between 3,200 and 3,300 feet. In ascending the valley, the gravel gradually
becomes entirely altered in character: high up, we have pebbles of
crystalline feldspathic rocks, compact clay-slate, quartzose schists, and
pale-coloured porphyries; these rocks, judging both from the gigantic
boulders in the surface and from some small pebbles embedded beneath 700
feet in thickness of the old tertiary strata, are the prevailing kinds in
this part of the Cordillera; pebbles of basalt from the neighbouring
streams of basaltic lava are also numerous; there are few or none of the
reddish or of the gallstone-yellow porphyries so common near the coast.
Hence the pebbles on the 350 feet plain at the mouth of the Santa Cruz
cannot have been derived (with the exception of those of compact clay-
slate, which, however, may equally well have come from the south) from the
Cordillera in this latitude; but probably, in chief part, from farther
north.

Southward of the Santa Cruz, the gravel may be seen continuously capping
the great 840 feet plain: at the Rio Gallegos, where this plain is
succeeded by a lower one, there is, as I am informed by Captain Sulivan, an
irregular covering of gravel from ten to twelve feet in thickness over the
whole country. The district on each side of the Strait of Magellan is
covered up either with gravel or the boulder formation: it was interesting
to observe the marked difference between the perfectly rounded state of the
pebbles in the great shingle formation of Patagonia, and the more or less
angular fragments in the boulder formation. The pebbles and fragments near
the Strait of Magellan nearly all belong to rocks known to occur in Fuegia.
I was therefore much surprised in dredging south of the Strait to find, in
latitude 54 degrees 10' south, many pebbles of the gallstone-yellow
siliceous porphyry; I procured others from a great depth off Staten Island,
and others were brought me from the western extremity of the Falkland
Islands. (At my request, Mr. Kent collected for me a bag of pebbles from