S4 PALAEONTOLOGY OF NEW-YORK. 



A comparison of the species shows that the fossils of the Lower 

 Helderberg i*ocks are analogous to those of the Niagara group, and 

 contain among them certain species which we regard as representative 

 forms of the Silurian species in Europe ; and we cannot do otherwise 

 than retain this series as a member of the Silurian system. 



It would seem, therefore, a very natural inference, since the pre- 

 gence of the Genus Eurypteros is regarded as piarking the uppermost 

 strata of the Silurian system of Great Britain, that our Lower Helder- 

 berg group constitutes a series of strata not recognized, and probably 

 not existing in the British islands. 



The sequence of these groups, as occurring in the eastern and western 

 parts of New- York, and their equivalents in Great Britain, is as fol- 

 lows in the descending order : 



Eatttrn Nevi-York. Wettern Ntvs-York, Ortat Britain.' 



Uppkr Heidebbiro OBOi-p, Upper Heldebberg group = Devonian or Old Red sandstone, 



with fi.th remains. 



OrISKANT SANnSTONE, ) „ . . „ „ . 



> Not occurring m Western New-Torlv. 

 Lower Ueldebbebq group. ) 



Wateblihe group, W^ATERiiiiiE oitocp, = Horizon of Lesinaliago, with 



Eurypterus. 



OnONDAOA-SALT group, 0.V0NDAGA-8ALT GROUP. 



XiAGARA GROUP, NIAGARA GROUP, = Wenlock limestouc. 



The relations of these crustacean beds to the earliest Ichthyolite 

 beds in Great Britain is very clearly shown by Sir Roderic Murchison, 

 with the important conclusions derived therefrom. But while there the 

 conditions of the ocean apparently admitted of the direct succession 

 of the two faunae, we have in the United States a very strongly marked 

 line, which has been traced over more than fifteen hundred miles of out- 

 crop, with no mingling of material or of the faunae.* 



It is interesting to observe these points of difference in the conditions 

 of the two continents, at the time of the distribution and deposition of 

 these sediments and the accumulation of calcareous material. These 

 discrepancies are moreover suggestive of enquiries as to how far the 



* Notwitbataudiug tkis evidence of separation over so wide an area, I am still diH|)0Hcd to believe that 

 we m»y find localities which, from the accumulation of material having been uninterrupted, will show a 

 gradual passage from one to the other of these formations, if not a mingling of the two fauna:. 



