The Acid-Andesites of Vanua Levu
(Comprising the hypersthene-andesites; hornblende-hypersthene-andesites;
and quartz-hornblende-hypersthene-andesites or dacites)
These rocks compose in mass numerous isolated hills that rise up abruptly in the interior of the central portion of the island. Such hills, or mountains, as they might be often termed, usually attain a height of from 700 to 1200 feet above the surrounding country, and possess precipitous slopes and frequently perpendicular cliff-faces. In the geological description of the island, I have referred in detail to these mountains, when speaking of Na Raro, Vatu Kaisia, Ndrandramea, Ngaingai, etc.; and illustrations of some of them are included in this work. It may, however, be here remarked that they are as a rule rudely conical with rounded or peaked summits. The ground-plan is generally elliptical in outline; and in consequence the profile often varies from different points of view, so that as in the case of Na Raro, it is that of a sharp conical peak when the mountain is viewed “end-on,” or of a broad truncated mass when seen from the side. A similar change of form is to be noticed in the illustrations of Ndrandramea. No traces of crateral cavities came under my notice. The rocks are neither vesicular nor scoriaceous, and are usually massive; but exhibit at times a rudely columnar structure.
Each hill or mountain has its peculiar variety of these rocks. This is well shown in the Ndrandramea district. Thus the rocks of Ngaingai and of Wawa-levu in carrying porphyritic quartz differ from those of all the other hills around. Those of Soloa Levu are distinguished by the orthophyric groundmass and by the absence of hornblende. Those of Mount Ndrandramea again have no porphyritic quartz, but little hornblende, and possess a micro-felsitic groundmass. The rocks of Na Raro and Vatu Kaisia differ as regards specific gravity, the “grain” of the felsitic groundmass, the presence of phenocrysts of rhombic pyroxene, etc. The characters of these rocks from various localities are contrasted in the table given on a later page, whilst the different sub-classes to which they belong are described in detail below.