Geological Observations on South America | Page 5

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