coral and coral reefs-第2部分

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great　expansion；　but　scattered　all　over　this　surface　there　were　polype
bodies　like　those　I　previously　described。　　Again；　when　this　great　cup
was　alive；　the　whole　surface　was　covered　with　a　beautiful　body　upon
which　were　set　innumerable　small　polype　flowers；　if　we　may　so　call
them；　often　brilliantly　coloured；　and　the　whole　cup　was　built　up　in　the
same　fashion　by　the　deposit　of　carbonate　of　lime　in　the　interior　of　the
combined　polype　body；　formed　by　budding　and　by　fission　in　the　way　I
described。　　You　will　perceive　that　there　is　no　necessary　limit　to　this
process。　　There　is　no　reason　why　we　should　not　have　coral　three　or　four
times　as　big；　and　there　are　certain　creatures　of　this　kind　that　do
fabricate　very　large　masses；　or　half　spheres　several　feet　in　diameter。
Thus　the　activity　of　these　animals　in　separating　carbonate　of　lime　from
the　sea　and　building　it　up　into　definite　shapes　is　very　considerable
indeed。

Now　I　think　I　have　said　sufficientas　much　as　I　can　without　taking　you
into　technical　details；　of　the　general　nature　of　these　creatures　which
form　coral。　　The　animals　which　form　coral　are　scattered　over　the　seas
of　all　countries　in　the　world。　　The　red　coral　is　comparatively　limited；
but　the　polypes　which　form　the　white　coral　are　widely　scattered。　　There
are　some　of　them　which　remain　single；　or　which　give　rise　to　only　small
accumulations；　and　the　skeletons　of　these；　as　they　die；　accumulate　upon
the　bottom　of　the　sea；　but　they　do　not　come　to　much；　they　are　washed
about　and　do　not　adhere　together；　but　become　mixed　up　with　the　mud　of
the　sea。　But　there　are　certain　parts　of　the　world　in　which　the　coral
polypes　which　live　and　grow　are　of　a　kind　which　remain；　adhere
together；　and　form　great　masses。　　They　differ　from　the　ordinary　polypes
just　in　the　same　way　as　those　plants　which　form　a　peat…bog　or
meadow…turf　differ　from　ordinary　plants。　　They　have　a　habit　of　growing
together　in　masses　in　the　same　place；　　they　are　what　we　call
〃gregarious〃　things；　and　the　consequence　of　this　is；　that　as　they　die
and　leave　their　skeletons；　those　skeletons　form　a　considerable　solid
aggregation　at　the　bottom　of　the　sea；　and　other　polypes　perch　upon
them；　and　begin　building　upon　them；　and　so　by　degrees　a　great　mass　is
formed。　　And　just　as　we　know　there　are　some　ancient　cities　in　which　you
have　a　British　city；　and　over　that　the　foundations　of　a　Roman　city；　and
over　that　a　Saxon　city；　and　over　that　again　a　modern　city；　so　in　these
localities　of　which　I　am　speaking；　you　have　the　accumulations　of　the
foundations　of　the　houses；　if　I　may　use　the　term；　of　nation　after　nation
of　these　coral　polypes；　and　these　accumulations　may　cover　a　very
considerable　space；　and　may　rise　in　the　course　of　time　from　the　bottom
to　the　surface　of　the　sea。

