the strata at [H] form a part) ranges N. by W. and S. by E., forming a
chain of mountains, apparently little inferior in height to the Cumbre:
the strata, as we have seen, dip at an average angle of 30 degrees to
the west. (At this place, there are some hot and cold springs, the
warmest having a temperature, according to Lieutenant Brand “Travels,”
page 240, of 91 degrees; they emit much gas. According to Mr. Brande,
of the Royal Institution, ten cubical inches contain forty-five grains
of solid matter, consisting chiefly of salt, gypsum, carbonate of lime,
and oxide of iron. The water is charged with carbonic acid and
sulphuretted hydrogen. These springs deposit much tufa in the form of
spherical balls. They burst forth, as do those of Cauquenes, and
probably those of Villa Vicencio, on a line of elevation.) The flanks
of the mountains are here quite bare and steep, affording an excellent
section; so that I was able to inspect the strata to a thickness of
about 4,000 feet, and could clearly distinguish their general nature
for 1,000 feet higher, making a total thickness of 5,000 feet, to which
must be added about 1,000 feet of the inferior strata seen a little
lower down the valley, I will describe this one section in detail,
beginning at the bottom.
1st. The lowest mass is the altered clay-slate described in the
preliminary discussion, and which in this line of section was here
first met with. Lower down the valley, at the R. de las Vacas, I had a
better opportunity of examining it; it is there in some parts well
characterised, having a distinct, nearly vertical, tortuous cleavage,
ranging N.W. and S.E., and intersected by quartz veins: in most parts,
however, it is crystalline and feldspathic, and passes into a true
greenstone often including grains of quartz. The clay-slate, in its
upper half, is frequently brecciated, the embedded angular fragments
being of nearly the same nature with the paste.
2nd. Several strata of purplish porphyritic conglomerate, of no very
great thickness, rest conformably upon the feldspathic slate. A thick
bed of fine, purple, claystone porphyry, obscurely brecciated (but not
of metamorphosed sedimentary origin), and capped by porphyritic
conglomerate, was the lowest bed actually examined in this section at
the Puente del Inca.
3rd. A stratum, eighty feet thick, of hard and very compact impure
whitish limestone, weathering bright red, with included layers
brecciated and re- cemented. Obscure marks of shell are distinguishable
in it.
4th. A red, quartzose, fine-grained conglomerate, with grains of
quartz, and with patches of white earthy feldspar, apparently due to
some process of concretionary crystalline action; this bed is more
compact and metamorphosed than any of the overlying conglomerates.
5th. A whitish cherty limestone, with nodules of bluish argillaceous
limestone.
6th. A white conglomerate, with many particles of quartz, almost
blending into the paste.
7th. Highly siliceous, fine-grained white sandstone.
8th and 9th. Red and white beds not examined.
10th. Yellow, fine-grained, thinly stratified, magnesian (judging from
its slow dissolution in acids) limestone: it includes some white quartz
pebbles, and little cavities, lined with calcareous spar, some
retaining the form of shells.
11th. A bed between twenty and thirty feet thick, quite conformable
with the underlying ones, composed of a hard basis, tinged lilac-grey
porphyritic with NUMEROUS crystals of whitish feldspar, with black mica
and little spots of soft ferruginous matter: evidently a submarine
lava.
12th. Yellow magnesian limestone, as before, part-stained purple.
13th. A most singular rock; basis purplish grey, obscurely crystalline,
easily fusible into a dark green glass, not hard, thickly speckled with
crystals more or less perfect of white carbonate of lime, of red
hydrous oxide of iron, of a white and transparent mineral like
analcime, and of a green opaque mineral like soap-stone; the basis is
moreover amygdaloidal with many spherical balls of white crystallised
carbonate of lime, of which some are coated with the red oxide of iron.
I have no doubt, from the examination of a superincumbent stratum (19),
that this is a submarine lava; though in Northern Chile, some of the
metamorphosed sedimentary beds are almost as crystalline, and of as
varied composition.
14th. Red sandstone, passing in the upper part into a coarse, hard, red
conglomerate, 300 feet thick, having a calcareous cement, and including
grains of quartz and broken crystals of feldspar; basis infusible; the
pebbles consist of dull purplish porphyries, with some of quartz, from
the size of a nut to a man’s head. This is the coarsest conglomerate in
this part of the Cordillera: in the middle there was a white layer not
examined.
15th. Grand thick bed, of a very hard, yellowish-white rock, with a
crystalline feldspathic base, including large crystals of white
feldspar, many little cavities mostly full of soft ferruginous matter,
and numerous hexagonal plates of black mica. The upper part of this
great bed is slightly cellular; the lower part compact: the thickness
varied a little in different parts. Manifestly a submarine lava; and is
allied to bed 11.
16th and 17th. Dull purplish, calcareous, fine-grained, compact
sandstones, which pass into coarse white conglomerates with numerous
particles of quartz.
18th. Several alternations of red conglomerate, purplish sandstone, and
submarine lava, like that singular rock forming bed 13.
19th. A very heavy, compact, greenish-black stone, with a fine-grained
obviously crystalline basis, containing a few specks of white
calcareous spar, many specks of the crystallised hydrous red oxide of
iron, and some specks of a green mineral; there are veins and nests
filled with epidote: certainly a submarine lava.
20th. Many thin strata of compact, fine-grained, pale purple sandstone.
21st. Gypsum in a nearly pure state, about three hundred feet in
thickness: this bed, in its concretions of anhydrite and layers of
small blackish crystals of carbonate of lime, exactly resembles the
great gypseous beds in the Peuquenes range.
22nd. Pale purple and reddish sandstone, as in bed 20: about three
hundred feet in thickness.
23rd. A thick mass composed of layers, often as thin as paper and
convoluted, of pure gypsum with others very impure, of a purplish
colour.
24th. Pure gypsum, thick mass.
25th. Red sandstones, of great thickness.
