the origin of species(物种起源)-第62章

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y　slowly　through　natural　selection。　It　is　hardly　possible　for　me　even　to　recall　to　the　reader；　who　may　not　be　a　practical　geologist；　the　facts　leading　the　mind　feebly　to　comprehend　the　lapse　of　time。　He　who　can　read　Sir　Charles　Lyell's　grand　work　on　the　Principles　of　Geology；　which　the　future　historian　will　recognise　as　having　produced　a　revolution　in　natural　science；　yet　does　not　admit　how　incomprehensibly　vast　have　been　the　past　periods　of　time；　may　at　once　close　this　volume。　Not　that　it　suffices　to　study　the　Principles　of　Geology；　or　to　read　special　treatises　by　different　observers　on　separate　formations；　and　to　mark　how　each　author　attempts　to　give　an　inadequate　idea　of　the　duration　of　each　formation　or　even　each　stratum。　A　man　must　for　years　examine　for　himself　great　piles　of　superimposed　strata；　and　watch　the　sea　at　work　grinding　down　old　rocks　and　making　fresh　sediment；　before　he　can　hope　to　comprehend　anything　of　the　lapse　of　time；　the　monuments　of　which　we　see　around　us。　
It　is　good　to　wander　along　lines　of　sea…coast；　when　formed　of　moderately　hard　rocks；　and　mark　the　process　of　degradation。　The　tides　in　most　cases　reach　the　cliffs　only　for　a　short　time　twice　a　day；　and　the　waves　eat　into　them　only　when　they　are　charged　with　sand　or　pebbles；　for　there　is　reason　to　believe　that　pure　water　can　effect　little　or　nothing　in　wearing　away　rock。　At　last　the　base　of　the　cliff　is　undermined；　huge　fragments　fall　down；　and　these　remaining　fixed；　have　to　be　worn　away；　atom　by　atom；　until　reduced　in　size　they　can　be　rolled　about　by　the　waves；　and　then　are　more　quickly　ground　into　pebbles；　sand；　or　mud。　But　how　often　do　we　see　along　the　bases　of　retreating　cliffs　rounded　boulders；　all　thickly　clothed　by　marine　productions；　showing　how　little　they　are　abraded　and　how　seldom　they　are　rolled　about！　Moreover；　if　we　follow　for　a　few　miles　any　line　of　rocky　cliff；　which　is　undergoing　degradation；　we　find　that　it　is　only　here　and　there；　along　a　short　length　or　round　a　promontory；　that　the　cliffs　are　at　the　present　time　suffering。　The　appearance　of　the　surface　and　the　vegetation　show　that　elsewhere　years　have　elapsed　since　the　waters　washed　their　base。　
He　who　most　closely　studies　the　action　of　the　sea　on　our　shores；　will；　I　believe；　be　most　deeply　impressed　with　the　slowness　with　which　rocky　coasts　are　worn　away。　The　observations　on　this　head　by　Hugh　Miller；　and　by　that　excellent　observer　Mr。　Smith　of　Jordan　Hill；　are　most　impressive。　With　the　mind　thus　impressed；　let　any　one　examine　beds　of　conglomerate　many　thousand　feet　in　thickness；　which；　though　probably　formed　at　a　quicker　rate　than　many　other　deposits；　yet；　from　being　formed　of　worn　and　rounded　pebbles；　each　of　which　bears　the　stamp　of　time；　are　good　to　show　how　slowly　the　mass　has　been　accumulated。　Let　him　remember　Lyell's　profound　remark；　that　the　thickness　and　extent　of　sedimentary　formations　are　the　result　and　measure　of　the　degradation　which　the　earth's　crust　has　elsewhere　suffered。　And　what　an　amount　of　degradation　is　implied　by　the　sedimentary　deposits　of　many　countries！　Professor　Ramsay　has　given　me　the　maximum　thickness；　in　most　cases　from　actual　measurement；　in　a　few　cases　from　estimate；　of　each　formation　in　different　parts　of　Great　Britain；　and　this　is　the　result：…　
