EVERYONEknowsthatthatsuperficialfilmoftheearth\'ssubstance，hardlytenmilesthick，whichisaccessibletohumaninvestigation，iscomposedforthemostpartofbedsorstrataofstone，theconsolidatedmudsandsandsofformerseasandlakes，whichhavebeendepositedoneupontheother，andhencearetheolderthedeepertheylie。Thesemultitudinousstratapresentsuchresemblancesanddifferencesamongthemselvesthattheyarecapableofclassificationintogroupsorformations，andtheseformationsagainarebrigadedtogetherintostilllargerassemblages，calledbytheoldergeologists，primary，secondary，andtertiary；bythemoderns，palaeozoic，mesozoic，andcainozoic:thebasisoftheformernomenclaturebeingtherelativeageofthegroupsofstrata；thatofthelatter，thekindsoflivingformscontainedinthem。
Thoughbutafilmifcomparedwiththetotaldiameterofourplanet，thetotalseriesofformationsisvastindeedwhenmeasuredbyanyhumanstandard，and，asallactionimpliestime，soarewecompelledtoregardthesemineralmassesasameasureofthetimewhichhaselapsedduringtheiraccumulation。Theamountofthetimewhichtheyrepresentis，ofcourse，intheinverseproportionoftheintensityoftheforceswhichhavebeeninoperation。If，intheancientworld，mudandsandaccumulatedonsea-bottomsattenfoldtheirpresentrate，itisclearthatabedofmudorsandtenfeetthickwouldhavebeenformedtheninthesametimeasastratumofsimilarmaterialsonefootthickwouldbeformednow，and\'viceversa\'。
Attheoutsetofhisstudies，therefore，thephysicalgeologisthadtochoosebetweentwohypotheses；either，throughouttheageswhicharerepresentedbytheaccumulatedstrata，andwhichwemaycall\'geologictime\'，theforcesofnaturehaveoperatedwithmuchsameaverageintensityasatpresent，andhencethelapseoftimewhichtheyrepresentmustbesomethingprodigiousandinconceivable，or，intheprimevalepochs，thenaturalpowerswereinfinitelymoreintensethannow，andhencethetimethroughwhichtheyactedtoproducetheeffectsweseewascomparativelyshort。
Theearliergeologistsadoptedthelatterviewalmostwithoneconsent。
Fortheyhadlittleknowledgeofthepresentworkingsofnature，andtheyreadtherecordsofgeologictimeasachildreadsthehistoryofRomeorGreece，andfanciesthatantiquitywasgrand，heroic，andunlikethepresentbecauseitisunlikehislittleexperienceofthepresent。
Evensotheearlierobserversweremovedwithwonderattheseemingcontrastbetweentheancientandthepresentorderofnature。Theelementalforcesseemedtohavebeengranderandmoreenergeticinprimevaltimes。Upheavedandcontorted，riftedandfissured，piercedbydykesofmoltenmatterorwornawayovervastareasbyaqueousaction，theolderrocksappearedtobearwitnesstoastateofthingsfardifferentfromthatexhibitedbythepeacefulepochonwhichthelotofmanhasfallen。
Butbydegreesthoughtfulstudentsofgeologyhavebeenledtoperceivethattheearliesteffortsofnaturehavebeenbynomeansthegrandest。AlpsandAndesarechildrenofyesterdaywhencomparedwithSnowdonandtheCumberlandhills；andtheso-calledglacialepoch——thatinwhichperhapsthemostextensivephysicalchangesofwhichanyrecordremainingoccurred——isthelastandthenewestoftherevolutionsoftheglobe。Andinproportionasphysicalgeography——whichisthegeologyofourownepoch——hasgrownintoascience，andthepresentorderofnaturehasbeenransackedtofindwhat，\'hibernice\'，wemaycallprecedentsforthephenomenaofthepast，sotheapparentnecessityofsupposingthepasttobewidelydifferentfromthepresenthasdiminished。
