Section from Illapel to Combarbala; gypseous formation with silicified
wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley;
fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas,
silicified wood.—Conglomerates, nature of former land, fossils,
thickness of strata, great subsidence.—Valley of Despoblado, fossils,
tufaceous deposit, complicated dislocations of.—Relations between
ancient orifices of eruption and subsequent axes of injection.—Iquique,
Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the
porphyritic conglomerate and gypseous formations.—Great subsidence with
partial elevations during the cretaceo-oolitic period.—On the elevation
and structure of the Cordillera.—Recapitulation on the tertiary
series.—Relation between movements of subsidence and volcanic
action.—Pampean formation.—Recent elevatory movements. Long-continued
volcanic action in the Cordillera.—Conclusion.
_Valparaiso to Coquimbo._ I have already described the general nature
of the rocks in the low country north of Valparaiso, consisting of
granites, syenites, greenstones, and altered feldspathic clay-slate.
Near Coquimbo there is much hornblendic rock and various dusky-coloured
porphyries. I will describe only one section in this district, namely,
from near Illapel in a N.E. line to the mines of Los Hornos, and thence
in a north by east direction to Combarbala, at the foot of the main
Cordillera.
Near Illapel, after passing for some distance over granite, andesite,
and andesitic porphyry, we come to a greenish stratified feldspathic
rock, which I believe is altered clay-slate, conformably capped by
porphyries and porphyritic conglomerate of great thickness, dipping at
an average angle of 20° to N.E. by N. The uppermost beds consist of
conglomerates and sandstone only a little metamorphosed, and
conformably covered by a gypseous formation of very great thickness,
but much denuded. This gypseous formation, where first met with, lies
in a broad valley or basin, a little southward of the mines of Los
Hornos: the lower half alone contains gypsum, not in great masses as in
the Cordillera, but in innumerable thin layers, seldom more than an
inch or two in thickness. The gypsum is either opaque or transparent,
and is associated with carbonate of lime. The layers alternate with
numerous varying ones of a calcareous clay-shale (with strong aluminous
odour, adhering to the tongue, easily fusible into a pale green glass),
more or less indurated, either earthy and cream-coloured, or greenish
and hard. The more indurated varieties have a compact, homogeneous,
almost crystalline fracture, and contain granules of crystallised oxide
of iron. Some of the varieties almost resemble honestones. There is
also a little black, hardly fusible, siliceo-calcareous clay-slate,
like some of the varieties alternating with gypsum on the Peuquenes
range.
The upper half of this gypseous formation is mainly formed of the
same calcareous clay-shale rock, but without any gypsum, and varying
extremely in nature: it passes from a soft, coarse, earthy, ferruginous
state, including particles of quartz, into compact claystones with
crystallised oxide of iron,—into porcellanic layers, alternating with
seams of calcareous matter,—and into green porcelain-jasper,
excessively hard, but easily fusible. Strata of this nature alternate
with much black and brown siliceo-calcareous slate, remarkable from the
wonderful number of huge embedded logs of silicified wood. This wood,
according to Mr. R. Brown, is (judging from several specimens) all
coniferous. Some of the layers of the black siliceous slate contained
irregular angular fragments of imperfect pitchstone, which I believe,
as in the Uspallata range, has originated in a metamorphic process.
There was one bed of a marly tufaceous nature, and of little specific
gravity. Veins of agate and calcareous spar are numerous. The whole of
this gypseous formation, especially the upper half, has been injected,
metamorphosed, and locally contorted by numerous hillocks of intrusive
porphyries crowded together in an extraordinary manner. These hillocks
consist of purple claystone and of various other porphyries, and of
much white feldspathic greenstone passing into andesite; this latter
variety included in one case crystals of orthitic and albitic feldspar
touching each other, and others of hornblende, chlorite, and epidote.
The strata surrounding these intrusive hillocks at the mines of Los
Hornos, are intersected by many veins of copper-pyrites, associated
with much micaceous iron-ore, and by some of gold: in the neighbourhood
of these veins the rocks are blackened and much altered. The gypsum
near the intrusive masses is always opaque. One of these hillocks of
porphyry was capped by some stratified porphyritic conglomerate, which
must have been brought up from below, through the whole immense
thickness of the overlying gypseous formation. The lower beds of the
gypseous formation resemble the corresponding and probably
contemporaneous strata of the main Cordillera; whilst the upper beds in
several respects resemble those of the Uspallata chain, and possibly
may be contemporaneous with them; for I have endeavoured to show that
the Uspallata beds were accumulated subsequently to the gypseous or
Neocomian formations of the Cordillera.
This pile of strata dips at an angle of about 20 degrees to N.E. by N.,
close up to the foot of the Cuesta de Los Hornos, a crooked range of
mountains formed of intrusive rocks of the same nature with the above
described hillocks. Only in one or two places, on this south-eastern
side of the range, I noticed a narrow fringe of the upper gypseous
strata brushed up and inclined south-eastward from it. On its
north-eastern flank, and likewise on a few of the summits, the
stratified porphyritic conglomerate is inclined N.E.: so that, if we
disregard the very narrow anticlinal fringe of gypseous strata at its
S.E. foot, this range forms a second uniclinal axis of elevation.
Proceeding in a north-by-east direction to the village of Combarbala,
we come to a third escarpment of the porphyritic conglomerate, dipping
eastwards, and forming the outer range of the main Cordillera. The
lower beds were here more jaspery than usual, and they included some
white cherty strata and red
sandstones, alternating with purple claystone porphyry. Higher up in
the Cordillera there appeared to be a line of andesitic rocks; and
beyond them, a fourth escarpment of the porphyritic conglomerate, again
dipping eastwards or inwards. The overlying gypseous strata, if they
ever existed here, have been entirely removed.
_Copper mines of Panuncillo._—From Combarbala to Coquimbo, I traversed
the country in a zigzag direction, crossing and recrossing the
porphyritic conglomerate and finding in the granitic districts an
unusual number of mountain-masses composed of various intrusive,
porphyritic rocks, many of them andesitic. One common variety was
greenish-black, with large crystals of blackish albite. At Panuncillo a
short N.N.W. and S.S.E. ridge, with a nucleus formed of greenstone and
of a slate-coloured porphyry including crystals of glassy feldspar,
deserves notice, from the very singular nature of the almost vertical
strata composing it. These consist chiefly of a finer and coarser
granular mixture, not very compact, of white carbonate of lime, of
protoxide of iron and of yellowish garnets (ascertained by Professor
Miller), each grain being an almost perfect crystal. Some of the
varieties consist exclusively of granules of the calcareous spar; and
some contain grains of copper ore, and, I believe, of quartz. These
strata alternate with a bluish, compact, fusible, feldspathic rock.
Much of the above granular mixture has, also, a pseudo-brecciated
structure, in which fragments are obscurely arranged in planes parallel
to those of the stratification, and are conspicuous on the weathered
surfaces. The fragments are angular or rounded, small or large, and
consist of bluish or reddish compact feldspathic matter, in which a few
acicular crystals of feldspar can sometimes be seen. The fragments
often blend at their edges into the surrounding granular mass, and seem
due to a kind of concretionary action.
These singular rocks are traversed by many copper veins, and appear to
rest conformably on the granular mixture (in parts as fine-grained as a
sandstone) of quartz, mica, hornblende, and feldspar; and this on
fine-grained, common gneiss; and this on a laminated mass, composed of
pinkish _orthitic_ feldspar, including a few specks of hornblende; and
lastly, this on granite, which together with andesitic rocks, form the
surrounding district.
_Coquimbo: Mining district of Arqueros._—At Coquimbo the porphyritic
conglomerate formation approaches nearer to the Pacific than in any
other part of Chile visited by me, being separated from the coast by a
tract only a few miles broad of the usual plutonic rocks, with the
addition of a porphyry having a red euritic base. In proceeding to the
mines of Arqueros, the strata of porphyritic conglomerate are at first
nearly horizontal, an unusual circumstance, and afterwards they dip
gently to S.S.E. After having ascended to a considerable height, we
come to an undulatory district in which the famous silver mines are
situated; my examination was chiefly confined to those of S. Rosa. Most
of the rocks in this district are stratified, dipping in various
directions, and many of them are of so singular a nature, that at the
risk of being tedious I must briefly describe them. The commonest
variety is a dull-red, compact, finely brecciated stone, containing
much iron and innumerable white crystallised particles of carbonate of
lime, and minute extraneous fragments. Another variety is almost
equally common near S. Rosa; it has a bright green, scanty basis,
including distinct crystals and patches of white carbonate of lime, and
grains of red, semi-micaceous oxide of iron; in parts the basis becomes
dark green, and assumes an obscure crystalline arrangement, and
occasionally in parts it becomes soft and slightly translucent like
soapstone. These red and green rocks are often quite distinct, and
often pass into each other; the passage being sometimes affected by a
fine brecciated structure, particles of the red and green matter being
mingled together. Some of the varieties appear gradually to become
porphyritic with feldspar; and all of them are easily fusible into pale
or dark-coloured beads, strongly attracted by the magnet. I should
perhaps have mistaken several of these stratified rocks for submarine
lavas, like some of those described at the Puente del Inca, had I not
examined, a few leagues eastward of this point, a fine series of
analogous but less metamorphosed, sedimentary beds belonging to the
gypseous formation, and probably derived from a volcanic source.
This formation is intersected by numerous metalliferous veins, running,
though irregularly, N.W. and S.E., and generally at right angles to the
many dikes. The veins consist of native silver, of muriate of silver,
an amalgam of silver, cobalt, antimony, and arsenic,[1] generally
embedded in sulphate of barytes. I was assured by Mr. Lambert, that
native copper without a trace of silver has been found in the same vein
with native silver without a trace of copper. At the mines of Aristeas,
the silver veins are said to be unproductive as soon as they pass into
the green strata, whereas at S. Rosa, only two or three miles distant,
the reverse happens; and at the time of my visit, the miners were
working through a red stratum, in the hope of the vein becoming
productive in the underlying green sedimentary mass. I have a specimen
of one of these green rocks, with the usual granules of white
calcareous spar and red oxide of iron, abounding with disseminated
particles of glittering native and muriate of silver, yet taken at the
distance of one yard from any vein,—a circumstance, as I was assured,
of very rare occurrence.
[1] See the Report on M. Domeyko’s account of those mines, in the
“Comptes Rendus,” tome xiv, p. 560.
