nd, 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.
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.
Moreover in sea-shells, it is known that the parts habitually
covered and shaded by the mantle of the animal, are of a paler
colour than those fully exposed to the light, just as is the case
with this incrustation. When we remember that lime, either as a
phosphate or carbonate, enters into the composition of the hard
parts, such as bones and shells, of all living animals, it is an
interesting physiological fact to find substances harder than the
enamel of teeth, and coloured surfaces as well polished as those of
a fresh shell, re-formed through inorganic means from dead organic
matter--mocking, also, in shape, some of the lower vegetable
productions. (1/6. Mr. Horner and Sir David Brewster have described
("Philosophical Transactions" 1836 page 65) a singular "artificial
substance resembling shell." It is deposited in fine, transparent,
highly polished, brown-coloured laminae, possessing peculiar
optical properties, on the inside of a vessel, in which cloth,
first prepared with glue and then with lime, is made to revolve
rapidly in water. It is much softer, more transparent, and contains
more animal matter, than the natural incrustation at Ascension; but
we here again see the strong tendency which carbonate of lime and
animal matter evince to form a solid substance allied to shell.)

We found on St. Paul's only two kinds of birds--the booby and the
noddy. The former is a species of gannet, and the latter a tern.
Both are of a tame and stupid disposition, and are so unaccustomed
to visitors, that I could have killed any number of them with my
geological hammer. The booby lays her eggs on the bare rock; but
the tern makes a very simple nest with seaweed. By the side of many
of these nests a small flying-fish was placed; which I su