40 A TREATISE ON METAMORPHISM. 



of the alterations; and contact metamorphism refers to the contiguity of 

 an igneous rock. 



As a matter of fact, all of these different kinds of metamorphism are 

 related in the most intricate manner, and certain metamorphic results 

 which have been attributed to one of these forces, agents, or processes 

 could equally well be attributed to another. For instance, in many cases 

 metamorphism known as thermo-metamorphism might just as well be 

 called hydro-metamorphism, or regional metamorphism be called dynamic 

 metamorphism, or contact metamorphism be called thermo-metamorphism 

 or chemical metamorphism. 



It follows from the above that a satisfactory classification of meta- 

 morphism based upon chemical forces alone, or physical forces alone, or 

 individual processes, is quite out of the question. It appears to me that 

 the only workable classification of metamorphism is geological. (See 

 pp. 43-44.) 



GEOLOGICAL FACTORS AFFECTING THE ALTERATIONS OF ROCKS. 



The more important geological factors affecting the alterations of rocks 

 are: Composition; structures and textures; porosity; water and gaseous 

 content; climatic and geographic conditions; time; environment; degree 

 of movement; depth. Many physical factors enter into each of these 

 geological factors. 



At present only general statements will be made with reference to 

 these factors, but on later pages the effect of each of them will more 

 clearly appear. 



composition — J n so f ar as rocks are composed of minerals which are 

 permanent under the existing conditions, or are composed of minerals 

 which may exht under a wide variety of conditions, this is favorable to 

 stability. 



structures and textures — J n so f ar as there are coarse structures and textures, 

 this is favorable to permanency, for it will be seen that fine material is 

 more readily altered than coarse material. 



porosity — Porosity has a very important influence upon the rapidity of 

 change. In proportion as rocks are porous the agents of alteration, gases 

 and water, may enter and rapidly circulate. In proportion as they are 

 dense, the amount of water present is small and the circulation is slow. 



