Coral Reefs; Volcanic Islands; South American Geology — Complete
by Charles Darwin · English · Gutenberg #4022 · 1,716,439 chars · public domain (US)
The Project Gutenberg eBook of Coral Reefs; Volcanic Islands; South American Geology — Complete
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: Coral Reefs; Volcanic Islands; South American Geology — Complete
Author: Charles Darwin
Release date: May 1, 2003 [eBook #4022]
Most recently updated: June 21, 2020
Language: English
Other information and formats: www.gutenberg.org/ebooks/4022
Credits: Produced by Sue Asscher
*** START OF THE PROJECT GUTENBERG EBOOK CORAL REEFS; VOLCANIC ISLANDS; SOUTH AMERICAN GEOLOGY — COMPLETE ***
Produced by Sue Asscher
Coral Reefs, Volcanic Islands, South American Geology
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.
CONTENTS
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
CRITICAL INTRODUCTION
INTRODUCTION
Chapter I—ATOLLS OR LAGOON-ISLANDS.
_Section I_—DESCRIPTION OF KEELING ATOLL.
Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.
_Section II_—GENERAL DESCRIPTION OF ATOLLS. General form and size of
atolls, their reefs and islets.—External slope.—Zone of
Nulliporæ.—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 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 I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS.
_Section 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.
GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
CRITICAL INTRODUCTION
Chapter I—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series.—A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent.—Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriæ.—Ancient and
obliterated orifices of eruption of small size.—Difficulty of tracing
over a bare plain recent streams of lava.—Inland hills of more ancient
volcanic rock.—Decomposed olivine in large masses.—Feldspathic rocks
beneath the upper crystalline basaltic strata.—Uniform structure and
form of the more ancient volcanic hills.—Form of the valleys near the
coast.—Conglomerate now forming on the sea beach.
Chapter II—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
rocks: their singular decomposition by steam of high temperature.
TAHITI.—Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
elevation.—Structure of its more ancient mountains; similarity with St.
Jago. ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular
mineralogical composition.
Chapter III—ASCENSION.
Basaltic lavas.—Numerous craters truncated on the same side.—Singular
structure of volcanic bombs.—Aeriform explosions.—Ejected granite
fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
formation.—Concretions in pumiceous tuff.—Calcareous deposits and
frondescent incrustations on the coast.—Remarkable laminated beds,
alternating with, and passing into obsidian.—Origin of
obsidian.—Lamination of volcanic rocks.
Chapter IV—ST. HELENA.
Lavas of the feldspathic, basaltic, and submarine series.—Section of
Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
and a parapet.—Cones of phonolite.—Superficial beds of calcareous
sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
land.—Denudation.—Craters of elevation.
Chapter V—GALAPAGOS ARCHIPELAGO.
Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
basaltic craters, with hollows at their bases.—Albemarle Island; fluid
lavas, their composition.—Craters of tuff; inclination of their
exterior diverging strata, and structure of their interior converging
strata.—James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected
fragments.—Concluding remarks on the craters of tuff, and on the
breached condition of their southern sides.—Mineralogical composition
of the rocks of the archipelago.—Elevation of the land.—Direction of
the fissures of eruption.
Chapter VI—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.
The sinking of crystals in fluid lava.—Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation.—Obsidian.—Apparent non-separation of the elements of
plutonic rocks.—Origin of trap-dikes in the plutonic
series.—Distribution of volcanic islands; their prevalence in the great
oceans.—They are generally arranged in lines.—The central volcanoes of
Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
volcanic islands, and their elevation in mass.—Eruptions on parallel
lines of fissure within the same geological period.
Chapter VII—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
Land.—Palæozoic formation.—Newer formation with volcanic
rocks.—Travertin with leaves of extinct plants.—Elevation of the
land.—New Zealand.—King George’s Sound.—Superficial ferruginous
beds.—Superficial calcareous deposits, with casts of branches; its
origin from drifted particles of shells and corals.—Their extent.—Cape
of Good Hope.—Junction of the granite and clay-slate.—Sandstone
formation.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.
CRITICAL INTRODUCTION
Chapter I—ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
shells.—Terrace-bounded valley of Santa Cruz, formerly a
sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.—Great shingle formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.
Chapter II—ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.
Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth or
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea-beach on San Lorenzo.—Human
remains, fossil earthenware, earthquake debacle, recent subsidence.—On
the decay of upraised shells.—General summary.
Chapter III—ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL
DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake-waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.—Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.
Chapter IV—ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution.—Microscopical structure.—Buenos Ayres,
shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S.
Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones, and infusoria of; co-existence of the recent shells and
extinct mammifers.—Buenos Ayres to St. Fe.—Skeletons of
Mastodon.—Infusoria.—Inferior marine tertiary strata, their
age.—Horse’s tooth. BANDA ORIENTAL.—Superficial Pampean
formation.—Inferior tertiary strata, variation of, connected with
volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia,
age of, subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—Contemporaneous with existing
mollusca.—Relations to underlying tertiary strata. Ancient deposit of
estuary origin.—Elevation and successive deposition of the Pampean
formation.—Number and state of the remains of mammifers; their
habitation, food, extinction, and range.—Conclusion.—Supplement on the
thickness of the Pampean formation.—Localities in Pampas at which
mammiferous remains have been found.
Chapter V—ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.—Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary
formations of the Western Coast.—Chonos and Chiloe groups, volcanic
rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.—Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coasts of South America.—On the
contemporaneous deposition and preservation of sedimentary formations.
Chapter VI—PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, palæozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.
Chapter VII—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
Central Chile.—Basal formations of the Cordillera.—Origin of the
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 to “Coral-Reefs”
Index to “Volcanic Islands”
Index to “South American Observations”
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 Südtirol 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
The object of this volume is to describe from my own observation and
the works of others, the principal kinds of coral-reefs, more
especially those occurring in the open ocean, and to explain the origin
of their peculiar forms. I do not here treat of the polypifers, which
construct these vast works, except so far as relates to their
distribution, and to the conditions favourable to their vigorous
growth. Without any distinct intention to classify coral-reefs, most
voyagers have spoken of them under the following heads:
“lagoon-islands,” or “atolls,” “barrier” or “encircling reefs,” and
“fringing” or “shore-reefs.” The lagoon-islands have received much the
most attention; and it is not surprising, for every one must be struck
with astonishment, when he first beholds one of these vast rings of
coral-rock, often many leagues in diameter, here and there surmounted
by a low verdant island with dazzling white shores, bathed on the
outside by the foaming breakers of the ocean, and on the inside
surrounding a calm expanse of water, which from reflection, is of a
bright but pale green colour. The naturalist will feel this
astonishment more deeply after having examined the soft and almost
gelatinous bodies of these apparently insignificant creatures, and when
he knows that the solid reef increases only on the outer edge, which
day and night is lashed by the breakers of an ocean never at rest. Well
did François Pyrard de Laval, in the year 1605, exclaim, “C’est une
mérueille de voir chacun de ces atollons, enuironné d’un grand banc de
pierre tout autour, n’y ayant point d’artifice humain.” The
accompanying sketch of Whitsunday island, in the South Pacific, taken
from Captain Beechey’s admirable “Voyage,” although excellent of its
kind, gives but a faint idea of the singular aspect of one of these
lagoon-islands.
Whitsunday Island is of small size, and the whole circle has been
converted into land, which is a comparatively rare circumstance. As the
reef of a lagoon-island generally supports many separate small islands,
the word “island,” applied to the whole, is often the cause of
confusion; hence I have invariably used in this volume the term
“atoll,” which is the name given to these circular groups of
coral-islets by their
inhabitants in the Indian Ocean, and is synonymous with “lagoon-
island.”
[Illustration: Whitsunday Island]
Barrier-reefs, when encircling small islands, have been comparatively
little noticed by voyagers; but they well deserve attention. In their
structure they are little less marvellous than atolls, and they give a
singular and most picturesque character to the scenery of the islands
they surround. In the accompanying sketch, taken from the “Voyage of
the _Coquille_,” the reef is seen from within, from one of the high
peaks of the island of Bolabola.[1] Here, as in Whitsunday Island, the
whole of that part of the reef which is visible is converted into land.
This is a circumstance of rare occurrence; more usually a snow-white
line of great breakers, with here and there an islet crowned by
cocoa-nut trees, separates the smooth waters of the lagoon-like channel
from the waves of the open sea. The barrier-reefs of Australia and of
New Caledonia, owing to their enormous dimensions, have excited much
attention: in structure and form they resemble those encircling many of
the smaller islands in the Pacific Ocean.
[1] I have taken the liberty of simplifying the foreground, and
leaving out a mountainous island in the far distance.
[Illustration: Island of Bolabola]
With respect to fringing, or shore-reefs, there is little in their
structure which needs explanation; and their name expresses their
comparatively
small extension. They differ from barrier-reefs in not lying so far
from the shore, and in not having within a broad channel of deep water.
Reefs also occur around submerged banks of sediment and of worn-down
rock; and others are scattered quite irregularly where the sea is very
shallow; these in most respects are allied to those of the fringing
class, but they are of comparatively little interest.
I have given a separate chapter to each of the above classes, and have
described some one reef or island, on which I possessed most
information, as typical; and have afterwards compared it with others of
a like kind. Although this classification is useful from being obvious,
and from including most of the coral-reefs existing in the open sea, it
admits of a more fundamental division into barrier and atoll-formed
reefs on the one hand, where there is a great apparent difficulty with
respect to the foundation on which they must first have grown; and into
fringing-reefs on the other, where, owing to the nature of the slope of
the adjoining land, there is no such difficulty. The two blue tints and
the red colour[2] on the map (Plate III), represent this main division,
as explained in the beginning of the last chapter. In the Appendix,
every existing coral-reef, except some on the coast of Brazil not
included in the map, is briefly described in geographical order, as far
as I possessed information; and any particular spot may be found by
consulting the Index.
Several theories have been advanced to explain the origin of atolls or
lagoon-islands, but scarcely one to account for barrier-reefs. From the
limited depths at which reef-building polypifers can flourish, taken
into consideration with certain other circumstances, we are compelled
to conclude, as it will be seen, that both in atolls and barrier-reefs,
the foundation on which the coral was primarily attached, has subsided;
and that during this downward movement, the reefs have grown upwards.
This conclusion, it will be further seen, explains most satisfactorily
the outline and general form of atolls and barrier-reefs, and likewise
certain peculiarities in their structure. The distribution, also, of
the different kinds of coral-reefs, and their position with relation to
the areas of recent elevation, and to the points subject to volcanic
eruptions, fully accord with this theory of their origin.[3]
[2] Replaced by numbers in this edition.
[3] A brief account of my views on coral formations, now published in
my Journal of Researches, was read May 31st, 1837, before the
Geological Society, and an abstract has appeared in the Proceedings.
In the several original surveys, from which the small plans on this
plate have been reduced, the coral-reefs are engraved in very different
styles. For the sake of uniformity, I have adopted the style used in
the charts of the Chagos Archipelago, published by the East Indian
Company, from the survey by Captain Moresby and Lieutenant Powell. The
surface of the reef, which dries at low water, is represented by a
surface with small crosses: the coral-islets on the reef are marked by
small linear spaces, on which a few cocoa-nut trees, out of all
proportion too large, have been introduced for the sake of clearness.
The entire _annular reef_, which when surrounding an open expanse of
water, forms an “atoll,” and when surrounding one or more high islands,
forms an encircling “barrier-reef,” has a nearly uniform structure. The
reefs in some of the original surveys are represented merely by a
single line with crosses, so that their breadth is not given; I have
had such reefs engraved of the width usually attained by coral-reefs. I
have not thought it worth while to introduce all those small and very
numerous reefs, which occur within the lagoons of most atolls and
within the lagoon-channels of most barrier-reefs, and which stand
either isolated, or are attached to the shores of the reef or land. At
Peros Banhos none of the lagoon-reefs rise to the surface of the water;
a few of them have been introduced, and are marked by plain dotted
circles. A few of the deepest soundings are laid down within each reef;
they are in fathoms, of six English feet.
_Plate I_—Map showing the resemblance in form between barrier
coral-reefs surrounding mountainous islands, and atolls or lagoon
islands.
[Illustration: Map showing the resemblance in form.]
Fig. 1—VANIKORO, situated in the western part of the South Pacific;
taken from the survey by Captain D’Urville in the _Astrolabe_; the
soundings on the southern side of the island, namely, from thirty to
forty fathoms, are given from the voyage of the Chev. Dillon; the other
soundings are laid down from the survey by D’Urville; height of the
summit of the island is 3,032 feet. The principal small detached reefs
within the lagoon-channel have in this instance been represented. The
southern shore of the island is narrowly fringed by a reef: if the
engraver had carried this reef entirely round both islands, this figure
would have served (by leaving out in imagination the barrier-reef) as a
good specimen of an abruptly-sided island, surrounded by a reef of the
fringing class.
Fig. 2—HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
“Atlas of the Voyage of the _Astrolabe,_” compiled from the surveys of
Captains Duperrey and D’Urville; the depth of the immense lagoon-like
space within the reef is not known.
Fig. 3—RAIATEA, in the Society Archipelago; from the map given in the
quarto edition of “Cook’s First Voyage;” it is probably not accurate.
Fig. 4—BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago,
from the survey by Captain Beechey, R.N.; the lagoon is choked up with
reefs, but the average greatest depth of about twenty fathoms, is given
from the published account of the voyage.
Fig. 5—BOLABOLA, in the Society Archipelago, from the survey of Captain
Duperrey in the _ Coquille_: the soundings in this and the following
figures have been altered from French feet to English fathoms; height
of highest point of the island 4,026 feet.
[Illustration: Map showing the resemblance in form.]
Fig. 6.—MAURUA, in the Society Archipelago; from the survey by Captain
Duperrey in the _ Coquille_: height of land about eight hundred feet.
Fig. 7—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from the
survey by Admiral Lutké.
Fig. 8—GAMBIER ISLANDS, in the southern part of the Low Archipelago;
from the survey by Captain Beechey; height of highest island, 1,246
feet; the islands are surrounded by extensive and irregular reefs; the
reef on the southern side is submerged.
Fig. 9—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
the Indian Ocean; from the survey by Captain Moresby and Lieutenant
Powell; not nearly all the small submerged reefs in the lagoon are
represented; the annular reef on the southern side is submerged.
Fig. 10—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
Ocean; from the survey by Captain Fitzroy; the lagoon south of the
dotted line is very shallow, and is left almost bare at low water; the
part north of the line is choked up with irregular reefs. The annular
reef on the north-west side is broken, and blends into a shoal
sandbank, on which the sea breaks.
Chapter I ATOLLS OR LAGOON-ISLANDS
_Section I_—KEELING ATOLL
Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.
KEELING or COCOS atoll is situated in the Indian Ocean, in 12° 5′ S.,
and longitude 90° 55′ E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. _Beagle_, is given
in Plate I, Fig. 10. The greatest width of this atoll is nine miles and
a half. Its structure is in most respects characteristic of the class
to which it belongs, with the exception of the shallowness of the
lagoon. The accompanying woodcut represents a vertical section,
supposed to be drawn at low water from the outer coast across one of
the low islets (one being taken of average dimensions) to within the
lagoon.
[Illustration: Vertical section of one of the low islets.]
A.—Level of the sea at low water: where the letter A is placed, the
depth is twenty-five fathoms, and the distance rather more than one
hundred and fifty yards from the edge of the reef.
B.—Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of
rugged points, like those a little farther seaward, beneath the water.
C.—A flat of coral-rock, covered at high water.
D.—A low projecting ledge of brecciated coral-rock washed by the waves
at high water.
E.—A slope of loose fragments, reached by the sea only during gales:
the upper part, which is from six to twelve feet high, is clothed with
vegetation. The surface of the islet gently slopes to the lagoon.
F.—Level of the lagoon at low water.
The section is true to the scale in a horizontal line, but it could not
be made so in a vertical one, as the average greatest height of the
land is only between six and twelve feet above high-water mark.
I will describe the section, commencing with the outer margin. I must
first observe that the reef-building polypifers, not being tidal
animals, require to be constantly submerged or washed by the breakers.
