CHAPTER II. 



THE FORCES OF METAMORPHISM. 



As already seen, the important forces of metamorphism are chemical 

 energy, gravity, and heat and light. 



CHEMICAL ENERGY. 



When different compounds are brought together molecular interchange 

 may occur between them. As a result the compositions of the compounds 

 are mutually changed. Such interchange is chemical action. Chemical 

 action usually involves expenditure of chemical energy, which is one of the 

 main original sources of energy; but it will be seen that other forms of 

 energy may be transformed into chemical energy, and chemical action in 

 this way be promoted. 



Chemical action may take place between gas and gas, gas and liquid, gas 

 and solid, liquid and liquid, liquid and solid, and solid and solid. Chemical 

 action, or molecular interchange, involves movement between the atoms and 

 molecules. Chemical action therefore never takes place without dynamic 

 action. So far as we know chemical action never takes place without the 

 presence of heat. Under the conditions obtaining in the crust of the earth 

 chemical action is usually promoted by heat and by mechanical action. As 

 chemical action always produces a heat effect, positive or negative, such 

 action may result in the liberation or in the absorption of heat. The heat 

 effect may hasten or retard further chemical action. In so far as chemical 

 action results in the liberation of heat, it usually hastens further chemical 

 action, and therefore promotes metamorphism; in so far as chemical action 

 results in the absorption of heat, it usually retards further chemical action, 

 and therefore stays metamorphism. It is shown (pp. 170-186) that both 

 classes of reactions take place on a very extensive scale. 



In consequence of chemical action material may be added to or sub- 

 tracted from a given mineral. A mineral may alter into two or more other 

 minerals with the simultaneous addition or subtraction of material. Two 

 or more minerals may unite to produce a single mineral. Either of these 



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