The Ndrandramea District
This hilly region of acid andesites is a continuation of the mountainous backbone of the island, being separated from the basaltic mountain of Seatura by the saddle formed by the Na Savu table-land. These acid andesites exhibit in nearly all cases a felsitic groundmass and phenocrysts of plagioclase and rhombic pyroxene; whilst many of them are characterised by brown hornblende more or less pseudomorphosed in the manner described on page [306], and a few display porphyritic quartz. Although these rocks have a common facies, they vary considerably among themselves; and it is difficult to find a term that would strictly include them all. A general description of their characters is given in the chapter on the Acid Andesites.
In this interesting region a number of hills or mountains formed in mass of acid andesites rise up abruptly without any regular arrangement within an area measuring 5 by 6 miles, and elevated 600 to 1,000 feet above the sea. Of these hills, thirteen in all, nine range in height between 1,600 and 2,500 feet above the sea, none of the others rising less than 1,000 feet above that level. But the actual height of each hill above the country at its base is much less than this. The height of the hill-mass, in five or six of the largest, ranges between 900 and 1,200 feet, whilst in the smaller hills it varies between 400 and 800 feet. (See accompanying plan.)
The NDRANDRAMEA District from the westward. The hills and mountains are of acid andesites and dacites. The foreground is elevated about 450 feet above the sea.
[Face p. 98.
Rough plan of the Ndrandramea district in Vanua Levu; made with prismatic compass and aneroid by H. B. Guppy.
These hills have sometimes a rounded profile, when their summits are usually wooded. Others again terminate in conical bare rocky peaks, either pointed or truncated. They have often precipitous slopes and display vertical cliff-faces high up their sides. Their arrangement is rather singular. To the south and apart from the others lies Soloa Levu (1,600 feet). Navuningumu (1,930 feet) is similarly isolated on the north. On the east rises Ngaingai (2,430 feet), the highest of the peaks, with Wawa Levu (2,000 feet), Vatu Kerimasi (1,900 feet), Vatu Vanaya (1,600 feet), and Mbona Lailai (2,100 feet) closely clustered by its side. On the west there is another group of hills, of which Ndrandramea (1,800 feet) is the highest and best known. Associated with it are Kala-Kala (1,600 feet), Mako-mako, Thoka-singa (1,300 feet), Vatu Mata (1,050 feet), and another unnamed peak (1,400 feet) lying west of Ndrandramea.
The districts between and among the hills are much cut up into lesser hills and ridges, the result of the very extensive denudation to which this region has been subjected. The greater part of this area is drained by the Tambu-lotu tributary of the Wainunu; but in the northern part we cross the watershed between the Wainunu and Ndreketi basins, and to reach Navuningumu we cross the valley of one of the tributaries of the Ndreketi. To the east of the Ndrandramea region extends a broken country, elevated rather more than 1,000 feet above the sea, and from it there rise one or two hills with bare cliff-faces, which are probably composed of similar acid andesites.
Although for the most part composed of these acid andesites, each hill, as far as my observations show, has as a rule its own type of the rock, differing from the others in specific weight, in the texture of the groundmass, and in the relative proportion of the porphyritic constituents. The petrological characters will be found more fully discussed in [Chapter XXI.]; and only some of the more distinctive features will be noticed here in the following description of the district.
The Ngaingai Group of Hills.—Within a space less than a mile square rise Ngaingai, Wawa Levu, and the other three hills above named, so closely clustered together that the collective name of “Hen and Chickens” might be aptly applied to the group.
The peculiar form of Ngaingai is shown in the accompanying profile-sketch. It is the Nangorongoro of the Admiralty chart. The height of the mountain from its base is 1,100 to 1,200 feet. Its ascent, which is not difficult, may be made from the west side. Above its wooded slopes rises its bare rocky peak, from which a magnificent panoramic view of the western half of Vanua Levu can be obtained. Characteristic dacites with porphyritic quartz came under my notice all the way up from the foot to the summit, being occasionally exposed in perpendicular cliff-faces. Specimens taken from the upper and lower portions are uniform in character, and have a specific gravity of 2·57. No other rocks were observed on its slopes. The whole hill-mass is in great part if not entirely formed of these acid andesites.
