more abundant and coarser quartz, carbonate, chlorite, and 

carbonaceous matter than the rest.

The important constituents, arranged in descending order of abundance, appear to 

be muscovite (sericite), quartz, carbonate, pyrite, chlorite, rutile, carbonaceous 

matter or graphite, magnetite.

The product of this quarry was formerly used largely for flagging, posts, steps, 

etc., and because of its hardness had to be cut with diamond saws which operated 

horizontally, but owing to the competition of sandstone flagging and the high price 

of diamonds this outlet is closed and the product is now used exclusively for roofing 

slate, for which, however, only selected material is available.

LEHIGH COTJXTY. ,

Geological relations. The slate quarries of Lehigh County all belong in the "soft 

vein" belt. The relation of the "soft vein" slate to the shale of the same formation 

is well exposed near Lehigh Gap, on the border of the county. On the eastern side 

of the gap the outcrop of the Hudson formation that is nearest'to the Oneida conglomerate is a black shale that occurs in alternating thick and thin beds. The thick 

ones, being entirely devoid of cleavage, weather into shelly fragments, while the thin 

ones, owing to the intersection of an incipient slaty cleavage with the bedding foliation,' break up into stick-like fragments. Under the microscope the thin beds prove 

to be an aggregate of muscovite scales, quartz grains, and carbonaceous particles, with 

some pyrite spherules and limonite specks. Here and there the beginning of a slaty 

cleavage foliation is indicated by short stringers of fibrous muscovite lying at right 

angles to the bedding, but otherwise there is no alignment of particles and the rock 

is a typical shale. A hundred feet south of the locality described a south warddipping'Slaty cleavage predominates, and the north ward-dipping beddingis only shown 

by an occasional ribbon. Therefore within a space of 200 feet the transition from a 

shale to a^ slate can be observed. In the slate the amount of secondary mica has 

greatly increased and the clastic particles of the shale have been forced into the same 

parallelism .of, arrangement.

In weathering, the\Lehigh slates undergo the same transformations as those of 

Northampton "Cquat'^;'' The black slate first passes into a soft yellowish-brown 

ochraceous rock;/which, later becomes nearly white, and finally passes into a white 

micaceous clay, the. "shale clay" of the limonite and fire-clay pits. But these 

changes probably required many centuries. See discussion of chemistry of slate, p. 37.

The Slatington quarries. The slate quarries of Lehigh County are now confined to 

an area comprising about 3 square miles along Trout Creek and its tributaries. This
82 SLATE DEPOSITS AND INDUSTKY OF UNITED STATES.

trearn empties into the Lehigh from the west at Slatington. The accompanying 

map (PI. XVI), includes all the quarries about Slatington which were in operation in 

1902, 45 in number, together with 50 abandoned ones. The older quarries have

Fig. 6. Sketch of anticline in Provident Slate Company's quarry, near Slatington, Pa. Looking 

west. Width represented, 50 feet.

rea'ched a depth of about 300 feet. The map shows the exact location of these quarries, their approximate dimensions at the surface, the strike of the beds, and both 

the direction and angle of the pitch of the folds. The lateral deflection of the strike 

amounts in places to 10°, if not a little over, and the pitch ranges from 5° to 10°.

Fig. 7. Sketch of anticline in old quarry north of Slatington, Pa. Looking west. Height represented, 25 feet.

The character of the folds in these quarries is shown in Pis. XVII, XVIII, and 

figs. 6, 7, 8, 9. The relation of the workable beds to the ribbons is shown in PI. 

XIX. The strike ranges from N. 40° E. to N. 90° E., and N. 86° W. The cleavage 

strike varies from N. 74° to 85° E., and its dip from S. 5° to 16° E., at angles ranging
U. S. GEOLOGICAL SURVEY 

BLUE MOUNTAIN SLATE QUARRY NEAR SLATINGTON, 

Seen looking east, showing the character of the syncline and the bending over southward of the cleavage at the s

the glacier.
U. S. GEOLOGICAL SURVEY .ULLETJN NO. 275 PL. XVIII

EUREKA SLATE QUARRY NEAR SLATINGTON, PA.

[North part, seen looking east, showing an eroded northward-inclined close syncline crossed by cleavage 

and curved joints, and overlain by glacial deposits. The cleavage within the syncline appears to be

curved also.
PENNSYLVANIA. 83

from 35° to 75°. There is sometimes a difference in the angle of the cleavage dip 

in adjacent beds. The curvature of the cleavage at the "Old William Hughes" 

quarry amounts to a change of 20° in 25 feet of slope. There is apt to be a little 

curvature of the cleavage on either side of the ribbon. There is a conspicuous system of dip joints striking about north-south, some of which are shown in Pis. XVII

Fig. 8. Sketch of syncline at William Hughes's quarry, near Slatington, Pa. Looking west. 

Length, 80 feet. Upper bed over 10 feet thick; lower bed 15 feet thick, but with marked curvature 

of cleavage.

and XVIII. There are also strike joints which sometimes undulate like bedding 

planes, as at the Eureka quarry (PI. XVIII).

The names of the Slatington beds are, from south to north, the Williamstown, 

Blue Mountain, Trout Creek, Washington, Little Franklin, Big Franklin, Mammoth, 

New Bangor, Snowdon, and Eureka. It is uncertain whether some of these are not

Fig. 9. Sketch of folds at Eureka quarry, near Slatington, Pa. Looking east. Length, Hf>0 feet; 

depth, 225 feet. The upper part of the syncline is shown in PI. XVIII.

duplications, for the structure of the belt, as shown in the section on PI. XVI, indicates that they may be, and their relations have never been worked out mathematically. Examples of these beds ("veins") are the Washington, which at the Hazel 

Dell quarry averages 27 feet in thickness or 40 feet measured along'the cleavage, and 

the Franklin, at the "Old Franklin" quarry, which consists of an upper bed of 28
84 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

feet and a lower one of 35 feet, separated by about 25 feet of small ribboned material, 

the measurements being taken along the cleavage. In general, the beds range from 

3 to 30 feet in thickness, measured at right angles to the bedding.

The Slatington slate is a very dark bluish gray. . To the unaided eye it has a somewhat fine texture and cleavage surface, but it is almost lusterless. It is carbonaceous 

or graphitic and slightly magnetitic. The sawn edges show little if any pyrite. It 

effervesces in cold dilute hydrochloric acid, is sonorous, and has some argillaceous 

odor. After prolonged exposure this slate becomes at first a dark gray and finally 

various shades of cream and coffee color, but the rapidity and degree of discoloration 

differ in different beds. Fissility fine.

Under the microscope it shows a matrix of muscovite (sericite) with a brilliant 

aggregate polarization, obscured, however, by much carbonate and carbonaceous 

matter. There are many angular grains of quartz up to 0.04; rarely one of plagioclase; a few scales of chlorite, interleaved with muscovite, or of each separately, up 

to 0.08 mm.; spherules of pyrite up to 0.012, rarely 0.02 mm., numbering about 130 

per square millimeter; very abundant rutile needles.

The principal constituents, arranged in descending order of abundance, appear to 

be muscovite (sericite), carbonate, quartz, kaolin, pyrite, rutile, carbonaceous or 

graphitic matter, magnetite.

The following analyses were made by Dr. W. F. Hillebrand, chemist of the United 

States Geological Survey A from the Lower Franklin bed, Old Franklin quarry, 

Slatington; B from the Washington bed, Hazel Dell quarry No. 1, Slatington.

Analyses of slates from Lehigh County, Pa.

Si02 (Silica)...... ............... .I.........................................

NaoO(Soda)................................................................

K.,0 (Potassa) ..............................................................

HoO (Water below 110° C.)........ ........ ............. ....................

HoO (Water above 110° C.) ........................... 1 .....................

TiOo (Titanium dioxide) ...................................................

PoOs (Phosphoric oxide) ..........:........................................

BaO (Baryta) ...............:............................................-..

LioO (Lithia) ...............................................................

 FeS2 (Pyrite) ................. I ...............................................

Specific gravity (21° C. )....................................................

A (complete).

56.38

15.27

bl.67

&3.23

2.84

4.23

1.30

3.51

.77

4.09

.78

3.67

.17

.09

.08

Trace.

TrticG

el. 72

.59

100. 39 

2.783

B (partial).

56.85

« 15. 24

1 c 5. 52

2.93

4.24

1.38

3.34

.45

.84

3.58

W

Trace '^

el. 72

96.12 

2.780

«Including P2 0G . 

t> Approximate.

c As Fe2 03 .

(l Included in A12O3 .

e Incl. 0.92 of sulphur.

The considerable amount of carbonate indicated both by the analyses and the 

microscopic examination and the discoloration due probably to the ferrous part of 

that carbonate areiobjectionable features in the Slatington slates, but their fine cleav-
U. S. GEOLOGICAL SURVEY ULLETIN NO. 275 PL. XIX

SLATE QUARRY NEAR SLATINGTON, PA.

Seen looking west, showing workmen engaged in clearing the " ribboned " slate from the surface of a thick

workable bed.
PENNSYLVANIA. . 85

age and the relative facility with which they are quarried make it possible to supply 

them at low figures. On the other hand they are remarkably well adapted by their 

softness and the thickness of the beds for mill-stock purposes and particularly for 

blackboards and other indoor objects.

All the more important features of the slates of Pennsylvania will be found in tabular form in the section on the comparative characteristics of slates (p. ]24).

PENNSYLVANIA AND MARYLAND. (PEACH BOTTOM) SLATE.

This slate deposit seems to have derived its name from that of a small village onefourth mile west of the Susquehanna River, formerly a sort of a port in the now 

disused canal which follows the river. This village is in a triangular township of 

the same name, which is bounded by the Maryland line on the south, the river on 

the northeast, and in part by Muddy Creek on the northwest, and which forms the 

southeast corner of York County, Pa.

Geological relations. The only available geological maps of the region are those 

of the two Pennsylvania geological surveys and. of the Maryland geological survey. ft

The slate forms a belt and low ridge from one-fifth to one-half mile wide, which 

extends from a point about a mile northeast of the Susquehanna, in the town of 

Fulton, in Lancaster County, Pa., in a south west direction across the river and across 

Peach Bottom township in York County, and continues for about 3 miles in the same 

direction into the town of Cardiff, in Harford County, Md. Its total length is about 

10 miles, of which 1 mile lies in Lancaster County, Pa., !<} miles are submerged by 

the Susquehanna, 4£ are in York County, Pa., and about 3 miles are in Harford 

County, Md. Most of the quarry population is congregated along one street, which 

follows the western foot of the ridge. The northeastern part of this street is Delta, 

Pa.; the southwestern is Cardiff, Md.

For the literature of Peach Bottom slate the reader is referred to the writings of 

Rogers, Lesley, Frazer, Williams, Mathews, Merrill, and Merriman. &

The slate belt has on its northwest side a sericite-chlorite-quartz-schist (containing rounded grains of zircon), with a foliation striking N. 45° E. and dipping 

southeast at an angle of 75°. About midway between Delta and Bryansville this 

schist is calcareous. According to the reports of the Second Pennsylvania Survey similar schists recur on thesoutheasternside of the slate belt, and the whole schist formation overlies in synclinal structure the older gneisses and serpentines which occur on 

either side of it; and these schists and the associated slate are classified'as Cambrian, 

although no typical Cambrian fossils have as yet been found in them. There is a . 

good exposure of the relations of the slate to the rocks on the southeast (referred to 

by Rogers) on the east side of Slate Point on the tow path along the Susquehanna. 

The slate here conformably underlies about 50 feet of slightly inuscovitic quartzite, 

which was formerly quarried for canal construction, both rocks striking N. 45°-50° 

E., and dipping southeast at 70°. A little farther east this quartzite is followed by 

a coarsely plicated gneiss consisting of quartz, plagioclase, muscovite, hornblende, 

and epidote, and having the same strike and dip. Still farther east, nearly a'quarter 

of a mile from the contact of the slate and quartzite, about 70 feet of slightly inuscovitic quartzite crop out with a strike of N. 45°-50° E., dip 42° SE., and are followed conformably by a gneiss like that previously described. These quartzites and

« Rogers, H. D., Geol. Pennsylvania, 1858; Frazer, Persifor, jr., York and Lancaster county sheets, 

Second Geol. Survey, Pennsylvania, Vol. OCC, Atlas, 1878,1879; Mathews, E. B., Maryland Geol. Survey, vol. 2, PI. XXX, 1898.

& Rogers, H. D., Geol. Pennsylvania, vol. 1,1858, pp. 188,189; Frazer, Persifor, jr., Geol. Lancaster 

County, Second Geol. Survey Pennsylvania, Vol. CCC, 1880, pp. 179-190, and Pis. VII, VIII; Williams, 

G. H., The petrography and structure of the Piedmont Plateau: Bull. Geol. Soc. Am. vol. 2,1891, p. 

304; Lesley, J. P., Second Geol. Survey Pennsylvania, Final Kept. vol. 1, 1892, pp. 136-141; Mathews, 

E. B., Maryland Geol. Survey, vol. 2,1898, pp. 215-231, 239-241; Merrill, G. P., Treatise on rocks, rock 

weathering and soils, 1897, p. 229; also Stones for building and decoration, 3d ed., 1903; Merriman, M., 

The strength and weathering qualities of roofing slates, Trails. Am. Soc. Civ. Engrs., vol. 27. 1892, pp. 

331-349; also vol. 32, 1894, pp. 529-539.

Bull. 275 06 7
86 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

gneisses apparently continue into Lancaster County, for in looking across the Susquehanna from Slate Point a considerable thickness of light-colored rock can be 

seen overlying the'slates there with a steep easterly dip.«

The structure of the slate belt proper is difficult to make out. The quartxite of the 

southeast side was not found on the northwest side during the reconnaissance. At 

the Foulk Jones & Sons' Slate Hill quarry, 2 miles northeast of Delta, there are 

some indications of anticlinal structure. Apparent bedding planes dip 30° to 40° SE. 

on the. southeast side of the quarry and similar planes curve over steeply to the northwest on the northwest side, both crossing the cleavage, which is nearly vertical. 

Whether this is a minor anticline in a complex synclineor anticline or the anticlinal 

axis of the entire belt could not be determined. There is also danger of confounding 

curved joints and bedding. Fraxer's section along the right bank of the Susquehanna 

in York County (sheet 4, Vol. CCC) represents the slate as interbedded with chlorite 

schist and forming part of the northwest limb of a syncline several miles wide.

The Delta and Cardiff quarries. In June, 1904, eleven concerns were operating quarries in the Peach Bottom belt. Five of these quarries or sets of quarries are located 

in Pennsylvania and six in Maryland. They lie in three parallel northeast-southwest 

lines and range from 75 to 120 feet across the cleavage and up to 200 feet in depth. 

The structural relations of these three belts of commercial slate are not yet apparent. 

The cleavage is uniformly vertical or dips steeply southeast with a strike ranging 

from N. 37° to 55° E. There is usually a horizontal joint "big flat joint" pitching 

gently south, sometimes from 40 to 60 feet below the surface and including 2 to 3 feet of 

crushed slate, which is evidently the result of a secondary crustal movement. Commercial slate occurs only below this joint. The " top " varies from 6 to 65 feet and 

in places includes 10 feet of slate in small fragments. Other flat joints dip about 20° 

NE. or 25° SW. Longitudinal joints strike N. 48° E. and dip 90°. Diagonal ones 

strike N. 25°-30° W. and also N. 45° W., dipping northeast at angles ranging from 

45° to 90° or 50° to 80°. Conspicuous dip joints strike about at right angles to the 

cleavage and dip 90° or southwest at angles of 25° or 45°. At Foulk Jones & Sons' 

there are vertical diagonal joints striking N. 10° E. to N. 10° W. The grain dips 

northeast from 20° to 50°. Quartz veins meander in the cleavage direction. The slate 

is apt to have along the joint planes toward the surface a brownish rim, called a hem, 

from 1 to 2 inches wide and more or less parallel to the joint face. The slate breaks 

off at the inner side of the "hem," and this part is discarded. Some of the joints are 

parted and contain red clay a half inch or an inch thick. The hems represent simply 

the initial stage of Aveathering caused by the percolation of water from the joints, of 

which process the final result is the red clay. The slate of the top has generally a 

reddish hematitic hue, and the whole belt is covered with reddish clay. The chemical processes involved in the passage of Peach Bottom slate into red clay as determined by Doctor Merrill are given on page 37. The effect of weathering; as shown 

under the microscope, will be given after the microscopic analyses of the slate.

The quarrymen complain of the occurrence of what they call "black stuff," a hard 

material up to an inch thick, occurring sometimes in vertical planes .diagonal to the 

cleavage. This proves to be an aggregate of much pyrite, carbonaceous matter, quartz, 

chlorite, and muscovite, and is probably vein matter segregated from the adjacent 

slate.

Peach Bottom slate. This is very dark gray with a slightly bluish tinge. To the 

unaided eye it has a minutely granular crystalline texture, and a slightly roughish 

but quite lustrous cleavage surface. It is markedly graphitic and contains magnetite. 

The sawn edges show a little pyrite. There is no effervescence in cold dilute hydrochloric acid. It is very sonorous and does not discolor. & Under the microscope it

«See also Frazer, as above, pp. 133,134.

bThe apparent contradiction of this last statement with the passage of this slate into reel clay in 

weathering is explained on p. 37.
PENNSYLVANIA. 87

shows a matrix of muscovite (sericite), of rather coarse almost schistose texture, but 

with brilliant aggregate polarization. There is much graphitic material in fine particles and also much quartz, some of which is probably secondary, in grains measuring up to 0.05 and sometimes 0.12 by 0.03 mm. Chlorite can in some specimens 

be made out mingled with the muscovite. There are tabular crystals of magnetite 

measuring up to 0.11 by 0.02 mm., and also lenses of magnetite of larger size, but 

the most conspicuous feature is the abundance of crystals of andalusite (A12O3 , SiO2 , 

silicate of alumina) measuring from 0.008 by 0.001 mm. and under, up to 0.11 by 

0.03 mm. and exceptionally 0.2 by 0.02 mm., and numbering from 15 to 80 persquare 

mm. These crystals contain opaque inclusions arranged diagonally to their vertical 

axes, and are often in lenses, measuring from 0.04 to 0.25 mm. long by 0.04 to 0.1 mm. 

wide, which consist variously of chlorite interleaved with muscovite with some quartz 

at the ends, or mainly of quartz, the andalusite crystal having a plate of muscovite 

parallel to it on one or both sides; or the lens consists of muscovite, the crystal being 

bordered by quartz, or, finally, entirely of secondary muscovite. When not in lenses, 

the andalusites are apt to have a plate of muscovite on one or both sides. These crysFIG. 10. Transverse section of Peach Bottom roofing slate, showing andalusite crystals in a rather 

coarse micaceous matrix. Enlargement about 113 diameters. Some of these crystals are in lenses 

of quartz and mnscovite and several show inclusions.

tals lie with their long axes across the cleavage at various angles and with or across 

the grain, but the axes of the lenses which inclose them are usually parallel to the 

cleavage (see fig. 10). Pyrite is difficult to distinguish under incident light from the 

andalusite when this is rendered opaque by inclusions. Rutile occurs in irregular 

crystalline masses up to 0.09 by 0.04 mm., but no needles were detected. There is 

rarely a rounded grain of zircon; no carbonate detected.

The important constituents of this slate, arranged in descending order of abundance, appear to be muscovite (sericite), quartz, graphite, andalusite, magnetite, 

pyrite, with accessory rutile and zircon. Fig. 10 will serve to show the andalusite 

crystals which are so characteristic of this slate. The highly crystalline character of 

the matrix and the absence of carbonate indicate a very durable slate.

The partially weathered slate of the "top" and "hems" shows, under the microscope, the magnetite crystals and lenses passing into hematite and the andalusite 

becoming limonitic from the oxidation of its inclusions, which must, therefore, be 

ferruginous and the matrix stained with hematite and limouite from the above 

alterations.
88 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Peach Bottom slate has been thoroughly tested by Professor Merriman,« with 

results as follows: f 

Modulus of rupture in pounds per square inch, 11.260; ultimate deflection in 

inches on supports 22 inches apart, 0.293; specific gravity, 2.894; softness (grains 

abraded by 50 turns of a small grindstone), 0.90; porosity (per cent of water 

absorbed in twenty-four hours), 0.224; corrodibility (per cent of weight lost in sixtythree hours in acid solution, consisting of 98 per cent H2Q, 1 per cent HC1,1 per cent 

S03 ), 0.226. ' ; 

The three published analyses show the following percentages of lime: 0.155, '° 0.3, c 

0.48.'* That by the second State Survey will be found in full on page 34. (Anal. 

VII.) This lime may be referable to the phosphate apatite, -which the analyses 

show to be present,

Peach Bottom slates are now used exclusively for roofing purposes. The chief 

features of this slate are stated in tabular form in the section on the comparative 

characteristics of slates (p. 124).

TENNESSEE.

By EDWIN C. ECKEL. .

Slate deposits occur in eastern Tennessee, notably southeast of Knoxville. The 

deposits here are described by Mr. Keith, e as follows:

Two formations in this region contain beds of slate the Wilhite and Pigeon slates. The VVilhite 

slate is too calcareous and soft for commercial use in the vicinity of Little Tennessee River, but has 

the necessary hardness, evenness, and cleavage along Little Pigeon River. Along this stream the 

slate is well exposed over great areas, but has never been developed. Quarries have been opened in 

the Pigeon slate along the Little Tennessee River at many points, and slates and flags taken out for 

local use. Recently a quarry has been opened on a small creek 2 miles from the river and much good 

material taken out for shipment. The slates are of fine, even grain, and split into slabs an inch thick, 

of any desirable size, or into roofing slates. In this particular quarry the cleavage crosses the bedding and produces ribbons in much of the slate. An old quarry about 2 miles north of this shows the 

cleavage and bedding coincident, and flags of gn'itt size are readily loosened. Some of the slate layers contain pyrite, necessitating selection of the material for use. There are a great number of available places for quarrying in the bluffs along the river and the adjacent small streams on either side. 

That this slate resists weathering is amply proved by the high, sharp slate cliffs that border the river 

along most of its course. 

The following analyses of slate quarried by the Southern Slate Company at Maryville, Blount County, were made by Mr. Gilbert McCulloch, of the University of 

Tennessee.

Analyses of Tennessee roofing slate.

Lime.(CaO). .......... ........

1.

58.45

21.88

6.04

1.86

.46

2.

59.00

23.44

6.28

1.30

.50

Potash (K2 0).................

Soda (Na2 0) ..................

Water....................'....

1.

1 60

2.34

65

6.66

2.

2 04

1.78

93

a Op. Cit.

& Second Pennsylvania Geol. Survey Rept. Progress for 1877, Vol. CCC, 1880; Anal, by A. S. McCreath, 

pp. 269, 270.

c Anal, by Booth, Garrett, and Blair, 1885; see Md. Geol. Survey, vol. 2,1898, p. 226. 

d Merrill, G. P., Rocks, rock weathering, and soils, 1897, p. 229. 

e Keith, Arthur, Knoxville folio: Geologic Atlas U. S., folio 16 T0- P,. Geol. Survey, 1895, p. 6.
U.S.GEOLOGICAL SURVEY 73°I5' BULLETIN NO.275 PL.XX

0 ~ iLowerSilurianlOrdovician). [~ _. I Lower Silurian.

5 I slate,shale,|ritetc. 3 D Berksnire schist

____(slate red, briBit green,blackl. I______I

Strike and dip of bedding

(vertical line-aipiWhen short-steep dip).

I I Lower Cambrian slate, quartette,

f "G I gritjimestone.and. shale

j______j (slate green/purple,orvariegated).

MI^H Red slate miarries with somel)right-green slate. 

Pm| Sea-greeiuxr purple slate cpiarries (includingvariegated).

Strike ancldip of cleavage (vertical lines 

-dip.when snort-steep dip.cleava^e sxiA 

bedding symbols are also contbined).

 ^_ Strike of1\.ogback."or sliear zone

^_^.. Bilte of cainptonite or augitecamptonite (basic eruptive). 

_ SEC-VI Section line s. (Tor sections see P1XXID

Note'. The numbers of the quarries refer to structural dAta on pages 72,102,103

Topography W J.H.Jennings G.E.Hvde and Jas.McConnlck.QuarriesitiosQy by,I.H.Jennmgs. 

G"e ology by T. tf els on D ale (I,.M. Prtndle and E H.Moffit, as sistantsU895 96. -W^W\v '' v - - -

.--' -->-X Trv\i...'. A . ,. .v\y.

JULUiSBIKN & Cb.^.V."

QUARRY MAP

SHOWING THE SITUATION AND SIZE OF THE SEA-GREEN, PURPLE,AND RED

SLATE QUARRIES IN THE TOWNS OF HAMPTON AND GRANVILLE NEW YORK AND

POULTNEYAND PAWT.ET.VERMONT,WITH THE GEOLOGY OF THE VICINITY

Scale 3ia5o

Contour interval 20feet. 

1905
U.S.GEOLOGICAL SURVEY BULLETIN N0.275 PLXX I

UL1U38IEN a CO.N.V.

QUARRY MAP

SHOWING THE SITUATIONAND SIZE OF THE MORE IMPORTANT SLATE QUARRffi S IN THE\INFADING GREEN SLATE BELT BETWEEN 

POULTNEY AND WEST CASILETON.VERMONT, TOGETHER WITH AFEW OF THE"SEA GREEN'SLATE QUARRIES NEAR POULTNEY

Topography andnearlyall quarries by J.EJennings. Geology by T.NelsonDale.LMPrindle and E.H.Moffit,assistants.

Surveyed in 1896. 

Scale 31250

SI

Lower Silurian (Ordovician) 

slate, shale, grit, etc. 

(slate r e^bngUt

c u Lower Silurian. 

Berkshire schist.

Contour interval 20feet,. 

1905

LEGEND

T Strike and dip of "bedding (vertic al li ne - dipp -when ;whesinort-steep dip». Strike of vertical joints.

u ___

<H ~j- I Horizontal "bedding. I "7*9 ' Strike of fold ftne curved line

| ~i I Lower Cambrian slate, quartzite. j I Strike and dip of cleavage (verticallines '

t/l Igritltmestone.andsliale | | -dip.wheri short-steep dip cleavage and

I. I (Sla'e Srecn.purple.orvariegated). j_____Jbeddin^ syinbols are also COTn"binldl.

snows cliaracter of fold).

Dite of camptonite or angitecamptonite ("basic eruptive.

Green,ptirple,orvariegated j 

slate (niarries. Strike of "hogback" or slxear zone j \ SEC.IIIJ Section lines. (For sections 

Note: The ntinibers oftlie quarries refer to structural data on pages 1O2,1O3
SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 89

UTAH. 

By EDWIN C. ECKEL.

For some years past a small amount of slate lias been annually gotten out, chiefly 

for samples and trial shipments, at various points in Utah. Deposits of slate, believed 

to be of workable extent and of good quality, have been described as occurring on 

the islands in Great Salt Lake, and some attempt has been made to develop these 

deposits.

