the Atlantic. The Amazonian basin which they traverse consists of millions of square miles of virgin forest. The first river is the Marañon, and being the most western and distant its source in the Andean lake of Lauricocha is considered to be the source of the Amazon. Next is the river Huallaga, flowing north until it joins the Marañon. Further to the east the great Ucayali tributary is formed by the Perene, Apurimac, and Vilcamayu, which all force a way through the Andes. Further south the Tono, Arasa, Inambari, Tambopata, and Beni rise on the eastern slopes of the Andes and do not penetrate the range. With the Mamoré and Itenez they form the great Madeira tributary. The rivers which have part of their courses within the Andean system, all have formidable rapids when they force their way through the mountains and enter the great Amazonian plains. These mountain rapids were called puncu, or doors, which the rivers had opened by their irresistible force. That of the Marañon is called the Puncu de Manseriche. On the Huallaga the rapid is known as the Salto de Aguirre, respecting which there is an interesting tradition: then the river is navigable for 160 miles. The Ucayali, a broad stream navigable for 1400 miles, breaks through the mountains at Canchaguayo. The Vilcamayu, navigable for 100 miles, enters the primeval forests by the Puncu de Mainique.
The vast Amazonian forests are approached by the descent of the eastern side of the Andes, down