26th. Pure gypsum, of great thickness.
27th. Alternating layers of pure and impure gypsum, of great thickness.
I was not able to ascend to these few last great strata, which compose
the neighbouring loftiest pinnacles. The thickness, from the lowest to
the uppermost bed of gypsum, cannot be less than 2,000 feet: the beds
beneath I estimated at 3,000 feet, and this does not include either the
lower parts of the porphyritic conglomerate, or the altered clay-slate;
I conceive the total thickness must be about six thousand feet. I
distinctly observed that not only the gypsum, but the alternating
sandstones and conglomerates were lens-shaped, and repeatedly thinned
out and replaced each other: thus in the distance of about a mile, a
bed 300 feet thick of sandstone between two beds of gypsum, thinned out
to nothing and disappeared. The lower part of this section differs
remarkably,—in the much greater diversity of its mineralogical
composition,—in the abundance of calcareous matter,—in the greater
coarseness of some of the conglomerates,—and in the numerous particles
and well-rounded pebbles, sometimes of large size, of quartz,— from any
other section hitherto described in Chile. From these peculiarities and
from the lens-form of the strata, it is probable that this great pile
of strata was accumulated on a shallow and very uneven bottom, near
some pre-existing land formed of various porphyries and quartz-rock.
The formation of porphyritic claystone conglomerate does not in this
section attain nearly its ordinary thickness; this may be PARTLY
attributed to the metamorphic action having been here much less
energetic than usual, though the lower beds have been affected to a
certain degree. If it had been as energetic as in most other parts of
Chile, many of the beds of sandstone and conglomerate, containing
rounded masses of porphyry, would doubtless have been converted into
porphyritic conglomerate; and these would have alternated with, and
even blended into, crystalline and porphyritic strata without a trace
of mechanical structure,—namely, into those which, in the present state
of the section, we see are unquestionably submarine lavas.
The beds of gypsum, together with the red alternating sandstones and
conglomerates, present so perfect and curious a resemblance with those
seen in our former section in the basin-valley of Yeso, that I cannot
doubt the identity of the two formations: I may add, that a little
westward of the P. del Inca, a mass of gypsum passed into a
fine-grained, hard, brown sandstone, which contained some layers of
black, calcareous, compact, shaly rock, precisely like that seen in
such vast masses on the Peuquenes range.
Near the Puente del Inca, numerous fragments of limestone, containing
some fossil remains, were scattered on the ground: these fragments so
perfectly resemble the limestone of bed No. 3, in which I saw
impressions of shells, that I have no doubt they have fallen from it.
The yellow magnesian limestone of bed No. 10, which also includes
traces of shells, has a different appearance. These fossils (as named
by M. d’Orbigny) consist of:—
Gryphaea, near to G. Couloni (Neocomian formation). Arca, perhaps A.
Gabrielis, d’Orbigny, “Pal. Franc.” (Neocomian formation).
Mr. Pentland made a collection of shells from this same spot, and Von
Buch considers them as consisting of:—
Trigonia, resembling in form T. costata. Pholadomya, like one found by
M. Dufresnoy near Alencon. Isocardi excentrica, Voltz., identical with
that from the Jura. (“Description Phys. des Iles Can.” page 472.)
Two of these shells, namely, the Gryphaea and Trigonia, appear to be
identical with species collected by Meyen and myself on the Peuquenes
range; and in the opinion of Von Buch and M. d’Orbigny, the two
formations belong to the same age. I must here add, that Professor E.
Forbes, who has examined my specimens from this place and from the
Peuquenes range, has likewise a strong impression that they indicate
the Cretaceous period, and probably an early epoch in it: so that all
the palaeontologists who have seen these fossils nearly coincide in
opinion regarding their age. The limestone, however, with these fossils
here lies at the very base of the formation, just above the porphyritic
conglomerate, and certainly several thousand feet lower in the series,
than the equivalent, fossiliferous, black, shaly rocks high up on the
Peuquenes range.
It is well worthy of remark that these shells, or at least those of
which I saw impressions in the limestone (bed No. 3), must have been
covered up, on the LEAST computation, by 4,000 feet of strata: now we
know from Professor E. Forbes’s researches, that the sea at greater
depths than 600 feet becomes exceedingly barren of organic beings,—a
result quite in accordance with what little I have seen of deep-sea
soundings. Hence, after this limestone with its shells was deposited,
the bottom of the sea where the main line of the Cordillera now stands,
must have subsided some thousand feet to allow of the deposition of the
superincumbent submarine strata. Without supposing a movement of this
kind, it would, moreover, be impossible to understand the accumulation
of the several lower strata of COARSE, well-rounded conglomerates,
which it is scarcely possible to believe were spread out in profoundly
deep water, and which, especially those containing pebbles of quartz,
could hardly have been rounded in submarine craters and afterwards
ejected from them, as I believe to have been the case with much of the
porphyritic conglomerate formation. I may add that, in Professor
Forbes’s opinion, the above-enumerated species of mollusca probably did
not live at a much greater depth than twenty fathoms, that is only 120
feet.
To return to our section down the valley; standing on the great N. by
W. and S. by E. uniclinal axis of the Puente del Inca, of which a
section has just been given, and looking north-east, greater tabular
masses of gypseous formation (KK) could be seen in the distance, very
slightly inclined towards the east. Lower down the valley, the
mountains are almost exclusively composed of porphyries, many of them
of intrusive origin and non-stratified, others stratified, but with the
stratification seldom distinguishable except in the upper parts.
Disregarding local disturbances, the beds are either horizontal or
inclined at a small angle eastwards: hence, when standing on the plain
of Uspallata and looking to the west or backwards, the Cordillera
appear composed of huge, square, nearly horizontal, tabular masses: so
wide a space, with such lofty mountains so equably elevated, is rarely
met with within the Cordillera. In this line of section, the interval
between the Puente del Inca and the neighbourhood of the Cumbre,
includes all the chief axes of dislocation.
