CIRCULATION OF GROUND WATER. 419 



surface, and separated from the belt of cementation by a belt of dry soil. 

 This holding of the water of precipitation near the surface, according to 

 Whitney, explains the greater efficiency of water in the production of crops 

 in the semiarid regions than in the humid regions. a 



In opposition to the above, where the subsoil is moist and where the rate 

 of precipitation is moderate, the water may make its way downward, drawn 

 by gravity and by molecular attraction, without forming an upper barrier 

 of saturated soil which prevents the escape of the air below and the down- 

 ward passage of the water. Where precipitation continues long and a 

 considerable part of the water makes its way downward into the soil, 

 through the belt of weathering to the belt of cementation, locally the belt 

 of saturation or the level of ground water may rise almost or quite to the 

 surface. 



The water which passes to the belt of cementation is carried down- 

 ward, as already explained, by gravity and molecular attraction. The 

 proportion of the water of the belt of weathering which on the average 

 passes into the belt of cementation is small. In the level arid regions the 

 proportion which thus passes downward may be almost zero, and even in 

 the humid regions only a small fraction of the water which gets into the 

 belt of weathering passes through this belt into the belt of cementation. 



UPWARD MOVEMENTS OF WATER. 



The water which enters the belt of weathering and does not join the 

 belt of cementation is brought to or above the surface (a) by molecular 

 attraction or (b) by vegetation. In either case this water is evaporated. 

 At times of abundant precipitation the evaporation is very small, but at 

 intervals between precipitation, especially at times of sunshine and in warm 

 climates, the water near the surface rapidly passes into the atmosphere. 

 The water thus brought to the surface, as already intimated, is derived 

 mainly from the belt of weathering, but a portion may come from the sea 

 of ground water. 



MOLECULAR ATTRACTION. 



The manner in which the water is brought to the surface by molecular 

 attraction, thus overcoming gravity, is full} T discussed on pages 150-152. 

 However, the proportion of the precipitation which is thus brought to 



"Whitney, cit, pp. 160-161. 



