GEOGRAPHY AND GEOLOGY 223 



streams running ultimately into the Tapajos ; and 

 between them, and close to them, were streamlets 

 which drained into the Diivida and Anands, whose 

 courses and outlets were unknown. 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 Guapord, 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 north-east 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 

 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 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, 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 con- 



