xv The Continental Area 281 



taries descending from the mountain borders. In the 

 rainy season, June to August, the plains are flooded, driving 

 the inhabitants to take refuge in houses built in the trees. 

 The Orinoco, from its source on the south-west of the 

 Guiana High Plain, flows along the watershed which parts 

 its basin from that of the Amazon. One branch, retaining 

 the name Orinoco, eventually flows down the northern slope 

 and sweeps east to the sea, while another, known as the 

 Casiquiare, breaks away down the southern slope and flows 

 rapidly into the Rio Negro, a tributary of the Amazon. 

 The two great river systems are thus connected by a natural 

 canal. 



361. Amazon Basin. At a distance of 1900 miles 

 from the sea the vast central plain only reaches an 

 elevation of 600 feet, and the basin of the Amazon pre- 

 sents the gentlest land-slope in the world. Nearly the whole 

 plain is covered with dense tropical forests, and it is there- 

 fore called the Selvas or Woods. On each side the Amazon 

 and its tributaries overflow in the rainy season ( 318), 

 covering the land for 20 or 30 miles from the banks, 

 so that the forests appear to be growing in the water ; 

 and depositing fine alluvial soil which, over the whole region, 

 does not contain a stone as large as a pea. Numerous 

 great tributaries, many exceeding 1000 miles in length, 

 converge to the main river from the slopes and high valleys 

 of the Andes. Of these the Maranon is generally con- 

 sidered the head stream, although the Ucayali is longer. 

 Other rivers flow in like veins joining a leaf-stem, from 

 the Guiana High Plain in the north and the Brazil 

 High Plain in the south. Two of the- largest rivers of 

 the latter region flow north in wide valleys but do not 

 reach the Amazon : one, the Tocantins, enters the sea 

 close to its mouth, and the other, the Rio San Fran- 

 cisco, curving sharply to the east, reaches the Atlantic 

 about 10 N. 



362. La Plata River System. From the temporary 

 lake which forms west of the flat low plateau of Matto Grosso 

 in the rainy season, and gives origin to some of the southern 

 tributaries of the Amazon, the river Paraguay flows south 



