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Chapter III
ON THE PLAINS AND VALLEYS OF CHILE:--SALIFEROUS SUPERFICIAL
DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.
Marks of sea-action on the eastern flanks of the Cordillera.
Sloping terrace-like fringes of stratified shingle within the valleys of
the Cordillera; their marine origin.
Boulders in the valley of Cachapual.
Horizontal elevation of the Cordillera.
Formation of valleys.
Boulders moved by earthquake-waves.
Saline superficial deposits.
Bed of nitrate of soda at Iquique.
Saline incrustations.
Salt-lakes of La Plata and Patagonia; purity of the salt; its origin.
The space between the Cordillera and the coast of Chile is on a rude
average from eighty to above one hundred miles in width; it is formed,
either of an almost continuous mass of mountains, or more commonly of
several nearly parallel ranges, separated by plains; in the more southern
parts of this province the mountains are quite subordinate to the plains;
in the northern part the mountains predominate.
The basin-like plains at the foot of the Cordillera are in several respects
remarkable; that on which the capital of Chile stands is fifteen miles in
width, in an east and west line, and of much greater length in a north and
south line; it stands 1,750 feet above the sea; its surface appears smooth,
but really falls and rises in wide gentle undulations, the hollows
corresponding with the main valleys of the Cordillera: the striking manner
in which it abruptly comes up to the foot of this great range has been
remarked by every author since the time of Molina. (This plain is partially
separated into two basins by a range of hills; the southern half, according
to Meyen ("Reise um Erde" Th. 1 s. 274), falls in height, by an abrupt
step, of between fifteen and twenty feet.) Near the Cordillera it is
composed of a stratified mass of pebbles of all sizes, occasionally
including rounded boulders: near its western boundary, it consists of
reddish sandy clay, containing some pebbles and numerous fragments of
pumice, and sometimes passes into pure sand or into volcanic ashes. At
Podaguel, on this western side of the plain, beds of sand are capped by a
calcareous tuff, the uppermost layers being generally hard and
substalagmitic, and the lower ones white and friable, both together
precisely resembling the beds at Coquimbo, which contain recent marine
shells. Abrupt, but rounded, hummocks of rock rise out of this plain: those
of Sta. Lucia and S. Cristoval are formed of greenstone-porphyry almost
entirely denuded of its original covering of porphyritic claystone breccia;
on their summits, many fragments of rock (some of them kinds not found in
situ) are coated and united together by a white, friable, calcareous tuff,
like that found at Podaguel. When this matter was deposited on the summit
of S. Cristoval, the water must have stood 946 feet above the surface of
the surrounding plain. (Or 2,690 feet above the sea, as measured
barometrically by Mr. Eck. This tuff appears to the eye nearly pure; but
when placed in acid it leaves a considerable residue of sand and broken
crystals, apparently of feldspar. Dr. Meyen ("Reise" Th. 1 s. 269) says he
found a similar substance on the neighbouring hill of Dominico (and I found
it also on the Cerro Blanco), and he attributes it to the weathering of the
stone. In some places which I examined, its bulk put this view of its
origin quite out of the question; and I should much doubt whether the
decomposition of a porphyry would, in any case, leave a crust chiefly
composed of carbonate of lime. The white crust, which is commonly seen on
weathered feldspathic rocks, does not appear to contain any free carbonate
of lime.)
To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable level gap in
the mountains, forming a true land-strait, and called the Angostura. It
then immediately expands into a second basin-formed plain: this again to
the south contracts into another land-strait, and expands into a third
basin, which, however, falls suddenly in level about forty feet. This third
basin, to the south, likewise contracts into a strait, and then again opens
into the great plain of San Fernando, stretching so far south that the
snowy peaks of the distant Cordillera are seen rising above its horizon as
above the sea. These plains, near the Cordillera, are generally formed of a
thick stratified mass of shingle (The plain of San Fernando has, according
to MM. Meyen and Gay "Reise" etc. Th. 1 ss. 295 and 298, near the
Cordillera, an upper step-formed plain of clay, on the surface of which
they found numerous blocks of rocks, from two to three feet long, either
lying single or piled in heaps, but all arranged in nearly straight
lines.); in other parts, of a red sandy clay, often with an admixture of
pumiceous matter. Although these basins are connected together like a
necklace, in a north and south line, by smooth land-straits, the streams
which drain them do not all flow north and south, but mostly westward,
through breaches worn in the bounding mountains; and in the case of the
second basin, or that of Rancagua, there are two distinct breaches. Each
basin, moreover, is not drained singly; thus, to give the most striking
instance, but not the only one, in proceeding southward over the plain of
Rancagua, we first find the water flowing northward to and through the
northern land-strait; then, without crossing any marked ridge or watershed,
we see it flowing south-westward towards the northern one of the two
breaches in the western mountainous boundary; and lastly, again without any
ridge, it flows towards the southern breach in these same mountains. Hence
the surface of this one basin-like plain, appearing to the eye so level,
has been modelled with great nicety, so that the drainage, without any
conspicuous watersheds, is directed towards three openings in the
encircling mountains. ((It appears from Captain Herbert's account of the
Diluvium of the Himalaya, "Gleanings of Science" Calcutta volume 2 page
164, that precisely similar remarks apply to the drainage of the plains or
valleys between those great mountains.) The streams flowing from the
southern basin-like plains, after passing through the breaches to the west,
unite and form the river Rapel, which enters the Pacific near Navidad. I
followed the southernmost branch of this river, and found that the basin or
plain of San Fernando is continuously and smoothly united with those
plains, which were described in the Second Chapter, as being worn near the
coast into successive cave-eaten escarpments, and still nearer to the
coast, as being strewed with upraised recent marine remains.
