Quartz Porphyries and Rhyolitic Rocks
Wichmann when he wrote in 1882 that no quartz-bearing younger eruptive rocks had hitherto been observed either in Fiji or in the South Sea Islands generally, had apparently overlooked Dana’s observations in the Fijian group. The American geologist[[124]] refers to a rock found on the north-east shores of Vanua Levu which exhibited in a greenish base thickly disseminated crystals of quartz (bipyramidal dodecahedrons, 1/8 of an inch in diameter) and glassy felspar, together with a few sphene crystals.
Quartz porphyries, akin to the rhyolites, are especially characteristic of the north-east part of the island, to which in fact they are entirely confined. They perhaps are best represented in the vicinity of Mount Thuku and in the neighbourhood of the mouth of the Wainikoro River. None of my specimens have the fresh appearance of the Lipari rhyolites and all are more or less altered. Their specific gravity does not exceed 2·4, and they are for the most part intrusive in character.
A rock frequently exposed between Tawaki and Mount Thuku[[125]] contains abundant phenocrysts of glassy felspar (oligoclase and sanidine) and quartz in a greenish opaque groundmass having a blurred microfelsitic structure. There appears to have been a secondary devitrification of the groundmass since consolidation. The porphyritic quartz crystals are rounded and about 2 mm. in diameter.
Another rock displayed in the coast-cliffs on the north side of Natewa Bay, a mile east of Mount Thuku, has a somewhat banded appearance. It shows crystals of quartz, more or less rounded and 3 to 4 mm. in diameter, together with phenocrysts of glassy felspar (oligoclase with lamellar extinction of 5° and sanidine). The groundmass displays traces of spherulites and is in places semi-isotropic; but for the most part it is microfelsitic.
The type of rock found in the Wainikoro district and in the adjacent sea-border, where it may be observed forming dykes in the pumice-tuffs, is light-grey and loose-textured with a specific gravity of 2·1. It exhibits small phenocrysts of quartz and of glassy felspar (oligoclase 5° to 12°, and sanidine), with, in one locality only, a scanty amount of dark green hornblende yielding extinctions up to 20°. The quartz crystals, which are 1 to 2 mm. in size, are sometimes bipyramidal; but are often rounded and have fused-like outer surfaces. The groundmass is semi-isotropic with a blurred aspect, and shows traces of spherulites and numerous crystallites, with occasional felspar-lathes giving a nearly straight extinction.
An extensively altered quartz porphyry of a different type is associated with other altered rocks at the base of Mount Nailotha. It has a specific gravity of 2·54, and displays large opaque crystals of plagioclase, 2 to 5 mm. in size with small quartz crystals 1 to 2 mm. across, in a grey compact matrix. The first-named shows the felspar to be mostly replaced by alteration products; but occasionally a lamellar extinction of 6° or 7° can be observed. The quartz crystals are rounded and penetrated by the magma, and contain numerous strings of fluid-cavities. The groundmass was originally spherulitic; but this structure is more or less disguised by the development of a mosaic of chalcedonic quartz. It shows some micro-porphyritic patches of viridite and calcite. A singular altered white rhyolitic rock is exposed on the north coast of Natewa Bay between Natasa and Sangani, where it is associated with altered tuffs. It is compact with a conchoidal fracture and has a specific gravity of 2·48. The hand-specimen has a banded appearance. Under the microscope it appears as a rhyolitic glass for the most part devitrified and rendered opaque by the formation of secondary silica. Much of it presents a microfelsitic structure, the bands appearing as semi-opaque streaks.
Glassy forms of the quartz porphyries or intrusive rhyolitic rocks are extensively represented in the pumice tuffs of the Undu Promontory and of the coasts between the Langa-langa river and Lambasa. These tuffs will be found described on page [336]. Fragments of a grey rhyolitic glass looking like perlite are inclosed in the pumice-tuffs near the mouth of the Wainikoro River. Under the microscope it is displayed as a colourless glass inclosing phenocrysts of sanidine, oligoclase (ext. 4°), and quartz, the last with rounded outlines and a fused-like outer surface. The glass shows in places perlitic cracks; but it is mainly characterised by a vacuolar structure, the minute cavities being lengthened out in the direction of the flow and displaying eddy-currents around the phenocrysts. The elongated steam-cavities sometimes contain water, but are usually more or less filled with granular materials.