FORCES OF METAMORPHISM. 51 



HEAT AND EIGHT. 



Heat and light are forms of energy of the first importance. It has 

 already been noted that their ultimate source is largely gravity. Heat 

 is always present as a factor in metamorphism, for nowhere upon the surface 

 of the earth nor within the earth is the temperature absolute zero. Other 

 things being equal, the higher the temperature the more rapidly do alterations 

 of rocks take place. Light also affects all parts of the earth at the surface. 

 In metamorphism heat and light should be considered from two points of 

 view — (1) sources of heat and light, and (2) effect of heat and light upon 

 the alterations of rocks. 



SOURCES OF HEAT AND LIGHT. 



Heat and light agential in the alteration of rocks are derived (a) from the 

 sun, (b) from deep within the earth by conduction or by convection through 

 water or magma, (c) from mechanical action, and (d) from chemical action. 

 The heat from all these sources is important; light, however, is derived 

 chiefly from the sun, that from the other three sources being of little 

 consequence. 



THE SUN AS A SOURCE OF HEAT AND LIGHT. 



The heat and light of the sun are forces of the first order of magnitude 

 in the alterations of rocks. The effect of these forces needs to be considered 

 in four cycles — the cycle of the solar system, that of the seasons, that of 

 the cyclone, and that of the day. 



The solar-system cycle is the most important. This cycle involves 

 two factors — the absolute temperature and change in temperature. 



As to the absolute temperature, were it not for the heat and light of 

 the sun it is certain that the temperature of the surface of the earth would 

 not greatly exceed that of the interstellar spaces. Probably it would be 

 — 200° C, or even lower. At the present time the temperature of the 

 surface of the earth averages 10° C. (283° C. absolute) or more. Therefore 

 the temperature of all the upper zone of the earth is 200° C, or more, 

 greater than it would be without the heat from the sun. Were it not for 

 this heat the water in the outer zone of the earth would be congealed, and 

 the atomic and molecular energy would be greatly diminished. As a 

 comparatively slight increase of temperature over that prevalent at the 

 surface of the earth increases greatly the speed of alteration of rocks, it is 



