100 COERELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
The difficulty found in discussing this problem has been due in 
large measure to the lack in common usage of any way to deal with 
the fauna independently of the name and classification of the 
geological formation to which it is said to belong. 
In the present case, in order to treat of the subject in hand with 
the nomenclature already in use, it is necessary to say that the rocks 
and their fossils appearing in the section of Chenango and adjacent 
counties, above the Oneonta sandstone, are either Ithaca, Oneonta, or 
Chemung. There seems to be no other way of designating them; the 
use of the word transition is only an avoidance of decision. But if one 
speak of the formation as Chemung, the necessity arises of assuming 
the fauna to be equivalent to some part of the fauna of the Chemung 
formation where typically exhibited. This, as has been shown, is not 
correct, if by the ' ' typical exhibition " be meant a case in which the sep- 
aration between the Ithaca and Chemung faunas is sharply defined. 
If a case be taken in which the mingling of the two faunas is evident, 
it is not properly a typical exhibition. But in the list of species from 
these rocks in Greene Township, Chenango County, there is an undis- 
puted mingling of a large number of species of the standard Tropi- 
doleptus fauna with a considerable number of species of the standard 
Spirifer disjunctus fauna, and a still larger number of species whose 
most central stratigraphical position is in the standard Ithaca for- 
mation. 
If now we are to deal with the formations as such, the evidence 
seems to be very strong for the opinion that the part of the actual col- 
umn of the Genesee section of western New York, called the Portage 
formation in the reports, when followed stratigraphically eastward is 
represented not only by the Oneonta formation of Otsego and adja- 
cent counties in the eastern part of the State, but by the f ossifer- 
ous beds lower down, and by some, at least, of the fossiliferous beds 
following the Oneonta. 
Even if we were to suppose, with Dr. Clarke, that the Oneonta sand- 
stone is the formational equivalent of the "Portage sandstone, " a 
this does not dispose of the essential problem; because the equiva- 
lency does not include likeness of species in the two formations. 
The fauna in the beds below the Oneonta sandstone is more diverse 
from the fauna immediately preceding the Portage sandstone of west- 
ern New York than it is from the fauna preceding the Genesee shale 
of the same column. The fauna following it is also less like the fauna 
following the Portage sandstone than it is like the fauna of the Ithaca 
formation, which is known to be stratigraphically below it. If the 
formational equivalency were in fact as Clarke supposed it to be, the 
term equivalency would not carry with it the meaning that the beds 
were deposited at the same epoch of geological time. 6 
The actual tracing of the beds step by step across from Otsego to 
Allegany County would settle the question as to time equivalency, 
a Thirteenth Ann. Rept. State Geologist New York, 1893. p. 557. '-> See p. 117. 
