полная версия

Alfred Russel Wallace
Travels on the Amazon

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.

PLATE II.

a. Fragments embedded in granite. b. Granite with twisted veins. c. Stratified rocks protruded through granite.

GRANITE ROCKS AND VEINS, ETC.

PLATE III.

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.

PLATE IV.

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.

PLATE V.

Mean Atmospheric Pressure at Para for 3 years.

CLIMATE

The climate of the Amazon valley seems remarkable for uniformity of temperature, and for a regular supply of moisture. There are, in most parts of it, six months' wet, and six months' dry season, neither of which are so severe as in some other tropical countries. From June to December is the dry, and from January to May the wet season. In the dry season there are a few occasional rains, especially about All Saints' day, in November; and during the wet season there are intervals of fine weather, and often bright mornings, and many days of gentle misty rain.

This is the general character of the climate over the whole of the main stream of the Amazon and its immediate neighbourhood. There are, however, remarkable deviations from this general routine, in particular localities. Pará itself is one of these exceptional places. Here the seasons are so modified, as to render the climate one of the most agreeable in the world. During the whole of the dry season, scarcely ever more than three days or a week passes without a slight thunderstorm and heavy shower, which comes on about four in the afternoon and by six has cleared off again, leaving the atmosphere delightfully pure and cool and all vegetable and animal life refreshed and invigorated. Had I only judged of the climate of Pará from my first residence of a year, I might be thought to have been impressed by the novelty of the tropical climate; but on my return from a three years' sojourn on the Upper Amazon and Rio Negro, I was equally struck with the wonderful freshness and brilliancy of the atmosphere, and the balmy mildness of the evenings, which are certainly not equalled in any other part I have visited.

The wet season has not so many stormy and cloudy days as in other parts. Sunshine and rain alternate, and the days are comparatively bright and cheerful, even when rainy. Generally, the variation of the thermometer in any one day does not exceed 15°; 75° being the lowest, and 90° the highest. The greatest variation in one day is not, I think, ever more than 20°; and in four years, the lowest and highest temperatures were 70° and 95°, giving only an extreme variation of 25°. A more equable climate probably does not exist on the earth. (See Diagram, Plate IV.)

On the Guiana side of the Amazon, in the islands of Mexiana and Marajo, the seasons are more strongly marked than even higher up the river. In the dry season, for about three months, no rain ever falls; and in the wet it is almost continual.

But it is in the country about the falls of the Rio Negro that the most curious modification of the seasons occur. Here the regular tropical dry season has almost disappeared, and a constant alternation of showers and sunshine occurs, almost all the year round. In the months of June, July, August, and September, when the Amazonian summer is in all its glory, we have here only a little finer weather about June, and then rain again as much as ever; till, in January or February, when the wet season in the Amazon commences, there is generally here a month or two of fine warm weather. It is then that the river, which has been very slowly falling since July, empties rapidly, and in March is generally at its lowest ebb. In the beginning of April it suddenly begins to rise, and by the end of May has risen twenty feet, and then continues slowly rising till July, when it reaches its highest point, and begins to fall with the Amazon. The district of the greatest quantity of rain, or rather of the greatest number of rainy days, seems to be very limited, extending only from a little below the falls of São Gabriel to Marabitanas at the confines of Brazil, where the Pirapocó and Cocoí mountains, and the Serra of Tunuhy, seem to form a separation from the Venezuela district, where there is a more regular summer in the months of December, January, and February.

The water of the Rio Negro in the month of September did not vary in temperature more than two degrees. I unfortunately lost my thermometers, or had intended making a regular series of observations on the waters of the higher parts of the rivers I ascended.

The extreme variation of the barometer at Pará for three years was only three-tenths of an inch (see diagram, Plate V.). The mean height, with all the necessary corrections, would seem to be almost exactly thirty inches; I have, however, already given my reasons for believing that there is a considerable difference in the pressure of the atmosphere in the interior of the country. In the month of May some very cold days are said to occur annually on the Upper Amazon and Rio Negro; but I never myself experienced anything of the kind worthy of notice. Many intelligent persons have assured me that the cold is sometimes so severe that the inhabitants suffer much, and, what is much more extraordinary, the fishes in the rivers die of it. Allowing this to be the fact, I am quite unable to account for it, as it is difficult to conceive that a diminution of temperature of five or ten degrees, which is as much as ever takes place, can produce any effect upon them.

I have an authentic account of hail having once fallen on the Upper Amazon, a remarkable occurrence at a place only three degrees south of the equator, and about two hundred feet above the level of the sea. The children were out at play, and brought it to their parents, astonished at a substance falling from the clouds quite new to them, and which was so remarkably cold. The person who told me was a Portuguese, and his information can be perfectly relied on.

