cemented together by a white aluminous substance, and I occasionally found
this to be the case with the gravel on the terrace D. I could not perceive
any trace of a similar deposition on the pebbles now thrown up by the
river, and therefore I do not think that terrace D was river-formed. As the
terrace E generally stands about twenty feet above the bed of the river, my
first impression was to doubt whether even this lowest one could have been
so formed; but it should always be borne in mind, that the horizontal
upheaval of a district, by increasing the total descent of the streams,
will always tend to increase, first near the sea-coast and then further and
further up the valley, their corroding and deepening powers: so that an
alluvial plain, formed almost on a level with a stream, will, after an
elevation of this kind, in time be cut through, and left standing at a
height never again to be reached by the water. With respect to the three
upper terraces of the Santa Cruz, I think there can be no doubt, that they
were modelled by the sea, when the valley was occupied by a strait, in the
same manner (hereafter to be discussed) as the greater step-formed, shell-
strewed plains along the coast of Patagonia.

To return to the shores of the Atlantic: the 840 feet plain, at the mouth
of the Santa Cruz, is seen extending horizontally far to the south; and I
am informed by the Officers of the Survey, that bending round the head of
Coy Inlet (sixty-five miles southward), it trends inland. Outliers of
apparently the same height are seen forty miles farther south, inland of
the river Gallegos; and a plain comes down to Cape Gregory (thirty-five
miles southward), in the Strait of Magellan, which was estimated at between
eight hundred and one thousand feet in height, and which, rising towards
the interior, is capped by the boulder formation. South of the Strait of
Magellan, there are large outlying masses of apparently the same great
tableland, extending at intervals along the eastern coast of Tierra del
Fuego: at two places here, 110 miles a part, this plain was found to be 950