THE IGNEOUS OR ERUPTIVE ROCKS 69 
One of the ways in which the igneous rocks differ 
from the other two classes, is in the general absence of 
a banding, though to this there are exceptions. Lavas 
that are nearly solidified flow like molasses, and just 
Fig. 24. 
Porous lava, on right, and amygdaloidal rock, on left. 
before they cease moving, there may be some minerals 
already formed, especially the porphyritic minerals. 
If so, these may be arranged in bands or layers, giving 
a kind of imitation stratification which is known as 
