712 A TREATISE ON METAMORPHISM. 



is certain that the batholiths of great magnitude which now show a large area 

 must at an earlier stage of erosion have been in the position j ust described 

 with reference to the surrounding rock. Therefore, while metamorphism is 

 a function of the size of the intrusive masses, it is not a function of the 

 size of the masses exposed. When the masses are small and do not rapidly 

 expand in size below, the appreciable metamorphic effect may extend only 

 a few centimeters or a few meters; but the metamorphosing effect of the 

 great batholiths, some of them scores of kilometers in diameter, may 

 extend from a kilometer to many kilometers. 



THE TEMPERATURE. 



The temperature effect depends largely upon the size of the intrusive 

 mass, and to a less extent upon its temperature. The greater the size of 

 the intrusive mass and the higher its temperature the larger the amount 

 of heat it is able to furnish to the surrounding rocks and the farther that 

 heat penetrates by conduction and convection. In the matter of heat the 

 intrusives in the zone of anamorphism are in a vastly more advantageous 

 position than those of the belt of cementation. In the latter belt the rocks 

 are comparatively near the surface. There is a somewhat rapid circulation 

 of the water, so that the heat is dispersed, slowly to be sure, but with rela- 

 tive speed as compared with the zone below; but in the zone of anamor- 

 phism the great store of heat of an intrusive mass is so far below the 

 surface, and the circulation is so slow, that it is a sustained factor in 

 increasing the temperature of the surrounding rocks, certainly for geo- 

 logical epochs, and frequently in the case of the larger masses for geological 

 periods. Thus there is ample time during high temperature for the altera- 

 tions to take place. Moreover, the heat through conduction and convection 

 slowly disperses itself from the central mass over a very wide area and 

 produces a metamorphic effect upon the rocks far beyond the distance to 

 which the materials of the magma can possibly penetrate. 



\MorNT OF WATKl; I'HI'SKVi. 



The amount of water, as fully explained in other connections (see 

 pp. 741-748), is of the utmost importance in determining whether the meta- 

 morphism is by granulation or by recrystallization. In proportion as water 

 is abundant recrystallization is likely to take place. Where the intrusive 



