160 A TREATISE ON METAMORPHISM. 



It has just been stated that the nature of the metamorphism varies 

 greatly with depth. The physical reasons for this are that, as depth 

 increases, temperature and pressure increase. It has been seen in Chapters 

 II and III that where the pressure is moderate chemical reactions are likely 

 to be such that heat is liberated, and this is a fact whether the reactions 

 decrease or increase the volume. It has also been seen that where the 

 pressure is great this is likely to be the controlling factor, and that under 

 such circumstances reactions take place which lessen the volume of the 

 materials. Whether the reactions take place with liberation of heat or 

 with absorption of heat is a subordinate matter; but very commonly the 

 reactions are of a kind that absorb heat. 



When the law of chemical affinity controls, and the reactions take 

 place with liberation of heat irrespective of the volume change, the reac- 

 tions may be said to be chemical-physical reactions. Where pressure is a 

 dominant factor and reactions take place with diminution of volume 

 irrespective of the heat change, the reactions may be said to be physical- 

 chemical. It is because variations in the geological factor of depth result 

 in these contrasting reactions that the lithosphere is divisible into a zone of 

 katamorphism and a zone of anamorphism. 



ZOSTE OF KATAMORPHISM. 



From the surface of the earth to a very considerable depth below the 

 surface (for strong rocks possibly 10,000 or 12,000 meters under quiescent 

 geological conditions) the rocks as originally formed may contain many 

 openings, as, for instance, those of sandstones, vesicular lavas, etc. Even if 

 not originally porous deformation may fracture the rocks and thus produce 

 many openings. Where the rocks contain openings chemical reactions may 

 take place, increasing .the volume of the material without rupturing the 

 rocks and without raising them to a higher position. In the outer litho- 

 sphere the pressures and temperatures are moderate. Under such circum- 

 stances the reactions which take place are controlled mainly by the laws 

 of chemical affinity, not by the influence of pressure. At low temperatures 

 the fundamental chemical law is that, on the whole, the preponderating 

 chemical reactions are those which take place with the liberation of heat in 

 accordance with the first part of van't Hoff's law. Therefore in this zone the 

 occurrence of a reaction in the alteration of a rock is favorable to further 



