Physical Features. 551 



CHAPTEE XLI. 



The Geological Structure of the Amazons Valley. 



The valley of the Amazons is a very shallow basin of 

 vast extent and of an oval shape, with the small end point- 

 ing eastward. Between December and June, a large part 

 of it resembles a huge, undraiued swamp, and people sail 

 half the year above districts where for the other half they 

 walk. Were the forest removed from the Lower Amazons, 

 a great mud flat would be exposed (lower than the island 

 of Marajo), threaded by a net- work of deep channels, par- 

 tially covered by every tide, and deluged by the annual 

 flood. From the marked feature (first noticed by Chaud- 

 less) that the tributaries enter the main stream at a very 

 acute angle, and have exceedingly tortuous courses, it is 

 inferred that the rest of the valley is a nearly level plain 

 gently inclined from west to east, and with very little slope 

 on either side toward the centre of drainage. 



From the diagram on page 341 it will be seen that be^ 

 tween Borja and Para, a distance of 29°, the inclination is 

 only 500 feet. A section from Exaltacion, on the Upper 

 Madeira, which has the same altitude as Borja, to San 

 Carlos, on the Upper Negro, which is elevated only 212 

 feet above the Atlantic, would show a depression at Fonte 

 Boa, on the Amazons, of only 150 feet in 1000 miles. The 

 Negro is a sluggish stream (San Carlos being on a level 

 with Tabatinga) ; the Napo is more rapid ; and the Pas- 

 tassa is a torrent. In the last thousand miles, the Ma- 

 deira descends 430 feet; the Purus, 225; and the Ucayah, 



