THE ORIGIN AND DESCENT OF ROCKS. 429 



general tendency in the upper zones penetrated by water is toward 

 hydration. In the lower zones, w^here the pressure is great, Van Hise 

 holds that there is a tendency toward dehydration, if the rocks have 

 been pre\aously hydrated. This may be the case if rocks have once 

 been near the surface and later deeply buried by the accumulation of 

 sediments on them. If the principle holds, rocks subjected to intense 

 lateral pressure may be dehydrated. 



Carbonation and decarbonation. — The igneous rocks are largely sili- 

 cates. The carbonic acid of the surface-waters and of the air acting 

 upon them, converts them, in part, into carbonates. In this wa^ has 

 arisen most of the original supply of calcium and magnesium carbon- 

 ates. Original carbonates formed in this way are precipitated and 

 redissolved again and again. The carbonates in river-waters are 

 much more largely solutions of previously sohd carbonates than original 

 carbonates formed from the silicates. The potassium and sochum of 

 the sihcates also form carbonates, but by preference they unite ^^ith 

 the sulphur and chlorine, and hence appear more largely as sulphates 

 and chlorides. 



Carbonation is usually accompanied by oxidation and hydration. 

 These several processes break up the complex and relatively unstable 

 silicates into simpler and more stable silicates, carbonates, and oxides. 

 This is illustrated by the following formulas illustrative of the changes 

 undergone by augite and labradorite, two common rock-forming min- 

 erals. 



The composition of augite may be represented by the formula £ 



CaO.(Mg,Fe)0.2Si02 

 (Mg,Fe)0.(Al,Fe)203.Si02. 



Assuming Mg and Fe to be equal in amomit in the first half of the above 

 formula, and Mg and Fe to be equal in the first part of the second half, 

 and Al and Fe to be equal in the last part of the second half, doubling 

 the whole and allowing it to be acted on by CO J and HjO, we have 



2Ca0.2Mg0.2FeO.AU03.FeA-6Si02 + 6C03+2H30 



= 2CaC03+ 2MgC03+ 2H3O. Al203.2Si02+ 2FeC03 + Fe 2O3+ 4Si02. 



The hydrous sihcate of the last part of the equation is kaolin. " 

 The composition of labradorite is represented by the formula 



