196 GEOLOGY 



regions where rainfall is deficient. The accompanying diagram 

 (Fig. 164) illustrates the flow here described. 



The ground-water surface. Water table. The water table has 

 already been defined (p. 117) as the upper surface 'of the ground- 

 water. In a flat region of uniform structure, the ground-water sur- 

 face is essentially level, though it rises and sinks with the rainfall. 



Fig. 164. Diagram showing how rainwater, falling in one place, may flow 

 underground to another and there be brought to the surface. The layer 

 a is porous, and water entering it in the mountains follows it to the plain. 



Where the topography of a region is not flat, the ground-water 

 surface is not level. As a rule it is higher (though farther belo^ 

 the surface) under an elevation than under surrounding lowland* 

 as illustrated by Fig. 165. The reason is understood readily. II 

 a hill of sand is exposed to rainfall, most of the water falling on i1 

 porous surface sinks into it. If the precipitation continues loi 

 enough, as in a protracted rain, the hill of sand will be filled wit I 





Fig. 165. Diagram illustrating the position of the ground-water surfs 

 (the dotted line) in a region of undulating topography. 



water, the water occupying the interstices between the 

 The water in the hill tends to spread, but since the movement in- 

 volves friction, the spreading is slow. With the spreading, tl 

 surface of the water in the sand sinks, and sinks fastest at tl 

 center where it is highest (Fig. 165). If no more water were added. 

 the surface of the water in the hill would, in time, sink nearly 

 the level of the water in the surrounding land; but at every st 

 preceding the last, the surface of the water would be higher bei 



