Chapter 6 (beginning)

CHAPTER VI.--ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.

(DESCRIPTION OF THE PLATES.

PLATE III.--MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE
VOLCANOES.

The principles, on which this map was coloured, are explained in the
beginning of Chapter VI.; and the authorities for each particular spot are
detailed in the Appendix to "Coral Reefs."  The names not printed in upper
case in the Index refer to the Appendix.)

Description of the coloured map.--Proximity of atolls and barrier-reefs.--
Relation in form and position of atolls with ordinary islands.--Direct
evidence of subsidence difficult to be detected.--Proofs of recent
elevation where fringing-reefs occur.--Oscillations of level.--Absence of
active volcanoes in the areas of subsidence.--Immensity of the areas which
have been elevated and have subsided.--Their relation to the present
distribution of the land.--Areas of subsidence elongated, their
intersection and alternation with those of elevation.--Amount and slow rate
of the subsidence.--Recapitulation.

It will be convenient to give here a short account of the appended map
(Plate III.)  [Inasmuch as the coloured map would have proved too costly to
be given in this series, the indications of colour have been replaced by
numbers referring to the dotted groups of reefs, etc.  The author's
original wording, however, is retained in full, as it will be easy to refer
to the map by the numbers, and thus the flow of the narrative is
undisturbed.]: a fuller one, with the data for colouring each spot, is
reserved for the Appendix; and every place there referred to may be found
in the Index.  A larger chart would have been desirable; but, small as the
adjoined one is, it is the result of many months' labour.  I have
consulted, as far as I was able, every original voyage and map; and the
colours were first laid down on charts on a larger scale.  The same blue
colour, with merely a difference in the depth of tint, is used for atolls
or lagoon-islands, and barrier-reefs, for we have seen, that as far as the
actual coral-formation is concerned, they have no distinguishing character.
Fringing-reefs have been coloured red, for between them on the one hand,
and barrier-reefs and atolls on the other, there is an important
distinction with respect to the depth beneath the surface, at which we are
compelled to believe their foundations lie.  The two distinct colours,
therefore, mark two great types of structure.

The DARK BLUE COLOUR [represented by (3) in our plate] represents atolls
and submerged annular reefs, with deep water in their centres.  I have
coloured as atolls, a few low and small coral-islands, without lagoons; but
this has been done only when it clearly appeared that they originally
contained lagoons, since filled up with sediment: when there were not good
grounds for this belief, they have been left uncoloured.

The PALE BLUE COLOUR [represented by (2)] represents barrier-reefs.  The
most obvious character of reefs of this class is the broad and deep-water
moat within the reef: but this, like the lagoons of small atolls, is
liable to become filled up with detritus and with reefs of delicately
branched corals: when, therefore, a reef round the entire circumference of
an island extends very far into a profoundly deep sea, so that it can
hardly be confounded with a fringing-reef which must rest on a foundation
of rock within a small depth, it has been coloured pale blue, although it
does not include a deep-water moat: but this has only been done rarely,
and each case is distinctly mentioned in the Appendix.

The RED COLOUR (4) represents reefs fringing the land quite closely where
the sea is deep, and where the bottom is gently inclined extending to a
moderate distance from it, but not having a deep-water moat or lagoon-like
space parallel to the shore.  It must be remembered that fringing-reefs are
frequently BREACHED in front of rivers and valleys by deepish channels,
where mud has been deposited.  A space of thirty miles in width has been
coloured round or in front of the reefs of each class, in order that the
colours might be conspicuous on the appended map, which is reduced to so
small a scale.

The VERMILLION SPOTS, and streaks (1) represent volcanoes now in action, or
historically known to have been so.  They are chiefly laid down from Von
Buch's work on the Canary Islands; and my reasons for making a few
alterations are given in the note below.

