THE WORK OF GROUND WATER 59 



exist " at a depth of at least eleven miles. " (Adams.) At depths 

 greater than eleven miles it does not seem possible that a crevice can 

 open, and if a fracture should occur, the parts would actually weld 

 together. It is evident from the above that water will not descend 

 a greater distance than eleven or twelve miles under the most favor- 

 able conditions, and usually far less than that. The temperature of 

 the rocks, and therefore of underground water, increases i° F. for 

 each 60 to 100 feet of descent, a fact which accounts for the warmth 

 of deep wells and springs coming from great depths (p. 273). 



Artesian Wells. — Strictly speaking, an artesian well is one in 

 which the water rises above the surface of the ground as a fountain, 

 but the term is now, unfortunately, frequently employed for any deep 

 well from which water is obtained, whether it flows to the surface or 

 not. This change in usage is doubtless due to the fact that often 



Ills* 



Fig. 34. — Block diagram showing the conditions favorable for artesian water. 

 The porous beds ( dotted ) receive water from the rain which falls on their outcrops, 

 and from the streams which lose somewhat in volume as they flow over them. Three 

 water-bearing beds ( aquifers ) are shown, from two of which water can be obtained 

 on the barrier island which is separated from the mainland by a salt-water lagoon. 



artesian wells, after flowing for some months or years, cease to do so 

 and must be pumped because of the excessive withdrawal of water 

 from the artesian basin. This was true of the first artesian well at 

 Artois, France (from which the name " artesian" was derived). Many 

 wells in the San Bernardino valley; California, which flowed strongly 

 when first drilled, are now pumped. The conditions favoring artesian 

 water (Fig. 34) are (1) a porous bed capable of absorbing and trans- 

 mitting large quantities of water ; (2) relatively impervious beds above 

 and below; (3) exposure of the porous stratum where it may absorb 

 water supplied either by rain or by streams flowing over it; (4) an 

 inclination of the water-bearing stratum so that gravity may force the 

 water down; (5) a lack of easy escape of the water at lower points; 

 and (6) a sufficient supply of water to maintain the " artesian head." 

 The artesian water of South Dakota (Fig. 35), for example, is derived 

 from a saturated sandstone bed which receives its water in the Black 



