|
Chapter VIII
NORTHERN CHILE. CONCLUSION.
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 degrees 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, generally embedded in
sulphate of barytes. (See the Report on M. Domeyko's account of those
mines, in the "Comptes Rendus" tome 14 page 560.) 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.
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 Gryphaea, 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 degrees 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 degrees, 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 aenigma, d'Orbigny, "Voyage" etc. (Pl. 22 Figures 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.
tetraedra, and that the varieties amongst them are such as are found in
that variable species. Generally speaking, the American specimens of T.
aenigma may be distinguished from the British T. tetraedra, 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 aenigma, 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. aenigma. 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 Terebratulae. 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 Gryphaea appears to Professor Forbes to be
identical with a species, associated in Southern India with unquestionably
cretaceous forms. On the other hand, the two Terebratulae 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. aenigma 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 Terebratulae 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, Gryphaea, Ostrea, Pecten,
Turritella, Nautilus, two Terebratulae, 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 (D'Orbigny "Voyage" Part Geolog. page 242.);--both of which
genera occur at the Puente del Inca.
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 aenigma, d'Orbigny, "Voyage" Part Pal.
This shell M. d'Orbigny does not consider identical with his T. aenigma,
but near to T. obsoleta. Professor Forbes thinks that it is certainly a
variety of T. aenigma: 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
Terebratulae, 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, Section 1/3 in Plate 1, 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, of sulphate of iron
under two forms, and of the sulphates of copper and alumina (Meyen's
"Reise" etc. Th. 1, s. 394.): 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.
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 degrees. 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 LOS 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 degrees. 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.
Gryphaea Darwinii, E. Forbes.
An intermediate form between G. gigantea and G. incurva.
Gryphaea 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 aenigma, 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. aenigma, and related to T. obsoleta.
Plagiostoma and Ammonites, fragments of.
The lower layers of the limestone contained thousands of the Gryphaea; and
the upper ones as many of the Turritella, with the Gryphaea (nov. species)
and Serpulae 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 aenigma 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. aenigma, 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 aenigma.
Spirifer linguiferoides.
Same bed, near Arqueros:
Hippurites Chilensis.
Gryphaea orientalis.
Collected by M. Domeyko from the same locality, apparently near Arqueros:
Terebratula aenigma 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 aenigma, 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 aenigma, var. as at Guasco.
Astarte Darwinii.
Gryphaea Darwinii.
Gryphaea nov. species?
Perna Americana.
Avicula, nov. species.
Main valley of Copiapo, apparently same formation with that of Amolanas:
Terebratula aenigma (true).
In the same bed, high up the great lateral valley of the Despoblado, in the
ravine of Maricongo:
Terebratula aenigma (true).
Pecten Dufreynoyi.
Gryphaea 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, Gryphaea 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 Gryphaea
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 aenigma,
var., and some Gryphites were embedded in these layers. The sandstone
varies in thickness from only twenty to eighty feet; and this variation is
caused by the inequalities in the upper surface of an underlying stream of
purple claystone porphyry. Hence the above fossils here lie at the very
base of the gypseous or cretaceo-oolitic formation, and hence they were
probably once covered up by strata about seven thousand feet in thickness:
it is, however, possible, though from the nature of all the other sections
in this district not probable, that the porphyritic claystone lava may in
this case have invaded a higher level in the series. Above the sandstone
there is a considerable mass of much indurated, purplish-black, calcareous
claystone, allied in nature to the often-mentioned black calcareous slate-
rock.
Eastward of the broad andesitic axis of this sixth line, and penetrated by
many dike
< Back
|
