第12章

Wehavenowonlytomentiontheturkey-buzzard(Vulturaura)，andtheGallinazo。Theformerisfoundwhereverthecountryismoderatelydamp，fromCapeHorntoNorthAmerica。DifferentlyfromthePolyborusBrasiliensisandChimango，ithasfounditswaytotheFalklandIslands。Theturkey-buzzardisasolitarybird，oratmostgoesinpairs。Itmayatonceberecognisedfromalongdistance，byitslofty，soaring，andmostelegantflight。ltiswellknowntobeatruecarrion-feeder。OnthewestcoastofPatagonia，amongthethickly-woodedisletsandbrokenland，itlivesexclusivelyonwhattheseathrowsup，andonthecarcassesofdeadseals。Wherevertheseanimalsarecongregatedontherocks，therethevulturesmaybeseen。TheGallinazo(Cathartesatratus)hasadifferentrangefromthelastspecies，asitneveroccurssouthwardoflat。41degs。Azarastatesthatthereexistsatraditionthatthesebirds，atthetimeoftheconquest，werenotfoundnearMonteVideo，butthattheysubsequentlyfollowedtheinhabitantsfrommorenortherndistricts。AtthepresentdaytheyarenumerousinthevalleyoftheColorado，whichisthreehundredmilesduesouthofMonteVideo。

ItseemsprobablethatthisadditionalmigrationhashappenedsincethetimeofAzara。TheGallinazogenerallyprefersahumidclimate，orrathertheneighbourhoodoffreshwater；henceitisextremelyabundantinBrazilandLaPlata，whileitisneverfoundonthedesertandaridplainsofNorthernPatagonia，exceptingnearsomestream。ThesebirdsfrequentthewholePampastothefootoftheCordillera，butIneversaworheardofoneinChile；inPerutheyarepreservedasscavengers。Thesevulturescertainlymaybecalledgregarious，fortheyseemtohavepleasureinsociety，andarenotsolelybroughttogetherbytheattractionofacommonprey。

Onafinedayaflockmayoftenbeobservedatagreatheight，eachbirdwheelingroundandroundwithoutclosingitswings，inthemostgracefulevolutions。Thisisclearlyperformedforthemerepleasureoftheexercise，orperhapsisconnectedwiththeirmatrimonialalliances。

Ihavenowmentionedallthecarrion-feeders，exceptingthecondor，anaccountofwhichwillbemoreappropriatelyintroducedwhenwevisitacountrymorecongenialtoitshabitsthantheplainsofLaPlata。

Inabroadbandofsand-hillockswhichseparatetheLagunadelPotrerofromtheshoresofthePlata，atthedistanceofafewmilesfromMaldonado，Ifoundagroupofthosevitrified，siliceoustubes，whichareformedbylightningenteringloosesand。ThesetubesresembleineveryparticularthosefromDrigginCumberland，describedintheGeologicalTransactions。[10]Thesand-hillocksofMaldonadonotbeingprotectedbyvegetation，areconstantlychangingtheirposition。Fromthiscausethetubesprojectedabovethesurface，andnumerousfragmentslyingnear，showedthattheyhadformerlybeenburiedtoagreaterdepth。Foursetsenteredthesandperpendicularly：byworkingwithmyhandsItracedoneofthemtwofeetdeep；andsomefragmentswhichevidentlyhadbelongedtothesametube，whenaddedtotheotherpart，measuredfivefeetthreeinches。Thediameterofthewholetubewasnearlyequal，andthereforewemustsupposethatoriginallyitextendedtoamuchgreaterdepth。Thesedimensionsarehoweversmall，comparedtothoseofthetubesfromDrigg，oneofwhichwastracedtoadepthofnotlessthanthirtyfeet。

Theinternalsurfaceiscompletelyvitrified，glossy，andsmooth。A

smallfragmentexaminedunderthemicroscopeappeared，fromthenumberofminuteentangledairorperhapssteambubbles，likeanassayfusedbeforetheblowpipe。Thesandisentirely，oringreaterpart，siliceous；butsomepointsareofablackcolour，andfromtheirglossysurfacepossessametalliclustre。Thethicknessofthewallofthetubevariesfromathirtiethtoatwentiethofaninch，andoccasionallyevenequalsatenth。

