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THE 


^STRUCTURE  AND   DISTRIBUTION 


OF 


COEAL  REEFS 


\  BY 

CHARLES   DARWIN,  M.A.,  R  R.  S.,  R  G.  S. 


THIRD  EDITION 
WITH  AN  APPENDIX  BY  PROF.  T.  G.  BONNEY,  D.Sc,  P.R.S.,  F.G.S. 


WITH  ILLUSTRATIONS 


NEW    YORK 
O.   APPLETON    AND    COMPANY 

1897 


Authorized  Edition, 


2)2 

/2e7 


PEEEACE 

TO 

THE    THIED    EDITION. 


For  all  that  distinguishes  the  present  from  the  second 
edition  the  reader  has  to  thank  Professor  Bonney. 
He  has  added  occasional  footnotes  (distinguished  by 
square  brackets),  and  he  has  given,  in  the  form  of  an 
appendix,  a  careful  summary  of  the  more  important 
memoirs  published  since  1874. 

My  own  contribution  is  merely  the  fulfilment  of  a 
pleasant  duty — the  expression  of  my  sincere  gratitude 
to  Professor  Bonney  for  the  ready  kindness  with  which 
he  undertook  a  difficult  task,  and  for  the  care  and 
skill  with  which  he  has  completed  it. 

I  must  also  be  allowed  the  satisfaction  of  expressing 
my  obligations  to  Captain  Wharton,  R.N.,  Hydro- 
grapher  to  the  Admiralty,  for  an  interesting  series  of 
'^otes,  which  ye  embodied  by  Professor  Bonney  in  the 
present  edition. 

Francis  Darwin. 

oAMBBiDGE  i  February  28, 1889. 


\ 


PEEFAOE 

TO 

TEE     SECOND    EDITION. 


The  fikst  edition  of  this  book  appeared  in  1842,  and 
since  then  only  one  important  work  on  the  same 
subject  has  appeared,  namely,  in  1872,  by  Professor 
Dana,  on  Corals  and  Coral-Reefs.  In  this  work  he 
justly  says  that  I  have  not  laid  sufficient  weight  on 
the  mean  temperature  of  the  sea,  in  determining  the 
distribution  of  coral-reefs ;  but  neither  a  low  tempera- 
ture nor  the  presence  of  mud-banks  accounts,  as  it 
appears  to  me,  for  the  absence  of  coral-reefs  throughi 
out  certain  areas ;  and  we  must  look  to  some  more 
recondite  cause.  Professor  Dana,  also,  insists  that| 
volcanic  action  prevents  the  growth  of  coral-reefs 
much  more  effectually  than  I  had  supposed ;  but  ^ 
how  the  heat  or  poisonous  exhalations  from  a  volcano 


A 


s 


PREFACE   TO   THE    SECOND   EDITION.  Vll 

can  affect  the  whole  circumference  of  a  large  island 
is  not  clear.  Nor  does  this  fact,  if  fully  established, 
falsify  my  generalisation  that  volcanos  in  a  state  of 
action  are  not  found  within  the  areas  of  subsidence, 
whilst  they  are  often  present  within  those  of  elevation ; 
for  I  have  not  been  influenced  in  my  judgment  by  the 
absence  or  presence  of  coral-reefs  round  active  volcanos ; 
I  have  judged  only  by  finding  upraised  marine  remains 
within  the  areas  of  elevation,  and  by  the  vicinity  of 
atolls  and  barrier-reefs  with  reference  to  the  areas 
of  subsidence.  Professor  Dana  apparently  supposes 
(p.  320)  that  I  look  at  fringing-reefs  as  a  proof  of 
the  recent  elevation  of  the  land ;  but  I  have  ex- 
pressly stated  that  such  reefs,  as  a  general  rule, 
indicate  that  the  land  has  either  long  remained  at  the 
same  level  or  has  been  recently  elevated.  Neverthe- 
less, from  upraised  recent  remains  having  been  found 
in  a  large  number  of  cases  on  coasts  which  are  fringed 
by  coral-reefs,  it  appears  that  of  these  two  alternatives 
recent  elevation  has  been  much  more  frequent  than  a 
statioL  iry  condition.  Professor  Dana  further  believes 
that  many  of  the  lagoon-islands  in  the  Paumotu 
or  Low  Archipelago  and  elsewhere  have  recently 
been  elevated  to  a  height  of  a  few  feet,  although 
originally  formed  during  a  period  of  subsidence ;  but  I 
shall  endeavour  to  show  in  the  sixth  chapter  of  the 
present  edition  that  lagoon-islands  which  have  long 


Vlll  PREFACE   TO 

remained  at  a  stationary  level  often  present  the  false 
appearance  of  having  been  slightly  elevated. 

Although  I  thus  demur  to  some  of  the  remarks  and 
criticisms  made  by  this  eminent  naturalist,  who  has 
examined  more  coral  formations  than  almost  any  other 
man,  yet  I  do  not  the  less  admire  his  work.^  It  has 
also  afforded  me  the  highest  satisfaction  to  find  that 
he  accepts  the  fundamental  proposition  that  lagoon- 
islands  or  atolls,  and  barrier-reefs,  have  been  formed 
during  periods  of  subsidence. 

The  late  Professor  Jukes,  in  his  account  of  the 
voyage  of  H.M.S.  Fly,  published  in  1847,  devoted  a 
chapter  to  the  Barrier-Reefs  of  Australia,  and  thus 
concludes  :  *  After  seeing  much  of  the  Great  Barrier- 
reefs,  and  reflecting  much  upon  them,  and  trying  if  it 
were  possible  by  any  means  to  evade  the  conclusions  to 
which  Mr.  Darwin  has  come,  I  cannot  help  adding  that 
his  hypothesis  is  perfectly  satisfactory  to  my  mind,  and 
rises  beyond  a  mere  hypothesis  into  the  true  theory  of 
coral-reefs.' 

On  the  other  hand,  a  distinguished  naturalist, 
Professor  Semper,  differs  much  from  me,  although  he 
seems  willing  to  admit  that  some  atolls  and  barrier- 
reefs  have  been  formed  in  the  manner  in  which  I 
suppose.     I  will  give  in  the  Appendix,  under  the  head 

'  A  friendly  reply  from  Professor  Dana,  contesting  some  of  the 
points  mentioned  above,  will  be  found  in  Nature,  Sept.  1874,  p.  408. 


THE   SECOND    EDITION.  IX 

of  the  Pelew  Islands,  which  were  carefully  examined 
by  him,  some  account  of  his  objections,  and  I  will  here 
only  state  that  his  view  does  not  differ  essentially  from 
that  of  Chamisso,  which  will  hereafter  be  discussed. 
It  will  be  seen  that  the  evidence  in  favour  of  atolls 
and  barrier-reefs  having  been  formed  during  sub- 
sidence is  of  a  cumulative  nature ;  and  that  it  is 
very  difficult  to  judge  with  safety  respecting  any 
single  lagoon-island  or  barrier-reef,  or  small  group  of 
them,  even  if  the  depth  outside  the  reef  and  the  slope 
of  the  encircled  land  are  both  known. 

In  the  present  edition  I  have  added  some  new  facts 
and  have  revised  the  whole  book ;  the  latter  chapters 
having  been  almost  re- written.  The  appended  map  of 
the  Pacific  and  Indian  Oceans  remains  in  nearly  the 
same  state  as  before,  for  I  have  added  only  two  red 
and  two  blue  circles.  I  have  removed  an  active  vol- 
cano, which  was  formerly  supposed  to  exist  in  Torres 
Straits.  An  account  of  a  remarkable  bar  of  sandstone 
off  Pernambuco  on  the  Brazilian  coast  has  been  added 
to  the  Appendix,  as  this  bar  is  protected  from  the 
wear  and  tear  of  the  waves  by  a  coating  of  organic 
bodies,  in  the  same  manner  as  are  most  coral-reefs. 
It  also  resembles  a  coral-reef  in  shape  or  outline  to  a 
curiously  deceptive  degree.  If  I  had  been  better 
situated  during  the  last  thirty  years,  for  hearing  of 
recent  discoveries  in  the  Pacific,  and  for  consulting 


X  PREFACE   TO    THE    SECOND    EDITION. 

charts  published  in  various  countries,  my  map  might 
have  been  greatly  improved.  But  I  hope  that  before 
long  some  one  may  be  induced  to  colour  a  map  on  a 
large  scale,  on  nearly  the  same  principles  as  I  have 
done,  and  in  accordance  with  our  advanced  state  of 
geographical  knowledge ;  for  I  believe  that  he  would 
thus  arrive  at  some  new  and  striking  generalisations. 

Down,  Beckenham,  Kent  : 
February  1874. 


-^ 


PEEFAOE 

TO 

THE    FIEST    EDITION, 


I  SHALL  HAVE  OCCASION,  in  many  parts  of  the  follow* 
ing  volume,  to  acknowledge  the  valuable  information 
I  have  received  from  several  persons;  but  I  must, 
more  particularly  express  my  obligations  to  Captain 
R.  Moresby,  I.N.,  who  conducted  the  survey  of  the- 
Red  Sea,  and  of  the  archipelagoes  of  low  coral-islands 
in  the  Indian  Ocean.  I  beg,  also,  to  be  permitted  tO' 
return  my  best  thanks  to  Captain  Beaufort,  R.N.,  for 
having  given  me  free  access  to  the  charts  in  the  Ad- 
miralty, as  well  as  to  Captain  Beecher,  R.N.,  for  most 
kmdly  aiding  me  in  consulting  them.  My  thanks  are 
likewise  especially  due  to  Captain  Washington,  R.N.,, 
for  his  invariable  desire  to  assist  me  in  every  possible 
manner.  Having  in  former  publications  had  the 
pleasure  of  acknowledging  how  much  I  owe  to  Captain 


XU  PREFACE   TO   THE   FIRST    EDITION. 

FitzEoy,  for  having  permitted  me  to  volunteer  my 
services  on  board  H.M.S.  Beagle,  and  for  his  uniform 
kindness  in  giving  me  assistance  in  my  researches,  I 
can  here  only  repeat  my  obligations  to  him.  The 
materials  for  this  volume  were  nearly  ready  two  years 
ago  ;  but  owing  to  ill  health,  its  publication  has  been 
delayed.  The  two  succeeding  Parts — one  on  the  vol- 
canic islands  visited  during  the  voyage  of  the  B eagle ^ 
.and  the  other  on  South  America — will  appear  as  soon 
:as  they  can  be  prepared. 

2.  1849« 


CONTENTS. 


Introduction        ••••••••.    paqe  1 

CHAPTER  I. 

ATOLLS    OR  LAGOON   ISLANDS. 

SECTION   I. — DESCBIPTION   OF   KEELING  ATOLL. 

Corals  on  the  outer  margin — Zone  of  NulliporsB — Exterior  reef — Isleta 
— Coral-conglomerate— Lagoon — Calcareous  sediment—  Scari  and 
Holuthuriffi  subsisting  on  corals — Changes  in  the  condition  of  the 
reefs  and  islets— Probable  subsidence  of  the  atoll— Future  state 
of  the  lagoons 7  to  27 

SECTION    II.— GENEBAL    DESCRIPTION    OF   ATOLLS. 

General  form  and  size  of  atolls,  their  reefs  and  islets — External  slope 
— Zone  of  Nulliporae — Conglomerate — Depth  of  lagoons — Sedi- 
ment— Reefs  submerged  wholly  or  in  part— Breaches  in  the  reef 
— Ledge -formed  shores  round  certain  lagoons — Conversion  of 
lagoons  into  land 27  to  43 

SECTION   m. — ATOLLS   OF   THE    MALDIVA  ARCHIPELAGO — 
GREAT   CHAGOS   BANK. 

Maldiva  Archipelago — Ring-formed  reefs,  marginal  and  central — 
Great  depth  in  the  lagoons  of  the  southern  atolls — Reefs  in  the 
lagoons  all  rising  to  the  surface — Position  of  islets  and  breaches 
in  the  reefs,  with  respect  to  the  prevalent  winds  and  action  of  the 
waves — Destruction  of  islets— Connection  in  the  position  and 
submarine  foundation  of  distinct  atolls — The  apparent  dissever- 
ment  of  large  atolls — The  Great  Chagos  Bank — Its  submerged 
condition  and  extraordinary  structure  .        .        .        .    43  to  55 


XIV  CONTENTS. 

CHAPTEK  II. 


Ti. 


BAERIEB-BEEFS. 

Closely  resemble  in  general  form  and  structure  atoll-reefs — vfil .  "^ 
and  depth  of  the  lagoon-channels — Breaches  through  the  reef  in 
front  of  valleys,  and  generally  on  the  leeward  side— Checks  to 
the  filling  up  of  the  lagoon-channels — Size  and  constitution  of 
the  encircled  islands — Number  of  islands  within  the  same  reef — 
Barrier-reefs  of  New  Caledonia  and  Australia— Position  of  the 
reef  relative  to  the  slope  of  the  adjoining  land — Probable  great 
thickness  of  barrier-reefs paqu  56  to  68 

CHAPTER  m. 

FRINGING   OR   SHORE   REEFS. 

Eeefs  of  Mauritius— Shallow  channel  within  the  reef— Its  slow 
filling  up — Currents  of  water  formed  within  it — Upraised  reefs 
— Narrow  fringing-reefs  in  deep  seas — Keefs  on  the  coast  of  E. 
Africa  and  of  Brazil — Fringing-reefs  in  very  shallow  seas,  round 
banks  of  sediment  and  on  worn-down  islands — Fringing-reefs 
affected  by  currents  of  the  sea — Coral  coating  the  bottom  of  the 
sea,  but  not  forming  reefs 69  to  79 

CHAPTER  IV. 

ON   THE   GROWTH   OF   CORAL-REEFS. 

SECTION  I. — ON  THE  DISTRIBUTION  OP  CORAL-REEFS,  AND  ON  THE  CON- 
DITIONS  FAVOURABLE    TO   THEIR   INCREASE  .  .  .      80  to  95 

SECTION   n. — ON    THE    RATE    OF   GROWTH    OF    CORAL-REEFS     .         95  tO  108 

SECTION     in. —  ON     THE     DEPTHS     AT     WHICH     REEF  -  BUILDING     CORALS 

LIVE 108  to  118 

CHAPTER  V. 

THEORY   OF   THE    FORMATION   OP  THE   DIFFERENT 
CLASSES   OF   CORAL-REEFS. 

The  atolls  of  the  larger  archipelagoes  are  not  formed  on  submerged 
craters,  or  on  banks  of  sediment— Immense  areas  interspersed 
with  atolls — Their  subsidence — The  effects  of  storms  and  earth- 
quakes on  atolls — Recent  changes  in  their  state — The  origin  of 
barrier-reefs  and  of  atolls — Their  relative  forms — The  step-formed 


CONTENTS.  XV 

ledges  and  walls  round  the  shores  of  some  lagoons — The  ring- 
formed  reefs  of  the  Maldiva  atolls — The  submerged  condition  of 
parts  or  of  the  whole  of  some  annular  reefs — The  disseverment 
of  large  atolls — The  union  of  atolls  by  linear  reefs — The  Great 
Chagos  Bank — Objections,  from  the  area  and  amount  of  subsi- 
dence required  by  the  theory,  considered — The  probable  composi- 
tion of  the  lower  parts  of  atolls         .        .        .    page  119  to  157 

CHAPTER  VI. 

ON    THE   DISTRIBUTION   OF   COEAL-REEFS  WITH  EEFEEENCB 
TO    THE   THEORY   OF   THEIR   FORMATION. 

Description  of  the  coloured  map — Proximity  of  atolls  and  barrier- 
reef  s —Eelation  in  form  and  position  of  atolls  with  ordinary 
islands — Direct  evidence  of  subsidence  difficult  to  be  detected — 
Proofs  of  recent  elevation  where  fringing-reefs  occur — Oscilla- 
tions of  level — Absence  of  active  volcanos  in  the  areas  of  subsi- 
dence— Immensity  of  the  areas  which  have  been  elevated  and 
have  subsided — Their  relation  to  the  present  distribution  of  the 
land — Areas  of  subsidence  elongated,  their  intersection  and  alter- 
nation with  those  of  elevation — Amount,  and  slow  rate  of  sub- 
sidence— Recapitulation 158  to  196 

APPENDIX   [I.] 

Containing  a  detailed  description  of  the  Reefs  and  Islands  in  the 
coloured  Map,  Plate  III 199  to  280 

[APPENDIX  II.] 

[Summary  of  the  principal  contributions  to  the  History  of  Coral- 
Reefs  since  the  year  1874] 281  to  332 

Qenbbal  Indbx.        •        •        •        • 333 


; 


DESCEIPTION  OF  THE  PLATES. 


PLATE   I.  at  e7id  of  Volume. 


In  the  several  original  surveys,  from  which  the  small  plans  on  this 
plate  have  been  reduced,  the  coral-reefs  are  engraved  in  very  dif- 
ferent styles.  For  the  sake  of  uniformity,  I  have  adopted  the' 
style  used  in  the  charts  of  the  Chagos  Archipelago,  published  by 
the  East  India  Company,  from  the  survey  by  Capt.  Moresby  and 
Lieut.  Powell.  The  surface  of  the  reef,  which  dries  at  low  water,, 
is  represented  by  a  stippled  surface  with  small  crosses :  the  coral- 
islets  on  the  reef  are  marked  by  small  linear  unstippled  spaces,, 
on  which  a  few  cocoa-nut  trees,  out  of  all  proportion  too  large,, 
have  been  introduced  for  the  sake  of  clearness.  The  entire 
annular  reef,  which  when  surrounding  an  open  expanse  of  water,, 
forms  an  '  atoll,'  and  when  surrounding  one  or  more  high  islands,, 
forms  an  encircling  '  barrier-reef,'  has  a  nearly  uniform  structure, 
and  has  been  tinted,  in  order  to  catch  the  eye,  of  a  pale  blue 
colour.  The  reefs  in  some  of  the  original  surveys  are  represented, 
merely  by  a  single  line  with  crosses,  so  that  their  breadth  is  not 
given ;  I  have  had  such  reefs  engraved  of  the  width  usually  at- 
tained by  coral-reefs.  I  have  not  thought  it  worth  while  to 
introduce  all  those  small  and  very  numerous  reefs,  which  occur 
within  the  lagoons  of  most  atolls  and  within  the  lagoon-channels, 
of  most  barrier-reefs,  and  which  stand  either  isolated,  or  are 
attached  to  the  shores  of  the  reef  or  land.  At  Peros  Banhos 
none  of  the  lagoon-reefs  rise  to  the  surface  of  the  water ;  a  few 
of  them  have  been  introduced,  and  are  marked  by  plain  dotted 
circles.  A  few  of  the  deepest  soundings  are  laid  down  within 
each  reef ;  they  are  in  fathoms,  of  six  English  feet. 

Fig.  1. — Vanikoro,  situated  in  the  western  part  of  the  S.  Pacific; 

taken  from  the  survey  by  Capt.  D'Urville  in  the  Astrolabe ;  the' 

scale  is  ^  of  an  inch  to  a  geographical  mile ;  the  soundings  on 

the  southern  side  of  the  island,  namely  from  30  to  40  fathoms, 

2 


XVlll  DESCRIPTION    OF    PLATES. 

are  given  from  the  Voyage  of  the  Chev.  Dillon  ;  the  other  sound- 
ings are  laid  down  from  the  survey  by  D'Urville ;  height  of  the 
summit  of  the  island  is  3,032  feet.  The  principal  small  detached 
reefs  within  the  lagoon-channel  have  in  this  instance  been  repre- 
sented. The  southern  shore  of  the  island  is  narrowly  fringed  by 
a  reef ;  if  the  engraver  had  carried  this  reef  entirely  round  both 
islands,  this  figure  would  have  served  (by  leaving  out  in  imagina- 
tion the  barrier-reef)  as  a  good  specimen  of  an  abruptly-sided 
island,  surrounded  by  a  reef  of  the  fringing  class. 

Pig.  2.— HoGOLEU,  or  Roug,  in  the  Caroline  Archipelago ;  taken  from 
the  atlas  of  the  Voyage  of  the  Astrolabe,  compiled  from  the 
surveys  of  Captains  Duperrey  and  D'Urville ;  scale  i  of  an  inch 
to  a  mile ;  the  depth  of  the  immense  lagoon-like  space  within  the 
reef  is  not  known. 

]Fig.  3. — Raiatea,  in  the  Society  Archipelago ;  from  the  map  given  in 
the  quarto  edition  of  Cook's  First  Voyage;  it  is  probably  not 
accurate ;  scale  ^  of  an  inch  to  a  mile. 

:Fig.  4. — Bow,  or  He  you  atoll  (or  lagoon-island),  in  the  Low  Arch 
pelago ;  from  the  survey  by  Capt.  Beechey,  R.N. ;  scale  ^^  of  an 
inch  to  a  mile;  the  lagoon  is  choked  up  with  reefs,  but  the 
average  greatest  depth  of  about  20  fathoms,  is  given  from  the 
published  account  of  the  voyage. 

IFig.  5. — BoLABOLA,  in  the  Society  Archipelago ;  from  the  survey  of 
Capt.  Duperrey,  in  the  Coquille ;  scale  }  of  an  inch  to  a  mile ; 
the  soundings  in  this  and  the  following  figure  have  been  altered 
from  French  feet  to  English  fathoms  ;  height  of  highest  point  of 
the  island  4,026  feet. 

'Fig.  6. — Maurua,  in  the  Society  Archipelago;  from  the  survey  by 
Capt.  Duperrey  in  the  Coquille \  scale  -}  of  an  inch  to  a  mile; 
height  of  land  about  800  feet. 

U'ig.  7. — PouYNiPETE,  or  Seniavine,  in  the  Caroline  Archipelago  { 
from  the  survey  by  Admiral  Lutk6 ;  scale  j  of  an  inch  to  a  mile. 

iFig.  8, — Gambier  Islands,  in  the  southern  part  of  the  Low  Archi- 
pelago ;  from  the  survey  by  Capt.  Beechey ;  scale  j  of  an  inch  to 
a  mile;  height  of  highest  island,  1,246  feet;  the  islands  are  sur- 
rounded by  extensive  and  irregular  reefs ;  the  reef  on  the  southeri'. 
Bide  is  submerged. 

(Fig.  9.— Peros  Banhos  atoll  (or  lagoon-island),  in  the  Chagos  grou}) 
in  the  Indian  Ocean;  from  the  survey  by  Capt.  Morepby  auJ 


DESCRIPTION   OF   PLATES.  xix 

Lieut.  Powell ;  scale  |  of  an  inch  to  a  mile ;  not  nearly  all  tho 
small  submerged  reefs  in  the  lagoon  are  represented ;  the  annular 
reef  on  the  southern  side  is  submerged. 

Fig.  10. — Keeling,  or  Cocos  atoll  (or  lagoon-island),  in  the  Indiau 
Ocean  ;  from  the  survey  by  Capt.  FitzKoy ;  scale  ^  of  an  inch  to 
a  mile ;  the  lagoon  south  of  the  dotted  line  is  very  shallow,  and 
is  left  almost  bare  at  low  water;  the  part  north  of  the  line  is 
choked  up  with  irregular  reefs.  The  annular  reef  on  the  N.W. 
side  is  broken,  and  blends  into  a  shoal  sand-bank,  on  which  the 
Bea  breaks. 

PLATE  XL  at  end  of  Volume. 

Fig.  1. — Great  Chagos  Bank,  in  the  Indian  Ocean ;  taken  from  the 
survey  by  Capt.  Moresby  and  Lieut.  Powell ;  scale  ^  of  an  inch 
to  a  mile  (same  scale  as  Hogoleu,  in  Plate  I.) ;  the  parts  which 
are  shaded,  with  the  exception  of  two  or  three  islets  on  the 
western  and  northern  sides,  do  not  rise  to  the  surface,  but  are 
submerged  from  4  to  10  fathoms;  the  banks  bounded  by  the 
dotted  lines  lie  from  15  to  20  fathoms  beneath  the  surface,  and 
are  formed  of  sand ;  the  central  space  is  of  mud,  and  from  30  to 
50  fathoms  deep. 

Fig.  2. — A  vertical  section,  on  the  same  scale,  in  an  E.  and  W.  line 
across  the  Great  Chagos  Bank,  given  for  the  sake  of  exnibiting 
more  clearly  its  structure. 

Fig.  3. — Menchicoff  atoll  (or  lagoon-island),  in  the  Marshall  Archi- 
pelago, northern  Pacific  Ocean ;  from  Krusenstern's  atlas  of  the 
Pacific ;  originally  surveyed  by  Capt.  Hagemeister ;  scale  ^^  of  an 
inch  to  a  mile ;  the  depth  within  the  lagoons  is  unknown. 

Fig.  4.— Mahlos  Mahdoo  atoll,  together  with  Horsburgh  atoll,  in 
the  Maldiva  Archipelago ;  from  the  survey  by  Capt.  Moresby  and 
Lieut.  Powell ;  scale  ^^  of  an  inch  to  a  mile  ;  the  white  spaces  in 
the  middle  of  the  separate  small  reefs,  both  on  the  margin  and 
in  the  middle  part,  are  meant  to  represent  little  lagoons ;  but  it 
was  found  not  possible  to  distinguish  them  clearly  from  the  small 
iulets,  which  have  been  formed  on  these  same  small  reefs ;  many 
of  the  smaller  reefs  could  not  be  introduced ;  the  nautical  mark 
(  — )  over  the  figures  250  and  200  between  Mahlos  Mahdoo  and 
Horsburgh  atoll  and  Powell's  Island,  signifies  that  soundings  were 
not  obtained  at  these  depths. 


XX  DESCKIPTION   OF   PLATES. 

Fig.  5. — New  Caledonia,  in  the  western  part  of  the  Pacific;  from 
Krusenstern's  atlas,  compiled  from  several  surveys ;  I  have 
slightly  altered  the  northern  point  of  the  reef,  in  accordance 
with  the  atlas  of  the  Voyage  of  the  Astrolabe.    In  Krusenstern's 

■r  atlas,  the  reef  is  represented  by  a  single  line  with  crosses ;  I  have 
for  the  sake  of  uniformity  added  an  interior  line ;  scale  ^j  of  an 
inch  to  a  mile. 

Fig.  6. — Maldiva  Archipelago,  in  the  Indian  Ocean ;  from  the  survey 
by  Capt.  Moresby  and  Lieut.  Powell;  scale  ^^  of  an  inch  to  a 
mile. 

PLATE  III.  at  beginning  of  Volume, 

The  principles  on  which  this  map  is  coloured  are  explained  in  the 
beginning  of  Chapter  VI. ;  and  the  authorities  for  colouring  each 
particular  spot  are  detailed  in  the  Appendix.  The  names  printed 
hi  italics  in  the  Index  refer  to  the  Appendix. 


THE 

STRUCTUEE  AND  DISTEIBUTION 

or 

COEAL-EEEFS. 


INTEODUCTION. 

The  object  of  this  volume  is  to  describe  from  my  own 
observation  and  the  works  of  others,  the  principal 
kinds  of  coral-reefs,  and  to  explain  the  origin  of  their 
pecuhar  forms.  I  shall  not  here  treat  of  the  poly- 
pifers,  which  construct  these  vast  works,  except  as 
to  their  distribution,  and  the  conditions  favourable 
to  their  vigorous  growth. 

Without  any  distinct  intention  to  classify  coral- 
reefs,  most  voyagers  have  spoken  of  them  under  the 
following  heads :  *  lagoon-islands '  or  *  atolls,*  *  barrier  * 
or  *  encircling  reefs,'  and  *  fringing '  or  *  shore  reefs.* 
The  lagoon-islands  have  received  much  the  most  atten- 
tion ;  and  it  is  not  surprising,  for  everyone  must  be 
struck  with  astonishment,  when  he  first  beholds  one  of 


Z  INTRODUCTION. 

these  vast  rings  of  coral-rock,  often  many  leagues  in 
diameter,  here  and  there  surmounted  by  a  low  verdant 
island  with  dazzling  white  shores,  bathed  on  the  out- 
side by  the  foaming  breakers  of  the  ocean,  and  on  the 
inside  surrounding  a  calm  expanse  of  water,  which, 
from  reflection,  is  generally  of  a  bright  but  pale  green 
colour.  The  naturalist  will  feel  this  astonishment 
more  deeply  after  having  examined  the  soft  and  almost 
gelatinous  bodies  of  these  apparently  insignificant 
coral-polypifers,  and  when  he  knows  that  the  solid  reef 
increases  only  on  the  outer  edge,  which  day  and  night 


No.  1. 


is  lashed  by  the  breakers  of  an  ocean  never  at 
rest.  Well  did  Francois  Pyrard  de  Laval,  in  the 
year  1605,  exclaim,  *  C'est  une  merueille  de  voir 
chacun  de  ces  atollons,  enuironne  d'un  grand  banc  de 
pierre  tout  autour,  n'y  ayant  point  d'artifice  humain.' 
The  above  sketch  of  Whitsunday  Island,  in  the 
S.  Pacific,  taken  from  Capt.  Beechey's  admirable 
Voyage,  although  excellent  of  its  kind,  gives  but  a 


-\  1 


INTRODUCTION.  6 

faint  idea  of  the  singular  aspect  of  one  of  these 
lagoon-islands.  Whitsunday  Island  is  of  small  size, 
and  the  whole  circle  has  been  converted  into  land, 
which  is  a  comparatively  rare  circumstance.  As  the 
reef  of  a  lagoon-island  generally  supports  many  sepa- 
rate small  islands,  the  word  *  island,'  applied  to  the 
whole,  is  often  the  cause  of  confusion ;  hence  I  have 
invariably  used  in  this  volume  the  term  *  atoll,'  which 
is  the  name  given  to  these  circular  coral  formations  by 
their  inhabitants  in  the  Indian  Ocean,  and  is  syn- 
onymous with  *  lagoon-island.' 

Barrier-reefs,  when  encircling  small  islands,  have 
been  comparatively  little  noticed  by  voyagers;    but 

No.  2. 


they  well  deserve  attention.  In  their  structure  they 
are  little  less  marvellous  than  atolls,  and  they  give  a 
singular  and  most  picturesque  character  to  the  scenery 
of  the  islands  they  surround.  In  the  accompanying 
sketch,  taken  from  the  Voyage  of  the  Coquille,  the  reef 
is  seen  from  within,  from  one  of  the  high  peaks  of  Bo- 


4  INTRODUCTION. 

labola,^  one  of  the  Society  Islands.  Here,  as  in  Whit- 
sunday Island,  the  whole  of  that  part  of  the  reef  which 
is  visible  is  converted  into  land.  This  is  a  circum- 
stance of  rare  occurrence  ;  more  usually  a  snow-white 
line  of  great  breakers,  with  here  and  there  an  islet 
crowned  by  cocoa-nut  trees,  separates  the  smooth 
waters  of  the  lagoon-like  channel  from  the  waves  of 
the  open  sea.  The  barrier  reefs  of  Australia  and  of 
New  Caledonia,  owing  to  their  enormous  dimensions, 
have  excited  much  attention :  in  structure  and  form 
they  resemble  those  encircling  many  of  the  smaller 
islands  in  the  Pacific  Ocean. 

With  respect  to  fringing,  or  shore  reefs,  there  is 
little  in  their  structure  which  needs  explanation ;  and 
their  name  expresses  their  comparatively  small  ex- 
tension. They  differ  from  barrier  reefs  in  not  lying 
far  from  the  shore,  and  in  not  having  within  them  a 
broad  channel  of  deep  water.  Eeefs  also  occur  around 
submerged  banks  of  sediment  and  of  worn-down  rock  ; 
and  others  are  scattered  quite  irregularly  where  the 
sea  is  very  shallow ;  these  are  allied  in  most  respects 
to  fringing  reefs,  but  are  of  comparatively  little 
interest. 

I  have  given  a  separate  chapter  to  each  of  the 
above  classes,  and  have  described  some  one  reef  or 
island,  on  which  I  possessed  most  information,  as 
typical ;  and  have  afterwards  compared  it  with  others 
of  a  like  kind.     Although  this  classification  is  useful 

'  I  have  taken  the  liberty  of  simplifying  the  foreground,  and 
leaving  out  a  mountainous  island  in  the  far  distance. 


INTEODUCTION.  5 

from  being  obvious,  and  from  including  most  of  the 
coral-reefs  existing  in  the  open  sea,  it  admits  of  a  more 
fundamental  division  into  barrier  and  atoll-formed 
reefs  on  the  one  hand,  where  there  is  a  great  apparent 
difficulty  with  respect  to  the  foundation  on  which  they 
must  first  have  grown  ;  and  into  fringing  reefs  on  the 
other,  where,  owing  to  the  nature  of  the  slope  of  the 
adjoining  land,  there  is  no  such  difficulty.  The  two 
blue  tints  and  the  red  colour  on  the  map  (Plate  III.) 
represent  this  main  division,  as  explained  in  the  be- 
ginning of  the  last  chapter.  In  the  Appendix,  every 
existing  coral-reef,  except  some  on  the  coast  of  Brazil 
not  included  in  the  map,  is  briefly  described  in  geo- 
graphical order,  as  far  as  I  possessed  information ;  and 
any  particular  spot  may  be  found  by  consulting  the 
Index. 

Several  theories  have  been  advanced  to  explain  the 
origin  of  atolls  or  lagoon-islands,  but  scarcely  one  to 
account  for  barrier-reefs.  From  the  limited  depths  at 
which  reef-building  polypifers  can  flourish,  taken  into 
consideration  with  certain  other  circumstances,  we  are 
compelled  to  conclude,  as  it  will  be  seen,  that  both  in 
atolls  and  barrier-reefs,  the  foundation  to  which  the 
coral  was  primarily  attached,  has  subsided  ;  and  that 
during  this  downward  movement,  the  reefs  have  grown 
upwards.  This  conclusion,  it  will  be  further  seen, 
explains  most  satisfactorily,  the  outline  and  general 
form  of  atolls  and  barrier-reefs,  and  likewise  certain 
peculiarities  in  their  structure.  The  distribution,  also, 
of  the  different  kinds  of  coral-reefs,  and  their  position 


b  INTRODUCTION. 

with  relation  to  the  areas  of  recent  elevation,  and  to 
the  points  subject  to  volcanic  eruptions,  fully  accord 
with  this  theory  of  their  origin.* 

'  A  brief  account  of  my  views  on  coral  formations,  now  published 
ill  my  Journal  of  Eesearches,  was  read  May  31, 1837,  before  the  Geo- 
logical Society,  and  an  abstract  has  appeared  in  the  Proceedings. 


;• 


CHAPTER   I. 

ATOLLS   OB   LAGOON-ISLANDS. 


SECTION   FIRST,    KEELING   ATOLL. 

Corals  on  the  outer  margin — Zone  of  NidliporcB — Exterior  reef — 
Islets  — Co7'al-conglomerate  — Lagoon — Calcareous  sediment  — 
Scari  and  Holuthurice  subsisting  on  corals — Changes  in  the  con- 
dition of  the  reefs  and  islets — Probable  subsidence  of  the  atoll — 
Future  state  of  the  lagoon. 

Keeling  or  Cocos  atoll  is  situated  in  the  Indian  Ocean, 
in  12°  5'  S.  and  long.  90°  55'  E. :  a  reduced  chart  of  it, 
from  the  survey  of  Capt.  EitzRoy  and  the  officers  of 
H.M.S.  Beagle,  is  given  in  Plate  I.  fig.  10.  The 
greatest  width  of  this  atoll  is  nine  miles  and  a  half. 
Its  structure  is  in  most  respects  characteristic  of  the 
class  to  which  it  belongs,  with  the  exception  of  the 
shallowness  of  the  lagoon.  The  accompanying  wood- 
cut (No.  3)  represents  a  vertical  section,  supposed  to  be 
drawn  at  low  water  from  the  outer  coast  across  one  of 
the  low  islets  (one  being  taken  of  average  dimensions) 
to  within  the  lagoon.  The  section  is  true  to  the  scale 
in  a  horizontal  line,  but  it  could  not  be  made  so  in  a 
vertical  one,  as  the  average  greatest  height  of  the  land 
is  only  between  six  and  twelve  feet  above  high-water 


8  ATOLLS.  Ch.  L 

mark.  I  will  describe  the  section,  commencing  with 
the  oute»*  margin.  But  I  must  first  observe  that  the 
reef-building  polypifers,  not  being  tidal  animals,  require 
to  be  constantly  submerged  or  washed  by  the  breakers. 
I  was  assured  by  Mr.  Liesk,  an  inteUigent  resident  on 
these  islands,  as  well  as  by  some  chiefs  at  Tahiti  (Ota- 


A — Level  of  the  sea  at  low  water :  where  the  letter  A  is  placed, 
the  depth  is  25  fathoms,  and  the  distance  rather  more  than  150  yards 
from  the  edge  of  the  reef. 

B — Outer  edge  of  that  flat  part  of  the  reef,  which  dries  at  low 
water:  the  edgo  either  consists  of  a  convex  mound,  as  represented, 
or  of  rugged  points,  like  those  a  little  farther  seaward,  beneath  the 
water. 

C — A  flat  of  coral -rock,  covered  at  high  water. 

D — A  low  projecting  ledge  of  brecciated  coral-rock,  washed  by  the 
waves  at  high  water. 

E — A  slope  of  loose  fragments,  reached  by  the  sea  only  during 
gales :  the  upper  part,  which  is  from  six  to  twelve  feet  high,  is 
clothed  with  vegetation.  The  surface  of  the  islet  gently  slopes  to 
the  lagoon. 

F — Level  of  the  lagoon  at  low  water. 

neite),  that  an  exposure  to  the  rays  of  the  sun  for  a  very 
short  time  invariably  causes  their  destruction.^    Hence 

•  [This  would  be  true  of  certain  genera  or  species,  but  according 
to  the  observations  made  during  the  voyage  of  the  Challenger  (Re- 
ports xvi.  pp.  23,  36),  and  by  Mr.  Guppy  (Proc.  R.  S.  Edin.  xiii.  p.  863), 
there  are  some  which  can  bear  exposure  for  a  limited  time,  perhaps 
two  or  three  hours,  especially  if  occasionally  wetted  by  the  spray.] 


Sect.  I.  KEELING  ATOLL.  9 

it  is  possible  only  under  the  most  favourable  circum- 
stances, afforded  by  an  unusually  low  tide  and  smooth 
water,  to  reach  the  outer  margin,  where  the  coral  is 
alive.  I  succeeded  only  twice  in  gaining  this  part,  and 
found  it  almost  entirely  composed  of  a  living  Porites, 
which  forms  great  irregularly  rounded  masses  (like 
those  of  an  Astrsea,  but  larger)  from  four  to  eight  feet 
broad,  and  little  less  in  thickness.  These  mounds  are 
separated  from  each  other  by  narrow  crooked  channels, 
about  six  feet  deep,  most  of  which  intersect  the  line  of 
reef  at  right  angles.  On  the  furthest  mound,  which  I 
was  able  to  reach  by  the  aid  of  a  leaping-pole,  and  over 
which  the  sea  broke  with  some  violence,  although  the 
day  was  quite  calm  and  the  tide  low,  the  polypifers  in 
the  uppermost  cells  were  all  dead,  but  between  three 
and  four  inches  lower  down  on  its  side  they  were  living, 
and  formed  a  projecting  border  round  the  upper  and 
dead  surface.  The  coral  being  thus  checked  in  its  up- 
ward growth,  extends  laterally,  and  hence  most  of  the 
masses,  especially  those  a  little  further  inwards,  had 
broad  flat  dead  summits.  On  the  other  hand  I  could 
see,  during  the  recoil  of  the  breakers,  that  a  few  yards 
further  seaward,  the  whole  convex  surface  of  the 
Porites  was  alive :  so  that  the  point  where  we  were 
standing  was  almost  on  the  exact  upward  and  shore- 
ward limit  of  existence  of  those  corals  which  form  the 
outer  margin  of  the  reef.  We  shall  presently  see 
that  there  are  other  organic  productions,  fitted  to  bear 
a  somewhat  longer  exposure  to  the  air  and  sun. 

Next,  but   much   inferior   in  importance  to  the 


10  ATOLLS.  Ch.  L 

Porites,  is  the  MiUepora  complanata}  It  grows  in 
thick  vertical  plates,  intersectiiig  each  other  at  various 
angles,  and  forms  an  exceedingly  strong  honey- combed 
mass,  which  generally  assumes  a  circular  form,  the 
marginal  plates  alone  being  alive.  Between  these  plates 
and  in  the  protected  crevices  on  the  reef,  a  multitude 
of  branching  zoophytes  and  other  productions  flourish, 
but  the  Porites  and  Millepora  alone  seem  able  to  resist 
the  fury  of  the  breakers  on  its  upper  and  outer  edge ; 
at  the  depth  of  a  few  fathoms  other  kinds  of  stony 
corals  live.  Mr.  Liesk,  who  was  intimately  acquainted 
with  every  part  of  this  reef,  and  likewise  with  that  of 
North  Keeling  atoll,  assured  me  that  these  corals  in- 
variably compose  tlie  outer  margin.  The  lagoon  is 
inhabited  by  quite  a  distinct  set  of  corals,  generally 
brittle  and  thinly  branched ;  but  a  Porites,  apparently 
of  the  same  species  with  that  on  the  outside,  is  found 
there,  although  it  does  not  seem  to  thrive,  and  cer- 
tainly does  not  attain  the  thousandth  part  in  bulk  of 
the  masses  opposed  to  the  breakers. 

The  wood-cut  (No.  3)  shows  the  form  of  the  bot- 
tom outside  the  reef :  the  water  deepens  very  gradually 
for  a  space  of  between  one  and  two  hundred  yards 
wide,  to  a  depth  of  25  fathoms  {A  in  section),  beyond 
which  the  sides  plunge  into  the  unfathomable  ocean 
at  an  angle  of  45°  2.     To  the  depth  of  ten  or  twelve 

'  This  Millepora,  (Palmipora  of  Blainville,)  as  well  as  the  M.  alci- 
cornis,  possesses  the  singular  property  of  stinging  the  skin  where  it 
is  delicate,  as  on  the  face  and  arm. 

2  The  soundings  from  which  this  section  is  laid  down  were  taken 
with  great  care  by  Captain  FitzKoy  himself :  he  used  a  bell-shaped 


Sect.  I.  KEELING   ATOLL.  11 

fathoms,  the  bottom  is  exceedingly  rugged  and  seems 
formed  of  great  masses  of  living  coral,  similar  to 
those  on  the  margin.  The  arming  of  the  lead  here 
invariably  came  up  quite  clean,  but  deepl;5f  indented, 
and  chains  and  anchors  which  were  lowered,  in  the 
hopes  of  tearing  up  the  coral,  were  broken.  Many 
small  fragments,  however,  of  Millepora  alcicornis 
were  brought  up ;  and  on  the  arming  from  an  eight- 
fathom  cast,  there  was  a  perfect  impression  of  an 
Astraea,  apparently  alive.  I  examined  the  rolled 
fragments  cast  on  the  beach  during  gales,  in  order 
further  to  ascertain  what  corals  grew  outside  the  reef. 
The  fragments  consisted  of  many  kinds,  of  which  the 
Porites  already  mentioned  and  a  Madrepora,  appa- 
rently the  M.  corymbosay  were  the  most  abundant.  As  I 
searched  in  vain  in  the  hollows  on  the  reef  and  in  the 
lagoon,  for  a  living  specimen  of  this  Madrepore,  I  con- 
clude that  it  is  confined  to  a  zone  outside,  and  beneath 
the  surface,  where  it  must  be  very  abundant.  Frag- 
ments of  the  Millepora  alcicornis  and  of  an  Astrsea 
were  also  numerous ;  the  former  is  found,  but  not  in 
proportionate  numbers,  in  the  hollows  on  the  reef ;  but 
the  Astraea  I  did  not  see  living.  Hence  w^e  may  infer, 
that  these  are  the  kinds  of  coral  which  form  the 
rugged  sloping  surface  (represented  in  the  wood-cut 


lead,  having  a  diameter  of  four  inches,  and  the  armings  each  time 
were  cut  off  and  brought  on  board  for  me  to  examine.  The  arming 
is  a  preparation  of  tallow,  placed  in  the  concavity  at  the  bottom  of 
the  lead.  Sand,  and  even  small  fragments  of  rock  will  adhere  to  it ; 
and  if  the  bottom  be  of  rock,  it  brings  up  an  exact  impression  of  its 
surface. 


12  ATOLLS.  Ch.  L 

by  an  uneven  line)  round  and  beneath  the  external 
margin.  Between  12  and  20  fathoms  the  arming  came 
up  an  equal  number  of  times  smoothed  with  sand,  and 
indented  with  coral :  an  anchor  and  lead  were  lost  at 
the  respective  depths  of  13  and  16  fathoms.  Out  of 
twenty-five  soundings  taken  at  a  greater  depth  than 
20  fathoms,  every  one  showed  that  the  bottom  was 
covered  with  sand ;  whereas  at  a  less  depth  than 
12  fathoms,  every  sounding  showed  that  it  was 
exceedingly  rugged,  and  free  from  all  extraneous 
particles.  Two  soundings  were  obtained  at  the  depth 
of  360  fathoms,  and  several  between  200  and  300 
fathoms.  The  sand  brought  up  from  these  depths 
consisted  of  finely  triturated  fragments  of  stony 
zoophytes,  but  not,  as  far  as  I  could  distinguish,  of  a 
particle  of  any  lamelliform  genus  :  fragments  of 
shells  were  rare. 

At  a  distance  of  2,200  yards  from  the  breakers, 
Captain  FitzKoy  found  no  bottom  with  a  line  7,200 
feet  in  length ;  hence  the  submarine  slope  of  this  coral 
formation  is  steeper  than  that  of  any  volcanic  cone. 
Off  the  mouth  of  the  lagoon,  and  likewise  off  the 
northern  point  of  the  atoll,  where  the  currents  act 
violently,  the  inclination,  owing  to  the  accumulation  of 
sediment,  is  less.  As  the  arming  of  the  lead  from  all 
the  greater  depths  showed  a  smooth  sandy  bottom,  I  at 
first  concluded  that  the  whole  consisted  of  a  vast  conical 
pile  of  calcareous  sand,  but  the  sudden  increase  of  depth 
at  some  points,  and  the  fact  of  the  line  having  been 
cut,  when  between  500  and  600  fathoms  were  out, 


Sect.  I.  KEELING   ATOLL.  13 

indicates  the  probable  existence  of  submarine  cliffs 
of  rock. 

On  the  margin  of  the  reef,  close  within  the  line 
where  the  upper  surface  of  the  Porites  and  of  the 
Millepora  is  dead,  three  species  of  Nullipora  flourish. 
One  grows  in  thin  sheets,  like  a  lichen  on  old  trees ;  the 
second  in  stony  knobs,  as  thick  as  a  man's  finger, 
radiating  from  a  common  centre ;  and  the  third,  which 
is  less  common,  in  a  moss-like  reticulation  of  thin,  but 
perfectly  rigid  branches.^  The  three  species  occur 
either  separately  or  mingled  together ;  and  they  form 
by  their  successive  growth  a  layer  two  or  three  feet  in 
thickness,  which  in  some  cases  is  hard,  but  where  formed 
of  the  lichen-like  kind,  readily  yields  an  impression  to 
the  hammer  :  the  surface  is  of  a  reddish  colour.  These 
Nulliporse,  although  able  to  exist  above  the  limit  of 
true  corals,  seem  to  require  to  be  bathed  during  the 
greater  part  of  each  tide  by  breaking  water,  for  they  are 
not  found  in  any  abundance  in  the  protected  hollows  on 
the  back  part  of  the  reef,  where  they  might  be  immersed 
during  either  the  whole  or  an  equal  proportional  time 
of  each  tide.  It  is  remarkable  that  organic  productions 
of  such  extreme  simplicity,  for  the  Nulliporae  undoubt- 
edly belong  to  one  of  the  lowest  classes  of  the  vegetable 
kingdom,  should  be  limited  to  a  zone  so  peculiarly  cir- 

*  This  last  species  is  of  a  beautiful  bright  peach -blossom  colour. 
Its  branches  are  about  as  thick  as  crow-quilla;  they  are  slightly 
flattened  and  knobbed  at  the  extremities.  The  extremities  only  are 
alive  and  brightly  coloured.  The  two  other  species  are  of  a  dirty 
purplish  white.  The  second  species  is  extremely  hard;  its  short 
knob-like  branches  are  cylindrical,  and  do.  not  gyow  thicker  at  their 
extremities. 

3 


14  ATOLLS.  Cn.  L 

cumstanced.  Hence  the  layer  composed  by  their  growth, 
merely  fringes  the  reef  for  a  space  of  about  20  yards  in 
width,  either  under  the  form  of  separate  mammillated 
projections,  where  the  outer  masses  of  coral  are  separate, 
or  more  commonly,  where  the  corals  are  united  into  a 
solid  margin,  as  a  continuous  smooth  convex  mound 
(B  in  wood-cut)  like  an  artificial  breakwater.  Both 
the  mound  and  mammillated  projections  stand  about 
three  feet  higher  than  any  other  part  of  the  reef,  by 
which  term  I  do  not  include  the  islets,  formed  by  the 
accumulation  of  rolled  fragments.  We  shall  hereafter 
see  that  other  coral  reefs  are  protected  by  a  similar 
thick  growth  of  NulliporsB  on  the  outer  margin,  the  part 
most  exposed  to  the  breakers,  and  this  must  effectually 
aid  in  preserving  it  from  being  worn  down. 

The  wood-cut  (at  p.  8)  represents  a  section  across 
one  of  the  islets  on  the  reef,  but  if  all  that  part  which 
is  above  the  level  of  C  were  removed,  the  section 
would  be  that  of  the  reef,  as  it  occurs  where  islets 
have  not  been  formed.  It  is  this  reef  which  essen- 
tially forms  the  atoll.  In  Keeling  atoll  the  ring 
encloses  the  lagoon  on  all  sides  except  at  the  northern 
end,  where  there  are  two  open  spaces,  through  one 
of  which  ships  can  enter.  The  reef  varies  in  width 
from  250  to  500  yards ;  its  surface  is  level,  or  very 
slightly  inclined  towards  the  lagoon,  and  at  high-tide 
the  sea  breaks  entirely  over  it :  the  water  at  low  tide 
thrown  by  the  breakers  on  the  reef,  is  carried  by  the 
many  narrow  and  shoal  gullies  or  channels  on  its  sur- 
face, into  the  lagoon :  a  return  stream  sets  out  of  the 


Sect.  I.  KEELING   ATOLL.  15 

lagoon  through  the  main  entrance.  The  most  frequent 
coral  in  the  hollows  on  the  reef  is  Pocillopora  ver* 
n^cosflt,  which  grows  in  short  sinuous  plates,  or  branches, 
and  when  alive  is  of  a  beautiful  pale  lake -red :  a  Mad- 
repora,  closely  allied  or  identical  with  M,  pocillifera,  is 
also  common.  As  soon  as  an  islet  is  formed,  and  the 
weaves  are  prevented  from  breaking  entirely  over  the 
reef,  the  channels  and  hollows  become  filled  up  with 
fragments  cemented  together  by  calcareous  matter ;  and 
the  surface  of  the  reef  is  converted  into  a  hard  smooth 
floor  (C  of  wood-cut),  like  an  artificial  one  of  free- 
stone. This  flat  surface  varies  in  width  from  100  to 
200,  or  even  800  yards,  and  is  strewed  with  a  few  large 
fragments  of  coral  torn  up  during  gales  :  it  is  uncovered 
only  at  low  water.  I  could  with  dijBficulty,  and  only 
by  the  aid  of  a  chisel  procure  chips  of  rock  from 
its  surface,  and  therefore  could  not  ascertain  how 
much  of  it  is  formed  by  the  aggregation  of  detritus, 
and  how  much  by  the  outward  growth  of  mounds  of 
corals,  similar  to  those  now  living  on  the  margin.  No- 
thing can  be  more  singular  than  the  appearance  at  low 
tide  of  this  '  flat  *  of  naked  stone,  especially  where  it  is 
externally  bounded  by  the  smooth  convex  mound  of 
Nulliporse,  appearing  like  a  breakwater  built  to  resist 
the  waves,  which  are  constantly  throwing  over  it  sheets 
of  foaming  water.  The  characteristic  appearance  of 
this  '  flat '  is  shown  in  the  foregoing  wood-cut  of  Whit- 
sunday Atoll. 

The  islets  on  the  reef  are  first  formed  between  200 
and  300  yards  from  its  outer  edge^  through  the  accu- 


16  ATOLLS.  Cn.  I. 

mulation  of  a  pile  of  fragments,  thrown  together  by 
some  unusually  strong  gale.  Then-  ordinary  width  is 
under  a  quarter  of  a  mile,  and  their  length  varies  from 
a  few  yards  to  several  miles.  Those  on  the  S.E.  and 
windward  side  of  the  atoll,  increase  solely  by  the  addi- 
tion of  fragments  on  their  outer  side ;  hence  the  loose 
blocks  of  coral,  of  which  their  surface  is  composed,  as 
well  as  the  shells  mingled  with  them,  almost  exclu- 
sively consist  of  those  kinds  which  live  on  the  outer 
coast.  The  highest  part  of  the  islets  (excepting  hil- 
locks of  blown  sand,  some  of  which  are  30  feet  high),  is 
close  to  the  outer  beach  {E  of  the  wood-cut)  and  aver- 
ages from  six  to  ten  feet  above  ordinary  high- water 
mark.  From  the  outer  beach  the  surface  slopes  gently 
to  the  shores  of  the  lagoon  ;  and  this  slope  no  doubt 
is  due  to  the  breakers,  the  further  they  have  rolled 
over  the  reef,  having  had  less  power  to  throw  up 
fragments.  The  little  waves  of  the  lagoon  heap  up 
sand  and  fragments  of  thinly-branched  corals  on  the 
inner  side  of  the  islets  on  the  leeward  side  of  the  atoll ; 
and  these  islets  are  broader  than  those  to  windward, 
some  being  even  800  yards  in  width ;  but  the  land  thus 
added  is  very  low.  The  fragments  beneath  the  surface 
are  cemented  into  a  solid  mass,  which  is  exposed  as  a 
ledge  (D  of  the  wood-cut),  projecting  some  yards  in 
front  of  the  outer  shore,  and  from  two  to  four  feet  high. 
This  ledge  is  just  reached  by  the  waves  at  ordinary 
high-water  :  it  extends  in  front  of  all  the  islets,  and 
everywhere  has  a  water- worn  and  scooped  appearance. 
The  fragments  of  coral  which  are  occasionaUy  cast  on 


Sect.  I.  KEELING   ATOLL.  17 

the  *  flat '  are  during  gales  of  unusual  violence  swept 
together  on  the  beach,  where  the  waves  each  day  at 
high-water  tend  to  remove  and  gradually  wear  them 
down ;  but  the  lower  fragments  are  firmly  cemented 
together  by  percolated  calcareous  matter,  and  they  resist 
the  daily  tides  longer  than  the  loose  upper  fragments  ; 
and  thus  a  projecting  ledge  is  formed.  The  cemented 
mass  is  generally  of  a  white  colour,  but  in  some  few 
parts  reddish  from  ferruginous  matter  :  it  is  very  hard 
and  sonorous  under  the  hammer :  it  is  obscurely  divided 
by  seams,  dipping  at  a  small  angle  seaward  :  it  consists 
of  fragments  of  the  corals  which  grow  on  the  outer 
margin,  some  quite  and  others  partially  rounded,  some 
small  and  others  between  two  and  three  feet  across ;  and 
of  masses  of  previously  formed  conglomerate,  torn  up, 
rounded,  and  recemented :  or  it  consists  of  a  calcareous 
sandstone,  entirely  composed  of  rounded  particles  of 
shells,  corals,  the  spines  of  echini,  and  other  organic 
bodies  generally  almost  blended  together; — rocks, 
of  this  latter  kind,  occur  on  many  shores,  where  there 
are  no  coral-reefs.  The  structure  of  the  coral  in  the 
conglomerate  has  generally  been  much  obscured  by  the 
infiltration  of  spathose  calcareous  matter  ;  and  I  col- 
lected an  interesting  series,  beginning  with  fragments 
of  unaltered  coral,  and  ending  with  others,  where  it  was 
impossible  to  discover  with  the  naked  eye  any  trace  of 
organic  structure.  In  some  specimens  I  was  unable, 
even  with  the  aid  of  a  lens,  and  by  wetting  them,  to 
distinguish  the  boundaries  of  the  altered  coral  and 
spathose  limestone.     Many  even  of  the  blocks  of  coral 


18  ATOLLS.  Ch.  L 

lying  loose  on  the  Leach,  had  their  central  parts  altered 
and  infiltrated.^ 

The  lagoon  alone  remains  to  be  described;  it  is 
much  shallower  than  that  of  most  atolls  of  considerable 
size.  The  southern  part  is  almost  filled  up  with  banks 
of  mud  and  fields  of  coral,  both  dead  and  alive ;  but 
there  are  considerable  spaces,  from  three  to  four 
fathoms,  and  smaller  basins  from  eight  to  ten  fathoms 
deep.  Probably  about  half  its  area  consists  of  sedi- 
ment, and  half  of  coral-reefs.  The  corals  composing 
these  reefs  have  a  very  different  aspect  from  those  on 
the  outside :  they  are  numerous  in  kind,  and  most 
of  them  are  thinly  branched.  Meandrina,  however, 
lives  in  the  lagoon,  and  many  great  rounded  masses 
of  this  coral  lie  loose  or  almost  loose  on  the  bottom. 
The  other  most  common  species  are  three  closely 
allied  species  of  true  Madrepora  with  thin  branches  ; 
Seriatapora  suhulata;  two  species  of  Porites^  with 
cylindrical  branches,  one  of  which  forms  circular  clumps, 
with  only  the  exterior  branches  alive;  and  lastly,  a 
coral  something  like  an  Explanaria,  but  with  stars  on 
both  surfaces,  growing  in  thin,  brittle,  stony,  foliaceous 

•  [Dead  coral  still  lying  on  the  beach  has  been  found  to  contain 
at  least  5  per  cent,  of  carbonate  of  magnesia,  though  only  a  very 
small  quantity  is  present  in  fresh  coral  (usually  less  than  1  per  cent.). 
In  old  coral-rock  as  much  as  3807  per  cent,  has  been  found  (Dana's 
Corals  and  Coral  Islands,  ch.  vi,  §  9).] 

*  This  Porites  has  somewhat  the  habit  of  P.  clavaria,  but  the 
branches  are  not  knobbed  at  their  ends.  When  alive  it  is  of  a 
yellow  colour,  but  after  having  been  washed  in  fresh  water  and 
placed  to  dry,  a  jet-black  slimy  substance  exuded  from  the  entire 
Burface,  so  that  the  specimen  now  appears  as  if  it  had  been  dipped 
in.  ink. 


Sect.  I.  KEELING  ATOLL.  19 

expansions,  especially  in  the  deeper  basins  of  the 
lagoon.  The  reefs  on  which  these  corals  grow  are 
very  irregular  in  form,  are  full  of  cavities,  and  have 
not  a  solid  flat  surface  of  dead  rock,  like  that  surround- 
ing the  lagoon ;  nor  can  they  be  nearly  so  hard,  for 
the  inhabitants  by  the  aid  of  crowbars  made  a  channel 
of  considerable  length  through  these  reefs,  in  which  a 
schooner,  built  on  the  S.E.  islet,  was  floated  out.  It  is 
a  very  interesting  circumstance,  pointed  out  to  us  by 
Mr.  Liesk,  that  this  channel,  although  made  less  than 
ten  years  before  our  visit,  was  then,  as  we  saw,  almost 
choked  up  with  Hving  coral,  so  that  fresh  excavations 
would  be  absolutely  necessary  to  allow  another  vessel 
to  pass  through  it. 

The  sediment  from  the  deepest  parts  in  the  lagoon, 
when  wet,  appeared  chalky,  but  when  dry,  Hke  very  fine 
sand.  Large  soft  banks  of  similar,  but  even  finer 
grained  mud,  occur  on  the  S.E.  shore  of  the  lagoon, 
affording  a  thick  growth  of  a  Fucus,  on  which  turtle 
feed;  this  mud,  although  discoloured  by  vegetable 
matter,  appears  from  its  entire  solution  in  acids  to  be 
purely  calcareous.  I  have  seen  in  the  Museum  of  the 
Geological  Society,  a  similar  but  more  remarkable  sub- 
stance, brought  by  Lieut.  Nelson  from  the  reefs  of 
Bermuda,  which,  when  shewn  to  several  experienced 
geologists,  was  mistaken  by  them  for  true  chalk.  On 
the  outside  of  the  reef  much  sediment  must  be  formed 
by  the  action  of  the  surf  on  the  rolled  fragments  of 
coral ;  but,  in  the  calm  waters  of  the  lagoon,  this  can 
take  place  only  in  a  small  degree.    There  are,  however, 


20  ATOLLS.  Ch.  L 

other  and  unexpected  agents  at  work  here :  large  shoals 
of  two  species  of  Scarus,  one  inhabiting  the  surf  outside 
the  reef  and  the  other  the  lagoon,  subsist  entirely,  as  I 
was  assured  by  Mr.  Liesk,  the  intelligent  resident  before 
referred  to,  by  browsing  on  the  living  polypifers.  I 
opened  several  of  these  fish,  which  are  very  numerous 
and  of  considerable  size,  and  I  found  their  intestines 
distended  by  small  pieces  of  coral,  and  finely  ground  cal- 
careous matter.  This  must  daily  pass  from  them  as  the 
finest  sediment ;  much  also  must  be  produced  by  the 
infinitely  numerous  vermiform  and  molluscous  animals 
which  make  cavities  in  almost  every  block  of  coral.  Dr. 
J.  Allan  of  Forres,  who  has  enjoyed  the  best  means  of 
observation,  informs  me  in  a  letter,  that  the  Holuthuriae 
(a  family  of  Eadiata),  subsist  on  living  coral ;  ^  and  the 
singular  structure  of  bone  within  the  anterior  extremity 
of  their  bodies,  certainly  appears  well  adapted  for  this 
purpose.  The  number  of  the  species  of  Holuthuria, 
and  of  the  individuals  which  swarm  on  every  part  of 
these  coral-reefs,  is  extraordinarily  great ;  and  many 
ship-loads  are,  as  is  well  known,  annually  freighted 
for  China  with  trepang,  which  is  a  species  of  this 
genus.  The  amount  of  coral  yearly  consumed,  and 
ground  down  into  the  finest  mud,  by  these  several  crea- 
tures, and  probably  by  many  other  kinds,*  aiust  be 
immense.  These  facts  are,  however,  of  more  importance 

•  [Mr.  Guppy,  Proc.  R.  S.  Edin.  xiii.  p.  894,  expresses  the  opinion 
that  the  Holothurians  do  not  subsist  on  the  living  coral,  but  obtain 
nutriment  from  swallowing  the  sand  and  detrital  material,  oi  whicb 
broken  coral  forms  a  large  constituent.] 


Sect.  I.  KEELING   ATOLL.  21 

in  another  point  of  view,  as  showing  us  that  there  are 
living  checks  to  the  growth  of  coral-reefs,  and  that  the 
almost  universal  law  of  *  consume  and  be  consumed,' 
holds  good  even  with  the  polypifers  forming  those 
massive  bulwarks,  which  are  able  to  withstand  the  force 
of  the  open  ocean. 

Considering  that  Keeling  atoll,  like  other  coral  for- 
mations, has  been  entirely  formed  by  the  growth  of 
organic  beings,  and  the  accumulation  of  their  detritus, 
one  is  naturally  led  to  enquire,  how  long  it  has  con- 
tinued, and  how  long  it  is  likely  to  continue,  in  its 
present  state.  Mr.  Liesk  informed  me  that  he  had 
seen  an  old  chart  in  which  the  present  long  island  on 
the  S.E.  side  was  divided  by  several  channels  into  as 
many  islets ;  and  he  assures  me  that  the  channels  can 
still  be  distinguished  by  the  smaller  size  of  the  trees 
on  them.  On  several  islets,  also,  I  observed  that  only 
young  cocoa-nut  trees  were  growing  on  the  extremities, 
and  that  older  and  taller  trees  rose  in  regular  succession 
behind  them :  which  shows  that  these  islets  have  very 
lately  increased  in  length.  In  the  upper  and  south- 
eastern part  of  the  lagoon,  I  was  much  surprised  by 
finding  an  irregular  field  of  at  least  a  mile  square  of 
branching  corals,  still  upright^  but  entirely  dead.  They 
consisted,  of  the  species  already  mentioned ;  they  were 
of  a  brown  colour,  and  so  rotten,  that  in  trying  to 
stand  on  them,  I -sank  half  way  up  the  leg,  as  if 
through  decayed  brushwood.  The  tops  of  the  branches 
were  barely  covered  by  water  at  the  time  of  lowest  tide. 
Several  facts  having  led  me  to  disbelieve  in  any  eleva- 


22  ATOLLS.  Cn.  I. 

tion  of  the  whole  atoll,  I  was  at  first  unable  to  imagine 
what  cause  could  have  killed  so  large  a  field  of  coral. 
Upon  reflection,  however,  it  appeared  to  me  that  the 
closing  up  of  the  above  mentioned  channels  would  be 
a  sufficient  cause ;  for  before  this,  a  strong  breeze 
by  forcing  water  through  them  into  the  head  of  the 
lagoon,  would  tend  to  raise  its  level.  But  now  this 
cannot  happen,  and  the  inhabitants  observe  that  the 
tide  rises  to  a  less  height,  during  a  high  S.E.  wind,  at 
the  head  than  at  the  mouth  of  the  lagoon.  The  corals, 
which,  under  the  former  condition  of  things,  had  at- 
tained the  utmost  possible  limit  of  upward  growth, 
would  thus  occasionally  be  exposed  for  a  short  time  to 
the  sun,  and  be  killed. 

Besides  the  increase  of  dry  land,  indicated  by  the 
foregoing  facts,  the  exterior  solid  reef  appears  to  have 
grown  outwards.  On  the  western  side  of  the  atoll,  the 
*  flat '  lying  between  the  margin  of  the  reef  and  the 
beach,  is  very  wide  :  and  in  front  of  the  regular  beach 
with  its  conglomerate  basis,  there  is,  in  most  parts,  a 
bed  of  sand  and  loose  fragments  with  trees  growing 
out  of  it,  which  apparently  is  not  reached  even  by  the 
spray  at  high  water.  It  is  evident  some  change  has 
taken  place  since  the  waves  formed  the  inner  beach  ; 
that  they  formerly  beat  against  it  with  violence  was 
evident,  from  a  remarkably  thick  and  water-worn  point 
of  conglomerate  at  one  spot,  now  protected  by  vegeta- 
tion and  a  bank  of  sand ;  that  they  beat  against  it  in 
the  same  peculiar  manner  in  which  the  swell  from 
windward  now  obliquely  curls  round  the  margin  of  the 


Sect.  I.  KEELING  ATOLL.  23 

reef,  was  evident  from  the  conglomerate  having  been 
worn  into  a  point  projecting  from  the  beach  in  a  simi- 
larly oblique  manner.  This  retreat  in  the  line  of 
action  of  the  breakers  may  have  resulted,  either  from 
the  surface  of  the  reef  in  front  of  the  islets  having 
formerly  been  submerged,  and  afterwards  having  been 
raised  by  accumulated  fragments,  or  from  the  mounds 
of  coral  on  the  margin  having  grown  outwards.  That 
an  outward  growth  of  this  part  is  in  process,  can 
hardly  be  doubted  from  the  existence  of  the  mounds 
of  Porites  with  their  summits  apparently  lately  killed, 
and  their  sides  only  three  or  four  inches  lower  down 
thickened  by  a  fresh  layer  of  living  coral.  But  there 
is  a  difficulty  in  this  supposition  which  I  must  not 
pass  over.  If  the  whole,  or  a  large  part  of  the  *  flat,* 
had  been  formed  by  the  outward  growth  of  the  margin, 
each  successive  margin  would  naturally  have  been 
coated  by  the  Nullipora),  and  so  much  of  the  surface 
would  have  been  of  equal  height  with  the  existing  zone 
of  living  Nulliporae  :  this  is  not  the  case,  as  may  be 
seen  in  the  wood-cut.  It  is,  however,  evident  from 
the  abraded  state  of  the  *  flat,'  with  its  original  ine- 
qualities filled  up,  that  its  surface  has  been  much 
modified ;  and  it  is  possible  that  the  inner  portions  of 
the  zone  of  Nulliporae,  perishing  as  the  reef  grows  out- 
wards, might  be  worn  down  by  the  surf.  If  this  has 
not  taken  place,  the  reef  can  in  no  part  have  increased 
outwards  in  breadth  since  its  formation,  or  at  least 
since  the  Nulliporae  formed  the  convex  mound  on  its 
margin:    for  the  zone  thus  formed,  which  stands 


24  ATOLLS.  Ch.  L 

between  two  and  three  feet  above  the  other  parts  of 
the  reef,  is  nowhere  much  above  twenty  yards  in 
width. 

Thus  far  we  have  considered  facts,  which  indicate, 
with  more  or  less  probabiHty,  an  increase  in  the 
diameter  of  the  atoll ;  but  there  are  others  having  an 
opposite  tendency.  On  the  S.E.  side,  Lieut.  Sulivan, 
to  whose  kindness  I  am  indebted  for  many  interesting 
observations,  found  the  conglomerate  (D,  in  wood-cut 
p.  8)  projecting  on  the  reef  nearly  fifty  yards  in  front  of 
the  islets :  we  may  infer  from  what  we  elsewhere  see 
that  the  conglomerate  was  not  originally  so  much 
exposed,  but  formed  the  base  of  an  islet,  the  front  and 
upper  part  of  wiiich  has  since  been  swept  away.  The 
degree  to  which  the  conglomerate,  round  nearly  the 
whole  atoll,  has  been  scooped,  broken  up,  and  the  frag- 
ments cast  on  the  beach,  is  certainly  very  surprising, 
even  on  the  view  that  it  is  the  office  of  occasional  gales 
to  pile  up  fragments,  and  of  the  daily  tides  to  wear 
them  away.  On  the  western  side,  also,  of  the  atoll, 
where  I  have  described  a  bed  of  sand  and  fragments 
with  trees  growing  out  of  it,  in  front  of  an  old  beach, 
it  struck  both  Lieut.  SuHvan  and  myself,  from  the 
manner  in  which  the  trees  were  being  washed  down, 
that  the  surf  had  lately  recommenced  an  attack  on  this 
Ime  of  coast.  Appearances  indicating  a  slight  eu- 
croachment  of  the  water  on  the  land,  are  plainer  within 
the  lagoon :  I  noticed  in  several  places,  both  on  its 
windward  and  leeward  shores,  old  cocoa-nut  trees 
falling  with  their  roots  undermined,  and  the  rotten 


Sect.  I.  KEELING  ATOLL.  25 

stumps  of  others  on  the  beach,  where  the  inhabitants 
assured  us  the  cocoa-nut  could  not  now  grow.  Capt. 
FitzEoy  pointed  out  to  me,  near  the  settlement,  the 
foundation  posts  of  a  shed,  now  washed  by  every  tide, 
but  which  the  inhabitants  stated,  had  seven  years 
before  stood  above  high  water-mark.  In  the  calm 
waters  of  the  lagoon,  directly  connected  with  a  great, 
and  therefore  stable  ocean,  it  seems  very  improbable 
that  a  change  in  the  currents,  sufficiently  great  to 
cause  the  water  to  eat  into  the  land  on  all  sides,  should 
have  taken  place  within  a  limited  period.  From  these 
considerations  I  inferred,  that  probably  the  atoll  had 
lately  subsided  to  a  small  amount ;  and  this  inference 
was  strengthened  by  the  circumstance,  that  in  1834, 
two  years  before  our  visit,  the  island  had  been  shaken 
by  a  severe  earthquake,  and  by  two  slighter  ones  during 
the  ten  previous  years.  If,  during  these  subterranean 
disturbances,  the  atoll  did  subside,  the  downward 
movement  must  have  been  very  small,  as  we  must  con- 
clude from  the  fields  of  dead  coral  still  lipping  the 
surface  of  the  lagoon,  and  from  the  breakers  on  the 
western  shore  not  having  yet  regained  the  line  of  their 
former  action.  The  subsidence  must,  also,  have  been 
preceded  by  a  long  period  of  rest,  during  which  the 
islets  extended  to  their  present  size,  and  the  living 
margin  of  the  reef  grew  either  upwards,  or  as  I  believe 
outwards,  to  its  present  distance  from  the  beach. 

Whether  this  view  be  correct  or  not,  the  above 
facts  are  worthy  of  attention,  as  showing  how  severe  a 
struggle  is  in  progress  on  these  low  coral- formations 


26  ATOLLS.  Ch.  L 

between  the  two  nicely  balanced  powers  of  land  and 
water.  With  respect  to  the  future  state  of  Keeling 
atoll,  if  left  undisturbed,  we  can  see  that  the  islets 
may  still  extend  in  length ;  but  as  they  cannot  resist 
the  surf  until  it  is  broken  by  rolHng  over  a  wide  space, 
their  increase  in  breadth  must  depend  on  the  in- 
creasing breadth  of  the  reef;  and  this  must  be  limited 
by  the  steepness  of  the  submarine  flanks,  which  can  be 
added  to  only  by  sediment  derived  from  the  wear  and 
tear  of  the  coral.  From  the  rapid  growth  of  the  coral 
in  the  channel  cut  for  the  schooner,  and  from  the 
several  agents  at  work  in  producing  fine  sediment,  it 
might  be  thought  that  the  lagoon  would  necessarily 
become  quickly  filled  up.  Some  of  this  sediment, 
however,  is  transported  into  the  open  sea,  as  appears 
from  the  soundings  off  the  mouth  of  the  lagoon,  in- 
stead of  being  deposited  within  it.  The  deposition, 
moreover,  of  sediment,  checks  the  growth  of  coral  reefs, 
BO  that  these  two  agencies  cannot  act  together  with  full 
effect  in  filling  up  the  lagoon.  We  know  so  little  of 
the  habits  of  the  many  different  species  of  corals  which 
form  the  lagoon-reefs,  that  we  have  no  more  reason  for 
supposing  that  their  whole  surface  would  grow  up  as 
quickly  as  the  coral  did  in  the  schooner-channel,  than 
for  supposing  that  the  whole  surface  of  a  peat-mosa 
would  increase  as  quickly  as  parts  are  known  to  do  in 
holes,  where  the  peat  has  been  cut  away.  These 
agencies,  nevertheless,  tend  to  fill  up  the  lagoon ;  but 
in  proportion  as  it  becomes  shallower,  so  must  the 
polypifers  be  subject  to  many  injurious  agencies,  such 


Sect.  IL  ATOLLS.  27 

as  impure  water  and  loss  of  food.  For  instance,  Mr. 
Liesk  informed  me,  that  some  years  before  our  visit 
unusually  heavy  rain  killed  nearly  all  the  fish  in  the 
lagoon,  and  probably  the  same  cause  would  likewise 
injure  the  corals.  The  reefs  also,  it  must  be  remem- 
bared,  cannot  possibly  rise  above  the  level  of  the 
lowest  spring-tide,  so  that  the  final  conversion  of  the 
lagoon  into  land  must  be  due  to  the  accumulation  of 
sediment :  and  in  the  midst  of  the  clear  water  of  the 
ocean,  and  with  no  surrounding  high  land,  this  process 
must  be  exceedingly  slow. 


SECTION   SECOND. 

General  form  and  size  of  atolls,  their  reefs  and  islets — External 
slope—Zone  of  Nullijoorce— Conglomerate — Depth  of  lagoons — 
Sediment— Reefs  submerged  wholly  or  in  part — Breaches  in  the 
reef — Ledge-formed  shores  round  certain  lagoons — Conversion  of 
lagoons  into  lai  d, 

I  WILL  here  give  a  sketch  of  the  general  form  and 
structure  of  the  many  atolls  and  atoll-formed  reefs 
which  occur  in  the  Pacific  and  Indian  oceans,  compar- 
ing them  with  Keeling  atoll.  The  Maldiva  atolls  and 
the  Great  Chagos  Bank  differ  in  so  many  respects,  that 
I  shall  devote  to  them,  besides  occasional  references,  a 
third  section  of  this  chapter.  Keeling  atoll  may  be 
considered  as  of  moderate  dimensions  and  of  regular 
form.  Of  the  thirty-two  islands  surveyed  by  Capt. 
Beechey  in  the  Low  Archipelago,  the  longest  was  found 
to  be  thirty  miles,  and  the  shortest  less  than  a  mile  ; 
but  Vliegen  atoll,  situated  in  another  part  of  the  same 


28  ATOLLS.  Ch.  I. 

group,  appears  to  be  sixty  miles  long  and  twenty  broad. 
Most  of  the  atolls  in  this  group  are  of  an  elongated 
form ;  thus  Bow  Island  is  thirty  miles  in  length,  and 
on  an  average  only  six  in  width  (See  Fig.  4,  Plate  I.), 
and  Clermont  Tonnere  has  nearly  the  same  proportions. 
In  the  Marshall  Archipelago  (the  Kalick  and  Eadack 
group  of  Kotzebue)  several  of  the  atolls  are  more  than 
thirty  miles  in  length,  and  Kimsky  Korsacoff  is  fifty- 
four  long,  and  twenty  wide  at  the  broadest  part  of  its 
irregular  outline.  Most  of  the  atolls  in  the  Maldiva 
Archipelago  are  of  great  size,  one  of  them  (which,  how- 
ever, bears  a  double  name),  measured  in  a  medial  and 
slightly  curved  line,  is  no  less  than  eighty- eight  geo- 
graphical miles  long,  its  greatest  width  being  under 
twenty,  and  its  least  only  nine  and  a  half  miles.  Some 
atolls  have  spurs  projecting  from  them  ;  and  in  the 
Marshall  group  there  are  atolls  united  together  by 
linear  reefs,  for  instance  Menchioif  Island  (See  Fig.  3, 
Plate  II.),  which  is  sixty  miles  in  length,  and  consists 
of  three  loops  tied  together.  In  far  the  greater  num- 
ber of  cases  an  atoll  consists  of  a  simple  elongated  ring, 
with  its  outline  moderately  regular. 

The  average  width  of  the  annular  reef  may  be  taken 
at  about  a  quarter  of  a  mile.  Capt.  Beechey  *  says 
that  in  the  atolls  of  the  Low  Archipelago  it  exceeded 
in  no  instance  half  a  mile.  The  description  given  of 
the  structure  and  proportional  dimensions  of  the  reef 
and  islands  of  Keeling  atoll,  appears  to  apply  perfectly 
to  nearly  all   the  atolls  in   the   Pacific   and   Indian 

'  Beeohey's  Voyage  to  the  Pacific  and  Behring's  Straits,  chap.  viiL 


Sect.  II.  ATOLLS.  29 

oceans.  The  islets  are  first  formed  some  way  back 
either  on  the  projecting  points  of  the  reef,  especially  if 
its  form  be  angular,  or  on  the  sides  of  the  main  en- 
trances into  the  lagoon — that  is  in  both  cases,  on  points 
where  the  breakers  can  act  during  gales  of  wind  in 
somewhat  different  directions,  so  that  the  matter 
thrown  up  from  one  side  may  accumulate  against  that 
before  thrown  up  from  another.  In  Lutke's  chart  of 
the  Caroline  atolls,  we  see  many  instances  of  the  former 
case ;  and  the  occurrence  of  islets,  as  if  placed  for 
beacons,  on  the  points  where  there  is  a  gateway  or 
breach  through  the  reef,  has  been  noticed  by  several 
authors.  There  are  some  atoll-formed  reefs,  rising  to 
the  surface  of  the  sea  and  partly  dry  at  low  water,  on 
which  from  some  cause  islets  have  never  been  formed ; 
and  there  are  others,  on  which  they  have  been  formed, 
but  have  subsequently  been  worn  away.  In  atolls  of 
small  dimensions  the  islets  frequently  become  united 
into  a  single  horse-shoe  or  ring- formed  strip ;  but 
Diego  Garcia,  although  an  atoll  of  considerable  size, 
being  thirteen  miles  and  a  half  in  length,  has  its 
lagoon  entirely  surrounded,  except  at  the  northern  end, 
by  a  belt  of  land,  on  an  average  a  third  of  a  mile  iu 
width.  To  show  how  small  the  total  area  of  the  annu- 
lar reef  and  the  land  is  in  islands  of  this  class,  I  may 
quote  a  remark  from  the  voyage  of  Lutke,  namely,  that 
if  the  forty-three  rings,  or  atolls,  in  the  Caroline  Archi- 
pelago were  put  one  within  another,  and  over  a  steeple 
in  the  centre  of  St.  Petersburg,  the  whole  would  not 
cover  that  city  and  its  suburbs. 
4 


30  ATOLLS.  Cii.  I. 

The  form  of  the  bottom,  as  given  by  Captain 
Beechey  in  his  sections  of  the  atolls  in  the  Low 
Archipelago,  exactly  coincides  with  that  already  de- 
scribed in  Keeling  atoll :  it  gradually  slopes  to  about 
twenty  fathoms,  at  the  distance  of  between  one  and 
two  hundred  yards  from  the  edge  of  the  reef,  and 
then  plunges  at  an  angle  of  45°  into  unfathomable 
depths.^  The  nature,  however,  of  the  bottom  seems 
to  differ,  for  this  officer  ^  informs  me  that  all  the 
soundings,  even  the  deepest,  were  on  coral,  but  he 
does  not  know  whether  dead  or  alive.  The  slope 
round  Christmas  atoU  (Lat.  1°4'  N.,  157°  45'  W.),  de- 
scribed by  Cook,3  jg  considerably  less ;  at  about  half  a 
mile  from  the  edge  of  the  reef,  the  average  depth  was 
about  fourteen  fathoms  on  a  fine  sandy  bottom,  and  at 
a  mile,  only  between  twenty  and  forty  fathoms.  It  has 
no  doubt  been  owing  to  this  gentle  slope,  that  the  strip 
of  land  surrounding  its  lagoon,  has  increased  in  one 
part  to  the  extraordinary  width  of  three  miles;  it  is 
formed  of  successive  ridges  of  broken  shells  and  corals, 
like  those  on  the  beach.  I  know  of  no  other  instance 
of  such  width  in  the  reef  of  an  atoll ;  but  Mr.  F.  D. 

'  The  slope  of  the  bottom  round  the  Marshall  atolls  in  the 
Northern  Pacific  is  probably  similar :  Kotzebue  (First  Voyage,  vol,  ii. 
p.  16)  says,  *  We  had  at  a  small  distance  from  the  reef,  forty  fatlioms 
depth,  which  increased  a  little  further  so  much  that  we  could  find  no 
bottom.' 

"^  I  must  be  permitted  to  express  my  obligation  to  Captain  Beechey, 
for  the  very  kind  manner  in  which  he  has  given  me  information  on 
several  points,  and  to  own  the  great  assistance  I  have  derived  from 
his  excellent  published  work. 

8  Cook's  Third  Voyage,  vol.  ii.  chap.  10. 


Sect.  II.  ATOLLS.  31 

Bennett  informs  me  that  the  inclination  of  the  bottom 
round  Caroline  atoll  in  the  Pacific,  is  like  that  off 
Christmas  island,  very  gentle.  Off  the  Maldiva  and 
Chagos  atolls,  the  inclination  is  much  more  abrupt ; 
thus  at  Heawandoo  Pholo,  Lieut.  Powell  ^  found  50  and 
60  fathoms  close  to  the  edge  of  the  reef,  and  at  300 
yards  distance  there  was  no  bottom  with  a  300  yard 
line.  Capt.  Moresby  informs  me,  that  at  100  fathoms 
from  the  mouth  of  the  lagoon  of  Diego  Garcia  he  found 
no  bottom  with  150  fathoms :  this  is  the  more  remark- 
able, as  the  slope  is  generally  less  abrupt  in  front  of 
channels  through  a  reef,  owing  to  the  accumulation 
of  sediment.  At  Egmont  Island,  also,  at  150  fathoms 
from  the  reef,  soundings  were  struck  with  150  fathoms. 
Lastly,  at  Cardoo  atoll,  only  sixty  yards  from  the  reef, 
no  bottom  was  obtained,  as  I  am  informed  by  Captain 
Moresby,  with  a  line  of  two  hundred  fathoms  !  The 
currents  run  with  great  force  round  these  atolls,  and 
where  they  are  strongest,  the  inclination  appears  to  be 
most  abrupt.  I  am  informed  by  the  same  authority, 
that  wherever  soundings  were  obtained  off  these  is- 
lands, the  bottom  was  invariably  sandy :  nor  was  there 
any  reason  to  suspect  the  existence  of  submarine  cliff's, 
as  there  was  at  Keeling  Island.^    Here,  then,  occurs  a 

'  This  fact  is  taken  from  a  MS.  account  of  these  groups  lent  me 
by  Capt.  Moresby.  See  also  Capt.  Moresby's  paper  on  the  Maldiva 
atolls  in  the  Geographical  Journal,  vol.  v.  p.  401. 

2  Off  some  of  the  atolls  in  the  Low  Archipelago  the  bottom 
appears  to  descend  by  ledges.  Off  Elizabeth  Island,  which  consists 
of  raised  coral-rock,  Capt.  Beechey  (p.  45,  quarto  ed.)  describes  three 
ledges :  the  first  slopes  gently  from  the  beach  to  a  distance  of  about 
fifty  yards ;  the  second  extends  two  hundred  yards  with  a  depth  of 


32  ATOLLS.  Ch.  I. 

difficulty; — can  sand  accumulate  on  a  slope,  which, 
in  some  cases,  appears  to  exceed  fifty-five  degrees  ?  It 
must  be  observed,  that  I  speak  of  slopes  where  sound- 
ings were  obtained,  and  not  of  such  cases,  as  that  of 
Cardoo,  where  the  nature  of  the  bottom  is  unknown, 
and  where  its  inclination  must  be  nearly  vertical.  M. 
Elie  de  Beaumont  ^  has  argued,  and  there  is  no  higher 
authority  on  this  subject,  from  the  inclination  at  which 
snow  slides  down  in  avalanches,  that  a  bed  of  sand  or 
mud  cannot  be  formed  at  a  greater  angle  than  thirty 
degrees.  Considering  the  number  of  soundings  on 
sand,  obtained  round  the  Maldiva  and  Chagos  atolls, 
which  appear  to  indicate  a  greater  angle,  and  the  ex- 
treme abruptness  of  the  sand-banks  in  the  West  Indies 
as  will  be  mentioned  in  the  Appendix,  I  must  conclude 
that  the  adhesive  property  of  wet  sand  counteracts  its 
gravity,  in  a  much  greater  ratio  than  has  been  allowed 
for  by  M.  filie  de  Beaumont.  From  the  facility  with 
which  calcareous  sand  becomes  agglutinated,  it  is  not 
necessary  to  suppose  that  the  bed  of  loose  sand  is  thick. 
Capt.  Beechey  has  observed,  that  the  submarine 
slope  is  much  less  at  the  extremities  of  the  more 
elongated  atolls  in  the  Low  Archipelago,  than  at  their 
sides ;  in  speaking  of  Ducie's  Island  he  says  -  the 
buttress,  as  it  may  be  called,  which  *  has  the  most 

twenty -five  fathoms,  and  then  ends  abruptly,  like  the  first;  and 
immediately  beyond  this  there  is  no  bottom  with  two  hundred 
fathoms. 

'  M6moires  pour  servir  k  une  description  G^olog.  de  France,  tome 
£v.  p.  216. 

*  Beechey's  Voyage,  4to.  ed.  p.  44, 


Seci.  II.  ATOLLS.  83 

powerful  enemy  (the  S.W.  swell)  to  oppose,  is  carried 
out  much  further,  and  with  less  abruptness,  than  the 
other.'  In  some  cases,  the  less  inclination  of  a  certain 
part  of  the  external  slope,  for  instance  of  the  northern 
extremities  of  the  two  Keeling  atolls,  is  caused  by  a 
prevailing  current  which  there  accumulates  a  bed  of 
sand.  Where  the  water  is  perfectly  tranquil,  as  within 
a  lagoon,  the  reefs  generally  grow  up  perpendicularly, 
and  sometimes  even  overhang  their  bases :  on  the 
other  hand,  on  the  leeward  side  of  Mauritius,  where 
the  water  is  generally  tranquil  although  not  invariably 
so,  the  reef  is  very  gently  inclined.  Hence  it  appears 
that  the  exterior  angle  is  much  varied.  We  can, 
however,  discern  the  effects  of  uniform  laws  in  the 
close  similarity  in  form  between  the  sections  of  Keel- 
ing atoll  and  of  the  atolls  in  the  Low  Archipelago — 
in  the  general  steepness  of  the  reefs  of  the  Maldiva 
and  Chagos  atolls — and  in  the  perpendicularity  of 
those  rising  out  of  water  always  tranquU ;  but  from 
the  complex  action  of  the  surf  and  currents  on  the 
growing  powers  of  the  coral  and  on  the  deposition  of 
sediment,  we  can  by  no  means  follow  out  all  the 
results. 

Where  islets  have  been  formed  on  the  reef,  that 
part  which  I  have  called  the  *  flat,'  and  which  is  partly 
dry  at  low  water,  appears  similar  in  every  atoll.  In 
the  Marshall  group  in  the  N.  Pacific,  it  may  be 
inferred  from  Chamisso's  description,  that  the  reef, 
where  islets  have  not  been  formed  on  it,  slopes  gently 
from  the  external  margin  to  the  shores  of  the  lagoon : 


Si  ATOLLS.  Ch.  L 

Flinders  states  that  the  Australian  barrier  has  a 
similar  inclination  inwards,  and  I  have  no  doubt  it  is 
of  general  occurrence,  although,  according  to  Ehren- 
berg,  the  reefs  of  the  Eed  Sea  offer  an  exception. 
Chamisso  observes  that  *  the  red  colour  of  the  reef  (at 
the  Marshall  atolls)  under  the  breakers  is  caused  by  a 
NuUipora,  which  covers  the  stone  wherever  the  waves 
heat ;  and,  under  favourable  circumstances,  assumes  a 
stalactitical  form,' — a  description  perfectly  applicable 
to  the  margin  of  Keeling  atoll.  ^  Although  Chamisso 
does  not  state  that  the  masses  of  Nulliporae  form 
points  or  a  mound,  higher  than  the  flat,  yet  I  believe 
that  this  is  the  case ;  for  Kotzebue,^  in  another  part, 
speaks  of  the  rocks  on  the  edge  of  the  reef  *  as  visible 
for  about  two  feet  at  low- water,'  and  these  rocks  we 
may  feel  certain   are   not    formed    of   true    coral.^ 

*  Kotzebue's  First  Voyage,  vol.  iii.  p.  142.  Near  Porto  Praya,  in 
the  Cape  de  Verde  Islands,  some  basaltic  rocks,  lashed  by  no  incon- 
siderable surf,  were  completely  enveloped  with  a  layer  of  Nullipora. 
The  entire  surface  over  many  square  inches,  was  coloured  of  a  peach - 
blossom  red ;  the  layer,  however,  was  of  no  greater  thickness  than 
paper.  Another  kind,  in  the  form  of  projecting  knobs,  grew  in  the 
same  situation.  These  NuUiporse  are  closely  related  to  those  de- 
scribed on  the  coral-reefs,  but  I  believe  are  of  different  species. 

2  Kotzebue's  First  Voyage,  vol.  ii.  p.  16.  Lieut.  Nelson,  in"  his 
excellent  memoir  in  the  Geological  Transactions  (vol.  ii.  p.  105), 
alludes  to  the  rocky  points  mentioned  by  Kotzebue,  and  infers  that 
they  consist  of  Serpulffi,  which  compose  incrusting  masses  on  the 
reefs  of  Bermudas,  as  they  likewise  do  on  a  sandstone-bar  off  the 
coast  of  Brazil,  as  described  by  me  in  the  London  Phil.  Journal, 
Oct.  1841.  I  have  added  my  description  as  a  short  supplement  to 
the  present  volume.  These  masses  of  Serpulas  hold  the  same  posi- 
tion, relatively  to  the  action  of  the  sea,  with  the  Nulliporse  on  the 
coral-reefs  in  the  Indian  and  Pacific  oceans. 

»  Capt.  Moresby,  in  his  valuable  paper  *  On  the  Northern  Atolla 


k 


Sect.  H.  ATOLLS.  35 

Whether  a  smooth  convex  mound  of  NulKporae,  like 
that  which  appears  as  if  artificially  constructed  to 
protect  the  margin  of  Keeling  Island,  is  of  frequent 
occurrence  round  atolls,  I  know  not ;  but  we  shall 
presently  meet  with  it  under  precisely  the  same  form, 
on  the  outer  edge  of  the  *  barrier  reefs '  which  en- 
circle the  Society  Islands. 

There  appears  to  be  scarcely  a  feature  in  the 
structure  of  Keeling  reef,  which  is  not  of  common,  if 
not  of  universal  occurrence,  in  other  atolls.  Thus 
Chamisso  describes^  a  layer  of  coarse  conglomerate, 
outside  the  islets  round  the  Marshall  atolls,  which 
*  appears  on  its  upper  surface  uneven  and  eaten 
away.'  From  drawings  with  appended  remarks,  of 
Diego  Garcia  in  the  Chagos  group  and  of  several  of 
the  Maldiva  atolls,  shown  me  by  Captain  MorcRby,^  it 
is  evident  that  their  outer  coasts  are  subject  to  the 
same  round  of  decay  and  renovation  as  those  of 
Keeling  atoll.  From  the  description  of  the  atolls  in 
the  Low  Archipelago,  given  in  Captain  Beechcy's 
Voyage,  it  is  not  apparent  that  any  conglomerate 
coral-rock  was  there  observed. 

I^he  lagoon  in  Keeling  atoll  is  shallow :  in  the 
atolls  of  the  Low  Archipelago  the  depth  varies  from  20 
to  38  fathoms,  and  in  the  Marshall  Group,  according 
to  Chamisso,  from  30  to  35  :  in  the  Caroline  atolls  it 

of  Maldivas'  (Geographical  Journal,  vol.  v.),  says  that  the  edges  of 
the  reefs  there  stand  above  water  at  low  spring  tides. 

*  Kotzebue's  First  Voyage,  vol.  iii.  p.  144. 

2  See  also  Moresby  on  the  Northern  Atolls  of  the  Maldivas,  Geo- 
graphical Journal,  vol.  v.  p.  400. 


36  ATOLLS.  Ch.  L 

is  only  a  little  less.  Within  the  Maldiva  atolls  there 
are  large  spaces  with  45  fathoms,  and  some  soundings 
are  laid  down  at  49  fathoms.  The  greater  part  of  the 
bottom  in  most  lagoons,  is  formed  of  sediment ;  large 
spaces  have  exactly  the  same  depth,  or  the  depth 
varies  so  insensibly,  that  it  is  evident  that  no  other 
means  excepting  aqueous  deposition,  could  have 
levelled  the  surface  so  equally.  In  the  Maldiva  atolls 
this  is  very  conspicuous,  and  likewise  in  some  of  the 
Caroline  and  Marshall  Islands.  In  the  former,  large 
spaces  consist  of  sand  and  soft  clay  ;  and  Kotzebue 
speaks  of  clay  having  been  found  within  one  of  the 
Marshall  atolls.  No  doubt  this  clay  is  calcareous  mud, 
similar  to  that  at  Keeling  Island,  and  to  that  at  Ber- 
muda already  referred  to,  as  undistinguishable  from 
disintegrated  chalk,  and  which  Lieut.  Nelson  says  is 
called  there  pipe-clay.^ 

Where  the  waves  act  with  unequal  force  on  the 
two  sides  of  an  atoll,  the  islets  appear  to  be  first 
formed,  and  are  generally  of  greater  length  on  the  more 
exposed   shore.      The  islets,   also,  which  are  placed 

'  I  may  here  observe  that  on  the  coast  of  Brazil,  where  there  is 
much  coral,  the  soundings  near  the  land  are  described  by  Admiral 
Boussin,  in  the  Pilote  du  Br^sil,  as  siliceous  sand,  mingled  with 
much  finely  comminuted  particles  of  shells  and  coral.  Further  in 
the  offing,  for  a  space  of  1,300  miles  along  the  coast,  from  the 
Abrolhos  islands  to  Maranham,  the  bottom  in  many  places  is  com- 
posed of  *tuf  blanc,  mele  ou  form6  de  madrepores  broy6s.'  This 
white  substance,  probably  is  analogous  to  that  which  occurs  within 
the  above-mentioned  lagoons ;  it  is  sometimes,  according  to  Koussin, 
firm,  and  he  compares  it  to  mortar.  [Probably  the  clay  is  commonly 
similar  to  that  mentioned  by  Mr.  Guppy  (Proc.  R.  S.  Edin.  vol.  xiii. 
p.  879  n.)  and  others.    See  the  abstract  of  his  paper  in  Appendix  IL] 


Seot.  n.  ATOLLS.  37 

to  leeward  as  regards  the  trade-wind,  are  in  most 
parts  of  the  Pacific  Hable  to  be  occasionally  swept 
entirely  away  by  gales,  equalling  hurricanes  in  vio- 
lence, which  blow  in  the  opposite  direction.  The 
absence  of  islets  on  the  leeward  side  of  atolls,  or, 
wlien  present,  their  lesser  dimensions  compared  with 
those  to  windward,  is  a  comparatively  unimportant 
fact ;  but  it  is  remarkable  that  in  several  instances 
the  reef  itself,  although  retaining  its  usual  defined 
outline,  does  not  rise  to  the  surface  by  several  fathoms 
on  the  leeward  side.  This  is  the  case  with  the 
southern  side  of  Peros  Banhos  (Plate  I.  fig.  9)  in  the 
Chagos  group,  with  Mourileu  atoll  ^  in  the  Caroline 
Archipelago,  and  with  the  barrier  reef  (Plate  I.  fig.  8) 
of  the  Gambler  Islands,  where  Captain  Beechey  was 
first  led  to  observe  the  peculiarity  in  question.  At 
Peros  Banhos  the  submerged  part  is  nine  miles  in 
length,  and  lies  at  an  average  depth  of  about 
five  fathoms ;  its  surface  is  nearly  level,  and  consists 
of  hard  stone  with  a  thin  covering  of  loose  sand. 
There  is  scarcely  any  living  coral  on  it,  even  on  the 
outer  margin,  as  I  have  been  particularly  assured  by 
Captain  Moresby :  it  is,  in  fact,  a  wall  of  dead  coral-rock, 
having  the  same  width  and  transverse  section  with  the 
reef  in  its  ordinary  state,  of  which  it  is  a  continuous 
portion.  The  living  and  perfect  parts  terminate 
abruptly,   and   abut  on   the  submerged  portions,  in 

'  Frederic  Lutk^'s  Voyage  autour  du  Monde,  vol.  ii.  p.  291.  See 
also  his  account  of  Namonouito,  at  pp.  97  and  105,  and  the  chart  oi 
OuUeay  in  the  Atlas. 


38  ATOLLS.  Ch.  I. 

the  same  manner  as  occurs  where  there  is  a  passage 
through  the  reef.  The  reef  to  leeward  in  other  cases  is 
nearly  or  quite  obliterated,  and  one  side  of  the  lagoon 
is  left  open ;  for  instance,  at  Oulleay  (Caroline  Archi- 
pelago), where  a  crescent-formed  reef  is  fronted  by  an 
irregular  bank,  on  which  the  other  half  of  the  annular 
reef  probably  once  stood.  At  Namonouito  in  the  same 
Archipelago,  both  these  modifications  of  the  reef  concur  ; 
it  consists  of  a  great  flat  bank,  with  from  20  to  25 
fathoms  of  water  on  it ;  for  a  length  of  more  than  40 
miles  on  its  southern  side  it  is  open  and  without  any 
reef,  whilst  on  the  other  sides  it  is  bounded  by  a  reef,  in 
parts  rising  to  the  surface  and  perfectly  characterised, 
in  parts  lying  some  fathoms  submerged.  In  the  Chagos 
group  there  are  annular  reefs  entirely  submerged,  which 
have  the  same  structure  as  the  submerged  and  defined 
portions  just  described.  The  Speaker's  Bank  offers  an 
excellent  example  of  this  structure  ;  its  central  expanse, 
which  is  about  22  fathoms  deep,  is  24  miles  across  ;  the 
external  rim  is  of  the  usual  width  of  annular  reefs,  and 
is  well-defined ;  it  lies  between  six  and  eight  fathoms 
beneath  the  surface,  and  at  the  same  depth  there  are 
scattered  knolls  in  the  lagoon.  Captain  Moresby  believes 
that  the  rim  consists  of  dead  rock  thinly  covered  with 
sand,  and  he  is  certain  that  this  is  the  case  with  the 
external  rim  of  the  Great  Chagos  Bank,  which  is  also 
essentially  a  submerged  atoll.  In  both  these  cases,  as  in 
the  submerged  portion  of  the  reef  at  Peros  Banhos,  Capt. 
Moresby  feels  sure  that  the  quantity  of  living  coral, 
even  on  the  outer  edge  overhanging  the  deep-sea  water, 


Sect.  II.  ATOLLS.  39 

is  quite  insignificant.  Lastly,  in  several  parts  of  the 
Pacific  and  Indian  Oceans  there  are  banks,  lying  at 
greater  depths  than  in  the  cases  just  mentioned,  of  the 
same  form  and  size  with  the  neighbouring  atolls,  but 
with  their  atoll-like  structure  wholly  obliterated.  It 
appears  from  the  survey  of  Freycinet,  that  there  are 
banks  of  this  kind  in  the  Caroline  Archipelago,  and,  as 
is  reported,  in  the  Low  Archipelago.  When  we  discuss 
the  origin  of  the  different  classes  of  coral  formations, 
we  shall  see  that  the  submerged  state  of  the  whole  of 
some  atoll-formed  reefs,  and  of  portions  of  others 
generally  but  not  invariably  on  the  leeward  side,  and 
the  existence  of  more  deeply  submerged  banks  now 
possessing  little  or  no  signs  of  their  original  atoll-like 
structure,  are  probably  the  effects  of  a  uniform  cause, — 
namely,  the  death  of  the  coral,  during  the  subsidence  of 
the  area,  in  which  the  atolls  or  banks  are  situated. 

