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Geological Observations On South America

C >> Charles Darwin >> Geological Observations On South America




The mountains near Las Amolanas, composed of the cretaceo-oolitic strata,
are interlaced with dikes like a spider's web, to an extent which I have
never seen equalled, except in the denuded interior of a volcanic crater:
north and south lines, however, predominate. These dikes are composed of
green, white, and blackish rocks, all porphyritic with feldspar, and often
with large crystals of hornblende. The white varieties approach closely in
character to andesite, which composes as we have seen, the injected axes of
so many of the lines of elevation. Some of the green varieties are finely
laminated, parallel to the walls of the dikes.

SIXTH AXIS OF ELEVATION (VALLEY OF COPIAPO).

This axis consists of a broad mountainous mass [O] of andesite, composed of
albite, brown mica, and chlorite, passing into andesitic granite, with
quartz: on its western side it has thrown off, at a considerable angle, a
thick mass of stratified porphyries, including much epidote [NN], and
remarkable only from being divided into very thin beds, as highly
amygdaloidal on their surfaces as subaerial lava-streams are often
vesicular. This porphyritic formation is conformably covered, as seen some
way up the ravine of Jolquera, by a mere remnant of the lower part of the
cretaceo-oolitic formation [MM], which in one part encases, as represented
in the coloured section, the foot of the andesitic axis [L], of the already
described fifth line, and in another part entirely conceals it: in this
latter case, the gypseous or cretaceo-oolitic strata falsely appeared to
dip under the porphyritic conglomerate of the fifth axis. The lowest bed of
the gypseous formation, as seen here [M], is of yellowish siliceous
sandstone, precisely like that of Amolanas, interlaced in parts with veins
of gypsum, and including layers of the black, calcareous, non-fissile
slate-rock: the Turritella Andii, Pecten Dufreynoyi, Terebratula aenigma,
var., and some Gryphites were embedded in these layers. The sandstone
varies in thickness from only twenty to eighty feet; and this variation is
caused by the inequalities in the upper surface of an underlying stream of
purple claystone porphyry. Hence the above fossils here lie at the very
base of the gypseous or cretaceo-oolitic formation, and hence they were
probably once covered up by strata about seven thousand feet in thickness:
it is, however, possible, though from the nature of all the other sections
in this district not probable, that the porphyritic claystone lava may in
this case have invaded a higher level in the series. Above the sandstone
there is a considerable mass of much indurated, purplish-black, calcareous
claystone, allied in nature to the often-mentioned black calcareous slate-
rock.
Eastward of the broad andesitic axis of this sixth line, and penetrated by
many dikes from it, there is a great formation [P] of mica-schist, with its
usual variations, and passing in one part into a ferruginous quartz-rock.
The folia are curved and highly inclined, generally dipping eastward. It is
probable that this mica-schist is an old formation, connected with the
granitic rocks and metamorphic schists near the coast; and that the one
fragment of mica-slate, and the pebbles of quartz low down in the gypseous
formation at Las Amolanas, have been derived from it. The mica-schist is
succeeded by stratified porphyritic conglomerate [Q] of great thickness,
dipping eastward with a high inclination: I have included this latter
mountain-mass in the same anticlinal axis with the porphyritic streams
[NN]; but I am far from sure that the two masses may not have been
independently upheaved.

SEVENTH AXIS OF ELEVATION.

Proceeding up the ravine, we come to another mass [R] of andesite; and
beyond this, we again have a very thick, stratified porphyritic formation
[S], dipping at a small angle eastward, and forming the basal part of the
main Cordillera. I did not ascend the ravine any higher; but here, near
Castano, I examined several sections, of which I will not give the details,
only observing, that the porphyritic beds, or submarine lavas, preponderate
greatly in bulk over the alternating sedimentary layers, which have been
but little metamorphosed: these latter consist of fine-grained red tuffs
and of whitish volcanic grit-stones, together with much of a singular,
compact rock, having an almost crystalline basis, finely brecciated with
red and green fragments, and occasionally including a few large pebbles.
The porphyritic lavas are highly amygdaloidal, both on their upper and
lower surfaces; they consist chiefly of claystone porphyry, but with one
common variety, like some of the streams at the Puente del Inca, having a
grey mottled basis, abounding with crystals of red hydrous oxide of iron,
green ones apparently of epidote, and a few glassy ones of feldspar. This
pile of strata differs considerably from the basal strata of the Cordillera
in Central Chile, and may possibly belong to the upper and gypseous series:
I saw, however, in the bed of the valley, one fragment of porphyritic
breccia-conglomerate, exactly like those great masses met with in the more
southern parts of Chile.