I might have given descriptions of numerous other plains of the same
general form, some at the foot of the Cordillera, some near the coast, and
some halfway between these points. I will allude only to one other, namely,
the plain of Uspallata, lying on the eastern or opposite side of the
Cordillera, between that great range and the parallel lower range of
Uspallata. According to Miers, its surface is 6,000 feet above the level of
the sea: it is from ten to fifteen miles in width, and is said to extend
with an unbroken surface for 180 miles northwards: it is drained by two
rivers passing through breaches in the mountains to the east. On the banks
of the River Mendoza it is seen to be composed of a great accumulation of
stratified shingle, estimated at 400 feet in thickness. In general
appearance, and in numerous points of structure, this plain closely
resembles those of Chile.
The origin and manner of formation of the thick beds of gravel, sandy clay,
volcanic detritus, and calcareous tuff, composing these basin-like plains,
is very important; because, as we shall presently show, they send arms or
fringes far up the main valleys of the Cordillera. Many of the inhabitants
believe that these plains were once occupied by lakes, suddenly drained;
but I conceive that the number of the separate breaches at nearly the same
level in the mountains surrounding them quite precludes this idea. Had not
such distinguished naturalists as MM. Meyen and Gay stated their belief
that these deposits were left by great debacles rushing down from the
Cordillera, I should not have noticed a view, which appears to me from many
reasons improbable in the highest degree--namely, from the vast
accumulation of WELL-ROUNDED PEBBLES--their frequent stratification with
layers of sand--the overlying beds of calcareous tuff--this same substance
coating and uniting the fragments of rock on the hummocks in the plain of
Santiago--and lastly even from the worn, rounded, and much denuded state of
these hummocks, and of the headlands which project from the surrounding
mountains. On the other hand, these several circumstances, as well as the
continuous union of the basins at the foot of the Cordillera, with the
great plain of the Rio Rapel which still retains the marks of sea-action at
various levels, and their general similarity in form and composition with
the many plains near the coast, which are either similarly marked or are
strewed with upraised marine remains, fully convince me that the mountains
bounding these basin-plains were breached, their islet-like projecting
rocks worn, and the loose stratified detritus forming their now level
surfaces deposited, by the sea, as the land slowly emerged. It is hardly
possible to state too strongly the perfect resemblance in outline between
these basin-like, long, and narrow plains of Chile (especially when in the
early morning the mists hanging low represented water), and the creeks and
fiords now intersecting the southern and western shores of the continent.
We can on this view of the sea, when the land stood lower, having long and
tranquilly occupied the spaces between the mountain-ranges, understand how
the boundaries of the separate basins were breached in more than one place;
for we see that this is the general character of the inland bays and
channels of Tierra del Fuego; we there, also, see in the sawing action of
the tides, which flow with great force in the cross channels, a power
sufficient to keep the breaches open as the land emerged. We can further
see that the waves would naturally leave the smooth bottom of each great
bay or channel, as it became slowly converted into land, gently inclined to
as many points as there were mouths, through which the sea finally
retreated, thus forming so many watersheds, without any marked ridges, on a
nearly level surface. The absence of marine remains in these high inland
plains cannot be properly adduced as an objection to their marine origin:
for we may conclude, from shells not being found in the great shingle beds
of Patagonia, though copiously strewed on their surfaces, and from many
other analogous facts, that such deposits are eminently unfavourable for
the embedment of such remains; and with respect to shells not being found
strewed on the surface of these basin-like plains, it was shown in the last
chapter that remains thus exposed in time decay and disappear.
(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN
CORDILLERA.
From Cordillera (left) through Talus-plain and Level surface, 2,700 feet
above sea, to Gravel terraces (right).)
I observed some appearances on the plains at the eastern and opposite foot
of the Cordillera which are worth notice, as showing that the sea there
long acted at nearly the same level as on the basin-plains of Chile. The
mountains on this eastern side are exceedingly abrupt; they rise out of a
smooth, talus-like, very gentle, slope, from five to ten miles in width (as
represented in Figure 13), entirely composed of perfectly rounded pebbles,
often white-washed with an aluminous substance like decomposed feldspar.
This sloping plain or talus blends into a perfectly flat space a few miles
in width, composed of reddish impure clay, with small calcareous
concretions as in the Pampean deposit,--of fine white sand with small
pebbles in layers,--and of the above-mentioned white aluminous earth, all
interstratified together. This flat space runs as far as Mendoza, thirty
miles northward, and stands probably at about the same height, namely,
2,700 feet (Pentland and Miers) above the sea. To the east it is bounded by
an escarpment, eighty feet in height, running for many miles north and
south, and composed of perfectly round pebbles, and loose, white-washed, or
embedded in the aluminous earth: behind this escarpment there is a second
and similar one of gravel. Northward of Mendoza, these escarpments become
broken and quite obliterated; and it does not appear that they ever
enclosed a lake-like area: I conclude, therefore, that they were formed by
the sea, when it reached the foot of the Cordillera, like the similar
escarpments occurring at so many points on the coasts of Chile and
Patagonia.
The talus-like plain slopes up with a smooth surface into the great dry
valleys of the Cordillera. On each hand of the Portillo valley, the
mountains are formed of red granite, mica-slate, and basalt, which all have
suffered a truly astonishing amount of denudation; the gravel in the
valley, as well as on the talus-like plain in front of it, is composed of
these rocks; but at the mouth of the valley, in the middle (height probably
about three thousand five hundred feet above the sea), a few small isolated
hillocks of several varieties of porphyry project, round which, on all
sides, smooth and often white-washed pebbles of these same porphyries, to
the exclusion of all others, extend to a circumscribed distance. Now, it is
difficult to conceive any other agency, except the quiet and long-continued
action of the sea on these hillocks, which could have rounded and
whitewashed the fragments of porphyry, and caused them to radiate from such
small and quite insignificant centres, in the midst of that vast stream of
stones which has descended from the main Cordillera.
SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA.
(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA, bordered
by smooth, sloping gravel-fringes (AA), worn along the course of the river
into cliffs.)
