THE IGNEOUS OR ERUPTIVE ROCKS 63 
there are streams of lava extending into fissures in the 
earth, but not reaching the surface. These deeply buried 
Diabase, a black rock, on the left; rhyolite, 
PLATE 2. 
a light-colored rock, on the right. 
Two fine-grained or cryptocrystalline rocks. 
intrusions will be molten for many centuries after the 
surface lava has cooled; and slowly they will become 
