that；　within　the　area　of　what　we　may　call　the　〃coral　zone；〃　there
  are　no　coral　reefs　upon　the　west　coast　of　America；　nor　upon　the
  west　coast　of　Africa；　and　it　is　a　general　fact　that　the　reefs　are
  interrupted；　or　absent；　opposite　the　mouths　of　great　rivers。　　The
  causes　of　this　apparent　caprice　in　the　distribution　of　coral　reefs
  are　not　far　to　seek。　　The　polypes　which　fabricate　them　require　for
  their　vigorous　growth　a　temperature　which　must　not　fall　below　68
  degrees　Fahrenheit　all　the　year　round；　and　this　temperature　is　only
  to　be　found　within　the　distance　on　each　side　of　the　equator　which
  has　been　mentioned；　or　thereabouts。　　But　even　within　the　coral　zone
  this　degree　of　warmth　is　not　everywhere　to　be　had。　　On　the　west
  coast　of　America；　and　on　the　corresponding　coast　of　Africa；　the
  currents　of　cold　water　from　the　icy　regions　which　surround　the
  South　Pole　set　northward；　and　it　appears　to　be　due　to　their　cooling
  influence　that　the　sea　in　these　regions　is　free　from　the　reef
  builders。　　Again；　the　coral　polypes　cannot　live　in　water　which　is
  rendered　brackish　by　floods　from　the　land；　or　which　is　perturbed　by
  mud　from　the　same　source；　and　hence　it　is　that　they　cease　to　exist
  opposite　the　mouths　of　rivers；　which　damage　them　in　both　these
  ways。
  Such　is　the　general　distribution　of　the　reef…building　corals；　but
  there　are　some　very　interesting　and　singular　circumstances　to　be
  observed　in　the　conformation　of　the　reefs；　when　we　consider　them
  individually。　　The　reefs；　in　fact；　are　of　three　different　kinds；
  some　of　them　stretch　out　from　the　shore；　almost　like　a　prolongation
  of　the　beach；　covered　only　by　shallow　water；　and　in　the　case　of　an
  island；　surrounding　it　like　a　fringe　of　no　considerable　breadth。
  These　are　termed　〃fringing　reefs。〃　　Others　are　separated　by　a
  channel　which　may　attain　a　width　of　many　miles；　and　a　depth　of
  twenty　or　thirty　fathoms　or　more；　from　the　nearest　land；　and　when
  this　land　is　an　island；　the　reef　surrounds　it　like　a　low　wall；　and
  the　sea　between　the　reef　and　the　land　is；　as　it　were；　a　moat　inside
  this　wall。　　Such　reefs　as　these　are　called　〃encircling〃　when　they
  surround　an　island；　and　〃barrier〃　reefs；　when　they　stretch　parallel
  with　the　coast　of　a　continent。　　In　both　these　cases　there　is
  ordinary　dry　land　inside　the　reef；　and　separated　from　it　only　by　a
  narrower　or　a　wider；　a　shallower　or　a　deeper；　space　of　sea；　which
  is　called　a　〃lagoon；〃　or　〃inner　passage。〃　But　there　is　a　third　kind
  of　reef；　of　very　common　occurrence　in　the　Pacific　and　Indian
  Oceans；　which　goes　by　the　name　of　〃atoll。〃　　This　is；　to　all　intents
  and　purposes；　an　encircling　reef；　without　anything　to　encircle；　or；
  in　other　words；　without　an　island　in　the　middle　of　its　lagoon。　　The
  atoll　has　exactly　the　appearance　of　a　vast；　irregularly　oval；　or
  circular；　breakwater；　enclosing　smooth　water　in　its　midst。　　The
  depth　of　the　water　in　the　lagoon　rarely　exceeds　twenty　or　thirty
  fathoms；　but；　outside　the　reef；　it　deepens　with　great　rapidity　to
  two　hundred　or　three　hundred　fathoms。　　The　depth　immediately
  outside　the　barrier；　or　encircling；　reefs；　may　also　be　very
  considerable；　but；　at　the　outer　edge　of　a　fringing　reef；　it　does
  not　amount　usually　to　more　than　twenty　or　twenty…five　fathoms；　in
  other　words；　from　one　hundred　and　twenty　to　one　hundred　and　fifty
  feet。
  Thus；　if　the　water　of　the　ocean　should　be　suddenly　drained　away；　we
  should　see　the　atolls　rising　from　the　sea…bed　like　vast　truncated
  cones；　and　resembling　so　many　volcanic　craters；　except　that　their
  sides　would　be　steeper　than　those　of　an　ordinary　volcano。　　In　the
  case　of　the　encircling　reefs；　the　cone；　with　the　enclosed　island；
  would　look　like　Vesuvius　with　Monte　Nuovo　within　the　old　crater　of
  Somma；＇121＇　while；　finally；　the　island　with　a　fringing　reef　would
  have　the　appearance　of　an　ordinary　hill；　or　mountain；　girded　by　a　vast
  parapet；　within　which　would　lie　a　shallow　moat。　　And　the　dry　bed　of
  the　Pacific　might　afford　grounds　for　an　inhabitant　of　the　moon　to
  speculate　upon　the　extraordinary　subterranean　activity　to　which
  these　vast　and　numerous　〃craters〃　bore　witness！
  When　the　structure　of　a　fringing　reef　is　investigated；　the　bottom
  of　the　lagoon　is　found　to　be　covered　with　fine　whitish　mud；　which
  results　from　the　breaking　up　of　the　dead　corals。　　Upon　this　muddy
  floor　there　lie；　here　and　there；　growing　corals；　or　occasionally
  great　blocks　of　dead　coral；　which　have　been　torn　by　storms　from　the
  outer　edge　of　the　reef；　and　washed　into　the　lagoon。　　Shellfish　and
