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 Freeport 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 pf 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- 

 ginia twenty feet in thickness, though nearly half slate — the Pittsburgh, 

 the Redstone, the Sewickley, and the Waynesburg ; while the greatest 

 development of the limestones lies 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. I., Part I., p. 93), where I have shown 



