The sphaerulites and the little nodules of obsidian in these rocks so closely resemble, in general form and structure, concretions in sedimentary deposits, that one is at once tempted to attribute to them an analogous origin.They resemble ordinary concretions in the following respects: in their external form,--in the union of two or three, or of several, into an irregular mass, or into an even-sided layer,--in the occasional intersection of one such layer by another, as in the case of chalk-flints,--in the presence of two or three kinds of nodules, often close together, in the same basis,--in their fibrous, radiating structure, with occasional hollows in their centres,--in the co-existence of a laminary, concretionary, and radiating structure, as is so well developed in the concretions of magnesian limestone, described by Professor Sedgwick.
("Geological Transactions" volume 3 part 1 page 37.) Concretions in sedimentary deposits, it is known, are due to the separation from the surrounding mass of the whole or part of some mineral substance, and its aggregation round certain points of attraction.Guided by this fact, I have endeavoured to discover whether obsidian and the sphaerulites (to which may be added marekanite and pearlstone, both of them occurring in nodular concretions in the trachytic series) differ in their constituent parts, from the minerals generally composing trachytic rocks.It appears from three analyses, that obsidian contains on an average 76 per cent of silica;from one analysis, that sphaerulites contain 79.12; from two, that marekanite contains 79.25; and from two other analyses, that pearlstone contains 75.62 of silica.(The foregoing analyses are taken from Beudant "Traite de Mineralogie" tome 2 page 113; and one analysis of obsidian from Phillips "Mineralogy.") Now, the constituent parts of trachyte, as far as they can be distinguished consist of feldspar, containing 65.21 of silica;or of albite, containing 69.09; of hornblende, containing 55.27 (These analyses are taken from Von Kobell "Grundzuge der Mineralogie" 1838.), and of oxide of iron: so that the foregoing glassy concretionary substances all contain a larger proportion of silica than that occurring in ordinary feldspathic or trachytic rocks.D'Aubuisson ("Traite de Geogn." tome 2 page 535.), also, has remarked on the large proportion of silica compared with alumina, in six analyses of obsidian and pearlstone given in Brongniart's "Mineralogy." Hence I conclude, that the foregoing concretions have been formed by a process of aggregation, strictly analogous to that which takes place in aqueous deposits, acting chiefly on the silica, but likewise on some of the other elements of the surrounding mass, and thus producing the different concretionary varieties.From the well-known effects of rapid cooling (This is seen in the manufacture of common glass, and in Gregory Watts's experiments on molten trap; also on the natural surfaces of lava-streams, and on the side-walls of dikes.) in giving glassiness of texture, it is probably necessary that the entire mass, in cases like that of Ascension, should have cooled at a certain rate; but considering the repeated and complicated alterations of nodules and thin layers of a glassy texture with other layers quite stony or crystalline, all within the space of a few feet or even inches, it is hardly possible that they could have cooled at different rates, and thus have acquired their different textures.
The natural sphaerulites in these rocks very closely resemble those produced in glass, when slowly cooled.(I do not know whether it is generally known, that bodies having exactly the same appearance as sphaerulites, sometimes occur in agates.Mr.Robert Brown showed me in an agate, formed within a cavity in a piece of silicified wood, some little specks, which were only just visible to the naked eye: these specks, when placed by him under a lens of high power, presented a beautiful appearance: