THE PHYSICAL GEOGRAPHY AND GEOLOGY OF THE AMAZON VALLEY.
The basin of the Amazon surpasses in dimensions that of any other
river in the world. It is entirely situated in the Tropics, on both
sides of the Equator, and receives over its whole extent the most
abundant rains. The body of fresh water emptied by it into the ocean,
is therefore far greater than that of any other river; not only
absolutely, but probably also relatively to its area, for as it is
almost entirely covered by dense virgin forests, the heavy rains which
penetrate them do not suffer so much evaporation as when they fall
on the scorched Llanos of the Orinooko or the treeless Pampas of La
Plata. For richness of vegetable productions and universal fertility of
soil it is unequalled on the globe, and offers to our notice a natural
region capable of supporting a greater population, and supplying it
more completely with the necessaries and luxuries of life, than any
other of equal extent. Of this wonderful district we will now describe
the principal physical peculiarities.
From about 4° north latitude to 20° south, every stream that flows down
the eastern slope of the Andes, is a tributary of the Amazon. This is
as if every river, from St. Petersburg to Madrid, united their waters
into one mighty flood.
The Marañon, which is generally considered the main stream of the
Amazon, deserves that title on several accounts. It rises to the
westward of all the other great tributaries, and it receives all the
waters which flow nearest to the Pacific, and most remote in a direct
line from the mouth of the river. It flows for a considerable distance
in the most westerly valley of the Andes, separated by one range only
from the Pacific, and at the point where it breaks through the eastern
chain of the Andes, in 78° west longitude, is already a large river,
on a meridian where all the other streams which can lay a claim to be
considered the head-waters of the Amazon have as yet no existence. On
going up the Amazon from its mouth, it is that branch on which you can
keep longest in the general east and west direction of the river; and
if the actual length of its course is considered, it still keeps its
place, for I find that there is not more than ten or twenty miles’
difference between it and the Uaycali, reckoning to the most distant
source of the latter; and its course is at present so uncertain, that
future surveys may increase or diminish it considerably.
These considerations, I think, decide the question as to the propriety
of considering the Marañon as the true source of the Amazon. We find
that from its origin in the Lake Lauricocha, to its mouth in longitude
50° west, its length, following the main curves, but disregarding the
minuter windings, is 2740 English miles.
Its extent, in a straight line from east to west, is about 2050 miles;
and from north to south, its tributary streams cover a space of 1720
miles.
The whole area of its basin, not including that of the Tocantíns, which
I consider a distinct river, is 2,330,000 English square miles, or
1,760,000 nautical square miles. This is more than a third of all South
America, and equal to two-thirds of all Europe. All western Europe
could be placed in it without touching its boundaries, and it would
even contain our whole Indian empire.
The numerous tributary streams of the Amazon, many of them equal to
the largest rivers of Europe, differ remarkably in the colour of
their waters, the character of the vegetation on their banks, and the
animals that inhabit them. They may be divided into three groups,—the
white-water rivers, the blue-water rivers, and the black-water rivers.
The main stream of the Amazon itself is a white-water river, this
name being applied to those waters which are of a pale yellowish
olive-colour. This colour does not seem to depend entirely on free
earthy matter, but rather on some colouring material held in solution;
for in lakes and inlets, where the waters are undisturbed and can
deposit all their sediment, they still retain the colour.
The waters of the Amazon continue of the same colour up to the mouth of
the Uaycali, when they become blue or transparent, and the white waters
are extended up that branch.
This has been taken as an evidence of the Uaycali being the main stream
of the Amazon; but I cannot consider that it has anything to do with
the question. It is evident that if equal quantities of clear and
muddy water are mixed together, the result will differ very little
from the latter in colour, and if the clear water is considerably
more in quantity the resulting mixture will still be muddy. But the
difference of colour between the white- and blue-water rivers, is
evidently owing to the nature of the country they flow through: a rocky
and sandy district will always have clear-water rivers; an alluvial or
clayey one, will have yellow or olive-coloured streams. A river may
therefore rise in a rocky district, and after some time flow through an
alluvial basin, where the water will of course change its colour, quite
independently of any tributaries which may enter it near the junction
of the two formations.
