544 THE ILLINOIS GLACIAL LOBE. 



Illinois. The estimates of average thickness are restricted to Illinois, inas- 

 much as the data collected there are fuller than in portions of neighboring 

 States covered by the Illinois lobe. It is also a geographic unit to which 

 the addition of the neighboring' districts would be of no advantage. Fur- 

 thermore, the amount of drift in these neighboring districts apparently differs 

 so little from that of the adjacent portions of Illinois that the result would 

 not be materially affected by including them in the calculation. The 

 greater part of the borings upon which these computations are based are 

 presented in the detailed discussion of wells below, while their relative fre- 

 quency is shown in the accompanying map (PI. XX). 



If the points at which the thickness of drift has been determined were 

 distributed uniformly over the region, it would be an easy matter to ascer- 

 tain the average thickness, but it so happens that in some parts of the region 

 the thickness has been tested at much more frequent intervals than at other 

 parts. This necessitates a discriminative averaging, by which a district 

 having many borings shall not be made of undue importance in determin- 

 ing the average for the State. For example, 254 of the 1,179 borings are 

 found in five counties of northeastern Illinois, bordering the Kankakee River 

 and Chicago Outlet, and represent an average area of only 16 square miles 

 for each boring, while the average for the State is 45 square miles for each 

 boring. In the remaining area occupied by the Wisconsin drift, there are 

 only 233 borings and each boring represents an average area of 61^ square 

 miles. In the portion outside the Wisconsin drift, the district north of 

 Green River Basin has one boring for every 36 square miles; that south 

 of Green River Basin one boring for 76.2 square miles; that between the 

 Illinois and Kaskaskia one boring for 61.1 square miles, and that east of 

 the Kaskaskia one boring for 78 square miles. In view of these differences 

 it was decided to make an average of townships since they have uniform 

 areas. This was done by averaging the wells entering rock in each town- 

 ship and dividing the sum of the averages for each township by the number 

 of townships in which rock is struck. This method seems preferable to an 

 average by counties (which was the first method attempted), since it is an 

 average of small areas of uniform size and includes only the townships in 

 which rock has been struck. It is found that 644 townships are represented 

 out of a total of 1,474 townships in the glaciated portion of the State, and 



