232 THROUGH THE BRAZILIAN WILDERNESS 



At Vilhena we were on a watershed which drained into 

 the Gy-Parana, which itself runs into the Madeira nearly 

 midway between its sources and its mouth. A little far- 

 ther along and northward we again came to streams run- 

 ning ultimately into the Tapajos; and between them, and 

 close to them, were streamlets which drained into the 

 Duvida and Ananas, whose courses and outlets were un- 

 known. This point is part of the divide between the basins 

 of the Madeira and Tapajos. A singular topographical 

 feature of the Plan Alto, the great interior sandy plateau 

 of Brazil, is that at its westernmost end the southward- 

 flowing streams, instead of running into the Paraguay as 

 they do farther east, form the headwaters of the Guapore, 

 which may, perhaps, be called the upper main stream of 

 the Madeira. These westernmost streams from the south- 

 ern edge of the plateau, therefore, begin by flowing south; 

 then for a long stretch they flow southwest; then north, 

 and finally northeast into the Amazon. According to some 

 exceptionally good geological observers, this is probably 

 due to the fact that in a remote geologic past the ocean 

 sent in an arm from the south, between the Plan Alto and 

 what is now the Andean chain. These rivers then emp- 

 tied into the Andean Sea. The gradual upheaval of the 

 soil has resulted in substituting dry land for this arm of 

 the ocean and in reversing the course of what is now the 

 Madeira, just as, according to these geologists, in some- 

 what familiar fashion the Amazon has been reversed, it 

 having once been, at least for the upper two thirds of its 

 course, an affluent of the Andean Sea. 



From Vilhena we travelled in a generally northward 

 direction. For a few leagues we went across the chapadao, 



