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。
　　Inallreefs，owingtothesedimentbeingwashedoutofthelagoon-channeltoleeward，thatsideisleastfavourabletothelong-continuedvigorousgrowthofthecorals；hencedeadportionsofreefnotunfrequentlyoccurontheleewardside；andthese，thoughstillretainingtheirproperwall-likeform，arenowinseveralinstancessunkseveralfathomsbeneaththesurface。
　　TheChagosgroupappearsfromsomecause，possiblyfromthesubsidencehavingbeentoorapid，atpresenttobemuchlessfavourablycircumstancedforthegrowthofreefsthanformerly：oneatollhasaportionofitsmarginalreef，ninemilesinlength，deadandsubmerged；asecondhasonlyafewquitesmalllivingpointswhichrisetothesurface，athirdandfourthareentirelydeadandsubmerged；afifthisamerewreck，withitsstructurealmostobliterated。Itisremarkablethatinallthesecases，thedeadreefsandportionsofreeflieatnearlythesamedepth，namely，fromsixtoeightfathomsbeneaththesurface，asiftheyhadbeencarrieddownbyoneuniformmovement。Oneofthese“half-drownedatolls。”socalledbyCapt。MoresbytowhomIamindebtedformuchinvaluableinformation，isofvastsize，namely，ninetynauticalmilesacrossinonedirection，andseventymilesinanotherline；andisinmanyrespectseminentlycurious。
　　Asbyourtheoryitfollowsthatnewatollswillgenerallybeformedineachnewareaofsubsidence，twoweightyobjectionsmighthavebeenraised，namely，thatatollsmustbeincreasingindefinitelyinnumber；andsecondly，thatinoldareasofsubsidenceeachseparateatollmustbeincreasingindefinitelyinthickness，ifproofsoftheiroccasionaldestructioncouldnothavebeenadduced。Thushavewetracedthehistoryofthesegreatringsofcoral-rock，fromtheirfirstoriginthroughtheirnormalchanges，andthroughtheoccasionalaccidentsoftheirexistence，totheirdeathandfinalobliteration。
　　Inmyvolumeon“CoralFormations“Ihavepublishedamap，inwhichIhavecolouredalltheatollsdark-blue，thebarrier-reefspale-blue，andthefringingreefsred。Theselatterreefshavebeenformedwhilstthelandhasbeenstationary，or，asappearsfromthefrequentpresenceofupraisedorganicremains，whilstithasbeenslowlyrising：atollsandbarrier-reefs，ontheotherhand，havegrownupduringthedirectlyoppositemovementofsubsidence，whichmovementmusthavebeenverygradual，andinthecaseofatollssovastinamountastohaveburiedeverymountain-summitoverwideocean-spaces。Nowinthismapweseethatthereefstintedpaleanddark-blue，whichhavebeenproducedbythesameorderofmovement，asageneralrulemanifestlystandneareachother。Againwesee，thattheareaswiththetwobluetintsareofwideextent；andthattheylieseparatefromextensivelinesofcoastcolouredred，bothofwhichcircumstancesmightnaturallyhavebeeninferred，onthetheoryofthenatureofthereefshavingbeengovernedbythenatureoftheearth’smovement。Itdeservesnoticethatinmorethanoneinstancewheresingleredandbluecirclesapproachneareachother，Icanshowthattherehavebeenoscillationsoflevel；forinsuchcasestheredorfringedcirclesconsistofatolls，originallybyourtheoryformedduringsubsidence，butsubsequentlyupheaved；
　　andontheotherhand，someofthepale-blueorencircledislandsarecomposedofcoral-rock，whichmusthavebeenupliftedtoitspresentheightbeforethatsubsidencetookplace，duringwhichtheexistingbarrier-reefsgrewupwards。
　　Authorshavenoticedwithsurprise，thatalthoughatollsarethecommonestcoral-structuresthroughoutsomeenormousoceanictracts，theyareentirelyabsentinotherseas，asintheWestIndies：wecannowatonceperceivethecause，forwheretherehasnotbeensubsidence，atollscannothavebeenformed；andinthecaseoftheWestIndiesandpartsoftheEastIndies，thesetractsareknowntohavebeenrisingwithintherecentperiod。Thelargerareas，colouredredandblue，areallelongated；andbetweenthetwocoloursthereisadegreeofrudealternation，asiftherisingofonehadbalancedthesinkingoftheother。Takingintoconsiderationtheproofsofrecentelevationbothonthefringedcoastsandonsomeothersforinstance，inSouthAmericawheretherearenoreefs，weareledtoconcludethatthegreatcontinentsareforthemostpartrisingareas：
