THE AMAZON VALLEY. 293
group. The Cocof 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 Pirapocd, and the serras of the Cababuris, 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 different 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 6o° or 70 south-south-west. (Plate IX. c.) Near the same place a large slab of granite rock exhibits quantities of curiously twisted or folded quartz veins (Plate IX. b.), 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 stratified crystalline rocks, dipping south from 70 to vertical, and sometimes waved and twisted.
The granite often exhibits a concentric arrangement of lamina?, particularly in the large dome-shaped masses in the bed of the river (Plate X. a. c), or in portions protruding from
