The Valanga Range
This range, which trends N.W. and S.E. between the Mariko mountain-ridge and the head of the valley of Na Kula, attains a height of 1,880 feet at its N.W. and of 1,710 feet at its S.E. end. The average elevation, however, is probably not over 1,300 or 1,400 feet. My acquaintance with the range is only partial, but it is sufficient to bring to light some of its leading structural features. Those who follow me will find in these mountains a very interesting region for their geological explorations.
(1) Traverse of the Valanga Range.—In making the journey from Valanga to Vunimbua, I crossed the range where its elevation was about 1,300 feet. Basic agglomerates, containing sometimes amygdaloidal blocks, are displayed in the low district between Valanga and the foot of the range. In the stream-course at the base of the slope the deeper seated rocks of the range are at once exposed. Large masses, 5 or 6 feet across, of altered grey pyroxene andesites lie in the bed of the stream. Some of them show opaque porphyritic felspar and have the appearance of porphyrites (sp. gr. 2·67). They belong to the type described on page [271] under genus 6 of the augite-andesites. Others are grey propylitic varieties of a basic semi-doleritic andesite penetrated by cracks containing calcite, and displaying in a groundmass, exhibiting much viridite and a little pyrites, calcitic pseudomorphs of the felspar phenocrysts and more or less parallel felspar-lathes, ·15 mm. long and somewhat altered. Another of the deeper-seated rocks commonly exposed on the upper west slopes of the range is a dark grey rock showing much porphyritic pyroxene (sp. gr. 2·72). It has a micro-felsitic groundmass and is referred to the fourth order of the hypersthene-augite-andesites described on page [291].
About two-thirds of the way up the western slope of the range, there is exposed a coarse palagonite-tuff, evidently an incrusting deposit. Stout crystals of augite can be picked out of it, and it contains also lapilli up to an inch in size of a basic vesicular semi-vitreous basalt.
Descending the eastern slopes one observes between 1,200 and 1,000 feet large blocks of the same grey hypersthene-augite-andesite above mentioned and of a grey granitoid rock of the gabbro type. This last is a hypersthene-gabbro with specific gravity of 2·75, and belongs to the group of plutonic rocks described on page [250]. Its pyroxene phenocrysts are often represented by fibrous bastite. One can scarcely doubt that this gabbro is the plutonic equivalent of the prevailing grey pyroxene-andesites.
Lower down the slope only small fragments of rocks were exposed, probably derived from an agglomerate. One of the specimens here obtained is a doleritic basaltic andesite (sp. gr. 2·77). Another is a very interesting rock displaying large porphyritic crystals of a mineral like bronzite in a groundmass originally to a large degree vitreous; but the glass is now replaced by viridite and secondary crystalline silica. The “bronzite” is the result of the conversion of associated rhombic and monoclinic pyroxene into fibrous bastite.
From the results of the traverse across this part of the Valanga Range it may be inferred that more or less altered grey basic andesites passing into gabbros chiefly compose it. No doubt at one time it was largely covered with basic tuffs and agglomerates, but these deposits have been almost completely stripped off by the denuding agencies, and were only noticed in one place on the western flank.
That the northern part of the range towards the Mariko ridge has a similar structure is shown by the character of the loose blocks in the upper course of the Vunimbua River, which takes its rise on these slopes. Amongst those in the river above the village I noticed a solitary block of a coarsely crystalline diorite containing prisms of brown hornblende a centimetre in length.[[83]] But the rocks most frequently represented were propylitic grey hypersthene-andesites, in which the pyroxene is mostly changed into bastite, whilst the surface often sparkles with pyrites (see page [297]).
(2) Ngone Hill.—This is a curious conical hill, about 700 feet in height, that rises up on the right side of the Vunimbua River about 1½ miles above the village of that name and near the foot of the range. It evidently represents a “volcanic neck,” and doubtless this vent was the source of the large blocks forming the basic agglomerate that occurs in huge masses in the river-course in the vicinity of this hill. On its lower flanks is exposed a hard compacted tuff, showing pyroxene crystals, which is composed principally of fragments of a palagonitised vacuolar basic glass, the minute cavities being often filled with opal. In the upper part of the hill is displayed a massive altered augite-andesite penetrated by fine veins of chalcedony. Numerous irregular cavities filled with the same material occur in its dark opaque glassy groundmass.
The blocks of the agglomerates found in the vicinity of the hill vary in size from 4 to 18 inches. They are composed of a compact blackish semi-vitreous basic andesite (sp. gr. 2·73) of the type characteristic of the basic agglomerates over most of the island. The matrix of the agglomerate is hard and somewhat altered, and is chiefly made up of fragments, ranging up to 5 mm. in size, of a vacuolar basic glass, sometimes but slightly changed, though usually converted into palagonite, the vacuoles being filled with chalcedonic opal. The large masses of coarse tuffs displayed in the bed of a stream-course close to Ngone Hill are non-calcareous and composed of palagonitic materials. Palagonite-tuff clays are also exposed in the river-course a little above Vunimbua. About half-way between the village and the hill there occurs in position at the river-side an amygdaloidal basic rock, the amygdules being formed of chalcedonic opal.