_Section eastward, up the Valley of Coquimbo._—After passing for a few
miles over the coast granitic series, we come to the porphyritic
conglomerate, with its usual characters, and with some of the beds
distinctly displaying their mechanical origin. The strata, where first
met with, are, as before stated, only slightly inclined; but near the
Hacienda of Pluclaro, we come to an anticlinal axis, with the beds much
dislocated and shifted by a great fault, of which not a trace is
externally seen in the outline of the hill. I believe that this
anticlinal axis can be traced northwards, into the district of
Arqueros, where a conspicuous hill called Cerro Blanco, formed of a
harsh, cream-coloured euritic rock, including a few crystals of reddish
feldspar, and associated with some
purplish claystone porphyry, seems to fall on a line of elevation. In
descending from the Arqueros district, I crossed on the northern border
of the valley, strata inclined eastward from the Pluclaro axis: on the
porphyritic conglomerate there rested a mass, some hundred feet thick,
of brown argillaceous limestone, in parts crystalline, and in parts
almost composed of _Hippurites Chilensis,_ d’Orbigny; above this came a
black calcareous shale, and on it a red conglomerate. In the brown
limestone, with the Hippurites, there was an impression of a Pecten and
a coral, and great numbers of a large Gryphæa, very like, and,
according to Professor E. Forbes, probably identical with _G.
Orientalis,_ Forbes MS.,—a cretaceous species (probably upper
greensand) from Verdachellum, in Southern India. These fossils seem to
occupy nearly the same position with those at the Puente del
Inca,—namely, at the top of the porphyritic conglomerate, and at the
base of the gypseous formation.
A little above the Hacienda of Pluclaro, I made a detour on the
northern side of the valley, to examine the superincumbent gypseous
strata, which I estimated at 6,000 feet in thickness. The uppermost
beds of the porphyritic conglomerate, on which the gypseous strata
conformably rest, are variously coloured, with one very singular and
beautiful stratum composed of purple pebbles of various kinds of
porphyry, embedded in white calcareous spar, including cavities lined
with bright-green crystallised epidote. The whole pile of strata
belonging to both formations is inclined, apparently from the
above-mentioned axis of Pluclaro, at an angle of between 20 and 30
degrees to the east. I will here give a section of the principal beds
met with in crossing the entire thickness of the gypseous strata.
Firstly: above the porphyritic conglomerate formation, there is a
fine-grained, red, crystalline sandstone.
Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly
rock, often marked with dendritic manganese, and having, where most
compact, the external appearance of honestone. It is easily fusible. I
shall for the future, for convenience’ sake, call this variety
pseudo-honestone. Some of the varieties are quite black when freshly
broken, but all weather into a yellowish-ash coloured, soft, earthy
substance, precisely as is the case with the compact shaly rocks of the
Peuquenes range. This stratum is of the same general nature with many
of the beds near Los Hornos in the Illapel section. In this second bed,
or in the underlying red sandstone (for the surface was partially
concealed by detritus), there was a thick mass of gypsum, having the
same mineralogical characters with the great beds described in our
sections across the Cordillera.
Thirdly: a thick stratum of fine-grained, red, sedimentary matter,
easily fusible into a white glass, like the basis of claystone
porphyry; but in parts jaspery, in parts brecciated, and including
crystalline specks of carbonate of lime. In some of the jaspery layers,
and in some of the black siliceous slaty bands, there were irregular
seams of imperfect pitchstone, undoubtedly of metamorphic origin, and
other seams of brown, crystalline limestone. Here, also, were masses,
externally resembling ill-preserved silicified wood.
Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum
concealed by detritus.
Sixthly: a thinly stratified mass of bright green, compact,
smooth-grained, calcareo-argillaceous stone, easily fusible, and
emitting a strong aluminous odour: the whole has a highly
angulo-concretionary structure; and it resembles, to a certain extent,
some of the upper tufaceo-infusorial deposits of the Patagonian
tertiary formation. It is in its nature allied to our pseudo-honestone,
and it includes well characterised layers of that variety; and other
layers of a pale green, harder, and brecciated variety; and others of
red sedimentary matter, like that of bed Three. Some pebbles of
porphyries are embedded in the upper part.
Seventhly: red sedimentary matter or sandstone like that of bed One,
several hundred feet in thickness, and including jaspery layers, often
having a finely brecciated structure.
Eighthly: white, much indurated, almost crystalline tuff, several
hundred feet in thickness, including rounded grains of quartz and
particles of green matter like that of bed Six. Parts pass into a very
pale green, semi-porcellanic stone.
Ninthly: red or brown coarse conglomerate, three or four hundred feet
thick, formed chiefly of pebbles of porphyries, with volcanic
particles, in an arenaceous, non-calcareous, fusible basis: the upper
two feet are arenaceous without any pebbles.
Tenthly: the last and uppermost stratum here exhibited, is a compact,
slate-coloured porphyry, with numerous elongated crystals of glassy
feldspar, from one hundred and fifty to two hundred feet in thickness;
it lies strictly conformably on the underlying conglomerate, and is
undoubtedly a submarine lava.
This great pile of strata has been broken up in several places by
intrusive hillocks of purple claystone porphyry, and by dikes of
porphyritic greenstone: it is said that a few poor metalliferous veins
have been discovered here. From the fusible nature and general
appearance of the finer-grained strata, they probably owe their origin
(like the allied beds of the Uspallata range, and of the Upper
Patagonian tertiary formations), to gentle volcanic eruptions, and to
the abrasion of volcanic rocks. Comparing these beds with those in the
mining district of Arqueros, we see at both places rocks easily
fusible, of the same peculiar bright green and red colours, containing
calcareous matter, often having a finely brecciated structure, often
passing into each other, and often alternating together: hence I cannot
doubt that the only difference between them, lies in the Arqueros beds
having been more metamorphosed (in conformity with their more
dislocated and injected condition), and consequently in the calcareous
matter, oxide of iron and green colouring matter, having been
segregated under a more crystalline form.
The strata are inclined, as before stated, from 20° to 30° eastward,
towards an irregular north and south chain of andesitic porphyry and of
porphyritic greenstone, where they are abruptly cut off. In the valley
of Coquimbo, near to the H. of Gualliguaca, similar plutonic rocks are
met with, apparently a southern prolongation of the above chain; and
eastward of it we have an escarpment of the porphyritic conglomerate,
with the strata inclined at a small angle eastward, which makes the
third escarpment, including that nearest the coast. Proceeding up the
valley we come to another north and south line of granite, andesite,
and blackish porphyry, which seem to lie in an irregular trough of the
porphyritic conglomerate. Again, on the south side of the R. Claro,
there are some irregular granitic hills, which have thrown off the
strata of porphyritic conglomerate to the N.W. by W.; but the
stratification here has been much disturbed. I did not proceed any
farther up the valley, and this point is about two-thirds of the
distance between the Pacific and the main Cordillera.
I will describe only one other section, namely, on the north side of
the R. Claro, which is interesting from containing fossils: the strata
are much dislocated by faults and dikes, and are inclined to the north,
towards a mountain of andesite and porphyry, into which they appear to
become almost blended. As the beds approach this mountain, their
inclination increases up to an angle of 70°, and in the upper part, the
rocks become highly metamorphosed. The lowest bed visible in this
section, is a purplish hard sandstone. Secondly, a bed two or three
hundred feet thick, of a white siliceous sandstone, with a calcareous
cement, containing seams of slaty sandstone, and of hard
yellowish-brown (dolomitic?) limestone; numerous, well-rounded, little
pebbles of quartz are included in the sandstone. Thirdly, a dark
coloured limestone with some quartz pebbles, from fifty to sixty feet
in thickness, containing numerous silicified shells, presently to be
enumerated. Fourthly, very compact, calcareous, jaspery sandstone,
passing into (fifthly) a great bed, several hundred feet thick, of
conglomerate, composed of pebbles of white, red, and purple porphyries,
of sandstone and quartz, cemented by calcareous matter. I observed that
some of the finer parts of this conglomerate were much indurated within
a foot of a dike eight feet in width, and were rendered of a paler
colour with the calcareous matter segregated into white crystallised
particles; some parts were stained green from the colouring matter of
the dike. Sixthly, a thick mass, obscurely stratified, of a red
sedimentary stone or sandstone, full of crystalline calcareous matter,
imperfect crystals of oxide of iron, and I believe of feldspar, and
therefore closely resembling some of the highly metamorphosed beds at
Arqueros: this bed was capped by, and appeared to pass in its upper
part into, rocks similarly coloured, containing calcareous matter, and
abounding with minute crystals, mostly elongated and glassy, of reddish
albite. Seventhly, a conformable stratum of fine reddish porphyry with
large crystals of (albitic?) feldspar; probably a submarine lava.
Eighthly, another conformable bed of green porphyry, with specks of
green earth and cream-coloured crystals of feldspar. I believe that
there are other superincumbent crystalline strata and submarine lavas,
but I had not time to examine them.
The upper beds in this section probably correspond with parts of the
great gypseous formation; and the lower beds of red sandstone
conglomerate and fossiliferous limestone no doubt are the equivalents
of
the Hippurite stratum, seen in descending from Arqueros to Pluclaro,
which there lies conformably upon the porphyritic conglomerate
formation. The fossils found in the third bed, consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.”
This species, which occurs here in vast numbers, according to M.
D’Orbigny, resembles certain cretaceous forms.
Ostrea hemispherica, d’Orbigny, “Voyage” etc.
Also resembles, according to the same author, cretaceous forms.
Terebratula ænigma, d’Orbigny, “Voyage” etc. (Pl. XXII, Figs. 10-12.)
Is allied, according to M. d’Orbigny, to T. concinna from the Forest
Marble. A series of this species, collected in several localities
hereafter to be referred to, has been laid before Professor Forbes; and
he informs me that many of the specimens are almost undistinguishable
from our oolitic T. tetrædra, and that the varieties amongst them are
such as are found in that variable species. Generally speaking, the
American specimens of T. ænigma may be distinguished from the British
T. tetrædra, by the surface having the ribs sharp and well-defined to
the beak, whilst in the British species they become obsolete and
smoothed down; but this difference is not constant. Professor Forbes
adds, that, possibly, internal characters may exist, which would
distinguish the American species from its European allies.
Spirifer linguiferoides, E. Forbes.
Professor Forbes states that this species is very near to S. linguifera
of Phillips (a carboniferous limestone fossil), but probably distinct.
