26 
PALAEONTOLOGY OF NEW-YORK. 
The Niagara group, which is the most important formation included in the 
second volume of the Palaeontology, preserves, apparently, its most com¬ 
plete development in the State of New-York : there it consists of a calca¬ 
reous shale and succeeding limestone, the former of which contains its 
most marked and peculiar fossils, particularly the Crinoidea, Brachiopoda, 
and Trilobites; while the limestone, often for miles in extent, bears the 
character of a coral reef, with few fossils except corals. In tracing this 
group eastward through New-York, we find it gradually diminishing in 
thickness, until, before reaching the Helderberg mountains, it is reduced to 
a band of limestone, sometimes brecciated, and often associated with a 
concretionary calcareous shale which is nearly or quite destitute of fossils. 
Its most easterly recognized extension is on the Hudson river, where it is 
very obscurely developed, and not everywhere continuous. In a southwest 
direction, along the Appalachian range, it is not conspicuous; and although 
I have traced it in its varying phases as far as Virginia, still in no locality 
examined has it attained any important thickness*. 
In the northeast, while there are certain indications in the fauna of the 
existence of this group, it does not seem to have acquired a full develop¬ 
ment ; for though a great abundance of fossils have been there obtained 
from the horizon of Niagara or the upper part of the Clinton group, there 
is a constant absence of certain forms which are peculiarly Niagarian, while 
many of the most predominant are such as occur* in the limestones which 
in New-York we include in the upper part of the Clinton group, but which 
in some instances bear stronger relations to the Niagara than to the Clinton 
group in its lower members. 
Probably no line of separation between our several formations requires 
so much careful examination and revision, as that between the Clinton and 
Niagara groups, when studied with a view to the true grouping of the 
fauna. The want of constancy in physical conditions during the period of 
the Clinton group, and particularly at the close of the epoch, induced the 
New-York geologists to include in that group all the variable formations 
• Both the Niagara and the Lower Helderberg groups, as well as the Onondaga-salt group, are included 
by Prof. Rogers in No. VI of his Pennsylvania formations. 
