PALiEONTOLOGY OF NEW-YORK. 



which bear the character and hold the position of the Onondaga-salt 

 group. 



During the explorations carried on in Iowa in 1855, we discovered 

 along the Mississippi river, at Le Claire and above, beds of drab lime- 

 stone having the chemical composition, litliological aspect, and position 

 of the higher beds of the Onondaga-salt group ; while the thin-bedded 

 lower portions present numerous small arched cavities, precisely of the 

 character of those which in New-York contain the gypsum beds. In 

 tracing these into the interior, they soon become lost beneath the accu- 

 mulated drift. From all these circumstances it is apparent that this 

 formation has once extended over the intervening space, and that its 

 continuity is broken only by the extensive denudation which has swept 

 over the entire lake country with such resistless force. 



In the next superior strata, the Waterlime group of New- York, we 

 find the horizon in this country of those peculiar crustaceans, the 

 Eurypterus and Pterygotus, which will be described and illustrated in 

 this volume. 



The relations of strata bearing similar organisms in Great Britain 

 has lately been discussed by Sir Roderick Murchison*, and they acquire 

 an importance from being regarded as the uppermost beds of Silui'ian 

 age in that country. 



Limiting our comparison to Western New- York, and the continuation 

 of the same beds westward, this view of the sequence would be entirely 

 applicable ; for we find the Waterlime group succeeded everywhere by 

 strata bearing iclithyic remains. When we turn to Eastern New-York, 

 however, we find the Eurypterus beds succeeded by a fossiliferous 

 formation, which is intercalated between them and the strata bearing 

 the earliest known evidences of the existence of fishes. Here the 

 Lower Helderberg limestones, not known at the west, rest upon the 

 Waterlime group, and to that group succeeds the Oriskany sandstone ; 

 all these preceding the fish-bearing strata. 



• Mi-RcniBoii's " Lesmahago Silurians :" Quart. Jour, of the Gool. Soc. London, Vol. xil, p. 15 et 

 fa$tim. 1856. 



