DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

V 3

BULLETIN 586

SLATE IN THE UNITED STATES

BY

T. NELSON DALE AND OTHERS
I >
CONTENTS.

Page.

Introduction ............................................................ 7

Fart I. Origin, composition, texture, and structure of slate.................. 9

Definition and classification of slate.................................... 9

Origin of slate....................................................... 11

Minerals of slate and their arrangement. ............................... 14

History of research............................................... 14

Spotted slates................................................... . 23

General character of spots..................................... 23

Microscopic analyses of spots.................................. 24

Chemical analyses of spots in red slate.......................... 25

Discussion of the spots........................................ 25

Igneous slates................................................... 27

Structure of slate..................................................... 27

Bedding........................................................ 27

Slaty cleavage.................................................... 33

Slip cleavage ("false cleavage ").................................. 37

Grain..................................'.......................... 40

Joints............................................................ 41

Faults.......................................................... 43

Shear zones..................................................... 44

Cleavage bands.................................................. 45

Veins............................................................ 48

Nodules......................................................... 49

Dikes........................................................... 49

Geologic relations of slate.............................................. 49

Chemistry of slate.................................................... 50

Chemical composition............................................. 50

Chemical changes in weathering................................... 53

Chemical composition of the roofing slates of eastern New York and

western Vermont, by W. F. Hillebrand........................... 55

Part II. Slate deposits of the United States................................ 60

Arizona .............................................................. 60

Arkansas............................................................ 61

Topography, geology, and character of deposits..................... 61

Chemical composition of the slatea of Arkansas....................'.. 62

Microscopic analyses of slates from Arkansas....................... 63

Physical and electric tests......................................... 65

California............................................................ 65

Eldorado County, by E. C. Eckel. ................................ 65

Merced County................................................... 70

Colorado............................................................. 70

Georgia.............................................................. 71

Bartow County.................................................. 71

Polk County....................................................... 72

230916
4 CONTENTS.

Part II, Slate deposits of the United States Continued. page.

Maine................................................................. 74

Maryland.............................................................. 83

Minnesota, by E. C. Eckel............................................. 86

Nevada................................................................ 87

New Jersey........................................................... 87

New York............................................................. 88

Geologic relations................................................. 88

The slates......................................................... 91

Pennsylvania......................................................... 93

General features................................................... 93

Northampton*County.............'................................. . 96

Lehigh County..................................................... 104

Carbon County..................................................... 108

Dauphin County.................................................. 109

York and Lancaster counties....................................... 110

Tennessee............................................................ 116

Blount County.. ................................................... 115

Monroe County..................................................... 115

Sullivan County. .......................................... ^....... 118

Washington County................................................ 119

Utah, by E. C. Eckel.................................................. 119

Vermont. ............................................................. 120

Geographic relations................................................ 120

Northfield district ................................................. 120

Western Vermont.................................................. 123

Geologic relations ............................................ 123

Geologic sections..................................... '. ......... 125

General structural features...................................... 133

Geologic and geographic distribution............................. 133

Quarry structure............................................... 135

Minor structural features........................................ 139

The Western Vermont slates.................................... 139

Virginia............................................................... 146

West Virginia.......................................................... 164

Part III. Economic geology of slate......................................... 167

Difficulties in slate quarrying.......................................... 167

Bedding and cleavage how distinguished.............................. 167

"Flints" their nature and cause....................................... 168

Relations of joints, dikes, and shear zones................................ 169

Use of geologic map and compass in prospecting......................... 169

Technical description of a slate quarry................................... 171

Methods of testing slate................................................ 171

Tests of certain Maine, New York, Pennsylvania, Vermont, and Virginia

slates, by Mansfield Merriman........................................ 181

Tests of slates from Arkansas........................................... 184

Comparative characteristics of slates.................................... 188

Slate machinery..............................:......................... 189

Uses of slate.......................................................... 191

Slate waste........................................................... 192

Statistics of production, imports, .and exports, by A. T. Coons............. 193

Prodxiction--.._...................................................' 193

Imports-..-....................................-.:.............--- 203

Exports........................................................... 203
CONTENTS. 5

Page!

Appendix................................................................. 205

Bibliography of slate.................................................. 205

Glossary of geologic and slate-quarry terms............................... 213

Index.................................................................... 217

ILLUSTRATIONS.

Page. 

PLATE I. Map of the United States showing location of slate deposits......... 7

IT. A, lied slate from quarries south of Hatch Hill, Whitehall, N. Y.;

B, Sandstone resting on slate at Eddy Hill, Fair Haven, Vt....... 28

III. Syncline in banded slate at West Castleton, Vt...................... 29

IV. Plicated and faulted beds in slate................................. 30

V. A, Quartz veins in slate at Jamesville, N. Y.; B, Bedding and

cleavage in Phoenix quarry, Windgap, Pa....................... 32

VI. A, Banded slate from Phoenix quarry, Windgap, Pa.; B, Slatesplitting machine in operation...................(............... 33

VII. Slip cleavage after slaty cleavage.................................. 38

VIII. Shear zones...................................................... 46

I.. Map showing slate area of Arkansas................................. 62

X. Geologic map of Placerville slate district, California................ 66

XI. A, Thin section of slate from Merrill quarry, Brownville, Maine; B,

Cleavage banding in shales partly altered to schist, Rupert, Vt.... 80

XII. Merrill slate quarry, Brownville, Maine............................ 81

XIII. Old Bangor quarry (north-northeast end), Bangor, Pa............... 96

XIV. Bangor Union quarry, Bangor, Pa.................................. 97

XV. Map showing location of quarry sites in Northampton County, Pa... 100

XVI. Map of the Slatington slate quarries, Lehigh County, Pa............ 104

XVII. Blue Mountain slate quarry, near Slatington, Pa................... 104

XVIII. Northern part of Eureka slate quarry, near Slatington, Pa........... 105

XIX. A, Cedar Point quarry, on Lake Bomoseen, Castleton, Vt.; B, Clay

quarry, Pawlet, Vt............................................. 124

XX. Map .of quarries in New York and Vermont slate belt, southern part. 124'

XXI. Map of quarries in New York and Vermont slate belt, northern part. 124

XXII. Sections across the New York and Vermont slate belt............... 124

XXIII. Quarry diagrams, New York and Vermont slate belt................ 126

XXIV. Quarry diagrams, New York and Vermont slate belt................ 127

XXV. Slate syncline, West Pawlet, Vt.................................... 132

XXVI. A, Lower quarry of Portland-Monson Slate Co., Monson, Maine; B,

Quarry of Standard Slate Corporation, Esmont, Va............... 156

FIGURE 1. Sketches of the cleavage surfaces of roofing slates from Rockmart,Ga. 32

2. Diagram showing relations of cleavage of slate to dip of inclosing hard

beds......................................................... 34

3. Cleavage banding showing slippage of bands, Poultney, Vt......... 47

4. Section through Fork Mountain, Ark............................ 62

5. Map of slate region in Maine..................................... 75

6. Map of parts of Frederick and Montgomery counties, Md., showing

slate prospects and abandoned quarries......................... 84

7. Sketch of anticline in Provident Slate Co.'s quarry, near Slatington, Pa...................................................... 106

8. Sketch of anticline in old quarry north of Slatington, Pa......... 106
CONTENTS.

9. Sketch of syncline at William Hughes's quarry, near Slatington, Pa.. 107

10. Sketch of folds at Eureka quarry, near Slatington, Pa.............. 107

11. Transverse section of Peach Bottom slate.......................... 112

12. Map of Virginia, showing distribution of slate areas in the crystalline

region........................................................ 148

13. Map of slate region in Buckingham and Fluvanna counties, Va..... 149

14. Map of slate region in Albemarle County, Va..................... 156

15. Map showing location of slate quarries and prospects in Amherst

County, Va................................................... 160

16. Map of slate prospects in Fauquier County, Va................... 162 -

17. Diagram illustrating the effect of divergence in strike of slate bedding and cleavage............................................ 168

18. Diagram showing electric connections made in testing slate....... 174

INSERT.

, Page. 

Comparative characteristics of various slates................................ 188
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE I

Topelia City r X,' ^X, 

S v» \Scflcrson pHjV'st Louis

MAP OF THE UNITED STATES 

SHOWING LOCATION OF SLATE DEPOSITS

BY T. NELSON DALE

 Productive districts

Prospective deposits

Scale

100 0 100 200 300 400 600 600 Milco
SLATE IN THE UNITED STATES.

By T. NELSON DALE and others.

INTRODUCTION.

By T. NELSON DALE.

Bulletin 275 of the United States Geological Survey, "Slate deposits 

and slate industry of the United States," issued in 1906, included, 

besides the results of the writer's field and microscopic work on the 

slates of New England, New York, Pennsylvania, Maryland, and 

Virginia, a series of elaborate analyses of slate and a note by W. F. 

Hillebrand, the results of E. C. Eckel's visits to the slate districts of 

California, Utah, and Arkansas, and a series of physical tests of slates 

made for the Survey by Mansfield Merriman. 4?

Since the publication of that bulletin, the edition of which was 

exhausted several years ago, slates of more or less economic value 

have been found in Merced County, Cal., in Colorado, in Bartow 

County, Ga., in Nevada, in Lancaster and Dauphin counties, Pa., 

and in Monroe County, Tenn., and all of them have been examined 

microscopically by the writer. He has also recently visited some of 

the prospects and quarries in Georgia and Tennessee, and the quarries 

of Penlan and Esmont, Va., of Sussex County, N. J., and of Aquashicola 

and Windgap, Pa., and has revisited some of the Maine and Vermont 

slate districts. Since the publication of Bulletin 275, A. H. Purduo's 

studies of the slate region of Arkansas have also appeared and two 

papers by T. L. Watson on the slates of Virginia.

The present bulletin is not only a corrected and revised edition of 

Bulletin 275, but contains also the results of all these recent discoveries and studies. Its plan is to set forth in Part I, in succinct form, 

the present state of scientific knowledge as to the origin, structure, 

texture, and chemical and mineral composition of slate; in Part II 

to describe in more or less detail the slates and quarries of each district; and in Part III to consider the economic geology of slate, 

including the scientific prospecting for slate, the methods of testing 

it, slate machinery, the uses of slate and slate waste, together with a 

tabular summary of all the slates described.

Geologists will find in Part II geologic notes on many of the slate 

districts, data as to the weathering of the slates of Pennsylvania,

7
8 SLATE IN THE UNITED STATES.

Maryland, and Georgia, and petrographic descriptions of all the slates. 

Those of Maine, Maryland, New York, Pennsylvania, Vermont, and 

Virginia are treated most fully. The accounts of the various lenses 

in slate, given on pages 70, 86, 91, 93, 112, 122, and 157, will probably interest petrographers.

A bibliography of slate, both economic and scientific, and a glossary 

of geologic and slate-quarry terms are appended.

Mr. C. A. Bonine, now of this Survey, has kindly made contributions from his Lehigh University thesis, "The slate deposits of Northampton County, Pa.," written prior to his connection with this Survey. 

These include the map (PI. XV) showing the locations of the slate 

quarries of that county, and also several paragraphs on its stratigraphy, which are credited to him in their place.

In the geologic mapping of the slate belt of western Vermont and 

eastern New York the writer was assisted in 1895-96 by Messrs. 

L. M. Prindle and F. H. Moffit. The quarry sites on the map of 

Slatington, Pa. (PL XVI), were located by Mr. Moffit.

As in Bulletin 275, Miss A. T. Coons contributes a chapter on the 

statistics of slate production.

The portions of this bulletin not specifically credited to others 

have been prepared by the writer.
PART I. ORIGIN, COMPOSITION, TEXTURE, AND STRUCTURE OF SLATE.

DEFINITION AND CLASSIFICATION OF SLATE.

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

less perfect cleavage, being thus adapted 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 that may be of identical chemical and mineral 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 defined 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 their mineral and chemical composition and in their 

physical properties.

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

variety of uses roofing, flooring, electric switchboards, blackboards, 

hand slates, billiard and laboratory table tops, vats, tubs, 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 necessities of life and death.

The appearance of large masses of fresh slate of good quality in 

cross fracture.is shown in Plates XIII, XIV, and XIX.

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, 

textural, for the cost of their production and the degree of their 

strength depend primarily upon their texture; second, it must be 

mineralogic, for their durability depends upon their content of certain 

mineral constituents.

Slates in which the particles have been merely compressed by 

weight or pressure and cemented by carbonates of lime and magnesia, by kaolin, or by different combinations of iron, and whose 

fissility, strength, and elasticity are therefore low, must be distin9
10 . SLATE IN THE UNITED STATES.

guished 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 partly formed 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 slate of Pennsylvania and Maryland are all mica slate.

But mica slate includes slates of very different composition and 

structure, and therefore of very different 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. But each of the color groups of fading and unfading mica slate embraces slates having 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 slate have a. texture approaching that of a 

schist. In some slates the sericite fibers are remarkably even and 

parallel; in others they make up minute elongated lenses, so that the 

slate in cross section is seen to consist of tiers of lenses. Such differences in texture can not but result in differences in 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. 

(6) Chloritic (greenish), 

(c) Hematitic and chloritic (purplish).
ORIGIN OF SLATE. 11

I. Aqueous sedimentary Continued.

B. Mica slates: Matrix with marked aggregate polarization Continued.

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).

(e) Ilematitic, specular, and graphitic (bluish blackish).

II. Igneous.

A. Ash slates. 

B. Dike slates.

The scientific basis for these subdivisions will be seen by consulting 

the table facing page 188 and the microscopic and chemical analyses of 

the slates there named. The application of this scheme to 20 kinds 

of American slates will be found on page 189.

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 such granitic land masses with sandy sediments consisting of coarser material from 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 brought 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, and 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 periods of such changing conditions in the 

water also alternated with- 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. Red slates owe their color to the 

access of ferruginous matter from the land, and purplish slates to an 

admixture of such matter and a green magnesian mineral (chlorite) of 

secondary origin. In both the reddish and the purplish slates the 

iron is supposed to have been originally precipitated in the form of the 

rust-colored limonite (2Fe2O3 .3H2O) from iron-bearing solutions and
12 SLATE IN THE UNITED STATES.

to have been afterward altered by loss of water (H20) to the reddish 

hematite (Fea08 ). 1

An accumulation of several hundred feet of such clayey and sandy 

sediments when buried under several thousand feet 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 be 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 outer portion. This corrugation, for reasons not yet perfectly evident, took place, so far as observation extends, chiefly within 

certain belts in which the mountain systems were formed from 

the lateral compression of a great mass of parallel strata. The first 

effect of this compression was to bend at least the lower portion of 

the strata into wavelike 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 slate regions of Pennsylvania 

and western Vermont. (See Pis. Ill; XIII; XIV; XVII; XVIII; 

XIX, A; 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. Each individual sedimentary particle was rotated 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 the heat 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 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 ribbonlike 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

1 See on these changes Van Hise, C. R., A treatise on metamorphism: U. S. Geol. Survey Mon. 47, pp. 

225, 232,1904.
ORIGIN OF SLATE. 13

of inclination governed by that pressure. A small but variable 

proportion, however, of these scales took such a position that their 

flat sides became parallel to the direction of the pressure. - 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 a mica 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 may-show this interlacing of the two sets of scales on their 

attenuated edges. It is to this microscopic texture that the 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 were formed by the constituents of the shale, 

which 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. Lenses 

consisting of some one mineral surrounded by one or two others were 

also formed and concentrically or radiately arranged.. The limonite 

became hematite. During these changes in the fine sediments the 

intercalated beds of sandstone passed into quartzite and metamorphic 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 Dauhre"e, the French 

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 experiment consisted in inclosing clay and water 

in a closed wrought-iron 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.
14 SLATE IN THE UNITED STATES.

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. The familiar one of Tyndall 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 texture.

The general microscopic texture 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 are more fully considered in the 

following section on the petrographic characters of slate. The above 

notes 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 clay slate 

is similar, but the metamorphism to which it has been subjected has 

been insufficient to produce the micaceous texture.

After the slate was formed the mass was subjected to various 

stresses (tensions, shears, contractions), which resulted in several 

systems of joints, in faults, slip cleavage ("false cleavage"), shear 

zones ("hogbacks"); and irregular openings in which veins of quartz 

and calcite were formed by infiltration. Some masses were also 

' 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 then- upper portions have generally been removed 

through various erosive processes the decomposition due to atmospheric agencies, the action of streams, and, in places, the erosion of 

the continental glacier. Plates III, XIII, XIV, XVII, XVIII, and 

XXV all illustrate this feature. In the Appalachian region generally the thickness of this eroded material was very considerable.

From the foregoing statements it may be seen that a piece of slate 

is in itself a record of a long series of complex geographic, geologic, 

chemical, mineralogic, and physical processes of great scientific 

interest.

MINERALS OF SLATE AND THEIR ARRANGEMENT.

HISTORY OF RESEARCH.

However uncertain the structural, textural, mineral, 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 the structure 'of slates in
HISTORY OF EESEAECH. 15

the field or the laboratory or in both, and Sorby, Zirkel, Renard, 

and others have investigated their mineral 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 Rosenbusch in 1898. Chemical analyses are given in 

Roth's "Chemical geology" (1890), and in ZirkeTs "Petrography" 

(1894). Hillebrand, Reade, and Holland made a number of complete analyses between 1899 and 1901. Van Hise, in his "Treatise 

on metamorphism" 1 (1904), discusses the theory of metamorphism 

in schists and slates, and Leith, in his paper on rock cleavage 2 

(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 205-210 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, Beoker, and Leith. 

Van Hise * 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; or it may 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 particles 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.

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

1 U. S. Geol. Survey Mon. 47, pp. 752, 753,1904. 2 U. S. Gcol. Survey Bull. 239,1905.
16 SLATE IN THE UNITED STATES.

block of ceresin 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. 1

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. 2

This question of the relation of the dip of slaty cleavage to the 

direction of pressure is not purely speculative 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. 3

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 slates at Brownville, in the 

same State, which have a micaceous luster.

An interesting transition from a shale to a mica slate, described on 

page 104, occurs at Lehigh Gap, Pa. The new muscovite (sericite)

1 Experiments on schistosity and slaty cleavage: U. S. Geol. Survey Bull. 241, p. 21, 1904.

2 Rock cleavage: U. S. Geol Survey Bull. 239, p. 138,1905.

3 Sorby, H. C., On the structure and origin of noncalcareous stratified rocks: Geol. Soc. London Q.uart. 

Jour., vol. 36, p. 76,1880.
HISTORY OF RESEARCH. .17

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.,1 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., possess fissility without any matrix of 

muscovite whatever, but others show the beginning of slaty cleavage 

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 2 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 

following 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 ? Was schist of sedimentary origin at 

one stage roofing slate ? 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 3 in the best Welsh slates as one twothousandth of an inch in breadth by one six-thousandth in thickness.4 

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 place 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 transverse 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 lamellae of 

muscovite or in places of biotite. Some He transverse to the cleavage and parallel to the grain, and others lie parallel to the cleavage. 5 

Chlorite and quartz may make up minute lenses'with their long

1 See U. S. Geol. Survey Bull. 242, PI. II, B, 1904.

2 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."

3 See Laspeyres, H., Sericit: Zeitschr. Kryst. Min., vol. 4, p. 224,1880.

 "Sorby, H.C., op. cit.

6 See Kenard, A. P., llecherches sur la composition et la structure des phyllades ardennais: Mus. roy. 

hist. nat. Belgique Bull., vol. 3, p. 235, PI. XII, 1882; Zirkel, F., Lehrbuch der Petrographie, p. 298,1894; 

and Hose, G., liber die regelmilssige Verwachsung der verschiedenen Glimmcrarten in it cinrmdcr sov/ic 

mitPcnnin und Eisenglanz: K. Akad. Berlin Monatsber., 1809.

40620° Bull. 586 14 2
18 SLATE IN THE UNITED STATES.

axes parallel to the cleavage, or chlorite surrounded by sericite 

makes lenses with their long axes in the grain direction. Some of 

these scales of chlorite contain needle-like crystals, probably rutile, 

crossing one another at angles of 60° and 120°. (See p. 102.) This 

chloritic mineral is regarded as of secondary origin. Rutile needles 

are generally but not invariably present, and in great abundance. 1 

This mineral occurs also in irregular masses 0.05 millimeter or less in 

diameter, consisting of a network of prisms-forming angles of 60° 

and 120° (sagenite twins); as at Peters Creek, Lancaster County, Pa., 

at Bremo, Va., and in Humboldt County, Nev. (See p. 155.) 

Hematite occurs in very minute scales and dots and very plentifully in: the reddish and purplish slates. At Esmont, Va., the 

hematite is specular, giving an alm'ost metallic luster to the slate 

when wet.2

Slate may contain also cubes, lenses, or spherules of pyrite. The 

spherules abound in black slates. The slate of Northfield, Vt., contains lenses of magnetic iron pyrites (Fe7S8 ). 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 unusual.

Calcite and other carbonates, particularly one of linie, iron, and 

magnesia, are likely to be evenly disseminated in rhombs and plates. 

Rarely calcite fossils appear, as at Arvonia, Va. Rhodochrosite 

(MnC03 ) probably also occurs. (See p. 91.) The following have 

also been identified: Ottrelite, staurolite, garnet, andalusite, sphene, 

anatase, biotite, hornblende, epidote, apatite, gypsum, magnetite, 

limonite, pyrophyllite, and talc.

Many slates are 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, or of these and carbonate, they may consist of an octahedron 

of magnetite partly surrounded by quartz and that entirely by 

chlorite.3 This quartz is then regarded as a later infiltration into 

a cavity formed between the magnetite and the chlorite by pressure. 

Others consist of chlorite surrounded by calcite and that by quartz.4

1 On these see Zirkel, F., Ueber die mikroskopische Zusammensetzung von Thonschiefern und Dachschiefem: Poggendorff's Annalen, vol. 144, p. 319, 1891; Van Werveke, Neues Jahrb., 1881, vol. 1, p. 178; 

Sauer, A., idem, pp. 227-238; Cathrein, A., Ueber das Vorkommen mikroskopischer Zirkone und TitanMineralien in den Gesteinen, Wiirzburg, 1884; Kalkowsky, E., Elemente der Lithologie, p. 257, 1886, 

Sorby (op. cit., p. 68) gives the diameter of the slate needles as less than -^fo^ inch.

2 Renard gives the size of the granules of FesOa as 0.020-0.005 millimeter (op. cit., vol. 3, p. 234). See also 

his PI. XII, fig. 2, of purple and green slate, in same volume. J. Gosselet (Etudes sur 1'origine de 1'ottrelite: 

Soc. ge'ol. du Nord Ann., vol. 15, pp. 188-189, Lillie, 1888) describes hematite as occurring in three forms in the 

reddish slates: (1) In irregular grains 0.01 to 0.02 millimeter 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 

brick-red under reflected light.

3 See Geinitz, E., Der Phyllit von Rimognein der Ardennen: Min. pet. Mitt., new ser., vol. 3, also Rermrp, 

A. F., op. cit., vol. 2, pp. 133 et seq. and Pi. VI, 1883. 

< Renard, A. F., op. cit., p. 248.
HISTOEY OF RESEAECH. 19

The"slates of Esmont, Va., have lenses of carbonate, chlorite, quartz, 

and muscovite radially arranged along the cleavage foliation/

Others have a central crystal of pyrite instead of magnetite, 1 or 

the pyrite may have been changed to limonite.2 Still others consist 

of chalcedony surrounded by chlorite scales,3 or of quartz surrounded 

by radial plates of muscovite,4 or of biotite surrounded by quartz. 

(See p. 80.) These lenses may, as in the Peach Bottom slate (p. Ill), 

be so minute as not to result in any visible speckling of the surface, 

and they 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 oxidation of a ferrous oxide has been shown to be due, in 

some slates at least, to the alteration of a ferrous carbonate to limonite, and it i::- thought to be probably due to this in most slates. 

(See pp. 55, 140.)

The presence of kaolin is, of course, to be assumed in all clay 

slates and also in all mica slates in which the micasization of the 

matrix is incomplete. Its presence in minute quantities in those 

mica slates which have little or no luster, like the slates of Monson, 

Maine, and probably the 'slates of Lehigh and Northampton, Pa., 

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, Renard computes 

the mineral percentages as follows:

Percentages of minerals in slates of Ardennes, France.

Mineral.

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

Purple.

40.69

7.75

40.41

6.23

1.55

Green.

39.54

5.81

45.78

2.90

i rtd

Bluish 

black.

07 7C

12.55

40.58

4

01

Grayish 

green.

37.97

17.99

OA Q7

1.44

3.09

In round numbers this amounts to 

Muscovite.................................................... 38-40

Chlorite...................................................... 6-18

Quartz........................................................ 31-45

Hematite...................................................... 3- 6

Rutile......................................................... 1- 1£

1 See Barker, Alfred, on "eyes" of pyrite and other minerals in slate: Geol. Mag., dec. 3, vol. 6, No. 103, 

p. 396, London, Sept., 1889.

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

cited see Bibliography, pp. 205-210.)

8 Mallard, E., Sur 1'examen microscopique de quelques schistes ardoisiers: Soc. min. France Bull., voL 

3, No. 4, p. 101, 1880.

* Zirkel, F., Lehrbuch der Petrographie, Thonschiefer, p. 744,1894.
20 SLATE IN THE UNITED STATES.

Renard * calls attention to the fact that the green slates of Furnay, 

in the Ardennes, contain 4 per cent more Si02 than the.purple ones 

and about 3£ per cent less Fe203 .

Rosenbusch, 2 in comparing 18 analyses of clay slates from different 

parts of Europe, calls attention to the strikingly characteristic preponderance of the MgO 4s pompared with the CaO, along with a 

uniformly high percentage of ir.on pxides and of A1203 , and also to 

the like preponderance of the 1^0 .over the Na20, 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.

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 shredlike 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 apparently 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 place by an alteration of partly decomposed feldspar, 'yet he admitted that the structure was just such 

as would result from the deposition of material sorted by gentle currents and subsequently compressed. The chlorite he considered as 

undoubtedly secondary.3

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.4

Hutchins expressed his conclusions thus:

This fine 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, tempera-

.1 Op. cit., vol. 2, p. 147, 1883.

2 Elements der Gesteinslelire, p. 424.

3 Sorby, H. C., On the structure and origin of noncalcarcous stratified rocks: Geol. Soc. London Quart. 

Jour., vol. 36, pp. 67-77, 1880. 

< Neues Jahrb., 1SS1, vol. 1, p. 399.
HISTORY OF RESEARCH. 21

ture, 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 

dynamometam orphic 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. 1

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 hy decomposition. As a 

matter of observation, fine-grained mica slates do contain, scattered 

scales of muscovite apart from the matrix, which may be of clastic 

origin, but the micaceous matrix of mica slate is regarded as mainly 

of metamorphic origin.

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

Roth 2 is decidedly of the opinion that they belong to the original 

sediment. Thurach,3 Pfaff,4 and Credner 5 find an abundance of 

them in clay. Rosenbusch 0 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 

62 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 slate .of Pennsylvania 

and the slates of Penlan, Va., and of Maine) they are hard to find, and 

in some less crystalline slates (those of Arkansas and 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 all the 

roofing slates described in this bulletin, are here summarized. The 

dimensions are given in millimeters.

Quartz, grains 0.013 to 0.1 by 0.004 to 0.03 (exceptionally 0.347 long).

Feldspar (plagioclase), grains up to 0.04.7 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 63 per square millimeter. 7

1 Hutchins, W. M., Clays, shales, and slates: Gcol. Mag., vol. 7, p. 317, 1890.

2 "Doubtless some of the constituents are of secondary origin (as quartz,pyrite and the products of its 

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

needles does not seem to me convincing. Unstratifted. sands also contain a scries of minerals." Allgem. 

chcm. Geologic, vol. 2, p. 586, 1S87.

3 Thiirach, H., Ueber das Vorkomraen mikroskopischer Zirkone und Titan Mineralien in den Gesteinen: 

Phys.-math. Gesellsch. Wurzburg Verh., N. F., vol. 18, No. 10, 1884.

4 Pfaff, E., Pctrographische Untersuchungen iiber die eocencn Thonschiefcr der Glarner Alpen: K. k. 

Akad. Wiss. Munich Math.-phys. Classo Sitzungsber., vol. 10, p. 479, 1880.

6 Credner, G. R., Die Krystallinischen Gemengtheile gewisser Schiefcrthone und Thoiie: Zeitschr. gcs. 

Naturwiss./i-lalle, 1874. 

«Elemente der Gesteinslehre, p. 424,1910.

7 The square millimeter given comprises only the thickness of the thin section.
22 SLATE IN THE UNITED STATES.

Chlorite, scales 0.047 to 0.17 by O.OOG to 0.1 (exceptionally 0.38 by 0.25) and numbering up to 380 per square millimeter.

Carbonate, rhombs 0.002 to 0.065.

Rutile "needles" 0.0017 to 0.095 by 0.0006 to 0.006 and averaging in certain Vermont slates about 65,000 per square millimeter, or about 40,000,000 per square inch. 

In a slate from Lancaster County, Pa., groups of intergrown rutile crystals measure up 

to 0.04 in diameter and average about 15 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 1,160 per square millimeter.

Pyrite, spherules 0.0017 to 0.027 and numbering up to 18,000 per square millimeter.

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

numbering up to 49 per square millimeter.

Hermatite, dots 0.0004 to 0.01.

Tourmaline, prisms 0.005 to 0.076 by 0.001 to 0.009.

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

numbering up to 310 per square millimeter.

Lenses of chalcedonic quartz and rhodochrosite: 0.32 by 0.15 (exceptionally 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.

Lenses of carbonate, chlorite, quartz, and muscovite measure from 0.2 to 2 in length 

and up to 0.7 in width and number up to 4 per square 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.

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:

Clarification of mineral constituents of slates.

Clastic.

Quartz grains. 

Feldspar grains. 

Zircon grains. 

Muscovite scales.

Kaolin.

Apatite.

Magnetite? 

Carbonates, granular.

Clastic or authigenous.

Rutile needles. 

Tourmaline.

Autliigenous.

Quartz, chalcedonic. 

Quartz, vein. 

Muscovite (sericite). 

Biotite.

Chlorite, interleaved with

muscovite or biotite.

Pyrite, pyrrhotite. 

Magnetite.

Hematite.

Carbonates of lime, iron,

magnesia.

Carbonate of manganese.

Andalusite.

Barite.

Gypsum.

Talc.

Organic.

Carbonaceous matter. 

Graphite. 

Calcite (fossils).

Of course these secondary or authigenous minerals were merely 

new combinations, sometimes in crystalline form of clastic minerals. 

Thus the hematite originated in sedimentary limonite. The andalusite (Al2 03 .SiO2 ) probably originated in kaolin.
.SPOTTED SLATES. 23

The principal elements in roofing .slates, as determined by the 

chemical analyses given in the section on the chemistry of slate 

(p. 50), are to be attributed to the mineral constituents in the following way:

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

and kaolin. 

Ti02 to rutile.

A1203 to muscovite, biotite, feldspar, chlorite, tourmaline^ andalusite, and kaolin. 

Fe203 to hematite, magnetite, muscovite, and biotite: 

FeO to chlorite, magnetite, carbonate, and biotite. 

MnO to rhodochrosite..

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

BaO to barite and some silicate. 

MgO to chlorite, biotite, and carbonate. 

K20 to muscovite and orthoclase. 

Na-jO to muscovite and plagioclase. 

Li2 0 to tourmaline.

H20 to muscovite, chlorite, limonite, and kaolin. 

P205 to apatite.

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

FeS2 to pyrite. : . 

S03 to barite and gypsum. 

C to graphite and coaly matter. 

Zr20 to zircon.

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

and ammonia detected by Dr. Hillebrand in the slates of Vermont 

and New York have not been determined.

SPOTTED SLATES. 

GENERAL CHARACTER OF SPOTS.

The spots in roofing slates have long attracted attention.1 In the 

slate region of eastern New York and -western Vermont many of the 

purple slates have green spots of circular, oval, or irregular outline. 

These spots locally 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. 

Some of the red slates are also spotted. The spots are commonly 

circular or oval, measure from a fraction of an inch to several inches 

in diameter, and are 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

1 Tyndall, John, Comparative view of the cleavage of crystals and slate rocks: Philos. Mag., vol. 12, July, 

1856. Maw, George, On the disposition of iron in variegated strata: Oeol. Soc. London Quart. Jour., vol. 24, 

p. 379, PI. XIV, figs. 23, 31, 32, 1868. Gosselet, J., Les schistes de Fumay, Soc. geol. Nord Ann., vol. 10. 

pp. 63-86, Lille, 1884; L'Ardenne, p. 35/ 1888. Geikie, Archibald, Text-book of geology, 4th ed., vol. 1, 

p. 451 (2), 1893. Zirkel, Ferdinand, Lehrbuch der Petrographie: 2d ed., vol. 3, pp. 296-297,1894.
24 SLATE IN THE UNITED STATES.

to and across the cleavage, and chemical analyses were made by 

Dr. Hillebrand of the green center of a spot from the spotted red 

slates, of its purple rim, and of the outer red slate.

MICROSCOPIC ANALYSES OF SPOTS.

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

the Lake Bomoseen Slate Co.'s quarry at Cedar Point, Castleton, 

Vt., in a section cut parallel to the 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 

surrounded 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 rim the same elements recur, but the 

pyrite is much more abundant, measuring up to 0.021 millimeter. 

There are also many dots of Fe203 from less than 0.003 to 0.009 

millimeter, and rutile needles up to 0.012 millimeter in length. .

An elliptical green spot, 3 inches long, with a purple rim, in Ordovician red slate from the National Red Slate Oo.'s quarry northnorthwest of Raceville; N. Y. (specimen D. XIV, '95, 397a), when 

cut transversely to the cleavage, measures half an 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 quartz 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 like those in the center, and also quartz 

grains. In the purple rim there is a decrease of carbonate and the 

hematite fragments begin to appear, and in the surrounding red slate 

hematite becomes still more abundant.

A green spot in Ordovician red slate (D. XIV, '95, 201c), from the 

Empire Red Slate Co.'s quarry, 1 mile north of Granville, N. Y., cut 

parallel to the cleavage, shows slate needles (TiO,) up to 0.043 millimeter long, carbonate rhombs up to 0.030 millimeter, chlorite scales 

up to 0.030 millimeter, angular quartz grains up to 0.039 millimeter, 

and prisms of tourmaline up to 0.021 by 0.002 millimeter. The surrounding red slate, that of analysis K (p. 92), is described on page 91.

Another spot, almost circular, 0.44 inch in diameter, from a piece 

of red slate (D. XIV, '95, 2011) 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, consisting 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
SPOTTED SLATES. 25

disseminated areas and rhombs of carbonate; then a very narrow 

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

another zone of green slate, 0.08 inch wide, like the first, but with 

very little carbonate. The angular quartz grains measure up to 0.030 

millimeter. There are also slender tourmaline prisms. Outside of 

all comes the red slate, full of Fe203 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 Dr. Hillebrand came from the 

same quarry (near Raceville) 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 92, is repeated for comparison.

Chemical analyses of spotted red slate.

Silica (SiOs).. .............

(TiOa). .................

Ferric oxide (Fe20s). ......

Manganous oxide (MnO). . 

Nickelous oxide (NiO) . . . .

Limo(CaO)...............

Baryta (BaO).............

M

63.88

.47

9.77

3. 86

1.44

.21 

Trace. 

Trace.

3 CO

.05

Q

64.59

.51

10.23

1.79

1.19

.26 

Trace.

.05

R

65.44

.52

9.38

1.09

1.06

.32 

Trace. 

Trace.

4.53

.06

Lithia(Li20)............

Water below 110° C.(H2 0)

Water abo ve 1 10 ° C . ( H20 )

Phosphoric oxide (P?.0 6 ).. 

Carbon dioxide (CO2 ) .... 

Pyrite (FeSa)............

M

5.37

3.45

.20

Str. tr.

.27

2.48

.08 

5.08

100. 14

Q

5.12

3.70

.23

Str. tr.

.28

2.29

.08 

5.84

100. 23

R

4 09

3

C7

99

Str. tr.

.25

2.10

.08 

6.55 -.04

100. 13

M (=D. XIV, '95, 397 a), 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.

Dr. Hillebrand adds this observation:

Calculation shows that with no C02 there would be only enough CaO for the P205 , 

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 MnC03 , and CaC03 , the 

remainder of the C02 is charged to MgO, we find the proportions shown in the columns 

below. [Seep. t5S.]

Ca03 ...........................................................................

MgCO3 (in part FeCO3 ). .........................................................

MnCOs.........................................................................

M

.6.14

4.22

.38

Q

7.11

4.77

..47

R

7.93

5.36

.57

DISCUSSION OY THE SPOTS.

From Dr. HLllebrand'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 Fe203 in the same direction.

The main results of the microscopic and chemical analyses agree 

even as to the relative amount of pyrite. The difference in color from 

the green to purple and red is manifestly due to the differences in the
26 SLATE IN THE UNITED STATES.

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 

locality the effect of organic matter, whether the carbonaceous matter 

of the lining of an annelid boring or matter from a marine alga, has 

been to diminish the quantity of Fe203 in the slate and possibly to 

increase the amount of chlorite.1 Gosselet regards the spots as the 

result of the reduction of the hematite (Fe203 ) 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 he attributes to infiltration.2

In the spots from the slates of New York and Vermont examined 

by Dr. Hillebrand and the writer the marked decrease of Fe203 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.3 The increase of the carbonates may 

be directly connected with the production of C02 by decaying organisms and the consequent decrease of the Fe203 . Not impossibly the 

organism may have had a calcareous exoskeleton which was dissolved 

and then redeposited as crystalline CaC03 . The infiltration of SiO2 

and the formation 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 occurred in some 

fossiliferous sediments. At any rate, the run of intermediate composition 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 is the. correct view, 

the green ribbons, which traverse both purple and red slate, would 

correspond to small deposits of decomposing organic material that 

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 this may have 

been the very condition which proved favorable to marine life.

Not to be confounded with the spots described above are certain 

white or whitish spots on the cleavage surfaces of some slates (Northfield, Vt., and Penlan, Va.) measuring up to 2 inches in diameter and

1 See Tyndall, Maw, Gosselet, Geikie, and Zirkel, as indicated by titles given in footnote on p. 23.

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

the ribbons contained 6 per cent more Si02, 7 per cent more Al2 0s, 4.5 per cent more McO (= 7 times as 

much), but 4 per cent less Fe 203,1 per cent less FeO, and 3.5 per cent less K2O than the adjacent blue beds. 

Under the microscope the green ribbons showed more feldspar and chlorite. Maw attributes these differences to change in sedimentation.

3 Zirkel, Ferdinand, loc. cit.
STR.UCTUKE OF SLATE. 27

consisting of a fine film of calcite, with or without other carbonates, 

deposited by water percolating along the cleavage, the water, originally^ acid, having dissolved the carbonate from the slate itself. 

The cause of these spots is analogous to that of some of the minerals 

found coating the joint planes in slate. (See p. 42.)

IGNEOUS SLATES.

Some very remarkable slates of igneous origin 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 been chemically 

and microscppically analyzed and described * and were found to 

consist chiefly of chlorite, calcite, quartz (mostly secondary), and 

muscovite, but to contain also andesitic lapilli, feldspar, garnets, 

sphene, and anatase.' Slate needles and tourmaline are conspicuously absent. The chemical analyses show the following important 

constituents:

FeO.......................... 5.97-7.06

CO............................ 2.45-5.41

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

A1203 ...................... 11. 94-17. 85

CaO....................... 3.67- 6.46

The C02 if calculated to CaC03 would give from 5. 56 to 12.29 'per 

cent of CaC03 . The 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. 142). ,.' -

Still more remarkable are the slates first described by E. C. Eckel 

in 1903,2 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. 50). These slates are fully described by Mr. Eckel on page 68.

In this connection E. B. Mathews now regards some of the slates 

of Maryland as at least partly of volcanic origin (p. 85), and J. E. 

Pogue, jr., describes a schistose slate from North Carolina as of 

mixed sedimentary and volcanic origin (p. 208).

STRUCTURE OF SLATE. 

BEDDING.

Ordinary planes of bedding may be defined as those which are 

approximately parallel to the surface of the water in which the sediment was formed. If the sediment changes in character, then a hori1 Hutchins, Maynard, The ash slates of the lake district: Geol. Mag., 1892. Heade, T. M., and Holland, 

Philip, The green slates of the lake district, with a theory of slate structure and slaty cleavage: Liverpool 

Geol. Soc. Proc. (1900-1901), 1901.

2 U. S. Geol. Survey Bull. 225, p. 419,1904; On a California roofing slate of igneous origin: Jour. Geology, 

vol. 12, p. 15, Jan.-Peb., 1904.
28 SLATE IN THE UNITED STATES.

zontal bed of different material results. If, deposition being interrupted, annelids creep over the bottom or algae decompose there, 

and the same kind of sedimentation is afterward 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, and some 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 that is, cleavage and bedding 'are parallel but in all 

other places cleavage is necessarily more or less transverse to bedding. 

This statement, of course, assumes that the slate beds still retain 

the position they had when slaty cleavage was set up in them.

Plate II, B, shows a ledge of green roofing slate with a cleavage 

striking N. 15° W. and dipping 20° E. and joints striking N. 30° E. 

and dipping 45° E. The upper part is a quartz sandstone or grit, 

with calcareous concretions containing Lower Cambrian trilobites. 

The direction of the axial planes of the calcareous bodies and the 

direction of the line of contact between the slate and sandstone show 

that the direction of the bedding is horizontal.

The "ribbons" of the Pennsylvania quarries (shown in figs. 9-12 

and Pis. V, B; XIII; XIV; and XVIII) consist of small beds of 

quartz sandstone or grit in which the particles are held together 

by a calcareous and locally a sericitic cement, the original clay 

having gone into muscovite. These beds are also very carbonaceous 

(PI. VI, A). Here and there 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 Plate XXV. In 

some places 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 Plate 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 beds.

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

the brachiopods, crinoids, and trilobites at the Arvonia quarries in 

Virginia, or by bifurcating impressions, possibly made by seaweeds,
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE I

A. RED SLATE FROM QUARRIES U MILES SOUTH OF HATCH HILL, WHITEHALL, N. Y.

Whitish streak is bed of greenish dolomite, rimmed by very thin green slate, and this by purple slate, the whole 

an inch wide. It crosses the cleavage diagonally.

SANDSTONE RESTING ON GREENISH SLATE AT EDDY HILL, FAIR HAVEN, VT.

Calcareous concretions in sandstone have their longer axes parallel to the bedding. Cleavage of slate dips 20° 

and joints 45°, An irregular cleavage continues into the sandstone. One foot of sledge handle is in sight.
SYNCLINE IN BLACK AND GRAY BANDED SLATE AT WEST CA

Showing effects of erosion on the more calcareous beds, Cleavage dips 30° E, (to 
STRUCTURE OF SLATE. 29

and serpentine tracks made 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.

Plate II, A, from a photograph of a piece of red slate from the old 

quarries of the Fair Haven Red Slate Co., 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 dolomite, 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 found to be this: The 

central green bed consists of dolomite and of rhombs of sideritic 

dolomite; the siderite is altered to limonite. There are large quartz 

grains, niuscovite scales without parallel orientation, and scattered 

plagioclase grains. The purple material consists chiefly of scales 

of niuscovite and chlorite, lying in two directions, at right angles to 

each other, irregular dots of hematite, some carbonate rhombs, 

quartz grains, and rarely a grain of plagioclase. The thin green 

strips on the sides contain less hematite than the purple material, 

and a large number of the niuscovite 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 dolomite is 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 25, 26 to be probably true 

of the green and purple spots in the same slate. Locally 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 purple slate of Vermont has in many places small beds of 

green slate, with or without a central band, crossing the cleavage; 

and some of the village sidewalks in that region are flagged with such 

purplish, green-banded slate. The roofing slates of this kind, whose 

grain, owing to folding and pitch, is not at right angles to the bedding, have diagonal green ribbons. These green ribbons, both in the 

red slate of New York and in the purple slate of Vermorit, run into 

planes or rows of spots of various sizes, usually more or less oval in 

outline. Where a series of such spots are in line they indicate the 

course of the bedding.

The small beds or ribbons may be plicated, as shown in Plate 

XXIV, Q, R. Plate IV, E, is a microscopic drawing of a thin section
.PLATE IV. 

PLICATED AND FAULTED BEDS IN SLATE.

A. Extremely plicated bed of quartzose limestone or .calcareous quartzite in Cambrian slate at Fair Haven, Vt. Natural size. The slate on both sides of the 

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

B. Extremely plicated bed of quartzose dolomite or calcareous quartzite in Cambrian slate at Fair Haven, Vt. Reduced one-half.

C. Faulted quartzite bed in slate at Meadow Slate Co.'s quarry, Fair Haven, Vt. 

Normal fault.

D. Faulted quartzite bed in slate at Eureka quarry, Poultney, Vt. Reversed fault.

E. Thin section of faulted calcareous bed at Pawlet, Vt., given in Plate XXIII, D 

(p. 126), showing adjustment of cleavage to faulting and production of secondary cleavage. Enlarged 3£ diameters.

F. Thin section of plicated and faulted beds of calcite and quartz separating beds of 

purple and green Cambrian slate at Blissville, Castleton, Vt. Enlarged 2 

diameters. Both calcite and quartz beds are bordered on both sides with 

chlorite scales.

G. Dislocated beds of quartzite in purple Cambrian slate at old quarry about 1 mile 

south of West Castleton,. Vt. A fragment of a bed of quartzite has been turned 

about into the cleavage foliation and across smaller beds of quartzite. By L. 

M. Prindle.

H. Plicated and. folded quartzite in Cambrian slate at Fair Haven, Vt. Height, 40 

feet. Minor plioations somewhat enlarged.

30
U. S. QEOLOQICAL SURVEY BULLETIN 586 PLATE IV

PLICATED AND FAULTED BEDS IN SLATE.
STRUCTURE OF SLATE. 31

of the plicated bed of Plate 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 slip cleavage 

caused by the dislocation. The plication of some of these small beds 

of quartzose limestone and calcareous quartzite is extreme, as shown 

in Plate IV, A, B, H. The folding in such places 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.1

Plate IV, F, 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 

tho 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 Castle ton, Vt., shows that 

the bed consists mainly of quartzite, but this contains grains of 

plagioclase, rhombs of dolomite, 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, and these scales lie at 

right angles to the cleavage that is, 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. 

The slaty material itself was also somewhat plicated, and a secondary, 

cleavage (slip cleavage) was produced in it. The plication and 

overturning of quartzite beds are shown on a large scale in Plate 

XXIV, T.

i See Hitchcock, C. H., Second annual report on the natural history and geology of Maine, p. 285, fig. 45, 

1SG2, where he describes a bed of strongly plicated limestone lying between implicated beds of slate. See 

also the classic, figure of H.C. Sorby given in his paper on the origin of slaty cleavage; Edinburgh New Philos. 

Jour., vol. 55, pp. 139,140, July, 1853, which was reproduced by Tyndall in his Eoyal Institute lecture, and 

also by Phillips in his British Association report on cleavage.
32 SLATE IN THE UNITED STATES.

Exceptionally the quartzite beds seem to have been pushed out of 

their normal parallelism, even without folding or faulting. Brecciation on a somewhat large scale is shown in Plate IV, G, from a photograph taken near the place shown in Plate XXIV, T.

The slate of Kockmart, Ga. (p. 73), shows very unusual ribbons, 

single compressed folds covering an area of the cleavage surface 10 

inches square. (See fig. 1.)

In many places, 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 in parts of theWestPawlet syncline (PL 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 that is, beds 

of muscovite and chlorite 

scales grains of quartz, 

spherules of pyrite, and 

some carbonate, but there 

is more carbonate in the

,0X6 inches gra7 bands ^^ "1 the 

FIGURE 1. Sketches of tho cleavage surfaces of two roofing slates black Ones, WfllCn Contain 

from the Prltchard & Davis qnarry, Rockmart, Ga., showing . more Carbonaceous matgreatly plicated bedding crossing the cleavage at a very acute , TTf>rA onrl thmvi i 

angle, with the grain nearly at right angles to both. The ier- -"-ore ana mere IS a 

little beds contain more quartz and carbonate than the inter- minute bed consisting

veningslale- . largely of .calcite. This 

explains why the beds are so clearly and yet so delicately brought 

out on the joint face. The original sediments contained varying 

amounts of calcareous material. 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 parts 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 may pass into the green along the
U. S. GEOLOGICAL SURVEY

A. QUARTZ VEINS IN SLATE AT JAMESVILLE, N, Y.

Shows quartz vein of segregation crossing the cleavage of greenish slate. Sledge 

handle is 30 inches long.

B. BEDDI

The more cal
U. S. GEOLOGICAL SURVEY

 : " A. BLACK-BANDED GRAY SLATE FROM PHOENIX QUARRY, WINDGAP, PA. 

Smoothly ground slabs showing variation in width of ribbons. Camera case 1 2 inches square.

B. SLATE-SPLITTING MACHINE IN OPERATION. 

The wide blade of the chisel is entering the slate block.
STRUCTURE OF SLATE. 33

same bed, and there is no reason why the Cambrian purple should 

not likewise pass into the green of the same formation.

In some places 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 may be made out 

in a thin section cut transverse to the cleavage when examined under 

the microscope. There may be 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. Plate VII, C, illustrates slates of this kind.

Gosselet 1 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. The great 

flat slate syncline of Bangor, Pa. ; is shown in Plates XIII and XIV.

SLATY CLEAVAGE.

The causes and the structure of slaty cleavage have already been 

discussed under the headings "Origin of slate" and "Petrographic 

characters."

Relation to bedding. In most slate regions cleavage is not coincident 

with bedding. Its relations to bedding are illustrated in figs. 9-12 

and Plates III; V, B; XII-XIV; XVII; XVIII; XIX, A;. XXIIIXXV. The lowest inclination- of slaty cleavage in slate districts 

visited by the writer is 5°-10° and occurs in. Pennsylvania. (See 

Pis. XIII, XIV.) 2 The lowest in the eastern New York and western 

Vermont belt is 20°. Where the cleavage approaches horizontality 

within 5° to 10° its position is probably due in part to a secondary 

crustal movement, and the local curvature of both joints and cleavage 

in the Pennsylvania region also points to such a movement.

Amount of compression in the formation of slaty cleavage. Sorby 

calculates that on a small bed of intensely plicated sandy slate, 

inclosed in ordinary slate, 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 th,e 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.

1 Gosselet, J., L'Ardenne, p. 41, fig. 7 (Les rapports do Ste. Marie avec les Trdsfossds), 1888; Les schistes de 

Fumay, PI. Ill, fig. 1 (Bond dans les schistes de Fumay), 1884.

' See also U. S. Geol. Survey Thirteenth Ann. Kept., pt. 2, I3 ]. XC, 1898, showing a slate quarry near 

Lebanon, N. Y., with cleavage dipping 10°.

40620° Bull. 586 14 3
34 SLATE IN THE UNITED STATES.

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 geometric relation between the cleavage dip of the slate and 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 against 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 (that is, vertical to the horizon) and the oblique movement of the wall 

(that is, 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°.

Gosselet's figure is here repeated (see fig. 2) with the construction 

added.1 The gist of his statement is that if we knew the dip of the

S/ate

FIGURE 2. Diagram showing relations of cleavage of slate to dip of inclosing hard beds. After J. Gosselet.

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 affect 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 small areas interfere with one another and both of which are 

intersected by a cleavage with a constant strike different from that 

of each. 2 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.3 

1 Gosselet, J., Les schistes de Fumay, pp. 68, 69, fig. 5,1884.

2 Loretz, H., Ueber Schieferung, pp. 69-70,1880.

3 Idem, pp. 83, 84..
STRUCTURE OF SLATE. 35

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

main direction of the great anticlinal axes bufrare not affected by the small undulations 

or contortions of these lines. 1

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

strike of the beds. 2

To these facts should be added this that in 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 3 gives a section 

from Sedgwick, which he calls a local exception, in which the cleavage 

planes, while coinciding in strike with that of the anticline that they 

traverse, incline on either side of it toward its axis. Rogers 4 

describes a case of this f anlike 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 the fold the fan structure would radiate downward.

Sorby 5 figures, from Ilfracombe, North Devon, a small, highly 

plicated bed of coarse-grained light-colored sandy slate traversing a 

mass of vertically cleft shaly slate. The gritty beds show a coarse 

and imperfect fanlike 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 cleavage 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 material has become rudely fanlike.

Effect of frost on cleavage. As all slate quarrymen 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, the difference in microscopic structure produced 

by freezing and thawing this experiment was tried: A fresh and unfrozen specimen was obtained from the quarry early in the winter 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

1 Phillips, Prof., quoted in Sharpe, Daniel, Contributions to the geology of North Wales: Ocol. Soc. 

London Quart. Jour., vol. 2, p. 309,1846.

2 Sharpe, .Daniel, idem.

3 Phillips, John, Report on cleavage, p. 374, 1857.

4 Geology of Pennsylvania, vol. 2, pt. 2, p. 903, fig. 715, 1858. 

& Sorby, H. C., On the origin of slaty cleavage, p. 138,1853.
36 SLATE IN THE UNITED STATES.

means GO recover it have been sought, and a process has recently been 

patented by W. A. McLaughlin, of Delta, Pa., for restoring the fissility 

of slate by the use of liquid air.

Curvature of the cleavage. As far back as 1839 De la Beche 1 called 

attention to the curvature of cleavage planes when approaching a 

bedding plane. Baur 2 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. 

John Phillips,3 in his British Association report, ascribed the curvature to the differing density of the beds. Harker 4 attributes it to a 

gradual change in the texture of the beds. Hughes 5 explains it as 

due to a secondary motion. Curvature of the cleavage is not uncommon in the Lehigh and Northampton County quarries in Pennsylvania, where it is plainly not dependent on change of texture. (See 

p. 105.) Slates from the beds so affected are also used for roofing towers.

In some of the Pennsylvania and New Jersey quarries (p. 88) the 

cleavage, instead of being curved, is.deflected in dip at the ribbon; 

and thus where ribbons abound a roofing slate of ordinary size may 

have several large-angled zigzags in it.

Phillips 8 gives a figure, the original authorship of which he does 

not mention, 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.

1 De la Beche, H. T., Report on the geology of Cornwall, Devon, and West Somerset, p.- 620, fig. si, London, 1839.

2 Ueber die Lagerung der Dachschiefer, etc., pp. 392, 393, fig. 9,1846. (For full titles of works cited see 

Bibliography, pp. 205-210.)

8 Op. cit., p. 384, fig. 23. See also Jukes, Geol. Soc. London Quart. Jour., vol. 22, p. 359, 1800. That 

the angle and amount of cleavage change with the density of the rock was shown by Phillips in 1828. 

See also Harkness, Robert, Edinburgh New Philos. Jour., new ser., vol. 2, No. 4,1855.

* On slaty cleavage, etc., 1886.

B Hughes, T. M., quoted in Lyell's Students' elements, 7th ed., pp. 53, 573, fig. 625,1871.

6 Report on the geology of Cornwall, etc., p. 372, fig. 2.
STRUCTURE OF SLATE. 37 

SLIP CLEAVAGE ("FALSE CLEAVAGE").

Several writers Sedgwick, 1 Phillips, 2 De la Beche, 3 Zirkel, 4 

Loretz 5 describe a striation or an extremely fine plication that 

appears on the cleavage surfaces of some slates. This is the "bate" 

or "false cleavage }J of quarrymen. Two systems 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. They consist 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. VII 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 half an inch square as certainly as from tests applied to pieces of commercial size. The specimen shown in Plate VII 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 slate from Arizona, described 

on page 60, shows two slip cleavages crossing each other, as well as 

botl* bedding and slaty cleavage.

False cleavage has received in recent years a variety of technical 

names, such as close-joint cleavage, strain-slip cleavage, Ausweichungs-clivage, 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, which is of 

Anglo-Saxon origin; is likely to be misleading. "Close-joint cleavage" 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 two just mentioned. Slip cleavage is a common feature in schist and is particularly characteristic of the Taconic region in western Massachusetts, 

Vermont, and eastern New York, where it seems to be due to a con1 "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 strife 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 strisc of a second cleavage on the surface of the Bangor slates which have been brought to Cambridge.'' 

A synopsis of classification of the British Paleozoic rocks, p. xxxv, London, 1855. Also Oeol. Soc. Trans., 

1840, p. C55.

2 On a group of slate rocks, p. 1,1829.

3 Geological observer, 2d ed., p. 588, fig. 239,1853. 

* Lehrbuch der Pctrographie, pp. 307, 308,1894.

