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Phyllastraea tubitex; a, animal, partly expanded.

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CORAL bhSiAN DS.

BY

JAMES D. DANA, aes

PROFESSOR OF GEOLOGY AND MINERALOGY IN YALE COLLEGE; AUTHOR OF REPORTS IN
CONNECTION WITH THE WILKES UNITED STATES EXPLORING EXPEDITION,
ON GEOLOGY, ZOOPHYTES, AND CRUSTACEA; OF A SYSTEM OF
MINERALOGY ; MANUAL OF GEOLOGY, ETC.

Third Loition,
WITH VARIOUS EMENDATIONS, LARGE ADDITIONS, THREE NEW
MAPS, AND FOUR NEW COLORED PLATES.

“ We wandered where the dreamy palm
Murmured above the sleeping wave ;
And, through the waters clear and calm,
Looked down into the coral cave.”

J. C. P., U.S. N. Expl. Expd.

NEW. YORK:
DODD, MEAD, AND COMPANY,

753 AND 755 BROADWAY.

Entered according to Act of Congress, in the year 1872, by
JAMES D. DANA,
in the Office of the Librarian of Congress, at Washington.

Copyright, 1890,
By Dopp, MEAD, AND COMPANY.

Gniversity Yress :
JouNn WILSON AND SON, CAMBRIDGE.

PREFACE TO THE THIRD EDITION.

HIS third edition of the “Corals and Coral Islands”
contains a full discussion of the views and arguments
which have recently been brought forward in opposition to
the theory of coral reefs proposed by Darwin and explained
in the following pages. Besides this, changes and additions
have been made in all parts of the work in order to bring
it up to date of publication. Moreover, four new maps have
been introduced: one, of the Central Pacific ; the second, of
the large coral-reef region of the Louisiade Archipelago, in
the southwest Pacific; the third, a new map of the Florida
and Bahama coral-reef banks, from the charts of the United
States Hydrographic Department ; and the fourth, a copy of
part of the Hawaiian Government map of the vicinity of
Honolulu, showing the coral reefs off the shores, and the
positions of the many artesian borings on this border of Oahu
that are throwing light on the thickness of the shore reef.
The work has its value enhanced also by four new colored
plates, one representing Actinias of the Pacific, and the others
living corals, selected from the author’s Atlas of his Explor-

ing Expedition Report on Zodphytes.

JAMES D. DANA.

New Haven, Cr.,
February 12, 1890.

eee ivak
Be

PREFACE TO THE SECOND EDITION.

In the preparation of this work fora second edition, a few
emendations have been made, and new facts introduced from
recent publications on the subject. Among the latter, an
account is given of the arrangements by MM. Le Clere and De
Benazé, on Tahiti, for marking, in the future, the rate of
growth of the Dolphin Shoal or reef.

The Preface to the first edition alludes to some points
in which the author differs from Mr. Darwin. The reader
will find some additional remarks on these differences in the
American Journal of Science for October, 1874, called out
by the discussion of the subject in the new edition of Mr,

Darwin’s “ Coral Reefs.”

NEw HAVEN, Conn., October 1, 1874.

PRHEFACH.

HE object in view in the preparation of this work has been
to present a popular account of “Corals and Coral
Islands,” without a sacrifice of scientific precision, or, on the
main topic, of fulness. Dry details and technicalities have
been avoided so far as was compatible with this restriction,
explanations in simple form have been freely added, and
numerous illustrations introduced, in order that the subject
may have its natural attractiveness to both classes of readers.
I have opened the volume with a chapter on “ Corals and
Coral makers,” describing, under it, the forms and structure
of polyps; how they live and grow and hold their own in a
world of enemies; how coral-making species secrete their
coral; how they multiply, and develop their large clusters,
spreading leaves and branching forms, so much like those
among plants; and in what seas they thrive, and under what
conditions produce the coral plantations,
All this is prefatory to the following part of the volume on
Coral Reefs and Islands, which comprises a description of the
features and structure of these reef-formations, an account of
~ their mode of accumulation and increase, and a discussion of

4 PREFACE.

the origin of the included channels and lagoons, and of the
distribution of reefs, together with a review of the facts with
reference to their geological bearing.

The observations forming the basis of the work were made
in the course of the cruise of the Wilkes Exploring Expedi-
tion, around the world, during the four years from 1838 to
1842. The results then obtained are published in my Report
on Zodphytes, which treats at length of Corals and Coral
Animals, and in a chapter on Coral Reefs and Islands form-
ing part of my Geological Report.

The opportunities for investigations in this department.
afforded by the Expedition, were large. We visited a number
of the coral islands of the Paumotu Archipelago, to the north
of east from Tahiti; also, some of the Society, Navigator, and
Friendly Islands, all remarkable for their coral reefs; the
Feejee Group, one of the grandest regions of growing corals
in the world, where we spent three months; several islands
north of the Navigator and Feejee Groups, including the Gil-
bert or Kingsmill Group; the Sooloo sea, between Borneo
and Mindanao, abounding in reefs; and, finally, Singapore,
another East India reef-region.

Most agreeable are the memories of events, scenes and
labors, connected with the cruise:—of companions in travel,
both naval and scientific; of the living things of the sea,
gathered each morning by the ship’s side, and made the study
of the day, foul weather or fair; of coral islands with their
groves, and beautiful life, above and within the waters; of
exuberant forests, on the mountain islands of the Pacific,
where the tree fern expands its cluster of large and graceful
fronds in rivalry with the palm, and eager vines or creepers
intertwine and festoon the trees, and weave for them hangings
of new foliage and flowers ; of lofty precipices, richly draped,

PREFACE. ‘ 5

even the sternest fronts made to smile and be glad as delights
the gay tropics, and alive with waterfalls, gliding, leaping, or
plunging, on their way down from the giddy heights, and,
as they go, playing out and in amid the foliage; of gorges ex-
plored, mountains and volcanic cones climbed, and a burning
crater penetrated a thousand feet down to its boiling depths;
and, finally,—beyond all these.—of man emerging from the
depths of barbarism through christian self-denying, divinely-
aided, effort, and churches and school-houses standing as cen-
tral objects of interest and influence in a native village.

On the other hand, there were occasional events not so
agreeable.

Even the beauty of natural objects had, at times, a dark
back-ground. When, for example, after a day among the
corals, we came, the next morning, upon a group of Fee
jee savages with human bones to their mouths, finishing off
the cannibal feast of the night; and as thoughtless of any im-
propriety as if the roast were of wild game taken the day be
fore. In fact, so it was.

Other regions gave us some harsh scenes. One—that of
our vessel, in a tempest, fast drifting toward the rocks of
Southern Fuegia, and finding anchorage under Noir Island,
but not the hoped-for shelter from either winds or waves; the
sea at the time dashing up the black cliffs two and three hun-
dred feet, and shrouding in foam the high rocky islets, half-
obscured, that stood about us; the cables dragging and clank-
ing over the bottom; one breaking; then another, the storm
still raging; finally, after the third day, near midnight, the
last of the four cables giving way, amid a deluge of waters
over the careering vessel from the breakers astern, and an in-
stant of waiting among all on board for the final crash; then,
that instant hardly passed, the loud calm command of the

6 PREFACE.

Captain, the spring of the men to the yard-arms, and soon the
ship again on the dark, stormy sea, with labyrinths of islands,
and the Fuegian cliffs to leeward; but, the wind losing some-
what of its violence and slightly veering, the ship making a
bare escape as the morning dawned with brighter skies.

And still another scene, more than two. vears later, on a
beautiful Sunday, in the summer of 1841, when, after a cruise
of some months through the tropics, we were expecting soon
to land on the shores of the Columbia; of the vessel sud-
denly stopped on the grinding sands; there, as the waves
passed, rising and falling with heavy blows on the fatal
bar that made the timbers to quiver and creak; and thus
helpless through a long night, the waters gaining in spite
of the pumps;— morning come, the old craft, that had
been a home for three eventful years, deserted, the boats
carrying us, empty handed, to ‘Cape Disappoimtment ’ —
a name that tells of other vessels here deceived and wrecked ;
and, twenty hours later, the old “ Peacock” gone, her upper
decks swept off by the waves, the hulk buried in the sands.

But these were only incidents of a few hours in a long and
always delightful cruise. If this work gives pleasure to any,
it will but prolong in the world the enjoyments of the “ Ex-
ploring Expedition.”

In explanation of some allusions in the following pages,
I may here state with regard to the Exploring Expedi-
tion, that Captain (now Admiral) Cuartes Wixkes, U.S. N.,
the Commander of the Expedition, was in charge of the Sloop-
of-war Vincennes; Capt. Wm. L. Hupson, U. 8. N., of the
Sloop-of-war Peacock; Capt. A. K. Lone, U. 5S. N., of the
Storeship Relief (the vessel which encountered the dangers
in the Cape Horn sea, above related) ; and Lieut. Command-
ant C. Rineeorp, of the Brig Porpoise; and that my associates

PREFACE. 7

in the ‘Scientific Corps” were Dr. Cuaries Pickertna, J. P.
Cournouy, and Trrtan R. Prax, Zodlogists; Wu. Ricu and
J. D. Brecxenripe®, Botanists; Horatio Hare, Philologist;
JosEpH Drayton and A. T. Acars, Artists.

Our cruise led us partly along the course followed by Mr.
Cuarurs Darwin during the years 1831 to 1836, in the Voyage
of the Beagle, under Captain Firzroy ; and, where it diverged
from his route, it took us over scenes, similar to his, of coral and
voleanic islands. Soon after reaching Sydney, Australia, in
1839, a brief statement was found in the papers of Mr. Dur-
win’s theory with respect to the origin of the atoll and barrier
forms of reefs. 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 when-
ever 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 Feejces, six’
months later, in 1840, I found there similar facts on a still
grander scale and of 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 man-
uscript. 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 there-
from 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.

In the preparation of the present work my former chapter

8 PREFACE.

on Coral Reefs and Islands has been greatly extended by the
addition of facts from numerous sources. The authorities
cited from are stated in the course of the volume, and need
not here be re-mentioned. I have occasion, however, for special
acknowledgments to our excellent Yale Zodlogist, Professor
A. E. Verrini, who now stands first in the country in the de-
partment of Zoéphytes. Through his recent memoirs on the
subject, and also by his personal advice, I have been greatly
aided in acquainting myself with the present state of the sci-
ence :—my own special labors in this branch of zoélogy having
ended in 1850, when both the Reports, referred to above, had
been published, and the last of my Expedition departments—
that of the Crustacea—forced my studies in another direction.

The illustrations of the following pages have been drawn
mainly from my Expedition Reports. Those not my own are
from the works or memoirs of Gossr, Ménrus, VerriLL, Pour-
TALES, L. Acassiz, A. Acassiz, Swirr, Enwarps and Hare,
Wixes, and Harrr. In addition, the volume is indebted for
a few cuts to the beautifully illustrated popular works, ‘ Le
Monde du Mer” and “ La Vie et les Mceurs des Animaux;”
but nearly half of these were engraved from my plates. The
sources of all the figures are given in the List of Illustrations.

James D, Dana.

New Haven, Conn., February 12, 1872.

CON DHOIN TS.

CHAPTER I.

CORALS AND CORAL MAKERS.

Page

PNM ALORA EVA TIONS (4. 0* Rk ae sek ui oes te. Wt eee) ecltes ee Gl

PPA Ok ee eet ey es Wa Laine kee ay allele. ta ey QO

I. Actinoid Polyps . “ : P|

I. Non-Coral-Making hee : 21

II. Coral-Making Actinoid Polyps 49

III. Classification : Bg EEG LH Peis ei) om pea ed

II. Cyathophylloids, or Rugosa Tetr Pana SM Ed oe on Wher Lio i's riers 78

III. Alcyonoid Polyps .. . SU ACA komt ORES BOC. Oe

IV. Life and Death in Gace ee SBN Matic a yioN Koo eel atte bet ioe oe

We omposition oi Coralsi eyes ce) ers Pease he! fs) So velit ey oe 298

ieLeyD ROU Si eerat sakes }- a TNL Pee nem Ls Bie ah lat wo le) toe Wl Oil

Ra COZOANE Ci, ie ee Se ee ee ee et eb he em ae LOD

IV. NuLirporrs. . ree: ie ee seated ss) et, at a es LOE
V. Tue Reer-FormMING Caacs, AND THE CAUSES INFLUENCING THEIR

Crowley AND DISTRIBUTION: ia cs le cals) 2 es ate ws LOS

ia Distnibusoncin Watitider meer ls Mares ee el e's Ye LOS

II. Distribution in Depth . . . . RSS SS ae were cs 2) 2 C

III. Local Causes influencing Distribution SURE egc cng stank = oe, va)’ Vee LO
iver hats. oimGrowinOr WOralsecl oie We ck te este ge seeds

CHAPTER II.

STRUCTURE OF CORAL REEFS AND ISLANDS.

IL (CRS IMSISED Gob Ge Ses a Es Se a a em ccs bf)

GSnera lehleauurese meet a ae vont aa wee raat eet yikes eee 198
MORON CTM CDi ae eer etl mE ra kd) Cea We Po S186

10 CONTENTS.

Ill. Formations in the Sea outside of Barrier Reefs .
IV. Inner Reefs.
V. Channels among Reefs .
VI. Beach Sand-Rock
VIL. Drift Sand-Rock .
VIII. Thickness of Reefs ;
IX. A Good Word for Coral Reefs

II. Cora IsLaAnps

I. Forms and General Features .

II. Soundings about Coral Islands

III. Structure of Coral Islands .

IV. Notices of some Coral Islands
Maldive Archipelago
Great Chagos Bank . j
Metia, and other elevated stands
Birnie’s, Enderbury’s
Hall’s, Swain’s
Oatafu, Fakaafo . :
Washington, Otuhu, Mocseee Teku, aes
Taiara, Ahii
Raraka .
Kawehe . Pea Se, we
Manhii, Aratica, Nairsa or Dean’s .
Florida Reefs and Keys

Between the Florida Reefs and Gules Salt ae Hane

Bahama Islands
Bermuda or Somers’ Islands .

CHAPTER III.

PAGE
139
144
148
152
154
156
159

161

161
Wel
174
185
186
192
193
196
197
198
199
200
201
202
203
204
210
214
218

FORMATION OF CORAL REEFS AND ISLANDS, AND CAUSES OF THEIR

FEATURES.

I. ForMATION OF REEFs .

I. Origin of Coral Sands and the Reef-Rock
II. Origin of the Shore Platform .
Ill. Effects of Winds and Gales
Il. Causes Mopiry1nc THE Forms AND GROWTH OF REEFS
I. Barrier and Fringing Reefs
II. Atoll Reefs . : ;
Ill. Rate or GrowtTH oF Ree:

227
224

CONTENTS. 11

Pace

IV. ORIGIN OF THE BARRIER CONDITION OF REEFS, AND OF THE ATOLL
HGEMIORICORALCISLANDS): (ia fii. Bowe) es Bo. Aloe |. 258
be Old. Vidws: i « mele 258
II. Darwin’s Theory of the ovina of Baitions ana ‘Aechii diel fet AOR
III. Objections to the Subsidence Theory . . . . . . . . . 277
MebnEeOComprernD ATOLE «<<... 0-2 Se 6. 8 a ew ee s. BOD

CHAPTER IV.

GEOGRAPHICAL DISTRIBUTION OF CORAL REEFS AND ISLANDS. 335

CHAPTER V.

CHANGES OF LEVEL IN THE PACIFIC OCEAN,

I. Evidences of Change of Level. . . . Eat ta 2st Prat Nal) oe oO:
II. Subsidence indicated by Atolls and Baeeiee Reefs By te Bn mh A re eens OT
Bbeerbitect.of the Subsidence =. . . 2 (fies. 6 + ties ws «O06
IV. Period of the Subsidence . . . SORA, Picea, vot, en ee eerO
VY. Elevations of Modern Eras in the Pacific Sra batetss ct cart ee Vawe eat er OOS

CHAPTER VI.

GEOLOGICAL CONCLUSIONS.

SCHORMATION ORULIMESTONES Ys oe.) a ss se 8 lw Ud te BBB
II. Beps or LimMEsTONE witH Livinc MarGins ..... . . 9887
TI. Maxine or Tuick STRATA OF LIMESTONE. ...... =. 938
IV. SuBSIDENCE ESSENTIAL YO THE MAKING OF THICK STRATA. . . 387
V. Deep-Sea LIMESTONES SELDOM MADE FROM CoRAL ISLAND OR
levaioims IDR aah Sn RBU Se Vy lee SO Oo ane ee Retr foo
VI. ABSENCE oF FossILs FROM LIMESTONE STRATA ..... . 9899
VII. THe wipE RANGE OF THE OLDER LIMESTONES NOT EXEMPLIFIED
IN MopERN CORAL-REEF FORMATIONS ... -..- - 999
WITI. CoNSOLIDATION OF CoRAL Rocks . .... .; BESS aes sen 0) |
IX. Formation oF DOLOMITE OR MAGNESIAN Cee oF LIME . 393
Moe NORMATIONGONS OHIATIKCn er yes Ny ee) fae ee ge BOA
XI. Rate or IncREASE OF LIMESTONE FORMATIONS . .. .. -. .9896
PU IMUSTONM CA VINE NS ONG cys os) cy we Nelle) ey ese! we ote OOF
Mele tOCKANIC LHMPERATURE | . «= ¢4.. Bre cs Cera aati euimermane 1210)

Miva Lun OCHANIC) CORAL-ISLAND SUBSIDENCE. . «= «|. . . «+ 401

12 CONTENTS.

APPENDIX.

i. ARTESIAN WELLS ON SOUTHERN OAHU . :
Il. Rare or GrRowTH OF CORALS AND CORAL REEFs .

lll. NAMEs oF SPECIES IN THE AUTHOR’S REPORT ON ZOOPHYTES .

INDEX .

PaGe

411
417
420

431

LIST OF ILLUSTRATIONS.

Tue following list contains a statement of the original sources of the illustra-
tions through the volume. By ‘‘ Author’s Atlas’’ is to be understood the Atlas
of his Report on Zoophytes. The figures are of natural size, except when other-
wise stated. The new figures included have been made by Mr. Lockwood San-
ford, a New Haven wood-engraver of most of the wood-cuts in this volume.

I. PLATES.

Plate I., frontispiece. Fig. 1, la. Phyllastreea tubifex. Author’s Zoodphyte
Atlas, Plate 16.

II., facing page 20. Fig. 1, Phymactis veratra: Actinia veratra, Author’s
Zoophyte Atlas, Plate 1. Fig. 2, Phymactis clematis: Actinia clem-
atis, Ibid., Plate 1.

VIII, page 31. Lasso-cells, K. Mobius, Abh. Nat. Ver. Hamburg, vol. v.,
1866.

IV., facing page 54. Fig. 1, Caulastraea furcata. Zoophyte Atlas, Plate 9.
Fig. 2, Tridacophyllia peonia, Ibid. ‘Fig. 3, Leptoria tenuis: Me-
andrina tenuis, Ibid., Plate 14 ; the tentacles are arranged along the
sides of the trench, with the mouths of the polyps between them.

V., page 73. Madrepora formosa. Zoophyte Atlas, Plate 58.

‘VL, facing page 82. Fig. 1,15. Merulina regalis. Zoophyte Atlas, Plate 15.
Fig. 2, Telesto trichostemma: Gorgonia trichostemma of Zoophyte
Atlas, Plate 59; referred to Telesto by Verrill.

VII., page 133. Louisiade Group. Proceedings of the R. Geograph. Soc.,
Sept., 1889.

VIII, page 165. Gilbert or Kingsmills Group. Author’s Expl. Geol. Rep.
from Expl. Exped. Maps.

IX., facing page 172. Phoenix Group. U. S. Hydrographic Maps of the
Pacific.

X., page 187. Maldive Archipelago. Darwin on Coral Reefs.

XI., facing page 204. Map of Florida, Bahamas, and Bermudas; U. S.
Hydrographic Maps of the Atlantic.

XIL.,. facing page 262. Map of the Feejee Islands. Wilkes’s Narrative of
the Expl. Expedition.

XIII., facing page 310. Cocoanut Grove, on Bowditch Island. Wilkes’s Nar-
rative, vol. v.

14

LIST OF ILLUSTRATIONS.

Plate XIV., facing page 314. Village of Utiroa. Wilkes’s Narrative, vol. v.
XV., facing page 312. Scene on the Lagoon side of Oatafu or Duke of

York’s Island. Ibid.

XVI, atend. Isocrymal Chart of the Oceans. Author’s Expedition Report

Page 23,

24,

26,

27,

42,

43,

45,

~~

46,

AT,

69

~

on Crustacea.

II. FIGURES IN THE TEXT.

Paractis rapiformis. From a drawing by the Author, made in 1852.
Cancrisocia expansa. Verrill, Amer. Naturalist, from Proc. Essex In-

stitute, vol. vi.
fig. 1. Peachia hastata. Gosse’s Actin. Brit., Plate viii.

fig. 2. Edwardsia callimorpha; and 3. Halocampa chrysanthellum. Ibid.,
Plate 7.

Section of Actinia. From a drawing by the Author, made in 1856, on
the basis of a study (1852) of the Actinia figured on p. 23.

Caryophyllia cyathus. Le Monde du Mer.

Thecocyathus cylindraceus. Pourtales on Deep-Sea Corals, Plate 2.

Flabellum spheniscus. Euphyllia spheniscus of Author’s Atlas, Plate 6.

Ctenactis echinata, one-third natural size. La Vie et les Mceurs des
Animaux.

Fungia lacera, living and expanded. F. echinata of Author’s Atlas,
Plate 18.

Enlarged view of tentacle of F. lacera, and profile, natural size, of one
of the calcareous septa. Ibid., Plate 18.

Madrepora aspera, living and expanded. Author’s Atlas, Plate 38.

Dendrophyllia nigrescens, living and expanded. Author’s Atlas,
Plate 30.

Goniopora columna. Ibid., Plate 56.

Porites mordax. Ibid., Plate 53.

Cladocora arbuscula. Caryophyllia arbuscula of Ibid., Plate 27.

Orbicella cavernosa. L. Sanford, from specimen.

Spontaneous fission. Author’s Report on Zoophytes.

Astrea pallida. Author’s Atlas, Plate 10.

Epizoanthus Americanus. Verrill, Amer. Naturalist, vol. iii., p. 248.
View of single polyp. From a drawing by Prof. Verrill.

Antipathes arborea, with enlarged view of polyp. Author’s Atlas,
Plate 56.

Astrea pallida. Author’s Atlas, Plate 10.

Diploria cerebriformis. Le Monde du Mer.

Fungia Dane. L. Sanford, one-sixth the natural size. From a photo-
graph by Prof. A. E. Verrill.

Caryophyllia Smithii, one of the figures with the animal expanded; the
other with it contracted. Gosse’s Actinologia Britannica, Plate 10.
Astrangia Dane ; fig. a. one of the polyps enlarged; c. coral with the

polyps expanded, natural size. Agassiz, Seaside Studies.
fig. b. surface of corallum, natural size. L. Sanford, from specimen.
Phyllangia Americana, Florida. Edwards & Haime, Corallieres.

LIST OF ILLUSTRATIONS. 15

Page 69, fig- 1. Oculina varicosa, extremity of a branch. Author’s Report on
Zoophytes, page 67, corrected from specimen.

fig. 2,3. Stylaster erubescens; 2. corallum, natural size; 3. extremity of
a branch enlarged. Pourtales, Deep-Sea Corals.

fig. 4, 5. Stylophora Danex; 4. extremity of a branch ; 5. one of the
calicles enlarged. Sideropora palmata of Author’s Atlas, Plate 49.

fig. 6. Polyp, enlarged, of St. mordax. Author’s Atlas, Plate 49.

fig. 7. Pocillipora grandis. L. Sanford; from an Exploring Expedition
specimen; portion of one of the large, flattened branches of the coral-
lum. An entire clump is figured in the Author’s Atlas, Plate 51.

fig. 8. Cell, enlarged, of Pocillipora elongata. Author’s Atlas, Plate 50.

fig. 9. Cell, enlarged, of Pocillipora plicata. Ibid., Plate 50.

fig. 10. Vertical section of corallum of P. plicata, showing the tabular
structure. Ibid., Plate 59.

72, Polyp of Madrepora cribripora, enlarged. Author’s Atlas, Plate 31.

75, Polyp of Dendrophyllia nigrescens, enlarged. Ibid., Plate 30.

76, Dendrophyllia nigrescens, natural size. Ibid., Plate 30.

77, Alveopora Verrilliana, natural size; the corallum covered below with a
peritheca. Alveopora dedalea in part of Author’s Atlas, Plate 48.
The species is here named after Prof. A. E. Verrill, as it is not the
true A. dedalea.

Alveopora spongiosa, vertical section of corallum, and upper view of
calicle, much enlarged; the diameter of the cell being about a
fifteenth of an inch. Author’s Atlas, Plate 48.

78, Polyp of Porites levis, enlarged. Author’s Atlas, Plate 54.

79, Porites levis, with the polyps of one of the branches expanded, natural
size. Author’s Atlas, Plate 54.

82, Xenia elongata. Author’s Atlas, Plate 57.

83, Anthelia lineata. Verrill, Proceedings of the Essex Institute, vol. iv.,
Plate 5. From a drawing by Dr. Stimpson.

84, Telesto ramiculosa. Verrill, Proc. Essex Inst., vol. iv., Plate 6; the
second figure, an enlarged view of expanded polyp. From drawings
by Dr. Stimpson.

Tubipora syringa; fig. 1. part of a clump, natural size; 2. one of the
polyps expanded. Author’s Atlas, Plate 59.

Tubipora fimbriata (3d figure), polyp, expanded. Author’s Atlas,
Plate 59.

85, Gorgonia flexuosa, part of zoophyte, natural size. Author’s Atlas,
Plate 60.

86, Spicules of Gorgoniw, much enlarged. Verrill, Transactions of the
Connecticut Academy of Sciences, vol. i., Plates 4 and 5.

88, Isis Hippuris. La Vie et les Moeurs des Animaux.

89, Corallium rubrum, the coral, natural size. L. Sanford, from specimen.

Extremity of branch of C. rubrum, enlarged, with some of the ani-
mals expanded. Lacaze-Duthiers, from La Vie et les Mceurs des
Animaux.

91, Cophobelemnon clavatum: the small figure, enlarged view of one of the
polyps. Verrill, Proc. Essex Institute, vol. iv., Plate 5. From a
drawing by Dr. Stimpson.

16
Page 91,

95,
101,
103,
104,

105,
106,

130,

140,
149,

162,
168,

170,

176,
179,

189,

191,
192,
193,

219,
235,
943,

247,
248,
250,
263,
264,
266,
267,
268,
311,
413,

LIST OF ILLUSTRATIONS.

Veretillum Stimpsoni, enlarged three diameters. Verrill, Proc. Essex
Institute, vol. iv.. Plate 5. From a drawing by Dr. Stimpson.

Caulastrea furcata. Author’s Atlas, Plate 9.

Hydra. Le Monde du Mer.

Hydrallmania faleata. Le Monde du Mer.

Animals of M. alcicornis, enlarged. L. Agassiz, Contributions to the
Natural History of the United States, vol. iii. Plate 15.

Millepora alcicornis. La Vie et les Mceurs des Animaux.

Hornera lichenoides: 1. natural size; 2. part of branch enlarged.
Smitt’s Mém. des Bryozoaires.

Discosoma Skenei, part of a group much enlarged. Ibid.

High Island, with Barrier and Fringing Reefs. Author’s Exp. Geol.
Report.

The Lixo Coral Reef, Abrolhos. Hartt’s Brazil, p. 202.

Coral Reefs off the North Shore of Tahiti. Author’s Exp. Geological
Report, from the Wilkes Expl. Exp. Maps.

Coral Island or Atoll. Wilkes’s Narr. Expl. Exped.

Maps of Taiara, Henuake, Swain’s Island, Jarvis Island, and Fakaafo.
Author’s Geol. Rep.; from Exp]. Exp. Maps.

Map of Menchicoff Atoll. Darwin on Coral Reefs; from Kotzebue’s
Atlas.

Section of the rim of an Atoll. Author’s Exp. Geol. Report.

Blocks of Coral on the shore platform of Atolls, Author’s Exp. Geol.
Report.

Map of Mahlos Mahdoo Atoll, one of the Maldives. Darwin on Coral
Reefs.

Map of Great Chagos Bank, Darwin on Coral Reefs.

East and West Section across the Great Chagos Bank. Ibid.

Metia, an elevated Coral Island. Wilkes’s Narrative of Expl. Exp.,
vol. i.

Map of the Bermuda Islands; reduced from an English Chart.

The ‘‘Old Hat.’’ Author’s Exp. Geol. Report.

Harbor of Apia. Author’s Exp. Geol. Rep.; from Charts of the Wilkes
Expl. Exped.

Part of North Shore of Tahiti. Ibid.

Harbor of Falifa. Ibid.

Whippey Harbor. Ibid.

Section illustrating the Origin of Barrier Reefs. Ibid.

Map and Ideal Section of Aiva Island. Ibid.

Map of Gambier Islands. Darwin on Coral Reefs.

Section illustrating the Origin of Atolls. Author’s Exp. Geol. Rep.

Menchicoff Atoll. Darwin on Coral Reefs.

Fakaafo. Author’s Exp. Geol. Rep.; from Charts of Expl. Exped.

Map of part of Oahu; reduced from chart of Hawaiian Government
Survey.

CORALS AND CORAL ISLANDS.

CHAPTER I.
CORALS AND CORAL MAKERS.

SINGULAR degree of obscurity has possessed the popu-
lar mind with regard to the growth of corals and coral
reets, in consequence of the readiness with which speculations
have been supplied and accepted in place of facts; and to the
present day the subject is seldom mentioned without the qual-
ifying adjective mysterious expressed or understood. Some
writers, rejecting the idea which science had reached, that
reefs of rocks could be due in any way to ‘ animalcules,”
have talked of electrical forces, the first and last appeal of ig-
norance. One author, not many years since, made the fishes
of the sea the masons, and in his natural wisdom supposed
that they worked with their teeth in building up the great
reef. Many of those who have discoursed most poetically on
zodphytes have inagined that the polyps were mechanical
workers, heaping up the piles of coral rock by their united la-
bors; and science is hardly yet rid of such terms as polypary,
polypidom, which imply that each coral is the constructed
hive or house of a swarm of polyps, like the honey-comb of
the bee, or the hillock of a colony of ants.

Science, while it penetrates deeply the system of things
2

18 CORALS AND CORAL ISLANDS.

about us, sees everywhere, in the.dim limits of vision, the word
mystery. Surely there is no reason why the simplest of organ-
isms should bear the impress most strongly. If we are aston-
ished that so great deeds should proceed from the little and
low, it is because we fail to appreciate that little things, even
the least of living or physical existences in nature, are, under
God, expressions throughout of comprehensive laws, laws that
govern alike the small and the great.

It is not more surprising, nor a matter of more difficult
comprehension, that a polyp should form structures of stone
(carbonate of lime) called coral, than that the quadruped
should form its bones, or the mollusk its shell. The pro-
cesses are similar, and so the result. In each case it is a sim-
ple animal secretion ; a secretion of stony matter from the
aliment which the animal receives, produced by the parts of
the animal fitted for this secreting process; and in each, car-
bonate of lime is a constituent, or one of the constituents, of
the secretion.

This power of secretion is then one of the jist and most
common of those that belong to living tissues ; and though dif-
fering in different organs according to their end or function, it
is all one process, both in its nature and cause, whether in the
Animalcule or Man. It belongs eminently to the lowest kinds
of life. These are the best stone-makers ; for in their simplici-
ty of structure they may be almost all stone and still carry on
the processes of nutrition and growth. Throughout geological
time they were the agents appointed to produce the material
of limestones, and also to make even the flint and many of the
siliceous deposits of the earth’s formations.

Coral is never, therefore, the handiwork of the many-
armed polyps; for it is no more a result of labor than bone-
making in ourselves. And again, it is not a collection of cells

CORAL AND CORAL MAKERS. 19

into which the coral animals may withdraw for concealment
any more than the skeleton of a dog is its house or cell; for
every part of the coral—or corallum as it is now called in sci-
ence—of a polyp, in most reef-making species, is enclosed
more or less completely within the polyp, where it was
formed by the secreting process.

It is not, perhaps, within the sphere of science to criticise
the poet. Yet we may say in this place, in view of the frequent
use of the lines even by scientific men, that more error in the
same compass could scarcely be found than in the part of
Montgomery’s ‘“‘ Pelican Island” relating to coral formations.
The poetry of this excellent author is good, but the facts nearly
all errors—if literature allows of such an incongruity. There
is no ‘‘toil,” no “skill,” no ‘‘ dwelling,” no “ sepulchre” in the
coral plantation any more than in a flower-garden ; and as lit-
tle are the coral polyps shapeless worms that “writhe and
shrink their tortuous bodies to grotesque dimensions.”

The poet oversteps his license, and besides devrades his
subject, when downright false to nature.

Coral is made by organisms of four very different kinds.
These are: st, Potyps, the most important of coral-making
animals, the principal source of the coral reefs of the world.

Second, Animals related to the littie Hydra of fresh waters,
and called Hyprorps (a division under the Acalephs), which,
as Agassiz has shown, form the very common and often large
corals called Millepores.

Third, The lowest tribe of Mollusks, called Bryozoans,
which produce delicate corals, sometimes branching and moss-
like (whence the name from the Greek for moss animal), and at
other times in broad plates, thick masses, and thin incrusta-
tions. Although of small importance as reef-makers at the

20 CORALS AND CORAL ISLANDS.

present time, in a former age of the world—the Paleozoic—
they so abounded over the sea bottom that some beds of lime-
stone are half composed of them.

Fourth, Alge or sea-weeds, some kinds of which would
hardly be distinguished from corals, except that they have no
cells or pores.

fT POLYES:

A. good idea of a polyp may be had from comparison with
the garden aster; for the likeness to many of them in external
form as well as delicacy of coloring is singularly close. The
aster consists of a tinted disk bordered with one or more series
of petals. And, in exact analogy, the polyp flower, in its
most common form, has a disk fringed around with petal-like
organs called tentacles. Below the disk, in contrast with the
slender pedicel in the ordinary plant, there is a stout cylindri-
cal pedicel or body, often as broad as the disk itself, and some-
times not much longer, which contains the stomach and inter-
nal cavity of the polyp; and the mouth, which opens into the
stomach, is at the centre of the disk. Here then the flower-
animal and the garden-flower diverge in character, the dif
ference being required by the different modes of nutrition and
other characteristics in the two kingdoms of nature. ‘The cor-
al polyp is as much an animal as a cat or a dog.

The figures of the frontispiece, and others on pages 23, 24,
26, sustain well the description here given, and afford some
idea also of the diversity of form among them.

The prominent subdivisions of polyps here recognized are
the following: )

I. Acrinom Potyps.—Related to the Actinia, or Sea-anem-

one, in tentacles and interior structure, and having, as in

PLATE 11.

1.Phymactis veratra ; 2, P. clematis.

CORAL AND CORAL MAKERS. 21

them, the number of tentacles and interior septa a multiple of
six. The name Actinza is from the Greek for ray.

II. CyatuopHyLioip Potyps.—Nike the Actinoids in tenta-
cles and interior structure, except that the number of tentacles
and interior septa is a multiple of four. Ludwig and De
Pourtales state that the number in the earliest young state is
siz, and that therefore the fundamental ratio is the same as in
the Actinoids; and that they pass from this ratio by develop-
ments of tentacles and septa more rapidly on one side than the
opposite, and in such a manner that the number becomes after
the first stage a multiple of four. The Cyathophylloid polyps
hence combine this characteristic of the Actinoids with one
feature of the Alcyonoids. The Cyathophylloids were the ear-
liest of polyps, and the most abundant species in Paleozoic
time.

III. Aucyonorw Potyrs.—Having eight fringed tentacles,
and other characters mentioned beyond; as the Gorgoniz and

Alcyonia.

I -ACTINOID: POLYPS:

The highest of Actinoid Polyps are those of the Actinra
TRIBE—the species that secrete no coral to clog vital action
and.prevent all locomotion. ‘The details of structure may be
best described from the Actinia or Sea-anemone, and after-—
ward the distinguishing characters of the coral-making polyps
may be mentioned. In external aspect and in internal charac-
ters all are essentially identical.

1. NON-CORAL-MAKING POLYPS.

As the colored figures on Plate II., and also the following.

show, the external parts of an Actinia are —a subcylindrical

22 CORALS AND CORAL ISLANDS.

body—a disk at top—one or more circular series of tentacles
making a border to the disk—a mouth, a merely fleshy, toothless
opening, at the centre of the disk, sometimes at the summit
of a conical prominence—a basal disk for attachment. The
upper extremity is called the acténal end, since it bears the
tentacles or rays, and the lower or base, the abactinal.
Sea-anemones vary greatly in color, and in the distri-
bution of their tints. This is finely illustrated on the first
four plates of the Author’s Atlas of Zodphytes. Two figures
of Plate I. are reproduced on the accompanying Plate IL:
one, Phymactis clematis, from Valparaiso, and the other, Phy-
mactis veratra, from Wollongong, New South Wales. An-
other variety of P. clematis has a pink disk, wine-red tenta-
cles, and the body reddish with dots of dark green. The
P. florida, from the coast of Peru, one variety of which 1s
shown on the second plate of the Atlas, has blue tentacles
and a paler disk; another has a bluish green disk with pur-
plish tentacles and the papillee of the body dark sap-green on
a pale reddish ground; and another is green throughout.
While often brilliantly colored, especially in the tropics,
other Actiniz are nearly colorless. This was the case with
that represented in the following cut, a species from Long
Island Sound near the New Haven Light-house, figured
some twenty years since by the author, but left undescribed.
The body in this species had a delicate texture throughout, its
walls being so transparent that the crgans within could be
seen through them. It was exceedingly flexible and passed
through various shapes, imitating vases of many forms, wine
glasses, goblets, etc. It was generally very slow in its
changes, and sometimes continued in the same vase-attitude

for a whole day.
Actinie vary immensly in size,—from an eighth of an inch

ACTINIA AND OTHER ACTINOID POLYPS. 25

and smaller in the diameter of the disk to over a fout,—
though commonly between half an inch and three inches.
One species from the Paumotu Coral Archipelago in the

PARACTIS RAPIFORMIS, EDW.

Pacific, a colored figure of which is given in the Atlas of the
Author’s Report on Zodphytes (Plate III.), had a diameter
across its disk of fowsteen inches; and it was also one of the
most beautiful in those seas, having multitudes of tentacles
with carmine tips and yellowish bases, around the open centre,
gathered into a number of large groups or lobes.

With rare exceptions, Actiniz live attached to stones,
shells, or the sea bottom, or are buried at. base in the sand or
mud. The attached species have the power of locomotion,

through the muscles of the base, but only with extreme slow-

24 CORALS AND CORAL ISLANDS.

ness. The loose stones on a sea-shore near low tide level
often have Actiniz fixed to their under surface. A very few
species swim or float at large in the ocean.

Now and then an Actinia puts itself on the back of a
crab, and thus secures rapid locomotion, but only at the will
of the crab, which inay at times give it some hard rubs:—a

CANCRISOCIA EXPANSA ST., ON THE BACK OF DORIPPE FACCHINO.

kind of association styled conumensalism by Van Beneden, as
the two in a sense live at the same table, without preying
one upon the other. In the above example, from the China
seas, the Actinia has mounted a Dorippe. The figure is from
the Proceedings of the Essex Institute, where an account of it
is published by Prof. Verrill; the specimen was collected by
the zodlogist, Dr. W. Stimpson. As Prof: Verrill states, the
Dorippe carries, for its protection when young, a small shell over
its back, which it holds in this position by means of its two
reversed pairs of hind legs. The Actinia appears to have fixed
itself, when young, to the shell, and afterward, by its growth,
spread over the back of the crab, taking the place of the shell.

This case of commensalism, like most others, is not a mere

chance association of species; for the two always go together,

ACTINILA AND OTHER ACTINOID POLYPS. 25

the Actinia, according to Dr. Stimpson, never being seen
except upon the crab’s back, and the crab never without its
Actinia. The fact shows an instinctive liking on the part of
the Actinia for a Dorippe courser, and for the roving life
thus afforded it. And the crab is undoubtedly conscious that
he is carrying his fortress about with him. It is not a soli-
tary case; for there are many others of Actiniz attaching
themselves to locomotives—to the claws or backs of crabs, or
to shells in possession of soldier crabs, or to a Medusa; and
frequently each Actinia has its special favorite, proving an
inherited instinctive preference for rapid change of place, and
for just that kind of change, or range of conditions, which the
preferred commensal provides. Prof. Verrill has an interest-
ing article on this subject, with especial reference to crustace-
ans, in the third volume of the American Naturalist.

Species living in sand are often unattached; and then the

g, and sometimes balloon-shaped ;

base is rounded or tapering,

some of them are long and almost worm-like, and even burrow
hke worms. |

The following are figures of three species: one, figure 3,
exhibiting simply the tentacles and disk of the Actinia, the
only parts visible above the sand; the others showing the
whole body removed from the sand, and consequently a little
out of shape. They are from Gosse’s “ British Sea-Anem-
ones,” in which they are given with the natural colors.
Figure 1 represents the Peachia hastata of Gosse, a beautiful
species having twelve large tentacles; fig. 2, the Adwardsia
callimorpha.G.; fig. 83, Halocampa chrysanthellum G. Most
of these sand-dwellers bury themselves like the Halocampa,
and often hide all the disk but the mouth. The Edwardsia
is peculiar in having, above the hollow bladder-like basal
portion, a firm opaque exterior to the body, making for it

26 CORALS AND CORAL ISLANDS.

a kind of case or jacket, into which the upper extremity,
which is soft and delicate in texture, may be retracted. The
thickening of the epidermis in this middle portion is produced

through the entangling of disintegrated cells and minute for.

1. PEACHIA HASTATA, G.; 2. EDWARDSIA CALLIMORPHA, G.; 3. HALOCAMPA

CHRYSANTHELLUM, G.

eign particles, sometimes in part spores of Confervx, by
means of the mucus of the surtace; and if the layer is re-
moved, as it may be, the skin will again become covered.
This species, like others of the genus, lives buried to its neck
in the sand, that is, with the soft upper extremity protrud-
ing If disturbed, the head is suddenly drawn in, together
with more or less of the following jacketed part of the body.

The warty prominences on some warty species have the
power of clinging by suction to a surface, and such Actiniz
often cover their sides thus with bits of shell or of other sub-

stances at hand. Where there are no warts the contracted

ACTINIAA AND OTHER ACTINOID POLYPS. rie |

exterior skin, reticularly corrugated, occasionally becomes a
surface of suction-warts, as in many Sagartie.

The znternal structure of the Actinia is radiate like the ex-
ternal, and more profoundly and constantly so. The mouth,
a fleshy toothless opening in the disk, opens directly into a
stomach, which descends usually about a third of the way to
the base of the body; its sides are closed together unless
it be in use. The general cavity of the body around and be-
low the stomach is divided radiately by fleshy partitions, or
septa, into narrow compartments; the larger of these septa

connect the stomach to the sides of the animal, and, besides
holding it in place, serve to pull it open or distend it for the
reception of food. The above figure represents in a gener-
al way a horizontal section of the body through the stomach,
and shows the position of the radiating septa and the interme-
diate compartments. It presents to view the fact that these
are in pairs, and another fact that the number of pairs of par-
titions in the ordinary Actinoid polyps is regularly some mul-
tiple of six, although other numbers occur during the succes-
sive developments that take place in the growth of a polyp,
and are occasionally persistent in the adult state. There are six
pairs in the first series; s¢z in the second; twelve in the third;
twenty-four in the fourth; forty-erght in the fifth, and so on.

28 CORALS AND CORAL ISLANDS.

The compartment between the two septa of each pair opens
at top into the interior of a tentacle, and thus the cavity in
each tentacle has its special corresponding compartment below.
This tentacular compartment is properly, as first recognized by
Prof. Verrill, the a@mbulacral, since each corresponds in posi-
tion and function to an ambulacral or tentacle-bearing section
in the Echinoderms and other Radiate animals.

Although polyps are true Radiates, they have something
of the antero-posterior (or head-and-tail) polarity, with also the
right-and-left, which is eminently characteristic of the animal
type. This is manifested in the occurrence in some polyps of
aray on the disk different in color from the general surface:
of one tentacle larger than the others, and sometimes peculiar
in color; of two opposite septa in a calicle or polyp-cell larger
than the others, and sometimes meeting so as to divide the cell
into halves. ‘The first of these marks the author has observed
in a Zoanthid, as mentioned in his Report on Zodphytes at
page 419, and represented on plate 30: and the last is very
strongly developed in the cells of many Pocillopore (ib. p. 523).
Gosse and many other authors have drawn attention to the
one large tentacle, and the fact that it lies in the direction of
the line of the mouth. Prof. H. James Clark, in his Mind in
Nature, states that the order in which the fleshy septa and the
tentacles in an Actinia are developed has direct reference to the
right and left sides of the body, and that there is only one
plane in which the body can be divided into two halves, and
this is that corresponding with the longer diameter of the stom-
ach. or the direction of the mouth. Mr. A. Agassiz has
shown that in Actinie of the genus Arachnactis, the new
septa and tentacles are developed either side of the one chief
or anterior tentacle; and Prof. Verrill, that in Zoanthids,
they are formed principally either side of this anterior tentacle

ACTINLH AND OTHER ACTINOID POLYPS. 29

and also of the opposite or posterior one, and much less
rapidly, if at all, along the sides intermediate. ‘This chief:
tentacle marks properly the true front or anterior side of
the polyp. A fore-and-aft structure is also very strongly
marked in some of the ancient cyathophylloid corals, and
hence it belonged to the type from early Paleozoic time.

The way leading out from the Radiate structure is thus
manifested by these tlower-like polyps. In fact perfect circu-
lar series in organs or parts do not belong to any living organ-
ism, not even to the true flower; for growth is fundamentally
spiral in its progress, and there must be always an advance
end to the spiral of growth; all apparent circles are only dis-
guised spirals.

The walls of the body contain two sets of muscles, a circu-
lar and a longitudinal, the latter becoming radial in the disk
and base. Similar muscles exist also in the tentacles, and cor-
responding muscles in the fleshy partitions or septa of the in-
ternal cavity.

By means of these muscles an Actinia, whenever disturbed,
contracts at once its body; and most species make of them-
selves a spheroidal or conoidal lump, showing neither disk
nor tentacles. One example of this contracted state is presented
on the frontispiece in figure 3a. After a brief period of quiet
the polyp commonly reassumes its full expansion. The ex-
pansion depends on an injection of the structure with salt wa-
ter, which is taken in mainly by the mouth. As the whole body
is thus filled and injected, the flower slowly opens out, and
shows its petal-like tentacles. On contraction the water is
suddenly expelled through the mouth, and by pores in the sides
of the polyps, and at the extremity of the tentacles, and the
tentacles disappear, along with the disk, beneath the adjoining
sides of the body which are drawn or rolled in over them.

30 CORALS AND CORAL ISLANDS.

The Actinia appears, at first thought, to be well prepared
for securing its prey through its numerous tentacles. But
these are generally too short for prehension. Yet the disk often
aids them by rolling over the captured animal, and pushing it
down into the stomach. At the same time, the mouth and
stomach are both very extensile, so that an Actinia may swal-
low an animal nearly as large as itself; it gradually stretches
the margins of the mouth over the mollusk or crab, until the
whole is enclosed and passed into the digestive sac; and when
digestion is complete, the shell and any other refuse matters
are easily got rid of by reversing the process.

But the Actinia owes nearly all its power of attack to its
concealed weapons, which are carried by myriads. These
are what Agassiz has called dasso-cells, because the little cell-
shaped sheath contains a very long slender tubular thread
coiled up, which can be darted out instantly when needed.
As first observed by Agassiz, the tubular lasso escapes from
the cell by turning itself inside out, the extremity showing it-
self last, and this is usually done “ with lightning-like rapidi-
ty.” Then follows the poison. The lasso-cells (called often
nettling cells, and by Gosse cnide, and thread capsules) are
usually less than a two-hundredth of an inch in length; but
they are thickly crowded in the larger part of the skin or walls
of the tentacles, and about the mouth; also in the walls of the
stomach, and within the visceral cavity in white cords hanging
in folds from the edge of the septa. Thus the polyp is armed
inside and out. The mollusk or crab that has the ill luck to
tall, or be thrown by the waves, on the surface of the pretty
flower is at once pierced and poisoned by the minute lassos,
and is rendered incapable of resistance.

The following figures, by Dr. Karl Mobius, of Hamburg, il-
lustrate admirably these organs. The views are magnified

Plate Ill.

Lear
SS,

er ™, .
ii.
© Srey

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= . * “
pie ar, aa Eh Ry, Pant "ane “alas ean eeewe Aap sbbegyetety taa tk tly DE ene t Cease Prue CUT PrCEUNTLETTUVOTe VY RYU DTS TRICE EN TDEVET TYRESE TTRTTT| ol
" 5 , ~ * ‘ ~ se * — Pa pe Gala Pa ee i lee
ee a tec ees ea nD Selig eae aaa aes, SR =e Seale, mig a mies Be

“ : ere
Sages ? .
a ein es Se Chi RAT

a

LASSO-CELLS,

‘HARVARD UNIVERSHY
CAMBRIDGE. MA USA

“mice WBRARY

bet

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*
i* .

ACTINLGi AND OTHER ACTINOID POLYPS. oD

700 diameters. Figure 1 represents one of the lasso-cells of
the Actinia, Corynactis viridis, with its lasso coiled up within,
its actual length is about a 350th of an inch. Figure 2 is the
same with the lasso out, though less than half of the long
thread is shown. Figure 3 is the lasso-cell of the polyps of a
European coral, the Caryophyllia Smithii. Jt differs from
figure | in having the basal part of the lasso within the cell or
sheath strait and stout; it is this part which makes the first
portion of the extended lasso. <A view of part of the latter is
represented in figure 4, and of the extremity of the same in
figure 5. The lasso-cells in the above species are from a 240th
to a 360th of an inch in length. In the Metridium margina-
tum, an American Actinia occurring along the coast of the Uni-
ted States, north of New York, the length of one of the lasso-
cells, according to Dr. Leidy, was about a 400th of an inch,
and the character of the extended lasso was much like that of
figure 4. The lower part of the lasso, for a length 14 times
or more longer than the cell or sheath, is usually thickened,
and sometimes slenderly spindle-shaped, while the rest is an
_ even slender thread; and the thickened part and sometimes
all the rest, as above shown, is spirally wound by a slender
line, sometimes elevated, set with short hairs or bristles. The
thread-like portion nay be wanting or very short. The lasso
is often twenty times as long as the cell or sheath, and occa-
sionally forty times; but if the thread-like part is absent, only
one and a half to two times.

A lasso-cell once used is afterward worthless; for the tube
cannot be returned to the sheath. But those thus expended
are not missed, as the polyp has indefinite supplies of such
weapons, and also ready means of refurnishing itself.

Figures 6, 7, 8, 9, 10, on the preceding page, illustrate

different stages in the development of a lasso-cell (fig. 10)
3

34 CORALS AND CORAL ISLANDS.

out of a common spherical cell, as ade out by Dr. Mobius in
his careful microscopic investigations. The Actinia affording
the results was the Urticina crassicornis, found in both Euro-
pean and American seas. The actual size of the cell represent-
ed in figure 6 is about a 5,000th of an inch. In fig. 7 the
lasso-cell has already taken form but is folded on itself; in 8,
there is a second infolding; 9 shows a return to a single fold,
and further progress in the forming cell; and 10, the straight-
ened lasso-cell. Thus the work of replenishing, throughout
the body wherever lassos are used, is always going on.

The radiating partitions or septa in the internal cavity
of the polyp have along the outer free edge what looks like a
slender white cord attached to it by a much convoluted or
mesentery-like membrane; and this cord contains vast num-
bers of lasso-cells radiately arranged. These white cords
through the multiplied plaitings of the mesenteric membrane
have great length; and they sometimes extend up through the
stomach and pass out of the mouth; or they are extended in
loops through the walls wherever they may happen to be torn.

There are often also bunches of somewhat similar white cords
full of lasso-cells appended to the septa, which are extended
from the body through some natural orifices near the base of the
Actinia (especially those of the Sargartia family). Gosse calls
these cords Acontia. ‘They extend out usually two or three
inches, and sometimes six inches, and thereby widen much the
stinging range of an Actinia, both for the purposes of defence
and attack.

Gosse, in his “‘ British Sea-Anemones,”

gives the results of
some experiments with regard to the action of these lasso-cells
(cnide), from which a few paragraphs may be here cited.

“Tt has long been known, that a very slight contact with

the tentacles of a polyp is sufficient to produce, in any minute

ACTINIA AND OTHER ACTINOID POLYPS. ao

animal so touched, torpor and speedy death. Since the discoy-
ery of these cnid@ (lasso-cells) the fatal power has been sup-
posed to be lodged in them. Baker, a century ago, in speak-
ing of the Hydra, suggested that “there must be something
eminently poisonous in its grasp ;” and this suspicion received
confirmation from the circumstance that the Hntomostraca
which are enveloped in a shelly covering frequently escape un-
hurt after having been seized. The stinging power possessed
by many Meduse, which is sufficiently intense to be formida-
able even to man, has been reasonably attributed to the same
organs, which the microscope shows to be accumulated by mil-
lions in their tissues.

“Though I cannot reduce this presumption to actual cer-
tainty, I have made some experiments, which leave no reason-
able doubt on the subject. First—I have proved that the ecthor-
eum (tubular thread of the lasso-cell) when shot out, has the
power of penetrating, and does actually penetrate, the tissues
of even higher animals. Several years ago, I was examining
one of the purple acontia of Adamsia palliata ; no pressure
had been used, but a considerable number of cnidw had been
spontaneously dislodged. It happened that I had just before
been looking at the-sucker-foot of an Asterina, which remained
still attached to the glass of the aquatic box, by means of its
terminal disk. ‘The cilia of the acontéwm had, in their rowing
action, brought it into contact with the sucker, round which it
then continued slowly to revolve. The result I presently dis-
covered to be, that a considerable number of the cnidw@ had
shot their ecthorea into the flesh of the sucking disk of the
Echinoderm, and were seen sticking all round its edge, the
wires (lassos) being embedded in its substance even up to the
very capsules, like so many pins stuck around a toilet pin-

cushion.

36 CORALS AND CORAL ISLANDS.

“To test this power of penetration still farther, as well as
to try whether it is brought into exercise on the contact of a
foreign body with the living Anemone, I instituted the follow-
ing experiment. With a razor I took shavings of the cuticle
from the callous part of my own foot. One of these shavings
I presented to the tentacles of a fully expanded Tealia crassi-
cornis ( Urticina crassicornis of Europe and America). After
contact, and momentary adhesion, I withdrew the cuticle, and
examined it under a power of 600 diameters. I found, as 1
had expected, enide standing up endwise, the wires in every
ease shot into the substance. They were not numerous—in a
space of .01 inch square, I counted about a dozen. * * *

“These examples prove that the slightest contact with the
proper organs of the Anemone is sufficient to provoke the dis-
charge of the cnid@; and that even the densest condition of
the human skin offers no impediment to the penetration of the
ecthorea.

“ As to the injection of a poison, it is indubitable that pain,
and in some cases death, ensues even to vertebrate animals
from momentary contact with the capsuliferous organs of the
Zodphyta. * * * I have elsewhere recorded an instance
in which a little fish, swimming about in health and vigor,
died in a few minutes with great agony through the momen-
tary contact of its lip with one of the emitted acontia of Sa-
gartia parasitica. It is worthy of observation, that, in this
case, the fish carried away a portion of the acontium sticking
to its lip; the force with which it adhered being so great, that
the integrity of the tissues yielded first. The acontium severed, —
rather than let go its hold.

‘* Now, in the experiments which I have detailed above, we
have seen that this adhesion is effected by the actual impene
tration of the foreign body by a multitude of the ecthorea,

ACTINL# AND OTHER ACTINOID POLYPS. 37

a

whose barbs resist withdrawal. So that we can with certainty
associate the sudden and violent death of the little fish with
the intromission of barbed ecthorea.”

The following observation Ly J. P. Couthouy, from the
author's Report on Zodphytes (p. 128), if it is beyond ques
tion, shows power even in the Actinia’s presence. “ Having a
number of Monodontas (a genus of univalve Mollusca allied
to our Trochi) too much crowded in a large jar of water. I
took out half-a-dozen, and placed them in a jar with an Ac-
tinia (Anthea flagellifera).- On looking at them about three
hours after, | found that, instead of climbing like the others
to the top of the water, they remained just where they had
fallen, closely withdrawn into their shells. Supposing them
to be dead, they were taken out, when they directly began to
emerge; and when returned to the jar with the other Mono
dontas, they were in less than five minutes clustered round
its mouth. On placing them again in the jar with the Ac-
tinia, though kept there for two hours, they did not once
show themselves out of the shell. Once more placing them
along with the other shells, they exhibited their former signs of
life and activity. The experiment was repeated several times
with a large Littorina, with the same result, evincing fear of
the Actinia on the part of the Mollusks.~

Gosse states the following fish story, which is much to the
point. Speaking of the Anthea cereus, or Opelet, a British
species, he says (p. 168): “I one day saw an amusing example
of its power of passive resistance. A beautiiul little speci-
men of the variety alabastrina, which had been sent to me
by Mr. Gatehouse, I had occasion to remove from one tank
to another. There was a hali-grown Bullhead (Cottus bubalis)
at the bottom, which had been in captivity rather more than

a fortnight. As he had not been fed during that time, I pre

38 CORALS AND CORAL ISLANDS,

sume he was somewhat sharp-set. He marked the Anthea
falling,
ern of a mouth and sucked in the bonne bouche. It was not

and before it could reach the bottom, opened his cay-

to his taste, however, for as instantly he shot it out again. Not
discouraged, he returned to the attack, and once more sucked
it in, but with no better success; for, after a moment’s rolling
of the morsel around his mouth, out it shot once more; and
now the Bullhead, acknowledging his master, turned tail, and
darted into a hole on the opposite side of the tank in manifest
discomfiture.”

He adds: ‘‘ But if you, my gentle reader, be disposed for
exploits in gastronomy, do not be alarmed at the Bullhead’s
failure: only take the precaution to “‘cook your hare.” Risso

calls this species ‘‘ edulis,”

and says of it,—‘t On le mange en
friture,” and I can say, “‘probatum est.” No squeamishness
of stomach prevents our volatile friends, the French, from
appreciating its excellence; for the dish called Aastegna,
which is a great favorite in Provence, is mainly prepared from
Anthea cereus. 1 would not dare to say that an Opelet is as
good as an Omelet; but chacun a son gout—try for your-
selves, ‘The dish is readily achieved.”

The stomach, although without a proper sphincter muscle
at its inner extremity, appears to be closed below during the
process of digestion. When digestion is complete, the refuse
from the food is pushed out through the mouth, the only ex-
ternal opening to the alimentary cavity, and the digested ma-
terial passes downward, into the interior cavity; and there,
mixed with sea-water from without, it is distributed through
all the interior cavities of the polyp for its nutrition, The
polyp has no circulating fluid but the results of digestion
mixed with salt water, no blood-vessels but the vacuities among

the tissues, and no passage-way for excrements excepting the

ACTINIG AND OTHER ACTINOID POLYPS. 39

mouth and the pores of the body that serve for the escape of
water on the contraction of the animal.

Actiniz have usually no gills or branchie for the aeration
of the blood, the whole surface of the body being ordinarily
sufficiently soft and delicate to serve in this function. Some spe-
cies live half buried in the sand, and, as this in large species
would prevent the skin of the sides from aiding in respiration,
there are sometimes very much lobed and crimpled organs,
attached to, or alongside of, the tentacles, which give the an-
imal-flower much greater beauty, and at the same time increase
the extent of surface for the purposes of eration; they are
set down as branchial by Prof. Verrill.

In one tribe of polyps closely related to the Actinic, the
Zoanthids, in which the outer skin is usually somewhat cor-
riaceous, or is filled with grains of sand, there are narrow gills
arranged vertically, one either side of the larger radiating sep-
ta, figures of which are given in the author’s Zodphyte Atlas.

As to senses, Actiniz, or the best of them, are not quite
as low as was once supposed. For, besides the general sense
of feeling, some of them have a series of eyes, placed like a neck-
lace around the body, just outside of the tentacles. The yel-
low prominences in this position on the larger figures in the
frontispiece are these eyes. They have crystalline lenses, and
a short optic nerve. Yet Actinie are not known to have a
proper nervous system: their optic nerves, where they exist,
are apparently isolated, and not connected with a nervous
ring such as exists in the higher Radiate animals.

Reproduction is carried forward both by ova and by buds,
though the latter method is mostly confined to the coral-mak:-
ing polyps.

The ovarian and spermatic functions belong to the radia-
ting septa in the interior cavity of the Actinia, and to the part

40 CORALS AND CORAL ISLANDS.

of a septum, mesenteric in character, at or near the outer mar-
gin. They have the aspect of a pulpy mass, or look like clus-
ters of ovules. The ova have no chance for escape except
through the stomach and mouth. They are covered with vi-
bratile cilia, and rove about free for a while. As the develop-
ment of the embryo goes forward, a depression begins at one
end, which deepens and becomes a stomach, with the entrance
to itas a mouth. Concurrently, septa grow out from the inner
wall, and a few tentacles commence to rise around the mouth.
Not unfrequently, the young has already some of its tentacles
before it leaves the parent. There is at first but a single row
of tentacles; the number increases with the size until the full
adult limit is reached, the newer series being successively the
outer.

In the budding process, which is of rare occurrence, Acti-
niz grow young ones on their sides near the margin of the
base. A protuberance begins to rise and soon shows a mouth,
and then becomes surrounded by tentacles; and, thus begun,
the new Actinia continues to grow, usually until its tentacles
have doubled their number, when finally it separates from the
parent, an independent animal. At times, as Prof. H. James
Clark has observed, small pieces of the base of an Actinia sep-
arate by a natural process before a trace of a tentacle has ap-
peared, and in this case ‘‘ they do not at first show any signs
of activity, but on the contrary, remain for a long time in a
quiet state, having the appearance of artificially separated
pieces, seeming to be undergoing, as in the latter, a recupera-
tive process after the shock of a separation.” After a while
they commence to develop and grow into perfect individuals.
Prof. Verrill mentions the case of an Actinia from Puget’s
Sound (the Hpzactis prolifera, V.) which had three rows of
young individuals attached to it around the middle of its

CORAL-MAKING ACTINOID POLYPS. 4]

body; but whether the young Actiniz were produced by bud-
ding from this part of the body, or whether they had colonized
there after being produced in the ordinary way, he was un-
able to determine. In all cases the young ultimately sepa-
rate from the parent.

These polyps have also the faculty of reproducing lost
parts; and to such an extent that a mere fragment, if it be
from the lower part and include a portion of the base, will re-
produce all the rest of the Actinia, even to the disk, tentacles
and stomach. Thus the mere forcible tearing of an Actinia
from the rock to which it is attached may result in starting a
crop of new Actiniz.

Although Actiniz have no internal coral secretions, they
sometimes make a thickened epidermic plate at the base, and
also in a few cases around a part of the body. ‘This is how-
ever not a result simply of an epidermic secretion, but arises
from an exudation of mucus from the surface, and the entan-
gling thereby of minute particles of foreign or dead matters.
A case of the kind, in an Edwardsia where the body is thus
encased, is mentioned and explained on page 25.

The above are the more prominent characters of the Actin-
ia tribe of polyps. The special features distinguishing them
from the coral-making polyps are the following: (1), They are
simple animals, or, if they bud, the buds early separate from
the parent ; (2), They have a muscular base ; (3), They are gen-
erally capable, more or less perfectly, of locomotion on the base
by means of its muscles ; (4), They sometimes possess rudimen-
tary eyes; (5), They have no internal coral secretions. Each
of these characters is evidence of the superior grade of this di-
vision of Polyps.

42 CORALS AND CORAL ISLANDS.

Il. CORAL-MAKING ACTINOID POLYPS,

Of the form, tentacles, mouth, stomach, fleshy septa, lasso.
cells, food, digestion and respiration of the coral-making polyps
here included, nothing need here be said, these characters being
the same as in the Actinie. Their more striking peculiarities
depend on the secretion of coral, making them fixed species,
and involving an absence of the base; and, in the case of the
majority of the species, on the extent to which they multiply
by buds, in imitation of species in the vegetable kingdom.

The coral skeleton which the secretions of polyps form is
called the corallum. 'These secretions take place among the tis-

CARYOPHYLLIA CYATHUS.

sues of the sides and lower part of the polyp, but never in the
disk or stomach, as this would interfere with the functions of
these organs. In the above sketches of a simple coral,
from the Mediterranean, the upper extremity is a depression,

or calicle, enclosed by a series of radiating calcareous (coral)

CORAL-MAKING POLYPS. 43

septa. Each of these septa is secreted between a pair of the
radiating fleshy partitions, or septa, of the polyp (see figure
p. 27); and thus the radiate structure of ordinary corals is
nothing but an expression of the internally radiate structure
of the polyp. When alive, the top, and usually the sides, of
the coral were concealed by the outer skin of the polyp, in-
cluding, above, the disk and tentacles; and into the depression
or calicle at top, descended the stomach.

Whether these radiating septa of the coral are secreted
from the surfaces of the fleshy septa, or from a prolongation
inward of the membrane forming the walls of the internal
cavity, has not been directly ascertained. The latter view is
sustained by Prof. Verrill, on the ground that the coral
septa contain fibres of animal tissue. The secretion does not
always commence at the central plane of a septum, for the
septa are sometimes hollow within, just as the surface spines
of some species (@. g., Hchinopora reflea) are hollow. The

THECOCYATHUS CYLINDRACEUS, Pout: ; FLABELLUM SPHENISCUS, 1Dy

exterior surface of the corallum, that is, the part outside of
the calicles, is often ribbed, and the ribs are ordinarily only
an outer extension of the interior septa; so that surface
spines are in fact but the outer margins of septa.

The first of the preceding figures exhibits another of the ,

forms of these simple corals. It is described by Pourtales

44 CORALS AND CORAL ISLANDS.

from specimens collected by him at a depth of 100 to 200
fathoms off the Florida reef. The actual size was one-third
that of the figure. The second figure represents a living
species.

The bottom of the calicle, or polyp-cell, in the corallum is
sometimes made simply by the meeting of the radiating sep-
ta; occasionally by the same, with the addition of a point
or columella at the centre; often by a twisting together of
this part of the radiated septa. Very often, also, it is a mere
porous mass. Sometimes there is a circle of prominent points
about the centre, as seen in the figure of a Caryophyllia on
page 42, which are the extremities of narrow vertical strips
(called palz) lying in the planes of the septa. Similar points
exist in the Thecocyathus on the preceding page, though not
in sight in the figure.

in many cases the bottom is quite solid; and this may be
so either (1), because the coral secretions fill up all the pores
as the polyp increases in age, and thus make the inte-
rior of the corallum solid or nearly so; or (2), because there are
formed periodically, as the polyp grows upward, solid horizon-
tal plates across the bottom, so that beneath, in the interior of
the corallum, there is a series of plates or tables with spaces
between. The Pocilloporze, among recent corals (p. 70), and the
Favosites among ancient, are examples. Increasing solidity
with the increasing age of the polyps is also produced at
times by additions to the exterior ef a corallum. In many
species, the skin, over part or all of the exterior, gradually
disappears or dies away and leaves the corallum bare, while
all is living within; and, in such cases, the skin, before disap
pearing, often adds a layer of stony material to the exterior,
viving greater firmness to the whole. An example is shown

in the figure on p. 42. In such a case, there is no skin or

CORAL-MAKING POLYPS. 45

aninal tissue over the outside of the corallum, excepting at
its upper extremity, above this calcareous coating,

Another form of a corallum, the secretion of a single
polyp, is illustrated in the following figure of a species of the
Fungia family, so-called in allusion to a resemblance to the
mushroom. ‘The long mouth occupied a considerable part of

the longitudinal central line. From the line at the centre,

CTENACTIS ECHINATA, AG.

there is the same radiated arrangement of calcareous septa
as in the preceding species, though the animal differs greatly
in its extreme shortness in proportion to the breadth. The
corals of this group are also peculiar in having the radiated
upper surface flat, or nearly so, instead of concave. The fig-
ure is a fourth the natural size. These corals, of the genus
Fungia, often exceed a foot in length; and thus coral animals
are sometimes as large as the largest of Actinie.

Another species of this genus, the Hungia lacera V. (for-
merly Pungia echinata D., trom the Feejees), is represented as it
appears when living (excepting a part left off to suit the page)
in the following figure. The coral in the perfect state of the an-
imal, is wholly concealed, though often showing the points of the
teeth of the septa in consequence of the skin being broken.

46 CORALS AND CORAL ISLANDS.

An enlarged view of one of the tentacles is given on the

FUNGIA LACERA, D: (V.)

opposite page. They are very small, compared with the size of

CORAL-MAKING POLYPS. 47

the polyp; and this is true of all the living Fungie studied
by the author. It is plain that the power of such tentacles
must reside wholly in their lasso-cells.

~ vy 2H pag vi t
“ol WE wy’ hi Wi WY a4

aga Lt

TENTACLE OF FUNGIA LACERA, ENLARGED.

The tentacles are scattered over the disk. instead of being
in regular circles. Nevertheless, there is a regular order of
development, as stated on page 27; a fact which shows that
the apparent irregularity is a consequence of the unusual
size of the polyp and the consequent larger number of the
radiating lamellae and polyps.

The Fungie, unlike most corals, are not fixed animals
except in the young state. They are common in coral-reef
seas, lying over the sandy or rocky bottom between the other
corals.

Other varieties of corals and coral animals are illustrated
in the figures on the following pages. They represent com-

pound groups, in which great numbers of polyps are con-

nected in a single zobphyte—a result, in part, of the process
of budding already alluded to, and partly of different modes
of growth connected therewith.

This budding is very similar to the budding process in
vegetation. One common method is the same that is occa-
sionally met with in Actinixw, the description of which is

briefly given on page 40. The bud commences as a slight

48 CORALS AND CORAL ISLANDS.

prominence on the side of the parent. ‘The prominence en-
larges, a mouth opens, a circle of tentacles grows out around
it, and increase continues till the young finally equals the
parent in size. Since in these species the young does not
separate from the parent, this budding produces a compound
group; and the process often continues until in some instances
thousands, or hundreds of thousands, have proceeded from a
single germ, and the colony has increased to a large size,
sometimes many feet, or even yards, in breadth or height
Such is the species of Dendrophyllia represented in the fig-
ure on page 51, and the Madrepora figured on page 50; in
both of which, and in all such coral zodphytes, each stellate
cavity or prominence over the surface corresponds to a sepa-
rate one of the united polyps.

The compound mass produced by budding—which con-
sists of the united polyps with the corallum.as their united
secretion—was called in the Author’s Report, a Zodphyte, it
being truly animal in nature, though under a plant-like form
through the plant-like process of budding. But the word to
many minds conveys the idea that the species is something
between a plant and an animal, which is totally false; and
besides, it is often used distinctively for the division of ani-
mals including the sponges. As a substitute the term Zod-
thome may be employed, derived from the Greek gwov, ani-
mal, and Owuoc, a heap—a term applicable also to compound
groups in other classes, as, for example, those of Rhizopods,
Bryozoans and Ascidians. The term zoOphyte, where employ-
ed beyond, signifies a zoéthome formed of united polyps, or a
polyp-zoothome. The coral of the zodthome being the coral-
lwm, that of each polyp in the compound corallum may be
called a corallet—the term calicle, formerly used by the

author for the same, being now restricted to the polyp-cell.

CORAL-MAKING POLYPS. 49

It is obvious that the connection of the polyps in all com-
pound groups must be of the most intimate kind. The sever-
al polyps have separate mouths and tentacles, and separate
stomachs; but beyond this there is no individual property.
They coalesce, or are one, by intervening tissues; and there is
a free circulation of fluids through the many pores or lacunes.
The zodthome is like a living sheet of animal matter, fed and
nourished by numerous mouths and as many stomachs.

Polyps thus clustered, constitute the greater part of the
flowering zodphytes of coral reefs. Only a few are simple
animals, like the Caryophyllia figured on page 42, or the
Thecocyathus, page 43, or the Fungia, page 46.

This kind of budding may take place from the sides of the
polyp at different heights ; either (1), from the base, as in the
Actinia mentioned on page 40, when it is basal ; or (2), above
the base, when it is called lateral; or (3), at the upper mar-
gin outside of the tentacles, when it is called marginal or sw
perior ; or (4), from the disk inside of the tentacles.

Sometimes a shoot grows out from one point only of the
base of a polyp, like the stoloniferous stem from a strawberry
plant, and at short intervals gives off buds; and thus makes a
linear zo6phyte with a row above of flower-animals. In other
cases, the base spreads in all directions and buds at the edge,
or in the upper surface near the edge, and so makes an in-
crusting plate, consisting of a multitude of polyps.

If the germ polyp, or that from which the compound zo6-
phyte proceeds, has the property of growing upward beyond
the adult height—which the existence of coral renders a possi-
bility, and even to an indefinite degree—various other forms
may result.

Sometimes the first polyp gives out buds from its sides,
and continues so to do while it grows upward; and thus a

4

50 CORALS AND CORAL ISLANDS.

rising stem is formed with one parent polyp at the extremity
of the stem, and a terminal corallet to the corallum, or to each
branch of it. This is the case in the genus Madrepora, a

MADREPORA ASPERA, D.

species of which is here represented. Each branch in the
living state had at its extremity the parent polyp of the
branch, or that whose budding made the other polyps of the
branch. In such species, a new lateral branch is commenced
by one, among the many polyps over the surface of a branch,
beginning to grow and bud. Thus branch after branch is
added, and the little tree produced.

Another kind of coral, growing and budding in the same
manner, is represented on page 51. It is a species of Dendro-
phyllia, from the Feejees—a genus often presenting tree-like
forms, as the name implies.

In other cases, budding goes on until a cluster of some size

VORAL-MAKING POLYPS. Dil

is formed, and then the older or marginal polyps of the cluster

DENDROPHYLLIA NIGRESCENS, D.

cease budding while the rest continue the process ; in this way

52 CORALS AND CORAL ISLANDS.

a stem rises, with the budding cluster of polyps at its summit,
and the more aged, or non-budding polyps, about its sides ; and
the breadth of the stem depends on the size of the budding

MS Me y
wh NH H
_ Ill I

=
i i ——

IS! ih
Deh

ult
i ]

GONIOPORA COLUMNA, D.

cluster. Above a case of this kind is represented, in which

the stem is a large column. .
The polyps, in this beautiful Pacific species, as seen, stand

up prominently over the coral when expanded, which is due

CORAL-MAKING ACTINOID POLYPS. 53

to the fact that only the lower parts of the polyp secrete coral,
as a moment's consideration will make apparent.

In other cases, the budding cluster is small, and hence

PORITES MORDAX, D.

makes small branches, as in the annexed figure of a species of
Porites, from the Feejees. The cells in this genus are very

small and nearly or quite superficial, as the figure shows.

54 CORALS AND CORAL ISLANDS.

4

New branches are made in such species by a forking of an
old one, The budding cluster enlarges as it grows, and, when
it is just beginning to pass the regular or normal size for the
species, a subdivision of the budding cluster commences at the
extremity of the branch. It is a process of spontaneous fis-
sion of a branch or stem. In this way the forking in the coral
of the figure on page 52 was produced, and also the branching
in that on page 53.

Sometimes, again, the budding cluster is a linear series ;
and then a coral with erect, flattened or lamellar branches is
made.

Again, sometimes each branch of the corallum is only
the corallet of a single polyp ; and new branches are added by
the budding of new polyps trom its sides, each to lengthen out

into a new branchlet. In this manner the coral here figured,

CLADOCORA ARBUSCULA, LEs.

a common species of the West Indies, and also that of fig. 1 on
Plate LV., a Caulastrea, from the Feejee group, were formed.
When the budding is not confined to any particular polyp,
or cluster of polyps, but takes place universally through the
growing mass, the coral formed is more or less nearly hemi-

spherical; and often the process goes on with such extreme

PLATE IV

ss

Caulastraea furcata; 2. Tridacophyllia pzonia;
3. Mezandrina tenuis.

CORAL-MAKING ACTINOID POLYPS. 55

regularity that these hemispheres are perfectly symmetrical,
even when enlarged to a diameter of ten or fifteen feet. A
portion of the surface of one of these massive species, called
Orbicella cavernosa, from the West Indies, is represented in
the annexed figure. In the growth of these hemispheres, the
enlargement takes place in the spaces between the polyps; and

ORBICELLA CAVERNOSA.

whenever these spaces begin to exceed the width usual to the
species, a new mouth opens, commencing a new polyp; and
thus the growth of the mass involves multiplication by buds.
The small calicle near the centre of the figure is from one of
the new interstitial buds.

Species of Porites also grow into hemispheres and rude hil-
lock-like forms, through the same method of budding, and
some of the masses in the tropical Pacific have a diameter of
even twenty feet. Myriads of living polyps are combined in a
single such mass, for each is but a fifteenth or a twentieth of an
inch in diameter.

Often there is a lateral growth of the polyp and thereby of
the zodphyte without much upward growth; and spreading
leaves are thus made, and bowl-like shapes. Where there is
lateral budding, the leaves have generally an edge of young
polyps from the new buds that are there opening, as in the

56 CORALS AND CORAL ISLANDS.

Gemmipores, and some foliaceous Madrepores. Where there is
superior budding, and sometimes in the case of inferior, the
new polyps appear some distance from the edge, the growing
margin spreading on in advance of the buds that open in it,
as in the Echinopores, the Phyllastreea represented on Plate I.
(frontispiece), and also in the Merulina of Plate VI., figure 1

(facing page 82).

Besides the method of budding explained in the above re-
marks, there is also a kind of superior budding called sponta-
neous fission, which consists in a spontaneous subdivision of a
polyp, by which two are made out of one. In such cases the
disk of the polyp has not a distinct limit of growth, as in the
above, but tends to enlarge indefinitely ; and when there isa
beginning of an increase beyond the proper adult size, a new
mouth opens in the disk, a short distance from the old one, and
at the same time its edges extend downward and make a new
stomach beneath it; finally tentacles are developed between
the two mouths, and then each polyp separates with its part of
the old tentacles as illustrated in the following figure. It is not,

SPONTANEOUS FISSION IN POLYPS.

as is seen, a subdivision strictly into halves, as one carries off
the old mouth and stomach. ‘The figure to the left represents
a polyp of the Astra tribe, with already two mouths, through
a commencement of the process of subdivision. In the next

figure there are tentacles between the two mouths, so that each

CORAL-MAKING AUTINOID POLYPS. ie

mouth has its own circle; and in tlie third, the separation has
gone so far as to complete the circles and make two independ-
ent polyps. This dividing one’s self in two, for the sake of an
increase of population, is the process called spontaneous fis-
sion or fissiparity.

This mode of budding does not belong exclusively to coral
polyps, for it has been observed among a few Actiniz. Grosse
describes its occurrence in a British species, the Anthea cereus,
in which it results in two distinct animals. He says “the
fission begins at the margin of the disk, and gradually extends
downward until the separation is complete, when each moiety
soon closes and forms a perfect animal.” The same author al-
ludes to the occurrence of double-disked individuals of the gen-
era Actinoloba, and Actinia as illustrating the process without
a separation of the spontaneously developed pair.

_ This spontaneous fission is the common kind of budding
in the large Astrzea tribe.

ASTRAA PALLIDA, D.

The preceding figure represents a species of living coral of
the Astrea family, from the Feejees, the Astrea pallida D.

58 CORALS AND CORAL ISLANDS.

which grew, and multiplied its polyps as it grew, by this meth-
od. In such species some of the disks of the polyps will be
found to have two mouths. This is the first step in the pro-
cess In others, the two mouths will be found to be partly
divided from one another by new-tormed tentacles ; and finally
each will have its own circle complete and all else in polyp
perfection.

Many of the Astrea hemispheres of the Pacific, grown by
this method, have a diameter of ten to fifteen feet.

In other Astrzea-like species, this spontaneous fission ends
in a complete separation of the two polyps formed ; and conse-
quently in a forking of an old branch. The figure of a Cau-
lastreea, on Plate IV. (figure 1), illustrates this mode of
branching. In the left hand polyp there are already two
mouths, and the work of subdivision is consequently begun ;
while in those to the right, which have a single mouth, the
subdivision has just been completed, and also the forking of
the old branch. Thus spontaneous fission goes forward, and
branches accordingly multiply. By this method some of the
most magnificent clumps of coral zodphytes found in tropical
seas have been, and are being, developed each from a single
germ. Many of them have the perfect hemispherical sym-
metry of the solid Astraas.

Sometimes, when a new mouth forms in an enlarging disk,
there is not at once a separation of the two, but the disk con-
tinues to enlarge in one direction and another, and then an-
other mouth opens, and so on until a string of mouths exists
in one elongated disk; and finally, a separation occurs, but
only to commence or carry forward another long series. In
this way the corals with meeandrine furrows are made, some
kinds of which are popularly called Brain coral, and pertain
to the Meeandrina family (figure on page 65). The same may

CORAL-MAKING ACTINOID POLYPS. 59

take place in the ramose corals, and so make flat branches,
each with a long sinuous line of polyp mouths at top. In
all such species the tentacles stand in a line either side of
the line of mouths as represented in figure 3 on Plate IV.,
between pages 54 and 55.

By the simple methods here explained all of the various
forms of Actinoid zodphytes have been produced ; and, equally
so, those of the Aleyonoids described beyond. The tree, shrub,
clusters of coral leaves, hemispheres, and coral net-work re-
quire for the explanation of their origin only the few princi-
ples which have been mentioned. The germ-polyp, growing
upward and more or less outward, and budding as it grows,
makes thus the rismg stem —that of the Madrepore or Den-
drophyllia, with its summit polyp (figures p. 50, 51), or that of
the Porites, with its terminal budding clusters (p. 53); or the
rising, massive dome of the Astrea and Meeandrina (pp. 57,
65), in case budding is symmetrical in all directions; or, if
growth in the germ-polyp is upward exclusively, it forms a ris-
ing stem bearing at top the single polyp that originated it, or
crowded clusters of such stems branching variously and having
each branch surmounted with its one polyp (figure p. 54); or,
if there is lateral growth and but little of upward, it produces
leaf-like forms and graceful groups or clusters of leaves, vases,
and other shapes; or, if the germ-polyp is capable of lateral
growth alone, the results are simple lines of polyps creep-
ing over the supporting rock, like the creeping stolons of
a plant, or else encrusting plates, spreading outward like a
lichen.

In the descriptions of corals the following terms have the
significations annexed. Those already mentioned are here
repeated to bring them all together.

60 . CORALS AND CORAL ISLANDS.

Zovthome. — The compound animal mass produced by budding.

Corallum. — The coral either of the compound mass, or of the solitary
polyp.

Corallet (In Latin, corallulum). — The coral of a single polyp in a com-
pound corallum.

Calicle. —The polyp cell in the top of a corallet, or of a solitary
corallum, within the walls of the cells; it is sometimes flat at top, that is,
without the usual depression.

Septa. — The radiated plates of the cell or calicle.

Dissepiments. — Small cross plates between adjoining septa, approxi-
mately horizontal; sometimes wanting.

Synapticule.— Calcareous bars extending across the interseptal spaces,
or loculi, and so uniting the surfaces of adjoining septa.

Tabule. — Horizontal plates dividing the interior of a cell into a
series of chambers, as in the ancient Favosites, and in the Pocilliporee.

Tabulate. — Waving tabule. The Tabulate include the Favosites and
some other ancient corals.

Columella. — The prominent axis of a corallet projecting at the centre
of @ calicle. It is generally absent.