(I have also made considerable use of the geological part of Berghaus'
"Physical Atlas."  Beginning at the eastern side of the Pacific, I have
added to the number of the volcanoes in the southern part of the
Cordillera, and have coloured Juan Fernandez according to observations
collected during the voyage of the "Beagle" ("Geological Transactions,"
volume v., page 601.)  I have added a volcano to Albemarle Island, one of
the Galapagos Archipelago (the author's "Journal of Researches," page 457).
In the Sandwich group there are no active volcanoes, except at Hawaii; but
the Rev. W. Ellis informs me, there are streams of lava apparently modern
on Maui, having a very recent appearance, which can be traced to the
craters whence they flowed.  The same gentleman informs me, that there is
no reason to believe that any active volcano exists in the Society
Archipelago; nor are there any known in the Samoa or Navigator group,
although some of the streams of lava and craters there appear recent.  In
the Friendly group, the Rev. J. Williams says ("Narrative of Missionary
Enterprise," page 29) that Toofoa and Proby Islands are active volcanoes.
I infer from Hamilton's "Voyage in the 'Pandora'" (Page 95), that Proby
Island is synonymous with Onouafou, but I have not ventured to colour it.
There can be no doubt respecting Toofoa, and Captain Edwards (Von Buch,
page 386) found the lava of recent eruption at Amargura still smoking.
Berghaus marks four active volcanoes actually within the Friendly group;
but I do not know on what authority: I may mention that Maurelle describes
Latte as having a burnt-up appearance: I have marked only Toofoa and
Amargura.  South of the New Hebrides lies Matthews Rock, which is drawn and
described as an active crater in the "Voyage of the 'Astrolabe'."  Between
it and the volcano on the eastern side of New Zealand, lies Brimstone
Island, which from the high temperature of the water in the crater, may be
ranked as active (Berghaus "Vorbemerk," II Lief. S. 56).  Malte Brun,
volume xii., page 231, says that there is a volcano near port St. Vincent
in New Caledonia.  I believe this to be an error, arising from a smoke seen

on the OPPOSITE coast by Cook ("Second Voyage," volume ii., page 23) which
smoke went out at night.  The Mariana Islands, especially the northern
ones, contain many craters (see Freycinet's "Hydrog. Descript.") which are
not active.  Von Buch, however, states (page 462) on the authority of La
Peyrouse, that there are no less than seven volcanoes between these islands
and Japan.  Gemelli Creri (Churchill's "Collect." volume iv., page 458),
says there are two active volcanoes in latitude 23 deg 30', and in latitude
24 deg: but I have not coloured them.  From the statements in Beechey's
"Voyage" (page 518, 4to edition) I have coloured one in the northern part
of the Bonin group.  M. S. Julien has clearly made out from Chinese
manuscripts not very ancient ("Comptes Rendus," 1840, page 832), that there
are two active volcanoes on the eastern side of Formosa.  In Torres
Straits, on Cap Island (9 deg 48' S., 142 deg 39' E.) a volcano was seen
burning with great violence in 1793 by Captain Bampton (see Introduction to
Flinders' "Voyage," page 41).  Mr. M'Clelland (Report of Committee for
investigating Coal in India, page 39) has shown that the volcanic band
passing through Barren Island must be extended northwards.  It appears by
an old chart, that Cheduba was once an active volcano (see also "Silliman's
North American Journal", volume xxxviii., page 385).  In Berghaus'
"Physical Atlas," 1840, No. 7 of Geological Part, a volcano on the coast of
Pondicherry is said to have burst forth in 1757.  Ordinaire ("Hist. Nat.
des Volcans," page 218) says that there is one at the mouth of the Persian
Gulf, but I have not coloured it, as he gives no particulars.  A volcano in
Amsterdam, or St. Paul's, in the southern part of the Indian Ocean, has
been seen ("Naut. Mag." 1838, page 842) in action.  Dr. J. Allan, of
Forres, informs me in a letter, that when he was at Joanna, he saw at night
flames apparently volcanic, issuing from the chief Comoro Island, and that
the Arabs assured him that they were volcanic, adding that the volcano
burned more during the wet season.  I have marked this as a volcano, though
with some hesitation, on account of the possibility of the flame arising
from gaseous sources.)

