218 



GEOLOGY 



greatly outnumber the geysers. From many of the vents but little 

 steam issues, and from some, little else. 



A few geysers have somewhat definite periods of eruption. Of 

 such "Old Faithful" is the type; but even this geyser, which 

 formerly erupted at regular intervals of about an hour, is losing 

 the reputation on which its name is based. Not only is its period 

 of eruption lengthening, but it is becoming irregular, and the 

 irregularity appears to be increasing. In the short time during 

 which this geyser has been under observation its period has changed 



Fig. 178. Deposts in the hot waters about Biscuit Geyser, Yellowstone 

 Park. (Fairbanks.) 



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. 



In the Yellowstone Park the geysers are mainly in the bottoms 

 of valleys (Fig. 179), but the deposits characteristic of goysrrs are 

 found in not a few places well above the present bottoms. Then- 

 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 deepened, the ground-water has found lower 



