88 
The A iitcricidi (ieoloijisl. 
Auf,'ust, 1)S9G- 
Fici. 1. Ideal Section ACROSS A portion of the I'ecatonica basin near Free- 
port, 111. 
a, Peneplain No. 1. h, Peneplain No. 2. c, I'cneiilain No. ;i. (I, Ba.selcvol No. 4. 
e, Baselevel No. .5. f, Present valley bottom. 
cessary to know {(i) whether the several uplifts were slow or 
rapid and whether they occupied the entire cycle or only a 
small portion of it; (/>) the amount of average annual rain- 
fall in the district during the several periods; (c) the state 
of the vegetation and the climate; [d) the lines of drainage; 
(e) the resistant ])roperties of the several formations eroded^ 
etc. Ignoring all the factors excepting the amount of mater- 
ial removed and the evidences of age presented by the diifer- 
ent topographic forms, the writer would consider the relative 
lengths of the several cj^cles about as follows: Kepresenting; 
the last cycle by 1, the upper canon valleys would require T), 
and the basins constituting the so-called peneplain No. 3 the 
figure 25, to indicate the ratio between them. T'ycle No. 2. 
ending in the completion of peneplain No. 2, might be repre- 
sented by 200; and the first and probtibly most important 
cycle, that which resulted in the formation of the supposed 
peneplain of the summit of the "mounds," might require a 
figure exceeding 500. It is needless to add that these figures 
are mere guesses, liable to modification at any time. 
Correlation. 
It is not proposed to definitely correlate the baselevels of 
northwestern Illinois wuth the series developed and studied in 
other distant districts. Such correlation can be effected hy 
direct tracing from one district to another, but in the Miss- 
issippi basin a diftlculty is encountered in the following facts : 
(«) the several baselevels are so close in vertical distance 
that a very tiiorough study is recpured to distinguish them ; 
(b) in ])assing away from tiie centers of uplift, the various 
