162 A TREATISE ON METAMORPHISM. 



and carbon dioxide. Analyses of rocks in the upper zone of metamorphism 

 show that the amount of combined water runs as high as 4.42 per cent in 

 shales (see p. 744), and it probably averages as high as 1.64 a per cent. 

 When it is remembered that the zone of katamorphism extends to a depth 

 of thousands of meters, it is apparent that the amount of water which is 

 thus fixed in the rocks by the process of hydration is enormous. However, 

 it will be seen that the process of hydration, like that of carbonation, is 

 reversed in the zone of anamorphism. 



By the statement that oxidation, carbonation, and hydration are the 

 very important characteristic reactions of the zone of katamorphism it is 

 not meant to imply that the reverse reactions do not take place to some 

 extent. In fact, deoxidation, decarbonation, and dehydration all occur; 

 but oxidation, carbonation, and hydration are greatly preponderant, and 

 indeed dominant over the reverse reactions. 



Summarizing so far as the energy factors are concerned, the changes in 

 volume commonly absorb heat, the chemical reactions dominantly liberate 

 heat and only exceptionally absorb heat. The heat liberated by the chem- 

 ical reactions is certainly very much greater than the sum of that absorbed 

 by the volume changes and that absorbed by the exceptional chemical 

 reactions. Therefore, so far as the rocks of the zone of katamorphism are 

 concerned, the total of the volume and chemical changes results in the 

 liberation of heat and the dissipation of energy. 



The minerals formed in the zone of katamorphism are comparative^ 

 few in number, with low specific gravities and probably for the most part 

 comparatively simple molecules; hence the propriety of calling this zone 

 the zone of katamorphism, or katamorphic, zone. This use of the term 

 katamorphism is parallel to the use of the term katabolism in biology to 

 designate those chemical changes within a living body which result in the 

 production of simple compounds from more complex ones. The zone of 

 katamorphism may therefore be defined as the zone in which alterations of 

 rocks result in the production of simple compounds from more complex ones. 



The zone of katamorphism is divisible into two belts, (1) an upper 

 belt of weathering, and (2) a lower belt of cementation. The belts are 



ffl This is the average taken from analyses of shales, sandstones, limestones, and volcanic and 

 crystalline rocks, given by F. W. Clarke in Bulls. U. S. Geol. Survey No. 78, pp. 36-37, and No. 168, 

 pp. 16-17. 



