166 A TREATISE ON METAMORPHISM. 



rocks. The material deposited is derived from the belt of weathering- or 

 from the alterations within the belt of cementation itself. Much of the 

 material dissolved in the belt of weathering is continuously transferred to 

 the belt of cementation by the downward movement of water. The total 

 amount of material which is thus derived from the belt of weathering is not 

 limited to the thin belt which exists at any given time; for, as a result of 

 denudation, the belt of weathering is constantly migrating downward and 

 encroaching iipon the upper part of the belt of cementation; and thus 

 there is never a lack of material for solution in the belt of weathering which 

 may be dissolved and transferred to the belt of cementation. Within 

 the belt of cementation itself the reactions of oxidation, carbonation, and 

 hydration all increase the volume, provided all the compounds formed, or 

 a large part of them, remain as solids. The material added to the belt of 

 cementation from the belt of weathering, and the reactions within the belt 

 of cementation, furnish an abundant supply of material for deposition in 

 the openings of the rocks, whether these openings be those originally 

 present or produced by orogenic forces. And, as a matter of fact, in the 

 belt of cementation the openings are continuously filled by mineral 

 matter and finally closed; but this does not show that solution may not 

 preponderate over deposition in this belt if the effect upon the original rocks 

 and the openings both be considered. (See pp. 612-617.) The mechanical 

 result of the various processes is to indurate the rocks. The processes of 

 the belt of cementation are constructive. The belt of cementation, from a 

 geological point of view, is fully considered in Chapter VII. 



BELTS OF WEATHERING AND CEMENTATION CONTRASTED. 



The alterations in the belts of weathering and cementation, while not 

 so fundamentally different as those in the zones of katamorphism and 

 anamorphism, contrast strongly. In the belt of weathering, of the great 

 reactions characteristic of the zone of katamorphism — -oxidation, carbonation, 

 and hydration — all are important, but carbonation is most characteristic. In 

 the belt of cementation, of these reactions hydration is most important. In 

 the belt of weathering, solution greatly dominates over deposition. In the 

 belt of cementation solution and deposition are more nearly balanced, but 

 because of reactions which increase the volume of the rocks the openings are 



