1879.] 1°' [Derby. 



stroyed by the great cuts made by the Rios Paraguay and Madeira in 

 the south, and the Rios Negro and Orinoco in the north, and only a com- 

 paratively slight continental depression would be required to entirely sepa- 

 rate these regions. In fact, the region of Guiana may be considered an 

 island, in consequence of the existence of that geographical phenomenon, 

 the Cassiquiari, uniting the Orinoco and Rio Negro. 



The Amazonas, unlike the Orinoco and the La Plata in this respect, has 

 relations with all three of the mountainous regions above indicated. The 

 upper part, or Marahon, belongs exclusively to the Andes ; the middle, or 

 Solimoes portion, is in the region intermediate between the Andes and the 

 highlands of Brazil and of Guiana ; and the Lower Amazonas, from the 

 mouth of the Rio Negro to the sea, is between these tw r o last masses of 

 highlands. 



From a purely geographical point of view, the Lower Amazonas and the 

 Solimoes might be united in a single section, because the differences be- 

 tween these two portions are, at present, much less than those between the 

 Marahon and the rest of the great stream. Taking into consideration, 

 however, the geological structure, and especially the conditions which 

 geology shows to have existed in former times, it will be seen, as I hope to 

 prove, that this division of the valley into three sections is a natural one. 



An examination of the hydrography of the Amazonian basin, taken as a 

 whole, reveals much more noticeable differences in the three portions than 

 are seen in the valley properly so called. The Maranon and its great 

 southern tributaries in the Andean region, the Huallaga and the Ucayael, 

 descend from great elevations in the cordilheiras, and flow northerly in 

 the general direction of the trend of the mountains, until, escaping from 

 them, the Marahon takes an easterly direction, in which it presents a no- 

 table contrast with theUcayale which, although it has descended to a com- 

 paratively low level, a long distance above its mouth, still continues to 

 flow in a northerly direction, as if it were forced for some reason, to follow 

 the margin of the mountainous region. The northern tributaries of the 

 Maranon, including the Napo which empties nearly opposite the mouth 

 of the Ucayale, descend from the Andes of Equador in a south-easterly 

 direction, directed by the slope of the mountains. The area drained by 

 the Marahon and its tributaries is very long in the direction north-south, 

 but very narrow in the direction east-west. 



In the Solimoes region, on the contrary, the region drained on the north 

 is rectangular in shape, the longest axis of the. rectangle extending east- 

 west, parallel with the river, and the tributaries in this region, including 

 the Rio Negro, flow in valleys of slight elevation in an easterly direction, 

 subparallel with the Solimoes, as if they were crowded down towards the 

 south, and directed in their courses by a line of highlands, uniting the 

 mountains of Guiana with the Andes. The southern area, drained by the 

 Solimoes and included between the Ucayale, the Madeira, and the eastern 

 prolongation of the Andes in Bolivia, is of triangular shape. The tribu- 

 taries in this area rise in the plateau east of the Andes, at moderate eleva- 