The Iça and Japurá have waters very similar in colour to the Amazon.
The Rio Branco, a branch of the Rio Negro from the north, is remarkable
for its peculiar colour: till I saw it, I had not believed it so well
deserved its name. The Indians and traders had always told me that it
was really white, much more so than the Amazon; and on descending the
Rio Negro in 1852, I passed its mouth, and found that its waters were
of a milky colour mixed with olive. It seemed as if it had a quantity
of chalk in solution, and I have little doubt of there being on its
banks considerable beds of the pure white clay which occurs in many
parts of the Amazon, and which helps to give the waters their peculiar
whiteness. The Madeira and Purús have also white waters in the wet
season, when their powerful currents bring down the alluvial soil
from their banks; but in the dry season they are a dark transparent
brownish-olive.
All the rivers that rise in the mountains of Brazil have blue or clear
water. The Tocantíns, the Xingú, and the Tapajóz, are the chief of this
class. The Tocantíns runs over volcanic and crystalline rocks in the
lower parts of its course, and its waters are beautifully transparent;
the tide however enters for some miles, and renders it turbid, as also
the Xingú. The Tapajóz, which enters the Amazon about five hundred
miles above Pará, is clear to its mouth, and forms a striking contrast
to the yellow flood of that river.
It is above the Madeira that we first meet with the curious phenomenon
of great rivers of black water. The Rio Negro is the largest and most
celebrated of these. It rises in about 2° 30´ N. lat., and its waters
are there much blacker than in the lower part of its course. All its
upper tributaries, the smaller ones especially, are very dark, and,
where they run over white sand, give it the appearance of gold, from
the rich colour of the water, which, when deep, appears inky black.
The small streams which rise in the same district, and flow into the
Orinooko, are of the same dark colour. The Cassiquiare first pours
in some white or olive-coloured water. Lower down, the Cababurís,
Maravihá, and some smaller white-water streams help to dilute it, and
then the Rio Branco adds its flood of milky water. Notwithstanding all
this, the Rio Negro at its mouth still appears as black as ink; only
in shallow water it is seen to be paler than it is up above, and the
sands are not dyed of that pure golden tint so remarkable there.
On the south of the Amazon there are also some black-water streams—the
Coary, the Teffe, the Juruá, and some others. The inhabitants have
taken advantage of these, to escape from the plague of the mosquitoes,
and the towns of Coary and Ega are places of refuge for the traveller
on the Upper Amazon, those annoying insects being scarcely ever found
on the black waters. The causes of the peculiar colour of these rivers
are not, I think, very obscure; it appears to me to be produced by the
solution of decaying leaves, roots, and other vegetable matter. In the
virgin forests, in which most of these streams have their source, the
little brooks and rivulets are half choked up with dead leaves and
rotten branches, giving various brown tinges to the water. When these
rivulets meet together and accumulate into a river, they of course have
a deep brown hue, very similar to that of our bog or peat water, if
there are no other circumstances to modify it. But if the streams flow
through a district of soft alluvial clay, the colour will of course be
modified, and the brown completely overpowered; and I think this will
account for the anomalies observed, of streams in the same districts
being of different colours. Those whose sources are pretty well known
are seen to agree with this view. The Rio Negro, the Atabapo, the
Isanna, and several other smaller rivers, have their sources and their
whole course in the deep forest; they flow generally over clean granite
rocks and beds of sand, and their streams are gentle, so as not to wear
away the soft parts of their banks.
The Iça, Japurá, and Upper Amazon, on the contrary, flow through a long
extent of alluvial country, and, having their sources on the slopes of
the Andes, are much more liable to sudden floods, and by their greater
velocity bring down a quantity of sediment. In fact it seems clear,
that a thorough knowledge of the course of each river would enable us
to trace the colour of its waters to the various peculiarities of the
country through which it flows.
