Structure of the Amazons Valley. 363 



we have purely sedimentary strata, contorted, but dipping 

 easterly — conglomerate, sandstone, slate, and Jurassic lime- 

 stone. ,Then follow, in succession : — Triassic beds (remarkably 

 like those in the Moyobamba valley, capped with white sand- 

 stone, and broken by protruding igneous rocks) ; Carboniferous, 

 at the south end of the lake, and reappearing east of Cocha- 

 bamba on the headwaters of the Chapara ; and the Devonian 

 and Silurian, forming the mass of the high Andes. 



If now we examine the valley of the Amazons, we shall 

 be struck with its remarkably uniform character, such as is 

 presented by no other region on the globe of equal area. From 

 the Andes to the Atlantic, and from the Falls of the Madeira 

 to the Orinoco, scarcely any thing is visible but clays and 

 sandstones. 



The fundamental rock is metamorphic, chiefly gneiss and 

 granite. It is exposed at the falls of the tributaries, especially 

 on the Madeira ; it is greatly disturbed, and frequently broken 

 through by porphyritic dykes. The granite contains little 

 mica and much quartz. The valley is bounded on the north 

 and south by immense metamorphic areas. The low water- 

 shed between the Amazons and Paraguay is covered with 

 Tertiary beds ; but the still lower region of the Upper Rio 

 Negro is one great undulating sheet of granite and gneiss 

 completely denuded of the stratified rocks that once overlay 

 it, save here and there a thin covering of white sand and red 

 loam filling the hollows, and abrupt peaks that suddenly rise 

 from the plain. 



Silurian formations are rarely visible. The gold and topaz- 

 bearing rocks of Minas Geraes probably belong to this age ; 

 but they are greatly altered. In the Bolivian Andes, facing 

 the Madeira valley, is an extensive development of Silurian 

 slates and sandstones. The only undoubted Devonian forma- 

 tion in the valley is the plain north of the Serra of Erere, 

 discovered by Hartt. 



The horizontal limestone strata at Itaituba on the Tapajos, 

 and on the Trombetas across the Amazons, abound with 

 Brachiopods of the Coal-measures. D'Orbigny and Forbes 

 have pointed out isolated Carboniferous deposits in the Titi- 

 caca basin and near Santa Cruz, on the Mamore - . From the 

 Pichis, which flows directly from the Cerro de Sal (a spur of 

 the eastern Cordillera), I obtained several fossils of limited 

 vertical range which go to show that the Pichis, Bolivian, and 

 Itaituban beds are identical. The Pichis, Titicaca, Oruro, and 

 Guaco (province of San Juan) deposits lie in the same line, 

 north-west south-east, along the Andes. The altitude of the 

 Tapajos beds is 125 feet, of the Pichis over 700 feet, of the 



