238 Through the Brazilian Wilderness 



the basins of the Madeira and Tapajos. A singular topo- 

 graphical 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 southern edge of the plateau, therefore, begin 

 by flowing south ; then for a long stretch they flow south- 

 west ; then north, and finally northeast into the Amazon. 

 According to some exceptionally good geological observ- 

 ers, this is probably due to the fact that in a remote geo- 

 logic past the ocean sent in an arm from the south, be- 

 tween the Plan Alto and what is now the Andean chain. 

 These rivers then emptied 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 somewhat familiar fashion the Am- 

 azon 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 chap- 

 adao, the sands or clays of the nearly level upland plateau, 

 grassy or covered with thin, stunted forest, the same 

 type of country that had been predominant ever since we 

 ascended the Parecis table-land on the morning of the 

 third day after leaving the Sepotuba. Then, at about 

 the point where the trail dipped into a basin containing 

 the headsprings of the Ananas, we left this type of coun- 