The contrast between the narrow crested peak of Ngaingai and the dome-shaped summit of Wawa Levu is seen in the sketch; and this is the more remarkable because it is not associated, as far as I could ascertain, with any important difference in geological character. Wawa Levu rises precipitously to a height of 900 or 1,000 feet above its base, and displays often perpendicular cliff-faces on its sides. Its broad level soil-covered summit is mostly covered with young wood, few of the trees having trunks more than 4 inches in diameter, whilst they are usually clothed with damp moss, and are often decayed and rotten.[[49]] True dacites, closely similar to those of the neighbouring Ngaingai and having a specific gravity of 2·61, were displayed often in slab-like blocks from the base to near the top. The rudely columnar structure to be observed in some of the other hills is rarely exhibited. No other rocks came under my notice. The remains of the stone walls of two old “war-towns,” one of them named “Ndaku-i-tonga,” occur on its south and south-east slopes.
Profiles of Ngaingai and Wawa Levu from Nambuna to the south-west. Both are dacitic mountains.
The other three hills of the Ngaingai group were not ascended by me. They show the same bare cliff-faces and have to all appearance the same geological character. Mbona Lailai and Vatu Kerimasi are two blunt-topped conical hills with precipitous slopes that rise respectively about 900 and 700 feet above the country at their base. Vatu Vanaya, about 500 feet in height, has a rounded summit.
The Ndrandramea Group of Hills.—A view of these hills from the westward is given in the accompanying illustration. They have a lower elevation than the hills of the Ngaingai group, none of them rising to over 1,800 feet above the sea, whilst their height from the base is also less, ranging between 400 and 900 feet. They rise, as the illustration shows, in the midst of a densely-wooded broken country.
Ndrandramea, which is 1,800 feet above the sea, has an individual height of about 900 feet. Fijians in distant parts of the island are familiar with the name of this remarkable peak. It has a legendary fame; and like Wawa Levu in the old time it served as a mountain stronghold in times of war. The remains of a stonewall of a “koro-ni-valu” or “town of war,” known as Mata-mei-ndami-ndami, occur on its side, 300 or 350 feet below its summit; whilst among the wild lemon trees that cover the slopes below large ovoid sling-stones 4 or 5 inches in length may still be found. Viewed from the south-east, as shown in the [frontispiece], Ndrandramea has the shape of a woman’s breast; and evidently the origin of its name is connected with this resemblance. But seen from the west and south-west, as in the other general view of the district (page [98]), it has a broadly truncated conical outline, its form being indeed somewhat elongated or elliptical.
This hill presents precipitous slopes, and on the south side it shows bare rocky faces. As seen in the illustration, it might appear inaccessible; but the ascent is not difficult on the west side. It is composed in mass of an acid andesite allied to the dacites of Ngaingai and Wawa Levu, but differing in the hemicrystalline character of the groundmass (except at the base), in the porphyritic development of rhombic pyroxene, and in the absence of porphyritic quartz. As remarked on page [301], the rock becomes more basic as one descends the hill. At the top its specific weight is 2·44, about 300 feet below it is 2·58, at 700 feet from the top it is 2·68, and at the base of the hill where it is holocrystalline and has a dioritic appearance it is 2·71. That it possesses a rudely columnar structure is shown by the occurrence here and there on the slopes and at the base of the hill of portions of prostrate columns, 3 to 4 feet broad and sometimes 20 to 25 feet long, which have a rounded surface and look like fossil tree-trunks. Masses of agglomerate of the same andesitic rocks lie about in places on the lower slopes, the included blocks, which are a few inches across, being sometimes rounded.
The neighbouring hills lying south and west of Ndrandramea are, as far as my observations show, of the same acid type of andesite. It is connected with those nearest by a saddle, 1,100 feet above the sea, where the same holocrystalline form of the rock occurs, having a specific gravity of 2·7 and being often rudely columnar in structure. Kala-kala, about 1,600 feet above the sea, is an imposing-looking hill with perpendicular cliff-faces on some of its sides. I did not ascend it, but found at its base a rock of the same andesitic type, differing from that of Ndrandramea in the more crystalline character of the groundmass, and having a specific gravity of 2·61. West of Kala-kala is the outlying hill of Vatu Mata with a flat top and rising only about 400 feet from its base. It has all the appearance of being composed of the same andesitic rocks. It is shown on the left-hand in the illustration.