All the main valleys on both flanks of the Chilean Cordillera have formerly
had, or still have, their bottoms filled up to a considerable thickness by
a mass of rudely stratified shingle. In Central Chile the greater part of
this mass has been removed by the torrents; cliff-bounded fringes, more or
less continuous, being left at corresponding heights on both sides of the
valleys. These fringes, or as they may be called terraces, have a smooth
surface, and as the valleys rise, they gently rise with them: hence they
are easily irrigated, and afford great facilities for the construction of
the roads. From their uniformity, they give a remarkable character to the
scenery of these grand, wild, broken valleys. In width, the fringes vary
much, sometimes being only broad enough for the roads, and sometimes
expanding into narrow plains. Their surfaces, besides gently rising up the
valley, are slightly inclined towards its centre in such a manner as to
show that the whole bottom must once have been filled up with a smooth and
slightly concave mass, as still are the dry unfurrowed valleys of Northern
Chile. Where two valleys unite into one, these terraces are particularly
well exhibited, as is represented in Figure 14. The thickness of the gravel
forming these fringes, on a rude average, may be said to vary from thirty
to sixty or eighty feet; but near the mouths of the valleys it was in
several places from two to three hundred feet. The amount of matter removed
by the torrents has been immense; yet in the lower parts of the valleys the
terraces have seldom been entirely worn away on either side, nor has the
solid underlying rock been reached: higher up the valleys, the terraces
have frequently been removed on one or the other side, and sometimes on
both sides; but in this latter case they reappear after a short interval on
the line, which they would have held had they been unbroken. Where the
solid rock has been reached, it has been cut into deep and narrow gorges.
Still higher up the valleys, the terraces gradually become more and more
broken, narrower, and less thick, until, at a height of from seven to nine
thousand feet, they become lost, and blended with the piles of fallen
detritus.
I carefully examined in many places the state of the gravel, and almost
everywhere found the pebbles equally and perfectly rounded, occasionally
with great blocks of rock, and generally distinctly stratified, often with
parting seams of sand. The pebbles were sometimes coated with a white
aluminous, and less frequently with a calcareous, crust. At great heights
up the valleys the pebbles become less rounded; and as the terraces become
obliterated, the whole mass passes into the nature of ordinary detritus. I
was repeatedly struck with the great difference between this detritus high
up the valleys, and the gravel of the terraces low down, namely, in the
greater number of the quite angular fragments in the detritus,--in the
unequal degree to which the other fragments have been rounded,--in the
quantity of associated earth,--in the absence of stratification,--and in
the irregularity of the upper surfaces. This difference was likewise well
shown at points low down the valleys, where precipitous ravines, cutting
through mountains of highly coloured rock, have thrown down wide, fan-
shaped accumulations of detritus on the terraces: in such cases, the line
of separation between the detritus and the terrace could be pointed out to
within an inch or two; the detritus consisting entirely of angular and only
partially rounded fragments of the adjoining coloured rocks; the stratified
shingle (as I ascertained by close inspection, especially in one case, in
the valley of the River Mendoza) containing only a small proportion of
these fragments, and those few well rounded.
I particularly attended to the appearance of the terraces where the valleys
made abrupt and considerable bends, but I could perceive no difference in
their structure: they followed the bends with their usual nearly equable
inclination. I observed, also, in several valleys, that wherever large
blocks of any rock became numerous, either on the surface of the terrace or
embedded in it, this rock soon appeared higher up in situ: thus I have
noticed blocks of porphyry, of andesitic syenite, of porphyry and of
syenite, alternately becoming numerous, and in each case succeeded by
mountains thus constituted. There is, however, one remarkable exception to
this rule; for along the valley of the Cachapual, M. Gay found numerous
large blocks of white granite, which does not occur in the neighbourhood. I
observed these blocks, as well as others of andesitic syenite (not
occurring here in situ), near the baths of Cauquenes at a height of between
two and three hundred feet above the river, and therefore quite above the
terrace or fringe which borders that river; some miles up the valleys there
were other blocks at about the same height. I also noticed, at a less
height, just above the terrace, blocks of porphyries (apparently not found
in the immediately impending mountains), arranged in rude lines, as on a
sea-beach. All these blocks were rounded, and though large, not gigantic,
like the true erratic boulders of Patagonia and Fuegia. M. Gay states that
the granite does not occur in situ within a distance of twenty leagues
("Annales des Science Nat. " 1 series tome 28. M. Gay, as I was informed,
penetrated the Cordillera by the great oblique valley of Los Cupressos, and
not by the most direct line.); I suspect, for several reasons, that it will
ultimately be found at a much less distance, though certainly not in the
immediate neighbourhood. The boulders found by MM. Meyen and Gay on the
upper plain of San Fernando (mentioned in a previous note) probably belong
to this same class of phenomena.
These fringes of stratified gravel occur along all the great valleys of the
Cordillera, as well as along their main branches; they are strikingly
developed in the valleys of the Maypu, Mendoza, Aconcagua, Cachapual, and
according to Meyen, in the Tinguirica. ("Reise" etc. Th. 1 s. 302.) In the
valleys, however, of Northern Chile, and in some on the eastern flank of
the Cordillera, as in the Portillo Valley, where streams have never flowed,
or are quite insignificant in volume, the presence of a mass of stratified
gravel can be inferred only from the smooth slightly concave form of the
bottom. One naturally seeks for some explanation of so general and striking
a phenomenon; that the matter forming the fringes along the valleys, or
still filling up their entire beds, has not fallen from the adjoining
mountains like common detritus, is evident from the complete contrast in
every respect between the gravel and the piles of detritus, whether seen
high up the valleys on their sides, or low down in front of the more
precipitous ravines; that the matter has not been deposited by debacles,
even if we could believe in debacles having rushed down EVERY valley, and
all their branches, eastward and westward from the central pinnacles of the
Cordillera, we must admit from the following reasons,--from the distinct
stratification of the mass,--its smooth upper surface,--the well-rounded
and sometimes encrusted state of the pebbles, so different from the loose
debris on the mountains,--and especially from the terraces preserving their
uniform inclination round the most abrupt bends. To suppose that as the
land now stands, the rivers deposited the shingle along the course of every
valley, and all their main branches, appears to me preposterous, seeing
that these same rivers not only are now removing and have removed much of
this deposit, but are everywhere tending to cut deep and narrow gorges in
the hard underlying rocks.
