CHAPTER VIII

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)

MOUNT VATU KAISIA AND DISTRICT

THIS peak, 1,880 feet in height, starts up suddenly in the mountainous interior of the island. Being situated in the valley of the Yanawai river, which opens to the south, it forms a conspicuous landmark for vessels off the south coast; but from most other points of view, on account of its peculiar situation, it is usually difficult and often impossible to obtain even a glimpse of it.[53] From its remarkable blunt-topped conical shape it has received the not very appropriate name of Marling Spike in the Admiralty charts. The natives name it Vatu Kaisia, the first word signifying “rock,” whilst the second is the name of a demon.

Some idea may be formed of its situation and of the character of the neighbouring country from the profile-sketch and photograph here produced. I was unable for reasons given below to take a photograph of the mountain itself, as it was either too near or too far away. Vatu Kaisia is approached either from Ndrawa on the north or from Ndawara on the south, the ascent being best made from the west side. The regions traversed on the way are so densely wooded that the mountain does not become visible until the traveller is right upon it. He becomes suddenly aware that there is some huge mass close to him looming above his head through the trees; and it is with a feeling of awe that he first looks upon a mountain that although only a few hundred yards away nearly escaped his search. He is startled by its proximity, and wonders what strange forces have been at work to place it there; but his view is transitory, and whether proceeding north or south he sees it no more, unless he essays to climb its slopes.

Vatu Kaisia lies not in the centre but towards the west side of the Yanawai valley the river flowing as an impetuous stream around the foot of its eastern slope. In the profile-sketch the mountain itself conceals the peculiar feature of its position, which is, however, shown in an exaggerated form in the geological section below. On its west side rises a broad ridge running south which in places is not much higher than the basaltic plateau of Wainunu to the west of it. This ridge is only separated from Vatu Kaisia by a dark narrow gorge not many hundred yards in width, across which my natives were able to make themselves heard when near the summit. The mountain rises 1,100 or 1,200 feet above the gorge on its west side, which is 700 feet above the sea, and some 1,400 or 1,500 feet above the Yanawai river on the east, which is 300 or 400 feet above the sea. It possesses two peaks, of which the western one is smaller and lateral and has a height of 1,600 or 1,650 feet, whilst the eastern is the main peak and rises to 1,880 feet. The saddle between the peaks has an elevation of about 1,500 feet. It is very difficult to obtain a distant view of the two peaks, which lie about N.W. and S.E. with each other. They are either merged into one as in the view from the south, or else the highest portion of the main peak is alone visible.

On the lower slopes of the mountain as high as 1,100 or 1,200 feet is exposed a porphyritic doleritic basalt showing semi-ophitic augite and abundant interstitial glass. Its specific gravity is 2·8, but there is no olivine. It belongs to a type of basalt described under genus 9, sub-genus A, of the augite-andesites. The upper double-peaked portion rises precipitously, displaying bare rocky cliff-faces with a drop of 100 or 150 feet, and formed in mass of a grey andesitic rock with a specific gravity of 2·71 and showing abundant small porphyritic crystals of hornblende and rhombic pyroxene. It represents a type of the hornblende-hypersthene-andesites described on page 301. I was unable, through want of a rope-ladder, to accomplish the last hundred feet of the summit; but the general uniformity of structure was evident. No detrital rocks came under my observation.

That the porphyritic basalt represents a later flow around this old andesitic mountain is indicated amongst other things by this absence of tuffs and agglomerates. Vatu Kaisia is undoubtedly the core of an ancient cone of hornblende-andesite, and as in the case of Mount Soloa Levu, which is formed of somewhat similar andesites (see page 103), it has been more or less completely surrounded by later basaltic flows. Vatu Kaisia and Soloa Levu occupy similar positions with respect to the great basaltic table-land of Wainunu, the first lying just within its eastern border, the second lying partly within its western margin.

