REPORT OF THE CHIEF ASTRONOMER 787 



SESSIONAL PAPER No. 25a 



fluid (though it be relatively cool) residual magma, extreme differentiation 

 might take place, giving, for example, alaskite in the one pole, minette in the 

 other, as the latest phases of granitic differentiation. On this view the com- 

 plementary bodies must always be relatively small though they may be very 

 numerous. 



Their injection into the frozen portion of the batholith, or into its country- 

 rocks, is often a mere hydrostatic process, perhaps aided by the expansive force 

 and fluxing power of the gases, which are specially abundant in residual 

 magmas. Harker's suggestion that the aplites are squeezed out is a valuable 

 one. (See page 776). The pegmatites are of coarser grain than the aplites 

 probably because the magmatic gases have been yet more concentrated in the 

 pegmatite-filled fissures than in the fissures carrying the finer-grained com- 

 plementary dikes. 



The geology of the Forty-ninth Parallel seems to favour this hypothesis. 

 A full display of complementary dikes is not often seen in the section. Most 

 of those exposed are found in the Selkirk and Columbia ranges, where they are 

 very abundant. The difficulty of discussing their origin is great because this 

 region has been invaded by such different magmas as granodiorite, alkaline 

 syenite, and monzonite. It is, therefore, often impossible to say which batholithic 

 type has produced a given dike. In general, here as elsewhere, the minettes 

 and kersantites seem to have been chiefly derived from granitic and granodioritic 

 masses. Vogesite in one case at least, like the rare odinite, has been derived 

 from granodiorite. (See page 348). Camptonite is usually associated with 

 large alkaline intrusives, and the rule may apply in the Boundary section also, 

 though the occurrences are too few to justify a more decisive conclusion. It 

 ha3 been noted that the peculiar ' olivine syenite ' of the Selkirk and Columbia 

 ranges possibly represents a minettic magma, which has crystallized with special 

 coarseness because of the unusual size of the injections (see page 358). 



The more acid complementary dikes are common and of the same charac- 

 ter in the Boundary section that they have elsewhere. At several localities the 

 alaskitic dikes are associated with stocks and other large intrusions of 

 essentially similar composition. Examples are seen in the bodies of the Sheppard 

 granite, which is chemically like the true aplites emanating from the Trail 

 granodiorite. This suggests that the differentiation in batholithic cupolas is on 

 the same principle a3 that postulated for complementary dikes. 



The Abnormal Gablros. — This division of the Forty-ninth Parallel rocks 

 includes the staple types of the Purcell sills and dikes, and of the Basic Complex 

 in the Okanagan range. 



The argument of Chapter X. has been made without any necessary reference 

 to the origin of the very peculiar gabbro of the Purcell injections. Its com- 

 position before it reached the visible chamber in the sedimentary series, offers a 

 problem much more difficult than that of the granite layer in a Moyie sill. The 

 chemical and mineralogical analyses shown on page 224 are typical of the many 

 occurrences of the rock except where it has been plainly acidified by solution 

 of quartzite. The chemical analysis is again stated in the following table, which 



25a — vol. iii — 51 



