168 GEOLOGY OF OHIO. 
We see that in Ohio the forces that distributed the Mahoning sand- 
stone were local in their action, and it seems hardly probable that this 
epoch should have been so strongly marked over so great an area as Mr. 
Lesquereux supposes, and yet:-have left so inconstant a record here. 
The Cincinnati arch is also a serious impediment to the acceptance of 
the theory that the Alleghany and Illinois coal fields are identical in 
structure. We have learned in the progress of the Survey that this is a 
very old topographical feature; that it has existed since the close of the 
Silurian age, and that during the Coal Measure epoch it was a barrier 
which somewhat widely separated the two coal basins. It seems almost 
impossible, therefore, that they should have anything more than a gen- 
eral similarity of structure. 
It should be remembered that both these great troughs have been filled 
for the most part by mechanical sediments washed from their margins. 
The Alleghany coal field was filled to the depth of at least 3,000 feet at 
its center, mainly by materials washed from its eastern and northern 
shores; the Illinois coal basin to only half that depth, and it received 
its clays and sands—now sandstones and shales—from the north. Its 
structure must necessarily, therefore, be very different. 
As I have shown on the preceding pages, the Pittsburgh coal thins to 
an edge on the north and west within the margin of our coal area, and 
this is also true of its associate, the “Great limestone”—facts which 
afford us ocular demonstration that these strata never reached through to 
Illinois. It may be said that the two coal basins were once united at the 
southern end of the Cincinnati arch—that is, in Alabama; but there are 
strong geological reasons for doubting this. The Illinois coal field ter- 
minates in western Kentucky several hundred miles further north than 
the extremity of the Alleghany basin, and we have no proof that the 
connection has been cut away by erosion. But even if this were true, 
Safford shows that the southern extremity of the Alleghany coal field 
has a very different structure from that which we find in the Coal Meas 
ures of Pennsylvania and Ohio and in Illinois. Hence, if the coal basins 
were once united at the far south, the connecting links in structure—such 
as the “Great limestone,” Pittsburgh coal, Nelsonville coal, and Putnam 
Hill limestone—were never formed there. It is true that in the Cum- 
berland Mountains we have only the lower coal seams, but these are cer- 
tainly very different from ours. Our Coal Measure limestones are there 
entirely wanting, and they have almost disappeared in Kentucky. This 
is a fact which has somewhat surprised and puzzled me, for we have 
heretofore supposed that the limestones of the Coal Measures marked 
