MINERALOGY OF BLACK LAKE AREA. 
11 
an opportunity of examining them at many of the pits; as a result of 
these observations, they are of the opinion that the minerals were deposited 
in their present positions as a direct result of the last phases of the intru- 
sion, and that the lime they contain was probably extracted by magmatic 
waters from the already consolidated portions of the igneous mass. 
The processes involved are outlined below. 
It may be assumed that the large bodies of molten magma, which 
gave rise to the rocks of the serpentine belt, contained certain amounts 
of aqueous vapour and other volatile constituents which remained in the 
fluid state until after the main mass of the magma had crystallized. In 
the present case, the magma, during crystallization, underwent pro- 
gressive differentiation, until the portion last remaining in the molten 
condition had approximately the composition of a granite; this then 
invaded the already cooling, solid mass, giving rise to dykes and other 
intrusive forms. All the aqueous vapour and other volatile constituents 
of the original magma (the so-called magmatic “extract”) must have 
been concentrated in this last acidic differentiate and expelled when 
it consolidated. Although this was the last phase of the intrusion, 
and represents a period of relatively low temperature, these residual 
magmas were sufficiently fluid from the presence of hydrous and other 
volatile constituents, to permit their circulation through the enclosing 
rocks. 
It is not believed that this fluid, as it originally separated from the 
magma, was sufficiently rich in lime to afford such minerals as diopside, 
vesuvianite, etc.; its chemical nature, however, suggests that it would 
have a very considerable solvent action on the walls of the fissures through 
which it passed, and it is believed that it has been responsible for the 
serpen tinization of the rock in the neighbourhood of these fissures. 
The peridotite, however, always contains some monoclinic lime-bearing 
pyroxene, and the amount of this mineral is considerable in the rock 
associated with the chromite deposits, which is about intermediate in 
composition between a peridotite and pyroxenite. It is significant, there- 
fore, that the dykes bearing the above-mentioned silicates are more 
frequently met with, so far as is at present known, in the chromite pits 
than in the asbestos quarries. As there is an almost complete absence of 
caldte, in veins or otherwise, traversing these rocks, the writers are of the 
opinion that the lime (and also alumina) present in the original rock 
and not required for the formation of serpentine, has been carried away 
in solution as silicate. Then, as these residual magmatic fluids rose in 
the fissures, there arrived a time when, either owing to their cooling or 
to the fact that they were under diminished pressure, they became 
unstable and commenced to crystallize out, forming “dykes” or veins 
of 9uch highly calcareous minerals as vesuvianite, grossularite, and 
