380 DEPARTMENT OF THE INTERIOR 



2 GEOROE V, A. 1912 



The western contact of the batholith is well exposed at several points in 

 the five-mile belt and it clearly illustrates many of the familiar phenomena 

 of batholithic intrusion with the basic schists, gneisses, and greenstones about 

 Grand Forks. One of the best and most accessible localities for observing this 

 relation occurs on the railway, four miles east of Grand Forks station. 



The batholith has been so intensely crushed that it is generally gneissic in 

 high degree. So prominent is this structural feature that the whole mass has 

 been mapped, in the reconnaissance West Kootenay sheet of the Canadian Geo- 

 logical Survey, as belonging to a group of Archean crystalline schists. The 

 writer believes, however, that the batholith was intruded possibly, if not probably, 

 long after the Cambrian period and that the gneissic structure was developed in 

 post-Paleozoic time. 



So thorough has been the shearing and mashing of the granite that not a 

 single ledge of undeformed rock was recognized in the whole five-mile belt. In 

 places the granite appears fairly massive, but a careful examination of the out- 

 crop and especially the microscopic study of typical hand-specimens of this 

 phase, show that the constituent minerals have been strained, warped, granulated, 

 or recrystallized. The specimens which seem most nearly to approximate the 

 original granite are light gray, medium-grained, gneissic, though not banded, 

 aggregates of quartz, feldspar, and biotite, with a small, variable amount of 

 accessory apatite, magnetite, titanite, and rare zircon. The original rock was 

 thus most probably a biotite granite. It was a type differing from the most 

 common mica granite only in carrying rather more plagioclase (andesine-labra- 

 dorite, near Ab 4 An 3 ) than orthoclase. Quartz was present in large amount. 

 If hornblende or pyroxene were essential, such a rock would form a typical 

 granodiorite. The specific gravity of four specimens of the fairly massive 

 phase varied from 2-674 to 2-718 and averaged 2-689. 



Nature and Origin of Banding. — Much of the batholitic mass has, however, 

 been metamorphosed into a well banded gneiss (Plate 34). The bands differ, 

 in mineralogical and chemical composition, not only from each other but 

 also from the rarer, more massive and less metamorphosed phase. Representa- 

 tive samples of the banded gneiss were taken at several localities and subjected 

 to microscopic examination. The bands are found everywhere to belong to 

 either one of two kinds, respectively light-coloured and dark-coloured. Except 

 for a few isolated grains of epidote and yet rarer garnets the bands are com- 

 posed of the same minerals that form a massive gneissic granite. The banding 

 is here simply produced by the varying concentration of the mineral. 



In the light-coloured and more acid bands the constituents are chiefly 

 quartz, orthoclase, and andesine-labradorite, with quite subordinate biotite and 

 only the barest traces of the accessories, apatite, magnetite, and titanite. The 

 last two almost entirely or quite fail to appear in the thin sections. By the 

 Posiwal method a rough estimate of the weight percentages was made for a 

 typical specimen collected on the railway track about two miles west of Cascade. 

 The proportions are as follows : — 