I was assured by Mr. Liesk, a very intelligent resident on these
islands, as well as by some chiefs at Tahiti (Otaheite), that an
exposure to the rays of the sun for a very short time invariably causes
their destruction. Hence it is possible only under the most favourable
circumstances, afforded by an unusually low tide and smooth water, to
reach the outer margin, where the coral is alive. I succeeded only
twice in gaining this part, and found it almost entirely composed of a
living Porites, which forms great irregularly rounded masses (like
those of an Astræa, but larger) from four to eight feet broad, and
little less in thickness. These mounds are separated from each other by
narrow crooked channels, about six feet deep, most of which intersect
the line of reef at right angles. On the furthest mound, which I was
able to reach by the aid of a leaping-pole, and over which the sea
broke with some violence, although the day was quite calm and the tide
low, the polypifers in the uppermost cells were all dead, but between
three and four inches lower down on its side they were living, and
formed a projecting border round the upper and dead surface. The coral
being thus checked in its upward growth, extends laterally, and hence
most of the masses, especially those a little further inwards, had
broad flat dead summits. On the other hand I could see, during the
recoil of the breakers, that a few yards further seaward, the whole
convex surface of the Porites was alive; so that the point where we
were standing was almost on the exact upward and shoreward limit of
existence of those corals which form the outer margin of the reef. We
shall presently see that there are other organic productions, fitted to
bear a somewhat longer exposure to the air and sun.
Next, but much inferior in importance to the Porites, is the _
Millepora complanata._[1]
[1] This Millepora (Palmipora of Blainville), as well as the _M.
alcicornis_, possesses the singular property of stinging the skin
where it is delicate, as on the face and arm.
It grows in thick vertical plates, intersecting each other at various
angles, and forms an exceedingly strong honeycombed mass, which
generally affects a circular form, the marginal plates alone being
alive. Between these plates and in the protected crevices on the reef,
a multitude of branching zoophytes and other productions flourish, but
the Porites and Millepora alone seem able to resist the fury of the
breakers on its upper and outer edge: at the depth of a few fathoms
other kinds of stony corals live. Mr. Liesk, who was intimately
acquainted with every part of this reef, and likewise with that of
North Keeling atoll, assured me that these corals invariably compose
the outer margin. The lagoon is inhabited by quite a distinct set of
corals, generally brittle and thinly branched; but a Porites,
apparently of the same species with that on the outside, is found
there, although it does not seem to thrive, and certainly does not
attain the thousandth part in bulk of the masses opposed to the
breakers.
The woodcut shows the form of the bottom off the reef: the water
deepens for a space between one and two hundred yards wide, very
gradually to twenty-five fathoms (A in section), beyond which the sides
plunge into the unfathomable ocean at an angle of 45°.[2] To the depth
of ten or twelve fathoms the bottom is exceedingly rugged, and seems
formed of great masses of living coral, similar to those on the margin.
The arming of the lead here invariably came up quite clean, but deeply
indented, and chains and anchors which were lowered, in the hopes of
tearing up the coral, were broken. Many small fragments, however, of _
Millepora alcicornis_ were brought up; and on the arming from an
eight-fathom cast, there was a perfect impression of an Astræa,
apparently alive. I examined the rolled fragments cast on the beach
during gales, in order further to ascertain what corals grew outside
the reef. The fragments consisted of many kinds, of which the Porites
already mentioned and a Madrepora, apparently the _M. corymbosa_, were
the most abundant. As I searched in vain in the hollows on the reef and
in the lagoon, for a living specimen of this Madrepore, I conclude that
it is confined to a zone outside, and beneath the surface, where it
must be very abundant. Fragments of the _Millepora alcicornis_ and of
an Astræa were also numerous; the former is found, but not in
proportionate numbers, in the hollows on the reef; but the Astræa I did
not see living. Hence we may infer, that these are the kinds of coral
which form the rugged sloping surface (represented in the woodcut by an
uneven line), round and beneath the external margin. Between twelve and
twenty fathoms the arming came up an equal number of times smoothed
with sand, and indented with coral: an anchor and lead were lost at the
respective depths of thirteen and sixteen fathoms. Out of twenty-five
soundings taken at a greater depth than twenty fathoms, every one
showed that the bottom was covered with sand; whereas, at a less depth
than twelve fathoms, every sounding showed that it was exceedingly
rugged, and free from all extraneous particles. Two soundings were
obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms. The sand brought up from these depths
consisted of finely triturated fragments of stony zoophytes, but not,
as far as I could distinguish, of a particle of any lamelliform genus:
fragments of shells were rare.
[2] The soundings from which this section is laid down were taken with
great care by Captain Fitzroy himself. He used a bell-shaped lead,
having a diameter of four inches, and the armings each time were cut
off and brought on board for me to examine. The arming is a
preparation of tallow, placed in the concavity at the bottom of the
lead. Sand, and even small fragments of rock, will adhere to it; and
if the bottom be of rock it brings up an exact impression of its
surface.
At a distance of 2,200 yards from the breakers, Captain Fitzroy found
no bottom with a line of 7,200 feet in length; hence the submarine
slope of this coral formation is steeper than that of any volcanic
cone. Off the mouth of the lagoon, and likewise off the northern point
of the atoll, where the currents act violently, the inclination, owing
to the accumulation of sediment, is less. As the arming of the lead
from
all the greater depths showed a smooth sandy bottom, I at first
concluded that the whole consisted of a vast conical pile of calcareous
sand, but the sudden increase of depth at some points, and the
circumstance of the line having been cut, as if rubbed, when between
five hundred and six hundred fathoms were out, indicate the probable
existence of submarine cliffs.
On the margin of the reef, close within the line where the upper
surface of the Porites and of the Millepora is dead, three species of
Nullipora flourish. One grows in thin sheets, like a lichen on old
trees; the second in stony knobs, as thick as a man’s finger, radiating
from a common centre; and the third, which is less common, in a
moss-like reticulation of thin, but perfectly rigid branches.[3] The
three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour. These Nulliporæ, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each
tide by breaking water, for they are not found in any abundance in the
protected hollows on the back part of the reef, where they might be
immersed either during the whole or an equal proportional time of each
tide. It is remarkable that organic productions of such extreme
simplicity, for the Nulliporæ undoubtedly belong to one of the lowest
classes of the vegetable kingdom, should be limited to a zone so
peculiarly circumstanced. Hence the layer composed by their growth
merely fringes the reef for a space of about twenty yards in width,
either under the form of separate mammillated projections, where the
outer masses of coral are separate, or, more commonly, where the corals
are united into a solid margin, as a continuous smooth convex mound (B
in woodcut), like an artificial breakwater. Both the mound and
mammillated projections stand about three feet higher than any other
part of the reef, by which term I do not include the islets, formed by
the accumulation of rolled fragments. We shall hereafter see that other
coral reefs are protected by a similar thick growth of Nulliporæ on the
outer margin, the part most exposed to the breakers, and this must
effectually aid in preserving it from being worn down.
[3] This last species is of a beautiful bright peach-blossom colour.
Its branches are about as thick as crow-quills; they are slightly
flattened and knobbed at the extremities. The extremities only are
alive and brightly coloured. The two other species are of a dirty
purplish-white. The second species is extremely hard; its short
knob-like branches are cylindrical, and do not grow thicker at their
extremities.
The woodcut represents a section across one of the islets on the reef,
but if all that part which is above the level of C were removed, the
section would be that of a simple reef, as it occurs where no islet has
been formed. It is this reef which essentially forms the atoll. It is a
ring, enclosing the lagoon on all sides except at the northern end,
where there are two open spaces, through one of which ships can enter.
The reef varies in width from two hundred and fifty to five
hundred yards, its surface is level, or very slightly inclined towards
the lagoon, and at high tide the sea breaks entirely over it: the water
at low tide thrown by the breakers on the reef, is carried by the many
narrow and shoal gullies or channels on its surface, into the lagoon: a
return stream sets out of the lagoon through the main entrance. The
most frequent coral in the hollows on the reef is _Pocillopora
verrucosa_, which grows in short sinuous plates, or branches, and when
alive is of a beautiful pale lake-red: a Madrepora, closely allied or
identical with _M. pocillifera_, is also common. As soon as an islet is
formed, and the waves are prevented breaking entirely over the reef,
the channels and hollows in it become filled up with cemented
fragments, and its surface is converted into a hard smooth floor (C of
woodcut), like an artificial one of freestone. This flat surface varies
in width from one hundred to two hundred, or even three hundred yards,
and is strewed with a few large fragments of coral torn up during
gales: it is uncovered only at low water. I could with difficulty, and
only by the aid of a chisel, procure chips of rock from its surface,
and therefore could not ascertain how much of it is formed by the
aggregation of detritus, and how much by the outward growth of mounds
of corals, similar to those now living on the margin. Nothing can be
more singular than the appearance at low tide of this “flat” of naked
stone, especially where it is externally bounded by the smooth convex
mound of Nulliporæ, appearing like a breakwater built to resist the
waves, which are constantly throwing over it sheets of foaming water.
The characteristic appearance of this “flat” is shown in the foregoing
woodcut of Whitsunday atoll.
The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile
of fragments, thrown together by some unusually strong gale. Their
ordinary width is under a quarter of a mile, and their length varies
from a few yards to several miles. Those on the south-east and windward
side of the atoll, increase solely by the addition of fragments on
their outer side; hence the loose blocks of coral, of which their
surface is composed, as well as the shells mingled with them, almost
exclusively consist of those kinds which live on the outer coast. The
highest part of the islets (excepting hillocks of blown sand, some of
which are thirty feet high), is close to the outer beach (E of the
woodcut), and averages from six to ten feet above ordinary high-water
mark. From the outer beach the surface slopes gently to the shores of
the lagoon, which no doubt has been caused by the breakers the further
they have rolled over the reef, having had less power to throw up
fragments. The little waves of the lagoon heap up sand and fragments of
thinly-branched corals on the inner side of the islets on the leeward
side of the atoll; and these islets are broader than those to windward,
some being even eight hundred yards in width; but the land thus added
is very low. The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (D of the woodcut), projecting
some yards in front of the outer shore and from two to four feet high.
This ledge is just reached by the waves at ordinary high-water:
it extends in front of all the islets, and everywhere has a water-worn
and scooped appearance. The fragments of coral which are occasionally
cast on the “flat” are during gales of unusual violence swept together
on the beach, where the waves each day at high-water tend to remove and
gradually wear them down; but the lower fragments having become firmly
cemented together by the percolation of calcareous matter, resist the
daily tides longer, and hence project as a ledge. The cemented mass is
generally of a white colour, but in some few parts reddish from
ferruginous matter; it is very hard, and is sonorous under the hammer;
it is obscurely divided by seams, dipping at a small angle seaward; it
consists of fragments of the corals which grow on the outer margin,
some quite and others partially rounded, some small and others between
two and three feet across; and of masses of previously formed
conglomerate, torn up, rounded, and re-cemented; or it consists of a
calcareous sandstone, entirely composed of rounded particles, generally
almost blended together, of shells, corals, the spines of echini, and
other such organic bodies; rocks, of this latter kind, occur on many
shores, where there are no coral reefs. The structure of the coral in
the conglomerate has generally been much obscured by the infiltration
of spathose calcareous matter; and I collected a very interesting
series, beginning with fragments of unaltered coral, and ending with
others, where it was impossible to discover with the naked eye any
trace of organic structure. In some specimens I was unable, even with
the aid of a lens, and by wetting them, to distinguish the boundaries
of the altered coral and spathose limestone. Many even of the blocks of
coral lying loose on the beach, had their central parts altered and
infiltrated.
The lagoon alone remains to be described; it is much shallower than
that of most atolls of considerable size. The southern part is almost
filled up with banks of mud and fields of coral, both dead and alive,
but there are considerable spaces, between three and four fathoms, and
smaller basins, from eight to ten fathoms deep. Probably about half its
area consists of sediment, and half of coral-reefs. The corals
composing these reefs have a very different aspect from those on the
outside; they are very numerous in kind, and most of them are thinly
branched. Meandrina, however, lives in the lagoon, and great rounded
masses of this coral are numerous, lying quite or almost loose on the
bottom. The other commonest kinds consist of three closely allied
species of true Madrepora in thin branches; of _Seriatapora subulata_;
two species of Porites[4] with cylindrical branches, one of which forms
circular clumps, with the exterior branches only alive; and lastly, a
coral something like an Explanaria, but with stars on both surfaces,
growing in thin, brittle, stony, foliaceous expansions, especially in
the deeper basins of the lagoon. The reefs on which
these corals grow are very irregular in form, are full of cavities, and
have not a solid flat surface of dead rock, like that surrounding the
lagoon; nor can they be nearly so hard, for the inhabitants made with
crowbars a channel of considerable length through these reefs, in which
a schooner, built on the S.E. islet, was floated out. It is a very
interesting circumstance, pointed out to us by Mr. Liesk, that this
channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh
excavations would be absolutely necessary to allow another vessel to
pass through it.
[4] This Porites has somewhat the habit of _P. clavaria_, but the
branches are not knobbed at their ends. When alive it is of a yellow
colour, but after having been washed in fresh water and placed to dry,
a jet-black slimy substance exuded from the entire surface, so that
the specimen now appears as if it had been dipped in ink.
The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand. Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its entire
solution in acids to be purely calcareous. I have seen in the Museum of
the Geological Society, a similar but more remarkable substance,
brought by Lieutenant Nelson from the reefs of Bermuda, which, when
shown to several experienced geologists, was mistaken by them for true
chalk. On the outside of the reef much sediment must be formed by the
action of the surf on the rolled fragments of coral; but in the calm
waters of the lagoon, this can take place only in a small degree. There
are, however, other and unexpected agents at work here: large shoals of
two species of Scarus, one inhabiting the surf outside the reef and the
other the lagoon, subsist entirely, as I was assured by Mr. Liesk, the
intelligent resident before referred to, by browsing on the living
polypifers. I opened several of these fish, which are very numerous and
of considerable size, and I found their intestines distended by small
pieces of coral, and finely ground calcareous matter. This must daily
pass from them as the finest sediment; much also must be produced by
the infinitely numerous vermiform and molluscous animals, which make
cavities in almost every block of coral. Dr. J. Allan, of Forres, who
has enjoyed the best means of observation, informs me in a letter that
the Holothuriæ (a family of Radiata) subsist on living coral; and the
singular structure of bone within the anterior extremity of their
bodies, certainly appears well adapted for this purpose. The number of
the species of Holothuria, and of the individuals which swarm on every
part of these coral-reefs, is extraordinarily great; and many shiploads
are annually freighted, as is well-known, for China with the trepang,
which is a species of this genus. The amount of coral yearly consumed,
and ground down into the finest mud, by these several creatures, and
probably by many other kinds, must be immense. These facts are,
however, of more importance in another point of view, as showing us
that there are living checks to the growth of coral-reefs, and that the
almost universal law of “consumed and be consumed,” holds good even
with the polypifers forming those massive bulwarks, which are able to
withstand the force of the open ocean.
Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation
of their detritus, one is naturally led to inquire how long it has
continued, and how long it is likely to continue, in its present state.
Mr. Liesk informed me that he had seen an old chart in which the
present long island on the S.E. side was divided by several channels
into as many islets; and he assures me that the channels can still be
distinguished by the smaller size of the trees on them. On several
islets, also, I observed that only young cocoa-nut trees were growing
on the extremities; and that older and taller trees rose in regular
succession behind them; which shows that these islets have very lately
increased in length. In the upper and south-eastern part of the lagoon,
I was much surprised by finding an irregular field of at least a mile
square of branching corals, still upright, but entirely dead. They
consisted of the species already mentioned; they were of a brown
colour, and so rotten, that in trying to stand on them I sank halfway
up the leg, as if through decayed brushwood. The tops of the branches
were barely covered by water at the time of lowest tide. Several facts
having led me to disbelieve in any elevation of the whole atoll, I was
at first unable to imagine what cause could have killed so large a
field of coral. Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause;
for before this, a strong breeze by forcing water through them into the
head of the lagoon, would tend to raise its level. But now this cannot
happen, and the inhabitants observe that the tide rises to a less
height, during a high S.E. wind, at the head than at the mouth of the
lagoon. The corals, which, under the former condition of things, had
attained the utmost possible limit of upward growth, would thus
occasionally be exposed for a short time to the sun, and be killed.
