86 The Aviericait Geologist. Aufjust, 1^96 
entire ])eriod, while the uplift of cycle No. 4 was Yn\nd aiul 
soon terminated, so that during the greater portion of the 
period the earth movement was quiescent. The canon-like 
characteristic of tlie erosion product of (-ycle No. 4 is an im- 
portant ])oint to be remembered, as it will be of great value 
in correlating the different baselevels in different districts of 
the upper Mississippi basin. 
The position of the sea coast at the close of cycle No. 4 is 
unknown, but I will anticipate future studies sufficiently to 
consider an extension of its drainage system to a distance at 
least twice as great as formerly, and, in determining the alti- 
tude of the district at that time, will add, besides the 100 feet 
for the depth of the valley at Freeport, 300 feet on account of 
the increased length of the streams. In this wa}^ we obtain 
a maximum altitude for the "mounds" of 941 feet, and an 
altitude for the baselevel in the Pecatonica valley of 466 feet. 
The present altitude of baselevel No. 4 at Freeport is 720 feet, 
and, as the present water level is not materially different from 
its position at the close of cycle No. 4, it may be urged that 
our estimated altitudes are very much too low. But the 
streams of the Mississippi basin are not on a perfect baselevel 
of erosion. They are depositing extensive alluvial plains. 
and so are the streams of the central Ozarks which are far 
above a baselevel. Our streams are capable of eroding at a 
greater deptli tlian their flood-plains, and I consider the whole 
country above the head of the embayment region to be above 
a true erosion baselevel. So I do not think that 456 feet is 
mnc]i too low an estimate of the altitude of baselevel No. 4 in 
the Pecatonica valley at the end of the fourth cycle. 
JJdseJerel J\^o. 5. This last of the preglacial cycles of erosion 
in our district was instituted by a renewed uplift and a revi- 
val of energy in the streams, which quickly trenched new val- 
leys or canons in the bottom of the former system. An abun- 
dant water sui)i)ly being secured from glacial gravels in the 
valleys, there is little inducement to send down wells to the 
rock-floor of the caiions. Consequently, we know but little 
more about them than their existence, and also that they are 
usually bounded by slopes and rarely by per])endicular blutfs. 
In the few jilaces where these slopes have been tletermined by 
studying them in relation to tlie de|)tii it would seem that the 
