168 GEOLOGY OF OHIO. 



We see that in Ohio the forces' that distributed the Mahoning sand- 

 stone were local in their action, t 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 