I have stated that these fringes of gravel, the origin of which are
inexplicable on the notion of debacles or of ordinary alluvial action, are
directly continuous with the similarly-composed basin-like plains at the
foot of the Cordillera, which, from the several reasons before assigned, I
cannot doubt were modelled by the agency of the sea. Now if we suppose that
the sea formerly occupied the valleys of the Chilean Cordillera, in
precisely the same manner as it now does in the more southern parts of the
continent, where deep winding creeks penetrate into the very heart of, and
in the case of Obstruction Sound quite through, this great range; and if we
suppose that the mountains were upraised in the same slow manner as the
eastern and western coasts have been upraised within the recent period,
then the origin and formation of these sloping, terrace-like fringes of
gravel can be simply explained. For every part of the bottom of each valley
will, on this view, have long stood at the head of a sea creek, into which
the then existing torrents will have delivered fragments of rocks, where,
by the action of the tides, they will have been rolled, sometimes
encrusted, rudely stratified, and the whole surface levelled by the
blending together of the successive beach lines. (Sloping terraces of
precisely similar structure have been described by me "Philosophical
Transactions" 1839 page 58, in the valleys of Lochaber in Scotland, where,
at higher levels, the parallel roads of Glen Roy show the marks of the long
and quiet residence of the sea. I have no doubt that these sloping terraces
would have been present in the valleys of most of the European ranges, had
not every trace of them, and all wrecks of sea-action, been swept away by
the glaciers which have since occupied them. I have shown that this is the
case with the mountains ("London and Edinburgh Philosophical Journal"
volume 21 page 187) of North Wales.) As the land rose, the torrents in
every valley will have tended to have removed the matter which just before
had been arrested on, or near, the beach-lines; the torrents, also, having
continued to gain in force by the continued elevation increasing their
total descent from their sources to the sea. This slow rising of the
Cordillera, which explains so well the otherwise inexplicable origin and
structure of the terraces, judging from all known analogies, will probably
have been interrupted by many periods of rest; but we ought not to expect
to find any evidence of these periods in the structure of the gravel-
terraces: for, as the waves at the heads of deep creeks have little erosive
power, so the only effect of the sea having long remained at the same level
will be that the upper parts of the creeks will have become filled up at
such periods to the level of the water with gravel and sand; and that
afterwards the rivers will have thrown down on the filled-up parts a talus
of similar matter, of which the inclination (as at the head of a partially
filled-up lake) will have been determined by the supply of detritus, and
the force of the stream. (I have attempted to explain this process in a
more detailed manner, in a letter to Mr. Maclaren, published in the
"Edinburgh New Philosophical Journal" volume 35 page 288.) Hence, after the
final conversion of the creeks into valleys, almost the only difference in
the terraces at those points at which the sea stood long, will be a
somewhat more gentle inclination, with river-worn instead of sea-worn
detritus on the surface.
I know of only one difficulty on the foregoing view, namely, the far-
transported blocks of rock high on the sides of the valley of the
Cachapual: I will not attempt any explanation of this phenomenon, but I may
state my belief that a mountain-ridge near the Baths of Cauquenes has been
upraised long subsequently to all the other ranges in the neighbourhood,
and that when this was effected the whole face of the country must have
been greatly altered. In the course of ages, moreover, in this and other
valleys, events may have occurred like, but even on a grander scale than,
that described by Molina, when a slip during the earthquake of 1762 banked
up for ten days the great River Lontue, which then bursting its barrier
"inundated the whole country," and doubtless transported many great
fragments of rock. ("Compendio de la Hist." etc. etc. tome 1 page 30. M.
Brongniart, in his report on M. Gay's labours "Annales des Sciences" 1833,
considers that the boulders in the Cachapual belong to the same class with
the erratic boulders of Europe. As the blocks which I saw are not gigantic,
and especially as they are not angular, and as they have not been
transported fairly across low spaces or wide valleys, I am unwilling to
class them with those which, both in the northern and southern hemisphere
"Geological Transactions" volume 6 page 415, have been transported by ice.
It is to be hoped that when M. Gay's long-continued and admirable labours
in Chile are published, more light will be thrown on this subject. However,
the boulders may have been primarily transported; the final position of
those of porphyry, which have been described as arranged at the foot of the
mountain in rude lines, I cannot doubt, has been due to the action of waves
on a beach. The valley of the Cachapual, in the part where the boulders
occur, bursts through the high ridge of Cauquenes, which runs parallel to,
but at some distance from, the Cordillera. This ridge has been subjected to
excessive violence; trachytic lava has burst from it, and hot springs yet
flow at its base. Seeing the enormous amount of denudation of solid rock in
the upper and much broader parts of this valley where it enters the
Cordillera, and seeing to what extent the ridge of Cauquenes now protects
the great range, I could not help believing (as alluded to in the text)
that this ridge with its trachytic eruptions had been thrown up at a much
later period than the Cordillera. If this has been the case, the boulders,
after having been transported to a low level by the torrents (which exhibit
in every valley proofs of their power of moving great fragments), may have
been raised up to their present height, with the land on which they
rested.) Finally, notwithstanding this one case of difficulty, I cannot
entertain any doubt, that these terrace-like fringes, which are
continuously united with the basin-shaped plains at the foot of the
Cordillera, have been formed by the arrestment of river-borne detritus at
successive levels, in the same manner as we see now taking place at the
heads of all those many, deep, winding fiords intersecting the southern
coasts. To my mind, this has been one of the most important conclusions to
which my observations on the geology of South America have led me; for we
thus learn that one of the grandest and most symmetrical mountain-chains in
the world, with its several parallel lines, has been together uplifted in
mass between seven and nine thousand feet, in the same gradual manner as
have the eastern and western coasts within the recent period. (I do not
wish to affirm that all the lines have been uplifted quite equally; slight
differences in the elevation would leave no perceptible effect on the
terraces. It may, however, be inferred, perhaps with one exception, that
since the period when the sea occupied these valleys, the several ranges
have not been dislocated by GREAT and ABRUPT faults or upheavals; for if
such had occurred, the terraces of gravel at these points would not have
been continuous. The one exception is at the lower end of a plain in the
Valle del Yeso (a branch of the Maypu), where, at a great height, the
terraces and valley appear to have been broken through by a line of
upheaval, of which the evidence is plain in the adjoining mountains; this
dislocation, perhaps, occurred AFTER THE ELEVATION of this part of the
valley above the level of the sea. The valley here is almost blocked up by
a pile about one thousand feet in thickness, formed, as far as I could
judge, from three sides, entirely, or at least in chief part, of gravel and
detritus. On the south side, the river has cut quite through this mass; on
the northern side, and on the very summit, deep ravines, parallel to the
line of the valley, are worn, as if the drainage from the valley above had
passed by these two lines before following its present course.)
