A long spur descends to the south from Ulu-i-ndali to form the rocky promontory of Vatu Vono or “Stone turtle,” so-named from the fanciful resemblance of the large rounded blocks of basalt on the shore to the backs of turtles. To the south-east extend the low tuff-formed Ravi-ravi plains which are but slightly elevated above the sea. The Ulu-i-ndali range is apparently connected by a “col” with a range of similar height to the eastward, the highest peak of which is about 3 miles distant.
A more or less coarse doleritic grey olivine-basalt forms the mass of this hill and is chiefly exposed in its upper portion. Around its slopes, extending from the coast usually halfway up the hill, are blackish-brown olivine-basalts; they differ amongst other points from the grey basalts—which are practically holocrystalline, in their greater amount of interstitial glass, to which, doubtless, is due their dark colour. These dark basalts also occur scantily on the summit; but from their greater prevalence on the lower slopes and from some other of their characters, it may be inferred that they are in the main formed at the surface. Outside all, on the north and south sides of the hill, are exposed coarse tuffs composed of fragments of palagonitised vacuolar basic glass and containing much secondary zeolitic and calcitic materials. They are purely of eruptive origin, and although containing no organic remains were doubtless, as in the case of the precisely similar tuffs of the neighbouring district of Nandua, deposited under the sea. A description of their characters is given on page [335]. Such tuffs extend as high as 300 feet above the sea on the north-west slopes, where there are exposures, 10 to 12 feet in thickness, in the dry stream courses; and here they may be seen overlying the basalt and rudely bedded, dipping away from the summit at an angle of 15 degrees.
The grey olivine-basalts of Ulu-i-ndali, which often look like clinkstone, range generally in specific gravity from 2·9 to 2·95. They contain microporphyritic olivine in abundance, which is usually more or less hæmatised and in extreme cases of the change looks like brown mica. Most of them are referred to genus 16 of the olivine class and their characters will be found described on page [258]. The felspar-lathes are stout and show sometimes lamellar twinning, and on account of their large size (·2 to ·5 mm in average length) the rock acquires a doleritic texture. They display as a rule a flow arrangement around the olivine crystals. Augite granules occur in great abundance, and there is rarely any interstitial glass.
These grey olivine-basalts are as a rule non-vesicular, but rocks with minute irregular cavities, though without glass, occur scantily on the upper slopes. They come near to the grey olivine-basalts of the hill of Koro-i-rea in the Solevu district, as described on page [77]; but they differ in their doleritic or coarser texture, the felspar-lathes in the last-named locality being much smaller, their average length being ·12 mm.
The blackish basalts, mostly characteristic of the lower slopes of Ulu-i-ndali, vary somewhat in character; but they may on the whole be regarded as surface forms of the more deeply situated grey basalts which are practically holocrystalline. The rock of this kind that prevails on the south and west sides has a specific gravity of 2·96. It is referred to the same genus (16) as the grey basalts, but differs from them in the circumstance that the microporphyritic olivine is serpentinised and not hæmatised, and in the occurrence of a fair amount of devitrified interstitial glass, to which probably the dark colour of the rock is due.... The dark aphanitic basalt, with flinty fracture and a specific gravity of 3·00, that is displayed in Vatu Vono Point, is merely a compact surface variety of the more coarse-textured grey basalts, being referred to the same genus. Here there is a great abundance of microporphyritic olivine in a groundmass of parallel felspar-lathes and augite grains; but the felspars are unusually small, averaging ·1 mm. in length; and there is a much larger amount of fine magnetite than in the grey basalts. There seems to be no interstitial glass; and the olivine when not fresh is usually serpentinised but occasionally hæmatised.
The dark basalts of Ulu-i-ndali when they occur on its upper slopes become ophitic. A specimen lying beside me has a specific gravity of 2·91. Allowing for the structural differences, it appears as an ophitic surface variety of the deeper seated grey basalts. A description of it is given under genus 12 on page [256], of which it forms the type.
From the data above given, the hill of Ulu-i-ndali is to be regarded as the basal portion of a submarine volcano still retaining part of its ash-coverings. The grey doleritic basalts probably represent the core and the dark fine-grained basalts represent the flows of this ancient vent.
The Kumbulau Peninsula.—South-east of Ulu-i-ndali stretches a remarkable “talasinga” district which for convenience I will call the peninsula of Kumbulau. Its south or seaward border is broken and hilly, and presents an irregular line of hills 300 to 470 feet in height, extending from Kumbulau Point to Soni-soni Island, which is almost connected with the coast. The rest of the peninsula is a low-lying and often marshy plain, which, though elevated in some places 20 to 25 feet above the sea, is usually much lower. On the north-east side of the isthmus is the narrow Nandi inlet, bordered by low mangrove-belts, which represents the broad channel that in a very recent period of the island’s history cut through the present neck of the peninsula between the head of the Nandi inlet and Ravi-ravi.
Stratified and often steeply inclined tuff-sandstones and clays, more or less basic and palagonitic in character, form together with basaltic agglomerates the prevailing rocks of the peninsula, whether in the hilly portion or in the plains. They belong to the basic tuffs of mixed composition described on page [330]; and though the agency of eruptions can be recognised in their components they are also the products of marine erosion.
Some of the hills represent volcanic “necks”; whilst the low narrow promontory between Kiombo and Soni-soni Island has been formed by an old basaltic flow.