356 The American Geologist, December, 1894 
Report on the invention and construction of the electro-chronograph 
clock ai the National Observatory, Washington. 1651. 
Catalogue of rocks, minerals', ores, and fossils collected by John 
Locke during 1847. Smithsonian Report, 1854, pp. 367-383. 
The devotee of Science and the National Institute. [Quoted by Dr. 
M. I'.. Wrighl in his memorial address, p. 12. 1857.] 
THE "SLATE BINDERS"* OF THE "PITTSBURG" 
COAL-BED. 
By W. S. Gresi.ey, F. G. S., Erie, Pa. 
Given, a "bench" or layer of good bituminous coal, of very 
uniform qualit} T , varying in thickness from sa} 7 22 to 27 inches, 
with one or two more or less irregular slaty partings or bind- 
ers here and there in it; and imagine such a deposit spread 
out over at least 15,000 square miles. The edges or outcrop- 
pings of this layer of coal reveal no signs of a beginning or of 
an end; in other words, there is nothing to indicate that this 
coal did not originally extend hundreds of miles beyond any 
of its existing limits. We will not now discuss the question. 
How did this layer of coal get where it is? bat proceed at 
once to observe that it has a practically dead-level and even 
surface or top.f Suppose this vast expanse of dead-level coal 
vegetation to be completely covered or sealed over by a thin 
layer or band of shale, or "slate," as miners call it. We will 
suppose the thickness of this film of shale to be from £ to 4 of 
an inch only. Imagine a practically unbroken 15,000+ square 
mile sheet of shale only f of an inch thick ! On top of this 
shale-band let a second and equally uniform layer of the same 
coal as the thicker one below, be deposited, whose thickness is 
about 4 inches — a layer of coal practically free from impuri- 
ties, and, in every respect, similar to the rest of the seam, re- 
garded as a whole. Again, on top of this 4-inch band of coal 
conceive a second layer of shale to exist, in thickness and kind 
just about the same as the shale-layer 4 inches below it. Then 
above this suppose we have a uniform bench of coal 3 feet to 
♦Binders, in coal, are thin intersl ratified layers of shale, more or less 
mixed with pyrites, etc. 
tThe only breaks or interruptions of the continuity of t he seam being 
a few small and local banks or ridges of rock, called "horse-backs."" 
rising from i he floor. 
