CHAPTER V

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)

THE SEATURA MOUNTAIN.—In my description of the profile of this part of Vanua Levu, reference has already been made (p. 3) to the great mass of this mountain which occupies five-sixths of the breadth of the island. Viewed from seaward it looks like a huge table-topped mountain-ridge, and as such it is represented in the Admiralty charts; but when its true contours are distinguished it appears, when defined by the 300-feet level in the map, as a somewhat rounded mass, measuring 12 miles in length and 10 miles in breadth and attaining a maximum height of 2,812 feet. Seen from the deck of a passing ship it displays more or less regular volcanic slopes, especially on the east, where there is a gradual descent at an angle of 3 or 4 degrees for some 10 miles, and on the north towards the Lekutu lowlands. It also shows a fairly regular descent towards Mbua Bay on the west. (See profile, p. 62.)

On the west side, however, there is a great gap in the mountain-mass (the Ndriti Gap), marking, as I hold, an old crateral cavity of large dimensions, and now occupied by the Ndama River and its tributaries.

The adjacent Seatovo Range to the southward obscures the profile of the mountain on the south; and it is in fact not at all easy for this reason to get a view with all the slopes displayed. It is only at times, when viewed in its complete mass with uninterrupted outlines, as from off the mouth of the Ndreketi River to the north-east, or when the symmetry of its long eastern slope is observed from Wainunu Bay that Seatura displays itself as a gentle-sloped mountain-mass of the Mauna Loa type. Dense forest clothes the greater part of it, except on the north and north-west, where it lies within the limits of the scantily vegetated “talasinga” region.

The slopes of this mountain are deeply furrowed by river-valleys which radiate like the spokes of a wheel from its central elevated mass. Down its northern slopes flow the Lekutu River and its tributaries and the principal tributaries of the Sarawanga River. The large western affluents of the Wainunu River descend from its eastern side, whilst the Korolevu, Tongalevu, and other small rivers flow south into Wainunu Bay, and the Ndama River drains its western slopes. In all these cases, excepting that of the Ndama River, the rivers have worn deep valleys into the mountain-mass, valleys of denudation that represent the work of ages. That of the Lekutu is a deep cut almost into the heart of the mountain; at Nandroro in this valley, which lies 6 to 7 miles inland and 800 feet above the sea, the hills rise steeply on either side of the river to an elevation of 1,100 and 1,200 feet and more. Some of the large tributaries of the Sarawanga and the Wainunu flow through gorge-like valleys 200 to 300 feet in depth. On the western slopes north of the Ndama river, the mountain-side presents an alternating series of lofty spurs and deep broad valleys. In fact, all around Seatura its slopes are deeply furrowed through the denudation and erosion of ages.

The rocks of this ancient volcanic mountain are almost all of the massive basic type, and except at the mouth of the Ndriti Gap hardly ever display a scoriaceous character. It is also noteworthy that no detrital rock, whether agglomerate, tuff, or tuff-clay came under my observation. The rocks exposed on the surface are mostly blackish brown olivine-basalts and porphyritic basaltic andesites, the former much prevailing. In the northern portion, however, grey olivine basalts of a different type occur. In the great crateral hollow, which I have named the Ndriti Gap, are displayed numerous dykes formed of highly altered basaltic rocks that may be classed among the propylites.

The dense forest that clothes the greater part of this mountain offers many serious hindrances to geological exploration. Except in the northern portion, views of the surroundings are very limited, and one has often to rely mainly on the aneroid and the compass to obtain correct ideas of the contours and general configuration. During most of the time spent in the southern part of the mountain, my work was greatly impeded by heavy rains, and from this cause and from the frequent necessity of following up the stream-courses and of crossing rivers in flood, I was usually wet through all the day.

_(a) The Eastern Slopes of Seatura._—The basaltic flows, of which this mountain is principally composed, are best observed on the eastern side where the original volcanic slopes are preserved. Although the rivers have worn such deep valleys into the mountain sides, it is however not often that any great exposure of rock occurs, on account of the dense forest-growth over much of this region. It is only occasionally that the columnar structure of these old basaltic flows is displayed. It is especially well exhibited in the face of a waterfall, distant about two miles in a straight line from Tembenindio and elevated about 700 feet above the sea. Here there is an exposure to the extent of 25 feet of huge basaltic vertical columns, four to five feet across, and pentagonal in form. The rock is a blackish basalt with scanty olivine and a specific gravity of 2·87. It is referred to genus 25 of the olivine-basalts which is described on page 259. Micro-phenocrysts of plagioclase and a few of augite occur, the olivine being mostly replaced by pseudomorphs. The felspar-lathes of the groundmass average ·18 mm. in length, and there is a little brown opaque interstitial glass. Boulders and fragments of a closely similar basalt, with a specific gravity of 2·9, lie about on the surface in this region. The Seatura slopes here abut on the plateau of Na Savu, formed largely of volcanic agglomerates, to be subsequently described.

