Rock Alterations. 
511 
When the environment changes, different minerals behave very 
differently, some being capable of existing under many condi¬ 
tions, others under only particularly favorable ones. Such 
minerals as leucite, nepheline, and olivine, are changed with 
comparative rapidity. Quartz, or rock crystal, upon the other 
hand, is comparatively permanent. 
This mineral is one of the commonest ones in the abundant 
rock granite. Granite crystallizes from a molten magma far be¬ 
low the surface of the earth. When a quartz individual below 
the surface passes through a mountain-making period, it is re¬ 
crystallized, or crushed into numerous granules, but in either 
case the material is of essentially the same character as the orig¬ 
inal. At the surface, weathering agencies have little effect upon 
quartz, except very slowly to dissolve it. However, they may 
break the quartz from its setting. The particles may be ground 
against one another as they are carried down the river, or they 
may beat against one another on the sea shore until they are 
rounded. These dirt-covered grains may be deposited in a sand 
bed and become deeply buried by newer deposits. Through the 
pores of this sand-rock later solutions may pass, bearing the ma¬ 
terial out of which quartz can be made. The mineral particles, 
notwithstanding all the vicissitudes through which they have 
gone, notwithstanding their millions of years of age, are able to 
take material like themselves and add it to themselves, and build 
up new, perfect crystals. 
The total effects of the interior alterations of minerals and 
the transportation of mineral material by underground waters 
are enormous. If one looked only at the result, and thought 
not of the vast time it took to accomplish the work, he would 
conclude that such refractory minerals as quartz are as soluble as 
sugar. In the cementation of a great sand-rock formation to a 
quartzite, thousands of cubic miles of quartz are deposited from 
the mineral-bearing solutions. This almost incredible statement 
is fully justified when the extensiveness of the quartzite forma¬ 
tions, and the amount of material required for the cementation 
of the original sandstones are considered. Some of the single 
quartzite formations cover thousands of square miles, and are 
several thousands of feet thick. If the original sandstones from 
