•• PALEONTOLOGY OF NEW-YORK. 



The Niagara group, which is the most important formation included in the 

 second voUime of the Palaeontology, preserves, apparently, its most com- 

 plete development in the State of New- York : there it consists of a calca- 

 reous shale and succeeding limestone, the former of which contains its 

 most marked and peculiar fossils, particularly the Crinoidea, Brachiopoda, 

 and Trilobites ; while the limestone, often for miles in extent, bears the 

 character of a coral reef, with few fossils except corals. In tracing this 

 group eastward through New- York, we find it gradually diminishing in 

 thickness, until, before reaching the Helderberg mountains, it is reduced to 

 a band of limestone, sometimes brecciated, and often associated with a 

 concretionary calcareous shale which is nearly or quite destitute of fossils. 

 Its most easterly recognized extension is on the Hudson river, where it is 

 very obscurely developed, and not everywhere continuous. In a southwest 

 direction, along the Appalachian range, it is not conspicuous ; "and although 

 I have traced it in its varying phases as far as Virginia, still in no locality 

 examined has it attained any important thickness*. 



In the northeast, while there are certain indications in the fauna of the 

 existence of this group, it does not seem to have acquired a full develop- 

 ment; for though a great abundance of fossils have been there obtained 

 from the horizon of Niagara or the upper part of the Clinton group, there 

 is a constant absence of certain forms which are peculiarly Niagarian, while 

 many of the most predominant are such as occur in the limestones which 

 in New-York we include in the upper part of the Clinton group, but which 

 in some instances bear stronger relations to the Niagara than to the Clinton 

 group in its lower members. 



Probably no line of separation between our several formations requires 

 so much careful examination and revision, as that between the Clinton and 

 Niagara groups, when studied with a view to the true grouping of the 

 fauna. The want of constancy in physical conditions during the period of 

 the Clinton group, and particularly at the close of the epoch, induced the 

 New- York geologists to include in that group all the variable formations 



* Both the Niagara and the I^wer Helderberg groups, as well as the Onondaga-salt group, are included 

 by Prof. Rooms in Mo. VI of his Pennsylrania formations. 



