28 L. Lesquereux on the Coal Formations of North America. 
But it is clear, at least to my mind, that the great ridge of 
Devonian and Silurian by which the Appalachian and the Ih- 
nois coal-fields are separated toa distance of from one to two 
undred miles, cannot be regarded simply as one of those hills - 
which separates two parts of a peat bog. We can discuss only 
these two alternatives: either the Silurian axis was not upraised 
at the epoch of the formation of the coal, and this formation, 
being in active progress upon the whole surface occupied now 
by the coal-fields and the Silurian and Devonian, was continu- 
ous, and consequently presented the same general features; or, 
the coal was formed on both sides of the ridge, and therefore 
in two separate basins, and then both formations, though of the 
same age, would have ‘been subjected to some peculiar influences, 
and each of them would be characterized by — Espns 
either in the relative position of their coal beds, o the com 
position of the strata, and especially in the beirfoation of theig 
flora. The of the Kentucky Survey shows on the con- 
trary: that in both coal-fields, the coal beds are exactly in the 
same relative position; that at the same geological level, their 
shales contain the same species of plants; that fro m Eastern 
Pennsylvania to Western Illinois, the thinning of some strata 
reserves a perfectly regular progression, and does not show any 
change on one or the other side of the great ridge. 
But there are some other reasons which may appear more con- 
clusive. 
1. The conglomerates, as also some beds of sandstone, espe- 
cially the great Mahoning sandstone, are developed near the 
eastern limits of the coal-fields to a prodigious thickness. This 
heaping of loose materials, sand or gravel, evidently shows the 
prolonged action of the sea against its shores. Supposing that 
the Silurian ridge had been elevated before the formation of the 
coal, it would have necessarily served as a shore, and we should 
nd somewhere a marked difference in the thickness of the 
transported materials abutting against it. No geologist has ever 
seen anything of the kind, and the conglomerates like some 
of coal and of sandstone, go thinning to the west with a co 
and uniform decrease 
2. All the peat bogs are formed in basins, as also all the de- 
posits of coal, and the outlines of these basins are of-course gen- 
erally broken and irregular. This fact is observable in the of | 
ern = pare borders of the er But on the sie 