M. d’Orbigny considers it as perhaps indicating the Jurassic period.
Ammonites, imperfect impression of.
M. Domeyko has sent to France a collection of fossils, which, I
presume, from the description given, must have come from the
neighbourhood of Arqueros; they consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal.
Ostrea hemispherica, d’Orbigny, “Voyage” Part Pal.
Turritella Andii, d’Orbigny, “Voyage” Part Pal. (Pleurotomaria
Humboldtii of Von Buch).
Hippurites Chilensis, d’Orbigny, “Voyage” Part Pal.
The specimens of this Hippurite, as well as those I collected in my
descent from Arqueros, are very imperfect; but in M. d’Orbigny’s
opinion they resemble, as does the Turritella Andii, cretaceous (upper
greensand) forms.
Nautilus Domeykus, d’Orbigny, “Voyage” Part Pal.
Terebratula ænigma, d’Orbigny, “Voyage” Part Pal.
Terebratula ignaciana, d’Orbigny, “Voyage” Part Pal.
This latter species was found by M. Domeyko in the same block of
limestone with the T. ænigma. According to M. d’Orbigny, it comes near
to T. ornithocephala from the Lias. A series of this species collected
at Guasco, has been examined by Professor E. Forbes, and he states that
it is difficult
to distinguish between some of the specimens and the T. hastata from
the mountain limestone; and that it is equally difficult to draw a line
between them and some Marlstone Terebratulæ. Without a knowledge of the
internal structure, it is impossible at present to decide on their
identity with analogous European forms.
The remarks given on the several foregoing shells, show that, in M.
d’Orbigny’s opinion, the Pecten, Ostrea, Turritella, and Hippurite
indicate the cretaceous period; and the Gryphæa appears to Professor
Forbes to be identical with a species, associated in Southern India
with unquestionably cretaceous forms. On the other hand, the two
Terebratulæ and the Spirifer point, in the opinion both of M. d’Orbigny
and Professor Forbes, to the oolitic series. Hence M. d’Orbigny, not
having himself examined this country, has concluded that there are here
two distinct formations; but the Spirifer and T. ænigma were certainly
included in the same bed with the Pecten and Ostrea, whence I extracted
them; and the geologist M. Domeyko sent home the two Terebratulæ with
the other-named shells, from the same locality, without specifying that
they came from different beds. Again, as we shall presently see, in a
collection of shells given me from Guasco, the same species, and others
presenting analogous differences, are mingled together, and are in the
same condition; and lastly, in three places in the valley of Copiapo, I
found some of these same species similarly grouped. Hence there cannot
be any doubt, highly curious though the fact be, that these several
fossils, namely, the Hippurites, Gryphæa, Ostrea, Pecten, Turritella,
Nautilus, two Terebratulæ, and Spirifer all belong to the same
formation, which would appear to form a passage between the oolitic and
cretaceous systems of Europe. Although aware how unusual the term must
sound, I shall, for convenience’ sake, call this formation
cretaceo-oolitic. Comparing the sections in this valley of Coquimbo
with those in the Cordillera described in the last chapter, and bearing
in mind the character of the beds in the intermediate district of Los
Hornos, there is certainly a close general mineralogical resemblance
between them, both in the underlying porphyritic conglomerate, and in
the overlying gypseous formation. Considering this resemblance, and
that the fossils from the Puente del Inca at the base of the gypseous
formation, and throughout the greater part of its entire thickness on
the Peuquenes range, indicate the Neocomian period,—that is, the dawn
of the cretaceous system, or, as some have believed, a passage between
this latter and the oolitic series—I conclude that probably the
gypseous and associated beds in all the sections hitherto described,
belong to the same great formation, which I have
denominated—cretaceo-oolitic. I may add, before leaving Coquimbo, that
M. Gay found in the neighbouring Cordillera, at the height of 14,000
feet above the sea, a fossiliferous formation, including a Trigonia and
Pholadomya;[2]—both of which genera occur at the Puente del Inca.
[2] D’Orbigny, “Voyage,” Part Géolog., p. 242.
_Coquimbo to Guasco._—The rocks near the coast, and some way inland, do
not differ from those described northwards of Valparaiso: we have
much greenstone, syenite, feldspathic and jaspery slate, and grauwackes
having a basis like that of claystone; there are some large tracts of
granite, in which the constituent minerals are sometimes arranged in
folia, thus composing an imperfect gneiss. There are two large
districts of mica-schists, passing into glossy clay-slate, and
resembling the great formation in the Chonos Archipelago. In the valley
of Guasco, an escarpment of porphyritic conglomerate is first seen high
up the valley, about two leagues eastward of the town of Ballenar. I
heard of a great gypseous formation in the Cordillera; and a collection
of shells made there was given me. These shells are all in the same
condition, and appear to have come from the same bed: they consist of:—
Turritella Andii, d’Orbigny, “Voyage,” Part Pal.
Pecten Dufreynoyi, d’Orbigny, “Voyage,” Part Pal.
Terebatula ignaciana, d’Orbigny, “Voyage,” Part Pal.
The relations of these species have been given under the head of
Coquimbo.
Terebratula ænigma, d’Orbigny, “Voyage,” Part Pal.
This shell M. d’Orbigny does not consider identical with his T. ænigma,
but near to T. obsoleta. Professor Forbes thinks that it is certainly a
variety of T. ænigma: we shall meet with this variety again at Copiapo.
Spirifer Chilensis, E. Forbes.
Professor Forbes remarks that this fossil resembles several
carboniferous limestone Spirifers; and that it is also related to some
liassic species, as S. Wolcotii.
If these shells had been examined independently of the other
collections, they would probably have been considered, from the
characters of the two Terebratulæ, and from the Spirifer, as oolitic;
but considering that the first species, and according to Professor
Forbes, the four first, are identical with those from Coquimbo, the two
formations no doubt are the same, and may, as I have said, be
provisionally called cretaceo-oolitic.
_Valley of Copiapo._—The journey from Guasco to Copiapo, owing to the
utterly desert nature of the country, was necessarily so hurried, that
I do not consider my notes worth giving. In the valley of Copiapo some
of the sections are very interesting. From the sea to the town of
Copiapo, a distance estimated at thirty miles, the mountains are
composed of greenstone, granite, andesite, and blackish porphyry,
together with some dusky-green feldspathic rocks, which I believe to be
altered clay-slate: these mountains are crossed by many brown-coloured
dikes, running north and south. Above the town, the main valley runs in
a south-east and even more southerly course towards the Cordillera,
where it is divided into three great ravines, by the northern one of
which, called Jolquera, I penetrated for a short distance. The section,
Fig. 3 in Plate V, gives an eye-sketch of the structure and composition
of the mountains on both sides of this valley: a straight east and west
line from the town to the Cordillera is perhaps
not more than thirty miles, but along the valley the distance is much
greater. Wherever the valley trended very southerly, I have endeavoured
to contract the section into its true proportion. This valley, I may
add, rises much more gently than any other valley which I saw in Chile.
To commence with our section, for a short distance above the town we
have hills of the granitic series, together with some of that rock [A],
which I suspect to be altered clay-slate, but which Professor G. Rose,
judging from specimens collected by Meyen at P. Negro, states is
serpentine passing into greenstone. We then come suddenly to the great
gypseous formation [B], without having passed over, differently from,
in all the sections hitherto described, any of the porphyritic
conglomerate. The strata are at first either horizontal or gently
inclined westward; then highly inclined in various directions, and
contorted by underlying masses of intrusive rocks; and lastly, they
have a regular eastward dip, and form a tolerably well pronounced north
and south line of hills. This formation consists of thin strata, with
innumerable alternations, of black, calcareous slate-rock, of
calcareo-aluminous stones like those at Coquimbo, which I have called
pseudo-honestones of green jaspery layers, and of pale-purplish,
calcareous, soft rotten-stone, including seams and veins of gypsum.
These strata are conformably overlaid by a great thickness of thinly
stratified, compact limestone with included crystals of carbonate of
lime. At a place called Tierra Amarilla, at the foot of a mountain thus
composed there is a broad vein, or perhaps stratum, of a beautiful and
curious crystallised mixture, composed, according to Professor G.
Rose,[3] of sulphate of iron under two forms, and of the sulphates of
copper and alumina: the section is so obscure that I could not make out
whether this vein or stratum occurred in the gypseous formation, or
more probably in some underlying masses [A], which I believe are
altered clay-slate.
[3] Meyen’s “Reise,” etc., Th. I, s. 394.
_Second axis of elevation._—After the gypseous masses [B], we come to a
line of hills of unstratified porphyry [C], which on their eastern side
blend into strata of great thickness of porphyritic conglomerate,
dipping eastward. This latter formation, however, here has not been
nearly so much metamorphosed as in most parts of Central Chile; it is
composed of beds of true purple claystone porphyry, repeatedly
alternating with thick beds of purplish-red conglomerate with the
well-rounded, large pebbles of various porphyries, not blended
together. _Third axis of elevation._—Near the ravine of Los Hornitos,
there is a well-marked line of elevation, extending for many miles in a
N.N.E. and S.S.W. direction, with the strata dipping in most parts (as
in the second axis) only in one direction, namely, eastward at an
average angle of between 30° and 40°. Close to the mouth of the valley,
however, there is, as represented in the section, a steep and high
mountain [D], composed of various green and brown intrusive porphyries
enveloped with strata, apparently belonging to the upper parts of the
porphyritic
conglomerate, and dipping both eastward and westward. I will describe
the section seen on the eastern side of this mountain [D], beginning at
the base with the lowest bed visible in the porphyritic conglomerate,
and proceeding upwards through the gypseous formation. Bed 1 consists
of reddish and brownish porphyry varying in character, and in many
parts highly amygdaloidal with carbonate of lime, and with bright green
and brown bole. Its upper surface is throughout clearly defined, but
the lower surface is in most parts indistinct, and towards the summit
of the mountain [D] quite blended into the intrusive porphyries. Bed 2,
a pale lilac, hard but not heavy stone, slightly laminated, including
small extraneous fragments, and imperfect as well as some perfect and
glassy crystals of feldspar; from one hundred and fifty to two hundred
feet in thickness. When examining it in situ, I thought it was
certainly a true porphyry, but my specimens now lead me to suspect that
it possibly may be a metamorphosed tuff. From its colour it could be
traced for a long distance, overlying in one part, quite conformably to
the porphyry of bed 1, and in another not distant part, a very thick
mass of conglomerate, composed of pebbles of a porphyry chiefly like
that of bed 1: this fact shows how the nature of the bottom formerly
varied in short horizontal distances. Bed 3, white, much indurated
tuff, containing minute pebbles, broken crystals, and scales of mica,
varies much in thickness. This bed is remarkable from containing many
globular and pear-shaped, externally rusty balls, from the size of an
apple to a man’s head, of very tough, slate-coloured porphyry, with
imperfect crystals of feldspar: in shape these balls do not resemble
pebbles, _and i believe that they are subaqueous volcanic bombs_; they
differ from _subaerial_ bombs only in not being vesicular. Bed 4; a
dull purplish-red, hard conglomerate, with crystallised particles and
veins of carbonate of lime, from three hundred to four hundred feet in
thickness. The pebbles are of claystone porphyries of many varieties;
they are tolerably well rounded, and vary in size from a large apple to
a man’s head. This bed includes three layers of coarse, black,
calcareous, somewhat slaty rock: the upper part passes into a compact
red sandstone.
