280 g. k. mansfield steuctube of rocky moitstaixs 



Hypotheses of AIuuvxaix-buildlsg 



Kobers hypothesis, which has been ably outlined by Dr. Longwell in 

 a preceding paper, is presented in a very interesting and plausible way. 

 The general arrangement of western and eastern mottntain chains with 

 intervening plateatts and ranges has a superficial agreement with his plan. 

 The Bocky Mountains in Idaho and northward show eastward overtam- 

 ing and overthmsting, which ac-c-ords widi Kober's hypothesis; but the 

 case for the Coast ranges, which by hypothesis should be orertumed west- 

 ward, is not so clear. The structure sections studied by the writer, 

 though showing intense folding and faultingj do not indic-ate a predom- 

 inant direction of overturning or overthrusting. Moreover, the distribu- 

 tion of igneotis intmsives in the Coast ranges is not very favorable to the 

 hypothesis. It may be questioned, too, how far the interior plateau and 

 ranges ac-cord with the conditions of his plan. 



Professor Hobbs, in the book^^ to which he referred in his paper, postu- 

 lates a two-sided symmetry for the Pacific mountain region, due to east- 

 ward underthrustinor on the west side, caused bv the subsidence of the 

 Pacific Ocean, and to westward underthrusting on the east side, caused 

 by the subsidence of the "Xaramide Oc-ean." The subse<^uent settling of 

 the Great Basin and its extension to the north caused further under- 

 thrusting on the east and west sides of this area, represented respectively 

 by the Wasatch and Sierra Xevada escarpments, and produced bilateral 

 svmmetry for the Bocky Mountain group on the east and for the Sierra 

 Xevaiia-Coast Bange group on the west. Objections to this hvpothesis, 

 so far as the Coast ranges are conc-emed, have been state«i in the preced- 

 ing paragraph. The view that the Wasatch and Sierra Xevada Moun- 

 tains are bounded by thrust faults inclined respectively east and west 

 reqtiires further demonstration. The writer's views regarding under- 

 thrusting in the eastern ranges of the Bocky Mountains have already been 

 stated (see page 2T2). All geologists will agree that the sea extended in 

 Cretac-eous times along the eastern front of the Bocky Mountains and 

 even covered areas that are now included in the motmtainous belt. This 

 sea, however, was in no sense an oc-ean, as the term is now understood. 

 It was rather an epic-ontinental sea of relatively shallow water. This is 

 clearly indicated by the nature of the deposits. 



Professor Willis's discoidal hypothesis, which has been set forth in an 

 earber paper in this symposium, though based on careful c-omputations 

 and plausibly presented, is dependent on the validity of its fundamental 



" W. H. Hobbs : Eartb evolution and its facial expression. The Macmillan Co., New 

 York, 1921, pp. 119-134. 