Ontheoutsidethegrainsofsandarerounded，andhaveaslightlyglazedappearance：Icouldnotdistinguishanysignsofcrystallization。InasimilarmannertothatdescribedintheGeologicalTransactions，thetubesaregenerallycompressed，andhavedeeplongitudinalfurrows，soascloselytoresembleashrivelledvegetablestalk，orthebarkoftheelmorcorktree。Theircircumferenceisabouttwoinches，butinsomefragments，whicharecylindricalandwithoutanyfurrows，itisasmuchasfourinches。

Thecompressionfromthesurroundingloosesand，actingwhilethetubewasstillsoftenedfromtheeffectsoftheintenseheat，hasevidentlycausedthecreasesorfurrows。Judgingfromtheuncompressedfragments，themeasureorboreofthelightning(ifsuchatermmaybeused)musthavebeenaboutoneinchandaquarter。AtParis，M。HachetteandM。Beudant[11]succeededinmakingtubes，inmostrespectssimilartothesefulgurites，bypassingverystrongshocksofgalvanismthroughfinely-powderedglass：whensaltwasadded，soastoincreaseitsfusibility，thetubeswerelargerineverydimension，Theyfailedbothwithpowderedfelsparandquartz。Onetube，formedwithpoundedglass，wasverynearlyaninchlong，namely。982，andhadaninternaldiameterof。019ofaninch。WhenwehearthatthestrongestbatteryinPariswasused，andthatitspoweronasubstanceofsucheasyfusibilityasglasswastoformtubessodiminutive，wemustfeelgreatlyastonishedattheforceofashockoflightning，which，strikingthesandinseveralplaces，hasformedcylinders，inoneinstanceofatleastthirtyfeetlong，andhavinganinternalbore，wherenotcompressed，offullaninchandahalf；

andthisinamaterialsoextraordinarilyrefractoryasquartz！

Thetubes，asIhavealreadyremarked，enterthesandnearlyinaverticaldirection。One，however，whichwaslessregularthantheothers，deviatedfromarightline，atthemostconsiderablebend，totheamountofthirty-threedegrees。Fromthissametube，twosmallbranches，aboutafootapart，weresentoff；onepointeddownwards，andtheotherupwards。Thislattercaseisremarkable，astheelectricfluidmusthaveturnedbackattheacuteangleof26degs。，tothelineofitsmaincourse。BesidesthefourtubeswhichIfoundvertical，andtracedbeneaththesurface，therewereseveralothergroupsoffragments，theoriginalsitesofwhichwithoutdoubtwerenear。Alloccurredinalevelareaofshiftingsand，sixtyyardsbytwenty，situatedamongsomehighsand-hillocks，andatthedistanceofabouthalfamilefromachainofhillsfourorfivehundredfeetinheight。Themostremarkablecircumstance，asitappearstome，inthiscaseaswellasinthatofDrigg，andinonedescribedbyM。RibbentropinGermany，isthenumberoftubesfoundwithinsuchlimitedspaces。AtDrigg，withinanareaoffifteenyards，threewereobserved，andthesamenumberoccurredinGermany。InthecasewhichIhavedescribed，certainlymorethanfourexistedwithinthespaceofthesixtybytwentyyards。Asitdoesnotappearprobablethatthetubesareproducedbysuccessivedistinctshocks，wemustbelievethatthelightning，shortlybeforeenteringtheground，dividesitselfintoseparatebranches。

TheneighbourhoodoftheRioPlataseemspeculiarlysubjecttoelectricphenomena。Intheyear1793，[12]oneofthemostdestructivethunderstormsperhapsonrecordhappenedatBuenosAyres：thirty-sevenplaceswithinthecitywerestruckbylightning，andnineteenpeoplekilled。