Coste. — The vertical ridges over the exterior of some corals; they
usually correspond to the sept, and are an external extension of them;
but in other cases they are opposite the intermediate chambers, or are
interseptal Joculi, as they are often called.

Ceenenchyma.—'The common mass of the corallutm between its dif-
ferent polyp cells or corallets, as in the Madrepore, Gemmipore, and
Dendrophylliz.

Epitheca.— The coral layer sometimes deposited over the exterior of

the corallum during the life of the polyp by the outer skin before it dries,

away, as explained on page 44.

Peritheca. — The epitheca of a compound group or zoothome (fig. p. 71).

Exotheca.— The portion of the corallum outside of the walls of cells
in many coralla of the Astreea family, and some others, in which the polyps
of the mass are properly in contact, and there is consequently no true
coenenchyma.

Endotheca. —'The portion of the corallum inside of the walls of the
cell.

We may now state briefly the characteristics of the
grander divisions of the Actinoid polyps, several of which
have been illustrated in the preceding figures.

SUBDIVISIONS OF ACTINOID POLYPS. 6]

The tribes adopted are those recognized by Prof. Vegrill,
and have the limits he has assigned tothem. The classification
diverges from his system in uniting the non-coral-making
and coral-making species into one grand division, that of the
Actinoids (on the ground of the close resemblance of the
polyps), and also in separating from the latter the Cyatho-
phylloid corals, for the reasons mentioned on page 21. Some
of the figures of corals on former pages are here repeated in
order to present together those of like relations.

1. Species without internal Coral Secretions. AcrinarRia of
Verrill.

1. The Actinza tribe, or ActTINACEA, secrete no coral inter-
nally, and moreover have a muscular base, with some degree
of locomotion by means of it. The Actinie of the frontis-
piece, and of pages 23, 24, 26, are examples.

2. The Zoanthus tribe, or Zoantuacea. The species here
included are like the Actiniz in secreting nocoral. But while
they have a base, it is not muscular, and they are never capable
of locomotion. The polyps have a thick or somewhat leath-
ery exterior, and, as already observed (p. 39), have gills, or
branchiz. Some of the species are solitary polyps; but
generally they form compound masses or zodthomes,
by budding; sometimes making simple lines of polyps
over a supporting surface; at other times incrusting plates,
or irregular masses. The following figure (from Verrill)
represents a species found in American seas off the coast of
New Jersey, in deep water, and also in Massachusetts Bay,
which has a habit of fixing ona shell for its support and of
always taking one containing a soldier crab. The shell
finally becomes dissolved away—how, it is not known, by

62 CORALS AND CORAL ISLANDS.

the growing Zoanthid; but the crab holds on to its house
although at the expense of transporting wherever it goes a

EPIZOANTHUS AMERICANUS, V., WITH EUPAGURUS PUBESCENS, ST.

colony of flowering polyps. The polyps are but partly ex-
panded in figure 1, and wholly so in figure 2.

The animals of the Zoanthus tribe have broad, radiated
disks, with an edging of short tentacles, in one or more
rows. Although not secreting coral, the mucus of the sur-
face in some of the species entangles the sand that falls
on it, and thus gives a degree of firmness to tie mass of the
zoophyte.

3. The Antipathus tribe, or Antrpatuacea. In this tribe
the polyps never have locomotion, and, so far as known, al-
ways produce compound groups by budding. These groups
have the forms of delicate shrubs and long twigs ; and some of
them are three feet or more in height. The branches consist of
a horny axis, usually spiny or hispid over its surface, surrounded
by an animal coating, which is made up of united polyps. An
example is shown in the following figure of a living species
from the Feejees. A view of one of the polyps, much enlarged, is
given in the following figure. Its tentacles are closely like
those of the Actinia. The height of the entire shrub, collected
by the author, was three feet, and the trunk at base was
half an inch thick. The polyps had a brownish-yellow color,
not particularly beautiful, and the tentacles were in general, as

SUBDIVISIONS OF ACTINOID POLYPS. 63

in another species described by the author, rather awkwardly

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ANTIPATHES ARBOREA, 1D}:

The number is commonly sza ; but

handled by the polyp.

in one genus, Gerardia, it is as great as twenty-four.

POLYP OF A. ARBOREA, MUCH ENLARGED,

64 CORALS AND CORAL ISLANDS.

2. Polyps having internal calcareous secretions. MApDREPO-
RARIA Of VERRILL (the Cyathophylloid species excluded).
HEXACORALLA of some authors.

4. Astrea tribe, or ASrraacEa.—In this tribe the polyp-
cells or calicles are distinctly lamello-radiate within, and gen-
erally so outside. Moreover, budding is always by division
of the disks, or spontaneous fission. The figure of the Cau-
lastreea, on page 58, illustrates one section of this family, that
in which each branch of the corallum is made by a single
polyp, and branching is by furcation through spontaneous fis-
sion. In other related genera, as Mussa, the polyps sometimes
have a diameter of two inclies, being as large as ordinary
Actinie.

ASTRA PALLIDA, D.

The Ast7@a pallida is a good representative of the massive
Astreas. The color of the polyps in this species is quite pale,
the disks being bluish-gray, and the tentacles whitish. In oth-
ers, the tentacles are emerald-green, or deep purple, or of other

shades.

SUBDIVISIONS OF ACTINOID POLYPS. 65

Another range of forms is represented by the following
figure of one of the Meandrine corals, already reterred to as
often called ‘‘ Brain coral.” In the figure, the coral is re-
duced one-half lineally. The difference between its mode of
formation and that of an Astraea, has been stated on page 59.
This species is common at the Bermudas, where it grows to a
diameter of three feet. It is also found in the West. Indies. The
ridges in this species are double, and hence the name Diplorza,

DIPLORIA CEREBRIFORMIS, E. anp H. &K}

from the Greek for double. A view of part of a living speci-
men of a related species is shown on Plate IV. A common
large West India species of Brain coral is called Meandrina
labyrinthica.

Still other forms of the Astrzea tribe are foliaceous, or such
as would result if the growing margin of an Astreea, or of a
Meeandrina, were to spread out into folia instead of thickening

upward in the ordinary way. The groups of gracefully cury-

66 CORALS AND CORAL ISLANDS.

ing leaves are sometimes very large and symmetrical, as in
the Tridacophyllia, Plate IV., and Merulina, Plate VI.

2. Fungia tribe, or Functacea.— The general character
of the simple species of this tribe is mentioned on page 45, and
the character of the living Fungia, with its tentacles, is shown
in the figure of a Feejee species on page 46. Large, com-
pound groups, both massive and foliaceous, are formed by
budding, and the budding is always superior. There are
no margins to the disk in this tribe, and in the corallum
of the compound kinds no wall or partition between the ad-
jacent stars, and no walls to adjoining polyps, or only im-
perfect ones. “The polyps consequently coalesce throughout
by their disks. The simple Fungi are attached when young,

FUNGIA DANA, BE. & H., REDUCED TO ONE-SIXTH LINEALLY; @,), TEETH OF UPPER
AND LOWER MARGINS OF SEPTUM, NATURAL SIZE.

and then would hardly be distinguished from a simple or
solitary species of the Astrea tribe.

3. Oculina tribe, or Ocutrnacra.—These species occur
either simple or compound, and the latter are often branched,
massive, or encrusting, never thin, foliaceous. Budding is
either superior, lateral, or basal ; never by spontaneous fission.
The coralla are remarkable for the solid walls and lamelle

of the cells; and often for having the ceenenchyma nearly or

SUBDIVISIONS OF ACTINOID POLYPS. 67

quite solid. ‘Transverse septa between the lamellz are some-
times wanting. ‘The calicles are usually striated externally
but seldom derttate. The polyps, moreover, are small; and
very commonly they stand prominent above the corallum when
expanded. ‘The Orbicella, figured on page 55, is an example
of one of the massive Astrea-like forms, constituting the Or-
bicella family, or Orbicellide, in the Oculina tribe.

The Caryophyllia here figured is one of the solitary species

CARYOPHYLLIA SMITHII, STOKES.

of the tribe found in European Seas, and on the coast of
Great Britain. The figure is from Gosse’s British Actinology.
It also grows much longer in proportion to the breadth. The
figure to the right is of one unexpanded. One of its lasso-
cells, in different states, is shown in figures 3, 4, 5, on page 31.

The corallum of a related species, Caryophyllia cyathus,
is given on page 42. The walls and septa are remarkably
solid. The Caryophyllia flavus has been found not only in
the Mediterranean, but also as far north as the British Isles,
and in the Florida Straits.

Another example of this tribe, as defined by Prof. Verrill,
is the species of Astrangia occurring alive along the southern
shores of New England, and on the coast of New Jersey.
Specimens are not uncommon in the vicinity of New Haven,
on the rocks by the Light-House, and at other places in Long
Island Sound, and when alive it is an exceedingly beautiful

68 CORALS AND CORAL ISLANDS.

object. The accompanying figures of the animal are from the
drawings made to illustrate a yet unpublished memoir by
Prot. Agassiz. They are copied from the “Sea-Side Studies ”
of Mrs. Agassiz and Alexander Agassiz. In fig. c, the polyps
are of the natural size, while fig. @ represents one of them en-
larged. The polyps, as is observed, stand very prominent
above the cells of the corallum, because only the bases of
them secrete coral; and the buds, which open between the
ealicles, are hence lateral buds; the coral has much resem-

blance to that of an Orbicella, in which budding is margin-

c

~ WW (i
\ AN
pW

ASTRANGIA DANA, AG.

al. The tentacles have minute warty prominences over
them, which are full of lasso-cells. each about a 500th of an
inch in length, or about two-thirds larger than those of the
white cords that edge the internal septa. ‘The corallum,
though massive, is somewhat irregularly lobed above, and
grows to a diameter of two or three inches. It is covered
with stars an eighth of an inch to a sixth across (fig. 6), which
are usually crowded together, the intervening wall being very
thin and solid. ‘The author alluded to the crowd of stars in
the name Pleiadia, which he proposed for the genus in his
Report on Zocphytes (p. 722).

The genus Cladocora, containing slenderly branching ra-

SUBDIVISIONS OF ACTINOID POLYPS. 69

mose zodphytes, is closely related in its polyps, according to
Prof. Verrill, to the Astrangiw, and belongs to that family
Its cylindrical stems are gathered into crowded clumps. The
C. arbuscula is figured on page 54.

SAIN
«

AWS

PHYLLANGIA AMERICANA, E. & H.

A West India species of another genus of the group, the
Phyllangia Americana, is represented in the annexed figure.
In the following cut, figure 1 represents the extremity of a
branch of an Oculina, the O. varicosa, of the family Oculinide.

CORALS OF THE OCULINA TRIBE.

The species of this genus grow in clumps of round branches,

and have very solid coralla, so white and firm when bleached

70 CORALS AND CORAL ISLANDS.

as to go by the popular name of ‘‘ white coral,” and to be some-
times polished for beads and other such ornamental purposes.

Figure 2 is a branch of a beautiful little coral called Sty-
laster erubescens Pourt., and 3, a portion of the same enlarged.
It has the firmness, and something of the habit of an Oculina,
but is rather like a miniature Oculina, its calicles never exceed-
ing a twentieth of an inch in breadth. The Stylasteride
have been shown, however, to be Hydrocorailine, like the
Millepores (page 103).

Figure 4, in the same cut, represents a portion of a branch of
the Stylophora Danw KE. und H. The corals of the genus are
remarkable for their small, crowded calicles, and for the very
distinct six-rayed star in each calicle (as shown magnified in
figure 5), and usually have a prominent point or columella at
the centre of the star. The polyp of a Feejee species, S.
mordax, is represented in figure 6. The name of the family,

tylophoride (signifying style-bearer), alludes to this colu-
mella. The corals grow in regular hemispherical clumps con-
sisting of flattened or rounded branches, and are sometimes a
foot or more across.

In another family under this tribe, the Pociliporide, very
common in coral-reef seas, the cells of the corallum are always
very small and crowded, as shown in figure 7. The corals are
branching, and in Pocillipora, the surface 1s often irregular and
warty, the little prominences, like the rest, being covered with
polyp cells; while in Seriatopora, the branches are slender,
even, and pointed. The corallum in both is very firm and sol-
id. In the larger part of them the number of tentacles is only
twelve, and formerly they were referred on this account to the
Madrepore tribe; a few have as many as twenty-four tenta-

cles.
The Pocilliporze form hemispherical clumps like the Stylo-

SUBDIVISIONS OF ACTINOID POLYPS. ral

phore; and the branches vary from the flattened and broad
form shown in figure 7 (which represents the upper part of
a branch of the P. grandis D.), to irregularly cylindrical
branches, looking rough on account of the very short branch-
lets. The cells are usually stellate, as in figure 8, from ge
elongata D., and often one of the septa, and sometimes two
opposite ones, extend to a columella at the centre, as illustra-
ted in figure 9, from P. plicata D.; dividing the cell into
halves. The cell in the interior of the corallum is crossed by
thin plates or tables, as shown in figure 10, and hence they
have been called tabulate corals. Agassiz, after the discovery
of the Hydroid character of the animals of the Millepore corals,
whose cells also are tabulate, referred the Pocillipore to the
same Hydroid type. But the recent study of the polyps has
shown that they are true polyps; and Prof. Verrill remarks
on the resemblance of the tentacles to those of the Oculine.
‘The stellate character of the calicle also proves that the ani-
mals must be polyps. ;

Madrepore tribe, or Mapreporacea.—In this tribe the cor-
alla, even to the walls of the corallets, are remarkable for be-
ing porous, and the radiating lamelle of the polyp-cells are
narrow, often perforated or imperfectly developed, and fre-
quently mere points. The coralla are either branched, mas-
sive, or foliaceous. Budding is lateral, and in the branching
species there is either a parent polyp, as in Madrepora and
Dendrophyllia, or a terminal budding cluster. This peculiar-
ity has been already illustrated in the figure of Madrepora
aspera,on page 50. On the following page there is an out-
line sketch of another species, the Madrepora formosa D.,
common in the Feejees, and also in the Kast Indies. The two
species here mentioned give a good idea of the ordinary char-
acter of the Madrepore corals. One of the polyps of the Mad.

ie CORALS AND CORAL ISLANDS.

repora cribripora D., a species collected in the Feejees, is rep.
resented much enlarged in the accompanying figure. The nat-

POLYP OF M. CRIBRIPORA, D.

ural size of the expanded polyp in this genus is generally
from an eighth to a twelfth of an inch across the star. The
disk of the polyp is quite small, and the number of tentacles is
always twelve. The most common color of the polyps is green,
while that of the general surface between is ordinarily a pale
or a dark umber. In many species of Madrepora the branch-
es spread out laterally from a central or lateral trunk, and co-
alesce together into a complete net-work, having the form of a
shallow vase; and the interior of the vase is filled with multi-
tudes of short, cylindrical coral stems, rising from the reticula-
ting branches, which, when alive, have literally the aspect of
sprigs of flowers in the vase.

In certain kinds, closely related to Madreporie, the calicles
are reduced to points, or spiniform or angular prominences, or
fail altogether, and there are sometimes rounded promi-
nences between the cells; these degraded Madrepores belong
to the genus Montipora (Manopora of the Author’s Report).

The genus Dendrophyllia is also referred to the Madre-
pore tribe. The budding, as already explained, is of the
same kind as in the Madrepores. But the tentacles exceed
twelve. One of the polyps of D. nigrescens D., enlarged, is
shown in the figure, on page 75. This Pacific species grows to
a height of at least three feet, and is peculiar in having a very

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SUBDIVISIONS OF ACTINOID POL YPS. 75

dark blackish green or almost black color, while the polyps
have the tentacles nearly colorless, and the disk has a circle
of emerald green around the mouth. Dendrophyllia arborea is
the name of a common species of this genus found in deep

POLYP OF DENDROPHYLLIA NIGRESCENS.

water in the Mediterranean ; it is equally large with the pre-
ceding, and somewhat similar in its mode of branching, but
a little stouter. It has also been found in the Atlantic about
the Azores. Another common Mediterranean species is
the D. cormgera. It is sparingly branched, and has very
long and stout corallets, sometimes as long and large as the
finger.

The genus Gemmipora contains porous corals, of foliaceous,
bowl-like, and massive forms, covered by prominent cylindrical,
porous calicles, and having many short tentacles to the polyps,
usually in a single circle.

Here belongs also the large Porites family (Poritidz), the
corals of which are very porous, and sometimes almost spongy,
and whose polyp-cells are exceedingly shallow, and usually only
imperfectly radiated.

One of the genera in this family is Alveopora. It con-
tains the lightest of known corals, the texture being exceeding-

76 CORALS AND CORAL ISLANDS.

ly porous, and the walls of the cells, which are continned reg-

Peay
RW).

DENDROPHYLLIA NIGRESCENS, D.

ularly through the corallum, are like delicate lace-work. As

stated long since by the author, “they are intermediate in char-

SUBDIVISIONS OF ACTINOID POLYPS. oe

acter between the Montipore and the Favosites group ”—as
shown by the texture and the horizontal partitions across the

ALVEOPORA VERRILLIANA, D.

cells, giving them the “tabulate” character of the ancient

Favosites, as represented by the author in the annexed figure

VERTICAL SECTION OF CORALLUM, AND UPPER VIEW OF CALICLES, ENLARGED, OF
ALVEOPORA SPONGIOSA, D.

exhibiting a section of the corallum of a Feejee species. On
account of this tabulate structure, the genus was referred by
the author to the Favosites family. A related species, of un-
known locality, has been made the type of a new genus, called
Favositipora, by Mr. W. S. Kent, on the ground of its tabu-
late character (Ann. Mag. Nat. Hist., 1870), thus confirming,

though overlooking, the author’s conclusions.

78 CORALS AND CORAL ISLANDS.

In the genus Porites, the corals are frequently branching, as.

in the Porites mordax D., sometimes more slenderly, but oftener
less so, and at times massive and monticulose in form. An-
other species of Porites is represented on the following page,
with one of the branches fully expanded, but the others in
outline; a polyp, much enlarged, having twelve tentacles as

POLYP OF PORITES LEVIS.

in the Madrepore, is shown in the following figure. |The cells
of the corallum are superficial, and hence the name of the
species, Porites levis.

Another form, different in the size and character of its
polyps, is exemplified in the genus Goniopora. In the species
figured on p. 52, the color of the projecting polyps was lilac
or pale purple, and the number of tentacles eighteen to twenty-
four, yet all were in a single series. The columns grow to a
height of two feet or more, with only the summits for two or
three inches alive. The dead portion is usually encrusted with
nullipores, sponges, serpule and various shells, which protect
the very porous corallum within from wear and solution by
the moving waters.

3. CYATHOPHYLLOIDS, 07 RuGosa. TETRACORALLA.

It is not necessary to dwell here at length upon the an-
cient Cyathophylloids. The corals have a close resemblance
to those of the Astreea tribe in general aspect, varieties of form,
and range of size; the methods of multiplication by buds were
the same that are now known in the Oculina tribe. Some

Ack
ig

SUBDIVISIONS OF ACTINOID POLYPS. 79

of the larger kinds of simple corals, such as those of the gen-

era Zaphrentis and Heliophyllum, had at times a diameter of

PORITES LEVIS, D.

three or four inches, so that the breadth of the polyp flower

was probably at least six inches. Hemispherical masses of

80 CORALS AND CORAL ISLANDS.

solid corals attained, in some species, a diameter of several
feet. No doubt the colors, among the coral polyps and other
life of the ancient seas, were as brilliant as now exist-

Nature’s economist here puts the question—Why all this
beauty when there were no eyes to enjoy it? But beauty ex-
ists because, ‘in the beginning,” ‘the Spirit of God moved
upon the face of the waters ;” and man finds delight therein in-
asmuch as he bears the image of his Maker.

A single recent species has been obtained by Mr. L. F. de
Pourtales, in dredging at a depth of 324 fathoms, near the
Florida reef, which may be a Cyathophylloid, although it has
been supposed that the species of the tribe have been extinct
since the middle of the Mesozoic era. It was half an inch
high and broad, and the polyp-cell had eight septa—a mul-
tuple of four, as in the true Cyathophylloids. The discoverer
has named it //aplophyllia paradoxa. But he observes that
it may after all be only an abnormal Actinoid.

Il. ALCYONOID POLYPS.

The name Alcyonewm, given to some of the species of this
croup, is derived from Alcyone, the fabled daughter of Nep-
tune. It is sometimes written with an initial H, in conform-
ity with the aspirate of the Greek word; but Latin authors
usually omitted the H, and this has been good enough author-
ity for Linneus and the majority of later writers.

The Alcyonoids include some of the gayest and most deli-
cate of coral shrubs. Almost all are flexible, and wave with the

motion of the waters. They contribute but little to the mate-.

rial of coral reefs, but add largely to the beauties of the coral
landscape. Not only are the polyps of handsome tints, but
the whole shrub is usually of a brilliant orange, yellow, scarlet,

ALCYONOID POLYPS. 81

crimson or purple shade. Dun colors also occur, as ash-
gray, and dark brown, and almost black. Some kinds, the
Sponggodiz, are too flexible to stand erect, and they hang
from the coral ledges, or in the coral caves, in gorgeous clus-
ters of scarlet, yellow, and crimson colors.

The species of this order spread from the tropics through
the colder seas of the globe, and occur at various depths, down
to thousands of feet.

The two following are the most striking external peculiari-
ties of the polyps: the number of tentacles is always eight;
and these tentacles are always fringed with papille, though the
papillz are sometimes mere warts. Some of the various forms
of the polyps are shown in the figures on the following pages.

But besides these characteristics, there is also the follow-
ing: the existence of only eight internal septa, and these septa
not in pairs; consequently, the interior is divided into only
eight compartments (octants), and with each a tentacle is con-
nected. Hence in the Alcyonoids, as Prof. Verrill has ob-
served, the areas externally, and the compartments within,
are all ambulacral, or tentacular, which makes a wide dis-
tinction between them and the Actinoids (p. 28) in which only
the alternate are tentacular.

The solid secretions of these polyps are of two kinds: Ki-
ther (1), internal and calcareous; or (2), epidermic, from the
base of the polyp. The latter make an axis to the stem or
branch, which is either horney (like that in Antipathus, p. 62) or
calcareous. A few species have no solid secretions.

Allthe species are incapable of locomotion on the base ;
yet there are some that sometimes occur floating in the open
ocean.

The three following divisions of the Alcyonoids are those

now generally recognized :
6

82 CORALS AND CORAL ISLANDS.

1. The Alcyonium tribe or Axucyonacea.—One of the
forms under this tribe is represented in the annexed figure.
It is from the Feejees (like most of the zodphytes figured
by the author), and in the living state the polyps had the mid-
dle portion of the tentacles pale brown, with the fringe deep
brown. In another more beautiful species of the genus, from
the same region, the Xenia florida D. (made Xenia Dane by
Verrill, as it proved to be distinct from Lamarck’s species to

XENIA ELONGATA, D.

which the author referred it), the polyps are as large, but short-
er, and the color is a.shade of lilac. These species differ from
the larger part of the Alcyonia in having the polyps not re-
tractile; the tentacles fold together, if the zodphyte is disturb-
ed, but cannot hide themselves.

The following figure represents another related species

ae
oat

7 1 eo |

sea *

1, 1a, Merulina pegalis; 2. 2a, Telesto trichostemma.

2 TR Se eee ee

MCZ LIBRARY
HARVARD UNIVERSETY
CAMBRIDGE. MA USA

the

ALCYONOID POLYPS. 83

obtained by Dr. W. Stimpson, near Hong Kong, and called by
its discoverer Anthelia lineata ; the polyps are but partly ex-
panded.

Other Alcyonoids are much branched, with the branches
thick and finger-like, and soft or flexible, and the polyps small
and wholly retractile into the mass. The branches, bare of
polyps, are usually of some dull pale color, and on account of
this fact some of these Alcyonia go by the common name of
dead-men’s fingers.

ANTHELIA LINEATA, ST.

The above kinds secrete granules or spicules of carbonate
of lime in the tissues, and are harsher or softer in texture ac-
cording to the proportion of these granules.

Some species form branching tubes, rising from an in-
crusting base, which are rather firm owing to the calcareous
spicules present. Such species are referred to the genus Te-
lesto—one of which, from Hong Kong, from the collection
made by Dr. Stimpson, is here figured (from Verrill). The
second figure shows the form of the expanded polyps.

Another species of Telesto, 7. trichostemma, is represented
colored, as in life, in figures 1, 2a, on Plate VI. It encrusts
the dead axis of a branching Antipathes. The polyp is re-
markable for its size and beauty.

In one family of this tribe the polyps form red calcareous
tubes; sometimes a slender, creeping tube, with polyps at
intervals, as in a species referred by the author to the venus

54 CORALS AND CORAL ISLANDS.

Aulopora; but generally vertical tubes, grouped into large red
masses, called, popularly, Organ-pipe coral. A portion of one
of the latter—Tubipora syrnga D.—is represented in the

TELESTO RAMICULOSA, V.

first of the following figures, with its expanded polyps; and a
polyp from the group much enlarged in the second figure.
The papilla of the fringe are arranged closely together in a

9

1, 2. TUBIPORA SYRINGA, D.; 3. T. FIMBRIATA, D.

,

plane, so that it is not at first apparent that there is a fringe.
The third figure represents, enlarged, the polyp of another Fee-
jee species, the Zubspora fimbriata D. Such coral masses

.re sometimes a foot or more in diameter, and the living zo0-

ALCYONOID POLYPS. 85

phyte, with its lilac or purple polyps fully exparided, looks
much like a large cluster of flowers from a lilac bush. The
tubes are united by cross plates at intervals.

2. Gorgonia tribe, or Gorconacea.—The following figure
represents a species of this tribe from the Kingsmill or Gilbert

GORGONIA FLEXUOSA, D.

Islands. It is one of the net-like or reticulated species, the
reticulation being a result of the coalescence of the branchlets.
The general color of the species was crimson; but when alive
and expanded it was covered throughout with yellowish polyps
of the form in figure a, though much smaller, the natural size
not exceeding a twelfth of an inch. The common sea-fan of
the West Indies, Gorgonia flabellum, is much more finely
reticulated, the meshes of the net-work being ordinarily not
over a fourth of an inch in breadth; while the fan often grows
to a height and breadth of a yard.

Other species of the Gorgonia family are like clusters

86 CORALS AND CORAL ISLANDS.

of slender twigs, and others like many-branched shrubs or
miniature trees.

The exterior of the stem or branch in a Gorgonia is a
layer of united polyps, with minute calcareous spicules dis-
tributed through the tissues and giving the layer some firm-
ness. Itislikea bark to the axis of the stem or branch, and may
be peeled off without difficulty, and hence is often called the
cortex. The outer surface of the dried cortex is often smooth,
or nearly so; but sometimes covered with small prominences.
Over it there may be seen numerous oblong points (one to
each of the prominences if there are any; each of these is
the spot where a polyp opened out its tentacles when the zo6-
phyte was alive.

Kolliker and others have shown that genera, and some-
times species, of the Gorgonacea, may be distinguished by the

SPICULES OF GORGONIZ, MUCH ENLARGED.

forms of the calcareous spicules. Some of these knobby spi-
cules are represented in the annexed cut, from figures published
by Prof. Verrill. The most common forms are those of figures
1, 4, 5; they occur, with small differences, in the genera Gor-
gonia, Eugorgia, Leptogorgia, etc. Figure 1 is from the Lep-
togorgia eximia V. Figure 2,in which one side is smooth
(from the Gorgonia quercifolia V), is characteristic of the
genus Gorgonia, but occurs in the species along with forms

much like fig. 1. The forms represented in figures 3, 4, 5,

ALCYONOID POLYPS. 87

are all from Hugorgia aurantiaca V., the peculiar kind shown
in fig. 3 occurring with the other more common form, in species
of this genus. In species of Plexaurella many of the spi-
cules are beautiful crosses of various fancy shapes. In Eu-
nicellz the cortex is covered with an outside layer, in which
the spicules are club-shaped, though ornately so, and have the
smaller end pointed inward. These spicules afford valuable dis-
tinguishing characters also in all Alcyonoids.

The spicules are often brilliantly colored, and sometimes
variously so in the same individual. Yellow, crimson, scar-
let and purple are common colors, and they occur both of
dark and pale shades. Viewed under a compound micro-
scope by transmitted light, a group of these spicules from
some species, part bright yellow and part crimson, or of
some other tints, produces an exceedingly beautiful effect.
It gives still greater interest to this subject that all Gor-
goniz owe the various colors they present to the colors of
their spicules.

Spicules are usually wholly internal, or they only come to
the surface so as to make the exterior slightly harsh. But in
other cases, as in the genus Muricea, they project and give
a somewhat bristly look to the coral.

The calcareous spicules are internal secretions, like those
of ordinary coral, and the constitution is the same,—mere
carbonate of lime. But the secretion of the axis of the
branches is epidermic, from the inner surface of the cortex,
asin the Antipathus before described (p. 62). In the ordinary
Aleyonoids that make no horny axis, the stolons, or budding
stem or mass, creeps or spreads over the supporting body.
But in these Gorgoniz, the budding cluster, which would make
a stolon if there were no horny secretions, has the form of a
tube about a horny axis; and as this tube elongates and se-

88 CORALS AND CORAL ISLANDS.

cretes the axis within, it gives out buds externally; thus the
branch rises. New branches commence at intervals over the
sides of the rising stem or branch through the starting of new

TALE y)
1 Mian
Ui

ISIS HIPPURIS, LINN.

budding centres, and so, finally, the Gorgonia zodphyte is
completed.

In a few species, the axis is partly or wholly calcareous.
In the Isis family, it is made up of a series of nodes and
internodes. The former, in the genus Isis, are white, calcare.
ous, furrowed or fluted pieces; and the latter are smaller and
horn-like in nature, as illustrated in the preceding figures.
In the branching stem here figured, the main stem and the
branch on the left are simply the axis, bare of the polyp-layer

ALCYONOID POLYPS. 89

or cortex; while the branch on the right, with the surface
dotted, has the cortex complete, and the dots are the sites of
the contracted polyps. ‘The circular figure below is a trans-
verse section of the stem enlarged, showing the cavities occu-
pied by the retracted polyps.

In the genus Melitza, and some others related, the inter.

CORALLIUM RUBRUM.

nodes are porous and somewhat cork-like or suberose instead
of horny. ‘The species of this group are often bright-colored
and much branched, and resemble, in aspect, ordinary Gor-
gonie ; but they are very brittle, breaking easily at the

internodes.

90 VORALS AND CORAL ISLANDS.

In the Corallide, the axis is wholly calcareous, and firm and
solid throughout, with usually a red color, varying from crim-
son to rose-red. Here belongs the Coralliwm rubrum, or pre-
cious coral. The polyp-crust or cortex, which covers the red
axis or coral, is thin, and contains comparatively few calcare-
ous spicules, and consequently it readily disappears when the
dried specimens are handled. In an uninjured state, the polyp
centres may be distinguished over it by a faint six-rayed star.
A branch from a specimen obtained by the author at Naples,
is represented, of natural size, in the cut on page 89. The pol-
yps, as the enlarged view, by Lacaze Duthiers, shows, are sim-
ilar to those of other Alcyonoids—the tentacles being eight in
number and fringed. The figure represents the extremity of
a branch, magnified about four times lineally, with one polyp
fully expanded, two partly, and the rest unexpanded. In the
living Corallium, they open out thickly over the branches, and
make it an exceedingly beautiful object. The coral grows in
branching forms, spreading its branches nearly in a plane; and
sometimes the little shrub is over a foot in height. The au-
thor just mentioned states that, among the polyps, those of the
same branch are often all of one sex alone, and that, besides
males and females, there are a few that combine both sexes.
The red caleareons axis consists really of united spicules.

The precious coral is gathered from the rocky bottom of
the borders of the Mediterranean, or its islands, and most
abundantly at depths of 25 to 50 feet, though occurring
also even down to 1,000 feet. There are important fisheries
on the coast of southern Italy ; of the island of Ponza, off the
Gulf of Gaeta; of Sicily, especially at Trapani, its western ex-
tremity ; of Corsica and Sardinia, in the straits of Bonifacio ;
of Algeria, south of Sardinia, near Bona, Oran, and other
places, which in 1853 afforded 80,000 pounds of coral; and on

ALCYONOID POLYPS. 91

the coast of Marseilles. The rose-colored is the most highly
valued, because the rarest.

Another species of Corallium was obtained by the author
at the Sandwich Islands (Atlas of Zodphytes, plate 60) ; but,
while probably from the seas of that region, its precise locality
is not known.

3. Pennatula tribe, or Pennarunacea. These are com-

COPHOBELEMNON CLAVATUM, V., AND VERETILLUM STIMPSONI, V.

pound Alcyonoids, that, instead of being attached to rocks or
some firm support, have the base or lower extremity free from
polyps and buried in the sand or mud of the sea-bottom, or
else live a floating life in the ocean. Their forms are very va-
rious.

In the Veretillum family (Veretillide) they are stout
and short club-shaped. One of the species from Hong Kong,
is shown in the figure on the left, with its polyps fully ex-

92 CORALS AND CORAL ISLANDS.

panded, and the small figure represents one of the polyps en-
larged. The third figure represents a polyp of another spe-
cies, from Hong Kong, a true Veretillum, enlarged three di-
ameters; the specimens, obtained by Dr. Stimpson, and de-
scribed by Prof. Verrill, were six to eight inches in length, and,
where thickest, were three inches or more in diameter.

A common Mediterranean species is the Veretillum cynomo-
roum ; and it has been recently found, of a length of ten in-
ches, in the depths of the Atlantic off the coast of Spain. Mr.
W. S. Kent observes, with regard to its polyps and their
phosphorescent qualities, as follows :

‘“‘ Nothing can exceed the beauty of the elegant opaline pol-
yps of this zodphyte when fully expanded, and clustered
like flowers on their orange-colored stalk ; a beauty, however,
almost equalled by night, when, on the slightest irritation, the
whole colony glows from one extremity to the other with un-
dulating waves of pale green phosphoric light. A large buck-
etful of these Alcyonaria was experimentally stirred up one
dark evening

oO?
spectacle too brilliant for words to describe. The supporting

and the brilliant luminosity evolved produced a

stem appeared always to be the chief seat of these phosphor-
escent properties, and from thence the scintillations travelled
onward to the bodies of the polyps themselves. Some of the
specimens of this magnificent zodphyte measured as much as
ten inches from the proximal to the distal extremity of the
supporting stalk, while the individual polyps, when fully ex-
serted, protruded upward of an inch and a half from this in-
flated stalk, and measured as much as an inch in the diameter
of their expanded tentacular discs.”

In several genera of the Pennatula tribe there are two
kinds of polyps over the surface, and this was the case with the
Veretillum Stimpsoni, as observed by Prof. Verrill. Between

ALCYONOID POLYPS. 93

the large and well-developed polyps, there were multitudes
of small wart-like prominences, each of which proved to be
a polyp, but very small and imperfectly developed, having
only two lamelle in the interior instead of the usual eight,
and without distinct tentacles, or the ordinary nettling cords
within.

Among the other forms of Zoéphytes in the Pennatula tribe
are those having a stout axis. with branches either side,
arranged regularly in plume-like style (the Pennatulide);
or a very slender stem and very short lateral polyp-bearing
pinnules or processes along it (the Virgularide); or a thin
reniform shape (Renillidz). Others differ from the preceding
in having the polyps not retractile; and some of these have
a slender stem and the polyps arranged along one side of it
(the Pavonaridz) ; and still others a terminal cluster of polyps
(the Umbellularide).

The most of the species secrete a slender, horny axis, and
have slender calcareous spicules among the tissues, somewhat
like those of the Gorgonidz. By the thickened base of the
stem these species anchor the corallum in the mud. Many
species occur in the deep seas, some at depths of two thou-
sand fathoms. Moreover, they are brilliantly phosphores-
cent; and Moseley says that the depths may be in places
lighted by patches of these species and “ possibly the animals
with eyes congregate around these sources of light.”

The Heliopore are peculiar among the Alcyonoids in havy-
ing a solid compound corallum, of rather large size; and
they are alone among corals in having a blue color within.
The corallum consists of slender tubes with intervening cel-
lular coenenchyma; and as the tubes are crossed by tabula,
though distantly, Heliopora has been referred to the Tabu-
late, and also to the Milleporids. It was shown to belong

Q4 CORALS AND CORAL ISLANDS.

here by Moseley of the Challenger Expedition. The eight
tentacles are pinnately fringed as in other Alcyonoids, and
are wholly retractile by mtroversion. /eliolites, a Paleozoic
genus, is supposed to be related to Heliopora. The most
common species of Heliopora is the //. cwrulea of the East

Indies.

IV. LIFE AND DEATH IN CONCURRENT PROGRESS IN CORAL
ZOOPHYTES.

The large, massive forms of stony corals would not exist,
and the tree-shaped and other kinds would be of diminutive size,
were it not for the fact that, in the living zodphyte, death and
life are going on together, pari passu. This condition of
growth is favored by the coral secretions; for these give a
chance for the polyp to mount upward on the coral, as it
lengthens it by secretions at the top. But, to be successful in
this ascending process, either the polyp must have the power
of indefinite elongation, or it must desert the lower part of
the corallum as growth goes forward; and this last is what
happens. In some instances, a polyp, but a fourth of an inch
long, or even shorter, is finally found at the top of a stem
many inches in height. ‘The following figure represents a case
of this kind; for all is dead coral, excepting less than an
inch at the extremity of each branch. The tissues that once
filled the cells of the rest of the corallum have dried away,
as increase went on above. Another example is shown
on page 54, in which the living part had a length of one
eighth of an inch. The Goniopora, on page 52, is still an-
other example of the process; but here the living part com-
bines a great number of polyps: these are growing and _ bud-
ding with all the exuberance of life, while below, the old _pol-

LIFE AND DEATH IN CONCURRENT PROGRESS. 95

yps gradually disappear, and even their cells become superfi-
cial and fade out. Trees of Madrepores may also have their
limits—all below a certain distance from the summit being
dead; and this distance will differ for different species. Bur
this is not a limit to the existence of the zodthome, even

CAULASTR#A FURCATA, D.

though a slender tree or shrub, or of its flourishing state; for the
dead coral below is firm rock itself, often stronger than ordinary
limestone or marble, and serves as an ever-rising basement
for the still expanding and rising zodphyte.

But this death is not in progress alone at the base of the
column orbranch. Generally the whole interior of a corallum
is dead, a result of the same process with that just explained.
Thus, a Madrepora, although the branch may be an inch in
diameter, is alive only to the depth of a line or two, the grow-
ing polyps of the surface having progressively died at the low-
er or inner extremity as they increased outward.

The large domes of Astrzeas, which have been stated to
attain sometimes a diameter of ten or fifteen feet, and are

96 CORALS AND CORAL ISLANDS.

alive over the whole surface, owing to a symmetrical and un-
limited mode of budding, are nothing but lifeless coral
throughout the interior. Could the living portion be sepa-
rated, it would form a hemispherical shell of polyps, in most
species about half an inch thick. In some Porites of the same
size, the whole mass is lifeless, excepting the exterior for a
sixth of an inch in depth.

With such a mode of increase, there is no necessary limit
to the growth of zodphytes. The rising column may increase
upward indefinitely, until it reaches the surface of the sea, and
then death will ensue simply from exposure, and not from any
failure in its powers of life. The huge domes may enlarge till
the exposure just mentioned causes the death of the summit,
and leaves only the sides to grow, and these may still widen,
it may be indefinitely. Moreover, it is evident that if the
land supporting the coral domes and trees were gradually
sinking, the upward increase might go on without limit.

In the following of death after life ‘“ aquo pede,” there is
obedience to the universal law. And yet the polyps, through
this ever yielding a little by piecemeal, seem to get the better
of the law, and in some instances secure for themselves almost
perpetual youth, or at least a very great age. Of the polyps
over an Astraea hemisphere, none ever die as long as the dome
is in a condition of growth; and the first budding individual,
or at least its mouth and stomach, is among the tens of
thousands that constitute the living exterior of the dome of
fifteen feet diameter. In the Madrepore, the terminal parent-
polyp of a branch grows on without being reached by the
death-warrant that takes off at last the commoners about the
base of the tree; it keeps growing and budding, and the tree
thus continues its increase.

The death of the polyps about the base of a coral tree

PROTECTION AGAINST INJURY. Sel

_ would expose it, seemingly, to immediate wear from the waters
around it, especially as the texture is usually porous.
But nature is not without an expedient to prevent to some
extent this catastrophe.

In the first place, there is often a perdtheca over the
dead corallum—that is, an outer impervious layer of carbonate
of lime, secreted by the lower edge of the series of dying pol-
yps, a fact in the Goniopora columna figured on page 52.
Then, further, the dead surface becomes the resting-place of
numberless small encrusting species of corals, besides Nulli-
pores, Serpulas, and some Mollusks. In many instances, the
lichen-like Nullipore grows at the same rate with the rate of
death in the zodphyte, and keeps itself up to the very limit of
the living part. The dead trunk of the forest becomes covered
with lichens and fungi, or in tropical climes, with other foliage
and flowers; so among the coral productions of the sea, there
are forms of life which replace the dying polyp. The process
of wear is frequently thus prevented.

The older polyps, before death, often increase their coral se-
cretions also within, filling the pores as the tissues occupying
them dwindle, and thus render the corallum nearly solid; and
this is another means by which the trees of coral growth,
though of slender form, are increased in strength and endur-
ance.

The facility with which polyps repair a wound, aids in
carrying forward the results above described. The breaking
of a branch is no serious injury to a zoodphyte. ‘There is often
some degree of sensibility apparent throughout a clump even
when of considerable size, and the shock, therefore, may occa-
sion the polyps to close. But, in an hour, or perhaps much
less time, their tentacles will again have expanded; and such
as were torn by the fracture will be in the process of com-

-
i

98 CORALS AND CORAL ISLANDS.

plete restoration to their former size and powers. The frag-
ment broken off, dropping in a favorable place, would become
the germ of another coral plant, its base cementing by means
of new coral secretions to the rock on which it might rest; or,
if still in contact with any part of the parent tree, it would be
reunited and continue to grow as before. The coral zodphyte
may be levelled by transported masses swept over it by the
waves; yet, like the trodden sod, it sprouts again, and contin-
ues to grow and flourish as before. The sod, however,
has roots which are still unhurt; while the zodphyte, which
may be dead at base, has a root—a source or centre of life—in
every polyp that blossoms over its surface. Hach animal
might live and grow if separated from the rest, and would ul-
timately produce a mature zodphyte.

V. COMPOSITION OF CORAL.

Ordinary corals have a hardness a little above that of com-
mon limestone or marble. The ringing sound given, when cor-
al is struck with a hammer, indicates this superior hardness.
It is possible that it may be owing to the carbonate of lime be-
ing in the state of aragonite, whose hardness exceeds a little
that of ordinary carbonate of lime or calcite. It is a common
error of old date to suppose that coral when first removed from
the water is soft, and afterward hardens on exposure. For, in
fact, there is scarcely an appreciable difference; the live coral
may have a slimy feel in the fingers ; but if washed clean of the
animal matter, it is found to be quite firm. ‘The waters with
which it is penetrated may contain a trace of lime in solution,
which evaporates on drying, and adds slightly to the strength
of the coral ; but the change is hardly appreciable. A branched
Madrepore rings on being struck when first collected ; and a
blow in any part puts in hazard every branch throughout it,

COMPOSITION OF CORALS. 99

on account of its elasticity and brittleness. The specific gravi-
ty of coral varies from 2°5 to 2°8: 2523 was the average from
fifteen specimens examined by Prof. Silliman.

Chemically, the common reef-corals, of which the branch-
ing Madrepora and the massive Astreas are good exam-
ples, consist almost wholly of carbonate of lime, the same in-
gredient which constitutes ordinary limestone. In 100 parts,
95 to 98 parts are of this constituent ; of the remainder, there
are 14 to 4 parts of organic matter, and some earthy ingredi-
ents amounting usually to less than 1 per cent. These earthy
ingredients are phosphate of lime, with sometimes a trace of
silica. A trace of fluorine also has been observed.

S. P. Sharples found the following constitution for the spe-
cies below named (Am. Jour. Scz., IIIL., 1. 168).

CARBONATE PHOSPHATE WATER AND OR-

OF LIME. OF LIME. GANIC MATTERS.
Ocalina arbuscula, N. Car.. . . 95.37 . . 0.84 . . . 38.79
Manicina areolata, Florida. . . 96.54 . . 0.50 . . . 2.96
Agaricia agaricites ieee eo Tao. |). 0.8) OLDS! a 20.5 | cee Mal 4
Siderastrea radians Meme O30) cos, MOLLBe sree stn (eee
Meaitopora cervicornis . /. . .- 98.07 . .. 0.82 . . = 1.93
Madrepora palmata Mee aM OTTO et it OL 10h ty aif swans Ol

Forchhammer found 2:1 per cent. of magnesia in Coral-
lium rubrum, and 6°36 in Isis hippuris.

The sea-water, and the ordinary food of the polyps, are evi-
dently the sources from which the ingredients of coral are ob-
tained. The same powers of elaboration which exist in other
animals belong to polyps; for this function, as has been re-
marked, is the lowest attribute of vitality. Neither is it at all
necessary to inquire whether the lime in sea-water exists as
carbonate, or sulphate or whether chloride of calcium takes the
place of these. The powers of life may make from the ele-

100 CORALS AND CORAL ISLANDS.

ments present whatever results the functions of the animal re-
quire.

The proportion of lime salts which occurs in the water of
the ocean is about | to % of all the ingredients in solution. The
lime is mainly in the state of sulphate. Bischof states that the
proportion of salts of all kinds in sea-water averages 3°527 per
cent.; and in 100 parts of this, 75°79 are chloride of sodium,
916 chloride of magnesium, 3°66 chloride of potassium, 1°18
bromide of sodium, 4°62 sulphate of lime or gypsum, and 5°597
sulphate of magnesia,=100. This corresponds to about 164
parts of sulphate of lime to 10,000 of water.

Fluorine has also been detected in sea-water; so that all the
ingredients of coral are actually contained in the waters of the
ocean.

It has been common to attribute the origin of the lime of
corals to the existence of carbonic-acid springs in the vicinity
of coral islands. But it is an objection to such a hypothesis,
that, in the first place, the facts do not require it; and, in the
second, there is no foundation for it. The islands have been
supposed to rest on volcanic summits, thus making one hy-
pothesis the basis of another. Carbonic-acid springs are by no
means a universal attendant on volcanic action. The Pacific
affords no one fact in support of such an opinion. ‘There are
none on Hawaii, where are the most active fires in Polynesia;
and the many explorations of the Society and Navigator Isl-
ands have brought none to light. Some of the largest reefs
of the Pacific, those of Australia and New Caledonia, oc-
cur where there is no evidence of former volcanic action.

The currents of the Pacific are constantly bearing new sup-
plies of water over the growing coral beds, and the whole ocean
is thus engaged in contributing to their nutriment. Fish, mol-
lusks, and zoéphytes are thus provided with earthy ingredi-

HYDROIDS. 101

ents for their calcareous secretions, if their food fails of giving
the necessary amount; and, by means of the powers of animal
life, bones, shells, and corals alike are formed.

The origin of the lime in solution throughout the ocean is
an inquiry foreign to our present subject. It is sufficient here
to show that this lime, whatever its source, is adequate to ex-
plain all the results under consideration.

I. HYDROIDS.

The annexed sketch represents a Hydra as it often occurs
attached to the under surface of a floating leaf—that of a spe-
cies of Lemna. The animal is seldom over half an inch

long. It has the form of a polyp, with long slender tentacles ;
and, besides these tentacles with their lasso-cells, it has no spe-
cial organs except a mouth and a tubular stomach. Like the
fabled Hydra, if its head be cut off another will grow out; and
any fragment will, in the course of a short time, become a per-
fect Hydra, supplying head, or tail, or whatever is wanting:
and hence the name given to the genus by Linnzus.

102 CORALS AND CORAL ISLANDS.

The Hydroids were long considered polyps. But they
have been found to give origin, with few exceptions, to Weduse,
or jelly-fishes, and it is now proved that they are only an
intermediate stage in the development of Medusz, between
the embryo state and that of the adult or Medusa state. The
Millepores afford, therefore, examples of coral-making by spe-
cies of the class of Acalephs. Many of these Meduse and
their Hydroids will be found illustrated in the admirable work
of Alexander and Mrs. L. Agassiz entitled “ Sea-Side Studies,”
—an excellent companion for all who take pleasure in sea-
shore rambles.

The Hydra is the type of a large group of species. It buds,
but the buds drop off soon, and hence its compound groups
are always small, and usually it is single. But other kinds
multiply by buds that are persistent, and almost indefinitely
so; and they thus make membranous coralla of considerable
size and often of much beauty.

The species here figured, Hydrallmania falcata (formerly
called Plumularia falcata), is one of them. Along the
branches there are minute cells, each of which was the seat of
one of the little Hydra-like animals (in this not a fourth of a
line long) having usually short tentacles spread out star-like.
Other kinds are simple branching threads, and sometimes the
cells are goblet-shaped and terminal. The Tubulariz grow
in tufts of thread-like tubes, and have a star-shaped flower
at top often half an inch in diameter, with a proboscis-like
mouth at the centre. In Coryne, a closely-related genus, the
tentacles are shorter, and somewhat scattered about the club-
shaped or probosciform head of the stem, so that the animal
at top is far from star-shaped or graceful in form.

To the animal of the Coryne, that of the very common, and
often large, corals, called Millepores, is closely related, as first

HYDROIDS. 103
detected by Agassiz on one of his cruises to the reefs of Flori-
da. The coral-making Hy

rdroids have been named /7ydroco-

HYDRALLMANIA FALCATA,

ralline. The group includes also, as Moseley first showed, the

Stylasteridz mentioned on page 70 and other related species.

The corals of the Milleporz are solid and stony, as much

so as any in coral seas. They have generally a smooth sur-

104 CORALS AND CORAL ISLANDS.

face, and are always without any prominent calicles, there
being only very minute rounded punctures over the surface,
from which the animals show themselves. The cells in the
corallum are divided parallel to the surface, but irregularly,
by very thin plates or tables, approaching in this character
the Pocilliporee and Favosites.

Each coral is a group or colony of Hydroids in the Hydri-
form state. Agassiz observes that the animals of Millepora
are very slow in expanding themselves. When expanded, they

have no resemblance to true polyps; there is simply a fleshy

ANIMALS OF MILLEPORA ALCICORNIS, MUCH ENLARGED.

tube with a mouth at top and a few small rounded promi-
nences in place of tentacles, four of them sometimes largest.
The preceding figure, from Agassiz, shows, much enlarged, a
portion of a branch of the AMillepora alcicornis with the ani-
mals expanded; and the small figure a, near the top of the
cut, gives the natural size of the same. But it has been fur-
ther observed by Moseley that in the Millepores the animals
have two forms: one is the tentacle-like kind here figured,
and the other shorter and mouth-bearing; and the former
are sometimes arranged around the latter in more or less

perfectly circular groups, called “ cyclosystems.”

BRYOZOANS. 105

In the Stylasteride, the cyclosystems occupy usually stel-
late cells which stand out prominently along the branches, and
look much like the calicles of an Oculina (Figs. 2, 3, p. 69).

8. P. Sharples found the coral of M. alcicornis to consist
of 97:46 per cent of carbonate of lime, 0:27 of phosphate of
lime, and 2°54 of water and organic matters. The Millepores

contribute largely to the material of coral reefs.

MILLEPORA ALCICORNIS, LINN.

The ancient corals of the Cheetetes family may be Hydro-
coralline, as suggested by Agassiz, but more probably were
Bryozoan.

Ill. BRYOZOANS.

The Bryozoans are very small animals, and look much like
Hydroids. Although belonging to the sub-kingdom of Mol-

lusks, they are externally polyp-like, having a circle or ellipse of

106 CORALS AND CORAL ISLANDS.

slender tentacles around the mouth. But, in internal struc-
ture, and all of the animal below the head, they are Mollusks.
They form delicate corals, membranous or calcareous, made up
of minute, cabin-like cells, which are either very thin crusts on
sea-weeds, rocks, or other supports, or slender moss-like tufts,
or graceful groups of thin, curving plates, or net-like fronds ;
and sometimes thread-like lines, or open reticulations.

Occasionally they make large, massive corals, from the
growing of plate over plate.

The first of the following figures, represents one of the
delicately branching species, of natural size; and the second,
a portion of the same, much enlarged. The latter figure
shows that the branches are made up of minute cells. From
each cell, when alive, the bryozoum extends a circlet of ten-

tacles, less than a line in diameter.

1, 2, HORNERA LICHENOIDES; 3, DISCOPORA SKENEI, SMITT.

The encrusting kinds are common in all seas. The crust
of cells they make is often thinner than paper. A portion of
such a crust is represented, enlarged, in figure 3. When ex-
panded, the surface is covered over with the delicate flower-like
bryozoa. A low magnifying power is necessary to observe them

NULLIPORES. 107

distinctly. The animals, unlike true polyps and the Hydroids,
have two extremities to the alimentary canal, and in this, and
other points, they are Molluscan in type.

The cells of a group never have connection with a common
tube, as in the Hydroids; on the contrary, each little Bryozoum,
in the compound group or zoédthome, is wholly independent of
the rest in its alimentary canal.

Bryozoans occur in all seas and at all depths; and in early
Paleozoic time they contributed largely to the making of lime-
stone strata,

IV. NULLIPORES.

The more important species of the Vegetable Kingdom that
afford stony material for coral reefs are called Nullipores.
They are true Algze or sea-weeds, although so completely stony
and solid that nothing in their aspect is plant-like. They form
thick, or thin, stony incrustations over surfaces of dead corals,
or coral rock, occasionally knobby or branching, and often
spreading lichen-like.

They have the aspect of ordinary coral, especially the Mil-

lepores, but may be distinguished from these species by their
having no cells, not even any of the pin-punctures of those
species.
Besides the more stony kinds, there are delicate species, of-
ten jointed, called Corallines, which secrete only a little lime in
their tissues, and have a more plant-like look. Even these
grow so abundantly on some coasts, that, when broken up and
accumulated along the shore by the sea, they may make thick
calcareous deposits. Agassiz has described such beds as hav-
ing considerable extent in the Florida seas.

108 CORALS AND CORAL ISLANDS.

V. THE REEF-FORMING CORALS AND THE CAUSES INFLU-
ENCING THEIR GROWTH AND DISTRIBUTION.

I. DISTRIBUTION IN LATITUDE.

Reef-forming species are the warm-water corals of the
globe. A general survey of the facts connected with the tem-
perature of the ocean in coral-reef seas appears to sustain the
conclusion that they are confined to waters which, through even
the coldest winter month, have a mean temperature not below
68° F. Under the equator, the surface waters in the hotter
part of the ocean have the temperature of 85° F’. in the Pacific,
and 83° F. inthe Atlantic. The range from 68° F. to 85° F-.
is, therefore, not too great for reef-making species.

An isothermal line, crossing the ocean where this winter-
temperature of the sea is experienced, one north of the equator,
and another south, bending in its course toward or from the
equator wherever the marine currents change its position,
will include all the growing reefs of the world; and the area
of waters may be properly called the coral-reef seas.

This isothermal boundary line, the isocryme (or cold-water
line) of 68° F., extends, through mid-ocean, near the parallel of
28°; but in the vicinity of the continents it varies greatly from
this, as explained beyond in the course of remarks on the geo-
graphical distribution of reefs. It is to be observed that the
temperature of 68° F. is a temporary extreme—not that under
which the polyps will flourish. Except for a short period, the
waters near the limits of the coral seas are much warmer; the
mean for the year is about 734° F. in the North Pacific, and
70° F. in the South; from which it may be inferred that the
summer mean would be as high at least as 78° and 74° F.

Over the sea thus limited coral reefs grow luxuriantly, yet

GEOGRAPHICAL DISTRIBUTION OF UORALS. 109

in greatest profusion and widest variety through its hotter por-
tions. Drawing the isocryme of 74° F’. (that is, the isotherm
for 74° F. as the mean for the coldest month) around the
globe, the coral-reef seas are divided, both north and south of
the equator, into two regions, a torrid, and a subtorrid, as thev
are named by the author (see Chart beyond, from the Author’s
Report on Crustacea); and these correspond, as seen below,
to a marked difference in the corals which they grow.

Further, the torrid region should be divided, as the distri-
bution of corals show, into a warmer and a cooler torrid, the
isocryme separating the two being probably that of 78°.

But, before considering the facts connected with the geo-
graphical distribution of existing coral-reef species, it is impor-
tant to have a correct apprehension of what are these reef spe-
cies as distinct from those of colder and deeper seas,

The coral-reef species of corals are the following.—

1. In the Astrea tribe (Astrzeacea), all the many known
species. -

2. In the Fungia tribe (Fungacea), almost all known spe-
cies, the only exceptions at present known being two free spe-
cies found much below coral-reef depths, in the Florida seas,
by 0. I’. de Pourtales, one of them, at a depth of 450 fathoms.

3. In the Oculina tribe (Oculinacea), all of the Orbicellids; ‘

part of the Oculinids and Stylasterids ; some of the Caryophyl-
lids, Astrangids and Stylophorids; all of the Pocilloporids.

4, Inthe Madrepora tribe (Madreporacea), all of the Madre-
porids and Poritids; many of the Dendrophyllia family or
Eupsammids.

5. Among Alcyonoids, numerous species of the Aleyonium
and Gorgonia tribes, and some of the Pennatulacea.

6. Among Hydroids, the Millepores and Stylasterids.

7. Among Algze, many Nullipores and Corallines.

110 CORALS AND CORAL ISLANDS.

The corals of colder waters, either outside of the coral-reet
seas, or at considerable depths within them, comprise, accord-
ingly, the following :—

1. A very few Fungids.

2. Some of the Oculinids; many of the Astrangids and
Caryophyllids; a few Stylophorids.

3. Many of the Eupsammids.

4. Some of the Gorgonia and Pennatula tribes, and a few
of the Aleyonium tribe.

5. Milleporids of the genus Pliobothrus; many Stylasterids.

A large proportion of the cold water species are solitary
polyps.

Through the torrid region, in the central and western Pa-
cific, that is, within 15° to 18° of the equator, where the tem-
perature of the surface is never below 74°F. for any month of
the year, all the prominent genera of reef-forming species are
abundantly represented—those of the Astraacea, Fungacea,
Oculinacea, Madreporacea, Aleyonoids, Millepores and Nulli-
pores. The Feejee seas afford magnificent exainples of these
torrid region productions. Astras and Meandrinas grow
there in their fullest perfection; Madrepores add flowering
shrubbery of many kinds, besides large vases and spreading
folia; some of these folia over six feet in expanse. Musse
and related species produce clumps of larger flowers ; Meru-
line, Echinopore, Gemmipore and Montipore form groups
of gracefully infolded or spreading leaves; Pavoniz, Pocilli-
pore, Seriatopore and Porites branching tufts of a great vari-
ety of forms; Tubipores and Xenie, beds or masses of the
most delicately-tinted pinks; Sponggodiz, large pendant clus-
ters of orange and crimson; and Fungie display their broad
disks in the spaces among the other kinds. Many of the
species may be gathered from the shallow pools about the reefs.

GEOGRAPHICAL DISTRIBUTION OF CORALS. itt

But with a native canoe, and a Feejee to paddle and dive, the
scenes in the deeper waters may not only be enjoyed, but boat-
loads of the beautiful corals be easily secured.

The Hawaian Islands, in the north Pacific, between the

latitudes 19° and 22°, are outside of the torrid zone of oceanic
| temperature, in the sudtorrid, and the corals are consequently
less luxuriant and much fewer in species. There are no Mad-
repores, and but few of the Astrea and Fungia tribes; while
there is a profusion of corals of the hardier genera, Porites and
Pocillipora.

The genera of corals occurring in the East Indies and Red
Sea are mainly the same as in the Central Pacific; and the
same also occur on the coast of Zanzibar.

At the eastern of the Pacific coral islands, the Paumotus,
which are within the limits of the torrid region, the variety of
species and genera is large, but less so than to the westward.
Special facts respecting this sea have not been obtained. The
author’s observations were confined to the groups of islands
farther west, the department of corals having been in the hands
of another during the earlier part of the cruise of the Govern-
ment Expedition with which he was connected.

The Gulf of Panama and the neighboring seas, north to the
extremity of the California peninsula and south to Guayaquil,
lie within the torrid region; but in the cooler part of it. The
species have throughout a Pacific character, and nothing of
the West Indian; but they are few in number, and are much
restricted in genera. There are none, yet known, of the As-
treeacea, and no Madrepores. Prof. Verrill, through the study
of collections made by F. H. Bradley and others, has observed
that there are, near Panama, a few species of Porites and Den-
drophylliz, a Stephanaria (near Pavonia), two species of Po-
cilliporz, two of Pavonis, one of them very large and named

12 CORALS AND CORAL ISLANDS.

P. gigantea V., several Astrangids, and a few other small
species, besides a large variety under the Gorgonia tribe. At
La Paz, on the California peninsula at the entrance to the Gulf,
occur a small but beautiful Fungia (7. elegans V.), three Pori-
tes, a Dendrophyllia, a Pocillipora, some Astrangids, and
many fine Gorgonie. The character of the species is that. of
the cooler torrid region, rather than that of the warmer
torrid.

Owing to the cold oceanic currents of the eastern border
of the Pacific—one of which, that up the South American
coast, is so strong and chilling as to push the southern isocryme
of 68°, the coral-sea boundary, nearly to the Galapagos,
and north of the equator—the coral-reef sea, just east of Pan-
ama, is narrowed to 20°, which is 36° less of width than it has
in mid ocean; and this suggests that these currents, by their
temperature, as well as by their usual westward direction, have
proved an obstacle to the transfer of mid-ocean species to the
Panama coast.

In the West Indies the reefs lie within the limits of the
isocryme of 74° F., or the torrid region; and yet the variety
of species and genera is very small compared with the same in
the central Pacific. The region contains some large Madre-
pores, the M. palmata, a spreading foliaceous species that
forms clumps two yards in diameter; JL. cervicornis, a stout,
sparsely-branched tree-like species, which attains a height of
fifteen feet; JL prolifera, a handsome shrub-like species, of rathi-
er crowded branches; besides others ; and these are marks of the
existence of the warmer torrid region; yet the sea has not as
high a temperature as the hottest part of the Pacific. The species
of the Astraa tribe are few in number, and among the largest
kinds are the Meandrine (the Diploria being here included).
None of the free Fungide are known excepting the two spe-

GHOGRAPHICAL DISTRIBUTION OF CORALS. 113

cies In deep water, and none of the Pavonie among the com-
pound species; but the massive Siderine (Siderastraez) are
common, and the foliaceous Agaricie and Mycedia. Of the
Oculina tribe, species of Oculina, Cladocora and Astrangia are
relatively more numerous than in the central Pacific; but
there are none of the Pocilliporids, which are common both in
the torrid and subtorrid regions of the Pacific. Millepores are
very common. Gorgoniz, are of many species.

Prof. Verrill observes that not a single West Indian coral
occurs on the Panama coast, although, on the opposite coast, at
Aspinwall, there are found nearly all the reef-building species
of Florida, viz.: Porites astrwoides Lmk., P. clavaria Luwnk.,
Madrepora palmata V.., M. cervicornis L., M. prolifera L.,
Meandrina clivosa V., M. labyrinthica, M. sinuosa Les., with
other species of Mzandrina, Manicina areolata Ehr., Sider-
astrea (Siderina) radiata V., S. galaxea Bl., Agaricia agari-
cites, Orbicella cavernosa V., O. annularis D. Moreover no
West Indian species is known to be identical with any from
the Pacific or Indian ocean.

The reefs of the Brazilian coast south of Cape Roque lie in
the subtorrid region of oceanic temperature, or between the is-
ocrymes ot 74° and 68°. The reef corals extend as far south
as Cape Frio, according to Prot. C. F. Hartt. The species, as
determined by Prof. Verrill, from Prof. Hartt’s collections, re-
semble the West Indian. All species of Madrepora, Mzan-
drina, Diploria, Manicina, Oculina, genera eminently charac-
teristic of the West Indies, appear to be wanting, while the
most important reef-making genera are Huvia, Acanthastrea,
Orbicella, Siderastrea, Porites, and Millepora, and also, of
less importance, Mussa and some others. A few species, viz. :
Siderastrea stellata V., Orbicella aperta V., Astrea gravida

V., and Porites solida V., are very close to West Indian spe-
8

114 CORALS AND CORAL ISLANDS.

cies; and Millepora alcicornis is an identical species, though
different in variety.

The Bermudas are in the North Atlantic subtorrid region,
in the range of the Gulf Stream. The few reef-making spe-
cies that occur there are all West Indian. The principal among
them are: Lsophyllia dipsacea, f. rigida, Astrea ananas, Di-
ploria cerebriformis, D. Stokesi, Meandrina labyrinthica, M.
strigosa, Orbicella cavernosa, Oculina diffusa, Oculina varicosa,
Oculina pallens, Oculina Valenciennes, O. speciosa, Siderastreea
radians, Mycedium fragile, Porites clavaria, P. astreoides,
Millepora alcicornis ; and the common West India Aleyonoids,
Gorgoma flabellum, Plexaura crassa Lx., Pl. flecuosa Lx., Pl.
homomalla Lx., Pterogorgia Americana EKhr., Pt. acerosa Ehr.

The facts presented are sufficient to show that temperature
has much to do with the distribution of reef-corals in latitude,
while proving also that regional peculiarities exist that are not
thus accounted for.

UW. DISTRIBUTION IN DEPTH.

(uoy and Gaymard were the first authors who ascertained
that reef-forming corals were confined to small depths, contrary
to the account of Foster and the early navigators. The mis-
take of previous voyagers was a natural one, for coral reefs
were proved to stand in an unfathomable ocean; yet it was
from the first a mere opinion, as the fact of corals growing at
such depths had never been ascertained. The few species which
are met with in deep waters appear to be sparsely scattered,
and nowhere form accumulations or beds.

The above-mentioned authors, who explored the Pacific in
the Uranie under D’Urville (and afterward also in the As-
trolabe), concluded from their observations that five or six
fathoms (30 or 86 feet) limited their downward distribution.

RANGE IN DEPTH OF CORALS. 115

Ehrenberg, by his observations on the reefs of the Red Sea,
confirmed the observations of Quoy and Gaymard; he conclud-
ed that living corals do not occur beyond six fathoms. Mr.
Stutchbury, after a visit to some of the Paumotus and Tahiti,
remarks, in Volume I. of the West of England Journal, that
the living clumps do not rise from a greater depth than 16 or
17 fathoms.

Mr. Darwin, who traversed the Pacific with Captain Fitz-
roy, R. N., gives 20 fathoms as not too great a range.

In his soundings off the fringing reefs of Mauritius, in
the Indian ocean, on the leeward side of the island, he ob-
served especially two large species of Madrepores, and two
of Astrea; and a Millepora down to fifteen fathoms, with
also, in the deeper parts, Seriatopora; between fifteen and
twenty fathoms a bottom mostly of sand, but partly covered
with the Seriatopora, with a fragment of one of the Madre-
pores at twenty fathoms. He states that Capt. Moresby, in
his survey of the Maldives and Chagos group, found, at seven
or eight fathoms, great masses of living coral; at ten fathoms,
the same in groups with patches of white sand between ; and,
at a little greater depth, a smooth steep slope without any
living coral; and further, on the Padua Bank, the northern
part of the Laccadive group, which had a depth of twenty-five
to thirty-five fathoms, he saw only dead coral, while on other
banks in the same group.ten or twelve fathoms under water,
there was growing coral.

In the Red Sea, however, according to Capt. Moresby and
Lient. Wellstead, there are, to the north, large beds of living
corals at a depth of twenty-five fathoms, and the anchors were
often entangled by them; and he attributes this depth, so
much greater than reported by Ehrenberg, to the peculiar pu-
rity, or freedom from sediment, of the waters at that place. Kot-

116 CORALS AND CORAL ISLANDS.

zebue states that in some lagoons of the Marshall group he ob.
served living corals at a depth of twenty-five fathoms, or one
hundred and fifty feet.

Prof. Agassiz observes that about the Florida reefs, the
reef-building corals do not extend below 10 fathoms. Mr. L.
F. de Pourtales states that he found species of Oculina and Clad-
ocora off the Florida reefs living to a depth of 15 fathoms.

It thus appears that all recent investigators since Quoy and
Gaymard have agreed in assigning a comparatively small depth
to growing corals. The observations on this point, made dur-
ing the cruise of the Wilkes Exploring Expedition, tend to
confirm this opinion.

The conclusion is borne out by the fact that soundings in the
course of the various and extensive surveys afford no evidence
of growing coral beyond twenty fathoms. Where the depth
was fifteen fathoms, coral sand and fragments were almost uni-
formly reported. Among the Feejee Islands, the extent of
coral-reef grounds surveyed was many hundreds of square
miles, besides the harbors more carefully examined. The reefs
of the Navigator Islands were also sounded out, with others
at the Society Group, besides numerous coral islands; and
through all these regions no evidence was obtained of corals
living at a greater depth than fifteen or twenty fathoms.
Within the reefs west of Viti Lebu and Vanua Lebu, the anchor
of the Peacock was dropped sixty times in water from twelve
to twenty four fathoms deep, and in no case struck among
growing corals; it usually sunk into a muddy or sandy bottom.
Patches of reef were encountered at times, but they were at a
less depth than twelve fathoms. By means of a drag, occasion-
ally dropped in the same channels, some fleshy Alcyonia
and a few Hydroids were brought up, but no reef-forming
species.

RANGE IN DEPTH OF CORALS. 1s

Outside of the reef of Upolu, corals were seen by the writer
growing in twelve fathoms. Lieutenant Emmons brought up
with a boat-anchor a large Dendrophyllia from a depth of
fourteen and a half fathoms at the Feejees; and this species
was afterward found near the surface. But Dendrophyllia, it
may be remembered, is one of the deep-water genera.

These facts, it may be said, are only negative, as the sound-
ing-lead, especially in the manner it is thrown in surveys, would
fail of giving decisive results. The character of a growing
coral bed is so strongly marked in its uneven surface, its deep
holes and many entangling stems, to the vexation of the sur-
veyor, that in general the danger of mistake is small. But al-
lowing uncertainty as great as supposed, there can be little
doubt as to the general fact after so numerous observations
over so extended regions of reefs.

The depth of the water in harbors and about shores where
there is no coral, confirms the view here presented. At Upo-
lu, the depth of the harbors varies generally from twelve to
twenty fathoms. On the south side of this island, off Falealili,
one hundred yards from the rocky shores, Lieutenant Perry
found bare rocks in eighteen and nineteen fathoms, with no ev-
idence of coral. There is no cause here which will explain the
absence of coral, except the depth of water; for corals and
coral reefs abound on most other parts of Upolu. Below Fa-
lelatai, of the same island, an equal depth was found, with no
coral. Off the east cape of Falifa harbor, on the north side of
Upolu, Lieutenant Emmons found no coral, although the depth
was but eighteen fathoms. About the outer capes of Funga-
sa harbor, Tutuila, there was no coral, with a depth of fifteen
to twenty fathoms; and a line of soundings across from capc
to cape, afforded a bottom of sand and shells, in fifteen to
twenty-one and a half fathoms. About the capes of Oafonu

118 CORALS AND CORAL ISLANDS.

harbor, on the same island, there was no coral, with a depth
of fifteen fathoms.

Similar results were obtained about all the islands surveyed,
as the charts satisfactorily show. ‘There is hence little room to
doubt that twenty-five fathoms, or 150 feet, may be received
as the limit in depth of flourishing banks of reef corals.

It may however be much less, possibly not over half this, on
the colder border of the coral-reef seas, as, for example, at the
Hawaian Islands and the atolls northwest of that group. It
is natural that regions so little favorable for corals on account
of the temperature should differ in this respect from those in
the warmer tropics.

It may be here remarked, that soundings with reference to
this subject are liable to be incorrectly reported, by persons
who have not particularly studied living zodphytes. It is of
the utmost importance, in order that an observation supposed
to prove the occurrence of living coral should be of any value,
that fragments should be brought up for examination, in order
that it may be unequivocally determined whether the corals
are living or not. Dead corals may make impressions on a
lead as perfectly as living ones.

As to the origin of this narrow limit in depth, tempera-
ture may be one cause through the colder parts of the coral
seas, it having been proved to be predominant with regard
to distribution of lfe throughout the extent of the ocean.
Yet it is not the only cause. The range of temperature 85°
to 74° gives sufficient heat for the development of the greater
part of coral-reef species; and yet the temperature at the
100 foot plane in the middle Pacific is mostly above 74°.
The chief cause of limitation in depth is the diminished light,

as pointed out by Prof. T. Fuchs.*

1 Verh. k. k. geologischen Reichsanstalt, 1882, and Ann. Mag. N. H. Jan. 1883.

CAUSHS AFFEUTING THE GROWTH OF CORALS. 119

III. LOCAL CAUSES INFLUENCING DISTRIBUTION.

Coral making species generally require pure ocean water,
and they especially abound in the broad inner channels among
the reefs, within the large lagoons, and in the shallow waters
outside of the breakers. It is therefore an assertion wide from
the fact that only small corals grow in the lagoons and chan-
nels, though true of lagoons and channels of small size, or of
such parts of the larger channels as immediately adjoin the
mouths of freshwater streams.

There are undoubtedly species especially fitted for the open
ocean; but as peculiar conveniences are required for the col-
lection of zodphytes outside of the line of breakers, we have
not the facts necessary for an exact list of such species. From
the very abundant masses of Astrzeas, Mzandrinas, Porites,
and Madrepores thrown up by the waves on the exposed reefs,
it was evident that these genera were well represented in the
outer seas. In the Paumotus, the single individuals of Porites
lying upon the shores were at times six or eight feet in diam-
eter. Around the Duke of York’s Island the bottom was ob-
served to be covered with small branching and foliaceous
Montipores, as delicate as any of the species in more protected
waters.

Species of the same genera grow in the face of the breakers,
and some are identical with those that occur also in deeper wa-
ters. Numerous Astras, Meeandrinas and Madrepores grow
at the outer edge of the reefs where the waves come tumbling
in with their full force. There are also many Millepores and
some Porites and Pocillipores in the same places. But the
weaker Montipores, excepting incrusting species, are found in
stiller waters either deep or shallow.

120 CORALS AND CORAL ISLANDS.

Again, the same genera occur in the shallow waters of the
reef inside of the breakers. Astrzeas, Meeandrinas and Pocilli-
pores are not uncommon, though requiring pure waters. There
are also Madrepores, some growing even in impure waters.
One species was the only coral observed in the lagoon of Hon-
den Island (Paumotus), all others having disappeared, owing
to its imperfect connection with the sea. Upon the reefs en-
closing the harbor of Rewa (Viti Lebu), where a large river.
three hundred yards wide empties, which during freshets en-
ables vessels at anchor two and a half miles off its mouth to
dip up fresh water alongside, there is a single porous species
of Madrepora (M. cribripora), growing here and there in
patches over a surface of dead coral rock or sand. In similar
places about other regions, species of Porites are most com-
mon. In many instances, the living Porites were seen stand-
ing. six inches above low tide, where they were exposed to sun-
shine and to rains; and associated with them in such exposed
situations, there were usually great numbers of Alcyonia and
Xeniz. The Siderinz endure well exposure to the air.

The exposure of six inches above low tide, where the tide
is six feet, as in the Feejees, is of much shorter duration than
in the Paumotus, where the tide is less than half this amount ;
and consequently the height of growing coral, as compared
with low-tide level, varies with the height of the tides.

Porites also occur in the impure waters adjoining the
shores; and the massive species in such places commonly
spread out into flat disks, the top having died from the depo-
sition of sediment upon it.

The effects of sediment on growing zoéphytes are strongly
marked, and may be often perceived when a mingling of fresh
water alone produces little influence. We have mentioned
that the Porites are reduced to flattened masses by the lodg-

CAUSHS AFFECTING THE GROWTH OF CURALS. by |

ment of sediment. The same takes place with the hemispheres
of Astrea; and it is not uncommon that in this way large
areas at top are deprived of life. The other portions still live
unaffected by the injury thus sustained. Even the Fungia,
which are broad simple species, are occasionally destroyed over
a part of the disk through the same cause, and yet the rest re-
mains alive. It is natural, therefore, that wherever streams or
currents are moving or transporting sediment, there no corals
grow ; and for the same reason we find few living zodphytes
upon sandy or muddy shores.

The small lagoons, when shut out from the influx of the
sea, are often rendered too salt for growing zodphytes, in con-
sequence of evaporation,—a condition of the lagoon of Ender-
by’s Island.

They also are liable to become highly heated by the sun,
which likewise would lead to their depopulation.

Coral zodphytes sometimes suffer injury from being near
large fleshy Alcyonia, whose crowded drooping branches lying
over against them, destroy the polyps and mar the growing
mass. Again, the dead parts of a zodphyte, though in very many
cases protected by incrusting nullipores, shells, bryozoans, etc.,
as already explained, in others is weakened by boring shells
and sponges. Agassiz states, in his paper on the Florida
Reefs (Coast Survey Report for 1851): ‘‘ Innumerable bor-
ing animals establish themselves in the lifeless stem, piercing
holes in all directions into its interior, like so many augurs,
dissolving its solid connection with the ground, and even pen-
etrating far into the living portion of these compact communi-
ties. The number of these |horing animals is quite incredible,
and they belong to different families of the animal kingdom ;
among the most active and powerful we would mention the
date-fish or Lithodomus, several Saxicave, Petricole, Arce,

129 CORALS AND CORAL ISLANDS.

and many worms, of which the Serpula is the largest and most
destructive, inasmuch as it extends constantly through the liv-
ing part of the coral stems, especially in the Meandrina. On
the loose basis of a Meandrina, measuring less than two feet
in diameter, we have counted not less than fifty holes of the
date-fish—some large enough to admit a finger—besides hun-
dreds of small ones made by worms. But however efficient
these boring animals may be in preparing the coral stems for
decay, there is yet another agent, perhaps still more destruc-
tive. We allude to the minute boring-sponges, which pene-
trate them in all directions, until they appear at last com- ,

ae

|

pletely rotten through.”

On the other hand Serpulas and certain kinds of barnacles
(of the genus Creusia, etc.) penetrate living corals without in-
jury to them. ‘They attach themselves when young to the sur-
face of the coral, and finally become imbedded by the increase
of the zodphyte, without producing any defacement of the sur-
face, or affecting its growth. Many of these Serpulas grow with
the same rapidity as the zodphyte, and finally produce a long
tube, which penetrates deep within the coral mass ; and, when
alive, they expand a large and brilliant circle or spiral of deli-
cate rays, making a gorgeous display among the coral polyps.
Instinct seems to guide these animals in selecting those corals
which correspond with themselves in rate of growth; and
there is in general a resemblance between the markings of a
Creusia and the character of the radiations of the Astrea it in-
habits. |

In recapitulation, the three most influential causes of the
exclusion of reef-forming corals from coasts are the following:

I. The too low temperature of the waters along shores.

I]. The too great depth of the waters.

IIL. The proximity of the mouths of rivers, on account of

RATE OF GROWTH OF CORALS. 123

which sediment is distributed along the coast adjoining and
over the sea bottom.

IV. RATE OF GROWTH OF CORALS.

The rate of growth of coral is a subject but little under-
stood. We do not refer here to the progress of a reef in for-
mation, which is another question complicated by many co-op-
erating causes; but simply to the rapidity with which partic-
ular living species increase in size. There is no doubt that
the rate is different for different species. It is moreover prob-
able that it corresponds with the rate of growth of other al-
lied polyps that do not secrete lime. The rate of growth of
Actinie might give us an approximation to the rate of growth
in coral animals of like size and general character; for the ad-
ditional function of secreting lime would not necessarily re-
tard the maturing of the polyp; and from the rate of growth
of the same animals in the young state, we might perhaps
draw some inferences as to the rate in polyps of corresponding
size. But no satisfactory observations on this point have yet
been made.

Although the rapidity is undoubtedly far less than was
formerly reported, the following facts from different sources
seem to show that the rate is greater than has been of late be-
lieved. Mr. Darwin, citing from a manuscript by Dr. Allan,
of Forres, some experiments made on the east coast of Mada-
gascar, states that, in December, 1830, twenty corals were
weighed, and then placed by him apart on a sandbank, in three
feet water (low tide), andin the July following, each had nearly
reached the surface and was quite immovable; and some had
grown over the others. Mr. Darwin mentions also a state-
ment made to him by Lieut Wellstead, that ‘in the Persian

124 CORALS AND CORAL ISLANDS.

Gulf a ship had her copper bottom encrusted in the course of
twenty months, with a layer of coral two feet thick,—evi-
dently to be accepted hesitatingly. He also speaks of a chan-
nel in the lagoon of Keeling atoll having been stopped up in
less than ten years; and of the natives of the Maldives find-
ing it necessary occasionally to root out, as they express it,
coral knolls from their harbors.

Mr. Stutchbury describes a specimen consisting of a spe-
cies of oyster whose age could not be over two years, encrust-
ed by an Agaricia weighing two pounds nine ounces; but he
does not state whether the shell was that of a living oyster
or not.

Dr. D. F. Weinland states that on Hayti, in a small coral
basin between the town of Corail and the island Caymites,
never disturbed by vessels on account of the small depth of
water, he observed several branches of the Madrepora cervi-
cornis projecting above the surface of the water from three to
five inches, all of which, down to the water level, were dead,
as a result evidently of exposure to the air. This was in the
month of June. He adds that all along the north shore of
Hayti, the water level is from four to six feet higher in the
winter season than during summer; and suggests that the
growth of three to five inches, above referred to, might. have
been made during the three winter months.

Duchassaing (in L’Institut, 1846, p. 117) observes that in
two months some large individuals of Madrepora prolifera
which he broke away, were restored to their original size.
More definite and valuable is the observation of Mr. L. F. de
Pourtales, that a specimen of Meandrina labyrinthica, meas:
uring a foot in diameter, and four inches thick in the most
convex part, was taken from a block of concrete at Fort Jet:
terson, Tortugas, which had been in the water only twenty

RATE OF GROWTH OF CORALS. 125

years. Again, Major E. B. Hunt mentions, in the American
Journal of Science for 1863, the fact of the growth of a Mean-
drina at Key West, Florida, to a radius of six inches in twelve
years, showing an average upward increase in this hemispherical
coral of half an inch a year, if, as is evidently implied, this
radius was a vertical radius. Major Hunt deposited speci-
mens of corals of his collection near Fort Taylor, Key West,
in the Yale College Museum, and three of these are labelled
by him as having grown to their present size between the
years 1846 and 1860, or in fourteen years. ‘Two are speci-
mens of Oculina diffusa; one is a clump four inches high
and eight broad; and the other has about the same height.
The weight of the first of these clumps is forty-four ounces.
The rate of four inches in fourteen years would be equal to
about 34 twelfths of an inch a year in height, or three and one-
seventh ounces a year of solid coral. The other specimen is
of the Meandrina clivosa V.; it has a height of two and a
quarter inches and a breadth of seven and a half inches. This
is equivalent to about a sixth of an inch of upward growth
in fourteen years. ‘The specimen weighs about eighteen ounces.
It is not certain that with either of these specimens the germs
commenced to grow the first year of this interval, and hence
there is much doubt with regard to these calculations.

The following observations are from a paper read by
Prof. Verrill before the Boston Society of Natural History
in 1862. The wreck of a vessel, supposed to have been the
British frigate Severn, lost in 1793 near “Silver Bay,” off
Turk’s Islands, is covered with growing corals. It lies (accord-
ing to the journal of Mr. J. A. Whipple, by whom specimens
were collected in 1857) in about four fathoms of water. One
of the specimens was a mass of the species Orbicella annula-
ris, shaped somewhat like a hat; it is attached to the top of a

126 CORALS AND CORAL ISLANDS.

bell and spreads outward on all sides. The thickness of the
coral at the centre is about eight inches, and the breadth fit:
teen. Another specimen consisted of an olive jar and glass
decanters cemented together by a mass, of like size, of the same
species of coral. The interval since the wrecking of the ves-
sel, to 1857, was sixty-four years, and if the corals commenced
their growth immediately after the wreck the increase of this
species of coral is very slow.

