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Geological Observations On South America

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Etext Prepared by Sue Asscher asschers@dingoblue.net.au
Urangan, 17 June, 2001





GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA

by CHARLES DARWIN




EDITORIAL NOTE.

Although in some respects more technical in their subjects and style than
Darwin's "Journal," the books here reprinted will never lose their value
and interest for the originality of the observations they contain. Many
parts of them are admirably adapted for giving an insight into problems
regarding the structure and changes of the earth's surface, and in fact
they form a charming introduction to physical geology and physiography in
their application to special domains. The books themselves cannot be
obtained for many times the price of the present volume, and both the
general reader, who desires to know more of Darwin's work, and the student
of geology, who naturally wishes to know how a master mind reasoned on most
important geological subjects, will be glad of the opportunity of
possessing them in a convenient and cheap form.

The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.

G.T.B.




PLATE I. GEOLOGICAL SECTIONS THROUGH THE CORDILLERAS.

SECTION 1/1. SECTION OF THE PEUQUENES OR PORTILLO PASS OF THE CORDILLERA.

SECTION 1/2. SECTION OF THE CUMBRE OR USPALLATA PASS.

SECTION 1/3. SECTION OF THE VALLEY OF COPIAPO TO THE BASE OF THE MAIN
CORDILLERA.


PLATE II. MAP OF SOUTHERN PORTION OF SOUTH AMERICA.



TABLE OF CONTENTS.


CRITICAL INTRODUCTION.


CHAPTER I.--ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.

Upraised shells of La Plata.--Bahia Blanca, Sand-dunes and Pumice-pebbles.-
-Step-formed plains of Patagonia, with upraised shells.--Terrace-bounded
valley of Santa Cruz, formerly a sea-strait.--Upraised shells of Tierra del
Fuego.--Length and breadth of the elevated area.--Equability of the
movements, as shown by the similar heights of the plains.--Slowness of the
elevatory process.--Mode of formation of the step-formed plains.--Summary.-
-Great shingle formation of Patagonia; its extent, origin, and
distribution.--Formation of sea-cliffs.


CHAPTER II.--ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.

Chonos Archipelago.--Chiloe, recent and gradual elevation of, traditions of
the inhabitants on this subject.--Concepcion, earthquake and elevation of.-
-VALPARAISO, great elevation of, upraised shells, earth or marine origin,
gradual rise of the land within the historical period.--COQUIMBO, elevation
of, in recent times; terraces of marine origin, their inclination, their
escarpments not horizontal.--Guasco, gravel terraces of.--Copiapo.--PERU.--
Upraised shells of Cobija, Iquique, and Arica.--Lima, shell-beds and sea-
beach on San Lorenzo.--Human remains, fossil earthenware, earthquake
debacle, recent subsidence.--On the decay of upraised shells.--General
summary.


CHAPTER III.--ON THE PLAINS AND VALLEYS OF CHILE:--SALIFEROUS SUPERFICIAL
DEPOSITS.

Basin-like plains of Chile; their drainage, their marine origin.--Marks of
sea-action on the eastern flanks of the Cordillera.--Sloping terrace-like
fringes of stratified shingle within the valleys of the Cordillera; their
marine origin.--Boulders in the valley of Cachapual.--Horizontal elevation
of the Cordillera.--Formation of valleys.--Boulders moved by earthquake-
waves.--Saline superficial deposits.--Bed of nitrate of soda at Iquique.--
Saline incrustations.--Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.


CHAPTER IV.--ON THE FORMATIONS OF THE PAMPAS.

Mineralogical constitution.--Microscopical structure.--Buenos Ayres, shells
embedded in tosca-rock.--Buenos Ayres to the Colorado.--S. Ventana.--Bahia
Blanca; M. Hermoso, bones and infusoria of; P. Alta, shells, bones, and
infusoria of; co-existence of the recent shells and extinct mammifers.--
Buenos Ayres to St. Fe.--Skeletons of Mastodon.--Infusoria.--Inferior
marine tertiary strata, their age.--Horse's tooth. BANDA ORIENTAL.--
Superficial Pampean formation.--Inferior tertiary strata, variation of,
connected with volcanic action; Macrauchenia Patachonica at S. Julian in
Patagonia, age of, subsequent to living mollusca and to the erratic block
period. SUMMARY.--Area of Pampean formation.--Theories of origin.--Source
of sediment.--Estuary origin.--Contemporaneous with existing mollusca.--
Relations to underlying tertiary strata. Ancient deposit of estuary
origin.--Elevation and successive deposition of the Pampean formation.--
Number and state of the remains of mammifers; their habitation, food,
extinction, and range.--Conclusion.--Supplement on the thickness of the
Pampean formation.--Localities in Pampas at which mammiferous remains have
been found.


