with innumerable veins, and vein-like masses, both small and large. Of
these vein-like masses, some calcareous ones, which contain minute
fragments of shells, are clearly of subsequent origin to the others.

A GLOSSY INCRUSTATION.

Extensive portions of these rocks are coated by a layer of a glossy
polished substance, with a pearly lustre and of a greyish white colour; it
follows all the inequalities of the surface, to which it is firmly
attached. When examined with a lens, it is found to consist of numerous
exceedingly thin layers, their aggregate thickness being about the tenth of
an inch. It is considerably harder than calcareous spar, but can be
scratched with a knife; under the blowpipe it scales off, decrepitates,
slightly blackens, emits a fetid odour, and becomes strongly alkaline: it
does not effervesce in acids. (In my "Journal" I have described this
substance; I then believed that it was an impure phosphate of lime.) I
presume this substance has been deposited by water draining from the birds'
dung, with which the rocks are covered. At Ascension, near a cavity in the
rocks which was filled with a laminated mass of infiltrated birds' dung, I
found some irregularly formed, stalactitical masses of apparently the same
nature. These masses, when broken, had an earthy texture; but on their
outsides, and especially at their extremities, they were formed of a pearly
substance, generally in little globules, like the enamel of teeth, but more
translucent, and so hard as just to scratch plate-glass. This substance
slightly blackens under the blowpipe, emits a bad smell, then becomes quite
white, swelling a little, and fuses into a dull white enamel; it does not
become alkaline; nor does it effervesce in acids. The whole mass had a
collapsed appearance, as if in the formation of the hard glossy crust the
whole had shrunk much. At the Abrolhos Islands on the coast of Brazil,
where also there is much birds' dung, I found a great quantity of a brown,