The structure of the ridge immediately west of Vatu Kaisia lends support to this view of the formation of this region. The ridge is here, it is true, elevated a hundred feet or so above the table-land which is about 1,000 feet above the sea; but whilst on its slopes facing the mountain the same porphyritic basalt prevails, there is a limited exposure on its top of the same rock (sp. gr. 2·68), differing only in the larger size of its porphyritic crystals of hornblende and rhombic pyroxene.

The narrow gorge isolating the mountain on the west is occupied by a tributary of the Yanawai River. It has a depth of 400 feet below the ridge; and as illustrated in the section below it has evidently been largely formed by the eroding agency of the stream. However, at the bottom of the gorge there is exposed a heavy aphanitic basalt showing no olivine and having a specific gravity of 2·85. Though of much finer texture, the felspar microliths only measuring ·05 mm. in length, it differs conspicuously from the overlying porphyritic basalt in possessing little or no interstitial glass. It is referred to genus 16, species A, sub-species 1, of the augite-andesites (page 280).

The probable structure of this district is shown in the geological section here given. It is assumed from the limited exposure of the same rock on the top of the ridge that the basaltic flows which surrounded the lower portion of Vatu Kaisia at the same time covered over another similar peak lying immediately west of it. Through stream-erosion Vatu Kaisia has now been isolated on its west side; and since the basaltic rocks rise to about the same height on both sides of the gorge thus produced, the original surface was probably as indicated by the dotted line in the diagram.

By following the summit of the ridge, as it runs south on the right side of the Yanawai valley towards Ndawara, some interesting rocks are observed. For the first mile from the camping-place opposite Vatu Kaisia the elevation increased from 1,100 to 1,300 feet, and blocks of a blackish basaltic andesite (sp. gr. 2·76) lay on the ground. About a mile further on fragments of white quartz-rock appeared on the surface having been thrown out of a shaft close to the track which had been sunk to a depth of 15 or 20 feet by a gold miner[54] a few years before. I could not descend the shaft to examine it: but the specimens picked up are evidently a white vein-quartz, some of them having a striated “slickenside” surface on one side.[55] There is evidently a “contact” in this locality, probably of a basaltic rock with an acid andesite.

Leaving the shaft, the track proceeds southward and eastward, and one descends gradually from a height of 1,100 feet down to the Yanawai river where the elevation is only about 150 feet above the sea. Occasional blocks of basaltic rocks lie on the surface of the ridge, and in one locality there is exposed a curious-looking agglomerate formed of fragments of a greenish altered augite-andesite, somewhat scoriaceous, the cavities being filled with a zeolite. At the crossing of the river a black basalt (sp. gr. 2·82) occurs _in situ_; whilst loose blocks of basalt and of an acid andesite occur in the river-bed. Continuing the journey from the Yanawai crossing to Ndawara near the mouth of the river, one follows the track across a range of hills, 500 to 600 feet in height, basaltic rocks prevailing on the surface.

THE NANDRONANDRANU DISTRICT.

Lying north-west of Vatu Kaisia is an elevated district which I have named after its highest summit, a square-topped peak rather higher than Vatu Kaisia and probably about 2,100 feet above the sea. Koro-ni-yalewa, which signifies “town of the women,” is another name of this peak. It is shown in the sketch given on page 113, and is situated about two miles north-west of Vatu Kaisia. I did not ascend this mountain, which from its form would seem to be made of an acid andesite like the Ndrandramea peaks. Much of this elevated region varies between 1,000 and 1,500 feet in elevation. It is connected with the Ndrandramea district by somewhat broken country not much over 1,000 feet in height, which is the “divide” between the river systems of the Ndreketi and Wainunu. A long tongue-like extension of similar elevation projects to the north-west between the Ndrawa and Navuningumu branches of the Ndreketi. This elevated region is continuous to the eastward with the Tavia Range which is described below. For convenience the valleys of the upper course of the Ndrawa river have been included in this district as their geological features can in this connection be best explained.

