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-southwest. ([Plate I.] c.) Near the same place a large slab of granite rock exhibits quantities of curiously twisted or folded quartz veins ([Plate I.] 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 II.] a, c), or in portions protruding from the ground ([Plate II.] 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 I.] a); I did not notice this so distinctly at any other locality.
High up the river Uaupés there is a very curious formation. All along the river-banks there are irregular fragments of rocks, with their interstices filled up with a substance that looks exactly like pitch. On examination, it is found to be a conglomerate of sand, clay, and scoriæ, sometimes very hard, but often rotten and easily breaking to pieces; its position immediately suggests the idea of its having been liquid, for the fragments of rocks appear to have sunk in it.
Coarse volcanic scoriæ, with a vitreous surface, are found over a very wide area. They occur at Caripé, near Pará,—above Baião, in the Tocantíns,—at the mouth of the Tapajóz,—at Villa Nova, on the Amazon,—above Barra, on the Rio Negro, and again up the Uaupés. A small conical hill behind the town of Santarem, at the mouth of the Tapajóz, has all the appearance of being a volcanic cone.
Plate II.
Forms of Granite rocks.