On the south-eastern slopes of the mountain between Ndawathumi (inland) and Korolevu (at the coast), somewhat similar basalts with scanty olivine are exposed (sp. gr. 2·86-2·91). Some of them show the felspar-lathes of the groundmass arranged in a plexus (genus 25), whilst others exhibit flow-structure (genus 37), the average length of the lathes varying in different localities between ·15 and ·21 mm. All display scanty residual glass. On the shores of Wainunu Bay between the Wainunu and Korolevu rivers occur porphyritic basaltic andesites with a considerable amount of glass in the groundmass. There is exposed on the right side of the mouth of the last-named river a highly basic variety of olivine-basalt with a specific gravity of 3·07. It is referred to genus 15 (described on page 258), which includes the most basic rocks in my collection. There are in this rock no plagioclase phenocrysts and the felspar-lathes of the groundmass are relatively infrequent, whilst olivine and augite occur in abundance. There is little or no residual glass. In the district of Tongalevu blackish olivine-basalts and basaltic andesites of the usual character are found. In the Na Suva range, which lies two miles inland from the shores of Nasawana Bay and forms the southerly extension of the mountain, a somewhat compact variety of olivine-basalt (sp. gr. 2·92) prevails up to the summit, 1,550 feet above the sea. It is included in genus 37 of the olivine-basalts. In the length of the felspar-lathes (·15 mm.) it belongs to the Seatura type of these dark basalts.

_(b) The Western Slopes of Seatura._—Here overlooking the plains north of the Ndama River the same olivine-basalts and porphyritic basaltic andesites occur. The vegetation is of the scanty “talasinga” character, and since there is little or no soil-cap the disintegration of the rocks has been very great, often extending to a depth of 10 or 12 feet. It is remarkable that this disintegration is most marked in the “talasinga” and similar scantily wooded districts of the mountain. On the densely wooded eastern and southern sides where there is a thick soil-cap, it is by no means so evident. Here on the western slopes have been carved out deep broad valleys and lofty spurs, the last in their turn furrowed on their flanks, without any apparent sufficient cause. The shallow streams at the bottom of the valleys appear quite incompetent to produce such great erosion; and doubtless these results are partly due to the action on the crumbling rock-surface of temporary torrents formed during the rains.

_(c) The Northern Slopes of Seatura._—Here within the scantily vegetated “talasinga” region the conformation of the land is well displayed. Broad, deep and nearly parallel valleys, separated by level-topped spurs and occupied by the Lekutu and its tributaries, score the mountain’s slopes. The prevailing rocks are blackish-brown olivine-basalts and porphyritic basaltic andesites, such as occur around the other parts of Seatura; but grey olivine-basalts also occur, possessing opaque plagioclase-phenocrysts and looking like porphyrites. They are essentially holocrystalline and are probably more deeply situated than the other basaltic rocks. They are referred to genera 26 and 38 described on pages 261, 263, and have a specific gravity of 2·75-2·83. Dark doleritic basalts distinct from all the others are exposed in places.

A good idea of this region may be obtained by following the road westward from Tavua on the head-waters of the Sarawanga River to Wailevu on the westernmost tributary of the Lekutu River, a distance of about 6 miles. Leaving Tavua one at once begins to ascend and cross the long spur that descends from Seatura and divides the valleys of these two river-systems. On its slopes are exposed much decomposed blackish basalts possessing scanty olivine and showing large porphyritic crystals of plagioclase. They have a specific gravity of 2·84 and are assigned to the porphyritic sub-genus of genus 25 (page 259). At the summit, 800 feet above the sea, occur blocks of a grey holocrystalline basalt with scanty olivine and semi-opaque plagioclase-phenocrysts referred to genus 26 and having a specific gravity of 2·76. It appears to form the axis of the spur. Descending to the main Lekutu River, just below Kavula, where the elevation is about 300 feet above the sea, one observes exposed in mass in the river-bed a dark semi-ophitic doleritic basalt similar to the doleritic rocks without olivine prevailing on the coast between Wailea Bay and Lekutu (see page 50), but differing in the absence of felspar-phenocrysts. It displays a considerable amount of opaque interstitial glass and is assigned to genus 12 of the augite-andesites (page 275). The specific gravity is 2·78, but there are a few minute irregular cavities in its substance.

On leaving Kavula one crosses another of the Seatura spurs at a level of 650 feet, descending then into a smaller river-valley occupied by a tributary of the Lekutu, on the banks of which lies the village of Nawai, 350 feet above the sea. Then another spur is crossed at an elevation of 450 feet and the descent is made into the valley of the Wailevu tributary of the Lekutu. Crossing the valley, which at the town of Wailevu is elevated 300 feet, one rises to a height of 700 feet and then descends into the Mbua plains. These three almost parallel valleys of the Lekutu and its two tributaries are worthy of a detailed examination.

The rocks on the surface between Kavula and Wailevu vary in character. Nearer Kavula there appears a blackish compact olivine-basalt (spec. grav. 2·88), showing a little microporphyritic plagioclase and belonging to genus 37 of the olivine rocks. Further on is exposed one of the holocrystalline grey olivine-basalts with porphyritic plagioclase-phenocrysts and specific gravity 2·83. It belongs to the type described in genus 38 of the rocks on page 263. Nearer Wailevu there occurs a blackish porphyritic basalt with scanty olivine and specific gravity 2·81. It contains but little residual glass and is referred to the porphyritic sub-genus of genus 25. In some cliffs at the river-side close to Wailevu, there is displayed a semi-vitreous basaltic andesite, showing large porphyritic plagioclase crystals, 3 to 8 mm. Its low specific gravity (2·68) is to be attributed to the large amount of glass in the groundmass. There is a loose mesh-work of felspar-lathes, but the augite is not differentiated. Westward of Wailevu commence the decomposing basaltic rocks of the Mbua plains.

