iron, together with a black powder, which is not soluble in heated acids.
This latter substance seems to be carbonaceous, and is evidently the
colouring matter. The sulphate of lime is extraneous, and occurs in
distinct, excessively minute, lamellar plates, studded on the surface of
the fronds, and embedded between the fine layers of which they are
composed; when a fragment is heated in the blowpipe, these lamellae are
immediately rendered visible. The original outline of the fronds may often
be traced, either to a minute particle of shell fixed in a crevice of the
rock, or to several cemented together; these first become deeply corroded,
by the dissolving power of the waves, into sharp ridges, and then are
coated with successive layers of the glossy, grey, calcareous incrustation.
The inequalities of the primary support affect the outline of every
successive layer, in the same manner as may often be seen in bezoar-stones,
when an object like a nail forms the centre of aggregation. The crenulated
edges, however, of the frond appear to be due to the corroding power of the
surf on its own deposit, alternating with fresh depositions. On some smooth
basaltic rocks on the coast of St. Jago, I found an exceedingly thin layer
of brown calcareous matter, which under a lens presented a miniature
likeness of the crenulated and polished fronds of Ascension; in this case a
basis was not afforded by any projecting extraneous particles. Although the
incrustation at Ascension is persistent throughout the year; yet from the
abraded appearance of some parts, and from the fresh appearance of other
parts, the whole seems to undergo a round of decay and renovation, due
probably to changes in the form of the shifting beach, and consequently in
the action of the breakers: hence probably it is, that the incrustation
never acquires a great thickness. Considering the position of the encrusted
rocks in the midst of the calcareous beach, together with its composition,
I think there can be no doubt that its origin is due to the dissolution and
subsequent deposition of the matter composing the rounded particles of
shells and corals. (The selenite, as I have remarked is extraneous, and