The locality which has been most widely discussed, however, is that near Provo. 

This has been exploited to some extent by F. W. C. Hathenbuck, of Provo. The 

slate deposits occur about 2 miles southeast of Provo station, in Slate Canyon. The 

slate here covers a considerable area, but that exposed at the surface is so badly 

broken up that large slabs cannot be obtained. It is possible, however, that this 

condition will disappear if the deposits are worked deeper.

The Provo deposits furnish green and purple slates, the latter being apparently 

present in greater quantity. The green slates show little tendency to cleavage in 

their surface outcrops, aixd will probably be less satisfactory for roofing purposes than 

the purple. The green slates rub very smooth, however, and would make good slabs 

or mill stock if obtainable in masses of sufficient size.

The purple slates split well, with a surface about as smooth as that of Peach- Bottom (Pennsylvania-Maryland) slate. From samples seen it appears that they also 

bear punching well.

A specimen of the purple slate selected by the writer was analyzed by Mr. .W. T. 

Schaller in the laboratory of the United States Geological Survey, the results, being 

as follows:

Analysis of purple slate, Prow, Utah.

Silica (SiOj) ............................................................. 54. 05

Alumina (A12 08 ) -----------------.-------------------------------------- 20. 95

Iron oxides (FeO, Fe2 03 ) ................................................. .28

Lime (CaO)................................... ^......'.................... .22

Magnesia (MgO) ......................................................... 9.12

Carbon dioxide (CO.,)) ,, nr. Water ..............} - - ? 90

Of a series of 36 analyses of American roofing slates collected and discussed" 

recently by the writer, the above slate from Provo stands lowest in its percentages of 

silica and magnesia, while its lime is very far below the average.

Nothing definite is known as to the geologic age of these slates, though they ai-e 

supposed to be Ordovician, or even older.

VERMONT.

By T. NELSON DALE.

GEOGRAPHICAL, RELATIONS.

There are at least four distinct slate districts in Vermont. The most easterly 

extends along the Connecticut River for more than two-thirds the length of the 

State. The slate is black or dark gray, and has been worked in Guilford, in Windham County, at the extreme southern end of the State, and also inThetford, in Orange 

County, and at Waterford, in Caledonia County. The next extends along the east 

flank of the Green Mountain range from the Canada line to about the middle of the

"Jour. Geol., vol. 12,1904, p. 20.
90 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

State, and has been extensively worked at Northfield, in Washington County. The 

most important district, which furnishes the well known "green" and "purple" 

slates, lies between the Taconic range and Lake Champlain, extending from the 

town of Sudbury, in Rutland County, southward to Rupert, in Bennington County, 

a distance of 26 miles. This belt also passes south-southwest into AVash'ington 

County, N. Y., where, however, it has thus far proved of less economic importance. 

The fourth is black slate, as yet undeveloped, and covering only from 10- to 12 

square miles in the town of Benson, in Rutland County, near Lake Champlain.

Geological relations. The Vermont geological map a represents a belt of clay slate, 

from 1 to 7 miles wide, as extending from Lake Memphremagog along the east side 

of the Green Mountain axis as far south as Barnard, in Windsor County, a distance 

of 90 miles. A recent paper and map by Richardson & represents a portion of this 

slate belt 52 miles long, extending from North Calais, in Washington County, to a 

point in Windsor County nearly 10 miles west of White River Junction. The age 

of this slate formation was thought to be probably Devonian by the authors of the 

older report, c but Richardson regards it as Lower Trenton. Conclusive paleontological evidence on the subject is yet lacking.

This slate has been quarried at Montpelier and Northfield, which are 10 miles 

apart. The strike of bedding at both places ranges from N. 10° to 20° E. At the 

Vermont Black Slate Company's quarry, 2 miles south of Northfield, the beds occur 

in minor very acute folds, as shown in fig. 11 which strike N. 10° E., while the cleavage, with an almost identical strike, dips W. tit 75°. This indicates that the general course of the bedding is not far from horizontal and that lateral repetitions of 

strata are to be looked for. The interpretation given by Richardson of PI. XV in 

his paper, referred to above, requires corroborative microscopic evidence. The low 

easterly (by error printed west) dipping planes resemble a secondary cleavage, while 

the steep westerly ones have the characteristics of bedding. . The slate continues about 300 feet west of this quarry, and about a third of a mile 

east, making its total width in that vicinity about 2,000 feet. The general relations 

of this slate on the west are these: The western part contains a bed of novaculite up 

to 12 feet thick, and is followed by slate of no commercial value, which is followed 

at the foot of the range, about three-fourths mile southwest of the village, by a 

sericite-chlorite-quartz-schist with grains of plagioclase, with a vertical slip cleavage 

striking about north and a plicated bedding with a northerly pitch. After a covered 

interval, at a point on the range 2 miles Avest of the village, there is a large exposure 

of thin-bedded, more or less muscovitic quartzite (with grains of zircon and plagioclase), so intricately folded as to strike N. 20° E., N. 20° W., N. 50° E., and N. 90° E. 

An area of a few acres of muscovite granite (formerly quarried, Moses King property) lies within this quartzite, or at least with this quartzite on both its east and 

west sides. This granite in places becomes orbicular, carrying nodules of concentrically arranged mica an inch in diameter. ^

Northfield slate. In 1904 but one quarry was in operation at Northfield, that of the 

Vermont Black Slate Company, which measures about 100 feet along the strike, 60 

feet across it, and 55 feet in depth. The relations of cleavage and bedding are shown 

in fig. 11. The cleavage strikes N. 12° E. and dips 75° W. Strike joints strike 

N. 15° E., dip 50° W., dip joints N. 77° W., and dip 85° S. Diagonal joints strike 

N. 55° E. and dip 65° NW. The grain dips 70° N. An exceptional false cleavage 

dips 15°-20° N. As there are no ribbons in the bedding nor horizontal joints crossa Hitchcock and Hager Kept. Geol. Vermont, vol. 2, 1861, PI. I, p. 794.

& llichardson, C. H., The terranes of Orange County, Vt., Slate, p. 77-79, and map, Pis. IX, IXA, in 

Kept. State Geologist Vermont, (3) of new series, 1901-1902. 

c Vol. 1, p. 497. 

dRept. Geol. Vermont, vol. 2, pp. 563, 564.
U. S. GEOLOGICAL SURVEY

I. Jamesville §(i£''-J "" - 

 Hampton '' '

II. Middle Granville

BULLETIN NO. 275 PL. XXII

E.

III. Lake Bomoseen

Flint Bill ., S

IV. Blissville

Lake 

i

E.

E.

_400 ft.

V. Wells

V.I. Pawlet Dike sic,

VII. West Pawlet 

ooo it.

E.

,500 ft.

N.Y.Vt.W.Pawlet 

I 

I

0

Vertical and horizontal scale

'A V* % 1 mile

LEGEND

Section lines on Quarry maps, Pis. XX and XXI.

Vertical dotted lines indicate places where dip was observed; otherwise hypothetical. 

Vertical dashes indicate boundaries between Cambrian and Silurian, or refer to names.

H||^i||||| = 0rdovician (Lower Silurian) shales, grits, and red and green slate.

All other areas =Lower Cambrian shale, limestone, quartzite, and green and purple slate.

SECTIONS ACROSS THE NEW YORK AND VERMONT SLATE BELT.
VERMONT. 91

ing the cleavage, channeling in a horizontal direction becomes necessary. It is, 

therefore, easier to obtain very large slabs for mill stock than small ones for roofing 

slates.

The slate is very dark gray; to the unaided, eye has a very fine texture. The 

cleavage surface is very smooth and very lustrous, but has here and there minute 

longish crystals of pyrite pointing in the direction of the grain, also some very minute 

nonmetallic lenses. It appears to be slightly graphitic and contains a very little 

magnetite. The sawn edge shows lenses and crystals of pyrite from 1 to 3 mm. long. 

There is no effervescence in cold dilute hydrochloric acid. It is very sonorous, very 

fissile, and does not discolor.

FIG. 11. Syncline at Vermont Black Slate Company's quarry, near 

Northfield, Vt.; looking S. 12° W. Depth of syncline as figured, 44 

feet.

Under the microscope shows a matrix" of muscovite (sericite), with a very brilliant 

aggregate polarization and a very even texture. There are abundant quartz fragments up to 0.06 mm., exceptionally 0.1 by 0.03 mm. The lenses and crystals of pyrite 

(including a small percentage of magnetite) number from 7 to 8 per square millimeter and measure from 0.05 by 0.03 up to 0.06 by 0.1 mm. These figures, however, do 

not include the exceptionally large ones, visible without a glass. The long axes of 

these lenses and crystals lie in the cleavage but parallel to the grain. They are 

usually surrounded by a rim of secondary quartz and muscovite, sometimes also 

of chlorite, radiating from the nucleus a distance of 0.04 to 0.08 mm. In some cases

"See Glossary, p. M(>.
PLATE XXIII.

STRUCTURAL DIAGRAMS OF SLATE QUARRIES IN EASTERN NEAV YORK 

AND .WESTERN VERMONT.

(A) Rising & Nelson's "sea-green" quarry No. 2, West Pawlet, Vermont. 

(jB) Rising & Nelson's "sea-green" quarry No. 2, West Pawlet, Vermont, with measurements.

(C) Hughes's "sea-green" quarry No. 7, West Pawlet, Vermont. 

(Z>) Robert R. Roberta's "sea-green" quarry, Pawlet, Vermont. 

( E) Hughes's western quarry "sea-green," Pawlet, Vermont. 

(F) McCarty's "sea-green" and purple quarry, Poultney, Vermont. 

(G] Schmidt & Williams's "sea-green" quarry, Pawlet, Vermont. 

(H) Empire slate quarry (red), Granville, New York.

(J) Auld & Conger's quarry ("sea-green" and variegated), Wells, Vermont. 

(K) Griffith & Nathaniel's quarry ("sea-green" and purple), Poultney, Vermont. 

92
U. S. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. XXIII

E.

(J3)

(E)

E.

(F)

E. ^^_ green 75ft. W.

purple, 

25ft.

(H)

W

W

joint

QUARRY DIAGRAMS, NEW YORK AND VERMONT SLATE BELT.
VERMONT. 93

these secondary minerals radiate only in the /.one of the cleavage of the slate. There 

are a few lenses of interleaved muscovite and chlorite. Considerable dark-gray, 

probably graphitic, matter in extremely'fine particles, very little carbonate, prisms 

of rutile up to 0.04 by O.OOC mm. and more rarely irregular angular masses of the 

same mineral, and finally some crystals of tourmaline, 0.36 by 0.008 mm. The nonmetallic lenses referred to above are evidently the quartz-coated pyrite lenses.

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite (sericite), quartz, pyrite, graphite, magnetite, chlorite, carbonate, besides accessory rutile and tourmaline.

For the reason given the product of this quarry is destined almost entirely for mill 

stock, but the examination shows it to be a superior roofing slate.

The following data were gathered at the abandoned quarry of the Union Slate 

Company a half mile east-southeast of Northfield: Bedding strike, N. 10° E., dip80° 

\V; cleavage strike, N. 4° E., dip 70° W. Strike joints clip, undulating, low west; 

diagonal ones strike N. 60°-65° W. and dip 75° NE.

MONTPELJER.

At the abandoned quarry three-fourths mile southeast of Montpelier and about 10 

miles north-northeast of the Vermont Black Slate Company's quarry at Nortbh'eld 

the strike of both bedding and cleavage was found to be N. 15°-20° E., dip 70°-75° 

W. This quarry was operated by means of three wide openings at intervals across 

the strike and communicating with each other by a 10-foot open cut, which also 

served as a drain. The quarry is said to have been abandoned on account of the 

large percentage of waste, which may have been the result of the complex opening. 

The slate appears to be essentially the same as that of the Northfield quarries.

WESTERN VERMONT.

Geological relations. The broader geographical and geological relations of the 

western Vermont slate belt are shown in the map of the slate belt of eastern New 

York and western Vermont," and still more fully in the economic and structure section sheets of the forthcoming Mettawee and Fort Ticonderoga folios.

The Ordovician (Hudson) schist of the Taconic range is bordered on the west, 

except along a stretch of 6 miles in Pawlet and Rupert, by a belt of Lower Cambrian 

rocks estimated as at least 1,400 feet in thickness, which include about 240 feet of 

greenish and purplish roofing slates. This boundary between the Ordovician and 

Lower Cambrian has recently been shown to represent not only an unconformity 

but a folded overlap. & In Pawlet and Rupert the schists of the Taconic range merge 

at the west through decrease of metamorphism into an irregular area of shales and 

grits of the same (Hudson) age not less than 1,200 feet thick, which include about 

50 feet of commercial reddish and greenish slate. These have long been quarried in 

Granville and Hampton, N. Y., and are described in detail on page 70. In places 

the Lower Cambrian slate protrudes through the Ordovician slate areas; in others 

lenticular remnants of Ordovician slate overlie the Lower Cambrian slate. The 

relations of these two formations are more intricate in the New York part of the slate 

belt than in the Vermont part. In Pis. XX and XXI their relations in the vicinity 

of the chief quarrying centers are shown, as well as the location and form of nearly 

all the slate quarries. c

"The slates of western Vermont were fully described by the writer in part 3 of Nineteenth Ann. 

Kept. U. S. Geol. Survey in-1899, and the matter appears here in revised form, but the. general map, 

PI. XIII, is not republished.

6See Am. Jour. Sci., 4th ser., vol. 17,1904, p. 185.

"It will be noticed that PI. XXI fits onto the eastern half of the northern end of PI. XX. Thescale 

of these maps is so large that they can be used for prospecting purposes by studying the relations of 

the quarry locations to the symbols which designate the strike of the beds. See also p. 44.
94 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

The Lower Cambrian slates of western A^erm out are greenish gray, purplish, and 

"variegated," i. e., greenish gray and purplish mixed. These occur in alternations. 

The quarry diagrams (Pis. XXIII and XXIV) show that there is little regularity in 

these alternations. In the main, however, this horizon seems to consist of from 100 

to 140 feet or even 200 feet of greenish and purplish slates, the greenish ones predominating, with from 40 to 50 feet of variegated or mottled overlying, but possibly 

replacing the purple in places. On the west side of Lake Bomoseen nearly 100 feet 

of purple are exposed. The purple sometimes contains a few inches of dark-reddish 

slate not unlike the red of the Ordovician. There is some difference in the shade of 

the different beds of green in the same quarry, some being more greenish, others 

more grayish. There are also differences in the amount of discoloration produced 

by weathering in beds of the same locality. Although some quarries produce only 

the so-called "unfading green" and others only the "sea green," these differences 

appear not to belong to strata of different ages, but to occur at different points in 

strata of the same age.

Interbedded with the slates are beds of calcareous quartzite of very different areas, 

ranging in thickness from a few inches up to 5 feet. This quartzite contains a few 

grains of plagioclase and more muscovite scales, and is veined with quartz, which 

crystallizes in cavities. The quartzite sometimes weathers brown; its calcite, therefore, probably contains some siderite.

Associated with the slates are also beds of limestone conglomerate or breccia ranging from a few feet to 40 feet in thickness, carrying the trilobite Olenellus and other 

fossils characteristic of the Lower Cambrian. One of these beds of limestone breccia 

is of frequent occurrence in the quarries, overlying the slate. (See quarry diagram 

P, PI. XXIV.)

The slate-bed surfaces are generally covered with annelid trails or impressions of 

algte, or both. The purple slates are often ribboned or banded with light-green slate 

beds an inch or more in thickness, or have oval or rqundish light-green spots, frequently in rows. Similarly the "sea-green" slates have grayish ribbons crossing 

them.

The position of these commercial slates in the Lower Cambrian series is shown in 

the following table, in which the beds are arranged in their natural order:

Lower Cambrian rocks of slate belt of western Vermont.

Feet. . 

(E) Ferruginous quartzite and sandstone ................................ 25-100

(D) Black shale and slate with Lower Cambrian fossils................... 50-250

(C) Black patch grit; a dark-gray grit or sandstone with black shaly 

patches, sometimes with calcareous nodules carrying Lower Cambrian fossils ..................................................... 10-40

(B) Roofing slate, grayish green, purplish or mixed green and purplish, 

alternating with beds of calcareous quartzite and limestone breccia 

up to 40 feet thick, the latter carrying Lower Cambrian fossils ...... 200-240

(A) Olive grit or graywacke, more or less massive, with minute scales of 

hematite or graphite, sometimes with small quartzite beds, frequently 

calcareous, generally weathering a pale brick red. Under the microscope shows grains of quartz, feldspar, large scales of muscovite and 

of chlorite interleaved with muscovite. Associated with this is a bed 

of quartzite 12-55 feet thick........................................ 50-200

The base of the formation is not certainly exposed. There may be a bed of commercial, slate in the olive grit.

The probable structure and structural relations of the two slate formations of 

western Vermont are shown in.PI. XX11. Each section is described beyond in 

detail. Those parts of the sections which are well substantiated are indicated by
VEEMONT. 95

vertically dotted lines. The typical features of the region are finely shown at certain 

points, and these have been utilized in drawing the hypothetical parts of the sections. Symbols showing the structure at a number of the quarries are given on the 

 large-scale quarry maps (Pis. XX and XXI), upon which the section lines have 

been drawn.

GEOLOGICAL. SECTIONS.

SECTION I. .1A MESV1LLE-HAMPTON.

This crosses the Jamesville Cambrian slate belt. The occurrence of the Ordovician 

red slate on the east of the Jamesville belt is shown at the Matthews quarries, a mile 

west of Poultney, in Hampton, and on the west at those of the National Red Slate 

Company, about a mile north-nQrthwest of Raceville, in the town of Granville. 

About.a mile due north of Raceville there is a small opening in red slate in close 

proximity to the Cambrian. The red slate dips easterly under the Cambrian by 

overturn. From 500 to 600 feet north-northwest of this the Hudson grits crop out, 

and a mile north-northwest, on the east side, three-fourths of a mile north of section, 

Mr. Walcott found Hudson graptolites in eastwardly dipping shales.

The eastern base of the Jamesville Ridge consists of the Cambrian black slates and 

shales with small beds of limestone (D).« These also crop out at the bend in road 

south of section. These slates and shales form the uppermost strata of the ridge, 

(E) being absent, but that outcrop probably belongs to those on the west side of the 

ridge. Arising from beneath (D) are the green and purple roofing slates, with 10 

feet of limestone carrying Lower Cambrian fossils. There are seven or eight old 

quarries on the hillside. In the largest 20 feet or more of black and gray slates 

Overlie the green and purple slates, which dip 22° E., with a cleavage of 35° E. A 

low easterly bedding and a steeper easterly cleavage (up to 45°) are well shown 

at several quarries. Beds of calcareous quartzite occur. The purple slates now 

underlie, now overlie, the green. In the main the ridge appears to be an anticline 

of (B) and. (D), very much overturned to the west, with the red slates and the 

Hudson graptolite shales and the Hudson grits on both sides of it. In the section 

the only well-observed things are the relations of cleavage and bedding on the eastern 

slope. The overturn at the west where -the Ordovician underlies is inferential and 

the other folds are hypothetical.

SECTION II. MIDDLE GRANVILLE.

This crosses the Middle Granville "sea-green" and red-slate quarries. Judging 

from the dovetailing of the Cambrian and Ordovician a few miles south of Middle 

Granville, we should expect, immediately west of Middle Granville, first, an anticline, then a syncline. The Jamesville belt crossed by the eastern end of the section 

would be, as in Section 1, anticlinal, and the intervening broad Ordovician belt 

would be synclinal fn structure.

There are about a dozen Cambrian slate quarries north of the village. Some measurements were taken about these quarries by the writer, but the locality is perplexing. 

There is a fault, and possibly much folding and faulting. The following succession, 

however, is clear, beginning above: Black.shale and slaty shale (D), 70-100 feet; 

limestone with Lower Cambrian fossils, 4 feet; green and purple slate (B), 50-60 

feet. The/e is an open drainage cut 213 feet long, east of one of the larger quarries, 

which crosses the black shales and exposes one fault plane. There is a tunnel 180 

feet long west of the quarry, with a shaft at the end also in black shales. The men 

who worked in the tunnel report that it also traversed black shales. West of the 

shaft is another slate quarry, and southeast of the east end of the open cut still

See table on page 94.
PLATE XXIV.

STRUCTURAL DIAGRAMS OF SLATE QUARRIES IN EASTERN NEW YORK 

AND WESTERN VERMONT.

(L) Eureka slate quarries ("unfading green" and variegated), Poultney, Vermont.

(M) Valley Slate Company ("unfading green"), Poultney, Vermont.

(N) Lake Bomoseen Slate Company (mill stock, purple and green), Cedar Point,

Castleton, Vermont.

(0) Old quarry (purple with green ribbons), near Blissville, Castleton, Vermont. 

(P) Old quarry near and north of Eagle Quarry, purple with green ribbons, Bliss-

"ville, Castleton, Vermont. . 

( Q) Meadow Slate Company's quarry (mill stock, variegated and green), Fair Haven,

Vermont. Calcareous quartzite beds 4 to 12 inches thick and 18 inches apart. 

(R) Meadow Slate Company's quarry, Fair Haven, Vermont, at north end. 

(S] Scotch Hill Slate Quarry Company's quarry (mill stock, purple, variegated, and

green), Fair Haven, Vermont. Dark beds are said to underlie the green. 

(T) Old quarry (purple), 1 mile south of West Castleton, Vermont. 

(U) National Red Slate Company's quarry, southwest of Jamesville, in Granville,

New York.

( V] Williams and Ed wards's quarry ("sea-green"), Wells, Vermont. 

96
U. S. GEOLOGICAL SURVEY BULLETIN NO. 275 PL. XXIV

QUARRY DIAGRAMS, NEW YORK-AND VERMONT SLATE BELT.

Bull. 275 06 8
VEKMONT. 97

another one. There appear, therefore, to be two masses of black beds and three of 

roofing slate. Mr. Prindle finds some evidence of an Ordovician area at the top of 

the hill. It would seem, therefore, possible that between that point and the red slate 

of the valley on the east we have an anticline consisting of several minor folds, and 

that the Cambrian green and purple slate occupy the centers of the lesser anticlines 

and the black shales, the sides, as shown in the section. But the structure might be 

interpreted as consisting of two beds of green and purple slates alternating with two 

of black shale, or a fault could be supposed between the central body of green slate 

and the black slate west of it. Several quarries in this line show about'70 feet of the 

black rock (D).

Only occasional observations were made in the valley east of the ridge, and these 

all indicate low easterly dips, and so do-the observations at Nixon's and Pr.itchard's 

quarries, with slight indications of a westerly dip at one of the latter. The folds 

are probably all overturned to the west, but their dimensions may vary greatly from 

those shown in this part of.the section. The only data as to the Cambrian ridge at 

the east end of the section are easterly dips on both its east and west sides, indicating the usual overturn.

SECTION III. rLAKE BOMOSEEN.

This crosses Cedar Point and extends to Glen Lake. At Cedar Point, Lake Bomoseen Slate Company's quarry, there are 127 feet of purple, overlain by 50 of green, 

and these by 5 to 10 feet of limestone, but all doubled over into a close syncline with 

an axial plane almost, if not quite, horizontal and traversed by a cleavage foliation 

dipping 20° east (see PI. XXIV, N). For that part of the section which lies east of 

the Ordovician strip and west of the bend in the section line the data have been 

taken from the.old quarries a mile south (see PI. XXIV, fig. T, and PI. XXII). The 

folds are many and small and overturned to the west. At the West Castleton quarries, north and south of this section and east of the Ordovician strip, the slates dip 

45° to 50° east, and' consist of about 25 feet of purple overlain by 8 feet of green 

slate, and these by 8 feet of limestone, followed by poor slates and quartzite beds. 

The relations of Ordovician and Cambrian are not well exposed. The Cambrian 

slates must either turn steeply to the west or be faulted. At the extreme north end 

of the Ordovician strip Mr. Prindle made out the following relations: The Black 

Cambrian slates (D) and the Ferruginous quartzite (E) occur between the Cambrian 

limestone and roofing slates (B) and the Ordovician graptolite shales. About a half 

mile north of West Castleton the graptolite shales occur 300 feet west of the Cambrian slates with some thin quartzites.

The structure of the Ordovician strip itself is beautifully shown at a ledge by 

the roadside between West Castleton and Glen Lake (see PI. III). The rock is a 

grayish, more or less calcareous shaly or arenaceous slate, banded with black beds 

.from a fraction of an inch to 2 inches in width. On the west side of the syncline the 

beds dip very slightly east or are horizontal. Farther east, at the top, the dip is 55° 

W. and still farther 90°. The cleavage throughout is about 35° E. The ledge is 

evidently the center of the syncline and gives the key to the structure of the whole 

strip, which is 3$- miles in length, while the fossils leave no doubt as to its age.

SECTION IV.-1 BLISSV1LLE.

This crosses several lines of quarries near Blissville, in Castleton, Vt. There are 

20 quarries hereabouts, including the old Eagle quarry. The northeasterly line of 

quarries shows a minimum of about 50 feet of green and variegated slate (strike N. 

10° to 15° E., dip 20° to 30° E.) overlain by 20 feet of black slate and shale and thinbedded limestone, followed by a few feet of green slate and a bed of quart/ite 10 to 

12 feet thick, which is exposed for 350 feet along the strike. The quartzite contains 

calcareous nodules which weather out and overlies and, at the south, runs into a
98 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

few feet of a green grit like that of Flint Hill. The black shales crop out at several 

points along the road south, and also on the ridge east of it. These are classed in 

(D) and the quartzite in (E) of the table.

Between this line of quarries and the road running south the Olive grits (A.) crop 

out at several points and extend to the road corner north all with an easterly dip. 

The first quarry on the west side of the north and south road shows a very gentle 

syncline at its south end and the beginning of an anticline at its northeast corner. 

The next line of quarries west includes the Eagle. The strike here changes to N. 15°- 

20°-30°-40° W. The dips are 25°-30° E. and SE. The cleavage strikes N. 5° E. 

and dips 20° E. There are about 70 feet of purple, overlain by 10 feet of green and 

these by 15 feet of thin-bedded Olenellus limestone. At the most northerly quarry 

but one of this line the beds are folded and 'Overturned almost as much as 'at Cedar 

Point (see fig. 0, PI. XXIV). The strike of axis of fold is N. 40° W.; cleavage dip, 

low, east. Small beds of light green, with or without a quartzose limestone in the 

center, produce bands on the cleavage surface.

At a quarry intermediate between the second and third lines of quarries, and onefourth of a mile north of this section, the strike changes to N. 75° W., dip 15°-20° S.; 

cleavage strike, N. 35°-50° E., dip 15° E. The complication is probably due to a 

southerly pitch.

In the third line of quarries the beds are nearly horizontal. The fourth line, a 

half mile south of section, shows very low westerly dips and at the extreme west 

end of the section a gentle syncline is exposed crossed by a cleavage dipping 35° E.

SECTION V. WELLS.

This starts at the west shore of Lake St. Catherine and crosses the slate ridge west.

