CHAPTER XIX. 

 METAMORPHISM OF ROCKS. 



General Definitions. The term metamorphism, from the 

 Greek jaera, interchange, and fJ^op<t>T] form (+ism), has been 

 appHed to the process of alteration of rock masses in any manner 

 whatsoever. The product of such metamorphism is metamorphic 

 rock, often an entirely new substance so far as form and other 

 physical characters are concerned. It is true that the term is sel- 

 dom used in the comprehensive sense, metamorphism being gener- 

 ally considered as brought about by the agencies of heat and pres- 

 sure, due to contact with heated bodies, or from being subject to 

 dynamic disturbances. There can be no question, however, that 

 these are only special phases of metamorphism, and that they can- 

 not be separated genetically from the more quiet processes of altera- 

 tion to which rocks are subject under ordinary physical conditions. 

 As such, the weathering of rocks must be included. Metamorphism 

 may bring about a change in one or more of the following charac- 

 ters : composition, texture, structure. The change in composition 

 may be chemical alteration or mineralogical change, new minerals 

 appearing in a rock as a result of metamorphism. and old minerals 

 may disappear. Texture may be changed completely, as from compact 

 limestone to crystalline marble, or special textures, characteristic of 

 metamorphic rocks, may be produced, such as cataclastic, parallel 

 orientation, etc. Original structures, such as flow structure, stratifi- 

 cation, etc., may be obliterated and new ones produced, such as 

 cleavage, fissility, joints, slatiness, schistosity and gneissosity (Van 

 Hise-34) . 



The Forces Producing Metamorphism. The forces at work 

 in the production of metamorphism are given by Van Hise (34:^5) 

 as: I, chemical energy; 2, gravity; 3, heat and light; while the 

 agents through which these work upon the rocks are: (a) gases, 

 (b) liquids, and (c) organic compounds. Organic compounds are 

 mainly active in the surface belt where weathering of rocks takes 



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