The uncoloured coasts consist, first and chiefly, of those, where there are
no coral-reefs, or such small portions as to be quite insignificant.
Secondly, of those coasts where there are reefs, but where the sea is very
shallow, for in this case the reefs generally lie far from the land, and
become very irregular, in their forms: where they have not become
irregular, they have been coloured.  thirdly, if I had the means of
ascertaining the fact, I should not colour a reef merely coating the edges
of a submarine crater, or of a level submerged bank; for such superficial
formations differ essentially, even when not in external appearance, from
reefs whose foundations as well as superficies have been wholly formed by
the growth of coral.  Fourthly, in the Red Sea, and within some parts of
the East Indian Archipelago (if the imperfect charts of the latter can be
trusted), there are many scattered reefs, of small size, represented in the
chart by mere dots, which rise out of deep water: these cannot be arranged
under either of the three classes: in the Red Sea, however, some of these
little reefs, from their position, seem once to have formed parts of a
continuous barrier.  There exist, also, scattered in the open ocean, some
linear and irregularly formed strips of coral-reef, which, as shown in the
last chapter, are probably allied in their origin to atolls; but as they do
not belong to that class, they have not been coloured; they are very few in
number and of insignificant dimensions.  Lastly, some reefs are left
uncoloured from the want of information respecting them, and some because
they are of an intermediate structure between the barrier and fringing
classes.  The value of the map is lessened, in proportion to the number of
reefs which I have been obliged to leave uncoloured, although, in a
theoretical point of view, few of them present any great difficulty: but
their number is not very great, as will be found by comparing the map with
the statements in the Appendix.  I have experienced more difficulty in
colouring fringing-reefs than in colouring barrier-reefs, as the former,
from their much less dimensions, have less attracted the attention of
navigators.  As I have had to seek my information from all kinds of
sources, and often from indirect ones, I do not venture to hope that the
map is free from many errors.  Nevertheless, I trust it will give an
approximately correct view of the general distribution of the coral-reefs
over the whole world (with the exception of some fringing-reefs on the
coast of Brazil, not included within the limits of the map), and of their
arrangement into the three great classes, which, though necessarily very
imperfect from the nature of the objects classified, have been adopted by
most voyagers.  I may further remark, that the dark blue colour represents
land entirely composed of coral-rock; the pale blue, land with a wide and
thick border of coral-rock; and the red, a mere narrow fringe of
coral-rock.

Looking now at the map under the theoretical point of view indicated in the
last chapter, the two blue tints signify that the foundations of the reefs
thus coloured have subsided to a considerable amount, at a slower rate than
that of the upward growth of the corals, and that probably in many cases
they are still subsiding.  The red signifies that the shores which support
fringing-reefs have not subsided (at least to any considerable amount, for
the effects of a subsidence on a small scale would in no case be
distinguishable); but that they have remained nearly stationary since the
period when they first became fringed by reefs; or that they are now rising
or have been upraised, with new lines of reefs successively formed on them:
these latter alternatives are obviously implied, as newly formed lines of
shore, after elevations of the land, would be in the same state with
respect to the growth of fringing-reefs, as stationary coasts.  If during
the prolonged subsidence of a shore, coral-reefs grew for the first time on
it, or if an old barrier-reef were destroyed and submerged, and new reefs
became attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of error,
any coast has been coloured wrongly with respect to movement indicated.
Well characterised atolls and encircling barrier-reefs, where several occur
in a group, or a single barrier-reef if of large dimensions, leave scarcely
any doubt on the mind respecting the movement by which they have been
produced; and even a small amount of subsequent elevation is soon betrayed.
The evidence from a single atoll or a single encircling barrier-reef, must
be received with some caution, for the former may possibly be based upon a
submerged crater or bank, and the latter on a submerged margin of sediment,
or of worn-down rock.  From these remarks we may with greater certainty
infer that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or have
been upraised.

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Critical introduction by John W. Judd

THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.


CRITICAL INTRODUCTION.