Thetransportingpowerofthegreatestdelugewhichcanbeimaginedsinksintoinsignificancebesidethatoftheslowlyfloating，slowlymeltingiceberg，ortheglaciercreepingalongatitssnail\'spaceofayardaday。ThestudyofthedeltasoftheNile，theGanges，andtheMississippihastaughtushowslowisthewearingactionofwater，howvastitseffectswhentimeisallowedforitsoperation。ThereefsofthePacific，thedeep-seasoundingsoftheAtlantic，showthatitistotheslow-growingcoralandtotheimperceptibleanimalcule，whichlivesitsbriefspaceandthenaddsitstinyshelltothemuddycairnleftbyitsbrethrenandancestors，thatwemustlookastheagentsintheformationoflimestoneandchalk，andnottohypotheticaloceanssaturatedwithcalcareoussaltsandsuddenlydepositingthem。
Andwhiletheinquirerhasthuslearntthatexistingforces——\'givethemtime\'——arecompetenttoproduceallthephysicalphenomenawemeetwithintherocks，so，ontheotherside，thestudyofthemarksleftintheancientstratabypastphysicalactionsshowsthattheseweresimilartothosewhichnowobtain。Ancientbeachesaremetwithwhosepebblesarelikethosefoundonmodernshores；thehardenedsea-sandsoftheoldestepochsshowripple-marks，suchasmaynowbefoundoneverysandycoast；nay，more，thepitsleftbyancientrain-dropsprovethatevenintheveryearliestages，the\"bowintheclouds\"musthaveadornedthepalaeozoicfirmament。SothatifwecouldreversethelegendoftheSevenSleepers，——ifwecouldsleepbackthroughthepast，andawakeamillionagesbeforeourownepoch，inthemidstoftheearliestgeologictimes，——thereisnoreasontobelievethatsea，orsky，ortheaspectofthelandwouldwarnusofthemarvellousretrospection。
Sucharethebeliefswhichmodernphysicalgeologistshold，or，atanyrate，tendtowardsholding。But，insodoing，itisobviousthattheybynomeansprejudgethequestion，astowhatthephysicalconditionoftheglobemayhavebeenbeforeourchaptersofitshistorybegin，inwhatmaybecalled（withthatlicencewhichisimpliedintheoften-usedterm\"prehistoricepoch\"）\"pre-geologictime。\"Theviewsindicated，infact，arenotonlyquiteconsistentwiththehypothesis，that，inthestillearlierperiodreferredto，theconditionofourworldwasverydifferent；buttheymaybeheldbysometonecessitatethathypothesis。
Thephysicalphilosopherwhoisaccuratelyacquaintedwiththevelocityofacannon-ball，andtheprecisecharacterofthelinewhichittraversesforayardofitscourse，isnecessitatedbywhatheknowsofthelawsofnaturetoconcludethatitcamefromacertainspot，whenceitwasimpelledbyacertainforce，andthatithasfollowedacertaintrajectory。Inlikemanner，thestudentofphysicalgeology，whofullybelievesintheuniformityofthegeneralconditionoftheearththroughgeologictime，mayfeelcompelledbywhatheknowsofcausation，andbythegeneralanalogyofnature，tosupposethatoursolarsystemwasonceanebulousmass；thatitgraduallycondensed，thatitbrokeupintothatwonderfulgroupofharmoniouslyrollingballswecallplanetsandsatellites，andthattheneachoftheseunderwentitsappointedmetamorphosis，untilatlastourownshareofthecosmicvapourpassedintothatconditioninwhichwefirstmeetwithdefiniterecordsofitsstate，andinwhichithassince，withcomparativelylittlechange，remained。
Thedoctrineofuniformityandthedoctrineofprogressionare，therefore，perfectlyconsistent；perhaps，indeed，theymightbeshowntobenecessarilyconnectedwithoneanother。