30 
PALAEONTOLOGY OF NEW-YORK. 
In New-York, the upper part of the Niagara limestone is well deter¬ 
mined by the sequence of the soft, marly, and shaly deposit of the 
Onondaga-salt group. In proceeding westward, however, the contrast 
in the succeeding formation is not so great; the beds are more calca¬ 
reous, and finally become fossiliferous, so that we must either admit an 
intermediate member of the series, or recognize the lower member of 
the Onondaga-salt group to be fossiliferous. In the present state of our 
knowledge, this horizon is one promising interesting results to investi¬ 
gation. 
In the second volume of the Palaeontology of New-York, I have 
given some fossils from Canada West, as coming from the Onondaga- 
salt group. These were of species not recognized in the Niagara group, 
and some of them of genera unknown in that period. Moreover, 
some of them were similar to a few forms obtained in New-York, from 
excavations made at the base of the Onondaga-salt group; and con¬ 
sequently I inferred these to be of the same age. A farther examination 
of the fossils themselves induced some doubts regarding the propriety 
of this reference; and my examinations in the west have rendered it 
quite certain that there is intercalated between the Niagara and the 
Onondaga-salt group a distinct set of strata, marked by a fauna which, 
partaking in part of the character of the Niagara, is yet quite distinct. 
At some points on the northern shore of Lake Michigan and elsewhere 
in the lake region, there occurs a light-colored limestone lying above 
the Niagara strata, containing generally few fossils, and among them 
some forms not unlike those of Galt in Canada West. 
In Iowa, at the Rapids of Le Claire, and extending northwesterly 
into the State, we find resting upon well determined Niagara limestone 
a light, gray, porous limestone, with numerous casts of fossils, all of 
which are unlike those of the Niagara group, and of species so peculiar 
as to render it necessary to refer the rock and its fauna to an epoch 
distinct from the Niagara period. 
Although the facts yet collected have not that perfect connection 
which we could desire, there is still sufficient known to warrant 
