The Va-lili Range
This range extends from the Na Raro Gap before mentioned to the Ndreke-ni-wai river. It is partly isolated on the north-east from the Korotini Range, the extension eastward of the mountainous axis of the island, by a depression or saddle which at its lowest part is not more than 1,200 or 1,300 feet above the sea; but there is no real break in the line of mountains. It is, however, convenient to make this distinction, and I have named the dip between the two ranges, the Waisali Saddle. The range now to be described attains its greatest elevation in the summit of Va-lili, which is 2,930 feet above the sea. There are two or three other peaks that exceed 2,000 feet, and much of the range is not under 1,700 feet. My acquaintance with this range is not extensive; but it will serve to illustrate its general geological structure.
The summit of Va-lili is very conspicuous from most points of view. From the north, east, and south-east, it has a remarkable broad and square-topped profile with a little conical elevation in the centre. From the south-west, it displays a different outline with a solitary squarish block on the top, and this is the form most familiar to the navigator. Unfortunately, for reasons given below, I did not quite reach the summit, and although I was able to obtain sufficient data for forming a general idea of the structure of this part of the range, the structure of the actual summit has yet to be ascertained.
(1) Ascent of Va-lili from Narengali.—This village, which is elevated 400 feet above the sea, lies about two miles in a direct line, N.N.E. from the peak. In traversing the intervening country, one crosses the Loma-loma ridge, elevated 1,000 feet, on the top of which was once situated the village of Loma-loma visited by Horne in 1878. The rocks exposed on the surface are scanty, a hard palagonite-tuff, which owes its induration to a calcitic cement, occurring on the upper part of the ridge; the original site of the village being marked by a large block of this stone.[[65]] The track then descends into the valley of the Loma-loma river, about 400 feet above the sea, in the bed of which occur blocks of an amygdaloidal basaltic andesite, containing phenocrysts of both rhombic and monoclinic pyroxene, and referred to genus 1 of the rhombic pyroxene andesites. The amygdules are formed of calcite.
Profile-sketches of the Va-Lili Range.
View from the south-east near Savarekareka.
View from the south-west.
Beyond the river the ascent of the northern slope of Va-lili begins. As high as 1,100 feet occur basic agglomerates overlying fine and coarse palagonite-tuffs, which are at times horizontally bedded, the finer kinds being sometimes calcareous, and like that of the Loma-loma ridge above mentioned. At 1,300 feet is a line of tall cliffs which extend for some distance at intervals along the mountain-slope, and are indicated by some fine waterfalls. My track struck these cliffs at a place named “Nangara-ravi” (the leaning cave-rock) where they have a height of 150 feet or more. The tall cliff leans slightly forward, so that it forms a shelter at its foot, and hence the name. It is composed of a tuff-agglomerate, the blocks, which are formed of a semi-vitreous basaltic andesite of the augite class, being not generally more than 3 or 4 inches across. These blocks, which are rounded on the outer exposed side and angular on the imbedded side, are inclosed in a hard, probably calcareous matrix. The whole face of the cliff has the appearance of having been worn smooth by attrition, and there are not to be observed the projecting blocks from its surface which are so characteristic of other agglomerate-cliffs. It shows no stratification; but at its base flush with the cliff-face are large masses of a basic massive rock. But few portions of rock have been detached from the cliff. However, I found in the midst of a huge fallen fragment of the agglomerate a dyke-like mass of a basaltic andesite, which differs chiefly from the rock forming the blocks of the agglomerate in being more crystalline. This dyke must have been about 15 feet thick.
Having regard to these various features, I am inclined to consider that this leaning cliff represents one side of a large fissure in the agglomerates which was occupied by a dyke. Reference has been above made to the fact that the agglomerates may be seen overlying the tuffs farther down the slope, so that the conditions favourable for landslips exist. I have shown on page [111] that the origin of the Mbenutha cliffs where agglomerates lie on clayey tuffs may be thus attributed to a landslip. In the case of the Nangara-ravi cliffs, the occurrence of this fragment of a large basaltic dyke is of some importance in connection with the origin of the basic agglomerates of this locality.