The altered clay-slate formation, already described, is seen in several
parts of the valley as far down as Las Vacas, underlying the
porphyritic conglomerate. At the Casa de Pujios [L], there is a hummock
of (andesitic?) granite; and the stratification of the surrounding
mountains here changes from W. by S. to S.W. Again, near the R. Vacas
there is a larger formation of (andesitic?) granite [M], which sends a
meshwork of veins into the superincumbent clay-slate, and which locally
throws off the strata, on one side to N.W. and on the other to S.E. but
not at a high angle: at the junction, the clay-slate is altered into
fine-grained greenstone. This granitic axis is intersected by a green
dike, which I mention, because I do not remember having elsewhere seen
dikes in this lowest and latest intrusive rock. From the R. Vacas to
the plain of Uspallata, the valley runs N.E., so that I have had to
contract my section; it runs exclusively through porphyritic rocks. As
far as the Pass of Jaula, the claystone conglomerate formation, in most
parts highly porphyritic, and crossed by numerous dikes of greenstone
porphyry, attains a great thickness: there is also much intrusive
porphyry. From the Jaula to the plain, the stratification has been in
most places obliterated, except near the tops of some of the mountains;
and the metamorphic action has been extremely great. In this space, the
number and bulk of the intrusive masses of differently coloured
porphyries, injected one into another and intersected by dikes, is
truly extraordinary. I saw one mountain of whitish porphyry, from which
two huge dikes, thinning out, branched DOWNWARDS into an adjoining
blackish porphyry. Another hill of white porphyry, which had burst
through dark- coloured strata, was itself injected by a purple,
brecciated, and recemented porphyry, both being crossed by a green
dike, and both having been upheaved and injected by a granitic dome.
One brick-red porphyry, which above the Jaula forms an isolated mass in
the midst of the porphyritic conglomerate formation, and lower down the
valley a magnificent group of peaked mountains, differs remarkably from
all the other porphyries. It consists of a red feldspathic base,
including some rather large crystals of red feldspar, numerous large
angular grains of quartz, and little bits of a soft green mineral
answering in most of its characters to soapstone. The crystals of red
feldspar resemble in external appearance those of orthite, though, from
being partially decomposed, I was unable to measure them; and they
certainly are quite unlike the variety, so abundantly met with in
almost all the other rocks of this line of section, and which, wherever
I tried it, cleaved like albite. This brick-red porphyry appears to
have burst through all the other porphyries, and numerous red dikes
traversing the neighbouring mountains have proceeded from it: in some
few places, however, it was intersected by white dikes. From this
posteriority of intrusive origin,—from the close general resemblance
between this red porphyry and the red granite of the Portillo line, the
only difference being that the feldspar here is less perfectly
granular, and that soapstone replaces the mica, which is there
imperfect and passes into chlorite,—and from the Portillo line a little
southward of this point appearing to blend (according to Dr. Gillies)
into the western ranges,—I am strongly urged to believe (as formerly
remarked) that the grand mountain-masses composed of this brick-red
porphyry belong to the same axis of injection with the granite of the
Portillo line; if so, the injection of this porphyry probably took
place, as long subsequently to the several axes of elevation in the
gypseous formation near the Cumbre, as the injection of the Portillo
granite has been shown to have been subsequent to the elevation of the
gypseous strata composing the Peuquenes range; and this interval, we
have seen, must have been a very long one.
The Plain of Uspallata has been briefly described in Chapter 3; it
resembles the basin-plains of Chile; it is ten or fifteen miles wide,
and is said to extend for 180 miles northward; its surface is nearly
six thousand feet above the sea; it is composed, to a thickness of some
hundred feet of loosely aggregated, stratified shingle, which is
prolonged with a gently sloping surface up the valleys in the mountains
on both sides. One section in this plain [Z] is interesting, from the
unusual circumstance of alternating layers of almost loose red and
white sand with lines of pebbles (from the size of a nut to that of an
apple), and beds of gravel, being inclined at an angle of 45 degrees,
and in some spots even at a higher angle. (I find that Mr. Smith of
Jordan Hill has described (“Edinburgh New Philosophical Journal” volume
25 page 392) beds of sand and gravel, near Edinburgh, tilted at an
angle of 60 degrees, and dislocated by miniature faults.) These beds
are dislocated by small faults: and are capped by a thick mass of
horizontally stratified gravel, evidently of subaqueous origin. Having
been accustomed to observe the irregularities of beds accumulated under
currents, I feel sure that the inclination here has not been thus
produced. The pebbles consist chiefly of the brick-red porphyry just
described and of white granite, both probably derived from the ranges
to the west, and of altered clay-slate and of certain porphyries,
apparently belonging to the rocks of the Uspallata chain. This plain
corresponds geographically with the valley of Tenuyan between the
Portillo and Peuquenes ranges; but in that valley the shingle, which
likewise has been derived both from the eastern and western ranges, has
been cemented into a hard conglomerate, and has been throughout tilted
at a considerable inclination; the gravel there apparently attains a
much greater thickness, and is probably of higher antiquity.
THE USPALLATA RANGE.
