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beach-heads of long narrow arms of the sea, first high up the
valleys, then lower and lower down as the land slowly rose. If this
be so, and I cannot doubt it, the grand and broken chain of the
Cordillera, instead of having been suddenly thrown up, as was till
lately the universal, and still is the common opinion of
geologists, has been slowly upheaved in mass, in the same gradual
manner as the coasts of the Atlantic and Pacific have risen within
the recent period. A multitude of facts in the structure of the
Cordillera, on this view receive a simple explanation.

(PLATE 74. SOUTH AMERICAN BIT.)

The rivers which flow in these valleys ought rather to be called
mountain-torrents. Their inclination is very great, and their water
the colour of mud. The roar which the Maypu made, as it rushed over
the great rounded fragments, was like that of the sea. Amidst the
din of rushing waters, the noise from the stones, as they rattled
one over another, was most distinctly audible even from a distance.
This rattling noise, night and day, may be heard along the whole
course of the torrent. The sound spoke eloquently to the geologist;
the thousands and thousands of stones which, striking against each
other, made the one dull uniform sound, were all hurrying in one
direction. It was like thinking on time, where the minute that now
glides past is irrevocable. So was it with these stones; the ocean
is their eternity, and each note of that wild music told of one
more step towards their destiny.

It is not possible for the mind to comprehend, except by a slow
process, any effect which is produced by a cause repeated so often
that the multiplier itself conveys an idea not more definite than
the savage implies when he points to the hairs of his head. As
often as I have seen beds of mud, sand, and shingle, accumulated to
the thickness of many thousand feet, I have felt inclined to
exclaim that causes, such as the present rivers and the present
beaches, could never have ground down and produced such masses.
But, on the other hand, when listening to the rattling noise of
these torrents, and calling to mind that whole races of animals
have passed away from the face of the earth, and that during this
whole period, night and day, these stones have gone rattling
onwards in their course, I have thought to myself, can any
mountains, any continent, withstand such waste?

In this part of the valley, the mountains on each side were from
3000 to 6000 or 8000 feet high, with rounded outlines and steep
bare flanks. The general colour of the rock was dullish purple, and
the stratification very distinct. If the scenery was not beautiful,
it was remarkable and grand. We met during the day several herds of
cattle, which men were driving down from the higher valleys in the
Cordillera. This sign of the approaching winter hurried our steps,
more than was convenient for geologising. The house where we slept
was situated at the foot of a mountain, on the summit of which are
the mines of S. Pedro de Nolasko. Sir F. Head marvels how mines
have been discovered in such extraordinary situations, as the bleak
summit of the mountain of S. Pedro de Nolasko. In the first place,
metallic veins in this country are generally harder than the
surrounding strata: hence, during the gradual wear of the hills,
they project above the surface of the ground. Secondly, almost
every labourer, especially in the northern parts of Chile,
understands something about the appearance of ores. In the great
mining provinces of Coquimbo and Copiapo, firewood is very scarce,
and men search for it over every hill and dale; and by this means
nearly all the richest mines have there been discovered.
Chanuncillo, from which silver to the value of many hundred
thousand pounds has been raised in the course of a few years, was
discovered by a man who threw a stone at his loaded donkey, and
thinking that it was very heavy, he picked it up, and found it full
of pure silver: the vein occurred at no great distance, standing up
like a wedge of metal. The miners, also, taking a crowbar with
them, often wander on Sundays over the mountains. In this south
part of Chile the men who drive cattle into the Cordillera, and who
frequent every ravine where there is a little pasture, are the
usual discoverers.

MARCH 20, 1835.

As we ascended the valley, the vegetation, with the exception of a
few pretty alpine flowers, became exceedingly scanty; and of
quadrupeds, birds, or insects, scarcely one could be seen. The
lofty mountains, their summits marked with a few patches of snow,
stood well separated from each other; the valleys being filled up
with an immense thickness of stratified alluvium. The features in
the scenery of the Andes which struck me most, as contrasted with
the other mountain chains with which I am acquainted, were, - the
flat fringes sometimes expanding into narrow plains on each side of
the valleys, - the bright colours, chiefly red and purple, of the
utterly bare and precipitous hills of porphyry, the grand and
continuous wall-like dikes, - the plainly-divided strata which,
where nearly vertical, formed the picturesque and wild central
pinnacles, but where less inclined, composed the great massive
mountains on the outskirts of the range, - and lastly, the smooth
conical piles of fine and brightly coloured detritus, which sloped
up at a high angle from the base of the mountains, sometimes to a
height of more than 2000 feet.

