PLEASE NOTE:  Only a few portions of this book will be presented in this document.  Eventually, the entire book will be presented. At this time not all of the photographs and maps will be presented here, although the photographs and maps have been placed in the "Quarries & Quarry Links, Photographs and Articles" section of the Maine state section according to the location of the quarry.  If you have questions about any portion of the book not presented here, please contact me.  Peggy B. Perazzo

 

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Difficulties in Slate Quarrying.

The difficulties in Slate quarrying are many.  Assuming that the commercial value and the grade of fissility of the slate itself have been fully determined by scientific and practical tests, the opening of a quarry offers serious problems.  These concern the thickness of the deposit of the weathered "top," the character of the jointing, the presence of faults, shear-zones, and dikes.  There are also the practical matters of drainage, of the location of dumps, of transportation facilities, and of fuel.  The cost of slate at some quarries is increased by the necessity of removing the dumps of former workings, which, for lack of capital or of good judgment, were placed on good slate.  Sometimes the only way to remove these old dumps is to throw them into the quarry on one side and hoist them up at the other.  In places where the beds are steeply inclined or vertical as the quarry deepens one of the walls falls in and the removal of this material entails great expense.  The occasional employment of a consulting engineer would obviate mechanical accidents of this nature by the mathematical determination of the points at which supporting walls should be left.  There are also uncertainties of a stratigraphic character which the use of the core drill would eliminate from many a slate-quarrying venture.

The percentage of waste is generally high in slate quarrying.  Watrin (see Bibliography, p. 145), referring to the Ardennes region, gives the total waste at from 70 to 75 per cent in weight, of which 20 to 25 occur in the quarry and 50 in splitting.  Merrill estimates the waste in the Peach Bottom region as 88 per cent.  In the Maine quarries it is also large, owing to the frequent interbedding of the slate with quartzite..."

(The rest of the section will not be presented in this document.  I will skip over to the section Maine slate.  Peggy B. Perazzo.)

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The slate region in Maine lies about in the center of the State, in the southern part of Piscataquis County, south and southeast of Moosehead Lake and east and west of Sebec Lake, in the towns of Monson, Blanchard, and Brownville.  (See map, fig. 4.)  Commercial slate occurs also in the town of Forks, Somerset County.

Fig. 4.  Map of slate region in Maine.  From Post-route map. 
The chief quarrying centers are shown by crossed hammers.

Geological relations.-The slate occurs in a belt consisting largely of slaty rocks, represented by Prof. Charles H. Hitchcock as from 15 to 20 miles wide, and extending from the Kennebec River, between Bingham and Dead rivers, northeastward to the sources of the Mattawamkeag River.  The rocks are probably of early Paleozoic age.[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. dip 80° ESE.  Near Blanchard and the Piscataquis River the strike is N. 55°-60° E. and the dip, 40 feet below the surface, is south-southeast at about 80°; but at the top, owing either to the glacier, which moved here S. 20-40° E., or else to the beginning of an anticline, the dip curves over to the north-northwest.  Within 1 1/2 miles southwest of Monson the strike is N. 60° E., dip 90°.  At Monson the strike is N. 47°-54° E., dip 80° SE; but at Brownville, 20 miles east of Monson, the strike is N. 78° E., dip 75° NNW.  As the grain is horizontal at Brownville and at points 3 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 1904 three quarries were in full operation in the town of Monson.  The Monson Pond quarry of the Monson Maine Slate Company; the newly opened one of the Maine Slate Company, of Monson, about 3 1/2 miles west-southwest of the village; and the West Monson quarry, about 1 1/2 miles southwest of it, operated by the Monson Consolidated Slate Company.

At the Monson Pond quarry the following series is exposed:  15 beds of slate, measuring altogether from 79 feet to 93 feet 6 inches, alternating with 15 beds of dark gray or black quartzite ("hards"), measuring altogether from 48 feet 5 inches to 49 feet 5 inches, both slate and quartzite amounting to from about 127 to about 142 feet.  The deposit has been prospected for 200 feet farther southeast, but the 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 biotite and pyritiferous, with a little magnetite and muscovite and a few grains of zircon.  In order to convey an idea of the great irregularity of the interbedding which marks the entire belt the following measurements of the Pond quarry series at the north edge of a quarry are here given:

 

Section at Monson Pond quarry, Monson, Me.
[Furnished by the courtesy of Mr. F. H. Crane, superintendent.]