  worms　of　various　kinds　abound；　and　fish；　some　of　which　prey　upon
  the　coral；　sport　in　the　deeper　pools。　　But　the　corals　which　are　to
  be　seen　growing　in　the　shallow　waters　of　the　lagoon　are　of　a
  different　kind　from　those　which　abound　on　the　outer　edge　of　the
  reef；　and　of　which　the　reef　is　built　up。　　Close　to　the　seaward　edge
  of　the　reef；　over　which；　even　in　calm　weather；　a　surf　almost　always
  breaks；　the　coral　rock　is　encrusted　with　a　thick　coat　of　a　singular
  vegetable　organism；　which　contains　a　great　deal　of　limethe　so…
  called　Nullipora。　　Beyond　this；　in　the　part　of　the　edge　of　the　reef
  which　is　always　covered　by　the　breaking　waves；　the　living；　true；
  reef…polypes　make　their　appearance；　and；　in　different　forms；　coat
  the　steep　seaward　face　of　the　reef　to　a　depth　of　one　hundred　or
  even　one　hundred　and　fifty　feet。　　Beyond　this　depth　the　sounding…
  lead　rests；　not　upon　the　wall…like　face　of　the　reef；　but　on　the
  ordinary　shelving　sea…bottom。　　And　the　distance　to　which　a　fringing
  reef　extends　from　the　land　corresponds　with　that　at　which　the　sea
  has　a　depth　of　twenty　or　five…and…twenty　fathoms。
  If；　as　we　have　supposed；　the　sea　could　be　suddenly　withdrawn　from
  around　an　island　provided　with　a　fringing　reef；　such　as　the
  Mauritius；＇122＇　the　reef　would　present　the　aspect　of　a　terrace；
  its　seaward　face；　one　hundred　feet　or　more　high；　blooming　with　the
  animal　flowers　of　the　coral；　while　its　surface　would　be　hollowed
  out　into　a　shallow　and　irregular　moat…like　excavation。
  The　coral　mud；　which　occupies　the　bottom　of　the　lagoon；　and　with
  which　all　the　interstices　of　the　coral　skeletons　which　accumulate
  to　form　the　reef　are　filled　up；　does　not　proceed　from　the　washing
  action　of　the　waves　alone；　innumerable　fishes；　and　other　creatures
  which　prey　upon　the　coral；　add　a　very　important　contribution　of
  finely…triturated　calcareous　matter；　and　the　corals　and　mud
  becoming　incorporated　together；　gradually　harden　and　give　rise　to　a
  sort　of　limestone　rock；　which　may　vary　a　good　deal　in　texture。
  Sometimes　it　remains　friable　and　chalky；　but；　more　often；　the
  infiltration　of　water；　charged　with　carbonic　acid；　dissolves　some
  of　the　calcareous　matter；　and　deposits　it　elsewhere　in　the
  interstices　of　the　nascent　rock；　thus　glueing　and　cementing　the
  particles　together　into　a　hard　mass；　or　it　may　even　dissolve　the
  carbonate　of　lime　more　extensively；　and　re…deposit　it　in　a
  crystalline　form。　　On　the　beach　of　the　lagoon；　where　the　coral　sand
  is　washed　into　layers　by　the　action　of　the　waves；　its　grains　become
  thus　fused　together　into　strata　of　a　limestone；　so　hard　that　they
  ring　when　struck　with　a　hammer；　and　inclined　at　a　gentle　angle；
  corresponding　with　that　of　the　surface　of　the　beach。　　The　hard
  parts　of　the　many　animals　which　live　upon　the　reef　become　imbedded
  in　this　coral　limestone；　so　that　a　block　may　be　full　of　shells　of
  bivalves　and　univalves；　or　of　sea　urchins；　and　even　sometimes
  encloses　the　eggs　of　turtles　in　a　state　of　petrification。　　The
  active　and　vigorous　growth　of　the　reef　goes　on　only　at　the　seaward
  margins；　where　the　polypes　are　exposed　to　the　wash　of　the　surf；　and
  are　thereby　provided　with　an　abundant　supply　of　air　and　of　food。
  The　interior　portion　of　the　reef　may　be　regarded　as　almost　wholly
  an　accumulation　of　dead　skeletons。　　Where　a　river　comes　down　from
  the　land　there　is　a　break　in　the　reef；　for　the　reasons　which　have
  been　already　mentioned。
  The　origin　and　mode　of　formation　of　a　fringing　reef；　such　as　that
  just　described；　are　plain　enough。　　The　embryos　of　the　coral　polypes
  have　fixed　themselves　upon　the　submerged　shore　of　the　island；　as
  far　out　as　they　could　live；　namely；　to　a　depth　of　twenty　or　twenty…
  five　fathoms。　　One　generation　has　succeeded　another；　building
  itself　up　upon　the　dead　skeletons　of　its　predecessor。　　The　mass　has
  been　consolidated　by　the　infiltration　of　coral　mud；　and　hardened　by
  partial　solution　and　redeposition；　until　a　great　rampart　of　coral
  rock　one　hundred　or　one　hundred　and　fifty　feet　high　on　its　seaward
  face　has　been　formed　all　round　the　island；　with　only　such　gaps　as
  result　from　the　outflow　of　rivers；　in　the　place　of　sally…ports。
  The　structure　of　the　rocky　accumulation　in　the　encircling　reefs　and
  in　the　atolls　is　essentially　the　same　as　in　the　fringing　reef。
  But；　in　addition　to　the　differences　of　depth　inside　and　out；　they
  present　some　other　peculiarities。　　These　reefs；　and　especially　the
  atolls；　are　usually　interrupted　a