Basalt No. 1.
This rock occurs on the slopes of Abbott's and Kaikorai Hills. It is the lowest volcanic outpouring in the Abbotsford Valley, and rests on an eroded surface of the marine sedimentaries. There are two good outcrops—one in a cliff below Kaikorai Trig., and the other at the back of Mr. Meechan's house. At the former place the basalt, which is traversed by vertical and horizontal joints, is well weathered. At the latter locality the upper portion is so greatly weathered that it would be difficult to recognize it were it not for the more solid rock below.
Macroscopically the rock is dark grey in colour. Where weathered the augite crystals can be seen, but they do not protrude. This basalt may be microscopically described thus:—
Phenocrysts.—Labradorite (2.6 mm. × 0.66 mm.) is twinned on the albite law, but at times there is a combination of the albite and pericline twinning. Frequently it has magnetite inclusions in the centre. Augite is in large idiomorphic crystals (3.3 mm. × 1.5 mm.), and of a very light brown colour. It exhibits slight pleoehroism. Often this mineral has orthopinacoidal twinning. Some crystals show zoning, the outer border being a little more inclined to a violet colour, owing probably to the presence of titanium. Olivine (1.5 mm. × 1.2 mm.) is allotriomorphic. Along fracture-lines the mineral has changed to serpentine, and some crystals have been entirely altered to this mineral. One crystal has inclusions of microlites of feldspar. Magnetite occurs in grains. The ferro-magnesian minerals are not plentiful, and there is a greater amount of augite than olivine. Spherosiderite, with its irregular pleochroism, is in patches, and in one section is seen as a narrow vein.
Groundmass.—Feldspar laths (0.24 mm. × 0.88 mm.) are of labradorite twinned once or twice. They show a flow-structure. A few grains of olivine and magnetite are also present.
The following is an analysis of the basalt:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Ferrous oxide (FeO)||7.35|
|Ferric oxide (Fe2O3)||6.80|
|Water (H2O) Moisture||1.08|