Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 / Vanua Levu, Fiji

by H. B. Guppy

This hill presents precipitous slopes, and on the south side it shows bare rocky faces. As seen in the illustration, it might appear inaccessible; but the ascent is not difficult on the west side. It is composed in mass of an acid andesite allied to the dacites of Ngaingai and Wawa Levu, but differing in the hemicrystalline character of the groundmass (except at the base), in the porphyritic development of rhombic pyroxene, and in the absence of porphyritic quartz. As remarked on page [301], the rock becomes more basic as one descends the hill. At the top its specific weight is 2·44, about 300 feet below it is 2·58, at 700 feet from the top it is 2·68, and at the base of the hill where it is holocrystalline and has a dioritic appearance it is 2·71. That it possesses a rudely columnar structure is shown by the occurrence here and there on the slopes and at the base of the hill of portions of prostrate columns, 3 to 4 feet broad and sometimes 20 to 25 feet long, which have a rounded surface and look like fossil tree-trunks. Masses of agglomerate of the same andesitic rocks lie about in places on the lower slopes, the included blocks, which are a few inches across, being sometimes rounded.

The neighbouring hills lying south and west of Ndrandramea are, as far as my observations show, of the same acid type of andesite. It is connected with those nearest by a saddle, 1,100 feet above the sea, where the same holocrystalline form of the rock occurs, having a specific gravity of 2·7 and being often rudely columnar in structure. Kala-kala, about 1,600 feet above the sea, is an imposing-looking hill with perpendicular cliff-faces on some of its sides. I did not ascend it, but found at its base a rock of the same andesitic type, differing from that of Ndrandramea in the more crystalline character of the groundmass, and having a specific gravity of 2·61. West of Kala-kala is the outlying hill of Vatu Mata with a flat top and rising only about 400 feet from its base. It has all the appearance of being composed of the same andesitic rocks. It is shown on the left-hand in the illustration.

Lying south of Kala-kala are the two peaks of Mako-mako and Thoka-singa, rising respectively 1,400 and 1,300 feet above the sea. I ascended the last-named, which has a rounded summit covered with trees. Approaching it from Nambuna on the east, I found at its foot a large mass of pitchstone-agglomerate, formed of fragments of vitreous basic rocks, such as occurs around the lower part of Soloa Levu on the other side of the valley. The slopes of Thoka-singa, between 200 and 450 feet below the summit, are strewn with masses of another kind of agglomerate made up of blocks 3 to 8 inches across, occasionally rounded, and composed of the same felsitic andesite, of which the mass of the hill is formed. This last-named rock is exposed in bulk in the upper part, but on the summit the agglomerate reappears. It has a granitoid appearance, and is distinguished from the acid andesites of the other hills of the Ndrandramea district by its greater specific gravity (2·72 to 2·74), by its holocrystalline texture, and by the coarse grain of the mosaic of its felsitic groundmass, which is probably quartz-bearing but is relatively scanty. It is, however, referable to the same group of felsitic andesites, but is to be placed at the basic end of the series. (Its description is given on page [302].) In Thoka-singa we have therefore a hill which is evidently formed in mass of these holocrystalline felsitic andesites but covered in places with an agglomerate of the same materials. I have already referred to this feature in the structure of Ndrandramea. Since the blocks are sometimes rounded, such agglomerates may represent the result of marine erosion during the emergence of this part of the island. In the case of Navuningumu, where they lie abruptly on calcareous clays containing tests of foraminifera and shells of pteropods, a different explanation appears to be needed.

The Hill of Soloa Levu.—This isolated hill, which presents another type of these acid andesites, has a broad rounded summit; and though elevated about 1,600 feet above the sea, the hill itself rises only 800 or 900 feet above the country at its base. It is not easy to obtain a view of the profile of this hill and to ascertain its relation to its surroundings; and it was only when I viewed it from near the top of Vatu Kaisia six miles to the eastward that I was able to understand its position. Looking from that standpoint across the basaltic table-land of Wainunu one observed Soloa Levu rising as a dome-shaped hill at the western margin of the table-land and apparently not separated from it. The examination of the district shows that on the east and south-east sides this hill was in part surrounded by the great basaltic flows by which the table-land was built up. Basic tuffs and agglomerates, however, occur on the lower slopes on the north-west, west, and south-west sides, so that Soloa Levu in fact lies in the midst of an area of basic rocks.

