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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.
The views put forward by Darwin on the origin of the crystalline schists
found an able advocate in Mr. Daniel Sharpe, who in 1852 and 1854 published
two papers, dealing with the geology of the Scottish Highlands and of the
Alps respectively, in which he showed that the principles arrived at by
Darwin when studying the South American rocks afford a complete explanation
of the structure of the two districts in question.
But, on the other hand, the conclusions of Darwin and Sharpe were met with
the strongest opposition by Sir Roderick Murchison and Dr. A. Geikie, who
in 1861 read a paper before the Geological Society "On the Coincidence
between Stratification and Foliation in the Crystalline Rocks of the
Scottish Highlands," in which they insisted that their observations in
Scotland tended to entirely disprove the conclusions of Darwin that
foliation in rocks is a secondary structure, and entirely independent of
the original stratification of the rock-masses.
Now it is a most significant circumstance that, no sooner did the officers
of the Geological Survey commence the careful and detailed study of the
Scottish Highlands than they found themselves compelled to make a formal
retraction of the views which had been put forward by Murchison and Geikie
in opposition to the conclusions of Darwin. The officers of the Geological
Survey have completely abandoned the view that the foliation of the
Highland rocks has been determined by their original stratification, and
admit that the structure is the result of the profound movements to which
the rocks have been subjected. The same conclusions have recently been
supported by observations made in many different districts--among which we
may especially refer to those of Dr. H. Reusch in Norway, and those of Dr.
J. Lehmann in Saxony. At the present time the arguments so clearly stated
by Darwin in the work before us, have, after enduring opposition or neglect
for a whole generation, begun to "triumph all along the line," and we may
look forward confidently to the near future, when his claim to be regarded
as one of the greatest of geological discoverers shall be fully vindicated.
JOHN W. JUDD.
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