. Van Rise — Metamorphism of Rocks and Rock Flowage. 77 



In order that crystals shall grow during the metamorphism 

 of rocks, it is necessary that the solutions shall be saturated or 

 supersaturated at the immediate place of crystal growth. As 

 underground there is always a superabundance of material 

 present as compared with the amount of water, we may sup- 

 pose that at a moderate depth below the surface, and especially 

 in the smaller spaces, where movement is slow, the solutions 

 are often saturated. In the laboratory it is a well known fact 

 that under conditions of saturation, with a superabundance of 

 material, the larger crystals grow at the expense of the smaller 

 ones, and that this process goes on more rapidly in propor- 

 tion as the temperature is high and the pressure is great. In 

 rocks this principle explains the growth of large mineral parti- 

 cles at the expense of smaller ones. 



Forces of Metamorphism. 



The work of underground water is accomplished by the 

 forces of mechanical action, heat, and chemical action. 



Dynamic action. — No changes in rocks take place without 

 movements of materials, small or great, for short or long dis- 

 tances. Wherever there is rearrangement of the elements, 

 there must be movements ; even in the case of a mineral pass- 

 ing from one form to an allotropic form, there is movement of 

 the molecules. 



Mechanical action assists water in its work by producing in 

 substances a state of strain which may pass to the stage of 

 pulverization. Moreover, dynamic action produces effects 

 through chemical forces and heat and by the agency of water. 

 The more important laws of the relations between pressure 

 and chemical action are as follows : " If we compress a chem- 

 ical system at constant temperature, there follows a displace- 

 ment of the equilibrium in that direction which is associated 

 with a diminution of volume. . . . Thus the solubility of a 

 salt in water, e. g., will increase with the pressure, provided 

 that the dissolving is associated with a contraction of the solu- 

 tion plus the salt, and, conversely, the solubility will decrease 

 if the separation of the salt (from the solution) is associated 

 with a diminution of the volume of the system." The first 

 of these cases is that applicable to underground water solu- 

 tions. " Moreover, those chemical forces are strengthened by 

 compression which condition a diminution of volume ; and 

 those chemical forces are weakened by compression which con- 

 dition an increase in volume." 



Heat. — All metamorphism takes place through the assistance 

 of heat. Nowhere upon the surface of the earth, nor within 

 the earth, is the temperature absolute zero. The activity of 

 the molecules, or their kinetic energy, increases in proportion 