A scientific discovery is the outcome of an interesting process of
evolution in the mind of its author.  When we are able to detect the germs
of thought in which such a discovery has originated, and to trace the
successive stages of the reasoning by which the crude idea has developed
into an epoch-making book, we have the materials for reconstructing an
important chapter of scientific history.  Such a contribution to the story
of the "making of science" may be furnished in respect to Darwin's famous
theory of coral-reefs, and the clearly reasoned treatise in which it was
first fully set forth.

The subject of corals and coral-reefs is one concerning which much popular
misconception has always prevailed.  The misleading comparison of coral-rock
with the combs of bees and the nests of wasps is perhaps responsible
for much of this misunderstanding; one writer has indeed described a
coral-reef as being "built by fishes by means of their teeth."  Scarcely
less misleading, however, are the references we so frequently meet with,
both in prose and verse, to the "skill," "industry," and "perseverance" of
the "coral-insect" in "building" his "home."  As well might we praise men
for their cleverness in making their own skeletons, and laud their assiduity
in filling churchyards with the same.  The polyps and other organisms, whose
remains accumulate to form a coral-reef, simply live and perform their
natural functions, and then die, leaving behind them, in the natural course
of events, the hard calcareous portions of their structures to add to the
growing reef.

While the forms of coral-reefs and coral-islands are sometimes very
remarkable and worthy of attentive study, there is no ground, it need
scarcely be added, for the suggestion that they afford proofs of design on
the part of the living builders, or that, in the words of Flinders, they
constitute breastworks, defending the workshops from whence "infant
colonies might be safely sent forth."

It was not till the beginning of the present century that travellers like
Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others, began to
collect accurate details concerning the forms and structure of coral-masses,
and to make such observations on the habits of reef-forming polyps,
as might serve as a basis for safe reasoning concerning the origin of
coral-reefs and islands.  In the second volume of Lyell's "Principles of
Geology," published in 1832, the final chapter gives an admirable summary
of all that was then known on the subject.  At that time, the ring-form of
the atolls was almost universally regarded as a proof that they had grown
up on submerged volcanic craters; and Lyell gave his powerful support to
that theory.

Charles Darwin was never tired of acknowledging his indebtedness to Lyell.
In dedicating to his friend the second edition of his "Naturalist's Voyage
Round the World," Darwin writes that he does so "with grateful pleasure, as
an acknowledgment that the chief part of whatever scientific merit this
journal and the other works of the author may possess, has been derived
from studying the well-known and admirable 'Principles of Geology.'"

The second volume of Lyell's "Principles" appeared after Darwin had left
England; but it was doubtless sent on to him without delay by his faithful
friend and correspondent, Professor Henslow.  It appears to have reached
Darwin at a most opportune moment, while, in fact, he was studying the
striking evidences of slow and long-continued, but often interrupted
movement on the west coast of South America.  Darwin's acute mind could not
fail to detect the weakness of the then prevalent theory concerning the
origin of the ring-shaped atolls--and the difficulty which he found in
accepting the volcanic theory, as an explanation of the phenomena of
coral-reefs, is well set forth in his book.

In an interesting fragment of autobiography, Darwin has given us a very
clear account of the way in which the leading idea of the theory of
coral-reefs originated in his mind; he writes, "No other work of mine was
begun in so deductive a spirit as this, for the whole theory was thought
out on the west coast of South America, before I had seen a true
coral-reef.  I had therefore only to verify and extend my views by a
careful examination of living reefs.  But it should be observed that I had
during the two previous years been incessantly attending to the effects on
the shores of South America of the intermittent elevation of the land,
together with the denudation and deposition of sediment.  This necessarily
led me to reflect much on the effects of subsidence, and it was easy to
replace in imagination the continued deposition of sediment by the upward
growth of corals.  To do this was to form my theory of the formation of
barrier-reefs and atolls."

On her homeward voyage, the "Beagle" visited Tahiti, Australia, and some of
the coral-islands in the Indian Ocean, and Darwin had an opportunity of
testing and verifying the conclusion at which he had arrived by studying
the statements of other observers.