Lying south of Kala-kala are the two peaks of Mako-mako and Thoka-singa, rising respectively 1,400 and 1,300 feet above the sea. I ascended the last-named, which has a rounded summit covered with trees. Approaching it from Nambuna on the east, I found at its foot a large mass of pitchstone-agglomerate, formed of fragments of vitreous basic rocks, such as occurs around the lower part of Soloa Levu on the other side of the valley. The slopes of Thoka-singa, between 200 and 450 feet below the summit, are strewn with masses of another kind of agglomerate made up of blocks 3 to 8 inches across, occasionally rounded, and composed of the same felsitic andesite, of which the mass of the hill is formed. This last-named rock is exposed in bulk in the upper part, but on the summit the agglomerate reappears. It has a granitoid appearance, and is distinguished from the acid andesites of the other hills of the Ndrandramea district by its greater specific gravity (2·72 to 2·74), by its holocrystalline texture, and by the coarse grain of the mosaic of its felsitic groundmass, which is probably quartz-bearing but is relatively scanty. It is, however, referable to the same group of felsitic andesites, but is to be placed at the basic end of the series. (Its description is given on page [302].) In Thoka-singa we have therefore a hill which is evidently formed in mass of these holocrystalline felsitic andesites but covered in places with an agglomerate of the same materials. I have already referred to this feature in the structure of Ndrandramea. Since the blocks are sometimes rounded, such agglomerates may represent the result of marine erosion during the emergence of this part of the island. In the case of Navuningumu, where they lie abruptly on calcareous clays containing tests of foraminifera and shells of pteropods, a different explanation appears to be needed.
The Hill of Soloa Levu.—This isolated hill, which presents another type of these acid andesites, has a broad rounded summit; and though elevated about 1,600 feet above the sea, the hill itself rises only 800 or 900 feet above the country at its base. It is not easy to obtain a view of the profile of this hill and to ascertain its relation to its surroundings; and it was only when I viewed it from near the top of Vatu Kaisia six miles to the eastward that I was able to understand its position. Looking from that standpoint across the basaltic table-land of Wainunu one observed Soloa Levu rising as a dome-shaped hill at the western margin of the table-land and apparently not separated from it. The examination of the district shows that on the east and south-east sides this hill was in part surrounded by the great basaltic flows by which the table-land was built up. Basic tuffs and agglomerates, however, occur on the lower slopes on the north-west, west, and south-west sides, so that Soloa Levu in fact lies in the midst of an area of basic rocks.
The type of acid andesite which is displayed in the upper two-thirds of the hill is distinguished from those of the other hills of the Ndrandramea district by its orthophyric groundmass. Instead of a fine mosaic, the matrix displays as a rule an arrangement of short stout plagioclase prisms; but in one of my slides the two forms of groundmass are associated. In their general characters as described on page [296], they cannot be separated from the acid andesites of the Ndrandramea district. Their specific weight ranges between 2·54 and 2·62, and like most of the other acid andesites they contain little, if any, interstitial glass. Huge blocks of these rocks lie about on the slopes, often assuming a columnar form, the fragments of such columns being sometimes 5 or 6 feet in diameter, and 12 to 15 feet in length. I found one such block standing erect like a solitary obelisk.
The best way to observe the basic rocks that invest the lower slopes of Soloa Levu is to follow the track that skirts it on the south side on the way from Tambu-lotu to Vunivuvundi. Palagonitic tuffs containing in places a little lime[[50]] and composed of fragments of basic glass of varying size and more or less palagonitised extend from Tambu-lotu and Nuku-ni-tambua (two villages lying about a mile to the westward) to the west and south-west slopes of Soloa Levu. A pitchstone-agglomerate, formed of fragments of a basic glass inclosing large crystals of plagioclase felspar one-third of an inch in length, is associated with these tuffs on the lower north-west, west, and south-west slopes of the hill. The tuffs are formed of the same materials as the pitchstone-agglomerates, but differ in their character of being more or less palagonitised. However, on the north-west side the latter have also undergone this change. On page [312] will be found a description of the basic glass of these agglomerates in its fresh and in its altered condition. Huge blocks of these rocks strew the surface on the south-west slopes of Soloa Levu, and in one place the underlying acid andesite that forms the mass of the hill is exposed in a stream-course.
