188 On the Silurian System of the Lake Superior Region. 
After examining the eastern shore of Green Bay, I took up my 
observations upon the same limestones at Depere, tracing them 
along the Fox river to the outlet of Lake Winnebago. Subse- 
quently, in order to connect the geology of the Lake Superior 
district with that of the Chippewa district, I continued my exam- 
inations across.the state of Wisconsin to the Mississippi river, and 
thence, at intervals, along that river to the falls of St. Anthony. 
At Mineral Point, the lower members of the series have become 
very argillaceous, weathering into a light drab color, and are char- 
acterized by numerous fossils. At Plattsville, the Birds-eye lay- 
ers are pretty well:defined, though associated with much shaly 
. matter and some layers of dark shale. The rock, on being 
freshly fractured, is very dark-colored, but weathers to an ashen 
hue. The Trenton limestone, which succeeds, is thin-bedded 
and light-colored, and all that remains is scarcely more than twen- 
ty-five feet in thickness. ‘The same features are observed at Ga- 
lena, and again at Dubuque. At the latter place, the connection 
of this group with the succeeding limestone is very obvious. 
all these localities, the entire thickness of these lower limestones, 
which can clearly be identified with the Trenton and associat 
limestones of the east, is less than fifty feet; but it is possible 
that some better exposures would give a greater thickness. 
I conceive that there can be no’ longer a doubt in reference to 
the age of the limestones under consideration. Their identity 
with those of New York and of Canada has been established, 
not only by a comparison of the fossils, but also by tracing al- 
most continuously their range from the Mohawk, Champlain, and 
Black-river valleys, throngh Canada, to the eastern limits of this 
district, and thence westward, continuously, to the Mississipp! 
river. 
These remarks also apply to other groups, concerning which 
some difference of opinion has heretofore prevailed. Feeling the 
necessity of adopting some recognized standard, we have refer 
these subdivisions, so well marked, to those which have already 
been made by the New York geologists, 
Before leaving the subject of these limestones, it will be neces 
sary to recall to mind some observations made on the Escanaba 
— 
i=) 
have observed farther east. In going westward, I had not an 0p- 
portunity of observing the overlying deposits of the Trenton 
limestone until I arrived in Wiscousin. Here, in numerous local- 
ities, as well as in Illinois and Iowa, the deposit above that which 
is marked by an abundance of fossils characteristic of the Tre™ 
ton, is a grey, or drab-colored limestone, and very friable, forming — 
