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Geological Observations On South America

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




Northward of Maldonado, and south of Las Minas, there is an E. and W. hilly
band of country, some miles in width, formed of siliceous clay-slate, with
some quartz, rock, and limestone, having a tortuous irregular cleavage,
generally ranging east and west. E. and S.E. of Las Minas there is a
confused district of imperfect gneiss and laminated quartz, with the hills
ranging in various directions, but with each separate hill generally
running in the same line with the folia of the rocks of which it is
composed: this confusion appears to have been caused by the intersection of
the [E. and W.] and [N.N.E. and S.S.W.] strikes. Northward of Las Minas,
the more regular northerly ranges predominate: from this place to near
Polanco, we meet with the coarse-grained mixture of quartz and feldspar,
often with the imperfect hornblende, and then becoming foliated in a N. and
S. line--with imperfect clay-slate, including laminae of red crystallised
feldspar--with white or black marble, sometimes containing asbestus and
crystals of gypsum--with quartz-rock--with syenite--and lastly, with much
granite. The marble and granite alternate repeatedly in apparently vertical
masses: some miles northward of the Polanco, a wide district is said to be
entirely composed of marble. It is remarkable, how rare mica is in the
whole range of country north and westward of Maldonado. Throughout this
district, the cleavage of the clay-slate and marble--the foliation of the
gneiss and the quartz--the stratification or alternating masses of these
several rocks--and the range of the hills, all coincide in direction; and
although the country is only hilly, the planes of division are almost
everywhere very highly inclined or vertical.

Some ancient submarine volcanic rocks are worth mentioning, from their
rarity on this eastern side of the continent. In the valley of the Tapas
(fifty or sixty miles N. of Maldonado) there is a tract three or four miles
in length, composed of various trappean rocks with glassy feldspar--of
apparently metamorphosed grit-stones--of purplish amygdaloids with large
kernels of carbonate of lime (Near the Pan de Azucar there is some greenish
porphyry, in one place amygdaloidal with agate.)--and much of a harshish
rock with glassy feldspar intermediate in character between claystone
porphyry and trachyte. This latter rock was in one spot remarkable from
being full of drusy cavities, lined with quartz crystals, and arranged in
planes, dipping at an angle of 50 degrees to the east, and striking
parallel to the foliation of an adjoining hill composed of the common
mixture of quartz, feldspar, and imperfect hornblende: this fact perhaps
indicates that these volcanic rocks have been metamorphosed, and their
constituent parts rearranged, at the same time and according to the same
laws, with the granitic and metamorphic formations of this whole region. In
the valley of the Marmaraya, a few miles south of the Tapas, a band of
trappean and amygdaloidal rock is interposed between a hill of granite and
an extensive surrounding formation of red conglomerate, which (like that at
the foot of the S. Animas) has its basis porphyritic with crystals of
feldspar, and which hence has certainly suffered metamorphosis.

MONTE VIDEO.

The rocks here consist of several varieties of gneiss, with the feldspar
often yellowish, granular and imperfectly crystallised, alternating with,
and passing insensibly into, beds, from a few yards to nearly a mile in
thickness, of fine or coarse grained, dark-green hornblendic slate; this
again often passing into chloritic schist. These passages seem chiefly due
to changes in the mica, and its replacement by other minerals. At Rat
Island I examined a mass of chloritic schist, only a few yards square,
irregularly surrounded on all sides by the gneiss, and intricately
penetrated by many curvilinear veins of quartz, which gradually BLEND into
the gneiss: the cleavage of the chloritic schist and the foliation of the
gneiss were exactly parallel. Eastward of the city there is much fine-
grained, dark-coloured gneiss, almost assuming the character of hornblende-
slate, which alternates in thin laminae with laminae of quartz, the whole
mass being transversely intersected by numerous large veins of quartz: I
particularly observed that these veins were absolutely continuous with the
alternating laminae of quartz. In this case and at Rat Island, the passage
of the gneiss into imperfect hornblendic or into chloritic slate, seemed to
be connected with the segregation of the veins of quartz. (Mr. Greenough
page 78 "Critical Examination" etc., observes that quartz in mica-slate
sometimes appears in beds and sometimes in veins. Von Buch also in his
"Travels in Norway" page 236, remarks on alternating laminae of quartz and
hornblende-slate replacing mica-schist.)

