INTRODUCTION. 
Tue publication of the Final Reports on the Geology of the State of New-York made 
known, in a very satisfactory manner, the true order and sequence among the strata, from 
the older crystalline, or azoic rocks, to the Carboniferous system. The subdivisions then 
proposed had been wrought out with much care, and though in their main features based 
upon lithological characters, had nevertheless been studied in reference to their fossil 
organic contents, and each group was illustrated by a few typical species. Still it remained 
to ascertain more conclusively whether the divisions proposed were, in all cases, important 
and distinct groups, characterized by an assemblage of fossils sufficiently numerous and 
constant to be reliable over wide areas, and under considerable lithological variations. 
With this view, the study of the paleozoic features of each group has been pursued with 
attention ; and though the entire series is yet incompletely made out, there is still enough 
known to prove more conclusively the propriety of retaining the subdivisions heretofore 
established. 
In proposing the groups at that time adopted, and uniting the whole series of strata as a 
single system, it was not for want of means of distinguishing the limits of the more impor- 
tant subdivisions. The entire succession was regarded as forming a series intimately linked 
together by the nature of its organic contents, and showing no very important changes, 
till we arrive at the base of the Old Red Sandstone. It is true, we found no difficulty in 
recognizing, in a general manner, the subdivisions of the Silurian system, and also, in the 
higher part of the series, a large number of fossil species identical with Devonian forms. 
Still, if we are to admit among species of Silurian date, those forms from our Hamilton 
group, so analogous, and in several cases identical, with those of the Ludlow rocks of 
England, it becomes impossible to recognize, as distinct from these, any Devonian rocks. 
Indeed we find the species here alluded to, so mingled with others of Devonian character, 
that we are forced to admit the whole as belonging to the latter system. Tracing the suc- 
cession downwards, also, from this latter point, we find, in the organic contents, no evidence 
of a change of sufficient importance to indicate the commencement of another great division 
of the system, till we pass below all those rocks (the Corniferous and Onondaga limestones) 
charged with Favosites, Cyathophylli, and other forms, of which many appear to be 
