Onemorningtheviewwassingularlyclear；thedistantmountainsbeingprojectedwiththesharpestoutlineonaheavybankofdarkblueclouds。
　　Judgingfromtheappearance，andfromsimilarcasesinEngland，Isupposedthattheairwassaturatedwithmoisture。Thefact，however，turnedoutquitethecontrary。Thehygrometergaveadifferenceof29。6degs。，betweenthetemperatureoftheair，andthepointatwhichdewwasprecipitated。
　　ThisdifferencewasnearlydoublethatwhichIhadobservedonthepreviousmornings。Thisunusualdegreeofatmosphericdrynesswasaccompaniedbycontinualflashesoflightning。Isitnotanuncommoncase，thustofindaremarkabledegreeofaerialtransparencywithsuchastateofweather？
　　Generallytheatmosphereishazy；andthisiscausedbythefallingofimpalpablyfinedust，whichwasfoundtohaveslightlyinjuredtheastronomicalinstruments。ThemorningbeforeweanchoredatPortoPraya，Icollectedalittlepacketofthisbrown-colouredfinedust，whichappearedtohavebeenfilteredfromthewindbythegauzeofthevaneatthemasthead。Mr。Lyellhasalsogivenmefourpacketsofdustwhichfellonavesselafewhundredmilesnorthwardoftheseislands。ProfessorEhrenberg[note3]findsthatthisdustconsistsingreatpartofinfusoriawithsiliceousshields，andofthesiliceoustissueofplants。InfivelittlepacketswhichIsenthim，hehasascertainednolessthansixty-sevendifferentorganicforms！Theinfusoria，withtheexceptionoftwomarinespecies，areallinhabitantsoffresh-water。
　　IhavefoundnolessthanfifteendifferentaccountsofdusthavingfallenonvesselswhenfaroutintheAtlantic。Fromthedirectionofthewindwheneverithasfallen，andfromitshavingalwaysfallenduringthosemonthswhentheharmattanisknowntoraisecloudsofdusthighintotheatmosphere，wemayfeelsurethatitallcomesfromAfrica。Itis，however，averysingularfact，that，althoughProfessorEhrenbergknowsmanyspeciesofinfusoriapeculiartoAfrica，hefindsnoneoftheseinthedustwhichIsenthim。Ontheotherhand，hefindsinittwospecieswhichhithertoheknowsaslivingonlyinSouthAmerica。Thedustfallsinsuchquantitiesastodirtyeverythingonboard，andtohurtpeople’seyes；vesselsevenhaverunonshoreowingtotheobscurityoftheatmosphere。Ithasoftenfallenonshipswhenseveralhundred，andevenmorethanathousandmilesfromthecoastofAfrica，andatpointssixteenhundredmilesdistantinanorthandsouthdirection。Insomedustwhichwascollectedonavesselthreehundredmilesfromtheland，Iwasmuchsurprisedtofindparticlesofstoneabovethethousandthofaninchsquare，mixedwithfinermatter。
　　Afterthisfactoneneednotbesurprisedatthediffusionofthefarlighterandsmallersporulesofcryptogamicplants。
　　Thegeologyofthisislandisthemostinterestingpartofitsnaturalhistory。Onenteringtheharbour，aperfectlyhorizontalwhiteband，inthefaceoftheseacliff，maybeseenrunningforsomemilesalongthecoast，andattheheightofaboutforty-fivefeetabovethewater。Uponexaminationthiswhitestratumisfoundtoconsistofcalcareousmatterwithnumerousshellsembedded，mostorallofwhichnowexistontheneighbouringcoast。Itrestsonancientvolcanicrocks，andhasbeencoveredbyastreamofbasalt，whichmusthaveenteredtheseawhenthewhiteshellybedwaslyingatthebottom。Itisinterestingtotracethechangesproducedbytheheatoftheoverlyinglava，onthefriablemass，whichinpartshasbeenconvertedintoacrystallinelimestone，andinotherpartsintoacompactspottedstoneWherethelimehasbeencaughtupbythescoriaceousfragmentsofthelowersurfaceofthestream，itisconvertedintogroupsofbeautifullyradiatedfibresresemblingarragonite。Thebedsoflavariseinsuccessivegently-slopingplains，towardstheinterior，whencethedelugesofmeltedstonehaveoriginallyproceeded。Withinhistoricaltimes，nosignsofvolcanicactivityhave，Ibelieve，beenmanifestedinanypartofSt。Jago。Eventheformofacratercanbutrarelybediscoveredonthesummitsofthemanyredcinderyhills；yetthemorerecentstreamscanbedistinguishedonthecoast，forminglinesofcliffsoflessheight，butstretchingoutinadvanceofthosebelongingtoanolderseries：theheightofthecliffsthusaffordingarudemeasureoftheageofthestreams。
