762 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



that the lower part of the melt carried 59-20 per cent of silica, the upper part 

 73-65 per cent; the original granite showed 68-9 per cent.* 



It is, however, to be expected, on the stoping hypothesis, that the primary 

 basaltic magma may close an entire petrogenic cycle, since the latest phase of 

 a batholith, after crystallizing, may be fissured and injected with a small volume 

 of the substratum. The common occurrence of diabase or porphyrite dikes in 

 granite may be thus explained. 



Eruptive Sequence. — The various eruptive sequences observed in the 

 Boundary section all seem to accord with this general deduction from the stop- 

 ing hypothesis. The longest series is that in the Okanagan igneous complex, 

 where the order of eruption for the batholiths is clearly that of decreasing 

 specific gravity of the rocks. (See page 471). Seven other sequences are here 

 tabulated; in each case the eruptives are named in order, beginning with the 

 youngest. 



Skagit Range. 



(a) Sumas granite. 

 Sumas diorite. 



(b) Chilliwack granodiorite. 

 Sleese diorite. 



(c) Acid monzonite. 



Skagit volcanics, chiefly basic andesite. 

 Columbia Range. 



(a) Rock Creek granodiorite. 

 Rock Creek gabbro and diorite. 



(b) Smelter granite. 

 Cascade granodiorite. 



(c) Syenite-porphyry chonolith and dikes, cutting more basic Coryell 



syenite. 

 Rossland monzonite and latites. 

 Fife and Baker gabbros. 



(d) Sheppard granite. 

 Trail granodiorite. 



Basic intrusives and older Rossland (basic) lavas. 



The discussion of the meaning of any eruptive sequence must be based on 

 a more or less definite idea as to what constitutes a petrogenic cycle. Certain 

 it is that much confusion has resulted from the common reference of all the 

 eruptives in a given region to one cycle. This view is one product of the pure- 

 differentiation theory, which excludes any essential amount of assimilation in 

 the formation of rock magmas. The hypothesis of assimilation by a primary 

 basaltic magma involves the possibility of several or many petrogenic cycles in a 

 province. Each cycle opens with an abyssal injection of pure basalt. According 



* J. Morozewicz. Tschermak's Min. und Petrog. Mitt., Vol. 18, 1898, p. 232. Cf- 

 C. Doelter, Petrogenesis, Braunschweig, 1906, p. 79. 