The top of the mountain-ridge is about 700 feet above Nangara-ravi, or 2,000 feet above the sea. The tuffs and agglomerates that once existed here have been stripped off to a great extent and the deeper rocks of the range are in part exposed. The upper part of this ridge (1,700 to 2,000 feet) is formed of a rubbly pitchstone where a basic glass has been broken up and then consolidated, the interstices being filled up with palagonite as described in other cases on page [313]. Though non-vesicular, it is just such a rock as one would expect to find on the surface of a lava-flow or on the sides of a dyke.
The crest of the range is here only a narrow ridge. I followed it along in a north-west direction, gradually ascending on the way, and in time the rubbly pitchstone gave place to a hardened palagonitic clay rock, which was observed as high as 2,300 feet. It apparently contains a little lime, and probably was at one time foraminiferous; but it is now much affected by hydration. Soon after this, we arrived at the foot of the steep ascent leading to the summit of Va-lili. We were now rather over 2,400 feet above the sea; but my natives refused to go on, the heavy rain having made the slope too slippery for a safe ascent. With much reluctance I retraced my steps; and as the bad weather continued for several days after, I did not make another attempt. There would, however, be no difficulty in dry weather.
(2) Traverse of the Va-lili Range from the old site of Loma-loma to Waiwai on the coast of Savu-savu Bay.—This route, which was taken by Mr. Horne, the botanist, in the reverse direction in 1878, is thus described in his book, A Year in Fiji (p. 19): “The path, rough and apparently not much used, ran along streams, up steep ascents and down awkward descents, over slippery boulders and fallen trees, up the sides and along the crests of densely wooded mountains.”
Ascending the north slope of the range I found at the Tangi-nandreli cave, which is 1,050 feet above the sea, a palagonite-tuff-sandstone underlying the basic agglomerate. This tuff, which is of the type described just below, does not effervesce with an acid, and shows no tests of foraminifera when examined with a lens. Further up the slopes large masses of agglomerate occur. At 1,350 feet I came upon a large mass of a fine-grained compacted palagonite-tuff made up chiefly of fragments of palagonitized vacuolar basic glass. Among the mineral fragments occur plagioclase, augite, and rhombic pyroxene, and a little fresh olivine, which is of very rare occurrence in these palagonite-tuffs. It contains little or no lime, and shows no tests of foraminifera in the slide. The summit of the range is here about half a mile broad, and is relatively level. I placed its elevation at 1,760 feet, which is not far from Horne’s estimate of 1,800 feet. The southern slope, which is the rainy side of the range, is much cut up into gorges. In the upper 200 feet palagonite-tuffs, similar to those above referred to, are displayed, and basic agglomerates occur lower down.... This part of the range is remarkable through being completely covered over by palagonite-tuffs and agglomerates. It has been pointed out above that this is not the case with the range close to the highest peak, where the underlying rocks are in part exposed at the crest of the range.
(3) The Eastern Peak of the Va-lili Range.—This hill, about 1,100 feet in height, overlooks the Mbale-mbale branch of the Ndreke-ni-wai river. At its foot near the river there is exposed at the roadside a rubbly pitchstone formed of a basic glass, inclosing porphyritic crystals of plagioclase, augite, and olivine, which is described on page [313]. Here also occurs an agglomerate made up of blocks of a semi-vitreous basaltic andesite (sp. gr. 2·78), showing prismatic pyroxene in the groundmass, and referred to genus 20 of the augite-andesites.
The upper part of the hill displays the same agglomerate, and a tuff-agglomerate in which small fragments of the basaltic andesite are inclosed in a matrix largely made up of fine debris of basic glass. There protrudes through these detrital rocks at the top of the hill a broad dyke-like mass of the same basaltic andesite that forms the agglomerate around; and the structure of the hill is thus displayed as that of an old volcanic neck. It has evidently an axis of massive basaltic rocks, more or less covered over with agglomerates and tuffs.
(4) The Nambuni Spur.—This singular spur runs down to the coast between Waiwai and Wailevu; but it is partly separated by a deep gap from the main range. It attains a height of 550 to 600 feet, and has two little peaks which the natives call Vatu-tolutolu and Vatu-tangitangiri. Its position is shown in one of the profile-sketches of Va-lili, given on page [141]. The crest of the spur is formed by a dyke-like mass, 25 to 30 feet thick, which is composed of a basic agglomerate passing down into a palagonite-tuff. The blocks of the agglomerate are composed of a semi-vitreous basaltic andesite, showing minute felspar-lathes in flow-arrangement in an abundant smoky glass, the fine pyroxene being not differentiated. The tuff, into which the agglomerate passes down is non-calcareous, and displays no organic remains. It is, however, composed of fragments, which do not generally exceed a millimetre in size, of palagonitised vacuolar glass, basic andesites, plagioclase, monoclinic and rhombic pyroxene, &c.
