A FRONDESCENT CALCAREOUS INCRUSTATION.

(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the
tidal-rocks at Ascension.)

In many respects this is a singular deposit; it coats throughout the year
the tidal volcanic rocks, that project from the beaches composed of broken
shells. Its general appearance is well represented in Figure 5; but the
fronds or discs, of which it is composed, are generally so closely crowded
together as to touch. These fronds have their sinuous edges finely
crenulated, and they project over their pedestals or supports; their upper
surfaces are either slightly concave, or slightly convex; they are highly
polished, and of a dark grey or jet black colour; their form is irregular,
generally circular, and from the tenth of an inch to one inch and a half in
diameter; their thickness, or amount of their projection from the rock on
which they stand, varies much, about a quarter of an inch being perhaps
most usual. The fronds occasionally become more and more convex, until they
pass into botryoidal masses with their summits fissured; when in this
state, they are glossy and of an intense black, so as to resemble some
fused metallic substance. I have shown the incrustation, both in this
latter and in its ordinary state to several geologists, but not one could
conjecture its origin, except that perhaps it was of volcanic nature!

The substance forming the fronds has a very compact and often almost
crystalline fracture; the edges being translucent, and hard enough easily
to scratch calcareous spar. Under the blowpipe it immediately becomes
white, and emits a strong animal odour, like that from fresh shells. It is
chiefly composed of carbonate of lime; when placed in muriatic acid it
froths much, leaving a residue of sulphate of lime, and of an oxide of