XX ABSTRACT OF VOLUME. 



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 discussion of the present beach of Lake 

 Michigan. 



Chapter XII. Influence of the drift on drainage systems and drain- 

 age conditions. — 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 on the Iowan and Illiuoian drift sheets 

 than on the Wisconsin. This is found to be due to differences in age and not to 

 natural advantages for discharge. The Wisconsin is, ou the whole, more favored by 

 une,ven surface for the rapid development of drainage lines than the Illiuoian. The 

 several drainage systems are discussed in considerable detail. 



Chapter XIII. Average thickness of the drift in Illinois. — Illinois 

 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 compara- 

 tive 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 arc thus two quite differ- 

 ent 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 out- 

 crops. 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. Con- 

 sideration 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 cor- 

 rections are made. By this method the thickness of the drift is found to be not less 

 than 100 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. Combining the two methods, 

 the average thickness of the drift of Illinois can be placed at not more than 130 feet 

 and not less than 100 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. 



Chapter XIV. The wells of Illinois. — This chapter aims to present all 

 the reliable well records obtained within the State which throw light upon the 

 deposits penetrated as well as upon the character of the water supplies. In addition 

 to the wells which terminate in the drift there are included many which extend 



