present　some　other　peculiarities。　　These　reefs；　and　especially　the
  atolls；　are　usually　interrupted　at　one　part　of　their　circumference；
  and　this　part　is　always　situated　on　the　leeward　side　of　the　reef；
  or　that　which　is　the　more　sheltered　side。　　Now；　as　all　these　reefs
  are　situated　within　the　region　in　which　the　tradewinds　prevail；　it
  follows　that；　on　the　north　side　of　the　equator；　where　the　trade…
  wind　is　a　northeasterly　wind；　the　opening　of　the　reef　is　on　the
  southwest　side：　while　in　the　southern　hemisphere；　where　the　trade…
  winds　blow　from　the　southeast；　the　opening　lies　to　the　northwest。
  The　curious　practical　result　follows　from　this　structure；　that　the
  lagoons　to　these　reefs　really　form　admirable　harbours；　if　a　ship
  can　only　get　inside　them。　　But　the　main　difference　between　the
  encircling　reefs　and　the　atolls；　on　the　one　hand；　and　the　fringing
  reefs　on　the　other；　lies　in　the　fact　of　the　much　greater　depth　of
  water　on　the　seaward　faces　of　the　former。　　As　a　consequence　of　this
  fact；　the　whole　of　this　face　is　not；　as　it　is　in　the　case　of　the
  fringing　reef；　covered　with　living　coral　polypes。　　For；　as　we　have
  seen；　these　polypes　cannot　live　at　a　greater　depth　than　about
  twenty…five　fathoms；　and　actual　observation　has　shown　that　while；
  down　to　this　depth；　the　sounding…lead　will　bring　up　branches　of
  live　coral　from　the　outer　wall　of　such　a　reef；　at　a　greater　depth
  it　fetches　to　the　surface　nothing　but　dead　coral　and　coral　sand。
  We　must；　therefore；　picture　to　ourselves　an　atoll；　or　an　encircling
  reef；　as　fringed　for　one　hundred　feet；　or　more；　from　its　summit；
  with　coral　polypes　busily　engaged　in　fabricating　coral；　while；
  below　this　comparatively　narrow　belt；　its　surface　is　a　bare　and
  smooth　expanse　of　coral　sand；　supported　upon　and　within　a　core　of
  coral　limestone。　　Thus；　if　the　bed　of　the　Pacific　were　suddenly
  laid　bare；　as　was　just　now　supposed；　the　appearance　of　the　reef…
  mountains　would　be　exactly　the　reverse　of　that　presented　by　many
  high　mountains　on　land。　　For　these　are　white　with　snow　at　the　top；
  while　their　bases　are　clothed　with　an　abundant　and　gaudily…coloured
  vegetation。　　But　the　coral　cones　would　look　grey　and　barren　below；
  while　their　summits　would　be　gay　with　a　richly…coloured　parterre　of
  flowerlike　coral　polypes。
  The　practical　difficulties　of　sounding　upon；　and　of　bringing　up
  portions　of；　the　seaward　face　of　an　atoll　or　of　an　encircling　reef；
  are　so　great；　in　consequence　of　the　constant　and　dangerous　swell
  which　sets　towards　it；　that　no　exact　information　concerning　the
  depth　to　which　the　reefs　are　composed　of　coral　has　yet　been
  obtained。　　There　is　no　reason　to　doubt；　however；　that　the　reef…cone
  has　the　same　structure　from　its　summit　to　its　base；　and　that　its
  sea…wall　is　throughout　mainly　composed　of　dead　coral。
  And　now　arises　a　serious　difficulty。　　If　the　coral　polypes　cannot
  live　at　a　greater　depth　than　one　hundred　or　one　hundred　and　fifty
  feet；　how　can　they　have　built　up　the　base　of　the　reef…cone；　which
  may　be　two　thousand　feet；　or　more；　below　the　surface　of　the　sea？
  In　order　to　get　over　this　objection；　it　was　at　one　time　supposed
  that　the　reef…building　polypes　had　settled　upon　the　summits　of　a
  chain　of　submarine　mountains。　　But　what　is　there　in　physical
  geography　to　justify　the　assumption　of　the　existence　of　a　chain　of
  mountains　stretching　for　one　thousand　miles　or　more；　and　so　nearly
  of　the　same　height；　that　none　should　rise　above　the　level　of　the
  sea；　nor　fall　one　hundred　and　fifty　feet　below　that　level？
  How；　again；　on　this　hypothesis；　are　atolls　to　be　accounted　for；
  unless；　as　some　have　done；　we　take　refuge　in　the　wild　supposition
  that　every　atoll　corresponds　with　the　crater　of　a　submarine
  volcano？　　And　what　explanation　does　it　afford　of　the　fact　that；　in
  some　parts　of　the　ocean；　only　atolls　and　encircling　reefs　occur；
  while　others　present　none　but　fringing　reefs？
  These　and　other　puzzling　facts　remained　insoluble　until　the
  publication；　in　the　year　1840；　of　Mr。　Darwin's　famous　work　on　coral
  reefs；＇123＇　in　which　a　key　was　given　to　all　the　difficult　problems
  connected　with　the　subject；　and　every　difficulty　was　shown　to　be
  capable　of　solution　by　deductive　reasoning　from　a　happy　combination
  of　certain　well…established　geological　and　biological　truths。　　Mr。
  Darwin；　in　fact；　showed　that；　so　long　as　the　level　of　the　sea
  remains　unaltered　in　any　area　in　which　coral　reefs　are　being
  formed；　or　if　the　level　of　the　sea　relatively　to　that　of　the　land
