autobiography and selected essays-第34章

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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　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　gaud