Erosion C'l/cle.s- in Xorth irasfern Illinois. — I/ersfte//. 81 
that the streams which baselevelecl our district flowed into 
the sea at the liead of tiie Mississippi enibayment. Approxi- 
mating- the distance as 400 miles, and allf)wing a gradient of 
two inches as a minimum and five inches as a raaximvim per 
mile, we should arrive at an altitude for the plain in the posi- 
tion now occupied by the city of Freeport of respectively 66 
feet and 166 feet. The latter figure is probably more nearl}'' 
correct than the former, and by adding 275 feet for the maxi- 
mum hight of the ''mounds" we should have 441 feet above 
the sea as the altitude of the highest point of our district 
(and probably of all districts in the present state of Illinois) 
at the close of cycle No. 2. Although little reliance can be 
placed on the accuracy of the foregoing (and subsequent) es- 
timates of the altitude of northwestern Illinois during differ- 
ent erosion cycles, they may be considered as indicating, in a 
general way, the amount of elevation during each uplift of 
the region. It is proposed to prove that our district was up- 
lifted by a series of movements of elevation, perhaps epeiro- 
genic in quality, culminating at the close of the five preglaeial 
erosion cycles. If there were movements of depression, their 
record has been destroyed by subsequent erosion. 
Peneplain JVo. 3. The main features of this peneplain have 
been already described. It is only quite recently that the 
writer first recognized its existence. As suggested before, 
many observers on a hasty reconnoissance of the region might 
deny its existence entirely. But upon proceeding eastward 
from Freeport and taking a position on the blulfs of the Pec- 
atonica valley, the facts can not be controverted that all the 
ridges adjacent to the present canon-like valley rise to the 
same hight, which is about 80 feet above the river level; and 
that this hilltop i)lane is about 100 feet lower than the ordi- 
nary upland surface several miles back from the river. These 
80-foot ridges undoubtedly represent a base level of erosion. 
The peneplain of the higher upland ridges, the basin-like val- 
leys above the level of the 80-foot ridges, and the present 
canon-like valleys, are all carved from a homogeneous fornui- 
tion, the Galena limestone. Furthermore, the facts that 
similar broad and shallow basins have been excavated below 
the surface of peneplain No. 2 throughout the region, and that 
the bottom of these basins always bears about tlie same rela- 
