Without noticing the minor branches, which would swell this article to an inconvenient length, it will be sufficient to review the great basins, not in their order of extent, but in respect to their position, relatively advanc ing from north to south.
The basin of Magdalena is long and narrow, and ex tending from south to north, nearly along the 2d and 3d meridian E. of Washington City, is in effect only an ex tension of the great vallies of the Andes. From Po payan, near the head of the basin, at an elevation of 5900 feet, the valley sinks gradually, but renders the rivers very unnavigable, from the great general descent. The entire basin stretches from N. lat. 2° to 11°, length 700; mean breadth about 120, area 84,000 square miles. Down this basin flow the various confluents of the two great constituent branches of the Magdalena, the Cauca and Magdalena proper. These streams unite at N. lat. 9°, and flowing thence one degree of latitude more northward, separate into two branches, which encircle the city of Carthagena, and enter the Caribbean sea 70 miles asunder.
The two minor basins of Atrato to the west, and Ma racaibo to the east of that of Magdalena, belong to the Caribbean slope of South America, which is continued eastward to the gulf of Paria or Trinidad by a narrow strip.
From the sources of the Magdalena, a vast arm of the Andes extends north-eastward upwards of eight hundred miles, until imperceptibly merged in the plains of Coro. From this chain flow westward the confluents of the Magdalena, and to the eastward those of the Orinoco.
Entering the basin of Orinoco, introduces us to the most interesting river system of the earth. In this sys tem, though only the third in extent, the basin of the Orinoco embraces an area of nearly four hundred thou sand square miles, between N. lat. and 10°, and from 2° to 17° long. E. from Washington City.
The particular confluents of this great river are too numerous to admit individual notice: it may suffice to observe, that the north-western branch, the Apure, draws its sources from the mountains of Merida and Paramo de la Rosa, and from the chain of Venezuela ; some of its northern fountains rise within 40 miles of the Carib bean sea in the provinces of Maracaibo and Caracas.
The Meta, a longer, though perhaps in volume not so great a body of water, follows the Apure, and is again succeeded by the Guaviare. The latter rises in the main Cordilleras of the Andes, interlocking sources with the Rio Negro and the Magdalena.
The Orinoco properly so called, rises in the mountains of Guyana, N. lat. and long. 11° E. from Washington ; flowing thence eastward, bends gradually to the south, south-west, and west, to where a stream flows from it into the Rio Negro. At the efflux of the ('assiquiari, the Orinoco has already flowed by comparative courses upwards of four hundred miles. The junction of the Amazon and Orinoco by the Cassiquiari and Rio Negro on an elevated plateau, is certainly the most singular and important fact in the natural history of rivers. 'I his connexion, long denied after discovery, has been added to ascertained fact and science by Humboldt and Born pland, who actually passed from the Rio Negro by the Cassiquiari into the Orinoco.
With its great extent, variety of feature, and peculiar structure, the Orinoco is still an humble stream if com pared to the Amazon. The latter basin sweeping from the Atlantic to the verge of the Pacific Ocean, embraces the central and southern equatorial regions of South America, and comprises the great superficies of two mil lions eight hundred thousand square miles. Similar to all very large rivers, there is an idle dispute respecting the source particularly deserving of the title given to the basin. Measured from the mouth, by the valley of the Negro, the length is about 1800 miles; by the Madeira, 2300 miles; but following the main volume and the Ucayale, it is 2900 miles. In a nearly western and east ern direction from the sources of the Tunguragua, the chord of the basin is about 2600 miles. lf, therefore, we regard length of course as decisive of the question, the Ucayale is the main stream of the Amazon basin. The Ucayale and Madeira rise together from the Andes in the province of Cochabamba ; but pursuing different directions, the former flows 2100 miles, and the latter 1800 miles before their union.