In a formation so highly variable in mineralogical nature, any division
not founded on fossil remains, must be extremely arbitrary:
nevertheless, the beds below the last conglomerate may, in accordance
with all the sections hitherto described, be considered as belonging to
the porphyritic conglomerate, and those above it to the gypseous
formation, marked [E] in the section. The part of the valley in which
the following beds are seen is near Potrero Seco. Bed 5, compact,
fine-grained, pale greenish-grey, non-calcareous, indurated mudstone,
easily fusible into a pale green and white glass. Bed 6, purplish,
coarse-grained, hard sandstone, with broken crystals of feldspar and
crystallised particles of carbonate of lime; it possesses a slightly
nodular structure. Bed 7, blackish-grey, much indurated, calcareous
mudstone, with extraneous particles of unequal size; the whole being in
parts finely brecciated. In this mass there is a stratum, twenty feet
in thickness, of impure gypsum. Bed 8, a greenish mudstone, with
several layers of gypsum. Bed 9,
a highly indurated, easily fusible, white tuff, thickly mottled with
ferruginous matter, and including some white semi-porcellanic layers,
which are interlaced with ferruginous veins. This stone closely
resembles some of the commonest varieties in the Uspallata chain. Bed
10, a thick bed of rather bright green, indurated mudstone or tuff,
with a concretionary nodular structure so strongly developed that the
whole mass consists of balls. I will not attempt to estimate the
thickness of the strata in the gypseous formation hitherto described,
but it must certainly be very many hundred feet. Bed 11 is at least 800
feet in thickness: it consists of thin layers of whitish, greenish, or
more commonly brown, fine-grained, indurated tuffs, which crumble into
angular fragments: some of the layers are semi-porcellanic, many of
them highly ferruginous, and some are almost composed of carbonate of
lime and iron with drusy cavities lined with quartzf-crystals. Bed 12,
dull purplish or greenish or dark-grey, very compact and much indurated
mudstone: estimated at 1,500 feet in thickness: in some parts this rock
assumes the character of an imperfect coarse clay-slate; but viewed
under a lens, the basis always has a mottled appearance, with the edges
of the minute component particles blending together. Parts are
calcareous, and there are numerous veins of highly crystalline
carbonate of lime charged with iron. The mass has a nodular structure,
and is divided by only a few planes of stratification: there are,
however, two layers, each about eighteen inches thick, of a dark brown,
finer-grained stone, having a conchoidal, semi-porcellanic fracture,
which can be followed with the eye for some miles across the country.
I believe this last great bed is covered by other nearly similar
alternations; but the section is here obscured by a tilt from the next
porphyritic chain, presently to be described. I have given this section
in detail, as being illustrative of the general character of the
mountains in this neighbourhood; but it must not be supposed that any
one stratum long preserves the same character. At a distance of between
only two and three miles the green mudstones and white indurated tuffs
are to a great extent replaced by red sandstone and black calcareous
shaly rocks, alternating together. The white indurated tuff, bed 11,
here contains little or no gypsum, whereas on the northern and opposite
side of the valley, it is of much greater thickness and abounds with
layers of gypsum, some of them alternating with thin seams of
crystalline carbonate of lime. The uppermost, dark-coloured, hard
mudstone, bed 12, is in this neighbourhood the most constant stratum.
The whole series differs to a considerable extent, especially in its
upper part, from that met with at [BB], in the lower part of the
valley; nevertheless, I do not doubt that they are equivalents. _Fourth
axis of elevation (Valley of Copiapo)._—This axis is formed of a chain
of mountains [F], of which the central masses (near La Punta) consist
of andesite containing green hornblende and coppery mica, and the outer
masses of greenish and black porphyries, together with some fine
lilac-coloured claystone porphyry; all these porphyries being injected
and broken up by small hummocks of andesite. The
central great mass of this latter rock, is covered on the eastern side
by a black, fine-grained, highly micaceous slate, which, together with
the succeeding mountains of porphyry, are traversed by numerous white
dikes, branching from the andesite, and some of them extending in
straight lines, to a distance of at least two miles. The mountains of
porphyry eastward of the micaceous schist soon, but gradually, assume
(as observed in so many other cases) a stratified structure, and can
then be recognised as a part of the porphyritic conglomerate formation.
These strata [G] are inclined at a high angle to the S.E., and form a
mass from fifteen hundred to two thousand feet in thickness. The
gypseous masses to the west already described, dip directly towards
this axis, with the strata only in a few places (one of which is
represented in the section) thrown from it: hence this fourth axis is
mainly uniclinal towards the S.E., and just like our third axis, only
locally anticlinal.
The above strata of porphyritic conglomerate [G] with their
south-eastward dip, come abruptly up against beds of the gypseous
formation [H], which are gently, but irregularly, inclined westward: so
that there is here a synclinal axis and great fault. Further up the
valley, here running nearly north and south, the gypseous formation is
prolonged for some distance; but the stratification is unintelligible,
the whole being broken up by faults, dikes, and metalliferous veins.
The strata consist chiefly of red calcareous sandstones, with numerous
veins in the place of layers, of gypsum; the sandstone is associated
with some black calcareous slate-rock, and with green
pseudo-honestones, passing into porcelain-jasper. Still further up the
valley, near Las Amolanas [I], the gypseous strata become more regular,
dipping at an angle of between 30 and 40 degrees to W.S.W., and
conformably overlying, near the mouth of the ravine of Jolquera, strata
[K] of porphyritic conglomerate. The whole series has been tilted by a
partially concealed axis [L], of granite, andesite, and a granitic
mixture of white feldspar, quartz, and oxide of iron.
_Fifth axis of elevation (Valley of Copiapo, near Las Amolanas)._—I
will describe in some detail the beds [I] seen here, which, as just
stated, dip to W.S.W., at an angle of from 30° to 40°. I had not time
to examine the underlying porphyritic conglomerate, of which the lowest
beds, as seen at the mouth of the Jolquera, are highly compact, with
crystals of red oxide of iron; and I am not prepared to say whether
they are chiefly of volcanic or metamorphic origin. On these beds there
rests a coarse purplish conglomerate, very little metamorphosed,
composed of pebbles of porphyry, but remarkable from containing one
pebble of granite;—of which fact no instance has occurred in the
sections hitherto described. Above this conglomerate, there is a black
siliceous claystone, and above it numerous alternations of
dark-purplish and green porphyries, which may be considered as the
uppermost limit of the porphyritic conglomerate formation.
Above these porphyries comes a coarse, arenaceous conglomerate, the
lower half white and the upper half of a pink colour, composed chiefly
of pebbles of various porphyries, but with some of red sandstone
and jaspery rocks. In some of the more arenaceous parts of the
conglomerate, there was an oblique or current lamination; a
circumstance which I did not elsewhere observe. Above this
conglomerate, there is a vast thickness of thinly stratified,
pale-yellowish, siliceous sandstone, passing into a granular
quartz-rock, used for grindstones (hence the name of the place _ Las
Amolanas_), and certainly belonging to the gypseous formation, as does
probably the immediately underlying conglomerate. In this yellowish
sandstone there are layers of white and pale-red siliceous
conglomerate; other layers with small, well-rounded pebbles of white
quartz, like the bed at the R. Claro at Coquimbo; others of a greenish,
fine-grained, less siliceous stone, somewhat resembling the
pseudo-honestones lower down the valley; and lastly, others of a black
calcareous shale-rock. In one of the layers of conglomerate, there was
embedded a fragment of mica-slate, of which this is the first instance;
hence perhaps, it is from a formation of mica-slate, that the numerous
small pebbles of quartz, both here and at Coquimbo, have been derived.
Not only does the siliceous sandstone include layers of the black,
thinly stratified, not fissile, calcareous shale-rock, but in one place
the whole mass, especially the upper part, was, in a marvellously short
horizontal distance, after frequent alternations, replaced by it. When
this occurred, a mountain-mass, several thousand feet in thickness was
thus composed; the black calcareous shale-rock, however, always
included some layers of the pale-yellowish siliceous sandstone, of the
red conglomerate, and of the greenish jaspery and pseudo-honestone
varieties. It likewise included three or four widely separated layers
of a brown limestone, abounding with shells immediately to be
described. This pile of strata was in parts traversed by many veins of
gypsum. The calcareous shale-rock, though when freshly broken quite
black, weathers into an ash- colour: in which respect and in general
appearance, it perfectly resembles those great fossiliferous beds of
the Peuquenes range, alternating with gypsum and red sandstone,
described in the last chapter.
The shells out of the layers of brown limestone, included in the black
calcareous shale-rock, which latter, as just stated, replaces the white
siliceous sandstone, consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage,” Part Pal.
Turritella Andii, d’Orbigny, “Voyage,” Part Pal.
Astarte Darwinii, E. Forbes.
Gryphæa Darwinii, E. Forbes.
An intermediate form between G. gigantea and G. incurva.
Gryphæa nov. spec.?, E. Forbes.
Perna Americana, E. Forbes.
Avicula, nov. spec.
Considered by Mr. G. B. Sowerby as the A. echinata, by M. d’Orbigny as
certainly a new and distinct species, having a Jurassic aspect. The
specimen has been unfortunately lost.
Terebratula ænigma, d’Orbigny, (var. of do. E. Forbes.)
This is the same variety, with that from Guasco, considered by M.
D’Orbigny to be a distinct species from his T. ænigma, and related to
T. obsoleta.
Plagiostoma and Ammonites, fragments of.
