Purcell 



Average 



Gabbro. 



Basalt. 



51-92 



49-06 



•83 



1-36 



14-13 



15-70 



2-97 



5-38 



6-92 



6-37 



•14 



•31 



8-22 



6-17 



11-53 



8-95 



1-38 



341 



•47 



1-52 



1-17 



1-62 



-04 



•45 



•06 





99-78 



100-00 



788 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



also shows the average basalt, including diabase, etc., calculated from 198 

 analyses : — 



Si0 2 .. 

 TiO... 

 A1 2 3 . 

 Fe s 3 . 

 FeO.. 

 MnO. 

 MgO. 

 CaO. . 

 Na 2 0. 

 K 2 0.. 

 H 2 0.. 

 PA. . 

 CO... 



The gabbro is much the poorer in each of the alkalies. The microscope 

 shows that nearly 60 per cent of the intrusive is composed of hornblende. The 

 specific gravity of the freshest rock is 3-0 or over. There has evidently been a 

 special concentration of cafemic material in the preparation of the magma. The 

 result is a gabbro of peridotitic tendencj^. Apparently its origin cannot be 

 stated in other than very doubtful terms. The relatively low total of the iron 

 oxides and the high alumina do not favour the view that the gabbro is a direct 

 basic differentiate of primary basalt. On the other hand, it is explicable as a 

 syntectic composed of argillaceous and dolomitic sediments with primary basalt; 

 or as a differentiate of such a syntectic. Needless to say, we have no data for 

 testing this or any allied hypothesis. The only useful conclusion is that the 

 abnormal gabbro does not lie outside the domain of the general theory. 



The same statement may be made regarding the gabbroid complex of the 

 Okanagan mountains. The petrogenic problem is there complicated by the 

 intense dynamic (and perhaps thermal) metamorphism which has affected the 

 complex. 



The Alkaline Rocks. — There remains for brief discussion that group of 

 igneous types which has long claimed the particular attention of petrographers, — 

 the rocks rich in soda or potash, or in both alkalies. This richness is relative. 

 Nephelite syenites usually carry more alkali than either granite or basalt. Monzo- 

 nite is placed among the alkaline types because it contains a higher percentage 

 of potash than rocks of the basaltic, gabbroid, or dioritic families, with which 

 the monzonites may be compared as to silica percentage; all four types have 

 nearly the same average content of soda. 



Many, perhaps most petrologists have been of opinion that the alkaline rocks 

 are products of primary reservoirs of alkaline magma. Rosenbusch's great 

 system of classification has been soundly built on the basis of objective facts 

 regarding the composition of igneous rocks; but he has coupled with his sys- 

 tematic statement a theoretical conception of rock origins which is at variance 



