老托福阅读100篇纯阅读版

PASSAGE1

Bythemid-nineteenthcentury,theterm"icebox"hadenteredtheAmericanlanguage,butice

wasstillonlybeginningtoaffectthedietofordinarycitizensintheUnitedStates.Theicetrade

grewwiththegrowthofcities.Icewasusedinhotels,taverns,andhospitals,andbysome

forward-lookingcitydealersinfreshmeat,freshfish,andbutter.AftertheCivilWar(1861-1865),

asicewasusedtorefrigeratefreightcars,italsocameintohouseholduse.Evenbefore1880,half

theicesoldinNewYork,Philadelphia,andBaltimore,andone-thirdofthatsoldinBostonand

Chicago,wenttofamiliesfortheirownuse.Thishadbecomepossiblebecauseanewhousehold

convenience,theicebox,aprecursorofthemodernrefrigerator,hadbeeninvented.

Makinganefficienticeboxwasnotaseasyaswemightnowsuppose.Intheearlynineteenth

century,theknowledgeofthephysicsofheat,whichwasessentialtoascienceofrefrigeration,

wasrudimentary.Thecommonsensenotionthatthebesticeboxwasonethatpreventedtheice

frommeltingwasofcoursemistaken,foritwasthemeltingoftheicethatperformedthecooling.

Nevertheless,earlyeffortstoeconomizeiceincludedwrappingtheiceinblankets,whichkeptthe

icefromdoingitsjob.Notuntilneartheendofthenineteenthcenturydidinventorsachievethe

delicatebalanceofinsulationandcirculationneededforanefficienticebox.

Butasearlyas1803,aningeniousMarylandfarmer,ThomasMoore,hadbeenontheright

track.HeownedafarmabouttwentymilesoutsidethecityofWashington,forwhichthevillage

ofGeorgetownwasthemarketcenter.Whenheusedaniceboxofhisowndesigntotransporthis

buttertomarket,hefoundthatcustomerswouldpassuptherapidlymeltingstuffinthetubsofhis

competitorstopayapremiumpriceforhisbutter,stillfreshandhardinneat,one-poundbricks.

Oneadvantageofhisicebox,Mooreexplained,wasthatfarmerswouldnolongerhavetotravelto

marketatnightinordertokeeptheirproducecool.

PASSAGE2

ThegeologyoftheEarth'ssurfaceisdominatedbytheparticularpropertiesofwater.Present

onEarthinsolid,liquid,andgaseousstates,waterisexceptionallyreactive.Itdissolves,transports,

andprecipitatesmanychemicalcompoundsandisconstantlymodifyingthefaceoftheEarth.

Evaporatedfromtheoceans,watervaporformsclouds,someofwhicharetransportedbywind

overthecontinents.Condensationfromthecloudsprovidestheessentialagentofcontinental

erosion:rain.Precipitatedontotheground,thewatertricklesdowntoformbrooks,streams,and

rivers,constitutingwhatarecalledthehydrographicnetwork.Thisimmensepolarizednetwork

channelsthewatertowardasinglereceptacle:anocean.Gravitydominatesthisentirestepinthe

cyclebecausewatertendstominimizeitspotentialenergybyrunningfromhighaltitudestoward

thereferencepoint,thatis,sealevel.

Therateatwhichamoleculeofwaterpassesthoughthecycleisnotrandombutisameasure

oftherelativesizeofthevariousreservoirs.Ifwedefineresidencetimeastheaveragetimefora

watermoleculetopassthroughoneofthethreereservoirs—atmosphere,continent,andocean一

weseethatthetimesareverydifferent.Awatermoleculestays,onaverage,elevendaysinthe

atmosphere,onehundredyearsonacontinentandfortythousandyearsintheocean.Thislast

figureshowstheimportanceoftheoceanastheprincipalreservoirofthehydrospherebutalsothe

rapidityofwatertransportonthecontinents.

Avastchemicalseparationprocesstakesplacesduringtheflowofwateroverthecontinents.

