THE IGNEOUS OR ERUPTIVE ROCKS 57 
a volcano is a steam explosion (Figs. 15 and 203). 
Sometimes the explosion is very violent and the molten 
lava is blown into shreds 
and cast high in the air, 
forming an ash-like prod- 
uct which is called vol- 
canic ash or pumice (Fig. 
16). So in an eruptive 
rock we may have two 
extremes, either a_ solid 
lava or a porous pumice, 
blown full of holes by 
the expansion of steam ; 
and between these ex- 
tremes there is every 
gradation. Sometimes 
the lava is blown into 
shreds, and even into 
hair-like threads of natu- 
ral glass. The cavities 
caused by the expansion 
of the steam usually have 
smooth rounded walls 
(Fig. 16). 
Fia. 16. 
Photograph of a piece of pumice. 
Texture of Igneous Rocks. —In the molten lava 
there are many elements, perhaps a score or two, 
which because of the heat, are prevented from combin- 