With the exception of the streams rising in the Andes, the boundaries
of the Amazon basin, or the most distant sources of its tributaries
on the north and south, are comparatively little elevated above the
level of the sea. The whole basin, with the exception of a very small
portion, is one great plain of the most perfect and regular character.
The true altitude of the source in the Lake Lauricocha has not been
ascertained. At Tomependa Humboldt states it to be 1320 feet above the
sea: this is as near as possible 2000 miles in a straight line from the
mouth; so that the average rise is only eight inches in a mile. But
if we take the height at Tabatinga, on the boundary of Brazil, which,
according to Spix and Martius is 670 feet, we shall find, the distance
being about 1400 miles, that the rise is only five and a half inches
per mile. If we had the height of Barra do Rio Negro accurately, we
should no doubt find the rise to that point not more than two or three
inches in a mile. The distance is, in a straight line, about seven
hundred miles, and we may therefore probably estimate the height at
less than 200, and perhaps not more than 150 feet.
This height I am inclined to believe quite great enough, from some
observations I made with an accurate thermometer, reading to tenths
of a degree, on the temperature of boiling water. This instrument
I received from England, after leaving Pará. The mean of five
observations at Barra, some with river- and some with rain-water,
gave 212·5° as the temperature of boiling water; a remarkable result,
showing that the barometer must stand there at more than thirty inches,
and that unless it is, in the months of May and August, considerably
more than that at the sea-level, Barra can be but very little elevated
above the sea.
For the height of the country about the sources of the Rio Negro,
Humboldt is our only authority. He gives 812 feet as the height of São
Carlos; he however states that the determination is uncertain, owing
to an accident happening to the barometer; I may therefore, though
with great diffidence, venture to doubt the result. The distance, in a
straight line, from the mouth of the Rio Negro to São Carlos, is rather
less than from the same point to Tabatinga, whose height is 670 feet.
The current however from Tabatinga is much more rapid than down the Rio
Negro, the lower part of which has so little fall, that in the month
of January, when the Amazon begins to rise, the water enters the mouth
of the Rio Negro, and renders that river stagnant for several hundred
miles up. The falls of the Rio Negro I cannot consider to add more than
fifty feet to the elevation, as above and below them the river is not
very rapid. Thus, from this circumstance alone, we should be disposed
to place São Carlos at a rather less elevation than Tabatinga, or
at about 600 feet. My observations up the Rio Negro gave consistent
results. At Castanheiro, about five hundred miles up, the temperature
of boiling water was 212·4°, at the mouth of the Uaupés 212·2°, and at
a point just below São Carlos, 212·0°. This would not give more than
250 feet for the height of São Carlos above Barra; and, as we have
estimated this at 200 feet above the sea, the height of São Carlos will
become 450 feet, which I think will not be found far from the truth.
The velocity of the current varies with the width of the stream and the
time of the year; we have little accurate information on this subject.
In a Brazilian work on the Province of Pará, the Madeira is stated to
flow 2970 braças, or about three and a half miles, an hour in the wet
season. At Obidos I made an observation in the month of November, when
the Amazon is at the lowest level, and found it four miles an hour;
but this by no means represents the current in the rainy season. On
descending to Pará, in the month of June, 1852, I found that we often
floated down about five miles an hour, and as the wind was strong
directly up the river, it probably retarded us, rather than helped us
on, our vessel not being rigged in the best manner.
Martius calculates that 500,000 cubic feet of water per second pass
Obidos. This agrees pretty well with my own calculations of the
quantity in the dry season; when the river is full, it is probably much
greater. If we suppose, on a moderate calculation, that seventy-two
inches, or six feet, of rain fall annually over the whole Amazon
valley, it will amount to 1,500,000 cubic feet per second, the whole of
which must either evaporate, or flow out of the mouth of the Amazon;
so that if we increase the amount given by Martius by one-half, to take
in the lower part of the Amazon and to allow for the whole year, we
shall have the evaporation as one-half of the rain falling annually.
