404 
PALAEONTOLOGY OF NEW-YORK. 
only possible to conceive, but highly probable, that as we approach the 
source of the sedimentary matter marking the beginning of the Oriskany 
period, we may find the same or similar physical conditions prevailing 
throughout a long period, and uniting by insensible gradations and alter- 
iiations of the sedimentary material the lower and the higher groups. 
When, therefore, we observe the absence of the Upper Helderberg lime¬ 
stones in some eastern localities, and the coarser sedimentary character 
of much of the Harhilton group and succeeding formations, we are pre¬ 
pared to understand how, in the absence of calcareous deposits, we may 
have a continuation of sedimentary formations from the base of the 
Oriskany sandstone to the Coal period; a condition which actually does 
exist in some parts of Pennsylvania, though marked at intervals by 
changes indicating lines of separation between groups which are else¬ 
where more strongly characterized. 
From the Reports of the Canadian Geological Survey, we learn that 
the physical conditions in the northeastern part of that territory, from 
the beginning of the Oriskany period, continued with little change through 
a long interval, and so uniform as to have prevented, up to the present 
time, the establishment of any lines of subdivision among the strata, 
which, in their lower part, bear fossils characteristic of the Oriskany 
sandstone, and in their higher members those which mark the period of 
the Hamilton and Chemung groups of New-York. 
Therefore while.in the central part of their extent we must regard the 
Oriskany sandstone as more closely allied in its fauna to the lower rocks, 
we find, in other localities both in Pennsylvania and in Canada, in the 
absence of the calcareous members of the higher groups, a more intimate 
relation between the Oriskany sandstone and the succeeding sedimentary 
formations ; making it, in fact, under these circumstances, the base of a 
set of strata which culminate in a period favorable to the production of 
land plants, and which again in other parts, in the absence of calcareous 
beds, form an unbroken series to the base of the Coal measures. 
These relations are important for consideration ; since, in the classifica¬ 
tion of the palaeozoic rocks, geologists are at the present time inclined 