There  are  seldom  (with  the  exception  of  the  Maldiva 
atolls),  more  than  two  or  three  channels,  and  generally 
only  one  leading  into  the  lagoon,  of  sufficient  depth  for 
a  ship  to  enter.  In  small  atolls,  there  is  usually  not 
oven  one.  Where  there  is  deep  water,  for  instance 
above  20  fathoms,  in  the  middle  of  the  lagoon,  tho 
channels  through  the  reef  are  seldom  as  deep  as  the 
centre, — it  may  be  said  that  the  rim  only  of  the  saucer- 
shaped  hollow  forming  the  lagoon  is  notched.  Sir  C. 
Lyeir  has  observed  that  the  growth  of  the  coral  would 
tend  to  obstruct  all  the  channels  through  a  reef,  except 
those  kept  open  by  discharging  the  water,  which  during 

'  Principles  of  Geology,  vol.  iii.  p.  289.    [Vol.  ii.  p.  609,  ed.  1872.] 


40  ATOLLS  Ch.  1. 

high  tide  and  the  greater  part  of  each  ebh  is  thrown 
over  a  large  portion  of  its  circumference.  Several  facts 
indicate  that  a  considerable  quantity  of  sediment  is 
likewise  discharged  through  these  channels  ;  and 
Captain  Moresby  has  observed,  during  the  change  of 
the  monsoon,  that  the  sea  is  discoloured  to  some  dis- 
tance off  the  entrances  into  the  Maldiva  and  Chagos 
atolls.  This  would  probably  check  the  growth  of  the 
coral  in  the  channels,  far  more  effectually  than  if  they 
merely  discharged  a  current  of  water.  Where  there 
is  not  any  channel,  as  in  the  case  of  many  small  atolls, 
these  causes  have  not  prevented  the  entire  ring  attain- 
ing the  surface.  The  channels,  like  the  submerged  and 
effaced  parts  of  the  reef,  occur  very  generally,  though 
not  invariably  on  the  leeward  side  of  the  atoll,  or  on 
that  side,  according  to  Beechey,'  which,  from  extending 
in  the  same  direction  with  the  prevalent  wind,  is  not 
fully  exposed  to  it.  Passages  between  the  islets  on 
the  reef  through  which  boats  can  pass  at  high- water, 
must  not  be  confounded  with  ship-channels  by  which 
the  annular  reef  itself  is  breached.  The  passages 
between  the  islets  occur,  of  course,  on  the  windward 
as  well  as  on  the  leeward  side ;  but  they  are  more 
frequent  and  broader  to  leeward,  owing  to  the  lesser 
dimensions  of  the  islets  on  that  side. 

At  Keeling  atoll  the  shores  of  the  lagoon  shelve 
gradually  where  the  bottom  is  of  sediment,  and  irregu- 
larly or  abruptly  where  there  are  coral  reefs  ;  but  this 
is  by  no  means  the  universal  structure  in  other  atolls. 

'  Beechey's  Voyage,  4to  ed.  vol.  i.  p.  189. 


Sect.  IL  ATOLLS.  41 

^  Chamisso,^  speaking  in  general  terms  of  the  lagoons  in 
the  Marshall  atolls,  says  the  lead  generally  sinks  'from 
a  depth  of  two  or  three  fathoms  to  twenty  or  twenty- 
four,  and  you  may  pursue  a  line  in  which  on  one  side 
of  the  boat  you  may  see  the  bottom,  and  on  the  other 
the  azure-blue  deep  water.'  The  shores  of  the  lagoon- 
like channel  within  the  barrier- reef  at  Vanikoro  have 
a  similar  structure.  Captain  Beechey  has  described  a 
modification  of  this  structure  (and  he  believes  it  is  not 
uncommon)  in  two  atolls  in  the  Low  Archipelago,  in 
which  the  shores  of  the  lagoon  descend  by  a  few  broad, 
slightly  inclined  ledges  or  steps :  thus  at  Matilda 
atoll,2  the  great  exterior  reef,  the  surface  of  which  is 
gently  inclined  inwards,  ends  abruptly  in  a  little 
submarine  cliff  three  fathoms  deep ;  at  its  foot,  a 
ledge  40  yards  in  width  also  shelves  gently  in- 
wards, like  the  surface-reef,  and  terminates  in  a 
second  little  cliff  five  fathoms  deep ;  beyond  this,  the 
bottom  of  the  lagoon  slopes  to  20  fathoms,  which 
is  the  average  depth  of  its  centre.  These  ledges 
seem  to  be  formed  of  coral  rock ;  and  Captain  Beechey 
says  that  the  lead  often  descended  several  fathoms 
through  holes  in  them.  In  some  atolls,  all  the  coral 
reefs  or  knolls  in  the  lagoon  come  to  the  surface  at 
low-water ;  in  other  cases  of  rarer  occurrence,  all 
lie   at  nearly  the  same  depth  beneath  it,  but  most 

•  Kotzebue's  First  Voyage,  vol.  iii.  p.  142. 

2  Beechey 's  Voyage,  4to  ed.  vol.  i.  p.  160.  At  Whitsunday  Island 
the  bottom  of  the  lagoon  slopes  gradually  towards  the  centre,  and 
then  deepens  suddenly,  the  edge  of  the  bank  being  nearly  perpen- 
dicular.   This  bank  is  formed  of  coral  and  dead  shells. 


42  ATOLLS.  Ch.  I. 

frequently  they  are  quite  irregular — some  with  per- 
pendicular, some  with  sloping  sides— some  rising 
to  the  surface,  and  others  lying  at  all  intermediate 
depths  from  the  bottom  upwards.  I  cannot,  there- 
fore, suppose  that  the  union  of  such  reefs  could  pro- 
duce even  one  uniformly  sloping  ledge,  and  much 
less  two  or  three  one  beneath  the  other,  and  each  ter- 
minated by  an  abrupt  wall.  At  Matilda  Island,  which 
offers  the  best  example  of  the  step-like  structure^ 
Captain  Beechey  observes  that  the  coral  knolls  within 
the  lagoon  are  quite  irregular  in  their  height.  We 
shall  hereafter  see  that  the  theory  which  accounts  for 
the  ordinary  form  of  atolls,  apparently  includes  thisj 
occasional  peculiarity  in  their  structure. 

In  the  midst  of  a  group  of  atolls,  there  sometimes 
occur  small,  flat,  very  low  islands  of  coral  formation, 
which  probably  once  included  a  lagoon,  since  filled 
up  with  sediment  and  coral-reefs.  Captain  Beethey 
entertains  no  doubt  that  this  has  been  the  case  with 
the  two  small  islands,  which  alone  of  thirty-on6  sur- 
veyed by  him  in  the  Low  Archipelago,  did  not  con- 
tain lagoons.  Komanzoff  Island  (in  lat.  15°  S.)  is 
described  by  Chamisso^  as  formed  by  a  dam  of  madre- 
poritic  rock  inclosing  a  flat  space,  thinly  covered  with 
trees,  into  which  the  sea  on  the  leeward  side  occasion- 
ally breaks.  North  Keeling  atoll  appears  to  be  in  a 
rather  less  forward  stage  of  conversion  into  land  :  it 
consists  of  a  horse-shoe  shaped  strip  of  land  surround- 
ing a  muddy  flat,  one  mile  in  its  longest  axis,  which  ia 
*  Kotzebue's  First  Voyage,  vol.  iii,  p.  221. 


Sect.  II.  ATOLLS.  43 

covered  by  the  sea  only  at  high-water.  When  de- 
scribing South  Keehng  atoll,  I  endeavoured  to  show 
how  slow  the  final  process  of  filling  up  a  lagoon  must 
be  ;  nevertheless,  as  all  causes  do  tend  to  produce  this 
effect,  it  is  very  remarkable  that  not  one  instance,  as  I 
believe,  is  known  of  a  moderately  sized  lagoon  being 
filled  up  even  to  the  low-water  line  at  spring-tides, 
much  less  of  such  a  one  being  converted  into  land.  It 
is,  likewise,  in  some  degree  remarkable,  how  few  atolls, 
except  small  ones,  are  surrounded  by  a  single  linear 
strip  of  land  formed  by  the  union  of  separate  islets. 
We  cannot  suppose  that  the  many  atolls  in  the  Pacific 
and  Indian  oceans  all  have  had  a  late  origin,  and  yet 
should  they  remain  at  their  present  level,  subjected 
only  to  the  action  of  the  sea  and  to  the  growing  powers 
of  the  coral,  during  as  many  centuries  as  must  have 
elapsed  since  any  of  the  earlier  tertiary  epochs,  it  can- 
not, I  think,  be  doubted  that  their  lagoons  and  the 
islets  on  their  reef,  would  present  a  totally  different 
appearance  from  what  they  now  do.  This  considera- 
tion leads  to  the  suspicion  that  some  agency  (namely, 
subsidence)  comes  into  play  at  intervals,  and  renovates 
their  original  structure. 


44  ATOLLS.  Ch.  I. 


SECTION   THIRD, 

Maldiva  Archipelago — Bing-formed  reefs,  marghidl  and  central-^ 
Great  depth  in  the  lagoons  of  the  southern  atolls — Beefs  in  the 
lagoons  all  reach  the  surface — Position  of  islets,  and  breaches  in 
the  reefs  with  respect  to  the  prevalent  winds  a7id  action  of  tlie 
waves — Destruction  of  islets — Belation  in  position  between  dis- 
tinct atolls — The  apparent  disseverment  of  large  atolls — The 
Great  Chagos  Bank — Its  submerged  condition  and  extraordinary 
striccture. 

Although  occasional  references  have  been  made  to  the 
Maldiva  atolls  and  to  the  banks  in  the  Chagos  group, 
some  points  of  their  structure  deserve  further  consi- 
deration. My  description  is  derived  from  an  exami- 
nation of  the  admirable  charts  lately  published  from 
the  survey  of  Captain  Moresby  and  Lieut.  Powell, 
and  more  especially  from  information  which  Captain 
Moresby  has  communicated  to  me  in  the  kindest 
manner. 

The  Maldiva  Archipelago  is  470  miles  in  length, 
with  an  average  breadth  of  about  50  miles.  The  form 
and  dimensions  of  the  atolls,  and  their  singular  posi- 
tion in  a  double  line,  may  be  seen,  though  imperfectly, 
in  the  greatly  reduced  chart  (fig.  6)  in  Plate  II.  The 
dimensions  of  the  longest  atoll  in  the  group  (called  by 
the  double  name  of  Milla-dou-Madou  and  Tilla-dou- 
Matte)  have  already  been  given ;  it  is  88  miles  in  a 
medial  and  slightly  curved  line,  and  is  less  than  20 
miles  in  its  broadest  part.  Suadiva,  also,  is  a  noble 
atoll,  being  44  miles  across  in  one  direction,  and  34  in 


Sect.  III.  MALDIVA   ATOLLS.  45 

another,  and  the  great  included  expanse  of  water  has  a 
depth  of  between  250  and  300  feet.  The  smaller  atolls 
in  this  group  differ  in  no  respect  from  ordinary  ones  ; 
but  the  larger  ones  are  remarkable  from  being  breached 
by  numerous  deep-water  channels  leading  into  the 
lagoon ;  for  instance,  there  are  42  channels  through 
which  a  ship  could  enter  the  lagoon  of  Suadiva.  In 
the  three  southern  large  atolls,  the  separate  portions  of 
reef  between  these  channels  have  the  ordinary  structure 
and  are  linear ;  but  in  the  other  atolls,  especially  the 
northern  ones,  these  portions  are  ring-formed  like 
miniature  atolls.  Other  ring- formed  reefs  rise  out  of 
the  lagoons,  in  the  place  of  those  irregular  ones  which 
ordinarily  occur  there.  In  the  reduction  of  the  chart 
of  Mahlos  Mahdoo  (Plate  II.  fig.  4),  it  was  not  found 
easy  to  define  the  islets  and  the  little  lagoons  within 
each  reef,  so  that  the  ring- formed  structure  is  very  im- 
perfectly shown :  in  the  large  published  charts  of  Tilla- 
dou-Matte,  the  appearance  of  these  rings,  from  stand- 
ing further  apart  from  each  other,  is  very  remarkable. 
The  rings  on  the  margin  are  generally  elongated ; 
many  of  them  are  three,  and  some  even  five  miles  in 
diameter  ;  those  within  the  lagoon  are  usually  smaller, 
few  being  more  than  two  miles  across,  and  the  greater 
number  rather  less  than  one.  The  depth  of  the  little 
lagoon  within  these  small  annular  reefs  is  generally  from 
five  to  seven  fathoms,  but  occasionally  more ;  and  in  Ai'i 
atoll  many  of  the  central  ones  are  twelve,  and  some 
even  more  than  twelve  fathoms  deep.  These  rings  rise 
abruptly  from  the  platform  or  bank  on  which  they 


46  ATOLLS.  Ch.  I. 

stand  ;  their  outer  margins  are  invariably  bordered  by 
living  coral,  ^  within  which  there  is  a  flat  surface  of 
coral  rock ;  on  this  flat,  sand  and  fragments  have  in 
many  cases  accumulated  and  been  converted  into  isk-ta 
clothed  with  vegetation.  They  are  indeed  larger,  and 
contain  deeper  lagoons  than  many  true  atolls  standing 
in  the  open  sea ;  and  I  can  point  out  no  essential 
difference  between  these  little  ring- formed  reefs  and 
the  most  perfectly  characterised  atolls,  excepting  that 
they  are  based  on  a  shallow  foundation,  instead  of  on 
the  floor  of  the  ocean,  and  that  instead  of  being 
scattered  irregularly,  they  are  grouped  closely  together 
with  the  marginal  rings  arranged  in  a  rudely- formed 
circle. 

The  perfect  series  which  can  be  traced  from  a  linear 
reef  like  that  surrounding  an  ordinary  atoll,  to  others 
which  are  ring-formed  and  much  elongated  but  con- 
taining only  a  very  narrow  lagoon,  and  to  others  which 
are  oval  or  almost  circular,  renders  it  probable  that  the 
latter  are  merely  modifications  of  a  linear  and  normal 
reef.  The  fact  that  the  marginal  annular  reefs 
generally  have  their  longest  axes  directed  in  the  line 
which  the  exterior  linear  reef  would  have  held,  agrees 
with  this  view.  We  may  also  infer  that  the  central 
annular  reefs  are  modifications  of  those  irregular  onep, 
which  are  found  in  the  lagoons  of  all  common  atolls. 
It  appears  from  the  charts  on  a  large  scale,  that  the 

'  Captain  Moresby  informs  me  that  Millepora  complanata  is  one 
of  the  commonest  kinds  on  the  outer  margin,  as  it  is  at  Keeling 
«coll. 


Sect.  III.  MALDIVA  ATOLLS.  47 

ring-like  structure  in  these  central  reefs  is  con- 
tingent 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  mar- 
ginal portions  of  reef,  and  especially  those  close  to  the 
larger  channels,  are  ring-formed,  but  the  central  ones 
are  not  so ;  where  they  are  broadest,  almost  every 
reef  throughout  the  atoll  is  more  or  less  perfectly  ring- 
formed.  Although  their  presence  is  thus  contingent 
on  the  openness  of  the  marginal  channels,  the  theory  of 
their  formation,  as  we  shall  hereafter  see,  is  included 
in  that  of  the  parent  atolls  of  which  they  form  the 
separate  portions. 

The  lagoons  of  all  the  atolls  in  the  southern  part  of 
the  Archipelago  are  from  10  to  20  fathoms  deeper  than 
those  in  the  northern  part.  This  is  well  exemplified  in 
the  case  of  Addoo,  the  southernmost  atoll  in  the  group, 
for  although  only  9  miles  in  its  longest  diameter,  it  has 
a  depth  of  39  fathoms,  whereas  all  the  other  small  atolls 
have  comparatively  shallow  lagoons  ;  I  can  assign  no 
adequate  cause  for  this  difference  in  depth,  excepting 
that  the  southern  part  of  the  Archipelago  has  subsided 
to  a  greater  degree  or  at  a  quicker  rate  than  the 
northern  part ;  and  this  conclusion  agrees  well  with  the 
fact  that,  in  the  Chagos  group,  lying  280  miles  still 
further  southwards,  most  of  the  atolls  are  sunken  and 


48  ATOLLS.  Ch,  L 

half  destroyed  with  the  dead  corals.  In  the  central  and 
deepest  part  of  the  Maldiva  lagoons,  the  bottom  consists, 
as  I  am  informed  by  Captain  Moresby,  of  stiff  clay 
(probably  a  calcareous  mud) ;  nearer  the  border  it  con- 
sists of  sand,  and  in  the  channels  through  the  reef,  of 
hard  sand-banks,  sandstone,  conglomerate  rubble,  and  a 
little  live  coral.  Close  outside  the  reef  the  bottom  is 
sandy,  and  slopes  abruptly  into  unfathomable  depths. 
In  most  lagoons  the  depth  is  considerably  greater 
in  the  centre  than  in  the  channels ;  but  in  Tilla- 
dou-Matte,  where  the  marginal  ring-formed  reefs 
stand  far  apart,  the  same  depth  is  carried  across  the 
entire  atoll,  from  the  deep-water  line  on  one  side  to 
that  on  the  other.  I  cannot  refrain  from  once  again 
remarking  on  the  singular  structure  of  these  atolls, 
— a  great  sandy  and  generally  concave  disk  rises 
abruptly  from  the  unfathomable  ocean,  with  the  central 
expanse  studded  and  the  margins  symmetrically  fringed 
with  oval  basins  of  coral- rock,  just  lipping  the  surface 
of  the  sea,  sometimes  clothed  with  vegetation,  and  each 
containing  a  little  lake  of  clear  salt  water. 

In  the  southern  Maldiva  atolls,  of  which  there  are 
nine  large  ones,  all  the  small  reefs  within  the  lagoons 
come  to  the  surface,  and  are  dry  at  low-water  spring- 
tides ;  hence  in  navigating  them  there  is  no  danger 
from  submarine  banks.  This  circumstance  is  very 
remarkable,  as  within  some  atolls,  for  instance  those  of 
the  neighbouring  Chagos  group,  not  a  single  reef  comes 
to  the  surface,  and  in  most  other  cases  a  few  only  do, 
and  the  rest  lie  at  all  intermediate  depths  from  the 


Sect.  III.  MALDIVA   ATOLLS.  49 

bottom  upwards.  When  treating  of  the  growth  of  coral 
I  shall  again  refer  to  this  subject. 

Although  in  the  neighbourhood  of  the  Maldiva 
Archipelago  the  winds,  during  the  monsoons,  blow 
during  nearly  an  equal  time  from  opposite  quarters, 
and  although,  as  I  am  informed  by  Captain  Moresby, 
the  westerly  winds  are  the  strongest,  yet  the  islets  are 
almost  all  placed  on  the  eastern  side  of  the  northern 
atolls,  and  on  the  south-eastern  side  of  the  southern 
atolls.  That  the  formation  of  the  islets  is  due  to 
detritus  thrown  up  from  the  outside,  as  in  the  ordinary 
manner,  and  not  from  the  interior  of  the  lagoons,  may, 
I  think,  be  safely  inferred  from  several  considerations 
which  it  is  hardly  worth  while  to  detail.  As  the  easterly 
winds  are  not  the  strongest,  their  action  probably  is 
aided  by  some  prevailing  swell  or  current. 

In  groups  of  atolls  exposed  to  the  trade  wind,  the 
ship-channels  into  the  lagoons  are  almost  always 
situated  on  the  leeward  or  less  exposed  side  of  the  reef, 
and  the  reef  itself  is  sometimes  either  wanting  there,  or 
is  submerged.  A  strictly  analogous,  but  different,  fact 
may  be  observed  at  the  Maldiva  atolls— namely,  that 
where  two  atolls  stand  near  together,  the  breaches  in 
the  reef  are  most  numerous  on  the  sides  which  face  each 
other,  and  are  therefore  less  exposed  to  the  waves.  Thus 
on  the  sides  of  Ari  and  the  two  Nillandoo  atolls  which 
face  S.  Male,  Phaleedoo,  and  Moloque  atolls,  there  are 
seventy-three  deep-water  channels,  and  only  twenty-five 
on  the  outer  sides ;  on  the  three  latter-named  atolls  there 
are  fifty-six  openings  on  the  near  side,  and  only  thirty- 


50  ATOLLS.  Ch.  L 

seven  on  the  outside.  It  is  scarcely  possible  to  attri- 
bute this  difference  to  any  other  cause  than  the  some- 
what different  action  of  the  sea  on  the  two  sides,  which 
would  ensue  from  the  mutual  protection  afforded  by 
the  two  rows  of  atolls.  I  may  here  remark  that  in 
most  cases,  the  conditions  favourable  to  the  greater 
accumulation  of  fragments  on  the  reef  and  to  its 
more  perfect  continuity  on  one  side  of  the  atoll  than 
on  the  other,  have  concurred,  but  this  has  not  been 
the  case  with  the  Maldivas ;  for  we  have  seen  that  the 
islets  are  placed  on  the  eastern  or  south-eastern  sides, 
whilst  the  breaches  in  the  reef  occur  indifferently  on 
any  side  where  protected  by  an  opposite  atoll.  The  reef 
being  more  continuous  on  the  outer  and  more  exposed 
sides  of  those  atolls  which  stand  near  each  other, 
accords  with  the  fact,  that  the  reefs  of  the  southern 
atolls  are  more  continuous  than  those  of  the  northern 
ones,  for  the  former,  as  I  am  informed  by  Captain 
Moresby,  are  more  constantly  exposed  to  a  heavy  surf 
than  are  the  northern  atolls. 

The  date  of  the  first  formation  of  some  of  the  islets 
in  this  Archipelago  is  known  to  the  inhabitants ;  on  the 
other  hand,  several  islets,  and  even  some  of  those  which 
are  believed  to  be  very  old,  are  now  fast  wearing  away. 
The  work  of  destruction  has,  in  some  instances,  been 
completed  in  ten  years.  Captain  Moresby  found  on  one 
water- washed  reef  the  marks  of  wells  and  graves,  w^hich 
were  excavated  when  it  supported  an  islet.  In  South 
Nillandoo  atoll,  the  natives  say  that  three  of  the  islets 
were  formerly  larger  :  in  North  Nillandoo  there  is  one 


Sect.  JII.  MALDIVA   ATOLLS.  51 

now  being  washed  away ;  and  in  this  latter  atoll  Lieut. 
Prentice  found  a  reef,  about  six  hundred  yards  in 
diameter,  which  the  natives  positively  affirmed  was 
lately  an  island  covered  with  cocoa-nut  trees.  It  is  now 
only  partially  dry  at  low- water  spring  tides,  and  is  (in 
Lieut.  Prentice's  words)  *  entirely  covered  with  live 
coral  and  madrepore.'  In  the  northern  part,  also,  of 
the  Maldiva  Archipelago  and  in  the  Chagos  group,  it 
is  known  that  some  of  the  islets  are  disappearing. 
The  natives  attribute  these  effects  to  variations  in  the 
currents  of  the  sea.  For  my  own  part  I  cannot  avoid 
suspecting,  that  there  must  be  some  further  cause, 
which  gives  rise  to  such  a  cycle  of  change  in  the  action 
of  the  currents  of  the  great  and  open  ocean. 

Several  of  the  atolls  in  this  Archipelago  are  so 
related  to  each  other  in  form  and  position,  that  at  the 
first  glance  one  is  led  to  suspect  that  they  have 
originated  in  the  disseverment  of  a  single  one.  Male 
consists  of  three  perfectly  characterised  atolls,  of  which 
the  shape  and  relative  position  are  such,  that  a  line 
drawn  closely  round  aU  three  gives  a  symmetrical 
figure  ;  but  to  see  this,  a  larger  chart  is  required  than 
that  of  the  Archipelago  in  Plate  II.  The  channel 
separating  the  two  northern  Male  atolls  is  only  httle 
more  than  a  mile  wide,  and  no  bottom  was  found  in  it 
with  100  fathoms.  Powell's  Island  is  situated  at  the 
distance  of  two  miles  and  a-half  off  the  northern  end 
of  another  atoll,  namely  Mahlos  Mahdoo  (fig.  4),  at 
the  exact  point  where  the  two  sides  of  the  latter, 
if    prolonged,   would    meet:    no    bottom,    however, 


52  ATOLLS.  Ch.  I. 

was  found  in  the  channel  with  200  fathoms :  in  the 
wider  channel  between  Horsburgh  atoll  and  the  south- 
ern end  of  Mahlos  Mahdoo,  no  bottom  was  found  with 
250  fathoms.  In  these  cases,  the  relation  consists 
only  in  the  form  and  position  of  the  atolls.  But 
in  the  channel  between  the  two  Nillandoo  atolls, 
although  three  miles  and  a- quarter  wide,  soundings 
were  struck  at  the  depth  of  200  fathoms :  the  channel 
between  Eoss  and  Ari  atolls  is  four  miles  wide,  and  only 
150  fathoms  deep.  Here  then  we  have  a  submarine 
connection,  besides  a  relation  in  position  and  form.  The 
fact  of  soundings  having  been  obtained  between  two 
separate  and  perfectly  characterised  atolls  is  in  itself 
interesting,  as  it  has  never,  I  believe,  been  effected  in 
any  of  the  many  other  groups  of  atolls  in  the  Pacific 
and  Indian  seas.  In  continuing  to  trace  the  con- 
nection of  adjoining  atolls,  if  a  hasty  glance  be  taken 
at  the  chart  (fig.  4,  Plate  II.)  of  Mahlos  Mahdoo  and 
the  line  of  unfathomable  water  be  followed,  no  one 
will  hesitate  to  consider  it  as  one  atoll.  But  a  second 
look  will  show  that  it  is  divided  by  a  bifurcating 
channel,  of  which  the  northern  arm  is  about  one  mile 
and  three-quarters  in  width,  with  an  average  depth 
of  125  fathoms,  and  the  southern  one  three-quarters 
of  a  mile  wide,  and  rather  less  deep.  These  channels 
resemble  in  the  slope  of  their  sides  and  general  form, 
those  which  separate  atolls  in  every  respect  distinct  ; 
and  the  northern  arm  is  wider  than  that  dividing  two 
of  the  Male  atolls.  The  ring-formed  reefs  on  the 
northern  and  southern  sides  of  this  bifurcating  channel 


Sect.  m.  GREAT   CHAGOS   BANK.  53 

are  elongated,  and  so  continuous  that  the  northern  and 
southern  portions  of  Mahlos  Mahdoo  may  claim  to 
be  considered  as  distinct  atolls.  But  the  reefs  of 
the  intermediate  portion  are  less  perfect,  so  that  this 
portion  hardly  yet  resembles  a  distinct  atoll.  Mahlos 
Mahdoo,  therefore,  is  in  every  respect  in  an  inter- 
mediate condition,  so  that  it  may  be  considered  either 
as  a  single  atoll  nearly  dissevered  into  three  portions, 
or  as  three  atolls  almost  perfect  and  intimately  con- 
nected. This  is  an  instance  of  a  very  early  stage  of 
the  apparent  disseverment  of  an  atoll,  and  another 
is  exhibited  at  Tilla-dou-Matte.  In  one  part  of 
this  atoll,  the  ring-formed  reefs  stand  so  far  apart 
from  each  other,  that  the  inhabitants  have  given 
different  names  to  the  northern  and  southern  halves : 
nearly  all  the  rings,  moreover,  are  so  perfect,  and 
stand  so  separate,  and  the  space  from  which  they  rise 
is  so  level  and  unlike  a  true  lagoon,  that  we  can  easily 
imagine  the  conversion  of  this  one  great  atoll,  not  into 
two  or  three  portions,  but  into  a  whole  group  of 
miniature  atolls.  A  series  such  as  we  have  here 
traced,  impresses  the  mind  with  the  idea  of  actual 
change  ;  and  it  will  hereafter  be  seen,  that  the  theory 
of  subsidence  together  with  the  upward  growth  of  the 
coral-reefs,  modified  by  accidents  of  probable  occur- 
rence, accounts  for  the  occasional  disseverment  of  large 
atolls. 

The  great  Chagos  Bank  alone  remains  to  be  de- 
scribed.^    In  the  Chagos  group  there  are  some  ordi- 
*  [See  Appendix  II.] 


54  ATOLLS.  Ch.  I. 

nary  atolls,  some  annular  reefs  rising  to  the  surface 
but  without  any  islets  on  them,  and  some  atoll- formed 
banks  either  quite  or  nearly  submerged.  Of  the 
latter,  the  Great  Chagos  Bank  is  much  the  largest,  and 
differc  in  its  structure  from  the  others ;  a  plan  of  it 
is  given  in  Plate  II.  fig.  1,  in  which,  for  the  sake  of 
clearness,  I  have  had  the  parts  under  ten  fathoms  deep 
finely  shaded :  an  east  and  west  vertical  section  is  given 
in  fig.  2,  in  which  the  vertical  scale  has  been  neces- 
sarily exaggerated.  Its  longest  axis  is  ninety  nautical 
miles,  and  another  line  drawn  across  the  broadest  part, 
at  right  angles  to  the  first,  is  seventy.  The  central 
part  consists  of  a  level  muddy  flat  between  forty  and 
fifty  fathoms  deep,  which  is  surrounded  on  all  sides, 
with  the  exception  of  some  breaches,  by  the  steep 
edges  of  a  set  of  banks  rudely  arranged  in  a  circle. 
These  banks  consist  of  sand  with  a  very  little  live 
coral ;  they  vary  in  breadth  from  five  to  twelve  miles, 
and  on  an  average  lie  about  sixteen  fathoms  beneath 
the  surface  ;  they  are  bordered  by  the  steep  edges  of  a 
third  narrow  and  upper  bank,  which  forms  the  rim  to 
the  whole.  This  rim  is  about  a  mile  in  width,  and,  with 
the  exception  of  two  or  three  spots  where  islets  have 
been  formed,  is  submerged  between  five  and  ten  fathoms. 
It  consists  of  smooth  hard  rock,  covered  with  a  thin 
layer  of  sand,  but  with  scarcely  any  live  coral;  it  is 
steep  on  both  sides,  and  slopes  abruptly  outwards  into 
unfathomable  depths.  At  the  distance  of  less  than  half 
a  mile  from  one  part,  no  bottom  was  found  with  190 
fathoms ;  and  off  another  point,  at  a  somewhat  greater 


Sect.  III.  GREAT   CHAGOS   BANK.  55 

distance,  there  was  none  with  210  fathoms.  Small 
steep-sided  banks  or  knolls,  covered  with  luxuriantly- 
growing  coral,  rise  from  the  interior  expanse  to  the 
same  level  with  the  external  rim,  which,  as  we  have  seen, 
is  formed  only  of  dead  rock.  It  is  impossible  to  look  at 
the  plan  (fig.  1,  Plate  II.),  although  reduced  to  so 
small  a  scale,  without  at  once  perceiving  that  the  Great 
Chagos  Bank  is,  in  the  words  of  Captain  Moresby,^  'no- 
thing more  than  a  half-drowned  atoll.*  But  of  what 
great  dimensions,  and  of  how  extraordinary  an  internal 
structure  !  We  shall  hereafter  have  to  consider  both 
the  cause  of  its  submerged  condition,  a  state  common 
to  other  banks  in  the  group,  and  the  origin  of  the 
singular  submarine  terraces  which  bound  the  central 
expanse ;  these,  I  think  it  can  be  shown,  have  resulted 
from  a  cause  analogous  to  that  which  has  produced  the 
bifurcating  channel  across  Mahlos  Mahdoo. 

'  This  officer  has  had  the  kindness  to  lend  me  an  excellent  MS. 
account  of  the  Chagos  Islands  ;  from  this  paper,  from  the  published 
charts,  and  from  verbal  information  communicated  to  me  by  Captain 
Moresby,  the  above  acooant  of  the  Great  Chagos  Bank  ia  taken. 


CHAPTEK   11. 

BABRIER-REEFS. 

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

The  term  *  barrier '  has  been  generally  applied  to  that 
vast  reef  which  fronts  the  N.E.  shore  of  Australia,  and 
by  most  voyagers  likewise  to  that  on  the  western  coast 
of  New  Caledonia.  At  one  time  I  thought  it  con- 
venient thus  to  restrict  the  term,  but  as  these  reefs  are 
similar  in  structure  and  in  position  relatively  to  the 
land,  to  those,  which,  like  a  wall  with  a  deep  moat 
within,  encircle  many  smaller  islands,  I  have  classed 
them  together.  The  reef,  also,  on  the  west  coast  of 
N.ew  Caledonia,  circling  round  the  extremities  of  the 
island,  is  an  intermediate  form  between  a  small  en- 
cii'cling  reef  and  the  Australian  barrier,  which  stretches 
for  a  thousand  miles  in  nearly  a  straight  Hne. 

The  geographer  Balbi  has  in  effect  described  those 
barrier-reefs  which  encircle  moderately  sized  islands, 
by  calling  them  atolls  with  high  land  rising  from 
within  their  central  expanse.    The  general  resem- 


Ch.  ii.  barrier-reefs.  57 

blance  between  the  reefs  of  the  barrier  and  atoll 
classes  may  be  seen  in  the  small,  but  accurately  re- 
duced charts  on  Plate  I.,'  and  this  resemblance  can  be 
further  shown  to  extend  to  every  part  of  their  struc- 
ture. Beginning  with  the  outside  of  the  reef ;  many 
scattered  soundings  off  Gambier,  Ualan,  and  some 
other  encircled  islands,  show  that  close  to  the  breakers 
there  exists  a  narrow  shelving  margin,  beyond  which 
in  most  cases,  the  ocean  suddenly  becomes  unfathom- 
able. Off  the  west  coast  of  New  Caledonia,  Captain 
Kent  2  found  no  bottom  with  150  fathoms,  at  two  ship's 
lengths  from  the  reef ;  so  that  the  slope  here  must  be 
nearly  as  precipitous  as  off  the  Maldiva  atolls. 

I  can  give  little  information  regarding  the  kinds 
of  corals  which  live  on  the  outer  margin.  When  I 
visited  the  reef  at  Tahiti,  although  it  was  low-water, 
the  surf  was  too  violent  for  me  to  see  the  living 
masses ;  but,  according  to  what  I  heard  from  some  in- 
telligent native  chiefs,  they  resemble  in  their  rounded 
and  branchless  forms,  those  on  the  margin  of  Keeling 
atoll.  The  extreme  verge  of  the  reef  which  was 
visible  between  the  breaking  waves  at  low-water,  con- 
sisted of  a  rounded,  convex,  artificial-like  breakwater, 
entirely  coated  with  Nulliporae,  and  absolutely  similar 
to  that  which  I  have  described  at  Keeling  atoll. 
From  what  I  heard  when   at   Tahiti,-  and  from  the 

'  The  authorities  from  which  these  charts  have  been  reduced, 
together  with  some  remarks  on  them,  are  given  in  a  separately  ap- 
pended page,  descriptive  of  the  Plates. 

'^  Dalrymple,  Hydrog.  Mem.  vol.  iii. 


58  BARRIER-KEEFS.  Ch.  IT. 

writings  of  the  Eevs.  W.  Ellis  and  J.  Williams,  I  con- 
clude that  this  peculiar  structure  is  common  to  most 
of  the  encircled  islands  of  the  Society  Archi^Delago. 
The  reef  within  this  mound  or  breakwater,  has  an  ex- 
tremely irregular  surface,  even  more  so  than  between 
the  islets  on  the  reef  of  Keeling  atoll,  with  which 
alone  (as  there  are  no  islets  on  the  reef  of  Tahiti)  it 
can  properly  be  compared.  At  Tahiti  the  reef  is  very 
irregular  in  width ;  but  round  many  other  encircled 
islands,  for  instance  Vanikoro  or  Gambler  Islands 
(figs.  1  and  8,  Plate  I.),  it  is  quite  as  regular,  and  of 
the  same  average  width,  as  in  true  atolls.  Most 
barrier-reefs  on  the  inner  side  slope  irregularly  into 
the  lagoon-channel,  (as  the  space  of  deep  water  sepa- 
rating the  reef  from  the  included  land  may  be  called,) 
but  at  Vanikoro  the  reef  slopes  only  for  a  short  dis- 
tance, and  then  terminates  abruptl}^  in  a  submarine 
wall  forty  feet  high, — a  structure  absolutely  similar  to 
that  described  by  Chamisso  in  the  Marshall  atolls. 

In  the  Society  Archipelago,  Ellis'  states  that  the 
reefs  generally  lie  at  the  distance  of  from  one  to  one 
and  a-half  miles,  and,  occasionally,  even  at  more  than 
three  miles  from  the  shore.  The  central  mountains 
are  generally  bordered  by  a  fringe  of  flat,  and  often 
marshy  alluvial  land,  from  one  to  four  miles  in  width. 
This  fringe  consists  of  coral- sand  and  detritus  thrown 
up  from  the  lagoon-channel,  and  of  soil  washed  down 
from  the  hills  ;  it  is  an  encroachment  on  the  channel, 

•  Consult,  on  this  and  other  points,  the  Polynesian  Researches 
by  the  Rev.  W.  Ellis,  an  admirable  work,  full  of  curious  information. 


I 


Ch.  II.  BARRIER-REEFS.  59 

analogous  to  that  low  and  inner  part  of  the  islets  in 
many  atolls,  which  is  formed  by  the  accumulation  of 
matter  from  the  lagoon.  At  Hogolen  (fig.  2,  Plate  I.), 
in  the  Caroline  Archipelago,^  the  reef  on  the  south 
side  is  no  less  than  twenty  miles;  on  the  east  side, 
five ;  and  on  the  north  side,  fourteen  miles  from  the 
encircled  islands. 

The  lagoon- channels  may  be  compared  in  every 
respect  with  true  lagoons.  In  some  cases  they  are 
open,  with  a  level  bottom  of  fine  sand  ;  in  others  they 
are  choked  up  with  reefs  of  delicately  branched  corals, 
which  have  the  same  general  character  as  those  within 
Keeling  atoll.  These  internal  reefs  either  stand  sepa- 
rately, or  more  commonly  skirt  the  shores  of  the  in- 
cluded high  islands.  The  depth  of  the  lagoon-channel 
round  the  Society  Islands  varies  from  two  or  three, 
to  thirty  fathoms ;  in  Cook's  ^  chart  of  Ulietea,  how^ 
ever,  there  is  one  sounding  laid  down  of  48  fathoms : 
at  Vanikoro  there  are  several  of  54  and  one  of  56J 
fathoms  (English),  a  depth  which  even  exceeds  by  a 
little  that  of  the  interior  of  the  great  Maldiva  atolls. 
Some  barrier-reefs  have  very  few  islets  on  them ;  whilst 
others  are  surmounted  by  numerous  ones ;  and  those 
round  part  of  Bolabola  (Plate  I.,  fig.  5),  form  a  single 
linear  strip.  The  islets  first  appear  either  on  the 
angles  of  the  reef,  or  on  the  sides  of  the  breaches 

'  See  Hydrographical  Mem.  and  the  Atlas  of  the  Voyage  of  the 
Astrolabe,  by  Capt.  Dumont  D'Urville,  p.  428. 

2  See  the  chart  in  vol.  i.  of  Hawkesworth's  4to  ed.  of  Cook's  First 
Voyage 


60  BARRIER-REEFS.  Cn.  U. 

through  it,  and  are  generally  most  numerous  on  the 
windward  side.  The  reef  to  leeward  retaining  its  usual 
width,  sometimes  lies  submerged  several  fathoms  be- 
neath the  surface ;  I  have  already  mentioned  Gambler 
Island  as  an  instance  of  this  structure.  Submerged 
reefs,  dead,  covered  with  sand,  and  with  a  less  defined 
outhne,  have  been  observed  (see  Appendix  I.)  off  some 
parts  of  Huaheine  and  Tahiti.  The  reef  is  more  fre- 
quently breached  to  leeward  than  to  windward,  although 
this  is  not  so  frequent  as  in  the  case  of  atolls.  Thus  I 
find  in  Krusenstern's  Memoir  on  the  Pacific,  that  there 
are  passages  through  the  encircling  reef  on  the  lee- 
ward side  of  the  seven  Society  Islands,  which  possess 
ship-harbours ;  but  that  there  are  openings  to  wind- 
ward through  only  three  of  them.  The  breaches 
in  the  reef  are  seldom  as  deep  as  the  interior 
lagoon-like  channel ;  they  generally  occur  in  front  of 
the  main  valleys,  a  circumstance  which  can  be  ac- 
counted for,  as  will  be  seen  in  the  fourth  chapter, 
without  much  difficulty.  The  breaches  being  generally 
situated  in  front  of  the  valleys  which  descend  on 
all  sides,  explains  their  more  frequent  occurrence 
through  the  windward  side  of  barrier-reefs  than 
through  the  windward  side  of  atolls, — for  in  atolls 
there  is  no  included  land  to  influence  the  position  of 
the  breaches. 

It  is  remarkable  that  the  lagoon-channels  round 
mountainous  islands  have  not  in  every  instance  been 
long  ago  filled  up  with  coral  and  sediment ;  but  it  is 
accounted  for  without  much  difficulty.     In  cases  like 


Ch.  n.  BARRIEK-REEFS.  61 

that  of  Hogoleu  and  the  Gambier  Islands,  where  a  few 
small  peaks  rise  out  of  a  great  lagoon,  the  conditions 
scarcely  differ  from  those  of  an  atoll;  and  I  have  already- 
shown  at  some  length,  that  the  filling  up  of  a  true 
lagoon  must  be  an  extremely  slow  process.  Where 
the  lagoon-channel  is  narrow,  that  agency,  which  on 
unprotected  coasts  is  the  most  productive  of  sediment, 
namely  the  force  of  the  breakers,  is  here  entirely  ex- 
cluded ;  and  owing  to  the  reef  being  breached  in  the 
front  of  the  main  valleys,  much  of  the  finer  mud  from 
the  rivers  must  be  transported  into  the  open  sea.  The 
water  which  is  thrown  over  the  edges  of  atoll-formed 
reefs  causes  a  current  which  carries  sediment  from  the 
lagoon  through  the  breaches  into  the  sea ;  and  the 
same  thing  probably  takes  place  in  barrier-reefs.  This 
would  greatly  aid  in  preventing  the  lagoon- channels 
from  being  filled  up.  The  low  alluvial  border,  how- 
ever, at  the  foot  of  the  encircled  mountains,  shows 
that  the  work  of  filling  up  is  in  progress ;  and  at 
Maurua  (Plate  I.,  fig.  6),  in  the  Society  group,  it  has 
been  almost  effected,  so  that  there  remains  only  one 
harbour  for  small  craft. 

If  we  look  at  a  set  of  charts  of  barrier-reefs,  and 
leave  out  in  imagination  the  encircled  land,  we  shall 
see  that  besides  the  many  points  already  noticed  of 
resemblance  or  rather  of  identity  in  structure  with 
atolls,  there  is  a  close  general  agreement  in  form,  aver- 
age dimensions,  and  grouping.  Encircling  reefs,  like 
atolls,  are  generally  elongated,  and  have  an  irregularly 
rounded,  though  sometimes  angular  outline.  There  are 
6 


62  BARRIER-REEFS.  Ch.  IL 

atolls  of  all  sizes,  from  less  than  two  miles  in  diameter 
to  sixty  miles  (excluding  Tilla-dou-Matte,  which  consists 
of  a  number  of  almost  independent  atoll- formed  reefs) ; 
and  there  are  encircling  barrier-reefs  from  three  miles 
and  a-half  to  forty-six  miles  in  diameter, — Turtle 
Island  being  an  instance  of  the  former,  and  Hogoleu  of 
the  latter.  At  Tahiti  the  encircled  island  is  thirty-six 
miles  in  its  longest  axis,  whilst  at  Mam'ua  it  is  only  a 
little  more  than  two  miles.  It  will  also  be  shown  in 
the  last  chapter,  that  there  is  the  strictest  resemblance 
between  the  grouping  of  atolls  and  of  common  islands, 
and  there  is  the  same  resemblance  between  atolls  and 
encircling  barrier-reefs. 

The  islands  lying  within  reefs  of  this  class,  are  of 
very  various  heights.  Tahiti '  is  7,000  feet ;  Maurua 
about  800 ;  Aitutaki  360,  and  Manouai  only  50.  The 
geological  nature  of  the  included  land  also  varies  ;  in 
most  cases  it  is  of  ancient  volcanic  origin,  owing  appa- 
rently to  the  fact  that  islands  of  this  nature  are  the  most 
frequent  within  all  great  seas ;  some,  however,  are  of 
madreporitic  limestone,  and  others  of  primary  forma- 
tion, of  which  latter  kind  New  Caledonia  offers  the  best 
example.  The  central  land  consists  either  of  one  island, 
or  of  several;  thus  in  the  Society  group,  Eimeo  stands  by 
itself;  while  Taha  and  Eaiatea  (fig.  3,  Plate  I.),  both 

J  The  height  of  Tahiti  is  given  from  Captain  Beechey ;  Maunia 
from  Mr.  F.  D.  Bennett  (Geograph.  Journ.  vol.  viii.  p.  220) ;  Aitutaki 
from  measurements  made  on  board  the  Beagle;  and  Manouai,  or 
Harvey  Island,  from  an  estimate  by  the  Rev.  J.  Williams.  The  two 
latter  islands,  however,  are  not  in  some  respects  well  characterised 
examples  of  the  encircled  class. 


Ch.  n.  BARRIER-REEFS.  63 

moderately  large  islands,  of  nearly  equal  size,  are  in- 
cluded in  one  reef.  Within  the  reef  of  the  Gambler 
group  there  are  four  large  and  some  smaller  islands 
(fig.  8,  Plate  I.) ;  within  that  of  Hogoleu  (fig.  2, 
Plate  I.)  nearly  a  dozen  small  islands  are  scattered  over 
the  expanse  of  one  vast  lagoon. 

After  the  details  now  given,  it  may  be  asserted  that 
there  is  not  one  point  of  essential  difference  between 
encircling  barrier-reefs  and  atolls ; — the  latter  enclose 
a  simple  sheet  of  water,  the  former  encircle  an  expanse 
with  one  or  more  islands  rising  from  it.  I  was  much 
struck  with  this  fact,  when  viewing,  from  the  heights 
of  Tahiti,  the  distant  island  of  Eimeo  standing  within 
smooth  water,  and  encircled  by  a  ring  of  snow-white 
breakers.  Eemove  the  central  land,  and  an  annular 
reef  like  that  of  an  atoll  in  an  early  stage  of  its  forma- 
tion is  left;  remove  Bolabola,  and  there  remains  a 
circle  of  linear  coral-islets  crowned  with  tall  cocoa- 
nut  trees,  like  one  of  the  many  atolls  scattered  over  the 
Pacific  and  Indian  oceans. 

The  barrier-reefs  of  Australia  and  of  New  Caledonia 
deserve  a  separate  notice  from  their  great  dimensions. 
The  reef  on  the  west  coast  of  New  Caledonia  (fig.  5, 
Plate  II.)  is  400  miles  in  length  ;  and  for  a  length  of 
many  leagues  seldom  approaches  within  eight  miles  of 
the  shore.  Near  the  southern  end  of  the  island,  the 
space  between  the  reef  and  the  land  is  sixteen  miles  in 
width.  The  Australian  barrier  extends,  with  a  few 
interruptions,  for  about  eleven  hundred  miles;  its 
average  distance  from  the  land  is  between  twenty  and 


64  BARRIER-REEFS.  Cn.  11. 

thirty  miles,  but  in  parts  from  fifty  to  ninety.  The 
great  arm  of  the  sea  thus  included,  is  from  ten  to 
twenty-five  fathoms  deep,  with  a  sandy  bottom ;  but 
towards  the  southern  end  where  the  reef  is  further 
from  the  shore,  the  depth  gradually  increases  to  forty, 
and  in  some  parts  to  more  than  sixty  fathoms.  Flinders 
has  described  the  surface  of  the  reef  as  consisting  of  a 
hard  white  agglomerate  of  different  kinds  of  coral, 
with  rough  projecting  points.  A  few  low  islets  have 
been  formed  on  it.  The  outer  edge  is  the  highest 
part ;  it  is  traversed  by  narrow  gullies^  and  at  intervals 
by  ship-channels.  The  sea  close  outside  is  in  most 
parts  profoundly  deep ;  but  to  the  north,  near  New 
Cruinea,  and  to  the  south,  the  depth  is  much  less,  and 
here  the  bottom  slopes  gradually  from  the  reef,  as  it 
generally  does  in  front  of  the  ship-channels.* 

There  is  one  important  point  in  the  structure  of 
barrier -reefs  which  must  here  be  considered.  The 
accompanying  diagrams  represent  north  and  south  ver- 
tical sections,  taken  through  the  highest  points  of  Yani- 
koro.  Gambler,  and  Maurua  islands,  as  well  as  through 
their  encircling  reefs.  The  scale  both  in  the  horizontal 
and  vertical  direction  is  the  same,  namely,  a  quarter  of 
an  inch  to  a  nautical  mile.  The  height  and  width  of 
these  islands  are  known ;  and  I  have  attempted  to  repre- 
sent the  form  of  the  land  from  the  shading  of  the  hills 

*  The  foregoing  details  are  taken  chiefly  from  Flinders'  Voyage 
to  Terra  Australis,  vol.  ii.  p.  88 ;  but  these  have  been  corrected  by 
the  account  given  by  Prof.  Jukes,  Narrative  of  the  Voyage  of  the  Fly, 
vol.  i.  1847,  chap.  xiii. 


Ch.  IL 


BARRIER   REEFS. 


65 


in  the  large  published  charts.  It  has  long  been, 
remarked,  even  from  the  time  of  Dampier,  that  a 
considerable  degree  of  relation  subsists  between  the 
inclination  of  that  part  of  the  land  which  is  beneath 
water  and  that  above  it :  hence  the  dotted  line  in  the 
three  sections  is  probably  a  moderately  accurate  repre- 
sentation of  the  actual  submarine  prolongation  of  the 
land.    If  we  now  look  at  the  outer  edge  of  the  reef 


No.  4. 


^fggm^l»""""y --.■ ,„^^ 

South.  >rorth» 

1 — Vanikoro,  from  the  Atlas  of  the  Voyage  of  the  Astrolabe,  by       ^ 
D.  D'Urville. 

2 — Gambier  Island,  from  Beechey. 

3— Maurua,  from  the  Atlas  of  the  Voyage  of  the  Coguille,  by 
Duperrey. 

The  horizontal  line  is  the  level  of  the  sea,  from  which  on  the 
right  hand  a  plummet  descends,  representing  a  depth  of  200  fathoms, 
or  1,200  feet.  The  vertical  shading  shows  the  section  of  the  land, 
and  the  horizontal  shading  that  of  the  encircling  barrier-reef ;  from 
the  smallness  of  the  scale,  the  lagoon-channel  could  not  be  repre- 
sented. 

A  A— Outer  edge  of  the  coral-reefs,  where  the  sea  breaks. 

B  B — The  shore  of  the  encircled  islands. 


66  BARRIER    REEFS.  Ch.  II. 

(A  A),  and  bear  in  mind  that  the  plummet  on  the  right 
hand  represents  a  depth  of  1,200  feet,  we  must  conclude 
that  the  vertical  thickness  of  these  barrier  coral-reefs  is 
very  great. 

1  must  observe,that  if  the  sections  had  been  taken  in 
any  other  direction  across  these  islands,  or  across  other 
encircled  islands,^  the  result  would  have  been  the  same. 
In  the  succeeding  chapter  it  will  be  shown  that  reef- 
building  polypifers  cannot  flourish  at  great  depths, — for 
instance,  it  is  highly  improbable  that  they  could  exist 
at  above  one-eighth  of  the  depth  represented  by  the 
plummet  on  the  right  hand  of  the  woodcut.  Here  then 
is  a  great  apparent  difficulty — how  were  the  basal  parts 
of  these  barrier-reefs  formed.  It  will  perhaps  occur  to 
some  that  the  actual  reefs  formed  of  coral  are  not  of 
great  thickness,  but  that  before  their  first  growth  the 
sea  had  deeply  eaten  into  the  coasts  of  these  encircled 
islands,  and  had  thus  left  a  broad  but  shallow  sub- 
marine ledge,  on  the  edges  of  which  the  corals  grew ; 
but  if  this  had  been  the  case,  the  shore  would  have 
been  invariably  bounded  by  lofty  cliffs,  and  not  have 
sloped  down  to  the  lagoon- channel,  as  it  does  in 
many  instances.  On  this  view,^  moreover,  the  cause 
of  the  reef  springing  up  at  such  a  great  distance  from 

*  An  East  and  West  section  across  the  Island  of  Bolabola  and  its 
barrier-reefs  is  given  in  the  fifth  chapter,  for  the  sake  of  illustrating 
another  point.  The  scale  is  -57  of  an  inch  to  a  mile ;  it  is  taken 
from  the  Atlas  of  the  Voyage  of  the  Coguille,  by  Duperrey.  The 
depth  of  the  lagoon-channel  is  exaggerated. 

2  The  Rev.  D.  Tyerman  and  Mr.  Bennett  (Journal  of  Voyage  and 
Travels,  vol.  i.  p.  215)  have  briefly  suggested  this  explanation  of  the 
origin  of  the  encircling  reefs  of  the  Society  Islands. 


Ch.  II.  BARRIER  REEFS.  67 

the  land,  leaving  a  deep  and  broad  moat  within, 
remains  altogether  unexplained.  A  supposition  of  the 
same  nature  and  appearing  at  first  more  probable,  is, 
that  the  reefs  have  risen  from  banks  of  sediment, 
\\'hich  had  accumulated  round  the  shore  previously  to 
the  growth  of  the  coral ;  but  the  extension  of  a  bank 
to  the  saiae  distance  round  an  unbroken  coast,  and 
in  front  of  deep  arms  of  the  sea  (as  in  Eaiatea,  see 
Plate  II.,  fig.  3),  which  penetrate  nearly  to  the  heart  of 
some  encircled  islands,  is  exceedingly  improbable.  And 
why,  again,  should  the  reef,  in  some  cases  steep  on  both 
sides  like  a  wall,  spring  up  at  a  distance  of  two,  three,  or 
more  miles  from  the  shore,  leaving  a  channel  often  be- 
tween 200  and  300  feet  deep — a  depth  which,  we  have 
good  reason  to  believe,  is  too  great  for  the  growth  of 
coral  ?  The  existence,  also,  of  this  same  channel  pre- 
cludes the  idea  of  the  reef  having  grown  outwards,  on  a 
foundation  slowly  formed  by  the  accumulation  of  its  own 
detritus  and  sediment.  Nor,  again,  can  it  be  asserted 
that  the  reef-building  corals  will  not  grow,  excepting  at 
a  great  distance  from  the  land ;  for,  as  we  shall  soon  see, 
there  is  a  whole  class  of  reefs  which  take  their  name 
from  growing  (especially  where  the  sea  is  deep)  closely 
attached  to  the  shore.  At  New  Caledonia  (see  Plate 
II.,  fig,  5),  the  reefs  which  run  in  front  of  the  west  coast 
are  prolonged  in  the  same  line  for  150  miles  beyond  the 
northern  extremity  of  the  island,  and  this  shows  that 
some  explanation,  quite  different  from  any  one  of  those 
just  suggested  is  requisite.  If  the  island  had  been 
originally  prolonged  to  this  distance,  and  if  the  northern 


68  BARRIER   REEFS.  Ch.  IL 

end  had  been  worn  away  until  it  was  a  little  beneath 
the  level  of  the  sea,  why  should  the  coral-reefs  have 
become  attached,  not  on  the  central  crest,  but  in  the 
same  line  with  the  reefs  which  still  front  the  existinj^ 
shores  ?  We  shall  hereafter  see,  that  there  is  one,  and 
I  believe  only  one  solution  of  this  difficulty. 

One  other  supposition  to  account  for  the  position 
of  encircling  reefs  remains,  but  it  is  almost  too  pre- 
posterous to  be  mentioned ; — namely,  that  they  rest 
on  enormous  submarine  craters  surrounding  the  in- 
cluded islands.  When  the  size,  height,  and  form  of 
the  islands  in  the  Society  group  are  considered, 
together  with  the  fact  that  all  are  thus  encircled, 
such  a  notion  will  be  rejected  by  everyone.  New 
Caledonia,  moreover,  besides  its  size,  is  composed  of 
primitive  formations,  as  are  some  of  the  Comoro 
Islands ;  ^  and  Aitutaki  consists  of  calcareous  rock. 
We  must,  therefore,  reject  the  several  explanations, 
and  conclude  that  the  vertical  thickness  of  barrier- 
reefs,  from  their  outer  edges  to  the  foundation  on 
which  they  rest  (from  A  A  in  the  sections  No.  4  to 
the  dotted  lines),  is  really  great:  but  this  presents 
no  real  difficulty,  as  I  hope  to  show  hereafter  when 
the  upward  growth  of  coral-reefs,  during  the  slow 
subsidence  of  their  foundation,  is  discussed. 

*  I  have  been  informed  that  this  is  the  case  by  Dr.  Allan  oi 
Forres,  who  has  visited  this  group. 


CHAPTER  ni. 

FRINGING   OR   SHORE   REEFS. 

Beefs  of  Mauritius— Shallow  channel  within  the  reef— Its  slow 
■filling  up  -Currents  of  water  formed  within  it — Upraised  reefs 
— Narrow  fringing -reefs  in  deep  seas— Reefs  on  the  coast  of  E. 
Africa  a7id  of  Brazil — Fringing -reefs  in  very  shallow  seas,  round 
banks  of  sediment,  and  on  worn-down  islands — Fringing -reefs 
affected  by  currents  of  the  sea — Coral  coating  bottom  of  the  sea, 
but  not  forming  reefs, 

Fringing-reefs,  or,  as  they  have  been  called  by  some 
voyagers,  shore-reefs,  whether  skirting  an  island  or 
part  of  a  continent,  at  first  appear  to  differ  little  from 
barrier-reefs,  except  that  they  are  generally  of  less 
breadth.  As  far  as  the  superficies  of  the  actual  reef 
is  concerned,  this  is  the  case ;  but  the  absence  of  an 
interior  deep-water  channel,  and  the  close  relation  in 
their  horizontal  extension  with  the  probable  slope  of 
the  adjoining  land  beneath  the  sea,  present  essential 
points  of  difference. 

The  reefs  which  fringe  the  island  of  Mauritius  offer 
a  good  example  of  this  class.  They  extend  round  its 
whole  circumference,  with  the  exception  of  two  or 
three  parts '  where  the  coast  is  almost  precipitous,  and 

*  This  fact  is  stated  on  the  authority  of  the  Ofi&cier  du  Eoi,  in 
his  extremely  interesting  *  Voyage  k  I'lsle  de  France,'  undertaken  in 
1768.  According  to  Captain  Carmichael  (Hooker's  Bot.  Misc.,  vol.  ii. 
p.  316),  on  one  part  of  the  coast  there  is  a  space  of  sixteen  miles 
without  a  reef. 


70  FRINGING-REEFS.  Ch.  IH. 

where,  if  as  is  probable  the  bottom  of  the  sea  has  a 
similar  inclination,  the  coral  would  have  no  foundation 
on  which  to  become  attached.  A  similar  fact  may 
sometimes  be  observed  even  in  reefs  of  the  barrier 
class,  which  follow  much  less  closely  the  outline  of 
the  adjoining  land ;  as,  for  instance,  on  the  S.E.  and 
precipitous  side  of  Tahiti,  where  the  encircling  reef  is 
interrupted.  On  the  western  side  of  the  Mauritius, 
which  was  the  only. part  I  visited,  the  reef  generally 
lies  at  the  distance  of  about  half  a  mile  from  the 
shore ;  but  in  some  parts  it  is  distant  from  one  to  two, 
and  even  three  miles.  Even  in  this  last  case,  as  the 
coast-land  is  gently  inclined  from  the  foot  of  the 
mountains  to  the  sea-beach,  and  as  the  soundings 
outside  the  reef  indicate  an  equally  gentle  slope 
beneath  the  water,  there  is  no  reason  for  supposing 
that  the  basis  of  the  reef,  formed  by  the  prolongation 
of  the  strata  of  the  island,  lies  at  a  greater  depth  than 
that  at  which  the  polypifers  could  begin  constructing 
the  reef.  Some  allowance,  however,  must  be  made  for 
the  outward  extension  of  a  foundation  formed  of  sand 
and  detritus,  from  the  wear  of  the  corals ;  and  this 
would  give  to  the  reef  a  somewhat  greater  vertical 
thickness  than  would  otherwise  be  possible. 

The  outer  edge  of  the  reef  on  the  western  or 
leeward  side  of  the  island,  is  tolerably  well  defined, 
and  is  a  little  higher  than  any  other  part.  It  chiefly 
consists  of  large  strongly  branched  corals  of  the  genus 
Madrepora,  which  also  form  a  sloping  bed  some  way 
out  to  sea:  the  kinds  of  coral  growing  in  this  part 


Ch.  in.  FRINGING   REEFS.  71 

will  be  described  in  the  ensuing  chapter.  Between 
the  outer  margin  and  the  beach,  there  is  a  flat  space 
with  a  sandy  bottom  and  a  few  tufts  of  living  coral ;  in 
some  parts  it  is  so  shallow,  that  people,  by  avoiding 
the  deeper  holes  and  gullies,  can  wade  across  it  at  low 
water ;  in  other  parts  it  is  deeper,  seldom,  however, 
exceeding  ten  or  twelve  feet,  so  that  it  offers  a  safe 
coasting  channel  for  boats.  On  the  eastern  and 
windward  side  of  the  island  which  is  exposed  to  a 
heavy  surf,  the  reef  was  described  to  me  as  having 
a  hard  smooth  surface,  very  slightly  inclined  inwards, 
just  covered  at  low- water,  and  traversed  by  gullies ; 
it  appears  to  be  quite  similar  in  structure  to  the  reefs 
of  the  barrier  and  atoll  classes. 

The  reef  of  Mauritius,  in  front  of  every  river  and 
streamlet,  is  breached  by  a  straight  passage :  at  Grand 
Port,  however,  there  is  a  channel  like  that  within  a 
barrier-reef :  it  extends  parallel  to  the  shore  for  four 
miles,  and  has  an  average  depth  of  ten  or  twelve 
fathoms ;  its  presence  may  probably  be  accounted  for 
by  two  rivers  which  enter  at  each  end  of  the  channel, 
and  bend  towards  each  other.  The  fact  of  reefs  of 
the  fringing  class  being  always  breached  in  front  of 
streams,  even  of  those  which  are  dry  during  the 
greater  part  of  the  year,  will  be  explained,  when  the 
conditions  unfavourable  to  the  growth  of  coral  are 
considered.  Low  coral-islets,  like  those  on  barrier- 
reefs  and  atolls,  are  seldom  formed  on  reefs  of  this 
class,  apparently  owing  in  some  cases  to  their  narrow- 
ness, and  in  others  to  the  gentle  slope  of  the  reef 


72  '  FRINGING   REEFS.  Ch.  III. 

outside  not  yielding  many  fragments  to  the  breakers. 
On  the  windward  side,  however,  of  the  Mauritius,  two 
or  three  small  islets  have  been  formed. 

It  appears,  as  will  be  shown  in  the  ensuing  chapter, 
that  the  action  of  the  surf  is  favourable  to  the  vigorous 
growth  of  the  stronger  corals,  and  that  sand  or  sedi- 
ment, if  agitated  by  the  waves,  is  injurious  to  them. 
Hence  it  is  probable  that  a  reef  on  a  shelving  shore, 
like  that  of  Mauritius,  would  at  first  grow  up,  not 
attached  to  the  actual  beach,  but  at  some  little  distance 
from  it ;  and  the  corals  on  the  outer  margin  would  be 
the  most  vigorous.  A  shallow  channel  would  thus  be 
formed  within  the  reef ;  and  this  channel  could  be  filled 
up  only  very  slowly  with  sediment,  for  the  breakers 
cannot  act  on  the  shores  of  the  island,  and  they  do 
not  often  tear  up  and  cast  inside  fragments  from  the 
outer  edge  of  the  reef,  whilst  every  streamlet  carries 
away  its  mud  in  a  straight  line  through  breaches  in 
the  reef.  But  a  beach  of  sand  and  of  fragments  of 
the  smaller  kinds  of  coral  seems,  in  the  case  of  Mauri- 
tius, to  be  slowly  encroaching  on  the  shallow  channel. 
On  many  shelving  and  sandy  coasts,  the  breakers  tend 
to  form  a  bar  of  sand  a  little  way  from  the  beach,  with 
a  slight  increase  of  depth  within  it — for  instance.  Cap- 
tain Grey^  states  that  the  west  coast  of  Australia,  in  lat. 
24°,  is  fronted  by  a  sand  bar  about  200  yards  in  width, 
on  which  there  is  only  two  feet  of  water ;  but  within 
it  the  depth  increases  to  two  fathoms.     Similar  bars, 

'  Captain  Grey's  Journal  of  Two  Expeditions,  vol.  i.  p.  369. 


Ch.  in.  FRINGING  REEFS.  73 

more  or  less  perfect,  occur  on  other  coasts.  In  these 
cases  I  suspect  that  the  shallow  channel,  (which  no 
iloubt  during  storms  is  occasionally  obliterated,)  is 
scooped  out  by  the  flowing  away  of  the  water  thrown 
beyond  the  line  on  which  the  waves  break  with  the 
greatest  force.  At  Pernambuco  the  bar  of  hard  sand- 
stone, before  alluded  to,  has  the  same  external  form 
and  height  as  a  coral  reef,  and  extends  nearly  parallel  to 
the  coast ;  within  this  bar  currents,  apparently  caused 
by  the  water  thrown  over  it  during  the  greater  part  of 
each  tide,  run  strongly,  and  are  wearing  away  its  inner 
wall.  From  these  facts  it  can  hardly  be  doubted  that 
within  most  fringing  reefs,  especially  within  those 
lying  some  distance  from  the  land,  a  return  stream 
must  carry  away  the  water  thrown  over  the  outer  edge ; 
and  the  current  thus  produced  would  tend  to  prevent 
the  channel  being  filled  up  with  sediment,  and  might 
even  deepen  it  under  certain  circumstances.  To 
this  latter  belief  I  am  led,  by  finding  that  channels  are 
almost  universally  present  within  the  fringing  reefs  of 
those  islands  which  have  undergone  recent  elevatory 
movements ;  and  this  could  hardly  have  been  the  case 
if  the  conversion  of  the  very  shallow  channel  into  land 
had  not  been  counteracted  to  a  certain  extent. 

A  fringing-reef,  if  elevated  in  a  perfect  condition 
above  the  level  of  the  sea,  would  present  the  singular 
appearance  of  a  broad  dry  moat  bounded  by  a  low  wall 
or  mound.     The  author  *  of  an  interesting  pedestrian 

'  Voyage  k  I'lsle  de  France,  par  un  Officier  du  Roi,  Part  L 
pp.  192,  200. 


74  FRINGING   REEFS.  Ch.  III. 

tour  round  the  Mauritius  seems  to  have  met  with  a 
structure  of  this  kind  :  he  says,  *  J'observai  que  la,  ou 
la  mer  etale  independamment  des  rescifs  du  large, 
il  y  a  a  terre  une  espece  d'effoncement,  ou  chemin 
convert  naturel.  On  y  pourrait  mettre  du  canon,' 
&c.  In  another  place  he  adds,  *  Avant  de  passer  le 
Cap,  on  remarque  un  gros  banc  de  corail  eleve 
de  plus  de  quinze  pieds :  c'est  une  espece  de  res- 
cif,  que  la  mer  a  abandonne :  il  regne  au  pied 
une  longue  flaque  d'eau,  dont  on  pourrait  faire  un 
bassin  pour  de  petits  vaisseaux.'  But  the  margin  of 
the  reef,  although  the  highest  and  most  perfect  part, 
from  being  most  exposed  to  the  surf,  would  generally 
during  a  slow  rise  of  the  land  be  either  partially  or 
entirely  worn  down  to  that  level  at  which  corals  could 
renew  their  growth  on  its  upper  edge.  On  some  parts 
of  the  coast-land  of  Mauritius  there  are  little  hillocks 
of  coral-rock,  which  are  either  the  last  remnants  of  a 
continuous  reef,  or  of  low  islands  formed  on  it.  I 
observed  two  such  hillocks  between  Tamarin  Bay  and 
the  Great  Black  Kiver ;  they  were  nearly  20  feet 
high,  about  200  yards  from  the  present  beach,  and 
about  30  feet  above  its  level.  They  rose  abruj)tly 
from  a  smooth  surface,  strewn  with  worn  fragments 
of  coral.  They  consisted  in  their  lower  part  of  hard 
calcareous  sandstone,  and  in  their  upper  of  great 
blocks  of  several  species  of  Astraea  and  Madrepora, 
loosely  aggregated ;  they  were  divided  into  irregular 
beds,  dipping  seaward,  in  one  hillock  at  an  angle  of  8°, 
and  in  the  other  at  18**.     The  upraised  reefs  round 


Ch.  m.  FRINGING   REEFS.  75 

this  island  have  been  much  less  worn  and  modified  by 
the  action  of  the  sea  than  in  most  other  cases. 

Many  islands  ^  are  fringed  by  reefs  quite  similar  to 
those  of  Mauritius:  but  on  coasts  where  the  sea 
deepens  very  suddenly,  the  reefs  are  much  narrower, 
and  their  limited  extension  seems  evidently  to  depend 
on  the  high  inclination  of  ^.he  submarine  slope; — a 
relation  which,  as  we  have  seen,  does  not  exist  in  reefs 
of  the  barrier  class.  The  fringing-reefs  on  steep  coasts 
are  frequently  not  more  than  from  50  to  100  yards  in 
width:  they  have  a  nearly  smooth,  hard  surface, 
scarcely  uncovered  at  low-water,  and  without  any 
interior  shoal  channel  like  that  within  those  fringing- 
reefs  which  lie  at  a  greater  distance  from  the  land. 
The  fragments  torn  up  during  gales  from  the  outer 
margin  are  thrown  over  the  reef  on  the  shores  of  the 
island.  I  may  give  as  instances,  Wateeo,  where  the 
reef  is  described  by  Cook  as  being  100  yards  wide ; 
and  Mauti  and  Elizabeth  ^  Islands,  where  it  is  only 
50  yards  in  width:  the  sea  round  these  islands  is 
very  deep. 

Fringing-reefs,  like  barrier-reefs,  surround  islands 

*  I  may  give  Cuba,  as  another  instance ;  Mr.  Taylor  (Loudon's 
Mag.  of  Nat.  Hist.,  vol.  ix.  p.  449)  has  described  a  reef  several  miles 
in  length  between  Gibara  and  Vjaro,  which  extends  parallel  to  the 
shore  at  the  distance  of  between  half  and  the  third  part  of  a  mile, 
and  encloses  a  space  of  shallow  water,  with  a  sandy  bottom  and 
tufts  of  coral.  Outside  the  edge  of  the  reef,  which  is  formed  of 
great  branching  corals,  the  depth  is  six  and  seven  fathoms.  Thia 
coast  has  been  upheaved  at  no  very  distant  geological  period. 

2  Mauti  is  described  by  Lord  Byron  in  the  Voyage  of  H.M.S. 
Blonde,  and  Elizabeth  Island  by  Captain  Beechey. 


76  FRINGING    REEFS.  Ch.  IIL 

and  front  the  shores  of  continents.  In  the  charts 
of  the  eastern  coast  of  Africa,  by  Captain  Owen, 
many  extensive  fringing-reefs  are  laid  down ; — thus, 
for  a  space  of  nearly  40  miles,  from  lat.  1°  5'  to 
1°  45'  S.,  a  reef  fringes  the  shore  at  an  average 
distance  of  rather  more  than  one  mile,  and  therefore 
at  a  greater  distance  than  is  usual  in  reefs  of  this 
class ;  but  as  the  coast-land  is  not  high,  and  as  the 
bottom  shoals  very  gradually,  (the  depth  being  only 
from  8  to  14  fathoms  at  a  mile  and  a-half  outside  the 
reef),  its  extension  thus  far  from  the  land  offers  no 
difficulty.  The  external  margin  of  this  reef  is  de- 
scribed as  formed  of  projecting  points;  and  within 
it  there  is  a  channel  from  six  to  twelve  feet  deep, 
with  patches  of  living  coral.  At  Mukdeesha  (lat. 
2°  1'  N.)  *  the  port  is  formed,'  it  is  said,'  *  by  a  long 
reef  extending  eastward  four  or  five  miles,  within 
which  there  is  a  narrow  channel,  with  ten  to  twelve 
feet  of  water  at  low  spring  tides : '  it  lies  at  the  distance 
of  a  quarter  of  a  mile  from  the  shore.  Again,  in  the 
plan  of  Mombas  (lat.  4°  S.)  a  reef  extends  for  thirty- 
six  miles,  at  the  distance  of  from  half  a  mile  to  one 
mile  and  a-quarter  from  the  shore;  within  it,  there 
is  a  channel  navigable  *  for  canoes  and  small  craft,' 
between  six  and  fifteen  feet  deep  :  outside  the  reef  the 
depth  is  about  30  fathoms  at  the  distance  of  nearly 
half  a  mile.  Part  of  this  reef  is  very  symmetrical,  and 
has  a  uniform  breadth  of  200  yards. 

•  Owen's  Africa,  vol.  i.  p.  367 ;  from  which  work  the  foregoing 
facts  are  likewise  taken. 


Ch.  in.  FRINGING   REEFS.  77 

The  coast  of  Brazil  is  in  many  parts  fringed  by  reefs. 
Of  these,  some  are  not  of  coral  formation  -x.for  instance, 
those  near  Bahia  and  in  front  of  Pernambuco ;  but  a 
few  miles  south  of  this  latter  city,  the  reef  follows  ^  every 
turn  of  the  shore  so  closely,  that  I  can  hardly  doubt  it 
is  of  coral.  It  runs  at  the  distance  of  three-quarters 
of  a  mile  from  the  land,  and  within  it  the  depth  ia 
from  ten  to  fifteen  feet.  I  was  assured  by  an  intelli- 
gent pilot,  that  at  Ports  Frances  and  Maceio,  the  outer 
part  of  the  reef  consists  of  living  coral,  and  the  inner 
of  a  white  stone  full  of  large  irregular  cavities  com- 
municating with  the  sea.^  The  bottom  of  the  sea  off 
the  coast  of  Brazil  shoals  gradually  to  between  thirty 
and  forty  fathoms,  at  the  distance  of  between  nine  and 
ten  leagues  from  the  land. 

From  the  description  now  given,  we  may  conclude 
that  the  dimensions  and  structure  of  fringing-reefs 
depend  entirely  on  the  greater  or  less  inclination  of  the 
submarine  slope,  conjoined  with  the  fact,  that  reef- 
building  polypifers  can  exist  only  at  limited  depths. 
It  follows  from  this,  that  where  the  sea  is  very  shallow, 
as  in  the  Persian  Gulf  and  in  parts  of  the  East  Indian 
Archipelago,  the  reefs  lose  their  fringing  character, 

'  Baron  Roussin's  Pilote  du  Brdsil,  and  the  accompanying  hydro- 
graphical  memoir.  See  also  the  supplement  to  this  volume  on  a  Bar 
of  Sandstone  off  Pernambuco. 

2  [Rathbun  (Amer.  Nat.,  xiii.  539-551)  describes  a  reef  on  the 
Brazilian  coast.  The  lower  part  of  the  reef  consists  of  true  corals, 
the  upper  of  nullipores  and  annelid  tubes.  The  reef  has  a  loose 
structure  near  the  surface,  compact  below.  The  coral  fragments  cover- 
ing the  channel  within  the  reef  '  form  beds  of  considerable  thickness 
in  places,  often  more  or  less  consolidated.'] 
7 


V^  FRINGING  REEFS.  Oh.  HI. 

and  appear  as  separate  and  irregularly  scattered 
patches  otikjxx  of  considerable  area.  As  the  conditions 
are  less  favourable  in  several  respects  on  the  inner 
side  of  these  patches,  the  growth  of  the  coral  is  more 
vigorous  on  the  outside ;  thus  causing  the  reefs  to  be 
generally  higher  and  more  perfect  in  their  marginal 
than  in  their  central  parts.  Hence  these  reefs  some- 
times assume  (and  this  circumstance  ought  not  to  be 
overlooked)  the  appearance  of  atolls ;  but  as  they  are 
based  on  a  shallow  foundation,  and  as  their  central 
expanse  is  much  less  deep  and  their  form  less  defined, 
this  resemblance  is  easily  seen  to  be  merely  superficial. 
On  the  other  hand,  when,  in  a  deep  sea,  banks  of  sedi- 
ment have  accumulated  round  islands  or  submerged 
rocks,  and  they  become  fringed  with  reefs,  they  are  dis- 
tinguished with  difficulty  from  encircling  barrier-reefs 
or  atolls.  In  the  West  Indies  there  are  reefs,  which  I 
should  probably  have  arranged  under  these  two  classes, 
if  the  existence  of  large  and  level  banks,  lying  a  little 
beneath  the  surface  and  ready  to  serve  as  the  basis 
for  the  attachment  of  coral,  had  not  been  present ;  the 
formation  of  such  banks  through  the  accumulation  of 
sediment  being  sufficiently  evident.  Fringing-reefs 
sometimes  coat,  and  thus  protect  the  foundations  of 
islands,  which  have  been  worn  down  by  the  surf  to  the 
level  of  the  sea.  According  to  Ehrenberg,  this  has  been 
extensively  the  case  with  the  islands  in  the  Eed  Sea, 
which  formerly  ranged  parallel  to  the  shores  of  the 
mainland,  with  deep  water  within  them:  hence  the 
reefs  now  coating  their  bases,  are  situated  relatively 


Ch.  in.  FRINGING   REEFS.  79 

to  the  land  like  barrier-reefs,  although  not  belonging 
to  that  class ; — but  there  are,  as  I  believe,  in  the  Eed 
Sea  some  true  barrier-reefs.  The  reefs  of  this  sea  and 
of  the  West  Indies  will  be  described  in  the  Appendix, 
In  some  cases,  fringing-reefs  appear  to  be  considerably 
modified  in  outline  by  the  course  of  the  prevailing 
currents ;  Dr.  J.  Allan  informs  me  that  on  the  east 
coast  of  Madagascar,  almost  every  headland  and  low 
point  of  sand  has  a  coral-reef  extending  from  it  in 
a  S.W.  and  N.E.  line,  parallel  to  the  currents  on  that 
shore.  I  should  think  the  influence  of  the  currents 
chiefly  consisted  in  causing  an  extension,  in  a  certain 
direction,  of  a  proper  foundation  for  the  attachment 
of  the  coral.  Eound  many  intertropical  islands,  for 
instance  the  Abrolhos  on  the  coast  of  Brazil  sur- 
veyed by  Captain  FitzEoy,  and,  as  I  am  informed  by 
Mr.  Cuming,  round  the  Philippines,  the  bottom  of  the 
sea  is  entirely  coated  by  irregular  masses  of  coral, 
which  although  often  of  large  size,  do  not  reach  the 
surface  and  form  proper  reefs.  This  must  be  owing 
either  to  insufficient  growth,  or  to  the  absence  of  those 
kinds  of  corals  which  can  withstand  the  breaking  of 
the  waves. 

The  three  classes,  atoll-formed,  barrier,  and  fringing 
reefs,  together  with  the  modifications  just  described  of 
the  latter,  include  all  the  most  remarkable  coral-forma- 
tions anywhere  existing.  At  the  commencement  of 
the  last  chapter  in  the  volume,  where  I  detail  the 
principles  on  which  the  map  (Plate  III.)  is  coloured, 
the  exceptional  cases  will  be  enumerated. 


CHAPTEE  IV. 

ON  THE   GROWTH   OF   CORAL-REEFS. 

In  this  chapter  I  will  give  all  the  facts,  which  I  havo 
collected,  relating  to  the  distribution  of  coral-reefs, — 
to  the  conditions  favourable  to  their  increase, — to  the 
rate  of  their  growth, — ^and  to  the  depth  at  which  they 
are  formed. 

These  subjects  have  an  important  bearing  on  the 
theory  of  the  origin  of  the  different  classes  of  coral- 
reefs. 


Section  !• 

On  the  Distribution  of  Coral-Eeefs,  and  on  the  Conditions 
favourable  to  their  Increase. 

With  regard  to  the  limits  of  latitude  over  which  coral- 
reefs  extend,  I  have  nothing  new  to  add.  The  Ber- 
muda Islands  in  32°  15'  N.,  is  the  point  furthest  re- 
moved from  the  equator  in  which  they  appear  to  exist; 
and  their  extension  here  so  far  northward  no  doubt  is 
due  to  the  warmth  of  the  Gulf  Stream.  In  the  Pacific, 
the  Loo  Choo  islands,  in  lat.  27°  N.,  have  reefs  on  their 
shores,  and  there  is  an. atoll  in  28°  30',  situated  N.W. 


Sect.  I.      DISTRIBUTION   OF   CORAL-REEFS.  81 

of  the  Sandwich  Archipelago.  In  the  Ked  Sea  there 
are  coral-reefs  in  lat.  30°.  In  the  Southern  Hemisphere 
coral-reefs  do  not  extend  so  far  from  the  equatorial  sea. 
In  the  Southern  Pacific  there  are  only  a  few  reefs 
beyond  the  line  of  the  tropic,  but  Houtmans  Abrolhos, 
on  the  western  shores  of  Australia,  in  lat.  29°  S.,  are  of 
coral-formation. 

The  proximity  of  volcanic  land,  owing  to  tho  lime 
generally  evolved  from  it,  has  been  thought  to  be 
favourable  to  the  increase  of  coral-reefs.  There  is, 
however,  no  foundation  for  this  view;  for  nowhere 
are  coral-reefs  more  extensive  than  on  the  shores  of 
New  Caledonia  and  of  north-eastern  Australia,  which 
consist  of  primary  formations ;  and  the  Maldiva, 
Chagos,  Marshall,  Gilbert,  and  Low  Archipelagoes, 
the  largest  groups  of  atolls  in  the  world,  are  formed 
exclusively  of  coral. 

The  entire  absence  of  coral-reefs  in  certain  large 
areas  within  the  tropical  seas,  is  a  remarkable  fact. 
Thus  no  coral-reefs  were  observed  during  the  survey- 
ing voyages  of  the  Beagle  on  the  west  coast  of  South 
America  south  of  the  equator,  or  round  the  Galapagos 
Islands.  It  appears,  also,  that  there  are  none '  on  this 
coast  north  of  the  equator ;  Mr.  Lloyd,  who  surveyed 
the  isthmus  of  Panama,  remarked  to  me,  that  although 
he  had  seen  corals  living  in  the  Bay  of  Panama,  yet  he 
had  never  observed  any  reefs  formed  by  them.  I  at  first 
attributed  this  absence  of  reefs  on  the  coasts  of  Peru  and 

*  I  have  been  informed  that  this  is  the  case,  by  Lieut.  Ryder,  R.N., 
and  others  who  have  had  ample  opportunities  for  observation. 


82  DISTRIBUTION   OF   CORAL-REEFS.       Cn.  IV. 

of  the  Galapagos  Islands,*  to  the  coldness  of  the  cur- 
rents from  the  south,  but  the  Gulf  of  Panama  is  one 
of  the  hottest  pelagic  districts  in  the  "world.^  In  the 
central  parts  of  the  Pacific  there  are  islands  entirely 
free  from  reefs ;  and  in  some  of  these  cases  this  appears 
to  be  due  to  recent  volcanic  action :  but  the  existence 
of  reefs,  though  scantily  developed,  and  according  to 
Dana,  confined  to  one  part  of  Hawaii  (one  of  the  Sand- 
wich Islands),  shows  that  recent  volcanic  action  does 
not  absolutely  prevent  their  growth. ^ 

*  The  mean  temperature  of  the  surface  sea,  from  observations 
made  by  the  direction  of  Captain  FitzRoy  on  the  shores  of  the 
Galapagos  Islands,  between  the  16th  of  September  and  the  20th  of 
October,  1835,  was  68°  Fahr.  The  lowest  temperature  observed  was 
58°-5  at  the  S.W.  end  of  Albemarle  Island ;  and  on  the  west  coast 
of  this  island,  it  was  several  times  62°  and  63°.  The  mean  tem- 
perature of  the  sea  in  the  Low  Archipelago  of  atolls,  and  near  Tahiti, 
from  similar  observations  made  on  board  the  Beagle,  was  (although 
further  from  the  equator)  77°'5,  the  lowest  any  day  being  76°*5. 
Therefore  we  have  here  a  difference  of  9°*5  in  mean  temperature, 
and  18°  in  extremes ;  a  difference  doubtless  quite  sufficient  to  affect 
the  distribution  of  organic  beings  in  the  two  areas. 

*  Humboldt's  Personal  Narrative,  vol.  vii.  p.  434. 

>  [Mr.  S.  J.  Whitmee  (Nature,  August  12,  1875,  p.  291)  states  that 
in  Savaii  (Samoan  group),  one  of  four  examples  of  islands  which 
Professor  Dana  brings  forward  as  instances  indicating  that  recent 
volcanic  action  has  prevented  the  formation  of  extensive  coral-reefs, 
the  cause  is  more  probably  the  depth  of  water  on  the  coast.  More- 
over, parts  of  Savaii  differ  in  change  of  level  from  the  rest  of  the 
island,  and  it  is  in  these  (the  upheaved  regions)  that  coral-reefs  are 
almost  wanting.  He  also  says  that  after  examining  *  a  good  many 
intertropical  islands  of  the  Pacific  belonging  to  the  three  orders— 
(1)  Volcanic  islands  with  fringing  coral-reefs,  such  as  Samoa,  the  New 
Hebrides,  &c. ;  (2)  Atolls,  such  as  the  Low  Archipelago,  Ellice,  Gilbert 
Islands,  &c. ;  (3)  Upraised  coral-islands,  such  as  Niu6  or  Savage 
Island,  part  of  the  Friendly,  the  Loyalty  Islands,  &c,*— he  has  been 
the  more  firmly  convinced,  the  further  he  has  gone,  of  the  correctness 
of  Mr.  Darwin's  theory.    Mr.  R.  Webb  (Nature,  id.  p.  475)  disputes 


Sect.  I.      DISTRIBUTION   OF   CORAL-REEFS.  83 

In  the  last  chapter  I  stated  that  the  bottom  of  the 
sea  round  some  islands  is  thickly  coated  with  living 
corals,  \»hich  nevertheless  do  not  form  reefs,  either 
from  insufficient  growth,  or  from  the  species  not  being 
adapted  to  contend  with  the  breaking  waves. 

I  have  been  assured  by  several  navigators  that 
there  are  no  coral-reefs  on  the  west  coast  of  Africa,^  or 
round  the  islands  in  the  Gulf  of  Guinea.  This  perhaps 
may  be  attributed  to  the  sediment  brought  down  by 
the  many  rivers  debouching  on  that  coast,  and  to  the 
extensive  mud-banks  which  line  great  part  of  it.  But 
the  islands  of  St.  Helena,  Ascension,  the  Cape  Verdes, 
St.  Paul's,  and  Fernando  Noronha,  are,  also,  entirely 
destitute  of  reefs,  although  they  lie  far  out  at  sea,  are 
composed  of  the  same  ancient  volcanic  rocks,  and  have 
the  same  general  form  with  those  islands  in  the  Pacific, 
the  shores  of  which  are  surrounded  by  gigantic  walls  of 
coral-rock./^ With  the  exception  of  Bermuda,  there  is 
not  a  single  coral-reef  in  the  central  expanse  of  the 
Atlantic  ocean.  It  will,  perhaps,  be  suggested  that 
the  quantity  of  carbonate  of  lime  in  different  parts  of 
the  sea  may  regulate  the  presence  of  reefs.  But  this 
cannot  be  the  case,  for  at  Ascension,  the  waves,  charged 
to  excess,  precipitate  a  thick  layer  of  calcareous  matter 

Mr.  Whitmee's  statement  as  to  the  upheaval  of  the  above-mentioned 
part  of  Savaii.     (From  materials  collected  by  Mr.  Darwin.)] 

'  It  might  be  concluded,  from  a  paper  by  Captain  Owen  (Geo- 
graph.  Journ.,  vol.  ii.  p.  89),  that  the  reefs  off  Cape  St.  Anne  and  the 
Sherboro'  Islands  were  of  coral,  although  the  author  states  that  they 
are  not  purely  coralline.  But  I  have  been  assured  by  Lieut.  Hol- 
land, R.N.,  that  these  reefs  are  not  of  coral,  or  at  least  that  they  do 
not  at  all  resemble  those  in  the  West  Indies. 


84  CONDITIONS   FAVOURABLE   TO  Ch.  IV. 

on  the  tidal  rocks ;  and  at  St.  Jago  in  the  Cape  Yerdes, 
carbonate  of  lime  not  only  is  abundant  on  the  shores, 
but  it  forms  the  chief  part  of  some  upraised  post- 
tertiary  strata.  The  apparently  capricious  distribution, 
therefore,  of  coral-reefs,  cannot  be  fully  explained  by 
any  of  the  above  obvious  causes  ;  but,  as  the  study  of 
the  terrestrial  and  better-known  half  of  the  world, 
must  convince  everyone  that  no  station  capable  of 
supporting  life  is  lost, — nay  more,  that  there  is  a 
struggle  for  each  station  between  different  organisms, 
— pe  may  conclude  that  in  those  parts  of  the  inter- 
tropical sea  in  which  there  are  no  coral-reefs,  there 
are  other  organic  beings,  supplying  the  place  of  the 
reef-building  polypifers.  It  has  been  shown  in  the 
chapter  on  Keeling  atoll  that  there  are  some  species  of 
large  fish,  and  the  whole  tribe  of  Holothurise,^  which 
prey  on  the  tenderer  parts  of  the  corals.  On  the  other 
hand,  the  polypifers  in  their  turn  must  prey  on  other 
organic  beings  ;  and  they  would  suffer  by  the  diminu- 
tion of  their  prey  through  any  cause.  The  relations, 
therefore,  which  determine  the  formation  of  reefs  on 
any  shore,  by  the  vigorous  growth  of  the  efficient  kinds 
of  coral,  must  be  very  complex,  and  with  our  imperfect 
knowledge  inexplicable.  From  these  considerations, 
we  may  infer  that  changes  in  the  condition  of  the  sea, 
not  obvious  to  our  senses,  might  destroy  all  the  coral- 
reefs  in  one  area,  and  cause  them  to  appear  in  another  : 
thus,  the  Pacific  or  Indian  ocean  might  become  aa 
barren  of-  coral-reefs  as  now  is  the  Atlantic,  without 

*  [See  Appendix  ii.] 


Sect.  I.      THE   GROWTH   OF   CORAL-REEFS.  85 

our  being  able  to  assign  any  adequate  cause  for  such  a 
change.^ 

It  has  been  a  question  with  some  naturalists,  which 
part  of  a  reef  is  most  favourable  to  the  growth  of  coral. 
The  great  mounds  of  living  Porites  and  of  Millepora 
round  Keeling  atoll  occur  exclusively  on  the  extreme 
verge  of  the  reef,  which  is  washed  by  a  constant  suc- 
cession of  breakers;  and  living  coral  nowhere  else 
forms  solid  masses.  At  the  Marshall  islands  the  larger 
kinds  of  corals  (chiefly  a  species  of  Astraea,  si,  genus 
closely  allied  to  Porites),  *  which  form  rocks  measuring 
several  fathoms  in  thickness,'  prefer,  according  to 
Chamisso,^  the  most  violent  surf.  I  have  stated 
that  the  outer  margin  of  the  Maldiva  atolls  consists  of 
living  corals,  (some  of  which,  if  not  all,  are  of  the  same 
species  with  those  at  Keeling  atoll),  and  here  the  surf 
is  so  tremendous,  that  even  large  ships  have  been 
thrown,  by  a  single  heave  of  the  sea,  high  and  dry  on 
the  reef,  all  on  board  thus  escaping  with  their  lives. 

Ehrenberg^  remarks,  that  in  the  Eed   Sea  the 

*  I  have  left  the  foregoing  paragraphs  nearly  as  they  stood  in  the 
first  edition ;  but,  as  stated  in  the  Preface  to  the  present  work,  Dana 
has  shown  that  I  have  undervalued  the  importance  of  the  mean 
temperature  of  the  sea  during  the  coldest  season  of  the  year,  on  the 
distribution  of  coral-reefs,  as  well  as  perhaps  the  injurious  effects  of 
recent  volcanic  action.  But  I  cannot  see  that  the  absence  of  coral- 
reefs  round  certain  islands  in  the  Atlantic,  for  instance  Ascension, 
St.  Paul's  Eock,  and  Fernando  Noronha,  or  from  the  shores  of  the 
Gulf  of  Panama,  is  explicable  through  any  known  cause. 

2  Kotzebue's  First  Voyage  (Eng.  Transl.),  vol.  iii.  pp.  142, 143, 
831. 

■  Ehrenberg,  Ueber  die  Natur  und  Bildung  der  Corallen  Banke 
im  rothen  Meere,  p.  49. 


86  CONDITIONS   FAVOURABLE   TO  Cn.  IV. 

strongest  corals  live  on  the  outer  reefs,  and  appear  to 
love  the  surf ;  he  adds,  that  the  more  branched  kinds 
abound  a  little  way  within,  but  that  these  in  still 
more  protected  places  become  smaller.  Many  other 
facts  having  a  similar  tendency  might  be  adduced.^  It 
has,  however,  been  doubted  by  MM.  Quoy  and  Gaimard, 
whether  any  kind  of  coral  can  even  withstand,  much 
less  flourish  in,  the  breakers  of  an  6pen  sea ;  ^  they 
affirm  that  the  saxigenous  lithophytes  flourish  only 
where  the  water  is  tranquil,  and  the  heat  intense. 
This  statement  has  passed  from  one  geological  work  to 
another  ;  nevertheless,  the  protection  of  the  whole  reef 
is  undoubtedly  due  to  those  kinds  of  coral,- which 
cannot  even  exist  in  the  situations  thought  by  these 
naturalists  to  be  most  favourable  to  them.  For  should 
the  outer  and  living  margin  perish,  of^ny  one  of  the 
many  low  coral-islands,  round  which  a  line  of  great 
breakers  is  incessantly  foaming,  the  whole,  it  is 
scarcely  possible  to  doubt,  would  be  washed  away  and 
destroyed  in  less  than  half  a  century.  But  the  vital 
energies  of  the  corals  conquer  the  mechanicat"power  of 
the  waves ;  and  the  large  fragments  of  reef  torn  up 
by  every  storm,  are  replaced  by  the  slow  but  steady 
growth  of  the  innumerable  polypifers  which  form  the 
living  zone  on  its  outer  edge. 

•  In  the  West  Indies,  as  I  am  informed  by  Captain  Bird  Allen, 
E.N.,  it  is  the  common  belief  of  those  who  are  best  acquainted  with 
the  reefs,  that  the  coral  flourishes  most  where  freely  exposed  to  the 
swell  of  the  open  sea. 

'^  Annales  des  Sciences  Naturelles,  tom.  vi.  pp.  276,  278. — '  La  o\\ 
les  ondes  sont  agit^es,  les  Lytophytes  ne  peuvent  travailler,  paroe 
qu'elles  d^truiraient  leurs  fragiles  ^diflces,'  (&c. 


Se(T.  I.       THE    GROWTH   OF   CORAL-REEFS.  87 

From  these  facts,  it  is  certain,  that  the  strongest 
and  most  massive  corals  flourish  where  most  exposed. 
The  less  perfect  state  of  the  reef  of  most  atolls  on  the 
leeward  and  less  exposed  side,  compared  with  its  state 
to  wii:dward ;  and  the  analogous  case  of  the  greater 
number  of  breaches  on  the  near  sides  of  those  atolls 
in  the  Maldiva  Archipelago  which  afford  some  pro- 
tection to  each  other,  are  obviously  explained  by  this 
circumstance.  If  the  question  had  been,  under  what 
conditions  the  greater  number  of  species  of  coral,  not 
regarding  their  bulk  and  strength,  were  developed,  I 
should  answer, — probably  in  the  situations  described 
by  MM.  Quoy  and  Gaimard,  where  the  water  is 
tranquil  and  the  heat  intense.  The  total  number  of 
species  of  coral  in  the  circumtropical  seas  must  be 
very  great ;  in  the  Eed  Sea  alone,  120  kinds,  accord- 
ing to  Ehrenberg,^  have  been  observed. 

The  same  author  has  observed  that  the  recoil  of 
the  sea  from  a  steep  shore  is  injurious  to  the  growth 
of  coral,  although  waves  breaking  over  a  bank  are 
not  so.  Ehrenberg  also  states  that  where  there  is 
much  sediment,  placed  so  as  to  be  liable  to  be  moved 
by  the  waves,  there  is  little  or  no  coral ; .  and  a  col- 
lection of  living  specimens  placed  by  him  on  a  sandy 
shore  died  in  the  course  of  a  few  days.^  An  experi- 
ment, however,  will  presently  be  related  in  which 
some  large  masses  of  living  coral  increased  rapidly  in 
size,  after  having  been  secured  by  stakes  on  a  sand- 
bank. That  loose  sediment  ^hould  be  injurious  to 
*  Ehrenberg  Ueber  die  Natur,  &c.  &c.,  p.  46.  *  Ibid  p.  49. 


88  CONDITIONS  FAVOURABLE   TO  Ch.  IV. 

the  living  polypifers,  appears  at  first  sight  probable ; 
and  in  sounding  off  Keeling  atoll  and  Mauritius,  the 
arming  of  the  ^ead  invariably  came  up  clean,  where 
the  coral  was  growing  vigorously.  A  strange  belief, 
which,  according  to  Captain  Owen,^  is  general  amongst 
the  inhabitants  of  the  Maldiva  atolls,  namely,  that 
corals  have  roots,  and  therefore  grow  up  again  if 
merely  broken  down  to  the  surface,  but  if  rooted 
out,  are  permanently  destroyed, — I  am  inclined  to 
believe  arises  from  the  fact  that  loose  sand  injures 
the  polypifers.  For  it  is  probable  that  sand  would 
accumulate  in  the  hollows  formed  by  tearing  out  the 
corals,  but  not  on  the  broken  and  projecting  stumps ; 
and  therefore,  in  the  former  case,  the  fresh  growth 
of  the  coral  would  be  prevented.  By  this  means 
the  inhabitants  keep  their  harbours  clear ;  and  thus 
the  French  governor  of  St.  Mary's,  in  Madagascar, 
*  cleared  out  and  made  a  beautiful  little  port  at  that 
p\ace.* 

In  the  last  chapter  I  remarked,  that  fringing- 
reefs  are  almost  universally  breached  where  streams 
enter  the  sea.^  Most  authors  have  attributed  this  fact 
to  the  injurious  effects  of  the  fresh  water,  even  where 

'  Captain  Owen  on  the  Geography  of  the  Maldiva  Islands,  Geo- 
graph.  Journal,  vol.  ii.  p.  88. 

2  Lieut.  Wellstead  and  others  have  remarked  that  this  is  the  case 
in  the  Red  Sea :  Dr.  Riippell  (Reise,  in  Abyss.  Band.  i.  s.  142)  says 
that  there  are  pear-shaped  harbours  in  the  upraised  coral-coast,  into 
which  periodical  streams  enter.  From  this  circumstance,  I  presume, 
we  must  infer  that,  before  the  upheaval  of  the  strata  now  forming 
the  coast-land,  fresh  water  and  sediment  entered  the  sea  at  these 
points ;  and  the  coral  being  thus  prevented  growing,  the  pear-shaped 
harbours  were  produced. 