Solubleionssuchascalcium,sodium,potassium,andsomemagnesiumaredissolvedand

transported.Insolubleionssuchasaluminum,iron,andsiliconstaywheretheyareandformthe

thin,fertileskinofsoilonwhichvegetationcangrow.Sometimessoilsaredestroyedand

transportedmechanicallyduringflooding.Theerosionofthecontinentsthusresultsfromtwo

closelylinkedandinterdependentprocesses,chemicalerosionandmechanicalerosion.Their

respectiveinteractionsandefficiencydependondifferentfactors.

PASSAGE3

TheNativeAmericansofnorthernCaliforniawerehighlyskilledatbasketry,usingthereeds,

grasses,barks,androotstheyfoundaroundthemtofashionarticlesofallsortsandsizes—not

onlytrays,containers,andcookingpots,buthats,boats,fishtraps,babycarriers,andceremonial

objects.

Ofalltheseexperts,noneexcelledthePomo—agroupwholivedonornearthecoastduring

the1800*s,andwhosedescendantscontinuetoliveinpartsofthesameregiontothisday.They

madebasketsthreefeetindiameterandothersnobiggerthanathimble.ThePomopeoplewere

mastersofdecoration.Someoftheirbasketswerecompletelycoveredwithshellpendants;others

withfeathersthatmadethebaskets1surfacesassoftasthebreastsofbirds.Moreover,thePomo

peoplemadeuseofmoreweavingtechniquesthandidtheirneighbors.Mostgroupsmadealltheir

basketworkbytwining—thetwistingofaflexiblehorizontalmaterial,calledaweft,around

stifferverticalstrandsofmaterial,thewarp.Othersdependedprimarilyoncoiling-aprocessin

whichacontinuouscoilofstiffmaterialisheldinthedesiredshapewithtightwrappingof

flexiblestrands.OnlythePomopeopleusedbothprocesseswithequaleaseandfrequency.In

addition,theymadeuseoffourdistinctvariationsonthebasictwiningprocess,oftenemploying

morethanoneoftheminasinglearticle.

Althoughawidevarietyofmaterialswasavailable,thePomopeopleusedonlyafew.The

warpwasalwaysmadeofwillow,andthemostcommonlyusedweftwassedgeroot,awoody

fiberthatcouldeasilybeseparatedintostrandsnothickerthanathread.Forcolor,thePomo

peopleusedthebarkofredbudfortheirtwinedworkanddyedbullrushrootforblackincoiled

work.Thoughothermaterialsweresometimesused,thesefourwerethestaplesintheirfinest

basketry.

IfthebasketrymaterialsusedbythePomopeoplewerelimited,thedesignswereamazingly

varied.EveryPomobasketmakerknewhowtoproducefromfifteentotwentydistinctpatterns

thatcouldbecombinedinanumberofdifferentways.

PASSAGE4

Theterm"HudsonRiverschool"wasappliedtotheforemostrepresentativesof

nineteenth-centuryNorthAmericanlandscapepainting.Apparentlyunknownduringthegolden

daysoftheAmericanlandscapemovement,whichbeganaround1850andlasteduntilthelate

1860's,theHudsonRiverschoolseemstohaveemergedinthe1870'sasadirectresultofthe

strugglebetweentheoldandthenewgenerationsofartists,eachtoassertitsownstyleasthe

representativeAmericanart.Theolderpainters,mostofwhomwerebombefore1835,practiced

inamodeoftenself-taughtandmonopolizedbylandscapesubjectmatterandweresecurely

establishedinandfosteredbythereigningAmericanartorganization,theNationalAcademyof

Design.TheyoungerpaintersreturninghomefromtraininginEuropeworkedmorewithfigural

subjectmatterandinaboldandimpressionistictechnique;theirprospectsforpatronageintheir

owncountrywereuncertain,andtheysoughttoattractitbyattainingacademicrecognitionin

NewYork.Oneoftheresultsoftheconflictbetweenthetwofactionswasthatwhatinprevious

yearshadbeenreferredtoastheuAmerican","native",or,occasionally,"NewYork"school—the

mostrepresentativeschoolofAmericanartinanygenre—hadby1890becomefirmly

establishedinthemindsofcriticsandpublicalikeastheHudsonRiverschool.