The lower layers of the limestone contained thousands of the Gryphæa;
and the upper ones as many of the Turritella, with the Gryphæa (nov.
species) and Serpulæ adhering to them; in all the layers, the
Terebratula and fragments of the Pecten were included. It was evident,
from the manner in which species were grouped together, that they had
lived where now embedded. Before making any further remarks, I may
state, that higher up this same valley we shall again meet with a
similar association of shells; and in the great Despoblado Valley,
which branches off near the town from that of Copiapo, the Pecten
Dufreynoyi, some Gryphites (I believe G. Darwinii), and the _ true_
Terebratula ænigma of d’Orbigny were found together in an equivalent
formation, as will be hereafter seen. A specimen also, I may add, of
the true T. ænigma, was given me from the neighbourhood of the famous
silver mines of Chanuncillo, a little south of the valley of the
Copiapo, and these mines, from their position, I have no doubt, lie
within the great gypseous formation: the rocks close to one of the
silver veins, judging from fragments shown me, resemble those singular
metamorphosed deposits from the mining district of Arqueros near
Coquimbo.
I will reiterate the evidence on the association of these several
shells in the several localities.
_Coquimbo._
In the same bed, Rio Claro:
Pecten Dufreynoyi.
Ostrea hemispherica.
Terebratula ænigma.
Spirifer linguiferoides.
Same bed, near Arqueros:
Hippurites Chilensis.
Gryphæa orientalis.
Collected by M. Domeyko from the same locality, apparently near
Arqueros:
Terebratula ænigma and Terebratula ignaciana, in same block of
limestone.
Pecten Dufreynoyi.
Ostrea hemispherica.
Hippurites Chilensis.
Turritella Andii.
Nautilus Domeykus.
_Guasco._
In a collection from the Cordillera, given me: the specimens all in the
same condition:
Pecten Dufreynoyi.
Turritella Andii.
Terebratula ignaciana.
Terebratula ænigma, _var._
Spirifer Chilensis.
_Copiapo._
Mingled together in alternating beds in the main valley of Copiapo near
Las Amolanas, and likewise higher up the valley:
Pecten Dufreynoyi.
Turritella Andii.
Terebratula ænigma, _var._, as at Guasco.
Astarte Darwinii.
Gryphæa Darwinii.
Gryphæa nov. species?
Perna Americana.
Avicula, nov. species.
Main valley of Copiapo, apparently same formation with that of
Amolanas:
Terebratula ænigma (true).
In the same bed, high up the great lateral valley of the Despoblado, in
the ravine of Maricongo:
Terebratula ænigma (true).
Pecten Dufreynoyi.
Gryphæa Darwinii?
Considering this table, I think it is impossible to doubt that all
these fossils belong to the same formation. If, however, the species
from Las Amolanas, in the Valley of Copiapo, had, as in the case of
those from Guasco, been separately examined, they would probably have
been ranked as oolitic; for, although no Spirifers were found here, all
the other species, with the exception of the Pecten, Turritella, and
Astarte, have a more ancient aspect than cretaceous forms. On the other
hand, taking into account the evidence derived from the cretaceous
character of these three shells, and of the Hippurites, Gryphæa
orientalis, and Ostrea, from Coquimbo, we are driven back to the
provisional name already used of cretaceo-oolitic. From geological
evidence, I believe this formation to be the equivalent of the
Neocomian beds of the Cordillera of Central Chile.
To return to our section near Las Amolanas:—Above the yellow siliceous
sandstone, or the equivalent calcareous slate-rock, with its bands of
fossil-shells, according as the one or other prevails, there is a pile
of strata, which cannot be less than from two to three thousand feet in
thickness, in main part composed of a coarse, bright red conglomerate,
with many intercalated beds of red sandstone, and some of green and
other coloured porcelain-jaspery layers. The included pebbles are
well-rounded, varying from the size of an egg to that of a
cricket-ball, with a few larger; and they consist chiefly of
porphyries. The basis of the conglomerate, as well as some of the
alternating thin beds, are formed of a red, rather harsh, easily
fusible sandstone, with crystalline calcareous particles. This whole
great pile is remarkable from the thousands of huge, embedded,
silicified trunks of trees, one of which was eight feet long, and
another eighteen feet in circumference: how marvellous it is, that
every vessel in so thick a mass of wood should have been converted into
silex! I brought home many specimens, and all of them, according to Mr.
R. Brown, present a coniferous structure.
Above this great conglomerate, we have from two to three hundred feet
in thickness of red sandstone; and above this, a stratum of black
calcareous slate-rock, like that which alternates with and
replaces the underlying yellowish-white, siliceous sandstone. Close to
the junction between this upper black slate-rock and the upper red
sandstone, I found the Gryphæa Darwinii, the Turritella Andii, and vast
numbers of a bivalve, too imperfect to be recognised. Hence we see
that, as far as the evidence of these two shells serves—and the
Turritella is an eminently characteristic species—the whole thickness
of this vast pile of strata belongs to the same age. Again, above the
last-mentioned upper red sandstone, there were several alternations of
the black, calcareous slate-rock; but I was unable to ascend to them.
All these uppermost strata, like the lower ones, vary extremely in
character in short horizontal distances. The gypseous formation, as
here seen, has a coarser, more mechanical texture, and contains much
more siliceous matter than the corresponding beds lower down the
valley. Its total thickness, together with the upper beds of the
porphyritic conglomerate, I estimated at least at 8,000 feet; and only
a small portion of the porphyritic conglomerate, which on the eastern
flank of the fourth axis of elevation appeared to be from fifteen
hundred to two thousand feet thick, is here included. As corroborative
of the great thickness of the gypseous formation, I may mention that in
the Despoblado Valley (which branches from the main valley a little
above the town of Copiapo) I found a corresponding pile of red and
white sandstones, and of dark, calcareous, semi-jaspery mudstones,
rising from a nearly level surface and thrown into an absolutely
vertical position; so that, by pacing, I ascertained their thickness to
be nearly two thousand seven hundred feet; taking this as a standard of
comparison, I estimated the thickness of the strata _above_ the
porphyritic conglomerate at 7,000 feet.
The fossils before enumerated, from the limestone-layers in the whitish
siliceous sandstone, are now covered, on the least computation, by
strata from 5,000 to 6,000 feet in thickness. Professor E. Forbes
thinks that these shells probably lived at a depth of from about 30 to
40 fathoms, that is from 180 to 240 feet; anyhow, it is impossible that
they could have lived at the depth of from 5,000 to 6,000 feet. Hence
in this case, as in that of the Puente del Inca, we may safely conclude
that the bottom of the sea on which the shells lived, subsided, so as
to receive the superincumbent submarine strata: and this subsidence
must have taken place during the existence of these shells; for, as I
have shown, some of them occur high up as well as low down in the
series. That the bottom of the sea subsided, is in harmony with the
presence of the layers of coarse, well-rounded pebbles included
throughout this whole pile of strata, as well as of the great upper
mass of conglomerate from 2,000 to 3,000 feet thick; for coarse gravel
could hardly have been formed or spread out at the profound depths
indicated by the thickness of the strata. The subsidence, also, must
have been slow to have allowed of this often-recurrent spreading out of
the pebbles. Moreover, we shall presently see that the surfaces of some
of the streams of porphyritic lava beneath the gypseous formation, are
so highly amygdaloidal that it is scarcely possible to believe that
they flowed under the vast pressure of a deep ocean. The conclusion of
a
great subsidence during the existence of these cretaceo-oolitic
fossils, may, I believe, be extended to the district of Coquimbo,
although owing to the fossiliferous beds there not being directly
covered by the upper gypseous strata, which in the section north of the
valley are about 6,000 feet in thickness, I did not there insist on
this conclusion.
The pebbles in the above conglomerates, both in the upper and lower
beds, are all well rounded, and, though chiefly composed of various
porphyries, there are some of red sandstone and of a jaspery stone,
both like the rocks intercalated in layers in this same gypseous
formation; there was one pebble of mica-slate and some of quartz,
together with many particles of quartz. In these respects there is a
wide difference between the gypseous conglomerates and those of the
porphyritic-conglomerate formation, in which latter, angular and
rounded fragments, almost exclusively composed of porphyries, are
mingled together, and which, as already often remarked, probably were
ejected from craters deep under the sea. From these facts I conclude,
that during the formation of the conglomerates, land existed in the
neighbourhood, on the shores of which the innumerable pebbles were
rounded and thence dispersed, and on which the coniferous forests
flourished—for it is improbable that so many thousand logs of wood
should have drifted from any great distance. This land, probably
islands, must have been mainly formed of porphyries, with some
mica-slate, whence the quartz was derived, and with some red sandstone
and jaspery rocks. This latter fact is important, as it shows that in
this district, even previously to the deposition of the lower gypseous
or cretaceo-oolitic beds, strata of an analogous nature had elsewhere,
no doubt in the more central ranges of the Cordillera, been elevated;
thus recalling to our minds the relations of the Cumbre and Uspallata
chains. Having already referred to the great lateral valley of the
Despoblado, I may mention that above the 2,700 feet of red and white
sandstone and dark mudstone, there is a vast mass of coarse, hard, red
conglomerate, some thousand feet in thickness, which contains much
silicified wood, and evidently corresponds with the great upper
conglomerate at Las Amolanas: here, however, the conglomerate consists
almost exclusively of pebbles of granite, and of disintegrated crystals
of reddish feldspar and quartz firmly recemented together. In this
case, we may conclude that the land whence the pebbles were derived,
and on which the now silicified trees once flourished, was formed of
granite.
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 ænigma, 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 ænigma, 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;[4] 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.
[4] I have endeavoured to show in my “Journal,” etc. (2nd edit.), p.
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.
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;[5] 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.
[5] 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.
No. 40
[Illustration]
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 no. 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° 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°: so that at this point the beds
have been turned through an angle of 135°; and here there is a kind of
anticlinal axis, with the strata on both sides dipping to opposite
points at an angle of 45°, 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.[6] 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,[7]consists of laminated, impure, argillaceous,
purplish-grey limestone, associated, I believe, with some purple
sandstone. In the limestone shells are found: the three following
species were given me:—
Lucina Americana, E. Forbes.
Terebratula inca, E. Forbes.
Terebratula ænigma, D’Orbigny.
[6] The lowest point, where the road crosses the coast-escarpment, is
1,900 feet by the barometer above the level of the sea.