　　Duringourstay，Iobservedthehabitsofsomemarineanimals。AlargeAplysiaisverycommon。Thissea-slugisaboutfiveincheslong；andisofadirtyyellowishcolourveinedwithpurple。
　　Oneachsideofthelowersurface，orfoot，thereisabroadmembrane，whichappearssometimestoactasaventilator，incausingacurrentofwatertoflowoverthedorsalbranchiaeorlungs。Itfeedsonthedelicatesea-weedswhichgrowamongthestonesinmuddyandshallowwater；andI
　　foundinitsstomachseveralsmallpebbles，asinthegizzardofabird。
　　Thisslug，whendisturbed，emitsaveryfinepurplish-redfluid，whichstainsthewaterforthespaceofafootaround。Besidesthismeansofdefence，anacridsecretion，whichisspreadoveritsbody，causesasharp，stingingsensation，similartothatproducedbythePhysalia，orPortugueseman-of-war。
　　Iwasmuchinterested，onseveraloccasions，bywatchingthehabitsofanOctopus，orcuttle-fish。Althoughcommoninthepoolsofwaterleftbytheretiringtide，theseanimalswerenoteasilycaught。Bymeansoftheirlongarmsandsuckers，theycoulddragtheirbodiesintoverynarrowcrevices；andwhenthusfixed，itrequiredgreatforcetoremovethem。
　　Atothertimestheydartedtailfirst，withtherapidityofanarrow，fromonesideofthepooltotheother，atthesameinstantdiscolouringthewaterwithadarkchestnut-brownink。Theseanimalsalsoescapedetectionbyaveryextraordinary，chameleon-likepowerofchangingtheircolour。
　　Theyappeartovarytheirtintsaccordingtothenatureofthegroundoverwhichtheypass：whenindeepwater，theirgeneralshadewasbrownishpurple，butwhenplacedontheland，orinshallowwater，thisdarktintchangedintooneofayellowishgreen。Thecolour，examinedmorecarefully，wasaFrenchgrey，withnumerousminutespotsofbrightyellow：theformerofthesevariedinintensity；thelatterentirelydisappearedandappearedagainbyturns。Thesechangeswereeffectedinsuchamanner，thatclouds，varyingintintbetweenahyacinthredandachestnut-brown[note4]，werecontinuallypassingoverthebody。Anypart，beingsubjectedtoaslightshockofgalvanism，becamealmostblack：asimilareffect，butinalessdegree，wasproducedbyscratchingtheskinwithaneedle。
　　Theseclouds，orblushesastheymaybecalled，aresaidtobeproducedbythealternateexpansionandcontractionofminutevesiclescontainingvariouslycolouredfluids[note5]。
　　Thiscuttle-fishdisplayeditschameleon-likepowerbothduringtheactofswimmingandwhilstremainingstationaryatthebottom。Iwasmuchamusedbythevariousartstoescapedetectionusedbyoneindividual，whichseemedfullyawarethatIwaswatchingit。Remainingforatimemotionless，itwouldthenstealthilyadvanceaninchortwo，likeacatafteramouse；
　　sometimeschangingitscolour：itthusproceeded，tillhavinggainedadeeperpart，itdartedaway，leavingaduskytrainofinktohidetheholeintowhichithadcrawled。
　　Whilelookingformarineanimals，withmyheadabouttwofeetabovetherockyshore，Iwasmorethanoncesalutedbyajetofwater，accompaniedbyaslightgratingnoise。AtfirstIcouldnotthinkwhatitwas，butafterwardsIfoundoutthatitwasthiscuttle-fish，which，thoughconcealedinahole，thusoftenledmetoitsdiscovery。Thatitpossessesthepowerofejectingwaterthereisnodoubt，anditappearedtomethatitcouldcertainlytakegoodaimbydirectingthetubeorsiphonontheundersideofitsbody。Fromthedifficultywhichtheseanimalshaveincarryingtheirheads，theycannotcrawlwitheasewhenplacedontheground。IobservedthatonewhichIkeptinthecabinwasslightlyphosphorescentinthedark。
