GABBROS OF EAST SOOKE AND ROCKY POINT. 
It 
ing to size, and almost identical in composition so far as can be 
determined microscopically, with the gabbro which they cut. 
They are composed of 45 to 55 per cent bytownite, varying in 
different specimens from Ab 15 An 85 to Ab 40 An 60 , but averaging 
about Ab 20 An 8j> . The only other essential mineral is a colourless 
augite, but olivine is almost constantly accessory. The porph-y- 
ritic dykes vary somewhat from the non-porphyritic, both in 
composition and texture. They are distinguished by the presence 
of phenocrysts of feldspar which, in a dyke 6 feet or more in 
width, may attain a diameter of ^ to 1 inch. These phenocrysts 
were invariably developed in place, as they do not appear in the 
chilled edge of the dyke for a distance of an inch or two from 
the margin, and from this point inward they become progress- 
ively larger until their full size is reached about 2 feet from the 
border. So far as could be determined, the composition of the 
phenocrysts was approximately that of the feldspar of the 
groimdmass, although in some of the sections they were zoned. 1 
The feldspar is uniformly a bytownite, about Ab 15 An 85 , as in 
the non-porphyritic dykes, and is present in approximately the 
same proportion, 45 to 55 per cent. The ferromagnesian mineral 
is the usual colourless augite, frequently altered to a greenish 
amphibole, but, unlike the non-porphyritic dykes, olivine was not 
identified. 
The granite dykes, although few in number, form a fairly 
complete series, characterized by the presence of free quartz, in 
amounts varying from 2 to 3 per cent in the most basic, to 35 
per cent in the most siliceous. The nature of the feldspar 
varies with the quartz content; in the dykes low in quartz the 
feldspar is andesine, Ab 60 An 40 , and as the proportion of quartz 
increases, the feldspar becomes progressively more sodic until 
in the most acid it becomes Ab 90 An 10 . The ferromagnesian min- 
eral undergoes a corresponding change. In the basic types a 
little pyroxene may be present, but hornblende predominates in 
all; a small amount of biotite, up to 4 or 5 per cent, is always 
I One case was observed in which the outer zones of the feldspar phenocrysts 
were more calcic than the inner. 
66673—31 