The portion of the slate belt most largely worked of late years is the ridge between 

West Pawlet and Poultney. Away from the quarries it is difficult to obtain satisfactory observations, and within the quarries bedding is generally obscured by cleavage.

Beginning at the lake, purple and green Cambrian slates dip 45° E. Judging from 

scattering observations along the west side of the ridge, it must be largely composed 

of roofing slate. From a point about a half mile west of the lake there is a line of 

quarries and prospect holes extending northward for 2 miles. The cleavage dip is 

uniformly east. Farther west the section crosses a strip of red Ordovician slate almost 

a half a mile long and 180 feet wide at the broadest part, but tapering out both north 

and south. The dip is 35° E. This is probably a small compressed and overturned 

syncline. The Ordovician grit is absent here, but some of the small quartzite beds 

occur between the Cambrian slates and the red slates. All the slates exposed between 

the strip of red and the road on the west should recur on the east of the red, but 

in inverse order. At Auld & Conger's quarries there are 170 feet of slate of various 

qualities, green and variegated, exposed. Strike N. 5° W., dip 35° E., with cleavage 

dipping 40°-45° E. Dipping toward and under the slate, but with greenish and grayish beds intervening, are the Hudson grits. Strike N. 5° E., dip 40° E. Between the 

syncline of red slate and the Hudson grits there is probably an anticline, as drawn in 

the section, and all the slates on the west side, of the red ought to recur east of the 

grits in inverse order. The folds shown in the grit are hypothetical, but the first one 

west of the slate would naturally be a syncline.

SECTION VI. PAWLET.

This begins near the Mettawee and crosses the slate ridge. The boundary between 

Ordovician and Cambrian at the east end of the section is uncertain. In the gorge of 

the Mettawee green and purple Cambrian slates of no commercial value are finely 

exposed. At the sawmill the axial planes of the folds stand erect, and the anticlinal 

parts of some of the folds have been pinched out. The cleavage is vertical. The
U. S. GEOLOGICAL SURVEY IULLETIN NO. 275 PL. XXV

SLATE SYNCLINE AT WEST PAWLET, VT,

Rising and Nelson's sea-green slate quarry, No. 2, at West Pawlet, Vt,, seen looking north, showing an eroded 

inclined syncline. The discoloration of the dip-joint face below is due to ferruginous water exuding at 

the bedding plane. Depth about 100 feet.
VERMONT. 99

eastern side" of the ridge for a mile and more south consists of folded schists interbedded with quartzite, possibly belonging to (A). West of this is a strip three-fourths 

of a mile wide of unknown character, but from the situation of two quarries about 2} 

miles south of a point a half mile west of the Mettawee sawmill, and from the direction of the strike of the slate at these quarries, slate probably occurs in the western. 

half of this blank space of the section. The roofing-slate quarries from this latitude 

to West Pawlet lie almost all within a strip one-fourth of a mile wide along the eastern 

side of the Ordovician grits on the western slope of the ridge. The structure at the 

quarries is difficult to make out. Bedding, where observed, dips east, as does also 

the cleavage. From the observed relations of the Cambrian and Ordovician wherever 

they occur very near each other in this region, the Cambrian overlies the latter 

through an overturn. An anticline should therefore occur on the Cambrian side of 

the boundary and a syncline on the Ordovician'side of it. Not far from the Columbia quarry is a dike of camptonite, 5 feet 9 inches wide, running northeast to southwest, dipping 90° or steeply to the northwest. The slate on the east side of the dike 

strikes north and dips 70° E. and on the west side 55° E. The dike has a rough 

jointing parallel to its sides and weathers in spherical nodules. The. cleavage at the 

quarries dips about 55° E. West of the quarries are greenish shales and schists with 

small quartzite beds striking N. 5° E. and dipping 45°-50° E., measuring, apparently, 

about 'i 25 feet, but possibly less, if close folded. These probably belong in the Ordovician. West of these come the Hudson grits, dipping 65° E.

SECTION VII. WEST .PAWLET.

This crosses the West Pawlet quarries and reaches the other Cambrian belt west of 

Indian River. The West Pawlet belt ends abruptly south of the village, with Hudson 

grits south, east, and west.

Some of the quarries are over 175 feet in depth. At the Hughes quarry No. 7 there 

is a syncline with an anticline east of it (see fig. C on PI. XXIII), and the foreman 

stated to the writer that another one was found later east of it. The pinching out 

of the material between the folds observed at the Mettawee gorge recurs here. At 

the Rising & Nelson quarry No. 2 the syncline is finely shown (PI. XXV and fig. 

A on PI. XXIII). We have here an isoclinal syncline with its axial plane dipping 

east with the cleavage, in such a structure the same beds of course occur on either 

side of the fold in opposite order and also at the bottom, but there in greater thickness. The structure indicated is a syncline with an anticline on either side of it, but, 

unless faulting occurred, these must be parts of an anticline and the Hudson grits, 

which crop out in the village and at the foot of the dumps, should be part of a syncline. The strike of the slates ranges from N. 12° to N. 25° E., and that of the cleavage N. 5° W. to N. 5° E., dip 70°,but in places 40° to 50° E. The thickness exposed, 

measured across two synclines and one anticline, is about 100 feet. There are some 

dark-gray or "black" beds on the east side of the syncline which, according to this 

construction, would belong not on top, but within the roofing slates (B). The Hudson grits west of the Cambrian slates strike N. 10° to 20° E. and dip 55° to 60° E., 

but at the West Pawlet railroad depot this changes to N. 5° W. and the dip to 80° E.

In the Indian River Valley neither the few red slate quarries nor the scattering 

outcrops afford very satisfactory data. The folds are probably numerous and overturned so as to give only easterly dips. Mr. Walcott has indicated a graptolite 

locality on the west side of the valley.

The Cambrian ridge at the west has a few old purple slate quarries. Some Cambrian fossils occur in the limestone. Along the eastern side of the ridge the 

Cambrian shales (D) occur. About a half mile east of South Granvilie, near the Ordovician boundary, west of this Cambrian belt, the purple slates dip 40° W., with a 

cleavage dipping 30° E. and striking N. 15° E., the bedding forming green bands on 

the cleavage surfaces, The second Cambrian ridge is clearly anticlinal in structure.
100 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

GENERAL STRUCTURAL, FEATURES.

From the foregoing sections, necessarily more or less incomplete, the general structural characteristics of the western Vermont and eastern New York slate belt can be 

grasped. The structure of the Cambrian slate mass, which usually adjoins the Ordovician schist, is illustrated in sections VI and VII. There are close folds, more or less 

 overturned to the west, with eastward-dipping slaty cleavage obscuring the bedding. 

The folds are so close and the cleavage is so pronounced that the cores of adjoining 

synclines and anticlines are brought very near together, or the anticlinal portions of 

several adjacent folds do not appear. In the northern part of the Cambrian area 

quartzite beds are more abundant within the roofing slates than in the southern. 

Although the entire mass of slate and quartzite is in places thrown into folds so 

greatly overturned that their axial planes are nearly horizontal, yet the slate itself 

is less distorted in the direction of the cleavage than in the southern area, and the 

cleavage itself is also less perfect in the northern than in the southern. Series of suchvarious folds form compound anticlines and these minor Cambrian anticlinoria alternate with Ordovician synclinoria overlying Cambrian ones in apparent but not real 

conformity. As the Ordovician areas consist of shales, slates, grits, and small quartzite beds, the beds being more heterogeneous, slaty cleavage is less prevalent, but the 

folds are also overturned toward the west (see sections I, II, V, VI, VII, PI. XXII).

A north-northwesterly strike appears in the Cambrian rocks about Blissville and 

again at Cedar Point (see map, PL XX). Large beds of quartzite overlie the roofing 

slate and are typical of this portion (section III, PL XXII).

GEOLOGICAL, AND GEOGRAPHICAL, DISTRIBUTION OF THE VARIETIES

OF SLATE.

The quarry maps, Pis. XX and XXI, show that the slate quarries situated within 

the Cambrian areas are generally very near- to; or not far from the edge of the Ordovician belts. In some places (as in Pawlet, Wells, West Castleton) the Cambrian 

slates occur within 100 or 200 feet of the "Hudson" grits. This is the case in such a 

variety of situations that the proximity of the two formations can hardly be explained 

by faulting. The Cambrian roofing slates are, therefore, regarded as occurring not 

far from the top of the Lower Cambrian series as exposed in this region and very 

near the overlying Ordovician. As the Cambrian belts are made up of numerous 

folds, generally close and overturned, the slates also occur toward the center of the 

belts, but their stratigraphical position is still the same. The first place to look for 

the Cambrian roofing slates is near the Cambro-Ordovician boundary. Where the 

red slate occurs in close proximity to and on the west side of the Cambrian green and 

purple slates and the dip is easterly, as it usually is, the red slate may be found 

underlying the "sea green," "unfading green," or purple slates, and vice versa; on 

the eastern side of the Cambrian areas the green and purple slates of the Cambrian 

may be found underlying the red of the Ordovician when both dip easterly. At 

several points (Blissville, Eureka, etc.) away from the Ordovician boundary the rock 

which appears to immediately underlie the Cambrian slates is the Olive grit (A), one 

of the.so-called "wild rocks" of the quarrymen. It is uncertain whether there may 

not be one or more beds-of slate interbedded with this. The rock which overlies the 

Cambrian slate is either the Black Patch grit (C) or the Cambrian black shale (D) 

or the Ferruginous quartzite and sandstone (E). Perhaps most generally there is a 

bed of limestone conglomerate or breccia, followed by black shales or slates (D), see 

page 94. These vertical relations are pretty well established.

The areal relations of the "sea green" and the "unfading green" are not at all 

clear. There is nothing as yet to show that the stratigraphical position of these two 

varieties of Cambrian slatea is not identical. It seems probable that, at the latitude
VERMONT. 101

of a point within 2 miles north of Poultney, a change in the sediments occurred in 

Cambrian time sufficient to account for the diminished percentage of carbonate and 

the increase of chlorite and pyrite." Whether this difference in composition is alone 

sufficient to account for the difference in the cleavage is uncertain. There may have 

been some difference in the resistance to pressure which-would, account for more perfect cleavage at the south than at the north. Possibly, as has al ready been suggested, 

the greater abundance of grains of quartz at the north may have restrained the 

cleavage structure, and so with more lime deposited at the south and more quart/, 

sand at the north the whole structural difference may be traced back to changes in 

sedimentation.

Even this demarcation between the fading and unfading green slate areas is not 

absolute, for fading green slates occur well within the unfading green area, as at an 

old quarry 1£ miles southwest of West Castleton and again 1|- miles south of Castleton 

and also a half mile south of Boinoseen. Slates which fade little are reported as 

occurring on the ridge west of Lake St. Catherine. In an old quarry about a half 

mile east of Jamesville, in a belt which seems to be directly continuous with that in 

which lie the Eureka and adjacent quarries, the slates fade comparatively little. In 

the Jamesville belt, at a quarry about 180 feet above the road and west of the chapel, 

there is a purple bed, probably overlying a green one; both purple and green fade 

badly, but on the west side of the purple, i. e., underlying it, is a green bed which 

scarcely fades and which, under the microscope, shows very few carbonate rhombs.

A. few things should be noted. The continuation of the West Pawlet slate is to be 

looked for in the lenticular Cambrian area which begins 2 miles south of'West Pawlet 

and stretches across the New York State line into Hebron. Sea green and purple 

slates also occur in the Cambrian area southeast of West Pawlet. The Jamesville 

belt continues south into the village of Granville. About the north end of Lake 

St. Catherine the Cambrian slate belt divides in two, one part passing a half mile 

east of Poultney and the-other a half mile east of East Poultney, where it erops out 

in the small gorge of the Poultney River. North of Castleton the strikes frequently 

change to the northwest or the north-northwest, and the be'ds of slate sometimes 

follow this direction. Barker Hill and Wallace Ledge both have Cambrian slates 

about them.

QUAlMtY STRUCTURE.

The types of structure exhibited at the quarries are shown in diagrams A.-G, 

K-T, and V, on Pis. XXIII and XXIV, and also in PI. XXV, reproduced from a 

photograph. The following table contains the more important compass observations 

taken at the quarries. The numbers in the first column will be found on the maps, 

Pis. XX and XXI. Where cleavage is given, but not bedding, the bedding is very 

nearly or quite the same as the cleavage. The usual dip joints may be assumed when 

not given. Approximations are indicated by the signs, plus ( + ), minus ( ), more 

or less (±).

To the data should be added a few observations at quarries.outside the areas 

shown in the maps. An abandoned quarry three-fourths mile south of Fair Haven 

shows 24 feet of purple slate overlain by about 9 feet of greenish slate, dipping 15°-20° 

E. and containing three beds of quartzite, each 2 inches thick. The cleavage strike? 

N. 10° W., dips 25°-30° E. Joints strike N. 35° E., dip 60° E., also N. 50° JS., clip 

90°. There'is a fatal secondary cleavage "false cleavage" here, striking about 

north-south. See PI. VI, A, and p. 24. At the Meadow Slate Company (Diagrams 

Q, R, PI. XXIV) and at the quarry just north of it bedding strikes N. 15° E, cleavage 

N. 7°-15° W., dip 15°-20° E. Vertical strike joints strike N. 15° E. and vertical dip 

joints N. 85° W. Diagonal ones strike NW., dip 65° NE., also N. 58° E., dip 90°.

aThe most southerly outcrop of decidedly unfading green observed by the writer occurs 2£ miles 

north-northeast of Poultney and three-fourths of a mile east of the railroad.
102 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

c^si

o Eij

ww

(4&=i [4

O

O i.

OOOOOOOO 

52 52 52

%. tq H fci Eij

o°ooo4-4-

°o +

IN £

52

+ +

!2 52

to t^ co

El) W 

o i--

4y + 

52 J5

- + - + +

IM CN C*4 <M
45......".....

46...........

47........... 

48...........

49...........

50...........

51 ...........

52...........

53...........

54...........

56...........

57...........

58............

N.+ '-

N. 15° E. 

N.55°E.

N. 25° E.

(a)

N.-

N.40°W. 

N.17° W.

N. 75° W.

N.+ 

N.+

f 3o°- \ 

\ 65° E. /

60° E. 

20° E.

20° E.

20° E. 

0° 

0° 

Fold. 

30° E.

7° W. 

15° \V.

N. 7° W.

N. 15° E. 

N. 30° E.

N. 7° E.

N. 25° W. 

N. 27° E. 

N.+ 

N.25°E. 

N.15°E. 

N. 5° E. 

N.42°E.

N.+ 

N.10°E.

f 50° to 1 

\ 65° E. f

60° E. 

25° E.

25° E. '

20° E. 

30° E. 

17° E. 

15° E. 

E.low. 

20° E.

17° E. 

17° E.

N.

N.20°E. 

N. 25° E.

N.5° W.

N.22° E.

90°

60° E.

45° E;

30° E.

60° E.

E.-W.

N. 80° W.

E.-W.

N. 65° W.

N. 70° E.

90°

80° N.

90° 

90°

90°

f N.30°E. 

1 N.40°E.

N. 35° E.

45° 

50° 

60° 
104 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

The grain strikes N. 35° W. At the Scotch ?Iill quarries (Diagram S, PI. XXIV), 2 

miles north-northeast of Fair Haven, the beds strike N. 25° W., dip 5° E., cleavage 

with same strike dips 15° E. The joints are vertical, striking N. 5°-17° E. and N. 

67° W. At the recently opened quarries of the Brandon Slate Company, 4 miles 

southwest of Brandon village, in Sudbury, the beds strike N. 55° E., dip 35° SE.. 

and in places the cleavage is parallel to it. In some of the slate ledges, east-southeast 

of their first opening, there is a false cleavage striking N. 25° W.

MINOR STRUCTURAL. FEATURES.

The purplish slates frequently have greenish ribbons calcareous, quartzitic, and 

chloritic an inch or two in thickness, and such ribbons run into rows or planes of 

green spots. The cause of these colored beds and spots has been discussed on page 

17. The ribboned slates here, as elsewhere, are used for flagging.

In some of the quarries there are pyritiferous quartzite nodules a few inches in 

diameter, of lenticular form, lying in the bedding foliation and of sedimentary origin 

.(seep. 33).

Faults of no great displacement are not uncommon. These may be reversed or 

normal (see PI. IV, C, E, and p. 29).

Veins of milky quartz are abundant and sometimes attain large dimensions (see 

PI. V and p. 32). Their course is often very irregular. They are apt to contain 

calcite and chlorite.

As shown by the maps, Pis. .XX, XXI, the region is traversed by dikes. These 

measure from 12 to 40 and more feet.in width and up to several miles in length. 

They are of Ordovician or later date and are usually of carnptonite, consisting chiefly 

of plagioclase feldspar and hornblende, with or without augite.«

These dikes are sometimes parallel to the strike or the dip joints, but more frequently to the diagonal joints.

Shear zones ("hogbacks") are of frequent occurrence, as is shown by the quarry 

diagrams, Pis. XXIII, XXIV. From their parallelism to the dikes and some of the 

joints they may all be due to the same earth movement (see, on shear zones, 

PI. VII and p. 29, and on their relation to dikes, p. 44).

The quartzite beds, the veins, dikes, shear zones, "posts," faults, and diagonal 

joints add seriously to the difficulties presented by the overturned close folding and 

the pinching out of folds which characterize this region. False cleavage is, however, 

exceptional.

THE WESTERN VERMONT SLATES.

"Sea-green" slate. This, when freshly quarried, varies from a light gray'to a slightly 

greenish gray. In some beds it is crossed by ribbons of a dark gray or, when bedding 

and cleavage are parallel, it bears traces of organisms in dark gray. To the unaided 

eye the texture is fine and the cleavage surface somewhat so, with a waxy luster. 

The sawn edges show a little pyrite. It is very slightly magnetitic, effervesces slightly 

under cold dilute hydrochloric acid applied to the edges, is quite sonorous and very 

fissile. After a few years exposure it changes its color to a brownish gray, and as 

the beds discolor differently a roof covered with."sea-green" slates from different 

beds acquires a mottled color.

Under the microscope it shows a matrix of muscovite (sericite), with brilliant 

aggregate polarization and of very fine texture, crossed here and there by obscure 

traces of bedding. This matrix incloses more or less angular grains of quartz measuring from 0.05 to 0.34 by 0.004 to 0.035 mm., but usually 0.035 by 0.013 mm., an 

occasional grain of plagioclase (lime-soda) feldspar up to 0.04 by 0.05 mm. Sections parallel to the cleavage show many plates and rhombs of carbonate, the latter

aSee Miss P. Bascom's study of these dike rocks in Nineteenth Ann. Rept., pt. 3, 1899, p. 223-226.
VERMONT. 105

measuring from 0.003 to 0.015 and even 0.05 mm. in diameter. These sometimes 

consist of two crystals, an inner rhomb and an outer one, with different orientation, 

possibly in twinned position. In some cases the central rhomb has fallen out, in 

others the rhomb has an opaque mineral nucelus which is not pyrite (magnetite 

or graphite?). There are also a few scales of chlorite with interleaved mnscovite 

up to 0.13 nun., with their laininaj across both grain and cleavage, and some equally 

large mnscovite scales, possibly of sedimentary origin. There ar.e some irregularly 

distributed pyrite spherules from 0.003 to 0.02 mm., also minute lenses of pyrite. 

.Rutile needles, measuring from 0.0017 to 0.009, rarely 0.012, by 0.002 mm., average 

from 1,000 to 1,850 per square millimeters, which is equivalent (o from 645,000 to 

1,200,000 per square inch; rarely a grain of zircon or prism of tourmaline 0.022 by 

0.008 mm. '

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite (sericite), quartz, carbonate, chlorite, rutile, pyrite, magnetite.«

In order to ascertain-the cause of the discoloration thin sections across the discolored surface of a slate which had been exposed three years were examined and it was 

found that while the carbonate rhombs within the body of the slate were transparent 

in ordinary light those at the surface were changed to the color of limonit'e. These 

rhombs measured 0.047 mm. A cleavage surface of the same slate was also affixed 

to the glass slide and the other side was ground down. This showed a multitude of 

rhombs, generally ranging in size from 0.008 to 0.013 mm., entirely or partially 

altered to limonite. In some cases there was a yellowish brown zone of alteration, 

surrounding an unaltered nucleus. These rhombs are regarded by Doctor Hillebrand as an isomorphous mixture of dolomite and siderite, i. e., a carbonate of 

lime, magnesia, and iron, in which the iron (ferrous carbonate) oxidizes into limonite. & The only way to prevent this discoloration would be to coat the slate with 

some preparation which- would protect it from oxidation.

Some of the sea-green slates are termed hard, others soft. The difference, judging 

from microscopic evidence, seems to be due to the greater percentage of carbonate in 

the soft ones and the larger size of the quartz grains in the hard ones.

The following complete analyses of sea-green slate were^ made by Dr. W. F. Hi liebrand, chemist of the United States Geological Survey:

Analyses of Vermont;'" sea-greet i." roofing date.

H-0 ( water below 110° C. ) ....................................

A.

07.70

.71

14. 12

.81

4.71

.10

Trace ?

.63

.04

2.38

3.52

1.39

trace. 

.23

B.

02. 37

.74

15. 43

1.34

5. 34

.22

.77

.07

3.14

4.20

1.14

.34

C.

59. 84

.74

.1.5. 02

1. 23

4.73

.34

2.20

.09

3.41

4.48

trace. 

.41

D.

"For colored lithographs of magnified thin sections of this slate as seen under both ordinary and 

polarized light, see Nineteenth Ann. Kept. U. S. Geol. Survey, pt. 3, PI. XXXV.

<>See Doctor Hillebrand's Chemical notes on the composition of the roofing slates of eastern New 

York and western Vermont, p. 39.
106 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Analyses of Vermont "sea-green" roofing slate Continued.

v ' \

A.

2.98

.07

.40

.22

100. 07

.12

B.

3.71

.06

.87

.06

trace.

99.80

.032

C.

3.44

.09

2.' 98 '

.05

Trace

.11

100.28

.024

D.

A (=D. XIV, 1895, 230o), Rising & Nelson's quarry No. 2, West Pawlet, Vermont; 13-foot bed. 

B (=D. XIV, 1895, 225/), Griffith & Nathaniel's quarry, 9 miles north of A, South Poultney, Vermont. 

C (=D. XIV, 1895, 256e), Wm. H. Hughes's quarry No. 10 (Brownell), 2 miles north of A, Pawlet, 

Vermont. D (=D. XIV, 1895, 35J), Auld & Conger's quarry, 8 miles north of A, in Wells, Vermont; 

22-foot bed. Determination of silica only. These are all fro*n the West Pawlet and South Poultney 

belt.

Specific gravity: C, 2.7910; D, 2.7627.

This slate is used exclusively for roofing purposes. The results of Professor Merriman's recent tests of it are stated on page 123.

"Unfading green" slate. This slate is greenish gray in color, and to the unaided 

eye has a somewhat fine texture and a roughish lusterless cleavage surface. The 

sawn edge shows some pyrite. It is magnetitic, does not effervesce with cold dilute 

hydrochloric acid, and is sonorous. Several years' exposure produces so little change 

of color that only when a fresh slate is put beside it is any change perceptible, and 

that is but slight. Its fissility is inferior to that of the "sea green." Under the 

microscope it shows a matrix of muscovite (sericite), with a brilliant aggregate 

polarization but considerable inequality in texture, coarser quartzose bands, with 

imperfect cleavage, alternating with finer more sericitic ones. In both there is 

irregularity in the size of the particles. Angular quartz grains measure from 0.01 by 

0.008 to 0.04 by 0.017, rarely 0.07 by 0.017 mm. There are a few lenses up to 0.2 by 

0.1 mm. of quartz fragments and muscovite scales, also lenses up to 0.4 by 0.1 mm. 

of chalcedonic quartz?. A few grains of plagioclase feldspar, scales of chlorite up to 

0.039 by 0.006 mm. parallel to the grain and also scales of muscovite in similar position. Not a few specks and lenses of pyrite, some octahedra of pyrite with rim of 

chlorite, carbonate rhombs 0.026 to 0.065 mm. but in very much smaller number 

than in the sea-green slate. Abundant rutile needles from 0.003 to 0.008 mm. long,

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite (sericite), quartz, chlorite, carbonate, rutile, pyrite, magnetite.«

These slates are relatively unfading, because they have fewer rhombs and plates 

of carbonate. 6 The sections also show why they cleave less perfectly than the 

sea-green slates.

o For colored lithographs of magnified thin sections of this slate, as seen both under ordinary and 

polarized light, see Nineteenth Ann. Kept., pt. 3, PI. XXXVI. 

*>See Doctor Hillebnuid's note, p. 39.
VERMONT. 107

The following analyses were made in the laboratory of the United States Geological 

Survey by Dr. W. F. Hillebrand:

Analyse* of Vermont "unfading green" roofing dales. '

TiOa( titanium dioxide, rutile)

FeO (ferrous oxide) ..........

MnO (raanganous oxide) .....

CaO(lime)....................

E.

59.27

.99

18.81 

1.12 

6.58 

.13 

Trace? 

Trace?

.42 

.05 

2.21 

3.75 

1.88

F.

59. 48

1.02 

Trace? 

18.22 

1. 24 

6.81 

.07 

Truce.

Trace? 

.56 

.05 

2.50 

3.81 

1.55

Li.,0 (lithia) .................

H2 0 (water below 110° C.) ... 

H2 0 (water above 110° C.) .. . 

PoOs (phosphoric oxide) .....

S0a (sulphuric oxide) ........

S (total sulphur included 

above) .....................

Specific gravity ..............

E.

Trace. 

0.32 

3.98 

.11. 

.21 

.15 

Trace. 

None.

99. 98

.08 

2.795

F.

Trace. 

0.1.7 

4.05 

.10 

.39 

.13

None. 

-.08

100. 23

.07

E (=D.XIV, '95, 314/), Eureka quarries, 3£ miles north of Poultney, in Ponltney Township, Vermont. F (=D.XV, '96, 645o), Valley Slate Company quarry, 2£ miles north of Poultney, in Poultney 

Township, Vermont.

The results of Professor Merriman's recent tests of Vermont unfading green slate 

will be found on page 123.

This slate is largely used for roofing purposes. The mill stock slate is described 

beyond.

"Purple" and "variegated" slate. The "purple" slate is dark purplish brown; 

the "variegated" is like the "sea green "'and the "unfading green," but is irregularly patched with purplish brown. These purplish slates are interbedded both with 

the sea green and the unfading green, as shown in the quarry diagrams (Pis. XXIII, 

XXIV). The texture, surface, and luster correspond generally to those of the slates 

with which they are interbedded. The purple of the sea-green areas discolors less 

than the sea green, but effervesces with cold dilute hydrochloric acid, and is very 

slightly magnetitic. The variegated of the Eureka quarry does not effervesce with 

cold dilute hydrochloric acid. All these slates are sonorous.