This region is well distinguished from most of the other districts of the island by the prevalence of aphanitic augite-andesites. These rocks have also supplied the agglomerates of the locality, and the palagonite-tuffs which are in places extensively represented are evidently in great part derived from vitreous forms of the same rocks. We seem to get nearer to supra-marine eruptions in this region than in most others. The palagonitic-tuffs and agglomerates appear to have rapidly accumulated in shallow water, and there is reason for regarding one exposure of the aphanitic augite-andesites as at all events a shallow-water lava-flow. The aphanitic character of the massive rocks, however it may have arisen, is here, as I take it, associated with the shallow-water habit of the tuffs and agglomerates.

(1) EAST SIDE OF THE NANDRONANDRANU DISTRICT.—By following the track leading from the ridge on the west side of Vatu Kaisia northward to Ndrawa one rises gradually to a more elevated region. The rocks exposed on the surface for the first mile are for the most part altered hypersthene-augite-andesites possessing a micro-felsitic groundmass. When a height of about 1,400 feet was attained, the track could not have been far from the peak of Nandronandranu, but on account of the wood no view was obtainable. In this locality between 1,300 and 1,400 feet soapy palagonitic clay-rocks and coarser palagonite-tuffs are displayed on the surface. No organic remains are to be noticed in the specimens collected here, but they are much affected by hydration. Judging from the fossiliferous character of similar deposits over a large part of the island, it is highly probable that these tuffs and clays are also submarine.

Afterwards a descent was made to an undulating region about 1½ miles across and elevated between 750 and 850 feet. The blocks there displayed on the surface are composed of a dark rather compact augite-andesite with a specific gravity of 2·75 (see genus 13) and of an altered greenish aphanitic augite-andesite with a specific gravity of 2·59 in which calcite occurs as an alteration product (genus 16). Aphanitic rocks of this character as shown below, are very prevalent in the north-west and north parts of the Nandronandranu district, but are not usually altered.

(2) THE NORTH-WEST PART OF THE NANDRONANDRANU DISTRICT.—The best route to follow here is to take the track from Nambuna to Ndrawa. After crossing the upper portion of the Wainunu table-land one reaches the headwaters of the Ndavutu River and then ascends the watershed between the Ndreketi and Wainunu river-systems, reaching Savulu, about 1,050 feet above the sea, where a solitary house marks the site of an old mountain town. This region is much cut up in deep valleys usually 200 to 300 feet deep, which are occupied by affluents of the Ndrawa branch of the Ndreketi, flowing north. The valley of the main affluents is from 400 to 500 feet in depth; and this constant ascent and descent of steep and often slippery valley sides makes the journey very tedious.

At Savulu one stands within the Nandronandranu district. Behind lies the Wainunu table-land with its olivine basalts; but here aphanitic augite-andesites prevail and extend to Ndrawa and beyond. They are exposed in position in the stream-courses and furnish most of the blocks and pebbles found in the bed of the main Ndrawa River for miles down its course towards the sea. They are dark, compact, and non-porphyritic rocks and are all referred to genus 16 of the augite-andesites as described on page 279. They vary, however, in certain features, as in the specific gravity, the amount of glass, &c. The residual glass is, however, usually small; but in a stream-course east of Savulu I found in position at an elevation of 750 feet a semi-vitreous scoriaceous variety of these rocks, in which the steam-pores had been drawn out into long tubular cavities half an inch and more in length. The scoriaceous character is infrequent; but reference should here be made to another exposure of a slaggy semi-vitreous rock showing abundant steam-pores in the tuff-district of the river valley above Ravuka. It differs in some respects from the prevailing rock, since it displays prismatic augite as well as felspar microliths in its glassy groundmass, and is for this reason referred to genus 20 of the augite-andesites. In the elevated region east of Savulu the aphanitic augite-andesites are in places overlain by tuffs and agglomerates formed of the same materials. There is a very good exposure of the tuffs in the Nganga-turuturu cliffs about 2 miles west of Savulu.