Besides the increase of dry land, indicated by the foregoing facts, the
exterior solid reef appears to have grown outwards. On the western side
of the atoll, the “flat” lying between the margin of the reef and the
beach, is very wide; and in front of the regular beach with its
conglomerate basis, there is, in most parts, a bed of sand and loose
fragments with trees growing out of it, which apparently is not reached
even by the spray at high water. It is evident some change has taken
place since the waves formed the inner beach; that they formerly beat
against it with violence was evident, from a remarkably thick and
water-worn point of conglomerate at one spot, now protected by
vegetation and a bank of sand; that they beat against it in the same
peculiar manner in which the swell from windward now obliquely curls
round the margin of the reef, was evident from the conglomerate having
been worn into a point projecting from the beach in a similarly oblique
manner. This retreat in the line of action of the breakers might
result, either from the surface of the reef in front of the islets
having been submerged at one time, and afterward having grown upwards,
or from the mounds of coral on the margin having continued to grow
outwards. That an outward growth of this part is in process, can hardly
be doubted from the fact already mentioned of the mounds of Porites
with their summits apparently lately killed, and their sides only three
or four inches lower down thickened by a fresh layer of living coral.
But there is a difficulty on this supposition which I must not pass
over. If the
whole, or a large part of the “flat,” had been formed by the outward
growth of the margin, each successive margin would naturally have been
coated by the Nulliporæ, and so much of the surface would have been of
equal height with the existing zone of living Nulliporæ: this is not
the case, as may be seen in the woodcut. It is, however, evident from
the abraded state of the “flat,” with its original inequalities filled
up, that its surface has been much modified; and it is possible that
the hinder portions of the zone of Nulliporæ, perishing as the reef
grows outwards, might be worn down by the surf. If this has not taken
place, the reef can in no part have increased outwards in breadth since
its formation, or at least since the Nulliporæ formed the convex mound
on its margin; for the zone thus formed, and which stands between two
and three feet above the other parts of the reef, is nowhere much above
twenty yards in width.
Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there
are others having an opposite tendency. On the south-east side,
Lieutenant Sulivan, to whose kindness I am indebted for many
interesting observations, found the conglomerate projecting on the reef
nearly fifty yards in front of the beach: we may infer from what we see
in all other parts of the atoll, that the conglomerate was not
originally so much exposed, but formed the base of an islet, the front
and upper part of which has since been swept away. The degree to which
the conglomerate, round nearly the whole atoll, has been scooped,
broken up, and the fragments cast on the beach, is certainly very
surprising, even on the view that it is the office of occasional gales
to pile up fragments, and of the daily tides to wear them away. On the
western side, also, of the atoll, where I have described a bed of sand
and fragments with trees growing out of it, in front of an old beach,
it struck both Lieutenant Sulivan and myself, from the manner in which
the trees were being washed down, that the surf had lately recommenced
an attack on this line of coast. Appearances indicating a slight
encroachment of the water on the land, are plainer within the lagoon: I
noticed in several places, both on its windward and leeward shores, old
cocoa-nut trees falling with their roots undermined, and the rotten
stumps of others on the beach, where the inhabitants assured us the
cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near
the settlement, the foundation posts of a shed, now washed by every
tide, but which the inhabitants stated, had seven years before stood
above high watermark. In the calm waters of the lagoon, directly
connected with a great, and therefore stable ocean, it seems very
improbable that a change in the currents, sufficiently great to cause
the water to eat into the land on all sides, should have taken place
within a limited period. From these considerations I inferred, that
probably the atoll had lately subsided to a small amount; and this
inference was strengthened by the circumstance, that in 1834, two years
before our visit, the island had been shaken by a severe earthquake,
and by two slighter ones during the ten previous years. If, during
these subterranean disturbances, the atoll did subside, the downward
movement must have been very small, as we must
conclude from the fields of dead coral still lipping the surface of the
lagoon, and from the breakers on the western shore not having yet
regained the line of their former action. The subsidence must, also,
have been preceded by a long period of rest, during which the islets
extended to their present size, and the living margin of the reef grew
either upwards, or as I believe outwards, to its present distance from
the beach.
Whether this view be correct or not, the above facts are worthy of
attention, as showing how severe a struggle is in progress on these low
coral formations between the two nicely balanced powers of land and
water. With respect to the future state of Keeling atoll, if left
undisturbed, we can see that the islets may still extend in length; but
as they cannot resist the surf until broken by rolling over a wide
space, their increase in breadth must depend on the increasing breadth
of the reef; and this must be limited by the steepness of the submarine
flanks, which can be added to only by sediment derived from the wear
and tear of the coral. From the rapid growth of the coral in the
channel cut for the schooner, and from the several agents at work in
producing fine sediment, it might be thought that the lagoon would
necessarily become quickly filled up. Some of this sediment, however,
is transported into the open sea, as appears from the soundings off the
mouth of the lagoon, instead of being deposited within it. The
deposition, moreover, of sediment, checks the growth of coral-reefs, so
that these two agencies cannot act together with full effect in filling
it up. We know so little of the habits of the many different species of
corals, which form the lagoon-reefs, that we have no more reasons for
supposing that their whole surface would grow up as quickly as the
coral did in the schooner-channel, than for supposing that the whole
surface of a peat-moss would increase as quickly as parts are known to
do in holes, where the peat has been cut away. These agencies,
nevertheless, tend to fill up the lagoon; but in proportion as it
becomes shallower, so must the polypifers be subject to many injurious
agencies, such as impure water and loss of food. For instance, Mr.
Liesk informed me, that some years before our visit unusually heavy
rain killed nearly all the fish in the lagoon, and probably the same
cause would likewise injure the corals. The reefs also, it must be
remembered, cannot possibly rise above the level of the lowest
spring-tide, so that the final conversion of the lagoon into land must
be due to the accumulation of sediment; and in the midst of the clear
water of the ocean, and with no surrounding high land, this process
must be exceedingly slow.
_Section II_—GENERAL DESCRIPTION OF ATOLLS.
General form and size of atolls, their reefs and islets.—External
slope.—Zone of Nulliporæ.—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.
I will here give a sketch of the general form and structure of the many
atolls and atoll-formed reefs which occur in the Pacific and Indian
Oceans, comparing them with Keeling atoll. The Maldiva atolls
and the Great Chagos Bank differ in so many respects, that I shall
devote to them, besides occasional references, a third section of this
chapter. Keeling atoll may be considered as of moderate dimensions and
of regular form. Of the thirty-two islands surveyed by Captain Beechey
in the Low Archipelago, the longest was found to be thirty miles, and
the shortest less than a mile; but Vliegen atoll, situated in another
part of the same group, appears to be sixty miles long and twenty
broad. Most of the atolls in this group are of an elongated form; thus
Bow Island is thirty miles in length, and on an average only six in
width (See Fig. 4, Plate I), and Clermont Tonnere has nearly the same
proportions. In the Marshall Archipelago (the Ralick and Radack group
of Kotzebue) several of the atolls are more than thirty miles in
length, and Rimsky Korsacoff is fifty-four long, and twenty wide, at
the broadest part of its irregular outline. Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, however,
bears a double name) measured in a medial and slightly curved line, is
no less than eighty-eight geographical miles long, its greatest width
being under twenty, and its least only nine and a half miles. Some
atolls have spurs projecting from them; and in the Marshall group there
are atolls united together by linear reefs, for instance Menchikoff
Island (See Fig. 3, Plate II), which is sixty miles in length, and
consists of three loops tied together. In far the greater number of
cases an atoll consists of a simple elongated ring, with its outline
moderately regular.
The average width of the annular wreath may be taken as about a quarter
of a mile. Captain Beechey[5] says that in the atolls of the Low
Archipelago it exceeded in no instance half a mile. The description
given of the structure and proportional dimensions of the reef and
islets of Keeling atoll, appears to apply perfectly to nearly all the
atolls in the Pacific and Indian Oceans. The islets are first formed
some way back either on the projecting points of the reef, especially
if its form be angular, or on the sides of the main entrances into the
lagoon—that is in both cases, on points where the breakers can act
during gales of wind in somewhat different directions, so that the
matter thrown up from one side may accumulate against that before
thrown up from another. In Lutké’s chart of the Caroline atolls, we see
many instances of the former case; and the occurrence of islets, as if
placed for beacons, on the points where there is a gateway or breach
through the reef, has been noticed by several authors. There are some
atoll-formed reefs, rising to the surface of the sea and partly dry at
low water, on which from some cause islets have never been formed; and
there are others on which they have been formed, but have subsequently
been worn away. In atolls of small dimensions the islets frequently
become united into a single horse-shoe or ring-formed strip; but Diego
Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in
width. To show how small the total area of the annular reef and the
land is in islands of this class,
I may quote a remark from the voyage of Lutké, namely, that if the
forty-three rings, or atolls, in the Caroline Archipelago, were put one
within another, and over a steeple in the centre of St. Petersburg, the
whole world would not cover that city and its suburbs.
[5] Beechey’s “Voyage to the Pacific and Beering’s Straits,” chapter
viii.
The form of the bottom off Keeling atoll, which gradually slopes to
about twenty fathoms at the distance of between one and two hundred
yards from the edge of the reef, and then plunges at an angle of 45°
into unfathomable depths, is exactly the same[6] with that of the
sections of the atolls in the Low Archipelago given by Captain Beechey.
The nature, however, of the bottom seems to differ, for this officer[7]
informs me that all the soundings, even the deepest, were on coral, but
he does not know whether dead or alive. The slope round Christmas atoll
(Lat. 1° 4′ N., 157° 45′ W.), described by Cook,[8] is considerably
less, at about half a mile from the edge of the reef, the average depth
was about fourteen fathoms on a fine sandy bottom, and at a mile, only
between twenty and forty fathoms. It has no doubt been owing to this
gentle slope, that the strip of land surrounding its lagoon, has
increased in one part to the extraordinary width of three miles; it is
formed of successive ridges of broken shells and corals, like those on
the beach. I know of no other instance of such width in the reef of an
atoll; but Mr. F. D. Bennett informs me that the inclination of the
bottom round Caroline atoll in the Pacific, is like that off Christmas
Island, very gentle. Off the Maldiva and Chagos atolls, the inclination
is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell[9]
found fifty and sixty fathoms close to the edge of the reef, and at 300
yards distance there was no bottom with a 300-yard line. Captain
Moresby informs me, that at 100 fathoms from the mouth of the lagoon of
Diego Garcia, he found no bottom with 150 fathoms; this is the more
remarkable, as the slope is generally less abrupt in front of channels
through a reef, owing to the accumulation of sediment. At Egmont
Island, also, at 150 fathoms from the reef, soundings were struck with
150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef,
no bottom was obtained, as I am informed by Captain Moresby, with a
line of 200 fathoms! The currents run with great force round these
atolls, and where they are strongest, the inclination appears to be
most abrupt. I am informed by the same authority, that wherever
soundings were obtained off these islands, the bottom was invariably
sandy: nor was there any reason to suspect the existence of submarine
cliffs, as there was at Keeling Island.[10] Here then occurs a
difficulty; can sand accumulate on a slope, which, in some cases,
appears to exceed fifty-five degrees? It must be observed, that I speak
of slopes where soundings were obtained, and not of such cases, as that
of Cardoo, where the nature of the bottom is unknown, and where its
inclination must be nearly vertical. M. Elie de Beaumont[11] has
argued, and there is no higher authority on this subject, from the
inclination at which snow slides down in avalanches, that a bed of sand
or mud cannot be formed at a greater angle than thirty degrees.
Considering the number of soundings on sand, obtained round the Maldiva
and Chagos atolls, which appears to indicate a greater angle, and the
extreme abruptness of the sand-banks in the West Indies, as will be
mentioned in the Appendix, I must conclude that the adhesive property
of wet sand counteracts its gravity, in a much greater ratio than has
been allowed for by M. Elie de Beaumont. From the facility with which
calcareous sand becomes agglutinated, it is not necessary to suppose
that the bed of loose sand is thick.
[6] The form of the bottom round the Marshall atolls in the Northern
Pacific is probably similar: Kotzebue (“First Voyage,” vol. ii, p. 16)
says: “We had at a small distance from the reef, forty fathoms depth,
which increased a little further so much that we could find no
bottom.”
[7] I must be permitted to express my obligation to Captain Beechey,
for the very kind manner in which he has given me information on
several points, and to own the great assistance I have derived from
his excellent published work.
[8] Cook’s “Third Voyage,” vol. ii, chap. 10.
[9] This fact is taken from a MS. account of these groups lent me by
Captain Moresby. See also Captain Moresby’s paper on the Maldiva
atolls in the _Geographical Journal_, vol. v, p. 401.
[10] Off some of the islands in the Low Archipelago the bottom appears
to descend by ledges. Off Elizabeth Island, which, however, consists
of raised coral, Captain Beechey (page 45, 4to ed.) describes three
ledges: the first had an easy slope from the beach to a distance of
about fifty yards: the second extended two hundred yards with
twenty-five fathoms on it, and then ended abruptly, like the first;
and immediately beyond this there was no bottom with two hundred
fathoms.
[11] “Memoires pour servir à une description Geolog. de France,” tome
iv, p. 216.
Captain Beechey has observed, that the submarine slope is much less at
the extremities of the more elongated atolls in the Low Archipelago,
than at their sides; in speaking of Ducie’s Island he says[12] the
buttress, as it may be called, which “has the most powerful enemy (the
S.W. swell) to oppose, is carried out much further, and with less
abruptness than the other.” In some cases, the less inclination of a
certain part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a prevailing
current which there accumulates a bed of sand. Where the water is
perfectly tranquil, as within a lagoon, the reefs generally grow up
perpendicularly, and sometimes even overhang their bases; on the other
hand, on the leeward side of Mauritius, where the water is generally
tranquil, although not invariably so, the reef is very gently inclined.
Hence it appears that the exterior angle varies much; nevertheless in
the close similarity in form between the sections of Keeling atoll and
of the atolls in the Low Archipelago, in the general steepness of the
reefs of the Maldiva and Chagos atolls, and in the perpendicularity of
those rising out of water always tranquil, we may discern the effects
of uniform laws; but from the complex action of the surf and currents,
on the growing powers of the coral and on the deposition of sediment,
we can by no means follow out all the results.
[12] Beechey’s “Voyage,” 4to ed., p. 44.
Where islets have been formed on the reef, that part which I have
sometimes called the “flat” and which is partly dry at low water,
appears similar in every atoll. In the Marshall group in the North
Pacific, it may be inferred from Chamisso’s description, that the reef,
where islets have not been formed on it, slopes gently from the
external margin to the shores of the lagoon; Flinders states that the
Australian barrier has a similar inclination inwards, and I have no
doubt it is of general occurrence, although, according to Ehrenberg,
the reefs of the Red Sea offer an exception. Chamisso observes that
“the red colour of the reef (at the Marshall atolls) under the breakers
is caused by a Nullipora, which covers the stone _wherever the waves
beat_; and, under favourable circumstances, assumes a stalactical
form,”—a description perfectly applicable to the margin of Keeling
atoll.[13] Although Chamisso does not state that the masses of
Nulliporæ form points or a mound, higher than the flat, yet I believe
that this is the case; for Kotzebue,[14] in another part, speaks of the
rocks on the edge of the reef “as visible for about two feet at low
water,” and these rocks we may feel quite certain are not formed of
true coral,[15] Whether a smooth convex mound of Nulliporæ, like that
which appears as if artificially constructed to protect the margin of
Keeling Island, is of frequent occurrence round atolls, I know not; but
we shall presently meet with it, under precisely the same form, on the
outer edge of the “barrier-reefs” which encircle the Society Islands.
[13] Kotzebue’s “First Voyage,” vol. iii, p. 142. Near Porto Praya, in
the Cape de Verde Islands, some basaltic rocks, lashed by no
inconsiderable surf, were completely enveloped with a layer of
Nulliporæ. The entire surface over many square inches, was coloured of
a peach-blossomed red; the layer, however, was of no greater thickness
than paper. Another kind, in the form of projecting knobs, grew in the
same situation. These Nulliporæ are closely related to those described
on the coral-reefs, but I believe are of different species.
[14] Kotzebue’s “First Voyage,” vol. ii, p. 16. Lieutenant Nelson, in
his excellent memoir in the Geological Transactions (vol. ii, p. 105),
alludes to the rocky points mentioned by Kotzebue, and infers that
they consist of Serpulæ, which compose incrusting masses on the reefs
of Bermudas, as they likewise do on a sandstone bar off the coast of
Brazil (which I have described in _London Phil. Journal,_ October
1841). These masses of Serpulæ hold the same position, relatively to
the action of the sea, with the Nulliporæ on the coral-reefs in the
Indian and Pacific Oceans.
[15] Captain Moresby, in his valuable paper “on the Northern atolls of
Maldivas” (_Geographical Journal_, vol. v), says that the edges of the
reefs there stand above water at low spring-tides.
There appears to be scarcely a feature in the structure of Keeling
reef, which is not of common, if not of universal occurrence, in other
atolls. Thus Chamisso describes[16] a layer of coarse conglomerate,
outside the islets round the Marshall atolls which “appears on its
upper surface uneven and eaten away.” From drawings, with appended
remarks, of Diego Garcia in the Chagos group and of several of the
Maldiva atolls, shown me by Captain Moresby,[17] it is evident that
their outer coasts are subject to the same round of decay and
renovation as those of Keeling atoll. From the description of the
atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it
is not apparent that any conglomerate coral-rock was there observed.
[16] Kotzebue’s “First Voyage,” vol. iii, p. 144.
[17] See also Moresby on the Northern atolls of the Maldivas,
_Geographical Journal_, vol v, p. 400.
The lagoon in Keeling atoll is shallow; in the atolls of the Low
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
Group, according to Chamisso, from 30 to 35; in the Caroline atolls it
is only a little less. Within the Maldiva atolls there are large spaces
with 45 fathoms, and some soundings are laid down of 49 fathoms. The
greater part of the bottom in most lagoons, is formed of sediment;
large spaces have exactly the same depth, or the depth varies so
insensibly, that it is evident that no other means, excepting aqueous
deposition, could have leveled the surface so equally. In the Maldiva
atolls this is very conspicuous, and likewise in some of the Caroline
and Marshall Islands. In the former large spaces consist of sand and
_soft clay_; and Kotzebue speaks of clay having been found within one
of the Marshall atolls. No doubt this clay is calcareous mud, similar
to that at Keeling Island, and to that at Bermuda already referred to,
as undistinguishable from disintegrated chalk, and which Lieutenant
Nelson says is called there pipe-clay.[18]
[18] I may here observe that on the coast of Brazil, where there is
much coral, the soundings near the land are described by Admiral
Roussin, in the _Pilote du Brésil_, as siliceous sand, mingled with
much finely comminuted particles of shells and coral. Further in the
offing, for a space of 1,300 miles along the coast, from the Abrolhos
Islands to Maranham, the bottom in many places is composed of “tuf
blanc, mêlé ou formé de madrépores broyés.” This white substance,
probably, is analogous to that which occurs within the above-mentioned
lagoons; it is sometimes, according to Roussin, firm, and he compares
it to mortar.
Where the waves act with unequal force on the two sides of an atoll,
the islets appear to be first formed, and are generally of greater
continuity on the more exposed shore. The islets, also, which are
placed to leeward, are in most parts of the Pacific liable to be
occasionally swept entirely away by gales, equalling hurricanes in
violence, which blow in an opposite direction to the ordinary
trade-wind. The absence of the islets on the leeward side of atolls, or
when present their lesser dimensions compared with those to windward,
is a comparatively unimportant fact; but in several instances the reef
itself on the leeward side, retaining its usual defined outline, does
not rise to the surface by several fathoms. This is the case with the
southern side of Peros Banhos (Plate 1, Fig. 9) in the Chagos group,
with Mourileu atoll,[19] in the Caroline Archipelago, and with the
barrier-reef (Plate I, Fig. 8) of the Gambier Islands. I allude to the
latter reef, although belonging to
another class, because Captain Beechey was first led by it to observe
the peculiarity in the question. At Peros Banhos the submerged part is
nine miles in length, and lies at an average depth of about five
fathoms; its surface is nearly level, and consists of hard stone, with
a thin covering of loose sand. There is scarcely any living coral on
it, even on the outer margin, as I have been particularly assured by
Captain Moresby; it is, in fact, a wall of dead coral-rock, having the
same width and transverse section with the reef in its ordinary state,
of which it is a continuous portion. The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same
manner as on the sides of an ordinary passage through the reef. The
reef to leeward in other cases is nearly or quite obliterated, and one
side of the lagoon is left open; for instance, at Oulleay (Caroline
Archipelago), where a crescent-formed reef is fronted by an irregular
bank, on which the other half of the annular reef probably once stood.
At Namonouito, in the same Archipelago, both these modifications of the
reef concur; it consists of a great flat bank, with from twenty to
twenty-five fathoms water on it; for a length of more than forty miles
on its southern side it is open and without any reef, whilst on the
other sides it is bounded by a reef, in parts rising to the surface and
perfectly characterised, in parts lying some fathoms submerged. In the
Chagos group there are annular reefs, entirely submerged, which have
the same structure as the submerged and defined portions just
described. The Speaker’s Bank offers an excellent example of this
structure; its central expanse, which is about twenty-two fathoms deep,
is twenty-four miles across; the external rim is of the usual width of
annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon. Captain Moresby believes the rim consists of dead
rock, thinly covered with sand, and he is certain this is the case with
the external rim of the Great Chagos Bank, which is also essentially a
submerged atoll. In both these cases, as in the submerged portion of
the reef at Peros Banhos, Captain Moresby feels sure that the quantity
of living coral, even on the outer edge overhanging the deep-sea water,
is quite insignificant. Lastly, in several parts of the Pacific and
Indian Oceans there are banks, lying at greater depths than in the
cases just mentioned, of the same form and size with the neighbouring
atolls, but with their atoll-like structure wholly obliterated. It
appears from the survey of Freycinet, that there are banks of this kind
in the Caroline Archipelago, and, as is reported, in the Low
Archipelago. When we discuss the origin of the different classes of
coral formations, we shall see that the submerged state of the whole of
some atoll-formed reefs, and of portions of others, generally but not
invariably on the leeward side, and the existence of more deeply
submerged banks now possessing little or no signs of their original
atoll-like structure, are probably the effects of a uniform
cause,—namely, the death of the coral, during the subsidence of the
area, in which the atolls or banks are situated.
[19] Frederick Lutké’s “Voyage autour du Monde,” vol. ii, p. 291. See
also his account of Namonouito, at pp. 97 and 105, and the chart of
Oulleay in the Atlas.
There is seldom, with the exception of the Maldiva atolls, more than
two or three channels, and generally only one leading into the lagoon,
of sufficient depth for a ship to enter. in small atolls, there is
usually not even one. Where there is deep water, for instance above
twenty fathoms, in the middle of the lagoon, the channels through the
reef are seldom as deep as the centre,—it may be said that the rim only
of the saucer-shaped hollow forming the lagoon is notched. Mr.
Lyell[20] has observed that the growth of the coral would tend to
obstruct all the channels through a reef, except those kept open by
discharging the water, which during high tide and the greater part of
each ebb is thrown over its circumference. Several facts indicate that
a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed,
during the change of the monsoon, the sea discoloured to a distance off
the entrances into the Maldiva and Chagos atolls. This, probably, would
check the growth of the coral in them, far more effectually than a mere
current of water. In the many small atolls without any channel, these
causes have not prevented the entire ring attaining the surface. The
channels, like the submerged and effaced parts of the reef, very
generally though not invariably occur on the leeward side of the atoll,
or on that side, according to Beechey,[21] which, from running in the
same direction with the prevalent wind, is not fully exposed to it.
Passages between the islets on the reef, through which boats can pass
at high water, must not be confounded with ship-channels, by which the
annular reef itself is breached. The passages between the islets occur,
of course, on the windward as well as on the leeward side; but they are
more frequent and broader to leeward, owing to the lesser dimensions of
the islets on that side.
[20] “Principles of Geology,” vol. iii, p. 289.
[21] Beechey’s “Voyage,” 4to ed., vol. i, p. 189.
At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other
atolls. Chamisso,[22] speaking in general terms of the lagoons in the
Marshall atolls, says the lead generally sinks “from a depth of two or
three fathoms to twenty or twenty-four, and you may pursue a line in
which on one side of the boat you may see the bottom, and on the other
the azure-blue deep water.” The shores of the lagoon-like channel
within the barrier-reef at Vanikoro have a similar structure. Captain
Beechey has described a modification of this structure (and he believes
it is not uncommon) in two atolls in the Low Archipelago, in which the
shores of the lagoon descend by a few, broad, slightly inclined ledges
or steps: thus at Matilda atoll,[23] the great exterior reef, the
surface of which is gently inclined towards and beneath the surface of
the lagoon, ends abruptly in a little cliff three fathoms deep; at its
foot, a ledge forty yards wide extends, shelving gently inwards
like the surface-reef, and terminated by a second little cliff five
fathoms deep; beyond this, the bottom of the lagoon slopes to twenty
fathoms, which is the average depth of its centre. These ledges seem to
be formed of coral-rock; and Captain Beechey says that the lead often
descended several fathoms through holes in them. In some atolls, all
the coral reefs or knolls in the lagoon come to the surface at low
water; in other cases of rarer occurrence, all lie at nearly the same
depth beneath it, but most frequently they are quite irregular,—some
with perpendicular, some with sloping sides,—some rising to the
surface, and others lying at all intermediate depths from the bottom
upwards. I cannot, therefore, suppose that the union of such reefs
could produce even one uniformly sloping ledge, and much less two or
three, one beneath the other, and each terminated by an abrupt wall. At
Matilda Island, which offers the best example of the step-like
structure, Captain Beechey observes that the coral-knolls within the
lagoon are quite irregular in their height. We shall hereafter see that
the theory which accounts for the ordinary form of atolls, apparently
includes this occasional peculiarity in their structure.
[22] Kotzebue’s “First Voyage,” vol. iii, p. 142.
[23] Beechey’s “Voyage,” 4to ed., vol. i, p. 160. At Whitsunday Island
the bottom of the lagoon slopes gradually towards the centre, and then
deepens suddenly, the edge of the bank being nearly perpendicular.
This bank is formed of coral and dead shells.
In the midst of a group of atolls, there sometimes occur small, flat,
very low islands of coral formation, which probably once included a
lagoon, since filled up with sediment and coral-reefs. Captain Beechey
entertains no doubt that this has been the case with the two small
islands, which alone of thirty-one surveyed by him in the Low
Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15 deg
S.) is described by Chamisso[24] as formed by a dam of madreporitic
rock inclosing a flat space, thinly covered with trees, into which the
sea on the leeward side occasionally breaks. North Keeling atoll
appears to be in a rather less forward stage of conversion into land;
it consists of a horse-shoe shaped strip of land surrounding a muddy
flat, one mile in its longest axis, which is covered by the sea only at
high water. When describing South Keeling atoll, I endeavoured to show
how slow the final process of filling up a lagoon must be;
nevertheless, as all causes do tend to produce this effect, it is very
remarkable that not one instance, as I believe, is known of a
moderately sized lagoon being filled up even to the low water-line at
spring-tides, much less of such a one being converted into land. It is,
likewise, in some degree remarkable, how few atolls, except small ones,
are surrounded by a single linear strip of land, formed by the union of
separate islets. We cannot suppose that the many atolls in the Pacific
and Indian Oceans all have had a late origin, and yet should they
remain at their present level, subjected only to the action of the sea
and to the growing powers of the coral, during as many centuries as
must have elapsed since any of the earlier tertiary epochs, it cannot,
I think, be doubted that their lagoons and the islets on their reef,
would present a totally different appearance from what they now do.
This consideration leads to the suspicion that some renovating agency
(namely subsidence) comes into play at intervals, and perpetuates their
original structure.
[24] Kotzebue’s “First Voyage,” vol. iii, p. 221.
_Section 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.
Although occasional references have been made to the Maldiva atolls,
and to the banks in the Chagos group, some points of their structure
deserve further consideration. My description is derived from an
examination of the admirable charts lately published from the survey of
Captain Moresby and Lieutenant Powell, and more especially from
information which Captain Moresby has communicated to me in the kindest
manner.
The Maldiva Archipelago is 470 miles in length, with an average breadth
of about 50 miles. The form and dimensions of the atolls, and their
singular position in a double line, may be seen, but not well, in the
greatly reduced chart (Fig. 6) in Plate II. The dimensions of the
longest atoll in the group (called by the double name of
Milla-dou-Madou and Tilla-dou-Matte) have already been given; it is 88
miles in a medial and slightly curved line, and is less than 20 miles
in its broadest part. Suadiva, also, is a noble atoll, being 44 miles
across in one direction, and 34 in another, and the great included
expanse of water has a depth of between 250 and 300 feet. The smaller
atolls in this group differ in no respect from ordinary ones; but the
larger ones are remarkable from being breached by numerous deep-water
channels leading into the lagoon; for instance, there are 42 channels,
through which a ship could enter the lagoon of Suadiva. In the three
southern large atolls, the separate portions of reef between these
channels have the ordinary structure, and are linear; but in the other
atolls, especially the more northern ones, these portions are
ring-formed, like miniature atolls. Other ring-formed reefs rise out of
the lagoons, in the place of those irregular ones which ordinarily
occur there. In the reduction of the chart of Mahlos Mahdoo (Plate II,
Fig. 4), it was not found easy to define the islets and the little
lagoons within each reef, so that the ring-formed structure is very
imperfectly shown; in the large published charts of Tilla-dou-Matte,
the appearance of these rings, from standing further apart from each
other, is very remarkable. The rings on the margin are generally
elongated; many of them are three, and some even five miles, in
diameter; those within the lagoon are usually smaller, few being more
than two miles across, and the greater number rather less than one. The
depth of the little lagoon within these small annular reefs is
generally from five to seven fathoms, but occasionally more; and in Ari
atoll many of the central ones are twelve, and some even more than
twelve fathoms deep. These rings rise abruptly from the platform or
bank, on which they are placed; their outer margin is
invariably bordered by living coral[25] within which there is a flat
surface of coral rock; of this flat, sand and fragments have in many
cases accumulated and been converted into islets, clothed with
vegetation. I can, in fact, point out no essential difference between
these little ring-formed reefs (which, however, are larger, and contain
deeper lagoons than many true atolls that stand in the open sea), and
the most perfectly characterised atolls, excepting that the ring-formed
reefs are based on a shallow foundation, instead of on the floor of the
open sea, and that instead of being scattered irregularly, they are
grouped closely together on one large platform, with the marginal rings
arranged in a rudely formed circle.
[25] Captain Moresby informs me that _Millepora complanata_ is one of
the commonest kinds on the outer margin, as it is at Keeling atoll.
The perfect series which can be traced from portions of simple linear
reef, to others including long linear lagoons, and from these again to
oval or almost circular rings, renders it probable that the latter are
merely modifications of the linear or normal state. It is conformable
with this view, that the ring-formed reefs on the margin, even where
most perfect and standing furthest apart, generally have their longest
axes directed in the line which the reef would have held, if the atoll
had been bounded by an ordinary wall. We may also infer that the
central ring-formed reefs are modifications of those irregular ones,
which are found in the lagoons of all common atolls. It appears from
the charts on a large scale, that the ring-like structure is contingent
on the marginal channels or breaches being wide; and, consequently, on
the whole interior of the atoll being freely exposed to the waters of
the open sea. When the channels are narrow or few in number, although
the lagoon be of great size and depth (as in Suadiva), there are no
ring-formed reefs; where the channels are somewhat broader, the
marginal portions of reef, and especially those close to the larger
channels, are ring-formed, but the central ones are not so; where they
are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on
the openness of the marginal channels, the theory of their formation,
as we shall hereafter see, is included in that of the parent atolls, of
which they form the separate portions.
The lagoons of all the atolls in the southern part of the Archipelago
are from ten to twenty fathoms deeper than those in the northern part.
This is well exemplified in the case of Addoo, the southernmost atoll
in the group, for although only nine miles in its longest diameter, it
has a depth of thirty-nine fathoms, whereas all the other small atolls
have comparatively shallow lagoons; I can assign no adequate cause for
this difference in depth. In the central and deepest part of the
lagoons, the bottom consists, as I am informed by Captain Moresby, of
stiff clay (probably a calcareous mud); nearer the border it consists
of sand, and in the channels through the reef, of hard sand-banks,
sandstone, conglomerate rubble, and a little live coral. Close outside
the reef and the line joining its detached portions (where intersected
by many channels), the bottom is sandy, and it slopes abruptly into
unfathomable depths.
In most lagoons the depth is considerably greater in the centre than in
the channels; but in Tilla-dou-Matte, where the marginal ring-formed
reefs stand far apart, the same depth is carried across the entire
atoll, from the deep-water line on one side to that on the other. I
cannot refrain from once again remarking on the singularity of these
atolls,—a great sandy and generally concave disc rises abruptly from
the unfathomable ocean, with its central expanse studded and its border
symmetrically fringed with oval basins of coral-rock, just lipping the
surface of the sea, sometimes clothed with vegetation, and each
containing a little lake of clear water!
In the southern Maldiva atolls, of which there are nine large ones, all
the small reefs within the lagoons come to the surface, and are dry at
low water spring-tides; hence in navigating them, there is no danger
from submarine banks. This circumstance is very remarkable, as within
some atolls, for instance those of the neighbouring Chagos group, not a
single reef comes to the surface, and in most other cases a few only
do, and the rest lie at all intermediate depths from the bottom
upwards. When treating of the growth of coral I shall again refer to
this subject.
Although in the neighbourhood of the Maldiva Archipelago the winds,
during the monsoons, blow during nearly an equal time from opposite
quarters, and although, as I am informed by Captain Moresby, the
westerly winds are the strongest, yet the islets are almost all placed
on the eastern side of the northern atolls, and on the south-eastern
side of the southern atolls. That the formation of the islets is due to
detritus thrown up from the outside, as in the ordinary manner, and not
from the interior of the lagoons, may, I think be safely inferred from
several considerations, which it is hardly worth while to detail. As
the easterly winds are not the strongest, their action probably is
aided by some prevailing swell or current.
In groups of atolls, exposed to a trade-wind, the ship-channels into
the lagoons are almost invariably situated on the leeward or less
exposed side of the reef, and the reef itself is sometimes either
wanting there, or is submerged. A strictly analogous, but different
fact, may be observed at the Maldiva atolls—namely, that where two
atolls stand in front of each other, the breaches in the reef are the
most numerous on their near, and therefore less exposed, sides. Thus on
the near sides of Ari and the two Nillandoo atolls, which face S. Mãle,
Phaleedoo, and Moloque atolls, there are seventy-three deep-water
channels, and only twenty-five on their outer sides; on the near side
of the three latter named atolls there are fifty-six openings, and only
thirty-seven on their outsides. It is scarcely possible to attribute
this difference to any other cause than the somewhat different action
of the sea on the two sides, which would ensue from the protection
afforded by the two rows of atolls to each other. I may here remark
that in most cases, the conditions favourable to the greater
accumulation of fragments on the reef and to its more perfect
continuity on one side of the atoll than on the other, have concurred,
but this has not been the case with the Maldivas; for we have seen that
the islets are placed on the eastern or south-eastern
sides, whilst the breaches in the reef occur indifferently on any side,
where protected by an opposite atoll. The reef being more continuous on
the outer and more exposed sides of those atolls which stand near each
other, accords with the fact, that the reef of the southern atolls is
more continuous than that of the northern ones; for the former, as I am
informed by Captain Moresby, are more constantly exposed than the
northern atolls to a heavy surf.
The date of the first formation of some of the islets in this
Archipelago is known to the inhabitants; on the other hand, several
islets, and even some of those which are believed to be very old, are
now fast wearing away. The work of destruction has, in some instances,
been completed in ten years. Captain Moresby found on one water-washed
reef the marks of wells and graves, which were excavated when it
supported an islet. In South Nillandoo atoll, the natives say that
three of the islets were formerly larger: in North Nillandoo there is
one now being washed away; and in this latter atoll Lieutenant Prentice
found a reef, about six hundred yards in diameter, which the natives
positively affirmed was lately an island covered with cocoa-nut trees.
It is now only partially dry at low water spring-tides, and is (in
Lieutenant Prentice’s words) “entirely covered with live coral and
madrepore.” In the northern part, also, of the Maldiva Archipelago and
in the Chagos group, it is known that some of the islets are
disappearing. The natives attribute these effects to variations in the
currents of the sea. For my own part I cannot avoid suspecting that
there must be some further cause, which gives rise to such a cycle of
change in the action of the currents of the great and open ocean.
Several of the atolls in this Archipelago are so related to each other
in form and position, that at the first glance one is led to suspect
that they have originated in the disseverment of a single one. Mãle
consists of three perfectly characterised atolls, of which the shape
and relative position are such, that a line drawn closely round all
three, gives a symmetrical figure; to see this clearly, a larger chart
is required than that of the Archipelago in Plate II; the channel
separating the two northern Male atolls is only little more than a mile
wide, and no bottom was found in it with 100 fathoms. Powell’s Island
is situated at the distance of two miles and a half off the northern
end of Mahlos Mahdoo (see Fig. 4, Plate II), at the exact point where
the two sides of the latter, if prolonged, would meet; no bottom,
however, was found in the channel with 200 fathoms; in the wider
channel between Horsburgh atoll and the southern end of Mahlos Mahdoo,
no bottom was found with 250 fathoms. In these and similar cases, the
relation consists only in the form and position of the atolls. But in
the channel between the two Nillandoo atolls, although three miles and
a quarter wide, soundings were struck at the depth of 200 fathoms; the
channel between Ross and Ari atolls is four miles wide, and only 150
fathoms deep. Here then we have, besides the relation of form, a
submarine connection. The fact of soundings having been obtained
between two separate and perfectly characterised atolls is in itself
interesting, as it has never, I believe, been effected in any of the
many
other groups of atolls in the Pacific and Indian seas. In continuing to
trace the connection of adjoining atolls, if a hasty glance be taken at
the chart (Fig. 4, Plate II) of Mahlos Mahdoo, and the line of
unfathomable water be followed, no one will hesitate to consider it as
one atoll. But a second look will show that it is divided by a
bifurcating channel, of which the northern arm is about one mile and
three-quarters in width, with an average depth of 125 fathoms, and the
southern one three-quarters of a mile wide, and rather less deep. These
channels resemble in the slope of their sides and general form, those
which separate atolls in every respect distinct; and the northern arm
is wider than that dividing two of the Mãle atolls. The ring-formed
reefs on the sides of this bifurcating channel are elongated, so that
the northern and southern portions of Mahlos Mahdoo may claim, as far
as their external outline is concerned, to be considered as distinct
and perfect atolls. But the intermediate portion, lying in the fork of
the channel, is bordered by reefs less perfect than those which
surround any other atoll in the group of equally small dimensions.
Mahlos Mahdoo, therefore, is in every respect in so intermediate a
condition, that it may be considered either as a single atoll nearly
dissevered into three portions, or as three atolls almost perfect and
intimately connected. This is an instance of a very early stage of the
apparent disseverment of an atoll, but a still earlier one in many
respects is exhibited at Tilla-dou-Matte. In one part of this atoll,
the ring-formed reefs stand so far apart from each other, that the
inhabitants have given different names to the northern and southern
halves; nearly all the rings, moreover, are so perfect and stand so
separate, and the space from which they rise is so level and unlike a
true lagoon, that we can easily imagine the conversion of this one
great atoll, not into two or three portions, but into a whole group of
miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter
be seen, that the theory of subsidence, with the upward growth of the
coral, modified by accidents of probable occurrence, will account for
the occasional disseverment of large atolls.
_Plate II_—Great Chagos Bank, New Caledonia, Menchikoff Atoll, etc.
[Illustration: Great Chagos Bank]
Fig. 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey
by Captain Moresby and Lieutenant Powell; the parts which are shaded,
with the exception of two or three islets on the western and northern
sides, do not rise to the surface, but are submerged from four to ten
fathoms; the banks bounded by the dotted lines lie from fifteen to
twenty fathoms beneath the surface, and are formed of sand; the central
space is of mud, and from thirty to fifty fathoms deep.
Fig. 2.—A vertical section, on the same scale, in an eastern and
western line across the Great Chagos Bank, given for the sake of
exhibiting more clearly its structure.
[Illustration: New Caledonia, Menchikoff Atoll, etc.]
Fig. 3.—Menchikoff Atoll (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the
Pacific;” originally surveyed by Captain Hagemeister; the depth within
the lagoons is unknown.
Fig. 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs,
both on the margin and in the middle part, are meant to represent
little lagoons; but it was found not possible to distinguish them
clearly from the small islets, which have been formed on these same
small reefs; many of the smaller reefs could not be introduced; the
nautical mark (dot over a dash) over the figures 250 and 200, between
Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that
soundings were not obtained at these depths.
Fig. 5.—NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern’s “Atlas,” compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the “Atlas
of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef
is represented by a single line with crosses; I have for the sake of
uniformity added an interior line.
Fig. 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.
The Great Chagos bank alone remains to be described. In the Chagos
group there are some ordinary atolls, some annular reefs rising to the
surface but without any islets on them, and some atoll-formed banks,
either quite submerged, or nearly so. Of the latter, the Great Chagos
Bank is much the largest, and differs in its structure from the others:
a plan of it is given in Plate II, Fig. 1, in which, for the sake of
clearness, I have had the parts under ten fathoms deep finely shaded:
an east and west vertical section is given in Fig. 2, in which the
vertical scale has been necessarily exaggerated. Its longest axis is
ninety nautical miles, and another line drawn at right angles to the
first, across the broadest part, is seventy. The central part consists
of a level muddy flat, between forty and fifty fathoms deep, which is
surrounded on all sides, with the exception of some breaches, by the
steep edges of a set of banks, rudely arranged in a circle. These banks
consist of sand, with a very little live coral; they vary in breadth
from five to twelve miles, and on an average lie about sixteen fathoms
beneath the surface;
they are bordered by the steep edges of a third narrow and upper bank,
which forms the rim to the whole. This rim is about a mile in width,
and with the exception of two or three spots where islets have been
formed, is submerged between five and ten fathoms. It consists of
smooth hard rock, covered with a thin layer of sand, but with scarcely
any live coral; it is steep on both sides, and outwards slopes abruptly
into unfathomable depths. At the distance of less than half a mile from
one part, no bottom was found with 190 fathoms; and off another point,
at a somewhat greater distance, there was none with 210 fathoms. Small
steep-sided banks or knolls, covered with luxuriantly growing coral,
rise from the interior expanse to the same level with the external rim,
which, as we have seen, is formed only of dead rock. It is impossible
to look at the plan (Fig. 1, Plate II), although reduced to so small a
scale, without at once perceiving that the Great Chagos Bank is, in the
words of Captain Moresby,[26] “nothing more than a half-drowned atoll.”
But of what great dimensions, and of how extraordinary an internal
structure? We shall hereafter have to consider both the cause of its
submerged condition, a state common to other banks in the group, and
the origin of the singular submarine terraces, which bound the central
expanse: these, I think, it can be shown, have resulted from a cause
analogous to that which has produced the bifurcating channel across
Mahlos Mahdoo.
[26] This officer has had the kindness to lend me an excellent MS.
account of the Chagos Islands; from this paper, from the published
charts, and from verbal information communicated to me by Captain
Moresby, the above account of the Great Chagos Bank is taken.
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.
The term “barrier” has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to
that on the western coast of New Caledonia. At one time I thought it
convenient thus to restrict the term, but as these reefs are similar in
structure, and in position relatively to the land, to those, which,
like a wall with a deep moat within, encircle many smaller islands, I
have classed them together. The reef, also, on the west coast of New
Caledonia, circling round the extremities of the island, is an
intermediate form between a small encircling reef and the Australian
barrier, which stretches for a thousand miles in nearly a straight
line.
The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high
land rising from within their central expanse. The general resemblance
between the reefs of the barrier and atoll classes may be seen in the
small, but accurately reduced charts on Plate I,[1] and this
resemblance can be further shown to extend to every part of the
structure. Beginning with the outside of the reef; many scattered
soundings off Gambier, Oualan, and some other encircled islands, show
that close to the breakers there exists a narrow shelving margin,
beyond which the ocean becomes suddenly unfathomable; but off the west
coast of New Caledonia, Captain Kent[2] found no bottom with 150
fathoms, at two ships’ length from the reef; so that the slope here
must be nearly as precipitous as off the Maldiva atolls.
[1] The authorities from which these charts have been reduced,
together with some remarks on them are given in a separately appended
page, descriptive of the Plates.
[2] Dalrymple, “Hydrog. Mem.” vol. iii.
I can give little information regarding the kinds of corals which live
on the outer margin. When I visited the reef at Tahiti, although it was
low water, the surf was too violent for me to see the living masses;
but, according to what I heard from some intelligent native chiefs,
they resemble in their rounded and branchless forms, those on the
margin of Keeling atoll. The extreme verge of the reef, which was
visible between the breaking waves at low water, consisted of a
rounded, convex, artificial-like breakwater, entirely coated with
Nulliporæ, and absolutely similar to that which I have described at
Keeling atoll. From what I heard when at Tahiti, and from the writings
of the Revs. W. Ellis and J. Williams, I conclude that this peculiar
structure is common to most of the encircled islands of the Society
Archipelago. The reef within this mound or breakwater, has an extremely
irregular surface, even more so than between the islets on the reef of
Keeling atoll, with which alone (as there are no islets on the reef of
Tahiti) it can properly be compared. At Tahiti, the reef is very
irregular in width; but round many other encircled islands, for
instance, Vanikoro or Gambier Islands (Figs 1 and 8, Plate I), it is
quite as regular, and of the same average width, as in true atolls.
Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for
a short distance, and then terminates abruptly in a submarine wall,
forty feet high,—a structure absolutely similar to that described by
Chamisso in the Marshall atolls.
In the Society Archipelago, Ellis[3] states, that the reefs generally
lie at the distance of from one to one and a half miles, and,
occasionally, even at more than three miles, from the shore. The
central mountains are generally bordered by a fringe of flat, and often
marshy, alluvial
land, from one to four miles in width. This fringe consists of
coral-sand and detritus thrown up from the lagoon-channel, and of soil
washed down from the hills; it is an encroachment on the channel,
analogous to that low and inner part of the islets in many atolls which
is formed by the accumulation of matter from the lagoon. At Hogoleu
(Fig. 2, Plate I), in the Caroline Archipelago,[4] the reef on the
south side is no less than twenty miles; on the east side, five; and on
the north side, fourteen miles from the encircled high islands.
[3] Consult, on this and other points, the “Polynesian Researches,” by
the Rev. W. Ellis, an admirable work, full of curious information.
[4] See “Hydrographical Mem.” and the “Atlas of the Voyage of the
_Astrolabe_,” by Captain Dumont D’Urville, p. 428.
The lagoon channels may be compared in every respect with true lagoons.
In some cases they are open, with a level bottom of fine sand; in
others they are choked up with reefs of delicately branched corals,
which have the same general character as those within the Keeling
atoll. These internal reefs either stand separately, or more commonly
skirt the shores of the included high islands. The depth of the
lagoon-channel round the Society Islands varies from two or three to
thirty fathoms; in Cook’s[5] chart of Ulieta, however, there is one
sounding laid down of forty-eight fathoms; at Vanikoro there are
several of fifty-four and one of fifty-six and a half fathoms
(English), a depth which even exceeds by a little that of the interior
of the great Maldiva atolls. Some barrier-reefs have very few islets on
them; whilst others are surmounted by numerous ones; and those round
part of Bolabola (Plate I, Fig. 5) form a single linear strip. The
islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the
windward side. The reef to leeward retaining its usual width, sometimes
lies submerged several fathoms beneath the surface; I have already
mentioned Gambier Island as an instance of this structure. Submerged
reefs, having a less defined outline, dead, and covered with sand, have
been observed (see Appendix) off some parts of Huaheine and Tahiti. The
reef is more frequently breached to leeward than to windward; thus I
find in Krusenstern’s “Memoir on the Pacific,” that there are passages
through the encircling reef on the leeward side of each of the seven
Society Islands, which possess ship-harbours; but that there are
openings to windward through the reef of only three of them. The
breaches in the reef are seldom as deep as the interior lagoon-like
channel; they generally occur in front of the main valleys, a
circumstance which can be accounted for, as will be seen in the fourth
chapter, without much difficulty. The breaches being situated in front
of the valleys, which descend indifferently on all sides, explains
their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,—for in atolls
there is no included land to influence the position of the breaches.
[5] See the chart in vol. i of Hawkesworth’s 4to ed. of “Cook’s First
Voyage.”
It is remarkable, that the lagoon-channels round mountainous islands
have not in every instance been long ago filled up with coral and
sediment; but it is more easily accounted for than appears at first
sight. In cases like that of Hogoleu and the Gambier Islands, where a
few
small peaks rise out of a great lagoon, the conditions scarcely differ
from those of an atoll, and I have already shown, at some length, that
the filling up of a true lagoon must be an extremely slow process.
