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Updated: June 19, 2025
The obsidians are finely zoned, in quite straight or curved lines, with exceedingly slight differences of tint, of cellularity, and of more or less perfect degrees of glassiness.
But whether obsidians be primitive rocks which have undergone the action of volcanic fire, or lavas repeatedly melted within the crater, the origin of the pumice-stones contained in the obsidian of the Peak of Teneriffe is not less problematic.
Don Juan de Larea, one of those men lately sacrificed to the fury of faction, had been struck with the phenomena exhibited by obsidians exposed to a white heat.
The experiments of M. da Camara, and those I made in 1802, tend to support the opinion, that the pumice-stones adherent to the obsidians of the Peak of Teneriffe do not unite to them accidentally, but are produced by the expansion of an elastic fluid, which is disengaged from the compact vitreous matter.
The oblong bubbles observed in the obsidians of every continent are incontestible proofs of their ancient state of igneous fluidity; and Dr. Thompson possesses specimens from Lipari, which are very instructive in this point of view, because fragments of red porphyry, or porphyry lavas, which do not entirely fill up the cavities of the obsidian, are found enveloped in them.
We might suppose that they had been thrown out in a softened state, and had afterwards been subject to a rotary motion. They contain a quantity of vitreous feldspar, of a snow-white colour, and the most brilliant pearly lustre. These obsidians are, nevertheless, but little transparent on the edges; they are almost opaque, of a brownish black, and of an imperfect conchoidal fracture.
Nor were there any igneous stones or red-hot cinders mingled with the smoke that crowned the summit; a circumstance that quite accorded with the absence of the pumice-stones, obsidians, and other minerals of volcanic origin with which the base of a burning mountain is generally strewn. Captain Servadac was of opinion that this peculiarity augured favorably for the continuance of the eruption.
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