I well recollect a remarkable conversation I had with Darwin, shortly after
the death of Lyell.  With characteristic modesty, he told me that he never
fully realised the importance of his theory of coral-reefs till he had an
opportunity of discussing it with Lyell, shortly after the return of the
"Beagle".  Lyell, on receiving from the lips of its author a sketch of the
new theory, was so overcome with delight that he danced about and threw
himself into the wildest contortions, as was his manner when excessively
pleased.  He wrote shortly afterwards to Darwin as follows:--"I could think
of nothing for days after your lesson on coral-reefs, but of the tops of
submerged continents.  It is all true, but do not flatter yourself that you
will be believed till you are growing bald like me, with hard work and
vexation at the incredulity of the world."  On May 24th, 1837, Lyell wrote
to Sir John Herschel as follows:--"I am very full of Darwin's new theory of
coral-islands, and have urged Whewell to make him read it at our next
meeting.  I must give up my volcanic crater forever, though it cost me a
pang at first, for it accounted for so much."  Dr. Whewell was president of
the Geological Society at the time, and on May 31st, 1837, Darwin read a
paper entitled "On Certain Areas of Elevation and Subsidence in the Pacific
and Indian oceans, as deduced from the Study of Coral Formations," an
abstract of which appeared in the second volume of the Society's
proceedings.

It was about this time that Darwin, having settled himself in lodgings at
Great Marlborough Street, commenced the writing of his book on "Coral-Reefs."
Many delays from ill-health and the interruption of other work,
caused the progress to be slow, and his journal speaks of "recommencing"
the subject in February 1839, shortly after his marriage, and again in
October of the same year.  In July 1841, he states that he began once more
"after more than thirteen month's interval," and the last proof-sheet of
the book was not corrected till May 6th, 1842.  Darwin writes in his
autobiography, "This book, though a small one, cost me twenty months of
hard work, as I had to read every work on the islands of the Pacific, and
to consult many charts."  The task of elaborating and writing out his books
was, with Darwin, always a very slow and laborious one; but it is clear
that in accomplishing the work now under consideration, there was a long
and constant struggle with the lethargy and weakness resulting from the sad
condition of his health at that time.

Lyell's anticipation that the theory of coral-reefs would be slow in
meeting with general acceptance was certainly not justified by the actual
facts.  On the contrary the new book was at once received with general
assent among both geologists and zoologists, and even attracted a
considerable amount of attention from the general public.

It was not long before the coral-reef theory of Darwin found an able
exponent and sturdy champion in the person of the great American
naturalist, Professor James D. Dana.  Two years after the return of the
"Beagle" to England, the ships of the United States Exploring Expedition
set sail upon their four years' cruise, under the command of Captain
Wilkes, and Dana was a member of the scientific staff.  When, in 1839, the
expedition arrived at Sydney, a newspaper paragraph was found which gave
the American naturalist the first intimation of Darwin's new theory of the
origin of atolls and barrier-reefs.  Writing in 1872, Dana describes the
effect produced on his mind by reading this passage:--"The paragraph threw
a flood of light over the subject, and called forth feelings of peculiar
satisfaction, and of gratefulness to Mr. Darwin, which still come up afresh
whenever the subject of coral islands is mentioned.  The Gambier Islands in
the Paumotus, which gave him the key to the theory, I had not seen; but on
reaching the Feejees, six months later, in 1840, I found there similar
facts on a still grander scale and of a more diversified character, so that
I was afterward enabled to speak of his theory as established with more
positiveness than he himself, in his philosophic caution, had been ready to
adopt.  His work on coral-reefs appeared in 1842, when my report on the
subject was already in manuscript.  It showed that the conclusions on other
points, which we had independently reached, were for the most part the
same.  The principal points of difference relate to the reason for the
absence of corals from some coasts, and the evidence therefrom as to
changes of level, and the distribution of the oceanic regions of elevation
and subsidence--topics which a wide range of travel over the Pacific
brought directly and constantly to my attention."

