Many of the Volcanic Islands thrown up above the sea-level soon
disappear, because the lavas and conglomerates of which they are formed
spread over flatter surfaces, through the weight of the incumbent
fluid; and the constant levelling process goes on below the sea by the
action of tides and currents. Such islands as have effectually resisted
this action are found to possess a solid framework of lava, supporting
or defending the loose fragmentary materials.
Among the most celebrated of these phenomena in our times may be
mentioned the Isle of Sabrina, which rose off the coast of St.
Michael’s in 1811, attained a circumference of one mile and a
height of 300 feet, and disappeared in less than eight months; in
the following year there were eighty fathoms of water in its place.
In July 1831 appeared Graham’s Island off the coast of Sicily,
which attained a mile in circumference and 150 or 160 feet in
height; its formation much resembled that of Sabrina.
The line of ancient subterranean fire which we trace on the
Mediterranean coasts has had a strange attestation in Graham’s Island,
which is also described as a volcano suddenly bursting forth in the mid
sea between Sicily and Africa; burning for several weeks, and throwing
up an isle, or crater-cone of scoriæ and ashes, which had scarcely been
named before it was again lost by subsidence beneath the sea, leaving
only a shoal-bank to attest this strange submarine breach in the
earth’s crust, which thus mingled fire and water in one common action.
Floating islands are not very rare: in 1827, one was seen twenty
leagues to the east of the Azores; it was three leagues in width, and
covered with volcanic products, sugar-canes, straw, and pieces of wood.