I frequently observed, both in Tierra del Fuego and within the
Andes, that where the rock was covered during the greater part of
the year with snow, it was shivered in a very extraordinary manner
into small angular fragments. Scoresby has observed the same fact
in Spitzbergen. (15/1. Scoresby's "Arctic Regions" volume 1 page
122.) The case appears to me rather obscure: for that part of the
mountain which is protected by a mantle of snow must be less
subject to repeated and great changes of temperature than any other
part. I have sometimes thought that the earth and fragments of
stone on the surface were perhaps less effectually removed by
slowly percolating snow-water than by rain, and therefore that the
appearance of a quicker disintegration of the solid rock under the
snow was deceptive. (15/2. I have heard it remarked in Shropshire
that the water, when the Severn is flooded from long-continued
rain, is much more turbid than when it proceeds from the snow
melting on the Welsh mountains. D'Orbigny tome 1 page 184, in
explaining the cause of the various colours of the rivers in South
America, remarks that those with blue or clear water have their
source in the Cordillera, where the snow melts.) Whatever the cause
may be, the quantity of crumbling stone on the Cordillera is very
great. Occasionally in the spring great masses of this detritus
slide down the mountains, and cover the snow-drifts in the valleys,
thus forming natural ice-houses. We rode over one, the height of
which was far below the limit of perpetual snow.

As the evening drew to a close, we reached a singular basin-like
plain, called the Valle del Yeso. It was covered by a little dry
pasture, and we had the pleasant sight of a herd of cattle amidst
the surrounding rocky deserts. The valley takes its name of Yeso
from a great bed, I should think at least 2000 feet thick, of
white, and in some parts quite pure, gypsum. We slept with a party
of men, who were employed in loading mules with this substance,
which is used in the manufacture of wine. We set out early in the
morning (21st), and continued to follow the course of the river,
which had become very small, till we arrived at the foot of the
ridge that separates the waters flowing into the Pacific and
Atlantic Oceans. The road, which as yet had been good with a steady
but very gradual ascent, now changed into a steep zigzag track up
the great range dividing the republics of Chile and Mendoza.

I will here give a very brief sketch of the geology of the several
parallel lines forming the Cordillera. Of these lines, there are
two considerably higher than the others; namely, on the Chilian
side, the Peuquenes ridge, which, where the road crosses it, is
13,210 feet above the sea; and the Portillo ridge, on the Mendoza
side, which is 14,305 feet. The lower beds of the Peuquenes ridge,
and of the several great lines to the westward of it, are composed
of a vast pile, many thousand feet in thickness, of porphyries
which have flowed as submarine lavas, alternating with angular and
rounded fragments of the same rocks, thrown out of the submarine
craters. These alternating masses are covered in the central parts
by a great thickness of red sandstone, conglomerate, and calcareous
clay-slate, associated with, and passing into, prodigious beds of
gypsum. In these upper beds shells are tolerably frequent; and they
belong to about the period of the lower chalk of Europe. It is an
old story, but not the less wonderful, to hear of shells which were
once crawling on the bottom of the sea, now standing nearly 14,000
feet above its level. The lower beds in this great pile of strata
have been dislocated, baked, crystallised and almost blended
together, through the agency of mountain masses of a peculiar white
soda-granitic rock.

