ENERGY REQUIRED FOR DEFORMATION. 773 



This implies crustal deformation which extends to the surface, and therefore to 

 places where the conditions are not such that recrystallization can take place. 



From the foi-egoing considerations, I believe that the amount of work 

 done, in order to produce the same mass deformation of the rocks, increases 

 to a certain depth and then decreases until the belt of the perfect schists is 

 reached. How far down this holds I am unable to conjecture, but believe it 

 is probable that it does so at least as deep as the zone in which the schists 

 formed by recrystallization develop, and that it may continue much farther. 



The deformation of this deep-seated zone may or may not require the 

 elevation of the superincumbent mass. Where the superincumbent mass is 

 not elevated it is concluded that the energy required for deformation by 

 recrystallization per unit mass is probably less than that required for defor- 

 mation by granulation, and may be less than that required to produce the 

 spaced fractures which occur near the surface. Where the deformation is 

 of the kind which requires the elevation of the superincumbent material, 

 energy is needed, not only to do interior work of deformation, but to elevate 

 this material. Where these conditions obtain it may be that the amount of 

 energy required to produce the deformation steadily increases with depth 

 on account of the energy required for lifting the load in addition to that 

 required for the interior deformation. 



So far as I know, the region in America which best illustrates all the 

 phenomena from deformation by widely spaced thrust faults and by joints to 

 the interior deformation of recrystallization is the Southern Appalachians. 

 In the Great Valley the Paleozoic rocks are little deformed except by thrust 

 faults and joint folds. Eastward, deeper into the mountains, the faults and 

 joints become closer together, and are finally replaced by numerous closely 

 distributed fractures. At the same time the rocks show more and more 

 evidence of metamorphism, first by granulation, and second by recrystalliza- 

 tion. In the cores of the mountains are rocks which have largely or 

 completely recrystallized, being slates, schists, and gneisses, with well- 

 developed cleavage. In the valley we find the alterations to affect but a 

 small part of the mass of the rocks now exposed. The deeper we go into 

 the mountains the larger is the proportion of the material which was affected 

 by the alterations, and in the schists in the core the entire mass was modi- 

 fied, both in a mechanical and in a chemical way. In this passage it 

 becomes clear that for a certain distance the amount of energy required for 

 deformation per unit mass increased, but this tendency may have been 