Fromfactsstatedinseveralbooksoftravels，Iaminclinedtosuspectthatthunderstormsareverycommonnearthemouthsofgreatrivers。Isitnotpossiblethatthemixtureoflargebodiesoffreshandsaltwatermaydisturbtheelectricalequilibrium？EvenduringouroccasionalvisitstothispartofSouthAmerica，weheardofaship，twochurches，andahousehavingbeenstruck。BoththechurchandthehouseIsawshortlyafterwards：

thehousebelongedtoMr。Hood，theconsul-generalatMonteVideo。Someoftheeffectswerecurious：thepaper，fornearlyafootoneachsideofthelinewherethebell-wireshadrun，wasblackened。Themetalhadbeenfused，andalthoughtheroomwasaboutfifteenfeethigh，theglobules，droppingonthechairsandfurniture，haddrilledinthemachainofminuteholes。Apartofthewallwasshattered，asifbygunpowder，andthefragmentshadbeenblownoffwithforcesufficienttodentthewallontheoppositesideoftheroom。Theframeofalooking-glasswasblackened，andthegildingmusthavebeenvolatilized，forasmelling-bottle，whichstoodonthechimney-piece，wascoatedwithbrightmetallicparticles，whichadheredasfirmlyasiftheyhadbeenenamelled。[1]Hearne’sJourney，p。383。

[2]Maclaren，art。“America。”Encyclop。Brittann。

[3]Azarasays，“Jecroisquelaquantiteannuelledespluiesest，danstoutescescontrees，plusconsiderablequ’enEspagne。”

[4]InSouthAmericaIcollectedaltogethertwenty-sevenspeciesofmice，andthirteenmoreareknownfromtheworksofAzaraandotherauthors。ThosecollectedbymyselfhavebeennamedanddescribedbyMr。WaterhouseatthemeetingsoftheZoologicalSociety。ImustbeallowedtotakethisopportunityofreturningmycordialthankstoMr。

Waterhouse，andtotheothergentlemanattachedtothatSociety，fortheirkindandmostliberalassistanceonalloccasions。

[5]InthestomachandduodenumofacapybarawhichIopenedIfoundaverylargequantityofathinyellowishfluid，inwhichscarcelyafibrecouldbedistinguished。Mr。Oweninformsmethatapartoftheoesophagusissoconstructedthatnothingmuchlargerthanacrowquillcanbepasseddown。Certainlythebroadteethandstrongjawsofthisanimalarewellfittedtogrindintopulptheaquaticplantsonwhichitfeeds。

AttheR。Negro，inNorthernPatagonia，thereisananimalofthesamehabits，andprobablyacloselyalliedspecies，butwhichIneversaw。ItsnoiseisdifferentfromthatoftheMaldonadokind；itisrepeatedonlytwiceinsteadofthreeorfourtimes，andismoredistinctandsonorous；whenheardfromadistanceitsocloselyresemblesthesoundmadeincuttingdownasmalltreewithanaxe，thatIhavesometimesremainedindoubtconcerningit。

Geolog。Transact。vol。ii。p。528。InthePhilosoph。Transact。(1790，p。294)Dr。Priestlyhasdescribedsomeimperfectsiliceoustubesandameltedpebbleofquartz，foundindiggingintotheground，underatree，whereamanhadbeenkilledbylightning。

[Azara’sVoyage，vol。i。p。36。PreviousChapterNextChapterTheVoyageoftheBeagle-Chapter4TheVoyageoftheBeaglebyCharlesDarwinPreviousChapterNextChapterChapter4-RioNegrotoBahiaBlancaJULY24th，1833——TheBeaglesailedfromMaldonado，andonAugustthe3rdshearrivedoffthemouthoftheRioNegro。ThisistheprincipalriveronthewholelineofcoastbetweentheStraitofMagellanandthePlata。ItenterstheseaaboutthreehundredmilessouthoftheestuaryofthePlata。Aboutfiftyyearsago，undertheoldSpanishgovernment，asmallcolonywasestablishedhere；anditisstillthemostsouthernposition(lat。41degs。)onthiseasterncoastofAmericainhabitedbycivilizedman。

Thecountrynearthemouthoftheriveriswretchedintheextreme：

onthesouthsidealonglineofperpendicularcliffscommences，whichexposesasectionofthegeologicalnatureofthecountry。Thestrataareofsandstone，andonelayerwasremarkablefrombeingcomposedofafirmly-

cementedconglomerateofpumicepebbles，whichmusthavetravelledmorethanfourhundredmiles，fromtheAndes。Thesurfaceiseverywherecoveredupbyathickbedofgravel，whichextendsfarandwideovertheopenplain。

Waterisextremelyscarce，and，wherefound，isalmostinvariablybrackish。