On the Silurian System of the Lake Superior Region. 193 
In this estimate, it must be understood that the elevated por- 
tions of the island and peninsula of Mackinae are not included, 
for they are occupied only to a small extent, if at all, by strata of 
this age. 
[The Onondaga Salt Group occurs at Mackinac and St. Ignace, 
but only in thin beds, having diminished in thickness to the west. 
North of St. Ignace Mr. Whittlesey found a marly bed about 50 
feet thick, containing gypsum in isolated masses occurring under 
the same forms as in New York and Canada West. ‘This marly 
bed with some higher and more calcareous ones, represent the 
Onondaga salt group. This bed is not recognized along Green 
Bay or at Milwaukee, and has probably entirely thinned out. 
The Upper Helderberg series, including the Schoharie grit, the 
Onondaga and Corniferous limestone, is largely represented in the 
west, though those subordinate groups cannot always be made 
out. The series, like others, is marked by an increase of calca- 
reous matter on passing westward. ‘The limestones have been 
Ce 
Scribed, but are identical, however, with species which occur in 
the ocks in New York. Some of the smaller bryozéoid 
Skconp Seems, Vol. XVII, No. 50.—March, 1854. 25 
