66 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
grained. Under the microscope it proves to be chloritic, pyritiferous, and slightly 
biotitic, with rare grains of zircon. 
The Merrill quarry measures about 450 feet along the strike, between 165 and 200 
feet across it, and 250 in depth. The bedding strikes N. 78° E., dips 75° NNW.; 
the cleavage strikes N. 68° E., and dips 70° NNW. Dip joints strike N. 20° E., dip 
82° WNW.; diagonal joints strike N. 60° W., dip 90°. There are also horizontal 
joints, to which the grain is parallel. Quartz veins are not conspicuous, but there 
are some 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 cleav- 
age 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 conspic- 
uous 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, .1 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 illus- 
trate "false cleavage" as well the fineness of slaty cleavage. The principal con- 
stituents, 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 Brown- 
ville 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 rup- 
ture 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, 
