INTRODUCTION. 4^1 



from those below on certain palseontological 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 suft'ered 

 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 gToup, 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 • Spiri/er with dlcbotoiuiziog ribs in strata of about the age of the Niagara 

 group. 



[ PALiKONTOLOOY III.] 6 



