6o8 ROLLIN T. CHAM BERLIN 



which might otherwise have developed in response to the local 

 tension developed by the torsion, might be prevented from forming 

 by the Rocky Mountain compressive stresses, provided the torsion 

 came at a time when the Cordilleran compressive stresses were 

 still operative. This would not seem an unreasonable assumption 

 since the Rocky Mountain province has been subjected to such 

 stresses during much of Cenozoic time. 



But, according to Becker, if faulting takes place because of 

 torsion, the faults should be inclined in the same sense as the 

 thread of a right-handed screw, when the twisting has been clock- 

 wise, as it seems to have been in the Lake Basin field case."^ This 

 would be in consequence of the nature of the strain at or near the 

 surface of the earth. There the direction of elongation should be 

 N.E.-S.W. and fracturing on this principle should occur along 

 lines trending N.W.-S.E. On the other hand, an analysis by the 

 strain-ellipsoid method shows that if a body having two parallel 

 sides, like Daubree's glass plate, be twisted, the axes of strain on 

 the opposite sides are exactly reversed. In Daubree's experiment 

 both sets of torsion fractures were about equally developed. This 

 was because the strains developed on the two faces of the glass plate 

 were about equally effective in causing the fracturing of the plate. 



For confirmation of these principles, paraffine was molded into 

 strips having the dimensions 12X2XI inches. The ends of these 

 strips were then twisted clockwise as in the Lake Basin case. 

 Each strip snapped apart along a single fracture surface. The 

 fracture in 10 different tests was in accordance with the orientation 

 observed in Montana. In 1 1 tests it was contrary, the fracturing 

 belonging to the cross set. In every test, without exception, the 

 fracture occurred not far from 45° to the axis of torsion. These 

 tests would seem to indicate that in strips, or plates, of the sort 

 used, neither set of fractures takes real precedence over the other 

 in forming. But in the earth the underside of the paraffine 

 block finds no exact counterpart, and the strain conditions on the 

 upper surface should dominate. Hence, other things being equal, 

 there is greater likelihood that the right-hand-screw fractures 



^ G. F. Becker, "The Torsional Theory of Joints," Trans. Amer. Inst. Min. 

 Eng., XXIV (1894), pp. 130-38. 



