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Volume 31, 1898
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Art. XLV.—Notes on a Hornblende Trachyte from Tawhetarangi.

[Read before the Auckland Institute, 10th October, 1898.]

Plate XLVIII.

A Sample of this rock was left me by Mr. S. A. R. Mair, surveyor; and, on account of its peculiar nature and appearance, I have, as a slight addition to the petrography of the Hauraki Peninsula, written the following outline concerning it.

My specimens were obtained at Tawhetarangi, or Amoteo Bay, about ten miles north of Coromandel. Nearly two miles inland, at an elevation of some 650 ft., Mr. Mair discovered some boulders in the hillside díbris along with others of a coaly substance, and from appearances, and on account of their being found near the top of the ridge, they could not have travelled far, though he could obtain no trace of them in situ, the country being covered with dense bush.

A somewhat similar, though darker-coloured, rock forms a dyke in Castle Bock, and quite lately Mr. J. M. Maclaren, Director of the Coromandel School of Mines, discovered a like rock forming a dyke in Moehau.

The microscopic characteristics are very striking, especially the colour and arrangement of the constituent minerals. The feldspathic portion is almost pure white, and crystalline to semi-vitreous. In it are set the porphyritic hornblendes, some of which are over ½in. in length, whilst here and there star-shaped twins of the same mineral stand out conspicuously. At first sight the rock might be taken for one of the granitic to syenitic type, but on closer investigation, by aid of the microscope and chemical analysis, it is seen to be a trachyte. Its fracture is uneven and massive, and its specific gravity, which I determined both by the ordinary method of weighing in air and water and also by the specific-gravity bottle, is abnormally low, being 2.52 to 2.53. This is probably due partly to infiltrated chalcedonic quartz and partly to the numerous gas-pores contained in the feldspars. Chemical analysis shows the following composition:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

SiO2 70.34
Al2O2 10.13
Fe2O2 0.56
FeO 4.02
CaO 5.18
MgO 1.74
Na2O 5.01
K2O 2.06
Ignition loss 2.14
101.18
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In this analysis the high percentage of SiO2, which is a noted feature in many of our rocks, is most likely due to infiltrated silica, patches of which are clearly discernible under the microscope.

Under the microscope the constituent minerals are seen to be feldspars and hornblendes set in a base, and following the natural order of consolidation. The hornblendes vary greatly in size, some being quite microscopic, whilst others are over ½ in. in length. T-shaped twins are visible in some sections. Their colour by ordinary light is brown, with a faint tinge of green, and on inserting the polariser the pleochroism is very marked, and is as follows:—

  • γ = Dense green-brown.

  • β = Yellow-brown.

  • α = Honey-yellow.

The cleavage is well defined, especially the prismatic, which is well shown both in longitudinal and cross sections. The polarisation colours vary from yellow-brown to dark green-brown, and the extinction angle C: c varies from about 14° to 16°. The hornblendes show a well-marked dark border-line, due to corrosion in the molten magma previous to consolidation.

The feldspars vary in size from quite microscopic plates and laths up to 5 mm. to 6 mm. long. By ordinary light they appear as colourless plates, with here and there a few colourless laths. Under polarised light they appear much twinned, and exhibit zonary banding, and the extinction angles varying from about. 4° to 10° point to their, being oligoclase, though the analysis showing some 2 per cent. K2O would point to the probability of some of the feldspar being anorthoclase.

The chief accessory minerals are magnetite, occurring in the hornblendes, and infiltrations of chalcedonic silica in cavities in the base. Negative crystals and gas inclusions are very numerous in the feldspars, whilst inclusions of the latter mineral are found in the large hornblendes.

The base consists chiefly of a crypto-crystalline aggregate of feldspathic matter, with here and there small granules of feldspar scattered through it.

Appended (Plate XLVIII.) are two water-colour sketches of sections of this rock—No. 1; under ordinary light; No. 2, under crossed nicols. i = inclusions of silica; f =. feldspars; h = hornblendes.