THE CARBONIFEROUS SYSTEM. 165 
that its elevation took place long anterior to that of the Alleghanies, 
and at the close of the Lower Silurian. Hence, during the Devonian 
and Carboniferous ages, it constituted a long, narrow island, which ex- 
tended as far south as Tennessee, and the basin in which our coal beds 
were formed was bounded at the west by that ridge. This proves that 
there was no connection between the Illinois and Alleghany-coal fields, 
and therefore our limestones, beds of coal, and even sheets of mechanical 
material—sandstones, shales, and conglomerates—all came to an edge, 
or ran out on the flanks of the old Cincinnati arch. For a very good 
reason, therefore, the limestones could not thicken in that direction 
indefinitely. 
The reason why the limestones lie nearest to the western margin of 
the great trough which stretched from the Blue Ridge to the Cincinnati 
arch is, in my judgment, simply this: on its eastern and northern 
sides the coal basin received the drainage of quite an extensive conti- 
nental surface, and great quantities of mechanical sediments were 
brought down and spread along that shore. The western border, how- 
ever, was formed by a narrow ridge, composed of lime rock, from which 
the drainage was insignificant and calcareous; therefore, on this side, 
the clear and quiet water necessary for the deposition of limestone set 
close up to the shore. On the north and east, deltas and mud-flats were 
forming like those on the shores of the Bay of Fundy, and at the mouths 
of our great rivers. A still better example of the mode of accumula- 
tion of clay, sand, etc., on that shore is.seen about the head of the Guli 
of California, where the physical condition is not unlike that of the 
Alleghany trough in Carboniferous times. There the head of the bay is 
filling up with sediment, and shallows and mud-flats, many miles in 
width, line the shore, over which it is next to impossible to pass from 
ship to land or land to ship. 
THE EXTENT OF COAL SEAMS. 
Two very different, and even antagonistic, theories are entertained 
by geologists in regard to the area over which any individual coal seam 
may be traced. One of these is that advocated by Mr. Leo Lesquereux, 
who claims that certain coal strata extend not only across the entire 
breadth of the Alleghany coal field, but that these may be identified in 
Indiana, western Kentucky, and Illinois.* 
* Geological Survey of Illinois, Vol. I., p. 208; American Journal of Science, 2d 
Series, Vol. XXX., p. 367. 
