CHANGES IN TEMPERATURE. 53 



as rook alterations now taking- place are concerned, the sun may be regarded 

 as furnishing to the earth a uniform amount of heat. 



The seasonal changes of temperature are very important, at the surface 

 ranging from 30° C. or less to as much as 80° C. However, the depth to 

 which the seasonal change produces an effect is not great, probably about 

 15 meters. 



The cyclonic changes of temperature may be very great, ranging from 

 a few, degrees to about 70°, but the depth to which these changes extend is 

 slight, probably less than 3 meters. 



The diurnal changes in temperature are scarcely less than the cyclonic, 

 ranging from to 50° C. or more; but the depth to which the diurnal 

 changes extend is insignificant, probably but a fraction of a meter. 



From the foregoing it is plain that the heat and light derived from the 

 sun are of very great direct importance in the chemical and mechanical 

 changes that rocks undergo. It will also be seen that the various changes 

 of temperature as well as the absolute temperatures are of great consequence. 

 Moreover, the heat and light of the sun exert a very important indirect 

 influence upon metamorphism by reason of their being the sole source 

 of the energy which produces plants and animals, and these agents will be 

 seen to have a far-reaching effect upon the alterations of rocks. The effects 

 of the heat and light derived from the sun are fully considered in Chapters 

 VI, VII, and VIII, on "The belt of weathering," "The belt of cementation," 

 and "The zone of anamorphism." 



HEAT DERIVED FROM WITHIN THE EARTH BY CONDUCTION OR CONVECTION THROUGH 



WATER OR MAGMA. 



The amount of heat derived by the crust of the earth from the interior 

 depends upon the conductivity of the various rocks and upon the convec- 

 tional movements of magma and water. 



The heat conductivity of the majority of rocks is between 0.4 and 0.6, 

 silver having a conductivity of 100. It is apparent that the conductivity 

 of rocks is very low as compared with that of the metals, but it can not be 

 doubted that there is a steady but slow flow of heat by conduction from the 

 interior of the earth to the zone of observation. 



The amount of heat derived by the crust of the earth from intrusions 

 of igneous rocks is very great. So far as this heat passes into the adjacent 