CHAPTER V.--ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.

Rio Negro.--S. Josef.--Port Desire, white pumiceous mudstone with
infusoria.--Port S. Julian.--Santa Cruz, basaltic lava of.--P. Gallegos.--
Eastern Tierra del Fuego; leaves of extinct beech-trees.--Summary on the
Patagonian tertiary formations.--Tertiary formations of the Western Coast.-
-Chonos and Chiloe groups, volcanic rocks of.--Concepcion.--Navidad.--
Coquimbo.--Summary.--Age of the tertiary formations.--Lines of elevation.--
Silicified wood.--Comparative ranges of the extinct and living mollusca on
the West Coast of S. America.--Climate of the tertiary period.--On the
causes of the absence of recent conchiferous deposits on the coasts of
South America.--On the contemporaneous deposition and preservation of
sedimentary formations.


CHAPTER VI.--PLUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.--Strike of
foliation.--Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.--La Plata, metamorphic and old volcanic rocks of.--S.
Ventana.--Claystone porphyry formation of Patagonia; singular metamorphic
rocks; pseudo-dikes.--Falkland Islands, palaeozoic fossils of.--Tierra del
Fuego, clay-slate formation, cretaceous fossils of; cleavage and foliation;
form of land.--Chonos Archipelago, mica-schists, foliation disturbed by
granitic axis; dikes.--Chiloe.--Concepcion, dikes, successive formation
of.--Central and Northern Chile.--Concluding remarks on cleavage and
foliation.--Their close analogy and similar origin.--Stratification of
metamorphic schists.--Foliation of intrusive rocks.--Relation of cleavage
and foliation to the lines of tension during metamorphosis.


CHAPTER VII.--CENTRAL CHILE:--STRUCTURE OF THE CORDILLERA.

Central Chile.--Basal formations of the Cordillera.--Origin of the
porphyritic clay-stone conglomerate.--Andesite.--Volcanic rocks.--Section
of the Cordillera by the Peuquenes or Portillo Pass.--Great gypseous
formation.--Peuquenes line; thickness of strata, fossils of.--Portillo
line.--Conglomerate, orthitic granite, mica-schist, volcanic rocks of.--
Concluding remarks on the denudation and elevation of the Portillo line.--
Section by the Cumbre, or Uspallata Pass.--Porphyries.--Gypseous strata.--
Section near the Puente del Inca; fossils of.--Great subsidence.--Intrusive
porphyries.--Plain of Uspallata.--Section of the Uspallata chain.--
Structure and nature of the strata.--Silicified vertical trees.--Great
subsidence.--Granitic rocks of axis.--Concluding remarks on the Uspallata
range; origin subsequent to that of the main Cordillera; two periods of
subsidence; comparison with the Portillo chain.


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.



INDEX.




GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA

BY

CHARLES DARWIN.




CRITICAL INTRODUCTION.

Of the remarkable "trilogy" constituted by Darwin's writings which deal
with the geology of the "Beagle," the member which has perhaps attracted
least attention, up to the present time is that which treats of the geology
of South America. The actual writing of this book appears to have occupied
Darwin a shorter period than either of the other volumes of the series; his
diary records that the work was accomplished within ten months, namely,
between July 1844 and April 1845; but the book was not actually issued till
late in the year following, the preface bearing the date "September 1846."
Altogether, as Darwin informs us in his "Autobiography," the geological
books "consumed four and a half years' steady work," most of the remainder
of the ten years that elapsed between the return of the "Beagle," and the
completion of his geological books being, it is sad to relate, "lost
through illness!"

Concerning the "Geological Observations on South America," Darwin wrote to
his friend Lyell, as follows:--"My volume will be about 240 pages,
dreadfully dull, yet much condensed. I think whenever you have time to look
through it, you will think the collection of facts on the elevation of the
land and on the formation of terraces pretty good."