6 Ueber Transversalschieferung, pp. 2C3, 264, 1882. J. E. Spurr, Am. Jour. Sci., 3d ser., vol. 48, p. 159, 

1894, describes slates in Minnesota with two and three cleavages and bedding.
PLATE VII.

THIN SECTIONS OF ROOFING SLATE WITH SLIP CLEAVAGE ("FALSE

CLEAVAGE").

A. 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. 

Enlarged 55 diameters.

B. Microscopic view of a thin section of green Cambrian roofing slate from, the 

Huckleberry Hill quarry, 2 miles southeast of West Pawlet, Vt., showing slip 

or "false" cleavage. Section transverse to both cleavages. Enlarged 55 

diameters. The black spots are pyrite. This figure represents a small part of 

the dark band shown in C.

C. Microscopic view of the same thin section, entire, showing alternation of fine 

and coarse beds, some of which are pyritiferous, and in only one. of which is the 

slip cleavage pronounced. Enlarged 4 diameters.

38
U. 6. GEOLOGICAL SURVEY BULLETIN 686 PLATE VII

SLIP CLEAVAGE AFTER SLATY CLEAVAGE.
STRUCTURE OP SLATE. 39

tinued if not, in some instances at least, a secondary crustal movement consequent upon the first metamorphism. H. B. Muff 1 finds 

in a folded Scotch slate two sets of slip cleavage, each at about 45° 

to the slaty cleavage. He regards all three cleavages as concomitant but not simultaneous, although a difference in the strike of the 

slip cleavage from that of the other structures requires a slight veering of the direction of maximum pressure toward the close of the 

earth stress.

Good illustrations of slip cleavage will be found in the following 

publications of the United States Geological Survey: Monograph 

XXIII (1894), figures 44, 45, 46, 53, 56; Thirteenth Annual Report, 

part 2 (1893), page 319, figure 25; Fourteenth Annual Report, part 2 

(1894), page 537, figure 57; Sixteenth Annual Report, part 1 (1896), figures 89, 96, 97; and Bulletin 521 (1912), Plates V, A; VIII, £(b). A 

careful study of these microscopic drawings and photographs will show 

that the 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 was as much due to a lateral compression 

as was the major folding of the Appalachian system. Slip cleavage 

should therefore not be confounded with 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.2 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 foMed, 

the schist in smaller folds, the quartzite in larger ones, but slip cleavage has been confined to the schist.3

The faults or fractures in slip cleavage belong rather to a process 

of "rock flowage" than to one of "rock fracture." As Heim 4 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." A light-green slate from Bartow County, Ga., shows 

planes 0.05 to 0.5 inch apart, carrying secondary chlorite along which 

the ribbon has been displaced 0.1 to 0.9 inch by faulting. The slaty 

cleavage, which is at right angles to the bed, does not seem to have 

been affected by these faults, nor has the slate been weakened. Such 

faulting would not come under Heim's definition of slip cleavage.

1 Peach, B. N., Kynaston, H., and Muff, H. B., Geol. Survey Scotland Mem., Expl. Sheet 36, p. 16, fig. 

3, 1909.

8 PI. XIX, A, in U. S. Geol. Survey Bull. 239, clearly shows close jointing, not slip cleavage, 

3TJ. S. Geol. Survey Thirteenth Ann. Rept., pt. 2, fig. 25,1893. ' 

< Helm, A., Mechanismus der Gebirgsbildung, vol. 2, p. 59. ' '
40 SLATE IN THE UNITED STATES.

GRAIN.

 Sharpe's explanation for slates splitting more readily along the 

"grain" than across it is that the mineral particles he 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. 1

Renard 2 states that the scales of chlorite lie perpendicular to the 

cleavage that is, about in the direction of the grain. Jannetaz's 

experiments 3 in reproducing, grain have already been referred to 

(p. 14), and his experiments showing that the direction of the grain 

is that of the greatest elasticity are given on page 172.

Daubre'e,4 in one of his experiments, produced a lamination in the 

direction of pressure and motion, which is the relation of grain to 

pressure. 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 planes0. 5 The variation of 

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°.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.7 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 quarrymen to ascertain 

its direction by the magnetic needle. A like arrangement of lenses 

and distorted cubes of pyrite characterizes the slate at Northfield, 

Vt. (see p. 122), and of folia of biotite hi the slate of Arvonia, Va. 

(See p. 151.)

The same feature is occasionally seen in schists. Thus scales of 

biotite in a schist in Beekman and Pawling, Dutchess County, 

N. Y., and actinolite crystals in schist in Hubbardton, Vt., have 

their axes in any olirection, but their flat sides are transverse to 

bedding and cleavage.

1 Sharpe, Daniel, Geol. Soc. London Quart. Jour., vol. 5, p. 114, 1849.

2 Renard, A. F., Mus. roy. hist. nat. Eelgique Bull., vol. 3, p. 235,1884.

3 Jannetaz, E., Mdmoire sur les clivages des roches (schistositd, longrain), et sur leur reproduction: Soc. 

geol. France Bull., 3d ser., vol. 12, p. 211,1884. 

< Daubre'e, A., Etudes synthe'tiques, p. 422,1879.

 6 Rosenbusch (Elemente der Gesteinslehre, fig. 73, p. 437,1910) figures biotite scales transverse to cleavage. 

o Renard, A. F., op. cit., vol. 3, p. 3. 

7 Daubre'e, A., op. cit., p. 336.
STRUCTURE OF SLATE. 41

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 locally to the dip joints. In some slates the grain is very conspicuous, as in that from Georgia sketched in figure 1.

In several of the Maine quarries the grain is abnormally nearly 

horizontal, but 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, arid 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 for this bulletin 

the writer obtained fresh specimens 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 

lensesv 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, however, points 

to a sudden rupture of large masses of rock affected in all its parts 

by one and the same mechanical expression of energy.1 The same 

stress may produce several sets of joints.

Sedgwick 2 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 ,tobe 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.

1 Loretz, H., tiber Schieferung, pp. 98-100, 1880.

2 Sedgwick, Adam, A synopsis of classification of the British Paleozoic rocks, p. 35, London, 1885.
42 SLATE IN THE UNITED STATES.

Kogers 1 called attention to the parallelism of joints to dikes in 

Pennsylvania, a parallelism previously observed in England. 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. Some 

joints are open and filled with vein matter; more commonly, when 

scarcely parted, they have their sides coated with crystals of chlorite, 

calcite, gypsum, pyrite, barite, anatase, etc.

Curved joints. Exceptionally joint planes undulate like bedding 

planes across both bedding and cleavage. (See PI. XVIII; also p. 157.) 

This may be attributed to a compression similar to that which produced the undulating sheet structure in granite. The joints shown 

in Plate XVIII could hardly have been produced by the flexure 

of a flat joint by secondary movement without faulting the syncline 

itself. The flat joint near the surface of the Peach 'Bottom slate 

belt, with, its mass of crushed slate, is probably also of secondary 

origin.2

At Foulk Jones & Sons' quarry at Slate Hill, 2 miles northeast of 

Delta, Pa., a curvature in jointing has 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 slate belt of western Vermont (more exactly, onefourth 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 beds, and also by minute beds 

with quartz grains as much as 0.1 millimeter in diameter alternating 

with beds without any quartz. All 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

1 Rogers, H. D. ; Geology of Pennsylvania, vol. 2,pt. 2, p. 912, fig. 718 (joints in red shale parallel to dike), 

' fig. 719 (joints in argillaceous sandstone), 1858. ' '

2 See, on curved joint planes in the Peach Bottom district, Mathews, E. B., Maryland Geol. Survey, 

vol. 2, p. 223, PI. XXII, fig. 1,1898. .
STRUCTURE OF SLATE. 43

bedding foliation and slaty cleavage. Under the microscope these 

joint veinsv are seen to consist on either side of a border of vein quartz, 

followed by a band of calcite fibers, then a band of quartz, and 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 resulting in slip cleavage; (3) the elongation of the mass 

along 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 the 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 common in slate 

than normal faults.1 Minor faults in slates from the western Vermont region are shown in Plate IV, C, D. One of these is a normal 

fault, the part overlying the fault plane having slid 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 slabs in the sidewalks of the villages of the Vermont-New York slate belt are finely faulted. The slate of Bolivar, 

Ga., is similarly faulted, but diagonally to the grain, and possibly 

the faulting occurred prior to the setting up of slaty cleavage. Faulting may occur both along the cleavage and along the gram that is, 

in two directions at right angles to each other.2

1 See Herbert, E. J., Reversed faults in bedded slates: Geol. Mag., new ser., dec. 2, vol. 4, p. 441,1877.

2 Such faulting occurs in the Hudson slates of Rensselaer County, N. Y. (U. S. Geol. Survey Thirteenth 

Ann. Kept., pt. 2, PI. CI, p. 320, fig. 26,1893). See also, in this connection,Teall, J. J. H., A faulted slate 

slab: Geol. Mag., dec. 3, vol. 1, PI. I, London, 1884.
44 . SLATE IN THE UNITED STATES.

The dropping of a block of some super] acent 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.

The terms " shear zones/' " zones of shearing," " Knickungsebenen," 

and "Querknicke" apply to one and the same feature. Rosenbusch, 1 

Brogger,2 Reusch,3 and Turner 4 describe shear zones in Norway, 

Saxony, Alsatia, and California. Their occurrence in the slate 

belt of eastern New York and western Vermont is fully described 

below. The writers cited explain this feature 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. 

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, which consist of two angular bends in opposite 

directions and near each other, traversing a mass of slate. These 

flexures may merge into fractures, and the slate between the two 

planes of fracture is broken into small fragments. The two bends 

or fractures may be from one-sixteenth of an inch to 4. feet apart. At 

one of the old quarries at Middle Granville, N. Y., a slate surface 

shows within a space of 4 inches four shear zones from one-sixteenth 

to one-fourth inch in width. Another has six in a space of 6 inches. 

At one of the quarries at Arvonia, Va., a mass 5 feet thick is traversed by numerous shear zones. As these zones 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. Figures E, G, and /, Plate XXIII, and figures L, U, and 

V, Plate XXIV, show the relations of shear zones at several 

quarries. Plate VIII, A, shows the microscopic structure of the 

"hogback" of Plate XXIV, V. The section was made where the 

bends had not as yet developed into complete fractures. The entire

1 Rosenbusch, H., Die Steiger Schiefer, p. 95, 1877.

2 Brogger, W. C., Die silurischen Etagen 2 u. 3 ira Kristiania Gebie't auf Eker, p. 216, fig. 31, Kristiania, 

1882.

8 Reusch, H., Die fossilienfilhrenden krystallinischen Schiefervon 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: U. S. Geol. Survey Seventeenth Ami. Rept., pt. 1, p. 662, fig. 22,1896.
STEUCTUBE OF SLATE. 45

width of the shear zone 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 one 

side. This may be the original bedding. Both of these systems of 

cracks are filled with secondary calcite. Another set, likewise filled 

with calcite, crosses the cleavage here and there about at right angles, 

but in zigzag. (See figs. B, C, D, PI. VIII.) These cracks, 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°-45° E.; strike of shear 

zone N. 35°-40° E., dip 65° NW. In this quarry the strike of the 

shear zone corresponds to that of the diagonal joints of the region 

and of a number of the dikes. Figures E to L, Plate VIII, illustrate 

the development of a shear zone still further. In figure L the diagonal fractures are filled with quartz, and a slip cleavage also occurs. 

Other observations of shear zones are given in the quarry tables, 

pages 137, 138.

CLEAVAGE BANDS.

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 shear zones.

Plate XI, B, is made from a photograph taken by the writer at a 

locality in Rupert, Vt., found by L. M. Prindle. Cleavage banding is 

of common occurrence 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. The rock shown in Plate XI, B, is divided 

into alternate bands of hard, uncleft quartzose shale and 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 that is, 

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 limomte having taken place along the 

more highly cleft bands.

A similar structure in purple slate is shown in figure 3 (p. 47), but 

here the bands themselves have slipped.1 Small beds of green slate 

indicate the course of the stratification and show the amount of 

slippage suffered by the bands.

Along Poultney River about a mile east of East Poultney certain 

hard bright-green and purple slates near the schist mass show the

> See also U. S. Geol. Survey Sixteenth Ann. Rept., pt. 1, pp. 561-564, figs. 88-91,1896.
PLATE VIII.

SHEAR ZONES.

A. Diagram of thin section of "sea-green" Cambrian slate from Williams & Edwards's 

quarry in Wells, Vt. Enlarged 2 diameters. Section across the cleavage and 

an incipient shear zone, showing the two bends in opposite directions and 

developed in but two places into fractures. Secondary fractures at 15° to 

cleavage cross the zone. A calcareous strip, possibly a trace of bedding, crosses 

the cleavage at 45°. All the fractures filled with calcite.

B, C, D. Microscopic drawings from thin section shown in A, enlarged 40 diameters, 

showing in reversed position the outlines of several of the vertical and diagonal 

fractures within and at edge of zone.

E. Diagram from specimen of shear zone, from E. E. Lloyd's sea-green slate quarry, 

Poultney, Vt.

F. Slate from within a fully developed shear zone, showing outline of fracture. Same 

location as E.

G. Slate showing two shear zones. Same location as E.

H. Diagram from specimen of Cambrian slate from Eddy Hill, Fair Haven, Vt., 

showing development of shear zones. Secondary quartz occurs along the fractures. One-half natural size.

/. Shear zone in Ordovician schist 1£ miles east of Rupert, Vt.

K. Diagram from specimen from top of the Pattern, in Pawlet, Vt.

L. Diagram from thin section of Cambrian slate from Eddy Hill, Fair Haven, Vt., 

enlarged 2 diameters, showing one of the main fractures of a shear zone with 

diagonal fractures which are filled with quartz, and beside it slip cleavage. 

46
U. 8. GEOLOGICAL SURVEY BULLETIN 588 PLATE VIII

SHEAR ZONES.
STRUCTURE OF SLATE. 47

cleavage banding well. In breaking up such a rock the denser uncleft parts come out in slablike blocks, the larger surfaces of which 

lie transverse to the bedding.

Becker 1 explains this structure by the alternate interference and 

coincidence of waves of vibration produced by shock. Where the 

waves of vibration coming from opposite directions coincide, cleavage 

fractures that is, planes of slip cleavage will be numerous.

Van Hise 2 regards such structures simply as the result of the concentration or sparseness of slip cleavage.

In the ledge figured in Plate XI, B, there are between 360 and 530 

such planes to the inch in the cleft bands. The hard bands in the 

same ledge show as many fractures, but they are discontinuous or 

merely incipient. Both hard and soft bands consist of stratified

FIGURE 3. Cleavage banding showing slippage of bands, at brook in Gorham, Poultney, Vt.

shale made up of quartz fragments, muscovite and chlorite scales, etc., 

but in the soft bands 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 slip-cleavage planes meander about and 

run into one another, though having a general parallelism. The pli1 Finite homogeneous strain, flow, and rupture of rocks: Geol. Soc. America Bull., vol. 4, Jan., 1895. 

On page 16 ho says: "Thus there seems sufficient reason to believe that a pressure very rapidly applied, 

producing primary ruptures attended by schock, 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."

2 Principles of North American pre-Cambrian geology: U. S. Geol. Survey Sixteenth Ann. Hept.,pt. 1, 

pp. 662-664,1896.
48 SLATE IN THE UNITED STATES.

cation is much more intense and irregular in the cleft strips and also 

appears to be more sericitic there.

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 beds and a cause of perplexity in quarrying. They 

appear at the most unexpected points, ramify very irregularly, and 

disappear as abruptly. Plate V, A, from a photograph taken at the 

old quarries at Jamesville, N. Y., shows a good type of such veins of 

segregation. Some of these veins show a rough parallelism. 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 e'chelon." The 

fibers or prisms lie transverse to the course of the vein. Daubre"e 1 

regards such veins as due to stretching. It is evident that the material of the veins described, of whatever sort, must have come from the 

adjacent slate itself, 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 some veins appear to be 

quite lawless, crossing in every direction, anastomosing, intersecting 

one another, and in places inclosing fragments of the adjacent slate 

and constituting the cement of a brecciated mass. 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 complex also.

The material which filled the openings thus made is chiefly quartz, 

commonly "milky" in color and in many places finely crystallized in 

small cavities. With the quartz are often associated chlorite,. biotite, 

calcite, and possibly dolomite. The chlorite locally 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, like 

those in plicated joints described on page 42. Here and there the 

quartz contains cavities measuring from 0.002 to 0.005 millimeter in 

diameter, filled with fluid, each containing a moving vacuole, as in

i Daubrde, A., G6ologie exp<5rimentale, p. 144, fig. 166,1879.
GEOLOGIC RELATIONS. 49

the quartz of granites. 1 Galenite in small particles was found in 

veins in the quarries at Jamesville, N. Y.

NODULES.

In some of the western Vermont quarries very hard lenticular 

nodules, a few inches in diameter, occur along the bedding planes. 

They consist of a quartzite nucleus containing -much calcite and large 

scales of muscovite surrounded by slaty layers carrying calcite, quartz, 

and muscovite scales. Pyrite is disseminated throughout both 

nucleus and outer zone. Such nodules are evidently of sedimentary 

origin.

Muller 2 describes concretions of pyrite and quartz in the slate 

quarries of Thuringen 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 uncommon in slate deposits. Aside from 

the very exceptional one described by Mr. Eckel on page 68, dikes 

are almost as detrimental as veins. They are usually more regular 

than veins of segregation, and, as stated under "Joints," their proximity is likely to be indicated by an abundance of joints parallel to 

them. For several feet on each side of the dike these joints become 

very close and are in places crossed by an equally numerous set of 

horizontal joints, thus breaking up the slate into small blocks. At 

Arvonia, Va., 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.

GEOLOGIC 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 metamorphism. 

The slate belt of Maine lies not far south of a granitic area. Two of 

the slate belts of Vermont are 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 slate of Virginia is 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

1 Dale, T. N., The chief commercial granites of Massachusetts, New Hampshire, and Rhode Island: 

U. S. Geol. Survey Bull. 354, pp. 42-47,1908.

2 Mtiller, F. E., Die normalen Schiefer des Hennbergs: Neues Jahrb., 1882, vol. 2, p. 218.

40620° Bull. 586 14 4
50 SLATE IN THE UNITED STATES.

than one crustal movement. Along the entire Appalachian province, 

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 geologic periods occur in proximity, and where one is even, 

superposed on another, the question of their mutual relations becomes of economic and scientific importance. 1 There was here at 

the close of Lower Cambrian time a crustal movement which resulted 

in the cleavage of the Lower Cambrian sediments and their emergence 

They were afterward 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 Fe203 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.

Silica (SiOjj).........

Titanium dioxide 

(TiOa)- ...........

Alumina (AlaOs).... 

Ferricoxide(Fe2 03 ).

Ferrousoxide (FeO)

Baryta (BaO).......

Magnesia (MgO). ... 

Potassa(K2 0)...... 

Soda(Na2O)........ 

Carbon dioxide(C02 ) 

Carbon (C).......... 

Manganous oxide 

(MnO)............

Phosphoric oxide 

(P*0s). ...........

Sulphuric oxide 

(SOs).. ...........

Water (H2O)....... 

Pyrite(FeS.>).....-. 

Cobaltous oxide 

(CoO).... .........

1

58.30

.23 

21.89 

7.05 

2.57 

.39

1.09 

2.45 

1.18

4.61

99.76

fl.81

2

61.57

1.31 

19.22 

6.63 

1.20 

.22

2.00 

3.63 

.93

3.25

99.96

3

65.63

.94 

20.20 

2.72

.85 

.19

1.54 

3.81 

.71

3". 17

99.76

4

61.43

.73 

19.10 

4.81 

3.12 

.31

2.29 

3.24 

.83

3.52

99.38

5

67.56

"ii" 23

2.87 

6.99 

.27

3.03 

1.76 

1.28

"a.'ii

0.10 

1.00

100. 20

6

59.35

1.00 

13.56 

'1.10 

4.75 

5.20

3.60 

1.77 

1.48 

4.45

0.31 

3.41

99.98

7

55.880

1.270 

21.849

"9.'633 

.155

1.495 

3.640 

.460

T794 

0.586

.022 

3.385 

.051

Trace

99.801

c2.89

8

55.06

"22."55

1.97 

5.96 

1.30

2.92 

3.82 

2.17

"4."35

100.10

2.78

9

60.68

.59 

21.20 

5.68 

.46 

1.71

.88 

3.64 

2.09

"~.'07

0.16

2.88

100.04

10

60.17

1.15

18.89

6.17 

.95 

1.75 

04

1.85 

2.76 

1.39 

1.04

0.20 

.11

03" 70

100. 17

11

56.71

.70 

14.43 

1.98 

3.65 

3.83 

.04

3.47 

2.61 

2.59 

3.71

.77

0.06 

.05

.12 

02.74 

2.64

100. 10

12

56.92

.90 

16.41 

.53 

3.52 

2.94

ftQ

3.14 

3.27 

1.47 

2.68 

1.54

0.16 

.09

.10 

o2.21 

3.97

99.88

13

57.23

.89 

20.43 

1.33 

5.64 

1.54 

.02

2.49 

2.39 

3.97

Trace 

0.05 

.54

.03 

3.94

100.09

14

63.06

.73 

18.03 

2.24 

4.07 

.81

2.21 

3.07 

1.51 

None

0.30 

.06

.09 

o3.62

99.80

2.83

o Combined water.

6 The specific gravity of the Festiniog black slates of Wales was determined as 2.751 at the physical 

laboratory, of Williams College in 1899. Since the publication of Dr. Hillebrand's note on the slates of 

New York and Vermont, Reade and Holland find as a mean of 13 Welsh slates Cr2O3 , 0.003; V2O5j 0.032; 

ZrO2 .0.022. See Liverpool Geol. Soc. Proc., 1900-1901, p. 101 and table.

c Specific gravity from Merriman.

i See U. S. Geol. Survey Bull. 242,1905.
CHEMISTRY OF SLATE. 51

1. Gray roofing slate, best quality, Delabole, Camelford, Cornwall; two analyses by J. A. Phillips 

London, Edinburgh, and Dublin Philos. Mag., 4th ser., No. 27, pp. 95-96, Feb.. 1871. '

2. Purple roofing slate, Fumay, Ardennes, northwest France; by A. Renard, Recherches sur la composition et la structure des phyllades ardennais; Mus. roy. hist. nat. Belgique Bull., vol. 1, p. 239,1882.

3. Green roofing slate beds from purple, Fumay, Ardennes, as above.

4. Blue-gray roofing slate, La Richolle quarry, Rimogne, Ardennes, northwest France; by Element; 

Renard, A. F., op. cit., p. 233.

5. Roofing slate (probablv black, Devonian), Westphalia; by H. von Dechen; Roth, J., Allgemeine 

und chemische Geologie, vol. 2, pp. 586, 587,1884. (No. 107.)

6. Roofingslate (color not given, Devonian). Frankenberg, near Goslar, in Prussia; by A. von Groddeck; 

Preuss. geol. Landesanstalt Jahrb. 1885-86; quoted by Roth, J., op. cit., pp. 586,587.

7. Black roofing slates (Peach Bottom) from J. Humphreys Co.'s quarry half a mile east of Delta, York 

County, Pa.; by Andrew S. McCreath, Second Geol. Survey Pennsylvania Rept. Prog., vol. CCC, pp. 

269,270,1880.

8. Bluish roofing slate of Carboniferous age, Mohradorf, near Wigstadl, Austrian Silesia; by Nikolic, 

Min. pet. Mitt., 1871, p. 207; quoted,by Roth, J., op. cit., pp. 588-589.

9. Blue slate, Glyn quarries, Llanberis, Wales; analysis made at Museum of Practical Geology, London, for Ge9rge Maw, Geol. Mag., vol. 5, p. 123, London, 1868.

10. Reddish roofing slate (best), Alexandra quarry, Moel Tryfaen, North Wales; by Reade and Holland, Liverpool Geol. Soc. Proc. 1899-1900. (No. 1.)'

11. Very dark banded roofing slate, Llansantfraid, Glyn Ceiriog, near Llangollen, North Wales; by 

Reade and Holland, op. cit. (No. 2.)

12. Very dark roofing slate, Moel Ferna. North Wales; by Reade and Holland, op. cit. (No. 3.) .

13. Green roofing slate from quarry between Camlyn and Cemmaes, Anglesey; by Reade and Holland, 

op. cit. (No. 4.)

14. Green roofing slate, Velenhelli, Wales; by Reade and Holland, op. cit. (No. 6.)

The 15 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.

Summarized analyses of roofing slates from New York, Vermont, and Pennsylvania.

Silica (SiOa)... .........:.....

Titanium dioxide (Ti02 ) 

Alumina (A1 2 03).............

Ferric oxide (FejOg) ..........

Lime (CaO). .................

Magnesia (MgO)............ .

Potassa(K2 0)................

Soda (Na2 0)..... .............

Carbon dioxide (CO2 ).....

Pvrito (FeSo)............. . . Water above 110° C. (H2O) . . . 

Carbon (C). ............... .

Sundry and water below 

110°C. ................... .

New York.

Bright 

green.

(1)0

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

Red. (4)

63.89 

.52 

11.80 

4.56 

J.33 

2.25 

4.57 

3.95 

.50 

3.15 

.02 

2.82

.77

100. 13 

2.796

Vermont.

Seagreen. 

(3)

63.33 

.73 

14.86 

1.12 

4.93 

1.20 

2.98 

4.06 

1.22 

1.41 

.11 

3.37

.69

100. 01 

2.776

Unfading 

green. 

(2)

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

Variegated (Eureka). (1)

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)

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

Black. 

(1)

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

Lehigh 

County 

Pa.

Black. 

(1)

56.38 

.78 

15.27 

61.67 

&3.23 

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

Figures in parentheses indicate the number of analyses averaged. t> Approximate.

If analysis K2 of red slate on page 92 is included with the four others, 

the Fe203 in the red slates would range from 3.48 to 7.10 per cent and 

would average 5.08. Comparing, then, the amount of Fe203 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.
52 SLATE IN THE UNITED STATES.

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 other 

slates.

There is less pyrite (FeS2 ) in the red and most in the black.

Dr. Hillebrand's discussion of the analyses of slates from New York 

and Vermont and of the causes of the discoloration of the sea-green 

slate is given on pages 55-59. While his 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 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.

Silica....................... 55-67

Alumina..................... 11-23

Ferric oxide................. 0. 52-7

Ferrous oxide................. 0. 46-9

Soda........................ 0. 50-3. 97

Magnesia.................... 0. 88-4. 57

Lime........................ 0. 33-5. 20

Water above 110° C........... 2.82-4.09

Potash........................ 1.76-5.27

For comparison a composite analysis of 51 American Paleozoic 

shales made by H. N. Stokes, of the United States Geological Survey,1 

is added, the unessential elements being omitted.

Composite analysis of 51 Paleozoic shales.

Soda.............:.............. 1. 01

Magnesia. ....................... 2. 32

Lime............................ 1. 41

Water above 110° C............... 3. 82

Silica........................... 60.15

Alumina......................... 16. 45

Ferric oxide..................... 4. 04

Ferrous oxide..................... 2. 90

Potash............................ 3. 60

As these figures come between the extremes of the slate analyses, 

and as the analyses of numerous clays and some schists do also,2 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 three out of the 29 analyses, and the high minima for com1 U. S. Geol. Survey Bull. 168, p. 17 (B), 1900.

2 For clays see Hies, Heinrich, Technology of the clay industry: U. S. Geol. Survey Sixteenth Ann! 

Kept., pt. 4, pp. 523-575,1895; also Prof. Paper 11, 1903. For schist, see Clarke, F. W., and Hillebrand, 

W. F., Analyses of rocks, etc.: U. S. Geol. Survey Bull. 148, p. 221 (L) and p. 99 (K), 1897.
CHEMISTRY OF SLATE. 53

bined water, many granites, syenites, porphyries, diorites, and 

basalts could pass chemically as roofing slate. It follows from the 

foregoing discussion that while considerable scientific interest attaches, to the chemistry of slate there is little correlation between its 

chemical composition and its physical properties. These depend 

primarily on its texture and secondarily on its mineral composition, 

and both of these are best determined by microscopic examination. 

That there should be 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 mineralogic 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 (p. 180).

CHEMICAL CHANGES IN 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; the black slate of 

Lehigh and Northampton counties, Pa., passes into a yellowish 

ocherous clay and finally into a white clay; and the black slate of 

Rockmart, Ga., passes into a bright yellow ocherous clay. These 

alterations will be referred to in the descriptions of 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 required tens of centuries.

G. P. Merrill l has investigated the weathering of the Peach Bottom 

slate and has 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 the lime, 28.16 per cent of the MgO, 

77.95 per cent of the K20, and 99.64 per cent of the Na2 0. All 

the A1203 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., Dr. Hillebrand made some partial analyses of a piece 

of black slate from a ledge near the Berks and Lehigh county line, 

1^ miles north-northwest of Rothrocksville, on the road-to Hynemansville, in which the stages of weathering were finely shown.

i Rocks, rock weathering, and soils, New York, 1897.
54 SLATE IN THE UNITED STATES.

Analysis A represents the black but slightly altered part of the slate; 

B 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 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. 108), 14 miles farther north-northeast, show a probable loss of 

4 per cent of CaO during a still earlier stage of weathering.

Analyses oj weathered slates from Pennsylvania.

Silica (Si02). . ................................................................

Ferric oxide (Fe203 ). ........................................................

Ferrous oxide (FeO) ........................................................

Lime (CaO). ................................................................

Soda (Na2O).. ..............................................................

Potassa (K20 ) .............................................................

Water (H2 0)c. ..............................................................

Titanium dioxide (TiO2 ). . ..................................................

Carbon dioxide (C02 ) .......................................................

Sulphur ( S) .................................................................

A

65.56

} 17.06

} 4.19

1.31

.20

.26

3.81

7.09

.68

None.

.02=.04FeS2

100. 18

B

68.81

16.44

3.14

1 00

.20

.27

4.32

5.33

.77

100. 28

a 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 parts, the carbonate 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 parts.

4. Muscovite (sericite) much more conspicuous in the light than 

in the dark part.

To the above data should be added two analyses, by Dr. Hillebrand, 

of white "shale clays/' which are regarded as the residual product 

of the weathering of the black slate.

Analyses of shale clays from Lehigh County, Pa.

Silica (Si02 ).. .......................................................................

Alumina ( A12 03 ) ....................................................................

Lime (CaO). ........................................................................

Soda (Na2O)................................................. .......................

Potassa (K2O) . .....................................................................

Titanium dioxide (TiO2) ............................................................

C

64.50

a21.67

1.83

1.09

.18

.20

4.26

C5.-65

.85

100. 23

D

75.77

615.30

.81

.20

Trace.

2.85

4.69

.36

99.98

a Including P2O5 . 6 With trace of Fe2O3 and (perhaps) P2O5 . « Ignition loss.

C. White "shale clay" quarried for fire clay half a mile south of Fogelsvillc, Upper Macungie, Lehigh 

County, Pa.

D. White "shale clay" associated with limonite one-fourth mile northwest of Guth station, South 

Whitehall, Lehigh County, Pa.
CHEMISTRY OF SLATE. 55

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 f eldspath'ic 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 differs from that in which the slate must have originated, 

in some places 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.

CHEMICAL COMPOSITION OP 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 undimmed; 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 very soluble condition. Even when carbonates 

were absent, a large proportion of it dissolved in moderately dilute
56 SLATE IN THE UNITED STATES.

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/,1 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 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 calcite, 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 cf CO2 uniformly 

accompanies the lowest percentages of CaO, and, that increase of 

CaO carries with it an approximately corresponding increase of 

CO2 . The CO2 is usually in molecular excess of the CaO, and in some 

slates of the MnO and FeO as well (compare 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 MnCO3 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, the slates high in C02 show a relatively high MnO content. 

Analysis 201^ shows also that manganese carbonate is found as a 

vein formation in the slates, and Mr. 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 FeCO3 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 abovementioned 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

1 Numbers refer to specimens whose analyses are given in the local descriptions.
CHEMISTRY oi? SLATE. 57

absent. The needed proof, however, is supplied by the microscope, 

as detailed in the text of Mr. Dale's report (p. 140).

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, for the original composition of the slateforming materials must have differed in each locality. This being 

so, it follows that a mere test as to the relative amount of C02 can 

furnish no guide in advance as to tne fading qualities of a slate, as it 

might were the composition of the mixed carbonates always constant.

Though pyrite is never a visible constituent of the red slates, its 

presence seems necessary to account for the sulphur found, for careful tests failed to show an equivalent amount by far of S03 . Hence

pyrite is reported in all the analyses.

The phenomenal percentage of barium in 20le (K), as compared 

with other slates, has been of service, as showing that, in this slate at 

least, it is mainly if not altogether a constituent of some silicate or 

silicates and not of barite only, which, according to Mr. Dale, has been 

observed on joint planes and in crystalline concretions, for the sul- 

'phur is totally inadequate to form barite with all the barium.

The condition of the nickel and cobalt has not been definitely ascertained. From the analysis of the manganese carbonate 20ld 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 with arsenic and sulphur, as arsenides and sulpharsenideSj 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 test their combined percentage 

as oxides, probably a maximum, was 0.025.

Heating of the slates in tubes closed at one end gave without exception alkaline vapors and whitish sublimates, and the latter reacted 

for S03 , Cl, and strongly for NH3 . In eight tests' the amount of 

ammonia thus given off in both conditions was ascertained by nesslerization and found to range from 0.0075 per cent for 760a (H) to 0.04 

per cent for 305d! (P). The latter 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 NH3 , 

or 314 of 1895 (I); with 0.03 per cent NH3 .

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
58 SLATE IN THE UNITED STATES.

rocks and minerals. It seems probable that its presence in these 

slates is to be attributed to 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 230& (A) and the red slates 201e (K) and 397e (L) were examined. 

In 2010 there was found 0.017 per cent of vanadium as V205 and 

0.007 per cent of chromium as Cr203 . A mixture of equal parts of 

the two red slates was found to contain 0.008 per cent V205 and 

about as much Cr203 as in 201e.

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 358^ (J) and 397e (L). Yet, after deducting all CaO as phosphate and carbonate, all MnO as carbonate, and enough MgO as 

carbonate to satisfy the remainder of the C02 , also the trace of 

pyrite, amounting to 16.58 per cent in all, and calculating the remaining 83.80 per cent to 100, we obtain the figures given in the first 

column below, for comparison with which analyses 358o5 (J) and 

20 le (K) of slates naturally altogether or nearly free from carbonates 

are partly reproduced:

8iOt. ...............

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

A1 203 . ..............

FesOa. ..............

FeO ................

BaO. ...............

MgO................

397c (L).

67.40

.57

13.84

4.15

1.70

.07

3.34

358d (J).

67.61

.56

IQ on

5.36

1.20

.04

3.20

201e(K).

67.55

.58

12.59

5.61

.31

3.27

K^O. ........ ... .

NU20......

H2O above 110° C..

397e (L).

4.50

' CO

3,37

inn nn

358d (J).

4 4T

.45

2.97

99 71

201<KK).

4 11

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 K, 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

i 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, p. 209,1898.
CHEMISTEY OF SLATE. 59

into special prominence their relatively high silica contents as compared with all the other slates analyzed:

SiOs. ...............

Tios. .............. .

AlsOs. ..............

FQ^OS. ..............

FeO. ...............

BaO. ...............

MgO.. ..............

397<i (R).

76.04

.60

10.90

1.27

1.23

.06

2.75

397o (Q).

73.64

.58

11.66

2.04

1 3fi

.05

3.25

397a (M).

71.59

.53

-10. 95

4.33

1.61

.05

3.77

K^O. ..............

NasO.. ............

H2Obp.lowllO°C..

397a (R).

.26

9Q

2.45

100. 00

397a (Q).

.26

99

99.99

397o (M).

3

Q7

.22

.30

2 7ft

100.00

The foregoing calculations for 397e 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:

CaCOa...............

MgCO ...............

MiiCOs..............

397e (L).

Percentage.

8.95 

6.93 

.48

16.38

Molecular ratio.

1.03 

| 1.00

397a (R).

Percentage.

7.93 

5.36 

.57

13.86

Molecular ratio.

1.15 

| 1.00

397a (Q).

Percentage.

 7.11 

4.77 

.47

12.35

Molecular ratio.

1.16 

| 1.00

397o (M).

Percentage.

6.14 

4.22 

.38

10.74

Molecular ratio.

1.15 

} 1.00

If, as L 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.
PART II. SLATE DEPOSITS OF THE UNITED STATES.

In Part II of this paper each State is taken up in alphabetic 

order. In the description of a deposit that has not been especially 

examined by the writer or some other geologist the result of the 

writer's microscopic examination is accompanied by such geologic 

data as could be found in the geologic literature of the region.

ARIZONA.

A specimen of roofing slate received from Arizona came from a 

deposit reported as about 6| miles north of Phoenix and said to 

measure 800 to 1,000 feet in width and about 5,000 feet in length. 

. Lee x refers to the geology of this locality as follows:

In the Phoenix Mountains, immediately north of the city of Phoenix, occurs an 

extensive series of metamorphosed sediments, many thousands of feet in thickness. 

The layers rest in a nearly vertical position.

Lee's map shows areas of Algonkian rocks along the foot of the 

Phoenix Mountains at points ranging from 6 miles N. 25° E. to 7 

miles north of Phoenix.

The examination of the specimen yielded the following results:

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 set 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 fissllity.

Under the microscope this slate shows a matrix 2 consisting of muscovite (sericite) 

with a brilliant aggregate polarization. 2 Cleavage and bedding foliations are parallel but are crossed by two incipient slip cleavages (see p. 37 and Pis. VII, XI, A), 

the plications of which have not resul-ted 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 quartz, which appears to be.mostly secondary. The conspicuous feature, however, is the greaf. abundance of lenses and 

particles of pyrite and hematite. In the cross sections these measure from 0.0043 to 

0.043 millimeter in length by 0.002 to 0.008 millimeter in width and number about 

800 per square millimeter. 3 In parallel section they have irregular angular-roundish 

outlines.

There are also large plates of muscovite, a few crystals of tourmaline, a few lenses 

of decomposed crystals of an undetermined mineral up to 0.0425 by 0.025 to 0.075

1 Lee, W. T., Underground waters of Salt River valley, Arizona; U. S. Geol. Survey Water-Supply 

Paper 136, p. 96, Pis. VII, VIII, 1905.

2 For definitions of technical terms see Glossary, pp. 213-215.

s The square millimeter referred to comprises the thickness of the thin section.

60
ARKANSAS. 61

millimeter, and considerable rutile in very minute prisms. No carbonate was 

detected.

The constituents of this slate, arranged in descending order of abundance, appear 

to be muscovite, quartz, pyrite, hematite, kaolin, magnetite, rutile, and tourmalineIf properly cut with reference to the direction of weakness this may prove a serviceable slate, but slip cleavage is generally objectionable. In this slate the slip 

cleavages are, however, only incipient.

AKKANSAS.

TOPOGRAPHY, GEOLOGY, AND CHARACTER OF DEPOSITS. 1

The slate area of Arkansas covers part of the Ouachita Mountains, 

which lie south of Arkansas River and have a general east-west trend. 

The area extends from the vicinity of Little Rock about 100 miles 

westward nearly to Mena, and has an average width of 15 miles. The 

rocks include shales, slates, chert, novaculite (a deposit of extremely 

fine quartz grains), sandstone, and a little limestone. The entire 

sedimentary series is estimated at 11,400 feet in thickness, of which 

the upper 8,825 feet contains some commercial slate, as shown in the 

table below:

Section in Arkansas showing the relations of slates.

Carboniferous:

Stanley shale: Greenish clay shale, locally black slate near Feet, 

the base, and greenish quartzitic sandstone ............. 6, 000

Unconformity. 

Age unknown:

Fork Mountain slate: Gray slate with thin beds of siliceous 

material.............................................. 125

Arkansas novaculite: Massive white and variegated novaculite with alternating flint and shale layers in the upper 

half................................................... 800

Missouri Mountain slate: Mainly red slate with green slate in 

basal part............................................. 75-300

Ordovician:

Blaylock sandstone: Greenish quartzitic sandstone alternating with brownish-black shale........................ 0-1, 5.00

Polk Creek shale: Black fissile and sandy graptolitic shale.. 07-100 

Bigfork chert. 

8,825

The areal distribution of these formations is shown on the geologic 

map (PI. Ill) of Purdue's report on the slates published by the 

Geological Survey of Arkansas in 1909. The general situation of the 

slate district is shown in E. C. EckeFs map (Plate IX of this bulletin).

These strata He in parallel close folds with the east-west axes which 

pitch alternately but always together east and west. The folds are 

either erect or overturned to the north or south. The peculiar

i Summary from Purdue, A. H., The slates of Arkansas: U. S. Geol. Survey Bull. 430, pp. 317-334,1910.
62 SLATE IN THE UNITED STATES.

topography of the region is due to these parallel pitching folds and to 

the unequal resistance offered to erosion by the soft shales and slates 

and by the hard quartzose beds.

Slate of possible economic value occurs in the four formations 

mentioned below.

The Polk Creek shale passes here and there into a black graptolitic 

sonorous slate, in places banded. The smallness of the amount of 

slate and the abundance of joints throughout the formation make 

the deposit of doubtful economic value.

The Missouri Mountain slate has been extensively prospected and 

is now quarried at Slatington. Its structure on Fork and Missouri 

mountains is shown in figure 4. It produces both red and green slate, 

the former predominating. In some places the cleavage is parallel

to the bedding; in others it is 

oblique. The slate is defective in sonorousness and in 

many places shows two sets 

of slip cleavage.

This slate is used for mill 

stock, particularly laundry

FIGURE 4. North-south section through Fork Mountain, tubs, lavatories, WainSCOtmg,

Ark., T. 3 S., R. 28 W., showing the folded structure of an(J electric Switchboards.

the Missouri Mountain slate, a, Bigfork chert; 6, Polk ~, , ,. . .

Creek shale; c, Missouri Mountain slate; d, Arkansas J-£e results 01 tests 01 ItS

novacul'ite; e, Stanley shale. From A. H. Purdue, U. S. electric Conductivity are

Geol. Survey Bull. 430, fig. 6,1910. , , T ,, ^

given beyond. In the preparation of millstock there is a certain loss from worked pieces cracking either along or at right angles to the cleavage.

The Fork Mountain slate consists of a hard slate, generally gray 

but weathering in places green or chocolate color and containing 

many thin sandy beds. The slate usually has a good cleavage and is 

ribboned, highly sonorous, strong, and tough. So far as prospected 

it is too much jointed to be of economic value.

The Stanley shale is almost everywhere a shale, rarely a slate. It 

has been extensively prospected near Slatington and in the southwestern part of Polk County. The color of the slate is blue to black 

and its cleavage remarkably fine, with very smooth cleavage surfaces. 

It is not regarded as sufficiently durable for roofing. Two quarries 

have been opened in it.

CHEMICAL COMPOSITION OF THE SLATES OF ARKANSAS.

The following analyses of slates from Arkansas, published in E. C. 

Eckel's note on slates of Arkansas in Bulletin 275, are fairly representative of the composition of some of these slates:
U. 8. GEOLOGICAL SURVEY BULLETIN 886 PLATE IX

(JOHNSON.J

RANKLINj Clarksville \

Scale of miles

35 50 75

MAP SHOWING SLATE AREA OF ARKANSAS. 

By E. C. Eckel.
ARKANSAS.

Analyses of slates from Montgomery County, Ark. 

.[Analyst, W. G. Waring.]

63

Silica (SiOj) ..................................

Ferrous oxide (FeO) . .......:................

Lime (CaO )..................................

Potash ( KjO). . ..............................

Soda (Na2O) .................................

Carbon (C )...................................

Water (H2O) .................................

i

66.16 

8.62 

9.04 

3.44 

1.77 

.78 

4.96 

.64 

.08 

.02 

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 

1.30 

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 

6.22

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

1.75 

1.52 

1.43 

.70 

.05 

.03 

3.70 

.01 

1.01

1. Green slate, quarry No. 2. 5. Green slate from State House Cove. 

2. Red slate, quarry No. 2. 6. Black slate from west end of Crooked Creek vein. 

3. Red slate from State House Cove. 7. Black slate from Crooked Creek Falls. 

4. Hard red slate from State House Cove.

MICROSCOPIC ANALYSES OF SLATES FROM ARKANSAS.

The microscopic analyses of slates from Arkansas published in 1904 1 

are here repeated and supplemented by several others of specimens 

collected by Mr. Eckel during his visit to the quarries of that State 

in 1905. All have been revised.

Black slate from Mena, near Big Fork. 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 millimeter 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 muecovite, carbonaceous or graphitic matter, quartz, pyrite, magnetite, and rutile.

This is a mica 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 Fork. 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 millimeter, muscovite and chlorite scales, and abundant hematite in minute 

dots. There are also rhombs, from 0.1 to 0.2 millimeter, of chlorite and rhodochrosite, 

probably pseudomorphs. No other carbonate.

The constituents, arranged in descending order of abundance, appear to be muscovite, hematite, kaolin, quartz, chlorite, rhodochrosite (?), and magnetite.

This slate compares favorably in texture with the "red" slate of New York.

Reddish slate from the Missouri Mountain slate ('' Mammoth red " and'' Lost Hannah " 

of quarrymen; exact locality not given). Color lighter than above, but not quite

»U. S. Geol. Survey Bull. 225, p. 414,
64 SLATE IN" THE UNITED STATES.

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 millimeter, muscovite and chlorite 

scales, abundant hematite pigment, and no carbonate.

The constituents, arranged in descending order of abundance, appear to be muscovite, 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 13° 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 millimeter, and has 

a slight argillaceous odor.

The constituents, arranged in descending order of abundance, appear to be muscovite, quartz, kaolin, chlorite, and magnetite.

The two extra foliations are likely to prove directions of weakness.

Light-greenish slate from the Missouri Mountain slate ("Mammoth red"), 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 millimeter. Rutile 

needles abound from 0.0028 by 0.0009 up to 0.014 by 0.0014 millimeter. Muscovite and 

chlorite scales occur, the latter producing the green color. There are some opaque 

granules (limonite? and pyrite), occasional lenses, 0.14 millimeter long, of a central 

mass (probably rhodochrosite), with secondary muscovite at both ends. No other carbonate. Shows a number of tourmaline prisms up to 0.025 by 0.008 millimeter.

The chief constituents, arranged in descending order of abundance, appear to be 

muscovite, quartz, kaolin, chlorite, rutile, pyrite, magnetite,*and tourmaline.

This mica slate probably possesses more petrographic interest than economic value. 

Its fissility, freedom from carbonate, and color are very favorable, but it will probably 

be found too delicate for use on a roof.

Very dark bluish-gray slate from sec. 25, T. 3 S., R. 29 W. (Specimen collected by 

E. C. Eckel.) To the unaided eye this slate has a fine texture and a smooth cleavage 

surface with a little luster, and shows a little pyrite on the sawn edge. It contains 

very little magnetite, considerable carbonaceous or graphitic matter, does 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 millimeter. Pyrite spherules, 

measuring up to 0.008 millimeter in diameter, number about 120 per square millimeter. 

There are rutile needles but no carbonate.
CALIFORNIA. 65

The chief constituents, arranged in order of decreasing abundance, appear to be 

muscovite, 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 the black slate from Mena (p. 63), but may rjrove more durable.

Light-gray slate with a slightly greenish tinge from sec. 30, T. 3 S., R. 28. W. (Specimen collected by E. C. Eckel.) To the unaided eye this has a fine texture, but a 

lusterless, roughish surface, and shows a little pyrite on 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 millimeter, scales of chlorite and of muscovite transverse to cleavage, some 

pyrite cubes up to 0.063 millimeter (generally with a rim of secondary quartz), passing 

into lirnonite and staining the matrix. Rutile needles abound.

The chief constituents, arranged in decreasing order of abundance, appear to be 

muscovite, quartz, chlorite, kaolin, pyrite, rutile, and lirnonite, 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 Co. (Specimen collected by 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 with 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, up to 0.27 by 0.17 millimeter; also a few of plagioclase feldspar, scales of muscovite and biotite up to 0.2 by 0.1 millimeter, some lenses of carbonate up to 0.4 millimeter long, a few grains of tourmaline, and lenses of secondary 

quartz. There is a faint incipient slip 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.

PHYSICAL AND ELECTRIC TESTS.

The results of elaborate physical and electric tests of some of these 

slates, made for Purdue's report, will be found in full in Part III, 

pages 184-187.

CALIFORNIA.

ELDORADO COUNTY.

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 slate-producing area in the State is located in 

40620° Bull. 586 14 5
66 SLATE IN THE UNITED STATES.

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 ill a belt of 

the Mariposa slate, of Jurassic age. The quarries which have been 

opened are all situated near the western boundary of this belt of 

Mariposa slate where it is bordered by a large area of diabase. This 

diabase, according to Lindgren and Turner, is "of the age of the 

Mariposa slate, or older." A number of linear areas of amphibolite 

occur in the Mariposa slate. These ampbibolites 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 * 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 slateproducing areas are shown on Plate X.

Structural relations in Eureka quarry. At present the most 

important quarry is that of the Eureka Slate Co., and this is now being 

worked on a large scale. This quarry is at Slatington, about half a 

mile southwest of Kelsey. The cleavage planes of the slates 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 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 numerous but narrow "ribbons." 

These ribbons are bands (from one-sixteenth to one-half inch thick

1 U. S. Geol. Survey (Jeol. Atlas, Placerville folio (No. 3), 1894.
U. 8. GEOLOGICAL SURVEY BULLETIN 586 PLATE X

120 50

120 50

GEOLOGIC MAP OF PLACEEV1LLE SLATE DISTRICT, CALIFORNIA.

Scale 

0______1_______2______3______4. miles

Mariposa slates Basic igneous rocks Slate quarries 

From U. S. Geol. Survey Geol. Atlas, folio 3.

"Line of cable
CALIFORNIA. 67

usually, but locally 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 in 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 seems 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 70°-80° NW. Joints across 

the grain of 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 

some 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, mucji like that of the slates 

of Lehigh and Northampton counties, Pa. The abundance 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 W. T. Schaller, follow:

Analysis of black slate from Eureka quarry, Slatington, Cal.

Silica (Si02)................................................... 63. 52

Alumina (A1203) and titanic oxide (Ti02)....................... 16. 34

Iron oxides (FeO, Fe203)...................................... 6. 79

Lime (CaO).................................................... .98

Magnesia (MgO).................................................. 2.50

Carbon dioxide (C02)........................................... "i

Water........................................................./ 4' 86

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 miscroscope 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 3 millimeters long by about 0.5 millimeter 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
68 SLATE IN THE UNITED STATES.

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 millimeter, 

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, 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.

i Green slate of Eureka quarry. 1 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 slate. 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 t considered. Two analyses of the green slates 

are presented below. The first is of a sample selected by the 

writer and analyzed in the laboratory to the United States Geological Survey by 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 slate, and that their average, 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 igneous green slate from Eureka quarry, Slatington, Cal.

Silica (Si02)... .................................................................

Iron oxides (FeO, F2O3 ) .................... . r ..................................

Sulphur (S)................................ ....................................

Alkalies (K2O, Na2O) . .........................................................

Water (H2 0) ..................................................................

1

45.15

16.33

8.42

6.42

8.72

W («)

| 11.28

2

47.30

15.53

8.00

7.83

7.86

19

3.17

9.92

Average.

4fi 99

11 CM

8.21

7 19

8 9Q

12

3.17

10.60

a Not determined.

These analyses have been compared 2 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 certain "basic"

1 For a more detailed discussion of this interesting "igneous slate" the reader is referred to a paper by 

the writer in the Journal of Geology, vol. 12, pp. 15-29,1904. 

? Jour. Geology, vol. 12, p. 26,1904. o
CALIFORNIA. 69

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.

CJiili Bar Slate Co.'s guarry. 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. It is the oldest quarry in the district, having been opened about 28 years ago. It has been shut 

down 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 and consists of 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 areas available near by. 

Neither quarry has gone deep enough to get really good slate, which 

might have been found at a greater depth.

San Francisco Slate Go's guarry. This quarry is located in T. 11 N., 

R. 10 E., within a quarter of a mile of that of the Chili Bar Slate Co., 

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 yard, which was 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.

Transportation and market. The slates of Eldorado County have 

practically no competition on or near the Pacific coast, and the 

Eureka quarry has 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-mile stretch of very hilly road. 

In 1905, however, the Eureka Slate Co. installed an aerial tramway
70 SLATE IN THE UNITED STATES.

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.

MERCED COUNTY.

A slate deposit was discovered in 1912 about 11 miles east of 

Planada, probably the Mariposa slate and of Jurassic age.1 It is 

reported as 1,500 feet in width at the surf ace, 3 miles in length, and 

300 feet in vertical thickness. The California Slate Co., of Planada, 

has commenced to develop the property. A specimen of the slate 

examined by the writer yields these results:

This slate is of dark bluish-gray color, with a somewhat lustrous, very finely granular surface with minute striations along the grain. Its fissility is good. It is very 

sonorous, indicating a high degree of metamorphism, does not effervesce with cold 

dilute hydrochloric acid nor show any carbonate in thin section, indicating durability 

of color. It is graphitic and contains many bronze-colored magnetic particles. 

Magnetite is not certainly present.

Under the microscope it shows a matrix of muscovite (sericite) very even in the 

grain direction with brilliant aggregate polarization, also conspicuous lenses with a 

nucleus of black mica (biotite). The biotite scales lie mostly parallel to the grain 

direction and the lenses, partly biotite and partly quartz, are elongated in the grain 

direction, but the biotite nucleus is crossed by parallel bands of dark inclusions that 

lie in the cleavage direction. It also contains many bronze-colored metallic particles 

and minute lenses of a highly magnetic mineral, presumably pyrrhotite (magnetic 

pyrites). The chief constituents of this slate, named in descending order of abundance, appear to be muscovite, quartz, biotite, kaolin, graphite, and pyrrhotite. 

Magnetite and rutile are not certainly present.

This is a mica slate of sedimentary origin, free from carbonate, with good cleavage. 

It should discolor but little if at all, but its content of magnetic particles makes it 

unsuitable for electric uses.

COLORADO.

The Colorado Slate Co. opened a quarry in 1910 in a deposit of 

black slate at Marble} in Gunnison County. The age of these slates 

is probably Ordovician. A specimen from this quarry examined by 

the writer yielded these results:

The slate has an extremely dark bluish-gray color. To the unaided eye its texture 

is fine, with a slightly uneven lusterless cleavage surface. It contains carbonaceous 

matter and extremely little magnetite. The sawn edge shows some pyrite. It 

effervesces with cold dilute hydrochloric acid, is sonorous, and has medium fissility.

Under the microscope it shows a matrix of muscovite (sericite) with feeble aggregate 

polarization. The cleavage is rendered uneven by alternating beds of fine and 

coarse particles, which are parallel to the cleavage. Carbonate is very abundant 

and helps to obscure the aggregate polarization. Pyrite is plentiful in spherules 

and irregular particles up to 0.16 millimeter across. Opaque carbonaceous matter 

also abounds, and there are several forms of organic origin. Rutile needles are very 

scarce or absent. Quartz grains measure up to 0.03 millimeter across.

i See Turner, H. W., and Ransome, F. L., U. S. Geol. Survey Geol. Atlas, Sonora folio (No. 41), p. 2, 

1897.
GEOEGIA. 71

The constituents of this slate, named in descending order of abundance, appear 

to be muscovite, carbonate, quartz, carbon, kaolin, pyrite, chlorite, and rutile (?).

As shown by the defective aggregate polarization and confirmed by the scarcity of 

slate needles, this is an incompletely formed mica slate and it also belongs to the 

fading series. Its strength shSuld be tested.

GEORGIA.

Roofing slates have been quarried in Bartow and Polk counties, 

in the northeastern part of Georgia.

BARTOW COUNTY.

Bolivar. The Georgia Green Slate Co., of Fairmount, Ga., in 1910, 

opened a quarry in the northern part of Bartow County about 15 

miles north-northeast of Cartersville and 3 miles northeast of Pinelog 

village, near Bolivar station of the Louisville & Nashville Railroad. 

The geologic age of the t?late is as yet undetermined. It may be 

Cambrian.

The quarry lies in a hollow trending N. 60° W. along the south foot 

of a ridge about half a mile from the railroad. The opening is 175 by 

50 to 100 feet in width, and in June, 1912, was idle and full of water. 

On the N. 60° W. side slate wag exposed for 40 feet above the water 

and on the S. 60° E. side for 25 feet. A* the hollow is nairow and the 

quarry was opened in the brook bed or close to it, the excavation 

soon fills.

