CONCERNING THE PROCESS OF THRUST FAULTING 419 



segments or sectors, according to the older and the newer theories 

 respectively, they act as individual components of a single mem- 

 ber and as such are subject to separate analyses. Curved, rigid, 

 sheetlike members under lateral compression fail in the center. 

 Apparently the earth members fail by rupture and buckling at 

 the ends or edges/ from which it follows, either that the conception 

 of sheetlike members is erroneous, or that the members are not 

 rigid bodies, or both. It is surely a fact that the earth members 

 are not rigid under the conditions of mountain folding; the manner 

 of their failure proves that beyond question. There is still a pos- 

 sibility that under some conditions the forces are transmitted 

 through shell-like members, and that at other times the forces 

 are distributed throughout deep earth sectors. It seems to be 

 probable that both conditions have prevailed repeatedly at differ- 

 ent times. Chamberhn's conclusion that the mountain ranges 

 are of two generic types, one with deep and the other with shallow 

 roots, supports this idea. But, however deep the strains may be, 

 it is possible that there is so distinct a zone of shearing between 

 the frangible, nearly rigid crust and the interior which is deformed 

 by flow that the crustal part in any case fairly may be considered 

 an individual member. It is probable that deep-seated strains 

 affect a discontinuous member, plastico-rigid at depth, but more 

 frangible toward the surface,^ wherefore the term "plastico- 

 frangible," perhaps, might be used to denote the characteristic 

 quality of the outer crust. Objections to these terms arise readily, 

 especially by comparison with the terminology of other writers. 

 T. C. ChamberHn prefers the term " elastico-rigid "^ because that 

 expression indicates rigidity associated with elasticity in distinc- 

 tion from solidity due to high viscosity. The word "fluidable"'' 

 has been used to denote the potential fluidity of the earth's interior. 



'Bailey Willis, "Mechanics of Appalachian Structure," 13th Ann. Rept. U.S. 

 Geol. Surv., Part II (1893), p. 247. 



= Cf. Bailey Willis, U.S. Geol. Surv., Prof. Paper No. 19 (1903), p. 97, and Joseph 

 BarreU, Jour. Geol., XXIII (1915), 438. 



3 T. C. Chamberlin, Jour. Geol., XXVI (191S), p. 194, and personal communi- 

 cations. 



'I J. W. Gregory, Geology of Today (London, 1915), p. 156. 