_(d) Traverse of the Northern Part of the Summit of Seatura from Kavula South-West to Narawai._—The track first lay up the picturesque valley of the Lekutu River to Nandroro, 2½ miles distant and 800 feet above the sea. On the way blackish basaltic rocks of the prevailing Seatura type, with or without scanty olivine, were displayed often in a decomposing condition. At one place a characteristic grey olivine-basalt, showing opaque porphyritic plagioclase (sp. gr. 2·87), and looking like a porphyrite, was exposed. On account of the abundance of the olivine, it is placed in genus 2 of the olivine-rocks. After Nandroro the path lay up the steep mountain-side to a height of 1,500 feet: and afterwards across the summit of the northern part of Seatura, which is here about two miles in breadth. This elevated region is well wooded with here and there a patch of “talasinga” land; but it is by no means level, its elevation varying between 1,400 and 1,800 feet, and it soon became evident that we were crossing the heads of valleys, sometimes 200 or 300 feet in depth, that could only have been excavated by the torrential rains. These streamless valleys afford another indication of the denudation to which this ancient mountain has been subjected.

The rocks prevailing in this elevated northern portion of Seatura, at heights of 1,500 to 1,800 feet above the sea, are: (_a_) blackish basalts with scanty olivine, a little interstitial glass, and belonging to the porphyritic and non-porphyritic sub-genera of genus 25 of the olivine-rocks: (_b_) grey olivine-basalts with porphyritic opaque plagioclase, containing but little residual glass, but varying greatly in the amount of olivine and belonging to the genera 2 and 26 of the olivine-basalts; they would be classed, as far as appearance goes, as porphyrites; their specific gravity ranges 2·85 to 2·90. The rock exposures were, however, scanty; and but little information could be obtained of the mode of occurrence. No scoriaceous rocks were found except in the instance of a compact dark basalt without plagioclase phenocrysts, apparently a dyke rock, and belonging to genus 40 of the olivine-basalts.

_(e) Ascent to the Summit of Seatura from Ndriti._—The town of Ndriti lies in the great gap in the south-west side of the mountain which has been previously mentioned as probably an old crateral cavity. After traversing a district of highly altered basic rocks or propylites, to be subsequently described, and reaching an elevation of about 400 feet above the sea, I came to the long slope that leads up to the summit. A dense forest hid everything from view, so that the compass and aneroid had alone to be relied on.

At first one traversed a series of step-like alternations of level ground and steep “rises,” until the old site of the village of Seatura, about 1,200 feet above the sea, was reached. There are some strange legends connected with this old mountain-village, which is now only indicated by little piles of stones and the debris of a wall, and was evidently abandoned long ago. We finally reached the summit by following up a spur or ridge in a northerly direction from Seatura. There was a precipitous descent on either side of the ridge with evidently a broad, deep valley to the eastward. The summit was rounded; but on account of the forest no view could be obtained. There was never any extensive exposure of rock noticed during the ascent; but all the way up occasional small blocks of a blackish olivine-basalt were observed on the surface, of the same general type as that found all around the mountain and referred to genus 37 in the synopsis.

_(f) The Ndriti Basin or Gap._—This great hollow in the side of Seatura, which I have named after the town in its midst, is apparently a crateral cavity now drained by the Ndama river, and its tributaries, and covered with dense forest to such a degree that a general view of the whole is impracticable. The glimpses, however, that one obtains of the mountain scenery are very grand, the town of Ndriti lying in the midst of mountains that rise almost on all sides of it except on the west. This great cavity is contracted at its mouth a little below the town and expands in its interior, where it must be two or three miles in width. Its floor is fairly level and is elevated only about 200 feet above the sea;[36] whilst its mountainous sides rise to 2,000 feet and over.

As shown in the map there are two breaks in the outline of this ancient crater, the one on the west through which the Ndama river flows, the other on the south where the dividing ridge, separating it from the Nandi Valley is under 700 feet in elevation. The Nandi Gorge, as I will term the last-named, is a narrow picturesque ravine leading through the mountains from Nandi to Ndriti. One follows up a rocky stream-course hemmed in by precipitous sides until the top of the gorge is reached, when the watershed is crossed, and the descent is then made to Ndriti by one of the tributary stream-courses of the Ndama river.

Two or three large rapid streams, after draining its mountainous slopes, unite within the basin to form the Ndama river, which, as it issues from its mouth, becomes a comparatively placid stream rolling sluggishly along to the sea, some five or six miles away, with an average drop of about thirty feet in a mile. In the course of ages the original configuration of this great hollow has doubtless been extensively modified by the denuding agencies. The rainfall on the mountain-slopes must be very great, probably not under 250 inches in the year[37]; and Ndriti, though only 200 feet above the sea, is in all probability on account of its situation one of the wettest places in the island. The rivers have evidently been important factors in reshaping the original cavity.