Thesobriquetwasfirstappliedaround1879.Whileitwasnotintendedasflattering,itwas

hardlyinappropriate.TheAcademiciansatwhomitwasaimedhadworkedandsocializedinNew

York,theHudson'sportcity,andhadpaintedtheriveranditsshoreswithvaryingfrequency.Most

important,perhaps,wasthattheyhadallmaintainedwithacertainfidelityamanneroftechnique

andcompositionconsistentwiththoseofAmerica'sfirstpopularlandscapeartist,ThomasCole,

whobuiltacareerpaintingtheCatskillMountainsceneiyborderingtheHudsonRiver.Apossible

implicationinthetermappliedtothegroupoflandscapistswasthatmanyofthemhad,likeCole,

livedonornearthebanksoftheHudson.Further,theriverhadlongservedastheprincipalroute

toothersketchinggroundsfavoredbytheAcademicians,particularlytheAdirondacksandthe

mountainsofVermontandNewHampshire.

PASSAGE5

Perhapsthemostobviouswayartisticcreationreflectshowpeopleliveisbymirroringthe

environment—thematerialsandtechnologiesavailabletoaculture.Stone,wood,treebark,clay,

andsandaregenerallyavailablematerials.Inaddition,dependingonthelocality,otherresources

maybeaccessible:shells,horns,gold,copper,andsilver.Thedifferentusestowhichsocietiesput

thesematerialsareofinteresttoanthropologistswhomayask,forexample,whypeoplechooseto

useclayandnotcopperwhenbothitemsareavailable.Althoughtherearenoconclusiveanswers

yet,thewayinwhichasocietyviewsitsenvironmentissometimesapparentinitschoiceanduse

ofartisticmaterials.Theuseofcertainmetals,forexample,maybereservedforceremonial

objectsofspecialimportance.Orthebeliefinthesupernaturalpowersofastoneortreemaycause

asculptortobesensitivetothatmaterial.

Whatisparticularlymeaningfultoanthropologististherealizationthatalthoughthematerials

availabletoasocietymaytosomeextentlimitorinfluencewhatitcandoartistically,the

materialsbynomeansdeterminewhatisdone.WhydotheartistsinJapanesesocietyrakesand

intopatterns;andtheartistsinRomansocietymeltsandtoformglass?Moreover,evenwhenthe

samematerialisusedinthesamewaybymembersofdifferentsocieties,theformorstyleofthe

workvariesenormouslyfromculturetoculture.Asocietymaysimplychoosetorepresentobjects

orphenomenathatareimportanttoitspopulation.AnexaminationoftheartoftheMiddleAges

tellsussomethingaboutthemedievalpreoccupationwiththeologicaldoctrine.Inadditionto

revealingtheprimaryconcernsofasociety,thecontentofthatsociety'sartmayalsoreflectthe

culture'ssocialstratification.

PASSAGE6

Potash(theoldnameforpotassiumcarbonate)isoneofthetwoalkalis(theotherbeingsoda,

sodiumcarbonate)thatwereusedfromremoteantiquityinthemakingofglass,andfromtheearly

MiddleAgesinthemakingofsoap:theformerbeingtheproductofheatingamixtureofalkali

andsand,thelatteraproductofalkaliandvegetableoil.Theirimportanceinthecommunitiesof

colonialNorthAmericaneedhardlybestressed.

Potashandsodaarenotinterchangeableforallpurposes,butforglass-orsoap-makingeither

woulddo.SodawasobtainedlargelyfromtheashesofcertainMediterraneanseaplants,potash

fromthoseofinlandvegetation.HencepotashwasmorefamiliartotheearlyEuropeansettlersof

theNorthAmericancontinent.

ThesettlementatJamestowninVirginiawasinmanywaysamicrocosmoftheeconomyof

colonialNorthAmerica,andpotashwasoneofitsfirstconcerns.Itwasrequiredforthe

glassworks,thefirstfactoryintheBritishcolonies,andwasproducedinsufficientquantityto

permittheinclusionofpotashinthefirstcargoshippedoutofJamestown.Thesecondshipto

arriveinthesettlementfromEnglandincludedamongitspassengersexpertsinpotashmaking.

