GEOLOGY.
Fully to elucidate the Geology of the Amazon valley, requires much more time and research than I was able to devote to it. The area is so vast, and the whole country being covered with forests renders natural sections so comparatively scarce, that the few distant observations one person can make will lead to no definite conclusions.
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.[4]
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 the most lofty. They consist of 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 above the surrounding forest like dead trunks of giant trees. Up the river Isanna, the Tunuhy mountains are a similar isolated 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 Pirapocó, and the serras of the Cababurís, which seem rather more extensive, and form something more like a connected range of hills.
a. Fragments embedded in granite. b. Granite with twisted veins. c. Stratified rocks protruded through granite.
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.
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.
The neighbourhood of Pará consists entirely of a coarse iron sandstone, which is probably a continuation of the rocks observed by Mr. Gardner at Maranham and in the Province of Piauhy, and which he considered to belong to the chalk formation. Up the Tocantíns we found fine crystalline stratified rocks, coarse volcanic conglomerates, and fine-grained slates. At the falls were metamorphic slates and other hard crystalline rocks; many of these split into flat slabs, well adapted for building, or even for paving, instead of the stones now imported from Portugal into Pará. In the serras of Montealegre, on the north bank of the Amazon, are a great variety of rocks,—coarse quartz conglomerates, fine crystalline sandstones, soft beds of yellow and red sandstones, and indurated clay rocks. These beds are all nearly horizontal, but are much cleft and shattered vertically; they are alternately hard and soft, and by their unequal decay have formed the hanging stones and curious cave described in my Journal.
The general impression produced by the examination of the country is, that here we see the last stage of a process that has been going on, during the whole period of the elevation of the Andes and the mountains of Brazil and Guiana, from the ocean. At the commencement of this period, the greater portion of the valleys of the Amazon, Orinooko, and La Plata must have formed a part of the ocean, separating the groups of islands (which those elevated lands formed on their first appearance) from each other. The sediment carried down into this sea by the rapid streams, running down the sides of these mountains, would tend to fill up and level the deeper and more irregular depressions, forming those large tracts of alluvial deposits we now find in the midst of the granite districts. At the same time volcanic forces were in operation, as shown by the isolated granite peaks which in many places rise out of the flat forest district, like islands from a sea of verdure, because their lower slopes, and the valleys between them, have been covered and filled up by the sedimentary deposits. This simultaneous action of the aqueous and volcanic forces, of submarine earthquakes and marine currents, shaking up, as it were, and levelling the mass of sedimentary matter brought down from the now increasing surface of dry land, is what has produced that marvellous regularity of surface, that gradual and imperceptible slope, which exists over such an immense area.[5]
At the point where the mountains of Guiana approach nearest to the chain of the Andes, the volcanic action appears to have been continued in the interval between them, throwing up the serras of Curicuriarí, Tunuhy, and the numerous smaller granite mountains of the Uaupés; and it is here probably that dry land first appeared, connecting Guiana and New Granada, and forming that slightly elevated ridge which is now the watershed between the basins of the Amazon and Orinooko. The same thing occurs in the southern part of the continent, for it is where the mountains of Brazil, and the eastern range of the Bolivian Andes, stretch out to meet each other, that the sedimentary deposits in that part appear to have been first raised above the water, and thus to have determined the limits of the basin of the Amazon on the south. The Amazon valley would then have formed a great inland gulf or sea, about two thousand miles long and seven or eight hundred wide.
The rivers and mountain-torrents pouring into it on every side, would gradually fill up this great basin; and the volcanic action still visible in the scoriæ of the Tocantins and Tapajoz, and the shattered rocks of Montealegre, would all tend to the levelling of the vast area, and to determining the channels of the future rivers. This process, continuing for ages, would at length narrow this inland sea, almost within the limits of what is now gapó, or flooded land. Ridges, gradually elevated a few feet above the waters, would separate the tributary streams; and then the eddies and currents would throw up sandbanks as they do now, and gradually define the limits of the river, as we now see it. And changes are yet going on. New islands are yearly forming in the stream, large tracts of flooded land are being perceptibly raised by the deposits upon them, and the numerous great lakes are becoming choked with aquatic plants, and filled up with sediment.
The large extent of flat land on the banks of the river will still continue to be flooded, till some renewed earthquakes raise it gradually above the waters; during which time the stream will work for itself a wider and deeper bed, capable of containing its accumulated flood. In the course of ages perhaps this might be produced by the action of the river itself, for at every annual inundation a deposit of sediment is formed, and these lands must therefore be rising, and would in time become permanently elevated above the highest rise of the river. This, however, would take a very long time, for as the banks rose, the river, unable to spread its waters over the adjoining country, would swell higher, and flow more rapidly than before, and so overflow a country elevated above the level of its former inundations.
The complete history of these changes,—the periods of elevation and of repose, the time when the dividing ridges first rose above the waters, and the comparative antiquity of the tributary streams,—cannot be ascertained till the country has been more thoroughly explored, and the organic remains, which must doubtless exist, be brought forward, to give us more accurate information respecting the birth and growth of the Amazon.
Comparative Climates of Pará and London.
The three upper curves show the Means of the highest lowest and mean temperatures at Para for four years. The two lower curves show the highest and lowest monthlhy mean temperatures at London.
Mean Atmospheric Pressure at Para for 3 years.
