I will begin the description of this peninsula with the eastern extremity north of Kumbulau Point, the interior of which is cut up into ridgy hills 300 to 350 feet in height. On its eastern coast are exposed volcanic agglomerates, composed of large blocks, which from their dimensions given below would weigh between one-third and two-thirds of a ton, a size indicating the immediate vicinity of the vent, now obliterated, from which they were originally ejected. Near Kumbulau Point the blocks, which are made of basaltic andesite, measure five or six cubic feet. Further north in the vicinity of Vatu-Ndamu, the precipitous coast cliffs are composed of agglomerates, the large blocks of which, often ten cubic feet in dimension, are formed, not of the prevailing basaltic andesites, as in other parts of the peninsula, but of a grey hornblende-andesite. This singular appearance of an acid andesite in a region of basic rocks has no doubt given rise to the native name of Vatu-Ndamu, “the red or brown stone.” It belongs to the second order of the hornblende-hypersthene-andesites, and is described on page [298].
Proceeding along the south coast westward from Kumbulau Point, before arriving at the village of Na Tokalau we pass from the district of agglomerates into that of the bedded tufaceous sandstones and clays which are exposed all along the coast to Kiombo about three miles away. The transition is indicated by the agglomerates becoming interstratified with the tuff-beds. These sedimentary tuffs are as a rule steeply inclined at angles of 20 to 40 degrees, the prevailing direction of the dip being to the north-east, its uniformity for such a length of coast being noteworthy. These beds however are occasionally “crumpled”; and here and there a globular structure is developed.
The hills of this region of sedimentary tuffs between Na Tokalau and Kiombo are the highest of the peninsula. They usually attain a height of 400 feet, but do not reach 500 feet. From each of them descends to the coast a spur terminating in a rocky point; whilst between these points lie low sandy flats, where the native villages of Levuka, Kiombo, &c., are situated. The tuff-rocks extend to the top of the hills behind Na Tokalau, and probably this will be found true of most of the other hills. Agglomerates are not common in the district. In the point west of Na Tokalau, however, they are overlaid by basaltic agglomerates, some of the blocks being scoriaceous. In the point east of Levuka, a chocolate-coloured somewhat calcareous tuff-clay occurs interstratified in thin beds with the coarser deposits.
The general characters of these tuff-sandstones and tuff-clays have already been briefly referred to. The former are much more prevalent and non-calcareous; the latter are sometimes a little calcareous and look like marl, and may perhaps contain a few tests of foraminifera. Both are formed of the debris of basic rocks and are more or less palagonitic. The coarser deposits are described as sample A on page [330]. At times these tuffs are composed of much coarser fragments of the same materials, some of them a centimetre in size. A type of tuff intermediate in character is not uncommon.
The promontory that lies between Kiombo and Soni-soni Island has been formed by a remarkable basaltic flow. The low tongue, about 50 feet high and 200 to 300 yards across, in which it terminates, was originally severed by a passage worn by the sea from the main portion; but it is now joined by a low tract only 2 or 3 feet above the beach and partly occupied by mangroves.
The structure of the flow is well exhibited in the shore-flat and coast-cliffs west of Kiombo, and extending to the end of the point. The waves have here cut into its mass and exposed its structure. Its lower part, as exposed in the shore-flat, is made of a compact hemicrystalline basalt; whilst its upper portion, as displayed in the cliffs, 30 or 35 feet in height, is composed of vitreous and semi-vitreous forms of the same rock looking like pitchstone. The upper vitreous part is sometimes massive; but usually it is rubbly, with a tendency to form spheroidal masses. All transitions can there be traced between the hemicrystalline rock of the shore-flat and the vitreous rock of the cliffs.
The rock of the shore-flat, which has a specific gravity of 2·83, is a blackish porphyritic basalt with scanty olivine, and on account of the semi-ophitic character of the augites of the groundmass it is placed in genus 33 of the olivine class. The plagioclase phenocrysts are 3 to 5 mm. in size. About half of the groundmass is made up of felspar-lathes (·17 mm. long) and large augites (·11 mm.), the rest consisting of a smoky devitrified glass containing a few irregular “lacunæ” filled with the residual magma in the form of a reddish-brown opaque palagonite-like material. The rock intermediate between the lower and upper portions of the flow is also intermediate in character, having a specific gravity of 2·77, whilst quite three-fourths of the groundmass are of smoky glass.
The vitreous rocks of the cliffs, though usually rubbly in appearance, have also the aspect in places of brecciated pitchstone tuffs with the interstices filled with waxy palagonite; but the microscopical examination shows that we have not to deal with a rock of detrital origin. We have here the effects of the breaking up and crushing in situ of a dark-brown isotropic basic glass[[47]] carrying porphyritic plagioclase. The interspaces then became partially filled with the finer fragments of the glass and of the crushed felspar; but they were in the main occupied by a still liquid magma which penetrated into the cracks of the glass-fragments and into those of the felspars, where the fractured portions in some cases remained in position. There it has become devitrified and often palagonitised. Whether this liquid magma was produced by a partial remelting resulting from the heat developed during the crushing of the glassy upper portion of the flow during the contracting process, or whether it was squeezed upwards from the less consolidated lower portion, I cannot determine, although the last supposition seems more probable. At all events the edges of the glass-fragments are peculiarly eroded as if by the magma. (The bearing of these facts on the origin of palagonite is discussed in [Chapter XXIV.])
I infer that this flow has descended from the hills west of Kiombo. Huge masses of agglomerate are exposed in the lower third of the hill marked “470 feet” in the chart, and immediately north of the town. Fine clayey tuffs are exposed in the hill at the back and to the westward of this place; but the locality requires a more detailed examination. The absence to all appearance of vesicular and scoriaceous rocks in the case of this basaltic flow is remarkable. This would not have been expected in the case of a supra-marine flow; and indeed the testimony of the tuffs of this peninsula sufficiently indicates that during their deposition the whole district was submerged.
The future inquirer will doubtless discover some old volcanic “necks” in the hills of this peninsula. One such hill overlooks the Soni-soni inlet about a mile west of Kiombo. It is a singular isolated hill which I have named Bare-poll Peak for descriptive purposes. In my notes its height is stated as 120 feet, but it appeared to me to be rather higher than this. It is capped by two huge masses, 14 or 15 feet high, of a dark grey slightly scoriaceous augite-andesite with a cryptocrystalline groundmass, which apparently form the uppermost portion of a volcanic “neck” or pipe. According to the size of these rock-masses the “neck” would have a circumference of 80 or 90 feet. These masses are in part incrusted with agglomerate.