Finally, I must observe, that though I have described between the town of
Copiapo and the western flank of the main Cordillera seven or eight axes of
elevation, extending nearly north and south, it must not be supposed that
they all run continuously for great distances. As was stated to be the case
in our sections across the Cordillera of Central Chile, so here most of the
lines of elevation, with the exception of the first, third, and fifth, are
very short. The stratification is everywhere disturbed and intricate;
nowhere have I seen more numerous faults and dikes. The whole district,
from the sea to the Cordillera, is more or less metalliferous; and I heard
of gold, silver, copper, lead, mercury, and iron veins. The metamorphic
action, even in the lower strata, has certainly been far less here than in
Central Chile.

VALLEY OF THE DESPOBLADO.

This great barren valley, which has already been alluded to, enters the
main valley of Copiapo a little above the town: it runs at first northerly,
then N.E., and more easterly into the Cordillera; I followed its dreary
course to the foot of the first main ridge. I will not give a detailed
section, because it would be essentially similar to that already given, and
because the stratification is exceedingly complicated. After leaving the
plutonic hills near the town, I met first, as in the main valley, with the
gypseous formation, having the same diversified character as before, and
soon afterwards with masses of porphyritic conglomerate, about one thousand
feet in thickness. In the lower part of this formation there were very
thick beds composed of fragments of claystone porphyries, both angular and
rounded, with the smaller ones partially blended together and the basis
rendered porphyritic; these beds separated distinct streams, from sixty to
eighty feet in thickness, of claystone lavas. Near Paipote, also, there was
much true porphyritic breccia-conglomerate: nevertheless, few of these
masses were metamorphosed to the same degree with the corresponding
formation in Central Chile. I did not meet in this valley with any true
andesite, but only with imperfect andesitic porphyry, including large
crystals of hornblende: numerous as have been the varieties of intrusive
porphyries already mentioned, there were here mountains composed of a new
kind, having a compact, smooth, cream-coloured basis, including only a few
crystals of feldspar, and mottled with dendritic spots of oxide of iron.
There were also some mountains of a porphyry with a brick-red basis,
containing irregular, often lens-shaped, patches of compact feldspar, and
crystals of feldspar, which latter to my surprise I find to be orthite.

At the foot of the first ridge of the main Cordillera, in the ravine of
Maricongo, and at an elevation which, from the extreme coldness and
appearance of the vegetation, I estimated at about ten thousand feet, I
found beds of white sandstone and of limestone including the Pecten
Dufreynoyi, Terebratula aenigma, and some Gryphites. This ridge throws the
water on the one hand into the Pacific, and on the other, as I was
informed, into a great gravel-covered, basin-like plain, including a salt-
lake, and without any drainage-exit. In crossing the Cordillera by this
Pass, it is said that three principal ridges must be traversed, instead of
two, or only one as in Central Chile.

