REVIEW OF GEOLOGICAL STRUCTURE. 11 
have called the Columbus limestone, a bluish gray and thin-bedded stratum 
above, opened in the quarries at Sandusky and Delaware, and designated 
in Volume I of this report, the Sandusky limestone. Professor N. H. 
Winchell, who made the surveys of a number of counties in the central 
and northwestern portions of the State, in his reports on Delaware and 
Paulding counties (Geol., Vol. II, pp. 272, 335), proposes to place the 
Sandusky limestone in the Hamilton Group, and divide the Corniferous 
into two members, which he identifies with the Onondaga and Corniferous 
limestones of New York. There seems, however, to be no good ground 
for this classification. The distinction between the Onondaga and Cor- 
niferous limestones of New York is not marked nor constant there, and 
the whole formation is now generally regarded by geologists as one, and 
to this the term Corniferous is applied. It is quite certain that no evi- 
dence has yet been obtained which can be relied upon for identifying the 
Onondaga limestone in Ohio. 
In regard to the position of the Sandusky limestone, it must be said 
that the weight of evidence is in favor of retaining it in the Corniferous. 
It is true that the Hamilton period is but a continuation of the Cornif- 
erous; the Hamilton strata being deposited in the same basin, and from 
the same sea, but at a time when this had become somewhat shallowed, 
and its sediments were more earthy and carbonaceous. There is even 
in New York much in common between the fossils of the two groups, and 
all the fossils which Professor Winchell relies upon as criteria for distin- 
guishing the Hamilton from the Corniferous, are found in both; hence, 
their presence in the Sandusky limestone is no proof of its Hamilton 
age. It should also be said that quite a number of fossils are found in 
the Sandusky limestone which are regarded as characteristic of the 
Corniferous, such as: Spirifera acuminata, 8. gregaria, Strophodonia hemis- 
pherica, Pentamerus aratus, Tentaculites scalaris, as well as the fishes, 
Onuchodus sigmoides, Macropztalichthys Sullivanti, Rhynchodus secans, Machx- 
racanthus major, ete. 
THE HAMILTON GROUP. 
As originally defined by the New York geologists, the Hamilton group 
of New York consisted chiefly of blue calcareous shales traversed by a 
thin band of impure limestone—the Encrinal limestone—and capped by 
another, the last of the caleareous sediments of the Devonian sea, called 
the Tully limestone. This group rests on a black shale—the Marcellus— 
and is overlain by a similar carbonaceous deposit, to which the name 
Genesee slate was given. These strata are highly fossiliferous, containing 
many species which are peculiar to the group, with also a considerable 