The road by the Villa Vicencio Pass does not strike directly across the
range, but runs for some leagues northward along its western base: and
I must briefly describe the rocks here seen, before continuing with the
coloured east and west section. At the mouth of the valley of Canota,
and at several points northwards, there is an extensive formation of a
glossy and harsh, and of a feldspathic clay-slate, including strata of
grauwacke, and having a tortuous, nearly vertical cleavage, traversed
by numerous metalliferous veins and others of quartz. The clay-slate is
in many parts capped by a thick mass of fragments of the same rock,
firmly recemented; and both together have been injected and broken up
by very numerous hillocks, ranging north and south, of lilac, white,
dark and salmon- coloured porphyries: one steep, now denuded, hillock
of porphyry had its face as distinctly impressed with the angles of a
fragmentary mass of the slate, with some of the points still remaining
embedded, as sealing-wax could be by a seal. At the mouth of this same
valley of Canota, in a fine escarpment having the strata dipping from
50 to 60 degrees to the N.E. (Nearly opposite to this escarpment, there
is another corresponding one, with the strata dipping not to the
exactly opposite point, or S.W., but to S.S.W.: consequently the two
escarpments trend towards each other, and some miles southward they
become actually united: this is a form of elevation which I have not
elsewhere seen.), the clay-slate formation is seen to be covered
by—(1st) a purple, claystone porphyry resting unconformably in some
parts on the solid slate, and in others on a thick fragmentary mass;
(2nd), a conformable stratum of compact blackish rock, having a
spheroidal structure, full of minute acicular crystals of glassy
feldspar, with red spots of oxide of iron; (3rd), a great stratum of
purplish-red claystone porphyry, abounding with crystals of opaque
feldspar, and laminated with thin, parallel, often short, layers, and
likewise with great irregular patches of white, earthy,
semi-crystalline feldspar; this rock (which I noticed in other
neighbouring places) perfectly resembles a curious variety described at
Port Desire, and occasionally occurs in the great porphyritic
conglomerate formation of Chile; (4th), a thin stratum of greenish
white, indurated tuff, fusible and containing broken crystals and
particles of porphyries; (5th), a grand mass, imperfectly columnar and
divided into three parallel and closely joined strata, of
cream-coloured claystone porphyry; (6th), a thick stratum of
lilac-coloured porphyry, which I could see was capped by another bed of
the cream-coloured variety; I was unable to examine the still higher
parts of the escarpment. These conformably stratified porphyries,
though none are either vesicular are amygdaloidal, have evidently
flowed as submarine lavas: some of them are separated from each other
by seams of indurated tuff, which, however, are quite insignificant in
thickness compared with the porphyries. This whole pile resembles, but
not very closely, some of the less brecciated parts of the great
porphyritic conglomerate formation of Chile; but it does not probably
belong to the same age, as the porphyries here rest unconformably on
the altered feldspathic clay-slate, whereas the porphyritic
conglomerate formation alternates with and rests conformably on it.
These porphyries, moreover, with the exception of the one blackish
stratum, and of the one indurated, white tufaceous bed, differ from the
beds composing the Uspallata range in the line of the Villa Vicencio
Pass.
I will now give, first, a sketch of the structure of the range, as
represented in the section, and will then describe its composition and
interesting history. At its western foot, a hillock [N] is seen to rise
out of the plain, with its strata dipping at 70 degrees to the west,
fronted by strata [O] inclined at 45 degrees to the east, thus forming
a little north and south anticlinal axis. Some other little hillocks of
similar composition, with their strata highly inclined, range N.E. and
S.W., obliquely to the main Uspallata line. The cause of these
dislocations, which, though on a small scale, have been violent and
complicated, is seen to lie in hummocks of lilac, purple and red
porphyries, which have been injected in a liquified state through and
into the underlying clay-slate formation. Several dykes were exposed
here, but in no other part, that I saw of this range. As the strata
consist of black, white, greenish and brown-coloured rocks, and as the
intrusive porphyries are so brightly tinted, a most extraordinary view
was presented, like a coloured geological drawing. On the gently
inclined main western slope [PP], above the little anticlinal ridges
just mentioned, the strata dip at an average angle of 25 degrees to the
west; the inclination in some places being only 19 degrees, in some few
others as much as 45 degrees. The masses having these different
inclinations, are separated from each other by parallel vertical faults
[as represented at Pa], often giving rise to separate, parallel,
uniclinal ridges. The summit of the main range is broad and undulatory,
with the stratification undulatory and irregular: in a few places
granitic and porphyritic masses [Q] protrude, which, from the small
effect they have locally produced in deranging the strata, probably
form the upper points of a regular, great underlying dome. These
denuded granitic points, I estimated at about nine thousand feet in
height above the sea. On the eastern slope, the strata in the upper
part are regularly inclined at about 25 degrees to the east, so that
the summit of this chain, neglecting small irregularities, forms a
broad anticlinal axis. Lower down, however, near Los Hornillos [R],
there is a well-marked synclinal axis, beyond which the strata are
inclined at nearly the same angle, namely from 20 to 30 degrees,
inwards or westward. Owing to the amount of denudation which this chain
has suffered, the outline of the gently inclined eastern flank scarcely
offers the slightest indication of this synclinal axis. The stratified
beds, which we have hitherto followed across the range, a little
further down are seen to lie, I believe unconformably, on a broad
mountainous band of clay-slate and grauwacke. The strata and laminae of
this latter formation, on the extreme eastern flank, are generally
nearly vertical; further inwards they become inclined from 45 to 80
degrees to the west: near Villa Vicencio [S] there is apparently an
anticlinal axis, but the structure of this outer part of the clay-slate
formation is so obscure, that I have not marked the planes of
stratification in the section. On the margin of the Pampas, some low,
much dislocated spurs of this same formation, project in a north-
easterly line, in the same oblique manner as do the ridges on the
western foot, and as is so frequently the case with those at the base
of the main Cordillera.
I will now describe the nature of the beds, beginning at the base on
the eastern side. First, for the clay-slate formation: the slate is
generally hard and bluish, with the laminae coated by minute micaceous
scales; it alternates many times with a coarse-grained, greenish
grauwacke, containing rounded fragments of quartz and bits of slate in
a slightly calcareous basis. The slate in the upper part generally
becomes purplish, and the cleavage so irregular that the whole consists
of mere splinters. Transverse veins of quartz are numerous. At the
Calera, some leagues distant, there is a dark crystalline limestone,
apparently included in this formation. With the exception of the
grauwacke being here more abundant, and the clay-slate less altered,
this formation closely resembles that unconformably underlying the
porphyries at the western foot of this same range; and likewise that
alternating with the porphyritic conglomerate in the main Cordillera.
