44 



VOLCANOES 



are acid lavas, and are composed of potash felspars (p. 16), with 

 quartz, hornblende, or mica.' Lavas which, like those of the Sand- 

 wich Islands, are notably fluid, are always of basic composition. 



When a lava stream reaches the surface of the ground, the 

 imprisoned vapours immediately begin to escape and the surface 

 of the molten mass to cool and harden. The surface layers are 

 blown by the steam bubbles into a light, frothy or slaggy consist- 



FlG. 12.— Lava-tunnel, and "Spatter-cone" formed by escaping steam, Kilauea. 

 (Photograph by Libbey.) 



ency, forming " scoriae " or cindery masses. The motion of the 

 lava breaks up this thin crust into loose slabs and blocks, and on 

 the advancing front of the stream these loose masses rattle down 

 over one another in the wildest confusion. The less perfectly 

 fused lavas are soon covered with heaped-up cindery blocks, while 

 the more completely fluid lavas are characterized by curiously 

 twisted, ropy surfaces, such as may be observed in the slag from an 

 iron furnace. 



