CHAPTER IV. 



THE ZONES AND BELTS OF METAMORPHISM. 



GENERAL CONSIDERATIONS. 



The various geological factors which bear upon metamorphism have 

 been briefly discussed in the introductory chapter. It is there held that 

 the geological factor of dominating importance is depth. Upon the basis 

 of depth it is stated that the known crust of the earth is divisible into upper 

 and lower zones of metamorphism; the first is called the zone of katamor- 

 phism, and the second the zone of anamorphism. It is further stated that 

 the zone of katamorphism is divisible into two belts, an upper belt of 

 weathering and a lower belt of cementation. 



While in the introductory chapter these general statements were made, 

 there was no attempt to show that they are correct. It is one of the pur- 

 poses of this and the following chapters to furnish evidence of the utility 

 of this classification, and to show that very different metamorphic effects 

 follow from the work of the same forces and agents in the different belts and 

 zones. In the present chapter a brief general statement will be made as to 

 the characteristic reactions of the different zones and belts. This statement 

 is primarily from physical and chemical points of view. The next chapter 

 will treat of the alterations of minerals with reference to the different zones 

 and belts. In succeeding chapters the alterations of the rocks in the belt 

 of weathering, the belt of cementation, and the zone of anamorphism will 

 be taken up in detail. The treatment will be primarily from the geological 

 point of view, but with reference to physical and chemical principles. 

 Finally, the alterations of the individual rocks will be considered. This 

 and the following chapters might be regarded as a consideration of the 

 metamorphism of the crust of the earth from the point of view of the 

 physical-chemical principles developed in Chapters II and III. 



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