There is sometimes a transition from the quartzite to the slate, a quartzitic slate intervening.  In the above lists such beds are classified as quartzite.

The Pond quarry measures about 500 feet along the strike and nearly 10 feet across it at the top, and from 250 to 300 feet in depth.  The beds strike N. 47° E., dips 78°-80° SE, without any indication of turning, and the cleavage strikes N. 45° E., dips 90°, thus intersecting the bedding at a very acute angle.  The grain strikes N. 45°-50° W. and dips 90°.  The slate is traversed at intervals by horizontal joints, which are more frequent in the quartzite-in places from 1 to 4 feet apart.  The quartzite also has joints, striking N. 65° W. and dipping 25° N. 65° E., which often are veined with quartz.  There are also vertical diagonal joints striking about northwest thus parallel to the grain.  The northeast half of the quarry is much broken up by diagonal jointing and faulting, but in the southwest half conditions are more normal, although veining is there more frequent.  The difference between the jointing of the quartzite and the slate results from the differences in their rigidity.  Their behavior under the same stress must needs have been very dissimilar.  The quartz veins traversing the slate sometimes contain biotite, chlorite, and a little calcite.  The surface of the formation is glaciated and covered with from 5 to 10 feet of glacial clay and pebbles.

The slate itself is very dark gray, but at the glaciated surface some of the beds have in bright sunlight a very slightly purplish hue.  The fifth bed from the north edge is slightly brownish.  To the unaided eye both texture and surface are fine, but the latter is almost lusterless.  It is slightly carbonaceous or graphitic, and has very little magnetite.  The sawn edge shows a little pyrite.  No effervescence in cold dilute hydrochloric acid.  It is very sonorous.

Under the microscope this slate shows a matrix of muscovite (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 mm.  Occasional quartz lenses measure 0.094 by 0.047 mm.  There are to each square millimeter from 30 to 40 scales of chlorite (interleaved with muscovite, rarely with biotite) measuring up to 0.047 by 0.03 mm. and lying transverse to the cleavage; also about ten scales of biotite to each square millimeter measuring up to 0.086 by 0.02 mm., lying both across and with the cleavage, and longish crystals and lenses of pyrite with their long axes parallel to the grain, numbering about fifty to the square millimeter and measuring up to 0.075 by 0.028 mm.  These crystals are mostly distorted cubes, but mingled with them are probably some distorted octahedra of magnetite.  Scattered throughout is dark-gray carbonaceous or graphitic matter in extremely minute particles, to which and to the biotite the slate owes its blackness.  Finally, a few delicate rutile prisms, 0.001 mm. long, some specks of hematite, and a few tourmaline prisms up to 0.036 by 0.004 mm.  No carbonate detected.

The constituents of this slate, arranged in the order of their decreasing abundance, appear to be muscovite (sericite), quartz, chlorite, biotite, pyrite, carbonaceous or graphitic matter, magnetite, rutile, and apatite.

The only available chemical analysis of this slate is that by L. M. Norton,[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 Bailey as 2.851.  Tests of the crushing weight and strength made at the Massachusetts Institute of Technology show that a cubic inch of this slate yields to the pressure of 30,425 pounds when applied at right angles to the cleavage, and that a slab, 12 by 6 by 1 inches, supported on knife edges 10 inches apart, breaks under a stress of from 3,950 to 4,000 pounds applied at the center of a steel rod, five-sixteenth inch in diameter, placed between the slate and the pressure block.  This gives a modulus of rupture of 9,937; pounds per square inch.

This slate is split to seven thirty-seconds of an inch for roofing.  It is also used for electric purposes, register borders, blackboards, refrigerator shelves, etc.

At the quarry of the Maine Slate Company of Monson, 3 1/2 miles west-southwest of the village, opened in 1903, there are about 30 feet of slate and interbedded quartzite exposed.  The thickest bed of slate measures about 8 feet.  In 1904, the quarry measured about 100 feet across the strike, 30 feet across it, and 40 feet in depth.  The bedding strikes N. 55°-60° E., and dips about 80° SSE., but at the surface curves over steeply north-northwest.  The cleavage strikes about the same, but dips 90°.  There are vertical dip joints, horizontal jointing, so that the slate has to be sawn in a northwest and southeast vertical direction and split ("sculped") in a horizontal one.