The Mount, a hill believed to be 450 feet in height, from which the place
takes its name, is much the highest land in this neighbourhood: it consists
of hornblendic slate, which (except on the eastern and disturbed base) has
an east and west nearly vertical cleavage; the longer axis of the hill also
ranges in this same line. Near the summit the hornblende-slate gradually
becomes more and more coarsely crystallised, and less plainly laminated,
until it passes into a heavy, sonorous greenstone, with a slaty conchoidal
fracture; the laminae on the north and south sides near the summit dip
inwards, as if this upper part had expanded or bulged outwards. This
greenstone must, I conceive, be considered as metamorphosed hornblende-
slate. The Cerrito, the next highest, but much less elevated point, is
almost similarly composed. In the more western parts of the province,
besides gneiss, there is quartz-rock, syenite, and granite; and at Colla, I
heard of marble.

Near M. Video, the space which I more accurately examined was about fifteen
miles in an east and west line, and here I found the foliation of the
gneiss and the cleavage of the slates generally well developed, and
extending parallel to the alternating strata composed of the gneiss,
hornblendic and chloritic schists. These planes of division all range
within one point of east and west, frequently east by south and west by
north; their dip is generally almost vertical, and scarcely anywhere under
45 degrees: this fact, considering how slightly undulatory the surface of
the country is, deserves attention. Westward of M. Video, towards the
Uruguay, wherever the gneiss is exposed, the highly inclined folia are seen
striking in the same direction; I must except one spot where the strike was
N.W. by W. The little Sierra de S. Juan, formed of gneiss and laminated
quartz, must also be excepted, for it ranges between [N. to N.E.] and [S.
to S.W.] and seems to belong to the same system with the hills in the
Maldonado district. Finally, we have seen that, for many miles northward of
Maldonado and for twenty-five miles westward of it, as far as the S. de las
Animas, the foliation, cleavage, so-called stratification and lines of
hills, all range N.N.E. and S.S.W., which is nearly coincident with the
adjoining coast of the Atlantic. Westward of the S. de las Animas, as far
as even the Uruguay, the foliation, cleavage, and stratification (but not
lines of hills, for there are no defined ones) all range about E. by S. and
W. by N., which is nearly coincident with the direction of the northern
shore of the Plata; in the confused country near Las Minas, where these two
great systems appear to intersect each other, the cleavage, foliation, and
stratification run in various directions, but generally coincide with the
line of each separate hill.

SOUTHERN LA PLATA.

The first ridge, south of the Plata, which projects through the Pampean
formation, is the Sierra Tapalguen and Vulcan, situated 200 miles southward
of the district just described. This ridge is only a few hundred feet in
height, and runs from C. Corrientes in a W.N.W. line for at least 150 miles
into the interior: at Tapalguen, it is composed of unstratified granular
quartz, remarkable from forming tabular masses and small plains, surrounded
by precipitous cliffs: other parts of the range are said to consist of
granite: and marble is found at the S. Tinta. It appears from M.
Parchappe's observations, that at Tandil there is a range of quartzose
gneiss, very like the rocks of the S. Larga near Maldonado, running in the
same N.N.E. and S.S.W. direction; so that the framework of the country here
is very similar to that on the northern shore of the Plata. (M. d'Orbigny's
"Voyage" Part. Geolog. page 46. I have given a short account of the
peculiar forms of the quartz hills of Tapalguen, so unusual in a
metamorphic formation, in my "Journal of Researches" 2nd edition page 116.)

The Sierra Guitru-gueyu is situated sixty miles south of the S. Tapalguen:
it consists of numerous parallel, sometimes blended together ridges, about
twenty-three miles in width, and five hundred feet in height above the
plain, and extending in a N.W. and S.E. direction. Skirting round the
extreme S.E. termination, I ascended only a few points, which were composed
of a fine-grained gneiss, almost composed of feldspar with a little mica,
and passing in the upper parts of the hills into a rather compact purplish
clay-slate. The cleavage was nearly vertical, striking in a N.W. by W. and
S.E. by E. line, nearly, though not quite, coincident with the direction of
the parallel ridges.

The Sierra Ventana lies close south of that of Guitru-gueyu; it is
remarkable from attaining a height, very unusual on this side of the
continent, of 3,340 feet. It consists up to its summit, of quartz,
generally pure and white, but sometimes reddish, and divided into thick
laminae or strata: in one part there is a little glossy clay-slate with a
tortuous cleavage. The thick layers of quartz strike in a W. 30 degrees N.
line, dipping southerly at an angle of 45 degrees and upwards. The
principal line of mountains, with some quite subordinate parallel ridges,
range about W. 45 degrees N.: but at their S.E. termination, only W. 25
degrees N. This Sierra is said to extend between twenty and thirty leagues
into the interior.

