1 
SING ULAR I NCR US TA TIONS 
9 
found to consist of numerous exceedingly thin layers, its total 
thickness being about the tenth of an inch. It contains much 
animal matter, and its origin, no doubt, is due to the action of 
the rain or spray on the birds’ dung. Below some small 
masses of guano at Ascension, and on the Abrolhos Islets, I 
found certain stalactitic branching bodies, formed apparently in 
the same manner as the thin white coating on these rocks. 
The branching bodies so closely resembled in general appearance 
certain nulliporae (a family of hard calcareous sea-plants), that in 
lately looking hastily over my collection I did not perceive the 
difference. The 
globular 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 accu¬ 
mulation 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 (Marchantiae) often seen on 
damp walls. The surface of the fronds is beautifully glossy ; and 
those parts formed where fully exposed to the light, are of a 
jet-black colour, but those shaded under ledges are only gray. 
I have shown specimens of this incrustation to several geologists, 
and they all thought that they were of volcanic 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-shells, it is known that the 
parts habitually covered' and shaded by the mantle of the 
animal, are of a paler colour than those fully exposed to the 
light, just as is the case with this incrustation. When we 
remember that lime, either as a phosphate or carbonate, enters 
into the composition of the hard parts, such as bones and 
shells, of all living animals, it is an interesting physiological 