　　St。Paul’sRocksIncrossingtheAtlanticwehove-toduringthemorningofFebruary16th，closetotheislandofSt。Paul’s。Thisclusterofrocksissituatedin0degs。58’northlatitude，and29degs。15’westlongitude。Itis540
　　milesdistantfromthecoastofAmerica，and350fromtheislandofFernandoNoronha。Thehighestpointisonlyfiftyfeetabovethelevelofthesea，andtheentirecircumferenceisunderthree-quartersofamile。Thissmallpointrisesabruptlyoutofthedepthsoftheocean。Itsmineralogicalconstitutionisnotsimple；insomepartstherockisofacherty，inothersofafelspathicnature，includingthinveinsofserpentine。Itisaremarkablefact，thatallthemanysmallislands，lyingfarfromanycontinent，inthePacific，Indian，andAtlanticOceans，withtheexceptionoftheSeychellesandthislittlepointofrock，are，Ibelieve，composedeitherofcoraloroferuptedmatter。Thevolcanicnatureoftheseoceanicislandsisevidentlyanextensionofthatlaw，andtheeffectofthosesamecauses，whetherchemicalormechanical，fromwhichitresultsthatavastmajorityofthevolcanoesnowinactionstandeithernearsea-coastsorasislandsinthemidstofthesea。
　　TherocksofSt。Paulappearfromadistanceofabrilliantlywhitecolour。Thisispartlyowingtothedungofavastmultitudeofseafowl，andpartlytoacoatingofahardglossysubstancewithapearlylustre，whichisintimatelyunitedtothesurfaceoftherocks。This，whenexaminedwithalens，isfoundtoconsistofnumerousexceedinglythinlayers，itstotalthicknessbeingaboutthetenthofaninch。Itcontainsmuchanimalmatter，anditsorigin，nodoubt，isduetotheactionoftherainorsprayonthebirds’dung。BelowsomesmallmassesofguanoatAscension，andontheAbrolhosIslets，Ifoundcertainstalactiticbranchingbodies，formedapparentlyinthesamemannerasthethinwhitecoatingontheserocks。
　　Thebranchingbodiessocloselyresembledingeneralappearancecertainnulliporaeafamilyofhardcalcareoussea-plants，thatinlatelylookinghastilyovermycollectionIdidnotperceivethedifference。Theglobularextremitiesofthebranchesareofapearlytexture，liketheenamelofteeth，butsohardasjusttoscratchplate-glass。Imayheremention，thatonapartofthecoastofAscension，wherethereisavastaccumulationofshellysand，anincrustationisdepositedonthetidalrocksbythewaterofthesea，resembling，asrepresentedinthewoodcut，certaincryptogamicplantsMarchantiaeoftenseenondampwalls。Thesurfaceofthefrondsisbeautifullyglossy；andthosepartsformedwherefullyexposedtothelightareofajetblackcolour，butthoseshadedunderledgesareonlygrey。Ihaveshownspecimensofthisincrustationtoseveralgeologists，andtheyallthoughtthattheywereofvolcanicorigneousorigin！Initshardnessandtranslucency——initspolish，equaltothatofthefinestoliva-shell——inthebadsmellgivenout，andlossofcolourundertheblowpipe——itshowsaclosesimilaritywithlivingsea-shells。Moreover，insea-shells，itisknownthatthepartshabituallycoveredandshadedbythemantleoftheanimal，areofapalercolourthanthosefullyexposedtothelight，justasisthecasewiththisincrustation。Whenwerememberthatlime，eitherasaphosphateorcarbonate，entersintothecompositionofthehardparts，suchasbonesandshells，ofalllivinganimals，itisaninterestingphysiologicalfact[note6]tofindsubstancesharderthantheenamelofteeth，andcolouredsurfacesaswellpolishedasthoseofafreshshell，reformedthroughinorganicmeansfromdeadorganicmatter——mocking，also，inshape，someofthelowervegetableproductions。
　　WefoundonSt。Paul’sonlytwokindsofbirds——theboobyandthenoddy。Theformerisaspeciesofgannet，andthelatteratern。Bothareofatameandstupiddisposition，andaresounaccustomedtovisitors，thatIcouldhavekilledanynumberofthemwithmygeologicalhammer。