PATAGONIA.

With the exception perhaps of the hill of S. Antonio (600 feet high) in the
Gulf of S. Matias, which has never been visited by a geologist, crystalline
rocks are not met with on the coast of Patagonia for a space of 380 miles
south of the S. Ventana. At this point (latitude 43 degrees 50 minutes), at
Points Union and Tombo, plutonic rocks are said to appear, and are found,
at rather wide intervals, beneath the Patagonian tertiary formation for a
space of about three hundred miles southward, to near Bird Island, in
latitude 48 degrees 56 minutes. Judging from specimens kindly collected for
me by Mr. Stokes, the prevailing rock at Ports St. Elena, Camerones,
Malaspina, and as far south as the Paps of Pineda, is a purplish-pink or
brownish claystone porphyry, sometimes laminated, sometimes slightly
vesicular, with crystals of opaque feldspar and with a few grains of
quartz; hence these porphyries resemble those immediately to be described
at Port Desire, and likewise a series which I have seen from P. Alegre on
the southern confines of Brazil. This porphyritic formation further
resembles in a singularly close manner the lowest stratified formation of
the Cordillera of Chile, which, as we shall hereafter see, has a vast
range, and attains a great thickness. At the bottom of the Gulf of St.
George, only tertiary deposits appear to be present. At Cape Blanco, there
is quartz rock, very like that of the Falkland Islands, and some hard, blue
siliceous clay-slate.

At Port Desire there is an extensive formation of the claystone porphyry,
stretching at least twenty-five miles into the interior: it has been
denuded and deeply worn into gullies before being covered up by the
tertiary deposits, through which it here and there projects in hills; those
north of the bay being 440 feet in height. The strata have in several
places been tilted at small angles, generally either to N.N.W. or S.S.E. By
gradual passages and alternations, the porphyries change incessantly in
nature. I will describe only some of the principal mineralogical changes,
which are highly instructive, and which I carefully examined. The
prevailing rock has a compact purplish base, with crystals of earthy or
opaque feldspar, and often with grains of quartz. There are other
varieties, with an almost truly trachytic base, full of little angular
vesicles and crystals of glassy feldspar; and there are beds of black
perfect pitchstone, as well as of a concretionary imperfect variety. On a
casual inspection, the whole series would be thought to be of the same
plutonic or volcanic nature with the trachytic varieties and pitchstone;
but this is far from being the case, as much of the porphyry is certainly
of metamorphic origin. Besides the true porphyries, there are many beds of
earthy, quite white or yellowish, friable, easily fusible matter,
resembling chalk, which under the microscope is seen to consist of minute
broken crystals, and which, as remarked in a former chapter, singularly
resembles the upper tufaceous beds of the Patagonian tertiary formation.
This earthy substance often becomes coarser, and contains minute rounded
fragments of porphyries and rounded grains of quartz, and in one case so
many of the latter as to resemble a common sandstone. These beds are
sometimes marked with true lines of aqueous deposition, separating
particles of different degrees of coarseness; in other cases there are
parallel ferruginous lines not of true deposition, as shown by the
arrangement of the particles, though singularly resembling them. The more
indurated varieties often include many small and some larger angular
cavities, which appear due to the removal of earthy matter: some varieties
contain mica. All these earthy and generally white stones insensibly pass
into more indurated sonorous varieties, breaking with a conchoidal
fracture, yet of small specific gravity; many of these latter varieties
assume a pale purple tint, being singularly banded and veined with
different shades, and often become plainly porphyritic with crystals of
feldspar. The formation of these crystals could be most clearly traced by
minute angular and often partially hollow patches of earthy matter, first
assuming a FIBROUS STRUCTURE, then passing into opaque imperfectly shaped
crystals, and lastly, into perfect glassy crystals. When these crystals
have appeared, and when the basis has become compact, the rock in many
places could not be distinguished from a true claystone porphyry without a
trace of mechanical structure.

In some parts, these earthy or tufaceous beds pass into jaspery and into
beautifully mottled and banded porcelain rocks, which break into splinters,
translucent at their edges, hard enough to scratch glass, and fusible into
white transparent beads: grains of quartz included in the porcelainous
varieties can be seen melting into the surrounding paste. In other parts,
the earthy or tufaceous beds either insensibly pass into, or alternate
with, breccias composed of large and small fragments of various purplish
porphyries, with the matrix generally porphyritic: these breccias, though
their subaqueous origin is in many places shown both by the arrangement of
their smaller particles and by an oblique or current lamination, also pass
into porphyries, in which every trace of mechanical origin and
stratification has been obliterated.