This formation is a considerable one, and extends several leagues
southward to near Mendoza: the mountains composed of it rise to a
height of about two thousand feet above the edge of the Pampas, or
about seven thousand feet above the sea. (I infer this from the height
of V. Vicencio, which was ascertained by Mr. Miers to be 5,328 feet
above the sea.)
Secondly: the most usual bed on the clay-slate is a coarse, white,
slightly calcareous conglomerate, of no great thickness, including
broken crystals of feldspar, grains of quartz, and numerous pebbles of
brecciated claystone porphyry, but without any pebbles of the
underlying clay-slate. I nowhere saw the actual junction between this
bed and the clay-slate, though I spent a whole day in endeavouring to
discover their relations. In some places I distinctly saw the white
conglomerate and overlying beds inclined at from 25 to 30 degrees to
the west, and at the bottom of the same mountain, the clay-slate and
grauwacke inclined to the same point, but at an angle from 70 to 80
degrees: in one instance, the clay-slate dipped not only at a different
angle, but to a different point from the overlying formation. In these
cases the two formations certainly appeared quite unconformable:
moreover, I found in the clay-slate one great, vertical, dike-like
fissure, filled up with an indurated whitish tuff, quite similar to
some of the upper beds presently to be described; and this shows that
the clay-slate must have been consolidated and dislocated before their
deposition. On the other hand, the stratification of the slate and
grauwacke, in some cases gradually and entirely disappeared in
approaching the overlying white conglomerate; in other cases the
stratification of the two formations became strictly conformable; and
again in other cases, there was some tolerably well characterised
clay-slate lying above the conglomerate. (The coarse, mechanical
structure of many grauwackes has always appeared to me a difficulty;
for the texture of the associated clay-slate and the nature of the
embedded organic remains where present, indicate that the whole has
been a deep-water deposit. Whence have the sometimes included angular
fragments of clay-slate, and the rounded masses of quartz and other
rocks, been derived? Many deep-water limestones, it is well known, have
been brecciated, and then firmly recemented.) The most probable
conclusion appears to be, that after the clay-slate formation had been
dislocated and tilted, but whilst under the sea, a fresh and more
recent deposition of clay-slate took place, on which the white
conglomerate was conformably deposited, with here and there a thin
intercalated bed of clay-slate. On this view the white conglomerates
and the presently to be described tuffs and lavas are really
unconformable to the main part of the clay-slate; and this, as we have
seen, certainly is the case with the clay-stone lavas in the valley of
Canota, at the western and opposite base of the range.
Thirdly: on the white conglomerate, strata several hundred feet in
thickness are superimposed, varying much in nature in short distances:
the commonest variety is a white, much indurated tuff, sometimes
slightly calcareous, with ferruginous spots and water-lines, often
passing into whitish or purplish compact, fine-grained grit or
sandstones; other varieties become semi-porcellanic, and tinted faint
green or blue; others pass into an indurated shale: most of these
varieties are easily fusible.
Fourthly: a bed, about one hundred feet thick of a compact, partially
columnar, pale-grey, feldspathic lava, stained with iron, including
very numerous crystals of opaque feldspar, and with some crystallised
and disseminated calcareous matter. The tufaceous stratum on which this
feldspathic lava rests is much hardened, stained purple, and has a
spherico-concretionary structure; it here contains a good many pebbles
of claystone porphyry.
Fifthly: thin beds, 400 feet in thickness, varying much in nature,
consisting of white and ferruginous tuffs, in some parts having a
concretionary structure, in others containing rounded grains and a few
pebbles of quartz; also passing into hard gritstones and into greenish
mudstones: there is, also, much of a bluish-grey and green
semi-porcellanic stone.
Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a
nature that I do not believe a score of specimens would show all the
varieties; much is highly amygdaloidal, much compact; there are
greenish, blackish, purplish, and grey varieties, rarely including
crystals of green augite and minute acicular ones of feldspar, but
often crystals and amygdaloidal masses of white, red, and black
carbonate of lime. Some of the blackish varieties of this rock have a
conchoidal fracture and resemble basalt; others have an irregular
fracture. Some of the grey and purplish varieties are thickly speckled
with green earth and with white crystalline carbonate of lime; others
are largely amygdaloidal with green earth and calcareous spar. Again,
other earthy varieties, of greenish, purplish and grey tints, contain
much iron, and are almost half composed of amygdaloidal balls of dark
brown bole, of a whitish indurated feldspathic matter, of bright green
earth, of agate, and of black and white crystallised carbonate of lime.
All these varieties are easily fusible. Viewed from a distance, the
line of junction with the underlying semi-porcellanic strata was
distinct; but when examined closely, it was impossible to point out
within a foot where the lava ended and where the sedimentary mass
began: the rock at the time of junction was in most places hard, of a
bright green colour, and abounded with irregular amygdaloidal masses of
ferruginous and pure calcareous spar, and of agate.
Seventhly: strata, eighty feet in thickness, of various indurated
tuffs, as before; many of the varieties have a fine basis including
rather coarse extraneous particles; some of them are compact and
semi-porcellanic, and include vegetable impressions.
Eighthly: a bed, about fifty feet thick, of greenish-grey, compact,
feldspathic lava, with numerous small crystals of opaque feldspar,
black augite, and oxide of iron. The junction with the bed on which it
rested, was ill defined; balls and masses of the feldspathic rock being
enclosed in much altered tuff.
Ninthly: indurated tuffs, as before.
Tenthly: a conformable layer, less than two feet in thickness, of
pitchstone, generally brecciated, and traversed by veins of agate and
of carbonate of lime: parts are composed of apparently concretionary
fragments of a more perfect variety, arranged in horizontal lines in a
less perfectly characterised variety. I have much difficulty in
believing that this thin layer of pitchstone flowed as lava.
