114 THE ROYAL SOCIETY OF CANADA 



still greater; but, though the resulting pile of overthrust segments be great, the 

 maximum thickness of strata involved is probably less than 1,000 feet. 



Above the zone of minor thrusting as limited by the upper major thrust the 

 strata are not notably dislocated, if at all, on planes of overthrusting. Nevertheless, 

 it is important to state, as bearing on the distribution of that stress which produced 

 the thrusts, the fact that dividing planes which are parallel to the Lewis overthrust, 

 traverse the higher Algonkian strata in the heart of the syncline. The appearance 

 of these planes which may be called X planes, is given in photographs from near 

 Swift Current looking southwest. They were also sketched from Trapper peak 

 looking south. In both cases they appeared as elements of the profile or as snow- 

 covered benches on the faces of the cliffs. They cross the stratification, indifferent 

 to the direction of dip. With the field glass no displacement along them could be 

 made out. Nevertheless, whether the strain exceeded the limit of rupture or not, it 

 follows from the parallelism of the X planes and the Lewis overthrust that the 

 stress which produced the system was effective throughout the mass. Between the 

 highest X planes in Mount Reynolds, in the upper part of the Siyeh limestone, and 

 the Altyn limestone at the Lewis thrust, the thickness of strata is something more 

 than 8,000 feet. 



Structure Below the Thrust. — The structure of the Cretaceous strata 

 below the thrust can be actually observed in a relatively narrow 

 zone adjacent to it, and partly overlain by the overthrust block. 

 This zone has certainly been overridden by the superposed strata. 

 The strata beneath the overthrust block west of the outcrop of the 

 thrust can only be conjectured, but for some distance it can be reason- 

 ably supposed to be similar to that in the visible zone. The structure 

 of the country east of the certainly overlapped zone can be deciphered 

 accurately, but the extent to which this country was formerly covered 

 by the overthrust mass can be stated only as a matter of theory. 

 It may be accepted as beyond doubt, however, on the basis of the 

 evidence now in hand, that the overthrust mass formerly extended 

 farther eastward than its present position. This former eastward 

 extension has been removed by denudation. 



In northern Montana, Willis considered the attitude of the 

 Cretaceous underneath the thrust to be a monocline of simple struc- 

 ture dipping southwestward (45, g), though he recognized some 

 complications, which, owing to the paucity of outcrops, could not be 

 worked out. Later more detailed work by Stebinger in the same 

 region has demonstrated that immediately adjacent to the moun- 

 tains the structure is more complicated than Willis had supposed. 

 He states (40, a) 



Minor undulations of the strata in the area of nearly horizontal rocks can be 

 seen in detail only along the principal stream valleys. They are gentle monoclinal 

 flexures in which the inclined beds are on the west, although in a few places reverse 

 dips produce slight anticlinal folds in the generally westward-dipping rocks. The 

 change in structure from the nearly horizontal rocks in the eastern half of the reser- 