Some highly porphyritic though coarse-grained masses, evidently of
sedimentary origin, and divided into thin layers, differing from each other
chiefly in the number of embedded grains of quartz, interested me much from
the peculiar manner in which here and there some of the layers terminated
in abrupt points, quite unlike those produced by a layer of sediment
naturally thinning out, and apparently the result of a subsequent process
of metamorphic aggregation. In another common variety of a finer texture,
the aggregating process had gone further, for the whole mass consisted of
quite short, parallel, often slightly curved layers or patches, of whitish
or reddish finely granulo-crystalline feldspathic matter, generally
terminating at both ends in blunt points; these layers or patches further
tended to pass into wedge or almond-shaped little masses, and these finally
into true crystals of feldspar, with their centres often slightly drusy.
The series was so perfect that I could not doubt that these large crystals,
which had their longer axes placed parallel to each other, had primarily
originated in the metamorphosis and aggregation of alternating layers of
tuff; and hence their parallel position must be attributed (unexpected
though the conclusion may be), not to laws of chemical action, but to the
original planes of deposition. I am tempted briefly to describe three other
singular allied varieties of rock; the first without examination would have
passed for a stratified porphyritic breccia, but all the included angular
fragments consisted of a border of pinkish crystalline feldspathic matter,
surrounding a dark translucent siliceous centre, in which grains of quartz
not quite blended into the paste could be distinguished: this uniformity in
the nature of the fragments shows that they are not of mechanical, but of
concretionary origin, having resulted perhaps from the self-breaking up and
aggregation of layers of indurated tuff containing numerous grains of
quartz,--into which, indeed, the whole mass in one part passed. The second
variety is a reddish non-porphyritic claystone, quite full of spherical
cavities, about half an inch in diameter, each lined with a collapsed crust
formed of crystals of quartz. The third variety also consists of a pale
purple non-porphyritic claystone, almost wholly formed of concretionary
balls, obscurely arranged in layers, of a less compact and paler coloured
claystone; each ball being on one side partly hollow and lined with
crystals of quartz.

PSEUDO-DIKES.

Some miles up the harbour, in a line of cliffs formed of slightly
metamorphosed tufaceous and porphyritic claystone beds, I observed three
vertical dikes, so closely resembling in general appearance ordinary
volcanic dikes, that I did not doubt, until closely examining their
composition, that they had been injected from below. The first is straight,
with parallel sides, and about four feet wide; it consists of whitish,
indurated tufaceous matter, precisely like some of the beds intersected by
it. The second dike is more remarkable; it is slightly tortuous, about
eighteen inches thick, and can be traced for a considerable distance along
the beach; it is of a purplish-red or brown colour, and is formed chiefly
of ROUNDED grains of quartz, with broken crystals of earthy feldspar,
scales of black mica, and minute fragments of claystone porphyry, all
firmly united together in a hard sparing base. The structure of this dike
shows obviously that it is of mechanical and sedimentary origin; yet it
thinned out upwards, and did not cut through the uppermost strata in the
cliffs. This fact at first appears to indicate that the matter could not
have been washed in from above (Upfilled fissures are known to occur both
in volcanic and in ordinary sedimentary formations. At the Galapagos
Archipelago "Volcanic Islands" etc., there are some striking examples of
pseudo-dikes composed of hard tuff.); but if we reflect on the suction
which would result from a deep-seated fissure being formed, we may admit
that if the fissure were in any part open to the surface, mud and water
might well be drawn into it along its whole course. The third dike
consisted of a hard, rough, white rock, almost composed of broken crystals
of glassy feldspar, with numerous scales of black mica, cemented in a
scanty base; there was little in the appearance of this rock, to preclude
the idea of its having been a true injected feldspathic dike. The matter
composing these three pseudo-dikes, especially the second one, appears to
have suffered, like the surrounding strata, a certain degree of metamorphic
action; and this has much aided the deceptive appearance. At Bahia, in
Brazil, we have seen that a true injected hornblendic dike, not only has
suffered metamorphosis, but has been dislocated and even diffused in the
surrounding gneiss, under the form of separate crystals and of fragments.

FALKLAND ISLANDS.

