general form, some at the foot of the Cordillera, some near the coast, and
some halfway between these points. I will allude only to one other, namely,
the plain of Uspallata, lying on the eastern or opposite side of the
Cordillera, between that great range and the parallel lower range of
Uspallata. According to Miers, its surface is 6,000 feet above the level of
the sea: it is from ten to fifteen miles in width, and is said to extend
with an unbroken surface for 180 miles northwards: it is drained by two
rivers passing through breaches in the mountains to the east. On the banks
of the River Mendoza it is seen to be composed of a great accumulation of
stratified shingle, estimated at 400 feet in thickness. In general
appearance, and in numerous points of structure, this plain closely
resembles those of Chile.

The origin and manner of formation of the thick beds of gravel, sandy clay,
volcanic detritus, and calcareous tuff, composing these basin-like plains,
is very important; because, as we shall presently show, they send arms or
fringes far up the main valleys of the Cordillera. Many of the inhabitants
believe that these plains were once occupied by lakes, suddenly drained;
but I conceive that the number of the separate breaches at nearly the same
level in the mountains surrounding them quite precludes this idea. Had not
such distinguished naturalists as MM. Meyen and Gay stated their belief
that these deposits were left by great debacles rushing down from the
Cordillera, I should not have noticed a view, which appears to me from many
reasons improbable in the highest degree--namely, from the vast
accumulation of WELL-ROUNDED PEBBLES--their frequent stratification with
layers of sand--the overlying beds of calcareous tuff--this same substance
coating and uniting the fragments of rock on the hummocks in the plain of
Santiago--and lastly even from the worn, rounded, and much denuded state of
these hummocks, and of the headlands which project from the surrounding
mountains. On the other hand, these several circumstances, as well as the