790 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



diopsidic pyroxene in alkaline rocks; and the regular association of alkaline 

 types with rocks of basaltic composition. The general conclusion is that all 

 alkaline rocks are of secondary origin; their existence goes to strengthen belief 

 in a primary basaltic substratum. 



For the Boundary belt there are special difficulties in the way of applying 

 tests to the hypothesis. The chief difficulty is the lack of sufficient exposure of 

 the formations cut by the various alkaline bodies. This is, of course, no ante- 

 cedent objection to the main principle, which remains as a good working hypo- 

 thesis even if the field evidence at the Forty-ninth Parallel were nil. The follow- 

 ing short review of field relations refers specially to the role played by limestone 

 absorption in subalkaline magma, but it is to be understood that other sediments, 

 or even basic crystalline rocks, may play a similar part. As pointed out in the 

 general paper on this subject a relatively small proportion of dissolved carbonate 

 may have great effect in the redistribution of chemical elements in a magma. 



The most easterly of the alkaline terranes is the assemblage of latites and 

 monzonitic intrusions at and near Rossland. These are intimately associated 

 with basalts, augite andesites, and gabbros. The country-rocks include phyllite, 

 greenstones, and serpentines, besides large bodies of Carboniferous limestone 

 referred to the Pend D' Oreille group. That the limestone contacted with the 

 magma at the Rossland vent or vents, is shown by its abundance in the frag- 

 ments of the agglomerates on Sophie mountain and Sheep Creek valley. The 

 Salmon River monzonite stock is partially surrounded by the very thick Pend 

 D'Oreille limestone, though the two rocks do not show visible contacts. 



There are few known bodies of typical syenite as large as the Coryell batho- 

 lith, which is more than 100 square miles (about 225 square kilometres) in area. 

 The general theory assumes that this batholithic mass is a differentiate from a 

 large-scale syntectic. It must have lower silica than the average batholith — a 

 granite — because of the enormous volume of basic volcanics, serpentines, argil- 

 lite, and limestones, which the magma has so evidently replaced. Compared to 

 granite the Coryell mass is somewhat desilicated. This desilication and the 

 high alkalies are explained on the hypothesis now considered. The satellitic 

 syenite porphyry is clearly a late salic differentiate from the main body. 



The rhomb-porphyries and the shackanite of the Midway district are differ- 

 entiates from one or more magmatic chambers not exposed. Their country-rocks 

 are very seldom visible but in part at least have the same lithological character 

 as those of the Rossland district. Heavy masses of limestone crop out at the 

 few places where the Midway volcanics have been eroded off the Paleozoic 

 formations. 



The country-rocks of the Kruger alkaline body are also poorly exposed. 

 On the west side they have been assimilated by the younger Similkameen batho- 

 lith. On the east a small area remains, between the 1 Kruger body and the 

 Osoyoos granodiorite. South of the Boundary line the country-rock area 

 broadens out, there showing an average lithological character identical with that 

 of the Anarchist series* This series, composed of argillite (phyllite), quartzite, 



••Of. G. O. Smith and F. C. Calkins, Bull. 235, U.S. Geol. Surv., 1904, p. 22 and 

 Plate I. 