Eleventhly: sedimentary and tufaceous beds as before, passing into
sandstone, including some conglomerate: the pebbles in the latter are
of claystone porphyry, well rounded, and some as large as
cricket-balls.
Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of
bed No. 8, divided by numerous joints into large angular blocks.
Thirteenthly: sedimentary beds as before.
Fourteenthly: a thick bed of greenish or greyish black, compact basalt
(fusing into a black enamel), with small crystals, occasionally
distinguishable, of feldspar and augite: the junction with the
underlying sedimentary bed, differently from that in most of the
foregoing streams, here was quite distinct:—the lava and tufaceous
matter preserving their perfect characters within two inches of each
other. This rock closely resembles certain parts of that varied and
singular lava-stream No. 6; it likewise resembles, as we shall
immediately see, many of the great upper beds on the western flank and
on the summit of this range.
The pile of strata here described attains a great thickness; and above
the last-mentioned volcanic stratum, there were several other great
tufaceous beds alternating with submarine lavas, which I had not time
to examine; but a corresponding series, several thousand feet in
thickness, is well exhibited on the crest and western flank of the
range. Most of the lava- streams on the western side are of a jet-black
colour and basaltic nature; they are either compact and fine-grained,
including minute crystals of augite and feldspar, or they are
coarse-grained and abound with rather large coppery-brown crystals of
an augitic mineral. (Very easily fusible into a jet-black bead,
attracted by the magnet: the crystals are too much tarnished to be
measured by the goniometer.) Another variety was of a dull- red colour,
having a claystone brecciated basis, including specks of oxide of iron
and of calcareous spar, and amygdaloidal with green earth: there were
apparently several other varieties. These submarine lavas often exhibit
a spheroidal, and sometimes an imperfect columnar structure: their
upper junctions are much more clearly defined than their lower
junctions; but the latter are not so much blended into the underlying
sedimentary beds as is the case in the eastern flank. On the crest and
western flank of the range, the streams, viewed as a whole, are mostly
basaltic; whilst those on the eastern side, which stand lower in the
series, are, as we have seen, mostly feldspathic.
The sedimentary strata alternating with the lavas on the crest and
western side, are of an almost infinitely varying nature; but a large
proportion of them closely resemble those already described on the
eastern flank: there are white and brown, indurated, easily fusible
tuffs,—some passing into pale blue and green semi-porcellanic
rocks,—others into brownish and purplish sandstones and gritstones,
often including grains of quartz,— others into mudstone containing
broken crystals and particles of rock, and occasionally single large
pebbles. There was one stratum of a bright red, coarse, volcanic
gritstone; another of conglomerate; another of a black, indurated,
carbonaceous shale marked with imperfect vegetable impressions; this
latter bed, which was thin, rested on a submarine lava, and followed
all the considerable inequalities of its upper surface. Mr. Miers
states that coal has been found in this range. Lastly, there was a bed
(like No. 10 on the eastern flank) evidently of sedimentary origin, and
remarkable from closely approaching in character to an imperfect
pitchstone, and from including extremely thin layers of perfect
pitchstone, as well as nodules and irregular fragments (but not
resembling extraneous fragments) of this same rock arranged in
horizontal lines: I conceive that this bed, which is only a few feet in
thickness, must have assumed its present state through metamorphic and
concretionary action. Most of these sedimentary strata are much
indurated, and no doubt have been partially metamorphosed: many of them
are extraordinarily heavy and compact; others have agate and
crystalline carbonate of lime disseminated throughout them. Some of the
beds exhibit a singular concretionary arrangement, with the curves
determined by the lines of fissure. There are many veins of agate and
calcareous spar, and innumerable ones of iron and other metals, which
have blackened and curiously affected the strata to considerable
distances on both sides.
Many of these tufaceous beds resemble, with the exception of being more
indurated, the upper beds of the Great Patagonian tertiary formation,
especially those variously coloured layers high up the River Santa
Cruz, and in a remarkable degree the tufaceous formation at the
northern end of Chiloe. I was so much struck with this resemblance,
that I particularly looked out for silicified wood, and found it under
the following extraordinary circumstances. High up on this western
flank, at a height estimated at 7,000 feet above the sea, in a broken
escarpment of thin strata, composed of compact green gritstone passing
into a fine mudstone, and alternating with layers of coarser, brownish,
very heavy mudstone, including broken crystals and particles of rock
almost blended together, I counted the stumps of fifty-two trees. (For
the information of any future traveller, I will describe the spot in
detail. Proceeding eastward from the Agua del Zorro, and afterwards
leaving on the north side of the road a rancho attached to some old
goldmines, you pass through a gully with low but steep rocks on each
hand: the road then bends, and the ascent becomes steeper. A few
hundred yards farther on, a stone’s throw on the south side of the
road, the white calcareous stumps may be seen. The spot is about half a
mile east of the Agua del Zorro.) They projected between two and five
feet above the ground, and stood at exactly right angles to the strata,
which were here inclined at an angle of about 25 degrees to the west.
Eleven of these trees were silicified and well preserved; Mr. R. Brown
has been so kind as to examine the wood when sliced and polished; he
says it is coniferous, partaking of the characters of the Araucarian
tribe, with some curious points of affinity with the Yew. The bark
round the trunks must have been circularly furrowed with irregular
lines, for the mudstone round them is thus plainly marked. One cast
consisted of dark argillaceous limestone; and forty of them of coarsely
crystallised carbonate of lime, with cavities lined by quartz crystals:
these latter white calcareous columns do not retain any internal
structure, but their external form plainly shows their origin. All the
stumps have nearly the same diameter, varying from one foot to eighteen
inches; some of them stand within a yard of each other; they are
grouped in a clump within a space of about sixty yards across, with a
few scattered round at the distance of 150 yards. They all stand at
about the same level. The longest stump stood seven feet out of the
ground: the roots, if they are still preserved, are buried and
concealed. No one layer of the mudstone appeared much darker than the
others, as if it had formerly existed as soil, nor could this be
expected, for the same agents which replaced with silex and lime the
wood of the trees, would naturally have removed all vegetable matter
from the soil. Besides the fifty-two upright trees, there were a few
fragments, like broken branches, horizontally embedded. The surrounding
strata are crossed by veins of carbonate of lime, agate, and oxide of
iron; and a poor gold vein has been worked not far from the trees.
