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

by H. B. Guppy

The Volcanic Agglomerates

In this place my remarks will be chiefly confined to a summary of some of the leading features of these formations. The agglomerates, which pass by all gradations through the tuff-agglomerates into the submarine tuffs, rank amongst the most prevalent and the most conspicuous of the rocks exposed at the surface in this island. Their lithological characters vary according to the type of the massive rocks of the district. Thus in the Ndrandramea district the blocks are composed of the prevailing acid andesites. In the Koro-mbasanga district they are formed of hypersthene-augite-andesites. In the Korotini and Va-lili ranges they are composed of olivine basalts and basaltic andesites. The agglomerates derived from basaltic rocks and basic andesites are by far the most frequent, and it is to them that the following general observations apply.

The basic agglomerates and tuff-agglomerates are found almost everywhere and at all elevations up to 2,500 feet above the sea and over. They compose the inland cliffs and the long lines of precipitous declivities that give character to the valleys and gorges of the mountainous interior. The blocks are often scoriaceous and semi-vitreous, but the characters of the rocks will be found described on page [316]. They are generally sub-angular and vary in size from a few inches to one or two feet; and, though sometimes heaped together in confusion, they will generally be found in the case of any extensive exposure to be rudely sorted according to size, or to present a rude horizontal arrangement.

The matrix varies much in amount, being sometimes barely appreciable and at other times so abundant that the deposit may be termed a tuff-agglomerate. Typically it has the character of the palagonite-tuffs of mixed composition described on page [326], being made up of fragments of palagonitised vacuolar basic glass, portions of crystals of plagioclase and augite, with the debris of the basic semi-vitreous and hemi-crystalline rocks forming the blocks. When it is scanty it contains neither carbonate of lime nor organic remains; but in the tuff-agglomerates it may be calcareous and may inclose tests of foraminifera and molluscan shells.

From the circumstance that the basic agglomerates overlie submarine sedimentary tuffs and clays almost everywhere, their submarine origin could alone be safely postulated. There are one or two localities that throw especial light on the conditions under which these accumulations occurred. They are dealt with at some length in the general description of each district and only a brief reference can be made to some of their indications here.

The testimony supplied by the interesting exposures on the slopes of Mount Thambeyu (page [178]) goes to show that after the deposition of the foraminiferous tuffs and clays the stage of the agglomerates was ushered in gradually. The tuffs increased in coarseness, and afterwards they were covered up with an agglomerate formed of blocks at first only one or two inches in size, but afterwards of larger dimensions.... Curious evidence is afforded by the agglomerates of Mount Vungalei (page [213]), where two beds of palagonite-tuff, at elevations of 900 and 1,700 feet, mark two pauses in the accumulation of the agglomerates. In each case the pause was introduced by the gradual decrease of the agglomerates which gave place by gradation to the tuffs. In each case also the pause was followed by a sudden renewal of the deposition of agglomerates.

With reference to the maximum thickness of these deposits, it would appear that on the slopes of the Korotini Range this amounts to some hundreds of feet, if we also include the tuff-agglomerates. Their origin is to be attributed partly to eruptions and partly to marine erosion. The two agencies although often associated were in their turns predominant in their different phases, and it is not too much to suppose that the agglomerates without arrangement, with scanty matrix, and composed of scoriaceous blocks, belong more to an eruptive period, and that those with abundant tufaceous matrix and sorted blocks are mainly the product of marine erosion. In either case the deposition was submarine.

But the history of these agglomerates and of their associated foraminiferous tuffs and clays must of necessity be a complicated one, since they indicate a minimum emergence of 2,500 feet. Their accumulation first began when a number of vents, in linear arrangement, were striving to raise their heads above the surface of the sea. It was continued after the waves had ultimately worn the volcanic islets down to below the sea-level, and the shoals became covered over with submarine deposits. Again and again no doubt this struggle between the eruptive agencies and the waves was renewed, until at length the great emergence began, and probably from that date the agency of marine erosion was predominant.

When on the island of Stromboli I had presented for my observation at least two modes of agglomerate-building under the sea. There was the ordinary work of the marine erosion of the lava-cliffs, of which the beach represents but a small part of the result; and there were the dribbling eruptions of the crater, from which at intervals of only a few minutes masses of semi-molten lava bounded down the steep slopes into the sea.

Note on the general characters of the rocks of the basic agglomerates.—In appearance the basic rocks forming the blocks are often very similar. They are usually compact blackish with a semi-vitreous aspect and display some plagioclase phenocrysts. But to enumerate the types to which they belong would be to go over much of the ground traversed in the classification of the basic rocks, whether olivine basalts, basaltic andesites, ordinary augite-andesites, or hypersthene-augite-andesites. The groundmass as a rule contains much smoky glass, but the hemi-crystalline portions of it vary considerably in character. Whilst fine granular augite prevails, semi-ophitic coarser augites are not uncommon, and prismatic pyroxene, sometimes of the rhombic form, is represented in the groundmass of the rocks composing the agglomerates of Mount Thambeyu and of the Sokena Cliffs. In some localities, as on the south-west slopes of the Korotini Range, rocks of the basic pitchstone kind are predominant.

CHAPTER XXIII
CALCAREOUS FORMATIONS, VOLCANIC MUDS, PALAGONITE-TUFFS

The classification that is adopted in my work on the geology of the Solomon group with respect to the calcareous formations and volcanic muds of those islands is only in part applicable to the calcareous rocks and volcanic deposits of Vanua Levu. Deposits strictly comparable with those of the Solomon Islands here exist, and have in some places an extensive distribution; but many others cannot be referred to that classification. In addition to the calcareous oozes and volcanic muds, such as are now forming off these reef-bound coasts, the result partly of marine erosion and partly of sub-aerial denudation, there are many kinds of submarine deposits in Vanua Levu that have been largely formed from the materials ejected by volcanic vents. Basic glasses, for instance, often finely vesicular and usually converted into palagonite, enter largely into the composition of submarine deposits that frequently form the surface from the sea-borders to the summits of the mountain-ranges; and it is by the degradation of a land-surface formed of such materials that the volcanic muds comparable to those of the Solomon Islands are mainly produced. It is therefore apparent that we have to distinguish here between the deposits of sedimentary and eruptive origin, a distinction, however, which is not always easy to make, since they are in both cases submarine, and doubtless were often in process of forming together. The deposits most prevalent in the island are the submarine tuffs partly sedimentary and partly eruptive in their origin and the overlying volcanic agglomerates. The first are usually palagonitic and calcareous and often contain organic remains, being usually associated with volcanic muds and clays mainly the product of marine erosion.

In connection with the employment of the terms “upraised” and “elevated” in the case of the Vanua Levu deposits I will take this opportunity to remark that I do not thereby commit myself to the view that there has been an actual upheaval of this region. This is a matter, however, that will be found discussed in [Chapter XXVII].