(3) THE NGANGA-TURUTURU CLIFFS.—These picturesque cliffs, 50 to 70 feet in height, rise up at the head of the Liwa-liwa valley between Savulu and Ndrawa. They are elevated about 1,200 feet above the sea; and probably derive their name from a small waterfall which, after descending over their face, drops into the valley below. At its bottom is situated the hamlet of Liwa-liwa, which is about 600 feet above the sea. This is the Fijian word for “cold,” and doubtless it has allusion to the coolness of the valley. On account of the more rapid weathering of the tuffs in the lower part of the cliffs, there is a rude shelter afforded by the overhanging portion which is the main feature of interest that the cliffs present from a native’s point of view.

The tuffs composing the cliffs are horizontally bedded and overlie the prevailing aphanitic augite-andesite exposed on the valley-slopes below. Originally grey in colour, they have been largely affected by the hydration accompanying the weathering process. They are fine in texture and somewhat friable, but contain no lime, and are chiefly made up of the palagonitised fine detritus of vitreous varieties of the aphanitic augite-andesites of the district. No organic remains came under my notice. Some of the beds contain a number of lapilli of basic pumice, 1 to 3 centimetres in size, which are often in the last stage of the disintegration produced during palagonitisation. It would seem probable that these lapilli, after having been ejected from some supra-marine vent, were deposited with the tuffs in the sea around. It should, however, be not forgotten that vesicular and pumiceous materials may be discharged during a submarine eruption. When I visited the museum at Catania, Prof. Platania showed me portions of a bomb, highly vesicular, that had been thrown up in a submarine eruption off Vulcano in the Lipari Islands.

(4) THE UPPER VALLEYS OF THE NDRAWA RIVER.—The two valleys of Liwa-liwa and Ndrawa meet at Ravuka, where their two streams unite to form the main Ndrawa River. The former is the largest; and its large impetuous stream, during its descent of about two miles from Liwa-liwa past Lutu-kina to Ravuka, which is between 200 and 250 feet above the sea, has a drop of 300 or 350 feet. The main stream flows with a gentle gradient to the coast about ten miles away. I did not descend its course for more than two miles below Ravuka, where some hot springs well up through the gravel on the left bank (see page 31.)

This is a region of palagonite-tuffs which like those of the Nganga-turuturu cliffs are mainly derived from vitreous and semi-vitreous aphanitic augite-andesites. They do not effervesce with an acid, and neither foraminiferous tests nor other organic remains occur. The palagonitic material is usually vacuolar, the vacuoles being filled with palagonitic glass or with a zeolite as in the more altered rocks. Where bedding is shown, the beds are generally horizontal. These tuffs are extensively displayed in the sides and beds of the rivers from Liwa-liwa and Ndrawa to Ravuka and as far as I went down the main river, namely to the hot springs. They are associated with agglomerates, formed of the aphanitic augite-andesites, below Ravuka and in the Ndrawa valley.

(5) THE VICINITY OF NDRAWA.—The village of Ndrawa, which is not elevated more than 300 feet above the sea, is situated in the heart of the island in a deep valley more or less hemmed in by the mountains. This is one of the wettest localities in Vanua Levu, and probably, as in the case of that of Ndriti in the Seatura basin, the rainfall is not far under 300 inches in the year. In the river-gorge descending westward to Ravuka are displayed horizontally bedded palagonite-tuffs and agglomerates above referred to in the description of the Ravuka district, and the same rocks are exposed on the mountain-slopes to the south of the village.[56]

Immediately to the north lies a broken hilly country, about 800 feet above the sea, which has to be crossed on the way to Mbatiri and is much cut up by streams descending from the vicinity of Na Raro to join the Ndrawa River below Ravuka. The prevailing rocks are tuff-breccias and agglomerates. The first are made up chiefly of angular fragments, less than an inch in size, of aphanitic augite-andesites, some of them being more or less vitreous and in different stages of palagonitisation, whilst the finer material derived from the same rocks contains some carbonate of lime. The agglomerates are composed of the same type of these augite-andesites, with however but little interstitial glass. It should be added that pebbles of a kind of jasper or iron-flint occur in the stream-beds in this locality. (The microscopical characters are described on page 355.)

