Art. XLIII.—Notes on a West Coast Dolerite.
[Read before the Auckland Institute, 15th August, 1898.]
This is a rock forming a large dyke running parallel to the strike of the lodes now being prospected by the Anglo-Continental Syndicate on Victoria Range, Westland.
Microscopically it is a dense, dark-coloured, basaltic-looking rock, and from its appearance might be any one of several of the more basic rocks. The fracture is on the whole even, though slightly inclined to be rough and hackley. Specific gravity, 2.88. The following is a. chemical analysis:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
The percentage of SiO2 in the above is slightly high, and on the other hand that of the MgO is a little-low, probably, due to the rather small proportion of augite present.
Under the microscope the constituent minerals are augite and plagioclase feldspar, a little base of a micro-crystalline nature being present, but, as the rock is almost holocrystal line in nature, I have termed it a dolerite, and not a basalt.
The augites in ordinary light are colourless, or of a faint violet-brown tint, in which latter case pleochroism is just visible. The crystalline form is generally rudely developed, though sometimes well shown. The prismatic cleavage is only moderately developed, and many of the crystals show an irregularly fractured surface. In size they vary from 0.05 mm. to 1 mm., though larger crystals may be found in other sections. The polarisation colours are brilliant, and resemble those of olivine. The angle of extinction C: c = 41° (approximately).
The feldspars occur abundantly in laths and needles, varying in. size-from about 0.05 mm. to 0.5 mm: By ordinary light
they appear as colourless rods or narrow plates, generally twinned. The extinction angles, varying from 20° to 35°, indicate that the feldspars are chiefly labradorite, with perhaps a little anorthite.
The chief accessory minerals, are magnetite (probably titaniferous), occurring probably as a decomposition product of the augites, and carbonate of lime in small thread-like veins. A little apatite is present in the augites, and gas-pores and negative crystals Occur in the feldspars.
The various constituents follow the normal order of consolidation, and the almost holocrystalline nature of the rock points to its having consolidated slowly and under pressure, and to its probably being a dyke rock, though perhaps at its original surface (now denuded away) it may gradually have merged into a basalt.
Appended (Plate XLVI.) are two diagrammatic water-colour drawings—No. 1, under ordinary light; No. 2, under crossed nicols. a = augites; f = feldspars.