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.
At Savulu one stands within the Nandronandranu district. Behind lies the Wainunu table-land with its olivine basalts; but here aphanitic augite-andesites prevail and extend to Ndrawa and beyond. They are exposed in position in the stream-courses and furnish most of the blocks and pebbles found in the bed of the main Ndrawa River for miles down its course towards the sea. They are dark, compact, and non-porphyritic rocks and are all referred to genus 16 of the augite-andesites as described on page [279]. They vary, however, in certain features, as in the specific gravity, the amount of glass, &c. The residual glass is, however, usually small; but in a stream-course east of Savulu I found in position at an elevation of 750 feet a semi-vitreous scoriaceous variety of these rocks, in which the steam-pores had been drawn out into long tubular cavities half an inch and more in length. The scoriaceous character is infrequent; but reference should here be made to another exposure of a slaggy semi-vitreous rock showing abundant steam-pores in the tuff-district of the river valley above Ravuka. It differs in some respects from the prevailing rock, since it displays prismatic augite as well as felspar microliths in its glassy groundmass, and is for this reason referred to genus 20 of the augite-andesites. In the elevated region east of Savulu the aphanitic augite-andesites are in places overlain by tuffs and agglomerates formed of the same materials. There is a very good exposure of the tuffs in the Nganga-turuturu cliffs about 2 miles west of Savulu.
(3) The Nganga-turuturu Cliffs.—These picturesque cliffs, 50 to 70 feet in height, rise up at the head of the Liwa-liwa valley between Savulu and Ndrawa. They are elevated about 1,200 feet above the sea; and probably derive their name from a small waterfall which, after descending over their face, drops into the valley below. At its bottom is situated the hamlet of Liwa-liwa, which is about 600 feet above the sea. This is the Fijian word for “cold,” and doubtless it has allusion to the coolness of the valley. On account of the more rapid weathering of the tuffs in the lower part of the cliffs, there is a rude shelter afforded by the overhanging portion which is the main feature of interest that the cliffs present from a native’s point of view.
The tuffs composing the cliffs are horizontally bedded and overlie the prevailing aphanitic augite-andesite exposed on the valley-slopes below. Originally grey in colour, they have been largely affected by the hydration accompanying the weathering process. They are fine in texture and somewhat friable, but contain no lime, and are chiefly made up of the palagonitised fine detritus of vitreous varieties of the aphanitic augite-andesites of the district. No organic remains came under my notice. Some of the beds contain a number of lapilli of basic pumice, 1 to 3 centimetres in size, which are often in the last stage of the disintegration produced during palagonitisation. It would seem probable that these lapilli, after having been ejected from some supra-marine vent, were deposited with the tuffs in the sea around. It should, however, be not forgotten that vesicular and pumiceous materials may be discharged during a submarine eruption. When I visited the museum at Catania, Prof. Platania showed me portions of a bomb, highly vesicular, that had been thrown up in a submarine eruption off Vulcano in the Lipari Islands.
(4) The Upper Valleys of the Ndrawa River.—The two valleys of Liwa-liwa and Ndrawa meet at Ravuka, where their two streams unite to form the main Ndrawa River. The former is the largest; and its large impetuous stream, during its descent of about two miles from Liwa-liwa past Lutu-kina to Ravuka, which is between 200 and 250 feet above the sea, has a drop of 300 or 350 feet. The main stream flows with a gentle gradient to the coast about ten miles away. I did not descend its course for more than two miles below Ravuka, where some hot springs well up through the gravel on the left bank (see page [31].)
This is a region of palagonite-tuffs which like those of the Nganga-turuturu cliffs are mainly derived from vitreous and semi-vitreous aphanitic augite-andesites. They do not effervesce with an acid, and neither foraminiferous tests nor other organic remains occur. The palagonitic material is usually vacuolar, the vacuoles being filled with palagonitic glass or with a zeolite as in the more altered rocks. Where bedding is shown, the beds are generally horizontal. These tuffs are extensively displayed in the sides and beds of the rivers from Liwa-liwa and Ndrawa to Ravuka and as far as I went down the main river, namely to the hot springs. They are associated with agglomerates, formed of the aphanitic augite-andesites, below Ravuka and in the Ndrawa valley.
(5) The Vicinity of Ndrawa.—The village of Ndrawa, which is not elevated more than 300 feet above the sea, is situated in the heart of the island in a deep valley more or less hemmed in by the mountains. This is one of the wettest localities in Vanua Levu, and probably, as in the case of that of Ndriti in the Seatura basin, the rainfall is not far under 300 inches in the year. In the river-gorge descending westward to Ravuka are displayed horizontally bedded palagonite-tuffs and agglomerates above referred to in the description of the Ravuka district, and the same rocks are exposed on the mountain-slopes to the south of the village.[[56]]
Immediately to the north lies a broken hilly country, about 800 feet above the sea, which has to be crossed on the way to Mbatiri and is much cut up by streams descending from the vicinity of Na Raro to join the Ndrawa River below Ravuka. The prevailing rocks are tuff-breccias and agglomerates. The first are made up chiefly of angular fragments, less than an inch in size, of aphanitic augite-andesites, some of them being more or less vitreous and in different stages of palagonitisation, whilst the finer material derived from the same rocks contains some carbonate of lime. The agglomerates are composed of the same type of these augite-andesites, with however but little interstitial glass. It should be added that pebbles of a kind of jasper or iron-flint occur in the stream-beds in this locality. (The microscopical characters are described on page [355].)
By following up the valley that extends to the east from Ndrawa, one enters after about a mile into the region of Na Raro, which is described on page [123].