The cleavage foliation strikes N. 30° E. and dips 5°-25° S. 60° E. 

The cleavage faces in the "top" are very limonitic and the region is 

thickly covered with rusty residual clays. The exact course of bedding could not be made out, but blocks on the dumps and the finished 

slates show ribbons intersecting the cleavage at 20°. Joints strike 

N. 63° E. and form a heading 15 feet wide at one end of the quarry, 

dipping 65° N. 27° W. Some of the slates show close joints filled 

with chlorite dislocating the ribbon but not affecting the cleavage. 

(See p. 39.)

The slate is of light blue-greenish gray color with some dark bluish-green ribbons, 

streaks, and lenses. It has a smooth to roughish, slightly lustrous surface, shows a 

little pyrite on the sawn edge, and contains extremely little magnetite. It effervesces 

slightly with acid test, is rather sonorous, and has a fair grade of fissility. The ribbons 

are somewhat calcareous.

Under the microscope it shows a matrix of muscovite (sericite) with marked aggregate polarization and a fine and regular cleavage, also abundant rutile needles. It 

contains a little carbonate in fine particles or rhombs and a Little pyrite in spherules 

and irregular grains. There are lenses of chlorite up to 0.34 by 0.009 millimeter. The 

chlorite scales measure as much as 0.009 millimeter and the quartz grains 0.047 millimeter.

The constituents, named in descending order of abundance, appear to be muscovite, 

quartz, chlorite, carbonate, pyrite, magnetite, and rutile.

This is a mica slate with a fair fissility and attractive color. Although the amount 

of carbonate is not so large as in the sea-green slate of Vermont (p. 140) it is probably
72 SLATE IN THE UNITED STATES.

sufficient to produce in time some discoloration. The slates about the quarry, exposed 

not over two years, show but slight discoloration. 

The following analysis of slate from Bolivar by Dr. Edgar Everhart, 

chemist of the Geological Survey of Georgia, is here give a for reference;

Analysis of slate from Bolivar, Bartow County, Ga.

Silica (Si02)................................................ 56.02

Titanium dioxide (Ti02)..................................... .74

Alumina (A1203 )............................................. 21. 61

Ferric oxide (Fe203 )......................................... 1. 36

Ferrous oxide (FeO)......................................... 5. 97

Manganous oxide (MnO)..................................... .05

Lime (CaO)................................................. 1. 22

Magnesia (MgO)............................................. 2. 96

Potash (K20)................................................ 2. 83

Soda (Na20)................................................ 1. 26

Water(H20)................................................ .14

Phosphorus pentoxide (P205)................................. .02

Carbon dioxide (C02)........................................ .91

Sulphur (S)................................................. .35

Loss on ignition, less carbonic acid (CO)....................... 4. 76

100. 20 

Less oxygen. ..............................^.................. .09

99.11 

POLK COUNTY.

Rockmart. The Rockmart slate, which belongs to the Ordovician 

period, was described by Haye? l in 1902 in these words:

The Rockmart 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 black slate, 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 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 finegrained 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,

1 Hayes, C. W., U. S. Geol. Survey Geol. Atlas, Rome folio (No. 78), 1902.
GEOEGIA. 73

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.

East and southeast of the village of Rockmart, on Euharlee Creek, 

is an east-northeast ridge of slate rising 100 to 200 feet above the 

creek, or 900 feet above sea level. The prevalent strike of the beds 

is N. 20°-40° E. The limestone which underlies the slate occupies 

the valley on the northwest and crops out in contact with the slate 

at the disused easternmost quarry. The slate is covered in places 

with 20 to 30 feet of weathered slate and this is overlain by 5 to 10 

feet of reddish clay. Within this weathered zone all transitions from 

a black mica slate to a bright-yellow Ocher can be studied. In an 

early stage of decarbonization the slate consists of alternating ribbons of grayish slate from 0.02 to 0.2 inch thick and of a buff-colored 

material, and both are crossed diagonally by a pronounced slaty 

cleavage. In a later stage of alteration the little beds can still be 

traced by pale-yellow bands in the bright-yellow clay, which still 

splits most readily in the direction of the original cleavage. In the 

final stage the slate has passed into an almost homogeneous structureless yellow ocherous clay.

The most important economic consideration suggested by the 

stratigraphy is that as the slate is limited in depth by the underlying 

limestone and in places in height by the zone of residual ocher the 

actual thickness of workable slate can not be very considerable. 

It should 3e carefully determined at any point before commencing 

operations involving large expenditures.

The Rockmart Brick & Slate Co. is engaged in manufacturing 

shale brick out of the weathered slate with the possibility, when the 

supply of ocher fails, of quarrying the underlying fresh slate for 

roofing.

The slate has been extensively quarried for roofing on the southeast side of the ridge, but the quarries are now disused. Pritchard 

& Davis, however, in 1912, reopened a small quarry between the 

Hood and Southern Slates Co. quarries, in a small northwest-southeast 

ravine. The opening is 20 by 25 feet by 10 feet deep and the "top" 

consists of 2 feet of red clay and soil. The beds strike about N. 40° E. 

and dip 20°-25° S. 50° E. The ribbons are wide and in places 

extremely plicated, as explained on page 32 and shown in figure 1. 

The thickness between ribbons in the Rockmart quarries is generally 

reported as from 2 to 5 feet. The cleavage strikes with the bed but 

dips 40°-45° S. 50° E. Joints strike N. 10° E. and N. 75° W., dipping steeply or vertical, both sets being discontinuous and spaced 15' 

to 18 feet.

The slate is of very dark bluish-gray color arid of slightly roughish, lusterless surface.' 

It is sonorous and effervesces with acid test. The sawn face shows minute 'magnetite;
74 SLATE IN THE UNITED STATES.

the powder shows very little magnetite but much carbon and effervesces very freely 

with acid test.

Under the microscope aggregate polarization is somewhat obscured by carbonaceous 

matter. The general texture is lenticular and somewhat coarse, the quartz grains 

measuring up to 0.047 millimeter. Small beds down to 0.12 millimeter thick (in the 

slates of fig. 1 to 0.1 inch), with quartz grains to 0.09 millimeter and much carbonate, 

intersect the cleavage at 14°. The matrix contains much fine carbon, much carbonate, 

chlorite scales interleaved with muscovite, abundant spherules and grains of pyrite, 

and rutile needles. The section parallel to cleavage is crowded with carbonate.

The constituents, named in descending order of abundance, appear to be mus-. 

covite, quartz (detrital), carbonate, carbon, chlorite, kaolin, pyrite, magnetite, rutile, 

and plagioclase (detrital).

This is a mica slate of the fading series, although some obtained from a tunnel at 

one of the quarries is reported to have kept its color for many years. Its fissility is fair.

MAINE.

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. 5.) Commercial 

slate occurs also in the town of Forks, Somerset County.

Geologic relations. The slate occurs in a belt consisting largely 

of slaty rocks, represented by Hitchcock as 15 to 20 miles wide 

and extending from Kennebec River, between Bingham and Dead 

rivers, northeastward to the sources of Mattawamkeag River. The 

rocks are probably of early Paleozoic age.1 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. and the dip 80°ESE. Near Blanchard and Piscataquis River the 

strike is N. 55°-62° 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 to the north-northwest. Within 1£ miles 

southwest of Monson the strike is N. 60° E. and the dip 90°. A 

mile south the strike is N. 63° E. and the dip still 90°. At Monson 

the strike is N. 47°-54° E. and the dip 80° SE.; but at Brownville, 20 

miles east of Monson, the strike is N. 78° E. and the dip 75° NNW. 

As the grain is horizontal at Brownville and at points 3£ miles westsouthwest 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.

Monson. In 1912 three companies were operating quarries in the 

township of Monson. The Monson Maine Slate Co. was operating two

i See Hitchcock, C. H., The geology of northern New England, 1886; also his Preliminary report on the 

natural history and geology of Maine, including geologic map of northern Maine, pt. 1, pp. 316,319,1861, 

and Second Ann. Kept., pt. 2, pp. 280, 282, 360,1862.
MAINE. 75

quarries east-southeast of its Pond quarry; the Maine Slate Co. of 

Monson its Mathews quarry, about 3$ miles west northwest of the 

village; and the Portland-Monson Slate Co. four narrow openings 

about a mile south of the village.

At the Monson Pond quarry the following series is exposed: Fifteen 

beds of slate, measuring altogether from 79 feet to 93 feet 6 inches, 

alternating with fifteen beds of dark-gray or black quartzite ("hards"), 

measuring altogether from 48 feet 5 inches to 49 feet 5 inches, both 

slate and quartzite aggregating from about 127 to about 142 feet. The 

deposit has been prospected for 200 feet farther southeast, but the

19 miles

FIGURE 5. Map of slate region in Maine. From post-route map. The chief quarry^g centers are shown

by crossed hammers.

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 

section at the north edge of the quarry are given.
76 SLATE IN THE UNITED STATES.

Section at Monson Pond quarry, Monson, Maine. 

 [Furnished by the courtesy of Mr. F. H. Crane, superintendent.]

Slate. .............

Quartzite..........

Slate. ............

Quartzite..........

Slate. ............

Quartzite..........

Slate. ............

Slate.. ...........

Slate. ............

Slate.. ..:........

Slate.............

Slate. ............

Ft.

........ 7-18

........ 1

........ 1

........ 2

........ 4

......... 1

........ 4

........ 4

........ 15-18

......... 4

......... 5

in. 

0 

6 

8 

2-8 

10 

8 

0 

3 

0 

6 

< 0 

8 

0 

6 

0 

0 

0

Quartzite............

Slate. ..............

Quartzite............

Slate. ...............

Slate...............

Slate. ..............

Quartzite............

Slate. ..............

Slate...............

Ft. in. ' 

........ 1 4.

........ 7-8 0

........ 4 0

........ 3 0

........ 1 2-4

........ 3 0

........ - 8-12

........ 1 0

........ 20 0

........ 7 0

........ 2 0

........ 6 0

11 0

, Total slate, 79-93 feet 6 inches. 

Total quartzite, 48 feet 5 inches to 49 

feet 5 inches.

There is in places a transition from the quartzite to the slate, a 

quartzitic slate intervening. In the above section such beds are 

classified as quartzite.

The Pond quarry, now abandoned, measures about 500 feet along 

the strike and nearly 100 feet across at the top, and from 250 to 400 

.feet in depth. The beds strike N. 47° E. and dip 78°-80° SE., without any indication of turning, and the cleavage strikes N. 45° E. 

and 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 numerous 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., many of which

are veined with quartz. There are also vertical diagonal joints 

striking about northwest and thus parallel to the grain. The northeast half of the quarry is much broken up by diagonal jointing and 

faulting, but hi the southwest half conditions are more normal, 

although veining is there more common. The difference between the 

jointing of the quartzite and the slate results from the difference in 

their rigidity. Then* behavior under the same stress must needs have 

been very dissimilar. Some of the quartz veins traversing the slate 

contain biotite, chlorite, and a little calcite. The surface of the formation is glaciated and covered with 5 to 10 feet of glacial clay and 

pebbles.

In 1909 a later and much longer opening next to the Pond quarry, 

on the east-southeast, had a length of 200 feet of its southern wall 

fall in, and the cost of removing the debris is reported to have been 

very great. It is difficult to see how such accidents can be avoided
MAINE.. ... 77

except by leaving supporting masses of unworked slate at short 

intervals. The horizontal close jointing of the quartzite greatly 

facilitates such falls.

The slate itself is very dark-bluish 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 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 (sericite) with a brilliant aggregate polarization, but there is considerable irregularity in the size of the 

particles. Quartz fragments measure up to 0.017 by 0.008 millimeter. Some quartz 

lenses measure 0.094 by 0.047 millimeter. There are to each square millimeter about 

100 scales of chlorite (interleaved with muscovite, rarely with biotite) measuring up 

to 0.047 by 0.03 millimeter and lying transverse to the cleavage; also about 18 scales 

of biotite to each square millimeter measuring up to 0.086 by 0.02 millimeter, lying 

both across and with the cleavage, and longish crystals and lenses of pyrite with their 

long axes parallel to the grain, numbering about 60 to the square millimeter and 

measuring up to 0.075 by 0.028 millimeter. 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 millimeter long, some specks of hematite, and a few tourmaline 

prisms up to 0.036 by 0.004 millimeter. No carbonate detected.

The constituents of this slate, named in the order of decreasing abundance, appear 

to be muscovite, 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, 1 which 

shows 56.42 per cent of Si02, 24.14 per cent of A1203 , 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 Bayley 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 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 to the 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 Mathews quarry (Maine Slate Co. of Monson), opened in 

1903, the thickest slate bed measures about 8 feet. The percentages 

of quartzite and slate are about 53.3 and 46.6. The strike of bedding 

and cleavage, taken 175 feet below the surface, is N. 62° E. and the 

dip 85° S. 28° E. There are two openings, the larger of which measures about 100 feet along the strike, 75 feet across it, and 180 feet 

in depth. On the north side of the east-northeast end is a tunnel 

extending 100 feet along the beds and 15 feet in width. Opposite

1 See Bayley, W.'S., U. S; Geol. Survey Bull. 150, p. 313,1898, and Twentieth Ann. Kept;, pt. 6 (continued),?. 394,1899. 
78 SLATE IN THE UNITED STATES.

to it on the west-northwest side is another of the same width, 55 feet 

in length and 40 feet high. In such tunnels in nearly vertical slate 

reliance is placed on lateral compression contracting the walls sufficiently to make a keystone of the roof. The grain dips 5° N. 62° E. 

so that the slate has to be sawn in a northwest-southeast vertical 

direction and "sculped" horizontally. There are vertical dip joints, 

horizontal joints, and one diagonal joint. The product of this quarry 

is used for roofing.

The slate is very dark bluish gray; to the unaided eye it 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 millimeter, rarely 1 by 0.075. 

Quartz fragments, unusually abundant in sections parallel to cleavage, measure up 

to 0.064 millimeter; biotito scales, about 63 to each square millimeter, measuring up 

to 0.13 by 0.028 millimeter, lie across as well as in the cleavage. A little less abundant 

than these are scales of chlorite interleaved with muscovite, lying across the cleavage, 

and measuring up to 0.13 by 2 millimeters. There are also about 1,160 lenses of pyriteto each square millimeter, with their long axes in the cleavage, and measuring from 

0.002 to 0.094 millimeter in length, and up to 0.047 millimeter in width and breadth. 

This number probably includes a few crystals of magnetite. Some of these lenses are 

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 millimeter. No carbonate was detected.

The relative abundance of these minerals appears to be, in descending order, muscovite, quartz, biotite, chlorite, pyrite, graphite, and magnetite.

The quarries of the Portland-Monson Slate Co., of 25 Central 

Wharf, Portland, are-about a mile south of Monson village and 1,600 

feet west of the railroad. (See PL XXVI, -A.) The largest opening 

measures 225 feet along the bed, 18 feet across it, and 180 feet in 

depth; another is 90 feet long and of the same width and depth; 

and two others are 75 feet long, 18 feet wide, and 60 and 40 feet deep, 

respectively. The slate surface is glaciated and covered with 10 

feet of gravel. The strike of the quartzite and the cleavage of the 

slate are both N. 63° E., the dip 90°, and the grain vertical and at 

right angles to the cleavage.

A channeler is used on horizontal joints for vertical cuts along 

and across the cleavage to obtain working faces. In order to avoid 

the falling in of the lateral walls, steel I beams 20 inches in height are 

used as braces in groups of three or four, set 2 feet into the wall on 

either side and inclosed in concrete. These reinforced braces measure 

about 5 feet square and are placed 30 feet apart and in alternate 

superposition. A supporting mass of slate, which can be tunneled 

later, is left every 60 feet.

The slate of these quarries is very dark bluish gray and has a lusterless roughish 

cleavage surface with marked grain. Its fissility is good. Pyrite very minute and
MAINE. 79

sparse. The slate does not effervesce with acid test. The powdered slate effervesces 

slightly, is very graphitic, and shows some magnetite.

Under the microscope the matrix has a brilliant aggregate polarization, an even 

(not lenticular) texture, but with some paasages of coarser quartz grains (up to 0.06 

millimeter). Meandering incipient plications cross the cleavage and bedding at 

right angles in the grain direction. Quartz (detrital) is generally very minute, not 

over 0.04 and mostly under 0.03 millimeter; it appears to be less plentiful than in the 

slate of Northfield, Vt. (See p. 122.) Chlorite scales plentiful and larger than quartz 

grains. No rutile was detected, or carbonate, although test with powdered slate shows 

its presence. Rare tourmaline prisms. Sparse biotite scales, not over 0.06 millimeter across, number about 14 per square millimeter. Conspicuous magnetite crystals 

or lenses (with a few particles of pyrite), 0.047-0.094 millimeter long and 0.009-0.03 

millimeter wide, number about 31 per square millimeter. The constituents, named in 

descending order of abundance, are muscovite, quartz, chlorite, magnetite, biotite, 

pyrite, carbonate, and tourmaline.

The slate is a mica slate of the unfading series and is used entirely 

for mills took, mostly for electric purposes. The president of the 

company states that electrical tests of this slate show that its insulating qualities are not affected by any magnetite content it may 

have. The amount of magnetite may also vary in different beds.

Tfflest Morison. At the quarry of the Monson Consolidated Slate Co. 

is exposed 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, the whole series measuring perhaps 50 feet. 

The quarry in 1904 measured 300 feet along the strike, 15 feet across 

it, and 160 feet in depth. The walls are supported by three pillars of 

slate. Bedding and cleavage both strike N. 60° E. and dip 90°. 

There are vertical dip joints striking N. 15° W.; also horizontal joints, 

to which the grain is parallel. About 15 feet of till lies 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 eys its tsxture and cleavage surface 

are very fine. It has more luster than the slate of the Pond quarry, but not so much 

as that of the Mathews quarry. 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 millimeter, some lying in the cleavage, others in the 

direction of the grain. The quartz fragments measure up to 0.02 by 0.012 millimeter. 

There are about 17 biotite scales to each square millimeter, measuring up to 0.08 by 

0.02 millimeter; also about 248 chlorite scales, the larger ones measuring from 0.047 

by 0.02 to 0.085 by 0.03 millimeter, with their longer axes and laminae usually parallel 

to the cleavage and across the grain; and finally, 30 to 88 lenses and crystals of pyrite 

to each square millimeter, measuring, in sections across the cleavage, up to 0.066 

by 0.02 millimeter, 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 millimeter. No carbonate was detected.

The probable relative abundance of these constituents, in descending order, is muscovite, quartz, chlorite, pyrite, biotite, and carbonaceous matter or graphite. Merriman's tests of this slate are given on page 183.
80 SLATE IN THE UNITED STATES.

In 1904 the slate was used both for roofing and for electric purposes. The quarry was idle in 1912.

North Blanchard. There are two quarries at North Blanchard 

which were both operated in 1904 by the Lowell Slate Co. At the 

State of Maine or Blanchard quarry 50 feet of slate and quartzite, 

10 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 80° ESE. The slate has vertical dip joints striking N. 70° W. 

and diagonal ones striking N. 40° W. and dipping 32° SSW. There 

are also joints confined to the quartzite, dipping 65°-70° SSW. and 

also 65°-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 half a 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 80° ESE. 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 millimeter and are not abundant. There are about 380 scales of chlorite, 

interleaved with muscovite or biotite, to each square millimeter, with their laminae 

across the cleavage and measuring up to 0.066 by 0.028 millimeter; also, about seven 

biotite scales to each square millimeter, measuring up to.0.085 by 0.047 millimeter, 

many of thsm bordered by secondary quartz or muscovite in the direction of the 

slaty cleavage, but with their laminae transverse to it. There are about 850 lenses of 

pyrite to each square millimeter, measuring from 0.004 to 0;03 millimeter in length and 

up to 0.01 millimeter in width; much dark-gray carbonaceous or graphitic matter in 

exceedingly fine particles; tourmaline prisms up to 0.07 by 0.009 millimeter are 

plentiful. No carbonate or slate needles found. The chief constituents, named in 

descending order of abundance, appear to be muscovite, chlorite, quartz, pyrite, 

graphite, and biotite.

This slate was used in 1904 for roofing and mill stock, including 

electric appliances. In 1912 the quarries were idle.
RILL QUARRY, 

ra of magnetite 

ng in slip cleav

ovite or of mus

ut 50 diameters.

BULLETIN sae PLATE XI

B, CLEAVAGE BANDING IN SHALES PARTLY ALTERED TO SCHIST, RUPERT, 

VT., NEAR THE HEBRON (N. Y.) LINE.

Showing alternate bands of finely cleft and uncleft rock, dipping 80° across the bedding 

foliation, which is visible at several points and dips at a low angle. Slip cleavage. 

Th e lower block in the foreground has three uncleft and two cleft bands. Sledge 

handle is 30 inches long.
ME. BROWNVILLE, QUARRY, SLATE MERRILL 

jointed. diagonally is that quartzite is wall left The quartzite. with alternating slate of beds 42 has face, 

g 
MAINE. . . 81

Brownville. The Old Merrill quarry at Brownville, operated in 1904 

by the Merrill Brownville Slate Co., lies less than a mile about northeast 

of the station. (See PL XII.) Here are exposed 42 beds of slate, 

alternating with as many of quartzite, the whole measuring 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 quaitzite in these calculations.1

At the Hughes quarry, owned by the same company, 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 this series of beds. 

Some of the quartzite is grayish and medium 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 feet in depth. The 

bedding strikes N. 78° E., and dips 75° NNW.; the cleavage strikes 

N. 68° E. and dips 70° NNW. Dip joints strike N. 20° E. and dip 

82° WNW.; diagonal joints strike N. 60° W. and dip 90°. There 

are abo horizontal joints, to which the grain is parallel. Quartz 

veins are not conspicuous, but there are some quartz lenses from 

2 to 3 feet in diameter. These veins contain a little biotite. The 

south wall of the quarry (see PI. XII), which is formed by a quartzite 

bed, ie divided into rhombic" blocks about 10 feet in their longer 

diameter, owing to the intersection of joints dipping 25° W. and 30° E. 

The surf ace 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 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 

and fissile.

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 millimeter; about five biotite plates to each square millimeter, 

measuring up to 0.076 by 0.03 millimeter, lying across the slaty cleavage. Many of 

these plates form the nuclei of quartz lenses which measure up to 0.4 by 0.03 millimeter. 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, from 29 to 49 to each square millimeter, and measuring from 

0.009 to 0.17 millimeter in length and up to 0.04 millimeter in width. Some of 

these crystals are bordered by secondary quartz and muscovite or chlorite, on one or

i Measurements obtained through the courtesy of C. H. Dunning, superintendent. 

40620° Bull. 586 14 6
82 SLATE IN THE UNITED STATES.

both sides, particularly wherever 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 0.75 millimeter long and 0.12 millimeter wide, 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 millimeter. No carbonate.

Plate 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 as the fineness of slaty cleavage. The principal constituents, named in descending order of abundance, appear to be muscovite, quartz, 

magnetite, pyrite, graphite, biotite, chlorite, tourmaline. This is a highly crystalline 

mica slate of the unfading series with a bright cleavage face.

W. 0. Crosby found that the slate of the East Brownville Slate Co. 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 to the square inch. 

The results of Merriman's recent tests of slate from Brownville will be found on page 

183.

The product of the Merrill quarry was used exclusively for roofing 

purposes; its magnetite is thought to prevent its use for electric appliances. However, a piece 6 by 4 inches by one-half inch makes no 

impression whatever on the magnetic needle, and the section-photographed in Plate XI, A, came from that piece. The quarry was 

idle in 1912.

Forks, Somerset County. A slate prospect opened in the town of 

Forks 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 

Railway 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 slate at Brownville. 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 15 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 millimeter in length and 0.004 to 0.016 

millimeter in width. In sections parallel to the cleavage these lenses have a very 

irregular outline and many of them are as broad as long. These lenses account for the 

limonitic staining on cleavage surfaces of water-soaked specimens. Quartz is abun-
MARYLAND. ' 83

dant but minute. No carbonate was detected. A few tourmaline prisms up to 0.11 

millimeter in length. Some scales of chlorite with interleaved muscovite measure 

up to 0.09 millimeter. There are rare zircon fragments and aggregations of rutile 

crystals.

The constituents of this slate, named in descending order of abundance, appear to 

be muscovite, quartz, chlorite, pyrite, and graphite, with accessory tourmaline, 

zircon, and rutile.

This slate of Pleasant Pond differs from the slates of Monson in 

having a lustrous and smooth surface, and from the slate at BrownviJle in having much less magnetite and a little less luster. It would 

prove suitable for roofing or mill stock.

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 the slates of Maine as brought out 

in the above descriptions are set forth in tabular form opposite 

page 188. 

MARYLAND.

The slate of Cardiff, in Harford County, Md.; about 30 miles northeast of Baltimore, extends up into Pennsylvania and will be considered under the heading "York and Lancaster counties" (pp. 110- 

115). Slate occurs also in Montgomery and Frederick counties, 

about 40 miles west of Baltimore and 33 miles northwest of Washington, where it has been prospected and quarried to a small extent. 

(See fig. 6.)

Geologic relations. For the geology of the region the reader is 

referred to the writings of Williams,1 Keyes,2 and Mathews.3 Keyes's 

section passes through Sugarloaf Mountain and along the Baltimore 

& Ohio Railroad (Metropolitan branch) to 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 

(phylh'tes), which 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.

This slate belt, whatever may be its exact bounds, is well exposed 

at Ijamsville, on Bush Creek and the Baltimore & Ohio Railroad, 

in Frederick County, where it has a well-defined strike of N. 10° E. 

Between Ijamsville and a point 2$ miles south-southwest of that place 

it is at least 1£ miles wide, and reappears west of Hyattstown, 3$ 

miles farther south, in Montgomery County; and from that point its

i Williams, G. H., The petrography and structure of the Piedmont Plateau in Maryland: Geol. Soc. 

America Bull., vol. 2, pp. 301-318, PI. XII, 1891.

a Keyes, G. E., A geological section across the Piedmont Plateau in Maryland: Idem, pp.319, 322, flg.3. 

« Mathews, E. B., On Ijamsville slate: Maryland Geol. Survey, vol. 2, pp. 231-232, PI. XXX, 1898.
84 SLATE IN THE UNITED STATES.

exposures and cleavage strike range from S. 20° W. to S. 37° W, 

It thus passes between Sugarloaf Mountain and the village of Mount

Contour interval SQ.fect

FIGURE 6. Map of parts of Frederick and Montgomery counties, Md., showing (by dots) slate prospects 

 and. abandoned quarries:

Ephraim on the west and Hyattstown and Barnesville on the east, 

its minimum length and width being about 12 and 1^ miles respec-
MARYLAND. 85

tively. Immediately east of this belt are chloritic schists and altered 

diabases.

E. B. Mathews 1 has recently concluded that some of the slates of 

Frederick County are of volcanic origin or contain material of such 

origin. .

Ijamsville. One-fourth mile west of Ijamsville station is an abandoned quarry in dark, slightly reddish purple slate.2 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 half a mile eastsoutheast of Thurston, in Frederick County, is another quarry 

(opening 70 by 50 feet) operated in 1885 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 60° E. There is a 2-foot quartz vein in the "top."

About 600 feet south-southwest of this quarry and half a mile 

southeast of Thurston is the quarry opened or reopened by the 

Bennett Creek Slate Co. about 1904, measuring 75 feet along the 

cleavage by 65 feet across it. In May, 1904, this quarry had a 

depth of 40 feet. The upper 25 feet is "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 the 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 half an 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 

0.094 millimeter. There are conspicuous scales of chlorite, often interleaved with 

muscovite lying transverse to the cleavage, measuring as much as 0.38 by 0.25 milli1 Maryland Geol. Survey, vol. 8, pp. 133-130,356-357,1900. 

'Idem, vol. 2, pp. 231, 232, PI. XXIV, fig. 2,1898.
86 SLATE IN THE UNITED STATES.

meter, or exceptionally 0.66 by 0.14 millimeter, and forming the centers of lenses of 

sericite or talc (7) 1 measuring up to 1.5 millimeters in length. There are also lenses 

of sericite or talc (7) 1 measuring up to 0.56 by 0.09 millimeter with their long axes in 

the grain direction.

These scales and lenses number about 88 per square millimeter, of which about 

18 are chlorite. Multitudinous minute dots of bright-red hematite occur throughout the slate, and to these and the chlorite it owes its purplish color. Among these 

dots are many particles of pyrite, some of which measure 0.038 millimeter. A few of 

these have passed into limonite. Slate needles (Ti02) are somewhat plentiful. No 

carbonate. Tourmaline 0.076 by 0.009 millimeter. The important constituents of 

this slate, named 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 

slightly talcose mica slate.

Mount Ephraim. About 4 miles southwest of the Bennett Creek 

quarries and a quarter of a mile south-southeast of the village of 

Mount Ephraim, in Montgomery County, east of the road to Barnesville and north of Little Monocacy River, on the property of Abraham 

Harris, a similar slate has been prospected. The cleavage strikes 

N. 30°-35° E. and dips 70° E. The bedding probably strikes N. 30° 

E.; the dip is uncertain.

The characteristics of the slates of Maryland are shown in the table

facing page 188.

MINNESOTA.

By EDWIN C. ECKEL.

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 from the old dumps, is hardly such as to justify reopening.

The following reports and papers, while not economic in intention, 

contain data of interest on the distribution and character of the 

slates of Minnesota:

HALL, C. W., Keewatin area of eastern and central Minnesota: Geol. Soc. America,

Bull., vol. 12, pp. 345-376, 1901. 

SPURB, J. E., The stratigraphic position of the Thomson slates: Am. Jour. Sci., 3d ser.,

vol. 48, pp. 159-166, 1894. 

WINCHELL, N. H., The geology of Carlton County: Minnesota Geol. Survey Final

Repts., vol. 4, pp. 1-24, 1899.

 The geology of the southern portion of St. Louis County: Idem, pp. 212-221.

 The geology of the Carlton plate: Idem, pp. 550-565.

 The geology of the Duluth plate: Idem, pp. 566-580.

« Mathews (Maryland Geol. Survey, vol. 2, p. 232), following some determination by G. H. Williams, 

regarded these slates as containing talc. A chemical test made by George Steiger at the United States 

Geological Survey laboratory shows 0.27 per cent of SiO2 soluble in 1-20 solution of NazSOs 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 SiO2 from the talc if talc is present. It would be one-fourth of the SiO» 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.
SLATE IN THE UNITED STATES. 87 

NEVADA.

The discovery of slate in the Blue Mountains in Humboldt County, 

Nev., was reported to the Geological Survey in 1908. The deposit 

occurs about 21 miles northwest of Winnemucca and is said to be 

extensive.

This slate may belong to the Mariposa slate, of Jurassic age. 

Clarence King 1 described a slate of the region in these words:

Over the Jurassic limestone on the northern points of tjie West Humboldt Kange 

is a very heavy body of variable but generally argillaceous slate. The exposure on 

Humboldt Canyon is of over 2,000 feet and on the north side of the Humboldt 

valley the same slate group is exposed with even greater thickness.

On page 268 of the same work King correlates these Jurassic 

slates with similar rocks at Mariposa, Cal. 2

Specimens reported as having been obtained from more or less 

weathered outcrops on the Gibraltar group of claims were examined 

with the following result:

The slate is of dark bluish-gray color. To the unaided eye it has an extremely 

fine texture and a very smooth, slightly lustrous cleavage surface. It contains some 

carbonaceous or graphitic matter and, as shown by the magnet, a little magnetite. 

The sawn edges show neither pyrite nor magnetite. It does not effervesce with cold 

dilute hydrochloric acid. It is sonorous and has a very high grade of fissility. The 

cleavage face shows traces of "grain," but the slate breaks usually at angles of 50° to 

70° and 30° to the apparent grain.

Under the microscope this slate shows a matrix of muscovite (sericite) with welldefined aggregate polarization and is thus a mica slate. The cleavage is fine, although 

showing some minor irregularities in the size of particles. The quartz particles measure up to 0.06 millimeter in diameter; one plagioclase feldspar measured 0.04 millimeter. No pyrite was detected. The dark particles are nonmetallic and probably 

carbonaceous. Rather abundant scales of chlorite and of interleaved chlorite and 

muscovite measure up to 0.14 millimeter in length and to 0.05 millimeter in width. 

Butile is very abundant both in single needles and more in net-shaped groups of 

twinned needles (sagenite) measuring 0.02 millimeter across. There is a little limonite proceeding possibly from the magnetite and due to incipient weathering. No 

carbonate could be found.

Its constituents, in descending order of abundance, appear to be muscovite, quartz 

(detrital), chlorite, carbon (probably graphite), rutile, magnetite, and plagioclase 

(detrital).

The fissility of this slate is equal to that of the slates of Pennsylvania, but its freedom from carbonate indicates that its color will prove more durable.

NEW JERSEY.

The slate belt of Lehigh and Northampton counties, Pa., is prolonged eastward into New Jersey, where roofing-slate quarries have 

been opened at several points, notably near Newton and Lafayette, 

in Sussex County.

1 King, Clarence, Systematic geology: U. S. Geol. Expl. 40th Par., vol. 1, p. 295,1878.

2 See Sinith, J. P., Age of the auriferous slates of the Sierra Nevada: Geol. Soc. America Bull., vol. 5, 

pp. 243-258,1894; also Turner, H. W., and Eansome, F. L., U. S. Geol. Survey Geol. Atlas, Sonora folio 

(No. 41)., p. 2, 1897.
88 SLATE IN THE UNITED STATES.

Lafayette. The quarry of the Lafayette Slate Co., of Newton, 

N. J., the only slate quarry in operation in this State in 1912, is 

about 2 miles N. 25° E. from Lafayette village and nearly 28 miles 

northeast of the Delaware Water Gap. The opening measures about 

200 feet in a N. 20° W. direction by 85 feet across and 70 to 100 

feet in depth. The "top" consists of 5 to 15 feet of weathered slate 

and till. The beds (ribbons) strike N. 45° E. and dip 20°-30° N. 

45° W. The cleavage strikes N. 70° E. and dips 20° S. 20° E. There 

are two sets of joints. Those of one set strike N. 10°-45° E., dip 

70° S. 63° E., and are spaced 2 to 20 feet. Those of the other set 

strike N. 30° W., dip 90°, and are spaced 5 to 40 feet. Owing to 

secondary motion the slate is more or less bent (not curved) at each 

ribbon. The weathered slate of the "top" is of light-brownish color.

The slate is very dark bluish gray. To the unaided eye it has a fine texture and a 

slightly roughish, lusterless cleavage surface. The sawn edge shows pyrite. It effervesces with acid test, and the ribbon still more. The powdered slate is very effervescent, carbonaceous, and but very- slightly magnetitic. It has a high grade of 

fissility and is sonorous.

Under the microscope this slate shows a matrix of muscovite (sericite) with fair 

aggregate polarization somewhat obscured by abundant carbonate, and with very 

elongate lenticular texture. Beds 0.75 millimeter thick cross the cleavage at 36°. 

Quartz grains (detrital) up to 0.034 millimeter. Chlorite scales, some with interleaved 

muscovite, up to 0.08 millimeter, lie at right angles to the cleavage. Sparse pyrite 

spherules 0.004-0.012 millimeter across. Carbonaceous matter in short streaks parallel to beds. In these beds the carbonate is coarser and the quartz up to 0.66 millimeter. Rutile needles.

The constituents, named in descending order of abundance, appear to be muscovite, quartz, carbonate, chlorite, carbon, kaolin, pyrite, magnetite, and rutile.

This is a mica slate of the fading series. It is reported as being harder than slate 

of Bangor, Pa., and as preserving its color on the roof for at least 30 years.

It is supplied to the market in two qualities No. 1, generally very 

slightly bent; No. 2, with one Or two very obtuse bends, giving to each 

slate a zigzag outline in longitudinal section. When in full operation 

this quarry employs eight splitters.

There is an abandoned quarry about half a mile S. 70° W. from 

the quarry described above. The opening is triangular, about 75 

feet on a side and about 90 feet deep. The beds strike N. 60° E. 

and dip 20° N. 30° W.; the cleavage strikes N. 55° E. and dips 35° 

S. 35° E. Although the quarry has not been worked for many years 

the slate on the dumps, which cover several acres, is generally very 

dark, but a quarried cleavage face at the edge of the quarry is light 

brownish gray.

NEW YORK.

GEOLOGIC 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 139,
NEW YORK. 89

which were once extensively quarried at Middle Granville and Jamesville. Overlying the Lower Cambrian, in isolated lenticular synclinal 

areas or in long, ramifying masses of complex structure, are Ordovician grits and black, red, and green shales; and these red and green 

shales pass here and there into roofing slates. 1 (See PL XX.)

Although the Lower Cambrian greenish and purplish slates are at 

present quarried almost exclusively on 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 slate-bearing formations on the New York side are shown in Sections I, II, VII of Plate 

XXII and are explained hi detail on pages 125, 128, 132.

As to mode of occurrence, these red and green slates occur in places 

in proximity to white-weathering greenish or black, more or less 

quartzose shales or slates, and they also seem at several points to

replace the Ordovician grit along the strike. Certainly these slates 

occur in as close proximity to the Cambrian slates as the grit. Black 

shales abounding with graptolites of the Normanskill fauna, as determined recently by Ruedemann, dip under the red slates on the west 

side of the now disused quarry a mile northeast of Middle Granville 

(locality 36, PI. XX).

The red and associated bright-green slates are regarded as of the 

same general horizon as the Ordovician grits and their interbedded 

graptolite shales; and on the east the entire group (red and black 

slates and shales and grits) appears to pass along a meandering line 

extending from a point in Rupert, Vt., about a mile south of the 

Bennington County, line, to the village of North Pawlet, Vt., on the 

north, merging gradually into the more highly metamorphosed sediments of the Taconic Range, the typical Berkshire schist, which is 

micaceous, chloritic, and in places also hematitic or graphitic. As 

the red slate appears to correspond to the lower part of the schist 

formation it is probably of middle Trenton age.

The cause of the abundant hematite to which the red slate owes 

its color may be sought in the beginning of a gradual elevation of a 

land surface that owing to long-continued weathering had become 

covered with somewhat thick deposits of rusty clays, which were 

then carried into the sea. These fine limonitic clayey sediments, 

when later .subjected to the compression that metamorphosed the 

clay into fibrous muscovite and set up slaty cleavage, became reddish 

by the loss of the combined water and the change of limonite into 

hematite. The beds of bright-green slate may be attributed, like 

the spots and small greenish beds (p. 25), to the effect of decomposing 

marine organisms reducing the iron in the clays or to the original 

absence of iron therefrom.

i U. S. Geol. Survey Nineteenth Ann. Kept., pt. 3, PI. XIII, 1899.
90 SLATE IN THE UNITED STATES.

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 after subtracting that 

which is too hard or too soft or badly veined, there remains in places 

but 10 feet, rarely more than 25 feet, and at a maximum 42 feet, of 

good red slate exposed at any one quarry. 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 slate is in some 

places interbedded with the red. Beds of greenish quartzite, locally 

calcareous, bordered by a purple slate, the whole ribbon measuring 

an inch or two in thickness, are not uncommon (PI. II, A), also beds 

half an inch thick of rhodochrosite (manganese carbonate) with 

crystalline calcite.

An analysis of this slate (specimen D. XIV, 1895, 201(Z), made by 

George Steiger, yielded the following:

Analysis of rhodochrosite bed.

Al£>9 ............................ 0.68

Fe208 ............................ .14

FeO............................ 1.13

MnO........................... 32.22

NiO and CoO..................... .10

CaO.............................. 3.81

MgO.............................. 2.61

C02 .............................. 25.06

Insoluble matter, including all silica from dissolved silicates. .... 32. 75

98.50

Under the microscope thin sections of this bed show, with polarized 

light, a fine-grained 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.

Both red and green slates also contain veins of quartz, some of 

which is crystallized, and in the red slate are light-green spots with 

or without a purple rim. Both red and green slates are in many 

places speckled with lenses. (See p. 91.) The bedding planes are 

locally covered with glistening annelid trails and with impressions 

that may have been made by algae. 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 is shown on Plate XX. The structure at 

two of the quarries is shown diagrammatically in figs. H of Plate 

XXIII and U of Plate XXIV, and the compass courses at the larger 

quarries, with reference numbers to their location on the map (PI. 

XX), are'given below, but Nos. 33, 39, and 41 lie west of the area 

mapped; 33 is 3£ miles north-northeast of Middle Granville, in Granville Township; 41 is 2|- miles northeast of Middle Granville, also in 

Granville; and 39 is 2£ miles west of Jamesville, in Whitehall.
NEW YOBK.

Compass observations at New York red-slate quarries.

91

Quarry No. on Plate XX.

33...................................

34...................................

35...................................

36...................................

37...................................

38...................................

39...................................

40...................................

41...................................

64...................................

66...................................

Quarry No. on Plate XX.

11

34..... ..............................

35...................................

36...................................

37...................................

38...................................

39...................................

40...................................

41...................................

64...................................

66...................................

Bedding.

Strike.

N. 

N. 

N.+. 

N. 5° E. 

N. 12° W. 

N. 10° W. 

N. 5°E. 

N. 

N. 

N. 15° W. 

N. 30° W.

Dip.

30± E. 

45° E. 

15° E. 

45° E. 

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.

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. 

35° E. 

30° E. 

45° E. 

40° E. 

40° E. 

25° E. 

25° E.

Diagonal joints.

Strike.

N. 30° E.

Dip.

70° W.

Strike joints.

Strike.

N.+

N. 10° W.

N. 15° W.

Dip.

55° E.

25° E.

50° E.

Shear zones 

("hogbacks").

Strike.

N. 7°E.

N. 55° W.

Dip.

77° E.

70° NE.

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 magnetitic, sonorous, and has an argillaceous odor. 

Some of it effervesces with cold dilute hydrochloric acid. Under the microscope it 

shows a matrix of muscovite (sericite) with somewhat faint aggregate polarization, 

but this faintness is due partly 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 millimeter. There is much irregularity in the size of the particles, 

indicating but a fair grade of fissility. The larger particles consist of quartz fragments 

up to 0.06 by 0.03 millimeter, rare grains of plagioclase feldspar and of zircon, all 

detrital, considerable carbonate in rhombs or plates up to 0.047 millimeter, scales of 

chlorite interleaved with muscovite, up to 0.075 by 0.036 millimeter, a very variable 

number of lenses up to 0.34 by 0.15, rarely 6.5 millimeter long, consisting probably 

of rhodochrosite (carbonate of manganese) and chalcedonic quartz (containing carbonate rhombs and muscovite scales and in some places partial pseudomorphs of 

chlorite after carbonate), and finally some tourmaline prisms up to 0.005 by 0.001 

millimeter.

The chief constituents of the red slate, named in descending order of abundance, 

appear to be muscovite, quartz, hematite, kaolin, carbonate, chlorite, magnetite, 

and rhodochrosite. For colored lithographs of magnified thin sections of this slate, 

as seen under both ordinary and polarized light, see Nineteenth Annual Beport U. S. 

Geological Survey, part 3, Plate XXVIII, A, B, C. For a discussion of the causes
92 SLATE IN THE UNITED STATES.

of the green and purplish spots which occur here and there in this slate see page 23 

of this bulletin. Associated with the red slate is generally a little purplish slate, 

some of it speckled, of no commercial importance. Under the microscope it shows 

less hematite, and possibly more chlorite than the red slate. 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 W. F. Hillebrand, the partial ones by 

George Steiger:

Analyses of red slates from New York.

Silica (SiOs). ............ ..................

Ferrous oxide (FeO) . .....................

Lime (CaO). ...............................

Baryta (BaO). ............................

Potassa (K20) ............................

Soda (Na20) . .............................

Lithia (Li2 0) .............................

Water below 110° C (H2 0) . ...............

Pyrite (FeS2 ) .............................

Carbon (C). ...............................

Total sulphur (S). ........................

J

67.61

.56

13.20

5.36

1.20

.10

Trace?

.11

.04

3.20

4.45

.67

Trace.

.45

.05

.03

Trace.

100. 00

.016

K

67.55

.58

12.59

5.61

1.24

.19

.26

.31

3.27

4.13

.61

Trace.

.40

3 no

.10 .11

.04

Trace.

100. 02

.02

L

56.49

.48

11.59

3.48

1.42

.30

Trace.

5.11

.06 fi di

3.77

.52

Str.tr.

.37

09

7.42

.03

Trace.

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

.08

5.08

Trace.

100. 14

K2

7.10

1.00

2. 8085

N

1.02

1.67

J (D. XIV, 1895.3584), red slate, H. H. Matthews's quarry, 1 mile west of Poultney, in Hampton, Washington County, N. Y.

K (D. XIV, 1895, 201<!), red slate, Empire Red Slate Co.'s quarry, 1 mile north of Granville, in Granville, 

Washington County, N. Y.

L (D. XIV, 1895,397e), red slate, National Red Slate Co.'s quarry, 1 mile north-northwest of Raceville, 

in Granville, Washington County, N. Y.

M (D. XIV. 1895,397a), red slate, same locality as L, but near a green and purple spot.

KZ (D. XIV, 1895, 201ft), red slate, same as K, but finer grained.

N (D. XIV, .1895, 284a), purple bed in red slate at Fair Haven Red Slate Co.'s quarry (not worked), 2 

miles north of Truthville, in East Whitehall, Washington County, N. Y.

For presence of chromium and vanadium in these see Dr. Hillebrand's note, p. 58.

Some points in these analyses are discussed by Dr. Hillebrand on pages 58, 59.

As these red slates do not discolor and yet show a considerable amount of carbonate 

under the microscope, and as the discoloration of the "sea-green " slates of Vermont 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 to the 

square inch, and Merriman 126.66 foot-pounds of work to the pound of slate as a result 

of impact tests. The results of Merriman's other tests of this slate are given on page 183.

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 those of the "red," and the surface 

is also in places 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 it 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
PENNSYLVANIA. 93

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 millimeter, chlorite scales up to 

0.043 millimeter, some carbonate rhombs from 0.002 to 0.056 millimeter, and lenses 

measuring up to 0.385 by 0.128 millimeter, 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.

Th'e chief constituents of this slate, named in descending order of abundance, appear 

to be muscovite,, quartz, chlorite, carbonate, magnetite, rutile.

Its color is due to the abundance of chlorite and is permanent.

The following analysis (specimen 0=D. XIV, 1895, 397c), by W. F. Hillebraud, is 

of a bright-greenish speckled slate from the National Red Slate Co.'s quarry, 1 mile 

north of Raceville, in Granville, Washington County, N. Y.

Analysis of bright-greenish slate from Neio York.

Silica (Si02 )..................... 67.89

Titanium dioxide (Ti02)......... .49

Alumina (A12 03 )................. 11. 03

Ferric oxide (Fe203 )............. 1,47

Ferrous oxide (FeO)............. 3. 81

Manganous oxide (MnO)......... .16

Nickelous oxide CNiO)........... Trace?

Cobaltous oxide (CoO)........... Trace?

Lime (CaO)...................... 1.43

Baryta (BaO)................... .04

Magnesia (MgO).................. 4. 57

.................. 2.82

Soda (Na^O)..................... .77

Lithia (Li2 0).................... Trace.

Water below 110° C. (H20)....... .' 36

Water above 110° C. (H20)....... 3.21

Phosphoric oxide (P205 ).......... .10

Carbon dioxide (C02 )............ 1. 89

Pyrite (FeS2 )................... .04

Sulphuric oxide (S03)........... Trace.

Carbon (C)...................... None.

100. 08 

Sulphur, total (S)............... .022

Specific gravity, 2.7171.

The relatively high percentage of magnesia is attributed to the abundance of both 

dolomite and chlorite.

The results of Merriman's tests of the bright-greenish slates quarried by the Mathews 

Consolidated Slate Co. are given on page 183.

Black slate. Black roofing slate of Ordovician age was quarried many years ago, in 

a small way, 3 miles south of Hoosick. Falls, near Hoosick, and also, at a later time, 

2 miles south of Stephen town, near Lebanon Springs, in Rensselaer County, N. Y., 

but did not prove to be of economic importance at either locality. 1

The principal features of the slates of New York as brought out in 

the above descriptions will be found in tabular form opposite page 188.

PENNSYLVANIA. 

GENERAL FEATURES.

The slates of Pennsylvania, aside from those of Lancaster County 

and the southeastern part of York County, which are known as the 

Peach Bottom slate (pp. 110-115), occur chiefly in Northampton and 

Lehigh counties in a strip from 2 to 4 miles wide on the south side of 

Blue Mountain, extending from Delaware Water Gap in a westsouthwest direction to a point 4 miles west of Lehigh Gap, a distance

1 PI. CI and fig. 26, Thirteenth Ann. Kept. U. S. Cool. Survey, pt. 2, 1892, show the structure at the 

Lebanon Springs quarry.
94 SLATE IN THE UNITED STATES.

of about 32 miles. The chief centers of the slate industry here are at 

Bangor and Slatington. The geographic 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, in Merrill's "Stones for building and decoration" (third edition), and in 

Merriman's article in Stone for July, 1898.

The map, Plate XV, compiled from that of the Second Pennsylvania Geological Survey by C. A. Bonine, shows the quarry sites of 

Northampton County as approximately determined by him in 1911.

The general geologic relations of this slate are these: On the southeast, forming an east-northeasterly belt between Easton and Heading 

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 belt and parallel to it is a great Cambrian and 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 (the 

Martinsburg shale), 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 Blue Mountain. 

The boundary between the shale and slate formation and the limestone is roughly parallel to the general course of the Shawangunk conglomerate boundary but passes a little north of Nazareth. In Carbon 

County, northwest of Blue Mountain, lies a strip of shales and slates, 

much resembling the Ordovician beds which dip under the Silurian 

conglomerate on the southeast, but which Rogers (section 2, Pennsylvania Survey) represented as overlying that conglomerate conformably and thus as of Silurian age. Slate has also been prospected 

in Dauphin County about 55 miles southwest of Lehigh County and 

southeast of Blue Mountain.

The shale and slate formation' measures from a minimum exposure 

of 1,600 feet to an estimated maximum .of, 6,000 feet, but of this.only 

a few hundred feet is commercial slate. In structure this formation 

consists of a succession of minor close folds, generally overturned 

northward, so that their axial planes have a general southerly dip, 

but usually the synclines so formed have a steeply inclined southern 

and a gently inclined northern limb. In places the fold is extremely 

close and the overturn so complete that its axial plane has a very 

low southeasterly dip. The cleavage dips southward at various angles, 

as low as 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 been crushed and
PENNSYLVANIA. ' 95

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 2 feet in thickness, characterize the slate belt throughout. 

(See Pis. V, B; VI, A; XVII; and XVIII.) 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, and 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 11, 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. . The lower and southerly belt, the "hard vein," near the 

base of the Ordovician formation ("Hudson"), 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." To judge from the map of the 

Second Geological Survey of 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 following notes on the general stratigraphy of this slate belt 

and on the relations of the slate to the overlying Silurian conglomerate are contributed by C. A. Bonine, of this Survey, from his 

Lehigh University thesis on "The slate deposits of Northampton 

County, Pa.," written in 1912:

In going up either the Delaware or the Lehigh Eiver at right angles to the strike 

it is evident even to the most superficial observer that there are three lithologically 

different types represented in the formation. Farthest south the beds of slate are 

hard and close together, possess a good cleavage, and are occasionally interbedded 

with sandstone. Northward the formation changes gradually into beds of almost 

pure sandstone with very few shaly beds, and still farther north up to the Shawangunk 

conglomerate the material is again nearly all slate, which is soft, thick bedded, highly 

jissile, and interlayered with only occasional sandy beds. In the lowe°r portion the 

"hard vein" slate is found and in the upper the "soft vein." The Delaware River 

section, along which the most reliable exposures were found, shows clearly that the 

Martinsburg shale is divisible into three parts a lower hard thin-bedded shale and 

slate, a middle member of sandstone, and an upper member of shale and slate, which 

is soft and thick bedded. From this section it will be seen that the " soft vein " slate 

is found only near the mountains, because the land farther south along the a'xis of the 

syncline has been reduced by erosion to such an extent that the upper soft member
96 SLATE IN THE UNITED STATES.

of the formation has been entirely removed, leaving exposed only the middle sandy 

portion.

A close examination of the beds of the. Shawangunk conglomerate near the base 

shows included fragments of the underlying shale. The contact between this formation and the Martinsburg shale underneath is well exposed in a new cut of the Lehigh 

& New England Railroad at Lehigh Gap. The dips here are not conformable, that of 

the conglomerate being 20° N. 45° W. and that of the slate 40° N. 45° W. There are 

also strong indications of a considerable erosional interval, as shown by the transitional bed of soil material and pebbles.

All the more important features of the slates of Pennsylvania are 

set forth in tabular form in the section on the comparative characteristics of slates (p. 188). The literature of the slate of Northampton 

and Lehigh counties is listed in the bibljography on pages 205-212, 

under the names Chance, Lesley, Merrill, Merriman, Rogers (H. D.), 

and Sanders.

A few typical quarries and slates from these belts will now be 

described.

NORTHAMPTON COUNTY.

Bangor. The "Old Bangor" quarry at Banger, 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 feet across it, and from 200 to 300 feet in depth. (See PI. XIII 

and map of Delaware Water Gap quadrangle, United States Geological 

Survey.) The general structure is that of a close overturned syncline, striking about N. 25° E., with an axial plane dipping eastsoutheast at a low angle and pitching 5°-10° SSW., crossed by 

cleavage dipping 5°-10° S. 32° E. The same gritty bed appears at 

both the top and the bottom of the quarry, and this bed is regarded as the lower limit of the slate, 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. In 1912 that part of the 

gritty bed overlying the good slate of the syncline was being blasted 

out and thrown down upon the gritty bed below it as fast as the good 

slate was quarried out. Vertical diagonal joints striking N. 52°-57° 

E. are conspicuous. There are also vertical dip joints and horizontal joints. Curvature of the cleavage occurs here and there. 

Certain diagonal or longitudinal joints intersecting the bedding planes 

at the northern corner of the quarry produced an optical illusion, for 

the beds tnere seemed 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. This peculiar 

feature, however, is fast disappearing with the growth of the excavation. The ribbons are markedly more calcareous than the slate 

and in weathering become white from incrustation of lime. Veins
U. S. GEOLOGICAL SURVEY 

OLD BANGOR QUARRY (NORTH-NORTHEAST END), BANG

Showing close, flat syncline crossed by nearly horizontal cleavage. Taken in 1 91 2. The shanty at t
U. S. GEOLOGICAL SURVE'

BANGOR UNION QUARRY, BANGOR. PA.

Looking N. 80° W. Showing inner part of flat syncline of Plate Xlli, Distance between the strike of the two 

about 500 feet. Taken in 1 91 2.
PENNSYLVANIA. 97

of white calcite and quartz, 4 inches thick, occur in the strike joints. 

Crystalline films of gypsum form on the horizontal joint faces.

Mr. Bonine obtained.the following names and order of the beds at 

this quarry from the foreman in 1912:

1. Surface (glacial d<§bris, weathered elate).

2. Hard roll.

3. Big bed, 4 feet.

4. Thirteen beds.

5. Seven beds, 25 feet.

6. Five beds.

7. Hogback run, 10 feet.

8. Gray bed run, 13 feet.

9. Big bed, 3£ feet.

10. Schuman run.

11. Feathery run.

12. Black ribbon run.

13. Five beds. f

14. Mill run, 3 feet.

15. Big bed, 4 feet.

16. Nine-foot bed, 9£ feet. 

Sand slate rock.

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 scattered 

chlorite scales up to 0.124 by 0.066 millimeter; quartz grains not very abundant up 

to 0.047 millimeter. Pyrite spherules number about 14,000 per square millimeter, 

measuring up to 0.01 millimeter in diameter. Kutile needles are very plentiful. 

Carbonaceous or graphitic matter occurs in fine particles throughout.

A transverse 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 millimeter, feldspar grains 

(plagioclase, orthoclase) up to 0.14 by 0.09 millimeter, the quartz and feldspartogether making up no more than 25 per cent of the ribbon; also spherules of pyrite 

up to 0.07 millimeter, some of them in lenses of chlorite; some large plates of muscovite and of chlorite interleaved with muscovite, up to 0.06 millimeter, and rarely 

a zircon fragment, 0.03 millimeter.

The constituents of this slate, named in descending order of abundance, appear 

to be muscovite, carbonate, quartz, kaolin, pyrite, chlorite, rutile, magnetite, and 

carbonaceous matter.