"Much condensed" is the verdict that everyone must endorse, on rising from
the perusal of this remarkable book; but by no means "dull." The three and
a half years from April 1832 to September 1835, were spent by Darwin in
South America, and were devoted to continuous scientific work; the problems
he dealt with were either purely geological or those which constitute the
borderland between the geological and biological sciences. It is impossible
to read the journal which he kept during this time without being impressed
by the conviction that it contains all the germs of thought which
afterwards developed into the "Origin of Species." But it is equally
evident that after his return to England, biological speculations gradually
began to exercise a more exclusive sway over Darwin's mind, and tended to
dispossess geology, which during the actual period of the voyage certainly
engrossed most of his time and attention. The wonderful series of
observations made during those three and a half years in South America
could scarcely be done justice to, in the 240 pages devoted to their
exposition. That he executed the work of preparing the book on South
America in somewhat the manner of a task, is shown by many references in
his letters. Writing to Sir Joseph Hooker in 1845, he says, "I hope this
next summer to finish my South American Geology, then to get out a little
Zoology, and HURRAH FOR MY SPECIES WORK!"

It would seem that the feeling of disappointment, which Darwin so often
experienced in comparing a book when completed, with the observations and
speculations which had inspired it, was more keenly felt in the case of his
volume on South America than any other. To one friend he writes, "I have of
late been slaving extra hard, to the great discomfiture of wretched
digestive organs, at South America, and thank all the fates, I have done
three-fourths of it. Writing plain English grows with me more and more
difficult, and never attainable. As for your pretending that you will read
anything so dull as my pure geological descriptions, lay not such a
flattering unction on my soul, for it is incredible." To another friend he
writes, "You do not know what you threaten when you propose to read it--it
is purely geological. I said to my brother, 'You will of course read it,'
and his answer was, 'Upon my life, I would sooner even buy it.'"

In spite of these disparaging remarks, however, we are strongly inclined to
believe that this book, despised by its author, and neglected by his
contemporaries, will in the end be admitted to be one of Darwin's chief
titles to fame. It is, perhaps, an unfortunate circumstance that the great
success which he attained in biology by the publication of the "Origin of
Species" has, to some extent, overshadowed the fact that Darwin's claims as
a geologist, are of the very highest order. It is not too much to say that,
had Darwin not been a geologist, the "Origin of Species" could never have
been written by him. But apart from those geological questions, which have
an important bearing on biological thought and speculation, such as the
proofs of imperfection in the geological record, the relations of the later
tertiary faunas to the recent ones in the same areas, and the apparent
intermingling of types belonging to distant geological epochs, when we
study the palaeontology of remote districts,--there are other purely
geological problems, upon which the contributions made by Darwin are of the
very highest value. I believe that the verdict of the historians of science
will be that if Darwin had not taken a foremost place among the biologists
of this century, his position as a geologist would have been an almost
equally commanding one.

But in the case of Darwin's principal geological work--that relating to the
origin of the crystalline schists,--geologists were not at the time
prepared to receive his revolutionary teachings. The influence of powerful
authority was long exercised, indeed, to stifle his teaching, and only now,
when this unfortunate opposition has disappeared, is the true nature and
importance of Darwin's purely geological work beginning to be recognised.

The two first chapters of the "Geological Observations on South America,"
deal with the proofs which exist of great, but frequently interrupted,
movements of elevation during very recent geological times. In connection
with this subject, Darwin's particular attention was directed to the
relations between the great earthquakes of South America--of some of which
he had impressive experience--and the permanent changes of elevation which
were taking place. He was much struck by the rapidity with which the
evidence of such great earth movements is frequently obliterated; and
especially with the remarkable way in which the action of rain-water,
percolating through deposits on the earth's surface, removes all traces of
shells and other calcareous organisms. It was these considerations which
were the parents of the generalisation that a palaeontological record can
only be preserved during those periods in which long-continued slow
subsidence is going on. This in turn, led to the still wider and more
suggestive conclusion that the geological record as a whole is, and never
can be more than, a series of more or less isolated fragments. The
recognition of this important fact constitutes the keystone to any theory
of evolution which seeks to find a basis in the actual study of the types
of life that have formerly inhabited our globe.

