216 



GEOLOGY 



tube, or which increases the viscosity of the water, hastens ;ui 

 eruption. 1 



Geysers often build up crater-like basins or cones (Figs. 174 

 to 176) about themselves, the cone being of material deposited 

 from solution. In the Yellowstone Park, the precipitation of the 

 matter in solution (chiefly silica) is partly due to cooling, but 

 largely to the algae which abound even in the boiling water, and the 



Fig. 175. Cone (or crater) of Grotto Geyser, Yellowstone Park. (Detroii 



Photo. Co.) 



brilliant colors of some of the deposits about the springs are at- 

 tributable to these plants. When the water from any geyser or 

 hot spring ceases to flow, the plants die and the colors disappear. 

 The heating of geyser water must cool the lava or other source 

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

 becomes longer and longer. In the course of time, therefore, the 

 geyser must cease to be eruptive, and when this climmc is brought 

 about the geyser becomes a hot spring. Within historic time 



1 Weed. Ninth Ann. Kept. U. S. Geol. Surv., pp. 613-76, and Am. Jour. 

 Sci., Vol. XXXVII, 1889, pp. 351-59. 



