five hundred. The puma with the condor in its train, follows
and preys upon these animals. The footsteps of the former
were to be seen almost every where on the banks of the river;
and the remains of several guanaco, with their necks dislocated,
and bones broken, showed how they had met their
death.
A p r i l 2 4 t i i .—Like the navigators of old when approaching
an unknown land, we examined and watched for the most
trivial sign of a change. The drifted trunk of a tree, or a
boulder of primitive rock, was hailed with joy, as if we had
seen a forest growing on the flanks of the Cordillera. The
top, however, of a heavy bank of clouds, which remained
almost constantly in one position, was the most promising
sign, and eventually turned out true. At first the clouds
were mistaken for the mountains themselves, instead of the
masses of vapour condensed by their icy summits.
2 6 t i i .— We this day met with a marked change in the
geological structure of the plains. From the first starting I
had carefully examined the gravel in the river, and for
the two last days had noticed the presence of a few small
pebbles of a very cellular basalt. These gradually increased
in number and in size, but none equalled in dimensions a
man’s head. This morning, however, pebbles of the same
rock, but more compact, suddenly became abundant, and in
the course of half an hour, we saw at the distance of five or
six miles the angular edge of a great basaltic platform.
When we arrived at its base we found the stream bubbling
among the fallen blocks. For the next twenty-eight miles,
the river-course was encumbered with these basaltic masses.
Above that limit, immense fragments belonging to a primitive
formation, but derived from the surrounding alluvium,
were equally numerous. In both cases no fragments at all
remarkable in size or number had been washed down the
stream, more than three or four miles below either the
parent rock, or the mass of alluvium from which they were
derived. Considering the singular rapidity of the great body
of water in the St. Cruz, and that no still reaches occur in
any part, these examples are most striking of the inefficiency
of rivers in transporting even moderately-sized fragments.
The basaltic cliffs are obscurely divided by lines of more
cellular or amygdaloidal varieties, and the strata appear to
the eye perfectly horizontal. They overlie the great tertiary
deposits, and are covered (except where denuded in some
of the lower terraces) by the usual beds of gravel. Ih e
basalt is clearly nothing more than lava, which has flowed
beneath the sea; but the eruptions must have been on the
grandest scale. At the point where we first met this formation,
the mass was about 120 feet in thickness; following
the river-course, it imperceptibly rose and became thicker,
so that at forty miles above the first station it was 320
feet. What the thickness may be close to the Cordillera, I
have no means of knowing, but the platform there attains
an elevation between two and three thousand feet above
the level of the sea ; we must therefore look to the mountains
of that great chain for its source ; and worthy of such a
source are streams, that have flowed over the bed of an ocean
to a distance of one hundred miles.
A fine section of the basaltic platform is presented by the
cliffs on both sides of the vaUey, At the first glance it is
evident the strata must at one time have been united. What
power then has removed along a whole line of country, a
solid mass of very hard rock, which had an average thickness
of about three hundred feet, and a breadth varying from rather
less than two to four miles ? The river, though it has so
little power in transporting even inconsiderable fragments,
yet in the lapse of ages might produce an effect by its gradual
erosion, of which it is difficult to judge the limit. But
in this case, independently of the insignificance of sueli
agency, good reasons can be assigned for believing that tins
valley was formerly occupied by an arm of the sea. It is
needless in this work to detail arguments, which chiefly rest
on the form and nature of the banks, on the manner in which
the valley near the foot of the Andes expands into a great
bay, and on the occurrence of a fciv sea-shells lying in the