30 
MUSEUM BULLETIN NO.- 30. 
PROOF OF DIFFERENTIATION. 
. The following facts, which have been already taken up in 
detail, indicate that the various rock types have been derived 
from a common magma by differentiation : 
The principal types occupy definite positions in space, the 
most basic at the centre, the most siliceous at the periphery. 
In undisturbed masses, such as the Empress Mountain body, the 
siliceous part lies at the top. 
The different types and sub-varieties grade into one another 
at their contacts, where disturbance during consolidation has 
been small or lacking. Where disturbance has been great, the 
more acid types intrude the more basic. 
Although the end members of the series are of widely differing 
chemical composition, the proximate members differ only slightly 
from one another. The mineralogical composition shows a 
similar gradation, except at two “ triple points,” where there are 
sudden changes accompanying the appearance of a new mineral 
or the disappearance of one already present. 
COMPOSITION OF THE ORIGINAL MAGMA. 
At present, olivine gabbro occupies approximately 90 per 
cent of the exposed area of the East Sooke intrusive, and augite 
gabbro practically all of the remainder; the relative area of 
anorthosite and granite exposed is very small. The proportion 
of granite was formerly probably larger, as on account of its 
peripheral position it was the first to be exposed to erosion; 
but from the examination of the Empress Mountain and other 
stocks, and from the results, already quoted from Collins and 
Wright it seems probable that it never formed more than 10 to 15 
per cent of the whole. The conclusion seems justified, therefore, 
that the magma was originally basaltic, and not far different in 
composition from the normal olivine gabbro. 
This inference is strengthened by a consideration of the 
relations of the Sooke gabbro to the Metchosin basalts. The 
composition of the basalt is identical with that of the gabbro ; 
in places olivine is present, in other places absent. The intrusions 
of the gabbro are confined to the same area as the extrusions of 