FORMATION OF VALLEYS.
The bulk of solid rock which has been removed in the lower parts of the
valleys of the Cordillera has been enormous. It is only by reflecting on
such cases as that of the gravel beds of Patagonia, covering so many
thousand square leagues of surface, and which, if heaped into a ridge,
would form a mountain-range almost equal to the Cordillera, that the amount
of denudation becomes credible. The valleys within this range often follow
anticlinal but rarely synclinal lines; that is, the strata on the two sides
more often dip from the line of valley than towards it. On the flanks of
the range, the valleys most frequently run neither along anticlinal nor
synclinal axes, but along lines of flexure or faults: that is, the strata
on both sides dip in the same direction, but with different, though often
only slightly different, inclinations. As most of the nearly parallel
ridges which together form the Cordillera run approximately north and
south, the east and west valleys cross them in zig-zag lines, bursting
through the points where the strata have been least inclined. No doubt the
greater part of the denudation was affected at the periods when tidal-
creeks occupied the valleys, and when the outer flanks of the mountains
were exposed to the full force of an open ocean. I have already alluded to
the power of the tidal action in the channels connecting great bays; and I
may here mention that one of the surveying vessels in a channel of this
kind, though under sail, was whirled round and round by the force of the
current. We shall hereafter see, that of the two main ridges forming the
Chilean Cordillera, the eastern and loftiest one owes the greater part of
its ANGULAR upheaval to a period subsequent to the elevation of the western
ridge; and it is likewise probable that many of the other parallel ridges
have been angularly upheaved at different periods; consequently many parts
of the surfaces of these mountains must formerly have been exposed to the
full force of the waves, which, if the Cordillera were now sunk into the
sea, would be protected by parallel chains of islands. The torrents in the
valleys certainly have great power in wearing the rocks; as could be told
by the dull rattling sound of the many fragments night and day hurrying
downwards; and as was attested by the vast size of certain fragments, which
I was assured had been carried onwards during floods; yet we have seen in
the lower parts of the valleys, that the torrents have seldom removed all
the sea-checked shingle forming the terraces, and have had time since the
last elevation in mass only to cut in the underlying rocks, gorges, deep
and narrow, but quite insignificant in dimensions compared with the entire
width and depth of the valleys.
Along the shores of the Pacific, I never ceased during my many and long
excursions to feel astonished at seeing every valley, ravine, and even
little inequality of surface, both in the hard granitic and soft tertiary
districts, retaining the exact outline, which they had when the sea left
their surfaces coated with organic remains. When these remains shall have
decayed, there will be scarcely any difference in appearance between this
line of coast-land and most other countries, which we are accustomed to
believe have assumed their present features chiefly through the agency of
the weather and fresh-water streams. In the old granitic districts, no
doubt it would be rash to attribute all the modifications of outline
exclusively to the sea-action; for who can say how often this lately
submerged coast may not previously have existed as land, worn by running
streams and washed by rain? This source of doubt, however, does not apply
to the districts superficially formed of the modern tertiary deposits. The
valleys worn by the sea, through the softer formations, both on the
Atlantic and Pacific sides of the continent, are generally broad, winding,
and flat-bottomed: the only district of this nature now penetrated by arms
of the sea, is the island of Chiloe.
Finally, the conclusion at which I have arrived, with respect to the
relative powers of rain and sea water on the land, is, that the latter is
far the most efficient agent, and that its chief tendency is to widen the
valleys; whilst torrents and rivers tend to deepen them, and to remove the
wreck of the sea's destroying action. As the waves have more power, the
more open and exposed the space may be, so will they always tend to widen
more and more the mouths of valleys compared with their upper parts: hence,
doubtless, it is, that most valleys expand at their mouths,--that part, at
which the rivers flowing in them, generally have the least wearing power.
When reflecting on the action of the sea on the land at former levels, the
effect of the great waves, which generally accompany earthquakes, must not
be overlooked: few years pass without a severe earthquake occurring on some
part of the west coast of South America; and the waves thus caused have
great power. At Concepcion, after the shock of 1835, I saw large slabs of
sandstone, one of which was six feet long, three in breadth, and two in
thickness, thrown high up on the beach; and from the nature of the marine
animals still adhering to it, it must have been torn up from a considerable
depth. On the other hand, at Callao, the recoil-wave of the earthquake of
1746 carried great masses of brickwork, between three and four feet square,
some way out seaward. During the course of ages, the effect thus produced
at each successive level, cannot have been small; and in some of the
tertiary deposits on this line of coast, I observed great boulders of
granite and other neighbouring rocks, embedded in fine sedimentary layers,
the transportal of which, except by the means of earthquake-waves, always
appeared to me inexplicable.
