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Critical Introduction
The preparation of the series of works published under the general title
"Geology of the Voyage of the 'Beagle'" occupied a great part of Darwin's
time during the ten years that followed his return to England. The second
volume of the series, entitled "Geological Observations on Volcanic
Islands, with Brief Notices on the Geology of Australia and the Cape of
Good Hope," made its appearance in 1844. The materials for this volume were
collected in part during the outward voyage, when the "Beagle" called at
St. Jago in the Cape de Verde Islands, and St. Paul's Rocks, and at
Fernando Noronha, but mainly during the homeward cruise; then it was that
the Galapagos Islands were surveyed, the Low Archipelago passed through,
and Tahiti visited; after making calls at the Bay of Islands, in New
Zealand, and also at Sydney, Hobart Town and King George's Sound in
Australia, the "Beagle" sailed across the Indian Ocean to the little group
of the Keeling or Cocos Islands, which Darwin has rendered famous by his
observations, and thence to Mauritius; calling at the Cape of Good Hope on
her way, the ship then proceeded successively to St. Helena and Ascension,
and revisited the Cape de Verde Islands before finally reaching England.
Although Darwin was thus able to gratify his curiosity by visits to a great
number of very interesting volcanic districts, the voyage opened for him
with a bitter disappointment. He had been reading Humboldt's "Personal
Narrative" during his last year's residence in Cambridge, and had copied
out from it long passages about Teneriffe. He was actually making inquiries
as to the best means of visiting that island, when the offer was made to
him to accompany Captain Fitzroy in the "Beagle. " His friend Henslow too,
on parting with him, had given him the advice to procure and read the
recently published first volume of the "Principles of Geology," though he
warned him against accepting the views advocated by its author. During the
time the "Beagle" was beating backwards and forwards when the voyage
commenced, Darwin, although hardly ever able to leave his berth, was
employing all the opportunities which the terrible sea-sickness left him,
in studying Humboldt and Lyell. We may therefore form an idea of his
feelings when, on the ship reaching Santa Cruz, and the Peak of Teneriffe
making its appearance among the clouds, they were suddenly informed that an
outbreak of cholera would prevent any landing!
Ample compensation for this disappointment was found, however, when the
ship reached Porta Praya in St. Jago, the largest of the Cape de Verde
Islands. Here he spent three most delightful weeks, and really commenced
his work as a geologist and naturalist. Writing to his father he says,
"Geologising in a volcanic country is most delightful; besides the interest
attached to itself, it leads you into most beautiful and retired spots.
Nobody but a person fond of Natural History can imagine the pleasure of
strolling under cocoa-nuts in a thicket of bananas and coffee-plants, and
an endless number of wild flowers. And this island, that has given me so
much instruction and delight, is reckoned the most uninteresting place that
we perhaps shall touch at during our voyage. It certainly is generally very
barren, but the valleys are more exquisitely beautiful, from the very
contrast. It is utterly useless to say anything about the scenery; it would
be as profitable to explain to a blind man colours, as to a person who has
not been out of Europe, the total dissimilarity of a tropical view.
Whenever I enjoy anything, I always look forward to writing it down, either
in my log-book (which increases in bulk), or in a letter; so you must
excuse raptures, and those raptures badly expressed. I find my collections
are increasing wonderfully, and from Rio I think I shall be obliged to send
a cargo home."
The indelible impression made on Darwin's mind by this first visit to a
volcanic island, is borne witness to by a remarkable passage in the
"Autobiography" written by him in 1876. "The geology of St. Jago is very
striking, yet simple; a stream of lava formerly flowed over the bed of the
sea, formed of triturated recent shells and corals, which it has baked into
a hard white rock. Since then the whole island has been upheaved. But the
line of white rock revealed to me a new and important fact, namely that
there had been afterwards subsidence round the craters which had since been
in action, and had poured forth lava. It then first dawned on me that I
might perhaps write a book on the geology of the various countries visited,
and this made me thrill with delight. That was a memorable hour to me, and
how distinctly I can call to mind the low cliff of lava beneath which I
rested, with the sun glaring hot, a few strange desert plants growing near
and with living corals in the tidal pools at my feet."
