"Applying these facto," Mr. Hall continues, "to the case of the prairies of larger dimensions farther south, we Infer, on what seem to be reasonable grounds, that the whole region now occupied by the prairies of the north-west was once an Immense lake, in whose basin sediment of almost impalpable fineness gradually accumulated : that this basin was drained by the elevation of the whole region; but, at first, so slowly, that the finer particles of the superficial deposits were not washed away, but allowed to remain where they wore originally deposited. After the more elevated portions of time former basin had been laid bare, the drainage becoming concentrated in com p arstively narrow channels, the current thus produced, aided perhaps more rapid rise of the region, acquired sufficient velocity to wear down through the finer material on the surface, wash away a portion of it altogether, and mix the rest so effectually with the underlying drift materials, or with abraded fragments of the rocks in place, as to give rise to a different character of soil in the valleys from that of the elevated land. This valley soil, being much less homogeneous in Its composition and containing a larger proportion of coarse material than that of the uplands, seems to have been adapted to the growth of forest vegetation ; and, in consequence of this, we find such localities covered with an abundant growth of timber.
" Wherever there has been a variation from the usual conditions of soil, on the prairie or hi the river bottom, there is a corresponding change In the character of the vegetation. Thus, on the prairie, we sometimes meet with ridges of coarse materiel, apparently deposits of drift, on which, from some local cause, there has never been an accumulation of fine sediment : In such localities we invariably find a growth of timber. This is the origin of the groves scattered over the prairies, for whose isolated position and peculiar circumstances of growth we are unable to account in any other way.
" The condition of things in the river valleys themselves seem. to add to the plitudbility of this theory. In the district which we have more particularly examined, we have found that where the rivers have worn deep and comparatively narrow valleys bordered by precipitous bluffs, there is almost always a growth of forest ; but where the valley widens out, and the bluffs become leas conspicuous, indicating a less rapid erosion and currents of diminished strength, there deposition has taken place under circumstances favourable to the accumulation of a prairie soil, and the result has been the formation of the ' bottom prairie,' which becomes so important a feature of the valleys of the Mississippi and Missouri below the limits of Iowa. Where these
bottom prairies hare become, by any recent change in the course of the river currents, covered with coarser materials, a growth of forest trees may be observed springing up, and indicating by their rapid develop ment the presence of a congenial soil." Vanes. —The whole interior of South America, from the mountains of Caracas on the north to the Straits of Magalhaens on the south, is divided by comparatively low transverse ridges, running east and west into three great basins; that of the Orinoco on the north, that of the Amazon or Maranon in the centre, and that of the La Plata on the south. The first comprises the Danes, vast plains occupying a surface of 260,000 square miles. They may be divided into two principal portions : the first, beginning at the mouths of the Orinoco, extends westward as far as the Andes of New Granada, being bounded on the north by the Caracas, and on the south by the mountainous group of Parim6 and the Rio Apure, an affluent of the lower Orinoco. The other portion of the llama, which is twice as extensive as the first, reaches from the Apure on the north to the Caqueta (an affluent of the Maranon) on the south : having the Andes on the west, and the sierra of Parim6 and the Orinoco on the cast. The inclination of these plains is to the cast and south, and they are traversed by many streams, which, taking their rise from the eastern slope of the Andes, bear their tributary waters to the Orinoco. As the medium height of the Hanes does not exceed 200 feet, the course of the rivers is very slow and often scarcely perceptible.
The chief characteristic of the Harms, says Humboldt, is the abso lute want of hills and inequalities, the perfect level of every part of the soil. Often in the space of 270 square miles there is not an eminence of a foot high. This resemblance to the surface of the sea strikes the imagination most powerfully where the plains are altogether destitute of palm-trees, and where the mountain's of the shore and of the Orinoco are so distant that they cannot be seen. This unvarying equality of surface reigns without interruption from the mouths of the Orinoco to the Villa do Auroro and Ospinos under a parallel of 540 miles in length, and from San Carlos to the Caqucta, on a meridian of 600 miles.
There are however, notwithstanding this uniformity of surface, two I kinds of inequalities in the Banos. The first, called brasses, are hori zontal banks of sandstone or limestone standing four or five feet higher than the rest of the plain, and sometimes many leagues in length. The second kind of inequality, called elm, consists of convex eminences rising to the height of a few fathoms.