Sect.  I.      THE   GROWTH   OF   CORAL-REEFS.  89 

it  enters  the  sea  only  in  small  quantity  and  during  a 
part  of  the  year.  No  doubt  brackish  water  would  pre- 
vent or  retard  the  growth  of  coral ;  but  I  believe  that 
the  mud  and  sand,  which  is  deposited,  even  by  smaU 
rivulets  when  flooded,  is  a  much  more  efficient  check. 
The  reef  on  each  side  of  the  channel  leading  into  Port 
Louis  at  Mauritius,  ends  abruptly  in  a  wall,  at  the 
foot  of  which  I  sounded,  and  found  a  bed  of  thick  mud. 
This  steepness  of  the  sides  appears  to  be  a  general 
character  in  such  breaches :  Cook,^  speaking  of  one 
at  Eaiatea,  says,  *  like  all  the  rest,  it  is  very  steep  on 
both  sides.*  Now,  if  it  were  the  fresh  water  mingling 
with  the  salt,  which  prevented  the  growth  of  coral, 
the  re^f  certainly  would  not  terminate  abruptly;  but  as 
the  polypifers  nearest  the  impure  stream  would  grow 
less  vigorously  than  those  farther  off,  so  would  the 
reef  gradually  thin  away.  On  the  other  hand,  the 
sediment  brought  down  from  the  land  would  only 
prevent  the  growth  of  the  coral  in  the  line  of  its 
deposition,  but  would  not  check  it  on  the  side,  so  that 
the  reefs  might  increase  till  they  overhung  the  bed  of 
the  channel.  The  breaches  are  much  fewer  in  number, 
and  front  only  the  larger  valleys  in  reefs  of  the  en- 
circling barrier  class.  They  probably  are  kept  open 
in  the  same  manner  as  those  into  the  lagoon  of  an 
atoll,  namely,  by  the  force  of  the  currents  and  the 
drifting  outwards  of  fine  sediment.  Their  position  in 
front  of  valleys,  although  often  separated  from  the 
land  by  deep-water  lagoon-channels,  which  it  might 
'  Cook's  First  Voyage,  vol.  ii.  p.  271.     (Hawkesworth's  edit.) 


90  CONDITIONS  FAVOURABLE   TO  Ch.  IV. 

be  thought  would  entirely  remove  the  injurious  effects 
both  of  the  fresh  water  and  the  sediment,  will  receive 
a  simple  explanation  when  we  discuss  the  origin  of 
barrier-reefs. 

In  the  vegetable  kingdom  every  different  station 
has  its  peculiar  group  of  plants,  and  similar  relations 
appear  to  prevail  with  corals.  We  have  already  de- 
scribed the  great  difference  between  the  corals  within 
the  lagoon  of  an  atoll  and  those  on  its  outer  margin. 
The  corals,  also,  on  the  margin  of  Keeling  Island  oc- 
curred in  zones  :  thus  the  Porites  and  Millepora  complu' 
nata  grow  to  a  large  size,  only  where  they  are  washed  by 
a  heavy  sea,  and  are  killed  by  a  short  exposure  to  the 
air ;  whereas,  three  species  of  Nullipora  also  live  amidst 
the  breakers,  but  are  able  to  survive  uncovered  for  a 
part  of  each  tide :  at  greater  depths,  a  strong  Madre- 
pora  and  Millepora  alcicornis  are  the  commonest  kinds ; 
the  former  appearing  to  be  confined  to  this  part :  be- 
neath the  zone  of  massive  corals,  minute  encrusting 
corallines  and  other  organic  bodies  live.  If  we  com- 
pare the  external  margin  of  the  reef  at  Keeling  atoll 
with  that  on  the  leeward  side  of  Mauritius,  which  are 
very  differently  circumstanced,  we  shall  find  a  corre- 
sponding difference  in  the  appearance  of  the  corals.  At 
the  latter  place,  the  genus  Madrepora  is  preponderant 
over  every  other  kind ;  and  beneath  the  zone  of  massive 
corals  there  are  large  beds  of  Seriatopora.  There  is 
also  a  marked  difference,  according  to  Captain  Moresby,^ 

*  Captain  Moresby  on  the  Northern  Maldiva  Atolls.    Geograph. 
Journ.,  vol.  v.  p.  401. 


Sect.  I.      THE   GROWTH   OF   CORAL-REEFS.  91 

between  the  great  branching  corals  of  the  Eed  Sea  and 
those  on  the  reefs  of  the  Maldiva  atolls. 

These  facts,  which  in  themselves  are  deserving  of 
notice,  bear,  perhaps,  not  very  remotely  on  a  remarkable 
circumstance  which  has  been  pointed  out  to  me  by 
Captain  Moresby,  namely,  that  with  very  few  excep- 
tions, none  of  the  coral-knolls  within  the  lagoons  of  Peros 
Banhos,  Diego  Garcia,  and  the  Great  Chagos  Bank  (all 
situated  in  the  Chagos  group),  rise  to  the  surface  of  the 
water ;  whereas,  with  equally  few  exceptions,  all  those 
within  Solomon  and  Egmont  atolls  in  the  same  group, 
and  likewise  those  within  the  large  southern  Maldiva 
atolls,  reach  the  surface.  I  make  these  statements,  after 
having  examined  the  charts  of  each  atoll.  In  the  lagoon 
of  Peros  Banhos,  which  is  nearly  twenty  miles  across, 
there  is  only  one  single  reef  which  rises  to  the  surface : 
in  Diego  Garcia  there  are  seven,  but  several  of  these  lie 
close  to  the  margin  of  the  lagoon,  and  need  scarcely 
have  been  reckoned :  in  the  Great  Chagos  Bank  there  is 
not  one.  On  the  other  hand,  in  the  lagoons  of  some  of 
the  great  southern  Maldiva  atolls,  although  thickly 
studded  with  reefs,  every  one  without  exception  rises  to 
the  surface ;  and  on  an  average  there  are  less  than  two 
submerged  reefs  in  each  atoll :  in  the  northern  atolls, 
however,  the  submerged  lagoon-reefs  are  not  quite  so 
rare.  The  submerged  reefs  in  the  Chagos  atolls  gene- 
rally have  from  one  to  seven  fathoms  water  on  them,  but 
some  have  from  seven  to  ten.  Most  of  them  are  small 
with  very  steep  sides ;  ^  at  Peros  Banhos  they  rise  from 

1  Some  of  these  statements  were  not  communicated  to  me  verb- 


92  CONDITIONS   FAVOURABLE   TO  Ch.  IV: 

a  depth  of  about  thirty  fathoms,  and  some  of  them  in 
the  Great  Chagos  Bank  from  above  forty  fathoms :  they 
are  covered,  Captain  Moresby  informs  me,  with  Uving 
and  healthy  coral  two  and  three  feet  high,  consisting 
of  several  species.  Why  then  have  not  these  lagoon- 
reefs  reached  the  surface,  like  the  innumerable  ones  in 
the  atolls  above  named  ?  If  we  attempt  to  assign  any 
difference  in  their  external  conditions,  as  the  cause  of 
this  diversity,  we  are  at  once  baffled :  the  lagoon  of 
Diego  Garcia  is  not  deep,  and  is  almost  wholly  sur- 
rounded by  its  reef ;  Peros  Banhos  is  very  deep,  much 
larger,  with  many  wide  passages  communicating  with 
the  open  sea.  On  the  other  hand,  of  those  atolls  in 
which  all,  or  nearly  all  the  lagoon-reefs  have  reached 
the  surface,  some  are  small,  others  large,  some  shallow, 
others  deep,  some  well  enclosed,  and  others  open. 

Captain  Moresby  informs  me  that  he  has  seen  a 
French  chart  of  Diego  Garcia  made  eighty  years  before 
his  survey,  and  apparently  very  accurate ;  and  from  it  he 
infers,  that  during  this  interval  there  has  not  been  the 
smallest  change  in  the  depth  on  any  of  the  knolls  within 
the  lagoon.  It  is,  also,  known  that  during  the  last  fifty- 
one  years,  the  eastern  channel  into  the  lagoon  has 
neither  become  narrower,  nor  decreased  in  depth ;  and 
as  there  are  numerous  small  knolls  of  living  coral  within 
it,  some  change  might  have  been  anticipated.  Moreover, 
as  the  whole  reef  round  the  lagoon  of  this  atoll  has  been 
converted  into  land — an  unparalleled  case,  I  believe,  in 

ally  by  Captain  Moresby,  but  are  taken  from  the  MS.  account,  before 
alluded  to,  of  the  Chagos  Group. 


Sect.  T.       THE   GROWTH   OF   CORAL-REEFS.  93 

an  atoll  of  such  large  size, — and  as  the  strip  of  land  is  for 
considerable  spaces  more  than  half  a  mile  wide — also  a 
very  unusual  circumstance, — we  have  the  best  possible 
evidence  that  Diego  Garcia  has  remained  a|  its  present 
level  for  a  very  long  period.  With  this  fact,  and  with 
the  knowledge  that  no  sensible  change  has  taken  place 
during  eighty  years  in  the  coral  knolls,  and  considering 
that  every  single  reef  has  reached  the  surface  in  other 
atolls,  which  do  not  present  the  smallest  appearance  of 
being  older  than  Diego  Garcia  and  Peros  Banhos,  and 
which  are  placed  under  the  same  external  conditions 
with  them,  one  is  led  to  conclude  that  these  submerged 
reefs,  although  covered  with  luxuriant  coral,  have  no 
tendency  to  grow  upwards,  and  that  they  would  remain 
at  their  present  levels  for  an  indefinite  period. 

From  the  number  of  these  knolls,  from  their  posi- 
tion, size,  and  form, — many  of  them  being  only  one  or 
two  hundred  yards  across,  with  a  rounded  outline  and 
precipitous  sides, — it  is  indisputable  that  they  have  been 
formed  by  the  growth  of  coral ;  and  this  makes  the  case 
much  more  remarkable.  In  Peros  Banhos  and  in  the 
Great  Chagos  bank,  some  of  these  almost  columnar 
masses  are  200  feet  high,  and  their  summits  lie  only  from 
two  to  eight  fathoms  beneath  the  surface;  therefore, 
a  little  greater  proportional  amount  of  growth  would 
cause  them  to  attain  the  surface,  like  those  numerous 
knolls  which  rise  from  an  equally  great  depth  within 
the  Maldiva  atolls.  We  can  hardly  suppose  that  time 
has  been  wanting  for  the  upward  growth  of  the  coral ; 
as  in  Diego  Garcia,  the  broad,  annular,  strip  of  land, 
8 


94  CONDITIONS    OF    GROWTH.  Ch.  IV. 

formed  by  the  continued  accumulation  of  detritus, 
shows  how  long  this  atoll  has  remained  at  its  present 
level.  We  must  look  to  some  other  cause  than  the 
rate  of  growth ;  and  I  suspect  it  will  be  found  in  the 
reefs  being  formed  of  different  species  of  corals,  adapted 
to  live  at  different  depths. 

The  Great  Chagos  bank  is  situated  in  the  centre  of 
the  Chagos  group,  and  the  Pitt  and  Speaker  banks  at 
its  two  extreme  points.  These  banks  resemble  atolls, 
except  in  their  external  rim  being  about  eight  fathoms 
submerged,  and  in  being  formed  of  dead  rock,  with  very 
little  living  coral  on  it :  a  portion  nine  miles  long  of 
the  annular  reef  of  Peros  Banhos  atoll  is  in  the  same 
condition.  These  facts,  as  will  hereafter  be  shown, 
render  it  probable  that  the  whole  group  at  some 
former  period  subsided  seven  or  eight  fathoms;  and 
that  the  coral  perished  on  the  outer  margins  of  those 
atolls  which  are  now  submerged,  but  that  it  continued 
alive  and  grew  up  to  the  surface  on  the  others  now 
perfect.  If  all  these  atolls  did  formerly  subside,  and  if 
from  the  suddenness  of  the  movement  or  from  any  other 
cause,  those  species  of  corals  which  are  best  adapted 
to  live  at  a  certain  depth,  once  got  possession  of  the 
knolls,  supplanting  their  former  occupants,  they  would 
have  little  or  no  power  to  grow  upwards.  To  illustrate 
this,  I  may  observe  that  if  the  corals  of  the  upper 
zone  on  the  outer  edge  of  Keeling  atoll  were  to  perish, 
it  is  improbable  that  those  of  the  lower  zone  would  grow 
to  the  surface,  and  thus  become  exposed  to  conditions 
for  which  they  do  not  appear  to  be  adapted.    The  con- 


Sect.  H.  RATE   OF   GROWTH.  95 

jecture  that  tlie  corals  on  the  submerged  knolls  within 
the  Ohagos  atolls  have  analogous  habits  with  those  of 
the  lower  zone  outside  Keeling  atoll,  receives  some  sup- 
port from  a  remark  by  Captain  Moresby,  namely,  that 
they  have  a  different  appearance  from  those  on  the  reefs 
in  the  Maldiva  atolls,  which,  as  we  have  seen,  all  rise  to 
the  surface :  he  compares  the  kind  of  difference  to  that 
of  the  vegetation  under  different  climates.  I  have 
entered  at  considerable  length  into  this  case,  although 
unable  to  throw  much  Hght  on  it,  in  order  to  show  that 
coral-reefs  situated  in  different  places  or  at  different 
depths,  whether  forming  the  ring  of  an  atoll  or  the 
knolls  within  a  lagoon,  need  not  all  be  supposed  to 
have  an  equal  tendency  to  upward  growth.  The  infer- 
ence, therefore,  that  one  reef  could  not  grow  to  the 
surface  within  a  given  time,  because  another,  not 
known  to  be  covered  with  the  same  species  of  corals, 
and  not  known  to  be  placed  under  exactly  the  same 
conditions,  has  not  w^ithin  the  same  time  reached  the 
surface,  is  unsound. 

Section  II. 
On  the  Rate  of  Growth  of  Coral-reefs, 

The  remark  made  at  the^  close  of  the  last  section, 
naturally  leads  to  this  division  of  our  subject,  which 
has  not,  I  think,  hitherto  been  considered  under  a 
right  point  of  view.  Ehrenberg  ^  has  stated  that  in 
the  Red  Sea,  the  corals  only  coat  other  rocks  in  a 

»  Ehrenberg,  as  before  cited,  pp.  39,  46,  and  60. 


96  RATE    OF    GROWTH.  Ch.  IV. 

layer  from  one  to  two  feet  in  thickness,  or  at  most  to 
a  fathom  and  a-half ;  and  he  disbelieves  that,  in  any 
case,  they  form  by  their  own  proper  growth  great 
masses,  stratum  over  stratum.  A  nearly  similar  ob- 
servation has  been  made  by  MM.  Quoy  and  Gaimard,' 
with  respect  to  the  thickness  of  some  upraised  beds  of 
coral,  which  they  examined  at  Timor  and  some  other 
places.  Ehrenberg^  saw  certain  large  massive  corals 
in  the  Red  Sea,  which  he  imagines  to  be  of  such  vast 
antiquity,  that  they  might  have  been  beheld  by 
Pharaoh;  and  according  to  Sir  C.  LyelP  there  are 
certain  corals  at  Bermuda,  which  are  known  by  tra- 
dition to  have  been  living  for  centuries.'*  To  show 
how  slowly  coral-reefs  grow  upwards.  Captain  Beechey  •' 
has  adduced  the  case  of  the  Dolphin  Reef  off  Tahiti, 
which  has  remained  at  the  same  depth  beneath  the 
surface,  namely,  about  two  fathoms  and  a-half,  for  a 
period  of  sixty-seven  years.  There  are  reefs  in  the 
Red  Sea,  which  certainly  do  not  appear  ^  to  have  in- 

*  Annales  des  Sciences  Nat.,  torn.  vi.  p.  28. 
»  Ehrenberg,  ut  sup.  p.  42. 

•  Lyell's  Principles  of  Geology,  book  iii.  ch.  xviii. 

*  Since  the  preceding  pages  (of  the  first  edit.)  have  been  printed 
off,  I  have  received  from  Sir  C.  Lyell  an  interesting  pamphlet,  en- 
titled Remarks  upon  Coral-Formations,  &c.,  by  J.  Couthouy,  Boston, 
United  States,  1842.  A  statement  (p.  6)  is  here  given  on  the  Jiutho- 
rity  of  the  Eev.  J.  Williams,  corroborating  the  above  remarks  on  the 
antiquity  of  certain  individual  corals,  namely,  that  at  Upolu,  one  of 
the  Navigator  islands,  *  particular  clumps  of  coral  are  known  to  the 
fishermen  by  name,  derived  from  either  some  particular  configuration 
or  tradition  attached  to  them,  and  handed  down  from  time  imme- 
morial.' 

»  Beechey's  Voyage  to  the  Pacific,  oh.  viu. 

•  Ehrenberg,  ut  sup.  p,  43. 


Sect.  U.  KATE   OF   GROWTH.  97 

creased  in  dimensions  during  the  last  half  century,  and 
from  the  comparison  of  old  charts  with  recent  surveys, 
probably  not  during  the  last  two  hundred  years. 
These,  and  other  similar  facts,  have  so  strongly  im- 
pressed many  with  the  belief  of  the  extreme  slowness 
of  the  growth  of  corals,^that  they  have  even  doubted 
the  possibility  of  islands  in  the  great  oceans  having 
been  formed  by  their  agency.  Others  again,  who  have 
not  been  overwhelmed  by  this  difficulty,  have  ad- 
mitted that  it  would  require  thousands,  and  tens  of 
thousands  of  years,  to  form  a  mass  even  of  incon- 
siderable thickness :  but  the  subject  has  not,  I  believe, 
been  viewed  in  the  proper  light. 

That  masses  of  considerable  thickness  have  been 
formed  by  the  growth  of  coral,  may  be  inferred  with 
certainty  from  the  following  facts.  In  the  deep 
lagoons  of  Peros  Banhos  and  of  the  Great  Chagos 
bank,  there  are,  as  already  described,  small  steep- 
sided  knolls  covered  with  living  coral.  There  are 
similar  knolls  in  the  southern  Maldiva  atolls,  some  of 
which,  as  Captain  Moresby  assures  me,  are  less  than 
a  hundred  yards  in  diameter,  and  rise  to  the  surface 
from  a  depth  of  between  250  and  300  feet.  Con- 
sidering their  number,  form,  and  position,  it  would  be 
preposterous  to  suppose  that  they  are  based  on  pin- 
nacles of  rock,  or  on  isolated  cones  of  sediment.  As 
no  kind  of  Uving  coral  grows  above  the  height  of  a 
few  feet,  we  are  compelled  to  suppose  that  these  knolls 
have  been  formed  by  the  successive  growth  and  death 


98  KATE   OF    GROWTH.  Ch.  IV.  ^ 

of  many  individuals, — first  one  being  broken  off  or 
killed  by  some  accident,  and  then  another,  and  one  set 
of  species  being  replaced  by  another  set  with  different 
habits,  as  the  reef  rose  nearer  the  surface,  or  as  other 
changes  supervened.  The  spaces  between  the  corals 
would  become  filled  up  with  fragments  and  sand,  and 
such  matter  would  probably  soon  be  consolidated,  for 
we  learn  from  Lieut.  Nelson's^  observations  at  Bermuda 
that  a  process  of  this  kind  takes  place  beneath  water, 
without  the  aid  of  evaporation.  In  reefs,  also,  of  the 
barrier  class,  we  may  feel  sure,  as  I  have  shown,  that 
masses  of  great  thickness  have  been  formed  by  the 
growth  of  coral.  In  the  case  of  Vanikoro,  judging 
only  from  the  depth  of  the  moat  between  the  land 
and  the  reef,  the  wall  of  coral-rock  must  be  at  least 
800  feet  in  vertical  thickness. 

So  again  some  of  the  upraised  islands  in  the  Pacific 
show  what  thick  masses  of  coral-rock  have  been 
formed.  Dana^  states  that  Metia,  in  the  Paumotu 
or  Low  Archipelago,  consists  of  white  solid  limestone 
with  some  disseminated  corals ;  and  this  island  once 
existed  as  an  atoll,  though  now  surrounded  by  cliffs 
250  feet  in  height.  The  cliffs  round  Elizabeth  Island 
in  the  same  archipelago  are  80  feet  high,  and  are 
composed,  according  to  Beechey,  of  homogeneous  coral- 
rock.  Mangaia  in  the  Hervey  Group,  and  Eurutu,  * 
appear  both  to  have  once  existed  as  encircled  islands, 

'  Geological  Transactions,  vol.  v.  p.  113. 

*  Corals  and  Coral  Islands,  1872,  p.  193.    See  also  Mr.  Couthouy'a 
pamphlet  above  referred  ta 


Sect.  II.  RATE   OF   GROWTH.  99 

and  their  barrier-reefs  are  now  in  parts  300  feet  above 
the  level  of  the  sea.* 

Some  attempts  have  been  made,  with  but  little 
success,  to  ascertain  by  boring  the  thickness  of  coral 
formations.  At  Bow  Island,  in  the  Low  Archipelago, 
Sir  E.  Belcher 2  bored. to  a  depth  of  45  feet,  and 
below  the  first  20  found  only  coral-sand.  During 
Wilke's  Expedition,^  in  a  boring  of  21  feet  in  depth 
on  one  of  the  islands  in  the  same  archipelago,  coral- 
sand  was  passed  through  for  the  first  10  or  11  feet, 
and  then  solid  reef  rock.  On  one  of  the  Maldiva 
atolls  in  the  Indian  Ocean,  Captain  Moresby  bored  to 
a  depth  of  26  feet,  when  his  auger  broke.  He  gave 
me  the  matter  brought  up,  and  it  was  perfectly  white 
like  finely  triturated  coral-rock. 

In  my  description  of  Keeling  atoll,  I  have  ad- 
vanced some  facts  showing  that  the  reef  has  probably 
grown  outwards ;  and  I  found,  just  within  the  outer 
margin,  the  great  mounds  of  Porites  and  of  Millepora, 
with  their  summits  lately  killed,  and  their  sides  sub- 
sequently thickened  by  the  growth  of  the  coral:  a 
layer,  also,  of  Nullipora  had  already  coated  the  dead 
surface.  As  the  external  slope  of  the  reef  is  the  same 
round  the  whole  of  this  and  many  other  atolls,  the 
angle  of  inclination  must  result  from  an  adaptation 

*  Dana,  Corals  and  Coral  Islands,  p.  336.  Also  Forster's  Voyage 
round  the  World  with  Cook,  vol.  ii.  pp.  163,  167.  Williams's  Narra- 
tive of  Missionary  Enterprise,  pp.  30,  48,  and  249. 

2  Voyage  Round  the  World,  vol.  i.  1843,  p.  369. 

*  Narrative  U.S.  Exploring  Expedition,  vol.  iv.  p.  268.  Dana, 
Corals  and  Coral  Islands,  p.  184. 


100  RATE    OF    GROWTH.  Ch.  IV 

between  the  growing  powers  of  the  coral  and  the 
force  of  the  breakers,  and  their  action  on  the  loose 
sediment.  The  reef,  therefore,  could  not  increase  out- 
wards without  a  nearly  equal  addition  to  every  part 
of  the  slope,  so  that  the  original  inclination  might  be 
preserved,  and  this  would  require  a  large  amount  of 
sediment,  all  derived  from  the  wear  of  corals  and 
shells,  to  be  added  to  the  lower  part.  Moreover,  at 
Keeling  atoll  and  probably  in  many  other  cases,  the 
different  kinds  of  coral  would  have  to  encroach  on  each 
other;  thus  the  Nulliporse  cannot  increase  outwards 
without  encroaching  on  the  Porites  and  Millepora 
complanata,  as  is  now  taking  place;  nor  these  latter 
without  encroaching  on  the  strongly  branched  Madre- 
pora,  the  Millepora  alcicornis,  and  some  Astrseas  ;  nor 
these  again  without  a  foundation  being  formed  for 
them  within  the  requisite  depth,  by  the  accumulation 
of  sediment.  How  slow,  then,  must  be  the  ordinary 
lateral  or  outward  growth  of  such  reefs  !  But  off 
Christmas  atoll,  where  the  sea  is  much  more  shallow 
than  is  usual,  we  have  good  reason  to  believe  that, 
within  a  period  not  very  remote,  the  reef  has  in- 
creased considerably  in  width.  The  land  has  the 
extraordinary  breadth  of  three  miles  ;  it  consists  of 
parallel  ridges  of  shells  and  broken  corals,  which 
furnish  *  an  incontestable  proof,*  as  observed  by  Cook,* 
*  that  the  island  has  been  produced  by  accessions  from 
the  sea,  and  is  in  a  state  of  increase.'  The  land  is 
fronted  by  a  coral-reef,  and  from  the  manner  in  which 

•  Cook's  Third  Voyage,  book  iii.  ch.  x. 


Sect.  IT.  RATE   OF   GROWTH.  101 

islets  are  known  to  be  formed,  we  may  feel  confident 
that  the  reef  was  not  three  miles  wide  when  the  first, 
or  most  inward  ridge,  was  thrown  up ;  and,  there- 
fore, we  must  conclude  that  the  reef  has  grown  out- 
wards during  the  accumulation  of  the  successive  ridges. 
Here  then,  a  wall  of  coral-rock  of  very  considerable 
breadth  has  been  formed  by  the  outward  growth  of 
the  living  margin,  within  a  period,  during  which 
ridges  of  shells  and  corals,  lying  on  the  bare  surface, 
have  not  decayed.  There  can  be  little  doubt,  from  the 
account  given  by  Captain  Beechey,  that  Matilda  atoll  in 
the  Low  Archipelago  has  been  converted  in  the  space 
of  thirty-four  years,  from  being,  as  described  by  the 
crew  of  a  wrecked  whaling  vessel,  a  *reef  of  rocks,' 
into  a  lagoon-island  fourteen  miles  in  length,  with 
*  one  of  its  sides  covered  nearly  the  whole  way  with 
high  trees.*  ^  The  islets,  also,  on  Keeling  atoll,  it  has 
been  shown,  have  increased  in  length,  and  since  the 
construction  of  an  old  chart,  several  of  them  have 
become  united  into  one  long  islet :  but  in  this  case, 
and  in  that  of  Matilda  atoll,  we  have  no  proof  that 
the  foundation  of  the  islets,  namely  the  reef,  has 
increased  in  breadth,  although  it  must  be  allowed  that 
this  is  probable. 

I  think,  therefore,  in  regard  to  the  possible  rate  of 
ouUvard  growth  of  coral-reefs,  but  little  importance 
need  be  attached  to  the  fact  that  certain  reefs  in  the 
Bed  Sea  have  not  increased  during  a  long  interval  of 
time,  or  to  other  similar  cases,  such  as  that  of  Ouluthy 

*  Beechey's  Voyage  to  the  Pacific,  ch.  vii  and  viii. 


102  RATE    OF    GROWTH.  Ch.  IV. 

atoll  in  the  Caroline  group,  where  every  islet,  described 
a  hundred  years  before  by  Cantova,  was  found  in  the 
same  state  by  Lutke.^  For  it  cannot  be  shown  that, 
in  these  cases,  the  conditions  were  favourable  to  the 
vigorous  and  unopposed  growth  of  the  corals  living 
in  the  different  zones  of  depth,  and  that  a  proper  basis 
for  the  extension  of  the  reef  was  present.  These 
conditions  must  depend  on  many  contingencies,  and 
a  basis  within  the  requisite  depth  can  rarely  be  pre- 
sent in  the  deep  oceans  where  coral  formations  most 
abound. 

Nor  do  I  think,  when  we  consider  the  rate  of  the 
upward  growth  of  reefs  under  favourable  circumstances, 
that  we  should  be  influenced  by  the  fact  that  certain 
submerged  reefs,  such  as  those  off  Tahiti  or  those  within 
Diego  Garcia,  are  not  now  nearer  the  surface  than  they 
were  many  years  ago.  For  it  has  been  shown  that  all 
the  reefs  have  grown  to  the  surface  in  some  of  the 
Chagos  atolls,  but  that  in  neighbom-ing  atolls  which 
appear  to  be  of  equal  antiquity  and  to  be  exposed  to 
the  same  external  conditions,  every  reef  remains  sub- 
merged ;  we  are,  therefore,  almost  driven  to  attribute 
this  to  a  difference,  not  in  the  rate  of  growth,  but  in 
the  habits  of  the  corals  in  the  two  cases. 

In  an  old-standing  reef,  the  corals,  which  greatly 
differ  in  kind  on  different  parts  of  it,  are  probably 

'  F.  Lutk6's  Voyage  autour  du  Monde.  In  the  group  Elato, 
however,  it  appears  that  what  is  now  the  islet  Falipi,  is  called  in 
Cantova' 8  Chart,  the  Banc  de  Falipi.  It  is  not  stated  whether  this 
has  been  caused  by  the  growth  of  coral,  or  by  the  accumulation  oi 
Band. 


Sect.  H.  BATE   OF   GROWTH.  103 

all  adapted  to  the  stations  they  occupy,  and  hold  their 
places,  like  other  organic  beings,  by  a  struggle  one 
^^^h  another  and  with  external  nature  ;  hence  we  may 
'i*ftfer  that  their  growth  would  be  slow  except  under 
peculiarly  favourable  circumstances.  Almost  the  only 
natural  condition,  allowing  a  quick  upward  growth  of 
the  whole  surface  of  a  reef,  would  be  a  slow  subsidence 
of  the  area  in  which  it  stood ; — if,  for  instance.  Keeling 
atoll  were  to  subside  two  or  three  feet,  can  we  doubt 
that  the  projecting  margin  of  live  coral,  about  half  an 
inch  in  thickness,  which  surrounds  the  dead  upper  sur- 
faces of  the  mounds  of  Porites,  would  in  this  case  form 
a  concentric  layer  over  them,  and  the  reef  thus  increase 
upwards,  instead  of,  as  at  present,  outwards  ?  The 
NulliporaB  are  now  encroaching  on  the  Porites  and 
Millepora,  but  in  this  case  might  we  not  confidently 
expect  that  the  latter  would,  in  their  turn,  encroach 
on  the  Nulliporse  ?  After  a  subsidence  of  this  kind,  the 
sea  would  gain  on  the  islets,  and  the  great  fields  of  dead 
but  upright  corals  in  the  lagoon  would  be  covered  by  a 
sheet  of  clear  water;  and  might  we  not  then  expect 
that  these  reefs  would  rise  to  the  surface,  as  they  an- 
ciently did  when  the  lagoon  was  less  confined  by  islets, 
and  as  they  did  within  a  period  of  ten  years  in  the 
schooner-channel  cut  by  the  inhabitants.  In  one  of  the 
Maldiva  atolls,  a  reef,  which  within  a  very  few  years 
existed  as  an  islet  bearing  cocoa-nut  trees,  was  found 
by  Lieut.  Prentice  *  entirely  covered  with  live  coral  and 
Madrepore.*  The  natives  believe  that  the  islet  was 
washed  away  by  a  change  in  the  currents,  but  ifj 


104  RATE    OF    GROWTH.  Ch.  IV. 

instead  of  this,  it  had  quietly  subsided,  surely  every 
part  of  the  island  which  offered  a  solid  Toundation, 
would  in  a  like  manner  have  become  coated  with  living 
coral. 

Through  steps  such  as  these,  any  thickness  of  rock 
composed  of  a  singular  intermixture  of  various  kinds 
of  corals,  shells,  and  calcareous  sediment,  might  be 
formed ;  but  without  subsidence,  the  thickness  would 
necessarily  be  determined  by  the  depth  at  which  the 
reef-building  polypifers  can  exist.  If  it  be  asked,  at 
what  rate  in  years  I  suppose  a  reef  of  coral  favourably 
circumstanced  could  grow  up  from  a  given  depth ;  I 
must  answer  that  we  have  no  precise  evidence  on 
this  head.  It  will,  however,  be  hereafter  shown  that 
in  certain  large  areas  where  subsidence  has  probably 
been  long  in  progress,  the  growth  of  the  corals  has 
been  sufficient  to  keep  the  reefs  up  to  the  surface  ;  and 
this  is  a  much  more  important  standard  of  comparison 
than  any  cycle  of  years. 

It  may,  however,  be  inferred  from  the  following 
facts,  that  the  rate  under  favourable  circumstances 
would  be  far  from  slow.  Dr.  Allan  of  Forres  has 
given,  in  his  MS.  Thesis  deposited  in  the  library  of 
the  Edinburgh  University,  the  following  account  of 
some  experiments,  which  he  tried  during  his  travels 
in  the  years  1830  to  1832  on  the  east  coast  of 
Madagascar.  *  To  ascertain  the  rise  and  progress 
of  the  coral  family,  and  fix  the  number  of  species 
met  with  at  Foul  Point  (lat.  17°  40'),  twenty  spe- 
cies of  coral  were  taken  off  the  reef  and    planted 


Sect.  H.  RATE   OF   GROWTH.  105 

apart  on  a  sand-bank  three  feet  deep  at  low  water. 
Each  portion  weighed  ten  pounds,  and  was  kept  in 
its  place  by  stakes.  Similar  quantities  were  placed 
in  a  clump  and  secured  as  the  rest.  This  was  done 
in  December  1830.  In  July  following,  each  detached 
mass  was  nearly  level  with  the  sea  at  low  water,  quite 
immovable,  and  several  feet  long,  stretching,  like  the 
parent  reef,  in  the  line  of  the  coast-current  from  north 
to  south.  The  masses  accumulated  in  a  clump  were 
•found  equally  increased,  but  some  of  the  species  in 
such  unequal  ratios  as  to  be  growing  over  each  other.'  ^ 
The  loss  of  Dr.  Allan's  magnificent  collection  by 
shipwreck,  unfortunately  prevents  its  being  known 
to  what  genera  these  corals  belonged ;  but  from  the 
numbers  experimented  on,  it  is  certain  that  all  the 
more  conspicuous  kinds  must  have  been  included. 
Dr.  Allan  informs  me,  in  a  letter,  that  he  believes 
it  was  a  Madrepora  which  grew  most  vigorously. 
One  may  be  permitted  to  suspect  that  the  level  of  the 
sea  might  possibly  have  been  somewhat  different  at  the 
two  stated  periods;  nevertheless,  it  is  quite  evident 
that  the  growth  of  the  ten-pound  masses,  during  the 
six  or  seven  months  at  the  end  of  which  they  were 
found  to  be  immovably  fixed  ^  and  several  feet  in 
length,  must  have  been  very  great.     The  fact  of  the 

*  I  owe  the  above  extract  to  the  kindness  of  Dr.  Malcolmson. 

2  It  is  stated  by  Mr.  De  la  Beche  (Geological  Manual,  p.  148),  on 
the  authority  of  Mr.  Lloyd,  who  surveyed  the  Isthmus  of  Panama, 
that  some  specimens  of  Polypifers,  placed  by  him  in  a  sheltered 
pool  of  water,  were  found  in  the  course  of  a  few  days  firmly  fixed  by 
the  secretion  of  a  stony  matter,  to  the  bottom. 


106  RATE    OF    GROWTH.  Ch.  IV. 

different  kinds  of  coral,  when  placed  in  one  clump, 
having  increased  in  extremely  unequal  ratios,  is  very 
interesting,  as  it  shows  the  manner  in  which  a  reef, 
supporting  many  species  of  coral,  would  probably  be 
affected  by  a  change  in  the  external  conditions 
favouring  one  kind  more  than  another.  The  growth 
of  the  masses  of  coral  in  N.  and  S.  lines  parallel  to 
the  prevailing  currents,  whether  due  to  the  drift- 
ing of  sediment  or  to  the  simple  movement  of  the 
water,  is,  also,  an  interesting  circumstance. 

Lieut.  Wellstead,  I.N.,  informed  me  that  in  the 
Persian  Gulf  a  ship  had  her  copper  bottom  en- 
crusted in  the  course  of  twenty  months  with  a  layer 
of  coral  two  feet  in  thickness,  which  it  required  great 
force  to  remove  when  the  vessel  was  docked :  it  was 
not   ascertained   to  what  order  this  coral  belonged.* 

'  Mr.  Stutchbury  (West  of  England  Journal,  No.  I.  p.  50)  has 
described  a  specimen  of  Agaricia,  '  weighing  2  lbs.  9  oz.,  which  sur- 
rounds a  species  of  oyster,  whose  age  could  not  be  more  than  two 
years,  and  yet  is  completely  enveloped  by  this  dense  coral.'  I  pre- 
sume that  the  oyster  was  living  when  the  specimen  was  procured ; 
otherwise  the  fact  tells  nothing.  Mr.  Stutchbury  also  mentions  an 
anchor,  which  had  become  entirely  encrusted  with  coral  in  fifty 
years;  other  cases,  however,  are  recorded  of  anchors  having  long 
remained  amidst  coral-reefs  without  having  become  coated.  The 
anchor  of  the  BeagU,  in  1832,  after  having  been  down  exactly  one 
month  at  Rio  de  Janeiro,  was  so  thickly  coated  by  two  species  of 
Tubularia,  that  large  spaces  of  the  iron  were  entirely  concealed ;  the 
tufts  of  this  horny  zoophyte  were  between  two  and  three  inches  in 
length.  Spallanzani  states  (Travels,  Eng.  Translat.  vol.  iv.  p.  313) 
that  in  the  Mediterranean,  the  red  coral  of  commerce  is  usually 
dredged  every  ten  years,  during  which  time  it  grows  to  a  height  of 
one  foot.  It  grows,  however,  at  different  rates  in  different  places. 
It  has  been  erroneously  attempted  to  compute  the  rate  of  growth  of 
a  reef,  from  the  fact  mentioned  by  Captain  Beechey  of  the  Chama 


Sect.  n.  RATE   OF    GROWTH.  107 

This  fact  in  some  degree  corroborates  the  result  of 
Dr.  Allan's  experiments.  The  case  of  the  schooner- 
channel,  choked  up  with  coral  in  an  interval  of  less 
than  ten  years,  in  the  lagoon  of  Keeling  atoll,  should 
be  here  borne  in  mind.  We  may  also  infer,  from 
the  trouble  which  the  inhabitants  of  the  Maldiva 
atolls  take  to  root  out,  as  they  express  it,  the  coral- 
knolls  from  their  harbours,  that  their  growth  can 
hardly  be  very  slow.  I  may  add,  that  M.  Duchassaing 
broke  off  all  the  Madrepores  growing  on  a  marked 
place  in  a  bay  at  Guadaloupe ;  and  in  the  course  of 
two  months  he  found  there  a  greater  number  of 
corals  than  before.* 

From  the  facts  given  in  this  section,  it  may  be 
concluded,  first,  that  considerable  thicknesses  of  rock 
have  certainly  been  formed  within  the  present  geo- 
logical era  by  the  growth  of  corals  and  the  accumu- 
lation of  thefr  detritus  ;  and,  secondly,  that  the 
increase  of  individual  corals  and  of  reefs,  both  out- 
wards or  horizontally,  and  upwards  or  vertically,  under 
conditions  favourable  to  such  increase,  is  not  slow, 
when  referred  either  to  the  standard  of  the  average 

gigas  being  embedded  in  coral  rock.  But  it  should  be  remembered, 
that  some  species  of  this  genus  invariably  live,  both  whilst  young 
and  old,  in  cavities,  which  the  animal  has  the  power  of  enlarging 
with  its  growth.  I  saw  many  of  these  shells  thus  embedded  in  the 
outer  *  flat '  of  Keeling  atoll,  which  is  composed  of  dead  rock ;  and 
therefore  the  cavities  in  this  case  had  no  relation  whatever  to  the 
growth  of  coral.  M.  Lesson,  also,  speaking  of  this  shell  (Partie 
Zoolog.,  Voyage  de  la  Coquille),  has  remarked,  'que  constamment 
ses  valves  6taient  engag^es  compl^tement  dans  la  masse  des  Madre- 
pores.' 

'  L'Institut,  1846,  p.  111. 


108  DEPTH   AT   WHICH  Ch.  IV. 

oscillations  of  level  in  the  earth's  crust,  or  to  the 
more  precise  but  less  important  one  of  a  cycle  of 
years.* 


Section  III. 

On  the  Depths  at  which  Reef-building  Corals  live, 

I  HAVE  already  described  in  detail  the  nature  of  the 
bottom  of  the  sea  immediately  surrounding  Keeling 
atoll ;  and  I  will  here  describe  with  almost  equal  care, 
the  soundings  off  the  fringing-reefs  of  Mauritius.  I 
sounded  with  the  wide  bell- shaped  lead  which  Captain 
FitzEoy  used  at  Keeling  Island.  My  examination 
of  the  bottom  was  confined  to  a  few  miles  of  coast 
(between  Port  Louis  and  Tomb  Bay)  on  the  leeward 
side  of  the  island.  The  edge  of  the  reef  is  formed 
of  great,  shapeless  masses  of  branching  Madrepores, 
which  chiefly  consist  of  two  species, — apparently 
M,  corymhosa  and  pocillifera^ — mingled  with  a  few 
other  kinds  of  coral.  These  masses  are  separated  from 
each  other  by  the  most  irregular  gullies  and  cavities, 

•  [See  Dana,  Corals  and  Coral  Islands,  ch.  i.  sec.  iv.  for  additional 
facts  relating  to  rate  of  growth  of  corals.  Le  Conte  (Amer.  Jour.  Sci. 
Ser.  3,  vol.  x.  pp.  34  6)  estimates  that  a  Madrepora  {cervicornis  ?)  in 
shoal  water  at  the  Tortugas  grew  upwards  at  the  rate  of  3^  inches 
per  annum.  Duncan  (Proc.  Eoy.  Soc.  xxvi.  133)  estimates  in  the 
case  of  Lophohelia pvlifera  and  Desynophyllum  eristagalli  growing  in 
deep  water  to  the  north-west  of  Spain  (522  to  550  fathoms)  an  in- 
crease upwards  at  the  rate  of  0-29  inches  per  annum.  The  result  of 
later  researches  indicates  considerable  variation  in  the  rate  of 
growth,  depending  probably  on  species,  locality,  (fee,  and  confirms  the 
general  conclusions  of  this  paragraph,] 


Sect.  III.  REEF-BUILDING    CORALS   LIVE.  109^ 

into  which  the  lead  sinl^s  many  feet.      Outside  this 
irregular   border  of  Madrepores,  the  water  deepens 
gradually  to   20   fathoms,  which  depth  generally  is 
found  at  the  distance  of  from  half  to  three-quarters 
of  a  mile  from  the  reef.    A  little  further  out  the  depth 
is  30  fathoms,  and  thence  the  bank   slopes  rapidly 
into  the  depths  of  the  ocean.    This  inclination  is  very 
gentle  compared  with  that  outside  Keeling  and  other 
atolls,  but  compared  with  most  coasts  it  is  steep.   The 
water   was   so   clear   outside  the  reef,  that  I   could 
distinguish  every  object  forming  the  rugged  bottom. 
In    this    part,    and   to   a   depth   of    8   fathoms,    I 
sounded   repeatedly,  and  at  each   cast  pounded   the 
bottom  with  the  broad  lead  ;  nevertheless  the  arming; 
invariably   came   up  perfectly  clean,  but  deeply  in- 
dented.     From    8    to    15    fathoms    a  little   calca- 
reous sand  was  occasionally  brought  up,   but  more 
frequently  the  arming  was  simply  indented.     In  alli 
this  space  the  two  Madrepores  above  mentioned,  and. 
two  species  of  Astrsea  with  rather  large  stars,  seemed, 
the  commonest  kinds;  and  it  must  be  noticed  that, 
twice  at  the  depth  of  15  fathoms,  the  arming  was 
marked  with    a    clean    impression    of    an    Astraea.. 
Besides   these   lithophytes,   some   fragments    of  the 
Millepora  alcicornis  which  occurs  in  the  same  relative 
position  at  Keeling  Island,  were  brought  up ;  and  in 
the  deeper  parts  there  were  large  beds  of  a  Seriate- 
pora,  different  from  S.  subulata,  but  closely  alhed  to 
it.    On  the  beach  within  the  reef,  the  rolled  fragments 
consisted  chiefly  of  the  corals  just  mentioned,  and  of 


110  DEPTH   AT    WHICH  Ch.  IV. 

a  massive  Poritcs  like  that  at  Keeling  atoll,  of  a 
I^Ieandrina,  Pocillopora  verrucosa,  and  of  numerous 
fragments  of  Nullipora.  From  15  to  20  fathoms  the 
bottom  was,  with  few  exceptions,  either  formed  of 
sand,  01  thickly  covered  with  Seriatopora :  this  delicate 
coral  seems  to  form  at  these  depths  extensive  beds 
unmingled  with  any  other  kind.  At  20  fathoms,  one 
sounding  brought  up  a  fragment  of  Madrepora,  ap- 
parently M.  pocilliferaj  and  I  believe  it  to  be  the 
same  species  as  that  which  mainly  forms  the  upper 
margin  of  the  reef;  if  so,  it  grows  in  depths  varying 
from  0  to  20  fathoms.  Between  20  and  23  fathoms  I 
-obtained  several  soundings,  and  they  all  showed  a  sandy 
bottom,  with  one  exception  at  30  fathoms,  when  the 
arming  came  up  scooped  out  as  if  by  the-  margin  of  a 
large  Caryophyllia.  Beyond  33  fathoms  I  sounded 
only  once;  and  from  86  fathoms,  at  the  distance  of  one 
mile  and  a  third  from  the  edge  of  the  reef,  the  arming 
'brought  up  calcareous  sand  with  a  pebble  of  volcanic 
;rock.  The  circumstance  of  the  arming  having  in- 
variably come  up  quite  clean  when  sounding  within  a 
certain  number  of  fathoms  off  the  reefs  of  Mauritius 
and  Keeling  atoll  (8  fathoms  in  the  former  case, 
and  12  in  the  latter),  and  of  its  having  always 
come  up  (with  one  exception)  smoothed  and  covered 
with  sand  when  the  depth  exceeded  20  fathoms, 
tprobably  indicates  a  criterion,  by  which  the  limits 
of  the  vigorous  growth  of  coral  might  in  all  cases  be 
readily  ascertained.  I  do  not,  however,  suppose  that 
iif  a  vast  number  of  soundings  were  obtained  round 


Sect.  III.  REEF-BUILDING   CORALS   LIVE.  Ill 

these  islands,  the  limit  above  assigned  would  be  found 
never  to  vary,  but  I  conceive  the  facts  are  sufficient  to 
show  that  the  exceptions  would  be  few.  The  circum- 
stance of  a  gradual  change,  in  the  two  cases,  from  a 
field  of  clean  coral  to  a  smooth  sandy  bottom,  is  far 
more  important  in  indicating  the  depth  at  which  the 
larger  kinds  of  coral  flourish,  than  almost  any  number 
of  separate  observations  on  the  depth  at  which  certain 
species  have  been  dredged  up.  For  we  can  understand 
the  gradation  only  as  a  prolonged  struggle  against 
unfavourable  conditions.  If  a  person  were  to  find  the 
soil  clothed  with  turf  on  the  banks  of  a  stream  of 
water,  but  on  going  to  some  distance  on  one  side  of  it 
he  observed  the  blades  of  grass  growing  thinner  and 
thinner  with  intervening  patches  of  sand,  until  he 
entered  a  desert  of  sand,  he  would  safely  conclude, 
especially  if  changes  of  the  same  kind  were  noticed  in 
other  places,  that  the  presence  of  the  water  was  abso- 
lutely necessary  to  the  formation  of  a  thick  bed  of 
turf :  so  may  we  conclude,  with  the  same  feeling  of 
certainty,  that  thick  beds  of  coral  are  formed  only  at 
small  depths  beneath  the  surface  of  the  sea. 

I  have  endeavoured  to  collect  every  fact  which 
might  either  invalidate  or  corroborate  this  conclusion. 
( /aptain  Moresby,  whose  opportunities  for  observation 
during  his  survey  of  the  Maldiva  and  Chagos  Archi- 
pelagoes were  unrivalled,  informs  me,  that  the  upper 
part  or  zone  of  the  steep-sided  reefs  on  the  inner  and 
outer  coasts  of  the  atolls  in  both  groups,  invariably 
consisted  of  coral,  and  the  lower  parts  of  sand.     At 


112  DEPTH   AT   WHICH  Ch.  IV. 

7  or  8  fathoms  depth,  the  bottom  is  formed,  as  could 
be  seen  through  the  clear  water,  of  great  livmg 
masses  of  coral,  which  at  about  10  fathoms  generally 
stand  some  way  apart  from  each  other,  with  patches  of 
white  sand  between  them,  and  at  a  little  greater  depth 
these  patches  become  united  into  a  smooth  steep  slope 
without  any  coral.  Captain  Moresby,  also,  informs  me 
in  support  of  the  above  statement,  that  he  only  found 
decayed  coral  on  the  Padua  Bank  (northern  part  of  the 
Laccadive  group),  which  has  an  average  depth  of  25 
to  35  fathoms ;  but  that  on  some  other  banks  in  the 
same  group,  with  a  depth  of  only  10  or  12  fathoms 
(for  instance,  the  Tillacapeni  bank)  the  coral  was 
living. 

Professor  Dana  likewise  states  that  during  the  various 
and  extensive  surveys  in  the  Pacific  Ocean,  made  during 
the  United  States  exploring  expedition,  no  evidence 
was  found  of  corals  growing  beyond  the  depth  of  20 
fathoms.^  I  may  here  add  that  Sir  E.  Belcher,  though 
he  does  not  state  to  what  depth  living  corals  extended, 
says  that  many  soundings  were  taken  off  Bow  atoll,  at 
depths  ranging  from  50  to  960  fathoms,  and  that  the 
bottom  always  consisted  of  coral  sand.^ 

With  regard  to  the  coral-reefs  in  the  Ked  Sea, 
Ehrenberg  has  the  following  passage.  *The  living 
corals  do  not  descend  there  into  great  depths.  On  the 
edges  of  islets  and  near  reefs,  where  the  depth  was 
small,  very  many  lived ;  but  we  found  no  more  even  at 

'  Corals  and  Coral  Islands,  1872,  p.  116. 

2  Voyage  Round  the  World,  1843,  p.  379,  vol.  i. 


SEcr.  m.      REEF-BUILDING   CORALS  LIVE.  113 

Bix  fathoms.  The  pearl-fishers  at  Yemen  and  Massaua 
asserted  that  there  was  no  coral  near  the  pearl-banks  at 
nine  fathoms  depth,  but  only  sand.  We  were  not  able 
to  institute  any  more  special  researches.'  *  I  am,  how- 
ever, assured  both  by  Captain  Moresby  and  Lieut.  Well- 
stead,  that  in  the  more  northern  parts  of  the  Eed  Sea, 
there  are  extensive  beds  of  living  coral  at  a  depth  of  25 
fathoms,  in  which  the  anchors  of  their  vessels  were 
frequently  entangled.  Captain  Moresby  attributes  the 
less  depth  at  which  the  corals  are  able  to  live  in  the 
places  mentioned  by  Ehrenberg,  to  the  greater  quantity 
of  sediment  there;  the  situations,  where  they  were 
flourishing  at  the  depth  of  25  fathoms,  were  protected, 
and  the  water  was  extraordinarily  limpid.  On  the 
leeward  side  of  Mauritius,  where  I  found  the  coral 
growing  at  a  somewhat  greater  depth  than  at  Keeling 
atoll,  the  sea,  owing  apparently  to  its  tranquil  state, 
was  likewise  very  clear.  Within  the  lagoons  of  some 
of  the  Marshall  atolls,  where  the  water  can  be  but  little 
agitated,  there  are,  according  to  Kotzebue,  living  beds 
of  coral  in  25  fathoms.  From  these  several  facts,  and 
considering  the  manner  in  which  the  beds  of  clean  coral 
off  Mauritius,  Keeling  Island,  the  Maldiva  and  Chagos 
atolls,  graduated  into  a  sandy  slope,  it  appears  very 
probable  that  the  depth  at  which  reef-building  poly- 
pifers  can  exist,  is  partly  determined  by  the  extent  of 
inclined  surface  which  the  currents  of  the  sea  and 
the  recoiling  waves  have  the  power  to  keep  free  from 
sediment. 

'  Ehrenberg,  Ueber  die  Natur,  Ac  p.  50. 


114  DEPTH   AT   WHICH  Ch.  IV. 

MM.  Quoy  and  Gaimard  ^  believe  that  the  growth 
of  coral  is  confined  within  very  limited  depths ;  and 
they  state  that  they  never  found  any  fragment  of  an 
Astrsea  (the  genus  they  consider  most  efficient  in  form- 
ing reefs)  at  a  depth  above  25  or  30  feet.  But  we 
have  seen  that  in  several  places  the  bottom  of  the  sea 
is  paved  with  massive  corals  at  more  than  twice  this 
depth  ;  and  at  15  fathoms  (or  thrice  this  depth)  off  the 
reefs  of  Mauritius  the  arming  was  marked  with  the 
distinct  impression  of  a  living  Astraea.  Millepora 
alcicornis  lives  in  from  0  to  12  fathoms,  and  the  genera 
Madrepora  and  Seriatopora  from  0  to  20  fathoms. 
Captain  Moresby  has  given  me  a  specimen  of  Sideropora 
scahra  (Porites  of  Lamarck)  brought  up  alive  from  17 
fathoms.  Mr.  Couthouy  ^  states  that  on  the  Bahama 
banks  he  dredged  up  considerable  masses  of  Mean- 
drina  from  16  fathoms,  and  has  seen  this  coral  growing 
in  20  fathoms. 

Captain  Beechey  informs  me  that  branches  of  pink 
and  yellow  coral  were  frequently  brought  up  from  be- 
tween 20  and  25  fathoms  off  the  Low  atolls ;  and  Lieut. 
Stokes,  writing  to  me  from  the  N.W.  coast  of  Australia, 
says  that  a  strongly  branched  coral  was  procured  there 
from  80  fathoms :  unfortunately  it  is  not  known  to  what 
genera  these  corals  belong. 

Although  the  limit  of  depth,  at  which  each  particular 
kind  of  coral  ceases  to  exist,  is  thus  far  from  being 
accurately  known :   yet  when  we  bear  in  mind  the 

'  Annales  des  Sci.  Nat.  torn.  vi. 

■  Bemarks  on  Coral  Formations,  p.  12. 


Sect.  III.      REEF-BUILDING   CORALS  LIVE.  115 

manner  in  which  the  clumps  of  coral  gradually  became 
infrequent  at  about  the  same  depth,  and  wholly  dis- 
appeared at  a  greater  depth  than  20  fathoms  on  the 
slope  round  Keeling  atoll,  off  the  reefs  in  the  Pacific 
(according  to  Dana),  on  the  leeward  side  of  the  Mauri- 
tius, and  at  rather  less  depth  both  within  and  without 
the  atolls  of  the  Maldiva  and  Chagos  Archipelagoes ; 
and  when  we  know  that  the  reefs  round  these  islands  do 
not  differ  from  other  coral  formations  in  their  form  and 
structure,  we  may,  I  think,  conclude  that  in  ordinary 
cases  reef-building  polypifers  do  not  flourish  at  greater 
depths  than  between  20  and  30  fathoms,  and  rarely  at 
above  15  fathoms.^ 

It  has  been  argued  * — that  reefs  may  possibly  rise 
from  very  great  depths  through  the  means  of  small 
corals  first  making  a  platform  for  the  growth  of  the 
stronger  kinds.  This,  however,  is  an  arbitrary  supposi- 
tion :  it  is  not  always  remembered,  that  in  such  cases 
there  is  an  antagonistic  power  at  work,  namely,  the 
decay  of  organic  bodies  when  not  protected  by  a  cover- 
ing of  sediment  or  by  their  own  rapid  growth.  We  have, 
moreover,  no  right  to  calculate  on  unlimited  time  for 

'  [The  general  conclusions  of  this  paragraph  do  not  appear  to  have 
been  disturbed , by  recent  researches,  though  Mr.  Guppy  (Proc.  Roy. 
Sec.  Edin.  xiii.  p.  857,  see  Appendix  II.)  argues  in  favour  of  the 
possibility  of  reefs  occasionally  beginning  to  grow  at  depths  of  at 
least  60  fathoms ;  and  in  the  Reports  of  the  Challenger  Expedition 
(Report  on  the  Reef -building  Corals,  p.  35)  cases  of  species  which 
build  reefs,  living  at  a  depth  of  40  fathoms,  and  in  two  instances  even 
at  70  fathoms,  are  recorded.  Still  even  here  it  is  admitted  that  '  the 
zone  of  most  active  growth  is  from  1  to  20  fathoms.'] 

«  Journal  of  the  Royal  Geograph.  Soc.  1831,  p.  218. 


116 


DEPTH   AT   WHICH 


Ch.  IV. 


the  accumulation  of  small  organic  bodies  into  great 
masses.'  Every  fact  in  geology  proclaims  that  neither 
the  dry  land  nor  the  bed  of  the  sea  retains  the  same  level 
for  indefinite  periods.  As  well  might  it  be  imagined 
that  the  British  seas  would  in  time  become  choked  up 
with  beds  of  oysters,  or  that  the  numerous  small  coral- 
lines off  the  inhospitable  shores  of  Tierra  del  Fuego 
would  in  time  form  a  solid  and  extensive  coral- 
reef.2 


'  [This  remark  has,  by  anticipation,  a  direct  bearing  on  an 
important  part  of  Mr.  Murray's  hypothesis.     See  Appendix  H.] 

■^  I  will  here  record  the  few  facts  which  I  have  been  able  to  collect 
as  to  the  depths,  both  within  and  without  the  tropics,  inhabited  by 


Depth 

COUXTRY  AND 

NAME  OF  ZOOPHYTB. 

ia 

fathoms. 

8.  LATITUDE. 

AUTTORITT. 

Sertularia       .... 

40 

Cape  Horn  66 

[\Vhere   none 

Cellaria 

ditto 

ditto 

is   given    the 

„    A  minute  scarlet  en- 

observation is 

crusting  species,  found 

my  own.] 

living. 

190 

Keeling  At.  12° 

„    An  allied,  small  stony 

sub -generic  form 

48 

S.CruzRiv.60° 

A  coral  allied  to  Vincularia, 

with  eight  rows  of  cells 

40 

Cape  Horn 

Tubulipora,  near  to  T.  patina 

ditto 

ditto 

Do.               do. 

94 

EastChiIoe43° 

Cellepora,  several  species  and 

allied  sub-generic  forms 

40 

Cape  Horn 

Ditto 

40 
and 
67 

Chonos  Arch. 
f  45° 

Ditto  .        ,        ,        .        . 

48 

S.  Cruz  50° 

Eschara.        *        •        •        . 

30 

J  Tierra  del 
1    Fuego  53° 

Ditto  ,        s        4        »        . 

48 

S.  Cruz  E.  50° 

Betepora        •        .        «        . 

40 

Cape  Horn 

QuoyandGai- 

Ditto 

100 

(C.  Good  Hope 
1     34° 

mard,   Ann. 
1    Scien.  Nat., 
[  t.  vi.  p.  284. 

Sect.  III.  REEF-BUILDING   CORALS   LIVE. 


117 


NAME  OF  ZOOPHYTE. 


Depth 

in 

fathoms. 


Millepora,  a  strong  coral  with^ 
cylindrical  branches,  of  a 
pink  colour,  about  two 
inches  high,  resembling  in 
the  form  of  its  orifices  M. 
aspera  of  Lamarck 


Coralium  •  •  • 
Antipathea  ,  ,  , 
Gorgonia  (or  an  allied  form) 


94 
and 
30 


120 
16 
160 


COUNTRY  AND 
S.  LATITUDE. 


E.  Chiloe  43° 
Tierra   del 
Euego  53° 


Barbary  33°  N 


Chonos  45° 

[Abrolhos,  on 
\  the  coast  of 
I    Brazil,  18' 


AUTHORITY. 


Peyssonel,  in 
paper  read  to 
Koyal  Society 
May,  1752. 

Capt.  Beechey 
informed  me 
of  this  fact 
in  a  letter. 


those  corals  and  corallines  which  we  have  no  reason  to  suppose  ever 
materially  aid  in  the  construction  of  a  reef.  Mr.  Stokes  also  showed 
me  a  Caryophyllia  which  was  dredged  up  alive  by  Captain  King 
from  a  depth  of  80  fathoms  off  Juan  Fernandez,  in  lat.  33°  S.  Ellis 
(Nat.  Hist,  of  Coralline,  p.  96)  states  that  Ombellularia  was  pro- 
cured in  lat.  79°  N.  sticking  to  a  line  from  the  depth  of  236  fathoms  ; 
hence  this  coral  either  must  have  been  floating  loose,  or  was  en- 
tangled in  a  stray  line  at  the  bottom.  Off  Keeling  atoll  a  compound 
Ascidia  (Sigillina)  was  brought  up  from  39  fathoms,  and  a  piece  of 
sponge,  apparently  living,  from  70,  and  a  fragment  of  Nullipora, 
also  apparently  living,  from  92  fathoms.  At  a  greater  depth  than 
90  fathoms  the  bottom  was  thickly  strewed  with  joints  of  a  Halimeda 
and  small  fragments  of  other  Nulliporae,  but  all  dead.  Captain  B. 
Allen,  R.N.,  informs  me  that  in  the  survey  of  the  West  Indies  it  was 
noticed,  that  between  the  depth  of  10  and  200  fathoms,  the  sounding- 
lead  very  generally  came  up  coated  with  the  dead  joints  of  a  Hali- 
meda, of  which  he  showed  me  specimens.  Off  Pernambuco,  in 
Brazil,  in  about  12  fathoms,  the  bottom  was  covered  with  fragments, 
dead  and  alive,  of  a  dull  red  Nullipora,  and  I  infer  from  Eoussin's 
chart,  that  a  bottom  of  this  kind  extends  over  a  wide  area.  On  the 
beach,  within  the  coral-reefs  of  Mauritius,  vast  quantities  sf  frag- 
ments of  NulliporaB  were  piled  up.  From  these  facts,  it  appears  that 
these  simply  organised  bodies,  belonging  to  the  vegetable  kingdom, 
are  amongst  the  most  abundant  productions  of  the  sea.  [Of  late  years 
corals,  commonly  solitary,  have  been  found  at  much  greater  depths 


118  DEPTH.  Ch.  IV. 

than  those  mentioned  in  this  note ;  for  instance,  Caryophyllia, 
down  to  at  least  1,000  fathoms  (The  Depths  of  the  Sea,  pp.  28,  431). 
True  corals  referable  to  the  Madreporaria  are  not  very  abundant 
in  deep  water.  According  to  Mr.  Moseley's  report,  about  ten  genera 
reach  a  depth  of  1,000  fathoms ;  four  genera  are  found  at  1,500 
fathoms  ;  and  a  single  species  extends  practically  through  all  depths, 
ranging  from  30  to  2,900  fathoms.  Challenger  Reports,  vol.  il 
(Z>ology),  'On  Corals,'  pp.  132,  133.  See  also  Reports,  vol.  xvi., 
•OnReef-CoraU.'p.  36. 


CHAPTER  Y. 

THEORY   OF   THE   FORMATION    OF    THE  DIFFERENT  CLASSES 
OF    CORAL-REEFS. 

The  atolls  of  the  larger-  archipelagoes  not  formed  on  submerged  craters, 
or  on  banks  of  sediment  -Immense  areas  interspersed  with  atolls 
— Their  subsidence— The  effects  of  storms  and  earthquakes  on 
atolls— Becent  changes  in  their  state — The  origin  of  barrier-reefs 
and  of  atolls — Their  relative  forms — The  step-formed  ledges  and 
walls  round  the  shores  of  some  lagoons — The  ring-formed  reefs  of 
the  Maldiva  atolls — The  submerged  condition  of  parts  or  of  the 
whole  of  some  annular  reefs—  The  disseverment  of  large  atolls — ■ 
The  union  of  atolls  by  linear  reefs — The  great  Chagos  Bank — 
Objections  considered  arising  from  the  area  and  amount  of  sub- 
sidence required  by  the  theory — The  probable  composition  of  the 
lower  parts  of  atolls. 

The  naturalists  who  have  visited  the  Pacific,  seem  to 
have  had  their  attention  riveted  by  the  lagoon-island  a 
or  atolls,— those  singular  rings  of  coral-land  which 
rise  abruptly  out  of  the  unfathomable  ocean, — and 
have  passed  over,  almost  unnoticed,  the  scarcely  less 
wonderful  encircling  barrier-reefs.  The  theory  most 
generally  received  on  the  formation  of  atolls,  is  that 
they  are  based  on  submarine  craters :  but  where  can 
we  find  a  crater  of  the  shape  of  Bow  atoll,  which  is  five 
times  as  long  as  it  is  broad  (Plate  I.,  fig.  4) ;  or  like 
that  of  Menchicoff  Island  (Plate  II.,  fig.  3),  with  ita 


120      THEORY  OF  THE  FORMATION     Ch.  V, 

three  loops,  together  sixty  miles  in  length ;  or  like 
EimskyKorsacoff,  narrow,  crooked,  and  j&fty-four  miles 
long ;  or  like  the  northern  Maldiva  atolls,  made  up  of 
numerous  ring-formed  reefs,  placed  on  the  margin  of 
a  disk, — one  of  which  disks  is  eighty-eight  miles  in 
length,  and  only  from  ten  to  twenty  in  breadth  ?  A 
further  difficulty  on  this  theory  of  the  origin  of  atolls 
arises  from  the  necessary  assumption  of  so  large  a 
number  of  immense  craters  crowded  together  beneath 
the  sea.  But,  as  we  shall  presently  see,  a  greater  diffi- 
culty is  involved,  namely,  that  all  these  craters  must 
lie  within  nearly  the  same  level  beneath  the  sea. 
Nevertheless,  if  the  rim  of  a  crater  afforded  a  basis 
at  the  proper  depth,  I  am  far  from  denying  that  a 
reef  like  a  perfectly  characterized  atoll  might  not  be 
formed  on  it.  Some  such,  perhaps,  now  exist;  but 
it  is  incredible  that  the  greater  number  could  have 
thus  originated. 

An  earlier  and  better  theory  was  proposed  by 
Chamisso :  ^  he  supposes  that  as  the  more  massive  kinds 
of  corals  prefer  the  surf,  the  outer  portions  of  a  reef  will 
first  reach  the  surface  and  consequently  form  a  ring.^ 
I  remarked  in  the  third  chapter  that  a  reef,  growing  on 
a  detached  bank,  would  tend  to  assume  an  atoll-Uke 
structure ;  if,  therefore,  corals  were  to  grow  up  from  a 
bank  some  fathoms  submerged  in  a  deep  sea,  having 
steep  sides  and  a  level  surface,  a  reef  not  to  be  dis- 

•  Kotzebue's  First  Voyage,  vol.  iii.  p.  331. 

*  [By  anticipation,  some  of  the  objections  which  have  been  raised 
of  late  years  (see  Appendix  II.)  are  considered  in  this  section.] 


Ch.  V.  OF   CORAL-REEFS.  121 

tinguished  from  an  atoll  might  be  formed  ;  and  I 
believe  some  such  exist  in  the  West  Indies.  But  on 
this  view  it  must  be  assumed,  that  in  every  case  the 
basis  consists  of  a  flat  bank ;  for  if  it  were  conically 
formed  like  a  mountainous  mass,  we  can  see  no  reason 
why  the  corals  should  spring  up  from  the  flanks  instead 
of  from  the  central  and  highest  parts.  As  the  lagoons 
of  atolls  are  sometimes'  even  more  than  40  fathoms 
deep,  it  must,  also,  be  assumed  on  this  view,  that  at  a 
depth  at  which  the  waves  do  not  break,  the  coral  grows 
more  vigorously  on  the  edges  of  a  bank  than  on  its 
central  part :  and  this  is  an  assumption  without  any 
evidence.*  If  we  consider,  moreover,  the  number  of 
the  atolls  in  the  midst  of  the  Pacific  and  Indian 
Oceans,  this  assumption  of  so  many  submerged  banks 
is  in  itself  very  improbable. 

No  theory  worthy  of  notice  has  been  advanced  to 
account  for  those  barrier-reefs  which  encircle  islands 
of  moderate  dimensions.  The  great. reef  which  fronts 
the  coast  of  Australia  has  been  supposed,  but  without 
any  evidence,  to  rest  on  the  edge  of  a  submarine 
precipice  parallel  to  the  shore.  The  origin  of  the 
third  class,  or  of  fringing-reefs,  presents,  I  believe, 
scarcely  any  difficulty,  and  arises  simply  from  the  poly- 
pifers  growing  in  moderate  depths,  and  not  flourishing 
close  to  gently  shelving  beaches  where  the  water  is 
often  turbid. 

*  [The  more  vigorous  growth  of  the  coral  on  the  outward  part  of  a 
reef  is,  however,  asserted  and  advocated  by  Mr.  Murray  as  producing 
the  atoll  form.    See  Appendix  II.] 


122       THEORY  OF  THE  FORMATION     Ch.  V. 

What  cause,  then,  has  given  to  atolls  and  barrier- 
reefs  their  characteristic  forms  ?  Let  us  see  whether 
an  important  deduction  does  not  follow  from  the 
following  facts, — first,  that  reef-building  corals  only 
flourish  at  a  very  limited  depth, — and  secondly,  that 
throughout  areas  of  vast  dimensions,  none  of  the 
coral-reefs  and  coral-islets  rise  to  a  greater  height 
above  the  level  of  the  sea  than  that  attained  by  matter 
thrown  up  by  the  waves  and  winds.  I  do  not  make 
this  latter  statement  vaguely ;  I  have  carefully  sought 
for  descriptions  of  every  island  in  the  inter-tropical 
eeas ;  and  my  task  has  been  in  some  degree  facili- 
tated by  a  map  of  the  Pacific,  corrected  in  1834  by 
MM.  D'Urville  and  Lottin,  in  which  the  low  islands 
are  distinguished  from  the  high  ones  (even  from  those 
much  less  than  a  hundred  feet  in  height)  by  being 
written  without  a  capital  letter.^  I  have  also  ascer- 
tained, chiefly  from  the  writings  of  Cook,  Kotzebue, 
Bellingshausen,  Duperrey,  Beechey,  and  Lutke  regard- 
ing the  Pacific ;  and  from  Moresby  ^  with  respect  to  the 
Indian  Ocean,  that  in  the  following  cases  the  term 
*  low  island  '  strictly  means  land  of  the  height  com- 

'  I  have  detected  a  few  errors  in  this  map,  respecting  the  heights 
of  some  of  the  islands,  which  will  be  noticed  in  the  Appendix,  where 
I  treat  of  coral-formations  in  geographical  order.  To  the  Appendix, 
also,  I  must  refer  for  a  more  particular  account  of  the  data  on  which 
the  following  statements  are  grounded. 

"^  See  also  Captain  Owen's  and  Lieut.  Wood's  papers  in  the  Geo- 
graphical Journal  on  the  Maldiva  and  Laccadive  Archipelagoes. 
These  officers  particularly  refer  to  the  lowness  of  the  islets ;  but  I 
chiefly  ground  my  assertion  respecting  these  two  groups,  and  the 
Chagos  group,  from  information  communicated  to  me  by  Captain 
Moresby. 


Ch.  V.  OF   CORAL-REEFS.  123 

monly  attained  by  matter  thrown  up  by  the  winds 
and  the  waves  of  an  open  sea.  If  we  draw  a  Une 
joining  the  external  atolls  of  that  part  of  the  Low 
Archipelago  in  which  the  islands  are  numerous — the 
plan  always  adopted — the  figure  will  be  a  pointed 
ellipse  (reaching  from  Hood  to  Lazaref  Island),  of 
which  the  longer  axis  is  840  geographical  miles,  and 
the  shorter  420  miles :  in  this  space, ^  none  of  the 
innumerable  islets,  united  into  great  rings,  rise  above 
the  stated  level.  The  Gilbert  group  is  very  narrow, 
and  300  miles  in  length.  In  a  prolonged  line  from 
this  group,  at  the  distance  of  240  miles,  is  the 
Marshall  Archipelago,  the  figure  of  which  is  an 
irregular  square,  one  end  being  broader  than  the 
other ;  its  length  is  520  miles  with  an  average  width 
of  240  :  these  two  groups  together  are  1,040  miles  in 
length,  and  all  their  islets  are  low.  Between  the 
southern  end  of  the  Gilbert  and  the  northern  end  of 
Low  Archipelago,  the  ocean  is  thinly  strewed  with 
islands,  all  of  which,  as  far  as  I  have  been  able  to 
ascertain,  are  low :  so  that  from  nearly  the  southern 
end  of  the  Low  Archipelago,  to  the  northern  end  of 
the  Marshall  Archipelago  there  is  a  narrow  band  of 
ocean  more  than  4,000  miles  in  length,  containing  a 

•  Metia  or  Aurora  Island  has  been  upraised;  but  it  lies  N.E.  of 
Tahiti,  and  in  the  map  appended  to  this  volume  is  close  without 
the  line  bounding  the  space  here  referred  to.  I  shall  have  occasion 
hereafter  to  make  some  remarks  on  the  supposed  slight  elevation  (of 
about  three  feet)  of  the  atolls  of  the  Low  Archipelago,  subsequently 
to  their  original  formation.  [Other  cases  of  upheaval  have  since  been 
recorded.     See  Appendix  II.] 


124       THEORY  OF  THE  FORMATION     Cii.  V. 

vast  number  of  islands,  all  of  which  are  low.  In  the 
western  part  of  the  Caroline  Archipelago,  there  is  a 
space  of  480  miles  in  length,  and  about  100  in  breadth, 
thinly  interspersed  with  low  islands.  Lastly,  in  the 
Indian  Ocean,  the  archipelago  of  the  Maldivas  is  470 
niiltrs  in  length,  and  60  in  breadth  ;  that  of  the  Lac- 
cadives  is  150  by  100  miles  :  as  there  is  a  low  island 
between  these  two  groups,  they  may  be  considered  as 
one  group  of  a  thousand  miles  in  length.  To  this 
may  be  added  the  Chagos  group  of  low  islands, 
situated  280  miles  distant,  in  a  line  prolonged  from 
the  southern  extremity  of  the  Maldivas.  This  group, 
including  the  submerged  banks,  is  170  miles  in  length 
and  80  in  breadth.  So  striking  is  the  uniformity  in 
direction  of  these  three  archipelagoes,  all  the  islands 
of  which  are  low,  that  Captain  Moresby,  in  one  of  his 
papers,  speaks  of  them  as  parts  of  one  great  chain 
nearly  1,500  miles  long.  I  am,  then,  fully  justified 
in  repeating  that  immense  spaces,  both  in  the 
Pacific  and  Indian  Oceans,  are  interspersed  with 
islands,  of  which  none  rise  above  the  height  to  which 
the  waves  and  winds  in  an  open  sea  can  heap  up 
matter. 

On  what  foundations,  then,  have  these  reefs  and 
islets  of  coral  been  constructed  ?  A  foundation  must 
originally  have  been  present  beneath  each  atoll,  at 
that  limited  depth  which  is  indispensable  for  the 
first  growth  of  the  reef-building  polypifers.  A  con- 
jecture will  perhaps  be  hazarded,  that  the  requisite 
bases  may  have  been  afforded  by  the  accumulation  of 


N   <i   v-^ 


Ch.  V.  OF   CORAL-REEFS.  1^5 

great  banks  of  sediment,  which  did  not  quite  reach 
the  surface  owing  to  the  action  of  superficial  currents, 
aided  possibly  by  the  undulatory  movement  of  Iho 
sea.  This  appears  actually  to  have  been  the  case  in 
some  parts  of  the  West  Indian  sea.  But  in  the  form 
and  disposition  of  the  groups  of  atolls,  there  is  nothing 
to  countenance  this  notion  ;  and  the  assumption  that 
a  number  of  immense  piles  of  sediment  have  been 
heaped  on  the  floor  of  the  great  Pacific  and  Indian 
Oceans  in  their  central  parts,  far  remote  from  land, 
where  the  dark  blue  colour  of  the  limpid  water 
bespeaks  its  purity,  cannot  for  one  moment  be 
admitted.^ 

The  many  widely  scattered  atolls  must,  therefore, 
rest  on  rocky  bases.  But  we  cannot  believe  that  a 
broad  mountain  summit  lies  buried  at  the  depth  of 
a  few  fathoms  beneath  every  atoll,  and  nevertheless 
that  throughout  the  immense  areas  above-named,  not 
one  point  of  rock  projects  above  the  level  of  the  sea. 
For  we  may  judge  of  mountains  beneath  the  sea  by 
those  on  the  land ;  and  where  can  we  find  a  single 
chain,  much  less  several  such  chains,  many  hundred 

•  [This  accumulation,  it  will  be  observed,  is  an  integral  part  of  Mr. 
Murray's  hypothesis.  See  Appendix  II.  for  a  sketch  of  the  arguments 
by  which  it  is  supported.  Perhaps  I  may  be  permitted  to  add  that, 
in  my  opinion,  the  perusal  of  the  observations  of  Mr.  Murray,  Mr. 
Cruppy,  and  others  would  probably  have  led  Mr.  Darwin  to  modify 
slightly  some  of  the  clauses  in  these  pages,  and  allow  a  more  important 
rdle  to  the  accumulation  of  organisms,  other  than  corals,  on  submarine 
banks.  I  do  not,  however,  anticipate  that  they  would  have  seriously 
modified  his  general  conclusions,  or  led  him  to  regard  modes  of 
formation,  which  these  authors  consider,  to  he  normal,  as  other  than 
exceptional.— T.  G.  B.] 

10 


i 


126       THEORY  OF  THE  FORMATION     Ch.  V. 

miles  in  length  and  of  considerable  breadth,  with 
broad  summits  attaining  the  same  height,  from  within 
120  to  180  feet  ?  Even  if  it  be  assumed  without  any 
evidence  that  the  reef-building  corals  can  flourish  at  a 
depth  of  100  fathoms,  yet  the  weight  of  the  above  argu- 
ment is  but  little  diminished;  for  it  is  almost  equally  im- 
probable, that  as  many  submarine  mountains,  as  there 
are  low  islands  in  the  several  great  and  widely-separated 
areas  above-specified,  should  all  rise  within  600  feet  of 
the  surface  of  the  sea  and  not  one  above  it,  as  that  they 
should  be  of  the  same  height  within  the  smaller  hmit 
of  one  or  two  hundred  feet.  So  highly  improbable  is 
this  supposition,  that  we  are  compelled  to  believe,  that 
the  rocky  foundations  of  the  many  atolls  did  never  at 
any  one  period  all  lie  submerged  within  the  depth  of  a 
few  fathoms  beneath  the  surface,  but  that  they  were 
brought  into  the  requisite  position  or  level,  some  at  one 
period  and  some  at  another,  through  movements  in  the 
earth's  crust.  But  this  could  not  have  been  effected 
by  elevation ;  for  the  belief  that  points  so  numerous  and 
so  widely-separated  were  successively  uplifted  to  a  cer- 
tain level,  but  that  not  one  point  was  raised  above  that 
level,  is  quite  as  improbable  as  the  former  supposition, 
and  indeed  differs  little  from  it.  It  will  probably  occur 
to  those  who  have  read  Ehrenberg's  account  of  the  reefs 
of  the  Bed  Sea,  that  many  points  in  these  great  areas 
may  have  been  elevated,  but  that  as  soon  as  raised,  the 
protuberant  parts  were  cut  off  by  the  destroying  action 
of  the  waves  :  a  moment's  reflection,  however,  on  the 
basin-like  form  of  the  .atoll^  will  show  that  this  is 


Ch.  V.  OF   CORAL-REEFS.  127 

impossible ;  for  the  upheaval  and  subsequent  abrasion 
of  an  island  would  leave  a  flat  disk,  which  might  become 
coated  with  coral,  but  not  a  deeply  concave  surface ; 
moreover,  we  should  expect  to  see,  at  least  in  some 
parts,  the  rock  of  the  foundation  brought  to  the  surface. 
If,  then,  the  foundations  of  the  many  atolls  were  not 
uplifted  into  the  requisite  position,  they  must  of  neces- 
sity have  subsided  into  it ;  and  this  at  once  solves  every 
difficulty,'  for  we  may  safely  infer  from  the  facts  given 
in  the  last  chapter,  that  during  a  gradual  subsidence 
the  corals  would  be  favourably  circumstanced  for  build- 
ing up  their  solid  frameworks  and  reaching  the  surface, 
as  island  after  island  slowly  disappeared.  Thus  areas 
of  immense  extent  in  the  central  and  most  profound 

'  The  additional  difficulty  on  the  crater  hypothesis  before  alluded 
to,  will  now  be  evident ;  for  on  this  view  the  volcanic  action  must 
be  supposed  to  have  formed  within  the  areas  specified  a  vast  number 
of  craters,  all  rising  within  a  few  fathoms  of  the  surface,  and  not  one 
above  it.  The  supposition  that  the  craters  were  at  different  times 
upraised  above  the  surface,  and  were  there  abraded  by  the  surf  and 
subsequently  coated  by  corals,  is  subjected  to  nearly  the  same  ob- 
jections with  those  given  at  the  top  of  the  page;  but  I  consider 
it  superfluous  to  detail  all  the  arguments  opposed  to  such  a  notion. 
Chamisso's  theory,  from  assuming  the  existence  of  so  many  banks, 
all  lying  at  the  proper  depth  beneath  the  water,  is  also  vitally  de- 
fective. The  same  observation  applies  to  an  hypothesis  of  Lieut. 
Nelson's  (Geolog.  Trans,  vol.  v.  p.  122),  who  supposes  that  the  ring- 
formed  structure  is  caused  by  a  greater  number  of  germs  of  corals 
becoming  attached  to  the  declivity,  than  to  the  central  plateau  of  a 
submarine  bank ;  it  likewise  applies  to  the  notion  formerly  enter- 
tained (Forster's  Observ.  p.  151),  that  lagoon-islands  owe  their  pecu- 
liar form  to  the  instinctive  tendencies  of  the  polypifers.  According 
to  this  latter  view,  the  corals  on  the  outer  margin  of  the  reef  in- 
stinctively oppose  themselves  to  the  surf  in  order  to  afford  protection 
to  corals  living  in  the  lagoon  which  belong  to  other  genera  and  to 
other  families  1 


128       THEORY  OF  THE  FORMATION     Ch.  V. 

parts  of  the  great  oceans  might  become  interspersed 
with  coral-islets,  none  of  which  would  rise  to  a  greater 
height  than  that  attained  by  detritus  heaped  up  by  the 
sea,  and  nevertheless  they  might  all  have  been  formed 
by  corals,  which  absolutely  require  for  their  growth  a 
solid  foundation  within  a  few  fathoms  of  the  surface. 

It  would  be  out  of  place  here  to  do  more  than 
allude  to  the  many  facts,  showing  that  the  supposition 
of  a  gradual  subsidence  over  large  areas  is  by  no  means 
improbable.  We  have  the  clearest  proof  that  a  move- 
ment of  this  kind  is  possible,  in  the  upright  trees 
buried  under  strata  many  thousand  feet  in  thickness ; 
we  have  also  every  reason  for  believing  that  there  are 
now  large  areas  gradually  sinking,  in  the  same  manner 
as  others  are  rising.  And  when  we  consider  how  many 
parts  of  the  surface  of  the  globe  have  been  elevated 
within  recent  geological  periods,  we  must  admit  that 
there  have  been  subsidences  on  a  corresponding  scale, 
for  otherwise  the  whole  globe  would  have  swollen.  It 
is  very  remarkable  that  Sir  C.  Lyell,'  even  in  the  first 
edition  of  his  Principles  of  Geology,  inferred  that  the 
amount  of  subsidence  in  the  Pacific  must  have  exceeded 
that  of  elevation,  from  the  area  of  land  being  very 
small  relatively  to  the  agents  there  tending  to  form  it, 
namely,  the  growth  of  coral  and  volcanic  action.  But, 
although  subsidence  may  explain  a  phenomenon  other- 
wise inexplicable,  it  may  be  asked,  are  there  any  direct 
proofs  of  a  subsiding  movement  in  these  areas  ?    This, 

*  Principles  of  Geology,  sixth  edition,  vol.  iii.  p.  386.  [Ch.  xlix. 
ToU  ii.  p.  604,  eleventh  edition.] 


Ch.  V.  OF   CORAL-REEFS.  129 

however,  can  hardly  be  expected,  for  it  must  ever  be 
most  difficult,  excepting  in  countries  long  civilized,  to 
detect  a  movement  the  tendency  of  which  is  to  conceal 
the  part  affected.  In  barbarous  and  semi- civilized 
nations  how  long  might  not  a  slow  movement,  even  of 
elevation  such  as  that  now  affecting  Scandinavia,  have 
escaped  attention ! 

Mr.  Williams^  insists  strongly  that  the  traditions  of 
the  natives,  which  he  has  taken  much  pains  in  collect- 
ing, do  not  indicate  the  appearance  of  any  new  islands : 
but  on  the  theory  of  a  gradual  subsidence,  all  that  would 
be  apparent  would  be,  the  water  sometimes  encroaching 
slowly  on  the  land,  and  the  land  again  recovering  by 
the  accumulation  of  detritus  its  former  extent,  and 
perhaps  sometimes  the  conversion  of  an  atoll  with  coral 
islets  on  it,  into  a  bare  or  into  a  sunken  annular  reef. 
Such  changes  would  naturally  take  place  at  the  periods 
when  the  sea  rose  above  its  usual  limits  during  a  gale 
of  more  than  ordinary  strength ;  and  the  effects  of  the 
two  causes  would  be  hardly  distinguishable.  In  Kotze- 
bue's  Voyage  there  are  accounts  of  islands,  both  in  the 
Caroline  and  Marshall  Archipelagoes,  which  have  been 
partly  washed  away  during  hurricanes ;  and  Kadu,  the 
native  who  was  on  board  one  of  the  Eussian  vessels, 
said  *  he  saw  the  sea  at  Kadack  rise  to  the  feet  of  the 
cocoa-nut  trees ;  but  it  was  conjured  in  time.'  ^  A  storm 
lately  entirely  swept  away  two  of  the  CaroHne  Islands 
and  converted  them  into  shoals ;  it  also  partly  destroyed 

*  Williams's  Narrative  of  Missionary  Enterprise,  p.  31. 
'  Kotzebue's  First  Voyage,  vol.  iii.  p.  168. 


130       THEORY  OF  THE  FORMATION     Ch.  V. 

two  other  islands.*  According  to  a  tradition  which  was 
communicated  to  Captain  FitzEoy,  it  is  believed  in  the 
Low  Archipelago  that  the  arrival  of  the  first  ship  caused 
a  great  inundation  which  destroyed  many  lives.  Mr. 
Stutchbury  relates  that  in  1825,  the  western  side  of 
Chain  Atoll  in  the  same  group,  was  completely  de- 
vastated by  a  hurricane,  and  not  less  than  300  lives 
lost :  *  in  this  instance  it  was  evident,  even  to  the 
natives,  that  the  hurricane  alone  was  not  sufficient  to 
account  for  the  violent  agitation  of  the  ocean.'  ^  That 
considerable  changes  have  taken  place  recently  in  some 
of  the  atolls  in  the  Low  Archipelago,  appears  certain 
from  the  case  of  Matilda  Island  given  in  the  last  chapter. 
With  respect  to  Whitsunday  and  Gloucester  Islands  in 
this  same  group,  we  must  either  attribute  great  inac- 
curacy to  their  discoverer,  the  famous  circumnavigator 
Wallis,  or  believe  that  they  have  undergone  a  consider- 
able change  in  the  period  of  fifty-nine  years  between  his 
voyage  and  that  of  Captain  Beechey.  Whitsunday 
Island  is  described  by  Wallis  as  '  about  four  miles  long, 
and  three  wide,*  now  it  is  only  one  mile  and  a-half 
long.  The  appearance  of  Gloucester  Island,  in  Captain 
Beechey's  words,^  *  has  been  accurately  described  by  its 
discoverer,  but  its  present  form  and  extent  differ  mate- 
rially.' Blenheim  reef  in  the  Chagos  group,  consists  of  a 
water-washed  annular  reef  thirteen  miles  in  circum  Ter- 
ence, surrounding  a  lagoon  ten  fathoms  deep ;  on  its  sur- 

'  M.  Desmoulins  in  Comptes  Rendus,  1840,  p.  837. 
«  West  of  England  Journal,  No.  1,  p  35. 

■  Beechey's  Voyage  to  the  Pacific,  chap,  vii.,  and  Wallia's  Voyage 
In  the  Dolphin,  chap.  iv. 


Ch.  V.  OF   CORAL-REEFS.  131 

face  there  are  a  few  worn  patches  of  conglomerate  coral- 
rock  of  about  the  size  of  hovels ;  and  these  Captain 
Moresby  considers  as  being,  without  doubt,  the  last 
remnants  of  islets ;  so  that  here  an  atoll  has  been  con- 
verted into  an  atoll-formed  reef.  The  inhabitants  of 
the  Maldiva  Archipelago,  as  long  ago  as  1605,  declared, 
*  that  the  high  tides  and  violent  currents  were  always 
diminishing  the  number  of  the  islands  : '  ^  and  I  have 
already  shown,  on  the  authority  of  Captain  Moresby,  that 
the  work  of  destruction  is  still  in  progress ;  but  that  on 
the  other  hand  the  first  formation  of  some  islets  is 
known  to  the  present  inhabitants.  In  such  cases,  it 
would  be  exceedingly  difficult  to  detect  a  gradual  sub- 
sidence of  the  foundation  on  which  these  mutable 
structures  rest. 

Some  of  the  archipelagoes  of  low  coral-islands  are 
subject  to  earthquakes :  Captain  Moresby  informs  me 
that  they  are  frequent,  though  not  very  strong,  in  the 
Chagos  group,  which  occupies  a  central  position  in  the 
Indian  Ocean,  and  is  far  from  any  land  not  of  coral 
formation.  One  of  the  islands  in  this  group  was 
formerly  covered  by  a  bed  of  mould,  which  disap- 
peared after  an  earthquake,  and  was  believed  by  the 
residents  to  have  been  washed  by  the  rain  into  the 
underlying  fractured  rock :  the  island  was  thus  ren- 
dered unproductive.     Chamisso^  states  that  earth- 

•  See  an  extract  from  Pyrard's  Voyage  in  Captain  Owen's  paper 
on  the  Maldiva  Archipelago,  in  the  Geographical  Journal,  vol.  ii. 
p.  84. 

*  See  Chamisso,  in  Kotzebue's  First  Voyage,  vol.  iii.  pp.  132  and 
136. 


132      THEORY  OF   THE  FORMATION     Ch.  V. 

quakes  are  felt  in  the  Marshall  atolls,  which  are  far 
from  any  high  land,  and  likewise  in  the  islands  of 
the  Caroline  Archipelago.  On  Oulleay  atoll,  one  of 
the  latter,  Admiral  Lutke  informs  me  that  he  ob- 
served several  straight  fissures  about  a  foot  in  width, 
running  for  some  hundred  yards  obliquely  across  the 
whole  width  of  the  reef.  Fissures  indicate  a  stretching 
of  the  earth's  crust,  and,  therefore,  probably  changes 
in  its  level ;  but  these  coral-islands,  which  have  been 
shaken  and  fissured,  certainly  have  not  been  elevated, 
and,  therefore,  probably  have  subsided.^  We  shall 
hereafter  see  that  the  position  of  certain  ancient  build- 
ings in  the  Caroline  Archipelago  clearly  indicates  recent 
subsidence.  In  the  chapter  on  Keeling  atoll,  I  have 
also  attempted  to  show,  by  direct  evidence,  that  the 
island  subsided  during  the  earthquakes  lately  felt  there. 
The  facts  then  stand  as  follows  ; — there  are  many 
large  spaces  of  ocean,  without  any  high  land,  inter- 
spersed with  reefs  and  islets  formed  by  the  growth 
of  those  kinds  of  coral  which  cannot  live  at  great 
depths;  and  the  existence  of  these  reefs  and  low 
islets  in  such  numbers  and  at  such  distant  points, 
is  inexplicable,  excepting  on  the  theory  that  their 
rocky  bases  slowly  and  successively  sank  beneath  the 
level  of  the  sea,  whilst  the  corals  continued  to  grow 
upwards.  No  positive  facts  are  opposed  to  this  view, 
and  some  direct  evidence,  as  well  as  general  considera- 
tions, render  it  probable.     There  is  also  evidence  of 

'  [It  seems  to  me  doubtful  whether  the  argument  from  the  ex- 
istence of  fissures  can  be  pressed.— T.  G.  B.] 


Ch.  V. 


OF    COEAL-REEFS. 


133 


change  in  form,  whether  or  not  from  subsidence,  on 
some  of  these  coral-islands  ;  and  there  is  evidence  of 
subterranean  disturbances  beneath  them.  Will  then 
the  theory,  to  which  we  have  thus  been  led,  solve  the 
curious  problem — what  has  given  to  each  class  of  reef 
its  peculiar  form  ? 

Let  us  in  imagination  place  within  a  subsiding  area, 


No.  5. 


A  A — Outer  edge  of  the  reef  at  the  level  of  the  sea. 

BB— Shores  of  the  island. 

A'A' — Outer  edge  of  the  reef,  after  its  upward  growth  during  a  period 

of  subsidence. 
C  C — The  lagoon-channel  between  the  reef  and  the  shores  of  the  now 

encircled  land. 
B'B' — The  shores  of  the  encircled  island. 

N.B. — In  this,  and  the  following  woodcut,  the  subsidence  of  the 
land  could  only  be  represented  by  an  apparent  rise  in  the  level  of 
the  sea. 


an  island  surrounded  by  a  *  fringing-r^ef ' — that  kind 
of  which  the  origin  alone  offers  no  difficulty.  Let  the 
unbroken  lines  in  the  woodcut  (No.  5)  represent  a 
vertical  section  through  the  land  and  water ;  and  the 
horizontal  shading  a  section  through  the  reef.  Now,  as 
the  island  sinks  down,  either  a  few  feet  at  a  time  or  quite 


134       THEORY  OF  THE  FORMATION     Ch.  V. 

insensibly,  we  may  infer,  from  what  we  know  of  the 
conditions  favourable  to  the  growth  of  coral,  that  the 
living  masses  bathed  by  the  surf  on  the  margin  of  the 
reef,  will  soon  regain  the  surface.  The  water,  however, 
will  encroach  little  by  little  on  the  shore,  the  island 
becoming  lower  and  smaller,  and  the  space  between  the 
edge  of  the  reef  and  the  beach  proportionally  broader. 
A  section  of  the  reef  and  island  in  this  state,  after  a 
subsidence  of  several  hundred  feet,  is  given  by  the 
dotted  lines :  coral-islets  are  supposed  to  have  been 
formed  on  the  new  reef,  and  a  ship  is  anchored  in 
the  lagoon-channel.  This  section  is  in  every  respect 
that  of  an  encircling  barrier-reef,  and  is,  in  fact, 
taken  E.  and  W.  through  the  highest  point  of  the 
encircled  island  of  Bolabola,*  of  which  a  plan  is  given 
in  Plate  I.,  fig.  5.  The  same  section  is  more  clearly 
shown  in  the  following  woodcut  (No.  6)  by  the  im- 
broken  lines.  The  width  of  the  reef  and  its  slope 
both  on  the  outer  and  inner  side,  will  have  been 
determined  by  the  growing  powers  of  the  coral,  under 
different  conditions,  for  instance,  of  the  force  of  the 
breakers  and  currents  to  which  it  has  been  exposed  ; 
and  the  lagoon-channel  will  be  deeper  or  shallower,  in 
proportion  to  the  growth  of  the  delicately  branched 
corals  within  the  reef,  and  to  the  accumulation  of 
sediment ;    relatively,  also,    to   the   rate    of    subsi- 

'  The  section  has  been  made  from  the  chart  given  in  the  Atlas 
of  the  Voyage  of  the  Coquille.  The  scale  is  -57  of  an  inch  to  a  mile. 
The  height  of  the  island,  according  to  M.  Lesson,  is  4,026  feet.  The 
deepest  part  of  the  lagoon-channel  is  162  feet ;  its  depth  is  exag- 
gerated in  the  woodcut  for  the  sake  of  clearness. 


Ch.  V.  OF    CORAL-REEFS.  135 

dence  and  the  length  of  the  intervening  stationary 
periods. 

It  is  evident  in  this  section,  that  a  hne  drawn  per- 
pendicularly down  from  the  outer  edge  of  the  new  reef 
to  the  foundation  of  solid  rock,  exceeds,  by  as  many 
feet  as  there  have  been  feet  of  subsidence,  that  small 
limit  of  depth  at  which  the  effective  polypifers  can 
live,— the  corals  having  grown  up,  as  the  whole  sank 
down,  from  a  basis  formed  of  other  corals  and  their  con- 
solidated fragments.  Thus  the  difficulty  on  this  head, 
which  before  seemed  so  great,  disappears. 

As  the  space  between  the  reef  and  the  subsiding 
shore  continued  to  increase  in  breadth  and  depth,  and 
as  the  injurious  effects  of  the  sediment  and  fresh  water 
borne  down  from  the  land  were  consequently  lessened, 
the  greater  number  of  the  channels  with  which  the  reef 
in  its  fringing  state  must  have  been  breached,  especially 
those  which  fronted  the  smaller  streams,  will  have 
become  choked  up  by  the  growth  of  coral :  on  the  wind- 
ward side  of  the  reef  where  the  coral  grows  most 
vigorously,  the  breaches  will  probably  have  first  been 
closed.  In  barrier-reefs,  therefore,  the  breaches  kept 
open  by  draining  the  tidal  waters  of  the  lagoon-channel, 
will  generally  be  placed  on  the  leeward  side,  and  they 
will  still  face  the  mouths  of  the  larger  streams,  although 
removed  beyond  the  influence  of  their  sediment  and 
fresh  water ; — and  this,  it  has  been  shown,  is  commonly 
the  case. 

Keferring  to  the  following  diagram  (No.  6),  in 
which  the  newly-formed  barrier-reef  is  represented  by 


136       THEORY  OF  THE  FORMATION     Ch.  V. 

unbroken  lines,  instead  of  by  dots  as  in  the  former  wood- 
cut, let  the  work  of  subsidence  go  on,  and  the  doubly- 
pointed  hill  will  form  two  small  islands  included  within 
one  annular  reef.  Let  the  island  continue  to  subside, 
and  the  coral-reef  will  continue  growing  up  on  its  own 
foundation,  whilst  the  water  gains  inch  by  inch  on  the 
land,  until  the  last  and  highest  pinnacle  is  covered,  and 
there  remains  a  perfect  atoll.   A  vertical  section  of  this 

No.  6. 


A'A' — Outer  edges  of  the  barrier-reef  at  the  level  of  the  sea.  The 
cocoa-nut  trees  represent  coral-islets  formed  on  the  reef. 

C  C— The  lagoon-channel. 

B'B'— The  shores  of  the  island,  generally  formed  of  low  alluvial  land 
and  of  coral  detritus  from  the  lagoon-channel. 

A'A" — The  outer  edges  of  the  reef,  now  forming  an  atoll. 

C — The  lagoon  of  the  newly-formed  atoll.  According  to  the  scale 
the  depth  of  the  lagoon  and  of  the  lagoon-channel  is  exaggerated. 

atoll  is  shown  in  the  woodcut  by  the  dotted  lines  ; — a 
ship  is  anchored  in  its  lagoon,  but  islets  are  not  supposed 
yet  to  have  been  formed  on  the  reef.  The  depth  of  the 
lagoon  and  the  width  and  slope  of  the  reef,  will  depend 
on  the  different  circumstances  to  which  it  has  been 
exposed,  as  just  stated  with  respect  to  barrier -reefs. 
Any  further  subsidence  will  produce  no  change  in  the 


Ch.  V.  OF   CORAL-REEFS.  137 

atoll,  except  a  diminution  in  its  size,  from  the  reef  not 
growing  vertically  upwards.  I  may  here  observe,  that 
a  bank  either  of  rock  or  of  hardened  sediment,  level 
with  the  surface  of  the  sea  and  fringed  with  living 
coral,  would  be  immediately  converted  by  subsidence 
into  an  atoll,  without  passing,  as  in  the  case  of  a  reef 
fringing  the  shore  of  an  island,  through  the  inter- 
mediate form  of  a  barrier-reef.  As  before  remarked,  if 
such  a  bank  lay  a  few  fathoms  submerged,  the  simple 
growth  of  the  coral,  without  the  aid  of  subsidence,  would 
produce  a  structure  scarcely  to  be  distinguished  from  a 
true  atoll;  for  the  corals  on  the  outer  margin,  from  being 
freely  exposed  to  the  open  sea,  would  grow  vigorously 
and  tend  to  form  a  continuous  ring,  whilst  the  growth 
of  the  less  massive  kinds  on  the  central  expanse,  would 
be  checked  by  the  sediment  formed  there,  and  by  that 
washed  inwards  by  the  breakers  ;  and  as  the  space  be- 
came shallower,  their  growth  would  also  be  checked  by 
the  impurities  of  the  water,  and  probably  by  the  small 
amount  of  food  brought  to  them  by  the  enfeebled  cur- 
rents. The  subsidence  of  a  reef  based  on  a  bank  of  this 
kind,  would  give  depth  to  the  central  expanse  or  lagoon, 
steepness  to  the  flanks,  and  through  the  free  growth  of 
the  coral,  symmetry  to  the  whole  outline ;  but,  as  we 
have  seen,  the  larger  groups  of  atolls  in  the  Pacific  and 
Indian  Oceans  cannot  have  been  formed  on  banks  of 
this  nature. 