In his third chapter, Darwin gives a number of interesting facts, collected
during his visits to the plains and valleys of Chili, which bear on the
question of the origin of saliferous deposits--the accumulation of salt,
gypsum, and nitrate of soda. This is a problem that has excited much
discussion among geologists, and which, in spite of many valuable
observations, still remains to a great extent very obscure. Among the
important considerations insisted upon by Darwin is that relating to the
absence of marine shells in beds associated with such deposits. He justly
argues that if the strata were formed in shallow waters, and then exposed
by upheaval to subaerial action, all shells and other calcareous organisms
would be removed by solution.

Following Lyell's method, Darwin proceeds from the study of deposits now
being accumulated on the earth's surface, to those which have been formed
during the more recent periods of the geological history.

His account of the great Pampean formation, with its wonderful mammalian
remains--Mastodon, Toxodon, Scelidotherium, Macrauchenia, Megatherium,
Megalonyx, Mylodon, and Glyptodon--this full of interest. His discovery of
the remains of a true Equus afforded a remarkable confirmation of the fact-
-already made out in North America--that species of horse had existed and
become extinct in the New World, before their introduction by the Spaniards
in the sixteenth century. Fully perceiving the importance of the microscope
in studying the nature and origin of such deposits as those of the Pampas,
Darwin submitted many of his specimens both to Dr. Carpenter in this
country, and to Professor Ehrenberg in Berlin. Many very important notes on
the microscopic organisms contained in the formation will be found
scattered through the chapter.

Darwin's study of the older tertiary formations, with their abundant
shells, and their relics of vegetable life buried under great sheets of
basalt, led him to consider carefully the question of climate during these
earlier periods. In opposition to prevalent views on this subject, Darwin
points out that his observations are opposed to the conclusion that a
higher temperature prevailed universally over the globe during early
geological periods. He argues that "the causes which gave to the older
tertiary productions of the quite temperate zones of Europe a tropical
character, WERE OF A LOCAL CHARACTER AND DID NOT AFFECT THE WHOLE GLOBE."
In this, as in many similar instances, we see the beneficial influence of
extensive travel in freeing Darwin's mind from prevailing prejudices. It
was this widening of experience which rendered him so especially qualified
to deal with the great problem of the origin of species, and in doing so to
emancipate himself from ideas which were received with unquestioning faith
by geologists whose studies had been circumscribed within the limits of
Western Europe.

In the Cordilleras of Northern and Central Chili, Darwin, when studying
still older formations, clearly recognised that they contain an admixture
of the forms of life, which in Europe are distinctive of the Cretaceous and
Jurassic periods respectively. He was thus led to conclude that the
classification of geological periods, which fairly well expresses the facts
that had been discovered in the areas where the science was first studied,
is no longer capable of being applied when we come to the study of widely
distant regions. This important conclusion led up to the further
generalisation that each great geological period has exhibited a
geographical distribution of the forms of animal and vegetable life,
comparable to that which prevails in the existing fauna and flora. To those
who are familiar with the extent to which the doctrine of universal
formations has affected geological thought and speculation, both long
before and since the time that Darwin wrote, the importance of this new
standpoint to which he was able to attain will be sufficiently apparent.
Like the idea of the extreme imperfection of the Geological Record, the
doctrine of LOCAL geological formations is found permeating and moulding
all the palaeontological reasonings of his great work.

In one of Darwin's letters, written while he was in South America, there is
a passage we have already quoted, in which he expresses his inability to
decide between the rival claims upon his attention of "the old crystalline
group of rocks," and "the softer fossiliferous beds" respectively. The
sixth chapter of the work before us, entitled "Plutonic and Metamorphic
Rocks--Cleavage and Foliation," contains a brief summary of a series of
observations and reasonings upon these crystalline rocks, which are, we
believe, calculated to effect a revolution in geological science, and--
though their value and importance have long been overlooked--are likely to
entitle Darwin in the future to a position among geologists, scarcely, if
at all, inferior to that which he already occupies among biologists.