Under the microscope the purplish slate closely resembles the green, with which 

it is associated, both in structure and composition, excepting that the purplish contains a large amount of hematite (Fe2 03 ) in irregular bright red dots from 0.001 to 

0.003 mm. in diameter, exceptionally in hexagonal scales. This mineral obscures 

the aggregate polarization. There are quartz fragments up to 0.047 and even 0.07 

mm., rare plagioclase grains; lenses of quartz a millimeter long, muscovite and 

chlorite scales lying at an angle to the cleavage, others without arrangement. 

Carbonate is less abundant than in the respective green .slates. Spherules of pyrite 

from 0.007 to 0.027 mm., rutile needles in abundance, a few-prisms of tourmaline 

and rare fragments of zircon.

The chief constituents of the "purple" slate, arranged in descending order of 

abundance, appear to be muscovite (sericite), quartz, chlorite, hematite, carbonate, 

rutile, pyrite, magnetite. The purplish color is due to the mixture of the red from 

the hematite and the bluish green from the chlorite.« In the variegated slates the 

mottling is produce'd by the irregular distribution of the hematite.

"For colored lithographs of magnified thin sections of the "purple" slate, as seen under both ordinary and polarized light, see Nineteenth Ann. Kept. U. S. Geol. Survey, pt. 3, PI. XXXVII,
108 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

The following analyses were made by Dr. \V. F. Hillebrand in the laboratory of 

the United States Geological Survey:

Analyses of Vermont "purple" and "variegated" roofing slates.

SiOo (silica) ...................................................

BaO (baryta) ..................................................

LinO (lithia) ....... ... .1 ......................................

HoO (water below 110° C.) ....................................

HoO (water above 110° C.) ....................................

Total....................................................

G.

6.1. 03

.68 '

16.33

4.10

2.71

.09

Trace?

Trace?

.50

.06

2.92

5.54

1.26

Str. tr.

.31

3.24

.16

.04

99.98

.02

2. 8064

H.

60.96

.86

Triicc^

16. 15

5.16

2.54

.07

.71

.04

3.06

5.01

1.50

3.08

 .23

.68

100.22

.07

I.

60. 24

.92

Trice1-*

18.46

2.56

5.18

.07

Trace.

.33

.03

2.33

4.09

1.57

i Q

3.81

.11

.08

.16

100. 12

.087

2. 8053

H,

5.28

2. 30

G (=D. XIV, '95, 260a), purple roofing slate,.McCarty quarry, eastof center of Lake Saint Catherine, 

South Poultney, Vermont. H (=D. XV, '95, 760a), purple roofing slate, Francis & Sons quarry, nearly 

a mile south of Hydeville, in (Jastleton, Vermont. I ( = D. XV, '95, 314), variegated roofing slate, from 

Eureka quarry, 3£ miles north of Poultney, in Poultney Township, Vermont, "unfading-green" aren. 

H2 (=D. XIV,-'95, 614a), dark reddish bed a few inches thick in purple of sea-green area, west of 

Lake Saint Catherine; determination of iron oxides only.

The results of certain tests of the strength of Fair Haven "purple" slate are given 

on page 124.

The purple and variegated slates are used mainly for roofing.

Mill-stock, date. In the northern and western part of the green-slate belt those 

beds which have a less perfect cleavage are used as mill stock, and find a market as 

blackboards, billiard-table tops, electric switchboards, tiles, vats, mantels, etc., and 

for some purposes are subjected to the process known as "marbleizing." They are 

purple or green. The purple is frequently paler than the purple roofing slate, and 

also spotted with green, while the green is sometimes brighter than the unfading 

green roofing slate.

Specimens from the Scotch Hill quarries, 2 miles north-northeast of Fair Haven; 

from the Meadow quarry, one-fourth mile east of Fair Haven; from the Lake Bomoseen Slate Company's quarry at Cedar Point, on Lake Bomoseen,« and from the 

J. Jones quarry, 2£ miles north of Castleton, were examined microscopically, with 

these resul-ts:

The matrix is muscovite (sericite), with aggregate polarization, but shows a cleav-

"See PI. XXIV, iV, for the structure at this quarry.
VERMONT. 109

age much inferior to that of the sea-green slate and somewhat inferior to that of the 

Eureka of the unfading green. There are about 6 chlorite plates, interleaved with 

muscovite, to each square millimeter, measuring up to 0.087 by 0..043 millimeter 

and lying at right angles to the cleavage. Quartz fragments are very abundant, 

measuring up to 0.06 by 0.03 millimeter, with a few of plagioclase feldspar; muscovite scales in various orientation 9; a few carbonate rhombs, the usual abundance 

of rutile needles, and, finally, pyrite, which seems to be more abundant in the purple 

than in the green. The purp', 3 differs from the green by the addition of hematite.

The chief constituents, arranged in descending order of abundance, appear to be 

muscovite (sericite), quartz, chlorite (hematite in the purple), carbonate, rutile 

(kaolin in the purple, at least), pyrite, and, as shown by a magnet test, magnetite.

The specific gravity of the purple, from Cedar Point was found to be 2.83 and of 

the green from the J. Jones quarry 2.84, both being a little higher than the figures 

for any of the other western Vermont slates.

SIdle-pencil date. In the unfading green-slate portion of the belt, about 1£ miles 

north of Bomoseen and a little cast of the lake, is an abandoned quarry where certain greenish slates were obtained and made into slate pencils. In Europe slate 

pencils have long been made.by utilizing a secondary cleavage, which breaks the 

rock up into squarish sticks which are easily rounded. Here, however, the method 

was to take tile-shaped blocks of slate and carve out first on one side, then on the 

other, by means of set gauges, a whole series of hemicylindrical pencils which readily 

broke apart into roundish pencils. A microscopic section of this rock shows essentially the same composition as the unfading green slates, excepting that sections parallel to the cleavage show no carbonate whatever, but a greater abundance and larger 

scales of muscovite (probably sedimentary), some limonite? specks, and a cleavage 

perhaps not quite so good as that of the Eureka quarries. The usual.quartz, sericite, 

chlorite, rutile needles, and lenses are present.

The general substitution of paper tablets for school slates in this country has almost 

stopped the manufacture of slate pencils.

Minerals associated with the green and purple slates. As the minerals of visible size 

associated with the slates throw light on the nature and origin of the 'microscopic 

constituents of the slate itself, they are here given:

Quartz is the most common accessory mineral. It is usually segregated in the veins 

already described, but occurs also as an infiltrated cement between the quartz grains 

in the beds of quartzite or in veins traversing the quartzite. In both of these modes 

it is crystallized whenever cavities admit of it.

Next in abundance is calcil/', occurring also in veins with or without quartz, or as 

delicate films on joint planes, or as a sediment in the beds of quartzite. The quartzite beds sometimes contain minute rhombs which effervesce readily with hydrochloric acid and weather a limonite brown, and are therefore probably a double 

carbonate of iron and lime.

Squarish or oval concretions an inch by three-fourths of an inch and one-half inch 

thick, consisting of radiating crystalline lamella; of barite, with the inter veiling spaces 

filled with slate and calcite and with many minute cubes of pyrite round about, occur 

in the Cambrian green slates of Middle Granville. Barite also occurs with calcite in 

crystalline films on joint planes.

Chlorite is common in quartz veins, or almost alone makes up small veins, or coats 

slickensided joint or bedding planes.

Pyrite occurs in cubes up to one-fourth inch across or in botryoidal concretions, 

coated with fibrous quartz (chalcedony) or with calcite, or, more rarely, chlorite. 

This coating of chalcedony is often confined to some of the sides, filling a space produced by motion or compression, as described by Eenard. Pyrite may collect in the 

vicinity of calcareous and quartzose veins or beds, or form dendritic crystallizations
110 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

on cleavage planes, or minute cub. s on joint faces. That this mineral is pyrite and 

not marcasite is shown by its not decomposing readily after long exposure on the 

slate dumps.«

Rarely a littlegalenite occurs in the quartz veins.

It will be observed that nearly all of the above minerals have already been mentioned as occurring in the slates, as shown either by the microscopic or the chemical 

analyses.

Benson black slate. T-Ialf a mile east of Benson village and 7£ miles north-northwest of Fair Haven, in Rutland county, black roofing slate was quarried in 1895, but 

the quarry was abandoned not long afterwards. The slate is of Ordovician age (Hudson-Utica), and belongs in a shale, slate, and schist belt which extends nearly 4 

miles westto within a mile of Lake Champlain, and 4^ miles south-southwest to the 

village of West Haven, and also, with bends and ramifications, 20 miles north-northeast to Weybridge Center, in Addison county. Almost the entire area and the quarry 

location will be shown on the forthcoming Fort Ticonderoga folio, Economic geology sheet. The slate-bearing area, however, probably does not exceed 12 square 

miles.

At the quarry the beds dip about east at angles ranging from 15° to 20°; cleavage 

strikes about N. 7° E., dipping east at 35°, and vertical joints strike N. 75° W.

The slate is a bluish black; to the unaided eye has a somewhat fine texture and a 

somewhat smooth, slightly lustrous cleavage surface. It is carbonaceous or graphitic, 

contains a little magnetite, effervesces with cold dilute hydrochloric acid, is sonorous, 

and has a fair degree of fissility.

Under the microscope it shows a matrix of muscovite (sericite), with a brilliant 

aggregate polarization, somewhat obscured by much carbonate and carbonaceous 

matter in fine and coarse particles. The quartz fragments measure from 0.013 to 

0.03 millimeter; carbonate rhombs from 0.004 to 0.035 millimeter. Pyrite spherules 

from 0.0017 to 0.007 millimeter number about 200 per square millimeter, often 

occurring in rows along the cleavage. Rutile needles from 0.0017 to 0.0952 millimeter long are abundant; chlorite scales are very few and small, zircon fragments rare.

The constituents of this slate, arranged in descending order of abundance, appear 

to be muscovite (sericite), quartz, carbonate, pyrite, rutile, carbonaceous or graphitic 

matter, and magnetite.

This slate is thus closely related, both in composition and quality, to the "soft 

vein'' slates of Lehigh and Northampton counties, Pa. Its large amount of carbonate 

indicates its probable discoloration on continued exposure. Its appearance in magnified thin sections under both ordinary and polarized light is very well shown in 

PL XXXIX, part 3, Nineteenth Annual Report United States Geological Survey, 

which would answer almost equally well for many of the Pennsylvania "soft vein" 

slates.

The following analysis (specimen P = D. XIV, '95, 305d) of black slate from the 

abandoned quarry, one-fourth mile east of Benson Village, Rutland County,-Vt., was 

also made by Dr. W. F. Hillebrand:

« See footnote on page 40 on pyrite and marcasite.
VERMONT VIRGINIA. Ill

Analysis of .Benaon black roofing slate.

Si02 (silica).................... 59. 70

Ti02 (titanium dioxide, rutile) .. .79

A1 2 03 (alumina)................ 16. 98

Fe2 03 (ferric oxide)............. .52

FeO (ferrous oxide) ............ 4. 88

MnO (manganous oxide)........ .16

NiO (nickelous oxide).......... Trace?

CoO (cobaltous oxide).......... Trace?

CaO (lime) .................. 1. 27

BaO (baryta) .................. .08

MgO (magnesia) ............... 3. 23

K2 0 (potassa) .................. 3. 77

Na2 0(soda).................... 1.35

Li2 0 (lithia) ............. Strong trace.

H2 0 (water below 110° C.)...... .30

H2 0 (water above 110° C.)...... 3. 82

P2 05 (phosphoric oxide) ........ .16

C02 (carbon dioxide)........... 1. 40

FeSj (pyrite)..........'......... 1.18

SO,, (sul phuric oxide)........... Trace.

C (carbon)..................... .46

Total .................... 100.05

S (total sulphur included above). . 63

Specific gravity................. 2. 7748

The more important features of all these Vermont slates, as brought out in the 

above descriptions, will be found in tabular form opposite page 124.

VIRGINIA.

By T. NELSON DALE.

W. B. Kogers, in his reports to the legislature of Virginia during the years 1835 to 

1841, a called attention to the slate deposits east of- the Blue Ridge in Buckingham, 

Fluvanna, and Fauquier counties. Slate also occurs in the Blue Ridge in Amherst 

County.

Geological relations in Buckingham and Fluvanna counties. Rogers' reference to this 

slate 6 may well be repeated here:

This (roofing slate) makes its appearance on both sides of the James River. ... In Buckingham the bed is largely exposed in the neighborhood of New Canton, on Slate River. . . . In texture, density, and capacity of resisting atmospheric agents it can scarcely be excelled by a similar 

material in any part of the world. This quarry was first opened to procure slate for roofing the Capitol, and notwithstanding it has been thus long known and its value established, but little further 

use has been made of it,,until the activity of the present owner has again brought it into notice. The 

building of the university will soon be furnished with a complete covering of slate from this quarry.

Slate River empties into the James about 40 miles west-northwest of Richmond 

and 52 miles northeast of Lynchburg. The extent and structure of the formation 

which includes these slate beds require further study, but fig. 12 will serve to show 

some of its important features. The width of the formation north of the James, as far 

as explored by the writer during a very brief visit, extends from a point one-half 

mile west of Brenio Bluff to a small creek entering the James If miles east-southeast of Shores, a distance of If miles. It strikes about south-southwest across the 

James, having a minimum length of 4 miles. 'South of the James its eastern boundary lies about 3,000 feet west of the toll bridge opposite Brerno Bluff, and its western 

boundary is roughly one-fourth mile east of the Virginia Mills, on Slate River. The 

"Bremo Bluffs," on the north side of the James, and those at New Canton, on the 

south side, both consist of quartzite with like strike and dip N. 17°-23° E., dip 65° 

E. and 90°, on the north, and N. 13°-18° E., dip 90°, on the south. The exposure of 

quartzite on the north is about 950 feet across the strike and on the south about 3,000 

feet. These facts and the jagged structure of the 75-foot high "Bremo Bluff" show 

that the James here flows through a broad transverse cut in the quartzite and the 

slate formation west of it. This quartzite is fine grained, muscovitic, and biotitic, 

with an occasional plate of calcite and a few fragments of plagioclase. It is interBull. 27E

"See Bibliography, p. 145. &0p. cit., p. 79,
112 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

bedded here and there with a few inches of slate consisting of muscovite (sericite)',, 

quartz, chlorite, and biotite, and containing garnets and lenses of quartz, muscovite,, 

and chlorite, some of which are probably altered garnets. The quartzite also has; 

slate of this character on both sides of it. On the east the quartzite dipping 80° E.,, 

is followed conformably by 40 feet of such slate, then by about 300 feet of micaceous 

quartzite, followed by an undetermined thickness of ferruginous slate full of pseudoScale of miles -

Z 3

Conto'ur interval 50 feet

FIG. 12. Map of slate region in Buckingham and Fluvanna counties, Va. Quarries shown by crossed 

hammers; slate prospect hy round dot; strike of commercial slate by arrows; quartzite areas by 

fine dots.

morphs or crystals of mica (muscovite?) 1£ millimeters across, the plates of which lie 

across the foliation of the rock. This slate was originally bluish gray, but weathers 

a hematite red.

The general structure of .the belt in which the commercial slate beds lie, judging 

from the railroad cuttings between New Canton and Slate (River, appears to consist 

of minor folds. It ia traversed by numerous dikes of olivine basalt, ranging from a
VIRGINIA. 113

,few inches to 12 feet in thickness (plagioclase, olivine, magnetite, augite), weathering in conchoidal masses with a yellowish-brown surface. The formation is bounded 

on the east by the quartzite cut by the James River. If this quartzite be a westwardly overturned syncline, it would overlie the slate; if an anticline of like character, 

it would underlie-the elate,- in either case in apparent conformity. On the west of the 

formation at Virginia Mills there are chloritic, sericitic, quartzose, and feldspathic 

schists striking N. 25° E. and dipping 90°, crossed diagonally by a small basaltic dike, 

also certain bright-red weathering chloritic, quartzose, arid feldspathic schists, possibly 

the same as the above, but both of uncertain age.

The commercial slate itself appears to occupy a belt two-fifths of a mile wide along 

Hunts Creek, which is a southern tributary of Slate River. At Arvonia its course ia 

N. 35° E., roughly like that of Hunts Creek, which meanders through it; but on the 

north side of the James, 3£ miles north-northeast of Arvonia, the strike of the slate 

is N. 20° E., like that of the quartzite at Bremo Bluff. There appears thus to be a 

bend in the slate beds between Arvonia and the north side of the James. The discovery of certain crinoids and more recently of brachiopods, trilobites, etc., in the 

quarried slate at Arvonia shows it to be of Ordovician age.« Mr. E. 0. Ulrich, to 

whom recent collections of these fossils were referred, determines them as unquestionably of Upper Ordovician age, but he finds that the fossils are of such strange 

types that it will require a very critical study to determine whether the beds are to 

be correlated with the Mohawkian or with the Cincinnatian formations. The dikes 

are probably of Mesozoic age.

Arvonia. The quarries near Arvonia are scattered along the sides of Hunts Creek 

for a mile northeast of that place. The principal quarries in operation in May, 1904, 

were the "Old or Big" quarry and the "Middle" quarry of the John R. Williams 

Slate Company, and the,Fontaine quarry of A. L. Pitts. Several others were just 

being opened or were temporarily abandoned.

The "Big quarry" measures 300 feet along the cleavage, 200 across it, and 125 in 

depth. Bedding and cleavage are identical, striking N. 37° E., dipping 86° SE. 

There are vertical dip joints striking about northwest, strike joints northeast, southwest, dipping 75°-80° SE., also two sets of diagonal joints, one dipping 30° E. and 

the other 55° W. The latter system has a "post" 5 feet thick full of shear zones. 

There are also gently undulating '' flat joints,'' to which the grain is parallel. Quartz 

veins in this quarry contain calcite, chlorite, and biotite.

The "Middle quarry" of the same firm is 500 feet along the cleavage, 350 feet 

across, and in places 350 feet deep. Bedding and cleavage strike N. 33° E., dip 85° 

SE., and are crossed by northwesterly vertical joints.« At the south end is a vertical 

dike of olivine basalt, 12 feet thick, traversing the slate diagonally, with joints parallel 

to it on both sides and, for a few feet on its north side, with horizontal joints from 4 

to 6 inches apart. The quality of the slate is said to be better a little beyond the zone 

of these joints than it is at a greater distance from the dike. A horizontal joint in 

this quarry meanders as much as 10 feet from the horizontal in a distance of 50 feet.

The slate from the Williams quarries is a very dark gray, with a slightly greenish 

hue. To the unaided eye it has a minutely granular crystalline texture and a slightly 

roughish, but very lustrous, surface. It is slightly graphitic and magnetitic, shows 

pyrite on the sawn edge, does not effervesce in cold dilute hydrochloric acid, and is 

very sonorous.

Under the microscope it shows a matrix of minute alternating beds, chiefly of fine 

niuscovite, with coarser ones, chiefly of quartz, the former with a brilliant aggregate 

polarization, the latter with a faint one. These beds are parallel to the cleavage. 

The quartz fragments measure, up to 0.085 mm. Scattered throughout both the

aSee Barton, N. I-I., Fossils in the "Archean" rocks of central Piedmont, Virginia: Am. Jour. Sci., 

8d ser., vol. 44,1892, pp. 50-52.
114 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

more micaceous and the more quartzose beds are crystals, lenses, and particles of 

pyrite, numbering about 25 to each square millimeter and measuring up to 0.09 mm., 

rarely 0.15 and 0.42, with their longer axes parallel to the cleavage. These probably 

include a little magnetite. There are also biotite scales transverse to the cleavage, 

about 22 per square millimeter, and measuring up to 0.12, rarely 0.2 mm. Almost, if 

not quite, as abundant are plates and rhombs of carbonate. There are occasional 

scales of chlorite interleaved with muscovite, a few grains of plagioclase feldspar 0.047 

mm., rarely one of zircon, some tourmaline prisms 0.014 mm. long, much extremely 

fine graphitic (or carbonaceous?) material, a few particles of hematite, and some 

rutile needles. vSections parallel to the cleavage are unusually brilliant in polarized 

light, owing to the abundance of quartz, biotite, and carbonate.

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite and sericite, quartz, biotite, carbonate, graphite (or carbonaceous matter), pyrite, chlorite, magnetite, with accessory plagioclase, zircon, hematite, tourmaline, and rutile.

The Fontaine quarry of A. L. Pitts has a length of 300 feet along the cleavage, a 

width of 200, and a depth of about 100. Bedding and cleavage strike N. 34° E.; dip 

80° SE. The quartz veins also contain calcite.

The slate from this quarry is very dark gray with a greenish hue, and to the 

unaided eye has a granular sparkling crystalline texture and a roughish, but very 

lustrous, cleavage surface. It is slightly graphitic (or carbonaceous). The sawn 

edge shows pyrite. It contains rare particles of magnetite, does not effervesce in 

cold dilute hydrochloric acid, and is very sonorous.

Under the microscope the Fontaine quarry slate shows alternating bedlets of fine, 

more muscovitic, and coarser, more quartzose, material, the former having a brilliant 

aggregate polarization. Much of the mica is in the form of muscovite rather than 

sericite, particularly in the quartzose beds. These beds are parallel to the cleavage. 

Quartz fragments abundant, measuring up to 0.09 and 0.12 inm. Most conspicuous 

scales of biotite lie across the cleavage, usually up to 0.2 by 0.09 mm. and averaging 

about 10 per square millimeter. Lenses and crystals of pyrite, often surrounded by 

secondary quartz, about 60 per square millimeter and measuring up to 0.076 by 0.03, 

rarely 0.34 by 0.03 mm., with their longer axes in the cleavage. A few scales of 

chlorite, interleaved with muscovite, lying across the cleavage and up to 0.3 mm. 

Some carbonate in the coarser bedlets, occasionally with inclusions of an opaque 

mineral (graphite, pyrite, or magnetite). Much graphitic (or carbonaceous) matter 

i.i very minute particles. Some hematite and a few crystals of tourmaline, 0.02 by 

0.006 mm.

The chief constituents of this slate, arranged in descending order. of abundance, 

appear to be muscovite and sericite, quartz, biotite, carbonate, pyrite, graphite (or 

carbonaceous matter), chlorite, magnetite, hematite, with accessory tourmaline.

A specimen from an exceptional bed in the same quarry shows a wrinkled surface 

from the abundance of biotite lenses measuring a half inch in length by a twentieth 

in width at the center. George H. Williams" found the joint faces at the old Roberts quarry covered with pyrite and crystals 1 mm. long, which when analyzed proved 

to be Ti02 and which he determined optically as anatase.

Although the Arvonia slates, as shown by the above determinations, contain some 

carbonate, that does not include any appreciable amount of ferrous carbonate, for 

some of these slates put on the old Richmond theater over sixty years ago, when 

removed within the last three years, did not show any discoloration whatever, and 

some have been -on buildings near the quarries over a century without losing any 

of their blackness. Their highly crystalline character also implies strength and 

durability.

a Anatase from the Arvon slate quarries. Buckingham County, Va.: Am. Jour. Sci., 3d ser., vol. 42, 

1891, p. 431,
VIRGINIA. 115

The results of Professor Merriman's recent tests of slate from the Williams and 

Pitt quarries are given on page 123.

Bremo. Slate lias been prospected on the estate of the late Dr. Casey Charles 

Cocke, 2 miles west-northwest of Bremo Bluff, in Fluvanna County. It occurs in the 

ravine east of the house and also on the hill northeast of it. The strike is N. 18°-23° 

E., dip nearly 90°. A little northwest of the house is a dike of olivine basalt 5 to 10 

feet wide.' The following results are from an examination of specimens obtained 

within a few feet of the surface which, therefore, belong to the "top."

This slate is dark gray, but its lightness of shade is in part the result of weathering. It will prove to be darker below the zone of weathering. To the unaided eye 

it has a fine texture with a somewhat line and very lustrous cleavage surface more 

or less dotted with .minute lenses or crystals. It is slightly graphitic, but not magnetitic, shows pyrite on sawn edges, does not effervesce in cold dilute hydrochloric 

acid, has an argillaceous odor, is sonorous and very fissile. The slate from one of 

the openings has exceedingly minute veins of mnscovite.

Under the microscope the Bremo slate shows a matrix of muscovite (sericite), 

with a brilliant aggregate polarization.

There are abundant quartz grains up to 0.09 by 0.03 and 0.13 mm. long, the larger 

ones surrounded by secondary quartz, radiating along the cleavage; about 50 lenses 

and crystals of pyrite per square millimeter, measuring up to 0.09 by 0.02 (exceptionally the lenses are 1.5 mm. long), with their longer axes in the cleavage. There 

are also square and rhombic spaces lined with secondary quartz, measuring up to 

0.6 mm., left by the dissolution of cubes or distorted cubes of pyrite. In some specimens there are 65 such cavities to the square inch, in others none. There are lenses 

up to 0.56 by 0.11 mm., consisting of quartz and muscovite or of these and chlorite 

and pyrite, or of chlorite and muscovite, the folia of muscovite and chlorite lying 

across the cleavage; also muscovite scales up to 0.09 by 0.02 mm. Throughout the 

matrix much dark-gray material occurs in exceedingly fine dots (graphite?). There 

are also some dots of hematite. Rutile needles are not very plentiful up to 0.01 mm. 

long; also a number of very irregular particles of rutile up to 0.05 mm., consisting of 

a network of crystals ("sagerute twinning") forming angles of 120°and 60°. A few 

fragments of zircon, an occasional crystal of dark tourmaline up to 0.05 by 0.02 nun., 

rare flakes of biotite, no carbonate.

The chief constituents of Bremo slate,- arranged in descending order of abundance, 

appear to be muscovite, quartz, pyrite, kaolin, chlorite, graphite (or carbonaceous 

material), rutile, with accessory tourmaline, zircon, biotite, and hematite.

This slate differs from the Arvonia slate in its finer texture, probable slightly 

lighter shade, and slightly higher percentage of pyrite. It has less biotite and, at 

least in the "top," no carbonate.

Core drilling or excavation deep enough to penetrate the "top" is amply warranted by the above determinations.«

Snowden, Amherst County. The Snowden slate deposit is on the southeast side of 

the axis of the Blue Ridge, north of the cut through which the James River flows. 

It is situated about 18 miles north-northwest of Lynchburg. (See map, fig. 1.3.) The 

slate crops out in a longitudinal valley with Rocky Row Mountain on its northwest 

side and a spur of Big Piney Mountain on its southeast side. It strikes N. 65° E. 

and has quartzite southeast of it. The general structure and the areal relations of 

the slate have not yet been exhaustively investigated. 6

The only quarry in operation in 1904 was that known as the Williams Brothers 

slate quarry, on the property of the Virginia Slate Mining Company, which lies 3

«Slate outcrops on this property were referred to by J. L. Campbell, Tenth Census, special report on 

building stones, vol. 30,1884, p. 181.

''See Campbell, Bibliography, p. 143, also Geology of the Blue Ridge, near Balcony Falls, Va.: Am. 

