I. Division of the Olivine-Basalts
Characters.—Abundant olivine.
Formula.—Oliv, matr, cop.
These rocks are characterised by abundant olivine usually as phenocrysts but sometimes represented in the groundmass. When a basalt presents much of this mineral in an ordinary hand-specimen and displays at least five or six phenocrysts in a slide, it is placed in this division. Olivine-basalts of this character are well exhibited in the hills around Solevu Bay and in the neighbouring Seatovo Range. They are also fairly represented on the northern slopes of Mount Seatura, on the coast between the Wainunu River and Nandi Bay, and on the Wainunu basaltic tableland. In the Ulu-i-ndali Range, which lies east of the Wainunu estuary, they are especially frequent. Whilst confined mostly to the portion of the island west of Savu-savu Bay, they occur sporadically in other localities to the eastward, as in Na Suva-suva Hill and in some parts of the Natewa Peninsula. The grey basalts, which form one-third of the total, are chiefly characteristic of the hill of Ulu-i-ndali, of the Solevu district, and of the northern slopes of Seatura. Whilst the blackish basalts usually compose the flows, the grey basalts form dykes and volcanic necks.
Two-thirds of these olivine-basalts belong to the order presenting flow-structure and almost all (28 out of 29) are included in the sub-order exhibiting granular augite in the groundmass. The ophitic structure is displayed in only one case; and the prismatic form of the augites is never a predominant feature.
1. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, non-flu, gran, phen, vitr.
Characters.—Abundant olivine. Felspar-lathes of the groundmass not in flow arrangement. Pyroxene of the groundmass granular. Phenocrysts of glassy plagioclase.
Description.—Dark-brown or blackish rocks. Sp. gr. 2·88 to 2·93. Phenocrysts of pyroxene occur in fair quantity in addition to those of the olivine and plagioclase. The groundmass displays a plexus of felspars and augite-granules with much magnetite in grains and irregular patches. The interstitial glass is scanty or almost absent. The olivine phenocrysts, of which the larger are 3 to 4 mm. in size, are as a rule hematised at the borders and in the fissures, and are sometimes partially serpentinised. In some cases small crystals of olivine are enclosed in the pyroxene-phenocrysts. The plagioclase phenocrysts do not usually exceed 2 mm. in size. They give lamellar extinctions of 15°-28°, and are often cross-macled. They generally contain magma-inclusions, which may be arranged in zones, and they sometimes inclose small pyroxene crystals. Their borders are often eroded. The pyroxene-phenocrysts, which frequently are 3 to 4 mm. in size, give extinctions of 30° and over, and may be described as composed of brown augite. It is only at times that intergrowths of rhombic pyroxene occur. They are often twinned and are sometimes eroded and may contain magma and other inclusions. The felspars of the groundmass, which for the most part form a plexus, are small and stout, their average length varying from ·08 to ·13 mm., whilst they frequently display lamellar twinning and give extinctions of about 15° (oligoclase-andesine). The pyroxene-granules of the groundmass, which are of brown augite, vary in average size from ·02 to ·04 mm.
Two of the four species, where the felspar-lathes are less than ·1 mm. and between ·1 and ·2 mm. in length, are represented in this collection.