Page:Science vol. 5.djvu/321

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��#Sefpn«, N". 11:;. They aic, however, still more drpendenl on Ibejteiieral struct lire of South AmeHea; tinee xlnio»t alJ of the great Urazilian rivers belong to hvdrographic syatems which interesC other parls of the continent oiitdiile of the Brazilian plateau.

South America is niiide up of three great masses of higbtands, in great part mouDtalnnus, more or le^s completely separated by depressed areas, in which flow the great rivers Ainazonas. Orinoco, ami Para- guay; the latter, rather than the Paranii, being taken as IbR dotnlnant feature of the La Plata syatBin.' Time masses of highlands are, the long and narrow Andean plateau, the Brazilian plateau, and the pla- leku of Giilana. The Andean plate.iii, being very near llie Pacific coaat, throws nearly all the dr^nage of the continent eastwards to the Atlantic; while the pUteaiii of Bnull and Guiana force the waters to flow northward to the Caribbean Sea. soulliward to the South Atlantic, or eastward through the central baiiti, or great Amazonian depression which sepa- ratps them. Thus the Paraguay ha.^ a southerly course in the centre of the great depression between the Anilean liTgihlands and those of Brazil, receiving a cont!ideralilR part of the drainage of both; the Ori- noco bears the same relation to the highlands of the Andes and of fjuiana, which give a northerly course to the drainage; wlilie the vaster Amazonas lias rela- tions with all three of the continental plateaus, ris- ing in the Andes, and flowing between the higlilands of Braxll and of Guiana, receiving tribute from 1>oth of them, while by means of its great tributaries, the Madeira, Rio Negro, and others above Iheia, i( In- cludes in its haiiu a considerable portion of the great depreraion between the Andes and the two detached eastern plateaus of the continent.

With few eiceptlons, all the great rivers of South America belong to one or the other of these basins, which may be called continental, because they per- Ujn to more than one of the ^reat component parts of the continent. The otiier rivers belong to one or another of the three plateaus; and of these, those of Brazil are the larged and most important, because the Brazilian plate.iu is lai^nr thau that of Gulans, and belter watered than the part of the Andes that drains Into the central depressions. Considering the Unigu^ty 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 thni. have their course In the country, but also those that commence or terminate iu it. are considered as Brazilian, the rivers group themselves naturally Into three great ilivisions; viz.. those that flow directly to the Atlantic, and those that form part of the Amazonian and Platine system*.

The great watershed of the empire, that which separates the indirect from the direct Atlaatir drain- age, is determined by Llie orographic features already described. It does not, however, follow continu- ously the culminating orographic lines, but rather passea from one to another of these lines by means

t the transverM ridges which unite tliera. Thus in

���the south the Atlantic- Parana divide is formed by the culminating ridge* of the southern part of the HerradoMar; in the central portion the Parana-Sio Francisco divide is formed by the Sernt da Ctnastra, or Malta da Curda, in western Minas, and by the transverseridges which unite this chain with the Man- tii|ueira branch of the Serra do Mar, and with the tnountairis of Goyan; hi the northern portion of the great watershed the divide Is fonneil liy the exten- sive ridge, which, branchlngoEf from the (ioyaz Moun- tains, accompanies all the course of the Tocantlns, — 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 deflned and regular in the pai't between the Araguaya-Tocanllns and the Parana, being formed by the mountains of southern Goyax. which extenrl from south-west to north-east; but farther west, be- tween the Pan^^ay on the one side, and th>- Xingu, Tapajiis, and Guapor€. of the Amazonian system, on the other, the divide is near the irregular jagged margin of the .\mazonian tableland, and Is not marked by any notable elevation of the surface; and the passi^ from one system to the other Is compara- tively easy. Thus in the detached SerraUoAguapehy. which seems to be an outlier of the southern margin of the tableland, rise the Rio Alegre — one of the heail waters of the Guaporfi — and the Aguapehy, which, through the Jauru, discharges Into (he Paraguay, lu the lower lands at the base of the serro. and after both theso streams have become navigable for small craft, they flow for a certain distance near together; and the intervening land affonls two practicable portages of the extension of 8,(t4ll metres and II.IOK metres respectively, over which boats have been passed from the waters of the I'ar.iguay to those of the Amazonai, or nice vema. In ITTI an attempt was made to open a canal across the shortest of these portages, which, like the Cassiqiiiari, hIiouM serve to unite two great basins, and affird uninterrupted fluvial communication from the uioutli of the La Plata to that of the Orinoco. The ntleinpt was, how- ever, abandoned ; and accurate levelling will probably Bhow that the project Is impracticable. There are aiso two practicable portals between the atlluents of the upper Tapajiis and the CuyabS, a tributarj- of the Paraguay. One of Lliese Is only l,28-i metres wide; and canoes with cargoes of Amazonian products have frequently been transported to the waters of the Paraguay. As In this region the Tapajtis flows at a much higher level than the CnyaiKi (at least, iu Its navigable portion], the difference of level to be overcome is probably much gre.iler than in the case of the .Vlegre and Aguapehy.

In consequence of the disposition of the highlands and lowlands above Indicated, there is a great diSer- eiice 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 rtf- pre^sions (theHrst deacnnds to a level of less than 10) metres verv near to the foot of the Andes, and Ilie

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