If,  instead  of  an  island,  as  in  the  diagram,  the  shore 
of  a  continent  fringed  by  a  reef  were  to  subside,  a  great 
barrier-reef  like  that  on  the  N.E.  coast  of  Australia, 


138       THEORY  OF  THE  FORMATION     Ch.  V. 

would  be  the  necessary  result ;  and  it  would  be  sepa- 
rated from  the  main  land  by  a  deep-water  channel, 
broad  in  proportion  to  the  amount  of  subsidence,  and 
to  the  less  or  greater  inclination  of  the  bed  of  the  sea. 
The  effect  of  the  continued  subsidence  of  a  barrier- 
reef,  and  its  probable  conversion  into  a  chain  of 
separate  atolls,  will  be  considered  when  we  discuss  the 
progressive  disseverment  of  the  larger  Maldiva  atolls. 
We  now  are  able  to  perceive  that  the  close  similarity 
in  form,  dimensions,  structure,  and  relative  position 
between  fringing  and  encircling  barrier-reefs,  and  be- 
tween these  latter  reefs  and  atolls,  is  the  necessary  result 
of  the  transformation,  during  subsidence,  of  the  one 
class  into  the  other.  On  this  view,  the  three  classes  of 
reefs  ought  to  graduate  into  each  other.  Eeefs  having 
an  intermediate  character  between  those  of  the  fring- 
ing and  barrier  classes  do  exist ;  for  instance,  on  the 
S.W.  coast  of  Madagascar,  a  reef  extends  for  several 
miles,  within  which  there  is  a  broad  channel  from 
7  to  8  fathoms  deep,  but  the  sea  does  not  deepen 
abruptly  outside  the  reef.  Such  cases,  however,  are 
open  to  doubt,  for  an  old  fringing-reef  which  had 
extended  itself  on  a  basis  of  its  own  formation,  would 
hardly  be  distinguishable  from  a  barrier-reef  produced 
by  a  small  amount  of  subsidence,  and  with  its  lagoon- 
channel  nearly  filled  up  with  sediment  during  a  long 
stationary  period.  Between  barrier-reefs,  encircling 
either  a  single  lofty  island  or  several  small  low  ones, 
and  atolls  including  a  mere  expanse  of  water,  a  striking 
series  can  be  shown :  and  in  proof  of  this,  I  need  only 


Ch.  V.  OF   CORAL-REEFS.  139 

refer  to  Plate  I.,  which  speaks  more  plainly  to  the  eye, 
than  any  description  to  the  ear.  The  authorities  from 
,  which  the  figures  have  been  copied,  together  with  some 
remarks  on  them,  are  given  on  a  separate  page  descrip- 
tive of  the  plate.  At  New  Caledonia  (Plate  II.,  fig.  5) 
the  barrier-reefs  extend  for  150  miles  on  each  side  of 
the  submarine  prolongation  of  the  island ;  and  at  the 
northern  extremity  these  reefs  appear  broken  up  and 
converted  into  a  vast  atoll  supporting  a  few  low  coral- 
islets.  We  may  imagine  that  we  see  in  New  Caledonia 
the  effects  of  subsidence  actually  in  progress,— the 
water  always  encroaching  on  the  northern  end  of  the 
island,  towards  which  the  mountains  slope  down,  and 
the  reefs  steadily  building  up  their  massive  fabrics  in 
the  line  of  their  ancient  growth. 

We  have  as  yet  only  considered  barrier-reefs  and 
atolls  in  their  simplest  form ;  but  there  remain  some 
peculiarities  in  structure  and  some  special  cases,  which 
were  described  in  the  two  first  chapters,  to  be  accounted 
for  by  our  theory.  These  consist,  firstly,  in  the  presence 
of  an  inclined  ledge  terminated  by  a  wall,  and  some- 
times succeeded  by  a  second  ledge  with  a.  wall,  round 
the  shores  of  certain  lagoons  and  lagoon-channels ;  for 
this  structure  cannot  be  explained  by  the  mere  growth 
of  the  corals; — secondly,  in  the  ring  or  basin-like  form 
of  the  central  and  circumferential  reefs  of  the  northern 
Maldiva  atolls, — thirdly,  in  the  disseverment  of  some 
of  the  Maldiva  atolls,— fourthly,  in  the  existence  of 
irregularly  formed  atolls,  some  tied  together  by  linear 
reefs,  and  others  with  spurs  projecting  from  them, — 


140      THEORY  OF  THE  FORMATION     Ch.  V. 

fifthly,  in  the  submerged  condition  of  the  whole,  or  of 
parts  of  certain  barrier  and  atoll-formed  reefs,  and 
in  the  submerged  parts  being  generally  to  leeward, — 
and,  lastly,  in  the  structure  and  origin  of  the  Great 
Chagos  bank. 

Step-formed  ledges  round  certain  lagoons. — If  we 
suppose  an  atoll  to  subside  at  an  extremely  slow  rate, 
the  living  corals  would  grow  up  on  the  outer  margin 
and  on  the  deeper  parts  of  the  bare  and  hard  surface 
of  the  annular  reef.  Detritus  would  soon  accumulate 
and  become  agglomerated  on  this  surface,  after  a  time 
forming  islets.  Consequently  the  whole  atoll  before  long 
would  recover  its  former  structure  and  appearance.  If, 
however,  an  atoll  were  to  subside  somewhat  suddenly 
to  the  depth  of  a  few  fathoms,  the  whole  annular  reef, 
consisting  of  solid  rock,  would  constitute  an  excellent 
basis  for  the  attachment  and  subsequent  upward  growth 
of  a  great  bed  of  living  corals.  But  the  corals  would 
not  be  able  to  grow  up  from  the  sandy  shores  of  the  old 
lagoon  ^ ;  consequently  the  new  annular  reef  would  be 
separated  from  the  new  lagoon  by  an  abrupt  edge  or 
wall.  As  the  corals  would  grow  upwards  much  more 
vigorously  on  the  outer  side,  and  more  detritus  would 
be  accumulated  there,  the  surface  of  the  new  annular 
reef  would  slope  gently  inwards.  Hence  the  summit 
of  the  new  annular  reef  on  the  inner  side  would  pro- 
bably never  rise  above  the  level  of  the  new  lagoon, 
and  in  this  case  would  be  covered  with  sand.     If  now 

•  [This  would  seem  possible  in  certain  cases,  though  obviously  the 
position  would  be  an  unfavourable  one.     See  Appendix  II.] 


Ch.  V.  OF    CORAL-REEFS.  141 

a  second  and  somewhat  sudden  subsidence  were  to 
occur,  the  same  results  would  follow  as  during  the 
previous  and  similar  subsiding  movement.  Conse- 
quently the  new  lagoon  would  be  surrounded  by  two 
inwardly  sloping  ledges,  which  once  existed  as  the' 
summits  of  two  successive  annular  reefs,  both  termi- 
nated on  the  inner  side  by  vertical  walls  or  cliffs.^ 

The  ring  or  basin-formed  reefs  of  the  northern 
Maldiva  atolls. — I  must  first  observe  that  small  reefs- 
within  large  lagoons  or  within  broad  lagoon-channels,, 
would  grow  up  during  subsidence;  and  therefore  such 
reefs  would  sometimes  be  found  rising  abruptly  from 
a  greater  depth  than  that  at  which  the  eflficient  poly- 
pifers  can  flourish.  We  see  this  well  exemplified  in- 
the  small  abruptly-sided  reefs  with  which  the  deep, 
lagoons  of  the  Chagos  and  Southern  Maldiva  atolls  are 
studded.  With  respect  to  the  ring  or  basin-formed  reefs 
of  the  Northern  Maldiva  atolls  (see  Plate  II.,  fig.  4),. 
it  is  evident  from  the  perfectly  continuous  series  which' 
exists,  that  the  rings  on  the  margin,  although  broader 
than  the  exterior  or  bounding  reef  of  an  ordinary 
atoll,  are  only  modified  portions  of  such  a  reef ;  it  is 

'  According  to  Mr.  Couthouy  (p.  26)  the  external  slope  round' 
many  atolls  descends  by  a  succession  of  ledges  or  terraces.  He 
attempts,  but  I  doubt  whether  successfully,  to  explain  this  structure ■ 
somewhat  in  the  same  manner  as  I  have  attempted,  with  respect  to 
the  internal  ledges  round  the  lagoons  of  certain  atolls.  More  facta 
are  wanted  regarding  the  nature  both  of  the  interior  and  exterior 
step-like  ledges.  Are  all  the  ledges,  or  only  the  upper  ones,  covered 
with  living  coral  ?  If  they  are  all  so  covered,  do  the  species  differ 
on  the  different  ledges  ?  Do  ledges  occur  on  the  inside  and  outside 
round  the  same  atolls  ?  &c.  [Some  further  information  on  this  sub- 
ject has  been  obtained  by  recent  investigations.    See  Appendix  II.] 

11 


142      THEORY  OF  THE  FORMATION     Ch.  V.. 

also  evident  that  the  central  rings,  although  broader 
than  the  knolls  or  reefs  which  commonly  occur  within 
lagoons,  occupy  the  same  relative  position.  The  ring- 
like structure  has  been  shown  to  be  contingent  on 
the  breaches  into  the  lagoon  being  wide  and  numerous, 
thus  causing  the  inner  side  of  the  marginal  reef  and 
the  central  reefs  to  be  placed  under  nearly  the  same 
conditions  with  the  outside  of  an  ordinary  atoll  which 
is  exposed  to  the  open  sea.  Hence  the  margins  of  these 
reefs  have  been  favourably  circumstanced  for  growing 
•outwards  and  increasing  beyond  their  usual  breadth; 
and  the  conditions  have  likewise  been  favourable  for 
their  growing  vigorously  upwards,  during  that  subsi- 
ding movement  to  which  by  our  theory  the  whole 
^archipelago  has  been  subjected ;  and  subsidence  toge- 
ther with  the  upward  growth  of  the  margin  would 
convert  the  central  space  of  each  little  reef  into  a 
small  lagoon.  This,  however,  could  only  take  place 
with  reefs  which  had  increased  in  breadth  sufficiently 
to  prevent  their  central  spaces  from  being  almost  im- 
mediately filled  up  with  the  sand  and  detritus  driven 
inwards  by  the  waves  from  all  sides.  We  can  thus 
understand  how  it  is  that  few  reefs  less  than  half 
a  mile  in  diameter,  even  in  the  atolls  where  perfect 
ring-formed  reefs  are  found,  include  lagoons.  This 
.remark,  I  may  add,  applies  to  all  coral-formations. 
The  basin-formed  reefs  of  the  Maldiva  Archipelago 
may,  in  fact,  be  briefly  described  as  small  atolls 
iormed  during  subsidence  over  separate  portions  of  a 
'large  and  broken  atoll,  in  the  same  manner  as  the 


Ch.  V.  OF   COEAL-EEEFS.  14o 

latter  was  originally  formed  over  a  reef  encircling  one 
or  more  mountainous  islands. 

The  disseverment  of  the  larger  Maldiva  atolls, — 
The  apparent  progressive  disseverment  of  large  atolls 
into  smaller  oiTes  in  the  Maldiva  Archipelago,  demands 
an  explanation.  The  graduated  series  which  markSj 
as  I  believe,  this  process,  can  be  observed  only  in  the 
northern  half  of  the  group,  where  the  atolls  have  im- 
perfect margins  consisting  of  detached  basin-formed 
reefs.  The  currents  of  the  sea  flow  across  these  atolls 
with  considerable  force,  as  I  am  informed  by  Captain 
Moresby,  and  drift  the  sediment  from  side  to  side 
during  the  monsoons,  transporting  much  of  it  seaward; 
yet  the  currents  sweep  with  greater  force  round  their 
flanks.  It  is  historically  known  that  these  atolls  have 
long  existed  in  their  present  state;  it  is  intelligible, 
that  they  might  thus  remain,  even  during  a  slow  sub- 
sidence, owing  to  the  continued  growth  of  the  corals, 
and  to  the  lagoon  being  kept  at  nearly  its  original  depth 
by  the  accumulation  of  sediment.  But  during  the 
action  of  such  nicely  balanced  forces,  it  would  be  strange 
if  the  currents  of  the  sea  had  never  made  a  direct 
passage  across  some  of  these  atolls,  through  the  many 
wide  breaches  in  their  margins.  As  soon  as  this  oc- 
curred the  channels  would  be  deepened  by  the  removal 
of  the  finer  sediment,  and  by  the  check  to  its  further 
accumulation.  The  sides  also  of  the  channels  would 
soon  be  worn  into  a  slope  like  that  on  the  outer  coasts, 
from  being  exposed  to  the  same  force  of  the  currents. 
In  fact,  a  channel  like  that   bifurcating   one   which 


144      THEORY  OF  THE  FORMATION     Ch.  V, 

divides  Mahlos  Mahdoo  (Plate  II.,  fig.  4)  would  almost 
necessarily  be  formed.  The  scattered  reefs  situated 
near  the  borders  of  the  new  channel,  from  being  favour- 
ably placed  for  the  growth  of  coral,  would,  by  their 
extension,  tend  to  produce  fresh  margins  to  the  dis- 
severed portions :  and  a  tendency  of  this  kind  is  evident 
in  the  elongated  reefs  which  border  the  two  channels 
intersecting  Mahlos  Mahdoo.  Such  channels  would 
become  deeper  with  continued  subsidence,  and,  from 
the  reefs  on  both  sides  not  growing  up  perpendicularly, 
somewhat  broader.  In  this  case,  and  more  especially 
if  the  channels  had  been  originally  formed  of  consider- 
able breadth,  the  dissevered  portions  would  soon  be- 
come perfect  and  distinct  atolls  like  Ari  and  Koss  atolls 
(Plate  II.,  fig.  6),  or  like  the  two  Nillandoo  atolls,  which 
must  be  considered  as  distinct,  although  plainly  related 
to  each  other  in  form  and  position,  and  separated  only 
by  moderately  deep  channels.  Further  subsidence 
would  render  such  channels  unfathomable,  and  the 
dissevered  portions  would  then  resemble  Phaleedoo  and 
Moluque  atolls,  or  Mahlos  Mahdoo  and  Horsburgh  atolls 
(Plate  II.,  fig.  4),  which  are  related  to  each  other  only 
in  proximity  and  position.  Hence,  on  the  theory  of 
subsidence,  the  disseverment  of  large  atolls  which  are 
exposed  to  strong  currents  and  which  have  imperfect 
margins  (for  otherwise  their  disseverment  would  be 
scarcely  possible)  is  far  from  being  an  improbable 
event ;  and  the  several  stages,  from  a  close  connection 
to  the  entire  isolation  of  some  of  the  atolls  in  the 
Maldiva  Archipelago,  are  readily  explicable. 


Ch.  V.  OF   CORAL-REEFS.  145 

It  is  even  probable  that  the  Maldiva  Archipelago 
originally  existed  as  a  barrier-reef  of  nearly  the  same 
dimensions  as  that  of  New  Caledonia  (Plate  II.  fig.  5) : 
for  if  we  complete  in  imagination  the  subsidence  of 
this  great  island,  we  may  infer  from  the  broken  condi- 
tion of  the  northern  portion  of  the  reef,  and  from  the 
almost  entire  absence  of  reefs  on  the  eastern  coast,  that 
the  present  barrier,  after  repeated  subsidences,  would 
become,  during  its  subsequent  upward  growth,  separated 
into  distinct  portions ;  and  these  portions  would  tend 
to  assume  an  atoll-like  structure,  owing  to  the  corals 
growing  with  vigour  where  freely  exposed  to  the  open 
sea.  As  some  large  islands  have  subsided  to  a  certain 
amount  and  are  partly  encircled  by  barrier-reefs,  so  our 
theory  makes  it  probable  that  there  should  be  other 
large  islands  wholly  submerged ;  and  these,  as  we  can 
now  see,  would  be  surmounted,  not  by  one  enormous 
atoll,  but  by  several  large  ones  like  the  atolls  of  the 
Maldiva  group ;  and  these  again,  during  long  periods 
of  subsidence,  would  sometimes  become  dissevered  into 
smaller  ones.  In  the  Marshall  and  Caroline  Archipela- 
goes, there  are  atolls  standing  close  together  which 
have  an  evident  relationship  in  form;  and  we  may 
suppose  that  either  two  or  more  encircled  islands  ori- 
ginally stood  close  together  and  afforded  bases  for  two 
or  more  atolls,  or  that  one  large  atoll  has  been  dis- 
severed. But  from  the  position  as  well  as  the  forms  of 
three  atolls  in  the  Caroline  Archipelago  (the  Namourrek 
and  Elato  groups),  which  are  placed  in  an  irregular 
circle,  I  am  strongly  inclined  to  believe  that  they  owe 


y 


/, 


146      THEORY  OF  THE  FORMATION     Ch.  V. 

their  origin  to  the  disseverment  of  a  single  large 
atoU.i 

Irregularly -formed  Atolls, — In  the  Marshall  group, 
Musquillo  atoll  consists  of  two  loops  united  by  a  single 
point ;  and  Menchicoff  atoll  is  formed  of  three  loops, 
two  of  which  (as  may  be  seen  in  fig.  3,  Plate  II.)  are 
connected  by  a  mere  ribbon-shaped  reef;  the  three 
together  being  60  miles  in  length.  In  the  Gilbert  group 
some  of  the  atolls  have  narrow  reefs  like  spurs,  pro- 
jecting from  them.  Linear  and  straight,  or  crescent- 
formed  reefs  with  their  extremities  more  or  less  curled 
inwards,  may  sometimes  be  found  standing  by  them- 
selves in  the  open  ocean.  All  these  irregular  forms 
would  naturally  follow  from  continued  subsidence, 
combined  with  the  upward  growth  of  reefs  fronting 
one  side  alone  of  a  high  island,  the  reefs  on  the  op- 
posite side  having  perished  or  never  having  existed. 

Submerged  and  Bead  Beefs. — In  the  second  section 
of  the  first  chapter,  I  have  shown  that  there  some- 
times exist  in  the  neighbourhood  of  atolls,  deeply 
submerged  banks  with  level  surfaces ;  that  there  are 
others,  less  deeply  but  yet  wholly  submerged,  having 
all  the  characters  of  a  perfect  atoll,  but  consisting 

*  The  same  remark  is,  perhaps,  applicable  to  the  islands  of  Ollap, 
Fanadik,  and  Tamatam  in  the  Caroline  Archipelago,  of  which  charts 
are  given  in  the  atlas  of  Duperrey's  voyage ;  a  line  drawn  through 
the  linear  reefs  and  lagoons  of  these  three  islands  form  a  semicircle. 
Consult  also  the  atlas  of  Lulk6's  voyage ;  and  for  the  Marshall  group 
that  of  Kotzebue ;  for  the  Gilbert  group  (which  is  re^^rred  to  in  the 
ensuing  paragraph)  consult  the  atlas  of  Duperrey's  voyage.  Most  of 
the  points  here  referred  to  may,  however,  be  seen  in  Krusenstern*a 
general  Atlas  of  the  Pacific. 


Ch.  V.  OF   CORAL-REEFS.  147 

merely  of  dead  coral-rock ;  that  there  are  barrier-reefs 
and  atolls  with  only  a  portion  of  the  reef,  generally  on 
the  leeward  side,  submerged  ;  and  that  such  portions 
eitlier  retain  their  perfect  outline,  or  appear  to  be 
more  or  less  completely  effaced,  their  former  place 
being  marked  only  by  a  bank,  conforming  in  general 
outline  with  that  part  of  the  reef  which  remains 
perfect.  These  several  cases  are,  I  believe,  intimately 
related,  and  can  all  be  explained  by  the  same  agency 
of  subsidence. 

We  see  that  in  those  parts  of  the  ocean  where 
coral-reefs  are  most  abundant,  one  island  is  fringed 
and  another  neighbouring  one  is  not  fringed,  and  that 
in  the  same  archipelago,  all  the  reefs  are  more  perfect 
in  one  part  than  in  another, — for  instance,  in  the 
southern  compared  with  the  northern  half  of  the 
Maldiva  Archipelago,  and  likewise  on  the  outer  as 
compared  with  the  inner  coasts  of  the  double  row  of 
atolls  in  this  same  archipelago.  The  existence  of  tho 
innumerable  polypifers  forming  a  reef  depends  on 
their  finding  sustenance,  and  we  know  that  they  are 
preyed  on  by  other  organic  beings,  and  that  some 
inorganic  causes  are  highly  injurious  to  their  growth. 
Can  it,  therefore,  be  expected  that  the  reef-building 
polypifers  should  keep  alive  for  perpetuity  in  any  one 
place,  during  the  round  of  change  to  which  earth, 
air,  and  water  are  subjected ;  and  still  less  can  this 
be  expected  during  progressive  subsidence,  to  which 
by  our  theory  these  reefs  and  islands  have  been  liable  *? 
Should  such  subsidence  be  at  any  time  greater  than 


148      THEORY  OF  THE  FORMATION     Ch.  V, 

the  rate  of  upward  growth  of  the  polypifers,  the  death 
of  the  reef  must  ensue,  and  it  would  have  been  strange 
had  we  found  no  evidence  of  this.  It  is,  then,  not 
at  all  improbable  that  the  corals  should  sometimes 
perish  either  on  the  whole  or  on  part  of  a  reef. 
If  only  on  a  part,  the  dead  portion,  after  a  small 
amount  of  subsidence,  would  still  retain  its  proper 
outline  and  position  beneath  the  water.  After  a  more 
prolonged  subsidence,  it  would  form,  owing  to  the 
accumulation  of  sediment,  a  more  or  less  level  bank 
marking  the  limits  of  the  former  lagoon.  Such  dead 
portions  of  a  reef  would  generally  lie  on  the  leeward 
side,^  for  the  impure  water  and  line  sediment  are 
driven  out  from  the  lagoon  over  this  side  of  the  reef, 
where  the  force  of  the  breakers  is  less  than  to  wind- 
ward, and  where  the  corals  are,  in  consequence,  less 
vigorous  and  less  able  to  resist  any  destroying  agency. 
It  is  owing  to  this  same  cause  that  reefs  are  fre- 
quently breached  to  leeward  by  channels  which  serve 

»  Sir  C.  Lyell,  in  the  first  edition  of  his  Principles  of  Geology, 
offered  a  somewhat  different  explanation  of  this  structure.  He  sup- 
poses that  there  has  been  subsidence ;  but  he  was  not  aware  that  the 
submerged  portions  of  reef  were  in  most  cases,  if  not  in  all,  dead ; 
and  he  attributes  the  difference  in  height  in  the  two  sides  of  most 
atolls  chiefly  to  the  greater  accumulation  of  detritus  to  windward 
than  to  leeward.  But  as  matter  is  accumulated  only  on  the  back- 
ward part  of  the  reef,  the  front  part  would  remain  of  the  same 
height  on  both  sides.  I  may  here  observe  that  in  most  cases  (for 
instance  at  Peros  Banhos,  the  Gambler  group  and  the  Great  Chagos 
bank),  and  I  suspect  in  all  cases,  the  dead  and  submerged  portions 
do  not  blend  or  slope  into  the  living  and  perfect  parts,  but  are  sepa- 
rated from  them  by  an  abrupt  line.  In  some  instances  small  patches 
of  living  reef  rise  to  the  surface  from  the  middle  of  the  submerged 
and  dead  parts. 


Ch.  V.  OF  CORAL-REEFS. 

as  ship-channels.  If  the  corals  perished  enu^ 
on  the  greater  part  of  the  circumference  of  an  atoll, 
the  result  would  be  an  atoll-shaped  bank  of  dead 
rock  more  or  less  entirely  submerged;  and  further 
subsidence,  together  with  the  accumulation  of  sedi- 
ment, would  obliterate  its  atoll-like  structure,  and 
leave  only  a  bank  with  a  nearly  level  surface. 

We  meet  with  all  these  cases  in  the  Chagos  group 
of  atolls.  Here  within  an  area  of  160  miles  by  60, 
there  are  two  atoll-formed  banks  of  dead  rock  (besides 
another  very  imperfect  one)  entirely  submerged;  a 
third  bank  with  merely  two  or  three  small  pieces  of 
living  reef  which  rise  to  the  surface ;  and  a  fourth, 
namely,  Peros  Banhos  (Plate  I.  fig.  9),  with  a  por- 
tion nine  miles  in  length  dead  and  submerged.  As 
by  our  theory  this  area  has  subsided,  and  as  there  is 
nothing  improbable  in  the  death  of  the  corals  on  por- 
tions  or  over  the  whole  surface  of  a  reef,  either  from 
changes  in  the  state  of  the  surrounding  sea  or  from  the 
subsidence  being  great  or  sudden,  these  Chagos  banks 
present  no  difficulty.  Sofar,  indeed,  are  any  of  the  above- 
mentioned  cases  of  dead  submerged  reefs  from  offering 
any  difficulty,  that  their  occurrence  might  have  been 
anticipated  on  our  theory  ;  and  as  fresh  atolls  are  sup- 
posed to  be  in  progressive  formation  by  the  sub- 
sidence of  encircling  barrier-reefs,  a  weighty  ob- 
jection might  even  have  been  raised,  namely  that 
atolls  must  increase  indefinitely  in  number,  unless 
proofs  of  their  occasional  destruction  could  have  been 
adduced. 


Y   OF   THE   FORMATION  Ch.  V. 

.  KJreat  Chagos  Bank} — I  have  already  shown 
that  the  submerged  condition  of  the  Great  Chagos 
bank  (Plate  II.  fig.  1,  with  its  section,  fig.  2),  and  of 
some  other  banks  in  the  Chagos  group,  may  in  all  pro- 
bability be  attributed  to  the  corals  having  perished 
during  an  unusually  rapid  or  sudden  subsidence.  The 
external  rim  or  upper  ledge  (shaded  in  the  chart)  con- 
sists of  dead  coral-rock  thinly  covered  with  sand  ;  it 
lies  at  an  average  depth  of  between  5  and  8  fathoms, 
and  perfectly  resembles  in  form  the  annular  reef  of  an 
atoll.  The  banks  of  the  second  level,  the  boundaries 
of  which  are  marked  by  dotted  lines  in  the  chart,  lie 
from  about  15  to  20  fathoms  beneath  the  surface; 
they  are  several  miles  in  breadth,  and  terminate  in 
a  very  steep  slope  round  the  central  expanse.  This 
central  expanse  consists  of  a  level  muddy  flat  between 
30  and  40  fathoms  deep.  The  banks  of  the  second 
level  appear  at  first  sight  to  resemble  the  internal  step- 
like ledges  of  dead  coral-rock  which  border  the  lagoons 
of  certain  atolls,  but  their  much  greater  width,  and 
their  being  formed  of  sand,  are  points  of  essential  dif- 
ference. On  the  eastern  side  of  the  atoll  some  of  the 
banks  are  linear  and  parallel,  like  islets  in  a  great  river, 
and  they  point  directly  towards  a  great  breach  on  the 
opposite  side  of  the  atoll :  these  are  best  seen  in  the 
large  published  chart.  I  inferred  from  this  circum- 
stance, that  strong  currents  sometimes  set  directly 
across  this  great  bank ;  and  I  hear  from  Captain  Mores})y 
that  this  is  the  case.  I  observed,  also,  that  the  channels, 

*  [See  Appendix  II.] 


Ch.  V.  OF   CORAL-REEFS.  151 

or  breaches  through  the  rim,  were  all  of  the  same  depth 
as  khe  central  expanse  into  which  they  lead ;  whereas 
the  channels  into  the  other  atolls  of  the  Chagos  group, 
and  as  I  believe  into  most  other  large  atolls,  are  not 
nearly  as  deep  as  the  lagoons.  For  instance  at  Peros 
Banhos,  the  channels  as  well  as  the  bottom  of  the  lagoon 
for  a  space  about  a  mile  and  a-half  round  its  shores,  are 
only  between  10  and  20  fathoms  in  depth,  whilst  the 
central  expanse  is  from  35  to  40  fathoms  deep.  Now, 
if  an  atoll  during  a  gradual  subsidence  once  became 
entirely  submerged  like  the  Great  Chagos  bank,  and 
therefore  no  longer  exposed  to  the  surf,  very  little 
sediment  could  any  longer  be  formed  from  it ;  conse- 
quently the  channels  leading  into  the  lagoon  would  be 
no  longer  filled  up  with  drifted  sand  and  coral  detritus, 
and  would  continue  increasing  in  depth,  as  the  whole 
sank  down.  In  this  case  we  might  expect  that  the 
currents  of  the  open  sea,  instead  of  sweeping  as  at 
first  round  the  submarine  flanks,  would,  as  the  many 
breaches  in  the  reef  increased,  flow  directly  across 
the  lagoon,  thus  removing  the  finer  sediment  from 
the  channels,  and  preventing  its  further  accumulation. 
The  submerged  reef  would  thus  ultimately  consist  of  an 
upper  and  narrow  broken  rim  of  rock,  surrounded  on 
the  inner  side  by  banks,  the  remnants  of  the  sandy 
bed  of  the  old  lagoon,  now  intersected  by  many  deep 
channels ;  these  channels,  with  their  sides  worn  steep  by 
the  oceanic  currents,  uniting  in  the  centre  and  forming 
tlie  central  deep  expanse.    By  such  means  the  Great 


152      THEORY  OF  THE  FORMATION     Ch.  V. 

Chagos  bank — the  most  anomalous  structure  which  I 
have  met  with — appears  to  have  originated. 

If  this  bank  should  continue  to  subside,  a  mere 
wreck  of  an  atoll  would  be  left;  for  the  corals  are 
almost  everywhere  dead.  Pitt's  bank,  situated  not  far 
southward,  appears  to  be  in  this  actual  condition :  it 
consists  of  a  moderately  level,  oblong  bank  of  sand, 
lying  from  10  to  20  fathoms  beneath  the  surface,  with 
two  sides  protected  by  a  narrow  ledge  of  rock  submerged 
between  5  and  8  fathoms.  A  little  to  the  south  of  this 
ledge,  at  about  the  same  distance  as  the  southern  rim  of 
the  Great  Chagos  bank  lies  from  the  northern  rim,  there 
are  two  other  small  banks  with  from  10  to  20  fathoms 
on  them ;  and  not  far  eastward,  soundings  were  struck 
on  a  sandy  bottom  with  between  110  and  145  fathoms. 
The  northern  portion  of  Pitt's  bank  with  its  ledge-like 
margin,  thus  closely  resembles  any  one  segment  of  the 
Great  Chagos  bank  between  two  of  the  deep-water 
channels,  and  the  scattered  banks  southward  and  east- 
w^ard  appear  to  be  the  last  wreck  of  the  less  perfect 
portions  of  one  great  and  now  ruined  atoll. 

I  have  examined  with  care  the  charts  of  the  Indian 
and  Pacific  Oceans,  and  have  now  laid  before  the  reader 
all  the  cases  which  I  have  met  with,  of  reefs  differing 
from  the  class  to  which  they  belong ;  and  I  think  it  has 
been  shown  that  they  are  all  included  in  our  theory, 
modified  by  occasional  accidents,  such  as  might  have 
been  anticipated.  We  have  thus  seen,  that  in  the  lapse 
of  ages  encircling  barrier-reefs  are  converted  into  atoUs, 
—the  term  atoll  being  applicable  as  soon  as  the  last 


Ch.  V.  OF    CORAL-REEFS.  153 

pinnacle  of  encircled  land  sinks  beneath  the  surface  of 
the  sea.  We  have  seen  that  large  atolls,  during  the  pro- 
gressive subsidence  of  the  areas  in  which  they  stand, 
sometimes  become  dissevered  into  smaller  ones.  At  other 
times,  when  the  reef-building  polypifers  perish,  atolls 
are  converted  into  atoll- formed  banks  of  dead  rock ;  and 
these  again,  through  further  subsidence  and  the  accu- 
mulation of  sediment,  pass  into  level  banks  with  scarcely 
any  distinguishing  character.  Thus  may  the  history  of  an 
atoll  be  followed  from  its  birth,  through  the  occasional  ac- 
cidents of  its  existence,  to  its  death  and  final  obliteration. 
Objections  to  our  theory  of  the  formation  of  Atolls 
and  Barrier-reefs. — The  vast  amount  of  subsidence 
both  in  area  and  depth,  necessary  to  have  submerged 
every  mountain,  even  the  highest,  throughout  the 
immense  spaces  of  ocean  now  interspersed  with  atolls, 
will  probably  strike  most  persons  as  a  formidable  objec- 
tion to  the  theory.  But  as  continents,  as  large  as  the 
spaces  supposed  to  have  subsided,  have  been  raised 
above  the  level  of  the  sea, — as  whole  regions  are  now 
rismg,  for  instance,  in  Scandinavia  and  South  America, 
—and  as  no  reason  can  be  assigned  why  subsidence 
should  not  have  occurred  in  some  parts  of  the  earth's 
crust  on  as  great  a  scale  as  elevation,  this  objection  has 
little  force.  The  remarkable  point  is,  that  a  subsiding 
movement  to  such  an  extent  and  amount  should  have 
taken  place  within  a  period,  during  which  the  corals 
have  continued  to  add  matter  to  the  same  reefs.  An- 
other and  less  obvious  objection  to  the  theory  may 
perhaps  be  advanced,  namely,  that,  although  atolls  and 


154      THEORY  OF  THE  FORMATION     Ch.  V. 

barrier-reefs  are  supposed  to  have  gone  on  subsiding 
for  a  long  period,  yet  that  their  lagoons  and  lagoon - 
channels  have  only  rarely  come  to  exceed  40  and 
never  60  fathoms  in  depth.  But  if  our  theory  is  worth 
consideration,  we  already  admit  that  the  rate  of  sub- 
sidence has  not  ordinarily  exceeded  that  of  the  upward 
growth  of  the  massive  corals  which  live  on  the  margins 
of  the  reefs,  so  that  we  have  only  further  to  suppose 
that  the  rate  has  never  exceeded  that  at  which  lagoons 
and  lagoon-channels  are  filled  up  by  the  growth  of  the 
delicate  corals  which  live  there,  and  by  the  accumula- 
tion of  sediment.  As  the  filling- up  process,  in  the  case 
of  barrier-reefs  lying  far  from  the  land,  and  of  the  larger 
atolls,  must  be  an  extremely  slow  one,  we  are  led  to 
conclude  that  the  subsiding  movement  has  always  been 
equally  slow.  And  this  conclusion  accords  well  with 
what  is  known  of  the  rate  of  recent  movements  of 
elevation. 

It  has,  I  think,  been  shown  in  this  chapter,  that 
subsidence  explains  both  the  normal  structure  and 
the  less  regular  forms  of  those  two  great  classes  of 
reefs  which  have  justly  excited  the  astonishment  of  all 
the  naturalists  who  have  sailed  through  the  Pacific  and 
Indian  Oceans.  The  necessity,  also,  that  a  foundation 
should  have  existed  at  the  proper  depth  for  the  growth 
of  the  corals  over  certain  large  areas,  almost  compels 
us  to  accept  this  theory.  But  further  to  test  its  truth, 
a  crowd  of  questions  may  be  asked.  Do  the  different 
kinds  of  reefs  which  have  been  produced  by  the 
same  kind  of  movement,   generally  lie  within   the 


Ch.  V.  OF   COEAL-REEFS.  155 

Bame  or  closely  adjoining  areas  ?  How  are  such  reefs 
related  to  each  other  in  form  and  position, — for 
instance,  do  neighbouring  groups  of  atolls,  and  the 
separate  atolls  in  each  group,  bear  the  same  relation 
to  each  other  as  do  ordinary  islands  ?  Although  coral- 
reefs  which  have  just  begun  to  re-grow,  after  having 
been  killed  by  too  rapid  a  subsidence,  would  at  first 
belong  to  the  fringing  class,  yet,  as  a  general  rule, 
reefs  of  this  class  indicate  that  the  land  has  either 
long  remained  at  a  stationary  level,  or  has  been  up- 
raised. Of  a  stationary  level  it  is  hardly  possible 
to  find  any  evidence  except  of  a  negative  kind ;  but 
of  recent  elevation,  upraised  marine  remains  afford 
a  sure  proof :  it  may  therefore  be  asked,  do  fringed 
coasts  often  afford  such  evidence?  Do  the  areas 
which  have  subsided,  as  shown  by  the  presence  of 
atolls  and  barrier-reefs,  and  the  areas  which  have 
either  remained  stationary  or  have  been  upraised, 
as  indicated  by  fringing-reefs,  bear  any  determinate 
relation  to  each  other  ?  Is  there  any  relation  between 
the  areas  of  recent  subsidence  or  elevation,  and  the 
presence  of  active  volcanic  vents?  These  several 
questions  will  be  considered  in  the  following  chax:)ter.' 

'  I  may  take  this  opportunity  of  briefly  considering  the  appear- 
ance .which  would  probably  be  presented  by  a  vertical  and  deep 
section  across  a  coral  formation  (referring  chiefly  to  an  atoll)  formed 
by  the  upward  growth  of  coral  during  successive  subsidences.  This 
is  a  subject  worthy  of  attention,  as  a  means  of  comparison  with 
ancient  coral  strata.  The  circumferential  parts  would  consist  of 
massive  species  in  a  vertical  position,  with  their  interstices  filled  up 
with  detritus ;  but  this  would  be  the  part  most  subject  to  subsequent 
denudation  and  removal.     It  is  useless  to  speculate  how  large  a 


156      THEORY  OF  THE  FORMATION     Ch.  V. 

proportion  of  the  exterior  annular  reef  would  consist  of  upright  coral, 
and  how  much  of  fragmentary  rock,  for  this  would  depend  on  many 
contingencies, — such  as  on  the  rate  of  subsidence  occasionally  allow- 
ing a  fresh  growth  of  coral  to  cover  the  whole  surface,  and  on  the 
breakers  having  force  sufficient  to  throw  fragments  over  this  same 
space.  The  conglomerate  which  composes  the  base  of  the  islets, 
would  (if  not  removed  by  denudation  together  with  the  exterior  reef 
on  which  it  rests)  be  conspicuous  from  the  size  of  the  fragments, — ■ 
the  different  degrees  in  which  they  have  been  rounded, — the  presence 
of  ragments  of  conglomerate  torn  up  rounded  and  re-cemented,^ 
and  from  the  oblique  stratification.  The  corals  which  lived  in  the 
lagoon-reefs  at  each  successive  level,  would  be  preserved  upright, 
and  they  would  consist  of  many  kinds,  generally  much  branched. 
In  this  part,  however,  a  very  large  proportion  of  the  rock,  and  in 
some  cases  nearly  all  of  it,  would  be  formed  of  sedimentary  matter, 
being  in  an  excessively  fine  or  moderately  coarse  state,  with  the  par- 
ticles almost  blended  together.  The  conglomerate  which  was  formed 
of  rounded  pieces  of  the  branched  corals  on  the  shores  of  the  lagoon, 
would  differ  from  that  formed  on  the  islets  and  derived  from  the 
outer  coast ;  although  both  might  have  been  accumulated  very  near 
each  other.  The  stratification,  taken  as  a  whole,  would  be  hori- 
zontal :  but  the  conglomerate  beds  resting  on  the  exterior  reef,  and 
the  beds  of  sandstone  on  the  shores  of  the  lagoon  and  on  the  ex- 
ternal flanks  of  the  reef,  would  probably  be  divided  (as  at  Keeling 
atoll  and  at  Mauritius)  by  numerous  layers  dipping  at  considerable 
angles  in  different  directions.  The  calcareous  sandstone  and  coral 
rock  would  almost  necessarily  contain  innumerable  shells,  echini, 
and  the  bones  of  fish,  turtle,  and  perhaps  of  birds :  possibly,  also, 
the  bones  of  small  saurians,  as  these  animals  find  their  way  to 
islands  far  remote  from  any  continent.  The  large  shells  of  some 
species  of  Tridacna  would  be  found  vertically  imbedded  in  the  solid 
rock,  in  the  position  in  which  they  lived.  We  might  expect,  also, 
to  find  a  mixture  of  the  remains  of  pelagic  and  littoral  animals  in 
the  strata  formed  in  the  lagoon,  for  pumice  and  the  seeds  of  plants 
are  floated  from  distant  countries  into  the  lagoons  of  many  atolls ; 
on  the  outer  coast  of  Keeling  atoll  near  the  mouth  of  the  lagoon,  the 
shell  of  a  pelagic  Pteropodous  animal  was  brought  up  on  the  arming 
of  the  sounding-lead.  All  the  loose  blocks  of  coral  on  Keeling  atoll 
were  burrowed  by  vermiform  animals ;  and  as  every  cavity,  no  doubt, 
ultimately  becomes  filled  with  spathose  limestone,  slabs  of  the  rock 
would,  if  polished,  probably  exhibit  the  excavations  of  such  burrow- 
ing animals.  The  conglomerate  and  fine-grained  beds  of  coral-rock 
would  be  hard,  sonorous,  white,  and  composed  of  nearly  pure  cal 


Ch.  V.  OF   CORAL-REEFS.  157 

careous  matter;  in  some  few  parts,  judging  from  the  specimens  at 
Keeling  atoll,  they  would  probably  contain  a  small  quantity  of  iron. 
I  have  seen  a  conglomerate  now  forming  on  the  shores  of  the  Mal- 
diva  atolls,  resembling  conglomerate  limestone  from  Devonshire. 
Floating  pumice  and  scoriae,  and  occasionally  stones  transported  in 
the  roots  of  trees  (see  my  Naturalist's  Voyage,  p.  461)  appear  the 
only  sources  through  which  foreign  matter  is  brought  to  coral-for- 
mations standing  in  the  open  ocean.  The  area  over  which  sediment 
is  transported  from  coral-reefs  must  be  considerable;  Captain 
Moresby  informs  me  that  during  the  change  of  monsoons,  the  sea 
is  discoloured  to  a  considerable  distance  off  the  Maldiva  and  Chagos 
atolls.  The  sediment  off  fringing  and  barrier  coral-reefs  must  be 
mingled  with  the  mud  which  is  brought  down  from  the  land,  and  is 
transported  seaward  through  the  breaches  which  occur  in  front  of 
almost  every  valley.  If  the  bed  of  the  ocean  were  to  be  upraised 
and  converted  into  land,  the  atolls  of  the  larger  archipelagoes  would 
form  flat-topped  mountains,  varying  in  diameter  from  a  few  to  sixty 
miles — for  the  smallest  atolls  would  probably  be  worn  quite  away ; 
and  from  being  horizontally  stratified  and  of  similar  composition, 
they  would,  as  Sir  C.  Lyell  has  remarked,  falsely  appear  as  if  they 
had  originally  been  united  into  one  vast  continuous  mass.  Such 
great  strata  of  coral-rock  would  rarely  be  associated  with  erupted 
volcanic  matter,  for  this  could  only  take  place,  as  may  be  inferred 
from  what  follows  in  the  next  chapter,  when  the  area  in  which  they 
were  situated,  commenced  to  rise,  or  at  least  ceased  to  subside. 
During  the  enormous  period  necessary  to  effect  an  elevation  of  the 
kind  just  alluded  to,  the  surface  would  necessarily  be  greatly  de- 
nuded ;  hence  it  is  highly  improbable  that  any  fringing-reef ,  or  even 
any  barrier-reef,  at  least  those  encircling  small  islands,  would  be 
preserved  to  a  distant  period.  From  this  same  cause,  the  strata 
which  were  formed  within  the  lagoons  of  atolls  and  the  lagoon- 
channels  of  barrier-reefs,  and  which  must  consist  in  a  large  part  of 
sedimentary  matter,  would  more  often  be  preserved  to  future  ages, 
than  the  exterior  solid  reef  composed  of  massive  corals  in  an  upright 
position ;  although  it  is  on  this  exterior  part  that  the  present  exist- 
ence and  further  growth  of  atolls  and  barrier-reefs  depend. 


12 


CHAPTEE  VI. 

ON    THE    DISTRIBUTION    OF  CORAL-REEFS  WITH  REFERENCH 
TO   THE    THEORY   OF   THEIR   FORMATION. 

Description  of  the  coloured  map — Proximity  of  atolls  and  barrier- 
reefs — Relation  inform  and  position  of  atolls  with  ordinary  islands 
— Direct  evidence  of  subsidence  difficult  to  be  detected — Proofs  oj 
recent  elevation  where  fringing-reefs  occur — Oscillations  of  level 
— Absence  of  active  volcanoes  in  the  areas  of  subsidence — Immen- 
sity of  the  areas  which  have  been  elevated  and  have  subsided — 
Their  relation  to  the  present  distribution  of  the  land — Areas  of 
subsidaice  elongated — Their  intersection  and  alternation  with 
those  of  elevation — Amountt  and  slow  rate  of  the  requisite  sub- 
sidence— Becapitulation. 

It  will  be  convenient  first  to  give  a  short  account  of 
the  appended  map  of  the  Pacific  and  Indian  Oceans 
(Plate  III.) ;  a  fuller  one,  with  the  data  for  colouring 
each  spot,  is  reserved  for  the  Appendix,  and  every 
place  there  referred  to  may  be  found  in  the  Index. 
A  larger  chart  would  have  been  desirable ;  but,  small 
as  the  adjoined  one  is,  it  is  the  result  of  many  months* 
labour.  I  have  consulted,  as  far  as  I  was  able,  every 
original  voyage  and  map ;  and  the  colours  were  first 
laid  down  on  charts  on  a  large  scale.  The  same  blue 
colour,  with  merely  a  difference  in  the  tint,  is  used 
for  atolls   or   lagoon-islands,  and   for   barrier-reefs; 


Ch.  VI.      DISTRIBUTION   OF    CORAL-REEFS.  159 

these  being  in  all  essential  respects  closely  related. 
Fringing-reefs,  on  the  other  hand,  have  been  coloured 
dull  red,  for  there  is  an  important  distinction  between 
them  and  barrier-reefs  and  atolls  with  respect  to  the 
depth  beneath  the  surface,  at  which,  as  we  must  believe, 
their  foundations  He.  The  two  distinct  colours,  there- 
fore, mark  two  great  types  of  structure. 

The  dark  blue  colour  represents  atolls  and  sub- 
merged annular  reefs  with  deep  water  in  their  centres. 
I  have  coloured  a  few  low  and  small  coral-islands 
as  if  they  had  been  atolls,  although  not  including  a 
lagoon ;  but  this  has  been  done  only  when  it  clearly 
appeared  that  they  had  originally  contained  one. 
When  no  such  evidence  exists  they  have  been  left 
uncoloured. 

The  pale  blue  colour  represents  barrier-reefs.  The 
most  obvious  character  of  reefs  of  this  class  is  the 
broad  and  deep-water  moat  within  the  reef ;  but  this, 
like  the  lagoon  of  a  small  atoll,  is  liable  to  become 
filled  up  with  detritus  and  with  reefs  of  delicately- 
branched  corals.  When,  therefore,  a  reef  round  the 
entire  circumference  of  an  island  extends  far  into  a 
profoundly  deep  sea,  so  that  it  can  hardly  be  con- 
founded with  a  fringing-reef  which  must  rest  on  a 
foundation  of  rock  within  a  small  depth,  it  has  been 
coloured  pale  blue,  although  it  does  not  now  include 
a  deep-water  moat.  But  this  has  been  rarely  done, 
and  each  case  is  distinctly  mentioned  in  the  Appendix. 

The  red  colour  represents  reefs  which  fringe  the 
land  closely  where  the  sea  is  deep,   and   extend  to 


160  DISTRIBUTION   OF   CORAL-REEFS.      Ch.  VI. 

a  moderate  distance  from  it  where  the  bottom  i3 
gently  incHned ;  but  they  never  include  a  deep-water 
moat  or  lagoon-like  channel  running  parallel  to  tha 
shore.  It  must,  however,  be  remembered  that 
fringing-reefs  are  frequently  breached  by  deep-water 
channels,  where  mud  has  been  deposited  in  front  of 
rivers  and  valleys. 

In  all  cases,  a  space  of  30  miles  in  width  has 
been  coloured  round  or  in  front  of  the  reefs  of  each 
class,  in  order  that  the  colours  might  be  made  con- 
spicuous in  a  map  on  so  small  a  scale. 

The  vermilion  spots  and  streaks  represent  vol- 
canos  now  in  action,  or  historically  known  to  have 
been  so.  They  are  laid  down  chiefly  from  Von  Buch's 
work  on  the  Canary  Islands ;  and  my  reasons  for 
making  a  few  alterations  are  given  in  the  note  below.* 

•  I  have  also  made  considerable  use  of  the  geological  part  ol 
Berghaus'  Physical  Atlas.  Beginning  at  the  eastern  side  of  the 
Pacific,  I  have  added  to  the  number  of  the  volcanos  in  the  southern 
part  of  the  Cordillera,  and  have  coloured  Juan  Fernandez  according 
to  observations  collected  during  the  voyage  of  the  Beagle  (Geol. 
Trans,  vol.  v.  p.  601).  I  have  added  a  volcano  to  Albemarle  Island, 
one  of  the  Galapagos  Archipelago  (see  my  Journal  of  Researches, 
p.  457).  In  the  Sandwich  group  there  are  no  active  volcanos,  except 
at  Hawaii ;  but  the  Rev.  W.  Ellis  informs  me  there  are  streams  of 
lava  apparently  modern  on  Maui,  having  a  very  recent  appearance, 
which  can  be  traced  to  the  craters  whence  they  flowed.  The  same 
^;entleman  informs  me  that  there  is  no  reason  to  believe  that  any 
active  volcano  exists  in  the  Society  Archipelago ;  nor  are  there  any 
known  in  the  Samoa  or  Navigator  group,  although  some  of  the 
streams  of  lava  and  craters  there  appear  recent.  In  the  Friendly 
group,  the  Rev.  J.  Williams  says  (Narrative  of  Missionary  Enter- 
prise, p.  29)  that  Toofoa  and  Proby  Islands  are  active  volcanos.  I 
infer  from  Hamilton's  Voyage  in  the  Pandora  (p.  95),  that  Proby 
Island  is  synonymous  with  Onouafou,  but  I  have  not  ventured  to 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  161 

The  uncohured  parts  consist,  first  and  chiefly,  of 
coasts  where  no  coral-reefs,  or  quite  insignificant  ones, 

colour  it.  There  can  be  no  doubt  respecting  Toofoa ;  and  Captain 
Edwards  (Von  Buch,  p.  386)  found  the  lava  of  a  recent  eruption 
at  Amargura  still  smoking.  Berghaus  marks  four  active  volcanos 
actually  within  the  Friendly  group ;  but  I  do  not  know  on  what 
authority ;  I  may  mention  that  Maurelle  describes  Latte  as  having 
a  burnt-up  appearance ;  I  have  marked  only  Toofoa  and  Armagura. 
South  of  the  New  Hebrides  lies  Matthews  Rock,  which  is  described 
as  an  active  crater  in  the  voyage  of  the  Astrolabe.  Between  it  and 
the  volcano  on  the  eastern  side  of  New  Zealand  lies  Brimstone 
Island,  which  from  the  high  temperature  of  the  water  in  the  crater 
may  be  ranked  as  active  (Berghaus  Vorbemerk,  II.  Lief.  S.  66). 
Malte  Brun,  vol.  xii.  p.  231,  says  that  there  is  a  volcano  near  Port 
St.  Vincent,  in  New  Caledonia :  I  believe  this  to  be  an  error,  arising 
from  smoke  seen  on  the  opposite  coast  by  Cook  (2nd  voyage,  vol.  ii. 
p.  23),  which  smoke  went  out  at  night.  The  Mariana  Islands, 
especially  the  northern  ones,  contain  many  craters  (see  Freycinet's 
Hydrog.  Descript.)  which  are  not  active.  Von  Buch,  however,  states. 
(p.  462),  on  the  authority  of  La  Peyrouse,  that  there  are  no  less  than 
seven  volcanos  between  these  islands  and  Japan.  Gemelli  Careri 
(Churchill's  Collect,  vol.  iv.  p.  458)  says  there  are  two  active  volcanos 
in  lat.  23°  30'  and  in  lat.  24°  ;  but  I  have  not  coloured  them.  From 
the  statements  in  Beechey's  Voyage  (p.  518,  4to  edit.)  I  have  coloured 
one  in  the  northern  part  of  the  Bonin  group.  M.  S.  Julien  has 
clearly  made  out  from  Chinese  manuscripts  not  very  ancient  (Comptes 
Rendus,  1840,  p.  832),  that  there  are  two  active  volcanos  on  the 
eastern  side  of  Formosa.  In  the  map  appended  to  the  first  edition 
I  marked  an  active  volcano  in  Torres  Straits,  and  gave  my  authority ; 
but  Mr.  Jukes  informs  me  that  there  certainly  is  no  volcano  there ; 
a  wooded  island  on  fire  having  been  mistaken  for  one.  Mr.  M'Clel- 
land  (Report  of  Committee  for  Investigating  Coal  in  India,  p.  39) 
has  shown  that  the  volcanic  band  which  passes  through  Barren 
Island  must  be  extended  northwards.  It  appears  by  an  old  chart, 
that  Cheduba  was  once  an  active  volcano  (see  also  Silliman's  North 
American  Journal,  vol.  xxxviii.  p.  385).  In  Berghaus'  Phys.  Atlas, 
1840  (No.  7  of  Geological  Part)  a  volcano  on  the  coast  of  Pondi- 
cherry  is  said  to  have  burst  forth  in  1757.  Ordinaire  (Hist.  Nat. 
des  Volcans,  p.  218)  says  that  there  is  one  at  the  mouth  of  the 
Persian  Gulf,  but  I  have  not  coloured  it,  as  he  gives  no  particulars. 
A.  volcano  in  Amsterdam,  or  St.  Paul's,  in  the  southern  part  of  tlia 


162  DISTRIBUTION   OF    CORAL-REEFS.      Ch.  \a 

exist.  Secondly,  of  coasts  where  the  sea  is  extremely 
shallow ;  and  the  reefs  in  this  case  generally  He  far 
from  the  land,  and  are  very  irregular,  so  that  they 
cannot  always  be  classed.  Thirdly,  reefs  which  appear 
merely  to  coat  submerged  banks  of  rock  or  of  sedi- 
ment ;  for  such  reefs  differ  in  some  essential  respects 
from  those  which  owe  their  whole  thickness  to  the 
growth  of  corals.  Fourthly,  in  the  Eed  Sea,  and 
within  some  parts  of  the  East  Indian  Archipelago 
(if  the  imperfect  charts  of  the  latter  can  be  trusted), 
there  are  many  scattered  reefs  of  small  size,  repre- 
sented by  mere  dots,  which  rise  out  of  deep  water ; 
and  these  have  likewise  been  left  uncoloured.  In  the 
Eed  Sea,  however,  some  such  reefs  seem  once  to  have 
formed  parts  of  a  continuous  barrier.  There  exist, 
also,  scattered  in  the  open  ocean,  some  linear  and 
irregularly-formed  reefs  which  are  probably,  as  shown 
in  the  last  chapter,  remnants  of  atolls ;  but  as  they 
cannot  safely  be  placed  in  this  class,  they  have  not 
been  coloured ;  they  are,  however,  few  in  number,  and 
of  insignificant  dimensions.  Lastly,  some  reefs  have 
been  left  uncoloured  from  the  want  of  sufficient  in- 
formation ;  and  some  because  they  are  intermediate 
in  character  between  barrier  and  fringing-reefs.     The 


Indian  Ocean,  has  been  seen  (Naut.  Mag.  1838,  p.  842)  in  action. 
Dr.  J.  Allan,  of  Forres,  informs  me  in  a  letter  that,  when  he  was  at 
Joanna,  he  saw  flames  at  night,  apparently  volcanic,  issuing  from 
the  Chief  Comoro  Island,  and  that  the  Arabs  assured  him  that  they 
were  volcanic,  adding  that  the  volcano  burnt  more  during  the  wet 
season  :  I  have  marked  this  as  a  volcano,  though  with  some  hesita- 
tion, as  the  flames  may  have  arisen  from  gaseous  sources. 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  163 

value  of  the  map  is  lessened,  in  proportion  to  the 
number  of  reefs  which  I  have  thus  been  obliged  to 
leave  uncoloured;  but  their  number  is  not  very  great, 
as  will  be  seen  by  comparing  the  map  with  the  state- 
ments in  the  Appendix.  I  have  experienced  more 
difficulty  in  colouring  fringing-reefs  than  in  colouring 
barrier-reefs,  as  the  former,  from  their  small  size, 
have  not  much  attracted  the  attention  of  navigators. 
As  I  have  had  to  seek  my  information  from  all  kinds 
of  sources,  I  do  not  venture  to  hope  that  the  map  is 
free  from  errors.  Nevertheless,  I  trust  it  will  give 
an  approximately  correct  view  of  the  general  distri- 
bution of  the  coral-reefs  throughout  the  world,  (with 
the  exception  of  some  fringing-reefs  on  the  coast  of 
Brazil,  not  included  within  the  limits  of  the  map,)  and 
of  their  arrangement  into  the  three  great  classes  which, 
though  necessarily  ill- defined  from  the  nature  of  the 
objects  classified,  have  been  adopted  by  most  voyagers. 
I  may  further  remark,  that  the  dark-blue  colour  repre- 
sents land  entirely  composed  of  coral-rock ;  the  pale 
blue,  land  with  a  wide  and  thick  border  of  coral-rock ; 
and  the  red,  land  with  a  mere  narrow  fringe  of  coral-rock. 
Looking  now  at  the  map  under  a  theoretical  point 
of  view,  the  two  blue  tints  signify  that  the  foundations 
of  the  reefs  thus  coloured  have  largely  subsided,  and 
that  the  rate  of  subsidence  has  been  less  than  the  up- 
ward growth  of  the  corals.  It  is  also  probable  that  in 
many  cases  the  foundations  are  still  subsiding.  The  red 
signifies  that  the  shores  thus  coloured  support  fringing- 
reefs  ;  and  they  have  not,  as  a  general  rule,  recently 


164  DISTRIBUTION   OF    CORAL-REEFS.       Ch.  YI. 

Bubsided,  at  least  to  any  considerable  amount,  for  tlie 
effects  of  subsidence  on  a  small  scale  would  hardly  be 
distinguishable.  Such  shores  must  either  have  remained 
stationary  since  the  period  when  they  were  first  fringed  ; 
or  they  may  have  been  repeatedly  upraised,  with  new 
lines  of  reefs  successively  formed  round  them.  If,  how- 
ever, coral-reefs  became  attached  for  the  first  time  to  a 
shore  which  was  subsiding,  or  if  a  barrier-reef  was  de- 
stroyed and  submerged  with  a  new  reef  re-attached 
to  the  shore,  this  would  necessarily  belong  at  first 
to  the  fringing  class,  and  would  be  coloured  red, 
although  the  land  was  sinking.  So  it  would  be  with 
a  subsiding  shore,  if  it  plunged  at  a  very  high  angle 
beneath  the  sea,  for  in  this  case  the  reef  would  remain 
closely  attached  to  the  land  as  it  grew  upwards,  and 
would  resemble  in  all  respects  a  fringing-reef.  This 
source  of  doubt  applies  especially  to  atolls  which 
have  been  upraised  (such  as  Metia  and  Elizabeth 
Islands),  for  from  the  steepness  of  their  sub-marine 
flanks,  a  reef  growing  up  during  a  subsequent 
period  of  subsidence  round  them,  would  still  continue 
closely  to  skirt  the  land,  and  would  therefore  be 
coloured  red.  Well-characterised  atolls  or  encircling 
reefs,  where  several  occur  together  in  a  group,  or  a 
single  barrier-reef  if  of  large  dimensions,  clearly  indicate 
a  movement  of  subsidence.  The  evidence  from  a  single 
atoll,  or  from  a  single  encircling-reef,  must  be  received 
with  caution,  for  the  former  may  be  based  upon  a  sub- 
merged crater  or  bank,  and  the  latter  on  a  submerged 
margin  of  sediment  or  of  worn-down  rock. 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  165 

On  the  distribution  of  the  different  classes  of  reejs. 
• — Having  made  the  foregoing  preliminary  remarks,  I 
will  now  consider  how  far  the  distribution  of  the  dif- 
ferent kinds  of  coral-islands  and  reefs  corroborates  our 
theory.  A  glance  at  the  map  shows  that  the  reefs 
which  are  coloured  blue  and  red,  and  which  are  believed 
to  owe  their  origin  either  to  widely  different  move- 
ments, or  in  the  case  of  the  red  to  a  stationary  condition, 
are  not  indiscriminately  mingled  together.  Atolls  and 
barrier-reefs,  as  may  be  seen  by  the  two  blue  tints, 
generally  lie  near  each  other ;  and  this  would  be  the 
natural  result  of  both  having  been  produced  by  the  same 
movement  of  subsidence.  Thus,  all  the  Society  Islands 
are  encircled  by  barrier-reefs ;  and  to  the  N.W.  and 
S.E .  there  are  several  scattered  atolls.  To  the  eastward 
lies  the  great  Paumotu  or  Low  Archipelago  consisting 
entirely  of  atolls ;  and  still  further  to  the  N.E.,  we  meet 
with  the  Mendana  or  Marquesas  Islands,  which,  from 
their  abrupt  and  deeply  indented  shores,  Dana  ^  be- 
lieves have  probably  subsided ;  though  hardly  any  coral- 
reefs  exist  there,  which  might  have  afforded  additional 
evidence  of  subsidence.  In  the  midst  of  the  Caroline 
atolls,  there  are  three  fine  encircled  islands.  The 
northern  point  of  the  barrier-reef  of  New  Caledonia 
apparently  forms,  as  before  remarked,  a  great  atoll. 
The  Australian  barrier  is  described  as  including  both 
atolls  and  small  encircled  islands.     Captain  King  ^ 


'  Corals  and  Coral  Islands,  1872,  p.  325. 

^  Sailing  Directions,  appended  to  vol.  ii.  of  his  Surveying  Voyage 
to  Australia. 


166  DISTRIBUTION   OF    CORAL-REEFS.      Ch.  VI. 

mentions  many  atoll-formed  and  encircling  coral-reefs, 
some  of  which  lie  within  the  barrier,  and  others  may  be 
said  (for  instance,  between  lat.  16°  and  13°)  to  form 
part  of  it.  Flinders^  has  described  an  atoll-formed 
reef  in  lat.  10°,  seven  miles  long  and  from  one  to  three 
broad,  resembling  a  boot  in  shape,  and  apparently  in- 
cluding a  deep  lagoon.  Eight  miles  westward  of  this, 
and  forming  part  of  the  barrier,  lie  the  Murray  Islands, 
which  are  high  and  are  encircled.  In  the  Corallian  sea, 
between  the  two  great  barrier-reefs  of  Australia  and 
New  Caledonia,  there  are  many  low  islets  and  coral- 
reefs,  some  of  which  are  annular,  cr  like  a  horse-shoe. 
Bearing  in  mind  the  smallness  of  the  scale  of  our  map 
(the  lines  of  latitude  being  900  miles  apart),  we  see  that 
none  of  the  larger  groups  of  reefs  and  islands  which  are 
coloured  blue,  and  which  are  supposed  to  have  been 
produced  by  long-continued  subsidence,  lie  near  exten- 
sive lines  of  coast  coloured  red ;  these  latter  having 
either  long  remained  stationary,  or  having  been  upraised 
with  new  reefs  re-formed  on  them.  Where  red  and  blue 
circles  do  occur  near  each  other,  I  am  able,  in  several 
instances,  to  show  that  there  have  been  oscillations  of 
level ;  subsidence  having  preceded  the  elevation  of  the 
red  spots ;  and  elevation  having  preceded  the  subsidence 
of  the  blue  spots ;  and  in  this  case  the  juxtaposition  of 
reefs  belonging  to  the  two  great  types  of  structure  is 
little  surprising.  We  find,  therefore,  that  atolls  and 
barrier-reefs,  which  both  owe  their  origin  to  subsidence, 
lie  near  together  and  are  as  a  general  rule  separated 

'  Voyage  to  Terra  Australts,  vol.  ii.  p.  336. 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  167 

from  fringing-reefs,  which  show  that  the  land  is  sta- 
tionary or  rising ;  and  all  this  holds  good  to  the  full 
extent  which  might  have  been  anticipated  by  our 
theory. 

As  atolls  have  been  formed  during  the  sinking  of 
the  land  by  the  upward  growth  of  the  reefs  which 
primarily  fringed  the  shores  of  ordinary  islands ;  so  we 
might  expect  that  these  rings  of  coral,  like  so  many 
rude  outline  charts,  would  still  retain  traces  of  the 
general  form,  or  at  least  of  the  general  range,  of  the 
islands  round  which  they  were  first  modelled.  That  this 
is  the  case  with  the  atolls  in  the  Southern  Pacific,  as 
far  as  their  range  is  concerned,  seems  highly  probable, 
when  we  observe  that  the  principal  groups  are  directed 
in  nearly  N.W.  and  S.E.  lines,  and  that  nearly  all  the 
mountainous  islands  and  shores  in  the  S.  Pacific  range 
in  this  same  direction;  namely, N. -Eastern  Australia, 
New  Caledonia,  the  northern  half  of  New  Zealand,  the 
New  Hebrides,  Saloman,  Navigator,  Society,  Marquesas, 
and  Austral  Archipelagoes.  In  the  Northern  Pacific, 
the  Caroline  atolls  almost  abut  against  the  N.W.  line  of 
the  Marshall  atolls,  much  in  the  same  manner  as  the 
E.  and  W.  line  of  islands  extending  from  Ceram  to  New 
Britain  abuts  against  New  Ireland.  In  the  Indian 
Ocean  the  Laccadive  and  Maldiva  atolls  extend  nearly 
parallel  to  the  western  mountains  of  India.  There  is 
also  a  close  resemblance  between  atolls  and  ordinary 
islands  in  the  manner  in  which  they  are  grouped,  as 
well  as  in  their  shapes.  Thus  the  outline  of  all  the 
larger  groups  of  atolls  is  elongated;  and  the  atolls 


168  DISTRIBUTION   OF    CORAL-REEFS.       Ch.  VI. 

themselves  are  generally  elongated  in  the  same  direc- 
tion with  the  group.  The  Chagos  group  is  less  elon- 
gated than  is  usual,  and  the  individual  atolls  in  it  are 
likewise  hut  little  elongated ;  this  is  strikingly  seen  by 
comparing  them  with  the  neighbouring  Maldiva  atolls. 
In  the  Marshall  and  Maldiva  archipelagoes,  the  atolls 
are  ranged  in  two  parallel  lines,  like  a  great  double 
mountain- chain.  Some  of  the  atolls  in  the  larger  archi- 
pelagoes stand  so  near  to  each  other,  and  have  such 
an  evident  relationship,  that  they  compose  little  sub- 
groups ;  in  the  Caroline  Archipelago,  one  such  sub-group 
consists  of  Pouynipete,  a  lofty  island  encircled  by  a 
barrier-reef,  and  separated  by  a  channel  only  four  miles 
and  a  half  in  width  from  Andeema  atoll,  with  a  second 
atoll  a  little  further  removed. 

On  the  direct  evidence  of  the  blue  spaces  in  the  map 
having  subsided  during  the  upward  growth  of  the  reefs 
thus  coloured^  and  of  the  red  spaces  having  remained 
stationary y  or  having  been  upraised. — With  respect  to 
subsidence,  we  cannot  expect  to  obtain  in  semi-civil- 
ised countries  proofs  of  a  movement  which  tends  to 
conceal  its  own  evidence.  But  on  coral-islands  we  see 
plain  signs  of  a  round  of  decay  and  renovation— on 
some,  the  last  vestiges  of  land — its  first  commence- 
ment on  others :  we  hear  of  storms  washing  away  and 
desolating  the  islets  to  an  extent  which  astonished  the 
inhabitants ;  we  know  by  the  great  fissures  with  which 
some  of  ihese  islands  are  traversed,  and  by  the  earth- 
quakes felt  under  others,  that  subterranean  disturbances 
are  in  progress.  All  these  appearances  accord  well  with 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  169 

the  belief  that  these  islands  have  recently  subsided ; 
though  not  proving  the  fact.  At  Keeling  atoll,  however, 
I  have  described  certain  appearances,  which  seem 
directly  to  show  that  the  surface  subsided  there  during 
the  late  earthquakes.  In  the  Caroline  Archipelago, 
the  island  of  Pouynipete  (Plate  I.  fig.  7),  from  being  en- 
circled by  a  great  barrier-reef,  must  have  subsided,  in 
accordance  with  our  theory;  and  in  the  New  South 
Wales  Lit.  Advert.  Feb.  1835,  there  is  an  account  of 
this  island,  (subsequently  confirmed  by  Mr.  Campbell,) 
in  which  it  is  said,  '  At  the  N.E.  end,  at  a  place  called 
Tamen,  there  are  ruins  of  a  town,  noiv  only  accessible 
by  boats,  the  waves  reaching  to  the  steps  of  the  houses,* 
Hence  it  would  appear  that  the  island  must  have  sub- 
sided since  these  houses  were  built.  Mr.  Hales  also  states, 
from  information  acquired  during  the  U.S.  Exploring 
Expedition,  that  certain  buildings  on  this  island  are 
now  in  the  water :  *  what  were  once  paths  are  now 
passages  for  canoes,  and  when  the  walls  are  broken  down 
the  water  enters  the  enclosures.'  ^  Vanikoro,  according 
to  the  Chevalier  Dillon,  is  often  violently  shaken  by 
earthquakes,  and  there,  the  unusual  depth  of  the  channel 
between  the  shore  and  the  reef,  the  wall-hke  structure 
on  the  inner  side  of  the  reef,  the  small  quantity  of  low 
alluvial  land  at  the  foot  of  the  mountains,  and  the  almost 
entire  absence  of  islets  on  the  reef,  all  seem  to  show 
that  this  island  has  not  remained  long  at  its  present 
level.^    At  the  Society  Archipelago,  on  the  other  hand, 

*  Professor  Dana  also  concludes  from  these  facts  that  the  island 
is  subsiding ;  see  Corals  and  Coral  Islands,  1872,  p.  330. 

'^  See  Captain  Dillon's  Voyage  in  search  of  La  Peyrouss.    M. 


170  DISTRIBUTION   OF   CORAL-REEFS.       Ch.  VI, 

where  a  slight  tremor  is  only  rarely  felt,  the  shoalness 
of  the  lagoon- channels  round  some  of  the  islands,  the 
number  of  islets  formed  on  the  reefs  of  others,  and  the 
broad  belt  of  low  land  at  the  foot  of  the  mountains,  all 
indicate  that  these  islands  have  not  undergone  for  a 
long  period,  any  movement  of  subsidence,  although 
their  encircling  reefs  must  on  our  theory  have  been 
originally  produced  through  subsidence.^ 

Although  Dana  admits  that  atolls  and  barrier-reefs 
must  have  been  originally  formed  by  the  subsidence  of 
their  foundations,  he  believes  that  a  large  number  of 
atolls,  situated  between  the  Paumotu  or  Low  group  to  the 
east  and  the  Feejees  to  the  west,  and  northward  nearly 
as  far  as  the  equator,  have  recently  been  uplifted  to 
the  height  of  a  very  few  feet.^  Mr.  Couthouy  came  to  a 
similar  conclusion  during  the  same  expedition  with  re- 
spect to  many  of  the  Paumotu  atolls.  These  observers 
ground  their  belief  chiejfly  from  having  found  the  great 
shells  of  the  Tridacna  vertically  embedded  in  coral- 
rock,  at  a  height  at  which  they  cannot  now  exist.  Mr. 
Couthouy  also  states  that  he  found  corals  standing  on 

Cordier,  in  his  Report  on  the  Voyage  of  the  Astrolabe  (vol.  i.  p.  cxi.), 
speaking  of  Vanikoro,  says  the  shores  are  surrounded  by  reefs  of 
madrepore,  *  qu'on  assure  Stre  de  formation  tout-d-fait  modeme.' 

'  Mr.  Couthouy  states  (Remarks,  p.  44)  that  at  Tahiti  and  Eimeo 
the  space  between  the  reef  and  the  shore  has  been  nearly  filled  up 
by  the  extension  of  coral-reefs  of  the  kind  which  within  most  barrier- 
reefs  merely  fringe  the  land.  From  this  circumstance,  he  arrives  at 
the  same  conclusion  as  I  have  done,  namely,  that  the  Society  Islands 
have  remained  stationary  during  a  long  period. 

2  Corals  and  Coral  Islands,  1872,  pp.  199,  346.  See  also  Mr. 
Couthouy's  Remarks  on  Coral  Formations.  [See  Wilkes'  Exploring 
Expedition,  vol.  i.  chap,  xv.] 


Ch.  VI.      DISTRIBUTION   OF    CORAL-REEFS.  171 

the  shores  and  in  the  midst  of  the  lagoons,  from  12  to 
30  inches  above  the  sea-level,  with  the  tips  of  their 
branches  dead.  He  also  refers  to  masses  of  coral-rock 
which  he  thinks  could  not  have  been  carried  into  their 
present  positions  and  subsequently  been  water-worn, 
whilst  the  land  stood  at  its  present  level.  Nevertheless 
it  might,  I  think,  have  been  anticipated  that  many 
atolls  would  have  presented  the  above  appearance,  if 
they  had  long  remained  at  a  stationary  level.  The  sea, 
after  the  land  had  at  some  former  period  subsided  a  few 
feet,  would  have  continued  for  a  long  time  breaking 
over  the  whole  reef,  even  after  the  living  corals  had 
grown  up  to  their  full  height  on  the  outer  margin.  The 
waters  of  the  lagoon  would  thus  have  been  disturbed  and 
raised,  so  that  shells  and  corals,  from  being  bathed  by 
the  troubled  waters,  could  have  existed  at  a  greater 
height  than  that  at  which  they  could  exist  after  the 
reef  had  been  raised  by  the  agglutination  of  fragments 
and  sand,  and  after  islets  had  been  formed  on  its  sur- 
face. Even  the  mere  outward  growth  of  a  reef,  and  the 
consequent  increase  of  its  breadth,  by  checking  the 
inward  rush  of  the  breakers,  would  tend  to  lower  the 
level  in  the  lagoon  at  which  corals  and  shells  can  live. 
We  have  seen  that  at  the  Keeling  Islands  there  are 
fields  of  rotten  coral  with  the  tips  of  their  branches  pro- 
jecting above  the  surface  of  the  lagoon, — the  result  of 
the  tides  not  rising  so  high  as  formerly  (as  is  said  to  be 
the  case  by  the  inhabitants),  from  the  closing  of  the 
channels  between  the  islets  on  the  outer  reef,  and  from 
the  lagoon  being  partially  choked  up  by  the  growth 


1 72  DISTRIBUTION   OF   CORAL-REEFS.       Ch.  VL 

of  the  corals.  Here,  so  far  from  there  having  been 
any  recent  elevation  of  the  land,  we  have  reason  to  be- 
lieve that  there  has  been  subsidence.  Messrs.  Dana 
and  Couthouy's  observations  relate  chiefly  to  the 
Paumotu  atolls,  and  here  again  some  facts  indicate 
recent  subsidence  rather  than  elevation :  I  refer  to  the 
manner  in  which  Chain  atoll  suffered  during  a  storm, 
and  to  SirE.  Belcher's  statement,'  that  after  an  interval 
of  fourteen  years,  a  well-known  islet  had  disappeared, 
and  the  lagoon  at  a  particular  spot  had  become  deeper 
than  it  was  before. 

There  are  other  causes  of  change  which  might,  as  it 
appears  to  me,  easily  lead  to  a  mistaken  belief  in  the 
recent  elevation  of  low  coral  formations.  We  must  re- 
member that  the  outer  and  living  margin  of  the  reef 
grows  up  to  a  height  determined  by  the  constant  break- 
ing of  the  waves.  Outside  this  margin  there  is  a  sloping 
surface  also  covered  with  living  corals,  but  belonging  to 
species  which  do  not  grow  to  the  surface  ;  and  beyond 
this,  there  is  a  much  steeper  slope,  consisting  of  coral- 
sand.  Now  after  a  somewhat  rapid  subsidence  of,  for 
instance,  one  or  two  fathoms,  we  may  feel  almost  sure 
that  the  corals  on  the  outer  margin  would  grow  up 
quickly  to  the  surface  and  form  a  nearly  vertical  wall. 
This  would  be  succeeded  outside  by  a  steeply  sloping 
surface  of  living  corals,  which  would  likewise  sooner  or 
later  grow  up  to  their  former  level ;  but  outside  this, 
the  much  steeper  slope,  formed  by  the  slow  accumula- 
tion of  fine  detritus,  would  not  recover  for  a  very  long 

>  Voyage  Round  the  World,  vol.  i.  1843,  p.  382. 


Cn.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  173 

time  its  former  angle  relatively  to  the  upper  bank  of 
living  corals.  Now  it  seems  highly  probable  that  a 
change  of  any  kind  in  the  outer  submarine  slope  of 
an  island  would  influence  the  height  to  which  the  living 
corals  on  the  margin  would  be  constantly  bathed  by  the 
surf,  and  to  which  they  would  consequently  be  able  to 
grow.  Again,  it  seems  possible  that  if  during  one  season 
of  the  year  the  currents  of  the  sea  and  the  prevalent 
winds  coincided  in  direction,  the  waves  would  then  reach 
to  a  higher  level  and  the  corals  grow  higher,  than  at 
another  season  when  the  currents  and  the  winds  did  not 
coincide  in  direction.  The  result  would  be  that  the 
corals  which  during  the  one  season  had  grown  to  their 
full  height,  would  at  the  other  season  expose  their  dead 
summits,  and  give  the  appearance  of  the  land  having 
been  slightly  elevated.  I  have  referred  to  these  possi- 
bilities merely  to  show  how  difficult  it  must  ever  be  to 
judge  whether  low  coral  formations  have  really  been 
raised  to  a  height  of  only  two  or  three  feet,  as  Dana 
believes  to  have  been  the  case  with  several  groups  of 
atolls.  To  me  it  seems  more  probable  that  all  the  above- 
mentioned  appearances  merely  indicate  that  the  atolls 
in  question  have  long  remained  at  the  same  level.  If, 
however,  the  conclusion  arrived  at  by  so  excellent  an 
observer  as  Professor  Dana,  should  hereafter  be  con- 
firmed, the  question  will  arise,  seeing  how  immense  an 
area  has  been  thus  affected,  whether  those  geologists 
are  not  right  who  believe  that  the  level  of  the  ocean 
is  subject  to  secular  changes  from  astronomical 
causes. 

13 


174  DISTRIBUTION   OF    CORAL-REEFS.       Ch.  VI. 

Evidence  that  many  coasts  fringed  with  coral-reefs 
and  coloured  red  on  the  map,  have  been  recently  ele- 
vated.— As  the  areas  which  have  slowly  subsided  with- 
in the  period  of  existing  corals  are  msiny  and  large, 
we  might  have  expected  that  such  movements  would 
have  been  counterbalanced  by  the  recent  elevation  of 
other  equally  large  areas  ;  and  this,  as  we  shall  see, 
apparently  holds  good.  Corals  attached  to  a  rising 
coast  would  necessarily  form  a  fringing-reef ;  and  this 
reef  would  be  upraised  at  each  successive  elevation,  with 
a  new  one  formed  on  the  coast  at  a  lower  level.  Such 
reefs  would  differ  only  by  their  smaller  breadth  from 
those  attached  to  a  shore  which  had  long  remained 
stationary  ;  for  they  would  not  have  had  sufficient  time 
to  form  a  foundation  of  their  own  detritus  and  grow  far 
outwards.  Fringing-reefs  indicate  as  a  general  rule 
that  the  land  to  which  they  are  attached  has  not  re- 
cently subsided.  But  they  do  not  tell  us  whether  the 
land  is  rising  or  stationary.  Nevertheless,  the  crust 
of  the  earth  seems  liable  to  such  incessant  changes  of 
level  that  a  long- continued  stationary  condition  ap- 
parently is  rare.  We  may  infer  that  this  is  so  from 
the  number  of  cases,  within  the  limits  of  our  map,  in 
which  upraised  corals  or  other  organic  remains  have 
been  found  on  the  shores  which  are  fringed  with  reefs, 
and  are,  therefore,  coloured  red.  It  may  be  mentioned 
as  bearing  on  this  subject,  that  I  was  much  surprised 
on  first  reading  a  memoir  on  coral  formations  by 
MM.  Quoy  and  Gaimard,*  by  finding  that  their  de- 
*  Annales  des  Sciences  Nat,  torn.  vi.  p.  279,  Ac. 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  175 

scriptions  applied  only  to  reefs  of  the  fringing  class,  for 
I  knew  that  they  had  crossed  both  the  Pacific  and  Indian 
Oceans ;  but  my  surprise  ended  in  satisfaction,  when  I 
discovered  that  all  the  islands  which  they  had  visited, 
though  several  in  number — namely,  Mauritius,  Timor, 
New  Guinea,  the  Mariana  and  Sandwich  Archipelagoes 
— could  be  shown  by  their  own  statements  to  have 
been  elevated  within  a  recent  geological  period. 

I  will  now  enter  on  some  details,  showing  how  many  of 
the  islands  and  coasts  which  from  being  fringed  with  reefs 
are  coloured  red  on  our  map,  have  been  recently  upraised. 

Sandwich  Islands. — Several  of  these  islands  are  fringed 
with  reefs,  though  Dana  found  very  few  corals  at  Hawaii ; 
and  almost  every  naturalist  who  has  visited  them  has  there 
observed  upraised  corals  and  shells,  apparently  identical 
with  living  species.  The  Eev.  W.  Ellis  informs  me  that 
he  noticed  round  several  parts  of  Hawaii,  beds  of  coral 
detritus,  about  twenty  feet  above  the  level  of  the  sea,  and 
where  the  coast  is  low  they  extend  far  inland.  Upraised 
coral-rock  forms  a  considerable  part  of  the  borders  of  Oahu  ; 
and  at  Elizabeth  Island  ^  it  composes  three  strata,  each 
about  ten  feet  thick.  Nihau,  which  forms  the  northern,  as 
Hawaii  does  the  southern  end  of  the  group  (350  miles  in 
length),  likewise  seems  to  consist  of  coral  and  volcanic 
rocks.  Mr.  Couthouy  ^  has  lately  described  several  upraised 
beaches  and  ancient  reefs  with  their  surfaces  perfectly  pre- 
served, as  well  as  beds  of  recent  shells  and  corals,  at  the 
Islands  of  Maui,  Morokai,  Oahu,  and  Tauai  (or  Kauai),  all 
in  this  group.  Mr.  Pierce,  an  intelligent  resident  at  Oahu, 
is  convinced,  from  changes  which  have  taken  place  within 
his  memory  during  the  last  sixteen  years,  *  that  the  eleva- 

*  Zoology  of  Captain  Beechey's  Voyage,  p.  176.     See  also  MM. 
Quoy  and  Gaimard  in  Annales  des  Sciences  Nat.  torn.  vi. 
^  Remarks  on  Coral  Formations,  p.  51. 


176  DISTRIBUTION   OF   CORAL-REEFS.       Ch.  VL 

tion  is  at  present  going  forward  at  a  very  perceptible  rate.' 
The  natives  at  Kauai  state  that  the  land  is  there  gaining 
rapidly  on  the  sea  ;  and  Mr.  Couthouy  has  no  doubt,  from 
the  nature  of  the  strata,  that  this  is  the  result  of  elevation. 

Elizabeth  Island,  in  the  southern  part  of  the  Low  or 
Paumotu  Archipelago,  and  Metia  in  the  northern  part, 
consist  of  upraised  coral-rock,  closely  fringed  by  living 
reefs.^  In  cases  like  these,  where  islands  have  the  appear- 
ance which  one  of  the  smaller  surrounding  atolls  with  a 
shallow  lagoon  would  present  if  elevated,  we  are  led  to  con- 
clude that  the  elevation  has  taken  place  at  an  epoch  not 
geologically  remote ;  for  it  is  improbable  that  such  small 
and  low  fabrics  should  have  resisted  for  an  immense  period 
all  the  many  destroying  agents  of  nature.  When  the  sur- 
face of  an  ordinary  island  is  strewed  with  marine  remains, 
from  the  beach  to  a  certain  height,  and  not  above  that 
height,  it  is  exceedingly  improbable  that  these  remains, 
although  they  may  not  have  been  specifically  examined, 
should  belong  to  any  very  ancient  period.  It  is  necessary 
to  bear  these  remarks  in  mind  in  considering  the  evidence 
of  the  elevatory  movements  in  the  Pacific  and  Indian 
Oceans,  as  it  does  not  often  rest  on  specific  determinations, 
and  therefore  should  be  received  with  caution.  Six  of  the 
Cook  and  Austral  Islands  (S.W.  of  the  Society  group)  are 
fringed  ;  of  these,  five  were  described  to  me  by  the  Eev.  J. 
Wilhams,  as  formed  of  coral-rock  (associated  with  some 
basalt  in  Mangaia),  and  the  sixth  as  lofty  and  basaltic. 
Mangaia  is  nearly  300  feet  high  with  a  level  summit ;  and, 
according  to  Mr.  S.  Wilson,^  is  an  upraised  reef;  *and 
there  are  in  the  central  hollow,  formerly  the  bed  of  the 
lagoon,  many  scattered  patches  of  coral-rock,  some  of  them 
raised  to  a  height  of  forty  feet.'     These  knolls  of  coral-rock 

'  Beechey's  Voyage  in  the  Pacific,  p.  46,  4to  edit.  Dana,  Corals 
and  Coral  Islands,  p.  193.  Wilkes,  U.S.  Exploring  Expedition,  vol.  i. 
p.  337. 

'^  Couthouy's  Remarks,  p.  34. 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  177 

were  evidently  once  reefs  within  the  lagoon  of  an  atoll. 
Mr.  Martens,  at  Sydney,  informed  me  that  this  island  is 
surrounded  by  a  terrace-like  plain  at  about  the  height  of 
100  feet,  which  probably  marks  a  pause  in  its  elevation, 
from  these  facts  we  may  infer  that  the  Cook  and  Austral 
Islands  have  been  upheaved  at  a  not  very  remote  period. 

Savage  Island  (S.E.  of  the  Friendly  group)  is  according 
to  Forster  about  40  feet  in  height,  and  according  to 
Williams  about  100  feet.  Forster  ^  describes  the  plants  as 
already  growing  out  of  the  dead  but  still  upright  and 
spreading  trees  of  coral ;  and  the  younger  Forster  ^  believes 
that  an  ancient  lagoon  is  now  represented  by  a  central 
plain  :  here  we  cannot  doubt  that  the  elevatory  forces  have 
recently  acted.  The  same  conclusion  may  be  extended  to 
the  islands  of  the  Friendly  Group,  which  have  been  well 
described  in  the  second  and  third  voyages  of  Cook,  and 
recently  by  Dana.  The  surface  of  Tongatabou  is  low  and 
level,  but  with  parts  50  or  60  feet  high  ;  the  whole  consists 
of  coral-rock,  '  which  yet  shows  the  cavities  and  irregular- 
ities worn  into  it  by  the  action  of  the  tides.'  ^  On  Eoua 
the  same  appearances  were  noticed  at  an  elevation  of 
between  200  and  300  feet.  Vavao,  also,  at  the  opposite  or 
northern  end  of  the  group,  consists,  according  to  the  Kev. 
J.  Williams,  of  coral-rock.  Tongatabou,  with  its  northern 
extensive  reefs,  resembles  either  an  upraised  atoll  with  one 
half  originally  imperfect,  or  one  unequably  elevated  ;  and 
Anamouka,  an  atoll  equably  elevated.  This  latter  island 
contains^  in  its  centre  a  salt-water  lake,  about  a  mile  and  a 
half  in  diameter,  without  any  communication  with  the  sea, 
and  around  it  the  land  rises  gradually  like  a  bank  :  the 
highest  part  is  only  between  twenty  and  thirty  feet;  but 

*  Observations  made  during  Voyage  Round  the  World,  p.  147. 
'  Voyage,  vol.  ii.  p.  16.S. 

*  Cook's  Third  Voyage  (4to  edit.),  voi.  i.  p.  314. 

*  Ibid.  vol.  i.  p.  2U5. 


178  DISTRIBUTION   OF    CORAL-REEFS.       Ch.  VL 

on  this  part,  as  well  as  on  the  rest  of  the  land,  (which,  as 
Cook  observes,  rises  above  the  height  of  a  true  lagoon- 
island,)  coral-rock  like  that  on  the  beach  was  found.  In  the 
Navigator  or  Samoan  Archipelago,  Mr.  Couthouy  ^  found 
on  Manua  many  large  fragments  of  coral  at  the  height  of 
eighty  feet,  *  on  a  steep  hill-side,  rising  half  a  mile  inland 
from  a  low  sandy  plain  abounding  in  marine  remains.' 
The  fragments  were  embedded  in  a  mixture  of  decomposed 
lava  and  sand.  It  is  not  stated  whether  they  were  accom- 
panied by  shells,  or  whether  the  corals  resembled  recent 
species  ;  as  these  remains  were  embedded,  they  possibly 
may  belong  to  a  remote  epoch ;  but  I  presume  this  was 
not  the  opinion  of  Mr.  Couthouy.  On  the  other  hand,  Mr. 
Dana  says  expressly  in  one  place,  that  *  no  satisfactory 
evidences  of  elevation  were  detected  about  these  islands ; ' 
and  in  another  place  he  says  (p.  326)  that  some  of  the 
islands  have  probably  subsided.  Earthquakes  are  very 
frequent  in  this  archipelago. 

Still  proceeding  westward  we  come  to  the  New  Hebrides, 
On  these  islands,  Mr.  G.  Bennett  (author  of  "Wanderings 
in  New  South  Wales)  informs  me  that  he  found  much  coral 
at  a  great  altitude,  which  he  considered  of  recent  origin.^ 
The  Loyalty  Islands  are  situated  west  of  the  New  Hebrides, 
and  not  far  from  New  Caledonia  ;  and  one  of  these  islands 
has  been  clearly  shown  by  the  Rev.  W.  B.  Clarke  (Journal 
of  Geolog.  Soc.  1847,  p.  61)  to  consist  wholly  of  coral-rock, 
and  to  have  been  raised  within  a  recent  period  by  at  least 
two  distinct  elevations  to  the  height  of  250  feet.  The 
shores  are  now  fringed  by  reefs.  Respecting  Santa  Cruz 
and  the  Salomaji  Archipelago  ^  I  have  no  information  ;  but 
at  New  Ireland,  which  forms  the  northern  point  of  the 

*  Remarks  on  Coral  Formations,  p.  50. 

'  [Prof.  Moseley,  Notes  of  a  Naturalist  in  the  Challenger,  speaks  ol 
ind 'cations  of  elevation  to  an  extent  of  about  5  feet.] 
"  [See  Mr.  Guppy's  description,  Appendix  II.] 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  179 

latter  chain,  both  Labillardiere  and  Lesson  have  described 
large  beds  of  an  apparently  very  modern  madreporitic  rock, 
with  the  form  of  the  corals  little  altered.  The  latter 
author  *  states  that  this  formation  composes  a  newer  line 
of  coast,  modelled  round  an  ancient  one.  There  only  re- 
mains to  be  described  in  the  Pacific,  that  curved  line  of 
fringed  islands,  of  which  the  Marianas  form  the  main  part. 
Of  these  Guam,  Rota,  Tinian,  Saypan,  and  some  islets 
farther  north,  are  described  by  Quoy  and  Gaimard,^  and 
Chamisso,^  as  chiefly  composed  of  madreporitic  limestone, 
which  attains  a  considerable  elevation,  and  is  in  several 
cases  worn  into  successively  rising  cliffs :  the  two  former 
naturalists  seem  to  have  compa.red  the  corals  and  shells 
with  the  existing  ones,  and  state  that  they  are  of  recent 
species.  Peel  Island,  one  of  the  Bonin  or  Arzobispo  group, 
between  the  Marianas  and  Japan,  has  f ringing-reefs  ;  and 
it  has  clearly  been  upraised  to  a  height  of  at  least  60  feet, 
as  shown  by  the  ridges  of  corals  and  shells  extending 
uniformly  at  this  level.'*  Fais,  which  lies  in  the  prolonged 
line  of  the  Marianas,  between  this  group  and  the  Pellews, 
is  fringed  by  reefs ;  it  is  90  feet  high,  and  consists  entirely 
of  madreporitic  rock.-^ 

In  the  East  Indian  Archipelago,  many  authors  have 
recorded  proofs  of  recent  elevation.  M.  Lesson**  states  that 
near  Port  Dory,  on  the  north  coast  of  New  Guinea,  the 
shores  are  flanked,  to  the  height  of  150  feet,  by  madre- 
poritic strata  of  a  modern  date.  He  mentions  similar  for- 
mations at  Waigiou,  Amboina,  Bourou,  Ceram,  Sonda,  and 
Timor :  at  this  latter  place,  MM.  Quoy  and  Gaimard  ^  have 

*  Voyage  de  la  Coquille,  Part.  Zoolog. 

2  Freycinet's  Voyage  autour  du  Monde.  See  also  the  Hydro- 
graphical  Memoir,  p.  215.  '  Kotzebue's  First  Voyage. 

*  P.  W.  Graves,  Journal  of  Geological  Soc.  1855,  p.  532. 

*  Lutk^'s  Voyage,  vol.  ii.  p.  304. 

*  Partie  Zoolog.  Voyage  de  la  Coquille. 
''  Ann.  des  Scien.  Nat.,  torn.  vi.  p.  281. 


180  DISTRIBUTION   OF   CORAL-REEFS.       Ch.  VI. 

likewise  described  the  primitive  rocks,  as  coated  to  a  con- 
Biderable  height  with  coral.  Some  small  islets  eastward  of 
Timor  are  said  in  Kolff 's  Voyage  *  to  resemble  small  coral 
islets  upraised  some  feet  above  the  sea.  Dr.  Malcolmson 
informs  me  that  Dr.  Hardie  found  in  Java  an  extensive 
formation,  containing  an  abundance  of  shells,  of  which  the 
greater  part  appear  to  be  of  existing  species.  Dr.  Jack  "^ 
has  described  some  upraised  shells  and  corals,  apparently 
recent,  on  Pulo  Nias  off  Sumatra ;  and  Marsden  relates  in 
his  history  of  this  great  island,  that  the  names  of  many 
promontories  show  that  they  were  originally  islands.  On 
part  of  the  west  coast  of  Borneo  and  at  the  Sooloo  Islands, 
the  form  of  the  land,  the  nature  of  the  soil,  and  the  water- 
washed  rocks,  present  appearances  ^  (although  it  is  doubt- 
ful whether  such  vague  evidence  is  worthy  of  mention)  of 
having  recently  been  covered  by  the  sea ;  and  the  inhabi- 
tants of  the  Sooloo  Islands  believe  that  this  has  been  the 
case.  Mr.  Cuming,  who  has  lately  investigated  with  so 
much  success  the  mollusca  of  the  Phillippines,  found  near 
Cabagan,  in  Luzon,  about  50  feet  above  the  level  of  the 
R.  Cagayan  and  70  miles  from  its  mouth,  a  large  bed  of 
fossil  shells :  these,  as  he  informs  me,  are  certainly  of  the 
same  species  with  those  now  existing  on  the  shores  of  the 
neighbouring  islands.    From  the  accounts  given  by  Captain 

'  Translated  by  Windsor  Earl,  chaps,  vi.  and  vii. 

'  Geolog.  Transact.  2nd  series,  vol.  i.  p.  403.  On  the  Peninsula 
of  Malacca,  in  front  of  Penang,  5°  30'  N.,  Dr.  Ward  collected  some 
shells  which  Dr.  Malcolmson  informs  me,  although  not  compared 
with  existing  species,  had  a  recent  appearance.  Dr.  Ward  describes 
in  this  neighbourhood  (Trans.  Asiat.  Soc.  vol.  xviii.,  part  2,  p.  166)  a 
single  water-worn  rock,  with  a  conglomerate  of  sea-shells  at  its  base, 
situated  six  miles  inland,  which,  according  to  the  traditions  of  the 
natives,  was  once  surrounded  by  the  sea.  Captain  Low  has  also 
described  (ibid.  Part  i.  p.  131)  mounds  of  shells  lying  two  milea 
inland  on  this  line  of  coast. 

'  Notices  of  the  East  Indian  Arch.,  Singapore,  1828,  p.  6,  and 
Append,  p.  43. 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  181 

Basil  Hall  and  Captain  BeecHey  ^  of  the  lines  of  inland 
reefs,  and  walls  of  coral  rock  worn  into  caves,  above  the 
present  reach  of  the  waves,  at  the  Loo  Choo  Islands,  there 
can  be  little  doubt  that  they  have  been  upraised  at  no  very 
remote  period. 

Dr.  Davy  ^  describes  the  northern  province  of  Ceylon  as 
being  very  low,  and  composed  of  a  limestone  with  shells 
and  corals  of  very  recent  origin  ;  he  adds,  that  it  does  not 
admit  of  a  doubt  that  the  sea  has  retired  from  this  district 
even  within  the  memory  of  man.  There  is  also  some 
reason  for  believing  that  the  eastern  shores  of  India,  north 
of  Ceylon,  have  been  upraised  within  the  recent  period.* 
On  the  opposite  side  of  the  Gulf  of  Bengal,  Captain  Hal- 
stead  everywhere  found  during  his  survey  of  the  Burmese 
coast  (as  he  informed  Sir  C.  Lyell),  proofs  of  recent  eleva- 
tion in  upraised  beaches  and  beds  of  shells  and  corals.  In 
the  Indian  Ocean  Mauritius  has  been  recently  upraised, 
as  I  have  shown  in  the  chapter  on  fringing-reefs.  The 
northern  extremity  of  Madagascar  is  described  by  Captain 
Owen  ^  as  formed  of  madreporitic  rock,  as  likewise  are  the 

'  Captain  B.  Hall,  Voyage  to  Loo  Choo,  Append,  pp.  xxi.  and  xxv. 
Captain  Beechey's  Voyage,  p.  496. 

'^  Travels  in  Ceylon,  p.  13.  This  madreporitic  formation  is  men- 
tioned by  M.  Cordier  in  his  report  to  the  Institute  (May  4,  1839)  on 
the  voyage  of  the  Chevrette,  as  one  of  immense  extent,  and  belonging 
to  the  latest  tertiary  period. 

3  Dr.  Benza,  in  his  Journey  through  the  N.  Circars  (the  Madras 
Lit.  and  Scient.  Journal,  vol.  v.),  has  described  a  formation  with 
recent  freshwater  and  marine  shells,  occurring  at  the  distance  of 
three  or  four  miles  from  the  present  shore.  Dr.  Benza,  in  conver- 
sation with  me,  attributed  their  position  to  a  rise  of  the  land.  Dr. 
Malcolmson,  however,  (and  there  cannot  be  a  higher  authority  on 
the  geology  of  India,)  informs  me  that  he  suspects  that  these  beds 
may  have  been  formed  by  the  mere  action  of  the  waves  and  currents 
accumulating  sediment.  From  analogy  I  should  much  incline  to 
Dr.  Benza's  opinion. 

*  Owen's  Africa,  vol.  ii.  p.  37,  for  Madagascar ;  and  for  S.  Africa, 
wol.  i.  pp.  412  and  426.     Lieut.  Boteler's  narrative  contains  fuller 


182  DISTRIBUTION   OF   COTIAL-EEEFS.       Ch.  VI. 

shores  and  outlying  islands  along  an  immense  space  of 
Eastern  Africa,  from  a  little  north  of  the  equator  for  900 
miles  southward.  Nothing  can  be  more  vague  than  the 
expression  *  madreporitic  rock  ;'  but  at  the  same  time  it  is, 
I  think,  scarcely  possible  to  look  at  the  chart  of  the  linear 
islets  running  in  front  of  the  coast  from  the  equator  far 
southward,  and  rising  to  a  greater  height  than  can  be  ac- 
counted for  by  the  growth  of  coral,  without  feeling  con- 
vinced that  a  line  of  fringing-reefs  has  been  elevated  at  a 
period  so  recent,  that  no  great  changes  have  since  taken 
place  on  the  surface  of  this  part  of  the  globe.  Some,  also, 
of  the  higher  islands  of  madreporitic  rock  on  this  coast,  for 
instance  Pemba,  are  singularly  shaped,  apparently  showing 
the  combined  effect  of  the  growth  of  coral  on  submerged 
banks,  together  with  their  subsequent  upheaval.  Dr.  Allan 
informs  me  that  he  never  observed  any  elevated  organic 
remains  on  the  Seychelles,  which  come  under  our  fringed 
class. 

The  nature  of  the  formations  round  the  shores  of  the 
Bed  Sea^  as  described  by  several  authors,  proves  that  the 
whole  of  this  large  area  has  been  elevated  within  a  very 
recent  tertiary  epoch.  A  part  of  this  space  in  the  ap- 
pended map  is  coloured  blue,  indicating  the  presence  of 
barrier-reefs;  on  which  circumstance  I  shall  presently 
make  some  remarks.  Kiippell '  states  that  the  tertiary 
formation,  of  which  he  has  examined  the  organic  remains, 
forms  a  fringe  along  the  shores  with  a  uniform  height  of 
from  30  to  40  feet,  from  the  mouth  of  the  Gulf  of  Suez  to 
about  lat.  26° ;  but  that  south  of  26°,  the  beds  attain  only 
the  height  of  from  12  to  15  feet.  This,  however,  can 
hardly  be  quite  accurate ;  although  possibly  there  may  be 

particulars  regarding  the  coral  rock,  vol.  i.  p.  174,  and  vol.  ii.  pp.  4J 
and  54.  See  also  Kuschenberger's  Voyage  round  the  World,  vol.  I 
p.  GO. 

'  Riippell,  Reise  in  Abyssinien,  Band  i.  s.  141. 