CHAPTER XV

VEGETATION OF THE AMAZON VALLEY

Perhaps no country in the world contains such an amount of vegetable matter on its surface as the valley of the Amazon. Its entire extent, with the exception of some very small portions, is covered with one dense and lofty primeval forest, the most extensive and unbroken which exists upon the earth. It is the great feature of the country,—that which at once stamps it as a unique and peculiar region. It is not here as on the coasts of southern Brazil, or on the shores of the Pacific, where a few days' journey suffices to carry us beyond the forest district, and into the parched plains and rocky serras of the interior. Here we may travel for weeks and months inland, in any direction, and find scarcely an acre of ground unoccupied by trees. It is far up in the interior, where the great mass of this mighty forest is found; not on the lower part of the river, near the coast, as is generally supposed.

A line from the mouth of the river Parnaiba, in long. 41° 30´ W., drawn due west towards Guayaquil, will cut the boundary of the great forest in long. 78° 30´, and, for the whole distance of about 2,600 miles, will have passed through the centre of it, dividing it into two nearly equal portions.

For the first thousand miles, or as far as long. 56° W., the width of the forest from north to south is about four hundred miles; it then stretches out both to the north and south, so that in long. 67° W. it extends from 7° N., on the banks of the Orinoko, to 18° S., on the northern slope of the Andes of Bolivia, a distance of more than seventeen hundred miles. From a point about sixty miles south-east of Tabatinga, a circle may be drawn of 1,100 miles in diameter, the whole area of which will be virgin forest.

Along the Andes of Quito, from Pasto to Guancabamba, it reaches close up to the eastern base of the mountains, and even ascends their lower slopes. In the moderately elevated country between the river Huallaga and Marañon, the forest extends only over the eastern portion, commencing in the neighbourhood of Moyobamba. Further on, to the east of Cuzco and La Paz, it spreads high up on the slopes of the Bolivian Andes, and passing a little to the west of Santa Cruz de la Sierra, turns off to the north-east, crossing the Tapajóz and Xingú rivers somewhere about the middle of their course, and the Tocantins not far above its junction with the Araguàya, and then passes over to the river Parnaíba, which it follows to its mouth.

The Island of Marajó, at the mouth of the Amazon, has its eastern half open plains, while in the western the forest commences. On the north of the Amazon, from its mouth to beyond Montealegre, are open plains; but opposite the mouth of the Tapajóz at Santarem, the forest begins, and appears to extend up to the Serras of Carumaní, on the Rio Branco, and thence stretches west, to join the wooded country on the eastern side of the Orinoko. West of that river, it commences south of the Vicháda, and, crossing over the upper waters of the Guaviáre and Uaupés, reach the Andes east of Pasto, where we commenced our survey.

The forests of no other part of the world are so extensive and unbroken as this. Those of Central Europe are trifling in comparison; nor in India are they very continuous or extensive; while the rest of Asia seems to be a country of thinly wooded plains, and steppes, and deserts. Africa contains some large forests, situated on the east and west coasts, and in the interior south of the equator; but the whole of them would bear but a small proportion to that of the Amazon. In North America alone is there anything approaching to it, where the whole country east of the Mississippi and about the great lakes, is, or has been, an almost uninterrupted extent of woodland.

In a general survey of the earth, we may therefore look upon the New World as pre-eminently the land of forests, contrasting strongly with the Old, where steppes and deserts are the most characteristic features.

The boundaries of the Amazonian forest have not hitherto been ascertained with much accuracy. The open plains of Caguan have been supposed much more extensive than they really are; but I have very nearly determined their limits to the south and east, by the observations I made, and the information I obtained in my voyage up the Uaupés. Again, on the Uaycáli there is a district marked on the maps as the "Pampas del Sacramento," which has been supposed to be an open plain; but the banks of the Amazon up to the mouth of the Uaycáli are clothed with thick forest, and Messrs. Smyth and Lowe, who crossed the Pampa in two places, found no open plains; and from their observations and those of Lieut. Mawe we must extend the forest district up to near Moyabamba, west of the Huallaga, and to the foot of the mountains east of Pasco and Tarma. I was informed by a native of Ecuador, well acquainted with the country, that the Napo, Tigre, Pastaza, and the adjacent rivers all flow through dense forest, which extends up even to Baéza and Canélos and over all the lower slopes of the Andes. Tschudi informs us that the forest districts commence on all the north and east slopes of the Andes of Peru, near Huánta, and at Urubámba north of Cúzco. I have learnt from a gentleman, a native of La Paz, that immediately on crossing the Bolivian Andes from that city and from Oropéssa and Santa Cruz, you enter the great forests, which extend over all the tributaries of the Madeira. Traders up the Purús and all the southern branches of the Upper Amazon, neither meet with, nor hear accounts of, any open land, so that there is little doubt but that the extent here pointed out is one vast, ever-verdant, unbroken forest.

