CIRCULATION OF GROUND WATER. 423 



of ground water is near the surface, the roots tap this water stratum and 

 carry water from it to the surface. 



In summary, while the lateral movements of water in the belt of weath- 

 ering are of consequence, it is clear that the upward and downward 

 movements are those of dominant importance. Gravity is always at work 

 carrying the water downward; the roots are always at work carrying it 

 upward. Molecular attraction following periods of precipitation works with 

 gravity in carrying the water downward, and at intervals between precipi- 

 tation works with the roots in carrying the water surfaceward, in opposition 

 to gravity. 



Thus, a large proportion of the water of precipitation takes a downward 

 journey for a certain distance into the belt of weathering, and then returns 

 to the surface. The chief impelling forces for the first part of the journey 

 are gravity and molecular attraction, and the impelling forces in the return 

 journey are roots and molecular attraction working against gravity. Where 

 vegetation is sparse or wanting, the upward movement by which water is 

 brought to the surface is chiefly or wholly molecular attraction. 



VARIATION IN LEVEL OF GROUND WATER. 



Variation in the level of ground water is of great importance in rock 

 alteration, since due to this a certain horizon is alternately under conditions 

 of the belt of weathering and under those of the belt of cementation. It 

 follows that the contrasting reactions of these two belts are superimposed 

 upon each other. Variation of the level of ground water has an exceptional 

 importance in connection with ore deposits. (See Chapter XII.) 



Variation in the level of ground water depends upon many factors, of 

 which (1) variable precipitation, seepage, and evaporation, (2) uplift and 

 subsidence, (3) denudation and valley filling, and (4) man, produce impor- 

 tant results, and (5) barometric pressure and (6) temperature produce 

 unimportant effects. 



PRECIPITATION, SEEPAGE, AND EVAPORATION. 



The surface of the ground water rises and falls (a) with climatic change 

 resulting in variations in precipitation, (b) with variations in precipitation 

 limning through several years, (c) with seasonal variations in precipitation, 

 and finally (d) with the cyclonic and diurnal variations in precipitation. 