[7] Mr. Bollaert has described (“Geolog. Proceedings,” vol. ii, p.
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.
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.[8] 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.
[8] 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’s
“Anniversary Address to Geolog. Soc., 1843,” p. 65.)
_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.];[9] 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 laminæ 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.[10] 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[11] 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. 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.
[9] 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.].
[10] The same fact has been observed by Mr. Taylor in Cuba: _London
Phil. Journ.,_ vol. xi, p. 21.
[11] “Travels in Chile,” p. 29.
Silver, in the form of a chloride, sulphuret, or an amalgam, or in its
native state, and associated with lead and other metals, and at
Arqueros with pure native copper, occurs chiefly in the upper great
gypseous or cretaceo-oolitic formation which forms probably the richest
mass in Chile. We may instance the mining districts of Arqueros near
Coquimbo, and of nearly the whole valley of Copiapo, and of Iquique
(where the principal veins run N.E. by E. and S.W. by W.), in Peru.
Hence comes Molina’s remark, that silver is born in the cold and
solitary deserts of the Upper Cordillera. There are, however,
exceptions to
this rule: at Paral (S.E. of Coquimbo) silver is found in the
porphyritic conglomerate formation; as I suspect is likewise the case
at S. Pedro de Nolasko in the Peuquenes Pass. Rich argentiferous lead
is found in the clay-slate of the Uspallata range; and I saw an old
silver-mine in a hill of syenite at the foot of the Bell of Quillota: I
was also assured that silver has been found in the andesitic and
porphyritic region between the town of Copiapo and the Pacific. I have
stated in a previous part of this chapter, that in two neighbouring
mines at Arqueros the veins in one were productive when they traversed
the singular green sedimentary beds, and unproductive when crossing the
reddish beds; whereas at the other mine exactly the reverse takes
place; I have also described the singular and rare case of numerous
particles of native silver and of the chloride being disseminated in
the green rock at the distance of a yard from the vein. Mercury occurs
with silver both at Arqueros and at Copiapo: at the base of C. de los
Hornos (S.E. of Coquimbo, a different place from Los Hornos, before
mentioned) I saw in a syenitic rock numerous quartzose veins,
containing a little cinnabar in nests: there were here other parallel
veins of copper and of a ferrugino-auriferous ore. I believe tin has
never been found in Chile.
From information given me by Mr. Nixon of Yaquil,[12] and by others, it
appears that in Chile those veins are generally most permanently
productive, which, consisting of various minerals (sometimes differing
but slightly from the surrounding rocks), include parallel strings
_rich_ in metals; such a vein is called a _veta real._ More commonly
the mines are worked only where one, two, or more thin veins or strings
running in a different direction, intersect a _poor_ “veta real:” it is
unanimously believed that at such points of intersection (_cruceros_),
the quantity of metal is much greater than that contained in other
parts of the intersecting veins. In some _ cruceros_ or points of
intersection, the metals extend even beyond the walls of the main,
broad, stony vein. It is said that the greater the angle of
intersection, the greater the produce; and that nearly parallel strings
attract each other; in the Uspallata range, I observed that numerous
thin auri-ferruginous veins repeatedly ran into knots, and then
branched out again. I have already described the remarkable manner in
which rocks of the Uspallata range are indurated and blackened (as if
by a blast of gunpowder) to a considerable distance from the metallic
veins.
[12] At the Durazno mine, the gold is associated with copper-pyrites,
and the veins contain large prisms of plumbago. Crystallised carbonate
of lime is one of the commonest minerals in the matrix of the Chilean
veins.
Finally, I may observe, that the presence of metallic veins seems
obviously connected with the presence of intrusive rocks, and with the
degree of metamorphic action which the different districts of Chile
have undergone.[13] Such metamorphosed areas are generally accompanied
by numerous dikes and injected masses of andesite and various
porphyries: I have in several places traced the metalliferous veins
from
the intrusive masses into the encasing strata. Knowing that the
porphyritic conglomerate formation consists of alternate streams of
submarine lavas and of the debris of anciently erupted rocks, and that
the strata of the upper gypseous formation sometimes include submarine
lavas, and are composed of tuffs, mudstones, and mineral substances,
probably due to volcanic exhalations,—the richness of these strata is
highly remarkable when compared with the erupted beds, often of
submarine origin, but _not metamorphosed,_ which compose the numerous
islands in the Pacific, Indian, and Atlantic Oceans; for in these
islands metals are entirely absent, and their nature even unknown to
the aborigines.
[13] Sir R. Murchison and his fellow travellers have given some
striking facts on this subject in their account of the Ural Mountains
(“Geolog. Proc.,” vol. iii, p. 748.
_Summary of the Geological History of the Chilean Cordillera, and of
the Southern Parts of South America._
We have seen that the shores of the Pacific, for a space of 1,200 miles
from Tres Montes to Copiapo, and I believe for a very much greater
distance, are composed, with the exception of the tertiary basins, of
metamorphic schists, plutonic rocks, and more or less altered
clay-slate. On the floor of the ocean thus constituted, vast streams of
various purplish claystone and greenstone porphyries were poured forth,
together with great alternating piles of angular and rounded fragments
of similar rocks ejected from the submarine craters. From the
compactness of the streams and fragments, it is probable that, with the
exception of some districts in Northern Chile, the eruptions took place
in profoundly deep water. The orifices of eruption appear to have been
studded over a breadth, with some outliers, of from fifty to one
hundred miles: and closely enough together, both north and south, and
east and west, for the ejected matter to form a continuous mass, which
in Central Chile is more than a mile in thickness. I traced this
mould-like mass, for only 450 miles; but judging from what I saw at
Iquique, from specimens, and from published accounts, it appears to
have a manifold greater length. In the basal parts of the series, and
especially towards the flanks of the range, mud, since converted into a
feldspathic slaty rock, and sometimes into greenstone, was occasionally
deposited between the beds of erupted matter: with this exception the
uniformity of the porphyritic rocks is very remarkable. At the period
when the claystone and greenstone porphyries nearly or quite ceased
being erupted, that great pile of strata which, from often abounding
with gypsum, I have generally called the gypseous formation was
deposited, and feldspathic lavas, together with other singular volcanic
rocks, were occasionally poured forth: I am far from pretending that
any distinct line of demarcation can be drawn between this formation
and the underlying porphyries and porphyritic conglomerate, but in a
mass of such great thickness, and between beds of such widely different
mineralogical nature, some division was necessary. At about the
commencement of the gypseous period, the bottom of the sea here seems
first to have been peopled by shells, not many in kind,
but abounding in individuals. At the P. del Inca the fossils are
embedded near the base of the formation; in the Peuquenes range, at
different levels, halfway up, and even higher in the series; hence, in
these sections, the whole pile of strata belongs to the same period:
the same remark is applicable to the beds at Copiapo, which attain a
thickness of between seven and eight thousand feet. The fossil shells
in the Cordillera of Central Chile, in the opinion of all the
palæontologists who have examined them, belong to the earlier stages of
the cretaceous system; whilst in Northern Chile there is a most
singular mixture of cretaceous and oolitic forms: from the geological
relations, however, of these two districts, I cannot but think that
they all belong to nearly the same epoch, which I have provisionally
called cretaceo-oolitic.
The strata in this formation, composed of black calcareous shaly-rocks
of red and white, and sometimes siliceous sandstone, of coarse
conglomerates, limestones, tuffs, dark mudstones, and those singular
fine-grained rocks which I have called pseudo-honestones, vast beds of
gypsum, and many other jaspery and scarcely describable varieties, vary
and replace each other in short horizontal distances, to an extent, I
believe, unequalled even in any tertiary basin. Most of these
substances are easily fusible, and have apparently been derived either
from volcanoes still in quiet action, or from the attrition of volcanic
products. If we picture to ourselves the bottom of the sea, rendered
uneven in an extreme degree, with numerous craters, some few
occasionally in eruption, but the greater number in the state of
solfataras, discharging calcareous, siliceous, ferruginous matters, and
gypsum or sulphuric acid to an amount surpassing, perhaps, even the
existing sulphureous volcanoes of Java,[14] we shall probably
understand the circumstances under which this singular pile of varying
strata was accumulated. The shells appear to have lived at the
quiescent periods when only limestone or calcareo-argillaceous matter
was depositing. From Dr. Gillies’ account, this gypseous or
cretaceo-oolitic formation extends as far south as the Pass of
Planchon, and I followed it northward at intervals for 500 miles:
judging from the character of the beds with the _Terebratula ænigma,_
at Iquique, it extends from four to five hundred miles further: and
perhaps even for ten degrees of latitude north of Iquique to the Cerro
Pasco, not far from Lima: again, we know that a cretaceous formation,
abounding with fossils, is largely developed north of the equator, in
Colombia: in Tierra del Fuego, at about this same period, a wide
district of clay-slate was deposited, which in its mineralogical
characters and external features, might be compared to the Silurian
regions of North Wales. The gypseous formation, like that of the
porphyritic breccia-conglomerate on which it rests, is of
inconsiderable breadth; though of greater breadth in Northern than in
Central Chile.
[14] Von Buch’s “Descript. Physique des Iles Canaries,” p. 428.
As the fossil shells in this formation are covered, in the Peuquenes
ridge, by a great thickness of strata; at the Puente del Inca, by at
least five thousand feet; at Coquimbo, though the superposition there
is less plainly seen, by about six thousand feet; and at Copiapo,
certainly by five or
six thousand, and probably by seven thousand feet (the same species
there recurring in the upper and lower parts of the series), we may
feel confident that the bottom of the sea subsided during this
cretaceo-oolitic period, so as to allow of the accumulation of the
superincumbent submarine strata. This conclusion is confirmed by, or
perhaps rather explains, the presence of the many beds at many levels
of coarse conglomerate, the well-rounded pebbles in which we cannot
believe were transported in very deep water. Even the underlying
porphyries at Copiapo. with their highly amygdaloidal surfaces, do not
appear to have flowed under great pressure. The great sinking movement
thus plainly indicated, must have extended in a north and south line
for at least four hundred miles, and probably was co-extensive with the
gypseous formation.
