I cannot leave the pampas without a few words upon their geological features, and upon the remarkable contrast exhibited in the appearance of the country on the northern and on the southern shores of the Plata. On the north side the formation is of clay-slate, gneiss, and granite, of which the islands in the river above Buenos Ayres are also composed, particularly Sola, Las Hermanas, and Martin Garcia, where the granite is quarried for the pavement of the city. On the southern side every trace of rock is entirely lost, and for hundreds of miles inland not even the smallest pebble is to be met with. As far as we are yet acquainted with it, the whole of that vast level called the pampas, reaching from the eastern terminations of the Andes to the shores of the Plata, appears to be one immense bed of alluvium tranquilly deposited during the imperceptible lapse of ages; the delta perhaps, not of one, but of numerous rivers, originating in a once more general diffusion of the waters from the Andes before their courses were defined by their present channels. Some such process of formation appears still to be going on in many parts of the pampas, where muddy streams and streamlets, the collections from the mountains in the south and of the rainy seasons, too sluggish to force a way through the level country, inundate the plains, and gradually deposit the alluvial sediment, together with a prodigious quantity of decomposed vegetable matter, in the swamps and morasses, until accumulations of fresh soil take place in sufficient quantity to throw off the waters again in some other direction. The bed of the Plata, itself the reservoir of a hundred rivers, is, from all I could learn, gradually silting up, and, wide as it is at the present day, along its shores, and particularly above Buenos Ayres, may be distinctly traced the evidences of the waters having once occupied a bed of infinitely greater extent. Every observation tends to the inference that this now mighty estuary may, centuries hence, be reduced to similar bounds and rules to those which govern the outlets of the Amazons, the Mississippi, the Nile, and the Ganges. Nor will this require, perhaps, so long a period as might at first be imagined.[46] If we except the narrow channel between the Chico and Ortiz banks, below Buenos Ayres, the average depth of the river between that city and Monte Video does not exceed twenty feet. The prodigious quantity of mud and detritus brought down by it is well known,--the whole river, wide as it is, is at times discoloured by it. Now, if but enough of this sediment is deposited to cause the small annual increase of only half an inch in the bed of the river, it will not require 500 years to form a delta, which, in the language of the country, will be nothing more or less than an extension of the existing pampas.