Supplementary note on the occurrence of palagonite in the glassy matrix of pitchstone-agglomerates and in rubbly pitchstones.—In my last revision of the proofs I find that I have not laid sufficient stress on the production of palagonite under these conditions. The evidence of crushing is often very evident, and especial references to this point will be found in the index under “Pitchstone,” and on page [334] under “Crush-tuffs.”
CHAPTER XXV
SILICIFIED CORALS AND FLINTS
Silicified corals, together with siliceous minerals (quartz, chalcedony, jasper, &c.) and siliceous concretions are evidently widely distributed in these islands. Kleinschmidt in his journal refers to large blocks of flint on the island of Ono, from which the natives used to obtain their musket-flints,[[133]] and he collected from this island as well as from Viti Levu, Ovalau, &c., numerous specimens of these and other siliceous minerals and rocks, such as hornstone, chalcedony and jasper, which were examined by Wichmann and described in his paper.[[134]] Mr. Andrews observed silicified corals on the summits and higher slopes of Vanua Mbalavu.[[135]] The Fijian name for flints, “ngiwa” (thunderbolt) or “vatu-ngiwa” (stone-thunderbolt), affords a good instance of that curious superstition connected with the origin of these stones, which came also under my notice in the Solomon Islands,[[136]] and in fact is widely spread.
In Vanua Levu these siliceous rocks and minerals are in places abundant. They are especially frequent on the surface of the extensive low plains on the north side of the island which constitute the basins of the Sarawanga, Ndreketi, Wailevu, and Lambasa rivers; but it is in the low-lying district of Kalikoso, in the north-eastern part, that they exist in the greatest quantity. They do not occur usually at greater elevations than 300 feet, and are found as a rule at much lower levels.
It must be understood that reference is not here made to quartz-veins, such as are found in certain localities and of which mention is made on pages [106], [116]. It is not with the ordinary products of contact or general metamorphism that we have here to deal; but with the remarkable surface-collections of silicified corals, nodules and flints of chalcedony, fragments of white quartz-rock, bits of jasper, and certain curious siliceous concretions, that occur often in association with fragments of limonite in these low-lying regions. All the siliceous materials above named have, as the microscope indicates, a common character, chalcedonic silica in a greater or less degree being the basis of all of them, whether coral, flint, white quartz-rock, or jasper. It soon became apparent whilst examining these districts that one general condition prevailed whilst this extensive deposition of silica and the formation of the beds of limonite were in progress. It cannot, however, be pretended that these processes are actually in operation on the plains now. Except in the case of the limonite in a few localities the processes have been suspended; but they were in active operation not long ago: and an examination of the general characters of the districts will probably disclose some of the conditions under which these products have been formed.
On the surface of the Kalikoso plains, where these materials are most abundant, we find silicified corals associated with fragments and nodules of chalcedony, flints, white quartz-rock, limonite, concretions of carbonate of iron, &c., in the low-lying and often swampy district around the fresh-water lake, the whole region being only elevated between 20 and 60 feet above the sea. This is an area of decomposing acid rocks (quartz-porphyries, trachytes).[[137]] On the other hand in most of the regions where these materials occur on the surface we have areas of basic rocks (basalts and basaltic andesites) incrusted in places with submarine tuffs and foraminiferous clays, the volcanic rocks undergoing extensive disintegration. Such for instance are the Lekutu, Sarawanga, Ndreketi, and Lambasa plains. In the Lambasa plains, which are described in this connection on page [139], we find besides the corals and flints and nodules of chalcedony, fragments of jasper. In the Sarawanga and Lekutu lowlands, we find silicified corals and limonite; but here the crystallised silica of the corals contains a large quantity of water, whilst in its lesser degree of hardness and in its low specific gravity it comes near to semi-opal. In these and other localities, as in the level country around Ndranimako on the right side of the Yanawai estuary, we find curious concretions of the same kind of hydrous silica more or less crystalline. These concretions are described below.
It may be remarked that nearly all the districts in which the silicified corals and concretions, siliceous minerals, and limonite occur, are scantily vegetated “talasinga” lands[[138]] with reddish soil. Except in the instance of the Kalikoso plains, the swamps and lakes have as a rule long since disappeared, their sites being alone indicated by the limonite on the surface. In the Mbua plains, however, there are occasional small ponds and swamps, and there is no doubt that the limonite so bountifully represented on the dry districts is still in process of formation.
Before drawing some general inferences as to the conditions under which this deposition of silica and iron took place, I will refer to the characters of the materials thus produced.
The silicified corals include massive corals of the Astraean and “Porites” kinds and branching specimens of the Madrepore type or habit. The former are rarely larger than 7 or 8 inches across and are merely fragments. The latter are always portions of branches, never exceeding 3 or 4 inches in length. In the last case it is sometimes possible to show, as in the case of a specimen found on the Kalikoso plains, that before silicification occurred the dead fragment of branching coral had been extensively eroded by solvent agencies and had been penetrated by burrowing molluscs. The larger blocks of massive corals have usually been extensively chipped by the natives in obtaining flints. In past times they were carried from one place to another, the result being that occasionally they were brought to me in the mountain-villages, all showing evidence of their having supplied flints to a past generation.
These corals are as a rule completely silicified. When a massive specimen is broken across it is not infrequently found that whilst the coral structure is preserved in its outer part, the inner portion is composed of a compact seemingly structureless mass of bluish-white or pale-grey flint, which has the characteristic microscopical appearance of chalcedony and a specific gravity of 2·59.[[139]] It is from the more compact parts of the silicified massive corals that the “worked” flints found on the surface were obtained, though in some of them, as in the case of a “scraper” in my collection, the traces of coral structure are still apparent to the eye. Wichmann observed in the case of the silicified corals from Fiji that the whole petrifying process appears to consist in the saturation of the coral with silica, the coral structure being usually distinct, whilst the septa, often still calcitic, show the points of the calcite crystals projecting into the chalcedony which forms the mass. Lime however rarely occurs in the silicified corals of Vanua Levu. It was only in the case of one or two localities that the corals displayed any effervescence with an acid. In the microscope slide the massive specimens appear to be entirely of chalcedonic silica, the outlines of the cells and of the septa being indicated by ferruginous material. In a specimen of Porites by my side the crystallization of the silica has advanced beyond the chalcedonic stage and the coral is composed entirely of minute quartz-crystals, ·2 to ·4 mm. in size, often irregular, but sometimes forming doubly-terminated prisms. This has produced a somewhat crumbling rock, which is easily powdered by the finger; and in this case, therefore, the complete crystallization of the silica is resulting in the disintegration of the silicified coral.