The type of acid andesite which is displayed in the upper two-thirds of the hill is distinguished from those of the other hills of the Ndrandramea district by its orthophyric groundmass. Instead of a fine mosaic, the matrix displays as a rule an arrangement of short stout plagioclase prisms; but in one of my slides the two forms of groundmass are associated. In their general characters as described on page [296], they cannot be separated from the acid andesites of the Ndrandramea district. Their specific weight ranges between 2·54 and 2·62, and like most of the other acid andesites they contain little, if any, interstitial glass. Huge blocks of these rocks lie about on the slopes, often assuming a columnar form, the fragments of such columns being sometimes 5 or 6 feet in diameter, and 12 to 15 feet in length. I found one such block standing erect like a solitary obelisk.

The best way to observe the basic rocks that invest the lower slopes of Soloa Levu is to follow the track that skirts it on the south side on the way from Tambu-lotu to Vunivuvundi. Palagonitic tuffs containing in places a little lime[^[50]] and composed of fragments of basic glass of varying size and more or less palagonitised extend from Tambu-lotu and Nuku-ni-tambua (two villages lying about a mile to the westward) to the west and south-west slopes of Soloa Levu. A pitchstone-agglomerate, formed of fragments of a basic glass inclosing large crystals of plagioclase felspar one-third of an inch in length, is associated with these tuffs on the lower north-west, west, and south-west slopes of the hill. The tuffs are formed of the same materials as the pitchstone-agglomerates, but differ in their character of being more or less palagonitised. However, on the north-west side the latter have also undergone this change. On page [312] will be found a description of the basic glass of these agglomerates in its fresh and in its altered condition. Huge blocks of these rocks strew the surface on the south-west slopes of Soloa Levu, and in one place the underlying acid andesite that forms the mass of the hill is exposed in a stream-course.

These pitchstone-agglomerates and palagonitic pitchstone-tuffs are elevated between 600 and 750 feet above the sea. As one proceeds on the road to Vunivuvundi and skirts the south-east side of the hill one ascends the western border of the basaltic Wainunu table-land which, however, is much cut up by rivers in this locality. Here the tuffs and agglomerates give place to a basaltic andesite, and on reaching an elevation of 1,000 feet we arrive at the top of the table-land from which an ascent of Soloa Levu is easily made. The road then lies on, but parallel to, the border of this plateau for some distance until it descends into a deep valley worn by one of the tributaries of the Wainunu River.

This hill of Soloa Levu is in fact a mass of acid andesite situated in the midst of an area of basic rocks. I found basaltic rocks exposed in the stream courses to the north and similar rocks prevail on the north-west on the way between Nambuna and Tambu-lotu. It has been above remarked that on the east and south it has been in part surrounded by the basaltic flows of the Wainunu table-land, and that pitchstone-tuffs and agglomerates cover its lower slopes on the west and south-west, yet it is not easy to find any trace of the vent from which they flowed or were ejected.

It may be here remarked that the occurrence here and there of basic rocks in the midst of this region suggests the vicinity of dykes. For instance, in a deep gulley about half a mile south-west of Kalakala, where a dacitic rock was exposed in situ, I came upon a single large mass of an aphanitic augite-andesite of the type described under genus 16, species A, of the augite-andesites.

The Altered Acid Andesites of the Ndrandramea District.—One of the most important features of the geological structure of this district lies in the fact that the bed-rock exposed in the lower region between the hills is a highly altered acid andesite of the type found in the hills around. By referring to the map of this locality, it will be observed that between the Ndrandramea hills on the west and the Ngaingai hills on the east is the valley of the Tambu-lotu river and its tributaries, an open broken country deeply eroded by the streams, and elevated 600 to 700 feet above the sea. These altered rocks are well exposed in the deep gorge-like channel of the river between the village of Nambuna and the foot of Ndrandramea, and in fact in all places in this district where the streams have worn deeply into the surface.