CONCERNING THE PROCESS OF THRUST FAULTING 425 



side of the continent or may give rise to further slow adjust- 

 ments affecting the major part of the continental member. 

 In short, the mutual bearing of the crustal members upon one 

 another may be partly responsible for the usual, very wide- 

 spread epirogenic movements which follow mountain folding. 

 These conceptions may appear at first glance to be clearly reac- 

 tionary and out of accordance with modern views based on the 

 research and studies of the last twenty years, especially with the 

 conceptions of T. C. ChamberUn,^ who discusses deep, rigid earth 

 cones separated from one another by shear zones, but so far as is 

 known to the writer these ideas are not necessarily in conflict with 

 such conceptions of the earth's interior. It should be emphasized 

 that the writer's suggestions are based on the assumption, which 

 seems to be widely accepted, that there is a crustal member, sub- 

 ject to fracture and flow, separated from the underlying plastico- 

 rigid interior by a zone of shearing or flow. The recognition of 

 this zone of separation causing the crust and the inner mass at 

 certain times to be a discontinuous member, the parts of which 

 are subject to separate analysis, is absolutely fundamental to this 

 discussion. It may be that this zone of shearing is merely an 

 occasional phenomenon coming into being only as the result of 

 more fundamental processes, and that its depth (and in conse- 

 quence its competence) may vary from time to time with differ- 

 ent periods of stress, and it is likely that the development of such 

 a zone to an extent comparable with the width of a continent is 

 an extreme and unusual condition. And it must be recognized 

 that the writer advances this study as a contribution to the analysis 

 of low-angle faulting, in its proper relation to the other factors 

 Hsted by Chamberhn and Miller' in the resume of their paper 

 previously quoted, with this difference, arising largely from the 

 different manner of approach, that the writer would not Ust 

 ''length of deformed area with respect to its other dimensions 

 (after analogy of long column)" among the minor factors, but 

 rather among the major factors in low-angle faulting. Neverthe- 

 less, it is held to be but one of several major factors. The revision 



' The Origin of the Earth (1916), chap, viii, and Jour. Geol.,'XXNl (1918), 197. 

 2 Jour. Geol., XXVI (1918), 44. 