It is a fact which has been frequently stated, and which seems fully
established, that the Amazon carries its fresh waters out into the
ocean, which it discolours for a distance of a hundred and fifty miles
from its mouth. It is also generally stated that the tide flows up the
river as far as Obidos, five hundred miles from the mouth. These two
statements appear irreconcilable, for it is not easy to understand how
the tides can flow up to such a great distance, and yet no salt water
enter the river. But the fact appears to be, that the tide never does
flow up the river at all. The water of the Amazon rises, but during
the flood as well as the ebb the current is running rapidly down. This
takes place even at the very mouth of the river, for at the island of
Mexiana, exposed to the open sea, the water is always quite fresh, and
is used for drinking all the year round. But as salt water is heavier
than fresh, it might flow up at the bottom, while the river continued
to pour down above it; though it is difficult to conceive how this
could take place to any extent without some salt water appearing at the
margins.
The rising of the water so far up the river can easily be explained,
and goes to prove also that the slope of the river up to where the tide
has any influence cannot be great; for as the waters of the ocean rose,
the river would of course be banked up, the velocity of its current
still forcing its waters onward; but it is not easy to see how the
stream could be thus elevated to a higher level than the waters of
the ocean which caused the rise, and we should therefore suppose that
at Obidos, where the tidal rise ceases to be felt, the river is just
higher than the surface of the ocean at the highest spring-tides.
A somewhat similar phenomenon is seen at the mouth of the Tapajóz.
Here, at the end of the dry season, there is but a small body of water,
and the current is very sluggish. The Amazon however rises considerably
with the tides, and its waters then become higher than those of the
Tapajóz, and they therefore enter into that river and force it back;
we then see the Amazon flowing rapidly down, at the same time that the
Tapajóz is flowing up.
It seems to be still a disputed question among geographers, whether
the Pará river is or is not a branch of the Amazon. From my own
observation, I am decidedly of opinion that it is not: it appears to me
to be merely the outlet of the Tocantíns and of numerous other small
streams. The canal or channel of Tagipurú, which connects it with the
Amazon, and by which all the trade between Pará and the interior is
carried on, is one of a complete network of channels, along which the
tide ebbs and flows, so as in a great measure to disguise the true
direction and velocity of its current. It seems probable that not a
drop of Amazon water finds its way by this channel into the Pará river,
and I ground my opinion upon the following facts.
It is well known, that in a tidal river the ebb-tide will continue
longer than the flood, because the stream of the river requires to
be overcome, and thus delays the commencement of the flood, while it
facilitates that of the ebb. This is very remarkable in all the smaller
rivers about Pará. Taking this as our guide, we shall be able to
ascertain which way the current in the Tagipurú sets, independently of
the tide.
On my journey from Pará to the Amazon, our canoe could only proceed
with the tide, having to wait moored to the bank while it was against
us, so that we were of course anxious to find the time of our tedious
stoppages diminished. Up to a certain point, we always had to wait more
time than we were moving, showing that the current set against us and
towards Pará; but after passing that point, where there was a bend, and
several streams met, we had but a short time to wait, and a long ebb in
our favour, showing that the current was with us or towards the Amazon,
whereas it would evidently have been different had there been any
permanent current flowing from the Amazon through the Tagipurú towards
Pará.
I therefore look upon the Tagipurú as a channel formed by the small
streams between the Tocantíns and Xingú, meeting together about
Melgáço, and flowing through a low swampy country in two directions,
towards the Amazon, and towards the Pará river.
At high tides the water becomes brackish, even up to the city of Pará,
and a few miles down is quite salt. The tide flows very rapidly past
Pará, up all the adjacent streams, and as far as the middle of the
Tagipurú channel; another proof that a very small portion, if any, of
the Amazon water is there to oppose it.
