The ascent at first is fairly steep, dacitic tuffs prevailing up to 1,000 or 1,100 feet above the sea and forming in places precipitous cliff-faces. Large masses of hornblende-andesite lie on the slopes. The dacitic tuffs distinguish Na Raro from all the other peaks of hornblende-hypersthene-andesite rocks that I examined. They seem generally to have been stripped off by the denuding agencies; and only at times, as around the slopes of Ndrandramea and Thokasinga, are to be found the remains of agglomerates of the same formation. In the case of Na Raro, however, the tuffs differ somewhat in their components from the rocks forming the mountain mass. The tuffs are derived from a hornblende-andesite of dacitic type; whilst the massive rocks of the mountain are of hornblende-hypersthene-andesites, without porphyritic quartz, but approaching the dacitic habit.
The tuffs of Na Raro, which are sometimes compacted and at other times rather friable, do not display bedding. They contain a little lime; but I found no tests of foraminifera. They are composed of fragments, up to a centimetre in size, of a dacite displaying brown hornblende, plagioclase, and quartz in a microfelsitic groundmass, together with a few fragments of a semi-vitreous basic andesite.
Above 1,100 feet the tuffs give place to the massive hornblende-hypersthene-andesite. At an elevation of 1,450 feet, a shoulder of the mountain is reached, near the top of which is the cave above mentioned. Crossing the shoulder one descends for 100 or 150 feet into a gap, thus reaching the foot of the precipitous northern peak, which rises up like a wall for a height of from 900 to 1000 feet overhead. It is in mass of the andesite just mentioned, many of its faces presenting inaccessible cliffs displaying seemingly no structure. This peak is somewhat lower than the southern peak. I placed its height at 2,270 feet, which, taking the total elevation of the mountain at 2,420 feet, as given in the chart, makes the difference 150 feet. A deep and broad cleft, that goes half-way down the mountain, separates the two peaks. The southern one, which appears to be inaccessible, is evidently formed of the same acid andesite.
These hornblende-andesites, with or without porphyritic quartz, appear to be for the most part restricted to the immediate vicinity of Na Raro, except to the south-west, where at a distance of about a mile and a half from the mountain at an elevation of 500 feet occur a rubbly hornblende-andesite and agglomerates of the same materials. Though the rock is of the Na Raro type, its presence here is suggestive of a distinct vent of small size, of which most of the traces have been swept away during the emergence of the island. About half a mile south-east of this locality at an elevation of 450 feet occur some singular banded palagonite-tuffs which, although they do not show foraminifera in the section examined, contain a little calcite and are probably of submarine origin.... In this locality I found a large white mass, measuring 4 × 4 × 5 feet, formed of a siliceous rock appearing in thin sections as granular chalcedonic quartz (see page [355]).
The hornblende-andesite of Na Raro, as in the case of the rocks of most of the other peaks of acid andesites, has its peculiar characters. It differs, for instance, from that of Vatu Kaisia in the larger grain of the felsitic groundmass (N. R. ·021 mm.; V. K. ·013 mm.), in the absence or rarity of phenocrysts of rhombic pyroxene, in its lower specific gravity (2·6 N. R.: 2·7 V. K.), in the presence of a little interstitial glass, and in other particulars. Both, however, belong to the sub-class of hornblende-hypersthene-andesites, and are described on page [301]. In the Na Raro rock the rhombic pyroxene is represented in the groundmass.
With regard to the relative age of Na Raro I am inclined to think that it is the most recent of the acid andesite peaks of the island. Neither vitreous nor vesicular rocks came under my notice in its vicinity; whilst the tuffs that clothe its lower slopes are non-pumiceous, though of dacitic origin, but containing also a few fragments of a semivitreous basic andesite showing tiny felspar lathes and augite-granules. Since the everywhere prevailing submarine palagonite-tuffs and foraminiferous clays do not extend over its area, we may assign to it a later date. It is evidently also posterior in time to the basaltic andesites and aphanitic augite-andesites around, which are covered by these submarine deposits. Relatively recent as it apparently is, this mountain bears the impress of a high antiquity. There is nothing to indicate that this “core” of a volcanic mountain belonged to a subaerial vent. Na Raro has shared in all the later stages of the submergence and emergence of the island. Though it presents the final page in the history of the hornblende-andesite volcanoes, that chapter has been for unknown ages closed.
The Na Raro Gap.—Between the Tavia and Va-lili Ranges there is a break in the mountainous backbone of the island, to which I have given this name. The greatest elevation is probably not over 800 feet. It is from the south side of this watershed that the Lango-lango river takes its rise.
CHAPTER IX
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Basaltic Lowlands of Sarawanga and Ndreketi.
One of the most striking features of the north side of the island is the extensive undulating plain that stretches from the Lekutu river to near Sealevu on the head-waters of the Ndreketi, a distance of almost 30 miles. In its western half this plain slopes gradually to the sea-coast, where it is bordered by a broad belt of mangroves. In its eastern half, from the mouth of the Ndreketi eastward, the lofty Nawavi coast range intervenes between it and the sea-shore. Its breadth varies usually between 4 and 6 miles, and its elevation, though it reaches a maximum of about 300 feet, is as a rule between 100 and 200 feet above the sea.