equable heights of the plains; that there have been at least eight periods
of denudation, and that the land, up to a height of from 950 to 1,200 feet,
has been similarly modelled and affected: that the area elevated, in the
southernmost part of the continent, extended in breadth to the Cordillera,
and probably seaward to the Falkland Islands; that northward, in La Plata,
the breadth is unknown, there having been probably more than one axis of
elevation; and finally, that, anterior to the elevation attested by these
upraised shells, the land was divided by a Strait where the River Santa
Cruz now flows, and that further southward there were other sea-straits,
since closed. I may add, that at Santa Cruz, in latitude 50 degrees S., the
plains have been uplifted at least 1,400 feet, since the period when
gigantic boulders were transported between sixty and seventy miles from
their parent rock, on floating icebergs.

Lastly, considering the great upward movements which this long line of
coast has undergone, and the proximity of its southern half to the volcanic
axis of the Cordillera, it is highly remarkable that in the many fine
sections exposed in the Pampean, Patagonian tertiary, and Boulder
formations, I nowhere observed the smallest fault or abrupt curvature in
the strata.

GRAVEL FORMATION OF PATAGONIA.

I will here describe in more detail than has been as yet incidentally done,
the nature, origin, and extent of the great shingle covering of Patagonia:
but I do not mean to affirm that all of this shingle, especially that on
the higher plains, belongs to the recent period. A thin bed of sandy earth,
with small pebbles of various porphyries and of quartz, covering a low
plain on the north side of the Rio Colorado, is the extreme northern limit
of this formation. These little pebbles have probably been derived from the