148 ELEMENTARY GEOLOGY 
exposed at the surface, water easily entered and passed 
down into the earth, being enclosed there by the imper- 
vious rocks above and below, and thus prevented from 
either rising or descending. By the time it reached the 
centre of the depression, the water was under consider- 
able pressure from the weight of the water contained 
in the upper ends of the layer. When this water-bear- 
Section showing conditions favoring formation of artesian wells (A), where the 
strata are bent in a U-shaped manner. SS, porous layer. J, J, impervious 
strata. 
, 
ing stratum was pierced by a boring, the water rushed 
to the surface, forming an artesian well. 
For an artesian well, it is necessary to have all the 
above conditions excepting the double slope of the 
water-bearing layer, which may or may not be present. 
In New Jersey, South Dakota, Texas, and many other 
parts of the world, a sandy layer of this kind dips into 
