THE IGNEOUS OR ERUPTIVE ROCKS 59 
certain centres, the feldspar elements around others, 
etc. Each growing mineral is made of molecules defi- 
nitely arranged by the laws of crystal growth, and thus 
a rock is made which is composed of crystalline parti- 
cles. So we speak of most igneous rocks as crystalline ; 
Fig. 18. 
A section of gabbro, a coarse-grained rock, enlarged by the microscope. 
for, even when no crystals can be seen with the eye, 
the microscope will usually reveal a beautiful mass of 
minute crystal forms, sometimes with definite outlines, 
but more commonly without (Figs. 17 and 18). 
If we have a hot solution of alum (or salt) and place 
it upon ice, it cools quickly, and there is not time for 