Jour. Sci., 3d ser., vol. 28,1884, pp. 221-223.
116 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

miles north-northeast of Snowden station, where Rocky Row Run enters the James 

River, and about 1,200 feet above it. About 250 feet of slate are exposed with a 

bedding strike of N. 65° E., forming a flat-topped anticline, whose northern limb 

dips 35° at the surface, but becomes vertical at the bottom of the quarry, and for a 

space of a few feet clips steeply south, indicating a possible overturn structure. The 

cleavage strikes N. 45° E., dips southeast at 32°. The bedding forms finely plicated 

ribbons of quartz and calcite several inches wide on the cleavage surfaces. Joints 

strike N. 18° E., dip 70° W., also N. 18° W., dip 65° E. The grain makes an angle 

of from 90° to 104° with the bedding and is said to be very marked.

This slate is very dark gray, to the unaided eye has a minutely granular texture, a 

moderately smooth cleavage surface, with very little luster. It is not graphitic,

79°25

Scale of miles

Contour interval lOOfefft

FIG. 13. Map showing location of Snowden quarries, Amherst County, Va. Quarries shown by crossed 

hammers; slate prospects by round dots; strike of commercial slate by arrows.

shows no pyrite on sawn edge, has no magnetite, does not effervesce with cold dilute 

hydrochloric acid, is quite sonorous, has a very slight argillaceous odor and is said 

not to fade.

Under the microscope it shows a matrix of muscovite (sericite), with aggregate 

polarization not very brilliant owing partly to the coarseness of many of the other 

constituents. Quartz fragments measure up to 0.08 mm.j'chlorite scales and lenses 

up to 0.28 in length, rarely 0.42 by 0.2 mm., numbering about 6 per square millimeter; also muscovite scales up to 0.09 mm.; some carbonate, but in exceedingly 

minute rhombs and plates; grayish carbonaceous (?) matter; about 55 spherules and 

pyritohedrons of pyrite per square millimeter, measuring up to 0.005 mm.; abundant 

rutile needles, and occasional fragments of zircon.

The chie'f constituents of this slate, arranged in descending order of abundance,
VIKGINIA. 117

appear to be muscovite (sericite), quartz, chlorite, kaolin, py'rite', carbonate, rutile,' 

and carbonaceous matter. This is a superior slate, but is less crystalline'than that of 

Arvonia. It is used exclusively for roofing, having been found less well adapted1 tot 

mill stock.

About 1$ miles northeast of the WiJlianis Brothers quarry slate was prospected 

nineteen years ago on the Thompson property.. This lies on a small tributary of 

Otter Creek, which flows into the James 1J miles downstream from Snowden. (See 

fig. 13.) The thickness exposed is 20 feet. Bedding strikes N. 53° E., dips 20° SE.; 

cleavage is nearly horizontal. Joints strike N. 38° E. and dip 20° W. This slate 

closely" resembles that of the Williams Brothers quarry both in its external and 

microscopic characteristics and i ls ronorousness.

77V

77°50'

77°50'

Scale of miles.

Contour interval 50feet

PIG. 14. Map of slate prospects in Fauquier County, Va. Slate prospects are shown by round dots.

Warrenton, Fauquier County. There are several prospects in the neighborhood of 

White Sulphur Springs on the Rappahannock, 6 miles southwest of Warrenton, to 

which Rogers referred," from which roofing slate was obtained as early as 1837. 

(See map, fig. 14.)

The following data afford some indications as to the geological relations: About li 

miles south of the Springs and 200 feet east and as far west of the road to Routsville 

there are outcrops of muscovitic quartzite with grains and pebbles of blue quartz up 

to one-fourth inch across.- These beds have an apparent east-west strike and a gentle 

southerly dip. A little north of this quartzite slate crops out and continues northward for 2 miles to near the "colored church." Its cleavage strikes N. 25°-30° E., 

but the slate three-fourths of a mile south of the Springs, at the river, strikes about

«Op, cit, p. 460.
118 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

east-west, like the quartzite farther south. Slate is also said to occur at points west 

of the road from the Springs to the church. The minimum width of this slate belt 

is half a mile. North of the church is a greenish metamorphic eruptive consisting 

of actinolite, zoisite, titanite, and sericitized .plagioclase. South of the quartzite are 

greenish schists, possibly of similar origin. Between the church and Warrenton 

greenish schists also crop out at several points. They are epidotic actinolite schists 

in places with lenses up to 3 by 2 feet, and stratiform' masses of an epidotic rock 

containing actinolite, titanite, magnetite, chlorite, and quartz.

All these hornblendic and epidotic rocks weather into a bright reddish soil. This 

slate is thus related to a quartzitic formation, as well as to certain hornblendic and 

epidotic rocks, which must be regarded as metamorphosed eruptives." The proximity of the slate to these other formations is so close that nothing can be assumed 

as to its thickness.

Slate has been prospected at three points, indicated by black dots on the map, 

tig. 14. One is seven-eighths of a mile north-northeast of the Springs, east of road to 

Warrenton and west of beginning of small brook flowing south into the Rappahannock. Cleavage strikes N. 30° E., dips 25° W. The plowed fields on the south 

show a strip of dark-gray soil, from the weathering of the slate, which measures 125 

feet across. The slate is black, and, to the unaided eye, has a coarsish texture,' a 

rough, speckled cleavage surface with little luster. It is very carbonaceous; shows 

pyrite on sawn edges, and has about 220 cubes and lenses of pyrite to the square 

inch, measuring one-fiftieth of an inch and under; no magnetite; nor effervesence 

with cold dilute hydrochloric acid. It has an argillaceous odor.

Under the microscope this shows no aggregate polarization, but a coarse cleavage, the 

matrix mainly of carbonaceous matter and quartz fragments with a little muscovite. 

The quartz grains measure from 0.009 to 0.3 mm.; quite a number of plagioclase 

feldspar fragments up to 0.2 mm. long. Lenses and cubes of pyrite coated with 

quartz, about 5 per square millimeter; a few scales of chlorite; rare fragments of 

zircon. No carbonate or rutile detected.

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be: Carbonaceous matter, quartz, muscovite, feldspar, pyrite, and chlorite. 

This is a clay-slate.

At the locality about a half mile northeast of the Springs the cleavage strikes 

N. 25° E., dips 20° W., bedding probably N. 12° W. Strike joints strike N. 15° E., 

dip 90°; dip joints N. 75° W., dip 90°, but variable. The slate is here covered with 

2 feet of black clay, and these by 3 feet.of brownish yellow clay, both of which still 

show a lamination parallel to the cleavage of the underlying slate, and therefore 

represent different stages in the decomposition of the slate.

This slate is bluish black, to the unaided eye has a moderately fine texture and 

cleavage surface with very few lenses of pyrite and but little luster. It is very carbonaceous, shows pyrite on sawn edge, has no magnetite, no effervescence in cold 

dilute hydrochloric acid, is sonorous, and has an argillaceous odor.

Under the microscope this has a matrix of muscovite (sericite) with distinct aggregate polarization but abundant irregular quartz grains from 0.009 to 0.37 mm.; also 

some lenses of secondary quartz, much carbonaceous matter, a few lenses of pyrite 

up to 0.14 mm., passing into limonite, a few grains of plagioclase feldspar. No 

carbonate or rutile detected.

The chief constituents of this slate, arranged in descending order of abundance, 

appear to be muscovite (sericite), quartz, carbonaceous matter, pyrite. Recent 

excavations are said to show a larger amount of pyrite at a depth of 20 feet than 

the specimen above described indicates. This is a mica slate.

The third locality is about a half mile south of the Springs, on the east bank of the 

Rappahannock. About 40 feet of "slate " are exposed in a small cliff. The strike of

a Rogers, op. cit., refers to the epidotic ones.
VIRGINIA WEST VIRGINIA. 119

cleavage and bedding (?) ranges from N. 80° E. to N. 85° W., dipping south at angles 

varying from 12° to 15°.

The slate is bluish black, has a coa-rse crystalline texture, and, as accurately 

described by Rogers,« a knotty, wrinkly surface. This is due to the presence of 

cubical and lenticular cavities once occupied by pyrite but now partially filled with 

quartz and graphite. These measure from one to three-fiftieths of an inch and number from 12 to 20 per square inch. The surface has very little luster. The slate is 

carbonaceous or graphitic, shows pyrite on the sawn edge, has very little magnetite, 

in places scales of a micaceous mineral up to one twenty-fifth of an inch, does not 

effervesce in cold dilute hydrochloric acid, has an argillaceous odor, and is very 

sonorous.

Under the microscope it shows a matrix of carbonaceous or graphitic material, 

quart/, and inuscovite. As the first two predominate the aggregate polarization is quite 

faint. The cleavage is serpentine, owing to the many large quartz grains (up to 0.14 

millimeter) and the cubes and lenses or distorted cubes of pyrite already refer red to, 

most of which seem to be partial psendomorphs of quartz and graphite after pyrite, 

the remaining pyrite having been oxidized and dissolved. There are some plagioclase 

feldspar grains almost as large as the quartz grains, with graphite inclusions parallel 

to the multiple twinning. There are veinlets of quartz and lenses of secondary 

quartz usually about or on.either side of the cubes and lenses of pyrite; rare grains 

of zircon. No carbonate or rutile detected.

The chief constituents of this, arranged in descending order of abundance, appear 

to be carbonaceous matter and graphite, quartz, muscovite, kaolin, pyrite, and feldspar. 

It seems to be more metamorphic than that of the other localities, yet less micaceous. 

This is properly a graphite-quartz-muscovite-schist, or in process of becoming such. 

A piece of this "slate" a half inch thick from the roof of one of the brick outbuildings at the White Sulphur Springs, where it was placed at least sixty-eight years 

ago, does not show any decomposition, but in places has a little limonite coating. 

. The pyritiferous character of all these Fauquier County slate prospects and the 

sulphurous character of the springs are probably intimately related. Such springs 

are common in the vicinity of pyritiferous shale and obtain their sulphur from the 

decomposition of the pyrite by surface water. The indications from the openings 

and the microscopic examinations are not sufficiently promising to warrant investments. '

The principalj features of Virginia slate, as given in the foregoing descriptions, will 

be found in tabular form opposite page 124.

WEST VIRGINIA.

By T. NELSON DALE.

This recently prospected slate district lies near Martinsburg, in Berkeley County, 

within the geological belt designated in the Harpers Ferry folio of the United States 

Geological Survey as "Martinsburg shale." Brief preliminary notices of it have 

appeared in Bulletin No. 213 (p. 363) and again in Bulletin No. 260 (p. 538). This 

belt lies about 13 miles west of the Blue Ridge and mostly on the western side of 

Opequon Creek, a small tributary of the Potomac. It measures at least 14 miles in 

length from north-northeast to south-southwest and from 2'to 3 miles in-width. 

The accompanying map (fig. 15) shows the geological relations and the principal 

outcrops. This shale and clay-slate formation, estimated at from 700 to 1,000 feet in 

thickness, is of Ordovician age and overlies the Cambro-Ordovician "Shenandoah 

limestone" in a series of folds represented in the folio as overturned to the west. 

The rock is generally a dark-grayish shale, weathering into a yellowish or white 

clay, known locally as "soapstone." At several points, usually near the Opequon

«0p. cit., p. 460.
120 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

or its tributary "runs," where the mass has been denuded of its weathered zone, it 

has a well-marked easterly dipping (exceptionally westerly) slaty cleavage crossing

Scale of miles

Contourjnterva! 100 feet

PIG. 15. Map of slate region about Martinsburg, W. Va. (From Geologic 

Polio No. 10, Harpers Terry, TJ. S. Geol. Survey.) Shading represents area 

underlain by Martinsburg shale. Slate quarry is shown by crossed hammers; 

slate prospects by round dots; strike of commercial slate by arrow.

the bedding at various angles, and pieces, when struck with a hammer, give the 

ty.pical ring of a slate. The slatiness of the formation is, however, inconstant.
WEST VIRGINIA. 121

The only quarry yet opened is that of the Shenandoah Slate Company, about 3 

miles northeast of Martinsburg, near a small run flowing into the Opequon. When 

visited in May, 1904, the quarry measured about 100 feet along the strike, 70 feet 

across it, and 75 feet in depth, of which the "top" took up 25 feet. The bedding 

strikes N. 25° E., dips about 15° E. The cleavage, with the same strike, dips 75° E. 

There are joints striking with the cleavage and dipping 35° E., dip joints striking N. 

50° W., dipping 90°. The beds are small and are separated by darker ribbons. The 

thickest bed exposed measured 3 feet 6 inches. A diamond drill core from down to 

40 feet below the bottom of the quarry shows several 3-foot beds.

The slate is black, with a slightly brownish hue. The texture is somewhat fine 

and the cleavage surface roughish without any luster whatever. The material is 

carbonaceous rather than graphitic, contains a little magnetite, shows pyrite on the 

sawn edge, effervesces with cold dilute hydrochloric acid, but less in the ribbon, is 

somewhat sonorous, and has an argillaceous odor. Under the microscope it shows a 

matrix consisting of carbonate and carbonaceous matter, and therefore without 

aggregate polarization, but a cleavage consisting in the parallel arrangement of the 

carbonate and carbonaceous matter in alternating bands. A very carbonaceous bed 

(ribbon) crosses the cleavage at an angle of 36°. There are abundant angular quartz 

grains up to 0.05 millimeter; scales of chlorite interleaved with muscovite, and some 

of muscovite only; spherules of pyrite up to 0.01 millimeter, numbering about 240 

per square millimeter. Rutile not observed. The amount of carbonate differs in dif-. 

ferent beds. It may be so abundant as to obscure completely the sericitic matrix. 

Some of the sections parallel to the cleavage show almost as much muscovite as carbonate and, curiously, a faint aggregate polarization parallel to the bedding or the grain. 

. The constituents of the Shenandoah Slate Company's slate, arranged in descending 

order of abundance, appear to be carbonate, muscovite (in places almost equal in 

amount to carbonate), quartz, kaolin, pyrite, carbonaceous matter, chlorite, magnetite. 

This is a clay-slate. The amount of C02 (carbon dioxide) in this slate was determined 

by Mr. George Steiger, chemist of the United States Geological Survey, at 1.94 per cent.

An analysis of slate from this quarry, made for the company by Dr. W. C. Tilden, 

of Washington, shows Si02 , 62.71; Al.,03 , 19.40, and CaO, 1.11 per cent. It is proposed to use the product of this quarry for mill stock, for which it seems betteradapted thari;for roofing.

The following prospects were noted: One 2 miles south of Martinsburg, on John 

Rowe's land, \vhere an opening 30 by 20 feet has been made. Bedding strikes N. 32° E., 

clips 60°-65°'S'E.; cleavage strikes N. 23° E., dips 80° E. A 3-foot bed is in sight. 

Another opening has been made on the Opequon, on Light's land, a half mile southsoutheast of Bedington (a little north of the north limit of map). Bedding and 

cleavage strike N. 20° E., the former dipping 30° E., the latter 55° E. A 3-foot bed 

is in sight. Under the microscope this shows a faint aggregate polarization, fragments of feldspar, and rutile needles. At an opening on T. F. Bowers's farm, about 4 

miles northeast of Martinsburg, the cleavage strikes N. 27° E. and dips 73° W., but 

bedding dips east. Under the microscope this also shows a faint aggregate polarization, but carbonate is unusually abundant and evidently obscures the cleavage. 

In the brook on John Shedd's farm, 2$ miles south-southeast of Martinsburg, the 

cleavage strikes N. 20° E. Under the microscope this shows a slight aggregate polarization and but little carbonate. Clay-slate has also been found on theMcKownand 

Busey farm, 2 miles N. 10° W. from Middleway, in brook 2£ miles southeast of 

Martinsburg, and also on land of J. W. Snyderon the Opequon, 3 miles southeast of 

Martinsburg; and it will be found in many other places.

Although the proportions of carbonate and muscovite vary in these slates, none of 

them show a complete sericitization of the matrix. They are all clay-slates. The 

material can therefore hardly possess sufficient fissility or prove sufficiently strong or 

elastic to compete with mica-slates for roofing purposes. Furthermore, the amount 

of carbonate shown by the microscope, as well as the mode of weathering by the
122 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

outcrops, indicates its probable discoloration on prolonged exposure, so that it belongs 

in the "fading" group of clay-slates. But these characteristics do not affect its 

serviceableness for indoor purposes.

The principal features of these West Virginia slates are stated in tabular form 

opposite page 124.

MERRIMAN'S RECENT TESTS OF CERTAIN MAINE, NEW YORK, 

PENNSYLVANIA, VERMONT, AND VIRGINIA SLATES.

. In view of the incompleteness of published data as to the physical properties of 

American roofing slates, Doctor Day, chief of the division of mineral statistics of the 

United States Geological Survey, induced Prof. Mansfield Merriman, of Lehigh University, to apply his well-known methods of testing roofing slates to typical specimens from nine important localities. The specimens were .obtained from various 

firms and forwarded to him. Professor Merrhnan's communication to Doctor Day, 

containing his full report on the results of these tests, follows:

The following is a report of the results of the tests of thirty-three specimens of roofing slate received 

at our laboratory from May 6 to May 13, 1905. The tests have been made according to the methods 

described in my paper on "The strength and weathering qualities of roofing slates," in Transactions 

of American Society of Civil Engineers, September, 1892.

All the specimens were 12 by 24 inches in size. The following is a statement of the marks placedupon them and of their mean thicknesses:

Cl, C2, C3, C4 designate four blue slates from the Chapman Slate Company, Chapman Quarries, Pa., 

the mean thicknesses being 0.228, 0.204, 0.216, 0.222 inches,.respectively.

Wl, W2, W3, \V4 designate four blue slates from the Williams Slate Company, Arvonia, Va., the 

mean thicknesses being 0.202, 0.195, 0.206, 0.263 inches, respectively.

PI, P2, P3, P4 designate four blue slates from A. L. Pitts, Arvonia, Va., the mean thicknesses being 

0.232, 0.200, 0.204, 0.194 inches, respectively.

Bl, B2, B3 designate three blue slates from the Merrill Brovvnville Slate Company, Brownville, Me., 

the mean thicknesses being 0.220, 0.195, 0.210 inches, respectively. A fourth piece sent by this firm 

was accidently broken.

Mol, Mo2, Mo3, Mo4 designate four blue slates from Monson Consolidated Slate Company, Monson, 

Me., their mean thicknesses being 0.238, 0.227, 0.237, 0.225 inches, respectively.

VI, V2 designate two green slates from Vermont Unfading Green Slate Company, Fair Haven, Vt., 

their mean thicknesses being 0.222,0.262 inches, respectively. Two other pieces sent by this firm were 

found broken in the package.

Rl, R2, R3, R4 designate four green slates from Rising & Nelson Slate Company, West Pawlet, Vt., 

their mean thicknesses being 0.260, 0.167, 0.224, 0.213 inches, respectively. :

M5, M6, M7, M8 designate four green slates from Mathews Consolidated Slate Company, Boston, 

Mass., their mean thicknesses being 0.194, 0.222, 0.187, 0.205 inches, respectively. The labels on these 

slates were marked "Quarry N. E."

Ml, M'2, M3, M4 designate four red slates from Mathews Consolidated Slate Company, Boston, Mass., 

their mean thicknesses being 0.190, 0.180, 0.174, 0.186 inches, respectively. The labels on these slates 

were marked " Quarry E."

The following table gives the results of the tests of the above 33 specimens for strength, toughness, 

density, softness, porosity, and corrodibility, and also the means for each of the nine different kinds 

of slate:

Color and firm.

Blue;" Chapman Slate Co .

Mark on specimen.

Cl......

C2......

C3......

C4......

Means ..

Strengthmodulus of 

rupture in 

pounds per 

square 

inch.

8,410

12, 490

7,420

9,540

9,460

Toughness ultimate deflection in 

inches on 

supports 

22 inches

0.21

.24

.16

.24

.212

Density 

specific 

gravity.

2. 775

2.780

2.734

2.766

2.764

Softnessamount in 

grams 

abraded 

by 50 turns 

of a small grindstone.

0.171

.218

'.234

.213

.208

Porosity 

per cent of 

water absorbed in 

24 hours.

0. 221

.290

.150

.263

. .231

Corrodibility 

per cent 

of weight 

lost in 

acid solution in 63 

hours.

0.336

.301

.560

.334

.383

"Designated as very dark gray in table facing page 124.
TESTS OF SLATES BY MEKRIMAN. 123

Color and firm.

*

Blue;« Williams Slate Co.

Blue;*1 Merrill Brownville

Blue;& Monson Consolidated Slate Co. ..........

Green; Vermon't Unfading

Green; Rising & Nelson 

Slate Co .................

Green ;« Mathews Consolidated Slate Co...........

Red; Mathews Consolidated Slate Co..........

Mark on 

specimen.

Wl ..... 

W2 ..... 

W3 ..... 

W4 .....

Means . .

PI......

 P2......

P3 ......

P4......

Means . .

Bl ......

B2......

B3......

Means..

Mol .... 

Mo2 .... 

Mo3 .... 

Mo4 ....

Means ..

VI......

V2......

Means . .

Rl ......

R2......

R3......

R4 ......

Means..

M5...... 

M6......

M7......

M8......

Means ..

Ml......

M2...... 

M3......

M4......

Means..

Strengthmodulus of 

rupture in 

pounds per 

square 

inch.

8,590 

8,250 

10, 700 

8, 620

9,040

8,540

9,010

11,970

9,880

9,850

8,860

9,050

' 11, 720

9,880

9,500 

11,370 

7, COO 

8,640

9,130

6,250

6,580

6, 410

6, 790

11,040

3,470

7,700

7, 250

7,090 

5,470

10,130

9 

' 490

8,050

6, 850

9,870 

11,340 

8,820

9,220

Toughness ultimate 

deflection in 

inches on 

supports 

22 inches 

apart.

0. 17 

.19 

.31 

.24

.227

.25

.18

'.25

.22

.225

.17

.21

.22

.200

.24 

.22 

.19

.17

.205

.19

.26

.225

.24

.22

.13

.24

.207

.22

IK

.20

.190

.18

.27 

.25 

.23

.232

Densityspecific 

gravity.

2.788 

2.754 

2. 795 

2. 788

2. 781

2.805

2. 790

2.788

2.781

2.791

2.790

2.804

2.799

2. 798

2.796 

2.795 

2.794 

2. 791

2.794

2.783

2.759

 2. 771

2. 750

2.740

2. 733

2.720

2. 736

2.783 

2. 782

2.786

2.781

2.783

2.834

2.853 

2.853 

2.853

2.848

Softnessamount in 

grams 

abraded 

by 50 turns 

of a small grindstone.

0.073 

.038 

.097 

.033

..060

.095

.159

.138

.038

.108

.243

.360

.192

.265

.234 

.302 

.271 

.218

.256

.326

.356

.341

.184

.183

.209

.185

.190

.286

.244

.172

.226

.069

.099 

.119 

.304

.148

Porosity 

per cent of 

water absorbed in 

24 hours.

0.105 

.090 

.209 

.167

.143

.332

.154

.163

. 216

.189

.128

128

.148

.181 

.181 

i» . 206 

.186

.188

.300

162

.231

429

O/'K

.286

.325

.376 

409

.360

.351

.374

.153

,.251 

.334 

.235

.243

Corrodibility 

per cent 

of weight 

lost in 

acid solution in 63 

hours.

0. 281 

.801 

.223 

.270

.394

.248

.552

.275

.218

.323

.366

9Rd

.264

.305

.282 

.384 

.234 

.243

.286

.277

.313

.295

433

1.067

849

.768

.413 

.265

Ann

.379

249

.425 

.507 

.311

.373

" Designated as very dark greenish gray in table facing page 124. 

t> Designated as very dark gray in table facing page 124. 

c Designated as bright green in table facing page 124.
124 SLATE DEPOSITS AND IKDUSTBY OF UNITED STATES.

The given results, with the exception of those for softness, may be compared with the results for 

Bangor, Pen Argyl, Lehigh, and Peach Bottom slates, given in my papers published in Transactions 

of American Society of Civil Engineers for September, 1892, and December, 1894, and in Stone for July, 

1898. The results above given for softness are comparable with each other, but can not be compared 

with those in the papers cited; this is due to the circumstance that the former grindstone has been 

destroyed by fire and that the one selected to replace it has a much smaller abrading capacity.

MANSFIELD MERRIMAN.

SOUTH BETHLEHEM, PA., July 21, 1905.

COMPARATIVE CHARACTERISTICS OF SLATES.

The following table shows the principal mineralogical, chemical, and physical 

characteristics of 38 kinds of slate described by the writer in this bulletin as far as 

these manifestly bear upon their economic value. These slates are from Arkansas, 

California, Maine, Maryland, New York, Pennsylvania, Vermont, Virginia, and West 

Virginia. For full particulars and for scientific details the descriptions should be 

consulted, by reference to the pages given in the first column. The columns headed 

"strength" and "toughness" refer to the tests by Merriman, whose methods of 

experimentation are described on page 47. Microscopic texture refers primarily to 

the matrix or body of the slate. By "crystalline" is meant that the matrix consists 

of interlacing and overlapping scales and fibers of muscovite and is, therefore, a micaslate or technically a phyllite-slate, although it may inclose unaltered particles of sedimentary origin. Such a slate should have, other things being equal, greater elasticity 

(toughness) and strength than one in which there is no such texture, or in which it is. 

only incipient. The fineness or coarseness of this crystalline texture probably has a 

bearing upon the strength and toughness of the slate, but physical data are not sufficient to show this. The coarse-textured Peach Bottom slates, which really approach 

a mica schist, are the strongest of the twelve kinds of American slates tested, but they 

are less flexible than all the other kinds tested. In the grade of fissility 1 signifies 

a perfect slaty cleavage, 4 a very imperfect one. The column of chief mineral constituents includes only the four or five principal ones seen under the microscope, or 

whose presence has been otherwise determined, and these are given in the descending 

order of their probable abundance. i

To these comparative data should be added the results of a few tests not easily 

tabulated.

Merriman's later corrosion tests showing the following percentages of loss in weight 

after immersion in acid solution for 360 hours: Pennsylvania slates, 1.68 to 2.76; 

Peach Bottom, 1.11 to 1.29; red of New York and Vermont, 0.25. During this test 

the Pennsylvania slates become a grayish white, some, of the Peach Bottom, slates 

change but slightly, others are almost unaffected; the red slates likewise remain 

almost unaffected. a

E. H. S. Bailey's tests of porosity give these indices of porosity: "Hard Vein" 

Pennsylvania Chapman, 0.11-0.14; Daniels quarry, 0.14; Belfast quarry, 0.25; red 

of New York and Vermont, 0.21.&

J. F. Williams's tests of the compression of columns of slate 10 inches long by an 

inch in section with the cleavage vertical, show that the purplish of the unfading 

green series of Vermont stands 20,000 pounds; the unfading green, 16,020 pounds, 

and the red of New York and Vermont, 17,730 pounds. c

The following results of various tests of Maine (Monson) slate made at the United 

States Arsenal at Watertown, Mass., were republished from the War Department 

reports in the Twentieth Annual Keport of the United States Geological Survey, 

Part VI (continued), 1899, p. 395:

aTrans. Am. Soc. Civ. Eng., vol. 32, p. 538. 

b Loc. cit., p. 542.

c. cit., p. 132 (see Bibliography, p. 145 of this bulletin).
r

Comparative, characteristics of various slates.

