58 ELEMENTARY GEOLOGY 
ing to form mimerals; but as the lava cools, these 
eventually find it possible to combine, and they come 
together, building up mimerals. If’ we have a hot, 
saturated solution of alum or of salt, and allow it to 
cool, little by little the dissolved substance begins 
Fic. 17. 
A section of diabase, a fine-grained rock, enlarged by the microscope. 
to come out of the solution, and crystals commence 
to form. From various parts of the liquid the alum 
(or salt) molecules draw toward the growing crystals, 
and build them up to larger and larger size. 
In a measure this is like the growth of minerals in 
cooling lava. The quartz elements combine around 
