By referring to the section across this part of the island, it will be observed that the basaltic lavas of this table-land must have issued from some fissure near the south side of the base of the Ndrandramea mountains. In crossing the head of this plateau on the way from Nambuna to Ndrawa one passes from the region of the acid andesites into that of the basalts. The track first skirts the base of Mount Wawa-Levu, where the prevailing altered dacitic rocks are exposed in a much decomposed condition in the stream-courses. Then there is a gradual ascent through somewhat broken country to reach the western slope of the table-land, and here are at first displayed the semi-vitreous basaltic andesites just referred to.
The Wainunu table-land is bisected in a singular fashion by the Ndavutu River. Since, however, the deep and often gorge-like channel of the river displays submarine deposits incrusting the basaltic slopes on its sides, it is evident that the break in the basaltic table-land existed in part at least before the emergence.
With regard to the total thickness of the basaltic flows of this plateau I have only a few data. In the bed of the Ndavutu River opposite Vunivuvundi, and about 400 feet above the sea, there is exposed a greyish porphyritic rock showing pyrites, apparently an altered andesite. If this is the bed-rock, the basaltic plateau in that locality would be 300 to 400 feet in thickness. This is rather over the thickness of the end of the table-land at Masusu.
I pass on now to consider briefly the submarine deposits that overlie the marginal slopes of this basaltic table-land in places. They are for the most part pteropod and foraminiferous ooze-rocks and are extensively represented on the surface and slopes of the Nandua flat to the north of Ndavutu, where they occur at all elevations up to 500 feet above the sea. They are also displayed on the eastern slopes overlooking the Yanawai but at rather lower heights; and little patches of them occur here and there in different places but not exceeding 500 feet in elevation. These friable clayey rocks, which contain from 30 to 40 per cent. of carbonate of lime, are described in detail on page [320]. It may however be remarked here that these deposits are but partly derived from the degradation of the submerged basaltic table-land or from the washings of a basaltic coast. They were formed in a clear sea-way, but probably at no great depth, at a time when the basaltic plateau was submerged below the level of breaker-action.
It is remarkable that these deposits do not repose directly on the basaltic rock. In one place below the Nandua tea-plantation, where there is a steep descent to the river of about 250 feet, the pteropod ooze-rock, which is exposed in the upper half, passes down into a chocolate-coloured marl that contains 5 per cent. of carbonate of lime and is horizontally bedded. It is composed in the main of fine palagonitic debris, with some fragments of minerals, &c., and contains a few microscopic tests of foraminifera. This deposit passes down into apparently a rock of pure palagonite. The succession of these beds and their characters are described more in detail on page [344]; and as indicated in the diagram there given it is to be inferred that a very extensive formation of palagonite has taken place on the surface of a submarine basaltic flow.
On a similar slope of the Nandua district, and about half a mile nearer Ndavutu, the pteropod ooze-rock overlies a coarse zeolitic palagonite-tuff composed in great part of fragments of a highly altered vacuolar basic glass, but without organic remains. These tuffs are horizontally stratified. Tuffs precisely similar occur on the northern slopes of Ulu-i-ndali three miles to the south. They are all described in detail on page [335].
Some miles up the valley of the Ndavutu River on the steep slope descending from Vunivuvundi to the river, and on the sides of the river lower down, are exposed dark palagonitic and sometimes calcareous clays and tuffs. I traced them as high as 450 feet above the sea where they were bedded and dipped gently to the west. In the river-channel they were mostly confined to the right bank, the slope on the other side being strewn with large fragments of columnar basalt. At the mouth of the Ndavutu River, there are exposed tufaceous sandstones and a tuff-conglomerate, probably in great part formed of palagonitic materials, but I have kept no specimens.
There is much that is puzzling about the tuffs of the region between Ndavutu and Vunivuvundi. The surface pteropod and foraminiferous ooze-rocks, that are found here and on the Yanawai or eastern border of the basaltic plateau and in other localities, offer no difficulties; but the origin of the palagonitic tuffs that in places lie beneath them is not so easy to explain. At Mr. Simpson’s old estate on the Nandua flat one finds numbers of huge blocks of columnar basalt scattered about on the slope descending to the river; and in places there is exposed in a small stream, up to a height of 500 feet, a fossiliferous ooze-rock containing marine shells. The ooze-rock is evidently an incrusting deposit; but when one goes down to the river-side, which is there about 200 feet above the sea, one finds displayed in situ in the river-bed an amygdaloidal basic lava with coarse tuffs and agglomerates a little lower down.
The Hill of Ulu-i-Ndali.—The meaning of the name of this hill is “Head of the rope.” It is noted on account of the dense growth of tall forest trees that clothes its surface, such as the Vesi (Afzelia bijuga), the Ndamanu (Calophyllum burmanni), the Ndakua (Dammara vitiensis), the Wathi-wathi (Sterculia sp.) &c.; and it may be that its name is connected with the launching of the large canoes that were at one time constructed on its slopes.
Ulu-i-ndali, which has a broad level summit 1,100 to 1,150 feet in height, rises on the left side of the mouth of the Wainunu estuary. Its relation to the surrounding region is partly shown in the rough sketch given on page [83]. It is separated from the basaltic table-land to the north by a deep and wide valley, the bottom of which is raised only a few feet above the sea; the small stream known as Ndawa-ndingo, that apparently flows through it, is merely a branch of the Wainunu estuary, the tide ascending it for some distance. This singular valley, like the main valley of the Wainunu, dates back in great part to the period preceding the emergence of this region. The steep basaltic slopes of Masusu, strewn with fragments of large columns, bound it on the north. On its south side are the lower slopes of Ulu-i-ndali which are composed of volcanic tuffs.