The slate is very dark gray; to the unaided eye has a finer texture and finer cleavage surface than that of the Monson Pond quarry, and also more luster.  It is slightly graphitic, has very little magnetite, but the sawn edges show considerable pyrite.  It does not effervesce with cold dilute hydrochloric acid, and is very sonorous.

Under the microscope this slate shows a matrix of muscovite (sericite), with a very brilliant aggregate polarization.  There are lenses of biotite and quartz, or of quartz with a nucleus of pyrite, measuring up to 0.565 by 0.14 mm., rarely 1 by 0.075.  Quartz fragments, usually abundant in sections parallel to cleavage, measure up to 0.064 mm.; biotite scales, about 18 to each square millimeter, measuring up to 0.13 by 0.028 mm., lie across as well as in the cleavage.  Little less abundant than these are scales of chlorite interleaved with muscovite, lying across the cleavage and measuring up to 0.13 by 2 mm.  There are also about 300 lenses of pyrite to each square millimeter, with their long axes in the cleavage, and measuring from 0.002 up to 0.094 mm. in length, and up to 0.047 in width and breadth.  This number probably includes a few crystals of magnetite.  These lenses are sometimes surrounded by secondary muscovite.  Generally distributed is a dark grayish or black material, probably graphitic, to which the slate owes its blackness.  Tourmaline prisms up to 0.047 by 0.009 mm.  No carbonate was detected.

The product is at present being prepared exclusively for roofing.

West Monson-At the quarry of the Monson Consolidated Slate Company a bed of black slate 9 feet thick, with a bed of quartzite 15 feet thick on its north side, and small alternating beds of quartzite and slate on its south side are exposed, the whole series measuring perhaps 50 feet.  The quarry in 1904 measured 300 feet along the strike, 15 across it, and 160 in depth.  The walls are supported by three pillars of slate.  Bedding and cleavage both strike N. 60° E., and dip 90° E.  There are vertical dip joints striking N. 15° W.; also horizontal joints to which the grain is parallel.  there are about 15 feet of till on the edges of the glaciated slate.  As only one bed of slate is worked, the percentage of waste at this quarry is very small.

The slate is a very dark gray.  To the unaided eye texture and cleavage surface are very fine.  It has more luster than the Pond quarry slate, but not as much as that of the Maine Slate Company of Monson.  It is slightly graphitic, but 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 biotite, measuring from 0.17 to 0.13 by 0.034 mm., some lying in the cleavage, others in the direction of the grain.  The quartz fragments measure up to 0.02 by 0.012.  There are about nine biotite scales to each square millimeter, measuring up to 0.08 by 0.02 mm.; also about fourteen chlorite scales, measuring 0047 by 0.02mm., but sometimes 0.085 by 0.03, with their longer axes and laminæ usually parallel to the cleavage and across the grain; and finally, twenty to fifty lenses and crystals of pyrite to each square millimeter, measuring, in sections across the cleavage, up to 0.066 by 0.02, with their longer axes parallel to the cleavage, and the usual finely disseminated carbonaceous matter; also tourmaline prisms up to 0.007 by 0.008 mm.  No carbonate was detected.

The probable relative abundance of these constituents, in descending order, is muscovite (sericite), quartz, chlorite, pyrite, biotite, and carbonaceous matter or graphite.  Professor Merriman's tests of this slate are given on page 123.  This slate is used both for roofing and mill stock, particularly for electric purposes. 

North Blanchard.-There are two quarries at North Blanchard, both operated by the Lowell Slate Company.  At the State of Maine or Blanchard quarry 50 feet of slate and quartzite, ten beds of each, in alternation, are exposed, and 200 or 300 feet more have been prospected east of the quarry.  The quarry measures between 250 and 300 feet along the strike, 40 to 50 feet across it, and 200 feet in depth.  Both bedding and cleavage strike N. 25° E. and dip east-southeast at 80°.  The slate has vertical dip joints striking N. 70° W. and diagonal ones striking N. 40° W., dipping 32° SSW.  There are also joints confined to the quartzite, dipping 65° to 70° SSW., and also 65° to 70° NNE.  The grain strikes N. 65° W. and dips 90°, almost like the dip joints.  The surface of the deposit is glaciated and covered with 10 feet of till.  Some of the quartzite surfaces show faint traces of marine life.  At the Moosehead quarry, which lies one-half mile southwest or south-southwest of the last, more than 65 feet of slate and quartzite are exposed.  The thickest beds of slate measure 4 and 7 feet.  The quarry measures about 500 feet along the strike, 50 feet across it, and 125 feet in depth.  Bedding and cleavage both strike about N. 37° E. and dip east-southeast at 80°.  Dip joints strike N. 55° W. and dip 90°.  The quartzite on the west side of the quarry is broken up by undulating horizontal joints from 1 to 4 feet apart.  The grain corresponds to the dip joints.

