426 A TREATISE ON METAMORPHISM. 



at which the evaporation is the most rapid. These two factors frequently 

 tend to neutralize each other so far as the level of ground water is 

 concerned, for the effect of precipitation is to raise the level of ground 

 water and the effect of evaporation is to lower that level. 



The variations in the level of ground water due to climatic variations 

 in precipitation are the greatest, but the variations of the level of ground 

 water due to seasonal variations in precipitation, or even to cyclonic and 

 diurnal variations in precipitation, are far more important than might at 

 first be thought, for these lesser variations largely or wholly compensate 

 for their smaller magnitude by their much greater frequency. 



UPLIFT AND SUBSIDENCE. 



Vertical movements of the earth's crust take place as a result of either 

 epeirogenic or orogenic movements, or the two combined. Uplift tends to 

 lower the ground water, and subsidence tends to raise it, in reference to a 

 fixed plane in the rocks. Only regions which have a considerable elevation 

 above the sea have the ground water far below the surface. Regions which 

 are near the level of the sea usually have the ground water near the surface ; 

 but this may not apply to interior low-lying deserts. 



DENUDATION AND VALLEY FILLING. 



Consequent upon denudation there is a steady downward movement 

 of the level of ground water. This continues, unless compensated for by 

 uplift, until base-level is reached, when the ground-water level approxi- 

 mately corresponds, on the average, with the ocean. The result of denuda- 

 tion is continuously to transfer the upper part of the belt of cementation 

 into the belt of weathering, and thus to change the conditions of alteration 

 from those of one belt to those of the other. This steady downward migra- 

 tion, in reference to a fixed plane in the rocks, of the boundary between 

 the belt of weathering and the belt of cementation is the most important of 

 all the changes in the variation of the level of ground water, leading to 

 profound geologic and economic consequences, which are fully discussed 

 elsewhere. Variation in the level of ground water may also result from 

 valley filling. When a valley is filled to a considerable depth, this may 

 result in corresponding rise in the level of ground water. If the valley 

 be again excavated, this may lower the ground water to its former level. 