　　andfromthenatureofthecoral-reefs，thatthecentralpartsofthegreatoceansaresinkingareas。TheEastIndianarchipelago，themostbrokenlandintheworld，isinmostpartsanareaofelevation，butsurroundedandpenetrated，probablyinmorelinesthanone，bynarrowareasofsubsidence。
　　Ihavemarkedwithvermilionspotsallthemanyknownactivevolcanoswithinthelimitsofthissamemap。Theirentireabsencefromeveryoneofthegreatsubsidingareas，colouredeitherpaleordarkblue，ismoststrikingandnotlesssoisthecoincidenceofthechiefvolcanicchainswiththepartscolouredred，whichweareledtoconcludehaveeitherlongremainedstationary，ormoregenerallyhavebeenrecentlyupraised。Althoughafewofthevermilionspotsoccurwithinnogreatdistanceofsinglecirclestintedblue，yetnotonesingleactivevolcanoissituatedwithinseveralhundredmilesofanarchipelago，orevensmallgroupofatolls。Itis，therefore，astrikingfactthatintheFriendlyarchipelago，whichconsistsofagroupofatollsupheavedandsincepartiallyworndown，twovolcanos，andperhapsmore，arehistoricallyknowntohavebeeninaction。Ontheotherhand，althoughmostoftheislandsinthePacificwhichareencircledbybarrier-reefs，areofvolcanicorigin，oftenwiththeremnantsofcratersstilldistinguishable，notoneofthemisknowntohaveeverbeenineruption。
　　Henceinthesecasesitwouldappear，thatvolcanosburstforthintoactionandbecomeextinguishedonthesamespots，accordinglyaselevatoryorsubsidingmovementsprevailthere。Numberlessfactscouldbeadducedtoprovethatupraisedorganicremainsarecommonwhereverthereareactivevolcanos；butuntilitcouldbeshownthatinareasofsubsidence，volcanoswereeitherabsentorinactive，theinference，howeverprobableinitself，thattheirdistributiondependedontherisingorfallingoftheearth’ssurface，wouldhavebeenhazardous。Butnow，Ithink，wemayfreelyadmitthisimportantdeduction。
　　Takingafinalviewofthemap，andbearinginmindthestatementsmadewithrespecttotheupraisedorganicremains，wemustfeelastonishedatthevastnessoftheareas，whichhavesufferedchangesinleveleitherdownwardsorupwards，withinaperiodnotgeologicallyremote。Itwouldappearalso，thattheelevatoryandsubsidingmovementsfollownearlythesamelaws。Throughoutthespacesinterspersedwithatolls，wherenotasinglepeakofhighlandhasbeenleftabovethelevelofthesea，thesinkingmusthavebeenimmenseinamount。Thesinking，moreover，whethercontinuous，orrecurrentwithintervalssufficientlylongforthecoralsagaintobringuptheirlivingedificestothesurface，mustnecessarilyhavebeenextremelyslow。Thisconclusionisprobablythemostimportantonewhichcanbededucedfromthestudyofcoralformations；——anditisonewhichitisdifficulttoimaginehowotherwisecouldeverhavebeenarrivedat。NorcanIquitepassovertheprobabilityoftheformerexistenceoflargearchipelagoesofloftyislands，wherenowonlyringsofcoral-rockscarcelybreaktheopenexpanseofthesea，throwingsomelightonthedistributionoftheinhabitantsoftheotherhighislands，nowleftstandingsoimmenselyremotefromeachotherinthemidstofthegreatoceans。Thereef-constructingcoralshaveindeedrearedandpreservedwonderfulmemorialsofthesubterraneanoscillationsoflevel；weseeineachbarrier-reefaproofthatthelandhastheresubsided，andineachatollamonumentoveranislandnowlost。Wemaythus，likeuntoageologistwhohadlivedhistenthousandyearsandkeptarecordofthepassingchanges，gainsomeinsightintothegreatsystembywhichthesurfaceofthisglobehasbeenbrokenup，andlandandwaterinterchanged。[1]ThesePlantsaredescribedintheAnnalsofNat。Hist。，vol。i。，1838，p。337。
　　[4]Thethirteenspeciesbelongtothefollowingorders：——IntheColeoptera，aminuteElater；Orthoptera，aGryllusandaBlatta；Hemiptera，onespecies；Homoptera，two；NeuropteraaChrysopa；
　　Hymenoptera，twoants；Lepidopteranocturna，aDiopaea，andaPterophorus？；Diptera，twospecies。
　　Thelargeclawsorpincersofsomeofthesecrabsaremostbeautifullyadapted，whendrawnback，toformanoperculumtotheshell，nearlyasperfectastheproperoneoriginallybelongingtothemolluscousanimal。Iwasassured，andasfarasmyobservationswentIfounditso，thatcertainspeciesofthehermit-crabalwaysusecertainspeciesofshells。
　　[7]SomenativescarriedbyKotzebuetoKamtschatkacollectedstonestotakebacktotheircountry。