第40章

The laminae of the beds, alternating with the obsidian at Ascension, dip at a high angle under the mountain, at the base of which they are situated;and they do not appear as if they had been inclined by violence.A high inclination is common to these beds in Mexico, Peru, and in some of the Italian islands (See Phillips "Mineralogy" for the Italian Islands page 136.For Mexico and Peru see Humboldt "Essai Geognostique." Mr.Edwards also describes the high inclination of the obsidian rocks of the Cerro del Navaja in Mexico in the "Proc.of the Geolog.Soc." June 1838.): on the other hand, in Hungary, the layers are horizontal; the laminae, also, of some of the lava-streams above referred to, as far as I can understand the descriptions given of them, appear to be highly inclined or vertical.Idoubt whether in any of these cases, the laminae have been tilted into their present position; and in some instances, as in that of the trachyte described by Mr.Scrope, it is almost certain that they have been originally formed with a high inclination.In many of these cases, there is evidence that the mass of liquified rock has moved in the direction of the laminae.At Ascension, many of the air-cells have a drawn out appearance, and are crossed by coarse semi-glassy fibres, in the direction of the laminae; and some of the layers, separating the sphaerulitic globules, have a scored appearance, as if produced by the grating of the globules.I have seen a specimen of zoned obsidian from Mexico, in Mr.Stokes' collection, with the surfaces of the best-defined layers streaked or furrowed with parallel lines; and these lines or streaks precisely resembled those, produced on the surface of a mass of artificial glass by its having been poured out of a vessel.Humboldt, also, has described little cavities, which he compares to the tails of comets, behind sphaerulites in laminated obsidian rocks from Mexico, and Mr.Scrope has described other cavities behind fragments embedded in his laminated trachyte, and which he supposes to have been produced during the movement of the mass.("Geological Transactions" volume 2 second series page 200 etc.These embedded fragments, in some instances, consist of the laminated trachyte broken off and "enveloped in those parts, which still remained liquid." Beudant, also, frequently refers in his great work on "Hungary" tome 3 page 386, to trachytic rocks, irregularly spotted with fragments of the same varieties, which in other parts form the parallel ribbons.In these cases, we must suppose, that after part of the molten mass had assumed a laminated structure, a fresh irruption of lava broke up the mass, and involved fragments, and that subsequently the whole became relaminated.) From such facts, most authors have attributed the lamination of these volcanic rocks to their movement whilst liquified.Although it is easy to perceive, why each separate air-cell, or each fibre in pumice-stone (Dolomieu "Voyage"page 64.), should be drawn out in the direction of the moving mass; it is by no means at first obvious why such air-cells and fibres should be arranged by the movement, in the same planes, in laminae absolutely straight and parallel to each other, and often of extreme tenuity; and still less obvious is it, why such layers should come to be of slightly different composition and of different textures.

In endeavouring to make out the cause of the lamination of these igneous feldspathic rocks, let us return to the facts so minutely described at Ascension.We there see, that some of the thinnest layers are chiefly formed by numerous, exceedingly minute, though perfect, crystals of different minerals; that other layers are formed by the union of different kinds of concretionary globules, and that the layers thus formed, often cannot be distinguished from the ordinary feldspathic and pitchstone layers, composing a large portion of the entire mass.The fibrous radiating structure of the sphaerulites seems, judging from many analogous cases, to connect the concretionary and crystalline forces: the separate crystals, also, of feldspar all lie in the same parallel planes.(The formation, indeed, of a large crystal of any mineral in a rock of mixed composition implies an aggregation of the requisite atoms, allied to concretionary action.The cause of the crystals of feldspar in these rocks of Ascension, being all placed lengthways, is probably the same with that which elongates and flattens all the brown sphaerulitic globules (which behave like feldspar under the blowpipe) in this same direction.) These allied forces, therefore, have played an important part in the lamination of the mass, but they cannot be considered the primary force; for the several kinds of nodules, both the smallest and largest, are internally zoned with excessively fine shades of colour, parallel to the lamination of the whole;and many of them are, also, externally marked in the same direction with parallel ridges and furrows, which have not been produced by weathering.