Nearly all the rocks exposed _in situ_ in the beds of the rivers and streams in the floor of the great Ndriti basin, and for 300 or 400 feet up its sides are more or less highly altered basic rocks, to which the old and the new names of greenstone and propylite may be fitly applied. They often sparkle with pyrites, and not uncommonly effervesce with an acid, so that one is apt to imagine one’s self in a region of limestone. The degree of alteration varies considerably, those most altered being light-coloured and greenish, whilst the others are darker, the specific gravity ranging from 2·69 to 2·79. In spite of these differences almost all of them appear to belong to the same eruptive series, being as a rule sharply distinguished from the prevailing unaltered surface basaltic rocks of the slopes of Seatura by the size of the felspars of the groundmass, which average about ·3 mm. in length, whilst those of the basaltic rocks just alluded to average only ·17 or ·18 mm. long. These rocks are also well displayed in the sides of the Nandi Gorge; and from their mode of exposure by river-erosion, as well as from their relatively coarse crystalline texture, and from their alteration, it may be inferred that they are older and more deeply situated than any of the Seatura rocks before referred to. Whether these rocks, which extend over an area of some square miles, have been altered by solfataric action or contact-metamorphism,[38] I will not now say. The fact remains, however, that they are best exposed wherever the streams have worn deeply into the floor, and lower slopes of the great basin, or have cut down into the mountain-mass as in the case of the Nandi Gorge. The rocks that lie in loose blocks on the surface either at the bottom of the basin or on its slopes extending even to the very summit of the mountain (see page 67), are characteristic blackish olivine-basalts of the type prevailing around the mountain’s slopes. These propylites are most frequently exposed as dykes in the beds of the rivers at the bottom of the basin. Such dykes vary from 4 to 6 feet in thickness, and they are very conspicuous when they stretch across the river’s breadth projecting more or less above the water. From their frequency it may be inferred that in many other small exposures, ill suited for displaying the mode of occurrence of the rock, we have also to deal with dykes. Judging from four dykes that were particularly examined, they are all vertical or nearly so, and all run in much the same direction, namely, N.N.W.—S.S.E. or N.W.—S.E., whether on the north or south side of the great basin. In one instance, a rudely columnar structure across the thickness of the dyke was observed. From their exposure in river-beds it was rarely possible to ascertain much more than is given above. However, in the bed of a river, a mile above Ndriti, there was an extensive exposure of a highly altered greenish rock which was crossed by a vertical dyke, 4 feet thick, formed of a dark grey less altered rock. I have referred these two propylites to two different genera of the augite-andesites, the dyke-rock to genus 2, and the other to genus 4. In the case of the dyke the rock is a little vesicular; whilst in the other it is densely charged with pyrites. Both have been subjected to the same alteration; but in a different degree; and it would thus seem that solfataric influences were here in operation before and after the intrusion of the dyke.

With reference to the characters of the alteration of these rocks of the Ndriti basin, it may be remarked that where the change is greatest the felspars of the groundmass are alone recognisable. The plagioclase phenocrysts are quite disguised by alteration products, and chlorite, viridite, epidote, calcite, pyrites, &c., occupy much of the groundmass. Other rocks are less affected and in a few the change is only slight.

With regard to the prevailing types of the propylites of the Ndriti Basin, it has already been observed that in most of them the felspar-lathes of the groundmass are unusually large, the average length being ·3 mm. From the rare occurrence of olivine in some of the rocks that are but slightly changed, it is to be inferred that most of them belong to the augite-andesites, and might be termed doleritic basaltic andesites. But in other respects they differ considerably, both as regards the presence or absence of flow-arrangement of the felspar-lathes, and in the occurrence and size of the plagioclase-phenocrysts, some having large porphyritic crystals, others small phenocrysts, and others none at all. Many of them contained a little interstitial glass. In my classification of the augite-andesites they are assigned to genera 2, 4, 16, &c., and additional particulars concerning their characters are given in the description of those genera. Judging from the average large size of the felspar-lathes it may be held that, although in other features they often differ, some of the general conditions under which they were produced were the same.

On the right bank of the Ndama river, opposite Ndriti, there is a singular association of a vertical dyke of a bluish-grey basic andesite with a reddish scoriaceous lava, apparently a flow. The dyke is about 4 feet thick and runs N.W. and S.E., like the other dykes of the basin, exhibiting also a rudely columnar structure across its breadth. Where the two rocks are in contact, the dyke has a vitreous border half an inch thick, and an offshoot of the dyke, four inches wide, has penetrated the lava, acquiring at the same time a more glassy texture. The small size of the felspar-lathes of both rocks distinguishes them from the dyke rocks of the basin, where the felspars are twice as long. Both rocks show some degree of alteration.[39]

In following the valley of the Ndama River from Ndriti to Telana, about three miles farther down, one traverses a picturesque region. Emerging from the great basin the river flows through the rolling plains of the “talasinga” district. Near Ndriti, and occasionally on the way to Telana, is exposed a scoriaceous grey basaltic rock; and between two and three miles below Ndriti there is to be observed in the river-bed evidence of a comparatively recent flow of a highly basic scoriaceous lava from the ancient crater of the Ndriti basin. The rock, which is dark and fresh-looking, shows large porphyritic crystals of augite and olivine but no plagioclase, whilst the groundmass contains a little brown interstitial glass. Its characters will be found described under genus 3 of the olivine-basalts (p. 255). Its specific gravity, notwithstanding its large empty steam-pores, is 2·91. It differs markedly from the basaltic rocks of the Seatura slopes and the Mbua and Ndama plains, in the great porphyritic development of augite and olivine, in the large size of the felspars and augite of the groundmass, and in its numerous steam-holes. But in the coarseness of its small felspars it belongs to the same type as the altered or propylitic basic rocks of the Ndriti basin. It is probably by some such lava flow from the old Ndriti crater that the submarine bank was formed off the adjacent coast on which the low Lekumbi promontory has been built up.