The crest of this first main ridge and the surrounding mountains, with the
exception of a few lofty pinnacles, are capped by a great thickness of a
horizontally stratified, tufaceous deposit. The lowest bed is of a pale
purple colour, hard, fine-grained, and full of broken crystals of feldspar
and scales of mica. The middle bed is coarser, and less hard, and hence
weathers into very sharp pinnacles; it includes very small fragments of
granite, and innumerable ones of all sizes of grey vesicular trachyte, some
of which were distinctly rounded. The uppermost bed is about two hundred
feet in thickness, of a darker colour and apparently hard: but I had not
time to ascend to it. These three horizontal beds may be seen for the
distance of many leagues, especially westward or in the direction of the
Pacific, capping the summits of the mountains, and standing on the opposite
sides of the immense valleys at exactly corresponding heights. If united
they would form a plain, inclined very slightly towards the Pacific; the
beds become thinner in this direction, and the tuff (judging from one point
to which I ascended, some way down the valley) finer-grained and of less
specific gravity, though still compact and sonorous under the hammer. The
gently inclined, almost horizontal stratification, the presence of some
rounded pebbles, and the compactness of the lowest bed, though rendering it
probable, would not have convinced me that this mass had been of subaqueous
origin, for it is known that volcanic ashes falling on land and moistened
by rain often become hard and stratified; but beds thus originating, and
owing their consolidation to atmospheric moisture, would have covered
almost equally every neighbouring summit, high and low, and would not have
left those above a certain exact level absolutely bare; this circumstance
seems to me to prove that the volcanic ejections were arrested at their
present, widely extended, equable level, and there consolidated by some
other means than simple atmospheric moisture; and this no doubt must have
been a sheet of water. A lake at this great height, and without a barrier
on any one side, is out of the question; consequently we must conclude that
the tufaceous matter was anciently deposited beneath the sea. It was
certainly deposited before the excavation of the valleys, or at least
before their final enlargement (I have endeavoured to show in my "Journal"
etc. (2nd edition) page 355, that this arid valley was left by the
retreating sea, as the land slowly rose, in the state in which we now see
it.); and I may add, that Mr. Lambert, a gentleman well acquainted with
this country, informs me, that in ascending the ravine of Santandres (which
branches off from the Despoblado) he met with streams of lava and much
erupted matter capping all the hills of granite and porphyry, with the
exception of some projecting points; he also remarked that the valleys had
been excavated subsequently to these eruptions.

This volcanic formation, which I am informed by Mr. Lambert extends far
northward, is of interest, as typifying what has taken place on a grander
scale on the corresponding western side of the Cordillera of Peru. Under
another point of view, however, it possesses a far higher interest, as
confirming that conclusion drawn from the structure of the fringes of
stratified shingle which are prolonged from the plains at the foot of the
Cordillera far up the valleys,--namely, that this great range has been
elevated in mass to a height of between eight and nine thousand feet (I may
here mention that on the south side of the main valley of Copiapo, near
Potrero Seco, the mountains are capped by a thick mass of horizontally
stratified shingle, at a height which I estimated at between fifteen
hundred and two thousand feet above the bed of the valley. This shingle, I
believe, forms the edge of a wide plain, which stretches southwards between
two mountain ranges.); and now, judging from this tufaceous deposit, we may
conclude that the horizontal elevation has been in the district of Copiapo
about ten thousand feet.

(FIGURE 24.)

In the valley of the Despoblado, the stratification, as before remarked has
been much disturbed, and in some points to a greater degree than I have
anywhere else seen. I will give two cases: a very thick mass of thinly
stratified red sandstone, including beds of conglomerate, has been crushed
together (as represented in Figure 24) into a yoke or urn-formed trough, so
that the strata on both sides have been folded inwards: on the right hand
the properly underlying porphyritic claystone conglomerate is seen
overlying the sandstone, but it soon becomes vertical, and then is inclined
towards the trough, so that the beds radiate like the spokes of a wheel: on
the left hand, the inverted porphyritic conglomerate also assumes a dip
towards the trough, not gradually, as on the right hand, but by means of a
vertical fault and synclinal break; and a little still further on towards
the left, there is a second great oblique fault (both shown by the arrow-
lines), with the strata dipping to a directly opposite point; these
mountains are intersected by infinitely numerous dikes, some of which can
be seen to rise from hummocks of greenstone, and can be traced for
thousands of feet. In the second case, two low ridges trend together and
unite at the head of a little wedge-shaped valley: throughout the right-
hand ridge, the strata dip at 45 degrees to the east; in the left-hand
ridge, we have the very same strata and at first with exactly the same dip;
but in following this ridge up the valley, the strata are seen very
regularly to become more and more inclined until they stand vertical, they
then gradually fall over (the basset edges forming symmetrical serpentine
lines along the crest), till at the very head of the valley they are
reversed at an angle of 45 degrees: so that at this point the beds have
been turned through an angle of 135 degrees; and here there is a kind of
anticlinal axis, with the strata on both sides dipping to opposite points
at an angle of 45 degrees, but those on the left hand upside down.

ON THE ERUPTIVE SOURCES OF THE PORPHYRITIC CLAYSTONE AND GREENSTONE LAVAS.

