THE AMAZON VALLEY. 



293 



group. The Cocoi is a quadrangular or cubical mass, about 

 a thousand feet in elevation, which forms the boundary between 

 Brazil and Venezuela; and behind it are the Pirapoco, and 

 the serras of the Cababurfs, which seem rather more extensive, 

 and form something more like a connected range of hills. 



But the great peculiarity of them all is, that the country 

 does not perceptibly rise to their bases ; they spring up 

 abruptly, as if elevated by some local isolated force. I ascended 

 one of the smaller of these serras as far as practicable, and 

 have recorded my impressions of it in my Journal. (See 

 page 153.) 



The isolation and abrupt protrusion of these mountains is 

 not, however, altogether without parallel in the Andes itself. 

 This mighty range, from all the information I can obtain, rises 

 with almost equal abruptness from an apparently level plain. 

 The Andes of Quito, and southward to the Amazon, is like a 

 hugh rocky rampart, bounding the great plain which extends in 

 one unbroken imperceptible slope from the Atlantic Ocean to 

 its base. It is one of the grandest physical features of the 

 earth,' — this vast unbroken plain, — that mighty and precipitous 

 mountain-range. 



The granitic rocks of the Rio Negro in general contain very 

 little mica ; in some places, however, that mineral is abundant, 

 and exists in large plates. Veins of pure quartz are common, 

 some of very great size ; and numerous veins or dykes of 

 granite, of a difterent colour or texture. The direction of 

 these is generally nearer east and west, than north and 

 south. 



Just below the falls of the Rio Negro are some coarse sand- 

 stone rocks, apparently protruding through the granite, dipping 

 at an angle of 60° or 70° south-south-west. (Plate IX. Near 

 the same place a large slab of granite rock exhibits quantities 

 of curiously twisted or folded quartz veins (Plate IX. 3.), which 

 vary in size from a line to some inches in diameter, and are 

 folded in a most minute and regular manner. 



On an island in the river, near this place, are finely stratifi';d 

 crystalline rocks, dipping south from 70° to vertical, and 

 sometimes waved and twisted. 



The granite often exhibits a concentric arrangement of 

 laminae, particularly in the large dome-shaped masses in the 

 bed of the river (Plate X. a, c), or in portions protruding from 



