From the Pongo to the ocean, a distance of more than 2,000 miles, no rocky barrier impedes the further course of the monarch of streams; and according to Herndon (Exploration of the Valley of the Amazons, 1851–1853), its depth constantly remains above eighteen feet, so that it is navigable for large ships all the way from Para to the foot of the Andes! No other river runs in so deep a channel at so great a distance from its mouth, and the tropical rains, spreading over a territory nearly equal in extent to one-half of Europe, are alone able to feed a stream of such colossal dimensions! The first considerable tributary of the Amazons is the Huallaga, which rises near the famous silver-mines of Cerro de Pasco, 8,600 feet above the level of the sea, and is 2,500 paces broad at the point where the rivers meet. Lower down at Nauta, the Ucayale, descending from the distant mountains of Cuzco, adds his waters to the growing stream, after a course nearly 400 miles longer than that of the Tunguragua itself. Where these mighty rivers meet, Lieutenant Lister Maw found a depth of thirty-five fathoms. From the Brazilian frontier, where it still flows at an elevation of 630 feet above the sea, to the influx of the Rio Negro, the Amazons is called the Solimoens, as if one name were not sufficient for its grandeur. During its progress between these two points it receives on the left, the Iça and the Yapura, on the right, the Xavari, the Jutay, the Jurua, the Teffe, the Coary, and the Purus, streams which, in Europe, would only be equalled by the Danube, but are here merely the obscure branches of a giant trunk. The Rio Negro is the most considerable northern vassal of the Amazons. It rises in the Sierra Tunuhy, an isolated mountain-group in the Llanos, and conveys part of the waters of the Orinoco to the Amazons, as if the latter were not already sufficiently great. After a course of 1,500 miles it flows into the vast stream, 3,600 paces broad and 19 fathoms deep. Brigs of war have already ascended the Amazons as far as the Rio Negro, and frigates would find no obstacle in their way. The Madeira, the next great tributary of the regal stream, has thus been named from the vast quantities of drift-wood floating on its waters. Farther on, after having with a side-arm embraced the island of Tupinambaranas, which almost equals Yorkshire in extent, the Amazons now reaches the strait of Obydos, where it narrows to 2,126 paces, and rolls along between low banks in a bed whose depth as yet no plummet hath sounded. The mass of waters which, during the rainy season, rushes in one second through the strait, is estimated by Von Martius at 500,000 cubic feet,--enough to fill all the streams of Europe with an exuberant current.