The journal of Mr. Whipple, in the library of the same
society, contains the records of his observations on the spot,
and the efforts made to remove the corals in order to examine
the wreck. The following are a few extracts made from it by
Prof. Verrill:

April 21, 1857.—Moored our boat over the remains of a
large wreck, * * its depth being from three to ten fath-
oms. I made the first descent in the armor. I found the bot-
tom very uneven and covered with the remains of a man-of-
war, what appeared to be the bow lying in a gulch, with the
shanks of three large anchors, the palm of only one of which
projected out of the coral rock.

April 22.—Made a second descent and commenced exam-
ining in six fathoms of water on what appeared to be mid-
ships. All astern of this is thick branching coral (Madrepora),
and it must have made very fast, the branches being twelve
inches in diameter and sixteen feet in height. ‘To look among
it from the bottom reminds one of a thick forest of a heavy
srowth of timber. * * * ‘This branched coral appears to
grow where there is but very little iron, as I could see no guns
or shot around its roots. Commenced examining the cannon
with hammer and chisel. * * * Near these cannon, which
must have been near the forward part of the ship, I com-
menced to work on a clear space between the cannon. After

RATH OF GROWTH OF CORALS. L237

breaking three inches of coral crust I found the collar bone
of a man, a brass regulating screw belonging to a quadrant,

sian eee fale

and some large lead bullets. The magazine must

be under the branch-coral, which has been sixty-four years
meawing, ~ ~ *

Here we have a height of sixteen feet in a Madrepora
attained in sixty-four years, or at the rate of three inches a
year. Observations of Prof. Joseph Le Conte on Madre-
pora growths at the Tortugas in 1851 (American Journal
of Science, 1875) lead to a rate of 3} inches a year.

Observations on the rate of growth of different species
might easily be made by those residing in coral seas, either in
the manner adopted by Mr. Allan (placing the specimens on
a platform which could be raised for examination from time
to time—say every five years), or by placing marks upon par-
ticular species where they are immovably fixed to the bottom.
By inserting slender glass pins a certain distance from the sum-
mit of a Madrepore, its growth might be accurately measured
from month to month. Two such pins in the surface of an
Astra, would in the same manner, by the enlarging distance
between, show the rate of increase in the circumference of
the hemisphere; or if four were placed so as to enclose an
area, and the number of polyps counted, the numerical in-
crease of polyps resulting from budding, might be ascer-
tained. If specimens are selected, as done by Mr. Allan, it is
important that they should be placed where other corals are
growing in luxuriance, so as to be sure that there are no dele-
terious influences to retard growth. It is to be hoped that
some of the foreign residents at the Sandwich, Society, Samo-
an or Feejee Islands will take this subject in hand. There arc
also many parts of the West Indies where these investiga-
tions might be conveniently made.

128 CORALS AND CORAL ISLANDS.

CHAPTER II.
STRUCTURE OF CORAL REEFS AND ISLANDS.

Corat reefs and coral islands are structures of the same
kind under somewhat different conditions. They are made
in the same seas, by the same means; in fact, a coral island
has in all cases been a coral reef through a large part of its

history, and is so still over much of its area. The terms how- —

ever are not synonymous. Coral islands are reefs that stand

isolated in the ocean, away from other lands, whether now

raised only to the water’s edge and half submerged, or covered
with vegetation ; while the term coral reef, although used for
reefs of coral in general, is more especially applied to those
which occur along the shores of high islands and continents.
There are peculiarities in each making it convenient to describe
them separately.

I. CORAL REEFS.

IL GENERAL FEATURES.

Coral reefs are bans of coral rock built upon the sea-bot-
tom about the shores of tropical lands. In the Pacitic, these
lands, with the exception of New Caledonia and others of
large size to the westward, are islands of volcanic or igneous
rocks. and they often rise to mountain heights. The coral
reefs which skirt their shores are ordinarily wholly submerged
at high tide; but, at the ebb, they commonly present to view a
broad, flat, bare surface of rock, just above the water level,

os

STRUCTURE OF CORAL REEFS. 129

strongly contrasting with the steep slopes of the encircled
island.

Nearing in a vessel a coral-bound coast, the first sign of the
reef, when the tide is well in, is a line of heavy breakers, per-
haps miles in length, off a great distance from the land. On
closer view, some spots of bare reef may be distinguished as
the waves retreat for another plunge; but the next moment
all again is an interminable line of careering waters. Happy
for the cruiser in untried reef-regions, if the surging waves con-
tinue to mark the line of reef; for a treacherous quiet some-
times intervenes, which seems to be evidence of deep waters
ahead, and the unsuspecting craft dashes onward ; but soon it
is grinding over the coral masses, then thumping heavily at
short intervals, and, in a few moments more, is landed helpless
on the coral reef. The heavier billows as they roll by a vessel
in such a plight—the author’s experience attesting—have a
way of lifting it and then letting it drop with all its
weight against the bottom, and hence, unless prompt escape is
in some way secured, the assaulting waves gain speedy posses-
sion, and soon after make complete the work of destruction.
At low tide the breakers often cease, or nearly so. But the
reef for the most part, is then in full view, and, with a good
lookout aloft, favorable winds, and plenty of daylight, navi-
gation is comparatively safe.

Some idea of the features of a tropical island thus bor-
dered, may be derived from the following sketch. The reef
to the right is observed to fringe the shore, making a simple
broad platform, as an extension, apparently, of the dry land.
To the left there is the same coral platform at the surface, but
it is divided by a channel into an inner and an outer reef—a
Jringing and a barrier reef, as these two parts are called. At

a single place the sea is faced by a cliff; and here, owing to
9

130 CORALS AND CORAL ISLANDS.

the boldness of the shores and depth of waters, the reef is
wanting. The barrier reef at one point has a passage through
it, which is an opening to a harbor; and many such harbors
exist about coral-girt islands.

HIGH ISLAND WITH BARRIER AND FRINGING REEFS.

While some islands have only narrow fringing reefs, others
are almost or quite surrounded by the distant barrier, which
stands off like an artificial mole to protect the land from an
encroaching ocean. ‘The barrier is occasionally ten or fifteen
miles from the land, and encloses not only one, but at times
several, high islands. From reefs of this large size, there are
all possible variations down to the simple fringing platform.

The inner channel is sometimes barely deep enough at low -
tide for canoes, or for long distances may be wanting entirely.
“hen again, it is a narrow intricate passage, obstructed by

1olls or patches of coral, rendering the navigation dangerous.

gain, it is for miles in length an open sea, in which ships
find room to beat against a head wind with a depth of ten,
twenty, or even thirty fathoms. Yet hidden reefs make caution
necessary. Patches of growing corals, from a few square feet
to many square miles in extent, are met with over the broad
area enclosed by these distant barriers.

These varieties of form and position are well exemplified
in a single group of islands—the Feejees; and the reader is
referred to the chart of this Archipelago at the close of this

volume. x

STRUCTURE OF CORAL REEFS. 131

Near the middle of the chart is the island Goro ; its shores,
excepting the western, are bordered by a fringing reef. The
island Angau, south of Goro, is encircled by a coral breakwa-
ter, which on the southern and western sides runs far from the
shores, and is a proper barrier reef, while on the eastern side,
the same reef is attached to the coast and is a fringing reef.
From these examples we perceive the close relation of barrier
and fringing reefs. While a reef is sometimes quite encircling,
in other instances it is interrupted, or wholly wanting, along
certain shores ; and occasionally it may be confined to asingle
point of an island.

Above Angau lies Naira: ; although a smaller island than
Angau, the barrier reef is of greater extent, and stretches off
far from the shores. *To the eastward of Nairai are Vatu
- Rera, Chichia, and Naiau, other examples of islands fringed
around with narrow reefs. Lakemba, a little more to the
southward, is also encircled with coral ; but on the east side
the reef is a distant barrier. In Azva, immediately south of
Lakemba, the same structure is exemplified; but the coral
ring is singularly large for the little spots of land it encloses.
The Argo Reef, east of Lakemba, is a still larger barrier, en-
circling two points of rock called Bacon’s Isles. It is actually
a large lagoon island, twenty miles long, with some coral islets
in the lagoon, and two of basaltic constitution, of which the
largest is only a mile in diameter. Aiva and Lakemba are in
fact other lagoon islands, in which the rocky islands of the in-
terior bear a larger proportion to the whole area. The same
view is further illustrated by comparing the Argo reef with
Nairai, Angau, or Moala: these cases differ only in the great-
er or less distance of the reef from the shores and the extent
of the enclosed land.

« Passing to the large islands Vanua Levu and Viti Levu,

132 CORALS AND CORAL ISLANDS.

we observe the same peculiarities illustrated on a much grand-
er scale. Along the southern shores of Viti Levu, the coral
reef lies close against the coast; and the same is seen on the
east side and north extremity of Vanua Levu. But on the
west side of these islands, this reef stretches far off from the
land, and in some parts is even twenty-five miles distant, with

a broad sea within. ‘This sea, however, is obstructed by reefs,

and along the shores there are proper fringing reefs.

The forms of encircling reefs depend evidently to a great
extent on that of the land they enclose. That this is the case
even in the Argo reef, and such other examples as offer now
but a single rock above the surface of the enclosed lagoon, we
shall endeavor to make apparent, if not already so, when the
cause of the forms of coral islands is under discussion. Yet it
is also evident that this correspondence is not exact, for many
parts of the shores, and sometimes more than half the coasts,
may be exposed to the sea, while other portions are protected
by a wide barrier.

In recapitulation, we remark, that reefs around islands may
be (1) entirely encircling ; or they may be (2) confined to a larg-
er or a smaller portion of the coast, either continuous or inter-
rupted; they may (3) constitute throughout a distant barrier ;
or (4) the reef may be fringing in one part and a barrier in
another; or (5) it may be fringing alone: the barrier may be
(6) at a great distance from the shores, with a wide sea within,
or (7) it may so unite to the fringing reef that the channel be-
tween will hardly float a canoe. These points are sustained
by all reef regions.

It is to be noted that the fringing and barrier reefs here
pointed out are not the whole of the coral reef; they are only
the portions that have been built up to the water's level. Be-
tween them, and also outside of all, there are the submerged

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STRUCTURE OF CORAL REEFS. 135

coral banks which are continuous with the higher portions, and
all together make up the coral reef-ground of an island.

A wide difference in the extent of reef-grounds, follows
from the above-mentioned facts. On some coasts there are only
scattered groups of corals, or rising knolls, or mere points of
emerged coral rock ; but again, as for example, west of the two
large Feejee Islands, there may be three thousand square miles
of continuous reef-ground, occupied with coral patches and in-
termediate channels or seas. ‘The enclosing barrier off Vanua
Levu alone is more than one hundred miles long. The Ex-
ploring Isles, in the eastern part of the Feejee group, have a
barrier eighty miles in circuit. New Caledonia has a reef
along its whole western shores, a distance of two hundred and
fifty miles, and it extends one hundred and fifty miles farther
north, adding this much to the length of the island. The
great Australian barrier forms a broken line, twelve hundred
and fifty miles in length, lying off the coast from the Northern
Cape to the tropical circle.

In the Louisiade Archipelago, Plate VII., the area within
the great reef, one hundred and twenty-five miles long, is
five sixths water, with depths of ten to two hundred feet ;
and the westernmost island is an atoll.

In the further description of reef-grounds, we note:

1. Outer reefs, or reets formed from the growth of corals
exposed to the open seas. Of this character are all proper
barrier reefs, and such fringing reefs as are unprotected by a
barrier.

2. Inner reefs, or reefs formed in quiet water between a
barrier and the shores of an island.

3. Channels, or seas within barriers, which may receive de-
tritus either from the reefs, or from the shores, or from both of

these sources combined

136 CORALS AND CORAL ISLANDS.

4, Beach and Drift formations, produced by coral accu-
mulations on the shores through the action of the sea and
winds,

The outer and inner reefs, channels, and beaches, act each
their part in producing the coral formations in progress about
islands.

Il. OUTER REEFS.

The barrier and other outer reefs are always submerged at
high tide, except where elevated at surface by accumulations
of beach sands. The level is generally that of about one third
tide. The coral rock is built up by the agencies at work to
this level, and hence the existence of the broad plattorm-like
top of the barrier. The surface is however not even, for there
are many pools of water over it, even at the lowest tides, espe-
cially toward its outer limits, where corals of various kinds are
crowing luxuriantly, with fit associates of shells, star-fishes,
echini, holothurias with their large flower-bearing heads,
sponges, corallines and sea-weeds, making scenes of rare beauty.
The growing corals are, however, most abundant along the outer
margin of the reef, and in the adjoming shallow seas. Here
they grow in profusion ; but yet the eager lover of coral land-
scapes will be often disappointed by finding among the crowd-
ed plantations, extensive areas of coral sand.

The outer margin of the reef receives the plunging waves,
and under this action, and the consequent unequal growth of
the corals, the outline is very irregular, being often deeply cut
into, and hence having sometimes long channels that give en-
trance to the surging tide, and to the currents that flow back
in preparation for the next breaker. From it, seaward, the
depth of water usually sinks off rapidly from three to six fath-
oms, and then falls away more gradually for many rods, or it

STRUCTURE OF CORAL REEFS. 137

may be some hundreds of yards ; over the bottom in these shal.
low waters are spread out the coral plantations, down to a
depth of 80 to 150 feet. Finally there is a rather abrupt de
scent to depths beyond the reach of an ordinary sounding-lead.
The great difference in the rapidity with which the water deep-
ens depends chiefly on the varied character of submarine
slopes. Shallow waters may extend out for miles, especially
off the prominent points or angles; but it is more common to
meet with the opposite extreme—great depths within a few
hundred feet.

The outer reef or coral platform is generally a little the
highest at its seaward margin, owing partly to the growth of
ordinary corals and other species on this part, and also to the
accumulations which naturally would there be piled up by
the waves and become cemented. ‘This part is therefore first
laid bare by the retreating tide; and though a tempting place
for a ramble, it is often a dangerous place on account of the
heavy breakers. There is not only greater height, but often
also a remarkably smooth surface to the reef-rock, looking as
if water-worn, and frequently a blotching of the rock with va-
rious shades of pink and purple. These colors and the smooth-
ness, as observed by Chamisso, are due to incrusting Nulli-
pores; and to the same calcareous sea-weeds, as Darwin first
observed, is often owing the increased height. The material
of the incrusting plant is more solid than ordinary coral, for it
is without a pore; and layer is added to layer until it has con-
siderable thickness. It is thus an important protection to the
reef against the wash of the waters.

Darwin states that on Keeling Island, the Nullipore bed
has a thickness of two or three feet and a breadth of twenty
feet. Nullipores are abundant on the Paumotu reefs. Still,
they are not essential to the formation or protection of an

138 CORALS AND CORAL ISLANDS.

outer reef, and are not always present; the outer margin is
higher than the rest of the reef when they are absent.

The Nullipores are not alone on this outer edge, for there
are always sprigs of Madrepores, small Astrzeas, and some oth-
er corals, lodged in the cavities, with many Echini, star-fishes
and sea-anemones, besides barnacles and serpulas; and fish of
many colors dart in and out of the numerous recesses.

Outer reefs are far more lable than the inner to become
covered with accumulations of coral fragments and sand
through the force and inward movement of the waves. The
debris gathered up by the waters finds a lodgment some dis-
tance back from the margin—it may be one or two hundred
fect, or as many yards, and gradually increases, until in many
instances dry land is formed, and an islet covered with vegeta-
tion appears. Such effects are confined chiefly to the reef on
the sides open to the prevailing wind, and the final result, a
green islet, is not of common occurrence. But occasionally,
the reef for miles has become changed from the coral bank,
bare at low or middle tide, to habitable land, and makes liter-
ally, as at Bolabola, a green belt to the island of volcanic rocks
and lofty hills within. The causes and the result are much the
same as in a coral island, and the steps in the process are
more particularly described beyond where treating of atolls.

The rock of the outer reef, wherever broken, exhibits usu-
ally a compact texture. In some parts it consists of coral
fragments, rounded or angular, of quite large size, firmly ce-
mented. Other portions are a finer coral breccia or conglom-
erate. Still others, more common, are solid white limestones,
as impalpable and homogeneous in texture as the old limestones
of our continents. There are also other regions where the
corals in the rock retain the original position of growth. But
the rock in general consists of the debris of the coral fields,

STRUCTURE OF CORAL REEFS. 139

consolidated by a calcareous cement; and the great abundance
of the finer variety of rock indicates that much of it has orig-
inated from coral sand or mud. Wherever broken, it usually
presents the character here described, a texture indicating a
detrital or conglomeritic origin. Such a reef-rock is formed
in the midst of the waves; and to this fact it owes many of its
peculiarities. Reef-rocks made of corals in the position of
erowth are formed about the outer reefs wherever the corals
grow undisturbed.

Besides corals, the shells of the seas contribute to it, and it
sometimes contains them as fossils, along with bones of fishes,
exuvia of crabs, spines and fragments of Echini, Orbitolites
(disk-shaped foraminifers), the tubes of Serpule or sea-worms,
and other remains of organic life inhabiting reef-grounds.

, 11. FORMATIONS IN THE SEA OUTSIDE OF THE BARRIER REEFS.

While barrier reefs are mostly made up of coarse coral ma-
terial, owing to the rough action of the waves, the region im-
mediately outside of the breakers, where of much width, is, to a
depth of 50 to 150 feet, one of growing patches of coral and
extended surfaces of coral sands.

Isolated islets of reef-rock are not however of common oc-
currence in the middle Pacific, though occurring in large groups
like the Feejees. They are most likely to occur where there
are great regions of shallow water extending outward from the
barrier, and where the tides are not heavy or there is partial pro-
tection from them. In some seas, such isolated patches are shaped
somewhat like a great mushroom—having a narrow trunk
or column below, supporting a broad shelf of reef above. Mr.
J. A. Whipple, in his Journal, referred to on page 126, figures
and describes one of these ‘coral heads” standing in water fit-
ty feet deep, near Turks Island. Its trunk, which made up

140 CORALS AND CORAL ISLANDS.

two thirds of its height (or of the fifty feet), was only fifteen
feet in diameter along its upper half; and it supported above a
great tabular mass one hundred feet in diameter, whose top was
bare at low tide. ‘The tide at this place is but two feet, and
this is favorable to the preservation of such top-heavy struc-
tures. In many places, he says, these tops have joined together,
leaving arches between them ; and in some parts of the reef-re-
gion such united coral-heads cover acres in extent, being joined
together above and supported by their pillars. A case is re-
ported of a whale having gone through one of these under
passages after being struck with a harpoon. Mr. Whipple
also states that there are cavernous recesses in some of these
heads, some that are 200 to 300 feet across; and ‘“‘ when there
is a heavy swell on, the water is one entire sheet of white foam,
caused by its being forced through them and the air entering
as the heavy sea recedes from them.”

THE LIXO CORAL REEF, ABROLHOS.

Professor C. F. Hartt, in his ‘‘ Geology, ete., of Brazil”
(1870), describes very similar coral-heads in his account of the
reefs of the Abrolhos, and represents a scene of coral-head tops
in a sketch, of which the preceding is a copy. Professor Hartt
speaks of it as giving simply a general view of the region with-

STRUCTURE OF CORAL REEFS. eI

out any attempt at accuracy of position. The patches of reef
in the view are of this coral-head kind, though not all as slen-
derly supported as that above described. A vessel is represent-
ed passing through a passage between two of them. Prof.
Hartt, after describing the fringing reefs of the Abrolhos, gives
the following account of the outside coral formations (p. 199).
“Corals grow over the bottom in small patches, 7n the open sea,
and, without spreading much, often rise to a height of forty or
fifty or more feet, like towers, and sometimes attain the level
of low water, forming what are called on the Brazilian coast
chapeiroes (signifying big hats). At the top these are usually
very irregular, and sometimes spread out like mushrooms, or,
as the fishermen say, like umbrellas. Some of these chapei-
rdes are only a few feet in diameter. A few miles to the east-
ward of the Abrolhos is an area, with a length of nine to ten
and in some places a breadth of four miles, over which these
structures grow abundantly, forming the well known Parcel
dos Abrolhos, on which so many vessels have been wrecked.”
‘“‘ Among these chapeirdes I measured a depth of sixteen to
twenty metres, and once, while becalmed, I found twenty me-
tres alongside of one and three metres on top. They are
rarely laid bare by the tide. They do not coalesce here to
form large reefs as they do to the west of the islands. * * *
Sometimes vessels striking heavily on small chapeirdes, break
them off and escape without injury, as has been remarked by
Mouchez. At other times a vessel may run upon one and stick
fast by the middle of the keel, to the amazement of the cap-
tain, who finds deep water all around, the vessel being perched
on the chapeirges like a weather-cock on the top of a tower.”
“In the northern part of the Parcel the chapeirées so close-
ly unite as to form an immense reef, which has grown upward
to a level a little above low water, and is quite uncovered at

142 CORALS AND CORAL ISLANDS.

low tide.” “The northeastern part of the reef is called the,

Recife do Lixo, that is, Reef of the /zxo, a shark-like ray which is
furnished with large crushing teeth and frequents the reef in
search of shell-fish.”

The rock of the submerged coral-heads is but a loose ag-
gregation of corals in the position of growth, except probably,
in their lower portion, where the open spaces may be filled
with sand and fragments and all cemented together.

The deposits of sand or coral mud over the bottom of the
seas outside of barrier reefs are sometimes of great extent.
These sands are the fine detritus which the return flow of the
breaker bears seaward ; and, in still deeper water, the deposits
should be mainly of the finest calcareous sand or mud—fit ma-
terial for impalpable compact limestones. The waters outside
of the reef, especially when moved by heavy tidal currents or
storms, are often milky with the coral sand; and while the
coarser sand is dropped near the shores, the finer may be
. carried for miles and distributed far out to sea. As Major
Hunt, in his observations on the Florida Reefs remarks, this
‘white water” is one of the signs of proximity to a coral reef.
After storms, the white coral material subsides and the waters
become clear again.

Mr. Jukes, who made special examinations of the Australi-
an reef region, and others in that vicinity, in H. M.S. Fly,
states that in the deeper waters outside of the great barrier,
‘and in all the neighboring East India seas, from Torres
Straits, north of Austraiia, to the Straits of Malacca, wher-
ever the bottom was brought up by the lead, it proved to
be a very fine-grained, impalpable, pale olive-green mud,
wholly soluble in dilute hydrochloric acid, and therefore essen-
tially carbonate of lime. The substance, when dried, looked
much like chalk, excepting in its greener tinge. How far this

STRUCTURE OF CORAL REEFS. 143

calcareous matter may be due to foraminifers, rather than cor-
als, is not known.”

Since the tidal waves on any coast that is gradually shal-
lowing have a landward propelling power, the coral sands are
mostly gathered about the reef, and generally are not to any
great extent lost in the depths of the ocean. The great ocean-
ic currents, like that of the Gulf stream, might bear away the
lighter material for long distances, if it swept with full strength
over the shore reefs; but it is generally true that such cur-
rents are little felt close in shore. Notwithstanding the prox-
imity of the Florida reets, and the strength of the Gulf stream
in the channel between the Keys and Florida, the adjoining
sea-bottom consists mainly of common inud, with relics of deep
water life, and only sparingly of coral débris. According to
Mr. L. F. de Pourtales, between twelve fathoms and one
hundred, in the Florida channel, outside of the reef, coral frag-
ments occur, but are rare; dead specimens of Cladocora and
Oculina occur to a depth of about 50 fathoms. But on the
other side of the channel, ‘‘along the Salt Key Bank, dead
corals were dredged up in 315 fathoms ; but this is at the foot
of a very steep slope washed by the edge of the Gulf stream ;
which is much better defined here than on the Florida side.”
The bottom, in the Florida channel, of 100 fathoms, is a rocky
plateau, and outside of 200 fathoms, a mud full of foraminifers,
Globigerina mud, as it is called from the species characterizing
it; and yet this channel is situated beneath the Gulf stream and
close by the Florida reefs. The facts seem to show that in most
regions the reefs contribute little calcareous matter to the deep
ocean. ‘This may be otherwise over the bottom, of compara-
tively little depth, of a great Archipelago like that of the East
Indies.

144 CORALS AND CORAL ISLANDS.

IV. INNER REEFS.

In the still waters of the inner channels or lagoons, when
of large extent, we find corals growing in their greatest per.
fection, and the richest views are presented to the explorer of
coral scenery. There are many regions—in the Feejees, ex-
amples are common—where a remote barrier encloses as pure
a sea as the ocean beyond; and the greatest agitation is only
such as the wind may excite on a narrow lake or channel.
This condition gives rise to some important peculiarities of
structure in the inner reefs, in which the inner margin of the
barrier reef participates.

In the general appearance of the surface, the inner gener-
ally much resemble the outer reefs. They are nearly flat, and,
though mostly bare of life, and much covered with coral sand,
there are seldom any large accumulations of coral débris. The
margin is generally less abrupt; yet there is every variety of
slope, from the gradually inclined bed of corals to the bluff de-
clivity with its clinging clumps. In different parts, there are
many: portions still under water at the lowest tides; and here
(as well as upon the outer banks) fine fishing sport is afforded
the natives, who wade out at ebb tide with spears, pronged
sticks, and nets, to supply themselves with food. The lover of
the marvellous may find abundant gratification by joining in
such a ramble; for besides living corals, there are myriads of
other beings which science alone has named, of various beauti-
ful forms and colors, as becomes the inhabitants of a coral world.

Between the large reefs, which spread a broad surface, at
the water’s edge, of lifeless coral rock, sometimes of great ex-
tent, there are other patches, still submerged, that are cov-
ered with growing corals throughout. ‘They are of different
elevations under the water’s surface; and though at times but

STRUCTURE OF CORAL REEFS. 145

a few yards in breadth, there is often alongside of them a
depth of many fathoms. The mushroom shape described
above is common among them; and a ship striking one with
her keel may crush it and glide on. More frequently, they
are at bottom like the solid reef above described, and the con-
test is more likely to be fatal to the vessel than to the coral
patch. Ina passage between two reefs near Tongatabu, called
the Astrolabe channel, the sloop-of-war Vincennes ran on a
coral patch, which had been laid down as a reef. It stopped
the ship for a moment, but broke away under her; and in the
survey of the passage afterward, says Captain Wilkes, “no shoal
was found in the place where the ship had struck, and we had the
satisfaction of knowing that we had destroyed it without injury
to the vessel.” Corals grow over these patches, as in the shal-
low waters about other reefs ; and, as elsewhere, there are deep
cavities among the congregated corals, in which a lead will some-
times sink to a depth of many feet, or even fathoms. These
holes about growing reefs often give much annoyance to the
boat which may venture to’anchor upon them; and in many
an instance diving is found to be the only resource left for free
ing the foul anchor.

The margins of the reefs in and about the inner channels
are often luxuriant with magnificent corals quite to the edge,
so that while the reef is elsewhere solid rock to its very top,
here at the margin it is alive and may be said literally to be
growing.

The rock of the inner reefs seldom consists of rolled or
broken fragments of coral like a large part of that of the
outer reef. It is often made of dead corals, standing to a
great extent as they grew; yet it is generally compact and
firm in texture. The cavities among the branches and masses

gradually become filled with coral sand, and the whole is
10

146 CORALS AND CORAL ISLANDS.

finally cemented and so made solid. At Tongatabu and
among the Feejee Islands, reefs thus formed of corals standing
in their growing positions are common. ‘Though now mere
dead rock, and exceedingly firm and compact, the limits of
the several constituent coral masses may be distinctly made
out. Some individual specimens of Porites in the rock of the
inner reef of Tongatabu are twenty-five feet in diameter ; and
Astras and Meeandrinas, both there and in the Feejees, meas-
ure twelve to fifteen feet. These corals, when growing be-
neath the water, form, as has been stated, solid hemispheres,
or rounded hillocks; but on reaching the surface, the top dies,
and enlargement takes place only on the sides; and in this
manner the hemisphere is finally changed to a broad cylinder
with a flat top. This was the condition of the Astraas and
Porites in the reef-rock referred to. Such a platform looks
like a Cyclopean pavement, except that the calcareous ce-
menting material, fillmg im between the huge masses, is more
solid than in any work of art: it even exceeds in compactness
the corals themselves. Other portions of reefs consist of
branching corals, with the intervals filled in by sand and small
fragments; for even in the stiller waters fragments are to some
extent produced. <A rock of this kind is often used for build-
ings and for walls on the island of Oahu. It consists mainly
of Porites, and in many parts is still cavernous, or but imper-
fectly cemented.

There is also to be found about inner reefs, over large
areas, the solid white limestone already described, showing
internally no evidence of its coral origin, and containing rarely
a shell or other imbedded fossil. It is a result of the consoli-
dation of the fine coral sand or mud that is made and accu-
mulated through the action of the light waves that work over
the inner reefs. It has been said that large regions of barren

\
\
}

STRUCTURE OF CORAL REEFS. 147

sands or mud occur among the patches of growing corals, and
these would give origin to this compact limestone.

The formation of the inner reefs goes on at a less rapid
rate than that of the outer, because the process depends on the
growth of the corals with comparatively little aid from the
action of the waves. Moreover, as is explained more par-
ticularly in another place, impure or fresh waters and cur-
rents often operate to destroy the living corals or retard their
progress.

Owing to the last mentioned cause, the inner reefs are not
usually joined directly to the beach. They stand off a little,
separated by an interval of shallow water. At Mathuata, in
the Feejees, however, the reef extends quite up; and it is
the more remarkable as the coast is flat, the site of a Feejee
village, and a mile or two back stands a high bluff. On an
island off this part of Vanua Lebu there is another exam-
ple of this fact, and many more might be cited. In such
cases, however, there is evidence that the shores upon which
the corals grew were bare rocks, instead of moving beach-
sands.

From these descriptions it appears that the main distinc-
tion between the inner and outer reefs consists in the less frag-
mentary character of the rock in the former case, the less fre-
quent accumulations of débris on their upper surface, and the
more varied features and slopes of the margin. Moreover,
the Nullipores, which seem to flourish best in the breakers,
are here but sparingly met with.

The variety of coral zodphytes is also greater in the stiller
waters, when these have great breadth, and there are species

peculiar to the different regions.

148 «~ \ CORALS AND CORAL ISLANDS. |

V. CHANNELS AMONG REEFS.

To complete this review of the general appearance and
constitution of reef formations, it remains to add some partic-
lars respecting the channels which intervene between coral
patches, or separate them from the shores of an island, and
also to describe the coral accumulations forming beaches.

The reef of Australia has been instanced as affording an
example of one of the larger reef-channels, varying from twenty
to sixty miles in width, and as many fathoms in depth. Its
average distance from the land is twenty to thirty miles, and
the ordinary depth ten to twenty-five fathoms; but toward the
southern end, where the channel is widest, the depth exceeds
sixty fathoms. ‘‘ The new Caledonia barrier reefs, 400 miles in
length,” says Darwin, “ seldom approach within eight miles of
the shore.” The reefs west of the large Feejee Islands are
another remarkable example, the reef-grounds being in some
parts twenty-five miles wide, and the waters within the bar-
rier, where sounded, twelve to forty fathoms in depth. ‘The
barrier in this instance may be from a few hundred yards to
half a mile in width; and some of the inner patches are of
the same extent; but by far the larger part of the reef-ground
is covered with deep waters, mostly blue like the ocean, and
as clear and pure. In the course of the cruise of the Wilkes
Exploring Expedition, the sloop of war Peacock sailed along
the west coast of both Viti Lebu and Vanua Lebu, within the
inner reefs, a distance exceeding two hundred miles.

The island of Tahiti, on its northern side, presents a good
illustration of a narrow channel, and at the same time one
that exhibits the usual broken or interrupted character of
reefs, This is seen in the following cut, in which the reefs,
both fringing and barrier, are the parts enclosed by dotted

STRUCTURE OF CORAL REEFS. 149

Jines. The outer reef extends half to two-thirds of a mile
from the shore. Within it, between Papieti and Matavai,
there is an irregular ship channel, varying from three to

CORAL REEFS OFF THE NORTH SHORE OF TAHITI,

twenty fathoms in depth. Occasionally it enlarges into har-
bors; and in other parts it is very intricate, though throughout
navigable by large vessels. The island of Upolu, of the Sa-
moan Group, is bordered by a reef nearly a mile wide on part
of its northern shore; but the waters within are too shallow
for a canoe at low tide; and therefore, notwithstanding its ex-
tent, the reef is rather a fringing than a barrier reef. Within
the green belt that encircles Bolabola (p. 138) there is a large
and deep channel navigable by ships.

Beneath these channels lies, in general, the coral rock of the
reef-region—the inferior part of the great reef formation whose
upper portions constitute the so-called barrier and fringing
reefs. The rock would necessarily resemble that of the inner
reefs already described; but there should be a larger propor-
tion of the white compact limestone made from the fine coral
sands carried off from the higher reefs by the currents.

150 CORALS AND CORAL ISLANDS.

Yet the bottoms of these channels are not always made up
of calcareous or coral sands and fragments; for the volcanic
or basaltic lands they adjoin are a source of ordinary mud;
and the river courses of the land and the tidal currents of the
sea will often determine the nature of the bottom, or may |
cause in it alternate variations.

At Upolu the white coral sands of the reefs (or in more
general terms the reef débris), forms the bottom. In some
places this coral material had the consistence of mud, and it
was seldom observed to be covered with coarse material ;
there were some small patches of coral over it, and here and
there a growing mass of Porites. The fresh waters of the
shores do not flow over these wide reefs, as there is no proper
inner channel, and there is consequently no shore detritus
mingled with the reef débris.

At Tahiti, the sounding lead, where dropped in the channels,
usually brought up sand, shells, and fragments of coral. At
Tongatabu, the bottom where the Peacock anchored was a
grayish blue calcareous mud, appearing as plastic as common
clay ; it consisted solely of comminuted corals and shells, with
coloring matter probably from vegetable and animal decom-
position.

But to the west of the larger Feejee islands, in the channels
near the land, soundings commonly indicated a bottom of mud
made from the material of the rocks of the mountains, and the
same was frequently brought up with our dredges. On the
north side of Vanua Lebu, a stream had so filled with its de-
tritus the wide channel into which it empties, that for a mile
the depth is but two to three fathoms, although elsewhere the
depth is mostly from twelve to twenty fathoms; and at least
half a dozen square miles of land had been added to the shores
trom this source. Though due principally to shore material.

STRUCTURE OF CORAL REEFS. 151

the reefs have probably added somewhat to these accumula-
tions; yet little coral sand could be detected in the mud by
the eye, and the proportion is certainly very small. In many
places where the ships of the Wilkes Exploring Expedition an-
chored, having the reef not more than five hundred yards from
the ship, the material of the bottom was wholly mud from
the land, as much so as if there were no corals or shells with-
in many miles.

When the materials from both sources, the shore and the
reef, are mingled, the proportion will necessarily depend on
the proximity to the mouths of streams, the breadth of the
inner waters or channels, and the direction and force of the
currents. These tidal currents often have great strength, and
are much modified and increased in force at certain places, or
diminished in others, by the position of the reef with reference
to the land. Sweeping on, they carry off the coral débris
from some regions to others distant; and again they bear along
and distribute only the shore detritus. It is thus seen that
the same region may differ widely in its adjacent parts, and
seemingly afford evidence in one place that there is no coral
near, and in another no high land, although either is within a
few rods, or even close alongside.

The extent of the land in proportion to the reef will have
an obvious effect upon the character of the channel or lagoon
depositions. When the island stands, like one of Bacon’s isles
in the Feejees, as a mere point of rock in a wide sea en-
closed by a distant barrier, the streams of the land are small
and their detritus quite limited in amount. In such a case,
the reef, and the growing patches scattered over the lagoon, are
the sources of nearly all the material that is accumulated upon
the bottom.

The bottom between the inner reefs within the great Aus-

1 Ly CORALS AND CORAL ISLANDS.

tralian barrier, according to Jukes, as brought up by the
dredge from depths of fifteen to twenty fathoms, often resem-
bles the unconsolidated mass of a shelly or coralline limestone.
At other times it consisted very largely of the small disk-shaped
foraminifers called Orbitolites, closely allied in form and na-
ture to the Nummulites of the Tertiary; and they seemed
in some places to make up the whole sand of the beaches, both
of the coral islets and of the neighboring Australian shores.

The facts show that the rock formed in such channels may
be of all the kinds that occur in reef regions—coral and shell
conglomerates, compact impalpable limestones, limestones full
of Orbitolites, or containing, as well, remains of other species of
the seas, and also rocks made of the clay, mud, sand or pebbles
of the mountains or high lands adjoining.

VI. BEACH SAND-ROCK.

Besides the ordinary coral rock, there are also beach for-
mations made of coral sands, worn shells, etc., thrown up by
the tides and waves. ‘Their mode of formation is like that
of any sea-beach. The material is mostly like common sand in
fineness, but often much coarser. When the beach is fronted
by a distant barrier to shield it from the force of the waves,
the material is usually sand and small pebbles; but if the reef
is narrow, so that the sea breaks over it with full force, it may
consist even of cobble stones, as on any other shore, and in-
clude also huge masses of coral rock.

These deposits become cemented by being alternately mois-
tened and dried, through the action of the recurring tides and
the wash of the sea on the shores. The waters take up some
carbonate of lime, and this is deposited and hardens among the
particles on the evaporation of the moisture at the retreat of
the tides. In some places the grains are loosely coherent, and

STRUCTURE OF CORAL REEFS. 53

seem to be united only by the few points in contact; and with
a little care the calcareous coating which caused the union
may be distinctly traced out. In other cases, the sand has
been consolidated into a solid limestone rock, the interstices
having been filled till a compact mass was formed. Generally
even the most solid varieties show evidence of a sand origin, and
in this they differ from the reef rock. The pebbly beds pro-
duce a pudding stone of coral.

In most localities the rock is an odlite or odlitic limestone.
The grains become coated by the agglutinating carbonate of
lime, and each enlarges thus into a minute sphere—a spherical
concretion ; and the aggregation of these concretions makes
the odlite. The grains are usually much smaller than the roe
of most fishes, a resemblance which is alluded to in the name,
from the Greek wor, egg.

These beach deposits consist of regular layers, commonly
from a few inches to a foot in thickness, and are generally con-
solidated up to a line a little above high-tide mark. In all in-
stances observed, the layers dip at an angle of six to eight de-
grees down the beach. ‘This dip is nothing but the slope of the
beach itself, and arises from the circumstance that the sands
are deposited by the incoming waves, or tides, on such a slop-
ing surface. Tutuila and Upolu, in the Navigator Group, and
Oahu in the Hawaiian, afford many examples of these beach
formations. At certain localities the beach sand-rock has been
washed away after it was formed ; and occasionally large mass-
es or slabs have been uplifted by the sea and thrown high up
on the beach.

Deposits of the same kind sometimes include detritus from
the hills. Black basaltic pebbles are thus cemented by the
white calcareous material, producing a rock of very singular
appearance. Near Diamond Hill, on Oahu, is a good locality

154 CORALS AND CORAL ISLANDS.

for observing the steps in its formation. Many of the pebbles
ot the beach are covered with a thin incrustation of carbonate
of lime, appearing as if they had been dipped in milk, and
others are actually cemented, yet so weakly that the fingers
easily break them apart.

The lime in solution in waters washing over these coral
shores is also at times deposited in the cavities or seams of the
volcanic rocks; thus the cavities of a lava or basalt become
filled with white calcareous kernels, and the cellular’ lava is
changed into an amygdaloid. In large cavities, or caverns, it
often forms stalactites or stalagmitic incrustations. Similar
facts are stated by Mr. Darwin as observed on the shores of
Ascension; and many interesting particulars are given respect-
ing calcareous incrustations on coasts in his work on Volcanic
Islands, some of which are cited beyond. They were observed
by the writer upon Madeira, in St. Jago, one of the Cape
Verds, as well as among the volcanic islands of the Pacific.

Jukes speaks of the odlitic character of the beach sand-rock
about islets connected with the Australian barrier, and states
“that the fact that the rock was not consolidated under wa-
ter was proved by nests of turtles’ eggs being found imbedded
in it, these evidently having been deposited by the animal
when the sand was above water and still loose and incoherent.”

Vil. DRIFT SAND-ROCK.

Still another kind of beach formation is going on in some
regions through the agency of the winds in connection with
the sea., It occurs only on the windward side of islands when
the reefs are narrow, and proceeds from the drifting of the
sand into hillocks or ridges by the winds.

The drifts resemble ordinary sand-drifts, and are often

STRUCTURE OF CORAL REEFS. Ley

quite extensive. On Oahu, they occur at intervals around the
eastern shores, from the northern cape to Diamond Point,
which forms the south cape of the island,—the part exposed to
the trades ; and they are in some places twenty to forty feet
in height. They are most remarkable on the north cape, a
prominent point exposed to the winds that blow occasionally
from the westward, as well as to the regular trades. They
also occur on Kauai, another of the Hawaian Islands. But
at Upolu (Samoa), where the protecting reefs are broad, the
author met with no instance worthy of mention.

These sand-banks, through the agency of infiltrating wa-
ters, fresh or salt, become cemented into a sand-rock, more or
less friable, which is frequently odlite. The rock consists of
thin layers or lamine, which are very distinct, and indicate,
generally, every successive drift of sand which puffs of wind
had added in the course of its formation: and where a heavier
gale had blown off the top of a drift, and new accumulations
again completed it, the whole history is distinctly displayed.

On northern Oahu, the elevated bluffs of coral-made
limestone near Kahuku Point, eighty feet or so high
above the sea-level, have a top of drift-sand rock, charac-
teristic in its structure, resting on a much thicker coral-
reef rock made up in part of large corals, some of them
in the position of growth. Views of the bluffs showing
the division between the two formations are given in the
author's notes on Oahu, in his work on “ Volcanoes and
the Hawaiian Islands.”

The thickness and extent of drift-sand deposits depends
on the character of the wind, the agent that makes them.
Winds from one direction add a little to the height of a
beach; from two or more, by sweeping off sands for con-
tributions from a wider range of surface, make the height

156 CORALS AND CORAL ISLANDS. aan

: | eI
often forty feet or more; and in a region subject to Vio- 4 |
lent tempests or cyclones, like those of the Bahamas and/ |
the Bermudas, they raise hills to a height of one or nce
hundred feet, that are sometimes one or two miles broad, | |

ae
|
|
|

and may make banks that are many miles in width. J

About cavities over the surface, the rock is usually very
compact to a depth of half an inch or more, almost horny
in texture, owing to the infiltration of lime from the
waters often occupying them; but this is an exceptional
variety of the rock.

Odlitic beds appear to be confined to the superficial forma-
tions of a reef, that is, to the beach and wind-drift accumulations. \"
No example has come under the notice of the author of odlite \ |
constituting the foundation rock of a reef orisland. Itis possi- .
ble that such beds might in some cases be the basement rocks to a
considerable depth below ; for a reef-island might subside so |
much more slowly than coral formations grow and accumulate, (
that a succession of beach-made beds would be produced even
to a great thickness. Yet the probability is that the subsi-
dence would sink the surface beneath the water, and put an
end to beach and wind-drift work. The beach slope of 6° to
8°. is an almost constant mark of beach-made beds.

Vill. THICKNESS OF REEFS.

We have considered in the preceding pages the peculiari-
ties of form and structure characterizing the reef formations |
bordering islands and continents, and their influence upon the
enclosed land. Could we raise one of these coral-bound islands
from the waves, we should find that the reefs stand upon the
submarine slopes, like massy structures of artificial masonry ;

some forming a broad flat platform or shelf ranging around

STRUCTURE OF CORAL ISLANDS. 157

the land, and others encircling it like vast ramparts, perhaps a
hundred miles or more in circuit. The reefs that were near
the water-line of the coast would be seen to have stood in the
shallowest water, while the outer ramparts rested on the more
deeply submerged slopes. Indeed, it is obvious that with a
given slope to the declivity of the land, the thickness of the
reef resting upon it nay be directly determined, as it would be
twice as great two hundred feet from the shore as at one hun-
dred feet. The only difficulty, therefore, in correctly determin-
ing the depth or thickness of any given reef, arises from the
uncertainty with regard to the submarine slope of the land.
It is, however, admitted as the result of extensive observation,
that in general these slopes correspond nearly with those of the
land above water. Mr. Darwin has thus estimated the thick-
ness of the reefs of the Gambier Group (p. 265) and some other
Pacific islands, and he arrives at the conclusion, as his figures
indicate, that some coral reefs, at their outer limits, are at least
two thousand feet in thickness.

The mountain slopes of the islands of the Pacific, except
when increased by degrading agents, do not exceed in angle
twelve or fourteen degrees, and they are often but half this
amount. The slopes of Mauna Kea and Mauna Loa, isl-
and of Hawaii, do not average over eight degrees. On the
north side of Upolu, where the reefs are wide, the inclination

is from three to six degrees. | Throughout the Pacific, the ~

steeper slopes of the mountains are due to agencies which can-
not be shown to have affected the submarine slopes, excepting
in cases of disruption of islands by forces below.

Assuming eight degrees as the mean inclination, we should
have for the depth of reef (or water), one mile from the shore,
740 feet; or assuming five degrees, 460 feet. Adopting the
first estimate, the Gambier Group would give for the outer

_-

158 CORALS AND CORAL ISLANDS.

reef a thickness of at least 1,750 feet; or with the second, 1,150
feet. The island of Tahiti (taking the north side for data)
would give in the same manner 250 feet by the last estimate,
which we judge to be most correct; Upolu, by the same esti-
mate, 440 feet. The deduction for Upolu, may be too large:
taking three degrees as the inclination, it gives 260 for the
thickness at the outer margin. The results are sufficiently ac-
curate to satisfy us of the great thickness of many barrier
reefs. j

These calculations, however, are liable to error from many
sources. Very different results might generally be obtained
from different sides of the same island; and the same group
often contains islands without reefs, and others with reefs one
or even several miles from the shores. But since we may show
that the absence of a reef, or its limited extent, may be traced
to some causes restricting or modifying its formation, it is ob-
vious that the error would probably be on the side of too low
an estimate.

Adjacent to the larger islands, such as those of Vanua
Levu, and Australia, the error might be of the opposite kind ;
for the slopes of the land are of a more complex or irregular ~
character than on the smaller islands. In the latter, they may
be shown to belong generally to a single elevation of igneous
origin, or,at the most, to two or three combined; while, in the
former, they may pertain to different ranges of hills or moun-
tains. | For correct results in any instance, the land and its
declivities should be carefully studied beforehand, and the sys-
tem in its inclinations determined by observation. With re-
gard to Tahiti and Upolu, information bearing upon this point
was obtained, and the above conclusions may be received with
much confidence. Many of the Feejee reefs, on the same prin-
ciple, cannot be less than 2,000 feet in thickness.

STRUCTURE OF CORAL REEFS. 159

Ix. A GOOD WORD FOR CORAL REEFS.

All coral-bound coasts, and especially those of islands in mid
ocean, derive great benefit from their reefs. The wide coral banks
and the enclosed channels greatly enlarge the limits tributary
to the lands they encircle. Besides being barriers against the
ocean, they are dykes to detain the detritus of the hills.
They stop the waters of the streams, and cause it to drop the
silt they were bearing off, and thus secure its addition to the land.
They prevent, therefore, the waste which is constantly going on
about islands without such barriers ; for the ocean not only en-
croaches upon the unprotected shores of small islands, but car-
ries off much of whatever the streams empty into it. The del-
ta of Rewa, on Viti Lebu, resulting from the detritus accumu-
lations of a large river, covers nearly sixty square miles. This
is an extreme case in the Pacific, as few islands are so large,
and consequently rivers of such magnitude are not common.
But there is rarely a coral-girt island which has not at least
some narrow plains from this source; and upon them the vil-
lages of the natives are usually situated. Around Tahiti these
plains are from half a mile to two or three miles in width, and
the cocoa-nut and bread fruit groves are mostly confined to
them.

The reefs also provide extensive fishing-grounds for the na-
tives, and afford abundant fish, their main reliance in the way
of animal food. They also supply large interior waters for
practice in navigation and for safe communication between dis-
tant settlements. And the effect is evident in the spirit of
maritime enterprise which characterizes the islanders; for
these circumstances have favored the construction of large sail-
canoes in which they venture beyond their own land, and often
undertake voyages hundreds of miles in length. Communica-

160 CORALS AND CORAL ISLANDS.

tion between the Friendly Islanders and the Feejees has long
been kept up by means of these large rudely-rigged sail-canoes.

Instead of a rock-bound coast, harborless and thinly hab-
itable, like St. Helena, in the tropics, and nearly all extra-
tropical islands, the shores of these reef-bound lands are bloom-
ing to the very edge, and wide plains are spread out with
bread fruit and other tropical productions. Harbors, safe for
scores of vessels, are also opened by the same means; and
some islands number a dozen, when the unprotected shores
would hardly have afforded a single good anchorage. Jukes
remarks that the sea within the great Australian barrier is
‘one great natural harbor;” and this harbor is as long as
from the extremity of Florida to Newfoundland.

Coral-reefs are sometimes viewed as only traps to sur-
prise and wreck the unwary mariner; but whoever has vis-
ited the dreary prison-house, St. Helena, will have some appre-
ciation of the benefits derived from the growing zoéphytes.

But in addition to these general, benefits, there are also
contributions from the larger reef regions to the commerce
of the world. Besides pearls, there is the biche de mar
(called also, béche de mer, sea-ginseng, and in China, tripang),
thousands of hundred-weight of which annually enter the
Chinese market from the reef-regions of the Kast Indies, Aus-
tralia, and the seas to the north, including the Feejee Archi-
pelago. This favorite material for Chinese dishes, either stews
or soups, etc, is dred holothuria—large slug-like animals,
called often sea slugs, and also sea cucumbers, from their form
in the contracted state. They are not slugs, but are most
nearly related to the echinus, though having a thick flexible
skin, while the echinus has for its exterior a firm shell, armed
about with spines. The largest are only nine or ten inches
long when contracted ; but they lengthen out sometimes to

STRUCTURE OF CORAL ISLANDS. 16]

two feet or more. They live just under the sand in the shal-
low waters, with the head projecting and bearing a beautiful
feathery rosette or flower which is branchial in nature. To
fit them for exportation, the holothuria, of which half a dozen
different kinds are taken, are slit open, boiled, and then dried,
in which last state they look like “‘smoked sausages. Dr. S.
Wells Williams says, in his ‘‘ Middle Kingdom,” that ‘ when
soaked in water, the material resembles pork rind, and is like
that in taste when stewed.” They are brought to China by the
Malays from Macassar, and elsewhere. ‘There are also large
drying-houses at the Feejees, and ships from America make
their occasional visits to collect them, with the aid of the Fee-
jees, and to dry and load up for China. The term biche de mar,
and also the French form of it, béche de mer, are corruptions of
the Portuguese bicho do mar, which means sea-worm or sea-slug.

Il. STRUCTURE OF CORAL ISLANDS.

I. FORMS AND GENERAL FEATURES.

Coral islands resemble the reefs just described, except that
a lake or lagoon is encircled instead of a mountainous island, _
A narrow rim of coral reef, generally but a few hundred yards
wide, stretches around the enclosed waters. In some parts
the reef is so low that the waves are still dashing over it into
the lagoon ; in others it is verdant with the rich foliage of the
tropics. ‘The coral-made land, when highest, is seldom more
than ten or twelve feet above high tide.

When first seen from the deck of a vessel, only a series of
dark points is descried just above the horizon. Shortly after
the points enlarge into the plumed tops of cocoa-nut trees, and
a line of green, interrupted at intervals, is traced along the

water's surface. Approaching still nearer, the lake and its belt
1]

162 CORALS AND CORAL ISLANDS.

of verdure are spread out before the eye, and a scene of more
interest can scarcely be imagined. ‘The surf, beating loud and
heavy along the margin of the reef, presents a strange contrast
to the prospect beyond,—the white coral beach, the massy
foliage of the grove, and the embosomed lake with its tiny
islets. The color of the lagoon water is often as blue as the
ocean, although but ten or twenty fathoms deep; yet shades
of green and yellow are intermingled, where patches of sand
or coral-knolls are near the surface ; and the green is a delicate
apple-shade, quite unlike the ordinary muddy tint of shallow

waters.

CORAL ISLAND, OR ATOLL.

The belt of verdure, though sometimes continuous around

the lagoon, is usually broken into islets separated by varying

intervals of bare reef; and through one or more of these in-
tervals, a ship-channel often exists opening into the lagoon.
The larger coral islands are thus a string of islets along a
line of reef. |

These lagoon islands are called atolls, a word of Maldive
origin. The king of the Maldives bears the high sounding
title of ‘“‘ Ibrahim Sultan, King of the thirteen Atollons and
twelve thousand Isles (see page 189); which Capt. W. F. W.
Owen, R. N., says is no exaggeration.

In the larger atolls, the waters within look like the ocean,
and are similarly roughened by the wind, though not to the

STRUCTURH OF CORAL ISLANDS. 163

|

same extent.. Standing on the north shore of the Raraka la-
goon and looking southwest, nothing is seen but blue waters.
Far in the distance to the right, and also to the left, a few
faint dots are observed; and as the eye sweeps around in
either direction, these dots gradually enlarge and pass into lines
of verdure, and finally, distinct groves near the observer. At
Dean’s Island, another of the Paumotus, and at some of the
Carolines, the resemblance to the ocean is still more striking.
The lagoon is in fact but a fragment of the ocean cut off by
more or less perfect walls of coral reef-rock; and the reef is
here and there surmounted by verdure, forming a series of
islets.

In many of the smaller coral islands, the lagoon has lost
its ocean character, and become a shallow lake, and the green
islets of the margin have coalesced in some instances into a
continuous line of foliage. Traces may perhaps be still de-
tected of the passage, or passages, over which the sea once com-
municated with the internal waters, though mostly concealed
by the trees and shrubbery which have spread around and
completed the belt of verdure. The coral island is now in its
most finished state; the lake rests quietly within its circle of
palms, hardly ruffled by the storms that madden the sur- __
rounding ocean. ibs

From the islands with small lagoons, there is every variety
in gradation down to those in which there is no trace of a la-
goon. These simple banks of coral are the smallest of coral
islands. In all the larger islands the windward side is the
highest ; and sometimes it is wooded and habitable through-
out when the leeward reef is bare. The entrances to the la-
goons are accordingly on the leeward side.

A single group of islands, the Gilbert or Kingsmill, af-
fords good examples of the principal varieties. It is at once

—

164 CORALS AND CORAL ISLANDS.

)

seen from these examples that atolls are not annular, In the
southernmost, Tapateuea, the form 1s very narrow, the
length being thirty-three miles, with the width of the southern
portion scarcely exceeding six miles, and that of the northern
more than one-half less. The emerged land is confined to one
side, the eastern or windward, and consists of a series of islets
upon the eastern line of coral reef. The western side is for
the most part several feet under water, and there is hardly a
proper lagoon. Sailing by the island, to windward, the patch-
es of verdure, thus strung together, seem to rise out of a long
white line of breakers, the sea surging violently against the un-
seen coral reef upon which they rest.

Nonouti, the next island north, is about twenty miles
long by eight broad. The rim of Jand, though in fewer islets,
is similar to that of Tapateuea in being confined to the reef
fronting northeast. The reef of the opposite side, though bare
of vegetation, stands near low-tide level, and the whole en-
closes a large lagoon.

Aranuka and Apamama, though smaller than Nonouti,
have the same general character. Aranuka is triangular in
shape, and has an islet on the western point or cape, which is
quite prominent. Apamama differs from either of the preced-
ing in having two narrow ship entrances to the lagoon, one
through thé northwestern reef, and another through the south-
western.

Kuria is a remarkable double island, without a proper
lagoon. It consists of two neighboring groves, each about a
square mile in extent, on adjacent patches of reef.

Maiana is quite regularly quadrangular, with an uninter-
rupted range of land on two of the four sides, and an exposed
reef constituting the other two.

Tarawa consists of two sides of a triangle. The western

Plate VIII.

x
~ Marana

Nonouti

,
e .

GILBERT OR KINGSMII.IL ISLANDS

STRUCTURE OF CORAL ISLANDS. 167

reef is wanting, and the sea and lagoon have unbroken com
munication. In place of it, there are two to ten fathoms of
water, and a bottom of coral sand. Small vessels may sail in
almost anywhere on this side to good anchorage, and there is
a passage for ships of the largest size. The depth within is
greater than on the bar, and these inner waters obviously cor-
respond to the lagoon of other islands.

Apaiang has much resemblance to Apamama in its forest
border and lagoon. Moreover, there is a ship entrance through
the southwestern reef.

Marakei is one of the prettiest coral islands of the Pacific.
The line of vegetation is unbroken. In a view from the mast-
head it lies like a garland thrown upon the waters; the un-
practiced eye scarcely perceives the variation from a circular
form, however great it may be. The grove is partially inter-
rupted at one point, where there are indications of a former
passage through the reef.

Tari-tari, lying to the north of Apia, is a large triangular
atoll. It is wooded almost continuously on the side facing

southeast, and has a few spots of verdure on the southwest, .

with three entrances to the extensive lagoon. The northern
side is a naked reef throughout, scarcely apparent from a ship’s
deck, except by the long line of breakers. Makin, just north
of Tari-tari, is a mere patch of coral reef without a lagoon.

Tapateuea is also called Drummond’s Island ; Nonouti
is Sydenham; Aranuka is fenderville, and Apamama is
Hopper Island.

We add a few more descriptions of Pacific islands, with
figures reduced from the maps of the Wilkes Expedition to a
scale of four-tenths of an inch to a mile.

Taiara and Henuake (figs. | and 2) are two small belts
of foliage, somewhat similar to Maraki.

dl

7a

168 CORALS AND CORAL ISLANDS.

Swain’s and Jarvis’s Islands (figs. 3 and 4), are of still
smaller size, and have no lagoon. ‘The former is densely
covered with foliage, while the surface of the latter is sandy.
Both islands are a little depressed about the centre, a fact
indicating that there was formerly a lagoon.

1

TAIARA, HENUAKE, OR HONDEN,

Fakaafo, or Bowditch (fig. 5), 200 miles north of the
Navigator Islands, is the type of a large part of coral islands.
The bank of reef has only here and there emerged from the

JARVIS’S ISLAND.

SWAIN’S ISLAND.

FAKAAFO, OR BOWDITCH’S ISLAND. |

waves and become verdant; in other portions the reef is of
the usual height,—that is, near low-tide level,—excepting a
few spots elevated a little by the accumulation of sand,

STRUCTURE OF CORAL ISLANDS. 169

The Paumotu. Archipelago, the crowded cluster of coral
islands east and northeast of ‘Tahiti, is a most instructive study
for the reader; and a map of these islands by the Wilkes Ex-
ploring Expedition, inserted in the Narrative of the Expedi-
tion, and also in the Hydrographical Atlas, will well repay
close examination. Sailing among these islands, over eighty
in number,—only four of which are over twelve feet high exclu-
sive of the vegetation,—two or three are almost constantly in
sight from the mast-head.

The small amount of habitable land on these reef-islands
is one of their most peculiar features. Nearly the whole sur-
face is water; and the land around the lagoon is but a narrow
rim, the greater part of which is usually under water at high
tide. This fact will be rendered more apparent from the fol-
lowing table, containing a statement of the sizes and areas
of several islands, with the amount of habitable land. The
measures are given in geographical miles.

GREATEST AREA IN “ABITABLE
LENGTH. GreaDra, 99, tus, PARTS IN
Carlshoff, Paumotus, 25, 13 200 10
Wolchonsky, ‘“ 15 3 40 3
Raraka, oe 15 10 90 8
Manhii, “ 14 64 50 9
Nairsa or Deans, Paumotus, 50 Me) 1000 16
Fakaafo, Union Group, + 43 20 24
Duke of Clarence, “ 84 5s 21 2
Tapateuea, Kingsmills, 33 6 60 6
Tarawa, “ 20 10 130 8
Nonouti, ie 22, 9 125 7
Tari-tari, e 18 11 110 +

The ten islands here enumerated have an ageregate area
of 1,852 square miles, while the amount of actual dry habit-
able land is but seventy-six miles, or less than one twenty-
fourth. In the Caroline Archipelago the proportion of land
is still smaller. Menchikoff atoll covers an area of 500 square

170 CORALS AND CORAL ISLANDS.

miles, and includes hardly six square miles of wooded land.
In the Marshall Islands the dry land is not over one-hundredth
of the whole surface; while in the Pescadores the proportion
of land to the whole area is about as 1 to 200.

The distribution of the land upon the reef is obvious from
the sketches already given. It is seen, as long since remarked.

MENCHICOFF ATOLL.
1-20 of an inch to a mile.

that the windward side is, in general, the highest. It is also
apparent that there are not only great irregularities of form,
but that on one side the reef may at times be wholly wanting
or deeply submerged.

In many islands there is a ship-entrance through the reef,
sometimes six or eight fathoms deep, to the lagoons, where
good anchorage may be ‘had; but the larger part have only
shallow passages, or none at all. In the Paumotus, out of the
twenty-eight visited by the Expedition, not one-half were
found to have navigable entrances. In the Carolines, where
the islands are large and not so much wooded, entrances are
of more common occurrence. About half of the Kingsmill
Islands afford a good entrance and ‘safe anchorage. Through

these openings in the reefs, there is usually a rapid outward

STRUCTURE OF CORAL ISLANDS. 171

current, especially during the ebbing tide. At Depeyster
Island, it was found to run at the rate of two and a half miles
an hour. It was as rapid at Raraka, in the Paumotus, and, as
Capt. Wilkes remarks, it was difficult to pull a boat against
tt into the lagoon.

Il. SOUNDINGS ABOUT CORAL ISLANDS.

The water around coral islands deepens as rapidly and in
much the same way as off the reefs about high islands. The
atoll usually seems to stand as if stilted up in a fathomless sea.
The soundings of the Expedition afford some interesting re-
sults.

Seven miles east of Clermont Tonnerre, the lead ran out to
1,145 fathoms (6,870 feet), without reaching bottom. Within
three quarters of a mile of the southern point of this island, the
lead, at another throw, after running out for a while, brought
up an instant at 350 fathoms, and then dropped off again and
descended to 600 fathoms without reaching bottom. On the
lead, which appeared bruised, a small piece of white coral was
found, and another of red; but no evidence of living z00-
phytes. On the east side of the island, three hundred feet
from the reef, a bottom of coral sand was found in 90 fathoms;
at one hundred and eighty feet, the same kind of bottom in 85
fathoms.

Off the southeast side of Ahii (another of the Paumotus),
about a cable’s length from the shore, the lead, after descend-
ing 150 fathoms, struck a ledge of rock, and then fell off and
finally brought up at a depth of 300 fathoms.

Two miles east of Serle’s Island, no bottom was found at
600 fathoms.

A mile and a half south of the larger Disappointment
Island, there was no bottom at 550 fathoms.

172 CORALS AND CORAL ISLANDS.

Near the eastern end of Metia, an island nearly north of
Tahiti, no bottom was found with a line of 150 fathoms; and,
a mile distant, no bottom was reached at 600 fathoms.

In general, for one to five hundred yards from the
margin of the shore reef, the water slowly deepens, and
then there is an abrupt descent at an angle of 40 or 50
degrees. The results of earlier voyagers correspond with
this statement.

Beechey, whose observations on soundings are the fullest
hitherto published, states many facts of great interest. At
Carysfort Island, he found the depth, 60 yards from the surf
line, 5 fathoms; 80 yards, 15 fathoms; 120 yards, 18 fath-
oms; 200 yards, 24 fathoms; and immediately beyond, no
bottom with 35 fathoms. At Henderson’s Island, soundings
continued out 250 yards, where the depth was 25 fathoms,
and then terminated abruptly. Off Whitsunday, 500° feet
out, there was no bottom at 1,500 feet.

Darwin states other facts bearing upon this subject, of
which we may cite the following: At Heawandoo Pholo (one
of the Maldives), Lieutenant Powell found 50 or 60 fathoms
close to the edge of the reef. One hundred fathoms from the
mouth of the lagoon of Diego Garcia, Captain Moresby found
no bottom with 150 fathoms. At Egmont Island, 50 fath-
oms from the reef, soundings were struck in 150 fathoms, At
Cardoo Atoll, only 60 yards from the reef, no bottom was ob-
tained with a line of 200 fathoms. Off Keeling Island, 2,200
yards from the breakers, Captain Fitzroy found no bottom
at 1,200 fathoms. Mr. Darwin also states that, at a depth
between five and six hundred fathoms, the line was partly
cut as if it had rubbed against a projecting ledge of rock;
and deduces from the fact “the probable existence of sub-

marine cliffs.”

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STRUCTURE OF CORAL ISLANDS. Wes

In the Phoenix Group (Plate IX.), depths of 3,000 to

3,300 fathoms occur, and 3,000 half way between Sydney’s and
Birnie’s, sixty miles apart; to the southwest of Hnderbury’s

slopes of 1 : 6 and 1: 3 exist, and to the northeast, of 1: 1:5
and 1:4. Off Swain’s Island, slopes of 1:7 and 1:13
were obtained; and off Danger Island (same Plate), 660
fathoms within half a mile southwest of the reef, and 985
fathoms one mile east, giving slopes of | : 1 and 1 : 0-75.

Off the Bahamas, for 400 miles, depths of 2,500 to 3,000
fathoms occur within twenty miles of the reefs, and at one
point 2,336 fathoms within two miles, a pitch down of
1: 0°75. South of central Cuba a depth of 3,428 fathoms exists
within twenty miles of the Grand Cayman reef, and 3,010
fathoms within fifteen miles of Swan Island reef.

There are examples also of less abrupt slopes. Northwest
of the Hawaian Group, Captain Lisiansky, who commanded
the Russian ship Neva in a voyage round the world in the
years 1860-61, at the island bearing his name found shallow
water for a distance of six or seven miles; the water deepened
to ten or eleven fathoms the first mile, fifteen the second, and at
the last throw of the lead there were still but twenty-five fath-
oms. Christmas Island affords on its western side another
example of gradually deepening waters. Yet these shallow
waters terminate finally in a rapid declivity of forty or
fifty degrees.

Off the prominent angles of an atoll, soundings gener-
ally continue much beyond the distance elsewhere, as was
first observed by Beechey. At Washington Island, mostly
abrupt in its shores, there is a bank, according to the sur-
veys of the Expedition, extending from the east point to
a distance of half a mile, and another on the west extend-

ing toa distance of nearly two miles. At Kuria, one of

et CORALS AND CORAL ISLANDS

the Kingsmills, soundings continue for three miles from the
north extremity, along a bank stretching off from this point
to the north-northwest.

Ill. STRUCTURE OF CORAL ISLANDS.

The descriptions of reefs and their islets already given apply ©
with equal force to coral islands. By transferring here the
statements respecting the former, we should have a nearly
complete account of the latter. ‘The same causes, with scarcely
an exception, are at work :—the growing of coral zodphytes, and
the action of the waves, of oceanic currents, and of the winds.
This resemblance will be rendered more apparent by a review
of their characters. The description will be found to be a sim-
ple recapitulation of a former paragraph.

The reef of the coral atoll, as it lies at the surface still
uncovered with vegetation, is a platform of coral rock, usually
two to four hundred yards wide, and situated so low as to be
swept by the waves at high tide. The outer edge, directly
exposed to the surf, is generally broken into points and
jagged indentations, along which the waters of the resurging
wave drive with great force. Though in the midst of the
breakers, the edge stands a few inches, and sometimes a foot,
above other parts of the platform; the incrusting Nullipores
cover it with varied tints, and afford protection from the
abrading action of the waves, There are usually three to five
fathoms water near the “margin ; and below, over the bottom,
which gradually degnans outward, beds of corals are growing
profusely among extensive patches of coral sand and frag-
ments. Generally the barren areas much exceed those flour-
ishing with zoéphytes, and not unfrequently the clusters are
scattered like tufts of vegetation in a sandy plain. The grow-
ing corals extend up the sloping edge of the reef, nearly to

STRUCTURE OF CORAL ISLANDS. DS

low-tide level. For ten to twenty yards from the margin, the
reef is usually very cavernous or pierced with holes or sinuous
recesses, a hiding-place for crabs and shrimps, or a retreat for
the echini, asterias, sea-anemones and mollusks ; and over this
portion of the platform, the gigantic Tridacna, sometimes over
two feet long, and 500 pounds in weight, is often found lying
more than half buried in the solid rock, with barely room to
gape a little its ponderous shell, and expose to the waters a
gorgeously colored mantle. Further in are occasional pools
and basins, alive with all that lives in these strange coral seas.

The reef-rock, when broken, shows commonly its detritus

origin. Parts are of compact homogeneous texture, a solid
white limestone, without a piece of coral distinguishable, and
rarely an imbedded shell. - But generally the rock is a breccia
or conglomerate, made up of corals cemented into a compact
mass, and the fragments of which it consists are sometimes
many cubic feet in size.

It is apparent that we are describing a second time an
outer reef. Without dwelling further upon its characters, we
may pass to the features of the reef when raised above the
waters and covered with vegetation.

Sections of coral islands and their lagoons have been given
by Captain Beechey and Mr. Darwin. We add another, by
way of illustration, although little may be presented that 1s
novel after the excellent descriptions of these authors. Sketch-
es of several of these islands, showing the general relation of
the rim of land to the reef and the lagoon within, are given
in the maps of islands on pages 165, 168. The following sketch
represents a section of the rim of land from the sea on one
side (the left), to the lagoon on the other. In the view, the
part m a represents the shallow sea bordering an island, and
abruptly deepening one to six hundred feet from the line of

{76 CORALS AND CORAL ISLANDS.

breakers. In these shallow waters are the growing corals;
yet, as before stated, a large part is often barren sand or coral
rock, especially where the depth is over fifty feet.

WMléuélllltétt-le

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is g SECTION’OF “THE RIM OF AN ATOLL.

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From « to 6 is the shore platform or reef-rock, nearly at
low-tide level, with the margin (@) slightly elevated, and usually
much incrusted at top with Nullipores. From the platform
there is a rise, by a steep beach () c), of six or eight feet, to
the wooded part of the coral belt represented between ¢ and d.
From d to ¢ there is a gently sloping beach bordering the
lagoon. Beyond e, the waters of the lagoon at first deepen
gradually, and then fall off more or less abruptly.

In the Paumotus, the shore platform, the steep beach, and
the more gently sloping shore of the lagoon are almost con-
stant characteristics.

The width of the whole rim of land, when the island gives
no evidence of late elevation, varies from three hundred yards
to one-third of a mile, excepting certain prominent points, more
exposed to the united action of winds and waves and often
from opposite directions, which occasionally exceed half a mile.

The shore platform is from one to three hundred feet in
width, and has the general features of a half-submerged outer
reef. Its peculiarities arise solely from the accumulations
which have changed the reef into an island. Much of it is
commonly bare at low tide, although there are places’where it
is always covered with a few inches or a foot of water; and
the elevated edge, the only part exposed, often seems like an

embankment. preventing the water from running off. The

STRUCTURE OF CORAL ISLANDS. LTP

tides, as they rise, cover it with water throughout, and bear
over it coral fragments and sand, comminuted shells and other
animal remains, to add them to the beach. ‘The heavier seas
transport larger fragments; and at the foot of the beach there
is often a deposit of blocks of coral, or coral rock, a cubic foot

or so in size, which low tide commonly leaves standing in a _

few inches of water. / On moving these masses, which gener-
ally rest on their projecting angles and have an open space
beneath, the waters at once become alive with fish, shrimps,
and crabs, escaping from their disturbed shelter; and beneath,
appear various Actiniz or living flowers, the spiny echini and
sluggish biche-de-mar, while swarms of shells, having a soldier
crab for their tenant, walk off with unusual life and stateliness.
Moreover, delicate corallines, Ascidiz and sponges tint with
lively shades of red, green and pink, the under surface of the
block of coral which had formed the roof of the little grotto.
Besides the deep channels. cutting into the margin of the
reef and giving it a broken outline, there are in some instances

long fissures intersecting its surface. On Aratica (Carlshoff),
and Ahii (Peacock Island), they extended along for a fourth
to half a mile, generally running nearly parallel with the
shore, and at top were from a fourth to half an inch wide.
These fissures are not essential features of the reef. They are
probably a result of a subterranean movement or shaking.

The beach consists of coral pebbles or sand, with some

worn shells, and occasionally the exuvie of crabs and bones
of fishes. Owing to its whiteness, and the contrast it affords
to the massy verdure above, it is a remarkable feature in the
distant view of these islands, and often seemed like an arti-

ficial wall or embankment running parallel with the shores. .

On Clermont Tonnerre, the first of these islands visited by us,

the natives seen from shipboard, standing spear in hand along
12

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x

178 CORALS AND CORAL ISLANDS.

the top of the beach, were believed by some to be keeping
patrol on the ramparts of a kind of fortification. This decep-
tion arose from the dazzling whiteness of the coral sand, in
consequence of which, the slope of the beach was not distin-
guished in the distant view.

The emerged land beyond the beach, in its earliest stage,
when barely raised above the tides, appears like a vast field of
ruins. Angular masses of coral rock, varying in dimensions
from one to a hundred cubic feet, lie piled together in the
utmost confusion; and they are so blackened by exposure, or
from incrusting lichens, as to resemble the clinkers of Mauna
Loa; moreover, they ring like metal under the hammer. / Such
regions may be traversed by leaping from block to block, with
the risk of falling into the many recesses among the huge
masses. On breaking an edge from the black masses, the
usual white color of coral is at once apparent. Some of the
blocks, measuring five or six feet in each of their dimensions,
were portions of single individual corals; while others had the
usual conglomerate character of the reef-rock, or, in other
words, were fragments torn by the waves from the reef-rock.

In the next stage, coral sand has found lodgment among
the blocks; and although so scantily supplied as hardly to
be detected without close attention, some seeds have taken
root, and vines, purslane, and a few shrubs have begun to
grow, relieving the scene, by their green leaves, of much of
its desolate aspect. :

Both of these stages are illustrated on the greater part of
coral islands.

In the last stage, the island stands six to ten feet out of
water. The surface consists of coral sand, more or less dis-
colored by vegetable or animal decomposition. Scattered
among the trees, stand, still uncovered, many of the larger

STRUCTURE OF CORAL. ISLANDS. 179

blocks of coral, with their usual rough angular features and
blackened surface. ‘There is but little depth of coral soil,
although the land may appear buried in the richest foliage.
In fact, the soil is scarcely any thing but coral sand. It is
seldom discolored beyond four or five inches, and but little of
it to this extent; there is no proper vegetable mould, but only
a mixture of darker particles with the white grains of coral
sand. It is often rather a coral gravel, and below a foot or
two, it is usually cemented together into a more or less com- .
pact coral sand-rock.

One singular feature of the shore platform, occasionally
observed, remains to be mentioned. Huge masses of reef-
rock are sometimes found upon it, some of which lie loose
upon the reef, while others are firmly imbedded in it below,
and so cemented to it as to appear to be actually a part of the
platform rock. Sketches of two of these masses are here given.

BLOCKS OF OORAL ROCK ON THE SHORE PLATFORM.

Figure 1 represents a mass on the island of Waterland
(one of the Paumotus), six-feet high and about five in diam-
eter; it was solid with the reef-rock below, as though a part
of it, and, about two feet above its base, it had been so nearly
worn off by the waters as to have become irregularly top-
shaped. Another mass, similarly attached to the reef at base,
observed on Kawehe (Vincennes Island), was six feet high
above low-water level, and seven feet in its longest diameter

180 CORALS AND CORAL ISLANDS.

Below, it had been worn like the one just described, though
to a less extent. Another similar mass was eight feet high.
Figure 2 represents a block six feet high and ten feet in its
longest diameter, seen on Waterland; it was unattached be-
low, and lay with one end raised on a smaller block. On
Aratica (Carlshoff), others were observed. One loose mass
like the last was eight feet high and fifteen feet in diameter,
and contained at least a thousand cubic feet. Raraka also
afforded examples of these attached and unattached blocks,
some standing with their tops six feet above high-water mark.

These masses are similar in character to many met with
among the fields of blocks just described, and differ only in
having been left on the platform instead of transported over it.
Some of them are near the margin of the reef, while others are
quite at itsinner limit. The second mass alluded to above, on
Kawehe, was a solid conglomerate, consisting of large fragments
of Astreeas and Madrepores, and contained some imbedded
shells, among which an Ostrea and a Cyprea were noticed.
This is their usual character. The other two were parts of
large individual corals (Porites); but there was evidence in the
direction of the cells that they did not stand as they grew; on
the contrary, they had been upthrown, and were afterward
cemented with the material of the rock beneath them, probably
at the time this rock itself was consolidated. Below some of
the loose masses the platform was at times six inches higher
than on either side of the mass, owing to the protection from
wear given to the surface beneath it. These blocks are always
extremely rough and uneven, like those of the emerging land
beyond; and the angular features are partly owing, in both
cases, to solution from rains and from the dashes of sea-water
to which, with every tide, they are exposed.

It should be distinctly understood that these masses here

¥ x
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STRUCTURE OF CORAL ISLANDS. 181

described were found isolated, and only at considerable inter-
vals. In no instance were they observed clustered. The loose
blocks and those cemented below had the same general charac-
ter, and must have been placed where they were by the same
cause, though it may have been at different periods.

Such blocks are of course not confined to coral island reefs,
but belong to barrier reefs generally.

Jukes says, ‘I once landed close to the edge of the Aus-
tralian barrier on the south side of the Blackwood channel, in
south latitude 11° 45’, on a continuous mass of Porites which
was at least twenty feet across, and it seemed to pass down-
wards into the mass of the reef below water without any dis-
connection. It was worn into pinnacles above, so that two or
three of us could stand in the different hollows without seeing
each other; and it was one of a line of such masses that at-
tracted our attention for a distance of three miles.”

The shore of the lagoon is generally low and gently in-
clined, yet in the larger islands, in which the waters of the
lagoon are much disturbed by the winds, there is usually a
beach resembling that on the seaward side, though of less
extent. A platform of reef-rock at the same elevation as

the shore platform sometimes extends out into the lagoon; |
but it is more common to find it a little submerged and coy- |
ered for the most part with growing corals; and in either case, |
the bank terminates outward in an abrupt descent, of a few.
yards or fathoms, to a lower area of growing corals, or a bot- |

tom of sand. Still more commonly, we meet with a sandy
bottom gradually deepening from the shores without growing
coral. These three varieties of condition are generally found
in the same lagoon, characterizing its different parts. The
lower area of growing corals slopes outward, and ceases where
the depth is 10 to 12 fathoms or -sooner; from this there

182 CORALS AND CORAL ISLANDS

is another descent to the depth which prevails over the lagoon.
On some small lagoons the shore is a thick plastic mud,
either white or brownish, and forms a low flat which is very
gently sloping. On Henuake, these mud deposits are quite ex-
tensive, and of a white color. At Enderbury’s Island, another
having a shallow lagoon, the mud was so deep and thick that
there was some difficulty in reaching the waters of the lagoon ;
the foot sunk in eight or ten inches and was not extricated
without some difficulty. It looked like a dirty brownish clay.
This mud is nothing but comminuted coral, so fine as to be
almost impalpable.

The lagoons of the smaller islands are usually very shallow ;
and in some, merely a dry bed remains, indicating the former
existence of water. Instances of the latter kind are met with
only in islands less than three miles in diameter; and those
with shallow lagoons are seldom much larger. ‘These shallow
waters, when direct communication with the sea is cut off, be-
come, in some instances, very salt by evaporation, and contain
no growing coral, with few signs of life of any kind; and in
other cases, they are made too fresh for marine life through
the rains. At Enderbury’s Island the water was not only ex-
tremely saline, but the shores of the lagoon were in some
places incrusted with salt. But when there is an open channel,
or the tides gain access over a bare reef, corals continue to
grow, and a considerable portion of the lagoon may be obstruct-
ed by them. At Henuake, the sea is shut out except at high
water, and there were consequently but few species of corals,
and those of small size. At Ahii (Peacock’s Island), there
was a small entrance to the lagoon, and though comparatively
shallow, corals were growing over a large part of it. Ee

In the larger islands, the lagoons contain but small reefs
compared with their whole extent; the greater part is an open

STRUCTURE OF CORAL ISLANDS. 1838

sea, with deep waters and a sandy or muddy bottom. | There
are instances, as at the southern Maldives, of a depth of 50
and 60 fathoms, From 20 to 35 fathoms is the usual depth
in the Paumotus. This was the result of Captain Beechey’s
investigations ; and those of the Expedition, though few, cor-
respond.. It is however probable that deeper soundings would
be found in the large island of Nairsa (Dean’s). In Gilbert’s
Group, southeast of the Carolines, the depth, where examined
by the Expedition, varied from 2 to 35 fathoms. Mr. Darwin

found the latter depth at Keeling Island. Chamisso found

25 to 35 fathoms at the Marshall Islands.
The bottom of these large lagoons is very nearly uniform,
varying but little except from the occasional abrupt shallow-

ings produced by growing patches of reef. Soundings bring

up sand, pebbles, shells, and coral mud; and the last men-
tioned material appears to be quite common, even in lagoons
of considerable size. It has the same character as above de-
may be classed with these deposits. Darwin describes this mud
as occurring at the Maldives, and at Keeling Island (op. cit.
p. 26); Kotzebue mentions it as common at the Marshall
atolls, and Lieutenant Nelson observed it at the Bermidas.
It appears, therefore, that the finer coral material of the
shores prevails throughout the depths of the lagoon. The
growing reefs within the lagoons are in the condition of the
inner reefs about high islands. The corals grow but little
disturbed by the waves, and the reef-rock often contains them
in the position of growth. At Taputeouea (Kingsmill’s or
Gilbert's Group), reefs very similar to those of the Feejees
occur; they contain similar large Astreeas ten to twelve feet
in diameter, which once were growing where they stand, but
are now a part of the solid lifeless rock.

184 CORALS AND CORAL ISLANDS.

Beach formations of coral sand-rock are common on the
coral islands, and they present the same features in every re-
spect as those described. They were observed among the Pau-
motus, on Raraka, Honden, Kawehe, and other islands. The
stratified character is always distinct, and the layers slope to-
ward the water at the usual small angle, amounting to 5-7
degrees bordering the lagoon, and 6—8 degrees on the seashore
side of the land. Agassiz gives the same angle for the sea-
ward slope of similar deposits at Key West. The rock is
largely a fine odlite. They often occupy a breadth of thirty
to fifty yards, appearing like a series of outcrops; yet they are
frequently covered by the sands of the steep part of the beach.
The rock is a fine or coarse sand-rock, or odlite, or coral
pudding-stone, and consists of beach materials. Occasion-
ally it is compact, and resembles common limestone, except-
ing in its whiter color; but generally its sand origin is
apparent. Deposits of sand and fragments of corals and
shells cover the top of a reef-bank underneath what there
is of soil; and they are horizontal instead of having the dip
of the beach.

In borings by Lieutenant Johnson, of the Wilkes Explor-
ing Expedition, on Aratica or Carlshoff’s Island, in the Pau-
motus, ten or eleven feet were passed through easily, and then
there was a sudden transition from this softer rock (probably
the beach sand-rock), to the solid reef-rock.

The drift sand-rock was not met with by the author on any of
the coral islands visited. The time for exploration on these
islands allowed by the Expedition was too short for thorough
work. It has been stated that the more exposed points toward
the trades, especially the northeastern and southwestern, are
commonly a little higher than other parts; and it is altogether
probable that some of the sand heaps there formed will prove

STRUCTURE OF CORAL ISLANDS. 185

on examination to afford examples of this variety of coral-
rock. Such situations are identical with those on Oahu,
where they occur on so remarkable a scale.