The beds of conglomerate just referred to, and the extraordinarily
numerous silicified trunks of fir-trees at Los Hornos, perhaps at
Coquimbo and at two distant points in the valley of Copiapo, indicate
that land existed at this period in the neighbourhood. This land, or
islands, in the northern part of the district of Copiapo, must have
been almost exclusively composed, judging from the nature of the
pebbles of granite: in the southern parts of Copiapo, it must have been
mainly formed of claystone porphyries, with some mica-schist, and with
much sandstone and jaspery rocks exactly like the rocks in the gypseous
formation, and no doubt belonging to its basal series. In several other
places also, during the accumulation of the gypseous formation, its
basal parts and the underlying porphyritic conglomerate must likewise
have been already partially upheaved and exposed to wear and tear; near
the Puente del Inca and at Coquimbo, there must have existed masses of
mica-schist or some such rock, whence were derived the many small
pebbles of opaque quartz. It follows from these facts, that in some
parts of the Cordillera the upper beds of the gypseous formation must
lie unconformably on the lower beds; and the whole gypseous formation,
in parts, unconformably on the porphyritic conglomerate; although I saw
no such cases, yet in many places the gypseous formation is entirely
absent; and this, although no doubt generally caused by quite
subsequent denudation, may in others be due to the underlying
porphyritic conglomerate having been locally upheaved before the
deposition of the gypseous strata, and thus having become the source of
the pebbles of porphyry embedded in them. In the porphyritic
conglomerate formation, in its lower and middle parts, there is very
rarely any evidence, with the exception of the small quartz pebbles at
Jajuel near Aconcagua, and of the single pebble of granite at Copiapo,
of the existence of neighbouring land: in the upper parts, however, and
especially in the district of Copiapo, the number of thoroughly
well-rounded pebbles of compact porphyries make me believe, that, as
during the prolonged accumulation of the gypseous formation the lower
beds had already been locally upheaved and exposed to wear and tear, so
it was with the porphyritic conglomerate. Hence in following thus far
the geological history of the Cordillera, it may be inferred that the
bed of a deep and open, or nearly open, ocean was filled up by
porphyritic
eruptions, aided probably by some general and some local elevations, to
that comparatively shallow level at which the cretaceo-oolitic shells
first lived. At this period, the submarine craters yielded at intervals
a prodigious supply of gypsum and other mineral exhalations, and
occasionally, in certain places poured forth lavas, chiefly of a
feldspathic nature: at this period, islands clothed with fir-trees and
composed of porphyries, primary rocks, and the lower gypseous strata
had already been locally upheaved, and exposed to the action of the
waves;—the general movement, however, at this time having been over a
very wide area, one of slow subsidence, prolonged till the bed of the
sea sank several thousand feet.
In Central Chile, after the deposition of a great thickness of the
gypseous strata, and after their upheaval, by which the Cumbre and
adjoining ranges were formed, a vast pile of tufaceous matter and
submarine lava was accumulated, where the Uspallata chain now stands;
also after the deposition and upheaval of the equivalent gypseous
strata of the Peuquenes range, the great thick mass of conglomerate in
the valley of Tenuyan was accumulated: during the deposition of the
Uspallata strata, we know absolutely, from the buried vertical trees,
that there was a subsidence of some thousand feet; and we may infer
from the nature of the conglomerate in the valley of Tenuyan, that a
similar and perhaps contemporaneous movement there took place. We have,
then, evidence of a second great period of subsidence; and, as in the
case of the subsidence which accompanied the accumulation of the
cretaceo-oolitic strata, so this latter subsidence appears to have been
complicated by alternate or local elevatory movement— for the vertical
trees, buried in the midst of the Uspallata strata, must have grown on
dry land, formed by the upheaval of the lower submarine beds. Presently
I shall have to recapitulate the facts, showing that at a still later
period, namely, at nearly the commencement of the old tertiary deposits
of Patagonia and of Chile, the continent stood at nearly its present
level, and then, for the third time, slowly subsided to the amount of
several hundred feet, and was afterwards slowly re-uplifted to its
present level.
The highest peaks of the Cordillera appear to consist of active or more
commonly dormant volcanoes,—such as Tupungato, Maypu, and Aconcagua,
which latter stands 23,000 feet above the level of the sea, and many
others. The next highest peaks are formed of the gypseous and
porphyritic strata, thrown into vertical or highly inclined positions.
Besides the elevation thus gained by angular displacements, I infer,
without any hesitation—from the stratified gravel-fringes which gently
slope up the valleys of the Cordillera from the gravel-capped plains at
their base, which latter are connected with the plains, still covered
with recent shells on the coast—that this great range has been upheaved
in mass by a slow movement, to an amount of at least 8,000 feet. In the
Despoblado Valley, north of Copiapo, the horizontal elevation, judging
from the compact, stratified tufaceous deposit, capping the distant
mountains at corresponding heights, was about ten thousand feet. It is
very possible, or rather probable, that this elevation in mass may not
have
been strictly horizontal, but more energetic under the Cordillera, than
towards the coast on either side; nevertheless, movements of this kind
may be conveniently distinguished from those by which strata have been
abruptly broken and upturned. When viewing the Cordillera, before
having read Mr. Hopkins’s profound “Researches on Physical Geology,”
the conviction was impressed on me, that the angular dislocations,
however violent, were quite subordinate in importance to the great
upward movement in mass, and that they had been caused by the edges of
the wide fissures, which necessarily resulted from the tension of the
elevated area, having yielded to the inward rush of fluidified rock,
and having thus been upturned.
The ridges formed by the angularly upheaved strata are seldom of great
length: in the central parts of the Cordillera they are generally
parallel to each other, and run in north and south lines; but towards
the flanks they often extend more or less obliquely. The angular
displacement has been much more violent in the central than in the
exterior _main_ lines; but it has likewise been violent in some of the
_minor_ lines on the extreme flanks. The violence has been very unequal
on the same short lines; the crust having apparently tended to yield on
certain points along the lines of fissures. These points, I have
endeavoured to show, were probably first foci of eruption, and
afterwards of injected masses of porphyry and andesite.[15] The close
similarity of the andesitic granites and porphyries, throughout Chile,
Tierra del Fuego, and even in Peru, is very remarkable. The prevalence
of feldspar cleaving like albite, is common not only to the andesites,
but (as I infer from the high authority of Professor G. Rose, as well
as from my own measurements) to the various claystone and greenstone
porphyries, and to the trachytic lavas of the Cordillera. The andesitic
rocks have in most cases been the last injected ones, and they probably
form a continuous dome under this great range: they stand in intimate
relationship with the modern lavas; and they seem to have been the
immediate agent in metamorphosing the porphyritic conglomerate
formation, and often likewise the gypseous strata, to the extraordinary
extent to which they have suffered.
[15] Sir R. Murchison and his companions state (“Geolog. Proc.,” vol.
iii, p. 747), that no true granite appears in the higher Ural
Mountains; but that syenitic greenstone—a rock closely analogous to
our andesite—is far the most abundant of the intrusive masses.
With respect to the age at which the several parallel ridges composing
the Cordillera were upthrown, I have little evidence. Many of them may
have been contemporaneously elevated and injected in the same
manner[16] as in volcanic archipelagoes lavas are contemporaneously
ejected on the parallel lines of fissure. But the pebbles apparently
derived from the wear and tear of the porphyritic conglomerate
formation, which are occasionally present in the upper parts of this
same formation, and are often present in the gypseous formation,
together with the pebbles from the basal parts of the latter formation
in its upper strata, render it almost certain that portions, we may
infer ridges,
of these two formations were successively upheaved. In the case of the
gigantic Portillo range, we may feel almost certain that a preexisting
granitic line was upraised (not by a single blow, as shown by the
highly inclined basaltic streams in the valley on its eastern flank) at
a period long subsequent to the upheavement of the parallel Peuquenes
range.[17] Again, subsequently to the upheavement of the Cumbre chain,
that of Uspallata was formed and elevated; and afterwards, I may add,
in the plain of Uspallata, beds of sand and gravel were violently
upthrown. The manner in which the various kinds of porphyries and
andesites have been injected one into the other, and in which the
infinitely numerous dikes of various composition intersect each other,
plainly show that the stratified crust has been stretched and yielded
many times over the same points. With respect to the age of the axes of
elevation between the Pacific and the Cordillera, I know little: but
there are some lines which must—namely, those running north and south
in Chiloe, those eight or nine east and west, parallel, far-extended,
most symmetrical uniclinal lines at P. Rumena, and the short N.W.-S.E.
and N.E.-S.W. lines at Concepcion—have been upheaved long after the
formation of the Cordillera. Even during the earthquake of 1835, when
the linear north and south islet of St. Mary was uplifted several feet
above the surrounding area, we perhaps see one feeble step in the
formation of a subordinate mountain-axis. In some cases, moreover, for
instance, near the baths of Cauquenes, I was forcibly struck with the
small size of the breaches cut through the exterior mountain-ranges,
compared with the size of the same valleys higher up where entering the
Cordillera; and this circumstance appeared to me scarcely explicable,
except on the idea of the exterior lines having been subsequently
upthrown, and therefore having been exposed to a less amount of
denudation. From the manner in which the fringes of gravel are
prolonged in unbroken slopes up the valleys of the Cordillera, I infer
that most of the greater dislocations took place during the earlier
parts of the great elevation in mass: I have, however, elsewhere given
a case, and M. de Tschudi[18] has given another, of a ridge thrown up
in Peru across the bed of a river, and consequently after the final
elevation of the country above the level of the sea.
[16] “Volcanic Islands,” etc.)
[17] I have endeavoured to show in my “Journal” (2nd edit., p. 321),
that the singular fact of the river, which drains the valley between
these two ranges, passing through the Portillo and higher line, is
explained by its slow and subsequent elevation. There are many
analogous cases in the drainage of rivers: see _ Edinburgh New Phil.
Journal,_ vol. xxviii, pp. 33 and 44.
[18] “Reise in Peru,” Band 2, s. 8: Author’s “Journal,” 2nd edit., p.
359.
