10 CHICAGO ACADEMY OF SCIENCES. 



C. Chamberliii, I have had the benefit of counsel and guidance, 

 which has been of great semce and which it is a pleasure to ac- 

 knowledge. 



The studies in Illinois, upon which the present paper is based, 

 were entered upon in 1886, near the beginning of my study of 

 Pleistocene deposits, and the greater part, of the work in the Chi- 

 cago Area Avas done in that and the following season. The study 

 in Northern Illinois was carried into greater detail than in most 

 of the territory subsequently covered, chiefly because of the com- 

 plexity of the features. Nearly every township was traversed at 

 intervals so close as to give opportunity to see practically all of 

 the surface, to examine the best natural exposures, as well as rail- 

 way cuttings and other artificial exposures, and to collect records 

 of nearly all the important borings. Portions of the district here 

 discussed have been recently re-examined. This has in a measure 

 made good ttie loss consequent upon the lapse of time, and has 

 brought the interpretation into harmony with the present state of 

 knowledsfe of Pleistocene events or stages. 



