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- 

 nations 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 Hamilton 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 

 Otiskany sandstone to the Coal period 5 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 tnote 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 



