WELLS OF ILLINOIS. 553 



Lower Magnesian limestone is also used for artesian supply. Portions of 

 the Coal Measures yield brackish water, and for this reason they have not 

 been extensively utilized for water supply. In some localities, however, it 

 has been found necessary to make use of these formations because of no 

 adequate supply from the drift or overlying rocks. In such places the 

 presence of beds yielding comparatively fresh water has been made known. 

 In the detailed discussion which follows, the degree of development of the 

 several rock formations is set forth. 



ATTITUDE OF THE STRATA. 



The attitude of the strata is very favorable for producing - a strong 

 hvdrostatic pressure in deep wells throughout much of the State. A north- 

 to-south section shows a gradual southward dip of the formations (see figs. 

 7 and 8), terminated at the south by an axis of upheaval which, as above 

 noted, leads eastward across the southern end of the State from Grand 

 Tower to Shawneetown. The descent probably amounts to 2,500 or 3,000 

 feet in the 350 miles from the north to the south' end of the State. There 

 are slight undulations carrying the strata up or down 100 to 200 feet or 

 more from a uniform grade, but so far as known no prominent west-to-east 

 axis of upheaval crosses the State north of the one just noted, none adequate 

 to prevent the' southeast passage of the water. 



East-to-west sections are less uniform in the inclination of strata than 

 the north-to-south sections. Sections across the northern part of the State 

 present two blocks of strata, each dipping gradually to the east, separated 

 by an abrupt fold or line of disturbance (see fig. 9). At this fold the 

 block on the east rises abruptly several hundred feet above the neighboring 

 portion of the western block. It is along this line of disturbance that the 

 St. Peter and Lower Magnesian strata are brought to view on the Illinois 

 and Rock rivers and on Elkkom Creek. Its trend from the Illinois River 

 northward is about southeast to northwest. Sections in the lead region 

 indicate that it continues in subdued form some distance into southwestern 

 Wisconsin. Its southward continuation from the Illinois is readily traceable 

 as far as Livingston County by disturbances shown in coal shafts, as noted 

 by the Illinois survey. Farther south its course is less definitely known. 



There is, over much of western Illinois, a gradual descent from the 

 western border of the State to this line of disturbance. In the latitude of 



