The uppermost bed, about twenty feet in thickness, consists of obliquely laminated, soft sandstone, including many pebbles of quartz, and falling at the surface into loose sand.The second bed, only six inches thick, is a hard, dark-coloured sandstone.The third bed is pale-coloured Pampean mud;and the fourth is of the same nature, but darker coloured, including in its lower part horizontal layers and lines of concretions of not very compact pinkish tosca-rock.The bottom of the sea, I may remark, to a distance of several miles from the shore, and to a depth of between sixty and one hundred feet, was found by the anchors to be composed of tosca-rock and reddish Pampean mud.Professor Ehrenberg has examined for me specimens of the two lower beds, and finds in them three Polygastrica and six Phytolitharia.
(The following list is given in the "Monatsberichten der konig.Akad.zu Berlin" April 1845:--POLYGASTRICA.
Fragilaria rhabdosoma.
Gallionella distans.
Pinnularia?
PHYTOLITHARIA.
Lithodontium Bursa.
Lithodontium furcatum.
Lithostylidium exesum.
Lithostylidium rude.
Lithostylidium Serra.
Spongolithis Fustis?)
Of these, only one (Spongolithis Fustis?) is a marine form; five of them are identical with microscopical structures of brackish-water origin, hereafter to be mentioned, which form a central point in the Pampean formation.In these two beds, especially in the lower one, bones of extinct mammifers, some embedded in their proper relative positions and others single, are very numerous in a small extent of the cliffs.These remains consist of, first, the head of Ctenomys antiquus, allied to the living Ctenomys Braziliensis; secondly, a fragment of the remains of a rodent;thirdly, molar teeth and other bones of a large rodent, closely allied to, but distinct from, the existing species of Hydrochoerus, and therefore probably an inhabitant of fresh water; fourth and fifthly, portions of vertebrae, limbs, ribs, and other bones of two rodents; sixthly, bones of the extremities of some great megatheroid quadruped.(See "Fossil Mammalia"page 109 by Professor Owen, in the "Zoology of the Voyage of the 'Beagle';"and Catalogue page 36 of Fossil Remains in Museum of Royal College of Surgeons.) The number of the remains of rodents gives to this collection a peculiar character, compared with those found in any other locality.All these bones are compact and heavy; many of them are stained red, with their surfaces polished; some of the smaller ones are as black as jet.
Monte Hermoso is between fifty and sixty miles distant in a S.E.line from the Ventana, with the intermediate country gently rising towards it, and all consisting of the Pampean formation.What relation, then, do these beds, at the level of the sea and under it, bear to those on the flanks of the Ventana, at the height of 840 feet, and on the flanks of the other neighbouring sierras, which, from the reasons already assigned, do not appear to owe their greater height to unequal elevation? When the tosca-rock was accumulating round the Ventana, and when, with the exception of a few small rugged primary islands, the whole wide surrounding plains must have been under water, were the strata at Monte Hermoso depositing at the bottom of a great open sea, between eight hundred and one thousand feet in depth? I much doubt this; for if so, the almost perfect carcasses of the several small rodents, the remains of which are so very numerous in so limited a space, must have been drifted to this spot from the distance of many hundred miles.It appears to me far more probable, that during the Pampean period this whole area had commenced slowly rising (and in the cliffs, at several different heights we have proofs of the land having been exposed to sea-action at several levels), and that tracts of land had thus been formed of Pampean sediment round the Ventana and the other primary ranges, on which the several rodents and other quadrupeds lived, and that a stream (in which perhaps the extinct aquatic Hydrochoerus lived) drifted their bodies into the adjoining sea, into which the Pampean mud continued to be poured from the north.As the land continued to rise, it appears that this source of sediment was cut off; and in its place sand and pebbles were borne down by stronger currents, and conformably deposited over the Pampean strata.
(FIGURE 15.SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT MAMMIFERS, ATPUNTA ALTA IN BAHIA BLANCA.(Showing beds from bottom to top: A, B, C, D.))Punta Alta is situated about thirty miles higher up on the northern side of this same bay: it consists of a small plain, between twenty and thirty feet in height, cut off on the shore by a line of low cliffs about a mile in length, represented in Figure 15 with its vertical scale necessarily exaggerated.The lower bed (A) is more extensive than the upper ones; it consists of stratified gravel or conglomerate, cemented by calcareo-arenaceous matter, and is divided by curvilinear layers of pinkish marl, of which some are precisely like tosca-rock, and some more sandy.The beds are curvilinear, owing to the action of currents, and dip in different directions; they include an extraordinary number of bones of gigantic mammifers and many shells.The pebbles are of considerable size, and are of hard sandstone, and of quartz, like that of the Ventana: there are also a few well-rounded masses of tosca-rock.
