556 The Andes and the Amazons. 



igneous rocks) ; Carboniferous, at the south end of the 

 lake, and re-appearing east of Cochabamba on the head- 

 waters 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 

 Madeii-a 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 dikes. The 

 granite contains little mica and much quartz. The valley 

 is bounded on the north and south by immense metamor- 

 phic areas. The low water-shed between the Amazons and 

 Paraguay is covered with tertiary beds ; but the still lower 

 region of the Upper Eio Negro is one great undulating 

 sheet of granite and gneiss completely denuded of the 

 stratified rocks that once overlaid it, save here and there 

 a thin covering of white sand and red loam filling the hol- 

 lows, and abrupt peaks that suddenly rise from the plain. 



Silurian formations are rarely visible. The gold and to- 

 paz 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 De- 

 vonian formation in the valley is the plain north of the 

 Serra of Erere, discovered by Ilartt. 



The horizontal limestone strata at Itaituba on the Ta- 

 pajos and on the Trombetas across the Amazons abound 

 with Brachiopods of the Coal Measures. D'Orbigny and 

 Forbes have pointed out isolated Carboniferous deposits 