Themethodofmakingpotashwassimpleenough.Logswaspiledupandburnedintheopen,

andtheashescollected.Theasheswereplacedinabarrelwithholesinthebottom,andwaterwas

pouredoverthem.Thesolutiondrainingfromthebarrelwasboileddowninironkettles.The

resultingmasswasfurtherheatedtofusethemassintowhatwascalledpotash.

InNorthAmerica,potashmakingquicklybecameanadjuncttotheclearingoflandfor

agriculture,foritwasestimatedthatasmuchashalfthecostofclearinglandcouldberecovered

bythesaleofpotash.SomepotashwasexportedfromMaineandNewHampshireinthe

seventeenthcentury,butthemarketturnedouttobemainlydomestic,consistingmostlyof

shipmentsfromthenortherntothesoutherncolonies.Fordespitethebeginningofthetradeat

Jamestownandsuchencouragementsasaseriesofacts“toencouragethemakingofpotash,"

beginningin1707inSouthCarolina,thesoftwoodsintheSouthprovedtobepoorsourcesofthe

substance.

PASSAGE7

(20)

AsPhiladelphiagrewfromasmalltownintoacityinthefirsthalfoftheeighteenthcentury,it

becameanincreasinglyimportantmarketingcenterforavastandgrowingagriculturalhinterland.

Marketdayssawthecrowdedcityevenmorecrowded,asfarmersfromwithinaradiusof24or

morekilometersbroughttheirsheep,cows,pigs,vegetables,cider,andotherproductsfordirect

saletothetownspeople.TheHighStreetMarketwascontinuouslyenlargedthroughouttheperiod

until1736,whenitreachedfromFrontStreettoThird.By1745NewMarketwasopenedon

SecondStreetbetweenPineandCedar.ThenextyeartheCallowhillMarketbeganoperation.

Alongwithmarketdays,theinstitutionoftwice-yearlyfairspersistedinPhiladelphiaeven

aftersimilartradingdayshadbeendiscontinuedinothercolonialcities.Thefairsprovideda

meansofbringinghandmadegoodsfromoutlyingplacestowould-bebuyersinthecity.Linens

andstockingsfromGermantown,forexample,werepopularitems.

Auctionswereanotherpopularformofoccasionaltrade.Becauseofthecompetition,retail

merchantsopposedtheseaswellasthefairs.Althoughgovernmentalattemptstoeradicatefairs

andauctionswerelessthansuccessful,theordinarycourseofeconomicdevelopmentwasonthe

merchants'side,asincreasingbusinessspecializationbecametheorderoftheday.Export

merchantsbecamedifferentiatedfromtheirimportingcounterparts,andspecialtyshopsbeganto

appearinadditiontogeneralstoressellingavarietyofgoods.

OneofthereasonsPhiladelphia'smerchantsgenerallyprosperedwasbecausethesurrounding

areawasundergoingtremendouseconomicanddemographicgrowth.Theydidtheirbusiness,

afterall,inthecapitalcityoftheprovince.Notonlydidtheycatertothegovernorandhiscircle,

butcitizensfromalloverthecolonycametothecapitalforlegislativesessionsoftheassembly

andcouncilandthemeetingsofthecourtsofjustice.

PASSAGE8

Thecanopy,theupperlevelofthetreesintherainforest,holdsaplethoraofclimbing

mammalsofmoderatelylargesize,whichmayincludemonkeys,cats,civets,andporcupines.

Smallerspecies,includingsuchrodentsasmiceandsmallsquirrels,arenotasprevalentoverallin

hightropicalcanopiesastheyareinmosthabitatsglobally.

Smallmammals,beingwarmblooded,sufferhardshipintheexposedandturbulent

environmentoftheuppermosttrees.Becauseasmallbodyhasmoresurfaceareaperunitof

weightthanalargeoneofsimilarshape,itgainsorlosesheatmoreswiftly.Thus,inthetrees,

whereshelterfromheatandcoldmaybescarceandconditionsmayfluctuate,asmallmammal

mayhavetroublemaintainingitsbodytemperature.