The final results of Merriman's physical and chemical tests of slate 

from the "Old Bangor" quarry are given here for convenience of 

reference: Strength (modulus.of rupture), 9,810 pounds to the square 

inch; toughness (ultimate deflection on supports 22 inches apart), 

0.312 inch; density (specific gravity), 2.780; softness (amount in

40620° Bull. 586 14 7
98 SLATE IN" THE UNITED STATES.

grains abraded by 50 turns of a small grindstone), 128; porosity (per 

cent of water absorbed in 24 hours), 0.145; corrodibility (per cent of 

weight lost in 63 hours in acid solution composed of 98 per cent 

H20, 1 per cent HC1, 1 per cent S03 ), 0.446. Per cent of lime, 4.38; 

computed carbonate of lime, 7.82 per cent; computed carbonate of 

magnesia, 5.65 per cent. 1

The product from the large beds is used for roofing, but that from 

the ribboned beds goes into mill stock.

The Bangor Union quarry, now operated by William E. Lloyd, of 

Bangor, lies about 500 feet northwest of the northwest wall of the 

"Old Bangor" quarry. The opening, estimated by Mr. Bonine as 

measuring 400 by 200 feet, exposes a completely overturned close 

fold on its northwest wall, crossed by an almost horizontal cleavage, 

as shown in Plate XIV. This fold is regarded as the inner northwesterly part of that exposed on the northeast wall of the "Old 

Bangor" quarry, shown in Plate XIII. The general strike is about 

N.45° E.

MX. Bonine has furnished the following note on the beds:

This quarry has some beds that are not present in the ' 'Old Bangor'' quarry because 

it is farther up in the fold. These new beds are:

1. Bangor Union big bed.

2. Spar ribbon.

3. Waste, 4 feet.

4. Pinhead run.

5. Magpie run.

6. "Waste, 6 feet.

?. Black ribbon big bed.

8. Shelibark run.

9. Spar ribbon.

10. Waste, 4 feet.

11. One-length factory.

12. Waste, 4 feet.

13. School and factory.

14. ;Hard roll.

15. Factory.

16. Three beds.

17. Two beds.

18. Hard roll.

19. Fine ribbon.

Below this the beds occur as named in the "Old Bangor" (p. 97). At the present 

time this quarry has only been worked down as far as the ' 'Gray bed run,'' No. 8 of the 

'.'Old Bangor" section.

The ribboned slates from this quarry reach the market in two 

grades. In grade No. 1 the ribbons occur only in one-half of the surface, which on the roof can be covered by the overlapping tier. Grade 

No. 2 has ribbons all over and the cleavage is likely to be bent slightly 

at the ribbon.

i Am. Soc. Civil Eng. Trans., vol. 27, pp. 342-347,1892.
PENNSYLVANIA. 99

North Bangor. The large quarry of the North Bangor Slate Co. is 

about 200 feet square and 350 feet deep. The folded beds strike 

about N. 60° E. and the cleavage dips 15° SW. 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. It is thought that the gritty bed which underlies and 

overlies the good slate at the "Old Bangor" quarry has been struck 

at the bottom of this one.

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 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 Co. '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 millimeter. There are some chlorite scales up to 0.09 millimeter and about 10,000 spherules of pyrite, measuring up to 0.01 millimeter, to 

each square millimeter, also carbonaceous matter or graphite and very abundant 

rutile needles.

The constituents, named in descending order of abundance, appear to be muscovite, carbonate, quartz, kaolin, pyrite, chlorite, magnetite, rutile, and 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 and 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, fixeboards, stationary tubs, well covers, tiles, lathe 

work, etc.

East Bangor. The large quarry of the East Bangor Consolidated 

Slate Co. measures 350 feet along the strike and 700 feet across by -130 

feet in depth. The beds dip.5° NNE. and the cleavage 15°-20° SSW. 

The main joints strike north and undulate in a general vertical direction. The grain strikes N. 50°-60° W. Ribbons are numerous.

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 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 the slate of the ' 'Old Bangor'' quarry, 

but carbonate as abundant as in that. Not very much quartz in grains measuring up 

to 0.04 by 0.02 millimeter; about 250 scales of chlorite, with interleaved muscovite, 

to each square millimeter, measuring up to 0.07 by 0.03 millimeter, exceptionally 

0.08 by 0.04 millimeter; about 12,400 spherules of pyrite to the square millimeter, 

measuring up to 0.008 millimeter; rutile needles very abundant; carbonaceous or 

graphitic matter in fine particles.
100 SLATE IN THE UNITED STATES.

The chief constituents of this slate, named in descending order of abundance, 

appear to be muscovite, carbonate, quartz, pyrite, chlorite, kaolin, rutile, carbonaceous matter or graphite, and magnetite.

The product of this quarry is used both for roofing and for millstock, but not for blackboards, as the 9-foot bed is crossed by ribbons. 

The roofing slates are said to show comparatively little discoloration 

after 30 years' exposure.

Windgap. The group of quarries known as the Windgap quarries 

is about a mile from the saddle in Blue Mountain known as the Windgap, which is about 11 miles southwest of the Delaware Water Gap. 

(See PI. XV.) Three of these quarries were visited by the writer.in 

1912.

The Alpha Slate Manufacturing Co.'s quarry, about a mile S. 35° E. 

from the gap, is. about 200 feet square and 70 to 150 feet deep. The 

light-buff "top," of decomposed slate, is 50 feet thick. The beds 

strike N. 55° E. and dip 70° N. 35° W. The cleavage strikes N. 75° W. 

and dips 25° S. 15° W. Beds free of ribbon are from 2 to 25 feet thick. 

About three-fourths of the product is used for mill stock and the rest 

for roofing. Channeling machines are used for both horizontal and 

vertical cutting.

The Imperial quarry, operated by William Harding & Co., is about 

S. 30° E. from the gap and measures 175 feet along the bed by 150 feet 

across it and 200 feet in depth. The "top " consists of 75 feet of decomposed and bad slate. The beds curve northward at a depth of 40 feet 

below the "top." The thickest clear bed measures 11 feet along the 

cleavage. The grain is diagonal to the ribbon. The product is used 

mostly for mill stock, including blackboards. Channeling machines 

are used altogether for vertical cuts, but powder is used for cuts along 

the grain.

The Phoenix quarry of the Phoenix Slate Co., of Windgap, Pa'., is 

about S. 20° E. from the gap and measures about 300 feet along the 

bed by 175 feet across and from 70 to 170 feet in depth. The decomposed slate of the top is 10 feet thick. The thicker beds free of 

ribbons measure 11 and 14 feet along the cleavage. The grain is 

diagonal to the ribbon. As shown in Plate V, B, the beds begin 

to curve southward at the rock surface. Slabs of banded (ribboned) 

slate fro'm this quarry are shown in Plate VI, A. The product is 

nearly all used for mill stock, including blackboards. At least one of 

the beds has been found suitable for electric switchboards.

Heimbach. The Heimbach quarry, about l£ miles northeast of 

Slatington, in Northampton County, is 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 the slate of the "Old Bangor" 

quarry.
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XV

A\ S/ H\t 1ST |O X O 3ST 

STatedale

SOFT-VEIN QUARRIES

1. Snpwden Bangor

2. Williams

3. Northampton

4. Cooper's

5. New York

6. Bangor Vein

7. Hoboken

8. Bangor Ridge

9. Shimer's

10. Consolidated No. 3

11. Consolidated No. 2

12. Consolidated No. 1 and Star

13. Bangor Eclipse

14. Bangor Central

15. Bangor Royal

16. Columbia Bangor

17. Bangor Excelsior

18. Old Bangor

19. Bangor Washington

20. Bangor Union

21. North Bangor

22. West Bangor

23. Bangor Superior

24. Crown

25. OldDorney

26. Old Pennsylvania

27. Master's

28. United States

29. Diamond

30. Valley No. 5

31. Valley No. 6

32. Valley No. 7

33. Stephen Jackson and Albion

34. West Albion

35. Parson's.

36. New Diamond

37. American Bangor

38. Bangor Southern

39. Delabole

40. Grand Central

41. Doney's

42. Harding's

43. Alpha

44. Imperial

45. Phoenix

46. Excelsior

47. Penn Bangor

48. Pelican

49. Avon

50. Blue Mountain

51. Bangor Rock

52. Dexter

53. Cambria

54. Gem

55.

56.

57.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

71.

72.

National

J. K. Hower

Hqwer

Seip's

Bryan

Prudential No. 1

Prudential No. 2

Washington

Berlin's

Kester's

Henry

Peters

Washington Standard

Old Heimbach

Heimbach Slate

Heimbach Quarries

Oak Hill

Laurella

Martins Creelc 

Station.

HARD-VEIN1QUARRIES

73. Lehr's'

74. White Oak

75. Globe

76. Pennsylvania

77. Northampton

78. Whiteselrs

79. Belfast

80. Chapman Superior

81. Keystone

82. Chapman v

83. Chapman Standard

84. Valley

85. True Blue

5 Miles

LEGEND

 Road 

11 i HH-. Electric railroad 

i i i H Steam railroad 

. Slate quarry , 

Compiled from Atlas of the Second Geological Survey

of Pennsylvania < with additions and quarry locations by C. A. Bonine

(Dimensions of quarries not exact)

1914

MAP SHOWING LOCATION OF QUARRY SITES IN NORTHAMPTON COUNTY, PA.
. PENNSYLVANIA. 101

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 slate of the "Old Bangor " quarry. 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 millimeter. Very little quartz is seen, in grains reaching only 

0.036 millimeter. Pyrite spherules measure up to 0.012 millimeter and number about 

18,000 to the square millimeter. Rutile needles are very abundant. The usual carbonaceous matter is seen.

The constituents of the "Heimbach big bed," named in descending order of abundance, appear to be muscovite, carbonate, quartz, pyrite, chlorite, rutile, carbonaceous 

matter, and magnetite, but the carbonate is more plentiful 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 blacker than any of the other 

slates of Northampton and Lehigh counties. 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 carbonaceous or graphitic and very slightly magnetitig, effervesces in cold 

dilute hydrochloric acid, is sonorous, and has a slight argillaceous odor.

Under the microscope the "Heimbach black bed " shows a fineimatrix 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 millimeter. There are chlorite scales up to 0.06 

millimeter. Spherules of pyrite measure up to 0.008 millimeter, exceptionally 0.017 

millimeter, and number about 6,800 to the square millimeter. Rutile needles abound. 

Barely a scale of hematite is seen.

The chief constituents of this slate, named in descending order of abundance, appear 

to be muscovite, carbonate, quartz, carbonaceous matter and graphite (?), pyrite, 

chlorite, rutile, and magnetite. The percentage of carbonate is large.

The principal difference betwen the slates of the Heimbach and "Old Bangor" 

quarries is in the larger amount of carbonate, lesser amount of pyrite, and the fine 

structure of the former, to judge from the microsopic 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, and 390 feet in depth. In 1904 it was 

operated by two companies, Wm. Lobb & Sons at the west end and 

Stephen Jackson & Co. at the east end. The structure is that of an 

S-shaped fold overturned to the north-northwest. 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 is seen to consist 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
102 SLATE IN THE UNITED STATES.

consists of vein quarcz 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 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 millimeter. Scales of chlorite, interleaved with muscovite, are 

conspicuous, lying transverse to the cleavage, in places forming lenses with their long 

axes parallel to the cleavage, measuring up to 0.14 by 0.07 millimeter and numbering 

about 310 to the square millimeter. Pyrite spherules occur up to 0.01 millimeter and 

average about 1,085 *o the square millimeter. There is finely disseminated carbonaceous (or graphitic) matter and a great abundance of rutile needles.

The mineral constituents, named in descending order of abundance, appear to be 

muscovite, carbonate, quartz, chlorite, pyrite, rutile, carbonaceous matter, and 

magnetite. The carbonate, although second, is not as abundant as in specimens 

from the "Old Bangor" quarry.

The results of Merriman's physical and chemical tests of the darkgray ("black") slate from this quarry are here given for convenience 

of reference: Strength (modulus of rupture), 7,150 pounds to the 

square inch; toughness (ultimate deflection on supports 22 inches 

apart), 0.270 inch; 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.1

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 millimeter. There is 

considerable carbonate. There are chlorite scales, interleaved with muscovite, lying 

transverse to the cleavage and parallel to the grain; also chlorite scales transverse both 

to grain and cleavage, with inclusions of needlelike crystals, probably rutile, crossing one another, usually at 60° or 120°. All these chlorite scales average about 28 

to the square millimeter, and measure up to 0.17 by 0.1 millimeter. The pyrite 

spherules measure up to 0.01 millimeter, and average about 968 to the square millimeter. Rutile needles also occur.

These constituents, named in descending order of abundance, appear to be mus- 

' covite, carbonate, quartz, chlorite, pyrite, rutile, and magnetite.

1 Merriman, Mansfield, loc. cit.
PENNSYLVANIA. 103

The product of the Albion quarry, at least that of the west end, is 

used for roofing.

Some of the other quarries at Pen Argyl show the passage of the 

black slate by weathering into a yellowish light-brown or even white 

shaly substance. The dumps afford specimens showing all gradations from black slate to white rock. The weathered zone 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° S. and the cleavage dips less than 10° in the same direction. 

The weathered top measures between 30 and 40 feet and is covered 

with 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. 53.)

In this connection it should be stated that hi the "soft vein" 

quarries generally, to judge from the dumps, the amount of iron and 

lime in the ribbon varies greatly. In some places pyrite or siderite 

predominates and limonitic staining ensues; in others lime is more 

abundant 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. The principal quarry at Chapman, which is in the 

"hard vein" belt, is operated by the Chapman Slate Co. 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 syncline or anticline, still visible on the south-southeast wall of the quarry, striking N. 75°-78° E. 

and pitching gently eastward; but the lower limb of this fold opens 

out on the east-northeast and west-southwest walls so as to dip steeply 

northwest. This fold is crossed by cleavage striking N. 65 ° E. and dipping 14 °-l 5° SSE. The principal joints are longitudinal, strikingN. 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
104 SLATE IN THE UNITED STATES.

slate, -measures 16 feet. The number of ribbons exposed on the eastnortheast wall, where the fold opens out, is 112, averaging a little 

over two to each foot of slate.1

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 many ribbons a trifle 

darker and measuring from one-eighth to one-half inch, which, however, scarcely deflect the cleavage.

To the unaided eye it lias 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 30 years' exposure. 

E. H. S. Bailey gives the specific gravity as 2.79. For Merriman's recent tests of this 

slate see page 182.

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 millimeter 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 the slate of the "Old Bangor" quarry. Quartz up to 0.06 millimeter, 

pyrite spherules up to 0.02 millimeter and numbering about 4,250 to the square millimeter; about 263 scales of chlorite, with interleaved muscovite to the square milli" 

meter, lying across the cleavage and measuring generally up to 0.08 by 0.04 millimeter; 

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, named in descending order of abundance, appear to 

be muscovite, quartz, carbonate, pyrite, chlorite, rutile, carbonaceous matter or 

graphite and magnetite.

The product of this quarry was formerly used largely for flagging, 

posts, steps, etc., and because of its hardness had to be out 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 COUNTY.

Geologic 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 east side of the gap the outcrop of the 

Martinsburg ("Hudson") shale, of Ordovician age, that is nearest to 

the so-called "Oneida" conglomerate is a black cshale that occurs in 

alternating thick and thin beds. The thick beds, being entirely 

devoid of cleavage, weather into shelly fragments, and the thin ones,

i Figures obtained through the courtesy of Mr. Richard Chapman, superintendent.
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XVI

70° 40'

SECTIONS ALONG LINES (A), (B), AND (C)

DOTTED LINES I.N SECTIONS SHOW PROBABLE COURSE OF BEDS

MAP OF

THE SLATIISTGTON 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.

Contour Interval 20 feet. 

St 1905.

Direction of strike of bed. 

P- '^ Direction and amount of pitch of axis of fold.

40
U. S. GEOLOGICAL SURVEY BULLETIN B86 PLATE XVII

BLUE MOUNTAIN SLATE QUARRY, NEAR SLATINGTON PA.

Looking east. Showing the character of the syncline, its relation to the cleavage, and the southward bonding of the cleavage at the surface : a bending probably due

to glacial action.
U. S. GEOLOGICAL SURVEY

NORTHERN PART OF EUREKA SLATE QUARRY, NEAR SLATINGTON, PA.

Looking east Eroded northward-inclined close syncline. crossed by southward-dipping cleavage and 

undulating joints and overlain by residual clay and glacial deposits. The cleavage within the syncline 

tppears to be curved also.
PENNSYLVANIA. 105

owing to the intersection of an incipient slaty cleavage with the bedding foliation, break up into sticklike 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 

farther south a southward-dipping slaty cleavage predominates, and 

the northward-dipping bedding is shown only 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 slates of Lehigh County undergo the same transformations as those of Northampton County. The black slate first 

passes into a soft yellowish-brown ocherous 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, page 53.

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 stream 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 had 

in 1902 reached 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°.

The character of the folds in these quarries is shown in Plates XVII 

and XVIII and figures 7, 8, 9; 10. The relation of the workable beds 

to the ribbons is shown in Plate XVII.' The strike ranges from N. 

40° E. to east and to N. 86° W. The cleavage strike is N. 74°-S5° 

E., and its dip 35°-75° S. 5°-16° E.

In places there is 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 usually 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 Plate XVII; also strike 

joints which locally undulate like bedding planes, as at the Eureka 

quarry (PL XVIII).
106 SLATE IN THE UNITED STATES.

The quarrymen's names of the slate beds in the Slatington area are, 

from south to north, Williamstown, Blue Mountain, Trout Creek, 

Washington, Little Franklin, Big Franklin, Mammoth, New Bangor,

FIGURE 7. Sketch of anticline in Provident Slate Co.'s quarry, near Slatington, Pa., looking west. Width

represented, 50 feet.

Snowdon, and Eureka. It is uncertain whether some of these beds 

are not duplications, for the structure of the belt, as shown in the 

section on Plate XVI, indicates that they may be, and their relations

FIGURE 8. Sketch of anticline in old quarry north of Slatington, Pa., looking west. Height represented,

25 feet.

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,
PENNSYLVANIA. 107

and the "Franklin," at the "Old Franklin" quarry, which consists of 

an upper bed of 28 feet and a lower one of 35 feet, separated by about 

25 feet of small ribboned material, the measurements being taken

FIGURE 9. 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.

along the cleavage. In general the beds range from 3 to 30 feet in 

thickness, measured at right angles to the bedding.

The slate of the Slatington area is very dark-bluish gray. To the unaided eye it 

has a somewhat fine texture and cleavage surface, but it is almost lusterless. It ia 

carbonaceous and slightly magnetitic. The sawn edges show little if any pyrite. It

FIGURE 10. Sketch of folds at Eureka quarry, near Slatington, Pa., looking east. Length, 350 feet; depth, 

225 feet. The upper part of the syncline is shown in Plate XVIII.

effervesces in cold dilute hydrochloric acid, is sonorous, and has some argillaceous 

odor. Alter 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.
108 SLATE IN" THE UNITED STATES.

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 millimeter; rarely one of plagioclase; a few scales of chlorite, interleaved with muscovite, or of each separately, up 

to 0.08 millimeter, and numbering about 7 to the square millimeter, spherules of 

pyrite up to 0.012, rarely 0.02 millimeter., numbering about 11,000 to the square millimeter; very abundant rutile needles.

The principal constituents, named in descending order of abundance, appear to be 

muscovite, carbonate, quartz, kaolin, pyrite, chlorite, rutile, carbonaceous or graphitic 

matter and magnetite.

The following analyses were made by Dr. W. F. Hillebrand 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 Lehiqh County, Pa.

Silica (Si02)...... ..........................................................

Ferric oxide (Fe2 03 ) ........................................................

Soda (Na2 0) ................................................................

Water below 110° C. (H2O).. . ...............................................

Water above 110° C. (H2 0).. . ...............................................

Carbon dioxide (CO2 ) .......................................................

Baryta (BaO). ..............................................................

Lithia (Li2O) ...............................................................

A (complete).

5G.38

15.27

61.67

63.23

2.84

4.23

. 1.30

3.51

.77

4.09

.78

Trace?

3.67

.17

Trace?

.09

.08

Trace.

el. 72

.59

100. 39 

2.783

B (partial).

56.85

015.24

} c5.52

2.93

4.24

1.38

3.34

.45

.84

3.58

(d)

Trace?

Not est.

Not est.

el. 72

96.12 

2.780

a Including PjOj. 

6 Approximate.

c As Fe203 .

d Included in Al2Os.

e Including 0.92 of sulphur.

The considerable amount of carbonate indicated both by the analyses and by the microscopic examination and the discoloration due 

probably to the ferrous part of that carbonate are objectionable 

features in the slates of Slatington, but their fine cleavage 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 and particularly for blackboards and other indoor objects.

CARBON COUNTY.

Aquasliicola. The only quarry in Carbon County is that of W. M. 

Bray, of Palmerton, Pa., less than one-fourth mile northeast of 

Aquashicola village, northeast of Lehigh Gap, and nearly 2 miles 

east-northeast from Palmerton station. The opening now worked
PENNSYLVANIA. 109

is about 275 feet along the cleavage by 150 feet across and from 90 

to 155 feet deep. The " top " is about 40 feet thick, of which a thickness of 17 feet is bent southward by glacial friction (creep) and is 

decomposed.

The. beds strike N. 70° E. and dip 80° N. 20° W. The cleavage 

strikes N. 70°-75° E. and dips 75° S. 17° E. On the west wall, at the 

north side, are curved joints striking about east, spaced 3 to 6 feet. 

Similar joints are very abundant on the east wall. The weathered 

" top " is buff, as in the quarries of Lehigh and Northampton counties, 

and decarbonization has operated chiefly along the bedding foliation. 

The course of the bed is shown in places by undefined calcareous 

nodules weathering light brown, which in thin section show very faint 

aggregate polarization, much carbonate in minute particles, quartz, 

chlorite, pyrite spherules up to 0.03 millimeter in diameter, and fine 

carbonaceous matter.

The slate is very dark bluish gray. To the unaided eye it has a very fine and 

finely banded texture and roughish lusterless cleavage surface. It contains much 

carbon and a little magnetite. The sawn edge shows pyrite. The rough edge effervesces with acid test. It is sonorous and has a high grade of fissility. The ribbons 

do not appear to be lines of weakness.

Under the microscope this slate shows a matrix of muscovite (sericite) with fairly 

good aggregate polarization. The cleavage is fine, crossing the bedding at a small 

angle. Quartz particles (detrital) measure up to 0.02 millimeter and abundant 

spherules of pyrite to 0.017 millimeter in diameter. Carbonate is plentiful but not 

excessive: Chlorite and rutile are present. The constituents, named in descending,, 

order of abundance, appear to be muscovite, quartz, carbonate, chlorite, carbon, 

kaolin, pyrite, magnetite, and rutile.

This is a mica slate of the fading series.

DAUPHIN COUNTY.

Some slate cores obtained in 1908 near Deny Church, in Dauphin 

County,. were examined, with the following results:

The slate is of dark bluish-gray color, with very fine, even, and somewhat lustrous 

cleavage surface.. It shows pyrite on the sawn edge, is graphitic, contains no magnetite, and effervesces freely with cold dilute hydrochloric acid. It is sonorous and 

has a superior grade of fissility.

Under the microscope it shows fine aggregate polarization and abundant rutile 

needles, much carbonate in rhombs and irregular plates up to 0.06 millimeter,' also 

abundant pyrite in spherules from 0.004 to 0.017 millimeter in diameter but mostly 

under 0.008 millimeter. The cleavage foliation is crossed by a fine zigzagging vein of 

calcite. It contains nodules of carbonate up to 0.1 by 0.04 millimeter and of chlorite 

up to 0.1 by 0.04 millimeter. The quartz grains measure up to 0.06 millimeter and 

there are a few prisms of black tourmaline.

Its constituents, named in descending order of abundance, appear to be muscovite, 

quartz, carbonate, pyrite, chlorite, graphite, rutile, and tourmaline.

This is a tru'e mica slate of fine fissility but of the fading series. The cores show 

some variation at different depths in the abundance of carbonate veinlets and also in 

what appear to be "ribbons," but the general quality of the slate is the same.
110 SLATE IN" THE UNITED STATES.

YORK AND LANCASTER COUNTIES.

The slate deposit of York and Lancaster counties is known as the 

Peach Bottom slate. The name seems to have been derived from 

that of a small village one-fourth mile west of 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.

PEACH BOTTOM SLATE.

Geologic relations. The only available geologic maps of the region 

are those of the two Pennsylvania geological surveys and of the Maryland Geological Survey.1

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, in a southwest 

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., 1^- miles is 

submerged by the Susquehanna, 4£ miles is in York County, Pa., and 

about 3 miles is 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 part 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. 2

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 75° SE. About midway between Delta and 

Bryansville this schist is calcareous. According to the reports of the 

Second Pennsylvania Survey similar schists recur on the southeast 

side of the slate belt, and the whole schist formation overlies in synclinal structure the older gneisses and serpentines which occur on

1 Rogers, H. D., Geology of Pennsylvania, 1858. Frazer, Persifor, jr., York and Lancaster county 

sheets, Second Geol. Survey Pennsylvania, vol. CCC, Atlas, 1878,1879. Mathews, E. B., Maryland Geol. 

Survey, vol. 2, PI. XXX, 1898.

2 Rogers, H. D., Geology of Pennsylvania, vol. 1, pp. 188,189,1858. Frazer, Persifor, jr., Geology of Lancaster County: Second Geol. Survey Pennsylvania, vol. CCC, pp. 179-190, Pis. VII, vni, 1880. Williams, 

G. H., The petrography and structure of the Piedmont Plateau: Geol. Soc. America Bull., vol. 2, p. 304, 

1891. Lesley, J. P., Second Geol. Survey Pennsylvania Final Rept., vol. 1, pp. 136-141,1892. Mathews, 

E. B., Maryland Geol. Survey, vol. 2, pp. 215-231, 239-241,1898. Merrill, G. P., Rocks, rock weathering, 

and soils, p. 229,1897; Stones for building and decoration, 3d ed., 1903. Merriman, Mansfield, The strength 

and weathering qualities of roofing slates: Am. Soc. Civil Eng. Trans., vol. 27, pp. 331-349, 1892; vol. 32, 

pp. 529-539,1894.
PENNSYLVANIA. Ill

either side of it. These schists and the associated slate are classified 

as Cambrian, although no typical Cambrian fossils have as yet been 

found in them. A good exposure of the relations of the slate to the 

rocks on the southeast (referred to by Eogers) may be seen on the 

east side of Slate Point on the towpath along the Susquehanna. The 

slate here conformably underlies about 50 feet of slightly muscovitic 

quartzite, which was formerly quarried for canal construction, both 

rocks striking N. 45°-50° E. and dipping 70° SE. A little farther 

east this quartzite is succeeded 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, slightly muscovitic 

quartzite about 70 feet thick crops out with a strike of N. 45°-50° E. 

and a dip of 42° SE. and is followed conformably by a gneiss like that 

previously described. These quartzites and 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.1

The structure of the slate belt proper is difficult to make out. The 

quartzite of the southeast side was not found on the northwest side 

during the reconnaissance. At Foulk Jones & Sons' Slate Hill 

quarry, 2 miles northeast of Delta, there are some indications of 

anticlinal structure. Apparent bedding planes clip 30°-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 conplex syncline or anticline or the anticlinal axis of the entire belt 

could not be determined. There is also danger of confounding 

curved joints and bedding. Frazer'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.

Characteristics. The color of the Peach Bottom slate 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 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.2 Under the 

microscope it 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 some of them 0.12 by 0.03 millimeter. In some specimens 

chlorite can be made out mingled with the muscovite. There are tabular crystals of 

magnetite measuring up to 0.11 by 0.02 millimeter, and also lenses of magnetite of 

larger size, but the most conspicuous feature is the abundance of crystals of andalusite

1 See also Frazcr, Persifor, jr., op. cit., pp. 133,134.

2 The apparent contradiction of this last statement with the passage of this slate into red clay in weathering is explained on page 53.
112 SLATE IN THE UNITED STATES.

(Al2O3 .SiO.,, silicate of alumina) measuring from 0.008 by 0.001 millimeter and under 

up to 0.11 by 0.03 millimett,r and exceptionally 0.2 by 0.02 millimeter, and numbering from 9 to the 310 square millimeter. These crystals contain opaque inclusions arranged diagonally to their vertical axes, and many of them are in lenses measuring from 0.04 to 0.25 millimeter long by 0.04 to 0.1 millimeter 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. Where not in lenses, the andalusites generally have a plate of muscovite on one or both sides. These crystals 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. 

11.) Pyrite is difficult to distinguish under incident light from the andalusite when 

this is rendered opaque by inclusions. Kutile occurs in irregular crystalline massf s

FIGUEE 11. Transverse section of Teach Bottom slate, showing andalusite crystals in a rather coarse 

micaceous matrix. Enlarged about 113 diameters. Some of these crystals are in lenses of quartz and 

muscovite and several show inclusions.

up to 0.09 by 0.04 millimeter, but no needles were detected. There is rarely a rounded 

grain of zircon; no carbonate was detected.

The important constituents of this slate, named in descending order of abundance, 

appear to be muscovite, quartz, graphite, andalusite, magnetite, and pyrite, with 

accessory rutile and zircon. Figure 11 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 partly 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 limonite from the alterations.

Peach Bottom slate has been thoroughly tested by Merriman, 1 

with results as follows: Modulus of rupture in pounds to the square 

inch, 11.260; ultimate deflection in inches on supports 22 inches

1 Op. cit.
PENNSYLVANIA. 113

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 24'hours), 0.224; corrodibility (per cent of weight lost in 63 hours 

in acid solution, consisting of 98 per cent H20, 1 per cent HC1, 1 per 

cent SO3), 0.226.

The three published analyses show the following percentages of 

lime: 0.155, 1 0.3,2 0.48.3 That by the Second Pennsylvania Survey 

will be found in full on page 50 (analysis 7). This lime may be referable to the phosphate, apatite, which the analyses show to be present.

The Peach Bottom slate is now used almost exclusively for roofing. 

It also makes excellent gravestones. The chief features of this slate 

are stated in tabular form in the section on the comparative characteristics of slates (p. 188).

Delta and Cardiff quarries. In 1912 seven concerns were operating quarries in the Peach Bottom slate belt. In June, 1904, there 

were 11. 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° E. to N. 55° E. There is usually a horizontal joint 

"big fiat joint" pitching gently south, locally 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 in thickness 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 joints 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 stand vertical or dip 25° or 45° SW. At 

Foulk Jones & Sons' quarry there are vertical diagonal joints striking N. 10° E. to N. 10° W. The grain dips 20°-50° NE. Quartz 

veins meander in the cleavage direction. The slate commonly has 

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 

half an inch to an inch thick. The hems represent simply the initial 

stage of weathering caused by the percolation of water from the joints,

1 Second Pennsylvania Geol. Survey Rept. Progress for 1877, Vol. CCC, 1880; analyses by A. S. McCreath, 

pp. 269, 270.

2 Analyses by Booth, Garrett, and Blair, 1885; see Maryland Geol. Survey, vol. 2, p. 226, 1898. 

s Merrill, G. P., Rocks, rock weathering, and soils, 1897, p. 229.

40620° Bull. 586 14 8
114 SLATE IN THE UNITED STATES.

and of this 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 Merrill 

are given on page 53. The effect of weathering, as shown under the 

microscope, will be discussed 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 locally 

in vertical planes diagonal to the cleavage. This material proves to 

be an aggregate of much pyrite, carbonaceous matter, quartz, chlorite, and muscovite, and is probably vein matter segregated from the 

adjacent slate.

The Baltimore Peach Bottom Slate Co., of Delta, Pa., has recently 

adopted the tunnel method in its Cardiff quarry in order to save the 

expense of removing the "top." It is found that this method also, 

has the advantages of affording not only a shelter to the workmen 

but a uniform temperature of 60° both day and night and of preserving the moisture in "quarried blocks for an indefinite period. The 

tunnel is 6 or 7 feet high, 12 feet wide, and about 600 feet long at 

right angles to the cleavage. It has lateral chambers, one on each 

side, measuring, respectively, 70 and 30 feet in length and 60 and 50 

feet in width. One is 60 feet deeper than the tunnel floor; the other 

is 12 feet higher than its roof. Channeling and overcutting machines 

operated by compressed air are used. 1 ^

Lancaster County prospect. Evidently belonging to the Peach 

Bottom slate is a deposit prospected in 1910 by P. B. Shank near 

Peters Creek, in Lancaster County. A specimen examined yielded 

these results:

The color of this elate is very dark gray, slightly bluish. The cleavage surface is 

fairly smooth, with minute striations in the grain direction and lustrous. The fissility 

is fair, grade (2). It is very sonorous. There is no effervescence with cold dilute 

hydrochloric acid nor any carbonate in thin section. It is very graphitic and shows 

no pyrite on sawn edge; there was a little magnetite in the powdered slate and in thin 

section.

Under the microscope it shows a matrix of muscovite (sericite), very even in grain 

direction, with marked aggregate polarization and containing considerable quartz and 

finely disseminated graphite. Its most conspicuous feature is the abundance of 

groups of intergrown rutile crystals (sagenite). These groups measure up to 0.04 millimeter in diameter and average about 15 to the square millimeter of thin section. A 

chemical analysis of the slate would probably show an unusually high percentage of 

Ti02 (titanium dioxide) for a slate. There are also andalusite prisms (aluminum silicate) up to 0.066 by 0.009 millimeter, more or less darkened by inclusions and lying 

mostly with their long axis in the grain direction. A little tourmaline is present and 

probably chlorite. The constituents, named in descending order of abundance, 

appear to be muscovite, quartz, graphite, andalusite, rutile, chlorite (?.), magnetite, 

tourmaline, pyrite, and zircon.

1 These particulars obtained through the courtesy of Mr! C. F. Guild, general manager.
TENNESSEE. 115

This is a graphitic mica slate free from carbonate. It will not discolor and ought 

to prove very durable. . Its. cleavage appears to correspond with that of the Peach 

Bottom slate, which it generally resembles.

TENNESSEE.

Slate deposits occur in eastern southeastern, and northeastern 

Tennessee.

BLOTJNT COUNTY.

The slate deposits southeast of Knoxville were described by 

Keith * in 1895 as follows:

Two formations in this region contain beds of slate the Wilhite and Pigeon slates. 

The Wilhite slate is too cr.lcareous 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 great 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 Southern Slate Co., of Columbus, Ohio, quarried a little slate 

near Chilhowie in 1910. There is also a slate prospect on the Virgil 

Grant farm, 3£ miles from Chilhowie.

The following analyses of slate quarried by the Southern Slate Co. 

at Maryville were made by Gilbert McCulloch, of the University of

Tennessee.

Analyses of Tennessee roofing slate.

Silica(SiOs). .................

1

58.45

21.88

6.04

1.80

.46

2

59.00

23.44

6.28

1.30

.50

Potash(IC2 0). ................

Soda (Na«0). .................

Water........................

1

1.60

2.34

.65

6.66

2

2.04

1 78

oo

4 64

MONBOE COUNTY.

Several outcrops and prospects on Tellico River and its tributaries 

were visited by the writer in 1912. The localities are among the 

western foothills of the Great Smoky Mountains at elevations between 

800 and 1,000 feet above sea level and in the drainage basin of

1 Keith, Arthur, U. S. Qeol. Survey Geol. Atlas, Knoxville folio (No. 16), p. 6,1895.
116 SLATE IN THE UNITED STATES.

Tennessee River.1 According to the folios on the Cleveland quadrangle,2 on the west, and the Nantahala quadrangle,3 on the east, the 

nearest areas that have been geologically mapped by the United 

States Geological Survey, these slates probably belong in the Hiwassee 

slate (Pigeon or Wilhite slate of Hayes) and are of Cambrian age. 

They consist of purplish, greenish, and black mica slates, interbedded 

here and there with quartzite and fine conglomerate.

The openings all lie within a radius of 3 miles of Tellico Plains 

station, the terminal of a branch of the Louisville & Nashville Railroad, and were made by the Tellico Co., of Tellico Plains.

Falls Branch prospect. This is 3 miles S. 5° E. of Tellico Plains 

station, on Falls Branch, a brook flowing northward intp Laurel 

Creek, a tributary of Tellico River. It is on the east side of Tellico 

Mountain, 400 feet above Tellico Plains station and about four-fifths 

of a mile south of the point where the Murphy pike crosses the branch. 

Ths opening is only 10 feet square and deep. Both bedding and 

cleavage strike N. 55° E. and dip S. 35° E., but the dip of the bedding 

is 25° and that of the cleavage 50°. The thickness exposed is about 

40 feet, but the total thickness may be much greater.

The slate is very dark bluish gray with a lustrous ribboned cleavage face of more or 

less smoothness. It does not effervesce with acid test, even when powdered. The 

sawn edge shows black metallic grains; the powder some magnetite and much graphite.

Under the microscope the matrix shows brilliant aggregate polarization and lenticular texture. Small ribbons cross the cleavage at an angle of 14°. These contain coarser 

particles of carbonate, much quartz, pyrite in particles and spherules, and here and 

there a plagioclase grain. The matrix has many lenses of chlorite and muscovite, 

mostly 0.04-0.14 millimeter, detrital quartz grains up to 0.22 millimeter, some lenses 

of carbonate as much as 0.14 millimeter, of pyrite up to 0.09 millimeter, and a lens of 

pyrite, quartz, and muscovite 0.56 millimeter. The section with the cleavage shows 

much carbonate in very minute rhombs and plates. Rutile is plentiful in short 

needles, also fine graphite, tourmaline, and zircon.

The constituents, named in descending order of abundance, appear to be muscovite, 

quartz, chlorite, carbonate, graphite, magnetite, pyrite, rutile, tourmaline, zircon, 

and plagioclase the last two and the quartz being detrital.

This is a mica slate of the fading series, but the discoloration will probably be 

slight.

Lyons Creek prospect. About 1^ miles southeast of Tellico Plains 

station, on the southwest side of Lyons Creek half a mile above its 

junction with Tellico River, is a cliff 40 to 50 feet high, of black slate, 

with a rusty-faced cleavage striking N. 50° E. and dipping 80° S. 40° E. 

Curved j obits or beds in the upper part of the cliff correspond to 

ribbons below, which dip about 20° N. The slate crops out for 200 

feet across the strike of the cleavage. About 325 feet N. 25° W. of 

the prospect is a bed of quartzite over 50 feet thick striking N. 50° E.

i See map of Murphy quadrangle, U. S. Geol. Survey.

« Hayes, C. W., U. S. Geol. Survey Geol. Atlas, Cleveland folio (No. 20), 1895.

a Keith, Arthur, idem, Nantahala folio (No. 143), 1907.
TENNESSEE. 117

and dipping 55° S. 40° E. The cleavage faces along the cliff are 

limonitic.

The slate is bluish black with more or less lustrous cleavage face. 

It is sonorous and does not effervesce with acid test but contains 

little whitish granular pyritiferous beds up to 0.1 inch thick which 

effervesce strongly with acid and appear to be calcareous sandstone.

Under the microscope this slate has a brilliant aggregate polarization. A 0.2-inch 

bed of coarser particles zigzags at right angles across the cleavage and abounds in 

quartz grains (mostly under 0.09 millimeter) and carbonate, has pyrite on the edges, 

and contains chlorite, a plagioclase, and a zircon grain. The matrix is more carbonaceous than the sandy beds and in sections with the cleavage shows lenses of pyrite to 

0.5 millimeter, with secondary fibrous quartz on opposite edges, much carbon and 

carbonate, some tourmaline, and rutile.

The constituents, named in descending order of abundance, are muscovite, quartz, 

carbonate, graphite, pyrite, chlorite, plagioclase, tourmaline, zircon, and rutile. Of

these, most of the quartz and all the zircon and plagioclase arc detrital.

This is a mica slate with much carbonate and pyrite and will probably discolor 

badly.

Laurel Creek quarry. This is but little over a mile southeast of 

Tellico Plains station on the southeast side of Laurel Creek about a 

fourth of a mile from its junction with Tellico River. The opening is 

25 feet square and 50 feet deep. The cleavage, which appears to 

coincide very nearly with the bedding, strikes N. 35° E. and dips 

55° S. 55° E. The slate is purple but incloses a bed of green 5 feet 

thick.

The purple (specimens D. XXXIV, 9, a-d, g) is of dark purplish-gray color and of 

lusterless smooth cleavage surface, in places with minute dark-green lenses. Some 

very delicate brownish beds and lenses up to 0.3 inch thick are calcareous, but the 

slate itself does not effervesce with acid test. The powdered slate effervesces strongly 

but shows no magnetite. Under the microscope it shows brilliant aggregate polarization and fine texture, with little beds of coarser material up to 0.37 millimeter thick 

crossing the cleavage at an angle of 5°. It has a very chloritic lens 0.62 millimeter 

thick. Minute irregular dots of very dark red hematite are scattered throughout and 

are coarser in the little beds, where they appear to be black and metallic. Much 

chlorite; no carbonate except in the little beds. The constituents, named in descending 

order of abundance, appear to be muscovite, quartz, chlorite, hematite, carbonate, 

kaolin, and rutile.

The green (specimen D. XXXIV, 9, h) is of light-blue to greenish-gray color and has 

a lusterless smooth cleavage surface with lenses of darker green 0.1 inch long and very 

thin, also brownish-weathering lenses effervescing much with acid test. The slate 

effervesces some, as does the powdered slate, which shows extremely little magnetite. 

The sawn face shows fine grayish bands and lenses. Under the microscope this slate 

shows a fine texture and brilliant aggregate polarization, but many lenses and beds 

up to 0.4 millimeter thick parallel to the cleavage containing quartz grains to 0.12 

millimeter and much carbonate in a matrix of chlorite. The general matrix contains 

much chlorite and fine quartz but no carbonate; also oxidized pyrite and abundant 

slate needles. The grain is well marked. The constituents, named in descending 

order of abundance, appear to be muscovite, quartz, chlorite, carbonate, pyrite, 

magnetite, and rutile.

These are mica slates. Although both contain carbonate, slates of both colors 

exposed for 10 years on the dumps have suffered scarcely any discoloration. The
118 SLATE IN" THE UNITED STATES.

quarry was given up because of the high cost of splitting, possibly caused by the acute 

intersection of bedding and cleavage and the little lenses.

Tellico River prospects. These are about half a mile east-southeast 

of Tellico Plains station on the north bank of the river a little east of 

a conspicuous road angle. The section seems to consist, beginning 

on the northwest, of 250 feet of slate, mostly purplish, striking N. 

15°-22° E. and dipping 45° S. 70° E., underlain by a bed of slightly 

limonitic quartzite 25 feet thick, and that in turn underlain by 75 

feet of green and purple slate, under which is still another bed of 

quartzite, all with the same dip and strike. The cleavage and bedding 

of the slate appear to be nearly identical, although the dip of the 

first bed of quartzite at the contact is 55°.

The purple slate (specimens D. XXXIV, 6, b, c, e) is of medium purplish-gray color 

and has an almost lusterless cleavage surface. It is dotted with minute lenses and 

contains very delicate limonitic films which do not effervesce with acid. The sawn 

slate shows pyrite and the powdered slate effervesces slightly and does not show 

magnetite. Under the microscope this has a brilliant aggregate polarization and a 

fine but somewhat uneven texture. There are chloritic lenses up to 0.5 millimeter 

thick and limonitic streaks up to 0.25 millimeter thick. The section with the cleavage 

shows no carbonate but dots of dark-red hematite, black metallic particles, and some 

pyrite, with limonite from both; rutile exceedingly minute. The constituents, 

named in descending order of abundance, appear to be muscovite, quartz, chlorite, 

hematite, kaolin, pyrite, carbonate, limonite, and rutile.

The green slate is light greenish gray and otherwise corresponds to the purple. Its 

color is identical with the green of the Laurel. Creek quarry described above and its 

composition presumably not far different.

SULLIVAN COUNTY.

A slate deposit 1^ miles south of Kingsport was visited in 1909, 

by C. H. Gordon, of this Survey. The following note is taken from 

his written report:

The slate belongs to the Athens shale of the Ordovician l and is exposed along a small 

branch that rises on the north slope of Bays Mountain and flows into Holston River. 

The bedding, to judge from, the relations to the underlying and overlying formations, 

probably dips 15°-20° S.. 10° E. The cleavage strikes N. 50° E. and dips 60° S. 40° E. 

The two principal sets of joints, both nearly vertical, strike N. 60° E. and N. 30° W. 

The slate is "black, " homogeneous, and even grained and effervesces with acid. In 

some places graptolites are very abundant. Small masses of pyrite are occasionally 

seen.

Thin sections of this slate, examined by the writer, yield these 

results:

Cleavage fair, but aggregate polarization feeble owing to incomplete micasization. 

Very carbonaceous; not graphitic. Carbonate very abundant; much pyrite in 

spherules, little quartz, rare grains of plagioclase, chlorite scales, Veinlets of calcite 

in the cleavage foliation.

This is not far from a clay slate, resembling somewhat that of Martinsburg, W. Va. 

(See p. 165.) It will probably fade badly. Its large percentage of carbonate and 

small content of quartz and mica make its commercial value doubtful.

i U. S. Geol. Survey Geol. Atlas, Estillville folio (No. 12), 1894.
UTAH. 119 

WASHINGTON COUNTY.

Specimens of "black" slate from the vicinity of Austin Springs, 

4 miles north of Johnson City, and also of the Athens shale, were 

examined microscopically with these results :

Better aggregate polarization than the slate near Kingsport, with much less carbonate and also more quartz. Some pyrite; carbonaceous matter probably graphite.

This is a mica slate the commercial value of which will depend mainly on the uniform quality of the cleavage and the distribution of the joints. There are no data as 

to the structure of the deposit.

UTAH.

By EDWIN C. ECKEL.

For some years past a small amount of slate has been annually 

quarried, 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 can not be obtained. It is possible, 

however, that this condition will disappear if the deposits are worked 

deeper.

The 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 and will probably be less satisfactory for roofing 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 the Peach Bottom slate of Pennsylvania and Maryland. 

From samples seen it appears that they also bear punching well.

A specimen of the purple slate selected by the writer was analyzed 

by W. T. Schaller in the laboratory of the United States Geological 

Survey, the results being as follows :

Analysis of purple slate from Provo, Utah.

Silica (Si02)......-. ........................................... 54.05 

Alumina (A1203 ). . ............................................ 20. 95

Iron.oxides (FeO, Fe2 03 ). . .................................... .28

Lime (CaO). ................................................. .22

Magnesia (MgO) .............................................. 9. 12

Carbon dioxide (C02)\ Q Qn ......../' 3-9° Water.
120 SLATE IN" THE UNITED STATES.

Of a series of 36 analyses of American roofing slates collected and 

discussed 1 by the writer, the above analysis of the slate from Provo 

stands lowest in its percentages of silica and magnesia, and its 

lime is very far below the average.

Nothing definite is known as to the geologic age of these slates, 

though they are supposed to be Ordovician, or even older.

VERMONT.

GEOGRAPHIC RELATIONS.

There are at least four distinct slate districts in Vermont. The 

most easterly extends along Connecticut River for more than twothirds the length of the State. The slate is black or dark gray and 

has been worked in Guilford, in Windham County, at the extreme 

south end of the State, and also in Thetford, in Orange County, and 

at Waterford, in Caledonia County. The next district extends 

along the east flank of the Green Mountain range from the Canada 

line to about the middle of the State, and its slate 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 Washington County, N. Y., 

where, however, it has thus far proved of less economic importance. 

The fourth district is an area of black slate, as yet undeveloped 

and covering only from 2 to 3 square miles in the town of Benson, in 

Rutland .County, near Lake Champlain.

NORTHFIELD DISTRICT.

The Vermont geologic map 2 represents a belt of clay slate, from 

1 to 7 miles wide, 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 paper and map by 

Richardson 3 represent 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 earlier report,4 but Richardson regards it as 

lower Trenton. Conclusive paleontologic evidence on the subject is 

yet lacking.

1 Eckel, E. C., Jour. Geology, vol. 12, p. 26,1904.

1 Hitchcock, C. H., and Eager, A. D., Report on the geology of Vermont, vol. 2, PI. I, p. 794,1861. 

3 Richardson, C. H., The terranes of Orange County, Vt.: Vermont State Geologist Rept., new ser., 

vol. 3, pp. 77-79, Pis. IX, IX, A, 1902. 

* Hitchcock, C. H., and Hager, A. D., op. cit., vol. 1, p. 497,1861.
VERMONT. 121

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° E. to N. 25° E. At the Vermont Black Slate Co.'s lower 

quarry, 2 miles south of Northfield, the beds occur in minor very 

acute folds, which strike N. 10° E., while the cleavage, with an 

almost identical strike, dips 75° W. At that company's upper quarry, 

a quarter of a mile to the east, the beds strike N. 25° E. and dip 75° 

N. 65° W., and the cleavage likewise. The interpretation given by 

Richardson of Plate XV in his paper, referred to above, requires 

corroborative microscopic evidence. The low easterly (by error 

printed west) dipping planes resemble a secondary cleavage, and 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 of a mile southwest of the 

village, by a sericite-chlorite-quartz schist containing grains of 

plagioclase, with a vertical slip cleavage striking about north and a 

plicated bedding having a northerly pitch. Beyond a covered interval, at a point on the range 2 miles west of the village, there is a large 

exposure of thin-bedded, more or less muscovitic quartzite, containing 

grains of zircon and plagioclase, so intricately folded as to strike N. 

20° E., N. 20° W., N. 50° E., and due east. 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.

In 1904 but one quarry was in operation in Northfield Township, 

that of the Vermont Black Slate Co., which then measured about 

100 feet along the strike, 60 feet across it, and 55 feet in depth. 

The company later reopened the "Clark quarry," on the ridge, a 

quarter of a mile east and 100 feet above the first. In 1909 both 

quarries became idle, and they were still idle in 1912. The Clark 

quarry has an east-west working face 100 feet long and 35 feet high 

and the beginning of a north-south face.

At the lower of these quarries the cleavage strikes N. 12° E. and 

dips 75° W. There are three sets of joints (a) striking N. 15° E., 

dipping 50° N. 75° W.; (6) striking N. 77° W., dipping nearly 90°; 

90°; (c) striking N. 55° E., dipping .65° N. 35° W. The grain dips 

70° N. An exceptional slip cleavage dips 15°-20° N.

At the Clark quarry the cleavage and beds strike N. 25° E. and 

dip 75° N. 65° W. Slip cleavage, which is confined to certain beds and 

to certain horizontal zones, strikes N. 60° E. and dips 15° N. 30° 

W. A quartz vein half an inch thick cuts the beds from east to
122 SLATE IN THE UNITED STATES.

west, meandering horizontally, and then dips about 45° W. across 

them. East of it is an 8-foot heading, much veined with quartz. 

In a height of 30 feet there are but three horizontal joints, and these 

undulate. The slate beds here, from 6 inches to 3 feet thick, are 

separated by quartzose beds one-fourth inch, to 3 inches thick 

and some very pyritiferous beds 1 or 2 inches thick, all of which 

weather a muddy brown, are void of cleavage, and effervesce freely 

with acid. In thin section these are a uniform compound of (1) 

carbonate plates and rhombs, (2) detrital quartz grains, (3) muscovite scales, and (4) fine carbonaceous material, with (5) a few 

lenses of secondary quartz, some with a nucleus of pyrite, these being 

numbered in descending order of abundance'.

The slate itself is very dark gray and has a very lustrous cleavage face, smooth or 

dotted with minute lenses or with longish crystals of pyrite pointing in the grain 

direction. It is very sonorous, very fissile, does not discolor, does not effervesce, 

even when powdered, with cold dilute hydrochloric acid, and contains much graphite, more or less magnetite, and some magnetic iron pyrites (Fe7 S8 ) with here and there 

a nodule of it as much as half an inch in diameter. Where slip cleavage occurs 

the plications that give rise to it average about 20 to the inch and, in the sections 

examined, cross the cleavage at nearly 60°.

Under the microscope the matrix of sericite has a brilliant aggregate polarization 

and a very even texture. Porphyritic crystals or lenses of magnetitite or of pyrite 

(some with rim of secondary quartz, or of chlorite, radiating to 0.08 millimeter along 

the cleavage) average 4 or 5 in a square millimeter, and measure 0.05 to 0.24 by 0.02 to 

to 0.06 millimeter, some with their long axis in the grain direction; rare lenses of 

carbonate up to -0.37 millimeter; and some entirely of rutile crystals. Detrital quartz 

up to 0.06 millimeter (mostly), and chlorite scales, some with interleaved muscovite, 

are plentiful. There are prisms of tourmaline, 0.36 by 0.008 millimeter, and of 

rutile, up to 0.04 by 0.006 millimeter, but no rutile needles.

The constituents named in descending order of abundance, appear to be muscovite, 

quartz, graphite, chlorite, magnetite, pyrite and magnetic iron pyrites, chlorite, rutile, 

carbonate, tourmaline, and limonite stain.

This is a superior mica slate of the unfading series resembling that 

of Brownville, Me. Its content of magnetic minerals is too high to 

make it suitable for electric uses. Owing to the scarcity of horizontal joints the company gave its attention almost entirely to the 

production of mill stock. The true method of operating such a quarry 

would be to use channelers for horizontal cuts and to make roofing 

slates of the blocks that have little or no slip cleavage and use the 

rest for mill stock. Roofing slates from the Clark quarry with not a 

little "false cleavage" are yet strong.

The following data were gathered at the abandoned quarry of the 

Union Slate Co. half a mile east-southeast of Northfield; bedding 

strike, N. 10° E., dip 80° W.; cleavage strike, N. 4° E., dip 70° W. 

Strike joints dip, undulating, low west; diagonal joints strike N. 

60°-65° W. and dip 75° NE.
VERMONT. 123

At the abandoned quarry three-fourths of a mile southeast of 

Montpelier and about 10 miles north-northeast of the Vermont Black 

Slate Co.'s quarry at Northfield the strike of both, bedding and cleavage was found to be N. 15°-20° E. ; and the dip 70°-75° W. This 

quarry was operated by means of three wide openings at intervals 

across the strike, communicating with with one another 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 fhe complex opening. The slate appears 

to be essentially the same as that of the Northfield quarries.

WESTERN VERMONT. 

GEOLOGIC KELATIONS.

The broader geographic and geologic relations of the slate belt of

western Vermont are shown in the map of the slate belt of eastern 

New York and western Vermont.1

The Ordovician (Berkshire) 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 a's at least 1,400 feet in 

thickness, which include about 240 feet of greenish and purplish 

roofing slates. 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 Ordovician 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 pages 89-93. 

In some places the Lower Cambrian slate protrudes through the 

Ordovician slate; 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 Plates 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 opened before 1904.2

The Lower Cambrian slates of western Vermont are greenish gray, 

purplish, and "variegated" that is, greenish gray and purplish 

mixed. These colors occur in alternations, in which, as is shown by 

the quarry diagrams (Pis. XXIII and XXIV) there is little regularity. 

In the main, however, the beds at this horizon seem to consist of 100 

to 140 feet or even 200 feet of greenish and purplish slates, the greenish 

beds predominating, with 40 to 50 feet of variegated or mottled slate 

overlying but possibly replacing the purple in places. On the west

1 The slates of western Vermont were fully described by the writer in U. S. Geol. Survey Nineteenth Ann. 

Kept., pt. 3, 1899, and the matter appears here in revised form, but the general map, PL XIII, is not 

republished.

3 It will bo noticed that Plate XXI fits onto the eastern half of the north end of Plate XX. The scale 

of these maps is so large that they can be xised for prospecting purposes by studying the relations of the 

quarry locations to the symbols which designate the strike of the beds. (See also p. 109.)
124 SLATE IN THE UNITED STATES.

side of Lake Bomoseen purple slate nearly 100 feet thick is exposed. 

The purple slate in places 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 

to occur not in strata of different ages but at different points in 

strata of the same age.

Interbedded with the slates are strata of calcareous quartzite 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. Here and there 

the quartzite 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 common occurrence 

in the quarries, overlying the slate. (See quarry diagram P, PL 

XXIV.)

The slate-bed surfaces are generally covered with annelid trails or 

impressions of algae, or both. Many of the purple slates are ribboned 

or banded with light-green slate beds an inch or more in thickness 

or have oval or roundish light-green spots, frequently in rows. Similarly the "sea-green" slates are crossed by grayish ribbons.

