GABBROS OF EAST SOOKE AND ROCKY POINT. 
33 
It has already been shown on field evidence that the 2 or 3 
per cent of the magma which forms the gneissic phases, was in a 
very viscous condition when movement took place. Nothing is 
known as to whether the remainder was viscous or fluid. Assum- 
ing a general advance of the whole magma after differentiation, 
let us consider both possibilities. 
If the greater part of the magma was fluid, the rest viscous, 
a general advance of the whole magma would almost certainly 
result in the complete remixing of these fluid portions and the 
viscous parts would be rendered gneissic; so that the present 
condition would be that of a matrix of uniform composition 
surrounding patches of gneissic differentiates. This is not the 
case. 
If the whole or greater part of the magma was in a viscous 
condition at the time of a general advance, then the whole or 
greater part would have been rendered gneissic. This is also not 
the case. 
It seems clear, therefore, that the movement which occurred 
after differentiation had reached its close, or nearly so, could not 
have been that through which the magma attained its present 
position; so that differentiation went on in situ. 
METHOD OF DIFFERENTIATION. 
N. L. Bowen has shown in a recent paper 1 that only one of 
the different methods suggested in the past appears entirely 
competent to effect the differentiation of an igneous magma; 
this is the process of crystallization, with separation of crystals 
and liquid by gravitative influence and zoning, aided sometimes 
in the later stages by the compressive action of regional move- 
ments. The facts observed by the writer which tend to prove 
Bowen’s contention will be taken up in detail, but may first be 
summarized as follows : 
(1) Spatial relations, imperfect as they are, show that the 
main differentiative processes have been controlled by gravitation 
influence. 
i Bowen, N. L,, op. clt. 
i 
