This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by dikes, many of which preserve a remarkable parallelism in a N.N.W. and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic with large crystals of augite; others are formed of a fine-grained and brown-coloured trap. Most of these dikes are coated by a glossy layer,[3] from one to two-tenths of an inch in thickness, which, unlike true pitchstone, fuses into a black enamel; this layer is evidently analogous to the glossy superficial coating of many lava streams. The dikes can often be followed for great lengths both horizontally and vertically, and they seem to preserve a nearly uniform thickness:[4] Mr. Seale states, that one near the Barn, in a height of 1,260 feet, decreases in width only four inches,—from nine feet at the bottom, to eight feet and eight inches at the top. On the ridge, the dikes appear to have been guided in their course, to a considerable degree, by the alternating soft and hard strata: they are often firmly united to the harder strata, and they preserve their parallelism for such great lengths, that in very many instances it was impossible to conjecture, which of the beds were dikes, and which streams of lava. The dikes, though so numerous on this ridge, are even more numerous in the valleys a little south of it, and to a degree I never saw equalled anywhere else: in these valleys they extend in less regular lines, covering the ground with a network, like a spider’s web, and with some parts of the surface even appearing to consist wholly of dikes, interlaced by other dikes. [3] This circumstance has been observed (Lyell, “Principles of Geology,” vol. iv, chap. x, p. 9) in the dikes of the Atrio del Cavallo, but apparently it is not of very common occurrence. Sir G. Mackenzie, however, states (p. 372, “Travels in Iceland”) that all the veins in Iceland have a “black vitreous coating on their sides.” Captain Carmichael, speaking of the dikes in Tristan d’Acunha, a volcanic island in the Southern Atlantic, says (“Linnæan Transactions,” vol. xii, p. 485) that their sides, “where they come in contact with the rocks, are invariably in a semi-vitrified state.” [4] “Geognosy of the Island of St. Helena,” plate 5.