AGENCIES CONCERNED IN ROCK CHANGES 39 



CLASSIFICATION OF METAMORPHISM. 



The forces of metamorphisin are chemical energy, gravity, and heat 

 and light. The agents of rnetamorphism are gases, liquids, and organic 

 compounds. 



A critical examination of the published classifications of metaniorphisni 

 shows that the kinds of rnetamorphism recognized are based upon the idea 

 that one force or agent or process is dominant in the production of a 

 particular kind of rock. But in all of the various kinds of rnetamorphism 

 ordinarily recognized in classifications, such as thermo-metamorpliism, 

 hydi-o-metamorphism, chemical rnetamorphism, static rnetamorphism, pres- 

 sure rnetamorphism, dynamo-metamorphism, regional rnetamorphism, and 

 contact rnetamorphism, all of the forces above mentioned are required, and 

 also the chief agent, water. There is no rnetamorphism of a rock without 

 the presence of heat, and hence all rnetamorphism is partly thermo- 

 rnetamorphisni; there is no rnetamorphism without the presence of water 

 solutions, and hence all rnetamorphism is partly hydro-metamorphism; there 

 is no rnetamorphism in which chemical action does not enter, and hence all 

 rnetamorphism is partly chemical rnetamorphism ; there is no rnetamorphism 

 without motion, and hence, in an exact sense, all rnetamorphism is dynamic. 

 In the alterations of rocks the forces of rnetamorphism in each case produce 

 atomic, molecular, and mechanical changes." When it is realized that in 

 all the varieties of rnetamorphism mentioned chemical action, heat, and 

 dynamic action enter as important factors, and that water is present and 

 active wherever rnetamorphism occurs, it becomes self-evident that the 

 classifications ordinarily given are not satisfactory. Moreover, the classifi- 

 cations involve different factors not belonging to the same category, some 

 being physical, some chemical, some geological, some referring to an ag*ent, 

 others to a cause. For instance, thermo-metamorphism refers to heat; 

 hydro-metamorphism refers to the presence of water; chemical rnetamor- 

 phism refers to the action of chemical forces; static rnetamorphism and 

 pressure rnetamorphism refer to quiescent conditions; dynamo-metamorphism 

 refers to conditions of motion ; regional rnetamorphism refers to the extent 



«If this be true, it is clear that a classification of rnetamorphism into paramorphism, metatrophy, 

 and rnetataxis, restricting these terms to atomic, molecular, and mechanical changes, respectively, 

 as proposed by A. Irving, is wholly impracticable. Irving, A., Metamorphism of rocks, London, 

 1889, pp. 4-5. 



