up a bar parallel to the shore, and at some distance from it; this bar
gradually becomes larger, affording a base for the accumulation of sand-
dunes, and the shallow space within then becomes silted up with mud. The
repetition of this process, without any elevation of the land, would form a
level plain traversed by parallel lines of sand-hillocks; during a slow
elevation of the land, the hillocks would rest on a gently inclined
surface, like that on the northern shore of Bahia Blanca. I did not observe
any shells in this neighbourhood at a greater height than twenty feet; and
therefore the age of the sea-drifted pebbles of pumice, now standing at the
height of 120 feet, must remain uncertain.

The main plain surrounding Bahia Blanca I estimated at from two hundred to
three hundred feet; it insensibly rises towards the distant Sierra Ventana.
There are in this neighbourhood some other and lower plains, but they do
not abut one at the foot of the other, in the manner hereafter to be
described, so characteristic of Patagonia. The plain on which the
settlement stands is crossed by many low sand-dunes, abounding with the
minute shells of the Paludestrina australis, d'Orbigny, which now lives in
the bay. This low plain is bounded to the south, at the Cabeza del Buey, by
the cliff-formed margin of a wide plain of the Pampean formation, which I
estimated at sixty feet in height. On the summit of this cliff there is a
range of high sand-dunes extending several miles in an east and west line.

Southward of Bahia Blanca, the river Colorado flows between two plains,
apparently from thirty to forty feet in height. Of these plains, the
southern one slopes up to the foot of the great sandstone plateau of the
Rio Negro; and the northern one against an escarpment of the Pampean
deposit; so that the Colorado flows in a valley fifty miles in width,
between the upper escarpments. I state this, because on the low plain at
the foot of the northern escarpment, I crossed an immense accumulation of