378 



GEOLOGY 



tons in weight are sometimes thrown out, and from masses of such 

 size, the fragments grade down to minute particles of dust. The 

 dust particles (often called ash) are thrown high into the air in 

 some cases, and, caught by the winds, are shifted incredible dis- 

 tances, as already noted (p. 89). While, therefore, the fluid lava 

 and the larger fragmental materials ejected from the volcano stay 

 near the vent, the fine materials are scattered broadcast. 



Liquid rock, lava. The term lava is applied to all kinds of 

 liquid rock which issue from a volcano, and also to the solid rock 



Fig. 296. The edge of an old stream of lava, showing (1) its broken char- 

 acter due to movenient after the outside had hardened, and (2) the stivj> 

 slope of the stream of stiffened lava. Near Flagstaff, Ariz. (Fairbanks.) 



formed when this congeals. Lava never flows so freely as water, 

 and it is sometimes very stiff or viscous. The distance to which 

 it flows depends on its liquidity, its amount, and the slope of the 

 surface on which it is poured out. The more fluid the lava, the 

 greater its amount, and the steeper the slope on which it flows, 

 the farther it will move. 



As lava flows, its upper surface may cool so much as to become 