I have described these islands in a paper published in the third volume of
the "Geological Journal." The mountain-ridges consist of quartz, and the
lower country of clay-slate and sandstone, the latter containing Palaeozoic
fossils. These fossils have been separately described by Messrs. Morris and
Sharpe: some of them resemble Silurian, and others Devonian forms. In the
eastern part of the group the several parallel ridges of quartz extend in a
west and east line; but further westward the line becomes W.N.W. and
E.S.E., and even still more northerly. The cleavage-planes of the clay-
slate are highly inclined, generally at an angle of above 50 degrees, and
often vertical; they strike almost invariably in the same direction with
the quartz ranges. The outline of the indented shores of the two main
islands, and the relative positions of the smaller islets, accord with the
strike both of the main axes of elevation and of the cleavage of the clay-
slate.

TIERRA DEL FUEGO.

My notes on the geology of this country are copious, but as they are
unimportant, and as fossils were found only in one district, a brief sketch
will be here sufficient. The east coast from the S. of Magellan (where the
boulder formation is largely developed) to St. Polycarp's Bay is formed of
horizontal tertiary strata, bounded some way towards the interior by a
broad mountainous band of clay-slate. This great clay-slate formation
extends from St. Le Maire westward for 140 miles, along both sides of the
Beagle Channel to near its bifurcation. South of this channel, it forms all
Navarin Island, and the eastern half of Hoste Island and of Hardy
Peninsula; north of the Beagle Channel it extends in a north-west line on
both sides of Admiralty Sound to Brunswick Peninsula in the St. of
Magellan, and I have reason to believe, stretches far up the eastern side
of the Cordillera. The western and broken side of Tierra del Fuego towards
the Pacific is formed of metamorphic schists, granite and various trappean
rocks: the line of separation between the crystalline and clay-slate
formations can generally be distinguished, as remarked by Captain King, by
the parallelism in the clay-slate districts of the shores and channels,
ranging in a line between [W. 20 degrees to 40 degrees N.] and [E. 20
degrees to 40 degrees S.]. ("Geographical Journal" volume 1 page 155.)

The clay-slate is generally fissile, sometimes siliceous or ferruginous,
with veins of quartz and calcareous spar; it often assumes, especially on
the loftier mountains, an altered feldspathic character, passing into
feldspathic porphyry: occasionally it is associated with breccia and
grauwacke. At Good Success Bay, there is a little intercalated black
crystalline limestone. At Port Famine much of the clay-slate is calcareous,
and passes either into a mudstone or into grauwacke, including odd-shaped
concretions of dark argillaceous limestone. Here alone, on the shore a few
miles north of Port Famine, and on the summit of Mount Tarn (2,600 feet
high), I found organic remains; they consist of:--

1. Ancyloceras simplex, d'Orbigny "Pal Franc" Mount Tarn.
2. Fusus (in imperfect state), d'Orbigny "Pal Franc" Mount Tarn.
3. Natica, d'Orbigny "Pal Franc" Mount Tarn.
4. Pentacrimus, d'Orbigny "Pal Franc" Mount Tarn.
5. Lucina excentrica, G.B. Sowerby, Port Famine.
6. Venus (in imperfect state), G.B. Sowerby, Port Famine.
7. Turbinolia (?), G.B. Sowerby, Port Famine.
8. Hamites elatior, G.B. Sowerby, Port Famine.

M. d'Orbigny states that MM. Hombron and Grange found in this neighbourhood
an Ancyloceras, perhaps A. simplex, an Ammonite, a Plicatula and Modiola.
("Voyage" Part Geolog. page 242.) M. d'Orbigny believes from the general
character of these fossils, and from the Ancyloceras being identical (as
far as its imperfect condition allows of comparison) with the A. simplex of
Europe, that the formation belongs to an early stage of the Cretaceous
system. Professor E. Forbes, judging only from my specimens, concurs in the
probability of this conclusion. The Hamites elatior of the above list, of
which a description has been given by Mr. Sowerby, and which is remarkable
from its large size, has not been seen either by M. d'Orbigny or Professor
E. Forbes, as, since my return to England, the specimens have been lost.
The great clay-slate formation of Tierra del Fuego being cretaceous, is
certainly a very interesting fact,--whether we consider the appearance of
the country, which, without the evidence afforded by the fossils, would
form the analogy of most known districts, probably have been considered as
belonging to the Palaeozoic series,--or whether we view it as showing that
the age of this terminal portion of the great axis of South America, is the
same (as will hereafter be seen) with the Cordillera of Chile and Peru.

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