Beagle Expedition (1831-1836); Geology -- South America; Volcanoes
particularly inquired from several intelligent countrymen whether they
had ever seen it alive; they all assured me that they had not, and they
would not even believe that it was a land animal: Mr. Seale, moreover,
who was a collector of shells all his life at St. Helena, never met
with it alive. Possibly some of the smaller species may turn out to be
yet living kinds; but, on the other hand, the two land- shells which
are now living on the island in great numbers, do not occur embedded,
as far as is yet known, with the extinct species. I have shown in my
“Journal” (“Journal of Researches” page 582.), that the extinction of
these land-shells possibly may not be an ancient event; as a great
change took place in the state of the island about one hundred and
twenty years ago, when the old trees died, and were not replaced by
young ones, these being destroyed by the goats and hogs, which had run
wild in numbers, from the year 1502. Mr. Seale states, that on
Flagstaff Hill, where we have seen that the embedded land-shells are
especially numerous, traces are everywhere discoverable, which plainly
indicate that it was once thickly clothed with trees; at present not
even a bush grows there. The thick bed of black vegetable mould which
covers the shell-bed, on the flanks of this hill, was probably washed
down from the upper part, as soon as the trees perished, and the
shelter afforded by them was lost.
ELEVATION OF THE LAND.
Seeing that the lavas of the basal series, which are of submarine
origin, are raised above the level of the sea, and at some places to
the height of many hundred feet, I looked out for superficial signs of
the elevation of the land. The bottoms of some of the gorges, which
descend to the coast, are filled up to the depth of about a hundred
feet, by rudely divided layers of sand, muddy clay, and fragmentary
masses; in these beds, Mr. Seale has found the bones of the tropic-bird
and of the albatross; the former now rarely, and the latter never
visiting the island. From the difference between these layers, and the
sloping piles of detritus which rest on them, I suspect that they were
deposited, when the gorges stood beneath the sea. Mr. Seale, moreover,
has shown that some of the fissure- like gorges become, with a concave
outline, gradually rather wider at the bottom than at the top; and this
peculiar structure was probably caused by the wearing action of the
sea, when it entered the lower part of these gorges. (A fissure-like
gorge, near Stony-top, is said by Mr. Seale to be 840 feet deep, and
only 115 feet in width.) At greater heights, the evidence of the rise
of the land is even less clear: nevertheless, in a bay-like depression
on the table-land behind Prosperous Bay, at the height of about a
thousand feet, there are flat-topped masses of rock, which it is
scarcely conceivable, could have been insulated from the surrounding
and similar strata, by any other agency than the denuding action of a
sea- beach.
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