They have a coarse felsitic groundmass, and are described under the felsitic order of the hypersthene-andesites on page [297]. They present all degrees of change from the hard dark grey mottled rocks, in which the phenocrysts of plagioclase and rhombic pyroxene are in part replaced by calcitic, viriditic, and chloritic materials, to those where the pseudomorphism and alteration is complete, when the decomposition products give their character to a pale yellowish rock, which sparkles with pyrites and often effervesces briskly with an acid. After this comes the final stage of disintegration, and we get a whitish rotten stone, often full of pyrites, the last condition of which is shown in a kaolin-like material exposed in the river-side.
The extensive alteration of these rocks is also indicated by the occurrence amongst the gravel of the river-bed and small stream courses near Nambuna of fragments of clear quartz prisms, half an inch across, and of nodules, three inches in size and sometimes hollow in the centre, formed of radiating quartz crystals that once filled cavities in the altered rock. Small masses of vein-quartz also occur in these streams, formed in a fissure by the growth of the crystals from the sides towards the centre. I was unable to find the source of the quartz; but it is probable that it was produced near the line of contact between the basaltic flows to the eastward and the older felsitic rocks of the district. The great alteration of the acid andesitic rocks exposed as the bed-rocks in this region may in all probability be attributed to the vicinity of these basaltic rocks. The two formations apparently come into contact about a mile east of Nambuna. In traversing this district on the road to Ndrawa one first observes in situ in the streams the decomposed felsitic bed-rock with occasional loose blocks of a quartzitic rock that displays in the thin section a mosaic of irregular grains of quartz. Afterwards, as one rises gradually to the top of the basaltic plateau, basaltic rocks are alone exposed in position.
In the character of the fine river sand a clue may be found to the exact locality of the contact. In the midst of the andesitic area between Nambuna and Ndrandramea, the sand, besides containing much magnetic iron, is also composed to a large extent of rhombic pyroxene prisms, clear quartz grains, and fragments of plagioclase, all derived from the porphyritic crystals of the dacites, &c. Near the basaltic district we find that the quartz and rhombic pyroxene have disappeared, the sand being largely made up of magnetic-iron grains mixed with fragments of plagioclase.
Profile and Geological Section of Vanua Levu, across the island from the Sarawanga (north) coast to the Yanawai (south) coast.
The Extent of the Area of Acid Andesite Rocks in the Ndrandramea District.—By referring to the map of this locality it will be observed that this region of andesites extends northward to the Navuningumu Range, and that on the south it would be separated from the district of tuffs and agglomerates, named the table-land of Na Savu, by a line joining the hills of Soloa Levu and Thokasinga. On the east it is bounded by the basaltic area of the Wainunu table-land. On the west it extends at the surface, with an occasional overlying patch of submarine tuffs and clays, for a distance of at least two or three miles from the base of the hills, and sometimes, as in the direction of Sarawanga, more than half way to the coast. I have endeavoured to show the relation of these acid rocks to the basalts and to the sedimentary deposits in the geological section.
When taking the track from Sarawanga to Nambuna by way of Ndrandramea one soon enters the region of these acid andesites. The prevailing rock exposed on the surface, where it is usually much decomposed, is a bluish-grey hypersthene-andesite with a specific gravity of 2·54, and displaying in a cryptocrystalline groundmass, where the felsitic texture can be recognised, abundant phenocrysts of plagioclase and rhombic pyroxene. As high as 500 feet above the sea it is occasionally capped by patches of palagonitised clays and tuffs scantily foraminiferous, and at one place I noticed a patch of agglomerate, the subangular blocks six to eight inches across being formed of the same acid andesite. In the same way by taking the road from Tembe-ni-ndio to Nambuna, passing the hill of Kala-kala on the way, we leave behind the foraminiferous tuffs and limestones of the lower coast regions; and when about 400 feet above the sea we enter the inland district of felsitic andesites which begin about two miles from Tembe-ni-ndio.
The Navuningumu Range.—By following the track from Nambuna to Navuningumu one skirts the bases of Wawa Levu and Ngaingai, where dacitic rocks are exposed. After passing the watershed[[51]] between the Wainunu and Ndreketi rivers, the track descends into the deep valley of one of the western tributaries of the Ndreketi, where a characteristic holocrystalline type of these felsitic andesites is exposed. Approaching Navuningumu one finds exposed at its base agglomerates, composed of scoriaceous and amygdaloidal semi-vitreous basic rocks, overlying a dark tufaceous sandstone which on examination proves to be a basic pumiceous tuff of the type described on page [333], and scantily foraminiferous.
We stand now in a region of basic rocks on the south-east side of the range, and before us rises abruptly the weird-looking magnetic peak of Navuningumu, which is well represented in the accompanying illustration. In the wet season its summit is usually enveloped in the thunder-clouds. Its elevation above the sea is 1,930 feet, but estimated from its base its height is 1,000 to 1,100 feet. The natives also name this peak Na Seyanga, after a town that once existed in this locality. It is the summit of a range that extends a mile or more to the north where it terminates in a lesser peak known as Mumu.
Ascending the peak of Navuningumu from the south-east one finds exposed in its lower part, up to 1,200 feet above the sea, pitchstone-agglomerates (composed of fragments of a vitreous basic rock) and white tufaceous sandstones (containing a few tests of foraminifera), such as are described below in the case of the neighbouring Mbenutha Cliffs. Between 1,300 and 1,500 feet there is displayed in position a typical dacite of the type described on page [303].