These remarks on the basaltic plains of Sarawanga and on their incrusting submarine deposits may be concluded with a brief reference to the siliceous concretions, 2 or 3 inches across, the silicified portions of corals, and the fragments of clay iron-stone and limonite resembling hæmatite, that occur frequently on the surface. They are common on the plains south of Lekutu and between Lekutu and Sarawanga, and up to elevations of 200 feet in the foraminiferous clay district east of Sarawanga, where fragments looking like portions of the silicified branches of Madrepores are to be found; but they are not limited to such localities, and may occur also where the surface is formed of decomposed basaltic rock. (These matters are generally discussed in [Chapter XXV].)
The Basaltic Plains of the Ndreketi.—This low-lying region of rolling “talasinga” country now serves as the basin of the Ndreketi river, the largest of the rivers of Vanua Levu. It is usually elevated between 100 and 300 feet above the sea, and its limits are well defined by the 300 feet contour line in the map of the island. On the east it is separated from the basin common to the Wailevu and Lambasa rivers by the Sealevu Divide, which is described on p. [136]. On the west, as before observed, it is only in part distinguished from the basin of the Sarawanga by the spur descending from the dacitic mountains of Ndrandramea. It meets the coast in the vicinity of the mouth of the Ndreketi; but for two-thirds of its length it is cut off from the sea by the great Nawavi range. It supports the characteristic vegetation of the “talasinga” or sun-burnt land. Whilst the Pandanus and the Casuarina are most conspicuous amongst the trees, bushes, herbs, grasses and ferns predominate. Here the native Ginger and the native Turmeric with species of Tacca are frequently to be recognised, and the waste-land bushes of Dodonæa viscosa and Mussænda frondosa are abundantly to be found.
As in the Sarawanga plains, the basaltic rocks are here often overlain or incrusted by submarine deposits, the former exposed in all the deeper river-beds, the latter frequently displayed in the sides of their tributaries.
I will deal first with the basaltic rocks. In the places where the surface deposits have been stripped off, these rocks are generally exposed as decomposing boulders, the spheroidal structure being well developed in the weathering process. Not infrequently, however, a rudely columnar structure is exhibited where the rivers have cut deeply into the basalt. The columns that I observed were usually vertical. In the river-bed at the landing-place at Mbatiri, for instance, the columns are from 2½ to 3 feet across and vertical. As exposed in the river-crossing about a mile above this town they are 12 to 15 inches in diameter and also vertical. However, at Na Kalou, a coast village about 1½ miles east of the mouth of the Ndreketi, where there is an unexpected exposure of basalt, the columns, about a foot in diameter, are inclined at an angle of about 20° from the vertical and face to the north.
These rocks are, as a rule, compact, only showing a typical scoriaceous structure in the case of specimens obtained near the foot of Nakambuta, an isolated hill about three miles to the southward of Natua, which probably represents a vent of more recent times. Often, however, they have a pseudo-vesicular appearance, from the occurrence in the midst of the patches of interstitial glass of minute irregular cavities that seem to have been formed during the last stage of consolidation of the magma.
The prevailing type of basalts is a blackish, doleritic, semi-ophitic rock without olivine, with specific gravity 2·78 to 2·80. They are characterised by the length of the felspars of the groundmass (·22-·35 mm.), by the large size of the augite granules (·1-·3 mm.), and by the quantity of dark interstitial glass. They present two forms, one with and the other without plagioclase phenocrysts. The first kind is referred to genus 9 of the augite-andesites (page [272]), some of the specimens being referred to the porphyritic sub-genus, and others to the non-porphyritic sub-genus, according to the size of the plagioclase phenocrysts. The second kind, without felspar phenocrysts, belongs to genus 12 of the same class (page [275]). A good example of the porphyritic rocks is afforded in the large blocks lying in the stream-beds during the first half of the way from Ndreketi to Sarawanga.
It may be pointed out here that these doleritic, semi-ophitic basaltic andesites of the Ndreketi plains differ conspicuously from the prevailing type found on the slopes of Seatura, on the Sarawanga and Mbua plains, and on the Wainunu table-land. There we have, as a rule, olivine-basalts, having a specific gravity of 2·86 to 2·90, and showing but scanty interstitial glass, the felspars of the groundmass being on the average not over ·2 mm. in length, whilst the augite granules are, as a rule, only ·02-·03 mm. in diameter, and the ophitic structure is infrequent.
The submarine deposits, consisting of foraminiferous clays and coarser tuff-sandstones, the former being usually beneath, are found at intervals all over this area. They occur inland as far as Vuinasanga and Nareilangi, near the base of the mountains of Va Lili and Na Raro, reaching as high as 300 feet, their place being taken on the mountain slopes by coarser tuffs and agglomerates. When not weathered they are more or less calcareous, and contain occasionally marine molluscan shells, whilst palagonitic debris enter largely into their composition. The foraminiferous clays, often much bleached by hydration, are well represented around Mbatiri and in the districts between that town and Natua and Nareilangi. They are relatively deep-water deposits, and belong to the type described on page [323]. Others, again, as exposed in the banks of the river at Natua, are chocolate coloured and of the kind referred to in detail on page [335]. These foraminiferous clays in the region between Natua and Mbatiri are overlain in places by coarse, almost brecciated, tuffs, formed in part of the debris of acid andesites, such as compose the not far distant mountain of Na Raro.
Since the massive basaltic rocks are exposed in all the deeper rock channels of these plains, it is apparent that the overlying submarine deposits can possess no great thickness. Probably they are never 100 feet thick, and usually far less. In many places, through their denudation, the underlying basaltic rocks are exposed, and in a decomposing condition largely form the surface. These deposits as a rule display bedding, the beds being horizontal or at least only inclined 2 or 3 degrees. This horizontality is a nearly constant feature of these submarine beds, as they overlie the basaltic rocks of the plains; and it is a feature we should expect to find where there has been emergence rather than upheaval.
Siliceous concretions and silicified coral fragments, so characteristic of the surface of some of these plains of Vanua Levu, did not frequently come under my notice here. They, however, occur occasionally, as in the district between Nanduri and Natua.