Ch.  vt.    distribution  of  coral-reefs.         183 

a  decrease  in  tlie  elevation  of  the  shores  in  the  middle  parts 
of  the  Red  Sea,  for  Dr.  Malcolmson  informs  me  that  he 
collected  shells  and  corals,  apparently  recent,  from  the 
cliffs  of  Camaran  Island  (lat.  15°  30'  N.)  at  a  height  of 
between  30  and  40  feet ;  and  Mr.  Salt  (Travels  in  Abyssinia) 
describes  a  similar  formation  a  little  southward  on  the  op- 
posite shore  at  Amphila.  Moreover,  near  the  mouth  of  the 
Gulf  of  Suez,  although  on  the  coast  opposite  to  that  on 
which  Dr.  Riippell  says  that  the  modern  beds  attain  a 
height  of  only  30  to  40  feet,  Mr.  Burton^  found  a  deposit 
replete  with  existing  species  of  shells,  at  the  height  of  200- 
feet.  In  an  admirable  series  of  drawings  by  Captain 
Moresby,  I  could  see  how  continuously  the  cliff-bounded, 
low,  tertiary  plains  extended  with  a  nearly  equable  height, 
both  on  the  eastern  and  western  shores.  The  southern 
coast  of  Arabia  seems  to  have  been  subjected  to  the  same 
elevatory  movement,  for  Dr.  Malcolmson  found  at  Sahar 
low  cliffs  containing  shells  and  corals  apparently  of  recent 
species. 

The  Persian  Gulf  abounds  with  coral-reefs ;  but  as  in 
this  shallow  sea  it  is  difficult  to  distinguish  reefs  from 
sandbanks,  I  have  coloured  only  some  near  the  mouth. 
Towards  the  head  of  the  gulf  Mr.  Ainsworth^  says  that  the 
land  is  worn  into  terraces,  and  that  the  strata  contain 
organic  remains  of  existing  forms. 

The  West  Indian  Archipelago  of  '  fringed  islands'  alone 
remains  to  be  mentioned :  evidence  of  an  elevation  within 
a  late  tertiary  epoch  of  nearly  the  whole  of  this  great  area, 
may  be  found  in  the  works  of  almost  all  the  geologists  w^ho 
have  visited  it.  I  will  give  some  of  the  principal  references 
in  a  note.^ 

'  Lyell's  Principles  of  Geology,  5th  edition,  vol.  iv.  p.  25. 

2  Ainsworth's  Assyria  and  Babylon,  p.  217. 

'  These  references  only  relate  to  works  published  before  1842 
the  date  of  the  first  edition  of  this  book.  On  Florida  and  the  north 
shores  of  the  Gulf  of  Mexico,  Rogers'  Report  to  Brit.  Assoc,  vol.  ill. 


184  DISTRIBUTION   OF   CORAL-REEFS.       Ch.  VL 

On  reviewing  the  above  details  it  is  impossible  not 
to  be  struck  with  the  number  of  cases  in  which  upraised 
organic  remains,  apparently  belonging  to  the  recent 
period,  have  been  found  on  the  shores  now  fringed  by 
reefs,  and  which  are  coloured  red  on  our  map.  It  may, 
however,  be  thought  that  similar  proofs  of  elevation 
could  be  found  on  the  coasts  coloured  blue,  and  which 
we  have  good  reason  to  believe  have  recently  subsided ; 
but  such  proofs  cannot  be  found,  with  the  few  follow- 
ing and  doubtful  exceptions. 

The  entire  area  of  the  Eed  Sea  appears  to  have  been 
upraised  within  a  late  tertiary  period ;  nevertheless  I 
have  been  compelled,  though  on  unsatisfactory  evidence 
(given  in  the  Appendix),  to  class  the  reefs  in  the  middle 
part  of  the  coast,  not  as  fringing,  but  as  barrier-reefs. 
If,  however,  the  statements  should  prove  accurate  re- 
specting the  less  height  of  the  tertiary  beds  in  the 
middle,  compared  with  the  northern  and  southern 
districts,  w^e  might  well  suspect  that  the  former  had 
subsided  subsequently  to  a  general  elevation  by  which 
the  whole  area  had  previously  been  upraised.  Several 
authors  ^  have  observed  shells  and  corals  high  up  on  the 

p.  14. — On  the  shores  of  Mexico,  Humboldt,  Polit.  Essay  on  New 
Spain,  vol.  i.  p.  62.  (I  have  also  some  corroborative  facts  with 
respect  to  the  shores  of  Mexico.) — Honduras  and  the  Antilles,  Lyell's 
Principles,  5th  ed.  vol.  iv.  p.  22. — Santa  Cruz  and  Barbadoes,  Prof. 
Hovey,  Silliman's  Journ.  vol.  xxxv.  p.  74. — St.  Domingo,  Courro- 
jolles  Jour,  de  Phys.  torn.  liv.  p.  106.- -Bahamas  United  Service 
Journ.  No.  Ixxi.  pp.  218  and  224.— Jamaica,  De  la  Beche,  Geol.  Man 
p.  142. — Cuba,  Taylor  in  Lond.  and  Edin.  Phil.  Mag.  vol.  xi.  p.  17. 
Dr.  Daubeney  also  at  a  meeting  of  the  Geolog.  Soc.  orally  described 
Bome  very  modern  beds  lying  on  the  N.W.  parts  of  Cuba.  I  might 
have  added  many  other  less  important  references.  [See  Appendix  II.] 
*  Ellis,  in  his  Polynesian  liesearches,  was  the  first  to  call  atten« 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  185 

mountains  of  the  Society  Islands, — a  group  of  islands 
encircled  by  barrier-reefs,  and  which,  therefore,  must 
have  recently  subsided.  Thus  at  Tahiti,  Mr.  Stutchbury 
found  on  the  summit  of  one  of  the  highest  mountains, 
between  5,000  and  7,000  feet  above  the  level  of  the  sea, 
*  a  distinct  and  regular  stratum  of  semi-fossil  coral ;  * 
but  we  cannot  infer  from  such  evidence  as  this  that  the 
island  has  been  elevated  within  the  recent  period ;  and 
on  the  other  hand,  several  naturalists,  including  Mr. 
Dana  and  myself,  have  in  vain  searched  near  the  coast 
for  upraised  shells  and  corals,  where  if  present  they 
could  not  have  been  overlooked.^     Two  of  the  Harvey 

tion  to  these  remains  (vol.  i.  p.  38)  and  the  tradition  of  the  natives 
concerning  them.  See  also  Williams,  Nar.  of  Miss.  Enterprise,  p. 
21 ;  also  Tyerman  and  G.  Bennett,  Journ.  of  Voyage,  vol.  i.  p.  213 ; 
also  Mr.  Couthouy's  Remarks,  p.  61 ;  but  his  principal  fact,  namely, 
that  there  is  a  mass  of  upraised  coral  on  the  narrow  peninsula  of 
Tiarubu,  is  from  hearsay  evidence;  also  Mr.  Stutchbury,  West  of 
England  Journ.  No.  1,  p.  64.  There  is  a  passage  in  Von  Zach, 
Corres.  Astronom.  vol.  x.  p.  266,  inferring  an  uprising  at  Tahiti, 
from  a  footpath  now  used,  which  was  formerly  impassable ;  but  I 
particularly  enquired  frorti  several  native  chiefs,  whether  they  knew 
of  any  change  of  this  kind,  and  they  were  unanimous  in  giving  me  an 
answer  in  the  negative. 

'  [Some  of  the  mountains  rise  to  7,000  feet.  A  depth  of  25  to  35 
fathoms,  which  is  the  limit  of  the  growing  corals,  is  reached  at  from 
100  to  150  fathoms  from  the  edge  of  the  reef.  The  slope  then  steepens 
rapidly  to  160  and  180  fathoms,  which  depth  is  reached  at  a  distance 
of  225  to  250  fathoms  from  the  edge  of  the  reef  ;  to  100  fathoms  the 
Blope  is  about  45°,  thence  to  about  200  it  is  about  30°,  and  then  it 
eases  off.  From  35  to  150  fathoms  sponges,  alcyonarians,  corals,  and 
other  invertebrates  were  obtained ;  beyond  the  latter,  coral-sand  with 
volcanic  minerals  and  pelagic  shells.  Inside  the  lagoons  the  reefs 
were  fringed  with  living  corals,  sloped  downwards  and  outwards  for 
a  few  feet,  then  plunged  at  once  to  depths  of  10  and  16  fathoms.  The 
deposit  in  the  lagoons  was  in  some  places  a  coral-sand,  in  others  a 
volcanic  mud.     There  is  evidenee  of  some  amount  of  upheaval. 


186  DISTRIBUTION    OF   CORAL-REEFS.       Ch.  VL 

Islands,  namely,  Aitutaki  and  Manouai,  are  formed  of 
upraised  coral  rocks,  and  have  probably  been  elevated 
within  a  recent  period ;  nevertheless  they  are  encircled 
by  reefs  extending  so  far  from  the  land,  that  I  have 
coloured  them  blue,  though  with  much  hesitation,  as 
the  space  within  the  reef  is  shallow,  and  the  encircled 
land  is  not  abrupt.  If  these  reefs  really  belong  to  the 
barrier  class,  we  have  here  another  instance  of  sub- 
sidence having  followed  elevation,  both  movements 
having  been  effected  apparently  within  the  recent 
period.  There  are  also  many  cases  of  coral-forma- 
tions, such  as  Elizabeth  Island,  Metia,  Mangaia, 
several  of  the  Friendly  and  one  of  the  Loyalty  Islands, 
which  it  can  hardly  be  doubted  once  existed  as  atolls, 
and  were  originally  formed  during  subsidence,  but 
have  since  been  elevated,  and  are  now  surrounded  by 
f ringing-reefs.  We  have,  however,  no  reason  to  feel 
surprise  at  occasional  or  even  frequent  alternations  of 
level  of  the  above  two  kinds. 

On  the  absence  of  active  Volcanos  in  the  areas  of 
subsidence,  and  on  their  frequent  presence  in  the  areas 
of  elevation,^ — The  absence  of  active  volcanos  through- 
out the  great  areas  of  subsidence  on  our  map,  as 
shown  by  the  pale  and  dark  blue  tints, — namely,  in 
the  central  parts  of  the  Indian  Ocean,  in  the  China 

Mr.  Murray  (p.  781)  regards  this  reef  as  favouring  his  theory.  Nar- 
rative of  Challenger  Voyage,  p.  778.] 

'  It  may  be  well  here  to  state  that  all  the  reefs  on  the  map  were 
coloured  either  red  or  blue  before  the  vermilion  spots  and  streaks, 
ghowing  the  position  of  the  active  volcanos  and  volcanic  chains, 
were  added ;  and  indeed  before  I  knew  of  the  existence  of  several  o! 
them. 


Ch.  VI.      DISTRIBUTION   OF  CORAL-REEFS.  187 

Sea,  in  the  sea  between  the  barriers  of  AustraHa  and 
New  Caledonia,  in  the  Caroline,  Marshall,  Gilbert,  and 
Low  Archipelagoes, — is  a  very  striking  fact.  So  is  the 
presence  of  active  volcanic  vents  and  chains  on  or  near 
many  of  the  shores  coloured  red  on  our  map,  and  which 
are  fringed  with  reefs  ;  for,  as  we  have  just  seen,  these 
fringed  coasts  have  been  recently  upheaved  in  a  large 
number  of  cases.  Active  volcanos  likewise  coincide 
with  proofs  of  recent  elevation  on  or  near  several 
other  long  lines  of  coast  within  the  limits  of  our  map, 
where  there  are  no  reefs  of  living  corals,  and  which 
consequently  are  not  coloured  red.  It  must  be  here 
remarked,  with  regard  to  the  proofs  of  both  subsidence 
and  elevation,  that  I  do  not  judge  by  the  absence  or  pre- 
sence or  nature  of  the  coral-reefs  round  the  volcanos 
themselves ;  for,  as  Dana  repeatedly  insists,  the  corals 
may  have  been  there  destroyed  or  injured  by  the  heat 
or  exhalations.  Nor  have  I  taken  into  account  the 
presence  of  upraised  organic  remains  on  the  flanks  of 
the  volcanos  themselves.  I  judge  from  the  position  of 
the  active  volcanic  vents  in  relation  to  neighbouring 
islands  and  coasts,  situated  at  too  great  a  distance  for 
any  corals  growing  there  to  be  injured  by  the  eruptions; 
and  where,  from  the  presence  of  atoll-formed  or  barrier- 
reefs,  or  of  upraised  marine  remains,  we  have  reason 
to  believe  that  either  subsidence  or  elevation  has 
occurred  within  a  recent  period. 

The  following  cases  offer  a  few  partial  exceptions  to 
the  rule  that  active  volcanos  are  situated  at  a  distance 
from  the  areps  of  subsidence.     Tlie  Great   Comoro 


188  DISTEIBUTION   OF   CORAL-REEFS.        Ch.  VI. 

Island  probably  contains  a  volcano,  and  it  is  only 
twenty  miles  distant  from  the  barrier-reef  of  Mohilla. 
Ambil  volcano,  in  the  Phillippine  Archipelago,  is  dis- 
tant only  a  little  more  than  sixty  miles  from  the  atoll- 
formed  Appoo  reef:  and  there  are  two  other  volcanos 
on  the  map  within  ninety  miles  of  circles  coloured 
blue.  But  there  is  not  a  single  active  volcano  within 
several  hundred  miles  of  a  group,  even  a  small  group,  of 
atolls  ;  and  it  is  clear  that  a  group  of  atolls,  surmount- 
ing a  number  of  islands  now  all  sunk  beneath  the  level 
of  the  sea,  implies  a  much  greater  amount  of  subsi- 
dence, than  does  a  single  atoll  or  a  single  encircling 
barrier-reef.  It  is  a  striking  fact  that  two  volcanos  are 
known  to  have  been  in  recent  action  in  the  Friendly 
Archipelago ;  and  the  islands  have  here  been  formed 
by  the  recent  elevation  of  a  group  of  atolls.  Again, 
extinct  craters  and  well-preserved  streams  of  lava  occur 
on  many  of  the  encircled  islands  in  the  Pacific,  and 
these  by  our  theory  have  subsided  at  no  very  remote 
period ;  but  although  thus  plainly  formed  of  volcanic 
matter,  they  do  not  offer  a  single  active  volcano.  In 
these  cases  the  volcanos  seem  to  have  come  into  action 
or  to  have  been  extinguished,  in  accordance  with  the 
latest  movements  of  elevation  or  subsidence. 

Within  the  limits  of  our  map,  active  volcanos  occur 
on  or  near  other  coasts  besides  those  which  are  fringed 
with  reefs  and  coloured  red ;  and  some  of  these  coasts 
are  known  to  have  been  upraised  within  the  recent 
period.  Thus  I  have  shown  in  my  Geological  Observa- 
tions on  S.  America  (1846)  that  the  whole  western  shore 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  189 

of  this  great  continent,  for  a  space  of  between  2,000  and 
8,000  miles  south  of  the  equator,  has  undergone  an  up- 
ward movement  during  the  period  of  existing  marine 
shells ;  and  the  Andes  here  form  the  grandest  volcanic 
chain  in  the  world.  The  islands  on  the  north-western 
side  of  the  Pacific,  forming  the  second  grandest  volcanic 
chain,  are  very  imperfectly  known  ;  but  Luzon,  in  the 
Phillippines,  and  the  Loo  Choo  islands,  have  been  re- 
cently elevated ;  and  at  Kamtschatka  ^  there  are  exten- 
sive tertiary  beds  of  modern  date.  The  co-existence 
in  other  parts  of  the  world,  of  active  volcanos  with 
upraised  beds  of  a  modern  origin,  will  occur  to  every 
geologist.  Nevertheless,  until  it  could  be  shown  that 
volcanos  were  absent  or  inactive  in  subsiding  areas, 
the  conclusion  that  their  distribution  depended  on  the 
nature  of  the  subterranean  movements  in  progress, 
would  have  been  hazardous.  But  now,  viewing  the 
appended  map,  it  may,  I  think,  be  considered  as 
almost  established,  that  volcanos  are  often  present  in 
the  areas  which  have  lately  risen  or  are  still  rising, 
and  are  invariably  absent  in  those  which  have  lately 
subsided  or  are  still  subsiding ;  and  this,  I  think,  is 
the  most  important  generalisation  to  which  the  study 
of  coral-reefs  has  indirectly  led  me.=^ 

On  the  dimensions  and  relative  positions   of   the 

'  Namely,  at  Sedanka,  in  lat.  58°  N.  (Von  Buch's  Descript.  dea 
Isles  Canaries,  p.  455). 

'^  We  may  infer  from  this  rule,  that  at  any  place  where  an  old 
formation  contains  inter  stratified  beds  of  erupted  matter,  the  surface 
of  the  land  or  the  bed  of  the  sea  formed,  at  the  period  of  eruption,  a 
rising,  at  least  not  a  subsiding  area. 

U 


190  DISTEIBUTIOX  OF   CORAL-KEEFS.       Ch.  \l 

snhsiding  areas  on  our  map,  as  indicated  hy  the  pre- 
sence of  atolls  and  barrier-reefs ;  and  of  the  rising  or  sta- 
tionary areas,  as  knoivn  hy  upraised  organic  remains, 
or  inferred  from  the  presence  of  fringing -reefs. — Tlie 
immense  surfaces  seen  on  the  map,  which  accord- 
ing to  our  theory,  or  from  the  plain  evidence  of  up- 
raised remains,  have  undergone  either  a  downward 
or  upward  change  of  level  within  a  geologically  late 
period,  is  a  highly  remarkable  fact.  The  existence  of 
continents  shows  that  the  areas  which  have  been  up- 
raised are  immense.  With  respect  to  South  x\merica 
we  may  feel  sure,  and  with  respect  to  the  western  shores 
of  the  Indian  Ocean  we  have  reason  to  suspect,  that 
this  rising  is  either  now  actually  in  progress,  or  has 
taken  place  quite  recently.  By  our  theory,  it  may 
safely  be  inferred  that  the  areas  which  have  lately  sub- 
sided are  likewise  immense ;  or,  judging  from  the  earth- 
quakes now  occasionally  felt  there,  and  from  other  ap- 
pearances, are  still  subsiding.  The  smallness  of  the 
scale  of  our  map  should  not  be  overlooked ;  each  square 
on  it  containing  810,000  square  miles.  If  we  take  the 
space  of  ocean  from  near  the  southern  end  of  the  Low 
Archipelago  to  the  northern  end  of  the  Marshall  Archi- 
pelago,— a  length  of  4,600  miles,  we  see  that,  as  far  as 
known,  every  island,  excepting  Metia,  is  atoll- formed. 
The  eastern  and  western  boundaries  of  our  map  are 
continents,  and  they  are  rising :  the  central  areas  of 
the  great  Indian  and  Pacific  Oceans,  are  mostly  sub- 
siding ;  between  them,  north  of  Australia,  lies  the  most 
broken  land  on  the  globe,  and  there  the  rising  parts 


Ch.  VI.      DISTRIBUTION   OF   CORAL-REEFS.  191 

are  surrounded  and  penetrated  by  areas  of  subsidence ;  * 
so  that  the  prevailing  movements  now  in  progress,  seem 
to  accord  with  the  present  state  of  the  great  terrestrial 
and  oceanic  divisions  of  the  world. 

The  blue  spaces  on  the  map  are  nearly  all  elongated ; 
Buch  as  the  great  north  and  south  line  of  atolls  in  the 
Indian  Ocean,  the  space  between  the  barrier-reefs  of 
Australia  and  New  Caledonia,  the  Caroline  Archi- 
pelago, &c.  Whether  adjoining  elongated  spaces,  run- 
ning in  different  directions,  have  subsided  by  one  com- 
mon movement,  or  independently  of  each  other,  we  do 
not  know.  In  the  case  of  the  Caroline  and  Marshall 
Archipelagoes,  situated  near  each  other,  but  extending 
Jn  different  directions,  it  seems  probable  that  they  have 
subsided  independently  of  each  other ;  for  the  Mc Askill 
Islands,^  lying  towards  the  eastern  end  of  the  Caroline 
Archipelago,  are  formed  of  upraised  coral-rock;  and  we 
thus  see  that  the  above  two  areas  of  subsidence  have 
been  at  one  time  interrupted  by  an  area  of  upheaval. 
The  curved  line  of  elevation  formed  by  the  Mariana 
Islands,  seems  to  cross  a  former  line  of  subsidence  pro- 
longed from  the  Caroline  Archipelago ;  for  the  island 
of  Fais,  apparently  an  upraised  atoll,  is  situated  nearly 
at  the  point  of  intersection  of  the  two  lines.  The 
Sandwich  Archipelago  is  530  miles  in  length,  from 
Hawaii  to  the  westernmost   rocky  islet,  but  is  pro- 

1  I  suspect  that  the  Arru  and  Timor -laut  Islands  present  an  in- 
cluded small  area  of  subsidence,  like  that  of  the  China  Sea ;  but  I 
have  not  ventured  to  colour  them  blue,  owing  to  the  want  of  sufficient 
information.     See  Appendix. 

'^  Dana,  Corals  and  Coral  Islands,  p.  306. 


192  DISTRIBUTION   OF   COKAL-REEFS.       Ch.  VI 

longed  by  numerous  reefs  to  a  point  2,000  miles  dis- 
tant from  Hawaii.  The  south-eastern  end  of  this  long 
line  is  one  of  elevation  and  of  volcanic  activity; 
whereas  the  north-western  end,  judging  from  the 
structure  of  the  reefs,  though  these  are  imperfectly 
known,  is  one  of  subsidence.^  So  that  here  we  ap- 
parently have  opposite  movements  in  progress  towards 
the  two  extremities  of  the  same  long  line.  The  com- 
monest case  seems  to  be  a  tendency  to  alternation 
between  the  areas  of  subsidence  and  elevation,  as 
if  the  sinking  of  one  had  counterbalanced  the  rising 
of  another. 

The  existence  in  many  parts  of  the  world  of  lofty 
table-land,  proves  that  large  surfaces  have  been  upraised 
in  mass  to  a  great  height  above  the  level  of  the  sea ; 
although  in  almost  every  country  the  highest  points 
consist  of  upturned  strata,  or  of  erupted  matter  :  and 
from  the  wide  spaces  over  which  atolls  are  scattered, 
although  not  one  pinnacle  of  land  now  remains  above 
the  level  of  the  sea,  we  may  conclude  that  immense 
areas  have  subsided  to  an  amount  sufficient  to  bury  not 
only  any  formerly  existing  lofty  table-land,  but  even  the 
heights  formed  by  fractured  strata  and  erupted  matter. 
The  effects  left  on  the  land  by  the  later  elevatory 
movements,  namely,  successively  rising  cliffs,  succes- 
sive lines  of  erosion,  and  great  beds  of  shells  and 
pebbles,  all  requiring  time  for  their  production,  prove 
that  these  movements  have  been  extremely  slow.    And 

'  Dana,  Corals  and  Coral  Islands,  pp.  307,  365.  See  also  my 
Appendix. 


Ch.  VI.  RECAPITULATION.  193 

with  respect  to  the  whole  amount  of  subsidence  neces- 
sary to  have  produced  the  many  atolls  widely  scattered 
over  immense  spaces,  the  movement,  as  already  shown, 
must  either  have  been  uniform  and  exceedingly  slow, 
or  effected  by  small  steps  separated  from  each  other  by 
long  intervals  of  time,  so  as  to  have  allowed  the  reef-con- 
structing polypifers  to  bring  up  their  solid  frameworks 
to  the  surface ;  and  this  is  one  of  the  most  interesting 
conclusions  to  which  we  are  led  by  the  study  of  coral- 
formations.  We  have  little  means  of  judging  whether 
many  considerable  oscillations  of  level  have  usually 
occurred  during  the  elevation  of  large  areas ;  but  we 
know  from  clear  geological  evidence,  such  as  trees  still 
standing  upright  at  successive  .levels  and  covered  by 
marine  strata,  that  this  has  frequently  been  the  case ; 
and  we  have  seen  on  our  map,  that  some  of  the  same 
islands  after  having  subsided,  have  been  upraised ;  and 
that  others  after  having  been  uplifted,  have  subsided. 
We  may  therefore  conclude  that  the  subterranean 
changes  which  cause  some  areas  to  rise  and  others  to 
sink,  have  generally  acted  in  a  closely  similar  manner. 

Recapitulation. — -In  the  three  first  chapters,  the 
principal  kinds  of  coral-reefs  were  described  in  detail, 
and  they  were  found  to  differ  little,  as  far  as  relates 
to  the  actual  surface  of  the  reef.  An  atoll  differs  from 
an  encircling  barrier-reef  only  in  the  absence  of  land 
within  its  central  expanse ;  and  a  barrier-reef  differs 
from  a  fringing-reef  only  in  being  placed,  relatively 
to  the  probable  inclination  of  its  submarine  foundation, 


194  EECAPITULATION.  Ch.  VI, 

at  a  much  greater  distance  from  the  land,  and  in 
the  presence  of  a  deep  lagoon-Hke  space  within  the 
reef.  In  the  fourth  chapter  the  growing  powers  of  tho 
reef-constructing  polypifers  were  discussed ;  and  it  was 
shown  that  they  cannot  flourish  beneath  a  very  limited 
depth.  In  accordance  with  this  limit,  there  is  no  diffi- 
culty respecting  the  foundation  on  which  a  fringing- 
reef  is  based ;  whereas,  with  barrier-reefs  and  atolls, 
there  is  the  greatest  difficulty  on  this  head  ; — in  bar- 
rier-reefs from  the  improbability  of  rock  or  banks  of 
sediment  having  extended,  in  every  instance,  so  far 
seaward  within  the  required  depth; — and  in  atolls, 
from  the  immensity  of  the  spaces  over  which  they  are 
interspersed,  and  the  apparent  necessity  for  believing 
that  they  are  all  based  on  mountain-summits,  which, 
although  rising  very  near  to  the  surface  of  the  sea,  in 
no  one  instance  rise  above  it.  To  escape  this  latter 
admission,  which  implies  the  existence  of  submarine 
chains  of  mountains  of  almost  exactly  the  same  height 
extending  over  many  thousand  square  miles,  there  is 
but  one  alternative ;  namely,  the  prolonged  subsidence 
of  the  foundations  on  which  the  atolls  first  became 
attached,  together  with  the  upward  growth  of  the 
reef-constructing  corals.  On  this  view  every  difficulty 
vanishes :  fringing-reefs  are  thus  easily  converted  into 
barrier-reefs ;  and  barrier-reefs  into  atolls,  as  soon  as 
the  last  pinnacle  of  land  sinks  beneath  the  surface  of 
the  sea. 

The  wall-like  structure  on  the  inner  sides  of  atolls 
and  barrier-reefs— the  basin  or  ring-like  shape  of  the 


Ch.  VI.  RECAPITULATION.  195 

marginal  and  central  reefs  in  the  Maldiva  atolls — the 
union  of  some  atolls  as  if  by  a  ribbon — the  apparent 
disseverment  of  others — the  ordinary  outline  of  groups 
of  atolls  and  their  forms — are  all  thus  explained.  We 
thus  understand  the  occurrence  in  both  atolls  and 
barrier-reefs  of  portions,  or  of  the  whole,  in  a  dead  and 
submerged  condition,  though  still  retaining  the  outline 
of  a  living  reef.  The  existence  of  breaches  through 
barrier-reefs  in  front  of  valleys,  though  separated  from 
them  by  wide  spaces  of  deep  water,  can  be  similarly  ex- 
plained. It  confirms  our  theory  that  we  find  the  two 
kinds  of  reefs  formed  through  subsidence  generally  situ- 
ated near  each  other  and  at  a  distance  from  the  spaces 
where  fringing-reefs  abound.  On  searching  for  other 
evidence  of  the  movements  assumed  by  the  theory, 
we  find  marks  of  change  in  atolls  and  in  barrier-reefs, 
and  of  subterranean  disturbances  beneath  them ;  but 
from  the  nature  of  things,  it  is  scarcely  possible  to 
find  direct  proofs  of  subsidence,  although  some  appear- 
ances are  strongly  in  favour  of  it.  On  the  fringed 
coasts,  however,  the  frequent  presence  of  upraised 
marine  remains  belongings  to  a  recent  epoch,  plainly 
shows  that  these  coasts  have  been  lately  elevated. 

Pinally,  when  the  two  great  types  of  structure, 
namely  barrier-reefs  and  atolls  on  the  one  hand, 
and  fringing-reefs  on  the  other,  are  laid  down  on  a 
map,  they  o£fer  a  grand  and  harmonious  picture  of 
the  movements  which  the  crust  of  the  earth  has 
undergone  within  a  late  period.  We  there  see  vast 
areas  rising,  with  volcanic  matter  every  now  and  then 


196  Recapitulation.  Ch.  yl, 

bursting  forth.  We  see  other  wide  spaces  sinking  with- 
out any  volcanic  outbursts ;  and  we  may  feel  sure  that 
the  movement  has  been  so  slow  as  to  have  allowed  the 
corals  to  grow  up  to  the  surface,  and  so  widely  extended 
as  to  have  buried  over  the  broad  face  of  the  ocean 
every  one  of  those  mountains,  above  which  the  atolls 
now  stand  like  monuments,  marking  the  place  of  their 
buriaL 


APPENDIX 


APPENDIX  CI.] 

CONTAIKING 

A  DETAILED  DESCEIPTION  OF  THE  BEEFS  AND  ISLANDS 
IN  THE   COLOUEED   MAP.  Plate  IIL 


In  the  beginning  of  the  last  chapter  I  stated  the  principles 
on  which  the  map  has  been  coloured.  There  only  remains 
to  be  said,  that  it  is  an  exact  copy  of  one  by  M.  C.  Gressier, 
published  by  the  Depot  G6n6ral  de  la  Marine,  in  1835. 
The  names  have  been  altered  into  English,  and  the  longi- 
tude has  been  reduced  to  that  of  Greenwich.  The  colours 
were  first  laid  down  on  accurate  charts,  on  a  large  scale. 
The  data,  on  which  the  volcanos  historically  known  to 
have  been  in  action,  have  been  marked  with  vermihon, 
were  given  in  a  note  to  the  last  chapter.  I  will  commence 
my  description  on  the  eastern  side  of  the  map,  and  will 
describe  each  group  of  islands  consecutively,  proceeding 
westward  across  the  Pacific  and  Indian  Oceans,  and  ending 
with  the  West  Indies. 

The  Western  Shores  of  America  appear  to  be 
entirely  without  coral-reefs  :  south  of  the  equator  the  survey 
of  the  Beagle,  and  north  of  it  the  published  charts  show 
that  this  is  the  case.  Even  in  the  Bay  of  Panama^  where 
corals  flourish,  there  are  no  true  coral-reefs,  as  I  have  been 
informed  by  Mr.  Lloyd.  There  are  no  coral-reefs  in  the 
Galapagos  archipelago,  as  I  know  from  personal  inspection  ; 
and  I  believe  there  are  none  on  the  Cocos,  Bevilla-gigedo, 
and  other  neighbouring  islands.  Clipperton^  rock,  10°  N., 
*  [Undoubtedly  an  atoll,  according  to  Sir  J.  Belcher's  cbart. — 
Captain  Wharton.] 


200  APPENDIX. 

109°  W.,  from  a  drawing  appended  to  a  MS.  plan  in  the 
Admiralty,  does  not  appear  to  be  an  atoll,  but  Sir  E. 
Belcher  (Voyage  round  the  World,  vol.  i.  1843,  p.  255) 
speaks  of  it  as  of  coral-formation,  with  deep  water  within 
the  lagoon ;  left  uncoloured.  The  eastern  part  of  the 
pacific  presents  an  enormous  area  without  any  islands, 
except  Easter  and  Gomez,  which  do  not  appear  to  be  sur- 
rounded by  reefs. 

The  Low  or  Paumotu  Aechipelago. — This  group 
consists  of  about  80  atolls  :  it  would  be  quite  superfluous 
to  refer  to  descriptions  of  each.  In  D'Urville  and  Lottin's 
chart,  one  island  (Wolchonsky)  is  written  with  a  capital 
letter,  signifying,  as  explained  in  a  former  chapter,  that  it 
is  a  high  island ;  but  this  must  be  a  mistake,  as  the  orginal 
chart  by  Bellingshausen  shows  that  it  is  a  true  atoll. 
Captain  Beechey  says  of  the  32  groups  which  he  examined 
(of  the  greater  number  of  which  I  have  seen  beautiful  MS. 
charts  in  the  Admiralty),  that  29  now  cgntain  lagoons,  and 
he  believes  the  other  three  orginally  did  so.  Bellingshausen 
(see  an  account  of  this  Russian  voyage,  in  the  Biblioth.  des 
Voyages,  1834,  p.  443)  says  that  the  17  islands  which  he 
discovered  resembled  each  other  in  structure,  and  he  has 
given  charts  on  a  large  scale  of  all  of  them.  Kotzebue  has 
given  plans  of  several ;  Cook  and  Bligh  mention  others  ;  a 
few  were  seen  during  the  voyage  of  the  Beagle ;  and  notices 
of  other  atolls  are  scattered  through  several  publications. 
The  Actceon  group  in  this  archipelago  has  lately  been  dis- 
covered (Geograph.  Journ.,  vol.  vii.  p.  454) ;  it  consists  of 
three  small  and  low  islets,  one  of  which  has  a  lagoon. 
Another  lagoon-island  has  been  discovered  (Naut.  Mag. 
1839,  p.  770)  in  22°  4'  S.  and  136°  20'  W.  Dana,  in  his 
work  on  Corals  and  Coral  Islands,  gives  a  full  account  of 
this  archipelago.  Towards  the  S.E.  there  are  some  islands 
of  a  different  nature  :  Elizabeth  Island  is  described  by 
Beechey  (p.  46,  4to  edit.)  as  fringed  by  reefs,  at  the  distance 


PACIFIC    OCEAN.  201 

of  between  two  and  three  hundred  yards  ;  coloured  red, 
Pitcairn  Island,  in  the  immediate  neighbourhood,  according 
to  the  same  authority,  has  no  reefs  of  any  kind,  althougli 
numerous  pieces  of  coral  are  thrown  up  on  the  beach  ;  the 
sea  close  to  its  shore  is  very  deep  (see  Zool.  of  Beechey's 
Voyage,  p.  164) ;  left  uncoloured.  Gamhier  Islands  (see 
Plate  I.  fig.  8)  are  encircled  by  a  barrier-reef;  the  greatest 
depth  within  is  38  fathoms  ;  coloured  pale  blue.  Metia  or 
Aurora  Island  lies  N.E.  of  Tahiti,  close  to  the  large  space 
coloured  dark  blue  in  the  map  ;  it  has  been  already  de- 
scribed as  an  upraised  atoll ;  as  it  is  said  by  Captain  Wilkes 
(Narrative  of  U.S.  Exploring  Expedition,  vol.  i.  p.  337)  to 
be  surrounded  by  fringing-reefs,  in  one  part  500  feet  in 
width,  it  has  been  coloured  red.  But  I  must  remind  the 
reader  of  the  discussion  in  the  sixth  chapter,  showing  that 
if  an  upraised  atoll  were  to  subside  again,  the  reef  would 
probably  retain  for  a  long  time  or  for  ever,  its  fringing 
character,  owing  to  the  steepness  of  the  submarine  flanks. 
The  Society  Aechipelago  is  separated  by  a  narrow 
space  from  the  Low  Archipelago ;  and  in  their  parallel 
direction  they  manifest  some  relation  to  each  other.  I 
have  already  described  the  general  character  of  the  reefs 
of  these  encircled  islands.  In  the  atlas  of  the  Coquille's 
Voyage  there  is  a  good  general  chart  of  the  group,  and 
separate  plans  of  some  of  the  islands.  TaMti,  the  largest 
island  in  the  group,  is  almost  surrounded,  as  seen  in  Cook's 
chart,  by  a  reef  from  half  a  mile  to  a  mile  and  a  half  from 
the  shore,  with  from  10  to  30  fathoms  within  it.  Some 
considerable  submerged  reefs,  lying  parallel  to  the  shore, 
with  a  broad  and  deep  space  within,  have  lately  been  dis- 
covered on  the  N.E.  coast  of  the  island,  (Naut.  Mag.  1836, 
p.  264,)  where  none  are  laid  down  by  Cook.  At  Eimeo  the 
reef,  *  which  like  a  ring  surrounds  it,  is  in  some  places  one 
or  two  miles  distant  from  the  shore,  in  others  united  to 
the  beach  '  (Ellis,  Polynesian  Researches,  vol.  i.  p.  18, 12mo. 


202  ArpENDix. 

edit.).  Cook  found  deep  water  (20  fathoms)  in  some  of  the 
harbours  within  the  reef.  Mr.  Couthouy,  however,  states 
(Remarks,  p.  45)  that  both  at  Tahiti  and  Eimeo,  the  space 
between  the  barrier-reef  and  the  shore  has  been  almost 
filled  up, — *  a  nearly  continuous  fringing-reef  surrounding 
the  island,  and  varying  from  a  few  yards  to  rather  more 
than  a  mile  in  width,  the  lagoons  merely  forming  canals 
between  this  and  the  sea-reef,'  that  is  the  barrier-reef. 
Tapamanoa  is  surrounded  by  a  reef  at  a  considerable 
distance  from  the  shore  ;  from  the  island  being  small, 
it  is  breached,  as  I  am  informed  by  the  Eev.  W.  Ellis, 
only  by  a  narrow  and  crooked  boat-channel.  This  is  the 
lowest  island  in  the  group,  its  height  probably  not  ex- 
ceeding 500  feet.  A  little  way  north  of  Tahiti,  the  low 
coral  islets  of  Teturoa  are  situated  ;  from  the  description  of 
them  given  me  by  the  Eev.  J.  Williams  (the  author  of  the 
Narrative  of  Missionary  Enterprise),  I  should  have  thought 
that  they  formed  a  small  atoll,  and  likewise  from  the  de- 
scription given  by  the  Eev.  D.  Tyerman  and  G.  Bennett 
(Journ.  ofVoy.  and  Travels,  vol.  i.  p.  183),  who  say  that  ten 
low  coral  islets  *  are  comprehended  within  one  general  reef, 
and  separated  from  each  other  by  interjacent  lagoons  ; '  but 
as  Mr.  Stutchbury  (West  of  England  Journal,  vol.  i.  p.  64) 
describes  it  as  consisting  of  a  mere  narrow  ridge,  I  have 
left  it  uncoloured.  Maitea,  eastward  of  the  group,  is  classed 
by  Forster  as  a  high  encircled  island ;  but  from  the  account 
given  by  the  Eev.  D.  Tyerman  and  G.  Bennett  (vol.  i.  p.  57) 
it  appears  to  be  an  exceedingly  abrupt  cone  rising  from 
the  sea  without  any  reef ;  left  uncoloured.  It  would  be 
superfluous  to  describe  the  northern  islands  in  this  group, 
as  they  may  be  well  seen  in  the  chart  accompanying  the 
4to.  edition  of  Cook's  Voyages,  and  in  the  atlas  of  the 
Coquille's  Voyage.  Maurua  is  the  only  one  of  the  northern 
islands  in  which  the  water  within  the  reef  is  not  deep,  being 
only  4|  fathoms  ;  but  the  great  width  of  the  reef,  stretching 


PACIFIC   OCEAIJ.  203 

three  miles  and  a  half  southward  of  the  land  (which  is 
represented  in  the  drawing  in  the  atlas  of  the  Coquille's 
Voyage  as  descending  abruptly  to  the  water),  shows,  on  the 
principle  explained  in  the  beginning  of  the  last  chapter, 
that  it  belongs  to  the  barrier  class.  I  may  here  mention, 
from  information  communicated  to  me  by  the  Eev.  W. 
Ellis,  that  on  the  N.E.  side  of  Huaheine  there  is  a  bank  of 
sand,  about  a  quarter  of  a  mile  wide,  extending  parallel  to 
the  shore,  and  separated  from  it  by  an  extensive  and  deep 
lagoon  :  this  bank  of  sand  rests  on  coral-rock,  which  un- 
doubtedly was  once  a  living  reef.  North  of  Bolabola  lies 
the  atoll  of  Toubai  (Motou-iti  of  the  Coqtdlle's  atlas),  which 
is  coloured  dark  blue  ;  all  the  islands  which  are  surrounded 
by  barrier-reefs  are  coloured  pale  blue  :  three  of  them  are 
represented  in  figures  3,  4,  and  5,  in  Plate  I.  There  are 
three  low  coral-groups  lying  a  little  W.  of  the  Society 
Archipelago,  and  almost  forming  part  of  it,  namely.  Bell- 
ingshauseiij  which  is  said  by  Kotzebue  (Second  Voyage, 
vol.  ii.  p.  255)  to  be  a  lagoon-island  ;  Mopeha,  which  from 
Cook's  description  (Second  Voyage,  book  iii.  chap,  i.)  no 
doubt  is  an  atoll ;  and  the  Scilly  Islands,  which  are  said 
by  Wallis  (Voyage,  chap,  ix.)  to  form  a  group  of  low  islets 
and  shoals,  and  which,  therefore,  probably  compose  an 
atoll ;  the  two  former  have  been  coloured  blue,  but  not  the 
latter. 

Mendana  or  Marquesas  Group. — These  islands  are 
almost  entirely  destitute  of  reefs,  as  may  be  seen  in 
Krusenstern's  Atlas,  making  a  remarkable  contrast  with 
the  adjacent  group  of  the  Society  Islands.  Mr.  F.  D. 
Bennett  has  ^ven  some  account  of  this  group,  in  the 
seventh  volume  of  the  Geograph.  Journ.  He  informs  me 
that  all  the  islands  have  the  same  general  character,  and 
that  the  water  is  very  deep  close  to  their  shores.  He 
visited  three  of  them,  namely,  Dominicana,  Christiana^ 
and  Boapoa,  their  beaches  are  strewed  with  rounded  masses 


204  APPENDIX. 

of  coral,  and  although  no  regular  reefs  exist,  yet  the  shore 
is  in  many  places  lined  by  coral  rock,  so  that  a  boat 
grounds  on  this  formation.  Hence  these  islands  ought 
perhaps  to  come  within  the  class  of  fringed  islands  and  be 
coloured  red  ;  but  as  I  am  determined  to  err  on  the  cautious 
side,  I  have  left  them  uncoloured.  Dana  infers  (Corals 
and  Coral  Islands,  p.  325),  from  their  steepness  and  deeply 
indented  outline,  that  they  have  subsided. 

Cook  or  Harvey  and  Austral  Islands. — Palmerston 
Island  is  minutely  described  as  an  atoll  by  Captain  Cook 
during  his  voyage  in  1774  ;  it  is  coloured  blue.  Aitutaki 
was  partially  surveyed  by  the  Beagle  (see  map  accompany- 
ing Voyages  of  Adventure  and  Beagle) ;  the  land  is  hilly, 
sloping  gently  to  the  beach  ;  the  highest  point  is  360  feet ; 
on  the  southern  side,  the  reef  projects  five  miles  from  the 
land :  off  this  point  the  Beagle  found  no  bottom  with  270 
fathoms :  the  reef  is  surmounted  by  many  low  coral-islets. 
I  am  informed  by  the  Kev.  J.  Williams,  that  within  the 
reef  the  water  is  exceedingly  shallow,  not  being  more  than 
a  few  feet  deep ;  nevertheless,  from  the  great  extension  of 
the  reef  into  a  profoundly  deep  ocean,  this  island  probably 
belongs,  on  the  principle  lately  adverted  to,  to  the  barrier 
class,  and  I  have  coloured  it  pale  blue,  although  with  much 
hesitation. — Manouai  or  Harvey  Island  :  the  highest  point 
is  about  50  feet :  the  Eev.  J.  Williams  informs  me  that 
although  the  reef  lies  far  from  the  shore,  it  is  less  distant 
than  at  Aitutaki,  but  the  water  within  the  reef  is  rather 
deeper :  I  have  likewise  coloured  this  island  pale  blue,  but 
with  many  doubts. — Eound  Mitiaro  Island,  as  I  am  in- 
formed by  Mr.  Williams,  the  reef  is  attached  to  the  shore ; 
coloured  red. — Mauki,  or  Maouti:  the  reef  round  this 
island  (under  the  name  of  Parry  Island  in  the  Voyage  of 
H.M.S.  Blonde,  p.  209)  is  described  as  a  coral  flat,  only 
50  yards  wide,  and  two  feet  under  water.  This  statement 
has  been  corroborated  by  Mr.  Williams,  who  calls  the  reef 


PACIFIC   OCEAN.  205 

attached;  coloured  red. — A tiu,  or  Wateeo:  a  moderately 
elevated,  hilly  island,  like  the  others  of  the  group ;  the 
reef  is  described  in  Cook's  Voyage  as  attached  to  the  shore, 
and  about  100  yards  wide;  coloured  red. — Fenoua-itii 
Cook  describes  this  island  as  very  low,  not  more  than  six 
or  seven  feet  in  height  (vol.  i.  book  ii.  chap.  iii.  1777) ;  in 
the  chart  published  in  the  Coquille's  atlas,  a  reef  is  en- 
graved close  to  the  shore  :  this  island  is  not  mentioned  in 
the  list  given  by  Mr.  Williams  (p.  16)  in  the  Narrative  of 
Missionary  Enterprise  ;  nature  doubtful ;  but  as  it  lies  so 
near  Atiu,  it  has  been  unavoidably  coloured  red. — Baro- 
tonga:  Mr.  Williams  informs  me  that  this  is  a  lofty 
basaltic  island,  with  an  attached  reef;  coloured  red. — 
There  are  three  other  islands,  Bourouti,  Boxburgh,  and 
Htill,  of  which  I  have  not  been  able  to  obtain  any  account, 
and  have  left  them  uncoloured.  Hull  Island,  in  the  French 
chart,  is  written  with  small  letters  as  being  low. — 
Mangaia :  height  about  300  feet ;  *  the  surrounding  reef 
joins  the  shore '  (Williams's  Narrative,  p.  18) ;  coloured  red. 
— Bimetara :  Mr.  Williams  informs  me  that  the  reef  is 
rather  close  to  the  shore ;  but,  from  information  given  me 
by  Mr.  ElUs,  the  reef  does  not  appear  to  be  quite  so  closely 
attached  to  it  as  in  the  foregoing  cases  :  the  island  is  about 
300  feet  high  (Naut.  Mag.  1839,  p.  738) ;  coloured  red.— 
Burutu:  Mr.  Williams  and  Mr.  Ellis  inform  me  that  this 
island  has  an  attached  reef;  coloured  red.  It  is  described 
by  Cook  under  the  name  of  Oheteroa :  he  says  it  is  not 
surrounded  like  the  neighbouring  islands,  by  a  reef ;  but 
he  must  mean  a  distant  veet^—Touhouai  :  in  Cook*s  chart 
(Second  Voyage,  vol.  ii.  p.  2)  the  reef  is  laid  down  in  a 
part  at  the  distance  of  one  mile,  and  in  another  part  at  the 
distance  of  two  miles  from  the  shore  ;  Mr.  Ellis  (Polynes. 
Res.  vol.  iii.  p.  381)  says  the  low  land  round  the  base  of 
the  island  is  very  extensive ;  and  this  gentleman  informs 
me  that  the  water  within  the  reef  appears  deep  ;  coloured 

15 


206  APPENDIX. 

blue. — Baivaivaiy  or  Vivitao :  Mr.  Williams  informs  me 
that  the  reef  is  here  distant  from  the  shore  ;  Mr.  Ellis, 
however,  says  that  this  is  certainly  not  the  case  on  one 
side  of  the  island ;  and  he  believes  that  the  water  witliin 
the  reef  is  not  deep ;  hence  I  have  left  it  uncoloured. — 
Lancaster  Eeef,  described  in  Naut.  Mag.  1833  (p.  693),  as 
an  extensive  crescent-formed  coral-reef,  has  not  been 
coloured. — Baim,  or  Oparree  :  from  the  accounts  given  of 
it  by  Ellis  and  Vancouver,  there  does  not  appear  to  be  any 
reef. — I.  de  Bass  is  an  adjoining  island,  of  which  I  cannot 
find  any  account. — Kemin  Island :  Krusenstern  seems 
hardly  to  know  its  position,  and  gives  no  further  par- 
ticulars. 

Islands  between  the  Low  and  Gilbert  Archipelagoes 

Caroline  Island  (10°  S.,  150°  W.)  is  described  by  Mr. 
F.  D.  Bennett  (Geograph.  Journ.  vol.  vii.  p.  225)  as  con« 
taining  a  fine  lagoon  ;  coloured  blue.  Westward  of  Caroline 
Island,  a  small  lagoon-island  is  described  in  the  U.S. 
Exploring  Expedition  in  lat.  10°  S.  and  152°  22'  W.  long. ; 
coloured  \A\kQ.— Flint  Island  (11°  S.,  151°  W.) :  Krusen- 
stern believes  that  it  is  the  same  with  Peregrino,  which  is 
described  by  Quiros  (Burney's  Chron.  Hist.  vol.  ii.  p.  283) 
as  '  a  cluster  of  small  islands  connected  by  a  reef,  and 
forming  a  lagoon  in  the  middle  ; '  coloured  blue. —  Wostock 
is  an  island  a  little  more  than  half  a  mile  in  diameter,  and 
apparently  quite  flat  and  low,  discovered  by  Bellingshausen; 
it  is  situated  a  little  west  of  Caroline  Island,  but  it  is  not 
placed  on  the  French  charts ;  I  have  not  coloured  it, 
although  I  entertain  little  doubt,  from  the  chart  of  Bellings- 
hausen, that  it  originally  contained  a  small  lagoon. — Pen- 
rhyn  Island  (9°  S.,  158°  W.) :  a  plan  in  the  atlas  of  the 
First  Voyage  of  Kotzebue,  shows  that  it  is  an  atoll,  which 
according  to  Wilkes  (U.S.  Exploring  Expedition,  vol.  iv. 
p.  277)  is  nine  miles  in  length  ;  coloured  blue.—  Starbuck 


PACIFIC   OCEAN.  207 

Island  (5°  S.,  158°  W.)  is  described  in  Byron's  Voyage  in 
the  Blonde  (p.  206)  as  formed  of  a  flat  coral-rock,  with  no 
trees  ;  the  height  not  given  ;  not  coloured. — Maiden  Island* 
(4°  S.,  154°  W.) :  in  the  same  Voyage  (p.  205)  this  island 
is  said  to  be  of  coral  formation,  and  no  part  above  40  feet 
high  ;  I  have  not  ventured  to  colour  it,  although  from  being 
of  coral  formation,  it  is  probably  fringed  ;  in  which  case  it 
should  he  xedi.—Jarvis,  or  Bunker  Island  (0°  20'  S.,  160° 
W.)  is  described  by  Mr.  F.  D.  Bennett  (Geograph.  Journ. 
vol.  vii.  p.  227)  as  a  narrow,  low  strip  of  coral  formation ; 
not  coloured. — Brook  is  a  small,  low  island  between  the  two 
latter ;  its  position,  and  perhaps  even  existence  is  doubtful ; 
not  coloured.  — Pescado  and  Humphrey  Islands :  I  can  find 
out  nothing  about  these  islands,  except  that  the  latter  ap- 
pears to  be  small  and  low ;  not  coloured. — Bearson^  or  Grand 
Duke  Alexander's  (10°  S.,  161°  W.) :  an  atoll,  of  which  a 
plan  is  given  by  Bellingshausen  ;  blue. — Souvoroff  Islands 
(13°  S.,  163°  W.) :  Admiral  Krusenstern,  in  the  most 
obhging  manner,  obtained  for  me  an  account  of  these  islands 
from  Admiral  Lazareff,  who  discovered  them.  They  con- 
sist of  five  very  low  islands  of  coral  formation,  two  of  which 
are  connected  by  a  reef,  with  deep  water  close  to  it.  They 
do  not  surround  a  lagoon,  but  are  so  placed  that  a  line 
drawn  through  them  includes  an  oval  space,  part  of  which 
is  shallow  ;  these  islets,  therefore,  probably  once  (as  is  the 
case  with  some  of  the  islands  in  the  Caroline  Archipelago) 
formed  a  single  atoll  ^ ;  but  I  have  not  coloured  them. — 
Danger  Island  (10°  S.,  166°  W.)  :  described  as  low  by 
Commodore  Byron,  and  more  lately  surveyed  by  Bellings- 
hausen ;  it  is  a  small  atoll  with  three  islets  on  it ;  blue. — 
Clarence  Island  (9°  S.,  172°  W.)  :  discovered  in  the  Pandora 
(G.  Hamilton's  Voyage,  p.  75) :  it  is  said,  *  In  running  along 
the  land,  we  saw  several  canoes  crossing  the  lagoons  ; '  as 

*  [Stavbuck  and  Maiden  Islands  are  fringed.— Captain  Wharton.] 
'  [SuverofE  is  a  complete  atoll  (French  chart),  -Captain  Wharton.] 


208  APPENDIX. 

this  island  is  in  the  close  vicinity  of  other  low  islands,  and 
as  it  is  said  that  the  natives  make  reservoirs  of  water  in 
old  cocoa-nut  trees  (which  shows  the  nature  of  the  land),  I 
have  no  doubt  it  is  an  atoll,  and  have  coloured  it  blue. — 
York  Island  (8°  S.,  172°  W.)  is  described  by  Commodore 
Byron  (chap;  x.  of  his  Voyage)  as  an  atoll;  blue. — Sydney 
Island  (4°  S.,  172°  W.)  is  about  three  miles  in  diameter,  with 
its  interior  occupied  by  a  lagoon  (Captain  Tromelin,  Annal. 
Marit.  1829,  p.  297) ;  coloured  blue. — HiUl  Island  is  situ- 
ated 60  miles  to  the  west  of  Sydney  Island,  and  is  described 
by  Wilkes  (U.S.  Exploring  Expedition,  vol.  iii.  p.  369)  as  a 
lagoon-island  ;  coloured  blue. — Phoejiix  Island  (4°  S.,  171° 
W.)  is  nearly  circular,  low,  sandy,  not  more  than  two  miles 
in  diameter,  and  very  steep  outside  (Tromelin,  Annal.  Marit. 
1829,  p.  297) :  it  may  be  inferred  that  this  island  originally 
contained  a  lagoon,  but  I  have  not  coloured  it. — New  Nan- 
tucket  (0°  15'  N.,  174°  W.) :  from  the  French  chart  it  must 
be  a  low  island ;  I  can  find  nothing  more  about  it,  or  about 
Mary  Island;  both  uncoloured. — Gardner  Island  (5°  S., 
174°  W.),  from  its  position,  is  certainly  the  same  as  Kemin 
Island,  and  is  described  (Krusenstern,  p.  435,  Appen.  to 
Mem.  published  1827)  as  having  a  lagoon  in  its  centre ; 
coloured  blue. 

Islands  south  of  the  Sandwich  Archipelago, 

Christmas  Island  (2°  N.,  157°  W.) :  Captain  Cook,  in  his 
Third  Voyage  (vol.  ii.  chap,  x.),  has  given  a  detailed  account 
of  this  atoll.  The  breadth  of  the  islets  on  the  reef  is  un- 
usually great,  and  the  sea  near  it  does  not  deepen  so  sud- 
denly as  is  generally  the  case.  It  has  more  lately  been 
visited  by  Mr.  F.  D.  Bennett  (Geograph.  Journ.  vol.  vii. 
p.  226) ;  and  he  assures  me  that  it  is  low  and  of  coral 
formation  :  I  particularly  mention  this,  because  it  is  en- 
graved with  a  capital  letter,  signifying  a  high  island,  in 
D'Urville  and  Lottin's  chart.      Mr.  Couthouy,  also,  has 


PACIFIC   OCEAN.  209 

given  some  account  of  it  (Eemarks,  p.  46)  from  the 
Hawaiian  Spectator ;  he  beheves  it  has  lately  undergone  a 
Bmall  elevation,  but  his  evidence  does  not  appear  to  me 
satisfactory ;  the  deepest  part  of  the  lagoon  is  said  to  be 
only  ten  feet ;  nevertheless,  I  have  coloured  it  blue. — Fan- 
ning Island  (4°  N.,  158°  W.),  according  to  Captain  Tromelin 
(Ann.  Maritim.  1829,  p.  283),  is  an  atoll :  his  account,  as 
observed  by  Krusenstern,  differs  from  that  given  in  Fan- 
ning's  Voyage  (p.  224),  which,  however,  is  far  from  clear ; 
coloured  blue. — Washington  Island  (4°  N.,  159°  W.)  is  en- 
graved as  a  low  island  in  D'Urville's  chart,  but  is  described 
by  Fanning  (p.  226)  as  having  a  much  greater  elevation  than 
Fanning  Island,  and  hence  I  presume  it  is  not  an  atoll 
not  coloured. — Palmyra  Island  (6°  N.,  162°  W.)  is  an  atoll 
divided  into  two  parts  (Krusenstern's  Mem.  Suppl.  p.  50 
also  Fanning's  Voyage,  p.  233)  ;  blue. — Smyth's,  or  John 
ston's  Islands  (17°  N.,  170°  W.) :  Captain  Smyth,  E.N. 
has  had  the  kindness  to  inform  me  that  they  consist  of  two 
very  low  small  islands,  with  a  dangerous  reef  off  the  east 
end  of  them  ;  Captain  Smyth  does  not  recollect  whether 
these  islets,  together  with  the  reef,  surrounded  a  lagoon ; 
uncoloured. 

Sandwich  Aechipelago. — Hawaii:  in  the  chart  in 
Freycinet's  Atlas  small  portions  of  the  coast  are  fringed 
by  reefs  ;  and  in  the  accompanying  Hydrog.  Memoir,  reefs 
are  mentioned  in  several  places,  and  the  coral  is  said  to 
injure  the  cables ;  but  Dana  saw  hardly  any  reefs  here.* 

'  pProf.  Dana,  noticing  this  remark  in  Silliman'a  Arier.  Jour.,  Deo. 
1874,  states  the  result  of  further  enquiries  on  his  part  from  the  Kev. 
Mr.  M'Coan,  long  a  resident  of  Hilo  : — '  With  respect  to  your  enquiry 
whether  there  is  any  elevated  coral-reef  rock  around  the  shores  of 
Hawaii,  I  would  reply  that  I  think  not.  .  . .  Honolulu,  on  the  island  of 
Oahu,  is  built  much  of  it  upon  the  elevated  coral-reef  rock,  and  there 
are  large  areas  in  the  district  of  Waiana  and  other  portions  of  the  Oahu 
shores :  but  there  is  nothing  of  this  kind  on  Hawaii.  You  are  awara 
that  corals,  even  under  the  water,  are  on  the  weather  [eastern]  side 


210  APPENDIX. 

On  one  side  of  the  islet  of  Kohaihai  there  is  a  bank  of 
sand  and  coral  with  five  feet  of  water  on  it,  running  parallel 
to  the  shore,  and  leaving  a  channel  of  about  fifteen  feet 
deep  within.  I  have  coloured  this  island  red,  but  it  13 
very  much  less  perfectly  fringed  than  others  of  the  group. 
— Maid  :  in  Freycinet's  chart  of  the  anchorage  of  Eaheina, 
two  or  three  miles  of  coast  are  seen  to  be  fringed  ;  and  in 
the  Hydrog.  Memoir  *  banks  of  coral  along  shore '  are 
spoken  of.  Mr.  F.  D.  Bennett  informs  me  that  the  reefs, 
on  an  average,  extend  about  a  quarter  of  a  mile  from  the 
beach  ;  the  land  is  not  very  steep,  and  outside  the  reefs  the 
sea  does  not  become  suddenly  deep  ;  coloured  red. — Morotoi, 
I  presume,  is  fringed :  Freycinet  speaks  of  the  breakers 
extending  along  the  shore  at  a  little  distance  from  it. 
From  the  chart,  I  believe  it  is  fringed ;  coloured  red. — Oahu: 
Freycinet,  in  his  Hydrog.  Memoir,  mentions  some  reefs. 
Mr.  F.  D.  Bennett  informs  me  that  the  shore  is  skirted  for 
forty  or  fifty  miles  in  length.  There  is  even  a  harbour  for 
ships  formed  by  the  reefs,  but  it  is  at  the  mouth  of  a  valley ; 
red. — Atooi,  in  La  Peyrouse's  charts,  is  represented  as 
fringed  by  a  reef,  in  the  same  manner  as  Oahu  and 
Morotoi ;  and  this,  I  am  informed  by  Mr.  Ellis,  is  of  coral- 
formation  on  part  at  least  of  the  shore  ;  the  reef  does  not 
leave  a  deep  channel  within ;  red. — Oneehow :  Mr.  Ellis 
believes  that  this  island  is  also  fringed  by  a  coral-reef: 
considering  its  close  proximity  to  the  other  islands,  I  have 
ventured  to  colour  it  red.  I  have  in  vain  consulted  the  works 
of  Cook,  Vancouver,  La  Peyrouse,  and  Lisiansky  for  any 
satisfactory  account  of  the  small  islands  and  reefs  which  lie 
scattered  in  a  N.W.  line  prolonged  for  a  great  distance  from 
the  Sandwich  group,  and  hence  have  left  them  uncoloured, 
with  one  exception  ;  for  I  am  indebted  to  Mr.  F.  D.  Bennett 
for  informing  me  of  an  atoll-formed  reef,  in  lat.  28°  22'  N., 

of  this  island  not  abundant.'  In  the  Narrative  of  the  Challenger 
Voyage  (p.  699)  reefs  are  mentioned  as  occurring  at  Honolulu.] 


PACIFIC    OCEAN.  211 

long.  178°  30'  W.,  on  which  the  Gledstanes  was  wrecked 
in  1837.  It  is  apparently  of  large  size,  and  extends  in  a 
N.W.  and  S.E.  line  :  very  few  islets  have  been  formed  on  it. 
The  lagoon  seems  to  be  shallow ;  at  least,  the  deepest  part 
which  was  surveyed  was  only  three  fathoms.  Mr.  Couthouy 
(Remarks,  p.  38)  describes  this  island  under  the  name  of 
Ocean  Island.  Considerable  doubts  should  be  entertained 
regarding  the  nature  of  a  reef  of  this  kind,  with  a  very 
shallow  lagoon,  and  standing  far  from  any  other  atoll,  on 
account  of  the  possibility  of  a  crater  or  flat  bank  of  rock 
lying  at  the  proper  depth  beneath  the  surface  of  the  water, 
having  afforded  a  foundation  for  a  ring-formed  coral- 
reef.  I  have,  however,  thought  myself  compelled,  from 
its  large  size  and  symmetrical  outline,  to  colour  it  blue. 
Some  information  and  references  are  given  by  Dana  (Corals 
and  Coral  Islands,  pp.  824,  365)  with  respect  to  the  reefs 
and  islets  extending  for  2,000  miles  in  a  N.W.  line  from 
Hawaii. 

Samoa  or  Navigator  Group. — Kotzebue,  in  his 
Second  Voyage,  contrasts  these  islands  with  many  others 
in  the  Pacific,  in  not  having  harbours  for  ships,  formed  by 
distant  coral-reefs.  The  Rev.  J.  Williams,  however,  informs 
me  that  coral-reefs  do  occur  in  irregular  patches  on  the 
shores ;  but  that  they  do  not  form  a  continuous  band  as  round 
Mangaia,  and  other  such  perfect  cases  of  fringed  islands. 
From  the  charts  accompanying  La  Peyrouse's  Voyage,  it 
appears  that  the  north  shore  of  Savaii,  Maouna,  Orosenga, 
and  Manua  are  fringed  by  reefs.  La  Peyrouse,  speaking 
of  Maouna  (p.  126),  says  that  the  coral-reef  surrounding  its 
shores  almost  touches  the  beach,  and  is  breached  in  front 
of  the  little  coves  and  streams,  forming  passages  for  canoes, 
and  probably  even  for  boats.  Further  on  (p.  159)  he  ex- 
tends the  same  observation  to  all  the  islands  which  he 
visited. — Mr.  Williams  in  his  Narrative,  speaks  of  a  reef 
going  round  a  small  island  attached  to  Oyolava,  and  return- 


212  APPENDIX. 

ing  again  to  it :  all  these  islands  have  been  coloured  red. — 
A  chart  of  Bose  Island,  at  the  extreme  [east]  end  of  the 
group,  is  given  by  Freycinet,  from  which  I  should  have 
thought  that  it  had  been  an  atoll ;  ^  but  according  to  LIr. 
Couthouy  (Eemarks,  p.  43)  it  consists  of  a  reef,  only  a 
league  in  circuit,  surmounted  by  a  very  few  low  islets  ;  the 
lagoon  is  very  shallow,  and  is  strewed  with  numerous  large 
boulders  of  volcanic  rock.  This  island,  therefore,  probably 
consists  of  a  bank  of  rock,  a  few  feet  submerged,  with  the  outer 
margin  fringed  with  reefs ;  hence  it  cannot  be  properly 
classed  with  atolls,  in  which,  as  we  have  reason  to  believe, 
the  foundations  always  lie  at  a  depth  greater  than  that  at 
which  the  reef- constructing  polypifers  can  live ;  not  coloured. 

Beveridge  Eeef,  20°  S.,  167°  W.,  is  described  in  the 
Naut.  Mag.  (May  1833,  p.  442)  as  ten  miles  long  in  a  N. 
and  S.  line,  and  eight  wide  ;  '  in  the  inside  of  the  reef,  there 
appears  deep  water ; '  there  is  a  passage  near  the  S.W. 
corner :  this  therefore  seems  to  be  a  submerged  atoll,  and 
is  coloured  blue. 

Savage  Island,  19°  S.,  170°  W.,  has  been  described  by 
Cook  and  Forster.  The  younger  Forster  (vol.  ii.  p.  163) 
says  it  is  about  40  feet  high  :  he  suspects  that  it  contains 
a  low  plain,  which  formerly  was  the  lagoon.  The  Kev.  J. 
Williams  gives  100  feet  as  its  height,  and  he  informs  me 
that  the  reef  fringing  its  shores  resembles  that  round 
Mangaia ;  coloured  red. 

Fbiendly  Aechipelago. — Pylstaart  Island:  judging 
from  the  chart  in  Freycinet's  Atlas  I  should  have  supposed 
that  it  had  been  regularly  fringed  ;  but  as  nothing  is  said  in 
the  Hydrog.  Memoir  (or  in  the  Voyage  of  Tasman,  the  dis- 

*  [It  is  an  atoll. — Capt.  Wharton.  Eose  Island  has  a  lagoon  six 
to  twelve  fathoms  deep  and  an  entrance  to  it  of  four  fathoms. 
Except  two  small  banks,  one  supporting  a  group  of  trees,  it  is  under 
water  at  high  tide. — Letter  from  Prof.  Dana  to  Mr.  Darwin,  July  21j 
1674.] 


PACIFIC    OCEAN.  213 

coverer)  about  coral-reefs,  I  have  left  it  uncoloured. — 
Tongatahou  :  in  the  atlas  of  the  Voyage  of  the  Astrolabe^ 
the  whole  south  side  of  the  island  is  represented  as 
narrowly  fringed  by  the  same  reef  which  forms  an  exten- 
sive platform  on  the  northern  side.  The  origin  of  this 
latter  reef,  which  might  have  been  mistaken  for  a  barrier- 
reef,  has  already  been  attempted  to  be  explained,  when 
giving  the  proofs  of  the  recent  elevation  of  this  island. — In 
Cook's  charts  the  little  outlying  island  of  Eoaigee  is  repre- 
sented as  fringed;  coloured  red. — Eoua:  I  cannot  make 
out  from  Captain  Cook's  charts  and  descriptions  that  this 
island  has  any  reef,  although  the  bottom  of  the  neighbour- 
ing sea  seems  to  be  covered  with  corals,  and  the  island 
itself  is  formed  of  coral-rock.  Forster,  however,  distinctly 
(Observations,  p.  14)  classes  it  with  the  high  islands 
having  reefs,  but  it  certainly  is  not  encircled  by  a  barrier- 
reef  ;  and  the  younger  Forster  (Voyage,  vol.  i.  p.  426)  says, 
that '  a  bed  of  coral  rocks  surrounded  the  coast  towards  the 
landing-place.'  I  have  therefore  classed  it  with  the  fringed 
islands,  and  coloured  it  red.  Dana  also  shows  (Corals  and 
Coral  Islands,  p.  337)  that  most  of  the  islands  of  this  group 
are  formed  of  upraised  coral-rock.  The  several  islands 
lying  N.W.  of  Tongatahou,  namely  Anamouha,  Komango^ 
Kotouj  Lefouga^  Foa,  &c.,  are  seen  in  Captain  Cook's  chart 
to  be  fringed  by  reefs,  and  several  of  them  are  connected 
together.  From  the  various  statements  in  the  first  volume 
of  Cook's  Third  Voyage,  and  especially  in  Chapters  IV.  and 
VI.,  it  appears  that  these  reefs  are  of  coral,  and  certainly 
do  not  belong  to  the  barrier  class  ;  coloured  red. — Toufoa 
and  Kao^  forming  the  western  part  of  the  group,  according 
to  Forster,  have  no  reefs  ;  the  former  is  an  active  volcano. 
— Vavao  :  there  is  a  chart  of  this  singularly-formed  island, 
by  Espinoza  :  according  to  Mr.  Williams  it  consists  of 
coral-rock  :  the  Chevalier  Dillon  informs  me  that  it  is  not 
fringed ;  not  coloured.    Nor  are  the  islands  of  Latte  and 


214  APPENDIX. 

Amargura  coloured,  for  I  have  not  seen  plans  of  them  on  a 
large  scale,  and  I  do  not  know  whether  they  are  frmged : 
Amargura  is  said  (Athenaeum,  1848,  p.  40)  to  have  been 
lately  in  violent  eruption. 

Niouha,  16°  S.,  174°  W.,  or  Eeppel  Island  of  Wallis,  or 
Cocos  Island :  from  a  view  and  chart  of  this  island,  given 
in  Wallis's  Voyage,  (4to  edit.)  it  is  evidently  encircled  by 
a  reef;  coloured  blue.  It  is,  however,  remarkable  that 
Boscaioen  Island,  immediately  adjoining,  has  no  reef  of 
any  kind ;  uncoloured. 

Wallis  Island,  13°  S.,  176°  W.:  a  chart  and  view  of 
this  island  in  Wallis's  Voyage  (4to  edit.)  shows  that  it  is 
encircled.^  A  view  of  it  in  the  Naut.  Mag.  July  1833,  p.  376, 
shows  the  same  fact.  Nine  islands,  most  of  them  high, 
are  said  in  Wilkes  U.S.  Exploring  Expedition  (vol.  ii. 
p.  157)  to  be  enclosed  within  the  same  reef,  through 
which,  it  is  asserted,  ships  can  enter  ;  coloured  blue. 

Alloufatoii,  or  Horn  Island,  Onouafu,  or  Pro 62/ Island,* 
and  Hunter  Islands,  lie  between  the  Navigator  and  Fidji 
groups.     I  (ian  find  no  distinct  accounts  of  them. 

Fidji  or  Feejee  or  Viti  Group.^ — Until  lately  the 

*  [Wallis  Island  is  encircled.  There  are  eleven  islands,  seven  of 
■whicli  are  on  the  outer  reef. — Note  sent  to  Mr.  Darwin  by  Lieut. 
Chas.  Smith,  H.M.S.  Fawn.] 

2  ['Horn  Islands,  comprising  Fotuna  andAlofa;  each  has  a  distinct 
fringing-reef.  Fotuna  is  about  2,500  feet  and  Alofa  1,200  feet  high. 
I  can  give  no  information  regarding  the  depth  of  water,  except  that 
there  is  a  deep  ship-channel  between  the  islands,  no  soundings  being 
obtainable  with  the  hand-line.  The  channel  is  hardly  a  mile  broad. 
Nina-fu,  or  Good  Hope  Island,  which  I  presume  to  be  the  same  aa 
that  called  Onouafu  in  Coral  Eeefs,  is  entirely  volcanic,  and  has  no 
reef  whatever.'  — Note  sent  to  Mr.  Darwin  by  Liout.  Chas.  Smith, 
H.M.S.  Fawn.] 

'  [This  group  contains  every  description  of  reef. — Capt.  Whartoa 
Makata  has  a  central  volcanic  peak,  according  to  the  Narrative  of  Chah 
lengerYoy&ge  (p.  487),  and  is  surrounded  by  a  barrier-reef,  or  one  inter- 
mediate between  that  and  a  fringing-reef.    The  shore  line  suggest! 


PACIFIC   OCEAN.  215 

best  chart  of  the  numerous  islands  of  this  group  was  that 
in  the  atlas  of  the  Astrolabe's  Voyage  ;  but  now  the  islands 
have  been  surveyed  during  the  U.S.  Exploring  Expedition, 
and  full  information  respecting  them  and  the  reefs  has 
been  given  by  Dana.  Many  of  the  islands  are  bold  and 
mountainous,  and  are  surrounded  by  reefs,  lying  far  from 
the  land,  and  outside  the  ocean  appears  very  deep.  The 
Astrolabe  sounded  with  90  fathoms  in  several  places  about 
a  mile  from  the  reefs,  and  found  no  bottom.  It  is  evident 
that  the  water  within  many  of  the  encircling  reefs  is  deep  : 
as  indeed  I  was  formerly  assured  was  the  case  by  Dillon. 
Beyond  the  high  and  encircled  islands  there  are  numerous 
atoll-formed  reefs.  Hence  the  whole  group  has  been 
coloured  blue.  In  the  S.E.  part  lies  Batoa,  or  Ttcrtle 
Island  of  Cook  (Second  Voyage,  vol.  ii.  p.  23,  and  chart ; 
4to  edit.),  surrounded  by  a  coral-reef,  'which  in  some 
places  extends  two  miles  from  the  shore  ; '  within  the  reef 
the  water  appears  to  be  deep,  and  outside  it  is  unfathom- 
able ;  coloured  pale  blue.  At  the  distance  of  a  few  miles, 
Captain  Cook  {ibid.  p.  24)  found  a  circular  coral-reef,  four 
or  five  leagues  in  circuit,  with  deep  water  within ;  *  in 
short,  the  bank  wants  only  a  few  little  islets  to  make  it 
exactly  like  one  of  the  half-drowned  isles  so  often  men- 
tioned,'— namely,  atolls.  South  of  Batoa  lies  the  high 
island  of  O710,  which  appears  in  Bellingshausen's  Atlas  to 
be  encircled  ;  as  do  some  other  small  islands  to  the  south  ; 
coloured  pale  blue :  near  Ono,  there  is  an  annular  reef, 
quite  similar  to  the  one  just  described  in  the  words  of 
Captain  Cook ;  coloured  dark  blue. 

Bubsidenee,  and  the  reef  is  breached  opposite  to  the  principal  inlet  on 
the  land.  At  Ngaloa  harbour,  Kandava,  the  map  of  this  part  of  the 
island  suggests  subsidence,  but  there  are  also  indications  of  slight 
upheaval.  There  is  a  barrier-reef.  The  soundings  150  fathoms 
from  the  edge  of  the  reef  were  80  fathoms,  the  slope  for  the  first  65 
fathoms  from  the  shore  being  1  in  1,  then  1  in  1'4,  diminishing  to  1  in 
2  till  300  fathQms  was  reached.] 


216  APPENDIX. 

BotoumaJi,  13°  S.,  179°  E.— From  the  chart  in 
Duperrey's  Atlas,  I  thought  that  this  island  was  encircled, 
but  the  Chevalier  Dillon  assures  me  that  the  reef  is  only  a 
shore  or  fringing  one  ;  coloured  red.^ 

Independence  Island,  10°  S.,  179°  E.,  is  described  by 
Mr.  G.  Bennett  (United  Service  Journ.  1831,  part  ii.  p.  197) 
as  a  low  island  of  coral  formation ;  it  is  small,  and  does 
not  appear  to  contain  a  lagoon,  although  an  opening 
through  the  reef  is  referred  to.  A  lagoon  probably  once 
existed,  and  has  since  been  filled  up  ;  left  uncoloured. 

Ellice  Group. — Oscar ^  Peyster,  and  Ellice  Islands 
are  figured  in  Arrowsmith's  Chart  of  the  Pacific  (corrected 
to  1832)  as  atolls,  and  are  said  to  be  very  low ;  blue.^ — • 
Nederlandisch  Island :  I  am  greatly  indebted  to  the  kind- 
ness of  Admiral  Krusenstem  for  sending  me  the  original 
documents  concerning  this  island.  From  the  plans  given 
by  Captains  Eeg  and  Khremtshenko,  and  from  the  detailed 
account  given  by  the  former,  it  appears  that  it  is  a  narrow 
coral-island,  about  two  miles  long,  containing  a  small 
lagoon.  The  sea  is  very  deep  close  to  the  shore,  which  is 
fronted  by  sharp  coral-rocks.  Captain  Eeg  compares  the 
lagoon  with  that  of  other  coral-islands  ;  and  he  distinctly 
says,  the  land  is  *  very  low.'  I  have  therefore  coloured  it 
blue.  Admiral  Krusenstern  (Supplement  au  Eecueil  des 
Memoires  Hydrographiques  publics  en  1826  et  1827,  &c.  &c. 
St.  Petersburg,  1835)  states  that  its  shores  are  80  feet  high ; 
this  probably  arose  from  the  height  of  the  cocoa-nut  trees, 
with  which  it  is  covered,  being  mistaken  for  land. — Grand 
Cocal  is  said  in  Krusenstern's  Memoir  to  be  low,  and  to  be 
surrounded  by  a  reef ;  it  is  small,  and  therefore  probably 

•  ['  There  is  an  extinct  volcano  on  the  island.' — Note  written  in 
Mr.  Darwin's  copy  of  this  work.] 

=*  ['Mitchell  Island,  to  the  south  of  the  Ellice  group,  is  a  very  low 
atoll  with  about  ten  small  islands  on  the  reef.  We  were  unable  to 
discover  any  entrance  to  the  lagoon.'— Lieut.  Cbab.  {Smith,  H.JM.S, 
Fawn.] 


PACIFIC    OCEAN.  217 

once  contained  a  lagoon;  uncolouredJ — St.  Augustin: 
from  a  chart  and  view  of  it,  given  in  the  atlas  of  the 
Coquille's  Voyage,  it  appears  to  be  a  small  atoll,  with  its 
lagoon  partly  filled  up  ;  coloured  blue. 

Gilbert  Geoup. — The  chart  of  this  group,  given  in  the 
atlas  of  the  Coquille's  Voyage,  shows  that  it  is  composed 
of  ten  well-characterized,  but  very  irregularly  shaped  atolls. 
In  D'Urville  and  Lottin's  chart,  Syde^iham  is  written  with 
a  capital  letter,  signifying  that  it  is  high  ;  but  this  certainly 
is  not  the  case,  for  it  is  a  perfectly  characterized  atoll,  and 
a  sketch,  showing  how  low  it  is,  is  given  in  the  Coquille's 
atlas.  Some  narrow  strip-like  reefs  project  from  the  south- 
ern side  of  Drummond  atoll,  and  render  it  irregular.  The 
southern  island  of  the  group  is  called  Chase  (in  some  charts, 
Botches) ;  of  this  I  can  find  no  account,  but  Mr.  F.  D.  Ben- 
nett discovered  (Geograph.  Journ.  vol.  vii.  p.  229)  a  low 
extensive  island  in  nearly  the  same  latitude,  about  three 
degrees  westward  of  the  longitude  assigned  to  Botches  ; 
and  this  probably  is  the  same  island.  Mr.  Bennett  informs 
me  that  the  man  at  the  masthead  reported  an  appearance 
of  lagoon-water  in  the  centre;  and,  therefore,  considering  its 
position,  I  have  coloured  it  blue. — Pitt  Island,  at  the  ex- 
treme northern  point  of  the  group,  is  left  uncoloured,  aa 
neither  its  exact  position  nor  nature  is  known. — Byron 
Island,  which  lies  a  little  to  the  eastward,  does  not  appear 
to  have  been  visited  since  Commodore  Byron's  voyage,  and 
it  was  then  seen  only  from  a  distance  of  18  miles  :  it  is  said 
to  be  low  ;  uncoloured. 

Ocean, Pleasant,  and.  Atlantic  Islands  all  lie  considerably 
to  the  west  of  the  Gilbert  group  :  I  have  been  unable  to  find 
any  distinct  account  of  them.     Ocean  Island  is  written  with 

'  ['  Grand  Cocal  was  searched  for  in  vain  by  H.M.S.  Basilisk,  and  as 
all  the  local  traders  deny  its  existence,  I  cannot  think  it  exists.  It 
has  long  been  marked  '  doubtful '  on  the  Admiralty  charts,  and  the 
description  leads  me  to  suppose  the  island  reported  to  have  been  St. 
^ugustin.' — Lieut.  Chas.  Smith,  H.M.S.  Fawn.] 


218  APPENDIX. 

small  letters  in  the  French  chart,  but  in  Krusenstem's 
Memoir  it  is  said  to  be  high. 

Marshall  Geoup. — We  are  well  acquainted  with  this 
group  from  the  excellent  charts  of  the  separate  islands,  made 
during  the  two  voyages  of  Kolzebue  :  a  reduced  one  of  the 
whole  group  may  be  seen  in  Krusenstern's  Atlas,  and  in 
Kotzebue's  Second  Voyage.  The  group  consists  (with  the 
exception  of  two  little  islands  which  probably  have  had  their 
lagoon  filled  up)  of  a  double  row  of  23  large  and  well-cha- 
racterized atolls,  from  the  examination  of  which  Chamisso 
drew  up  his  well-known  account  of  coral  formations.  I  in- 
clude in  this  group  Gaspar  Rico,  or  Gornwallis  Island,  which 
is  described  by  Chamisso  (Kotzebue's  First  Voyage,  vol.  iii. 
p.  179)  '  as  a  low  sickle-formed  group,  with  mould  only  on 
the  windward  side.'  Gaspard  Island  is  considered  by  some 
geographers  as  a  distinct  island  lying  N.E.  of  the  group, 
but  it  is  not  entered  in  the  chart  by  Krusenstern ;  left  un- 
coloured.  In  the  S.W.  part  of  this  group  lies  Baring  Island, 
of  which  little  is  known  (see  Krusenstern's  Appendix,  1835, 
p.  149).  I  have  left  it  uncoloured ;  but  Bostoii  Island  I 
have  coloured  blue,  as  it  is  described  (ibid.)  as  consisting  of 
1^  ^mall  islands,  which,  no  doubt,  inclose  a  lagoon,  as  re- 
presented in  a  chart  in  the  Coquille's  atlas. — Three  islands, 
Aur,  Kawen  and  Gaspar  Bico,  are  written  in  the  French 
chart  with  capital  letters  ;  but  this  is  an  error,  for  from  the 
account  given  by  Chamisso  in  Kotzebue's  First  Voyage,  they 
are  certainly  low.  The  nature,  position,  and  even  existence 
of  the  shoals  and  small  islands  north  of  the  Marshall  group 
are  doubtful. 

New  Hebrides. — Any  chart,  on  even  a  small  scale,  of 
these  islands  will  show  that  their  shores  are  almost  without 
reefs,*  presenting  a  remarkable  contrast  with  those  of  New 

*  [The  New  Hebrides  have  fringing-reefs  in  various  parts.  No 
barrier-reefs  are  yet  known,  but  the  charts  are  still  very  imperfect. 
—Capt.  Wharton.] 


PACIFIC    OCEAN.  219 

Caledonia  on  the  one  hand,  and  the  Fidji  group  on  the  other. 
Nevertheless,  I  have  been  assured  by  Mr.  G.  Bennett,  that 
coral  grows  vigorously  on  their  shores  ;  as,  indeed,  will  be 
further  shown  in  some  of  the  following  notices.  As,  there- 
fore, these  islands  are  not  encircled,  and  as  coral  grows 
vigorously  on  their  shores,  we  might  almost  conclude,  with- 
out further  evidence,  that  they  are  fringed,  and  hence  I 
have  applied  the  red  colour  with  rather  greater  freedom 
than  in  other  instances. — Matthew's  Eock,  an  active  volcano, 
some  way  south  of  the  group  (of  which  a  plan  is  given  in 
atlas  of  the  Astrolabe's  Voyage)  does  not  appear  to  have 
reefs  of  any  kind  about  it. — Ajinatom,  the  southernmost  of 
the  Hebrides  :  from  a  rough  woodcut  given  in  the  United 
Service  Journal  (1831,  part  iii.  p.  190),  accompanying  a 
paper  by  Mr.  Bennett,  it  appears  that  the  shore  is  fringed ; 
coloured  red. — Tanna  :  Forster,  in  his  Observations  (p.  22), 
says  Tanna  has  on  its  shores  coral-rock  and  madrepores ; 
and  the  younger  Forster,  in  his  account  (vol.  ii.  p.  269), 
speaking  of  the  harbour,  says  the  whole  S.E.  side  consists 
of  coral-reefs,  which  are  overflowed  at  high  water  :  part  of 
the  southern  shore  in  Cook's  chart  is  represented  as  fringed ; 
coloured  red. — Immer  is  described  (United  Service  Journ. 
1831,  part  iii.  p.  192)  by  Mr.  Bennett  as  being  of  moderate 
elevation,  with  cliffs  appearing  like  sandstone  ;  coral  grows 
in  patches  on  its  shore,  but  I  have  not  coloured  it ;  and 
I  mention  these  facts  because  Immer  might  have  been 
thought,  from  Forster's  classification  (Observations,  p.  14), 
to  have  been  a  low  island,  or  even  an  atoll. — Erromango 
Island  :  Cook  (Second  Voyage,  vol.  ii.  p.  45,  4to  edit.) 
speaks  of  rocks  everywhere  lining  the  coast,  and  the  natives 
offered  to  haul  his  boat  over  the  breakers  to  the  sandy 
beach  :  Mr.  Bennett,  in  a  letter  to  the  editor  of  the  Singa- 
pore Chron.,  alludes  to  the  reefs  on  its  shores.  It  may,  I 
think,  be  safely  inferred  from  these  passages  that  the  shore 
is  fringed  in  parts  by  coral-reefs  ;  coloured  red. — Sandwich 


220  APPENDIX. 

Island :  the  east  coast  is  said  (Cook's  Second  Voyage,  vol.  ii. 
p.  41)  to  be  low,  and  to  be  guarded  by  a  chain  of  breakers. 
In  the  accompanying  chart  it  is  seen  to  be  fringed  by  a  reef ; 
coloured  red. — Mallicollo :  Forster  speaks  of  the  reef- 
bounded  shore :  the  reef  is  about  30  yards  wide,  and  so 
shallow  that  a  boat  cannot  pass  over  it.  Forster,  also,  (Ob- 
servat.  p.  23,)  says  that  the  rocks  of  the  sea-shore  consist 
of  madrepore.  In  the  plan  of  Sandwich  harbour,  the  head- 
lands are  represented  as  fringed ;  coloured  red. — A2irora 
and  Pentecost  Islands,  according  to  Bougainville,  apparently 
have  no  reefs  ;  nor  has  the  large  island  of  S.  Espiritu,  nor 
Bligh  Island,  nor  Banks  Islands,^  which  latter  lie  to  the 
N.E.  of  the  Hebrides.  But  in  none  of  these  cases  have  I 
met  with  any  detailed  account  of  their  shores,  or  seen  plans 
on  a  large  scale  ;  and  it  will  be  evident  that  a  fringing-reef 
of  only  thirty,  or  even  a  few  hundred  yards  in  width  is  of 
so  little  importance  to  navigation,  that  it  will  seldom  be 
noticed,  excepting  by  chance ;  and  hence  I  do  not  doubt 
that  several  of  these  islands,  now  left  uncoloured,  ought 
to  be  red. 

Santa- Cruz  Group. — Vanikoro  (Fig.  1,  Plate  I.)  offers 
a  striking  example  of  a  barrier-reef :  it  was  first  described 
by  the  Chevalier  Dillon,  in  his  Voyage,  and  was  surveyed 
in  the  Astrolabe  ;  coloured  pale  blue. — Tikopia  and  Fataka 
Islands  appear,  from  the  descriptions  of  Dillon  and  D'Urville, 
to  have  no  reefs  :  Anouda  is  a  low,  flat  island,  surrounded 
by  cliffs,  {Astrolabe,  Hydrog.  and  Krusenstern  Mem.  vol.  ii. 
p.  432)  ;  these  are  uncoloured. — Toupoua  (Otooboa  of 
Dillon)  is  stated  by  Captain  Tromelin  (Annales  Marit.  1829, 
p.  289}  to  be  almost  entirely  included  in  a  reef,  lying  at  the 
distance  of  two  miles  from  the  shore.  There  is  a  space  of 
three  miles  without  any  reef,  which,  although  indented 
with  bays,  offers  no  anchorage  from  the  extreme  depth  of 
the  water  close  to  the  shore.  Captain  Dillon  also  speaka 
'  [Banks  Islands  are  fringed  in  parts.— Capt.  Wharton.] 


PACIFIC   OCEAN,  221 

of  tlie  reefs  fronting  this  island ;  coloured  hlne.^^-Santa 
Cruz :  I  have  carefully  examined  the  works  of  Carteret, 
Dentrecasteaux,  Wilson,  and  Tromelin,  and  I  cannot  dis- 
cover any  mention  of  reefs  on  its  shores  ;  left  uncoloured, 
— Tinahoro  is  a  constantly  active  volcano  without  reefs.^ 
Mendana  Isles  (mentioned  by  Dillon  under  the  name  of 
Mammee,  &c.)  are  said  by  Krusenstern  to  be  low  and 
intertwined  with  reefs.  I  do  not  believe  they  include  a 
lagoon ;  I  have  left  them  uncoloured. — Duff's  Islands  com- 
pose a  small  group  directed  in  a  N.W.  and  S.E.  band  ;  they 
are  described  by  Wilson  (p.  296,  Miss.  Voy.  4to  edit.)  as 
formed  by  bold  peaked  land,  with  the  islands  surrounded 
by  coral-reefs,  extending  about  half  a  mile  from  the  shore : 
at  the  distance  of  a  mile  from  the  reefs  he  found  only  seven 
fathoms.  As  I  have  no  reason  for  supposing  there  is  deep 
water  within  these  reefs,  I  have  coloured  them  red. — Kennedy 
Island,  N.E.  of  Duff's :  I  have  been  unable  to  find  any 
account  of  it. 

New  Caledonia. — The  great  barrier-reefs  on  the  shores 
of  this  island  have  already  been  described  (Fig.  5,  Plate  II.). 
They  have  been  visited  by  Labillardiere,  Cook,  and  the 
northern  point  by  D'Urville ;  this  latter  part  s^  closely 
resembles  an  atoll  that  I  have  coloured  it  dark  blue.  The 
Loyalty  group  is  situated  to  the  east  of  New  Caledonia ; 
some  at  least  of  the  islands  are  formed  of  upraised  coral- 
rock,  and  are  fringed  with  living  reefs  ;  see  Rev.  W.  B, 
Clarke,  in  Journal  of  Geolog.  Soc.  1847,  p.  61 ;  coloured 
red.  North  of  this  group  there  are  some  extensive  low 
reefs  (called  Astrolabe  and  Beaupr^^),  which  do  not  seem 
to  be  atoll-formed  :  these  are  left  uncoloured. 

•  [This  island  has  a  barrier-reef,  with  a  4-fathom  channel  through 
II,  which  leads  into  a  harbour  in  the  island  itself.  There  is  also 
deep  but  uneven  water  generally  inside  the  reef.  -  Lieut.  Chas.  Smith, 
H.M.S.  Fawn.-] 

■'  [This  is  an  atoll.— Capt.  Wharton.] 

16 


222  APPENDIX. 

AustealianBaerieb-Reef. — This  great  reef,  which  has 
already  been  described,  has  been  coloured  from  the  charts 
of  Flinders  and  King.  Jukes  has  given  many  details  re- 
specting it  in  the  Voyage  of  H.M.S.  Fly  (vol.  i.  1847,  chap, 
xiii.}.  In  the  northern  parts,  an  atoll-formed  reef,  lying 
outside  the  barrier,  has  been  described  by  Bligh,  and  is 
coloured  dark  blue.  In  the  space  between  AustraHa  and 
New  Caledonia,  called  by  Flinders  the  Corallian  Sea,  there 
are  numerous  reefs.  Of  these,  some  are  represented  in 
Krusenstern's  Atlas  as  having  an  atoll-like  structure  ;  * 
namely,  Bampton  Shoal,  Frederic,  Vine  or  Horse-shoe, 
and  Alert  Eeefs  ;  these  have  been  coloured  dark  blue. 

LouisiADE. — The  dangerous  reefs  which  front  and 
surround  the  western,  southern,  and  northern  coasts  of  this 
so-called  peninsula  and  archipelago,  seem  evidently  to 
belong  to  the  barrier  class.  The  land  is  lofty,  with  a  low 
fringe  on  the  coast ;  the  reefs  are  distant,  and  the  sea  out- 
side them  profoundly  deep.  Nearly  all  that  is  known  of 
this  group  is  derived  from  the  labours  of  Dentrecasteaux 
and  Bougainville  :  the  latter  has  represented  one  continuous 
reef  90  miles  long,  parallel  to  the  shore,  and  in  places  as 
much  as  10  miles  from  it ;  coloured  pale  blue.  A  little 
distance  northward  we  have  the  Laughlan  Islands,  the 
reefs  round  which  are  engraved  in  the  atlas  of  the  Voyage 
of  the  Astrolabe,  in  the  same  manner  as  round  the  encircled 
islands  of  the  Caroline  Archipelago  :  the  reef  is,  in  parts,  a 
mile  and  a  half  from  the  shore,  to  which  it  does  not  appear 
to  be  attached ;  coloured  blue.  At  some  little  distance  from 
the  extremity  of  the  Louisiade  lies  Wells  Reef,  described 
in  G.  Hamilton's  Voyage  in  H.M.S.  Pandora  (p.  100) : 
it  is  said,  *  We  found  we  had  got  embayed  in  a  double 
reef,  which  will  soon  be  an  island.'  As  this  statement  is 
only  intelligible  on  the  supposition  of  the  reef  being  crescent 

'  [There  are  many  atolls  in  this  sea. — Capt.  Wharton.] 


PACIFIC   OCEAN.  223 

or  horse-shoe  formed,  like  so  many  other  submerged  annular 
reefs,  I  have  ventured  to  colour  it  blue. 

Saloman  Aechipelago. — The  chart  in  Krusenstern's 
Atlas  shows  that  these  islands  are  not  encircled  ;  and  as 
coral  appears,  from  the  works  of  Surville,  Bougainville,  and 
Labillardiere,  to  grow  on  their  shores,  this  circumstance, 
Eb  in  the  case  of  the  New  Hebrides,  is  a  presumption  that 
they  are  fringed.  I  cannot  find  out  anything  from  Dentre- 
casteaux's  Voyage,  regarding  the  southern  islands  of  the 
group,  so  have  left  them  uncoloured. — Malayta  Island,  in 
a  rough  MS.  chart  in  the  Admiralty,  has  its  northern  shore 
fringed. — Ysabel  Island :  the  N.E.  part  of  this  island,  as 
shown  in  the  same  chart,  is  also  fringed :  Mendana  (Burney, 
vol.  i.  p.  280),  speaking  of  an  islet  adjoining  the  northern 
coast,  says  it  is  surrounded  by  reefs :  the  shores,  also,  of 
Port  Praslin  appear  regularly  fringed.  Choiseul  Island : 
parts  of  the  shores  are  fringed  by  coral-reefs,  in  Bougain- 
ville's chart  of  Choiseul  Bay. — Bougainville  Island  :  accord- 
ing to  Dentrecasteaux,  the  western  shore  abounds  with 
coral-reefs,  and  the  smaller  islands  are  said  to  be  attached 
to  the  larger  ones  by  reefs  ;  all  the  above-mentioned  islands 
have  been  coloured  red. — Bouha  Islands :  Captain  Duperrey 
has  kindly  informed  me  in  a  letter  that  he  passed  close 
round  the  northern  side  of  this  island  (of  which  a  plan  is 
given  in  his  atlas  of  the  Coqioille's  Voyage),  and  that  it 
was  '  garnie  d'une  bande  de  recifs  a  fleur  d'eau  adherentes 
au  rivage ; '  and  he  infers,  from  the  abundance  of  coral  on 
the  islands  north  and  south  of  Bouka,  that  the  reef  pro- 
bably is  of  coral ;  coloured  red.* 

Off  the  north  ^ast  of  the  Saloman  Archipelago  there 
are  several  <^cill  groups  which  are  little  known:  they 
appear  to  be  low,  and  of  coral  formation ;  and  some  of 
them  probably  have  an  atoll-like  structure  :  the  Chevalier 

'  [Bouka,  according  to  the  best  accounts,  has  a  barrier-reef,  but 
our  information  is  still  imperfect. — Capt.  Wharton.] 


224  ArPENDix. 

Dillon,  however,  informs  me  this  is  not  the  case  with  the 
r3axos  de  Cajidelaria^ — Outong  Java,  according  to  the 
Spanish  navigator,  Maurelle,  is  thus  characterized  ;  hut  this 
is  the  only  one  which  I  have  ventured  to  colour  blue. 

New  Ireland. — The  shores  of  the  S.W.  point  of  this 
island  and  some  adjoining  islets,  are  fringed  by  reefs,  as 
may  be  seen  in  the  atlases  of  the  Voyages  of  the  Coquille 
and  Astrolabe.  M.  Lesson  observes  that  the  reefs  are  open 
in  front  of  each  streamlet.  The  Duke  of  York's  Island  is 
also  fringed ;  but  with  regard  to  the  other  parts  of  New 
Ireland,  New  Hanover,  and  the  small  islands  lying  north- 
ward, I  have  been  unable  to  obtain  any  information.  I 
will  only  add  that  no  part  of  New  Ireland  appears  to  be 
fronted  by  distant  reefs.  I  have  coloured  red  only  the 
above  specified  portions. 

New  Britain  and  the  Northern  Shore  of  New 
Guinea. — From  the  charts  in  the  Voyage  of  the  Astrolabe, 
and  from  the  Hydrog.  Memoir,  it  appears  that  these  coasts 
are  entirely  without  reefs,  as  are  the  Schouten  Islands, 
lying  close  to  the  northern  shore  of  New  Guinea.  The 
western  and  south-western  parts  of  New  Guinea  will  be 
treated  of  when  we  come  to  the  islands  of  the  East  Indian 
Archipelago. 

Admiralty  Group.^ — From  the  accounts  given  by 
Bougainville,  Maurelle,  Dentrecasteaux,  and  the  scattered 

*  [This  is  a  perfect  atoll. — Capt.  Wharton.] 

'  [Narrative  of  Challenger  Voyage,  p.  699.  Admiralty,  or  Bosco 
Islands.  The  main  island  rises  to  nearly  3,000  feet.  The  coast  is  low 
and  indented  with  deep  bays.  There  are  many  coral-reefs  off  the  coast 
at  varying  distances,  not  forming  a  connected  barier-reef.  There  is 
convenient  anchorage  within  the  reef,  the  soundings  in  the  deeper  part 
of  the  channel  at  Nares  Harbour  being  from  25  to  34  fathoms,  and  very 
generally  nearly  or  over  20  fathoms.  Four  other  of  the  islands  attain 
an  elevation  of  from  600  to  800  feet ;  the  remainder  are  low  and  are 
situated  on  coral-reefs.  The  coast  line  of  the  main  island  is  a  plat- 
form of  coral-sand  rook,  and  the  low  outlying  islands  are  the  same, 
but  the  hills  are  presumed  to  be  of  volcanic  rock.] 


PACIFIC   OCEAN.  225 

notices  collected  by  Horsburgli,  it  appears  that  some  of  the 
many  islands  composing  it  are  high,  with  a  bold  outline ; 
and  others  are  low,  small,  and  interlaced  with  reefs.  All 
the  high  islands  appear  to  be  fronted  by  distant  reefs  rising 
abruptly  from  the  sea,  and  within  some  of  which,  there  is 
reason  to  believe  that  the  water  is  deep.  I  have  therefore 
little  doubt  that  they  belong  to  the  barrier  class.  In  the 
southern  part  of  the  group,  we  have  Elisabeth  Island, 
which  is  surrounded  by  a  reef  at  the  distance  of  a  mile ; 
and  two  miles  eastward  of  it  (Krusenstern,  Append.  1835, 
p.  42)  there  is  a  little  island  containing  a  lagoon.  Near 
here,  also,  lies  Circular  Eeef  (Horsburgh  Direct,  vol.  ii. 
p.  796,  8th  edit.),  *  three  or  four  miles  in  diameter,  having 
deep  water  inside  with  an  opening  at  the  N.N.W.  part :  the 
reef  on  the  outside  is  steep  to.'  I  have  from  these  data, 
coloured  the  group  pale  blue,  and  Circular  Eeef  dark  blue. 
. — The  Anachorites,  Echequier,  and  Hermites  consist  of  in- 
numerable low  islands  of  coral  formation,  which  probably 
are  atolls  ;  but  not  being  able  to  ascertain  this,  I  have  not 
coloured  them,  nor  Durour  Island,  which  is  described  by 
Carteret  as  low. 

