OCR Text |
Show 200 PATAGONIA. Jan. 1834. Roy on a long walk round the head of the harbour. We were eleven hours without tasting any water, a~d some ?f the party were quite exhausted. From the sum1~nt of a hill (since well named Thirsty Hill) a fine lake was spied, and two of the party proceeded with concerted signals to show whether it was fresh water. What was our disappointment to find a snow-white expanse of salt, crystallized in great cubes ! We attributed our extreme thirst to the dryness of the atmosphere ; but whatever the cause might be, we were exceedingly glad late in the evening to get back to the ?~ats. Although we could nowhere find, during our whole VISlt, a single drop of fresh water, yet some must exist; for by an odd chance I found on the surface of the salt water, near the head of the bay, a Colymbetes not quite dead, which in all prob.ability had lived in some not far distan~ pool. ~hr.ee other kmds of insects,-a Cincindela, like hyb1'Zda, Cymmdis, and a Harpalus which all live on muddy fiats occasionally overflowed by the s~a, and one other beetle found dead on the plain,-completes the list of coleoptera. A good-sized fly (Tabanus) was extremely numerous, and tormented us by its painful bite. The common horsefly, which is so troublesome in the shady lanes of England, belongs to this genus. We here have the puzzle, that so frequently occurs in the case of musquitoes ; on the blood of what animals do these insects commonly feed? The guanaco is nearly the only warmblooded quadruped, and they are present in numbers quite inconsiderable, compared to the multitude of :flies. The foundation of porphyry is not here present, as it was at Port Desire, and in consequence the tertiary ~eposits are arranged with greater regularity. Five successive plains of different altitudes are very distinct. The lower one is a mere fringe nearly on a level with the sea, but the upper one is elevated 950 feet. This latter is represented in this neighbourhom! only by a few truncate conical hills, of exactly the same height. It was very interesting to stand on one of these fiat patches of gravel, and viewing the wide surrounding country, to speculate on the enormous quantity of matter which must Jan. 1834. GEOLOGY. 201 have been removed, thus to leave these mere points, as meaures of the former table-land. I will now give a brief sketch of the geology of the grand tertiary formation of Patagonia, which extends from the Strait of Magellan to the Bay of S. Antonio. In Europe, deposits of the more recent eras have generally been accumulated in small basins or trough-shaped hollows. In South America, however, the entire plains of Patagonia extending seven hundred miles in length, and backed on the one hand by the chain of the Andes, and fronted on the other by the shores of the Atlantic, are thus constituted. Moreover the northern boundary is merely assumed in consequence of a mineralogical change in the strata : if organic remains were present, it probably would be found to be only an artificial limit. Again to the northward ( 1300 miles distant from the Strait of Magellan) we have the Pampas deposit, which though very different in composition, belongs to the same epoch with the superficial covering of the plains of Patagonia. The cliffs on the coast give the following section : The lower part consists of a soft sandstone, containing large concretions of a harder nature. These strata contain many organic remains-immense oysters nearly a foot in diameter, curious pectens, echini, turritellre, and other shells, of which the greater portion are extinct, but a few resemble those now existing on the coast.* Above these fossiliferous beds, a mass of soft friable stone or earth is superimposed, which, from its extreme whiteness, has been mistaken for chalk. It is, however, quite different ; and closely resembles the less argillaceous varieties of decomposed felspar. This substance . never contains organic remains. Lastly, the cliff is surmounted by a thick bed of gravel, almost exclusively derived from porphyritic rocks. For the sake of making the following description more easily intelligible, I have subjoined an imaginary section of the plains near the coast. • The geologist must recollect this is a mere sketch, and that the fossil shells have not yet been carefully examined. |