98 The America)! Geologist . Anf,'ust, iis9) 
The ancient Hock-Illinois river was prol)al)ly the largest 
original stream in the area of the j)resent state of Illinois. It 
obliijuely (crossed the Orand de Tour- La Salle axis of u])lift. 
and perhai)S j>ersisted from the first cycle. It was the only 
stream of our district that did cross tliis axis, which is still a 
prominent divide; and it is the only important exception to 
the rule of anticlinal drainage lines. The streams have cut 
l)ack across the axis, some on one side and some on the other, 
so that the watershed does not exactly coincide with the axis 
of uplift. 
In conclusion, I wish to add a few suggestions in regard to 
the age of the Mississippi river along the western boundary 
of our district. It is probably a comparatively recent creation. 
The origin of its present general course cannot be earlier than 
the Tertiary era. If any stream comparable in size with the 
present, Mississippi flowed south across or close to the district 
during pre-Cretaceous time, it must have been captured by 
the headwaters of smaller streams flowing into the Cretaceous 
sea a few score miles west of the ])reseiit course ; for the strong 
tilting of the peneplain along the coastal portion, requisite to 
give the necessary drainage gradient of a stream flowing into 
the head of the Mississippi embayment, would give the small 
coastal streams so great an advantage over the larger stream 
that they could not fail to cut back into its basin and to cap- 
ture it in a short time. Moreover, the distribution of the 
Cretaceous deposits indicates an incursion of the C'retaceous 
sea from the west as far as to the Mississippi river in south- 
eastern Minnesota and, ])erhaps, be3^ond to Adams and Han- 
cock counties, Illinois. Undoubtedly the Cretaceous drainage 
of the upper Mississippi region was mainly westward. 
I am, further, inclined to the opinion that the Mississippi 
river did not flow in its present course across the area occupied 
by the " mounds " until near the 'close of the Tertiar}" era. 
Had a stream so large as the present Mississippi baseleveled 
the district along the present course, it should have more com- 
pletely destroyed peneplain No. 1 in its vicinity. The present 
river passes the chain of "mounds" in a gap scarcely half a 
dozen miles wide. Residuals are generally found away from 
the main lines of drainage. In fact, if peneplain No. 1 is Cre- 
taceous in age, the preservation of any remnants of it in this 
