4i8 TERENCE T. QUIRKE 



character of a thin layer or sheet. In a later paper Chamberlin^ 

 concludes as the result of his work in the Rocky Mountains that 

 the western mountain mass has this generic difference from the 

 eastern range in that it is composed of broadly folded members 

 of very great depth as contrasted with severely folded and faulted 

 members of less depth. Thus following Chamberlin, it seems 

 that there ai^e some cases of mountain folding in which the strains 

 are relatively shallow as well as some cases in which deformation 

 has been much deeper, which may support the theory that faulting 

 and sharp folding may affect a shallow terrain at the same time that 

 deeper parts of the crust are undergoing flow deformation. The 

 observation of Daly^ that in south central British Columbia the 

 overlying sediments are much more sharply folded than the under- 

 lying pre-Cambrian rocks gives further support to this general 

 notion. 



In attempting to analyze earth deformation it is natural, 

 erroneously, to consider the deformed parts of the crust as com- 

 prising the whole members subjected to stress ; whereas such parts 

 are merely those portions of the structural member which failed.^ 

 In consequence of this, it follows that it is a matter of considerable 

 doubt as to what the nature and dimensions of the structual mem- 

 bers may have been. So it comes about that most discussions are 

 based upon an attempted analysis of the strains involved rather 

 than upon the basis of controlling forces. And this is logical 

 enough, in that here and there the strains remain clearly recorded, 

 whereas the stresses and the nature of the members involved can 

 only be inferred. Presumably, if we follow the traditional teach- 

 ings, the compressive forces are caused by a more rapid decrease 

 in volume of the inner part of the earth than can be accompanied 

 by the solid shell without buckling or breaking, and consequently 

 the whole shell of the earth constitutes the members concerned. 

 However, the earth's crust is sharply divided into continental and 

 oceanic members; whether these members are conceived to be 



^ R. T. Chamberlin, "The Building of the Colorado Rockies," Jour. GeoL, XXVII 

 (1919), 248. 



^ R. A. Daly, quoted by C. K. Leith, Structural Geology (1913), p. 127. 

 3 Cf. Chamberlin and Miller, op. cit., p. 21. 