By following up the valley that extends to the east from Ndrawa, one enters after about a mile into the region of Na Raro, which is described on page 123.

THE TAVIA RANGES.

North of Vatu Kaisia the elevated Nandronadranu district divides into two ranges, one of which stretches eastward to the south of Na Raro as far as the gap of that name, whilst the other extends southward on the east side of the Yanawai valley. Near the angle of bifurcation is situated Mount Tavia, a remarkable pyramidal peak marked 2,210 feet in the Admiralty chart and lying 1½ miles north-east (N33°E) of Vatu Kaisia. It is shown in the view facing page 108. All this region is densely wooded, and I had chiefly to rely on “course-and-distance,” and on my aneroid, to determine the surface-configuration.

(1) RANGE ON THE EAST SIDE OF THE YANAWAI VALLEY.—No ascent of these hills was made. They vary from 1,500 to 1,800 feet in height, and judging from the loose blocks and gravel in the bed of the Yanawai River below Vatu Kaisia they would seem to be mainly formed of basaltic rocks, acid andesites being also represented. However, I crossed the southern end of the range, where it is 500 to 600 feet in height, to the north of Ndawara, and found basaltic andesites prevailing at the surface.

(2) RANGE EXTENDING EASTWARD FROM MOUNT TAVIA ON THE SOUTH SIDE OF NA RARO.—Mount Tavia, which has the appearance of a dacitic peak, was not ascended; but the range was crossed in two places in going from Ndrawa to Vatu-vono and from Valeni to Nareilangi, its usual height varying between 1,200 and 1,500 feet, the extreme height being about 1,700 feet.

In making the traverse from Ndrawa to Vatu-vono, one first passes through a part of the hornblende-andesite region of Na Raro, which is described in a later page. Afterwards while ascending the north slopes of the range, basaltic andesites, often doleritic in texture and referred to genus 1 of the augite-andesites, are usually found as far as the summit 1,200 to 1,300 feet above the sea. On descending the south slopes one finds coarse and fine palagonite-tuffs and clays at 900 to 1,100 feet up, similar to those prevailing near the sea-border. They are probably submarine, but my specimens are weathered and give no effervescence with an acid. In the bed of the river above Vatu-vono, about 400 feet above the sea, there occurs in position an aphanitic augite-andesite (spec. grav. 2·77), referred to genus 16, species A; whilst blocks of a coarser grained basaltic andesite lie loose in the stream.

In my traverse across the range from Valeni to Nareilangi I noticed about a mile from Valeni and not much over 100 feet above the sea an agglomerate formed of blocks of an altered acid andesite possessing a micro-felsitic groundmass and showing microporphyritic rhombic pyroxene with dark alteration borders (spec. grav. 2·5). It is distinct from the Na Raro rocks; and its presence in an agglomerate seems to indicate the vicinity of some old acid andesite peak buried beneath later basic eruptive products. Ascending the south slopes of the range, I found decomposing basaltic andesites and basic tuffs, the prevailing rocks up to an elevation of 1,300 feet; but in one locality (800 feet) occurred large masses of what seemed to be a disintegrating dacitic rock penetrated by quartz veins less than an inch thick. An aphanitic augite-andesite, of a somewhat exceptional character (spec. grav. 2·63), was displayed at the top of the ridge, 1,500 feet above the sea.[57] Basic rocks were exposed in the spur running northward on the east side of Na Raro.

THE SEA-BORDER EXTENDING EAST FROM THE YANAWAI RIVER TO THE LANGO-LANGO RIVER.—In this district is included the area between the foot of the slopes of the Tavia Ranges and the shores of Savu-savu Bay. This undulating country, two to three miles in breadth, does not attain a greater elevation inland than 300 or 400 feet. Fine and coarse palagonite-tuffs, some of them with the texture of sandstone, are the characteristic rocks. They at times contain a little lime and probably a few tests of foraminifera. The palagonitised glass is often vacuolar, the vacuoles being filled with the same material. In places where they are well displayed these tuffs generally show bedding, as in a hill-slope just east of Vuni-evu-evu, where there are fine and coarse tuffs interstratified and dipping gently W. by S. Basic agglomerates also occur in this district.

