740 A TREATISE ON METAMOEPHISM. 



upon the gradation and relations between granulation and recrystallization 

 is furnished by the Algonkian rocks of the Black Hills." There conglom- 

 erates, quartzites, slates, schists, and gneisses occur in intimate relations to 

 one another, due largely to varying character of material. Nearly every 

 phase of deformation, from that in which granulation is the dominant 

 process to that in which recrystallization is the dominant process, is there 

 represented. 



TEMPERATURE. 



Low temperature is favorable to granulation; high temperature is 

 favorable to recrystallization. If»Barus's experiments upon the solubility 

 of glass 6 be a guide as to the silicates, the temperature of 185° C. is 

 more nearly crucial between the processes than any other. Below 185° 

 C granulation is likely to be prevalent, especially if the deformation be 

 rapid. Above 185° C, if sufficient water be present, recrystallization is 

 probabty so rapid that the mechanical strains do not go far before they are 

 largely obliterated by recrystallization. 



Temperature increases with depth: the less the depth the greater 

 the tendency to deformation by granulation, and the greater the depth the 

 greater the tendency to recrystallization. But it must be remembered 

 that the mechanical work of deformation itself develops heat, which can 

 escape only by conduction or convection. Therefore, during mountain- 

 making periods, temperatures sufficient for recrystallization may exist 

 much nearer the surface than under quiescent conditions, and consequently 

 recrystallization takes- place rapidly at no great depths. 



Furthermore, the temperature is raised by intrusive igneous rocks. 

 The heat of the intrusives is conveyed to the adjacent rocks, both by 

 conduction and hj convection through water. Hence the presence of 

 igneous rocks is favorable to reciystallization. This has already been 

 dwelt upon. The numerous broad zones of schists and gneisses in which 

 recrystallization is complete about great batholiths, with schistosity every- 

 where parallel to the sides of the intrusives, have been described. (See 

 pp. 716-717.) 



« Van Hise, C. R., The pre-Canibrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 

 1890, pp. 214-230. 



*Barus, C, Hot water and soft glass in their thernio-dynamic relations: Am. Jour. Sci., 4th ser., 

 vol. 9, 1900, pp. 161-175; vol. 6, 1898, p. 270. 



