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Show 216 PATAGONIA. April, 1834. five hundred. The puma with the condor in its train, follows and preys upon these animals. The footsteps of the fo~mer 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. APRIL 24TH.-Like the navigators of old when approach-ing 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 sicrn and eventually turned out true. At first the clouds 0 ' were mistaken for the mountains themselves, instead of the masses of vapour condensed by their icy summits. 26TH.-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 thad 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, l10wever, 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 o.ccur in April, 1834. BASALTIC PLATFORM, 217 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. The 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, l 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 valley. 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 tbis case, independently of the insignificance of such agency, good reasons can be assigned for believing that this 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 few sea-shells lying in the |