14 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
Maine, Maryland, New York, Pennsylvania, Virginia, Vermont, and West Virginia, 
are here summarized: 
Quartz, grains: 0.013 to 0.1 millimeter by 0.004 to 0.03 millimeter (exceptionally 
0.347 long). 
Feldspar (plagioclase), grains: up to 0.047 by 0.052. 
Muscovite (sericite) of matrix: thickness down to 0.00017, length from 0.006 to 
0.06 and over. 
Biotite, scales: up to 0.085 by 0.047 (exceptionally 0.2 by 0.09) and numbering up 
to 22 per square millimeter. a 
Chlorite, scales: 0.047 to 0.17 by 0.006 to 0.1 (exceptionally 0.38 by 0.25) and num- 
bering up to 100 per square millimeter. 
Carbonate, rhombs: 0.002 to 0.065. 
Rutile "needles:" 0.0017 to 0.095 by 0.0006 to 0.006 millimeter and numbering 
from 1,000 to 1,850 per square millimeter, or 900,000 per square inch. Those in 
Arkansas slates number up to about 2,400 per square millimeter. 
Pyrite, lenses and distorted cubes: 0.002 to 0.004 by 0.47 (rarely 0.6 by 0.1) and 
numbering up to 300 per square millimeter. 
Pyrite, spherules: 0.0017 to 0.027 millimeter and numbering up to 600 per square 
millimeter. 
Magnetite, distorted octahedra: 0.009 to 0.14 by 0.02 (rarely 0.17 by 0.04) and 
numbering up to 4:; per square millimeter. 
Hematite, dots: 0.0004 to 0.01 millimeter. 
Tourmaline, prisms: 0.005 to 0.076 by 0.001 to 0.009 millimeter. 
Andalusite, prisms: 0.008 by 0.001 to 0.11 by 0.03 (exceptionally 0.2 by 0.02) and 
numbering up to 80 per square millimeter. 
Lenses of chalcedonic quartz and rhodochrosite: 0.32 by 0.15 millimeter (sometimes 
1 millimeter long). 
Lenses of biotite and quartz: up to 0.565 by 0.14 ( rarely 1 by 0.075). 
Lenses of pyrite and quartz: up to 0.75 by 0.12 millimeter. 
Lenses of chlorite or quartz or muscovite or combinations of them, containing each 
an andalusite prism and measuring 0.04 to 0.25 by 0.04 to 0.1 millimeter. 
As the mineral constituents of aqueous-sedimentary slates were either (1) derived 
from older rocks and deposited either as mechanical sediments or chemical precipi- 
tates, or (2) formed during metamorphism, or (3) derived from marine organisms, 
they are to be classified as follows: 
Classification of mineral constituents of slates. 
Clastic. 
Clastic or authig- 
enous. 
Authigenous. 
Organic. 
Quartz grains. 
Rutile needles. 
Quartz, chalcedonic 
Carbonaceous matter. 
Feldspar grains. 
Tourmaline. 
Quartz, vein. 
Graphite. 
Zircon grains. 
Muscovite (sericite). 
Muscovite scales. 
Biotite. 
Kaolin. 
Chlorite, interleaved with 
Apatite. 
muscovite or biotite. 
Magnetite? 
Pyrite. 
Carbonates, granular. 
Magnetite. 
Hematite. 
Carbonates of lime, iron, 
magnesia. 
Carbonate of manganese. 
Andalusite. 
Barite. 
Gypsum. 
Talc. 
a The square millimet< r given comprises the thickness of the thin section. 
