TABLE OF CONTENTS.
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. depth of the lagoon-channels.--Breaches through the reef in front of
Closely resemble in general form and structure atoll-reefs.--Width and
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. up.--Currents of water formed within it.--Upraised reefs.--Narrow
Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling
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 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 Description of the coloured map.--Proximity of atolls and barrier-reefs.--
THEORY OF THEIR FORMATION.
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.
Containing a detailed description of the reefs and islands in Plate III.
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
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
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
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.