These pitchstone-agglomerates and palagonitic pitchstone-tuffs are elevated between 600 and 750 feet above the sea. As one proceeds on the road to Vunivuvundi and skirts the south-east side of the hill one ascends the western border of the basaltic Wainunu table-land which, however, is much cut up by rivers in this locality. Here the tuffs and agglomerates give place to a basaltic andesite, and on reaching an elevation of 1,000 feet we arrive at the top of the table-land from which an ascent of Soloa Levu is easily made. The road then lies on, but parallel to, the border of this plateau for some distance until it descends into a deep valley worn by one of the tributaries of the Wainunu River.
This hill of Soloa Levu is in fact a mass of acid andesite situated in the midst of an area of basic rocks. I found basaltic rocks exposed in the stream courses to the north and similar rocks prevail on the north-west on the way between Nambuna and Tambu-lotu. It has been above remarked that on the east and south it has been in part surrounded by the basaltic flows of the Wainunu table-land, and that pitchstone-tuffs and agglomerates cover its lower slopes on the west and south-west, yet it is not easy to find any trace of the vent from which they flowed or were ejected.
It may be here remarked that the occurrence here and there of basic rocks in the midst of this region suggests the vicinity of dykes. For instance, in a deep gulley about half a mile south-west of Kalakala, where a dacitic rock was exposed in situ, I came upon a single large mass of an aphanitic augite-andesite of the type described under genus 16, species A, of the augite-andesites.
The Altered Acid Andesites of the Ndrandramea District.—One of the most important features of the geological structure of this district lies in the fact that the bed-rock exposed in the lower region between the hills is a highly altered acid andesite of the type found in the hills around. By referring to the map of this locality, it will be observed that between the Ndrandramea hills on the west and the Ngaingai hills on the east is the valley of the Tambu-lotu river and its tributaries, an open broken country deeply eroded by the streams, and elevated 600 to 700 feet above the sea. These altered rocks are well exposed in the deep gorge-like channel of the river between the village of Nambuna and the foot of Ndrandramea, and in fact in all places in this district where the streams have worn deeply into the surface.
They have a coarse felsitic groundmass, and are described under the felsitic order of the hypersthene-andesites on page [297]. They present all degrees of change from the hard dark grey mottled rocks, in which the phenocrysts of plagioclase and rhombic pyroxene are in part replaced by calcitic, viriditic, and chloritic materials, to those where the pseudomorphism and alteration is complete, when the decomposition products give their character to a pale yellowish rock, which sparkles with pyrites and often effervesces briskly with an acid. After this comes the final stage of disintegration, and we get a whitish rotten stone, often full of pyrites, the last condition of which is shown in a kaolin-like material exposed in the river-side.
The extensive alteration of these rocks is also indicated by the occurrence amongst the gravel of the river-bed and small stream courses near Nambuna of fragments of clear quartz prisms, half an inch across, and of nodules, three inches in size and sometimes hollow in the centre, formed of radiating quartz crystals that once filled cavities in the altered rock. Small masses of vein-quartz also occur in these streams, formed in a fissure by the growth of the crystals from the sides towards the centre. I was unable to find the source of the quartz; but it is probable that it was produced near the line of contact between the basaltic flows to the eastward and the older felsitic rocks of the district. The great alteration of the acid andesitic rocks exposed as the bed-rocks in this region may in all probability be attributed to the vicinity of these basaltic rocks. The two formations apparently come into contact about a mile east of Nambuna. In traversing this district on the road to Ndrawa one first observes in situ in the streams the decomposed felsitic bed-rock with occasional loose blocks of a quartzitic rock that displays in the thin section a mosaic of irregular grains of quartz. Afterwards, as one rises gradually to the top of the basaltic plateau, basaltic rocks are alone exposed in position.
