Sub-Class Hypersthene-Andesites
These are dark and light grey rocks, sometimes granitoid in appearance. They pass on the one hand into the hypersthene-augite-andesites before described and on the other into the hornblende hypersthene-andesites to be subsequently dealt with. From the former they are distinguished by the great predominance of rhombic pyroxene both as phenocrysts and in the groundmass; whilst from the latter they are separated by the absence of brown hornblende or its pseudomorphs. These rocks are found in the Ndrandramea, Valanga, and Vunimbua districts. They may form isolated dome-shaped hills as in that of Soloa Levu, or they may constitute the deeper-seated rocks of the region from which these hills arise, as in the Ndrandramea district. In their general mode of occurrence, however, they cannot be treated apart from the allied hornblende-hypersthene-andesites and the dacites.
This sub-class may be divided like the hypersthene-augite-andesites into four orders according to the character of the groundmass; and these are enumerated in the Synopsis. Only the orthophyric and felsitic orders are represented in my collection. Of the former the most typical rocks are those composing the hill of Soloa Levu which is described on page [103].
These Soloa Levu rocks are lightish grey and granitoid in aspect, with specific gravity of 2·54-2·62, and displaying abundant porphyritic crystals of pyroxene, 2-3 mm. in size. In the slides they show a large number of plagioclase phenocrysts together with those of pyroxene in a relatively scanty groundmass, for the most part orthophyric in texture and without residual glass.... The plagioclase phenocrysts, which are not usually over 2 or 3 mm. in size, are often tabular and show distinct zone-lines. Though they are traversed by minute cracks and have frequently a semi-saussuritic appearance arising partly from change-products and partly from the abundance of colourless inclusions, they yield clear lamellar extinctions of medium and basic andesine (15°-25°).... The pyroxene phenocrysts, which are not much altered, are in most cases long pale-yellow rhombic prisms with rounded ends, behaving optically as described on page [285]; but intergrowths with monoclinic pyroxene may occur and even separate crystals of augite.... The scanty groundmass, though in the main formed of short and broad felspars, ·12 mm. long, of the orthophyric type, displays in places a rude mosaic, apparently of quartz and felspar. It also shows abundant small pyroxenes in the form of small prisms (·05 mm. in length), giving extinctions nearly always straight but occasionally oblique (30°-35°).
As examples of the felsitic order of these rocks, most of which are altered like the propylites, I will first take the case of those deep-seated rocks that are exposed in the river-bed above Nambuna in the Ndrandramea district. In the least altered state they are dark grey and mottled, and have a specific gravity of 2·66-2·69. In section they display tabular zoned plagioclase phenocrysts, usually more or less occupied by alteration products, but at times giving lamellar extinctions of basic andesine (20°-25°). The rhombic pyroxene is more or less replaced by chloritoid pseudomorphs; whilst the “grain” of the mosaic is often coarse (·03 mm.), and much of it is evidently quartz. The more advanced stages of alteration of these rocks are described in the account of the district given on page [106].... Similar rocks, showing pyrites, occur amongst the blocks of Vunimbua River; but here the rhombic pyroxene is mostly converted into bastite, and the groundmass is in part trachytic as well as felsitic in texture. The specific gravity is 2·7.
(In the last survey of my collection I have found a solitary specimen from an agglomerate in the Mbua-Lekutu “divide,” which must be referred to the order with felspar-lathes in flow-arrangement. It is a pale grey rock showing abundant macroscopic pyroxene prisms, 2 mm. long, mostly rhombic but showing also intergrowths with monoclinic pyroxene. The felspar-lathes do not average more than ·1 mm., and there is a quantity of small prisms of rhombic pyroxene in the groundmass, which also contains a little residual glass.)