Described 

on page 

53

54

54

54 

54 

55 

55 

55 

57

GO 

05 

03

03 

04 

09

72 

74 

70

77

79 

79

78

78 

78

S4

si

SG 

91 

104

107

100

'107 

110 

113

114

115 

110 

121

State.

.....do.....!.....

.....do...........

.....do...........

.....do...........

.....do...........

.....do........... 

.....do...........

Maine ........... 

.....do........... 

.....do.........'..

.....do........... 

.....do........... 

Maryland .......

New York .......

.....do...........

Pennsylvania. . . .

.....do....--...-- 

.....do...........

.....do...........

.....do...........

.....do........... 

.....do...........

.....do....-..--... 

.....do............

PennsylvaniaMaryland.

....do...'.........

....do............

....do............

....do............ 

....do............ 

Virginia ..........

....do............

....do............ 

...".do............ 

West Virginia ....

Locality, quarry, bed.

.....do.....................

Mammoth Red and Lost 

Hannah, Polk County.

Mena, Polk County........ 

Mammoth Red, PolkCountj

Sec. 25, T. 3 S., R. 29 W., 

Polk County.

Sec. 30, T. 3 S., R. 28 W., 

Polk County.

Southwest Slate Mfg. Co., 

Polk County.

Eureka quarry, Eldorado 

County.

Merrill quarry, Brown ville 

North Blanchard .......... 

Monson Pond, Monson ....

Maine Slate Co., of Monson 

West Monson .............. 

Thurston ..................

Gran ville and Hampton.. . 

.....do.....................

Old Bungor, Northampton 

County.

North Bangor, Northampton County.

Heimbach, Big bed, Northampton County.

Heimbach, Black bed .....

East Bangor Consolidated, 

Northampton County.

Slatington, Lehigh County.

Chapman "hard vein".....

 Peach Bottom ..............

Northfield, Vermont Black 

Slate Co.

"Sea green" ...............

Purplish of "sea green "...

" Unfading green ".........

Purplish of " unfading "... 

Benson (prospect) .......... 

Arvonia, Williams. . ........

Arvonia, Fontaiiie ..........

Snowden ...................

Color.

Black...... .

Reddish .........

Greenish gray . . .

Dark-bluish gray 

Light' gray ...... 

Very dark' gray . 

.....do...........

.....do...........' 

.....do........... 

.....do*:..........

.....do........... 

.....do........... 

Dark purplish ...

Reddish .........

Bright greenish .

.... .do ...........

.....do...........

Very dark greenish. '

Very dark gray, 

not bluish.

Bluish black .....

Very dark bluish' 

gray.

.....do............

Very dark gray . . .

rVery dark bluish 

I gray.

Very dark gray... 

Grav greenish ....

Purplish brown. . .

Greenish gray

Purplish brown... 

Bluish black .....

Very dark greenish gray.

....do............

Very dark gray . . .

Black, brownish 

hue.

Cleavage surface.

Remarkably fine .... 

Roughish, speckled .

Roughish. ...........

Fine...... ........... 

Roughish. . .......... 

Roughish, spangled . 

Fine. ................

Very fine ............

Roughish...... .;.... 

.....do................

..... .do.. ...... ....... 

Very fine ............ 

Slightly granular. . . .

{

Fine or roughish, 

speckled.

.....do...............

Fi 11 G

.....do................

Roughish...... .......

.....do................

Slightly roughish ....

 Minutely granular . . . 

Very fine .....'........

Fine...'...............

....do................

....do................ 

Somewhat- fine ....... 

Minutely granular ...

Granular .............

Minutely granular ...

Luster.

Slight..........'......

Waxy ...............

Slight ............... 

None ................

Very. bright ......... 

Slight ...............

Almost none.........

Bright '...............

Somewhat bright. . . . 

.....do................

.....do................

Almost none ......... 

.....do................

.....do................

.....do ................

.....do................

None .................

.....do................

Slight ................

Very bright .......... 

....do................

Waxv ................

None .................. 

....do'................

....do................ 

Slight ................ 

Very bright ..........

....do................ 

....do................

A Irnost none .........

Magnetite.

.....do.......

.....do.......

Very little... 

.....do....... 

.....do.......

Much........ 

None ........ 

Very little...

.....do....... 

None ........ 

.... .do .......

.....do........

Very little....

.....do........

little. 

Very little....

Little.........

Some .........

....do........

Very little....

....do........

....do........

....do........ 

Very little .... 

....do........

....do........

.....do........

Little.........

. Microscopic texture.

Crystalline, extremely 

fine, homogeneous.

.....do.....'.....:l.........

Crystalline, extremely 

fine, homogeneous.

.....do......................

Crystalline, fine ............ 

.....do....................;.

Crystalline, coarse, granular.

Crystalline, very fine. ....... 

Crystalline, fine ............

Crystalline, fine, but particles irregular.

Crystalline, very fine. ...... 

Crystalline, fine ............

Crystalline, fine, particles 

irregular.

.....do......................

.....do......................

Crystalline, fine ............

.....do.'.....................

.....do .......................

.....do ......................

.....do......................

.....do......................

.....do......................

.....do......................

Crystalline, coarse ......... 

Crystalline, very fine. ......

....do..:...................

....do...................... 

Crystalline, irregular. ......

..'..do......................

Crystalline, fine. ........... 

Crystalline, irregular ......

Crystalline, irregular, 

coarse.

Crystalline, fine, irregular..

Not crystalline or imperfectly so, coarse.

Grade of fissility.

1

3 

1

3 

1

2 

2 

3

2

1 

3 

3

2 

2 

3

3 

3 

1

1

1

1

1

1 

1

1

2

2 

1

1

1 

2

2 

2 

2

2

1

2 

3

Chief minerals, a 

Muscov., hematite, kaolin, qnartz, 

chlorite. .

Muscpv., quartz, pyrite, carbon ...... 

Muscov., quartz, chlorite, kaolin ....

Muscov., quartz, chlorite, pyrite.....

f Muscov., quartz, hematite, kaolin, 

t carb.

Muscov., quartz, chlorite, carbon, 

magnetite.

Muscov., carb., quartz, kaolin ........

Muscov., carb., quartz, chlorite ......

Muscov., carb., quartz, pyrite ........ 

Muscov., carb., quartz, carbon .......

Muscov., carb., quartz, pyritc, chlorite.

Muscov., carb., quartz, kaolin. .......

Muscov., quartz, carb., pyrite ........

FMuscov., quartz, graphite, andalusite, 

[ magnetite.

Muscov., quartz, chlorite, earb ........

Muscov., quartz, carb., pyrite ........

Muscov., quartz, biotite, carb., 

carbon.

Muscov., quartz, biotite, carb., 

pyrite.

Muscov., .quartz, chlorite, kaolin..... 

Carb., ninscov., quartz, kaolin, pyrite, carbon.

Carbonate.

.....do........

.....do........

.... .do ........

.....do........

.....do........ 

.....do........

.....do ........

.....do........

. . . .'.do ........

.....do........'

.....do ........ 

.....do........

JMuch.........

Less than red, 

usually.

Quite a little..

Quite a little.. 

.....do........

Very much . . . 

.:... do........

.....do........ 

Quite a little. .

Much.........

Very little.... 

.....do........

Much .........

.....do........

Little.........

From some to 

much.

Lime 

by 

analysis.

0.98

0.52

f 0. 11 

\ 5.11

1.43 

4. 38

4.09

4.23

f 2. 83- 

\ 3.40

1 0.155- 

\ 0.48

f 0. 03- 

\ 2.20

f 0.50-

f 0. 42- 

l 0.56

1.27

1.11 

and 

over.

Strength, 

pounds 

per 

square 

inch.b

9,880

9, 130

\ 9, 220 

8,050 

9,810

7, 150

[ 9, 400 

! 11,200

 7, 250 

}.........

> 6, 410

9,040 

9,850

Toughness, c

0.200

0.205

0.232 

0.190 

0,312

0.270

0.212 

0.93

0.207

' 0. 225

0.227 

0. 225

Remarks.

Strength, toughness, and behavior in freezing and thawing should be tested. ' .

Do.

Specimen shows two extra foliations, which will prove 

directions of weakness.

Strength, toughness, and behavior in freezing and thawing should be tested.

Some limonite staining from pyrite. 

Between a metamorphic grit and a slate. 

See test, p. 47.

See p. GO on amount of magnetite. See tests on p. OG. 

Very sonorous. 

Very sonorous. See tests on p. 03.

Very sonorous. 

Very sonorous. See tests on p. 123. 

Sonorousness very moderate; can be sawn with handsaw.

{

Becomes brighter on exposure. Little or no ferrous 

carbonate. Impact test shows 120.66 foot-pounds of 

work per pound of slate. See tests on pp. 49, 124.

Said to be unfading. See tests on p. 123.

Discolors on continued exposure. Impact tests of various 

Northampton and Lehigh County slates show from 3. 50 

to 5.44 foot-pounds of work per pound of slate. See tests 

on p. 124.

Do.

Do. (Seep. 79.)

Do.

Discolors on continued exposure.

In ribboned slate f rom this quarry the percentageof quartz 

would be higher than in the rest. Impact tests of various Northampton and Lehigh County slates show from 

3.50 to 5.44 foot-pounds of work per pound of slate. See 

tests on p. 124.

Discolors on continued exposure. Impact tests of various 

Northampton and Lehigh County slates show from 3.50 

to 5.44 foot-pounds of work per pound of slate. See tests 

on pp. 47, 124.

(Discolors less readily than any of the above Pennsylvania 

1 slates. Impact tests of various Northampton and Le- 

| high County slates show from 3.50 to 5.44 foot-pounds 

[ of work per pound of slate. See tests on p. 124.

/Very sonorous. Impact tests show from S.49 to 24.17 foot- 

\ pounds of work per pound of slate. See tests on p. 124.

Very sonorous.

Becomes brownish gray on continued exposure. See 

tests on p. 123.

Discoloration less pronounced than that of "sea green."

fPreserv.es nearly all its color on continued exposure. See 

L tests on p. 124".

Do. 

Sonorous. Probably " fading." 

Very sonorous. See tests on p. 123.

Do.

Very sonorous.

Analysis by W. C. Tilden. In some specimens muscovite 

equals trie carbonate in amount.

aCarb. = carbonate; carbon. = carbonaceous matter or graphite. 

Bull. 275 06. (To face page 124.)

6Tests bv Merrinian. e Deflection, in inches, on supports 22inches apart; tests by Merriman.
COMPARATIVE CHARACTERISTICS OF SLATES. 125

Pounds. 

Maximum fiber stress per square inch..................................... 7, 671

Shearing test per square inch............................................. 2, ] 92

Ultimate compressive strength per square inch .........I.................. 19, 510'

Coefficient of expansion, 0.000005.

The relative commercial value of several slates is an index of their physical characteristics. Mathews, in 1898, gave these prices for slates 14 by 7 inches, threesixteenths thick, per square: Peach Bottom, $4.85; Northampton County, Pa., $3.50; 

Lehigh County, Pa., $3.40-$3.95; Maine (No. 1), $6.40; Arvonia, Va., $3.60; unfading green, Vermont, $4.50; red, New York, $11.«

The following prices per square for slates, No. 1 quality, 16 by 8 inches, f. o. b., 

were obtained by Doctor Day from producers for January, 1905: Peach Bottom, 

$6.35; Monson, Me., $7.20; red, New York, $11; Bangor, Pa., $5.75; Albion, Pa., 

$5; Pen Argyl, Pa., $4.75; Chapman, Pa., hard-vein, $5.25; Slatington, Pa., $4.50 to 

$5; unfading green, Vermont, $4.50 to $5.25; sea green, Vermont, $3.50; Virginia, 

$5 to $5.50.

In accordance with the scheme of classification of slates given on page 6, most of 

the slates whose characteristics are given on the preceding table are here arranged 

systematically:

(A) Clay-slates (FADING) Martinsburg, W. Va.

r (a) Carbonaceousi r ,.,, ,, , A - -n

.... Lehigh and Northampton counties, Pa. 

or graphitic \ D Tr , / ui i UN [Benson, Vt. .. (blackish). J

(b) Chloritic (greenish). "Sea green," Vermont.

(c) Hematitic and Chloritic (purplish). Purplish of Pawlet and Poultney, Vt.

rPeach Bottom, Pa, and Md.

(B) Mica-slates

(FADING)'

(UNFADING)

(a) Graphitic or 

carbonaceous (blackish).

Arvonia, Va. 

Northfield, Vt. 

Brown ville, Monson, Me. 

North Blanchard, Me. 

West Monson, Me.

(b) Hemati ti cfGranville, Hampton, N. Y.; Polk 

(reddish). I County, Ark.

(c) Chloritic (greenish). "Unfading green," Vermont.

(d) H e mati ticl

and chlo-[Purplish of Fair Haven, Vt. 

ritic (pur-fThurston, Md. 

plish). J

STATISTICS OF PRODUCTION, IMPORTS, AND EXPORTS.

By A. T. COONS. '

PRODUCTION.

Although slate has been known and quarried in this country, more or less, since 

1750, when we find records of its use in Massachusetts, Maryland, and Pennsylvania, 

and since about 1850 in New York, Vermont, Georgia, California, Virginia, and other 

States, no statistics of production for the entire United States are obtainable for any 

year preceding 1879.

For the years 1879, 1881, 1882, and 1883 the production is largely estimated. The 

statistics for 1880 are taken from the Tenth Census and are very accurate. The 

figures from 1884 to 1888 are fairly reliable, and from 1889, when the figures are those

Maryland Geol. Survey, vol..2, p. 240.
126 SLATE DEPOSITS AND INDUSTRY OF'UNITED STATES.

of the Eleventh Census, to the present time, the data as to production have been 

obtained directly from the slate quarries, and are as accurate as can be procured. 

There is no record of production in New York from 1884 to 1890, but the operations 

in this State being closely allied to those of .Vermont the output is doubtless included 

in that of Vermont.

The following tables give the output of slate in the United States from 1879 to 1904, 

by States, showing, as nearly as possible, the number of squares of roofing slate produced, their value, and the value of slate used for other purposes, as far as figures are 

obtainable:

Production of slate in the United States, by years, from 1879 to 1904-

State or Territory.

Utah ...................

Other States ...........

Total..............

State or Territory.

Georgia ................

Minnesota .............

Tennessee

Utah...................

Other States ...........

Total.............

1879.

Roofing 

slate, 

squares.

14,000 

8,000

9,000 

245, 500

83,300 

8,057

367, 857

Value.

$42, 000 

36, 000

45,000 

826,093

249, 900 

32,228

1,231,221

1880.

Roofing 

slate, 

squares.

1,000 

26, 200 

12, 280 

1,550

4,683 

19,850 

271, 313

108, 891 

11,500

457, 267

1883.

Roofing 

slate, 

squares.

30, 000 

15,000

5,000

1,000 

10, 200 

325, 000

110,000 

10,000

506, 200

Value.

8120,000 

67, 500

18, 250

4,000 

51,000 

1, 147, 500

440, 000 

50,000

1,898,250

Value.

84,500 

84,800 

C6, 700 

7,000

15, 000 

95, 500 

862,877

352,608 

51, 000

1,529,985

1881.

Roofing 

slate, 

squares.

500 

25, 200 

12,000 

1,000

. 3, 000 

19, 000 

272,000

109, 370 

12,000

454, 070

Value.

82, 250 

77, 500 

54,000 

4,500

10, 500 

95, 000 

877, 978

368, 110 

54,000

1, 543, 838

1882.

Roofing 

slate, 

squares.

200 

26, 000 

15, 000 

500

4,000 

21,000 

292,000

127, 300 

15, 000

501,000

Value.

8900 

78,000 

67, 500 

2,000

12, 000 

105,000 

1,002,000

435, 100 

51,000

1,753,500

1884.

Roofing 

slate, 

squares.

41,000 

a 10, 000

7,000

329, 004

85,000 . 

9,000

481,004

Value.

8215, 000 

45, 000

35, 300

1,162,560

340,500 

45,505

1,843,865

Other purposes than 

roofing 

slate.

$8,000.

Total value.

$215,000 

' 45. 000

35, 000

1,162,560 ,.; 

340,500 ' 

45, 505 f

i

1,851 ,86"

a Includes Pen.ns.ylYa.nJEv Peach. Bottom.
PRODUCTION. 127

Production of slate in the United States, by year's, from 1879 to 1904 Continued.

State or Territory.

Utah...................

Other States ...........

Total .............

State or Territory.

Maryland..............

Utah...................

Other States ...........

Total.......:.....

1885.

Roofing 

slate, 

squares.

34, 000 

"14,500

10, 000

331,160

130, 000 

17,300

536, 9(10

Value.

8102, 000 

65, 250

40, 000

993,080

380, 000 

58, 137

1,638,467

Other

purposes 

than 

roof ng 

slate.

si 0,090

1888.

Roofing 

slate, 

squares.

37, 000 

a 19, 000

7,000

6,000

416,000

160,000 

17,400

'662,400

Value.

gill, 000 

85, 500

24,000

22, 000

1,270,400

480, 000 

60, 540

2,053,440

Total 

value.

5102,000 

Go, 250

40, 000

993,080

380,000 

58,137

1, 648, 467

1886.

Roofing 

slate, 

squares.

30, 000 

u 12, 000

12,000

'

348,805

111,385 

16,600

536, 790

Value.

$.118,000 

54,000

46,000

979,415

353,155 

59, 800

1,610,370

1887.

Roo/ ng 

slate, 

squares.

37, 000 

"20,000

7,000

370,439

120, 000 

19,000

573,439

Value.

 $81, 000 

90, 000

25, 000

1,111,317

350, 000 

57, 000

1,720,317

1889.

Roofing 

slate, 

squares.

3, 104 

3,050 

41,000 

23, 099

. 2, 700 

16,767 

476, 038

230, 350 

30, 457 

3,060

835, 625

Value.

518, 089 

14,850 

201,500 

105, 745

9,675 

81, 726 

1,641,003

596, 997 

113, 079 

15,240

2, 797, 904

Other 

purposes 

than 

roofing 

slate.

8480 

18,000 

4,263

1,250 

44,877 

370, 723

245, 016

684,609

Total value.

'(") 

818,089 

15,330 

219, 500 

110,008

CO

A 10,925 

126,603 

2, Oil, 726

(») 

842,013 

113, 079 

15,240

3,482,513

"Includes Pennsylvania Peach Bottom. 

i» Included in other States.

Bull. 275 06 10
128 SLATE DEPOSITS AND INDUSTRY'OF UNITED STATES.

Production of slate in tlie United States, by years, from 1879 to 1904- Continued.

State or Territory.

Tennessee

Utah...................

Other States ...........

Total.............

State or Territory.

New York

Utah...................

Other States ...........

Roofing 

slate, 

squares.

120

4,000

3,000

50, 000

25, 166

2,500

17, 000

507, 824

247,643

36,059

893, 312

Roofing 

slate, 

squares.

2,500

18, 184

7,422

900

AQ Adn

364,051

75

132,061

27, 106

189

Value.

8480

24,000

13, 500

250, 000

123,425

10, 000

136, 000

1,741,836

698, 350

127,819

3,125,410

189

Value.

111,250

124,200

VI CJU

3,653

204, 776

1,314,451

450

407,538

104, 817

1.

Other 

purposes 

than 

roofing 

slate.

12,000

40, 000

257, 267

700, 336

3.

Other 

purposes 

than 

roofing 

slate.-

$15, 000

206

157,824

400

128,194

12,500

*

Total 

value.

$480

24,000

13, 500

250, 000

125, 425

10, 000

176, 000

2, 142, 905

955, 617

127,819

3, 825, 746

Total 

value.

111,250

139,200

37, 884

3,653

204, 982

1,472,275

850

535, 732

117,347

Roofing 

slate, 

squares.

3,500

2,500

60,000

24,000

3,000

20, 000

550, 000

260, 000

40,000

953, 000

Roofing 

slate, 

squares.

900

5,000

24, 690

39,460

375

7,955

411, 550

214,337

33, 955

182

Value.

821,000

10, 625

250,000

114,000

12,000

160,000

1,925,000

754,000

150,000

3, 396, 625

189

Value.

85, 850

22, 500

123,937

150,568

1,050

42 092

1,380,430

455, 860

118,851

2.

Other 

purposes 

than 

roofing 

slate.

$2, 500

50, 000

408,000

260, 000

720, 500

4.

Other 

purposes 

than 

roofing 

slate.

822,901

2

CAA

2 4^)0

 ion 7')Q

202, 307

489, 186

Total 

value.

821,000

10,625

250,000

116,500

12, 000

210, 000

2,333,000

1,014,000

150,000

4,117,125

Total 

value.

85, 850

22, 500

146, 838

1 f^Q ftfift

1,050

44 542

1 ' 620 ' 158

658, 107

T oft 1 s, 1
PEODUCTION. 129

Production of slate in the United States, by years, from 1879 to 1904 Continued.

State or Territory.

Utah...................

Total.............

State or Territory.

Utah...................

Other States ...........

Total.............

Roofing slate,' 

squares.

1,500

23, 774

13,188

200

13, 624

42(1, 687

221, 359

27, 095

729, 927

Roofing 

slate, 

squares.

1,000

38, 367

11,592

400

250

9 197

657, 692

244, 575

38,375

1,001,448

18C

Value.

110,500

118,791

59, 157

700

90, 150

1,437,697

531, 482

92, 357

2, 351, 509

18

Value.

$7, 000

53, 049

1, 000*

775

52 799

2, 034, 958

656, 114

130, 495

3, 097, 452

)5.

Other 

purposes 

than 

roofing 

slate.

$21,363

1, 200

1, 725

210, 054

93,849

19,000

347, 191

37.

Other 

purposes 

than 

roofing 

slate.

$39,855

890

500

330, 341

39, 701

14, 875

427, 162

Total 

value.

$10, 500

10,675

140,154

60, 357

700

91, 875

1,647,751

625, 331

111,357

2, 698, 700

Total 

value.

87, 000

201,117

53, 939

1,500

775

53, 799

2, 365, 299

695,815

145, 370

3, 524, 614

Roofing 

slate, 

squares.

4,597

23, 078

15, 557

200

16,002

431 , 324

100

155, 523

26, 863

673, 304

Roofing 

slate, 

squares.

400

3,450

29, 834

18, 332

100

200

7,160

43,745

916,239

18

Value.

99, 831

70,194

700

78, 612

1,391,539

640

509, 681

92, 163

2, 263, 748

1S9S

Value.

82, 700

13,125

.131,752

80, 786

400

800

2, 097, 735

612 90^

96.

Other 

purposes 

than 

roofing 

slate.

§24,255

1,948

.1 , 200

3,880

334,779

780

99,915

.15,700

482,457

.

Other 

purposes 

than 

roofing 

slate.

1, 454

958

1,950

394,021

119, 782

8,500

594,150

Total 

value.

§20, 388

124,086

72, 142

1,200

700

82,492

1, 720, 318

1,420

009, 596

107,863

2, 746, 205

Total 

value.

82-, 700

13,125

199,237

958

800

2,491,750

150,940

3, 723, 540
130 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Production of slate in the United States, by years, from 1879 to 1904 Continued.

State or Territory.

New Jersey

Utah...................

Total.............

State or Territory.

Utah...................

Total.............

Roofing 

slate, 

squares.

928

2,000

24, 676

20, 196

400

10^912

711,338

50

200

277, 463

52, 550

1,100,513

Roofing 

slate, 

squares.

2,500

800

20, 791

20,153

7,500

15, 786

853,028

330, 191

53,630

1,304,379

189

 Value.

$6, 642

7,500

121, 640

90, 897

1, 600

69,525

2, 202, 742

250

1,100

777, 971

174,950

3,454,817

19

Value.

818,608

3,000

111,295

104,781

30, 000

91, 805

2,591,625

984, 317

178 979

9.

Other 

purposes 

than 

roofing 

slate.

$60, 126

2, 698

800

' 7, 150

334, 280

94, 702

8,160

507, 916

31.

Other 

purposes 

than 

roofing 

slate.

891,030

1,017

1,400

9,155

392, 639

177,874

673, 115

Total 

value.

86, 642

 7,500

381,766

93, 595

800

1,600

' 76,675

2, 537, 022

250

1,100

872, 673

183, 110

3, 962, 733

Total 

value.

$18,608

3,000

202, 325

105, 798

If400

30,000

100, 960

2, 984, 264

1, 162, 191

178,979

Roofing 

slate, 

squares.'

3,500

2,500

21, 771

27, 158

3,600

7,713

788, 571

50

282, 820

56,365

1,194,048

Roofing 

slate, 

squares.

500

4,500

1,000

26, 468

22,569

8,000

21,165

908,206

400,029

42, 731

1,435,168

190

 } 

Value.

826, 500

9,375

103, 949

126, 271

13, 600

58, 360

2, 277, 192

250

795, 474

185,211

3, 596, 182

190

Value.

84, 000

31, 500

4,000

143, 832

117, 155

32,000

116, 628

1,338,817

160, 951

4, 950, 428

).

Other 

purposes 

than 

roofing 

slate.

873, 393

2,402

700

4,395

436, 406

121, 988

5,000

644, 284

2.

Other 

purposes 

than 

roofing 

slate.

862, 726

929

10,090

545, 777

126, 101

745, 623

Total 

value.

826,500

£,375

177, 342

128, 673

700

13, 600

62, 755

2, 713, 598

250

917, 462

190,211

4,240,466

Total 

value.

84,000

31,500

4,000

206,558

118, 084

32,000

' 126, 718

3,547,322

1,464,918

160, 951

5, 696, 051
PRODUCTION. 131

Production of slate in the United States, by years, from 1879 to 1904 Continued.

State or Territory.

Utah ...................

Other States ...........

Total.............

Roofing 

slate, 

squares.

118

10, 000

27, 377

24,475

7,647

15, COO

871,875

391, 36C

29,640

1. 378. 194

IS

Value.

$709

70, 000

157, 911

135,424

33,403

89, 548

3,378,804

1, 363, 923

115,356

5. 345. 078

03.

Other 

purposes 

than 

roofing 

slate.

$4,000

73,319

2,207

22, 450

581, 102

228, 729

911. 807

Total 

value.

$4,709

70,000

231,230

137,631

33, 403

111,998

3, 959, 906

1,592,' 652

115,356

6. 256. 885

Roofing 

slate, 

squares.

1,750

5, 600

1,000

 20, 789

22, 628

10,022

778, 825

115

50

301, 126

31,852

1, 233, 757

190

Value.

$10, 300

39, 200

4,500

120, 838

131, 245

64, 102

2, 922, 259

G07

300

1 ., 245, 730

130, 208

4.669.289

(.

Other 

purposes 

than 

roofing 

slate.

$4,000

60, 330

2, 727

7,441

710, 987

162, 421

947, 906

Total 

value.

$14,300

39, 200

4,500

181,168

133, 972

71, 543

3,633,246

007

300

1,408,151

330,208

5.617.195

The following table shows the number of squares and value of. roofing slate, average 

value per square, and value of milled stock, by years, from 1879 to 1904, inclusive:

Values of roofing slate produced in the United States, 1879-1904-

Year.

