GROUND-WATER 



219 



and lower points of issue. In this respect the geysers probably 

 have had the same history as other springs. 



Unless new intrusions of lava occur, or unless heat is otherwise 

 renewed at the proper points, it is probable that all existing geysers 

 will become extinct within a time which is, geologically, short. 

 New geyser regions may, however, develop as old ones disappear. 



Artesian Wells 



The terms artesian wells and flowing wells were synonymous 

 originally; but any notably deep well is now called artesian. The 



Fig. 179. Hot springs and geysers. Norris Geyser Basin, Yellowstone Park. 



artesian well which does not flow does not differ from common wells 

 in principle, while the flowing well is really a gushing spring, the 

 opening of which was made by man. 



Flowing wells 1 depend upon certain relations of rock structure, 

 water supply, and elevation. Generally speaking, a flowing well 

 is possible in any place underlain by any considerable bed of porous 



1 Chamberlin. Geol. of Wis., Vol. I, pp. 689-97, and Fifth Ann. Kept., 

 . S. Geol. Surv., pp. 131-73. The former a brief, and the latter an elaborate, 

 exposition of the principles involved. The same principles in various forms 

 of statement have appeared in later publications of the U. S. Geol. Surv. 