The green and brown mudstone beds including the trees, are conformably
covered by much indurated, compact, white or ferruginous tuffs, which
pass upwards into a fine-grained, purplish sedimentary rock: these
strata, which, together, are from four to five hundred feet in
thickness, rest on a thick bed of submarine lava, and are conformably
covered by another great mass of fine-grained basalt, which I estimated
at 1,000 feet in thickness, and which probably has been formed by more
than one stream. (This rock is quite black, and fuses into a black
bead, attracted strongly by the magnet; it breaks with a conchoidal
fracture; the included crystals of augite are distinguishable by the
naked eye, but are not perfect enough to be measured: there are many
minute acicular crystals of glassy feldspar.) Above this mass I could
clearly distinguish five conformable alternations, each several hundred
feet in thickness, of stratified sedimentary rocks and lavas, such as
have been previously described. Certainly the upright trees have been
buried under several thousand feet in thickness of matter, accumulated
under the sea. As the trees obviously must once have grown on dry land,
what an enormous amount of subsidence is thus indicated! Nevertheless,
had it not been for the trees there was no appearance which would have
led any one even to have conjectured that these strata had subsided. As
the land, moreover, on which the trees grew, is formed of subaqueous
deposits, of nearly if not quite equal thickness with the
superincumbent strata, and as these deposits are regularly stratified
and fine-grained, not like the matter thrown up on a sea-beach, a
previous upward movement, aided no doubt by the great accumulation of
lavas and sediment, is also indicated. (At first I imagined, that the
strata with the trees might have been accumulated in a lake: but this
seems highly improbable; for, first, a very deep lake was necessary to
receive the matter below the trees, then it must have been drained for
their growth, and afterwards re-formed and made profoundly deep, so as
to receive a subsequent accumulation of matter SEVERAL THOUSAND feet in
thickness. And all this must have taken place necessarily before the
formation of the Uspallata range, and therefore on the margin of the
wide level expanse of the Pampas! Hence I conclude, that it is
infinitely more probable that the strata were accumulated under the
sea: the vast amount of denudation, moreover, which this range has
suffered, as shown by the wide valleys, by the exposure of the very
trees and by other appearances, could have been effected, I conceive,
only by the long-continued action of the sea; and this shows that the
range was either upheaved from under the sea, or subsequently let down
into it. From the natural manner in which the stumps (fifty-two in
number) are GROUPED IN A CLUMP, and from their all standing vertically
to the strata, it is superfluous to speculate on the chance of the
trees having been drifted from adjoining land, and deposited upright: I
may, however, mention that the late Dr. Malcolmson assured me, that he
once met in the Indian Ocean, fifty miles from land, several cocoa-nut
trees floating upright, owing to their roots being loaded with earth.)
In nearly the middle of the range, there are some hills [Q], before
alluded to, formed of a kind of granite externally resembling andesite,
and consisting of a white, imperfectly granular, feldspathic basis,
including some perfect crystals apparently of albite (but I was unable
to measure them), much black mica, epidote in veins, and very little or
no quartz. Numerous small veins branch from this rock into the
surrounding strata; and it is a singular fact that these veins, though
composed of the same kind of feldspar and small scales of mica as in
the solid rock, abound with innumerable minute ROUNDED grains of
quartz: in the veins or dikes also, branching from the great granitic
axis in the peninsula of Tres Montes, I observed that quartz was more
abundant in them than in the main rock: I have heard of other analogous
cases: can we account for this fact, by the long-continued vicinity of
quartz when cooling, and by its having been thus more easily sucked
into fissures than the other constituent minerals of granite? (See a
paper by M. Elie de Beaumont, “Soc. Philomath.” May 1839 “L’Institut.”
1839 page 161.) The strata encasing the flanks of these granitic or
andesite masses, and forming a thick cap on one of their summits,
appear originally to have been of the same tufaceous nature with the
beds already described, but they are now changed into porcellanic,
jaspery, and crystalline rocks, and into others of a white colour with
a harsh texture, and having a siliceous aspect, though really of a
feldspathic nature and fusible. Both the granitic intrusive masses and
the encasing strata are penetrated by innumerable metallic veins,
mostly ferruginous and auriferous, but some containing copper-pyrites
and a few silver: near the veins, the rocks are blackened as if blasted
by gunpowder. The strata are only slightly dislocated close round these
hills, and hence, perhaps, it may be inferred that the granitic masses
form only the projecting points of a broad continuous axis-dome, which
has given to the upper parts of this range its anticlinal structure.
CONCLUDING REMARKS ON THE USPALLATA RANGE.
I will not attempt to estimate the total thickness of the pile of
strata forming this range, but it must amount to many thousand feet.
The sedimentary and tufaceous beds have throughout a general
similarity, though with infinite variations. The submarine lavas in the
lower part of the series are mostly feldspathic, whilst in the upper
part, on the summit and western flank, they are mostly basaltic. We are
thus reminded of the relative position in most recent volcanic
districts of the trachytic and basaltic lavas,—the latter from their
greater weight having sunk to a lower level in the earth’s crust, and
having consequently been erupted at a later period over the lighter and
upper lavas of the trachytic series. (See on this subject, “Volcanic
Islands” etc. by the Author.) Both the basaltic and feldspathic
submarine streams are very compact; none being vesicular, and only a
few amygdaloidal: the effects which some of them, especially those low
in the series, have produced on the tufaceous beds over which they have
flowed is highly curious. Independently of this local metamorphic
action, all the strata undoubtedly display an indurated and altered
character; and all the rocks of this range—the lavas, the alternating
sediments, the intrusive granite and porphyries, and the underlying
clay- slate—are intersected by metalliferous veins. The lava-strata can
often be seen extending for great distances, conformably with the under
and overlying beds; and it was obvious that they thickened towards the
west. Hence the points of eruption must have been situated westward of
the present range, in the direction of the main Cordillera: as,
however, the flanks of the Cordillera are entirely composed of various
porphyries, chiefly claystone and greenstone, some intrusive, and
others belonging to the porphyritic conglomerate formation, but all
quite unlike these submarine lava-streams, we must in all probability
look to the plain of Uspallata for the now deeply buried points of
eruption.