Smallsizemakesiteasytoscrambleamongtwigsandbranchesinthecanopyforinsects,

flowers,orfruit,butsmallmammalsaresurpassed,inthecompetitionforfood,bylargeonesthat

havetheirowntacticsforbrowsingamongfood-richtwigs.Theweightofagibbon(asmallape)

hangingbelowabrancharchestheterminalleavesdownsothatfruit-bearingfoliagedropstoward

thegibbon'sface.Walkingorleapingspeciesofasimilarorevenlargersizeaccesstheoutertwigs

eitherbysnappingoffandretrievingthewholebranchorbyclutchingstiffbrancheswiththefeet

ortailandpluckingfoodwiththeirhands.

Smallclimbinganimalsmayreachtwigsreadily,butitisharderforthemthanforlarge

climbinganimalstocrossthewidegapsfromontreecrowntothenextthattypifythehighcanopy.

Amacaqueorgibboncanhurlitselffartherthanamousecan:itcanachievearunningstart,andit

canmoreeffectivelyuseabranchasaspringboard,evenbouncingonaclimbseveraltimesbefore

jumping.Theforwardmovementofasmallanimalisseriouslyreducedbytheairfrictionagainst

therelativelylargesurfaceareaofitsbody.Finally,forthemanysmallmammalsthatsupplement

theirinsectdietwithfruitsorseeds,aninabilitytospanopengapsbetweentreecrownsmaybe

problematic,sincetreesthatyieldthesefoodscanbesparse.

PASSAGE9

PrehistoricmammothshavebeenpreservedinthefamoustarpitsofRanchoLaBrea(Breais

theSpanishwordfortar)inwhatisnowtheheartofLosAngeles,California.Thesetarpitshave

beenknownforcenturiesandwereformerlyminedfortheirnaturalasphalt,ablackorbrown

petroleum-likesubstance.Thousandsoftonswereextractedbefore1875,whenitwasfirstnoticed

thatthetarcontainedfossilremains.Majorexcavationswereundertakenthatestablishedthe

significanceofthisremarkablesite.Thetarpitswerefoundtocontaintheremainsofscoresof

speciesofanimalsfromthelast30,000yearsoftheIceAge.

Sincethen,over100tonsoffossils,1.5millionfromvertebrates,2.5millionfrom

invertebrates,havebeenrecovered,oftenindenselyconcentratedandtangledmasses.The

creaturesfoundrangefrominsectsandbirdstogiantgroundsloth's,butatotalof17proboscides

(animalswithaproboscisorlongnose)—includingmastodonsandColumbianmammoths一

havebeenrecovered,mostofthemfromPit9,thedeepestbone-bearingdeposit,whichwas

excavatedin1914.Mostofthefossilsdatetobetween40,000and10,000yearsago.

TheasphaltatLaBreaseepstothesurface,especiallyinthesummer,andformsshallow

puddlesthatwouldoftenhavebeenconcealedbyleavesanddust.Unwaryanimalswouldbecome

trappedonthesethinsheetsofliquidasphalt,whichareextremelystickyinwarmweather.Stuck,

theunfortunatebeastswoulddieofexhaustionandhungerorfallpreytopredatorsthatoftenalso

becamestuck.

Astheanimalsdecayed,morescavengerswouldbeattractedandcaughtintheirturn.

Carnivoresgreatlyoutnumberherbivoresinthecollection:foreverylargeherbivore,thereisone

saber-toothcat,acoyote,andfourwolves.Thefactthatsomebonesareheavilyweatheredshows

thatsomebodiesremainedabovethesurfaceforweeksormonths.Bacteriaintheasphaltwould

haveconsumedsomeofthetissuesotherthanbones,andtheasphaltitselfwoulddissolvewhat

wasleft,atthesametimeimpregnatingandbeautifullypreservingthesaturatedbones,rendering

themdarkbrownandshiny.

