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settlement, and several papers discussing the beaches and the outlet have 
appeared. ‘The latter has long been known as the Chicago outlet, because it 
led away from the site of that city. The lake has recently been given a 
name in harmony with that of the outlet (Lake Chicago). 
After reviewing the previous reports and papers, the Chicago outlet is 
described in some detail. The several beaches of Lake Chicago are then 
taken up in order from highest to lowest. The chapter ends with a discus- 
sion of the present beach of Lake Michigan. 
Chapter XII, Influence of the adrift on drainage systems and drainage con- 
ditions.— It is shown that many drainage systems are entirely independent of 
the preglacial lines, while others are independent only in part, a considerable 
part of their courses being along the lines of old valleys. The development 
of drainage systems is shown to be much farther advanced en the Iowan and 
Illinoian drift sheets than on the Wisconsin. This is found to be due to dif- 
ferences in age, and not to natural advantages for discharge. The Wisconsin 
is, on the whole, more favored by uneven surface for the rapid development 
of drainage lines than the Illinoian. The several drainage systems are dis- 
cussed in considerable detail. 
Chapter XIII, Average thickness of the drift in [linots.— inois affords 
an especially good opportunity for the estimate of the thickness of the drift, 
because of the large number of well sections obtained, and because of the 
comparative smoothness of the region. The inequalities of the rock surface 
beneath drift plains may be estimated by the study of neighboring driftless 
tracts, as well as by borings and outcrops within the drift-covered area. 
There are thus two quite different methods by which the average thickness 
of the drift may be ascertained. 
The first method here used is that of averaging the results of borings and 
outcrops. These are averaged in each township in which the distance to 
rock is known, and the results are then combined for the average of all the 
explored townships. Consideration is then given to the distribution of the 
explored townships in reference to drift plains and moraines and to preglacial 
uplands and valleys, and necessary corrections are made. By this method 
the thickness of the drift is found to be not less than roo feet, and it may be 
120 feet or even more. 
The second method, based upon a comparison of the Illinois drift area 
with the neighboring driftless tracts, gives 129.3 feet as the average thickness, 
or slightly more than the highest results obtained by the first method. Com- 
bining the two methods, the average thickness of the drift of Illinois can be 
placed at not more than 130 feet and not less than 1oo feet. 
An attempt is made to estimate the part contributed by each ice invasion, 
but the data prove to be scarcely complete enough for a good estimate. It 
is found that the general thickness within the limits of the Wisconsin drift is 
40 to 45 feet greater than in the portion of the state outside. 
