THE WORK OF GROUND-WATER. 



241 



observation its period has changed from a regular one of sixty minutes, 

 or a little less, to an irregular one of seventy to ninety minutes. In 

 the case of some geysers years elapse between eruptions, and in some 

 the date of the last eruption is so distant that it is uncertain whether 

 the vent should be looked upon as a geyser or merely a hot spring. 



In the Yellowstone Park^ the geysers are mainly in the bottoms 

 of valleys (Fig. 219), but the deposits characteristic of geysers are found 

 in not a few places well above the present bottoms. These deposits 

 record the fact that in earlier times the geysers were at higher levels 

 than now. It is probable that they have been, at all stages in their 

 history, near the bottoms of the valleys, and that, as the valleys have 

 been deepened the ground-water has found lower and lower points of 



Fig 219 — Hot springs and geysers. Norris Geyser basin, Yellowstone Park. 



issue. In this respect the geysers have probably had the same history 

 as other springs. 



1 Geikie. Geological Sketches, pp. 206-38. Hayden. Amer. Jour. Sci., Vol. Ill, 

 1872, pp. 105-15 and 161-76. 



