164 GEOLOGY OF OHIO. 
posits toward the bottom of each basin, we find that in the epoch of the 
Putnam Hill limestone the basin in which it was deposited lay almost 
entirely in Ohio, and that in its center the blue or Zoar limestone is 
buried ninety feet deeper than on its sides. 
In the epoch which succeeded the deposit of the Pree pet t limestone 
the locality of greatest depression was east of Ohio, as this limestone, 
while covering a large area in western Pennsylvania, reaches continu- 
ously through but one tier of counties in Ohio. 
The center of the basin continued to be east of Ohio during the depo- 
sition of the Barren Coal Measures, as they are thickest and contain 
most limestone on or near our eastern border; are thinner, with less 
limestone and more coal, toward the west. 
During the Pittsburgh epoch, or that which immediately preceded and 
followed the deposition of the Pittsburgh coal, the area of open water 
was, as in the Upper Freeport epoch, nearly in the line of the center of 
the basin. The proof of this we find in the great deposit of limestone 
beneath and over the Pittsburgh coal at Wheeling, and other localities 
in West Virginia and western Pennsylvania. After passing the Ohio 
line these limestones rapidly thin out and are replaced by mechanical 
shore deposits. It is also shown by the thickening toward the east of 
the mechanical materials which separate the Pittsburgh, Redstone, 
Sewickley, and Waynesburg seams. And yet, after the filling up of the 
water basin in which the Pittsburgh limestone was deposited, but little 
more limestone accumulated at the east up to the close of the Carbonif- 
erous age, the open water and calcareous sediments preponderating, as 
we have seen, in Ohio. 
By tracing our Coal Measures into Pennsylvania, it will be found, as 
was shown by Prof. Rogers, that our most important coal seams thicken 
toward the east; as the Upper Freeport—which becomes in West Vir- 
einia twenty feet in thickness, though nearly half slate—the Pittsburgh, 
the Redstone, the Sewickley, and the Waynesburg; while the greatest 
development of the limestones hes relatively further west in the basin. 
This fact led Prof. Rogers to conclude that the limestones of the Coal 
Measures thickened westward toward the open sea, and he supposed that 
their relative importance constantly increased until the mechanical 
sediments ceased to have any place in the series. Yet, aS we learn by 
examination of the Coal Measures in Ohio, the limestones do not con- 
tinue to increase in thickness indefinitely toward the west, but, on the 
contrary, toward the western margin of the coal field they thin out and 
disappear. The reason of this I have given in the analysis of the struc- 
ture of the Cincinnati arch (Vol. J., Part I, p. 98), where I have shown 
