THE WORK OF WATERS UNDERGROUND 109 



all spaces, however small, are closed by the tremendous 

 weight of the overlying rocks. 



The temperature of the rocks, and therefore of the ground 

 water, increases with the depth. The rate of increase varies 

 in different places from one degree for about 17 feet of 

 descent to one degree for over 100 feet. The larger figure 

 is probably much nearer the average than the smaller one. 



Amount of ground water. If the average distance from 

 the water table to the base of the zone of fracture were 

 known definitely, and if, in addition, it were possible to 

 determine what proportion of the volume of the rocks be- 

 tween is made up of cracks, pores, etc., it would be possible 

 also to determine accurately the total amount of the ground 

 water. It is sufficient to form a layer of water over the 

 surface of the' earth, having a depth estimated variously at 

 from 800 feet, and less, 1 to 3500 feet, and more. The larger 

 figure would represent only about one third, and the 

 smaller figure but a small fraction of the water of the oceans. 

 The ground waters encircle the earth, forming a rude sphere. 

 The name applied to the waters of the earth, the hydro- 

 sphere, is accordingly an appropriate one. 



Form and position of the water table. The water table 

 is not a level surface. If the rocks beneath an uneven sur- 

 face such as that shown in Figure 101 were filled with water 

 by rains, the water 

 table would, of course, 

 coincide with the sur- 

 face of the ground, and 

 would be far from 



level Subsecmentlv *" IG ' 1( ^' Diagram showing the relation of 



' the level of ground water (the broken line) 



the water would move to the surface of the ground and to a lake and 



under gravity from river - 



the higher levels A and B toward the neighboring lower levels. 

 The ultimate tendency would be to make the ground-water 

 surface level. The movement of the water would, however, 



1 One of the latest estimates fixes the depth at only 96 feet. 



