76 Van Ilise — Metamorpkism of Rocks and Rock Flowage. 



A critical examination of the classifications of metamorphism 

 shows that the kinds of metamorphism recognized in the text- 

 books are based upon the idea that the particular force or agent 

 mentioned is dominant in the production of the particular 

 metamorphism. However, the classifications involve different 

 factors, not belonging to the same category. For instance, 

 thermo-metamorphism refers to heat; contact metamorphism 

 refers to the contiguity of an igneous rock ; hydro-metamor- 

 phism refers to the presence of water. As a matter of fact, 

 all of the different kinds of metamorphism are related in the 

 most intricate manner, and certain kinds of metamorphism 

 which have been called thermo-metamorphism might just as 

 well be called hydro-metamorphism. 



Underground Flowage of Water. 



Underground water, the agent of metamorphism, needs to 

 be considered from two points of view — (1) its movement and 

 (2) its work. 



(1) Movement of underground water. — The movements of 

 underground waters are dependent upon (a) head, (b) the 

 underground openings, and (<?) viscosity. 



(a) The flowage of underground water is caused by head. 

 Head is due chiefly to difference in the level at which the 

 water enters and issues from its underground course. It may, 

 however, be partly due to difference in temperature in the 

 descending and ascending columns. 



(b) Openings in rocks may be divided into (1) openings 

 which are larger than those of capillary size, (2) capillary open- 

 ings, and (3) subcapillary openings. 



To movements of water in openings larger than those of 

 capillary size the ordinary laws of hydrostatics apply. To 

 movements of water in capillary openings the laws of capillary 

 flow apply. By subcapillary openings are meant those in 

 which the attraction of the solid molecules extends from wall 

 to wall, and in these flowage is either exceedingly slow or does 

 not occur. 



(<?) The elements entering into viscosity are the concentra- 

 tion of the solutions and the temperature. The viscosity of 

 water decreases very rapidly with rise of temperature, and 

 hence the high temperature in the lower part of the zone of 

 fracture is very favorable to flowage. 



(2) Work of underground water. — The potency of water as 

 an agent through which metamorphism may take place is due, 

 according to the modern ideas of physical chemistry, to its 

 capacity to separate substances which it holds in solution into 

 their free ions. In this power of ionization it exceeds all 

 other solvents. 



