Geological Observations on South AmericaDarwin, Charles
Science
Geological Observations on South America
Darwin, Charles
Beagle Expedition (1831-1836); Geology -- South America; Mollusks, Fossil -- South America; Paleontology
The rise of Lima, having been eighty-five feet within the period of
man, is the more surprising if we refer to the eastern coast of the
continent, for at Port S. Julian, in Patagonia, there is good evidence
(as we shall hereafter see) that when the land stood ninety feet lower,
the Macrauchenia, a mammiferous beast, was alive; and at Bahia Blanca,
when it stood only a few feet lower than it now does, many gigantic
quadrupeds ranged over the adjoining country. But the coast of
Patagonia is some way distant from the Cordillera, and the movement at
Bahia Blanca is perhaps noways connected with this great range, but
rather with the tertiary volcanic rocks of Banda Oriental, and
therefore the elevation at these places may have been infinitely slower
than on the coast of Peru. All such speculations, however, must be
vague, for as we know with certainty that the elevation of the whole
coast of Patagonia has been interrupted by many and long pauses, who
will pretend to say that, in such cases, many and long periods of
subsidence may not also have been intercalated?
In many parts of the coast of Chile and Peru there are marks of the
action of the sea at successive heights on the land, showing that the
elevation has been interrupted by periods of comparative rest in the
upward movement, and of denudation in the action of the sea. These are
plainest at Chiloe, where, in a height of about five hundred feet,
there are three escarpments,—at Coquimbo, where in a height of 364
feet, there are five,— at Guasco, where there are six, of which five
may perhaps correspond with those at Coquimbo, but if so, the
subsequent and intervening elevatory movements have been here much more
energetic,—at Lima, where, in a height of about 250 feet there are
three terraces, and others, as it is asserted, at considerably greater
heights. The almost entire absence of ancient marks of sea-action at
defined levels along considerable spaces of coast, as near Valparaiso
and Concepcion, is highly instructive, for as it is improbable that the
elevation at these places alone should have been continuous, we must
attribute the absence of such marks to the nature and form of the
coast-rocks. Seeing over how many hundred miles of the coast of
Patagonia, and on how many places on the shores of the Pacific, the
elevatory process has been interrupted by periods of comparative rest,
we may conclude, conjointly with the evidence drawn from other quarters
of the world, that the elevation of the land is generally an
intermittent action. From the quantity of matter removed in the
formation of the escarpments, especially of those of Patagonia, it
appears that the periods of rest in the movement, and of denudation of
the land, have generally been very long. In Patagonia, we have seen
that the elevation has been equable, and the periods of denudation
synchronous over very wide spaces of coast; on the shores of the
Pacific, owing to the terraces chiefly occurring in the valleys, we
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