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, locally with calcareous nodules carrying Lower Cambrian fossils............................................... 10- 40

B. Roofing slate, grayish green, purplish or mixed green and purp- . 

 lish, alternating with beds of calcareous quartzite and of limestorie breccia up to 40 feet thick, the breccia carrying Lower 

Cambrian fossils........................................... 200-240

A. Olive-colored grit or graywacke, more or less massive, with minute 

scales of hematite or graphite, containing some small quartzite 

beds, much of it calcareous, generally weathering a pale brickred. Under the microscope shows grains of quartz, feldspar, 

and large scales of muscovite and of chlorite interleaved with 

i muscovite. Associated with this is a bed of quartzite 12 to

55 feet thick............................................... 50-200
U. S. GEOLOGICAL SURVEY

A. CEDAR POINT QUARRY, ON LAKE BOMOSEEN, CASTLETON, VT.

North face, showing good mill-stock slate in cross fracture. The flat syncline has 

a slight southerly pitch and the cleavage dips 15°-20° E. The board on the wall is 3£ feet long. Compare Piate^XXIV, N.

North face, s

with bedd

ing upon 

remnant o

is named.
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XX

SHOWING THE LOCATION AND SIZE OF THE QUARRIES 

OF "SEA-GREEN." PURPLE, AND RED SLATE IN THE 

TOWNS OF HAMPTON AND GRANVILLE, NEW YORK.

AND POULTNEY AND PAWLET, VERMONT. AND 

_ THE GEOLOGY OF THE VICINITY 

a Scale 027500 2 Miles

Contour interval 2Ofeet. 

________3914 ____1

78° 15'

LEGEND

Os

Ordovician slate, shale, [

grit, etc. l

(Slate, red, bright green, black) ' ' >

Ordovician 

Berkshire schist

 1 ;j

Lower Cambrian slate, 

quartzite, grit, limestone, and shale

(State, green, purple, or variegated}

\

Quarries of red slate 

with some bright-green 

slate

Quarries of "sea-green" 

or purple slate (including variegated)

I___... .____;

Strike and dip of bedding

(The shorter the arrow the 

steeper the dip)

Strike and dip of cleavage

( The shorter the lines the steeper 

the dip. Cleavage and bedding 

symbols are in places combined)

J

Strike of "hogback" or 

shear zone

Dike of eamptonite or 

augite eamptonite

(Basic eruptive)

L. ___ J

Sections along these lines 

given on Plate XXII

The numbers of the quarries refer 

to structural data given in the text.

Topography by J. H. Jennings. 

G. E. Hyde, and Jas. McCoimick. 

Quarries mostly by J. H. Jennings. 

Geology by T. Nelson Dale,assisted 

by L. M, Prindle and F. H, Moffit 

Surveyed in 1895-96
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XXI

QUARRY MAP

SHOWING THE LOCATION AND SIZE OF THE MOKE IMPORTANT SLATE

QUARRIES IN THE "UNFADING GREEN" SLATE BELT BEWEEN

POULTNEY AND WEST CASTLETON, VERMONT, AND A FEW OF

THE "SEA-GREEN" SLATE QUARRIES NEAR POULTNEY

i Scale eaSoo

LEGEND

Os

Ordovician slate, shale, ^ 

grit, etc. j

(Slate, red, bright green, black) ; 

Ob

Ordovician 

Berkshire schist

 ei

Lower Cambrian slate, 

quartzite, grit, limestone, and shale

(Slate, green, purple, or variegated)

Quarries of green, purple, or variegated slate

Horizontal bedding

Strike and dip of cleavage

( The shorter the lines the steeper 

the dip . Cleavage and bedding 

symbols are in places combined)

Strike of "hogback" or 

shear zone

Strike of vertical joints

Strike of fold

(The curved line shows character of fold)

Dike of camptonite or 

augite camptonite

(Basic eruptive)

Sections along these lines 

given on Plate XXII

The numbers of the quarries refer 

to structural data given in the text.

Topography and nearly all quarries 

by J. H. Jennings. 

Geology by T. Nelson Dale, assisted 

by L. M. Prindle and f. H. Moffit 

Surveyed in 1896

Contoxu? interval 2Ofe&t. 

1914
U. S. GEOLOGICAL SURVEY BULLETIN 686 PLATE XXII

I. Jamesville

 Hampton

JI. Middle Granville

E.

III. Lake Bomoseen

Flint Hill -C, S ,£_.,

IV. Blissville

Lake 

_L

E.

E. 

_400 ft.

V. Wells

S-C CS,C

ii Mi

E.

"VI. Pawlet Dike

,S| ,

Liiku 

477 ft;

Mettawee 

: -ClSb

VII. West Pawlet 

goo f

Vertical .and liorix.ontal sculo

A 1 '/z ?-t 1 mile

LEGEND

Section lines on Quarry maps, Pis. XX and XXT.

Vertical dotted lines indicate places where dip was observed; otherwise hypothetical. 

Vertical dashes indicate boundaries between Cambrian and Silurian, or refer to names.

= Ordovician (Lower Silurian) shales, grits, and red and green slate. 

other areas =Lower Cambrian shale, limestone, quartzite, and green and purple slate.

SECTIONS ACROSS THE NEW YORK AND VERMONT SLATE BELT.
VERMONT. 125

The base of the formation is not certainly exposed. There may be 

a bed of commercial slate in the olive-colored grit.

The probable structure and structural relations of the two slate 

formations of western Vermont are shown in Plate XXII. Each 

section is described beyond in detail. Those parts of the sections 

which are well substantiated are indicated by vertical 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), on which the section lines have been drawn.

GEOLOGIC SECTIONS. 

SECTION I. JAMESVILLE-HAMPTON.

Section I 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 Co., about a mile northnorthwest 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 

opening the Ordovician grits crop out, and a mile north-northwest, on 

the east side, three-fourths of a mile north of the section, Mr. Walcott 

found Ordovician graptolites in eastward-dipping shales.

The eastern base of the Jamesville Ridge consists of the Cambrian 

black slates and shales with small beds of limestone (D in section on 

p. 124). These also crop out at the bend in the road south of the 

section. These slates and shales form the uppermost strata of the 

ridge, the quartzite and sandstone (E) being absent, but that outcrop 

probably belongs to those on the west side of the ridge. Arising from 

beneath the black slates and shales are the green and purple roofing 

slates (B), with 10 feet of limestone carrying Lower Cambrian fossils. 

There are seven or eight old quarries on the hillside. In the largest 

one 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 here underlie, not overlie, the green. In the main the 

ridge appears to be an anticline of the roofing slates (B) and the black 

slates (D), very much overturned to the west, with the red slates and 

the Ordovician graptolite shales and Ordovician grits on both sides 

of it. In Section I the only well-observed features are the relations of 

cleavage and bedding on the eastern slope. The overturn at the
PLATE XXIII.

STRUCTURAL DIAGRAMS OF SLATE QUARRIES IN EASTERN NEW 

YORK AND WESTERN VERMONT.

A. Rising & Nelson's "sea-green" quarry No. 2, West Pawlet, Vt. 

B. Rising & Nelson's "sea-green" quarry No. 2, West Pawlet, Vt., with measurements.

C. Hughes's "sea-green" quarry No. 7, West Pawlet, Vt. 

D. Roberts R. Roberts's "sea-green" quarry, Pawlet, Vt. 

E. Hughes's western quarry "sea-green," Pawlet, Vt. 

F. McCarty's "sea-green" and purple quarry, Poultney, Vt. 

G. Schmidt & Williams's " sea-green " quarry, Pawlet, Vt. 

H. Empire slate quarry (red), Granville, N. Y. . . 

/. Auld & Conger's quarry ("sea-green" and variegated), Wells, Vt. 

K. Griffith & Nathaniel's quarry ("sea-green" and purple), Poultney, Vt. 

126
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XXIII

E.

(E)

(G)

(f-J)

\N

joint

QUARRY DIAGRAMS, NEW YORK AND VERMONT SLATE BELT.
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XXIV

QUARRY DIAGRAMS, NEW 'YORK AND VERMONT SLATE BELT.
PLATE XXIV.

STRUCTURAL DIAGRAMS OF SLATE QUARRIES IN EASTERN NEW 

YORK AND WESTERN VERMONT.

L. Eureka slate quarries ("unfading green" and variegated), Poultney, Vt.

M, Valley Slate Co. ("unfading green"), Poultney, Vt.

N. Lake Bomoseen Slate Co. (mil,! stock, purple and green), Cedar Point, Castleton, 

Vt.

0. Old quarry (purple with green ribbons), near Blissville, Castleton, Vt.

P. Old quarry near and north of Eagle quarry, purple with green ribbons, Blissville, Castleton, Vt.

Q. Meadow Slate Co.'s quarry (mill stock, variegated and green), Fair Haven, Vt. 

Calcareous quartzite beds 4 to 12 inches thick and 18 inches apart.

R. Meadow Slate Co.'s quarry, Fair Haven, Vt., at north end.

S. Scotch Hill Slate Quarry Co.'s quarry (mill stock, purple, variegated, and 

green), Fair Haven, Vt. Dark beds are said to underlie the green.

T. Old quarry (purple), 1 mile south of West Castleton, Vt.

V. National Red Slate Co.'s quarry, southwest of Jamesville, in Granville, N. Y.

V. Williams & Edwards's quarry ("sea-green"), Wells, Vt..

127
128 SLATE IN THE UNITED STATES.

west where the Ordovician underlies is inferential, and the other folds 

are hypothetical.

SECTION tl. MIDDLE GRANVILLE.

Section II 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 east end of the section 

would be, as in Section I, anticlinal, and the intervening broad 

Ordovician belt would be synclinal in 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. An open drainage cut 213 feet 

long, east of one of the larger quarries, 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 in black shales,, and 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 another. 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 possible that between that point and the red slate of the valley 

on the east there is 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 scattered observations were made in the valley east of the 

ridge, and these all indicate low easterly dips, and so do most of the 

observations at Nixon's and Pritchard's quarries, there being 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.
VERMONT. 129 

SECTION III. LAKE BOMOSEEN.

Section III crosses Cedar Point and extends to Glen Lake. At the 

Lake Bomoseen Slate Co.'s quarry, Cedar Point, purple slate 127 

feet thick is overlain by 50 feet of green slate, and this, in turn, 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° E. (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, 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°-50° E., and consist of about 25 

feet of purple slate overlain by 8 feet of green slate and 8 feet of 

limestone, followed by poor slates and quartzite beds. The relations 

of the Ordovician and Cambrian rocks 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, p. 124) 

and the ferruginous quartzite (E) occur between the Cambrian 

limestone and roofing slates (B) and the Ordovician graptolite 

shales. About half a 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 east it is 90 9 . 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. BLISSVILLE.

Section IV 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°-159 E., 

dip 20°-30° E.), overlain by 20 feet of black slate and shale and 

thin-bedded limestone (D, p. 124), a few feet of green slate, and a bed 

of quartzite 10 to 12 feet thick (E), which is exposed for 350 feet

40620° Bull. 586 14 9
130 SLATE IN THE UNITED STATES.

along the strike. The quartzite contains calcareous nodules which 

weather out. It overlies and, at the south, runs into a few. feet of 

a green grit like that of Flint Hill. The black shales crop oijt at 

several points along the road south, and also on the ridge east of it.

Between, this line of quarries and the road running south the 

olivercolored 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-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 to the west includes the Eagle. 

The strike here changes to N. 15°-40° W. The dips are 25°-30° 

E. and SE. -The cleavage strikes N. 5° E. and dips 20° E. -The 

beds include about 70 feet of purple slate, overlain by 10 feet of 

green slate, and 15 feet of thin-bedded Olenellus-bearing 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 

PLXXIV, 0.) The strike of the axis of the fold is N. 40° W.; the 

cleavage dips at a low angle to the east. Small, beds of light-green 

slate, with or without a quartzose limes tone, in the center, produce 

bands on the cleavage surface., <

At a quarry intermediate between the second and third lines of 

quarries, one-fourth of a mile north of this section, the strike changes 

to N., 75° W.,. dip 15°-20° S., and the cleavage strike to N. 35°-500 

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, half a mile south of the section, shows very Low westerly 

dips, and at the extreme west end of the section a. gentle syncliae 

is,.exposed crossed by a cleavage dipping 35° E.

SECTION V. WELLS.

.Section, Y starts at the west shore of Lake St. Catherine and'crosses 

.the ..slate 'ridge to the 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. 

3]o() ,judge from .scattering observations along v the west side of the 

ridge, it mu§# be.composed: largely of roofing slate. From a point 

about' !h.alf : a mile west of the lake there is .a,line of quarries and 

pr^Qspec.t,holes extending northward'for 2 miles., The cleavage dip 

^/.uniformly east. Farther west the section crosses a strip of red 

Ordovician slate almost half a mile long and 180 feet wide at the 

broadest part but tapering out both north and south. The dip is
VERMONT. 131

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 slate, but in inverse order. 

At Auld & Conger's quarries 170 feet of slates of various qualities, 

green and variegated, are exposed. The strike here is N. 5° W. 

and the dip 35° E.; the cleavage dips 40°-45° E. Dipping toward 

and under the slate, but with greenish and grayish beds intervening, 

are the Ordovician grits, which strike N. 5° E. and dip 40° E. Between the syncline of red slate and the Ordovician 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 in the grit shown in the section are hypothetical, 

but the first one west of the slate would naturally be a syncline.

SECTION VI. PAWLET.

Section VI begins near Mettawee River 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 east side of the ridge for a mile and 

more to the south consists of folded schists interbedded with quartzite, possibly belonging to A of the section on page 124. West of 

this is a strip three-fourths of a mile wide of unknown character, 

but from the situation of two quarries about 2J miles south of a 

point half a 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 east 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 Ordovician 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 and dipping 90° or steeply to the northwest. The 

slate on the east side of the dike strikes north and dips 70° E. and 

that on the west side dips 55° E. The dike has a rough jointing 

parallel to its sides and weathers in spherical nodules. The cleavage
132 SLATE IN THE UNITED STATES.

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 125 feet, but possibly 

less, if closely folded. These beds probably belong in the Ordovician. 

West of them come the Ordovician grits, dipping 65° E. .

SECTION VII. WEST PAWLET.

Section VII 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 Ordovician grits south, east, and 

west.

Some of the quarries are more than 175 feet in depth. At the 

Hughes quarry .No. 7 there is a syncline with an anticline east of it 

(see PL XXIII, G}, 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 PI. XXIII, A). 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 each side of the fold in opposite order 

and also at the bottom, but there they are thicker. The structure 

indicated is a syncline with an anticline on each side of it, but unless 

faulting occurred these must be parts of an anticline, and the Ordovician 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° E. to N. 25° E., and that of the cleavage from N. 5° 

W. to N. 5° E., the dip is generally 70°, but in places 40°-50° E. 

The thickness exposed, measured across two synclines and one 

anticline, is about 100 feet. There are on the east side of the syncline some dark-gray or "black" beds which, according to this construction, would belong not on top, but within the roofing slates (B, 

p. 124). The Ordovician grits west of the Cambrian slates strike 

N. 10°-20° E. and dip 55°-60° E., but at the West Pawlet railroad 

station the strike 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 

east side of the ridge lie the Cambrian shales (D). About half a 

mfle east of South Granville, near the Ordovician boundary, west 

of this Cambrian belt, the purple slates dip 40° W. and have a cleavage
:ULLETIN 586 PLATE XXV

SLATE SYNCLINE, WEST PAWLET, VT.

North face of Rising 8c Nelson's " sea-green " slate quarry No. 2, showing eroded eastward-inclined sync I inc. 

Deptn, 1 00 feet to lower syncllne.
VERMONT. 133

dipping 30° E. and striking N. 15° E., the bedding forming greenbands on the cleavage surfaces. The second Cambrian ridge is 

clearly anticlinal in structure.

GENERAL STRUCTURAL FEATURES.

From the sections above described, 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. It lies in 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 tlie northern part of the Cambrian 

area quartzite beds within the roofing slates are more abundant than 

in the southern part. 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 and the cleavage itself is also less perfect 

in the northern than in the southern, area. Series of such various 

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 are 

more heterogeneous and 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, PI. XX.) Large 

beds of quartzite overlie the roofing slate and are typical of this 

portion (Section III, PI. XXII).

The slate folds have suffered much erosion (Pis. Ill, XXV). The 

surfaces when freshly exposed are finely striated and in places deeply 

grooved by glacial action. The glaciated edges are covered either by 

morainal deposits or by finely bedded clays dating back to the postglacial submergence. (See ?1. XIX, B.}

GEOLOGIC AND GEOGRAPHIC DISTRIBUTION.

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, and West Castleton) the Cambrian slates occur within 

100 or 200 feet of the Ordovician grits. This proximity of the two
134 SLATE IN THE UNITED STATES.

formations occurs in such a variety of situations that it 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 stratigraphic position is still the same. The 

first place to look for the Cambrian roofing slates is near the Cambrian-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 on 

the east 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-colored grit (A, p. 124), 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). (Seep. 124). These vertical relations are pretty wBll 

established.

The areal relations of the "sea-green" and the "unfading green" 

are not at all clear. Nothing has yet been found to show that the 

stratigraphic position of, these two varieties of Cambrian slates is not 

identical. It seems probable that, at the latitude 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 

already 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 quartz sand at the north 

the whole structural difference may be traced back to changes in 

sedimentation. /

Even this demarcation between the areas of "fading" and "unfading" green slate 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
VERMONT. 135

half a mile south of Bomoseen. Slates which fade little are reported 

as occurring on the ridge west of Lake St. Catherine. In an old 

quarry about half a mile east of Jamesville, in a belt that seems to be 

directly continuous with that in which lies 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 that is, 

underlying, it is a green bed which scarcely fades and which, under 

the microscope, shows very few carbonate rhombs. '

The "sea-green" slate at two newly opened quarries in Bullfrog 

Hollow, in. Wells, 2£ miles east-northeast of Middle Granville, has 

less carbonate than many slates of that belt, but it fades. Some old 

quarries reopened in 1911 in the "unfading green" belt in Fa'r 

Haven, near Carvers Falls, contain eyen less carbonate than that of

the same sort in Poultney. The most southerly outcrop of the 

"unfading green " observed by the writer occurs 2$ miles N. 25° E. of 

Poultney and three-fourths of a mile east of the railroad.

A few things should be noted. The continuation of the slate of 

West Pawlet 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 half a mile east of Poultney and the other half a mile 

east of East Poultney, where it crops out in the small gorge of Poultney River. In many places north of Castleton' the strikes change to 

the northwest or the north-northwest, and the beds of slate locally 

follow this direction. Barker Hill and Wallace Ledge both have 

Cambrian slates about them.

QUARRY STRUCTURE.

The types of structure exposed at the quarries are shown in diagrams 

A-G, K-T, and V, on Plates XXIII and XXIV, and also in Plates 

XIX, A, and XXV.

The table on pages 137-138 contains the more important compass 

observations taken at the quarries. The numbers in the first column 

will be found on the maps, Plates 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 where not given. 

Approximations are indicated by the signs plus ( + ), minus ( ), 

more or less (±). Close jointing in the dip direction occurs in places. 

Such joints are spaced 4 to 24 inches over a length of 200 feet on the 

west wall of the Norton No. 6 quarry in Pawlet.
136 SLATE IN THE UNITED STATES.

To the data given in the table should be added a few observations 

at quarries outside of the areas shown on the maps. An abandoned 

quarry three-fourths of a 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 strikes N. 10° W. and dips 25°-30° E. Joints strike N. 35° 

E., dip 60° E.; also N. 50° E., dip 90°. There is a fatal secondary 

cleavage "false cleavage" here, striking about north. (See PI. 

VII. A, and p. 37.) At the Meadow Slate Co.'s quarry (PL XXIV, 

Q, E], and at the quarry just north of it the bedding strikes N. 

15° E. and the cleavage strikes N. 7°-15° W., and dips 15°-20° E. 

Vertical strike joints strike N. 15° E. and vertical dip joints N. 85° W. 

Diagonal joints strike northwest, dip 65° NE.; also N. 58° E., dip 90°. 

The grain strikes N. 35° W. At the Scotch Hill quarries (PI. XXIV, 

8), 2 miles north-northeast of Fair Haven, the beds strike N. 25° W. 

and dip 5° E., and the cleavage has the same strike and dips 15° E. 

The joints are vertical, striking N. 5°-17° E. and N. 67° W.

At the Hampshire Slate Co.'s quarries, about one-third of a mile 

east-southeast of Carvers Falls, in Fair Haven, the beds strike N. 15° 

E. and dip 33° S. 75° E., but the cleavage strikes north and dips 

40° E. At the Cedar Point quarry (No. 43, PL XXI) there is a 

marked southerly pitch, the syncline being found to rise as quarrying 

proceeds northward. There is also here a slip cleavage parallel to 

the axis of the fold, which prevents the slate from being used for 

roofing but does not interfere with its use for mill stock. On the 

hill north of the quarry this slip cleavage is vertical. At the now 

disused quarries 4 miles southwest of Brandon, in Sudbury, the beds 

strike N. 55° E. and dip 35° S. 35° E., and the cleavage is in places 

parallel, but some adjoining slate ledges have a slip cleavage that 

strikes N. 25° W., with the neighboring Ordovician schists.
Slate-quarry observations in Vermont.

Quarry No. 

on PI. XX 

or XXI.

10---........

11...........

12...........

13...........

14...........

15...........

16...........

17...........

18...........

19...........

20..........

21 .....

22.......

23:.......

24.........

25...........

26...:.......

27 ...........

28...........

29...........

30...........

31...........

43...........

44...........

45...........

46.......:'..-.

47...........

48...........

Bedding.

Strike.

N.+ 

N. 5° E.

N. 20° E. 

N.+ 

N.25°E.

N.+ 

N.+' 

N.+

N.+ 

N.+ 

(?) 

N.25°W.

(?) 

(?)

N. N.+- 

N. 

-N.+ 

N.+

(?) 

N.10°W.

N. 7° W.

N.+

N. 15° E. 

N. 55° E.

N.25°E.

Dip.

30° E. 

40° E.

(?) 

E. low. 

25° E.

37° E.

30° E.± 

20° ± E. 

50° E.?

30° E. 

17° E. 

(?) 

30° E.

J,

20° E. 

25° E. 

20° E. 

22° E. 

(?) 

Fold. 

45° E. 

/ 35°- 

\65°E. 

60° E. 

.20° E.

20° E.

Cleavage.

Strike.

N. 

N.

N. 

N. 

N. 15° E.

N.15°E.

N.+ 

N.+ 

N.

N.

N. 10° W. 

N.5° W. 

N.20° W.

N.25" W.

N.17° W.

N.25°E. 

N.15°E. 

N. 12° E. 

N. 15° E. 

N.+ 

N.+ 

N.10° W. 

N. 10° E. 

N.

IN. 7° w..

N. 15° E. 

N. 30° E.

N. 7°E.

Dip.

42° E.

40° E.

/40°-70° 

\ E. 

40° E.

50° E. 

55° E. 

50° E.± 

40° ± E. 

37° E.

35° E.

50° E. 

42° E. 

40° E. 

- 32° E.

40° E. 

' 65° E. 

45° E. 

40° E. 

37° E. 

27° E. 

35° E. 

45°'E. 

20° E. 

30° E. 

/ 50° to 

\ 65° E. 

60° E. 

25° E.

25° E.

Strike joints.

Strike.

N. 5°W..

fN.15°E.

i NN. 15° E.

N. 10° E. 

N.5°E. 

/N.7°E. 

\N.25°E.

N.+

/ N.+ 

\ N.+

N.12°W.

r

.lO° E. 

N. 

N.

N.5° W.

/ N" 

\ N.+ 

N.10°W.

N.+

N.

N.18° E.

} N

N. 20° E. 

N.25°E.

Dip.

40° E. 

65° E. 

45° W. 

62° E. 

30° W. 

65°E. 

50° E. 

45" W.

E. high.

90° 

30° W.

60° E.

55° E. 

40° E. 

60° W.

55° W. 

75° E. 

45°±W. 

50° E.

42° W.

75° E.

45°+ 

90°

,60°E. 

45° E.

Dip joints.

Strike.

JN.75°W.

JN.75°E. 

E.-W.

}.........

1

/ E.-W. 

\ E.-W.

JN.82°E.

E.-W. 

E.-W. 

N.80°E.

N. 65° E.

N. 70° W.

E.-W.

E.-W. 

E.-W.

N. 80° W.

Dip.

90° -

90° 

90°

55° W. 

90°

90°

90° 

90° 

90°

90°

70° ESE.

90°

90° 

90°

80° N.

Diagonal joint

Strike.

}...........

N. 50° W.

/N.30° E. 

\N. 40° E.

D

70° 

45

50° 
Slate-quarry observations in Vermont Contin

Quarry No. 

on PL XX 

or XXI.

49...........

50...........

51...........

52...........

53...........

54...........

56...........

57...........

58...........

59...........

60...........

61...........

Bedding.

Strike.

N.

N.40°W. 

N. 17° W. 

N. 75° W.

N.+ 

N.+ 

N. 

N. 

N.5°W.

Dip.

20° E. 

0° 

0° 

Fold. 

30° E. 

17° S. 

7° W. 

15° W. 

35° E. 

35° E. 

37° E.

Cleavage.

Strike.

N. 25° W. 

N. 27° E. 

N.+ 

N. 25° E. 

N.15°E. 

N. 5°E. 

N.42°E. 

N.+ 

N. 10° E. 

N.

N,

Dip.

20° E. 

30° E. 

17° E. 

15° E. 

E. low. 

20° E. 

15° E. 

17° E. 

17° E. 

42° E.

37° E.

Strike joints.

Strike.

N.5°W.

N.22° E.

Dip.

30° E.

60° E.

Dip joints.

Strike.

E.-W.

-N.65°W.

N. 70°'E.

Dip.

90° 

90°

90°

Diagonal join

Strike.

N.35°E.

D

60
VERMONT. 139 

MINOR STRUCTURAL FEATURES.

The purplish slates in many places include 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 23. 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. (See p. 49.)

Faults of no great displacement are not uncommon. These may be 

reversed or normal. (See PL IV, C, E, and p. 43.)

Veins of milky quartz are abundant, and some of them attain large 

dimensions. (See PL V, A, and p. 48.) Their course is in places 

very irregular. They generally contain calcite and chlorite. The 

west wall of the Briar Hill quarry (No. 62, PL XX), in Pawlet, has 

many parallel curved tapering veins evidently due to shearing 

motion and tension.

As shown by the maps (Pis. XX, XXI) the region is traversed by 

dikes. These measure from 12 to 40 feet or more in width and as 

much as several miles in length. They are probably of Triassic 

date and are usually made up of camptonite, consisting chiefly 

of plagioclase feldspar and hornblende, with or without augite.1 

These dikes are locally parallel to the strike or the dip joints, but 

more commonly to the diagonal joints.

Shear zones ("hogbacks") are numerous, 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, PL VIII and p. 44, and on their relation 

to dikes, p. 169.)

The quartzite beds, 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 general aspect of good commercial green slate as seen in a 

large mass broken across the cleavage is well shown in Plate XIX, A 

("sea-green," quarried for roofing) and B ("unfading green," quarried for mill stock).

THE WESTERN VERMONT SLATES.

"Sea-green" slate. When freshly quarried, the "sea-green" slate 

varies from a light gray to a slightly greenish gray. In some beds 

it is crossed by ribbons of a dark gray or, where bedding and cleavage 

are parallel, it bears traces of organisms in dark gray.

i See Bascom, F., U. S. Geol. Survey Nineteenth Ann. Kept., pt. 3, p. 223-226,1899.
140 SLATE IN THE UNITED STATES.

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 a little under cold dilute hydrochloric acid applied to the edges and much 

under acid applied to the powdered slate, is sonorous and very fissile. After a few_ 

years' exposure it changes its color to a brownish gray, and as the slates from different 

beds discotor differently a roof covered with ''sea-green" slates from different beds 

acquires ajnottled 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 or with very minute more quartzose beds parallel to the cleavage. 

This matrix incloses more or less angular grains of detrital quartz measuring from 

0.05 to, 0.34 by 0.004 to 0.035 millimeter, but usually 0.035 by 0.013 millimeter, and 

an occasional grain of plagioclase (lime-soda) feldspar up to 0.04 by 0.05 millimeter. 

Sections parallel to the cleavage show many plates and rhombs of carbonate, the rhombs 

measuring from 0.003 to 0.03 or even 0.05 millimeter in diameter. Some of these 

consist of two crystals, an inner rhomb and an outer one, with different orientation, 

possibly in twinned position. In some of the double rhombs the inner one has fallen 

out; in others the rhomb has an opaque mineral nucleus which is not pyrite but may 

be magnetite or graphite. There are also a few scales, of chlorite with interleaved - muscovite up to 0.13 millimeter, their laminae crossing the cleavage and mostly in the , 

grain direction, and some equally large muscovite scales, possibly of sedimentary 

origin. There are some irregularly distributed pyri'te spherules from 0.003 to 0.02 

millimeter, also minute lenses of pyrite. Rutile needles, measuring from 0.0017 to 

0.009, rarely 0.012, by 0.002 millimeter average about 65,000 to the square millimeter, 

which is equivalent'to about 40,000,000 to the square inch of thin section. Here a 

and there is a grain of zircon or prism of tourmaline 0.022 by 0.008 millimeter.

The chief constituents of this slate, named in descending order of abundance, 

appear to be muscovite, quartz, carbonate, chlorite, rutile, pyrite, and magnetite. 1

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 limonite. These 

rhombs measured 0.047 millimeter. 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 millimeter, entirely or partly altered to limonite. 

In some places there was a yellowish-brown zone of alteration surrounding an unaltered nucleus. These rhombs are regarded by 

Dr. Hillebrand (see p. 55) as an isomorphous mixture of dolomite and 

siderite that is, 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

1 For colored lithographs of magnified thin sections of this slate as seen under both ordinary and polarized 

light, see U. S. Geol. Survey Nineteenth Ann: Kept., pt. 3, PI. XXXV, 1899.
VEBMONT. 141

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 W. F. Hillebrand: " -

k-, Analyses of Vermont "sea-green" roofing slate.

Silica (Si03 ).. ..................................................

Rutile, titanium dioxide (Ti02 ). ...............................

Lime(CaO). ................................................. °.

Potassa(K2O).... .....:...............:........................ Soda(NajO).......................................,...........

Lithia(LiaO).... ..............................................

Water below 110° C. (H2O). ....................................

Water above 110° C. (H2 0). . .. . ................................

Pvrite(FeS2 ).... ..................................:...........

Carbon (C). ....................................................

Fluorine (Fl)..... .............................................

A

07. 76

.71

14.12

.81

4.71

.10

.63

.04

2.38

3.52

1.39

Strong

trace. 

.23

2.98

.07

.40

.22

Trace.

100. 07

.12

B

62.37

.74

15.43

1.34

5.34

.22

.77

3.14

4.20

1.14

Trace.

.34

 3.71

.06

.87

.06

Trace.

trace.

99.80

.032

C '

59.84

.74

15. 02

1.23

4.73

.34

2.20

.09

3. 41 ' 4.48

1.12

Strong

trace.

3.44 . 09

2.98

.05

 Trace.

.11

100.28

.024

D

65.29

: 

A (=D. XIV, 1895, 230a). Rising & Nelson's quarry No. 2, West Pawlet, Vt.; 13-foot bed. 

B (=D. XIV, 1895, 225/). Griffith & Nathaniel's quarry, 9 miles'north of A, South Poultnoy, VI:. 

C (=D. XIV. 1895, 256e). Wm. H. Hughes's quarry No. 10 (Brownell), 2 miles north of A, Pawlet, Vt. 

D (=D. XIV, 1895,35i!). Auld & Conger's quarry, 8 miles north of A, in Wells, Vt.; 22-loot bed. Determination of silica only. - 

These are all from the West Pawlet and South Poultney belt. Specific gravity: C, 2.7910; D, 2.7627.

This slate is used exclusively for roofing. The results of Merriman's tests of it are given on page 183.

" Unfading green" slate. The slate called " unfading green" is 

greenish gray in colori 

To the unaided eye it has a somewhat fine texture and a roughish lusterless cleavage 

surface. The sawn edges show some pyrite. It is magnetitic, does not effervesce 

with cold dilute hydrochloric acid, even when powdered, 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 millimeters. There are a few lenses up to 0.2 

by 0.1 millimeter of quartz fragments arid muscovite scales, also lenses up to 0.4 by 

0.1 millimeters of chalcedonic quartz (?) A few grains of plagioclase feldspar; scales of 

chlorite up to 0.039 by 0.006 millimeters 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 millimeter but in very much 

smaller number than in the "sea-green" elate; abundant rutile needles from 0.003 

to 0.008 millimeters long.
142 SLATE IN" THE UNITED STATES.

The chief constituents of this slate, named in descending order of abundance, appear 

to be muscovite, quartz, chlorite, carbonate, rutile, pyrite, and magnetite. 1

These slates are relatively unfading, because they have fewer rhombs and plates of 

carbonate. (See Dr. Hillebrand's note, p. 55.) The sections also show why they 

cleave less perfectly than the "seagreen" slates.

The following analyses were made in the laboratory of the United 

States Geological Survey by W. F. Hillebrand:

Analyses of Vermont "unfading green" roofing slates.

Sitic&(8iOs).. .................

Titanium dioxide, rutile (Ti02 ) 

Zirconia (Zr02 ) ................

Strontia (SrO). . .. . ............

Barvta(BaO). ................

Soda (Na2O)... ................

E

59.27

.99

18.81

1.12

G.58

.13

Trace?

.42

.05

2.21

3.75

1.88

F

59.48

1.02 

Trace?

18.22

1.24

6.81

.07

Trace?

TracG?

.56

.05

2.50

3.81

1.55

Lithia(Li2 0).................

Water below 110° C. (H2 0)... 

Water above 110° C. (H2 0). . .

Pyrite (FeS2 ). ................

Carbon (C).. .................

Fluorine (Fl).................

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

/Q\

E

0.32 

3.98

.11

.21

.15

99 98

.08

2.795

F

0.17 

4.05

.10

^0

.13

.08

100. 23

.07

E (=D. XIV, 1895, 314/). Eureka quarries, 3J miles north of Poultney, in Poultney Township, Vt. 

F (=D. XV, 1896, 645a). Valley .Slate Co. quarry, 2J miles north of Poultney, in Poultney Township, Vt.

The results of Merriman's recent tests of Vermont unfading green 

slate will be found on page 183.

This slate is largely used for roofing. The mill stock slate is described beyond.

"Purple" and "variegated" slate. The "purple" slate is darkpurplish 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" slate 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" slate 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 (Fe203) in irregular bright red dots from 0.001 to 0.003 

millimeter in diameter, exceptionally in hexagonal scales. This mineral obscures 

the aggregate polarization. There are quartz fragments up to 0.047 and even 0.07 

millimeter, rare plagioclase grains; lenses of quartz a millimeter long; muscovite and 

chlorite scales lying at an angle to the cleavage, others without arrangement. Carbon1 For colored lithographs of magnified thin sections of this slate, as seen both under ordinary and polarized 

light, see U. S. Geol. Survey Nineteenth Ann. Kept., pt. 3, PI. XXXVI, 1899.
VERMONT. 143

ate is less abundant than in the green slates. Spherules of pyrite from 0.007 to 0.027 

millimeter, rutile needles in abundance, a few prisms of tourmaline, and rare fragments of zircon.

The chief constituents of the "purple" slate, named in descending order of abundance, appear to be muscovite, quartz, chlorite, hematite, carbonate, rutile, pyrite, 

and magnetite. The purplish color is due to the mixture of the red from the hematite 

and the bluish green from the chlorite. 1 In the "variegated" slates the mottling is 

produced by the irregular distribution of the hematite.

The following analyses were made by W. F. Hillebrand in the 

laboratory of the United States Geological Survey:

Analyses of Vermont "purple" and ''variegated" roofing slates.

Ferric oxide (l^Os) ...........................................

Lime (CaO )....................................................

Baryta (BaO).. . ...............................................

Soda (Na2 0 ) ...... :............................................

Lithia (Li20) . .................................................

Water below 110° C. (H2 0) - ...................................

G

61.63 

.68

16.33 

4.10 

2.71 

.09 

Trace? 

Trace? 

.50 

.06 . 

2.92 

5.54 

1.26 

Str. tr. 

.31 

3.24 

.16 

.41 

.04 

Trace. 

None.

99 98 

!02 

2.8064

H

60.96

.86

Trace? 

16.15 

5.16 

2.54 

.07 

Trace. 

Trace. 

.71 

.04 

3.06 

5.01 

1.50 

Trace. 

.17 

3.08 

.23 

.68 

None.

None.

100.22 

.07

.

I

60.24 

.92

Trace? 

18.46

2.56 

5. IS 

.07 

Trace. 

Trace. 

.33 

.03 

2.33 

4.09 

1.57 

Str. tr. 

.18 

3.81 

.11 

 .08 

.16

None.

100. 12 

.087 

2.8053

H»

5.28 

2.36

G (=D. XIV, 1895, 260a), purple roofing slate, McCarty quarry, east of center of Lake St. Catherine, 

South Poultnoy, Vt.

H (=D. XV, 1895, 760o), purple roofing slate, Francis & Sons' quarry, nearly a mile south of Hydeville, 

in Castleton. Vt.

I (=D. XV, 1895, 314), variegated roofing slate, from Eureka quarry 3i miles north of Poultney, in 

Poultney Township, Vt, "unfading green" area.

H2 (=D. XIV, 1895, 614a), dark-reddish bed a few inches thick in purple of "sea-green" area, west of 

Lake St. Catherine; determination of iron oxides only.

The results of certain tests of the strength of Fair Haven "purple" 

slate are given on page 183. The "purple" and "variegated" slates 

are used mainly for roofing.

Mill-stock slate. In the northern and western parts of the greenslate 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. Much of the purple is paler than the "purple" 

roofing slate and also spotted with green, and some of the green is 

brighter than.the "unfading green" roofing slate.

1 For colored lithographs of magnified thin sections of the "purple" slate, as seen under both ordinary 

and polarized light, see U. S. Geol. Survey Nineteenth Ann. Rept., pt. 3, PI. XXXVII, 1899.
144 SLATE IN THE UNITED STATES.

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 Co.'s quarry at Cedar 

Point, on Lake Bomoseen;1 and from the J. Jones quarry,. 2£ miles 

north of Castleton, were examined miscroscopically, with these results:

The matrix is muscoyite (sericite), with aggregate polarization, but shows a cleavage 

much inferior to that of the "sea-green" slate and somewhat inferior to that of the 

Eureka of the "unfading green." There are about six 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 orientations; 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 purple differs from the green by the addition of hematite.

The chief constituents, named in descending order of abundance, appear to be 

muscovite, 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 littfe higher than the figures 

for any of the other slates of western Vermont.

Slate-pencil slate. In the portion of the belt containing the "unfading green" slate, about 1£ miles north of Bomoseen and a little east 

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 that are easily rounded. In the 

Vermont quarry the method used was to take tile-shaped blocks of 

slate and carve out first on one side, then on the other, by means of 

set gages, 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 that of

the "unfading green" slates, except 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 noted.

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

1 See Pis. XIX, A, and XXIV, N, for the structure at this quarry.
VERMONT. 145

traversing the quartzite. In both of these modes it is crystallized 

wherever cavities admit of crystallization.

Next in abundance is calcite, occurring also in veins with or without quartz, or as delicate films on joint planes, or as a sediment in 

the beds of quartzite. Some of the quartzite beds 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 lamellae of 

barite, with the intervening 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.

Chlofite 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 in places 

confined to some of the sides, filling a space produced by motion or 

compression, as described by Renard. Pyrite may collect in the 

vicinity of calcareous and quartzose veins or beds, or form dendritic 

crystallizations on cleavage planes, or minute cubes 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. 1

Rarely a little galenite occurs in the quartz veins.

It will be observed that nearly all the above-named minerals have 

already been mentioned as occurring in the slates, as shown either by 

the microscopic or the chemical analyses.

Black slate near Benson. Half 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 afterward. The slate is probably of Ordoviciaii age and belongs 

in a shale, slate, and schist belt which extends nearly 4 miles west to 

a point within a mile of Lake Champlain, and 4£ miles south-southwest to the viUage of West Haven, and also, with bends and ramifications, 20 miles north-northeast to Weybridge Center, in Addison 

County. The slate-bearing area, however, probably does not exceed 

3 square miles.

At the quarry the beds dip about east at angles ranging from 15° 

to 20°; cleavage strikes about N. 7° E., dipping 35° E., and vertical 

joints strike N. 75° W.

1 See footnote on pyrite and marcasite, p. 17'4. 

40620° Bull. 5S6 14 10
146 SLATE IN" THE UNITED STATES.

The slate is bluish black. To the unaided eye it has a somewhat fine texture and a 

rather 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 17,000 to the square millimeter, many of 

them occurring in rows along the cleavage. Butile 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, named in descending order of abundance, appear 

to be muscovite, 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 Plate 

XXXIX, Nineteenth Annual Report United States Geological 

Survey, part 3, a view which would answer almost equally well for 

many of the "soft vein" slates of Pennsylvania.

The following analysis (specimen P = D, XIV, 1895, 305cZ) of black 

slate from the abandoned quarry, one-fourth mile east of Benson 

Village, Rutland County, Vt., was also made by W. F. Hillebrand:

Analysis of black roofing slate from quarry near Benson, Vt.

Silica (Si02)................... 59.70

Titanium dioxide, rutile (Ti02).. .79

Alumina (A1203)................ 16. 98

Ferric oxide (Fe203)............ .52

Ferrous oxide (FeO)............ 4.88

Manganous oxide (MnO)........ .16

Nickelous oxide (NiO)........... Trace?

Cobaltous oxide (CoO.)........... Trace?

Lime (CaO).................... 1. 27

Baryta (BaO)................... .08

Magnesia (MgO)................ 3. 23

"Potassa (K20).................. 3! 77

Soda(NaaO)................... 1 1.35

The more important features of all these Vermont slates, as brought 

out in the above descriptions, will be found in tabular form opposite

page 188.

VIRGINIA.

W. B. Rogers,1 in his reports to the legislature of Virginia during 

the years 1835 to 1841, called attention to the slate deposits east of 

the Blue Ridge in Buckingham, Fluvanna, and Fauquier counties.

Lithia (Li20)............. Strong trace.

Water below 110° C. (H20)...... .30

Water above 110° C. (H20)...... 3. 82

Phosphoric oxide (P205)......... .16

Carbon dioxide (C02)........... 1.40

Pyrite (FeS2 ),.................. 1.18

Sulphuric oxide (S03)........... Trace.

Carbon (C)...................... .46

100.05 

Total sulphur (S)............... .63

Specific gravity................. 2. 7748

1 A reprint of annual reports and other papers on the geology of the Virginias [posthumous]; on the 

roofing slate of Buckingham County, pp. 78, 79; on that of Fauquier County, pp. 460,461, New York, 1884,
VIBGINTA. 147

Slate also occurs in the Blue Ridge in Amherst County and Albemaiie 

County.

Eecent field studies of these slate areas by T. L. Watson and S. L. 

Powell, of the Virginia Geological Survey, 1 have thrown more light on 

their geographic and geologic relations and their geologic age; and 

N. H. Darton's discoveries of fossils in the slate of the Arvonia 

district, announced in 1892,2 determined the general geologic age of 

that belt. According to the studies of Watson and Powell and their 

map, reproduced in figure 12, there .are three slate belts, of which 

the Amherst-Bedford County belt is the most westerly, the Buckingham-Fluvanna County and Quantico belt the most easterly, and 

the Albemarle-Fauquier County belt the central. Economically 

considered the slates in both the central and eastern belt vary considerably. The slate of the eastern belt is of Ordovician age; that

of the central and western belts belongs to the Cambrian.

Geologic relations in Buckingham and Fluvanna counties. Rogers's 

reference 3 to the slate of Buckingham and Fluvanna counties 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 figure 13 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 half a mile west of Bremo 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 and extends at least 

a mile south of Penlan, or 5J miles from the south bank'of the James. 

This, on the scale of the map, would' be three-fourths of an inch 

beyond its lower margin. South of the James its eastern boundary 

lies about 3,000 feet west of the toll bridge opposite Bremo Bluff, 

and its western boundary is roughly one-fourth mile east of the 

Virginia Mills, on Slate River. The bluffs on the north and south

1 Watson, T. L., and Powell, S. L., Fossil evidence of the age of the Virginia Piedmont slates: Am. Jour. 

Sci., 4th ser., vol. 31, pp. 33-44, and map, fig. 1, 1911.

2 Barton, N. H., Fossils in the " Archean" rocks of central Piedmont, Virginia: Am. Jour. Sci., 3d ser., 

vol. 44, pp. 50-52, 1892.

3 Op. cit., p. 79.
LEGEND

Arvonia belt (Buckingham and Fluvanna Counties) 

Quanticobelt (princeWilliam,Stafford,and

Spotsylvania counties)

Warrenton belt (Fauquier and Culpeper counties) 

Esmont belt (Albemarle county) 

Snowden belt (Amherst and Bedford counties)

FIGUKE 12. Map of Virginia, showing distribution of slate areas in the crystalline re
VIEGINIA. 149

sides of the James at Bremo Bluff, New Canton, consist of quartzite 

with like strike and dip on. the north, strike N. 17°-23° E., dip 

65° E. and 90°; on the south, strike N. 13°-18° E., dip 90°. The 

exposure of quartzite on the north is about 950 feet across the strike

Scale of miles 

2

Contour interval 50 feet

FIGURE 13. Map of slate region in Buckingham and Fluvanna counties, Va. Quarries shown by crossed 

hammers; slate prospect by round dot; strike of commercial slate by arrows; quartzite areas by fine 

dots.

and on the south about 3,000 feet. These facts and the jagged 

structure of the Bremo Bluff, which is 75 feet high, 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,
150 SLATE IET THE UNITED STATES.

muscovitic, and biotitic, with scattered plates of calcite and a few 

fragments of plagioclase. It is interbedded 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, about 300 feet of micaceous quartzite, and an 

undetermined thickness of ferruginous slate full of pseudomorphs 

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, to judge from the railroad cuttings between New Canton and 

Slate River, appears to consist of minor folds. It is traversed by 

numerous dikes of olivine basalt, ranging from a few inches to 12 

feet in thickness and consisting of plagioclase, olivine, magnetite, 

and augite. These dikes weather in conchoidal masses with a yellowish brown surface. The formation is bounded on the east by the 

quartzite cut by James River. If this quartzite is in a westward 

overturned syncline, it would overlie the slate; if in an anticline of 

like character, it would underlie the slate, 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 chloritic, quartzose, and feldspathic schists that weather 

bright red, possibly the same as the above, but both of uncertain age.

The commercial slate itself appears to occupy a belt two-fift'hs of 

a mile wide along Hunts Creek, which is a southern tributary of Slate 

River. At Penlan its strike is N. 30° E. At Arvonia its .course is 

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 crmoids and later of brachiopods, trilobites, etc., in the quarried slate at Arvonia shows it to be of Ordovician age. E. 0. TJlrich, 

to whom collections of these fossils were referred, determines them 

as unquestionably of 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 Mohawkian or Cincinnatian. The 

dikes, are probably of Mesozoic age> C. D. Walcott, as cited by Darton,1 regards the crinoids from the slates at Arvonia as allied to the 

genera Schizocrinus, Heterocrinus, and Poteriocrinus. Mr. T. E.

i Barton. N. H., loc. cit.
VIRGINIA. 151

Williams, of Arvonia, recently found in the Middle quarry an unusually well-preserved crinoid. The cup, as compressed, is 3 inches 

high and 2^ inches wide at the top. Seventeen tentacles, thickly 

fringed with pinnulse up to an inch in length, cover an area of the 

slab 14 by 8 inches. The specimen is at the National Museum.

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 Williams Slate Co., and the Fontaine quarry 

of A. L. Pitts. Several others were being opened or were temporarily 

abandoned. One opened later near the mouth of Hunts Creek had 

been abandoned in 1912.

The Big quarry in 1904 measured 300 feet along the cleavage, 

200 feet across it, and 125 feet in depth. Bedding and cleavage are 

identical, striking N. 37° E., and dipping 86°SE. There.are vertical 

dip joints striking about northwest; strike joints trending northeast, 

dipping 75°-80°SE.; also two sets of diagonal joints, one dipping 30° 

E. and the other 55° W. The latter system had 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 is 400 feet along 

the cleavage, 400 feet across, and in places 350 feet deep, with a wing 

150 by 50 feet and 100 feet deep. Bedding and cleavage strike N. 

33° E., dip 75° S. 57° E., and are crossed by northwesterly vertical 

joints. There are also horizontal joints spaced 1 to 25 feet. One 

of these meanders 10° from the horizontal in a distance of 50 feet. 

At the south end is a vertical dike of olivine basalt, 10 to 12 feet 

thick, trending N. 25° W., with horizontal joints, 4 to 6 inches apart, 

for a few feet on its nortn side and joints parallel to it on both sides. 

The slate is said to be of better quality a little beyond the zone of 

these joints than it is at a greater distance from the dike.

The slate from the Williams quarries is very dark gray, with a slightly greenish hue. 

To the unaided eye it has a minutely granular 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 and fairly fissile.

Under the microscope it shows a matrix of minute alternating beds, chiefly of fine 

muscovite, 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 qleavage. 

The quartz fragments measure up to 0.085 millimeter. Scattered throughout both 

the 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 millimeter, rarely 0.15 and 0.42 millimeter, with their longer axes parallel to the cleavage. 

These probably include a little magnetite. There are also biotite scales transverse 

to the cleavage, about 39 to the square millimeter, and.measuring up to 0.12, rarely 

0.2 millimeter. Almost if not quite as abundant are plates and rhombs of carbonate. 

There are scattered scales of chlorite interleaved with muscovite, a feAV grains ot plagioclase feldspar 0.047 millimeter, rarely one of zircon, some tourmaline prisms 0.014
152 SLATE IN THE UNITED STATES.

millimeter long, much extremely fine graphitic (or carbonaceous?) material, a few 

particles of hematite, and some-rutile needles. Sections 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, named in descending order of abundance, 

appear to be muscovite and sericite, quartz, biotite, carbonate, graphite, pyrite, 

chlorite, and magnetite, with accessory plagioclase, zircon, hematite, tourmaline, 

and rutile.

The Fontaine quarry of A. L. Pitts in 1904 had a length of 300 feet 

along the cleavage, a width of 200 feet, and a depth of about 100 

feet. Bedding and cleavage strike N. 34° E., and 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 slate of the Fontaine quarry shows alternating little beds 

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 are abundant, measuring up to 0.12 millimeter. Conspicuous scales of biotite lie across the cleavage, usually up to 0.2 by 0.09 millimeter and averaging about 12 to the squar.e millimeter. Lenses and crystals of 

pyrite, many of them surrounded by secondary quartz, about 27 to the square millimeter and measuring up to 0.076 by 0.03, rarely 0.34 by 0.03 millimeter, with their 

longer axes in the cleavage. A few scales of chlorite, interleaved with muscovite, 

lying across the cleavage and up to 0.3 millimeter. Some carbonate in the coarser 

beds, here and there with inclusions of an opaque mineral (graphite, pyrite, or. magnetite). Much graphitic matter in very minute particles. Some hematite and a few 

crystals of tourmaline, 0.02 by 0.006 millimeter.

The chief constituents of this slate, named in descending order of abundance, 

appear to be muscovite and sericite, quartz, biotite, carbonate, pyrite, graphite, 

chlorite, magnetite, and 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 half 

an inch in length by a twentieth of an inch in width at the center. 

George H. Williams 1 found the joint faces at the old Roberts quarry 

covered with pyrite - and crystals 1 millimeter long, which when 

analyzed proved to be Ti02 and which 'he determined optically as 

anatase..

Although the slates of Arvonia, as shown by the above determinations, contain some carbonate, they do not include any appreciable 

amount of ferrous carbonate, for some of these slates put on the old 

Richmond Theater over 70 years ago did not show any discoloration whatever when removed, about 1902, and some have been on 

buildings near the quarries over a century without losing any of their

i Anatase fiom the Arvon slate quarries, Buckingham County, Va.: Am. Jour. Sci., 3d ser., vol. 42, 

p. 431,1891.
VIRGINIA. 153

blackness. Their highly crystalline character also implies strength 

and durability.

The results of Merriman's tests of slate from the Williams and Pitt 

quarries are given on pages 182-183.

Penlan. Penlan station is 2 miles about south-southwest of 

Arvonia. The quarry of the Penlan Slate Co., of Perilan, is a mile 

farther south. It measures about 100 feet along the cleavage, 150 

feet across it, and 100 feet in depth. At 50 feet from the west 

edge, on the south side, there is a tunnel, 30 feet wide and 50 feet 

high measured from the quarry bottom, and extending 40 feet along 

the cleavage. In its upper 10 feet the tunnel has been extended 40 

feet still farther. Another tunnel has been made on the north side 

nearly in line with the west wall. This is 40 feet wide and 50 feet 

high and extends 40 feet northward along the cleavage. The company expects to extend the first tunnel 500 feet, to sink a shaft to 

meet its north end, and to work from both ends.

Both cleavage and bedding strike N. 30° E. and dip 80°-85° 

S. 60° E. There are three sets of joints (a) horizontal, spaced 

2 to 10 feet; (&) transverse, discontinuous, spaced 2 to 10 feet; (c) 

diagonal, open 3 inches and filled with quartz, graphite, or slate fragments (?). The weathered "top" is 50 feet thick. Whitish disks 

of calcite of extreme tenuity occur here and there on cleavage faces 

and have a diameter of 1 or 2 inches. They were evidently deposited 

by percolating water deriving the lime from the slate itself.. These 

disks are easily removed with dilute muriatic acid and have no 

economic importance.

The .slate has a very dark gray color with a faintly brownish, not bluish, hue. A 

specimen (D. XXXIV, 16, a), obtained from the company in 1912, and probably 

finer than the average, has a lustrous, very smooth cleavage surface, which only 

with a magnifier is seen to be very sparsely and minutely granular. It is very sonorous 

and very fissile, shows pyrite on the sawn edge, does not effervesce with acid test 

except when powdered, and contains graphite and some magnetite.

Under the microscope it shows a matrix of muscovite (sericite) with fine texture 

and brilliant aggregate polarization. Passages and little beds with more and coarser 

detrital quartz grains, parallel to the cleavage, make the texture somewhat uneven 

and show that bedding and cleavage are parallel. Particles and crystals of magnetite and of pyrite, 0.02-0.3 millimeter long, average about 5 to the square milJ^ 

meter. Some of the pyrite is inclosed in secondary quartz. Neither biotite nor 

chlorite was detected. Firj,e graphite occurs throughout; considerable carbonate 

in minute rhombs and plates; tourmaline and rare detrital zircon and plagioclase.- 

Butile was not detected.

The constituents, named in descending order of abundance, appear to be muscovite, 

quartz, graphite, carbonate, magnetite, pyrite, tourmaline, plagioclase, and zircon.

This is a highly crystalline mica slate generally identical with that of Arvonia.
154 SLATE IN THE UNITED STATES.

The following is a resume of the results of physical and magnetitic 

tests of four specimens of this slate made for the company by C. W. 

Hudson at the Cooper Union Laboratory in New York in 1908:

Tests of slate from Penlan, Fa.

Strength, modulus of rupture...pounds to the square inch.. 8,000-11,720 

Toughness, ultimate deflection, supports 18 to 22 inches 

apart...........................................inch.. 0.266-0. 366

Porosity, water absorbed in 24 hours............per cent.. 0.152-0. 235

Specific gravity........................:.................. 2.763-2.777

Content of magnetite...........................per cent.. . 0.05

The recently opened quarry of the Arvon Slate Manufacturing Co., 

of Penlan, is about a quarter of a mile southeast of Penlan station 

and 1,000 feet east of "the railroad. It measures about 130 feet 

along the cleavage, 50 feet across it, and 50 feet in depth. The 

weathered "top" is in places 10 feet thick.

The cleavage strikes N. 25° E. and dips 75° S. 65° E. and the attitude 

of the bedding is probably identical. The foliation has some slight 

plications, 6 to 12 inches wide, crossed by slip cleavage. There are 

transverse vertical joints spaced 1 to 10 feet with a heading 5 feet 

wide at the south end of the quarry. Irregular horizontal joints are 

spaced 10. to 20 feet and a diagonal joint strikes N. 20° W.

The slate is of very dark gray shade, like that of the Arvonia quarries, with a faintly 

brownish, not bluish, tinge, and a lustrous, very granular surface with marked minute 

plications. The slate is very fissile and sonorous, does not effervesce with acid test 

unless powdered, shows minute spangles on a rough edge and a little pyrite when 

sawn, and is graphitic and magnetitic.

