Beagle Expedition (1831-1836); Coral reefs and islands; Geology -- South America; Islands; Volcanoes
There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured
red from being fringed by coral-reefs. Thus I hope to show in a future
volume, that nearly the whole line of the west coast of South America,
which forms the greatest volcanic chain in the world, from near the
equator for a space of between 2,000 and 3,000 miles southward, has
undergone an upward movement during a late geological period. The
islands on the north-western shores of the Pacific, which form the
second greatest volcanic chain, are very imperfectly known; but Luzon,
in the Philippines, and the Loo Choo Islands, have been recently
elevated; and at Kamtschatka[36] there are extensive tertiary beds of
modern date. Evidence of the same nature, but not very satisfactory,
may be detected in Northern New Zealand where there are two volcanoes.
The co-existence in other parts of the world of active volcanoes, with
upraised beds of a modern tertiary origin, will occur to every
geologist.[37] Nevertheless, until it could be shown that volcanoes
were inactive, or did not exist in subsiding areas, the conclusion that
their distribution depended on the nature of the subterranean movements
in progress, would have been hazardous. But now, viewing the appended
map, it may, I think, be considered as almost established, that
volcanoes are often (not necessarily always) present in those areas
where the subterranean motive power has lately forced, or is now
forcing outwards, the crust of the earth, but that they are invariably
absent in those, where the surface has lately subsided or is still
subsiding.[38]
[36] At Sedanka, in latitude 58° N. (Von Buch’s “Descrip. des Isles
Canaries,” page 455). In a forthcoming part, I shall give the evidence
referred to with respect to the elevation of New Zealand.
[37] During the subterranean disturbances which took place in Chile,
in 1835, I have shown (“Geolog. Trans.” 2nd Ser., vol. v., page 606)
that at the same moment that a large district was upraised, volcanic
matter burst forth at widely separated points, through both new and
old vents.
[38] We may infer from this rule, that in any old deposit, which
contains interstratified beds of erupted matter, there was at the
period, and in the area of its formation, a _tendency_ to an upward
movement in the earth’s surface, and certainly no movement of
subsidence.
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