The curious phenomenon of the bore, or “piroróco,” in the rivers Guamá
and Mojú, I have described and endeavoured to explain in my Journal,
and need not now repeat the account of it. (See page 131.)
Our knowledge of the courses of most of the tributaries of the Amazon
is very imperfect. The main stream is tolerably well laid down in
the maps as far as regards its general course and the most important
bends; the details however are very incorrect. The numerous islands and
parallel channels,—the great lakes and offsets,—the deep bays,—and the
varying widths of the stream, are quite unknown. Even the French survey
from Pará to Obidos, the only one which can lay claim to detailed
accuracy, gives no idea of the river, because only one channel is
laid down. I obtained at Santarem a manuscript map of the lower part
of the river, much more correct than any other I have seen. It was,
with most of my other papers, lost on my voyage home; but I hope to be
able to obtain another copy from the same party. The Madeira and the
Rio Negro are the only other branches of the Amazon whose courses are
at all accurately known, and the maps of them are very deficient in
anything like detail. The other great rivers, the Xingu, the Tapajóz,
the Purús, Coarí, Teffe, Juruá, Jutaí, Jabarí, Içá, Japurá, etc.,
though all inserted in our maps, are put in quite by guess, or from the
vaguest information of the general direction of their course. Between
the Tocantíns and the Madeira, and between the Madeira and the Uaycali,
there are two tracts of country of five hundred thousand square miles
each, or each twice as large as France, and as completely unexplored as
the interior of Africa.
The Rio Negro is one of the most unknown in its characteristic
features; although, as before stated, its general course is laid down
with tolerable accuracy. I have narrated in my Journal how I was
prevented from descending on the north side of it, and thus completing
my survey of its course.
The most remarkable feature is the enormous width to which it
spreads,—first, between Barra and the mouth of the Rio Branco, and
from thence to near St. Isabel. In some places, I am convinced, it is
between twenty and thirty miles wide, and, for a very great distance,
fifteen to twenty. The sources of the rivers Uaupés, Isanna, Xié, Rio
Negro, and Guaviare, are very incorrectly laid down. The Serra Tunuhy
is generally represented as a chain of hills cutting off these rivers;
it is however a group of isolated granite peaks, about two thousand
feet high, situated on the north bank of the river Isanna, in about 1°
north latitude and 70° west longitude. The river rises considerably
beyond them, in a flat forest-country, and further west than the Rio
Negro, for there is a path across to the Inirizá, a branch of the
Guaviare which does not traverse any stream, so that the Rio Negro does
not there exist.
My own journey up the Uaupés extended to near 72° west longitude.
Five days further, in a small canoe, or about a hundred miles, is the
Juruparí caxoeira, the last fall on the river. Above that, traders have
been twelve days’ journey on a still, almost currentless river, which,
by the colour of its water, and the aspect of its vegetation, resembles
the Upper Amazon. In all this distance, which must reach very nearly to
the base of the Andes, is a virgin forest. The Indians however in the
upper part give account of campos, or plains, and cattle, further up;
and they possess Spanish knives, and other articles, showing that they
have communications with the civilized inhabitants of the country to
the east of Bogotá.
I am therefore strongly inclined to believe that the rivers Ariarí and
others, rising about a hundred miles south of Bogotá, are not, as shown
in all our maps, the sources of the Guaviare, but of the Uaupés, and
that the basin of the Amazon must therefore be here extended to within
sixty miles of the city of Bogotá. This opinion is strengthened by
information obtained from the Indians of Javíta, who annually ascend
the Guaviare to fish in the dry season, and who state that the river
is very small, and in its upper part, where some hills occur, and the
forest ends, it is not more than a hundred yards wide; whereas the
Uaupés, at the furthest point the traders have reached, is still a
large river, from a quarter of a mile to a mile in width.