In the numerous dykes of the Ndriti basin and in the great alteration which their rocks have frequently undergone, we have evidence in support of the view that this is an old crateral cavity, an opinion that is supported by the indications of lava-flows that have issued, apparently in later times, from the mouth of the basin. Reference has already been made to the locality where a dyke-rock and the rock-mass, into which it has been intruded, are both propylitic; and from this and other facts, such as the varying degrees of alteration in different parts of the basin, it is to be inferred that in the last stage of the activity of this vent its bottom and sides were extensively affected by solfataric influences. Since that period, the configuration of the crater-basin has been greatly modified through the denuding agencies.

The absence, or at least the great rarity, of tuffs and agglomerates in the case of Seatura is remarkable. The mountain has evidently been built up in the mass by flows of basic lava; and from this source have no doubt in an important degree been derived the basaltic flows of the Ndama, Mbua, and Sarawanga plains, great streams of basalt that further seaward have helped to form the submarine platform extending several miles from the coast. The submarine tuffs and agglomerates that occur at various elevations, reaching as high as 1,200 feet above the sea, in the Sesaleka, Lekutu, Sarawanga, and Ndrandramea districts lying to the north-west, north, and east, did not come under my notice on the Seatura slopes. On the other hand, except in the few localities, where scoriaceous rocks occur, the general type of the basalts is such as we would expect to find in submarine flows. In no part of the island, however, is the antiquity of the land-surface so well attested by the disintegration of the basaltic flows, which extends here to depths of ten and even twenty feet. This is in favour not only of the sufficiency of time, but also of the ability of the denuding agencies to strip off the surface-deposits.

However this may be, it is evident that the mountain of Seatura possesses a history quite independent of that of the rest of the island. I have pointed out in Chapter I. that it represents a mountain of the Tahitian type. In its radiating valleys and in its basaltic character it much resembles the mountainous island of Tahiti, which Dana describes as a gently sloping cone of the Hawaiian order that through the erosion of ages has become a dissected mountain.[40]

THE SEATOVO RANGE.—This remarkably situated mountain-range, which I have named after a town at the foot of its western slope, extends from the valley of the Ndama River to Solevu Bay. It attains a maximum height of about 1,800 feet, and varies between this elevation and 1,500 feet until in the vicinity of Solevu, where it descends as a mountainous headland to the coast. Its summit is narrow and ridge-shaped, and although the whole range is not interrupted by gaps it has a composite origin. At its north end, where it is cut off from the Seatura Range by the Nandi Gorge it helps to close in the large Ndriti basin. Towards the south an offshoot proceeds eastward and shuts in Solevu Bay. But, although apparently all the rocks are basic, considerable variety prevails, and there are many puzzling points in the geological structure of this region.

At the place where this range abuts on the Ndama valley, below Ndriti, the grey scoriaceous basalt, before referred to, is exposed at its foot. However, the usual blackish basaltic rocks, often carrying a little olivine, form in mass the mountainous southern headland that culminates in Solevu Peak (Ulu-i-matua); and the same rocks prevail in the lower regions on the west side of the range from Vuia Point to the valley of the Ndama River. The southern portion will be described in the account of Solevu Bay; and I will now give the results of my journey across the summit of the range about half a mile south of the Leading Peak of the chart.

The eastern slopes are steep and often precipitous, whilst on the western side there is a more or less gentle descent to the lower levels, suggestive of a volcanic slope; and it is remarkable that whilst the rocks exposed on the precipitous eastern side for the lower two-thirds are sometimes markedly altered, on the western side they are comparatively unchanged. These facts at once suggest that we have here the western rim of a large crateral cavity, though the topography of this district is not sufficiently well shown in the chart to enable one to define its original limits. This inference is also supported by the occasional scoriaceous character of the rocks below referred to.

The most frequent rocks in the upper two-thirds of the range are grey porphyritic olivine-basalts, displaying opaque plagioclase phenocrysts and more or less hematised olivine, the specific gravity being about 2·9. They approach in characters the grey porphyritic olivine-basalts of the northern part of Seatura (pages 65, 66); but differ amongst other features in the greater abundance of the olivine and in exhibiting flow-structure. They are usually almost holocrystalline, and are assigned for the most part to genus 14 of the olivine-basalts. They are extensively exposed in the stream-courses on the west side between 500 and 900 feet; and huge masses of the same rocks, but containing less olivine and more glass, and displaying much calcite, viridite, and other alteration products, are found near the base of the eastern slopes. The semi-vitreous condition of these rocks is represented in the large masses of a dark very scoriaceous porphyritic lava, possessing quite a cindery appearance, that occur on the narrow ridge-shaped summit. The groundmass shows a few scattered felspar microliths; but it is in the main composed of a dark opaque glass. Small cube-like crystals of chabazite line some of the cavities.

