252 



AMAZONIA AND LA PLATA. 



The divide between tlie sources of the Guaporé and the headwaters of the 

 Paraguay scarcely exceeds 1,G5(J feet in altitude, and the Brazilian uplands appear 

 to be connected with those of the Chiquitos territory only by a very narrow 

 isthmus of ancient rocks. Here is the true geographical centre of South America. 



On the maps a continuous chain of mountains is traced between the Madeira 

 and Tapajoz basins, then between the Tapajoz and Paraguay, and lastly between 

 the Tapajoz and the Araguaya. Yet it is certain that this semi-circular ridge 

 has but a fragmentary existence. The heights dominating the j^lains of the 

 upper Paraguay and its affluents are in reality merely the escarpments of a 

 plateau disposed in horizontal or very slightly inclined strata, and eroded by the 

 streams now descending towards the Amazons. The rampart itself has a mean 



Fig. 110. — Tapajoz and Paraguay Watershed. 



Scale 1 : .^.50'::,00U. 



125 Miles. 



elevation of no more than 1,650 feet, and above the edge of the plateau rise a 

 few isolated crests, attaining here and there a height of some 3,000 feet. 



Thus the orographic system of the Matto Grosso watershed, indifferently 

 called " cordilheira " or " campos " dos Parexi, from the local Indian tribe, 

 presents a mountainous aspect, only as seen from the south. On this steep side 

 the face of the escarpments is carved into rocky walls, sharp peaks, or needles. 

 Bat on the opposite side, facing the Tapajoz and Xingu basins, nothing is seen 

 except a long gently-inclined slope gradually merging in the Amazonian plains. 



The southern parts of the Araxa, as the edge of the plateau is generally called, 

 date probably from paleozoic times, and here are represented carboniferous, 

 devonian, and silurian formations. Farther north, in the zone of cataracts 

 traversed by the Madeira, Tapajoz, Xingu, Tocantins, and their affluents, the 

 rocks exposed by the erosions of these streams are all of crystalline character — 

 granites, gneiss, porphyries, and quartzites. 