Under the microscope this slate has a brilliant aggregate polarization. The cleavage 

is intersected at 36° by minute plications. Particles of magnetite and pyrite, the 

former far more abundant, 0.05-0.25 millimeter long, average about 5 to the square 

millimeter. Crystals and irregular scales of biotite, 0.05-0.3 millimeter long, average 

5 Or 6 to the square millimeter. The magnetite crystals lie mostly between plates 

of biotite, and these have secondary quartz on the outer side. Graphite in fine particles throughout; very little carbonate, 3 to 7 email plates to the square millimeter. 

Very little chlorite; tourmaline; rutile not detected.

The constituents, named in descending order of abundance, appear to be muscovite, quartz, graphite, biotite, magnetite, carbonate, pyrite, chlorite, and tourmaline.

This highly crystalline mica slate, although microscopically slightly different from 

that of the other Penlan quarry, ia generally identical with that of Arvonia.

Bremo. Bluff Slate has been prospected on the estate of the 

late Dr. Casey. <C, 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 q-23° E., and the dip 

nearly 90°. A little northwest of the h'6'use is a dike of olivine basalt 

5 to 10 feet wide. The results of an examination of specimens obtained within a few feet of the surface and therefore belonging to 

the "top" are given on page 155.
VIRGINIA. 155

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 fine 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, and is sonorous and very fissile. The slate from one of 

the openings has exceedingly minute veins of muscovite.

Under the microscope the slate from the C. C. Cocke estate shows a matrix of 

muscovite (sericite), with a brilliant aggregate polarization. There are abundant 

quartz grains up to 0..09 by 0.03 millimeter and 0.13 millimeter long, the larger 

ones surrounded by secondary quartz radiating along the cleavage; about 35 lenses 

and crystals of pyrite to the square millimeter, measuring up to 0.09 by 0.02 millimeter (exceptionally the lenses are 1.5 millimeters long), with their longer axes in 

the cleavage. There are also square and rhombic spaces lined with secondary quarte, 

measuring up to 0.6 millimeter, 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 millimeter, 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 

millimeter. 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 millimeter long; also a number of very irregular particles 

of rutile up to 0.05 millimeter, consisting of a network of crystals ("sagenite 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 millimeter; rare flakes of biotite; no 

carbonate.

The chief cons titutents of the slate, named in descending order of abundance, 

appear to be muscovite, quartz, pyrite, kaolin, chlorite, graphite, rutile, with accessory tourmaline, zircon, biotite, and hematite.

This slate differs from the slate of the Arvonia area 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.1

Slate has also been prospected at several other points in Fluvanna 

County between James River and Palmyra, but no quarry was yet 

in operation in 1912. A company has been organized and a quarry 

opened at a point 2 miles northeast of the Bremo mansion and 7 

miles south-southwest of Palmyra, a° little south of the road, with a 

course generally parallel to that of the James. (See fig. 13.)

A specimen (D XXXIV, 92, a) from this locality, obtained in June, 1913, from 

J. 0. Shepherd, of Palmyra, and reported as having been taken about 7 feet below 

the surface, is of medium bluish-gray color with a very slight purplish tinge. It has 

a lustrous roughish cleavage surface dotted with minute blackish lenses, is very fissile 

and sonorous, very graphitic and magnetitic, and with acid test effervesces very 

slightly in the powder, but not at all in the rock.

Under the microscope it shows a matrix of sericite with brilliant aggregate polarization. It contains lenses of carbonate up to 0.6 by 0.25 millimeter, numbering about

1 Slate outcrops on this property were referred to by J. L. Campbell, Tenth Census, vol. 10, Special report 

on building stones, p. 181,1884.
156 SLATE IN THE UNITED STATES.

23 to the square inch of transverse section, also crystals and lenses, mostly of magnetite with some of pyrite, up to 0.5 millimeter long by 0.07 millimeter thick and 

numbering about five to the square millimeter of section with the cleavage but seven 

to the square millimeter of transverse section. The lenses have radiating secondary 

quartz and muscovite about them, also chlorite. The pyrite shows limonite stain. 

The slate also contains prisms of tourmaline and of rutile but no slate needles, also 

a few grains of zircon. There is much fine graphite but no carbonate besides that in 

lenses.

The constituents of this slate, named in descending order of abundance, appear 

to be muscovite, quartz, graphite, chlorite, magnetite, carbonate, pyrite, tourmaline, rutile, zircon, and limonite. ,_ ' "*

This is a highly crystalline graphitic mica slate of the unfading series, resembling 

that of Arvonia but of a little lighter shade. It may be found darker as quarrying 

deepens.

Esmont, Albemcirle County. The slate outcrops of the Esmont district lie east of the Blue Ridge and 10 to 12 miles west of the slate of

Contour interval 200 feet

FIGURE 14. Map of slate region in Albemarle County, Va. Quarry shown l>y crossed hammers. 'Course

of railroad approximate.

Buckingham and Fluvanna counties. The quarries are on Ballinger 

Creek, a small tributary of James River, 5 miles northwest of Warren, 

on the James, and a mile west of Porter Precinct, as shown on figure 

14. A north-northeastward-trending ridge, Green Mountain, attaining an elevation of 800 feet, or 400 feet above the James, lies west of 

the slate. A diabase dike 50 feet thick, with a northerly course, cuts 

the slate east of the village and. creek.

The quarry of the Standard Slate Corporation, of Esmont, lies 

west of Ballinger Creek and north of the road to Porter and measures
U. S. GEOLOGICAL SURVEY BULLETIN 586 PLATE XXVI

A. LOWER QUARRY OF PORTLAND-MONSON SLATE CO., MONSON, ME.

Showing lateral walls of jointed quartzite kept in place by steel and concrete brace 

and slate pillar (in background). This has flat joints and has been tunneled 30 

feet to the next opening.

7>'. QUARRY OF STANDARD SLATE CORPORATION, ESMONT, VA. 

Showing benches from use of channeling and undercutting machines
VIRGINIA. 157

about 200 feet east to west, 80 feet north to south, and 170-feet in 

depth. (See PL XXVI, B.) Although the general structure at this 

place is much obscured it seems to consist of an acute syricline in 

- fehe center of the quarry and an acute anticline east of it, with probably another west of it and still another syncline beyond the west 

edge of the quarry in all, two synclines and two anticlines: The 

strike of the cleavage is N. 17°-22° E. and the dip 70° S. 80° E. The 

limbs of the folds strike with the cleavage and dip 50°-60° S. 80° E. 

There are vertical discontinuous joints striking N. 58° W. and spaced 

2 to 10 feet. Another set undulates horizontally and has a spacing 

of 3 to 30 feet. The slate within the upper part of the central syncline on the north wall is much broken up by many open joints of 

the horizontal set and the slate about the lower part of this syncline 

abounds in shear zones (see p. 44), slip cleavage, and quartz veins. 

A specimen from this place shows about eight planes of slip cleavage

to the inch. From the somewhat uncertain structure exposed there 

seems to be about 50 feet of black slate overlain by about the same 

thickness of green and "variegated," but in close eastward-inclined 

folds so that within the length of the quarry and the uncovered area' 

west of it each bed of slate is repeated, the green five times and the 

black four times. As quarrying proceeds more light will be thrown 

on the structure. 

There are four varieties of slate, as described below:

"No. 1, blue-black " (specimens D, XXXIV, 11, a, d) is of dark bluish-gray color, the 

bluish tint being marked and in the wet slate resembling that of specular iron. It 

has a very lustrous granular surface with an obscure foliation crossing the grain. It 

does not effervesce with acid test; the sawn edge shows pyrite; the powdered slate 

shows very little magnetite and still less graphite. It is sonorous and rather fissile. 

Under the microscope this slate shows a matrix of sericite with brilliant aggregate 

polarization containing on the average 2£ lenses to the square millimeter and measuring 

0.25-1.12 millimeters in length and up to 0.25 millimeter in thickness. These lenses 

consist of carbonate, mostly in rhombs with dark-brownish content, also of chlorite, 

muscovite, and quartz. In sections with the cleavage these rhombs and the muscovite 

are radially arranged, forming brilliant objects in polarized light. The proportions 

of the constituents of the lenses vary greatly. In one the carbonate amounted to 50 

per cent, chlorite 25 per cent, and muscovite and quartz 12.5 per cent each. The matrix abounds in minute bluish-black metallic grains of irregular size and form that are 

probably metallic hematite (Fe203). The carbonate is largely confined to the lenses. 

There are a few stains of limonite. The constituents, named in descending order of 

abundance, appear to be muscovite, quartz, chlorite, specular hematite, carbonate, 

graphite, pyrite, magnetite, rutile, tourmaline, and limonite. This is a mica slate 

of superior quality with a considerable content of specular hematite, to which its 

peculiar sheen is due. Its content of carbonate is so slight as not to appreciably affect 

the durability of its color. The color can not easily be confounded with that of the 

slates of the Arvonia district.

The "dark-green " slate (specimens D, XXXIV, 11, e, k) is of medium greenish-gray 

color and of lustrous, finely spangled or coarsely and sparsely granulated surface. It 

does not effervesce with the acid test and is sonorous. The color is devoid of the bluish 

tinge which marks some green slates. On exposure the color darkens somewhat.
158 SLATE IN" THE UNITED STATES.

There is also a "light-green" variety of this slate which is a dingy green. The "dark 

green" contains a little magnetite and the spangles are magnetite. The sawn face 

shows pyrite. It is very sonorous and fissile. Under the microscope this slate shows 

a matrix of sericite with brilliant aggregate polarization, containing lenses of chlorite, 

carbonate, and quartz, or of each separately with a little muscovite. The chlorite 

(interleaved with muscovite) lies in the grain direction, transverse to the axis of the 

lens. The lenses are 0.25-2 millimeters long, 0.07-0.1 millimeter thick, and 0.17-0.7 

millimeter wide and average 3 or 4 to the square millimeter. The carbonate and muscovite are concentrically and radiately arranged, the carbonate being within, as seen 

in sections parallel to the cleavage. There are secondary (?) quartz lenses up to 1.7 

millimeters wide, also scales and elongated crystals of magnetite, 0.006-0.05 millimeter 

wide and as much as 0.2 millimeter long, averaging about 13 to the square millimeter. 

Slate needles are very plentiful; a little sagenite, tourmaline, and zircon. The constituents, named in descending order of abundance, appear to be muscovite, quartz, 

chlorite, carbonate, magnetite, rutile, pyrite, tourmaline, and zircon. This is a mica 

slate with an amount of carbonate probably not large enough to affect the durability 

of its color appreciably.

The "variegated" slate is of purplish-gray color irregularly streaked with greenish 

gray, or of a peculiar very light brownish-purplish gray with meandering parallel 

bands ("ribbons") or beds of dark greenish gray, 0.1 to 0.3 inch wide. The banding 

and coloring vary greatly. The surface has but little luster and is granular. It is 

, sonorous, does not effervesce with acid test, and has a very little magnetite and pyrite. 

Under the microscope this slate shows a matrix with brilliant aggregate polarization 

containing little beds that are less micaceous and contain more and coarser quartz and 

carbonate. Particles of specular hematite are plentiful but not so abundant as in the 

black slate. There are lenses of radiately arranged carbonate rhombs and of chlorite, 

0.3 to 1.25 millimeter in diameter, in sections parallel to the cleavage and averaging 

nearly 2 to the square millimeter. Tourmaline, rutile needles, and limonite stains 

also appear. The bands are due to variations in the amount of quartz, chlorite, carbonate, and hematite. The constituents, named in descending order of abundance, 

appear to be muscovite, quartz, chlorite, carbonate, specular hematite, rutile, mag. 

netite, limonite, pyrite, and tourmaline.

The "ferrous" slate, a variety of the "dark blue" which discolors on continued 

exposure, has dark-brownish spots and indeed a general very slight brownish tinge, 

both attributable to a carbonate of iron, probably also of lime and magnesia. Its 

surface is lustrous and granular. It effervesces slightly with acid test and is sonorous. 

It contains much more carbonate than the black. Its strength is reported as fair.

The following analyses of the black and green slate, made for the 

company by the Lehigh Testing Laboratory at Allentown, Pa. ; are 

given here for reference:

Analyses of slates from Esmont, Fa.

Silica (Si02 )... .............................................................

Alkalies (Na2O. JT,0) . ......................................................

Sulphur (S)......... ........................................................

.

Dark blue.

56.33

22.26

.68

1 48

5.07

.004

1 69

2.86

99.814

Dark green.

53.58

24.53

8.81

.42

1.95

4.88

.008

1 QQ

3.39

99. 558
VIRGINIA. 159

The following physical tests were made by the division of tests, 

United States Department of Agriculture, in 1907, for the company:

Physical tests of slates from Esmont, Fa. 

Specific gravity.............................................. 2. 90

Weight .per cubic foot.....'......................... .pounds... 181

Water absorbed per cubic foot. ........................do.... 0.18

Toughness.................................................. 18

Crushing strength with grain....... pounds to the square inch.. 19,150 : Crushing strength across grain......................... do.... 29, 500 Modulus of. rupture, on a cross bending test, load at right angles 

to grain........................pounds tp the square inch.. 9, 360

Machinery. Channeling machines are used by this company for 

both horizontal and vertical cuts. The plant also includes a "carrier" with a continuous chain running through a trough into which 

the refuse slate from the cutters and splitters is thrown and is thus

automatically carried to the dumps. The slate blocks are cut across 

the grain by circular saws, and a gang of saws is being put in to cut 

the large blocks obtained by channeling to the size required by the 

splitters. 0 

An abandoned quarry east of Ballinger Creek and the dike and a 

little northeast of Esmont station is about 75 feet square, and shows 

steep eastward-dipping cleavage.

The slate is light greenish gray and has a slightly lustrous surface, two intersecting 

etriations, and minute dark spangles. The green is more bluish than the green slate 

of the Standard Slate Co.'s quarry. The sawn edge shows pyrite. Effervescence with 

acid test very slight. The pieces on the dumps show slip cleavage,.which in this and 

the other quarry may possibly be related to the intrusion of the diabase.

Under the microscope this slate shows a matrix of sericite with brilliant aggregate 

polarization and abundant chlorite. There are lenses of chlorite and carbonate; 

sparsely distributed magnetite in plates; quartz and carbonate; a passage of coarser 

quartz, carbonate, and chlorite; an obscure plication diagonal to the cleavage.

The constituents, named in descending order of abundance, appear to be rxmscovite, 

chlorite, quartz, carbonate, and magnetite.

Snowden, Amherst County. The slate deposit of Snowden is on 

the southeast side of the axis of the Blue Kidge, north of the cut 

through which James River flows. It is situated about 18 miles 

north-northwest of Lynchburg. (See map, fig. 15.) The slate crops 

out in a longitudinal valley between .Rocky Row Mountain on the 

northwest and a spur of Big Piney Mountain on the southeast. It 

strikes N. 65° E. and is bordered by quartzite on the southeast. 

The general structure and the areal relations of the slate have not 

yet been exhaustively investigated.1

The only quarry in operation in 1904 was that known as the Williams Brothers slate quarry, on the property of the Virginia Slate

1 See Campbell, J. L. and H. D., The Snowden slate quarries: The Virginias, vol. 5, pp. 162-163, 1884. 

Campbell, J. L., Geology of the Blue Kidge, near Balcony Falls, Va.: Am. Jour. Sci., 3d ser., vol. 28, pp. 

221-223,1884.
160 SLATE IN THE UNITED STATES.

Mining Co., 3 miles north-northeast of Snowden station and about 

1,200 feet above it, where Rocky Row Run enters James River. 

Slate about 250 feet thick is exposed with a bedding strike of N. 65° E., 

forming a flat-topped anticline, whose northern limb dips 35° at the 

surface but is vertical at th,e bottom of the quarry and for a few feet 

dips steeply south, indicating a possible overturn. The cleavage 

strikes N. 45° E. and dips 32° SE. 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 90° to 104° with the bedding 

and is said to be very marked.

79^5'

Scale of miles

Contour interval lOOfeet

FIGURE 15. Map showing location of slate quarries and prospects in Amhe'rst County,Va. Quarriesshown 

by crossed hammers; slate prospects by round dots; strike of commercial slate by arrows.

This slate is very dark gray. To the unaided eye it has a minutely granular texture 

and a moderately smooth cleavage surface, with very little luster. It is not graphitic, 

shows no pyrite on sawn edge, has no magnetite, does not effervesce with cold dilute 

hydrochloric acid, is fairly 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 millimeter; chlorite scales and 

lenses up to 0.28 millimeter in length, rarely 0.42 by 0.2 millimeter, numbering about 

126 to the square millimeter; also muscovite scales up to 0.09 millimeter; some carbonate in exceedingly minute rhombs and plates; grayish carbonaceous (?) matter;
VIRGINIA. 161

about 3,000 spherules and pyritohedrons of pyrite to the square millimeter, measuring up to 0.005 millimeter; abundant rutile needles, and scattered fragments of 

zircon.

The chief constituents of this slate, named in descending order of abundance, 

appear to be muscovite (sericite), quartz, chlorite, kaolin, pyrite, 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 adapted 

for mill stock. In 1912 this quarry was closed owing to litigation.

Thompson prospect. About 1£ miles northeast of the Williams 

quarry slate was prospected in 1886 on the Thompson property, 

which lies on a small tributary of Otter Creek that flows into the 

James 1$ miles downstream from Snowden, as shown in figure 15. 

The thickness exposed is 20 feet; the bed strikes N. 53° E. and dips 

20° S. 37° E.; the cleavage is nearly horizontal; joints strike N. 

38° E. and dip 20° N. 52° W.

A reexamination of the specimens collected by the writer in 1904 shows an aggregate 

polarization probably obscured by lenticular texture and the distribution'of carbon 

between the lenses as well as by the amount of carbonate. The quartz particles are 

noticeably angular; one detrital plagioclase; much carbonate; lamellae of chlorite 

ingrain direction; spherules of pyrite; abundant rutile needles.

The constituents, named in descending order of abundance, appear to be muscovite, 

quartz, chlorite, carbonate, carbon, pyrite, rutile, and plagioclase.

This is a mica slate, probably of the fading series, although it is 

asserted that slates obtained from this opening when it was made 

and then put on the courthouse at Amherst and on Kenmore College, 

in Amherst County, show no discoloration. In comparing the thin 

sections of this slate with those from the Williams quarry the latter 

are found to polarize less brightly. In sonorousness, color, and general characteristics this slate closely resembles that of that quarry.

Warrenton, Fauguier County. There are several prospects in the 

neighborhood df White Sulphur Springs on the Rappahannock, 6 

miles southwest of Warrenton, to which Rogers l referred and from 

which roofing slate was obtained as early as 1837. (See map, fig. 16.)

The following data afford some indications as to the geologic relations: About 1| 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 easterly strike and a 

gentle southerly dip. A little north of this point quartzite slate 

crops out and continues northward for 2 miles, nearly to the " colored 

church." Its cleavage strikes N. 25°-30° E., but the slate threefourths of a mile south of the springs, at the river, strikes about east, 

like the quartzite farther south. Slate is also said to occur at points

1 Op. cit.,p.460.

40620° Bull. 586 14 11
162 SLATE IN THE UNITED STATES.

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.

77V 77°50'

77°50'

Scale of miles

Contour interval 50feet 

FIGURE 16. Map of slate prospects in Fauquier County, Va. Slate prospects are shown by round dots.

All these hornblendic and epidotic rocks weather into a brightreddish 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.1 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 (fig. 16). One place is seven-eighths of a mile northnortheast of the springs, east of the road to Warrenton, and west of

i Rogers (op. cit.) refers to the epidotic rocks.
VIRGINIA. 163

the beginning of a small brook flowing south into the Rappahannock. 

The cleavage strikes N. 30° E. and 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 rather coarse texture, a rough speckled 

cleavage surface with little luster. It is very carbonaceous, shows pyfite 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; no 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 millimeter; a number of plagioclase 

feldspar fragments up to 0.2 millimeter long. There are lenses and cubes of pyrite 

coated with quartz, about 4 to the square millimeter; a few scales of chlorite; rare 

fragments of zircon. No carbonate or rutile detected.

The chief constituents of this slate, named in descending order of abundance, 

appear to be carbonaceous matter, quartz, muscovite, feldspar, pyrite, and chlorite. 

This is a clay slate.

At a second locality about half a mile northeast of the springs, the 

cleavage strikes N. 25° E. and dips 20° W., and the bedding probably 

strikes N. 12° W. Strike joints trend N. 15° E. and dip 90°; dip 

joints strike N. 75° W. and dip about 90° but variable. The slate 

is here covered by 2 feet of black clay and 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 it 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, does not effervesce 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 millimeter; 

also some lenses of secondary quartz, much carbonaceous matter, a few lenses of pyrite 

up to 0.14 millimeter, passing into limonite, and a few grains of plagioclase feldspar. 

No carbonate or rutile detected.

The chief constituents of this slate, named in descending order of abundance, 

appear to be muscovite, quartz, carbonaceous matter, and 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 half a mile south of the springs, on the 

east bank of the Rappahannock. "Slate" about 40 feet thick is exposed in a small cliff. The strike of cleavage and bedding (?) ranges 

from N. 80° E. to N. 85° W. and the dip from 12° to 15° S.

This slate is bluish black. It has a coarse crystalline texture and, as accurately 

described by Rogers, 1 a knotty, wrinkly surface. This is due to the presence of 

cubical and lenticular cavities once occupied by pyrite but now partly filled with 

quartz and graphite. These measure from one-fiftieth to three-fiftieths of an inch

i Op. cit., p. 460. . .....
164 SLATE IN THE UNITED STATES.

and number from 12 to 20 to the square inch. The surface has very little luster. 

The slate is carbonaceous or graphitic, shows pyrite on the sawn edge, has very little 

magnetite, contains 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, 

quartz, and muscovite. As the first two predominate the aggregate polarization is 

rather 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 referred 

to; most of which seem to be partial pseudomorphs of quartz and graphite after pyrite, 

the remaining pyrite having been oxidized and dissolved. There are some grains of 

plagioclase feldspar almost as large as the quartz grains, with graphite inclusions 

parallel to the multiple twinning. Veinlets of quartz and lenses of secondary quartz 

occur, 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 slate, named 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 two localities, yet 

to be less micaceous. This is properly a graphite-quartz-muscovite schist, or in process of becoming such.

A piece of this "slate" half an inch thick from the roof of one of 

the brick outbuildings at the White Sulphur Springs, where it was 

placed at least 75 years ago, does not show any decomposition but 

in places has a little limonite coating.

The pyritif erous character of the slate in all these prospects and 

the sulphurous character of the springs are probably intimately 

related. Such springs are common in the vicinity of pyritif erous 

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 principal features of the slate of Virginia, as given in the foregoing descriptions, will be found in tabular form opposite page 188.

WEST VIRGINIA.

The slate district near Martinsburg, in Berkeley County, W. Va., 

lies within the geologic belt designated Martinsburg shale in the 

Harpers Ferry folio of the United States Geological Survey. 

This belt lies about 13 miles west of the Blue Ridge and mostly on 

the west side of Opequon Creek, a small tributary of the Potomac. 

It measures at least 2 to 3 miles in width. This shale and clay slate 

formation, estimated at 700 to 1,000 feet in thickness, is of Ordovician 

age and overlies the Cambrian-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 or its tributary runs, where the mass has 

been denuded of its weathered zone, it has a well-marked eastward-
WEST VIRGINIA. 165

4

dipping (exceptionally westward) slaty cleavage crossing the bedding 

at various angles, and pieces wjien struck with a hammer give the 

typical ring of a slate.. The slatiness of the formation is, however, 

inconstant. ' The now disused quarry of the Shenandoah Slate Co., about 3 

miles northeast of Martinsburg, is near a small run flowing into the 

Opequon. In May, 1904, it 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. and dips about 15° 

S.- 65° E. The cleavage, with the same strike, dips 75° in the same 

direction. There are joints striking with the cleavage and dipping 

35° S. 65° E., also dip joints striking N. 50° W. and 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 a 

hole put 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 from 6,500 to 

11,000 to the square millimeter. Rutile was not observed. The amount of carbonate 

differs in different beds. It may be so abundant as to completely obscure 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 this slate, named in descending order of abundance, appear to 

be carbonate, muscovite (in places almost equal in amount to carbonate), quartz, 

kaolin, pyrite, carbonaceous matter, chlorite, and magnetite. This is a clay slate. 

The amount of C02 (carbon dioxide) in this slate was determined by George Steiger, 

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; A1203, 19-40; 

CaO, 1.11 per cent. It was proposed to use the product of this quarry 

for mill stock, for which it seems better adapted than for roofing.

The following prospects were noted:

One 2 miles south of Martinsburg, on John Rowe's land, where an 

opening 30 by 20 feet has been made. Bedding strikes N. 32° E. 

and dips 60°-65° E.; cleavage strikes N. 23° E. and dips 80° E. A 

3-foot bed is in sight.

Another opening has been made on the Opequon, on Light's land, 

half a mile south-southeast of Bedington. Bedding and cleavage
166 SLATE IN THE UNITED STATES.

strike N. 20° E., the former dipping 30° E., the latter 55° E. A 3-foot 

bed is in sight. Under the microscope this slate shows a faint aggregate polarization, fragments of feldspar, and rtitile 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 the 

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 

the slate from this place shows a slight aggregate polarization and 

but little carbonate.

Clay slate has also been found on the McKown & Busey farm, 2 

miles N. 10° W. from Middleway, in a brook 2£ miles southeast of 

Martinsburg, and also on land of J. W. Snyder on 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. Furthermore, the amount of carbonate 

shown by the microscope, as well as the mode of weathering in the 

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 uses.

The principal features of these West Virginia slates are stated in 

tabular form opposite page 188.
PAKT III. ECONOMIC GEOLOGY OF SLATE. 

DIFFICULTIES IN SLATE QUARRYING.

The difficulties in slate quarrying are many. Even after 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, 

and 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. The only way to remove some of these old dumps is 

to throw them into the quarry on one side and hoist them up on 

the other. In places where the beds are steeply inclined or vertical, 

one of the walls falls in as the quarry deepens, and the removal of 

this material entails great expense. The occasional employment of 

a consulting engineer for the mathematical determination of the 

points at which supporting walls should be left would obviate mechanical accidents of this nature. (See pp. 76, 78.) The recent introduction of the method of tunneling from the slate quarries of Angers, 

in France, is a step in advance, as it avoids the cost of removing the 

"top," furnishes shelter for the workmen in winter and rain, and preserves the moisture in the exposed slate. (See pp. 114,153.) When 

the tunnels attain large dimensions the French method of systematic 

daily inspection of marks set on the walls and roof to detect the slightest indication of any rock movement should also be adopted.1

There are also uncertainties of a stratigraphic nature which would 

be eliminated from-many a slate-quarrying venture by the liberal use 

of the core drill.

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 experience 

know that they have to do with beds and that the quality of the slate

i See Lariviere, G., Angers et 1'Anjou, Assoc. frangaise avanc. sci., pp. 1-51,1903.

167
168 SLATE IN THE UNITED STATES.

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 on 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 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 little change in direction but will be very much affected by 

great changes in the material or the grain of the sediment. (See 

PL II, B.} The prime factor is, then, the bed; the second factor is 

the cleavage.

In the southern 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 beN>I5° E< tween bedding and cleavage is not "always easily 

made. Quarrymen and prospectors sometimes 

regard them as identical where they really differ. Where the strikes of the bedding and 

cleavage are divergent, if that of the cleavage 

is mistaken for that of the bedding, a new 

opening may easily be made at the wrong point 

and the bed looked for may be missed. (See 

I \quarry fig. 17.) In such places the readiest means of 

' ' distinguishing cleavage and bedding are as follows: (1) The fossil impressions, trails, or algae 

("wavers") are always on a bed surface; so

I I

bed /

cleavage

symt>o/ aiso are crinoid stems and trilobites. (2) Miym o /-. nute plicated beds of quartz and calcite indicate

riGUEE i7.-Diagramiiiustrating bedding. (3) A microscopic section transverse

the effect of divergence in strike to the cleavage, if other means fail, may indiOf slate bedding and cleavage. ^^ ^e amount of divergence between bedding

and cleavage, as shown in Plate VII, C. Where both are parallel 

the course of the bedding may be shown by that of lenses or passages 

of coarser particles of quartz, carbonate, carbon, etc.

"FLINTS" THEIR NATURE AND CAUSE.

Beds of quartzite, which may be calcareous, micaceous, or pyritiferous (see PI. II, A], should never be confounded with veins of 

quartz (see PL V, A}. In some regions they are both indiscriminately 

designated by the quarrymen "flints." The quartzite beds are sediments consisting mainly of quartz sand, and, although varying considerably in thickness, are generally more persistent than the quartz
PEOSPECTING. 169

veins, which, as has been already shown, are chemical infiltrations 

into fractures produced at a much later time 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 of a quartz vein. It should be noted, however, that 

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.

RELATIONS OF JOINTS, DIKES, AND SHEAR ZONES.

From the similarity in the courses of the diagonal joints and many 

of the dikes and shear zones in western Vermont and eastern New 

York there seems to be a close relation between them. They may 

all have been produced by the same stress at the same time, in some 

places the strain resulting in a " hogback," in others in a diagonal joint, 

and these joints, where very deep, may have given rise to dikes. The 

practical application of this statement is that the possibility of such 

a relation 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 GEOLOGIC MAP AND COMPASS IN 

PROSPECTING.

Although the following paragraphs refer particularly to the slate 

belt of western Vermont and eastern New York and necessarily so 

because this is the only slate region that has been mapped by the 

writer in this way the general principles and method of procedure 

are -applicable to any slate region where the beds consist of close and 

more or less overturned folds.

The quarry maps, Plates XX and XXI, are designed to be of practical utility. The coloring shows where the Cambrian green and pur-
170 SLATE IN THE UNITED STATES.

pie 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 JamesvUle 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 at several 

quarries has been shown 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 geologic compass to determine the strike of 

any bed of good slate at any of the located quarries, and transferring 

the strike 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, and movable ring to set off magnetic variation 

and should have a clinometer attachment to indicate angle of dip.

Where, as at West Pawlet (see Section VII, PI. XXII, and diagrams 

A, B, C, PI. XXIII), the slate is closely folded, repetitions of the 

same series of beds, may be looked for in an east-west direction at 

varying intervals. The possibility of the pitching of the axis of a 

fold in a northerly or southerly direction should be kept in mind. In 

such folds older or newer beds are traversed in following the direction 

of the pitch. Where an Ordovician belt abruptly cuts off a Cambrian 

belt on the north or south, the Cambrian must ordinarily be supposed 

to plunge under the Ordovician.

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, and that skill 

appears to have been their only guide in prospecting in this region. 

It would be well if this skill were reinforced by the use of the following method in exploration:

1. Make reference to a geologic map for the areas in which the 

various slates may occur.

2. Determine on a quarry map or general map the good slate beds 

already exploited.

3. Make compass determination of the strike of such beds.

4. Explore along that strike.

5. Explore at right angles to that strike to see if the series is repeated by folding. (Note order of horizons in table on p. 124.)

6. Trench at promising localities across the strike to expose as 

large a series as possible.
 TECHNICAL DESCRIPTION OP A SLATE QUARRY. 171

'.'7. Where surface indications are favorable, make an opening large 

enough to determine the angle of dip of both bed and cleavage and 

to obtain specimens sufficient for the tests given on pages 172-179.

8. 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.

9. Measure thickness at right angles to bedding planes on the core. 

' The map of the slate district near Slatington, Pa. (PL XVI), upon 

which the quarries are carefully located and the strike and pitch 

shown, is also designed for practical use.

TECHNICAL DESCRIPTION OF A SLATE QUARRY.

The following outline of a description of a slate quarry is proposed 

as covering the features of practical geologic and economic importance, aside from the ordinary statistical matters as to amount and 

value of product, number of employees, etc.:

Quarry name or number. 

Location (exact).

Geologic formation as given on the geologic 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 hoists or of horse derricks. 

Number of carriers. 

Number of channelers. 

Number of slate saws. 

Capacity of air compressor. 

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. 

Bends or curvature of cleavage. 

Strike and dip of strike, dip, and diagonal joints. 

Curvature of joints. 

Strike and dip of grain. 

Strike and dip of slip cleavage. 

Strike and dip of shear zones. 

Location and diameter of "posts" and supports. 

Proximity of dikes. 

Course and thickness of veins,

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
172 SLATE IN THE UNITED STATES.

recent years, however, more exact methods of reaching these results 

have been devised. All such methods 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, are 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.1 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 tune-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 semivitreous 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.

Cleavability. The test for cleavability 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. The cleavage face of a freshly cleft slate with a high, 

grade of fissility usually shows extremely delicate partly detached 

scales of slate lying parallel with, the cleavage.

Cross fracture ("sculping"). This test to determine the character of 

the "grain" should also be applied by an experienced man to a large 

block several inches thick, with a stout chisel and a long-handled heavy 

mallet. Jannetaz 2 published a method for determining with scientific 

precision the direction of the grain in slate in which 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 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

1 Full titles are given in the economic bibliography, pp. 210-213.

2 Jannetaz, E., Relation entre la propagation de lachaleur et 1'e'lasticite' sonore dans les roches, p. 417,1877.
METHODS OF TESTING SLATE. 173

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 arid 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. If the gram is pronounced, it will appear in fine transverse lines. If slip cleavage, 

which is generally a cause of weakness, is 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. The presence of lime can be determined by the 

application of cold dilute hydrochloric acid to the edges of a freshly 

quarried slate or still better to the powdered slate. Rapid effervescence implies the 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 a high percentage of lime must, other things being equal, in time 

be acted upon more powerfully by the acids of the atmosphere than 

those with little or no lime.

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 yalue of 

slates is somewhat affected by the extent of their discoloration.

Presence of clay. By breathing upon a fresh and clean piece of 

slate and observing whether there is any argillaceous odor the presence 

of clay will be detected. 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,

i Reverdin, F., and De la Harpe, C., Chem. Zeitung, vol. 14, pp. 64-65,94-95,126-127.
174 SLATE IN THE UNITED STATES.

forming a yellowish-white film and rusty spots, is poor; x but pyrite 

in small quantity is not necessarily detrimental, for it does not decompose readily. It is present in nearly all slates, 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 

magnetite has become of practical importance, A cubic inch of the 

slate should be weighed, broken up, and reduced to a fine powder in 

an agate motor 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 quantity of magnetite, by 

weight, in a cubic inch of slate can thus be determined.

Electric resistance. The electric resistance of slate is governed in 

part by the scarcity of magnetite and in part probably also by the

So character of bedding, 

cleavage, and grain. The 

tests applied by Prof. 

W. N. Gladson, of the 

University of Arkansas, 

to Arkansas slates can 

be used. The object of 

these tests is to determine the adaptability of 

the slate for electric 

switchboards.

The following description of these tests is taken from a report by 

Purdue: 2

These pieces of slate were tested in comparison with three pieces of gray slate taken 

at random from old switch bases in the university electric laboratory. A piece 1 

centimeter cube was cut from each sample and these were numbered consecutively 

from 1 to 9, Nos. 1, 3, and 4 being gray slate. In preparing the cubes metallic particles were found in samples 4 and 6, and Nos. 5 and 6 were so easily split that it was 

difficult to obtain a centimeter cube.

The pieces of red slate as received were smooth blocks 4 by 5 inches by five-eighths 

of an inch, neither varnished nor in any way filled. They were red or reddish brown, 

were much softer than the gray slate, and split much more readily. All samples 

tested were dry and appeared to be seasoned. The method of measuring the resistance of these centimeter cubes was aa follows:

A block of paraffin wax was attached to the center of a glass plate, which in turn 

was thoroughly insulated from the table by glass strips piled across one another. In 

the top of the paraffin block an opening was cut 1 centimeter square and about 3 millimeters deep. In the bottom of the cavity thus formed four copper supports were 

embedded so that their top surfaces were in the same plane, about 1 millimeter below

1 See Julien, A. A., On the variation of decomposition in the iron pyrites; its cause, and its relation to 

density: New York Acad. Sci. Annals, vol. 3, pp. 365-403,1886; vol. 4, pp. 125-221 and Pis. VIII, IX, 1887. 

Brown, A. P., On marcasite and pyrite; a comparative study of the chemical behavior of pyrite and marcasite: Am. Philos. Soc. Proc., vol. 33, pp. 225-243, 1894. Stokes, H. N., On pyrito and marcasite: U. S. 

Geol. Survey Bull. 186,1901. 

8 Purdue, A. H., The slates of Arkansas: U. S. Geol. Survey Bull. 430, pp. 329-330,1910.

FIGURE 18. Diagram showing electric connections made in 

testing slate, by W. N. Gladson.
METHODS OF TESTING SLATE. 175

the top of the paraffin cup. A drop of mercury coming about flush with the copper 

supports in this cavity formed one terminal for making electric connection to the 

slate cube. Contact with the opposite face was made by placing a well-amalgamated 

zinc.plate 1 centimeter square on top of the cube. This arrangement insured equal 

contact with each slate cube under test.

The galvanometer used was of the D'Arsonval type and had a working constant of 

70,533 millimeters on the scale 1 meter distant through 1 megohm resistance. The 

electromotive force was furnished by storage cells and was kept constant at 42 volts 

during the experiment. The connections were made as shown in figure 18.

To avoid leakage over the surface of the slate a guard wire was connected as shown. 

All readings were taken after the deflections became constant; in some tests they did 

not become so until half an hour after electrification.

The results of the test are shown in the following table, from which we find the 

average resistance of all samples to be 1,224.2 megohms per cubic centimeter. The 

average resistance of the three gray samples was 1,180, and of the six red-slate samples 

1,267.8 megohms per cubic centimeter. . Each piece tested, except No. 7, shows a 

different resistance between each pair of opposite parallel faces, which seems to 

depend on the plane of cleavage. The gray-slate samples show a decidedly higher 

resistance between faces of cubes perpendicular to cleavage planes, but in individual 

samples the distribution of resistance would be greatly affected by the presence of 

foreign conducting particles or seams, which are likely to be present in all slate.

Results of tests of electric resistance of samples of slate from Arkansas.

Sample No.

2

-

3....................................

4....................................

5....................................

6....................................

7....................................

8....................................

9....................................

Galvanometer scale 

deflections.

Perpendicular to 

cleavage 

planes.

D

Mm. 

39.0 

98.0 

171.0 

35.0 

104.0 

338.9 

91.0 

57.0 

45.0

Parallel to cleavage 

planes.

D'

Mm. 

40.0 

174.0 

185.0 

94.0 

47.7 

28.0 

91.0 

51.0 

33.0

D"

Mm. 

44.0 

625.0 

283.0 

43.0 

39.9 

88.0 

48.0 

27.0 

36.0

Kesistance. <*

Perpendicular to 

cleavage 

planes.

R

Megohms. 

1, 808. 5 

719.7 

414.9 

2,015.3 

678.2 

208.1 

775.0 

1,500.7 

1,567.4

Parallel to cleavage 

planes.

R'

Megohms. 

1,763.3 

405.3 

381.2 

750.4 

1.476.3 

2,519.0 

775.0 

1,383.0 

2, 137. 3

R"

Megohms. 

1,603.0 

67.1 

249.2 

1,640.3 

1,767.1 

801.5 

1,469.4 

2,612.3 

1,959.2

a R, R', and R" correspond to the directions D, D', and D", respectively.

Average of Nos. 1,3, and 4 (gray slate), 1,180.6 megohms per centimeter cube. 

Average or Nos. 2, 5.6, 7, 8, and 9 (red slate), 1,267.8 megohms per centimeter cube. 

Average of all samples, 1,224.2 megohms per centimeter cube.

Strength. Merriman's apparatus for testing the strength of slate is 

described in his paper: 1

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 by means of sand running out of an orifice in a bgx

i Merriman, Mansfield, Am. Soc. Civil Eng. Trans., vol. 27, Nos. 3 and 6, 1892; vol. 32, pp. 529-539, 1894. 

An extended abstract of Merriman's paper is given by W. C. Day in part 6, continued, of -U. S. Oeol. Survey 

' Nineteenth Ann. Rept, pp. 257-263,1898. On p. 182 of this, bulletin the results of his recent tests of 33 

specimens of slate are given.
176 - SLATE IN THE UNITED STATES.

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 the formula 

£ = O2> in which w is the load in pounds which- causes the rupture,

Z the distance between the supports in inches, & the breadth, and d the 

thickness of the plate in inches. S is the number of pounds to the 

square inch. According to these tests the modulus of rupture in the 

best slate should range from 7,000 to 10,000 pounds.

J. F. "Williams 1 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 

to the square inch. Campbell and Donald 2 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. 3 Watrin 4 gives the maximum crushing 

weight of some French slates as 2,000 kilograms to the 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 and from 0.20 

to 0.28 inch thick, and repeats the blows until the slate breaks. The 

foot-pounds of work to the 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 of Eldorado County, Cal., when split seven to the inch and 18 inches square 

and fastened solidly at the two ends are said to -bend 3 inches in the 

center without any sign of fracture. 5 J. F. Williams 1 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 a pressure of 770 to 1,200 pounds, and when 

the supports were placed 3 inches apart by 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 anyone visiting 

the shanties where the splitting is done.

Density or specific gravity. The specific gravity is determined in 

the. usual way, by weighing a piece of the slate in and out of water and

\ Van Nostrand's Eng. Mag., vol. 31, pp. 101-103, 1884. 

'''l. Brit., 9th ed., 1887.

3 Quoted by Hull, Edward, A treatise on the building and ornamental stones of Great Britain, etc., 1872.

' f Walrinj &.,_ L/es "ardoisieres des Ardennes, pp. 192, 193, 1898.

'& Cra'wfbrd, JV J., California State Min. Bureau, Twelfth Rept., p. 400, 1894.

S! i.. ''-.» I " - > . ' 
METHODS OF TESTING SLATE. 177

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. The porosity is best determined by drying, then weighing, then immersing for 24 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 24 hours, 

cools to the normal temperature of the room, weighs, immerses it for 

24 hours, and weighs again. His tests of Pennsylvania 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.

Reverdin and De la Harpe * state that slates are liable to deterioration from the chemical action of gases arising from woodwork beneath 

the slate, as well 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, or 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 and then heated two hours in an oil bath at 170° C., air 

pressure is restored, and heating is 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 24 times, and the slate is then impregnated with diphenylamine and the procedure is as in the 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 24 

hours in a freezing mixture and heating another piece 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

1 Op. cit. 

40620° Bull. 586 14 12
178 SLATE IN THE UNITED STATES.

frost on the microscopic structure has already been referred to 

(p. 35).

Hardness or abrasion. Merriman has also de'vised a method of 

testing the relative hardness of slates by subjecting apiece 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 the destructive power of these acids. 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, but 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 120 hours, then dried for 40 hours, 

weighed, the solution strengthened, and the pieces reimmersed for 

another 120 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 slip cleavage, if any, the probable durability or nondurability of the color, as well as the presence of any mineral constituents likely to determine its other qualities. The specimen 

should be carefully selected, so as to fairly represent the general 

quality of the bed. It should be fresh, unfrozen, and half an inch 

thick across the cleavage. At least two sections should be 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 thinner 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 and then in polarized light, with powers ranging from 

140 to 700 or even 1,100 diameters. The transverse section will 

show the quality of the cleavage, the slip 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 not be misled,
METHODS OF TESTING SLATE. 179

however, 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. It 

could be eliminated by the application of hydrochloric acid to the bare 

section and then re-covering 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. If the carbonate contains no ferrous carbonate a thin 

section cut parallel to the cleavage of a long-exposed roofing slate and 

ground down almost entirely from the back so as to bring the exposed 

surface next to the slide will show the carbonate free from brownish 

discoloration.

The following passage translated from an abstract of J. Hirschwald's exhaustive work on the testing of building stones * will be 

found useful in the microscopic examination of roofing slate:

The microscopic examination of roofing slates,- which is best made with sections at 

right angles to the cleavage, shows that the textural variations of slate depend mainly 

on the development of the mica layers. The mica itself either occurs in thick-leaved 

lamellae of dark color with the optical properties of biotite (magnesia mica) or it appears 

faint yellowish to colorless and then always makes up fine scaly aggregates, but sometimes very much streaked thin lamellae. The scaly aggregates resemble phlogopite 

(the ironless magnesia mica); the streaked lamellae resemble sericite (fibrous muscovite). Besides these there are chloritic varieties. Some of the mica is of unitorm 

color, and some spotted to opaque. This may be due to magnetite or to coaly matter. 

In the latter case the black spots are not sharply bounded, but have merging outlines! 

and this is also true of the general outline of the mica lamellae when strongly impregnated with coal. If the mica is in continuous layers, as it usually is in the good slates, 

it prevents access of water from the surface. It not" only protects the other constituents that lie between the mica layers from the dissolving, softening, and decomposing 

effect of water, but the inner part of the slightly dampened slate is not as much exposed 

to the action of frost as it would be in slates greatly saturated with water. When the 

mica is less abundant it makes up discontinuous layers or in the inferior slates quite 

separate little lamellae. These admit water freely from the cleavage face and consequently more solution takes place, more decomposition of any content of pyrite, and 

especially more frost action. Those slates that contain mica in separate scales, even 

where continuous mica layers have been developed, show an increased absorption 

and thus greater evidence of decomposition, and are therefore among the least valuable slates.

But there is still another difference in the development of the mica layers. In 

some slates these possess a remarkable parallelism of arrangement; in others they are 

more or less fibrous, the mica enveloping the other constituents in lenses, and in this 

way a close connection is brought about at short intervals between the separate mica 

layers. In the slates of the first type, water gains free access laterally from the edge 

of each slate by capillary attraction and distributes itself between the mica layers, 

and such slates easily scale under the action of frost. But in the slates of fibrous texture such lateral admission of water can occur only to a very slight extent, and as the

1 Zeitschr. prakt. Qeologie, vol. 16, pp. 387-389,1908.
180 SLATE IN THE UNITED STATES.

continuous and mostly well-developed layers of mica'prevent the entrance of water 

from the cleavage surface any considerable frost action is shut out. When, after the 

lapse of centuries, such fibrous elates finally weather they show but little exfoliation 

and in many cases only scaly disintegration. Very micaceous slates of fibrous texture 

are among the best roofing materials if their mica is in homogeneous layers and not in 

the loose scaly aggregates above described.

But slates poor in mica may be tolerably durable if they are strongly silicified. 

The clayey material thus acquires considerable hardness and affords a corresponding 

resistance to the softening effect of water.

Coaly slates belong in a special class. In some the coaly substance is only interleaved with the mica layers, which, under the microscope, appear intensely black 

and opaque and have peculiar intergrown borders, while the other constituents are 

quite transparent. In others the coaly matter is distributed either evenly or in single 

irregularly disseminated spots throughout the slate mass. All coaly varieties of slate 

are inferior, because the coal is hygroscopic and thus opens the mica layers so that 

they fail to protect the intervening matter from the operation of water. As stated, 

the black inclusions may also consist of magnetite, which does not possess the deleterious property of the coaly constituent. As both of these substances and the usually 

associated pyrite are equally opaque it is important when other means fail to distinguish them microscopically. The simplest way is to examine the section under incident light, preferably lamplight. The coaly matter will then appear intensely black, 

the magnetite bright gray with metallic luster, but the pyrite a bright golden yellow. 

This method is also serviceable in distinguishing the other constituents of slate, as, 

for instance, quarlz and feldspar. The quartz is water clear, but the feldspar is more 

or less cloudy according to its degree of decomposition.

The above extract is an important supplement to the microscopic 

notes in this bulletin. Although some of the slates described probably differ from the American slates, they may yet be found here, 

and the observations are discriminate and the methods suggestive.

Chemical analysis. In order to give a correct idea of the composition of the slate, the chemical analysis should not be partial but 

complete.1 Such analysis should then be compared with complete 

analyses of the best slates of like color. Its microscopic analysis 

and the results of the tests of its strength, elasticity, porosity, and 

corrodibility should be considered in connection with its chemical 

analysis. Merriman 2 concludes' from six different kinds of tests 

applied to each of 24 specimens of slates from the "Old Bangor'7 

and Albion quarries of Pennsylvania, 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. ,

1 See, on the advantages of complete analysis, Hillebrand, W. F., Principles and methods of analysis 

applied to silicate rocks: U. S. Geol. Survey Bull. 148, pp. 1-64,1897; The analysis of silicate and carbonate' 

rocks: U. S. Geol. Survey Bull. 422,1910.

2 Op. cit.
TESTS OF CERTAIN SLATES. 181

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 the amount or absence of the 

carbonate, but this is just what a chemical analysis can not do with 

so complex a rock. It rarely happens that the chemical analysis of a 

slate affords conclusive evidence as to its commercial value. The 

value of such analysis is quite subsidiary to that of the microscopic 

examination. This was practically demonstrated in the West Virginia clay slate district, where investments were made and a quarry 

was opened largely on the basis of a chemical analysis, and what 

could have been learned from the study of a few thin sections was 

not ascertained until it was too late.

Hutchings 1 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.

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 were formerly made at the United States Arsenal at 

Watertown, Mass., and that transverse and absorption tests are now 

made by the United States Bureau of Standards. (See p. 189.)

TESTS OF CERTAIN MAINE, NEW YORK, PENNSYLVANIA, 

VERMONT, AND VIRGINIA SLATES.

By MANSFIELD MERRIMAN.

Physical tests of some of the slates described in this bulletin were 

made for the United States Geological Survey at Lehigh University, 

South Bethlehem, Pa., in 1905.

The following is a report of the results of the tests of 33 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." 2

All the specimens were 12 by 24 inches in size. The following is a 

statement of the marks placed upon them and of their mean thicknesses :

Cl, C2, C3, C4 designate four blue slates from the Chapman Slate 

Co., Chapman quarries, Pennsylvania, the mean thicknesses being 

0.228, 0.204, 0.216, 0.222 inch, respectively.

Wl, W2, W3, W4 designate four blue slates from the Williams 

Slate Co., Arvonia, Va., the mean thicknesses being 0.202, 0.195, 

0.206, 0.263 inch, respectively.

i Hutchings, W. M., Clays, shales, and slates: Qeol. Mag., dec. 4, vol. 3, Nos. and 8,1896. 

1 Am. Soc. Civil Eng. Trails., September, 1892.
182 SLATE IN THE UNITED STATES.

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 inch, 

respectively.

Bl, B2, B3 designate three blue slates from the Merrill Brownville 

Slate Co., Brownville, Me., the mean thicknesses being 0.220, 0.195, 

0.210 inch, respectively. A fourth piece sent by this firm was 

accidentally broken.

Mol, Mo2, Mo3, Mo4 designate four blue slates from Monson Consolidated Slate Co., Monson, Me., the mean thicknesses being 0.238, 

0.227, 0.237, 0.225 inch, respectively.

Vl, V2 designate two green slates from Vermont Unfading Green 

Slate Co., Fair Haven, Vt., the mean thicknesses being 0.222, 0.262 

inch, respectively. Two other pieces sent by this firm were found 

broken in the package.

Hi, R2, R3, R4 designate four green slates from Rising & Nelson 

Slate Co., West Pawlet, Vt., the mean thicknesses being 0.260, 0.167, 

0.224, 0.213 inch, respectively.

M5, M6, M7, M8 designate four green slates from Mathews Consolidated Slate Co., Boston, Mass., the mean thicknesses being 0.194, 

0.222, 0.187, 0.205 inch, respectively. The labels on these slates 

were marked "Quarry N. E."

Ml, M2, M3, M4 designate four red slates from Mathews Consolidated Slate Co., Boston, Mass., the mean thicknesses being 0.190, 

0.180, 0.174, 0.186 inch, respectively. The labels on these slates 

were marked " Quarry E."

The following table gives the results of the tests of the 33 specimens 

for strength, toughness, density, softness, porosity, and corrodibility,

and also the means for each of the nine different kinds of slate:

tTests of roofing slates.

Color and firm.

Blue;" Chapman Slate Co...

Blue; & Williams Slate Co...

Mark on 

specimen.

Cl.......

C2.......

C3.......

C4.......

Mean....

Wl......

W2......

W3.....

W4.....

Mean....

Strengthmodulus of 

rupture in 

pounds to 

the square 

inch.

8,410

12,490

7,420

9,540

9,460

8,590

8,250

10, 700

8,620

9,040

Toughness ultimate deflection in 

inches on 

supports

0.21

.24

.16

.24

.212

.17

.19

.31

.24

.227

Density 

specific 

gravity.

2,775

2.780

2.734

2.766

2.764

2.788

2.754

2.795

2.788

2.781

Softnessamount in 

grams 

abraded 

by 50 turns 

of a small 

grindstone.

0.171

.218

.234

.213

.208

.073

.038

.097

.033

.060

Porosity 

per cent of 

water absorbed in 

24 hours.

0.221

9QH

.150

.263

.231

.105

.090

.209

.167

.143

Corrodibility 

per cent 

of weight 

lost in 

acid solution in 63

hours.

0.336 ini

560

.334

.383

.281

.801

.223

.270

.394

Designated "very dark gray" in table opposite page 188. 

Designated " very dark greenish gray " in table opposite page 188.
TESTS OF CERTAIN SLATES. 

Tests of roofing slates Continued.

183

Color and firm.

Blue;«A.L. Pitta..........

Blue; 6 Merrill Brownville

Blue; 6 Monson Consolidated Slate Co. ...........

Green; Vermont Unfading 

Green Slate Co ...........

Green; Rising & Nelson

Green; c Mathews Consolidated Slate Co. ............

Red; Mathews Consolidated 

SlateCo. .................

Mark on specimen.

PI......

P2......

P3......

P4......

Mean....

Bl......

B2......

B3......

Mean....

Mol..... 

Mo2..... 

Mo3..... 

Mo4.....

Mean....

VI...... 

V2......

Mean....

Rl......

R2......

R3......

R4......

Mean....

M5...... 

M6......

M7......

M8......

Mean....

Ml......

M2...... 

M3...... 

M4......

Mean...

Strength 

.modulus of 

rupture in 

pounds to 

the square 

inch.

8,540

9.010

11,970

9,880

9,850

8,860

9,050

11,720

9,880

9,500 

11,370 

7,000 

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.

0.25

.18

.25

.22

.225

.17

.21

.22

.200

.24 

.22 

.19 

.17

.205

.19 

.26

.225

.24

.22

.13

.24

.207

.22

.15

.20

.190

.18

.27 

.25 

.23

.232

Densityspecific 

gravity.

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

2.700

2.736

2.783 

2.782

2.786

2.781

2.783

2.834

2.853 

2.853 

2.853

2.848

Softnessamount in 

frams 

raded 

by 50 turns 

of a small grindstone.

0.095

.159

.138

.038

.108

.243

.360

.192

.265

.234 

.302 

.271 

.218

.256

.326 

.356

.341

.184

.183

.209

.185

.190

.286 

.203

.244

.172

.226

.069

.099 

.119 

.304

.148

Porositypercent of 

water absorbed in 

24 hours.

0.332

.154

.163

.216

.189

.128

.128

.148

.181 

.181 

.206 

.186

.188

.300 

.162

.231

.429

.220

.365

.286

.325

.376 

.409

.360

.351

.374

i w

.251 

.334 

.235

.243

Corrodibility 

per cent 

of weight 

lost in 

acid solution In 63 

hours.

0.248

.552

.'275

.218

.323

.366

.284

.284

.305

.282 

.384 

.234 

.243

.286

.277 

.313

.295

.433

.722

1.067

.849

.768

.413 

.265

.428

.409

.379

.249

.425 

.507 

.311

.373

a Designated " very dark greenish gray " in table opposite page 188. 

b Designated " very dark gray" in table opposite page 188. 

c Designated " bright green" in table opposite page 188.

The results given, with the exception of those for softness, may be 

compared with the results for the slates of the Bangor, Pen Argyl, 

Lehigh, and Peach Bottom districts, given in my papers published 

in Transactions of the 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 one another, 

but can not be compared with those in the papers cited; this is due 

to the circumstance that the former grindstone has been destroyed
184 SLATE IF THE UNITED STATES.

by fire and that the one selected to replace it has a much smaller 

abrading capacity.

TESTS OF SLATES FROM ARKANSAS.

Specimens of slates from Arkansas, collected by Prof. A. H. Purdue, 

.were subjected to various physical tests at the United States Geological Survey laboratory at St. Louis. The results of these tests, 

published in Survey Bulletin 430, are repeated here for reference.

Source and color of slate samples tested.

No.

1

3

4

S 

6

7

8

9

10

11 

13

Owner.

Southwestern Slate Mfg. Co. . 

.....do.......................

M. J. Harrington. ...........

Southwestern Slate Co. ......