  is　falling；　the　only　reefs　which　can　be　formed　are　fringing　reefs。
  While　if；　on　the　contrary；　the　level　of　the　sea　is　rising
  relatively　to　that　of　the　land；　at　a　rate　not　faster　than　that　at
  which　the　upward　growth　of　the　coral　can　keep　pace　with　it；　the
  reef　will　gradually　pass　from　the　condition　of　a　fringing；　into
  that　of　an　encircling　or　barrier　reef。　　And；　finally；　that　if　the
  relative　level　of　the　sea　rise　so　much　that　the　encircled　land　is
  completely　submerged；　the　reef　must　necessarily　pass　into　the
  condition　of　an　atoll。
  For；　suppose　the　relative　level　of　the　sea　to　remain　stationary；
  after　a　fringing　reef　has　reached　that　distance　from　the　land　at
  which　the　depth　of　water　amounts　to　one　hundred　and　fifty　feet。
  Then　the　reef　cannot　extend　seaward　by　the　migration　of　coral
  germs；　because　these　coral　germs　would　find　the　bottom　of　the　sea
  to　be　too　deep　for　them　to　live　in。　　And　the　only　manner　in　which
  the　reef　could　extend　outwards；　would　be　by　the　gradual
  accumulation；　at　the　foot　of　its　seaward　face；　of　a　talus　of　coral
  fragments　torn　off　by　the　violence　of　the　waves；　which　talus　might；
  in　course　of　time；　become　high　enough　to　bring　its　upper　surface
  within　the　limits　of　coral　growth；　and　in　that　manner　provide　a
  sort　of　factitious　sea…bottom　upon　which　the　coral　embryos　might
  perch。　　If；　on　the　other　hand；　the　level　of　the　sea　were　slowly　and
  gradually　lowered；　it　is　clear　that　the　parts　of　its　bottom
  originally　beyond　the　limit　of　coral　growth　would　gradually　be
  brought　within　the　required　distance　of　the　surface；　and　thus　the
  reef　might　be　indefinitely　extended。　　But　this　process　would　give
  rise　neither　to　an　encircling　reef　nor　to　an　atoll；　but　to　a　broad
  belt　of　upheaved　coral　rock；　increasing　the　dimensions　of　the　dry
  land；　and　continuous　seawards　with　the　fresh　fringing　reef。
  Suppose；　however；　that　the　sea…level　rose　instead　of　falling；　at
  the　same　slow　and　gradual　rate　at　which　we　know　it　to　be　rising　in
  some　parts　of　the　world；not　more；　in　fact；　than　a　few　inches；　or；
  at　most；　a　foot　or　two；　in　a　hundred　years。　　Then；　while　the　reef
  would　be　unable　to　extend　itself　seaward；　the　sea…bottom　outside　it
  being　gradually　more　and　more　removed　from　the　depth　at　which　the
  life　of　the　coral　polypes　is　possible；　it　would　be　able　to　grow
  upwards　as　fast　as　the　sea　rose。　　But　the　growth　would　take　place
  almost　exclusively　around　the　circumference　of　the　reef；　this　being
  the　only　region　in　which　the　coral　polypes　would　find　the
  conditions　favourable　for　their　existence。　　The　bottom　of　the
  lagoon　would　be　raised；　in　the　main；　only　by　the　coral　debris　and
  coral　mud；　formed　in　the　manner　already　described；　consequently；
  the　margins　of　the　reef　would　rise　faster　than　the　bottom；　or；　in
  other　words；　the　lagoon　would　constantly　become　deeper。　　And；　at
  the　same　time；　it　would　gradually　increase　in　breadth；　as　the
  rising　sea；　covering　more　of　the　land；　would　occupy　a　wider　space
  between　the　edge　of　the　reef　and　what　remained　of　the　land。　　Thus
  the　rising　sea　would　eventually　convert　a　large　island　with　a
  fringing　reef　into　a　small　island　surrounded　by　an　encircling　reef。
  And　it　will　be　obvious　that　when　the　rising　of　the　sea　has　gone　so
  far　as　completely　to　cover　the　highest　points　of　the　island；　the
  reef　will　have　passed　into　the　condition　of　an　atoll。
  But　how　is　it　possible　that　the　relative　level　of　the　land　and　sea
  should　be　altered　to　this　extent？　　Clearly；　only　in　one　of　two
  ways：　either　the　sea　must　have　risen　over　those　areas　which　are　now
  covered　by　atolls　and　encircling　reefs；　or；　the　land　upon　which　the
  sea　rests　must　have　been　depressed　to　a　corresponding　extent。
  If　the　sea　has　risen；　its　rise　must　have　taken　place　over　the　whole
  world　simultaneously；　and　it　must　have　risen　to　the　same　height
  over　all　parts　of　the　coral　zone。　　Grounds　have　been　shown　for　the
  belief　that　the　general　level　of　the　sea　may　have　been　different　at
  different　times；　it　has　been　suggested；　for　example；　that　the
  accumulation　of　ice　about　the　poles　during　one　of　the　cold　periods
  of　the　earth's　history　necessarily　implies　a　diminution　in　the
  volume　of　the　sea　proportioned　to　the　amount　of　its　water　thus
  permanently　locked　up　in　the　Arctic　and　Antarctic　ice…cellars；
  while；　in　the　warm　periods；　the　greater　or　less　disappearance　of
  the　polar　ice…cap　implies　a　corresponding　addition　of　water　to　the
  ocean。　　And　no　doubt　this　reasoning　must　be　admitted　to　be　sound　in
  pr