THE ORIGIN AND DESCENT OF ROCKS. 427 



permeating and circulating water is now, and for long ages has been, 

 working changes in the rocks, partly by dissolving matter out of them, 

 partly by depositing matter in them, and partly by furnishing a medium 

 through which new combinations of their constituents may take place. 

 This outer fractured portion of the Hthosphere has been called the 

 zone of fracture} 



Oxidation and deoxidation. — At and above the surface of the under- 

 ground water, where the rocks are easily reached by atmospheric waters 

 carrying much free oxygen, and by the air itself, oxidation prevails. 

 Through oxidation the ferrous oxides are changed to ferric oxides, a 

 change which is usually manifested by a transition from a gray, green, 

 or blue color, to buff, brown, yellow, or red. The partial progress of such 

 oxidation is often shown in a fractured block or bowlder whose exterior 

 shows the latter colors, while the interior shows the former. The sul- 

 phides, of which common pyrites (FeS2) is the most famihar, are 

 oxidized into sulphates, and then sometimes pass on into the higher 

 oxides and other compounds. Thus copperas (FeSO^) arises from 

 pyrites (FeSa) by direct oxidation of both Fe and S. The sulphuric 

 acid of this compound, uniting with some base stronger than the fer- 

 rous oxide, gives rise to further oxidation and results in hematite 

 (FePa) and limonite (FeoOgjSHjO). In general, the mineral con- 

 stituents of the rocks in this upper zone take on their maximum states 

 of oxidation. This oxidation affects more or less profoundly the char- 

 acter of the rock as a whole. Deeper in the earth oxidation is less 

 prevalent, and the action is sometimes reversed and deoxidation takes 

 place. So also wherever organic matter is undergoing decomposition 

 deoxidation is likely to occur. 



Solution and deposition. — Solution preponderates in the upper part 

 of the zone of fracture, but deposition is prevalent in its deeper parts. 

 The calcium carbonate and silica dissolved near the surface are often 

 deposited below as calcite and quartz. The sulphates and other 

 sulphur compounds that are formed and dissolved near the surface are 

 apt to be changed into sulphides lower down by deoxidation. The 

 soluble oxides and other compounds formed near the surface are often 

 hkewise precipitated below. This is particularly true where the de- 

 scending waters encounter decomposing organic matter, and where they 

 mingle with waters that have followed other routes and have become 

 » Van Hise. 16th Ann. U. S. Geol. Surv., Pt. I, pp. 589-94. 