PASSAGE10

Oneareaofpaleoanthropologicalstudyinvolvestheeatinganddietaryhabitsofhominids,

erectbipedalprimates—includingearlyhumans.Itisclearthatatsomestageofhistory,humans

begantocarrytheirfoodtocentralplaces,calledhomebases,whereitwassharedandconsumed

withtheyoungandotheradults.Theuseofhomebasesisafundamentalcomponentofhuman

socialbehavior;thecommonmealservedatacommonhearthisapowerfulsymbol,amarkof

socialunity.Homebasebehaviordoesnotoccuramongnonhumanprimatesandisrareamong

mammals.Itisunclearwhenhumansbegantousehomebases,whatkindofcommunicationsand

socialrelationswereinvolved,andwhattheecologicalandfood-choicecontextsoftheshiftwere.

Workonearlytools,surveysofpaleoanthropologicalsites,developmentandtestingofbroad

ecologicaltheories,andadvancesincomparativeprimatologyarecontributingtoknowledgeabout

thiscentralchapterinhumanprehistory.

Oneinnovativeapproachtotheseissuesinvolvesstudyingdamageandwearonstonetools.

Researchersmaketoolsthatreplicateexcavatedspecimensascloselyaspossibleandthentryto

usethemastheoriginalsmighthavebeenused,inwoodcutting,hunting,orcultivation.

Dependingonhowthetoolisused,characteristicchippagepatternsandmicroscopically

distinguishablepolishesdevelopneartheedges.Thefirstapplicationofthismethodofanalysisto

stonetoolsthatare1.5millionto2millionyearsoldindicatesthat,fromthestart,animportant

functionofearlystonetoolswastoextracthighlynutritiousfood—meatandmarrow—from

largeanimalcarcasses.Fossilboneswithcutmarkscausedbystonetoolshavebeendiscovered

lyinginthesame2-million-year-oldlayersthatyieldedtheoldestsuchtoolsandtheoldest

hominidspecimens(includinghumans)withlargerthanape-sizedbrains.Thisdiscoveryincreases

scientists*certaintyaboutwhenhumanancestorsbegantoeatmoremeatthanpresent-day

nonhumanprimates.Butseveralquestionsremainunanswered:howfrequentlymeateating

occurred;whatthesocialimplicationsofmeateatingwere;andwhethertheincreaseduseofmeat

coincideswiththebeginningsoftheuseofhomebases.

PASSAGE11

Plantsaresubjecttoattackandinfectionbyaremarkablevarietyofsymbioticspeciesand

haveevolvedadiversearrayofmechanismsdesignedtofrustratethepotentialcolonists.These

canbedividedintopreformedorpassivedefensemechanismsandinducibleoractivesystems.

Passiveplantdefensecomprisesphysicalandchemicalbarriersthatprevententryofpathogens,

suchasbacteria,orrendertissuesunpalatableortoxictotheinvader.Theexternalsurfacesof

plants,inadditiontobeingcoveredbyanepidermisandawaxycuticle,oftencarryspikyhairs

knownastrichomes,whicheitherpreventfeedingbyinsectsormayevenpunctureandkillinsect

larvae.Othertrichomesarestickyandglandularandeffectivelytrapandimmobilizeinsects.

Ifthephysicalbarriersoftheplantarebreached,thenpreformedchemicalsmayinhibitorkill

theintruder,andplanttissuescontainadiversearrayoftoxicorpotentiallytoxicsubstances,such

asresins,tannins,glycosides,andalkaloids,manyofwhicharehighlyeffectivedeterrentsto

insectsthatfeedonplants.ThesuccessoftheColoradobeetleininfestingpotatoes,forexample,

seemstobecorrelatedwithitshightolerancetoalkaloidsthatnormallyrepelpotentialpests.

Otherpossiblechemicaldefenses,whilenotdirectlytoxictotheparasite,mayinhibitsome

essentialstepintheestablishmentofaparasiticrelationship.Forexample,glycoproteinsinplant

cellwallsmayinactivateenzymesthatdegradecellwalls.Theseenzymesareoftenproducedby

bacteriaandfungi.