In the character of the fine river sand a clue may be found to the exact locality of the contact. In the midst of the andesitic area between Nambuna and Ndrandramea, the sand, besides containing much magnetic iron, is also composed to a large extent of rhombic pyroxene prisms, clear quartz grains, and fragments of plagioclase, all derived from the porphyritic crystals of the dacites, &c. Near the basaltic district we find that the quartz and rhombic pyroxene have disappeared, the sand being largely made up of magnetic-iron grains mixed with fragments of plagioclase.
Profile and Geological Section of Vanua Levu, across the island from the Sarawanga (north) coast to the Yanawai (south) coast.
The Extent of the Area of Acid Andesite Rocks in the Ndrandramea District.—By referring to the map of this locality it will be observed that this region of andesites extends northward to the Navuningumu Range, and that on the south it would be separated from the district of tuffs and agglomerates, named the table-land of Na Savu, by a line joining the hills of Soloa Levu and Thokasinga. On the east it is bounded by the basaltic area of the Wainunu table-land. On the west it extends at the surface, with an occasional overlying patch of submarine tuffs and clays, for a distance of at least two or three miles from the base of the hills, and sometimes, as in the direction of Sarawanga, more than half way to the coast. I have endeavoured to show the relation of these acid rocks to the basalts and to the sedimentary deposits in the geological section.
When taking the track from Sarawanga to Nambuna by way of Ndrandramea one soon enters the region of these acid andesites. The prevailing rock exposed on the surface, where it is usually much decomposed, is a bluish-grey hypersthene-andesite with a specific gravity of 2·54, and displaying in a cryptocrystalline groundmass, where the felsitic texture can be recognised, abundant phenocrysts of plagioclase and rhombic pyroxene. As high as 500 feet above the sea it is occasionally capped by patches of palagonitised clays and tuffs scantily foraminiferous, and at one place I noticed a patch of agglomerate, the subangular blocks six to eight inches across being formed of the same acid andesite. In the same way by taking the road from Tembe-ni-ndio to Nambuna, passing the hill of Kala-kala on the way, we leave behind the foraminiferous tuffs and limestones of the lower coast regions; and when about 400 feet above the sea we enter the inland district of felsitic andesites which begin about two miles from Tembe-ni-ndio.
The Navuningumu Range.—By following the track from Nambuna to Navuningumu one skirts the bases of Wawa Levu and Ngaingai, where dacitic rocks are exposed. After passing the watershed[[51]] between the Wainunu and Ndreketi rivers, the track descends into the deep valley of one of the western tributaries of the Ndreketi, where a characteristic holocrystalline type of these felsitic andesites is exposed. Approaching Navuningumu one finds exposed at its base agglomerates, composed of scoriaceous and amygdaloidal semi-vitreous basic rocks, overlying a dark tufaceous sandstone which on examination proves to be a basic pumiceous tuff of the type described on page [333], and scantily foraminiferous.
We stand now in a region of basic rocks on the south-east side of the range, and before us rises abruptly the weird-looking magnetic peak of Navuningumu, which is well represented in the accompanying illustration. In the wet season its summit is usually enveloped in the thunder-clouds. Its elevation above the sea is 1,930 feet, but estimated from its base its height is 1,000 to 1,100 feet. The natives also name this peak Na Seyanga, after a town that once existed in this locality. It is the summit of a range that extends a mile or more to the north where it terminates in a lesser peak known as Mumu.
Ascending the peak of Navuningumu from the south-east one finds exposed in its lower part, up to 1,200 feet above the sea, pitchstone-agglomerates (composed of fragments of a vitreous basic rock) and white tufaceous sandstones (containing a few tests of foraminifera), such as are described below in the case of the neighbouring Mbenutha Cliffs. Between 1,300 and 1,500 feet there is displayed in position a typical dacite of the type described on page [303].
Mt. TAVIA (2,210 feet) from VATU KAISIA. It is probably formed of an acid andesite.
The magnetic peak of NAVUNINGUMU (1,931 feet) from the south. The summit represents a basaltic neck.
[Face p. 108.
