These rocks possess an extremely varying character; they consist of black,
brown, and grey, compact, basaltic bases, with numerous crystals of augite,
hornblende, olivine, mica, and sometimes glassy feldspar. A common variety
is almost entirely composed of crystals of augite with olivine. Mica, it is
known, seldom occurs where augite abounds; nor probably does the present
case offer a real exception, for the mica (at least in my best
characterised specimen, in which one nodule of this mineral is nearly half
an inch in length) is as perfectly rounded as a pebble in a conglomerate,
and evidently has not been crystallised in the base, in which it is now
enclosed, but has proceeded from the fusion of some pre-existing rock.
These compact lavas alternate with tuffs, amygdaloids, and wacke, and in
some places with coarse conglomerate. Some of the argillaceous wackes are
of a dark green colour, others, pale yellowish-green, and others nearly
white; I was surprised to find that some of the latter varieties, even
where whitest, fused into a jet black enamel, whilst some of the green
varieties afforded only a pale gray bead. Numerous dikes, consisting
chiefly of highly compact augitic rocks, and of gray amygdaloidal
varieties, intersect the strata, which have in several places been
dislocated with considerable violence, and thrown into highly inclined
positions. One line of disturbance crosses the northern end of Quail Island
(an islet in the Bay of Porto Praya), and can be followed to the mainland.
These disturbances took place before the deposition of the recent
sedimentary bed; and the surface, also, had previously been denuded to a
great extent, as is shown by many truncated dikes.

DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC
ROCKS.

This stratum is very conspicuous from its white colour, and from the
extreme regularity with which it ranges in a horizontal line for some miles