Only five years before, when listening to poor Professor Jameson's lectures
on the effete Wernerianism, which at that time did duty for geological
teaching, Darwin had found them "incredibly dull," and he declared that
"the sole effect they produced on me was a determination never so long as I
lived to read a book on Geology, or in any way to study the science."
What a contrast we find in the expressions which he makes use of in
referring to Geological Science, in his letters written home from the
"Beagle!" After alluding to the delight of collecting and studying marine
animals, he exclaims, "But Geology carries the day!" Writing to Henslow he
says, "I am quite charmed with Geology, but, like the wise animal between
two bundles of hay, I do not know which to like best; the old crystalline
group of rocks, or the softer and more fossiliferous beds." And just as the
long voyage is about to come to a close he again writes, "I find in Geology
a never-failing interest; as it has been remarked, it creates the same
grand ideas respecting this world which Astronomy does for the Universe."
In this passage Darwin doubtless refers to a remark of Sir John Herschel's
in his admirable "Preliminary Discourse on the Study of Natural
Philosophy,"--a book which exercised a most remarkable and beneficial
influence on the mind of the young naturalist.
If there cannot be any doubt as to the strong predilection in Darwin's mind
for geological studies, both during and after the memorable voyage, there
is equally little difficulty in perceiving the school of geological thought
which, in spite of the warnings of Sedgwick and Henslow, had obtained
complete ascendancy over his mind. He writes in 1876: "The very first place
which I examined, namely St. Jago in the Cape de Verde Islands, showed me
clearly the wonderful superiority of Lyell's manner of treating Geology,
compared with that of any other author, whose works I had with me, or ever
afterwards read." And again, "The science of Geology is enormously indebted
to Lyell--more so, as I believe, than to any other man who ever lived...I
am proud to remember that the first place, namely, St. Jago, in the Cape de
Verde Archipelago, in which I geologised, convinced me of the infinite
superiority of Lyell's views over those advocated in any other work known
to me."
The passages I have cited will serve to show the spirit in which Darwin
entered upon his geological studies, and the perusal of the following pages
will furnish abundant proofs of the enthusiasm, acumen, and caution with
which his researches were pursued.
Large collections of rocks and minerals were made by Darwin during his
researches, and sent home to Cambridge, to be kept under the care of his
faithful friend Henslow. After visiting his relations and friends, Darwin's
first care on his return to England was to unpack and examine these
collections. He accordingly, at the end of 1836, took lodgings for three
months in Fitzwilliam Street, Cambridge, so as to be near Henslow; and in
studying and determining his geological specimens received much valuable
aid from the eminent crystallographer and mineralogist, Professor William
Hallows Miller.
The actual writing of the volume upon volcanic islands was not commenced
till 1843, when Darwin had settled in the spot which became his home for
the rest of his life--the famous house at Down, in Kent. Writing to his
friend Mr. Fox, on March 28th, 1843, he says, "I am very slowly progressing
with a volume, or rather pamphlet, on the volcanic islands which we
visited: I manage only a couple of hours per day, and that not very
regularly. It is uphill work writing books, which cost money in publishing,
and which are not read even by geologists."
The work occupied Darwin during the whole of the year 1843, and was issued
in the spring of the following year, the actual time engaged in preparing
it being recorded in his diary as "from the summer of 1842 to January
1844;" but the author does not appear to have been by any means satisfied
with the result when the book was finished. He wrote to Lyell, "You have
pleased me much by saying that you intend looking through my 'Volcanic
Islands;' it cost me eighteen months!!! and I have heard of very few who
have read it. Now I shall feel, whatever little (and little it is) there is
confirmatory of old work, or new, will work its effect and not be lost." To
Sir Joseph Hooker he wrote, "I have just finished a little volume on the
volcanic islands which we visited. I do not know how far you care for dry
simple geology, but I hope you will let me send you a copy."
Every geologist knows how full of interest and suggestiveness is this book
of Darwin's on volcanic islands. Probably the scant satisfaction which its
author seemed to find in it may be traced to the effect of a contrast which
he felt between the memory of glowing delights he had experienced when,
hammer in hand, he roamed over new and interesting scenes, and the slow,
laborious, and less congenial task of re-writing and arranging his notes in
book-form.
