CONCERNING THE PROCESS OF THRUST FAULTING 423 



Let it be noted, however, that this band of great stress is supposed 

 to be but a part of the total member under compression. 



The depth of the crustal members. — According to Chamberh'n 

 and Salisbury^ (writing in 1904) the average thickness of the 

 folded shell is probably between three and five miles. From the 

 context it appears that the estimate was based, in part at least, 

 upon a reconnaissance report on the amount of uplift and shorten- 

 ing involved in the Appalachian folding, and the resulting figure 

 probably should be modified especially by the results of R. T. Cham- 

 berhn before quoted, as well as by the many contributions based 

 upon experiments made since 1904. Heim concludes from his 

 studies in the Alps that there the great deformations were rela- 

 tively shallow in extent. In 191 2 F. D. Adams reported that 

 open cavities might persist in rocks at depths of at least eleven 

 miles,^ from which it appears that the zone subject to fracture and 

 flow, or plastico-frangible deformation, might be deeper than 

 usually expected. But from his work in association with Ban- 

 croft,^ Adams finds that the amount of thrust required to produce 

 deformation increases rapidly with the increase in depth, and 

 therefore concludes that the transference of material at the earth's 

 crust must take place comparatively near the surface. Bridg- 

 man,"* who subjected metal pieces to pressures comparable to 

 those that are supposed to obtain at depths of between twenty 

 and seventy miles, agrees that substances tend to become more 

 rigid under high pressures, but reports further that under great 

 pressures there is no relation between the yield and rupture points, 

 for there is no rupture point. Materials (metals at least) deform 

 without rupture, although they remain highly rigid. This is the 

 condition supposed to be characteristic of the great interior of the 

 earth, which is therefore called plastico-rigid.^ 



'^Op. cit., p. 126; cf. Bailey Willis, U.S. Geol. Surv., ijth Ann. Kept., Part II 

 (1891-92), p. 228. 



^ F. D. Adams, Jour. Geo!., XX (191 2), 97-118. 



3 Frank D. Adams and Austin J. Bancroft, ibid., XXV (1917), 635. 



'• P. W. Bridgman, Phil. Mag. (July 19 12), p. 65; see also Pkys. Review, XXXIV 

 (1912), 1-24. 



5 A later paper by Bridgman has further bearing on the probable plastico-rigid 

 behavior of the interior, "On the Effect of General Mechanical Stress on the Tem- 

 perature of Transition of Two Phases, with a Discussion of Plasticity," Phys. Review, 

 New Series, VII (1916), 215-23. See also Joseph Barrell, op. cit., pp. 431-32. 



