SOURCE OF GROUND WATER. 413 



fractured, and there is therefore great difference between the topography 

 of the surface and that of the level of ground water. In contrast with this 

 may be mentioned the drift-covered regions of North America. Here, even 

 where the topography is only moderately accentuated, the level of ground 

 water may rise rather rapidly. This is illustrated by the rise of that level 

 where one passes from one of the numerous lakes of the region to the 

 adjacent higher lands. A good instance is found at Madison, Wis. About 

 360 meters from Lake Mendota and 25 meters above its surface, on the 

 slope of a hill, is a well in which the water is, on the average, about 15 

 meters above the surface of the lake." 



AMOUNT AND SOURCE OF WATER IN BELT OF WEATHERING. 



Under ordinary circumstances the water above the level of ground 

 water is mainly that of imbibition. (See p. 124.) This water is held by 

 the adhesion between the mineral particles and the water. But in the arid 

 regions a large part of the belt of weathering may not contain more than 

 a fraction of the water of imbibition. Whitney explains that not only the 

 surface soil but strata below the soil are often so dry as to be dusty. 6 



However, in contrast with this, for many regions, and for much of the 

 year, the water present is greater than that of imbibition and consequently 

 includes water of hygrometricit}' . (See p. 124.) Indeed, locally, and for 

 a time, the belt may be nearly or quite saturated with water. The upper 

 belt is therefore characterized by conditions ranging from almost complete 

 dryness, through water of imbibition, to almost complete saturation. 



It has been shown in Chapter III that the circulating ground water is 

 almost wholly derived from precipitation, although it is possible that a 

 small fraction of this water is derived from deep within the earth. Before 

 considering the circulation in the belt of weathering, it is necessary to 

 understand how the precipitation is divided among the surface water, the 

 belt of weathering, and the belt of cementation. 



In the greater part of the very humid region comprising the south- 

 eastern United States the rainfall varies from 100 to 150 cm., but runs up 

 to 175 cm. In this region, according to Newell, the run-off in streams is 



a King, F. H., cit.,p. 99. 



6 Whitney, Milton, Conditions in soils of the arid region: Yearbook of the Dept. of Agric., 1894, 

 pp. 159-160. 



