216 GEOLOGY. 



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

 level. As a rule it is higher, though farther below the surface, under 

 an elevation than under surrounding lowlands, as illustrated by Fig. 200. 

 The explanation is not far to seek. If a hill of sand be exposed to rain- 

 fall, most of the water falling on its porous surface will sink into it. 

 If the precipitation continues long enough, as in a protracted rain, 

 the hill of sand will be filled with water, the water occupying the 

 interstices between the grains. If the sand of the hill could be re- 

 moved, leaving the water which it contains on the same area, it 

 would constitute a mound perhaps a third or a fourth as liigh as 

 the hill itself. If unsupported, this mound of water would spread 

 promptly in all directions until its surface was level. While the sand 

 remains, the water in it constitutes a mound, and has a tendency to 

 spread. It does in fact spread, but since the process involves great 

 friction the spreading is slow. With the spreading the surface of 

 the water in the sand sinks, and sinks fastest at the center where it 

 is highest (6, Fig. 201). If the process were not interrupted the sur- 



FiG. 201. — Diagram to illustrate the relations of ground-water to the surface. 



face of the water in the hill would, in time, sink approximately to the 

 level of the water in the surrounding land {d, Fig. 201) ; but at every 

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

 beneath the summit of the hill than elsev/here, though farther from 

 the surface. In regions of even moderate precipitation the water 

 surface beneath the hills rarely sinks to the level of that in the lowlands 

 adjacent, before being raised b}^ further rains. 



The water-level beneath the lowlands also sinks. Some of it finds 

 its way into valleys, some of it sinks to greater depths, and some of 

 it evaporates ; but since the water surface beneath the elevation sinks 

 more rapidly than that beneath the lowland, the two approach a com- 

 mon level. Their difference will be least at the end of a drought, and 

 greatest just after heavy rains. 



Depth to which ground- water sinks. — The depth to which groimd- 

 water penetrates has not been determined empirically. No borings 



