332 



B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTATA 



Chief Mt. 



m° 



North 



Yef/owMt. 



ilar structure has been described by McConnell for the same range in 

 latitude 51 degrees.* 



Warded thrust surface. — The strike and dip of the thrust plane can not 

 be measured directly, but by graphic construction they may be deter- 

 mined for any triangular area provided the relative heights and hori- 

 zontal positions of its three corners are known. The topographic map 

 gives these facts within fairly satisfactory limits, and the results are 

 given in figure 4 for five areas between Flattop and Chief mountains. 



They show that the strike varies from 

 north 15 degrees 30 minutes west to 

 north, and then to north 59 degrees 

 west, and the dip ranges from 3 de- 

 grees to 7 degrees 45 minutes. f In 

 these solutions any such area, as that 

 of the triangle Flattop, Swift Current, 

 Appekunny, is considered a plane 

 which is assumed to coincide with 

 the fault surface. It was observed in 

 the field that the fault surface was 

 curved in cross-sections in the direc- 

 tion of the dip, the dips being ex- 

 ceedingly low, if not truly zero, under 

 East Flattop, Yellow mountain, and 

 Chief, but being also as steep as 10 

 degrees or more where the overthrust 

 approaches the falls of Swift Current, 

 South Kennedy and North Kennedy 

 creeks. The differences of strike in 

 different segments also show that it 

 is a warped surface. Thus the mathe- 

 matical determinations of dip and 

 strike are averages, true only of 

 planes which subtend curved surfaces. They nevertheless give valuable 

 indications of the form of the warped surface. 



So far as the figures go, they show that the strike of the fault surface 

 makes northerly and westerly in a step-like manner, which corresponds 

 to the offset of the mountains about Chief. The prominence and isola- 

 tion of Chief mountain is in large measure due to the fact that its mass 

 forms the northeastern corner of this offset. West of Chief the strike 



South Kennedy. 

 ^AppeHunny Mt. 



Figure 4. — Diagram of Strikes and Dips of Planes 

 Subtending Areas of the Lewis Thrust. 



♦ Canada Geol. and Nat. History Survey, Report 1886, Part II. 



fThe method of reaching these results is by solution of the simple problem of descriptive ge- 

 ometry: Given the horizontal and vertical projections of three points, to find the horizontal trace 

 and the inclination of their plane. 



