These deposits represent coarse kinds of the submarine tuffs of basic glass, in which the palagonitic change is far advanced, and where zeolites and at times secondary calcite have been produced in abundance as a result of the alteration. They present themselves in the mass as mottled grey rocks which when examined in thin sections are seen to be composed in great part of fragments of more or less palagonitised vacuolar basic glass, whilst zeolites are extensively developed in numerous irregular cavities and in the interspaces. Although displaying no organic remains, their submarine character is indicated as at Nandua by the circumstance of their occurring as horizontal beds overlaid by pteropod-ooze deposits, or as at Tembe-ni-ndio by their forming part of a series of horizontal beds with a shelly limestone and a foraminiferous palagonite clay overlying them.
The fragments of bottle green basic glass vary usually between 1 and 4 millimetres. They were originally vacuolar and at times fibrillar from the lengthening out of the minute steam-pores; but through the palagonitic change these characters have been often disguised, and it is only at times that the unaltered glass is observed. Plagioclase and sometimes augite and occasionally olivine formed phenocrysts in the original glass. The zeolites, which include chabazite and natrolite, may be so extensively developed that they make up a fourth or a fifth of the rock mass. One may observe them in cavities where the walls are lined by fibrous natrolite with the cube-like crystals of chabazite occupying the interior. The calcite is usually subordinate to the zeolites, but sometimes the tuff contains as much as 10 per cent. of this mineral, which is evidently of secondary origin.... The history of these tuffs in the district of Nandua and Ulu-i-ndali is no doubt applicable to these deposits in other localities. They are the products of submarine eruptions which shattered into fragments the extensive palagonite crusts of flows of basaltic lava. In [Chapter XXIV.] I have attempted to show how palagonite is formed on a large scale in the case of such submarine displays of volcanic activity.
Chocolate-coloured Foraminiferous Palagonite-marls
We have here hard, somewhat calcareous, clay-rocks which consist in great part (nine-tenths) of fine palagonite debris with some fragments of minerals and a little fine detritus of semi-vitreous basic rocks. Some hand-specimens would be taken for pure palagonite; but the fragmental nature appears at once in the slide. This is especially the case with a rock exposed in a stream-course near Rewa on the shores of Savu-savu Bay (see page [95]). The materials composing them are exceedingly fine, the largest fragments not usually exceeding ·2 mm. As a rule they contain a little carbonate of lime and sometimes as much as 10 per cent., whilst a few tests of minute foraminifera are to be noticed in the slide. These deposits are horizontally bedded, and underlie a pteropod-ooze rock at Nandua and a shelly impure limestone at Tembe-ni-ndio. They are not very frequent, and sometimes approach in characters the volcanic-mud rocks, which, however, are much more mixed in composition. I regard them in the main as sedimentary deposits derived from the disintegration of the palagonitised vitreous surface of a submarine basaltic flow. They pass downward at Nandua, as described on page [345], into a rock of pure palagonite; and they are only to be found in localities where basaltic plains or plateaux are covered over with submarine deposits.
Acid Pumice Tuffs
The general characters of these deposits are described on pages [10], [218], [220], [222], [223], [231], [233], &c. Such tuffs are restricted to the north-east part of the island east of Lambasa and Tawaki, and are well displayed in the coast cliffs. They are pale yellow or whitish, and are usually non-calcareous. They are composed of the debris of a vacuolar and fibrillar isotropic glass, nearly colourless and in some localities altered. Small crystals of quartz and of glassy felspar with bits of obsidian (up to 3 mm.) and lapilli of rhyolitic glass are inclosed in them. In places inclosed pieces of coral and coral rock indicate submarine deposition.
CHAPTER XXIV
PALAGONITE
From the sea-border to the mountain-top in almost every part of the island, palagonite occurs in a fragmental condition. It is only where tuffs are not found, as in the mountainous mass of Seatura, or where these deposits are formed of acid rocks as in the north-east portion of the island, that palagonite has not been observed. Perhaps, it is not too much to say that the later if not all the stages in the history of Vanua Levu are bound up with the history of this material. In this place I will only deal with certain features in the problem connected with the origin of palagonite which seem to receive further elucidation from my observations in this island. The literature is already extensive, and those interested in the matter will find in Zirkel’s Petrographie and in the Challenger Report on Deep-Sea Deposits by Murray and Renard a good introduction to the subject.
In Vanua Levu we are confronted with the same difficulty that has perplexed geologists in various parts of the world. If we expected to find in this island the source of the enormous quantities of the basic glass that are represented by the palagonite of the tuffs, we should look in vain. Basic or basaltic glass usually occurs in agglomerates in the form of tachylytic pitchstones, as described on page [312], and is also found at times in basic pumiceous tuffs, as described on page [333]; but it is far from frequent. Palagonite-rock, that is to say, a basaltic glass converted in mass into this substance, never came under my notice.
In order to clear the ground for the discussion of my own observations, I will quote from the report on deep-sea deposits above named. Fragments of basic glass undergoing the palagonite change are found everywhere in these deposits and especially in the red-clay areas. The hydro-chemical modifications determining the decomposition of these fragments into palagonite, and at the same time the formation of zeolites, have likewise resulted in the complete transformation of these lapilli into ferruginous argillaceous matter (p. [309]). The authors, however, of this report do not attribute the frequent occurrence of fragments of basic glass on the bottom of the ocean to the buoyant powers of basic pumice. Unfortunately, the problem does not permit of such a simple solution. Basic volcanic glass, writes Prof. Renard, though known only from a few geological formations and from a few eruptions of recent volcanoes at the surface of the continents, appears in abundance and in most typical form among the products of submarine eruptions, as if the deep oceans had been in some way specially favourable to the development of this lithological type (p. [299]).