3S 
PALAEONTOLOGY OF NEW-YORK. 
and southern parts of Tennessee, the collections of fossils are so like 
those from the Helderberg mountains, near Albany, that but for their 
color and here and there a difference in the development of certain 
forms, there would be little to distinguish the two localities. 
During the period of the deposition of the Upper Helderberg lime¬ 
stone, the area of accumulation corresponded to the direction of the 
ancient currents and spread far westward, as at the period of the 
deposition of the Iludson-river group; but during the accumulation of 
the Lower Helderberg formation, the condition .of the ocean on the 
west was such that no deposits were made, and, so far as we know, 
no fauna existed over a very large area. At the same time, along the 
line of the Lower Helderberg group, marine life was more prolific than 
at almost any previous period. We already know nearly three hundred 
species from this group; and this number does not include some 
forms known in Gaspe, and others which occur in greater numbers in 
Tennessee. 
Whether on the west there existed a deep or a shallow sea, or what 
the conditions were, we have no means of knowing; for the entire inter¬ 
val between Central New-York and the Mississippi river, and from the 
northern limits of the Silurian strata on the great lakes to' the mouth of 
the Ohio, afford no evidence of a fauna of the age of the Lower Helder¬ 
berg group. 
Nor is this absence due to subsequent denudation, for we are able 
clearly to trace the thinning out of the beds as well as the lines of the 
greatest accumulation and greatest vitality; and these lines are appa¬ 
rently correspondent and co-extensive. 
Influenced by considerations of the physical conditions alone, we 
would naturally incline to make the base of the Lower Helderberg 
group a line of separation between the geological systems. Great physi¬ 
cal changes had taken place ; the relative positions of sea and land, or 
at least the relations of the previously formed deposits to the sources 
of these deposits, had been materially changed. The older sediments 
had become in some degree consolidated, and had likewise suffered 
