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 Guaporé, 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 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 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 chapadão, 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 Parecís 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 Ananás, we left this type of coun-
