GABBKOS OF EAST SOOKE AND ROCKY POINT. 
35 
Changes in the Composition of the Constituent Minerals . 
The investigators of the geophysical laboratory have shown 1 
that in the crystallization of a gabbroid magma when crystals are 
allowed to sink as they form, the first step is the separation of 
* olivine. Much of this sinks away from its mother liquor, so that 
possibilities of its further reaction are lost ; but for what remains 
there will be a tendency to resorption when the separation of 
pyroxene later begins. As pyroxene forms, it changes in com- 
position, and, in the case of a lime-magnesia pyroxene poor in 
iron, becomes gradually more calcic. Feldspar commences its 
crystallization at nearly the same time as pyroxene, and its com- 
position gradually changes to more and more sodic. While these 
changes are going on, the liquid part of the magma^is being 
enriched in silica, together with water and other volatile con- 
stituents. After a time the volatile constituents attain a con- 
centration sufficient to cause an appreciable breaking down of 
the polysilicate molecules of the alkalis and the metasilicates of 
iron and magnesia into orthosilicates, with liberation of silica. 
The result is the still further enrichment of the liquid in silica, 
which presently begins to separate as quartz, whereas the ferro- 
magnesian elements tend first to form hornblende and then, as 
the influence of water and the other volatile constituents continues 
to increase, biotite. 
The observed changes in the constituent minerals of the 
Sooke gabbro follow the course outlined very closely, in so far 
as microscopic observation indicates. The variation of the feld- 
spar is the most accurately determinable. In every hand specimen 
such variation is to be found; if the majority of the cleavage 
flakes indicate an average composition -of Ab ra An n , where m + n 
= 100, there are always a number present both more calcic 
and more sodic; these usually vary from Ab m „ 5 An n+5 to 
Ab m+5 An n _ 5 , but occasionally the variations are greater than this, 
especially in the coarser-grained rocks. In some of the latter, 
zonary banding is observable under the microscope, and the 
outer bands are more sodic than the inner. Taking the series 
l Anderson, 0. ( Am. Jour. Sc., vol. xxxix, 1915, p. 407. 
Bowen, N. L., Jour. Geol., Supplement to vol. xxm, No. 8, 1915. 