The  Caeoline  Aechipelago  is  now  well  known,  chiefly 
from  the  hydrographical  labours  of  Lutke :  it  contains 
about  forty  groups  of  atolls,  and  three  encircled  islands, 
two  of  which  are  engraved  in  Figs.  2  and  7,  Plate  I. 
Commencing  with  the  eastern  part,  the  encircling  reef 
round  Ualan  appears  to  be  only  about  half  a  mile  from  the 
shore ;  but  as  the  land  is  low,  and  covered  with  mangroves 
(Voyage  autour  du  Monde,  par  F.  Lutke,  vol.  i.  p.  339), 
its  margin  has  not  probably  been  ascertained.  The  extreme 
depth  in  one  of  the  harbours  within  the  reef  is  33  fathoms 
(see  charts  in  Atlas  of  Coquille's  Voyage),  and  outside  at 
half  a  mile  distance  from  the  reef,  no  bottom  was  obtained 
with  250  fathoms.  The  reef  is  surmounted  by  many 
islets,  and  the  lagoon-like  channel  within  is  mostly  shallow, 


226  APPENDIX. 

and  appears  to  have  been  much  encroached  on  by  the  low 
land  surrounding  the  central  mountains  ;  these  facts  show 
that  time  has  allowed  much  detritus  to  accumulate ; 
coloured  pale  blue. — PouynipUe  or  Seniavme.  In  the  greater 
part  of  the  circumference  of  this  island,  the  reef  is  about 
one  mile  and  three  quarters  from  the  shore ;  but  on  the  north 
side  it  is  five  miles  distant  from  the  included  high  islets. 
The  reef  is  broken  in  several  places  ;  and  just  within  it, 
the  depth  in  one  place  is  30  fathoms,  and  in  another,  28, 
beyond  which,  to  all  appearance,  there  was  *  un  port  vaste 
et  sur  *  (Lutke,  vol.  ii.  p.  4).  Coloured  pale  blue. — Hogoleu 
or  Boug.  This  wonderful  group  contains  at  least  62  islands, 
and  its  reef  is  135  miles  in  circuit.  Of  the  islands,  only  a 
few,  about  six  or  eight  (see  Hydrog.  Description,  p.  428,  of 
the  Voyage  of  the  Astrolabe,  and  the  large  accompanying 
chart  taken  chiefly  from  that  given  by  Duperrey)  are  high, 
and  the  rest  are  all  small,  low,  and  formed  on  the  reef. 
The  depth  of  the  great  interior  lake  has  not  been  ascer- 
tained ;  but  Captain  D'Urville  appears  to  have  entertained 
no  doubt  about  the  possibility  of  taking  in  a  frigate.  The 
reef  lies  no  less  than  14  miles  distant  from  the  northern 
coasts  of  the  interior  high  islands ;  seven  miles  from  their 
western  sides,  and  20  from  the  southern :  the  sea  is  deep 
outside.  This  island  resembles  on  a  grand  scale  the 
Gambler  group  in  the  Low  Archipelago.  Of  the  low  ^ 
islands  forming  the  chief  part  of  the  Caroline  Archipelago, 
all  those  of  larger  size  (as  may  be  seen  in  the  Atlas  by 
Captain  Lutke),  and  some  even  of  the  small  ones  of  which 
plans  are  given  in  the  Atlas  of  the  Coquille's  Voyage,  are 
true  atolls.  There  are,  however,  some  low,  small  islands 
of  coral  formation,  namely,  Ollap,  Tamatam,  Bigaliy 
Satahoual,  which  do  not  contain  lagoons ;  but  it  is  probable 
that  lagoons  originally  existed,  but  have  since  filled  up : 

^  In  D'Urville  and  Lottin's  chart,  Pescrare  is  written  with  capital 
letters ;  but  this  evidently  is  an  error,  for  it  is  one  of  the  low  islets  on 
the  reef  of  Namonouyto  (see  Lutk6's  charts),  which  is  a  regular  atoll. 


PACIFIC   OCEAN.  227 

Lutke  (vol.  ii.  p.  304)  seems  to  have  thought  that  all  the  low 
islands,  with  only  one  exception,  contained  lagoons.  The 
most  southern  island  in  the  group,  namely,  Piguiram,  is  not 
coloured,  because  I  have  found  no  account  of  it.  Nougouor, 
or  Monte  Verdison,  which  was  not  visited  by  Lutk^,  is 
described  and  figured  by  Mr.  Bennett  (United  Service 
Journal,  Jan.  1832)  as  an  atoll.  All  the  before-mentioned 
islands  have  been  coloured  blue.  It  must,  however,  be 
stated  that  between  Ualan  and  Pouynipete,  the  three 
McAskill  Islands  rise  to  a  height  of  from  40  to  100  feet, 
and  consist,  according  to  Dana  (Corals  and  Coral  Islands, 
p.  306),  of  coral-rock;  whether  they  are  encircled  or  fringed 
by  coral-reefs  does  not  seem  to  be  known. 

Western  paet  of  the  Caeoline  Aechipelago.— i^ais 
Island  is  90  feet  high,  and  is  surrounded,  as  I  have  been  in- 
formed by  Admiral  Lutke,  by  a  narrow  reef  of  living  coral, 
of  which  the  broadest  part,  as  represented  in  the  charts,  is 
only  150  yards  ;  coloured  red. — Philip  Island,  I  believe,  is 
low  ;  but  Hunter,  in  his  Historical  Journal,  gives  no  clear 
account  of  it ;  uncoloured.  Elivi :  from  the  manner  in 
which  the  islets  on  the  reefs  are  engraved  in  the  Atlas  of 
the  Astrolabe's  Voyage,  I  should  have  thought  they  were 
above  the  ordinary  height ;  but  Admiral  Lutke  aCssures  me 
that  this  is  not  the  case  :  they  form  a  regular  atoll ;  co- 
loured blue.  Go2iap  {Eap  of  Chamisso)  is  a  high  island 
with  a  reef  (see  Chart  in  Voyage  of  Astrolabe)  in  most  parts 
more  than  a  mile  distant  from  the  shore,  and  two  miles  in 
one  part.  Captain  D'Urville  thinks  that  there  would  be 
anchorage  (Hydrog.  Descript.  Astrolabe  Voyage,  p.  43G) 
for  ships  within  the  reef,  if  a  passage  could  be  found  ;  co- 
loured pale  blue. — Goulou,  from  the  chart  in  the  Astrolabe's 
atlas,  appears  to  be  an  atoll :  D'Urville  (Hydrog.  Descript. 
p.  437)  speaks  of  low  islets  on  the  reef ;  coloured  dark  blue. 

Pelew  Islands. — Krusenstern  speaks  of  some  of  the 
islands  being  mountainous  ;  the  reefs  are  distant  from  the 


228  APPENDIX. 

shore,  and  there  are  spaces  within  them,  not  opposite  to 
any  valley,  from  10  to  15  fathoms  deep.     According  to  a 
MS.  chart  of  the  group  by  Lieut.  Elmer  in  the  Admiralty, 
there  is  a  large  space  within  the  reef  with  deepish  water  : 
although  the  high  land  does  not  hold  a  central  position 
with  respect  to  the  reefs,  as  is  generally  the  case,  I  have 
little  doubt  that  the  reefs  of  the  Pelew  Islands  ought  to  be 
ranked  in  the  barrier  class,  and  I  have  coloured  them  pale 
blue.     In  Lieut.  Elmer's  chart  there  is  a  horse-shoe-formed 
shoal,  13  miles  N.W.  of  Pelew,  with  15  fathoms  within  the 
reef,  and  some   dry  banks  on  it  ;   coloured  dark  blue. — 
Spanish,    Mar  tires,   Sa7iserot,   Pulo  Anna  and  Mariere 
Islands  are  not  coloured,  because  I  know  nothing  about 
them,  excepting  that  according  to  Krusenstern,  the  second, 
third,  and  fourth  mentioned,  are  low,  placed  on  coral-reefs, 
and  therefore  perhaps  include  a  lagoon ;  but  Pulo  Mariere  is 
a  little  higher.    Since  the  above  remarks  were  written  Prof. 
Semper  has  published  an  interesting  article  (Zeitschr.  f. 
Wissensch.     Zoologie,   Bd.  xiii.  1863,  p.  558)  on  these 
islands.     He   states  that  the  southern  islands  consist  of 
coral-rock,  upraised  to  the  height  of  from  400  to  500  feet  : 
and  some  of  them,  before  their  upheaval,  seem  to  have  ex- 
isted as  atolls.  They  are  now  merely  fringed  by  living  reefs. 
The  northern  islands  are  volcanic,  deeply  indented  by  bays, 
and  are  fronted  by  barrier-reefs.     To  the  north  there  are 
three  true  atolls.     Prof.  Semper  doubts  whether  the  whole 
group  has  subsided,  partly  from  the  fact  of  the  southern 
islands  being  formed  of  upraised  coral-rock  ;  but  there  seems 
to  me  no  improbability  in  their  having  originally  subsided* 
then  having  been  upraised  (probably  at  the  time  when  the 
volcanic  rocks  to  the  north  were  erupted),  and  again  having 
subsided.    The  existence  of  atolls  and  barrier-reefs  in  close 
proximity  is  manifestly  not  opposed  to  my  views.     On  the 
other  hand,  the  presence  of  reefs  fringing  the  southern 
islands  is  opposed  to  my  views,  as  such  reefs  generally  indi- 


PACIFIC    OCEAN.  229 

cate  that  the  land  has  either  long  remained  stationary,  or 
has  been  upraised.  It  must,  however,  be  borne  in  mii.d  (as 
remarked  in  our  sixth  chapter)  that  when  the  land  U  pro- 
longed beneath  the  sea  in  an  extremely  steep  slope,  reefs 
formed  there  during  subsidence  will  remain  closely  attached 
to  the  shore,  and  will  be  undistinguishable  from  fringing- 
reefs.  Now  we  know  that  the  submarine  flanks  of  most  atolls 
are  very  steep ;  and  if  an  atoll  after  upheaval  and  before  the 
sea  had  eaten  deeply  into  the  land,  and  had  formed  a  broad 
flat  surface,  were  again  to  subside,  the  reefs  which  grewto  the 
surface  during  the  subsiding  movement,  would  still  closely 
skirt  the  coast.  After  some  hesitation,  I  have  thought  my- 
self justified  in  leaving  these  islands  coloured  blue. 

Maeiana  Archipelago,  or  Ladrones. — Guahan : 
almost  the  whole  of  this  island  is  fringed  by  reefs,  which 
extend  in  most  parts  about  a  third  of  a  mile  from  the  land. 
Even  where  the  reefs  are  most  extensive,  the  water  within 
them  is  shallow.  In  several  parts  there  is  a  navigable 
channel  for  boats  and  canoes  within  the  reefs.  In  Frey- 
cinet's  Hydrog.  Mem.  there  is  an  account  of  these  reefs, 
and  in  the  atlas,  a  map  on  a  large  scale  ;  coloured  red. — 
Bota :  *  L'ile  est  presque  entierement  entouree  de  recifs ' 
(p.  212,  Freycinet's  Hydrog.  Mem.).  These  reefs  project 
about  a  quarter  of  a  mile  from  the  shore  ;  coloured  red. — 
Tinian :  the  eastern  coast  is  precipitous,  and  is  without  reefs ; 
but  the  western  side  is  fringed  like  the  last  island ;  coloured 
red.  Saypan :  theN.E.  coast,  and  likewise  the  western  shores 
appear  to  be  fringed  ;  but  there  is  a  great,  irregular,  horn- 
like reef  projecting  far  from  this  side  ;  coloured  red. — 
Farallon  de  Medinilla  appears  so  regularly  and  closely 
fringed  in  Freycinet's  charts,  that  I  have  ventured  to 
colour  it  red,  although  nothing  is  said  about  reefs  in  the 
Hydrographical  Memoir.  The  several  islands  which  form 
the  northern  part  of  the  group  are  volcanic  (with  the  excep- 
tion perhaps  of  Torres,  which  resembles  in  form  the  madre- 


230  APPENDIX, 

pomic  island  of  Medinilla),  and  appear  to  be  without 
Teeh.—Mangs,  however,  is  described  (by  Freycinet,  p.  219, 
Hydrog.)  from  some  Spanish  charts,  as  formed  of  small 
islands  placed  *  au  milieu  de  nombreux  recifs  ;  '  and  as 
these  reefs  in  the  general  chart  of  the  group  do  not  project 
so  much  as  a  mile  ;  and  as  there  is  no  appearance  from  a 
double  line,  of  the  existence  of  deep  water  within,  I  have 
ventured,  although  with  much  hesitation,  to  colour  them 
red.  Kespecting  Folger  and  Marshall  Islands,  which  lie 
some  way  east  of  the  Marianas,  I  can  find  out  nothing, 
excepting  that  they  are  probably  low.  Krusenstem  says 
this  of  Marshall  Island ;  and  Folger  Island  is  written  with 
small  letters  in  D'Urville's  chart ;  uncoloured. 

BoNiN  OB  Aezobispo  Group. — Peel  Island  has  been 
examined  by  Captain  Beechey,  to  whose  kindness  I  am 
much  indebted  for  giving  me  information  regarding  it :  *  at 
Port  Lloyd  there  is  a  great  deal  of  coral ;  and  the  inner 
harbour  is  entirely  formed  by  coral-reefs,  which  extend 
outside  the  port  along  the  coast.'  Captain  Beechey,  in 
another  part  of  his  letter  to  me,  aUudes  to  the  reefs  fring- 
ing the  island  in  all  directions ;  but  at  the  same  time  it 
must  be  observed  that  the  surf  washes  the  volcanic  rocks 
of  the  coast  in  the  greater  part  of  its  circumference.  This 
island  has  certainly  been  elevated  at  least  50  feet  within 
the  recent  period  (see  Journal  of  Geolog.  Soc.  1855,  p. 
532).  I  do  not  know  whether  the  other  islands  of  the 
archipelago  are  fringed ;  I  have  coloured  Peel  island  red. 
— Grampus  Island,  to  the  eastward,  does  not  appear 
(Meare's  Voyage,  p.  95)  to  have  any  reefs,  nor  does 
Bosario  Island  (from  Lutke's  chart),  which  lies  to  the 
westward.  Bespecting  the  few  other  islands  in  this  part 
of  the  sea,  namely  the  Sulphur  Islands,  with  an  active 
volcano,  and  those  lying  between  Bonin  and  Japan  (situated 
near  the  extreme  limit  in  latitude  at  which  reefs  can  grow), 
I  have  not  been  able  to  find  any  clear  account. 


PACIFIC    OCEAN.  231 

West  End  of  New  Guinea. — Port  Dory:  from  the 
charts  in  the  Voyage  of  the  Coquille,  it  would  appear  that 
the  coast  in  this  part  is  fringed  by  coral-reefs  ;  M.  Lesson, 
however,  remarks  that  the  corals  are  sickly  ;  coloured  red. 
— Waig'ou:  a  considerable  portion  of  the  northern  shore 
of  these  islands  are  seen  in  the  charts  (on  a  large  scale) 
in  Freycinet's  Atlas  to  be  fringed  by  coral-reefs.  Forrest 
(p.  21,  Voyage  to  New  Guinea)  alludes  to  the  coral-reefs 
lining  the  heads  of  Piapis  Bay ;  and  Horsburgh  (vol.  ii. 
p.  599,  4th  edit.),  speaking  of  the  islands  in  Dampier  Strait, 
says,  *  sharp  coral-rocks  line  their  shores  ; '  coloured  red. — 
In  the  sea  north  of  these  islands,  we  have  Gicedes  (or 
Freewill,  or  St.  David's),  which  from  the  chart  given  in 
the  4to  edit,  of  Carteret's  Voyage  must  be  an  atoll. 
Krusenstern  says  the  islets  are  very  low ;  coloured  blue. — 
Carteret's  Shoals,  in  2°  53'  N.,  are  described  as  circular, 
with  stony  points  showing  all  round,  with  deeper  water  in 
the  middle  ;  coloured  blue. — Aioic :  the  plan  of  this  group, 
given  in  the  atlas  of  the  Voyage  of  the  Astrolabe,  shows 
that  it  is  an  atoll ;  and,  from  a  chart  in  Forrest's  Voyage, 
it  appears  there  is  12  fathoms  within  the  circular  reef; 
coloured  blue. — The  S.W.  coast  of  New  Guinea  appears  to 
be  low,  muddy,  and  devoid  of  reefs.  The  Arru,  Timor-laut 
and  Tenimber  Groups  have  lately  been  examined  by 
Captain  Kolff,  the  MS.  translation  of  which,  by  Mr.  W. 
Earl,  I  have  been  permitted  to  read,  through  the  kindness 
of  Captain  Washington,  E.N.  These  islands  are  mostly 
rather  low,  and  are  surrounded  by  distant  reefs  (the  Ki 
Islands,  however,  are  lofty,  and,  from  Mr.  Stanley's  survey, 
appear  without  reefs) ;  the  sea  in  some  parts  is  shallow,  in 
others  profoundly  deep,  as  near  Larrat.  From  the  imper- 
fection of  the  published  charts,  I  have  been  unable  to 
decide  to  which  class  these  reefs  belong.  From  the  dis- 
tance to  which  they  extend  from  the  land  where  the  sea  is 
very  deep,  I  am  strongly  inclined  to  believe  they  ought  to 


232  APPENDIX. 

come  within  the  barrier  class,  and  be  coloured  blue ;  but  I 
have  been  forced  to  leave  them  uncoloured. — The  last- 
mentioned  groups  are  connected  with  the  east  end  of  Ceram 
by  a  chain  of  small  islands,  of  which  the  small  groups  of 
Ceram-laut,  Goram,  and  Keffing  are  surrounded  by  very 
extensive  reefs,  projecting  into  deep  water,  which,  as  in  the 
last  case,  I  strongly  suspect  belong  to  the  barrier  class ; 
but  I  have  not  coloured  them.  From  the  south  side  of 
Keffing,  the  reefs  project  five  miles  (Windsor  Earl's  Sailing 
Direct,  for  the  Arafura  Sea,  p.  9). 

Ceeam. — In  various  charts  which  I  have  examined, 
several  parts  of  the  coast  are  represented  as  fringed  by 
reefs. — Manipa  Island,  between  Ceram  and  Bourou,  in  an 
old  MS.  chart  in  the  Admiralty,  is  fringed  by  a  very  irregu- 
lar reef,  partly  dry  at  low  water,  which  I  do  not  doubt  is 
of  coral  formation  ;  both  islands  coloured  red. — Bourou : 
parts  of  this  island  appear  fringed  by  coral-reefs,  namely, 
the  eastern  coast  as  seen  in  Freycinet's  chart ;  and  Cajeli 
Bay,  which  is  said  by  Horsburgh  (vol.  ii.  p.  630)  to  be 
lined  by  coral-reefs,  that  stretch  out  a  little  way,  and  have 
only  a  few  feet  of  water  on  them.  In  several  charts, 
portions  of  the  islands  forming  the  Amboina  Geoup  ^  are 
fringed  by  reefs;  for  instance,  Noessa,  Harenca,  and 
Ucaster,  in  Freycinet's  charts.  The  above-mentioned 
islands  have  been  coloured  red,  although  the  evidence  is 
not  very  satisfactory. — North  of  Bourou  the  parallel  line 
of  the  Xulla  Isles  extends  :  I  have  not  been  able  to  find 
out  anything  about  them,  excepting  that  Horsburgh  (vol. 
ii.  p.  543)  says  that  the  northern  shore  is  surrounded  by  a 
reef  at  the  distance  of  two  or  three  miles  ;  uncoloured. — 
Mysol  Group  :  the  Kanary  Islands  are  said  by  Forrest 
(Voyage,  p.  180)  to  be  divided  from  each  other  by  deep 

'  [At  Amboina  coral-reef  rock  occurs  raised  many  hundred  feet 
above  sea  level,  forming  a  steep  hill  slope.  Narrative  of  CJiallengef 
Voyage,  vol.  i.  p.  580.  See  also  Moseley,  Notes  by  a  Naturalist,  p.  389.] 


EAST-INDIAN  ARCHIPELAGO.  233 

straits,  and  are  lined  with  coral-rocks ;  coloured  red. — 
Guehe,  lying  between  Waigiou  and  Gilolo,  is  engraved  as  if 
fringed ;  and  it  is  said  by  Freycinet,  that  all  the  soundings 
under  five  fathoms  were  on  coral ;  coloured  red.—  Gilolo  :  in 
a  chart  published  by  Dalrymple,  the  numerous  islands  on 
the  western,  southern  {Batchian  and  the  Strait  of  PatieU' 
tia),  and  eastern  sides  appear  fringed  by  narrow  reefs ; 
these  reefs,  I  suppose,  are  of  coral,  for  it  is  said  in  Malte 
Brun  (vol.  xii.  p.  156),  *  sur  les  cotes  (of  Batchian),  comme 
dans  la  phcpart  des  iles  de  cet  archipel,  il  y  a  des  rocs  de 
madrepores  d'une  beaute  et  d'une  variete  infinies.'  Forrest, 
also  (p.  50),  says  Seland,  near  Batchian,  is  a  little  island 
with  reefs  of  coral ;  coloured  red. — Marty  Island  (north  of 
Gilolo) :  Horsburgh  (vol.  ii.  p.  506)  says  the  northern  coast 
is  lined  by  reefs,  projecting  one  or  two  miles,  and  having 
no  soundings  close  to  them  ;  I  have  left  it  uncoloured, 
although,  as  in  some  former  cases,  it  ought  probably  to  be 
pale  blue. — Celebes.  The  western  and  northern  coasts 
appear  in  the  charts  to  be  bold  and  without  reefs.  Near 
the  extreme  northern  point,  however,  an  islet  in  the 
Straits  of  Limbe,  and  part  of  the  adjoining  shore,  appear 
to  be  fringed  :  the  east  side  of  the  bay  of  Manado  has  deep 
water,  and  is  fringed  by  sand  and  coral  {Astrol.  Voyage, 
Hydrog.  Part,  p.  453-4) ;  this  extreme  point,  therefore,  I 
have  coloured  red.  Captain  Keppell,  also,  speaks  (Expe- 
dition to  Borneo,  vol.  i.  p.  130)  of  the  shore  being  in  parts 
fringed  with  reefs  ;  he  found  upraised  coral-reefs  at  the 
height  of  from  80  to  100  feet  above  the  level  of  the  sea. — 
Of  the  islands  between  the  northern  point  of  Celebes  and 
the  Phihppines,  I  have  not  been  able  to  find  any  account, 
except  of  Serangani,  which  appears  surrounded  by  narrow 
reefs ;  and  Forrest  (Voyage,  p.  164)  speaks  of  coral  on 
its  shores  ;  I  have,  therefore,  coloured  this  island  red. 
To  the  eastward  of  this  chain  lie  several  islands  ;  of  which 
l  cannot  find  any  account,  except  of  Karkalang,  which  is 


234  APPENDIX. 

said  by  Horsburgh  (vol.  ii.  p.  504)  to  be  lined  by  a  dan- 
gerous reef,  projecting  several  miles  from  the  northern 
shore ;  not  coloured. 

Islands  near  Timor. — The  account  of  the  following 
islands  is  taken  from  Captain  D.  Kolff's  Voyage  in  1825, 
translated  by  Mr.  W.  Earl  from  the  Dutch. — Lette  has 
*  reefs  extending  along  shore  at  the  distance  of  half  a  mile 
from  the  land.' — Moa  has  reefs  on  the  S.W.  part. — Lahor 
has  a  reef  lining  its  shore ;  these  islands  are  coloured  red. 
— Still  more  eastward,  Luan,  differently  from  the  last- 
mentioned  islands,  has  an  extensive  reef ;  it  is  steep  out- 
side, and  within  there  is  a  depth  of  12  feet ;  from  these 
facts  it  is  impossible  to  decide  to  which  class  this  island 
belongs. — Kissa,  off  the  point  of  Timor,  has  its  *  shore 
fronted  by  a  reef,  steep  too  on  the  outer  side,  over  which 
small  proahs  can  go  at  the  time  of  higli  water ; '  coloured 
red. — Timor:  most  of  the  points,  and  some  considerable 
spaces  of  the  northern  shore,  are  seen  in  Freycinet's  chart 
to  be  fringed  by  coral-reefs  ;  and  mention  is  made  of  them 
in  the  accompanying  Hydrog.  Memoir ;  coloured  red. — 
Savu,  S.W.  of  Timor,  appears  in  Flinders'  chart  to  be 
fringed ;  but  I  have  not  coloured  it,  as  I  do  not  know  that 
the  reefs  are  of  coral. — Sandalwood  Island  has,  according 
to  Horsburgh  (vol.  ii.  p.  607),  a  reef  on  its  southern  shore, 
four  miles  distant  from  the  land ;  as  the  neighbouring  sea 
is  deep,  and  generally  bold,  this  probably  is  a  barrier-reef, 
but  I  have  not  ventured  to  colour  it. 

N.W.  Coast  of  Australia. — It  appears,  in  Captain 
King's  Sailing  Directions  (Narrative  of  Survey,  vol.  ii. 
pp.  325  to  369),  that  there  are  many  extensive  coral-reefs 
skirting,  often  a.t  considerable  distances,  the  N.W.  shores 
and  encompassing  the  small  adjoining  islets.  Deep  water 
in  no  instance  is  represented  in  the  charts  between  these 
reefs  and  the  land ;  and,  therefore,  they  probably  belong 
to  the  fringing  class.     But  as  they  extend  far  into  the  sea, 


EAST-INDIAN   ARCHIPELAGO.  235 

which  is  generally  shallow,  even  in  places  where  the  land 
seems  to  be  somewhat  precipitous,  I  have  not  coloured 
them.  Houtman's  Abrolhos  (lat.  28°  S.  on  west  coast) 
have  lately  been  surveyed  by  Captain  Wickham  (as  described 
in  Naut.  Mag.  1841,  p.  511)  :  they  He  on  the  edge  of  a 
steeply-shelving  bank,  which  extends  about  30  miles  sea- 
ward, along  the  whole  line  of  coast.  The  two  southern 
reefs,  or  islands,  enclose  a  lagoon-like  space  of  water, 
varying  in  depth  from  5  to  15  fathoms,  and  in  one  spot 
with  23  fathoms.  The  greater  part  of  the  land  has  been 
formed  on  their  inland  sides,  by  the  accumulation  of  frag- 
ments of  corals  ;  the  seaward  face  consisting  of  nearly  bare 
ledges  of  rock.  Some  of  the  specimens,  brought  home  by 
Captain  Wickham,  contained  fragments  of  marine  shells, 
but  others  did  not ;  and  these  closely  resembled  a  formation 
at  King  George's  Sound,  principally  due  to  the  action  of 
the  wind  on  calcareous  dust,  which  I  have  described  in  my 
work  on  Volcanic  Islands.  From  the  extreme  irregularity 
of  these  reefs  with  their  lagoons,  and  from  their  position 
on  a  bank,  the  usual  depth  of  which  is  only  30  fathoms, 
I  have  not  ventured  to  class  them  with  atolls,  and  hence 
have  left  them  uncoloured. — Bowley  Shoals :  these  lie 
some  way  from  the  N.W.  coast  of  Australia  :  according  to 
Captain  King  (Narrative  of  Survey,  vol.  i.  p.  60),  they  are 
of  coral-formation.  They  rise  abruptly  from  the  sea,  and 
Captain  King  found  no  bottom  with  170  fathoms  close  to 
them.  Three  of  them  are  crescent-shaped  ;  a  third  oval 
reef  of  the  same  group  is  entirely  submerged  (Lyell, 
Principles  of  Geolog.,  book  iii.  chap,  xviii.) ;  ^  coloured  blue. 
— Scott's  Reefs,  lying  north  of  Eowley  Shoals,  are  briefly 
described  by  Captain  Wickham  (Naut.  Mag.,  1841,  p.  440) 
as  of  great  size,  of  a  circular  form,  and  *  with  smooth 
water  within,  forming  probably  a  lagoon  of  great  extent.' 
There  is  a  break  on  the  western  side,  where  there  probably 
'   [Book  iii.  ch.  xlix.  11th  edition.] 


2o6  APPENDIX. 

is  an  entrance :  the  water  is  very  deep  off  these  reefs ; 
coloured  blue. 

Proceeding  westward  along  the  great  volcanic  chain  of 
the  East  Indian  or  Malay  Archipelago,  Solor  Strait  is 
represented  as  fringed  in  a  chart  published  by  Dalrymple 
from  a  Dutch  MS. ;  as  are  parts  of  Flores,  Adenara,  and 
Solor.  Horsburgh  speaks  of  coral  growing  on  these  shores, 
and  therefore  I  have  no  doubt  that  the  reefs  are  of  coral, 
and  have  coloured  them  red.  We  hear  from  Horsburgh 
(vol.  ii.  p.  602)  that  a  coral  flat  bounds  the  shores  of  Sapy 
Bay.  From  the  same  authority  it  appears  (p.  610)  that 
reefs  fringe  the  island  of  Timor -Young ,  on  the  N.  shore  of 
Sumbawa ;  and  likewise  (p.  600)  that  Bally  town  in 
Lomhock,  is  fronted  by  a  reef,  stretching  along  the  shore 
at  the  distance  of  a  hundred  fathoms,  with  channels 
through  it  for  boats  ;  these  places,  therefore,  have  been 
coloured  red. — Bally  Island :  in  a  Dutch  MS.  chart  on  a 
large  scale  of  Java,  which  was  brought  from  that  island 
by  Dr.  Horsfield,  who  had  the  kindness  to  show  it  me  at 
the  India  House,  its  western,  northern,  and  southern  shores 
appear  regularly  fringed  by  a  reef  (see  also  Horsburgh, 
vol.  ii.  p.  593) ;  and  as  coral  is  found  abundantly  there,  I 
have  no  doubt  that  the  reef  is  of  coral,  and  therefore  have 
coloured  it  red. 

Java. — My  information  regarding  the  reefs  of  this  great 
island  is  derived  from  the  chart  just  mentioned.  The  greater 
part  of  Madura  is  represented  in  it  as  regularly  fringed, 
and  likewise  portions  of  the  coast  of  Java  immediately  south 
of  it.  Dr.  Horsfield  informs  me  that  coral  is  very  abundant 
near  Sourabaya.  The  adjoining  islets,  and  parts  of  the  N. 
coast  of  Java,  west  of  Point  Buang,  or  Japara,  are  fringed 
by  reefs,  said  to  be  of  coral.  Ltcbeck,  or  Bavian  Islands, 
lying  at  some  distance  from  the  shore  of  Java,  are  regularly 
fringed  by  coral-reefs  :  Curimon  Java  appears  equally  so, 
though  it  is  not  directly  said  that  the  reefs  are  of  coral ; 


EAST-INDIAN   ARCHIPELAGO.  237 

there  is  a  depth  of  between  30  and  40  fathoms  round  these 
islands.  Parts  of  the  shore  of  Sunda  Straits,  where  the 
water  is  from  40  to  80  fathoms  deep,  and  the  islets  near 
Batavia  appear  in  several  charts  to  be  fringed.  In  the 
Dutch  chart  the  southern  shore,  in  the  narrowest  part  of 
the  island,  is  in  two  places  fringed  by  reefs  of  coral.  West 
of  Segorrowodee  Bay,  and  the  extreme  S.E.  and  E.  shores 
are  likewise  fringed  by  coral-reefs ;  all  the  above-mentioned 
places  coloured  red. 

Macassar  Strait :  the  east  coast  of  Borneo  appears,  in 
most  parts,  free  from  reefs,  and  where  they  occur,  as  on  the 
coast  of  Pamaroong,  the  sea  is  very  shallow  ;  hence  no  part 
is  coloured.  In  Macassar  Strait  itself,  in  about  lat.  2°  S., 
there  are  many  small  islands  with  coral  shoals  pro- 
jecting far  from  them.  There  are  also  (old  charts  by  Dal- 
rymple)  numerous  little  flats  of  coral,  not  rising  to  the 
surface  of  the  water,  and  shelving  suddenly  from  five 
fathoms  to  no  bottom  with  50  fathoms  ;  they  do  not  appear 
to  have  a  lagoon-like  structure.  There  are  similar  coral- 
shoals  a  little  farther  south  ;  and  in  lat.  4°  55'  there  are 
two,  which  are  engraved  from  modern  surveys,  in  a  manner 
which  may  represent  an  annular  reef  with  deep  water 
inside  :  Capt.  Moresby,  however,  who  was  formerly  in  this 
sea,  doubts  this  fact,  so  that  I  have  left  them  uncoloured : 
at  the  same  time  I  may  remark,  that  these  two  shoals  make 
a  nearer  approach  to  the  atoll-like  structure  than  any  other 
within  the  E.  Indian  Archipelago.  Southward  of  these 
shoals  there  are  other  low  islands  and  irregular  coral-reefs  ; 
and  in  the  space  of  sea,  north  of  the  great  volcanic  chain, 
from  Timor  to  Java,  we  have  other  islands,  such  as  the 
Postillions,  Kalatoa,  Tokan-Bessees,  &c.,  which  are  chiefly 
low,  and  are  surrounded  by  very  irregular  and  distant  reefs. 
From  the  imperfect  charts  I  have  seen,  I  have  not  been 
able  to  decide  whether  they  belong  to  the  atoll  or  barrier 
class,  or  whether  they  merely  fringe   submarine  banks, 

17 


238  APPENDIX. 

and  gently  sloping  land.  In  the  Bay  of  Bonin,  between 
the  two  southern  arms  of  Celebes,  there  are  numerous  coral- 
reefs  ;  but  none  of  them  seem  to  have  an  atoll-like  structure. 
I  have,  therefore,  not  coloured  any  of  the  islands  in  this 
part  of  the  sea ;  I  think  it,  however,  exceedingly  probable 
that  some  of  them  ought  to  be  blue.  I  may  add  that  there 
is  a  harbour  on  the  S.E.  coast  of  Bouton,  which,  according 
to  an  old  chart,  is  formed  by  a  reef,  parallel  to  the  shore, 
with  deep  water  within  ;  and  in  the  Voyage  of  the  Coquille, 
some  neighbouring  islands  are  represented  with  distant 
reefs,  but  I  do  not  know  whether  with  deep  water  within. 
I  have  not  thought  the  evidence  sufficient  to  permit  me  to 
colour  them. 

SuMATBA. — Commencing  with  the  west  coast  and  out- 
lying islands ;  Engano  Island  is  represented  in  the  published 
chart  as  surrounded  by  a  narrow  reef,  and  Napier,  in  his 
Sailing  Directions,  speaks  of  the  reef  being  of  coral  (also 
Horsburgh,  vol.  ii.  p.  115) ;  coloured  red.  Bat  Island  (3° 
61'  S.)  is  surrounded  by  reefs  of  coral,  partly  dry  at  low 
water  (Horsburgh,  vol.  ii.  p.  ^Q).— Trieste  Island  (4°  2'  S.) : 
the  shore  is  represented,  in  a  chart  which  I  saw  at  the  India 
House,  as  fringed  in  such  a  manner,  that  I  feel  sure  the 
fringe  consists  of  coral ;  but  as  the  island  is  so  low  that  the 
sea  sometimes  flows  quite  over  it  (Dampier,  Voyage,  vol.  i. 
p.  474)  I  have  not  coloured  it. — Pulo  Dooa  (lat.  3°) :  it  is 
said  in  an  old  chart  that  there  are  chasms  in  the  reef  round 
the  island,  admitting  boats  to  the  watering-place,  and  that 
the  southern  islet  consists  of  a  mass  of  sand  and  coral. — 
Pulo  Pisang  :  Horsburgh  (vol.  ii.  p.  86)  says  that  the  rocky 
coral-bank,  which  stretches  about  40  yards  from  the  shore, 
is  steep  aU  round :  in  a  chart,  also,  which  I  have  seen,  the 
island  is  represented  as  regularly  fringed. — Pulo  Mintao  is 
lined  with  reefs  on  its  west  side  (Horsburgh,  vol.  ii.  p.  107). 
— PuloBaniah  :  the  same  authority  (vol.  ii.  p.  105),  speaking 
of  a  part,  says  it  is  faced  with  coral-rocks. — Minguin  (3* 


EAST-INDIAN   AKCHIPELAGO.  239 

86'  N.) ;  a  ccralreef  fronts  this  place,  and  projects  into  the 
sea  nearly  a  quarter  of  a  mile  (Notices  of  the  Indian  Archi- 
pelago, published  at  Singapore,  p.  105). — Fulo  Brassa  (5° 
46'  N.) :  a  reef  surrounds  it  at  a  cable's  length  (Horsburgh, 
vol.  ii.  p.  60).  I  have  coloured  all  the  above  specified  points 
red.  I  may  here  add,  that  both  Horsburgh  and  Mr.  Moor 
(in  the  Notices  just  alluded  to)  frequently  speak  of  the 
numerous  reefs  and  banks  of  coral  on  the  west  coast  of 
Sumatra ;  but  they  nowhere  have  the  structure  of  a  barrier- 
reef,  and  Marsden  (History  of  Sumatra)  states  that  where 
the  coast  is  flat,  the  fringing-reefs  extend  far  from  it.  The 
northern  and  southern  points,  and  the  greater  part  of  the 
east  coast,  are  low,  and  faced  with  mud  banks,  and  there- 
fore without  coral. 

NicoBAE  Islands. — The  chart  represents  the  islands 
of  this  group  as  fringed  by  reefs.  With  regard  to  Great 
Nicobar,  Captain  Moresby  informs  me  that  it  is  fringed  by 
reefs  of  coral,  extending  between  200  and  300  yards  from 
the  shore.  The  Northern  Nicobars  appear  so  regularly 
fringed  in  the  published  charts,  that  I  have  no  doubt  the 
reefs  are  of  coral.     This  group,  therefore,  is  coloured  red. 

Andaman  Islands. — From  an  examination  of  the  MS. 
chart,  on  a  large  scale,  of  these  islands,  by  Captain  Arch. 
Blair,  in  the  Admiralty,  several  portions  of  the  coast  appear 
fringed;  and  as  Horsburgh  speaks  of  coral-reefs  being 
numerous  in  the  vicinity  of  these  islands,  I  should  have 
coloured  them  red,  had  not  some  expressions  in  a  paper  in 
the  Asiatic  Researches  (vol.  iv.  p.  402)  led  me  to  doubt  the 
:jtistence  of  reefs  ;  uncoloured. 

The  coast  of  Malacca,  Tanasserim,  and  the  coasts 
northward,  appear  in  the  greater  part  to  be  low  and  muddy  : 
where  reefs  occur,  as  in  parts  of  Malacca  Straits,  and  near 
Singapore,  they  are  of  the  fringing  kind ;  but  the  water  is 
BO  shoal,  that  I  have  not  coloured  them.  In  the  sea,  how- 
ever, between  Malacca  and  the  west  coast  of  Borneo,  where 


240  APPENDIX. 

there  is  a  greater  depth  from  40  to  50  fathoms,  I  have 
coloured  red  some  of  the  groups,  which  are  regularly 
fringed.  The  northern  Natunas  and  the  Anambas  Islands 
are  represented  in  the  charts  on  a  large  scale,  published  in 
the  atlas  of  the  Voyage  of  the  Favourite  [by  La  Place,  1831]» 
as  fringed  by  reefs  of  coral,  with  very  shoal  water  within  them. 
Tumhelan  and  Bunoa  Islands  (1°  N.)  are  represented  in  the 
English  charts  as  surrounded  by  a  very  regular  fringe. — 
St.  Barbes  (0°  15'  N.)  is  said  by  Horsburgh  (vol.  ii.  p.  279) 
to  be  fronted  by  a  reef,  over  which  boats  can  land  only  at 
high  water. — The  shore  of  Borneo,  at  Tunjong  Ajjee,  is  also 
fronted  by  a  reef,  extending  not  far  from  the  land  (Hors- 
burgh, vol.  ii.  p.  4G8).  These  places  I  have  coloured  red  ; 
although  with  some  hesitation,  as  the  water  is  shallow.  I 
might  perhaps  have  added  Pulo  Leat,  in  Gaspar  Strait, 
Lucepara  and  Carimata ;  but  as  the  sea  is  confined  and 
shallow,  and  the  reefs  not  very  regular,  I  have  left  them 
uncoloured. 

The  water  deepens  very  gradually  from  the  whole  west 
coast  of  Borneo ;  and  I  cannot  make  out  that  it  has  any 
reefs  of  coral.  The  islands,  however,  off  the  northern 
extremity,  and  near  the  S.W.  end  of  Palawan,  are  fringed 
by  very  distant  coral  reefs  :  thus  the  reefs  off  Balabac  are 
no  less  than  five  miles  from  the  land  ;  but  the  sea,  in  the 
whole  of  this  district,  is  so  shallow,  that  the  reefs  might  be 
expected  to  extend  very  far  from  the  land.  I  have  not, 
therefore,  thought  myself  authorized  to  colour  them.  The 
N.E.  point  of  Borneo,  where  the  water  is  very  shoal,  is 
connected  with  Magindanao  by  a  chain  of  islands  called 
the  Sooloo  Archipelago,  about  which  I  have  been  able  to 
obtain  very  little  information ;  Pangootaran,  although  ten 
miles  long,  entirely  consists  of  a  bed  of  coral-rock  (Notices 
of  E.  Indian  Arch.  p.  58)  :  I  believe  from  Horsburgh  that 
the  island  is  low ;  not  coloured. — Tahow  Bank,  in  some 
old  charts,  appears  like  a  submerged  atoll ;  not  coloured. 


EAST-INDIAN   ARCHIPELAGO.  241 

Forrest  (Voyage,  p.  21)  states  that  one  of  the  islands  near 
Sooloo  is  surrounded  by  coral-rocks  ;  but  there  is  no  dis- 
tant reef.  Near  the  S.  end  of  Basselan,  some  of  the  islets 
in  the  chart  accompanying  Forrest's  Voyage  appear  fringed 
with  reefs ;  hence  I  have  coloured,  though  unwillingly,  parts 
of  the  Sooloo  group  red.  The  sea  between  Sooloo  and 
Palawan,  near  the  shoal  coast  of  Borneo,  is  interspersed  with 
irregular  reefs  and  shoal  patches  ;  not  coloured  :  but  in  the 
northern  part  of  this  sea  there  are  two  low  islets,  Cagayanes 
and  Cavilli,  surrounded  by  extensive  coral-reefs ;  the  breakers 
round  the  latter  (Horsburgh,  vol.  ii.p.  513)  extend  five  or  six 
miles  from  a  sand-bank,  which  forms  the  only  dry  part ;  these 
breakers  are  steep  to  outside  :  there  appears  to  be  an  open- 
ing through  the  reef  on  one  side,  with  four  or  five  fathoms 
within:  from  this  description,  I  strongly  suspect  that 
Cavilli  ought  to  be  considered  an  atoll ;  but,  as  I  have  not  • 
seen  any  chart  of  it,  even  on  a  moderately  large  scale,  I 
have  not  coloured  it.  The  islets  off  the  northern  end  of 
Palawan  are,  like  those  off  the  southern  end,  fringed  by 
reefs,  some  way  distant  from  the  shore,  but  the  water  is 
exceedingly  shallow  ;  uncoloured.  The  western  shore  of 
Palawan  will  be  treated  of  under  the  China  Sea. 

Philippine  Aechipelago. — A  chart  on  a  large  scale 
of  ^^^00  Shoal,  which  lies  near  the  S.E.  coast  of  Mindoro, 
has  been  executed  by  Captain  D.  Ross :  it  appears  atoll- 
formed,  but  with  rather  an  irregular  outline ;  its  diameter 
is  about  ten  miles ;  there  are  two  well-defined  passages 
leading  into  the  lagoon ;  close  outside  and  all  round  the 
reef,  there  is  no  bottom  with  70  fathoms ;  coloured  blue. — 
Mindorc  :  the  N.W.  coast  is  represented  in  several  charts 
as  fringed  by  a  reef ;  and  Luhan  Island  is  said  by  Horsburgh 
(vol.  ii.  p.  436)  to  be  *  lined  by  a  reef.' — Luzon:  Mr. 
Cuming,  who  has  lately  investigated  with  so  much  success 
the  Natural  History  of  the  Philippines,  informs  me  that 
a  length  of  about  three  miles  of  the  shore  northward  of 


242  APPENDIX. 

Point  St.  Jago  is  fringed  by  a  reef ;  as  are  (Horsburgh, 
vol.  ii.  p.  437)  the  Three  Friars  off  Silanguin  Bay.  Between 
Point  Capones  and  Playa  Honda,  the  coast  is  *  lined  by  a 
coral-reef,  stretching  out  nearly  a  mile  in  some  places* 
(Horsburgh)  ;  and  Mr.  Cuming  visited  some  fringing-rcefg 
on  other  parts  of  the  coast,  namely,  near  Puebla,  Iba,  and 
Mansinglor.  In  the  neighbourhood  of  Solon-solon  Bay, 
the  shore  is  lined  (Horsburgh,  vol.  ii.  p.  439)  by  coral-reefs, 
stretching  out  a  great  way :  there  are  also  reefs  about  the 
islets  off  Solamague  ;  and  as  I  am  informed  by  Mr.  Cuming, 
near  St.  Catalina,  and  a  little  north  of  it.  The  same 
gentleman  informs  me  that  there  are  reefs  on  the  S.E. 
point  of  this  island  in  front  of  Samar,  extending  from 
Malalabon  to  Bulusan.  These  appear  to  be  the  principal 
fringing-reefs  on  the  coasts  of  Luzon ;  and  they  have  all 
been  coloured  red.  Mr.  Cuming  informs  me  that  none  of 
them  have  deep  water  within  ;  although  it  appears  from 
Horsburgh  that  some  few  extend  to  a  considerable  distance 
from  the  land.  Within  the  Philippine  Archipelago,  the 
shores  of  the  islands  do  not  appear  to  be  commonly  fringed, 
with  the  exception  of  the  S.  shore  of  Masbate,  and  nearly 
the  whole  of  Bohol ;  which  are  both  coloured  red.  On  the 
S.  shore  of  Magindanao,  Bunwoot  Island  is  surrounded 
(according  to  Forrest,  Voyage,  p.  253)  by  a  coral-reef, 
which  in  the  chart  appears  one  of  the  fringing  class.  With 
respect  to  the  eastern  coasts  of  the  archipelago,  I  have  not 
been  able  to  obtain  any  account.  Prof.  Semper  has  re- 
cently published  a  notice  (Zeitschr.  f.  Wissensch.  Zoologie, 
Bd.  xiii.  1863,  p.  558)  respecting  the  coral-reefs  of  tliia 
archipelago.  It  appears  that  some  of  them  come  under  the 
class  of  barrier-reefs ;  but  as  I  have  not  seen  a  chart  on  a 
large  scale,  and  know  nothing  about  the  depth  of  the  water 
outside  the  reefs,  nor  about  the  slope  of  the  encircled  land, 
I  cannot  judge  whether  they  properly  come  under  the  barriei 
class. 


EAST-INDIAN   ARCHIPELAGO.  243 

Babuyan  Islands. — Horsburgh  says  (vol.  ii.  p.  442) 
coral-reefs  line  the  shores  of  the  harbour  m  Fuga ;  and  the 
charts  show  there  are  other  reefs  about  these  islands. 
Camiguin  has  its  shore  in  parts  lined  by  coral-rock  (Hors- 
burgh, p.  443) ;  and  about  a  mile  off  shore  the  depth  is 
between 30  and35  fathoms.  The  planof  Port  San  Pio  Quinto 
shows  that  its  shores  are  fringed  with  coral ;  coloured 
red. — Bashee  Islands  :  Horsburgh,  speaking  of  the  south- 
ern part  of  the  group  (vol.  ii.  p.  445),  says  the  shores 
of  both  islands  are  fortified  by  a  reef,  and  through  some  of 
the  gaps  in  it  the  natives  can  pass  in  their  boats  in  fine 
weather;  the  bottom  near  the  land  is  coral-rock.  From 
the  published  charts,  it  is  evident  that  several  of  these 
islands  are  regularly  fringed  ;  coloured  red.  The  northern 
islands  are  left  uncoloured,  as  I  have  been  unable  to  find 
any  account  of  them. — Formosa  :  the  shores,  especially 
the  western  one,  seem  composed  chiefly  of  mud  and  sand, 
and  I  cannot  make  out  that  they  are  anywhere  lined  by 
reefs,  except  in  a  harbour  (Horsburgh,  vol.  ii.  p.  449)  at  the 
extreme  northern  point :  hence,  of  course,  the  whole  of 
this  island  is  left  uncoloured.  The  small  adjoining 
islands  are  in  the  same  case. — Patchow,  orMadjiko-sima 
Groups  :  Patchuson  :  Captain  Broughton  says  (Voyage  to 
the  N.  Pacific,  p.  191)  that  boats,  with  some  difficulty,  can 
pass  through  the  coral-reefs,  which  extend  along  the  coast, 
nearly  half  a  mile  off  it.  His  boats  were  well  sheltered 
within  the  reef ;  but  it  does  not  appear  that  the  water  is 
deep  tJiere.  Outside  the  reef  the  depth  is  very  irregular, 
varying  from  5  to  50  fathoms  ;  the  form  of  the  land  is  not 
very  abrupt ;  coloured  red. — Taypin-san :  from  the  de- 
scription given  by  the  same  author  (p.  195)  it  appears  that 
a  very  irregular  reef  extends  from  the  southern  island  to  the 
distance  of  several  miles  ;  but  whether  it  encircles  a  space 
of  deep  water  is  not  evident ;  nor,  indeed,  whether  these 
outlying  reefs  are  connected  with  those  more  immediately 


244  APPENDIX. 

adjoining  the  land  ;  left  uncoloured.  I  may  here  add  that 
the  shore  of  Kumi  (lying  west  of  Patchow)  has  a  narrow 
reef  attached  to  it  in  the  plan,  in  La  Peyrouse's  Atlas  ;  but 
in  the  account  of  the  voyage  it  is  not  stated  to  be  of  coral ; 
uncoloured. — Loo  Choc  :  the  greater  part  of  the  coast  of 
this  moderately  hilly  island  is  skirted  by  reefs,  which  do 
not  extend  far  from  the  shore,  and  which  do  not  leave  a 
channel  of  deep  water  within  them,  as  may  be  seen  in  the 
charts  accompanying  Captain  B.  Hall's  Voyage  to  Loo  Choc 
(see  also  remarks  in  Appendix,  p.  xxi.  and  xxv.)  There  are, 
however,  some  ports  with  deep  water,  formed  by  reefs,  in 
front  of  the  valleys,  in  the  same  manner  as  happens  at 
Mauritius.  Captain  Beechey,  in  a  letter  to  me,  compares 
these  reefs  with  those  encircling  the  Society  Islands  ;  but 
there  appears  to  be  a  marked  difference  between  them,  in 
the  less  distance  at  which  the  Loo  Choo  reefs  lie  from  the 
land  with  relation  to  the  probable  submarine  inclination  of 
the  land,  and  in  the  absence  of  an  interior  deep-water 
channel.  Hence  I  have  classed  these  reefs,  with  fringing- 
reefs,  and  coloured  them  red. — Pescadobes  (west  of 
Formosa) :  Dampier  (vol.  i.  p.  416)  has  compared  the  ap- 
pearance of  these  islands  to  the  southern  parts  of  England ; 
they  are  interlaced  with  coral-reefs  ;  but  as  the  water  is  very 
shoal,  and  as  spits  of  sand  and  gravel  (Horsburgh,  vol.  ii. 
p.  450)  extend  far  out  from  them,  it  is  impossible  to  decide 
whether  the  reefs  are  of  coral. 

China  Sea.^ — Proceeding  from  north  to  south,  we  first 
meet  the  Pratas  Shoal  (lafc.  20°  N.),  which,  according  to 
Horsburgh  (vol.  ii.  p.  335),  is  composed  of  coral,  is  of  a 
circular  form,  and  has  a  low  islet  on  it.  The  reef  is  on  a 
level  with  the  water's  edge,  and  when  the  sea  runs  high, 

*  [The  China  Sea  has  in  it  many  atolls.  They  are  of  large  size, 
though  not  complete,  most  of  them  having  the  rim  submerged,  with 
the  reef  awash  and  islands  on  parts.  Some  are  wholly  submerged 
like  Macclesfield  Bank.] 


EAST-INDIAN   ARCHIPELAGO.  245 

there  are  breakers  nearly  all  round;  *tlie  water  within 
Beems  pretty  deep  in  some  places  ;  although  steep  in  most 
parts  outside,  there  appear  to  be  several  parts  where  a 
ship  might  find  anchorage  outside  the  breakers ; '  coloured 
blue, — The  Faracells  have  been  accurately  surveyed  by 
Captain  D.  Eoss,  and  charts  on  a  large  scale  published : 
only  a  few  low  islets  have  been  formed  on  these  shoals,  and 
this  seems  to  be  a  general  circumstance  in  the  China  Sea  ; 
the  sea  close  outside  these  reefs  is  deep  ;  several  of  them 
have  a  lagoon-like  structure ;  in  other  cases  separate  islets 
(Prattle,  Bobert,  Drummondy  &c.)  are  so  arranged  round  a 
moderately  shallow  space  as  to  appear  as  if  they  had  once 
formed  one  large  atoll. — Bombay  Shoal  (one  of  the  Para- 
cells)  has  the  form  of  an  annular  reef,  and  is  *  apparently 
deep  within  ;  '  it  seems  to  have  an  entrance  (Horsburgh, 
vol.  ii.  p.  332)  on  the  west  side ;  it  is  very  steep  outside. — 
Discovery  Shoal,  also,  is  of  an  oval  form,  with  a  lagoon-like 
space  within,  and  three  openings  leading  into  it,  in  which 
there  is  a  depth  from  2  to  20  fathoms.  Outside,  at  the  dis- 
tance (Horsburgh,  vol.  ii.  p.  333)  of  only  twenty  yards  from 
the  reef,  soundings  could  not  be  obtained.  The  Paracells 
are  coloured  blue. — Macclesfield  Bank  is  a  coral  bank  of 
great  size,  lying  east  of  the  Paracells ;  some  parts  are  level, 
with  a  sandy  bottom,  but  generally  the  depth  is  very 
irregular,  and  intersected  by  deep  channels ;  not  coloured. 
— Scarborough  Shoal:  this  coral  shoal  is  engraved  with 
a  double  row  of  crosses,  forming  a  circle,  as  if  there 
was  deep  water  within  the  reef :  close  outside  no  bottom 
was  found  with  a  hundred  fathoms  ;  coloured  blue. — 
The  sea  off  the  west  coast  of  Palawan  and  the  northern 
part  of  Borneo  is  strewed  with  shoals  :  Siuallow  Shoal, 
according  to  Horsburgh  (vol.  ii.  p.  431),  *  is  formed,  like 
most  of  the  shoals  hereabouts,  of  a  belt  of  coral-rocks,  with 
a  basin  of  deeper  water  within.' — Half -Moon  Shoal  has  a 
similar  structure  ;  Captain  D.  Ross  describes  it  as  a  narrow 


216  APPENDIX. 

belt  of  coral-rock,  *  witli  a  basin  of  deeper  water  in  the 
centre,' and  deep  sea  close  outside. — Bombay  Shoal  appears 
(Horsburgh,  vol.  ii.  p.  432)  *  to  be  a  basin  of  smooth  water 
surrounded  by  breakers.'  I  have  coloured  these  three 
shoals  blue. — The  Paraquas  Shoals  are  of  a  circular  form, 
with  deep  gaps  running  through  them ;  not  coloured.  A 
bank,  gradually  shoaling  to  the  depth  of  30  fathoms,  ex- 
tends to  a  distance  of  about  20  miles  from  the  northern 
part  of  Borneo,  and  to  30  miles  from  the  southern  part  of 
Palaiuan ;  near  the  land  this  bank  appears  tolerably  free 
from  danger,  but  a  little  further  out  is  thickly  studded 
with  coral-reefs,  which  do  not  generally  rise  to  the  surface  ; 
some  of  them  are  very  steep,  whilst  others  have  a  fringe  of 
shoal-water  round  them.  I  should  have  thought  that  these 
shoals  had  level  surfaces,  had  it  not  been  for  a  statement 
made  by  Horsburgh,  *  that  most  of  the  shoals  hereabouts 
are  formed  of  a  belt  of  coral : '  I  have  not  coloured  them. — 
The  coasts  of  China,  Tonquin,  and  Cochin- Chifia,  forming 
the  western  boundary  of  the  China  Sea,  appear  to  be  with- 
out reefs  :  with  regard  to  the  two  last-mentioned  coasts,  I 
judge  from  an  examination  of  the  charts  on  a  large  scale  in 
the  atlas  of  the  Voyage  of  the  Favourite. 

Indian  Ocean. — South  Keeling  atoll  has  been  specially 
described  in  my  first  chapter.  Nine  miles  north  of  it  lies 
North  Keeling,  a  very  small  atoll,  surveyed  by  the  Beagle, 
the  lagoon  of  which  is  dry  at  low  waiter.— Christmas  Island,' 
lying  to  the  east,  is  a  high  island,  without,  as  I  have  been 

'  [This  island  is  described  in  letters  to  Nature  by  Captains  Wharton 
and  Maclear  (vol.  xxxvi.  pp.  12,  413,  and  vol.  xxxvii.  p.  203 ;  cf.  also 
p.  222).  It  is  190  miles  from  Java,  the  intervening  ocean  attaining 
a  depth  of  2,430  fathoms.  It  consists  of  coral  limestone,  no  other 
rock  being  visible,  which  rises  from  the  sea  to  a  height  of  about 
1,200  feet.  At  the  base  is  commonly  a  cliff  about  30  feet  high,  and 
there  are  two  upper  tiers  of  cliffs  :  one  is  described  as  being  from  200  to 
300  feet  high.  A  depth  of  100  fathoms  is  found  at  one  or  two  cables' 
length  from  the  water's  edge  at  the  base  of  the  lowest  line  of  cliffs.] 


INDIAN   OCEAN.  247 

informed,  any  reef. — Ceylon:  a  space  of  about  80  milea 
in  length  on  the  south-western  and  southern  shores  of  these 
islands  has  been  described  by  Mr.  Twynam  (Naut.  Mag. 
1836,  pp.  865  and  518) ;  and  parts  appear  to  be  regularly 
fringed  by  coral-reefs,  which  extend  from  a  quarter  to  half 
a  mile  from  the  shore.  These  reefs  are  in  places  breached, 
and  a£ford  safe  anchorage  for  the  small  trading  craft.  Out- 
side, the  sea  gradually  deepens  ;  there  is  40  fathoms  about 
six  miles  off  shore  :  I  have  coloured  these  reefs  red.  In  the 
published  charts  of  Ceylon,  reefs  also  appear  to  fringe 
several  parts  of  the  south-east_ern  shores,  coloured  red. — At 
Venloos  Bay  the  shore  is  likewise  fringed.  North  of 
Trincomalee  there  are  also  reefs  of  the  same  character. 
The  sea  off  the  northern  part  of  Ceylon  is  exceedingly 
shallow  ;  and  therefore  I  have  not  coloured  the  reefs  which 
partially  fringe  portions  of  the  shores,  and  the  adjoining 
islets,  as  well  as  the  Indian  promontory  of  Madura. 

Chagos,  Maldiva,  and  Laccadive  Archipelagoes.' — 
These  three  great  groups  of  atolls  and  atoll-formed  banks, 
have  been  often  referred  to  in  this  volume,  and  are  now 
well  known  from  the  admirable  surveys  of  Captain  Moresby 
and  Lieut.  Powell.  Their  published  charts  are  worthy  of 
the  most  attentive  examination.  In  the  Laccadive  group, 
the  atoll-like  structure  is  less  evident  than  in  the  Maldivas; 
nevertheless  the  islands  are  all  low,  not  exceeding  the  usual 
height  of  coral  formations  (see  Lieut.  Wood's  account, 
Geograph.  Journ.  vol.  vi.  p.  29),  and  most  of  the  reefs  are 
circular;  within  several  of  them,  as  I  am  informed  by 
Captain  Moresby,  there  is  deepish  water  ;  these,  therefore, 
have  been  coloured  blue.  Directly  north  of  the  Laccadives, 
and  almost  forming  part  of  the  same  group,  there  is  along, 
na]'row,  slightly-curved  bank,  rising  out  of  the  depths  of 
the  ocean,  composed  of  sand  shells  and  decayed  coral,  with 
from  23  to  30  fathoms  on  it.  I  have  no  doubt  that  it  has 
'  [See  Appendix  IL] 


218  APPENDIX. 

had  the  same  origin  with  the  other  atoll-like  banks ;  but  as  it 
does  not  deepen  towards  the  centre,  I  have  not  coloured  it. 
I  might  have  referred  to  other  authorities  regarding  these 
three  archipelagoes  ;  but  after  the  publication  of  the  charts 
by  Captain  Moresby  (to  whose  personal  kindness  in  giving 
me  much  information  I  am  exceedingly  indebted),  this 
would  have  been  superfluous. 

The  Sahia  de  Malha  Bank  consists  of  a  series  of  narrow 
banks,  with  from  8  to  16  fathoms  on  them;  they  are 
arranged  in  a  semi-circular  manner,  round  a  space  about 
40  fathoms  in  depth,  sloping  to  the  S.E.  to  unfathomable 
depths  ;  they  are  steep  on  both  sides,  but  more  especially 
on  the  ocean-side.  Hence  this  bank  closely  resembles  in 
structure,  and  I  may  add  from  Captain  Moresby's  informa- 
tion in  composition,  Pitt's  Bank  in  the  Chagos  group  ;  and 
Pitt's  Bank  must,  from  what  we  know  about  the  great 
Chagos  Bank,  be  considered  as  a  sunken,  half-destroyed 
atoll ;  hence  coloured  blue. — Cargados  Carajos  Bank :  its 
southern  portion  consists  of  a  large,  curved  coral-shoal, 
with  some  low  islets  on  the  eastern  edge,  and  likewise 
some  on  the  western  side,  between  which  there  is  a  depth 
of  about  12  fathoms  :  northward,  a  great  bank  extends.  I 
cannot  (probably  owing  to  the  want  of  perfect  charts)  refer 
this  reef  and  bank  to  any  class  ;  therefore  not  coloured. — 
He  de  Sable  is  a  little  island,  lying  west  of  C.  Carajos,  only 
some  toises  in  height  (Voyage  of  the  Favourite,  vol.  i. 
p.  130) ;  it  is  surrounded  by  reefs ;  but  its  structure  is  un- 
intelligible to  me.  There  are  some  small  banks  north  of 
it,  of  which  I  can  find  no  clear  account. — Mauritius  :  the 
reefs  round  this  island  have  been  described  in  the  chapter 
on  fringing-reefs  ;  coloured  red. — Bodriguez  :  the  coral- 
reefs  here  are  very  extensive  ;  in  one  part  they  project  even 
five  miles  from  the  shore.  As  far  as  I  can  make  out,  there 
is  no  deep-water  channel  within  them  ;  -and  the  sea  outside 
does  not  deepen  very  suddenly.     The  outline,  however,  of 


INDIAN   OCEAN.  249 

the  land  appears  to  be  (Life  of  Sir  J.  Mackintosh,  vol.  ii. 
p.  165)  hilly  and  rugged.  I  am  unable  to  decide  whetlier 
these  reefs  belong  to  the  barrier  class,  as  seems  probable 
from  their  great  extension,  or  to  the  fringing  class  ;  un- 
coloured.^ — Bourbon  :  the  greater  part  of  the  shores  of  this 
island  are  without  reefs  ;  but  Captain  Carmichael  (Hooker's 
Bot.  Misc.)  states  that  a  portion,  15  miles  in  length,  on  the 
S.E.  side,  is  imperfectly  fringed  with  coral-reefs  ;  I  have  not 
thought  this  sufficient  evidence  for  colouring  the  island. 

Seychelles. — The  rocky  islands  of  primary  formation, 
composing  this  group,  rise  from  a  very  extensive  and  toler- 
ably level  bank,  having  a  depth  of  between  20  and  40 
fathoms.  In  Captain  Owen's  chart,  and  in  that  in  the  atlas 
of  the  Voyage  of  the  Favourite^  it  appears  that  the  east 
side  of  Make  and  the  adjoining  islets  of  St.  Anne  and  Cer/, 
are  regularly  fringed  by  coral-reefs.  A  portion  of  the  S.E. 
part  of  Curieuse  Island,  the  N.  and  part  of  the  S.W.  shore 
of  Praslin  Island,  and  the  whole  west  side  of  Digue  Island, 
appear  fringed.  From  a  MS.  account  of  these  islands  by 
Captain  F.  Moresby,  in  the  Admiralty,  it  appears  that 
Silhouette  is  also  fringed ;  he  states  that  all  these  islands 
are  formed  of  granite  and  quartz,  that  they  rise  abruptly 
from  the  sea,  and  that  *  coral-reefs  have  grown  round  them, 
and  project  for  some  distance.'  Dr.  Allan  of  Forres,  who 
visited  these  islands,  informs  me  that  there  is  no  deep  water 
between  the  reefs  and  the  shore.  The  above  specified 
points  have  been  coloured  red.  Amirantes  Islands  :  the 
small  islands  of  this  neighbouring  group,  according  to  the 
MS.  account  of  them  by  Captain  F.  Moresby,  are  situated 
on  an  extensive  bank  ;  they  consist  of  the  debris  of  corals 
and  shells  ;  they  are  only  about  20  feet  in  height,  and  are 

'  [There  are  fringing-reefs  of  a  width  of  four  and  a  half  miles  to 
leeward  and  of  a  few  yards  to  windward.  Outside  them  the  water 
shoals  gradually.  The  island  is  high  and  basaltic,  with  upraised 
coral  in  many  places  up  to  a  height  of  about  50  feet  above  the  sea. — 
Capt.  Wharton.    See  Appendix  II,] 


250  APPENDIX. 

environed  by  reefs,  some  attached  to  the  shore,  and  some 
rather  distant  from  it. — I  have  taken  pains  to  procure 
plans  and  information  regarding  the  several  islands  which 
lie  to  the  S.E.  and  S.W.  of  the  Seychelles  ;  from  accounts 
given  me  by  Captain  F.  Moresby  and  Dr.  Allan,  it  appears 
that  the  greater  number — namely,  PZai^e,  Alphonse,  Coetivi, 
Galega,  Provideyice,  St.  Pierre,  Astova,  Assomption,  and 
Glorioso  ^ — are  low,  formed  of  sand  or  coral-rock,  and  irre- 
gularly shaped ;  they  are  situated  on  very  extensive  banks, 
and  are  in  connection  with  great  coral-reefs.  Galega  is 
Baid  by  Dr.  Allan  to  be  rather  higher  than  the  others ;  and 
St.  Pierre  is  described  by  Captain  F.  Moresby  as  being 
cavernous  throughout,  and  as  not  consisting  of  either  lime- 
stone or  granite.  These  islands,  as  well  as  the  Amirantes, 
certainly  are  not  atoll-formed,  and  they  seem  to  differ  from 
all  other  groups  ;  I  have  not  coloured  them ;  but  probably 
the  reefs  belong  to  the  fringing  class.  Their  formation  is 
attributed  both  by  Dr.  Allan  and  Captain  F.  Moresby,  to 
the  action  of  the  currents,  here  exceedingly  violent,  on 
banks  which  no  doubt  have  had  an  independent  geological 
origin.  They  resemble  in  many  respects  some  of  the 
islands  and  banks  in  the  West  Indies,  which  owe  their 
origin  to  a  similar  agency,  in  conjunction  with  an  eleva- 
tion of  the  entire  area.  In  close  vicinity  to  the  above 
several  islands,  there  are  three  others  of  an  apparently 
different  nature ;  first,  Juan  de  Nova,  which  appears  from 
some  plans  and  accounts  to  be  an  atoll,  but  from  others 
this  does  not  appear  to  be  the  case ;  **not  coloured.  Secondly, 
Cosmoledo :  '  this  group  consists  of  a  ring  of  coral,  ten 
leagues  in  circumference,  and  a  quarter  of  a  mile  broad  in 

'  [Platte,  Coetivi,  and  Galega  have  narrow  fringing-reefs. — Capfc. 
Wharton.] 

2  [Jiian  de  Nova  is  an  imperfect  atoll.  The  islands  on  its  eastern 
or  weather  side  have  been  raised  about  8  or  10  feet.  The  western  part 
is  submerged. — Capt.  Wharton.  A  coral-bank,  with  5  fathoms  water 
on  it  stretches  off  the  southern  end.— Lieut.  Chas.  Smith.] 


INDIAN    OCEAN.  251 

some  places,  inclosing  a  magnificent  lagoon,  into  wliich 
there  did  not  appear  a  single  opening '  (Horsburgh,  vol.  i. 
p.  151);  coloured  blue.^  Thirdly,  Aldahra:  consists  of 
three  islets,  about  25  feet  in  height,  with  red  cliffs  (Hors- 
burgh, vol.  i.  p.  176),  surrounding  a  very  shallow  basin  or 
lagoon.  The  sea  is  profoundly  deep  close  to  the  shore. 
Viewing  this  island  in  a  chart,  it  would  be  thought  to  be 
an  atoll ;  but  the  foregoing  description  shows  that  there 
is  something  different  in  its  nature  ;  Dr.  Allan  also  states 
that  it  is  cavernous,  and  that  the  coral-rock  has  a  vitrified 
appearance.  Is  it  an  upheaved  atoll,  or  the  crater  of  a 
volcano  ? — uncoloured.^ 

CoMOKO  Gboup. — Mayotta,  according  to  Horsburgh 
(vol.  i.  p.  216,  4th  edit.),  is  completely  surrounded  by  a 
reef,  which  runs  at  the  distance  of  three,  four,  and  in  some 
places  even  five  miles  from  the  land  ;  in  an  old  chart,  pub- 
lished by  Dalrymple,  a  depth  in  many  places  of  36  and  38 
fathoms  is  laid  down  within  the  reef.  In  the  same  chart, 
the  space  of  open  water  within  the  reef  is  in  some  parts 
even  more  than  three  miles  wide :  the  land  is  bold  and 
peaked;  this  island,  therefore,  is  encircled  by  a  well- 
characterized  barrier-reef,  and  is  coloured  pale  blue. — 
Johanna :  Horsburgh  says  (vol.  i.  p.  217),  this  island  from 
the  N.W.  to  the  S.W.  point,  is  bounded  by  a  reef,  at  the 

'  [The  islands  on  the  ring  have  been  upraised  about  10  feet. — 
Capt.  Wharton.] 

'^  [Aldabra  is  an  upraised  atoll  22  miles  long  ;  the  lagoon  is  nearly 
dry  at  low  water.  The  height  of  the  rock  on  the  encircling  islands 
is  20  feet,  and  it  descends  on  both  sides  to  the  water  for  that  distance 
in  a  cliff,  though  on  the  lagoon  side  the  coral  is  much  disintegrated  by 
the  mangroves.  This  is  the  only  island  in  the  Indian  Ocean  where 
the  gigantic  tortoise,  of  a  distinct  species,  exists. — Capt.  Wharton. 
Horsburgh 's  account  is  misleading,  as  neither  the  red  cliffs  nor  high 
forests  were  to  be  found.  It  is  entirely  composed  of  coral-rock  with 
a  fine  growth  of  mangroves,  inclosing  an  extensive  but  shallow  lagoon. 
There  is  a  narrow  riband  of  5  fathoms  water  running  3  miles  into  the 
*agoon  from  the  N.W.  corner.— Lieut.  Chas.  Smith.] 


252  APPENDIX. 

distance  of  two  miles  from  the  shore ;  in  some  parts,  how- 
ever, the  reef  must  be  attached,  since  Lieut.  Boteler  (Narr. 
vol.  i.  p.  IGl)  describes  a  passage  through  it,  within  which 
there  is  room  only  for  a  few  boats.  Its  height,  as  I  am 
informed  by  Dr.  Allan,  is  about  3,500  feet ;  it  is  very  pre- 
cipitous, and  is  composed  of  granite,  greenstone,  and 
quartz  ;  coloured  blue. — Mohilla :  on  the  S.  side  of  this 
island  there  is  anchorage  between  a  reef  and  the  shore 
in  from  30  to  45  fathoms  (Horsburgh,  vol.  i.  p.  214) ;  it 
appears  also  encircled  in  Captain  Owen's  chart  of  Mada- 
gascar ;  coloured  blue. — Great  Comoro  Island  is,  as  I  am 
informed  by  Dr.  Allan,  about  8,000  feet  high,  and  ap- 
parently volcanic  ;  ^  it  is  not  regularly  encircled ;  but  reefs 
of  various  shapes  and  dimensions  jut  out  from  ^very  head- 
land on  the  W.,  S.,  and  S.E.  coasts,  inside  of  which  reefs 
there  are  channels,  often  parallel  with  the  shore,  with  deep 
water.  On  the  N.W.  coasts  the  reefs  appear  attached  to 
the  shore.  The  land  near  the  coast  is  in  some  places  bold, 
but  generally  speaking  it  is  flat ;  Horsburgh  says  (vol.  i. 
p.  214),  the  water  is  profoundly  deep  close  to  the  shore, 
from  which  expression  I  presume  some  parts  are  without 
reefs.  From  this  description,  I  apprehend  the  reef  belongs 
to  the  barrier  class  ;  but  I  have  not  coloured  it,  as  most  of 
tlie  charts  which  I  have  seen  represent  the  reefs  round  it 
as  very  much  less  extensive  than  round  the  other  islands 
of  the  group. 

Madagascae. — My  information  is  chiefly  derived  from 
the  published  charts  by  Captain  Owen,  and  the  accounts 
given  by  him  and  by  Lieut.  Boteler.  Commencing  at  the 
S.W.  extremity  of  the  island  :  towards  the  northern  part  of 
Star  Bank  (in  lat.  25°  S.)  the  coast  for  ten  miles  is  fringed 
by  a  reef ;  coloured  red.     The  shore  immediately  S.  of  St. 

'  [Great  Comoro  is  volcanic  and  about  8,600  feet  high.  There  ia 
a  little  fringing-reef  on  the  north  and  on  the  south-east  side.— Lieut 
Chas.  Smith.] 


INDIAN   OCEAN.  25^ 

Augustin's  Bay  appears  fringed;  but  Tullear  Harbour, 
directly  N.  of  it,  is  formed  by  a  narrow  reef  ten  miles  long, 
extending  parallel  to  the  shore,  with  from  4  to  10  fathoms 
within  it.  If  this  reef  had  been  more  extensive,  it  must 
have  been  classed  as  a  barrier-reef ;  but  as  the  line  of  coast 
falls  inwards  here,  a  submarine  bank  perhaps  extends 
parallel  to  the  shore,  which  has  offered  a  foundation  for 
the  growth  of  the  coral ;  I  have  left  this  part  uncoloured. 
From  lat.  22°  16'  to  21°  37',  the  shore  is  fringed  by  coral- 
reefs  (see  Lieut.  Boteler's  Narrative,  vol.  ii.  p.  106),  less 
than  a  mile  in  width,  and  with  shallow  water  within. 
There  are  outlying  coral  shoals  in  several  parts  of  the  offing, 
with  about  10  fathoms  between  them  and  the  shore,  and 
the  depth  of  the  sea  one  mile  and  a  half  seaward,  is  only 
about  30  fathoms.  The  part  above  specified  is  engraved  on  a 
large  scale  ;  and  as  in  the  charts  on  rather  a  smaller  scale 
the  same  fringe  of  reef  extends  as  far  as  lat.  23°  15',  I 
have  coloured  the  whole  of  this  part  of  the  coast  red.  The 
islands  of  Juan  de  Nova  (in  lat.  17°  S.)  appear  in  the  charts 
on  a  large  scale  to  be  fringed,  but  I  have  not  been  able  to 
ascertain  whether  the  reefs  are  of  coral ;  uncoloured.  The 
main  part  of  the  west  coast  appears  to  be  low,  with  outlying 
sand  banks,  which  Lieut.  Boteler  (vol.  ii.  p.  106)  says,  *  are 
faced  on  the  edge  of  deep  water  by  a  line  of  sharp-pointed 
coral-rocks.'  Nevertheless  I  have  not  coloured  this  part, 
as  I  cannot  make  out  by  the  charts  that  the  coast  itself  is 
fringed.  The  headlands  of  Narrenda  and  Passandava  Bays 
(14°  40')  and  the  islands  in  front  of  Badama  harbour  are 
presented  in  the  plans  as  regularly  fringed,  and  have 
accordingly  been  coloured  red.  With  respect  to  the  East 
Coast  of  Madagascar  J  Dr.  Allan  informs  me,  that  the  whole 
line  of  coast,  from  Tamatave  in  18°  12'  to  G.  Amber  at  the 
extreme  northern  point  of  the  island,  is  bordered  by  coral- 
reefs.  The  land  is  low,  uneven,  and  gradually  rises  from 
the  coast.  From  Captain  Owen's  charts,  the  existence  of 
18 


254  APPENDIX. 

reefs,  which  evidently  belong  to  the  fringing  class,  N.  of 
British  Sound  and  near  Ngoncy,  might  also  have  been 
inferred.  Lieut.  Boteler  (vol.  i.  p.  155)  speaks  of  *  the  reef 
surrounding  the  island  of  St.  Mary's  at  a  small  distance 
from  the  shore.'  In  a  previous  chapter  I  have  described* 
from  the  information  of  Dr.  Allan,  the  manner  in  which 
the  reefs  extend  in  N.E.  lines  from  the  headlands  on  this 
coast,  thus  sometimes  forming  rather  deep  channels  within 
them :  this  seems  caused  by  the  currents,  the  reefs  spring- 
ing up  from  the  submarine  prolongations  of  the  sandy  head- 
lands. The  above  specified  portion  of  the  coast  is  coloured 
red.*  The  remaining  S.E.  portions  do  not  appear  in  any 
published  chart  to  possess  reefs  of  any  kind ;  and  the  Eev. 
W.  EUis  believes  that  there  are  none. 

East  Coast  of  Africa.— The  northern  parts  appear,  for 
a  considerable  space,  to  be  without  reefs.  My  information, 
I  may  observe,  is  derived  from  the  survey  by  Captain  Owen, 
together  with  his  Narrative ;  and  that  by  Lieut.  Boteler. 
At  Mukdeesha  (2°  1'  N.)  there  is  a  coral-reef  extending 
four  or  five  miles  along  the  shore  (Owen's  Nar.  vol.  i. 
p.  357),  which  in  the  chart  lies  at  a  distance  of  a  quarter  of 
a  mile  from  the  shore,  and  has  within  it  from  6  to  10  feet 
of  water:  this  then  is  a  fringing-reef  and  is  coloured  red. 
From  Jtiba,  a  httle  S.  of  the  equator,  to  Lamoo  (in  2°  2(y 
S.)  *  the  coast  and  islands  are  formed  of  madrepore  * 
(Owen's  Narrative,  vol.  i.  p.  363).  The  chart  of  this  part 
(entitled  Dundas  Islands)  presents  an  extraordinary  ap-. 
pearance  ;  the  coast  of  the  mainland  is  quite  straight,  and 
is  fronted  at  the  average  distance  of  two  miles,  by  exceed- 
ingly narrow,  straight  islets,  fringed  with  reefs.  Within 
this  chain  of  islets,  there  are  extensive  tidal  flats  and 
muddy  bays,  into  which  many  rivers  enter  :  the  depth  of 
these  spaces  varies  from  one  to  four  fathoms — the  latter 

*  [The  northern  end  of  Madagascar,  of  volcanic  origin,  has  upraised 
eoral,  nnd  is  fringed  with  living  coral. — Capt.  Wharton.] 


EAST  COAST  OF  AFRICA.         255 

depth  not  being  common,  and  about  12  feet  the  average. 
Outside  the  chain  of  islets,  the  sea,  at  the  distance  of  a 
mile,  varies  in  depth  from  8  to  15  fathoms.  Lieut.  Boteler 
(Nar.  vol.  i.  p.  369)  describes  the  muddy  bay  of  Patta, 
•which  seems  to  resemble  other  parts  of  the  coast,  as  fronted 
by  small,  narrow,  level  islets  formed  of  decomposing  coral, 
the  margin  of  which  is  seldom  of  greater  height  than  12 
feet,  overhanging  the  rocky  surface  from  which  the  islets 
rise.  Knowing  that  the  islets  "are  formed  of  coral,  it  is 
I  think  scarcely  possible  to  view  the  coast,  and  not  at  once 
conclude  that  we  here  see  a  fringing-reef,  which  has  been 
upraised  a  few  feet :  the  unusual  depth  of  from  two  to  four 
fathoms  within  some  of  these  islets,  is  probably  due  to  the 
mud  of  the  rivers  having  prevented  the  growth  of  coral 
near  the  shore.  As  several  parts  of  this  line  of  coast  are 
undoubtedly  fringed  by  living  reefs,  I  have  coloured  it 
red. — Maleenda  (3°  20'  S.) :  in  the  plan  of  the  harbour, 
the  south  headland  appears  fringed ;  and  in  Owen's  chart 
on  a  larger  scale,  the  reefs  are  seen  to  extend  nearly  30 
miles  southward ;  coloured  red.  Mombas  (4°  5'  S.) :  the 
island  which  forms  the  harbour  *  is  surrounded  by  cliffs  of 
madrepore,  capable  of  being  rendered  almost  impregnable  * 
(Owen's  Nar.  vol.  i.  p.  412).  The  shore  of  the  mainland, 
N.  and  S.  of  Mombas,  is  regularly  fringed  by  a  coral-reef 
at  a  distance  from  half  a  mile  to  a  mile  and  a  quarter  from 
the  land  ;  within  the  reef  the  depth  is  from  9  to  15  feet ; 
outside  the  reef  the  depth  at  rather  less  than  half  a  mile 
is  30  fathoms.  From  the  charts  it  appears  that  a  space 
about  36  miles  in  length,  is  here  fringed ;  coloured  red. — 
Pemba  (5°  S.)  is  an  island  of  coral  formation,  level,  and 
about  200  feet  in  height  (Owen's  Nar.  vol.  i.  p.  425) ;  it  is 
85  miles  long,  and  is  separated  from  the  mainland  by  a 
deep  sea.  The  outer  coast  is  represented  in  the  charts  as 
regularly  fringed  ;  coloured  red.  The  mainland  in  front  of 
Pemba  is  likewise  fringed. — Zanzibar  resembles  Pemba  in 


256  APPENDIX. 

most  respects ;  its  southern  half  on  the  western  side  and 
the  neighbouring  islets  are  fringed ;  coloured  red.*   On  the 

*  [The  following  interesting  account  of  Zanzibar  is  contained 
in  a  letter  from  Captain  Wharton,  found  among  Mr.  Darwin's 
papers  : — 

•  Zanzibar  seems  to  me  to  have  undergone  several  motions  of  sub- 
sidence and  upheaval,  the  latter  being  the  latest ;  it  appears  now  to 
have  been  for  many  years  nearly  stationary. 

•  The  island  at  present  is  surrounded  with  a  nearly  perfectly  flat, 
dead,  altered  coral  ledge,  more  or  less  dry  at  low  water,  without  doubt 
the  result  of  long  action  of  the  sea  on  the  upheaved  ancient  and  com- 
pressed coral  of  which  the  island  is  principally  formed.  This  action 
has  worn  away  the  sea  face  of  the  land  to  the  level  of  low  water  for  a 
distance,  in  some  instances,  of  1^  miles  inside  the  original  high-water 
line,  which  now  remains  as  a  steep  rim,  dropping  to  10  and  20  fathoms 
almost  immediately,  with  (on  the  outside  of  the  island)  100  fathoms 
within  a  quarter  of  a  mile.  I  could  see  no  sign  of  this  ledge  extend- 
ing seaward,  though  there  is  living  coral  on  its  steep  face  visible  a 
few  feet  below  at  low  water,  but  this  is  not  abundant,  as  it  is  on  some 
of  the  detached  reefs  off  the  island. 

•  The  present  high -water  line  of  the  island  at  the  back  of  this  flat 
area  is,  for  the  major  part  of  its  perimeter,  a  cliff  of  the  same  old 
coral  from  10  to  20  feet  in  height,  undermined  by  the  waves,  and 
overhanging,  in  some  places,  to  a  marvellous  extent,  showing  the 
hardness  and  cohesion  of  the  material,  and  giving  a  notion  of  the 
long  period  of  time  necessary  to  wear  it  away.  As  a  further  proof 
of  this  is  the  fact  of  very  few  lately  detached  pieces  being  seen  at  the 
foot  of  the  cliffs,  though  the  blocks,  when  they  do  fall,  must  be  large 
and  not  easily  moved  by  the  sea. 

'  In  most  parts  of  the  island  the  tops  of  these  low  cliffs  run  back 
from  the  sea  nearly  level  for  a  greater  or  less  distance,  showing  water- 
worn  coral  wherever  the  surface  rock  is  exposed,  and  indicating 
another  stationary  period  or  one  of  very  slow  upheaval.  Out  of  thin 
level  the  higher  lands  of  the  island  rise. 

*  Zanzibar  is  intersected  by  what  may  be  regarded  as  3  lines  of  hills 
running  north  and  south,  the  highest  of  them  being  450  feet  above 
the  sea. 

*  I  regret  to  add  that  I  cannot  say  of  what  formation  these  hills 
may  be ;  I  cannot  call  to  mind  any  rock  beyond  the  coral,  which 
crops  out  at  considerable  heights  (in  one  instance  250  feet),  but  there 
is  a  good  deal  of  hardened  clay  or  mudstone,  which  generally  appears 


EAST  COAST  OF  AFRICA.         257 

main  land,  a  little  S.  of  Zanzibar,  there  are  some  banks 
parallel  to  the  coast,  which  I  should  have  thought  had  been 

in  the  ravines,  &c.,  and  on  the  bare  sides  of  the  hills,  but  there  may 
be  other  rock  lying  under  this. 

♦  The  valleys,  or  rather  flat  plains,  between  the  ranges  of  hills,  are 
mostly  (particularly  to  the  south)  coral,  worn  and  roughened,  un- 
doubtedly by  water.  These  are  generally  about  50  feet  above  the 
sea.  Several  isolated  hills  of  coral  stand  on  these  plains,  their  bases 
being  undermined  and  worn  precisely  as  the  present  cliffs,  and  their 
flat  summits  present  the  same  appearance. 

♦  The  whole  thickness  of  the  coral  of  Zanzibar  must  be  very  great. 

•  The  coast  of  the  mainland  about  Zanzibar  is  similar  to  the  island, 
and,  as  far  as  I  know  them,  Pemba,  Monfia,  and  the  coast  far  north 
and  south  are  the  same. 

*  The  outlying  and  detached  reefs  are  of  two  kinds,  those  growing 
up  with  living  coral,  and  those  of  dead  coral,  like  the  island  washing 
gradually  away.  Of  these  latter  many  still  have  level  islets  and 
rocks  on  them,  remnants  of  a  former  upheaval ;  others  afford  a  found- 
ation to  coral  sand-banks  that  are  dry  high  at  low  water,  and  others 
are  perfectly  smooth  and  covered  at  high  water,  being  just  awash  at 
low  tide.  Of  the  second  of  these,  are  the  reefs  referred  to  by  you  at 
page  258  as  described  by  Lieut.  Boteler  as  sand-banks.  That  descrip- 
tion is  erroneous. 

'  One  island,  mentioned  in  the  beginning  of  the  century,  had  by 
Capt,  Owen's  time  (1825)  been  reduced  to  a  sand-head  always  visible. 
Now  (1874)  even  this  has  entirely  disappeared,  and  the  reef  on  which 
it  stood  is  flat  and  bare. 

*  This  is  the  only  instance  in  which  I  have  been  able  to  make  any 
reliable  comparison  between  Capt.  Owen's  chart  and  mine,  as  to 
reduction  of  reefs. 

•  As  to  the  perhaps  still  more  interesting  question  of  growing  coral, 
I  have  been  unable  to  make  any  such,  as  Owen's  work  was  so  cursory 
and  hurried  that  it  is  impossible  to  know  whether  he  struck  the 
shoalest  part  of  a  reef. 

*  There  is,  indeed,  one  instance  that,  if  not  isolated,  might  have  been 
of  use.  He  describes  a  particular  shoal  as  being  a  *  knoll  with  deep 
water  all  round,'  and  in  his  chart,  7  fathoms  is  marked  on  it  and  25 
fathoms  around.  That  patch  has  now  only  1^  fathoms  on  it  and  20 
fathoms  round. 

'  This,  altogether,  looks  like  upheaval  of  the  whole  bottom ;  but  aa 
in  most  instances  our  soundings  agree  remarkably  well,  I  cannot  think 


258  APPENDIX. 

formed  of  coral,  had  it  not  been  said  (Boteler*s  Nar.  vol.  iL 
p.  39)  that  they  were  composed  of  sand :  not  coloured. — 
Latham'' s  Bank  is  a  small  island  fringed  by  coral-reefs ; 
but  being  only  10  feet  high  it  has  not  been  coloured. — 
Mo7ifeea  is  an  island  of  the  same  character  as  Pemba :  its 
outer  shore  is  fringed,  and  its  southern  extremity  is  con- 
nected with  Keelwa  Point  on  the  main  land  by  a  chain  of 
islands  fringed  by  reefa ;  coloured  red.  The  four  last-men- 
tioned islands  resemble  in  many  respects  some  of  the  islands 
in  the  Red  Sea,  which  will  presently  be  described. — 
Keelwa :  in  a  plan  of  the  shore,  a  space  of  20  miles  N.  and 
S.  of  this  place  is  fringed  by  reefs,  apparently  of  coral ; 
these  reefs  are  prolonged  still  further  southwards  in  Owen's 
Ifeneral  chart.  In  the  plans  of  the  rivers  Lindy  and  Mong- 
how  (9°  59'  and  10°  7'  S.)  the  coast  seems  to  have  the  same 
structure,  coloured  red. — Qucrimha  Islands  (from  10°  40' 
to  13°  S.) :  a  chart  on  a  large  scale  is  given  of  these 
islands;  they  are  low  and  of  coral  formation  (Boteler's 
Nar.  vol.  ii.  p.  54) ;  and  generally  have  extensive  reefs  pro- 
jecting from  them,  which  are  dry  at  low  water,  and  which 
on  the  outside  rise  abruptly  from  a  deep  sea  ;  on  the  inside 
they  are  separated  from  the  continent  by  a  channel,  or 
rather  a  succession  of  bays,  with  an  average  depth  of  10 
fathoms.  The  small  headlands  on  the  continent  also  have 
coral  banks  attached  to  them  ;  and  the  Querimba  islands 
and  banks  are  placed  on  the  lino  of  prolongation  of  these 
headlands,  and  are  separated  from  them  by  very  shallow 
channels.    It  is  evident  that  whatever  cause,  whether  the 

that  that  can  be  bo.  On  the  other  hand,  the  reef  is  so  small  and  the 
bottom  so  clear,  that  it  is  difficult  to  understand  how  they  could 
have  missed  the  shoaler  water  if  it  existed  then,  as  it  is  very  plain 
to  see. 

*  Other  reefs,  with  from  7  to  10  fathoms  on  them,  seem  not  to  havo 
altered. 

*  W.  J.  S.  Whaeton,  Commander  R.N. 
'  Mauritius :  Sept.  15, 1874.] 


PERSIAN   GULF.  259 

drifting  of  sediment  or  subterranean  movements,  produced 
the  headlands,  likewise  produced,  as  might  have  been  ex- 
pected, submarine  prolongations  to  them  ;  and  these  to- 
wards their  outer  extremities  have  since  afforded  a  favour- 
able basis  for  the  growth  of  coral-reefs,  and  subsequently  for 
the  formation  of  islets.  As  these  reefs  clearly  belong  to  the 
fringing  class,  the  Querimba  Islands  have  been  coloured 
red. — Monabila  (13°  82'  S.) :  in  the  plan  of  this  harbour, 
the  headlands  outside  are  fringed  by  reefs  apparently  of 
coral;  coloured  red. — Moza7nbique{15°  B.):  the  outer  part 
of  the  island  on  which  the  city  is  built,  and  the  neighbour- 
ing islands  are  fringed  by  coral-reefs;  coloured  red. 
From  the  description  given  in  Owen's  Nar.  (vol.  i.  p.  162) 
the  shore  from  Mozambique  to  Delagoa  Bay  appears  to  be 
low  and  sandy :  many  of  the  shoals  and  islets  off  this  line 
of  coast  are  of  coral  formation  ;  but  from  their  small  size 
and  lowness,  it  is  not  possible,  from  the  charts,  to  know 
whether  they  are  truly  fringed.  Hence  this  portion  of 
coast  is  left  uncoloured,  as  are  likewise  those  parts  more 
northward,  of  which  no  mention  has  been  made  in  the 
foregoing  pages,  from  the  want  of  information.* 

Pebsian  Gulf. — From  the  charts  lately  published  on  a 
large  scale  by  the  East  Indian  Company,  it  appears  that 
several  parts,  especially  the  southern  shores,  are  fringed  by 
coral-reefs ;  but  as  the  water  is  very  shallow,  arid  as  there 
are  numerous  sand- banks,  which  are  difficult  to  distin- 

'  [The  whole  of  the  eastern  coast  of  Africa,  from  the  equator  to 
Mozambique  (at  least)  is  of  upraised  coral,  and  so  are  the  outlying 
islands.  Fringing-reefs  occur  everywhere,  partly  formed  by  the 
action  of  the  sea  wearing  back  the  upraised  coral,  and  partly  by 
living  coral.  In  Zanzibar  undoubted  coralline  limestone  exists  at 
100  feet,  and  a  limestone  of  origin  as  yet  undetermined  at  300  feet. 
— W.  From  Wasin  to  Pangani  (about  lat.  5°  S.)  there  is  a  barrier  of 
large  coral-reefs  from  2  to  5  miles  off  shore  with  a  deep  channel  inside, 
sometimes  as  much  as  20  fathoms  in  depth. — Lieut.  Ghas.  Smith.—' 
From  Mr.  Darwin's  papers.] 


260  APPENDIX. 

guish  on  the  chart  from  reefs,  I  have  not  coloured  the  upper 
part  red.  Towards  the  mouth,  however,  where  the  water  is 
rather  deeper,  the  islands  of  Ormuz  and  Larrack,  appear 
BO  regularly  fringed,  that  I  have  coloured  them  red.  There 
are  certainly  no  atolls  in  the  Persian  Gulf.  The  shores  of 
Immauniy  and  of  the  promontory  forming  the  southern  head- 
land of  the  Persian  Gulf,  seem  to  be  without  reefs.  The 
whole  S.W.  part  of  Arabia  FeliXy  except  one  or  two  small 
patches,  and  the  shores  of  Socotra  appear  from  the  charts 
and  the  memoir  of  Captain  Haines  (Geograph.  Journ.  1839, 
p.  125)  to  be  without  reefs.  I  believe  there  are  no  exten- 
sive coral-reefs  on  any  part  of  the  coasts  of  Indian  except  on 
the  low  promontory  of  Madura  (as  already  mentioned)  in 
front  of  Ceylon. 

Bed  Sea.. — My  information  is  chiefly  derived  from  the 
admirable  charts  published  by  the  East  India  Company  in 
1836,  from  personal  communication  with  Captain  Moresby, 
one  of  the  surveyors,  and  from  the  excellent  memoir^ 
*  Ueber  die  Natur  der  Corallen-BankendesRothen  Meeres/ 
by  Ehrenberg.  The  plains  immediately  bordering  the  Red 
Sea  seem  to  consist  chiefly  of  a  sedimentary  formation  of 
the  newer  tertiary  period.  The  shore  is,  with  the  exception 
of  a  few  parts,  fringed  by  coral-reefs.  The  water  is  gener- 
ally profoundly  deep  close  to  the  shore ;  but  this  fact,  which 
has  attracted  the  attention  of  most  voyagers,  seems  to  have 
no  necessary  connection  with  the  presence  of  reefs ;  for 
Captain  Moresby  particularly  observed  that,  in  lat.  24°  KV 
on  the  eastern  side,  there  is  a  piece  of  coast  with  very  deep 
water  close  to  it,  without  any  reefs,  but  not  differing  in  any 
other  respect  from  the  usual  coast  line.  The  most  remark- 
able feature  in  the  Red  Sea  is  the  chain  of  submerged  banks, 
reefs,  and  islands  lying  some  way  from  the  shore,  chiefly  on 
the  eastern  side ;  the  space  within  being  deep  enough  to 
admit  safe  navigation  in  small  vessels.  The  banks  are 
generally  of  an  oval  form,  and  some  miles  in  width ;  but 


RED   SEA.  261 

some  of  them  are  very  long  in  proportion  to  their  width. 
Captain  Moresby  informs  me  that  any  one  who  had  not 
made  actual  plans  of  them,  would  be  apt  to  think  that  they 
were  much  more  elongated  than  they  really  are.  Many  of 
them  rise  to  the  surface,  but  the  greater  number  lie  from  5 
to  30  fathoms  beneath  it,  with  irregular  soundings  on  them. 
They  consist  of  sand  and  living  coral ;  the  latter  in  most 
cases,  according  to  Captain  Moresby,  covering  the  greater 
part  of  their  surface.  They  extend  parallel  to  the  shore, 
and  are  not  unfrequently  connected  in  their  middle  parts 
by  short  transverse  banks  with  the  main  land.  The  sea  is 
generally  profoundly  deep  quite  close  to  them,  as  it  is  near 
most  parts  of  the  coast  of  the  main  land ;  but  this  is  not 
universally  the  case,  for  between  lat.  15°  and  17°  the  water 
deepens  quite  gradually  from  the  banks,  both  on  the  eastern 
and  western  shores.  In  many  parts  islands  rise  from  the 
banks  ;  they  are  low,  flat-topped,  and  consist  of  the  same 
horizontally  stratified  formation  with  that  forming  the 
plain-like  margin  of  the  main  land.  Some  of  the  smaller 
and  lower  islands  consist  of  mere  sand.  Captain  Moresby 
informs  me  that  small  masses  of  rock,  the  remnants  of 
islands,  are  left  on  many  of  the  banks  where  there  is  now 
no  dry  land.  Ehrenberg  also  asserts  that  most  of  the 
islets,  even  the  lowest,  have  a  flat  abraded  basis,  com- 
posed of  the  same  tertiary  formation  as  elsewhere:  he 
believes  that  as  soon  as  the  surf  wears  down  the  protube- 
rant parts  of  the  banks  to  just  beneath  the  level  of  the  sea, 
the  surface  becomes  protected  from  further  abrasion  by  the 
growth  of  coral,  and  he  thus  accounts  for  the  existence  of  so 
many  banks  standing  on  a  level  with  the  surface  of  this  sea. 
It  appears  that  most  of  the  islands  are  certainly  decreasing 
in  size. 

The  banks  and  islands  are  curiously  shaped  in  the  parts 
just  referred  to,  namely,  from  lat.  15"  to  17°,  where  the 
sea  deepens   quite   gradually :  the  DJialac  group,  on  the 


262  APrENDix. 

western  coast,  is  surrounded  by  an  intricate  archipelago  of 
islets  and  shoals  ;  the  main  island  is  irregular  in  outline, 
and  includes  a  bay  seven  miles  long,  by  four  across,  in 
which  no  bottom  was  found  with  252  feet ;  there  is  only 
one  entrance  into  it,  half  a  mile  wide,  and  with  an  island 
in  front.  The  submerged  banks  on  the  eastern  coast, 
within  the  same  latitudes,  round  Farsan  Island,  are,  like- 
wise, penetrated  by  many  narrow  creeks  of  deep  water ; 
one  is  twelve  miles  long,  in  the  form  of  a  hatchet,  and  close 
to  its  broad  upper  end,  soundings  were  not  struck  with 
860  feet;  its  entrance  is  only  half  a  mile  wide.  In 
another  creek  of  the  same  nature,  but  even  with  a  more 
irregular  outline,  there  was  no  bottom  with  480  feet.'  The 
island  of  Farsan  itself,  has  as  singular  a  form  as  any  of  its 
surrounding  banks.  The  bottom  of  the  sea  round  the 
Dhalac  and  Farsan  Islands  consists  chiefly  of  sand  and 
agglutinated  fragments  of  coral,  but,  in  the  deep  and  narrow 
creeks,  it  consists  of  mud;  the  islands  consist  of  thin, 
horizontally  stratified,  modern  tertiary  beds,  containing 
but  little  broken  coral ;  ^  their  shores  are  fringed  by  living 
eoral-reefa. 

From  the  account  given  by  Eiippell  ^  of  the  manner  in 
which  Dhalac  is  rent  by  fissures,  the  opposite  sides  of 
which  have  been  unequally  elevated  (in  one  instance  to 
the  amount  of  60  feet),  it  seems  probable  that  this  irregular 
form,  as  well  as  that  of  Farsan,  may  have  been  partly 
caused  by  unequal  elevation  ;  but,  considering  the  general 
form  of  the  banks,  and  of  the  deep-water  creeks,  together 
with  the  composition  of  the  land,  I  think  their  configura- 

•  [The  islands  of  this  group  are  of  upraised  coral,  as  is  the  fore- 
shore of  the  opposite  coast  of  Abyssinia.  In  many  parts  of  the  Red 
Sea  coast  the  low  coral  cliffs  give  evidence  of  upheaval.  There  are, 
nevertheless,  reefs  which  would  be  classed  as  barrier-reefs  on  both 
sides  of  the  central  part  of  the  Red  Sea.— Capt.  Whart*  n.] 

*  Riippell,  Reise  in  Abyssinie,  Band.  i.  s  247. 
»  Ibid.  s.  245. 


RED    SEA.  263 

tion  is  more  probably  due  in  great  part  to  currents  having 
drifted  sediment  over  an  uneven  bottom.  It  is  almost 
certain  that  their  form  cannot  be  attributed  to  the  growth 
of  coral.  The  greater  number  of  banks  on  the  eastern 
side  of  the  Eed  Sea  seems  to  have  originated  in  nearly  the 
same  manner,  whatever  this  may  have  been,  as  the  Dhalac 
and  Farsan  archipelagoes.  I  judge  of  this  from  their 
similar  configuration  (in  proof  of  which  I  may  instance  a 
bank  on  the  east  coast  in  lat.  22°)  and  from  their  similar 
composition.  The  depth,  however,  within  the  banks  north- 
ward of  lat.  17°  is  usually  greater,  and  their  outer  sides 
shelve  more  abruptly  (circumstances  which  seem  to  go 
together)  than  in  the  Dhalac  and  Farsan  archipelagoes ; 
but  this  may  have  been  caused  by  a  stronger  action  of  the 
currents  during  their  formation:  moreover,  the  greater 
abundance  of  living  coral  on  the  northern  banks,  tends  to 
give  them  steeper  margins. 

