erupted matter. The volcanic nature of these oceanic islands
is evidently an extension of that law, and the effect of those
same causes, whether chemical or mechanical, from which it
results that a vast majority of the volcanoes now in action
stand either near sea-coasts or as islands in the midst of the
sea.

The rocks of St. Paul appear from a distance of a brilliantly
white colour. This is partly owing to the dung of a
vast multitude of seafowl, and partly to a coating of a hard
glossy substance with a pearly lustre, which is intimately
united to the surface of the rocks. This, when examined
with a lens, is found to consist of numerous exceedingly
thin layers, its total thickness being about the tenth of an
inch. It contains much animal matter, and its origin, no
doubt, is due to the action of the rain or spray on the birds'
dung. Below some small masses of guano at Ascension, and
on the Abrolhos Islets, I found certain stalactitic branching
bodies, formed apparently in the same manner as the thin
white coating on these rocks. The branching bodies so closely
resembled in general appearance certain nulliporae (a family
of hard calcareous sea-plants), that in lately looking hastily
over my collection I did not perceive the difference. The
globular extremities of the branches are of a pearly texture,
like the enamel of teeth, but so hard as just to scratch plate-
glass. I may here mention, that on a part of the coast of
Ascension, where there is a vast accumulation of shelly sand,
an incrustation is deposited on the tidal rocks by the water
of the sea, resembling, as represented in the woodcut, certain
cryptogamic plants (Marchantiae) often seen on damp