THE WORK OF GROUND-WATER. 



215 



fication planes. Horizontally bedded rock, or rock which is not bedded 

 at all, may be so much jointed, and the joints so open, as to allow the 

 water to enter readily. 



The conditions favorable to the sinking of abimdant water below 

 the surface are therefore heavy precipitation, falHng slowly on a 

 surface with little relief, a soil of open texture underlain by rock which 

 is porous, or affected by vertical or highly inchned planes of cleavage. 

 The annual discharge of water by rivers is estimated to be about 22 per- 

 cent, of the rainfall on the land.^ 



Supply of ground-water not altogether dependent on local rainfall. 

 — The amount of ground-water in a given region is not always 

 entirely dependent on the local rainfall. Ground-water is in constant 

 movement, and entering the soil or rock at one point it may, after a 

 long subterranean journey, reach a point far from that at which it 

 entered. Thus beneath the Great Plains of the West there is much 

 subterranean water which fell on the eastern slopes of the mountains 

 to the west. It has flowed beneath the surface to the Plains, where 



Fig. 199. — Diagram illustrating the general point that ground-water is not dependent 

 entirely upon local supply. 



some of it is now withdrawn for the purposes of irrigation in regions 

 where rainfall is deficient. The accompanying diagram (Fig. 199) 

 illustrates the flow here described. 



Fig. 200. — Diagram illustrating the position of the ground-water surface (the dotted 

 line) in a region of undulatory topography, 



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

 already been defined (p. 71) as the upper surface of the ground- 

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

 is essentially level, but rises and falls with the rainfall. Where the 



* Murray. Scot. Geog. Mag., Vol. Ill, p. 70, 1887. 