In Central Chile, from the extreme metamorphic action, it is in most parts
difficult to distinguish between the streams of porphyritic lava and the
porphyritic breccia-conglomerate, but here, at Copiapo, they are generally
perfectly distinct, and in the Despoblado, I saw for the first time, two
great strata of purple claystone porphyry, after having been for a
considerable space closely united together, one above the other, become
separated by a mass of fragmentary matter, and then both thin out;--the
lower one more rapidly than the upper and greater stream. Considering the
number and thickness of the streams of porphyritic lava, and the great
thickness of the beds of breccia-conglomerate, there can be little doubt
that the sources of eruption must originally have been numerous:
nevertheless, it is now most difficult even to conjecture the precise point
of any one of the ancient submarine craters. I have repeatedly observed
mountains of porphyries, more or less distinctly stratified towards their
summits or on their flanks, without a trace of stratification in their
central and basal parts: in most cases, I believe this is simply due either
to the obliterating effects of metamorphic action, or to such parts having
been mainly formed of intrusive porphyries, or to both causes conjoined; in
some instances, however, it appeared to me very probable that the great
central unstratified masses of porphyry were the now partially denuded
nuclei of the old submarine volcanoes, and that the stratified parts marked
the points whence the streams flowed. In one case alone, and it was in this
Valley of the Despoblado, I was able actually to trace a thick stratum of
purplish porphyry, which for a space of some miles conformably overlay the
usual alternating beds of breccia-conglomerates and claystone lavas, until
it became united with, and blended into, a mountainous mass of various
unstratified porphyries.

The difficulty of tracing the streams of porphyries to their ancient and
doubtless numerous eruptive sources, may be partly explained by the very
general disturbance which the Cordillera in most parts has suffered; but I
strongly suspect that there is a more specific cause, namely, THAT THE
ORIGINAL POINTS OF ERUPTION TEND TO BECOME THE POINTS OF INJECTION. This in
itself does not seem improbable; for where the earth's crust has once
yielded, it would be liable to yield again, though the liquified intrusive
matter might not be any longer enabled to reach the submarine surface and
flow as lava. I have been led to this conclusion, from having so frequently
observed that, where part of an unstratified mountain-mass resembled in
mineralogical character the adjoining streams or strata, there were several
other kinds of intrusive porphyries and andesitic rocks injected into the
same point. As these intrusive mountain-masses form most of the axes-lines
in the Cordillera, whether anticlinal, uniclinal, or synclinal, and as the
main valleys have generally been hollowed out along these lines, the
intrusive masses have generally suffered much denudation. Hence they are
apt to stand in some degree isolated, and to be situated at the points
where the valleys abruptly bend, or where the main tributaries enter. On
this view of there being a tendency in the old points of eruption to become
the points of subsequent injection and disturbance, and consequently of
denudation, it ceases to be surprising that the streams of lava in the
porphyritic claystone conglomerate formation, and in other analogous cases,
should most rarely be traceable to their actual sources.

IQUIQUE, SOUTHERN PERU.

Differently from what we have seen throughout Chile, the coast here is
formed not by the granitic series, but by an escarpment of the porphyritic
conglomerate formation, between two and three thousand feet in height. (The
lowest point, where the road crosses the coast-escarpment, is 1,900 feet by
the barometer above the level of the sea.) I had time only for a very short
examination; the chief part of the escarpment appears to be composed of
various reddish and purple, sometimes laminated, porphyries, resembling
those of Chile; and I saw some of the porphyritic breccia-conglomerate; the
stratification appeared but little inclined. The uppermost part, judging
from the rocks near the famous silver mine of Huantajaya, consists of
laminated, impure, argillaceous, purplish-grey limestone, associated, I
believe, with some purple sandstone. (Mr. Bollaert has described
"Geological Proceedings" volume 2 page 598, a singular mass of stratified
detritus, gravel, and sand, eighty-one yards in thickness, overlying the
limestone, and abounding with loose masses of silver ore. The miners
believe that they can attribute these masses to their proper veins.) In the
limestone shells are found: the three following species were given me:--

Lucina Americana, E. Forbes.
Terebratula inca, E. Forbes.
Terebratula aenigma, D'Orbigny.

