124 THE ROYAL SOCIETY OF CANADA 



5.. A thrust-plane below may pass into an anticline above, and never reach 

 the surface. 



6. A major thrust-plane above may, and probably always does, originate in a 

 fold below. 



7. A thrust-plane may branch into smaller thrust-planes, or pass into an over- 

 fold along the strike. 



8. The front portion of a mass of rock being pushed along a thrust-plane tends 

 to bow forward and roll under the back portion. 



9. The more rigid the rock, the better will the phenomena of thrusting be 

 exhibited. 



10. Fan-structure may be produced by the continued compression of a single 

 anticline. 



11. Thrust-planes have a strong tendency to originate at the sides of the fan. 



12. The same movement which produces the fan renders its core schistose. 



13. The theory of a uniformly contracting substratum explains the cleavage 

 often found in the deeper parts of a mountain system, the upper portion of which is 

 simply plicated. 



14. This theory may also explain the origin of fan-structure, thrusting, and 

 its accompanying phenomena, including wedge structure. 



The significance of these experiments in relation to the region 

 under discussion is that they indicate the possibility of formation, 

 by compressive stress, of steeply inclined reverse faults in connection 

 with flat overthrusts or "soles." This is not meant to imply that 

 the same sort of stress formed the steep faults as formed the flat ones ; 

 indeed recent researches by Chamberlin and Miller (10) (.see also 

 41), indicate that the low angle thrusts are found under conditions 

 of rotational strain as contrasted with purely compressive stresses 

 which form the usual type of reverse fault with an angle of dip of 

 something under 45 degrees. The fact that the structures of the 

 foothill belt can be seen to pass under the great overthrust block 

 demonstrates that they antedate the Lewis thrust, though the over- 

 thrusting may have modified them. This difference in time of forma- 

 tion, and the difference in stratigraphie position shows that the 

 stresses forming the two types of fault were different. 



With the various considerations outlined above in mind, the 

 development of the structure may be treated in narrative style. 

 Two sections across the mountains are illustrated in Figures 2 and 3. 

 Each of these figures shows by means of serial sections the successive 

 stages by which it is imagined the mountains gained their present 

 structure. Both figures are generalized, the intention being to illus- 

 trate generalities rather than details. Figure 2 is an east- west section 

 just north of the International Boundary. In this latitude Cretaceous 

 rocks are confined to the Great Plains except for some small down- 

 faulted blocks in Flathead valley. Figure 3 represents an east-west 

 section north of the North Kootenay pass, in which latitude the Cre- 



