220 



GEOLOGY 



rock, if this rock outcrops at a sufficiently higher level in a region 

 of adequate rainfall, and is covered by a layer or bed of impervious 

 or relatively impervious rock. This statement involves four con- 

 ditions, all of which are illustrated by Fig. 180, where a is the bed 

 of porous rock. It is not necessary that the beds of rock form a 

 structural basin, nor is it usually necessary to take account of the 

 character of the rock beneath the porous bed which contains the 

 water. 



The bed of porous rock is the " reservoir " of the flowing well. 

 Formations of sand or sandstone, and of gravel or conglomerate, 



B 



Fig. 180. Diagrams illustrating conditions favorable for artesian wells. 

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



most commonly serve as the reservoirs. In order that they may 

 contain abundant water they must have some thickness, and their 

 outcropping edges must be so situated that the water may enter 

 freely and be replenished, chiefly by rain, as the water flows out 

 at the well. 



A relatively impervious layer of rock above the reservoir (h, 

 Fig. 180) is most important; otherwise the water in the reservoir 

 will leak out, and there will be little or no "head" at the well site. 

 Thus if the rock overlying stratum a (Fig. 180) were badly broken. 

 the fractures extending up to the surface, the conditions would l>e 

 unfavorable for flowing wells, for though wells in the positions of 

 those in Fig. 180 might get abundant water, they would not be 

 likely to flow. If the stratum next below the reservoir is not iin- 



