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Excerpts From

Slate in The United States, Bulletin 586

By T. Nelson Dale and Others

Department of the Interior, United States Geological Survey
Washington, Government Printing Office

1914

Title page


Plate I
Plate I

Slate in the United States - Introduction.

By T. Nelson Dale and others.

__________

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.

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. Purdue'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.


Maine.  (1914)

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.

Map of slate region in Maine
Figure 5.  Map of slate region in Maine.  From post-route map. 
The chief quarrying centers are shown by crossed hammers.

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 1/2 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 mile 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 1/2 miles west-southwest and 1 1/2 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 quarries east-southeast of its Pond quarry; the Maine Slate Co. of Monson its Mathews quarry, about 3 1/2 miles west northwest of the village; and the Portland-Monson Slate Co. four narrow openings about a mile south of the village.

  • 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 hasbeen tunneled 30 feet to the next opening.

    Lower Quarry of Portland-Monson Slate Co., Monson, ME.

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 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.

Section at Monson Pond quarry, Monson, Maine.
Section at Monson Pond quarry, Monson, Maine.
 
(Furnished by the courtesy of Mr. F. H. Crane, superintendent.)

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 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 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 in 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.  Their 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 much accidents can be avoided 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 traverse 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,[2] which shows 56.42 per cent of SiO2, 24.14 per cent of Al2O3, 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 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 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; biotite scales, about 63 to each square millimeter, measuring up to 0.13 by 0.28 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 pyrite to 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 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 of polarization, an even (not lenticular) texture, but with some passages 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.  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 millstock, 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.

West Monson.-At the quarry of the Monson Consolidated Slate Co. is exposed a bed of black slate 9 feet thick, with a bed 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 No. 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 eye its texture 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 liminę 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 0.077 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.

In 1904 the slate was used both for roofing and for electric purposes.  The quarry was idle in 1912.

North Blanchard.-These 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 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 parallel to produce a light 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, 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 them bordered by secondary quartz or muscovite in the direction of the 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.

  • Plate XI-A.  Thin section of black roofing slate from Merrill Quarry, Brownville, Me.  Showing fine matrix of muscovite (sericite) with distorted octahedra of magnetite and (exceptionally for this quarry) a secondary plication resulting in slip cleavage.  Lenses of chlorite and muscovite or of quartz and muscovite or of muscovite alone about some of the magnetite crystals.  Enlarged about 50 diameters.

    Thin section of black roofing slate from Merrill

  • 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.  The lower block in the foreground has three uncleft and two cleft bands.  Sledge handle is 30 inches long.

    Cleavage banding in shales partly altered to schist

  • Plate XII.  Merrill Slate Quarry, Brownville, Me.  Looking S. 60° W.  The end wall, working face, has 42 beds of slate alternating with quartzite.  The left wall is quartzite that is diagonally jointed.

     Merrill Slate Quarry, Brownville, Me.

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 quartzite slate is considered quartzite in these calculations.[3]

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 also 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 Pl. XII), which is formed by a quartzite bed, is divided into rhombic blocks about 10 feet in their longer diameter, owing to the intersection of joints dipping 25° W. and 30° E.  The surface of this bed has also what resembles a coarse ripple marking but is probably a minor effect of the rhombic jointing.

The slate is a very dark gray.  To the unaided eye it has a very fine texture and a very smooth cleavage surface, with a very bright luster.  It is slightly 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 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, B, 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. O. 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 effervescence 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 abundant 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 smooth surface, and from the slate at Brownville 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 188.



[1] Slate in The United States, 1914, page 74 footnote 1:  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. Rept. pt. 2, pp. 280, 282, 360, 1862.

[2] Ibid., page 77, footnote 1:  See Bayley, W. S., U. S. Geol. Survey Bull. 150, p. 313, 1898, and Twentieth Ann. Rept., pt. 6 (continued), p. 394, 1899.

[3] Ibid., page 81, footnote 1:  Measurements obtained through the courtesy of C. H. Dunning, superintendent.

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