Darwin's studies of the great rock-masses of the Andes convinced him of the
close relations between the granitic or Plutonic rocks, and those which
were undoubtedly poured forth as lavas. Upon his return, he set to work,
with the aid of Professor Miller, to make a careful study of the minerals
composing the granites and those which occur in the lavas, and he was able
to show that in all essential respects they are identical. He was further
able to prove that there is a complete gradation between the highly
crystalline or granitic rock-masses, and those containing more or less
glassy matter between their crystals, which constitute ordinary lavas. The
importance of this conclusion will be realised when we remember that it was
then the common creed of geologists--and still continues to be so on the
Continent--that all highly crystalline rocks are of great geological
antiquity, and that the igneous ejections which have taken place since the
beginning of the tertiary periods differ essentially, in their composition,
their structure, and their mode of occurrence, from those which have made
their appearance at earlier periods of the world's history.

Very completely have the conclusions of Darwin upon these subjects been
justified by recent researches. In England, the United States, and Italy,
examples of the gradual passage of rocks of truly granitic structure into
ordinary lavas have been described, and the reality of the transition has
been demonstrated by the most careful studies with the microscope. Recent
researches carried on in South America by Professor Stelzner, have also
shown the existence of a class of highly crystalline rocks--the
"Andengranites"--which combine in themselves many of the characteristics
which were once thought to be distinctive of the so-called Plutonic and
volcanic rocks. No one familiar with recent geological literature--even in
Germany and France, where the old views concerning the distinction of
igneous products of different ages have been most stoutly maintained--can
fail to recognise the fact that the principles contended for by Darwin bid
fair at no distant period to win universal acceptance among geologists all
over the globe.

Still more important are the conclusions at which Darwin arrived with
respect to the origin of the schists and gneisses which cover so large an
area in South America.

Carefully noting, by the aid of his compass and clinometer, at every point
which he visited, the direction and amount of inclination of the parallel
divisions in these rocks, he was led to a very important generalisation--
namely, that over very wide areas the direction (strike) of the planes of
cleavage in slates, and of foliation in schists and gneisses, remained
constant, though the amount of their inclination (dip) often varied within
wide limits. Further than this it appeared that there was always a close
correspondence between the strike of the cleavage and foliation and the
direction of the great axes along which elevation had taken place in the
district.

In Tierra del Fuego, Darwin found striking evidence that the cleavage
intersecting great masses of slate-rocks was quite independent of their
original stratification, and could often, indeed, be seen cutting across it
at right angles. He was also able to verify Sedgwick's observation that, in
some slates, glossy surfaces on the planes of cleavage arise from the
development of new minerals, chlorite, epidote or mica, and that in this
way a complete graduation from slates to true schists may be traced.

Darwin further showed that in highly schistose rocks, the folia bend around
and encircle any foreign bodies in the mass, and that in some cases they
exhibit the most tortuous forms and complicated puckerings. He clearly saw
that in all cases the forces by which these striking phenomena must have
been produced were persistent over wide areas, and were connected with the
great movements by which the rocks had been upheaved and folded.

That the distinct folia of quartz, feldspar, mica, and other minerals
composing the metamorphic schists could not have been separately deposited
as sediment was strongly insisted upon by Darwin; and in doing so he
opposed the view generally prevalent among geologists at that time. He was
thus driven to the conclusion that foliation, like cleavage, is not an
original, but a superinduced structure in rock-masses, and that it is the
result of re-crystallisation, under the controlling influence of great
pressure, of the materials of which the rock was composed.

In studying the lavas of Ascension, as we have already seen, Darwin was led
to recognise the circumstance that, when igneous rocks are subjected to
great differential movements during the period of their consolidation, they
acquire a foliated structure, closely analogous to that of the crystalline
schists. Like his predecessor in this field of inquiry, Mr. Poulett Scrope,
Charles Darwin seems to have been greatly impressed by these facts, and he
argued from them that the rocks exhibiting the foliated structure must have
been in a state of plasticity, like that of a cooling mass of lava. At that
time the suggestive experiments of Tresca, Daubree, and others, showing
that solid masses under the influence of enormous pressure become actually
plastic, had not been published. Had Darwin been aware of these facts he
would have seen that it was not necessary to assume a state of imperfect
solidity in rock-masses in order to account for their having yielded to
pressure and tension, and, in doing so, acquiring the new characters which
distinguish the crystalline schists.

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