32 
PALAEONTOLOGY 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, lithological 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 Silurian 
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 ichthyic 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. 
•Murchison’s “ Lesmahago Silurians:” Quart. Jour, of the Geol.’Soc. London, Vol. xii, p. 15 et 
passim. 1856. 
