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Updated: June 6, 2025
In the vicinity of the silicious springs are seen deposits of white, yellow, red, and bluish-grey clays, overlaying one another in narrow strata-like variegated marl, manifestly the disintegrated produce of volcanic rocks transported hither by rain and stained with oxide of iron. These clays perhaps come from the same rocks from the disintegration of which the silicious earth has been formed.
We began our descent into this great valley when we left the source of the Bua; and now these low hills, called Ngalé or Ngaloa, though only 100 feet or so above the level we had left, showed that we had come to the shore of an ancient lake, which probably was let off when the rent of Kebra-basa on the Zambesi was made, for we found immense banks of well-rounded shingle above or, rather, they may be called mounds of shingle all of hard silicious schist with a few pieces of fossil-wood among them.
"It is impossible to avoid associating such a formation with the fine, smooth, homogeneous clays and schists, poor in fossils, but showing worm- tubes and tracks, and bunches of doubtful branching things, such as Oldhamia, silicious sponges, and thin-shelled peculiar shrimps.
The paste is silicious, fine-grained, and quite hard. The color is slightly ferruginous and clouded with fire stains from the baking. The body is ornamented with the engraved figure of a bird apparently intended for an eagle. The head, with its notched and strongly curved beak and conventionalized crest, occupies one side.
"'The remarkable structure of the geological feature of this islet led me to examine the south-east part, which was the most exposed to the weather, and where the disposition of the strata was, of course, more plainly developed. The base is a coarse granular, silicious sandstone, in which large pebbles of quartz and jaspar are imbedded.
The characteristic rock is a conglomerate of large and small stones, compacted by hard silicious paste, and stained mauve-purple apparently by manganese: we had seen it on the way to Shuwak; and the next day's march will pave the uplands with it.
Mixed among the white substance is a quantity of silicious clay of all sorts and conditions of color. This produces a variation in the appearance, but is merely in addition to what is otherwise marvelous in the extreme. Pearl gray, with terra cotta, red and green tints is the basic color of this boiling, seething mass, which seems to be continually at unrest and in a course of worry.
Sometimes these little specks of protoplasm are surrounded with beautifully formed "silicious shells a skeleton of radiating spiculae or crystal-clear concentric spheres of exquisite symmetry and beauty. " The simplest amoeba however, has no definite form; but the little mass moves about, expands and contracts, throws out projections on one side and draws them in on the other.
These spicules are of very varied though regular shape, but ordinarily assume a rod-like needle shape or else a stellate form. In silicious sponges the spicules are composed of silica, and are generally deposited around axial rods in concentric layers.
This consisted of a dross, or, rather, of the collected pieces of ore themselves, which, on account of the flinty contents of the stones composing the funnel, were transformed by the decomposition of the sulphurous metal into a porous mass, and which could not be converted into dross nor form combinations with silicious acid, being deficient in the base as well as in the requisite heat; and also of a very impure "stone," of from four to five kilograms weight, and containing from fifty to sixty per cent of copper.
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