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Proceeding eastward, by the base of the mountain ranges of San Luis and Cordova, which bound the Pampas to the north-west, we have the testimony of Schmitmeyer, Helms, and other travellers, to the existence of water-worn rocks and beds of shells at Portezuela and on the banks of the Rio Tercero; whilst to the east of the Sierra de Cordova, on the great river Paraná, near Santa Fé, upwards of 300 miles from the sea, Mr. Darwin found in the cliff which skirts the river a stratum of marine shells distinctly exposed a little above the level of the water, surmounted by the alluvial bed, here forty or fifty feet thick, and containing bones of extinct mammalia. "On the cliff-formed shores of Entre Rios," he says, "the line can be distinguished where the estuary mud first encroached upon the deposits of the ocean." In the underlying strata are to be seen gigantic oyster and other marine shells; and thus, I think, we may trace almost continuously the shores of a gulf which must have been nearly as large as that of Mexico, and not very unlike it, perhaps, in general outline.

As the alluvial deposit approaches the great estuary of La Plata and the ocean, it gradually thins out, and the evidences of marine remains become still more frequently exposed to view. At distances varying from one to six leagues from the river, and from fifty to a hundred and fifty miles from the sea, extensive beds of marine shells are to be seen, which the inhabitants dig up and burn for lime. From these deposits I have had specimens of Voluta Colocynthis, Voluta Angulata, Buccinum Globulosum, Buccinum Nov. Spe., Oliva Patula; Cytherœa Flexuosa? Mactra? Venus Flexuosa, Ostrea, &c.* some places these shells are so compact as to form a sort of limestone, easily worked when first quarried, and hardening afterwards on exposure to the air. The church of La Magdalena upon the coast is built of this material. They are generally in good preservation, and some of the species appear almost identical with those now living on the coasts of Brazil; whilst others, on the contrary,

* Now in the Museum of the Geological Society.

In

associated with them are unknown. There is one, found generally by itself, which is particularly interesting, as strikingly proving the gradual growth of the Pampas; it is that

small mya * of which Sowerby has formed the genus Potamomya, usually found in estuaries at the junction of the fresh and salt water, and the type of which is now to be found living at the mouth of the Plata; but the bed from which my fossil specimens were taken is at the Calera de Arriola, to the north of Buenos Ayres, nearly 150 miles from its present habitat; and there (I think clearly indicated by these little shells) must have been once the mouth of the mighty estuary, which in the lapse of ages has been removed to where it now is, more than 50 marine leagues below it.

(Potamo-mya.)

Mr. Bland, one of the North American Commissioners sent to Buenos Ayres in 1818, reasoning upon the quantity of saline matter found in the Pampas, hazards the conjecture that the Pampa formation, as he says, "may have been gently lifted just above the level of the ocean, and left with a surface so unbroken and flat as not yet to have been sufficiently purified of its salt and acrid matter, either by filtration or washing:" and undoubtedly such saline matter does exist very extensively over this formation. Many of the streams, as their names denote, are rendered brackish by it; and lakes which have no outlet, becoming saturated with it, deposit it in regular beds, where in the dry season it may be collected in any quantity.

But is it not more likely to have been washed down from the secondary strata which form the base of the Andes, in which we know that enormous beds of salt abound, particularly in those parts of the Cordillera where the greater number of the rivers rise which run through

* Vide Sowerby, Min. Conch., tab. 263.

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the Pampas, and which are almost all more or less impregnated with it? Can we suppose the Pampas themselves to have originated in sedimentary deposits from those mountain chains, without equally admitting that the alluvial soil washed down may be impregnated with so soluble a substance as the salt which abounds in them ?*

In a country of more varied surface we might expect the briny particles to be carried off by the streams and lost in the sea; but in the dead levels of the Pampas the greater part of the streams themselves are lost long ere they reach the ocean. The waters deposit their sediment over the surface, and the salt is left mixed up with the mud of the marshes, until the rains again collect it, and either partially carry it off in brackish streams, or deposit it in the basins of the inland lakes, in which it is so abundantly found. That it is a superficial deposit I am disposed to infer from the fact that (as elsewhere noticed) in the immediate vicinity of some of the saline lakes and rivers in the Pampas, where the surface of all the surrounding country appears to be incrusted with salt, the people find perfectly fresh and potable water by digging wells to an inconsiderable depth.

