Geology of Johnson County. 409 
the preglacial river channels which have become obliterated, 
at least in part, and the condition of things during the Champlain 
period, are all subjects of the most profound interest to the 
geologist. 
In this paper I have endeavored to elucidate some of these phe- 
nomena as observed in Iowa, and it is hoped that it may not be 
without at least some value to the broad field of science. 
In the hard Devonian limestone in the west bank of the Iowa 
River, at Iowa City, are numerous (often large) very interesting 
potholes, which have been formed by the action of running water ; 
and as they shed so much light upon the condition of things at this 
locality at the beginning of the Carboniferous age, a short descrip- 
tion of a few of the more prominent ones here i is deemed not to be 
without interest and value. 
The rocks in which these potholes are formed, rises toa height of 
from twenty to forty feet above the water in the Iowa River. 
Pothole number one: Circular, sides smooth and symmetrical, 
sloping gradually to the bottom, upper margin rounded, depth five 
feet, diameter at the top two foal, and at the bottom one foot. 
Pothole No. 1. 
The following diagram will illustrate this description :— 
One foot from this is another smaller subcircular pothole, having 
a depth of two feet and a diameter at the top of one foot, and at the 
