It is remarkable that I was never able to find any fossil remains whatever,—not even a shell, or a fragment of fossil wood, or anything that could lead to a conjecture as to the state in which the valley existed at any former period. We are thus unable to assign the geological age to which any of the various beds of rock belong.
My notes, and a fine series of specimens of the rocks of the Rio Negro, were lost, and I have therefore very few materials to go upon.
Granite seems to be, in South America, more extensively developed than in any other part of the world. Darwin and Gardner found it in every part of the interior of Brazil, in La Plata, and Chile. Up the Xingú Prince Adalbert met with it. Over the whole of Venezuela and New Granada, it was found by Humboldt. It seems to form all the mountains in the interior of Guiana, and it was met with by myself over the whole of the upper part of the Rio Negro, and far up the Uaupés towards the Andes.
From what I could see of the granitic formation of the Upper Rio Negro, it appeared to be spread out in immense undulating areas, the hollows of which, being filled up with alluvial deposits, form those beds of earth and clay which occur, of various dimensions, everywhere in the midst of the granite formation. In these places grow the lofty virgin forests, while on the scantily covered granite rocks, and where beds of sand occur, are the more open catinga forests, so different in their aspect and peculiar in their vegetation. What strikes one most in this great formation, is its almost perfect flatness. There are no ranges of mountains, or even slightly elevated plateaus; all is level, except the abrupt peaks that rise suddenly from the plain, to a height of from one hundred to three thousand feet. In the Upper Rio Negro these peaks are very numerous. The first is the Serra de Jacamí, a little above St. Isabel; it rises immediately from the bank of the river, on the south side, to a height of about six hundred feet. Several others are scattered about, but the Serras de Curicuriarí are far the most lofty. They are a group of three or four mountains, rising abruptly to the height of near three thousand feet; towards their summits are immense precipices and jagged peaks. Higher up, on the same river, is another group of rather less height. On the Uaupés are numerous hills, some conical, others dome-shaped, but all keeping the same character of abrupt elevations, quite independent of the general profile of the country. About the falls of the river Uaupés there are small hills of granite, broken about in the greatest confusion. Great chasms or bowls occur, and slender pillars of rock rise up like dead trunks out of the surrounding forest. Up the river Isanna, the Tunuhy mountains are a similar isolated group. The Cocoí 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 Pirapocó, and the serras of the Cababurís, which seem rather more extensive, and form something more like a connected range of hills.
Plate I.
- Fragments imbedded in Granite.
- Granite with twisted veins.
- Stratified rocks protruded through Granite.
A. R. Wallace, del. et. lith.
F. Reeve, imp.
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 218].)