Other basic rocks are not infrequent and apparently represent dykes. Thus on the eastern side at 800 feet is exposed a dark-grey semi-ophitic doleritic rock (sp. gr. 2·77) assigned to genus 12 of the augite-andesites (page 275). The felspar-lathes average ·3 mm. in length, and there is a little interstitial glass containing viriditic and calcitic alteration products, the same materials filling small rounded vesicular cavities. On the same slope between 1,000 and 1,200 feet, there are displayed fresh-looking compact non-porphyritic basaltic andesites (sp. gr. 2·84), where the felspar-lathes average ·2 mm. and the interstitial glass is scanty. They are referred to genus 16, species C, of the augite-andesites. On this side also between 600 and 800 feet occur blocks of a highly altered slightly vesicular augite-andesite showing a little microporphyritic plagioclase. It is assigned to genus 13, species B, of the augite-andesites. In one place where it is in position it is scoriaceous, the steam-holes being round, empty and one to five mm. in size. In the less glassy rock it displays numerous small irregular cavities either filled with fibrous viridite or calcite or showing concentric zones of the two minerals. The felspar-lathes are ·15-·2 mm. in length. In blocks near the foot of the eastern slope occur a blackish olivine-basalt (sp. gr. 2·88) of the prevailing Seatura type, possessing a little interstitial glass and felspar-lathes with an average length of ·2 mm. It belongs to genus 25 of the olivine-rocks.... On the western slopes at a height of 500 feet occurs a dark compact rock (sp. gr. 2·89) with abundant olivine which is referred to genus 1 of the olivine basalts. There is a little residual glass, the felspar-lathes averaging only ·08 mm. in length. A similar-looking rock is exposed at 1,400 feet, which displays felspar-lathes averaging ·2 mm. long (sp. gr. 2·9). It belongs to genus 37 of the same olivine class. Here also is assigned an aphanitic basalt, with a few scattered large plagioclase phenocrysts and felspar-lathes averaging ·15 mm. long, which is displayed near the base of the slope.

I could not satisfy myself as to the presence of tuffs on the slopes of this range. Some fine argillaceous rocks exposed half-way up on either side show no lime and contain no organic remains. One specimen beside me is certainly a disintegrated basic rock. No agglomerates came under my notice. In the absence or rarity of detrital rocks this part of the range resembles the adjacent mountain of Seatura.

Although olivine-basalts prevail in this part of the Seatovo Range there is great variety in their characters; and it does not appear possible to explain such a diversity except to assume that we have here an old crateral ridge which has again and again been penetrated by dykes and has since been greatly denuded. We have here one of those singular mountain-ridges that characterise the central portion of the island, but differing in this respect that the submarine tuffs and agglomerates, which there occur on the surface, even in the higher levels, are here absent.

SOLEVU BAY.—There are few localities in the island where so many kinds of basic rocks are displayed as around Solevu Bay. In addition to the prevailing blackish porphyritic basalts and basaltic andesites, there are grey porphyritic basalts, grey non-porphyritic basalts, black basalts with abundant large crystals of olivine, &c., all of which have their distinctive characters.

This picturesque bay is surrounded by hills. On the west side it is inclosed by the promontory forming the southern extension of the Seatovo range which, culminating in Ulu-i-matua, or the “Head-of-the-Strong” peak, descends at first steeply and then gradually to the coast, where it projects as Vulavulandre Point. On the east side is a broken line of hills, of which Koro-i-rea, the hill known to the natives as the “Town of the Albinos,” is the most conspicuous. Beyond it stretches the eastern point of the bay, which the Fijians call “Ua-nguru,” that is, “the noise of the waves.” On the shores lie the village of Nawaindo, “the running-stream,” and the once populous town of Solevu, which has given its name to the bay. Solevu, as its name indicates, is the place of the “great assembly.” In the background rises the three-peaked mountain of Koro-tolutolu, “the three towns,” which forms a continuation inland of the eastern arm of the bay, and joins the Seatovo Range at the head of it. Between these two ranges inclosing the bay lies the valley of Solevu, down which descends the Solevu River to the sea. In ascending this valley from the shore, one rises only about 100 feet above the sea for the first mile or two.

The promontory, which in the even-topped Ulu-i-matua or Solevu Peak, attains a height of 1,100 feet above the sea, displays on its summit and on its eastern slopes descending to the Solevu river, and on its western slopes reaching down to the coast at Vuia, more or less porphyritic blackish olivine-basalts of the usual type with specific gravity 2·88-2·90. These basaltic rocks contain scanty olivine and only a little interstitial glass. The felspars of the groundmass vary in different localities from ·11 to ·15 mm. in average length. The rocks belong to genus 37 of the olivine class which is described on page 262.

They are in the lower regions often decomposed to a considerable depth, the spheroidal structure being well displayed during the weathering process. Where this promontory terminates in the low Vulavulandre point, these rocks give place in part to grey porphyritic olivine-basalts, with specific gravity 2·79-2·83, which from the abundance of the macroscopic opaque felspar look like porphyrites. They come near to the rocks exposed on the north slopes of Seatura and in the Seatovo Range. At the end of the point they become scoriaceous and more vitreous; but with this exception they contain but little glass. They vary somewhat in character and are referred to genera 2 and 38 of the olivine-class.

The prevailing rock in the interior of the Ua-nguru promontory to the south of Koro-i-rea is the blackish porphyritic basalt, containing a little olivine, and often much decomposed; but at the point and on the east shores of Solevu Bay, there is a considerable variation in the character of the basic rocks, of which the two following are the most conspicuous. Near the village of Nawaindo, there is an apparent intrusion of a black lava-like basalt of high basicity (specific gravity 3·01) showing abundant large olivine crystals, five or six mm. across, with some porphyritic augite, but no macroscopic felspar. At the point the rock is somewhat scoriaceous, with calcite occasionally filling the cavities, whilst the olivine is so thoroughly hæmatised that it glistens like brown mica. The compact rock contains a little devitrified interstitial glass, the felspar-lathes being unusually small, their average length being only ·07 mm. It belongs to genus 15, the most basic of the genera of the olivine class represented in the island. The second rock to be noticed is a slightly altered compact basalt without olivine forming apparently a dyke near the coast about half way between the village of Solevu and Ua-nguru Point. It has a specific gravity of 2·84, the felspar lathes (·15 mm.) presenting a marked flow-arrangement, whilst there is a fair amount of altered residual glass in irregular spaces, a millimetre in size. The rock, on account of its joint-structure, could be easily worked as a building-stone. It is referred to genus 16, species B, of the augite andesites.