SUPERFICIAL SALINE DEPOSITS.
This subject may be here conveniently treated of: I will begin with the
most interesting case, namely, the superficial saline beds near Iquique in
Peru. The porphyritic mountains on the coast rise abruptly to a height of
between one thousand nine hundred and three thousand feet: between their
summits and an inland plain, on which the celebrated deposit of nitrate of
soda lies, there is a high undulatory district, covered by a remarkable
superficial saliferous crust, chiefly composed of common salt, either in
white, hard, opaque nodules, or mingled with sand, in this latter case
forming a compact sandstone. This saliferous superficial crust extends from
the edge of the coast-escarpment, over the whole face of the country; but
never attains, as I am assured by Mr. Bollaert (long resident here) any
great thickness. Although a very slight shower falls only at intervals of
many years, yet small funnel-shaped cavities show that the salt has been in
some parts dissolved. (It is singular how slowly, according to the
observations of M. Cordier on the salt-mountain of Cardona in Spain "Ann.
des Mines, Translation of Geolog. Mem." by De la Beche page 60, salt is
dissolved, where the amount of rain is supposed to be as much as 31.4 of an
inch in the year. It is calculated that only five feet in thickness is
dissolved in the course of a century.) In several places I saw large
patches of sand, quite moist, owing to the quantity of muriate of lime (as
ascertained by Mr. T. Reeks) contained in them. From the compact salt-
cemented sand being either red, purplish, or yellow, according to the
colour of the rocky strata on which it rested, I imagined that this
substance had probably been derived through common alluvial action from the
layers of salt which occur interstratified in the surrounding mountains
("Journal of Researches" page 444 first edition.): but from the interesting
details given by M. d'Orbigny, and from finding on a fresh examination of
this agglomerated sand, that it is not irregularly cemented, but consists
of thin layers of sand of different tints of colour, alternating with
excessively fine parallel layers of salt, I conclude that it is not of
alluvial origin. M. d'Orbigny observed analogous saline beds extending from
Cobija for five degrees of latitude northward, and at heights varying from
six hundred to nine hundred feet ("Voyage" etc. page 102. M. d'Orbigny
found this deposit intersected, in many places, by deep ravines, in which
there was no salt. Streams must once, though historically unknown, have
flowed in them; and M. d'Orbigny argues from the presence of undissolved
salt over the whole surrounding country, that the streams must have arisen
from rain or snow having fallen, not in the adjoining country, but on the
now arid Cordillera. I may remark, that from having observed ruins of
Indian buildings in absolutely sterile parts of the Chilian Cordillera
("Journal" 2nd edition page 357), I am led to believe that the climate, at
a time when Indian man inhabited this part of the continent, was in some
slight degree more humid than it is at present.): from finding recent sea-
shells strewed on these saliferous beds, and under them, great well-rounded
blocks, exactly like those on the existing beach, he believes that the
salt, which is invariably superficial, has been left by the evaporation of
the sea-water. This same conclusion must, I now believe, be extended to the
superficial saliferous beds of Iquique, though they stand about three
thousand feet above the level of the sea.
Associated with the salt in the superficial beds, there are numerous, thin,
horizontal layers of impure, dirty-white, friable, gypseous and calcareous
tuffs. The gypseous beds are very remarkable, from abounding with, so as
sometimes to be almost composed of, irregular concretions, from the size of
an egg to that of a man's head, of very hard, compact, heavy gypsum, in the
form of anhydrite. This gypsum contains some foreign particles of stone; it
is stained, judging from its action with borax, with iron, and it exhales a
strong aluminous odour. The surfaces of the concretions are marked by
sharp, radiating, or bifurcating ridges, as if they had been (but not
really) corroded: internally they are penetrated by branching veins (like
those of calcareous spar in the septaria of the London clay) of pure white
anhydrite. These veins might naturally have been thought to have been
formed by subsequent infiltration, had not each little embedded fragment of
rock been likewise edged in a very remarkable manner by a narrow border of
the same white anhydrite: this shows that the veins must have been formed
by a process of segregation, and not of infiltration. Some of the little
included and CRACKED fragments of foreign rock are penetrated by the
anhydrite, and portions have evidently been thus mechanically displaced: at
St. Helena, I observed that calcareous matter, deposited by rain water,
also had the power to separate small fragments of rock from the larger
masses. ("Volcanic Islands" etc. page 87.) I believe the superficial
gypseous deposit is widely extended: I received specimens of it from
Pisagua, forty miles north of Iquique, and likewise from Arica, where it
coats a layer of pure salt. M. d'Orbigny found at Cobija a bed of clay,
lying above a mass of upraised recent shells, which was saturated with
sulphate of soda, and included thin layers of fibrous gypsum. ("Voyage
Geolog." etc. page 95.) These widely extended, superficial, beds of salt
and gypsum, appear to me an interesting geological phenomenon, which could
be presented only under a very dry climate.
The plain or basin, on the borders of which the famous bed of nitrate of
soda lies, is situated at the distance of about thirty miles from the sea,
being separated from it by the saliferous district just described. It
stands at a height of 3,300 feet; its surface is level, and some leagues in
width; it extends forty miles northward, and has a total length (as I was
informed by Mr. Belford Wilson, the Consul-General at Lima) of 420 miles.
In a well near the works, thirty-six yards in depth, sand, earth, and a
little gravel were found: in another well, near Almonte, fifty yards deep,
the whole consisted, according to Mr. Blake, of clay, including a layer of
sand two feet thick, which rested on fine gravel, and this on coarse
gravel, with large rounded fragments of rock. (See an admirable paper
"Geological and Miscellaneous Notices of Tarapaca" in "Silliman's American
Journal" volume 44 page 1.) In many parts of this now utterly desert plain,
rushes and large prostrate trees in a hardened state, apparently Mimosas,
are found buried, at a depth from three to six feet; according to Mr.