In the Atlantic, on the Bermudas and Bahamas, and Flor-
ida reefs, the drift-sand accumulations often have heights of
forty to one hundred feet and sometimes of one hundred and
fifty to two hundred and fifty feet. They make the dry land
or islands and cays of these regions as described beyond.

Although in these descriptions of atolls, some points have
been dwelt upon more at length than in the description of
barrier reefs, still it will be observed that the former have no
essential peculiarities of structure apart from such as necessar-
ily arise from the absence of high rocky lands. The encircling
atoll reef corresponds with the outer reefs that enclose hich
islands; and the green islands and the beach formations, in the
two cases, originate in the same manner.

The lagoons, moreover, are similar in character and posi-
tion to the inner channels within barrier reefs; they receive
coral material only from the action of degrading agents, be-
cause no other source of detritus but the reefs is at hand. The
accumulations going on within them are, therefore, wholly of
coral. ‘The reefs within the lagoons correspond very exactly
in mode of growth and other characters to the nner reefs un-
der the lee of a barrier.

IV. NOTICES OF SOME CORAL ISLANDS.

The preceding descriptions represent the general character
of atolls, but are more especially drawn from the Paumotus.
There are some peculiarities in other seas, to which we may
briefly allude.

Among the scattered coral islands north of the Samoan

186 CORALS AND CORAL ISLANDS.

Group, the shore platform is seldom as extensive as at the
Paumotus. It rarely exceeds fifty yards in width, and is cut
up by passages often reaching almost to the beach. In some
places the platform is broken into islets. Enderbury’s Island
is one of the number to which this description applies. The
beach is eleven or twelve feet high. or the first eight feet it
slopes very regularly at an angle of thirty to thirty-five degrees,
and consists of sand, coarse pebbles, or rounded stones of coral,
with some shells; and there is the usual beach conglomerate
near the water’s edge. After this first slope, it is horizontal
for eighty to two hundred feet, and then there is a gradual
rise of three to four feet. Over this portion there are large
slabs of the beach conglomerate, along with masses from the
reef-rock, and some thick plates of a huge foliaceous Madre-
pora; and these slabs, many of which are six feet square, lie
inclining quite regularly against one another, as if they had
been taken up and laid there by hand. They incline in the
same direction with the slope of the beach. The large Madre-
pora alluded to has the mode of growth of the Madrepora
palmata ; and probably the entire zodphyte extended over
an area twelve or fifteen feet in diameter. The fragments
are three to four inches thick, and thirty square feet in surface.

As a key to the explanation of the peculiarities here ob-
served, it may be remarked that the tides in the Paumotus are
two to three feet, and about Enderby’s Island five to six feet
in height.

Maldive Archipelago.—The Maldives have been often
appealed to in illustration of coral structures. They are par-
ticularly described by Mr. Darwin from information commu-
nicated to him by Captain Moresby, and from the charts of this
officer and Lieutenant Powell. A paper onthe northern Mal-
dives, by Captain Moresby, is contained in the Journal of the

Ly Se

Phoowa
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QF) Addoo Atoll

MALDIVE ARCHIPELAGO.
One urch to CO mites.

1 Ee } yr

Plate X.

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CAMBRIDGE. MA USA .

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STRUCTURE OF CORAL ISLANDS. 189

Royal Geographical Society, vol. v., p. 398; and another on
this group by J. J. Horsburgh, and W. F. W. Owen, in the
the same journal, vol. ii, pp. 72 and 81. As stated by Mr.
Darwin, the archipelago has a length of 470 miles, with an
average breadth of 50 miles; and it consists for the most of
its length of two parailel lines of atolls. The large atoll at
the north end has a length of 88 miles, while Suadiva, one of
the southernmost, is 44 miles long from north to south, and
34 miles across.

The point of special interest in their structure is the oc-

currence of atolls or annular reefs within the larger atolls.
yy Powells Is*

MAHLOS MAHDOO ATOLL, WITH HORSBURGH ATOLL.
Scale 1-20 of an inch to a mile.

The islets of the lagoon and those of the encircling reef, are
in many instances annular reefs, each with its own little lake.
Gems within gems are here clustered together.

190 CORALS AND CORAL ISLANDS.

This feature is well exhibited in the Mahlos Mahdoo atoll,
an enlarged map of which, from Darwin’s work, is here in-
serted. The atoll consists of three main atoll-shaped portions ;
but in each of these, the border is made up in part of atolls.
Many of the subordinate atolls of the border are “three, and
some even five miles in diameter, while those within the lagoon
are usually smaller, few being more than two miles across, and
the greater number less than one. The depth of the little
lagoons 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 subordinate atolls rise abruptly |
from the platform or bank on which they stand, with their
outer margin bordered by living corals.” ‘The small atolls |
of the border, even where most perfect and standing farthest
apart, generally have their longest axis directed in the line
which the reef would have held if the atoll had been bounded
by an ordinary wall.” (Darwin, on Coral Reefs, pp. 33, 34.)

The Maldives are among the largest atoll reefs known;
and they are intersected by many large open channels; and
Mr. Darwin observes, that the interior atolls occur only near
these channels, where the sea has free access. We may view
each large island in the archipelago as a sub-archipelago of
itself. Although thus singular in their features, they illus-
trate no new principles with regard to reef-formations.

Mr. Darwin thus remarks (Op. cit. pp. 33, 34),—“ TI can
in fact poimt 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 characterized 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

STRUCTURE OF CORAL ISLANDS. 19]

scattered irregularly, they are grouped closely together.” —‘ 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. A1-
though their presence is thus contingent on the openness of
the marginal channels, the theory of their formation, as we

GREAT CHAGOS BANE,

shall hereafter see, is included in that of the parent atolls, of
which they form the separate portions.”

192 CORALS AND CORAL ISLANDS.

The Great Chagos Bank, This bank lies about ten degrees
south of the Maldives, and is ninety miles long and seventy in
its greatest breadth. It is a part of the Chagos Group, in which
there are some true atolls, some bare atoll-reefs, and others, like
the Great Chagos Bank, that are quite submerged, or nearly
so. Its rim is mostly from four to ten fathoms under water.

Mr. Darwin confirms the opinion of Captain Moresby, that
this bank has the character of a lagoon reef, resembling one
of the Maldives; and he states, on the evidence of extensive
soundings, that, if raised to the surface, it would actually be-
come a coral island, with a lagoon forty fathoms deep. He
says that, in the words of Captain Moresby, it is in truth
“nothing more than a half-drowned atoll.”

The form of the bank, its margin of shoals, and a line
of soundings across it, giving the depth of the central or
lagoon portion, are shown in the map on p. 191, from Darwin,
and for which, as well as for other information about the

4to 10 fathoms.

2POr ms

Leveh of the Sea.

EOS HEL

J \bo
(

Poet

76 miles in length-

EAST AND WEST SECTION ACROSS THE GREAT CHAGOS BANK.

bank, he gives credit to Captain Moresby. ‘The cross section
is still further illustrated in the annexed cut. The whole
length of the section (or width of the bank in the line of .the
soundings) is seventy-six miles. From the outer rim of
the submerged atoll, there is a drop off to a deeper level, which
is mostly fifteen to eighteen fathoms below the surface ; and

STRUCTURE OF CORAL ISLANDS. 193

then to the bottom of what was once the lagoon, now for
the most part forty to fifty fathoms under water, though hay-
ing its shoals that are five to ten fathoms submerged. All
points in the map that are shaded, have a depth of less than
ten fathoms; the only emerged parts are three or four spots
on the western margin, as indicated on the map above. The
bottom over the interior is muddy; on the flat bordering it,
15 to 20 fathoms deep, there is coral sand with ‘‘a very little
live coral; the outer rim is coral rock with scarcely any live

” while the shoals or knolls of the interior are ‘ cov-

coral ;
ered with luxuriantly-growing corals.” Darwin states also
that the rim is steep on both sides, and outward slopes abruptly
to unfathomable depths; at a 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. For other similar facts see page
Metia and other elevated Coral Islands.—Metia, or Au-
rora Island, is one of the western Paumotus. It is a small
| island about four miles by two and a half in width, and two

, hundred and fifty feet in height; and it consists throughout

METIA, OR AURORA ISLAND

of coral limestone. Approached from the northeast, its high

vertical cliffs looked as if basaltic, resembling somewhat the
13

194 CORALS AND CORAL ISLANDS.

Palisades on the Hudson. This appearance of a vertical
structure was afterward traced to vertical furrowings by the
waters dripping down its front, and the consequent formation
of stalagmitic incrustations. Deep caverns were also seen.

The cliff, though vertical in some parts, is roughly sloping
in others, and on the west side, the surface of the island grad-
ually declines to the sea.

The rock is a white and solid limestone, seldom presenting
any traces of its coral origin. In some few layers there were
disseminated corals, looking like imbedded fossils, along with
beautiful casts of shells; but for the most part it was as com-
pact as any ancient limestone, and as uniform in texture. Oc-
casionally there were disseminated spots of crystallized calcite.

The caverns contain coarse stalactites, some of which are
six feet in diameter; and interesting specimens were obtained
containing recent land shells that had been enclosed by a cal-
eareous film while hibernating. |

It is probable that more extensive caverns would have
been found had there been more than a few hours for the ex-
amination of the island. The Rey. Mr. Williams, in his work
on Missionary Enterprises in the Pacific, gives very interesting
descriptions of caverns in the elevated coral rock of Atiu,
one of the Hervey Group. In one, he wandered two hours,
without finding a termination to its windings, passing through
chambers with ‘“ fretwork ceilings of stalagmite and stalactite
columns, which, ’mid the darkness, sparkled brilliantly with
the reflected torch-light.” This author remarks, ‘‘ that while
the madrepores, the brain and every other species of coral are
full of little cells, these islands (including those resembling
Atiu), appear to be solid masses of compact limestone, in
which nothing like a cell can be detected.”

Beechey, in his description of Henderson Island, another

STRUCTURE OF CORAL ISLANDS. 195

of this character, speaks of the rock as compact, and having
the fracture of a secondary limestone.

The surface of Metia is singularly rough, owing to ero-
sion by the rains. The paths that cross it wind through nar-
row passages among ragged needles and ridges of rock as high
as the head, the peaks and narrow defiles forming a miniature
model of the grandest Alpine scenery. ‘There is but little
soil, yet the island is covered with trees and shrubbery.

The shores at the first elevation of the island, must have
been worn away to a large extent by the sea; and the cliff
and some isolated pinnacles of coral rock still standing on the
coast are evidence of the degradation. But at present there
is a wide shore-platform of coral reef, two hundred or two
hundred and fifty feet wide, resembling that of the low coral
islands, and having growing coral, as usual, about its margin
and in the shallow depths beyond.

In the face of the cliff there are two horizontal lines,
along which cavities or caverns are most frequent, which con-
sequently give an appearance of stratification to the rock,
dividing it into three nearly equal layers.

We might continue this account of coral reefs and islands,
by particular descriptions of others in the Pacific. But the
similarity among them is so great, and their peculiarities are
already so fully detailed, that this would amount only to a
succession of repetitions. The characters of a few, briefly
stated, will suffice in this place. The first eight mentioned
beyond ‘are small islands situated within ten degrees of the
equator; Birnie’s, Enderbury’s, and Hull’s belonging to the
Pheenix Group, and Oatafu and Fakaafo to the Union Group.
Their positions are shown on Plate VIILf. The remainder are “7Y~ e

9°

a few islands of the Paumotu Archipelago, in latitudes 12

196 CORALS AND CORAL ISLANDS.

to 21° 8. Several of them are guano islands, as described
on pages 318 to 324.

Birnie’s. — Lat. 3° 35’ 8. Long. 171° 30’ W. Four-
fifths of a mile by one-third, trending northwest. No lagoon.
A sandy flat about ten feet high, except near the north-north-
east extremity, where it is about twelve feet. To the south-
southwest the submerged reef extends out nearly a mile, over
which the sea breaks. In passing it, distinguished no vegeta-
tion except the low purslane and some trailing plants.

Enderbury’s. — 3° 8S. 171° 15’ W. 4 miles by 1 mile
nearly, trending N. N. W., and 8. 8. E.; form trapezoidal or
nearly rectangular. Little vegetation on any part, and but
few trees. The lagoon very shallow, and containing no grow-
ing coral; its shores of coral mud, allowing the foot to sk in
eight or ten inches, and covered in places with saline incrus-
tations. Shore platform one hundred feet or less in width,
and surface inclined outward at a very small angle; covered
with three or.four feet of water at high tide, and with few
corals or shells; beyond this, falls off four to six feet, and then
the bottom inclines for one hundred yards or more. The beach
very high and regular; rises eight feet at an inclination of
thirty to thirty-five degrees ; then horizontal for eighty to two
hundred, after which another rise of three or four feet. It
consists of pebbles and fine sand, but above of slabs and
blocks of coral rock and of the beach sand-rock, those of the
latter nearly rectangular and flat. This beach sand-rock
occurs in layers from ten to twenty inches thick along the
shore, and is inclined from five to seven degrees seaward.
Some portions are very compact, and ring under the ham-
mer, while others enclose fragments of different sizes to a
foot or more in diameter. Large trunks of transported trees
lay upon the island, one of which was forty feet long and

STRUCTURE OF CORAL ISLANDS. 197

four in diameter. The shore platform was much intersected
by channels.

Captain Hudson obtained soundings half a mile off in two
hundred fathoms; the lead struck upon a sandy bottom, but
was indented by coral. |

Hull’s Island. — Lat. 4° 20’ 8. Long. 171° 15’ W.
Trends northeast and southwest. Well wooded nearly all
round ; but on leeward side the forest in patches, with breaks
of bare coral. Lagoon narrow, without entrance. Width of
island from sea to lagoon, one hundred to four hundred yards:
width greatest at south end. Beach ten feet high. The soil
of the island consisted of coral fragments and sand. Shore
platform fifty to eighty feet wide; five or six feet of water
over it at high tide. Cut up very irregularly by channels
three to eight or ten feet wide. Observed small corals grow-
ing on the bottom outside of the platform. Shores of lagoon
shallow for fifty yards, and consisting of coral sand. Beyond
this a slope covered with growing corals; the corals rather
tender species of Madrepores. In the interior of the lagoon
many knolls and large patches of coral.

Swain’s. — (Fig. 8, page 168.) Lat. 11° 10’ S. Long.
170° 52’ W. 1:3 miles by 2; shape nearly rectangular;
trends east and west. No lagoon, but the centre a little
lower than the sides. Surface covered with shrubbery and
large trees, among the latter many cocoanuts; the centre
more sparsely wooded. Height fifteen to eighteen feet, ex-
_cepting on the middle of the western side, where the surface
is covered with loose fragments of coral of small size; there
appears to have been a former entrance to the lagoon at this
place. Shore reef, or platform, one hundred yards in average
width, and one hundred and fifty yards at the place where
we landed. Beach high, ten to twelve feet. At lower part

198 CORALS AND CORAL ISLANDS.

of beach, for a height of two to three feet, the coral reef-rock
was exposed, indicating an elevation of the island. For three
or four feet above this, layers of the beach sand-rock were
often in view, consisting of coral pebbles firmly cemented, and
having the usual dip of seven or eight degrees seaward ; in
many places it was concealed by the beach sands and pebbles.
There was no growing coral on the platform, excepting Nulli-
pores. The outer margin of this platform was very uneven,
and much intersected by channels, though less so than at
Enderbury’s Island. Great numbers of Birgi (large Crustacea)
were burrowing over the island, some of which were six
inches in breadth.

Oatafu or Duke of York’s. — 8° 38'S. 172°27'W. Form
oblong, trending northwest. Length 33 miles; breadth 2 miles.
Circuit 94 miles, and about one-half wooded in patches. South-
west reef mostly bare. A lagoon, but without entrance except
for canoes at high tide, on leeward side. Island ten feet high.
Shore platform narrow, and intersected by channels. Shores
lined by reef-rock, two or three feet out of water, indicating
an elevation of the island. This reef-rock consists of various
corals firmly cemented. Within the lagoon, knolls of coral,
but none near the shore on the leeward side.

Fakaafo or Bowditch’s. — 9° 20'S. 171° 5’ W. 63 miles
by 4. Shape nearly triangular. Circuit seventeen miles,
about six of which are wooded in several patches, separated
by long bare intervals. A large lagoon, but no ship entrance.
Height of island, fifteen feet. Width to the lagoon, one hun-
dred to two hundred yards. Soil of the island coral sand,
speckled black with results of vegetable decomposition. Shore
platform narrow. At outer edge a depth of three fathoms,
and from thence gradually deepens, and abounds in corals
for fifty yards, when it deepens abruptly. Coral reef-rock

STRUCTURE OF CORAL ISLANDS. 199

elevated three or four feet, mdicating an elevation of the
island. . Lagoon shallow, with some growing coral, but none
near the shore. Some corals growing on the platform, near
its margin, mostly small Madrepores, Astraas, Nullipores.
Fragments of pumice were found among the natives, which
had floated to the island (see fig. 5, page 168).

Washington Island. — Lat. 4° 41’ N. Long. 160° 15’ W.
3 miles by 14, trending east and west. It is a dense cocoanut
grove with luxuriant shrubbery. No lagoon. The shore plat-
form is rather narrow. <A point of submerged reef, one and a
half miles long, stretches out from the southwest end. Did
not land on account of bad weather.

Otuhu, Paumotu Archipelago. — 14° 5’ 8S. 141° 30’ W.
1i miles by 2, trending north and south. No lagoon.
Wooded.

Margaret, Paumotu Archipelago. — 20° 42'S. 143° 4’ W.
Diameter one mile, nearly circular. A small shallow lagoon
with no entrance. Northeast side alone wooded, and in two
patches. |

Teku or Four Crowns, Paumotu Archipelago. — 20° 28'S.
143° 18’ W. Diameter 14 miles, nearly circular. <A small
lagoon with no entrance. Southwestern reef bare; five
patches of forest on the other part.

Honden or Henuake, Paumotu Archipelago. — Size 3!
miles by 2 miles. Oblong, five-sided; trending west-north-
west. A small, shallow lagoon, communicating with the sea
only at high tide, on the west side. There are two other
entrances which are seldom if ever covered with water, and
appeared merely as dry beds of coral rock. Height of the
island twelve feet ; lowest on the south side. Belt of verdure
complete, and consisting of large forest trees, with the Pan-

danus and other species, but no cocoanuts; its breadth half a

200 CORALS AND CORAL ISLANDS.

mile, and in some parts three-fourths. Among the trees large
masses of coral rock often exposed to view, and the surface in
many parts very rough. It seemed surprising at all these
islands that there should be so luxuriant a growth of trees
and shrubbery over so rocky a surface. Shores of the lagoon
nearly flat. On one side there was a large area of extremely
fine coral sand and mud, which extended a long distance into
the lagoon. Elsewhere about the centre of the island the
reef-rock was bare, and contained numerous shells of Tri-
dacnz. A few small Madrepores still growing in the lagoon.
Beach on the sea-shore side eight feet high. In lower part
of beach several layers of white limestone (the beach sand-
rock), formed of coral fragments or sand, shells, etc., much
of which was very compact. The layers inclined toward the
sea at an angle of about six degrees. Shore platform as else-
where in this archipelago.

The facts above stated are evidence of a slight ele-
vation, probably not exceeding three feet (see fig. 5, page
168). .

Taiara, or King’s, Paumotu Archipelago. — 15° 42’ S.
144° 46’ W. 22 miles by 13, trending northwest. Has a
small lagoon with no entrance. Reef almost continuously
wooded around, somewhat broken into patches.

Ahii, or Peacock’s Island, Paumotu Archipelago. — 14°
30’ S. 146° 20’ W. 13 miles by 6, trending N. E. by EH.
Shape irregularly oblong. A large lagoon, having an entrance
for small vessels on the west. Reef wooded throughout nearly
its whole circuit. Lagoon shallow, and much obstructed by
growing coral, the latter giving the water over it a clear light
green color. Platform, or outer coral shelf of the island,
about two hundred and fifty feet wide; under water except
at the lowest tides. Margin highest, and covered with Nulli-

STRUCTURE OF CORAL ISLANDS. 201

pore incrustations, which give it a variety of delicate shades of
color, mostly reddish, of peach-blossom red, rose, scarlet. For
thirty to fifty feet from the margin, very cavernous, and con
taining many Tridacne, lying half imbedded, with the variously
tinted mantle expanded when the surface is covered with wa-
ter. Rock of the platform either a compact white limestone or
a solid conglomerate; dead over its surface, excepting a few
Madrepore tufts or Astreas near the margin in pools. In this
shelf there were long fissures, extending nearly parallel with the
shore, a quarter to half an inch wide at top, and continuing
sometimes a fourth of a mile or more. These fissures were com-
monly filled with coral sand. ‘The higher parts of the island
either consisted of loose blocks of coral or were covered with
some soil ; the soil mostly of comminuted coral and shells, with
dark particles from vegetable decomposition intermingled. On
the bottom, exterior to the shore platform, observed the same
corals growing as occurred in fragments upon the island ; but
the larger part of the bottom was without coral, or consisted
only of sand.

Raraka, Paumotu Archipelago.—16° 10’S., 145° W. 14
miles by 8, trending east and west. Shape somewhat trian-
_ gular. North side nearly continuously wooded; south angle
and southwest reef bare. A large lagoon with an entrance
for small vessels on the north side. A rapid current flows
from the entrance, which it was difficult for a boat to pull
against. Shore platform, as usual, about a hundred yards
wide, with the edge rather higher than the surface back ; the
platform mostly bare of water at low tide. Several large
masses of coral and coral rock, one to four hundred cubic feet,
on the platform and upon the higher parts of the island, some
of which stood five and six feet above high-water mark; they
were cemented to the reef-rock below, and appeared like project-

202 CORALS AND CORAL ISLANDS.

ing parts of the reef. Layers of beach sand-rock on the lagoon
shores, as well as on the seaward side, inclined at an angle of
six or seven degrees: characters as already described. Grow-
ing coral in the entrance to the lagoon, within two feet of the
surface, mostly a species of Millepora (M. squarrosa). Inte-
rior of the lagoon not examined, no time being allowed for it by
the Expedition. The water looked as blue as the ocean, and
was much roughened by the winds.

Kawehe or Vincennes Island, Paumotu Archipelago, 15°
30’ S., 145° 10’ W. 13 miles by 9, trending north-north-
west. Shape irregularly oval. Having a large lagoon, and
mostly wooded around, least so to leeward. Between the
wooded islets (as on, Raraka and elsewhere), surface consisted
of angular masses of coral rock (among which the Porites pre-
vail), strewed in great numbers together; and in some parts
bearing a few vines and purslane among the blocks, though
scarcely any appearance of soil, or even of coral sand. In
other parts, not as high, no vegetation, and surface still wet
by high tide. A few large masses of coral on the shore plat-
form, either lying loose, or firmly attached below, as already
described; some of them were six feet cube, and one was
raised seven feet above high-water mark. Shore platform about
a hundred yards wide, rather highest at the edge, and much of
its surface two to four feet under water at low tide. As else-
where, this platform is nothing but a compact coral conglom-
erate or limestone, having no growing coral over it, except in
some shallow pools near its outer margin, where also there
are numerous holes in which crabs are concealed, with small
fish and other animals of the shores. On the lagoon shore,
layers of beach sand-rock, six or seven in number, dipping at
an angle of seven degrees toward the lagoon, and outcropping
one above the other. Similar layers on the sea-shore side

STRUCTURE OF OORAL ISLANDS. 203

Manhii, Wilson’s or Waterlandt, Paumotu Archipelago,
14° 25’ S., 146° W. 15 miles by 6, trending EK. N. EK. A
large lagoon with a deep entrance on the west side. Shape
oblong triangular.

Shore platform as usual; mostly under water at low tide.
Large masses of coral here and there, standing on this reef,
either cemented to it, or loose. One top-shaped mass is figured
on p.179. High water did not reach the part of it which was
most worn ; and this was evidently owing to the fact that the
action of the swell or waves is greatest above the actual level
of the tide at the time. The reef-rock is either a compact
limestone, showing no traces of its composite origin, or a con-
glomerate. Beach, regular as usual, six to ten feet high, con-
sisting of coral sand, and fragments of worn shells, with occa-
sional exuvie of crabs, remains of Echini, fish, etc. The en-
trance to the lagoon is deep and narrow, with vertical sides.

Aratica or Carlshof, Paumotu Archipelago, 15° 30’S.,
145° 30’ W. 17 miles by 10, trending N. E: Large lagoon
with a good entrance for vessels. The reef fronting south
bare for nine miles; on northwest side, mostly very low, with
only here and there a clump of trees; occasionally a line of
wooded land for a quarter of a mile on the east side; more
continuously wooded on the north. The bare parts mostly
covered with blocks of coral, one to thirty cubic feet and larg-
er, tumbled together as on the preceding. Some blocks of
coral on the shore platform very large; one eight feet high
and fifteen in diameter, containing at least 1,000 cubic feet.

Nairsa or Dean’s, Paumotu Archipelago, 15° S., 148° W.
44 miles by 17, trending W. N. W. Northern shore mostly
wooded; southern with only an occasional islet, connected by
long lines of bare reef. In these intervals, the reef stood
eight feet or so out of water, according to estimate from ship-

204 CORALS AND CORAL ISLANDS.

board, and was worn into a range of columns, or excavated
with caverns, so as to look very much broken, though quite
regularly even in the level of the top line.

We continue these descriptions with notes on some coral
reefs and islands of the Atlantic, derived from the charts of
the ocean and different publications mentioned beyond.

Florida Reefs. — The position of the Florida coral reefs,
and of the Bahama Islands with reference to Florida and
Cuba, and to one another, and the depths over and among
the reefs, are shown on Plate XI., which is taken from one
of the Atlantic Ocean charts of the United States Hydro-
graphic Department.’

The Florida coral formations are mainly great reef-made
banks. They commence on the east coast, at Cape Florida,
in latitude 25° 40’ N., continue around the south extremity
of the peninsula, and stretch westward to and beyond Key

1 The chart is Sheet One of the North Atlantic Ocean, Lower Part (No. 21).
It has great geological interest; for it includes all the West Indian and Gulf seas,
with also the Bermudas and northern South America to the equator, and gives all
soundings to date of publication. This chart and another of the Florida reefs and
Bahamas on a still larger scale (No. 944) are the best sources of information as to
the extent, positions, forms, general characters, and relations of the reef regions,
and the distributions of islands and keys; and detailed descriptions are of little
value without them.

The more important sciéntific accounts of the Florida reefs are the following:
Prof. Louis Agassiz’s Memoir, published, in part, in the Coast Survey Reports for
1851, and entire, with added illustrations of West India corals, in the Memoirs of
the Museum of Comparative Zodlogy, vol. vii. 1880; M. Tuomey’s paper in the
American Journal of Science, 1851, 2d ser., xi.; Publications of F. de Pourtales, in
the Bulletin of the Museum of Comparative Zodlogy for 1867, 1868, 1878, and his
Report on Deep-Sea Corals in the Illustrated Catalogue of the Museum, in 1871; the
excellent and finely illustrated work of Mr. Alexander Agassiz, entitled the “ Three
Cruises of the Blake,” in two volumes, 1888; besides papers by Mr. Agassiz in the
Bulletin of the Museum of Comparative Zoology, and in the Memoirs of the American
Academy, 1883, xi. Other papers are those of Prof. Joseph Le Conte, American
Journal of Science for 1857, xxiii., 46, on the Agency of the Gulf Stream in the For-
mation of the Peninsula and Keys of Florida; and Capt. E. B. Hunt, U.S. A., ibid.,
1863, xxxv., 197, on the Origin, Growth, and Chronology of the Florida Reef.

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FLORIDA BANKS. 205

West. They are broad reef-ground flats more or less swept
by the tides; the Biscayne Bay, two to eight miles wide, is
on the east side of Florida, and the larger Florida Bay on the
south. The width of the bank rapidly increases to the west-
ward, becoming thirty miles off Cape Sable. No subdivision
into a fringing and barrier reef is suggested by its features.
“The whole tract between Cape Sable and the Keys as far as
Cape Florida, at least as far as Soldier Key, is so shoal that
it is inaccessible except for very small vessels.” This shoal
region, as Professor Agassiz’s Memoir (1851) states, is literally
studded with mangrove islands, either In continuous ranges
of great extent, or making “innumerable small islets, so inti-
mately interwoven and separated by so narrow and shallow
channels as to be almost impenetrable.” The mangrove
bushes, which seem to be “striding out in the mud” in conse-
quence of the many root-making stems that grow downward
from the branches, serve to entangle the sand, sea-weed, and
drift-wood that float by, and thus aid in the process of emer-
gence. The material of the great mud-flats of the area on
the bottom of the shoal water is the finest of coral sand;
but it is darkened in color by the carbonaceous results of
animal and vegetable decomposition, so that much of it looks
like ordinary silt or mud.

Along the western side of Florida the coast-flats extend
from Cape Sable northward for fifty to sixty miles, with a
mean width of nearly ten miles; but the sands of the flats
are partly siliceous. Outside of the bank the waters are
deeper, but the bottom continues to be largely calcareous.'
The bed of coral rock at Tampa Bay containing silicified
corals has been supposed to be of recent origin; but it is

1 The “Three Cruises of the Blake,” of Mr. A. Agassiz, has, at page 286 of
the first volume, a colored map of much interest illustrating the nature of the
sea-bottom in the Florida region.

206 CORALS AND CORAL ISLANDS.

proved, by the observations of Dr. E. C. Stearns and Prof. A.
Heilprin, to be Tertiary and probably Miocene. At Tampa
Bay and along the coast south there is shell-rock, or coquina,
in process of formation in some places. The large accumu-
lations of worm-like tubes on this coast, which have been
referred to Serpule, have been recently found by Mr. W. H.
Dall to be the tubes of a Mollusk, — the Gastropod Vermetus
nigricans, Whose worm-like spirals are much like those of

sea-worms.!

Along the outer borders of the great Florida Bank thea

waters abound in growing corals. They cover its margin,
and also the bottom adjoining at depths of one to seven or
eight fathoms, this being the ordinary limit of reef-forming
corals in the Florida seas. Over this marginal belt of grow-
ing corals, toward its outer limit, there are many spots of
half-emerged reef and some islets; and these reefs make the
outer boundary of a navigable channel carrying four to seven
fathoms of water. This belt of growing corals is ordinarily
called the “ Florida Reefs.”

The islands and keys of the bank are almost wholly of
wind-drift origm. The sands of the outer margin of the reef,
where thrown up by the waves, have been taken up by the
winds and deposited in long, high drifts a mile or so back.
Key Largo, one of the eastern of the drift ridges, is thirty
miles long and between one and two miles wide. Key West
is near the southwestern extremity of the bank, and has the
rather unusual height of fifteen feet above tide-level. Hast
of Key West for thirty-five miles the Keys are cut into strips
that trend north and south, —the Pine Islands, — owing to
passages that have been kept open by the tides.

The drift-sands of the Keys are consolidated into a coral

1 Dall, American Journal of Science, 1887, xxxiv., 161.

| q

\

—

FLORIDA BANKS. 207

sand rock, and much of it into true odlite. The manner in
which this consolidation is produced is explained on page 153.

Pourtalés was the first to point out that the sand-made
reef off the coast of Northern Florida continues to consist of
siliceous sands quite down to Cape Floridi; that abruptly at
this pot it becomes a coral-made reef, and is so continued \
southward. The facts mdicate, as observed by Major E. ice
Hunt in 1863, that the drifting action of currents parallel
with the coast have had much to do in locating the coral-
built reef as well as the banks of siliceous sands; the
material supplied is the chief difference between the two,
coral sands failmg where growing corals fail. Major Hunt
pointed out also that the current along the Florida coral reefs
was a current counter to the Gulf Stream. cape

How far the encroachment of siliceous sediments from the _
north may have determined the limit of coral-growing in that
direction has not been ascertained. / If, as the author states
in his Wilkes Expedition Report on Crustacea, the isocryme of
68° F. terminates at Cape Canaveral, it would appear that the
coral seas, or those warm enough to grow corals, extend one
hundred and seventy-five miles north of Cape Florida, and that
the northern siliceous sands have encroached all this distance.

The limit may have been even north of Cape Canaveral ;
for, as the author has recently learned from Professor Verrill,
corals of the genus Oculina have been dredged up from a
rocky bottom off Charleston, South Carolina, in depths of
eight to ten fathoms. There is a question, however, whether
the Oculina was one of the reef-making species.

Little is seen over the flats or reefs of the true coral-reef
rock, or the under-water coral limestone, which is the main
material of all coral formations. It is generally concealed
beneath the drift-sands of the surface. Tuomey observes

KK

208 CORALS AND CORAL ISLANDS.

that — “the greater number, if not all the Keys, rest upon a
foundation of corals. At Sand Key large rugged masses of
dead coral are seen bordering the Key on the windward side,
and rising above low water; similar masses may be seen at
Sambo Key, and at other places along the outer reef. But
the Keys within this barrier present better opportunities for
studying the foundation upon which they rest. At Key
Vacca corals rise to a height of four feet above high water,
and present not the slightest evidence of disturbance, beyond
the upward movement which raised them to their present
position. The rocky mass of coral along the margin of the
Key is undermined by the waves, and otherwise worn into sin-
gularly rugged shapes, with sharp projecting points. Even at
some distance from the water, bunches of coral project above
the surface wherever the overlying sand is washed away.
“In the shallow water off many of the Keys very beauti-
ful patches of Alga, interspersed with living corals, are seen
within six or eight inches of the surface. Off Indian and
Plantation Keys dark knobs of coral are visible upon the
white mud of the bottom, which render the navigation
amongst these Keys dangerous. On lower Matacumba I
traced the rugged coral rocks for a mile in extent; I also
found them on Conch Key, as I did indeed on nearly every
island that I examined, where a section could be found on
the shore, from which the overlymg sands were washed.”
What is the actual thickness of the Florida coral-reef
formation, and what it rests upon, no one knows, as no at-
tempts to ascertain by borings have been made. The Missis-
sippi silt makes no encroachments on the region; for it is
carried westward by the tidal currents of the Gulf, and does

~ not reach eastward to within five hundred miles of the Flor-

ida reef region.

FLORIDA BANKS. 209

The banks continue’ westward of Key West to the Mar-
quesas Key, which is atoll-like in form and probably in ori-
gin. But it is so situated at present within the sand-made
area that there are no growing corals “on its weather-side.”
Farther west a true atoll—that of the Tortugas — stands
apart from the bank, a channel of eleven to sixteen fathoms
intervening between it and the Marquesas reef. Branching
corals and others are growing in abundance about it, which
are the source of the coral fragments that are thrown on the
beach. The bank may perhaps receive some light calcareous
silt from the reefs to the eastward ; but the intervening chan-
nel through which the tide sweeps must be filled before drift-
ings from the eastward will reach effectively the Tortugas.

Whether a bend in the south end of the peninsula of
Florida determined the direction of drift of the marine cur-
rents in the region, and the bend in the form of the skirting
reef, or whether the direction of current drift alone gave di-
rection to the belt of reef, is not determined by any positive
facts. The east-and-west trend of Cuba favors the former of
these explanations; and the position of the Tortugas leads
to the conclusion that it was probably an atoll off the ex-
tremity of the Florida Bank. An artesian boring in the
centre of the Tortugas with a diamond drill that would sup-
ply a large core for study, would give the facts for positive
conclusions as to the nature of the basement and the depth
to which it consists of coral-reef rock.

No good evidence of elevation of the Florida Bank has
been reported. The height of the islands and cays is not
satisfactory evidence, as it is so generally due to sand-drifts.
At the same time the opinion that there had been a rise of
half a dozen feet or so may be found to have much in its

favor.
14

210 CORALS AND CORAL ISLANDS.

Region between the Florida Reefs and Cuba.— Through
the labors of Pourtalés, in connection with the soundings
by the Coast Survey, interesting facts have been brought to
light respecting the sea between the Florida reefs and the
opposite shores or reefs along Cuba and the Bahamas (see
Plate XI.). A few paragraphs on the region, by Pourtalés,
are cited from his memoir.t

“Tn transverse sections of the channel the greatest depth
is nearest its southern or eastern shore, and in a longitudinal
section the depth diminishes in passing toward the north,
finding its minimum in the narrowest part between Cape
Florida and the Bemini Islands, after which it increases
again. In a transverse section between Key West and
Havana, the greatest depth is 855 fathoms; between Som-
brero Light and Elbow or Double-Headed Shot Key on the
Cay Sal, or Salt Key Bank, 500 fathoms; between Carys-
fort reef and Orange Key on the Great Bahama Bank, 475
fathoms; and between Cape Florida and the Bemini Islands,
370 fathoms.”

Pourtalés says of the bottom outside of the Florida reefs :

“‘ Although the deep blue color of the water after passing
the reef seems to indicate a very abrupt slope, there is in no
part of it anything to compare with the sudden deepening on
the edge of the coral reefs of the Pacific Ocean, or even of
the Bahamas or the coast of Cuba. The distance from the
reef to the 100-fathom line is not less than three miles, and
often as much as six. In this space the bottom consists of
calcareous mud, and is not particularly rich in animal life.
From ninety or a hundred fathoms to two hundred and fifty
or three hundred, the bottom slopes rather gently in the

1 Museum of Comparative Zodlogy, Illustrated Catalogue, Cambridge, Mass.,
1871.

BETWEEN FLORIDA AND CUBA. 911

shape of a rough, rocky floor, without great imequalities ;
this formation obtains its greatest breadth, of about eighteen
miles, a little to the east of Sombrero Light, and tapers off
to the west, where it ends in about the same longitude as the
end of the reef; toward the east and north it approaches
nearer the reef, and ends gradually between Carysfort reef
and Cape Florida. This bottom, which is called the ‘ Pourtalés
Plateau’ in Professor Agassiz’s report, is very rich in deep-sea
corals, and most of the species described in the memoir [of
Pourtalés] were dredged on this ground. Outside of the
rocky bottom the Globigerina mud prevails and fills the
trough of the channel.

“On the Cuba shore the bottom is rocky and the slope
very abrupt, particularly for the first four or five hundred
fathoms. Along the Salt Key and Bahama Banks the slope
is also exceedingly abrupt, but the underlying rock is often
covered with mud.”

The rock of the bottom of the Pourtalés Plateau is a lime-
stone made from the calcareous relics of the species (corals,
shells, echinoderms, etc.) living at those depths; and it is
still increasing, well exemplifying what limestone-making
may go on below the limit of reef-making corals. Mr. A.
Agassiz has a cut on page 287 of his “Cruises of the
Blake” representing a specimen of the material.

The Salt Key Bank is mostly a submerged atoll-like area,
setenty miles long, having emerged islets and cays along its
northeastern and northern sides, or those to windward, and
falling off southward to a depth of four and a half to six
and a half fathoms and then steeply to depths of two hun-
dred and fifty fathoms and beyond to those of the channel
along the north side of Cuba.

Prof. L. Agassiz describes the banks and keys embraced

a2, CORALS AND CORAL ISLANDS.

between Double-Headed Shot Key, Salt Key, and Anguilla
Key as a very mnstructive combination of the phenomena of
building and destruction. He says: ‘* The whole group is a
flat bank covered by four or five, and occasionally six, fathoms
of water, with fine sandy bottom, evidently corals reduced to
odlite, the grains, which are of various sizes, from fine powder
to coarse sand, mingled with broken shells, among which a
few living specimens are occasionally found. The margin of

the Bank is encireled on several points by rocky ridges of

the most diversified appearance, and at others edged by sand-

dunes. A close examination and comparison of the different
Keys show that these different formations are in fact linked
together, and represent various stages of the accumulation,
consolidation, and cementation of the same materials. On
the flat top of the bank the loose materials are pounded
down to fine sand; in course of time this sand is thrown up
upon the shoalest portions of the Bank, and it is curious to
notice that these shoalest parts are its very edge, along which
corals have formed reefs which have become the basis of the
dry banks. The foundation rock, as far as tide, wind, and
wave may carry the coarser materials, consists of a conglom-
eration of coarser odlitic grains, rounded fragments of corals,
or broken shells, and even larger pieces of a variety of corals
and conchs, all the species being those now found living upon
the Bank, among which Strombus gigas is the most com-
mon; besides that, Astrea [ Orbicella| annularis, Siderastrea
siderea, and Maandrina mammosa prevail. The shells of
Strombus are so common that they give great solidity and
hardness to the rock. The stratification is somewhat irregu-
lar, the beds slanting toward the sea at an angle of about
seven degrees. Upon this foundation immense masses of

Strombus, dead shells, and corals have been thrown in banks,

a

BETWEEN FLORIDA AND CUBA. Ds

evidently the beginning of deposits similar to those already
consolidated below; but there is this difference in their for-
mation, namely, that while the foundation rock is slightly
inclined, and never rises above the level of high water, the ac-
cumulation of loose materials above water-level forms steeper
banks, varying from fifteen to twenty and thirty degrees. In
some localities broken shells prevail; in others, coarse and
fine sand; and the ridges thus formed, evidently by the action
of high waves, rise to about twelve and fifteen feet. This is
evidently the foundation for the accumulation of finer sand
driven by the wind over these ridges, and forming high sand-
dunes, held together by a variety of plants, among which a
trailing vine (Latatas littoralis), various grasses, and shrubs
are the most conspicuous. These dunes rise to about twenty
feet ; on their lee side and almost to their summits grows a
little palmetto. The sand of the dunes is still loose, but here
and there shows a tendency to incrustation at the surface.
The slope of these dunes is rather steep, sometimes over
thirty degrees, and steeper to the seaward than on the land-
ward side.

‘In the interior of Salt Key there 1s a pool of intensely
salt water, the tint of which is pinkish or flesh-colored, owing
to the accumulation of a small alga. When agitated by the
wind, this pool is hedged all around by foam of the purest
white, arising from the frothing of the viscous water. Along
the edge the accumulation of this microscopic plant forms
large cakes, not unlike decaying meat, and of a very offensive
odor. The foundation rock of this Key is exactly like what
Gressly described as the ‘facies corallien’ of the Jurassic
formation; while the deposit in deep water, consisting chiefly
of muddy lime particles, answers to his ‘facies vaseux.’

“ Double-Headed Shot Key is a long, crescent-shaped

ANA: CORALS AND CORAL ISLANDS.

ridge of rounded knolls, not unlike ‘roches moutonnées,’ at
intervals interrupted by breaks, so that the whole looks like a
dismantled wall, broken down here and there to the water’s
edge. The whole ridge is composed of the finest odlite, pretty
regularly stratified, but here and there like torrential depos-
its; the stratification is more distinctly visible where the rocks
‘have been weathered at the surface into those rugged and
furrowed slopes familiarly known as ‘ karren’ in Switzerland.
\ It is plain that we have here the same formation as on Salt
Key, only older, with more thoroughly cemented materials.”

The Bahama Islands. —The Bahamas are, like the Flor-
ida reefs, great coral-made banks, having their emerged land
in the form of islands and cays. These dry portions are
situated mostly along the windward sides. To comprehend
the relations of the Bahamas to atolls they should be com-
pared with the Louisiade Group, Plate VII., and other broad
combinations of barriers and atolls, although true barrier
reefs are absent. They are more remarkable than the Flor-
ida reefs for their drift-sand accumulations, their height be-
ing sometimes two hundred to two hundred and thirty feet,
though generally under one hundred feet. A large part of
the banks, however, are in a lagoon condition, being covered
with water at depths ordinarily of two to five fathoms, and
the leeward margin is for the most part submerged.

The group extends for six hundred miles through the seas
north of Cuba and Hayti, with the Florida Straits — fifty miles
in mean width and three to five hundred fathoms in depth —
separating them from Florida. The two great western banks
are the Little Bahama, or the northwestern, one hundred and
fifty miles long, and the Great Bahama Bank, to the south
and east, three hundred and twenty-five miles long. The
latter is shaped like a letter 8, or a pothook, and occupies two

THE BAHAMA ISLANDS. 215

thirds of the whole Bahama area. Nassau, the principal sea-
port of the group, is situated on the island of New Provi-
dence, at the northwest angle of the second turn in the S.

The Bahama group continues eastward, in a number of
coral islands, to Turk’s Island, in longitude 71°-W.; but topo-
graphically and geologically it extends two degrees farther
west in a line of atolls and reefs to Navidad Island. The
small map to the right on Plate XI. contains Turk’s Island
and the reef islands east of it. Turk’s Island is situated near
the southeast angle of Caicos Island.

The western portion of the Bahamas is built up on the
broad submarine plateau, five hundred to six hundred fathoms
under water, that extends parallel with the continental coast-
line from off North Carolina southward. The plateau in this .
southern part has a breadth from east to west of about two
hundred miles. More to the eastward along the Bahamas
the water deepens, and at the same time the reef becomes
narrower; and from the east end of the S stretches a line
of independent coral islands, some of which are typical
atolls.

The peculiar form of the “ Great Bahama Bank” is partly
due to the coalescence of separate islands with the main west-
ern portion through coral growths. It probably secured thus
the addition of Long Island Bank, Eleuthera Bank, and Cat
Island; and almost certainly the last of the three, Cat Island,
whose connection with the part next west is made by a nar-
row submerged reef, fifty to seventy-five feet under water, the
northern side of which pitches off at an angle of about forty-
five degrees to a depth of fifty-five hundred feet.

The S-like form is dependent also on the intrusion into
the area of two deep and broad tongues of the ocean, one

going in from the northward, commencing with two thou-

216 CORALS AND CORAL ISLANDS.

sand fathoms at its entrance, and having depths inside of
one thousand to seven hundred and fifty fathoms; and the
other, Exuma Sound, entering from the east, and having
depths within of twelve hundred fathoms toward the en-
trance, and eight hundred near the mner extremity.
Beyond the Great Bank, eastward, the ocean’s depths and
the abruptness of the submarine declivities increase rapidly.
Even in the New Providence Channel, east of the Little Ba-
hama Bank, there are depths of twenty-two hundred and
seventy fathoms, over thirteen thousand five hundred feet.
The north shore of Eleuthera Island has nearly this depth
within seven miles, or thirty-seven thousand feet, of its emerged
reef; the pitch, therefore, 1:2°75. San Salvador, east of Cat
Island, has a depth of fourteen thousand feet within three
miles of the emerged reef, a pitch down of about 1:1:13, and
a depth of sixteen thousand six hundred and forty-four feet
within ten miles, the pitch 1:3; and almost eighteen thou-
sand feet within twenty-one miles. These steep northern un-
der-water slopes are continued along the course of the group
eastward. Besides this, the channel on the south side of the
group, Which is twelve hundred to fifteen hundred feet deep
against middle Cuba, is over thirteen thousand five hundred
feet north of Hayti. The line of reef-islands beyond Turk’s
Island eastward have less emerged land than those to the
westward. The last, Navidad, is mostly submerged reef, and
has depths of thirteen thousand five hundred feet within fif-
teen miles on the east and south, and seventeen thousand
five hundred within thirty-five miles to the east-northeast.
These facts as to the depths and steep submarine declivi-
ties along the coral-reef islands are as remarkable as any
yet observed in the Pacific Ocean, even among its equatorial

islands.

' .
j if} iv" a
ad

THE BAHAMA ISLANDS. 217

The rock of the Bahama Islands is, like that of the
Florida Keys, a coral-sand rock of wind-drift origin. It is
generally a poor building stone because of the numerous
sand-flaws. Its weight varies from sixty-five to one hundred
and forty-five pounds per cubic foot. In some of the basins
or lagoon-bottoms a chalk-like deposit occurs, and nowhere so
extensively as along the western coast of Andros Island. The
coarser fragments of corals are never found much beyond the
surf range of high tide. Captain Nelson says that “the south
side of Silver Cay and the beach extending westward from
Nassau afford rolled blocks, pebbles, and sand derived from
the more massive corals, mixed with remains of turtles, fish,
crustaceans, echinoderms, and mollusks. On the beach be-
tween Clifton Point and West Bay the shells of Strombus
gigas more especially accompany the rolled corals. At Hast
Point the sand is derived from corallines and nullipores ;
the finer sand being often in approximately spherical grains,
though not so perfectly as the White Cay, and between Ex-
uma and Long Cay. The beach near Charlotteville Pomt con-
sists principally of Lucina Pennsylvanica in various stages of
comminution. At Six Hills (Caicos Group) the mass of conch
shells (Strombus gigas) is so great and sufficiently cemented
together as to form not only rock, but an island several hun-
dred feet in length. Along the northwest beach at Gun Cay
a hard, coarse, stratified rock is formed of conch shells and
others, together with coral fragments.”

“A counterfeit odlite occurs near White Cay, Exuma, and
elsewhere, in which the spherules have been derived appar-
ently from the stems of corallines.” On the larger islands
the rocky surface of the hills is very thinly and partially cov-

?

ered with “red earth”? mixed in varying proportions with

vegetable matter. There are many large caverns in the

218 CORALS AND CORAL ISLANDS.

group, and those of Long Cay and Rum Cay are described as
equalling those of the Bermudas.

The Bahamas differ from ordinary atolls more in the
great size of the two western banks, and the wide distribu-
tion and high heapings of the wind-drift deposits, than in any
other characteristics. Some peculiarities are due also to the
position of the reefs, — one end resting on a sea-border pla-
teau and the other extending out into the deeper waters of

the ocean. To the westward, a rise of three thousand feet .

would make one land of Florida, Cuba, and the Bahamas; but
the Bahamas to the eastward would still be a line of oceanic
islands. As to evidences of recent elevation nothing is posi-
tively known. The great extent and height of the drift-made
dry land appears to indicate a long resting at the present

level.

~The Bermuda or Somers’ Islands. —The Bermudas are
what remains of a large atoll, as first announced by Lieutenant
Nelson ;* and this atoll is the most remote from the equator

of any existing. It lies in deep seas between the parallels

1 Transactions of the Geological Society of London, 1840, v., 103.

The following are other important publications on the structure of the Bermu-
das: The Reports of the Challenger Expedition of 1873 and 1876, by Sir Wyville
Thomson, London, vol. i.; ‘ The Naturalist in Bermuda,” by John Matthew Jones,
with a map and illustrations, London, 1859; also, by the same, “ A Visitor’s Guide
to Bermuda;” Observations on the Bermudas, in Nature for 1872; and on the geo-
logical features of the Bermudas in the Proceedings and Transactions of the Nova
Scotia Institute of Natural Science, 1867; A paper on the Bermuda Reefs by
Dr. J. J. Rein, in the Senckenberg. Ber. naturforsch. Gesellschaft, 1869-70, and
Verhandlung des I. deutsch. Geographentages fiir 1881, Berlin, 1882. There are
also two valuable American contributions to the Geology of the Bermudas, one by
Prof. William North Rice, of Middletown, Conn., 32 pp. 8vo, being Bulletin No. 25
of the U. S. National Museum, 1884; and a volume by A. Heilprin, of Philadel-
phia, entitled “ The Bermuda Islands: a contribution to the Physical History and
Zoology of the Somers’ Archipelago,” a handsomely illustrated work of 231 pp. 8vo,
treating of the coral reefs, and also of the zoology, and discussing at length the
coral-island problem, with conclusions favoring, like those of Professor Rice, the

Darwinian theory.

THE BERMUDA ISLANDS. 219

32° and 32° 35’, and the meridians 64° 30! and 65° 30’. The
principal species of corals are mentioned on page 114.

The general form and position of the reef and its islets
are shown on the accompanying map; and its position in the
ocean and the depths of the seas, on Plate XI. The longer
diameter of the elliptical area trends nearly northeast-by-east,

ed GES
Ve

THE BERMUDA ISLANDS.

and is about twenty-five miles in length, while the transverse
diameter is about fifteen miles. In the ocean about the Ber-
mudas, the depth descends to twenty-five hundred fathoms
and beyond. Within seventeen English miles west of the
Bermuda reef there is a sounding (by the-Challenger Expe-
dition) of twenty-four hundred and fifty fathoms, giving
a slope of 1:6:1; and fifteen miles east of the southern
submerged (in ten fathoms) reef, one of twenty-two hun-
dred and fifty fathoms. giving the slope 1:6. There is also
a sounding of twenty-six hundred and thirty-two fathoms

220 CORALS AND CORAL ISLANDS.

sixteen miles southwest of the same southern reef, giving a
slope of 1:5°4. The soundings are too few for a decision as
to the maximum slope.

The emerged land, about fifteen miles long, is confined
to the side facing southeast, excepting a single isolated rock,
or group of rocks, on the north side (between ¢ and d on the
map) called North Rock. It is broken into a hundred and
fifty or more islets, and the surface is made up of hills and
low basins. The highest point, Sears’ Hill (E), is, according
to Lieutenant Nelson, two hundred and sixty feet in eleva-
tion above the sea, and Gibbs Hill (D), the site of the light-
house, two hundred and forty-five feet. Wreck Hill (F), near
the western point of the principal island, is about one hundred
and fifty feet high, and North Rock 1s sixteen feet high, above
mean tide. H is the position of Hamilton, the seat of Govern-
ment, and G, of St. George’s, the other principal town. A
(Castle Harbor), B (Harrington Sound), and C (Great Sound)
are three encircled bays, looking as if once the lagoons of sub-
atolls in a Maldive-like compound atoll. The surface, about
half way between the sounds A and B, is low. Most of the
land is covered with cedars where not cultivated or given
over to loose sand. The last island of the southern hook is
Ireland Island.

The greater part of the old atoll is still a submerged reef.
But it is of the typical form, in having a large lagoon-like
depression enclosed within a relatively narrow border. Its
border is mostly from one to three fathoms under water at
low tide, though in some parts laid bare at the ebb. It has
open channels at a, called the Chub cut; b, Blue cut, shallow;
ce, N. W. Channel; d, N. E. Channel ; e, Mills’ Breaker Chan-
nel ; f, the channels affording the nearest routes to Murray
Anchorage and St. George’s Harbor; g, Channel by St.

THE BERMUDA ISLANDS. 221

David’s Head, shallow; and h, Hog-fish cut. The reef-
grounds, inside, are encumbered with countless clumps of
corals and coral-heads, one to four fathoms under water, with
intervals between of five to ten fathoms; some large tracts
are without corals, and these have a nearly uniform depth of
seven or eight fathoms. To a vessel entering, the positions
of the coral clumps are made known by the brownish or dis-
colored water above them. The bottom, over large areas, is
a calcareous clay or mud; that of Murray Anchorage, a fine
chalky clay.

Serpule have made large accumulations over parts of the
reef, as stated by Nelson. ‘The tubes are so heaped over one
another as to make rings or atoll-like elevations two feet or
so high and two to twenty feet wide. Nelson calls them
“Serpuline reefs.’ The reefs on the east and south sides
are narrow, not over a fourth of a mile wide, and the waters
abruptly deepen; we may consequently conclude that this
southeastern side of the original land was bold and _ high,
while off to the north and west the surface was relatively
low and flat.

The rock of the surface is a calcareous sand-rock of wind-
drift and beach origin like that of the Florida and Bahama
reefs. Prof. Wm. N. Rice says that no true coral-reef rock
is seen anywhere above the sea-level, and that the beach and
wind-drift formations graduate into one another and are not
easily distinguishable. The beach-made rock is in some
places eight to fifteen feet above tide-level. Professor Rice
says further : ' —

“The beach-rock is, on the average, more perfectly consoli-
dated than the drift-rock, but in this character both rocks
vary widely. Drift-rock, when submerged by a subsidence

} Rice, Bulletin No. 25, United States National Museum, 1884, pp. 10, 11, 14, 15.

Doan CORALS AND CORAL ISLANDS.

consequent to its deposition, may come to assume the degree
of consolidation usually observed in beach-rock. On the
south shore of the main island, near Spanish Rock, I ob-
served strata perfectly continuous dipping toward the water,
exceedingly hard at the margin of the water, but becoming
considerably softer as they were traced upward and landward.
Mr. Ebenezer Bell, who some years ago had charge of some
works in progress on Boaz Island, informed me that he found
that the reck, so soft as to crumble in one’s fingers, became
quite hard on immersion for a week or a fortnight im sea-
water. Some of the hardest rock which I observed in Ber-
muda was shown by other characters to be unmistakably
drift-rock. A more reliable distinction is found in the lami-
nation, the beach-rock showing a general and uniform dip
toward the water, while the drift-rock shows the high and
extremely irregular dips which are characteristic of wind-
blown sands. But not every section exhibits characters sufti-
ciently marked to settle the nature of the rock, since the
beach-structure admits of a considerable degree of irregu-
larity in dip, while wind-blown sands in a long ridge or dune
may have for long distances a gentle and nearly uniform dip.
The indication furnished by the fossil contents of the rock is
important. The beach-rock is often richly fossiliferous, con-
taining shells and pieces of coral of considerable size. The
drift-rock will, of course, ordinarily contain no relics of
marine animals except fragments so small as to be blown
by the wind. A high wind can, however, sweep along pieces
of shell and coral larger than one would at first suppose. .
“While the presence of marine fossils in a sand-rock Is
an indication that it is a beach-rock, the drift-rock 1s quite
apt to contain the shells of land snails. The presence of

snail shells cannot, however, be regarded as a sure proof of

THE BERMUDA ISLANDS. 28

drift-rock, smmce they may easily be washed down by rains
from a bank or bluff above the beach and imbedded in the
beach-sands.

“The usual softness of this drift-rock has made it a mat-
ter of small labor and expense to secure easy grades on most
of the roads in the islands, by making deep cuts wherever
they are required. ‘These cuttings are of great interest to
the geologist, from the beautiful illustration which they
afford of that extreme irregularity of lamination which is
characteristic of wind-drifts. Not only the country roads,
but also the streets of the towns, abound in these beautiful
and instructive sections. Fine exhibitions of this same struc-
ture are to be seen in the natural sections afforded by the
cliffs or pinnacles of the shore.”

Professor Heilprin’s work on the Bermuda Islands con-
tains three phototypes which show finely the general land-
scape features of the rock.

The great drift-sand hills, ridges, and flats, ike those of
the Bahamas, are results of the fiercer storm-winds, as
stated on page 155. At the Bermudas, the ordinary west-
erly winds are feeble at transportation. But cyclones, as the
“Sailing Directions” state, are very frequent, and “ especially

b

in the autumn;” and during the earlier and more furious
half of the storm, the wind is easterly. In addition, “ Ber-
muda squalls are sudden and violent tempests” of the winter,
and in them the winds are from all directions. By compar-
ing ordinary winds with violent tempests we may apprehend
the difference at these times in the amount of force at work,
and lose surprise over the differences in results. The “North
Rock” is apparently the remains of drift-hills built up on the
projecting northwest point of the reef-island ; but the surface

must have been higher than now, for their formation.

>

224 CORALS AND CORAL ISLANDS.

The Bahamas are still farther within the belt of Atlantic
storm tracks, and in the West Indian portion, as is well shown
on the Chart of Atlantic Storms by Wm. C. Redfield in Vol-
ume XXXI. of the American Journal of Science, 1837. They
are situated just outside of the continental line where the
tracks of many of the cyclones make the turn northward ;
and this is reason enough for high drift-heaps and the great
width of the areas.

The Florida region feels less powerfully the influence of
the storms, but their influence is sufficient for the accumula-
tion of extensive drift-ridges and a wide spread of the sands
over the bank.

The Bermudas have suffered greatly from erosion. There
are no running streams, but the coral sands and the limestones
made from them are easily dissolved and removed by carbon-
ated waters ; and consequently the rains, reinforced in their
carbonic acid by more from vegetable or animal decomposi-
tion in the soil, have done a large part of the erosion over
the surface and of that of cavern-making beneath it ; while the
waves have made cliffs, towers, and pinnacles, and caves too,
along the coasts. The winds, moreover, have aided both.

Professor Rice confirms the earlier accounts of Lieutenant
Nelson and others, and speaks of the “innumerable caves”
as “ranging in size from miniature grottos — the bijoux
of Nelson—to extensive caverns.” One of the miniature
caves had been opened at Paynter’s Vale in quarrying: its
horizontal diameter was about five feet, its height at middle
only two; but pigmy stalagmites rose from the floor toward
the slender stalactites that were pendent from the roof, and
along the sides the stalactites and stalagmites were in many
cases united to form little columns; and all was of most

exquisite finish.

THE BERMUDA ISLANDS. 22

Proofs of subsidence are reported to exist in the occur-
rence of shoreward dips of the sand-rock into and below the
water-level (Rice); in the existence of caverns submerged
over fifty feet, with stalagmites rismg from their floors, now
far beneath the surface (Heilprin); and the fact mentioned
in the Report of the Challenger Expedition, that a peat-bed,
stumps of cedar, land snails (//elic LBermudensis), and loose
masses of the drift-sand rock were found on Ireland Island at
a depth below tide-level of forty feet, in the excavation for
a floating dock. When these cedars were growing, the land
was consequently forty-five feet or more above its present
level. It is probable that at this time there was a long
period of rest or cessation in the subsidence, and that during
it the drift-sand formation of the island, including that of
“North Rock,” was chiefly made. Afterward, when the sub-
sidence was resumed, the work of degradation began.

A comparatively recent elevation is indicated, according
to Professor Rice, by the height of the beach-sand rock. fif-
teen feet, on part of the north side of the strip of dry land.

The origin of the “red earth” ' making much of the soil

1 Analysis of the coral sand of Bermuda, and mean of three analyses of the
red earth, by Mr. F. A. Manning (Agricultural Report, at Bermuda in 1873, of
Maj.-Gen. J. H. Lefroy).

1. Coral sand 2. Red earth.
Warhbonicaciduer erm aces oe eS 4.06
Ae ae gen eth tas Sich os rte oh ee OTT 5.95
Wapnegiaie vers War Gar ist acc eeu 52a 1 109 0.36
OLAS Estee testes cleat a ciate ae 2 BOLOG 0.14
SIQCAMMMEE ah A Seicsccist uss. waa) esl OLD4: 0.03
Alumina ‘ Cpe aera esate ¢ 16.94
Tron sesquioxide ( 19.58
SAUBNUME ACTON a 5a ees) Yee e 9 OL20 0.05
Chlorinehysesy city ease e el ee eane ty OL02 0.015
PhOSspHOLiclacid someway ee O08 0.70
Sauls stt aerrpie es stele ee sre 1 OL05 56.60
Oreanic substance’. 2) .) 2) 2) S180 15.41
102.00 99.81

Excluding the 15:41 per cent of organic substance in the red earth, 100 parts,
according to the above, contain about 23 per cent of iron oxide, 20 of alumina, and
43 of sand, besides some lime carbonate and small portions of the other ingredients
enumerated,

15

226 CORALS AND CORAL ISLANDS.

was first explained by Sir Wyville Thomson. The limestone
contains about half of one per cent of iron oxide and earthy
ingredients ; and these are left behind as “red earth” when
the rest is dissolved away by the carbonated waters.

Another source in some regions, if not at the Bermudas,
is the volcanic dust that is widely distributed by the winds,
or fragments of pumice and other volcanic rocks that may
have been brought by the sea and drifting logs. Pieces of
pumice and augitic lava have been found on the island ; and
from the sands Mr. Murray obtained magnetite, chrysolte,
augite. sanidin and other feldspars, mica, and perhaps quartz.

Twenty miles southwest-by-west from the Bermudas,
there are two submerged banks or shoals, both reported

’

as having a “corally and rocky bottom ;” one has over it
a minimum depth of twenty-four fathoms, and the other
of ten fathoms. Dredging on these banks might make some

interesting disclosures.

CORALS AND CORAL ISLANDS. 227

CHAPTER III.

FORMATION OF CORAL REEFS AND ISLANDS, AND CAUSES OF
THEIR FEATURES.

I. FORMATION OF REEFS.

I. ORIGIN OF CORAL SANDS AND THE REEF-ROCK.

Very erroneous ideas prevail respecting the appearance of
a bed or area of growing corals. The submerged reef’ is
often thought of as an extended mass of coral, alive uniform-
ly over its upper surface, and as gradually enlarging upward
through this living growth; and such preconceived views,
when ascertained to be erroneous by observation, have some-
times led to skepticism with regard to the zodphytic origin
of the reef-rock. Nothing is wider from the truth: and this
must have been inferred from the descriptions already given.
Another glance at the coral plantation should be taken by
the reader, before proceeding with the explanations which
follow.

Coral plantation and coral field are more appropriate ap-
pellations than coral garden, and convey a juster impression
of the surface of a growing reef. Like a spot of wild land,
covered in some parts, even over acres, with varied shrub-
bery, in other parts bearing only occasional tufts of vegeta-
tion in barren plains of sand, here a clump of saplings, and
there a carpet of variously-colored flowers in these barren
fields—such is the coral plantation. Numerous kinds of
zoophytes grow scattered over the surface, like vegetation

228 FORMATION OF CORAL REEFS AND ISLANDS.

upon the land; there are large areas that bear nothing, and
others of great extent that are thickly overgrown. There is,
however, no green sward to the landscape; sand and frag-
ments fill up the bare intervals between the flowering tufts:
or, where the zodphytes are crowded, there are deep holes
among the stony stems and folia.

These fields of growing coral spread over submarine
lands, such as the shores of islands and continents, where the
depth is not greater than theirhabits require, just as vegetation
extends itself through regions that are congenial. The germ
or ovule, which, when first produced, is free, finds afterward «

point of rock, or dead coral, or some support, to plant itself

upon, and thence springs the tree or other forms of coral growth.

The analogy to vegetation does not stop here. It is well
known that the débris of the forest, decaying leaves and
stems, and animal remains, add to the soil; that in the marsh
or swamp—where decaying vegetation is mostly under water,
and sphagnous mosses grow luxuriantly, ever alive and flour-
ishing at top, while dead and dying below,—accumulations
of such débris are ceaselessly in progress, and deep beds of
peat are formed. Similar is the history of the coral mead.
Accumulations of fragments and sand from the coral zo6-
phytes growing over the reef-grounds, and of shells and other
relics of organic life, are constantly making; and thus a bed
of coral débris is formed and compacted. ‘There is this dif-
ference, that a large part of the vegetable material consists of
elements which escape as gases on decomposition, so that there
is a great loss in bulk of the gathered mass; whereas coral is
an enduring rock material undergoing no change except the
mechanical one of comminution. The animal portion is but
a mere fraction of the whole zodphyte. The coral débris and
shells fill up the intervals between the coral patches, and the

CORALS AND CORAL ISLANDS. 229

cavities among the living tufts, and in this manner produce
the reef deposit; and the bed is finally consolidated, while
still beneath the water.

The coral zoéphyte is especially adapted for such a mode
of reef-making. Were the nourishment drawn from below, as
in most plants, the solidifying coral rock would soon destroy
all life: instead of this, the zoéphyte is gradually dying be-
low while growing above; and the accumulations of débris
cover only the dead portions.

But on land, there is the decay of the year, and that of old
age, producing vegetable debris; and storms prostrate forests.
And there are corresponding effects among the groves of the
sea. It has been shown that coral plantations, from which
reefs proceed, do not grow in the “calm and still” depths of
the ocean. They are to be found amid the waves, and usually
extend little below a hundred feet, which is far within the
reach of the sea’s heavier commotions. To a considerable ex-
tent they grow in the very face of the tremendous breakers
that strike and batter as they drive over the reefs. Here is
an agent which is not without its effects. The enormous
masses of uptorn rock found on many of the islands may give
some idea of the force of the lifting wave; and there are ex-
amples on record, to be found in various treatises on Geology,
of still more surprising effects.

During the more violent gales the bottom of the sea is
said, by different authors, to be disturbed to a depth of three
hundred, three hundred and fifty, or even five hundred feet,
and De la Beche remarks, that when the depth is fifteen fath-
oms, the water is very evidently discolored by the action of
the waves on the sand and mud of the bottom.

In an article on the Force of Waves, by Thomas Steven-
son, of Edinburgh, published in the Transactions of the Royal

230 CORALS AND CORAL ISLANDS.

Society of Edinburgh (vol. xvi., 1845), it is stated as a deduc-
tion from two hundred and sixty-seven experiments, extend-
ing over twenty-three successive months, that the average
force for Skerryvore, for five of the summer months, during
the years 1843, 1844, was six hundred and eleven pounds per *
square foot; and for six of the winter months of the same
year, it was two thousand and eighty-six pounds per square
foot, or three times as great as during the summer months.
During a westerly gale, at the same place, in March, 1845, a
pressure of six thousand and eighty-three pounds was regis-
tered by Mr. Stevenson’s dynamometer (the name of the in-
strument used). He mentions several remarkable instances
of transported blocks.

We must, therefore, allow that some effect will be pro-
duced upon the coral groves. There will be trees prostrated
by gales, as on land, fragments scattered, and fragmentary
and sand accumulations commenced. Besides, masses of the
heavier corals within ten to twenty feet of the surface may
be uptorn, and carried along over the coral plantation, which
will destroy and grind down every thing in their way. So
many are the accidents of this kind to which zodphytes ap-
pear to be exposed, that we might believe they would often be
exterminated, were they not singularly tenacious of life, and
ready to sprout anew on any rock where they may find quiet
long enough to give themselves again a firm attachment.

But it should be observed, that the sea would have far
less effect upon the slender forms characterizing many z00-
phytes, among which the water finds free passage, than on the
massive rock, against whose sides a large volume may drive
unbroken. Moreover, much the greater part of the strength
of the ocean is exerted near tide level, where it rises in break-
ers which plunge against the shores. Yet owing to the many

FORMATION OF CORAL REEFS AND ISLANDS. PAS

nooks and recesses deep among the corals, the rapidly moving
waters, during the heavier swells, must produce whirling ed-
dies of considerable force. tending to uproot or break the coral
clumps. Moreover, it is to be kept in mind that shells and
echinoderms make contributions as well as corals, and that all
life grows luxuriantly in the coral seas.

There is another process going on over the coral field, some-
what analogous to vegetable decay, though still very different.
Zodphytes have been described as ever dying while living. The
dead portions have the surface much smoothed, or deprived
of the roughening points which belong to the living coral, and
the cells are sometimes half obliterated, or the delicate lamelle
worn away. This may be viewed as one source of fine coral
particles ; and as the process is constantly going on, it is not
altogether unimportant. This material is in a fit condition to
enter into solution, and it cannot be doubted that the water
receives lime from this source, which is afterward yielded to
the reef.

In the Alcyonia family, which includes semi-fleshy corals,
and in the Gorgonie, the lime is often scattered through the
polyps in granules ; and the process of death sets these calca-
reous grains free, which are constantly added to the coral sands.
The same process has been supposed to take place in the more
common reef corals, the Madrepores and Astreeas, and it is
possible that this may be to some extent the case. Yet it
would seein, from facts observed, that after the secretion has
begun within the polyp, the secretion of lime going on takes
place against the portions already formed and in direct union
with them, and not as granules to be afterward cemented.

The mud-like deposits about coral reefs (pp. 142, 183, 205)
have been attributed to the causes just mentioned, but with-
out due consideration. There is an unfailing and abundant

232 CORALS AND CORAL ISLANDS.

source of this kind of material in the self-triturating sands of
the reefs acted upon by the moving waters. On the seaward
side of coral islands, and on the shores of the larger la-
goons, where the surface rises into waves of much magnitude,
the finer portions are carried off, and the coarser sand remains
alone to form the beaches. This making of coral sand and
mud is just like that of any other kind of sand or mud. It
takes place on all shores exposed to the waves, coral or not
coral, and in every case the gentler the prevailing movement
of the water, the finer the material on the shore. In the
smaller lagoons, where the water is only rippled by the winds,
or roughened for short intervals, the trituration is of the
gentlest kind possible, and, moreover, the finely pulverized
material remains as part of the shores. Thus the fine mater-
ial of the mud must be constantly forming on all the shores,
for the sands are perpetually wearing themselves out; but the
particles of the fine mud, which is washed out from the beach
sands, accumulates only in the more quiet waters some dis-
tance outside of the reef, and within the lagoons and channels,
where it settles. This corresponds exactly with the facts; and
every small lake or region of quiet waters over our continent,
illustrates the same point.

Mr. Darwin, in discussing the origin of the finer calcareous
mud, (op. cit., p. 14), supposes that it is derived in part from
fishes and Holothurians; and other authors have thrown out
the same suggestion. He cites as a fact, on the authority of
Mr. Liesk, that certain fish browse on the living zodphytes; and
from Mr. Allan, of Forres, he learned also that Holothurians
subsisted on them. The statement about the Holothurians
has been set aside by observation. Small fish swarm about
the branching clumps, and when disturbed, seek shelter at once
among the branches, where they are safe from pursuit. The

FORMATION OF CORAL REEFS AND ISLANDS. 2353

author has often witnessed this, and never saw reason to sup-
pose that they clustered about the coral for any other purpose.
It is an undoubted fact, as stated by Mr. Darwin, that frag-
ments of coral and sand may be found in the stomachs of
these animals, but this is not sufficient evidence of their
browsing on thecoral. Fish so carefully avoid polyps of all
kinds because of their power of stinging (as illustrated on
p. 37), that we should wait for further and direct evidence
on this point. The conclusion deduced by him from the facts,
may be justly doubted. The fish and Holothurians, though
numerous, are quite inadequate for the supply; and, more-
over, we have, as explained above, an abundant source of the
finest coral material without such aid. Motion of particle
over particle will necessarily wear to dust, even though the
particles be diamonds; and this incessant grinding action
about reefs accounts satisfactorily for the deposits of coral
mud, however great their extent.

The coral world, as we thus perceive, is planted, like the
land, with a variety of shrubs and smaller plants, and the el-
ements and natural decay are producing gradual accumula-
tions of material, like those of vegetation. The history of the
growing reef has consequently its counterpart among the or-
dinary occurrences of the land about us.

The progress of the coral formation is like its commence-
ment. The same causes continue, with similar results, and
the reader might easily supply the details from the facts al-
ready presented. The production of débris will necessarily
continue to go on: a part will be swept by the waves, across
the patch of reef, into the lagoon or channel beyond, while
other portions will fill up the spaces among the corals along
its margin, or be thrown beyond the margin and lodge on it-

234 CORALS AND CORAL ISLANDS.

surface. The layer of dead coral rock which makes the body
of the reef, has its border of growing corals, and is thus un-
dergoing extension at its margin, both through the increase in
the corals, and the débris dropped among them.

But besides the small fragments, larger masses will be
thrown on the reefs by the more violent waves, and commence
to raise them above the sea. The clinker fields of coral by
this means produced, constitute the first step in the formation
of dry land. Afterward, by further contributions of the |
coarse and fine coral material, the islets are completed, and
raised as far out of the water as the waves can reach—that is,
about ten feet, with a tide of three feet; and sixteen to
eighteen feet with a tide of six or seven.

The Ocean is thus the architect, while the coral polyps af
ford the material for the structure; and, when all is ready, it
sows the land with seed brought from distant shores, covering
it with verdure and flowers.

The growth of the reefs and islands around high lands is
the same as here described for the atoll. The reef-rock is
mainly a result of accumulations of coral and shell débris.
There are reefs where the corals retain the position of growth,
as has been described on a former page. But with these the
débris comes in to fill up the intervening spaces or cavities, and
make a compact bed for consolidation. There are other parts,
especially portions of the outer reef along the line of break-
ers, which are formed by the gradual growth of layer upon
layer of incrusting Nullipores; but such formations are of
small extent, and only add to the results from other sources.

II. ORIGIN OF THE SHORE PLATFORM.

Among the peculiarities of coral islands, the shore plat-
form appears to be one of the most singular, and its origin

FORMATION OF CORAL REEFS AND ISLANDS. 235

has not been rightly understood. It will be remembered that
it lies but little above low-tide level, and it is often over three
hundred feet in width, with a nearly flat surface throughout.

Though apparently so peculiar, the existence of this plat-
form is due to the simple action of the sea, and is a necessary
result of this action. On the shores of New South Wales,
Australia, near Sydney, as observed by the author, the same
structure is exemplified along the sandstone shores of this
semi-continent, where it is continued for scores of miles. At
the base of the sandstone cliff, in most places one or more hun-
dred feet in height, there is a layer of sandstone rock, lying,
like the shore platform of the coral island, near low-tide level,
and from fifty to one hundred and fifty yards in width. It is
continuous with the bottom layer of the cliff: the rocks which
once covered it have been removed by the sea. Its outer edge
is the surf-line of the coast. At low-tide it is mostly a naked
flat of rock, while at high tide it is wholly under water, and
the sea reaches the cliff.

New Zealand, at the Bay of Islands, affords a like fact in
ap argillaceous sand-rock; and there was no stratification in
this case to favor the production of a horizontal surface; it

THE OLD HAT.

was a direct result from the causes at work. The shore shelf
stands about five feet above low water. A small island in this
bay is well named the ‘Old Hat,” the platform encircling it,
as shown in the above figure, forming a broad brim to a rude

236 CORALS AND CORAL ISLANDS.

conical crown. The water, in these cases, has worn away the
cliffs, leaving a broad horizontal basement above the level of
low tide.

According to Professor Verrill, the same feature is exhib-
ited on a grand scale at the island of Anticosti im the Gulf
of St. Lawrence, and “Old Hats” are among the forms
produced. The cliff consists of limestone, and the “ shore-
platform” is in many places over four hundred yards wide.

A surging wave, as it comes upon a coast, gradually rears it-
self on the shallowing shores; finally, the waters at top, through
their greater velocity, plunge with violence upon the barrier
before it. The force of the ocean’s surge is therefore mostly
confined to the summit waters, which add weight to superior ve-
locity, and drive violently upon whatever obstacle is presented.
The lower waters of the surge advance steadily but more
slowly, owing to the retarding friction of the bottom; the
motion they have is directly forward, and thus they act with
little mechanical advantage ; moreover, they gradually swell
over the shores, and receive, in part, the force of the wpper
waters. ‘The wave, after breaking, sweeps up the shore till it
gradually dies away. Degradation from this source is conse-
quently most active where the upper or plunging portion of
the breaker strikes.

But, further, we observe that at low-tide the sea is compara-
tively quiet; it is during the influx and efflux that the surges
are heaviest. The action commences after the rise, is strongest
from half to three-fourths tide, and then diminishes again near
high tide. Moreover, the plunging part of the wave is raised
considerably above the general level of the water. From
these considerations, it is apparent that the line of greatest
wave-action must be above low-water level. ‘ Let us suppose a

tide of three feet, in which the action would probably be ~

FORMATION OF CORAL REEFS AND ISLANDS. 237

strongest when the tide had risen two feet out of the three;
and let the height of the advancing surge be four feet: the
wave, at the time of striking, would stand, with its summit,
three feet above high-tide level; and from this height would
plunge obliquely downward against the rock or any obstacle
before it. It is obvious that, under such circumstances, the
greatest force would be felt not far from the line of high tide,
or between that line and three feet above it; moreover, the
rise of the waters to half or two-thirds tide affords a protection
against the breaker to whatever is below this level. In re-
gions where the tide is higher than just supposed, as six feet
for example, the same height of wave would give nearly the
same height to the line of wave action, as compared with high-
tide level. Under the influence of heavier waves, such as are
common during storms, the line of wave-action would be at a
still higher elevation, as may be readily estimated by the
reader.

Besides a line of greatest wave-action, we also distinguish
a height of feeble action, —so feeble that the rock remains
unremoved along and below a nearly horizontal plane which
is often three hundred to four hundred yards in width. The
height, as is evident from the facts stated, is some distance
above low-tide level. The lower waters of the tide, besides
being protective, as above explained, are accumulative in their
ordinary action, when the material exposed to them is moy-
able; they transport shoreward, while the upper waters are
eroding, and preparing material to be carried off. The height
at which these two operations balance each other will be the
height, therefore, of the line of least degradation. Moreover,
it should vary with the height of the tides. This height, on
the eastern shores of Australia, is three feet above ordinary
low tide, and at New Zealand about five feet. With regard

238 CORALS AND: CORAL ISLANDS.

AK
to the height varying with the tides, we observe that in the |

Paumotus, where the water rises but two or three feet, the |
platform is seldom over four to six inches above low tide, >~
which is proportionally less than at Australia and New Zea-
land, where the tide is six and eight feet. From these ob-
servations it appears that the height of least wave-action.
as regards the degradation of a coast under ordinary seas,
is situated near one-fifth tide in the Paumotus, and above
half-tide at New Zealand, showing a great difference between
the effect of the comparatively quiet work of the middle Pa-
cific, and the more violent of New Zealand. Within the Bay
of Islands, where the sea has not its full force, the platform,
as around the ‘‘ Old Hat,” is but little above low-water level.
The exact relation of the height of the platform to the height
and force of the tides, and the force of wave-action, remains
to be determined more accurately by observation. While,
therefore, the height of the shore platform depends on the
tides, and the degree of exposure to the waves, the breadth
of it will be determined by the same causes in connection with
the nature of the rock material.

On basaltic shores it is not usual to find a shore platform,
as the rock scarcely undergoes any degradation, except from
the most violent seas; such coasts are consequently often cov-
ered and protected by large fragments of the basaltic rocks.
But on sandstone shores, if the rock is not too firm to yield
sensibly under the stroke of the breakers, this gradual action
keeps the platform of nearly uniform breadth. Moreover,
any masses torn from the edge of the platform and thrown
upon it by storm waves, or the heavier earthquake waves,
may be soon destroyed by the same action, and carried off ;
and thus the platform may be kept nearly clean of débris,
even to the base of the cliff.

}

FORMATION OF CORAL REEFS AND ISLANDS. 239

In the case of the coral island, the material of the coral
platform is piled up by the advancing surges, and cemented
through .the infiltrating waters. These surges, on reaching
the edge of the shelf, break upon it with more or less force

during low tide and the commencing rise; but later the
waters swell over it before breaking, and thus throw a pro-
tection about the exposed rocks; and as the tide continues to
rise, they sweep over the shelf, but only clear it of sand and
fragments, by bearing them to the beach on which they ex-
pend their force. Where the tides are five to six feet in
height, the shore platform of atolls is narrow.

The isolated blocks in the Paumotus which stand on the
platform, attached to it below, are generally most worn one
or two feet above high-tide level, —a fact which corresponds
with the statement in a preceding paragraph with regard to
the height of the greatest wave-action.

Il, EFFECTS OF WINDS AND GALES,

In addition to this ordinary wave-action, there are also
more violent effects from storms; and these are observed alike
on the Australian shores referred to, and on those of coral
islands. The waters as they move in, first draw away, and
then drive on with increased velocity up the shallowing shores,
or under shelving layers, and thus they easily break off great
rocks from the edge of the platform, and throw them on the
reef. From the observations of Mr. Stevenson, cited on a pre-
ceding page (p. 229), it appears that the force of the waves
during the summer and winter months differs at Skerryvore
more than 1,200 pounds to the square foot. The seasons are
not as unlike in the tropical] part of the Pacific. But in all

seas there is a marked difference and in some stormy months

240 CORALS AND CORAL ISLANDS.

increase this difference. Further, the winds work with the
waves, and bear the lighter part of the beach-making sands
to a higher level than can be reached by the waves, giving
the beach a top of wind-drift deposition as already explained.
Still more violent in action are the great earthquake-waves,
which move through the very depths of the ocean.

These principles offer an explanation also of the general
fact that the windward reef is the highest. The ordinary
seas both on the leeward and windward sides, are sufficient
for producing coral débris and building up the reef, and in
this work the two sides will go on together, though at different
rates of progress. We may often find no very great dif-
ference in the width of the leeward and windward reefs, es-
pecially as the wind for some parts of the year, has a course
opposite to its usual direction. But seldom, except on the
side to windward, is a sufficient force brought to bear upon the
edge of the platform, to detach and uplift the larger coral
blocks. The distance to which the waves may roll on without
becoming too much weakened for the transportation of up-
torn blocks, will determine the outline of the forming land.
With proper data as to the force of the waves, the tides, and
the soundings around, the extent of the shore platform might
be made a subject of calculation.

