CHAPTER XIII

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)

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.

THE MOUNTAIN-RIDGE OF MARIKO

This mountain-ridge, which trends nearly east and west and joins the Valanga Range, rises in mass to a height of rather over 2,000 feet. Above this elevation it terminates in several short conical peaks, of which the highest, 2,890 feet, is named Mariko, the Drayton Peak of the chart. One of the peaks, lying a little to the east of the summit, and apparently between 100 and 200 feet lower, is called the Vatu-mbutho or White Rock. In the profile of the range, as seen nearly “end-on” from the distant south shore of Natewa Bay, it would appear to be rounded in its upper part. Its true outline, however, when viewed in length, is, as described above, namely, a massive ridge with various peaks.

When viewed from the top of the hills behind Valanga, this mountainous range has a very imposing appearance. On the south side it rises precipitously to the summit, but the northern slopes below an elevation of 1,800 or 1,900 feet descend with a very easy gradient for 1½ or 2 miles into the valley of the river Ndreke-ni-wai. In the first case the average angle of the slope would be from 15 to 20 degrees and in places often more; whilst in the second case the average inclination would be about 7 degrees. The contrast between the two sides of the range is very striking and one ought, I think, to find a parallel in the broken-down rim of a large crater with a gentle outer slope and a precipitous inner face. When descending recently the outer slope of Monte Somma, the ancient Vesuvian vent, I found reproduced some of the features of the northern slope of Mariko. The tuffs and agglomerate-tuffs that cover their outer flanks are in both mountains deeply scored by the gorges and ravines worn by the torrents. After the description of the geological structure of the Mariko Range, we shall perhaps be in a better position to consider this question; but until a proper survey of the region has been made it will not be possible to give a final answer. There are also many other uncertainties which would be removed by the accurate mapping of the district, such for instance as the mode of connection between the Mariko and Valanga Ranges.

The highest peak of the Mariko Range is irregularly square-topped and is only a few paces across. It has a soil-cap and supports small trees and shrubs, whilst there is a precipitous rocky face on the east and south. Like most of the other lofty peaks of the island it is magnetic, and as remarked on page 368, it markedly deflects the compass-needle.

I made two ascents of this mountain from Vunimbua, one to the highest peak (2,890 feet), and the other across the range to Nukumbolo at a point half a mile or more to the west of the summit, where its elevation is 2,200 feet. Basic agglomerates and agglomerate-tuffs prevail on both the slopes up to 1,800 or 2,000 feet, the blocks being composed of a dark semi-vitreous basic andesite referred to the hypersthene-augite sub-class with specific gravity 2·75. It contains much glass in the groundmass, and since the pyroxene of the groundmass is prismatic, this rock belongs to the prismatic sub-order described on page 289. Ordinary basic tuffs are also well represented on the north flank. On the south or precipitous side they are usually more or less altered. Here, for instance, they may take the form of a hard breccia-tuff containing vesicular lapilli, up to half-an-inch in size, of a semi-vitreous basic rock, the small steam-holes being either empty or filled with opal or chalcedony. The matrix of the rock is made up of finer fragments of a basic vacuolar glass, showing a few felspar microliths, but often more or less palagonitised. Evidence of further alteration is afforded by the small cracks and crevices filled with chalcedony.

Other altered tuff-rocks are exposed on the south slope. At an elevation of 400-450 feet above the sea, and underlying the agglomerates and breccia-tuffs, I found exposed in a stream-course a hard dark rock looking like a compact andesite. Under the microscope, however, it is shown to be an altered palagonite-tuff composed in part of angular fragments of plagioclase and of rhombic and monoclinic pyroxene, not exceeding ·15 mm. in size, and containing also similar-sized fragments of a basic hemicrystalline rock. The base is made up of palagonitic material and contains a few “Globigerina” tests sometimes displaying calcite in their interior. Fine cracks filled with chalcedonic silica testify to a subsequent alteration of the deposit. At 1,500 feet occurs a hard red altered palagonite-tuff, having a similar composition and being altered in like fashion, but not displaying tests of foraminifera in the slide.

