of the trachytic series, or at least in rendering that structure apparent. [49] “Mém. pour servir,” etc., tome iv, p. 131. [50] _Edinburgh New Phil. Journal,_ 1842, p. 350. [51] I presume that this is nearly the same explanation which Mr. Scrope had in his mind, when he speaks (“Geolog. Transact.,” vol. ii, second series, p. 228) of the ribboned structure of his trachytic rocks, having arisen, from “a linear extension of the mass, while in a state of imperfect liquidity, coupled with a concretionary process.” [52] Basaltic lavas, and many other rocks, are not unfrequently divided into thick laminæ or plates, of the same composition, which are either straight or curved; these being crossed by vertical lines of fissure, sometimes become united into columns. This structure seems related, in its origin, to that by which many rocks, both igneous and sedimentary, become traversed by parallel systems of fissures. Whatever may be thought of the explanation here advanced of the laminated structure of the rocks of the trachytic series, I venture to call the attention of geologists to the simple fact, that in a body of rock at Ascension, undoubtedly of volcanic origin, layers often of extreme tenuity, quite straight, and parallel to each other, have been produced;—some composed of distinct crystals of quartz and diopside, mingled with amorphous augitic specks and granular feldspar,—others entirely composed of these black augitic specks, with granules of oxide of iron,—and lastly, others formed of crystalline feldspar, in a more or less perfect state of purity, together with numerous crystals of feldspar, placed lengthways. At this island, there is reason to believe, and in some analogous cases, it is certainly known, that the laminæ have originally been formed with their present high inclination. Facts of this nature are manifestly of importance, with relation to the structural origin of that grand series of plutonic rocks, which like the volcanic have undergone the action of heat, and which consist of alternate layers of quartz, feldspar, mica and other minerals. Chapter IV ST. HELENA Lavas of the feldspathic, basaltic, and submarine series.—Section of Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge and a parapet. Cones of phonolite. Superficial beds of calcareous sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the land.—Denudation.—Craters of elevation.