1879 ........................................:..

1880. ..........................................

1881...........................................

1882. ..........................................

1883...........................................

1884 ..........................................

1885...........................................

1886...........................................

1887. . .........................................

1888...........................................

1889. . .........................................

1891. ..........................................

1892. ..........................................

1893 ...........................................

1894 ...........................................

1895. ..........................................

1896.................. ........................

1897 ..................:........................

1898...........................................

1899. .........................................

1900...........................................

1901...........................................

1902...........................................

1903 ...........................................

1904 ...........................................

Roofing slate.

Number of 

squares.

367, 857 

457, 267 

454, 070 

501, 000 

506, 200 

481, 004 

536, 960 

536, 790 

. 573, 439 

662, 400 

835, 625 

893, 312 

953, 000 

621, 939 

738, 222 

729, 927 

673, 304 

1,001,448 

916,239 

1,100,513 

1,194,048 

1,304,379 

1,435,168 

1,378,19-1 

1 , 233, 757

Value.

o$l, 231, 221 

"1,529,985 

« 1, 543, 838 

« 1, 753, 500 

"1,898,250 

1, 843, 865 

'1,638,467 

1, 610, 370 

1, 720, 317 

2, 053, 440 

2, 797, 904 

3,125,410 

3, 396, 625 

2, 209, 049 

2, 301, 138 

2, 351, 509 

2,263,748 

3, 097, 452 

3, 129, 390 

3,454,817 

3,596,182 

4,114,410 

4,950,428 

5, 345, 078 

4, 669, 289

Average 

price per 

square.

$3.35 

3.35. 

3.40 

3.50 

3.75 

3.83 

3.05 

3.00 

3.00 

3.10 

3.35 

3.50 

3.56 

3.55 

3.12 

3.22 

3.36 

3.09 

3.42 

3.14 

3.01 

3.15 

3.45 

3.88 

3.78

Value of 

milled 

stock.

$8,000 

10,000

684, 609 

700, 330 

720, 500 

314,124 

489, 186 

347, 191 

482, 457 

427, 162 

594, 150 

507, 916 

644, 284 

673, 115 

745, 623 

911,807 

947, 906

Total 

value.

"$1,231,221 

"1,529,985 

« 1, 543, 838 

« 1, 753, 500 

a 1, 898, 250 

1,851,865 

1,648,467 

1,610,370 

1,720,317 

2, 053, 440 

3,482,513 

3,825,746 

4,117,125 

2, 523, 173 

2, 790, 324 

2, 698, 700 

2, 746, 205 

3, 524, 614 

3, 723, 540 

3, 962, 733 

4, 240, 466 

4, 787, 525 

5, 696, 051 

6,256,885 

5, 617, 195

" Estimated.
132 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

As will be seen from the above tables, the values from 1879 to 1889 do not represent the entire value of slate quarried, as no record of slate used for mantels, school 

slates, pencils, tombstones, etc., was kept. In 1885 there was a large decrease in the 

average value per square compared with 1884 from $3.83 in 1884 to $3.05 in 1885. 

From 1885 to 1892 both the price and production increased in value, but owing to 

the financial troubles of 1893 the output decreased considerably, .or from a total 

value of $4,117,125 in 1892 to $2,523,173 in 1893. Since that time the value of the 

output has steadily increased, being aided largely since 1896 by export trade, until 

in 1903 the value of the output was $6,256,885, the greatest yet obtained. In 1903, 

however, while the beginning of the year showed active operations in the slate 

industry, the latter part of the year showed a very decreased demand, due to strikes 

in the building trades. Several of the chief producing States showed decreased output of roofing slates, with increased price. This is shown in the increase of average 

price from $3.45 per square in 1902 to $3.88 in 1903, and by a decrease of 56,974 

squares from 1,435,168 squares in 1902 to 1,378,194 squares in 1903 while the value 

increased from $4,950,428 in 1902 to $5,345,078 in 1903, a gain of $394,650. In 1904 

the beginning of the year showed the same unsettled state, with less demand and 

slightly lower price, the total value of roofing slate decreasing $675,789 in 1904, or 

from $5,345,078 in 1903 to $4,669,289 in 1904. The number of squares also decreased 

from 1,378,194 squares in 1903 to 1,233,757 squares in 1904, or 144,437 squares. The 

value of. mill stock has steadily increased, the demand being continually increased 

by the increased use for which this class of slate is continually demanded.

IMPORTS AND EXPORTS.

The importation of slate into this country has never assumed very great proportions, 

and the largest quanity was of course imported before the slate quarries of the United 

States were fairly developed and in running order. The following tables, taken from 

the reports of the Bureau of Statistics, Department of Commerce and Labor, show the 

value of roofing slate and other slate, chiefly slate mantels, chimney pieces, etc., by 

fiscal years ending June 31 from 1867 to 1904. From 1867 to 1875 the total value per 

annum amounted to somewhat over $100,000, the value of the roofing slate averaging 

two-thirds of the whole. In 1875 there was a decided drop in the value of roofing 

slate imported, and since that time the imports of roofing slate have amounted to 

practically nothing. In the last twenty years the annual value of all imported slate 

has decreased from $50,739 in 1884 to $9,436 in 1904.

The following ;table shows the value of roofing and other slates imported and 

entered for consumption in the United States, by fiscal years ending June 30, from 

1867 to 1904, inclusive:

Value of roofing and other slate imported and entered for consumption in the United States,

1867 to 1904, inclusive.

Year.

1807 ................

18C8 ................

1869 ................

1870................

1871 ................

1872................

1873......'..........

1874................

Roofing 

slate.

$85, 204

118,776

85,364

107, 521

117,484

107, 192

91, 503

80.519

Other.

837, 510

16, 045

19, 602

19, 879

21,381

25, 925

'26, 643

27.519

Total.

8122,714

134, 821

104, 966

127, 400

138,865

133,117

118, 146

108. 038

Year.

1875.......:........

1876................

1877................

1878................

1879................

1880................

1881................

1882.;..............

Roofing 

slate.

116,342

2,051

4

275

620

72

2

154

Other.

842, 022

44,266

34, 479

39, 935

46, 260

51,165

46, 862

45. 774

Total.

858, 364

46, 317

34, 483

40, 210

46, 880

51,237

46, 864

45. 928
IMPORTS AND EXPORTS. 133

Value of roofing and other slate imported and entered for consumption, etc. Continued.

Year.

1883................

1884 ................

1885................

1886 ................

1887................

1888................

1889 ................

1890 ................

1891................

1892 ................

1893 ................

Roofing 

slate.

82,813

16, 099

5,196

4,366

529

765

481

369

1,148

1,615

1,878

Other.

844, 375

34, 640

56,913

 60, 512

46,188

44, 069

 35,828

34, 602

12,373

4,952

6,671

Total.

847,188

50', 739

62, 109

64,878

46, 717

44,834

36,309

34, 971

-13,521

6,567

8,549

Year.

1894................

1895................

1896................

1897.."..............

1898................

1899................

1900................

1901................

1902.................

1903................

1904................

Roofing 

slate.

82,198

4,418

224

72

58

0

0

3

 o

2,221

45

Other.

86, 342

1,212

' 6,191

4,997

4,866

4,025

6, 998

6,467

4,817

7,182

9,391

Total.

88,540

5,630

6,415

5,069

4,924

4,025

6,998

6,470

4,817

9,403

9,436

Almost from the beginning of the slate-quarrying industry in the United States 

slate has contributed to the export movement of this country. In 1876, 1877, 1878, 

and again in 1880 a large quantity of slate was exported, the greatest part going to 

Great Britain and the Continent, a remarkable feature being that several cargoes 

were shipped directly to Welsh ports, the base of the slate supply for Great Britain. 

From that time until 1896 Great Britain did not take much of the product, which 

found an outlet in South America, the West Indies, and Australia. In fact, British 

Australasia has always been one of the largest foreign consumers of American slate. 

Until 1884 no general record of the export movement was obtainable, but as probably 

90 per cent was shipped from the port of New York, the following table is given, 

showing the shipment of slate from that port from 1876 to 1888. The export of 

manufactured slate is also shown in this table, and is almost entirely the value of 

school slates.

Value of exports of roofing and other slate from the port of New York from 1876 to 1SS8,

inclusive.

Year.

1876 ................

1877................

1878 .................

1879 .................

1880................

1881.......'.........

1882................

Roofing 

slate.

$377, 233

646,272

308,852

166, 220

220, 292

138, 904

153, 318

Other.

887,500

68,437

88,215

74, 251

76, 709

62, 109

68, 150

Total.

8464, 733

714, 709

397, 067

240, 471

297, 001

201,013

221,468

Year.

1883................

1884................

1885................

188G................

1887................

1888................

Roofing 

slate.

. $54,063

90, 262

115,206

79, 064

62, 052

116,119

Other.

840, 674

53, 021

49, 965

40, 804

39, 560

46, 142

Total.

143, 283

165, 171

119,868

101,612

162, 261

Since 1884 the Bureau of Statistics, Department of Commerce and Labor, has shown 

by ports and custom districts the value of the roofing slate exported by fiscal years 

ending June 30, and these figures are given in the following table. No separate 

record has been kept of the manufactured product.
134 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Exports of slate from the United States, showing ports and customs districts from ^vhich and 

to which sent, in the fiscal years 1884-1904.

Port and customs district.

Total ....................

J^rRnCG

Dominion of Canada: 

Nova Scotia, New BrunsCentnil American States:

Costa Rica.................

Azores, Madeira, Cape Verde

West Indies:

Haiti ......................

Chile .................:........

Guianas: 

British.....................

Dutch '.....................

British Africa South .........

Total....................

188-1 .

$476

1,790

CO, 567

s^ft

10,301

167

jnn

5,609

oon

10, 301

250

8,054

15

26

1,817

2, 120

79, 464

1885.

$850

48, 859

302

1,000

51, Oil

850

204

1,000

75

2,631

76

1,225

44 950

51,011

1886.

8296

66

120, 804

35

1,350

666

348

123, fi63

442

42'

2,364

-\

999

2,405

490

m Q79

785

123° 565

1887.

1566

54,833

230

403

58

4,400

557

61, 047

5,310

58

1,130

30C

1, 200

316

72

52, 405

250

1888.

$16,835

80, 852

20

97, 707

20

4,633

152

390

121

1,140

2 210

425

88,444

172

97, 707

1889.

1679

22, 842

84,114

30

2,231

109,896

3,000

950

351

32

2,231

30

980

488

962

99 924

109,896

1890.

83, 012

89, 032

600

114

1,290

94,048

2,000

37

1,890

114

848

2,440

731

650

V7f\

84, 763

94 048
EXPORTS. 135

Exports of slate from the United States, showing ports and customs districts from which and 

to which sent, in the fiscal, years 1S84-1904 Continued.

Port and customs district.

Puso del Nortc TGX

Buffalo Creek, N. Y ...........

Total ....................

Bermuda ......................

Dominion of Canada:

Nova Scotia, New Bruns- 

 wick, etc ................

Central American* States:

Mexico ........................

West Indies: 

British.....................

Haiti ......................

Cuba ......................

Guianas: 

British.....................

Dutch .....................

East Indies British ...........

.N Total ....................

J891.

$252

r

82, 303

1,235

018

84,408

268

onn

1,235

7,013

427

16

55

74, 408

138

84, 408

1892.

?225

276

50, 038

975

57, 514

2

AO/TI

1,195

210

975

60

73

4,512

366

 160

660

47,089

178

57,514

1893.

51,086

30, 306

5

13,428

869

200

94

24

52,012

1,400

1, 046

119

14, 615

32

22

3,145

405

30, 362

866

52,012

1894.

$445

19,684

587

621

13, 690

1,869

 16°

133

37,195

4,800

' 336

445

15, 858

587

621

3,803

330

2,643

712

7,060

37,195

1895.

$443

31,092

192

105

4,748

1,961

65

200

38,806

25

3,000

1,550

406

6, 974

13

488

4,419

10

3, 258

702

340

17, 363

258

38, 806

1896.

$9, 860

350

609

242, 559

2, 300

174

5, 903

1,617

2,874

139

266, 385

12,000

910

197,440

2,312

1,278

10, 533

821

1, 159

90

259

440

417

1,628

34,970

245

1,883

266, 385

1897.

$101,581

1,020

18,170

557, 099

120

94,865

270

2, 378

613

2, 427

1,569

780, 112

. 5,850

2, 087

695, 980

1, 395

730

0, 977

150

1,860

100

165

1,640

810

60, 604

166

1,598

780,112
186 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

Exports of slate from the United Stales, showing ports and customs districts from which and 

to which sent, in the fiscal years 1884-1904 Continued.

Port and customs district.

Boston and Charlestown.Mass.

Memphremagog, Vt ...........

Total .......:............

Belgium ..........'.............

United Kingdom ..............

Denmark ........ .1 ............

Norway and Sweden ......... ;

Dominion of Canada: 

Nova Scotia, New B runsCentral American States:

West Indies: 

British ....................

Haiti ......................

Cuba ......................

Brazil .........................

Ecuador .......................

1.898.

8170, 916

385 

05, 290

98C, 638

136, 916

1,761

22

4,141

3,015

854

137

1,370,075

82, 916

25

1,213,377

8,150

270

157

8,147

22

1,755

1,872

35

2,356

26

673

1899.

899, 083

40, 622 

42, 220

968, 395

205, 779

14

67

7

6,364

937

.129

1,363,617

524

65, 974

520

1, 188, 962

25,323

669'

230

288

7,430

67

330

1,400

16

1900.

8110, 049

577

65, 531 

19, 950

592, 288

150, 254

6,584

2,320

1,441

424

612

487

26

950, 543

17, 921

813, 918

25,437

859

202

798

11, 894

30

2,049

332

285

1901.

$135,571

I,'l44

,93,972 

12, 910

388,590

236, 090

1,436

20

19, 193

2,446

380

644

942

4,915

9

898, 262

5,180

600

727, 088

43,344

1,857

3,222

1,269

27, 587

2,378

20

60

1,067

1902.

8240, 734

449

375

48, 299 

6,650

374, 264

160

243, 701

20

1,343

790

18,014

6,561

246

3,702

945, 352

1,555

1,400

731, 556

47 957

25

443

532

28, 600

1,343

1,268

854

6,609

1903.

8221,933

1,170

30, 273 

350

207,250

120, 240

1,504

1,222

35, 185

5,771

3,714

628, 612

89

477,251

17,376

765

1,183

44,670

1 ' 504

423

479

4,724

52

1904.

8275, 393

30

29, 175

284,750

97,247

378

505

580

1,445

29, 034

4,185

669

3,007

317

726,715

379

1, 440

8

OQQ

515,085

32, 512

378

240

37, 242

1,445

59

115

1,085

Y

4,212

132

761

251
EXPORTS. 137

Exports of slate from the United States, showing ports and customs districts from, which and 

to which sent, in the, fiscal years 1884-1904 Continued.

Port and customs district.

Guianas: 

British.....................

Total ....................

1898.

$000

1,325

807

110

550

44,042

2,218

42

1,370,075

1899.

$2, 275

700

04,434

 77

4,335

3

1,303,617

1900.

$050

1,000

829

71,881

2,458

950, 543

1901.

8424

79, 319'

4,847

898, 262

1902.

$365

195

50

121, 921

679

945, 352

1903.

$1 '399

75, 970

1,222

1,499

628,612

1904.

V

$49

120,882

2, 200

726, 715

It will be seen that the value of roofing slate exported decreased from $377,233 in 

1876 and $646,272 in 1877 to $37,195 in 1894 and $38,806 in 1895. In 1896, however, owing to strikes in the Welsh quarries, the United States gained a foothold 

again in England and in the English colonies. The value of slate exported in 1896 

was $266,385; in 1897, $780,112, and the greatest valuation was reached in 1899, when 

the figures were $1,363,617. The value of the exported slate in 1904 was $726,715, 

showing that, although the strikes are practically over, the demand for American 

slate remains. However, in the last two years French slates of a small size have 

found considerable market in England, at less price than American slates. Great 

Britain and Ireland, British Australasia, the Dominion ol Canada, and Denmark are, 

in the order named, the largest receivers of American slate.
BIBLIOGRAPHY OF SLATE,

The following bibliography is divided into three sections. The first includes only 

works of a general character, the second special scientific papers and memoirs, and 

the last works of an economic character.

WORKS OF A GENERAL CHARACTER.

CAMPBELL, (DONALD). Slate.

Encycl. Britannica, 9th eel., vol. 22; 1887.

CHAMBERLIN (T. C.), and SALISBURY (R. D.). Geology.

Vol. 1. Geologic processes and their results. Slaty cleavage and slate, pp. 420-422, fig. 364; New 

York, 1904.

GEIKIE (ARCHIBALD). Cleavage.

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138
BIBLIOGRAPHY OF SLATE. 139

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140 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

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142 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES.

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MERRIMAN (MANSFIELD). The strength and weathering qualities of roofing slates.

With discussion.

Trans. Am. Soc. Civ. Eng., vol. 27, No. 3, pp. 331-349, Sept., 1892, and No. 6, Dec. 1892, p. 685; also 

vol. 32, p. 529-539, 1894.

MERRIMAN (MANSFIELD). The slate regions of Pennsylvania.

Stone, vol. 17,pp. 77-90, New York, 1898 (has a map showing general boundaries of Lehigh, "hard 

vein," Pen Argyl, and Bangor regions).
BIBLIOGRAPHY OF SLATE. 145

MULLER (H.). Ueber die Dachschiefer in der Niihe von Lossnitz. 

Jahrb. fiir den Berg-und Hiittenmann, 1854, pp. 190-233.

NEVINS (3. NELSON). Roofing-slate quarries of Washington County, N. Y. 

Fifty-third Ann. Kept. N. Y. State Museum, 1899, vol. 1, pp. 135-150, Albany, 1902.

NIVOIT (EDMOND). Notions e'le'mentaires sur Findustrie dans le departement dea 

Ardennes (France), 1869.

PARRISH. History of the slate trade in America.

Am. Jour. Mining. 11,1866-1867. In Johns Hopkins Library.

PATTISON (S. R.). Slate and slate quarries geologically and commercially considered. 

Letters to the Mining Journal (Eng.), 1866.

PERKINS (GEORGE H.). Rept. State Geologist on the mineral industries of Vermont,

]899-1900. 

Slate, pp. 17-30.

PERKINS (GEORGE H.). Rept. State.Geologist on the mineral industries and geology 

of certain areas of Vermont. (IV), 1903-1904. .

Slate, pp. 47-51, Montpelier, 1904.

PONCELET (J. B.). Des gites ardoisiers de 1'Ardenne; Bruxelles, 1848.

REVERDIN (F.) and DE I,A HARPE (C.). The examination of roofing slates.

Chera. Zeitung, vol. 14, pp. G4-65, 94-95,126-127. Abstract in English in Jour. Soc. Chem. Industry, 

vol. 9, p. 394, London, Apr. 30, 1890.

RICHARDSON (C. H.). The terranes of Orange County, Vt.

Slate, pp. 77-81, pis. 9, 9 A, Rept. State Geologist Vt., 3d of new series (1901-1902) 1902.

ROGERS (WILLIAM B.). Reprint of annual report and other papers on the geology 

of the Virginias (posthumous). On the roofing slate of Buckingham Co., 

p. 78, 79. On thfk of Fauquier Co., p. 460, 461. New York, 1884.

RAUVAGE et BUVIGNIER. Statistique mine'ralogique et ge"ologique du de'parternent des 

Ardennes (France), 1842.

SHALER (N. S.). Slates, description of quarries and quarry regions.

Tenth Census U. S., 1880, vol. 10, pt. 2-, report on the building stones of the United States and 

statistics of the quarry industry, pp. 168-174.

SMITH (THOMAS COOPER). Slate quarries in Wales, 1860. 

Two later editions.

SPEKR (F. W.). Quarry methods: The working of slate. 

Tenth Census U. S., 1880, vol. 10, pt. 2, p. 38.

WATRIN (N.). Les ardoisieres des Ardennes. Description et exploitation du schiste 

ardoisier, fabrication des ardoises, lever des plans d'ardoisieres. Charleville, 

1898.

WILLIAMS (J. FRANCIS). Tests of Rutland and Washington County slates.

(Purple and green from Fair Haven, Vt. Red fromGranville, N. Y.) Van Nostrand's Engineering 

Magazine, vol. 31, pp. 101-103, July-Dec., 1884.

[ANONYMOUS] : The slate quarries of North Wales. 

Reprint of letters to Carnarvon Herald, 1873. .
GLOSSARY OF GEOLOGICAL AND SLATE-QUARRY TERMS,

As this bulletin is intended primarily for the use of persons unfamiliar with geological science, a number of the geological terms used in it, whose meaning is not selfevident, are here explained, and for the benefit of geologists who may have occasion 

to use the book some of the terms in use among slate quarrymen are also translated 

into scientific ones. 

AGGREGATE POLARIZATION. The optical property possessed by a thin section of slate cut transverse 

to its cleavage, which causes it, when viewed through a microscope under polarized light, to appear 

like a crystal of raica cut across its mineral cleavage. The aggregate polarization of a slate is due to 

the parallelism of the flakes of mica in the slate, and this, when marked, shows it to be a mica-slate 

and not a clay-slate. A very brilliant aggregate polarization, coupled with regularity in size of particles, indicates a high grade of fissility.

ANTICLINE. The arch part of a folded bed.

ANTICLINORIUM. A mountain mass arch-shaped in its general internal structure.

AUTHIGENOUS. Minerals originating chemically within a rock are calledauthigenous.

BACK JOINT. Joint plane more of less parallel to the strike of the cleavage and frequently vertical;

BED. A continuous mass of material deposited under water at aboutjgnc time.

BLIND JOINT. Obscure bedding plane.

BOTTOM JOINT. Joint or bedding plane horizontal or nearly so.

BRECCIA. Rock made up of angular fragments produced by crushing and then recemented by infiltrating mineral matter.

BRECCIATED. Applied to a rock made up of angular fragments but not transported.

CLASTIC. Constituted of rocks or minerals which are fragments derived from other rocks.

CLEAVE. Slaty cleavage.

CONFORMITY. When two beds overlie in parallelism without any disturbance of the crust having 

affected the first one before the deposition of the second, they are said to be in conformity.

DIAGONAL JOINTS. Joints diagonal to the strike of the cleavage.

DIP. The degree and the direction of the inclination of a bed, cleavage plane, joint, etc.

DIP JOINT. Vertical joints about parallel to the direction of the cleavage dip.

DIKE. Molten material erupted through a. narrow fissure.

END JOINT. Vertical joint about parallel to direction of the cleavage dip. . .

EROSION. The " wear" of a rock surface by natural mechanical or chemical agencies.

FALSE CLEAVAGE. A secondary slip cleavage superinduced on slaty cleavage. (See p. 24 and Pis. 

Vf, XI, A.) 

FAULT. A'fracture resulting in a dislocation of the bedding or cleavage, one part sliding up or 

down, or both changing positions along the fracture.

FLINTS. A term applied alike to quartz veins or beds or quartzite.

FORMATION. A larger group of beds possessing some common general characteristics or fossil forms 

differing from those of the beds above and below.

GRAIN. An obscure vertical cleavage usually more or less parallel to the end or dip joints.

HARDS. Term applied to beds of quartzite or quartzitic slate.

HCL. Hydrochloric or muriatic acid.

HOGBACKS. Shear zones. (Seep. 29and PI. VII.)

ISOCLINAL. Folds with sides nearly parallel are said to be isoclinal.

MATRIX. Term used in microscopic descriptions of slate, etc., to denote the chief substance of the 

slate itself, apart from the various mineral particles or crystals it may inclose.

METAMORPHISM. The process, partly physical, partly chemical, by which a rock is altered in the 

molecular structure of its constituent minerals and frequently in the arrangement of its particles.

OVERLAP. Where, owing to the depression of a coast and the consequent landward shifting of the 

shore line, the later marine sediments cover up the extremities of the older ones, there is said to be 

an overlap..

PITCH. The inclination of the axis of a fold of rock.

146
GLOSSARY OF TERMS. 147

POST. A mass of slate traversed by so many joints as to be useless. , This term is also used to denote 

bands of hard rock.

PSEUDOMORPH. A mineral that has assumed the crystal form of a different mineral as the result of 

the partial or entire alteration or replacement of the original mineral through chemical processes.

QUARTZITE. A sandstone in which the grains are held together by a siliceous (quartz) cement.

RIBBON. A line of bedding or a thin bed appearing on the cleavage surface, sometimes of a different 

color; or a small bed of quartzose'or calcTireous'material either crossing or parallel to the cleavage. 

When such ribbons are separated by beds of slate too.thin to be worked, the ribbons and the small 

beds are together designated as "ribbon."

SCULPING. Fracturing the slate along the grain, i. e., across the cleavage in the direction of the dip.

SERICITE. A ribbon-like or fibrous form of muscovite or potash mica.

SHEAR ZONE. Hogback. (See p. 29 and PI. VII.)

SLANT. Longitudinal joint more or less parallel to cleavage and often slickensided.

SLICKENSIDES. Surface of bed or joint plane along which the rock has slipped, polishing and grooving the surfaces.

SLIP. Occasional joint crossing the cleavage, but of no great continuity. Slips are not infrequently 

fault planes.

SLIP CLEAVAGE. Microscopic folding and fracture,accompanied by slippage; quarrymen's "false 

cleavage." (See p. 24 and Pis. VI, XI, A.)

SPLIT. Slaty cleavage.

STRATUM". A bed. ' 

STRATIFICATION, Bedding, in distinction from cleavage.

STRIKE. Direction at right angles to the inclination of a plane cf bedding, cleavage, jointing, etc.

STRIKE JOINT. Joint parallel to the strike of the cleavage.

SULPHUR. Iron pyrite.

SYNCLINE. The trough part of a fold of rock.

SYNCLINORIUM. A mountain mass, in general internal structure trough shaped.

THICK JOINT. Two or more parallel joints between which the slate has been broken up ot 

decomposed.

TILL. A mixture of clay and pebbles deposited by glaciers.

TOP. The weathered surface of a slate mass or the shattered upper part of it.

UNCONFORMITY. When the lower one of two contiguous deposits affords evidence of having been 

exposed to atmospheric erosion before the deposition of the upper one, there is said to be an unconformity between them.

VEIN. When correctly used, denotes a more or less irregular, sometimes ramifying, inineral mass, . 

often Of quartz, with calcite, etc., within the slate. Such veins are called veins of segregation, to 

show that they consist of matter collected from the adjacent rock by solvent waters. But, as generally used by slate quarrymen, " vein" is the equivalent of bed or stratum.

WAVERS. Annelid trails.

WILD ROCK. Anv rock not fit for commercial slate.

INDEX.

A.

Pago. 