This dyke-like mass forms the axis of the ridge and protrudes vertically about 100 feet, the bulk of the spur being composed of a compacted brecciated palagonite-tuff made up mainly of fragments a centimetre in size, of a basic vacuolar glass, sometimes fibrillar, which is extensively palagonitised.
The filling up of a fissure in a mass of tuff-breccia by palagonite-tuffs and agglomerates probably occurred during the submergence, the original dyke-rock having been removed by marine erosion. After the emergence the subaerial denuding agencies reshaped the surface, and as a result of the less yielding character of the materials filling the fissure, they protrude as a dyke-like mass from the crest.
In a cliff-face of the adjacent main range there are displayed an agglomerate of basaltic andesite and a pitchstone-breccia, composed of fragments of but little altered basic glass, the interstices being filled up with palagonite. In the case of the Kiombo flow I have endeavoured to explain the origin of a closely similar pitch-stone-breccia (page [92]).
(5) The Sea-border and the Low-lying Districts at the Base of the Va-lili Range.—It may be generally remarked that palagonite-tuffs and clays, often foraminiferous, prevail in these localities. Thus in the sea-border between Waiwai and the mouth of the Ndreke-ni-wai basic agglomerates are displayed where the mountains approach the coast; but further west a broad tract of undulating land, elevated usually 100 to 300 feet, intervenes between the range and the sea-border, and here coarse and fine palagonite-tuffs predominate.... On the north-west the foraminiferous tuffs and clays of the Ndreketi plains approach the Va-lili range in the vicinity of Vuinasanga, and extend for at least 200 or 300 feet up its sides.... At the east end of the range, where the slopes descend to the plains of the Waisali valley, a little west of Mbale-mbale, there are exposed bedded palagonite-tuffs, tilted up at an angle of about 20° to the south-west. They contain a little lime and display microscopic tests of foraminifera, the palagonite being minutely vacuolar, the cavities also being filled with the altered glass. I noticed those submarine deposits at an elevation of 100 feet, but probably they reach much higher.
The inference to be drawn from the data above given concerning the Va-lili range seems clearly to be this. We have here indicated the emergence of a submarine mountain-ridge covered over with palagonite-tuffs and agglomerates, the last being uppermost. These coverings have been in places stripped off by the denuding agencies and the underlying massive basic rocks exposed. These rocks, however, vary much in texture, some being vitreous, as in the case of the pitchstones, others hemi-crystalline as in the case of the basaltic andesites; and it is to be gathered from this and other similar indications that different submarine vents were formed along a fissure or fissures at the sea-bottom. No evidence of subaerial eruptions came under my notice. After the vents became extinct they were buried beneath the palagonite-tuffs and agglomerates. During and after the emergence the denuding agencies reshaped the surface of the range and left but little of its original form.
Since it is my object to build up a theory of the origin of the ridge-mountains as I proceed with the systematic description of the island, it will be here convenient to follow up the preceding remarks on the Va-lili Range by a preliminary reference to the great ridge district lying east of it.
When a panoramic view of this region is obtained, one observes a series of lofty ridges more or less parallel and running about N.W. and S.E. There are the Va-lili, Narengali, and Sealevu ridge-mountains with lesser ridges between. The intervening valleys are elevated about 400 feet above the sea, whilst the mountains rise up to over 2,000 feet. In many localities this configuration of the surface would be attributed mainly to subaerial denudation. In this island I will endeavour to show that these mountain-ridges existed before the emergence. They do not owe their form to the rivers that flow through the valleys, though no doubt river-erosion has brought these features into greater relief.
In Vanua Levu, as there will be frequent occasion of showing, rivers often flow in valleys that they have not made. This is especially pointed out on page [151]; and it is necessary to emphasise it here, before proceeding farther with the description of the geological structure of the mountain-ridges.