[30] Journal de Physique, tome 59 (1804), pp. 10, 12.
[31] Idem, tome 60 (1805), p. 418.
Comparison of the obsidian beds and alternating strata of ascension, with those of other countries.—I have been struck with much surprise, how closely the excellent description of the obsidian rocks of Hungary, given by Beudant,[[32]] and that by Humboldt, of the same formation in Mexico and Peru,[[33]] and likewise the descriptions given by several authors[[34]] of the trachytic regions in the Italian islands, agree with my observations at Ascension. Many passages might have been transferred without alteration from the works of the above authors, and would have been applicable to this island. They all agree in the laminated and stratified character of the whole series; and Humboldt speaks of some of the beds of obsidian being ribboned like jasper.[[35]] They all agree in the nodular or concretionary character of the obsidian, and of the passage of these nodules into layers. They all refer to the repeated alterations, often in undulatory planes, of glassy, pearly, stony, and crystalline layers: the crystalline layers, however, seem to be much more perfectly developed at Ascension, than in the above-named countries. Humboldt compares some of the stony beds, when viewed from a distance, to strata of a schistose sandstone. Sphærulites are described as occurring abundantly in all cases; and they everywhere seem to mark the passage, from the perfectly glassy to the stony and crystalline beds. Beudant’s account[[36]] of his “perlite lithoide globulaire” in every, even the most trifling particular, might have been written for the little brown sphærulitic globules of the rocks of Ascension.
[32] “Voyage en Hongrie,” tome i, p. 330; tome ii, pp. 221 and 315; tome iii, pp. 369, 371, 377, 381.
[33] “Essai Géognostique,” pp. 176, 326, 328.
[34] P. Scrope, in “Geological Transactions,” vol. ii (second series) p. 195. Consult, also, Dolomieu’s “Voyage aux Isles Lipari,” and D’Aubuisson, “Traité de Géogn.,” tome ii, p. 534.
[35] In Mr. Stokes’ fine collection of obsidians from Mexico, I observe that the sphærulites are generally much larger than those of Ascension; they are generally white, opaque, and are united into distinct layers: there are many singular varieties, different from any at Ascension. 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. Tracing some of the less perfectly glassy zones, they are seen to become studded with minute white sphærulites, which become more and more numerous, until at last they unite and form a distinct layer: on the other hand, at Ascension, only the brown sphærulites unite and form layers; the white ones always being irregularly disseminated. Some specimens at the Geological Society, said to belong to an obsidian formation from Mexico, have an earthy fracture, and are divided in the finest parallel laminæ, by specks of a black mineral, like the augitic or hornblendic specks in the rocks at Ascension.
[36] Beudant’s “Voyage,” tome iii, p. 373.
From the close similarity in so many respects, between the obsidian formations of Hungary, Mexico, Peru, and of some of the Italian islands, with that of Ascension, I can hardly doubt that in all these cases, the obsidian and the sphærulites owe their origin to a concretionary aggregation of the silica, and of some of the other constituent elements, taking place whilst the liquified mass cooled at a certain required rate. It is, however, well-known, that in several places, obsidian has flowed in streams like lava; for instance, at Teneriffe, at the Lipari Islands, and at Iceland.[[37]] In these cases, the superficial parts are the most perfectly glassy, the obsidian passing at the depth of a few feet into an opaque stone. In an analysis by Vauquelin of a specimen of obsidian from Hecla, which probably flowed as lava, the proportion of silica is nearly the same as in the nodular or concretionary obsidian from Mexico. It would be interesting to ascertain, whether the opaque interior portions and the superficial glassy coating contained the same proportional constituent parts: we know from M. Dufrénoy[[38]] that the exterior and interior parts of the same stream of lava sometimes differ considerably in their composition. Even should the whole body of the stream of obsidian turn out to be similarly composed with nodular obsidian, it would only be necessary, in accordance with the foregoing facts, to suppose that lava in these instances had been erupted with its ingredients mixed in the same proportion, as in the concretionary obsidian.
[37] For Teneriffe, see von Buch, “Descript. des Isles Canaries,” pp. 184 and 190; for the Lipari Islands, see Dolomieu’s “Voyage,” p. 34; for Iceland, see Mackenzie’s “Travels,” p. 369.