WELLS OF ILLINOIS. 555 



there is no marked disturbance along the Mississippi north from that point. 

 From the Cap au Gres disturbance southward to the Ozark Ridge, in south- 

 ern Illinois, a different field is found. Disturbances are frequent along the 

 Mississippi. There is also in this district a more abrupt descent in the floor 

 of the Coal Measures within a few miles east of the Mississippi. Thus, in 

 passing from the east bluff of the river in western St. Clair County to 

 Belleville a descent of 650 feet is made within a distance of 10 miles. 



ESSENTIAL CONDITIONS FOE ARTESIAN WELLS. 



Since the essential conditions for obtaining artesian wells have been 

 discussed at some length by Prof. T. C. Chamberlin in a report of this Sur- 

 vey, 1 only a brief outline of their conditions is here attempted. The essen- 

 tial conditions for artesian wells are: (1) A suitable exposure of a porous 

 rock in a humid region, i. e., a favorable absorbing area; (2) the extension 

 of the porous bed from the absorbing area out underneath regions having 

 a lower altitude, i. e., a favorable transmitting area; (3) a partial or full 

 obstruction to the escape of the waters at a lower level than the absorbing 

 area. The porous rock is usually confined between beds which are less 

 porous and which act as a partial or complete obstruction to the escape of 

 the waters. It is not necessary, however, that these beds should be per- 

 fectly water-tight; indeed, such is rarely the case. It is only necessary 

 that the confining beds should be such as to prevent most of the water from 

 escaping. In some cases the water contained in semiporous beds overlying 

 the porous rock aids in preventing- the escape of water from the porous bed 

 at points between the absorbing area, or fountain head, and the well. In 

 connection with this condition Professor Chamberlin remarks: 2 



I conceive that one of the most favorable conditions for securing a fountain is 

 found where thick, semiporous beds, constantly saturated with water to a greater 

 height than the fountain head, lie upon the porous stratum and occupy the whole 

 country between the well and its source. This is not only a good but an advan- 

 tageous substitute for a strictly impervious confining bed. Under these conditions 

 limestone strata reposing on sandstone furnish an excellent combination. 



This condition jirevails extensively in northern Illinois. The absorbing 

 area for the artesian waters of northern Illinois is found in southern Wis- 

 consin, the porous rock thence dipping southward to northern Illinois. 



'Fifth Anu. Kept. U. S. Geol. Survey, 1885, pp. 131-173. 

 2 Loc. cit., p. 140. 



