CHAPTER IV. 



THE WORK OF GROUND- (UNDERGROUND) WATER. 



Many familiar facts demonstrate the general presence of abundant 

 water beneath the surface of the land. The thousands of wells in 

 regions peopled by civihzed races, and the countless springs which 

 issue from the sides of mountains and valleys are a sufficient proof 

 both of the wide distribution of ground-water and of its great abundance. 



Certain well-known facts make it clear that ground-water is inti- 

 mately connected with rainfall. In a dry season the level of the water 

 in wells commonly sinks, and after a heavy rain it rises (p. 71) ; and 

 the amount of sinking is greater when the drought is long, and the 

 rise is most notable when the rainfall is heavy. Many springs which 

 discharge large quantities of water during a wet season flow with 

 reduced volume, or cease to flow altogether in periods of drought. Fur- 

 thermore, the water of springs and wells has the properties which rain- 

 water would possess after sinking beneath the surface and dissolving 

 mineral substances. Rain-water is seen to sink beneath the surface 

 with every shower, and since this source seems altogether adequate 

 for ground-water, and since no other source is known whence any con- 

 siderable amount of ground-water might come, it is concluded that 

 atmospheric precipitation is its chief source. 



Water gets beneath the surface by processes which are readily seen. 

 Wherever the soil is porous some of the rain which falls upon it is ab- 

 sorbed. Sinking through the soil to the solid rock it finds cracks and 

 pores through which it descends to great depths. Nowhere are the 

 rocks beneath the mantle rock so compact and so free from cracks, 

 when any considerable area is considered, as to prevent the percola- 

 tion of water through them. 



Conditions influencing descent of rain-water. — There are sev- 

 eral conditions which influence not only the amount of water which 

 sinks beneath the surface in a given area, but the proportion of the 

 precipitation which follows this course. These are as follows: (1) 

 Amount of precipitation. — In a general way it is true that the greater 



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