DESTRUCTIVE ACTION OF SPRINGS 95 



The quantity of water which can be annually drawn from an 

 artesian reservoir cannot exceed the annual supply through rain- 

 fall, though, at first, the stored water will cause a more abundant 

 flow. When several wells are bored near together, each new 

 well is apt to dimmish the flow from the older ones, though the 

 total discharge from all the wells increases until the limit of supply 

 is reached. New wells at lower levels may take all the water and 

 leave none for those at higher levels. 



While the basin-like arrangement of strata is the most favour- 

 able, it is not necessary. Along the New Jersey coast is a great 

 thickness of alternating beds of sands and clays, all dipping gently 

 toward the sea. Aside from levels of minor importance, no less 

 than six separate porous layers afford abundant supplies of water, 

 which are now very largely drawn upon. 



The depth necessary to obtain flowing wells differs, of course, 

 with the topography and structure of the country. In North 

 Dakota, for example, flowing wells have been obtained at depths 

 varying from less than 100 to more than 1500 feet. The Cre- 

 taceous sandstone which underlies immense areas of the Great 

 Plains region is the principal source of supply. 



In limestone districts ravines may intersect the course of con- 

 siderable underground streams, which thus reach the surface in 

 springs of unusual volume. A very striking and beautiful example 

 is the Giant Spring in the canon of the upper Missouri, near Great 

 Falls, Montana. 



Springs, as such, do little in the way of rock disintegration, but 

 they accomplish something by undermining the rocks at the point 

 where they issue, and thus working their way backward. This 

 process is known as the recession of spring-heads. The under- 

 ground streams, of which springs are the outlets, have often ef- 

 fected much in the way of dissolving rock-material, and hence 

 spring-water always contains dissolved minerals, principally the 

 carbonates and sulphates of lime and magnesia, and the chlo- 

 rides of magnesium and sodium. In mineral springs the 

 quantity of dissolved materials is larger and perceptible to the 

 taste. 



