All around the mountain, except on the upper steep portion on the south side where it is well-wooded, the slopes have the usual character of the “talasinga” districts, being occupied only by grass, ferns, cycads, and the ordinary scanty vegetation of such regions. Whilst on most sides the surface configuration is fairly regular and the ascent to the summit is more or less regular, on the south side bold spurs with valleys between them descend to the coast, and the central mass rises abruptly in the middle of the peninsula from a height between 300 and 500 feet above the sea. It is on this side that Naivaka has the appearance of having been originally a crateral mountain, of which, however, only the north segment in a much degraded condition now remains, whilst the other two-thirds have disappeared.
The prevailing rocks are a blackish compact olivine-basalt, having as a rule much smoky glass in the ground-mass and possessing a specific gravity of 2·92-2·94. They are referred to in the description of genus 25 of the olivine-basalts given on page [259].
These rocks compose the agglomerate and the agglomerate-tuffs that form the eastern portion of the summit and probably most of the elevated part of the mountain. Similar agglomerates occur along most of the north coast, the rock being in a few places scoriaceous or amygdaloidal; and they occur in huge fallen masses on the south side near the foot of the precipitous portion. The blocks in the agglomerate of the summit are usually six to eight inches across.
On the south-west side the massive rocks exposed are less basic with a specific gravity of 2·76 to 2·79. They are also more altered, the olivine being infrequent and the interstitial glass scanty. They differ besides in the parallel arrangement and in the length of the felspar-lathes (·18 mm.), which are on the average half as long again as those of the prevailing olivine basalts (·12 mm.). They are placed in a different order of these rocks and belong to genus 37 described on page [262].
Tuffs did not come frequently under my notice. At one part of the north coast the cliffs are formed of a palagonitic tuff-sandstone, effervescing with an acid, which is described on page [330]. Although no organic remains are to be noticed, it is probably a submarine deposit.
On a spur on the south-west side, at an elevation of 600 feet, there is exposed a hard red palagonitic tuff dipping away from the summit at an angle of 40°. It is mainly composed of the palagonitised débris of a vacuolar basic glass and incloses broken and entire crystals of plagioclase, augite, and olivine.
The augite crystals, which attain a length of five or six mm., project from the weathered surface and are easily detached, lying about in quantities on the ground in places. Although they are now imbedded in evidently a submarine tuff, these pyroxene crystals could only have been ejected as such from a subaerial vent; and it would therefore appear that they fell into the sea around the shores of a volcanic island in a state of activity. These crystals are often cracked and are as a rule not so perfect as those I have gathered from the slopes of Vesuvius, Stromboli, and Etna. They exhibit an unusual tabular form arising from the great development of the clinopinakoid at the expense of the orthopinakoid faces.
On the whole it may be inferred that the Naivaka volcano was submerged at the time of its origin, but that the eruptions continued after it began to show itself above the sea. In many of its features, especially in the character of the agglomerate that forms its upper portion, and in the palagonitic nature of the tuffs, Naivaka differs only from other elevated districts of the island, where organic remains occur, in the absence of such remains. Its form bears testimony to the extreme degradation we find in other districts, and the occurrence of foraminiferous tuffs high up the neighbouring slopes of Mount Sesaleka affords additional evidence of the original submergence of this district.
The Hill of Korolevu.[[33]]—About three miles east of Mount Naivaka there rises to a height of 800 feet, about a mile inland from the shores of Wailea Bay, the singular flat-topped hill of Korolevu. It displays vertical cliff-faces, with a drop often of 200 or 300 feet, which have become so deeply furrowed or fluted by the eroding atmospheric agencies that they appear at a distance to be made of columnar basalt. The hill is, however, formed in mass of a compacted tuff or agglomerate tuff built up of materials of a hyalomelan basic glass that has undergone partial conversion into palagonite. In the upper thirds these rocks show no bedding, but in the lower slopes on the seaward side they are bedded and dip to the north away from the summit at an angle of 15° or 20°. The form of this hill is well shown in the sketch attached, and there is little doubt that we have here an old volcanic “neck,” the remains of a submarine vent.
A specimen of the tuff from the summit is made up of compacted fragments, in size ranging up to one third of an inch, of a bottle-green vacuolar glass, which fuses readily in a lamp-flame and is not dissolved by hydrochloric acid. This glass is usually isotropic, but much of it is also palagonitic and feebly refractive, the vacuoles or steam-holes, which are often elongated, being in the last case filled with the same palagonitic material. Plagioclase crystals occur macroscopically in the glass; they are much eroded and contain numerous large inclusions both of the clear isotropic glass and of its palagonitised form.