From  this  account,  brief  and  imperfect  as  it  is,  we  can 
see  that  the  great  chain  of  banks  on  the  eastern  side  of  the 
Eed  Sea,  and  on  the  western  side  of  the  southern  portion, 
dijBfer  greatly  from  true  barrier-reefs,  which  are  wholly 
formed  by  the  growth  of  coral.  Ehrenberg  also  concludes 
(Ueber  die,  &c.  pp.  45  and  51)  that  these  banks  owe  their 
origin  in  a  quite  secondary  manner  to  the  growth  of  coral. 
He  remarks  that  the  islands  off  the  coast  of  Norway,  if 
worn  down  level  with  the  sea,  and  merely  coated  with 
living  coral,  would  present  a  nearly  similar  appearance. 
It  seems,  however,  from  information  given  me  by  Dr.  Mal- 
colmson  and  Caj)tain  Moresby,  that  Ehrenberg  has  rather 
under-rated  the  influence  of  corals  on  the  formation  of  the 
tertiary  deposits  of  the  Eed  Sea. 

The  West  Coast  of  the  Bed  Sea  between  Lat.l9°  and  22°. 
— Eeefs  exist  here,  which,  if  I  had  known  nothing  of  the 
others  in  the  Eed  Sea,  I  should  unhesitatingly  have  con- 
sidered as  barrier-reefs.     One  of  these  reefs,  in  20°  15^  ig 


2G4  APPENDIX. 

twenty  miles  long,  less  than  a  mile  in  width  (but  expanding 
at  the  northern  end  into  a  disk),  slightly  sinuous,  and 
parallel  to  the  main  land  at  the  distance  of  five  miles  from 
it,  with  very  deep  water  inside,  so  that  in  one  place  sound- 
ings were  not  obtained  with  205  fathoms.  Some  leagues 
further  south,  there  is  another  very  narrow  reef,  ten  miles 
long,  with  other  small  portions  of  reef,  north  and  south, 
almost  connected  with  it ;  and  within  this  line  of  reefs  (as 
well  as  outside)  the  water  is  profoundly  deep.  There  are 
also  some  small  linear  and  sickle-formed  reefs,  lying  a 
little  way  out  at  sea.  All  these  reefs  are  covered,  as  I  am 
informed  by  Captain  Moresby,  by  living  corals.  Here,  then, 
we  have  all  the  characters  of  reefs  of  the  barrier  class,  and 
some  of  the  outlying  reefs  partially  resemble  atolls.  My 
only  source  of  doubt  arises  from  the  narrowness  and 
straightness  of  the  spits  of  sand  and  rock  in  the  Dhalao 
and  Farsan  groups  ;  one  of  these  spits  in  the  former  group 
is  nearly  fifteen  miles  long,  only  two  broad,  and  is  bordered 
on  each  side  with  deep  water ;  so  that,  if  worn  down  by 
the  surf,  and  coated  with  living  corals,  it  would  form  a  reef 
nearly  similar  to  those  within  the  space  under  considera- 
tion. Nevertheless  I  cannot  believe  that  the  many  small, 
isolated,  and  sickle-formed  reefs,  as  well  as  others  long, 
nearly  straight,  and  very  narrow,  with  the  water  unfathom- 
ably  deep  close  round  them,  could  have  been  formed  by 
corals  merely  coating  banks  of  sediment  or  the  abraded 
surfaces  of  irregularly  shaped  islands.  It  seems  more  pro- 
bable that  the  foundations  of  these  reefs  have  subsided,  and 
that  the  corals,  during  their  upward  growth,  have  given  to 
them  their  present  forms.  I  have,  therefore,  with  much 
hesitation  coloured  this  part  blue. 

The  West  Coast,  from  Lat.  22°  to  24°.— This  part  of 
the  coast  (north  of  the  space  coloured  blue  on  the  map)  is 
fronted  by  an  irregularly  shelving  bank,  from  10  to  80 
fathoms  deep ;  numerous  little  reefs,  some  of  which  have 


RED    SEA.  265 

the  most  singular  shapes,  rise  from  this  bank.  Many  of 
them  may  have  been  formed  by  the  growth  of  coral  on 
Bmall  abraded  islets;  but  some  almost  atoll-formed  reefs 
rising  from  deep  water  near  a  promontory  in  lat.  24°,  aro 
probably  allied  to  the  barrier  class.  I  have  not,  however, 
ventured  to  colour  this  portion  of  coast  blue. — On  the  west 
coast,  from  lat.  19°  to  17°  (south  of  the  space  coloured  blue 
on  the  map),  there  are  many  low  islets  of  small  dimensions 
not  much  elongated,  and  rising  out  of  great  depths  at  a 
distance  from  the  coast:  these  cannot  be  classed  either 
with  atolls,  or  barrier,  or  fringing-reefs. 

Eastern  Coast. — There  are  many  small  outlying  coral- 
reefs  along  this  whole  line  of  coast ;  but  as  the  greater 
number  rise  from  banks  not  very  deeply  submerged,  their 
origin,  as  we  have  seen,  may  be  due  simply  to  the  growth 
of  corals  on  an  irregular  abraded  foundation.  But  between 
lat.  18°  and  20°  there  are  so  many  linear,  elliptic  and  ex- 
tremely small  reefs,  rising  abruptly  out  of  profound  depths, 
that  the  same  reasons  which  led  me  to  colour  a  portion  of 
the  west  coast  blue,  have  induced  me  here  to  do  the  same. 
There  are  some  small  outlying  reefs  on  the  east  coast,  north 
of  lat.  20°  (the  northern  limit  coloured  blue),  which  rise 
from  deep  water ;  but  as  they  are  not  numerous,  and  as 
scarcely  any  of  them  are  linear,  I  have  left  them  un- 
coloured. 

In  the  southern  parts  of  the  Eed  Sea,  considerable 
spaces  of  the  main  land,  and  some  of  the  Dhalac  islands, 
are  skirted  by  reefs,  which,  as  I  am  informed  by  Captain 
Moresby,  are  of  living  coral,  and  have  all  the  characters  of 
the  fringing  class.  As  there  are  here  no  outlying  linear 
or  sickle-formed  reefs,  rising  out  of  unfathomable  depths, 
I  have  coloured  these  parts  of  the  coast  red.  On  similar 
grounds  I  have  coloured  the  northern  parts  of  the  western 
coast  (north  of  lat.  24°  30')  red,  and  likewise  the  shores  of 


26G  APPENDIX. 

the  chief  part  of  the  Gulf  of  Suez}  In  the  Gulf  of  Acaha, 
as  I  am  informed  by  Captain  Moresby,  there  are  no  coral- 
reefs,  and  the  water  is  profoundly  deep. 

West  Indies. — My  information  regarding  the  reefs  of 
this  area  is  derived  from  various  sources,  and  from  an  ex- 
amination of  numerous  charts  ;  especially  of  those  lately 
executed  during  the  survey  under  Captain  Owen,  E.N.  I  lie 
under  particular  obligation  to  Captain  Bird  Allen,  R.N., 
one  of  the  members  of  the  late  survey,  for  many  personal 
communications  on  this  subject.  As  in  the  case  of  the 
Red  Sea,  it  is  necessary  to  make  some  preliminary  remarks 
on  the  submerged  banks  of  the  West  Indies,  which  are  in 
some  degree  connected  with  coral-reefs,  and  cause  consider- 
able doubts  in  their  classification.  That  large  accumula- 
tions of  sediment  are  in  progress  on  the  West  Indian  shores, 
will  be  evident  to  any  one  who  examines  the  charts  of  that 
sea,  especially  of  the  portion  north  of  a  line  joining 
Yucutan  and  Florida.  The  area  of  deposition  seems  less 
intimately  connected  with  the  debouchement  of  the  great 
rivers,  than  with  the  course  of  the  sea-currents  ;  as  is 
evident  from  the  vast  extension  of  the  banks  from  the  pro- 
montories of  Yucutan  and  Mosquito. 

Besides  the  coast -banks,  there  are  others  of  various 
dimensions  which  stand  isolated ;  these  closely  resemble 
each  other  ;  they  lie  from  2  or  3  to  20  or  30  fathoms  under 
water,  and  are  composed  of  sand,  sometimes  firmly  ag- 
glutinated, with  little  or  no  coral ;  their  surfaces  are  smooth 
and  nearly  level,  shelving  very  gradually  to  the  amount  of 
a  few  fathoms  all  round  towards  their  edges,  where  they 
plunge  abruptly  into  the  unfathomable  sea.  This  steep 
inclination  of  their  sides,  which  is  likewise  characteristic 
of  the  coast-banks,  is  very  remarkable  :  I  may  give  as  an 

'  [Wherever  I  have  seen  the  coast  of  the  Ked  Sea,  it  shows  clear 
signs  of  upheaval  in  low  coral  cliffs.  There  are,  nevertheless,  reefs 
which  would  be  classed  as  barrier  on  both  sides  of  the  central  part 
of  the  Red  Sea.— Capt.  Wharton.  For  Masamarhu  Island,  see  App.  II.] 


WEST   INDIES.  267 

instance,  the  Misteriosa  Bank,  on  the  edges  of  which  the 
soundings  change  in  250  fathoms  horizontal  distance,  from 
11  to  210  fathoms  ;  off  the  northern  point  of  Old  Provi- 
dence Bank,  in  200  fathoms  horizontal  distance,  the  change 
is  from  19  to  152  fathoms  ;  off  the  Great  Bahama  Bank,  in 
100  fathoms  horizontal  distance,  the  inclination  is  in  many- 
places  from  10  fathoms  to  no  bottom  with  190  fathoms. 
In  all  parts  of  the  world,  where  sediment  is  accumulat- 
ing, something  of  the  same  kind  may  be  observed ;  the 
banks  shelving  very  gently  far  out  to  sea,  and  then  termin- 
ating abruptly.  The  form  and  composition  of  the  banks  in 
the  middle  parts  of  W.  Indian  sea,  clearly  show  that  their 
origin  must  be  chiefly  attributed  to  the  accumulation  of 
sediment ;  and  the  only  obvious  explanation  of  their  iso- 
lated position  is  the  presence  of  a  nucleus,  round  which 
the  currents  have  collected  fine  drift  matter.  Any  one  who 
will  compare  the  bank  surrounding  the  hilly  island  of  Old 
Providence,  with  the  banks  in  its  neighbourhood  which 
stand  isolated,  will  scarcely  doubt  that  they  surround  sub- 
merged mountains.  We  are  led  to  the  same  conclusion  by 
examining  the  bank  called  Thunder  Knoll,  which  is  separ- 
ated from  the  Great  Mosquito  bank  by  a  channel  only  seven 
miles  wide,  and  145  fathoms  deep.  There  cannot  be  any 
doubt  that  the  Mosquito  bank  has  been  formed  by  the  ac- 
cumulation of  sediment  round  the  promontory  of  the  same 
name ;  and  Thunder  Knoll  resembles  the  Mosquito  bank, 
in  the  state  of  its  surface  submerged  20  fathoms,  in  the  in- 
clination of  its  sides,  in  composition,  and  in  every  other 
respect.  I  may  observe,  although  the  remark  is  here  irre- 
levant, that  geologists  should  be  cautious  in  concluding  that 
all  the  outlyers  of  any  formation  have  once  been  connected 
together,  for  we  here  see  that  deposits,  doubtless  of  exactly 
the  same  nature,  may  be  deposited  with  large  valley-like 
spaces  between  them. 

Linear  coral-reefs  and  small  knolls  project  from  many 


268  APPENDIX. 

of  the  isolated,  as  well  as  from  the  coast  banks ;  sometimes 
they  are  irregularly  placed,  as  on  the  Mosquito  bank,  but 
more  generally  they  form  crescents  on  the  windward  side, 
situated  some  little  distance  within  the  outer  edge  : — thus 
on  the  Serranilla  bank  they  form  an  interrupted  chain  which 
ranges  between  two  and  three  miles  within  the  windward 
margin :  generally  they  occur,  as  on  Eoncador,  Courtown 
and  Anegada  banks,  nearer  the  line  of  deep  water.  Their 
occurrence  on  the  windward  side  is  conformable  to  the 
general  rule,  of  the  efficient  kinds  of  corals  flourishing  best 
where  most  exposed ;  but  I  cannot  explain  their  position 
some  way  within  the  line  of  deep  water  unless  it  be  that  a 
depth  somewhat  less  than  that  close  to  the  outer  margin  is 
most  favourable  to  their  growth.  Where  the  corals  have 
formed  a  nearly  continuous  rim,  close  to  the  windward 
edge  of  a  bank  some  fathoms  submerged,  the  reef  closely 
resembles  an  atoll ;  and  if  the  bank  surrounds  an  island  (as 
in  the  case  of  Old  Providence),  the  reef  resembles  an  encir- 
cling barrier-reef.  I  should  undoubtedly  have  classed  some 
of  these  fringed  banks  as  imperfect  atolls,  or  barrier-reefs, 
if  the  sedimentary  nature  of  their  foundations  had  not  been 
evident  from  the  presence  of  other  neighbouring  banks,  of 
similar  forms  and  of  similar  composition,  but  without  the 
crescent-like  marginal  reef.  In  the  third  chapter,  I  re- 
marked that  some  atoll-like  reefs  probably  did  exist,  which 
had  originated  in  the  manner  here  supposed. 

Proofs  of  elevation  within  recent  tertiary  periods  abound, 
as  referred  to  in  the  sixth  chapter,  over  nearly  the  whole 
area  of  the  West  Indies.  Hence  it  is  easy  to  understand 
the  origin  of  the  low  land  near  those  coasts  where  sedi- 
ment is  now  accumulating ;  for  instance,  on  the  northern 
part  of  Yucutan,  and  on  the  N.E.  part  of  Mosquito.  Hence, 
also,  the  origin  of  the  great  Bahama  banks,  which  are 
bordered  on  their  western  and  southern  edges  by  narrow, 
long,  singularly- shaped  islands,  formed  of  sand,  shells  and 


WEST    INDIES.  269 

coral-rock,  some  of  them  being  about  a  hundred  feet  in 
height,  is  easily  explained  by  theelevaticn  of  banks  fringed 
on  their  windward  sides  by  coral-reefs.  On  this  view,  how- 
ever, we  must  suppose  either  that  the  great  Bahama  sand- 
banks were  all  originally  deeply  submerged,  and  were 
brought  up  to  their  present  level  by  the  same  elevatory 
action  which  formed  the  linear  islands  ;  or  that  during  the 
elevation  of  the  banks,  the  superficial  currents  and  swell 
of  the  waves  wore  them  down,  and  kept  them  at  a  nearly 
uniform  level.  But  this  level  is  not  quite  uniform ;  for  in 
proceeding  from  the  N.W.  end  of  the  Bahama  group  towards 
the  S.E.,  the  depth  of  the  banks  increases,  and  the  area  of 
land  decreases,  in  a  very  gradual  and  remarkable  manner. 
The  view  that  these  banks  have  been  worn  down  by  the 
currents  and  waves  of  the  sea  during  their  elevation,  seems 
to  me  the  most  probable  one.  This  view  is  also,  I  believe, 
applicable  to  many  of  the  submerged  banks,  in  widely  dis- 
tant parts  of  the  West  Indian  sea  ;  for,  on  any  other  view, 
the  elevatory  forces  must  have  acted  with  astonishing  uni- 
formity. 

The  shore  of  the  Gulf  of  Mexico,  for  a  space  of  many 
hundred  miles,  is  formed  by  a  chain  of  lagoons,  frpm  1  to  20 
miles  in  breadth  (Columbian  Navigator,  p.  178,  &c.),  con- 
taining either  fresh  or  salt  water,  and  separated  from  the 
sea  by  linear  strips  of  sand.  The  shores  of  southern  Brazil, 
and  of  the  United  States  from  Long  Island  (as  observed  by 
Professor  Eogers,)  to  Florida,  have  the  same  character. 
Professor  Eogers,  in  his  report  to  the  British  Association 
(vol.  iii.  p.  13),  speculates  on  the  origin  of  these  low,  sandy, 
linear  islets ;  he  states  that  the  layers  of  which  they  are 
composed  are  too  homogeneous,  and  contain  too  large  a 
proportion  of  shells,  to  permit  the  common  supposition  of 
their  formation  being  simply  due  to  matter  thrown  up, 
where  it  now  lies,  by  the  surf:  he  considers  these  islands 
as  upheaved  bars  or  shoals,  which  were  deposited  in  lines 
19 


270  APPENDIX. 

where  opposed  currents  met.  It  is  evident  that  these  islands 
and  spits  of  sand  parallel  to  the  coast  and  separated  from 
it  by  shallow  lagoons,  have  no  necessary  connection  with 
coral-formation. 

Having  now  endeavoured  to  remove  some  sources  of 
doubt  in  classifying  the  reefs  of  the  West  Indies,  I  will 
give  my  authorities  for  colouring  such  portions  of  coast  as 
I  have  thought  myself  warranted  in  doing.  Captain  Bird 
Allen  informs  me  that  most  of  the  islands  on  the  Bahama 
Banks  are  fringed,  especially  on  their  windward  sides,  with 
living  reefs  ;  and  hence  I  have  coloured  those,  which  are 
thus  represented  in  Captain  Owen's  chart,  red.  The  same 
officer  informs  me,  that  the  islets  along  the  southern  part 
of  Florida  are  similarly  fringed ;  coloured  red. — Cuba  : 
proceeding  along  the  northern  coast,  at  the  distance  of  40 
miles  from  the  extreme  S.E.  point,  the  shores  are  fringed 
by  reefs,  which  extend  westward  for  a  space  of  160  miles, 
with  only  a  few  breaks.  Parts  of  these  reefs  are  represented 
in  the  plans  of  the  harbours  on  this  coast  by  Captain  Owen ; 
and  an  excellent  description  is  given  of  them  by  Mr.  Taylor 
(Loudon's  Mag.  of  Nat.  Hist.  vol.  ix.  p.  449) ;  he  states 
that  they  enclose  a  space  called  the  *  haxo,'  from  half  to 
three-quarters  of  a  mile  in  width,  with  a  sandy  bottom,  and 
a  little  coral.  In  most  parts  people  can  wade,  at  low  water, 
to  the  reef;  but  in  some  parts  the  depth  is  between  two 
and  three  fathoms.  Close  outside  the  reef,  the  depth  is 
between  six  and  seven  fathoms  :  these  well-characterized 
fringing-reefs  are  coloured  red. — Westward  of  long.  77°  80', 
on  the  northern  side  of  Cuba,  a  great  bank  commences, 
which  extends  along  the  coast  for  nearly  four  degrees  of 
longitude.  In  its  structure,  and  in  the  *  cays,'  or  low 
islands  on  its  edge,  there  is  a  marked  correspondence  (as 
observed  by  Humboldt,  Pers.  Narr.  vol.  vii.  p.  88)  between 
it  and  the  great  Bahama  and  Sal  Banks,  which  lie  directly 
in  front.     Hence  one  is  led  to  attribute  the  same  origin  to 


f 
WEST   INDIES.  271 

all  these  banks ;  namely,  the  accumulation  of  sediment, 
conjoined  with  an  elevatory  movement,  and  the  growth  of 
coral  on  their  outer  edges.  The  parts  which  are  fringed 
by  living  reefs  are  coloured  red. — Westward  of  these  banks 
there  is  a  portion  of  coast  apparently  without  reefs,  except 
in  the  harbours,  the  shores  of  which  seem  in  the  published 
plans  to  be  fringed. — The  Colorado  Shoals  (see  Captain 
Owen's  charts),  and  the  low  land  at  the  western  end  of 
Cuba,  correspond  as  closely  in  relative  position  and  struc- 
ture to  the  banks  at  the  extreme  point  of  Florida,  as 
the  banks  above  described  on  the  north  side  of  Cuba 
do  to  the  Bahamas.  The  depth  within  the  islets  and 
reefs  on  the  outer  edge  of  the  Colorados,  is  generally  be- 
tween two  and  three  fathoms,  increasing  to  12  fathoms  in 
the  southern  part,  where  the  bank  becomes  nearly  open, 
without  islets  or  coral-reefs  ;  the  portions  which  are  fringed 
are  coloured  red. — The  southern  shore  of  Cuba  is  deeply 
concave,  and  the  included  space  is  filled  up  with  mud  and 
sand-banks,  low  islands  and  coral-reefs.  Between  the 
mountainous  Isle  of  Pines  and  the  southern  shore  of  Cuba, 
the  general  depth  is  only  between  two  and  three  fathoms  ; 
and  in  this  part,  small  islands,  formed  of  fragmentary  rocks 
and  broken  madrepores  (Humboldt,  Pers.  Narr.  vol.  vii.  pp. 
61,  86  to  90,  291,  309,  320),  rise  abruptly,  and  just  reach 
the  surface  of  the  sea.  From  some  expressions  used  in  the 
Columbian  Navigator  (vol.  i.  pt.  ii.  p.  94),  it  appears  that 
considerable  spaces  along  the  outer  coast  of  southern  Cuba 
are  bounded  by  cliffs  of  coral-rock,  formed  probably  by  the 
upheaval  of  coral-reefs  and  sand-banks.  The  charts  re- 
present the  southern  part  of  the  Isle  of  Pines  as  fringed  by 
reefs,  which  the  Columb.  Navig.  says  extend  some  way  from 
the  coast,  but  have  only  from  9  to  12  feet  of  water  on 
them  ;  these  are  coloured  red. — I  have  not  been  able  to  pro- 
cure any  detailed  description  of  the  large  group  of  banks  and 
*  cays '  further  eastward  on  the  southern   side  of  Cuba ; 


272  APPENDIX. 

within  them  there  is  a  large  expanse,  with  a  muddy  bottom, 
from  8  to  12  fathoms  deep  :  although  some  parts  on  this 
line  of  coast  are  represented  in  the  general  charts  of  the 
West  Indies,  as  fringed,  I  have  not  thought  it  prudent  to 
colour  them.  The  remaining  portion  of  the  south  coast  of 
Cuba  appears  to  be  without  coral-reefs. 

YucuTAN. — The  N.E.  part  of  the  promontory  appears, 
in  Captain  Owen's  charts,  to  be  fringed ;  coloured  red. 
The  eastern  coast  from  20°  to  18°  is  fringed.  South  of  lat. 
18°,  there  commences  the  most  remarkable  reef  in  the 
West  Indies  :  it  is  about  130  miles  in  length,  ranging  in  a 
N.  and  S.  line,  at  an  average  distance  of  fifteen  miles  from 
the  coast.  The  islets  on  it  are  all  low,  as  I  have  been  in- 
formed by  Captain  B.  Allen ;  the  water  deepens  suddenly  on 
the  outside  of  the  reef,  but  not  more  abruptly  than  off  many 
of  the  sedimentary  banks  :  within  its  southern  extremity 
(off  Honduras)  the  depth  is  25  fathoms ;  but  in  the  more 
northern  parts,  the  depth  soon  decreases  to  10  fathoms,  and 
within  the  northernmost  part,  for  a  space  of  20  miles,  the 
depth  is  only  from  one  or  two  fathoms.  In  most  of  these 
respects  we  have  the  characteristics  of  a  barrier-reef ;  never- 
theless, £rom  observing,  first,  tLat  the  channel  within  the 
reef  is  a  continuation  of  a  great  irregular  bay,  which  pene- 
trates the  mainland  to  the  depth  of  50  miles ;  and  secondly, 
that  considerable  spaces  of  this  barrier-like  reef  (for  instance, 
in  lat.  16°  45^  and  16°  12')  are  described  in  the  charts  as 
formed  of  pure  sand;  and  thirdly,  from  knowing  that  sedi- 
ment is  accumulating  in  many  parts  of  the  West  Indies  in 
banks  parallel  to  the  shore  ;  I  have  not  ventured  to  colour 
this  reef  as  a  barrier.  To  add  to  my  doubts,  close  outside 
this  barrier-like  reef ,  Turneffe,  Lighthouse,  and  Glover  veeia 
are  situated,  and  these  have  so  completely  the  form  of  atolls, 
that  if  they  had  occurred  in  the  Pacific,  I  should  not  have 
hesitated  to  colour  them  blue.  Turneffe  Beef  seems  almost 
entirely  filled  up  with  low  mud  islets ;  and  the  depth  within 


WEST   INDIES.  273 

the  other  two  reefs  is  only  from  one  to  three  fathoms.  From 
this  circumstance,  and  from  their  similarity  in  form,  struc- 
ture, and  relative  position,  both  to  the  bank  called  Northern 
Triangles,  on  which  there  is  an  islet  between  70  and  80 
feet  in  height,  and  to  Gozumel  Island,  the  level  surface  of 
which  is  likewise  between  70  and  80  feet  high,  it  is  probable 
that  the  three  foregoing  banks  are  the  worn-down  bases  of 
upheaved  shoals,  fringed  with  corals  ;  left  uncoloured. 

In  front  of  the  eastern  Mosquito  coast  there  are,  between 
lat.  12°  and  16°,  some  extensive  banks  (already  mentioned), 
with  high  islands  rising  from  their  centres,  and  others  wholly 
submerged,  both  kinds  being  bordered,  near  their  windward 
margins,  by  crescent -shaped  coral-reefs.  But  it  can  hardly 
be  doubted  that  these  banks  owe  their  origin,  like  the  great 
bank  extending  from  the  Mosquito  promontory,  almost  en- 
tirely to  the  accumulation  of  sediment, and  not  to  the  growth 
of  corals  ;  hence  I  have  not  coloured  them. 

Cayman  Island  :  this  island  appears  in  the  charts  to  be 
fringed ;  and  Captain  B.  Allen  informs  me  that  reefs  extend 
about  a  mile  from  the  shore,  and  have  only  from  5  to  12  feet 
of  water  within  them  ;  coloured  red. — Jamaica  :  judging 
from  the  charts,  about  15  miles  of  .the  S.E.  extremity,  and 
about  twice  that  length  at  the  S.W.  extremity,  and  some 
portions  on  the  S.  side  near  Kingston  and  Port  Eoyal,  are 
regularly  fringed,  and  are  therefore  coloured  red.  From  the 
plans  of  some  harbours  on  the  N.  side,  parts  of  the  coast  ap- 
pear to  be  there  fringed  ;  but  I  have  not  coloured  them. — 
St.  Domingo :  I  have  not  been  able  to  obtain  sufficient 
information,  either  from  plans  of  the  harbours,  or  from 
general  charts,  to  enable  me  to  colour  any  part  of  the  coast, 
except  60  miles  from  Port  de  Plata  westward,  which  seems 
regularly  fringed :  many  other  parts,  however,  of  the  coast 
are  probably  fringed,  especially  towards  the  eastern  end  of 
the  island. — Puerto  Rico  :  considerable  portions  of  tho 
southern,  western,  and  eastern  coasts,  and  some  parts  of  the 


274  APPENDIX. 

northern  coast,  appear  in  the  charts  to  be  fringed ;  coloured 
red.  Some  miles  in  length  of  the  southern  side  of  the  Island 
of  St.  Thomas  is  fringed  ;  most  of  the  Virgin  Gorda  Islands, 
as  I  am  informed  by  Sir  R.  Schomburgk,  are  fringed  ;  the 
shores  of  Anegada,  as  well  as  the  bank  on  which  it  stands, 
are  likewise  fringed  ;  these  islands  have  been  coloured  red. 
The  greater  part  of  the  southern  side  of  Santa  Cruz  ap- 
pears in  the  Danish  survey  to  be  fringed  (see  also  Professor 
Hovey's  account  of  this  island,  in  Silliman's  Journal, 
vol.  XXXV.  p.  74) ;  the  reefs  extend  along  shore  for  a  consider- 
able space,  and  project  rather  more  than  a  mile  ;  the  depth 
within  the  reef  is  three  fathoms  ;  coloured  red. — The  An- 
tilles,  as  remarked  by  Von  Buch  (Descrip.  lies  Canaries, 
p.  494),  maybe  divided  into  two  linear  groups,  the  western 
row  being  volcanic,  and  the  eastern  of  modern  calcareous 
origin ;  my  information  is  very  defective  on  the  whole  group. 
Of  the  eastern  islands,  Barbuda  and  the  western  coasts  of 
Antigua  and  Mariagalante  appear  to  be  fringed ;  this  is  also 
the  case  with  Barbadoes,  as  I  have  been  informed  by  a  resi- 
dent ;  these  islands  are  coloured  red.  On  the  shores  of  the 
western  Antilles,  of  volcanic  origin,  very  few  coral-reefs 
appear  to  exist.  The  island  of  Martinique,  of  which  there 
are  beautifully  executed  French  charts  on  a  very  large  scale, 
alone  presents  any  appearance  worthy  of  special  notice. 
The  south-western,  southern,  and  eastern  coasts,  together 
forming  about  half  the  circumference  of  the  island,  are 
skirted  by  very  irregular  banks,  projecting  generally  rather 
less  than  a  mile  from  the  shore,  and  lying  from  two  to  fivo 
fathoms  submerged.  In  front  of  almost  every  valley,  they 
are  breached  by  narrow ,  crooked,  steep-sided  passages.  The 
French  engineers  ascertained  by  boring,  that  these  sub- 
merged banks  consisted  of  madreporitic  rocks,  covered  in 
many  parts  by  thin  layers  of  mud  or  sand.  From  this  fact, 
and  especially  from  the  structure  of  the  narrow  breaches, 
these  banks  were  probably  formed  by  living  reefs,  which 


BERMUDA   ISLANDS.  275 

fringed  the  shores  of  the  island,  and  once  reached  the  sur- 
face. From  some  of  these  submerged  banks  reefs  of  living 
coral  still  rise  abruptly,  either  in  small  detached  patches, 
or  in  lines  parallel  to,  but  some  way  within,  the  margin. 
Besides  the  above  banks  which  skirt  the  shores  of  the  island, 
there  is  on  the  eastern  side  a  range  of  linear  banks,  similarly 
constituted,  20  miles  in  length,  extending  parallel  to  the 
coast-line,  and  separated  from  it  by  a  space  between  two  and 
four  miles  in  width,  and  from  5  to  15  fathoms  in  depth. 
From  this  range  of  detached  banks,  some  linear  reefs  of 
living  coral  likewise  rise  abruptly  ;  and  if  they  had  been  of 
greater  length  (for  they  do  not  front  more  than  a  sixth  part 
of  the  circumference  of  the  island)  they  would  necessarily 
from  their  position  have  been  coloured  as  barrier-reefs ;  as 
the  case  stands,  they  are  left  uncoloured. 

Floeida. — An  account  of  the  reefs  on  this  coast,  toge- 
ther with  references  to  various  authorities,  will  be  found  in 
Professor  Dana's  work  on  Corals  and  Coral  Islands,  1872, 
p.  204.1 

The  Bermuda  Islands  have  been  carefully  described 
by  Lieut.  Nelson,  in  an  excellent  memoir  in  the  Geol. 
Transactions  (vol.  v.  part  i.  p.  103). ^  In  the  form  of  the 
bank  or  reef,  on  one  side  of  which  the  islands  stand,  there 
is  a  close  general  resemblance  to  an  atoll ;  but  in  the  fol- 
lowing respects  there  is  a  considerable  difference, — first,  in 
the  margin  of  the  reef  not  forming  (as  I  have  been  informed 
by  Mr.  Chaffers,  E.N.)  a  flat,  solid  surface,  which  is  laid 
bare  at  low  water  ;  secondly,  in  the  water  gradually  shoal- 
ing for  nearly  a  mile  and  a  half  in  width  round  the  entire 
reef,  as  may  be  seen  in  Captain  Hurd's  chart ;  and  thirdly, 
in  the  size,  height,  and  extraordinary  form  of  the  islands, 
which  present  little  resemblance  to  the  long,  narrow,  simple 

*  [See  Appendix  II.] 

*  [An  interesting  account  will  also  be  found  in  Sir  \Vyviil« 
Thomson,  Voyage  of  the  Challenger,  vol.  i.  chap,  iv.] 


276  APPENDIX. 

islets,  seldom  exceeding  half  a  mile  in  breadth,  which  sur. 
mount  the  annular  reefs  of  almost  all  the  atolls  in  the 
Indian  and  Pacific  Oceans.  Moreover,  there  are  evident 
proofs  (Nelson,  ibid.  p.  118)  that  islands  similar  to  the  ex- 
isting ones  formerly  extended  over  other  parts  of  the  reef. 
It  would,  I  believe,  be  difficult  to  find  a  true  atoll  with 
land  exceeding  30  feet  in  height ;  whereas,  Mr.  Nelson  es- 
timates the  highest  point  of  the  Bermuda  Islands  at  260 
feet ;  if,  however,  Mr.  Nelson's  view,  that  the  whole  land 
consists  of  sand  drifted  by  the  winds  and  agglutinated  to- 
gether, is  correct,  this  difference  would  be  immaterial ;  but, 
from  his  own  account  (p.  118),  there  occur  in  one  place 
five  or  six  layers  of  red  earth,  interstratified  with  the  ordi- 
nary calcareous  rock,  and  including  stones  too  heavy  for 
the  wind  to  have  moved,  without  having  at  the  same  time 
utterly  dispeised  every  grain  of  the  accompanying  drifted 
matter.  Mr.  Nelson  attributes  the  origin  of  these  several 
layers,  with  their  embedded  stones,  to  violent  catastrophes; 
but  further  investigation  has  generally  succeeded  in  ex- 
plaining such  phenomena  by  simpler  means.  Finally,  I 
may  remark  that  these  islands  bear  a  considerable  resem- 
blance in  shape  to  Barbuda  in  the  West  Indies,  and  to 
Pemba  on  the  eastern  coast  of  Africa,  which  latter  island 
is  about  200  feet  in  height,  and  consists  of  coral-rock. 
I  beheve  that  the  Bermuda  Islands,  from  being  fringed  by 
living  reefs,  ought  to  have  been  coloured  red ;  but  I  have 
left  them  uncoloured,  on  account  of  their  general  resem- 
blance in  external  form  to  a  lagoon-island  or  atoll.  Pro- 
fessor Dana  (Corals  and  Coral  Islands,  pp.  218,  269)  ranks 
them  in  this  class.^ 

•  [The  following  particulars  relating  to  Bermuda,  taken  from  the 
Beport  of  the  Challenger  Voyage,  Narrative,  p.  138,  are  of  interest: — 

An  excavation  made  to  form  a  bed  for  the  floating  dock  went 
down  to  60  feet  below  low- water  mark.  It  cut  through  calcareous 
mud,  loose  beds  (coral-sands  mixed  with  mollusks,  smaller  corals 
and  other  organisms),  passing  into  a  loosely  coherent  freestone 


BAR   OF   SANDSTONE.  277 

Stipplement  on  a  remarhdble  Bar  of  Sandstone  off  Per' 
namhuco,  on  the  Coast  of  Brazil.  (Originally  published 
in  the  Philosophical  Magazine,  October  1841,  p.  257.) 

In  entering  the  harbour  of  Pernambuco,  a  vessel  passes 
close  round  the  point  of  a  long  reef,  which,  viewed  at  high 
water  when  the  waves  break  heavily  over  it,  would  natu- 
rally be  thought  to  be  of  coral  formation,  but  when  beheld 
at  low  water  might  be  mistaken  for  an  artificial  breakwater, 
erected  by  cyclopean  workmen.  At  low  tide  it  shows  itself 
as  a  smooth  level-topped  ridge,  from  BO  to  60  yards  in  width, 
with  even  sides,  and  extending  in  a  perfectly  straight  line, 
for  several  miles  parallel  to  the  shore.  Off  the  town  it  in- 
cludes a  shallow  lagoon  or  channel  about  half  a  mile  in 
width,  which  further  south  decreases  to  scarcely  more  than 
a  hundred  yards.  Close  within  the  northern  point,  ships 
lie  moored  to  old  guns  let  into  the  reef.  Here,  on  the 
inner  side,  at  low  water  spring-tides,  a  section  of  about 
seven  feet  in  height  is  exhibited.  This  consists  of  hard 
pale-coloured  sandstone  breaking  with  a  smooth  fracture, 
and  formed  of  siliceous  grains,  cemented  by  calcareous 
matter.  Well-rounded  quartz  pebbles,  from  the  size  of  a 
bean,  rarely  to  that  of  an  apple,  are  embedded  in  it, 
together  with  a  very  few  fragments  of  shells.     Traces  of 

formed  of  the  same  material  cemented ;  and  then,  at  a  depth  of  45  feet, 
through  an  old  peat  with  land  vegetation,  shells  of  Helix  hermiidensis, 
and  bones  of  birds,  beneath  which  was  the  ordinary  hard  '  base  rock.' 
Serpulse  are  very  abundant  on  the  Bermuda  reefs,  and  form  evi- 
dently, by  their  mode  of  growth,  miniature  atolls  from  2  to  20  feet  in 
diameter,  with  little  interior  lagoons.  It  was  found  by  soundings 
thai  on  the  S.E.  edge  of  the  bank  the  100-fathom  line  was  about  11- 
mile  from  the  rocks  awash.  Then  a  slope  of  about  20°  led  down  to 
350  or  400  fathoms,  after  which  it  varied,  from  T  to  15°,  to  1,000 
fathoms.  The  100-fathom  line  on  the  N.E.  edge  was  about  3  milea 
away ;  on  the  S. W.  still  further,  and  the  submarine  slopes  were  more 
gentle.  The  rock  of  the  island  appears  to  be  of  aeolian  origin,  but  it 
is  not  said  whether  this  also  forms  the  highest  ground,  which  is  256 
feet  above  the  sea»] 


278  APPENDIX. 

stratification  are  obscure,  but  in  one  spot  there  was  an  in- 
cluded layer  of  stalactitic  limestone,  an  eighth  of  an  inch  in 
thickness.  In  another  place  some  false  strata,  dipping 
landwards  at  an  angle  of  45°,  were  capped  by  a  horizontal 
mass.  On  each  side  of  the  ridge  quadrangular  fragments 
have  subsided;  and  the  whole  mass  is  in  some  places 
fissured,  apparently  from  the  washing  out  of  some  soft 
underlying  bed.  One  day,  at  low  water,  I  walked  a  full 
mile  along  this  singular,  smooth,  and  narrow  causeway, 
with  water  on  both  sides  of  me,  and  could  see  that  for 
nearly  a  mile  further  south  its  form  remained  unaltered. 
In  Baron  Eoussin's  beautiful  chart  of  Pernambuco  {Le 
Pilote  du  BrdsU)  it  is  represented  as  stretching  on,  in  an 
absolutely  straight  hne,  for  several  leagues;  how  far  its 
composition  remains  the  same,  I  know  not ;  but  from  the 
accounts  I  received  from  intelligent  native  pilots,  it  seems 
to  be  replaced  on  some  parts  of  the  coast  by  true  coral- 
reefs. 

The  upper  surface,  though  it  must  on  a  large  scale  be 
called  smooth,  yet  presents,  from  unequal  disintegration, 
numerous  small  irregularities.  The  larger  embedded  peb- 
bles stand  out  supported  on  short  pedestals  of  sandstone. 
There  are,  also,  many  sinuous  cavities,  two  or  three  inches 
in  width  and  depth,  and  from  six  inches  to  two  feet  in 
length.  The  upper  edges  of  these  furrows  sometimes 
slightly  overhang  their  sides  ;  and  they  end  abruptly  with 
a  rounded  outline.  A  furrow  occasionally  branches  into 
two  arms,  but  generally  they  run  nearly  parallel  to  each 
other,  in  a  line  transverse  to  the  sandstone  ridge.  I  know 
not  how  to  account  for  their  origin  except  through  the 
washing  to  and  fro  of  pebbles  in  originally  slight  depres- 
sions, by  the  waves  which  break  daily  over  the  bar.  Op- 
posed to  this  notion  is  the  fact  that  some  of  these  furrows 
were  lined  with  numerous  small  living  ActinicB.  The 
exterior  surface  of  the  bar  is  coated  with  a  thin  layer  oi 


BAR    OF    SANDSTONE.  279 

calcareous  matter;  this,  on  the  outer  subsided  masses, 
which  can  be  reached  only  at  low  water,  between  the 
successively  breaking  waves,  is  so  thick  that  I  could 
seldom  expose  the  sandstone  by  the  aid  of  a  heavy  hammer. 
I  procured,  however,  some  fragments,  which  were  between 
three  and  four  inches  in  thickness,  and  consisted  chiefly  of 
small  Serpulcs,  including  some  Balani,  with  a  few  thin 
paper-like  layers  of  Q,NziUijpora.  The  surface  alone  is  alive, 
and  all  within  consists  of  the  above  organic  bodies,  filled  up 
with  dirty  white  calcareous  matter.  The  layer,  though  not 
hard,  is  tough,  and  from  its  rounded  surface  resists  the 
breakers.  Along  the  whole  external  margin  of  the  bar,  I 
only  saw  one  very  small  point  of  sandstone  which  was  ex- 
posed to  the  surf.  In  the  Pacific  and  Indian  Oceans  the 
outer  and  upper  margin  of  the  coral-reefs  are,  as  we  have 
seen,  protected  by  a  similar  coating ;  but  formed  almost 
exclusively  of  several  species  of  NuUiporce,  Lieut.  Nelson, 
in  his  excellent  memoir  on  the  Bermudas  (Geol.  Trans, 
vol.  V.  part  1,  p.  117),  says  that  the  reefs  there  are  formed 
of  similar  masses  of  SerpulcB  ;  but  I  suspect  that  they  are 
only  thus  coated. 

I  enquired  from  some  old  pilots  at  Pernambuco  whether 
there  was  any  tradition  of  the  bar  having  undergone  any 
change  during  the  lapse  of  time  ;  but  they  were  unanimous 
in  answering  me  in  the  negative.  It  is  astonishing  to  re- 
flect, that  although  waves  of  turbid  water,  charged  with 
sediment,  are  driven  night  and  day  by  the  ceaseless  trade 
wind  against  the  abrupt  edges  of  this  natural  breakwater, 
yet  that  it  has  lasted  in  its  present  perfect  state  for  cen- 
turies, or  perhaps  for  thousands  of  years.  Seeing  that  the 
surface  on  the  inner  side  does  gradually  wear  away,  as 
shown  by  the  pebbles  on  the  little  sandstone  pedestals,  this 
durability  must  be  entirely  due  to  the  protection  afforded 
by  the  thin  coating  of  Scr;pul<B  and  other  organic  bodies, 


280  APPENDIX. 

This  is  a  fine  example  of  what  apparently  inefiScient  meana 
may  be  effectual.^ 

I  believe  that  similar  bars  of  rock  occur  in  front  of  some 
of  the  other  bays  and  rivers  on  the  coast  of  Brazil :  Baron 
Eoussin  states  that  at  Porto  Seguro  there  is  a  *  quay  '  simi- 
lar to  that  of  Pernambuco.  Spaces  of  several  hundred 
miles  in  length  on  the  shores  of  the  Gulf  of  Mexico,  the 
United  States,  and  of  Southern  Brazil  are  formed  by  long 
narrow  islands  and  spits  of  sand,  including  extensive  shal- 
low lagoons,  some  of  which  are  several  leagues  in  width. 
The  origin  of  these  linear  islets  is  rather  obscure :  Pro- 
fessor Rogers  (Report  to  British  Association,  vol.  iii.  p.  13) 
gives  reasons  for  suspecting  that  they  have  been  formed  by 
the  upheaval  of  sand-banks,  deposited  where  currents 
formerly  met.  The  bar  of  sandstone  at  Pernambuco  has 
probably  been  formed  in  an  analogous  manner.  The  town 
stands  partly  on  a  low  narrow  islet  and  partly  on  a  long 
spit  of  sand,  in  front  of  a  low  shore,  bounded  in  the  distance 
by  a  semicircle  of  hills.  By  digging  at  low  water  near  the 
town,  the  sand  is  found  consolidated  into  sandstone,  similar 
to  that  of  the  bar,  but  containing  many  more  shells.  If, 
then,  the  nucleus  of  a  spit  of  sand,  extending  in  front  of 
the  bay,  had  formerly  become  consolidated,  a  small  change, 
probably  of  level,  but  perhaps  merely  in  the  currents,  might 
have  given  rise,  by  washing  away  the  loose  sand,  to  a  struc- 
ture like  that  of  the  bar  in  front  of  Pernambuco  and  along 
the  coast  southward  of  it ;  but  without  the  protection  af- 
forded by  the  successive  growth  of  the  above-named  organic 
beings,  its  duration  would  have  been  short. 

•  [There  is  an  interesting  account  of  this  reef,  containing  particulars 
of  some  borings  undertaken  in  1874,  by  Mr.  J.  C.  Hawkshaw,  in  the 
Quarterly  Journal  of  the  Geological  Society  for  1879  (vol.  xxxv.  p.  239).] 


APPENDIX  II. 

SUMMAEY  OF  THE  PKINCIPAL  CONTKIBUTIQNS  TO  THH 
HISTOBY  OF  COKAL  EEEFS   SINCE   THE   YEAR  1874 

By  Professor  T.  G.  Bonney,  D.Sc,  LL.D.,  F.E.a 

Since  the  publication  of  the  last  edition  of  Mr.  Darwin' 3 
work  several  important  researches  have  been  ur^dertaken, 
which  have  added  largely  to  the  stock  of  knowledge  con- 
cerning marine  physical  geography  in  general  and  coral 
reefs  in  particular.  Of  the  valuable  material  thus  obtained 
Mr.  Darwin  would,  no  doubt,  have  availed  himself  had  his 
life  been  spared  and  his  health  allowed.  Probably  addi- 
tions would  have  been  made  to  the  text  of  this  work,  and 
not  a  few  pages  have  been  rewritten ;  the  older  and  less 
precise  information  being  replaced  by  the  more  ample  and 
exact  results  of  receiit  eApioraciunb.  The  criticism  to 
which  Mr.  Darwin's  theory  has  been  subjected  during  the 
last  few  years,  and  the  hypotheses  which  have  been  ad- 
vanced by  other  workers,  would,  no  doubt,  have  been  dis- 
cussed by  him  in  that  candid  and  philosophic  spirit  which  is 
so  evident  in  all  his  writings.  This  revision  may  be  counted 
as  one  of  the  heavy  losses  which  science  has  suffered  by  his 
death.  It  became,  then,  a  question,  when  a  new  edition 
of  this  book  was  called  for,  what  should  be  done  to  it. 
Simply  to  reprint  the  last  edition,  without  any  notice 
of  the  important  contributions  which  have  been  made  to 
the  knowledge  of  the  subject  during  the  last  few  years, 
seemed  undesirable ;  but  any  attempt  to  reconstruct  the 
work  seemed  yet  more  undesirable.    Mr.  F.  Darwin,  after 


*82  APPENDIX  n. 

consultation  with  several  of  his  friends  (including  myself, 
"wliom  he  had  asked  to  aid  him  in  preparing  the  new 
edition  for  the  press),  accordingly  decided  to  reprint  *  The 
Structure  and  Distribution  of  Coral  Keefs  '  from  the 
edition  of  1874,  subject  only  to  a  few  press  corrections,  and 
to  give  any  important  emendations  or  additions  in  the 
form  of  notes,  so  arranged  as  to  be  easily  distinguished 
from  those  written  by  the  late  author.  As  regards  the 
extent  and  amount  of  the  additional  matter,  we  thought 
that,  as  the  volume  was  never  intended  as  a  text-book 
for  examination  purposes,  it  was  needless  to  endeavour  to 
concentrate  within  its  pages  every  result  of  recent  work, 
and  it  would  suffice  to  call  attention  to  the  more  important 
points,  which  would  almost  certainly  have  been  noticed  by 
the  author  in  any  new  edition. 

Therefore,  from  a  few  papers  left  by  Mr.  Darwin,  from 
information  kindly  supplied  by  Capt.  Wharton  and  other 
friends,  and  from  my  own  reading,  I  have  added  a  few  foot- 
notes to  the  text,  and  have  given  in  this  appendix  a  summary 
of  the  papers  which  appeared  to  me  of  special  importance 
among  those  which  have  been  published  since  1874.  No 
atiempc  has  been  made  lo  compile  a  bibliography  of  the 
literature  of  *  Coral  Eeefs.*  This  was  a  task,  as  I  told  Mr. 
F.  Darwin  candidly  at  the  outset,  which  my  previous  studies 
and  present  occupations  would  not  permit  me  to  undertake, 
and  it  was  also  one  which,  for  the  reason  above  given,  seemed 
to  me  needless.  I  believe,  however,  that  I  have  looked 
through  most  of  the  recent  literature,  and  I  have  selected 
therefrom  certain  papers,  in  which,  as  it  seemed  to  me,  the 
arguments  for  and  against  Mr.  Darwin's  theory  were 
stated  with  considerable  fulness.  The  remainder  have  been 
passed  over,  either  because  they  did  not  contain  original 
information,  or  because  they  would  have  supplied  additional 
details,  on  the  one  side  or  the  other,  rather  than  fresh 
arguments.   In  making  this  selection  I  have  been  influenced 


VIEWS    OF  MR.    MURRAY.  283 

to  some  extent  by  the  adventitious  prominence  which, 
during  the  last  two  or  three  years,  has  been  given  to  certain 
valuable  and  interesting  communications  on  this  subject. 
Of  the  papers  selected  I  have  given  a  fairly  full  abstract, 
which  represents,  to  the  best  of  my  ability,  the  views  of 
their  authors,  whichever  side  they  may  have  espoused,  so 
that  I  trust  the  reader  will  be  enabled  to  understand  the 
present  state  of  this  difficult  question,  and  to  appreciate 
the  reasons  which  have  led  some  very  competent  authorities 
to  maintain,  and  others  to  reject,  the  theory  advanced  by  the 
late  Mr.  Darwin. 

It  is  true  that,  as  I  have  stated  in  the  conclusion,  the 
close  study  of  the  question  has  not  materially  altered  the 
view  which  I  entertained  when  I  began  the  task,  but  I  have 
done  my  best  to  make  my  abstract  a  fair  statement  of  each 
writer's  case.  If,  then,  it  should  appear  to  any  one  that  I 
ought  to  have  given  more  prominence  to  this  point  and 
less  to  that,  I  may  fairly  plead  that  this  has  resulted  from 
deficient  apprehension,  and  not  from  conscious  bias.  I 
have  placed  first,  arranging  them  chronologically,  the 
papers  which  are  more  or  less  unfavourable  to  the  dis- 
tinctive feature  of  Mr.  Darwin's  theory ;  then  those  which 
in  the  main  support  it. 

The  following  is  a  summary  of  Mr.  Murray's  ^  views : 
Very  nearly  all  oceanic  islands,  other  than  coral  atolls,  are 
now  known  to  be  of  volcanic  origin.  Hence  it  is  probable 
that  the  foundations  of  the  latter  are  volcanic  rocks  and  not 
those  indicative  of  an  ancient  and  pre-existent  land.  As 
shown  by  the  soundings  of  the  '  Tuscarora'  and  'Challenger,* 
numerous  submarine  elevations  exist  which  rise  from  depths 
of  2,500  to  3,000  fathoms  to  within  a  few  hundred  fathoms 
of  the  surface.     The  upper  water  of  the  ocean  (to  a  depth, 

•  On  the  structure  and  Origin  of  Coral  Reefs  and  Islands.  By 
John  Murray.     Proc.  R.  S.  Edin.  (1880),  vol.  x.  p.  505. 


284  ArrENDix  ii. 

probably,  of  about  100  fathoms)  teems  with  organism?, 
calcareous  and  siliceous ;  such  as  algae,  protozoa,  hydrozoa, 
niollusca  and  other  members  of  the  animal  kingdom  :  these 
are  drifted  by  the  currents  from  place  to  place ;  by  these 
the  reef-building  corals  are  supplied  with  food.  It  has  been 
estimated,  as  the  result  of  experiment,  that  a  mass  of 
ocean  water  one  mile  square  and  100  fathoms  deep  con- 
tains more  than  sixteen  tons  of  carbonate  of  lime. '  After 
death  the  *  skeletons '  of  these  organisms  are  showered 
down  upon  the  bed  of  the  ocean.  In  water  which  exceeds 
Bome  800  or  900  fathoms  in  depth  their  remains  are  more 
or  less  affected  by  the  solvent  power  of  the  carbonic  acid 
present  in  the  water,  but  at  less  depths  they  accumulate. 
Thus  any  submarine  bank  which  rose  within  the  above- 
named  depth  would  be  brought  nearer  to  the  surface,  and 
its  upper  part,  as  the  water  above  it  shallowed,  would  be 
colonised  by  larger  pelagic  organisms ;  these,  after  death, 
would  augment  by  their  remains  the  increasing  pile  of 
material,  which  at  last  would  arrive  within  the  bathy- 
metrical  zone  in  which  reef-building  corals  can  live  and  the 
formation  of  an  atoll  would  commence. 

As  already  pointed  out  by  Mr.  Darwin,^  the  corals  on 
the  outer  margin  of  a  bank  grow  vigorously,  while  the 
diminution  of  food  and  the  increase  of  sediment  tend  to 
check  the  development  of  those  in  the  inner  part.     Thus, 

'  I  estimate  that  this  amount  of  carbonate  of  lime  is  equivalent 
to  a  solid  layer  of  the  same  area  which  is  approximately  "00009  of  an 
inch  thick.  We  may  arrive  at  it  thus:  taking  2-7  as  the  specific 
gravity  of  carbonate  of  lime,  we  shall  find  the  volume  of  sixteen  tons 
to  be  about  212-4  cubic  feet,  or  7*8  cubic  yards.  This  has  to  be  spread 
out  over  3,097,600  square  yards  (the  number  of  square  yards  in  a 
mile),  giving  the  above  result.  Even  if  we  make  a  large  allowance 
for  the  fact  that  the  carbonate  of  lime  is  not  solid,  but  in  the  form 
of  an  aggregate  of  hollow  shells,  I  believe  '09  of  an  inch  is  in  excess 
rather  than  in  defect  of  the  truth. 

■'  Page  87 


DEVELOPMENT   OF   ATOLLS.  285 

while  the  reef  is  still  several  fathoms  below  the  surface, 
the  corals  in  the  central  part  are  placed  at  a  disadvantage, 
which  becomes  greater  as  they  are  left  behind  in  the  up- 
ward race  by  their  neighbours.  In  a  small  reef,  the  peri- 
phery for  the  supply  of  food  to  the  interior  is  relatively 
large  ;  thus  the  lagoons  in  small  atolls  are  also  small  and 
are  soon  filled  up,  while  long  and  narrow  banks  have  no 
lagoons.  As  the  reef  becomes  larger  the  conditions 
become  more  favourable  to  the  formation  of  lagoons,  for 
(as  is  shown  by  experiment)  the  lagoon  of  such  an  atoll 
is  less  rich  in  pelagic  life  than  the  exterior  water.  Thus 
growth  is  checked ;  many  species  of  coral  die,  and  their 
calcareous  *  skeletons '  are  exposed  to  the  solvent  action 
of  sea-water.  "When  the  water  outside  becomes  too  deep 
for  reef-building  corals  to  live,  the  d&hris  from  the  exist- 
ing reef,  aided  by  the  accumulation  of  organisms,  forms  a 
talus  at  the  foot  of  its  submarine  cliffs,  and  thus  the  reef 
spreads  slowly  outward,  *  like  a  fairy  ring,'  on  foundations 
to  which  its  own  materials  have  contributed.  Coral-reefs 
which  have  been  elevated  for  some  distance  above  sea- 
level  are  frequently  found  to  rest  upon  a  deposit  thus  con- 
stituted.^ The  lagoon  channels  have  in  many  cases  been 
subsequently  formed  by  the  solvent  action  of  sea-water,  and 
the  islets  in  the  lagoon  channel  are  parts  of  the  original 
veef  still  left  standing.  Where  the  reefs  rise  quite  up  to 
the  surface  and  are  nearly  continuous,  there  is  little  coral 
growth  in  the  lagoon  or  its  channels  ;  where  the  outer  reefs 
are  much  broken  up  the  growth  is  relatively  abundant. 

*  At  the  Admiralty  Islands,  on  the  lagoon  side  of  the 
islets  of  the  barrier-reefs,  the  trees  were  found  overhanging 
the  water,  and  in  some  cases  the  soil  was  washed  away  from 
their  roots.  It  is  a  common  observation  in  atolls  that  the 
islets  on  the  reefs  are  situated  close  to  the  lagoon  shore. 

'  The  case  of  Tahiti  is  here  described ;  see  p.  314,  where  it  is  aia- 
cusBed  by  Prof.  Dana. 

20 


286  APPENDIX  n. 

These  facts  point  out  the  removal  of  matter  which  is  going 
on  in  the  lagoons  and  lagoon  channels.' 

Elevation,  not  subsidence,  is  to  be  expected  in  a  volcanic 
region,  as  there  is  an  a  priori  reason  for  attributing  the 
phenomena  of  coral  reefs — as  resting  on  volcanic  foun- 
dations—to elevation  rather  than  to  subsidence.  The 
former  hypothesis  appears  to  Mr.  Murray  to  accord  with  all 
the  facts  indicated  by  the  published  charts  of  coral-reefs, 
and  thus  is  considered  by  him  preferable  to  the  latter. 

Mr.  Murray's  general  conclusions  may  be  briefly  enun- 
ciated as  follows : — 

1.  That  foundations  have  been  prepared  for  barrier-reef8 
and  atolls  by  the  disintegration  of  volcanic  islands,  and  by 
the  building  up  of  submarine  volcanoes,  and  by  the  depo- 
sition on  their  summits  of  organic  and  other  sediments. 

2.  That  the  chief  food  of  the  corals  consists  of  the  abun- 
dant pelagic  life  of  the  tropical  regions,  and  the  extensive 
solvent  action  of  sea-water  is  shown  by  the  removal  of  the 
carbonate  of  lime  shells  of  these  surface  organisms  from 
the  greater  depths  of  the  ocean. 

3.  That  when  coral  plantations  build  up  from  sub- 
marine banks  they  assume  an  atoll  form,  owing  to  the 
more  abundant  supply  of  food  to  the  outer  margins  and 
the  removal  of  dead  coral-rock  from  the  interior  portions 
by  currents  and  the  action  of  the  carbonic  acid  dissolved 
in  the  water. 

4.  That  barrier-reefs  have  built  out  from  the  shore  on  a 
foundation  of  volcanic  ddbris  or  on  a  talus  of  coral  blocks, 
coral  sediment  and  pelagic  shells,  and  the  lagoon  channel 
is  formed  in  the  same  way  as  a  lagoon. 

5.  That  it  is  not  necessary  to  call  in  subsidence  to 
explain  any  of  the  characteristic  features  of  barrier-reefs 
or  atolls,  and  that  all  their  features  would  exist  alike  in 
areas  of  slow  elevation,  of  rest,  or  of  slow  subsidence. 


VIEWS    OF    PROF.    A.   AGASSIZ.  287 

Professor  A.  Agassiz  ^  accepts  the  views  of  L.  Agassiz, 
Le  Conte,  and  E.  B.  Hunt,  that  the  Florida  reefs  can- 
not be  explained  by  subsidence,  but  that  the  southern 
extremity  of  Florida  is  of  comparatively  recent  growth, 
consisting  of  concentric  barrier-reefs  which  have  been 
gradually  converted  into  land  by  the  accumulation  of  in- 
tervening mud-flats,  and  thus  explains  the  details  of  the 
process  and  the  manner  in  which  the  foundations  of  the 
reefs  are  formed. 

He  rejects  Le  Conte's  explanation  that  the  substructure 
of  the  reefs  was  formed  by  the  mass  of  material  brought 
by  the  Gulf  Stream,  pointing  out  that  more  recent  investi- 
gations have  shown  that  it  ran  across,  not  parallel  with,  the 
peninsula,  the  curve  of  the  eastern  shore  of  the  latter  being 
due  to  a  counter-current  along  the  reef  running  westwards. 
The  Gulf  Stream,  however,  has  an  indirect  influence  by 
reason  of  the  abundant  food  which  it  supplies  to  animals 
living  on  the  Bank  of  Florida.  Across  the  reefs,  and 
through  the  channels  between  the  Keys,  the  tides  set 
strongly,  bearing  the  mud  derived  from  coral  and  other 
organisms ;  this  gradually  accumulates  to  form  the  inter- 
vening mud-flats,  and  when  swept  westwards  enlarges  the 
submarine  plateau  in  that  direction.  The  Tortugas,  the 
most  recent  cluster  of  Florida  reefs,  are  at  the  very  ex- 
tremity of  the  slope  upon  which  the  line  of  these  reefs 
has  been  built  up.    Nothing  among  them  corresponds 

*  Agassiz,  Alexander.  The  Tortugas  and  Florida  Eeefs.  Mem. 
Amer.  Acad.  Arts  and  Sci.,  vol.  xi.  p.  107,  1885.  In  Three  Cruises  of 
the  Blake,  vol  i.  (Bulletin  of  the  Museum  of  Comparative  Zoology, 
Harvard  College,  vol.  xiv.,  1888),  a  chapter  is  devoted  to  the  Florida 
Keefs.  As,  however,  the  line  of  argument  and  the  principal  facts  are 
identical  with  those  given  above,  I  have  not  thought  it  necessary  to 
give  a  separate  analysis.  A  convenient  and  clear  summary  of  the 
views  of  Semper,  Eein,  Murray,  and  Agassiz  is  given  by  Prof.  A.  Geikie 
in  his;  presidential  address  to  the  Royal  Physical  Society  of  Edinburgh 
in  1883  (Proc.  vol.  viii.  p.  1). 


288  APPENDIX   II. 

with  the  extensive  mud-flat  which  extends  at  a  depth  of  a 
few  feet  below  the  surface  northward  of  the  Keys.  Where 
there  is  a  larger  accumulation  of  material  than  usual  on 
the  submarine  plateau,  so  as  to  bring  its  surface  within  the 
depth  at  which  corals  can  flourish,  a  reef  begins  to  form  ; 
that  is  the  history  of  the  Tortugas.  West  of  it,  an  in- 
cipient reef  may  be  found  now  in  process  of  formation, 
east  of  it  all  the  reefs  in  their  turn  have  had  a  like  origin. 
Then  the  deposition  of  silt  produces  mud-flats,  and  material 
'accumulates,  till  at  last  the  channels  are  closed  and  the 
whole  is  added  to  the  land.  From  Everglades  to  Cape 
Sable  the  work  may  be  seen  completed ;  on  the  eastern 
coast,  and  beyond  the  latter  place  to  Marquesas  Key,  it 
occurs  in  its  various  stages,  until  at  last  it  is  shown  in  its 
beginning.  The  backbone,  however,  of  the  Florida  penin- 
sula is  ascribed  to  a  fold  in  the  earth's  crust  in  an  earlier 
geological  period.  As  a  secondary  result  of  this~  a  great 
submarine  plateau  was  formed  directly  in  the  track  of  the 
Gulf  Stream,  which  has  since  been  gradually  built  up  by 
the  accumulation  of  marine  organisms  of  various  kinds. 
The  area  within  the  100-fathom  line  on  the  west  coast  of 
the  great  Florida  plateau  is  extraordinarily  rich  in  organic 
life ;  large  fragments  of  the  modern  limestone  were  often 
brought  up  in  trawl  or  dredge,  consisting  of  the  solid  parts 
of  the  very  species  which  now  live  on  the  top  of  this 
recent  limestone.  West  of  the  western  shore-line  Florida 
now  stretches  out  as  an  immense  submarine  plateau, 
forming  a  huge  tongue,  coated  or  veneered  only  by 
coral  limestone  over  its  very  top.  The  eastern  and 
western  edges  of  Florida  consist  of  recent  limestones, 
predecessors  of  that  now  forming  on  the  western  and 
southern  slopes  of  the  Florida  plateau.  Very  probably 
the  part  of  the  peninsula  north  of  the  Everglades  has 
had  in  the  past  a  like  formation.  Pourtales  plateau  ia 
built  of  the  same  species  of  corals  and  shells  as  now  live 


THE  FLORIDA  PENINSULA.        289 

upon  it.  Of  like  origin  are  the  great  bank  east  of  the 
Mosquito  coast  and  the  reefs  on  the  south  coast  of  Cuba  ; 
the  Basse-Terre  of  Guadeloupe  is  the  same,  now  slightly 
elevated,  and  the  barrier-reefs  on  the  windward  side  of  the 
West  India  Islands  rest  on  plateaux  of  similar  origin.  At 
Barbados  the  nucleus  is  a  trachytic  mass  round  which 
are  terraces  formed  of  moUusca  and  radiata,  still  living  in 
the  sea,  which  have  been  successively  lifted.^  The  author 
considers  that  in  the  West  India  Islands  many  volcanic 
masses,  which  probably  have  never  reached  the  surface, 
form  the  foundation  of  these  banks  of  organisms. 

It  would  seem  probable  that  reef-building  corals  had 
little  to  do  with  building  the  peninsula  north  of  Cape 
Florida.  The  author  explains  the  Alacran  reef  (atoll- 
shaped)  by  a  growth  of  corals  upwards  from  a  submarine 
bank,  and  shows  that  the  slope  is  steep  down  to  a  depth  of 
thirty  fathoms,  then  more  gradual. 

He  lays  much  stress  on  the  importance  of  currents 
bringing  food,  and  points  out  that,  on  the  lee  side  of  a 
reef,  corals  may  be  killed  by  the  drift  of  sediment.  *  When 
Darwin  wrote,  and  when  we  knew  little  of  the  limestone 
deposits  formed  by  the  accumulation  of  the  ddbris  of  mol- 
luscs, echinoderms,  polyps,  and  the  like,  upon  folds  of  the 
earth's  crust,  the  basal  parts  of  barrier-reefs  were  difficult 
of  explanation.  The  evidence  gathered  by  Murray,  Semper, 
and  myself,  partly  in  districts  which  Darwin  had  already 
examined,  and  partly  in  regions  where  his  theory  of  reef- 
formation  never  seemed  to  find  its  proper  application,  has 
in  part  removed  this  difficulty.  It  tends  to  show  that  we 
must  look  to  many  other  causes  than  those  of  elevation 

•  In  Three  Cruises  of  the  Blake,  vol.  i.  p.  79,  Prof.  A.  Agassiz  says : 
•  111  some  instances  coral  reefs  have  unquestionably  been  upHfted. 
I  have  seen  the  elevated  reefs  of  Cuba,  of  San  Domingo,  and  other 
West  Indian  Islands,  and  of  Barbados,  which  are  perhaps  the  most 
striking  examples  of  elevated  reefs.' 


290  APPENDIX   II. 

and  subsidence  for  a  satisfactory  explanation  of  coral-reef 
formation.  All-important  among  these  causes  are  the 
prevailing  winds  and  currents,  the  latter  charged  with 
sediment  which  helps  to  build  extensive  plateaux  from 
lower  depths  to  levels  at  which  corals  can  prosper.  This 
explanation,  tested  as  it  has  been  by  penetrating  into  the 
thickness  of  the  beds  underlying  the  coral  reefs,  seems  a 
more  natural  one,  for  many  of  the  phenomena  at  least, 
than  that  of  the  subsidence  of  the  foundation  to  which  the 
great  vertical  thickness  of  barrier-reefs  has  been  hitherto 
referred.  It  is,  however,  difficult  to  account  for  the  great 
depth  of  some  of  the  lagoons — forty  fathoms— on  any  other 
theory  than  that  of  subsidence  '  (p.  121). 

The  author  also  describes  the  distribution  of  material, 
living  and  dead,  on  the  Tortugas,  the  action  of  the  waves 
in  pounding  up  dead  coral,  molluscs,  and  other  organisms. 
Thus  a  great  quantity  of  calcareous  ooze  is  formed  (aided 
by  the  material  which  passes  through  the  digestive  cavity  of 
holothurians,  echinoderms,  &c.).  This  silt,  by  iis  accumu- 
lation, kills  the  corals,  which  accordingly  can  only  flourish 
where  well  *  scoured.'  The  water  is  often  chalk  colour  for 
a  considerable  distance  from  the  reefs ;  it  is  sometimes, 
after  a  heavy  wind,  discoloured  for  six  to  ten  miles  from 
the  outer  reefs.  This  process  accounts  for  the  scarcity  of 
fossils.  He  also  expresses  the  opinion  that  in  this  region 
the  corals  do  not  flourish  at  depths  over  six  or  seven 
fathoms,  being  probably  choked  by  the  ooze.^ 

'  In  Three  Cruises  of  the  Blake,  vol.  i.  p.  74,  Prof.  A.  Agassiz 
states  that '  all  the  evidence  accumulated  by  Dana,  Darwin,  Ehrenberg, 
Quoy,  and  Gaimard  tends  to  show  that  the  limit  of  reef-building 
corals  is  to  be  found  at  about  twenty  fathoms.' 

Prof.  Agassiz's  views  in  regard  to  Florida  do  not  appear  to  have 
met  with  universal  acceptance  among  American  men  of  science  ;  for 
instance,  Mr.  W.  H.  Dall  (Geology  of  Florida,  Amer.  Journ.  Sci., 
3rd  ser.  xxxiv.  p.  161,  1877)  says  that  in  the  southern  part  of  Florida 
he  saw  no  coral-rock  or  coral-reef  formation  :  '  The  coral  formation 


VIEWS   OF   MK.    GUPPY.  291 

Mr.  Guppy '  describes  the  Solomon  ArcMpelago,  which 
includes  seven  or  eight  large  islands,  some  being  from 
seventy  to  eighty  miles  in  length,  and  the  highest  rising 
from  8,000  to  10,000  feet  above  the  sea,  with  a  great 
number  of  smaller  islands  and  islets,  some  of  volcanic  and 
others  of  recent  calcareous  formations.  The  author  found 
exploration  to  be  difficult  and  dangerous,  but  believes  that 
he  saw  enough  to  give  him  a  fair  idea  of  the  leading  types 
of  structure  among  these  islands.  The  observations  re- 
corded in  the  paper  may  be  summed  up  in  his  own 
words : — 

The  islands  examined  indicate  upheaval,  in  some  cases 
to  at  least  1,200  feet.  'There  are,  in  the  first  place, 
numerous  small  islands  and  islets,  less  than  a  hundred 
feet  in  height,  which  are  composed  entirely  of  coral  lime- 
stone. Then  there  are  islands  of  larger  size,  which  are 
composed  in  bulk  of  partially  consolidated  volcanic  muds, 
such  as  are  at  present  forming  around  oceanic  volcanic 
islands.  Coral  limestones  encrust  the  lower  slopes  of 
these  islands,  and  do  not  attain  a  greater  thickness  than 
150  feet.  In  the  next  place  we  have  islands  of  similar 
structure,  but  possessing  in  their  centre  some  ancient 
volcanic  peak  that  was  once  submerged.     Then  there  are 

observed  by  Agassiz  in  the  region  in  the  Keys  must  be  of  very 
limited  scope,  as  it  has  not  been  identified  from  the  mainland  of 
Florida  by  any  modern  geologist.'  Further,  Prof.  A.  Heilprin  in  a 
paper  on  Explorations  in  Florida  (Transactions  Wagner  Inst.  Sci. 
Philadelphia,  May  1887),  noticed  in  the  above-named  volume  (p.  230), 
Bays :  '  No  observed  facts  sustain  the  coral  theory  of  formation  pro- 
pounded by  Agassiz.  They  prove,  on  the  contrary,  that  the  coral 
tract  of  Florida  is  confined  to  a  border  region  on  the  south  and 
south-east,  and  there  are  no  tertiary  reefs  whatever.'  But  he  admits 
that  the  southern  area  is  one  of  shallow  sea  formation,  so  that  there 
has  been  a  gradual  uniform  progressive  elevation  over  the  whole. 

'  H.  B.  Guppy.  Observations  on  the  Kecent  Calcareous  Forma- 
tions of  the  Solomon  Group  made  during  1882-4.  Tr.  li.  S.  Edin, 
sxxii.  p,  545  (1884-5). 


292  APPENDIX   11, 

islands  in  which  the  volcanic  peak  has  become  an  eccentric 
nucleus,  from  which  line  after  line  of  barrier-reef  has  been 
advanced,  overlying  the  volcanic  muds ; '  islands  in  which 
he  did  not  find  the  coral  limestone  of  a  thickness  of  100 
feet.  Then  we  have  the  upraised  atoll,  such  as  Santa 
Anna,  which  within  the  small  compass  of  a  height  of 
470  feet  displavs  the  several  stages  of  its  growth :  *  first, 
the  originally  submerged  volcanic  peak,  then  the  investing 
soft  deposit,  and  over  all  the  ring  of  coral  limestone,  that 
cannot  far  exceed  150  feet  in  thickness ;  lastly,  we  come  to 
the  mountainous  islands  formed  of  old  volcanic  rocks,  such 
as  St.  Christoval,  which,  although  over  4,000  feet  in  height, 
showed  to  me  no  calcareous  envelopes  at  a  greater  height 
than  500  feet  above  the  sea,  the  coral  limestone  crust 
being  even  thinner  than  at  the  smaller  and  more  recent 
islands.'  From  these  considerations  the  author  concludes 
*  (1)  that  these  upraised  reef  masses,  whether  atoll,  barrier- 
reef,  or  fringing-reef,  were  formed  in  a  region  of  elevation ; 
(2)  that  such  upraised  reefs  are  of  moderate  thickness, 
their  virtual  measurement  not  exceeding  the  limit  of  the 
depth  of  the  reef-coral  zone,  i.e.  not  more  than  about  150 
feet ;  (3)  that  these  upraised  reef  masses  in  the  majority 
of  islands  rest  on  a  partially  consolidated  deposit  which 
possesses  the  characters  of  the  "  volcanic  muds  "  which 
were  found  during  the  *  Challenger '  expedition  to  be  at 
present  forming  around  volcanic  islands ;  (4)  that  this 
deposit  envelopes  anciently  submerged  volcanic  peaks.' 

The  earlier  part  of  the  next  paper  ^  is  occupied  by  a  de- 
scription of  a  reef  of  the  Solomon  group  and  the  distribution 
upon  it  of  coral  life.  According  to  Mr.  Guppy's  observations 
the  large  masses  of  corals  usually  flourish  below  the  wash 
of  the  breakers,  and  in  these  regions  corals  generally  do  not 

'  H.  B.  Guppy.  Notes  on  the  Character  and  Mode  of  Formation 
of  the  Coral  Reefs  of  the  Solomon  Islands.  Proc.  R.  S.  Edin.  xiii, 
p.  857  (Session  1885-6). 


SOLOMON   ISLANDS.  293 

thrive  in  the  break  of  the  trade-swell.  *  They  are  only  to  be 
found  in  luxuriance  on  the  slopes  of  the  declivity  that  is 
situated  in  depths  between  five  and  fifteen  fathoms,  a  decli- 
vity which  may  be  truly  termed  the  growing  edge  of  the  reef.' 
At  exceptionally  low  tides,  when  there  is  a  heavy  sea,  large 
branches  are  apt  to  be  torn  off  from  corals  growing  beyond 
the  usual  reach  of  the  breakers,  and  these  are  thrown  up  on 
the  upper  flat  of  the  reef.  But  in  cases  where  the  reefs  are 
protected  from  the  heavier  rollers,  the  corals  living  in  the 
wash  of  the  breakers  are  more  numerous  and  in  greater 
variety.  The  same  rule  holds  good  on  the  lee  sides  of  small 
coral  islands.  Here  the  corals  are  often  grouped  in  irre- 
gular patches  or  masses,  which  sometimes  rise  with  wall-like 
sides  from  depths  of  twelve  or  fifteen  feet  of  water.  A  large 
part  of  the  interior  both  of  lagoons  and  of  their  channels, 
is  occupied  by  sandy  and  chalky  mud  ;  but  in  the  shallower 
portions,  and  especially  in  those  situations  which  are  near 
the  breaks  in  the  reef,  corals,  especially  of  foliaceous  and 
branching  habit,  thrive  in  great  profusion.  As  a  rule 
corals  are  unable  to  sustain  exposure  to  the  air  for  long  ; 
from  one  to  two  hours  continuously  appears  the  maximum 
of  endurance,  and  that  is  reached  only  by  a  few  species. 

Mr.  Guppy  considers  that  in  this  group  the  numerous 
detached  submerged  reefs  or  shoals,  which  lie  at  depths  of 
from  4  to  10  fathoms  (that  is,  at  depths  which  vary  with 
the  amounts  of  disturbance  produced  by  the  breakers), 
represent  the  earliest  conditions  of  coral  reefs.  Numerous 
instances  of  such  reefs  are  given  in  this  memoir  :  one. 
Lark  Shoal,  covered  by  water  having  a  minimum  depth  of 
7  fathoms,  rises  from  a  depth  of  200  fathoms.  The  shoal 
within  the  20  fathom  line  measures  IJ  miles  in  one  direc- 
tion and  1  mile  in  the  other.  There  is  no  sign  of  a  central 
depression,  the  summit  being  comparatively  level  and 
covered  by  from  7  to  10  fathoms  of  water.  This  general 
flatness  of  the  upper  surface  is  not  peculiar  to  Lark  Shoal, 


2f)4  APPENDIX   n. 

but  was  observed  at  similar  depths  in  the  case  of  others. 
Between  such  submerged  reefs  and  those  marked  on  the 
surface  by  a  reef  flat,  with  its  accompanying  islet,  or  by  a 
sand  key,  intermediate  conditions  were  not  found,  and  Mr. 
Guppy  is  of  opinion  that  reefs  on  arriTing  within  from  4  to 
8  fathoms  from  the  surface  have  reached  the  hmit  of  their 
upward  growth,  and  afterwards  have  to  extend  laterally. 
Hence  he  infers  that  detached  submerged  reefs  are  unable 
to  raise  themselves  within  the  hmit  of  constructive  breaker- 
action  without  the  assistance  of  a  movement  of  elevation. 
Of  such  a  movement,  in  this  region,  there  are  certainly 
proofs,  and  the  same  is  the  case  in  the  Low  Archipelago, 
the  Fiji  and  Pelew  groups. 

Among  the  reefs  which  have  reached  the  surface  in 
the  Solomon  Archipelago,  fringing  and  barrier  reefs  are 
much  commoner  than  atolls.  *  A  line  of  barrier  reef  pro- 
bably not  much  under  60  miles  in  length,  and  having  in- 
numerable islets  on  its  surface,  fronts  the  eastern  coasts  of 
the  islands  of  New  Georgia  at  a  distance  of  from  1  to  3 
miles  from  the  shore.  At  St.  Christoval  the  fringing  reefs 
occasionally  reach  a  mile  in  breadth,  but  usually  do  not 
exceed  a  quarter  of  a  mile.  The  100  fathom  line  lies 
generally  about  1,200  yards  from  the  edge  of  the  reef  flat 
which  would  give  an  average  slope  of  10°.'  Upheaval  is  indi- 
cated by  a  recently  elevated  flat  of  coral  rock,  which  is  in 
some  places  15  feet  or  so  above  high  water  leveL  North 
of  St.  Christoval  are  three  small  islands  (named  the  Three 
Sisters).  They  commenced  their  growth,  according  to 
Mr.  Guppy,  as  submerged  flat-topped  reefs,  like  those 
already  mentioned.  They  were  then  elevated  to  about 
70  feet  above  the  sea,  and  have  since  assumed  an  atoll 
structure.  He  regards  them  as  *  based  on  three  submerged 
peaks  which  he  at  some  unknown  distance  below  the  sur- 
face.' They  are  enclosed  within  the  same  hundred  fathom 
lice;  the  submarine  slope  at  first  is  gradual  and  then 


SOLOMON   ISLANDS.  295 

descends  more  rapidly  ;  on  the  weather  side  at  an  angle  of 
rather  more  than  20°,  on  the  lee  side  usually  at  a  smaller 
angle.  The  highest  points  (of  coral  limestone)  on  the 
southernmost  island  rise  to  about  70  feet  above  the  sea. 
It  has  twc  lagoons;  the  deeper  is  about  9  fathoms,  and 
over  the  bar  at  its  mouth  the  water  is  less  than  a  fathom 
deep  at  low  water  neaps. 

Mr.  Guppy  is  of  opinion  that  this  island  commenced  its 
history  as  two  flat-topped  submerged  reefs,  and  that  the 
atoU  form  has  been  assmned  since  these  have  been  up- 
heaved. Santa  Anna  is  an  upheaved  atoll  with  shore-reefs 
ranging  from  150  to  600  yards  wide  according  to  the  steep- 
ness of  the  land.  On  the  westward  the  shore-reefs  enclose  a 
circular  lagoon  700  to  800  yards  wide  and  16  to  17  fathoms 
deep.  JJgi  island  has  shore-reefs  of  varying  width,  and  on 
the  east  coast  the  shore-reef  encloses  a  narrow  lagoon  a 
mile  long  and  10  fathoms  or  less  in  depth.  There  is  also 
in  that  on  the  south  coast  a  circular  lagoon  about  100  yards 
wide  and  6  fathoms  deep,  approached  by  a  narrow  chaimel. 
Biu  is  a  patch  of  coral  reef  which  has  been  raised  about 
100  feet  above  the  sea,  and  is  still  encircled  by  living 
reefs.  Biua  Sura  is  an  atoll  of  elongated  form  about  3  miles 
in  length.  Except  for  three  wooded  islets  on  the  south 
side,  its  circumference  is  either  just  awash  at  low  tide  or 
is  within  a  fathom  of  the  surface ;  but  soundings  in  the 
lagoon  to  a  depth  of  37  fathoms  failed  to  reach  the  bottom. 
The  islets  are  at  highest  only  15  or  20  feet  above  the  sea, 
*  cliffs  of  coral  rock  4  or  5  feet  in  height  *  in  one  *  betokens 
recent  elevation  of  a  small  amount.*  Eddy  stone  Island 
consists  of  *  two  distinct  islands  (volcanic)  which  have 
become  united  by  elevation  of  an  intervening  coral  reef. 
On  the  east  coast  the  submarine  slope  down  to  the  100 
fathom  line  is  from  30°  to  35°.  There  is  a  hole  in  the 
middle  of  the  reef  on  the  west  side,  about  150  yards  across 
and  18  fathoms  deep  which  Mr.  Guppy  thinks  may  mark 


296  APPENDIX   II. 

an  old  crater  cavity,  and  a  smaller  one  in  a  reef  on  the  east 
Bide,  where  also  there  is  an  elevated  barrier  reef.  Within 
a  mile  or  two  to  the  south  are  a  couple  of  submerged  coral 
patches  with  level  summits  covered  by  from  6  to  10  fathoms 
of  water,  and  in  each  case  measuring  within  the  latter 
contour  line  about  half  a  mile  in  length.  They  both  rise 
on  all  sides  from  water  in  which  casts  of  100  fathoms  did 
not  reach  the  bottom. 

East  of  Bougainville  Island,  in  the  strait  between  it  and 
Choiseul  Island,  is  a  submarine  plateau,  about  15  miles  in 
width,  extending  fi'om  the  former,  and  covered  by  from  30 
to  60  fathoms  of  water.  This,  at  its  outer  edge,  termin- 
ates abruptly  in  a  steep  slope  of  from  15°  to  25°, '  which  is 
sharply  delineated  on  the  charts,  by  the  100  fathom  line, 
and  descends  to  considerable  depths.'  There  is  a  hole  80 
fathoms  deep  in  its  generally  level  surface,  towards  the 
middle  of  the  strait,  and  another,  not  bottomed  by  100 
fathoms  of  line  off  Cyprian  Bridge  Island.  A  narrow  neck 
rather  over  2  miles  wide  links  this  plateau  to  a  smaller 
one  prolonging  Choiseul  Island.  *  Broken  lines  of  barrier 
reef  (sometimes  elevated)  and  elongated  coral  shoals, 
covered  by  4  to  10  fathoms  of  water,  which  may  be  re- 
garded as  incipient  barrier  reefs,  mark  the  edge  of  the  Bou- 
gainville plateau,  within  a  few  hundred  yards  of  the  100 
fathom  line.'  The  western  extremity  of  Choiseul  is  skirted 
by  a  broken  line  of  barrier  reef,  which  encloses  a  lagoon- 
like channel,  and  supports  islets  on  which  coral  rock  indicates 
an  elevation  of  at  least  a  few  feet ;  and  there  is  an  island 
in  the  lagoon  which  bears  similar  testimony,  while  'the 
hills  on  the  coast,  composed  as  they  are  of  foraminiferous 
and  pteropod  mud  encrusted  by  coral  limestones,  have  been 
antecedently  upheaved.'  Oima  atoll,  about  2  miles  long, 
has  been  built  up  above  a  group  of  islets  composed  of 
hornblende-andesite,  each  probably  indicating  four  separate 
volcanic  necks.     This  atoll  rises  from  depths  of  40  to  50 


BARRIER   REEFS.  297 

fathoms,  with  a  submarine  slope,  varying  from  12°  to  26°. 
The  erosion  line  on  the  volcanic  rocks  indicates  an  up- 
heaval of  some  four  or  five  feet,  prior  to  the  coral  growth. 
The  land  is  bordered  by  extensive  flats,  covered  by  less 
than  a  fathom  of  water,  on  which  the  coral  appears  to  be 
dead,  and  two  basins  or  lagoons  occur  within  them,  about 
20  fathoms  deep. 

Sections  are  given  of  some  barrier  reefs.  As  the  result 
of  his  investigations  of  these,  Mr.  Guppy  concludes  that, 
from  the  edge  of  the  reef  flat  for  the  first  70  or  80  yards, 
there  is  usually  a  gradual  slope  to  a  depth  of  from  4  to  5 
fathoms.  On  this  but  little  living  coral  is  found.  Beyond 
it,  there  is  generally  a  rapid  descent  to  a  depth  ranging 
between  12  and  18  fathoms,  on  the  face  of  which  the  corals 
flourish:  'this  is  in  fact  the  growing  edge  of  the  reef.' 
Below  this  descent  sand  and  gravel,  produced  by  the  action 
of  the  breakers  at  the  margin  of  the  reef,  collect  at  a  depth 
generally  of  from  15  to  20  fathoms,  though  sometimes  this 
occurs  at  greater  depths.  One  section,  that  of  Santa  Anna 
Island,  exhibits  two  submarine  cliffs, — the  one,  after  a 
rapid  slope,  occurs  between  the  depths  of  16  and  32 
fathoms ;  the  other,  after  reaching  a  depth  of  about  42 
fathoms,  gives  a  drop  of  25  fathoms,  after  which,  a  slope  at 
an  angle  of  18°  or  19°  descends  to  considerable  depths ; 
corals  thrive  in  this  case  at  a  depth  of  30  fathoms. 
Another  section  exhibits  a  second  but  less  strongly 
marked  drop  at  about  25  fathoms. 

In  an  explanation  of  the  formation  of  barrier  reefs, 
which  Mr.  Gruppy  regards  as  produced  successively  while 
the  ground  is  uprising,  it  is  admitted  that  on  this  hypo- 
thesis, lagoon  channels  should  never  be  deeper  than  the 
limit  at  which  reef -building  corals  can  grow.  *  But  as  a 
matter  of  fact  the  depths  inside  barrier  reefs  as  well  aa 
atolls,  not  unfrequently  exceed  '  25  fathoms — in  corrobora- 
tion of  which  statement  several  cases  of  soundings  in  these 


298  APPENDIX   II. 

positions  of  40,  60  and  even  60  fathoms  are  mentioned. 
This  difficulty  Mr.  Guppy  overcomes  by  the  hypothesis  that 
the  Hmit  for  the  development  of  reef-building  coral  is  really 
determined,  not  so  much  by  actual  depth  as  by  the  con- 
dition of  the  water,  especially  in  regard  to  the  presence  or 
absence  of  suspended  mud  (p.  888). 

Another  consideration  confirms  Mr.  Guppy  in  his 
opinion  that  reefs  are  often  begun  at  a  much  greater  depth 
than  25  fathoms.  The  usual  foundation,  so  far  as  his 
observations  go,  is  composed  of  partially  consolidated 
volcanic  mud  or  ooze,  more  or  less  foraminiferous,  and 
generally  abounding  in  recent  shells,  and  is  not  a  layer  of 
detrital  sand  and  gravel.  But  in  all  the  soundings  about 
the  reef,  which  often  extended  down  to  60  fathoms,  the 
armings  never  brought  up  any  indication  of  the  nature  of 
the  bottom  other  than  sand  and  gravel.  Hence  it  may 
be  presumed  that  such  reefs — as,  for  example,  those  in 
the  Shortland  Islands — began  at  depths  greater  than  60 
fathoms.  But  if  it  be  urged  that  in  this  case  the  reefs 
should  be  more  than  100  feet  thick — and  this  amount  is 
rarely  exceeded  in  the  Solomon  Islands — he  replies  that, 
as  a  rule,  reef  corals  will  be  confined  to  depths  of  25  or 
80  fathoms,  and  the  beginning  of  a  reef  in  deeper  waters 
wiU  be  an  exceptional  thing.  It  must  also  be  remembered 
that  the  rapid  subaerial  denudation  which  occurs  in  these 
regions  may,  in  some  cases,  have  reduced  the  vertical 
thickness  of  the  reef. 

In  support  of  Mr.  Murray's  view  that  reefs  spread  by 
an  outward  growth,  Mr.  Guppy  states  that  he  found  the 
corals  inside  the  lagoons  to  be  much  larger  than  those 
which  occur  near  the  outward  border  of  the  reef,  and  in 
the  barrier  reefs  the  corals  were  largest  near  the  inuer 
edge  of  the  flat,  and  diminished  in  size  as  the  outer  edge  of 
the  reef  was  approached.  *  These  facts  are  of  importance, 
since,  according  to  the  theory  of  subsidence,  the  central 


DEVELOPMENT   OF   REEFS.  299 

portion  of  the  lagoon  of  an  atoll,  and  the  inner  portions  of 
the  lagoon  channel  of  a  barrier  reef,  are  more  recently  pro- 
duced than  any  other  portion  of  the  area  of  such  reefs.* 
The  smallness  of  the  outer  corals  is  '  only  to  be  explained 
on  the  hypothesis  that  the  reef  has  gradually  grown  out- 
wards as  from  a  centre,  and  quite  independently  of  any 
movement  of  subsidence.'  Further,  the  low  coral  lime- 
stone cliffs,  which  not  seldom  back  the  present  reef  flats, 
are  probably  lines  of  erosion,  indicative  of  an  epoch  an- 
terior to  the  formation  of  the  shore  reefs,  when  these  cliffs 
were  washed  by  the  sea.  The  disposition  both  of  the 
vegetation  and  of  the  humus  on  the  wooded  islets  shows 
that  the  lee  side  of  such  an  islet  is  its  oldest  portion,  and 
its  weather  side  is  its  growing  margin. 

Illustrative  of  the  question  of  the  removal  of  dead  coral 
by  solution,  cases  are  mentioned  of  masses  of  madrepora 
and  pontes  several  feet  across,  the  centres  of  which  were 
dead  and  were  depressed  a  few  inches  below  the  living  ex- 
terior. During  the  bright  sunlight  the  increased  tempera- 
ture *  of  the  sea  water  covering  the  reef  flats  probably  assists 
in  the  solution  of  the  dead  coral ;  moreover  its  destruction 
by  organisms,  to  which  other  authors  have  called  attention, 
must  not  be  overlooked.  Holothurians  and  echinids  play 
an  important  part  in  this  respect. 

The  author  concludes  by  stating  that  the  calcareous 
sand  and  gravel  which  strew  the  outer  slopes  below  the 
zone  of  living  coral  are  largely  composed  of  reef  debris,  of 
the  tests  of  Orhitolites  complanata  and  0,  heterostegina, 
of  the  joints  of  the  calcareous  alga  Halimeda  opuntia  and 
of  nullipores.  Of  these  the  foraminifera  were  found  living 
between  2  feet  and  75  fathoms  ;  the  alga  does  not  appear 
to  live  below  10  fathoms.  At  greater  depths  than  100 
fathoms  the  sea  bottom  consists  generally  of  volcanic  mud 

'  About  16°  F.  higher  than  the  open  sea  where  the  water  was  only 
8  or  4  inches  deep,  and  8  higher  where  it  was  a  foot  deep. 


.^00  APPENDIX    II. 

(with  or  without  organisms),  which  however  forms  also  at 
all  depths,  from  a  few  feet  beneath  the  low  tide  level,  in 
the  case  of  large  islands,  the  coasts  of  which,  owinp:  to 
the  sediment  brought  down  by  streams,  are  bare  of  reefs. 

In  a  paper  printed  in  the  *  Proceedings  of  the  Eoyal 
Society  '  (vol.  xHii.  p.  440,  1888),  Mr.  G.  C.  Bourne  gives  a 
minute  description  of  the  atoll  of  Diego  Garcia,  and  discusses 
the  theories  of  coral-reef  formation  in  connection  with  the 
Chagos  group.  In  the  Laccadive,  Maldive  and  Chagoa 
group,  '  there  is  no  instance  of  a  fringing  or  a  barrier  reef; 
nothing  but  coral  structure  rises  above  the  waves  ;  all  the 
islands  are  atolls.'  The  three  groups  are  beheved  to  stand 
on  a  submarine  bank  lying  1,000  fathoms  below  the  surface, 
in  an  ocean  of  an  average  depth  of  2,000  fathoms.  At 
Diego  Garcia,  the  shores  externally  '  slope  away  very 
rapidly  to  considerable  depths,  the  sounding  line  giving 
depths  of  250  fathoms  and  upwards  at  a  distance  of  a  few 
hundred  yards  from  the  edge  of  the  reef,'  except  in  one 
case.  The  depths  inside  the  lagoon  vary  up  to  19  fathoms. 
Mr.  Bourne  describes  the  different  kinds  of  coral  rock,  and 
gives  reasons  for  supposing  that  there  has  been  a  recent 
elevation  of  a  few  feet.  He  calls  attention  to  the  changes 
produced  by  the  action  of  waves  and  currents,  and  to  the 
effect  of  the  latter  upon  the  growth  of  coral :  showing 
how  the  living  coral  may  be  killed  b'y  a  change  in  a  current 
which,  formerly  clear,  now  brings  sand.  This  material 
proceeds  to  entomb  the  dead  coral,  and  then,  on  a  return 
to  the  former  conditions,  a  new  growth  of  coral  may  take 
place  upon  the  stratum  of  sand.  He  is  of  opinion  that  the 
subsidence  theory  cannot  be  applied  to  explain  the  Great 
Chagos  Bank  (see  p.  53),  because  its  rim  is  *  on  an  average 
not  more  than  6  fathoms  below  the  surface,  and  therefore 
situated  in  a  depth  eminently  favourable  for  coral  growth, 
and  there  are  actually  six  islets  on  the  northern  and  western 


VIEWS   OF   MR.    BOURNK.  301 

edges,  rising  above  the  water  and  some  of  tliem  inhabited.' 
He  indicates  further  difficulties  in  applying  the  theory 
of  subsidence  to  the  Chagos  Bank,  especially  pointing  out 
that  the  Six  Islands  atoll,  within  a  few  miles  distance,  has- 
not  been  affected ;  still  he  admits  that  the  '  Saya  de  Malha 
Bank  appears  to  have  the  characters  of  a  submerged  atoll, 
having  a  central  depression  of  65  fathoms  surrounded  by  a. 
rim  which  has  only  8  to  16  fathoms  on  its  eastern  side,, 
but  22  fathoms  on  the  western.'  On  the  whole,  however, 
he  considers  that  *  most  of  the  coral  formations  of  the' 
Indian  Ocean  mark  areas  of  elevation  rather  than  of  rest ; 
certainly  they  are  not  evidence  of  subsidence.' 

In  regard  to  the  explanation  of  the  formation  of 
lagoons  by  solution  of  the  interior  parts  of  the  reef,  and 
by  the  more  rapid  growth  of  the  corals  on  its  periphery,  as^ 
being  more  directly  in  the  track  of  food -bearing  currents, 
Mr.  Bourne  observes : — *  Neither  of  these  explanations  has^ 
completely  satisfied  me.  That  sea-water  exercises  a  solvent 
action  upon  carbonate  of  lime  does  not  admit  of  doubt,  and 
that  the  scour  of  tides,  combined  with  this  solvent  action 
of  the  water,  does  affect  the  extent  end  depth  of  a  lagoon  is; 
obvious.  But  I  challenge  the  statement  that  the  destructive- 
agencies  within  an  atoll  or  a  submerged  bank  are  in  excess 
of  the  constructive.  It  would  be  nearer  the  mark  to  say 
that  they  nearly  balance  one  another.  In  the  first  place  the- 
carbonate  of  lime  held  in  solution  by  sea-water  is  deposited 
as  crystalline  limestone  in  the  interstices  of  dead  corals  or 
coral  debris.  Anyone  who  is  acquainted  with  the  struc- 
ture of  coralline  rock  knows  how  such  a  porous  mass  as  a 
mceandrina  head  becomes  perfectly  solid  by  the  deposition 
of  lime  within  its  mass.  This  deposition  can  only  be 
effected  by  the  infiltration  of  sea- water.  In  reckoning  the 
solvent  action  of  sea-water,  therefore,  account  must  be 
taken  of  the  fact  that  a  not  inconsiderable  proportion  of 
the  carbonate  of  lime  held  in  solution  is  re-deposited  in  the 
21 


302  APPENDIX    II. 

form  of  crystalline  limestone.  Of  this,  it  seems,  Mr. 
Murray  has  not  taken  sufficient  account,  and  has,  there- 
fore, overstated  the  destructive  agency  of  the  sea.  Secondly, 
the  growth  of  corals,  and  the  consequent  formation  of  coral 
rock  within  the  lagoon,  is  generally  overlooked. 

*  Whilst  diving  for  corals  at  Diego  Garcia  I  had  abundant 
opportunities  of  studying  the  formation  of  coral  rock  within 
the  lagoon,  in  depths  under  2  fathoms.  The  layers  of 
itolerably  compact  rock  thus  formed  are  of  no  mean  extent 
or  thickness;  they  soon  become  covered  with  sand,  and 
are  thus  protected  from  the  solvent  action  of  the  water.  I 
have  found  it  impossible  to  reconcile  Mr.  Murray's  views 
with  what  I  saw  of  coral  growth  within  a  lagoon.  Not  only 
•do  the  more  delicate  branching  species  of  the  madrcporaria 
flourish  in  considerable  numbers,  but  true  reef-building 
;species,  pontes,  mceandrina,  pocillopora,  and  various  stout 
•species  of  madrepora,  are  found  there.  It  is  a  mistake 
'to  suppose  that  certain  spesies  of  corals  are  restricted  to 
!the  external  shores,  others  to  the  lagoon.  My  collections 
proved  that  many  of  the  species  growing  in  the  lagoon  at 
•distances  of  five  miles  and  upwards  from  its  outlet  are 
identical  with  those  growing  on  the  outer  reef.  In  addi- 
tion to  them  are  numerous  species,  such  as  Seriatopora 
■stricta,  Mussa  corymbosa,  Favia  lobata,  Fungia  dentata, 
.and  many  others  that  are  not  found  on  the  outside.  The 
reason  is  that  the  last-named  are  either  free  forms  such  as 
fungia,  or  are  attached  by  such  slender  and  fragile  stems 
to  their  supports  that  they  could  not  possibly  obtain  a  foot- 
hold and  maintain  themselves  among  the  powerful  currents 
and  waves  of  the  open  ocean. 

*  These  various  species,  numbers  of  which  grow  cloie 
•together,  form  knolls  and  patches  within  the  lagoon,  and  it 
■cannot  be  doubted  that  their  tendency  is  to  fill  it  up. 
Again,  in  reefs  which  do  not  rise  above  the  surface,  or  are 
awash  for  the  greater  part  of  their  extent  at  low  tides, 


DIEGO    GARCIA   ATOLL.  303 

great  quantities  of  dSbriSj  torn  from  the  outer  slopes,  are 
constantly  carried  over  the  rim  of  the  reef  and  tend  to  fill 
it  up.  Hence  it  follows  that  in  a  lagoon  entirely  surrounded 
by  dry  land,  or  nearly  so,  as  is  the  case  at  Diego  Garcia, 
the  tendency  to  accumulation  of  material  within  the  lagoon 
would  be  less  than  in  submerged  or  incomplete  atolls,  for 
d&bris  cannot  be  swept  over  into  the  lagoon,  and  the  only 
constructive  agency  is  the  growth  of  coral.  If  the  power  of 
solution  of  sea-water  is  so  great,  it  must  be  supposed  that 
in  complete  or  nearly  complete  atolls  the  lagoon  would  be 
deepening  rather  than  shallowing ;  yet  at  Diego  Garcia  the 
lagoon  is  obviously  shallowing  in  many  places,  and  has 
nowhere  increased  in  depth  since  Captain  Moresby's  survey 
in  1837.  Indeed,  the  southern  part  seems  to  have  shoaled 
a  fathom  since  that  time,  and  this  is  the  more  remarkable, 
since  the  S.E.  trade  winds  are  by  far  the  most  constant 
and  strongest  winds  there  and  tend  to  accumulate  material 
at  the  northern  rather  than  at  the  southern  end.  The 
fact  is,  that  these  winds  sweep  the  sand  out  of  the 
southern  part,  and  thus  leave  an  area  particularly  favour- 
ably situated  for  the  growth  of  corals.  Mr.  Murray 
points  out  that  larger  atolls  generally  have  deeper  lagoons 
than  small  atolls,  and  urges  this  fact  in  support  of  his 
theory  ;  but  here  again  the  facts  in  the  Chagos  group  are 
against  him.  Victory  Bank  is  a  submerged  atoll,  the 
Solomons  is  an  atoll  with  a  large  extent  of  dry  land ;  in 
each  the  lagoon  attains  a  depth  of  17-18  fathoms,  and  in 
Diego  Garcia  the  lagoon,  although  far  larger,  does  not 
attain  a  greater  depth.  Peros  Banhos  is  far  smaller  than 
the  Great  Chagos  Bank,  yet  in  both  the  lagoons  attain 
nearly  the  same  maximum  depth,  viz.,  41  fathoms  for 
Peros  Banhos,  44  fathoms  for  the  Great  Chagos  Bank. 
Speaker's  Bank  is  very  little  larger  than  Peros  Banhos ; 
its  lagoon  is  far  shallower,  having  a  maximum  depth  of 
24  fathoms.' 


