512 
GEOLOGY OF THE FOURTH DISTRICT. 
It is in the Cliff formation, as before described, that the lead ores of Illinois, Wisconsin, 
and Iowa are found, a part only of the rock yielding these ores. This portion appears to 
correspond to the Niagara limestone ; the upper part of the “ cliff rock,” which represents 
the Helderberg series, being destitute of metallic veins. Although the rock contains few fos¬ 
sils, these indicate its position to be the same. The Catenipora, which in New-York occurs 
far more abundantly in the Niagara limestone than in any other rock, is found here in great 
abundance, with Aulopora and a few other fossils. The next fossiliferous group below the 
lead-bearing rock on the Misisssippi river, is that corresponding to the Hudson river group. If 
the Clinton group exists, it has there become so incorporated with the rocks above, as to be 
overlooked as a distinct formation. This point requires examination, and it is possible that 
some representative of the group may be found there. Being a very variable assemblage in 
New-York, it would not be surprising to find it under another aspect, or even incorporated 
with the Niagara group, as far west as the Mississippi. Its last appearance in New-York, 
on the Niagara river, is in the form of a mass of limestone twenty-five feet thick, with about 
four feet of shale below ; the limestone at this point has lost the sandy and impure character 
which it has further east, and assimilates more with the lower part of the Niagara limestone. 
The only fossil met with in this mass on the Niagara river, was the Delthyris radiata, which 
is found extending through the shale above, and in the lower part of the limestone. 
In crossing the country from Galena to Chicago, few opportunities offered of examining 
the strata except at detached points. The only rocks seen on the direct route are the two 
limestone formations, the lower including Niagara and Helderberg, and the upper the Carbo¬ 
niferous ; and, from the great extent of level country, I was unable to see the intervening 
rocks. 
I had an opportunity of examining the rocks on the northern and northeastern shores of the 
southern peninsula of Michigan, but my observations were too cursory to admit of any thing 
like a connected view of them. The limestones of the Helderberg series, principally the 
Corniferous and Onondaga masses, form the rocks of many of the bays and harbors ; charac¬ 
teristic fossils of these masses being recognizable in those examined. At Mackinac, the upper 
part of the Onondaga salt group, and possibly a small portion of the Water-lime group, form 
all the rocks seen. The former is partially altered from its usual characters in New-York, 
some parts exhibiting the large angular cavities, the whole appearance being that of a vesicular 
mass; and had the cavities been spherical instead of angular, it would have been referred to 
igneous origin. This mass being isolated, renders it difficult to trace its connection with those 
appearing to the south and east of it; but from its great similarity to the Salt group of New- 
York, and from its apparently passing beneath those representing the Helderberg series, no 
doubt remains as to the propriety of this reference. 
The exhibition of strata along the line of the section given, illustrates some very interesting 
points in the geology of our country, and proves the existence and order of succession in 
certain rocks, and their equivalents over wide areas ; offering us facts which will still further 
enable us to solve the problem of the condition and character of the ocean during the earlier 
geological periods. 
