This island is situated in the Atlantic ocean, in lat. 8° S. long. 14° W. It has the form of an irregular triangle, (see accompanying map,) each side being about six miles in length. Its highest point is 2,870 feet above the level of the sea. The whole is volcanic, and, from the absence of proofs to the contrary, I believe of subaërial origin. The fundamental rock is everywhere of a pale colour, generally compact, and of a feldspathie nature. In the S.E. portion of the island, where the highest land is situated, well characterized trachyte, and other congenerous rocks of that varying family, occur. Nearly the entire circumference is covered up by black and rugged streams of basaltic lava, with here and there a hill or single point of rock (one of which near the sea-coast, north of the Fort, is only two or three yards across) of the trachyte still remaining exposed.

Basaltic rocks.—The overlying basaltic lava is in some parts extremely vesicular, in others little so; it is of a black colour, but sometimes contains crystals of glassy feldspar, and seldom much olivine. These streams appear to have possessed singularly little fluidity; their side walls and lower ends being very steep, and even as much as between twenty and thirty feet in height. Their surface is extraordinarily rugged, and from a short distance appears as if studded with small craters.