294 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



In these Priest River rocks the thermal metamorphism has not developed new 

 types of minerals to any notable extent. The changes in the quartzitic beds con- 

 sist chiefly in their becoming micaceous, with the liberal generation of both 

 muscovite and biotite. The phyllites, metargillites, and quartz-sericite schists, 

 interbedded with the quartzites, have been converted into coarse, glittering mica 

 schists, in which the individual mica-plates average scores of times the size of 

 the original micaceous elements in the equivalent bands farther south and not 

 thermally metamorphosed. These metamorphosed schists are regularly composed 

 of dominant quartz, muscovite, and biotite in variable proportion, giving mus- 

 covite-quartz schist, muscovite-biotite-quartz schist, and biotite schist. Grains 

 of plagioclase and orthoclase are accessory in variable amount. Here and there 

 prisms of tourmaline are developed in abundance. In general, the metamorphic 

 effects along the trail are of a nature leading to a higher crystallinity and 

 ■coarser granularity in the ancient sediments rather than to the generation of new 

 minerals. Tbis effect is manifest for distances as great as three miles from the 

 main contact of the granite. Since the exomorphic collar was not thoroughly 

 studied in the part lying north of the Dewdney trail, it is possible that many 

 variations on the described simple scheme of metamorphism would be discovered 

 by one exploring the inner edge of the collar. 



On the other hand, the mineralogical changes in the Summit series of rocks 

 are often very marked. This is the case even at long distances from the grano- 

 diorite contact. 



One of the most remarkable instances is shown in the band of basal Irene 

 conglomerate. At the Dewdney trail, nearly two miles south of the batholithic 

 contact, this rock is exposed on a large scale. As usual it is intensely sheared, 

 with its quartzite, carbonate, and slate pebbles rolled out into flat lenses and 

 ribbons. The thermal metamorphic effects are most pronounced in the cement, 

 which is often abundant. In ledge and hand-specimen the cement is of a dark 

 green colour and of silky lustre, evidently due to abundant biotite and mus- 

 covite crystallized in minute individuals. In the less metamorphosed beds the 

 microscope shows that grains of quartz and carbonate are the other essential 

 ■constituents. There is considerable effervescence with cold dilute acid, showing 

 that the disseminated grains of carbonate are, in part, calcitic. The numerous 

 pebbles of carbonate are true dolomite. Through their mashing the cement has 

 become mechanically impregnated with grains and small, granular aggregates of 

 dolomite. The calcite may be, in part at least, of secondary origin and, in any 

 ■case, is subordinate to the magnesian carbonate. On the whole, the composition 

 of these few, relatively unaffected bands of the conglomerate is like that des- 

 cribed for the standard sections of the Irene formation. 



For hundreds of feet of thickness the cement has been very notably altered 

 through contact action. The chief effect consists in the extremely abundant 

 generation of dark green, actinolitic amphibole, forming long straight or curved 

 prisms. These often shoot irregularly through the quartz-mica ground-mass or 

 "form beautifully developed sheaves and rosettes, which are specially well exhibi- 

 ted on fractures parallel to the schistosity. The individual prisms run from 1 

 •cm. or less to 3 or 4 cm. in length, with widths usually under 1 mm. The 

 amphibole has the optical properties of actinolite. 