Blake, they have all fallen to the south-west. The bed of nitrate of soda
is said to extend for forty to fifty leagues along the western margin of
the plain, but is not found in its central parts: it is from two to three
feet in thickness, and is so hard that it is generally blasted with
gunpowder; it slopes gently upwards from the edge of the plain to between
ten and thirty feet above its level. It rests on sand in which, it is said,
vegetable remains and broken shells have been found; shells have also been
found, according to Mr. Blake, both on and in the nitrate of soda. It is
covered by a superficial mass of sand, containing nodules of common salt,
and, as I was assured by a miner, much soft gypseous matter, precisely like
that in the superficial crust already described: certainly this crust, with
its characteristic concretions of anhydrite, comes close down to the edge
of the plain.
The nitrate of soda varies in purity in different parts, and often contains
nodules of common salt. According to Mr. Blake, the proportion of nitrate
of soda varies from 20 to 75 per cent. An analysis by Mr. A. Hayes, of an
average specimen, gave:--
Nitrate of Soda.... 64.98
Sulphate of Soda.... 3.00
Chloride of Soda... 28.69
Iodic Salts......... 0.63
Shells and Marl..... 2.60
99.90
The "mother-water" at some of the refineries is very rich in iodic salts,
and is supposed to contain much muriate of lime. ("Literary Gazette" 1841
page 475.) In an unrefined specimen brought home by myself, Mr. T. Reeks
has ascertained that the muriate of lime is very abundant. With respect to
the origin of this saline mass, from the manner in which the gently
inclined, compact bed follows for so many miles the sinuous margin of the
plain, there can be no doubt that it was deposited from a sheet of water:
from the fragments of embedded shells, from the abundant iodic salts, from
the superficial saliferous crust occurring at a higher level and being
probably of marine origin, and from the plain resembling in form those of
Chile and that of Uspallata, there can be little doubt that this sheet of
water was, at least originally, connected with the sea. (From an official
document, shown me by Mr. Belford Wilson, it appears that the first export
of nitrate of soda to Europe was in July 1830, on French account, in a
British ship:--
In year, the entire export was in Quintals.
1830............................ 17,300
1831............................ 40,885
1832............................ 51,400
1833............................ 91,335
1834........................... 149,538
The Spanish quintal nearly equals 100 English pounds.)
THIN, SUPERFICIAL, SALINE INCRUSTATIONS.
These saline incrustations are common in many parts of America: Humboldt
met with them on the tableland of Mexico, and the Jesuit Falkner and other
authors state that they occur at intervals over the vast plains extending
from the mouth of the Plata to Rioja and Catamarca. (Azara "Travels" volume
1 page 55, considers that the Parana is the eastern boundary of the
saliferous region; but I heard of "salitrales" in the Province of Entre
Rios.) Hence it is that during droughts, most of the streams in the Pampas
are saline. I nowhere met with these incrustations so abundantly as near
Bahia Blanca: square miles of the mud-flats, which near that place are
raised only a few feet above the sea, just enough to protect them from
being overflowed, appear, after dry weather, whiter than the ground after
the thickest hoar-frost. After rain the salts disappear, and every puddle
of water becomes highly saline; as the surface dries, the capillary action
draws the moisture up pieces of broken earth, dead sticks, and tufts of
grass, where the salt effloresces. The incrustation, where thickest, does
not exceed a quarter of an inch. M. Parchappe has analysed it (M. d'Orbigny
"Voyage" etc. Part. Hist. tome 1 page 664.); and finds that the specimens
collected at the extreme head of the low plain, near the River Manuello,
consist of 93 per cent of sulphate of soda, and 7 of common salt; whilst
the specimens taken close to the coast contain only 63 per cent of the
sulphate, and 37 of the muriate of soda. This remarkable fact, together
with our knowledge that the whole of this low muddy plain has been covered
by the sea within the recent period, must lead to the suspicion that the
common salt, by some unknown process, becomes in time changed into the
sulphate. Friable, calcareous matter is here abundant, and the case of the
apparent double decomposition of the shells and salt on San Lorenzo, should
not be forgotten.
The saline incrustations, near Bahia Blanca, are not confined to, though
most abundant on, the low muddy flats; for I noticed some on a calcareous
plain between thirty and forty feet above the sea, and even a little occurs
in still higher valleys. Low alluvial tracts in the valleys of the Rivers
Negro and Colorado are also encrusted, and in the latter valley such spaces
appeared to be occasionally overflowed by the river. I observed saline
incrustations in some of the valleys of Southern Patagonia. At Port Desire
a low, flat, muddy valley was thickly incrusted by salts, which on analysis
by Mr. T. Reeks, are found to consist of a mixture of sulphate and muriate
of soda, with carbonate of lime and earthy matter. On the western side of
the continent, the southern coasts are much too humid for this phenomenon;
but in Northern Chile I again met with similar incrustations. On the
hardened mud, in parts of the broad, flat-bottomed valley of Copiapo, the
saline matter encrusts the ground to the thickness of some inches:
specimens, sent by Mr. Bingley to Apothecaries' Hall for analysis, were
said to consist of carbonate and sulphate of soda. Much sulphate of soda is
found in the desert of Atacama. In all parts of South America, the saline
incrustations occur most frequently on low damp surfaces of mud, where the
climate is rather dry; and these low surfaces have, in almost every case,
been upraised above the level of the sea, within the recent period.
SALT-LAKES OF PATAGONIA AND LA PLATA.