The slate from these quarries is a very dark gray.  To the unaided eye the texture and cleavage surface are fine, but the latter is only slightly lustrous.  The slate contains a little carbonaceous or graphitic matter and no magnetite, but the sawn edges show pyrite.  No effervescence in cold dilute hydrochloric acid.  It is very sonorous and very fissile.

Under the microscope this slate shows a matrix of muscovite (sericite), with brilliant aggregate polarization.  A thin section parallel to the cleavage shows muscovite scales sufficiently numerous and parallel to produce a slight aggregate polarization.  This may be due to an unusually pronounced grain.  The quartz fragments occasionally measure 0.028 mm., and are not abundant.  There are about one hundred scales of chlorite, interleaved with muscovite or sometimes biotite, to each square millimeter, with their laminæ across the cleavage and measuring up to 0.066 by 0.028; also, about seven biotite scales to each square millimeter, measuring up to 0.085 by 0.047 mm., often bordered by secondary quartz or muscovite in the direction of the slaty cleavage, but with their laminæ transverse to it.  There are about two hundred lenses of pyrite to each square millimeter, measuring from 0.004 to 0.03 mm. in length and up to 0.01 in width; much dark-gray carbonaceous or graphitic matter in exceedingly fine particles; tourmaline prisms up to 0.07 by 0.009 mm. are plentiful.  No carbonate or slate needles found.  The chief constituents, arranged in descending order of abundance, appear to be muscovite, chlorite, quartz, pyrite, carbonaceous matter or graphite, and biotite.

This slate is used for roofing and mill stock, including electric appliances.

Brownville.-Only one quarry is now in operation at Brownville, the "Old Merrill," operated by the Merrill Brownville Slate Company.  This quarry lies less than a mile about northeast of the station (see Pl. XII).  Here are exposed 42 beds of slate alternating with as many of quartzite, and measuring altogether 165 feet in thickness.  The slate beds range from 6 inches to 6 feet, and the quartzite beds from 6 inches to 5 feet 6 inches.  Any quartzitic slate is considered quartzite in these calculations.[3]

At the Hughes quarry, owned by the same company, but now idle, and situated a mile northwest of the Merrill, there are 28 beds of slate alternating with 28 of quartzite, measuring in all 161 feet 6 inches.  The slate beds range from 1 to 9 feet and the quartzite from 4 inches to 20 feet in thickness.  There is no evidence of duplication in these series of beds.  Some of the quartzite is grayish and medium grained.  Under the microscope it proves to be chloritic, pyritiferous, and slightly biotitic, with rare grains of zircon.

The Merrill quarry measures about 450 feet along the strike, between 165 and 200 feet across it, and 250 in depth.  The bedding strikes N. 78° E., dips 75° NNW.; the cleavage strikes N. 68° E., and dips 70° NNW.  Dip joints strike N. 20° E., dip 82 WNW.; diagonal joints strike N. 60° W., dip 90°.  There are also horizontal joints, to which the grain is parallel.  Quartz veins are not conspicuous, but there are some 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. respectively.  The surface of this bed has also what resembles a coarse ripple marking, but is probably a minor effect of the rhombic jointing.

 

The slate is a very dark gray.  To the unaided eye it has a very fine texture and a very smooth cleavage surface, with a very bright luster.  It is slightly carbonaceous or graphitic.  When powdered, it yields considerable magnetite to the magnet.  The sawn edges show lenses of pyrite a millimeter and less in length.  Some of the cleavage and other surfaces on the dumps show a very dark purplish coating.  There is no effervescence in cold dilute hydrochloric acid nor any discoloration whatever.  It is very sonorous.

