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Updated: June 20, 2025
There was something striking about him, thought Mr. Heard. He said little of consequence, but Denis listened enthusiastically to his abstruse remarks about fractures and so forth, and watched with eagerness as he poked his stick into the rough walls to dislodge some stone that seemed to be of interest. "So you don't know the difference between augite and hornblende?" he once enquired. "Really?
Their rusty brown or yellowish colour is partly due to the oxides of iron, but chiefly to innumerable, microscopically minute, black specks, which, when a fragment is heated, are easily fused, and evidently are either hornblende or augite.
The steel-blue mineral, which is abundant in the pinnacle, but which disappears in the streams derived from the pinnacle, has a fused appearance, and rarely presents even a trace of cleavage; I obtained, however, one measurement, which proved that it was augite; and in one other fragment, which differed from the others, in being slightly cellular, and in gradually blending into the surrounding matrix the small grains of this mineral were tolerably well crystallised.
Below this were strata of trachitic breccia and augite; the formation was then seamy to an unknown depth. Within the reef, the surface soil was red, and of a sandy nature. The next stratum was of a loose, yellow, gravelly lime, and the third blue, of a hard, slaty nature. This last was the real diamantiferous soil.
PORPHYRY is one of this class, and very characteristic of the volcanic formations. Thus trachyte is usually porphyritic; for in it, as in many modern lavas, there are crystals of feldspar; but in some porphyries the crystals are of augite, olivine, or other minerals.
A ribbon of beach not more than fifteen yards wide, composed of iron-sand, augite, and pyroxene, running along under the basaltic precipice upwards of a thousand feet high which serves as a kind of plinth to the mountain, was the only standing room this part of the coast afforded.
Their resemblance to the great trappean series of Western India, even in minute particulars, is referred to by Mr. Blanford, who suggests the view that they belong to one and the same great series of lava-flows extruded over the surface of this part of the globe. This view is inherently probable. Beds of volcanic ash or breccia also frequently occur, and often contain augite crystals.
Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other, by melting and recrystallising, does not perhaps demonstrate their absolute identity. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest.
Applying these facts to natural productions, we have reason to believe that clay-slate, and many kinds of greywacke, are amorphous feldspar, as transition limestone is amorphous marble, basalt and lava mixtures of amorphous zeolite and augite. Anything that influences the cohesion, must also in a certain degree alter the properties of bodies.
On the beach of this promontory, a quadrant-shaped segment of a small subordinate point of eruption stands exposed. It consists of nine separate little streams of lava piled upon each other; and of an irregular pinnacle, about fifteen feet high, of reddish-brown, vesicular basalt, abounding with large crystals of glassy albite, and with fused augite.
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