The coast as far as Bahia Blanca (in latitude 39 degrees S.) is formed
either of a horizontal range of cliffs, or of immense accumulations of
sand-dunes. Within Bahia Blanca, a small piece of tableland, about twenty
feet above high-water mark, called Punta Alta, is formed of strata of
cemented gravel and of red earthy mud, abounding with shells (with others
lying loose on the surface), and the bones of extinct mammifers. These
shells, twenty in number, together with a Balanus and two corals, are all
recent species, still inhabiting the neighbouring seas. They will be
enumerated in the Fourth Chapter, when describing the Pampean formation;
five of them are identical with the upraised ones from near Buenos Ayres.
The northern shore of Bahia Blanca is, in main part, formed of immense
sand-dunes, resting on gravel with recent shells, and ranging in lines
parallel to the shore. These ranges are separated from each other by flat
spaces, composed of stiff impure red clay, in which, at the distance of
about two miles from the coast, I found by digging a few minute fragments
of sea-shells. The sand-dunes extend several miles inland, and stand on a
plain, which slopes up to a height of between one hundred and two hundred
feet. Numerous, small, well-rounded pebbles of pumice lie scattered both on
the plain and sand-hillocks: at Monte Hermoso, on the flat summit of a
cliff, I found many of them at a height of 120 feet (angular measurement)
above the level of the sea. These pumice pebbles, no doubt, were originally
brought down from the Cordillera by the rivers which cross the continent,
in the same way as the river Negro anciently brought down, and still brings
down, pumice, and as the river Chupat brings down scoriae: when once
delivered at the mouth of a river, they would naturally have travelled
along the coasts, and been cast up during the elevation of the land, at
different heights. The origin of the argillaceous flats, which separate the
parallel ranges of sand-dunes, seems due to the tides here having a
tendency (as I believe they have on most shoal, protected coasts) to throw