they were perfectly circular, and consisted of the finest fibres of a brown colour, radiating with great exactness from a common centre.These little radiating stars are occasionally intersected, and portions are quite cut off by the fine, ribbon-like zones of colour in the agate.In the obsidian of Ascension, the halves of a sphaerulite often lie in different zones of colour, but they are not cut off by them, as in the agate.) In some fine specimens of partially devitrified glass, in the possession of Mr.Stokes, the sphaerulites are united into straight layers with even sides, parallel to each other, and to one of the outer surfaces, exactly as in the obsidian.These layers sometimes interbranch and form loops; but I did not see any case of actual intersection.They form the passage from the perfectly glassy portions, to those nearly homogeneous and stony, with only an obscure concretionary structure.In the same specimen, also, sphaerulites differing slightly in colour and in structure, occur embedded close together.Considering these facts, it is some confirmation of the view above given of the concretionary origin of the obsidian and natural sphaerulites, to find that M.Dartigues ("Journal de Physique" tome 59 1804pages 10, 12.), in his curious paper on this subject, attributes the production of sphaerulites in glass, to the different ingredients obeying their own laws of attraction and becoming aggregated.He is led to believe that this takes place, from the difficulty in remelting sphaerulitic glass, without the whole be first thoroughly pounded and mixed together; and likewise from the fact, that the change takes place most readily in glass composed of many ingredients.In confirmation of M.Dartigues' view, I may remark, that M.Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found that the sphaerulitic portions of devitrified glass were acted on both by nitric acid and under the blowpipe, in a different manner from the compact paste in which they were embedded.
COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, WITHTHOSE OF OTHER COUNTRIES.
I have been struck with much surprise, how closely the excellent description of the obsidian rocks of Hungary, given by Beudant ("Voyage en Hongrie" tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, 371, 377, 381.), and that by Humboldt, of the same formation in Mexico and Peru ("Essai Geognostique" pages 176, 326, 328.), and likewise the descriptions given by several authors (P.Scrope "Geological Transactions" volume 2second series page 195.Consult also Dolomieu "Voyage aux Isles Lipari" and D'Aubuisson "Traite de Geogn." tome 2 page 534.) 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.
(In Mr.Stokes' fine collection of obsidians from Mexico, I observe that the sphaerulites 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 sphaerulites, 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 sphaerulites 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 laminae, by specks of a black mineral, like the augitic or hornblendic specks in the rocks at Ascension.) 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.Sphaerulites 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 (Beudant "Voyage" tome 3 page 373.) of his "perlite lithoide globulaire" in every, even the most trifling particular, might have been written for the little brown sphaerulitic globules of the rocks of Ascension.
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 sphaerulites 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.