Mariners　have　a　name　which　they　apply　to　all　sorts　of　obstacles
consisting　of　hard　and　rocky　matter　which　comes　in　their　way　in　the
course　of　their　navigation；　they　call　such　obstacles　〃reefs；〃　and　they
have　long　been　in　the　habit　of　calling　the　particular　kind　of　reef；
which　is　formed　by　the　accumulation　of　the　skeletons　of　dead　corals；　by
the　name　of　〃coral　reefs；〃　therefore；　those　parts　of　the　world　in　which
these　accumulations　occur　have　been　termed　by　them　〃coral　reef　areas；〃
or　regions　in　which　coral　reefs　are　found。　　There　is　a　very　notable
example　of　a　simple　coral　reef　about　the　island　of　Mauritius；　which　I
dare　say　you　all　know；　lies　in　the　middle　of　the　Indian　Ocean。　　It　is　a
very　considerable　and　beautiful　island；　and　is　surrounded　on　all　sides
by　a　mass　of　coral；　which　has　been　formed　in　the　way　I　have　described；
so　that　if　you　could　get　upon　the　top　of　one　of　the　peaks　of　the　island；
and　look　down　upon　the　Indian　Ocean；　you　would　see　that　the　beach　round
the　Island　was　continued　outward　by　a　kind　of　shallow　terrace；　which　is
covered　by　the　sea；　and　where　the　sea　is　quite　shallow；　and　at　a
distance　varying　from　three…quarters　of　a　mile　to　a　mile　and　a　half　from
the　proper　beach；　you　would　see　a　line　of　foam　or　surf　which　looks　most
beautiful　in　contrast　with　the　bright　green　water　in　the　inside；　and
the　deep　blue　of　the　sea　beyond。　　That　line　of　surf　indicates　the　point
at　which　the　waters　of　the　ocean　are　breaking　upon　the　coral　reef　which
surrounds　the　island。　　You　see　it　sweep　round　the　island　upon　all
sides；　except　where　a　river　may　chance　to　come　down；　and　that　always
makes　a　gap　in　the　shore。

There　are　two　or　three　points　which　I　wish　to　bring　clearly　before　your
notice　about　such　a　reef　as　this。　　In　the　first　place；　you　perceive　it
forms　a　kind　of　fringe　round　the　island；　and　is　therefore　called　a
〃fringing　reef。〃　　In　the　next　place；　if　you　go　out　in　a　boat；　and　take
soundings　at　the　edge　of　the　reef；　you　find　that　the　depth　of　the　water
is　not　more　than　from　20　to　25　fathomsthat　is　about　120　to　150　feet。
Outside　that　point　you　come　to　the　natural　sea　bottom；　but　all　inside
that　depth　is　coral；　built　up　from　the　bottom　by　the　accumulation　of　the
skeletons　of　innumerable　generations　of　coral　polypes。　　So　that　you　see
the　coral　forms　a　very　considerable　rampart　round　the　island。　　What　the
exact　circumference　may　be　I　do　not　remember；　but　it　cannot　be　less
than　100　miles；　and　the　outward　height　of　this　wall　of　coral　rock
nowhere　amounts　to　less　than　about　100　or　150　feet。

