in it recent species of
shells, some of them proper to brackish water, and is believed by Mr.
Darwin to be an estuary or delta deposit. M. A. d'Orbigny, however, has
advanced an hypothesis referred to by M. E. de Beaumont, that the
agitation and displacement of the waters of the ocean, caused by the
elevation of the Andes, gave rise to a deluge, of which this Pampean
mud, which rises sometimes to the height of 12,000 feet, is the result
and monument.[253]

In studying many chains of mountains, we find that the strike or line of
outcrop of continuous sets of strata, and the general direction of the
chain, may be far from rectilinear. Curves forming angles of 20 degrees
or 30 degrees may be found in the same range as in the Alleghanies; just
as trains of active volcanoes and the zones throughout which modern
earthquakes occur are often linear, without running in straight lines.
Nor are all of these, though contemporaneous or belonging to our own
epoch, by any means parallel, but some at right angles, the one to the
other.

_Slow upheaval and subsidence._--Recent observations have disclosed to
us the wonderful fact, that not only the west coast of South America,
but also other large areas, some of them several thousand miles in
circumference, such as Scandinavia, and certain archipelagoes in the
Pacific, are slowly and insensibly rising; while other regions, such as
Greenland, and parts of the Pacific and Indian Oceans, in which atolls
or circular coral islands abound, are as gradually sinking. That all the
existing continents and submarine abysses may have originated in
movements of this kind, continued throughout incalculable periods of
time, is undeniable, and the denudation which the dry land appears
everywhere to have suffered, favors the idea that it was raised from the
deep by a succession of upward movements, prolonged throughout
indefinite periods. For the action of waves and currents on land slowly
emerging from the deep, affords the only power by which we can conceive
so many deep valleys and wide spaces to have been denuded as those which
are unquestionably the effects of running water.

But perhaps it may be said that there is no analogy between the slow
upheaval of broad plains or table-lands, and the manner in which we must
presume all mountain-chains, with their inclined strata, to have
originated. It seems, however, that the Andes have been rising century
after century, at the rate of several feet, whil