The Laguna of Santa Ana, in the Department of Huánuco, though only six square miles in circumference, is worthy of mention as the source of the Marañon, one of the parent streams of the great Amazon River. It is situated in the Nudo of Cerro de Pasco, a few leagues northwest of Junin. The Ucayali and the Madeira—the latter with its tributaries, the Beni and the Madre de Dios, being the longest of all the Amazon’s tributaries—have their source in the Nudo of Vilcanota, and, like the Marañon, have an inter-Andean course for a considerable distance before entering the vast Amazon plain. On the route from Puno to Cuzco, at a point called La Raya, the mountain summits surround a small basin, in the midst of which is a little pond; from this insignificant beginning, two streams take their course in opposite directions,—the Ramis, which flows into Lake Titicaca, and the Vilcanota, afterward the Urubamba, which, after forming a junction with the Apurimac, another important river of this region, becomes the chief affluent of the Ucayali. The Urubamba and the Apurimac run parallel along the separate valleys formed between the Occidental and Oriental ranges by the intermediary chain which accompanies them from the Nudo of Vilcanota northward, until the three Cordilleras join again in the Nudo of Cerro de Pasco. Separating north of Cerro de Pasco, these three great ranges form the valleys of the Marañon and the Huallaga. The rapids, or “pongos,” which interrupt navigation on nearly all the Amazon tributaries at some point of their course, usually indicate a break in the enclosing Andean walls, simultaneous with a change of their direction, the river resisting an effort to turn its channel, and plunging through the narrow gorge that affords it escape.