REPORT OF THE CHIEF ASTRONOMER 259 



SESSIONAL PAPER No. 25a 



priately given to this whole group as exposed in the southern Selkirks at the 

 Boundary. It appears to be the oldest series anywhere exposed on the Forty- 

 ninth Parallel. The group is of sedimentary origin but has been largely recry- 

 stallized. It is as yet entirely unfossiliferous. Its stratigraphic relation to 

 the Summit series leaves no room for doubt that the Priest River terrane is 

 both pre-Cambrian and pre-Beltian in age. 



Exposures and Conditions of Study.— Within the 10-mile Boundary belt 

 where it crosses the Selkirk range, this old terrane covers about one hundred 

 square miles. Such an area would seem sufficient to afford leading data as to 

 the composition and structure of the series. Yet a comparatively long and 

 certainly arduous field attack on the area has been exceedingly unsatisfactory 

 in its results. The difficulties of geological exploration in this area are unsur- 

 passed in the entire Boundary section. The intense metamorphism of the series 

 in almost every part, and its structural complexity would alone render the 

 solution of the main geological problems as difficult as in most typical Archean 

 terranes. The strong relief of the country and, above all, the heavy and con- 

 tinuous forest cap add special physical troubles in a field where the geologist's 

 mental troubles in interpretation are already of the first order. (Plate 26.) With 

 wearisome repetition outcrops failed at critical localities. For a mile or two 

 together the sections were often left quite blank where fallen timber, deep moss, 

 or humus effectually covered the rock ledges; so complete was this cover of 

 vegetation that even the ' wash/ frost-riven from the ledges, was invisible for 

 long stretches. 



Under these conditions it has proved impossible to treat the Priest River 

 terrane in anything like as satisfactory a manner as would be desirable. Though 

 its rocks are almost entirely of clearly sedimentary origin, not the slightest 

 clue was discovered as to the succession of beds. Neither top nor bottom, nor 

 certain indication of relative ages among individual members has yet been 

 determined. Four, more or less complete, traverses, besides several shorter 

 ones, were run across the area, and a tolerable idea of the lithological nature 

 of the series was obtained. The map and section as well as the following des- 

 cription of the series, indicate that the characters of the rock-members and the 

 attitudes of the beds are not favourable to the discernment of stratigraphic 

 sequence. It has thus seemed best to map the series on a purely lithological 

 basis. 



Compiling the data won from the several traverses it appears that the rocks 

 of the terrane may be grouped into seven irregular belts which will be hence- 

 forth referred to by the letters A to G. In general they run meridionally and 

 follow, more or less faithfully, the strike of the bedding planes, which appear 

 usually to He parallel to the planes of schistosity. Belts A, B, C, D, and E 

 have been most fully investigated. The relative inaccessibility of the area 

 covered by belts F and G has caused the information concerning them to be 

 very scant. Along the northern edge of the Boundary belt all the belts exposed 

 show specially complicated features as a residt of the intrusion of the great 

 25a— vol. ii — 17* 



