Erosio)) Cycles in North ircst em fUlnois. — Ilerxhetj. 99 
clistvict is remarkable, and is explainable only on the suppo- 
sition that they were far distant from the main lines of drain- 
age. Those who are engaged in a special stud}' of the Missouri 
region frequently express the opinion that the great elevation 
of the western half of the Mississippi basin was delayed until 
well on toward the end of the Tertiary era. The upper Mis- 
sissippi may well have flowed in some other course during this 
cycle. 
The comparative recency of the origin of the present Mis- 
sissippi may farther be inferred from the narrowness of the 
up])er trough or basin of the age of baselevel No. 3 in the 
Freejjort section. Ver}'- small creeks in the central portion of 
the district have eroded basins in Galena limestone which are 
rarely less than a mile in width. The Pec-atonica river above 
Freeport excavated a basin three miles wide in the same for- 
mation. The ancient Rock-Illinois river, in crossing the Ga- 
lena terrane, baseleveled an area at least several times as wide 
as the last. On this principle of vallej^s being proportional 
to the size of the streams, so large a river as the Mississippi 
(all other conditions being equal) should have completed a 
basin at the very least fifty miles wide. Instead, through a 
large i)ortion of the stream's course, where it crosses the Ga- 
lena terrane, the baselevel No, 3 is undistinguishable by means 
of the topographic forms. At Dubuque the valley is a canon 
about one mile in width and 250 feet in depth, reaching nearly 
to the general upland surface. If the basin of erosion cycle 
No. 3 is at all represented here, it must be by a 100-foot slope 
at the top of the precipitous bluff on the east side of the river. 
Again, in tracing the upper trough or basin of Carroll creek 
to the Mississippi river, baselevel No. 3 is found well defined 
all the way; but it cannot be located, without a close study, 
in tlie neighboring canon of the great river. In short, the 
weight of evidence tends to prove that no large stream occu- 
pied the position of the present Mississippi on the western 
border of our district during cycle No. 3. But the canon val- 
le}^ is of the proper size for a large river (althougli it does 
seem to be rather narrow at places, as at I)ubu(|ue), and it 
may have been excavated by the Mississippi in its present 
size. This is a very complex subject and will require much 