This latter species we have seen associated with the fossils of which lists
have been given in this chapter, in two places in the valley of Coquimbo,
and in the ravine of Maricongo at Copiapo. Considering this fact, and the
superposition of these beds on the porphyritic conglomerate formation; and,
as we shall immediately see, from their containing much gypsum, and from
their otherwise close general resemblance in mineralogical nature with the
strata described in the valley of Copiapo, I have little doubt that these
fossiliferous beds of Iquique belong to the great cretaceo-oolitic
formation of Northern Chile. Iquique is situated seven degrees latitude
north of Copiapo; and I may here mention, that an Ammonites, nov. species,
and an Astarte, nov. species, were given me from the Cerro Pasco, about ten
degrees of latitude north of Iquique, and M. D'Orbigny thinks that they
probably indicate a Neocomian formation. Again, fifteen degrees of latitude
northward, in Colombia, there is a grand fossiliferous deposit, now well
known from the labours of Von Buch, Lea, d'Orbigny, and Forbes, which
belongs to the earlier stages of the cretaceous system. Hence, bearing in
mind the character of the few fossils from Tierra del Fuego, there is some
evidence that a great portion of the stratified deposits of the whole vast
range of the South American Cordillera belongs to about the same geological
epoch.

Proceeding from the coast escarpment inwards, I crossed, in a space of
about thirty miles, an elevated undulatory district, with the beds dipping
in various directions. The rocks are of many kinds,--white laminated,
sometimes siliceous sandstone,--purple and red sandstone, sometimes so
highly calcareous as to have a crystalline fracture,--argillaceous
limestone,--black calcareous slate-rock, like that so often described at
Copiapo and other places,--thinly laminated, fine-grained, greenish,
indurated, sedimentary, fusible rocks, approaching in character to the so-
called pseudo-honestone of Chile, including thin contemporaneous veins of
gypsum,--and lastly, much calcareous, laminated porcelain jasper, of a
green colour, with red spots, and of extremely easy fusibility: I noticed
one conformable stratum of a freckled-brown, feldspathic lava. I may here
mention that I heard of great beds of gypsum in the Cordillera. The only
novel point in this formation, is the presence of innumerable thin layers
of rock-salt, alternating with the laminated and hard, but sometimes
earthy, yellowish, or bright red and ferruginous sandstones. The thickest
layer of salt was only two inches, and it thinned out at both ends. On one
of these saliferous masses I noticed a stratum about twelve feet thick, of
dark-brown, hard brecciated, easily fusible rock, containing grains of
quartz and of black oxide of iron, together with numerous imperfect
fragments of shells. The problem of the origin of salt is so obscure, that
every fact, even geographical position, is worth recording. (It is well
known that stratified salt is found in several places on the shores of
Peru. The island of San Lorenzo, off Lima, is composed of a pile of thin
strata, about eight hundred feet in thickness, composed of yellowish and
purplish, hard siliceous, or earthy sandstones, alternating with thin
layers of shale, which in places passes into a greenish, semi-porcellanic,
fusible rock. There are some thin beds of reddish mudstone, and soft
ferruginous rotten-stones, with layers of gypsum. In nearly all these
varieties, especially in the softer sandstones, there are numerous thin
seams of rock-salt: I was informed that one layer has been found two inches
in thickness. The manner in which the minutest fissures of the dislocated
beds have been penetrated by the salt, apparently by subsequent
infiltration, is very curious. On the south side of the island, layers of
coal and of impure limestone have been discovered. Hence we here have salt,
gypsum, and coal associated together. The strata include veins of quartz,
carbonate of lime, and iron pyrites; they have been dislocated by an
injected mass of greenish-brown feldspathic trap. Not only is salt abundant
on the extreme western limits of the district between the Cordillera and
the Pacific, but, according to Helms, it is found in the outlying low hills
on the eastern flank of the Cordillera. These facts appear to me opposed to
the theory, that rock-salt is due to the sinking of water, charged with
salt, in mediterranean spaces of the ocean. The general character of the
geology of these countries would rather lead to the opinion, that its
origin is in some way connected with volcanic heat at the bottom of the
sea: see on this subject Sir R. Murchison "Anniversary Address to the
Geological Society" 1843 page 65.) With the exception of these saliferous
beds, most of the rocks as already remarked, present a striking general
resemblance with the upper parts of the gypseous or cretaceo-oolitic
formation of Chile.