Among the Reports of the United States Exploring Expedition, two important
works from the pen of Professor Dana made their appearance;--one on
"Zoophytes," which treats at length on "Corals and Coral-Animals," and the
other on "Coral-Reefs and Islands."  In 1872, Dana prepared a work of a
more popular character in which some of the chief results of his studies
are described; it bore the title of "Corals and Coral-Islands."  Of this
work, new and enlarged editions appeared in 1874 and 1890 in America, while
two editions were published in this country in 1872 and 1875.  In all these
works their author, while maintaining an independent judgment on certain
matters of detail, warmly defends the views of Darwin on all points
essential to the theory.

Another able exponent and illustrator of the theory of coral-reefs was
found in Professor J.B. Jukes, who accompanied H.M.S. "Fly", as naturalist,
during the survey of the Great Barrier-Reef--in the years 1842 to 1846.
Jukes, who was a man of great acuteness as well as independence of mind,
concludes his account of the great Australian reefs with the following
words:--"After seeing much of the Great Barrier-Reefs, and reflecting much
upon them, and trying if it were possible by any means to evade the
conclusions to which Mr. Darwin has come, I cannot help adding that his
hypothesis is perfectly satisfactory to my mind, and rises beyond a mere
hypothesis into the true theory of coral-reefs."

As the result of the clear exposition of the subject by Darwin, Lyell,
Dana, and Jukes, the theory of coral-reefs had, by the middle of the
present century, commanded the almost universal assent of both biologists
and geologists.  In 1859 Baron von Richthofen brought forward new facts in
its support, by showing that the existence of the thick masses of dolomitic
limestone in the Tyrol could be best accounted for if they were regarded as
of coralline origin and as being formed during a period of long continued
subsidence.  The same views were maintained by Professor Mojsisovics in his
"Dolomit-riffe von Sudtirol und Venetien," which appeared in 1879.

The first serious note of dissent to the generally accepted theory was
heard in 1863, when a distinguished German naturalist, Dr. Karl Semper,
declared that his study of the Pelew Islands showed that uninterrupted
subsidence could not have been going on in that region.  Dr. Semper's
objections were very carefully considered by Mr. Darwin, and a reply to
them appeared in the second and revised edition of his "Coral-Reefs," which
was published in 1874.  With characteristic frankness and freedom from
prejudice, Darwin admitted that the facts brought forward by Dr. Semper
proved that in certain specified cases, subsidence could not have played
the chief part in originating the peculiar forms of the coral-islands.  But
while making this admission, he firmly maintained that exceptional cases,
like those described in the Pelew Islands, were not sufficient to
invalidate the theory of subsidence as applied to the widely spread atolls,
encircling reefs, and barrier-reefs of the Pacific and Indian Oceans.  It
is worthy of note that to the end of his life Darwin maintained a friendly
correspondence with Semper concerning the points on which they were at
issue.

After the appearance of Semper's work, Dr. J.J. Rein published an account
of the Bermudas, in which he opposed the interpretation of the structure of
the islands given by Nelson and other authors, and maintained that the
facts observed in them are opposed to the views of Darwin.  Although, so
far as I am aware, Darwin had no opportunity of studying and considering
these particular objections, it may be mentioned that two American
geologists have since carefully re-examined the district--Professor W.N.
Rice in 1884 and Professor A. Heilprin in 1889--and they have independently
arrived at the conclusion that Dr. Rein's objections cannot be maintained.

The most serious opposition to Darwin's coral-reef theory, however, was
that which developed itself after the return of H.M.S. "Challenger" from
her famous voyage.  Mr. John Murray, one of the staff of naturalists on
board that vessel, propounded a new theory of coral-reefs, and maintained
that the view that they were formed by subsidence was one that was no
longer tenable; these objections have been supported by Professor Alexander
Agassiz in the United States, and by Dr. A. Geikie, and Dr. H.B. Guppy in
this country.

