740 LHE JOURNAL OF GEOLOGY. 
ELEVATION AND SLOPE OF PREGLACIAL VALLEY FLOORS COMPARED 
WITH PRESENT STREAMS. 
The Mississippt Valley.—In this discussion we assume for con- 
venience that the upper Mississippi found its continuation from 
the mouth of the Wapsipinnicon through Scott and Muscatine 
counties, Iowa, to the preglacial valley below Muscatine. Ona 
subsequent page we discuss the comparative elevations and slopes 
of the valley floors of the upper Mississippi and the [Ilinois. 
In preparing the table which follows some difficulty was 
found in deciding upon distances. In the portion above Cairo 
there are few oxbows or other deflections from a direct course, 
hence the distances at high and low water stages are not greatly 
different, and the high water distance is usually taken. In the 
portion below Cairo the oxbows and deflections of the stream are 
numerous. The low water stage involves, therefore, a much 
greater distance than the high water stage. The distances given 
are the low water route. They are taken from Dana’s Manual. 
The distance from Cairo to Memphis at high water stage is about 
15G miles, or only two-thirds that of the low water stage. 
Between Memphis and the Gulf the distance at high water is 
estimated to be but 500 miles, or 350 less than the low water 
stage. Taking the direct course, the rate of fall between Cairo 
and Memphis at high water stage is 8.24 inches per mile, while 
between Memphis and the Gulf it is 5.23 inches per mile. Some 
interesting differences in the rate of fall between certain points 
at high water and low water stages appear in the upper Missis- 
sippi. Thus for a few miles above each of the rapids there is a 
greater fall at flood stage than at low water. The low water 
stage is below the usual rate of fall, owing probably to the 
barrier presented by the rock floor at the head of the rapids. 
This barrier has little effect upon high water stages, and the 
descent of the stream is, therefore, nearer the normal. 
It is probable that the borings at Fort Snelling, St. Paul, 
Dubuque, Sabula, Fort Madison, Bellefontaine and East St. 
Louis, enter the deep part of the preglacial channel, though they 
may not mark the very lowest limit of erosion. The other bor- 
ings probably strike shelves bordering the channel when not on 