In the promontory named Yanutha Point in the map there is displayed an old flow of basaltic lava, showing a columnar structure at the end of the point. The columns are 20 inches in diameter, and are inclined about 20 degrees from the vertical in such a direction that it may be inferred that the original flow, doubtless submarine, descended at that angle from N.N.W. The dark grey rock of the columns (spec. grav. 2·76) has a fair amount of interstitial glass, whilst a blackish compact rock (spec. grav. 2·78) that represents apparently a more superficial part of the flow has an abundance of smoky glass in the groundmass. These rocks are basaltic andesites and are neither vesicular nor scoriaceous, and come near the basalts of the Kiombo flow which, however, contain some olivine (see page 92). They are semi-ophitic and are referred to genus 21 of the augite-andesites which is described on page 283.

NA RARO.

In Na Raro we have one of the most interesting of the isolated hornblende-andesite mountains of Vanua Levu. Unlike Vatu Kaisia, which often eludes the observation, Na Raro is visible from most points of view. It is double-peaked, the two peaks lying in a north and south line and rising precipitously. It is this feature that gives the mountain such a variety in its profile. From the north and south it appears as shown in the accompanying sketch as a sharp conical peak. From the north-east and south-east, as illustrated in the two other sketches, it has the form of a blunt or square-topped mountain; and its true shape is only shown when it is seen from the east or west. In the photograph here reproduced which was taken about 1½ miles to the south-west, the two peaks are with difficulty distinguished. (See frontispiece.)

Not many ascents have been made of these precipitous peaks. Mr. A. Barrack, who kindly supplied me with some information about it, made the ascent some years ago; and Mr. Blyth (?), a magistrate, also reached the top. There are stories of some big officials being hauled up in baskets; and the natives told me of a white man who was seized with a shivering-fit when he arrived at the summit. It is certainly a rather hazardous climb; but the safest plan is to resign oneself into the hands of the natives, who “bundle” one up in an expeditious, if not in a very ceremonious, fashion. Nareilangi, near the foot of the mountain on the north side, is a convenient starting-point, and half a dozen stout Fijians will not prove too many to assist the climber in the difficult parts of the ascent. Since the top usually becomes clouded as the day progresses, it is best to spend a night in a cave about 1,400 feet above the sea from which the ascent can be made in the early morning. The view from Na Raro is panoramic and extends over a large part of the island from Naivaka to Savu-savu.

Na Raro rises up to a height of 2,420 feet in the midst of a region of basic rocks. Agglomerates and coarse tuffs formed of aphanitic augite-andesites prevail in the broken country on the north and west sides towards Nareilangi and Ndrawa. Immediately south rises the Tavia Range with its basaltic andesites and overlying palagonite-tuffs; whilst on the east lies a spur of this range.

Nareilangi, the village from which the start is made, is about 2½ miles distant from Na Raro, and though situated in the heart of the island it is only about 100 feet above the sea. The track first passes through a district of foraminiferous tuffs and clays reaching up to 200 or 250 feet. Afterwards a broken country extending up to 800 feet is traversed. Here prevail agglomerates and tuff-agglomerates derived from aphanitic augite-andesites.[58] One then descends into a valley about 600 feet above the sea, and from this place the ascent of the mountain proper begins.

The ascent at first is fairly steep, dacitic tuffs prevailing up to 1,000 or 1,100 feet above the sea and forming in places precipitous cliff-faces. Large masses of hornblende-andesite lie on the slopes. The dacitic tuffs distinguish Na Raro from all the other peaks of hornblende-hypersthene-andesite rocks that I examined. They seem generally to have been stripped off by the denuding agencies; and only at times, as around the slopes of Ndrandramea and Thokasinga, are to be found the remains of agglomerates of the same formation. In the case of Na Raro, however, the tuffs differ somewhat in their components from the rocks forming the mountain mass. The tuffs are derived from a hornblende-andesite of dacitic type; whilst the massive rocks of the mountain are of hornblende-hypersthene-andesites, without porphyritic quartz, but approaching the dacitic habit.