The other main line, namely, that of the Portillo, is of a totally
different formation: it consists chiefly of grand bare pinnacles of
a red potash-granite, which low down on the western flank are
covered by a sandstone, converted by the former heat into a
quartz-rock. On the quartz there rest beds of a conglomerate
several thousand feet in thickness, which have been upheaved by the
red granite, and dip at an angle of 45 degrees towards the
Peuquenes line. I was astonished to find that this conglomerate was
partly composed of pebbles, derived from the rocks, with their
fossil shells, of the Peuquenes range; and partly of red
potash-granite, like that of the Portillo. Hence we must conclude
that both the Peuquenes and Portillo ranges were partially upheaved
and exposed to wear and tear when the conglomerate was forming; but
as the beds of the conglomerate have been thrown off at an angle of
45 degrees by the red Portillo granite (with the underlying
sandstone baked by it), we may feel sure that the greater part of
the injection and upheaval of the already partially formed Portillo
line took place after the accumulation of the conglomerate, and
long after the elevation of the Peuquenes ridge. So that the
Portillo, the loftiest line in this part of the Cordillera, is not
so old as the less lofty line of the Peuquenes. Evidence derived
from an inclined stream of lava at the eastern base of the Portillo
might be adduced to show that it owes part of its great height to
elevations of a still later date. Looking to its earliest origin,
the red granite seems to have been injected on an ancient
pre-existing line of white granite and mica-slate. In most parts,
perhaps in all parts, of the Cordillera, it may be concluded that
each line has been formed by repeated upheavals and injections; and
that the several parallel lines are of different ages. Only thus
can we gain time at all sufficient to explain the truly astonishing
amount of denudation which these great, though comparatively with
most other ranges recent, mountains have suffered.

Finally, the shells in the Peuquenes or oldest ridge prove, as
before remarked, that it has been upraised 14,000 feet since a
Secondary period, which in Europe we are accustomed to consider as
far from ancient; but since these shells lived in a moderately deep
sea, it can be shown that the area now occupied by the Cordillera
must have subsided several thousand feet - in northern Chile as much
as 6000 feet - so as to have allowed that amount of submarine strata
to have been heaped on the bed on which the shells lived. The proof
is the same with that by which it was shown that, at a much later
period since the tertiary shells of Patagonia lived, there must
have been there a subsidence of several hundred feet, as well as an
ensuing elevation. Daily it is forced home on the mind of the
geologist that nothing, not even the wind that blows, is so
unstable as the level of the crust of this earth.

I will make only one other geological remark: although the Portillo
chain is here higher than the Peuquenes, the waters, draining the
intermediate valleys, have burst through it. The same fact, on a
grander scale, has been remarked in the eastern and loftiest line
of the Bolivian Cordillera, through which the rivers pass:
analogous facts have also been observed in other quarters of the
world. On the supposition of the subsequent and gradual elevation
of the Portillo line, this can be understood; for a chain of islets
would at first appear, and, as these were lifted up, the tides
would be always wearing deeper and broader channels between them.
At the present day, even in the most retired Sounds on the coast of
Tierra del Fuego, the currents in the transverse breaks which
connect the longitudinal channels are very strong, so that in one
transverse channel even a small vessel under sail was whirled round
and round.

About noon we began the tedious ascent of the Peuquenes ridge, and
then for the first time experienced some little difficulty in our
respiration. The mules would halt every fifty yards, and after
resting for a few seconds the poor willing animals started of their
own accord again. The short breathing from the rarefied atmosphere
is called by the Chilenos "puna;" and they have most ridiculous
notions concerning its origin. Some say "All the waters here have
puna;" others that "where there is snow there is puna;" - and this
no doubt is true. The only sensation I experienced was a slight
tightness across the head and chest, like that felt on leaving a
warm room and running quickly in frosty weather. There was some
imagination even in this; for upon finding fossil shells on the
highest ridge, I entirely forgot the puna in my delight. Certainly
the exertion of walking was extremely great, and the respiration
became deep and laborious: I am told that in Potosi (about 13,000
feet above the sea) strangers do not become thoroughly accustomed
to the atmosphere for an entire year. The inhabitants all recommend
onions for the puna; as this vegetable has sometimes been given in
Europe for pectoral complaints, it may possibly be of real
service: - for my part I found nothing so good as the fossil shells!

