784 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912" 



Granting at once that we can secure only partial information as to the 

 chemical nature of a batholith's country-rocks, it is still possible to believe that 

 the known facts suffice to show extensive assimilation. The same considerations 

 apply to those satellitic injections which are large enough and initially hot 

 enough to be ' capable of some assimilation. In some cases the contacts of 

 these injections are so exposed as to show all the important country-rocks; but 

 then the igneous bodies must always be small affairs when compared to a first- 

 class batholith. 



The Granites. — Only two batholiths of true granite occur in the Boundary 

 belt — the Bykert of the Selkirk range and the Cathedral of the Okanagan 

 range. 



The Bykert batholith makes visible intrusive contact only with the rocks of 

 the Briest Biver terrane. From the field relations it seems probable that the 

 contact at the roof was made with the same terrane. Beneath the terrane, which 

 is the oldest exposed in the Boundary belt, is probably the usual acid pre- 

 Cambrian complex. The assimilation of either the average Briest Biver rock 

 or the postulated underlying formation by a great abyssal injection would, after 

 gravitative differentiation, give an acid, granitic mass at the roof. 



The Cathedral granite has replaced the Similkameen and Bemrnel 

 granodiorites. The remelting of these might, by the theory, permit of a new 

 differentiation whereby the salic elements collect at and near the batholithic 

 roof in greater purity than was the case with the older magmatic chambers. 

 The consanguinity of the Cathedral and Similkameen can only be explained on 

 the view that such separation of the always * antagonistic ' salic and femic con- 

 stituents did take place in the Cathedral magma chamber. This is, of course, 

 no proof of the assimilation theory in the case. The theory is forced on us by 

 the field evidence of replacement, and" the generation of a younger, acid-alkaline 

 granite is an incident of a very advanced differentiation of the syntectic. 



The other true granites — the Sheppard granite in the Bossland and Bonning- 

 ton mountain groups, the Bunker Hill granite, the Lost Creek granite and 

 summit stocks of the Selkirks, the stock just east of Cascade, the Smelter granite 

 at Grand Forks, and the Sumas granite at the Fraser river — are all cupola-like 

 stocks probably satellitic to granodiorite batholiths. The more salic character 

 of the stocks is again the result of more advanced differentiation, which was, 

 perhaps, facilitated specially by the concentration of juvenile and resurgent 

 gases in the cupolas. Nevertheless, the fact of replacement by these stocks is 

 as indubitable as the replacement by the main batholiths. 



The abnormal granites of the Moyie and Corn Creek sills have already been 

 explained at length as due to assimilation of quartzose sediments by hornblende 

 gabbro. (See pages 238 and 283). 



The Granodiorites. — The granodiorite bodies are both larger and more 

 numerous than those composed of true granites. The list includes the Bayonne, 

 Trail, Cascade, Osoyoos, Similkameen, Bemrnel, and Chilliwack batholiths; 

 many stocks in the Columbia mountain system, and the Castle Beak stock of 