The effect of a windward reef in diminishing the force of
the sea, is sometimes shown in the influence of one island on
another. A striking instance of this is presented by the
northernmost of the Gilbert Islands (see map, on page 165.)
All the islands of this group are well wooded to windward—
the side fronting east. But the north and northeast sides of
Tari-tari are only a bare reef, through a distance of twenty
miles, although the southeast reef is a continuous line of ver-

FORMATION OF CORAL REEFS AND ISLANDS. DAW

dure. The small island of Makin, just north of Tari-tari, is
the breakwater which has protected the reef referred to from
the heavier seas.

Coral island accumulations have an advantage over all
other shore deposits, owing to the ready agglutination of cal-
careous grains, as explained on a following page. It has been
stated that coral sand-rocks are forming along the beaches,
while the reef-rock is consolidating in the water. A defence
of rock against encroachment is thus produced, and is in con-
tinual progress. Moreover, the structure built amid the |
waves, will necessarily have the form and condition best fitted
for withstanding their action. The atoll is, therefore, more
enduring than hills of harder basaltic rocks. Reefs of
zoophytic growth but ‘“‘mock the leaping billows,” while
other lands of the same height gradually yield to the assaults
of the ocean. There are cases, however, of wear from the
sea, owing to some change of condition in the island, or in
the currents about it, in consequence of which, parts once
built up are again carried off. Moreover, those devastating
earthquake-waves which overleap the whole land, may occa-
sion unusual degradation. Yet in ordinary seas these islands

have within themselves the source of their own repair, and
are secure from all serious mjury.

The change of the seasons is often apparent in the distri-
bution of the beach sands covering the prominent points of an
island. At Baker’s Island (near the equator, in long. 176°
23’, W.), this fact is well illustrated. J. D. Hague states
(Am. Jour. Scr, IL., xxxiv, 237), that the shifting sands
change their place twice a year. ‘The western shore of the
island trends nearly northeast and southwest; the southern
shore, east-by-north. At their junction there is a spit of sand

extending out toward the southwest. During the summer.
16

242 CORALS AND CORAL ISLANDS.

the ocean swell, like the wind, comes from the southeast, to
the force of which the south side of the island is exposed,
while the western side is protected. In consequence, the sands
of the beach that have been accumulating during the summer
on the south side, are all washed around the southwest point
and are heaped up on the western side, forming a plateau
along the beach two or three hundred feet wide, nearly cover-
ing the shore platform, and eight or ten feet deep. With
October and November comes the winter swell from the north-
northeast, which sweeps along the western shore, and from
the force of which the south side is in its turn protected.
Then the sand begins to travel from the western to the south-
ern side; and, after a month or two, nothing remains of the
great, sand plateau but a narrow strip; while on the south
side, the beach has been extended two hundred or three hun-
dred feet. This lasts until February or March, when the

operation is repeated.”

Il. CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS.

Coral reefs, although (1) dependent on the configuration of
the submarine lands for many of their features, undergo vari-
ous modifications of form, or condition, through the influence of
extraneous causes, such as (2) wnequal exposure to the waves ;
(8) oceanze or local currents ; (4) presence of fresh or impure
waters. In briefly treating of these topics, we may consider
first, reefs around high islands, and afterward, atoll reefs.
The effect of the waves on different sides of reefs has already
been considered, and we pass on, therefore, at once to the
influence of oceanic or local currents, and fresh or impure

waters.

FORMATION OF CORAL REEFS AND ISLANDS. 245

I, BARRIER AND FRINGING REEFS.

The existence of harbors about coral-bound lands, and of
entrances through reefs, is largely attributable to the action of
tidal or local marine currents. The presence of fresh-water
streams has some effect toward the same end, but much less
than has been supposed. ‘These causes are recognized by
Mr. Darwin in nearly the same manner as here: yet the
views presented may be taken as those of an independent wit-
ness, as they were written out before the publication of his
work.

There are usually strong tidal currents through the reef
channels and openings. These currents are modified in char-
acter by the outline of the coast, and are strongest wherever
there are coves or bays to receive the advancing tides. The
harbor of Apia, on the north side of Upolu, affords a striking
illustration of this general principle. The coast at this place

HARBOR OF APIA, UPOLU.

has an indentation 2,000 yards wide and nearly 1,000 deep,
as in the accompanying sketch, reduced from the chart by the
Expedition. The reef extends from either side, or cape, a mile
out to sea, leaving between an entrance for ships. The har-
bor averages ten feet in depth, and at the entrance is fifteen
feet. In this harbor there is a remarkable out-current along
the bottom, which, during gales, is so strong at certain states

244 CORALS AND CORAL ISLANDS.

of the tide that a ship at anchor, although a wind may be
blowing directly in the harbor, will often ride with a slack
cable; and in more moderate weather the vessel may tail out
against the wind. ‘Thus when no current but one inward is
perceived at the surface, there is an undercurrent acting
against the keel and bottom of the vessel, which is of sufficient
strength to counteract the influence of the winds on the rig-
ging and hull. The cause of such a current is obvious. The
sea is constantly pouring water over the reefs into the harbor,
and the tides are periodically adding to the accumulation ;
the indented shores form a narrowing space where these waters
tend to pile up: escape consequently takes place along the
bottom by the harbor-entrance, this being the only means of
exit. There are many such cases about all the islands. Ina
group like the Feejees, where a number of the islands are
large and the reefs very extensive, the currents are still more
remarkable, and they change in direction with the tides,
“Through the channels and among the inner reefs of the
Australian reef-region,” says Jukes, ‘“‘they run sometimes
with an impetuous sweep in the same direction even for two
or three days together, especially after great storms have
driven large quantities of water into the space between the
outer edge and the land.”

A current of the kind here represented will carry out much
coral débris, and strew it along its course. The transported
material will vary in amount from time to time, according to the
force and direction of the current. It is therefore evident
that the ground over which it runs must be wholly unfit for
the growth of coral, since most zodphytes are readily destroy-
ed by depositions of earth or sand, and require, for most spe-
cies, a firm basement. Or if the flow is very strong, it will
scour out the channels and so keep them open. The existence

FORMATION OF CORAL RHEFS AND ISLANDS. 245

of an opening through a reef may require, therefore, no other
explanation ; and it is obvious that harbors may generally be
expected to exist wherever the character of the coast is such
as to produce currents and give a fixed direction to them.

The currents, about the reef grounds west of the large
Feejee Islands, aid in distributing the debris both of the land
and the reefs. In some parts, the currents eddy and deposit
their detritus; in others they sweep the bottom clean. Thus,
under these varying conditions, there may be growing corals
over the bottom in some places and not in others; and the
reefs may be distributed in patches, when without such an
influence we mjght expect a general continuity of coral reef
over the whole reef-grounds.

The results from marine currents are often increased by
waters from the island streams; for the coves, where harbors
are most likely to be found, are also the embouchures of val-
leys and the streamlets they contain. The fresh waters poured
in add to the amount of water, and increase the rapidity of
the out-current. At Apia, Upolu, there is a stream thirty
yards wide; and many other similar instances might be men-
tioned. These waters from the land bring down also much
detritus, especially during freshets, and the depositions aid
those from marine currents in keeping the bottom clear of
growing coral. These are the principal means by which fresh-
water streams contribute toward determining the existence of
harbors; for little is due to their freshening the salt waters of
the sea.

The small influence of the last-mentioned cause—the one
most commonly appealed to—will be obvious, when we con-
sider the size of the streams of the Pacific islands, and the
fact that fresh water is lighter than salt, and therefore, in-
stead of sinking, flows on over its surface. The deepest rivers

DAG CORALS AND CORAL ISLANDS.

are seldom over six feet, even at their mouths; and three or
four feet is a more usual depth. They will have little effect,
therefore, on the sea water beneath this depth, for they can-
not sink below it; and corals may consequently grow even
in front of a river’s mouth.

Fresh-water streams, acting in all the different modes
pointed out, are of little importance in harbor-making about
the islands of the Pacific. The harbors, with scarcely an ex-
ception, would have existed without them. They tend, how-
ever, by the detritus which they deposit, to keep the bottom
more free from growing patches of coral, and keep channels
over the shore reef sutticiently deep and wide for a boat to
reach the land.

The map of the reef of North Tahiti, on page 149, and
the following map of Matavai Bay on a larger scale, afford
illustrations of this subject.

a. The harbor of Papieti is enclosed by a reef about three
fourths of a mile from the shore. The entrance through the
reef is narrow, with a depth of eleven fathoms at centre, six
to seven fathoms either side, and three to five close to the reef.
This fine harbor receives an unimportant streamlet, while a
much larger stream empties just to the east of the east cape,
opposite which the reef is close at hand and unbroken.

b. Toanoa is the harbor next east of Papieti. The en-
trance is thirty-five fathoms deep at middle. and three and
a half to five fathoms near the points of the reef. There is
no fresh-water stream, except a trifling rivulet.

c. Papaoa is an open expanse of water, harbor-lke in
character, but without any entrance; the reef is unbroken.
Yet two streams empty into it.

d. Off Matavai, the place next east, the reef is inter-
rupted for about two miles. The harbor is formed by an

FORMATION OF CORAL REEFS AND ISLANDS. 247

extension of the reef off Point Venus, the east cape. There
is no stream on the coast opposite this interruption in the
reef, except toward Point Venus; and at the present time
the waters find their principal exit east of the Point, behind

a large coral reef, a quarter of a mile distant.

_ Scale of 500 meters

100__200___300__400. “B00. iia

10) &
43° «12

Dolphin Shoal

PART OF NORTH SHORE OF TAHITI.

It is evident that the growth of coral reefs is not much
retarded about Tahiti by fresh-water streams. In fact none
of these so-called rivers are over three feet in depth; and the
most they can do is to produce a thin layer of brackish water

over the sea within the channels.

24S FORMATION OF CORAL REEFS AND ISLANDS.

e. The following figure of the harbor of Falita, Upolu,
represents another coral harbor, as surveyed by Lieutenant
Emmons. At its head there is a stream twenty-five or thirty
yards wide and three feet deep. Notwithstanding the unusual

size of the river, the coral reef lies near its mouth, and pro-

HARBOR OF FALIFA,

jects some distance in front of it. Its surface is dead, but
corals are growing upon its outer slope.

yj. The harbor of Rewa, in the Feejees, may be again al-
luded to. ‘The waters received by the bay amount to at least
500,000 cubic feet a minute. Yet there is an extensive reef
enclosing the bay, lying but three miles from the shores, and
with only two narrow openings for ships. The case is so re-
markable that we can hardly account for the facts without
supposing the river’s mouth to have neared the reef by depo-
sitions of detritus since the inner parts of the reef were formed ;
and there is some evidence that this was the case, though to
what distance we cannot definitely state. With this admis-
sion, the facts may still surprise us; yet they are explained on
the principle that fresh water does not sink in the ocean, but
is superficial, and runs on in a distinct channel; its effect is al-
most wholly through hydrostatic pressure, increasing the force
of the underwater currents, and through their depositions of.
detritus Besides these instances, there are many others in
the Feejees, as will be observed on the chart at the end of
this volume. Mokungai has a large harbor, without a
stream of fresh water ;—so also Vakea and Direction Island.

FORMATION OF CORAL REEFS AND ISLANDS. 949

The instances brought forward are a fair example of what
is to be found throughout coral seas; and they establish, be-
yond dispute, that while much in harbor-making should be at-
tributed to the transported sand or earth of marine and fresb-
water currents, in preventing the growth of coral, but little
is due to the freshening influence of the streams of islands.

But while observing that currents have so decided an in-
fluence on the condition of harbors, we should remember an-
other prevalent cause already remarked upon, which is perhaps
more wide in its effects than those just considered. I refer to
the features of the supporting land, or the character of sound-
ings off a coast. We need not repeat here the facts, showing
that many of the interruptions of reefs have thus arisen.
The wide break off Matavai may be of this kind. The widen-
ing of the inner channel at Papieti, forming a space for a har-
bor, may be another example of it; for the reef’ here extends
to a greater distance from the shores, as if because the waters
shallowed outward more gradually off this part of the coast.

The same cause—the depth of soundings, on the principle that
corals do not grow where the depth much exceeds a hundred
feet—has more or less influence about all reefs in determining
their configuration and the outlines of harbors. A remark-
able instance of the latter is exemplified in the annexed chart
of Whippey harbor, Viti Levu, reduced from the chart of the
Wilkes Expedition to the scale of half an inch to the mile.

The existence of harbors should therefore be attributed, to
a great extent, to the configuration of the submarine land;
while currents give aid in preventing the closing of channels,
and keeping open grounds for anchorage. This subject will be
further illustrated in the following pages.

The permanency of coral harbors follows directly from the
facts above presented They are secure against any immediate

250 CORALS AND CORAL ISLANDS.

- obstruction from reefs. Any growing patches within them
may still grow, and the margins of the enclosing reef may
gradually extend and contract their limits; yet only at an ex
tremely slow rate. Notwithstanding such changes, the chan-

nels will remain open, and large anchorage grounds clear, as

WHIPPEY HARBOR, VITI LEVU.

long as the currents continue in action. Coral harbors are
therefore nearly as secure from any new obstructions as those
of our continents. The growing of a reef in an adjoining
part of the coast, may in some instances diminish or alter the
currents, and thus prepare the way for more important chan-
ges in the harbor; but such effects need seldom be feared, and
results from them would be appreciable only after long periods,
since, even in the most favorable circumstances, the growth
of reefs is very slow.

When channels have a bottom of growing coral, they form
an exception to the above remark; for since the coral is acted
upon by no cause sufficient to prevent its growth, the reef will
continue to rise slowly toward the surface.

Again, when the channels are over twenty-five fathoms
in depth, they have an additional security beyond that from
currents, in the fact that reef-making corals rarely grow at
such a depth. The only possible way in which such channels
could close, without first filling up by means of shore mate-
rial, would be by the extension of the reefs from either side,

FORMATION OF CORAL REEFS AND ISLANDS. 251

till they bridge over the bottom below. But such an event
is not likely to happen in any but narrow channels.

In recapitulation, the existence of passages through reefs,
and the character of the coral harbors, may be attributed to
the following causes :

1. The configuration and character of the submarine land ;
—corals not growing where the depth exceeds certain limits,
or where there is no firm rocky basement for the plantation.

2. The direction and force of marine currents, with their
transported detritus ;—these currents having their course
largely modified, if not determined, as in other regions, by the
features of the land, the form of the sea-bottom, and the posi-
tions of the reefs, and being sometimes increased in force by
the contributions of island streams, which add to the detritus
and to the weight of accumulating waters.

3. Harbors which receive fresh-water streams, or submarine
springs of fresh-water, are more apt to be clear from sunken
patches; and the same causes keep open shallow passages to
the shores, where there are shore reefs.

It should be remembered, that while the effects from fresh-
water streams are so trifling around islands, they may be of
very wide influence on the shores of the continents where the
streams are large and deep, and transport much detritus.
This point is illustrated beyond.

UU. ATOLL REEFS,

The remarks on the preceding pages, respecting reefs around
other lands, apply equally to atoll reefs. There are usually
currents flowing to leeward through the lagoon, and out, over
or through the leeward reef, the waves with the rising tide
dashing over the windward side, and keeping up a large sup-

252, CORALS AND CORAL ISLANDS.

ply, which is greatly increased in times of storms; and this ac.
tion tends to keep open a leeward channel for the passage of
the water. This is the common explanation of the origin of
the channels opening into lagoons. These currents are strong-
est when a large part of the windward reef is low, so as to
permit the waves to break over it; andthe coral débris they
bear along will then be greatest. When a large part of the
leeward reef is under water, or barely at the water’s edge, the
waters may escape over the whole, and on this account large
reefs sometimes have no proper channels. When the land
to windward becomes raised throughout above the sea, so as
to form a continuous barrier which the waves cannot pass,
the current is less perfectly sustained, since it is then dependent
entirely upon the influx and efflux of the tides; and the leeward
channels, in such a case, may gradually become closed.

The action of currents on atolls is, therefore, in every way
identical with what has been explained. The absence of
coves of land to give force to the waters of currents, and to
divect their course, and the absence also of fresh-water streams,
are the only modifying causes not present. It is readily un-
derstood, therefore, why lagoon entrances are more likely to
become filled up by growing coral than the passages through
barrier reefs.

Although atolls in seas of moderate tides have the sta-
bility stated on page 257, yet in those having tides of six
feet or over and subject to the sweep of cyclones, they may
find it difficult to stand their ground and repair losses above
mean tide. But the larger reef islands may be increased in
height by the more powerful agencies, as is well exemplified
in the Bermuda Islands and the Bahamas.

FORMATION OF CORAL REEFS AND ISLANDS. Oi

Oo

Il. RATE OF GROWTH OF REEFS.

The formation of a reef has been shown to be a very dif-
ferent process from the growth of a zodphyte. Its rate of
progress is a question to be settled by a consideration of
many distinct causes, none of which have yet been properly
measured.

a. The rapidity of the growth of zodphytes is an element
in this question of great importance, and one that: should be
determined by direct observation with respect to each of the
species which contribute largely to reefs, both in the warmer
and colder parts of coral-reef seas.

b. The character of the coral plantation under consider-
ation should be carefully studied; for it is of the greatest con-
sequence to know whether the clusters of zodphytes are scat-
tered tufts over a barren plain, or whether in crowded profu-
sion. Compare the débris of vegetation on the semi-deserts of
California with that of regions buried in foliage; equally va-
rious may be the rate of growth of coral rock in different
places. An allowance should also be made for the shells and
other reef relics. The amount of reef-rock formed in a given
time cannot exceed, in cubic feet, the aggregate of corals and
shells added by growth—that is, if there are no additions from
other distant or neighboring plantations.

c. It is also necessary to examine all conditions that are
connected with, or can influence, the marine or tidal currents
of the region—their strength, velocity, direction, where they
eddy, and where not, whether they flow over reefs that may
afford débris or not. All the débris of one plantation may
sometimes be swept away by currents to contribute to other
patches, so that one will enlarge at the expense of others. Or,

954 CORALS AND CORAL ISLANDS.

a

currents may carry the detritus into the channels or deeper
waters around a coral patch, and leave little to aid the plan-
tation itself in its increase and consolidation.

d. The course and extent of fresh waters from the land,
and their detritus, should be ascertained.

e. The strength and height of the tides, and general force
of the ocean waves, will have some influence.

Owing to the action of these causes, barrier reefs enlarge
and extend more rapidly than inner reefs. ‘The former have
the full action of the sea to aid them, and are farther removed
from the deleterious influences which may affect the latter.

No results with reference to this question of the rate of
progress in reefs were arrived at by the author in the course
of his observations in the Pacific. The general opinion, —
that their progress is exceedingly slow, was fully sustained.
The facts with regard to the growth of zodphytes, give some
data.

Allowing that the large Madrepora of the wreck, men-
tioned on page 126, may grow three inches in height a year,
and other Madrepores probably three to four inches, it is still
not easy to deduce from it the rate of increase of the reef: In
the first place, the whole Madrepore is growing over the sides
of its branches, at the rate, if we may judge from the size of the
trunk at base, of a tenth of an inch a year, thus increasing
annually the diameter a fifth of an inch a year, which, in a
large species, is a very great addition to the three inches per
year at the extremities of the branches. Again, the branches of
the large Madrepore of the wreck were widely spaced, those of JZ,
cervicornis, having intervals of from six to eighteen inches or
more between the branches.

In fact it is impossible to make any exact estimate of the
amount of increase without a knowledge of the weight of the

RATE OF GROWTH OF CORAL REEFS. 255

part annually added. This ascertained, it would be easy to
calculate how much the added coral would, if ground up, raise
the area that is covered by the Madrepora. A rough esti-
mate gives the author an average increase to this surface of
a fourth of an inch a year. But this fourth must be much
reduced, if we would deduce the rate of growth of the reef;
because a large part of the reef-grounds—that is, of the region
of soundings receiving the coral débris—is bare of growing
corals. This is the case with much the larger portion of all
lagoons and channels among reefs, the bottoms of which, as
already explained, are often sandy or muddy, and to a great
extent so because too deep for living corals; and it is true
even of the coral plantations, these including many and large
barren areas. These unproductive portions of reef-grounds
constitute ordinarily at least two-thirds of the whole; and
making this allowance, the estimate of one-fourth of an inch
a year would become one-twelfth of an inch.

Again, shells add considerably to the amount of calcareous
material, perhaps one-sixth as much as the corals; but against
this we may set off the porosity of the coral.

The rate of growth of the Maandrina clivosa, stated on
page 125, would make the rate of increase in the reef very
much less rapid. ‘The specimen—the growth of fourteen
years—weighs 24 oz. avoirdupois, and has an average diameter
of 7 inches. ‘This gives for the amount of calcareous material
—the specific gravity being 2°523 (p. 99)—16°45 cubic inches ;
which is sufficient to raise a surface seven inches in diameter
to a height of 0-428 inch; and consequently the average yearly
increase would be about 1-33d of an inch. Allowing for two-
thirds of the reef-ground being unproductive in corals, the
rate of increase for the whole would become 1-100th of an
inch. But supposing that shells add one-fourth as much as

256 CORALS AND CORAL ISLANDS.

the corals to the reef material, the rate of increase would be.
come about 1-80th of an inch per year.

The specimen of Oculina diffusa, referred to on page 125,
weighs 44 ounces, which is five-sixths more than that of the
Meeandrina, while the average diameter of the clump is the
same. The average annual increase would consequently cover
a circular area of seven inches diameter 1-18th of an inch
deep. And making the same allowances as above, the rate
for the year for the whole reef-grounds would be 1-44th of an
inch. The specimen of Meandrina mentioned by Major
Hunt, is not here made the basis of a calculation, because we
have not the specimen for examination, and it is not certain
that the diameter stated by him was not the horizontal
diameter. For other facts see the Appendix.

These estimates from the Maandrina clivosa and Oculina
diffusa have this great source of uncertainty, that the growth
of the groups may not have been begun in the first year of the
fourteen. Further, the corals obtained by Major Hunt near
Fort Taylor, Key West, may not have been as favorably situ-
ated for growth as those of the outer margin of the reef.
Again, we have made no allowance for the carbonate of lime
that is supplied by the waters by way of cement, supposing
that this must come originally, for the most part, from the
reef itself. Besides, we have supposed, above, all the coral
reef-rock to be solid, free from open spaces; and, further,
it is not considered that much of it is a coral conglomerate,
in which the fragments have their original porosity.

On the other side, we have not allowed for loss of dé-
bris from the reef-grounds by transportation into the deep seas
adjoining, believing the amount to be very small.

Whatever the uncertainties, it is evident that a reef in-
creases its height or extent with extreme slowness. If the

RATE OF GROWTH OF CORAL REEFS. DRG

rate of upward progress is one-sixteenth of an inch a year, it
would take for an addition of a single foot to its height, one
hundred and ninety years, and for five feet a thousand years.

It is here to be considered, that the thickness of a growing
reef could not exceed twenty fathoms (except by the few feet
added through beach and wind-drift accumulations), even if
existing for hundreds of thousands of years, unless there were
at the same time a slowly progressing subsidence; so that if
we know the possible rate of increase in a reef, we cannot
infer from it the actual rate for any particular reef; for it may
have been very much slower than that. Without a subsidence
in progress, the reef would increase only its breadth.

In order to obtain direct observations on the rate of in-
crease of reefs, a slab of rock was planted, by the order of Cap-
tain Wilkes, on Point Venus, Tahiti, and by soundings, the
depth of Dolphin shoal, below the level of this slab, was care-
fully ascertained. By adopting this precaution, any error
from change of level in the island was guarded against. The
slab remains as a stationary mark for future voyagers to test
the rate of increase of the shoal. Betore, however, the results
can be of any general value toward determining the average
rate of growing reefs, it is still necessary that the growing
condition of the reef should be ascertained, the species of
corals upon it be identified, and the influence of the currents
investigated which sweep in that direction out of Matava
Bay. See the map, page 247, and Appendix, page 417,

The depth to which the shells of Tridacnas lie imbedded in
coral rock, has been supposed to afford some data for estima.
ting the growth of reefs. But Mr. Darwin rightly argues that
these mollusks have the power of sinking themselves in the
rock, as they grow, by removing the lime about them. ‘They

occur in the dead rock,—generally where there are no growing
i aa

258 CORALS AND CORAL ISLANDS.

corals, except rarely some small tufts. If they indicate any
thing, it must be the growth of the reef-rock, and not of the
corals themselves. But the shore-platform where they are
found is not increasing in height; its elevation above low-tide
being determined, as has been shown, by wave action (page
232). They resemble, in fact, other saxicavous mollusks, sev-
eral species of which are found in the same seas, some
buried in the solid masses of dead coral lying on the reef.
The bed they excavate for themselves is usually so complete
that only an inch or two in breadth of their ponderous shells
are exposed to’view. Without some means like this of secur-
ing their habitations, these mollusks would be destroyed by
the waves; a tuft of byssus, however strong, which answers
for some small bivalves, would be an imperfect security against
the force of the sea for shells weighing one to five hundred

pounds. vn

IV. ORIGIN OF THE BARRIER CONDITION OF REEFS, AND
OF THE ATOLL FORMS OF CORAL ISLANDS.

I. OLD VIEWS.

In the review of causes modifying the forms of reefs, no
reason is assigned for the most peculiar, we may say the most
surprising, of all their features,—that they so frequently take
a belt-like form, and enclose a wide lagoon; or, in other cases,
range along, at a distance of some miles, it may be, from
the land they protect, with a deep sea separating them from
the shores.

This peculiar character of the coral island was naturally
the wonder of early voyagers, and the source of many specu-
lations. The instinct of the polyp was made by some the sub-
ject of special admiration; for the “helpless animacules”

>

ORIGIN OF THE BARRIER REEF 259

were supposed to have selected the very form best calculated
to withstand the violence of the waves, and apparently with
direct reference to the mighty forces which were to attack the
rising battlements. ‘They had thrown up a breastwork as a
shelter to an extensive working ground under its lee, ‘‘ where,”
as Flinders observes, ‘‘their infant colonies might be safely
sent forth.”

It has been a more popular theory that the coral struc-
tures were built upon the summits of volcanoes ;—that the
crater of the volcano corresponded to the lagoon, and the rim
to the belt of land; that the entrance to the lagoon was over
a break in the crater, a common result of an eruption. This
view was apparently supported by the volcanic character of
the high islands in the same seas. But since a more satisfac-
tory explanation has been offered by Mr. Darwin, numerous
objections to this hypothesis have become apparent, such as
the following: }

a. The volcanic cones must either have been subaerial and
then have afterward sunk beneath the waters, or else they
were submarine from the first.- In the former case the cra-
ter would have been destroyed, with rare exceptions, during
the subsidence; and in the latter there is reason to believe
that a distinct crater would seldom, if ever, be formed.

b. The hypothesis, moreover, requires that the ocean’s bed
should have been thickly planted with craters—seventy in a
single archipelago,—and that they should have been of nearly
the same elevation; for if more than twenty fathoms below the
surface, corals could not grow upon them. But no records
warrant the supposition that such a volcanic area ever existed.
The volcanoes of the Andes differ from one to ten thousand
feet in altitude, and scarcely two cones throughout the world
are as nearly of the same height as here supposed. Mount

960 CORALS AND CORAL ISLANDS.

Loa and Mount Kea, of Hawaii, present a remarkable instance
of approximation, as they differ but two hundred feet ; but
the two sides of the crater of Mount Loa differ three hundred
and fourteen feet in height. Mount Kea, though of volcanic
character, has no large crater at top. Hualalai, the third
mountain of Hawaii, is 5,440 feet lower than Mount Loa.
The volcanic summit of East Maui is 10,000 feet high, and
contains a large crater; but the wall of the crater on one
side is 700 feet lower than the highest point of the mountain ;
and the bottom of the crater is 2,000 feet below the rim of
the crater. Similar facts are presented by all volcanic regions.

c. It further requires that there should be craters over
fifty miles in diameter, and that twenty and thirty miles
should be a common size. Facts give no support to such an
assumption.

d. It supposes that the high islands of the Pacific, in the
vicinity of the coral islands, abound in craters; while, on the
contrary, there are none, so far as is known, in the Marquesas,
Gambier, or Society Group, the three which lie nearest to
the Paumotus. Even this supposition fails, therefore, of giv-
ing plausibility to the crater hypothesis.

Thus at variance with tacts, the theory has lost favor, and
it is now seldom urged.

The question still recurs with regard to the basement
of coral islands, and the origin of their lagoon character.

ORIGIN OF BARRIER REEFS AND ATOLLS. 261

DARWIN’S THEORY OF THE ORIGIN OF BARRIER REEFS
AND ATOLLS.

Mr. Darwin, in his voyage around the world as natu-
ralist of the expedition of the “Beagle,” under Captain
Fitzroy, R. N., during the years 1832 to 1836, visited and
investigated the Keeling atoll in the Indian Ocean, and the
barrier and fringing reefs of Tahiti. With the facts thus
gathered, he had a key to all descriptions and maps of the
reefs and reef islands of the oceans, and through careful
study of the resources at hand, he arrived at a comprehen-
sive knowledge of the facts and a theory of their origin.'
The voyage of the author in 1858 to 1842 brought the sub-
ject to his attention, and afforded him abundant illustrations
of all sides of the subject and elucidations of some points
which had been deemed obscure; and he believes that the
collected facts place the theory on a firm basis of evidence.

Darwin's theory is this: that a fringing reef skirting
an ordinary island becomes changed by means of a slow
subsidence and the compensating upward growth of the
corals into a barrier reef; and that the barrier reef, by the
continuation of the sinking until the old island has dis-
appeared, and by the same process of growth, becomes finally
an atoll.

1 The third edition of Darwin’s work, issued in 1889, contains a valuable appen-
dix by Prof. T. G. Bonney, giving a full review of the new contributions to the sub-
ject of coral reefs, and his own views confirmatory of those of Darwin.

® The author, besides working among the reefs of Tahiti, the Samoan (or Navi-
gator) Islands, and the Feejees (at this last group staying three months), was also
twice at the Hawaiian Islands. In addition, he landed on and gathered facts from
fifteen coral islands, — seven of these in the Paumotu Archipelago; one, Tongatabu,
in the Friendly Group; two, Taputeuea and Apia, in the Gilbert Group; and five
others near the equator, east of the Gilbert Group, Swain’s, Fakaafo, Oatafu (Duke
of York’s), Hull, and Enderbury’s Island.

262 CORALS AND CORAL ISLANDS.

In sustaining the theory, the fact of the subsidence re-
quires proof, and secondly, its sufficiency for the result
claimed.

Darwin gives as evidence of the subsidence the near
identity of barrier-girt islands and atolls. He compares
the two, points out the fact that a slight change in the
former by submergence is all that is required to convert
it into an atoll, and enforces the argument by pointing to
transitions between the two states.

The facts from the Feejee Archipelago illustrate the sub-
ject well. On the map, Plate XII., let the reader glance
successively at the islands Goro, Angau, Nairai, Lakemba,
Argo Reef, Exploring Isles, and Nanuku. It will be ob-
served that in Goro the reef closely encircles the land upon
whose submarine shores it was built up. In the island next
mentioned, the reef has the same character, but 1s more dis-
tant from the shores, forming what has been termed a bar-
rier reef; the name implying a difference in position, but
none in mode of formation. In the last of the islands
enumerated, the barrier reef includes a large sea, and the
island it encloses is but a rocky peak within this sea.

If, now, the island Angau were sinking slowly, at a rate
not more rapid than that of the upward growth of the reef,
there would be a gradual disappearance of the land beneath
the waters, while the reef might keep its level unchanged.
Should the sinking go on until the land had mostly gone,
the condition would be like that of the Exploring Isles, in
which only a single ridge and a few isolated summits stand
above the waters; and, at a stage beyond when only a single
peak was left, the reef-girt island would have become a
Nanuku. The subsidence of Goro, on the same principle,

would produce an Angau.

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ORIGIN OF BARRIER REEFS AND ATOLLS. 263

The steps in the process are illustrated in the following
sections of an island and its reefs. If the water-level be at
I, the island enclosed would be, like Goro, one with a simple,
fringing reef f, f Suppose a submergence to have gone on
until II is the water line: —the reef growing upward may
then have the surface represented by 0’ f’, b’ f’. There is
here a fringing reef (/’), and also a barrier reef (6’), with a
narrow channel (c’) between, such as exists on the shores of
Tahiti (p. 149). Suppose a further submergence, till III is
the water line: then the channel (c” ¢”), within the barrier

has become quite broad, as in the island of Nairai or Angau;

SECTION ILLUSTRATING THE ORIGIN OF BARRIER REEFS.

on one side (f’”’) the fringing reef remains, but on the other
it has disappeared, owing, perhaps, to some change of cir-
cumstance as regards currents which retarded its growth
and prevented its keeping pace with the subsidence. With
the water at IV, there are two islets of rock in a wide lagoon,
along with other islets (¢” 2”) of reef over two peaks which
have disappeared. 6” b’” are sections of the distant enclos-
ing barrier, and c” c”, and other intermediate spots, of the
water within. The coral reef-rock by gradual growth has
attained a great thickness, and envelops nearly the whole of
the former land. Nanuku, the Argo Reef, and Exploring
Isles are here exemplified ; for the view is a good transverse
section of either of them.

The supposed similarity between these ideal sections and

264 CORALS AND CORAL ISLANDS.

existing islands is fully sustained by actual comparison. The
figure beyond is a map of the island of Aiva, in the Feejee
Group. There are two peaks in the lagoon, precisely as
above; and although we have no soundings of the waters in
and about it, nor sketches of peaks, facts observed elsewhere
authorize in every essential point the transverse section here
viven, which resembles closely, as is apparent, the preceding.
The section is made through the line b db, b’ b’, of the map.

It is unnecessary to add other illustrations. They may be made

MAP AND IDEAL SECTION OF AIVA ISLAND.

out from any of the eastern groups of the Feejees, the Gamb-
ier Group of the Paumotus, or Hogoleu in the Carolines.

It has been urged against the theory, that the process ap-
pealed to ought to fill the channels inside as the island sinks,
and thus a plane of coral result, instead of an outside barrier
reef and narrow belts within.

But the facts prove that the existence of ner passages
is a necessary feature of such islands. It has been shown
that the ocean acts an important part in reef-making, that
the outer reefs, exposed to its action and to its pure waters,
grow more rapidly than those within, which are under the in-
fluence of marine and fresh-water currents and transported
detritus. It is obvious, therefore, that the former may retain

themselves at the surface, when through a too rapid subsi-

ORIGIN OF BARRIER REEFS AND ATOLLS. 26d

dence the inner patches would disappear. Moreover, after
the barrier is once begun, it has growing corals on both its
inner and outer margins, while a fringing reef grows only on
one margin. Again, the detritus of the outer reefs is, to a
great extent, thrown back upon itself by the sea without and
the currents within, while the inner reefs contribute a large
proportion of their material to the wide channels between
them. These channels, it is true, are filled in part from the
outer reefs, but proportionally less from them than from the
inner. The extent of reef-grounds within a barrier, raised
by accumulations at the same time with the reefs, is often
fifty times greater than the area of the barrier itself. Owing
to these causes, the rate of growth of the barrier may be at
least twice more rapid than that of the inner reefs. If the
barrier increases one foot in height in a century, the inner
reef, according to this supposition, would increase but half a
foot ; and any rate of subsidence between the two mentioned,
would sink the inner reefs more rapidly than they could grow,
and cause them to disappear. There is therefore no objec-
tion to the theory from the existence of wide channels and
open seas; on the contrary, they afford an argument in its
favor.

From these remarks on the channels and seas within
barrier reefs, we pass to an illustration of the origin of an
atoll. The inference has probably been already made by the
reader, that the same subsidence which has produced the dis-
tant barrier, if continued a step farther, would produce the
lagoon island. Nanuku is actually a lagoon island, with a
single mountain peak still visible; and Nuku Levu, north of
it, is a lagoon island, with the last peak submerged. The
Gambier group, near the Paumotus, appears to have afforded

an early hint with regard to the origin of the atoll, or at

266 CORALS AND CORAL ISLANDS.

least the close relations of the two. Captain Beechey, in his
“ Voyage in the Pacific,” implies this resemblance, when he
says of the Gambier group, which he surveyed, “It consists

of five large islands and several small ones, all situated in a

GAMBIER ISLANDS.

lagoon, formed by a reef of coral.” Balbi, the geographer, as
Mr. Darwin remarks, describes those barrier reefs which
encircle islands of moderate size, by calling them atolls with
high lands rismg from their central expanse.

The manner in which a further subsidence results in
producing the atoll is illustrated in the following figures.
Viewing V as the water line, the land is entirely sub-
merged ; the barrier (b’” b””) then encloses a broad area of
waters, or a lagoon, with a few island patches of reef over
the peaks of the mountains. A continuation of the subsi-
dence would probably sink beneath the waters some of the
islets, because of their increasing in height less rapidly than
the barrier; and this condition is represented along the upper
line of the above Figure VI, subsidence having taken place
to that level. The lagoon has all the characters of those of
atoll reefs.

Should subsidence now diminish greatly or cease, the

ORIGIN OF BARRIER REEFS AND ATOLLS. 267

reefs, no longer increasing in height, would go on to widen,
and the accumulations produced by the sea would commence
the formation of dry land, as exhibited in figure 2. Verdure
may soon after appear, and the coral island will finally be
completed.

All the features of atolls harmonize completely with this
view of their origin. In form they are as various and irreg-
ular as the outlines of barrier reefs. Compare Angau of the
Feejees, with Tari-tari of the Gilbert Group (p. 165); Nairai

or Moala with Tarawa; Nanuku with Maiana or Apamama.

SECTION ILLUSTRATING THE ORIGIN OF ATOLLS.

The resemblance is close. In the same manner we find the
many forms of lagoon reefs represented among barrier reefs.
We observe, also, that the configurations are such as would
be derived from land of various shapes of outline, whether a
narrow mountain ridge (as in Taputeuea, one of the Gilbert
Islands), or wide areas of irregular slopes and mountain
ranges. Among the groups of high islands, we observe that
abrupt shores may occasion the absence of a reef on one side,
as on Moala; and a like interruption is found among coral
islands. Many of the passages through the reefs may be thus

accounted for.

268 CORALS AND CORAL ISLANDS.

The fact that the submerged reef is often much prolonged
from the capes or points of a coral island, accords well with
these views. These points or capes correspond to points in
the original land, and often to the lme of the prominent
ridge; and it is well known that such ridge limes often ex-
tend a long distance to sea, with slight inclination compared
with the slopes or declivities bounding the ridge on either
side.

The derivation of the forms of reef islands from a former
mountain range is further sustained, according to Darwin, by
the occurrence of coral islands or reefs in chains, like the peaks

20 of an ind to a mile.

MENCHICOFF ATOLL.

of such a range. He gives as an example Menchicoff atoll, of
the Caroline Archipelago, which consists of three long loops
or lagoon islands, united by their extremities, and which fur-
ther subsidence might reduce to three islands.

Darwin, in his account of the Maldives, points out indica-
tions of a breaking up of a large atoll into several smaller
atolls. The land with many summits or ranges of heights
may at first have had its single enclosing reef; but as it sub-
sided, this reef, contracting upon itself, may have encircled
separately the several ranges of which the island consisted.

ORIGIN OF BARRIER REEFS AND ATOLLS. 269

and thus several atoll reefs may have resulted in place of the
large one; and, further, each peak may have finally become
the basis of a separate lagoon island, under a certain rate
of subsidence or variations in it, provided the outer reef were
so broken as to admit the influence of waves and winds.
Some of the large atolls of the Maldives are properly atoll
archipelagoes.

The sizes of atolls offer no objection to these views, as
they do not exceed those of many barrier reefs.

All the conditions from fringing to barrier and from the
barrier island to the atoll are admirably illustrated in the
Louisiade Archipelago, Plate VII. The small amount of
included high land within the enormous barrier, the linear
form of the high islands, and the many islets which continue
the line westward, the appendage-like relation to the large
barrier-island of the islands at its northeast and northwest
ends, look as if all the pieces of high land were parts of a
nine-tenths-buried mountain-chain; and so much like it that
any other supposition is evidently unreasonable.

According to the principles explained and the facts illus-
trating them, an atoll that 1s wooded through the larger part
of its circuit, especially if not below medium size, bears evi-
dence in this fact that the subsidence through which it was
formed has probably ceased ; and on the contrary that atolls
which are wholly or mostly covered with the sea at high tide,
with few islets above high-water mark, are still undergoing
subsidence. On this principle we may infer that the larger
part of the Paumotus have passed to a period of cessation of
subsidence, and that Keeling atoll in the Indian Ocean is of
like character. Many of the northern Carolines, on the con-
trary, may be still subsiding.

It is of interest to follow still further the subsidence of a

270 CORALS AND CORAL ISLANDS.

coral island, the earlier steps in which are illustrated in the
preceding figures.

It is to be noted in this connection that if an atoll-reef is
not undergoing subsidence the coral and shell material pro-
duced that is not lost by currents serves: (1) to widen the
reef; (2) to steepen, as a consequence of the widening, the
upper part of the submarine slopes; (3) to accumulate, on
the reef, material for beaches and dry land; and (4) to fill
the lagoon. In regions of barrier reefs the inner channel
may be a large receiver, like the lagoon of the atoll. But if,
while subsidence is in progress, the contributions from corals
and shells exceed not greatly or feebly the loss by subsidence
and current waste, the atoll-reef, unable to supply sufficient
debris to raise the reef above tide-level by making beaches
and dry-land accumulations, would (1) remain mostly a bare
tide-washed reef; (2) lose in diameter or size hecause the
debris that is not used to keep the reef at tide-level is carried
over the narrow reef to the lagoon by the waves whose throw
on all sides is shoreward; (5) lose in irregularity of outline
and thus approximate toward an annular form; (4) lose the
channels through the reef into the lagoon by the growth of
corals and by consolidating debris; and (5) become at last a
small bank of reef-rock with a half obliterated lagoon-basin.
Some of the islands of the equatorial Pacific in this last con-
dition are described on page 198. |

Moreover, the subsidence, if more rapid than the increase
of the coral reef, would become fatal to the atoll, by gradu-
ally sinking it beneath the sea. Such a fate, as stated by
Darwin, has actually befallen several atoll-formed reefs of
the Chagos Group, in the Indian Ocean (p. 192); one of
them has only “two or three very small pieces of living reef
rising to the surface.’ Darwin calls such reefs dead reefs.

ORIGIN OF BARRIER REEFS AND ATOLLS. yy a |

=i

The southern Maldives have deeper lagoons than the northern,
fifty or sixty fathoms being found in them. This fact indi-
cates that subsidence was probably most extensive to the
south, and perhaps also most rapid. The sinking of the
Chagos Bank, which lies farther to the south in nearly
the same line, may therefore have had some connection with
the subsidence of the Maldives. Other drowned atoll reefs,
of similar character, exist in the China Sea and to the north
of Madagascar.

The submerged Macclesfield Bank, and the Tizard Bank
five degrees farther south, have been described by W. J. L.
Wharton! and Captain Aldrich? The Tizard Bank is 10 miles
broad, has depths of 30 to 47 fathoms in the lagoon part, and
4 to 10 fathoms over the border on which alone are growing
corals; but the border extends to the surface in eight places
and at three of them are islets. The Macclesfield Bank is
70 by 40 miles in area; it is like the Tizard, but lies deeper,
the lagoon in places being 40 to 60 fathoms under water and
the margin 4 to 10 fathoms. The Saya de Malha Bank, east
of northern Madagascar, measures two degrees across, has
depths of 60 to 70 fathoms within the lagoon part, and a
border on the north and northeast sides at a depth of 10 to
17 fathoms, a portion of which comes within 8 to 9 fathoms
of the surface. Darwin remarks on its close resemblance to
reefs of the Chagos Bank. To the south is the submerged
Nazareth Bank, and Cargados Carajos at the south end of
a reef region common to the two; and to the northeast, the
Seychelles, of great area. The Seychelles have some granite
islands near the centre, but constitute otherwise a great sub-
merged bank; on the western border are shoals 3 to 7 fath-

1 Nature, 1888, Feb. 23.
2 Bulletin of Hydrographic Department, London, for February, 1889.

Pie CORALS AND CORAL ISLANDS.

oms under water more or less covered with growing corals.
The seas about these sunken reefs of the Indian Ocean are
2,000 to 2,600 fathoms in depth. Nearer to northern Mada-
gascar there are coral islands that are not submerged.

The following are other evidences in favor of the theory
of subsidence.

The theory explains all the varying depths of lagoons,
from the condition of near obliteration to that of a basin
one to three hundred feet deep.

It gives a satisfactory reason for the existence of great
and abrupt depths about many atolls, and off great barriers,
and the steepest of submarine declivities. The powers of
growth in the reef, through the limestone material derived
from the waters by the polyps, enable it to keep itself at the
water-level in spite of the deepening that is going on. Such
facts as those from the depths alongside of the eastern Baha-
mas, the reef-islands southwest of Cuba, and many atolls in
the deeper oceans have here their full elucidation.

The sunken atolls of the ocean, like the Chagos Bank
(page 191), derive from Darwin’s theory their only explana-
tion. The basin-shaped reef with high borders, the bottom
dead because of the too great depth, the borders in places
growing corals and having some surface islets over spots
of more luxuriant growth where rate of progress was suffi-
cient to keep up with the subsidence, —all the facts are a
natural consequence of the method of origin. A model of
such a lagoon-bank with its raised margin and few tall and
steep islet towers, 20 to 30 feet above the rest of the border,
would be one of the best of demonstrations for the subsidence
theory. The coral-growing areas over the great lagoons of
atolls and the barrier-bounded channels of the Feejees and
other archipelagoes and those of the outer waters about

ORIGIN OF BARRIER REEFS AND ATOLLS. 273

islands or their barriers, show no tendency to grow with
large depressed centres, but rather with flat. tops, as vegeta-
tion might grow, or else with elevated centres. It is only
when nearing the surface where the waves can help vigor-
ously the growth and the accumulation of material on the
border and injure the interior corals, that anything like
a lagoon-basin begins and the atoll takes shape; and it is
only through continued subsidence under such conditions
that the margin can be made to grow so much faster than |
the interior as to produce thereby a basin-like interior 50 to
300 feet deep. Corals will grow most rapidly where food is
most freely supplied by currents, as observed by Mr. Agassiz.
The principle serves to explain the unequal progress in some

reef-banks; but food is seldom deficient. Darwin draws a
good argument for his theory, also, from the fact that lines

of coral islands are the continuations of lines of high islands,
and, also, that lines of the two kinds of islands often run
parallel with one another; for example, the Hawaiian
line of high islands four hundred miles long, ending off to
the westward in a longer line of atolls, and the many
parallel lines in the Pacific. |
There is, further, not merely probable but positive evi-
dence of subsidence in the deep coast-indentations of the
high islands within the great barriers. The long points and
deep fiord-like bays are such as exist only where a land, after
having been deeply gouged by erosion, has become half sub-
merged. The author was led to appreciate this evidence
when on the ascent of Mt. Aorai on Tahiti, in September of
1839." Sunk to any level above that of five hundred feet,

1 A map of Tahiti and an account of the ascent are contained in the author’s
“Volcanoes and Hawaiian Volcanic Phenomena.” ‘The map is published, also, in
the American Journal of Science, 1886, xxxii, 247.

18

274 CORALS AND. CORAL ISLANDS.

the erosion-made valleys of Tahiti would become deep bays,
and above that of one thousand feet, fiord-like bays, with the
ridges spreading in the water like spider’s legs; and this is a
common feature of the islands and islets‘within the lagoons
of barrier islands. The evidence of subsidence admits of no
doubt. It makes the conclusion from the Gambier group
positive ; and equally so that for Raiatea and Bolabola repre-
and that
for the Exploring Isles and others of the Feejee group; and

>

sented on the charts in Darwin’s “Coral Islands ;’
?

that for islands, great and small, in the Louisiade Archipelago
and in other similar groups over the oceans.

Other arguments for the thickening of reefs through sub-
sidence are afforded by the existence of elevated reefs, and of
sunken and buried fringing reefs.

The island of Metia is 250 feet in height (p. 195), full
twice the coral-growing depth, and consists of horizontally
stratified limestone. At the island of Mangaia, in the Her-
vey group, the coral rock is raised 300 feet out of water.
Such thick beds could not have been made by corals growing
in depths not exceeding 150 feet without a sinking of scores
of feet during their progress.

Christmas Island, in the eastern part of the Indian Ocean,
according to Mr. J. J. Lister and Captain Aldrich, R. N., al-
though 1,200 feet high, has a series of horizontal terraces of
coral-made rock to the top. There is at bottom a vertical
cliff of 30 feet; then above the 120-foot level, a cliff of 85
feet begins; above that of 475 feet, two cliffs together of 95
feet; next a steep, rough slope for 650 feet of the height,
ending in a top layer. Captain Aldrich, who ascended to the
summit, says that he saw no rock but coral rock, and implies
that the rock is in successive layers. The facts, taken as
stated, prove a thickness of reef-rock of 1,200 feet. One

ORIGIN OF BARRIER REEFS AND ATOLLS. ahh

writer has since said, that perhaps the coral rock encases a
voleanic mountain ; another has gone further and dropped the
perhaps. But these statements are at present unwarranted.
On Cuba, according to Prof. W. O. Crosby,’ coral rock
occurs in successive terraces up to a height of nearly 2,000
feet, and, excepting small breaks, makes the circuit of the
island. The terraces are described as a striking feature in
the view from the water. One terrace-plain, at 30 feet ele-
vation, is in places nearly a mile wide and extends almost
horizontally for hundreds of miles. <A second, of 200 to 250
feet, rises steeply from the imner edge of the first, and a ,

third reaches a level of 500 feet. Near Havana the eleva“

tion is over 1,200 feet, and the rock, as reported by Mr. A.
Agassiz, is true reef-rock. The mountain El Yunque, five
miles west of Baracoa, 1,800 feet in height, is_volcanic rock
below, but coral for the upper 1,000 feet. Further coral lime-
stone has been found by Mr. Sawkins, on J amaica, at a height
of 2,000 feet. “My. Crosby concludes that the great thickness
of the now elevated reefs could have been produced only
“ during a progressing subsidence,’ so that “we have appar-
ently no recourse but to accept Darwin’s theory.”

We are thus led to the conclusion that each coral atoll ~~ ‘\

once formed a fringing reef around a high island. The
fringing reef, as the island subsided, became a barrier reef,
which continued its growth while the land was slowly dis-
appearing. The area of waters within finally contained the
last sinking peak. Another period, and this had gone, leay-
ing only the barrier at the surface and an islet or two of
coral in the enclosed lagoon. Thus the coral wreath en-
circling the lofty island becomes afterward its monument,
and a record of its past existence. The Paumotu Archi-

1 Proceedings of the Boston Society of Natural History, 1882-1883, xxii. 124.

276 CORALS AND CORAL ISLANDS.

pelago is accordingly a vast island cemetery, where each
atoll marks the site of a buried island; and the whole Pacific
is scattered over with these simple memorials.

In view of the facts that have been presented, it is fur-
ther evident that a barrier reef indicates approximately the
former limits of the land enclosed. The Exploring Isles
(Feejee chart), instead of having an area of only siz square
miles. the whole extent of the existing land, once covered
three hundred square miles; and the outline of the former

land is indicated by the course of the enclosing reef. A still

greater extent may be justly inferred. For since a barrier,

as subsidence goes on, gradually contracts its area, owing to
the fact that the sea bears a great part of the material
inward over the reefs, the declivity forming the outer limit
of the sub-marine coral formation has a steep angle of in-
clination. In the same manner it follows that the island
Nanuku, instead of one square mile, extended once over two
hundred square miles, or had two hundred times the present
area of high land. Bacon’s Isles once formed a large trian-
gular island of equal extent, though now but two points of
rock remain above the water.

The two large islands in the western part of the group
Vanua Levu and Viti Levu, have distant barriers on the
western side. Off the north point of the former island, the
reef begins to diverge from the coast. and stretches off from
the shores till it is twenty and twenty-five miles distant ;
then, after a narrow interruption, without soundings, the
Asaua islands commence in the same line, and sweep around
to the reef which unites with the south side of Viti Levu;
and, tracing the reef along the south and east shores, we find
it at last nearly connecting with a reef extending southward
from Vanua Levu. Thus these two large islands are nearly

OBJECTIONS TO THE THEORY OF SUBSIDENCE. Oy,

encircled in a single belt; and it would be dog no violence
to principles or probabilities to suppose them once to have
formed a single island, which subsidence has separated by
inundating the low intermediate area. We may thus not
only trace out the general form of the land which once occu-
pied this large area (at least 10,000 square miles), but may
detect some of its prominent capes, as in Wakaia and Direc-
tion Island. The present area is not far from 4,500 square
miles. The Feejee Group, exclusive of coral islets, includes
an area of about 5,500 square miles of dry land; while, at
the period when the corals commenced to grow, there were
at least, as the facts show, 15,000 square miles of land, or
nearly three times the present extent of habitable surface.

» |
HAA A
oe

oT ee

“OBJECTIONS TO THE SUBSIDENCE THEORY.

The objections to the theory of coral reefs which have
been recently urged are mostly independent hypotheses which
are supposed to meet the facts without requiring subsidence,
and not strictly objections to Darwin’s theory. Two or three
real objections, however, are among them. The discussion
brings out many points of interest.

An improbability. — So extremely slow a subsidence, keep-
ing pace so well with the upward growth, is improbable.
This objection is put forth by those who are not aware that
so slow subsidences are those with which geology is most
familiar. A movement of the kind has been proved to be
in progress along the coast of New Jersey and some other
parts of the North American Atlantic border, and in western
Greenland ; and geology is now inquiring as to whether any
regions are absolutely stable, or wholly free from movement
up or down.

278 CORALS AND CORAL ISLANDS.

Another improbability. — So great a subsidence is declared
improbable. The thickness of coral limestone, attributed un-
der the theory to some coral-reef formations, is pronounced
by Dr. J. J. Rein* to be far beyond that of any reefs of earlier
time, and therefore improbable. But precedent, whether a
fact or not, settles nothing. Coral-made limestones are not
essentially different in origin from shell-made limestones ;
and the past year we have reported from Mexico, by Dr. C. A.
White, a Cretaceous limestone, having similar fossils from
bottom to top, and yet 4,000 feet thick. It could not have
been made, as Dr. White states, without 4,000 feet of slowly
progressing subsidence, for the fossils prove that it is not of
deep-water origin. Nevada has Devonian limestones, accord-
ing to Hague and Walcott, 6,000 feet thick, and some simi-
lar fossils in the upper and lower portions which are proof
of a gradual subsidence. No thick formation of any kind of
rock was ever made, or could be made, by shore or shallow-
sea operations without a slowly continued subsidence or a
corresponding change of water level.

But there is another improbability and it is the most im-
probable of all suppositions that can be made with regard
to the globe; that, while the continents have had their great
changes of level, both upward and downward by the thou-
sands of feet, the oceanic basins, of nearly three times the
area, and volcanic in the constitution of many high islands,
have had no changes of level except some local elevations,
for this is implied by the objectors; not a subsidence any-
where of a thousand feet or even a hundred, in the breadth
of thirty millions of feet between America and the East India
Seas. Whence this stability ?

1 Dr. Rein’s Memoir on Bermuda is mentioned in the note on page 218. The
above argument is given here from the citation by Dr. Geikie, the publication not
being accessible to the writer. .

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 279

The Elevation Theory. — The view has been presented
that, in place of subsidence, elevation is at the bottom in
the origin of barriers and atolls. Coral reefs may, like sea-
beaches, be made at different heights on the slopes of rising
land; but this is not the result of elevation which is implied ;
for barrier reefs and atolls are the objects whose origin is to
be accounted for.

a. Mr. G. C. Bourne’ found at Diego Garcia, an atoll of
the Indian Ocean southeast of the Chagos Bank, that in
wells sunk over the island, the rock, for a short way down,
consists of horizontal layers, eighteen inches to three feet
thick, of coral sand in two or three alternations with coral
shingle containing corals in coarse masses; and he concludes
that the coarser layer corresponds to a layer of growing corals
and the sand layer to a deposit of sand that was spread over
and killed the corals; and that this process was repeated two
or three times or more. Then he says: “ aise the forma-
tion to the surface and you get that stratification which you
see on so many parts of the island, a stratification which can-
not be explained on any theory of subsidence.” He does not
seem to be aware that the elevation did not make the strati-
fication; and that the stratification is on his view positive
evidence of a period of submergence; and that the thicken-
ing of such a series by additions above, as in the process
described, would require a continued subsidence previous to
the final elevation. In any case, the final emergence is all
there is in such facts to support an elevation theory of coral
islands.

The case of Christmas Island, 1,200 feet high (p. 274),
is one of upheaval; but the upheaval, if the horizontal ter-
races are parts of successive layers, did not make the layers.

1 Bourne, Nature, April 5, 1888.

280 CORALS AND CORAL ISLANDS.

If they are only fringing reefs marking different stages in
the elevation, they are like elevated sea-beaches, and if each
is no thicker than the depth of growing corals, they have no
bearing on any theory of coral islands. But the descriptions
say that only coral rock exists on the island.

b. The fact that elevated reefs and other evidences of
elevation occur at the Pelews, a region of wide barrier reefs
and atolls, has been presented by Prof. Karl Semper,’ after a
study of those islands, as bearing in the same direction; that
is, against the theory of subsidence, for by the subsidence
theory we have in such facts (in the words of another), “a
cumbrous and entirely hypothetical series of upward and
downward movements.” Professor Semper reports the exist-
ence of reefs raised 200 to 250 feet above the sea-level in the
southern third of the larger of the islands, while the other
two thirds exhibit evidence of but little, 1f any, elevation.
The facts are like those of other elevated reefs and atolls
discussed on former pages. The Pelew region is one of
comparatively modern volcanic rocks and this renders local
displacements a probability. The elevations are proof of
local change of level; and likewise of a recent change in each
case; for the top layer of the elevated reef-rock and all below
it through the coral-made structure were completed previous
to the uplift. They prove nothing as to the changes that
were in progress when all this coral-rock was in process of
formation.

c. The occurrence of great numbers of large and small
masses of coral rock, in some places crowded together, upon
the western or leeward reef of the several Pelew islands, and

1 First in 1868, Zeitschr. Wissensch. Zool. xiii. 558; additions in Die Philippi-
nen und ihre Bewohner, Wiirzburg, 1869, and still later in his “ Animal Life ” pub-
lished in Appleton’s International Scientific Series in 1881.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 281

of none on the eastern reef, is mentioned as evidence against
subsidence and in favor of some elevation: because, Professor
Semper says, the strongest wind-waves on the western side
are too feeble to break off and lift on the reef so large masses,
some of them (as his words imply rather than distinctly state)
ten feet thick.

But the difficulty does not exist in fact; for earthquakes
may have made the waves. The region just west of the
Pelews is one of the grandest areas of active volcanoes on
the globe. It embraces the Philippine Islands, Krakatoa and
other volcanic islands of the Sooloo Sea, Celebes, etc. The
agents that could do the work were there in force. To the
eastward, in contrast, lie the harmless islands of the Caroline
Archipelago, mostly atolls, serving, perhaps, as a breakwater
to the Pelews.

Professor Semper has other steps in his theory which are
considered beyond.

The idea of elevation as a requisite is not suggested by
the atolls of the ocean. In the Paumotu Archipelago, cover-
ing four hundred and fifty thousand square miles, the nearly
uniform height of the land, eight to ten feet, in the seventy
to eighty atolls, with the shore platform a hundred yards or
more wide near third tide-level, does not favor the assump-
tion that elevation had put them into their present uniform
positions.

1. The talus-theory. — Mr. John Murray, one of the able
naturalists of the Challenger Expedition, has proposed the
following theory: that shore-reefs extend themselves out
within coral-growing depths on the basement of debris de-
rived from the growing margins.

The fact that reefs widen by the process here mentioned

when subsidence ceases is recognized in the author’s Expedi-

282 CORALS AND CORAL ISLANDS.

tion Report; but that this is the means of attaining great
width and thickness with no aid from subsidence is the new
view of Mr. Murray. The method has been described in the
words: Reefs grow out on their own talus. The view is
supported by Dr. Guppy, Mr. A. Agassiz, and others.

a. Mr. Murray’s observations were made at Tahiti. He
reports the following facts derived from soundings off north-
ern Tahiti, made under his supervision and that of the sur-
_veying officer.

Along a line outward from the edge of the barrier reef
there were found: (1) for about 250 yards, a shallow region
covered partly with growing corals, which deepened seaward
to 40 fathoms; (2) for 100 yards, between the depths of 40
and 100 fathoms, a steeply but irregularly sloping surface
which commenced with a precipice of 75° and had a mean
angle exceeding 45°; then (3) for 150 yards a sloping bot-
tom 50° in angle; (4) then a continuation of this sloping
surface, diminishing in a mile to 6°, at which distance out
the depth found was 590 fathoms (3,540 feet). Over the
area (2), or the 100 yards between 40 and 100 fathoms, the
bottom was proved to be made of large coral masses, some of
them “20 to 30 feet in length,” along with finer debris; out-
side of this, of sand to where the slope was reduced to 6°;
and then of mud, composed ‘of volcanic and coral sand,
pteropods, pelagic and other foraminifers, coccoliths, etc.”

These observations have great significance. They show
(1) that the feeble currents off this part of Tahiti carry little
of the coral debris in that direction beyond a mile outside of
the growing reef; (2) that a region of large masses of coral

1 Dr. Archibald Geikie gives in his Presidential Address before the Royal
Physical Society of Edinburgh in 1883 (Proc. viii. 1, 1883) a section of the soundings
“on a true scale, vertical and horizontal,” and in it the upper steepest part of this
100 yards has a slope of about 75°.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 283

rock and finer material occurs at depths between 240 and
600 feet; (3) that, a mile out, the bottom has the slope
nearly of the adjoining land, and in this part is covered with
the remains of pelagic life. i

- From the second of these facts, —the great accumulation
of coral blocks below a level of 240 feet, — Mr. Murray draws
the conclusion that, in the making of fringing, barrier, and
atoll reefs, the widening goes forward (a) by making first
upon the submarine slopes outside of the growing reef a
pile of coral debris up to the lower limit of living reef-corals ;
and then (6) by building outward upon this accumulation as
a basement.

b. But these observations fail to prove that an accumu-
lation of coral blocks over the slopes below the reef-coral
limit is a result of the process appealed to. In truth they
set aside his conclusion by making the fact of a subsidence
unquestionable.

That belt of coarse debris — including “ masses 20 to 30
feet” long — was found over the steeply sloping bottom at

below the limit of forcible wave-action. They are depths
where the waters, however disturbed above by storms, have
no rending and lifting power, even when the bottom is grad-
ually shelving; depths, in this special case, against a slope
which for 100 yards is 75° in its upper part, and in no part
under 45°, the vertical fall being 360 feet in the 100 yards.
Strokes against the reef-rock thus submerged, and under such
conditions, would be extremely feeble. Waves advancing up
a coast, whether storm-driven waves or earthquake waves,
do little rock-rending below the depth to which they can bare
the bottom for a broadside plunge against the obstacle before
them, although the velocity gives them transporting power

/ .

depths between 240 and 600 feet. These depths are far ~

284 CORALS AND CORAL ISLANDS.

to a greater depth. It is the throw of an immense mass of
water against the front, with the velocity increased by
the tidal flow over a shelving bottom,—the rate some-
times amounting, according to Stevenson, to 36 miles an
hour or 52°8 feet a second, — together with the buoyant
action of the water, that produces the great effects.

A vertical surface below the sea-level of 20 feet made
bare for the broadside stroke is probably very rarely ex-
ceeded even in the case of earthquake-waves; and with
storm-waves, or recorded earthquake-waves, the displace-
ment of the water at a depth of 240 feet would be at
the most only a few inches. I saw on atoll reefs no up-
thrown masses of coral rock over ten feet in thickness and
twenty feet in length or breadth. It is therefore plainly
impossible that such a belt of debris should have been made
at its present level, or even at a depth of twenty feet; and
hence the debris affords positive proof of a large subsidence
during some part of the reef-making era.

ce. But if such accumulations of great blocks cannot result
from the dropping of masses from the edge of the reef, the
facts show that fine coral debris may be spread over the
bottom for a mile outside of the surface reef, and contrib-
ute thereby toward a rising basement. But the expression
that they grow out on their own talus is wholly fallacious ;
for there is nothing visible that is of this nature. Outside of
the emerged reef there is a region of growing corals, as has
been explained, and spots and areas of sand exist among the
luxuriant groups and plantations. Outside of this there is
more or less of deeper pelagic life and thinner deposits of
sand from the reef; but nowhere a talus of debris. The sea
makes debris as the waves and inflowing tides move toward
the shores; but the action is almost. wholly landward, trans-

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 285

porting nearly all the debris over the emerging reef and into
the inner channels or lagoons; little is carried off and dropped
in the deeper waters outside. The condition is like that on
other coasts which the sea is extending. But there is this
difference that the corals, the chief source of the debris, have
a hold on the bottom because attached species, and the cal-
careous sands are easily cemented; so that the reef they
make becomes a solid bank. And further, the waves and
tidal waters have this great area of reef and inner channels
to receive the sands they distribute. Little debris reaches
the outer slopes; and the most of this little finds lodgment
among the corals within coral-growing depths.

The Florida reefs well illustrate the facts. Off the great
emerged banks there is, first, the region of growing corals,
called the “ Florida reefs,” half a mile to a mile wide, and
one to nine fathoms deep, where some spots of reef reach the
surface ; and outside of this, on the south, there is the Pour-
talés plateau at depths down to two hundred and fifty fath-
oms, growing pelagic species. There is no talus. Some
debris drops over the plateau; yet, as Mr. Agassiz’s figure
shows, the amount is very small. The Florida Bank gets
nearly all, and thus it has derived its height and extent.

d. The view which Mr. Agassiz sustains that the Florida
corals grew out on a basement made by pelagic growth or on
the inner part of the Pourtalés plateau, and thus extended
the great bank, has more to sustain it than the talus-theory.
With only the facts that are open to view in the Florida seas
no other explanation would be thought of. But there is one
difference between banks over such a bottom and those in-
creasing through a subsidence. With the former, the slope
of the bottom would usually be gentle. While the explana-
tion may answer as well as the subsidence-theory for the

286 CORALS AND CORAL ISLANDS.

Florida Bank, it does not appear to suffice for the Eastern
Bahamas, with their steep submarine slopes toward the ocean,
or for the Grand Cayman and other reefs southwest of Cuba,
or for the great majority of the atoll and barrier reefs of
the Pacific. It is, however, not yet proved to be true for
Florida.

e. By Darwin’s theory, the growing reef increases its

thickness as the slow subsidence progresses; and the inside ~

channel, so common a feature, is a consequence in the way
that has been explained. But by the Murray method, after
the outer edge of the reef has a thickness equalling the
range in depth of reef-corals for the region, the reef, as it
further enlarges, keeps extending out into deeper and deeper
water. But the width cannot be doubled without using three
times as much material as for the preceding part in case the
slope of the bottom is unchanged and the coast is a straight
line, and much more than three times if the coast line is con-
vex like that of Tahiti, and still more if the slope of the
bottom increases. With the feeble amount of debris to be
had, the rate of growth would therefore be extremely slow, —
not over one hundredth of that for the first one thousand feet.
During all the long era of this extension the waves and tidal
movements and winds would be at their usual work, throw-
ing the chief part of the debris on the reef and beyond into
the channel; and thus the channel, the process being aided
by its own growing corals, would be sure to become filled
and nearly obliterated; for it is ever receiving, and has no
chance to increase its capacity or relieve itself of the debris
received.

Among examples of a reef undergoing such an extension
where there was little or no subsidence, is that of the north
side of Upolu, of the Samoa Group, which the author’s Expe-

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 287

dition Report describes as nearly a mile wide and yet having
no channel deep enough for a canoe. The whole amount of
subsidence estimated in the Report is one to two hundred feet
— and whatever the amount it may have long since ceased.
The same conclusion comes from the Florida Bank. The reef
of Tahiti, on the contrary, is far as possible from such a con-
dition. Its channel is partly a ship channel, as already de-
scribed; and such deep waters are common within Pacific
barrier reefs.

The thick coral-made beds reached by artesian borings on
Oahu (see Appendix), regarded as evidence of the subsidence
of the island attending their formation, are explained by Mr.
A. Agassiz on the Murray hypothesis: “the extension sea-
ward of a growing reef, active only within narrow limits
near the surface, which was constantly pushing its way sea-
ward upon the talus formed below the living edge.” ?

The above considerations may be deemed sufficient to set
aside the suggestion; for it is only a suggestion, since no
facts are mentioned in its support. The borings give it none.
Some of them commence in the elevated reef which is the
inner part of the fringing reef of the island, within three or
four hundred yards of the mountain slopes, and go down
through coral rock interruptedly for two hundred feet or more.
There is no reason for regarding any part of it talus-made.

jf. Although no sufficient reason is found for believing in
talus-made basements, a characteristic by which they might
be known if lifted into view may be mentioned. The debris
is laid down on a sloping surface, whence the beds would
have a corresponding pitch. The deposit would thus differ
from ordinary coral formations and, too, from all great ex-

amples of limestone strata of which we have knowledge. In

1 Bulletin of the Museum of Comparative Zodlogy, 1889, xvii. No. 3.

988 avs |, CORALS AND CORAL ISLANDS.

the elevated coral island, Metia, two hundred and fifty feet
high, the stratification is horizontal; the terraces of Christ-
mas Island are horizontal; and the same is true of the ele-
vated reefs of Cuba.

If a satistactory decision in any case is not arrived at by
the methods or criteria which have been described, there is
the best of evidence to be had by artesian borings that shall
go to the bottom and bring up a core six inches in diameter.
It may thus be decided whether the limestone passed through
is of reef-coral, or talus-debris, or partly or wholly of pelagic
constitution."

2. Courses of reefs and channels determined by marine
currents.— The view has been urged by Lieut. E. B. Hunt,
U.S. N. and Messrs. A. Agassiz, Murray, Semper, and Guppy
that the positions of reefs off coasts are sometimes and gener-
ally located by drift currents at considerable distances from
coasts, and hence that the wide intervals of water inside of
barrier reefs may result without aid from subsidence; and
Dr. Guppy has urged, as remarked on another page, that
atolls may be so shaped.

a. The facts, presented by Lieutenant Hunt, and more
fully by Mr. Agassiz with regard to the effects of the eddy
current of the Gulf Stream have been mentioned on page 207.
They show that coral reefs may be elongated, and also that

1 The kind of submarine slopes to be ordinarily looked for off coral reefs is
ilustrated by the Challenger soundings. And it is interesting to note that the facts
sustain instead of correcting those announced by earlier observers. Beechey and
Darwin make the mean slope about 45°, and my Report says 40° to 50°. I have
assumed for the slope of the bottom outside of the reef-limit the same angle as for
the surface-slope of the island just above the water-level ; 5° to 8° off Tahiti, of
which 5° is accepted as most correct, and 3° to 5° off Upolu; and the assump-
tion as regards Tahiti is sustained by the Challenger soundings. My Report
states (from the Expedition surveys) that off Upolu, the bottom “loses more and
more in the proportion of coral sand till we finally reach a bottom of earth,” and
introduces this as an argument against the indefinite drifting of coral sands into the
deep ocean; and this argument the Tahiti soundings sustain.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 289

inner channels may be made, by the drifting of coral sands.
The action with coral sands is essentially the same as with
other sands; and illustrations of this drifting process occur
along the whole eastern coast of North America from Florida
to Long Island. We there learn that drift-made beaches run
in long lines between broad channels or sounds and the ocean ;
that they have nearly the uniform direction of the drift of
the waters, with some irregularities introduced by the forms
of the coast and the outflow of the imner waters which are
tidal and fluvial and have much strength during ebb tide.
The easy consolidation of coral sands puts in a peculiar feature,
but not one that affects the direction of drift accumulation.

b. The great barrier reef off eastern Australia, a thou-
sand miles long, has some correspondence in position to the
sand-reets off eastern North America. But it is full of irreg-
ularities of direction and of interruptions, and follows in no
part an even line. In the southern half, it extends out one
hundred and fifty miles from the coast and includes a large
atoll-formed reef; in the northern half, the barrier, while
varying much in course, is hardly over thirty miles from the
land. There is very little in its form to suggest similarity
of origin to the drift-made barriers of sand.

c. In the Pacific Ocean, the trends of many of the coral
island groups, and of the single islands, do not correspond
with the direction of the oceanic currents, or with any eddy
currents, except such as are local and are determined by
themselves.

Near longitude 180°, as the map of the Central Pacific
(Plate IX.) illustrates, the equator is crossed by the long
Gilbert (or Kingsmill) Group, at an angle with the meridian
of 25° to 30°, and not in the direction of the Pacific current
which is approximately equatorial. This obliquely crossing

19

290 CORALS AND CORAL ISLANDS.

chain of atolls is continued northward in the Ratack and
Ralick Groups (or the Marshall Islands), making in all a
chain over 1,200 miles long; and, adding the concordant
Kllice Islands on the south, and extending the Ratack line to
Gaspar Rico, its northern outlier, the chain is nearly 2,000
miles long. Nothing in the direction of the long range, ex-
cepting local shapings of some of the points about the atolls,

‘can be attributed to the Pacific currents. Moreover, none of
, the diversified forms of atolls have any sufficient explanation

~ in the drift process.

Dr. Guppy urges the idea that banks of reef after reach-
ing the surface are converted into atolls by means of the
marine currents.’ In his last paper he speaks of the conver-
sion after the island has been thrown up by the waves; in the
earlier he appears to speak of elevation of some kind as
necessary ; but the currents would act the same, whatever
the means of reaching the surface. In the groups of atolls
above referred to, neither the forms of the atolls nor the
currents favor such an hypothesis. In the Paumotus, there
are no currents of strength enough for work of this kind,
as shown on page 296. He speaks of Keeling Atoll as a
horse-shoe or crescentic island, and states that such forms are
common among atolls. But the Keeling Atoll, according to
the best maps, is an ordinary atoll, the lagoon nearly encircled
with reef, and the application of such terms to it 1s wholly
misleading. The shaping of a reef by the horse-shoe method
of drifting and making an eddy to leeward which he describes,
is wholly inapplicable to the ordinary atolls of the ocean; for
there are no means for producing the result.

d. Further, drifting by currents may make beaches and

1 On the Solomon Islands, Proceedings of the Royal Society of Edinburgh,
1885-1886, xiii. 857; On the Keeling Atoll, Nature, January, 1889, xxxix. 236.

,

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 291 |

inner channels whether subsidence is going on in the region
or not, and are not evidence for or against either a move-
ment downward or upward. Sandy Hook, the long, sandy
point off the southern cape of New York harbor, has been
undergoing (as the United States Coast Survey has shown) an
increase in length, or rather variations in length, through the
drifting of sands by an outside and an inside current; and
this is no evidence that Prof. G. H. Cook is*wrong in his con-

clusion that the New Jersey coast is slowly subsiding.

3. Planting of corals on basements made and raised to the —

right level by the growth of pelagic limestone or by other means.
— The theory has been sustained by Mr. Semper, Dr. Rein,
Mr. Agassiz, Mr. Murray, Dr. Guppy, and others, that since
the growing calcareous deposits of the sea-bottom are slowly
rising toward the surface by successive accumulations of the
shells and other debris of pelagic species, they may have been
built up locally in various regions of the deep seas (as they
actually are now about some islands) until they were near
enough to the surface to become next a plantation of corals;
and that in this way, without any subsidence, atolls became
common within the area of the tropical oceans.

In support of this view, Dr. Guppy states that in elevated
coral reefs of the Solomon Islands, one hundred to twelve
hundred feet high, the coral reef rock forms a comparatively
thin layer over impure earthy limestone abounding in fora-
minifers, pteropods, and other pelagic organisms. Such ob-
servations have great interest, but they only prove that in
coral-reef seas corals will grow over any basis of rock that
may offer where the water is right in depth and other cir-
cumstances favor. They are not evidence against the subsi-
dence theory,_but simply local examples under the general
principle just stated. / Lr

™. . : ”

292 CORALS AND CORAL ISLANDS,

The wide oceans are, however, wonderfully free from
banks approaching the surface within one hundred fathoms.
The Indian Ocean and the China seas afford exceptions.
But the principal submerged banks of these waters have
the lagoon-basins and raised borders of an atoll and thus
bear, as has been explained (page 272), positive evidence of
a former atoll existence and therefore of subsidence. They
illustrate the fact ‘that the rate of subsidence has not always
been within the narrow maximum limit that is favorable to
‘the atoll, but sometimes has exceeded it to their destruction.
The regions of such sunken atolls in the China seas and
Indian Ocean have many spots of shallow soundings which
were probably sunk at the same time.

Mr. Murray observes that “the soundings of the ‘Tus-
carora’ and ‘Challenger’ have made known numerous sub-
marine elevations; mountains rising from the general level
of the ocean’s bed at a depth of 2,500 or 5,000 fathoms up
to within a few hundred fathoms of the surface.” But “a
few hundred fathoms,” if we make “few” equal two means
twelve hundred feet or more, which leaves a long interval
unfilled.’

The atoll of the Tortugas, and others in the West Indies,
are regarded by Mr. Agassiz as having a basement, up to the
coral-growing limit, of pelagic limestone or of some other
material. It may be so; but there is as yet no proof of it.

1 The actual depths over the elevations in the “ Tuscarora” section between the
Hawaiian Islands and Japan, numbering them from east to west are as follows: (1)
11,500 feet; (2) 7,500 feet; (3) 8,400 feet; (4) 12,000; (5) 9,000 (this seven miles
west of Marcus Island) ; (6) 9,600 feet. Whether ridges or peaks, the facts do not
decide; probably the former. No. 1 has a base of 185 miles with the mean east-
ward slope 40 feet per mile (= 1: 132) and the westward 128 feet per mile. No. 2
has a breadth of 396 miles, with the mean eastern slope mostly 37 feet per mile,
but 51 feet toward the top, and the westward 55 feet per mile (1:96). No.3
was the narrowest and steepest, it being about 100 miles broad at base, and having
the mean eastern slope 192 feet per mile and the mean western 200 feet.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 293

The basement, according to Mr. Murray, may be a vol-
-eanic cone or a submerged mountain-peak at the proper level;
and if too low, it may be raised up to it by pelagic life-relics ;
if too high, like the “emerged volcanic mountains situated in
the ocean basins,’ it may be razed by abrading agencies to
the water-level and finally lowered by the waves and cur-
rents to the needed depth. It is true that the basement for
growing corals may be anything that has the proper depth.
But this shaving off of a mountain and carrying the erosion
to a few fathoms below the sea without a change of level is
beyond physical possibility. Land-waters cut out valleys or
gorges, and indefinite time would be required for the final lev-
elling. Waves are too shallow in their action, the rising, tidal
waters too protective, and the rocks under a cover of water too
resisting, for the marine part of the degradation.

But this making of atolls and barrier reefs on banks sub-
merged at the right distance without submergence, encoun-
ters a fatal difficulty wherever there are deep lagoons and
deep channels, which difficulty has not been overcome though
various methods have been suggested.

4. Lagoon basins and channels made largely by abrading
and solvent action. It is urged, in agreement with Darwin,
that the outer portions of reefs increase faster than the inner,
owing to the purer water about them and the more abundant
life for food; that the inner parts are not only at a disadvan-
tage in these respects, but suffer also from coral debris thrown
over them. Some writers have considered this of itself suffi-
cient, saying that the edges of the bank will thus reach the
surface, while the interior makes little progress; and so
comes the lagoon basin. But the above authors add to the
causes of unequal growth mentioned by Darwin the solvent
and abrading action of the waters.

294 CORALS AND CORAL ISLANDS.

It is, hence, concluded by them that, under these conditions,
the simple bank of growing corals may have a depression
made at centre, which, as the process continues, will become
a lagoon basin, and the reef, thereby, an atoll with its lagoon;
that the atoll, so begun, may continue to enlarge through the
external widening of the reef and the further action of cur-
rent abrasion and solution within; or, in the case of fringing
reefs, that the change may go on until the reef has become a
barrier-reef with an inner channel and inner reefs. It is ad-
mitted that subsidence may possibly have helped in the case
of the deepest lagoons.

Dr. Geikie expresses his opinion on the subject thus :—
“ As the atoll increases in size the lagoon becomes proportion-
ally larger, partly from its waters being less supplied with
pelagic food, and therefore less favorable to the growth of
the more massive kinds of corals, partly from the imjurious
effects of calcareous sediment upon coral growth there, and
partly also from the solvent action of the carbonic acid of the
sea-water upon the dead coral.” The argument has been en-
forced by observations on the solvent power of carbonic acid.
But this is a point about which there is no ground for dispute.

a. Mr. Semper gives examples of the effects of currents
at the Pelew Islands, stating that by striking against or flow-
ing by the living corals they make the reef grow with steeper
sides and determine its direction, and urging that abrasion
and solution have made, not only the deep lagoon-like chan-
nels, but the deeper channels between the islands. He holds
that in Kriangle, which he describes as a true atoll with no
channel leading into the lagoon from the sea, that the lagoon
may have been “the result of the action of currents on the
porous soil during a period of slow upheaval.’* He says,

1 Animal Life, pp. 269, 270.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 295

further, that the large channel in the main island of the
group, “forty fathoms deep and many miles wide. . . . finds
an easy explanation on the assumption of an upheaval ;”’ it
became “ wider in proportion as the enclosed island, consist-
ing of soft stone [tufa], was gradually eaten away, and during
slow upheaval it would continue to grow deeper in propor-
tion as the old porous portions of the reef and the rock in
which it was forming were more and more worn down by
the combined action of boring animals and plants, and of the
currents produced by the tides and by rain.” ; Mr. Semper
refers to the dead depressed tops of some masses of Porites
near tide-level as the effects of the deposit of sediment over
the top of the living coral and of erosion by the waves and
exposure to rains while the sides continued to grow; and the
fact is made an example on a very small scale of atoll-making.
In addition, experiments on the solvent power of sea-water
are appealed to. Examples of the action of currents, sedi-
ment, boring species, and the solvent action of carbonic acid
in the waters, are mentioned by Mr. Agassiz, in his excellent
account of the “Tortugas and Florida reefs,’ and in his
“Three Cruises of the Blake ;” but in his paper on the Coral
Reefs of the Hawaiian Islands he concludes that the fact
that “the majority of reefs are of great width goes to show
also that solution alone is not active enough to remove great
masses and form lagoons.” ! —_

b. The theory, if satisfactory, accounts not only for the
origin of an atoll, but for the origin of atolls of all sizes,
shapes, and conditions, and for great numbers of them in
archipelagoes and chains; not only for channels through
fringing reefs, like those that abrasion in other cases makes,
but for all the irregular outlines of barriers, for the great

1 Bulletin of the Museum of Comparative Zoology, 1ssd, vol. xvii. No. 3.

296 CORALS AND CORAL ISLANDS.

barriers reaching far away from any land, and for the posi-
tions and indented coasts of the small included lands. Is it
a sufficient explanation of the facts?

c. The currents that influence the structure of reefs are:
(1) the general movement or drift of the ocean, in some parts
varying with seasonal variations in the winds; (2) the cur-
rents connected with wave action and the inflowing tide over
a shelving bottom; and (3) the currents during the ebb, flow-
ing out of channels; together with (4) counter-currents.
Each region must have its special study in order to mark out
all the local effects that currents occasion. Such effects are
produced whether a secular subsidence 1s in progress or not,
and hence a particular review of the subject in this place is
unnecessary.

The shaping of the outside of the reef and the determina-
tion of the width and level surface of the shore-platform are
due chiefly to the tidal flow and the accompanying action of
wind-waves as explained on preceding page.

