WELLS OF ILLINOIS. 551 



differs from the former class not only in the great lateral transmission, but 

 also in being fed partly by stream percolation. The level rises and falls 

 with that of the neighboring streams. This class should perhaps include 

 the waters of sand plains and gravel plains which have no surface streams 

 traversing them, for waters in such plains usually have great lateral trans- 

 mission and but little hydrostatic pressure. 



ci a ss 3 . — Water included in porous beds of glacial drift or other non- 

 indurated formations lying beneath impervious beds but without strong 

 hydrostatic pressure. Such water is supplied from more or less distant 

 absorption areas and is less directly influenced by rainfall than the preced- 

 ing - classes. 



ciass 4 . — Water with strong hydrostatic pressure included in porous beds 

 of glacial drift or of alluvium. This includes two subclasses, viz: («) 

 Flowing wells; and (&) wells in which water rises nearly to the surface. 

 Subclass (a) embraces wells to which the name "artesian" is considered 

 applicable, while the term is not extended to wells of subclass (b). 



ciass 5 . — Streams in caves and subterranean passages in the rock, fed by 

 sink holes and brooks and also by direct percolation from ground water. 



ciass 6. — Rock water with but little current and slight hydrostatic 

 pressure. 



ciass 7. — Rock water under strong hydrostatic pressure. This includes 

 two subclasses, viz: (a) Waters which overflow when tapped; (b) waters 

 which rise nearly to the surface. Subclass (a) embraces wells to which the 

 term "artesian" has long been applied, and it seems a convenient term for use 

 if properly restricted, but its extension to nonflowing wells tends to mislead 

 and confuse those interested in obtaining a flow of water. 1 



Discrimination in selection of records. — It should not be inferred that theaverage 

 depth of the wells reported in the ensuing discussion represents the average 

 of all the wells that the region affords. Attention is directed chiefly to the 

 deeper wells, since they throw more light than the shallow ones upon the 

 structure of the formations penetrated and upon the supplies of water. 

 The great majority of wells in the State are the seep or ground-water wells 

 (class 1). They are sunk to depths of but 10 to 20 feet. In seasons when 

 rainfall is normal or excessive such wells supply the needs of the residents, 



'Compare Chamberlin, Fifth Ann. Rept. U. S. Geol. Survey, p. 131. 



