REPORT OF TEE CHIEF ASTRONOMER 475 



SESSIONAL PAPER No. 25a 



The theoretical bearing of this double law underlying the evolution of the 

 Okanagan composite batholith will be discussed more fully in chapter XXVI. 

 xxt present it may only be pointed out that the proved facts regarding the 

 changes of acidity and density in the batholith are readily correlated with the 

 view that post-Cambrian granitic magmas are of secondary origin and have been 

 differentiated primarily through density stratification. On this view the basic 

 (gabbroid or basaltic) magma is the original carrier of the heat, and the granites 

 as a class have resulted from the interaction of the superheated basic magma 

 on acid gneisses, schists, and sediments or on pre-existing granitic terranes. 

 The Osoyoos-Eemmel granodiorite is the product of the assimilation of acid 

 Paleozoic and pre-Paleozoic terranes by invading basic magma. The Similka- 

 meen batholith is largely the product of the refusion of the Remmel and Osoyoos 

 batholiths. The Cathedral granite is a later differentiate of the magma which 

 had partly crystallized as the Siniilkanieen granite, or was a differentiate from 

 the Similkameen granite when partly remelted. 



Among other purposes Table XXXI. will serve to show the correlation of 

 the different formations described in the present chapter, excepting that the 

 oldest of all, the Anarchist series (Carboniferous?), is not entered^ nor is the 

 Tertiary (?) conglomerate at Osoyoos lake noted, for its relations are not 

 important to a treatment of the composite batholith. 



The principal cause of differentiation has been sought in gravitative adjust- 

 ment, stratifying the magmatic couche according to the law of upwardly decreas- 

 ing density (meaning, in general, increasing content of silica from below upward 

 in the magmatic strata). Some authors hold that large-scale differentiation 

 may develop basified contact zones by the diffusion of basic materials to the sur- 

 faces of cooling. In chapter XXVII., an alternative and preferable explanation 

 of the thicker basic contact-shells is outlined, again with primary emphasis laid 

 on gravitative differentiation. 



It has not proved possible to demonstrate a law of increase of density with 

 depth in the Similkameen granite. A series of fifteen fresh specimens of the 

 rock were collected at altitudes varying from 1,200 to 8,050 feet above sea, and 

 their specific gravities were carefully determined. The difference between the 

 densities of specimens taken near or at the two extremes of vertical distance was 

 found too small to allow of a definite conclusion, though the difference, small 

 as it is, favours the law of density stratification. It must be remembered, how- 

 ever, that the concentration of volatile matter, such as water vapour dissolved 

 in the magma but largely expelled during crystallization, would possibly be 

 greatest at the roof. The specific gravities of the crystallized rocks may there- 

 fore not afford direct values for the total density stratification during the fluid 

 state of the magma. Then, too, the observed relative uniformity of the Simil- 

 kameen granite is a function of the scale of the subcrustal magma couche. It 

 was unquestionably very thick; strong density differences are probably not, on 

 any hypothesis, to be expected in a vertical section less than several miles in 

 depth. 



The whole petrogenic cycle had already closed and the Cathedral batholith 

 was solidified when the dikes of vesicular basalt and andesite were injected into 



