2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER IV. 

 STEATIGEAPHY AND STEUCTUEE OF THE CLAEKE EANGE. 



ROCKY MOUNTAIN GEO SYNCLINAL PRISM. 



One of the least expected results of the Boundary survey consists in the 

 discovery that almost all of the mountains traversed by the Commission map 

 between the Great Plains and the summit of the Selkirk range — an air-line 

 distance of one hundred and fifty miles — are composed of a single group of 

 conformable strata. These rocks are as yet largely unfossiliferous but all of 

 them are believed to be of pre-Devonian age. For the most part they are water- 

 laid, well-bedded sediments but contain one important sheet of extrusive lava 

 which extends quite across the whole Eocky Mountain system and the eastern 

 part of the Purcell system. Though the sedimentary group is a unit, it has 

 been found that noteworthy lithological differences appear in the rocks as they 

 are followed along the Boundary line from the Front ranges westward. These 

 differences are due to gradual changes of composition and no two complete 

 sections taken five miles apart on an east-west line would be identical. Never- 

 theless it has been found possible to relate all the essential features of these 

 varying strata to four standard or type sections. 



The most easterly type section was made in the Clarke range. It agrees very 

 closely with the section already described by Willis from the Lewis range at 

 localities lying on the tectonic strike from the localities specially studied in 

 the Clarke range by the present writer. The rocks thus found to compose both 

 the Lewis and Clarke ranges belong to what may be called the Lewis series. 

 The type section constructed from traverses made in the Galton and Mac- 

 Donald ranges include strata which are here grouped as the Galton series. The 

 equivalents of the same series compose the entire Purcell mountain system at 

 the Forty-ninth Parallel and belong to a sedimentary group which may be called 

 the Purcell series. The fourth type section was constructed from magnificent 

 exposures occurring in the eastern half of the Selkirk mountain system. This 

 assemblage of beds will be referred to as the Summit series. The name is taken 

 from Summit creek along which a great part of the series is exposed; the creek 

 was itself named from the fact that it heads on the water-divide of the Selkirks. 

 Analogy with the other three series names suggests ' Selkirk series ' for this 

 fourth group of strata, but that designation has already been used by Dawson 

 for the related but lithologically distinct group described in his traverse on 

 the main line of the Canadian Pacific railway. 



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