autobiography and selected essays-第36章

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uniform　testimony　of　geology。　　Beds　of　sandstone　or　limestone；

thousands　of　feet　thick；　and　all　full　of　marine　remains；　occur　in

various　parts　of　the　earth's　surface；　and　prove；　beyond　a　doubt；

that　when　these　beds　were　formed；　that　portion　of　the　sea…bottom

which　they　then　occupied　underwent　a　slow　and　gradual　depression　to

a　distance　which　cannot　have　been　less　than　the　thickness　of　those

beds；　and　may　have　been　very　much　greater。　　In　supposing；

therefore；　that　the　great　areas　of　the　Pacific　and　of　the　Indian

Ocean；　over　which　atolls　and　encircling　reefs　are　found　scattered；

have　undergone　a　depression　of　some　hundreds；　or；　it　may　be；

thousands　of　feet；　Mr。　Darwin　made　a　supposition　which　had　nothing

forced　or　improbable；　but　was　entirely　in　accordance　with　what　we

know　to　have　taken　place　over　similarly　extensive　areas；　in　other

periods　of　the　world's　history。　　But　Mr。　Darwin　subjected　his

hypothesis　to　an　ingenious　indirect　test。　　If　his　view　be　correct；

it　is　clear　that　neither　atolls；　nor　encircling　reefs；　should　be

found　in　those　portions　of　the　ocean　in　which　we　have　reason　to

believe；　on　independent　grounds；　that　the　sea…bottom　has　long　been

either　stationary；　or　slowly　rising。　　Now　it　is　known　that；　as　a

general　rule；　the　level　of　the　land　is　either　stationary；　or　is

undergoing　a　slow　upheaval；　in　the　neighborhood　of　active

volcanoes；　and；　therefore；　neither　atolls　nor　encircling　reefs

ought　to　be　found　in　regions　in　which　volcanoes　are　numerous　and

active。　　And　this　turns　out　to　be　the　case。　　Appended　to　Mr。

Darwin's　great　work　on　coral　reefs；　there　is　a　map　on　which　atolls

and　encircling　reefs　are　indicated　by　one　colour；　fringing　reefs　by

another；　and　active　volcanoes　by　a　third。　　And　it　is　at　once

obvious　that　the　lines　of　active　volcanoes　lie　around　the　margins

of　the　areas　occupied　by　the　atolls　and　the　encircling　reefs。　　It

is　exactly　as　if　the　upheaving　volcanic　agencies　had　lifted　up　the

edges　of　these　great　areas；　while　their　centres　had　undergone　a

corresponding　depression。　　An　atoll　area　may；　in　short；　be　pictured

as　a　kind　of　basin；　the　margins　of　which　have　been　pushed　up　by　the

subterranean　forces；　to　which　the　craters　of　the　volcanoes　have；　at

intervals；　given　vent。



Thus　we　must　imagine　the　area　of　the　Pacific　now　covered　by　the

Polynesian　Archipelago；　as　having　been；　at　some　former　time；

occupied　by　large　islands；　or；　may　be；　by　a　great　continent；　with

the　ordinarily　diversified　surface　of　plain；　and　hill；　and　mountain

chain。　　The　shores　of　this　great　land　were　doubtless　fringed　by

coral　reefs；　and；　as　it　slowly　underwent　depression；　the　hilly

regions；　converted　into　islands；　became；　at　first；　surrounded　by

fringing　reefs；　and　then；　as　depression　went　on；　these　became

converted　into　encircling　reefs；　and　these；　finally；　into　atolls；

until　a　maze　of　reefs　and　coral…girdled　islets　took　the　place　of

the　original　land　masses。



Thus　the　atolls　and　the　encircling　reefs　furnish　us　with　clear；

though　indirect；　evidence　of　changes　in　the　physical　geography　of

large　parts　of　the　earth's　surface；　and　even；　as　my　lamented

friend；　the　late　Professor　Jukes；＇124＇　has　suggested；　give　us

indications　of　the　manner　in　which　some　of　the　most　puzzling　facts

connected　with　the　distribution　of　animals　have　been　brought　about。

For　example；　Australia　and　New　Guinea　are　separated　by　Torres

Straits；　a　broad　belt　of　sea　one　hundred　or　one　hundred　and　twenty

miles　wide。　　Nevertheless；　there　is　in　many　respects　a　curious

resemblance　between　the　land　animals　which　inhabit　New　Guinea　and

the　land　animals　which　inhabit　Australia。　　But；　at　the　same　time；

the　marine　shellfish　which　are　found　in　the　shallow　waters　of　the

shores　of　New　Guinea　are　quite　different　from　those　which　are　met

with　upon　the　coasts　of　Australia。　　Now；　the　eastern　end　of　Torres

Straits　is　full　of　atolls；　which；　in　fact；　form　the　northern

