Such, I conceive, may have been the origin of the far spread formation of the present pampas or plains, throughout which are to be found the fossil remains of gigantic animals of long lost species, such as the megatherium and mastodon, and other monsters yet unnamed, which in former ages may have grazed upon the abundant pastures produced in the rich loamy lands saved from the waters; whilst beneath, in strata of marine shells, are no less incontestable evidences of the ancient bed of the ocean. It cannot be expected that, in a country so uniformly level as the pampas, sections of sufficient depth will frequently occur to exhibit the underlying strata. They must be looked for at the outlying extremities of the formation, where the upper bed thins out,--to use a geological term. Now there is nothing that I know of to interrupt the uniformity of the stratum between the southern shore of the Plata on the one side, and the eastern base of the Andes on the other, and at both these extremes marine remains are strikingly exhibited. General Cruz, in his journey from Antuco to Buenos Ayres (noticed in chapter viii.), in passing through the valleys in the lower ranges of the Cordillera, immediately before reaching the pampas, was exceedingly struck with the abundance of marine remains thereabouts. He says, in his diary, "In all the hills and valleys under the Cordillera, as far as the river Chadileubu, a great quantity of marine remains are met with, some of them constituting a sort of limestone. Not only may these remains be observed upon the surface, but also at great depths below it, in the sections formed by the torrents as they descend from the mountains: there can, therefore, be no doubt that the waters of the sea once occupied the place of the land in those parts." Proceeding eastward, by the base of the mountain ranges of San Luis and Cordova, which bound the pampas to the north, we have the testimony of water-worn rocks and beds of shells in that direction, from Schmitmeyer, Helms, and other travellers, at Portezuela and on the banks of the Tercero; and beyond the Sierra de Cordova, on the great river Paranã, near Santa Fé, 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, and with the alluvial bed over it, forty or fifty feet thick, containing bones of extinct mammalia.