Albion quarry, Pen Argyl, Pa.............. 78-80

Analyses of slates, chemical... 16-18,34-38,41-42,

53,57-58,CO,71,73-74,88-89,105-108, 111

Analyses of slates, microscopic or mineral. 12,

14-16,51,53-55,57,63-67,124

Analysis, microscopic, method of.......... 48-49

Anatase in slate, occurrence of............ 11,114

Andalusite in slate, occurrence of....... 11,14,87

Apatite in slate, occurrence of............. 11

Aqueous sediments, slates derived from.... 5-6

Ardennes, France, slates of, character of... 6 

slates of, cleavage in ................... 27

minerals in......................... 12

waste in .....................s....... 42

Arizona, slate of, description of............ 51

Arkansas, slate deposits of, description of.. 52-53 

slate deposits of, location of............ 51-52

location of, map showing .......... 52,54

slate of, analysis of..................... 53

character and mineral composition

of........................... 53-55,124

mineral particles in ................ 13-14

production of..................... 126-131

Arvonia, Va., slate deposits at............. 19,31,

33,113-115,124-125 

Anld & Conger, slate quarries of........... 77,98

Ausweichungs ciivage. See Slip cleavage.

B.

Bailey, E. H. S., on slate characteristics... 81,124 

Bangor, Pa., slate quarries at.......... 75-77,125

Barite in slate, occurrence of.............. 109

Bate. See Slip cleavage.

Baur, , on cleavage .................... 24

Becker, G. F., on cleavage ................. 10

on cleavage bands...................... 32

Bedding, character of...................... 13-22

erosion of............................... 9

origin of................................ 18

relations of cleavage and ........ 22-23,42-43

figures showing .................... 23,43

views of.... 1........................... 18

Bennett Creek Slate Co., slate quarry of... 69-70 

Benson, Vt., slate deposits at .............. 110

slate from, analyses of .................. Ill

features of..................... 33,110,124

Bibliography of slate....... ............. 138-145

Biotite in slate, occurrence of.............. 11,14

Blissville, Vt., section at................... 97-98

section at, plate showing........... 20

Blue Mountain quarry, Pa., view of....... 82

Bottinger, C., on frost action as related to porosity in slates.................. 48

Bran don Slate Co., slate deposits of........ 104

Bremo, Va., slate deposits at.......... 11,115,124

Brennan & McElwen, slate deposits of..... 53

Brogger, on shear zones.................... 29

Brownville, Me., quarries at........... 65-67,124

quarry ac, plate showing............... 66

slate from, thin section of, plate showing .............................. 64

Buckley, E. R., on Arkansas slates......... 52

C.

Calcite in slates, occurrence of......... 11,76,109

California, geologic map of part of......... 56

igneous slate in, analyses of............ 58

roofing slate in, production of.......... 59

slate deposits of........................ 50-59

slate of, analyses of..................... 57-68

characteristics of............... 57-58,124

market for.......................... 59

production of..................... 126-131

-structure of......................... 56-57

transportation of................... 59

Cambrian time, crustal movements in ..... 34

slate deposits of.................... 70,93-110

bedding in, plateshowing.......... 20

shear zones in, plate showing...... 30

Campbell and Donald, on slate characteristics ........................... 47

Carbon, color of slates due to............... 7

Carbonaceous matter in slate, occurrence of. 11 

Carbonate in slate, occurrence of.......... 14

Castleton, Vt., slate deposits at............. 21

Cedar Point, Vt., slate deposits at....... 15-16,21

Chapman, Pa., slate deposits at........ 80-81,125

Chapman Slate Co., slate quarries of.. 80,122,124

Chemical analyses. Sec Analyses.

Chemistry of slates, account of............. 34-42

Chili Bar Slate Co., slate quarry of......... 58

Chlorite in slates, occurrence of..... 11,12,14,103

Classification of slates ...................... 5-6

Clay in slates,occurrence of................ 46

Clay slates,origin of........................ 5,8

See also Slates. 

Cleavage, cause of.......................\... 7-8

character of................ 5,8-9,18-19,22-25

curvature of............................ 24

relation of axes of folds to.............. 23

relation of bedding planes to..... 22-23,42-43

figures showing..................... 23,43

surface of, character of................. 46

149
150 INDEX.

47 

33

28,44 

39-42

46

12,46

47

Page. 

Cleavage bands, occurrence of.............. 31-32

plate showing .......................... 64

slipping of, figures showing ............ 32

Cocke, C. C., slate deposits of................ 115

Color of slates, causes of.................. 7,39-42

changes in.............................. 46

shades of ............................... 5

Compass, use of, in prospecting............. 44

Coons, A. T., on slate production ......... 125-132

Corrodibility, tests of....................... . 48

Credner, G. R., on rutile in slates........... 13

Crosby, VV.O.,on Brownville slates......... 66

D.

Danville, slate deposits of..................

Daubree, A., experiments of................

De la Beche, H. T., on cleavage of slates....

De la Harpe and Reverdin, on porosity of 

slate............................

Density of slate, testing of................:.

Dikes, occurrence of, in slate ..............

relation of joints and shear zones to... 

Discoloration, cause of............... 15-17,

effect of, on values.....................

occurrence of......................... 6,

Donald and Campbell, on slate characteristics ...........................

E. 

East Bangor Consolidated Slate Co., slate

quarries of...................... 77-78

East Brownville Slate Co., §late of.........' 66

East Poultney, Vt., slate deposits near..... 31

Eckel, E. C., on Arkansas slates............ 51-53

on Californiaslates....................... 56-59

on Georgia slates............. ......... 59-60

on Tennessee slates .................... 88-89

on Utah slates.......................... 89

Elasticity, character of...................... 47

Empire Red Slate Co., slates of............. 16

England, slates of, analyses of.......... 18,34-35

 slatesof, cleavage in................... 23

Epidote in slate, occurrence of............. 11

Eureka quarry, Pa., structure in, figure

showing........................ 83

view in.................................. 82

Eureka Slate Co., quarry and slate of...... 56-58

Exports of slate, table showing .......... 133-137

Fair Haven, Vt., shear zone at, plate showing ............................. 30

slate quarries near..................... 101

slip cleavage at, plate showing ........ 26

Fair Haven Red Slate Co., slatesof........ 19

slates of, bedding of, plate showing.... 20

False cleavage. See Slip cleavage. 

Faults, occurrence of........................ 29

Feldspar in slate, occurrence of............ 11,14

Fissility, definition of...................... 25

occurrence of........................... 5-6

See also Slip cleavage. 

Flints, definition of ........................ 32

nature and cause of.................... 43-44

occurrence of........................... 32-33

See also Veins, quartz. 

Folds, axes of, relations of cleavage and... 23

effects of................................ 7-8

Page.

Fontaine quarry. See Pitts, A. L. 

Forks, Me., slate deposits at................ 67

Fracture, character of...................... 45-46

Fracture cleavage. See Slip cleavage.

France, slates of............................ 6

slates of, analyses of....................' 34-35

Fresenius, R., on slate ..................... 48

Frost, effect of, on slate .................... 24

G.

Garnet in slate, occurrence of........... 11

Geologic map, of Martinsburg, W. Va., district............................ . 120

of Placerville, Cal., district ............ 56

of western Vermont slate district..... 88,100

use of................................... 44-45

Geology of slates ........................... 34

Georgia, slate of, analyses of............... 60

slate-of, geology 'of..................... 59

production of .................... 126-131

uses of.............................. 60

Germany, slates of, analyses of ............ 34-35

Glossary of terms used ................... 146-147

Gorham, Vt., cleavage bands in, figure 

showing.................................. 32

Gosselet, J., diagram'by.................... 23

on folding.............................. 22

on slaty cleavage....................... 22-23

figure explaining................... 23

Grain, character of................... 25,27,45-46

Granite, slate in proximity to.............. 34

Granville, N. Y., slate deposits at... 17,29,93,124 

Graphite in slate, occurrence of............ 11

Graptolites, occurrence of.................. 95,99

Guilford, Vt., slate deposits at ............. 89

Gulf Slate Co., slate deposits of............. 52

Gypsum in slate, occurrence of ............ 11,76

H.

Hampton, N. Y., slate deposits at......... 93,124

Hampton-Janesville, Vt., section through .. 95 

Hardness of slate; tests for.................. 48

Barker, A.,011 cleavage..................... 24

Harris, Abraham,slate prospect of ......... 70

Hathenbuck,F. W.C.,slate deposits of ..... 89

Hayes,C. W.,on Georgia slates ............. 59

Heimbach, Pa., slate qimrry at ............ 78,124

Hematite in slates,occurrence of........ 11,12,14

Hill, Lawrence, slate prospects of........... 67.

Hillebrand, W. F., analyses of slates by .... 15-17,

36-38,73-74,84,105-108,110 

on chemical composition of roofing

slates........................... 39-42

Hincum Pond.Vt., plicated joints in slate .

at............................... 28-29.

Hitchcock, C, H., on Maine slates........... CO

Hogbacks, description of .................... 29-30

See also Shear zones. 

Hoosick.N. Y., slate .quarry at.............. 74

Hornblende in slate, occurrence of......... 11

Hot Springs, Ark., slate deposits near ...... 52

Hudson grit, slate beds in ............ 71,75,95,98

Hughes, William, quarry of (Slatington, 

Pa.), structure in, figure showing ............................. 83

Hutchins, W. M.,on slates .................. 13,49
INDEX. 151

I.

 Page. 

Igneous slates, analyses of.................. 18

origin of................................ 5

Ijamsville, Md., slate deposits at............ 33,69

Imports of slate, table of.................. 132-133

Iron, color of slates due to.................. 7

Jackson (Stephen) & Co., slate quarry of... 79

Jauiesville, N. Y., slate quarries at......... 32-33

Jamesville-Hampton, Vt., section through. 95

Jannetaz, E., experiments by............... 8,27

Joints, description of ....................... 27-29

occurrence of........................... 27-29

relations of dikes, shear zones, and..... 28,44

Joints, curved, occurrence of................ 28

Joints, plicated, occurrence of.............. 28

Jones (Foulk) & Sons, quarry of, structure

in............................... 86

slate deposits of...................:.... 28

Kansas City Slate Quarrying Co., slate quarry of............................ 52

Kaolin in slates, occurrence of............. 12

Keith, Arthur, on Tennessee slates......... 88

Keyes, C. R., section by .................... 67-68

King, Moses, orbicular granite in quarry of. 90 

Knots in slate, occurrence of............ 11-12,14

L.

Lake Bomoseen, Vt., section at............. 97

Lake Bomoseen Slate Co., slate of....... 15-16,97

Lebanon Springs, N. Y., slate quarry near.. . 74 

Lehigh Gap, Pa., slates of.................. 10

Leith, C. K., on cleavage..;................ 9-10

Lenses in slate, occurrence of........... 11-12,14

Lime in slate, occurrence of ................. 46

'Limonite in slates, occurrence of........... 11,12

Lindgren and Turner, on California slates.. 56 

Little Missouri Creek, slate deposits of..... 52

Lloyd, E. E., slate quarry of, shear zone in,

plate showing ................... 30

Lobb, Win., & Sons, slate quarry of......... 79

Loretz, on slip cleavage .................... 24

Lost Hannah bed, Arkansas, slate from... 54,124 

Lowell Slate Co., slate quarries of.......... 65

McCulloch, Gilbert, analyses by............ 88

Magnesia, color of slate due to............. 67

Magnetite in slate, occurrence of.... 14,27,11,46

Maine, slate deposits of, description of.... 61-67

slate deposits of, location of............ 60

location of, map showing .......... 61

slate of, characteristics of............ 123-125

grain in ............................. 34

luster of................'........:... 10

prices of............................ 125

production of ..................... 126-131

structure of......................... 27

Mammoth Red Beds, Arkansas, slate from. 54,124 

Map of Arkansas slate localities............ 52,54

of Maine slate region................... 61

of Maryland slate region............... 68

of Martinsburg, \V. Va., slate region ... 120

Page.

Map of New York and Vermont slate belt.. 88 

of Slatington, Pa., quarries............. 80

of United States, showing location of

 . slate deposits................... 5

of Virginia slate region .:.............. 112

Map, geologic. See Geologic map.

Marcasite, occurrence of................... 46

Martinsburg, W. Va., clay-slate deposits

near ..................... 6,10,119,124

clay-slate deposits of, location of, map

showing.. ..................... 120

Martinsburg shale, slates of................ 119

Maryland, slate deposits of, description

of-............................:.. 67-70

slate deposits of, location of............ 67-69

location .of, map showing .......... 68

 slate of, characteristics of............:. 6,124

geology of.......................... 6,34'

. production of .................... 126-131

veins m ........................-'-^. 33

weathering of....................... 34

See also Peach.Bottom region. 

Massachusetts, slate deposits of, production

of....................:........ 126-131

Mathews, E. B., on talc- in Maryland

slate............'................ 69

Mathews Consolidated Slate Co., -slate

quarry of................... 74,95,123

Maw, George, on ribbons in Welsh slate.... 17

Meadow Slate Co., slate quarry of......... 101

Mena, Ark., slate deposits at........... 53-54,124

Merrill, G. P., on slate weathering......... 37

on waste in slate manufacture ......... 42

Merrill-Brownville Slate Co., quarry-of,

view of......................... 66

slate quarries of.................... 65-67,123

Merriman. Mansfield, on slate characteristics .............. 47-49, 73-74,77,79,88

recent tests of slates by............... 122-124;

Metamorphism, effects of .................. 7.

Mica, formation of .......................... 8

Mica slates, classification of................ 6

origin of................................ 5,7-8

See also Slates. 

Michigan, slate production of............ 126-131

Middle Granville, Vt., section at........... '95-97

Mil Istock slate, description of.........'... 108-109'

Minerals in slate,'Character and occurrence 

of;........................... 5-6,11-15

classification of.........................' 14

Minerals in green and purple Vermont slate,

 occurrence of................. 109-110

Mineral analyses/ See Analyses. 

Minnesota,' slate deposits of, bibliography

of.::..........:................. 70

slate deposits of-, locatiorrof......:..... 70

slate of; production of...::........... 126-131

Monson, -Me., slate deposits at....... 10,'61-64,125

Monson Consolidated-Slate Co;, slatequarry

Of:...:::;..;.;:............. 61, 64,123

Monson Maine Sla'e Co., slate quarry of... 61-63 

Monson Pond,-Me.,-quarryat,-section of:.. 62 

f quarry at, slates of................. 61-63,124

Montpelier, Vt., slate quarry at............ 90,93

Mount Ephraim, Md., slate prospect near:. 70 

Moyer (J. S.) & Co., quarry of, machinery

 of,-view of.....................:. 78
152 INDEX.

Page. 

Moyer (J. S.) & Co., quarry of, views of.... 78

slate quarries of.................... 76,78,124

Miiller, F. E., on concretions............... 33

Muscovite in slates, generation of.......... 7,13

occurrence of........................... 12,14

N.

National Red Slate Co., slate of......... 16,74,95

National Slate Co., slate quarry of.......... 53

New Jersey, slate deposits of............... 70

slate of, production of................ 120-131

New York, slate deposits of, location of.... 70-71

slate deposits of, location of, map showing............!................. 88

sections across, plate showing ...... 90

slate of, analyses of......... 36,41-42,71,73,74

characteristics of.............. 71,124-125

composition of................ 39-42,72-74

descriptions of...................... 72-74

faults in ............................ 29

geology of.......................... 7,70

prices of ............................ 125

production of..................... 126-131

structure of................... 10,19,22,44

veins in ............................ 32

slate quarries in, diagrams of .......... 92,96

structure at ........................ 72

Nodules, occurrence and character of...... 33

North Bangor, Pa., slate quarry at......... 77

North Bangor Slate Co., slate quarry of .... 77

North Blanchard, Me., slate quarries a ... 65,124 

Northfield, Vt., slate quarries at........ 90-93,124

structure at, figure showing ............ 91

Norton,L. M.,analysis by .................. 63

0.

Old Bangor quarry. See Moyer, J. S., & Co. 

Olenellus, occurence of..................... 94,98

Olive grits, occurrence of.................. 98,100

Ordovician time, crustal movements in.... 34

slate deposits of............ 71,95-110,119

Ottrelite in slate, occurrence of............ ll

P.

Paleozoic shales, analyses of............... 37

Pawlet, Vt., section at...................... 98-99

section at, plate showing............... 20

shear zone at, plate showing........... 30

Peach Bottom region, Md.-Pa., slate deposits of, description of............ 85-88

slate deposits of, location of............ 85

slate of, analyses of.................... 34-35

characteristics of........... -86-88,124-125

composition of ..................... 12-18

geology of........................ 6,85-S6

prices of............................ 125

section of, figure showing.......... 87

waste of............................ 42

weathering of...................... 37

Pen Argyl, Pa., slate quarries at.... 78-80,124-125

weathering at.......................... 80

Pennsylvania, geology of slate regions of.. 75-76^ 

Lehigh County, slate deposits of....... 81-85

slate deposits of, analyses of........ 84

Northampton County, slate deposits of. 76-81 

slate deposits of, location of........ 75,85

Page.

Pennsylvania, slate of, analyses of. 34-36,38,84,88 

slate of, characteristics of.............. 75-76,

. 84,86-88,122,124-125 

cleavage in......................... 24

composition of .................. 12-13,49

geology of............ 0,10,34,75,81,85-86

joints in ............................ 28'

prices of............................ 125

production of..................... 126-131

structure in...................... 19,75,76

figures showing ................ 82,83

weathering of................ 37,39,80-81

slate quarries in, map of............... '80

views of....'.. ..................... 82,84

See also Peach Bottom region. 

Penrhyn, Wales, slates of .................. 6

Petrography of slates, history of........... 9-11.

minerals in, character of............... 11-13

. sizesof ............................. 13-14

Pfaff, E., on rutile in slates................. 13

Phillips, John, on cleavage................ 23-24

Phillips and Sharpe, on slaty cleavage..... 23

Phoenix, Ariz., slates from near............ 51

Phyllite slates. See Mica slates.

Pigeon slate, character of.................. 88

Pitts, A. L., slate quarry of-...... 113-114,123-124

Placerville, Cal., slate deposits near....... 56,88

slate district of, geologic map of....... 56

Polarity of slate ...........................'. 27

Porosity of slate ............................ 47-48

Portland cement, slate waste used in manufacture of..................... 60

Potash-mica. See Muscovite. 

Poultney, Vt., shear zone at, plate showing. 31? 

slate deposits at, bedding of, plate

showing...................... 1. 20

Prindle, L. M., on Vermont slates.......... 97

Production of slates, statistics of......... 125-132

Prospecting, suggestions for ............... 44-45

Provident Slate Co., slate quarry of, structure in, figure showing......... 82

Provo, Utah, slate deposits at.............. 89

Purple slate, occurrence and character of. 107-108 

Pyrite in slate, occurrence of.......... 11,109-110

Pyrophyllite in slate, occurrence of........ 14

Pyrrhotite in slate, occurrence of.......... 11

Q.

Quarries, slate, technical description of ... 50 

Quarrying, difficulties of................... 42

Quartz in slates, occurrence of...... 11,12,14,109

plate showing quartz veins ............ 22

R.

Raceville, N. Y., slate of................... 16

Renard, A. F., on petrography of slates.. 11,12,27 

Reusch, H., on shear zones................. 29

Reverdin and de la Harpe, on slate porosity. 48 

Rhodochrosite, bed of, analysis of.......... 71

Ribbons, occurrence of..................... 19, 42

removal of, view of..................... 84

Richardson, C. H., on Vermont slates...... 90

Rising & Nelson Slate Co., slate quarry of.. 123 

Rockmartformation,slate deposits in...... 59

Rogers, H. D., on structure of slates........ 23,28

Rogers, \V. B., on Virginia slates............ Ill
INDEX. 153

Page. 

Roofing slates, analyses of.................. 34-36

 composition of.......................... 39-42

production of........................... 131

Rosenbusch, H., on slates.................. 12-13

on shear zones.......................... 29

Roth, Justus, on slates...................... 13

Rupert, Vt., shear zone at, plate showing.. 30

slate deposits at ........................ 31

Rutile in slates, occurrence of.............. 11-13

S.

San Francisco Slate Co., slate quarry of.... "58

Scales in slates., occurrence of.............. 11

Schists, definition of........................ 5

relations of slates and .................. 10-11

Sea-green roofing slate, occurrence and

character of.................. 104-106

Sedgwick, Adam, on joints................. 28

on slip cleavage ........................ 24

Scricite in slate, occurrence of............. H

Shale, transition of, to mica slate........... 10,81

Shales, Paleozoic, analyses of.............. 37

Sharpe, Daniel, on cleavage..........t..'.... 25,27

Sharpe and Phillips, on slaty cleavage..... 23

Shear zones, occurrence of................. 29,31

plate showing.......................... 30

relations of joints, dikes, and .......... 44

Shenandoah Slate Co., slate quarry of...... 121

Slate Hill, Pa., slate quarry at.............. 28

Slate needles, occurrence of................ 13-14

Slate-pencil slate, description of........... 109

Slates, characteristics of................... 124-125

chemistry of............................ 34-42

classification of........................ 6,125

cleavability of.......................... 45

See also Cleavage, 

compression of....................... 34-37,39

corrosion of............................. 48

definition of........................... 5-6,10

deposits of, location of, map showing.. 5 

formation of............................ 7

origin, theory of........................ 5-9-

unsettled questions concerning.. 10-11,13 

prices of.............. ................. 125

strength of, tests showing .............. 47

thin section of, plate showing.......... 64

toughness of............................ 47

uses of.................................. 5

See also Clay slates; Mica slates; Ash 

slates; Dike slates; Igneous slates; 

Roofing slates; Slate-pencil slates; 

Mill-stock slates. 

Slatington, Ark., slate deposits of.......... 52

Slatington,'Cal., slate deposits at.... ...... 56

Slatington, Pa., quarries near, structure in. 82-83 

ribbon at, removal of, view of.......... 84

slate deposits at................ 75,81,124-125

map of.............................. 80

structure in, figures shovving....... 82-83

slate of, analyses of..................... 84

Slaty cleavage, definition of................ 9

plate showing .......................... 26

production of........................ . 8

Slip cleavage, description of................ 24-25

plate'showing ........................... 26

synonyms of............................ 25

Page. 

Snowden, Va.,slate deposits at....... 115-117,124-

slate quarries at, location of, map showing ............................. 116

Sonorousness, lack of....................... 70

tests for. ................................. 45

Sorby, H. C.,on petrography of slates.... 10-11,13

on slaty cleavage....................... 22-23

Southern States Portland Cement Co., slate

and cement of.................. 60

Southwestern SlateManufacturingCo., slate

quarries of ..................... 52,55

slate deposits of....................... 55,124

Specific gravity, determination of.......... 47

Spotted slates, analyses of .................. 15-17

occurrence of........................... 15-18

specific gravity of...................... 47

Staurolitein slate.......................... 11

Steiger, George, analyses by ............... 73-121

Stokes,H.N.,analyses by................... 37

Strain-slip cleavage. See Slip cleavage. 

Stratton & Kelley, slate deposits of......... 53

Structure, account of ...................... 5,18-34

views of....................... 18,20,22,26,30

Sphene in slate, occurrence of......;....... 11

T.

Talc in slate,occurrence of................. 11,69

Tennessee, slate deposits of, description of.. 88 

slate of, analysis of..................... 88

production of..................... 126-131

Terms used, glossary of................... 146-147

Testing,manner of :........................ 45-49

Texture, character of....................... 6-8

Thetford, Vt., slate deposits at.............. 89

Thurach, H., on slate characteristics....... 13

Thurston, Md., slate quarries near ..... 69-70,124

Tilden, W. C., analysis by .................. 121

Toughness, determination'of..... °......... 47

Tourmaline in slates, occurrence of........ 11

Turner, H. Wv on shear zones.............. 29

Tyndall, John, experiment by ............. 8

U.

Ulrich, E. 0., on age of Arvonia slate formation ............................ 113

Unfading and seagreen slates, boundary of. 101

Unfading green roofing slate, description

of............................. 106-107

Utah, slate of, analysis of................... 89

production of..................... 126-131

V.

Van Hise, C. R., on cleavage bands........ 32

on slate................................. 9

Variegated slate, description of.......... 107,108

Veins, quartz, occurrence of............. 32-33,43

plate showing .......................... 22

Vermont, Northfield Township, slate quarries of........................... 90-93

slate deposits of, location of............ 89-90

slate deposits of, location of, map showing ............................. 88

sections across, descriptions of...... 94-99

plate showing.................. 90

structure of....................... 101-103
154 INDEX.

Page. 

Vermont, slate of, analyses of...... 36,105-108, 111

slate of, bedding of, plate showing..... 20

characteristics of.........."....... 123-125

cleavage bands in, figure showing.. 32

 composition of.. 3 3,18,39-44,90-93,104-110

distribution of.................... 100-101

geology of.................. 7,34,90,93-95

joints in..................i......... 28

minerals associated with......... 109-110

nodules in.......................... 33

prices of............................ 125

production of..................... 126-131

spots in............................. 15-17

structure in........... 19,21-22,29,100-104

plate showing.................. 98

slate quarries in, diagrams of/......... 92,96

western part of, slate deposits of ...... 93-95,

104-110,124 

slate deposits of, section of......... 94

Vermont Black Slate Co., slate quarry of. 90,124 

slate quarry of, structure in, -figure

showing........................ 91

Vermont Unfading Green Slate Go., slate

quarry of....................... 123

Virginia, slate deposits of, location of...... Ill

slate deposits of, location of, map showing ....................... 112,116,117

slate of, characteristics of.....'...:... 123-125

composition of ................ 11,113-119

geology of..................... 34,111-113

prices of............................ 125

production of..................... 126-131

structure of......................... 19

_ waste of............................ 37

Virginia Slate Mining Co., slate property of 115-117

W. 

Wales, slates of............................. 6,23

slates of, analyses of..................... 34-35

Page,

Walcott, Charles D., graptolites found by.. 95,99 

Warrenton, Va., slate prospects near..... 117-119

Waterford, Vt.-, slate deposits at ............ 89

Watrin,-N., on petrography of slate........ 27

on slate characteristics................. 47

on waste................................ 42

Weathering, chemical changes in.......... 37-39'

Wells, Vt., section at........................ 98

shear zone at, plate showing........... 30

West Castleton, Vt., slate quarries at....... 19,21

slate quarries at, bedding of, plate showing ............................. 20

West Monson, Me., slate quarry at........ 64,124

West Pawlet, Vt., section at............ ... 99

slate structure at.................... 19,44,98

slip cleavage at, plate showing......... 26

West Virginia, slate deposits of, location

of................................ 119

slate deposits of, location of, map showing .............................. 120

slate of, characteristics of ............... 124

composition of................. 10,121-122

geology of.......................... 6,119

weathering of ..................... 37,122

Wilhite slate, slate of....................... 88

Wilkinson, , on slate characteristics...... 47

Williams, George H., on Virginia slates.... 114 . Williams, J. F., on slate characteristics.. 47,73,124 

Williams and Edwards, slate quarry of,

shear zone in ................... 30

Williams Brothers, slate quarry of....... 115,116

Williams (John R.) Slate Co., slate quarry

of.......................... 113,123-124

Zircon in slate, occurrence of ............... 11

Zirkel, F., on slipcleavage................. 24