142 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



ridge running in an easterly direction from the triangulation station south, of 

 Monk creek on the Canadian side to the steep slope immediately overlooking 

 Priest river canyon. 



Through interbedding the conglomerate is transitional into the overlying 

 Irene Volcanic formation; the base of the conglomerate marks a profound 

 unconformity with the much older Priest River terrane. The width of the con- 

 glomerate belt, measured on the map, is about 1-5 miles. Everywhere the rock 

 shows evidence of exceedingly intense crushing and shearing. The true bedding 

 is thus masked by schistosity, especially in the coarser and more homogeneous 

 phases of the ancient gravel. The two structures were sometimes found in 

 the same ledge and then usually had the same strike but differed in dip from 

 ten to thirty degrees. The average strike of the bedding, to the southward 

 of the Dewdney trail is about 1ST. 5° E.; its average dip is at least 60°. In 

 spite of the obvious difficulties of mensuration the minimum thickness of the 

 formation must be placed at a very high figure. The apparent thickness based 

 on the average dip is nearly 8,000 feet. Since the beds of conglomerate were 

 probably not laid down horizontally but were built out in imbricate fashion 

 on a sloping sea-bottom, this estimate must be corrected by some, as yet 

 unknown, amount. To what extent the bedding was originally inclined is a 

 problem which, on account of the heavy subsequent metamorphism of the forma- 

 tion, it is doubtless impossible to solve in the area so far studied. Allowing 

 for a strongly inclined deposition a conservative minimum estimate of the 

 total thickness, an estimate based on three complete sections, is 5,000 feet; it 

 should,, perhaps, be many hundreds of feet greater. 



Coarse conglomerate is the highly dominant constituent of the formation. 

 It occurs in well-knit, very massive beds of squeezed pebbles, which, as a rule, 

 were well water-worn when they finally came to rest in their respective beds. 

 The pebbles range in size from coarse sand-grains to bouldery masses a foot or 

 more in diameter. More than one-half of them are composed of gray vitreous 

 or micaceous quartzite or of white sugary quartz. Next to them in abundance 

 are pale gray or white compact pebbles of dolomite-marble (specific gravity 

 2-833 — 2-875), often silicious to some extent. A few pebbles of phyllitic 

 slate and, yet more rarely, pebbles of a biotite granite may also be seen. The 

 top-most beds bear small angular fragments of altered porphyrite and diabase 

 which seem to have been directly derived from the contemporaneous, 

 locally interbedded lavas and tuffs of the Irene Volcanic formation. Some of 

 the larger, bouldery masses of the quartzites and especially of the dolomites, 

 are subangular and apparently were not long rolled on a beach. 



The majority of the pebbles have been deformed in the crush of mountain- 

 building. They are commonly flattened into lenses much longer than the 

 original pebbles. The mashing is wonderfully illustrated in the case of small 

 pebbles examined microscopically in thin section. A notable biproduct of this 

 metamorphism of the dolomitic pebbles is the common generation of many 

 glass-clear, twinned crystals of basic plagioelase (probably acid bytownite) 

 among the grains of carbonate. 



