STRUCTURE 333 



trends more strongly westward, according to field observations of its po- 

 sition at the forks of Belly, and again more northerly along the western 

 slope of Belly valley. About the northern end of the Lewis range, in 

 Canada (the Wilson range of Dawson), the strike is thought to be to the 

 westward again. From Waterton lake the outcrop of the fault surface 

 follows the base of the mountains northwestward, and the strike approx- 

 imately coincides with this direction. According to these observations, 

 the relation of the Lewis and Livingston ranges, en echelon at the 49th 

 parallel,' is an effect of step-like though very gentle flexure in the fault 

 surface of the Lewis thrust. 



As to the origin of the flexures in the Lewis thrust surface, several hy- 

 potheses suggest themselves. They may be original — that is, the surface 

 may never have been plane. They may have been developed during or 

 after the episode of thrusting movement. They may or may not coin- 

 cide with flexures of the Algonkian strata ; and if coincident as to axes 

 they may not equal the structure of the Algonkian in degree of flexure. 

 Only close studies of the relations with the aid of the complete topo- 

 graphic map will answer the questions thus raised.. 



Structure beneath the thrust surface. — The structure of Cretaceous strata 

 beneath the Lewis thrust was not connectedly observed. The rocks are 

 commonly covered with drift of talus, and they are much disturbed super- 

 ficially by landslides, to which the Benton shales give rise. Out of per- 

 haps twenty reliable observations of dip, distributed over the entire area 

 of Cretaceous subterrane, nine-tenths are to the southwest and vary from 

 a degree to 25 degrees. In the field the monoclinal southwestern dip 

 was taken to be a simple structure. From the determinations of Stanton 

 and Knowlton, however, it follows that in this supposed monocline the 

 younger, Laramie, strata underlie the older Benton and Dakota. Such 

 an apparent relation might result (a) from the existence of eastward dips 

 along Saint Mary valley, west of Maine, or (b) from an overthrust of 

 Dakota and Benton on Laramie, parallel to and beneath the Lewis thrust. 

 Two and a half miles southwest of Maine ridges of Cretaceous sandstone, 

 probably Dakota, exhibit an anticlinal attitude, which may represent an 

 important axis or a local incident. The thickness of the strata and more 

 precise dips must be observed before either of the above possible sug- 

 gestions can be confirmed or excluded. 



Structure above the thrust surface. — The detailed structure of the Algon- 

 kian mass above the Lewis overthrust is sometimes chaotic when con- 

 sidered in the small, yet simple when observed in the large. The cha- 

 otic structure is best exhibited in Chief mountain, where the lower 

 massive member of the Altyn limestone is crushed (see figure 2, plate 52, 

 and figure 5). The fractures divide the masses irregularly into blocks 