When　the　outward　face　of　the　reef　is　examined；　you　find　that　the　upper
edge；　which　is　exposed　to　the　wash　of　the　sea；　and　all　the　seaward
face；　is　covered　with　those　living　plant…like　flowers　which　I　have
described　to　you。　　They　are　the　coral　polypes　which　grow；　flourish；　and
add　to　the　mass　of　calcareous　matter　which　already　forms　the　reef。　　But
towards　the　lower　part　of　the　reef；　at　a　depth　of　about　120　feet；　these
creatures　are　less　active；　and　fewer　of　them　at　work；　and　at　greater
depths　than　that　you　find　no　living　coral　polype　at　all；　and　it　may　be
laid　down　as　a　rule；　derived　from　very　extensive　observation；　that
these　reef…building　corals　cannot　live　in　a　greater　depth　of　water　than
about　120　to　150　feet。　　I　beg　you　to　recollect　that　fact；　because　it　is
one　I　shall　have　to　come　back　to　by　and　by；　and　to　show　to　what　very
curious　consequences　that　rule　leads。　　Well　then；　coming　back　to　the
margin　of　the　reef；　you　find　that　part　of　it　which　lies　just　within　the
surf　to　be　coated　by　a　very　curious　plant；　a　sort　of　seaweed；　which
contains　in　its　substance　a　very　great　deal　of　carbonate　of　lime；　and
looks　almost　like　rock；　this　is　what　is　called　the　nulli　pore。　　More
towards　the　land；　we　come　to　the　shallow　water　upon　the　inside　of　the
reef；　which　has　a　particular　name；　derived　from　the　Spanish　or　the
Portugueseit　is　called　a　〃lagoon；〃　or　lake。　　In　this　lagoon　there　is
comparatively　little　living　coral；　the　bottom　of　it　is　formed　of　coral
mud。　　If　we　pounded　this　coral　in　water；　it　would　be　converted　into
calcareous　mud；　and　the　waves　during　storms　do　for　the　coral　skeletons
exactly　what　we　might　do　for　this　coral　in　a　mortar；　the　waves　tear　off
great　fragments　and　crush　them　with　prodigious　force；　until　they　are
ground　into　the　merest　powder；　and　that　powder　is　washed　into　the
interior　of　the　lagoon；　and　forms　a　muddy　coating　at　the　bottom。　　Beside
that　there　are　a　great　many　animals　that　prey　upon　the　coralfishes；
worms；　and　creatures　of　that　kind；　and　all　these；　by　their　digestive
processes；　reduce　the　coral　to　the　same　state；　and　contribute　a　very
important　element　to　this　fine　mud。　　The　living　coral　found　in　the
lagoon；　is　not　the　reef　building　coral；　it　does　not　give　rise　to　the
same　massive　skeletons。　　As　you　go　in　a　boat　over　these　shallow　pools；
you　see　these　beautiful　things；　coloured　red；　blue；　green；　and　all
colours；　building　their　houses；　but　these　are　mere　tenements；　and　not　to
be　compared　in　magnitude　and　importance　to　the　masses　which　are　built
by　the　reef…builders　themselves。　　Now　such　a　structure　as　this　is　what
is　termed　a　〃fringing　reef。〃　　You　meet　with　fringing　reefs　of　this　kind
not　only　in　the　Mauritius；　but　in　a　number　of　other　parts　of　the　world。
If　these　were　the　only　reefs　to　be　seen　anywhere；　the　problem　of　the
formation　of　coral　reefs　would　never　have　been　a　difficult　one。　Nothing
can　be　easier　than　to　understand　how　there　must　have　been　a　time　when
the　coral　polypes　came　and　settled　on　the　shores　of　this　island；
everywhere　within　the　20　to　25　fathom　line；　and　how；　having　perched
there；　they　gradually　grew　until　they　built　up　the　reef。

But　these　are　by　no　means　the　only　sort　of　coral　reefs　in　the　world；　on
the　contrary；　there　are　very　large　areas；　not　only　of　the　Indian　ocean；
but　of　the　Pacific；　in　which　many　many　thousands　of　square　miles　are
covered　either　with　a　peculiar　kind　of　reef；　which　is　called　the
〃encircling　reef；〃　or　by　a　still　more　curious　reef　which　goes　by　the
name　of　the　〃atoll。〃　　There　is　a　very　good　picture；　which　Professor
Roscoe　has　been　kind　enough　to　prepare　for　me；　of　one　of　these　atolls；
which　will　enable　you　to　form　a　notion　of　it　as　a　landscape。　　You　have
in　the　foreground　the　waters　of　the　Pacific。　　You　must　fancy　yourself
in　the　middle　of　the　great　ocean；　and　you　will　perceive　that　there　is
an　almost　circular　island；　with　a　low　beach；　which　is　formed　entirely
of　coral　sand；　growing　upon　that　beach　you　have　vegetation；　which　takes；
of　course；　the　shape　of　the　circular　land；　and　then；　in　the　interior　of
the　circle；　there　is　a　pool　of　water；　which　is　not　very　deepprobably
in　this　case　not　more　than　eight　or　nine　fathomsand　which　forms　a
strange　and　beautiful　contrast　to　the　deep　blue　water　outside。　　This
circular　island；　or　atoll；　with　a　lagoon　in　the　middle；　is　not　a
complete　circle；　upon　one　side　of　it　there　is　a　break；　exactly　like　the
entrance　into　a　dock；　and；　as　a　matter　of　course；　these　circular
islets；　or　atolls；　form　most　efficient　break…waters；　for　if　you　can　only
get　inside　your　ship　is　in　perfect　safety；　with　admirable　anchorage　in
the　interior。　　If　the　ship　were　lying　within　a　mile　of　that　beach；　the
water　would　be　one　or　two　thousand　feet　deep；　therefore；　a　section　of
that　atoll；　with　the　soundings　as　deep　as　this　all　round；　would　give
you　the　notion　of　a　great　cone；　cut　off　at　the　top；　and　with　a　shallow
cup　in　the　middle　of　it。　　Now；　w