Art. XXXV.—Note on the Eruptive Rocks of the Bluff Peninsula, Southland.
[Read before the Philosophical Institute of Canterbury, 7th August, 1890.]
The structure of the Bluff Peninsula has been lately described by Mr. James Park in a report to the Director of the Geological Survey of New Zealand.† It is composed partly of sedimentary sandstones and slates, which are referred by Mr. Park to the Te Anau series, from lithological characters, and partly of eruptive rocks which have usually been called syenites.‡ There are also diabasic-ash breccias, which, as Mr. Park points out, prove that volcanic activity was exhibited during the period of deposition of the sandstones. In fact, Bluff Hill is the stump of an old volcano.
[Footnote] † “Reports of Geological Explorations,” 1887-88, p. 72, with map and sections.
[Footnote] ‡ Dr. C. Forbes, Quar. Jour. Geol. Soc., vol. ii., 1855, p. 522; Hector, in Otago Provincial Gov. Gazette, 5th November, 1863; Hutton, Reports Geol. Expl., 1871-72, p. 102; Geology of Otago, Dunedin, 1875, p. 41; Hamilton, Trans. N.Z. Inst., vol. xix., p. 452.
A few years ago I examined two rocks in my possession, from the Bluff Peninsula, and found one to be an enstatite diorite, the other a hornblende porphyrite.*
Last summer I had an opportunity of visiting the Bluff for a few hours, and I then collected other specimens of rocks, of which I offer short descriptions.
Hornblende Diorite. From Bluff Hill.
A coarse-grained, granular, light-grey rock, composed of black hornblende and greyish-white feldspar, the latter usually the more abundant. Specific gravity = 2.835.
Section.—The feldspar is entirely plagioclase, in plates about 0.035in. square, or in broad laths 0.046in. long by 0.022in. to 0.03in. broad, and well twinned. The hornblende is allotriomorphic, greenish-brown, strongly pleochroic, and partly decomposed to a pale bluish-green, slightly pleochroic, fibrous chlorite. Magnetite is not in great quantity, and is generally associated with the hornblende. There is a little pyrites.
Enstatite Diorite. From Bluff Hill.
A fine- to medium-grained, granular, dark-grey rock, composed of black hornblende and colourless feldspar in nearly equal quantities. Specific gravity = 2.88.
Section.—The feldspar is entirely plagioclase, in crowded, well-twinned crystals, from 0.01in. to 0.05in. in length, and from 0.007in. to 0.001in. in breadth. The hornblende is brownish - green, moderately pleochroic, the polarisation colours not brilliant. Augite occurs in quite subordinate quantity to the hornblende; pale-greenish in colour, and forming intergrowths with the hornblende; not pleochroic, and with brilliant polarisation colours. Enstatite is in small quantity, intergrown with the hornblende; it is moderately pleochroic, changing from bluish-green to brownish-red; in ordinary light it is yellow-green. The polarisation colours are not brilliant. Magnetite occurs in crystals and in masses; it is nearly always associated with the ferro-magnesian minerals.
Hornblende Porphyrite. From Green Hills.
A dark greenish-grey, fine-grained rock, with porphyritic crystals of hornblende scattered through it rather abundantly. Specific gravity = 2.901.
Section—Ground-mass holocrystalline, composed of grains and crystals of feldspar and brown hornblende, with some chlorite. The porphyritic minerals are plagioclase and hornblende. The former is much altered. The hornblende is brown, and partly idiomorphic. There is a little magnetite and pyrites.
[Footnote] * Jour. of Roy. Soc. of N.S. Wales, vol. xxiii., 1889, pp. 128 and 129.
Olivine Gabbro. From Bluff Hill
A rather coarse-grained, dark-grey, granular rock. Specific gravity = 2.916.
Section.—Plagioclase in broad laths and plates composes about one-half of the rock; the other half is ferro-magnesian minerals. Diallage is allotriomorphic, pale-purple in colour, and forms ophitic plates enclosing the plagioclase and olivine; in places it is decomposed into chlorite. The olivine is colourless, in rounded crystals, much decomposed on the margins and in cracks; it is not abundant. Magnetite is in small quantity. There is also a little pyrites.
Greenstone Ash. From Green Hills.
Fine-grained, dark-green rocks, sometimes laminated with finer and coarser materials. These rocks are the diabasic ash of Mr. Park. Thin sections show them to consist of minute angular fragments of feldspar, much decomposed, but chiefly orthoclase, abundantly infiltrated with chlorite. There is no quartz. Pyrites occurs commonly.