My idea that I was dealing with a clay-rock affected by the proximity of an igneous intrusion was dispelled when the powdered material presented itself as pure palagonite with scarcely any mineral fragments. Unlike the marl above, it does not effervesce with an acid; and appears as a mass of compacted minute fragments of basic glass converted into palagonite, which is seemingly non-vacuolar, and containing about 15 per cent. of water.
In connection with the diagram it should be remarked that I did not find the palagonite-rock actually passing down into the basalt which, however, is exposed in the river-bed below. The whole district is characterised by columnar basalt, and the series of deposits here described have been formed on the flank of the great basaltic table-land of Wainunu. It is noteworthy that in the uppermost deposits of the pteropod-ooze palagonite forms a noticeable proportion (10-20 per cent.) of the residue; and perhaps most of the fine clayey material is thus derived. As noted on page [321], minute pellets of pure palagonite are not infrequent in the residue. Probably about 90 per cent. of the underlying marl consists of palagonite. In the lowest palagonite-rock the proportion would be quite 98 per cent.
Diagram showing the succession of deposits below the Nandua tea-estate. The total thickness is about 250 feet. The figures refer to the proportion of palagonite.
Whilst it is apparent that we have represented in this series the covering of a submarine basaltic flow with submarine deposits, it is also evident that the mode of junction between the flow and the overlying deposits is of an unexpected nature. Before drawing any inferences, it is necessary to point out that when we begin on à priori grounds to frame our notions as to the course of events on the surface of a submarine basaltic flow, we are entering a little known region of inquiry. I would, however, suggest in the light of the theory before advanced, the following explanation of the appearances presented by this series.
During the consolidation of the flow much of the magma-residuum that still retained its fluidity was extruded on the surface, where after solidification it became palagonitised. According to my view this would be the typical behaviour of submarine basaltic flows; but, owing to the unstable and perishable nature of the palagonitic crust of the flow, it would be rarely preserved in upheaved volcanic regions. There would probably be, as in the case of the Nandua series, no sharp line to be drawn between the palagonite-crust and the deposits subsequently covering it, deposits indeed that would derive no inconsiderable proportion of their materials from the disintegration of the crust itself. During and after the emergence of such a district of submarine eruptions the unstable palagonitic crust would be further subjected to the hydration resulting from weathering and similar agencies; and as a result of its final degradation there would often alone remain a bed of reddish argillaceous material.
In concluding these remarks on palagonite the following summary of the principal points here dwelt upon may be added:
(a) The basic glass, that undergoes the palagonitic change, is the vitreous form of the magma-residuum that in a particular type of basalt and under certain conditions remains fluid after the mass of the rock has solidified. During the last stage of the consolidation it is in part imprisoned in the “magma-lakelets” of the groundmass; whilst the rest of it is squeezed into cracks and fissures, or extruded on the surface of the flow.
(b) This glass differs from ordinary basic glass in its molecular condition, its mineral composition, its low degree of fusibility, and in its unstable character.
(c) The formation of palagonite in connection with the crushing of a basic glass is to be explained by the hypothesis that the heat developed during the crushing is sufficient to partially re-fuse the glass, the material thus produced corresponding to the magma-residuum of low degree of fusibility, which is above referred to.