along the coast. Its average height above the sea, measured from the upper
line of junction with the superincumbent basaltic lava, is about sixty
feet; and its thickness, although varying much from the inequalities of the
underlying formation, may be estimated at about twenty feet. It consists of
quite white calcareous matter, partly composed of organic debris, and
partly of a substance which may be aptly compared in appearance with
mortar. Fragments of rock and pebbles are scattered throughout this bed,
often forming, especially in the lower part, a conglomerate. Many of the
fragments of rock are whitewashed with a thin coating of calcareous matter.
At Quail Island, the calcareous deposit is replaced in its lowest part by a
soft, brown, earthy tuff, full of Turritellae; this is covered by a bed of
pebbles, passing into sandstone, and mixed with fragments of echini, claws
of crabs, and shells; the oyster-shells still adhering to the rock on which
they grew. Numerous white balls appearing like pisolitic concretions, from
the size of a walnut to that of an apple, are embedded in this deposit;
they usually have a small pebble in their centres. Although so like
concretions, a close examination convinced me that they were Nulliporae,
retaining their proper forms, but with their surfaces slightly abraded:
these bodies (plants as they are now generally considered to be) exhibit
under a microscope of ordinary power, no traces of organisation in their
internal structure. Mr. George R. Sowerby has been so good as to examine
the shells which I collected: there are fourteen species in a sufficiently
perfect condition for their characters to be made out with some degree of
certainty, and four which can be referred only to their genera. Of the
fourteen shells, of which a list is given in the Appendix, eleven are
recent species; one, though undescribed, is perhaps identical with a
species which I found living in the harbour of Porto Praya; the two
remaining species are unknown, and have been described by Mr. Sowerby.
Until the shells of this Archipelago and of the neighbouring coasts are
better known, it would be rash to assert that even these two latter shells