6 
MUSEUM BULLETIN NO. 30 . 
DETAILED DESCRIPTIONS. 
Olivine Gabhro. 
The olivine gabbro, which forms approximately 85 to 90 per 
cent of the exposed part of the intrusive mass, is a dark grey, 
greenish, or black rock, composed of plagioclase, augite, and 
olivine, with accessory ilmenite. The colour of the rock varies 
with that of the plagioclase, which ranges from pure white and 
greenish tints to purplish-black. When the feldspar is purplish 
black, it may easily be mistaken in the hand specimen, unless^ 
closely examined, for a ferromagnesian mineral, and the rock 
may be classified as augitite or hornblendite. The composition 
of the feldspar varies from pure anorthite to Ab 40 An fl0 , but in 
the great majority of cases it is bytownite, Ab lc An 25 to Ab 25 
An^. 1 2 The augite is almost colourless and non-pleochroic ; it is 
probably low in iron, and approaches diallage in composition. 
Partial alteration to hornblende has taken place, and the cleavage 
faces of the latter are so conspicuous in the hand specimen as to 
give the impression in the field that the gabbro is chiefly horn- 
blendic. No primary hornblende was found, however, in the thin 
sections. The olivine is also colourless, generally without much 
serpentinous alteration. Its axial angle varies about 90 degrees, 
so that some crystals are found to be optically positive, others in 
the same section optically negative. Such a behaviour, accord- 
ing to Rosenbusch, indicates an FeO content of about 12 per cent. 
The proportions of the different constituents in the average 
olivine gabbro are 45 to 50 per cent of feldspar by volume, about 
the same amount of augite, 5 per cent of olivine, and 1 per cent 
of ilmenite. Wide variations from these proportions are found, 
however. Feldspar may be present in any proportion from 35 to 
100 per cent ; pyroxene from 0 to 65 per cent, olivine from 0 to 
25 per cent. The variations, moreover, are fairly independent: 
1 Tte composition of the feldspar in these rocks was carefully determined from 
extinction angles given by the albite twinning lamellae on the (001) face. To obtain 
these values with certainty, 5-10 grams of each rock specimen were crushed, sifted, 
washed, and examined for suitable cleavage Hakes. All fragments were rejected in 
which the readings of the one set of extinction angles did not correspond within 
2 degrees with those of the complementary set. 
