autobiography and selected essays-第31节

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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