Under the microscope it shows a very fine grained matrix of muscovite (sericite), with a very brilliant aggregate polarization.  It contains much quartz in fragments up to 0.076 by 0.02 mm.; about 5 biotite plates to each square millimeter, measuring up to 0.076 by 0.03 mm., lying across the slaty cleavage.  These often form the nuclei of quartz lenses which measure up to 0.4 by 0.03 mm.  But the most conspicuous feature, next to the brilliant matrix, is the abundance of magnetite in tabular crystals, probably distorted octahedra, lying parallel to the cleavage, about 43 to each square millimeter, and measuring from 0.009 to 0.141 mm. in length and up to 0.02 in width, rarely 0.17 by 0.04. These crystals are sometimes bordered by secondary quartz and muscovite or chlorite, on one or both sides, particularly whenever they happen to diverge from the cleavage direction.  These secondary minerals occupy spaces resulting from a movement of the crystals after the commencement of slaty cleavage.  There are also, but in less abundance, lenses of pyrite, up to 0.62 long, but sometimes 0.75 and 0.12 wide, and consisting of a nucleus of pyrite surrounded by secondary quartz or by this and biotite, these minerals forming the tapering part of the lens.  There is also the usual abundance of dark gray graphitic? material in extremely fine particles.  Not a few prisms of tourmaline occur, up to 0.043 by 0.008 mm.  No carbonate.

Pl. XI-A will give some idea of the distribution of the magnetite crystals in this slate, but the "false cleavage" of the specimen is not typical of the product of the Merrill quarry-indeed it is quite exceptional.  The specimen was selected to illustrate "false cleavage" as well the fineness of slaty cleavage.  The principal constituents, arranged in descending order of abundance, appear to be muscovite (sericite), quartz, magnetite, pyrite, carbonaceous matter or graphite, biotite, chlorite, tourmaline.  Prof. W. O. Crosby found that the slate of the East Brownville Slate Company had an average crushing strength of 29,270 pounds to the square inch, the weight being applied perpendicular to the cleavage, and that it required 3,550 pounds to break a slab 6 inches wide, 1 inch thick, and 11 inches long between supports, the load being applied at the middle.  This would give a modulus of rupture of 9,762 pounds per square inch.  The results of Professor Merriman's recent tests of Brownville slate will be found on page 123.

The product of the Merrill quarry is now used exclusively for roofing purposes; its magnetite, it is thought, prevents its use for electric appliances.  However, a piece 6 by 4 by one-half inches makes no impression whatever on the magnetic needle, and the section photographed in Pl. XI-A came from that piece.  The Brownville slate is highly crystalline.

Forks, Somerset County.-A slate prospect opened in this town in 1890 was visited by the writer in 1905.[4]

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 Brownville slate.  It is graphitic, contains a very small amount of magnetite, has no argillaceous odor, does not effervesce in cold dilute hydrochloric acid, is sonorous, splits, and can be perforated readily.  Neither the ledge nor the fragments, said to have been exposed fifteen years, show discoloration.

Under the microscope the section shows a matrix of muscovite (sericite), with a brilliant aggregate polarization, proving it to be a mica slate.  The cleavage is fine and regular.  There are about 52 lenses of pyrite to each square millimeter, measuring (in transverse section) from 0.02 to 0.06 mm. in length by from 0.004 to 0.016 mm. in width.  In sections parallel to the cleavage these lenses have a very irregular outline and are often as broad as long.  These lenses have a very irregular outline and are often as broad as long.  these lenses account for the limonitic staining on cleavage surfaces of water-soaked specimens.  Quartz is abundant but minute.  No carbonate was detected.  A few tourmaline prisms up to 0.11 mm. in length.  Some scales of chlorite with interleaved muscovite measure up to 0.09 mm.  There are rare zircon fragments and aggregations of rutile crystals.

The constituents of this slate, arranged in descending order of abundance, appear to be muscovite, quartz, chlorite, pyrite, and graphite, with accessory tourmaline, zircon, and rutile.

This Pleasant Pond slate differs from the Monson slates in having a lustrous and smooth surface, and from the Brownville slate in having much less magnetite and a little less luster.  It would prove suitable for roofing or mill stock purposes.

Whether, like the other slates of this State, it is interbedded with quartzite at frequent intervals could not be determined.

The more important features of Maine slates as brought out in the above descriptions will be found set forth in tabular form opposite page 124.

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Commercial use of material within this site is strictly prohibited. It is not to be captured, reworked, and placed inside another web site ©. All rights reserved. Peggy B. and George (Pat) Perazzo.