54 A TREATISE ON METAMORPHISM. 



rocks by conduction, the coefficients are the same as in the transfer of heat 

 from the interior of the earth. The transfer of the heat of magma to adjacent 

 rocks is probably largely accomplished by convection. The magmas fre- 

 quently furnish heated solutions. Ordinary circulating waters approach or 

 come in contact with the igneous rocks; they thus become heated. The 

 heated waters move through the rocks controlled by the laws of under- 

 ground circulating waters (see pp. 146-153), and give up a part of their heat 

 to the surrounding rocks. The important metamorphosing effects of the 

 great igneous masses through water convection may extend several miles. 

 Contact metamorphism is sometimes restricted to the very marked effects 

 due to high temperature immediately adjacent to the igneous rock. How- 

 ever, the alterations thus produced by high temperature as the result of 

 direct conduction are probably small, compared with the widespread effects 

 resulting from the dispersal of heat and material by means of underground 

 waters. 



MECHANICAL ACTION AS A SOURCE OF HEAT. 



It is a well-known principle that when work is done involving strain 

 of solids within the elastic limit, or subdivision of solids, or differential 

 movement between solids in contact, the energy is partly transformed to 

 heat. Hence strain within the elastic limit, subdivision of the rocks, and 

 differential movement between rock masses and particles and within the 

 particles raise the temperature of the rocks, and this greatly increases the 

 speed and extent of the chemical reactions. Heat developed by mechanical 

 action is therefore an important factor in the metamorphism of rocks. 

 Indeed, the resultant metamorphic products are very different under con- 

 ditions of movement and under conditions of quiescence ; but heat is only 

 one of the factors entering into the differences. (See pp. 685-707.) 



CHEMICAL ACTION AS A SOURCE OF HEAT. 



Chemical action always produces a positive or negative heat effect, 

 and thus promotes or retards metamorphism. 



EFFECTS OF HEAT AND LIGHT ON ALTERATIONS OF ROCKS. 



The relations between metamorphism and heat and light may be gener- 

 ally stated as follows: The kinetic energy of the molecules of substances, 

 whether in the form of gas, liquid, or solid, is increased by heat and light, 



