44 A TREATISE ON METAMORPHISM. 



good classifications, viz, that a factor or factors of the same class shall be 

 used throughout as a primary basis. While the primary classification of 

 metamorphism will be based upon depth, it is recognized that there are no 

 sharp dividing lines between the zones and belts. In metamorphism, as in 

 every other branch of geology and of science, there is complete gradation 

 between the phenomena of the various classes. 



However, it has been seen that depth is not the only geolog-ical factor 

 of consequence in metamorphism. It is recognized that various other 

 geological factors enter into the alteration of a given rock. Moreover, these 

 various factors overlap. In the discussion of the zones of metamorphism- 

 the geological factors of subordinate importance will be given proper 

 consideration. 



Before considering the general alterations in the zones of katamor- 

 phism and anamorphism, and the alterations of the individual minerals 

 and rocks in these zones, it is necessary to consider the forces and the 

 agents of metamorphism from chemical and physical points of view. 



It should therefore be recalled that the forces of metamorphism are 

 chemical energy, gravity, and heat and light, and that the agents of 

 metamorphism are gases, liquids, and organic compounds. The rocks 

 are the materials upon which these forces and agents work. The forces 

 of metamorphism are considered in Chapter II, the agents of metamor- 

 phism in Chapter III, and the work of these forces and agents upon the 

 rocks in the later chapters. 