The forests of the Amazon are distinguished from those of most other countries, by the great variety of species of trees composing them. Instead of extensive tracts covered with pines, or oaks, or beeches, we scarcely ever see two individuals of the same species together, except in certain cases, principally among the Palms. A great extent of flooded land about the mouth of the Amazon, is covered with the Mirití Palms (Mauritia flexuosa and M. vinifera), and in many places the Assaí (Euterpe edulis) is almost equally abundant. Generally, however, the same species of tree is repeated only at distant intervals. On a road for ten miles through the forest near Pará, there are only two specimens of the Masserandúba, or Cow-tree, and all through the adjoining district they are equally scarce. On the Javíta road, on the Upper Rio Negro, I observed the same thing. On the Uaupés, I once sent my Indians into the forest to obtain a board of a particular kind of tree; they searched for three days, and found only a few young trees, none of them of sufficient size.

Certain kinds of hard woods are used on the Amazon and Rio Negro, for the construction of canoes and the schooners used in the navigation of the river. The difficulty of getting timber of any one kind for these vessels is so great, that they are often constructed of half-a-dozen different sorts of wood, and not always of the same colours or degrees of hardness. Trees producing fruit, or with medicinal properties, are often so widely scattered, that two or three only are found within a reasonable distance of a village, and supply the whole population. This peculiarity of distribution must prevent a great trade in timber for any particular purpose being carried on here. The india-rubber and Brazil-nut trees are not altogether exceptions to this rule, and the produce from them is collected over an immense extent of country, to which the innumerable lakes and streams offer a ready access.

The chief district from which india-rubber is procured, is in the country between Pará and the Xingú. On the Upper Amazon and the Rio Negro it is also found, but is not yet collected.

The Brazil-nuts, from the Bertholletia excelsa, are brought chiefly from the interior; the greater part from the country around the junction of the Rio Negro and Madeira with the Amazon rivers. This tree takes more than a whole year to produce and ripen its fruits. In the month of January I observed the trees loaded at the same time with flowers and ripe fruits, both of which were falling from the tree; from these flowers would be formed the nuts of the following year; so that they probably require eighteen months for their complete development from the bud. The fruits, which are nearly as hard and heavy as cannon-balls, fall with tremendous force from the height of a hundred feet, crashing through the branches and undergrowth, and snapping off large boughs which they happen to strike against. Persons are sometimes killed by them, and accidents are not unfrequent among the Indians engaged in gathering them.

The fruits are all procured as they fall from the tree. They are collected together in small heaps, where they are opened with an axe, an operation that requires some practice and skill, and the triangular nuts are taken out and carried to the canoes in baskets. Other trees of the same family (Lecythideæ) are very abundant, and are remarkable for their curious fruits, which have lids, and are shaped like pots or cups, whence they are called "pot-trees." Some of the smaller ones are called by the natives "cuyas de macaco,"—monkeys' calabashes.

The next most important vegetable product of the Amazon district, is the Salsaparilha, the roots of Smilax syphilitica, and perhaps of other allied species. This plant appears to occur over the whole forest-district of the Amazon, from Venezuela to Bolivia, and from the Lower Amazon to Peru. It is not generally found near the great rivers, but far in the interior, on the banks of the small streams, and on dry rocky ground. It is principally dug up by the Indians, often by the most uncivilised tribes, and is the means of carrying on a considerable trade with them.

The Brazilian nutmegs, produced by the Nectandrum Puchury, grow in the country between the Rio Negro and Japura.

The Cumarú, or Tonquin-beans, are very abundant on the Upper Rio Negro, and are also found near Santarem on the Amazon.

A highly odoriferous bark, called by the Portuguese "Cravo de Maranhño" (Cloves of Maranham), is produced by a small tree growing only on one or two small tributaries of the Rio Negro.

A peculiar transparent oil, with an odour of turpentine, called Sassafras by the Venezuelans, is abundantly obtained by tapping a tree, common on the Upper Rio Negro, whence it is exported to Barra, and used for mixing oil-colours. In the Lower Amazon, a bitter oil, called Andiróba, much used for lamps, is made from a forest fruit.

A whitish resin, with a strong camphorous smell, is produced very abundantly in the Rio Negro and the Amazon, and is commonly used as pitch for the canoes and all the larger vessels of the country; while the inner bark of young trees of the Bertholletia excelsa, or Brazil-nut tree, is used instead of oakum for caulking.

Among the forest-trees of the Amazon, the Leguminosæ are much the most abundant in species, and they also most attract attention from their curious bean-like fruits, often of extraordinary size or length. Some of the Ingás, and allied genera, have pods a yard long, and very slender; while others are short, and three or four inches wide. There are some curious fruits of this family, which grow on a stalk three to five feet long and very slender, appearing as if some one had suspended a number of pods from the branches by long strings.

The flowers of this family are among the most brilliant and conspicuous; and their often finely-cut pinnate foliage has a very elegant appearance.