Where the channel is narrow, the agency, which on unprotected coasts is
most productive of sediment, namely the force of the breakers, is here
entirely excluded, and the reef being breached in the front of the main
valleys, much of the finer mud from the rivers must be transported into
the open sea. As a current is formed by the water thrown over the edge
of atoll-formed reefs, which carries sediment with it through the
deep-water breaches, the same thing probably takes place in
barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up. The low alluvial border, however,
at the foot of the encircled mountains, shows that the work of filling
up is in progress; and at Maura (Plate I, Fig. 6), in the Society
group, it has been almost effected, so that there remains only one
harbour for small craft.
If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many
points already noticed of resemblance, or rather of identity in
structure with atolls, there is a close general agreement in form,
average dimensions, and grouping. Encircling barrier-reefs, like
atolls, are generally elongated, with an irregularly rounded, though
sometimes angular outline. There are atolls of all sizes, from less
than two miles in diameter to sixty miles (excluding Tilla-dou-Matte,
as it consists of a number of almost independent atoll-formed reefs);
and there are encircling barrier-reefs from three miles and a half to
forty-six miles in diameter,—Turtle Island being an instance of the
former, and Hogoleu of the latter. At Tahiti the encircled island is
thirty-six miles in its longest axis, whilst at Maurua it is only a
little more than two miles. It will be shown, in the last chapter in
this volume, that there is the strictest resemblance in the grouping of
atolls and of common islands, and consequently there must be the same
resemblance in the grouping of atolls and of encircling barrier-reefs.
The islands lying within reefs of this class, are of very various
heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and
Manouai only 50. The geological nature of the included land varies: in
most cases it is of ancient volcanic origin, owing apparently to the
fact that islands of this nature are most frequent within all great
seas; some, however, are of madreporitic limestone, and others of
primary formation, of which latter kind New Caledonia offers the best
example. The central land consists either of one island, or of several:
thus, in the Society group, Eimeo stands by itself; while Taha and
Raiatea (Fig. 3, Plate I), both moderately large islands of nearly
equal size, are included in one reef. Within the reef of the Gambier
group there are four large and some smaller islands (Fig. 8, Plate I);
within that of
Hogoleu (Fig. 2, Plate I) nearly a dozen small islands are scattered
over the expanse of one vast lagoon.
[6] The height of Tahiti is given from Captain Beechey; Maurua from
Mr. F. D. Bennett (_Geograph. Journ._ vol. viii, p. 220); Aitutaki
from measurements made on board the _Beagle_; and Manouai or Harvey
Island, from an estimate by the Rev. J. Williams. The two latter
islands, however, are not in some respects well characterised examples
of the encircled class.
After the details now given, it may be asserted that there is not one
point of essential difference between encircling barrier-reefs and
atolls: the latter enclose a simple sheet of water, the former encircle
an expanse with one or more islands rising from it. I was much struck
with this fact, when viewing, from the heights of Tahiti, the distant
island of Eimeo standing within smooth water, and encircled by a ring
of snow-white breakers. Remove the central land, and an annular reef
like that of an atoll in an early stage of its formation is left;
remove it from Bolabola, and there remains a circle of linear
coral-islets, crowned with tall cocoa-nut trees, like one of the many
atolls scattered over the Pacific and Indian Oceans.
The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions. The reef on the west coast of New
Caledonia (Fig. 5, Plate II) is 400 miles in length; and for a length
of many leagues it seldom approaches within eight miles of the shore;
and near the southern end of the island, the space between the reef and
the land is sixteen miles in width. The Australian barrier extends,
with a few interruptions, for nearly a thousand miles; its average
distance from the land is between twenty and thirty miles; and in some
parts from fifty to seventy. The great arm of the sea thus included, is
from ten to twenty-five fathoms deep, with a sandy bottom; but towards
the southern end, where the reef is further from the shore, the depth
gradually increases to forty, and in some parts to more than sixty
fathoms. Flinders[7] has described the surface of this reef as
consisting of a hard white agglomerate of different kinds of coral,
with rough projecting points. The outer edge is the highest part; it is
traversed by narrow gullies, and at rare intervals is breached by
ship-channels. The sea close outside is profoundly deep; but, in front
of the main breaches, soundings can sometimes be obtained. Some low
islets have been formed on the reef.
[7] Flinders’ “Voyage to Terra Australis,” vol. ii, p. 88.
There is one important point in the structure of barrier-reefs which
must here be considered. The accompanying diagrams represent north and
south vertical sections, taken through the highest points of Vanikoro,
Gambier, and Maurua Islands, and through their encircling reefs. The
scale both in the horizontal and vertical direction is the same,
namely, a quarter of an inch to a nautical mile. The height and width
of these islands is known; and I have attempted to represent the form
of the land from the shading of the hills in the large published
charts. It has long been remarked, even from the time of Dampier, that
considerable degree of relation subsists between the inclination of
that part of the land which is beneath water and that above it; hence
the dotted line in the three sections, probably, does not widely differ
in inclination from the actual submarine prolongation of the land. If
we now look at the outer edge of the reef (AA), and bear in mind that
the plummet on the right hand represents a depth of 1,200 feet, we must
conclude that the vertical thickness of these barrier coral-reefs is
very great.
[Illustration: Vertical thickness of Vanikoro, Gambier and Maurua.]
1. VANIKORO, from the “Atlas of the Voyage of the _Astrolabe_,” by D.
D’Urville.
2. GAMBIER ISLAND, from Beechey.
3. MAURUA, from the “Atlas of the Voyage of the _ Coquille_,” by
Duperrey.
The horizontal line is the level of the sea, from which on the right
hand a plummet descends, representing a depth of 200 fathoms, or 1,200
feet. The vertical shading shows the section of the land, and the
horizontal shading that of the encircling barrier-reef: from the
smallness of the scale, the lagoon-channel could not be represented.
AA.—Outer edge of the coral-reefs, where the sea breaks.
BB.—The shore of the encircled islands.
I must observe that if the sections had been taken in any other
direction across these islands, or across other encircled islands,[8]
the result would have been the same. In the succeeding chapter it will
be shown that reef-building polypifers cannot flourish at great
depths,—for instance, it is highly improbable that they could exist at
a quarter of the depth represented by the plummet on the right hand of
the woodcut. Here there is a great _apparent_ difficulty—how were the
basal parts of these barrier-reef formed? It will, perhaps, occur to
some, that the actual reefs formed of coral are not of great thickness,
but that before their first growth, the coasts of these encircled
islands were deeply eaten into, and a broad but shallow submarine ledge
thus left, on the edge of which the coral grew; but if this had been
the case, the shore would have been invariably bounded by lofty cliffs,
and not have sloped down to the lagoon-channel, as it does in many
instances. On this view,[9] moreover, the cause of the reef springing
up at such a great distance from the land, leaving a deep and broad
moat within, remains altogether unexplained. A supposition of the same
nature,
and appearing at first more probable is, that the reefs sprung up from
banks of sediment, which had accumulated round the shore previously to
the growth of the coral; but the extension of a bank to the same
distance round an unbroken coast, and in front of those deep arms of
the sea (as in Raiatea, see Plate II, Fig. 3) which penetrate nearly to
the heart of some encircled islands, is exceedingly improbable. And
why, again, should the reef spring up, in some cases steep on both
sides like a wall, at a distance of two, three or more miles from the
shore, leaving a channel often between two hundred and three hundred
feet deep, and rising from a depth which we have reason to believe is
destructive to the growth of coral? An admission of this nature cannot
possibly be made. The existence, also, of the deep channel, utterly
precludes the idea of the reef having grown outwards, on a foundation
slowly formed on its outside, by the accumulation of sediment and coral
detritus. Nor, again, can it be asserted, that the reef-building corals
will not grow, excepting at a great distance from the land; for, as we
shall soon see, there is a whole class of reefs, which take their name
from growing closely attached (especially where the sea is deep) to the
beach. At New Caledonia (see Plate II, Fig. 5) the reefs which run in
front of the west coast are prolonged in the same line 150 miles beyond
the northern extremity of the island, and this shows that some
explanation, quite different from any of those just suggested, is
required. The continuation of the reefs on each side of the submarine
prolongation of New Caledonia, is an exceedingly interesting fact, if
this part formerly existed as the northern extremity of the island, and
before the attachment of the coral had been worn down by the action of
the sea, or if it originally existed at its present height, with or
without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but
fronting its sides, in the same line with the reefs which front the
shores of the lofty island? We shall hereafter see, that there is one,
and I believe only one, solution of this difficulty.
[8] In the fifth chapter an east and west section across the Island of
Bolabola and its barrier-reefs is given, for the sake of illustrating
another point. The unbroken line in it (woodcut No. 5) is the section
referred to. The scale is .57 of an inch to a mile; it is taken from
the “Atlas of the Voyage of the _Coquille_,” by Duperrey. The depth of
the lagoon-channel is exaggerated.
[9] The Rev. D. Tyerman and Mr. Bennett (“Journal of Voyage and
Travels,” vol. i, p. 215) have briefly suggested this explanation of
the origin of the encircling reefs of the Society Islands.
One other supposition to account for the position of encircling
barrier-reefs remains, but it is almost too preposterous to be
mentioned; namely, that they rest on enormous submarine craters,
surrounding the included islands. When the size, height, and form of
the islands in the Society group are considered, together with the fact
that all are thus encircled, such a notion will be rejected by almost
every one. New Caledonia, moreover, besides its size, is composed of
primitive formations, as are some of the Comoro Islands;[10] and
Aitutaki consists of calcareous rock. We must, therefore, reject these
several explanations, and conclude that the vertical thickness of
barrier-reefs, from their outer edges to the foundation on which they
rest (from AA in the section to the dotted lines) is really great; but
in this, there is no difficulty, for it is not necessary to suppose
that the coral has sprung up from an immense depth, as will be evident
when the theory of the upward growth of coral-reefs, during the slow
subsidence of their foundation, is discussed.
[10] I have been informed that this is the case by Dr. Allan of
Forres, who has visited this group.
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 East 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.
Fringing-reefs, or, as they have been called by some voyagers,
shore-reefs, whether skirting an island or part of a continent, might
at first be thought to differ little, except in generally being of less
breadth, from barrier-reefs. As far as the superficies of the actual
reef is concerned this is the case; but the absence of an interior
deep-water channel, and the close relation in their horizontal
extension with the probable slope beneath the sea of the adjoining
land, present essential points of difference.
The reefs which fringe the island of Mauritius offer a good example of
this class. They extend round its whole circumference, with the
exception of two or three parts,[1] where the coast is almost
precipitous, and where, if as is probable the bottom of the sea has a
similar inclination, the coral would have no foundation on which to
become attached. A similar fact may sometimes be observed even in reefs
of the barrier class, which follow much less closely the outline of the
adjoining land; as, for instance, on the south-east and precipitous
side of Tahiti, where the encircling reef is interrupted. On the
western side of the Mauritius, which was the only part I visited, the
reef generally lies at the distance of about half a mile from the
shore; but in some parts it is distant from one to two, and even three
miles. But even in this last case, as the coast-land is gently inclined
from the foot of the mountains to the sea-beach, and as the soundings
outside the reef indicate an equally gentle slope beneath the water,
there is no reason for supposing that the basis of the reef, formed by
the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef.
Some allowance, however, must be made for the outward extension of the
corals on a foundation of sand and detritus, formed from their own
wear, which would give to the reef a somewhat greater vertical
thickness, than would otherwise be possible.
[1] This fact is stated on the authority of the Officier du Roi, in
his extremely interesting “Voyage à l’Isle de France,” undertaken in
1768. According to Captain Carmichael (Hooker’s _Bot. Misc._ vol. ii,
p. 316) on one part of the coast there is a space for sixteen miles
without a reef.
The outer edge of the reef on the western or leeward side of the island
is tolerably well defined, and is a little higher than any other part.
It chiefly consists of large strongly branched corals, of the genus
Madrepora, which also form a sloping bed some way out to sea: the
kinds of coral growing in this part will be described in the ensuing
chapter. Between the outer margin and the beach, there is a flat space
with a sandy bottom and a few tufts of living coral; in some parts it
is so shallow, that people, by avoiding the deeper holes and gullies,
can wade across it at low water; in other parts it is deeper, seldom
however exceeding ten or twelve feet, so that it offers a safe coasting
channel for boats. On the eastern and windward side of the island,
which is exposed to a heavy surf, the reef was described to me as
having a hard smooth surface, very slightly inclined inwards, just
covered at low-water, and traversed by gullies; it appears to be quite
similar in structure to the reefs of the barrier and atoll classes.
The reef of Mauritius, in front of every river and streamlet, is
breached by a straight passage: at Grand Port, however, there is a
channel like that within a barrier-reef; it extends parallel to the
shore for four miles, and has an average depth of ten or twelve
fathoms; its presence may probably be accounted for by two rivers which
enter at each end of the channel, and bend towards each other. The fact
of reefs of the fringing class being always breached in front of
streams, even of those which are dry during the greater part of the
year, will be explained, when the conditions unfavourable to the growth
of coral are considered. Low coral-islets, like those on barrier-reefs
and atolls, are seldom formed on reefs of this class, owing apparently
in some cases to their narrowness, and in others to the gentle slope of
the reef outside not yielding many fragments to the breakers. On the
windward side, however, of the Mauritius, two or three small islets
have been formed.
It appears, as will be shown in the ensuing chapter, that the action of
the surf is favourable to the vigorous growth of the stronger corals,
and that sand or sediment, if agitated by the waves, is injurious to
them. Hence it is probable that a reef on a shelving shore, like that
of Mauritius, would at first grow up, not attached to the actual beach,
but at some little distance from it; and the corals on the outer margin
would be the most vigorous. A shallow channel would thus be formed
within the reef, and as the breakers are prevented acting on the shores
of the island, and as they do not ordinarily tear up many fragments
from the outside, and as every streamlet has its bed prolonged in a
straight line through the reef, this channel could be filled up only
very slowly with sediment. But a beach of sand and of fragments of the
smaller kinds of coral seems, in the case of Mauritius, to be slowly
encroaching on the shallow channel. On many shelving and sandy coasts,
the breakers tend to form a bar of sand a little way from the beach,
with a slight increase of depth within it; for instance, Captain
Grey[2] states that the west coast of Australia, in latitude 24°, is
fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away
of the
water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone,[3] which has the
same external form and height as a coral-reef, extends nearly parallel
to the coast; within this bar currents, apparently caused by the water
thrown over it during the greater part of each tide, run strongly, and
are wearing away its inner wall. From these facts it can hardly be
doubted, that within most fringing-reefs, especially within those lying
some distance from the land, a return stream must carry away the water
thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even
deepen it under certain circumstances. To this latter belief I am led,
by finding that channels are almost universally present within the
fringing-reefs of those islands which have undergone recent elevatory
movements; and this could hardly have been the case, if the conversion
of the very shallow channel into land had not been counteracted to a
certain extent.
[2] Captain Grey’s “Journal of Two Expeditions,” vol. i, p. 369.
[3] I have described this singular structure in the _Lond. and Edin.
Phil. Mag.,_ October 1841.
A fringing-reef, if elevated in a perfect condition above the level of
the sea, ought to present the singular appearance of a broad dry moat
within a low mound. The author[4] of an interesting pedestrian tour
round the Mauritius, seems to have met with a structure of this kind:
he says “J’observai que là, où la mer étale, indépendamment des rescifs
du large, il y à terre _une espèce d’effoncement_ ou chemin couvert
naturel. On y pourrait mettre du canon,” etc. In another place he adds,
“Avant de passer le Cap, on remarque un gros banc de corail élevé de
plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné,
il regne au pied une longue flaque d’eau, dont on pourrait faire un
bassin pour de petits vaisseaux.” But the margin of the reef, although
the highest and most perfect part, from being most exposed to the surf,
would generally during a slow rise of the land be either partially or
entirely worn down to that level, at which corals could renew their
growth on its upper edge. On some parts of the coast-land of Mauritius
there are little hillocks of coral-rock, which are either the last
remnants of a continuous reef, or of low islets formed on it. I
observed that two such hillocks between Tamarin Bay and the Great Black
River; they were nearly twenty feet high, about two hundred yards from
the present beach, and about thirty feet above its level. They rose
abruptly from a smooth surface, strewed with worn fragments of coral.
They consisted in their lower part of hard calcareous sandstone, and in
their upper of great blocks of several species of Astræa and Madrepora,
loosely aggregated; they were divided into irregular beds, dipping
seaward, in one hillock at an angle of 8°, and in the other at 18°. I
suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been
much more modified by the wearing action of the sea, than those of
Mauritius.
