186 A TREATISE ON METAMORPHISM, 



In conclusion of this part of the subject it may be said that in the 

 zone of katamorphism the alterations are mainly controlled by the chemical 

 law that reactions take place with liberation of heat, and this ordinarily 

 results in increase of volume, provided the compounds which form remain 

 as solids. In the zone of anamorphism the reactions are mainly controlled 

 by the physical law that reactions take place with decrease of volume, and 

 this commonly results in chemical reactions with absorption of heat. In 

 the upper zone chemical law is the determinative factor in the reactions; 

 in the lower, physical law. In the upper zone the important chemical 

 reactions are those of oxidation, carbonation (involving desilication), and 

 hydration; in the lower zone the important reactions are those of deoxida- 

 tion, silication (involving decarbonation), and dehydration. In the upper 

 zone the minerals are few in number, of low specific gravity, and probably 

 of simple molecular structure; in the lower zone the minerals are 

 numerous, of high specific gravity, and probably of complex molecular 

 structure. 



If one were to select three words which roughly represent the charac- 

 teristics of the alterations in the belt of weathering, the belt of cementation, 

 and the zone of anamorphism, these three words would be, respectively, 

 destruction, construction, and reconstruction. 



GENERAL CONSIDERATIONS. 



It is now apparent that a geological classification of metamorphism 

 dependent upon depth carries with it profound chemical and physical 

 significance. Not only is the classification applicable to all parts of the 

 earth, but the alterations of the zones of katamorphism and anamorphism 

 are more fundamentally different than any distinction heretofore made with 

 reference to metamorphism. Also, the alterations of the belt of weathering 

 and the belt of cementation of the zone of katamorphism, again dependent 

 upon depth, are very different so far as the geological facts are concerned, 

 but are closely allied from the chemical and physical point of view, and 

 therefore belong together in a single zone. 



It is further to be noted that the classification is based upon one idea 

 throughout. There is no overlapping, although the different belts and 

 zones are side by side and grade into one another. 



After the universal geological applicability of the classification proposed 



