denudation of a more regular bed of gravel, capping the old tertiary
sandstone plateau of the Rio Negro. The gravel-bed near the Rio Negro is,
on an average, about ten or twelve feet in thickness; and the pebbles are
larger than on the northern side of the Colorado, being from one or two
inches in diameter, and composed chiefly of rather dark-tinted porphyries.
Amongst them I here first noticed a variety often to be referred to,
namely, a peculiar gallstone-yellow siliceous porphyry, frequently, but not
invariably, containing grains of quartz. The pebbles are embedded in a
white, gritty, calcareous matrix, very like mortar, sometimes merely
coating with a whitewash the separate stones, and sometimes forming the
greater part of the mass. In one place I saw in the gravel concretionary
nodules (not rounded) of crystallised gypsum, some as large as a man's
head. I traced this bed for forty-five miles inland, and was assured that
it extended far into the interior. As the surface of the calcareo-
argillaceous plain of Pampean formation, on the northern side of the wide
valley of the Colorado, stands at about the same height with the mortar-
like cemented gravel capping the sandstone on the southern side, it is
probable, considering the apparent equability of the subterranean movements
along this side of America, that this gravel of the Rio Negro and the upper
beds of the Pampean formation northward of the Colorado, are of nearly
contemporaneous origin, and that the calcareous matter has been derived
from the same source.

Southward of the Rio Negro, the cliffs along the great bay of S. Antonio
are capped with gravel: at San Josef, I found that the pebbles closely
resembled those on the plain of the Rio Negro, but that they were not
cemented by calcareous matter. Between San Josef and Port Desire, I was
assured by the Officers of the Survey that the whole face of the country is
coated with gravel. At Port Desire and over a space of twenty-five miles
inland, on the three step-formed plains and in the valleys, I everywhere