Coral Reefs; Volcanic Islands; South American Geology — CompleteDarwin, Charles
Science
Coral Reefs; Volcanic Islands; South American Geology — Complete
Darwin, Charles
Beagle Expedition (1831-1836); Coral reefs and islands; Geology -- South America; Islands; Volcanoes
The upper beds of the limestone, at the above-mentioned quarry on the
Sugar-Loaf Hill, are softer, finer-grained and less pure, than the
lower beds. They abound with fragments of land-shells, and with some
perfect ones; they contain, also, the bones of birds, and the large
eggs,[15] apparently of water-fowl. It is probable that these upper
beds remained long in an unconsolidated form, during which time, these
terrestrial productions were embedded. Mr. G. R. Sowerby has kindly
examined three species of land-shells, which I procured from this bed,
and has described them in detail. One of them is a Succinea, identical
with a species now living abundantly on the island; the two others,
namely, _ Cochlogena fossilis_ and _Helix biplicata_, are not known in
a recent state: the latter species was also found in another and
different locality, associated with a species of Cochlogena which is
undoubtedly extinct.
[15] Colonel Wilkes, in a catalogue presented with some specimens to
the Geological Society, states that as many as ten eggs were found by
one person. Dr. Buckland has remarked (“Geolog. Trans.,” vol. v, p.
474) on these eggs.
_Beds of extinct land-shells._—Land-shells, all of which appear to be
species now extinct, occur embedded in earth, in several parts of the
island. The greater number have been found at a considerable height on
Flagstaff Hill. On the N.W. side of this hill, a rain-channel exposes a
section of about twenty feet in thickness, of which the upper part
consists of black vegetable mould, evidently washed down from the
heights above, and the lower part of less black earth, abounding with
young and old shells, and with their fragments: part of this earth is
slightly consolidated by calcareous matter, apparently due to the
partial decomposition of some of the shells. Mr. Seale, an intelligent
resident, who first called attention to these shells, gave me a large
collection from another locality, where the shells appear to have been
embedded in very black earth. Mr. G. R. Sowerby has examined these
shells, and has described them. There are seven species, namely, one
Cochlogena, two species of the genus Cochlicopa, and four of Helix;
none of these are known in a recent state, or have been found in any
other country. The smaller species were picked out of the inside of the
large shells of the _Cochlogena aurisvulpina._ This last-mentioned
species is in many respects a very singular one; it was classed, even
by Lamarck, in a marine genus, and having thus been mistaken for a
sea-shell, and the smaller accompanying species having been overlooked,
the exact localities where it was found have been measured, and the
elevation of this island thus deduced! It is very remarkable that all
the shells of this species found by me in one spot, form a distinct
variety, as described by Mr. Sowerby, from those
procured from another locality by Mr. Seale. As this Cochlogena is a
large and conspicuous shell, I particularly inquired from several
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