1832.] SINGULAR INCRUSTATIONS. 9 
the thin white coating on these rocks. The branching bodies so 
closely resembled in general appearance certain nullipore (a 
family of hard calcareous sea-plants), that in lately looking hastily 
over my collection I did not perceive the difference. The glo- 
bular extremities of the branches are of a pearly texture, like the 
enamel of teeth, but so hard as just to scratch plate-glass. I 
may here mention, that on a part of the coast of Ascension, 
where there is a vast accumulation of shelly sand, an incrustation 
is deposited on the tidal rocks, by the water of the sea, resembling, 
as represented in the woodcut, certain cryptogamic plants (Mar- 
chantise) often seen on damp walls. The surface of the fronds is 
beautifully glossy ; and uhose 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 incrusta- 
tion to several geologists, and they all thought that they were of 
voleanic 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-shelis, 
it is known that the parts habitually covered and shaded by the 
mantle-of the animal, are of a paler colour than those fully ex- 
posed to the light, just as is the case with this incrustation. 
When we remember that lime, either asa phosphate or carbonate, 
enters into the comvosition of the hard parts, such as bones and 
