The narrow gorge isolating the mountain on the west is occupied by a tributary of the Yanawai River. It has a depth of 400 feet below the ridge; and as illustrated in the section below it has evidently been largely formed by the eroding agency of the stream. However, at the bottom of the gorge there is exposed a heavy aphanitic basalt showing no olivine and having a specific gravity of 2·85. Though of much finer texture, the felspar microliths only measuring ·05 mm. in length, it differs conspicuously from the overlying porphyritic basalt in possessing little or no interstitial glass. It is referred to genus 16, species A, sub-species 1, of the augite-andesites (page [280]).
The probable structure of this district is shown in the geological section here given. It is assumed from the limited exposure of the same rock on the top of the ridge that the basaltic flows which surrounded the lower portion of Vatu Kaisia at the same time covered over another similar peak lying immediately west of it. Through stream-erosion Vatu Kaisia has now been isolated on its west side; and since the basaltic rocks rise to about the same height on both sides of the gorge thus produced, the original surface was probably as indicated by the dotted line in the diagram.
By following the summit of the ridge, as it runs south on the right side of the Yanawai valley towards Ndawara, some interesting rocks are observed. For the first mile from the camping-place opposite Vatu Kaisia the elevation increased from 1,100 to 1,300 feet, and blocks of a blackish basaltic andesite (sp. gr. 2·76) lay on the ground. About a mile further on fragments of white quartz-rock appeared on the surface having been thrown out of a shaft close to the track which had been sunk to a depth of 15 or 20 feet by a gold miner[[54]] a few years before. I could not descend the shaft to examine it: but the specimens picked up are evidently a white vein-quartz, some of them having a striated “slickenside” surface on one side.[[55]] There is evidently a “contact” in this locality, probably of a basaltic rock with an acid andesite.
Leaving the shaft, the track proceeds southward and eastward, and one descends gradually from a height of 1,100 feet down to the Yanawai river where the elevation is only about 150 feet above the sea. Occasional blocks of basaltic rocks lie on the surface of the ridge, and in one locality there is exposed a curious-looking agglomerate formed of fragments of a greenish altered augite-andesite, somewhat scoriaceous, the cavities being filled with a zeolite. At the crossing of the river a black basalt (sp. gr. 2·82) occurs in situ; whilst loose blocks of basalt and of an acid andesite occur in the river-bed. Continuing the journey from the Yanawai crossing to Ndawara near the mouth of the river, one follows the track across a range of hills, 500 to 600 feet in height, basaltic rocks prevailing on the surface.
The Nandronandranu District.
Lying north-west of Vatu Kaisia is an elevated district which I have named after its highest summit, a square-topped peak rather higher than Vatu Kaisia and probably about 2,100 feet above the sea. Koro-ni-yalewa, which signifies “town of the women,” is another name of this peak. It is shown in the sketch given on page [113], and is situated about two miles north-west of Vatu Kaisia. I did not ascend this mountain, which from its form would seem to be made of an acid andesite like the Ndrandramea peaks. Much of this elevated region varies between 1,000 and 1,500 feet in elevation. It is connected with the Ndrandramea district by somewhat broken country not much over 1,000 feet in height, which is the “divide” between the river systems of the Ndreketi and Wainunu. A long tongue-like extension of similar elevation projects to the north-west between the Ndrawa and Navuningumu branches of the Ndreketi. This elevated region is continuous to the eastward with the Tavia Range which is described below. For convenience the valleys of the upper course of the Ndrawa river have been included in this district as their geological features can in this connection be best explained.
This region is well distinguished from most of the other districts of the island by the prevalence of aphanitic augite-andesites. These rocks have also supplied the agglomerates of the locality, and the palagonite-tuffs which are in places extensively represented are evidently in great part derived from vitreous forms of the same rocks. We seem to get nearer to supra-marine eruptions in this region than in most others. The palagonitic-tuffs and agglomerates appear to have rapidly accumulated in shallow water, and there is reason for regarding one exposure of the aphanitic augite-andesites as at all events a shallow-water lava-flow. The aphanitic character of the massive rocks, however it may have arisen, is here, as I take it, associated with the shallow-water habit of the tuffs and agglomerates.
(1) East Side of the Nandronandranu District.—By following the track leading from the ridge on the west side of Vatu Kaisia northward to Ndrawa one rises gradually to a more elevated region. The rocks exposed on the surface for the first mile are for the most part altered hypersthene-augite-andesites possessing a micro-felsitic groundmass. When a height of about 1,400 feet was attained, the track could not have been far from the peak of Nandronandranu, but on account of the wood no view was obtainable. In this locality between 1,300 and 1,400 feet soapy palagonitic clay-rocks and coarser palagonite-tuffs are displayed on the surface. No organic remains are to be noticed in the specimens collected here, but they are much affected by hydration. Judging from the fossiliferous character of similar deposits over a large part of the island, it is highly probable that these tuffs and clays are also submarine.
Afterwards a descent was made to an undulating region about 1½ miles across and elevated between 750 and 850 feet. The blocks there displayed on the surface are composed of a dark rather compact augite-andesite with a specific gravity of 2·75 (see genus 13) and of an altered greenish aphanitic augite-andesite with a specific gravity of 2·59 in which calcite occurs as an alteration product (genus 16). Aphanitic rocks of this character as shown below, are very prevalent in the north-west and north parts of the Nandronandranu district, but are not usually altered.
(2) The North-west Part of the Nandronandranu District.—The best route to follow here is to take the track from Nambuna to Ndrawa. After crossing the upper portion of the Wainunu table-land one reaches the headwaters of the Ndavutu River and then ascends the watershed between the Ndreketi and Wainunu river-systems, reaching Savulu, about 1,050 feet above the sea, where a solitary house marks the site of an old mountain town. This region is much cut up in deep valleys usually 200 to 300 feet deep, which are occupied by affluents of the Ndrawa branch of the Ndreketi, flowing north. The valley of the main affluents is from 400 to 500 feet in depth; and this constant ascent and descent of steep and often slippery valley sides makes the journey very tedious.