must have been derived from the sea-water. It is an interesting
circumstance thus to find the waves of the ocean, sufficiently charged with
sulphate of lime, to deposit it on the rocks, against which they dash every
tide. Dr. Webster has described ("Voyage of the 'Chanticleer'" volume 2
page 319) beds of gypsum and salt, as much as two feet in thickness, left
by the evaporation of the spray on the rocks on the windward coast.
Beautiful stalactites of selenite, resembling in form those of carbonate of
lime, are formed near these beds. Amorphous masses of gypsum, also, occur
in caverns in the interior of the island; and at Cross Hill (an old crater)
I saw a considerable quantity of salt oozing from a pile of scoriae. In
these latter cases, the salt and gypsum appear to be volcanic products.)
From this source it derives its animal matter, which is evidently the
colouring principle. The nature of the deposit, in its incipient stage, can
often be well seen upon a fragment of white shell, when jammed between two
of the fronds; it then appears exactly like the thinnest wash of a pale
grey varnish. Its darkness varies a little, but the jet blackness of some
of the fronds and of the botryoidal masses seems due to the translucency of
the successive grey layers. There is, however, this singular circumstance,
that when deposited on the under side of ledges of rock or in fissures, it
appears always to be of a pale, pearly grey colour, even when of
considerable thickness: hence one is led to suppose, that an abundance of
light is necessary to the development of the dark colour, in the same
manner as seems to be the case with the upper and exposed surfaces of the
shells of living mollusca, which are always dark, compared with their under
surfaces and with the parts habitually covered by the mantle of the animal.
In this circumstance,--in the immediate loss of colour and in the odour
emitted under the blowpipe,--in the degree of hardness and translucency of
the edges,--and in the beautiful polish of the surface (From the fact
described in my "Journal of Researches" of a coating of oxide of iron,
deposited by a streamlet on the rocks in its bed (like a nearly similar