STRUCTURE 



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upper major thrust which is at the base of nearly horizontal thin-bedded 

 limestones, constituting the upper member of the Altyn formation. 



The thickness of strata within which major and minor thrusts are 

 developed is by no means constant. As stated, near Altyn the lowest 

 beds of Altyn limestone present mural regularity of structure, whereas 

 in Yellow mountain probably not more than 500 feet of strata are so 

 repeated as to pile up 2,400 feet high. West of Waterton lake, in the 

 section seen by Dawson, the effects of minor thrusting are still greater ; 

 but, though the resulting pile of overthrust segments be great, the maxi- 

 mum 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 over- 



Figure 6. — Structure of Yellow Mountain, drawn from photograph from Chief Mountain, looking southeast. 



A A, upper major thrust under Appekunny argillite. AB, minor thrusts traversing and repeating 

 Altyn limestone, minor folding and faulting omitted. BB, Lewis thrust. Kb, Cretaceous-Benton, 

 much covered by talus. 



thrusting. Nevertheless, it is important to state, as bearing on the dis- 

 tribution 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 pass looking southwest (figures 1, plates 50 and 51). The}'' 

 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 over- 

 thrust that the stress which produced the system was effective through- 

 out the mass. Between the highest X planes in mount Reynolds, in the 



