10 



GEOLOaY OF OHIO. 



shallowing and locally complete withdrawal of the Silurian Sea. Such 

 material as composes the Oriskany might possibly be spread some dis- 

 tance from the shore over the bed of that sea in its retreat, but the very 

 decided contrast which the Oriskany presents, both in lithological char- 

 acter and in fossils, to the underlying Helderberg limestones, indicates that 

 it marks the dawning of a new era rather than the close of an old one, 

 and that it was the first product of an incoming sea — the Devonian — 

 rather than the last of the retreating Silurian. 



The Cauda Galli and Schoharie grits are universally regarded as De- 

 vonian, since they contain many of the fossils of the Corniferous lime- 

 stone; they are, in part, the shore equivalents of the open sea Corniferous. 

 In the judgment of the writer, the Oriskany is the product of the earlier 

 and wider spread of conditions similar to those in which the Cauda Galli 

 and Schoharie beds accumulated; and that it is the true base of the 

 Devonian system, corresponding in character and relative position to 

 the Medina and Potsdam sandstones below. 



THE C0ENIFER0U8 LIMESTONE. 



This has proved to be one of the most interesting of all the rocks of 

 Ohio, since it is a vast store-house of fossils, and these — especially the 

 fishes and land-plantS: — hold a prominent place among the objects de- 

 scribed in our volumes on palaeontology. The Corniferous corresponds in 

 general character with the Niagara and Cincinnati limestones below ; 

 that is, it is the organic sediment formed from the debris of animal life 

 which thronged the sea in the age of its deposition, and which slowly 

 accumulated, by the processes of growth and decay, over all the area where 

 deep and clear water prevailed in the submergence that occurred in this 

 age. 



Many new species of fossils have been found in the Corniferous lime- 

 stone since it was described in our first volume, but no facts have been 

 observed which require any important modification of the view presented 

 in that volume in regard to its distribution, history, and the life of the 

 period in which it was deposited. Some interesting additions have been 

 made by Professor Nelson to the collection of land-plants found in the 

 Corniferous limestone at Delaware and Sandusky, and which we have 

 conjectured once formed part of a luxuriant vegetation that covered the 

 Cincinnati island in the Devonian Age, the first land flora of which we 

 have any traces in the United States. These will be described more 

 fully in our third volume of Palseontology. 



It will be remembered that the Corniferous limestone in Ohio consists 

 of two divisions : a whiter and more massive member below, which we 



