"Slate Binders" of "Pittsburg" Coal Becl.—Gresley. 361 
erosive processes on land, flowed over or through rocks of 
marvelous uniformity, otherwise how could the finest sedi- 
ments derived therefrom preserve their sameness?; also that 
no changes or disturbances took place from an off-shore or in 
a landward direction meeting the waters from the rivers; a 
contemporaneous set of physical conditions, in short, are ap- 
parently demanded for the accumulation of this film or layer 
of mud, which is indeed hard to conceive. We ask, Can such 
a period be reasonably supposed to have actually happened? 
It must be borne in mind that it would take about 100 tons 
of sediment to cover each acre about -J- inch deep; also that a 
15,000 sq. mile sheet of shale ^ inch thick is in the same pro- 
portion as one sq. mile covered by a film ^qoot inch thick. 
Will or can transportation of sediment by water produce such 
wonderful uniformity of deposition or precipitation on the 
bottom as the application of the method calls for? 
(b) Take the eolian process. It seems to me that the ob- 
served facts in regard to these binders would entirely forbid 
the acceptance of the idea that this shaly material got there 
by the agenc} T of wind, i. e. that it was dust blown or carried 
off the land and dropped in an even manner upon the quiet 
waters of a great inland sea. 
(c) Concretionary. If these parallel bands of shale had 
had a concretionary origin, the}'' must have taken another 
form. Concretions are characterized by their irregular or 
nodular shapes, by a tendency to spherical or ringed structures, 
to differences in texture, hardness, fracture, etc., so that 
while the mottling ma} 7 in part be due to some slight action 
of the kind within the layers, the origin and formation of the 
la} r ers themselves cannot be attributed to concretion. 
(d) Substitution or replacement formations. This implies 
that the binders were originally part and parcel of the coal- 
bed, regarded as a whole, and that during solidification or the 
process of coal-forming most of the carbon and of the volatile 
matters made way for concentration (along definite horizons 
in the bed) of the inorganic matters in the vegetable mass; 
that in fact there was set up, at different bights in the coal- 
mass, molecular affinity, perhaps akin to the process of seg- 
regation or leaching, among the particles, resulting ultimately 
in the growth of definite sheets or bands to the nearly total 
