The plagioclase phenocrysts, 1 to 2 mm. in size, give extinctions indicating in some rocks oligoclase-andesine (10°-15°) and in others basic andesine (15°-25°). Their opacity in the porphyrites is sometimes due to multiple macling, but more usually it arises from the numerous fine cracks filled with decomposition products that traverse them. The phenocrysts of dark brown hornblende are generally abundant and give extinctions of 15 degrees. They as a rule have dark resorption borders in which the process of conversion into fine pyroxene is in active operation. The pyroxene phenocrysts are scanty and in most cases rhombic; but intergrowths with augite and separate crystals of the last-named may occur. In the altered rocks or porphyrites they are largely replaced by bastite and viridite. The felspars of the groundmass are broad and often rectangular and may give lamellar extinctions of oligoclase-andesine. The pyroxene in the groundmass of the porphyrites is often partly decomposed. It is as a rule prismatic. A little interstitial glass, altered in the porphyrites, is generally present.
Fourth Order of the Hornblende-Hypersthene-Andesites
(Groundmass felsitic, displaying a granular mosaic structure)
These are light and dark grey rocks showing usually macroscopic pyroxene and hornblende. They vary considerably in appearance from the open-textured rock to that with a granitoid coarsely crystalline aspect. They generally carry brown hornblende phenocrysts, but frequently these are represented by pseudomorphs; and they all have a felsitic groundmass. They are only separated by the absence of porphyritic quartz from the dacites of Vanua Levu, which are treated in the next sub-class. They present all stages from the crypto-crystalline to the holo-crystalline condition, but all show a groundmass which may be scanty in the more coarsely crystalline rocks.
These rocks are characteristic of some of the hills of the Ndrandramea district and of the isolated peaks of Vatu Kaisia and Na Raro. They include a large proportion of the acid andesites of the island, and all belong to the prismatic sub-order with prismatic pyroxene in the groundmass, and to the section with plagioclase phenocrysts. Their specific gravity ranges usually from 2·55 in the more acid and less crystalline types to 2·74 in the most crystalline and basic kinds.
In the typical slides they display phenocrysts of plagioclase, pyroxene, and brown hornblende in a microfelsitic groundmass formed evidently of felspar and quartz together with much prismatic pyroxene. They may be conveniently divided into three species according to the size of the “grain” of the groundmass.
Of the first species, where the “grain” is less than ·01 mm., the rocks of Mount Ndrandramea are typical. They have a crypto-crystalline groundmass where the felsitic structure is in process of development and where the pyroxene prisms or microliths are very minute. The plagioclase phenocrysts (1 mm. in size) give extinctions of acid and medium andesine (10°-20°), and are tabular, zoned, and contain abundant pale inclusions. The hornblende phenocrysts, except in the case of the rocks at the foot of the hill, are represented by pseudomorphs in various stages of dispersion, so that this character is likely to be overlooked. The pyroxene phenocrysts are in most cases of the pale yellow feebly pleochroic rhombic type, but they may present intergrowths of both the monoclinic and rhombic forms. These are light grey rocks with a specific gravity increasing as one descends from the summit, where it is about 2·5, to the base where it is 2·7, a change corresponding with increase of the ferro-magnesian minerals and with the more crystalline structure of the groundmass.
The second species, where the “grain” of the mosaic is between ·01 and ·02 mm., is represented by the Vatu Kaisia rock and by that exposed in the opposite side of the gorge. They are granitoid in appearance and have a specific gravity of 2·68 to 2·71. Large porphyritic crystals of pyroxene and hornblende, the last sometimes 7 mm. in length, occur in a dark grey base. In the slide these phenocrysts together with those of plagioclase are displayed in a somewhat scanty holo-crystalline groundmass, where the “grain” of the mosaic averages ·012 mm. The plagioclase phenocrysts are zoned, and give in different crystals extinctions in some cases of oligoclase-andesine (10°-12°) and in others of andesine-labradorite (25°-30°). The hornblende phenocrysts in their pseudomorphism illustrate the various stages of the process of conversion into fine granular and prismatic pyroxene. The least altered crystals have dark resorption borders and are at times deeply corroded. The pyroxene phenocrysts are for the most part rhombic; but intergrowths with the monoclinic form occur. The pyroxene of the groundmass consists for the most part of small rhombic prisms averaging ·05 mm. in length.
The third species, where the groundmass may be described as a coarse mosaic with a “grain” between ·02 and ·03 mm., is represented by the rocks of the peak of Na Raro and of Mount Thokasinga in the Ndrandramea district.
The Na Raro rocks are light grey with a specific gravity in the unweathered state of about 2·6, and display macroscopic crystals of glassy plagioclase and hornblende. In the slide they exhibit tabular phenocrysts of the plagioclase, together with dark pseudomorphs after hornblende, in a coarsely felsitic groundmass (grain ·022 mm.) where a little very fine prismatic pyroxene, apparently rhombic, occurs. There is also a little altered interstitial glass. The plagioclase phenocrysts are zoned and give extinctions of 15° to 25° (acid and basic andesine), whilst they often show magma inclusions. An interesting feature of these rocks is concerned with the rarity or absence of phenocrysts of pyroxene. They are to be seen, however, in the process of being built up within the substance of the hornblende pseudomorphs, which consist entirely of minute prisms and granules of pyroxene and of fine magnetite. The process seems to consist in the formation of parallel layers of rhombic and monoclinic pyroxene.