Patagonia, poor as she is in some respects, can howeve boast of a greater stock of small rodents [1] than perhaps an other country in the world. Several species of mice ar externally characterized by large thin ears and a very fin fur. These little animals swarm amongst the thickets in th valleys, where they cannot for months together taste a dro of water excepting the dew. They all seem to be cannibals for no sooner was a mouse caught in one of my traps tha it was devoured by others. A small and delicately shape fox, which is likewise very abundant, probably derives it entire support from these small animals. The guanaco i also in his proper district, herds of fifty or a hundred wer common; and, as I have stated, we saw one which mus have contained at least five hundred. The puma, with th condor and other carrion-hawks in its train, follows an preys upon these animals. The footsteps of the puma wer to be seen almost everywhere on the banks of the river and the remains of several guanacos, with their neck dislocated and bones broken, showed how they had met thei death.
April 24th. -- Like the navigators of old when approachin an unknown land, we examined and watched for the mos trivial sign of a change. The drifted trunk of a tree, or boulder of primitive rock, was hailed with joy, as if we ha seen a forest growing on the flanks of the Cordillera. Th top, however, of a heavy bank of clouds, which remaine almost constantly in one position, was the most promisin sign, and eventually turned out a true harbinger. At first th clouds were mistaken for the mountains themselves, instea of the masses of vapour condensed by their icy summits.
April 26th. -- We this day met with a marked change i the geological structure of the plains. From the first starting I had carefully examined the gravel in the river, an for the two last days had noticed the presence of a few smal pebbles of a very cellular basalt. These gradually increase in number and in size, but none were as large as a man' head. This morning, however, pebbles of the same rock but more compact, suddenly became abundant, and in th course of half an hour we saw, at the distance of five o six miles, the angular edge of a great basaltic platform When we arrived at its base we found the stream bubblin among the fallen blocks. For the next twenty-eight mile the river-course was encumbered with these basaltic masses Above that limit immense fragments of primitive rocks derived from its surrounding boulder-formation, wer equally numerous. None of the fragments of any considerable size had been washed more than three or four mile down the river below their parent-source: considering th singular rapidity of the great body of water in the Sant Cruz, and that no still reaches occur in any part, this example is a most striking one, of the inefficiency of rivers i transporting even moderately-sized fragments.
The basalt is only lava, which has flowed beneath the sea but the eruptions must have been on the grandest scale. A the point where we first met this formation it was 120 fee in thickness; following up the river course, the surfac imperceptibly rose and the mass became thicker, so that a forty miles above the first station it was 320 feet thick What the thickness may be close to the Cordillera, I hav no means of knowing, but the platform there attains a heigh of about three thousand feet above the level of the sea we must therefore look to the mountains of that great chai for its source; and worthy of such a source are streams tha have flowed over the gently inclined bed of the sea to distance of one hundred miles. At the first glance of th basaltic cliffs on the opposite sides of the valley, it wa evident that the strata once were united. What power, then has removed along a whole line of country, a solid mass o very hard rock, which had an average thickness of nearl three hundred feet, and a breadth varying from rather les than two miles to four miles? The river, though it has s little power in transporting even inconsiderable fragments yet in the lapse of ages might produce by its gradual erosio an effect of which it is difficult to judge the amount. Bu in this case, independently of the insignificance of such a agency, good reasons can be assigned for believing that thi valley was formerly occupied by an arm of the sea. It i needless in this work to detail the arguments leading to thi conclusion, derived from the form and the nature of th step-formed terraces on both sides of the valley, from th manner in which the bottom of the valley near the Ande expands into a great estuary-like plain with sand-hillock on it, and from the occurrence of a few sea-shells lying i the bed of the river. If I had space I could prove tha South America was formerly here cut off by a strait, joinin the Atlantic and Pacific oceans, like that of Magellan But it may yet be asked, how has the solid basalt bee moved?