4 i)i ■
I have called these step-like plains level, because they
appear to be absolutely so to the eye, but in truth they rise a
little between the edge of one line of cliff and the base of the
next above it. Their slope is about the same as that of the
gradually shoaling bottom of the neighbouring sea. The elevation
of 350 feet is gained by three step s; one of about 100
feet, the second 250, and the third 350. Over these three
plains marine remains are frequently scattered, but they are
especially abundant on the lower one. The shells are the
same as the now existing littoral species, and the muscle and
turbo yet partially retain their blue and purple colours.
We have now stated the problem, which is to be explained
so as to connect together these various phenomena. At first
I could only understand the grand covering of gravel, by the
supposition of some epoch of extreme violence, and the successive
lines of chff, by as many great elevations, the precise
action of which I could not however follow out. Guided by
the “ Principles of Geology,” and having under my view the
vast changes going on in this continent, which at the present
day seems the great workshop of nature, I came to another,
and I hope more satisfactory conclusion. The importance of
any view which may explain the agency by which such vast
beds of shingle have been transported over the surface of the
successive plains, cannot be doubted. Whatever the cause
may have been, it has determined the condition of this
desert country, with respect to its form, nature, and capabilities
of supporting life.
There are proofs, that the whole coast has been elevated
to a considerable height within the recent period; and on
the shores of the Pacific, where successive terraces likewise
occur, we know that these changes have latterly been very
gradual. There is indeed reason for believing, that the
uplifting of the ground during the earthquakes in Chile,
although only to the height of two or three feet, has been a
disturbance which may be considered as a great one, in comparison
to the series of lesser and scarcely sensible movements
which are hkewise in progress. Let us then imagine
the consequence of the shoaling bed of an ocean, elevated at
a perfectly equable rate, so that the same number of feet
should be converted into dry land in each succeeding century.
Every part of the surface would then have been exposed for
an equal length of time to the action of the beach-line, and
the whole in consequence equally modified. The shoaling bed
of the ocean would thus be changed into a sloping land, with
no marked line on it. If, however, there should occur a long
period of repose in the elevations, and the currents of tlie
sea should tend to wear away the land (as happens along this
whole coast), then there would be formed a line of cliff.
Accordingly as the repose was long, so would be the quantity
of land consumed, and the consequent height of such
cliffs. Let the elevations recommence, and another sloping
bank (of shingle, or sand, or mud, according to the nature of
the successive beach-lines) must be formed, which again will
be broken by as many lines of cliff, as there shall he periods
of rest in the action of the subterranean forces. Now this is
the structure of the plains of Patagonia; and such gradual
changes harmonize well with the undisturbed strata, extending
over so many hundred miles.
I must here observe, that I am far from supposing that
the entire coast of this part of the continent has ever been
lifted up, to the height of even a foot, at any one moment of
tim e ; but, drawing our analogies from the shores of the
Pacific, that the whole may have been insensibly rising, ivith
every now and then a paroxysmal or accelerated movement
in certain spots. With respect to the alternation of the
periods of such continued rise and those of quiescence, we
may grant that they are probable, because such alternation
agrees with what we see in the action, not only of a single
volcano, but likewise of the disturbances affecting whole
regions of the earth. At the present day, to the north of the
parallel 44°, the subterranean forces are constantly manifesting
their power over a space of more than one thousand
miles. But to the southward of that line, as far as Cape
Horn, an earthquake is seldom or never experienced, and