The current that accompanies the ebb is locally the strong-
est. Owing to the great width of many barrier reefs and of
the channels and harbors within them, the tide flows in over
a wide region. At the turn in the tide the waters escape at
first freely over the same wide region; but with a tide of
but two or three feet. there is little fall before the reef —
which lies at low tide level and a little above it — retards it
by friction; and thus escape by the open entrances is in-
creased in-amount and in rate of flow. The facts are the
same in atolls where the lagoons have entrances.)

1 The currents of the tropical Pacific Ocean are of very unequal rate in its dif-
ferent parts, and very feeble in the Paumotu Archipelago and the Tahitian and Sa-
moan regions. Captain Wilkes reports that in the cruise of the Expedition through
the Paumotu Archipelago to Tahiti, a distance of a thousand miles, during a month
from August 13 to September 13, 1839, the drift of the vessels was only seventeen
miles; and that during fourteen days in the first half of October, between Tahiti

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 297

d. Examples of massive corals having the top flat, or de-
pressed and lifeless, while the sides are living, are common
in coral-reef regions, wherever such corals are exposed to the
deposition of sediment, and where they have grown up to the
surface so that the top is bare above low tide. A disk of
Porites, having the top flat and the sides raised (owing to
growth) so as to give it an elevated border, is figured on
Plate LV. of the author’s Report on Zodphytes. Many such
were found in the impure waters of a shore reef at the
Feejees. At Tongatabu one flat-topped mass of Porites was
twenty-five feet in diameter; and both there and in the
Feejees, others of Astraeids and Mzandrinas measured twelve
to fifteen feet in diameter.

Over the dead surfaces, as Mr. Semper observes, the coral
may be eroded by the solvent action of the waters and espe-
cially where depressions occur to receive any deposits; and

and Upolu of the Samoan group, nearly eighteen hundred miles, the drift was only
forty-three miles.

The “Challenger,” on her route from the Hawaiian Islands to Tahiti, found, be-
tween the parallel of 10° S. and Tahiti, “ the general tendency of the current west-
erly, but its velocity variable; ”” between the parallel of 10° S. and 6° N., the direction
was westerly with “the average velocity thirty-five miles per day, the range seven-
teen to seventy miles per day,” the maximum occurring along the parallel of 2° N.
Farther west, about the Pheenix group, the equatorial current, as described by Mr.
Hague (loc. cit. p. 237) has “a general direction of west-southwest and a velocity
sometimes exceeding two miles per hour.” At times it changes suddenly and flows
as rapidly to the eastward. The drifting of the sands about Baker’s Island (in lati-
tude 0° 13’ N., longitude 176° 22’ E.) has much interest in connection with this sub-
ject of current action, and the facts are here cited from Mr. Hague’s.paper. The
west side of the little island (1 xX 2 m. in area) trends northeast, and the southern
east by north, and at the junction a spit of sand extends out. During the summer
the ocean swell, like the wind, comes from the southeast, and strikes the south side;
and consequently the beach sands of that side are drifted around the point and
heaped up on the western or leeward side, forming a plateau along the beach two
or three hundred feet wide, and eight or ten feet deep over the shore platform.
With October and November comes the winter swell from the northeast, which
sweeps along the western shore ; and in two or three months the sands of the pla-
teau are all drifted back to the south side, which is then the protected side, extend-
ing the beach of that side two or three hundred feet. This lasts until February or
March when the operation is repeated.

298 CORALS AND CORAL ISLANDS.

boring animals may riddle the coral with holes or tubes.
But generally the erosion is superficial; the large masses
referred to showed little of it. Such dead surfaces in corals
are generally protected by a covering of nullipores and other
incrusting forms of life, and the crusts usually spread over
the surfaces pari passu with the dying of the polyps.

e. Every stream, says Mr. Semper (when explaining, as
cited on a preceding page, the origin of the deep channel of
the large Pelew Island, whose depth is “ 35 to 45 fathoms”’),
‘has a natural tendency to deepen its bed.” But there is a
‘limit to this action. The eroding or deepening power of a
stream through abrasion and transportation is null, or nearly
so, below the level of its outlet. A basin or channel 45 fath-
oms (270 feet) deep. with an outlet of much less depth could
not be deepened by such means nor protect itself from shallow-
ing. The depth of the outlets is not stated except that they
are said to be ship-channels. Moreover, with a tufa bottom,
solution could not contribute to the removal, since carbonated
waters, although decomposing the tufa, dissolve very little of
its ingredients. An elevation in progress would result in
making the channel a closed lake and finally dry land.

For the same reason, the small atoll, Kriangle, having, as
described, a closed lagoon, could have no deepening of the
lagoon from abrasion by tidal currents or wave-action during
the progress of an elevation. And if a lagoon have an out-
let, the rapid current of the ebb would be confined to the
narrow passage-way and a portion of the bottom near it;
through the larger part of the lagoon, as in any other lake,
the waters would have scarcely perceptible motion, and there-
fore slight energy for any kind .of work. Hence a lagoon
would lose very little by this means, and shallowing would
go on unless there were great loss through the solvent action

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 299

of the waters. An elevation would only hurry the shallow-
ing and end in emptying the lagoon.

f. Erosion through solvent action is promoted by the
presence in the waters both of carbonic acid and organic
acids. The material within reach of the tides or waves ex-
posed to this action is dead corals and shells, or their debris,
and bare coral rocks, occurring over: (1) the outer region of
living corals and for a mile or so outside ; (2) the shore plat-
form and the reef, bare at low tide, on which there is com-
paratively little living coral; and (3) the lagoon basin.
There is nothing in the material within the lagoon to favor
solution more than in either of the other two regions; in
fact, the platform and bare reef are most exposed to the
action because of the small amount of living corals over
them. The outside waters take up what they can through
the carbonic acid they contain, and supply thereby the wants
of the lime-secreting polyps, shells, ete., and carry on the pro-
cess of solidification in the debris; the same waters move on
over the atoll reef and take up more lime as far as the acid
ingredient is present ; and then they pass to the lagoon for
work similar to that outside, with probably a diminished
amount of free carbonic acid, on account of the loss over the.
reef-ground previously traversed.

The lagoon-basin is not, therefore, the part of the atoll
that loses most by solution, any more than by abrasion and
transportation. The outer reefs suffer the most; and yet, if
the island is not subsiding at too rapid a rate, they keep ex-
tending and encroaching on the ocean, instead of wasting
through the drifting into the ocean at large of calcium car-
bonate in grains and solution, and the shore-platform also
preserves its unvaried level notwithstanding the daily sweep
of the tidal floods, and the holes that riddle its outer portion.

300 CORALS AND CORAL ISLANDS.

The remark: “It is a common observation in atolls that
the islets on the reefs are situated close to the lagoon shore;”
such “ facts point out the removal of matter which is going
on in the lagoons and lagoon channels,” I know nothing to
sustain. The width of the shore-platform on the seaward
side is always greater than that on the lagoon side; but the
outside shore-platform has its width determined by tidal and
wave action, and this action is powerful on the ocean side,
and feeble on the lagoon side; it produces a high coarse
beach on the outside as the inner limit of the platform, and
a finer, lower and much more gently sloping beach on the
inside. The amount of erosion is far greater, as it should be,
on the side of the powerful agencies.

g. The loss to the lagoon by abrasion and solution is re-
duced toa minimum, nm the majority of atolls, by the absence
of lagoon entrances, which leaves them with only concealed
leakage passages for slow discharge.

Nine tenths of atolls under six miles in length (or in
longer diameter), half of those between six and twenty
miles, and the majority of all atolls in the Pacific ocean,
have no entrances to the lagoon a fathom deep; and the
larger part of those included in each of these groups have
no open entrances at all.

For evidence on this subject, reference may be made to
the Wilkes Expedition Hydrographic Atlas. This atlas con-
tains maps of nearly sixty coral islands from the surveys of
its officers, drawn on a large scale — one or two miles, rarely
four, to the inch.

Out of the number, nine, ranging from 13 to 3 English
miles in the longer diameter of the reef, have no lagoon, but
only a small depression in its place; two of these take in
water at high tide, and the rest are dry.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 301

Of those under six miles im length having lagoons, seven-
teen in number, sixteen are represented as having no en-
trances to the lagoon at low tide; and the one having an
entrance is 5X4 miles in size. The smallest is about a mile
in diameter.

Of those that are six miles or over in length, twenty-nine
in number, seventeen have channels and twelve have none.
Those having channels are mostly over ten miles in length.
A list of them is here given with their sizes, and also the
proportion of the reef around the lagoon which is under
water above one third tide, and bare at low tide,—a feature
of much interest in this connection : — |

ELLIcE Group. — Depeyster’s, 6X6 m.; three fourths of
the encircling reef bare. llice’s, 9X5 m.; three fourths bare.

GILBERT Group. — Apia, 17X7 m.; half bare. Tarawa,
io m.; hali bare. ‘Taritari, 18X11 m.; two thirds bare.
Apamama, 12X5m.; half bare. Tapateuea, west side mostly
submerged.

MarsHaLut Istanps (northern). — Pescadores, 108 m. ;
four fifths bare. Korsakoff, 26 m.; four fifths bare.

Paumotus. — Peacock, 15X7 m.; nearly all wooded.
Manhii, 13X5 m.; nearly all wooded. Raraka, 6X9 m.;
three fourths wooded. Vincennes, 139 m.; mostly wooded.
Aratica, 18X11 m.; three fifths bare. Tiokea, 184 m.; two
thirds wooded. Kruesenstern’s, 16X10 m.; mostly wooded.
Dean’s (or Nairsa), 55X18 m.; half or more bare.

h. The absence of open channels in so large a proportion
of lagoons, and especially in lagoons of the smaller atolls, is
fatal to the abrasion-solution theory. The method of enlarg-
ing atolls through currents and solution can act only feebly,
if at all, where waters have no free outlet ; and this is emi-
nently so with the smaller atolls which have been assumed

4 ,

302 CORALS AND CORAL ISLANDS.

by the theory to be most favorable in purity of water and in
abundant life for progress; if the small cannot grow, the
large lagoons cannot be made from them by the proposed
method.

Reverse the method, letting the large precede the small,
and then, under the subsidence theory, we have a consistent
order of events. We have large atoll reefs with several large
entrances (like the great barrier reef about a high island in

trances concurrently narrowing through the growing corals
and the consolidating debris, in spite of the efforts of abra-
sion and solution to keep them open and make them deeper ;
and, afterwards, the atoll becoming still smaller until the
entrances close up; and finally the lagoon-basin reduced to
a dry depression with nothing of the old sea-water remaining
except, perhaps, some of its gypsum.

i. Instead of small lagoons having the purest waters, the
reverse is most decidedly and manifestly the fact, and this
accords with the reversal in the history just suggested. Since
atolls of middle and larger size commonly have one third
to two thirds of the encircling reef covered with the sea at
one third tide, making the ocean and lagoon for more than
half the time continuous, the large lagoon in such a case has
as pure water as the ocean, and commonly as good a supply
of food-life, and sometimes as brilliant a display of living
corals. But in the smaller atolls, the area of the lagoon has
little extent compared with the length and area of the encir-
cling reef ; coral sands and other calcareous material conse-
quently have possession of the larger part of the bottom,
and the waters, since they are less pure than those outside,
contain fewer and hardier kinds of corals and less life of
other kinds. They are exposed, also, to wider variations of

49
this and other respects) gradually contracting, and the ont

Aw
-

}

4

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 303

temperature than the outer, with injury to many species, and,
at lowest tides, may become destructively overheated by the
midday sun, as many a plantation of corals with dead tops
for a foot or more bears evidence. In the smallest atolls,
the lagoons are liable also to alternations of excessive saltness
from evaporation and excessive freshness from rains, and con-
sequently no corals can grow inside, though still flourishing
well in the shallow sea about the outer reef. The above are
the facts, not the suggestions of theory. __

j. We read: “So great is the destructive and transport-
ing influence of the sea under the combined or antagonistic
working of tides, currents, and wind-waves that the whole !
mass of the reef, as well as the flats and shoals inside, may
be said to be in more or less active movement.’* ‘This
description of the Tortugas reefs is not applicable to the
atolls of the Pacific nor to the Tortugas. Notwithstanding
the testimony of Captain Beechey and others about occa-
sional catastrophes — which are mostly catastrophes to the ~
islets and banks within the lagoons—I was led to look
upon a coral island as one of the most stable of structures.
Through the wind-made and tidal movements, the loose sands
are drifted along the shores and over the reef; edges of the
reef are broken off in gales or by earthquake waves; and
occasionally a mushroom islet in the lagoon, where growing
corals are not compacted by wave-action, is overthrown by
the same means; but beyond this. the structure is singularly
defiant of the encroaching waters. Earthquakes may bring
devastation ; and so they may to other lands.

5. Lagoon-basins made by caving in. — Mr. Fewkes has
suggested” that the lagoon-basin of the Bermudas may have

1 Dr. Geikie’s Address, p. 23.
2 Fewkes, Proceedings of the Boston Society of Natural History, 1888, p. 518.

304 CORALS AND CORAL ISLANDS.

been made by the caving in of caverns with which the reef-
rock is supposed to have been honey-combed, but does not
apply the theory to all other atolls. The view is opposed
by Mr. Heilprin, after a more recent study of the islands.
There is nothing to sustain it among the Pacific atolls
that were visited by the author. Ordinary atolls have no
caves ; for caves in reef-limestones are made only after their
elevation. The constant effort and action of the sea upon
submerged reefs is to fill up all cavities and consolidate all
sands. In the drift-sand ridges or hills, however, they may
be formed without an elevation of the island; but these do

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not exist in the region of the lagoon-basin. « ., //..:

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As a further reply to the arguments adverse to Darwin’s
theory from the West Indian seas, the following facts are
here added.

The arguments from the Florida and West Bahama reefs
in favor of no subsidence have more weight than for most
coral-reef regions. They appear to indicate that any subsi-
dence once in progress long since ceased; but they are far
from proving that the reefs of the seas have been formed
without help from subsidence. There is evidence that a
great subsidence occurred during the coral-reef era, affording
all that the Darwinian theory demands.

In a valuable paper by Mr. Alexander Agassiz, published
in 1879 in the Bulletin of the Museum of Comparative Zodl-
ogy,” the author points out that the South American conti-
nent, in comparatively recent geological times, had connection
with the West India islands through two lines: (1) one along
a belt from the Mosquito Coast to Jamaica, Porto Rico, and

1 Heilprin, Bermuda Islands, p. 44. 1889.
2 An abstract of the paper is contained in the American Journal of Science,
1880, xviii. 230.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 305

Cuba; and (2) the other through Trinidad to Anguilla, of
the Windward Islands. He sustains the conclusion by a
review of the soundings made by the steamer “ Blake” under
the command of J. R. Bartlett, U. S. N., and a consideration
of the facts connected with the distribution of marine and ter-
restrial species. As the soundings show, the former of the
two connections requires for completeness an elevation of the
region amounting to 4,060 feet over the part south of Jamaica,
4,830 feet between Jamaica and Hayti, and 5,240 feet between
Hayti and Cuba. The other line of connection requires an
elevation of 3,450 feet. An open channel, as he observes,
would thus be left between Anguilla and the Virgin Islands,
where there is now a depth of 6,400 feet. The close relations
in the existing fauna of the Gulf to that of the Pacific waters
prove that it continued to be a salt-water gulf through the
era of elevation.

Mr. Agassiz infers that the connection of the West India
islands with South America existed before the Quaternary era.
But there are other facts which seem to prove that it was con-
tinued into, or at least was a fact, in the Quaternary.

The opinion as to a connection of the Windward Islands
with South America in the Quaternary was presented by
Prof. E. D. Cope in 1868 (and earlier, as he states, by Pomel),
on the ground of the discovery in the caves of Anguilla of a
species of gigantic rodent related to the chinchilla, as large
as the Virginia deer, and nearly equalling the Quaternary
Castoroides of Ohio. Further, De Castro, as cited by Dr. J.

1 Proceedings of the Academy of Natural Sciences of Philadelphia, 1868, 313,
and of the American Philosophical Society, Philadelphia. 1869, 183; also Smithsonian
Contributions to Knowledge, 30 pp. 4to with 5 plates, Washington, 1883. The last
paper (prepared in 1878) contains descriptions of the following species from the
Anguilla bone-cave: Amblyrhiza inundata Cope (the large rodent announced in
1869), A. quadrans Cope, A. latidens Cope, an Artiodactyl, apparently of the Bovi-
de and a little smaller than Ovis aries. With them was obtained an implement
(“a spoon-shaped scraper or chisel ”?) made of the lip of the large Strombus gigas.

20

306 CORALS AND CORAL ISLANDS.

Leidy in his “ Mammalian Fauna of Dakota and Nebraska,”
1869, announced, in 1865, a gigantic sloth of the Quater-
nary, from Cuba, which he referred to the genus MMegalonyz,
and Dr. Leidy named Megalocnus rodens, proving a Quater-
nary connection between the continent and Cuba.

The fact of an elevated condition of the region sufficient
to make Cuba and Anguilla part of the continent during the
earlier Quaternary, 1f not in the Pliocene also, is thus made
quite certain. This is fully recognized by Wallace. Such a
condition could hardly have existed without a large elevation
also of Florida, though probably not, as Mr. Agassiz holds,
to the full amount of the depression between it and Cuba —
nearly 5,000 feet — because Cuba is most closely related in
fauna to South America. The subsidence which brought the
region to the present level was consequently within the coral-
reef period. It is hence hardly to be doubted that the mak-
ing of the Florida, Bahama, and other West India coral reefs
was going on during the progress of a great subsidence.
None of the facts mentioned by observers are opposed to
this view.

On this point Mr. Heilprin says :? —

“Dr. Supan, in reviewing Professor Dana’s paper in the
American Journal of Science for 1885,’ criticises the views
relative to subsidence in the Floridian region, since it is
claimed that even if direct connection did exist between the
West Indian Islands and the southern continent, there is no
proof that this connection extended northward to the North

American continent ; and he further denies — without, how-

ever, giving any reason for his denial —that there ever was
any (Quaternary ?) connection between the West Indies and
1 Geographical Distribution of Animals, ii. 60, 78.

2 «The Bermudas,” 227.
$ Petermann’s Mittheilungen, vol. xxxii. pl. 1; Litteraturbericht, p. 5. 1886.

OBJECTIONS TO THE THEORY OF SUBSIDENCE. 307

North America. This notion is probably based upon the old
idea (advanced by L. Agassiz and Le Conte) of the making
of the Floridian peninsula, in which no movements of either
elevation or subsidence were supposed to have been involved.
Since, however, this conception has proved to be a myth,
there is no further reason, except so far as the case may be
supported by fact, to adhere to the old views of continental

(or oceanic) stability in this region. My own observations ‘

have conclusively proved a peninsula uplift as late as the
Post-Pliocene period, and extending as far south as Lake
Okeechobee. But I am by no means convinced, as I have
elsewhere stated (in chapter on the Coral-reef Problem) that
a nearly simultaneous subsidence did not take place in (and
from) what are now known as the straits of Florida. The
existence of such a subsidence (Lruch) is considered likely by
Suess* who has paralleled it with (a supposed) similar occur-
rence in the eastern basin of the Mediterranean. This view
of the formation of the deep Gulf-channel, I must confess,
appears to me far more captivating than that which ascribes
it to the wash of the Gulf-current.”

Mr. Heilprin brings forward, also, additional evidence of
great weight.

“But I believe direct evidence pointing to (although by
no means proving) a former connection between the Floridian
peninsula and the mainland is not wanting. In a paper on
‘The Value of the “ Nearctic,” as one of the Primary Zodlogi-
eal regions,’ published in the Proceedings of the Academy of
Natural Sciences of Philadelphia for 1882, I pointed out cer-
tam facts in favor of considering the lower portion of the
peninsula as part of the Neotropical rather than the Nearctic
realm; more recent zodlogical researches have still further

1 Antlitz der Erde, vol. i.

308 CORALS AND CORAL ISLANDS.

demonstrated the correspondence existing between this south-
ern fauna and that of the tract lying to the south. But more
significant is the finding of the large assemblage of mamma-
lian remains which have been lately brought to light from
various parts of the peninsula. These have been determined
by Dr. Leidy* to be the skeletal parts of the elephant, masto-
don, llama, rhinoceros, tapir, Hippotherium, the sabre-toothed
tiger (Machzerodus), Glyptodon, etc. Neither the Sabre-tooth
nor the Glyptodon, both of which are so closely related to the
commoner South American forms as to be barely distinguish-
able from them, have heretofore been found in the south-
ern United States. Of course they may yet be found, and
indicate a passage over from South America by way of Mex-
ico and the southern United States. But the great abund-
ance of these remains on the Floridian peninsula, and their
absence, either in whole or part, from the Gulf States, are
facts which, so far as they go, point to a former direct land-
connection across what is now an arm of the Gulf.”

Concrusion. — With the theory of abrasion and solution
to make lagoon-basins and the deep channels inside of barriers
incompetent, and also that of current-drift, all the hypotheses
of objectors to Darwin’s theory are alike weak ; for they have
made these processes their reliance, whether appealing to a
calcareous, or volcanic, or pelagic-growth, or mountain-peak
basement for the structure. The subsidence which the Dar-
winian theory requires has not been opposed by the mention
of any fact at variance with it. nor by setting aside the argu-
ments in its favor; and it has found new support in the facts
from the “Challenger’s”’ soundings off Tahiti that had been
put in array against it, and strong corroboration in the evi-
dences of subsidence from the West Indies.

1 Leidy, Proceedings of the Academy of Natural Sciences of Philadelphia,
1884-1889.

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~ Darwin’s theory, therefore, still remains as the theory

af that accounts for the origin of atolls and barrier islands,

which is not true of any other that has been proposed.
Fringing reefs and isolated coral-reef banks may form in
shallow water within the growing depths of reef-making
corals, and on any kind of bottom. But atolls, barrier-reefs,
and coral formations of great thickness require, as a rule, the
aid of slow subsidence, —as has been true for nearly all the
thick rock formations over the continents.

V.—THE COMPLETED ATOLL.

The atoll, a quiet scene of grove and lake, is admirably
set off by the contrasting ocean. Its placid beauty rises to
grandeur when the storm rages, and the waves foam and
roar about the outer reefs; for the child of the sea still rests
quietly, in unheeding and dreamy content. This coral-made
land is firm, because as has been already explained, it is liter-
ally sea-born, it having been built out of sea-products, by the
aid of the working ocean. And so with the groves: they
were planted by the waves; and hence the species are those
that can defy the encroaching waters, and meet the various
conditions in which they are placed. The plants therefore
take firm hold of the soil, and grow in all their natural
strength and beauty.

Only an occasional coral island has a completely encir-
cling grove, and is hence a model atoll. But the many in
which a series of green islets surround the lagoon are often
but little less attractive, especially when the several islets pre-
sent varied groupings of palms and other foliage. To give per-
fection to the coral island landscape there ought to be, here
and there, beneath the trees, a pretty cottage or villa, and
other marks of taste and intelligence; and now and then a

310 CORALS AND CORAL ISLANDS.

barge should be seen gliding over the waters. As it is, the
inhabitants are swarthy and nearly naked savages, having
little about them that is pleasant to contemplate ; and their
canoes with a clumsy outrigger to keep them right side up,
as well as their thatched huts, are as little in harmony as
themselves with Nature’s grace and loveliness.

Where the islets of a coral reef are heaped up blocks of cor-
al rock, blackened with lichens, and covered with barely
enough of trailing plants and shrubs to make the surface green
in the distant view, the traveller, on landing, would be greatly
disappointed. But still there is enough that is strange and
beautiful, both in the life of the land and sea, and in the his-
tory and features of the island, to give enjoyment for many a
day.

The great obstacle to communication with a majority of
atolls, especially the smaller, is the absence of an entrance to
the lagoon, and hence of a good landing-place. In that case
landing can be effected only on the leeward side, and in good
weather; and best, when the tide is low. ven then, the sea
often rolls in, so heavily, over the jagged margin of the reef, that
it is necessary for the boat to take a chance to mount an in-go-
ing wave and ride upon it over the line of breakers, to a stop-
ping-place somewhere on the reef’ or shore-platform.

Less easy is the return through the breakers, especially if
the sea has risen during the ramble ashore. The boat, in or-
der to get off again, would naturally take one of the narrow
channels or inlets indenting the margin of the reef. But,
with the waves tumbling in one after another, roughly lifting
and dropping it, as they pass, and with barely room between
the rocks for the oars to be used, there is a fair chance of its be-
ing dashed against the reefs to its destruction, or thrown
broadside to the sea and swamped under a cataract of waters.

THE COMPLETED ATOLL. Lt

If another boat with its crew were lying at the time off the
reef, a line, carried to it through the surf by an expert swim-
mer, might prove a means of rescue:—and so, in 1840, we
safely reached our ship. To those approaching such a shore in
a boat, prudence would give the advice—first, drop, some dis-
tance outside of the breakers, a kedge or anchor, for aid both
in landing on, and leaving, the reef. But the bottom off a cor-
al island is often bad anchoring ground. And then, if the
kedge thus planted holds firm, in spite of the jerking waves,
well and good. If not

The accompanying plate represents a scene on Bowditch or
Fakaafo Island, sketched by Mr. A. T. Agate, one of the artists
of the Wilkes Exploring Expedition, and copied from Volume
V. of Wilkes’s Narrative of the Expedition. This island is

FAKAAFO, OR BOWDITCH ISLAND.

the easternmost of three small atolls, situated to the north of
the Samoan or Navigator Group, near the parallels of 84°, 9°,
and 93° S., and between the meridians of 171° and 1723°

12, CORALS AND CORAL ISLANDS.

(Su)

and has already been described (p. 168). The grove of cocoa-
nut trees contains the sacred or public house of the island—
a well-made structure measuring fifty feet by thirty-five in
length and breadth, and twenty feet in height. In front of
the building stands the deity of the place, consisting of a
block of stone fourteen feet high, enveloped in mats; and also
near by, a smaller idol, partially covered with matting. In
the left corner there is a young cocoanut palm—usually a more
beautiful object than the full-grown tree.

This island and the two others near it were among the
few, perhaps the last, examples that remained until 1840,
of Pacific lands never before visited by the white man.
The people therefore were in that purely savage state which
Captain Cook found almost universal through the ocean in
the latter part of last century. A few words respecting our
reception at this coral island, may not, therefore, be an im-
proper digression.

The islanders knew nothing of any other land or people:
—an ignorance not surprising, since the lagoons of the group
have no good entrances, and a nation cannot be great in nav-
igation or discovery without harbors. As a consequence, our
presence was to them like an apparition. The simple in-
habitants took us for gods from the sun, and, as we landed,
came with abundant gifts of such things as they had, to pro-
pitiate their celestial visitors. They, no doubt, imagined that
our strange ship had sailed off from the sun when it touched
the water at sunrise, or sunset, and any child among them
could see that this was a reasonable supposition. The king,
after embracing Captain Hudson, as the latter states in his
Journal (Wilkes’s Narrative), rubbed noses, pointed to the
sun, howled, moaned, hugged him again and again, put a mat
around his waist, securing it with a cord of human hair, and

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repeated the rubbing of noses and the howling; and the mo-
ment the captain attempted to leave his side, he set up again
a most piteous howl, and repeated in a tremulous tone, ‘‘ Nofo
ki lalo, mataku au,” ‘Sit down, I am afraid.” While thus
in fear of us, they showed a great desire that their dreaded
visitors should depart; some pointed to the sun, and asked
by their gestures about our coming thence, or hinted to us to
be off again.

But with all their reverence toward their mysterious guests,
they became after awhile quite familiar, and took advantage
of every opportunity to steal from us. Our botanist gave his
collecting-box to one of them to hold, and, the moment his
back was turned, off the native ran, and a hard chase was re-
quired to recover it—a most undignified run on the part of
the celestial.

While the men wore the maro, the equivalent of tight-fit-
ting breeches six inches or less in length, the women were at
tired in a simple bloomer costume, consisting solely of a petti-
coat or apron, twelve to eighteen inches long, made of a large
number of slit cocoanut leaves, and kept well oiled. Besides this
they had on, as ornaments, necklaces of shell or bone. The girls
and boys were dressed au naturel, after the style in the garden
of Eden. These primitive fashions, however, were not peculiar
to the group, being in vogue also in other parts of the Pacific.

As a set-off against the geographical ignorance of these
islanders, we may state that Captain Hudson and the best
map-makers of the age knew nothing of the existence of Bow-
ditch Island until he discovered it; and from him comes the
name it bears, given in honor of the celebrated author of
“ Bowditch’s Navigator” as well as of the translation of [a-
place’s Mécanique Céleste.

The annexed plate—also from Wilkes’s Narrative, Vol. V.

314 CORALS AND CORAL ISLANDS.

—represents a scene on Duke of York’s Island, another of the
same solitary group of atolls. The view was taken on the
lagoon side, and exhibits the placid lake, the border of verdure
far away in the distance, and, near by, the margin of a native
village beneath its cocoa-nut grove. A few young plants of
the Pandanus stand along the point. The houses are like
those of other islands to the west and northwest. The point
in front of the village is one of three small quays, two feet out
of water. The house, resting partly upon it and partly on
poles in the water, and thatched with leaves of the Pan-
danus, was apparently a shelter for canoes and fishing-tackle.
The Gilbert Group affords an example of a less isolated
coral-island people. A. beautiful view representing a part of
the village of Utiroa, on Drummond’s Island, is contained in
the same volume of Wilkes’s Narrative with the preceding.
The public-house of the island is even larger than that on
Bowditch’s Island, measuring one hundred and twenty feet in
length, forty-five feet in width, and forty in height to the
ridge-pole. This island, unlike the Duke of York’s, was
densely peopled, and, owing apparently to the scant supply of
fish and vegetables thus occasioned, many of the natives were
afflicted with leprosy, and also had bad teeth, both circum-
stances unusual for the Pacific. Lean in body and savage in
look and gesture, they strangely contrasted with their fat, jolly
kinsmen on some of the more northern islands of the same
group. An old, fat chief who came from one of these islands
to the ship’s side in his canoe was actually too large to have
reached the deck except by the use of a tackle. It was evi-
dent that infanticide—a necessity according to their system
of political economy—was more thoroughly practised than on
Drummond’s Island, and that the population was thus kept
from becoming uncomfortably numerous. The obesity was

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probably owing to their having nothing to do, and plenty,
in the vegetable way, to eat; for these equatorial islands,
somewhat elevated, as elsewhere observed, are unusually pro-
ductive for atolls,—just the place for a voluptuous barbarian.
The people on Drummond’s Island were great thieves, and
knew the pleasures of a cannibal feast. Without metals, or
any kind of hard stone, they make, out of the teeth of the
sharks caught about the reefs, a sharp, jagged edging for long
knives, swords and spears; and the women, jealous of one an-
other, sometimes, as Mr. Hale says, carry about with them for
months a small weapon of shark’s teeth concealed under their
dress, watching for an opportunity to use it; and desperate
fights sometimes take place. The same author mentions, also,
some good points in them: observing that the women are, for
the most part, better treated than is common among uncivil-
ized people; that the men do the hard out-door work, while
the women clear and weed the ground, and attend to the do-
mestic duties that naturally fall to them. ‘Custom also re-
quires that when a man meets a female he shall pay her the
same mark of respect that is rendered to a chief, by turning
aside to let her pass,’—a rule that probably does not al-
ways hold in practice. He adds: “The word manda sig-
nifies among the Gilbert Islanders a man thoroughly accom-
plished in all their knowledge and arts, and versed in every
noble exercise; a good dancer, an able warrior, one who has
seen life at home and abroad, and enjoyed its highest excite-
ments and delights—in short, a complete man of the world.
In their estimation this is the proudest character to which any
person can attain; and such a one is fully prepared to enter,
at his death, on the highest enjoyments of their elysium.”
Thus much for the human productions of coral islands.

Coral islands are exposed to earthquakes and storms like

316 CORALS AND CORAL ISLANDS.

the continents, and occasionally a devastating wave sweeps
across the land. During the heavier gales, the natives some-
times secure their houses by tying them to the cocoanut trees,
or toa stake planted for the purpose. A height of ten or
twelve feet, the elevation of their land, is easily overtopped
by the more violent seas; and great damage is sometimes
experienced. The still more extensive earthquake-waves,
such as those which have swept up the coast of Spain, Peru,
and the Sandwich Islands, would produce a complete deluge
over these islands. We were informed by both Grey and
Kirby that effects of this kind had been experienced at the
Gilbert Islands; but the statements were too indefinite to
determine whether the results should be attributed to storms,
or to this more violent cause.

But while coral islands have their storms, the region in
their vicinity is generally one of light winds and calms, even
when the trades are blowing strongly all around them. The
heated air which rises from the islands lifts the currents to a
considerable height above the island. J.D. Hague mentions
that on Jarvis’ and the two neighboring islands, under the
equator, near 180° in longitude from Greenwich, he “ often
observed the remarkable phenomenon of a rain squall ap-
proaching the island, and, just before reaching it, separat-
ing into two parts, one of which passed by on the north, the
other on the south side, the cloud having been cleft by the
column of heated air rising from the white coral sands.”

An occasional log drifts to the shores, and at some of the
more isolated atolls, where the natives are ignorant of any
land but the spot they inhabit, they are deemed direct gifts
from a propitiated deity. These drift-logs were noticed by
Kotzebue, at the Marshall Islands, and he remarked also that
they often brought stones in their roots. Similar facts have

THE COMPLETED ATOLL. 317

been observed at the Gilbert Group, and also at Enderbury’s
Island, and many other coral islands in the Pacific. The
stones at the Gilbert Islands, as far as could be learned, are
generally basaltic or volcanic, and they are highly valued
among the natives for whetstones, pestles, and hatchets. The
logs are claimed by the chiefs for canoes. Some of the logs
seen by the author, like those of Enderbury’s Island, were
forty feet or more long. Several large masses of compact
cellular lava occur on Rose Island, a few degrees east of the
Navigator Group; they were lying two hundred yards inside
of the line of breakers. The island is uninhabited, and the
origin of the stones is doubtful; they may have been brought
there by roots of trees, or perhaps by some canoe.

Fragments of pumice and resin are transported by the
waves to many of the islands in the Central Pacific. We
were informed at the Gilbert Islands that the pumice was
gathered from the shores by women and pounded up to fer-
tilize the soil of their taro patches; and that it is common
for a woman to pick up a peck a day.

Where this pumice comes from is not ascertained. It is ‘
probably drifted from the westward, and perhaps from vol-
canic islands of the Ladrones or Phillipines. In addition,
volcanic ashes are sometimes distributed over these islands,
through the atmosphere. In this manner the soil of the
Tonga Islands has been improved, and it may thus have de-
rived its reddish color. This group has its own active vol-
cano to supply the ashes, and the volcanic group of the
New Hebrides is not far distant to the southwest.

The mineralogy of an atoll is usually confined to one
species, — calcite, or carbonate of lime,— the material of
the coral rock. On some of the smaller islands, in the drier
equatorial part of the ocean, there are, in addition to this

318 CORALS AND CORAL ISLANDS.

and the stones brought by logs with the floating pumice, beds
of gypsum which have been made through the evaporation of
sea-water (which holds it in solution) in the gradually drying
lagoon-basins; and also large deposits of guano from the
multitudes of sea birds that occupy them. Such are Jarvis,
Baker’s, Howland’s, Malden’s, McKean’s, Birnie’s, Starbuck’s,
Enderbury’s, and probably other islands in the dry central
Pacific (Plate IX. and p. 168). As these deposits are con-
nected with the completion of the coral island and its accom-
panying reduction in size, and illustrate one of the ways by
which new minerals are added to a destitute land, a few facts
are here cited from an article in the “ American Journal of
Science,” volume xxxiv. (1862), by J. D. Hague, who resided
for several months on the islands he describes.

Baker’s Island is situated in lat. 0° 13’ north, and long.
176° 22’ west from Greenwich, and, excepting Howland’s
Island, forty miles distant, is very remote from any other
land. It is about one mile long and two thirds of a mile
wide. The surface is nearly level; the highest point is
twenty-two feet above the level of the sea, showing some
evidence of elevation.

Above the crown of the beach there is a sandy ridge
which encircles the guano deposit. This marginal ridge is
about one hundred feet wide on the lee side of the island,
and is there composed of fine sand and small fragments of
corals and shells, mixed with considerable guano; on the
eastern, or windward, side it is much wider, and formed of
coarser fragments of corals and shells, which, in their arrange-
ment, present the appearance of successive beach formations.
Encircled by this ridge lies the guano deposit, occupying the
central and greater part of the island. The surface of this
deposit is nearly even, but the hard coral bottom which forms

THE COMPLETED ATOLL. 319

its bed has a gradual slope from the borders toward the cen-
tre, or, perhaps more properly, from northwest to southeast,
giving the guano a variable depth from six inches at the
edges to several feet.

Howland’s Island is situated in lat. 0° 51’ north, and long.
176° 32’ west from Greenwich. It is about a mile and a half
long by half a mile wide, containing, above the crown of the
beach, an area of some four hundred acres. The highest
point is seventeen feet above the reef, and ten or twelve feet
above the level of high tide. The general features of the
island resemble those of Baker’s.

The main deposit of guano occupies the middle part of
the island, and stretches, with some interruptions of inter-
vening sand, nearly from the north to the south end. Its
surface is even, and in many places covered by a thick
growth of purslane, whose thread-like roots abound in the
guano where it grows. The deposit rests on a hard coral
bottom, and varies in depth from six inches to four feet.
The fact observed at Baker’s that vegetation flourishes most
where the guano is shallow, is also quite apparent here, and
the consequent characteristic difference between the 2uano
of the deep and shallow parts is distinctly marked.

Some interesting pseudomorphs occur buried in the guano
of this island. Coral fragments of various species were found
that had long been covered up under the deposit, and in some
of which the carbonic acid had been almost entirely replaced
by phosphoric acid. In such I have found seventy per cent
of phosphate of lime. In many others the change was only
partial, and, on breaking some of these, in the centre was
usually found a nucleus or core of coral.

Jarvis Island is situated in lat. 0° 22’ south, and long.
159° 58’ west from Greenwich. It isnearly two miles long by

320 CORALS AND CORAL ISLANDS.

one mile wide, and contains about 1,000 acres. Like Baker’s
and Howland’s, it has the general features of a coral island,
but it differs from them essentially in the fact that the
lagoon, which it once contained, has gradually been filled up
with sand and detritus, while the whole island has undergone
some elevation. “It therefore presents a basin-like form, the
surface being depressed from the outer edge toward the cen-
tre. It is encircled by a fringing reef, or shore platform,
about three hundred feet wide; from this a gradually sloping
beach recedes, the crown of which is from eighteen to twenty-
eight feet high, forming a ridge or border, of varying width,
which surrounds the island like a wall, from the im-shore
edge of which the surface of the island is gently depressed.

Within this depression there are other ridges, parallel
to the outer one, and old beach lines and water marks, the
remaining traces of the waters of the lagoon, marking its
gradual decrease and final disappearance.

This flat depressed surface in the centre of the island is
about seven or eight feet above the level of the sea. It bears
but little vegetation, consisting of long, coarse grass, Mesem-
bryanthemum, and Portulaca, and this is near the outer
edges of the island, where the surface is formed of coral sand,
mixed with more or less gtano. In the central and lower
parts the surface is composed of sulphate of lime (gypsum),
and it is on this foundation that the principal deposit of
guano rests.

In examining the foundation of the guano deposit on
Baker's or Howland’s Island by sinking a shaft vertically,
the hard conglomerate reef-rock is found directly underlying
the guano. Resting on this foundation the guano has under-
gone only such changes as the climate has produced. On

Jarvis Island, however, after sinking through the guano,

THE COMPLETED ATOLL. 321

one first meets with a stratum of sulphate of lime (sometimes
compact and crystalline, sometimes soft and amorphous) fre-
quently two feet thick, beneath which are successive strata
of coral sand and shells, deposited one above the other in the
gradual process by which the lagoon was filled up. These
horizontal strata were penetrated to a depth of about twenty
feet. They were composed chiefly of fine and coarse sand
with an occasional stratum of coral fragments and shells.

Of the origin of this sulphate of lime there can hardly be |
any doubt. As the lagoon was nearly filled up, while, by the
gradual elevation of the island, the communication between
the outer ocean and the inner lake was constantly becoming
less easy, large quantities of sea-water must have been evap-
orated in the basm. By this means deposits would be formed
containing common salt, gypsum, and other salts found in the
waters of the ocean. From these the more soluble parts would
gradually be washed out again by the occasional rains, leaving
the less soluble sulphate of lime as we find it here.

Some additional light is thrown on this matter by the dif-
ferent parts of the surface, which, though nearly flat, shows
some slight variety of level. The higher parts, particularly
around the outer edges, are composed chiefly of coral sand,
either mixed with or underlying guano. Nearer the centre ©
is a large tract, rather more depressed, forming a shallow
basin, in which the bulk of the sea-water must have been
evaporated, the surface of which (now partly covered with
guano) is a bed of sulphate of lime, while, further, there 1s
a still lower point, the least elevated of the whole, where the
lagoon waters were, without doubt, most recently concen-
trated. This latter locality is a crescent-shaped bed, about
six hundred feet long by two hundred or three hundred feet

wide, having a surface very slightly depressed from the outer
21

Say CORALS AND CORAL ISLANDS.

edge toward the middle. Around the borders are incrusta-
tions of crystallized gypsum and common salt, ripple-marks,
and similar evidences of the gradually disappearing lake.
The whole is composed of a crystalline deposit of sulphate
of lime, which, around the borders, as already observed, is
mixed with some common salt, while near the centre, where
rain water sometimes collects after a heavy shower, the salt
is almost entirely washed out, leaving the gypsum by itself.
It is closely, but not hard, packed, and is still very wet.- By
digging eighteen to twenty-four inches down, salt water may
generally be found.

These facts help us to understand the varying conditions
in which we now find the guano beds. The most important
part, and that from which the importations have thus far
come, rests on a bed of sulphate of lime, of an earlier but
similar origin to that just described above; part rests on a
coral formation; while still another part, covering a large
tract, has been by the action of water mixed with coral mud.

The first named deposit, lying on the sulphate of lime
bed, has a peculiar character. It is covered by, or consists
of, a hard crust, that is from one-fourth of an inch to an
inch and a half in thickness, beneath which hes a stratum of
~ guano, varying in depth from one inch to a foot. In many
places where the guano was originally shallow, the whole is
taken up and formed into the hard crust which then lies im-
mediately on the sulphate. This crust, when pure, is snow-
white, with an appearance somewhat resembling porcelain,
but is usually colored more or less by organic matter. Gen-
erally it is very hard, and strongly cohesive, though some-
times friable, and it lies unevenly on the surface in rough
fragments that are warped and curved by the heat of the
sun. It consists chiefly of phosphoric acid and lime, but,

THE COMPLETED ATOLL. 323

owing to the variable amount of sulphate of lime with which
it is mechanically mixed, there is a lack of uniformity in
different samples. Hence the percentage of phosphoric acid
varies from over fifty per cent to less than thirty per cent.

The gypsum, or sulphate of lime, is usually soft and amor-
phous, sometimes crystalline, and, at a depth of eighteen
inches to two feet, occurs in hard, compact, crystalline beds.
It is of a light snuff color, and where it underlies guano, is
mixed with considerable phosphate of lime, which has been
washed down from the surface. Similar deposits of sul-
phate of lime occur on many other elevated lagoon islands
of the Pacific.

Starbuck’s, Starve, or Hero Island is an elevated atoll, and
is worthy of mention, because like Jarvis, McKean’s, and
other islands of similar structure, it contains a large deposit
of gypsum. Its supposed guano I have found to consist of
the hydrated sulphate of lime, containing about twelve per
cent of phosphate of lime, and colored by a little organic
matter. So far as my observation extends, all elevated
lagoons have similar deposits of gypsum. ae
PAs regards the distribution of these phosphatic guano
deposits, I believe them, in this region of the Pacific, to be
confined to latitudes very near the equator, where rain is
comparatively of rare occurrence. In latitudes more remote
from the equator than 4° or 5°, heavy rains are frequent, and
this circumstance is not only directly unfavorable to the for-
mation of guano deposits, but it encourages vegetation, and
when an island is covered with trees and bushes, the birds
prefer to roost in them, and there is no opportunity for the
accumulation of guano deposits.

An article in the same Journal (vol. xl., 1865), by A. A.
Julien, gives an account of the various phosphatic minerals

324 CORALS AND CORAL ISLANDS.

formed from the guano deposits on a coral island, Sombrero,
in the Caribbean Sea.

Lord Byron, of the ‘ Blonde,” mentions that phosphate
of lime (apatite) was collected by him on Mauke, an elevated
coral island of the Hervey Group, west. of the Society Islands,
but its exact condition in the rock is not stated.

Water is to be found commonly in sufficient quantities for
the use of the natives, although the land is so low and flat.
They dig wells five to ten feet deep in any part of the dry
islets, and generally obtain a constant supply. These wells
are sometimes fenced around with special care; and the
houses of the villagers, as at Fakaafo, are often clustered
about them. On Aratica (Carlshoff) there is a. watering
place fifty feet in diameter, from which vessels of the Wilkes
Exploring Expedition obtained three hundred and ninety gal-
lons. The Gilbert Islands are generally provided with a sup-
ply sufficient for bathing, and each native takes his morning
bath in fresh water, which is esteemed by them a great lux-
ury. On Tari-tari (of the Gilbert Group, p. 165), as Mr.
Horatio Hale, philologist of the same ‘expedition, was in-
formed by a Scotch sailor by the name of Grey, taken from
the island, there is a trench or canal several miles long, and
two feet deep. They have taro plantations (which is possible
only where there is a large supply of water), and besides some
bread-fruit. He spoke of the taro as growing to a very large
size, and as being in great abundance; it was planted along
each side of the pond. Grey added further that ten ships of
the line might water there, though the place was not reached
without some difficulty. There were fish in the pond which
had been put in while young. The bottom was adhesive like
clay. These islands have been elevated a little, but are not

over fifteen feet above the sea.

THE COMPLETED ATOLL. 325

Kotzebue observes, that “in the inner part of Otdia (one
of the Marshall Islands) there is a lake of sweet water; and
in Tabual, of the Group Aur, a marshy ground exists. There
is no want of fresh water in the larger islands; it rises in
abundance in the pits dug for the purpose.” (Voyage, London,
1821, i. 145.)

The only source of this water is the rain, which, perco-
lating through the loose sands, settles upon the hardened
coral rock that forms the basis of the island. As the soil is
white or nearly so, it receives heat but slowly, and there is
consequently but little evaporation of the water that is once
absorbed.

Water is sometimes obtained by making a large cavity in
the body of a cocoanut tree, two feet or so from the ground.
At the Duke of York’s Island, and probably also at the adja-
cent Bowditch Island, this method is put in practice; the
cavities hold five or six gallons of water.

The extensive reefs about coral islands, as already stated,
abound in fish, which are easily captured, and the natives,
with wooden hooks, often bring in larger kinds from the
deep waters. From such resources a population of seven
thousand persons is supported on the single island of Tapu-
teuea, whose whole habitable area does not exceed six square
miles,

There are also shell-fish of edible kinds, and others that
are the source of considerable activity in pearl-fishing.

Although the vegetation of coral islands has the luxuri-
ance that characterizes more favored tropical lands, the num-
ber of species of land plants is small. A work on the
“Botany of the Paumotus” would contain descriptions of
only twenty-eight or thirty species. The most common kinds

are the following : —

326 CORALS AND CORAL ISLANDS.

Portulaca oleracea, L. (lutea Cassytha filiformis, Z.

of Solander). Gouldia Romanzoffiensis, A. Gr.
Triumfetta procumbens, Forst. Euphorbia Chamissonis, Boiss.
Tournefortia argentea, L. Boerhavia diffusa, L.
Scevola Konigii, Vahl. Boerhavia hirsuta, Wild.
Ipomea longiflora, R. Br. Achyranthes canescens, &. Br.
Lepidium piscidium, Forst. Heliotropium prostratum, &. Br.
Pemphis acidula, Lorst. Nesogenes euphrasioides, A. DC.
Pandanus odoratissimus, L. f. Asplenium Nidus, Z.
Pisonia grandis, Parkinson. A polypodium, and a species of
Morinda citrifolia, L. grass.

Guettarda speciosa, L.

Still, there is a better supply than might be supposed.
For the cocoanut, in view of its uses, is a dozen trees in
one. Its trunk furnishes timber for the houses of the natives,
and the best of wood, on account of its weight and strength,
for clubs and spears, — weapons much in use, besides serving
as ornamental side-arms. Its leaves supply material for
thatching; for coarse matting to sit on, and beautiful fine
mats for use in the way of occasional dress; also for the
short aprons or petticoats of the women, above alluded to.
The fruit, besides its delicately flavored hollow kernel, af-
fords, by the grating of this kernel, a milky juice that is
richer than cream for purposes of native cookery, and which
we explorers often used with satisfaction in coffee, cows be-
ing unknown in those regions; also, from each nut, a pint
of the thinner “cocoanut milk,’ a more agreeable drink in
the land of cocoanuts than in New York; also an abundant
oil, much valued for. sleeking down their naked bodies, and
sometimes offered to a friendly visitor whom they would
honor with a like anointing. Further, from the young fruit,
three fourths grown, comes a delightful beverage as brisk
nearly as soda-water, besides a rich creamy pulp; both of
these far better than the corresponding products of the ripe |

THE COMPLETED ATOLL. aan

fruit. The husk is excellent for cordage, twine, thread, fish-
ing-lines; and the smaller cord serves in place of nails for
securing together the beams of their domestic and public
buildings, and also for ornamenting the structure within, the
cord being often wound with much taste and diversity of fig-
ures. The nut, when opened, is a ready-made drinking-cup
or cooking utensil. Finally, the developing bud, before
blossoming, yields a large supply of sweet juice, from which
molasses is sometimes made, and then, by fermentation, a
spirituous liquor, called among the Gilbert Islanders by a
name that sounded very much lke toddy, and possessing
qualities that answer to the name; but this is procured at
the expense of the fruit, and the good of the tree, and also
of the best interests of the natives.

It is doubted whether the ocean is ever successful in plant-
ing the cocoanut on coral islands. The nut seems to be well
fitted for marine transportation, through its thick husk, which
serves both as a float and a protection; but there is no known
evidence that an island never inhabited has been found sup-
plied with cocoanut trees. The possibility of a successful
planting by the waves cannot be denied; but there are so
many chances that the floating nut will be kept too long in
the water, or be thrown where it cannot germinate, that the
probability of a transplanting is exceedingly small. This
palm — the Cocos nucifera of the botanists —is not included
in the above list of native Coral Island plants.

Another tree, peculiarly fitted for the region, is the Pan-
danus or Screw-Pine—well named as far as the syllable screw
goes, but having nothing of a pine in its habit. Its long
sword-like leaves, of the shape and size of those of a large
Iris, are set spirally on the few awkward branches toward
the extremity of each, and make a tree strikingly tropical in

328 CORALS AND CORAL ISLANDS.

character. It grows sometimes to a height of thirty feet. It
is well fitted for the poor and shallow soil of a coral island ;
for, as it enlarges and spreads its branches, one prop after
another grows out from the trunk and plants itself in the
ground ; and by this means its base is widened and the grow-
ing tree supported. The fruit, a large ovoidal mass made up
of oblong dry seed diverging from a centre, each near two
cubic inches in size, affords a sweetish husky article of food,
which, though little better than prepared corn stalks, admits
of being stored away for use when other things fail; and at
the Gilbert Islands and others in that part of the ocean it is
so employed.

The Pisonia is another of the forest trees, and one of
handsome foliage and large beautiful flowers, sometimes
attaining a height of forty feet, and the trunk a girt of
twenty feet. |

Among the species that are earliest in taking root in the
emerging coral debris over the reef, there are the Portulacee
(species of purslane); the Zriwmphetta procumbens, a creep-
ing, yellow-flowering plant of the Tilia family; the Tourne-
fortia sericea, a low, hoary shrub of the family Borraginacee,
and Scevola Konigii, a sub-fleshy seashore plant.

On Rose Island, just east of the Navigator Group, Dr. C.
Pickering, of the Wilkes Exploring Expedition, found only a
species of Pisonia and of Portulaca. This is a small atoll,
under water at high-tide, excepting two banks, one of which
is covered with trees.

In the Marshall Group, on the contrary, where the vege-
tation is more varied, and the islands have probably under-
gone some elevation since they were made, Chamisso observed
fifty-two species of land plants, and in a few instances the
banana, taro, and bread-fruit were cultivated. At the ele-

THE COMPLETED ATOLL. 399

vated coral island, Metia, north of Tahiti (p. 193), 250 feet
above the sea, sugar-cane and bread-fruit and many plants
of the Society Group occur.

The tropical birds of the islands are often more in keep-
ing with the beautiful scenery about them than the savage
inhabitants. On one atoll, — Honden Island, of the Paumo-
tus, — where no natives had ever dwelt, the birds were so
innocent of fear, that we took them from the trees as we
would fruit ; and many a songster lost a tail feather, as it sat
perched on a branch, apparently unconscious that the world
contained an enemy. Our ornithologist went ashore with
powder and shot. But the sportsman could find no pleasure
in shooting; indeed he could help himself without.

During a ramble over the island I came across a noble
bird as white as snow and nearly as large as an albatross.
In my zeal for science I began to contemplate it as a very
fine specimen — indeed, a magnificent specimen; and although
it was not my special line of research, it seemed a failure of
duty to neglect the opportunity to secure it. By a simple
process, the work of death is easily accomplished. I went
up to him. He stood still, not offering to fly. I commenced
to carry out my plan. A slight point of blood soiled the
white plumage, and my zeal gave out. It was another's part
to serve as executioner, not mine; and stroking down his
feathers and wishing him well, I hurried away. But as I
glanced back from time to time, on my retreat, there the
bird stood, his eye still fixed upon me, and that reproachful
look followed me until a far-off grove came in between us.
I take it the bird recovered, as I shared not the fate of the
“Ancient Mariner.”

Ever since, the words of the old Mariner have seemed

to rise in melody from that island Paradise : —

1S)
sy)
oS

CORALS AND CORAL ISLANDS.

“ He prayeth well who loveth well
Both man and bird and beast.
He-prayeth best, who loveth best

All things both great and small ;
For the dear God who loveth us
He made and loveth all.”

Mr. J. D. Hague gives an account of the birds of Jarvis
and some other uninhabited islands in the equatorial Pacific,
in which it appears that, after all, there are evil-doers even
among tropical birds. He gives the following facts : —

“From fifteen to twenty varieties of birds may be distin-
guished among those frequenting the islands, of which the
principal are gannets and boobies, frigate birds, tropic
birds, tern, noddies, petrels, and some game birds, as the
curlew, snipe, and plover. Of Terns there are several spe-
cies, the most numerously represented of which is what I be-
lieve to be the Sterna hirundo. These frequent the island
twice in the year for the purpose of breeding. They rest on
the ground, making no nests, but selecting tufts of grass,
where such may be found, under which to lay their eggs. I
have seen acres of ground thus thickly covered by these birds,
whose numbers might be told by millions. Between the
breeding seasons they diminish considerably in numbers,
though they never entirely desert the island. They are ex-
pert fishers and venture far out to sea in quest of prey. The
noddies (Sterna stolida) are also very numerous. They are
black birds, somewhat larger than pigeons, with much longer
wings, and are very simple and stupid. They burrow holes
in the guano, in which they live and raise their young, gen-
erally inhabiting that part of the deposit which is shallow-
est and driest. Their numbers seem to be about the same
throughout the year. The gannet and booby, two closely
allied species (of the genus Sula), are represented by two or

THE COMPLETED ATOLL. aol

three varieties. They are large birds, and great devourers of
fish, which they take very expertly, not only catching those
that leap out of the water, but diving beneath the surface for
them. They are very awkward and unwieldy on land, and
_may be easily overtaken and captured, if indeed they attempt
to escape at all on the approach of man. They rest on the
trees wherever there is opportunity, but in these islands they
collect in great groups on the ground, where they lay their
eggs and raise their young. One variety, not very numerous,
has the habit of building up a pile of twigs and sticks, twenty
or thirty inches in height, particularly on Howlands, where
more material of that sort is at hand, on which they make
their nest. When frightened, these birds disgorge the con-
tents of their stomachs, the capacity of which is sometimes
very astonishing. They are gross feeders, and I have often
seen one disgorge three or four large flying-fish fifteen or
eighteen inches in length.

“The frigate bird (Zachypetes aquilus) I have already
alluded to. It is a large, rapacious bird, the tyrant of the
feathered community. It lives almost entirely by piracy,
forcing other birds to contribute to its support. These frig-
ate birds hover over the island, constantly lying in wait for
fishing birds returning from the sea, to whom they give chase,
and the pursued bird escapes only by disgorging its prey,
which the pursuer very adroitly catches in the air. They
also prey upon flying fish and others that leap from sea to sea,
but never dive for fish and rarely even approach the water.

“The above are the kinds of birds most numerously repre-
sented, and to which we owe the existing deposits of guano.
Besides these are the tropic birds, which are found in consid-
erable numbers on Howland’s Island, but seldom on Jarvis
or Baker’s. They prefer the former because there are large

Bye CORALS AND CORAL ISLANDS.

blocks or fragments of beach rock scattered over the island’s
surface, under which they burrow out nests for themselves.
A service is sometimes required of this bird, which may, per-
haps, be worthy of notice. A setting bird was taken from
her nest and carried to sea by a vessel just leaving the island.
On the second day, at sea, a rag, on which was written a mes-
sage, was attached to the bird’s feet, who returned to the nest,
bringing with it the intelligence of the departed vessel. This
experiment succeeded so well that, subsequently, these birds
were carried from Howland’s to Baker’s Island (forty miles
distant), and on being liberated there, one after the other, as
occasion demanded, brought back messages, proving them-
selves useful in the absence of other means of communica-
tion. The game birds, snipe, plover, and curlew, frequent
the islands in the fall and winter, but I never found any evi-
dence of their breeding there. They do not leave the island
in quest of prey, but may be seen at low-tide picking up their
food on the reef which is then almost dry.

“Some of the social habits of these birds are worthy of
remark. The gannets and boobies usually crowd together in
a very exclusive manner. The frigate birds likewise keep
themselves distinct from other kinds. The tern appropriate
to themselves a certain portion of the island; each family
collects in its accustomed roosting-place, but all in peace and
harmony. The feud between the fishing birds and their
oppressors, the frigate birds, is active only in the air; if the
gannet or booby can but reach the land and plant its feet on
the ground, the pursuer gives up the chase immediately.”

Notwithstanding the products and the attractions of a
coral island, it is in its best condition but a miserable place
for human development, physical, mental, or moral. There

THE COMPLETED ATOLL. ja5

is poetry in every feature, but the natives find this a poor
substitute for the bread-fruit and yams of more favored lands.
The cocoanut and pandanus are, in general, the only products
of the vegetable kingdom afforded for their sustenance, and
fish, shellfish, and crabs from the reefs their only animal
food. Scanty too is the supply; and infanticide is resorted
to in self-defence, where but a few years would otherwise
overstock the half a dozen square miles of which their little
world consists — a world without rivers, without hills, in the
midst of salt water, with the most elevated point but ten to
twenty feet above high tide, and no part more than three
hundred yards from the ocean.

In the more isolated coral islands, the language of the
natives indicates their poverty, as well as the limited produc-
tions and unvarying features of the land. All words like
those for mountain, hill, river, and many of the implements
of their ancestors, as well as the trees and other vegetation
of the land from which they are derived, are lost to them ;
and as words are but signs for ideas, they have fallen off in
general intelligence. To what extent a race of men placed
in such circumstances is capable of mental improvement,
would be an interesting inquiry for the philosopher. Perhaps
the query might be best answered by another, How many
of the various arts of civilized life could exist in a land
where shells are the only cutting instruments, — the plants
of the land in all but twenty-nine in number, — minerals but
one, — quadrupeds none, with the exception of foreign rats or
mice, — fresh water barely enough for household purposes, —
no streams, nor mountains, nor hills? How much of the
poetry or literature of Europe would be intelligible to per-
sons whose ideas had expanded only to the limits of a coral
island; who had never conceived of a surface of land above

aod CORALS AND CORAL ISLANDS.

half a mile in breadth, — of a slope higher than a beach, —
of a change of seasons beyond a variation in the prevalence
of rains? What elevation in morals should be expected upon
a contracted islet, so readily over-peopled that threatened
starvation drives to infanticide, and tends to cultivate the
extremest selfishness? Assuredly there is not a more un-
favorable spot for moral or intellectual progress in the wide
world than the coral island. |

Still, if well supplied with foreign stores, including a good
stock of ice, they might become, were they more accessible, a
pleasant temporary resort for tired workers from civilized
lands, who wish quiet, perpetual summer air, salt-water bath-
ing, and boating or yachting; and especially for those who
could draw inspiration from the mingled beauties of grove,
lake, ocean, and coral meads and grottoes, where

“ Life in rare and beautiful forms
Is sporting amid the bowers of stone.”

Yet after all, the dry land of an atoll is so limited, its
features so tame, its supply of fresh water so small, and of
salt water so large, that whoever should build his cottage on
one of them would probably be glad, after a short experience,
to transfer it to an island of larger dimensions, like Tahiti or
Upolu, one more varied in surface and productions; that has
its mountains and precipices, its gorges and open valleys, leap-
ing torrents not less than surging billows, and forests spread-
ing up the declivities, as well as groves of palms and corals
by the shores.

GEOGRAPHICAL DISTRIBUTION. ooo

CHAPTER IV.

GEOGRAPHICAL DISTRIBUTION OF CORAL REEFS AND
ISLANDS.

THE distribution of coral reefs over the globe depends on
the following circumstances, arising from the habitudes of
polyps already explained.

1. The temperature of the ocean.

2. The character of coasts as regards (a) the depth of
water; (b) the nature of the shores; (c) the presence of
fresh-water streams ; (d) the direction and force of currents.

3. Liability to exposure to destructive agents, such as
volcanic heat.

It has been stated (p. 108) that reef-growing corals will
flourish in the hottest seas of the equator, and wherever the
average temperature of the waters of the ocean for the winter
is not below 68° F. The isothermal line of this temperature
(or isocryme) forms, therefore, the boundary line of the coral-
reef seas. Other corals not forming reefs grow in colder seas
(p. 109) and at great depths, but to those we do not now refer.

This line traverses the oceans between the parallels 26°
and 30°, or in general near 28°. But, as has been stated,
it undergoes in the vicinity of the continents remarkable
flexures from the influence of oceanic currents, the polar cur-
rents bending it toward the equator, while the tropical cause
a divergence. From a comparison of the thermometrical

336 CORALS AND CORAL ISLANDS.

observations of various voyages with those of the Expedition,
the author has been enabled to draw these boundary lines
with a considerable degree of accuracy, and they are laid
down on the chart, Plate XVI., from the author’s Exploring
Expedition Report on Crustacea.

In the Pacific Ocean, this coral boundary, or isocryme of
68°, passes near the Galapagos, but to the south of them,
and thus approaches closely if it does not cross the equator,
instead of being near the parallel of 28° south, its position
in mid-ocean. Captain Fitzroy, R.N., found the surface tem-

. perature of the sea at the Galapagos, from Sept. 16 to Oct.
2» 18, 1855, 62° to 70° F., while, under the equator, about the

middle of the Pacific, the range of surface temperature of the
sea through the year is 81° to 88° F.

Change of level would make great changes in the boun-
dary. Mr. A. Agassiz found modern coral limestone near
Tilibiche, Peru, at an elevation of two to three thousand feet ;
showing that, in the early part of the coral-reef era, the con-
tinent on the Pacific side stood this much lower than now,
and that then corals were growing on part of its Pacific
border.

On the side of Asia the boundary line bends far south-
ward, and reaches the coast of Cochin China within 15° cf
the equator, although 50° from the equator a little to the
eastward. On the west side of the Atlantic, the northern line
starts at Cape Florida, in latitude 15° N., stretches abruptly
northward, and bends around the Bermudas in latitude 33° N.
On the African coast opposite, the northern line curves down-
ward to the latitude of the Cape Verdes, and the southern
upward nearly to the equator. The following table gives the
positions of the coral boundary lines where they meet the
coasts of the continents.

GHOGRAPHICAL DISTRIBUTION. 3an

Pacific Ocean. Atlantic Ocean,
East side of ocean—Northern, Lat. 21° N Thats 7102" Ni.
Southern. 4° 8, SERS.
West side of ocean—Northern. 15° N. 26° N.

Southern.

30° S., N. Holland.
29° S., Africa.

29° §,

It follows from the above, that while the coral-reef seas
are about fifty-six degrees wide in mid-ocean, they are

In the Pacific twenty-five degrees wide on the west coast
of America, and forty-five degrees on the Asiatic side.

In the Atlantic about fifteen degrees wide on the African
coast, and forty-eight degrees on the coast of America.

If we reckon to the extremity of the bend in the Gulf
Stream, the whole width of the coral reef sea off the east
coast of America, will be over sixty-four degrees; while off
the west coast of America, the width is hardly eighteen degrees.
It is obvious that these facts enable us to explain many seem-
ing anomalies in the distribution of coral reefs.

The other causes which influence the distribution of reefs,
operate under this more general one of oceanic temperature,
that is, within the coral-reef boundary lines. ‘The effect of a
deep abrupt coast on the distribution of reefs has been pointed
out (p. 114). The unfavorable character of sandy or muddy
shores, and the action of detritus, marine currents, and fresh
waters have also been stated (p. 119), and it is not necessary
to touch again upon these points.

Not less striking are the effects of volcanic action in pre-
venting the formation of reefs; and instances of this influ-
ence are numerous throughout the Pacific. The existence of
narrow reefs, or their entire absence, may often be thus ac:
counted for. For example, in the Hawauan Group, the island
Hawaii, still active with volcanic fires, has but few patches
of reef about it, while the westernmost islands, which have

99

338 CORALS AND CORAL ISLANDS.

been longest free from such action, have reefs of considerable
extent. The island of Maui exemplifies well the same gen-
eral fact. The island consists of two peninsulas: one the
eastern, recent volcanic in character, with a large crater at
summit, and the other, the western, presenting every evidence,
in its gorges and peaks and absence of volcanic cones, of hav-
ing become extinct ages since. In conformity with the view
expressed, the coral reefs are confined almost exclusively to the
latter peninsula. Other examples are afforded by the Samoan
or Navigator Islands. Savaii abounds in extinct craters and
lava streams, and much resembles Hawaii in character; it
bears proof in every part of being the last seat of the vol-
canic fires of the Samoan Group. Its reefs are consequently
few and small: there is but a narrow line on part of the
northern shores, although on the other islands they are very
extensive.

This absence of corals results obviously from the destruc-
tion of the zodphytes by heat, consequent on volcanic action.
Submarine eruptions, which are frequent as long as a vol-
cano near the sea is ijn action, heat the waters and destroy
whatever of life they may contain. After the eruption of Ki-
lauea, in 1840, there were numerous dead fish thrown on the
beach; and many such instances in different regions are on
record.

The agencies affecting the growth of coral reefs being be-
fore the mind, we may proceed to notice the actual distribu-
tion of reefs through the coral seas. The review given is a
rapid one, as our present object is simply to explain the
absence or presence of reefs within the coral reef’ limits, by,
reference to the above facts.

In the valuable work by Mr. Darwin, the geographical
distribution of reefs is treated of at length. The facts here

GHOGRAPHICAL DISTRIBUTION. 339

detailed have been obtained from independent sources, except
where otherwise acknowledged. In accounting for the char-
acter and distribution of reefs, Mr. Darwin appears to attrib-
ute too much weight to a supposed difference in the change
of level in different regions, neglecting to allow the requis- ,
ite limiting influence to volcanic agency, and to the other
causes mentioned. His conclusion that the areas of active
volcanos in general, are areas of elevation, and not of subsi-
dence, and the inference that reefs are absent from the shores
of islands of recent volcanic action on this account, do not
accord with the facts above stated: for example, the condition
of Maui, that it has no reefs on the larger half, that of the
volcanic cone of recent action, but has them on the other
half whose fires were long since extinct; for it is not prob-
able that one end has been undergoing elevation, and the
other subsidence.

Pacific Ocean.— The west coast of South America is
known to be without coral reefs even immediately beneath
the equator ; but the seas of the Galapagos grow some corals.
The northward deflection of the coral boundary line accounts,
as has been shown, for their absence. In the Bay of Panama,
and elsewhere on the coast, north and south, corals occur in
patches but no reefs, and this is attributed by Verrill to the
rough tides of ten to twelve feet. Corals are living at La
Paz, on the Peninsula of California (p. 112).

In Captain Colnett’s voyage, allusion is made to a beach
of coral sand on one of the Revillagigedo Islands, in latitude
18°, and Clipperton Rock is described as an elevated coral
island.

Between the South American coast and the Paumotus are
two rocky islands, Easter or Waihu and Sala-y-Gomez, both
of which are stated to be without reefs.

340 CORALS AND CORAL ISLANDS.

Captain Beechey mentions, however, that at forty-one fath-
oms, near Sala-y-Gomez, he found a bottom of sand and
coral.

The Paumotus commence in longitude 130° W., and em
brace eighty coral islands, all of which, excepting about eight
of small size, contain lagoons. Besides these, there are, near
the southern limits of the archipelago, the Gambier Islands
and Pitcairn, of volcanic or basaltic constitution. The for-
mer in 23° §., have extensive reefs. About the latter, in 25°
S., there are some growing corals, but no proper reefs.

The Marquesas, in latitude 10° S., have but little coral
about them; and this is the more remarkable, since they are
in close proximity to the Paumotus. But their shores are
mostly very abrupt, with deep waters close to the rocks. An
island which, before subsidence has commenced, has some ex-
tent of shallow waters around, might have very bold shores
after it had half sunk beneath the waves. This would be the
case with the island of Tahiti; for its mountain declivities are
in general, singularly precipitous, except at base. The Mar-
quesas may, therefore, have once had barrier reefs, which were
sunk from too rapid subsidence; and afterward, on the ces-
sation of the subsidence, others failed to form again on ac-
count of the deep waters.

The Society Islands have extensive coral reefs, with dis-
tant barriers. The reefs of Tahiti extend, in some parts, a
mile from the shores. Tetuaroa, to the north of Tahiti, and
Tubuai, near Bolabola, are lagoon islands. Maitea, east of
Tahiti, is a sugar loaf truncated at summit, four miles in
compass, and is said by Forster to have an encircling reef.

South of the Society Islands, near 25° S., is Rapa, which
is represented as a collection of rugged peaks without coral
shores. The Rurutu and Hervey Islands, just northwest of

GEOGRAPHICAL DISTRIBUTION. 341

Rapa, have coral reefs fringing the shores. There is no evi-
dence of recent volcanic action among them. Some of them
are elevated coral islands, as Mitiaro, Atiu, Mangaia and
Mauki, and also, according to Stutchbury, Rurutu. Oka-
tutaia is a low coral island but six or seven feet out of
water. ‘

Between the Paumotus and the longitude of Samoa are
numerous small islands, all of coral origin.

The Samoan or Navigator Islands have extensive reefs,
About Tutuila, owing to its abrupt shores, they are somewhat
less extensive than around Upolu, and about Savaii they are
still smaller, as already explained. The influence of abrupt
shores may also be seen in some parts of Upolu; for example,
to the west of the harbor of Falifa, where, for several miles,
there is no reef, except in some indentations of the coast.
Manua is described as having only shore reefs.

The Tonga Islands, south of Samoa, for the most part
abound in coral reefs, and Tongatabu and the Hapai Group
are solely of coral. Hoa is a moderately high island, with a
narrow reef. ‘Tafoa an active volcano, and Kao, an extinct
cone, are without reefs. Vavau, according to Williams (M/s.
Enterprises, p. 427, Amer. ed.), is an elevated coral island.
Pylstaarts, near Koa, is a naked rock, with abrupt shores, and
little or no coral. Sunday Island, farther south (29° 12’S.),
is beyond the coral-reef limits.

North of Samoa are the Union group and other islands of
small size, all of coral.

/ The Feejee Group, already sufficiently described, abounds
in reefs of great extent. There are no active volcanoes,
and, where examined, no evidence of very recent volcanic
action. The many islands afford a peculiarly favorable re-
gion for the growth of zodphytes, and the displays of reefs

342 CORALS AND CORAL ISLANDS.

and hving corals were the most remarkable seen by the author
m the Pacific.

North of the Feejees are numerous islands leading up to
the Carolines. They are all of coral, excepting Rotuma,
Horne and Wallis’s Islands, which are high, and have fringing
or barrier reefs. The reefs of Wallis’s Island are very exten-
sive.

The Gilbert or Kingsmill Islands, the Marshall Islands,
and the Carolines, about eighty in number, are all atolls, ex-
cepting the three Carolines, Ponape (Pouynipete of Lutke),
Kusaie (or Ualan), and Truk (or Hogoleu). Between Ponape
and Ualan, the McAskill Islands, three in number, are of
coral, but 60 to 100 feet high (Miss. Herald, 1856, p. 193).

The westernmost of the Sandwich Islands, Kauai and
Oahu, have fringing reefs, while eastern Maui and the island
of Hawaii have but few traces of corals. On Hawaii, the
only spot of reef seen by us, was a submerged patch off the
southern cape of Hilo Bay. We have already attributed the.
absence of corals to the volcanic character of the island. The
small islands to the northwest of Kauai, are represented as
coral reefs, excepting the rocks Necker and Bird Island; the
line stretches on to 28° 30’ N., the northern limit of the coral
seas. Lisiansky’s Voyage, 1803-6, in the Neva, 4to., Lon-
don, 1814, pp. 254, 256, contains an account of some of these
islands; also the Hawaian Spectator, vol. i.; and also a Re-
port to the U. S. Bureau of Navigation, December, 1867, by
Capt. Wm. Reynolds, U. S. N., partially reproduced in the
American Journal of Science for 1871, vol. ii., p. 380.

The Ladrones, like the Hawaian Group, constitute a line
or linear series of islands, one end of which has been long
free from volcanic action, while the other has still its smok-

ing cones. The appearances of recent igneous action increase

GEOGRAPHICAL DISTRIBUTION. 343

therefore as we go northward, and the extent of the coral
reefs increase as we go southward; no reefs occur about the
northernmost islands, while they are quite extensive on the
shores of Guam. This group, consequently, like the Hawai-
an and Navigator, illustrates the influence of volcanic action
on the distribution of reefs.

A short distance southwest of the Ladrones, and nearly
in the same line, lie extensive reefs. Mackenzie’s is an atoll
of large size. Yap (or Hap), Hunter’s, Los Matelotas and
the Pelews (Palao), are high islands, with large reefs. In the
last mentioned, the reef grounds cover at least six times the
area occupied by the high land. Still farther south, toward
New Zealand, lie the large atolls Aiou, Asie and Los Guedes.

South of the equator again:—The New Hebrides consti-
tute along group of high islands, remarkable for the absence
of coral reefs of any extent, though situated between two of
the most extensive coral regions in the world,—the Feejees
and New Caledonia. But the volcanic nature of the group,
and the still active fires of two vents in its opposite extremi-
ties, are a sufficient reason for this peculiarity. ‘Tanna is one of
the largest volcanoes of the Pacific; and nearly all the islands
of the New Hebrides, as far as known, indicate comparatively
recent igneous action, in which respect they differ decidedly
from the Feejees.

The Vanikoro Group, north of the New Hebrides, accord-
ing to Quoy, has large barrier reefs about the southernmost
island, Vanikoro; but at the northern extremity of the range
there is an active volcano, Tinakoro, and no coral. Tikopia,
to the southeast of Vanikoro, is high and volcanic, according
to Quoy, though not now with active fires; and it appears from
the descriptions given, to have no reefs. Mendana, northeast
of Tinakoro, according to Kruesenstern, as stated by Darwin,

344 CORALS AND CORAL ISLANDS.

is low, with large reefs; Duff's Islands have bold summits
with wide reefs. |

New Caledonia, and the northeast coast of New Holland,
with the intermediate seas, constitute one of the grandest reef.
regions in the world. On the New Caledonia shores (p. 134),
the reefs are of great width, and occur not only along the
whole length of the western coast, a distance of 200 miles, but
extend to the south beyond the main land 50 miles, and north
150 miles, making in all a line of reef full 400 miles in length.
Toward the north extremity, however, it is interrupted or bro-
ken into detached reefs. This surprising extent is partly ex-
plained by the fact that New Caledonia is not a land of vol-
canoes; but on the contrary consists of older metamorphic
rocks. The streams of so large a land might be expected to
exclude reefs from certain parts: and in accordance with this
fact, we find the reefs of the windward or rainy side compara-
tively small, and scarcely indicated on the charts; while on
the dry or western side, they often extend thirty miles fromthe
shores. The theory of subsidence accounts fully for the great
prolongation of the New Caledonia reefs. The reefs indicate
moreover, the existence of a former land near three times the
area of the present island.

Between New Caledonia and the New Hebrides are sev-
eral high islands, one of which, Lafu, has been described
(Quart. J. Geol. Soc., 1847, p. 61) by Rev. W. B. Clarke as
an elevated coral island, with fringing reefs; it appears also
from the remarks of this writer, that the other islets of what
is called the Loyalty Group, are of the same kind. Lafu, the
largest of the number, is about ninety miles in circumference.

South of New Caledonia lies Norfolk Island, in latitude
29° S., about which there is said to be some coral, which is
occasionally thrown on the beach, but no reefs.

GHOGRAPHICAL DISTRIBUTION. 345

Between Australia and New Caledonia the islands are all
of coral. The Australian reef extending south to the east
cape, in latitude 24° S., has already been described. Such
long reefs on the shores of continents are not common. In
the case of Australia, the zodphytes are not exposed to the
destructive agents usual on continental shores, as the land has
a dry climate, the shores are mostly rocky, and there are no
streams of any extent emptying into the ocean. The east cape
is the southern limit, because here the tropical current, owing
to the direction of the coast above, trends off to the eastward
of south, away from the land, while a polar current follows up
the shores from the south as far as this cape. South of this
cape there are only a few scattered coral zodphytes.

The Louisiade Group is, as has been shown, a region of
extensive reefs.

The Solomon Islands, as far as ascertained, are but spar-
ingly fringed, except the two westernmost, which have some
large frmging and barrier reefs, and include also some atolls.
They are described by Dr. Guppy in the Transactions and
Proceedings of the Royal Society of Edinburgh, 1884-86.
North of the Solomon Islands are some large reef islands.
New Ireland, according to D’Urville, has distant reefs on
part of its shores.

The Admiralty Islands, farther west, are enclosed by bar-
rier reefs, and beyond this group there are a few lagoon
islands.

The north side of New Guinea is mostly without coral.
There are several islands off this coast, which are conical vol-
canic summits, and one of them, near New Britain, and an-
other, Vulcano, near longitude 145° E., are in action.

From the facts thus far detailed, the connection between
the prevalence or extent of reefs, and the various causes as-
signed as limiting or promoting their growth, is obvious.

346 CORALS AND CORAL ISLANDS.

The amount of subsidence determines in some cases the dis.
tance of barrier reefs from shore ; but it by no means accounts
for the difference in their extent in different parts of a single
croup of islands. Indeed, if this cause be considered alone,
every grade of extent, from no subsidence to the largest amount,
might in many instances be proved as having occurred on a
single island. Of far greater importance, as has appeared, is
the voleanic character of the land. At whatever time the ex-
isting reefs in the Pacific commenced their growth, they be-
gan about those of the igneous islands whose fires had become
nearly or quite extinct; and as others in succession were ex:
tinguished, these became in their turn, the sites of corals, and
of coral reefs. Those lands whose volcanoes still burn, are
yet without corals, or there are only limited patches on some
favored spots. Zodphytes and volcanoes are the land-making
agents of the Pacific... The latter prepare the way by pour
ing forth the liquid rock, and building up the lofty summit.

vu ‘Quiet succeeds, and then commences the work of the zoophyte

beneath the sea, while verdure covers the exposed heights. ©

“ia _ ¥%& We may add a few more illustrations from other parts of

the coral-reef seas.

Along the north and northwest coast of Australia, there
appears to be little or no coral in the Gulf of Carpentaria,
while some extensive patches occur on the shores west of this
Gulf, as far as the northwest cape in latitude 23° S.

In the East Indies, there are large, scattered reef-islands
south of Borneo and Celebes, about some of the Molluccas,
and near the west end of New Guinea. The islands of Timor-
laut, and Timor, with many of those intermediate, have large
rects. The Arru Grou» consists wholly of coral. This sea,
from Arru, to the islands south of Borneo, is more thriving in
corals than any other in the Kast Indies.

Y GHOGRAPHICAL DISTRIBUTION. 347

Another East Indian coral-reef region of some extent, is
the Sooloo Sea, between Mindanao and the north of Borneo.
Yet the reefs are mostly submerged. The author saw no wide
platforms bordering the high lands, like those of the Pacific.
There are, however, some small coral islets in the Balabac
Passage.

In other parts of the East Indies, coral reefs are quite in-
considerable. Occasional traces, sometimes amounting to a
fringing reef, occur along Luzon and the other Philippines.

The Wilkes Exploring Expedition coasted by the west
shore of Luzon to Manila, and thence by Luban, Mindoro,
Panay, to Caldera, near Samboangan in Mindanao; and
through this distance no reefs were distinguished, as would
have been the case, had there been any of much extent. At
the last-mentioned place we found coral pebbles on the beach,
and by dredging, obtained living specimens in six to eight
fathoms of water. The only large reefs were those between
Mindoro and the Calaminianes. There are fringing reefs at
Singapore. The islands of Borneo, Celebes, Java, and Su-
matra, according to all the authorities seen by the writer, have
but few coral patches about their shores, although affording
long lines of coast for their growth. In the China Seas,
there are numerous shoals, banks, and island reefs of coral.
Moreover, shore reefs occur about Loochoo, and the islands be-
tween it and Formosa. But the whole eastern coast of China
appears to be without coral. Quelpaert’s island, south of Co-
rea, in 34° N., is described as having coral about it; and this
has been confirmed by late information.

Why should the reefs of the Hast India Archipelago be so
limited in extent, and large parts be almost destitute, notwith-
standing their situation in the warmest seas of the ocean, and

in the most favorable region for tropical productions? We are

348 CORALS AND CORAL ISLANDS.

not prepared for a full answer to this inquiry; for it would de.
mand a thorough knowledge of the shores, as well as of the cur-
rents, and of the former and present condition of volcanic fires.
From personal observation we may reply satisfactorily, as far as
regards part of the southern half of the east coast of Su-
matra. This coast is low, and sandy, or muddy, and thus af:
fords the most unfavorable place for zodphytes. A strong
current sweeps through the Straits of Banka, which keeps the
water muddy, and the shores in constant change. The same
cause may operate on the coasts of other islands, but we can-
not say to what extent.

The East Indies have been remarkable for their volcanoes,
exceeding, for the area, everv other part of the world; and this
fact must. have had influence on the formation of coral reefs,
though there are not data for fixing the extent of the influence. -
Of the thousand vents which have been in action, several still
make themselves felt over wide areas. The Sooloo Islands are
about one hundred in number, and nearly all are pointed with
voleanic cones, and while some have the broken declivities that
are marks of age, others have regular slopes, as if but just now
extinguished; a dozen of these cones may sometimes be seen
on a single island. These volcanic peaks often rise out of the
sea, as if their formation had begun with a submarine erup-
tion. Ina region so extensively and so recently igneous, the
coral polyps would have found little chance for growth, until
volcanic action had become comparatively quiet, and deluges
of hot water ceased. There appears, therefore, to be some
reason for the fact that the reefs are small, and have seldom
reached the surface. The Sooloo Sea is but one of the volcan-
ic centers in these seas. Java, several of the Philippines, and
other islands south of these last, with the northern shore of
New Guinea, make up a wide region of fires, and it cannot be

GEOGRAPHICAL DISTRIBUTION. 349

doubted that the frequent eruptions prevented the growth of
any thing more than isolated corals, for a long period, over
each of these areas. For other causes we must look to the na-
ture of the coasts, fresh-water streams, and marine currents ;
we leave it for other investigators to apply the explanation to
particular coasts.

