In the extreme western parts of this province, between the Uruguay and a line drawn from Colonia to the R.Perdido (a tributary of the R.Negro), the formations are far more complicated.Besides primary rocks, we meet with extensive tracts and many flat-topped, horizontally stratified, cliff-bounded, isolated hills of tertiary strata, varying extraordinarily in mineralogical nature, some identical with the old marine beds of St.Fe Bajada, and some with those of the much more recent Pampean formation.
There are, also, extensive LOW tracts of country covered with a deposit containing mammiferous remains, precisely like that just described in the more eastern parts of the province.Although from the smooth and unbroken state of the country, I never obtained a section of this latter deposit close to the foot of the higher tertiary hills, yet I have not the least doubt that it is of quite subsequent origin; having been deposited after the sea had worn the tertiary strata into the cliff-bounded hills.This later formation, which is certainly the equivalent of that of the Pampas, is well seen in the valleys in the estancia of Berquelo, near Mercedes; it here consists of reddish earth, full of rounded grains of quartz, and with some small concretions of tosca-rock arranged in horizontal lines, so as perfectly to resemble, except in containing a little calcareous matter, the formation in the eastern parts of Banda Oriental, in Entre Rios, and at other places: in this estancia the skeleton of a great Edental quadruped was found.In the valley of the Sarandis, at the distance of only a few miles, this deposit has a somewhat different character, being whiter, softer, finer-grained, and full of little cavities, and consequently of little specific gravity; nor does it contain any concretions or calcareous matter: I here procured a head, which when first discovered must have been quite perfect, of the Toxodon Platensis, another of a Mylodon (This head was at first considered by Professor Owen (in the "Zoology of the 'Beagle's' Voyage") as belonging to a distinct genus, namely, Glossotherium.), perhaps M.Darwinii, and a large piece of dermal armour, differing from that of the Glyptodon clavipes.These bones are remarkable from their extraordinarily fresh appearance; when held over a lamp of spirits of wine, they give out a strong odour and burn with a small flame;Mr.T.Reeks has been so kind as to analyse some of the fragments, and he finds that they contain about 7 per cent of animal matter, and 8 per cent of water.(Liebig "Chemistry of Agriculture" page 194 states that fresh dry bones contain from 32 to 33 per cent of dry gelatine.See also Dr.Daubeny, in "Edinburgh New Philosophical Journal" volume 37 page 293.)The older tertiary strata, forming the higher isolated hills and extensive tracts of country, vary, as I have said, extraordinarily in composition:
within the distance of a few miles, I sometimes passed over crystalline limestone with agate, calcareous tuffs, and marly rocks, all passing into each other,--red and pale mud with concretions of tosca-rock, quite like the Pampean formation,--calcareous conglomerates and sandstones,--bright red sandstones passing either into red conglomerate, or into white sandstone,--hard siliceous sandstones, jaspery and chalcedonic rocks, and numerous other subordinate varieties.I was unable to mark out the relations of all these strata, and will describe only a few distinct sections:--in the cliffs between P.Gorda on the Uruguay and the A.de Vivoras, the upper bed is crystalline cellular limestone often passing into calcareous sandstone, with impressions of some of the same shells as at St.
Fe Bajada; at P.Gorda, this limestone is interstratified with and rests on, white sand, which covers a bed about thirty feet thick of pale-coloured clay, with many shells of the great Ostrea Patagonica (In my "Journal" page 171 1st edition, I have hastily and inaccurately stated that the Pampean mud, which is found over the eastern part of B.Oriental, lies OVER the limestone at P.Gorda; I should have said that there was reason to infer that it was a subsequent or superior deposit.): beneath this, in the vertical cliff, nearly on a level with the river, there is a bed of red mud absolutely like the Pampean deposit, with numerous often large concretions of perfectly characterised white, compact tosca-rock.At the mouth of the Vivoras, the river flows over a pale cavernous tosca-rock, quite like that in the Pampas, and this APPEARED to underlie the crystalline limestone; but the section was not unequivocal like that at P.Gorda.These beds now form only a narrow and much denuded strip of land; but they must once have extended much further; for on the next stream, south of the S.Juan, Captain Sulivan, R.N., found a little cliff, only just above the surface of the river, with numerous shells of the Venus Munsterii, D'Orbigny,--one of the species occurring at St.Fe, and of which there are casts at P.Gorda:
the line of cliffs of the subsequently deposited true Pampean mud, extend from Colonia to within half a mile of this spot, and no doubt once covered up this denuded marine stratum.Again at Colonia, a Frenchman found, in digging the foundations of a house, a great mass of the Ostrea Patagonica (of which I saw many fragments), packed together just beneath the surface, and directly superimposed on the gneiss.These sections are important: M.
d'Orbigny is unwilling to believe that beds of the same nature with the Pampean formation ever underlie the ancient marine tertiary strata; and Iwas as much surprised at it as he could have been; but the vertical cliff at P.Gorda allowed of no mistake, and I must be permitted to affirm, that after having examined the country from the Colorado to St.Fe Bajada, Icould not be deceived in the mineralogical character of the Pampean deposit.