Although Mr. Darwin did not live to bring out a third edition of his
"Coral-Reefs," I know from several conversations with him that he had given
the most patient and thoughtful consideration to Mr. Murray's paper on the
subject.  He admitted to me that had he known, when he wrote his work, of
the abundant deposition of the remains of calcareous organisms on the sea
floor, he might have regarded this cause as sufficient in a few cases to
raise the summits of submerged volcanoes or other mountains to a level at
which reef-forming corals can commence to flourish.  But he did not think
that the admission that under certain favourable conditions, atolls might
be thus formed without subsidence, necessitated an abandonment of his
theory in the case of the innumerable examples of the kind which stud the
Indian and Pacific Oceans.

A letter written by Darwin to Professor Alexander Agassiz in May 1881 shows
exactly the attitude which careful consideration of the subject led him to
maintain towards the theory propounded by Mr. Murray:--"You will have
seen," he writes, "Mr. Murray's views on the formation of atolls and
barrier-reefs.  Before publishing my book, I thought long over the same
view, but only as far as ordinary marine organisms are concerned, for at
that time little was known of the multitude of minute oceanic organisms.  I
rejected this view, as from the few dredgings made in the "Beagle", in the
south temperate regions, I concluded that shells, the smaller corals, etc.,
decayed and were dissolved when not protected by the deposition of
sediment, and sediment could not accumulate in the open ocean.  Certainly,
shells, etc., were in several cases completely rotten, and crumbled into
mud between my fingers; but you will know whether this is in any degree
common.  I have expressly said that a bank at the proper depth would give
rise to an atoll, which could not be distinguished from one formed during
subsidence.  I can, however, hardly believe in the existence of as many
banks (there having been no subsidence) as there are atolls in the great
oceans, within a reasonable depth, on which minute oceanic organisms could
have accumulated to the depth of many hundred feet."

Darwin's concluding words in the same letter written within a year of his
death, are a striking proof of the candour and openness of mind which he
preserved so well to the end, in this as in other controversies.

"If I am wrong, the sooner I am knocked on the head and annihilated so much
the better.  It still seems to me a marvellous thing that there should not
have been much, and long-continued, subsidence in the beds of the great
oceans.  I wish some doubly rich millionaire would take it into his head to
have borings made in some of the Pacific and Indian atolls, and bring home
cores for slicing from a depth of 500 or 600 feet."

It is noteworthy that the objections to Darwin's theory have for the most
part proceeded from zoologists, while those who have fully appreciated the
geological aspect of the question, have been the staunchest supporters of
the theory of subsidence.  The desirability of such boring operations in
atolls has been insisted upon by several geologists, and it may be hoped
that before many years have passed away, Darwin's hopes may be realised,
either with or without the intervention of the "doubly rich millionaire."

Three years after the death of Darwin, the veteran Professor Dana
re-entered the lists and contributed a powerful defence of the theory of
subsidence in the form of a reply to an essay written by the ablest
exponent of the anti-Darwinian views on this subject, Dr. A. Geikie.  While
pointing out that the Darwinian position had been to a great extent
misunderstood by its opponents, he showed that the rival theory presented
even greater difficulties than those which it professed to remove.

During the last five years, the whole question of the origin of coral-reefs
and islands has been re-opened, and a controversy has arisen, into which,
unfortunately, acrimonious elements have been very unnecessarily
introduced.  Those who desire it, will find clear and impartial statements
of the varied and often mutually destructive views put forward by different
authors, in three works which have made their appearance within the last
year,--"The Bermuda Islands," by Professor Angelo Heilprin; "Corals and
Coral-Islands," new edition by Professor J.D. Dana; and the third edition
of Darwin's "Coral-Reefs," with Notes and Appendix by Professor T.G.
Bonney.

