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.
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