The peak itself is formed of a dark-brown slightly vesicular semi-vitreous basaltic andesite, of which, in fact, for the upper 200 feet, the summit is composed. The rock is somewhat rubbly; and where it is exposed on the bare peak it is powerfully magnetic, displaying polarity in a marked degree, and rendering the compass useless (see page [368]). A specimen of the magnetic rock, which is a little vesicular, has a specific gravity of 2·82. It is referred to genus 1 of the augite-andesites described on page [267]. It displays in the slide porphyritic plagioclase, with a little augite, in a groundmass formed of a plexus of minute felspar-lathes (·06 mm. in length), and exhibiting a large amount of a brown opaque glass in which grains and rods of magnetite with a few pyroxene granules are developed. The magnetite in the groundmass, although abundant, is not in greater quantity than is usually found in semi-vitreous basaltic rocks without polarity.... This terminal mass of basic lava-rock evidently forms the “plug” of a volcanic pipe that pierces the acid andesitic rocks of the district; and from this ancient vent were doubtless ejected the basic tuffs and agglomerates that now cover the lower slopes of the mountain.
The conditions under which this volcano displayed its activity are further illustrated in a remarkable section exhibited on the east side of the mountain half a mile or more north of the summit. Here there is a line of bold cliffs, in which, as shown in the illustration, a bed of agglomerate, 60 or 70 feet thick, overlies a series of foraminiferous clays and tufaceous sandstones, which are elevated about 1,100 feet above the sea. The locality is named “Mbenu-tha” or “Rubbish-heap.” It is well known to the natives on account of its caves, which serve as a half-way resting-place on the road from Nambuna to Ndreketi. These caves have been produced by the more rapid weathering of the underlying clays and sandstones. The line of cliff extends northward to Mumu, the peak at that end of the range, and preserves there the same structure. The clays and tuff-sandstones are more or less stratified, and dip generally to the west or south-west at an angle perhaps of 20 degrees; but in more than one place they show signs of great disturbance, being contorted and steeply tilted.
The foraminiferous clays form a more or less compact rock and contain 15 or 16 per cent. of lime. They inclose pteropod shells in places and show many minute foraminiferous tests of the pelagic type. Their composition is given on page [323]; but it may be here remarked that the residue is made up mainly of palagonitic debris, fine clayey material and minerals. The mineral fragments form about 20 per cent. of the mass, and consist principally of glassy plagioclase, with some rhombic pyroxene, and magnetite, their size averaging ·1 mm. The tuff-sandstones interstratified with the clays contain only 2 or 3 per cent. of lime, and show only a few scattered microscopic tests of foraminifera. About two-thirds of the rock consist of fragments of a bottle green basic glass, vacuolar and but little altered, the rest being composed chiefly of glass debris, plagioclase, and a little pyroxene, the larger mineral and glass fragments averaging ·3 to ·5 mm. in size. They are in fact submarine hyalomelane tuffs very similar to those first met with at the foot of the mountain, which are referred to on page [108]. (They are described on page [333].)
These interbedded clays and tufaceous sandstones of the Mbenu-tha cliffs were deposited under somewhat different conditions. The clays represent the quiet deposition in fairly deep water of fine materials derived from the degradation of acid andesites as well as of basic rocks. The hyalomelane tuff-sandstones were formed more rapidly by the accumulation of fine volcanic ash consisting of fragments of a basic glass ejected from some neighbouring volcano that rose above the sea-surface.
Submarine hyalomelane-tuffs with basic agglomerates appear to be of common occurrence around the base of the Navuningumu mountain. As we leave the range behind and begin to descend the long spur that slopes northward to Ndreketi, we find for the first mile or two these agglomerates. But where the deeper rocks are exposed at an elevation of 600 feet, near the village of Singa-singa, there are displayed fine basic pumiceous tuffs and compact palagonitised clays containing little if any lime, the last, however, containing a few casts of microscopic foraminifera. The tuff is made up of minute fragments, the largest less than ·1 mm. in size, of a basic hyalomelane glass, which is vacuolar, and often fibrillar like ordinary pumice, and in places shows the early stage of alteration into palagonite. The clay principally consists of more or less palagonitised debris of the same basic glass, together with minute fragments of plagioclase and rhombic pyroxene. These tuffs and clays represent the two conditions of deposition above referred to, the last indicating a period of quiescence when the fine materials resulting from the degradation of both acid and basic andesites were slowly accumulating in deep water, the first denoting the activity of a neighbouring supra-marine vent from which fine dust and ash formed of basic pumice were ejected.