The coast of China owes its freedom from corals to the
cool temperature of the waters, the coast being wholly outside,
as has been stated, of the coral-reef seas.

One interesting fact should be noted:—the most extensive
reefs in the East Indies are to be found in the open seas, be-
tween the large islands; these islands, at the same time, often
being without proper reefs, or with mere traces of coral. This
is the case between Borneoand the range of large islands south ;
the China Sea is another instance of it; north of New Guinea,
a few degrees, is another. How far this is due to their being
distant from the scenes of igneous action, and from the detri-
tus and fresh water of island streams, remains to be deter-
mined. A sinking island becomes a more and more favorable
spot for the growth of coral, as it descends; for as its extent
diminishes, its streams of fresh water and detritus also de- |
crease. It might, therefore, be expected, on this account alone,
that such isolated spots of land, away from all impure waters,
in the open ocean, should become the bases of large reefs.
The existence of these reef-islands is, therefore, no necessary
proof of greater subsidence than the coast adjoining has un-
dergone. Still the fact of a greater subsidence is not im-
possible or improbable.

In the Indian Ocean, the Asiatic coast is mostly free from
growing coral. The great rivers of the continent are probably
the most efficient cause of their absence, both directly, through
their fresh waters, and through the detritus they transport and

300 GORALS AND CORAL ISLANDS.

distribute along the shores. It will be observed that this
agent, so ineffectual on small islands, is one of vast influence
upon larger lands. Mr. Darwin alludes to small patches of
coral in the Persian Gulf. Ceylon has some fringing reefs.

The islands of the Indian Ocean are, to a great extent,
purely of coral. Of this character are the Laccadives, Mal-
dives, Keeling’s, the Chagos Group; and, north and east of
northern Madagascar, Saya de Malha, Amirante, Cosmoledo ;
and also, nearer northern Madagascar, a series of raised atolls
from Farquhar Island to Aldabra Island.

Madagascar has a fringing reef upon its southwestern
point, according to Mr. Darwin, and on some parts of the
coast above: also on the north and eastern shores far down as
latitude 18° S. The Comoro Islands, between Madagascar and
the continent, have large barrier reefs.

The eastern coast of Africa has narrow reefs extending
north with some interruptions from Mozambique, in latitude
16° S., to a short distance from the equator. Corals also
abound in the Red Sea, occurring in some parts on both shores,
though most frequent on the eastern, from Tor, in the Gulf of
Suez, to Konfodah. This long continental reef may at first be
deemed a little remarkable, after what has been stated about
such reefs elsewhere. Yet the surprise is at once set aside by the
striking fact that this whole coast, from the isthmus of Suez
south, has no rivers, excepting some inconsiderable streams. It
affords, therefore, an interesting elucidation of the subject un-
der consideration, and confirms the view taken to account for
the absence of reefs from many continental coasts. It is a fact
almost universal, that where there are large fresh-water
streams, there are earthy, or sandy shores; and where there
are no such streams, rocky shores, though not uniformly occur:

ring, are common.

GEOGRAPHICAL DISTRIBUTION. 351

On the African coast there are coral reefs at Port Natal,
in latitude 30° S. ; and here, owing to the warm currents from
the tropical regions, the mean winter temperature of the water
is not below 68° F.

Passing from the Indian to the Atlantic Ocean, we find
little or no coral on the west coast of Africa. The islands of
Cape St. Ann and Sherboro, south of Sierra-Leone, are de-
scribed as coral by Captain Owen, R. N., in the Journal of
the Geographical Society (vol. ii, p. 89); but this has been
since denied. The Island of Ascension, in 7° 56’S.,and 14°
16’ W., must have been bordered by growing coral, as Quoy
and Gaymard mention that a bed of coral rock may be seen
buried beneath streams of lava. Quoy also states that the
corals which formed these reefs are no longer found alive, and
adds that volcanic eruptions have probably destroyed them.
The cold polar currents along the western African coast are the
cause of the absence of corals from it, to within six or seven de-
grees of the equator; and these coid waters may at times ex-
tend still farther north. The same obstacle to the diffusion of
species eastward, mentioned as occurring in the Pacific—that is,
westward currents—exists also in the Atlantic.

On the American shores of the Atlantic, north of the
equator, there are few reefs, except in the West Indies. The
waters of the Orinoco and Amazon, and the alluvial shores
they occasion, exclude corals from that part of the coast.

In the West Indies, the reefs of Florida (p. 204), Cuba,
the Bahamas (p. 213), and of many of the eastern islands are
well known. On the east coast of Florida they continue up
as far as Cape Florida, in latitude 25° 40’ N.; and reef-corals
are living off Charleston, S. C., according to Prof. Verrill.
There are also said to be patches at intervals along the coast

352 CORALS AND CORAL ISLANDS.

of Venezuela and Guatemala; but the west shores of the Gulf ~

of Mexico, as well as the northern, like West Florida, are
mostly low, and without reefs ; they are within the influence
of the Mississippi and other large rivers. Some species of
reef corals, however, occur in the vicinity of Aspinwall (p.
ig)

South of the equator, on the east coast of South America,
there are reefs at intervals, from the vicinity of Cape St.
Roque to the Abrolhos shoals in latitude 18°, as described by
Prof. C. F. Hartt, while reef corals extend south to Cape Frio.

Descriptions of part of the Abrolhos reefs are given on page ~

140. North of the Abrolhos reefs, there are others of coral
stretching on to Point Carumba; again, off the Bay of Porto

Securo, and across the Bay of Santa Cruz; in the vicinity of

Camamt, around Quieppe Island; along the shores of Ita-
parica Island; and at Bahia and Periperi; then, after an in-
terruption, off Maceid, in the vicinity of Pernambuco. More-
over the Roceas, a cluster of reefs in the latitude of Fernando
do Noronha, are, as Hartt observes, probably of coral.

It is thus seen that the earth is belted by a coral zone,
corresponding nearly to the tropics in extent, and that the
oceans throughout it abound in reefs, wherever congenial sites
are afforded for their growth. It has also been shown that
the currents of extra-tropical seas, which flow westward, and
are interrupted and trended toward the equator by the con-
tinents, contract the coral seas in width, narrowing them to a
few degrees on the western coasts of the continents ; while
the tropical currents flowing eastward, diverge from the equa-
tor, and cause the belt to widen near the eastern shores. The
polar currents flow also by the eastern coasts, preventing the
warmer waters from increasing the width of the coral zone as

GEOGRAPHICAL DISTRIBUTION. 353
much as it is contracted on the western coasts. / Moreover, the
trend of the coast and its capes produce other modifications in

the direction of the currents, the most of which are apparent

in the actual distribution of coral reefs. On the shores of /~

the continents it is observed that there are few extensive

reefs, and the coasts on which they occur are those which, ,
owing to the dryness of the climate, have no great rivers to °

pour freshwater and detritus into the sea. Thus the influence
of continental waters and detritus on the distribution of reefs,
is shown to be very marked. But about the Pacific Islands,
where streams are small, the same cause has had little effect,

seldom doing more than modifying somewhat the shores and _
bottom of a harbor. It has been further demonstrated that

in different groups, as the Ladrones, Sandwich Islands, Navi-

gators, New Hebrides, there is an inverse relation between |
° ° |
the extent of reefs and the evidences of recent volcanic ac- |

tion in the island; and that the largest reefs exist where |
there is no proof of former igneous action, or where it has
long ceased. ‘The existence of large reef-islands in open seas
where the neighboring lands are mostly destitute of coral
reefs, harmonizes with the conclusions announced, since such
islands are in general removed from the deleterious influences

|

|

ana} --

just mentioned ; yet it is very probable that in many cases
of this kind the region of the open sea may have undergone
a subsidence not experienced equally by the lands either side ;
for the cases in which such seas contain coral islands are
many.

The modifications of form and interruptions of reefs,
arising from abrupt or sloping shores, and tidal or local
currents, have also been exemplified.

23

304 CORALS AND CORAL ISLANDS.

CHAPTER V.

ON CHANGES OF LEVEL IN THE PACIFIC OCEAN.1

I. EVIDENCES OF CHANGE OF LEVEL.

It has been shown that atolls, and to a large extent other
coral reefs, are registers of change of level. From the evi-
dence thus afforded, the bottom of a large part of the Pacific
Ocean is proved to have undergone great oscillations in recent
geological time. In this direction, then, we find the grandest
teachings of coral formations. In treating the subject we
necessarily bring into connection with it evidences of change
of level from other sources. The proofs of change of level
here considered are the following : —

A. Evidences of elevation.

1. The existence on coral or other islands of patches of
coral reef, and deposits of shells and sand from the reefs,
above the level where they are at present forming.

The coral reef-rock has been shown occasionally to in-
crease, by growth of coral, to a height of four to six inches
above low-tide level when the tide is but three feet, and to
twice this height with a tide of six feet. It may, therefore,
be stated as a general fact, that the limit to which coral may
grow above ordinary low tide is about one sixth the height of
the tide, though it seldom attains this height. Its existence

1 The conclusions and nearly all the facts presented in this chapter are from
the author’s Exploring Expedition Report, in which the subject is illustrated by a
colored chart of the Pacific Ocean.

CHANGES OF LEVEL IN CORAL-REEF REGIONS. 355

on an island at a higher level would be proof of an elevation
of the land.

When the tide is three feet, beach accumulations of large
masses seldom exceed eight feet above high tide, and the finer
fragments and sand may raise the deposit to ten feet; but
with a tide of six feet twice this height may be attained.
With the wind and waves combined, or on prominent points |
where these agents may act froin opposite directions, such
accumulations may be fifteen to twenty feet in height, and
occasionally thirty to forty feet. These latter are drift de-
posits, finely laminated, generally with a sandy texture, and
commonly without a distinguishable fragment of coral or
shell; and in most of these particulars they are distinct from
reef-rocks.

2. On islands not coral, the existence of sedimentary de-
posits, or layers of rolled stones, interstratified among the
layers of igneous or other rocks constituting the halls.

B. Evidence of subsidence.

1. The existence of wide and deep channels between an
island and any of its coral reefs; or in other words, the exist-
ence of barrier reefs.

2. The existence of lagoon islands or atolls.

3. The existence of submerged atolls.

4. Deep bay-indentations in the coasts of high islands as
the terminations of valleys. —The kind of evidence here re-
ferred to has been fully explained on page 273. It may be
added that the absence of coves, or deep-valley bays, may be
evidence that no subsidence has taken place, or only one of
comparatively small amount.

C. Probable evidence of subsidence now in progress.

1. An atoll reef without green islets, or with but few small
spots of verdure.—The accumulation requisite to keep the

356 CORALS AND CORAL ISLANDS.

reef at the surface-level, during a slow subsidence, renders it
impossible for the reef to rise above the waves and supply
itself with soil, unless the subsidence is extremely slow, or
has wholly ceased.

From the above review of evidences of change of level, it
appears that where there are no barrier reefs, and only fringing
reefs, the corals may afford no evidence of subsidence. But
it does not follow that the existence of only fringing reefs, or
of no reefs at all, is proof against a subsidence having taken
place. For we have elsewhere shown that through volcanic
action, and at times other causes, corals may not have begun
to grow till a recent period, and, therefore, we learn nothing
from them as to what may have previously taken place.
While, therefore, a distant barrier is evidence of change of
level, we can draw no conclusion either one way or the other,
from the fact that the reefs are small or wholly wanting,
until the possible operation of the several causes limiting
their distribution has been duly considered.

The influence of volcanoes in preventing the growth of
zodphytes extends only so far as the submarine action may
heat the water, and it may, therefore, be confined within a
few miles of a volcanic island, or to certain parts only of its
shores.

There are two epochs of changes in elevation which may
be here distinguished and separately considered. 1. The sub-
sidence indicated by atolls and barrier reefs. 2, Hlevations

during more recent periods.

SUBSIDENCE IN PACIFIC CORAL REGIONS. 373)/(

II. SUBSIDENCE INDICATED BY ATOLLS AND BARRIER
REEFS.

Looking at atolls as covering buried islands, we observe
that through the equatorial latitudes such marks of. subsi-
dence abound from the Kastern Paumotus to the Western
~ Carolines, a distance of about six thousand geographical miles.
In the Paumotu Archipelago there are about eighty of these
atolls. Going westward, a little to the north of west they
are found to dot the ocean at irregular intervals; and the
| Kingsmill or Gilbert Group, the Marshall Group, and the
Carolines comprise seventy-five or eighty atolls.

If a line be drawn from Pitcairn’s Island, the southern-
most of the Paumotus, by the Gambier Group, the north of
the Society Group, the Navigators, and the Solomon Islands
to the Pelews, it will form nearly a straight boundary, trend-
ing N. 70° W., running between the atolls on one side and
the high islands of the Pacific on the other, the former lying
to the north of the line, and the latter to the south.

Between this boundary line and the Hawaiian Islands, an
area nearly two thousand miles wide and six thousand long,
there are two hundred and four islands, of which only three,
exclusive of the eight Marquesas, and the Ladrones with Yap,
Hunter’s, and Los Matelotas in the line of the Ladrones and
Pelews, are not of coral. These three are Kusaie or Ualan,
Ponape, and Truk or Hogoleu, all in the Caroline Archipelago.
South of the same line, within three degrees of it, there is an
occasional atoll; but beyond this distance, there are none
excepting the few in the Friendly Group, and one or two in
the Feejees. °

If each coral island scattered over this wide area indicates
the subsidence of an island, we may believe that subsidence

358 CORALS AND CORAL ISLANDS.

was general throughout the area. Moreover, each atoll, could

we measure the thickness of the coral constituting it, would
inform us nearly how much subsidence took place where it
stands; for they are actually so many registers placed over
the ocean, marking out, not only the site of a buried island,
but also the depth at which it lies covered. We have not the
means of applying the evidence ; but there are facts at hand,
which may give at least comparative results.

a. We observe, first, that the barrier reefs are, in general,

evidence of less subsidence than atoll reets (p. 266). Conse-
quently, the great preponderance of the former just below the
southern boundary line of the coral island area, and, farther
south, the entire absence of atolls, while atolls prevail so uni-
versally north of this line, are evidence of some depression
just below the line ; of less, farther south ; and of the greatest
amount, north of the line or over the coral area.
. b. The subsidence producing an atoll, when continued,
gradually reduces its size until it finally becomes so small that
the lagoon is obliterated ; and, consequently, a prevalence of
these small islands is presumptive evidence of the greater
subsidence. We observe, in application of this principle, that
the coral islands about the equator, five or ten degrees south,
between the Paumotus and the Gilbert Islands, are the small-
est of the ocean; several of them are without lagoons, and
some not a mile in diameter. At the same time, in the Pau-
motus, and among the Gilbert and Marshall Islands, there are
atolls twenty to fifty miles in length, and rarely one less than
three miles. It is probable, therefore, that the subsidence
indicated was greatest at some distance north of the boundary
line, over the region of small equatorial islands, between the
meridians of 150° and 180° W.

c. When, after thus reducing the size of the atoll, the

SUBSIDENCE IN PACIFIC CORAL REGIONS. 359

subsidence continues its progress, or when it is too rapid for
the growing reef, it finally smks the coral island, which,
therefore, disappears from the ocean. Now, it is a remarkable
fact that while the islands about the equator above alluded
to indicate greater subsidence than those farther south, there
is over a region north of these islands, — that is, between
them and the Hawaiian Group, —a wide blank of ocean
without an island, which is nearly twenty degrees in breadth.
This area lies between the Hawaiian, the Fanning and the
Marshall Islands, and stretches off, between the first and last
of these groups, far to the northwest. YT me,

Is it not, then, a legitimate conclusion that the subsidence,”
which was least to the south beyond the boundary line and
increased northward, was still greater or more rapid over this

open area ; that the subsidence which reduced the size of the

islands about the equator to mere patches of reef, was further we

continued, and caused the total disappearance of islands that
once existed over this part of the ocean ? en
_ d. That the subsidence gradually diminished southwest-
wardly from some point of greatest depression situated to the
northward and eastward, is apparent from the Feejee Group
alone. Its northeast portion (see chart) consists of immense
barriers, with only a few points of rock remaining of the
submerged land; while in the west and southwest there
are mountain islands of great magnitude. Again, along the
north side of the Vanikoro Group, Solomon Islands, and
New Ireland, there are coral atolls, but scarcely one to the
south.

In view of this combination of evidence, we are led to
believe that the subsidence increased from the south to the
northward or northeastward, and was greatest between the

Navigator and Hawaiian Islands, near the centre of the area

360 CORALS AND CORAL ISLANDS.

destitute of islands, about longitude 170° to 175° W., and
latitude 8° to 10° N.

But we may derive some additional knowledge respecting
this area of subsidence from other facts.

Hawauan Range.— We observe that the western islands
in the Hawaiian Range, beyond Bird Island, are atolls, and all
indicate a large participation in this subsidence. To the east-
ward in the range, Kauai and Oahu have only fringing reefs,
yet in some places these reefs are half a mile to three fourths
in width. They indicate a long period since they began to
grow, which is borne out by the features of Kauai showing a
long respite from volcanic action. We detect proof of sub-
sidence, but not of a large amount. Moreover, there are no
deep bays; and, besides, Kauai has a gently-slopime coast
plain of great extent, with a steep shore acclivity of one to
three hundred feet. The facts favor the idea of much less
subsidence since the time when the corals began to grow in
the region of Kauai and Oahu than along the range to the
westward. The rather small width of the reefs about these
two islands may be owing to the former action of their vol-
canoes, which may have been burning during the earlier part
of the coral-reef era. But deep-sea soundings must be made
along the whole chain, including the line to the westward,
before we can speak positively about the change of level.

The western islands of the range bear some evidence of
having, in recent times, commenced a new subsidence after a
temporary cessation. They all have little dry land and vege-
tation about the reefs. Brooks’s Island, in latitude 28° 15'N.,

—

and longitude 177° 20’ W., eighteen miles in circumference,
has on its north and east sides a compact coral wall of about
five feet elevation, which continues for four and a quarter

miles, and then becomes a line of detached rocks at tide level.

SUBSIDENCE IN PACIFIC CORAL REGIONS. 361

This bare wall, thus described by Capt. Wm. Reynolds,
U.S. N., appears to be an indication that the land was once
finished off under a cessation of subsidence, but that a sink-
ing of small amount has since taken place, amounting per-
haps to four or five feet.

Ocean Island, in 28° 25’ N., 178° 25’ W., another of this
range, is very similar to Brook’s in its wall of coral rock on
the east; and so also is Pearl and Hermes’ reef, in 27° 50’
N., 176° W., though the wall of the latter is more a series of
detached rocks than a continuous parapet.

Marquesas.—The Marquesas are remarkable for their ab-
rupt shores, often inaccessible cliffs, and deep bays. The ab-
sence of gentle slopes along the shores, their angular features,
abrupt soundings close alongside the island, and deep inden-
tations, all bear evidence of subsidence to some extent; for
their features are very similar to those which Kauai or Ta-
hiti would present, if buried half its height in the sea, leaving
only the sharper ridges and peaks out of water. They are
situated but five degrees north of the Paumotus, where eighty
islands or more have disappeared, including one at least fifty
miles in length. There is sufficient evidence that they partici-
pated in the subsidence of the latter, but not to the same ex-
tent. They are nearly destitute of coral, and apparently be-
cause of the depth of water about the islands.

Gambier Growp.—In the southern limits of the Paumotu
Archipelago, where, in accordance with the foregoing views,
the least depression in that region should have taken place,
there are actually, as we have stated, two high islands, Put-
cairn’s and Gambier’s. There is evidence, however, in the ex-
tensive barrier about the Gambier’s (see cut on page 266),
that this subsidence, although less than farther north, was by
no means of small amount. On page 157, we have estimated

362 CORALS AND CORAL ISLANDS.

it at 1,150 feet—possibly 1,750. These islands, therefore,
although toward the limits of the subsiding area, were still far
within it. The valley-bays of the islets of the lagoon are of
great depth, and afford additional evidence of the subsidence.

Tahitian Islands.—The Tahitian Islands, along with Sa-
moa and the Feejees, are near the southern limits of the area
pointed out. Twenty-five miles to the north of Tahiti, within
sight from its peaks, lies the coral island Tetuaroa, a register
of subsidence. Tahiti itself, by its barrier reefs, gives evidence
of the same kind of change; amounting, however, as we have
estimated, to a depression of but two hundred and fifty or
three hundred feet. The northwestern islands of the group
lie more within the coral area, and correspondingly, they have
wider reefs and channels, and deep bays, indicating a greater
amount of subsidence.

Samoan or Navigator Growp—tThe island of Upolu has
extensive reefs, which, in many parts are three-fourths of a
mile wide, but no inner channel. The subsidence is estimated
on page 158, at one or two hundred feet. The volcanic land
west of Apia, declines with an unbroken gradual slope of
one to three degrees beneath the sea. The absence of a low
cliff is probable evidence of a depression, as has been else-
where shown. The island of Tutuila has abrupt shores, deep
bays and little coral. It appears probable, therefore, that it
has experienced a greater subsidence than Upolu. Yet the
central part of Upolu has very similar bays on the north,
which would afford apparently the same evidence; and it is
quite possible that the facts indicate a sinking which either
preceded the ejections that now cover the eastern and west-
ern extremities of Upolu, or accompanied this change of level.
The large island of Savaii, west of Upolu, has small reefs,
small because, probably, of volcanic action; for it bears, every-

SUBSIDENCE IN PACIFIC CORAL REGIONS. 363

where, evidence of comparatively recent eruption; from it,
therefore, we gather no certain facts bearing on this subject.
East of Tutuila is the coral island, Rose. It may be, there-
fore, that the greatest subsidence in the group was at its

eastern extremity.
Feeee Islands. — We have already remarked upon the

change of level in this group. A large amount of subsidence
is indicated by the extensive reefs in every portion of the
group, but it was greatest beyond doubt, in the northeastern
part. The subsidence, where least, could hardly have been
less than 2,000 to 5,000 feet.

We have thus followed around the borders of the coral
area; and, besides proving the reality of the limits, have
ascertained some facts with reference to a gradual diminu-
tion of the subsidence toward, and beyond, these limits. A
Ine through the Hawaiian Group would pass along the
northern boundary line of the area; and, taking the southern
boundary as given on page 357, the oblong area narrows
eastward. An axis nearly bisecting this space, drawn from
the eastern Paumotus toward Japan, passes through the re-
gion of greatest subsidence, as above determined and now
proved by soundings, and may be considered the line of great-
est depression for the area of subsidence.

It is worthy of special note, that this axial line, or line
of greatest depression, coincides in direction with the mean
trend of the great ranges of islands, it having the course, N.
56° W.; and it also corresponds approximately with the
axial line of the Pacific Ocean. On the map of the Phoenix
Group, Plate XI, the axial line is drawn just as it was laid
down in the author’s Expedition Report. Moreover, this map
gives the soundings that have been recently made in the seas,
and it is interesting to find full confirmation of the conclu-

364 CORALS AND CORAL ISLANDS.

sion that was derived over fifty years since from the size and
distribution of the atolls; for the line, A A, lies in the 3,000-
4,000 area of the central Pacific, and, moreover, passes over
the deepest point in it yet observed, — the sounding giving
a depth of 3,448 fathoms. The deep central area of the
Pacific apparently extends, in the direction of this axial line,
to the still deeper waters east of Japan.

The southern boundary line of the coral area, as we have
laid it down, hes within the area of subsidence, although near
its limits. This area has been prolonged southeastward in
some places beyond the boundary line. One of the regions
of this prolongation lies between the Samoan or Navigator
Group and the Feejees and Tonga Group; another is east of —
Samoa, along by the Hervey Group. Each of these exten-
sions trends parallel with the groups of islands. Tt would
seem, therefore, that the Society and Samoan Islands were
regions of less change of level than the deep seas either side
of them; that, therefore, instead of a uniform subsidence over —
the subsiding area, shading off toward the borders, there were
troughs of greater subsidence, whose courses were parallel to
the ranges of islands; that, im other words, there were in the
ocean's bottom, a few broad synclinal and anticlinal flexures, |
having a common direction nearly parallel to the axial line
of the Pacific. The Marquesas and Fanning Groups lie in a
common line, and thus may mark the course of a great cen-
tral anticlinal in the oceanic basin.

The Hawaiian range has probably experienced its greatest
subsidence to the northwest, where the islands are all atolls,
and show some evidences of recent sinking; and this north-
western extremity of the range is nearer to the axis of the area
of subsidence, above laid down, than is the southwestern.

What is the extent of the subsidence indicated by the coral

SUBSIDENCE IN PACIFIC CORAL REGIONS. 365

reefs and islands of the Pacific? It is very evident that the
sinking of the Society, Samoan, and Hawaiian Islands has
been small compared with that required to submerge all the
lands on which the Paumotus and the other Pacific atolls rest.
One, two, or five hundred feet, could not have buried the
many peaks of these islands. Even the 1,200 feet of depres-
sion at the Gambier Group is shown to be at a distance from
the axis of the subsiding area. The groups of high islands
above mentioned contain summits from 4,000 to 14,000 feet
above the sea; and it is not probable that throughout this
large area, when the two hundred islands now sunken were
above the waves, there were none of them equal in altitude
to the mean of these heights, or 9,000 feet. Hence, however
moderate our estimate, there must still be allowed a sinking
of many thousand feet. Moreover, whatever estimate we
make that is within probable bounds, we shall not arrive at
a more surprising change of level than the continents show

that they have undergone; for since the Tertiary began (or ,

the preceding period, the Cretaceous, closed) more than 10,000

feet have been added to the Rocky Mountains, parts of the ~'

Andes, and Alps, and 19,000 feet to part of the Himalayas. ~

Between the New Hebrides and Australia, the reefs and
islands mark out another area of depression, which may have
been simultaneously in progress. The long reef of one hun-
dred and fifty miles from the north cape of New Caledonia,
and the wide barrier on the west, cannot be explained with-
out supposing a subsidence of one or two thousand feet at the
least. The distant barrier of Australia is proof of great sub-
sidence, even along the border of that continent. But the
ereatest amount of sinking took place, in all probability, over
the intermediate sea, called the “‘ Coral Sea,’”’ where there are

now a considerable number of atolls. fae

—

366 CORALS AND CORAL ISLANDS.

Il. EFFECT OF THE SUBSIDENCE.

The facts surveyed give us a long insight into the past,
and exhibit to us the Pacific once scattered over with lofty
lands, where now there are only humble monumental atolls.
Had there been no growing coral, the whole would have
passed without a record. These permanent registers, exhibit
in enduring characters some of the oscillations which the
“stable” earth has since undergone.

From the actual size of the coral reefs and islands, we
know that the whole amount of high land lost to the Pacific
by the subsidence was at the very least fifty thousand square —
miles. But since atolls are necessarily smaller than the land —
they cover, and the more so, the further subsidence has pro-
ceeded ;—since many lands, owing to their abrupt shores,
or to volcanic agency, must have had no reefs about them,
and have disappeared without a mark; and since others may
have subsided too rapidly for the corals to retain themselves
at the surface; it is obvious that this estimate is far below
the truth. It is apparent that in many cases, islands now
disjoined have been once connected, and thus several atolls
may have been made about the heights of a single subsiding
land of large size. Such facts show additional error in the
above estimate, evincing that the scattered atolls and reefs tell
but a small part of the story. Why is it, also, that the Pa-
cific islands are confined to the tropics, if not that beyond
thirty degrees the zodphyte could not plant its growing reeg-
isters ?

The island of Ponape, in the Caroline Archipelago, affords
evidence of a subsidence in progress, as Mr. Horatio Hale, the
Philologist of the Wilkes Expedition, gathered from a for-
eizner who had been for a while a resident on this island.

a

yr F Mie VN ;* vy iw. F ==, ;

SUBSIDENCE IN PACIFIC CORAL REGIONS. 367

Mr. Hale remarks, after explaining the character of certain
sacred structures of stone: “It seems evident that the con-
structions at Ualan and Ponape are of the same kind, and
were built for the same purpose. It is also clear that when
the latter were raised, the islet on which they stand was
in a different condition from what it now is. For at present
they are actually in the water; what were once paths are now
passages for canoes, and as O’Connell [his informant] says,
‘when the walls are broken down, the water enters the enclo-
sures.’” Mr. Hale, hence infers ‘“ that the land, or the whole
group of Ponape, and perhaps all the neighboring groups,
have undergone a slight depression.” He also states respect-
ing a small islet near Ualan, “ From the description given of
Leilei, a change of level of one or two feet would render it
uninhabitable, and reduce it, in a short time, to the same state
as the isle of ruins at Ponape.”

In some of the northern Carolines, the Pescadores, and
perhaps some of the Marshall Islands, the proportion of dry
land is so very small compared with the great extent of the
atoll, that there is reason to suspect a slow sinking even at the
present time; and it is a fact of special interest in connection
with it, that this region is near the axial line of greatest de-
pression, where, if in any part, the action should be longest
contanued.

Among the Kingsmills and Paumotus, there is no reason
whatever for supposing that a general subsidence is still in
progress; the changes indicated are of a contrary character.

IV. PERIOD OF THE SUBSIDENCE.

The period during which these changes were in progress,
extends back to the Tertiary era, and perhaps still farther back.

368 CORALS AND CORAL ISLANDS.

In the island of Metia, elevated two hundred and fifty feet,
the corals below were the same as those now existing, as far
as we could judge from the fossilized specimens. At the in-
ner margin of shore reefs, there is the same identity with ex-
isting genera. We do not claim to have examined the base-
ment of the coral islands, and offer these facts as the only ev-
idence on this point that is within reach. We cannot know
with absolute certainty that the present races of zodphytes
may not be the successors of others that flourished, on the saine
sites, even before the Tertiary era in Cretaceous and Jurassic
times; but as yet have little reason in facts observed, for such

om

a conclusion. For a long time volcanic action may have been

too general and constant over the Pacific, for the growth of
corals; and this may have continued to interfere till a com-
paratively late period, if we may judge from the appearance
of the rocks, even on Tahiti. The subsidence has probably tor
a considerable period ceased in most, if not all, parts of the
ocean, and subsequent elevations of many islands and groups

have taken place.

V. ELEVATIONS OF MODERN ERAS IN THE PACIFIC,

Since the period of subsidence discussed in the preceding
pages, there has been no equally general elevation. Yet va-
rious parts of the ocean bear evidence of changes confined to
particular islands, or groups of islands. While the former
exemplify one of the grander events in the earth’s history, in
which a large segment of the globe was concerned, the latter
exhibit its minor changes over limited areas. The instances
of these changes are so numerous and so widely scattered, that
they afford convincing evidence of a cessation in the previous

general subsidence.

ELEVATIONS IN PACIFIC CORAL REGIONS. 369

ed

lat. 10° 17 N. and long. 109° 19! W.. is an elevated “atoll at

least 100 feet, high, according to W. Harper Pease, as re-

ported in the Proceedings of the California Academy of °

Sciences, Vol. III., p. 199.

b. Paumotu Archipelago. — The islands of this archipelago
appear in general to have that height which the ocean may
give to the materials. Nothing was detected indicating any
general elevation in progress through the archipelago. The
large extent of wooded land shows only that the islands have
been long at their present level. There are examples of ele-
vation in particular islands, however, some of which are of

unusual interest. The instances examined by the Expedition
are those of Honden Island, Dean’s Island, and Clermont

Tonnerre. Besides these, Elizabeth Island has been described
by Beechey, and, according to the same author, Ducie’s
Island and Osnaburgh suggest some elevation.

Honden Island or Henuake.—This island is wooded on its
different sides, and has a shallow lagoon. The beach is eight
feet high, and the land about twelve. There are three entrances
to the lagoons, all of which were dry at low water, and one
only was filled at high water. Around the lagoon, near the
level of high tide, there were numerous deserted shells of the
huge Tridacna, often a foot long, lying in cavities in the coral
rock, precisely as they occur alive on the shore reef. As these
Tridacnas evidently lived where the shells remain, and do not
occur alive more than six or eight inches, or a foot at the most,
above low tide, they prove, in connection with the other facts,
an elevation of at least two feet.

Nairsa or Dean’s Island—The south side of Dean’s
Island, the largest of the Paumotus, was coasted along by the

Peacock, one of the Sloops of War of the Wilkes Exploring
24

70) CORALS AND CORAL ISLANDS.

Expedition, and from the vessel we observed that the rim of
land consisted for miles of an even wall of coral rock, appar-
ently six or eight feet above high tide. This wall was broken
into rude columns, or excavated with arches and caverns; in
some places the sea had carried it away from fifty to one hun-
dred rods, and then there followed again a line of columns,
and walls, with occasional arches as before. The reef, for-
merly lying at the level of low tide, had been raised above the
sea, and subsequently had undergone degradation from the
waves. ‘The standing columns had some resemblance in cer-
tain parts to the masses seen here and there on the shore plat-
forms of other islands; but the latter are only distantly scat-
tered masses, while on this island, for the greater part of the
course, there were long walls of reef-rock. The height, more-
over, was greater, and they occurred too on the leeward side
of the island, ranging along nearly its whole course, while the
north side, according to the map, 7s wooded throughout.

The elevation here indicated is at least sia feet; but it
may have been larger; the observations were made from ship-
board.

Thirty miles to the southward of Dean’s Island, we
come to

Metia.—This island has already been described, and its el-
evation stated at two hundred and fifty feet. (See page 193.)

Clermont Tonnerre shows the same evidence of elevation
from Tridacnas, as Honden Island. Clermont Tonnerre and
Honden are on the northeastern limits of the Paumotus.

Elizabeth Island was early shown to be an elevated coral
island by Beechey. This distinguished voyager represents
it as having perpendicular cliffs over fifty feet_in height,
From his description it is obviously like Metia; the elevation

ELEVATIONS IN PACIFIC CORAL REGIONS. 371

is erghty feet. It is one of the southeastern Paumotus, near
Ducie’s.

Ducie’s [sland is described by Beechey as twelve feet high,
which would indicate a probable elevation of one or two feet.

Osnaburgh Island, according to the same author, affords evi-
dence of having increased its height since the wreck of the Matil-
da, in 1792. He contrasts the change from a “reef of rocks,”
as reported by the crew, to ‘‘a conspicuously wooded island,”
the condition when he visited it; and states, further, that the
anchor, iron works, and a large gun (4-pounder) of this vessel
were two hundred yards inside of the line of breakers. Cap-
tain Beechey suggests that the coral had grown, and thus in-
creased the height. But this process might have buried the
anchor if the reef were covered with growing corals (which
is improbable), and could not have raised its level. If there
has been any increase of height (which we do not say is cer-
tain), it must have arisen from an upheaval.

ce. Tahitian Group.—tThe island of Tahiti presents no
conclusive evidence of elevation. The shore plains are said to
rest on coral, which the mountain débris has covered; but
they do not appear to indicate a rise of the land.

The descriptions by different authors of the other islands
of this group do not give sufficient reason for confidently be-
lieving that any of them have been elevated. The change,
however, of the barrier reef around Bolabola into a verdant
belt encircling the island, may be evidence that a long period
has elapsed since the subsidence ceased; and, as such a change
is not common in the Pacific, we may suspect that it has been
furthered by at least a small amount of elevation. The observa-
tion by the Rev. D. Tyerman with regard to the shells found at
Huahine high above the sea, may be proof of elevation; but
the earlier erroneous conclusions with regard to Tahiti (on

—

~—p

a712 CORALS AND CORAL ISLANDS.

which island masses of coral are carried by natives up the
mountain, to leave at the highest point reached, and also to
mark the limits between the land of different chiefs, and are
common from these causes, up to a height of fifteen hundred
feet), teach us to be cautious in admitting it without a more
particular examination of the deposit. Moreover, shells, even
large ones, are carried far away from the sea by Hermit
Crabs (Pagurids).

d. Hervey and Rurutu. Groups.—These groups lie to the
southwest and south of Tahiti.

Mangaia is girted by an elevated coral reef three hundred
feet in height. Mr. Williams, in his Missionary Enterprises,
pages 48, 50 and 249, speaks of it as coral, with a small quan-
tity of fine-grained basalt in the interior of the island; he states
again that a broad ridge (the reef) girts the hills.

Atiu (Wateoo of Cook) is a raised coral island. Cook
Voy., i. 180, 197, observes, that it is “nearly like Mangaia.”
The land near the sea is only a bank of coral ten or twelve feet
high, and steep and rugged. The surface of the island is cov-
ered with verdant hills and plains, with no streams. It is de-
scribed by Williamsin his Missionary Enterprises. Mauke is
a low elevated coral island according to Williams, and Mtiaro
resembles Mauke. Okatutaia is a low coral island, not more
than six or seven feet high above the beach, which is coral
sand. It has a light-reddish soil.

Furutu has an elevated coral reef one hundred and fifty
feet in height, as stated by Stutchbury, and also Williams.
Tyerman and Bennet describe the island as having a high cen-
tral peak with lower eminences, and speak of the coral rock as
two hundred feet high on one side of the bay and three hun-
dred on the other (ii, 102).—Ellis says that the rocks of the in-
terior are in part basaltic, and in part vesicular lava, il. 393.

| ELEVATIONS IN PACIFIC CORAL REGIONS. 373

With regard to the other islands of these groups, Manuai,
Aitutaki, Rarotonga, Rimetara, Tubuar, and Rawavai, the de-
scriptions by Williams and Ellis appear to show that they have
undergone no recent elevation.

é Tonga or Friendly Islands, and others in their vicinity.

All the islands of the Tonga group about which there are
reefs, give evidence‘of elevation: Tongatabu and the Hapait
islands consist solely of coral, and are elevated atolls.

Hua, at the south extremity of the line, has an undulated
mostly grassy surface, in some parts eight hundred feet in
height. Around the shores, as was seen by us from shipboard,
there is an elevated layer of coral reef-rock, twenty feet thick,
worn out into caverns, and with many spout-holes. Between
the southern shores and the highest part of the island, we ob-
served three distinct terraces. Coral is said to occur at a height
of three hundred feet. From the appearance of the land, we
judged that the interior was basaltic ; but nothing positive was
ascertained with regard to it.

Tongatabu (an island visited by us) lies near Eua, and is in
some parts fifty or sixty feet high, though in general but twen-
ty feet. It has a shallow lagoon, into which there are two en-
trances ; some hummocks of coral reef-rock stand eight feet out
of water.

Namuka and most of the Hapazi cluster, are stated by Cook
to have abrupt limestone shores, ten to twenty feet in height.
Namuka has a lagoon or salt lake at centre, one and a half miles
broad; and there is a coral rock in one part twenty-five feet
high. It is described by Williams, p. 296.

Vavau, the northern of the Group, according to Williams
(p. 427), is a cluster of elevated islands of coral limestone,
thirty to one hundred feet in height, having precipitous cliffs,
with many excavations along the coast.

374 CORALS AND CORAL ISLANDS.

Pylstaart’s Island, south of Tongatabu, is a small rocky
islet without coral. Zafua and Proby are volcanic cones, and
the former is still active.

Savage Island, a little to the east of the Tonga Group, re-
sembles Vavau in its coral constitution and cavernous cliffs. It
is elevated, according to Williams (pp. 275, 276), one hun-
dred feet.

Beveridge Reef, a hundred miles southeast of Savage, is low
coral,

f. Samoan or Navigator Islands. —No satisfactory evi-
dences of elevation were detected about these islands.

g. Atolls, north of Samoa.

On account of the high tides (four to six feet), the sea
may give a height of twelve to sixteen feet to the land.

Swain’s (Gente Hermosas ?), near latitude 11° S., is fifteen
to eighteen feet above the sea where highest, and the beach
is ten to twelve feet high. It is a small island, with a de-
pression at centre, but no lagoon. Probably an elevation of
two or three feet. This island was named Swain, by Capt.
Hudson, of the Wilkes Expedition, because not in the position
assigned to Gente Hermosas.

Fakaafo, ninety miles to the north, is fifteen feet high. The
coral reef-rock is raised in some places three feet above the
present level of the platform. Elevation at least three feet.

Nukunono, or Duke of Clarence, near Fakaafo, was seen
only from shipboard.

Oatafu, or Duke of York’s, is in some parts fourteen feet
high. Whether elevated or not is uncertain; probably as
much so as Fakaafo.

h. Scattered islands farther north, near the equator, east of
the Gilbert Group.

Of the Fanning Group, Washington Island, in lat. 4° 41'S.,

ELEVATIONS IN PACIFIC CORAL REGIONS. 375

and long. 160° 15’ W., is three miles in diameter, and is with-
out a proper lagoon; the whole surface is densely covered
with cocoanut and other trees. The height of the land is ten
or twelve feet. The unusual size of the island for one without
a lagoon, and the luxuriance of the forest vegetation, are prob-
able evidence of some elevation, but not beyond three feet.

Palmyra Island, northeast of Washington, is described by
Fanning as having two lagoons, the westernmost with twenty
fathoms water. |
~ Fanning’s Island, southeast of Washington, according to
the same voyager, is lower than that island. The accounts
give no evidence of elevation in either Fanning’s or Palmyra.

Christmas Island, in lat. 1° 53’ N., long. 157° 32’ W., is
thirty miles long. Cook speaks of the land as in some parts
three miles wide, and as having narrow ridges lying parallel
with the seacoast, which “ must have been thrown up by the
sea, though it does not reach within a mile of some of
these places.” The amount of elevation is uncertain. The
account of J. D. Bennett (Geogr. Journ., vii. 226), represents
it as a low coral island.

Jarvis’s Island, in 0° 22’ S., and 159° 58’ W., is, ac-
cording to J. D. Hague, eighteen to twenty-eight feet in
height, which would indicate an elevation of at least eight
or ten feet. See further page 291.

Malden’s, in 4° 15’ 8., 155° W., two hundred and fifty
miles southeast of Jarvis, visited by Lord Byron, is described
by him as not over forty feet high. It is ten miles long.

Starbuck's, or Hero Island, in 5° 40/ S., 155° 55’ W., is
an elevated lagoon island; but the amount of elevation is not
stated. Like Jarvis’s, it contains a large deposit of gypsum,
but not much guano.—(J. D. Hague.)

Penrhyn’s Island, near 9° 8. and 157° W., has a length
of nine miles, and an extensive lagoon with a boat entrance

376 CORALS AND CORAL ISLANDS.

into it. According to Captain Ringgold of the Wilkes Ex-
pedition, it has a height of fifty feet, which, if correct, would
indicate an elevation of full thirty-five feet. The northwest
side is, throughout, a cocoanut grove.

Fint’s Island, in 11° 26’ S., and 151° 48’ W., is only a
mile and a half long, but is thickly wooded, according to Cap: ‘
tain Ringgold, which is unusual for so small an island.

Staver’s Island, in 10° 05’ 8., and 152° 223’ W., is only
half a mile across, and yet is well wooded. Both of these
islands were passed by Captain Ringgold, but he does not
state the height.—(Wilkes’s Narr., iv. 277.)

Baker's Island, 0° 13’ N., 176° 22’ W., is one mile
long and two-thirds of a mile wide. The greatest height,
according to J. D. Hague, is twenty-two feet, “showing some
evidences of elevation.” (See further, p. 289.) It has prob-
ably been elevated at least s¢x feet.

Howland’s Island, 0° 51’ N., and 176° 32’ W., and about
forty miles north of Baker’s. It is about one and one-half
miles long, and one-half mile wide. The highest point, accord-
ing to Hague, is ten or twelve feet above high-tide level;
which is evidence of but little if any elevation. It is a guano
island like Baker’s.

McKean’s Island, of the Phenix Group (like Phenix, En-
derbury, and Birnie’s), in 38° 35’S., 174° 17’ W,, is a low
island, according to Hague, circular in form, one-quarter of a
mile in diameter, but less elevated than Jarvis Island, It has
a lagoon depression in which there is a gypsum and guano de-
posit; and at high tides the guano is sometimes two feet under
water. Phenia’s Island, near McKean’s, 3° 40’ 8.,170° 52’
W., is less than half a mile in diameter, and the border is only
eight or ten feet high; so that there is no evidence in the
heicht of an elevation. It is also a guano island.

ELEVATIONS IN CORAL PACIFIC REGIONS. ate

Enderbury’s, in 3° 8’ 8., 174° 14’ W., is eighteen feet
high. It has probably experienced some elevation. But the
height of the tides is such in the seas as to give the beach and
drift sands much greater height than they have in the Paumo-
tus. Birnie’s Island is a small bank of coral, only six feet
above the sea, according to Wilkes (Narr., V. 4).

Gardner's, Hulls, Sydney and Newmarket were visited
by the Wilkes Expedition. No satisfactory evidences of ele-
vation were observed on the first three. Newmarket is stated
by Captain Wilkes to have a height of twenty-five feet, which
would indicate an elevation of six or eight feet.

h. Sandwich or Hawaian Islands.—Oahu affords decisive
proof of an elevation of twenty-five or thirty feet. There is an
impression at Honolulu, derived from a supposed increasing
height in the reef off the harbor, that the island is slowly ris-
ing. Upon this point we have nothing satisfactory. The pres-
ent height of the reef is not sufficiently above the level to which
it might be raised by the tides, to render it certain, from this.
kind of evidence, that the suspected elevation is in progress.

Kauaz presents us with no evidence that the island, at the
present time, is at a higher level than when the coral reefs be-
gun ; or, at the most, no elevation is indicated beyond a foot or
two. The drift sand-rock of Koloa appears to be a proof of
elevation, from its resemblance to that of Northern Oahu; but
if so, there must have been a subsidence since, as it now forms
a cliff on the shore that is gradually wearing away.

Molokai, according to information from the Rev. Mr. An-
drews, has coral upon its declivities three hundred feet above
ie sea, We, Karbon

Mr. Andrews, in his communication, informed the author
that the coral occurs ‘“‘ upon the acclivity of the eastern or high-
est part of the island, over a surface of more than twenty or

378

OO

CORALS AND CORAL ISLANDS.

thirty acres, and extends almost to the sea. We had no means
of accurately measuring the height ; but the specimens were ob-
tained at least three hundred feet above the level of the sea,
and probably four hundred. The specimens have distinctly the
structure of coral. The distance from the sea was two to three
miles.”

Coral has been reported to occur on the western peninsula
of Maui, in some places eight hundred feet above the sea; but
according to C. I. Winslow, the supposed coral does not effer-
vesce with acids, and therefore is not calcareous.

On page 324, it is suggested that the westernmost coral
islands of the Hawaian range, Ocean and Brooks's Islands,
may have undergone a small subsidence. Should the bro-
ken wall of emerged rock turn out, on examination, to be
coral reef-rock, instead of the beach sand-rock, the facts would
prove an elevation of a few feet, instead of a subsidence. The
islands differ from Dean’s, in having no long range of wooded
land on the windward side.

7. Heejee Lslands.—The proofs of an elevation of four to six
feet about the larger Feejee Islands, Viti Lebu and Vanua
Lebu, and also Ovalau, are given in the author’s report on this
group. How far this rise affected other parts of the group, he
was unable definitely to determine ; but as the extensive bar-
rier reefs in the eastern part of the group, rarely support a green
islet, they rather indicate a subsidence in those parts than an
elevation.

j. Islands north of the Feejees.—Horne Island, Wallis, Er
lice, Depeyster, and four islands on the track toward the
Kingsmills, were passed by the sloop of war ‘“ Peacock,” of the
Wilkes Expedition ; but from the vessel no evidences of ele-
vation could be distinguished. The first two are high islands,
with barriers, and the others are low coral. Rotuma (177°

ELEVATIONS IN PACIFIC CORAL REGIONS. 379

15’ E., and 12° 30’ N.), is another high island, to the west of

Wallis’s. It has encircling reefs, but we know nothing as to
its changes of level. According to J. 8. Whitnell, Eilice’s
Island, or Funafuti, situated in latitude 9° 8. and longitude
179° E., has a small lagoon basin, dry at low water, which
is shut off from the sea by a wall twenty feet high, consist-
ing of large masses of coral. He regarded the facts as
proof of some elevation.

k. Kingsmill or Gilbert Group. (Map, p. 165.)

Tapateuea or Drummond.—This is one of the southern
islands of the group. The reef-rock, near the village of Utiroa,
is a foot above low-tide level, and consists of large massive As-
treas and Meandrinas. The tide in the Kingsmill seas is seven
feet ; and consequently this evidence of a rise might be doubted,
as some corals may grow to this height where the tide is so high.
But these Astreas and Meandrinas, as far as observed by the
writer, are not among the species that may undergo exposure
at low tide, except it be to the amount of three or four inches ;
and it is probable that an elevation of at least one foot has
taken place.

Apaiang or Charlotte's Island, one of the northernmost of
the group, has the re¢f-rock in some parts raised bodily toa height
of six or seven feet above low-water level, evidencing this
amount of elevation. This elevated reef was observed for long
distances between the several’ wooded islets ; it resembled the
south reef of Nairsa in the Paumotu Archipelago in its bare,
even top, and bluff, worn front. Anislet of the atoll, where we
landed, was twelve feet high, and the coral reef*rock was five
or six feet above middle tide. A wall of this rock, having the
same height extends along the reef from the islet. ‘There was
no doubt that it was due to an actual uplifting of the reef to a
height of full scx feet.

380 CORALS AND CORAL ISLANDS.

Nonouti, Kuria, Marana, and Tarawa, lying between the
two islands above mentioned, were seen only from the ship, and
nothing decisive bearing on the subject of elevation was ob-
served. On the northeast side of Nononti there was a hill
twenty or thirty feet in height covered with trees; but we had
no means of learning that it was not artificial. We were, how-
ever, informed by Kirby, a sailor taken from Kuria, that the
reef of Apamama was elevated precisely like that of Apaiang, to
a height of fiwe feet ; and this was confirmed by Lieutenant De
Haven, who was engaged in the survey of the reef. We were
told, also, that Kuria and Nononti were similar in having the
reef elevated, though to a less extent. It would hence appear
that the elevations in the group increase to the northward.

Marakei, to the north of Apaiang, is wooded throughout.
We sailed around it without landing, and can only say that it
has probably been uplifted like the islands south. Makin, the
northernmost island, presented in the distant view no certain
evidence of elevation.

The elevation of the Kingsmills accounts for the long con-
tinuity of the wooded lines of land, an unusual fact considering
the size of the islands. The amount of fresh water obtained
from springs is also uncommon. (p. 324).

1. The Marshall and Caroline Islands.—The facts in
reference to the islands of these groups, are not yet fully known.
The very small amount of wooded land on the Pescadores in-

clines us to suspect rather a subsidence than an elevation ; and.

the same fact might be gathered, with regard to some of the
islands south, from the charts of Kotzebue and Kruesenstern.
But McAskill’s, as stated on page 342, is an elevated coral
island, having a height of 100 feet.

m. Ladrones.—The seventeen islands which constitute this
group may all have undergone elevations within a recent pe-

ELEVATIONS IN CORAL PACIFIC REGIONS. 381

riod, but owing to the absence of coral from the northern, we
have evidence only with regard to the more southern,

Guam, according to Quoy and Gaymard, has coral rock upon
its hills more than six hundred feet (one hundred toises) above
the sea.

Rota, the next island north, afforded these authors similar
facts, indicating the same amount of elevation.

n. Pelews, and neaghboring Islands.—The island Feis, three
hundred miles southwest of Guam, of the Ladrone Islands, is.
stated by Darwin, on the authority of Lutke, to be of coral,
and ninety feet high. Mackenzie Island, seventy-five miles
south of Feis, is a low atoll, as ascertained by the Wilkes
Expedition. In the Pelews elevated reefs occur at various
heights up to 500 feet, according to Prof. Semper. The most
of the islands of the southern half of the group are coral
islands. and are more or less elevated. In Pelelew, the west-
ern coral cliffs are 250 feet high; the eastern 80 feet.

0. Melanesian Islands. —

New Hebrides. — ‘“‘ Much coral at a great altitude,” ac-
cording to G. Bennett, as reported by Darwin.

Loyalty Islands. — One of the islands is wholly of coral,
and is elevated 250 feet, according to Rev. W. B. Clarke, of
Sydney, in the Journal of the Geological Society, 1847, p. 61.

Bonin Group. — Peel Island has coral reefs raised 50 feet
above tide-level, according to P. W. Graves, in the Journal of
the Geological Society, 1855, p. 552.

Solomon Islands. — According to Dr. Guppy, there are
elevated coral reefs, varying in height from 100 to 1,200 feet.
On St. Christoval, coral occurs to a height not exceeding 500
feet.

The details given on the preceding pages are here pre-

sented in a tabular form.

382

Off North American coast .
Paumotu Archipelago .

Tahitian Group

6c ce

Hervey and Rurutu Groups

Savage Island. .

Samoan or Navigator Islands .

North of Samoa

Scattered Equatorial Islands

“e (74 ‘74

Feejee Islands .. .

North of Feejees .

CORALS AND CORAL ISLANDS.

Clipperton Rock : :
Bi Gridert syn es Vai hake aie noe a 2 or 3
Glermont Lonnerre | 2779). 2 or 3
INairsa or” Dean's) f.:6 a owe ape 6
Elizabeth . 80
Metia or Aurora 250
Ducie’s 1 or 2%
Tahiti . GE 5 amt ty SG Ae
Bolabolat. “cir. ai ee eee 2
Atiu 12%
Mauke . somewhat elevated.
Ma tisdto i.e aoe ea ee a &
Mangaia . 300
Rurutu 150
Remaining Islands 0?
Kua 300?
Tongatabu : ; 50 to 60
Namuka and the Hapaii ; 25
Vavau . 100
100
0
Swain’s Peet i 2 or 3
Fakaafo, or Bowditch, a Fela he 5)
Oatafu, or Duke of York’s 2 or 3
Washington . 2 or 32
Christmas: 2.4 59 "tc Woe oa ae ?
Jarvis . . 8 or 10
Malden’s . (a) er, Deas 20 SOE
Starbuck’ sic oc) (vee aun et ee ?
Penrhyn’s : 30
Flint’sand Stavers” os. eee ?
Baker's. “2.45 .2hi Shee ee ee
lowlands). 4 a ee ae MC ?
Phoenix and McKean’s Si. et ce 0
Enderbury’s. .. \2 =... 5. =270nnam
Newmarket 6 or 8?
Gardner’s, Hull’s, Beir Birnie’ S, 0?
Viti Levu and Vanua Levu, Ovalau, 5 or 6
Eastern of the Feejee Islands . 0?
Horne, Wallis, Depeyster 0?
or 6

Ge sere Mee CS: Pech ins OP Ae ee ae ae

ELEVATIONS IN PACIFIC CORAL REGIONS. ~

383

FEET,

Sandwich Islands. Kauai . 1 or 2
66 2 Oahu . 25 to 60

‘“ é“ Molokai . La 300 ?
“ 66 DU GUT, Fat I eae a 12
Gilbert Islands Tapateuea se 3 2 or 3

6 “6 Nonouti, Kuria, Maiana and

Tarawa . 3 or more.
‘ é PAPAMMAMIBy eh lites k ot sol wonots ce 9)
é «“ Apaiang or Charlotte 6 or 7
‘ ‘“ Marakei . . 3 or more.

‘“ «“ THOTT PANE By g Salear seed come Ua ?
Carolines IMO Naki si ihae feted sol ot teen cles 60
Ladrones Guam . 600
«“ Rota 600
Feis . Sie 90
Pelews . . 10 to 300
New Hebrides ... . several elevations.
Loyalty Islands, one island s 250
Solomon Islands, some islands - 100 to 1,200
Bonin Group, Peel Island 40-50

Several deductions are at once obvious : —

1. That the elevations have taken place in all parts of
the ocean.

2. That they have in some instances affected single islands,
and not those adjoining. Metia is 250 feet high, and yet the
other Paumotus in that part of the archipelago, and also the
Tahitian Islands, have been but little, or not at all, elevated.

3. That the amount is often very unequal in adjacent
islands.

4. That in a few instances the change has been experienced
by a whole group or chain of islands. The Gilbert Group is
an instance, and the rise appears to increase from the south-
ernmost island to Apaiang, and then to diminish again to the

other extremity.
The Feejees may be an example of a rise at the west side

384 CORALS AND CORAL ISLANDS.

of a group, and possibly a subsidence on the east; while a
little farther east, the Tonga Islands constitute another ex-
tended area of elevation. We observe that while the Samoan
Islands afford no evidences of elevation, the Tonga Islands on
the south have been raised, and also the Fakaafo Group and
others on the north.

We cannot, therefore, distinguish any evidence that a
general rise is, or has been, in progress ; yet some large areas
appear to have been simultaneously affected, although the
action has generally been isolated. Metia and Elizabeth
Island, in the Paumotu Archipelago, may have risen ab-
ruptly ;. but the changes of level in the Feejees and the
Friendly Islands appear to have taken place by gradual

action.
] Rat 7 ae BS Ct a

GEOLOGICAL CONCLUSIONS. 385

CHAPTER VI.
GEOLOGICAL CONCLUSIONS.

THE geological bearing of the facts that have been detailed
in the preceding pages may have been already perceived by
our readers. A brief review of the points of more special in-
terest may serve as a convenient recapitulation of the subject.

I. FORMATION OF LIMESTONES.

Coral reefs are beds of limestone made of corals, with the
help of shells and other calcareous relics of the life of the
sea. The mode of formation is essentially the same, which-
ever of the two kinds of organic products — corals or shells
— predominate ; although in one case the bed would be called
coral limestone, and in the other, shell limestone.

The reefs illustrate two different modes of origin of such
beds: (1), by undisturbed growth, with only additions of fine
material to fill up the intervals; (2), by the grinding of the
corals, etc., to fragments, sand, or mud, through the agency
of the waves.

Beds made by the former method, have many open spaces
between the grouped masses or branches, and could not be
turned into a solid layer of limestone, if situated too deep in
the ocean to feel sensibly the movement of the waves,—unless
Rhizopods, or minute shells of some kinds, multiplied so rap-
idly over the same sea-bottom, as to fill up the interstices.
There is no reason to believe that such aid from shells or

86 CORALS AND CORAL ISLANDS.

Od

Rhizopods is consistent with the grouping of living corals .

thickly enough to form reefs.

The other kind of limestone beds referred to, where un-
mixed with the former, grow up in compact layers to the
surface, as a necessary consequence of wave-action ; and lime-
stones are made in such regions, instead of sandstones and
shales, because the material exposed to degradation is corals
and shells, instead of common rocks.

The facts show that there are formed about coral reefs,
in indefinite amount, all the ordinary products of degrada-
tion by wave-action—fragments large and small, down to
sand, and even mud. With such an agent as the ocean’s
waves, driven often by the storm, so powerful and so per-
sistent at lifting, rending,
little account, at least about outer reefs, that some coral

grinding, and transporting, it is of

stems or masses are first weakened below by the boring
sponge or mollusk; and neither fish, nor holothurian, nor alcy-
onoid is needed, in order to keep up the supply of particles
for sand or mud-beds. In accordance with these facts, the reef-
formations illustrate that not only coral conglomerates, or coral
rag, may be made of corals, but also the very finest and most
compact unfossiliferous limestones; that fine compact lime-
stone, as flint-like in fracture as any of Silurian time, is one of
the most common of coral-reef rocks, and is nothing but con-
solidated mud, or fine sand, of coral origin.

The elevated portion of the island of Metia, which con-
sists largely of this kind of white, compact, coral-made lime-
stone, appears to correspond to the interior of the original la

goon of the island; it exemplifies the kind of rock-making ©

which is going forward in most coral island lagoons. In
archipelagos like that of the Feejees, where the reef chan-
nels are very broad, there is an opportunity for the formation

GHOLOGICAL CONCLUSIONS. 387

of very large areas of this compact white limestone, and also
for others of impure or argillaceous limestones.

Besides the kinds of coral rocks above mentioned, there are
also the Beach and Drift Sand-rocks, which are accumulated
and consolidated above low-tide level. These formations illus-
trate the common mode of origin of odlitéc limestones.
They also afford numerous examples of the formation of
coarse and fine conglomerates consisting of beach pebbles—
these pebbles being either worn corals, or shells, or sometimes
of other kinds, if other rocks are at hand.

The general slope of the beach sand-rock and odlite, and
the mixed stratification of the drift sand-rock, are identical re-
spectively with those of beach and drift-sand deposits in other
regions.

II. BEDS OF LIMESTONE WITH LIVING MARGINS.

The coral reef as it lies at the water’s level is in fact a bed
of limestone with living margins; and the living part fur-
nishes material for its horizontal extension outward, and also,
if a slow subsidence is in progress, for its increase upward.
It illustrates an ordinary mode of formation of coral, or of
shell, limestone, whatever the age.

Il. MAKING OF THICK STRATA OF LIMESTONE.

The coral reef-rock has been shown to have in some cases
a thickness of at least 2,000 feet (page 156.) The reefs are,
therefore, examples of great limestone strata, nearly as re-
markable in this respect as the largest of ancient times.

IV. SUBSIDENCE ESSENTIAL TO THE MAKING OF THICK STRATA.

The coral island reef-rock has been shown to depend for
its thickness on a slowly progressing subsidence (p. 263).

388 CORALS AND CORAL ISLANDS.

This is the only method by which any thick stratum of lime.
stone could be made out of a single set of species, for all
such species have a narrow range in depth ; and the only way,
from any succession of species, if those species are alike in
range of depth. |

In the case of existing coral reefs, there is yet no evidence
that the species of the lower beds differ from those of the top.
There is also no evidence, in any part of any ocean, that there
is a set of cold-water corals fitted to commence a reef in deep
water and build it up to such a level that another set of
species may take it and carry it up higher; the facts thus far
gathered are all opposed to such an idea. Should it be here-
after proved that the corals of the inferior beds differ in
species from those now existing, it will probably be found that
the predecessors of those now living were also shallow-water
species; so that the subsidence in any case was necessary.

_V. DEEP-SEA LIMESTONES SELDOM IF EVER MADE FROM CORAL
ISLAND OR REEF DEBRIS.
This point has been discussed on pages 143, 211. The facts
show that the sediment or débris from a shore is almost wholly
thrown back by the waves against the land where it originated,

or over its submerged part in the shallow waters, and that it —

is not transported away to make deep sea formations.

The facts have also a wider bearing, for they teach that
lands separated by a range of deep ocean cannot supply one
another with material for rocks. The existence of an Atlan-
tic ocean continent—an Atlantis—has sometimes been assumed
in order to make it a source of the mud, sand and gravel, out
of which the thick sedimentary formations of the Appalachian
region of North America were made. But if this Atlantis
were a reality, there would still have been needed, in addition

GHOLOGICAL CONCLUSIONS. 389

to the presence of such an ocean continent, a set of freight
carriers that could beat off the waves from their accustomed
work, and push aside the ordinary oceanic currents; or else
; a would get back all its own dirt.

VI. ABSENCE OF FOSSILS FROM LIMESTONE STRATA.

Absence of fossils has been mentioned as a frequent char-
acteristic of the fine compact coral reef-rock, and also of the
beach and drift sand-rock or odlite (pp. 153, 194). The rocks
are formed at the sea-level, and in the midst of abundant life,
and yet trituration by the action of the waves and winds has
in many places reduced all to the finest. material, so that an em-
bedded shell is seldom to be found in the beach or drift odlite,
and rarely too in inuch of the fine-grained coral reef-rock.

The interior basins appear to be eminently the place for
making these non-fossiliferous limestones. This is the case in
two widely different conditions: fist, over the portions that are
below the coral-growing depths, which are sometimes of great
area; and second, im lagoons that have become so small and
shallow that corals and large shells have all disappeared, and
the trituration is of the finest kind, producing calcareous mud ;
such lagoons being properly in a marsh condition. These last
appear to illustrate on a small scale the conditions under which
many of the ancient non-fossiliferous, or sparingly fossiliferous,
limestones were formed.

VII. THE WIDE RANGE OF THE OLDER LIMESTONES NOT EXEMPLIFIED
AMONG MODERN CORAL-REEF FORMATIONS.

Coral-reefs, though they may stretch along a coast for scores

of miles, are seldom a single mile in width at the surface ; and if

elevated above the sea, they would stand as broad ramparts

separated by passages mostly 20 to 200 feet deep, and often of

590 CORALS AND CORAL ISLANDS.

great width. The substratum, however, is, in general, contin-
uous coral-rock; and if these more elevated parts were re-
moved by any process, after an elevation, they would leave a
nearly level area of coral limestone often as extensive as the
whole reef-grounds. In an island like Dean’s, one of the Pau-
motus, these reef-grounds are 1,000 square miles in extent.
Still greater are the Bahama banks, the largest of them being
390 miles long and 200 wide, an intervening “tongue of the
ocean’ excepted.

But the most extensive reef-grounds of the oceans are after
all of small breadth compared with many of the ancient lime
stones of the continents; and the reef-rocks also are peculiar

in their very abrupt limits, the margins sometimes descending

at a steep angle a thousand feet or more. ‘These differences
between the new and the old arise in part from the fact that
the coral reefs of the present era are made about small oceanic
lands, or along the edges of the continents, while the limestones
of ancient time were generally formed over the broad surface
of a continent as it lay slightly submerged. 4

The Abrolhos reefs of the Brazilian coast, described on page
140, illustrate one of the methods by which the coral banks ex-
tend and finally coalesce into beds of wide extent; but these
are small compared with the great limestones of early time,
and owe their slight approximation to them as regards extent
to the wide range of shallow waters there afforded. These
Abrolhos reefs differ from most limestone beds also in being
formed largely of the corals in the position of growth.

The tendency of modern reefs to grow up to the surface in
narrow banks, separated by channels, appears to be unlike any
thing we discover in the old rocks; and it seems to be an un-
avoidable result of growth in the sea, where the waves pile up
barriers, and the currents make, and keep open, channels. The

case of the Australian and Feejee reefs are good examples It

GEOLOGICAL CONCLUSIONS. 391

is possible that such barriers may often have existed in ancient
time, and have disappeared through subsequent denudation of
the surface. But may not the difference between the great even
layers of the continental formations and those of a coral island
have proceeded from the difference in the depth of the seas?
Over the great shallow continental seas where the limestones
were in progress, the waves may have generally been feeble,
and therefore there may have been a less tendency to form nar-
row barriers and deep intervening channels.

The marsh condition of a drying-up lagoon with its forming
limestones has been compared above with that under which an-
cient unfossiliferous limestones were made. The narrow limits

of the former make the comparison unsatisfactory ; for, in the

coral island, coarsely fossiliferous beds are all the while form-
ing about the exterior of the island, but a few miles at the

most from the lagoon-marsh ; while the ancient limestones re-

tain their unfossiliferous character often through many thou-
sands of square miles. Still, the above mentioned difference be-
tween the continental sea and the existing deep oceans may
perhaps account for the diversity of results.

VIII. CONSOLIDATION OF CORAL ROCKS.

All true coral-reef rocks are examples of the consolidation
of material mainly of coral origin—either mud, sands, frag-
ments, or standing corals, the last with mud or sands inter-
mixed—by (1), an under-water process; (2), at the ordinary
temperature ; and they exemplify the mode in which all other
submarine limestones of organic origin have been consolidated.
The process appears to depend on the presence (proved by
chemical analysis) of carbonic acid in the sea waters that bathe
and penetrate the sands. This carbonic acid is derived from
three sources: from (1), the rains which wash it down from

392 CORALS AND CORAL ISLANDS.

the atmosphere; (2) the respiration of all the animal life in
the waters, even down to the simplest and minutest; and (3)
the decomposition of all vegetable or animal débris in the
waters or diffused through the sand or muds. This gas is set
free, therefore, just where it is needed for the work, and is
always ready to perform its part in the process of consolida-
tion. It enables the water to take up carbonate of lime from
the grains of the mass to be solidified, or from outside sources ;
and then the deposition of the same among the grains through
their attractions produces the cementation.

The beach and drift sand-rocks or odlites are different
from the reef-rock in being superficial deposits. The carbonic
acid of the waters performs the same part as in the latter; but
with these, there is alternate wetting and drying during the

ebb and flow of the tides and the succession of gales and quiet °

winds. By this means, the grains become incrusted, and every
new wetting and drying adds a new layer to the surface of each;
and thus the odlitic structure is produced. [acts are men-
tioned on page 153 of pebbles of volcanic or basaltic rocks,
lying loose on a seashore, becoming incrusted in this way with
a milky layer; and of basaltic conglomerates being made by
the same means, the carbonate of lime being added until all
the intervals between the stones were filled up and the whole
made solid; and of an amygdaloidal volcanic rock on a coast
having derived its little calcareous kernels or amygdules from
the same source. ‘The following additional facts are cited
from Mr. Darwin’s Journal (p. 588):

“ Lieutenant Evans informs me that during the six years
he has resided on this island (Ascension) he has always ob-
served that in the months of October and November, when
the sand [of a calcareous beach] commences travelling to-
ward the southwest, the rocks which are situated at the end

GEOLOGICAL CONCLUSIONS. 393

of the long beach become coated by a white, thick, and very
hard calcareous layer. I saw portions of this remarkable
deposit, which had been protected by an accumulation of
sand, In the year 1831 it was much thicker than during any
other period. It would appear that the water charged with
calcareous matter, by the disturbance of a vast mass of calca-
reous particles only partially cemented together, deposits this
substance on the first rocks against which it impinges. But
the most singular circumstance is that, in the course of a
couple of months, this layer is either abraded or redissolved,
so that after that period, it entirely disappears. It is curious
thus to trace the origin of a periodical incrustation, on certain
isolated rocks, to the motion of the earth with relation to the
sun; for this determines the atmospheric currents which give
direction to the swell of the ocean, and this again the arrange-
ment of the sea-beach, and this again the quantity of calcareous
matter held in solution by the waters of the neighboring sea.”

Mr. Darwin, speaking of a large beach of calcareous sand
composed of comminuted and rounded fragments of shells and
corals at Ascension, says, ‘‘The lower part of this, from the
percolation of water containing calcareous matter in solution,
soon becomes consolidated and is used as a building stone ;
but some of the layers are too hard for fracture, and when
struck by the hammer, ring like flint.”

The surface of hills of drift sand-rock often has small de-
pressions that are coated with a smooth, solid crust, as al-

ready explained.

IX. FORMATION OF DOLOMITE OR MAGNESIAN CARBONATE OF LIME.

Analyses of the coral limestone of the elevated coral island
Metia, by Prof. B. Silliman, Jr., have determined the singular
fact that, although the corals themselves contain very little

204 CORALS AND CORAL ISLANDS.

carbonate of magnesia, magnesia is largely present in some

specimens of the rock. The rock is hard (H. = 4), and splint. —

ery in fracture, with a specific gravity 2°690. It affords
on analysis, 38°07 per cent. carbonate of magnesia, and hence,
only 61°93 of carbonate of lime.

Another specimen from the same island, having the spe-
cific gravity 2°646, afforded 5:29 per cent. of carbonate of mag-
nesia.

The former was a compact homogeneous specimen, and the
latter was partly fragmentary. Recent examinations of coral
sand and coral mud from the islands, give no different com-
position, as regards the magnesia, from that for corals, which,
as the analyses on page 99 show, contain very little or no
magnesia. The coral sand from the Straits of Balabac, af-
forded Prof. Silliman carbonate.of lime 98°26, carbonate of
magnesia 1°38, alumina 0°24, phosphoric acid and silica a trace.

This introduction of magnesia into the consolidating
under-water coral sand or mud, has apparently taken place (1)
in sea waters at the ordinary temperature; and (2) without
the agency of any mineral waters except the ocean. But the
sand or mud may have been that of a contracting and evap-
orating lagoon, in which the magnesian and other salts of the

ocean were in a concentrated state. It has been already ob-

served (p. 349), that this was probably the actual condition
of the elevated portion of the island of Metia, every thing
about it looking as if it corresponded to the lagoon part of
the old atoll; and also that the idea of the existence of min-
eral springs there has no support in known facts.

X. FORMATION OF CHALK.

The formation of chalk from coral is known to be ex-
emplified at only one spot among the reefs of the Pacific

GEOLOGICAL CONCLUSIONS. ~\N 4395

The coral mud often looks as if it might be a fit material for
its production; moreover, when simply dried, it has much the
appearance of chalk, a fact pointed out by Lieutenant Nel-
son in his Memoir on the Bermudas (1834), and also by Mr.
Darwin, and suggested to the author by the mud in the lagoon
of Honden Island. Still this does not explain the origin of
chalk ; for, under all ordinary circumstances, this mud _solidi-
fies into compact limestone instead of chalk, a result which
would naturally be expected. What condition then is neces-
sary to vary the result, and set aside the ordinary process.

The one locality of chalk among the reefs of the Pacific,
referred to above, was not found on any of the coral islands,
but in the elevated reef of Oahu, near Honolulu, of which reef
it forms a constituent part. It is twenty or thirty feet
in extent, and eight or ten feet deep. The rock could not be
distinguished from much of the chalk of England : it is equal-
ly fine and even in its texture, as earthy in its fracture, and so
soft as to be used on the blackboard in the native schools,
Some imbedded shells look precisely like chalk fossils. It con-
tained, according to Professor Silliman, 92°80 per cent. of
carbonate of lime, 2°38 of carbonate of magnesia, besides
some alumina, oxyd of iron, silica, ete.

The locality is situated on the shores, just above high-tide
level, near the foot of Diamond Hill. This hill is an extinct
tufa cone, nearly seven hundred feet in height, rising from the
water’s edge, and in its origin it must have been partly sub-
marine. It is one of the lateral cones of eastern Oahu, and
was thrown up at the time of an eruption through a fissure,
the lavas of which appear at the base. ‘There was some coral
on the shores when the eruption took place, as is evident from
imbedded fragments in the tufa; but the reef containing the
chalk appeared to have been subsequent in formation, and

296 © CORALS AND CORAL ISLANDS.

afforded no certain proof of any connection between the fires
of the mountain and the formation of the chalk.

The fine earthy texture of the material is evidence that the
deposit was not a subaerial seashore accumulation, since only
sandstones and conglomerates, with rare instances of more
compact rocks, are thus formed. Sand-rock making is the
peculiar prerogative, the world over, of shores exposed to
waves, or strong currents, either of marine or fresh water. We
should infer, therefore, that the accumulation was produced
either in a confined area, into which the fine material from a
beach may have been washed, or on the shore of a shallow,
quiet sea; in other words, under the same conditions nearly
as are required to produce the calcareous mud of the coral
island. But, although the agency of fire in the result cannot
be proved, it is by no means improbable, from the position
of the bed of chalk, that there may have been a hot spring at
the spot occupied by it. That there were some peculiar cir-
cumstances distinguishing this from other parts of the reefs, is
evident.

This, if a true conclusion, is to be taken, however, only as
one method by which chalk may be made. For there is no
reason to suppose that the chalk of the Chalk formation has

been subjected to heat. On the contrary, it is now well ascer- —

tained that it is of cold-water origin, even to its flints, and that
it is made up largely of minute foraminifers, the shells of
Rhizopods. Professor Bailey found under his microscope
no traces of foraminifers, or of any thing distinctly organic,
in the Oahu chalk.

XI. RATE OF INCREASE OF LIMESTONE FORMATIONS.

On page 253 it is shown that coral-reef limestones are of
slow formation, the rate of increase in thickness, where all

GEOLOGICAL CONCLUSIONS. 397

is most favorable, not exceeding perhaps a sixteenth of an
inch a year, or five feet in a thousand years. And yet such
limestones probably form at a more rapid rate than those
made of shells, because the animals are to a larger extent
calcareous or make proportionally larger calcareous secretions ;
and in addition they have the property of rapid multiplica-
tion by budding. The mollusks that grow and multiply most
rapidly and have proportionally the largest shells are the
Lamellibranchs or bivalves, among which the oyster is a fa-
mous example; and the Brachiopods were once the full equals
of the ordinary bivalves. Large banks of bivalves seldom
occur in regions of corals, the species there being to a great
extent Gasteropods (or univalves) ; and hence the contributions
of shells to coral reefs from mollusks are small compared
with the extent of the beds which, by themselves, they make
on other coasts. The coral seas of Florida nowhere have
shore shell-beds like those of St. Augustine in northern
Florida outside of the coral-reef seas. There is reason for
this in the fact that these bivalves that grow in large banks live
in beds of ordinary sand or mud, such as reef-regions do not

generally supply.

XII. LIMESTONE CAVERNS.

The elevated coral limestone, although in general a hard
and compact rock, abounds in caverns. They may be due in
part to open spaces, or regions of loose texture, in or between
the strata. But in most cases they are a result of solution
and erosion by the fresh waters of the land, or the waves and
currents of the ocean, subsequent to the elevation.

On the island of Metia, many caverns open outward in
the coral limestone cliff and in some were large stalactites, as
stated on page 194.

398 CORALS AND CORAL ISLANDS.

In the raised coral rock of Oahu (p. 871) there are several
long winding horizontal chambers, some of which are the
sources of subterranean streams that open out on the shores
between the layers of the rock, or from the mouths of caverns.
These running waters, and others trickling from above, are
obviously the eroding agents that have made the caves.

As briefly remarked on page 194, caverns are still more re-
markable on the island of Atiu, on which the coral reef-rock
stands at about the same height above the sea as on Oahu,
Rev. John Williams states that there are seven or eight of
large extent on the island. Into one he entered by a descent
of twenty feet, and wandered a mile in one only of its branches
without finding an end ‘‘ to its interminable windings.” He says,
“TInnumerable openings presented themselves on all sides as
we passed along, many of which appeared to be equal in height,
beauty and extent to the one we were following. The roof, a
stratuin of coral rock fifteen feet thick, was supported by massy
and superb stalactitic columns, besides being thickly hung with
stalactites from an inch to many feet in length; some of these
pendents were just ready to unite themselves to the floor, or to
a stalagmitic column rising from it. Many chambers were
passed through whose fretwork ceilings and columns of stalac-
tites sparkled brilliantly, amid the darkness, with the reflected
light of our torches. The effect was produced not so much by
single objects, or groups of them, as by the amplitude, the
depth, and the complications of this subterranean world.”

Other similar caves exist on the neighboring island of
Mauke.

The Bermudas and Bahamas are also noted for their cay-
erns. The great height of the easily eroded drift sand-rock
gives these reef-regions a chance for caverns, large and small ;
a notice of the Bermuda caverns will be found on page 224.

...

GEOLOGICAL CONCLUSIONS. 399

These are examples of the comparatively rapid formation
of caverns. The waters which run or percolate through them
must be charged with carbonic acid to accomplish such work,
and yet they have no source for this ingredient except the atmos-
phere, animal respiration, and vegetable and animal decom-
position in the soil. The flutings and stalactitic incrustations
of a precipice facing the sea must depend on the former alone,
with the aid perhaps of the spray from the sea brought over
the reef by storms.

XIII. OCEANIC TEMPERATURE.

Facts seem to indicate—though perhaps not sufficient to
demonstrate—that the Gulf Stream has had, from the Juras-
sic period in Geological history onward, the same kind of in-
fluence on the temperature of the North Atlantic Ocean which
it now has.

The existence of a coral reef made out of corals of the As-
trea tribe and others, during part of the Odlitic era (middle
Jurassic), in England, as far north as the parallel of 52° to
55° is strong evidence that the isocryme of 68° F., the coral-
reef boundary, extended then even to that high latitude; for
species of the Astrea tribe are now confined to coral-reef seas
(p. 109). This isocryme now reaches along the course of the
Gulf Stream, to a point just north of the Bermudas, near
33° N.; and 55° is 22° beyond this.

There are no marine fossils in any rocks of that period on
the American side of the Atlantic, so that facts fail for defi-
nitely locating the western terminus of this odlitic isocryme of
68° F. But it is highly improbable that the whole ocean
across, on, or near, the parallel of 55° N., should have had, as the
mean temperature for the coldest month of the year, one so high
as 68° F’.; the present average position of the isocryme of 68°F...

400 CORALS AND CORAL ISLANDS.

through the middle of the two oceans, the Pacific and Atlan-
tic, is near the parallel of 27° or 28°, or one-half nearer the
equator than the parallel of 55°. It is difficult to account for
an oceanic temperature high enough to give England’s seas
68° FI’. as the average for the coldest winter month, even sup-
posing the Gulf Stream to have aided; but it is vastly more
difficult if no such northeastward current existed, and the high
temperature extended equably so far from the equator. The
probability is therefore strong that the existence of coral reefs
in the Odlitic era in England was owing to the extension, by
the aid of the Gulf stream, of the isocryme of 68° move than
30° in latitude (and over 3,000 miles in distance) beyond its
present most extra-tropical position, just outside of the Ber-
mudas; in other words, that the whole ocean was just enough
warmer, to allow this oceanic current (part of the great
water-circulation of the globe) to bear the heat required for
corals as far north as northern England.

The present isocryme of 44° F’., as drawn on the chart of
the world accompanying this volume, has approximately the
course which that of 68° F. probably had in Odlitic times. It
should have a little less northing, and the loop to the north
should lean more to the eastward. The latter would have been
a consequence of the submerged condition at the time of most
of the European continent.

The ocean’s waters seem to have cooled somewhat before
the next period—the Cretaceous—began, since evidence fails
of any Cretaceous coral reefs in the British seas; but such
reefs prevailed then in central and southern Europe, so that
the amount of cooling in the interval since the Odlitic era,
had not been large; and as late as the Miocene Tertiary, there
were reef corals in the seas of Northern Italy, above latitude
45° N., or that of Montreal, in Canada.

THE OCEANIC CORAL-ISLAND SUBSIDENCE. 40]

The absence from the American coast of the Atlantic,
of any coral reefs in the Cretaceous beds, and of any reef corals,
seem to show that the oceanic temperature off this coast was
not favorable for such corals; and if so, then the line of 68° |
F’. extended at least 20° farther north on the European side!
of the ocean, than on the Atlantic—an inequality to be ac-
counted for in part by the existence of the Gulf Stream. But,
in addition, the whole range of life in the European Creta-
ceous, and its vastly greater variety of species, leave no doubt}
as to the higher temperature of the ocean along its European
border; so that the idea of a Cretaceous Gulf Stream must be
accepted. And that of a Tertiary is demonstrated by similar
facts.

If the Gulf Stream had its present position and force in
Odlitic, Cretaceous and Tertiary times, then the ocean had,
throughout these eras, its present extension and oceanic char-
acter; and, further, no barrier of land extended across from
South America to the Canaries and Africa, dividing the South
from the North Atlantic, but all was one great ocean. |
Such a barrier would not annul entirely the flow of the Gulf
Stream; yet the North Atlantic is so small an ocean, that if
left to itself, its system of currents would be very feeble.

XIV. THE OCEANIC CORAL ISLAND SUBSIDENCE.

Coral islands have been shown to be literally monuments
erected over departed lands; and, through the evidence from
such records, it is discovered that the Pacific has its deep-water
mountain chains, or lines of volcanic summits, not merely
hundreds, but thousands of miles in leneth. Some of the
ranges of high islands are proved by such records to have an
under-water prolongation, longer than that above water: the
Hawaian Islands for example, which have a length of only

4()2 CORALS AND CORAL ISLANDS.

four hundred miles from Hawaii to Kauai, and five hundred
and thirty to Bird Island, the western rocky islet of the group,
stretch on westward, as the coral registers show, even to a dis-
tance of two thousand miles from Hawaii, or, as far as from
New York to Salt Lake City; and how much farther is un-
known, as the line of coral islands here passes the boundary
of the coral reef seas, or the region where coral records are
possible.

Other ranges of submerged summits are shown to extend
through the whole central Pacific, even where not a rocky
peak remains above the surface; for all the coral islands from
the eastern Paumotus to Wakes’ Island, near long. 170° E,.
and lat. 19° N., north of the Ralick and Radack (or Marshall)
groups, are in linear ranges; and they have, along with the
equally linear ranges of high islands just south, a nearly uni-
form trend, curving into northwest and north-northwest at the
western extremity. The coral islands consequently cap the sum-
mits of linear ranges of elevations, and all these linear ranges
together constitute a grand chain of heights, the whole over five
thousand miles in length. Thus, the coral islands are records
of the earth’s submarine orography, as well as of slow changes
of level in the ocean’s bottom.