In 1874, in writing an account of the ancient volcanoes of the Hebrides, I
had frequent occasion to quote Mr. Darwin's observations on the Atlantic
volcanoes, in illustration of the phenomena exhibited by the relics of
still older volcanoes in our own islands. Darwin, in writing to his old
friend Sir Charles Lyell upon the subject, says, "I was not a little
pleased to see my volcanic book quoted, for I thought it was completely
dead and forgotten."
Two years later the original publishers of this book and of that on South
America proposed to re-issue them. Darwin at first hesitated, for he seemed
to think there could be little of abiding interest in them; he consulted me
upon the subject in one of the conversations which I used to have with him
at that time, and I strongly urged upon him the reprint of the works. I was
much gratified when he gave way upon the point, and consented to their
appearing just as originally issued. In his preface he says, "Owing to the
great progress which Geology has made in recent times, my views on some few
points may be somewhat antiquated, but I have thought it best to leave them
as they originally appeared."
It may be interesting to indicate, as briefly as possible, the chief
geological problem upon which the publication of Darwin's "Volcanic
Islands" threw new and important light. The merit of the work consisted in
supplying interesting observations, which in some cases have proved of
crucial value in exploding prevalent fallacies; in calling attention to
phenomena and considerations that had been quite overlooked by geologists,
but have since exercised an important influence in moulding geological
speculation; and lastly in showing the importance which attaches to small
and seemingly insignificant causes, some of which afford a key to the
explanation of very curious geological problems.
Visiting as he did the districts in which Von Buch and others had found
what they thought to be evidence of the truth of "Elevation-craters,"
Darwin was able to show that the facts were capable of a totally different
interpretation. The views originally put forward by the old German
geologist and traveller, and almost universally accepted by his countrymen,
had met with much support from Elie de Beaumont and Dufrenoy, the leaders
of geological thought in France. They were, however, stoutly opposed by
Scrope and Lyell in this country, and by Constant Prevost and Virlet on the
other side of the channel. Darwin, in the work before us, shows how little
ground there is for the assumption that the great ring-craters of the
Atlantic islands have originated in gigantic blisters of the earth's
surface which, opening at the top, have given origin to the craters.
Admitting the influence of the injection of lava into the structure of the
volcanic cones, in increasing their bulk and elevation, he shows that, in
the main, the volcanoes are built up by repeated ejections causing an
accumulation of materials around the vent.
While, however, agreeing on the whole with Scrope and Lyell, as to the
explosive origin of ordinary volcanic craters, Darwin clearly saw that, in
some cases, great craters might be formed or enlarged, by the subsidence of
the floors after eruptions. The importance of this agency, to which too
little attention has been directed by geologists, has recently been shown
by Professor Dana, in his admirable work on Kilauea and the other great
volcanoes of the Hawaiian Archipelago.
The effects of subsidence at a volcanic centre in producing a downward dip
of the strata around it, was first pointed out by Darwin, as the result of
his earliest work in the Cape de Verde Islands. Striking illustrations of
the same principle have since been pointed out by M. Robert and others in
Iceland, by Mr. Heaphy in New Zealand, and by myself in the Western Isles
of Scotland.
Darwin again and again called attention to the evidence that volcanic vents
exhibit relations to one another which can only be explained by assuming
the existence of lines of fissure in the earth's crust, along which the
lavas have made their way to the surface. But he, at the same time, clearly
saw that there was no evidence of the occurrence of great deluges of lava
along such fissures; he showed how the most remarkable plateaux, composed
of successive lava sheets, might be built up by repeated and moderate
ejections from numerous isolated vents; and he expressly insists upon the
rapidity with which the cinder-cones around the orifices of ejection and
the evidences of successive outflows of lava would be obliterated by
denudation.
