GEOLOGICAL AND NATURAL HISTORY SURVEY. 57 



Davis. — Professor W. M. Davis has published a description 

 of the Chicago outlet in the Popular Science Monthly (2). His 

 paper was based upon a personal inspection of the channel with 

 the topographic maps in hand, and is a remarkably clear discussion 

 of the features. 



Guthrie. — Mr. Ossian Guthrie, of Chicago, has contributed 

 several newspaper articles and has published two or more pam- 

 phlets within the past few years which contain popular accounts of 

 local features in the region under discussion and which have served 

 to stimulate interest in its geological history. Having been a resi- 

 dent of the city for about 50 years and an enthusiastic student of 

 natural features he is in possession of a fund of information which 

 it is to be hoped will be placed on record in suitable form for refer- 

 ence. 



THE CHICAGO OUTLET. 



The name "Chicago Outlet" has come into use by geologists 

 and engineers without announcement or conference among 

 writers, to designate the line of southwestward discharge from the 

 basin of Lake Michigan across the low divides near Chicago and 

 thence down the Des Plaines and Illinois to the Mississippi. It 

 may appropriately embrace both the points of discharge from the 

 lake to the Des Plaines, namely, the one entering at Summit and 

 the one at Sag Bridge, 



When the lake was occupying the highest beach the north or 

 main outlet was entered about three miles southwest of Summit; 

 when occupying the second beach the outlet was entered at Summit ; 

 when occupying the third beach the point of entrance appears to 

 have been transferred eastward nearly to the present shore of Lake 

 Michigan, as explained below. Similarly the southern outlet was 

 lengthened eastward with the lowering of the lake. The point of 

 entrance at the time of the highest beach being about five miles 

 east of Sag Bridge, at the time of the second beach near Blue 

 Island, and at the time of the third beach at Riverdale. This 

 relationship of the several beaches to the outlets and the eastward 

 lengthening of the outlets may be readily understood by a glance at 

 the accompanying map. (Plate 3.) 



There have, been several surveys which have contributed con- 

 tour maps of portions of the Chicago outlet and the plain covered 

 by the lake in the vicinity of Chicago. The Chicago Drainage Com- 

 mission have prepared an excellent map with five-foot contours 



[21 The Ancient Outlet of Michigan,- by Prof. W. M. Davis. Popular Science 

 Monthly, December, 1894, pp. 218-229. 



