Paludestrinas, on the low plain near the Fort at this place, and that long
line on the edge of the cliff, sixty feet higher up; if he had crossed that
long and great belt of parallel sand-dunes, eight miles in width, standing
at the height of from forty to fifty feet above the Colorado, where sand
could not now collect,--I cannot believe he would have thought that the
elevation of this great district had been sudden. Certainly the sand-dunes
(especially when abounding with shells), which stand in ranges at so many
different levels, must all have required long time for their accumulation;
and hence I do not doubt that the last 100 feet of elevation of La Plata
and Northern Patagonia has been exceedingly slow.

If we extend this conclusion to Central and Southern Patagonia, the
inclination of the successively rising gravel-capped plains can be
explained quite as well, as by the more obvious view already given of a few
comparatively great and sudden elevations; in either case we must admit
long periods of rest, during which the sea ate deeply into the land. Let us
suppose the present coast to rise at a nearly equable, slow rate, yet
sufficiently quick to prevent the waves quite removing each part as soon as
brought up; in this case every portion of the present bed of the sea will
successively form a beach-line, and from being exposed to a like action
will be similarly affected. It cannot matter to what height the tides rise,
even if to forty feet as at Santa Cruz, for they will act with equal force
and in like manner on each successive line. Hence there is no difficulty in
the fact of the 355 feet plain at Santa Cruz sloping up 108 feet to the
foot of the next highest escarpment, and yet having no marks of any one
particular beach-line on it; for the whole surface on this view has been a
beach. I cannot pretend to follow out the precise action of the tidal-waves
during a rise of the land, slow, yet sufficiently quick to prevent or check
denudation: but if it be analogous to what takes place on protected parts
of the present coast, where gravel is now accumulating in large quantities,