758 THE JOURNAL OF GEOLOGY. 
the preglacial channels of east-central Illinois have a sufficiently 
low depth to correspond with the low altitude at Princeton or at 
Oxford. These borings which show so low a rock floor all occur 
in a district very heavily covered with drift. The cause sug- 
gested for the low altitude at Princeton may find application in 
the entire field of heavy drift between Princeton and Oxford. 
The Lake Michigan basin and outlet.—It is yet to be deter- 
mined whether the Lake Michigan basin is connected on the 
south with a preglacial tributary of the Mississippi by a channel 
so low even as the floor of the Oxford borings. We have 
already noted that the most probable line for such a connection 
would be southward from Michigan city to the Wabash, near 
La Fayette, and thence west to Oxford. But the lowest alti- 
tudes found along this line, north from La Fayette, are much 
above that of the rock floor at the Oxford boring. The boring 
at Monticello, Indiana enters mock aljy4 O7aete tae ens be meiniciiamret 
Winamac at 490 feet and that at North Judson at 497 feet. The 
lowest altitude yet found near the border of the southern portion 
of the Lake Michigan basin is 350 feet A. T. 
A line following the longer axis of the basin descends from 
350 feet at Michigan city to about sea level opposite Racine, 
then rises above 200 feet A. T. opposite Milwaukee, north from 
which it descends to 289 feet below sea level, and continues 
below sea level nearly to the northend of the lake. As the lake 
bottom is probably coated to considerable depth with glacial 
deposits and lake sediments, the rock floor will show even 
greater range. 
Cross sections of the basin show interesting variations. In 
the deep portion of the basin opposite Racine there is very little 
irregularity. The bottom is as smooth as it is in the southern 
end where the drift has concealed all the irregularities of the 
rock floor. East from Milwaukee the lake charts show a single 
narrow ridge in an otherwise smooth bottom. Between Port 
Washington, Wisconsin, and Muskegon, Michigan, a cross sec- 
tion presents the appearance of a series of escarpments facing 
westward like the Niagara escarpment of eastern Wisconsin. 
The highest points exceed 300 feet Av P.°) Mor some Wdistance 
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