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
walls. The surface of the fronds is beautifully glossy; and
those parts formed where fully exposed to the light are of a
jet black colour, but those shaded under ledges are only grey.
I have shown specimens of this incrustation to several
geologists, and they all thought that they were of volcanic
or igneous origin! In its hardness and translucency - in
its polish, equal to that of the finest oliva-shell - in the
bad smell given out, and loss of colour under the blowpipe - it
shows a close similarity with living sea-shells.
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