could not observe any increase in their size. We shall presently see that
the theory of a slow and almost insensible rise of the land, will explain
all the facts connected with the gravel-capped terraces, better than the
theory of sudden elevations of from one to two hundred feet.

M. d'Orbigny has argued, from the upraised shells at San Blas being
embedded in the positions in which they lived, and from the valves of the
Azara labiata high on the banks of the Parana being united and unrolled,
that the elevation of Northern Patagonia and of La Plata must have been
sudden; for he thinks, if it had been gradual, these shells would all have
been rolled on successive beach-lines. But in PROTECTED bays, such as in
that of Bahia Blanca, wherever the sea is accumulating extensive mud-banks,
or where the winds quietly heap up sand-dunes, beds of shells might
assuredly be preserved buried in the positions in which they had lived,
even whilst the land retained the same level; any, the smallest, amount of
elevation would directly aid in their preservation. I saw a multitude of
spots in Bahia Blanca where this might have been effected; and at Maldonado
it almost certainly has been effected. In speaking of the elevation of the
land having been slow, I do not wish to exclude the small starts which
accompany earthquakes, as on the coast of Chile; and by such movements beds
of shells might easily be uplifted, even in positions exposed to a heavy
surf, without undergoing any attrition: for instance, in 1835, a rocky flat
off the island of Santa Maria was at one blow upheaved above high-water
mark, and was left covered with gaping and putrefying mussel-shells, still
attached to the bed on which they had lived. If M. d'Orbigny had been aware
of the many long parallel lines of sand-hillocks, with infinitely numerous
shells of the Mactra and Venus, at a low level near the Uruguay; if he had
seen at Bahia Blanca the immense sand-dunes, with water-worn pebbles of
pumice, ranging in parallel lines, one behind the other, up a height of at
least 120 feet; if he had seen the sand-dunes, with the countless