When about half-way up we met a large party with seventy loaded
mules. It was interesting to hear the wild cries of the muleteers,
and to watch the long descending string of the animals; they
appeared so diminutive, there being nothing but the black mountains
with which they could be compared. When near the summit, the wind,
as generally happens, was impetuous and extremely cold. On each
side of the ridge we had to pass over broad bands of perpetual
snow, which were now soon to be covered by a fresh layer. When we
reached the crest and looked backwards, a glorious view was
presented. The atmosphere resplendently clear; the sky an intense
blue; the profound valleys; the wild broken forms: the heaps of
ruins, piled up during the lapse of ages; the bright-coloured
rocks, contrasted with the quiet mountains of snow, all these
together produced a scene no one could have imagined. Neither plant
nor bird, excepting a few condors wheeling around the higher
pinnacles, distracted my attention from the inanimate mass. I felt
glad that I was alone: it was like watching a thunderstorm, or
hearing in full orchestra a chorus of the Messiah.

On several patches of the snow I found the Protococcus nivalis, or
red snow, so well known from the accounts of Arctic navigators. My
attention was called to it by observing the footsteps of the mules
stained a pale red, as if their hoofs had been slightly bloody. I
at first thought that it was owing to dust blown from the
surrounding mountains of red porphyry; for from the magnifying
power of the crystals of snow, the groups of these microscopical
plants appeared like coarse particles. The snow was coloured only
where it had thawed very rapidly, or had been accidentally crushed.
A little rubbed on paper gave it a faint rose tinge mingled with a
little brick-red. I afterwards scraped some off the paper, and
found that it consisted of groups of little spheres in colourless
cases, each the thousandth part of an inch in diameter.

The wind on the crest of the Peuquenes, as just remarked, is
generally impetuous and very cold: it is said to blow steadily from
the westward or Pacific side. (15/3. Dr. Gillies in "Journal of
Natural and Geographical Science" August 1830. This author gives
the heights of the Passes.) As the observations have been chiefly
made in summer, this wind must be an upper and return current. The
Peak of Teneriffe, with a less elevation, and situated in latitude
28 degrees, in like manner falls within an upper return stream. At
first it appears rather surprising that the trade-wind along the
northern parts of Chile and on the coast of Peru should blow in so
very southerly a direction as it does; but when we reflect that the
Cordillera, running in a north and south line, intercepts, like a
great wall, the entire depth of the lower atmospheric current, we
can easily see that the trade-wind must be drawn northward,
following the line of mountains, towards the equatorial regions,
and thus lose part of that easterly movement which it otherwise
would have gained from the earth's rotation. At Mendoza, on the
eastern foot of the Andes, the climate is said to be subject to
long calms, and to frequent though false appearances of gathering
rain-storms: we may imagine that the wind, which coming from the
eastward is thus banked up by the line of mountains, would become
stagnant and irregular in its movements.

Having crossed the Peuquenes, we descended into a mountainous
country, intermediate between the two main ranges, and then took up
our quarters for the night. We were now in the republic of Mendoza.
The elevation was probably not under 11,000 feet, and the
vegetation in consequence exceedingly scanty. The root of a small
scrubby plant served as fuel, but it made a miserable fire, and the
wind was piercingly cold. Being quite tired with my days work, I
made up my bed as quickly as I could, and went to sleep. About
midnight I observed the sky became suddenly clouded: I awakened the
arriero to know if there was any danger of bad weather; but he said
that without thunder and lightning there was no risk of a heavy
snow-storm. The peril is imminent, and the difficulty of subsequent
escape great, to any one overtaken by bad weather between the two
ranges. A certain cave offers the only place of refuge: Mr.
Caldcleugh, who crossed on this same day of the month, was detained
there for some time by a heavy fall of snow. Casuchas, or houses of
refuge, have not been built in this pass as in that of Uspallata,
and therefore, during the autumn, the Portillo is little
frequented. I may here remark that within the main Cordillera rain
never falls, for during the summer the sky is cloudless, and in
winter snow-storms alone occur.

At the place where we slept water necessarily boiled, from the
diminished pressure of the atmosphere, at a lower temperature than
it does in a less lofty country; the case being the converse of
that of a Papin's digester. Hence the potatoes, after remaining for
some hours in the boiling water, were nearly as hard as ever. The
pot was left on the fire all night, and next morning it was boiled
again, but yet the potatoes were not cooked. I found out this by
overhearing my two companions discussing the cause, they had come
to the simple conclusion "that the cursed pot (which was a new one)
did not choose to boil potatoes."

MARCH 22, 1835.

