CHAPTER VIII. 



THE ZONE OF ANAMORPHISM. 



DEFINITION OF ZONE. 



\ 



In Chapter IV the zone of anamorphism has been treated from a 

 physical-chemical point of view. It is there shown that this is a zone in 

 which there is great pressure in all directions, and that this is the dominant 

 factor controlling the reactions, so that changes take place which diminish 

 the volume of the rocks. The chemical law of energy is subordinate to 

 the dominant physical demand of pressure for reactions which lessen the 

 volume. Chemical reactions therefore take place with liberation or absorp- 

 tion of heat, as demanded by the pressure; but in large measure pressure 

 demands that chemical reactions take place with absorption of heat, such 

 as silication, dehydration, deoxidation. 



It is the purpose of the present chapter to consider the zone of anamor- 

 phism from a geological point of view. The zone of anamorphism is 

 bounded above by the belt of cementation of the zone of katamorphism. 

 It has no assignable boundary below. The depth of the upper surface of 

 the zone of anamorphism is very variable, depending upon many factors, 

 among which the strength of the rocks and the speed of deformation are 

 very important. It is not practicable to assign a minimum limit to this 

 depth, but it is highly probable that in many cases the upper surface of this 

 zone is not at greater depth than 1,000 to 2,000 meters, for rocks have been 

 modified by the reactions characteristic of the zone which have apparently 

 not been buried to a greater depth than this. By making assumptions so 

 as to give a maximum depth for the belt of cementation, the upper surface 

 of the zone of anamorphism is calculated to be at a depth not greater than 

 10,000 to 12,000 meters/ (See pp. 189-190.) 



«See Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 1, 1896, p. 593. 



MON XLVII— 04 42 657 