The tuffs of Na Raro, which are sometimes compacted and at other times rather friable, do not display bedding. They contain a little lime; but I found no tests of foraminifera. They are composed of fragments, up to a centimetre in size, of a dacite displaying brown hornblende, plagioclase, and quartz in a microfelsitic groundmass, together with a few fragments of a semi-vitreous basic andesite.

Above 1,100 feet the tuffs give place to the massive hornblende-hypersthene-andesite. At an elevation of 1,450 feet, a shoulder of the mountain is reached, near the top of which is the cave above mentioned. Crossing the shoulder one descends for 100 or 150 feet into a gap, thus reaching the foot of the precipitous northern peak, which rises up like a wall for a height of from 900 to 1000 feet overhead. It is in mass of the andesite just mentioned, many of its faces presenting inaccessible cliffs displaying seemingly no structure. This peak is somewhat lower than the southern peak. I placed its height at 2,270 feet, which, taking the total elevation of the mountain at 2,420 feet, as given in the chart, makes the difference 150 feet. A deep and broad cleft, that goes half-way down the mountain, separates the two peaks. The southern one, which appears to be inaccessible, is evidently formed of the same acid andesite.

These hornblende-andesites, with or without porphyritic quartz, appear to be for the most part restricted to the immediate vicinity of Na Raro, except to the south-west, where at a distance of about a mile and a half from the mountain at an elevation of 500 feet occur a rubbly hornblende-andesite and agglomerates of the same materials. Though the rock is of the Na Raro type, its presence here is suggestive of a distinct vent of small size, of which most of the traces have been swept away during the emergence of the island. About half a mile south-east of this locality at an elevation of 450 feet occur some singular banded palagonite-tuffs which, although they do not show foraminifera in the section examined, contain a little calcite and are probably of submarine origin.... In this locality I found a large white mass, measuring 4 × 4 × 5 feet, formed of a siliceous rock appearing in thin sections as granular chalcedonic quartz (see page 355).

The hornblende-andesite of Na Raro, as in the case of the rocks of most of the other peaks of acid andesites, has its peculiar characters. It differs, for instance, from that of Vatu Kaisia in the larger grain of the felsitic groundmass (N. R. ·021 mm.; V. K. ·013 mm.), in the absence or rarity of phenocrysts of rhombic pyroxene, in its lower specific gravity (2·6 N. R.: 2·7 V. K.), in the presence of a little interstitial glass, and in other particulars. Both, however, belong to the sub-class of hornblende-hypersthene-andesites, and are described on page 301. In the Na Raro rock the rhombic pyroxene is represented in the groundmass.

With regard to the relative age of Na Raro I am inclined to think that it is the most recent of the acid andesite peaks of the island. Neither vitreous nor vesicular rocks came under my notice in its vicinity; whilst the tuffs that clothe its lower slopes are non-pumiceous, though of dacitic origin, but containing also a few fragments of a semivitreous basic andesite showing tiny felspar lathes and augite-granules. Since the everywhere prevailing submarine palagonite-tuffs and foraminiferous clays do not extend over its area, we may assign to it a later date. It is evidently also posterior in time to the basaltic andesites and aphanitic augite-andesites around, which are covered by these submarine deposits. Relatively recent as it apparently is, this mountain bears the impress of a high antiquity. There is nothing to indicate that this “core” of a volcanic mountain belonged to a subaerial vent. Na Raro has shared in all the later stages of the submergence and emergence of the island. Though it presents the final page in the history of the hornblende-andesite volcanoes, that chapter has been for unknown ages closed.

THE NA RARO GAP.—Between the Tavia and Va-lili Ranges there is a break in the mountainous backbone of the island, to which I have given this name. The greatest elevation is probably not over 800 feet. It is from the south side of this watershed that the Lango-lango river takes its rise.