The foregoing remarks refer to the main undivided mass of the range, that is, up to 2,000 feet. The highest peak of Mariko probably represents in structure the other peaks rising to various heights on either side of it. Here, at elevations between 2,000 feet and the summit, a rubbly agglomerate prevails of a somewhat different character from that occurring at lower levels. It is well exposed in some cave-cliffs at a height of 2,500 feet and also in the rocky face of the peak. The rock composing the blocks is a dark-grey aphanitic augite-andesite (sp. gr. 2·65), referred to genus 20 of that sub-class and displaying prismatic pyroxene in the groundmass. Smoky residual glass exists usually in fair amount; whilst in the blocks of the cave-cliffs it is so abundant that the rock may be termed semi-vitreous. In the locality just named the blocks are scoriaceous, the steam-pores, which are drawn out to a length of 5 or 6 mm. and more, being partially or completely filled with calcite and occasionally with opal. At times the steam cavities are much larger. In one of my specimens there is an elongated cavity 5 cm. (2 inches) in length, which has a thin lining of chalcedony, from the surface of which pyramidal crystals of calcite project into the interior. (I found the same grey andesite exposed _in situ_ lower down the south slope at an elevation of 1,800 feet, but non-scoriaceous.) The matrix of the agglomerate principally consists of fine palagonitic material with small fragments of plagioclase and pyroxene but apparently no lime.

At heights of about 2,800 feet on the south side of the peak, and of 1,600 feet on the north flank of the range, are exposed non-calcareous greyish tuffs remarkable for the quantity of crystals of rhombic pyroxene, entire and in fragments, that they contain. This is a characteristic feature of the more acid andesitic tuffs of the island, and it is to these deposits that the Mariko tuffs in question make a near approach. They contain at times subangular fragments of more basic rocks; and are true tuffs in the sense that although perhaps deposited on a sea-bottom they represent the ejected materials of a subaerial vent.

The crest of the range, where it is crossed by the road from Vunimbua to Nukumbolo and for 200 feet below, is formed of a decomposed rock, perhaps a breccia. A fragment of the rock obtained from the crest is a grey somewhat altered hypersthene-augite andesite (sp. gr. 2·75) with an orthophyric groundmass, and referred to the order described on page 290. This rock may be connected with the tuffs above alluded to.... Reference may here be made to a black basaltic rock (sp. gr. 2·88) of which, at an elevation of 2,500 feet at the foot of the peak, I found a portion of a columnar block about 18 inches across. It may prove to be an olivine-basalt; but no section has been made of it.

It is apparent from the foregoing description of the Mariko Range that in general structure it does not differ materially from the other mountain-ridges of the island, although in the types of the rocks it presents some variety. Here also we have agglomerates prevailing on the flanks and forming the summit. As far as the characters of the rocks can guide us, we cannot determine whether the range has been built up by a number of vents on a great fissure, or whether it represents the remains of a huge crater. In this uncertainty we can only appeal to the contrast between the gentle gradient of the north slopes and the precipitous descent of the south slopes as favouring the last supposition. We cannot, however, doubt that the agglomerates of the upper portion of the range are the products of an eruptive vent or of vents that rose above the surface of the sea, since the blocks are all of one kind of andesite and are often scoriaceous. We can be fairly certain that at such a time the lower slopes were in part submerged, seeing that foraminiferous tuffs underlying the agglomerates are now exposed. But we have to distinguish between these submarine basic tuffs of the lower slopes which may in part be the result of marine-erosion and the grey rhombic-pyroxene-tuffs of the upper levels which are probably derived from subaerial eruptions.

THE SAVU-SAVU PENINSULA

I include in this district the promontory west of Naindi Bay and Sava-reka-reka Bay. Although its surface is much cut up, it has, when viewed from a distance, a fairly even profile and attains a maximum height of rather over 800 feet. From the region east of it, it is separated by the Naindi Gap. Here one can cross the peninsula between the two bays above named without rising more than 50 feet above the sea. The elevated interior is divided into two parts, which are divided by a _col_, about 250 feet in elevation, which is ascended in crossing from Naithekoro on the south coast to Na Kama on the north coast. Much of the surface is clothed with the usual “talasinga” vegetation. Close to the north shore, with which it is connected by the reef-flat, rises the small island of Na-Wi, and off the extremity of the peninsula, which is known as Harman’s Point, is the islet of Naviavia, formed of raised reef-limestone as described on p. 8. The celebrated boiling springs known as Na Kama are situated on the north coast opposite Na-Wi. It may be remarked in passing that besides finding an exit in the springs, the hot water oozes through the beach and below the tide-marks for several hundred yards along the shore. These springs are described in detail on p. 25.

This is one of the few districts of the island in which elevated reef-masses occur at the sea-border. These old reefs, which attain a maximum elevation of 250 feet above the sea, are principally restricted to the neighbourhood of Naindi Bay. (They are referred to in detail in