coating at the great cataracts of the Orinoco and Nile), becoming finely
polished where the surf acts, I presume that the surf in this instance,
also, is the polishing agent.), rivalling when in a fresh state that of the
finest Oliva, there is a striking analogy between this inorganic
incrustation and the shells of living molluscous animals. (In the section
descriptive of St. Paul's Rocks, I have described a glossy, pearly
substance, which coats the rocks, and an allied stalactitical incrustation
from Ascension, the crust of which resembles the enamel of teeth, but is
hard enough to scratch plate-glass. Both these substances contain animal
matter, and seem to have been derived from water in filtering through
birds' dung.) This appears to me to be an interesting physiological fact.
(Mr. Horner and Sir David Brewster have described "Philosophical
Transactions" 1836 page 65 a singular "artificial substance, resembling
shell." It is deposited in fine, transparent, highly polished, brown-
coloured laminae, possessing peculiar optical properties, on the inside of
a vessel, in which cloth, first prepared with glue and then with lime, is
made to revolve rapidly in water. It is much softer, more transparent, and
contains more animal matter, than the natural incrustation at Ascension;
but we here again see the strong tendency which carbonate of lime and
animal matter evince to form a solid substance allied to shell.)

SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN.

These beds occur within the trachytic district, at the western base of
Green Mountain, under which they dip at a high inclination. They are only
partially exposed, being covered up by modern ejections; from this cause, I
was unable to trace their junction with the trachyte, or to discover
whether they had flowed as a stream of lava, or had been injected amidst
the overlying strata. There are three principal beds of obsidian, of which
the thickest forms the base of the section. The alternating stony layers