The Amazon and all its branches are subject, like most tropical rivers,
to an annual rise and fall of very great regularity.
In the main stream, and in all the branches which flow from the Andes,
the waters begin to rise in December or January, when the rains
generally commence, and continue rising till June, when the fine
weather has just set in. The time when the waters begin to fall is
about the 21st of June,—seldom deviating more than a few days from this
date. In branches which have their sources in a different direction,
such as the Rio Negro, the time of rising does not coincide. On that
river the rains do not commence steadily till February or March, when
the river rises with very great rapidity, and generally is quite filled
by June, and then begins to fall with the Amazon. It thus happens
that in the months of January and February, when the Amazon is rising
rapidly, the Rio Negro is still falling in its upper part; the waters
of the Amazon therefore flow into the mouth of the Rio Negro, causing
that river to remain stagnant like a lake, or even occasionally to flow
back towards its source. The total rise of the Amazon between high and
low water mark has not been accurately ascertained, as it cannot be
properly determined without a spirit-level; it is however certainly
not less than forty, and probably often fifty feet. If therefore we
consider the enormous water surface raised fifty feet annually, we
shall gain from another point of view an idea of the immense quantity
of water falling annually in the Amazon valley. We cannot take the
length of the Amazon and its main tributaries at less than ten thousand
miles, and their average width about two miles; so that there will be
a surface of twenty thousand square miles of water, raised fifty feet
every year. But it is not only this surface that is raised, for a great
extent of land on the banks of all the rivers is flooded to a great
depth at every time of high water. These flooded lands are called, in
the language of the country, “gapó,” and are one of the most singular
features of the Amazon. Sometimes on one side, sometimes on both, to
a distance of twenty or thirty miles from the main river, these gapós
extend on the Amazon, and on portions of all its great branches. They
are all covered with a dense virgin forest of lofty trees, whose
stems are every year, during six months, from ten to forty feet under
water. In this flooded forest the Indians have paths for their canoes,
cutting across from one river to another, and much used, to avoid the
strong current of the main stream. From the mouth of the river Tapajoz
to Coary, on the Solimões, a canoe can pass, without once entering the
Amazon: the path lies across lakes, and among narrow inland channels,
and through miles of dense flooded forest, crossing the Madeira, the
Purus, and a hundred other smaller streams. All along, from the mouth
of the Rio Negro to the mouth of the Iça, is an immense extent of gapó,
and it reaches also far up into the interior; for even near the sources
of the Rio Negro, and on the upper waters of the Uaupés, are extensive
tracts of land which are annually overflowed.
In the whole country around the mouth of the Amazon, round the great
island of Marajó, and about the mouths of the Tocantíns and Xingú,
the diurnal and semi-monthly tides are most felt, the annual rise and
fall being almost lost. Here the low lands are overflowed at all the
spring-tides, or every fortnight, subjecting all vegetation to another
peculiar set of circumstances. Considerable tracts of land, still
covered with vegetation, are so low, that they are flooded at every
high water, and again vary the conditions of vegetable growth.
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.
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.
[Illustration: _Plate I._
_a. Fragments imbedded in Granite._
_b. Granite with twisted veins._
_c. 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.)
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.
[Illustration: _Plate II._
_Forms of Granite rocks._
R. Wallace, del. et. lith. F. Reeve, imp.]
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.
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 Tocantíns 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 sand-banks 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.
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 im- mediate 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.
[Illustration: _Plate III._
_Comparative Climates of Pará and London._]
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 III.)
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 modifications 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 Amazon 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 keeping a regular series of observations
on the waters of the higher parts of the rivers I ascended.
[Illustration: _Plate IV._
_Mean Atmospheric Pressure at Para for 3 years._]
The extreme variation of the barometer at Pará for three years was only
three-tenths of an inch (see diagram, Plate IV.). 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 experienced anything of the kind very remarkable myself. Many
intelligent persons have assured me that they sometimes are 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.