The hill of Koro-i-rea, which rises on the east side of the bay to a height of 850 feet, has a ridge-shaped summit. Its upper half is composed of a bluish-grey rock looking like a phonolite and usually compact, except at the top of the hill, where it is a little scoriaceous. It has, however, a specific gravity of 2·91 or 2·92, and is in fact a pretty grey olivine-basalt studded with small olivine crystals about a millimetre in size and showing no other phenocrysts. This type of olivine-basalt occurs also at Ulu-i-ndali on the east side of Wainunu Bay, but is rare in the island. It differs amongst other features from the porphyritic olivine-basalts of the northern part of Seatura and of the Seatovo range in the absence of plagioclase phenocrysts. There is apparently no interstitial glass, whilst the average length of the more or less parallel felspar-lathes is ·13 mm.[41] On the lower slopes of the hill the common blackish porphyritic basalt or basaltic andesite is exposed. In the grey-basaltic upper portion of this hill we have probably an old volcanic “neck.”

Following the line of hills inland from Koro-i-rea, we cross the intervening saddle 450 feet above the sea, and ascend the slopes of Koro-tolutolu, a ridge-shaped mountain backing Solevu Bay, and having, as its name indicates, three peaks, of which the highest is 1,280 feet above the sea. My observations indicate that this mountain is formed in mass of the common blackish-basalts described under genus 37, their specific gravity being 2·88 to 2·94. But Koro-tolutolu has also the peculiarity that it appears to be in mass magnetic. The rocks obtained from its summit, half-way up its western slopes, and near its foot on the same side, all display polarity, a character also of the rocks of the neighbouring hills of Ulu-i-matua and Koro-i-rea, but in their cases seemingly confined to the higher levels.[42]

Neither tuffs nor agglomerates came under my notice at Solevu Bay. This appears to be an ancient corner of the island, from which denudation has stripped off nearly everything that could guide us in speculating as to its past. Although the hills of Koro-i-rea and Koro-tolutolu doubtless represent old volcanic necks, the relation of Ulu-i-matua to the very differently composed northern part of the same range, as described on page 73, is extremely puzzling. Then again in the opposite sides of Solevu Bay we see exposed the remains of lava-flows that bear no relation to the present configuration of the surface. We may suspect, however, that most of the volcanic energy was displayed under the sea.

NANDI BAY.—Lying north of Solevu Bay, this bay is situated between spurs, descending to the coast from the mountainous interior. The valley extends a long distance inland without much change of level, the elevation 1½ miles from the coast being not over 100 feet above the sea. At its head is the Nandi Gorge, which leads into the Ndriti Basin, the great crateral cavity of Seatura. There are some remarkable lofty, isolated hills in this valley that would be well worth examining.

That the bay represents the site of an old volcanic centre is indicated by the occurrence on the shore of two basaltic dykes, one on either side of the village of Na Savu and 300 to 400 yards apart. The eastern dyke is perhaps 30 feet thick, whilst that to the west is scarcely half this thickness. They exhibit an imperfect columnar structure, the columns, which are 6 to 12 inches across, being inclined at an angle of 15° or 20° from the vertical in such a way that it may be inferred that the molten material was ejected from some subterranean focus lying to the northward (or inland) at an angle of 15° or 20° above the horizon. The basalt is a compact bluish-black rock with specific gravity 2·95-2·99. It contains abundant olivine but no other phenocrysts and very scanty interstitial glass, whilst the felspar-lathes average ·1 mm. in length. It is referred to genus 16 of the olivine basalts, and is remarkable for the flow arrangement not only of the felspar-lathes but also of the smaller olivine crystals.

Blackish basaltic rocks of the prevailing type are exposed on the surface of the broad spur, not over 500 feet in height, that divides the Nandi and Nasawana valleys and descends to the coast between the two bays thus named. They belong to genus 37 of the olivine-basalts and display a few small plagioclase phenocrysts. The felspar-lathes average ·2 mm. in length, and there is a little interstitial glass. Entering Nasawana Bay we find ourselves on the southern slopes of Seatura, of which the high Na Suva range that backs the bay is the southern extension.

THE TABLE-LAND OF NA SAVU.—This remarkable plateau has an elevation varying usually between 700 and 800 feet above the sea and a maximum breadth of four or five miles. It is an area of basic agglomerates and basic tuffs and lies in the hollow between the basaltic mountain of Seatura and the acid andesitic hilly region of Ndrandramea. For the convenience of description I have named it after the picturesque falls of Na Savu[43] at its southern edge. These falls are celebrated in Fijian tradition; and from the brink in old time the native desirous of ending his life leapt into the gorge below.

After flowing sluggishly along on the surface of the table-land, the Mbutu-mbutu River arrives suddenly at the edge of a line of cliffs of volcanic agglomerate, that here form the southern border of the plateau, and with a volume 30 to 40 feet across, it plunges down into the ravine 150 feet below. As shown in the view from the gorge below, there is a break in the middle of the descent. These falls, however, are not easily accessible. They are best approached by proceeding from Wainunu to Ndawathumi and thence up the gorge of the Mbutu-mbutu River.

