Formula.—Aug, matr, flu, oph, phen, vitr.
Characters.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is ophitic or semi-ophitic. Glassy plagioclase phenocrysts.
Description.—Dark rocks, sp. gr. 2·76-2·8, forming ancient flows and displaying at times a columnar structure as at Yanutha Point (page [123]). The ophitic character is only in part developed, which may be connected with the flow-arrangement of the felspars. These rocks come near to the blackish ophitic basalts with scanty olivine (genus 33 of the olivine sub-class).
They all belong to the non-porphyritic sub-genus where the plagioclase phenocrysts are less than 3 mm. in size. These phenocrysts, which often contain abundant magma-inclusions, give extinctions of andesine labradorite (20°-30°). The augite phenocrysts are small and composite in character as often happens with these ophitic rocks. They sometimes invest the smallest felspar phenocrysts, and occasionally display intergrowths of rhombic pyroxene. The felspar lathes are ·1 to ·14 mm. in length, and give extinctions of medium and basic andesine. The augite granules are large (·02-·06 mm.), and tend to wrap around the lathes. Interstitial glass exists in fair amount.
CHAPTER XX
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
Hypersthene-Augite Class
II. Sub-Class. Hypersthene-Augite-Andesites
Formula.—Plag, hypersth-aug, matr.
Characters.—The pyroxene phenocrysts usually are represented by separate crystals of the monoclinic as well as the rhombic type, and the two forms are often associated in the same crystal. The monoclinic form prevails in the groundmass in most cases.
Remarks.—It is not possible to draw a sharp line between the augite and the hypersthene-augite-andesites; but where two or three phenocrysts of the rhombic type occur in a slide the rock may be placed in this division. Between this variety and that where rhombic pyroxene prevails, both among the phenocrysts and in the groundmass, numerous intermediate kinds exist. These rocks mostly occur in agglomerates and form small and large dykes or sills, but rarely are found in flows. They are distributed over most of the island except in the western portion (the basaltic districts of Wainunu, Seatura, and Solevu), but reappear again in the Mbua peninsula in places, as at Mount Koroma.
The pale yellow rhombic pyroxene is uniform in its optical behaviour. The prisms are noticeably pleochroic, being nearly colourless when lying across the long axis of the lower nicol and pale yellow when parallel with it. The intergrowths with monoclinic pyroxene often take the form of lamellar bands, whilst in some cases a nucleus of the one (usually rhombic) is invested by a growth of the other.