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 strata of most of these hills in the upper part, where alone the
planes of division are distinguishable, are inclined at a small angle
from the interior of the island towards the sea-coast. The inclination
is not the same in each hill; in that marked A it is less than in B, D,
or E; in C the strata are scarcely deflected from a horizontal plane,
and in F (as far as I could judge without ascending it) they are
slightly inclined in a reverse direction, that is, inwards and towards
the centre of the island. Notwithstanding these differences of
inclination, their correspondence in external form, and in the
composition both of their upper and lower parts,—their relative
position in one curved line, with their steepest sides turned
inwards,—all seem to show that they originally formed parts of one
platform; which platform, as before remarked, probably extended round a
considerable portion of the circumference of the island. The upper
strata certainly flowed as lava, and probably beneath the sea, as
perhaps did the lower feldspathic masses: how then come these strata to
hold their present position, and whence were they erupted?
In the centre of the island[7] there are lofty mountains, but they are
separated from the steep inland flanks of these hills by a wide space
of lower country: the interior mountains, moreover, seem to have been
the source of those great streams of basaltic lava which, contracting
as they pass between the bases of the hills in question, expand into
the coast-plains. Round the shores of St. Helena there is a rudely
formed ring of basaltic rocks, and at Mauritius there are remnants of
another such a ring round part, if not round the whole, of the island;
here again the same question immediately occurs, how came these masses
to hold their present position, and whence were they erupted? The same
answer, whatever it may be, probably applies in these three cases; and
in a future chapter we shall recur to this subject.
[7] I saw very little of the inland parts of the island. Near the
village of St. Domingo, there are magnificent cliffs of rather
coarsely crystallised basaltic lava. Following the little stream in
this valley, about a mile above the village, the base of the great
cliff was formed of a compact fine-grained basalt, conformably covered
by a bed of pebbles. Near Fuentes, I met with pap-formed hills of the
compact feldspathic series of rocks.
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