contrary, I believe of subaerial origin. The fundamental rock is everywhere
of a pale colour, generally compact, and of a feldspathic nature. In the
S.E. portion of the island, where the highest land is situated, well
characterised 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. These projections consist of broad,
irregularly conical, hillocks, traversed by fissures, and composed of the
same unequally scoriaceous basalt with the surrounding streams, but having
an obscure tendency to a columnar structure; they rise to a height between
ten and thirty feet above the general surface, and have been formed, as I
presume, by the heaping up of the viscid lava at points of greater
resistance. At the base of several of these hillocks, and occasionally
likewise on more level parts, solid ribs, composed of angulo-globular
masses of basalt, resembling in size and outline arched sewers or gutters
of brickwork, but not being hollow, project between two or three feet above
the surface of the streams; what their origin may have been, I do not know.
Many of the superficial fragments from these basaltic streams present
singularly convoluted forms; and some specimens could hardly be