INTRODUCTION. 
41 
from those below on certain paleontological grounds; giving, among 
other evidences, the occurrence of Spirifers with dichotomizing ribs*. 
In this place, before proceeding to the consideration of the higher 
groups, it may be useful to present some facts relative to the supposed 
equivalency of certain formations, and their relation to the lines of 
subdivision between great systems. 
It is acknowledged that we have very satisfactory evidence of the 
parallelism of the Niagara group with the Wenlock formation of Eng¬ 
land, from similarity of position, analogy, and, to some extent, iden¬ 
tity in fossil remains. We have seen that this group, together with the 
Onondaga-salt group, were deposited in an ocean which had suffered 
little change in its bed, or in the direction of its accumulations, from 
the period of the Trenton limestone : the general trend of the forma¬ 
tion and the line of accumulation have been similar. At the close of the 
latter epoch, there lived those peculiar crustaceans which are supposed 
by Sir Roderick Murchison to mark everywhere in the northern hemi¬ 
sphere a corresponding zone. Thus far we are able to find a pretty 
satisfactory parallelism with European formations; and thus far the 
depositions have gone on in the same general direction, and have been 
spread over the bottom of the same pre-continental ocean. 
We see, at the commencement of the Lower Helderberg group, that 
there are evidences of great physical changes in the bed of the ocean; 
so that the sediments of this epoch were confined to a narrower limit 
than the preceding group, and do not reach westward over the same 
area, but trend in a northeast and southwest direction. This condition 
continued through the period of the Oriskany sandstone, which followed 
with comparative quiet the previous area of deposition, and in some 
places forms with it almost a natural group of strata.. 
Notwithstanding, however, the great physical change which preceded 
the Lower Helderberg deposits, the materials are in character not 
* In the west, there is a Spirifer with dichotomizing ribs in strata of about the age of the Niagara 
group. 
[ Palaeontology III.] 6 