MBENUTHA Cliffs showing volcanic agglomerates overlying tuffs and clays, containing shells of pteropods and foraminifera, which are raised 1,100 feet above the sea.
[Face p. 111.
The bed of agglomerate, 60 to 70 feet thick, which overlies the foraminiferous tuffs and clays exposed in the line of cliff extending from Mbenu-tha to Mumu, is made up of subangular blocks, not usually over 6 inches in diameter, of an acid andesite of the general type found in the Ndrandramea region, but possessing a semi-vitreous groundmass.[[52]] By clambering up the steep slope on the south side of these cliffs, it will be observed that this thick bed of agglomerate is covered by bedded foraminiferous clays and tuffs similar to those that underlie it. It is therefore without doubt submarine, and presents the result of the more violent outbursts of some neighbouring vent. That this vent is now represented by the “plug” of basic lava forming the peak of Navuningumu is highly probable. It is, however, noteworthy that these beds of agglomerates, tuffs, and clays, as shown in the photograph of the cliffs, are all inclined at an angle of 20° towards the axis of eruption or to the westward. The tuffs and clays underlying the agglomerates are, as already remarked, much disturbed in places. It would seem that all the beds here exposed were originally horizontal, and were tilted up during the disturbances accompanying the outbursts of volcanic activity.
The natural section, which the Mbenu-tha cliffs present, is doubtless due to landslips. Similar exposures, displayed by cliffs of basic agglomerate with submarine tuffs and clays at their base, are common on the mountain-slopes of other parts of the island. Water oozes through the underlying soft deposits, and the result is seen in the occurrence of huge masses of agglomerate on the slopes below.
From the details here given respecting Navuningumu and its surroundings, it is apparent that there have been two stages in the history of this volcanic mountain. The first was submarine and was characterised by the discharge of acid lavas which consolidated around the vent and were afterwards covered over with deposits of foraminiferous clays. The second was in the last part supra-marine. With the renewal of activity, the overlying acid andesites were broken through and basic materials were discharged from the new vent. The bed of acid agglomerates exposed in the Mbenu-tha cliff belongs to that period of the second stage when the explosive agencies were most violent. It represents the extensive destruction of the overlying rocks. The foraminiferous tuff-sandstones are submarine accumulations of the finely comminuted fragments of basic pumice that constituted the dust and fine ash discharged from a supra-marine vent. The scoriaceous and amygdaloidal blocks of the basic agglomerates overlying these tuffs around the base of the mountain have had a similar origin. The original ash-cone that at one time rose above the surface of the sea has long since been destroyed by the denuding agencies; and its situation is alone indicated by the “neck” of basic lava-rock that forms the peak of Navuningumu.
A very long period must have elapsed since this last stage in the activity of the vent. The clays containing pteropod-shells and tests of foraminifera, with which the basic pumice tuffs and the acid agglomerates were interstratified, are now about 1,100 feet above the sea, and are situated in the centre of the island. During the emergence the denudation of the new land-surface was no doubt very great; and these submarine clays and tuffs, as displayed in the cliffs, owe their preservation in great part to the protection of the overlying mass of agglomerate.
Much light is thrown on the history of the whole Ndrandramea region of acid andesites by the examination of this old volcano of Navuningumu. Some of the hills, as in the case of Ngaingai and Wawa Levu, seem to have been stripped of everything that could give information to the geologist. Others again, like those of Thoka-singa and Ndrandramea, display here and there on their slopes agglomerates of the same materials, the rounded forms of some of the blocks being in part indicative of marine erosion during the emergence of this region from the sea. In Soloa Levu, however, we have one of these hills partially surrounded by later basaltic flows and covered in places on its lower slopes by basic tuffs and agglomerates, probably submarine. In Navuningumu the original mass of acid andesite is only scantily exposed. It is for the most part buried beneath submarine clays which are in their turn covered by the tuffs and agglomerates of later basic eruptions.