After eating our potato-less breakfast, we travelled across the
intermediate tract to the foot of the Portillo range. In the middle
of summer cattle are brought up here to graze; but they had now all
been removed: even the greater number of the guanacos had decamped,
knowing well that if overtaken here by a snow-storm, they would be
caught in a trap. We had a fine view of a mass of mountains called
Tupungato, the whole clothed with unbroken snow, in the midst of
which there was a blue patch, no doubt a glacier; - a circumstance
of rare occurrence in these mountains. Now commenced a heavy and
long climb, similar to that of the Peuquenes. Bold conical hills of
red granite rose on each hand; in the valleys there were several
broad fields of perpetual snow. These frozen masses, during the
process of thawing, had in some parts been converted into pinnacles
or columns, which, as they were high and close together, made it
difficult for the cargo mules to pass. (15/4. This structure in
frozen snow was long since observed by Scoresby in the icebergs
near Spitzbergen, and, lately, with more care, by Colonel Jackson
"Journal of Geographical Society" volume 5 page 12, on the Neva.
Mr. Lyell "Principles" volume 4 page 360, has compared the
fissures, by which the columnar structure seems to be determined,
to the joints that traverse nearly all rocks, but which are best
seen in the non-stratified masses. I may observe that in the case
of the frozen snow the columnar structure must be owing to a
"metamorphic" action, and not to a process during DEPOSITION.) On
one of these columns of ice a frozen horse was sticking as on a
pedestal, but with its hind legs straight up in the air. The
animal, I suppose, must have fallen with its head downward into a
hole, when the snow was continuous, and afterwards the surrounding
parts must have been removed by the thaw.

When nearly on the crest of the Portillo, we were enveloped in a
falling cloud of minute frozen spicula. This was very unfortunate,
as it continued the whole day, and quite intercepted our view. The
pass takes its name of Portillo from a narrow cleft or doorway on
the highest ridge, through which the road passes. From this point,
on a clear day, those vast plains which uninterruptedly extend to
the Atlantic Ocean can be seen. We descended to the upper limit of
vegetation, and found good quarters for the night under the shelter
of some large fragments of rock. We met here some passengers, who
made anxious inquiries about the state of the road. Shortly after
it was dark the clouds suddenly cleared away, and the effect was
quite magical. The great mountains, bright with the full moon,
seemed impending over us on all sides, as over a deep crevice: one
morning, very early, I witnessed the same striking effect. As soon
as the clouds were dispersed it froze severely; but as there was no
wind, we slept very comfortably.

The increased brilliancy of the moon and stars at this elevation,
owing to the perfect transparency of the atmosphere, was very
remarkable. Travellers having observed the difficulty of judging
heights and distances amidst lofty mountains, have generally
attributed it to the absence of objects of comparison. It appears
to me, that it is fully as much owing to the transparency of the
air confounding objects at different distances, and likewise partly
to the novelty of an unusual degree of fatigue arising from a
little exertion, - habit being thus opposed to the evidence of the
senses. I am sure that this extreme clearness of the air gives a
peculiar character to the landscape, all objects appearing to be
brought nearly into one plane, as in a drawing or panorama. The
transparency is, I presume, owing to the equable and high state of
atmospheric dryness. This dryness was shown by the manner in which
woodwork shrank (as I soon found by the trouble my geological
hammer gave me); by articles of food, such as bread and sugar,
becoming extremely hard; and by the preservation of the skin and
parts of the flesh of the beasts which had perished on the road. To
the same cause we must attribute the singular facility with which
electricity is excited. My flannel-waistcoat, when rubbed in the
dark, appeared as if it had been washed with phosphorus, - every
hair on a dog's back crackled; - even the linen sheets, and leathern
straps of the saddle, when handled, emitted sparks.

MARCH 23, 1835.

The descent on the eastern side of the Cordillera is much shorter
or steeper than on the Pacific side; in other words, the mountains
rise more abruptly from the plains than from the alpine country of
Chile. A level and brilliantly white sea of clouds was stretched
out beneath our feet, shutting out the view of the equally level
Pampas. We soon entered the band of clouds, and did not again



Online LibraryCharles DarwinThe Voyage of the Beagle → online text (page 32 of 51)