M. J. Harrington. ........... 

M. W. Jones .................

.....do.......................

.....do.......................

.....do.......................

.....do.......................

C.B. Baker. ....... .........

Locality.

East line of NE. J SE. J sec. 33, T. 3 S., R. 27 W. . 

.....do.. ...........................................

Sec. 24, T. 3S., E. 29 W............. . ........... 

SE. i SE. J sec. 33, T. 3 S., R. 27 W. ..............

Sec. 24, T. 3 S.,R. 29 W.. ......................... 

.....do.............................................

.....do.............................................

.....do.:..........................................

.....do.............................................

NE. i sec. 18, T. 3 S., E. 28 W .................... 

Sec. 24, T. 3 S.,E. 29 W.. .........................

Color.

Red.

Black. 

Red.

Black.

Do.

Red.

Red.

Buff. 

Red.
Results of physical tests on slates from Arkansa

Field 

No.

1 

2 

3

4 

5

Register No.

Misc. 141..... 

Average...

Misc. 141..... 

Average...

Misc. 143.....

Misc. 144.....

Misc. 145..... 

Average. .

Individual 

specimen 

No.

1 

2 

3

1 

2 

3

1 

2 

3

1 

2 

3

1 

2 

3

Transverse tests.

Dimensions.

Width.

Inches. 

2.00 

1.98 

1.98

1.94 

1.97 

1.99

1.98 

1.95 

2.05

2.07 

1.98 

1.99

1.94 

2.04 

1.94

Depth.

Inches. 

0.95 

.97 

.98

1.00 

.98 

.97

.28 

.26 

.27

.30 

.27 

.29

.85 

.77 

.78

Span. .

Inches. 

' 9 

9 

9

9 

9 

9

- 12 

12 

12

12 

12 

12

12 

12 

12

Conditions at maximum 

load.

Load at 

center.

Pounds. 

322 

674 

854

920 

959 

920

59 

56 

80

93 

64 

80

210 

279 

388

Deflection at 

center.

Inches. 

0.0190 

. 0148 

.0196

.0134 

.0150 

.0170

.0430 

.0790 

.0855

.0600 

.0560 

.0615

.0405 

.0335

Modulus of 

rupture.

Lbs. -per 

sq. inch. 

2,410 

4,880 

6,060

4,450.

6,400 

6,840 

6,630

6,620

6,840 

7,640 

9.640

8,040

8,990 

7,980 

8,600

8,520

2,700 

4,150 

5,920

4,260

Modulus of 

elasticity 

(constant 

. nearly to 

maximum).

2,100,000 

4,640,000 

4,250,000

3,660,000

6,430,000 

6,040,000 

5,470,000

.5,980,000

13,420,000 

8,820,000 

11,030,000

11,090,000

11,820,000 

12,420,000 

11,570,000

11,940,000

2,900,000 

3,150,000 

6,010,000

4,020,000

Absor

Weight ratio of absorp

tion.

30 

minutes.

0^0017 

.0005 

.0011

.0011

.0004 

.0004 

.0003

.0004

.0037 

.0012 

.0041

.0030

.0026 

.0009 

.0013

.0016

.0023 

.0033 

.0024

.0027

"24 

hours.

0. 0189 

.0175 

.0164

.0176

.0089 

.0075 

.0087

.0084

.0160 

.0114 

.0167

.0147

.0113 

.0087 

.0112

.0104

.0120 

.0153 

.0121

.0131

'48 

hour

0. 018

.019

.078

.018

.010

.009

.010

.010

.018

.014

.020

.017

:012

.011

.012

.011

.015

.019

.015

.016

a Apparent specific gravity-s-true specific gravity.
Results of physical tests on slates from Arkansas C

Field 

No.

6 

7 

9 

11 

8

Register No.

Misc. 146..... 

Average. . .

Misc. 147..... 

Average...

Misc. 149..... 

Average...

Misc. 150..... 

Average...

Misc. 148..... 

Average. . .

Individual 

specimen 

No.

1 

2 

3

1 

2 

3

1 

2 

3

- 1 

2 

3

1 

2 

3

Transverse tests.

Dimensions.

Width.

Inches. 

2.04 

2.03 

2.03

2.03 

2.07 

1.96

1.99 

2.00 

2.00

1.97 

1.94 

1.93

1.94 

2.04 

2.02

Depth.

Inches. 

1.09 

1.02 

1.01

.24 

.23 

.24

.17 

.14 

.22

.53 

.51 

.52

.15 

.17 

.17

Span.

Inches. 

12 

12 

12

12 

12 

12

7 

7 

4

9 

9 9'

7 

7 

7

Conditions at maximum 

load.

Load at 

center.

Pounds. 

400 

318 

791

35 

57 

40

48 

47 

120

139 

155 

140

20 

36 

32

Deflection at 

center.

Inches. 

.0252 

.0134 

.0233

.0660

.0740

.0317 

.0358

.0181 

.0181 

.0206

.0265 

.0330 

.0250

Modulus of 

rupture.

I/bs. per 

sq. inch. 

2,970 

2,710 

6,880

4,190

5,390 

9,370 

6,380

7,050

8,760 

12,590 

7,440

9,600

3,390 

4,150 

3,620

3,720

4,810 

6,410 

5,760

5,660

Modulus of 

elasticity 

(constant 

nearly to 

maximum).

2,620,000 

4,810,000 

6,920,000

4,780,000

9,240,000 

10,290,000 

8,060,000

9,200,000

13,150,000 

19,530,000

16,340,000

4,720,000 

6,090,000 

4,500,000

5,100,000

9,820,000 

9,410,000 

15,810,000

11,680,000

Abso

Weight ratio of abso

tion.

30 

' minutes.

0.0014 

.0010 

.0015

.0013

.0010 

.0013 

.0011

.0011

.0010 

.0010 

.0009

.0010

.0183 

.0185 

.0084

.0151

.0016 

.0010 

.0010

.0012

24 

hours.

0.0076 

.0081 

.0080

.0079

.0089 

.0089 

.0090

.0089

.0073 

.0081 

.0082

.0079

.0410 

.0402 

.0365

.0392

.0094 

.0081 

.0089

.0088

48 

hou

0.01

.00

.00

.01

.01

.01

.01

.01

.00

.00

.00

.00

.04

.04

.03

.03

.01

.00

.00

.00

Apparent specific gravity-r-true specific gravity.
10

13

Misc. 148.....

Average...

Misc. 148.....

1

2

3

1

2

3

1.99

1.93

1.97

2.03

2.00

1.93

.12

.13

.14

.15

.16

.18

7

 7

6

6

6

9.75

8

18 .

20

20

24

,.0430

.0378

.0440

.0440

.0300

.0180

3,570

2,570

4,890

3,680

3,940

3,520

3,450

3,640

5,730,000

4,250,000

6,340,000

5,440,000

3,550,000

4,420,000

6,480,000

4,820,000

.0088

.0072

.0067

.0076

.0046

.0062

.0061

.0056

.0251

.0257

.0251

.0253

.0228

.0243

.0241

.0237

.02

.02

.02

.02

.02

.02

.02

NOTE. Specific gravities corrected at 70° F.
188 SLATE IN" THE UNITED STATES.

COMPARATIVE CHARACTERISTICS OF SLATES.

The accompanying table shows the principal mineralogic, chemical, 

and physical characteristics of 56 kinds of slate described by the writer 

in this bulletin, so far as these characteristics manifestly bear on 

their economic value. These slates are from Arkansas, California, 

Colorado, Georgia, Maine, Maryland, Nevada, New Jersey, New York, 

Pennsylvania, Tennessee, 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 third colufim. 

The columns headed "Strength" and "Toughness" refer to the tests 

by Merriman, whose methods of experimentation are described on 

page 175. "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 mica slate 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 on the strength and toughness of the 

slate, but physical data are not sufficient to show this. The coarsetextured Peach Bottom slate, which really approaches a mica schist, 

is the strongest of the twelve kinds of American slates tested, but it 

is 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 minerals" includes only the four or five principal 

constituents which were seen under the microscope or whose presence 

has been otherwise determined, and these are given in the descending 

order of their probable abundance, In the column headed "Class" 

the technical economic designation of each slate is given with reference to the classification on page 10. To these comparative data 

should be added the results of a few tests not easily tabulated.

Merriman's later corrosion tests show the following percentages of 

loss in weight after immersion in acid solution for 360 hours: Pennsylvania slates (not including Peach Bottom slate), 1.68 to 2.76; 

Peach Bottom slate, 1.11 to 1.29; red slate of New York and Vermont, 

0.25. During these tests the Pennsylvania slates became a grayish 

white, some of the Peach Bottom samples changed but slightly, 

others were almost unaffected; the red slates likewise remained 

almost unaffected.1

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, 0.21.2 :

i Am. Soc. Civil Eng. Trails., vol. 32, p. 538,1894. 2 Idem, p. 542.
Comparative characteristics of various slates.

State.

Arkansas.......... 

Do.............

Do....... -----

Do.... 

Do...... ....

Do... 

Do. ... --------

Do.............

Do. ....... -----

Do.............

Do 

Do -----

Do.. _....... 

Do.............

Do............. 

Pennsylvania .....

Do. .........---

Do. ............

Do.........---

Do..........---

Do.............

Do.......-..--

Do.............

Do.............

Do.......--...-

PennsylvaniaMaryland.

Do.............

Do............

Do............

Do............

Do.... ........

Do............

Do............

Do............

Do............

Do............

Do............ 

Do............

Do.............

Do.............

Do.............

Locality, quarry, bed.

Mena, Polk County. ......... 

....do..............-----....

Missouri Mountain slate 

("Mammoth red" and 

"Lost Hannah"), Polk 

County.

Missouri Mountain slate 

("Mammoth red" and 

"Lost Hannah"), Polk 

County. 

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. 

Near Planada, Merced-County. 

Marble, Gunnison County..

Bolivar. Bartow County. .... 

Roekmart, Polk County....

Merrill quarry, Brownville, 

Piscataquis County. 

North Blanehard, JPiscataquis County. 

Monson Pond quarry, Monson, Piscataquis County. 

Portland-Monson quarry, 

Monson. 

Maine Slate Co., of Monson. . West Monson............... 

Thurston, Frederick County

Near Winnemucca, Humboldt County. 

Lafayette, Sussex County . . .

Granville and Hampton, 

Washington County.

.....do.......................

"Old Bangor" quarry, 

Northampton County.

North Bangor, Northampton County. 

Albion quarry, Pen Argyl, 

Northampton County. 

Albion quarry, gray bed, 

Northampton County. 

Heimbach, big bed, Northampton County.

East Bangor Consolidated, 

Northampton County.

Slatingtoii, Lehigh County..

Chapman "hard v e i n," 

Northampton County.

Aquashicola, Carbon County.

Derry Church, Dauphin 

County.

Peters Creek, Lancaster 

County. 

Falls Branch, Tellico, Monroe County. 

Laurel Creek, Tellico, Monroe County.

Ar*

Northfield, Washington 

County. 

"Sea-green," Rutland 

County. 

Purplish of "sea-green," 

Rutland County. 

"Unfading green/' Rutland 

Countjr. 

Purplish of "unfading," 

Rutland County. 

Benson (prospect), Rutland 

County. 

Arvonia, Williams, Buckingham County. 

Arvonia, Fontaine, Buckingham County. 

Penlan, Penian Slate Co., 

Buckingham County. 

Penlan, Arvon Slate Manufacturing Co., Buckingham County. 

Bremo (prospect), Fluvanna 

Countv. 

Esmont, Albemarle County

Snowden, Amherst County 

Martinsburg, B e r k e 1 ey 

County.

Described 

on page 

63

63 

63

64 

64

64 

65 

65 

67 

70 

70 

71 

73 

81 

80 

77 

78

78 

79 

85

87 

88 

91

92 

97

99 

102 

102

101

101

99

107 

104

109 

109 

111 

114 

116 

117 

117 

122 

140

141 

142 

146 

151 

152 

153 

154

155 

157

157 

158

160 

165

Color

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

Dark reddish.. .... 

Reddish..........

Greenish gray. .... 

Light greenish. . . .

Verv dark bluish 

grayLight gray........

Very dark gray. . . . 

....do.............

Dark bluish gray. .

Extremely dark 

bluish gray. 

Light blue-greenish gray. 

Very dark bluish 

gray. 

Very dark gray...

.....do............

Very dark bluish, 

gray.

.....do............ 

.....do............ 

Dark purplish. . . .

Dark bluish gray.

Very dark bluish 

gray.

.....do............

.....do............

Very dark greenish. 

Very dark gray, 

not bluish. 

Bluish black ..... 

Very dark bluish 

gray.

.....do............

Very dark gray...

Very dark bluish 

gray. 

Dark bluish gray.

Very dark bluish 

gray. 

Very dark, slightly 

bluish gray. 

Very dark bluish 

gray. 

Purplish gray....

Light blue-greenish gray. 

Very dark gray. . .

Greenish gray....

Bluish black.....

Very dark greenish gray.

Very dark gray, 

faintly brownish 

.....do............

Dark bluish gray, 

like specular 

iron when wet. 

Medium greenish 

gray. 

Purplish-brownish 

gray, banded. 

Very dark gray... 

Black, brownish 

hue.

Cleavage surface.

Remarkably fine. . . . . '.

Roughish, speckled. . . 

Fine ..................

Roughish. ............

Very fine .............

Fine.................. 

Roughish. ............

Roughish, spangled . . .

Fine ..................

Very finely granular...

Slightly uneven. ......

Smooth to roughish . . ; 

Slightly roughish. ....:

Very fine.............;.

.....do................

.....do.................

.....do................

Very smooth. .........

Fine or roughish, 

speckled.

.....do................

.....do.................

.....do.................

Slightly roughish. .....

Roughish, finely 

banded.

Minutely granular. ....

Fine to roughish ...... 

Fine or speckled ......

.....do................

.....do................

Minutely granular. . . . 

Very granular. ........

Fine, spangled, 

sparsely granular. 

Granular. .............

Minutely granular.

Luster.

Slight......... ........

Almost none ..........

Waxy. ................

Almost none. . ........

Slight.... ............. 

None. ................. 

Almost none ..........

Bright................

Some. .................

None..................

Slight................. 

None. .................

Very bright........... 

Slight.................

Almost none. .........

Bright...... . .

.....do.................

Slight................. 

None.............. .

.....do................

Almost none .......... 

. ....do.... ...... .......

.....do.................

.....do.................

.....do.................

.....do.................

Slight.................

Very bright ...........

.....do................. 

None.................. 

.....do.................

Very bright...........

.....do.................

Slight................. 

Bright...............

......do................

.....do................

.....do................ 

.....do................

Bright................

Little................

Magnetite.

Some. ..........

Very little...... 

.....do..........

.....do..........

Little..........

Very little ..... 

.....do.......... 

.....do..........

.....do..........

Magnetic pyrite. 

None...........

Extremely little 

Very little......

Much.......... 

None...........

Very little......

Some...........

None. .........

.....do.........

Very little.....

.....do..........

Some ..........

.....do.........

Very little.....

Some... .

.....do.........

Little.........

Extremely 

little. 

Very little.....

Some..........

Little..........

Some. _ ....... 

Little.........

.....do.........

Some........... 

None. ..........

Extremely little. 

More or less ....

.....do..........

.....do.........

Some. .........

.....do..........

Very little..... 

.....do.........

.....do.........

0.05 percent...

Very little..... 

Little.........

Very little..... 

.....do.........

Little.........

Microscopic texture.

Crystalline, extremely 

fine, homogeneous. 

Crystalline, fine ............. 

....do......................

Crystalline, extremely 

fine, homogeneous. 

.....do......................

Crystalline, fine............. 

.....do.......................

Crystalline, coarse, granular. 

Crystalline, firm

.....do.......................

Crystalline, irregular ........

Crystalline, fine .............

Crystalline, lenticular, coarse 

Crystalline, very fine. .......

Crystalline, fin R. t . t .

Crystalline, fine, but particles irregular. 

.....do.......................

Crystalling firm,

Crystalline, fine, particles 

irregular. 

.....do.....................

Crystalline, lenticular .......

.....do......................

.....do.......................

Crystalling firm., . LL . . ,

.....do......................

.....do......................

.....do......................

.....do......................

.....do......................

.....do......................

.....do.......................

.....do........................ 

.....do.......................

.....do......................

Crystalline, coarse. .......... 

.....do.......................

Crystalline, fine.............

Crystalline, crossed by beds 

at acute angle.

.....do.......................

.....do......................

Crystalline, irregular .......

.....do......................

Crystalline, fine ............

Crystalline, irregular .......

Crystalline, irregular, 

coarse. 

Crystalline, irregular, fine . -

Crystalline, with minute 

plications.

Crystalline, lenticular...... 

.....do......................

Crystalline, fine, irregular. 

Not crystalline or imperfectly so, coarse.

Grade offissility.

1

3

1

3 

1

2 

2 

3 

2 

2 

3 

2 

3 

1 

3 

3 

3

2

2 

3

1 

1 

3

3 

1

1 

1

1 

1

1 

1

1

2

1

1 

2 

2 

2 

3 

3 

1 

1 

1 

2 

2 

2 

2 

2 

2 

2

2

2 

2

2 

3

Chief minerals a (probable descending 

order of abundance).

Muscovite, carbon, quartz, pyrite, 

magnetite. 

Muscovite, hematite, kaolin, quartz..

Muscovite, hematite, kaolin, quartz, 

chlorite.

Muscovite, Quartz, kaolin, chlorite....

Muscovite, quartz, pyrite, carbon. ....

Muscovite, quartz, chlorite, kaolin. . . .

Muscovite, quartz, carbonate, pyrite, 

carbon. 

Muscovite, quartz, chlorite, carbonate, 

pyrite. 

Muscovite, quartz, biotite, kaolin, 

graphite. 

Muscovite, carbonate, quartz, carbon, 

kaolin. 

Muscovite, quartz, chlorite, carbonate.

Muscovite, quartz, carbonate, carbon. . 

Muscovite, quartz, magnetite, TDyrite. .

Muscovite, quartz, chlorite, biotite....

Muscovite, quartz, chlorite,. magnetite, 

biotite. 

Muscovite, quartz, biotite, chlorite. . . .

Muscovite, quartz, chlorite, pyrite

Muscovite, cnlorite, quartz, talc _ ..

Muscovite, quartz, chlorite, graphite 

(probably). 

Muscovite, quartz, carbonate, chlorite, 

carbon. 

Muscovite, quartz, hematite, kaolin, 

carbonate.

Muscovite, quartz, chlorite, carbon, 

magnetite. 

Muscovite, carbonate, quartz, kaolin.

Muscovite, carbonate, quartz, kaolin.

Muscovite, carbonate, quartz, chlorite 

Muscovite, carbonate, quartz, chlorite 

Muscovite, carbonate, quartz, pyrite. 

Muscovite, carbonate, quartz, carbon.

Muscovite, carbonate, quartz, pyrite, 

chlorite.

Muscovite, carbonate, quartz, kaolin. 

Muscovite, quartz, carbonate, Dyrite.

Muscovite, quartz, carbonate, chlorite, 

carbon. 

Muscovite, quartz, carbonate, pyrite, 

chlorite. 

Muscovite, quartz, graphite, andalusite, magnetite. 

Muscovite, quartz, graphite, andalusite, rutiie. 

Muscovite, quartz, chlorite, carbonate.

Muscovite, quartz, chlorite, hematite, 

carbonate. 

Muscovite, quartz, chlorite, carbonate, 

pyrite. 

Muscovite, quartz, graphite, chlorite, 

magnetite. 

Muscovite, quartz, carbonate, chlorite.

Muscovite, quartz, carbonate, hematite. 

Muscovite, quartz, chlorite, carbonate.

Muscovite, quartz, chlorite, hematite. .

Muscovite, quartz, carbonate, pyrite. .

Muscovite, quartz, biotite, carbonate, 

graphite. 

Muscovite, quartz, biotite, carbonate, 

pyrite, graphite. , Muscovite, quartz, graphite, carbonate, magnetite. 

Muscovite, quartz, graphite, biotite, 

magnetite.

Muscovite, quartz, pyrite, kaolin? 

graphite. 

Muscovite, quartz, chlorite, specular, 

hematite.

Muscovite, quartz, chlorite, carbonate, 

magnetite. 

Muscovite, quartz, chlorite, carbonate, 

specular hematite. 

Muscovite, quartz, chlorite, kaolin. . . .

Carbonate, muscovite, quartz, kaolin, 

pyrite, carbon.

Class.

.....do........... .

.....do.......................

.....do.................. .

.....do.......................

Mica slate, unfading (?) . . .

Mica slate, unfading..

Mica slate, slightly fading. . .

Mica slate, f ading. ......

Mica slate, unfading. ........

.....do.......................

.....do.......................

.....do......................

.....do.....................

.....do..... .

Talcose mica slate, unfading

Mica slate, unfading...

Mica slate, fading. ...

Mica slate, brightens on exposure.

Mica slate, fading. .......

.....do....................

.....do.............

.....do......................

.....do...................

.....do...............

.....do..........

.....do..................

.....do......................

.....do.......................

.....do......................

Mica slate, unfading ........

.....do.......... .. .

Mica slate, fading. ..........

Mica slate, unfading. ........

.....do.......................

.....do.......................

Mica slate, fading. ..........

.....do.......................

Mica slate, unfading.. . ......

.....do.......................

Mica slate, unfading. .......

.....do......................

.....do.......................

.....do......................

.....do......................

.....do......................

Mica slate, darkens a little 

on exposure. 

Mica slate, fading ?.........

Mica slate, fading ..........

Carbonate.

.....do.........

.....do.........

.....do.........

.....do.........

.....do.........

.....do.........

.....do.........

Little.........

.....do.........

.....do.........

.....do.........

.....do.........

.....do........

.....do........

.....do........

.....do........

Less than red, 

usually. 

Quite a little.

Quite a little. 

.....do........

.....do........

Much........

.....do........

A little.......

Much........

.....do........

None.........

.....do........

.....do........

.....do........

Very little....

Much........

Some.........

Very little....

..... do.. ......

Some......... 

.....do........

.....do........

Very little....

None.........

Very little.... 

.....do........

.....do........

Little........

From some to

much.

Lime 

by analysis.

0.98

1.22

0.52

0.11-

5.11

1.43

4.38

4.09

4.23

2.83- 

3.40

0. 155- 

0.48

0.63- 

2.20 

0.50-

0.71 

0.42- 

0.56

1.27

0.68 

0.42

1.11

and 

over.

Strength, 

pounds 

to the 

square 

inch.o

9,880

9,130

9,220

8,050 

9,810

7,150

..........

9,460

11, 260

7,250

6,410

9,040 

9,850

8,000- 

11, 720

9,360 

9,360

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

0.266- 

0.366

........

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. See p. 64.

Some limonite staining from pyrite. 

Between a metamorphic grit and a slate. 

See test, p. 176.

See p. 81 on amount of magnetite. See tests on pp. 82, 183. 

Very sonorous. 

Do. See tests on p. 77. 

Do. See p. 79 on amount of magnetite.

Do.

Do. See tests on p. 183. 

Sonorousness very moderate; can be sawn with handsaw.

Sonorous. 

Reported as harder than Bangor, Pa., slate.

Little or no ferrous carbonate. Impact test shows 126.66 

foot-pounds of work to the pound of slate. See tests on 

pp. 92, 183. 

See tests on p. 183.

finpact tests of various slates from Northampton and 

Lehigh counties show from 3.50 to 5.44 foot-pounds of 

work to the pound of slate. See tests on p. 97. 

Do.

Do. See p. 102.

Do.

In ribboned slate from this quarry the percentage of quartz 

would be higher than in the rest. Impact tests of various 

slates from Northampton and Lehigh counties show from. 

3.50 to 5.44 foot-pounds of work to the pound of slate. 

Discolors on continued exposure. Impact tests of various 

slates from Northampton and Lehigh counties show from. 

3.50 to 5.44 foot-pounds of work to the pound of slate. See 

tests on p. 188. 

Discolors less readily than any of the above Pennsylvania 

slates. Impact tests of various slates from Northampton 

and Lehigh counties show from 3.50 to 5.44 foot-pounds 

of work to the pound of slate. See tests on p. 182.

Very sonorous. Impact tests show from 8.49 to 24.17 foot, 

pounds of work per pound of slate. See tests on p. 112.

Sonorous. 

Do. 

Do. 

Very sonorous. Contains magnetic iron pyrites.

Becomes brownish gray on continued exposure. See tests 

on p. 183. 

Discoloration less pronounced than that of "sea-green."

Preserves nearly all its color on continued exposure. See 

tests on r>. 183. 

Do.

Sonorous. Probably "fading." 

Very sonorous. See tests on p. 182. 

Very sonorous. See tests on p. 183. 

Very sonorous. See tests on p. 154. 

Very sonorous.

Very sonorous. See tests on p. 159. 

Do.

Very sonorous. 

Analysis by W. C. Tiiden. In some specimens rauscovite 

equals the carbonate in amount.

a Carbon=carbonaceous matter or graphite. 6 Tests bv Merriman. c Deflection, in inches, on supports 22 inches apart; tests by Merriman and others.

40620° Bull. 586 14. (To face page 188.)
COMPARATIVE CHARACTERISTICS OF SLATES. 189

J. F. Williams's tests of the compression of cplumhs 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 tjie -red of New 

York and Vermont 17,730 pounds. 1 ' , ' , : 

The following results of'various tests of slate from Morisoii, Maine, 

were made at the United States Arsenal at Wajbertown^Mass.: 2

Pounds. 

Maximum fiber stress per square inch..'.......................' 7,671

Shearing test per square inch. ........'. .............'........ 2,192

Ultimate compressive strength .per square 'inch......... J...... 19,510

Coefficient of expansion, 0.000005. . . . ^

In accordance with the scheme of classification of slates given'on 

pages 10; 11 many of the slates whose characteristics are given in the 

preceding table are here arranged systematically.

A. Clay slates (fading) Martinsburg, W. Va.' '

(a) Carbonaceous] Lehigh, Northampton, and Carbon coiinorgraphitic| ties, Pa.; Sussex County, N. J'.; Ben- 

(blackish).'J son, "v^t.; Rockmart, Ga. :

' (fading)

B. Mica slates

(u n f a d-

(6) Chloritic (greenish). . "Sea-green,", Vermont,; green,- 

Bartow County, Ga. . , , 

(c) Hematitic and ctiloritic (purplish). Purplish of Pawlet 

and Poultney, Vt.

Peach Bottom, Pa. and Md. ' 

Arvonia and Penlan, Va.

Northfield, Vt. 

Brownville, Monson, Me. 

North Blanchard,'Me. 

Merced County, Gal. 

Humboldt County,

(a) Gra p h i t i 'c 

(blackish). 

(6) H e m a t i t i c fGranville, Hampton, N. Y.; Polk County, 

(reddish). \. , Ark. -.,....

(c) Chlori; ticf'Unfadillg ;g;reei1'" Vermont; " bright' 

(greenish). green,", Granville, N, Y..; green, Teil lied, Tenn. .

(d) Hematitic

. and chloritic (purplish). 

(e) Hematitic 

(specular) 

and graphitic (bluish 

black). . ..

Purplish of Fair. Haven, Vt.; Thurston, 

Md.; and Tellico, Tenn. . . 

'Blue-black " of Esmont,'Va.

SLATE MACHINERY.

A-description of all the various machines used in quarrying and 

finishing slate is not within the scope of this publication, but atteti1 Van Nostrand's Eng. Mag., vol. 31, p. 132, 1884.

2 War Dept. reports, republished in Twentieth Ann. Kept. U. S. Geol. Survey, pt. 6 (contd.), p. 395,1899.
190 SLATE IN THE UNITED STATES.

tion will be briefly called here to the growing substitution of cutting 

machinery for powder in quarrying slate and to the new devices 

for finishing it.

It will be noticed from the quarry descriptions that where the 

cleavage is nearly horizontal, as in some of the Pennsylvania quarries, 

or where it is nearly vertical, as in the Maine quarries, machines for 

vertical or horizontal cutting, constructed on the same principle as 

those used in the marble quarries, are being used, and in connection 

with them air compressors are also being introduced. The reduction 

of waste by these processes is obvious. The attempt, referred to on 

page 159, to do all the transverse cutting of large blocks by circular 

saws is of interest. Among the minor improvements is the automatic transportation of waste from the cutting sheds to dumps at a 

distance (p. 159) and the use of carborundum wheels in finishing 

slate moldings, etc.

The most important recent addition to the mechanical devices in the 

ndustry is the slate-splitting machine invented by Vincent F. Lake, 

which, in January, 1913, was on exhibition at the plant of the Genuine 

Washington Slate Co., at Berlinsville, Pa. The underlying principle 

of this machine is the substitution for the sudden jar given to the 

slate by the hand-splitter's hammer and chisel of a great number of 

blows, each of which moves the chisel a small fraction of an inch 

only, while the operator presses the vibrating chisel into the cleft 

slate as it parts. These vibrations, numbering as many as 16,000 a 

minute, are imparted to the chisel by means of a broad-edged iron 

cylinder 6 inches in diameter into which are sunk 24 small steel 

cylinders projecting but slightly above its edge. The rapid rotation 

of the main cylinder with its minor ones coming into contact with a 

single cylinder carried in a piston to which the chisel is attached 

imparts the vibratory motion. The slate block, preferably with one 

or more sawn edges, is placed with its long edge on an iron table and 

with its short edge facing the operator. (See PI. VI, B.} The chisel, 

a very thin blade of vanadium steel, 9 inches or more in width but 

made flexible by splits at intervals and with its edge vertical, is set 

loosely in the piston socket. The edge of the slate is held between 

gages mechanically adjusted and operated by a treadle so that the 

exact center of the slab is brought before the chisel. The operator 

controls the intensity of the hammer blow by means of a trigger and 

pushes the chisel horizontally through the block.

This machine can be operated with little skill and makes commercial slate from blocks which now find their way to the dumps. 

It also makes slate of unusual thinness. Mr. Lake has also perfected a punch for making beveled holes of any shape in slate 

with a view of utilizing slates of various thicknesses as a foundation for
USES OF SLATE. 191

plaster and stucco work. The perforated slates are nailed to the 

studding or furring, and the plaster, applied directly to the slate, 

passes through the perforations and clinches itself on the back.

The slate-splitting machine thus seems likely not only to reduce 

the cost of splitting but also the percentage of waste, and to make 

new demands for slate.

USES OF SLATE.

As will be seen by consulting the descriptions in this bulletin and 

the table opposite page 188, the slates of the United States have a very 

wide range of color, texture, and composition and therefore of 

adaptation. While nearly all of them possess one or two excellent 

features, few possess many such features and none possess them all. 

Several are so conspicuous for their well-nigh perfect adaptation to 

certain uses that the demand for them is likely to increase with the 

growth of the country. Such are the blackboard slates of the "softvein '' region of Pennsylvania, the red roofing slates of New York, which 

increase in brightness of color on exposure, and the fadeless brightgreen ones associated with them; the black unfading slates of the 

Peach Bottom district of Maryland and Pennsylvania, of Arvonia, 

Penlan, and Esmont, in Virginia, and of Monson and Brownville, in 

Maine; the almost unfading green slates of Vermont; and the millstock slates of Vermont, Maine, and Pennsylvania.

On account of their cheapness, which is due to their perfect cleavage, 

the "sea-green" slates of Vermont and the black slates of Lehigh and 

Northampton counties, Pa., will always be in demand in spite of their 

content of ferrous carbonate. Some architects are reported as preferring such fading slates for esthetic reasons.

The selection of a slate should manifestly be governed mainly by 

its adaptation to the purpose in view as well as by considerations of 

cost and transportation.

An increasing quantity of mill-stock slate is used for electric purposes, but this use requires a minimum content of magnetite. It 

would be well if the more durable slates were more widely used for 

inexpensive tablets and gravestones instead of marble. Facts showing the relative durability of inscriptions on slate and marble exposed 

to the weather will be found in Bulletin 521 of'the United States Geological Survey.1

There are also many structures where thick slate slabs with their 

great transverse strength could be used to advantage instead of concrete beams. Finally should be mentioned the proposed substitution 

of slate veneers for laths, described on page 190.

i Pale, T. N., The commercial marbles of western Vermont, pp. 37-39,1912,
192' SLATE IN THE UNITED STATES.

SLATE WASTE. , .

The percentage of waste is generally high in slate quarrying. 

Watrin, 1 referring to* the Ardennes region, gives the total waste at 70 

to 75 per cent in weight, of which 20 to 25 per cent occurs in the quarry 

and 50 per cent 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 interbedding of the slate with quartzite. Among the 

important'practical problems in the slate industry is that of the reduction of this large percentage of waste and the utilization of the 

unavoidable remainder. The substitution, wherever practicable, of 

mechanical cutting for blasting is probably the most available method 

of 'effecting this reduction. Much ingenuity has been expended in 

devising methods of utilizing slate waste. The slate-splitting machine 

described above aims to make a contribution to this end. The growing'use in roofing of "graduated slates," which near the ridge are onefourth inch thick'but at the cornice 1% inches thick, consumes slates 

with imperfe'ct cleavage. The Inlaid Slate Co. of Bangof, Pa., is utilizing slate 3 inches square embedded in a mixture of asphalt having a' 

high melting point with a backing of roofing felt for covering flat roofs.2 

A demand for waste slate has also arisen from the use of powdered 

slate for filling in oilcloth, etc., and of granulated slate for roofing. 

The particles, averaging about 0.1 inch in diameter, are pressed into 

a tarred paper (Staco Mills, Whitehall, N. Y.). The bright-red slate 

waste of Granville, N. Y., is being used by the Algonkian Red Slate 

Co., of Truthville, N. Y., for roofing and bridge paint by being combined with several superior oils and a small quantity of white lead. 

An article by one of the editors of the Slate Trades Gazette (England) 

describes the use being made of powdered slate in Norway.3

At the present time in Norway waste slate is being ground into a fine powder and 

mixed with casein, * * * the nitrogenous constituent (curd) of milk. The casein 

may be either solid or liquid, the proportion varying according to the nature of the 

slate. Hard and soft slates obviously require different treatment. The casein may 

be pure or mixed with other substances lime, soda, resins, or acids according to the 

character of the product required. If the final resultant is required to be of any 

particular tint, coloring matter is introduced at this stage. The composition thus 

obtained is plastic and is placed hot or cold in molds and subjected to pressure. The 

material is then exposed to the drying action of the air. ' The final product has properties identical with those of slate except that the casein, if anything, renders it harder, 

and tougher. The casein can be made insoluble by the application of formaldehyde. 

The compressed slate is then ready to undergo sawing, planing, polishing, etc. * * *

All manner of fancy designs may be imprinted on the plastic slate, which may be 

manufactured into blocks of uniform size or in large sheets, and the thickness may, of 

course, be regulated by the pressure. This material could be used as a covering for

A Watrin, N., Les ardoisieres.des Ardennes, 1898. .-

2 U. S. Geol. Survey Mineral Tlesources for 1910, pt. 2, p. 632,1911.

a Idem for 1909, pt. 2, p. 500,1910.
STATISTICS .OF PRODUCTION, IMPORTS, AND EXPORTS. 193

walls, .either internally or externally, in place .of- the ordinary wall paper or plaster, or 

to enhance the picturesqueness of the slate roof. Embossed mantelpieces could also 

be made and other uses would in course of time reveal themselves.

Chemists have also considered the feasibility of utilizing slate 

waste for the manufacture of aluminum, and when combined with 

lime for the manufacture of cement, but as clay would answer both 

purposes equally well and is abundant these schemes have been 

given up. ,

STATISTICS OF PRODUCTION, IMPORTS, AND EXPORTS.

Although slate has been known and quarried in this country more 

or less since 1750 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.

In the tables here presented the production for the years 1879> 

1881, 1882, and 1883 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; those for 1889 are the figures 

of the Eleventh Census; and from 1890 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 as the operations in this 

State are closely allied to those of Vermont the output is doubtless 

included in that of Vermont.

The statistics of the production of slate as here given represent the 

output of slate as reported by the slate quarrymen and include the 

quantity and value of roofing slate and of mill stock sold by them 

and the value of slate sold for other purposes. The values given,. for 

both mill stock and roofing slate represent prices f . o. b. at the point 

of shipment, the mill stock being classed as rough or manufactured, 

according to the condition in which it is sold by the quarrymen, 

whether as rough blocks to slate mills or. in a finished or partly finished state from mills at the quarries.

The tables give the output of slate in the United States from 1879 

to 1913 by States, showing as nearly as possible the number of squares 

of roofing slate produced, their value, and the quantity and value of 

slate used for other purposes so far as figures are obtainable. 

40620° Bull. 586 14 13
194 SLATE IN THE UNITED STATES.

Production of slate in the United States, by States and uses, from 1879 to 1913.

State.

 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

Value.

84,500 

84,800 

56,700 

7,000 

15, 000 

95, 500 

862, 877 

352, 608 

51,000

457,267 1,529,985

i

1881

Roofing slate.

Squares.

500 

25, 200 

12,000 

1,000 

3,000 

19,000 

272,000 

 109,370 

12,000

454,070

Value.

$2,250 

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.

,$900 

78,000 

67,500 

2,000 

12, 000' 

105,000 

1,002,000 

435, 100 

51,000

1,753,500

. . State. 

New York '

State.

Maine

1883

Roofing slate.

Squares.

30,000 

15,000 

5,000 

1,000 

10, 200 

325, 000 

110, 000 

10, 000

506, 200

Value.

$120,000 

67, 500 

18, 250 

4,000 

51, 000 

1,147,500 

440, 000 

50, 000

1,898,250

 ' 1884

Roofing slate.

Squares.

41,000 

« 10. 000 

7,000

329,004 

85,000 

9,000

481,004

1885

Roofing slate.

Squares.

34,000 

ol4,500 

10,000 

331, 160 

130, 000 

17,300

536,960

Value.

$102,000 

65,250 

40,000 

993,080 

380, 000 

58, 137

1,638,467

Other 

purposes 

than 

roofing 

slate.

$10,000

Total 

value.

$102,000 

65, 250 

40,000 

993, 080 

380, 000 

58, 137

1,648,467

Value.

3215,000 

45,000 

35,300

1, 162, 560 

1 340, 500 

45,505

1,843,865

Other purposes than 

roofing 

slate.

$8,000

1886

Roofing slate.

Squares.

36,000 

0.12,000 

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

Total value.

$215,000 

45,000 

35,300

1, 162, 560 

340,500 

45, 505

, 1,851,865

1887 .

Roofing slate.

Squares.

37,000 

020,000 

7,000 

370,439 

120, 000 

19,000

573, 439

Value.

$81,000 

90, 000 

25,000 

1.111.317, 

'356,000. 

57, 000

1,720,317

a Includes Pennsylvania Peach Bottom.
STATISTICS OF PRODUCTION, IMPORTS, AND EXPORTS. 195.

Production of slate in the United States, by States and uses, from 1879 to 1913 Con.

' State.

Utah...............'.

State.

State,

Utah................

1888

Roofing slate.

Squares.

37.000 

6 19, 000 

7.000 

6,000

416, 000

160, 000 

17,400

662,400

Value.

$111,000 

85, 500 

24,000 

22,000

1,270,400

480,000 

60,540

2,053,440

1889

Roofing slate.

Squares.

3,104 

3,050 

41, 000 

23,099

2,700 

16, 767 

476,038

236,350 

30, 457 

3,060

835,625

1891

Roofing slate.

Squares.

120 

4,000 

3,000 

50, 000 

25, 166 

2,500 

17, 000 

507, 824 

247, 643 

36, 059

893,312

Value.

$480 

24,000 

13,500 

250,000 

123,425 

10, 000 

136, 000 

1,741,836 

698, 350 

127, 819

3,125,410

Other 

purposes 

than 

roofing 

slate.

$2,000

40, 000 

401,069 

257, 267

700,336

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

1893

Roofing slate.

Squares.

2,500 

18, 184 

7,422 

900 

69,640 

364,051 

75 

132, 061 

27, 106

621,939

Value.

$11, 250 

124, 200 

37, 884 

3,653 

204, 776 

1,314,451 

450 

407,538 

104,847

2,209,049

Other 

purposes 

than 

roofing 

slate.

815,000

206 

157,824 

400 

128, 194 

12, 500

314, 124

Total 

value.

$11, 250 

139, 200 

37, 884 

3,653 

204,982 

1,472,275 

850 

535, 732 

117,347

2,523,173

Value.

$18,089 

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.

$480 

18, 000 

4,263

1,250 

44,877 

370, 723

245,016

684,609

Total value.

(a) 

$18,089 

15, 330 

219, 500 

110,^08 

(«) 

10,925 

126, 603 

2,011,726 

& 

842,013 

113,079 

15, 240

3,482,513

1892

Roofing slate.

Squares.

3,500 

2,500 

50, 000 

24,000 

3, 000 

20,000 

550, 000 

260, 000 

40, 000

Value.

$21,000 

10,625 

250, 000 

114, 000 

12, 000 

160, 000 

1,925,000 

754,000 

150, 000

953,000 3,396,625

Other 

purposes 

than 

roofing 

slate.

$2, 500

50,000 

408, 000 

260, 000

720, 500

Total 

value.

$21,000 

10, 625 

250,000 

116, 500 

12,000 

210,000 

2,333,000 

1,014,000 

150, 000

4, 117, 125

1894

Roofing slate.

Squares.

900 

5,000 

24, 600 

39, 460 

375 

7,955 

411,550

214,337 

33, 955

Value.

$5,850 

22, 500 

123,937 

150, 568 

1,050 

42, 092 

1,380,430

455, 860 

118, 851

738,222 2,301,138

Other 

purposes 

than 

roofing 

slate.

$22,901 

2,500

2, 450 

239, 728

202, 307 

19, 300

489, 186

Total 

value.

$5,850 

22,500 

146,838 

153,068 

1,050 

44,542 

1, 620, 158

658, 167 

138, 151

2,790,324

a Included in other States.

6 Includes Pennsylvania Peach Bottom.
196 .SLATE IN THE UNITED. STATES.

Production of slate in the United States, by,States and uses, from 1879 to 1913 Con.

State.

State.

State.

Massachusetts. .......

New Jersey ..........

New York ..........'.

Utah.:...............

1895

Roofing slate.

Squares.

1.500 

2,500 

23,774 

13,188

200 

13,624 

426,687

221,359 

27,095

729,92?

Value.

$10,500 

10, 675 

118, 791 

59, 157

700 

90, 150 

1,437,697

531, 482 

92,357

2,351,509

Other 

purposes 

than 

roofing 

slate.

821,363 

1,200

1,725 

210,054

93, 849 

19,000

347, 191

Total 

value.

$10,500

10, 675 

140, 154 

60,357

700 

91,875 

1,647,75.1

625,331 

111,357

2,698,700

1897

Roofing slate.

Squares.

1,000

, 38.367

11,592

400 

250 

9, 197 

657,692 

244,575 

38,375

1,001,448

Value.

$7,000

iei, 262 '< '53,049

1,000 

775 

52, 799 

2,034,958 

' 656,114 

130, 495

3,097,452

Other 

purposes 

than 

roofing 

slate.

" $39,855 

' 890

500

1,000 

330,341 

39, 701 

14, 875

427, 162

Total 

value.

$7,000

201,117'- 

53, 939

1,500 

775 

53, 799 

2,365,299 

695, 815' 

145,370

3,524,614

1899

Roofing slate.

Squares.

928 

;'2,000 

24, 676 

20, 196

400 

10,912 

711,138 

50 

200 

277,463 

52,550

1,100,513

Value. .

$6,642 

7,500 

121,640 

90,897

. 1,600: 

69,525 

2,202,742 

1 250 

 1,100 

777, 971 

174,950

3,454,817

Other 

purposes 

than 

.roofing 

slate.

$60, 126 

 2, 698 

 800

7,150 

' 334,280

94, 702 

8,160

507,916

Total 

.value.

$6,642 

7,500 

181, 766 

93,595 

800

1,600

76, 675 

2,537,022 

250

a, 100

872,673 

...183,110

3,962,733

1S96

Roofing slate.

Squares.--

-41-597 

23,078 

15,557

200' 16,002" 

431,324 

160 

155,523 

26,863

673,304

Value.

S20, 388 

. 99,831 

70, 194

700 

78,612 

1,391,539 640 ' 

 509,681 

' '92,163

2, 263, 748

Other 

purposes 

than 

roofing 

slate.

$24, 255 

1,948 

1,200

3,880' 

334, 779 

780 

99, 915 

15, 700

482,457'

Total 

value.

320,388 

"124,086 

72, 142 

1,200 

.,700 

82, 492 

1,726,318 

1,420 

609,596 

107,863-

2, 746, 205

1S98

Roofing slate.

Squares.

400 

3,450 ' '29,834' 

.18,332

. 100 ^ 200' 

7,160 

571,256 

241, 762* 

43, 745

916, 239

Value.

$2, 700 

13,125 

131, 752 

80,786

400 

800 

. 46,744 

2,097,735 

. 612, 902 

142, 446

3,129,390

Other 

purposes than 

roofing 

slate.

$67, 485 

1,454 

958

1,950 

394,021- 

119, 782 

8,500

594, 150

Total 

value.

$2,700 

13,125 

 199,-237 

82,240 

 958 

MOO 

.- 800 

48,.694 

2, 491; 756 

732, 684 

150,946

3,723,540

1900

 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

Value.

$26,500 

9,375 

103, 949 

126, 271

. 13,600

58, 360 

2, 277, 192

;.. 250

795, 474 

185,211

3,596,182

Other 

purposes 

than 

roofing 

slate.

. _ .....

$73,393 

2,402

700

4,395. 

436, 406

121,988 

5,000

644", 284

Total 

value.

$26,500 

- 9,375 

- 177,342 

.:128,673

 ;'WO 

.' IS; 600

' -.62^55 

2, 7% 598

. ;; i 2oo

917,462 

1GO, 211

'4~ 240, 460
STATISTICS OF PRODtJCTiON, IMPORTS, AND EXPORTS. 197

Production of slate in the United'State's, by States and uses, from 1879 to 1918 Con.

State.

Vermont..... .......

State.

Georgia..............

Utah.................

State.

Arkansas. ............

California............

Georgia..............

Alaine. ....... . .....

Maryland............

New York ..... . ...

Virginia. ..... . . . .

1901

Roofing slate.

Squares.

2,500 

800 

20, 791 

20, 153

. 7,500 

15, 786 

853,028 

330, 191 

53, 030

1,304,379

Value.

318,608 

3,000 

111,295 

104, 781

30,000 

91,805 

2,591,625 

984,317 

178,979

4,114,410

Other 

purposes 

than 

roofing 

slate.

891,030 

 1,017 

. 1,400

9, 155 

392, 639 

177, 874

673,115

Total 

value.

818, 608 

 3,000 

202,325 

105, 798 

1,400 

30,000 

100, 960 

2,984,264 

1, 162, 191 

178,979

4,787,525

1903 

Roofing slateSquares.

118 

10,000

27,377 

24,475 

7,647 

 15,690 

'871,875

391.366 

29; 646

1, 378, 194

Value.

S709 

70,000

i57,9li 

135,424 

33, 403 

89,548 

3, 378, 804

1, 363, 923 

115/350

5,345,078

Other 

purposes 

than 

roofing

slate.

34,000

73, 319 

2,207

22, 450 

581, 102

228, 729

911,807

Total 

value.

84, 709 

70,000

231, 230 

137,031 

33,403

111,998 

3,959,906

1,592,652 

115,356

6, 256, 885

" 1905

Roofing slate.

Squares.

50 

5,000 

1,500 

19,865 

25,845 

1,340 

10,354 

802, 170 

339, 001 

36, 102

1, 241, 227

Value.

S350 

40,000 

7,500 

106,271 

149,315 

5,360 

65, 051 

2, 879, 671 

1,174,246 

146, 786

4,574,550

Value of 

mill 

stock.

$9,650

117,983 

1,900

1,595 

612, 234 

178, 295

921,657

Total 

value.

810,000 

40,000 

7,500 

224, 254 

151, 215 

5,360

66, 646 

3,491,905 

1,352,541 

146, 786

5, 490, 207

1902

Roofing slate.

Squares.

500 

. 4,500 

1,000 

26, 468 

22,569

8,000 

21,165 

S08, 206 

400, 029 

42, 731

1, 435, 168

Value.

$4,000 

31,500 

4,000 

143, 832 

117,155

32,000 

116, 628 

3,001,545 

1,338,817 

100,951

4,950,428

Other 

purposes 

than 

roofing 

slate.

862, 726 

929

10,090 

545,777 

126, 101

745, 623

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

1904

.Roofing, slate.

Squares.

1,750 

5,600 

1,000 

20, 789 , 22, 628

10,022 

778, 825 

115 

50 

361,126 

31,852

1, 233, 757

Value.

810,300 

39,200 

4,500 

120,838 

131, 245

64, 102 

2, 922, 259 

607 

. 300 

1, 245, 730 

130, 208

4,669,289

Other 

purposes 

than 

roofing 

slate.

84,000

60.330 

2,727

7,441 

710, 987

162, 421

947, 906

Total 

value.

814,300 

39, 200 

4,500 

181. 168 

133,972

71,543 

3,633,246 

607 

300 

1,408,151 

130, 208

5,617,195

1906

Roofing slate.

Squares.

10,000 

1,000 

18, 498 

' 25,288

10, 788 

755, 966 

354, 134 

39,068

1,214,742

Value.

880,000 

5,000 

100, 916 

129,965

60,000 

2,710,249 

1, 189, 799 

172,857

4, 448, 786

Value of 

mill 

stock.

S5,000

137, 765 

1,004

12, 360 

811,900 

251,531

1,219,560

. Total 

value.

85,000 

80,000 

5,000 

238,681 

130,969

72, 360 

3,522,149 

1,441,330 

172,857

5,6613,346
198 SLATE IN THE UNITED STATES. 

Production of slate in the United States, by States and uses, from 1879 to 1913 Con.

State.

Arkansas. .....................

New Jersey ...................

1907

Roofing slate.

Squares.

7,000 

16, 879 

21,815 

2,000 

11,908 

793,466 

385, 314 

39, 172

1, 277, 554

Value.

$60,000 

91, 583 

113.665 

8,000 

81,535 

2,987,740 

1,301,576 

173, 670

4, 817, 769

Mill stock.

Quantity.

Square feet.

404, 829

13,000 

4, 597, 884 

963,911

5,979,624

Value.

$145,023

 1,950 

620, 753 

175, 683

943, 409

Other.

o$8,500

2,395

ft 247, 147

258,042

Total 

value.

$8,500 

60,000 

236, 606 

116, 060 

8,000 

83,485 

3, 855, 640 

1, 477, 259 

173,670

6,019,220

State.

New Jersey ..............

1908

Number of 

operators.

1 

1 

5 

6 

2 

11 

102 

51 

7

186

Roofing slate.

Squares.

7,000 

20, 151 

 18, 521

1 18, 485

825,078 

402, 258 

41,678

1, 333, 171

Value.

$60,000 

115, 682 

101, 204

130, 439

3,070,906 

1, 513, 580 

194, 356

5, 186, 167

Mill stock.

Quantity.

Square feet.

285,299

1,500

3, 565, 083 

941, 930

4, 793, 812

Value.

$98, 025

180

498, 188 

196,911

793, 304

Other.

$2,500

982

c 333, 864

337,346

Total 

value.

$2, 500 

60,000 

213, 707 

102, 186

130, 619

3,902,958 

1,710,491 

194, 356

6,316,817

State.

Georgia...................

Virginia ..................

Other States/............

1909

Number of 

operators.

1 

1

6 

6 

2 

8 

98 

51 

7

180

Roofing slate.

Squares.

18,024 

22,563

18,098 

626, 228 

397,441 

40, 880 

10, 479

1, 133, 713

Value.

$101,865 

128,227.

ioe, i75

2,281,779 

1,533,936 

180,775 

61,840

4, 394, 597

Mill stock.

Quantity.

Square feet.

372, 229

6,043 

3,389,119 

1,345,503

5, 112, 894

Value. .

$126, 017

1,261 

441,464 

307,347

876, 089

Other.

$i,3ii

e 169, 115 

306

170, 732

Total

value.

(<*) to

$227,882 

129, 538 

(d) 

107,436 

2,892,358 

1,841,589 

180, 775 

61,840

5,441,418

a Used chiefly for electric supplies.

6 Composed of 5,711,105 school slates valued at $48,152 and 1,531,330 square feet of blackboard material, 

valued at $198,995.

c Composed of 5,036,147 school slates, valued at $42,364, and 2,388,886 square feet of blackboard material, 

valued at $291,500.

d Included in '.' Other States."

« Composed of 3,650,831 school slates, valued at $32,319; 1,095,540 square feet of blackboard material, 

valued at $130,195; and $6,601 for slate used for structural and other purposes.

/ Includes California, Georgia, and New Jersey.
STATISTICS OF PRODUCTION, IMPORTS,-AND EXPORTS. 199

Production of slate in the United States, by States and uses, from 1879 to 1913 Con.. |

, State.

Virginia..................

Other States*: ............

1910

Number of 

operators.

1

2 6' 

4 

2 

6 

104 

1 

48 

7

181

Roofing slate.

Squares.

16, 708 

. 14, 529

17,618 

777, 190

395, 640 

31,787 

7,149

1,260,621

Value.

899,023 

77,791

84, 089 

2,809,593

1, 585, 324 

148,721 

40, 123

4,844,664

Mill stock.

Quantity.

Square feet.

385, 307

2,933 

3, 476, 526

1,316,732

5,181,498

Value.

$149, 982

733 

539,572

308, 811

999,098

Other.

$782

6 391, 641

524

50

392,997

Total 

value. |

8$249,005 

78,573

(0)

84, 822 

3, 740, 806 

(a) 

1,894,659 

148, 721 

40, 173

6,236,759

State.

New York................

Other States c. ...........

State.

Arkansas.................

New Jersey. . .............

New York. ...............

Virginia ..................

Other States c............

1911

Number of 

operators.

2 

1 

5 

4 

2 

9 

97 

49 

7

176

Roofing slate.

Squares.

14,879 

14,816

21, 452 

699, 344 

328, 760 

40, 040 

5,386

1,124,677

Value.

$98,074 

74,692

120, 359 

2,508,435 

1,335,244 

188, 808 

22, 959

.4,348,571

Mill stock.

Quantity.

Square feet.

394, 531

4,029,663 

1,320,383

5,744,577

Value.

$165,442

574,966 

287, 197

1,027,605

Other.

$1,343

d 347, 950 

2,500

50

351,843

Total 

value, i

$263, 516 

76,035 

(a) 

120,359 

3,431,351 

1,624,941 

188,808 

23,009

5, 728, 019

1912

Number of 

operators.

1

3 

4 

2 

10 

93 

54 

8

175

Roofing slate.

Squares.

16,640 

18, 236

27,024 

716, 770 

373, 638 

42, 220 

2,760

1, 197, 288

Value.

$96,079 

90,993

135, 136 

2, 528, 791 

1,576.294 

195; 392 

13, 500

4, 636, 185

Mill stock.

Quantity.

Square feet.

428, 689

4,101,200 

1,234,384

1,000

5, 765, 273

Value.

$186,599

552, 929 

273, 582

110

1,013,220

Other.

$1,191

71 

e 392, 527 

99

25

393,913

Total 

value.

(a) 

$282, 678 

92, 184 

(a) 

135, 207 

3,474,247 

1,849,975 

195, 392 

13,635

6,043,318

a Included in " Other States."

6 Composed of 5,610,518 school slates, valued at $47,075; 2,821,689 square feet of blackboard material, 

valued at $334,070; and $10,496 for slate used for structural and other purposes.

c Includes, in 1910, California, Georgia, New Jersey, and Tennessee; in 1911, Arkansas, Georgia, and 

New Jersey; in 1912. Arkansas and New Jersey.

d Composed of 4,308,292 school slates, valued at $35,157; 2,636,650 square feet of blackboard material, 

valued at $300,034; and $12,759 for slate used for structural and other purposes.

e Composed of 4,482,571 school slates, valued at $38,852; 2,898,742 square feet of blackboard material, 

valued at $352,109, and $1,566 for slate .used for structural and other purposes.
200 SLATE IN THE UNITED STATES.

Production of slate in the United States, by States and uses, from 1879 to 1913 Con.

State.

Maine ......................

Virginia....................

Other States c. .............

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

1913

Number of 

operators.

2 

4 

3 

2 

10 

90 

53 

7

171

Roofing slate.

Squares.

(a) 

15,593 

15,913 

(a) 

29, 868 

678,396 

332, 642 

38,330 

. 3,202

1,113.944

Value.

