passed over gravel which, where thickest, was between thirty and forty
feet. Here, as in other parts of Patagonia, the gravel, or its sandy
covering, was, as we have seen, often strewed with recent marine shells.
The sandy covering sometimes fills up furrows in the gravel, as does the
gravel in the underlying tertiary formations. The pebbles are frequently
whitewashed and even cemented together by a peculiar, white, friable,
aluminous, fusible substance, which I believe is decomposed feldspar. At
Port Desire, the gravel rested sometimes on the basal formation of
porphyry, and sometimes on the upper or the lower denuded tertiary strata.
It is remarkable that most of the porphyritic pebbles differ from those
varieties of porphyry which occur here abundantly in situ. The peculiar
gallstone-yellow variety was common, but less numerous than at Port S.
Julian, where it formed nearly one-third of the mass of the gravel; the
remaining part there consisting of pale grey and greenish porphyries with
many crystals of feldspar. At Port S. Julian, I ascended one of the flat-
topped hills, the denuded remnant of the highest plain, and found it, at
the height of 950 feet, capped with the usual bed of gravel.

Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet plain
is from twenty to about thirty-five feet in thickness. The pebbles vary
from minute ones to the size of a hen's egg, and even to that of half a
man's head; they consist of paler varieties of porphyry than those found
further northward, and there are fewer of the gallstone-yellow kind;
pebbles of compact black clay-slate were here first observed. The gravel,
as we have seen, covers the step-formed plains at the mouth, head, and on
the sides of the great valley of the Santa Cruz. At a distance of 110 miles
from the coast, the plain has risen to the height of 1,416 feet above the
sea; and the gravel, with the associated great boulder formation, has
attained a thickness of 212 feet. The plain, apparently with its usual
gravel covering, slopes up to the foot of the Cordillera to the height of