GRANITE ROCKS AND VEINS, ETC.
FORMS OF GRANITE ROCKS.
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 huge 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 sandstone rocks, apparently protruding through the granite, dipping at an angle of 60° or 70° south-south-west. ([Plate II.] c.) Near the same place a large slab of granite rock exhibits quantities of curiously twisted or folded quartz veins ([Plate II.] 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 laminæ, particularly in the large dome-shaped masses in the bed of the river ([Plate III.] a, c), or in portions protruding from the ground ([Plate III.] b). Near São Gabriel, and in the Uaupés, large masses of pure quartz rock occur, and the shining white precipices of the serras are owing, I have no doubt, to the same cause. At Pimichin, near the source of the Rio Negro, the granite contains numerous fragments of stratified sandstone rock imbedded in it ([Plate II.] a); I did not notice this so distinctly at any other locality.