REACTIONS OF ZONE OF ANAMORPHISM. 167 



filled. In the belt of weathering, the material continuously decreases in 

 volume due to solution; in the belt of cementation it continually increases in 

 volume due to deposition of material through reactions involving expansion 

 of volume. These changes of volume due to addition or subtraction of 

 material commonly involve decrease in specific gravity. In the belt of 

 weathering the mechanical results are disintegration and softening; in the 

 belt of cementation, cementation and induration. The belt of weathering 

 is therefore especially characterized by solution, decrease of volume, and 

 softening, resulting in physical degeneration. The belt of cementation is 

 especially characterized by deposition, increase of volume, and induration, 

 resulting in physical coherence. 



ZONE OF ANAMORPHISM. 



At a variable depth below the surface of the earth the pressure is so 

 great that it can not be supposed that considerable openings permanently 

 exist. The depth at which this condition of affairs is reached depends 

 largely upon the character of the rocks. For the strong rocks, as already 

 noted (p. 160), this depth, under quiescent geological conditions, may be as 

 great as 10,000 or 12,000 meters. If openings be originally present in 

 the rocks of the zone of anamorphism, as, for instance, sandstones, vesicular 

 lavas, etc., or be due to fracture while the rocks are not deeply buried, 

 when such rocks become sufficiently deeply buried to be in the zone of 

 anamorphism, it is certain that rock flowage will take place and the 

 openings will be closed, except possibly those of subcapillary size and 

 other minute openings in which water, carbon dioxide, or other liquids and 

 gases are occluded. In the zone of anamorphism there is great pressure in 

 all directions, and mechanical energy becomes the dominant factor which 

 controls the reactions. Changes consequently take place which diminish 

 the volume of the rocks. This volume change increases the specific grav- 

 ity, and contrasts with the volume changes of the zone of katamorphism. 

 The fundamental chemical law of energy in reference to heat is subordinate. 

 Reactions take place with the liberation or absorption of heat, depending 

 upon what is demanded by the pressure. Commonly, the preponderant 

 chemical reactions are those which take place with absorption of heat. 

 The depth at which pressure becomes dominant is variable, depending 



