think it scarcely possible that these coincidences in height should be
accidental. We must therefore conclude that the action, whatever it may
have been, by which these plains have been modelled into their present
forms, has been singularly uniform.

These plains or great terraces, of which three and four often rise like
steps one behind the other, are formed by the denudation of the old
Patagonian tertiary beds, and by the deposition on their surfaces of a mass
of well-rounded gravel, varying, near the coast, from ten to thirty-five
feet in thickness, but increasing in thickness towards the interior. The
gravel is often capped by a thin irregular bed of sandy earth. The plains
slope up, though seldom sensibly to the eye, from the summit edge of one
escarpment to the foot of the next highest one. Within a distance of 150
miles, between Santa Cruz to Port Desire, where the plains are particularly
well developed, there are at least seven stages or steps, one above the
other. On the three lower ones, namely, those of 100 feet, 250 feet, and
350 feet in height, existing littoral shells are abundantly strewed, either
on the surface, or partially embedded in the superficial sandy earth. By
whatever action these three lower plains have been modelled, so undoubtedly
have all the higher ones, up to a height of 950 feet at S. Julian, and of
1,200 feet (by estimation) along St. George's Bay. I think it will not be
disputed, considering the presence of the upraised marine shells, that the
sea has been the active power during stages of some kind in the elevatory
process.

We will now briefly consider this subject: if we look at the existing
coast-line, the evidence of the great denuding power of the sea is very
distinct; for, from Cape St. Diego, in latitude 54 degrees 30' to the mouth
of the Rio Negro, in latitude 31 degrees (a length of more than eight
hundred miles), the shore is formed, with singularly few exceptions, of