April 10, 1885.] 



SCIENCE 



297 



in Science, No. 112. They are, however, still more 

 dependent on the general structure of South America; 

 since almost all of the great Brazilian rivers belong to 

 hydrographic systems which interest other parts of 

 the continent outside of the Brazilian plateau. 



South America is made up of three great masses 

 of highlands, in great part mountainous, more or less 

 completely separated by depressed areas, in which 

 flow the great rivers Amazonas, Orinoco, and Para- 

 guay; the latter, rather than the Parana, being taken 

 as the dominant feature of the La Plata system. 1 

 These masses of highlands are, the long and narrow 

 Andean plateau, the Brazilian plateau, and the pla- 

 teau of Guiana. The Andean plateau, being very 

 near the Pacific coast, throws nearly all the drainage 

 of the continent eastwards to the Atlantic; while the 

 plateaus of Brazil and Guiana force the waters to 

 flow northward to the Caribbean Sea, southward to 

 the South Atlantic, or eastward through the central 

 basin, or great Amazonian depression which sepa- 

 rates them. Thus the Paraguay has a southerly 

 course in the centre of the great depression between 

 the Andean highlands and those of Brazil, receiving 

 a considerable part of the drainage of both; the Ori- 

 noco bears the same relation to the highlands of the 

 Andes and of Guiana, which give a northerly course 

 to the drainage; while the vaster Amazonas has rela- 

 tions with all three of the continental plateaus, ris- 

 ing in the Andes, and flowing between the highlands 

 of Brazil and of Guiana, receiving tribute from both 

 of them, while by means of its great tributaries, the 

 Madeira, Rio Negro, and others above them, it in- 

 cludes in its basin a considerable portion of the great 

 depression between the Andes and the two detached 

 eastern plateaus of the continent. 



With few exceptions, all the great rivers of South 

 America belong to one or the other of these basins, 

 which may be called continental, because they per- 

 tain to more than one of the great component parts 

 of the continent. The other rivers belong to one or 

 another of the three plateaus ; and of these, those of 

 Brazil are the largest and most important, because 

 the Brazilian plateau is larger than that of Guiana, 

 and better watered than the part of the Andes that 

 drains into the central depressions. Considering the 

 Uruguay as belonging to the La Plata system, the 

 exclusively Brazilian rivers (in a geographical sense) 

 are those that flow from the eastern watershed of 

 the plateau directly to the Atlantic. If, however, 

 not only those that have their course in the country, 

 but also those that commence or terminate in it, are 

 considered as Brazilian, the rivers group themselves 

 naturally into three great divisions; viz., those that 

 flow directly to the Atlantic, and those that form part 

 of the Amazonian and Platine systems. 



The great watershed of the empire, that which 

 separates the indirect from the direct Atlantic drain- 

 age, is determined by the orographic features already 

 described. It does not, however, follow continu- 

 ously the culminating orographic lines, but rather 

 passes from one to another of these lines by means 

 of the transverse ridges which unite them. Thus in 

 1 See the map on p. 274. 



the south the Atlantic-Parana divide is formed by 

 the culminating ridges of the southern part of the 

 Serra do Mar; in the central portion the Parana-Sao 

 Francisco divide is formed by the Serra da Canastra, 

 or Matta da Corda, in western Minas, and by the 

 transverse ridges which unite this chain with the Man- 

 tiqueira branch of the Serra do Mar, and with the 

 mountains of Goyaz; in the northern portion of the 

 great watershed the divide is formed by the exten- 

 sive ridge, which, branching off from the Goyaz Moun- 

 tains, accompanies all the course of the Tocantins, — 

 a ridge whose true orographic character is, as stated 

 in a previous article, very imperfectly known. 



The secondary watershed, which divides the waters 

 of the Amazonas from those of the La Plata system, 

 is well defined and regular in the part between the 

 Araguaya-Tocantins and the Parana, being formed 

 by the mountains of southern Goyaz, which extend 

 from south-west to north-east ; but farther west, be- 

 tween the Paraguay on the one side, and the Xingii, 

 Tapajos, and Guapore, of the Amazonian system, 

 on the other, the divide is near the irregular jagged 

 margin of the Amazonian tableland, and is not 

 marked by any notable elevation of the surface ; and 

 the passage from one system to the other is compara- 

 tively easy. Thus in the detached Serra do Aguapehy, 

 which seems to be an outlier of the southern margin 

 of the tableland, rise the Rio Alegre — one of the head 

 waters of the Guapore — and the Aguapehy, which, 

 through the Jaurii, discharges into the Paraguay. In 

 the lower lands at the base of the serra, and after 

 both these streams have become navigable for small 

 craft, they flow for a certain distance near together; 

 and the intervening land affords two practicable 

 portages of the extension of 8,640 metres and 11,708 

 metres respectively, over -which boats have been 

 passed from the waters of the Paraguay to those of 

 the Amazonas, or vice versa. In 1773 an attempt 

 was made to open a canal across the shortest of these 

 portages, which, like the Cassiquiari, should serve 

 to unite two great basins, and afford uninterrupted 

 fluvial communication from the mouth of the La 

 Plata to that of the Orinoco. The attempt was, how- 

 ever, abandoned; and accurate levelling will probably 

 show that the project is impracticable. There are 

 also two practicable portages between the affluents 

 of the upper Tapajos and the Cuyaba, a tributary of 

 the Paraguay. One of these is only 1,285 metres 

 wide ; and canoes with cargoes of Amazonian products 

 have frequently been transported to the waters of 

 the Paraguay. As in this region the Tapajos flows 

 at a much higher level than the Cuyaba (at least, in 

 its navigable portion), the difference of level to be 

 overcome is probably much greater than in the case 

 of the Alegre and Aguapehy. 



In consequence of the disposition of the highlands 

 and lowlands above indicated, there is a great differ- 

 ence between the rivers of these two divisions, which 

 is of capital importance with reference to the inter- 

 nal communications of the empire. The Amazonas 

 and Paraguay, being pre-eminently rivers of the de- 

 pressions (the first descends to a level of less than 103 

 metres very near to the foot of the Andes, and the 



