CONCERNING THE PROCESS OF THRUST FAULTING 421 



center, the part most susceptible in nearly rigid members. Never- 

 theless, the rocks within a mile of the earth's surface are, in general, 

 more nearly rigid in behavior than those near the zone of flow. 

 Furthermore, there is more force to be transmitted presumably 

 near the surface, because the length of the arc is greater at the 

 surface than near the center, and the accumulative forces are pro- 

 portional to the length of the arc to be accommodated to the 

 shrinking. Consequently, there is a greater thrust transmitted 

 in the rocks near the surface than through those near the zone of 

 flow for two reasons, because the rocks are more nearly rigid, 

 transmitting a greater proportion of the forces extant, and because 

 near the surface there is more total horizontal- force to be trans- 

 mitted. These conditions may be analyzed as an unequally dis- 

 tributed force of the rotational type. Since the strain is greatest 

 where relief of pressure is easiest, it follows that the strain is 

 greater near the surface than at depth, which would result in a 

 rotational strain even if the forces were translatory.^ Thus rota- 

 tional strain results from a ready relief of pressure near the 

 surface, and it appears reasonable that the original stresses applied 

 also are rotational. 



The length of the crustal members. — The commonly deformable 

 crust is considered to be a discontinuous structure in the form of 

 a supported hollow sphere composed of irregularly shaped, curved 

 strata plates which are geographically coincident with the con- 

 tinental and oceanic segments. These members of the structure 

 exert thrust forces on one another which are locaHzed in their 

 maximum application at the planes of contact, i.e., the border- 

 land of continents and oceans. Roughly speaking, the members 

 involved in the compression are as long as the continents and 

 ocean basins are wide. The forces, however, are not of equal 

 intensity everywhere along the length of each member; under 

 normal conditions they are least in the center and greatest at the 

 ends. If the earth's crust were truly rigid the intensity of thrust 

 would be equal throughout the length of each member, each seg- 

 ment would serve as a footing for its neighboring segments, result- 

 ing in failure of the segments near their centers. Apparently the 



' F. D. Adams and J. A. Bancroft, Jour. Geol., XXV (1917), 637; R. T. Cham- 

 berlin and W. Z. Miller, ihid., XXVI (1918), 35-37. 



