The first ridge, south of the Plata, which projects through the Pampean formation, is the Sierra Tapalguen and Vulcan, situated 200 miles southward of the district just described.This ridge is only a few hundred feet in height, and runs from C.Corrientes in a W.N.W.line for at least 150 miles into the interior: at Tapalguen, it is composed of unstratified granular quartz, remarkable from forming tabular masses and small plains, surrounded by precipitous cliffs: other parts of the range are said to consist of granite: and marble is found at the S.Tinta.It appears from M.
Parchappe's observations, that at Tandil there is a range of quartzose gneiss, very like the rocks of the S.Larga near Maldonado, running in the same N.N.E.and S.S.W.direction; so that the framework of the country here is very similar to that on the northern shore of the Plata.(M.d'Orbigny's "Voyage" Part.Geolog.page 46.I have given a short account of the peculiar forms of the quartz hills of Tapalguen, so unusual in a metamorphic formation, in my "Journal of Researches" 2nd edition page 116.)The Sierra Guitru-gueyu is situated sixty miles south of the S.Tapalguen:
it consists of numerous parallel, sometimes blended together ridges, about twenty-three miles in width, and five hundred feet in height above the plain, and extending in a N.W.and S.E.direction.Skirting round the extreme S.E.termination, I ascended only a few points, which were composed of a fine-grained gneiss, almost composed of feldspar with a little mica, and passing in the upper parts of the hills into a rather compact purplish clay-slate.The cleavage was nearly vertical, striking in a N.W.by W.and S.E.by E.line, nearly, though not quite, coincident with the direction of the parallel ridges.
The Sierra Ventana lies close south of that of Guitru-gueyu; it is remarkable from attaining a height, very unusual on this side of the continent, of 3,340 feet.It consists up to its summit, of quartz, generally pure and white, but sometimes reddish, and divided into thick laminae or strata: in one part there is a little glossy clay-slate with a tortuous cleavage.The thick layers of quartz strike in a W.30 degrees N.
line, dipping southerly at an angle of 45 degrees and upwards.The principal line of mountains, with some quite subordinate parallel ridges, range about W.45 degrees N.: but at their S.E.termination, only W.25degrees N.This Sierra is said to extend between twenty and thirty leagues into the interior.
PATAGONIA.
With the exception perhaps of the hill of S.Antonio (600 feet high) in the Gulf of S.Matias, which has never been visited by a geologist, crystalline rocks are not met with on the coast of Patagonia for a space of 380 miles south of the S.Ventana.At this point (latitude 43 degrees 50 minutes), at Points Union and Tombo, plutonic rocks are said to appear, and are found, at rather wide intervals, beneath the Patagonian tertiary formation for a space of about three hundred miles southward, to near Bird Island, in latitude 48 degrees 56 minutes.Judging from specimens kindly collected for me by Mr.Stokes, the prevailing rock at Ports St.Elena, Camerones, Malaspina, and as far south as the Paps of Pineda, is a purplish-pink or brownish claystone porphyry, sometimes laminated, sometimes slightly vesicular, with crystals of opaque feldspar and with a few grains of quartz; hence these porphyries resemble those immediately to be described at Port Desire, and likewise a series which I have seen from P.Alegre on the southern confines of Brazil.This porphyritic formation further resembles in a singularly close manner the lowest stratified formation of the Cordillera of Chile, which, as we shall hereafter see, has a vast range, and attains a great thickness.At the bottom of the Gulf of St.
George, only tertiary deposits appear to be present.At Cape Blanco, there is quartz rock, very like that of the Falkland Islands, and some hard, blue siliceous clay-slate.
At Port Desire there is an extensive formation of the claystone porphyry, stretching at least twenty-five miles into the interior: it has been denuded and deeply worn into gullies before being covered up by the tertiary deposits, through which it here and there projects in hills; those north of the bay being 440 feet in height.The strata have in several places been tilted at small angles, generally either to N.N.W.or S.S.E.By gradual passages and alternations, the porphyries change incessantly in nature.I will describe only some of the principal mineralogical changes, which are highly instructive, and which I carefully examined.The prevailing rock has a compact purplish base, with crystals of earthy or opaque feldspar, and often with grains of quartz.There are other varieties, with an almost truly trachytic base, full of little angular vesicles and crystals of glassy feldspar; and there are beds of black perfect pitchstone, as well as of a concretionary imperfect variety.On a casual inspection, the whole series would be thought to be of the same plutonic or volcanic nature with the trachytic varieties and pitchstone;but this is far from being the case, as much of the porphyry is certainly of metamorphic origin.Besides the true porphyries, there are many beds of earthy, quite white or yellowish, friable, easily fusible matter, resembling chalk, which under the microscope is seen to consist of minute broken crystals, and which, as remarked in a former chapter, singularly resembles the upper tufaceous beds of the Patagonian tertiary formation.
