(PLATE 4. INCRUSTATION OF SHELLY SAND.)
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 Plate 4,
certain cryptogamic plants (Marchantiae) often seen on damp walls.
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