should come to be of slightly different composition and of different textures. [45] See Phillips’ “Mineralogy,” for the Italian Islands, p. 136. For Mexico and Peru, see Humboldt’s “Essai Géognostique.” 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. [46] “Geological Transactions,” vol. ii (second series), p. 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 iii, p. 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. [47] Dolomieu’s “Voyage,” p. 64. 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 sphærulites 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.[48] 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. [48] 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 sphærulitic globules (which behave like feldspar under the blowpipe) in this same direction.