THE IGNEOUS OR ERUPTIVE ROCKS 61 
grained lavas, and even to coarsely crystalline granites. 
The glassy form of lava is known as natural glass, 
or obsidian (Fig. 
a black glass: 
nary lavas, are 
cryptocrystalline 
(Figs. 15 and 17, 
and Plate 2); the 
coarse-grained, 
like granite, are 
holocrystalline 
(Figs. 18 and 20). 
An ordinary 
lava cools at the 
surface while it is 
still flowing (Fig. 
14); and ina few 
days or months 
is solidified 
throughout. So 
these surface or 
volcanic lavas, be- 
cause they cool so 
19), and it looks very much like 
the fine-grained forms, like ordi- 
Fic. 20. 
Photograph of two pieces of granite, a plutonic, 
holocrystalline rock. 
rapidly, are usually either glassy or fine grained. Like- 
wise if blown to ash, the material is generally a glass. 
Those rocks which are intruded into the earth, and 
come to view only when the surface is worn down to 