This coral island subsidence is an example of one of the
great secular movements of the earth’s crust. ‘The axis of the
subsiding area—the position of which is stated on page 363,
has a length of more than six thousand miles—equal to one-
quarter of the circumference of the globe; and the breadth,
reckoning only from the Sandwich Islands to the Friendly
Group (or to Tongatabu) is over twenty-five hundred miles,
thus equalling the width of the North American continent.
A movement of such extent, involving so large a part of the
earth’s crust, could not have been a local change of level, but

Se

THE OCHANIC CORAL-ISLAND SUBSIDENCE. 4(1)3

one in which the whole sphere was concerned as a unit; for
all parts, whether participating or not, must have in some
way been in sympathy with it.

This subsidence was in progress, in all probability, during
the Glacial era, the thickness of the reefs proving that in their
origin they run back through a very long age, if not also
into the Tertiary. It was adownward movement for the Trop-
ical Pacific, and perhaps for the warmer latitudes of all the
oceanic areas, while the more northern continental lands, or at
least those of North America, were making their upward
movement, preparatory to, or during that era of ice.

The subsidence connected with the origin of coral islands
and barrier reefs in the Pacific has been shown (p. 357) to have
amounted to several thousands of feet, perhaps full ten thou-
sand. And, it may be here repeated that, although this
sounds large, the change of level is not greater than the eleva-
tion which the Rocky Mountains, Andes, Alps and Himalayas
have each experienced since the close of the Cretaceous era,
or the early Tertiary; and perhaps it does not exceed the
upward bulging in the Glacial era of part of northern North
America.

The author has presented reasons for beleving (Am. J. Sci.,
III. v. 1873) that in this Glacial era the watershed of Canada,
between the River St. Lawrence and Hudson’s Bay, was
raised somewhat above its present level (1,500 feet); and
that this plateau thus elevated was the origin of the great
glacier which moved southeastward over New England. This
region is the summit of the eastern arm of the great V-shaped
Archean area of the continent, the earliest elevated land of
North America ; and it is not improbable that the other arm of
the V, reaching from Lake Superior and Huron, northwest-

ward, to the Arctic, was the source of glacial movements over

404 CORALS AND CORAL ISLANDS.

the more central portions of the continent :—we cannot say
western portions also, since in the first place, the facts, accord-
ing to Prof. J. D. Whitney, do not sustain the statement ; and,
in the second, the great mountain ranges of the west would
have been a barrier to all influences from any central conti-
nental elevation, and, besides, the slopes of these ranges, even if
the Pacific border were higher to the north than now, would
have determined the course of all western glacial movements.

The idea that both arms of the great Archzean nucleus were
raised together, is not without some support. For the courses of
the two were the courses of great continental uplifts or move-
ments, again and again, through the successive subsequent ages;
and the present outline of the continent is but the final expres-
sion of the great fact; moreover, the elevations parallel to the
western arm of the V have been much the greatest. Even the —
exceptional courses, such as the nearly north and south trend of
the Green Mountains, were marked out first in the Archzean, the
Archzean peninsula of northern New York with the line of the
Adirondacks being an exhibition of it. And all this uniformity
of movement, from the laying of the first stone in the develop-
ing continent to the last, has been shown by the author to be
directly connected with the fact that the continent has always
been bordered by the same two great oceanic depressions, the
Atlantic, and the larger Pacific, the same in trend of axis as
now, the North Atlantic having a northeast and southwest
trend, parallel with one arm of the Archean, and the Pacific
a northwest and southeast parallel with the other arm of the
Archean. It is therefore reasonable that, late in geological
history, during the Glacial era, after the great mountain chains
of the continent had been made and raised to their full height,
and the surface crust thickened over all the continent, except
that of the Archzan nucleus, by successive beds to a thickness of

THE OCEANIC CORALISLAND SUBSIDENCE. A405

thousands of feet, even thirty-five thousand by the close of the
Paleozoic along the Appalachians, and much beyond this on
the Pacific border; and when these thick sediments had in
many regions been stiffened by crystallization or metamor-
phism; I say it is reasonable that, finally, changes of level,
through the working still of the old system of forces, should
again have affected most the old nucleal Archzan area of the
continent, where there had been no thickening except what
had taken place internally; and that, if one arm of the V,
that along the Canadian watershed, were raised at this time,
the other, northwestern in trend, should also have been raised.
This is at least probable enough to become a question for
special examination over the region. See further the author's
Manual of Geology, 1874.

The northern continental upward movements which intro-
duced the Glacial era, carrying Arctic cold toward the tropics,
may have been a balance to the downward oceanic move-
ments that resulted in the formation of the Pacific atolls. While
the crust was arching upward over the former (not rising into
mountains, but simply arching upward) it may have been
bending downward over the vast central area of the great ocean.

The changes which took place, cotemporaneously, in
the Atlantic tropics, are very imperfectly recorded. The
Bahamas show by their form and position that they cover
a submerged land of large area stretching over six hundred
miles from northwest to southeast. The long line of reefs
and the Florida Keys, trending far away from the land of
southern Florida, are evidences that this Florida region par-
ticipated somewhat in the downward movement, but to a
much less extent than the Bahamas. Again, the islands
of the West Indies diminish in size to the eastward being
quite small in the long line that looks out upon the blank

’ 406 ve CORALS AND CORAL ISLANDS.

ocean, just as if the subsidence increased in that direction.
Finally, the Atlantic beyond is water only, as if it had been
made a blank by the sinking of its lands.

Thus the size of the islands, as well as the existence of
coral banks, and also the blankness of the ocean’s surface, all
appear to bear evidence to a great subsidence.

The peninsula of Florida, Cuba, and the Bahamas look, as
they lie together, as if all were once part of a greater Florida
or southeastern prolongation of the continent. The northwest-
ern and southwestern trends, characterizing the great features
of the American continent, run through the whole like a warp
and woof structure, binding them together in one system; the
former trend, the northwest, existing in Florida and the Ba-
hamas, and the main line of Cuba; and the latter course, the
west-southwest, in cross lines of islands in the Bahamas (one
at the north extremity, another in the line of Nassau, and
others to the southeast), in the high lands of northwestern
and southeastern Cuba, and in the Florida line of reefs, and
even further, in asubmerged ridge between Florida and Cuba.
This combination of the two continental trends shows
that the lands are one in system, if they were never one in
continuous dry land.

We cannot here infer that there was a regular increase of
subsidence from Florida eastward, or that Florida and Cuba
participated in it equally with the intermediate or ad-
joining seas; for the facts in the Pacific have shown that
the subsiding oceanic area, had its nearly parallel bands of
greater and less subsidence, that areas of greatest sinking al-
ternated with others of less, as explained on page 326; and that
the groups of high islands are along the bands of least sinking.
So in the Atlantic, the subsidence was probably much greater
between Florida and Cuba than in the peninsula of Florida it

THE OCEANIC CORAL ISLAND SUBSIDENCE. 407

self; and greater along the Caribbean Sea parallel with Cuba,
as well as along the Bahama reefs, than in Cuba.

The conclusions of Mr. Thomas Bland, based on the distri-
bution of terrestrial mollusks in the Bahama Islands, a sub-
ject with which he had made himself familiar by study in
the region, have much interest in this connection. He
shows? that these mollusks, eighty in number of species,
prove that the alliance of the Great Bahama Bank with
Cuba is very close, as is apparent in the many species of
Polymeta and Strophia, and the occurrence in both of the
genera Polygyra, Thelidomus and Melaniella, not known in
Hayti; while Turk Island bears evidence of connection with
Hayti through the genus Plagioptycha and the species com-
mon to the two, P. Albertsiana and P. disculus.

After mentioning the views in this volume on the dimin-
ished size of the islands to the eastward and the evidence
thereby of subsidence, he says : —

“The facts regarding the diminution in size of the islands
of the West Indies to the eastward are of peculiar interest, not
only as affording conclusive evidence of greater subsidence in
that direction, but also in connection with geographical distri-
bution. The banks and islands forming the long Bahama
chain diminish in size to the southeast, where are situated at
its termination the submerged Mouchoir Carré, Silver and
Navidad Banks. In a similar manner, the submerged Virgin
Island Bank (with Anegada on its northeastern extremity
geologically resembling the Bahamas in the opinion of Dr.
Cleve), Sombrero and the Anguilla Bank, terminate the chain
of the West Indies eastward from Cuba, parallel with the Ba-
hama chain. In the caves of Anguilla the remains of large

1 Annals of the Lyceum of Natural History of New York, Vol. X., 1873, an
abstract of which appeared in the American Journal of Science, 1874, VIII. 231.

408 CORALS AND CORAL ISLANDS.

extinct Mammalia are found which must have inhabited a far
more extensive area subsequently broken up by subsidence.”

The position of the lonely Bermuda atoll confirms these
deductions. Its solitary state is reason for suspecting that
great changes have taken place about it; for it is not natu-
ral for islands to be alone. The tongue of warm water due
to the Gulf Stream, in which the Bermudas lie, is narrow,
and an island a hundred miles or more distant to the north-
east-by-east, or in the line of its trend (p. 219), if experiencing
the same subsidence that made the Bermuda land an atoll,
would have disappeared without a coral monument to bear
record to its former existence. Twenty miles to the south-
west-by-west from the Bermudas, there are two submerged
banks, ten and twenty-four fathoms under water, showing
that the Bermudas are not completely alone, and demonstrat-
ing that they cover a summit in a range of heights; and it
may have been a long range.

In the Indian ocean, again, there is evidence that the
coral-island subsidence was one that affected the oceanic area
more than the adjoining borders of the continent, and most,
the central parts of the ocean. For, in the first place, the
Archipelago of the Maldives narrows and deepens to the
southward (p. 186). Further, the large Chagos Group, lying
to the south of the Maldives, contains but very little dry
land in any of its extensive reefs, while some of them, includ-
ing the Great Chagos Bank, are sunken atolls. Again, still
other large reefs nearly bare lie to the southwest of the
Chagos Bank, and submerged banks exist in the seas north-
east and east of northern Madagascar.

The probability is, therefore, that both the central Atlan-
tic and Indian Oceans included regions of subsidence like the
central Pacific, and that the absence of islands over a large

THE OCHANIC CORAL-ISLAND SUBSIDENCE. 409

part of their interiors may be a consequence of it. A rate of
sinking exceeding five feet in a thousand years (if the estimate
on page 253 is right) would have buried islands and reefs to-
gether in the ocean; while, with a slower rate, the reefs might
have kept themselves at the water’s surface. So small may
have been the difference of rate in the great movement that
covered the Pacific with coral islands, but left the Indian
Ocean a region of comparatively barren waters, with some
“half-drowned” atolls, and the central Atlantic almost wholly
a blank.

While thus seeming to prove that all the great oceans have
their buried lands, we are far from establishing that these
lands were oceanic continents. For as the author has elsewhere
shown, the profoundest facts in the earth’s history prove that
the oceans have always been oceans. These lands in all proba-
bility were, for the most part, volcanic islands or summits of
volcanic ranges, for of this nature are all the islands over the
interior of either ocean that are not of coral origin.

The course of argument leads us to the belief that a very
large number of islands, more than has been supposed, lie
buried in the ocean. Coral islands give us the location of
many of these lands; but still we know little of the extent to
which the earth’s ranges of heights, or at least of volcanic
peaks, have disappeared through oceanic subsidence. Recent
dredgings and soundings have proved that the bottom of the
oceanic basin has little of the diversity of mountain chains
and vallies that prevails over the continents; and, through
this observation (and also by the discovery that some ancient
types of animal life, supposed to have been long extinct, are
perpetuated there), they have afforded new demonstration of
the proposition, above stated, that the oceans have always been
oceans. But while the facts do not imply the existence deep

q

in the ocean of many granitic mountain chains, they do teach —

410 CORALS AND CORAL ISLANDS.

that there are long ranges, or lines, of volcanic ridges and

peaks, and some of these may be among the discoveries of —
future dredging expeditions. A range of deep-sea cones, or —
sunken volcanic islands, would be as interesting a discovery —

as a deep-sea sponge or coral, even if it should refuse, ex-
cepting perhaps a mere fragment, to come to the surface in
the dredge.

We may also accept, with some confidence, the conclusion —

that atolls and barrier reefs originated in the same great bal-

ance-like movement of the earth’s crust that gave elevation —
and cold, in the Glacial era, to high-latitude lands. If so, the —

tropics and the colder latitudes were performing their several
works simultaneously in preparation for the coming era; and
it is a gain to us in our contemplations, that we hence may bal-
ance the beauty and repose of the tropics, through all the pro-
gressing changes, against the prolonged scenes of glacial deso-
lation that prevailed over large portions of the continents.

*
4
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San Fraherseo ©

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APPENDIX.

I. ARTESIAN WELLS ON SOUTHERN OAHU.

On page 287 reference is made to artesian borings on Oahu of the Ha-
waiian Islands, that descended through coral reef strata of much thickness
and at large depths, which are believed to be good evidence of a former
higher level of the island by some hundreds of feet, and therefore of a grad-
ual subsidence since, the only doubt coming from the possibility that the coral
rock at the lower levels may be not true reef rock but of pelagic origin. The
following are the details as to fifteen of these borings in the area between
the centre of the city of Honolulu and Diamond Head on the coast to
the eastward. The level of the surface from which they start is not above
thirty feet.

The positions of these wells may be learned from the following map, — a
reduced copy of a chart received from the surveyor general of the Hawaiian
Islands, Prof. W. D. Alexander. Part of the shore region of Southern
Oahu is here represented, from the city of Honolulu, on the west, to the
tufa cone, Leahi or Diamond Head, whose southern brow has a height of 761
feet. Puowaina, or Punchbowl, standing just back of Honolulu and 498 feet
high, is also a tufa-crater. Rocky Hill, near Oahu College, and some low
hills near by, appear to have been made by flows of lava from fissures of
comparatively recent date. The large and deep valleys of the mountains
Nuuanu and Manoa, and the narrower Palolo valley, open out on this shore-
region near the northern limit of the map.

The map also shows the position and width of the coral reef of the coast,
and of the harbor of Honolulu, which owes its existence to the reef. The
larger part of this shore-region is covered by the elevated fringing reef, which
has a height above the sea of fifteen to twenty-five feet. Its surface is cov-
ered by six to ten feet of a glassy black volcanic sand, which may have been
ejected from the Rocky Hill vent, and four to six feet of surface soil. The
borings descend through the soil and sand, and then the elevated fringing reef
of coral limestone, to a bottom of solid lava, and intersect at some levels,

besides coral rock, layers of tufa, lava, “clay,” sand or bowlders, which are

412 APPENDIX.

the results of occasional eruptions during the growth of the reef, or of depo-
sition from the streams of the valleys.

The positions of the artesian wells are indicated on the map by small
circles, and those referred to beyond have their names attached in small
capitals. They are also numbered within the circles. (A magnifying-glass
may be needed to read the names on the much-reduced map.) The records
of the superintendent of the borings, Mr. J. A. McCandless, were received
through Prof. W. D. Alexander, excepting that of the Atherton well, for which
and for specimens of the materials of the successive beds, the author is indebted
to Mr. W. C. Merritt, President of Oahu College.

The wells are topographically of three groups. Seven of them (numbers
1 to 7) are situated around the base of Punchbowl; four (8 to 11) southwest
of the base of Rocky Hill; and four (12 to 15) northwest to southwest of
Diamond Head, the last one at its base, and only a short distance from the
seashore. In order to interpret the sections it should be noted that the preva-

lent winds under whose action the tufa cones were made are the trades, or |

from the northeast. The thickness of beds and depth are given in feet. To
aid in the comparison of the results, the sections that are most similar are
placed in parallel columns.

1. ARTESIAN WELLS ABOUT THE BASE OF PUNCHBOWL

GNos:/ 120: 7):
1. Foster WELL, £. OF PUNCHBOWL. 2. PaLace WELL, s. or PUNCHBOWL.
Thick- |
et Depth.
Soil 4 ft., black sand4 . 8 Soil 4, black sand 4
Bowlders . . . A 6 14 Cora . A 72
Lava ; cae 40 54 Lava. cae 78
Clay 16, bowlders 12. . 28 82 WHITE CoRAL 138
Clay 20, bowlders8 . . 28 110
Claye ers *. se | 800 410 Clay . 378
CORAL.) §. 40 450 CorAL 452
Clay, gravel, ‘110; clay,
bowlders, 180. . 290 | 740 Clay and gravel . 707
Lava or bed-rock . . rat hoe Lava or bed-rock 762

8. ATHERTON WELL, Ss. OF PUNCHBOWL.

| |
whi | pep

Soil 6, black sand6 . . 12 Corat (clay 5 incl.)
Cora (clay 5 incl. ) a oo || Lets) 190 Gravel and clay .
uRediclayac) ue gaohs 10 200 CoraL Mei ae ts 565
CorRaL . 30 230 Clay, sand, some coral,
Brown clay (coral 6 5 incl. 60 290 gravel mAbs 595
CORATAS ee 3 320 | Bed-rock or lava 655
Browniclayaeem aces menes tS) 355 |

HAWAIIAN GOVERNMENT SURVEY

HONOLULU

AND VICINITY

ENTRANCE
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414

4. THomas SQuaRE WELL, S. OF PUNCH-

APPENDIX.

6. Ice Co. (Sass’) WELL, s. oF Puncu-

BOWL. BOWL.
Thick-_ Thick-
mene | Depth. age Depth
Soil 6, black sand 6, gas + 16 Soil 4, black sand 10 . 14 |
CoraL . : 200 216 CoraL . . j 200 214
Brown clay 44 | 260 IBroWwniClayee me. er ee 3 252
Cora 10 | 270 Cora. SRG areas 25 277
Brown clay 60 380 Clay 40 317
WHITE CoRAL 50 | 380 CorRAL 80 397
Brown clay 80 460 Clay . 53 450
Bed-rock or lava 49 509 Lava, bed-rock | 53 503
|
5. Warp’s, s. OF PUNCHBOWL. 7. WrxLcox, s. of PUNCHBOWL.
poe Depth. aS Depth.
Soil and sand. 15 Soil 4, black sand 6 10
Cora . 204 219 Coray . . 50 60
Yellow clay ; 4 260 Hard lava (fr. Rocky Hill ") 40 100
HARDICORAL. Gg. 51 cu 1 YAGI |) Clyro Ae 30 130
Yellow clay 109 570 Corau 120 250
CoraL . . 23 393 || Clay. 30 280
White and yellow clay, CoRAL . 70 500
sand . . 111 504 Clays 100 450
Lava. 4 508 Bowlders, clay, gravel. 85 | 535
Lava bowlders 40, clay 45 85 620
Lava or bed-rock 40 660
2. ARTESIAN WELLS SOUTHWEST OF ROCKY HILL.
8. DiztiincHam WELL. 9. Darry WELL (SOUTHERN).
ban Depth. apie Depth
Loam, gravel, cay» 90 Soil, sand, sash 60
Corat . : ‘ 40 130 CoRAL . ae iO) 70
Clay . 60 190 Clay. . as 35 105
CoraL . 20 210 Genus (clay 5 incl. ) 100 205
Black clay . : 25 230
Clay, black sand 40 250 Conran 2. 10 240
Clay 30, sand, “gravel . 40 | 280
Lava (fr. Rocky Hill *) 50 300 Lava, black (fr. Rocky
Hil yee beara ae 10 | 290
— — ——— — as — U -
10. Darry WELL Ueaeues aN 11. Marques WELL.
Thick Depth. br | Depth
Soil? claw, 4). eeke oe 30 | Earth 10 |
Bowlderclayies 0 = 90 120 | Sandy, soft Corat. 20 |. 730
Coratandclay. . . 15 135 Lava, layers of gravel. 40. |" 30
CoRAL . Bakes on 10 145 Clay. = ewes ahr) eee 3 100
Clay, sand, ‘gravel che: 25 170
Bed- rock, lata. ake. 3 213 Lava in layers 195 295

APPENDIX. 415

The above four wells all bear evidence of their nearness to Rocky Hill,
a lateral lava vent, and of their distance from the seashore.

3. ARTESIAN WELLS NORTHWEST TO SOUTHWEST OF
DIAMOND HEAD.

12. Panos WELL, 6300 FEET FROM THE Coast.

Thick- | Depth. | pee | Depth.
CIEST So ete RS aes an a7 Wet CORATy Rem hes! aN [3 50 175
Bowlders 17, clay 10 . . 27 64 Clayaneta seine es t's 20 195
Clay and coraL . : 8 72 CORA aerate eae 1. 80 275
Bowlders: 4) - 00s: 2. 8 80 Clay, sand. . Viet: 100 375
“COOTES pgek Re Geet Beate 26 106 Lava, or bed-rock . . . 32 407
va . 19 125

13. Goo Kim’s We tt, 4000 FEET FROM THE CoasT.

3 %, l :
Thick- | Thick-
Wak | Depth. | ness. | Depth.

| as

Soil 10, anyel 133 AAS aA 23 | Clays 203) en Gace te ee 20 290
Lava. . ee 43 GON > Consu = aa ee Pee 50 344
MEaPeE Un asi ne bor |) 124) Clays. co oo obs eh 20" S80
Cora 5 Sh) bop als ee 26 150 WORAT@ htcak se Be fee 0 430
eee et I 96> 1 ATEN “Clay Sis we ie vane eis
GRATES a ee 94 270 || Lava, bed- rock bu fetes male 65 540

14. Kine’s WELL, asoutr 1000 FEET FROM THE COAST.

Thick- |

ness, | Depth. | aS Depth.
Soilland coral ss). - 38 || Soft Cora, 30, white, 451 75 495
RVhite. CORAL’ 9. -<. 4 22 GOmN SClay-euert aa. : 30 525
ellow:sand) =) 0. * 43 103 Corarn, white; “27: 100 625
Wie = Mo ene 47 150) |, Clay. (tough). 9: 2° =. 5 630
White Conran... . 110 260 || Coranandclay. .. . 70 700
Clay . . canto f wae: 120 380 Clay 28, black sand2. . 30 73
Roe, hard'.0) 51 2s ee 40 420 || Lava,bed-rock . .. . | 120 850

15. CAMPBELL’s WELL, NEAR THE Foot or D1AmMonp HEAD, ABouT A MILE MORE
TO THE SoutTH THAN WELL No. 14.

ahi Depth. a ae Depth.
Gravel, beach-sand. . 50 WHITE CORAL, soft . . | 28 1048
Tufa, like that of Dia- Rock soft like soapstone. | 20 1068
mond Head . . . 270 329 Brown clay, with broken |
CoRAL, HARD, WHITE, | Corley has Renee ee eT O Sa eLnTS
LIKE MARBLE. 9. .) | 605 825 || Lava. . 45 1223
Dark brownclay .. . 75 900 Black clay 10, red pipe
Washed gravel . . . . 25 925 | clay, 18> 28 =| 1251
WP Prauaae ited, cumeeiact hme 249 1500

Mery red clay)... .%: 95 | 1020

1 5 foot layer of clay included.

416 APPENDIX.

The great distance of the Pahoa well from the coast is a reason for the
small amount of coral rock.

b]

The water of Campbell’s well was “as salt as brine ;” it stood in the

casing about a foot higher than the water of a surface well adjoining.

The artesian wells indicate great diversity in the thickness of the coral

formation, and a dependence, as regards thickness, on distance from the hills.
Campbell’s well, No. 15, is about 4,000 yards from the hills, and has a lime-
stone bed of “hard coral rock, like marble” over 500 feet thick, which at
bottom is 865 feet below the sea-level, besides a bed of probably similar origin
over 200 feet lower; while King’s well, about one half nearer the hills has the
bottom of the lowest coral bed at 700 feet. ‘The same level in Goo Kim’s
well, No. 13, is at 430 feet, and in the Pahoa well, No. 12, still nearer the
hills (the distance but 1,500 feet), 275 feet. All these levels are below the
limit of growing corals, and far below the Hawaiian limit, which, according
to Mr. Agassiz, is less than 100 feet. But the intercalation of beds of lava,
tufa, and clay (either tufa beds or decomposed lava) make a close comparison
of the sections in this respect with one another impracticable.

The wells about Punchbowl have great interest. The Foster and Palace
wells, Nos. 1 and 2, are about 600 yards from the base of Punchbowl, and
bear N. 70° W., and S. 65° W., from its centre. In each, the bottom bed of
coral extends to a depth of about 450 feet (450 in No. 1, and 452 in No. 2),
which, as No. 2 shows, is 442 feet below the top of the elevated fringing reef.

Hence, in the case of each, the coral reef rock at bottom is more than 350
feet below the Hawaiian limit of growing corals. Difference in position must
account for the difference in the upper portion of the two sections; and the
chief fact as to position is that the Foster well is within the Nuuanu Valley,
where clay and bowlders may have been carried down by its running waters.
But in an important feature they are alike, the coral bed just referred to hav-
ing over it, in one, 300 feet of “clay,” and in the other, 278 feet. This thick
bed of “clay ” was very probably made by cinder-ejections from Punchbowl;
for the tufa of Punchbowl (a kind of palagonite), if brought up from a boring,
would feel and look much like clay. This is further sustained by the fact that
in the wells more to the south of Punchbowl, little of the clay-bed was found,
the localities being too far to the eastward to receive much of the cinders;
their bearings from the centre of Punchbowl are between S. 30° W., and
Spiel De

In each of the five wells, Nos. 3 to 7, the top layer consists of 12 to 16
feet of soil and black sand. Below this, in Nos. 3 to 6, there are, severally,
178, 200, 200, 204 feet of the coral limestone of the elevated fringing reef ;
and the lower limits of the bottom layer of coral are, severally, at 515, 380,
397, 393 feet. In the Wilcox well, No.7, a bed of lava, 40 feet thick (perhaps

APPENDIX. 417

from Rocky Hill) with 30 feet of “ clay” below it, intervenes between the
first and second beds of coral limestone, which two have together a thickness
of 170 feet. It is an interesting example of the adverse circumstances attend-
ing coral growths about an island of active fires.

The author is unable to find in the facts from these wells evidence that
sustains, as urged by Mr. A. Agassiz,’ the Murray hypothesis, or anything
that sets aside the various objections to this hypothesis that have been pre-
sented. In two of the Oahu wells, the Jaeger well and one of the Govern-
meut wells on the Waikiki road, according to Professor Alexander, carbonized
cocoanut-wood was found at a depth of about 150 feet, beneath a 150-foot
stratum of coral. ‘This and all the other observations in connection with the
wells are fully explained by the theory of subsidence.

Oahu is an example of an island that has had an upward shove, notwith-
standing the progressing subsidence. The amount of elevation indicated by
the elevated coral reef is about 25 feet on the south side and 50 to 60 feet on
the north side. The Kahuku bluffs of the vicinity of the north cape are not
made wholly of drift sand, as Mr. Agassiz concluded from his observations.
The bluffs have a top layer of wind-drift origin, while the rest, 50 to 60 feet
high above the sea-level by estimate, is true coral reef rock, as the author has
illustrated elsewhere. Such a change of level, as already stated, is not
against the subsidence theory ; it is one of the common incidents of a volcanic
region.”

Il. RATE OF GROWTH OF CORALS AND CORAL REEFS.

Arrangements made at Tahiti for measuring the rate of growth of a coral
reef. — The arrangements made by Captain Wilkes for measuring the rate
of growth of coral reefs are mentioned on page 257. A memoir by MM. Le
Clerc and De Benazé was published at Paris in 1872, giving an account of
their attempts to make use of the stone planted by Captain Wilkes. They
made various measurements; but they observe that Wilkes does not state

1 Coral Reefs of the Hawaiian Islands, by Alexander Agassiz, Bulletin of the Mu-
seum of Comparative Zoology, 1889, X VII., 121.

2 The fact that corals were growing about Oahu during the time when tufa eruptions
were in progress over the southern border of the island, is proved by facts recently com-
municated (Dec. 11, 1889) to Professor Alexander, by Rey. 8. E. Bishop, of Honolulu.
He observes that in Halawa, Ewa, at the cutting for the railway across two small bays of
Pearl Lochs, and south of Kuahua Island, there occur, in the finely laminated tufa, layers
of comminuted shells and corals. Mr. Bishop concludes that these materials were “ prob-
ably torn off from the sides of the fissure of ejection that was presumably opened through
the anciently subsided strata of corals and shells.” Fragments of corals and shells were
still more abundant over the top of the tufa. Scattering pieces of coral were also observed

by Mr. Bishop imbedded in the tufa of the low craters southwest of Koko Head.
27

418 APPENDIX.

whether he measured from the top of a head of coral, or from the solid bank
on which the corals were growing: and, further, that the use of our “ excel-
lent spirit level” from a stone of so little length is not sufficiently exact for
correct results, and hence they draw no conclusions from their trials.

Before leaving the region, they made the following arrangements with
reference to future measurements. They planted two blocks of coral, cement-
ing them below, nearly burying them in the soil, placing them 0.21 metres
above the Wilkes’ stone, which is between them. ‘They then put a mark
upon them on plates of metal directed toward the place of observation on the

shoal. A third stone was placed forty metres from the southwest angle of

the Point Venus lighthouse, in order to give a second observation on the
position of the spot on which the soundings were to be made. This spot was
found to bear from the two new stones N. 77° 30’ E.; from the third stone,
N. 70° 55’ E ; from the bell of the new Mission Church, S. 81° 40! E. A
horizontal line passing from the mark on the new stone is 7.460 metres above
the madreporic heads.

They also made observations which satisfied them that Tahiti was not at
present undergoing any general elevation. ‘Two maps accompany the pam-
phlet: one is copied from Wilkes; the other is from a chart by Lieutenants
Le Clere and Minier, and contains lines showing the positions of the points
referred to above. See page 246.

The following letter on the Rate of Growth of Florida Corals, for which
the author is indebted to Mr. H. T. Woodman, the investigator of the subject,
was received too late for the use of the facts in the earlier part of this work.
It is dated :—

BurrFato, N. Y. Jan. 25, 1890.

** Herein I enclose a copy from the diary which I kept while on the Florida
Reef during the winter of 1881-82. It gives almost the exact growth of such
massive corals as I found or placed in a shallow channel across the reef just
east of the Tortugas Group in the winter of 1867-68. The water on the reef
at this point was only about three feet deep with two feet more in the chan-
nel at lowest tide. From the healthy appearance of the Madrepora cervi-
cornis on both sides of the channel, with Miilepora alcicornis and Porites
clavaria in close proximity, as well as good specimens of Meandrina labyrin-
thica, Orbicella cavernosa, and a Dichoccenia within a few feet of the centre
of the channel, I was inclined to think that no more favorable locality to
watch the growth of some of the massive corals could be found. I therefore
added to the above-named massive forms, a Siderastrea, Orbicella annularis,
Porites astreoides, Diploria cerebriformis, Manicina areolata, Meandrina

sinuosa, and M. clivosa.

APPENDIX. 419

In the winter of 1881-82, the last time they were examined, their growth
for the fourteen years was as follows : —

Dichoceenia had an upward growth of a fraction less than half an inch;
Orbicella cavernosa, seven eighths of an inch; O. annularis, one and a quarter
inches large; Siderastrzea, a fraction less than five eighths of an inch; Diplo-
ria cerebriformis, almost three fourths of an inch; Meandrina sinuosa, an
inch and a quarter large; M. labyrinthica, an inch and seven eighths; Mani-
cina areolata, a fraction less than five eighths of an inch.

We could not get the exact rate of growth of the Mzandrina clivosa for

‘the reason that it was not regular. The specimen, when placed in the chan-
nel, measured less than six inches and in the fourteen years had spread out
over eight inches, being, when last measured, about fourteen inches in diame-
ter, while its upward growth was three or four inches in some places and less
than an inch in others; and what appeared still more strange was the fact
that the thickest piece was near the edge. I regard the growth of this species
very uncertain. I have frequently seen it on bricks and other objects in the
form of an incrusting coral and measuring six or more inches in diameter with
perhaps less than half an inch in thickness. It is the most abundant coral at
Fort Taylor as well as at Fort Jefferson.

Madrepora cervicornis had so encroached upon the channel as to oblit-
erate all of my marks, hence I know but little of its rate of growth; but it is
certain that the channel had been narrowed from six to eight or perhaps ten
feet by this coral.

The temperature of the water has much to do with the growth of some
species of corals. I do not now recall a single instance of finding a specimen
of Dichoccenia or of Orbicella cavernosa, except in close proximity to the Gulf
Stream. The largest specimen of O. cavernosa I have ever found was in a
four foot channel where the waters of the Gulf Stream encroached upon the
reef. This specimen, as nearly as I can recall, was about eighteen or twenty
inches in diameter and is now in the Washington University, St. Louis, Mo.
Should I live until 1892-93 it is my intention to remove these corals, when I
shall be glad to give you the exact increase in twenty-five years.”

Respecting the supply of food for the growing corals of a reef, it is to be
considered that the amount of life is elsewhere unequalled. Prof. 'T. Fuchs,
in his paper on the Distribution of Oceanic Life (page 118), speaks of such
seas within the depth of twenty fathoms as “the gathering ground of an

>

extremely rich fauna;” a fauna that embraces “the whole splendor of the

animal life of the Indian and Pacific Oceans,” and as being of so peculiar

29

character that the terms “coral fishes” and “coral mollusks”? would not be

inappropriate.

Ill NAMES OF SPECIES IN THE AUTHOR’S REPORT ON
ZOOPHYTES.

TuE following catalogue contains the names that are now accepted for the species
of Actinoid Coral Zoéphytes described in the Author’s Report. The changes have
chiefly resulted from the subdivision of the old genera. The catalogue has been pre-

pared for this place by Prof. Verrill, and the explanatory notes have been added by
him.

NAMES IN THE AUTHOR’S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING

FROM THOSE OF THE REPORT,

Page 159. Euphyllia pavonina Flabellum pavoninum Lesson.

<< anthophyllum fe anthophyllum #. & H.
spheniscus spheniscus #. & IH.
o rubra = rubrum #. & H.
4 spinulosa Desmophyllum spinulosum Verridl.
a glabrescens unchanged.
Z gracilis Ud
fs aspera EKusmilia aspera 2. & H.
oh aperta <c fastigiata H. & H. (?)
“ costata(p. 720) “« _ costata Verrill.
ee rugosa unchanged.
rs turgida cg
se meandrina Euphyllia fimbriata #. & H.
sinuosa Pterogyra sinuosa H. & H.
6 cultrifera “ cultrifera 2. & 71.
170. Ctenophyllia pectinata (not of Zam.) Pectinia Dane FH. & H.
quadrata 2 quadrata #. & /I.
pachyphylla ss pachyphylla #. & H.
profunda sg profunda 7. & #.
174. Mussa! fastigiata
es carduus unchanged.
. angulosa -
a corymbosa us
‘i cactus zl
z costata

sinuosa (not of Hillis) Mussa tenuidentata #. & H.

NAMES OF SPECIES. 421

NAMES IN THE AUTHOR’S REPORT.

{74. Mussa cytherea

“

“ce

“

“

multilobata
cerebriformis
regalis

crispa (not of Lam.)

dipsacea
fragilis
gyros
recta
nobilis

189. Manicina amarantum

ce

“,

fissa
areolata *
meandrites
hispida
prerupta
dilatata

195. Tridacophyllia lactuca
196.

oe

“

pzeonia
manicina

198. Caulastrea furcata

199.

“ce

“

distorta
undulata

205. Orbicella * radiata

720.
205. Siderina galaxea
205. Astrea (Fissicell.) speciosa

6

argus

glaucopis

patula

curta

rotulosa

coronata

hyades

excelsa

pleiades

annularis

stellulata

stelligera

crispata

microplithalma (not of
Lam.)

ocellina

orion

uva
ananas
pandanus
puteolina
as pallida
dipsacea

NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT.

unchanged.

“ec

«

“e

Mussa Indica Verril/, and Mussa ra-
dians Verrill.
Isophyllia dipsacea Verrill.

& fragilis Verrill.
Colpophyllia gyrosa FE. & H.
unchanged.

Tracyphyllia amarantum #, & H.
Colpophylia.
Manicina areolata Hhr.

unchanged.

Orbicella radiata Dana.

<c cavernosa Verrill.

glaucopis Dana.
Acanthastrea patula 2. & 1
Plesiastrea curta H. & H.
Astrea rotulosa Lam.
Plesiastrea coronata #7. & H.
Solenastrza hyades Vervrdll.

Ke excelsa Pourtales

s pleiades Vervrill.
Orbicella annularis Dana. J
€¢ stellulata Dana.

Plesiastrea stelligera H. & H.
Ulastrea crispata H#. & H.

Cyphastrea Danie LH. & J.

ocellina #. & H.
Orbicella orion Dana.
Siderastrea radians Verril.
Astrea? speciosa Dana.
Dichocenia uva H. & H.
Astrea ananas Lam.

« _ pandanus Dana.
puteolina Dana.
pallida Dana.
Acanthastreea dipsacea Verritll.

“

oe

29

NAMES OF SPECIES.

NAMES IN THE AUTHOR'S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING

FROM THOSE OF THE REPORT.

205. Astrea (Fissicella) porcata (not of

ce “ee

“é oe
6c “6
“ “
“ «
ce sf
“e “<<
aN “
“ce «

“ its
ii “
“ “
“ “é
oe “ce
“ “é
“ oe
“ “
ee “<c
oe “e
“ it
“ “
oe “
“cc “
“e “
« ti
« “
“ ce
“ “

Esper) Astrea Dane Verrill.

flexuosa Prionastrza flexuosa Verrill.
fusco-viridis “ fusco-viridis #. & H.
virens < virens 1. & H.
echinata Acanthastrea echinata H. & HA.
fragilis Astrea fragilis Dana.

tenella Acanthastrea tenella Verriil.

magnifica (not
of Bv.) Prionastrea spectabilis Verrill.

filicosa Orbicella filicosa Verrill.
versipora Astrea versipora Lam.
ff (var.) « Putnami Verrill.

denticulata (not

of Lam.) « cellulosa Verriil.
pectinata « —_ pectinata Dana.
deformis Aphrastrea deformis 7. & H.
var. dedalina Cceloria deedalina Verrill.
varia Ceeloria spongiosa, var. H. & H.
rigida Isophyllia rigida Verriit.

reticularis (not
of Lam.) Prionastrea Agassizii L. & H.

petrosa Dichoccenia petrosa Verriil.
purpurea Leptastrzea purpurea Verrill.
pulchra ‘Ki pulchra Verrill.
pentagona Goniastrea pentagona Verriil.
iavistella « favistella Verrill.
var., from

Wakes Il. Astrea Pacifica Verrill.
eximia Goniastrea eximia 1. & H.
sinuosa Prionastrea sinuosa Verrill.
melicerum sf melicerum #. & H.
parvistella Goniastrea parvistella H. & H.
favulus Prionastrea favulus Verrill.
cerium Goniastrea cerium H. & H.

intersepta (not of
Lam.) Plesiastrea armata Verrill.

abdita Prionastreea abdita H. & H.
tesserifera S tesserifera H. & H.
robusta se robusta H. & H.
complanata (?) oe complanata #7. & H.
heliopora Orbicella heliopora Vervill.
“figured Prionastrea valida Verridl.
Hemprichii Prionastrea Hemprichii #. & H.
halicora sf halicora H. & H.
cyclastra Astrea cyclastrea Dana.
favosa Prionastrea favosa H. & £1.

254. Meandrina dedalea (not of Hillis) Ceeloria deedalina, var. Ver7idll.

“cc

spongiosa ai spongiosa #. & H.

oe labyrinthica Meandrina labyrinthiformis Verril..

NAMES IN THE AUTHOR S REPORT.

NAMES

205. Meandrina strigosa

interrupta
rustica
valida

phrygia (not of Hilzs)

gracilis
tenuis
filograna
cerebriformis
truncata
mammosa
cylindrus
caudex

266 Monticularia microcona

“e

“ce

lobata
polygonata

270. Phyllastrea tubifex
271. Merulina ampliata

“

regalis
speciosa
crispa
folium
scabricula
laxa
rigida

278. Echinopora undulata

“ce

«

ai

rosularia
ringens
reflexa
aspera
horrida

289. Fungia cyclolites

“

tenuis

glans

discus
agariciformis
var. tenuifolia
dentata

echinata (not of Pal/as)
var. from Feejees

repanda

integra
confertifolia
horrida
actiniformis
crassitentaculata
Paumotensis
dentigera
scutaria

OF SPECIES. 4323

NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT.

unchanged.

“e

Meeandrina rudis Verrill.
Leptoria gracilis #. & H.
es tenuis #. & H.
Meandrina clivosa (young) Verrill
Diploria cerebriformis H & H.
B truncata H. & A.
Meeandrina clivosa Verrili.
Dendrogyra cylindrus Hhr
ss caudex Hhr.
Hydnophora microconos #. & H.
¢ exesa Hi. & H.
i polygonata H. & H
unchanged.

“
[73
“

“e

Hydnophora Demidoffi Fischer.
Clavarina scabricula Verriill.
unchanged.

Hydnophora rigida #. & H.
unchanged.

“e
“ee

“e

Trachypora aspera Verriil.
Acanthopora horrida Verrilz.
Cycloseris cyclolites #. & H

ss tenuis Verrill
a glans Verrill.
unchanged.

Fungia patella H#. dé H.
Fungia tenuifolia Dana (not HL. & H.)
unchanged.
Fungia Dane #. & H.
« —_lacera Verriil.
unchanged.

«““

oe

Lobactis Paumotensis Verrili.
es Dane Verrill.
Pleuractis scutaria (Ag. MSS.) Ver-
rill.

+

4

NAMES

NAMES IN THE AUTHOR'S REPORT.

289.

307.

311.
813.

319.
821.

335.

Fungia pectinata
« Ehrenbergii
« var. gigantea
“_ asperata
“ — Ruppellii
“<"_ erassa
Herpetolithus limacinus
Herpetolithus interruptus
© foliosus
stellaris
strictus
crassus
Halomitra pileus (not of Linn.)
Polyphylla talpa
‘ leptophylla
sigmoides
dd pelvis
5. fungia
pileiformis
galeriformis
Zoépilus echinatus
Pavonia explanulata
ad crispa
oe papyracea

“cc

OF SPECIES.

NAMES NOW ACCEPTED, WHEN DIFFERING

FROM THOSE OF THE REPORT.

Ctenactis echinata (Ag. MSS.) Verridl

“ Ehrenbergii Verrild.
gigantea Verrill.
asperata Verriil.
echinata Verrill.
crassa Verrill.
Herpetolitha limax Esch.

“

“ “ce

unchanged.
Halomitra clypeus Verrill.
Cryptabacia talpina 7. & H.
se leptophylla Z. & A.
< sigmoides Verrill.
unchanged.

Lithactinia pileiformis Z. & H.

galeriformis #. & A.

unchanged.

Podabacia crustacea #. & H.
Haloseris crispa H. & H.
Leptoseris papyracea Verriill.

as elephantotus (not of Pallas) Mycedium elegans &. & H.

ae cactus unchanged.

a pretorta x

6 formosa s

se venusta se

os divaricata <s

“ boletiformis (not of Zam.) Pavonia Dane Verrill.
< frondifera unchanged.

= decussata 6

“ lata 6

ss crassa
cs var. loculata

ae siderea
Ke latistella
a clavus

Agaricia (Undaria) undata
“ “
Lam.)
speciosa
levicollis
planulata
“ (Mycedia) cucullata
ss ss purpurea
fragilis
gibbosa
agaricites

“ec “e

“ “

a “6

rugosa (not of

Pavonia loculata Verrill.
Siderastrea siderea Blainv.
unchanged.

Siderastrea clavus Verrill.
Undaria*® undata Dana.

‘s monticulosa Verriil.

« speciosa Dana.

<s levicollis Dana.
Asteroseris planulata Vervill.
Mycedium elephantotus #. & 7
Agaricia purpurea Les.
Mycedium fragile Verrild.
Agaricia gibbosa Dana.

rs agaricites E. dé H

ee
.

$35. Agaricia (Mycedia) cristata (not of
345. Psammocora obtusangula
ee plicata (not of Lam.)
< fossata
columna
sf exesa
349. Monomyces anthophyllum
ss eburneus
870. Cyathina cyathus
« pezita
«s Smithii
#6 turbinata
375. Desmophyllum dianthus
* stellaria
876. Culicia stellata
« tenella
“ truncata
879. Caryophyllia cespitosa
< conferta
i flexuosa
s arbuscula
cornigera
“ anthophyllum
t solitaria
sc pocillum
« dilatata
385. Dendrophyllia ° ramea
micrantha
% nigrescens
“ aurantiaca
se coccinea
“ diaphana
st rubeola
oh scabrosa
391. Oculina hirtella
ss horrescens

B29.

NAMES

NAMES IN THE AUTHOR'S REPORT.

oe prolifera
se axillaris

varicosa

s oculata

“ pallens

S virginea

s diffusa

Anthophyllum musicale

< fasciculatum
a astreatum
f cespitosum
a hystrix
“ cuspidatum)

OF SPECIES.

425

NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT.

Iam.) Agaricia Dane H. & H.

unchanged.
Psammocora frondosa Verrili
unchanged.

“

“se

Flabellum anthophyllum Z, & H.
Caryophyllia cyathus Lam.

“ce

“ec “e

< Smithii Stokes.

Os clavus Scaceni.
Desmophyllum crista-galli #. & H. (2)
unchanged.

Cladocora cespitosa Forbes.

¢ conferta H. & H.

‘ stellaria 1. & H. (2)

ss arbuscula Hdw.
Dendrophyllia cornigera.
Lophohelia anthophyllites #. & H.
Astrangia solitaria Verrill.
Phyllangia pocillum Verriii.

unchanged.

“ce

“ce

Dendrophyllia Danze Verrill.
unchanged.

Balanophyllia scabrosa Verrtll
Sclerohelia hirtella H. & H.
Acrohelia horrescens H. & H.
Lophohelia prolifera #. & H.
Cyathohelia axillaris #. & H.
unchanged.

unchanged,

Lophohelia oculata Powrt.
unchanged.

Galaxea musicalis Oken.
fascicularis Oken (in part)
ss astreata E. & H.
cespitosa Verrill.

hystrix Verrtll

cuspidata Oken.

426

NAMES OF SPECIES. i

NATIES IN THE AUTHOR’S REPORT.

399.

404.
406.

409.

414.

418.

420.

423,

435.

491.

Anthophyllum clavus
Stylina echinulata
Astroitis calicularis

ss viridis

Gemmipora palifera

ie peltata

wo patula

os crater

i cinerascens

es frondens

# brassica
Astreopora pulvinaria (not of

Lam.)

a punctifera

ee fungiformis

s stellulata

Isaura Hemprichii
“ — Savignii
«aster
“speciosa

Zoantha Ellisii

re sociata
Gi Solandri
ne dubia

i

" Bertholetii
Palythoa denudata

e auricula

be nymphea

< fuliginosa

s mammillosa

< ocellata

¢ glareola

a flavo-viridis

yy argus

‘2 ceesia
Madrepora

Names of species unchanged ex-

cept the following:

. 238, corymbosa (not of Lam.)
« 26, plantaginea (not of Lam.)
« $28, acervata
« 56, secunda
« 90, deformis (not of Mich.)

Manopora

NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT. *

Galaxea clavus H. & II.

unchanged.

Goniopora viridis H. & H.

Turbinaria palifera H. & H.
fs peltata H. & H.
‘s patula #. & H.

crater Oken.

s cinerascens Oken.

« frondens Verrill.

sc brassica #7. & H,

Astreopora profunda Verrill.

unchanged.

Turbinaria fungiformis Verrill.
Us stellulata #. & H.

unchanged.

“

Zoanthus Ellisii Lam.

“ sociatus Les.

‘¢ Solandri Les.

se dubius Les.

<< Bertholetii Hhr.
Mammillifera denudata HAr

e auricula Les.
‘ nymphea Les.
ee fuliginosa Hhr
unchanged.
Madrepora.

Madrepora convexa (young) Dana.

s appressa (var.) Dana.

ca plantaginea Lam.

‘ nobilis (var.) Dana.

‘s Dane Verrill.
Montipora.

Names of species unchanged, except

the following:

No. 1, gemmulata
« 6, crista-galli (not of Hr.)
« , spumosa (not of Lam.)

, circumvallata

Turpmaria gemmulata Verrid.
Montipora aspera Verrill.

M. hispida (var.) Dana.

M. monasteriata H. & H.

NAMES OF SPECIES. 427

NAMES IN THE AUTHOR’S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT.
No. 9, foliosa (not of Paillas) M. Ehrenbergii Verridl.
“« 21, nudiceps M. crista-galli #. & Z.
« 25, tuberculosa (not of Lam.) M. Dane F. & H.
511. Alveopora retepora unchanged.
“ dedalea (from Red Sea) -
e ce (specimen fig-
ured) Alveopora Verrilliana Dana.
ee spongiosa unchanged.
f rubra Montipora rubra 7. & H.
s fenestrata unchanged. -
*¢ viridis ss ,
515. Sideropora digitata Stylophora digitata #. & H.
ce elongata ss cs
6 pistillata ss pistillata Schweigger.
: subdigitata ée es
as palmata “ Dane Li. & H.
a mordax ss mordax Verriil.
619. Seriatopora subulata anchanged.
& lineata of
sf hystrix s
octoptera oe
us caliendrum se
oy var. gracilis Seriatopora gracilis Dana.
ss valida unchanged.
523. Pocillopora Pocillipora.
Names of species unchanged, ex-
cepting :
No. 3, brevicornis (var. from Sand-
wich Is.) Pocillipora cespitosa Dana.
“6, favosa (var. from Feejees) “s Dane Verrill.
ge « (var. from Sandwich
Islands) < aspera Verrill.
“7, verrucosa (var. from Sand- :
wich Islands) s avvilis Verril.
“ 15, plicata (var. from Sandwich
Islands) e aspera (var. lata) Verrill.
540. Heliopora coerulea unchanged. :
§43. Millepora alcicornis o
ae moniliformis Millepora alcicornis (var.) Linn.
ns ramosa unchanged.
pumila %
iy tortuosa e:
plicata .
a complanata A variety of plicata. (?)
squarrosa unchanged.
platyphylla <e
: mordax 4
va compressa ss
clavaria My

flexuosa “

428

NAMES IN THE AUTHOR'S REPORT.

NAMES OF

551. Porites furcata

“ee

recta
divaricata
conferta
nigrescens

var. mucronata
palmata

levis

cylindrica

contigua (not of Hsper)

astraoides

SPECIES.

NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT.

unchanged,
Porites furcata (var.) Lam.
unchanged.

ce

ce

Porites mucronata Daya.
unchanged,

«e

“c

Synarea Dane Verrill.
unchanged.

conglomerata (not of Zam.) Porites lutea @ & H.

lobata
fragosa
limosa
favosa
cribripora
informis
erosa
monticulosa
lichen
reticulosa
arenacea

569. Goniopora pedunculata

“ce

“cc

columna
Savignyi

571. Errina aspera
575 Antipathes 7 spiralis

oe

ct%

anguina
larix
eupteridea
pectinata

myriophylla

subpinnata
reticulata
flabellum
ericoides
mimosella
pinnatifida
cupressus
paniculata
pennacea
scoparia
foeniculum
corticata
lacerata

pyramidata

Boscii

alopecuroides

urborea

unchanged,

Synarea informis Verriii.

6 erosa Verrill.
6 monticulosa Verriill.
unchanged.

“cc

Porites arenosa H. & H.
unchanged.

it9
“
“
ce
&é
ce

“e

Hyalopathes pectinata H. & H.
unchanged.

“

Hyalopathes corticata 1. & H.
unchanged.

Hyalopathes pyramidata 4 & H.
unchanged.

“ee

«

NAMES OF SPECIES. 429

NAMES IN THE AUTHOR’S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING
FROM THOSE OF THE REPORT.
575. Antipathes dichotoma unchanged.
<4 glaberrima Leiopathes glaberrima /. & H.
fi compressa s compressa FY. « H.

' The genus, Mussa, as here restricted, includes both Mussa and Symphyllia of
Milne-Edwards and Haime,—different specimens of the same species sometimes dif-
fering in the same way, and to the same extent, as do these two so-called genera. The
only difference given, is dependant upon the mode of growth.

> It is probable that this, and some of those following it, are only varieties of one
species.

3’ The name Orbicella is now restricted to the genus of which 0. annularis and
O. cavernosa are types. This group is equivalent to Heliastrwa of Edwards and
Haime, of more recent date.

* The genus, Astrea, is here restricted to the group of which A. rotulosa is the
type. This was the original type named by Lamarck, in 1801, when the genus As.
tr@a was first established. The genus, thus limited, is equivalent to Aawvia of Oken,
1815.

5 The genus, Undaria, is equivalent to Pachyseris Edw. and Haime, of later date.

5 Cenopsammia is recombined with Dendrophyllia, because in certain species
part of the corallets have the structure of the former genus, and others that of the
latter, even in the same specimen. ‘The only distinction made is that the former
genus has a smaller number of lamellz,—a character that is by itself seldom of gen-
eric value.

7 The genus, Antipathes, as here adopted, includes Cirrhipathes, Arachnopathes,
and Rhipidopathes of Edwards and Haime. ‘Those divisions were based only upon
the modes of growth and branching, which are quite insufficient for establishing gen-
era among Polyps.

tN DEX:

ABACTINAL, 22. ;
Abrasion and solution in the making of
lagoon-basins and channels, 293.
Abrolhos reef, 140, 352.
Acontia, 54.
Actinacea, 61.
Actinal, 22.
Actinaria, 61.
Actinia, 20, 22.
Actinoid Polyps, 21.
Adamsia palliata, 35.
Admiralty Islands, 345.
Africa, reefs of eastern, 350, 351.
Agaricia agaricites, 99, 113.
Agassiz, A., on Arachnactis, 28.
Florida reefs, 204, 285.
Oahu artesian wells, 287, 417.
Murray hypothesis, 285.
effects of currents, 288.
on solution as a means of making
lagoon-basins, 295.
change of level in W. Indies, 304.
elevated coral rock in Peru, 336.
‘¢ Seaside Studies,’’ 68, 102.
‘« Three Cruises of the Blake,’’ 204,
205.
Agassiz, L., on Astrangia, 68.
depth of reef corals, 116.
coral borers, 121.
Florida reefs, 204, 205.
Salt Key Bank, 211.
Pourtalés Plateau, 211.
Ahii, 171, 177, 182, 200.
Aiou, 343.
Aitutaki, 375.
Aiva, 264.

Alcyonacea, 82.
Alcyonium, derivation of term, 80.
Alcyonoid Polyps, 21, 80.
Aldrich, Capt. R. N., on Christmas
Island, 274, 279.
on Macclesfield Bank, 271.
Alexander, W. D., records of artesian
borings and chart of Oahu received
from, 411.
Almirante, 350.
Alveopora, 75.
spongiosa, 77.
Verilliana, 77.
Anguilla Key, 212.
Anthea cereus, 37, 57.
flagellifera, 37.
Anthelia lineata, 83.
Antipathacea, 62.
Antipathes arborea, 63.
Apaiang, elevation of, 167, 383.
Apamama, 164, 380.
Apatite on Mauke, 324.
Apia, harbor of, 243.
Aratica, 177, 180, 203, 301.
Arru Group, 346.
Artesian borings on Oahu, 287, 411.
Ascension Island, 392.
Asia, temperature of ocean along the
east coast of, 336.
Asie, 343.
Astrea ananas, 114.
gravida, 113.
pallida, 57, 64.
Astreeacea, 64.
distribution of, 109.
Astrangia Dane, 68.

432 INDEX.
Atiu, 194, 341, 372, 398. Bourne, G. C., on Diego Garcia, 279.
Atlantic Ocean, subsidence in, 403. Bowditch’s Island, 168, 169, 198, 311.
Atolls, structure of, 162, 174. Branchie in Actiniz, 59.

origin of, 251, 266. Brazil, corals of, 113.

origin of lagoons of, 258, 293. reefs, 140, 352.

proportion of, having entrances to | Brooks’s Island, 360, 378.

lagoons, 300. Bryozoans, 19, 105.

completed, 309. Budding in Actiniz, 40.

submerged, 191, 271. Budding in Coral Polyps, 48.
Aurora Island, 193. Bunodes gemma, 22.
Australian reefs, 185, 142, 148, 289, 345,| Byron, of the ‘‘ Blonde,’’ apatite on

346, 365. Mauke, 324.

Bauwamas, 214. Caicos group, 215.

depths near, 173, 216. Calaminianes, 347.

drift sand rock, 185. Calicle, 42, 44, 48.
Bailey, J. W., chalk of Oahu, 396. California Gulf, corals of, 112.
Baker’s Island, 241, 297, 318, 376. Cancrisocia expansa, 24.
Balbi, on encircling reefs, 266. Cape St. Ann, 351.
Barrier reefs, origin of, 248, 258, 261. | Carlshoff, 169, 177, 180, 203.
Beach formations, 184, 221. Caroline Archipelago, 169, 170.
Beechey, on Henderson Island, 194. elevations in, 380.

soundings by, 172. Caryophyllia cyathus, 42.

on Gambier Islands, 266. Smithii, lasso cell, 33.

on Elizabeth Island, 370. Smithii, animal of, 67.

on Ducie’s and Osnaburgh Islands, | Carysfort Island, 172.

371. Cat Island, 215.

Bermudas, structure of islands, 183, 218. | Caulastrea furcata, 54, 58, 95, Plate IV.

corals of, 114. Caverns, 194, 224, 392, 397.

depths near, 173. Celebes, 346.

drift sand rocks, 185. Ceylon, reefs of, 350.

former extent of, 408. Cheetetes, 105.

caverns of, 224. Chagos Bank, 191, 192, 270, 350.

red earth of, 225. Chalk, origin of, 394. :

winds of, 225. of Oahu, 399.
Beveridge reefs, 374. Chamisso, plants of the Marshall Islands,
Biche-de-Mar, 160. 328.
Bird Island, 342. Charlotte’s Island, 379.
Birds of Coral Islands, 329, 330. China, coast of, free from corals, 349.
Birgi, large crustacea, 198. Christmas Island in Pacific, 178, 375.
Birnie’s Island, 173, 196, 318, 377. in Indian Ocean, 274, 279.

Bischof, composition of sea water, 100. | Cladocora arbuscula, 54, 69, 95.
Bishop, 8. E., on fragments of Coral in | Clarke, H. J., budding in Actiniz, 28.

Oahu tufas, 417. Clarke, W. B., on Lafu, 344.
Bland, T., on Bahama and W. India| Classification of Actinoid Polyps, 61.
mollusk, 407. Clermont Tonnerre, 171, 370.
Bolabola, 371. Clipperton Rock, 369.

Bonney, T. G., on Darwin’s theory, 261. | Cnidz, 30.
Borneo, 347. Cocoanut Grove on Bowditch Island, 311.

INDEX.

Cocoanut tree, uses of, 326.
Coenenchyma, 60.
Columella, 44, 60.
Commensalism in polyps, 24, 62.
Comoro Islands, 350.
Cook, Captain, on Christmas Island,
375.

Cope, E. D., on species of the Anguilla
caves, 305.

Cophobelemnon clavatum, 91.

Corals changed to a phosphate by guano,
319.

Corals, rate of growth of, 125, 253, 418.
temperature limiting, 108, 419.
influence of impure and fresh wa-

ters on distribution, 119.
injured by boring animals, 121.

Coral, precious, 90.

Coral heads, 139, 140, 145.
islands, forms and features, 161.
submerged, 191, 271.
poor place for human development,

332.
Coral mud and sand of bottom, 142,
150, 181, 182, 183.

Coral reefs, rate of growth of, 253.
benefits from, 159.
geographical distribution of, 335.
submarine slopes off reefs, 288.
forms determined by marine cur-

rents, 288.

Coral reef harbors, 160.
seas, extent of, 336.

Coral rock, 152, 206, 212, 217, 221, 385.
solid compact of Metia, 194, 386.

Coral sands, formation of, 142, 227, 385.

Coral sand-rocks, 152, 154, 392.

Corallet, 48, 60.

Corallide, 90.

Corallines, 107.

Corallium from the Sandwich Islands, 91.

Corallium rubrum, 89.

Corallum, 42, 48, 60.
composition of, 98, 105.
hardness of, 98.

Corynactics viridis, lasso-cells, 33.

Coryne, 102.

Cosmoledo, 350.

Costz, 160.

28

433

| Couthouy, J. P., on Anthea flagellifera,
37.
Crosby, W. O., Cuba elevated reefs,
Oe
Ctenactis echinata, 45, 66.
Cuba, elevated reefs of, 275.
Currents, as a means of giving atolls
their features, Guppy, 288, 290.
A. Agassiz, 288.
Pacific, 296.
Cyathophylloids, 21, 78.

Dati, W. H., Vermetus nigricans on
Florida shores, 206.
Dana’s Report on Zodphytes, names of
species of, 420.
Danger Island, depths near, 173.
Darwin, on Coral Reefs, 7, 261.
depth of reef corals, 115.
rate of growth of corals, 123.
origin of coral mud, 231.
thickness of reefs, 157.
on soundings, 172.
on the Maldives, 172, 186, 189,
268.
on the Chagos Bank, 192, 270.
on the Gambier Islands, 157, 265.
origin of barrier and atoll reefs,
261.
geographical distribution of coral
reefs, 339.
consolidation of coral sands, 393,
395.
objections to theory of, considered,
Dh
Dead men’s fingers, 83.
Dean’s Island, 169, 203, 301, 369.
Dendrophyllia arborea, 75.
cornigera, 75.
nigrescens, 51, 75.
Depeyster Island, 171, 378.
Depth of reef-corals, 114,
Depths near coral reefs, 173, 216, 219,
288.
Diego Garcia, 172, 279.
Diploria cerebriformis, 65, 418.
Stokesi, 114.
Disappointment Islands, 172.
Dissepiments, 60.

434

Distribution of corals, 108, 114.
of coral-reets and islands, 335.
Dolomite, formation of, 593.
Dorippe facchino, 24.
Drift sand-rocks, 154.
on Oahu, 155.
Bahamas and Bermudas, 155, 214,
221.
of Florida reefs, 185, 206.
Drift of sands changing with the sea-
sons at Baker’s Island, 297.
Drummond’s Island, 167, 314, 315, 379.
Duchassaing, growth of a Madrepora,
124.
Ducie’s Island, 371.
Duff’s Islands, 344.
Duke of Clarence’s Islands, 169, 374.
Duke of York’s Island, 198, 314.

Eap, 343.

Easter Island, 339.

Echinopora reflexa, 43.

Echthorza, 35.

Edwards & Haime, Phyllangia Ameri-
cana, 69.

Edwardsia callimorpha, 25, 41.

Egmont Island, 172.

Elevations in the Pacific, 368.

Elizabeth Island, an elevated coral isl-
and, 370, 384.

Ellice’s Island, 346, 378.

Ellice group, 301.

Enderbury’s Island, 182, 186, 196, 577.

Endotheea, 60.

Koa, 341.

Epiactis prolifera, 40.

Epitheca, 60.

Eugorgia aurantiaca, 87.

Eunicella, 87.

Eupagurus pubescens, 62.

Evans, Lieut., consolidation of coral
sands of Ascension Island, 392.

Exotheca, 60.

Exuma sound, 216.

FaKAAFo, 168, 169, 198, 391, 374, 384.
Fanning Group, 374, 375.
Favosites, 77, 104.

relation to Alveopora, 76, 77.

INDEX.

Feejees, corals of, 110.
delta of Rewa, 248.
reefs of, 262, 341, 363.
elevations among, 378, 383.
Whippey harbor, 249.
Feis, 381.
Fewkes, origin of lagoon-basin, 303.
Fission, fissiparity, 56.
Fissures in reef-rock, 177.
Fitzroy, Captain, soundings by, 172.
temperature about the Galapagos,
336.
Flabellum spheniscus, 43.
Flint’s Island, 376.
Florida Reefs, soundings, etc., 173, 185,
204, 285.
rate of growth of corals of, H. T.
Woodman, 418.
Florida region, subsidences in, 304.
former connection with Cuba and
South America, Heilprin, 306.
Flustra, 105.
Foraminifers of reefs, 152.
Forchammer, magnesia in corals, 99.
Four Crowns, 199.
Frigate bird, 331.
Fringing reefs, 129.
Fuchs, T., light a cause limiting the
depth of species, 118.
rich fauna of coral-reef seas, 419.
Fungacea, or Fungia tribe, 66.
distribution of, 109.
Fungia echinata, 45.
lacera, 45, 46.
Dane, 66.

GALAPAGOS, temperature about, 300.
Gambier group, 157, 266, 340, 361.
Gardner’s Island, 377.
Gemmipora, 75.
Gente Hermosas, 374.
Geographical distribution of coral reefs,
ODD:
Gilbert Islands, 163, 170, 183, 240, 301,
$15, 342)
elevations in, 379, 383.
toddy of, 327.
water of, 324.

Globigerina mud, 143.

INDEX.

Goniopora columna, 52, 94, 97.

Gorgonacea, 89.

Gorgonia flabellum, 85.

flexuosa, 85.
quercifolia, 86.

Gorgoniz, spicules of, 86.

Gosse, P. H., species of Peachia, Ed-
wardsia, etc., 25.

lasso-cells, 54.

on Anthea cereus, 37.

spontaneous fission in Anthea, 57.

mention of his work, British Sea-
Anemones, 95.

Grand Cayman Bank, depths near, 173.

Growth of corals, rate of, 125, 253, 418.

Guam, 343.

elevation of, 381.
Guano, birds contributing to, 318.
islands of Pacific, 318.

Gulf Stream, influence of, in the Juras-
sic and Cretaceous eras, on the tem-
perature of the Atlantic Ocean, 400.

Guppy, H. D., on the Solomon Islands,
290.

objections to theory of subsidence,
288, 290, 291.
Gypsum on coral islands, 318, 321.

HagukE, J. D., sands shifted in position
with the season, 241.
guano islands of Pacific, 318.
birds of Pacific coral islands, 530.
effect of heated air of coral islands
on winds, 316.

Hale, H., on Gilbert Islanders, 324.
on subsidence at Ponape, 367.

Halocampa chrysanthellum, 25.

Hapaii Group, 341, 373.

Haplophyllia paradoxa, 80.

Harbors and channels, conditions deter-
mining the formation and condition
of, 243.

Hartt, C. F., corals of Brazilian coast,
115.

Brazil reefs, 140, 352.

Hawaiian chain, length of, 401.
western, coral atolls of, 342, 360, 364.
northwestern part, soundings in, 173.
subsidence in, 360.

435

Hawaiian Islands, corals of, 111.
elevations at, 377.
Hawaii, reefs of, 342.
Heilprin, A., on the Bermudas, 218, 223,
225, 304, 307.
on connection of Florida and South
America, 306.
on the views of Mr. Fewkes, 304.
Heliolites, 94.
Heliopora, 93.
Henderson Island, 172, 194.
Henderville Island, 167.
Henuake, 167, 182, 199, 329, 369.
Hero Island, 525, 375.
Hervey Group, elevations in, 372.
Hexacoralla, 64.
Hogoleu, 342.
Holothuria, dried, 160.
Honden, see Henuake.
Hopper Island, 167.
Horne Island, 542, 378.
Horsburgh, J. J., on the Maldives, 189.
Howland’s Island, 319, 376.
Huahine, shells of, at elevations, 371.
Hull’s Island, 197, 377.
Hunt, E. B., rate of growth of corals, 125,
on Florida Reefs, 204, 207, 288.
Hunter’s Island, 343.
Hydra, 101.
Hydrallmania falcata, 102, 103.
Hydrocoralline, 70, 103.
Hydroids, 101, 105.

INDIAN OCEAN, reefs of, 347.
subsidence in, 408.
Ireland Island, Bermudas, 220.
Isis hippuris, 88.
Isothermal or isocrymal chart, 108, 335,
399.

JAMAICA, elevated reef of, 275.
Jarvis Island, 168, 316, 319, 375.
Jones, J. M., on the Bermudas, 218.

Jukes, Australian reefs, 142, 181.
Julien, on guano minerals, 323.

Kao, 341.
Kauai, 306, 324, 377.
| Kawehe, 179, 202.

436

Keeling’s Island, 172, 261, 350.

INDEX.

MAccLESFIELD Bank, 271.

Kent, W. S., on Veretillum cynomo- | Mackenzie Island, 343, 381.

rium, 92.
Key West, 205, 206.
Kingsmills, see Gilbert Group.
King’s Island, 200.
Kophobelemnon, see Cophobelemnon.
Kotzebue, water of coral islands, 325.
Kuria, 164, 173, 380.
Kusaie, 342.

LaccaDIVvEs, 350.
Ladrones, 342, 380.
Lafu, 344,
Lagoons of atolls, 151, 182, 293, 300, 303.
Lasso-cells, 30.
Leconte, Joseph, on Florida Reefs, 204.
growth of Madrepora, 127.
Lefroy, on Red Earth of Bermudas, 225.
Leidy, J., size of a lasso-cell, 33.
fossil mammals of the West Indies
and Florida, 306, 308.
Leptogorgia, 86.
Leptoria tenuis, Plate IV.
Level, changes of, in the Pacific, 357,
368.
Life and death in concurrent progress, 94.
Lime in sea-water, 100.
Limestones, formation of, 385.
beds of, with living margins, 387.
thick strata of, 387.
subsidence essential to the making
of thick strata of, 387.
deep sea, from coral reef debris,
rarely made, 388.
rate of increase of, 253, 396.
consolidation of, 591.
Lisiansky, soundings by, 173.
on islands northwest of Kauai, 341.
Lister, J. J., on Christmas Island, 274.
Lixo coral reef, 140.
Loculi, 60.
Logs on coral islands, 196, 316, 317.
Loochoo, 347.
Los Guedes, 342.
Los Matelotas, 342.
Louisiade Archipelago, 133, 135, 269,
345.
Loyalty Group, 344, 381.

Madagascar reefs, 350.
Madrepora from a wreck, growth of,
126, 254. ;
aspera, 50, 71.
cervicornis, 99, 1138, 124, 418.
cribripora, 72, 120.
formosa, 73.
palmata, 99, 113.
prolifera, 115, 124.

Madreporacea, distribution of, 109.

Madreporaria, 64.

Meandrina cerebriformis, see Diploria,
clivosa, 118, 125, 255, 418.
labyrinthica, 65, 113, 114, 418.
sinuosa, 113, 418.
strigosa, 114.

Magnesia in corals, 99.

Magnesian coral limestones, 393.

Mahlos Mahdoo atoll, 189.

Maiana, 164, 380.

Makin, 167, 380.

Malden’s Island, 291, 375.

Maldives, 162, 172, 186, 268, 350.

map of, 187.

Manicina areolata, 99, 113, 418.

Mangaia, 274.
elevation of, 372.

Manhil, 203.

Manuing, F. A., analyses of coral sand

a red earth, 225.

Manopora, 72.

Manuai, 373.

Marakei, 167, 380.

Margaret, 199.

Marquesas Key, Florida, 209.

Islands, 340, 361.
Marshall Islands, 170, 183, 301, 325,
328, 380.

Matea, see Metia.

Maui, elevation of, 378.

Mauke, 324, 341, 372.

McAskill Islands, 342.

McCandless, artesian borings on Oahu,

411.

McKean’s Island, 376.

Melitza, 89.

Menchicoff Island, 170, 268.

INDEX.

Mendana, 343.
Merulina, 56, 66, Plate VI.
Metia, 172, 193, 274, 329, 383, 386.
magnesian limestone of, 393.
Metridium marginatum, lasso-cell, 33.
Millepora alcicornis, 104, 105, 113, 114,
418.
Minerals of coral islands, 317.
Mitiaro, 341.
Mobius, K., on lasso-cells, 30.
Molluccas, 346.
Molokai, elevation of, 377.
Montipora, 72.
Moresby, Captain, on the Maldives and
Chagos Bank, 172, 186, 192.
Moseley, H. N., on Heliopore, 93.
on Millipores, 104.
Mud of channels and lagoons, 142, 150,
181, 182, 183.
Muricea, 87.
Murray, J, on the talus theory and
Tahiti, 281.
erosion of emerged lands, 293.
Mussa, 64.

Narrsa, 169, 203, 369.
Namuka, 373.
Navidad reef, 271.
Navigator Group, 341, 362, 374.
Nazareth Bank, 271.
Necker Island, 342.
Nelson, on Bermudas, 218.
on Bahamas, 217.
Nettling cells, 30.
New Britain, 345.
New Caledonia reefs, 135, 148, 344.
New Guinea, 345.
New Hebrides, 348, 381.
New Ireland, 345.
Newmarket, 377.
New Zealand Old Hat, 255.
Nonouti, 164, 169, 380.
Norfolk Island, 344.
Nukunono, 374.
Nullipores, 107, 174.

Oanu, 306, 324, 411.
caverns in elevated coral reef of, 398.
chalk of, 395.

437

Oahu, artesian borings on, 287, 411.
drift sand rock, 155, 417.
map of part of, 413.
tufa cones of, 411.
Oatafu, 198, 374.
Ocean, depths of, see Depths.
temperatures of, 335, 399.
Ocean Island, 362, 378.
Oceanic currents carry away little detri-
tus from islands, 143.
Oceanic subsidence, proofs of, 368, 411.
Oculina arbuscula, 99.
diffusa, 114, 125, 256.
pallens, 114.
speciosa, 114.
Valenciennesii, 114.
varicosa, 69, 114.
Oculinacea, 66, 109.
Okatutaia, 341, 372.
Old Hat, of New Zealand, 235.
of Anticosti, 236.
Oolite, 153, 156, 392.
Oolitic rocks of Florida Keys, 206.
Orbicella annularis, 113, 125, 418.
aperta, 113.
cavernosa, 55, 113, 114, 418.
Orbicellidee, 67.
Orbitolites about Australian reefs, 152.
Organ-pipe Coral, 84.
Otuhu, 199.

Paciric, elevations in, 368.
axis of subsidence in, 363.
subsidence by broad anticlinals and
synclinals, 363.
chain, length of central, 402.
Palao, see Pelews.
Pali, 44.
Palmyra Island, 375.
Panama, corals of, 111, 339.
Pandanus, 314, 327.
Paractis rapiformis, 23.
Paumotus, 111, 169, 301, 340.
elevations in, 369, 385.
botany of, 325.
Pavonaride, 93.
Peachia hastata, 25.
Peacock’s Island, 171, 177, 182, 200.

| Pearl and Hermes Reef, 361.

438

Pelews, 280, 307, 381.
Pennatulacea, 91.
Penrhyn’s Island, 375.
Peritheca, 60, 97.
Persian Gulf, reefs in, 350.
Pescadores, 170, 380.
Phillippine Islands, 348.
Phoenix Group, depths near, 173, 179.
on islands of, 195, 376.
Phyllastraea tubifex, 56, Plate I.
Phymactis clematis, 22, Plate II.
florida, 22.
veratra, 22, Plate II.
Pitcairn’s Island, 340.
Plants of Paumotus, list of, 326.
Plexaura crassa, 114.
flexuosa, 114.
homomalla, 114.
Plexaurella, 87.
Pliobothrus, 105.
Plumularia faleata, 102.
Pocillipora, 70.
Pocillipora grandis, 71.
elongata, cell of, 71.
plicata, cell of, 71.
Polyps, classification of, 20, 61, 80.
Ponape, 342
Porites family, Poritide, 75.
size of some, 146.
astreoides, 113, 114, 418.
clavaria, 113, 114, 418.
levis, 75, 79.
mordax, 53, 78.
solida, 113.
Port Natal, 351.
Pourtalés, L. F. de, on Thecocyathus, 43.
on Haplophyllia, 80.
rate of growth of corals, 124.
bottom outside of Florida reefs, 142,
207, 210.
depth of reef corals, 116.
on Florida region, 204.
Pourtalés Plateau, 211.
Pouynipete, 342.
Powell, Lieut., on the Maldives, 186.
Pterogorgia Americana, 114.
acerosa, 114.
Pumice on coral islands, 226, 317.
Pylstaarts, 341, 374.

INDEX.

QUATERNARY changes of level in West
Indies, 304.
Quelpaert’s Island, 347.
Quoy and Gaymard, depth of reef corals,
115.
Ascension Island, 351.

RAIVAVAT, 373.
Rapa, 340.
Raraka, 169, 171, 201, 301.
Rarotonga, 373.
Red Earth at Bermudas, 225.
at Tongatabu, 317.
Red Sea corals, 111, 350.
Reefs, formation of, 227.
causes modifying forms of, 242.
rate of growth of, 253.
windward side of highest, 240.
Rein, on Bermudas, 218.
Renillide, 93.
Revillagigedo Islands, 339.
Rice, Wm. N., on the Bermudas, 218,
221, 224; 295.
Aimetara, 373.
Ringgold, Captain, on Penrhyn’s and
other islands, 376.
Rivers, effects of, 245.
Rocks, see Coral.
Rose Island, 328.
Rota, elevation of, 381.
Rotuma, 342, 378.
Rugosa, 78.
Rurutu, 340.
an elevated island, 372.

SAGARTIA parasitica, 36.

St. Augustine shell rock, 397.

Sala-y-Gomez, 340.

Salomon Islands, see Solomon Islands.

Salt Key Bank, 210, 211.

Samoa, see Navigator Group.

Sandwich Islands, see Hawaiian.

San Salvador, 216.

Savage Island, 374.

| Savali, 862.

Sawkins, elevated reef of Jamaica, 275.

Saya-de-Malha, 271.

Schomburgh, R. H., drift sands of Ane-
gada, 185.

INDEX.

20, 22.

Sea-anemone,
Sea-cucumbers, 160.
Sea, depths of disturbance of, by waves,
229:
Sea-ginseng, 160.
Sea-slugs, 160.
Sea-water, composition of, 100.
Semper, Karl, on the Pelews, 289.
on making of lagoon basins, 294, 298.
Senses in Actiniz, 39.
Septa, 43, 60.
Seriatopora, 70.
Serle’s Island, 171.
Serpule in reef-making at Bermudas,
130,221.
supposed, of Florida, 206.
Seychelles, 271.
Sharples, 8 P., analyses of corals, 99.
Sherboro Island, 351.
Shore-platform, origin of, 235.
Siderastraea radians, 99.
galaxea, 113.
radiata, 113.
Silliman, B., analysis of dolomitic coral
rock of Metia, 394.
of coral sand of Straits of Balabac,
394.
of chalk of Oahu, 395.
Society Islands, 340.
Solomon Islands, 309, 345, 381.
Somers’ Islands, see Bermudas.
Sooloo‘Sea, 347.
Soundings about atolls, 171.
Spontaneous fission, 56.
Starbuck's Island, 523, 375.
Starve Islaad, 325.
Staver’s Island, 376.
Stevenson, force of waves, 229.
Stimpson, Win., observations by, 24, 83,
84, 92.
Strombus gigas in West India reefs,
212217,
Stutchbury, growth of an Agaricia, 124.
on Rurutu, 372.
Stylaster erubescens, 70.
Stylasteridz, 70, 105, 110.
Stylophora Danze 70.
Subsidence theory of coral reefs, 261.
objections to, 227.

439

Subsidence in the Pacific, 357, 401, 411.
amount of land lost by, 276.
period and extent of, and accom-
panying changes over the globe,
401.
thickening of reefs by, 387.
Sunday Island, 341.
Swain’s Island, 168, 173, 197, 374.
Swan Island Reef, 175.
Sydenham Island, 167.
Sydney Island, 377.
Synapticule, 60.

TABUL, 60.

Tabulate, 60, 77.

Tafoa, 341, 374.

Tahiti, north shore of, 149, 246.
thickness of reef, 158.
no elevation at, 571.
Murray’s observations at, 282.
valleys of, 273.
arrangements of Wilkes for deter-

mining rate of growth of reef,

257, 417.
same, of Le Clerc and De Benazé,
ANT.

Taiara, 167, 200.
Tampa Bay, 206.
Tanna, 343.
‘Tapateuea, 164, 167, 169, 183.
elevation of, 379.
Tarawa, 164, 169, 380.
Tarawan Archipelago, see Gilbert Islands,
Tari-tari, 167, 169.
Tealia crassicornis, see Urticina.
Teku, 199.
Telesto ramiculosa, 84.
trichostemma, 83.
Temperature limiting distribution of
corals, 108, 335.
of Atlantic Ocean in past time, 399.
chart, 335.
Tetracoralla, 78.
Thecocyathus cylindraceus, 43.
Thomson, Sir Wyville, on Bermudas,
218.
on red earth, 225.
Tikopia, 345.

| Timor, 346.

44.0

Timorlaut, 346.
Tinakoro, 343.
Tizard Bank, 271.
Tonga Islands, 341, 373, 384.
Tongatabu, 341, 373.
Tortugas, 209.
Tridacophyllia, 66, Plate 1V.
Tripang, 160.
Truk, 342.
Tubipora fimbriata, 84.
syringa, 84.
Tubuai, 340, 373.
Tubularia, 103.
Tuomey, M., on Florida reefs, 204, 207.
Turk’s Islands, 215.
Tuscarora section in the Pacific, 292.
Murray on, 292.
Tutuila, 305, 326.
Tyerman on Huahine, 371.
on Rurutu, 372.

Uatan, 306, 331.

Umbellularid, 93.

Upolu, 288, 341, 362.
thickness of reef, 158, 286.

harbor of, at Apia, 245; at Falifa, |

248.
Urticina crassicornis, lasso-cells, 34, 36.

VANIKORO Group, 348.

Vavau, 341, 373, 374.

Veretillum Stimpsoni, 91, 92.

cynomorium, phosphorescence of,
92.

Vermetus nigricans, Florida, 206.

Verrill, A. E., on Cancrisocia expansa,
24; Epiactis prolifera, 40; coral secre-
tion, 43; classification of actinoid cor-

als, 61; compartments in Alcyonia all

ambulacral, 81; Anthelia and Telesto,
84; spicules of Gorgoniz, 86; species
of Veretillum and Cophobelemnon,

91; corals, of Panama, 111; corals of
La Paz, 112; corals of the West in“|

INDEX.

cepted names for species in Dana’s
Zoophytes, 420.

Vincennes Island, 179, 202.

Virgularide, 93.

Volcanic action limiting the distribu-
tion of corals, 301, 337, 348.

Wattis’s IsLanp, 342, 378.
Washington Island, 173, 199, 374.
Wateoo, see Atiu.
Water on coral islands, 324.
Waterlandt, 205.
Waves, action of, on coasts, 236, 283.
force of, Stevenson, 229.
West Indies, corals of, 112.
changes of level in, Agassiz, 304;
Heilprin, 206.
Weinland, D. F., rate of growth of
corals, 124.
Wharton, W. J. L., on Macclesfield
Bank, 271.
Whippey Harbor, 250.
Whipple, J. A., corals from a wreck, 126.
coral heads of Turk’s Islands, 139.
Whitsunday Island, 172.
Williams’s Missionary Enterprises, 341.
rock and caverns of Atiu, 194, 598.
on Mangaia, Atiu, and Rurutu, 372.
Williams, S. W., on biche-de-mar, 161.
Wilson’s Island, 203.
| Winds about coral islands, 206, 217,
223, 316.
Wolchonsky, 169.
Woodman, H. T., rate of growth of
corals, 418,

XENIA Dana#, 82.
elongata, 82.
florida, 82.

Yap or Eap, 343.

ZOANTHACEA, 61.
Zoanthus Americana, 62.

dies and Brazilian coast, 113; corals | Zodphyte, 48.

of the Bermudas, 114; Whipple’s ob- |
servations on corals of a wreck, 126;

Anticosti shore-platform. 236;

ac-

Zoophytes, names now used for species
in Dana’s report on, Verrill, 420.
Zoothome, 48, 60.

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