Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 / Vanua Levu, Fiji

by H. B. Guppy

The Mountain-Ridge of Mariko

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

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

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

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

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

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

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

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

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