METALLIFEROUS VEINS.

I have only a few remarks to make on this subject: in nine mining
districts, some of them of considerable extent, which I visited in CENTRAL
Chile, I found the PRINCIPAL veins running from between [N. and N.W.] to
[S. and S.E.] (These mining districts are Yaquil near Nancagua, where the
direction of the chief veins, to which only in all cases I refer, is north
and south; in the Uspallata range, the prevailing line is N.N.W. and
S.S.E.; in the C. de Prado, it is N.N.W. and S.S.E.; near Illapel, it is N.
by W. and S. by E.; at Los Hornos the direction varies from between [N. and
N.W.] to [S. and S.E.]; at the C. de los Hornos (further northward), it is
N.N.W. and S.S.E.; at Panuncillo, it is N.N.W. and S.S.E.; and, lastly, at
Arqueros, the direction is N.W. and S.E.): in some other places, however,
their courses appeared quite irregular, as is said to be generally the case
in the whole valley of Copiapo: at Tambillos, south of Coquimbo, I saw one
large copper vein extending east and west. It is worthy of notice, that the
foliation of the gneiss and mica-slate, where such rocks occur, certainly
tend to run like the metalliferous veins, though often irregularly, in a
direction a little westward of north. At Yaquil, I observed that the
principal auriferous veins ran nearly parallel to the grain or imperfect
cleavage of the surrounding GRANITIC rocks. With respect to the
distribution of the different metals, copper, gold, and iron are generally
associated together, and are most frequently found (but with many
exceptions, as we shall presently see) in the rocks of the lower series,
between the Cordillera and the Pacific, namely, in granite, syenite,
altered feldspathic clay-slate, gneiss, and as near Guasco mica-schist. The
copper-ores consist of sulphurets, oxides, and carbonates, sometimes with
laminae of native metal: I was assured that in some cases (as at Panuncillo
S.E. of Coquimbo), the upper part of the same vein contains oxides, and the
lower part sulphurets of copper. (The same fact has been observed by Mr.
Taylor in Cuba: "London Philosophical Journal" volume 11 page 21.) Gold
occurs in its native form; it is believed that, in many cases, the upper
part of the vein is the most productive part: this fact probably is
connected with the abundance of this metal in the stratified detritus of
Chile, which must have been chiefly derived from the degradation of the
upper portions of the rocks. These superficial beds of well-rounded gravel
and sand, containing gold, appeared to me to have been formed under the sea
close to the beach, during the slow elevation of the land: Schmidtmeyer
remarks that in Chile gold is sought for in shelving banks at the height of
some feet on the sides of the streams, and not in their beds, as would have
been the case had this metal been deposited by common alluvial action.
("Travels in Chile" page 29.) Very frequently the copper-ores, including
some gold, are associated with abundant micaceous specular iron. Gold is
often found in iron-pyrites: at two gold mines at Yaquil (near Nancagua), I
was informed by the proprietor that in one the gold was always associated
with copper-pyrites, and in the other with iron-pyrites: in this latter
case, it is said that if the vein ceases to contain iron-pyrites, it is yet
worth while to continue the search, but if the iron-pyrites, when it
reappears, is not auriferous, it is better at once to give up working the
vein. Although I believe copper and gold are most frequently found in the
lower granitic and metamorphic schistose series, yet these metals occur
both in the porphyritic conglomerate formation (as on the flanks of the
Bell of Quillota and at Jajuel), and in the superincumbent strata. At
Jajuel I was informed that the copper-ore, with some gold, is found only in
the greenstones and altered feldspathic clay-slate, which alternate with
the purple porphyritic conglomerate. Several gold veins and some of copper-
ore are worked in several parts of the Uspallata range, both in the
metamorphosed strata, which have been shown to have been of probably
subsequent origin to the Neocomian or gypseous formation of the main
Cordillera, and in the intrusive andesitic rocks of that range. At Los
Hornos (N.E. of Illapel), likewise, there are numerous veins of copper-
pyrites and of gold, both in the strata of the gypseous formation and in
the injected hills of andesite and various porphyries.

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