112 THE ROYAL SOCIETY OF CANADA 



Highwood range. These relations are evidence that the Lewis over- 

 thrust is thus an integral part of the structure of the mountains 

 themselves, and is not found only where the mountains join the 

 plains. 



Wherever it has been observed, this overthrust has reversed the 

 normal superposition of the strata, and normally lower rocks have 

 been pushed up to and over the Cretaceous measures. For many 

 miles in Montana a considerable thickness of the Altyn limestone, one 

 of the oldest Precambrian formations, rests on the Cretaceous 

 (45, f), and a similar relation has been noted by Daly in Alberta 

 (12, h). In the North Kootenay pass, rocks correlated with the 

 Siyeh of the International Boundary section overlie the Kootenay 

 formation of the Cretaceous (24, a). The stratigraphie distance 

 between the Altyn and the upper Siyeh is 7,500 feet, so the relations 

 given above are evidence that the stratigraphie break along the thrust 

 surface is lessening toward the north. The Precambrian rocks do 

 not extend as far north as the Crowsnest pass, and Dr. Rose in a 

 personal communication states that the Lewis thrust "continues 

 northward, cutting across the formations in ascending order, and at 

 Crowsnest lake the Devonian lies on Belly River (Allison formation) 

 sandstone." He also states that the flat thrust under Crowsnest 

 mountain (50, a), if not the Lewis, is a lower, parallel thrust. It is 

 clear, therefore, that the break along the Lewis thrust lessens in 

 magnitude as it is traced north, and Dr. Rose suggests that the main 

 Lewis thrust of Gould Dome may be represented by some very 

 complex faulting and folding farther north (cf. 33, a). 



The nature of the thrust surface has been described by Willis as 

 warped, and he gives some graphic determinations of its attitude 

 south of Chief mountain in Montana (45, h). In general the thrust 

 surface has a very low westward dip. Although in any area of a 

 few square miles the surface is warped, when the very great extent 

 of the break is considered it is apparent that the surface taken as a 

 whole is a remarkably even one. The actual distance that the over- 

 thrust mass moved cannot now be demonstrated, but at the southern 

 end of the Glacier National Park it is at least 15 miles (9, a). Farther 

 north a shift of 7 miles has been observed (45, j) and Daly makes the 

 interesting suggestion "that the entire Clarke range in this region 

 represents a gigantic block loosed from its ancient foundations, like 

 the Mt. Wilson or Chief Mountain massifs, and bodily forced over the 

 Cretaceous or Carboniferous formations. In that case the thrust 

 would have driven the block at least 40 miles across country" (12, b). 



