Upon Examination
This White Stratum Is Found To Consist Of Calcareous Matter
With Numerous Shells Embedded, Most Or All Of Which Now
Exist On The Neighbouring Coast.
It rests on ancient volcanic
rocks, and has been covered by a stream of basalt, which
must have entered the sea when the white shelly bed was
lying at the bottom.
It is interesting to trace the changes
produced by the heat of the overlying lava, on the friable
mass, which in parts has been converted into a crystalline
limestone, and in other parts into a compact spotted stone
Where the lime has been caught up by the scoriaceous fragments
of the lower surface of the stream, it is converted into
groups of beautifully radiated fibres resembling arragonite.
The beds of lava rise in successive gently-sloping plains,
towards the interior, whence the deluges of melted stone
have originally proceeded. Within historical times, no signs
of volcanic activity have, I believe, been manifested in any
part of St. Jago. Even the form of a crater can but rarely
be discovered on the summits of the many red cindery hills;
yet the more recent streams can be distinguished on the
coast, forming lines of cliffs of less height, but stretching
out in advance of those belonging to an older series: the
height of the cliffs thus affording a rude measure of the age
of the streams.
During our stay, I observed the habits of some marine
animals. A large Aplysia is very common. This sea-slug
is about five inches long; and is of a dirty yellowish colour
veined with purple. On each side of the lower surface, or
foot, there is a broad membrane, which appears sometimes
to act as a ventilator, in causing a current of water to flow
over the dorsal branchiae or lungs. It feeds on the delicate
sea-weeds which grow among the stones in muddy and shallow
water; and I found in its stomach several small pebbles,
as in the gizzard of a bird. This slug, when disturbed, emits
a very fine purplish-red fluid, which stains the water for the
space of a foot around. Besides this means of defence, an
acrid secretion, which is spread over its body, causes a
sharp, stinging sensation, similar to that produced by the
Physalia, or Portuguese man-of-war.
I was much interested, on several occasions, by watching
the habits of an Octopus, or cuttle-fish. Although common
in the pools of water left by the retiring tide, these animals
were not easily caught. By means of their long arms and
suckers, they could drag their bodies into very narrow crevices;
and when thus fixed, it required great force to remove
them. At other times they darted tail first, with the rapidity
of an arrow, from one side of the pool to the other, at the
same instant discolouring the water with a dark chestnut-brown
ink. These animals also escape detection by a very
extraordinary, chameleon-like power of changing their colour.
They appear to vary their tints according to the nature
of the ground over which they pass:
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