One of the most striking parts of the book is that in which he deals with
the effects of denudation in producing "basal wrecks" or worn down stumps
of volcanoes. He was enabled to examine a series of cases in which could be
traced every gradation, from perfect volcanic cones down to the solidified
plugs which had consolidated in the vents from which ejections had taken
place. Darwin's observations on these points have been of the greatest
value and assistance to all who have essayed to study the effects of
volcanic action during earlier periods of the earth's history. Like Lyell,
he was firmly persuaded of the continuity of geological history, and ever
delighted in finding indications, in the present order of nature, that the
phenomena of the past could be accounted for by means of causes which are
still in operation. Lyell's last work in the field was carried on about his
home in Forfarshire, and only a few months before his death he wrote to
Darwin: "All the work which I have done has confirmed me in the belief that
the only difference between Palaeozoic and recent volcanic rocks is no more
than we must allow for, by the enormous time to which the products of the
oldest volcanoes have been subjected to chemical changes."
Darwin was greatly impressed, as the result of his studies of volcanic
phenomena, followed by an examination of the great granite-masses of the
Andes, with the relations between the so-called Plutonic rocks and those of
undoubtedly volcanic origin. It was indeed a fortunate circumstance, that
after studying some excellent examples of recent volcanic rocks, he
proceeded to examine in South America many fine illustrations of the older
igneous rock-masses, and especially of the most highly crystalline types of
the same, and then on his way home had opportunities of reviving the
impression made upon him by the fresh and unaltered volcanic rocks. Some of
the general considerations suggested by these observations were discussed
in a paper read by him before the Geological Society, on March 7th, 1838,
under the title "On the Connection of Certain Volcanic Phenomena, and On
the Formation of Mountain-chains, and the Effect of Continental
Elevations." The exact bearing of these two classes of facts upon one
another are more fully discussed in his book on South American geology.
The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in which
upward movements were taking place, and he was naturally led to contrast
them with the areas in which, as he showed, the occurrence of atolls,
encircling reefs, and barrier-reefs afford indication of subsidence. In
this way he was able to map out the oceanic areas in different zones, along
which opposite kinds of movement were taking place. His conclusions on this
subject were full of novelty and suggestiveness.
Very clearly did Darwin recognise the importance of the fact that most of
the oceanic islands appear to be of volcanic origin, though he was careful
to point out the remarkable exceptions which somewhat invalidate the
generalisation. In his "Origin of Species" he has elaborated the idea and
suggested the theory of the permanence of ocean-basins, a suggestion which
has been adopted and pushed farther by subsequent authors, than we think
its originator would have approved. His caution and fairness of mind on
this and similar speculative questions was well-known to all who were in
the habit of discussing them with him.
Some years before the voyage of the "Beagle," Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the foliated
crystalline schists. It does not appear that Darwin was acquainted with
this remarkable memoir, but quite independently he called attention to the
same phenomena when he came to study some very similar rocks which occur in
the island of Ascension. Coming fresh from the study of the great masses of
crystalline schist in the South American continent, he was struck by the
circumstance that in the undoubtedly igneous rocks of Ascension we find a
similar separation of the constituent minerals along parallel "folia."
These observations led Darwin to the same conclusion as that arrived at
some time before by Scrope--namely that when crystallisation takes place in
rock masses under the influence of great deforming stresses, a separation
and parallel arrangement of the constituent minerals will result. This is a
process which is now fully recognised as having been a potent factor in the
production of the metamorphic rock, and has been called by more recent
writers "dynamo-metamorphism."
In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was in
making out the true facts with regard to the rocks he studied by the simple
aid of a penknife and pocket-lens, supplemented by a few chemical tests and
the constant use of the blowpipe. Since his day, the method of study of
rocks by thin sections under the microscope has been devised, and has
become a most efficient aid in all petrographical inquiries. During the
voyage of H.M.S. "Challenger," many of the islands studied by Darwin have
been revisited and their rocks collected. The results of their study by one
of the greatest masters of the science of micropetrography--Professor
Renard of Brussels--have been recently published in one of the volumes of
"Reports on the 'Challenger' Expedition." While much that is new and
valuable has been contributed to geological science by these more recent
investigations, and many changes have been made in nomenclature and other
points of detail, it is interesting to find that all the chief facts
described by Darwin and his friend Professor Miller have stood the test of
time and further study, and remain as a monument of the acumen and accuracy
in minute observation of these pioneers in geological research.
JOHN W. JUDD.
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