"Slate Binders'''' of "Pittsburg" Coal Bed. — Gresley. 357 
5 feet high. Here, then, we have three separate and distinct 
benches or divisions of a coal-seam separated horizontally by 
a couple of thin, parallel-bedded layers of shale; or, looked 
at in another way, we have a, say, 15,000 square mile 4-inch 
band of excellent coal sandwiched between two very thin, but 
remarkably persistent layers of what is presumably hardened 
mud, these again being enclosed by thicker layers of the same 
kind of coal. Now, the foregoing is in reality a description 
of what actually occurs in nature ; it is the lower or workable 
division of the "great Pittsburg bed." These two "slate-bind- 
ers" seem to be so remarkable as regards their geographical 
extent, uniformity in thickness, composition, distance apart 
vertically, etc., that some special effort ought to be made 
to explain: 1 — What they are or signify; 2 — How they 
got there ; and, 3 — Whence they came, — three questions, so far 
as I know, not yet at all satisfactorily answered, and much 
less easy of solution than at first sight appears. My wish in 
this connection is that this paper may stir up sufficient in- 
terest in this matter to lead to further, extended, and closer 
observation; and such a detailed study of the Pittsburg bed 
as it (a typical one) surely deserves and ought to receive at 
the hands of all local geologists and men capable of doing 
useful work on it. Of course, the question of the origin and 
formation of the shale-bands in the coal opens up that of the 
whole question of the formation of coal-seams, for the bands 
are part and parcel of the seam ; the two substances (coal and 
shale) cannot be considered separately. 
Most geologists have access to about all that has been pub- 
lished in the way of detailed sections, etc., of the Pittsburg 
bed, and so are more or less familiar with its geographical 
extent as shown upon geological maps; so that to copy a host 
of sections and cite others would be superfluous here. We 
all know by this time how vastly greater in area] dimensions 
this seam of coal must originally have been, compared with 
what it is now. Prof. Lesley sees no reason why it may not 
have extended north far into what is now Canada, and north- 
east at least as Car as the hard coal region of Pennsylvania. 
That it is a typical coal-seam nobody disputes; that the coal 
is the remains of Carboniferous-period vegetation no geolo- 
gist denies; but lioir ii got there, nobody knows, although 
