116 THE ROYAL SOCIETY OF CANADA 



Heart Mountain overthrust between the middle and upper Eocene, 

 and in Montana the Lombard overthrust has been placed with some 

 uncertainty by Haynes in the very late Cretaceous or early Tertiary 

 (15). It is probable, as Hewett states (16, a), that these faults did 

 not take place simultaneously. It is, perhaps, not to be expected 

 that they should so form, even though they are all effects of the same 

 cause — the compressive stresses of the Laramide revolution. It is 

 reasonable to suppose that these stresses were relieved by periodic 

 slipping in certain regions at slightly different times. This may 

 have been the case, so that the Lombard and Bannock thrusts, 

 taking place before the Eocene (though not necessarily simultane- 

 ously) relieved the stresses for a time, and later, reaccumulated 

 stresses caused the Lewis, Absaroka, and Heart Mountain over- 

 thrusts to form. This interpretation agrees with the suggestion of 

 Dake that it is "quite improbable that these various faults will ulti- 

 mately be found to be part of one great overthrust" (11). 



Structural Geology. Development of the Structure 



The general structural features of the southern Rocky mountains 

 of Canada have been summarized in the foregoing statements in 

 order to present a basis of facts for the hypothesis which is put forward 

 to explain the present structures and physiography. The significant 

 structural elements to consider are three areas: (1) a western area, 

 with structures characteristic of tension following compression; 

 (2) a central area, with structures characteristic of severe compres- 

 sion ; and (3) an eastern area, the structures of which are not readily 

 to be explained. An hypothesis interpreting the development of 

 the structure of the mountains must explain each of these three 

 zones, and its relation to the others, and the structures found in 

 them. Such an hypothesis is presented in this paper. As Willis has 

 given an interpretation of the structure of the Clarke range, and the 

 Lewis thrust, his hypothesis will first be outlined, and the reasons 

 for dissenting from it stated. It must be remembered, however, that 

 his explanation considered only that region in Montana which is the 

 southern continuation of the central structural area of this paper, so 

 it is not as comprehensive a treatment as the one attempted here. 



Willis' Explanation of the Lewis Thrust 



The paper by Willis, so frequently referred to here, is worthy of 

 the closest study by ahy one interested in the geology of the region 

 here discussed. His hypothesis as to the existing relief of the Front 