(a) 

889, 933 

82,981

(a)

139,970 

2, 605, 882 

1,351,175 

175, 774 

15,347

4.461.062

Mill stock.

Quantity.

513, 745 

21,342 

4,210,515 

1,566,409

6.312.011

Value.

$234,065

4,912 

648,216 

346,645

1.233.838

Other.

$1,012 

(a)

6479,483

56 

25

480.576

Total 

value.

(a) 

$323,998 

83,993 

(a) 

144, 882 

3, 733, 581 

1, 697, 820 

175, 830 

15, 372

6,175.476

o Included in " Other States."

6 Composed of 6,174.526 school slates, valued at 851,313; 3,504,162 square feet of blackboard material, 

valued at $426,703; and slate used for structural and other purposes, valued at $1,467. 

c Includes Georgia and New Jersey. -

The following table shows the number of squares and value of 

roofing slate, average value per square, and value of mill stock, by 

years, from 1879 to 1913, inclusive:

' Values of'roofing slate produced in the United States, 1879-1912.

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.... 

1905.... 

1906.... 

1Q07.... 

1908.... 

1909.... 

1910. . . . 

1911.... 

1912.... 

1913....

Roofing slate.

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 

1,378,194 

1,233,757 

1,241,227 

1,214,742 

1,277,554 

1,333,171 

1, 133, 713 

1,260,621 

1,124,677 

1,197,288 

1,113,944

Value.

"81,231,221 

"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 

4,574,550 

4,448,786 

4,817,769 

5,186,167 

4,394,597 

4,844,664 

4,348,571 

4,636,185 

4,461,062

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 

3.69 

3.66 

3.77 

3.89 

3.87 

3.' 84 

3.87 

3.87 

4.00

Mill stock.

Quantity.

Squarefect.

 

"5,'979,"624" 

4,793,812 

5,112,894 

5,181,498 

5,744,577 

5,765,273 

6,312,011

Value.

.

$8,000 

10,000

684,609 

700, 336 

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

921,657 

1,219,560 

943, 409 

793,304 

876,089 

999, 098 

1,027,605 

1,013,220 

1,233,838

Average 

price 

(square 

foot).

'"$6." 157' 

.165 

.171 

.192 

.178 

.166 

.195

Other 

uses 

(value).

"6 $258," 042' 

6337,346 

6 170, 732 

6392,997 

6351,843 

6 393, 913 

6 480, 576

Total 

value. .

o $1,231, 221 

"1,529,985 

"1,543,838 

"1,753,500 

"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 

5,496,207 

5,668,346 

6,019,220 

6,316,817 

5,441,418 

6,236,759 

5,728,019 

6,043,318 

6,175,476

"Estimated.

61907 includes 5,711,105 school slates, valued at $48,152, and 1,531,330 square feet of blackboard material, 

valued at $198,995; 1908 includes 5,036,147 school slates, valued at $42.364, and 2,388,886 square feet of blackboard material, valued at $291,500; 1909 includes 3,650,831 school slates, valued at $32,319,.and 1,095,540 

square feet of blackboard material, valued at $130,195; 1910 includes 5,610,518 school slates, valued at $47,075, 

and 2,821,689 square feet of blackboard material, valued at $334,070; 1911 includes 4,308,292 school slates, 

valued at $35,157. and 2,636,650 square feet of blackboard material, valued at $300.034; 1912 includes 4,482,571 

school slates, valued at $38,852, and 2,898.742 square feet of blackboard material, valued at $352,109; 1913 

includes 6,174,526 school slates, valued at $51,313, and 3,504,162 square feet of blackboard material, valued 

at $426,703.
STATISTICS OF PRODUCTION, IMPORTS, AND EXPORTS. 201

As will be seen from the above tables, the values from 1879 to 

1889 do riot represent the entire value of slate quarried, as no record 

of slate used for mantles, school slates, pencils, tombstones, etc., 

was kept. In 1885 there was a large decrease in the average value 

per square 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 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 later part of the year showed a greatly decreased demand, 

owing to strikes in the building trades. Several of the chief producing States showed decreased output of roofing slates, with increased 

price.

In 1908 and 1910 the slate output was practically the same as in 

1903, owing to the increased production of mill stock. The output 

of roofing slate was larger in 1908 than in any other year for which 

statistics have been collected.

The largest average price per square obtained has been $4, in 

1913. The value of slate used in the manufacture of blackboards 

and school slates has been separated from other mill stock since 1906 

and included under "Other uses." This slate is obtained entirely 

from Pennsylvania, and a separation of these figures into quantity 

and value is shown in the footnote to the above table and also in 

the table showing the output of Pennsylvania by counties.

Pennsylvania, producing over 57 per cent of the total value of 

the slate in the United States, is practically the only State where the 

industry is not confined to one district, and the following table is 

given showing the production of this State by counties and by uses 

for the years 1909-1913:
Production of slate in Pennsylvania, by counties and us

County.

1909. 

Carbon. ...............

York (Peach Bottom).

1910. 

Carbon ................

York (Peach Bottom).

1911.

York (Peach Bottom).

1912. 

Lancaster .............

York (Peach Bottom).

T ohiVh

1913.

Lancaster. ..........

Number of 

operators.

1 

3

37 

57

98

1

4 

43 

56

104

1 

1 

5

34 

57

98

2 

3 

33 

55

93

2 

3 

31 

54

90

Roofing slate.

Squares.

1 12,843

185, 249 

428, 136

626, 228

| 14,291

216, 743 

546, 156

777, 190

I 17,354

160,150 

521, 840

699, 344

| 18,135

158, 603 

 540,032

716, 770

\ 9,965

143, 145 

525, 286

678, 396

Value.

877, 129

670, 443 

1,534,207

2, 281, 779

75, 191 

697,964 

2,036,438

2,809,593

91,573

566, 1C1 

1,850,761

2,508,435

99, 810

571, 269 

1,857,712

2, 528, 791

57, 059

513, 027 

2, 035, 796

2, 605, 882

Price 

per 

square.

/ S3. 69 

\ 6.19 

3.62 

3.58

3.64

f 2.25 

\ 5.60 

3.22 

3.73

3.62

f 3.58 

i 5.00 

[ 5.55 

3.53 

3.55

3.59

f 5.71 

\ 5.50 

3.60 

3.44

3.53

/ 6.00 

\ 5.71 

3.58 

3.88

3.84

Mill stock.

Manufactured.

Quantity.

}..'.........

147, 313 

2,272,592

2, 419, 905

}...........

. 294,730 

2,741,663

3,036,393

1

506, 461 

3, 188, 160

3,694,621

}...........

394, 419 

3,269,099'

3, 663, 518

}...........

353, 948 

3, 116, 338

3,470,'286

Value.

S40, 119 

324, 684

364, 803

73,092- 

435,369

508, 461

60,710 

487,015

547, 725

53," 764 

466,970

520, 734

 63,632 

535,284

598, 916

Rough.

Quantity.

575,903 

393, 311

969,214

173,052 

267,081

440, 133

194,941 

140,101

335,042

197,930 

239, 752

437, 682

191, 902

548, 327

740, 229

Value.

847, 208 

29,453

. 76,661

18,578 

12, 533

31, 11.1

19,078 

8,163

27, 241

19,619 

12, 576

32, 195

21, 193

28, 107

49, 300

B

Qua

2

8

1,0

1, 5

1,2

2, 8

1,0

1,5

2, 6

9

1,9

2, 8

1, 3

2, 1

3, 5
STATISTICS OF PRODUCTION, IMPORTS,- AND EXPORTS. 203

IMPORTS.

The importation of slate into this country has never assumed very 

great proportions, and the largest quantity 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, and 

Bureau of Foreign and Domestic Commerce, Department of Commerce, show the value of roofing slate and other slate, chiefly slate 

mantels, chimney pieces,.etc. From 1867 to 1875 the total annual 

value amounted to more than $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.. The 

tariff act of October, 1913, reduced the duty on imported slate by 

half. Formerly the duty on slates, slate chimney pieces, mantels, slabs 

for tables, roofing slates, and all other manufactures of slate was 20 per 

cent ad valorem; the present rate is 10 per cent ad valorem.

The following table shows the value of roofing and other slates 

imported and entered for consumption in the United States from 

1867 to 1913, inclusive:

Value of roofing and other slate imported and entered for consumption in the United States,

1867 to 1913, inclusive. , .

Year.

1868................

1869................

1870................

1871................

1Q79

i 070

1874................

1875................

1876. ...............

1Q77

1ft7ft

1879................

1880................

Iftftl

1 ftft9

1 QQO

1884................

1 QQE

1 OCR

1QO7

Iftfift

1889................

1890................

Roofing 

slate.

$85,204

118, 776

85,364

107, 521

117, 484

107, 192

91,503

80, 519

16,342

2,051

4

275

620

72

2

Tid

2,813

16,099

5,196

4,366

con

765

481

369

Other.

$37,510

16,045

19, 602

19, 879

21,381

25, 925

26, 643

27, 519

42,022

44,266

34, 479

39, 935

46, 260

51, 165

46, 862

45, 774

44,375

34,640

56, 913

46, 188

44, 069

35, 828

34, 602

Total.

$122, 714

134, 821

104, 966

127, 400

138, 865

133,117

118,146

108, 038

58,364

46,317

34,483

40, 210

46, 880

51,237

46, 864

45,928

47, 188

50, 739

62, 109

64, 878

46, 717

44,834

36,309

34,971

Year.

1891................

1832................

1893................

1894................

1895................

1896................

1897. '. ..............

1898................

1899................

1900..'..............

1901................

1902................

1903................

1Q04

1905................

1906................

1907................

1908

IQHQ

1910...'.............

1911

1912................

1913................

Roofing 

slate.

81, 148

1,615

1,878

2,198

4,418

224

72

58

0

0

3

0

2,221

45

913

228

208

Other.

$12,373

4,952

6,671

6,342

1,212

6,191

4,997

4,866

4,025

6,998

6,467

4,817

7,182

9,391

8,941

9,243

5, 196

Total.

$13,521

6,567

8,549

8,540

5,630

6,415

>; nr>o

4 924

4,025

6,998

6,470

4,817

9,403

9,436

9,854

9 471

5

Af\A

7 997

7,872

8,367

14,768

5,479

EXPORTS.

Almost from the beginning of the slate-quarrying industry in the 

United States slate has contributed to the export trade of this country. 

In 1876, 1877, 1878, and 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
204 SLATE IN THE UNITED STATES.

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 1888,

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.

$464,733

714, 709

397,067

240,471

297,001

201,013

221, 468

Year.

1883.......'.........

1S84................

1885................

1886. ...............

1887................

1888................

Roofing 

slate.

$54,063

90, 262

115,206

79,064

62,052

116,119

Other.

§40,674

53,021

49, 965

40,804

39,560

46,142

Total.

$94, 737

143,283

165, 171

119,868

101,612

162,261

The following table, taken from the report of the Bureau of Foreign 

and Domestic Commerce, Department of Commerce, shows the value 

of. exports of roofing slate from the United States, by fiscal years 

from 1884 to 1904 and by calendar years from 1903 to 1913. No 

separate record has been kept of the manufactured product.

Exports of slate from the United States, by fiscal years 1884-1904, by calendar years

1903-1913.

1884.......................... $79,464

1885......................... 51,011

1886......................... 123,565

1887......................... 61,047

1888......................... 97,707

1889......................... 109,896

1890.

1891.

1892.

1893.

1894.

1895.

................... 94,048

................... 84,408

................... 57,514

..................... 52,012

.................... 37,195

................... 38,806

1896......................... 266,385

1897......................... 780,112

1898......................... 1, 370,075

1899......................... .$1, 363, 617

1900......................... 950,543

1901.........:............... 898,262

1902.......................... 945,352

1 628,612

2 QQQ fiQQ 1903.

1904. 726, 715

449, 743

1905......................... 408,309

1906......................... 255,785

1907......................... 220,995

1908......................... 197,216

1909......................... 209,338

1912........................ 3 171,775

1913........................ 226,413

No figures are available for 1910 and 1911, the slate exports not 

being kept separate from those of other kinds of stone. A large part of 

the slate exported from the United States goes to Great Britain and 

Ireland, and in 1912, according to figures published by the Slate 

Trade Gazette of London, the imports of United States slate amounted 

to 4,441 tons, valued at £18,133; in 1913 these exports decreased to 

2,49.8 tons, valued at £9,362, or about one-fifth of the slate exported.

1 Fiscal year. 2 Calendar year. 3 July 1 to Dec/31 only.
APPENDIX.

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 ed., vol. 22, 1887. 

CHAMBERLIN, T. C., and SALISBURY, K. D., Geology, vol. 1, Geologic processes and

their results, New York, 1904. Slaty cleavage and slate, pp. 420-422, fig. 364. 

FLETT, J. S., Slate: Encycl. Britannica, llth ed., vol. 25, pp. 210, 211,1911. 

GEIKIE, ARCHIBALD, Cleavage: Encycl. Britannica,9th ed., vol. 10; article "Geology,

VI, "1887.

 Text-book of geology, vol. 1, 4th ed., London, 1903. Cleavage, pp. 417^419,

684-687. 

GREEN, H. H., Geology, Part I, Physical geology, London, 1882. Section 7, Some

structures impressed on rocks after their formation, pp. 269-279, figs. 100-103. 

HARKER, ALFRED, Petrology for students; an introduction to the study of rocks under

the microscope, 4th ed., Cambridge, England, 1908. 

HINNB, F., Praktische Gesteinslehre, 3d ed., 1908. Also a French translation with

additions, Etude pratique des roches, by L. Pervinquiere, 2d ed., 1912. 

PvoSENBUscH, H., Elemcnte der Gesteinslehre, 3d ed.,Thonschiefer, Phyllite, Analysen, 1910.

 Mikroskopische Physiographic der gesteinsbildende Mineralien, 4th ed.,

1905.

ROTH, JUSTUS, Allgemeine und chemische Geologic, vol. 2, pp. 585, 593, Berlin, 1887. 

ZIRKEL, FERDINAND, Lehrbuch der Petrographie, 2d ed., vol. 3, Leipzig, 1894. Phyllit, pp. 295-319; Thonschiefer, pp. 744-753.

SPECIAL SCIENTIFIC PAPERS AND MEMOIRS. -

ADIE, A. J., On the expansion of different kinds of stone 'from an increase of tempera- 

 ' ture, with a description of the pyrometer used in making the experiments: Roy. 

Soc. Edinburgh Trans., vol. 13, 1836. [Slate from Penrhyn- quarry, Wales 

... expands,0.00000576 inch linear for 1° F.]

ANGER, F. A., Mikroskopische Studien iiber klastische Gesteirie: Min.. Mittheil. 

.- .-(Tschermak), 1875, pp. 153-174. . ,'.-\v.- a tv5J 

ANTEN. J., Note pr61iminaire sur le metamorphisme d'un phyllade-' oligis'tifere

 - ' Balmien, au .contact de 1'arkose gedinienne: Soc. g6ol. BelgiqaecAiih., vol. 38, 

pt. 2, pp. M 27-29, PL II, . -' - ' .'. y.'v i

JBAYLEY; W.... S.., Slate from Monson, Piscataquis County, Maine: TJ.--S. Geol. Survey 

Bull. 150, pp..308-313, 1898. .'...-.

BARRQIS,-CHARLES, Recherches sur les terrains anciens des Asturies at de la Galice, 

Lille, 1882-. Phyllades-du terrain.Cambrieh,-pp. 22-28. - - -

.'-. .. - ,.... ....... . -.,......., .i ..... ..:. _ 205--- 
206 SLATE IN THE UNITED STATES.

BAUR, , Uber die Lagerung der Dachschiefer, iiber Wetzschiefer und tiber die 

von der Schichtungabweichende SchieferungderThonscbieferimnordwestliclien 

Theile des Diirener Bergamts-Bezirks: Arcbiv Min., Geogn., etc., Von Karsten 

und Von Dechen, vol. 20, p. 351, PL V, Berlin, 1846.

BECKBR, G. F., Finite homogeneous strain, flow, and rupture of rocks: Geol. Soc. 

America Bull., vol. 4, p. 13, 1893.

 Schistosity and slaty cleavage: Jour. Geology, vol. 4, p. 429, 1896.

 - Experiments on schistosity and slaty cleavage: U. S. Geol. Survey Bull. 241, 

1904.

Current theories of slaty cleavage: Am. Jour. Sci., 4th ser., vol. 24, pp. 1-17,

1907. 

BiscHor, GUSTAV, Lehrbuch der chemischen und physikalischen Geologic, 2d ed.,

Bonn, 1866. Cause of discoloration, vol. 2, pp. 350-351, footnote. 

CHANCE, H. M., The slate quarries [on Delaware and Lehigh rivers in Pennsylvania]

in 1875; with four cross sections: Second Geol. Survey Pennsylvania Rept.

Progress D3, vol. 1, Lehigh and Northampton counties pp. 148-160, 1883. 

CREDNER, G. R., Das Grunschiefersystem von Hainichen im Konigreich Sachsen:

Zietschr. ges. Naturwiss., vol. 47, Berlin, 1876.

DALE, T. N., The Rensselaer grit plateau in New York: U. S. Geol. Survey Thirteenth Ann. Rept., pt. 2, 1893. The structure in black slate at the Lebanon

Springs quarry, pp. 319-321 and PI. C.

 The slate belt of eastern New York and western Vermont: U. S. Geol. Survey

Nineteenth Ann. Rept, pt. 3, pp. 153-307, 1899. 

DARTON, N. H., Fossils in the "Archean" rocks of central piedmont Virginia: Am.

Jour. Sci., 3d ser., vol. 44, pp. 50-52, 1892.

 U. S. Geol. Survey Geol-. Atlas, Fredericksburg folio (No. 13), 1884.

DATJBREE, A., Experiences sur la schistosit6 des roches et sur les deformations des 

fossiles, correlatives de ce ph^nomene; consequences ge"ologiques de ces experiences: Soc. ge"ol. France Bull., 3d ser., vol. 4, p. 529,1876; also Compt. Rend., 

vol. 82, pp. .710-716, 1876.

 Etudes synthetiques de geologic exp^rimentale, pp. 333-336, 392-432, 442-445,

Paris, 1879. , DE LA BECHE, H. T., Geological observer, 2d ed., London, 1853. 

ECKEL, E. C., On a California roofing slate of igneous origin: Jour. Geology, vol. 12, 

, pp. 15-24, 1904. .

 On the chemical composition of American shales and roofing slates: Jour. Geology, vol. 12, pp. 25-29, 1904.

FISCHER, 0., On the cleavage of slates: Nature, vol. 72, pp. 55-66,1905.

FORBES, D., The structures of rockmasses (stratification, joints, cleavage, foliation, 

striation), London, 1870. . Fox, H., The variegated slates of North Cornwall: Boy. Geol. Soc. Cornwall Trails., 

vol. 13, 1906. . ;

GEINITZ, E., Der Phyllit von Rimogue in den Ardennen: Min. pet. Mitt., neue Folge, 

vol. 3, pp. 533-540, 18.80. . .

GQSSELET, J., Les schistes de Fumay: Soc. g£ol. Nord Annales, vol. 10, pp. 63-86,

. Lille, 1884. .... , '.

 r . L'.Ardenne: M^moires pour servir & 1'explication de la carte ge"ologique detaille"e

de la France, pp. 26-28, 33-35, 54, 60-62, Paris, 1888. 

GQSSELET, J., and MALAISE, Sur la terminaison occidentale du massif ardoisieur de

Fumay: Soc. g£ol. Nord Annales, vol. 22, pp. 59-64, Lille, 1898. 

GRUBENMANN, ,TJ., Die'kristallinen Schiefer, pt. I, Berlin, 1904. [Metamorphism in

schist and slate] Struktur, p. 71; texture, p.^ 84; kluftung, p. 92. 

GUMBEL, , Geognostische Beschreibung des Fichtelgebirges, Gotha, 1879.
BIBLIOGEAPHY. . 207

HALL, C. W., Keewatin area of eastern central Minnesota: Geol. Soc. America Bull.,

vol. 12, p. 362, PL XXXI, fig. 1, 1901. 

HARKER, ALFRED, On slaty cleavage and allied rock structures, with, special reference

to the mechanical theories of their origin: Brit. Assoc. Adv. Sci. Kept, for 1885,

1886.

 The cleavage of slates: Nature, vol. 72, pp. 20, 152, 1905.

HARKNESS, ROBERT, and BLYTHE, JOHN, On the cleavage of the Devonians of the 

southwest of Ireland: Edinburgh. New Philos. Jour., new ser., vol. 2, No. 4, 

October, 1855.

HAWES, G. W., Clay slate (argillite): Geology of New Hampshire, by C. H. Hitchcock 

and others, vol. 3, pt. 4, pp. 236-238, 1878.

HOPKINS, W., On the internal pressure to which rock masses may be subjected, and 

1 its possible influence on the production of the laminated structure: Cambridge 

Philos. Soc. Trans., vol. 8, pp. 456-470, Cambridge, England, 1849.

HOSKINS, L. M., Flow and fracture of rocks as related to structure: U. S. Geol. Survey Sixteenth Ann. Kept., pt. 1, 1896. Cleavage and fissility, pp. 868-874.

HUTCHINGS, W. M., Glays, shales, and slates:' Geol. Mag., dec. 4, vol. 3, No. 7, p. 309; 

No. 8, p. 343, London, 1896.

 Notes on the ash slates and other rocks of the Lake district: Geol. Mag., dec.

3, vol. 9, pp. 145-218, London, 1892. 

INOSTRANZEFF, A., Studien iiber metamorphosirteGesteineim Gouvernement Olonez

[Russia], Leipzig, 1879. Thonschiefer, pp. 137-144. 

JANNETTAZ, E., De la propagation de la chaleur dans les corps; de ses relations avec

la structure des mine'raux, le me'tamorphisme des roches: Soc. ge"ol. France Bull.,

3d ser., vol. 3, pp. 499-510, 1875. Experiments with slate, p. 505. See also

vol. 2, p. 264, 1874.

 Relation entre la propagation de la chaleur et I'elasticit6 sonore dans les roches 

et dans les corps cristallise's: Soc. ge"ol. France Bull., 3d ser., vol. 5, pp. 410-426, 

1877. Experiments with slate, p. 416.

 Me"moire sur les connexions de la propagation de la chaleur avec leurs diffe'rents clivages et avec les mouvements du sol qui les ontproduits: Soc. ge"ol. 

France Bull., 3d ser., vol. 9, p. 196, 1881.

Me"moire sur les clivages des roches (schistosite", longrain) et sur leur reproduction: Soc. gSol. France Bull., 3d ser., vol. 12, p. 211, 1S84.

KING , WILLIAM, On jointing and its relation to slaty cleavage:. Roy. Irish Acad. Trans., 

vol.- 25, 1875.

KRUGER, R., Die natiirlichen Gesteine, Leipzig, 1889. Slates, pp. 123-136.

LARIVIERE, G., Notes d'un voyage aux ardoisieres du pays de Galles: Annales dea 

mines, M<§m., 8th ser., vol. 6, pp. 505-564, Paris, 1884.

LAUGEL, AUGUSTE, Du clivage des roches: Soc. ge'ol. France Bull., 2d ser., vol. 12,- 

pp. 363-368, 1855. - 

LEITH, C. K., Rock cleavage: U. S. Geol. Survey Bull. 239, 1905. Summary statement of secondary rock cleavage, p. 135. Comparison of present statement with 

previous ones as to secondary cleavage, p. 140. Original cleavage, p. 143.

LESLEY, J. P., A summary description of the geology of Pennsylvania: Pennsylvania

- . Geol. Survey Final Rept., vol.-l, Harrisburg, 1892. The southern or Peach Bottom phyllite belt, pp. 136-141, 555, 556; chapters 45-49, Formation No. 3 and the 

 'roofing-slate beds. ' . . . . .

.', Ueber Schieferung: Senckenbergische naturf. Gesell. Ber., pp. 61-116, 

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,n mUeber Transversalschieferung und verwandte Erscheinungen in Thuringi-- 

scneV Schiefergebirge: K.- preuss. geol. Landesanstalt. und Bergakad.-Jahrb., 

1881, pp. 258-306, PL VII, Berlin, 1882.
208 SLATE IN THE UNITED STATES.

MALLARD, E., Sur 1'examen microscopique de quelques schistes ardoisiers: Soc. min.

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MAW, GEORGE, On a new section of the Cambrian rocks in a cutting of the Llanberis

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121-125, Pis. VI, VII, London, 1868.

: On the disposition of iron in variegated strata: Geol. Soc. London Quart. 

Jour., vol. 24,1868. On variegated Cambrian slates, p. 379, PI. XIV, figs. 29, 31, 32.

MCCREATH, A. S., Analysis of Peach Bottom slates: Pennsylvania Geol. Survey Kept. 

Progress for 1877, Vol. CCC, p. 269, 270, Harrisburg, 1880.

MERRILL, G. P., A treatise on rocks, rock weathering, and soils: New York, 1897. 

Gives analysis of Peach Bottom slate and remarks on their weathering, with 

analyses showing the incidental chemical changes.

MULLER, F. E., Die normalen Schiefer des Hennbergs: Neues Jahrb., 1882, vol. 2, pp. 

205-248.

NICOL, JAMES, On the slate rocks.and trap veins of Easdale and Oban: Geol. Soc. 

Quart. Jour., vol. 15, pp. 110-116, 1859. '

PEACH, B. N., KYNASTON, H., MUFF, H. B., and others, The geology of the seaboard 

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On the stratification of slate, pp. 16, 17; on "bate," p. 8.

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POGUE, J. E., jr., Geology and structure of the ancient volcanic rocks of Davidson

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RAMSAY, A. C., The geology of- North Wales: Geol. Survey Great Britain Mem.,

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BIBLIOGEAPHY. 209

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 Deformation of rocks, cleavage, and fissility; studies for students: Jour.

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40620 ° B u 11. 586 14 14
210 SLATE IN THE UNITED STATES.

VAN HISE, C. R., A treatise on metamorphism: U. S. Geol. Survey Mon. 47, 1904. 

Theory of recrystallization, pp. 690-694; rock flowage, pp. 748-762; slate and 

schist, pp. 778, 779; mud family, pp. 886-892; shale fanf.ly, pp. 892-894; pelite 

slate, pp. 894-898. ,

WALCOTT, C. D., Cambrian age of the roofing slates of Granville, Washington County, 

N. Y.: Am. Assoc. Adv. Sci. Proc., vol. 35, pp. 220-221, Salem, 1887.

WATSON, T. L., and POWELL, S. L., Fossil evidence of the age of the Virginia Piedmont slates: Am. Jour. Sci., 4th ser., vol. 31, pp. 33-44, 1911.

WILLIAMS, G. H., Anatase from the Arvon slate quarries, Buckingham County, Va.: 

Am. Jour. Sci., 3d ser., vol. 42, pp. 431-432, 1891.

ZIRKEL, FERDINAND, Tiber die mikroskopische Zusammensetzung von Thonschiefern 

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WORKS OF AN ECONOMIC CHARACTER.

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ser., vol. 11, pp. 19-86,133-197,1909. Abstractin Geol. Centralbl., vol. 13, pp. 606, 

607, 1910.

BARNUM, GEORGE, Slate mining versus quarrying: Stone, vol. 28, pp. 219-227, 1907.

BENNETT, H. D., United States vice consul at Nantes, France: The slate quarries of 

Angers: U. S. Cons. Repts., No. 24, p. 115, January, 1891.

BLAVIER, A., Essai sur 1'industrie ardoisiered'Angers, 1863. On methods of quarrying.

BOTTINGER, C., Zusammensetzung und Verhalten von Dachschiefer: Chem. Zeitung, 

vol. 19, p. 1144, 1895.

BOWER, JOHN, Slate quarries as an investment [England], 1865.

BROWN, H. Y. L., The Willunga slate quarries: Review of mining operations in the 

State of South Australia, Feb.-June, 1911, p. 24 and 3 pis.. Adelaide, 1911.

BRUNNER, H., Valuation of roofing slates: Soc. Chem. Ind. Jour., vol. 9, p. 393, London, Apr. 30,1890 (abstract in English from Wochenschr. Pharm., 1889).

CAMPBELL, J. L. and H. D., The Snowden slate quarries: The Virginias, vol. 5, pp. 

162, 163, 1884.

CLARK, W. B., and MATHEWS, E. B., Maryland mineral industries, 1896-97: Maryland 

Geol. Survey, vol. 8,1909. Slates, pp. 133-136, 355, 356.

COONS, A. T., Slate: U. S. Geol. Survey Mineral Resources, 1904-1910.

CRAWPORD, J. J., California slate and slate industry: California State Min. Bur. 

Twelfth Rept, 1894. (See also Eighth Rept.)

DAVIES, D. C., A treatise on slate and slate quarrying, scientific, practical, and commercial, 3d ed., p. 186, London, 1887.

DAY, D. T., Report on mineral industries in the United States at the Eleventh Census 

(1890), 1892. Slate, distribution of quarries, production, labor, and wages, pp. 

662-665.

 Slate: U. S. Geol. Survey Mineral Resources, 1893, 1900-1903.

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DRESSER, J. A., On the slate industry in southern Quebec: Canadian Min. Inst.

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The Quarry, London, February, 1912, pp. 46-48. 

ECKEL, E. C., The slate deposits of California and Utah;. U. S. Geol. Survey Bull.

225, pp. 417-422, 1903.

 Building stones and clays; their origin, characters, and examination, New

York, 1912. 

FRAIPONT, , De 1'exploitation des ardoises et du coticule au comte de Salm,

anterieurement & Fan 1625: Soc. g6ol. Belgique Annales, vol. 38, pt. 2, pp. BB

3, 4, 5, Liege, Sept. 30, 1911.
BIBLIOGRAPHY. 211

FERGUSON, E. G. W., Peach Bottom slate deposits, Pennsylvania: Min. World,

vol. 33, pp. 183, 184, 1910. 

FRAZER, PERSIFOR, jr., Geology of Lancaster County, Pa.: Second Geol. Survey

Pennsylvania Kept. Progress 1877, vol. CCC, Harrisburg, 1880. Roofing slate

quarries of Lancaster and York counties, pp. 179-190, Pis. VII, VIII. 

FUESENIUS, R., TJeber die Priifung der Dachschiefer auf den Grad Ihrer Verwitterbarkeit: Zeitschr. anal. Chemie, vol. 7, pp. 72-78, Wiesbaden, 1868. 

HAWES, G. W., MERRILL, G. P., and others, Special reports on building stones of the

United States for 1880: Tenth Census, vol. 10, 1884. Slate, pp. 38-41, 50-57,

60-77, 168-174, 180-181. 

HAYES, C. W., Slates of the Rockmart formation in Georgia: U. S. Geol. Survey

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HERMANN, 0., Steinbruchindustrie und Steinbruchgeologie, Berlin, 1899. Dachschiefer, pp. 64, 146, 176, 272-277; Fruchtscluefer, pp. 278-283. 

HIRSCHWALD, J., Die Priifung der natiirlichen Bausteine auf ihre Wetterbestandigkeit, Berlin, 1908. Abstract in Zeitschr. prakt. Geologic, vol. 16, pp. 387-389,

1908.

 Handbuch der bautechnischen Gesteinsprufung, Berlin, 1912. Chapter 9,

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HITCHCOCK, C. H., Roofing elate in Maine: Preliminary Rept. Nat. Hist, and Geol. 

Maine, pp. 316-319, 1861.

 Roofing slate in Piscataquis and Penobscot counties, Maine: Second Ann.

Rept. Nat. Hist, and Geol. Maine, pt. 2, Geology. Monson, p. 280; Shirley,

p. 282; Paten, p. 360, 1862. 

HOWE, J. A., The geology of building stones, London, 1910. Slates and other fissile

rocks, pp. 273-326. 

HULL, EDWARD, A treatise on the building and ornamental stones of Great Britain

and foreign countries, London, 1872. 

JACKSON, C. T., On the Waterville, Maine, slates: Am. Jour. Sci., 1st ser., vol. 41,

pp. 163, 164, 1841; also Assoc. Am. Geologists Trans., 1843, p. 16. 

JAMIESON, ROBERT, Mineralogy of the Scottish Isles, Edinburg, 1800. Slates, vol. 1,

p. 191. 

KEITH, ARTHUR, Slates of Little Pigeon River, Tennessee: U. S. Geol. Survey

Geol. Atlas, Knoxville folio (No. 16), p. 6, 1895. 

KINAHAN, G. H., The redevelopment of the slate trade in Ireland: Inst. (British)

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KNOCH, A., Der Dachschiefer, Berlin, 1895. 

LAMBETH, W. A., Notes on the geology of the Monticello area, Virginia, Charlottesville, 1901. University thesis; discusses mica slate for roofing. 

LANGLEY, A. A., and BELLAMY, C. J., Slate quarrying in the Festiniog district, North

Wales: Inst. Civil Eng. Proc., vol. 44, p. 211, PI. XXI, London, 1876. 

LARIVIERE, G., Industrie ardoisiere, carrieres et mines de Maine-et-Loire, 1903.

Extract from "Angers et 1'Anjou," published for meeting of Assoc. franc, avanc.

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LEIGHTON, HENRY, Slate [industry] in New York in 1909: New York State Mus.

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LINDGREN, WALDEMAR, and TURNER, H. W., Roofing slates in Eldorado County, Cal.:

U. S. Geol. Survey Geol. Atlas, Placerville folio (No. 3), 1894. 

MATHEWS, E. B., An account of the character and distribution of Maryland building

stones, together with a history of the quarrying industry: Maryland Geol. Survey,

vol. 2, 1898. Slate: The Peach Bottom area, pp. 214-231; Ijamsville, pp. 231,

232; prices, p. 239. 

MAYNARD, T. P., Slate in Georgia: Geol. Survey Georgia Bull. 23, pp. 183-186,1910.
212 SLATE IN THE UNITED STATES.

MCLEISH, JOHN, Slate quarried in Richmond County, Quebec, and near Jarvis Inlet,

north of Victoria, British Columbia: Ann. Kept. Min. Prod. Canada for 1906,

1909. 

MERRILL, 'G. P., The collection of building and ornamental stones in the United

States National Museum, a handbook and catalogue: Smithsonian Inst. Ann.

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p. 291; states of various States and Territories, pp. 466-471.

 Stones for building and decoration, 3d ed., New York, 1908.

 Among the Pennsylvania slate quarries: Sci. Am. Suppl., vol. 27, No. 681, 

p. 10874, 1889.

Stone: Twelfth Census special reports on mines and quarries (1902), 1905.

Slate, pp. 802-805.

MERRILL, F. J. H., Mineral resources of New York State: New York State Mus. Bull., 

vol. 3, Albany, 1895. Geology and geographic distribution of building stone in 

New York, by John C. Smock, revised by F. J. H. Merrill, Slate, pp. 421-423.

MERRIMAN, MANSFIELD, The strength and weathering qualities of roofing slates; with 

discussion: Am. Soc. Civil Eng. Trans., vol. 27, No. 3, pp. 331-349, September, 

1892; No. 6, p. 685, December, 1892; also vol. 32, pp. 529-539, 1894.

 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. 

MILLER, H., Ueber die Dachschiefer in der Nahe von Lossnitz: Jahrb. Berg-und

Hiittenmann, 1854, pp. 190-233. 

NEVINS, J. N., Roofing-slate quarries of Washington County, N. Y.: New York State

Mus. Fifty-third Ann. Kept., for 1899, vol. 1, pp. 135-150, Albany, 1902. 

NEWLAND, D. H., The mining and quarry industry of New York State; report of

operations and production during 1910: New York State Mus. Bull. 151, 1911. 

NIVOIT, EDMOND, Notions 616mentaires sur 1'industrie dans le d&partement des

Ardennes [France], 1869. 

PARRISH, , History of the slate trade in America: Am. Jour. Mining, vol. 2, 1866-

1867. In Johns Hopkins University library. 

PATTISON, S. R., Slate and slate quarries geologically and commercially considered:

Min. Jour. [England], 1866. 

PEACH, B. N., and others, The geology of the seaboard of Mid Argyll, etc.: Geol. Survey

Scotland Mem., Expl. Sheet 36, Glasgow, 1909. Economics of the roofing slates

pp. 102-110, Pis. I, VII. (See also bibliography of scientific papers.) 

PERKINS, G. H., Report of the State geologist on the mineral industries of Vermont,

1899-1900. Slate, pp. 17-30.

 Report of the State geologist on the mineral industries and geology of certain

areas of Vermont, IV, 1903-4, pp. 47-51. Idem, V, 1905-6, pp. 8-19. Idem, VI,

1906-7, pp. 46-49.

PONCELET, J. B., Des gltes ardoisiers de 1'Ardenne, Bruxelles, 1848. 

PURDUE, A. H., The slates of Arkansas, with a bibliography of the geology of Arkansas

by J. C. Branner, Geol. Survey Arkansas, 1909. See also U.S. Geol. Survey Bull.

430, pp. 317-334, 1910. 

REVERDIN, F., and DE LA HARPE, C., The examination of roofing slates: Chem.

Zeitung, vol. 14, pp. 64-65, 94-95, 126-127, Abstract in English in Jour. Soc.

Chem. Ind., vol. 9, p. 394, London, Apr. 30, 1890. 

RIES, HEINRICH, Building stones and clay products; a handbook for architects, New

York, 1912. Slate, pp. 225-242, in part quoted from U. S. Geol. Survey Bull. 275. 

ROGERS, W. B., Reprint of annual reports and other papers on the geology of the

Virginias [posthumous], New York, 1884. On the roofing slate of Buckingham

County, pp. 78, 79; on that of Fauquier County, pp. 460, 461.
GLOSSARY. 213

SAUVAGE, , and BUVIGNIER, , Statistique mine'ralogique et ge"ologique du

de"partement des Ardennes (France), 1842. 

SELWYN, A. R. C., Slates on Calabogie Lake, in Horton, Ross, and Lavant townships,

in southern Ontario: Canada Geol. Survey Kept. Progress, 1876-77, pp. 250, 253,

260, 262, 1878. 

SHALER, N. S., Slates, description of quarries and quarry regions: Tenth Census U.S.,

vol. 10, pt. 2, pp. 168-174, 1880.

SMITH, T. C., Slate quarries in Wales, 1860. Also, two later editions. 

SPEER, F. W., Quarry methods: The working of slate. Tenth Census U. S., vol. 10,

pt. 2, p. 38, 1880. 

STONE (editorial). History of slate and slate quarrying: Vol. 28, pp. 203-223, New

York, 1907.

 Australian slate: Vol. 28, p. 225, New York, 1907.

 A defence of slate mining: Vol. 28, pp. 332-334, New York, 1907.

 Slate in Norway: Vol. 28, pp. 377-378, New York, 1907.

 Quarrying innovation in the Peach Bottom region: Vol. 30, pp. 259-261, New

York, 1909. 

THOMAS, J. J., Westmorland slates; their geology, chemistry, and architectural

value: Royal Inst. British Arch. Jour., 3d ser., pp. 194-200, 1896. 

WATRIN, N., Les arcloisieres des Aidennes; description et exploitation du schiste

ardoisier, fabrication des ardoises, lever des plans d'ardoisieres, Charleville, 1898. 

WATSON, T. L., Slate in Virginia: Virginia Geol. Survey Bull. 6, pp. 86-88, 1911. 

WHEDON, M. D., The New York-Vermont slate belt: Stone, vol. 28, pp. 214-218,

New York, 1907. 

WILLIAMS, J. F., Tests of Rutland and Washington County slates: Van Nostrand's

Eng. Mag., vol. 31, pp. 101-103, 132, July-Dec., 1884. Purple and green from

Fair Haven, Vt.; red from Granville, N. Y.

GLOSSARY OF GEOLOGIC AND SLATE-QUARRY TERMS.

As this bulletin, is intended largely for the use of persons unfamiliar 

with geologic science, a number of the geologic terms used in it, whose 

meaning is not self-evident, 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 terms.

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 mica 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.

ANTICLINOEIUM. A mountain mass arch-shaped in its general internal structure.

AUTHIGENOUS. Minerals originating chemically within a rock are called authigenous.

BACK JOINT.' Joint plane more or less parallel to the strike of the cleavage and commonly vertical.

BATE. False cleavage.

BED. A continuous mass of material deposited under water at about one 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.
214 SLATE IN THE UNITED STATES.

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. 

CLEAVAGE. Cleavage in a rock is an arrangement of its particles that, gives it the

property of splitting readily in a certain direction into thin plates. (See on

slaty cleavage, p. 33.) 

CLEAVE. Slaty cleavage. 

CONFORMITY. When one bed overlies another in parallelism without any disturbance

of the crust having affected the lower one before the deposition of the upper

they are said to be in conformity. 

CREEP. Effect of the motion of the continental glacier on the upper edges of a mass

of steeply inclined thinly foliated rock, bending the foliation for a few inches

or feet from the surface in the direction of the motion of the glacier. Creep may

also result from gravity operating on steeply inclined weathered beds. 

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. 37 and Pis. VII, 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 of 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. 

HC1. Hydrochloric or muriatic acid. 

HOGBACKS. Shear zones. (See p. 44 and PI. VIII.) 

HYGROSCOPIC, Term denoting the absorption of moisture by a rock or chemical

compound.

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 in their arrangement. 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, they are said to overlap. 

PITCH. The inclination of the axis of a fold of rock. 

POST. A mass of slate traversed by so many joints as to be useless. The 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.
GLOSSARY. 215

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 calcareous material either 

crossing or parallel to the cleavage. If 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; that is, across the cleavage in the 

direction of the dip.

SERICITE. A ribbon-like or fibrous form of muecovite or potash mica.

SHEAR ZONE. Hogback. (See p. 44 and PL VIII.)

SLANT. Longitudinal joint more or less parallel to cleavage and commonly 

slickensided.

SLATY CLEAVAGE. See CLEAVAGE.

SLICKENSIDES. Surf ace of bed or joint plane along which the rock has slipped, polishing 

and grooving the surfaces.

SLIP. Joint crossing the cleavage, but of no great continuity. Many slips are fault 

planes.

SLIP CLEAVAGE. Microscopic folding and fracture, accompanied by slippage; quarrymen's "false cleavage." (See p. 37 and Pie. VII, 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 of bedding, cleavage, 

jointing, etc.

STRIKE JOINT. Joint parallel to the strike of the cleavage.

SULPHUR. Quarrymen's term for iron pyrites.

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 or 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. If 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, in places ramifying 

mineral mass, commonly 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. Any rock not fit for commercial slate.

INDEX.

A. Pa 

Acknowledgments........................... 7-8

Alpha Slate Manufacturing Co., quarry of... 100 

Analyses of shale clays...................... 54

of shales................................. 52

of slates....................... 50,51,54,03,72,

90,92,93,108,119,141,142,143,146,158 

Anticlines in slate deposits, figures showing............................ 106,107

Aquashicola, Pa., quarry at............... 108,109

Ardennes, France, slates of, mineral compo-'

sition of......................... 19-20

Arizona, slate from, description of........... 60-61

Arkansas, slates of, chemical composition of.. 62-03

slates of, comparative characteristics of. 188-189

electric resistance of................. 175

geology and description of........... 61-62

map showing area of................. 62

microscopic analyses of.............. 63-65

production of...................... 195-201

tests of............................ 184-187

Arvonia, Va., quarries near................ 151-153

Arvon Slate Manufacturing Co., quarry of... 154

B.

Baker, C. B., slate from, tests of.......... 184-187

Baltimore Peach Bottom Slate Co., quarry of. 114. 

Bands in slate, plate showing................ 33

Bangor, Pa., quarries at..................... 96-98

Becker, G. F., experiments by, on the production of cleavage.............. 15-16

on the cause of cleavage bands..... .footnote, 47

Bedding of slate, how distinguished from

cleavage....................... 167-168

plate showing........................... 32

production of............................ 27-33

Benson, Vt., black-slate area in............ 145-146

Bibliography.............................. 205-213

Blissville section in western Vermont, description of.................... 129-130

Bolivar station, Ga., quarry near............ 71-72

Bonine, C. A., acknowledgments to.......... 8

on the slate deposits of Northampton

County, Pa................ 95-96,97,98

Bray, W. M., quarry of.................... 108-109

Bremo Bluff, Va., slate deposits near...... 154-156

Brownsville, Me., quarries at................ 81-82

C.

California, slates of, comparative characteristics of........................ 188-189

slates of, location and description of...... 65-70

production of...................... 195-201

California Slate Co., specimen from.......... 70

Castleton, Vt., Cedar Point quarry at, plate

showing.......................... 124

Chapman Slate Co., quarry of............. 103-104

slate from, tests of..................... 181-184

Chemical analysis, value of................ ISO-181

Chemical composition of slates............... 50-59

Chili Bar Slate Co., quarry of................ 69

Clay, testfor................................ 173

Cleavability, test for......................... 172

Cleavage, slaty, curvature of................. 36

slaty, effect of frost on................... 35-36

how distinguished from bedding... 167-168 

nature and production of............ 15-16

,plate showing....................... 32

relation of, to axes of folds........... 34-35

to bedding...................... 33

to dip of inclosing hard beds..... 34

slip, nature and cause of................. 37-39

Cleavage banding, nature and cause of....... 45-48

plate showing........................... 80

Cleavage surface, examination of............. 173

Color, examination of........................ 173

Colorado, slates of, comparative characteristics of......................... 188-189

Colorado Slate Co., specimen from........... 70-71

Coons, A. T., statistics of production, imports, and exports............. 193-204

Corrodibility, testfor........................ 178

Cross fracture, testfor..................... 172-173

D.

Density, determination of................. 176-177

Derry Church, Pa., slate deposit near........ 109

Dikes, occurrence of................:........ 49

relations of, to joints and shear zones..... 169

E.

East Bangor Consolidated Slate Co., quarry

of.............................. 99-100

Eckel, E. C., on the slates of California..... 65-70

on the slates of Minnesota............... 86

on the slates of Utah.................. 119-120

Elasticity, test for........................... 176

Electric resistance, mode of testing........ 174-175

Esmont, Va., slate deposits near........... 156-159

Eureka Slate Co., shipment from............ 69-70

specimens from.......................... 66-69

Everhart, Edgar, analysis by................ 72

Exports, statistics of....................... 203-204

Eyes in slate, nature of...................... 18-19

F. 

Faults, occurrence of........................ 43-44

Flints, nature and cause of................ 168-169

Forks, Me., slate deposit at.................. 82-83

Frederick and Montgomery counties, Md.,

parts of, map showing........... 84

Frost, effect of, on cleavage.................. 35-36

217
218 INDEX.

G.

Geologic compass, use of................... 169-171

Geologic map, use of....................... 169-171

Geologic relations of slate.................... 49-50

Georgia, slates of............................ 71-74

slates of, comparative characteristics of. 188-189 

production of...................... 193-201

Georgia Green Slate Co., specimen from...... 71-72

Gladson, W. N., test for electric resistance

made by........................ 174

Glossary of geologic and slate-quarry terms. 213-215 

Gordon, C. H., on a slate deposit near Kingsport, Term.................... 118-119

Gosselet, J., on dip of slaty cleavage.......... 34

Grain, nature and direction of............... 40-41

H.

Harding, William, & Co., quarry of.......... 100

Hardness, test for........................... 178

Harrington, M. J., slate from, tests of...... 184-187

Hayes, C. W., on Rockmart slate............ 72-73

Heat and cold, effects of, method of testing . 177 

Hillebrand, W. P., analyses of slates by....... 25,

92> 93,108,141,142,143,146 

on the composition of slates of New York

and Vermont.................... 55-^59

Hirschwald, J., on the microscopic examination of slates................... 179-180

Hudson, C. W., tests of slate by............. 154

Hutchins, W. M., on the mineral composition

of slates......................... 20-21

I.

Igneous slates, composition and origin of.... 27

Ijamsville, Md., slate deposit near........... 85

Imports,statistics of......................... 203

J.

Jackson, Stephen, & Co.. quarry of........ 101-103

Janesville-Hampton section in western Vermont , description of... :....... 125-128

Johnson City, Tenn., slate deposit near...... 119

Joints, nature and kinds of.................. 41-43

relations of, to dikes and shear zones...... 1C9

Jones, M. W., slate from, testsof........... 184-187

K.

Keith, Arthur, on slate deposits near Knoxville, Tenn. ..................... 115

King, Clarence, on a slate deposit in Nevada. . 87 

Kingsport, Term., slate deposit near...... . 118-119

.Knots. See Eyes.

L.

Lafayette Slate Co. , quarry and slate of . . ^ . . . 88

Lake Bomoseen section in western Vermont,

description of. ................... 129

Lehigh Testing Laboratory, analyses made at 158 

Leith, C . K. , on the development of cleavage. . 16 

.Lime, test for. ............................... 173

Lloyd, William E.., quarry of................. 98

Lobb, William & Sons, quarry of. ......... 101-103

Lowell Slate Co., quarries and slate of. ....:... 80

M.

McCulloch, Gilbert, analysis by.............. 115

Machine for splitting slate, plate showing..... 33

Machinery, recent......................... 189-191

Magnetite, test for........................... 174

Maine, slates of, comparative characteristics

of.............................. 188-189

slates of, geology of...................... 74

map of region of..................... 75

production of...................... 193-201

tests of............................ 181-184

Maine Slate Co. of Monson, quarry and slate

of......................'.......... 77-78

Marble, Colo., quarry at..................... 70-71

Marcasite', test for......................... 173-174

Martinsburg, W. Va., slate deposits near.... 164-166

Maryland, slates of.......................... 83-S6

slates of, comparative characteristics of. 188-189 

production of...................... 193-201

Massachusetts, slates of, production of..... 194,196

Mathews Consolidated Slate Co., slate from,

tests of........................ 181-184

Mena, Ark., slates from..................... 63-64

Merrill Brownville Slate Co., quarries and

slate of.................. 81-82,181-184

Merriman, Mansfield, methods of, for testing

strength of slate............... 175-176

on the comparative value of chemical

analysis and other tests........ 180-181

tests of certain slates by............... 181-184

Michigan, slates of,production of........... 194,195

Microscopicanalysis, method for........... 178-180

Middle Granville section in western Vermont,

description of.................... 128

Minerals found in slate.....................: 17-23

Minnesota, slates of.......................... .86

slates of, production of............'.... 196,197

Missouri Mountain slate, specimens from...... 63-64

Mofflt, F. H., acknowledgments to.......... 8

Monson Consolidated Slate Co., quarry and

slate of.................. 79-80,181-184

Monson Maine Slate Co., quarries.of...'........ 74-77

specimen from........................... 77

Montpelier, Vt, slate deposit near...........,.....,. 123

Mount Ephraim, Md., slate deposit near. ..<... 86 

 Moyer, J. S., &Co., quarry andslateof. 96-98,100-101

N.

Nevada, slates of............................ 87

slates of, comparativ.e.characteristics of. 188-189 

New Jersey, slates of........................ 87-88

slates of, comparative characteristics of. 188-189 

production of...................... 193-201

New York, slates of......................... 88-93

slates of, comparative characteristics of. 188-189 

production of...................... 193-201

tests of............................ 181-184

New York slate belt, quarry diagrams in,

plates showing................ 126,127

maps of..........................;...... 124

sections across, plate showing............ 124

Nodules, occurrence and composition of..... 49

Northampton County, Pa., map showing location of quarry sites in.......... 100
INDEX. 219

Page. 

North Bangor Slate Co., quarry of........... 99

North Blanchard, Me., quarries at........... 80

NorthfieloV Vt., slate deposit at............ 120-122

P.

Pawlet, Vt., clay quarry at................... 124

section near, description of............ 131-132

Peach Bottom region (Pa., Md.), location of.. 110 

slate of, characteristics of............... 111-113

geologic relations of................ 110-111

quarries in........................ 113-115

Pen Argyl, Pa., quarries at................. 101-103

Fenian Slate Co., quarry of................ 153-154

Pennsylvania, slates of..................... 93-115

slates of, comparative characteristics of. 188-189 

production of...................... 193-202

tests of............................ 181-184

Phoenix Slate Co., quarry of................. 100

Physical tests of slates..................... 154,159

Pitts, A. L., quarry of.............-..----- 152-153

slate from, tests of..................... 181-184

Placerville, Cal., quarries near............... 69

slate district near, map of............... 66

Planada, Cal., slate deposit near............. 70

Plication of beds, examples of............... 29-33

Porosity, tests for........................... 177

Portland-Monson Slate Co., quarries and slate . of........................... 78-79,156

Prindle, L. M., acknowledgments to........ 8

Production, statistics of................... 193-202

Prospecting, suggestions for............... 169-171

Purdue, A. H., on tests of Arkansas slates for

electric resistance............. 174-1-75

Q.

Quarry, outline for technical description of.. 171 

Quarrying, difficulties in.................... 167

Quartz veins in slate, cause of.............. 168-169

occurrence of.......... ................ 48-49

plate showing........................... 32

R.

Ribbons, nature of.......................... 28-29

Rising & Nelson Slate Co., slate from, tests

of............................. 181-184

Rockmart, Ga., slate deposit near........... 72-74

Rogers, W. B., on the slate of Buckingham

and Fluvanna counties, Va...... 147

Rosenbusch, H., on the mineral composition

of slates......................... 20

Rutileneedles, origin of..................... 21

S.

Sandstone resting on slate, plate showing.... 28

San Francisco Slate Co., quarry of........... 69

Schaller, W. T., analyses by............. 67,68,119

Schist, nature of............................. 9

Shear zones, nature and occurrence of........ 44-45

plate showing........................... 46

relations of, to joints and dikes.......... 169

Page. 

Shenandoah Slate Co., quarry of............. 165

Sizes of mineral particles.................... 21-22

Slate, definition and classification of.... 9-11,16-17

formation of.......................... 11-14,17

mineral constituents of.................. 17-23

Slate pencils, methods of making............ 144

Slatington, Cal., quarries at................. 66-69

Slatington, Pa., quarries at........ 100-101,105-108

quarries at, map showing location of..... 104

Snowden, Va., slate deposit at............. 159-101

Sonorousness, test for........................ 172

Sorby, H. C., on the definition of slate....... 16

Southern Slate Co., slate of.................. 115

Southwestern Slate Manufacturing Co., slate

from............................. 65

slate from, tests of..................... 184-187

Specific gravity, determinaticn of.......... 176-177

Spots, colored, nature and origin of.......... 23-26

white, nature and cause of............... 26-27

Standard Slate Corporation, quarry of..... 156-159

Steiger, George, analyses by................. 90,92

Strength, modes of testing................. 175-176

Synclines in slate deposits, figures showing.. 107 

plates showing......................... 29,132

T.

Tellico Co., quarries of..................... 115-118

Tennessee, slates of........................ 115-119

slates of, comparative characteristics of. 188-189 

production of...................... 196-201

Testing, methods of....................... 171-181

Thurston, Md., slate deposit near........... 85

Toughness, test for.......................... 176

U.

Union Slate Co., quarry of................... 122

United States, map of, showing location of

slate deposits.................... 7

Uses of slate................................. 191

Utah, slates of............................. 119-120

slates of, production of................. 195-197

V.

VanHise,C. R.,ontheeffectsofpressure.... 15

Veins, occurrence of......................... 48-49

Vermont, slates of......................... 120-146

slates of, comparativecharacteristics .of. 188-189 

production of..................:... 193-201

tests of............................ 181-184

western district of, geology of...... 123-125

quarry structure in................ 135-138

slates of........................... 139-145

distribution of................. 133-135

structural features of.............. 133,139

Vermont Black Slate Co., quarries of...... 121-122

Vermont slate belt, maps of................. 124

quarry diagrams in, plates showing... 126,127 

sections across, plate showing............. 124

Vermont Unfading Green Slate Co., slate

from, tests of.................. 181-184
220 INDEX.

Virginia, slate areas in, maps showing........ 148

149,156,160,162 

slates of............................... 146-164

comparative characteristics of...... 188-189

production of...................... 193-201

tests of............................ 181-184

Virginia Slate Mining Co., quarry of....... 159-161

W.

Waring, W. B., analyses by................. 63

Warrenton, Va., slate deposits near........ 161-164

Waste, utilization of....................... 192-193

Weathering, chemical changes in............ 53-55

Wells section in western Vermont, description of......................... 130-131

West Pawlet section in western Vermont,

description of................. 132-133

West Virginia, slates of.................... 164-166

slates of, comparative characteristics of 188-189 

Whitehall, N. Y., red slate from, composition

of............................... 29

red slate from, plate showing........ 28

Williams Slate Co., quarries of............. 151-152

slate from, tests of..................... 181-184

Windgap, Pa., quarries at................... 100

Winnemucca, Nev., slate deposit northwest

of............................... 87

o