52 GROUND-WATER 



irregularity appears to be increasing. In the short time during 

 which this geyser has been under observation its period has changed 

 from a regular one of 60 minutes, or a little less, to an irregular one 

 of 60 to 90 minutes. In the case of some geysers, years elapse be- 

 tween eruptions, and in some the date of the last eruption is so 

 remote that it is uncertain whether the vent should be looked upon 

 as a geyser or merely a hot spring. 



Fig. 31- 

 Photo. Co.) 



Cone (or crater) of Grotto Geyser, Yellowstone Park. (Detroit 



Artesian wells. The terms artesian well and flowing well were 

 synonymous originally; but any notably deep well is now called 

 artesian. The artesian well which does not flow does not differ 

 from a common well in principle, while the flowing well is really a 

 gushing spring, the opening of which was made by man. 



Flowing wells l 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. Rept., U. S. 

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

 tion of the principles involved. 