It is apparent that this hill represents a lesser vent which probably dates back to the period before the emergence. All the products of its eruption are, however, more or less altered. From the absence of sorting in the blocks of the agglomerates, and from the character of the matrix, it may be inferred that these deposits have been accumulated directly from the ejected materials without the intervention of the agency of marine erosion.
(3) The Western Flank of the Valanga Range.—One of the boldest pieces of coast in the island lies on the eastern side of Savu-savu Bay, between the mouth of the Ndreke-ni-wai River and Valanga Harbour. Here a number of lofty headlands separated by broad valleys descend with precipitous fronts to the shore, some of them, as in the case of the Nambathi promontory on the north side of Valanga Harbour, retaining an elevation of 1,000 feet within a few hundred yards of the coast.
By following the coast-track from the Ndreke-ni-wai River to Valanga one crosses some of these headlands. As far as Vatu-lele altered red tuffs, basic agglomerates, and massive basaltic andesites are the prevailing rocks. The red tuffs exhibit a double alteration. They were originally composed of finely pulverised basic vacuolar glass, which subsequently became palagonitised, and afterwards there was an extensive deposition of chalcedonic silica and of red iron oxide. No organic remains appear to exist; whilst the scanty calcite present is evidently an alteration product. Where the road “tops” the headland on the north side of Vatu-lele Bay, there is exposed a dyke-like mass of a rubbly semi-vitreous basaltic rock penetrated in all directions by veins, 1 to 3 inches thick, of a tachylytic glass, splinters of which fuse readily in the ordinary spirit-lamp flame. The numerous fissures were doubtless produced during the consolidation of the rock; and subsequently they were filled with the still fluid residual portion of the magma, which would be composed of the most fusible constituents. This subject, which bears on the origin of palagonite, is discussed in [Chapter XXIV.]
Between Vatu-lele and Urata, palagonite-tuffs and basic agglomerates are chiefly displayed. On the north slope descending to Urata there is exposed in the foot-path a dyke-like mass of a dark-grey hornblende-pyroxene-andesite, an unusual type of rock which is described on page [298]. Just south of Urata I observed an agglomerate containing large blocks, 3 or 4 feet across, of the deeper-seated altered grey pyroxene-andesites that with the gabbros and diorites form the axis of the range.
(4) The Valley of Na Kula.—In crossing from Sava-reka-reka to Natewa Bay, one ascends the remarkable valley of the Kula and traverses the ridge at its head. This ridge, which is about 700 feet in height and forms the termination of the Valanga Range, is composed of altered grey hornblende-pyroxene-andesites and of similar holo-crystalline rocks representing the gabbro or plutonic type of the same. One of these rocks is described on page [250], under the head of hornblende-gabbro. Another is referred provisionally to the hypersthene-gabbros (page [249]); but it is extensively occupied by chlorite, viridite, and other alteration products. Here, as with the other rocks of the Na Kula Ridge, the plagioclase phenocrysts are opaque, the result of the numerous fine cracks with decomposition products in the interior of the crystals.... It is thus seen that in general structure the Na Kula Ridge represents the main axis of the Valanga Range to the north.
The valley of Na Kula is occupied by a river which does not empty itself, as one would expect, into Savu-savu Bay, but turns off sharply to the south at right-angles to its previous course, and after breaking through the coast range, opens into Naindi Bay. This peculiarity has attracted the attention of the natives. The village of Sawa-Ndrondro, which lies about 1½ miles up this valley, is not elevated more than 50 feet above the sea. The gradient is evidently not only very slight but is also irregular, so that in their upper course about 3 miles inland, where the elevation is only 130 feet, the waters of the river are partially checked in their flow and form extensive swamps where the “vitho” or wild sugar-cane flourishes.
(5) Concluding Remarks on the Valanga Range.—It may be inferred from the geological structure of the range that it is one of the oldest in the island. The agglomerates and tuffs that enter so largely into the formation of most of the other mountain-ridges are here to a great extent absent, except in the lower flanks; and we have exposed the axis of the range composed of more or less altered grey pyroxene and hornblende-pyroxene andesites passing, as appears to be the case, into gabbros and diorites. It is true that the exposure of the gabbros is limited and that only a single block of diorite came under my notice; but this might be looked for where the plutonic rocks are deeply seated. Although far overtopped by the neighbouring agglomerate mountain-ridge of Mariko, the Valanga Range would seem to date back to a much earlier stage in the history of the island.