Geological Observations of Volcanic Islands | Page 9

hills of a reddish
colour (like Red Hill in Figure 1 (Map 1). (The outline of the coast, the
position of the villages, streamlets, and of most of the hills in this
woodcut, are copied from the chart made on board H.M.S. "Leven."
The square-topped hills (A, B, C, etc.) are put in merely by eye, to
illustrate my description.)), and others less regular, flat-topped, and of a
blackish colour (like A, B, C,) rise from successive, step-formed plains
of lava. At a distance, a chain of mountains, many thousand feet in
height, traverses the interior of the island. There is no active volcano in
St. Jago, and only one in the group, namely at Fogo. The island since
being inhabited has not suffered from destructive earthquakes.
The lowest rocks exposed on the coast near Porto Praya, are highly
crystalline and compact; they appear to be of ancient, submarine,
volcanic origin; they are unconformably covered by a thin, irregular,
calcareous deposit, abounding with shells of a late tertiary period; and
this again is capped by a wide sheet of basaltic lava, which has flowed
in successive streams from the interior of the island, between the

square-topped hills marked A, B, C, etc. Still more recent streams of
lava have been erupted from the scattered cones, such as Red and
Signal Post Hills. The upper strata of the square-topped hills are
intimately related in mineralogical composition, and in other respects,
with the lowest series of the coast- rocks, with which they seem to be
continuous.
MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE
LOWEST SERIES.
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 along the coast. Its average height above the sea, measured from
the upper line of junction with the superincumbent basaltic lava, is
about sixty feet; and its thickness, although varying much from the
inequalities of the underlying formation, may be estimated at about
twenty feet. It consists of quite white calcareous matter, partly
composed of organic debris, and partly of a substance which may be
aptly compared in appearance with mortar. Fragments of rock and
pebbles are scattered throughout this bed, often forming, especially in
the lower part, a conglomerate. Many of the fragments of rock are
whitewashed with a thin coating of calcareous matter. At Quail Island,
the calcareous deposit is replaced in its lowest part by a soft, brown,
earthy tuff, full of Turritellae; this is covered by a bed of pebbles,
passing into sandstone, and mixed with fragments of echini, claws of
crabs, and shells; the oyster-shells still adhering to the rock on which
they grew. Numerous white balls appearing like pisolitic concretions,
from the size of a walnut to that of an apple, are embedded in this
deposit; they usually have a small pebble in their centres. Although so
like concretions, a close examination convinced me that they were
Nulliporae, retaining their proper forms, but with their surfaces slightly
abraded: these bodies (plants as they are now generally considered to
be) exhibit under a