Comparing our section of the Uspallata range with that of the Cumbre,
we see, with the exception of the underlying clay-slate, and perhaps of
the intrusive rocks of the axes, a striking dissimilarity in the strata
composing them. The great porphyritic conglomerate formation has not
extended as far as this range; nor have we here any of the gypseous
strata, the magnesian and other limestones, the red sandstones, the
siliceous beds with pebbles of quartz, and comparatively little of the
conglomerates, all of which form such vast masses over the basal series
in the main Cordillera. On the other hand, in the Cordillera, we do not
find those endless varieties of indurated tuffs, with their numerous
veins and concretionary arrangement, and those grit and mud stones, and
singular semi-porcellanic rocks, so abundant in the Uspallata range.
The submarine lavas, also, differ considerably; the feldspathic streams
of the Cordillera contain much mica, which is absent in those of the
Uspallata range: in this latter range we have seen on how grand a
scale, basaltic lava has been poured forth, of which there is not a
trace in the Cordillera. This dissimilarity is the more striking,
considering that these two parallel chains are separated by a plain
only between ten and fifteen miles in width; and that the Uspallata
lavas, as well as no doubt the alternating tufaceous beds, have
proceeded from the west, from points apparently between the two ranges.
To imagine that these two piles of strata were contemporaneously
deposited in two closely adjoining, very deep, submarine areas,
separated from each other by a lofty ridge, where a plain now extends,
would be a gratuitous hypothesis. And had they been contemporaneously
deposited, without any such dividing ridge, surely some of the gypseous
and other sedimentary matter forming such immensely thick masses in the
Cordillera, would have extended this short distance eastwards; and
surely some of the Uspallata tuffs and basalts also accumulated to so
great a thickness, would have extended a little westward. Hence I
conclude, that it is far from probable that these two series are not
contemporaneous; but that the strata of one of the chains were
deposited, and even the chain itself uplifted, before the formation of
the other:—which chain, then, is the oldest? Considering that in the
Uspallata range the lowest strata on the western flank lie
unconformably on the clay- slate, as probably is the case with those on
the eastern flank, whereas in the Cordillera all the overlying strata
lie conformably on this formation:—considering that in the Uspallata
range some of the beds, both low down and high up in the series, are
marked with vegetable impressions, showing the continued existence of
neighbouring land;—considering the close general resemblance between
the deposits of this range and those of tertiary origin in several
parts of the continent;—and lastly, even considering the lesser height
and outlying position of the Uspallata range,—I conclude that the
strata composing it are in all probability of subsequent origin, and
that they were accumulated at a period when a deep sea studded with
submarine volcanoes washed the eastern base of the already partially
elevated Cordillera.
This conclusion is of much importance, for we have seen that in the
Cordillera, during the deposition of the Neocomian strata, the bed of
the sea must have subsided many thousand feet: we now learn that at a
later period an adjoining area first received a great accumulation of
strata, and was upheaved into land on which coniferous trees grew, and
that this area then subsided several thousand feet to receive the
superincumbent submarine strata, afterwards being broken up, denuded,
and elevated in mass to its present height. I am strengthened in this
conclusion of there having been two distinct, great periods of
subsidence, by reflecting on the thick mass of coarse stratified
conglomerate in the valley of Tenuyan, between the Peuquenes and
Portillo lines; for the accumulation of this mass seems to me, as
previously remarked, almost necessarily to have required a prolonged
subsidence; and this subsidence, from the pebbles in the conglomerate
having been to a great extent derived from the gypseous or Neocomian
strata of the Peuquenes line, we know must have been quite distinct
from, and subsequent to, that sinking movement which probably
accompanied the deposition of the Peuquenes strata, and which certainly
accompanied the deposition of the equivalent beds near the Puente del
Inca, in this line of section.
The Uspallata chain corresponds in geographical position, though on a
small scale, with the Portillo line; and its clay-slate formation is
probably the equivalent of the mica-schist of the Portillo, there
metamorphosed by the old white granites and syenites. The coloured beds
under the conglomerate in the valley of Tenuyan, of which traces are
seen on the crest of the Portillo, and even the conglomerate itself,
may perhaps be synchronous with the tufaceous beds and submarine lavas
of the Uspallata range; an open sea and volcanic action in the latter
case, and a confined channel between two bordering chains of islets in
the former case, having been sufficient to account for the
mineralogical dissimilarity of the two series. From this correspondence
between the Uspallata and Portillo ranges, perhaps in age and certainly
in geographical position, one is tempted to consider the one range as
the prolongation of the other; but their axes are formed of totally
different intrusive rocks; and we have traced the apparent continuation
of the red granite of the Portillo in the red porphyries diverging into
the main Cordillera. Whether the axis of the Uspallata range was
injected before, or as perhaps is more probable, after that of the
Portillo line, I will not pretend to decide; but it is well to remember
that the highly inclined lava-streams on the eastern flank of the
Portillo line, prove that its angular upheavement was not a single and
sudden event; and therefore that the anticlinal elevation of the
Uspallata range may have been contemporaneous with some of the later
angular movements by which the gigantic Portillo range gained its
present height above the adjoining plain.
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.
A 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 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.