The surface of the plateau of Na Savu is densely wooded. In places it is marshy, and here thrives the Giant Sedge (Scirpodendron costatum). The Makita tree (Parinarium laurinum) also flourishes in the wet districts; and in the drier localities occur the Ndakua (Dammara vitiensis) and the Ndamanu (Calophyllum-burmanni) together with a palm of the genus Veitchia. Here on this level watershed between the basins of the Wainunu and Sarawanga rivers, the sluggish streams flow aimlessly along in but slightly eroded channels; and it is not always possible to determine the side of the island to which they ultimately direct their course. In their beds are pebbles and irregularly formed concretions of an impure reddish flint which I have described on page 354. On the north and south sides the table-land is much excavated by the tributaries of the Sarawanga and Wainunu rivers. On the west where it meets the foot of the Seatura slope portions of columns of basaltic rocks appear on the surface, and deep gorges are worn by the large streams descending from the mountain. On the east towards Nuku-ni-tambua and Tambu-lotu, the surface is also much cut up. The preservation of this table-land in a region, where the denuding agencies are very active in their operations all around it, is to be attributed to its being a level watershed, where the head-waters of the Wainunu and Sarawanga rivers in part take their rise but have little or no eroding power.

It is not easy to obtain a good general view of the district of the falls on account of the dense forest-growth. When making the traverse from Tambu-lotu to Ndawa-thumi, it is observed that there is here a singular hollow, about half a mile in length, which receives the falls at the western end. The river crosses this hollow and is at once received into the gorge below, but there is no stream to explain the origin of the cavity. On its north side the cliffs of agglomerate rise to a height of 150 to 200 feet from their base, but on the south the sides are much lower. Here there seem to be the remains of the crater of the ancient vent from which all the tuffs and agglomerates of the district were derived. We must look for their origin in the vicinity, and the only evidence of a crateral cavity is this streamless hollow extending east from the falls of Na Savu.

With reference to the basic tuffs and agglomerates of this plateau it may be observed that they cover the massive basic rocks and are probably not over 100 or 150 feet in maximum thickness. They are well exposed where the streams cut into the borders of the plateau. The tuffs are sometimes bedded and slightly inclined, and they may be fine or coarse grained. They are more or less palagonitised hyalomelane-tuffs, being composed mainly of fragments of a basic glass, often finely vesicular and even fibrillar, the vacuoles being filled with different materials, whilst the palagonitisation is well advanced. Sometimes they have a brecciated appearance, and in that case when the alteration of the basic glass is very extensive we find angular fragments, 1 to 2 inches across, of a greenish palagonite imbedded in a pale matrix of palagonitic debris, the whole rock having a soapy feel and a steatitic appearance. This is well shown on the sides of the stream-course at Ndawathumi which lies at the border of the table-land. These tuffs effervesce but slightly with an acid.

The basic agglomerate is displayed in the face of the falls and in the gorges. The blocks are as a rule composed of semi-vitreous basaltic andesites of varying type, showing no olivine and containing a fair amount of smoky glass in the groundmass. At times they are scoriaceous and display amygdules of calcite or a zeolite. In places the rock shows large phenocrysts of plagioclase and a semi-ophitic groundmass, when it is referred to the porphyritic group of genus 9 of the augite-class. In a few of the scoriaceous blocks the augite of the groundmass is for the most part prismatic and rarely granular (genus 5).

The massive rocks underlying the agglomerates in the vicinity of Na Savu are aphanitic augite-andesites, differing in important characters from the rocks of the agglomerates. They probably represent ancient lava flows of the Na Savu vent. They are compact (sp. gr. 2·72-2·76), and display a groundmass formed of a felt of felspar-lathes, averaging ·05 or ·06 mm. only in length, and in flow-arrangement. That occurring just below the falls is almost aphanitic, but is referred to genus 13, species A, sub-species _a_, of the augite-andesites. The rock from the gorge below is of the same character, but on account of its opaque plagioclase phenocrysts it is referred to genus 14, and is described on p. 279.

In one place on the plateau a tuff-agglomerate is penetrated by veins, a few inches thick, formed apparently of a finely brecciated tuff of basic glass fragments in a palagonitic matrix. It is, however, pointed out on p. 340 that they were originally veins of basaltic glass which have been subjected to crushing, and that the palagonite has since been produced.

In concluding this description of the table-land of Na Savu, it may be inferred that the source of its basic tuffs and agglomerates is to be found in the same locality; and probably the original vent is now represented by the hollow extending eastward from the falls. With the exception of a large block of silicified coral found in the vicinity of Ndawathumi and of the impure flints of the surface of the plateau, which are described on pages 354, &c., no direct testimony of its submarine origin offered itself to me. The palagonitic characters of the tuffs afford, however, indirect evidence in this connection; and indeed the occurrence of submarine tuffs and limestones in the vicinity of Tembenindio on its lower northern slopes (see page 131), and the existence at elevations of several hundred feet above the sea of fossiliferous tuffs and clays in the Wainunu and Ndrandramea districts to the eastward, afford strong presumptive evidence that the tuffs and agglomerates of the table-land were deposited under the sea, and I may add in a period subsequent to that of the formation of the great basaltic flows of Seatura and Wainunu.