Most readers will, I think, rise from the perusal of these works with the
conviction that, while on certain points of detail it is clear that,
through the want of knowledge concerning the action of marine organisms in
the open ocean, Darwin was betrayed into some grave errors, yet the main
foundations of his argument have not been seriously impaired by the new
facts observed in the deep-sea researches, or by the severe criticism to
which his theory has been subjected during the last ten years.  On the
other hand, I think it will appear that much misapprehension has been
exhibited by some of Darwin's critics, as to what his views and arguments
really were; so that the reprint and wide circulation of the book in its
original form is greatly to be desired, and cannot but be attended with
advantage to all those who will have the fairness to acquaint themselves
with Darwin's views at first hand, before attempting to reply to them.

JOHN W. JUDD.

Coral Reefs. Introduction. Table of Contents

This etext was prepared by Sue Asscher  





CORAL REEFS

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.



CORAL REEFS.


TABLE OF CONTENTS.


CRITICAL INTRODUCTION.


INTRODUCTION.


CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.

SECTION 1.I.--DESCRIPTION OF KEELING ATOLL.
Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.--
Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae
subsisting on corals.--Changes in the condition of the reefs and islets.--
Probable subsidence of the atoll.--Future state of the lagoon.

SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS.
General form and size of atolls, their reefs and islets.--External slope.--
Zone of Nulliporae.--Conglomerate.--Depth of lagoons.--Sediment.--Reefs
submerged wholly or in part.--Breaches in the reef.--Ledge-formed shores
round certain lagoons.--Conversion of lagoons into land.

SECTION 1.III.--ATOLLS OF THE MALDIVA ARCHIPELAGO--GREAT CHAGOS BANK.
Maldiva Archipelago.--Ring-formed reefs, marginal and central.--Great
depths in the lagoons of the southern atolls.--Reefs in the lagoons all
rising to the surface.--Position of islets and breaches in the reefs, with
respect to the prevalent winds and action of the waves.--Destruction of
islets.--Connection in the position and submarine foundation of distinct
atolls.--The apparent disseverment of large atolls.--The Great Chagos
Bank.--Its submerged condition and extraordinary structure.


CHAPTER II.--BARRIER REEFS.
Closely resemble in general form and structure atoll-reefs.--Width and
depth of the lagoon-channels.--Breaches through the reef in front of
valleys, and generally on the leeward side.--Checks to the filling up of
the lagoon-channels.--Size and constitution of the encircled islands.--
Number of islands within the same reef.--Barrier-reefs of New Caledonia and
Australia.--Position of the reef relative to the slope of the adjoining
land.--Probable great thickness of barrier-reefs.


CHAPTER III.--FRINGING OR SHORE-REEFS.
Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling
up.--Currents of water formed within it.--Upraised reefs.--Narrow
fringing-reefs in deep seas.--Reefs on the coast of E. Africa and of
Brazil.--Fringing-reefs in very shallow seas, round banks of sediment and
on worn-down islands.--Fringing-reefs affected by currents of the sea.
--Coral coating the bottom of the sea, but not forming reefs.


CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.

SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.

SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.

SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.


CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS.
The atolls of the larger archipelagoes are not formed on submerged craters,
or on banks of sediment.--Immense areas interspersed with atolls.--Recent
changes in their state.--The origin of barrier-reefs and of atolls.--Their
relative forms.--The step-formed ledges and walls round the shores of some
lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged
condition of parts or of the whole of some annular reefs.--The disseverment
of large atolls.--The union of atolls by linear reefs.--The Great Chagos
Bank.--Objections, from the area and amount of subsidence required by the
theory, considered.--The probable composition of the lower parts of atolls.


CHAPTER VI.--ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.
Description of the coloured map.--Proximity of atolls and barrier-reefs.--
Relation in form and position of atolls with ordinary islands.--Direct
evidence of subsidence difficult to be detected.--Proofs of recent
elevation where fringing-reefs occur.--Oscillations of level.--Absence of
active volcanoes in the areas of subsidence.--Immensity of the areas which
have been elevated and have subsided.--Their relation to the present
distribution of the land.--Areas of subsidence elongated, their
intersection and alternation with those of elevation.--Amount and slow rate
of the subsidence.--Recapitulation.


APPENDIX.
Containing a detailed description of the reefs and islands in Plate III.


INDEX.