andsofar，thisisindirectevidenceinfavourofourtheory。Iwasparticularlystruckwiththisfact，whenIfound，tomysurprise，thatthedescriptionsgivenbyMM。QuoyandGaimardwereapplicable，nottoreefsingeneralasimpliedbythem，butonlytothoseofthefringingclass；mysurprise，however，ceasedwhenIafterwardsfoundthat，byastrangechance，alltheseveralislandsvisitedbytheseeminentnaturalists，couldbeshownbytheirownstatementstohavebeenelevatedwithinarecentgeologicalera。
Notonlythegrandfeaturesinthestructureofbarrier-reefsandofatolls，andtotheirlikenesstoeachotherinform，size，andothercharacters，areexplainedonthetheoryofsubsidence——whichtheoryweareindependentlyforcedtoadmitintheveryareasinquestion，fromthenecessityoffindingbasesforthecoralswithintherequisitedepth——butmanydetailsinstructureandexceptionalcasescanthusalsobesimplyexplained。Iwillgiveonlyafewinstances。Inbarrier-reefsithaslongbeenremarkedwithsurprise，thatthepassagesthroughthereefexactlyfacevalleysintheincludedland，evenincaseswherethereefisseparatedfromthelandbyalagoon-channelsowideandsomuchdeeperthantheactualpassageitself，thatitseemshardlypossiblethattheverysmallquantityofwaterorsedimentbroughtdowncouldinjurethecoralsonthereef。Now，everyreefofthefringingclassisbreachedbyanarrowgatewayinfrontofthesmallestrivulet，evenifdryduringthegreaterpartoftheyear，forthemud，sand，orgravel，occasionallywasheddownkillsthecoralsonwhichitisdeposited。Consequently，whenanislandthusfringedsubsides，thoughmostofthenarrowgatewayswillprobablybecomeclosedbytheoutwardandupwardgrowthofthecorals，yetanythatarenotclosedandsomemustalwaysbekeptopenbythesedimentandimpurewaterflowingoutofthelagoon-channelwillstillcontinuetofrontexactlytheupperpartsofthosevalleys，atthemouthsofwhichtheoriginalbasalfringing-reefwasbreached。
Wecaneasilyseehowanislandfrontedonlyononeside，orononesidewithoneendorbothendsencircledbybarrier-reefs，mightafterlong-continuedsubsidencebeconvertedeitherintoasinglewall-likereef，orintoanatollwithagreatstraightspurprojectingfromit，orintotwoorthreeatollstiedtogetherbystraightreefs——allofwhichexceptionalcasesactuallyoccur。Asthereef-buildingcoralsrequirefood，arepreyeduponbyotheranimals，arekilledbysediment，cannotadheretoaloosebottom，andmaybeeasilycarrieddowntoadepthwhencetheycannotspringupagain，weneedfeelnosurpriseatthereefsbothofatollsandbarriersbecominginpartsimperfect。ThegreatbarrierofNewCaledoniaisthusimperfectandbrokeninmanyparts；hence，afterlongsubsidence，thisgreatreefwouldnotproduceonegreatatoll400milesinlength，butachainorarchipelagoofatolls，ofverynearlythesamedimensionwiththoseintheMaldivaarchipelago。Moreover，inanatolloncebreachedonoppositesides，fromthelikelihoodoftheoceanicandtidalcurrentspassingstraightthroughthebreaches，itisextremelyimprobablethatthecorals，especiallyduringcontinuedsubsidence，wouldeverbeableagaintounitetherim；iftheydidnot，asthewholesankdownwards，oneatollwouldbedividedintotwoormore。IntheMaldivaarchipelagotherearedistinctatollssorelatedtoeachotherinposition，andseparatedbychannelseitherunfathomableorverydeepthechannelbetweenRossandAriatollsis150fathoms，andthatbetweenthenorthandsouthNillandooatollsis200fathomsindepth，thatitisimpossibletolookatamapofthemwithoutbelievingthattheywereoncemoreintimatelyrelated。Andinthissamearchipelago，Mahlos-Mahdooatollisdividedbyabifurcatingchannelfrom100to132fathomsindepth，insuchamanner，thatitisscarcelypossibletosaywhetheritoughtstrictlytobecalledthreeseparateatolls，oronegreatatollnotyetfinallydivided。
Iwillnotenteronmanymoredetails；butImustremarkthatthecuriousstructureofthenorthernMaldivaatollsreceivestakingintoconsiderationthefreeentranceoftheseathroughtheirbrokenmarginsasimpleexplanationintheupwardandoutwardgrowthofthecorals，originallybasedbothonsmalldetachedreefsintheirlagoons，suchasoccurincommonatolls，andonbrokenportionsofthelinearmarginalreef，suchasboundseveryatolloftheordinaryform。Icannotrefrainfromonceagainremarkingonthesingularityofthesecomplexstructures——agreatsandyandgenerallyconcavediskrisesabruptlyfromtheunfathomableocean，withitscentralexpansestudded，anditsedgesymmetricallyborderedwithovalbasinsofcoral-rockjustlippingthesurfaceofthesea，sometimesclothedwithvegetation，andeachcontainingalakeofclearwater！
Onemorepointindetail：asinthetwoneighbouringarchipelagoescoralsflourishinoneandnotintheother，andassomanyconditionsbeforeenumeratedmustaffecttheirexistence，itwouldbeaninexplicablefactif，duringthechangestowhichearth，air，andwateraresubjected，thereef-buildingcoralsweretokeepaliveforperpetuityonanyonespotorarea。Andasbyourtheorytheareasincludingatollsandbarrier-reefsaresubsiding，weoughtoccasionallytofindreefsbothdeadandsubmerged。
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