304  APPENDIX    II, 

Mr.  Bourne  passes  on  to  consider  the  opinion  expressed 
by  certain  authors  that  the  favourable  conditions  for  coral 
growth  in  the  external  slopes  of  a  reef  consist  in  the  in- 
creased food  supply  brought  by  the  superficial  currents  of  the 
ocean.  This  explanation,  for  reasons  given,  he  regards  as 
incomplete,  being  of  opinion  that  the  direction  and  velocity 
of  currents  are  the  most  important  circumstances.  His 
observations,  he  states,  are  confirmed  in  every  particular  by 
those  made  by  Dr.  Hickson  in  Celebes,  and  communicated 
by  him  to  the  British  Association  in  1887.^  Mr.  Bourne 
expresses  the  result  of  his  observations  in  the  following 
words: — 

*  Corals  grow  best  in  places  where  a  moderate  current 
flows  constantly  over  them.  They  are  killed  in  still  water 
by  the  deposition  of  sediment,  and  they  will  not  grow  in 
places  where  a  strong  current  sets  directly  against  them. 
I  noticed  at  Diego  Garcia  in  many  places,  but  particularly 
at  the  east  end  of  East  Islet,  that  a  strong  and  direct 
ocean  current  is  most  unfavourable  to  coral  growth,  and 
that  the  reef  is  barren  and  suffering  rapid  erosion  at  such 
exposed  spots  as  allow  the  whole  force  of  the  current  to 
fall  directly  upon  them.  As  the  current  parts  and  flows 
round  the  obstacle,  one  meets  with  a  reef  covered  with 
d^brisy  but  barren  of  live  coral ;  further  on,  as  the  current 
moderates  in  force,  one  finds  a  few  growing  heads  of  coral ; 
and,  finally,  at  the  further  end  of  the  reef,  where  the 
current  has  abated  its  force  considerably,  there  is  a  luxu- 
riant bed  of  living  corals  and  Alcyonaria.  This  can  be 
seen  in  perfection  on  the  southern  reef  of  East  Islet.  Dr. 
IJickson  tells  me  that  he  has  observed  the  same  facts  at 
Celebes,  that  direct  and  strong  currents  are  unfavourable 
to  coral  growth,  that  moderate  tangential  currents  aro 
extremely  favourable,  and  sluggish  or  still  water  again 
unfavourable.  This  view,  which  both  of  us  can  support 
*  The  paper  is  not  printed  in  the  volume  for  that  year. 


MALADIVE   AND    KEELING   ATOLLS.  305 

by  many  observations,  is  mucli  at  variance  witli  the  old 
accepted  saying  that  corals  grow  best  where  the  breakers 
are  the  heaviest.  It  appeared  to  me  that  heavy  breakers 
are  not  favourable  to  coral  growth,  because  of  the  quantity 
of  shingle  which  they  dash  against  the  soft-bodied  polyps. 
Some  massive  forms  might  withstand  the  force  of  breakers 
and  violent  currents  if  the  polyps  could  be  sufficiently 
protected  from  the  shingle,  but  the  branching  madrepores 
are  soon  broken  off  and  swept  away,  and  even  the  more 
massive  mceandrina  soon  follows,  for  whilst  the  surface  of 
the  colony  grows  the  base  is  dead,  is  soon  riddled  by  boring 
sponges,  serpula3,  &c.,  and  is  no  longer  able  to  bear  the 
strain  put  upon  it.  The  great  mass  then  breaks  off  and  is 
rolled  along  the  reef,  pounding  other  corals  in  its  course.' 

Still,  as  a  rule,  the  outward  portions  of  the  reef  are 
the  most  favourable  for  coral  growth.  Hence,  if  a  bank  of 
coral  be  established  below  water,  there  is  a  tendency  in  the 
coral  at  its  margin  to  grow  both  outwards  and  upwards,  so 
that  at  last  an  atoll  form  is  developed.  As  the  rim 
approaches  the  surface,  it  is  raised  by  the  piling  up  of 
debris,  broken  off  by  the  waves,  and  may,  in  some  cases, 
also  be  upheaved.  But  the  waves,  tides,  currents,  &c., 
tend  also  to  destroy  parts  of  the  island,  so  that  there  is  a 
constant  struggle  going  on  between  the  constructive  and 
destructive  agencies.  The  author  then  proceeds  to  apply 
his  theories  to  the  formation  of  the  Maladive  atolls,  remark- 
ing in  conclusion : — *  However  one  looks  at  the  subject  one 
must  realise  that  the  laws  governing  the  formation  of  coral 
reefs  are  exceedingly  complex,  and  that  many  circum- 
stances have  to  be  taken  into  account  before  any  perfect 
explanation  of  their  structure  can  be  obtained.' 

While  these  sheets  were  passing  through  the  press  a 
letter  written  by  Mr.  H.  B.  Guppy  to  Mr.  J.  Murray  ap- 
peared in  Nature  (vol.  xxxix.  p.  23G),  giving  some  account 


306  APPENDIX   II. 

of  the  results  of  a  visit  to  Keeling  Atoll  *  (known  also  as  tha 
Cocos  Island).  As  the  letter  is  only  a  preliminary  note,  it 
is  difficult  to  analyse  or  appreciate  the  writer's  arguments, 
so  it  may  suffice  to  say  that  he  is  convinced  that  *  several 
important  characters  of  these  islands  escaped  the  attention 
of  Mr.  Darwin,*  that  *  these  features  throw  considerable 
light  on  the  mode  of  origin  of  lagoon  islands,  and  give 
no  support  to  the  theory  of  subsidence.'  According  to 
Mr.  Guppy's  description  *  Keeling  Atoll  consists  essentially 
of  a  ring  of  horseshoe  or  crescentic  islands,  including  a 
lagoon,  and  presenting  their  convexities  seawards.  The 
crescentic  form  is  possessed  in  varying  degrees  by  different 
islands ;  some  of  the  smaller  ones  are  perfect  horseshoe 
atoUons  and  inclose  a  shallow  lagoonlet,  others  again  ex- 
hibit only  a  semi-crescentic  form,  while  the  larger  islands 
have  been  produced  by  the  union  of  several  islands  of  this 
shape.'  He  states  that  from  the  effects  of  gales,  &c.,  the 
islands  are  constantly  altering  in  shape,  and  expresses  his 
decided  opinion  that  the  'small  atolls  and  horseshoe 
islands  only  assume  their  horseshoe  form  after  the  island 
has  been  thrown  up  by  the  waves.'  This  is  due  to  the 
sand  and  debris,  which  are  swept  along  by  a  current, 
accumulating  under  the  lee  of  the  ends  of  a  shoal  on  the 
face  of  which  the  current  impinges,  so  that  the  island 
tends  to  extend,  both  laterally  and  to  leeward,  and  thus 
gradually  to  assume,  more  or  less,  the  shape  of  a  crescent 
or  horseshoe.  Some  estimates  are  given  of  the  amount  of 
material  transported  by  the  currents. 

Outside  the  seaward  edge  of  the  present  reef,  Mr.  Guppy 
has  observed  a  series  of  submerged  lines  of  growing  corals 
separated  from  each  other  by  sandy  intervals.  Thus  the 
outward  extension  of  a  reef  is  effected  'not  so  much 
by  the  seaward  growth  of  the  present  edge  of  the  reef,  aa 
by  the  formation  outside  of  it  of  a  line  of  growing  corala 
'  See  oh.  i.  sect.  i.  of  the  present  work. 


VIEWS   OF   PROFESSOR   B.    BALFOUR.        307 

which,  when  it  reaches  the  surface,  reclaims,  so  to  speak, 
the  space  inside  it,  which  is  soon  filled  up  with  sand  and 
teei-d4brls.  The  evidence,  in  fact,  goes  to  show  that  a 
reef  grows  seaward  rather  by  jumps  than  by  a  gradual 
outward  growth.  This  inference  is  of  considerable  import- 
ance since  it  connects  all  classes  of  reefs  together  in  the 
matter  of  their  seaward  growth,  the  degree  of  inclination 
of  the  submarine  slope  being  the  chief  determining  factor.' 

Proceeding  now  to  the  papers  favourable  to  Mr,  Darwin's 
views  we  may  quote  first  a  passage  in  Professor  Bayley 
Balfour's  description  of  the  physical  features  of  Eodriguez  * 
which  has  an  important  bearing  on  one  point  in  recent 
controversies.  After  stating  that  the  island  is  substantially 
a  hilly  mass  of  volcanic  rock,  the  highest  point  being 
1,300  feet  above  sea  level ;  that  the  western  slopes  of  this 
terminate  in  a  wide  coralline  limestone  plain,  diversified 
with  elevations ;  and  that  a  fringing  reef  of  coral,  studded 
with  islets,  skirts  the  island  on  every  side,  extending  on 
the  west  about  three  miles  from  land,  but  with  its  edge  at 
the  eastern  end  within  about  one  hundred  yards  of  the 
beach  ;  he  proceeds  : — *  On  the  south-west  the  central  vol- 
canic ridge  gradually  descends,  the  ravines  become  less 
deep,  and  the  ground  spreads  out  into  a  large  coralline 
limestone  plain.  The  demarcation  betwixt  the  limestone 
and  the  volcanic  rock  is  very  sharp,  but  isolated  patches  of 
limestone  are  met  with  on  the  surface  of  the  volcanic 
region  in  the  vicinity  of  the  main  mass.  .  .  .  The  lime- 
stone is  not  found  along  the  northern  or  southern  shores, 
until  we  near  the  eastern  extremity,  where  patches  occur 
at  the  mouths  of  valleys,  and  even  at  some  distance  from 
the  shore.  ...  On  the  southern  shore  between  Riviere 
Palmiste  and  Eiviere  Poursuite,  indications  of  raised 
beaches  are  seen,  reaching  about  20  feet  above  the  sea 
»  Philosoph.  Transact.  K.  S.  vol.  olxviii.  (1879),  p.  289. 


308  APPENDIX   II, 

level.  The  existence  of  these  masses  of  coralline  lime- 
stone indicates  clearly  a  lower  level  of  the  island,  and  the 
evidence  of  raised  beaches  confirms  this.  But  a  considera- 
tion of  the  coral  reefs  points  as  clearly  to  a  time  when 
the  island  stood  at  a  higher  level.  The  present  coral  reef 
fringes  the  coast,  extending,  as  I  have  mentioned,  about 
three  miles  on  the  south-west  side,  but  coming  close  in 
shore  on  the  east.  An  older  reef,  however,  exists,  now 
quite  submerged  in  some  places  to  a  depth  of  over  90 
fathoms.  Upon  it  the  present  reef  rests,  and  it  extends 
westwards  nearly  fifteen  miles  from  the  present  coast, 
while  on  the  east  it  stretches  about  six  miles.  We  have 
thus  proof  of  great  and  intermittent  oscillations  of  the 
level  of  the  island.  Of  the  islets  scattered  over  the  reef 
some  are  volcanic  and  the  others  are  composed  of  coralline 
limestone  and  sand.  They  are  all  within  the  compass  of 
the  present  reef,  and  only  occur  on  its  wider  parts.'  Eight 
islets  are  of  volcanic  origin  ;  the  coralline  limestone  and 
small  islets  are  more  numerous,  and  are  confined  to  the 
southern  and  western  reefs ;  none  occur  on  the  north. 

Mr.  W.  0.  Crosby  ^  states  that  level  terraces  with 
vertical  walls,  resting  against  the  rugged  mountains  of  the 
interior,  and  forming  the  shore  of  the  island,  are  conspi- 
cuous features  in  the  scenery  of  Cuba.  They  may  be  ob- 
served at  various  levels  up  to  nearly  2,000  feet.  The  first 
preserves  a  sensibly  uniform  altitude  of  about  30  feet  for 
hundreds  of  miles.  It  is  breached  by  the  rivers  which  flow 
into  the  sea,  and  is  seen  to  be  composed  of  coral :  in  she  rt, 
it  is  an  elevated  fringing  reef,  similar"  to  that  which  is  now 
forming  on  the  adjacent  sea  bottom.  It  varies  in  width 
from  a  few  rods  to  a  mile  or  more.  Sand  and  gravel  are 
occasionally  interstratified,  especially  near  the  rivers, 
showing  that  they  are  older  than  the  reef. 

»  On  the  elevated  Coral  Eeefs  of  Cuba,  by  W  0.  Crosby.  Proo. 
Boston  Nat.  Hist.  Soc.  vol.  xxii.  (1882-3),  p.  124. 


Crosby's  description  of  cuba.        309 

The  second  reef  rises  steeply,  often  vertically  from  the 
inner  edge  of  the  first,  and  along  the  north  coast  varies 
from  200  to  250  feet,  being  more  affected  by  atmospheric 
denudation.  It  is  older  than  the  other,  and  the  organic 
structures  therein  are  in  part  obliterated  by  crystallisation, 
but  of  their  identity  of  origin  there  can  be  no  doubt.  The 
third  has  an  altitude  of  about  500  feet,  and  is  yet  older, 
more  soHd  and  more  crystalline.  A  fourth  reef  has  an  ele- 
vation probably  of  not  less  than  800  feet.  These  ancient 
coast-reefs,  with  slight  interruption,  extend  round  the  whole 
coast  of  Cuba.  Moreover,  the  limestone  plateau  of  El 
Yunque  is  considered  by  Mr.  Crosby  to  be  an  old  coral  reef. 
Its  top  is  about  1,800  feet  above  the  sea  ;  its  sides  for  the 
upper  500  or  600  feet  are  an  almost  continuous  wall  of 
cliffs. 

Now,  these  terraces,  of  which  the  lowest  is  the  most 
recent,  obviously  prove  that  Cuba  has  been  elevated,  and 
they  mark  stages  in  the  uprising.  But  there  is  also  evi- 
dence that,  at  the  present  day,  the  coast  is  sinking.  This 
is  indicated  by  the  condition  of  the  lower  part  of  the  valleys, 
which  are  invaded  by  the  sea  and  are  filled  to  a  consider- 
able depth  with  land  detritus.  Moreover,  if  El  Yunque 
be  an  ancient  reef,  it  is  even  now,  after  undergoing  con- 
siderable denudation,  more  than  1,000  feet  thick,  and  in  any 
case,  the  third  reef,  mentioned  above,  consists  of  not  less 
than  400  feet  (in  vertical  thickness)  of  coral  rock.  But  the 
reef-building  corals  do  not  flourish,  generally  speaking,  in 
water  deeper  than  about  25  fathoms.  Hence,  the  maximum 
thickness  of  a  coral  reef  would  be  about  150  feet,  and  to 
obtain  even  this  we  must  assume  that,  from  the  time  when 
its  growth  became  possible,  till  it  reached  the  surface,  the 
bed  of  the  sea  remained  at  rest.  Thus  the  conclusion  seems 
inevitable  that  the  elevation  of  the  island  of  Cuba  was  inter- 
rapted  and  diversified  by  periods  of  movement  in  the 
opposita  direction. 


310  APPENDIX   II. 

Professor  J.  D.  Dana's  paper  *  *  On  the  Origin  of  Coral 
Reefs  and  Islands>'  though  it  deals  with  facts  already 
pubhshed  more  than  it  adduces  those  which  are  novel,  is 
so  important,  as  the  work  of  a  naturalist  whose  personal 
knowledge  of  coral  reefs  is  perhaps  unequalled,  that  it  calls 
for  a  rather  full  abstract.  Professor  Dana  obtained  the  ex- 
perience, upon  which  his  independent  testimony  is  founded, 
in  the  course  of  three  years  spent  in  travelling  among 
coral  reefs  and  islands  in  the  Pacific,  during  which  the 
reefs  of  Tahiti,  the  Samoan  (or  Navigator)  Islands,  the 
Hawaian  Islands,  and  the  Feejees  were  examined  with 
care,  and  fifteen  other  coral  islands  visited,  *  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  Island.' 

Professor  Dana  calls  special  attention  to  the  eastern 
half  of  the  Feejee  Archipelago,  where  several  of  the  great 
barrier  reefs,  from  ten  to  twenty  miles  long,  have  but  one 
or  two  emergent  peaks  of  land.  Nanuku,  for  instance,  has 
one  little  point  near  its  south-eastern  angle,  *  a  mile  of 
peak  within  a  barrier  island  200  square  miles  in  area. 
Bacon's  Isles  are  the  last  two  little  peaks  of  a  si  ill  larger 
lagoon  ...  a  dozen  of  the  easternmost  islands  are  actual 
atolls — the  last  peak  gone.'  But  in  case  it  should  be 
answered  that  these  are  the  emergent  portions  of  sub- 
marine volcanos,  in  which  case  the  ring-shaped  barriers 
become  difficult  of  explanation,  while  they  are  easy  on  the 
theory  of  subsidence,  Professor  Dana  adds,  that  move- 
ment in  this  direction  is  proved  by  the  existence  of  deep 
fiord-like  indentations  in  the  rocky  coasts  of  islands,  both 
of  those  inside  of  barriers,  and  those  not  bordered  by  reefs. 
As  examples  of  this  structure,  generally  admitted  to  be 
»  Amer.  Jour.  Sci.  (1885),  Ser.  III.  vol.  xxx.  pp.  89,  169. 


VIEWS    OF   PROFESSOR   DANA.  311 

one  of  the  strongest  evidences  of  subsidence  all  the  world 
over,  he  quotes  the  Marquesas  Islands  with  the  Gambier 
and  Hogolen  Islands,  Eaiatea  and  Bolabola  of  the  Tahiti 
Group  and  the  Exploring  Isles  of  the  Feejees.  Professor 
Dana  also  calls  attention  to  the  general  parallelism  between 
the  average  trends  of  coral  islands  and  the  courses,  not 
only  of  the  groups  of  which  they  form  part,  but  also  of  the 
groups  of  high  islands  not  far  distant,^  and  refers  to  the 
arguments  drawn  by  Mr.  Darwin  from  the  fact  that  the 
larger  coral  islands  have  the  same  diversity  of  form  as 
is  found  in  the  barrier-reefs  of  high  islands  and  exhibit 
groupings  such  as  would  result  from  the  sinking  of  a  large 
island  of  ridges  and  peaks  with  encircling  reefs.  The 
depth  of  the  lagoon,  and  of  the  channels  inside  of  barrier 
reefs — in  many  cases  two  or  three  times  greater  than 
twenty  fathoms — is  very  difficult  to  explain  if  there  has 
been  no  subsidence ;  so  is  that  of  the  ocean  near  to  atolls. 
Professor  Dana,  after  noticing  one  or  two  considerations 
of  a  general  nature,  points  out  that  *  if  an  atoll  reef  is  not 
undergoing  subsidence,  the  coral  and  shell  material  pro- 
duced which  is  not  swept  away  and  distributed  by  currents 
serves:  (1)  to  widen  the  reef;  (2)  to  steepen,  as  a  conse- 
quence of  the  widening,  the  upper  parts  of  the  submarine 
slopes;  (3)  to  accumulate,  on  the  reef,  material  for  beaches 
and  dry  land;  and  (4)  to  fill  the  lagoon.  But  if,  while 
subsidence  is  in  progress,  the  contributions  from  corals 
and  shells  barely  compensate  for  the  loss  by  subsidence 
and  current  waste,  the  atoll-reef,  unable  to  supply  suffi- 
cient 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  because  the  debris  that  is  not  used  to  keep  the  reef  at 

*  This,  however,  I  conceive,  would  not  offer  a  difficulty  to  those 
who  advocate  submarine  volcanic  masses  as  a  foundation  for  the 
reefs. 


312  APPENDIX   II. 

tide-level  is  carried  over  tlie  narrow  reef  to  the  lagoon  by 
the  waves  whose  throw  on  all  sides  is  shoreward ;  (3)  lose 
in  irregularity  of  outline  and  thus  approximate  towards  an 
annular  form  ;  (4)  lose  the  channels  through  the  reef  into 
the  lagoon  by  the  growth  of  corals  and  by  consoHdating 
ddbris ;  and  (5)  become  at  last  a  small  bank  of  reef-rock 
with  a  half- obliterated  lagoon  basin. 

*  The  Pacific  contains  reefs  of  the  three  kinds :  (1)  atolls 
with  much  of  the  reef  overgrown  by  trees  and  shrubbery ; 
(2)  others,  of  large  and  small  size,  with  the  reefs  mostly  or 
wholly  tide-washed ;  (3)  others,  only  two  or  three  square 
miles  in  area  without  lagoons.  Further,  the  different  kinds 
are  generally  grouped  separately  and  gradationally :  (1) 
the  islands  of  the  Paumotu  and  Gilbert  Archipelagos  have 
usually  half  or  more  of  the  reef  dry  and  green ;  (2)  the 
northern  Carolines  and  the  northern  Marshall  Islands  and 
the  eastern  Feejees,  although  in  fact  of  large  size,  are 
mostly  bare  reefs ;  while  (3)  the  islands  of  the  Phoenix 
Group,  of  the  equatorial  Pacific  east  of  the  hne  of  180°, 
are,  with  one  exception  (Canton  or  Mary),  not  over  four 
miles  long.  The  three  more  southern  of  the  Phoenix 
Islands,  Gardner's,  Hull's  and  Sydney,  between  4°  25'  S. 
and  4°  85'  S.,  are  two  to  four  miles  long  and  have  lagoons; 
five  (islands),  including  Phoenix,  Birnie's  and  Kean's  be- 
tween 3°  10'  S.  and  3°  30'  S.,  and  Rowland  and  Baker's, 
north  of  the  equator,  are  a  mile  and  a  half,  and  less,  in 
length,  and  have  depressions  at  the  centre  but  no  lagoons. 
The  depressions  contain  guano,  and  one  of  them,  Kean's, 
has  much  gypsum  mixed  with  the  guano ;  Kean's  and 
Phoenix  have  a  foot  or  two  of  water  at  high  tide,  the  tide 
rising  six  feet.  Another  of  the  number,  Enderbury's,  is 
three  miles  long,  and  has  a  half-dried  lagoon  which  is 
very  shallow  and  has  no  growing  corals.  To  the  north 
of  these  islands  for  fifteen  degrees  of  latitude,  the  sea  is 
an  open  one,  and  in  the  next  ten  degrees,  to  the  line  of 


EVIDENCE    OF   SUBSIDENCE.  313 

the  Hawaian  chain,  the  only  islets  not  marked  doubtful 
are  "  Coral  Eeef,  Awash  "  and  Johnston  Island.  A  similar 
gradation  in  size  takes  place  in  the  Ellice,  Ratack,  and 
many  other  groups  of  the  ocean.'  Smallness  of  size  and 
dried  lagoon  basins,  with  occasionally  a  deposit  of  gypsum 
from  evaporated  sea  water,  are  just  the  results  which  should 
be  expected  if  the  cause  which  had  regulated  the  coral 
growth  had  been  subsidence;  and  gradation  in  it  would 
result  from  gradation  in  the  amount  of  subsidence. 

Professor  Dana  states  that  he  also  came  to  the  conclu- 
sion (and  this  appeared  to  be  sustained  by  the  *  Tuscarora ' 
and  '  Challenger '  soundings)  that  the  belt  of  maximum 
subsidence  in  the  Pacific  ran  from  the  south  of  Japan  in 
a  south-easterly  direction,  passing  south  of  the  Marquesas 
Group  towards  Easter  Island.  The  '  Tuscarora '  sound- 
ings indicated  that  transverse  to  the  trend  of  the  Phoenix 
Islands  (i.e.  transverse  to  the  belt  of  maximum  sub- 
sidence), the  mean  submarine  slopes  appear  to  be  1  to  1*5 
or  1  to  1*7  (the  former  being  nearly  the  maximum  slopes 
of  Cotopaxi,  Mount  Shasta,  and  several  other  volcanic 
summits  of  Western  America),  while  the  slopes  along  the 
trend  are  much  less.  This  fact  is  more  in  accordance 
with  a  theory  of  extensive  subsidence  than  of  extensive 
upheaval. 

Subsidence  also  is  indicated  by  the  deeply  indented 
shores  of  the  Marquesas  Islands,  but  here,  probably  owing 
to  the  boldness  of  the  coast  line,  reefs  are  few.  Tahiti  on 
the  contrary  affords  no  direct  proof  of  subsidence,  and 
none  of  elevation,  beyond  that  of  one  or  two  feet.  But  its 
broad  reefs  are  favourable,  in  Professor  Dana's  opinion,  to 
the  idea  of  subsidence,  and  he  suggests  that  it  has  amounted 
to  about  45  or  50  fathoms.  In  one  island  of  the  Navigator 
Group  the  indented  shores  seem  to  favour  a  local  sub- 
sidence, but  in  the  others  there  is  no  direct  proof  of 
movement  in  either  direction.      Subsidence,  however,  is 


314  APPENDIX    II. 

indicated  by  the  broad  reefs,  barrier  islands,  and  atolls 
of  the  Feejee  groups. 

Elevation  undoubtedly  has  occurred  in  several  localities, 
e.g.  in  tLe  Austral,  Hervey,  Friendly,  and  even  in  the 
Sandwich  Islands,  but  in  all  the  amount  is  small — not,  so 
far  as  he  knows,  anywhere  exceeding  300  feet.  These  Pro- 
fessor Dana  considers  to  be  merely  local  phenomena,  and  he 
passes  in  review  several  facts  showing  the  uncertainty  of 
evidence  as  to  little  or  no  subsidence,  or  as  to  recent  ele- 
vation from  narrow  reefs  and  the  volcanic  character  of 
islands.  Further,  these  local  elevations  in  coral  seas, 
where  they  do  occur,  are  spread  over  very  large  areas.  For 
instance,  the  Paumotu  Archipelago,  consisting  of  more  than 
eighty  atolls  and  two  barrier  islands,  contains  only  three 
or  four  atolls  that  are  over  12  feet  high.  Of  these  Metia 
is  250  feet  high.  Dean's,  probably  at  its  highest  15  or  20 
feet,  is  60  miles  to  the  N.N.E.;  far  to  the  S.E.  of  that, 
nearly  1,450  miles  from  Metia,  is  Elizabeth  Island,  80  feet. 
*  Locate  these  points  on  a  continent,  and  Pacific  distances 
and  the  length  of  Pacific  chains  of  atolls  will  be  appre- 
ciated.' 

Professor  Dana  next  reviews  the  arguments  in  favour 
of  hypotheses  other  than  that  of  subsidence,  and  deals  with 
the  soundings  of  the  *  Challenger '  off  Northern  Tahiti,  upon 
which  great  stress  has  been  laid  by  Mr.  Murray  and  others. 
Here,  from  the  edge  of  the  barrier  reef,  the  sea  bottom, 
covered  partly  with  growing  corals,  deepened  gradually  in 
about  250  yards  to  40  fathoms  {i.e.  to  considerably  below 
the  depth  at  which  reef-building  corals  usually  can  grow), 
then  from  this  limit  the  bottom  dropped  down  in  about 
100  yards  to  100  fathoms  ;  at  first  precipitately  at  an  angle 
of  75°,  then  more  gradually,  but  above  45° ;  and  for 
another  150  yards  the  sea  bed  still  shelved  down  at  30°, 
but  beyond  this  the  slope  diminished  in  the  course  of  a 
mile  to  6°,  where  at  last  the  depth  was  590  fathoms. 


TAHITI.  315 

In  otlier  words,  we  may  regard  this  part  of  Tahiti  as  a 
submerged  mountain  3,600  feet  high.  Up  to  a  contour 
line  of  3,000  feet  the  ground  shelves  upwards,  at  first 
gently,  then  more  steeply,  till  it  attains  a  slope  of  30° ; 
from  the  top  of  this  rises  a  hne  of  cliffs  about  350  feet  high 
crowned  by  a  slope  of  which  the  angle  is  18°.  The  craggy 
zone  is  strewn,  we  are  told,  with  large  masses  of  coral — 
like  a  talus  beneath  a  line  of  cliff — mingled  with  fine  debris  ; 
in  about  100  yards  there  is  only  sand,  which  continues  to 
the  lower  part  of  the  mountain,  where  it  gives  place  to  mud, 
composed  of  volcanic  and  coral  sand  and  various  organisms, 
generally  minute. 

Great  stress  is  laid  on  the  occurrence  of  this  area  of 
coral  crags  and  *  screes '  as  indicative  of  the  mode  in  which 
a  reef  is  enabled  to  grow  outwards  on  a  foundation,  built 
from  its  own  ruins.  Such  a  mode  of  enlargement  (as  Pro- 
fessor Dana  points  out)  had,  however,  been  obviously  ad- 
mit ed  as  possible  in  particular  cases,  ^  and  so  cannot  be 
regarded  as  contrary  to  the  general  hypothesis  put  forward 
by  Mr.  Darwin.  But  he  calls  attention  to  the  fact  that  the 
above  observations  prove :  (1)  that  the  currents  round  Tahiti 
are  evidently  weak  because  they  carry  little  coral  d&hris  so 
far  as  a  mile  from  the  edge  of  the  reef ;  (2)  that  very  large 
masses  of  coral  are  lying  about  below  the  submarine  cliffs 
at  depths  of  from  240  to  600  feet,  i.e.  far  below  the  depth 
at  which  the  waves  could  exert  any  serious  rending  force. 
The  position  of  these  blocks,  always  below  240  feet— too 
far  from  the  edge  of  the  reef  to  have  been  borne  from  it 
and  washed  at  last  over  the  brow  of  the  steep  declivity — 
seems  only  to  be  explicable  when  it  is  regarded  as  indica- 
ting a  stage  in  the  past  history  of  the  reef,  and  is  a  me- 
morial of  a  time  when  this  declivity  was  the  edge  of  a 
growing  reef,  and  its  brow  was  beaten  by  the  waves.^ 

*  See  pp.  22,  67,  &c.  of  the  present  volume. 

*  Professor  Dana  considers  that  waves  do  little  rending  below  the 


316  APPENDIX   II. 

The  case  at  Tahiti  appears  to  be  confirmed  by  other 
instances ;  such  as  Captain  Fitzroy's  sounding  (in  the 
*  Beagle '  Voyage)  at  Keehng  Atoll,  2,200  yards  from  the 
breakers,  when  no  bottom  was  found  at  a  depth  of  1,200 
fathoms  ;  and  the  sounding  by  Captain  Wilkes  off  Clermont 
Tonnerre  (Paumotu  Archipelago),  where  'the  lead  brought 
up  an  instant  at  350  fathoms,  then  dropped  off  and  de- 
scended to  600  fathoms,  coming  up  bruised  with  small 
pieces  of  red  and  white  coral  attached  ' ;  as  well  as  that  by 
the  same  'a  cable's  length  from  Ahii,'  where  the  lead 
struck  a  ledge  of  rock  at  150  fathoms,  and  brought  up 
finally  at  300  fathoms.  Still,  it  would  be  well  that  the 
older  soundings  should  be  repeated,  and  the  subject  be 
more  fully  investigated.* 

In  regard  to  Professor  Agassiz's  argument  that  the 
Florida  reefs  are  the  result  of  drifting  material  (see  p.  287), 
and  in  no  way  require  or  indicate  subsidence,  Professor 
Dana  points  out  that  there  is  little  in  the  great  barrier  reef 
of  Eastern  Australia,  which  has  some  correspondence  in 
position  with  the  sand  reefs  off  eastern  North  America,  to 
suggest  a  similarity  of  origin.  Full  of  irregularities  of 
direction  and  of  interruptions,  it  follows  in  no  part  an  even 
line.  In  the  northern  part,  the  barrier,  while  varying 
much  in  its  course,  is  barely  30  miles  from  the  land  ;  in  the 
southern  half  it  extends  out  150  miles  from  the  coast,  and 
includes  a  large  atoll-formed  reef.  Further,  in  the  Pacific 
Ocean,  the  trends,  whether  of  coral  island  groups,  or  of  the 
single  islands,  frequently  do  not  correspond  with  the  direc- 
tion of  the  oceanic   currents,  or  indeed  of  any  current 

depth  to  which  they  can  bare  the  bottom,  so  as  to  obtain  an  effective 
broadside  stroke,  which  he  thinks  rarely  exceeds  in  the  most  extreme 
cases  20  feet  vertical.  At  240  feet  he  believes  the  displacement  of 
the  water  would  be  at  most  only  a  few  inches,  and  thus  the  battering 
power  would  obviously  be  nil. 

•  See  below,  p.  319,  for  an  account  of  Mas&marhu  Island  in  the 
Red  Sea. 


GROWTH    OF    REEFS.  317 

which  IS  not  determined  by  their  existence.  Moreover, 
to  prove  formation  from  drifting  does  not  suffice  to  dis- 
prove subsidence.  The  length  of  Sandy  Hook  varies  in 
consequence  of  the  action  of  currents,  yet  this  does  not 
disprove  Professor  Cook's  conclusion  that  the  New  Jersey 
Coast  is  subsiding. 

Further,  it  must  be  borne  in  mind  that  subsidence  has 
undoubtedly  taken  place  in  the  region  around  Florida, 
though  at  present  we  can  only  prove  this  to  have  lasted  into 
the  earlier  Pleistocene,  the  difficulty  of  dealing  with  this 
being  augmented  by  the  occurrences  of  elevated  coral  reefs, 
in  Jamaica  at  2,000  feet  above  the  sea,  and  in  Cuba  perhaps 
even  at  the  same,  and  certainly  at  1,000  feet.  The  former, 
while  obviously  proving  elevation,  are  considered  by  Mr. 
Crosby  to  prove  also  the  occurrence  of  epochs  when  move- 
ment was  in  the  opposite  direction  (see  p.  809). 

The  next  section  calls  attention  to  the  vague  character 
of  the  evidence  adduced  for  the  building  up  from  deep 
waters  of  important  banks,  composed  of  organic  debris,  to 
serve  as  the  foundations  of  a  coral  reef,  though  doubtless 
such  a  thing  may  occasionally  occur.  But  in  regard  to  the 
existence  of  submerged  mountain  masses,  which  have  been 
indicated  as  suitable  for  the  like  purpose,  it  may  be  well 
to  give  Professor  Dana's  quotation  of  the  argument,  and 
his  comment  thereon.  Mr.  Murray  *  observes  that  the 
"  soundings  of  the  Tuscarora  and  Challenger  have  made 
known  numerous  submarine  elevations  :  mountains  rising 
from  the  general  level  of  the  ocean's  bed,  at  a  depth  of 
2,500  or  3,000  fathoms  up  to  within  a  few  hundred  fathoms 
of  the  surface."  But  "  a  few  hundred  fathoms,"  if  we  make 
**  a  few  "  equal  2,  means  1,200  feet  or  more,  which  leaves  a 
long  interval  yet  unfilled.' 

Lastly,  Professor  Dana  reviews  the  proposed  explana- 
tion of  the  ring-like  shape  of  atolls,  and  of  the  channels 
which  exist  in  the  enclosure  of  atolls  or  between  atolls ; 
22 


318  APPENDIX   II. 

and  points  out  that  the  larger  the  atoll  the  purer  the  sea- 
water  of  the  lagoon,  so  that  the  latter  would  speedily  reach 
a  limit  to  its  expansion  in  consequence  of  the  non-growth 
of  coral ;  indeed,  as  a  matter  of  fact,  these  larger  lagoona 
contain  plenty  of  living  coral.  The  second  (and  ultimately 
more  important)  factor  in  the  enlargement  of  a  lagoon, 
viz.  the  corrosion  of  the  dead  coral  by  the  solvent  action 
of  water,  he  regards  as  a  hypothesis  which  has  little  direct 
evidence  in  its  favour  and  much  indirect  against  it.  The 
connection  of  channels  with  prevaiHng  currents  cannot  be 
shown,  and  the  former  in  many  cases  are  sufficiently  deep 
to  be  well  below  the  limit  of  abrasion.  Indeed,  as  a  rule, 
so  far  from  these  channels  being  enlarged  by  solution  and 
abrasion,  they  tend  to  be  closed  by  the  growth  of  living 
coral,  and  many  of  the  lagoons  in  the  smaller  islands  are 
without  channels.  Hence  in  them,  as  there  can  be  no 
appreciable  transference  of  water,  the  action  of  solution 
must  be  reduced  to  a  minimum.  Yet  these  closed  atolls 
are  by  no  means  exceptional.  For  instance,  in  the  case  of 
about  sixty  coral  islands  mapped  by  the  Wilkes  expedition, 
of  those  which  range  from  1^  to  3  miles  in  the  longer 
diameter  of  the  reef,  nine  have  no  lagoon,  but  only  a  small 
depression  in  its  place,  which  is  dry,  except  in  the  case  of 
two  where  water  gets  in  at  high  tide.  Of  those  under  6 
miles  in  length,  having  lagoons,  17  in  number :  16  have 
no  entrance  to  the  lagoon  at  low  tide  ;  the  other  has  an 
entrance  of  large  size.  Of  those  6  miles  or  over  in  length, 
29  in  number :  17  have  channels  and  12  have  none ;  those 
having  channels  are  generally  over  10  miles  long.  It 
must  also  be  understood  that  the  opponents  of  the  subsid- 
ence theory  are  compelled  to  admit  it  in  order  to  explain 
the  depth  of  certain  lagoons.  Hence  Professor  Dana  con- 
siders that  the  hypothesis  of  elevation  or  lateral  spreading 
during  a  period  of  rest  is  inadequate  as  a  general  expla- 
nation  of  the  problem. 


masImarhu  island.  319 

To  the  arguments  advanced  by  the  author  already 
quoted  we  add  the  diagrams  annexed,  for  the  use  of  which 
we  are  indebted  to  the  courtesy  of  Messrs.  Macmillan  &  Co. 
They  appeared  in  Nature  (vol.  xxxvi.  p.  413),  illustrating  a 
communication  from  Captain  Wharton,  and  represent  two 
sections  on  a  true  scale,  made  by  Captain  Maclear  (H.M.S. 
*  Flying  Fish,'  of  the  slope  of  the  coral  reef'^surrounding 
the  small  island  of  Masamarhu,  situated  in  the  Red  Sea 
in  lat.  18°  49'  N.,  long.  38°  45'  E.  The  dotted  Unes  show 
where  the  soundings  were  obtained,  and  the  words  indicate 
the  nature  of  the  bottom. 

It  will  be  observed  that  there  is  a  remarkable  and 
significant  correspondence  between  these  two  sections, 
which,  as  the  plan  indicates,  are  taken  nearly  half  a  mile 
apart.  In  each  the  surface  of  the  fringing-reef,  after 
shelving  very  gently  downwards  to  a  depth  of  about  three 
or  four  fathoms,  is  bounded  by  a  submarine  cliff.  This  in 
one  section  (No.  I.)  continues  almost  unbroken  to  a  depth 
of  about  600  feet,  except  that  a  kind  of  ledge  or  terrace  is 
clearly  indicated  at  a  depth  of  rather  less  than  100  feet. 
In  the  other  section  (No.  II.)  the  foot  of  a  great  submarine 
cliff  is  found  at  about  500  feet,  but  in  this  case  the  cliff  is 
distinctly  divided  into  two  precipices  by  a  shelving  bank  oi 
coral  and  sand,  which  begins  at  a  depth  of  about  140  feet 
and  reaches  the  brow  of  the  lower  precipice  at  about  260 
feet.  This  bank  is  covered  by  *  sand  and  coral.'  At  this 
depth  in  each  section  the  island  is,  as  it  were,  defended 
by  a  deep  and  narrow  ditch,  the  edge  of  its  steep  glacis 
being  formed  by  a  sharp  arete  of  coral  which  in  one  case 
rises  into  soundings  of  about  250  feet.  From  this  the 
former  section  shows  a  second  rapid  fall  down  to  another 
ditch,  the  bottom  of  which  lies  more  than  1,200  feet  below 
the  sea  level.  This  in  section  resembles  the  other  one, 
and  the  height  of  its  counterscarp  is  more  than  300  feet. 
From  the  edge  of  this  the  glacis  for  a  short  distance  is  nearly 


No.  I. 


No.  n. 


Soalo  of  Feet 

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Sketch  of  MasAmarhu  I. 

^  Tn! 

showing:  approximate 

position  of  Sections 

1 

Lat.iS'.So'.N.rong.sS^ 

1    SEA   MILE 

.._ 

masXmaehu  island.  321 

level,  and  then  descends  at  an  angle  of  some  thirty  degrees. 
In  the  lower  diagram  we  find  no  indication  of  this  second 
ditch,  but  a  long  slope  begins  at  the  foot  of  the  submarine 
cliff  at  a  depth  of  about  850  feet,  which  is  very  nearly 
identical  with  that  of  the  flat  part  of  the  glacis  in  the 
former  section. 

It  will  be  observed  that  the  upper  ditch  (-that  common 
to  both  sections)  has  its  bottom  at  a  depth  of  full  600  feet, 
or  about  85  fathoms — that  is,  at  more  than  three  times  the 
average  depth  at  which  reef-building  corals  cease  to  live, 
while  the  least  depth  of  the  final  submarine  slope  is  850 
feet,  or  more  than  140  fathoms.  These  ditches  seem  irre- 
concilable with  any  idea  of  an  outward- spreading  growth 
of  the  reef,  and  must,  I  think,  be  indicative  of  a  subsidence 
which  isolated  the  outward  and  more  flourishing  edge  of 
a  shore  reef,  and  progressed  rather  too  rapidly  to  allow 
its  corals  to  extend  across  the  trench  thus  formed  and  effect 
a  union  with  the  main  mass.  Of  course  if  a  fissure-like 
hollow  were  once  established  between  two  masses  of  growing 
coral  in  a  subsiding  area,  it  would  not  be  readily  filled  up, 
unless  the  edge  of  its  outer  wall  were  sufficiently  near  the 
surface  to  suffer  much  from  the  violence  of  the  waves. 

The  former  section  seems  to  me  inexplicable  under  the 
conditions  ordinarily  admitted  for  coral  growth,  unless  we 
suppose  that  the  bottom  of  the  lower  ditch,  now  at  a 
depth  of  over  1,200  feet  (200  fathoms),  was  formerly  situ- 
ated within  about  25  fathoms  of  the  surface ;  so  that  a  sub- 
sidence of  more  than  1,000  feet  may  fairly  be  claimed  for 
the  coral  reef  of  Masdmarhu. 

Professor  Dana,^  in  an  article  which  appeared  while 
this  sheet  was  in  the  press,  adduces  some  new  and  very 

^  Points  in  the  Geological  History  of  the  islands  Maui  and  Oahu, 
By  J.  D.  Dana,  Amer.  Jour.  Sci.  vol.  xxxvii.  p.  81  (February  1889). 
I  am  indebted  to  my  friend  Professor  Judd  for  calling  my  attention 
to  this  article. 


322 


APPENDIX   II. 


important  evidence  in  regard  to  the  Sandwich  Islands. 
Oahu,  the  island  on  which  is  the  town  of  Honolulu,  gives 
indications  of  a  recent  upward  change  of  level,  amounting 
to  60  feet  at  least  on  its  northern,  and  about  20  feet  less 
on  its  southern  side.  But  this  is  not  all.  Several  deep 
borings  have  been  made  in  different  parts  of  the  island, 
the  particulars  of  which  are  recorded.  The  following  may 
be  taken  as  examples. 


I.  James  CamphelVs  Well,  at  west  foot  of  Diamond  Head, 
not  far  from  sea-level. 


II. 


Ihickness 

Depth 

Feet 

Feet 

Gravel  and  beach  sand .        .        • 

.      60    . 

Tufa  like  that  of  Diamond  Head  . 

270    . 

320 

Hard  coral  rock,  like  marble        .        < 

605    . 

825 

Dark  brown  clay 

.      75    . 

900 

Washed  gravel 

25     . 

925 

Deep  red  clay       •        •        .        •        i 

95     . 

1,020 

Soft  white  coral 

28     . 

1,048 

Soapstone-like  rock       .        .        • 

20     . 

1,068 

Brown  clay  and  broken  coral 

110    . 

1,178 

Hard  blue  lava 

45    . 

1,223 

Black  and  red  clay       .        •        •        . 

28     . 

1,251 

Brown  lava 

249    . 

1,600 

King's  Well  No.  2,  about  half  a  mile  west  of  Diamond 
Hillf  and  350  yards  from  the  sea-shore. 


Sand  and  coral 

White  coral  rock  • 

Yellow  sand     .  • 

Hard  lava        ,  • 

White  coral  rock  . 

Blue  clay 

Tough  clay  and  coral 

Blue  clay 

Hard  coral  rock  j 

Soft  coral  • 


Thickness 

Depth 

Feet 

Feet 

38     . 

22    . 

60 

,      43    . 

103 

.      47    . 

150 

110    . 

260 

25    . 

285 

65    . 

360 

30    . 

380 

40    . 

420 

30    . 

450 

masJCmat^hu  island. 


323 


King's  Well,  etc. — continued. 

Thickness 
reet 

Depth 
Feet 

Tougli  clay 5    . 

455 

White  coral  rock 

.      40    . 

495 

Tough  clay 

30     . 

525 

White  coral  rock 

100     . 

625 

Tough  clay      . 

5     . 

630 

Coral  and  clay 

.      70     . 

700 

Tough  clay 

.      28     . 

728 

Black  sand      .        . 

2     , 

730 

Lava 

120     . 

850 

III.  Well  in  Thomas  Square,  Honolulu 

. 

Thickness 

Depth 

Feet 

Feet 

Soil  6  feet,  with  6  feet  of  black  sand,  and 

clay  4  feet 16     . 

White  coral  rock 

200     . 

216 

Brown  clay 

44    . 

260 

Coral  rock       .        , 

10    . 

270 

Brown  clay 

60    . 

330 

White  coral  rock 

60    . 

380 

Brown  clay 

.      80     . 

460 

Bed  rock  or  lava,  pe 

netra 

ted 

, 

.      49    . 

609 

The  evidence  of  these  borings,  which  is  corroborated 
by  others  quoted  in  the  paper,  points  to  a  very  consider- 
able subsidence  in  this  region,  to  the  amount  of  at  least 
800  feet,  and  in  all  probability  of  considerably  more  than 
1,000  feet.  Moreover,  the  'hard  coral  rock,  like  marble' 
(No.  L)  can  hardly  be  anything  but  a  *  fossil  reef ' ;  the  base 
of  this,  it  will  be  observed,  after  some  upheaval,  is  even  now 
at  a  depth  of  825  feet  (137^  fathoms),  and  the  reef  has  a 
continuous  thickness  of  505  feet  (full  84  fathoms). 


The  above  abstracts  may  suffice,  I  hope,  to  give  a  fair 
representation  of  the  arguments  for  and  against  Mr.  Dar- 
win's theory,  which  have  been  advanced  during  the  last 
fourteen  years.  That  theory  is  regarded  by  some  enthu- 
siastic opponents,  as  already  on  the  threshold  of  the  limbo 


324  APPENDIX   II. 

appointed  for  exploded  hypotheses.  To  this  opinion  I 
cannot  declare  myself  a  convert,  for  reasons  which,  in 
conclusion,  I  shall  endeavour  to  indicate. 

First,  however,  I  may  remark  that  certain  of  Mr. 
Darwin's  critics  occasionally  appear  to  have  perused  liia 
book  with  overmuch  haste,  and  to  have  overlooked  the 
fact  that  he  admits  such  possibilities  as  local  upheavals, 
the  lateral  growth  of  reefs,  and  modes  of  formation  similar 
io  those  asserted  for  the  Florida  reefs  ;  *  that  in  short,  most 
of  the  causes  on  which  stress  has  been  laid  by  his  critics 
have  been  already  noticed  by  him,  so  that  he  differs  from 
them,  not  in  overlooking  such  causes,  but  in  assigning 
to  them  a  subordinate  value.  Moreover,  it  may  not  be 
unfair  to  call  attention  to  the  want  of  unanimity  among 
his  opponents  :  some  advocating  solution  as  a  primary  cause 
in  the  shaping  of  atolls,  while  others  rely  chiefly  on  the 
mode  of  growth  of  the  polyps.  Such  a  divergence  obviously 
does  not  prove  Mr.  Darwin  right,  but  it  does  indicate  that 
as  yet  no  other  hypothesis  has  been  able  to  secure  a  general 
acceptance,  and  that  the  problem  still  demands  the  exercise 
of  cautious  induction,  which  was  his  method  of  procedure, 
and  does  not  justify  the  over-confident  boldness  of  assertion 
which  has  characterised  at  least  one  critic  of  his  work. 

The  chief  arguments  which  have  been  advanced  against 
Mr.  Darwin's  theory,  as  it  appears  to  me,  may  be  thus 
summarised : — 1.  That  such  evidence  as  can  be  obtained 
in  regions  where  extensive  coral  reefs  exist  is  favourable 
to  upheaval  rather  than  to  subsidence.  2.  That  lateral 
growth  is  a  most  important  factor  in  the  formation  of  a  reef, 
the  polyps,  as  they  advance,  being  supported  on  a  founda- 
tion composed  partly  of  the  broken  fragments  of  the  reef, 
partly  of  other  marine  organisms,  and  that  by  means  of  the 
latter  deeply  submerged  banks  are  sometimes  augmented 
vertically  until  they  are  brought  within  the  zone  of  reef-coral 

>  ^ee  pp.  22,  23,  79,  120,  121,  174,  etc. 


SUMMARY   OF   ARGUMENTS.  325 

life.  8.  That  lagoons  and  lagoon  channels  are  materially 
enlarged  by  the  destruction  of  dead  coral  through  the 
solvent  effects  of  sea- water.  4.  That  in  the  past  history 
of  the  earth  we  find  no  evidence  in  favour  of  the  formation 
of  coral  reefs  in  areas  of  subsidence,  or  in  other  words 
that  fossil  coral  reefs  are  less  than  some  25  fathoms  thick. 

1.  Much  stress  is  evidently  laid  upon  the  fact  that 
many  coral  islands  afford  evidences  of  a  certain  amount  of 
upheaval.  This  amount,  in  most  cases,  is  but  slight,  and 
its  significance  appears  to  me  to  have  been  exaggerated. 
Undoubtedly,  it  proves  that  the  record  which  is  the  most 
obvious  indicates  an  upward  and  not  a  downward  motion, 
but  in  so  doing  it  introduces  a  difficulty  which  will 
presently  be  noticed.  These  indications,  however,  do  not 
of  themselves  prove  a  general  upheaval,  but  only  oscilla- 
tion. Every  geologist  is  aware  that  movements  in  any 
given  direction  are  frequently  neither  uniform  nor  contin- 
uous. For  instance,  no  one  doubts  that  the  western  coast 
of  Scandinavia,  and,  in  a  less  degree,  that  of  Great  Britain, 
have  very  considerably  subsided  since  the  sculpture  of  their 
leading  physical  features,  and  yet  from  the  Land's  End  to 
the  North  Cape  we  constantly  find  proofs  that  the  latest 
movements  have  been  in  an  upward  direction.  Even  in 
the  case  of  the  more  important,  but  much  rarer,  upheaval 
of  reefs,  as  at  the  island  of  Cuba,  the  coral  masses  are  so 
thick  that  we  must  assume  the  practical  arrest  of  all  up- 
ward movement  during  the  growth  of  the  reef.  In  this 
case  also,  if  the  coral  reef  be  only  a  sort  of  cap  concealing 
a  hill  of  pre-existent  rock,  we  may  reasonably  be  surprised 
that  the  '  ashlar- work '  of  coral  limestone  has  in  no  case 
so  far  yielded  to  the  action  of  the  atmospheric  agencies  as 
to  lay  bare  its  inner  support. 

Doubtless  there  are  many  reefs  to  which  either  explan- 
ation might  be  applied,  but  there  are  some  which,  unless 
eoral  polyps  can  build  at  depths  much  greater  than  26 


326  APPENDIX  n. 

fathoms,  can  only  be  explained  by  subsidence.  It  is  sought 
to  elude  this  difiSculty  by  supposing  that  the  reef  builders, 
under  specially  favourable  circumstances,  may  commence 
operations  at  depths  considerably  greater  than  the  usual 
limit.  It  is  indeed  true  that  reef  corals  are  sometimes 
dredged  alive  from  depths  much  exceeding  25  fathoms, 
but  the  result  of  all  recent  researches  has  certainly  been 
to  confirm  the  general  correctness  of  this  bathymetrical 
limit,  and  the  proposed  evasion  of  the  difficulty  is  at 
present  a  mere  hypothesis,  which  bears  a  suspicious  resem- 
blance to  the  epicycles  devised  to  prop  up  the  Ptolemaic 
system  of  astronomy. 

While  the  existence  of  *  continental  rocks,'  as  they  may 
be  called,  in  oceanic  islands  would  have  almost  proved  a 
general  subsidence,  I  do  not  see  that  the  frequent  occur- 
rence of  volcanic  rocks  is  seriously  opposed  to  it.  The 
arrangement  of  the  majority  of  coral  islands,  whether 
wholly  composed  of  organic  material,  or  incrusting  a 
nucleus  of  volcanic  rocks,  is  indicative  of  lines  of  weakness 
in  the  earth's  crust,  which  would  give  rise  to  movements 
in  either  direction,  and  in  each  case  the  islands  would  be 
connected  with  extruded  masses  of  volcanic  rocks,  ejected 
at  various  points  along  these  lines.*  Thus,  we  have  to 
consider  which  of  two  hypotheses  is  the  more  probable : 
(a)  that  mounds  thus  formed  have,  in  the  majority  of 
cases,  failed  to  reach  the  surface,  but  have  nevertheless 
generally  arrived  within  a  comparatively  short  distance  of 
that  goal;  or  (6),  that  they,  after  having  in  many  cases 
overtopped  the  surface,  have  again  subsided.     The  latter, 

'  It  must  not  be  forgotten  that  though  the  peaks  of  mountain 
ranges  are  frequently  composed  of  '  continental  rocks,'  instances  are 
by  no  means  wanting,  as  in  the  Andes,  Caucasus,  &c.,  where  the 
higher  portions  are  volcanic.  In  more  insulated  mountain  masses, 
as  those  of  Etna,  Kilimanjaro,  Ararat,  of  some  of  the  islands  of  the 
Malay  Archipelago  and  on  the  western  coast  of  North  America,  we 
have  instances  of  volcanoes  forming  the  highest  part  of  the  land. 


SUMMARY   OF    AKGUMENTS.  327 

I  must  confess,  seems  to  me  the  more  probable,  especially 
when  we  remember  that  subsidence  very  commonly  occurs 
in  a  district  when  it  has  recently  ceased  to  be  the  scene  of 
volcanic  disturbances  on  a  large  scale. 

2.  In  regard  to  the  lateral  spreading  of  reefs,  like  a  *  fairy 
ring,'  as  it  has  been  happily  expressed,  there  is  no  doubt 
that,  as  has  been  admitted  by  Mr.  Darwin,'  some  augmen- 
tation may  occur  in  this  way ;  but  to  regard  this  as  a  factor 
of  prime  importance  in  the  development  of  a  reef  seems  tome 
to  import  new  and  serious  difficulties.  Let  us  assume  that 
the  submarine  mound  or  shoal  on  which  the  reef  is  founded 
remains  at  rest  during  the  whole  period  of  the  growth  of  the 
latter,  and  that  this  commences  on  the  area  (regarded,  for 
simplicity,  as  a  plain)  included  within  the  bathymetrical 
contour  line  of  25  fathoms.  For  a  considerable  period, 
until  the  edge  of  the  reef  arrives  within  a  few  fathoms,  pro- 
bably less  than  ten  (see  p.  315),  of  the  surface  of  the  sea — 
that  is,  for  full  three-fifths  of  its  whole  vertical  growth — 
the  exterior  slopes  will  only  be  augmented  by  the  accu- 
mulation of  marine  organisms,  a  process  which  cannot  be 
rapid.  Hence,  for  a  considerable  time,  until  the  reef 
itself  has  completed  the  greater  part  of  its  growth,  and 
begins  to  augment  the  talus  with  its  own  ruins,  the  process 
of  laying  the  foundation  for  a  new  coral  growth,  and  thus 
the  lateral  spreading  of  the  reef,  will  be  slow. 

Consider,  then,  the  case  of  a  reef  where  this  process  hag 
begun,  and  for  simplicity  regard  it  as  a  cylinder  cap- 
ping a  flat-topped  cone.  Obviously,  if  the  reef  begin  to 
spread  laterally,  the  volume  of  the  foundation  required  to 
support  the  new  growth  increases  far  more  rapidly  than 
the  area  from  which  material  can  be  supplied.  Hence, 
as  the  reef  advances  outwards,  the  rate  of  increase  will 
rapidly  diminish,  unless  we  suppose  either  an  extraordi- 
nary annual  destruction  of  growing  coral,  or  an  increased 
*  See  pp,  22,  67,  70  of  this  work. 


328  APPENDIX   II. 

accumulation  of  other  organisms.  Moreover,  unless  we 
rely  on  solution  for  enlarging  the  lagoon,  this  will  remain 
of  its  original  size,  and  thus  will  be  small  in  comparison 
with  the  ultimate  area  of  the  atoll.  No  doubt,  for  a  time, 
as  the  reef  is  approaching  the  surface  of  the  sea,  the  more 
rapid  growth  of  the  coral  at  its  outer  margin  will  cause 
it  to  be  saucer-like  in  section,  and  thus  somewhat  enlarge 
the  lagoon,  but  as  soon  as  the  upward  growth  ceases  this 
process  is  arrested  and  the  atoll  can  only  spread  laterally 
and  thus  must  increase  in  breadth,  while  the  lagoon,  if 
there  be  no  solution,  tends  rather  to  diminish  in  size. 

It  is,  however,  stated  on  good  authority^  that  coral 
growth,  as  a  rule,  is  by  no  means  entirely  arrested  in 
a  lagoon,  and  we  cannot  suppose  that  so  long  as  there  is 
free  passage  for  a  considerable  stratum  of  water  above  the 
reef — i.e.  so  long  as  there  are  soundings  of  8  or  10  fathoms 
over  it — the  polyps  on  its  inner  part  will  suffer  materially 
from  want  of  food  or  properly  aerated  water.  Hence  the 
lagoon  will  not  be  formed  at  all  until  the  reef  has  made 
some  progress  upwards,  so  that  it  should  always  be  com- 
paratively shallow,  not  exceeding  a  few  fathoms  in  maxi- 
mum depth.  From  the  above  considerations  it  appears  to 
me  that  the  *  fairy-ring  '  hypothesis  is  inadequate  unless 
it  be  inseparably  linked  with  that  of  '  solution.' 

At  this  period  we  may  not  unfitly  notice  another  con- 
sideration which  has  been  urged,  viz.  that  many  shoals, 
chiefly  of  volcanic  origin,  which  lie  at  too  great  a  depth  to 
be  colonised  by  reef-building  polyps,  may  be  raised  up  to 
the  proper  level  by  the  accumulation  of  marine  organisms. 
That  this  may  sometimes  occur  cannot  be  denied,  but  it 
must  be  remembered  that,  unless  the  shoal  lie  at  a  very 
moderate  depth  below  the  required  level,  the  process 
of  accumulation  will  be  extremely  slow.  Mr.  Murray's 
estimate  of  the  quantity  of  carbonate  of  lime  present  in  the 
»  See  pp.  302,  318  of  this  work. 


THE   SOLUTION   THEORY.  329 

minute  organisms  wMch  inhabit  the  upper  stratum  of  the 
ocean  water  seems  at  first  considerable,  but  when  we 
estimate  its  thickness  in  a  given  area,  this  proves  to  be 
extremely  small.  Hence,  unless  we  assign  a  very  brief 
existence  to  each  individual,  and  thus  suppose  a  heavy  rain 
of  sliells  on  the  ocean  floor,  the  foundation  for  the  future  reef 
will  rise  but  slowly,  and  its  initiation,  in  the  case  of  those 
which  now  exist,  must  be  carried  back  to  a  rather  remote 
epoch.  Here,  again,  we  may  inquire  whether  a  cause,  which 
must  not  be  wholly  overlooked,  has  not,  through  an  error 
in  mental  perspective,  been  brought  into  undue  prominence. 
3.  The  solution  theory,  which  indeed  by  no  mea,ns 
meets  with  universal  acceptance  among  Mr.  Darwin's 
critics,  appears  to  me  beset  with  considerable  difficulties. 
The  solubility  of  carbonate  of  lime  in  ocean-water  cannot 
of  course  be  denied ;  but  is  there  satisfactory  evidence  that 
this  is  a  factor  of  primary  importance  to  the  case  of  a 
coral  reef?  The  apparently  rapid  solution  of  calcareous 
organisms  at  great  depths  has  but  little  bearing  on  what 
occurs  at  small  depths,  and  the  good  preservation  of  the 
*globigerina  ooze '  down  to  depths  of  some  2,000  fathoms,  in 
itself  indicates  that  solution  to  any  important  amount  takes 
place  under  very  exceptional  conditions.  The  rottenness 
frequently  noted  in  dead  coral  is  mainly  due  to  the  decom- 
position of  the  animal  tissues  with  which  the  mineral 
constituent  is  incorporated :  thus  the  process  is  one  of 
disintegration  more  than  of  solution.  The  dead  coral  is 
no  doubt  to  some  extent  dissolved,  but  it  mainly  forms  a 
sand  or  mud.  This  of  course,  in  some  cases,  will  be  swept 
out  by  currents  into  the  open  ocean,  and  thus  the  coral 
will  be  removed  from  its  place  of  growth,  but  it  may  well 
be  doubted  whether  this  substitute  for  a  true  solution  will 
be  for  long  a  factor  of  prime  importance  in  the  genesis 
of  a  lagoon.  There  is  moreover  some  evidence  directly 
opposed  to  the  theory  of  solution  at  a  moderate  depth,  as,  fox 


330  APPENDIX   II. 

example,  the  blocks  of  recent  limestone  which  were  dredged 
by  Professor  A.  Agassiz  off  the  Florida  reef.^  Under  what 
circumstances,  then,  will  the  sea- water  act  as  a  solvent  on 
the  dead  coral  ?  I  think  we  must  reply,  When  the  fluid 
is  rather  rapidly  altering  its  position  in  regard  to  the  sub- 
stance attacked.  Thus  rain  and  streams  are  important 
solvents,  and  so  might  be  breaking  waves  or  tidal  ebb  and 
flow,  but  when  the  water  is  at  rest  or  is  only  spreading 
with  a  slow,  diffusive  movement,  its  solvent  action  is  ex- 
tremely slight.  For  instance,  chalk  often  is,  and  must 
often  have  been,  saturated  with  water,  yet  numbers  of  the 
minute  organisms  which  enter  into  its  composition  are  still 
perfectly  distinguishable.  The  same  is  true  of  many 
other  limestones ;  indeed  the  effect  of  water  often  seems 
conservative  rather  than  destructive.  It  sinks  down  into 
the  body  of  the  rock,  carrying  with  it  the  carbonate  of  lime 
which  has  been  obtained  from  the  exposed  superficial  part 
of  the  mass,  but  on  reaching  the  level  of  saturation,  when 
it  only  percolates  by  diffusion,  it  commonly  deposits  its 
burden,  filling  up  with  mineral  calcite  the  interstices  of  the 
organic  materials.  Hence  the  comparatively  quiet  waters 
of  a  lagoon  would  be  favourable  to  the  consolidation  rather 
than  to  the  destruction  of  the  dead  coral,  save  only  within 
a  very  limited  distance  from  the  surface.  Moreover,  the 
remains  of  organisms,  when  once  the  interstitial  animal 
tissues  have  been  replaced,  appear  to  be  less  soluble  than 
the  other  parts  of  a  rock,  as  is  indicated  by  the  familiar 
*  weathering  out '  of  fossils.  Eeef  rock  also  appears  very 
apt  to  assume  a  solid  and  semi-crystalline  condition 
(p.  17),  and  in  regard  to  this  we  must  not  overlook  a 
peculiarity  of  coral  which,  as  it  seems  to  me,  has  an 
important  bearing  on  the  subject.  Dead  coral  is  very 
readily  converted  into  dolomite,  which  is  a  much  less 
Boluble  salt  than  calcite.  Further,  the  conditions  which 
>  See  p.  288  of  this  work. 


GEOLOGICAL  EVIDEXCE.         331 

would  prevail  in  a  lagoon,  when  its  waters  had  become 
unsuitable  for  coral  life,  would  be  those  which  would  be 
exceptionally  favourable  to  the  formation  of  dolomite.  It 
seems,  then,  from  the  above  considerations  that  we  cannot 
regard  the  corrosive  effect  of  sea-water  as  an  agent  of  more 
than  very  secondary  importance  in  modifying  the  structure 
of  an  atoll. 

4.  In  regard  to  the  negative  geological  evidence.  Here 
we  must  not  overlook  two  considerations — one  that  the 
structure  of  a  coral  reef  is  very  commonly  more  or  less 
composite ;  broken  coral,  shells,  &c.,  forming  a  part,  and 
sometimes  predominating  when  from  one  cause  or  another 
the  growth  of  the  polyps  is  temporarily  checked  (p.  155) ; 
Jience  in  some  cases,  what  is  really  a  continuous  reef  may 
be  supposed,  if  only  an  occasional  section  be  visible,  to  be 
a  series  of  thin  reefs — the  other  (the  more  important  and 
general)  that  the  characteristic  structure  of  dead  coral 
becomes  rapidly  inconspicuous  and  may  be  only  discover- 
able in  thin  sections  under  the  microscope.  Where  dolo- 
/mitisation  has  occurred  it  may  be  actually  obliterated,  for 
the  molecular  changes  involved  in  the  process  are  often 
sujBScient  to  destroy  every  trace  of  an  organism.  We  may 
thus  be  prevented  from  recognising  many  ancient  coral 
reefs.  Moreover,  the  aporosa  and  madre;poraria,  which 
are  now  the  chief  reef-builders,  have  only  become  common 
since  the  conclusion  of  Paleozoic  ages,  so  that  the  largest 
volume  of  the  geological  history  of  the  earth  is  excluded 
from  consideration,  because  in  the  times  which  it  covers 
the  habits  of  the  reef-builders  may  have  been  different. 
Eeefs  also,  it  must  be  remembered,  are  restricted  at  the 
present  day  to  almost  tropical  regions,  so  that,  notwithstand- 
ing any  variation  of  climate,  they  must  always  have  been 
less  frequent  and  less  luxuriant  in  northern  latitudes — that 
is  to  say,  in  those  regions  with  which  geologists  are  best 
acquainted.     Still,  instances  of  thick  reefs  of  comparatively 


332  APPENDIX  n. 

late  date  are  on  record,^  and  if  those  geologists  are  right  who 
consider  the  Schlem  dolomites  as  being  to  a  great  extent 
due  to  reef-building  corals,  we  have  in  the  Triassic  deposits 
of  the  Italian  Tyrol  reefs  thick  enough  to  satisfy  the  most 
exacting  requirements. 

It  is  then,  I  think,  premature  to  regard  the  theory 
which  was  advanced  by  Mr.  Darwin,  and  has  received  the 
approval  of  an  observer  of  such  an  exceptional  experience 
as  Professor  Dana,  as  conclusively  disproved  by  the  results 
of  the  more  recent  investigations.  That  this  theory  may 
have  been  expressed  in  terms  a  little  too  comprehensive, 
that  there  may  be  a  larger  number  of  exceptional  cases 
than  was  at  first  supposed,  is  quite  possible.  This,  however, 
is  the  almost  inevitable  lot  of  every  great  generalisation.  Its 
author  concentrates,  and  rightly  concentrates,  his  atten- 
tion on  the  salient  features,  as  one  who  gazes  first  at  a 
mountain  group  fixes  his  eyes  upon  the  principal  peaks 
and  for  a  time  pays  little  attention  to,  perhaps  even 
under-estimates  the  importance  of,  the  subordinate  ranges ; 
nevertheless  his  conception  of  the  physical  structure  of  the 
region,  though  modified,  is  not  overthrown  by  the  work  of 
subsequent  travellers.  This  may  prove  to  be  the  case  in 
regard  to  the  present  controversy.  It  may  very  possibly 
be  found  that,  as  remarked  by  Mr.  Bourne,  the  history  of 
coral  reefs  is  more  varied  and  complicated  than  was  at  first 
supposed,  but  it  seems  to  me  that,  as  the  evidence  at 
present  stands,  it  is  insufficient  to  justify  a  decision  adverse 
to  Mr.  Darwin's  theory  as  a  general  explanation. 

t  gee  pp.  309,  322  of  this  work. 


INDEX. 


The  names  In  italics  are  all  names  of  places,  and  refer  exclusively  to  the  Appendix;  lo 
well-defined  archipelagoes,  or  groups  of  islands,  the  name  of  each  separate  island  i& 
not  given.    References  in  square  brackets  refer  to  the  new  appendix. 


ABB 

Abrolhos,  Brazil,  coated  by  corals, 

79 
Abrolhos  [Australia),  235 
Absence  of  coral-reefs  from  cer- 
tain coasts,  81 
Acaba,  gulf  of,  266 
ActcBon  group,  200 
Admiralty  group,  224 
Admiralty  islands,  [285] 
Africa,  east  coast,  f ringing-reef  of, 
76.  Madreporitic  rocks  of, 
181 
Africa,  east  coast,  254 
Agassiz,  Prof.  A. 

on    Tortuga    and    Florida 

reefs,  [287] 
effect  of  Gulf  stream,  [287] 
growth  of  Florida  reefs,  [288] 
effect  of  currents  on  reefs, 

[289] 
depth  of  lagoons,  [290] 
formation  of  silt  [291] 
Age  of  individual  corals,  96 
Aiou,  231 
Aitutaki,  204 
Aldabra,  251 
Alert  reef,  222 
Alexander,  Grand  Duke,  island^ 

207 
Allan,  Dr. 

on  HoluthuriaB  feeding   on 
corals,  20 


AS6 

Allan,  Dr. 

on  quick  growth  of  corals  at 

Madagascar,  104 
on  reefs  affected  by  currents, 

79 
Alloufatou,  214 
Alphonse,  250 
Amargura,  214 
Amboina,  232 
America,  west  coast,  199 
Amirantes,  249 
Anachorites,  225 
Anambas,  240 

Anamouka,  description  of,  177 
Anamouka,  213 
Andaman  islands,  239 
Antilles,  274 
Appoo  reef,  241 
Arabia  Felix,  260 
Areas, 

great  extent  of,  interspersed 

with  low  islands,  122 
of  subsidence  and  of  eleva- 
tion, 191 
of  subsidence  appear  to  be 

elongated,  191 
of    subsidence     alternating 

with   areas  of   elevation, 

192 
Ami  group,  231 
Arzobispo,  230 
Ascension,  no  reef  at,  83 


384 


INDEX. 


^  A80 

A.scidia,  depth   at  which  found, 

117 
Assovvption,  250 
Astova,  250 

Atlantic  islands,  83,  217 
AtoUs, 

breaches  in  their  reefs,  ?9, 

145 
dimensions  of,  27 
dimensions  of  groups  of,  123 
not  based  on  craters,  or  on 
banks  of  sediment,  or  on 
rock,  119, 124, 125, 126, 194 
of  irregular  forms,  28,  146 
steepness  of  their  fianks,  31, 

164,  229 
width  of  their  reef  and  islets, 

28 
their  lowness,  122 

lagoons,     85,     [285], 

[318] 
general  range,  167 
with  part  of  their  reef  sub- 
merged, and  theory  of,  37, 
146,  147 
whole  reef  submerged,  and 
theory  of,  38,146, 147,  [28^ 
Augttstin,  St.,  217 
Aurora  island,  an  upraised  atoll, 

123 
Aurora,  201 
Austral  islands,  recently  elevated, 

167,  177,  186 
Austral  islands,  204 
Australia,  N.W.  coast,  234 
Australian  barrier-reef,  63,  166 
Australian  barrier,  222 


Babuyan  group,  243 
Bahama  banks,  268,  270 
Balabac,  240 

Balfour,  Prof.  Bayley,  descrip- 
tion of  Eodriguez,  [307] 
Bally,  236 
Bampton  shoal,  222 
Banks*  islands,  220 
Banks  in  the  West  Indies,  267 
Barbes,  St.,  240 
Baring,  218 


BON 

Barrier-reef 

of  Australia,  63,  166 
of  New  Caledonia,  63,  67 
Barrier-reefs, 

breaches  through,  135 
not    based    on    worn-down 
margin  of  rock,  66 

on  banks  of  sediment, 

67 
on  submarine  craters, 
68 
steepness  of  their  flanks,  57, 

164 
their  probable  vertical  thick- 
ness, 64,  134 
.    theory  of    their  formation, 
133,  137,  [297] 
Bashee  islands,  243 
Bass  island,  206 
Batoa,  215 
Beaupri  reef,  221 
Beechey,  Capt., 

obligations  of  the  author  to, 

30 
on  submerged  reefs,  37 
account  of  Matilda  island, 
101 
Belcher,  Sir  E. 

on    boring    through    coral 

reefs,  99 
on  changes  in  Chain  atoll, 

172 
on    Clipperton    rock,    199, 
200 
BelUnghausen,  203 
Bengal,  gulf  of,  elevation  of  east- 
ern shores  of,  181 
Bermuda  islands,  275 
Beveridge  reef,  212 
Bligh,  220 
Bolabola,  view  of,  3 
Bombay  shoal,  245 
Bonin  bay,  238 
Bonin  group,  230 
Bonney,  Prof.  T.  G., 

discussion     of      arguments 
against      Mr.     Darwin's 
theory,  [325] 
movements  of  upheaval  and 
subsidence,  [326] 


INDEX, 


335 


BON 

Bonney,  Prof.  T.  G., 

on  lateral  spreading  of  reef, 

[327] 
on  the  solution  theory,  [329] 
on  the  geological  evidence, 
[331] 
Borings  through  coral  reefs,  99, 

[322] 
Borneo,  W.  coast,  recently  ele- 
vated, 180 
Borneo, 

E.  coast,  237 
S.W.and  W.  coast, 'iAQ 
N.  coast,  240 
western  hank,  240 
Boscawen,  214 
Boston,  218 
Bouka,  223 
Bourbon,  249 
Bourne,  Mr.  G.  0., 

Chagos  group,  [300] 

on  solution  of  dead  coral, 

[301] 
on  lateral  spreading  of  reefs, 

[301] 
coral  growth  in  lagoons,  [302] 
importance  of  currents,  [304] 
conclusions,  [305] 
Bourou,  232 
Bouton,  238 
Brazil,  fringing-reefs  on  coast  of, 

77 
Breaches  through  barrier -reefs, 

135 
Brook,  207 
Bunker,  207 
Bunoa,  240 
Byron,  217 


Cagayanes,  241 
Candelaria,  224 
Carbonate     of    lime    in    ocean 

water,  [284],  [328] 
Cargados  Carajos,  248 
Caroline  archipelago,  225 
Caroline  island,  206 
Carteret  shoal,  231 
Caryophyllia,  depths  at  which  it 

lives,  117 


CON 

Cavilli,  241 
Cayman  island,  273 
Celebes,  233 
Ceram,  232 

Ceylon,  recently  elevated,  171 
Ceylon,  247 

Chagos  Great  hank,  [300] 
Chagos,  Great  bank,  description 
and  theory  of,   53,   149, 
[300] 
Chagos  group,  149 
Chagos  group,  247,  [300] 
Chama  shells  embedded  in  coral 

rock,  106 
Chamisso  on  corals  preferring  the 

surf,  85 
Changes 

in  the  state  of  Keeling  atoll, 

20 
of  atolls,  130,  170 
Channels 

leading  into  the  lagoons,  59 
of  atolls,  39,  148,  [312] 
the  Maldiva  atolls, 
48,49 
through  barrier-reefs,  135 
Chase,  217 
China  sea,  244 
Christmas  atolh/  jO 
Christmas  atoy,  208 
Christmas  isl&nd  (Indian  OceanSy 

246 
Clarence,  207 

Clarke,  W.  B.,  on  recent  elevation 
of  the  Loyalty  islands,  178 
Clipperton  rock,  199 
Cochin  China,  246 
Cocos,  or  Keeling  atoll,  7,  [305] 
Cocos  {or  Keeling),  246 
Cocos  island  (Pacific),  199,  214 
Coetivi,  250 
Comoro  group,  251 
Composition  of  coral  formations, 

155 
Conglomerate 

coral  rock  on  Keeling  atoll, 

17 
coral  rock  on  other  atolls, 

35 
coral  rock,  156 


336 


INDEX. 


COO 

Cook  islands,  recently  elevated, 

177 
Cook  islands,  204 
Corallian  sea,  166 
Corallian  sea,  222 
Coral-reefs, 

their   distribution    and  ab- 
sence from  certain  areas, 
80 
destroyed  by  loose  sediment, 
87 
Coral-rock, 

at  Keeling  atoll,  16 
Mauritius,  74 
Metia,  98 
organic  remains  of,  156 
at  Oahu,  [322] 
Coral-rocks  bored  by  vermifonu 

animals,  20,  156 
Corals, 

dead  but  upright  in  Keeling 

lagoon,  21 
depths  at  which  they  live, 

108,  [293],  [298],  [326] 
off  Keeling  atoll,  11 
killed  by  a  short  exposure, 

8 
living  in  the  lagoon  of  Keel- 
ing atoll,  18 
quick  growth  of,  in  Keeling 

lagoon,  17 
merely  coating  the  bottom 

of  the  sea,  79 
standing  exposed  in  the  Low 
archipelago,  170 
Comwallis,  218 
Cosmoledo,  250 
Couthouy,  Mr. 

alleged  proofs  of  recent  elevii- 
tion  of  the  Low  archipo 
lago,  170 
on    external    ledges    round 
coral  islands,  140 
Crescent- formed  reefs,  146 
Crosby,  Mr.  W.O.,  on  the  raised 
reefs  of  Cuba,  [308] 
proofs     of     ancient     subsi- 
dence, [309] 
Cuba,  270,  [808] 
Cuming,  Mr.,  on  the  recent  olo- 


DEP 

vation  of  the  Philippines, 
180 


Dana,    Prof.,    on   the    reef    of 
Hawaii,  82,  175 

distribution  of  coral-reefs  as 
affected  by  the  tempera- 
ture of  the  sea,  85 

upraised  coral-rock  of  Metia, 
98 

boring  through  coral -rock, 
99,  [322] 

depth  at  which  corals  live, 
112 

Bubsidence  of  the  Mendana 
island,  165,  204 

Bubsidence  in  the  Caroline 
archipelago,  169 

slight  recent  subsidence  of 
the  Paumotu  archipelago, 
170 

extension  of  the  Hawaii  ar- 
chipelago, 192,  211 

Feejee  islands,  215 

outline  of  some  islands  in- 
dicative of  subsidence, 
[310] 

distribution  of  reef -materials, 
[311] 

different  kinds  of  reef  in 
Pacific,  [312] 

submarine  slopes,  [313] 

local  elevations,  [314] 

on  soundings  at  Tahiti, 
[314] 

on  lateral  spreading  of  reefs, 
[315] 

on  Florida  reefs,  [316] 

on  Mr.  Murray's  explanation 

of  the  foundation  of  reefs 

and  of  ring-shaped  atolls, 

[317] 

Danger  islands,  207 

Dangerous  or  Low  arcliipelago^ 

200 
Depths, 

at  which  reef-building  corals 
Hve,     108,    [293],     [298], 
[326] 


INDEX. 


337 


DEP 

Depths   at   Mauritius,  the  Eed 

Sea,  and  in  the  Maldiva 

archipelago,  113 
at  which  other  corals  and 

corallines  can  live,  116 
Dhalac  group,  261 
Diego    Garcia,    slow  growth  of 

reefs,  92 
Dimensions  of  the  larger  groups 

of  atolls,  123 
Discovery  shoal,  245 
Disseverment    of     the    Maldiva 

atolls,  and  theory  of,  50, 

143 
Distribution  of  coral-reefs,  80 
Dolomitisation  of  coral,  18,  [330] 
Domingo,  St.,  273 
Dory  Port,  recently  elevated,  179 
Dory  Port,  231 
Duchassaing  on  rapid  growth  of 

corals,  107 
Duff's  islands,  221 
Durour,  225 


Eap,  227 
Earthquakes, 

at  Keeling  atoll,  25 

in  groups  of  atolls,  131 

in  Navigator  archipelago,178 
East  Indian  archipelago  recently 

elevated,  179 
Easter,  200 
Echequier,  225 
Ehrenberg, 

on  the  banks  of   the   Bed 
Sea,  78,  260 

on  depth   at   which   corals 
live  in  the  Eed  Sea,  113 

on  corals  preferring  the  surf, 
85 

on  the  antiquity  of  certain 
corals,  96 
Eimeo,  201 
Elevated  Keef  of  Mauritius,  74 

of  Rodriguez,  [307] 

of  Cuba,  [308] 
Elevations,  recent, 

proofs  of,  175,  [291] ,  [314] 

immense  areas  of,  190 


FBI 

Elivi,  227 
Elizabeth  island,  98 

recently   elevated,  176,  186 
Elizabeth  island,  200 
Ellice  group,  216 
Encircled  islands, 
their  height,  62 
their  geological  composition, 

62,68 
Eoua,  description  of,  177 
Eoua,  213 
Erupted    matter,   probably    not 

associated      with      thick 

masses    of    coral     rock, 

155-157 


Fais  recently  elevated,  179,  191 

Fais,  227 

Fanning,  209 

Farallon  de  Medinilla,  229 

Farsan  Group,  262 

Fataka,  220 

Fidji  archipelago,  214,  [310] 

Fish, 

feeding  on  corals,  20 
killed  in  Keeling  lagoon  by 
heavy  rain,  27 

Fissures    across    coral    islands 
132,  262 

FitzRoy,  Capt., 

on    a    submerged    shed    at 

Keeling  atoll,  25 
on  an  inundation  in  the  Low 
archipelago,  130 

Flint,  206 

Flores,  236,  259 

Florida,  270,  275,  [287J 

Folger,  230 

Formosa,  243 

Forster,  theory  of  coral  forma- 
tions, 127 

Frederic  reef,  222 

Freewill,  231 

Friendly  group  recently  elevated, 
177,  186 

Friendly  archipelago,  212 

Fringing-reefs 

absent  where    coast    preci- 
pitous, 69 


388 


INDEX. 


FBI 

Fringing-reefs 

breached  in  front  of  streams, 
88 

described  by  MM.  Quoy  and 
Gaimard,  175 

not  closely  attached  to  shel- 
ving coasts,  72 

of  east  coast  of  Africa,  76 

of  Cuba,  75 

of  Mauritius,  69,  71 

on  worn-down  banks  of  rock, 
78 

on  banks  of  sediment,  78 

their  appearance  when  ele- 
vated, 74 

their  growth  influenced  by 
currents,  79,  [304] 

by  shallowness  of  sea, 
77 


Galapagos  archipelago,  199 

Galega,  250 

Gambier  islands,  section  of,  66 

Qambier  islands,  201 

Gardner,  208 

Gaspar  Bico,  218 

Geological  composition  of  coral 
formations,  156,  [331] 

Gilbert  archipelago,  217 

Gilalo,  233 

Glo'rioso,  250 

Gloucester  island,  130 

Glover  reef,  272 

Gomez,  200 

Go^iap,  227 

Goulou,  227 

Grampus,  230 

Gra7id  Cocal,  216 

Graves,  on  the  recent  elevation 
in  the  Bonin  archipelago, 
179 

Great  Chagos  bank,  description 
and  theory  of ,  53, 150,  [300] 

Grey,  Capt.,  on  sand-bars,  72 

Guedes,  231 

Guppy,  Mr.,  on  Solomon  archi- 
pelago, [291] 
proofs  of  upheaval,  [291] 
summary  of  opinion,  [292] 


rail 

Guppy,  Mr., 

growth  of  corals,  [292] 

development  of  reefs,  "293] 

different  kinds  of  reei'a, 
[294] 

description  of  varioua 
islands,  [294] 

barrier  reefs  and  their  for- 
mation, [297] 

depth  at  which  reefs  begin, 
[298] 

lateral  spreading  of  reefs, 
[298] 

removal  of  dead  coral,  [299] 

nature  of  sea-bed  near  reefs, 
[299] 

on  Keeling  atoll,  [306] 


Hales,  Mr.,  on  subsidence  in  the 

Caroline  archipelago,  169 
Hall,  Capt.  B.,  on  Loo  Choo,  181 
Halstead,    Capt.,     elevation     of 

eastern  shore  of  Gulf  of 

Bengal,  181 
Harvey  islands  recently  elevated, 

185,  186 
Harvey  or  Cook  islands,  204 
Height  of  encircled  islands,  62 
Hermites,  225 
Hogoleu,  226 
HoluthurisB    feeding  on    corals, 

20 
Honduras,  reef  off,  272 
Honolulu,  boring  at,  [323] 
Horn,  214 

Houtman's  Abrolhos,  235 
Huaheine,  203 
Hull  island,  208 
Humphrey,  207 
Hunter,  214 
Hurricanes,  effects  of  on  coral 

islands,  129 


Immaum,  260 
Independence,  216 
India,    east   coast  recently  ele- 
vated, 181 
India,  260 


INDEX. 


339 


IBB 

Irrt-gular  reefs  in  shallow  seas, 
77,  78 

Islets  of  coral-rock,  their  forma- 
tion, 15 
their  destruction  in  the  Mal- 
diva  atolls,  50 

Jamaica,  273 
Jarvis,  207 

Java  recently  elevated,  180 
Java,  236 

Johnston  island,  209 
Juan  de  Nova,  250 
Jtia7i  de  Nova  (Madagascar),  253 
Jukes,  P  of.,  the  barrier  reef  of 
Australia,  64 

Kalatoa,  237 

Kamtschatka,  proofs  of  its  recent 

elevation,  189 
Karhalang,  233 
Keeling  atoll,  section  of  reef,  7, 8, 

[305] 
Keeling, 

twrth  atoll,  246 
south  atoll,  246 
Kcffi7ig,  232 
Kemin,  206,  208 
Kennedy,  221 
Kepvel  island,  214 
Keppell,  Capt.,  on  the  reefs  and 

elevation  of  Celebes,  233 
King,  Capt.,  on  distribution  of  the 

different  classes  of  reefs, 

165 
Kumi,  244 

Laccadive  group,  247 

Ladrones  or  Marianas,  recently 

elevated,  172 
Ladrones  archipelago,  229 
Lagoon  of  Keeling  atoll,  18 
Lagoons, 

bordered  by  inclined  ledges 

and  walls,  and  theory  of 

their  formation,  41,   139, 

[285],  [301] 
of  small  atolls  filled  up  with 

Eediment,  42 


MAO 

Lagoon-channels  within   barrier 

reefs,  59 
Lagoon-reefs,  all  submerged  in 

some  atolls  and  all  rising 

to  the  surface  in  others,  90 
Lancaster  reef,  206 
Larrack,  260 
Latte,  213 

Laughlan  islands,  222 
Ledges  round  certain  lagoons,  41, 

139 
Lette,  234 

Lighthouse  reef,  272 
Lloyd,   Mr.,   on   corals  refixing 

themselves,  105 
Loo  Choo  recently  elevated,  181 
Loo  Choo,  244 
Louisiade,  222 
Low  archipelago,  alleged  proofs 

of  its  recent  elevation,  170 
Low  archipelago,  200 
Lowness  of  coral  islands,  122 
Loyalty  group,  221 
Loyalty  islands, 

recently  elevated,  186,  221 
Lucepara,  240 
Lutk6,  Adm.,  on  fissures  across 

coral  islands,  132 
Luzon  recently  elevated,  180 
Luzon,  241 
Lyell,  Sir  C, 

on  channels  into  the  lagoons 

of  atolls,  39 
on  the  lowness  of  their  lee- 
ward sides,  148 
on  the  antiquity  of  certain 

corals,  96 
on  the  apparent  continuity 

of   distinct  coral-islands, 

157 
on  the  recently  elevated  beds 

of  the  Bed  Sea,  184 


MacAshill  Islands,  formed  of  up- 
raised coral  rock,  191, 227 

Macassar  strait,  237 

Macclesfield  bank,  245 

Maclear,  Capt.,  on  Masaraarhu 
Island,  [319] 


340 


INDEX. 


MAD 

Madagascar, 

quick  growth  of   corals  at, 
104 

madreporitic  rock  of,  181 
Madagascar,  252 
Madjiko-sima,  243 
Madura  (India),  247,  260 
Madura  (Java),  236 
Mahlos  Mahdoo,  theory  of  for- 
mation, 144 
Maitea,  202 

Malacca  recently  elevated,  180 
Malacca,  239 
Malcolm  son,  Dr., 

on   recent  elevation   of    E. 
coast  of  India,  181 

on  recent  elevation  of  Cama- 
ran  island,  183 
Maiden,  207 
Maldiva  atolls, 

and  theory  of  their  forma- 
tion, 44,  141,  142 

steepness  of  their  flanks,  30 

growth  of  coral  at,  103 
Maldiva  archipelago,  247 
Mangaia  island, 

recently  elevated,  176, 186 
Mangaia,  205 
Mangs,  230 
Manouai  island,  204 
Marianas,  recently  elevated,  179 
Mariana  archipelago,  229 
Mariere,  228 

Marquesas,  subsidence  of,  165 
Marquesas,  203 
Marshall  archipelago,  218 
Marshall  island,  230 
Martinique,  L74 
Martires,  228 

Mary's,  St.,  in  Madagascar,  har- 
bour made  in  reefs,  88 
Maiy  island,  208 
Masdmarhu  island,  sections  of, 

[319] 
Matilda  atoll,  101 
Mauki  island,  204 
Mauritius, 

fringing-reefs  of,  69,  71 

depths  at  which  corals  there 
live.  110 


NEW 

Mauritius, 

recently  elevated,  181 
Mauritius,  248 
Maurua,  section  of,  65 
Maurvxi,  202 
Menchicoff  atoll,  28,  146 
Mendana  isles,  203,  221 
Mendana  island,  subsidence   of, 

165 
Metia,  98,  123,  176,  186 
Mexico,  gulf  of,  269 
Millepora  complanata  at  Keeling 

atoll,  10 
Mindoro,  241 
Mohilla,  252 
Mopeha,  203 
Moresby,  Capt.,  on  boring  through 

coral  reefs,  99 
Morty,  233 
Mosquito  coast,  273 
Murray,   Mr.,  on  the   structure 

and   origin  of  coral  reefs 

and  islands,  [283] 
quantity    of    carbonate    of 

lime     present    in    ocean 

water,  [284] 
lateral    spreading  of    coral 

reefs,  [285] 
solvent  action  of  sea-water, 

[285] 
summary    of     conclusions, 

[286] 
Musquillo  atoll,  146 
Mysol,  232 


Namourrek  group,  146 

Nattmas,  240 

Navigator  archipelago,  elevation 

of,  178 
Navigator  archipelago,  211 
Nederlandisch  islands,  216 
Nelson,  Lieut., 

on  the  consolidation  of  coral- 
rock,  under  water,  98 
theory  of  coral  formations, 

127 
on    the    Bermuda    islands, 
275 
New  Britain,  224 


INDEX. 


341 


NEW 

New  Caledonia, 

steepness  of  its  reef,  57 
barrier-reef  of,  63,  67,  139, 
145,  166 

New  Caledonia,  221 

Neio  Guinea  {E.  end),  224 

New  Guinea  {W.  end),  231 

New  Hanover,  224 

New  Hebrides  recently  elevated, 
178 

New  Hebrides,  218 

New  Ireland  recently   elevated, 
178 

New  Ireland,  224 

New  Nantucket,  208 

Nicobar  Islands,  239 

Niouha,  214 

NulliporsB, 

at  Keeling  atoll,  13 
on  the  reefs  of  atolls,  34 
on  barrier  reefs,  67 
their  wide  distribution  and 
abundance,  117 


Oahu,  borings  at,  [322] 
Objections  to  the  theory  of  subsi- 
dence, 153 
Ocean  islands,  211,  217 
One,  215 
Onouafu,  214 
Ormuz,  260 
Oscar  group,  216 
Oscillations  of  level,  166, 184, 193 
Oualari  or  Ualan,  225 
Ouluthy  atoll,  101 
Outo7ig  Java,  224 


Palawan, 

S.W.  coast,  14.^ 
N.W.  coast,  241 
western  bank,  245 

Palmerston,  204 

Palmyra,  209 

Paracells,  245 

Paraquas,  246 

Patchow,  243 

Paumotu  archipelago,  170 

Paumotu  archipelago,  200 


QUO 

Peel  island,  230 

Pelew  islands,  227 

Pemba  island,  singular  form  of, 

182 
Pemba,  255 
Penrhyn,  206 
Peregri7io,  206 
Pernambuco,  bar  of  sandstone  at, 

73,  277 
Persian  gulf,   recently  elevated, 

183 
Persian  gulf,  259 
Pescado,  207 
Pescadores,  244 
Peyster  group,  216 
Philip,  227 
Philippine  archipelago,   recently 

elevated,  180 
Philippine  archipelago,  241 
Phoenix,  208 
Pierre,  St.,  250 
Piguiram,  227 
Pitcairn,  201 
Pit  island,  217 
Pitt's  bank,  152 
Platte,  250 
Pleasa7it,  217 
Porites,  chief  coral  on  margin  of 

Keeling  atoll,  9 
Postillions,  237 
Pouynipete,  168 

its  probable  subsidence,  169 
Pouynipdte,  226 
Pratas  shoal,  244 
Proby,  214 
Providence,  250 
Puerto  Rico,  273 
PwZo  Anno,  228 
Pulo  Leat,  240 
Pumice  floated  to  coral  islands, 

157 
Pylstaart,  212 
Pyrard  de  Laval,   astonishment 

at  the  atolls  in  the  Indian 

Ocean,  2 


Quoy  and  Gaimard, 

depths  at  which  corals  live, 
lU 


342 


INDEX. 


QUO 

Quoy  and  Gaimard, 

description  of  reefs  appli- 
cable only  to  fringing- 
roefs,  174 


Raivaivaif  206 
Eange  of  atolls,  167 
Bapa,  206 
Rearson,  207 
Bed  Sea, 

banks  of  rock  coated  by  reefs, 

78 
proofs  of  its  recent  elevation, 

182 
supposed      subsidence     of, 
184,  [319] 
Red  Sea,  260 
Reefs 

irregular    in    shallow   seas, 

77 
rising  to  the  surface  in  some 
lagoons  and  all  submerged 
in  others,  91 
their  distribution,  80 
their    absence    from     some 

coasts,  81 
lateral  spreading  of,  22,  [285], 

[298],  [301],  [315],  [327] 
formation  of,  [^291] 
Revilla-gigedo,  199 
Ring-formed  reefs  of  the  Maldiva 
atolls,  and  theory  of,  45, 
139 
Rodriguez,  248,  [307] 
Rosario,  230 
Rose  island,  212 
Rotches,  217 
Roug,  226 
Routoumah,  216 
Rowley  shoals,  235 
Riippell,  Dr.,  on  the  recent  d&' 
posits  of  Red  Sea,  183 


Sable,  He  de,  248 
Sahia  de  Malha,  248 
Salomon  archipelago,  223 
Samoan,  or  Navigator  archipe- 
lago, elevation  of,  178 


SOL 

Samoa  archipelago,  211 
Sandalwood,  234 
Sandbars,  parallel  to  coasts,  73 
Sandwich    archipelago  recently 
elevated,  175 
extension  of,  191,211 
Sandtvich  archipelago,  209,  [322] 
Sanserot,  228 
Santa  Cruz,  220,  274 
Savage  island  recently  elevated, 

177 
Savage,  212 
Savu,  234 

Saya  or  Sahia  de  Malha,  248 
Scarborough  shoal,  245 
Scarus  feeding  on  corals,  20 
Schouion,  224 
Scilly,  203 
Scoriae  floated  to  coral  islands, 

157 
ScoWs  reef,  235 
Sections, 

of  islands  encircled  by  bar- 
rier reefs,  66,  133 
of  Bolabola,  134 
Sediment, 

in  Keeling  lagoon,  19 
in  other  atolls,  36,  48 
injurious  to  corals,  87 
transported      from      coral- 
islands  far  seaward,  157 
formation  of,  [291] 
Semper,  Prof., 

on  the  Pelew  islands,  228 
on  the  reef  of  the  Philippine 
archipelago,  242 
Seniavine,  227 
Serangani,  233 
Seychelles,  249 
Ship-bottom  quickly  coated  with 

coral,  106 
Smyth  island,  209 
Society  archipelago,  165 

stationary  condition  of,  169 
alleged  proofs  of  recent  ele 
vation,  185 
Society  archipelago,  201 
Socotra,  260 

Solomon  archipelago,  [291] 
Solor,  236 


INDEX. 


343 


SOL 

Solution  of    dead    coral,   [286], 

[299],  [301],  [329] 
Sooloo  islands,  recently  elevated, 

180 
Sooloo  islands,  240 
Soiivoroff,  207 
Spallanzani, 

on  growth  of  coral,  106 
Spanish,  228 
Starbuck,  206 
^Stones   transported  in  roots  of 

trees,  157 
Storms,  effects  of  on  coral-islands, 

129 
Stutchbury,  Mr., 

on  the  growth  of  an  Agari- 

cia,  106 
on  upraised  corals  in  Society 
archipelago,  185 
Subsidence 

of  Keeling  atoll,  23-25 
extreme  slowness  of,  193 
areas    of,   apparently  elon- 
gated, 191 
areas  of,  immense,  190 
great  amount  of,  193 
indicated  by  shape  of  coast, 

[313] 
at  Masamarhu  island,  [319] 
at  Oahu,  [322] 
Suez,  gulf  of,  266 
Sulphur  islands,  230 
Sumatra,  recently  elevated,  180 
Sumatra,  238 
Sumhawa,  236 
Surf  favourable  to  the  growth  of 

massive  corals,  85 
Swallow  shoal,  245 
Sydney  island,  208 


Tahiti,  alleged  proofs  of  its  re- 
cent  elevation,  185,  [314] 

Tahiti,  201 

Tanasserim,  239 

Tapamanoa,  202 

Temperature  of  the  sea  at  the 
Galapagos  archipelago,  82 

Tenimber  island,  231 

Teturoa,  202 


VOL 

Theories  on  coral  formations, 
119,  127,  [286],  [290], 
[298],  [305] 

Theory  of  subsidence,  and  objec- 
tions to,  126,  153,  [283], 
[325] 

Thickness,  vertical,  of  barrier- 
reefs,  66,  135 

TJwmas,  St.,  274 

Tikopia,  220 

Timor  recently  elevated,  180 

Timor,  234 

Timor-laut,  231 

Tokan-Bessees,  237 

Tongatabou,  description  of,  177 

Tongatabou,  213 

Tonquin,  246 

Toubai,  203 

Toufoa,  213 

Toupotia,  220 

Traditions  of  change  in  coral- 
islands,  129 

TridacnflB, 

embedded  in  coral-rock,  156 
left    exposed    in    the    Low 
archipelago,  170 

Tubularia,  quick  growth  of,  106 

Tumbelan,  240 

Turneffe  reef,  272 

Turtle,  215 


TJalan,  225 


Vanikoro, 

section  of,  65 

its  state  and  changes  in  its 
reefs,  169 
Vanikoro,  220 
Vavao,  213 
Vine  reef,  222 
Virgin  Oorda,  274 
Viti  archipelago,  214 
Volcanic 

islands,  with  living  corals  on 

their  shores,  81 
matter,    rarely     associated 
with  thick  masses  of  coral- 
rock,  157 


344 


INDEX. 


VOL 

Volcanoes, 

authorities  for  their  position 
on  the  map,  160 

their  presence  determined 
by  the  movements  in  pro- 
gress, 189 

absent  or  extinct  in  the  areas 
of  subsidence,  186 


Waigiou,  231 

Wallis  island,  214 

Washington,  209 

Wells'  reef,  222 

Wellstead,  Lieut.,  account  of  a 

ship  coated  with   corals, 

106 
West  Indies, 

banks  of  sediment,  fringed 

by  reefs,  78 
recently  elevated,  183 
West  Indies,  266 
Wharton,  Capt.,  on  Masimarbu 

island,  [329] 


ZON 

Whitsunday  island, 
view  of,  2 

changes  in  its  state,  130 
Williams,  Kev.  J., 

on  traditions  of  the  nativeo 
regarding      coral-islands, 
129 
on  antiquity  of  certain  corals, 
96 
Wolchonsky,  200 
Wostock,  206 


Xulla  islands,  232 


York  island,  208 
Yucutan,  coast  of,  272 

Zones  of  different  kinds  of  corals, 
outside  the  same  reefs, 
74, 90, 100 


BINDING  LIST   ...lil    m 


(ilC) 


^ 


"m