324 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



•phase ' augite porpliyrite/ One of the chief difficulties in mapping these rocks 

 lies in the fact that the distinct and much younger augite latites are extremely 

 difficult to distinguish in the field from the older augite andesites. There are, 

 moreover, true augite andesites and basalts belonging to the younger series 

 of lavas and the problem of differentiating them from the Carboniferous lavas 

 is in many cases not to be solved. 



Since, therefore, most of the volcanic belt has defied clear-cut division 

 on the map, the writer has followed McConnell and Brock in colouring under 

 one legend, the ' Bossland Volcanic Group,' most of the volcanic formations 

 occurring in the Boundary belt between the Salmon river and Christina lake. 

 Between the Columbia river and Christina lake the larger part of the volcanic 

 masses have been found to belong to the family of latites, although there are 

 some flows of true basalt and augite andesite associated with them. In the 

 Beaver Mountain region there is a considerable area of relatively unaltered 

 lavas and tuffs which nowhere seem to have any latitic phase. Chiefly because 

 of their relatively fresh and recent appearance, Brock has already separated 

 this series of volcanics, and he has given the series the name, ' Beaver Moun- 

 tain Group.' The petrographie distinction just noted further justifies our fol- 

 lowing Brock in his mapping, and this part of the whole volcanic area will be 

 separately described, as well as separately mapped in the accompanying sheet. 

 If, in the future, the Bossland volcanic group can be analyzed with sufficient 

 accuracy to permit of its subdivision on the map, it would be appropriate to 

 reserve the name ' Bossland Volcanic Group ' for the latitic lavas and asso- 

 ciated pyroclastics, for these seem to be the dominant extrusives of the area. 



PETROGRAPHY OF THE LAVAS AND PYROCLASTICS. 



The writer has collected about one hundred specimens of the freshest and 

 most typical rocks of the volcanic belt and from them about eighty-five thin 

 sections were cut. It was not until these had been microscopically examined 

 that the lithological diversity of the lavas became fully apparent. Seven 

 varieties of latite, olivine basalt, olivine-free basalt, augite andesite, and 

 possibly picrite (corresponding to harzburgite among the plutonic rocks and 

 described among the latter) have been recognized among the less altered lavas. 

 The most abundant types are probably the augite latite and biotite-augite 

 latite. These are respectively transitional into olivine-augite latite and biotite 

 latite. Hornblende-biotite latite and hornblende (-augite) latite and a specially 

 femic augite latite are of more local occurrence. The true basalts are far less 

 common than one would suspect in the field, since so many of the latites have 

 basaltic habit. True augite andesite is probably more abundant than the 

 basalts. 



Augite Latite. — Massive lava belonging to this variety was found at widely 

 spaced localities, among which are specially noted the area between Castle moun- 



