It must be observed, that the width of each plain is in
nature very much greater in proportion to the height, than
here represented.
The whole series is horizontally stratified, and I do not recollect
ever seeing any signs of violence, not even such as a
fault. The gravel covers the entire surface of the land, from the
Rio Colorado to the Strait of Magellan, a space of 800 miles,
and is one chief cause of the desert character of Patagonia.
Judging from a section across the continent at the St. Cruz
river, and from some other reasons, I believe the gravel beds
gradually thickening as they ascend, every where reach the
L s e of the Cordillera. It is to these mountains we must
look for the parent rocks, of at least a large portion of the
well-rounded fragments. I apprehend so great an area covered
by shingle, could scarcely be pointed out in any other
part of the world.
Having said thus much of the constitution, let us look at
the external configuration of the mass. The level plains are
cut off along the whole line of coast by perpendicular cliffs,
which are necessarily of different altitudes, because any one of
the successive terraces, which, as I have already noticed, rise
like steps one above the other, may form the sea cliff.
These steps are often several miles broad; but from one
point of view I have seen four very distinct lines of escarpment
abutting one over the other. Having observed that
the plains appeared to run for great distances along the
coast at the same level, I measured barometrically the elevation
of some of them, and compared these measurements,
and took all those made by the officers employed in the
survey. I was astonished to find at how great distances,
even of 600 miles, plains occurred that had, within a few
feet of difference, the same elevation. I believe I can distinguish
seven or eight distmct terraces which occur along
the line of coast, and which include heights between 1200
feet and the level of the sea. It will be understood that they
are not always present, for the lower ones have in some parts
been removed by the action of the sea sooner than in others.
When any broad valley enters the country, the terraces sweep
round and run up on each side ; in which case, the correspondence
on the opposite sides is beautifully illustrated.