608 



GEOLOGY. 



Formation of Cones. 



Lava-cones. — The lava usually flows away from the vent in short 

 streams which solidify before running far. As the lava-streams flow in 

 different directions at different times^ the total effect is a low cone formed 

 of radiating tongues surrounding the point of exit. Occasionally the 

 streams run a dozen or a score of miles, but such cases, except in the 

 gigantic volcanoes of Hawaii and a few others, are rare. Often the 

 streams congeal before they reach much beyond the base of the cone, 

 and quite often while they are yet on its slope. So far, therefore, as 

 the volcanic cone is formed of lava, it has a radiate structure made up 

 of a succession of congealed lava-streams. In these cases the slopes 

 are low, because the fluidity of the lava prevents the development of 

 high gradients. It is, however, rather the exception than the rule, that 

 the cone is made up mainly of lava-streams, though the great Hawaiian 

 volcanoes are of this class. 



Fig. 465. — Typical cinder-cone, Clayton valley, Cal. (Turner, U. S. Geol. Surv.) 

 Cinder-cones. — The larger portion of the lava blown into the air 

 by the expanding gas-bubbles falls back in the immediate vicinity of 

 the vent and builds up a cinder-cone. From the nature of the case, this 

 often takes on a beautiful symmetry and assumes a steep slope (Fig. 465) . 

 The ragged cinders lend themselves readily to the formation of an acute 

 cone, quite different from the flatter cone formed by lavas. Sometimes 



