20 GEOLOGY OF OHIO. 



.9. The Olentangy Shale. 



At. Prout's Station, seven miles south of Sandusky, Dr. Newberry 

 found fifteen to twenty feet of a highly fossiliferous blue shale, interven- 

 ing between the Corniferous limestone and the great black shale. The 

 fossils that he found in this exposure are proved to be all of Hamilton 

 age, unmingled with those of the underlying Corniferous limestone; 

 and he accordingly described this stratum, of which there are several 

 other exposures in the same region, as the only Hamilton formation of 

 the entire scale of the state. 



It seems probable that this blue shale of northern Ohio is the exten- 

 sion and equivalent of a deposit of shale which Professor N. H. Win- 

 chell found in Delaware county and which he named the Olentangy. 

 This stratum is twenty or thirty feet in thickness, is blue in color, cal- 

 careous in composition, but almost destitute of fossils. Its stratigraph- 

 ical position is exactly that of Newberry's Hamilton. It is found in 

 comparatively few sections of three or four counties of central Ohio. 

 When the rocks of this part. of the series are traversed by the drill of 

 the well-borer, this stratum is likely to be classed with the limestone be- 

 low, rather than with the black shale above, and, as already suggested, 

 the incorporation of this element with the limestone might easily serve 

 to expand the measurement of the latter by a small amount. 



With this formation the great limestones of Ohio were completed. 

 While they are built into the foundations of almost the whole state, they 

 constitute the surface rocks only in its western half. The Upper Silu- 

 rian and Devonian limestones of our scale which were formerly known 

 collectively as the Cliff limestone, have an aggregate thickness of seven 

 hundred and fifty to eleven hundred and fifty feet where found under 

 cover; and though differences exist among them by which it has already 

 been shown they can be divided into four or more main divisions, there 

 is still no reason to believe that any marked change occurred in the char- 

 acter of the seas in which they were formed during the protracted 

 periods of their growth. The life which these seas contained was slowly 

 changing from age to age, so that we can recognize three or more distinct 

 faunas or assemblages of animal life in them. 



Differences are also indicated in the several strata, as to the depth 

 of the water in which they were formed, and as to the conditions 

 under which the sedimentary matter that enters into them was supplied, 

 but no marked physical break occurs in the long history. No part of the 

 entire series indicates more genial conditions of growth than those that 

 the Devonian limestone, last described, and the latest in order of them 

 all, shows. It is the purest limestone of Ohio. The formation consists 

 almost exclusively of the beautifully preserved fragments of the life of 

 these ancient seas. In particular, the corals and crinoids that make a 

 large element in many of the beds could only have grown in shallow but 

 clear water of tropical warmth. 



