168 A TREATISE ON METAMORPHISM. 



upon the character of the rock and upon whether the conditions are mass- 

 static or mass-mechanical. 



It has been seen that at the moderate temperatures of the zone of kata- 

 morphism the preponderant chemical reactions are those which take place 

 with the liberation of heat. As the depth below the surface increases, the 

 temperature ever becomes higher; and consequent^ the temperature may 

 become so high that the tendency for chemical reactions to take place 

 with the liberation of heat is less' dominant, and at sufficiently great depths 

 the heat may be so great that this tendency ceases, or is even reversed. 

 Or, using the words of van't Hoff, at high temperatures the preponderating 

 chemical reactions, or associations, which take place at lower temperatures 

 with the development of heat are replaced by preponderating chemical 

 reactions, or dissociations, which take place with the absorption of heat." 

 However, at moderate depths in the zone of anamorphism under ordinary 

 conditions the temperatures are not very high. For instance, at a depth of 

 9,000 meters the temperature is probably in the neighborhood of 300° C. 

 Therefore, so far as the temperature is concerned, for that part of the crust 

 of the earth within observation the preponderant chemical reactions would 

 probably take place under the first part of van't Hoff 's law, rather than under 

 the second part, if it were not for the pressure. But the pressure is the 

 dominant factor which controls the reactions. The rocks in this zone are 

 under so great pressure in all directions that this fact demands chemical 

 reactions which produce diminished volumes irrespective of whether heat is 

 liberated or absorbed by them. 



The very important reactions in the zone of anamorphism are silica- 

 tion, or union of silicic acids with bases producing silicates, and dehydra- 

 tion. Deoxidation is subordinate. The process of silication commonly 

 takes place upon carbonates, and consequently involves decarbonation and 

 the liberation of the carbon dioxide, which may escape and thus the volume 

 be decreased. To what extent the pressure is the controlling factor in the 

 production of this reaction is difficult to say. Probably it is the dominant 

 cause, but it is possible that at the temperatures which prevail in this zone 

 silicic acid may be relatively more active than at the lower temperatures of 

 the zone of katamorphism, where carbonic is the stronger acid. 



«Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., Loudon, 1895, 

 p. 583. 



