240 



GEOLOGY. 



other source of heat below. As this takes place, the time between 

 eruptions becomes longer and longer. In the course of time, there- 

 fore, the geyser must cease to be eruptive, and when this change is 

 brought about the geyser becomes a hot spring. Within historic 

 times several geysers have ceased to erupt and new ones have been 

 developed. In the Yellowstone Park, where there are said to be some- 

 thing like 3000 vents of all sorts, hot springs which are not eruptive 

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

 steam issues, and from some, little else 



Fig. 218. — Hot springs deposits. Terraces of Mammoth Hot Springs, Yellowstone Park. 



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 lengthen- 

 ing, but it is becoming irregular, and the irregularity appears to be 

 increasing. In the short time during which this geyser has been under 