Salinas, or natural salt-lakes, occur in various formations on the eastern
side of the continent,--in the argillaceo-calcareous deposit of the Pampas,
in the sandstone of the Rio Negro, where they are very numerous, in the
pumiceous and other beds of the Patagonian tertiary formation, and in small
primary districts in the midst of this latter formation. Port S. Julian is
the most southerly point (latitude 49 degrees to 50 degrees) at which
salinas are known to occur. (According to Azara "Travels" volume 1 page 56,
there are salt-lakes as far north as Chaco (latitude 25 degrees), on the
banks of the Vermejo. The salt-lakes of Siberia appear (Pallas "Travels"
English Translation volume 1 page 284) to occur in very similar depressions
to those of Patagonia.) The depressions, in which these salt-lakes lie, are
from a few feet to sixty metres, as asserted by M. d'Orbigny, below the
surface of the surrounding plains ("Voyage Geolog." page 63.); and,
according to this same author, near the Rio Negro they all trend, either in
the N.E. and S.W. or in E. and W. lines, coincident with the general slope
of the plain. These depressions in the plain generally have one side lower
than the others, but there are no outlets for drainage. Under a less dry
climate, an outlet would soon have been formed, and the salt washed away.
The salinas occur at different elevations above the sea; they are often
several leagues in diameter; they are generally very shallow, but there is
a deep one in a quartz-rock formation near C. Blanco. In the wet season,
the whole, or a part, of the salt is dissolved, being redeposited during
the succeeding dry season. At this period the appearance of the snow-white
expanse of salt crystallised in great cubes, is very striking. In a large
salina, northward of the Rio Negro, the salt at the bottom, during the
whole year, is between two and three feet in thickness.
The salt rests almost always on a thick bed of black muddy sand, which is
fetid, probably from the decay of the burrowing worms inhabiting it.
(Professor Ehrenberg examined some of this muddy sand, but was unable to
find in it any infusoria.) In a salina, situated about fifteen miles above
the town of El Carmen on the Rio Negro, and three or four miles from the
banks of that river, I observed that this black mud rested on gravel with a
calcareous matrix, similar to that spread over the whole surrounding
plains: at Port S. Julian the mud, also, rested on the gravel: hence the
depressions must have been formed anteriorly to, or contemporaneously with,
the spreading out of the gravel. I was informed that one small salina
occurs in an alluvial plain within the valley of the Rio Negro, and
therefore its origin must be subsequent to the excavation of that valley.
When I visited the salina, fifteen miles above the town, the salt was
beginning to crystallise, and on the muddy bottom there were lying many
crystals, generally placed crossways of sulphate of soda (as ascertained by
Mr. Reeks), and embedded in the mud, numerous crystals of sulphate of lime,
from one to three inches in length: M. d'Orbigny states that some of these
crystals are acicular and more than even nine inches in length ("Voyage
Geolog." page 64.); others are macled and of great purity: those I found
all contained some sand in their centres. As the black and fetid sand
overlies the gravel, and that overlies the regular tertiary strata, I think
there can be no doubt that these remarkable crystals of sulphate of lime
have been deposited from the waters of the lake. The inhabitants call the
crystals of selenite, the padre del sal, and those of the sulphate of soda,
the madre del sal; they assured me that both are found under the same
circumstances in several of the neighbouring salinas; and that the sulphate
of soda is annually dissolved, and is always crystallised before the common
salt on the muddy bottom. (This is what might have been expected; for M.
Ballard asserts "Acad. des Sciences" October 7, 1844, that sulphate of soda
is precipitated from solution more readily from water containing muriate of
soda in excess, than from pure water.) The association of gypsum and salt
in this case, as well as in the superficial deposits of Iquique, appears to
me interesting, considering how generally these substances are associated
in the older stratified formations.
Mr. Reeks has analysed for me some of the salt from the salina near the Rio
Negro; he finds it composed entirely of chloride of sodium, with the
exception of 0.26 of sulphate of lime and of 0.22 of earthy matter: there
are no traces of iodic salts. Some salt from the salina Chiquitos, in the
Pampean formation, is equally pure. It is a singular fact, that the salt
from these salinas does not serve so well for preserving meat, as sea-salt
from the Cape de Verde Islands; and a merchant at Buenos Ayres told me that
he considered it as 50 per cent less valuable. The purity of the Patagonian
salt, or absence from it of those other saline bodies found in all sea-
water, is the only assignable cause for this inferiority; a conclusion
which is supported by the fact lately ascertained, that those salts answer
best for preserving cheese which contain most of the deliquescent
chlorides. ("Horticultural and Agricultural Gazette" 1845 page 93.) (It
would probably well answer for the merchants of Buenos Ayres (considering
the great consumption there of salt for preserving meat) to import the
deliquescent chlorides to mix with the salt from the salinas: I may call
attention to the fact, that at Iquique, a large quantity of muriate of
lime, left in the MOTHER-WATER during the refinement of the nitrate of
soda, is annually thrown away.)
With respect to the origin of the salt in the salinas, the foregoing
analysis seems opposed to the view entertained by M. d'Orbigny and others,
and which seems so probable considering the recent elevation of this line
of coast, namely, that it is due to the evaporation of sea-water and to the
drainage from the surrounding strata impregnated with sea-salt. I was
informed (I know not whether accurately) that on the northern side of the
salina on the Rio Negro, there is a small brine spring which flows at all
times of the year: if this be so, the salt in this case at least, probably
is of subterranean origin. It at first appears very singular that fresh
water can often be procured in wells, and is sometimes found in small
lakes, quite close to these salinas. (Sir W. Parish states "Buenos Ayres"
etc. pages 122 and 170, that this is the case near the great salinas
westward of the S. Ventana. I have seen similar statements in an ancient
MS. Journal lately published by S. Angelis. At Iquique, where the surface
is so thickly encrusted with saline matter, I tasted water only slightly
brackish, procured in a well thirty-six yards deep; but here one feels less
surprise at its presence, as pure water might percolate under ground from
the not very distant Cordillera.) I am not aware that this fact bears
particularly on the origin of the salt; but perhaps it is rather opposed to
the view of the salt having been washed out of the surrounding superficial
strata, but not to its having been the residue of sea-water, left in
depressions as the land was slowly elevated.
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