the beach of White Rock harbour, in the northern part of the sound, between
the two Falkland Islands. Out of these well-rounded pebbles, varying in
size from a walnut to a hen's egg, with some larger, thirty-eight evidently
belonged to the rocks of these islands; twenty-six were similar to the
pebbles of porphyry found on the Patagonian plains, which rocks do not
exist in situ in the Falklands; one pebble belonged to the peculiar yellow
siliceous porphyry; thirty were of doubtful origin.) The distribution of
the pebbles of this peculiar porphyry, which I venture to affirm is not
found in situ either in Fuegia, the Falkland Islands, or on the coast of
Patagonia, is very remarkable, for they are found over a space of 840 miles
in a north and south line, and at the Falklands, 300 miles eastward of the
coast of Patagonia. Their occurrence in Fuegia and the Falklands may,
however, perhaps be due to the same ice-agency by which the boulders have
been there transported.

We have seen that porphyritic pebbles of a small size are first met with on
the northern side of the Rio Colorado, the bed becoming well developed near
the Rio Negro: from this latter point I have every reason to believe that
the gravel extends uninterruptedly over the plains and valleys of Patagonia
for at least 630 nautical miles southward to the Rio Gallegos. From the
slope of the plains, from the nature of the pebbles, from their extension
at the Rio Negro far into the interior, and at the Santa Cruz close up to
the Cordillera, I think it highly probable that the whole breadth of
Patagonia is thus covered. If so, the average width of the bed must be
about two hundred miles. Near the coast the gravel is generally from ten to
thirty feet in thickness; and as in the valley of Santa Cruz it attains, at
some distance from the Cordillera, a thickness of 214 feet, we may, I
think, safely assume its average thickness over the whole area of 630 by
200 miles, at fifty feet!