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[1] since the
time of Molina. 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[2] above the surface of the
surrounding plain.
[1] This plain is partially separated into two basins by a range of
hills; the southern half, according to Meyen (“Reise um Erde,” Th. i,
s. 274), falls in height, by an abrupt step, of between fifteen and
twenty feet.
[2] 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. i, 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;[3] 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.[4] 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.
[3] The plain of San Fernando has, according to MM. Meyen and Gay
“Reise,” etc., Th. i, 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.
[4] It appears from Captain Herbert’s account of the Diluvium of the
Himalaya, (“Gleanings of Science,” Calcutta, vol. ii, p. 164), that
precisely similar remarks apply to the drainage of the plains or
valleys between those great mountains.
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.
No. 13
Section of the plain at the eastern foot of the Chilian Cordillera.
[Illustration: Section of the plain at the eastern foot of the Chilian
Cordillera.]
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._—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.
[Illustration: Ground plan of a bifurcating valley in the Cordillera.]
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[5] states that the granite does not occur in situ within
a distance of twenty leagues; 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.
[5] “Annales des Science Nat.” (I. séries, 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.
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,[6] in the Tinguirica. XIn 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.
[6] “Reise,” etc., Th. I, s. 302.
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.[7] 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.[8] 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.
[7] Sloping terraces of precisely similar structure have been
described by me (“Philosoph. Transactions,” 1839, p. 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 Edin. Phil. Journal,_ vol. xxi,
p. 187) of North Wales.
[8] I have attempted to explain this process in a more detailed
manner, in a letter to Mr. Maclaren, published in the _Edinburgh New
Phil. Journal,_ vol. xxxv, p. 288.
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,[9] 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. 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,[10] 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.
[9] “Compendio de la Hist.,” etc., tome i, p. 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 (“Geolog. Transac.,” vol. vi, p. 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.
[10] 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.[11] 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[12] substance had probably been derived
through common alluvial action from the layers of salt which occur
interstratified in the surrounding mountains: 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[13] observed analogous
saline beds extending from Cobija for five degrees of latitude
northward, and at heights varying from six hundred to nine hundred
feet: 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.
[11] It is singular how slowly, according to the observations of M.
Cordier on the salt-mountain of Cardona in Spain (“Ann. des Mines,
Transl. of Geolog. Mem.” by De la Beche, p. 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.
[12] “Journal of Researches,” p. 444, first edit.
[13] “Voyage,” etc., p. 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 edit., p. 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.
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.[14] 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[15] 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. 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.
[14] “Volcanic Islands,” etc., p. 87.
[15] “Voyage Géolog.,” etc., p. 95.
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,[16] 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. 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.
[16] See an admirable paper “Geolog. and Miscell. Notices of
Tarapaca,” in _Silliman’s American Journal_, vol. xliv, p. 1.
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[17] to contain much muriate of lime. 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.[18]
[17] _Literary Gazette_, 1841, p. 475.
[18] (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:—
Entire export 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[19] state
that they occur at intervals over the vast plains extending from the
mouth of the Plata to Rioja and Catamarca. 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[20] has analysed it;
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.
[19] Azara (“Travels,” vol. i, p. 55) considers that the Parana is the
eastern boundary of the saliferous region; but I heard of “salitrales”
in the Province of Entre Rios.
[20] M. d’Orbigny’s “Voyage,” etc., Part. Hist., tome i, p. 664.
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 (lat. 49° to 50°) at which salinas are known to
occur.[21] The depressions, in which these salt-lakes lie, are from a
few feet to sixty metres, as asserted by M. d’Orbigny,[22] below the
surface of the surrounding plains; 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.
[21] According to Azara (“Travels,” vol. i, p. 56) there are
salt-lakes as far north as Chaco (lat. 25°), on the banks of the
Vermejo. The salt-lakes of Siberia appear (Pallas’s “Travels,” English
Trans., vol. i, p. 284) to occur in very similar depressions to those
of Patagonia.
[22] “Voyage Géolog.,” p. 63.
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.[23] 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[24] states that some of these crystals are
acicular and more than even nine inches in length; 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.[25] 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.
[23] Professor Ehrenberg examined some of this muddy sand, but was
unable to find in it any infusoria.
[24] “Voyage Géolog.,” p. 64.
[25] This is what might have been expected; for M. Ballard asserts
(_Acad. des Sciences_, Oct. 7, 1844, that sulphate of soda is
precipitated from solution more readily from water containing muriate
of soda in excess, than from pure water.
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,[26] that those salts answer best for
preserving cheese which contain most of the deliquescent chlorides.[27]
[26] _Hort. and Agricult. Gazette_, 1845, p. 93.
[27] 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,[28] and is sometimes found in small lakes, quite close to these
salinas. 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.
[28] Sir W. Parish states (“Buenos Ayres,” etc., pp. 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.