Activeplantdefensemechanismsarecomparabletotheimmunesystemofvertebrateanimals,

althoughthecellularandmolecularbasesarefundamentallydifferent.Both,however,are

triggeredinreactiontointrusion,implyingthatthehosthassomemeansofrecognizingthe

presenceofaforeignorganism.Themostdramaticexampleofaninducibleplantdefensereaction

isthehypersensitiveresponse.Inthehypersensitiveresponse,cellsundergorapidnecrosis-that

is,theybecomediseasedanddie—afterbeingpenetratedbyaparasite;theparasiteitself

subsequentlyceasestogrowandisthereforerestrictedtooneorafewcellsaroundtheentrysite.

Severaltheorieshavebeenputforwardtoexplainthebasisofhypersensitiveresistance.

PASSAGE12

(25)

AmongthespeciesofseabirdsthatusethewindsweptcliffsoftheAtlanticcoastofCanadain

thesummertomate,layeggs,andreartheiryoungarecommonmurres,Atlanticpuffins,

black-leggedkittiwakes,andnortherngannets.Ofallthebirdsonthesecliffs,theblack-legged

kittiwakegullisthebestsuitedfornestingonnarrowledges.Althoughitsnestinghabitsare

similartothoseofgullsthatnestonflatground,thereareanumberofimportantdifferences

relatedtothecliff-nestinghabit.

Theadvantageofnestingoncliffsistheimmunityitgivesfromfoxes,whichcannotscalethe

sheerrocks,andfromravensandotherspeciesofgulls,whichhavedifficultyinlandingon

narrowledgestostealeggs.Thisimmunityhasbeenfollowedbyarelaxationofthedefenses,and

kittiwakesdonotreacttopredatorsnearlyasfiercelyasdoground-nestinggulls.Acolonyof

Bonaparte'sgullsrespondstotheappearanceofapredatoryherringgullbyflyingupasagroup

withaclamorofalarmcalls,followedbyconcertedmobbing,butkittiwakessimplyignoreherring

gulls,sincetheyposelittlethreattonestsoncliffs.Neitherdokittiwakesattempttoconcealtheir

nest.Mostgullskeepthenestareaclearofdroppings,andremoveemptyeggshellsafterthechicks

havehatched,sothatthelocationofthenestisnotgivenaway.Kittiwakesdefecateovertheedge

ofthenest,whichkeepsitclean,butthispractice,aswellastheirtendencytoleavethenest

litteredwitheggshells,makesitslocationveryconspicuous.

Ontheotherhand,nestingonanarrowledgehasitsownpeculiarproblems,andkittiwake

behaviorhasbecomeadaptedtoovercomethem.Thefemalekittiwakesitswhenmating,whereas

othergullsstand,sothepairwillnotoverbalanceandfallofftheledge.Thenestisadeepcup,

madeofmudorseaweed,toholdtheeggssafely,comparedwiththeshallowscrapeofothergulls,

andthechicksareremarkablyimmobileuntilfullygrown.Theydonotrunfromtheirnestswhen

approached,andiftheyshouldcomeneartothecliffedge,theyinstinctivelyturnback.

PASSAGE13

Anyrockthathascooledandsolidifiedfromamoltenstateisanigneousrock.Therefore,if

theEarthbeganasasuperheatedsphereinspace,alltherocksmakingupitscrustmaywellhave

beenigneousandthustheancestorsofallotherrocks.Eventoday,approximately95percentof

theentirecrustisigneous.Periodically,moltenmaterialwellsoutoftheEarth'sinteriortoinvade

thesurfacelayersortoflowontothesurfaceitself.Thismaterialcoolsintoawidevarietyof

igneousrocks.Inthemoltenstate,itiscalledmagmaasitpushesintothecrustandlavawhenit

runsoutontothesurface.

Allmagmaconsistsbasicallyofavarietyofsilicateminerals(highinsilicon-oxygen

compounds),butthechemicalcompositionofanygivenflowmaydifferradicallyfromthatofany

other.Theresultingigneousrockswillreflectthesedifferences.Igneousrocksalsovaryintexture

aswellaschemistry.Granite,forinstance,isacoarse-grainedigneous