The rocks exposed in the sides of the gorge are basic agglomerates overlying palagonitic tuffs of mixed composition and evidently sedimentary. On the bottom lie huge masses, some of them 70 or 80 tons in weight, of altered grey aphanitic or non-porphyritic augite-andesites, penetrated in some cases by thin veins of white quartz, and at times displaying a rudely columnar structure, the columns being 12 to 14 inches across. Sometimes the alteration is mainly confined to the filling of the fissures with chalcedonic quartz, minute nests of the same material occurring in the groundmass. At other times the small augite granules are also decomposing. The specific gravity varies from 2·64 to 2·73; the rocks being referred to genus 16, species A, of the augite-andesites. Occasional detached masses of a propylitic basic andesite, displaying porphyritic plagioclase and pyroxene, also occur in this gorge, the felspar phenocrysts being largely occupied by calcitic and other alteration products, whilst much viridite occurs in the groundmass. It exhibits both monoclinic and rhombic pyroxene; and on account of the prism form of the groundmass pyroxene it is placed in the 2nd sub-order of the hypersthene-augite andesites. These altered rocks are deep-seated intrusive masses that were originally covered over by the basic agglomerates and palagonite-tuffs exposed in the sides of the gorge.
Below the gorge there is an extensive exposure in the sides and bed of the river of light-coloured calcareous tuffs which were originally composed of palagonitic materials; but owing partly to hydration, and partly to other secondary changes, the original structure is much disguised.
Crossing the river in the midst of these tuffs there is a dyke, 15 feet thick, formed of a propylitic basaltic andesite, a semi-vitreous rock in which calcitic and zeolitic materials have been developed in quantity. The dyke, which is not columnar, is steeply inclined at an angle of 45° to the north-east.... Further down the river-valley as far as Narengali, occur basic tuff-agglomerates.
The Tract of Nakambuta
This is a tract of broken country that projects from the mountainous backbone of the island (between the Va-lili and Koro-tini ranges) into the heart of the Ndreketi plains in the vicinity of Natua. As limited by the 300-feet contour line, it is indicated in the map attached to this work. Its general level varies between 300 and 600 feet in elevation; but a number of isolated peaks are included within this area. More than one of these hills attain a height of 1,000 feet, Nakambuta a very conspicuous hill being as much as 1,500 feet. Basaltic andesites with basic agglomerates and palagonite-tuffs prevail.
Towards Natua the basaltic andesites, which are often much decomposed, are of the doleritic type referred to in the account of the Ndreketi Plains on page [133]. Inland, towards Narengali and Va-lili, these rocks are often more or less glassy and take the form of pitchstones; whilst the agglomerates have the same character. The first probably represent submarine flows of basaltic lava which have spread far and wide over the Ndreketi plains. The inland rocks are, as is pointed out below, the products of vents that, as in the case no doubt of Mount Nakambuta, rose out of a shallow sea. The palagonite-tuffs and clays, often foraminiferous, which cover the Ndreketi plains, are extensively represented in the lower levels up to 400 feet or more.
Between one and two miles to the westward of the Narengali valley, and immediately north of Va-lili, the agglomerates, overlying palagonite-tuffs, form lofty precipices. The agglomerates are composed of blocks of more or less vitreous basaltic-andesites, some of them semi-vitreous and amygdaloidal, some in the form of pitchstone, and others again as tachylyte that fuses in the lamp-flame. The underlying palagonite-tuffs are bedded, and are composed of fragments of basic glass that originally inclosed porphyritic crystals of plagioclase. In the slide it is observed that the glass and mineral fragments have often been re-fractured as they lie in the tuff and that the former have rounded angles and eroded edges. The interstices are filled with a more or less palagonitised magma. Similar rocks occur in other localities, and they will all be found described on page [334]. It may, however, be remarked here that in all cases these rocks would seem to have undergone some crushing, the heat developed in the process being sufficient to partially remelt the glass. A high temperature was not required to effect this fusion, since splinters of the tachylyte occurring in the overlying agglomerate fuse in an ordinary lamp-flame. It is pointed out on page [341] that tuffs of this character differ in origin and in characters from the prevailing foraminiferous palagonite-tuffs.
The road from Narengali to Natua traverses the length of this district. At and near the mouth of the Narengali valley there are exposed basic tuffs and agglomerates, the blocks in the last case being formed of a semi-vitreous, vesicular or almost scoriaceous basaltic andesite. In this neighbourhood the track passes across the top of a waterfall which is the result of the existence of a huge dyke-like mass of a compact basaltic andesite showing a little interstitial glass and referred to genus 13 of the augite-andesites. It lies in a district of tuffs and agglomerates. Farther on, about two miles north-west of Narengali, the track crosses some rounded hills, elevated about 600 feet, on the top of which is displayed a concretionary pitchstone, showing little nodular concretions of the size of filberts, and having the microscopical characters of “variolite,” as described on page [313]. This is the only locality of this rock that is known to me.
My acquaintance with the tract of Nakambuta is, however, very imperfect. But it is apparent that in the pitchstones and in the semi-vitreous basic rocks, sometimes vesicular and amygdaloidal, we get a nearer approach to the products of subaerial eruptions than is to be observed in most other portions of the island. The examination of the Nakambuta peak by some future investigator will bring to light some interesting facts concerning this region. It is not unlikely that during a late stage of the emergence of this region Nakambuta and the other peaks around protruded as active vents above the surface of a shallow sea, at the bottom of which the products of their eruptions accumulated.