774 A TREATISE ON METAMORPHISM. 



reversed in the deepest seated zone. However, since this latter zone is a 

 region of uplift, and the work required for the raising 1 of the superincumbent 

 strata must be added to that required for the interior deformation of the 

 rocks which we now see, no positive statement can be made as to whether 

 the total energy increased or decreased per unit mass in the deformation 

 of the deeper seated rocks. 



The conclusions of the previous pages concerning the energy required 

 for a given mass deformation at different depths gives a possible explana- 

 tion of the concentration of superficial deformation found in mountain 

 ranges. If the energy of deformation be less at the depth at which the 

 slates, schists, and gneisses develop than in more superficial belts, it is 

 possible that the more rigid outer shell of the earth may shear over the 

 nucleus in the zone at which the schists develop, the deformation being 

 widely distributed. Such shearing for a considerable area may require 

 less expenditure of energy than would be demanded for the similar defor- 

 mation of the rocks above; but during the earth movements, as a result 

 of cooling and other changes, the superficial material must certainly be 

 deformed and shortened," and at such places deformation is concentrated 

 and mountain ranges are formed. This subject is, however, better dis- 

 cussed in Chapter X, under the heading " Relations of rock flowage to 

 mountain making." 



CONCLUSION. 



The energy requLed to produce a given mass deformation increases 

 downward to the bottom of the zone where deformation is chiefly by 

 fracture. In deformation by recrystallization the energy required to pro- 

 duce a given mass deformation is probably less and may be much less than 

 that in the lower part of the zone of fracture. By observation we see that 

 recrystallization does take place wherever that process can occur. We have 

 shown that the amount of water present in the rocks wherever recrystalliza- 

 tion takes place is adequate to accomplish the process. In the fact that less 

 energy is required for recrystallization than for granulation lies the most 

 fundamental answer to the question why recrystallization rather than defor- 

 mation by fracture takes place wherever the conditions are such that the 

 former process can occur. Nature is a great economist, and expends the 

 minimum amount of energy to accomplish her work. 



"Van Hise, 0. E., Estimates and causes of crustal shortening: Jour. Geol., vol. 6, 1898, pp. 41-64. 