[4] “Voyage à l’Isle de France, par un Officier du Roi,” part i, pp.
192, 200.
Many islands[5] are fringed by reefs quite similar to those of
Mauritius; but on coasts where the sea deepens very suddenly the reefs
are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation,
which, as we have seen, does not exist in reefs of the barrier class.
The fringing-reefs on steep coasts are frequently not more than from
fifty to one hundred yards in width; they have a nearly smooth, hard
surface, scarcely uncovered at low water, and without any interior
shoal channel, like that within those fringing-reefs, which lie at a
greater distance from the land. The fragments torn up during gales from
the outer margin are thrown over the reef on the shores of the island.
I may give as instances, Wateeo, where the reef is described by Cook as
being a hundred yards wide; and Mauti and Elizabeth[6] Islands, where
it is only fifty yards in width: the sea round these islands is very
deep.
[5] I may give Cuba, as another instance; Mr. Taylor (_Loudon’s Mag.
of Nat. Hist.,_ vol. ix, p. 449) has described a reef several miles in
length between Gibara and Vjaro, which extends parallel to the shore
at the distance of between half and the third part of a mile, and
encloses a space of shallow water, with a sandy bottom and tufts of
coral. Outside the edge of the reef, which is formed of great
branching corals, the depth is six and seven fathoms. This coast has
been upheaved at no very distant geological period.
[6] Mauti is described by Lord Byron in the voyage of H.M.S. _
Blonde_, and Elizabeth Island by Captain Beechey.
Fringing-reefs, like barrier-reefs, both surround islands, and front
the shores of continents. In the charts of the eastern coast of Africa,
by Captain Owen, many extensive fringing-reefs are laid down; thus, for
a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a
reef fringes the shore at an average distance of rather more than one
mile, and therefore at a greater distance than is usual in reefs of
this class; but as the coast-land is not lofty, and as the bottom
shoals very gradually (the depth being only from eight to fourteen
fathoms at a mile and a half outside the reef), its extension thus far
from the land offers no difficulty. The external margin of this reef is
described, as formed of projecting points, within which there is a
space, from six to twelve feet deep, with patches of living coral on
it. At Mukdeesha (lat. 2° 1′ N.) “the port is formed,” it is said,[7]
“by a long reef extending eastward, four or five miles, within which
there is a narrow channel, with ten to twelve feet of water at low
spring-tides;” it lies at the distance of a quarter of a mile from the
shore. Again, in the plan of Mombas (lat. 4° S.), a reef extends for
thirty-six miles, at the distance of from half a mile to one mile and a
quarter from the shore; within it, there is a channel navigable “for
canoes and small craft,” between six and fifteen feet deep: outside the
reef the depth is about thirty fathoms at the distance of nearly half a
mile. Part of this reef is very symmetrical, and has a uniform breadth
of two hundred yards.
[7] Owen’s “Africa,” vol. i, p. 357, from which work the foregoing
facts are likewise taken.
The coast of Brazil is in many parts fringed by reefs. Of these, some
are not of coral formation; for instance, those near Bahia and in front
of Pernambuco; but a few miles south of this latter city, the reef
follows[8] so closely every turn of the shore, that I can hardly doubt
it is of coral; it runs at the distance of three-quarters of a mile
from the land, and within it the depth is from ten to fifteen feet. I
was assured by an intelligent pilot that at Ports Frances and Maceio,
the outer part of the reef consists of living coral, and the inner of a
white stone, full of large irregular cavities, communicating with the
sea. The bottom of the sea off the coast of Brazil shoals gradually to
between thirty and forty fathoms, at the distance of between nine and
ten leagues from the land.
[8] See Baron Roussin’s “Pilote du Brésil,” and accompanying
hydrographical memoir.
From the description now given, we must conclude that the dimensions
and structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows
from this, that where the sea is very shallow, as in the Persian Gulf
and in parts of the East Indian Archipelago, the reefs lose their
fringing character, and appear as separate and irregularly scattered
patches, often of considerable area. From the more vigorous growth of
the coral on the outside, and from the conditions being less favourable
in several respects within, such reefs are generally higher and more
perfect in their marginal than in their central parts; hence these
reefs sometimes assume (and this circumstance ought not to be
overlooked) the appearance of atolls; but they differ from atolls in
their central expanse being much less deep, in their form being less
defined, and in being based on a shallow foundation. But when in a deep
sea reefs fringe banks of sediment, which have accumulated beneath the
surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling
barrier-reefs, and on the other from atolls. In the West Indies there
are reefs, which I should probably have arranged under both these
classes, had not the existence of large and level banks, lying a little
beneath the surface, ready to serve as the basis for the attachment of
coral, been occasionally brought into view by the entire or partial
absence of reefs on them, and had not the formation of such banks,
through the accumulation of sediment now in progress, been sufficiently
evident. Fringing-reefs sometimes coat, and thus protect the
foundations of islands, which have been worn down by the surf to the
level of the sea. According to Ehrenberg, this has been extensively the
case with the islands in the Red Sea, which formerly ranged parallel to
the shores of the mainland, with deep water within them: hence the
reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as
I believe, in the Red Sea some true barrier-reefs. The reefs of this
sea and of the West Indies will be described in the Appendix. In some
cases, fringing-reefs appear to be considerably modified in outline by
the course of the prevailing currents. Dr. J. Allan informs me that on
the east coast of Madagascar almost every headland and low point of
sand has a coral-reef extending from it in a S.W. and N.E. line,
parallel
to the currents on that shore. I should think the influence of the
currents chiefly consisted in causing an extension, in a certain
direction, of a proper foundation for the attachment of the coral.
Round many intertropical islands, for instance the Abrolhos on the
coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by
Mr. Cuming, round the Philippines, the bottom of the sea is entirely
coated by irregular masses of coral, which although often of large
size, do not reach the surface and form proper reefs. This must be
owing, either to insufficient growth, or to the absence of those kinds
of corals which can withstand the breaking of the waves.
The three classes, atoll-formed, barrier, and fringing-reefs, together
with the modifications just described of the latter, include all the
most remarkable coral formations anywhere existing. At the commencement
of the last chapter in the volume, where I detail the principles on
which the map (Plate III) is coloured, the exceptional cases will be
enumerated.
Chapter IV ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS
In this chapter I will give all the facts which I have collected,
relating to the distribution of coral-reefs,—to the conditions
favourable to their increase,—to the rate of their growth,—and to the
depth at which they are formed.
These subjects have an important bearing on the theory of the origin of
the different classes of coral-reefs.
_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE
With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32° 15′ N., is the
point furthest removed from the equator, in which they appear to exist;
and it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27° N., have reefs on their shores, and
there is an atoll in 28° 30′, situated N.W. of the Sandwich
Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In
the southern hemisphere coral-reefs do not extend so far from the
equatorial sea. In the Southern Pacific there are only a few reefs
beyond the line of the tropics, but Houtmans Abrolhos, on the western
shores of Australia in latitude 29° S., are of coral formation.
The proximity of volcanic land, owing to the lime generally evolved
from it, has been thought to be favourable to the increase of
coral-reefs.
There is, however, not much foundation for this view; for nowhere are
coral-reefs more extensive than on the shores of New Caledonia, and of
north-eastern Australia, which consist of primary formations; and in
the largest groups of atolls, namely the Maldiva, Chagos, Marshall,
Gilbert, and Low Archipelagoes, there is no volcanic or other kind of
rock, excepting that formed of coral.
The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the _Beagle_ and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none[1] north of the equator;
Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that
although he had seen corals living in the Bay of Panama, yet he had
never observed any reefs formed by them. I at first attributed this
absence of reefs on the coasts of Peru and of the Galapagos Islands,[2]
to the coldness of the currents from the south, but the Gulf of Panama
is one of the hottest pelagic districts in the world.[3] In the central
parts of the Pacific there are islands entirely free from reefs; in
some few of these cases I have thought that this was owing to recent
volcanic action; but the existence of reefs round the greater part of
Hawaii, one of the Sandwich Islands, shows that recent volcanic action
does not necessarily prevent their growth.
[1] I have been informed that this is the case, by Lieutenant Ryder,
R.N., and others who have had ample opportunities for observation.
[2] The mean temperature of the surface sea from observations made by
the direction of Captain Fitzroy on the shores of the Galapagos
Islands, between the 16th of September and the 20th of October, 1835,
was 68° Fahr. The lowest temperature observed was 58.5° at the
south-west end of Albemarle Island; and on the west coast of this
island, it was several times 62° and 63°. The mean temperature of the
sea in the Low Archipelago of atolls, and near Tahiti, from similar
observations made on board the _Beagle_, was (although further from
the equator) 77.5°, the lowest any day being 76.5°. Therefore we have
here a difference of 9.5° in mean temperature, and 18° in extremes; a
difference doubtless quite sufficient to affect the distribution of
organic beings in the two areas.
[3] Humboldt’s “Personal Narrative,” vol. vii, p. 434.
In the last chapter I stated that the bottom of the sea round some
islands is thickly coated with living corals, which nevertheless do not
form reefs, either from insufficient growth, or from the species not
being adapted to contend with the breaking waves.
I have been assured by several people, that there are no coral-reefs on
the west coast of Africa,[4] or round the islands in the Gulf of
Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks,
which line great part of it. But the islands of St. Helena, Ascension,
the Cape Verdes, St. Paul’s, and Fernando Noronha, are, also, entirely
without reefs, although they lie far out at sea, are composed of the
same ancient volcanic rocks, and have the same general form, with those
islands in the Pacific, the shores of which are surrounded by gigantic
walls of coral-rock. With the exception of Bermuda, there is not a
single coral-reef in the central expanse of the Atlantic Ocean. It
will, perhaps, be suggested that the quantity of carbonate of lime in
different parts of the sea, may regulate the presence of reefs. But
this cannot be the case, for at Ascension, the waves charged to excess
precipitate a thick layer of calcareous matter on the tidal rocks; and
at St. Jago, in the Cape Verdes, carbonate of lime not only is abundant
on the shores, but it forms the chief part of some upraised
post-tertiary strata. The apparently capricious distribution,
therefore, of coral-reefs, cannot be explained by any of these obvious
causes; but as the study of the terrestrial and better known half of
the world must convince every one that no station capable of supporting
life is lost,—nay more, that there is a struggle for each station,
between the different orders of nature,—we may conclude that in those
parts of the intertropical sea, in which there are no coral-reefs,
there are other organic bodies supplying the place of the reef-building
polypifers. It has been shown in the chapter on Keeling atoll that
there are some species of large fish, and the whole tribe of Holothuriæ
which prey on the tenderer parts of the corals. On the other hand, the
polypifers in their turn must prey on some other organic beings; the
decrease of which from any cause would cause a proportionate
destruction of the living coral. The relations, therefore, which
determine the formation of reefs on any shore, by the vigorous growth
of the efficient kinds of coral, must be very complex, and with our
imperfect knowledge quite inexplicable. From these considerations, we
may infer that changes in the condition of the sea, not obvious to our
senses, might destroy all the coral-reefs in one area, and cause them
to appear in another: thus, the Pacific or Indian Ocean might become as
barren of coral-reefs as the Atlantic now is, without our being able to
assign any adequate cause for such a change.
[4] It might be concluded, from a paper by Captain Owen (_Geograph.
Journ._, vol. ii, p. 89), that the reefs off Cape St. Anne and the
Sherboro’ Islands were of coral, although the author states that they
are not purely coralline. But I have been assured by Lieutenant
Holland, R.N., that these reefs are not of coral, or at least that
they do not at all resemble those in the West Indies.
It has been a question with some naturalists, which part of a reef is
most favourable to the growth of coral. The great mounds of living
Porites and of Millepora round Keeling atoll occur exclusively on the
extreme verge of the reef, which is washed by a constant succession of
breakers; and living coral nowhere else forms solid masses. At the
Marshall islands the larger kinds of coral (chiefly species of Astræa,
a genus closely allied to Porites) “which form rocks measuring several
fathoms in thickness,” prefer, according to Chamisso,[5] the most
violent surf. I have stated that the outer margin of the Maldiva atolls
consists of living corals (some of which, if not all, are of the same
species with those at Keeling atoll), and here the surf is so
tremendous, that even large ships have been thrown, by a single heave
of the sea, high and dry on the reef, all on board thus escaping with
their lives.
Ehrenberg[6] remarks, that in the Red Sea the strongest corals live on
the outer reefs, and appear to love the surf; he adds, that the more
branched kinds abound a little way within, but that even these in still
more protected places, become smaller. Many other facts having a
similar tendency might be adduced.[7] It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand,
much less flourish in, the breakers of an open sea:[8] they affirm that
the saxigenous lithophytes flourish only where the water is tranquil,
and the heat intense. This statement has passed from one geological
work to another; nevertheless, the protection of the whole reef
undoubtedly is due to those kinds of coral, which cannot exist in the
situations thought by these naturalists to be most favourable to them.
For should the outer and living margin perish, of any one of the many
low coral-islands, round which a line of great breakers is incessantly
foaming, the whole, it is scarcely possible to doubt, would be washed
away and destroyed, in less than half a century. But the vital energies
of the corals conquer the mechanical power of the waves; and the large
fragments of reef torn up by every storm, are replaced by the slow but
steady growth of the innumerable polypifers, which form the living zone
on its outer edge.
[5] Kotzebue’s “First Voyage” (Eng. Trans.), vol. iii, pp. 142, 143,
331.
[6] Ehrenberg, “Über die Natur und Bildung der Corallen Bänke im
rothen Meere,” p. 49.
[7] In the West Indies, as I am informed by Captain Bird Allen, R.N.,
it is the common belief of those, who are best acquainted with the
reefs, that the coral flourishes most, where freely exposed to the
swell of the open sea.
[8] “Annales des Sciences Naturelles,” tome vi, pp. 276, 278.—“Là où
les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
qu’elles détruiraient leurs fragiles édifices,” etc.
From these facts, it is certain, that the strongest and most massive
corals flourish, where most exposed. The less perfect state of the reef
of most atolls on the leeward and less exposed side, compared with its
state to windward; and the analogous case of the greater number of
breaches on the near sides of those atolls in the Maldiva Archipelago,
which afford some protection to each other, are obviously explained by
this circumstance. If the question had been, under what conditions the
greater number of species of coral, not regarding their bulk and
strength, were developed, I should answer,—probably in the situations
described by MM. Quoy and Gaimard, where the water is tranquil and the
heat intense. The total number of species of coral in the
circumtropical seas must be very great: in the Red Sea alone, 120
kinds, according to Ehrenberg,[9] have been observed.
[9] Ehrenberg, “Über die Natur,” etc., p. 46.
The same author has observed that the recoil of the sea from a steep
shore is injurious to the growth of coral, although waves breaking over
a bank are not so. Ehrenberg also states, that where there is much
sediment, placed so as to be liable to be moved by the waves there is
little or no coral; and a collection of living specimens placed by him
on a sandy shore died in the course of a few days.[10] An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on
a sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in
sounding off Keeling atoll, and (as will hereafter be shown) off
Mauritius, the arming of the lead invariably came up clean, where the
coral was growing vigorously. This same circumstance has probably given
rise to a strange belief, which, according to Captain Owen,[11] is
general amongst the inhabitants of the Maldiva atolls, namely that
corals have roots, and therefore that if merely broken down to the
surface, they grow up again; but, if rooted out, they are permanently
destroyed. By this means the inhabitants keep their harbours clear; and
thus the French Governor of St. Mary’s in Madagascar, “cleared out and
made a beautiful little port at that place.” For it is probable that
sand would accumulate in the hollows formed by tearing out the corals,
but not on the broken and projecting stumps, and therefore, in the
former case, the fresh growth of the coral might be thus prevented.
[10] _Ibid_., p. 49.
[11] Captain Owen on the Geography of the Maldiva Islands, _Geograph.
Journal_, vol. ii, p. 88.