Ascending to the older tertiary formations, I will not again
recapitulate the remarks already given at the end of the Fifth
Chapter,—on their great extent, especially along the shores of the
Atlantic—on their antiquity, perhaps corresponding with that of the
eocene deposits of Europe,—on the almost entire dissimilarity, though
the formations are apparently contemporaneous, of the fossils from the
eastern and western coasts, as is likewise the case, even in a still
more marked degree, with the shells now living in these opposite though
approximate
seas,—on the climate of this period not having been more tropical than
what might have been expected from the latitudes of the places under
which the deposits occur; a circumstance rendered well worthy of
notice, from the contrast with what is known to have been the case
during the older tertiary periods of Europe, and likewise from the fact
of the southern hemisphere having suffered at a much later period,
apparently at the same time with the northern hemisphere, a colder or
more equable temperature, as shown by the zones formerly affected by
ice-action. Nor will I recapitulate the proofs of the bottom of the
sea, both on the eastern and western coast, having subsided seven or
eight hundred feet during this tertiary period; the movement having
apparently been co-extensive, or nearly co-extensive, with the deposits
of this age. Nor will I again give the facts and reasoning on which the
proposition was founded, that when the bed of the sea is either
stationary or rising, circumstances are far less favourable than when
its level is sinking, to the accumulation of conchiferous deposits of
sufficient thickness, extension, and hardness to resist, when upheaved,
the ordinary vast amount of denudation. We have seen that the highly
remarkable fact of the absence of any _ extensive_ formations
containing recent shells, either on the eastern or western coasts of
the continent,—though these coasts now abound with living
mollusca,—though they are, and apparently have always been, as
favourable for the deposition of sediment as they were when the
tertiary formations were copiously deposited,—and though they have been
upheaved to an amount quite sufficient to bring up strata from the
depths the most fertile for animal life—can be explained in accordance
with the above proposition. As a deduction, it was also attempted to be
shown, first, that the want of close sequence in the fossils of
successive formations, and of successive stages in the same formation,
would follow from the improbability of the same area continuing slowly
to subside from one whole period to another, or even during a single
entire period; and secondly, that certain epochs having been favourable
at distant points, in the same quarter of the world for the synchronous
accumulation of fossiliferous strata, would follow from movements of
subsidence having apparently, like those of elevation,
contemporaneously affected very large areas.
There is another point which deserves some notice, namely, the analogy
between the upper parts of the Patagonian tertiary formation, as well
as of the upper possibly contemporaneous beds at Chiloe and Concepcion,
with the great gypseous formation of Cordillera; for in both
formations, the rocks, in their fusible nature, in their containing
gypsum, and in many other characters, show a connection, either
intimate or remote, with volcanic action; and as the strata in both
were accumulated during subsidence, it appears at first natural to
connect this sinking movement with a state of high activity in the
neighbouring volcanoes. During the cretaceo-oolitic period this
certainly appears to have been the case at the Puente del Inca, judging
from the number of intercalated lava-streams in the lower 3,000 feet of
strata; but generally, the volcanic orifices seem at this time to have
existed as submarine solfataras, and were certainly quiescent compared
with their state
during the accumulation of the porphyritic conglomerate formation.
During the deposition of the tertiary strata we know that at S. Cruz,
deluges of basaltic lava were poured forth; but as these lie in the
upper part of the series, it is possible that the subsidence may at
that time have ceased: at Chiloe, I was unable to ascertain to what
part of the series the pile of lavas belonged. The Uspallata tuffs and
great streams of submarine lavas, were probably intermediate in age
between the cretaceo-oolitic and older tertiary formations, and we know
from the buried trees that there was a great subsidence during their
accumulation; but even in this case, the subsidence may not have been
strictly contemporaneous with the great volcanic eruptions, for we must
believe in at least one intercalated period of elevation, during which
the ground was upraised on which the now buried trees grew. I have been
led to make these remarks, and to throw some doubt on the strict
contemporaneousness of high volcanic activity and movements of
subsidence, from the conviction impressed on my mind by the study of
coral formations,[19] that these two actions do not generally go on
synchronously;—on the contrary, that in volcanic districts, subsidence
ceases as soon as the orifices burst forth into renewed action, and
only recommences when they again have become dormant.
[19] “The Structure, etc., of Coral Reefs.”
At a later period, the Pampean mud, of estuary origin, was deposited
over a wide area,—in one district conformably on the underlying old
tertiary strata, and in another district unconformably on them, after
their upheaval and denudation. During and before the accumulation,
however, of these old tertiary strata, and, therefore, at a very remote
period, sediment, strikingly resembling that of the Pampas, was
deposited; showing during how long a time in this case the same
agencies were at work in the same area. The deposition of the Pampean
estuary mud was accompanied, at least in the southern parts of the
Pampas, by an elevatory movement, so that the M. Hermoso beds probably
were accumulated after the upheaval of those round the S. Ventana; and
those at P. Alta after the upheaval of the M. Hermoso strata; but there
is some reason to suspect that one period of subsidence intervened,
during which mud was deposited over the coarse sand of the Barrancas de
S. Gregorio, and on the higher parts of Banda Oriental. The mammiferous
animals characteristic of this formation, many of which differ as much
from the present inhabitants of South America, as do the eocene mammals
of Europe from the present ones of that quarter of the globe, certainly
co-existed at B. Blanca with twenty species of mollusca, one balanus,
and two corals, all now living in the adjoining sea: this is likewise
the case in Patagonia with the Macrauchenia, which co-existed with
eight shells, still the commonest kinds on that coast. I will not
repeat what I have elsewhere said, on the place of habitation, food,
wide range, and extinction of the numerous gigantic mammifers, which at
this late period inhabited the two Americas.
The nature and grouping of the shells embedded in the old tertiary
formations of Patagonia and Chile show us, that the continent at that
period must have stood only a few fathoms below its present level, and
that afterwards it subsided over a wide area, seven or eight hundred
feet. The manner in which it has since been rebrought up to its actual
level, was described in detail in the First and Second Chapters. It was
there shown that recent shells are found on the shores of the Atlantic,
from Tierra del Fuego northward for a space of at least 1,180 nautical
miles, and at the height of about 100 feet in La Plata, and of 400 feet
in Patagonia. The elevatory movements on this side of the continent
have been slow; and the coast of Patagonia, up to the height in one
part of 950 feet and in another of 1,200 feet, is modelled into eight
great, step-like, gravel-capped plains, extending for hundreds of miles
with the same heights; this fact shows that the periods of denudation
(which, judging from the amount of matter removed, must have been long
continued) and of elevation were synchronous over surprisingly great
lengths of coasts. On the shores of the Pacific, upraised shells of
recent species, generally, though not always, in the same proportional
numbers as in the adjoining sea, have actually been found over a north
and south space of 2,075 miles, and there is reason to believe that
they occur over a space of 2,480 miles. The elevation on this western
side of the continent has not been equable; at Valparaiso, within the
period during which upraised shells have remained undecayed on the
surface, it has been 1,300 feet, whilst at Coquimbo, 200 miles
northward, it has been within this same period only 252 feet. At Lima,
the land has been uplifted at least 80 feet since Indian man inhabited
that district; but the level within historical times apparently has
subsided. At Coquimbo, in a height of 364 feet, the elevation has been
interrupted by five periods of comparative rest. At several places the
land has been lately, or still is, rising both insensibly and by sudden
starts of a few feet during earthquake-shocks; this shows that these
two kinds of upward movement are intimately connected together. For a
space of 775 miles, upraised recent shells are found on the two
opposite sides of the continent; and in the southern half of this
space, it may be safely inferred from the slope of the land up to the
Cordillera, and from the shells found in the central part of Tierra del
Fuego, and high up the River Santa Cruz, that the entire breadth of the
continent has been uplifted. From the general occurrence on both coasts
of successive lines of escarpments, of sand-dunes and marks of erosion,
we must conclude that the elevatory movement has been normally
interrupted by periods, when the land either was stationary, or when it
rose at so slow a rate as not to resist the average denuding power of
the waves, or when it subsided. In the case of the present high
sea-cliffs of Patagonia and in other analogous instances, we have seen
that the difficulty in understanding how strata can be removed at those
depths under the sea, at which the currents and oscillations of the
water are depositing a smooth surface of mud, sand, and sifted pebbles,
leads to the suspicion that the formation or denudation of such cliffs
has been accompanied by a sinking movement.
In South America, everything has taken place on a grand scale, and all
geological phenomena are still in active operation. We know how violent
at the present day the earthquakes are, we have seen how great
an area is now rising, and the plains of tertiary origin are of vast
dimensions; an almost straight line can be drawn from Tierra del Fuego
for 1,600 miles northward, and probably for a much greater distance,
which shall intersect no formation older than the Patagonian deposits;
so equable has been the upheaval of the beds, that throughout this long
line, not a fault in the stratification or abrupt dislocation was
anywhere observable. Looking to the basal, metamorphic, and plutonic
rocks of the continent, the areas formed of them are likewise vast; and
their planes of cleavage and foliation strike over surprisingly great
spaces in uniform directions. The Cordillera, with its pinnacles here
and there rising upwards of twenty thousand feet above the level of the
sea, ranges in an unbroken line from Tierra del Fuego, apparently to
the Arctic circle. This grand range has suffered both the most violent
dislocations, and slow, though grand, upward and downward movements in
mass; I know not whether the spectacle of its immense valleys, with
mountain-masses of once liquified and intrusive rocks now bared and
intersected, or whether the view of those plains, composed of shingle
and sediment hence derived, which stretch to the borders of the
Atlantic Ocean, is best adapted to excite our astonishment at the
amount of wear and tear which these mountains have undergone.
The Cordillera from Tierra del Fuego to Mexico, is penetrated by
volcanic orifices, and those now in action are connected in great
trains. The intimate relation between their recent eruptions and the
slow elevation of the continent in mass,[20] appears to me highly
important, for no explanation of the one phenomenon can be considered
as satisfactory which is not applicable to the other. The permanence of
the volcanic action on this chain of mountains is, also, a striking
fact; first, we have the deluges of submarine lavas alternating with
the porphyritic conglomerate strata, then occasionally feldspathic
streams and abundant mineral exhalations during the gypseous or
cretaceo-oolitic period: then the eruptions of the Uspallata range, and
at an ancient but unknown period, when the sea came up to the eastern
foot of the Cordillera, streams of basaltic lava at the foot of the
Portillo range; then the old tertiary eruptions; and lastly, there are
here and there amongst the mountains, much worn and apparently very
ancient volcanic formations without any craters; there are, also,
craters quite extinct, and others in the condition of solfataras, and
others occasionally or habitually in fierce action. Hence it would
appear that the Cordillera has been, probably with some quiescent
periods, a source of volcanic matter from an epoch anterior to our
cretaceo-oolitic formation to the present day; and now the earthquakes,
daily recurrent on some part of the western coast, give little hope
that the subterranean energy is expended.
[20] On the Connection of certain Volcanic Phenomena in South America:
“Geolog. Transact.,” vol. v, p. 609.