110 PHYSICAL GEOLOGY 



be extremely slow, because of the small passages through 

 which it must move and the friction it would develop with 

 the rocks. Long before the surface of the water became 

 level, further rains would be likely to raise it again beneath 

 the uplands. In keeping with these considerations, the 

 water table is found to repeat, in a general way, the topog- 

 raphy of the surface above. As Figure 101 suggests, how- 

 ever, it is nearer the surface below the valley bottoms than 

 it is beneath the hilltops. 



As implied in the preceding paragraph, the position of the 

 water table varies not only from place to place, but also 

 from time to time at any given place. It is higher after 

 heavy rains, and lower during periods of drought. 



How ground water is disposed of. On the average the 

 amount of water withdrawn from the ground in the course 

 of a year probably balances that which enters. It is with- 

 drawn in various ways. It issues as springs and as seepage, 

 is pumped out through wells, flows underground to the sea, 

 is taken up by plants, and evaporates into the ah* which 

 fills the rock cavities above the water table. It sometimes 

 enters into chemical combination with rocks (pp. 103-104). 

 The deeper water is probably imprisoned underground for long 

 periods. 



SPRINGS AND UNDERGROUND CIRCULATION 



Kinds of springs. When water issues from the ground 

 in volume sufficient to form a distinct current, it constitutes 

 a spring. When it issues in less quantity, it is known as 

 seepage. Springs differ in many respects, and these differ- 

 ences have led to numerous classifications. Thus there are 

 hot (thermal) and cold springs, intermittent and constant 

 springs, deep and shallow springs, and many others. 

 Medicinal springs are those whose waters have real or sup- 

 posed medicinal value. The springs at Saratoga Springs, 

 New York ; Hot Springs, Arkansas ; Vichy, in central France ; 

 and Karlsbad, in Bohemia, are famous examples. These 



