托福英语阅读每日一练托福英语阅读每日精选一练Athousand-lijourneyisstartedbytakingthefirststep.以下是小编为大家搜索整理的托福英语阅读每日精选一练，希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网!PerhapsoneofthemostdramaticandimportantchangesthattookplaceintheMesozoiceraoccurredlateinthatera,amongthesmallorganismsthatpopulatetheuppermost,sunlitportionoftheoceans—theplankton.Theterm"plankton"isabroadone,designatingallofthesmallplantsandanimalsthatfloataboutorweaklypropelthemselvesthroughthesea.InthelatestagesoftheMesozoicera,duringtheCretaceousperiod,therewasagreatexpansionofplanktonthatprecipitatedskeletonsorshellscomposedoftwotypesofmineral:silicaandcalciumcarbonate.Thisdevelopmentradicallychangedthetypesofsedimentsthataccumulatedontheseafloor,because,whiletheorganicpartsoftheplanktondecayedaftertheorganismsdied,theirmineralizedskeletonsoftensurvivedandsanktothebottom.ForthefirsttimeintheEarth'slonghistory,verylargequantitiesofsilicaskeletons,whichwouldeventuallyhardenintorock,begantopileupinpartsofthedeepsea.Thickdepositsofcalcareousoozemadeupofthetinyremainsofthecalciumcarbonate-secretingplanktonalsoaccumulatedasneverbefore.ThefamouswhitechalkcliffsofDover,inthesoutheastofEngland,arejustoneexampleofthehugequantitiesofsuchmaterialthatamassedduringtheCretaceousperiod;therearemanymore.JustwhythecalcareousplanktonweresoprolificduringthelatterpartoftheCretaceousperiodisnotfullyunderstood.Suchmassiveamountsofchalkysedimentshaveneversincebeendepositedoveracomparableperiodoftime.ThehighbiologicalproductivityoftheCretaceousoceansalsoledtoidealconditionsforoilaccumulation.Oilisformedwhenorganicmaterialtrappedinsedimentsisslowlyburiedandsubjectedtoincreasedtemperaturesandpressures,transformingitintopetroleum.SedimentsrichinorganicmaterialaccumulatedalongthemarginsoftheTethysSeaway,thetropicaleast-westoceanthatformedwhenEarth'ssinglelandmass(knownasPangaea)splitapartduringtheMesozoicera.Manyoftoday'simportantoilfieldsarefoundinthosesediments—inRussia,theMiddleEast,theGulfofMexico,andinthestatesofTexasandLouisianaintheUnitedStates.1.Whatd