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Show ZOOLOGY OF TilE VOYAGE OF THE DEAGLE. th nature of the soil. The formation consists of a reddish argillaceous earth, enerally containing irregular concretions of a pale brown, indurated marl. This tone, where most compact, is traversed by small linear cavities, and in several respects resembles the less pure fresh-water limestones of Europe. The concretions not unfrequently become so numerous, that they unite and form a continu-ous stratum, or even the entire mass. At Bajada de Se. F', in the Province of Entre Rios, beds of sand, limestone, and clay of different qualities, containing sharks' teeth and sea-shells, underlie the Pampas deposit. The shells, although numerous, are few in kind. Mr. George B. Sowerby informs me that they appear to belong to one of the less ancient tertiary epochs; they consist of Venus nov. spec. near to V. cancellata; Area nov. spec. near to A. antiquata; a very large oyster, probably an extinct species; an imperfect specimen of a second species of oyster near to 0. eduLis; and a Pecten near toP. opercularis. These beds pass upwards into an indurated marl, and this again into the red argillaceous earth of the Pampas, containing the remains of those extinct quadrupeds, which every where characterize that deposit. To the southward of the Plata level plains of an uniform composition, interrupted only at wide intervals by hills of crystalline rock, extend to a distance of about three hundred miles; and to the northward for at least an equal space, and probably much further. A might have been expected from the perfectly level surface, wherever a continuous section is presented on the banks of the great rivers, very slight chano-es of colour show, that the deposit has been accumulated in strata as horizontal as the land, or as the water-line at the base of the cliffs. In the province of Banda Oriental (to theN. and N. E. of the Plata), and in part of that of Entre Rios, the land, though very low and level, has a foundation of granitic and other primary rocks. These older formations are partially covered, in mo t part , by a reddi h arthy mass containing a few small calcareous concretion ; while in other parts, they are concealed by more regular strata, of indurated marl passing into lime tone, of conglomerates, and ferruginous sandstone. The entire formation probably belongs to the same epoch with that of the Pampa depo it. In the earthy mas , even wh re it is of little thickness and wher~ it migh.t readily be mi taken for detritus produced from the unde1:lying gramte 'remams oflarge quadrupeds have several times been discovered. FOSSIL MAMMALIA. 5 On the shores of the Plata and in the neighbouring districts, proofs of a change of level having taken place between the land and the water within a recent period, may be observed. Both near Monte Video and Colonia del Sacramiento, beds of shells are lying on the beach at the height of several feet above the present tidal action. Near Maldonado I saw estuary shells of recent species embedded in clay, and raised above the level of a neighbouring fresh-water lake. On the banks of the Parana, a shell identical with, or most closely resembling an estuary species (Potamomya labiata, now living in that part of the Plata, where the water is brackish) is accumulated in great masses, which are found some miles inland, and are elevated several yards above the level of the river. Sir Woodbine Parish, also, has in his possession, shells procured from an extensive formation near Ensenada de Barragan (south of Buenos Ayres), which is quarried for lime. Mr. George Sowerby has examined these fossils, and says the following are identical with living kinds ; Voluta colocyntltis, Dillwyn : V. angttlata, Swainson : Buccinum globttlosum, Kiener : a variety of Oliva patttla: a Cytltera!a closely resembling or identical with C. jlexuosa, and a fragment of a second species, probably C. ptt1pw·ascens; Potarnomya labiata; and fragments of oysters. There is, however, a species of Mactra in very great numbers, with which Mr. Sowerby is wholly unacquainted. I may observe that I found recent shells of the first five species inhabiting the coast, a short distance to the southward. Some shelly limestone from the same place, which Sir Woodbine Parish had the kindness to show me, resembles that which I saw at Bajada, and in Banda Oriental. These beds, therefore, probably form parts of the Pampas deposit, and are not merely indicative of the period of its elevation. N evertheless, on the opposite shores of the Plata, near the mouth of the Uruguay, I found lines of sand dunes, where the JJtlactm and Cytltera!a jlexuosa were lying in such quantities on the bare surface, that the inhabitants, by merely sifting the sand, collect them for burning into lime. After these facts we may feel certain, that at a period not very remote, a great bay occupied the area both of the Pampas and of the lower parts of Banda Oriental. Into this bay the rivers which are now united iu the one great stream of the Plata, must formerly have carried down (as happens at the present day) the carcasses of the animals, inhabiting the surrounding countries ; and their |