Although the ancient bed of an ocean lies no doubt beneath it, the Pampa Formation, so called, is of a very different origin, as is manifestly proved by the skeletons of terrestrial animals, discovered in it in such abundance, and in so perfect a state as to negative any idea that they did not live where they died, and near to where they have been subsequently entombed.

Remains of the megatherium have been found in all parts of the Pampas, from the river Carcaraña, in the province of Santa Fé, to the south of the Rio Salado, à distance of nearly 300 miles in a direct line, and might be met with in still greater numbers if searched for during the dry season, or after long droughts, either in the banks of the rivers, or in the beds of some of the numerous lakes

* See account of beds of fossil salt in the higher ranges of the Sierras of Tucuman and Salta, in the works of Helms and other travellers.-Rock salt abounds in Alto Peru, in the New Red Sandstone

series, one of the best characterized of the Secondary formatious, in the great chain of the Andes, from Panamá to the Straits of Magellan.

which are then dried up. All the remains I sent home were so discovered; and so were those which were found in the bed of the river Luxan, a short distance to the north of the city of Buenos Ayres, and which were sent to Madrid by the Marquis of Loreto in 1789.

The portions of the great skeleton which I obtained, and which are now deposited in the Museum of the Royal College of Surgeons of London, were discovered in the river Salado, to the south of Buenos Ayres, after a drought of unusually long continuance, by a countryman, who, attempting to cross the river at an unfrequented spot, was struck by the appearance of a large mass of something standing above the surface of the water, which, supposing it to be part of the trunk of a tree, he determined to get out if possible: in this he was assisted by some of his brother gauchos, who, throwing their lassoes over it, succeeded in dragging it out, fortunately without injury, for it proved to be nearly the entire pelvis of the megatherium: with it were also brought up several of the other bones, and amongst them some of the vertebræ. To the gauchos the pelvis luckily appeared to be useless: turn it which way they would, they all agreed that it did not make half so comfortable a seat as a bullock's head, the arm-chair of the Pampas; but the vertebræ did not so easily escape, and, in a place where not a stone is to be seen, were eagerly seized upon as substitutes whereon to boil their campkettles. The smaller ones being best suited to the purpose were the first to disappear, which may account for the deficiency of almost all the cervical vertebræ as well as of many of the smaller bones of the feet and other parts.

After a time the pelvis and some of the largest bones were sent as curiosities to the owner of the estancia on which they were found, Don Hilario Sosa, at whose house in Buenos Ayres I first saw them, and who eventually agreed to place them at my disposal, and to allow me to send people to his estancia to search for the remainder of the skeleton. By their exertions many other portions of it were saved; and but for the destruction of some by the country people, as described, and of others which, having been taken out in the first instance, had remained exposed

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for some months to the sun, and had become so brittle in consequence as not to bear removal, the skeleton would have been tolerably perfect.

Further researches, set on foot after finding the great skeleton in the river Salado, led to the discovery of the remains of other gigantic animals hitherto unknown, and no less extraordinary than the megatherium.

When the country people saw the eagerness with which the big bones were sought for, they were not backward in speaking of other places where similar remains had been met with, and were still to be found. Upon this information I once more despatched an intelligent agent, with directions to make a further examination of the low lands to the south of the Salado, and General Rosas, the Governor, taking an interest in the matter, furnished him with a recommendation to the local authorities, desiring them to give him every assistance he might need to ensure his success.

In little less than three weeks we were repaid by the discovery of two more enormous skeletons on the estancias belonging to the Governor, called Villanueva and Las Averias, and in both instances with the novelty of their being encased in a thick coating or shell resembling that of the armadillo. The first, found at Villanueva, though still of gigantic proportions, was of a smaller animal than that taken out of the Salado: it was discovered in the bed of a rivulet, and upon exposure to the air nearly all crumbled to dust; the only portions it was possible to preserve being part of a scapula, a portion of the lower jaw with one small but perfect tooth remaining in it, and a fragment of a hind leg, with some of the bones of the foot. The shell lay a little below the principal mass of the bones, looking like the section of a huge cask: the form of it when first discovered appeared natural and perfect, but it would not bear to be lifted out of its bed, and broke into small pieces and crumbled away immediately.

The other skeleton was of larger proportions. It lay in a bed of hard clay on the side of the lake of Las Averias, partly exposed to view by the action of the water