In the last chapter I remarked that fringing-reefs are almost
universally breached, where streams enter the sea.[12] Most authors
have attributed this fact to the injurious effects of the fresh water,
even where it enters the sea only in small quantity, and during a part
of the year. No doubt brackish water would prevent or retard the growth
of coral; but I believe that the mud and sand which is deposited, even
by rivulets when flooded, is a much more efficient check. The reef on
each side of the channel leading into Port Louis at Mauritius, ends
abruptly in a wall, at the foot of which I sounded and found a bed of
thick mud. This steepness of the sides appears to be a general
character in such breaches. Cook,[13] speaking of one at Raiatea, says,
“like all the rest, it is very steep on both sides.” Now, if it were
the fresh water mingling with the salt which prevented the growth of
coral, the reef certainly would not terminate abruptly, but as the
polypifers nearest the impure stream would grow less vigorously than
those farther off, so would the reef gradually thin away. On the other
hand, the sediment brought down from the land would only prevent the
growth of the coral in the line of its deposition, but would not check
it on the side, so that the reefs might increase till they overhung the
bed of the channel. The breaches are much fewer in number, and front
only the larger valleys in reefs of the encircling barrier class. They
probably are kept open in the same manner as those into the lagoon of
an atoll, namely, by the
force of the currents and the drifting outwards of fine sediment. Their
position in front of valleys, although often separated from the land by
deep water lagoon-channels, which it might be thought would entirely
remove the injurious effects both of the fresh water and the sediment,
will receive a simple explanation when we discuss the origin of
barrier-reefs.
[12] Lieutenant Wellstead and others have remarked that this is the
case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i, p. 142)
says that there are pear-shaped harbours in the upraised coral-coast,
into which periodical streams enter. From this circumstance, I
presume, we must infer that before the upheaval of the strata now
forming the coast-land, fresh water and sediment entered the sea at
these points; and the coral being thus prevented growing, the
pear-shaped harbours were produced.
[13] Cook’s “First Voyage,” vol. ii, p. 271 (Hawkesworth’s edit.)
In the vegetable kingdom every different station has its peculiar group
of plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the
lagoon of an atoll and those on its outer margin. The corals, also, on
the margin of Keeling Island occurred in zones; thus the _Porites_ and
_Millepora complanata_ grow to a large size only where they are washed
by a heavy sea, and are killed by a short exposure to the air; whereas,
three species of Nullipora also live amidst the breakers, but are able
to survive uncovered for a part of each tide; at greater depths, a
strong Madrepora and _Millepora alcicornis_ are the commonest kinds,
the former appearing to be confined to this part, beneath the zone of
massive corals, minute encrusting corallines and other organic bodies
live. If we compare the external margin of the reef at Keeling atoll
with that on the leeward side of Mauritius, which are very differently
circumstanced, we shall find a corresponding difference in the
appearance of the corals. At the latter place, the genus Madrepora is
preponderant over every other kind, and beneath the zone of massive
corals there are large beds of Seriatopora. There is also a marked
difference, according to Captain Moresby,[14]between the great
branching corals of the Red Sea, and those on the reefs of the Maldiva
atolls.
[14] Captain Moresby on the Northern Maldiva atolls, _Geograph.
Journal_, vol. v, p. 401.
These facts, which in themselves are deserving of notice, bear,
perhaps, not very remotely, on a remarkable circumstance which has been
pointed out to me by Captain Moresby, namely, that with very few
exceptions, none of the coral-knolls within the lagoons of Peros
Banhos, Diego Garcia, and the Great Chagos Bank (all situated in the
Chagos group), rise to the surface of the water; whereas all those,
with equally few exceptions, within Solomon and Egmont atolls in the
same group, and likewise within the large southern Maldiva atolls,
reach the surface. I make these statements, after having examined the
charts of each atoll. In the lagoon of Peros Banhos, which is nearly
twenty miles across, there is only one single reef which rises to the
surface; in Diego Garcia there are seven, but several of these lie
close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand,
in the lagoons of some of the great southern Maldiva atolls, although
thickly studded with reefs, every one without exception rises to the
surface; and on an average there are less than two submerged reefs in
each atoll; in the northern atolls, however, the submerged lagoon-reefs
are not quite so rare. The submerged reefs in the Chagos atolls
generally have from one to seven fathoms water on them, but some have
from seven to ten. Most of them are small with
very steep sides;[15] at Peros Banhos they rise from a depth of about
thirty fathoms, and some of them in the Great Chagos Bank from above
forty fathoms; they are covered, Captain Moresby informs me, with
living and healthy coral, two and three feet high, consisting of
several species. Why then have not these lagoon-reefs reached the
surface, like the innumerable ones in the atolls above named? If we
attempt to assign any difference in their external conditions, as the
cause of this diversity, we are at once baffled. The lagoon of Diego
Garcia is not deep, and is almost wholly surrounded by its reef; Peros
Banhos is very deep, much larger, with many wide passages communicating
with the open sea. On the other hand, of those atolls, in which all or
nearly all the lagoon-reefs have reached the surface, some are small,
others large, some shallow, others deep, some well-enclosed, and others
open.
[15] Some of these statements were not communicated to me verbally by
Captain Moresby, but are taken from the MS. account before alluded to,
of the Chagos Group.
Captain Moresby informs me that he has seen a French chart of Diego
Garcia made eighty years before his survey, and apparently very
accurate; and from it he infers, that during this interval there has
not been the smallest change in the depth on any of the knolls within
the lagoon. It is also known that during the last fifty-one years, the
eastern channel into the lagoon has neither become narrower, nor
decreased in depth; and as there are numerous small knolls of living
coral within it, some change might have been anticipated. Moreover, as
the whole reef round the lagoon of this atoll has been converted into
land—an unparalleled case, I believe, in an atoll of such large
size,—and as the strip of land is for considerable spaces more than
half a mile wide—also a very unusual circumstance,—we have the best
possible evidence, that Diego Garcia has remained at its present level
for a very long period. With this fact, and with the knowledge that no
sensible change has taken place during eighty years in the
coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance
of being older than Diego Garcia and Peros Banhos, and which are placed
under the same external conditions with them, one is led to conclude
that these submerged reefs, although covered with luxuriant coral, have
no tendency to grow upwards, and that they would remain at their
present levels for an almost indefinite period.
From the number of these knolls, from their position, size, and form,
many of them being only one or two hundred yards across, with a rounded
outline, and precipitous sides,—it is indisputable that they have been
formed by the growth of coral; and this makes the case much more
remarkable. In Peros Banhos and in the Great Chagos Bank, some of these
almost columnar masses are 200 feet high, and their summits lie only
from two to eight fathoms beneath the surface; therefore, a small
proportional amount more of growth would cause them to attain the
surface, like those numerous knolls, which rise from an equally great
depth within the Maldiva atolls. We can hardly suppose that time has
been wanting for the upward growth of
the coral, whilst in Diego Garcia, the broad annular strip of land,
formed by the continued accumulation of detritus, shows how long this
atoll has remained at its present level. We must look to some other
cause than the rate of growth; and I suspect it will be found in the
reefs being formed of different species of corals, adapted to live at
different depths. The Great Chagos Bank is situated in the centre of
the Chagos Group, and the Pitt and Speaker Banks at its two extreme
points. These banks resemble atolls, except in their external rim being
about eight fathoms submerged, and in being formed of dead rock, with
very little living coral on it: a portion nine miles long of the
annular reef of Peros Banhos atoll is in the same condition. These
facts, as will hereafter be shown, render it very probable that the
whole group at some former period subsided seven or eight fathoms; and
that the coral perished on the outer margin of those atolls which are
now submerged, but that it continued alive, and grew up to the surface
on those which are now perfect. If these atolls did subside, and if
from the suddenness of the movement or from any other cause, those
corals which are better adapted to live at a certain depth than at the
surface, once got possession of the knolls, supplanting the former
occupants, they would exert little or no tendency to grow upwards. To
illustrate this, I may observe, that if the corals of the upper zone on
the outer edge of Keeling atoll were to perish, it is improbable that
those of the lower zone would grow to the surface, and thus become
exposed to conditions for which they do not appear to be adapted. The
conjecture, that the corals on the submerged knolls within the Chagos
atolls have analogous habits with those of the lower zone outside
Keeling atoll, receives some support from a remark by Captain Moresby,
namely, that they have a different appearance from those on the reefs
in the Maldiva atolls, which, as we have seen, all rise to the surface:
he compares the kind of difference to that of the vegetation under
different climates. I have entered at considerable length into this
case, although unable to throw much light on it, in order to show that
an equal tendency to upward growth ought not to be attributed to all
coral-reefs,—to those situated at different depths,—to those forming
the ring of an atoll or those on the knolls within a lagoon,—to those
in one area and those in another. The inference, therefore, that one
reef could not grow up to the surface within a given time, because
another, not known to be covered with the same species of corals, and
not known to be placed under conditions exactly the same, has not
within the same time reached the surface, is unsound.
_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS
The remark made at the close of the last section, naturally leads to
this division of our subject, which has not, I think, hitherto been
considered under a right point of view. Ehrenberg[16] has stated, that
in
the Red Sea, the corals only coat other rocks in a layer from one to
two feet in thickness, or at most to a fathom and a half; and he
disbelieves that, in any case, they form, by their own proper growth,
great masses, stratum over stratum. A nearly similar observation has
been made by MM. Quoy and Gaimard,[17] with respect to the thickness of
some upraised beds of coral, which they examined at Timor and some
other places. Ehrenberg[18] saw certain large massive corals in the Red
Sea, which he imagines to be of such vast antiquity, that they might
have been beheld by Pharaoh; and according to Mr. Lyell[19] there are
certain corals at Bermuda, which are known by tradition, to have been
living for centuries. To show how slowly coral-reefs grow upwards,
Captain Beechey[20] has adduced the case of the Dolphin Reef off
Tahiti, which has remained at the same depth beneath the surface,
namely about two fathoms and a half, for a period of sixty-seven years.
There are reefs in the Red Sea, which certainly do not appear[21] to
have increased in dimensions during the last half-century, and from the
comparison of old charts with recent surveys, probably not during the
last two hundred years. These, and other similar facts, have so
strongly impressed many with the belief of the extreme slowness of the
growth of corals, that they have even doubted the possibility of
islands in the great oceans having been formed by their agency. Others,
again, who have not been overwhelmed by this difficulty, have admitted
that it would require thousands, and tens of thousands of years, to
form a mass, even of inconsiderable thickness; but the subject has not,
I believe, been viewed in the proper light.
[16] Ehrenberg, as before cited, pp. 39, 46, and 50.
[17] “Annales des Sciences Nat.” tom. vi, p. 28.
[18] Ehrenberg, _ut sup._, p. 42.
[19] Lyell’s “Principles of Geology,” book iii, ch. xviii.
[20] Beechey’s “Voyage to the Pacific,” ch. viii.
[21] Ehrenberg, _ut sup._, p. 43.
That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are,
as already described, small steep-sided knolls covered with living
coral. There are similar knolls in the southern Maldiva atolls, some of
which, as Captain Moresby assures me, are less than a hundred yards in
diameter, and rise to the surface from a depth of between two hundred
and fifty and three hundred feet. Considering their number, form, and
position, it would be preposterous to suppose that they are based on
pinnacles of any rock, not of coral formation; or that sediment could
have been heaped up into such small and steep isolated cones. As no
kind of living coral grows above the height of a few feet, we are
compelled to suppose that these knolls have been formed by the
successive growth and death of many individuals,—first one being broken
off or killed by some accident, and then another, and one set of
species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The
spaces between the corals would become filled up with fragments and
sand, and such matter would probably soon be consolidated, for we learn
from
Lieutenant Nelson,[22] that at Bermuda a process of this kind takes
place beneath water, without the aid of evaporation. In reefs, also, of
the barrier class, we may feel sure, as I have shown, that masses of
great thickness have been formed by the growth of the coral; in the
case of Vanikoro, judging only from the depth of the moat between the
land and the reef, the wall of coral-rock must be at least three
hundred feet in vertical thickness.
[22] “Geological Transactions,” vol. v, p. 113.
It is unfortunate that the upraised coral-islands in the Pacific have
not been examined by a geologist. The cliffs of Elizabeth Island, in
the Low Archipelago, are eighty feet high, and appear, from Captain
Beechey’s description, to consist of a homogeneous coral-rock. From the
isolated position of this island, we may safely infer that it is an
upraised atoll, and therefore that it has been formed by masses of
coral, grown together. Savage Island seems, from the description of the
younger Forster,[23] to have a similar structure, and its shores are
about forty feet high: some of the Cook Islands also appear[24] to be
similarly composed. Captain Belcher, R.N., in a letter which Captain
Beaufort showed me at the admiralty, speaking of Bow atoll, says, “I
have succeeded in boring forty-five feet through coral-sand, when the
auger became jammed by the falling in of the surrounding _ creamy_
matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth
of twenty-six feet, when his auger also broke: he has had the kindness
to give me the matter brought up; it is perfectly white, and like
finely triturated coral-rock.
[23] Forster’s “Voyage round the World with Cook,” vol. ii, pp. 163,
167.
[24] Williams’s “Narrative of Missionary Enterprise,” p. 30.
In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just
within the outer margin, the great mounds of Porites and of Millepora,
with their summits lately killed, and their sides subsequently
thickened by the growth of the coral: a layer, also, of Nullipora had
already coated the dead surface. As the external slope of the reef is
the same round the whole of this atoll, and round many other atolls,
the angle of inclination must result from an adaption between the
growing powers of the coral, and the force of the breakers, and their
action on the loose sediment. The reef, therefore, could not increase
outwards, without a nearly equal addition to every part of the slope,
so that the original inclination might be preserved, and this would
require a large amount of sediment, all derived from the wear of corals
and shells, to be added to the lower part. Moreover, at Keeling atoll,
and probably in many other cases, the different kinds of corals would
have to encroach on each other; thus the Nulliporæ cannot increase
outwards without encroaching on the Porites and _ Millepora
complanata,_ as is now taking place; nor these latter without
encroaching on the strongly branched Madreporet, the _ Millepora
alcicornis,_ and some Astræas; nor these again without a foundation
being formed for them within the requisite depth, by the accumulation
of sediment. How slow, then, must be the ordinary lateral or outward
growth of such reefs. But off
Christmas atoll, where the sea is much more shallow than is usual, we
have good reason to believe that, within a period not very remote, the
reef has increased considerably in width. The land has the
extraordinary breadth of three miles; it consists of parallel ridges of
shells and broken corals, which furnish “an incontestable proof,” as
observed by Cook,[25] “that the island has been produced by accessions
from the sea, and is in a state of increase.” The land is fronted by a
coral-reef, and from the manner in which islets are known to be formed,
we may feel confident that the reef was not three miles wide, when the
first, or most backward ridge, was thrown up; and, therefore, we must
conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the
living margin, within a period during which ridges of shells and
corals, lying on the bare surface, have not decayed. There can be
little doubt, from the account given by Captain Beechey, that Matilda
atoll, in the Low Archipelago, has been converted in the space of
thirty-four years, from being, as described by the crew of a wrecked
whaling vessel, a “reef of rocks” into a lagoon-island, fourteen miles
in length, with “one of its sides covered nearly the whole way with
high trees.”[26] The islets, also, on Keeling atoll, it has been shown,
have increased in length, and since the construction of an old chart,
several of them have become united into one long islet; but in this
case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets,
has increased as well as the islets themselves.
[25] Cook’s “Third Voyage,” book III, ch. x.
[26] Beechey’s “Voyage to the Pacific,” ch. vii and viii.
After these considerations, I attach little importance, as indicating
the ordinary and still less the possible rate of _ outward_ growth of
coral-reefs, to the fact that certain reefs in the Red Sea have not
increased during a long interval of time; or to other such cases, as
that of Ouluthy atoll in the Caroline group, where every islet,
described a thousand years before by Cantova was found in the same
state by Lutké,[27]—without it could be shown that, in these cases, the
conditions were favourable to the vigorous and unopposed growth of the
corals living in the different zones of depth, and that a proper basis
for the extent of the reef was present. The former conditions must
depend on many contingencies, and in the deep oceans where coral
formations most abound, a basis within the requisite depth can rarely
be present.
[27] F. Lutké’s “Voyage autour du Monde.” In the group Elato, however,
it appears that what is now the islet Falipi, is called in Cantova’s
Chart, the Banc de Falipi. It is not stated whether this has been
caused by the growth of coral, or by the accumulation of sand.
Nor do I attach any importance to the fact of certain submerged reefs,
as those off Tahiti, or those within Diego Garcia not now being nearer
the surface than they were many years ago, as an indication of the rate
under favourable circumstances of the _upward_ growth of reefs; after
it has been shown, that all the reefs have grown to the surface in some
of the Chagos atolls, but that in neighbouring atolls which appear to
be of equal antiquity and to be exposed to the same external
conditions, every reef remains submerged; for we are almost driven to
attribute this to a difference, not in the rate of growth, but in the
habits of the corals in the two cases.
In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a str
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/4022