termination　of　the　Great　Barrier　Reef　which　skirts　the　eastern

coast　of　Australia。　　It　follows；　therefore；　that　the　eastern　end　of

Torres　Straits　is　an　area　of　depression；　and　it　is　very　possible；

and　on　many　grounds　highly　probable；　that；　in　former　times；

Australia　and　New　Guinea　were　directly　connected　together；　and　that

Torres　Straits　did　not　exist。　　If　this　were　the　case；　the　existence

of　cassowaries　and　of　marsupial　quadrupeds；　both　in　New　Guinea　and

in　Australia；　becomes　intelligible；　while　the　difference　between

the　littoral　molluscs　of　the　north　and　the　south　shores　of　Torres

Straits　is　readily　explained　by　the　great　probability　that；　when

the　depression　in　question　took　place；　and　what　was；　at　first；　an

arm　of　the　sea　became　converted　into　a　strait　separating　Australia

from　New　Guinea；　the　northern　shore　of　this　new　sea　became　tenanted

with　marine　animals　from　the　north；　while　the　southern　shore　was

peopled　by　immigrants　from　the　already　existing　marine　Australian

fauna。



Inasmuch　as　the　growth　of　the　reef　depends　upon　that　of　successive

generations　of　coral　polypes；　and　as　each　generation　takes　a

certain　time　to　grow　to　its　full　size；　and　can　only　separate　its

calcareous　skeleton　from　the　water　in　which　it　lives　at　a　certain

rate；　it　is　clear　that　the　reefs　are　records　not　only　of　changes　in

physical　geography；　but　of　the　lapse　of　time。　　It　is　by　no　means

easy；　however；　to　estimate　the　exact　value　of　reef　chronology；　and

the　attempts　which　have　been　made　to　determine　the　rate　at　which　a

reef　grows　vertically　have　yielded　anything　but　precise　results。　　A

cautious　writer；　Mr。　Dana；＇125＇　whose　extensive　study　of　corals　and

coral　reefs　makes　him　an　eminently　competent　judge；　states　his

conclusion　in　the　following　terms：



〃The　rate　of　growth　of　the　common　branching　madrepore　is　not　over

one　and　a　half　inches　a　year。　　As　the　branches　are　open；　this　would

not　be　equivalent　to　more　than　half　an　inch　in　height　of　solid

coral　for　the　whole　surface　covered　by　the　madrepore；　and；　as　they

are　also　porous；　to　not　over　three…eighths　of　an　inch　of　solid

limestone。　　But　a　coral　plantation　has　large　bare　patches　without

corals；　and　the　coral　sands　are　widely　distributed　by　currents；

part　of　them　to　depths　over　one　hundred　feet　where　there　are　no

living　corals；　not　more　than　one…sixth　of　the　surface　of　a　reef

region　is；　in　fact；　covered　with　growing　species。　　This　reduces　the

three…eighths　to　ONE…SIXTEENTH。　　Shells　and　other　organic　relics

may　contribute　one…fourth　as　much　as　corals。　　At　the　outside；　the

average　upward　increase　of　the　whole　reef…ground　per　year　would　not

exceed　ONE…EIGHTH　of　an　inch。



〃Now　some　reefs　are　at　least　two　thousand　feet　thick；　which　at　one…

eighth　of　an　inch　a　year；　corresponds　to　one　hundred　and　ninety…two

thousand　years。〃*





*　Dana；　Manual　of　Geology；　p。　591。





Halve；　or　quarter；　this　estimate　if　you　will；　in　order　to　be

certain　of　erring　upon　the　right　side；　and　still　there　remains　a

prodigious　period　during　which　the　ancestors　of　existing　coral

polypes　have　been　undisturbedly　at　work；　and　during　which；

therefore；　the　climatal　conditions　over　the　coral　area　must　have

been　much　what　they　are　now。



And　all　this　lapse　of　time　has　occurred　within　the　most　recent

period　of　the　history　of　the　earth。　　The　remains　of　reefs　formed　by

coral　polypes　of　different　kinds　from　those　which　exist　now；　enter

largely　into　the　composition　of　the　limestones　of　the　Jurassic

period；＇126＇　and　still　more　widely　different　coral　polypes　have

contributed　their　quota　to　the　vast　thickness　of　the　carboniferous

and　Devonian　strata。　　Then　as　regards　the　latter　group　of　rocks　in

America；　the　high　authority　already　quoted　tells　us：



〃The　Upper　Helderberg　period　is　eminently　the　coral　reef　period　of

the　palaeozoic　ages。　　Many　of　the　rocks　abound　in　coral；　and　are　as

truly　coral　reefs　as　the　modern　reefs　of　the　Pacific。　　The　corals

are　sometimes　standing　on　the　rocks　in　the　position　they　had　when

growing：　others　are　lying　in　fragments；　as　they　wer