high sand-dunes, estimated by the Gauchos at no less than eight miles in
breadth. These dunes range westward from the coast, which is twenty miles
distant, to far inland, in lines parallel to the valley; they are separated
from each other by argillaceous flats, precisely like those on the northern
shore of Bahia Blanca. At present there is no source whence this immense
accumulation of sand could proceed; but if, as I believe, the upper
escarpments once formed the shores of an estuary, in that case the
sandstone formation of the river Negro would have afforded an inexhaustible
supply of sand, which would naturally have accumulated on the northern
shore, as on every part of the coast open to the south winds between Bahia
Blanca and Buenos Ayres.

At San Blas (40 degrees 40' S.) a little south of the mouth of the
Colorado, M. d'Orbigny found fourteen species of existing shells (six of
them identical with those from Bahia Blanca), embedded in their natural
positions. ("Voyage" etc. page 54.) From the zone of depth which these
shells are known to inhabit, they must have been uplifted thirty-two feet.
He also found, at from fifteen to twenty feet above this bed, the remains
of an ancient beach.

Ten miles southward, but 120 miles to the west, at Port S. Antonio, the
Officers employed on the Survey assured me that they saw many old sea-
shells strewed on the surface of the ground, similar to those found on
other parts of the coast of Patagonia. At San Josef, ninety miles south in
nearly the same longitude, I found, above the gravel, which caps an old
tertiary formation, an irregular bed and hillock of sand, several feet in
thickness, abounding with shells of Patella deaurita, Mytilus Magellanicus,
the latter retaining much of its colour; Fusus Magellanicus (and a variety
of the same), and a large Balanus (probably B. Tulipa), all now found on
this coast: I estimated this bed at from eighty to one hundred feet above