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.

THE UPRAISED CORAL LIMESTONES

These reef-limestones are scantily represented in the island, though one can scarcely doubt that they were once far more extensive, having been largely stripped off by the denuding agencies. They are mostly found on the south coast between Naindi Bay and Fawn Harbour, and rarely extend to heights greater than 20 or 30 feet above the sea, usually composing the sea cliffs and not occurring as a rule inland. Massive corals are often to be seen imbedded in their position of growth, as described in