GROUND-WATER 



37 



wells. They are the following: (1) A porous layer or bed of rock, 

 a, underlying one which is not porous, and which prevents the 

 water from escaping upward until it is penetrated by the well hole, 

 w. The porous bed should come to the surface in a region which 



Fig. 23. Diagrams illustrating the conditions favorable for artesian wells. 

 In A, the porous bed a is in the form of a basin; in B, it merely dips. 



is somewhat higher than the site of the well. (2) Enough rainfall, 

 where the porous bed comes to the surface, to keep that bed well 

 filled with water. Under these conditions, the water beneath w 

 will gush up, if a hole is made down to it (Fig. 1, PL XII, p. 33). 



Flowing wells may be but a few feet deep, or they may be thou- 

 sands of feet deep. Thus there is one in St. Louis nearly 4,000 feet, 

 and many in New Jersey less than 100 feet in depth. Many villages 

 and small cities get their water from artesian wells; but great cities, 

 such as New York, Chicago, Philadelphia, etc., could not get enough 

 water in this way. In the semi-arid region of the Great Plains, 

 and at various other places on the West, water from deep wells is 

 extensively used for irrigation. 



THE WORK OF GROUND- WATER 



Ground-water does two kinds of work: (1) It dissolves mineral 

 matter, and it changes the character of the rock through which it 

 flows in more ways than one. Changes of this sort are chemical 

 changes. (2) Where it flows in streams, as it sometimes does, it 

 wears the channels where it flows, much as streams on the surface 

 do. Most ground-water is in the pores of rock, not in channels, 

 and its mechanical work is far less important than its chemical 

 work. 