Feet　
Palaeozoic　strata　（not　including　igneous　beds）　57；154　
Secondary　strata　13；190　
Tertiary　strata　2；240　
making　altogether　72；584　feet；　that　is；　very　nearly　thirteen　and　three…quarters　British　miles。　Some　of　these　formations；　which　are　represented　in　England　by　thin　beds；　are　thousands　of　feet　in　thickness　on　the　Continent。　Moreover；　between　each　successive　formation；　we　have；　in　the　opinion　of　most　geologists；　enormously　long　blank　periods。　So　that　the　lofty　pile　of　sedimentary　rocks　in　Britain；　gives　but　an　inadequate　idea　of　the　time　which　has　elapsed　during　their　accumulation；　yet　what　time　this　must　have　consumed！　Good　observers　have　estimated　that　sediment　is　deposited　by　the　great　Mississippi　river　at　the　rate　of　only　600　feet　in　a　hundred　thousand　years。　This　estimate　may　be　quite　erroneous；　yet；　considering　over　what　wide　spaces　very　fine　sediment　is　transported　by　the　currents　of　the　sea；　the　process　of　accumulation　in　any　one　area　must　be　extremely　slow。　
But　the　amount　of　denudation　which　the　strata　have　in　many　places　suffered；　independently　of　the　rate　of　accumulation　of　the　degraded　matter；　probably　offers　the　best　evidence　of　the　lapse　of　time。　I　remember　having　been　much　struck　with　the　evidence　of　denudation；　when　viewing　volcanic　islands；　which　have　been　worn　by　the　waves　and　pared　all　round　into　perpendicular　cliffs　of　one　or　two　thousand　feet　in　height；　for　the　gentle　slope　of　the　lava…streams；　due　to　their　formerly　liquid　state；　showed　at　a　glance　how　far　the　hard；　rocky　beds　had　once　extended　into　the　open　ocean。　The　same　story　is　still　more　plainly　told　by　faults；　those　great　cracks　along　which　the　strata　have　been　upheaved　on　one　side；　or　thrown　down　on　the　other；　to　the　height　or　depth　of　thousands　of　feet；　for　since　the　crust　cracked；　the　surface　of　the　land　has　been　so　completely　planed　down　by　the　action　of　the　sea；　that　no　trace　of　these　vast　dislocations　is　externally　visible。　
The　Craven　fault；　for　instance；　extends　for　upwards　of　30　miles；　and　along　this　line　the　vertical　displacement　of　the　strata　has　varied　from　600　to　3000　feet。　Prof。　Ramsay　has　published　an　account　of　a　downthrow　in　Anglesea　of　2300　feet；　and　he　informs　me　that　he　fully　believes　there　is　one　in　Merionethshire　of　12；000　feet；　yet　in　these　cases　there　is　nothing　on　the　surface　to　show　such　prodigious　movements；　the　pile　of　rocks　on　the　one　or　other　side　having　been　smoothly　swept　away。　The　consideration　of　these　facts　impresses　my　mind　almost　in　the　same　manner　as　does　the　vain　endeavour　to　grapple　with　the　idea　of　eternity。　
I　am　tempted　to　give　one　other　case；　the　well…known　one　of　the　denudation　of　the　Weald。　Though　it　must　be　admitted　that　the　denudation　of　the　Weald　has　been　a　mere　trifle；　in　comparison　with　that　which　has　removed　masses　of　our　Palaeozoic　strata；　in　parts　ten　thousand　feet　in　thickness；　as　shown　in　Prof。　Ramsay's　masterly　memoir　on　this　subject。　Yet　it　is　an　admirable　lesson　to　stand　on　the　North　Downs　and　to　look　at　the　distant　South　Downs；　for；　remembering　that　at　no　great　distance　to　the　west　the　northern　and　southern　escarpments　meet　and　close；　one　can　safely　picture　to　oneself　the　great　dome　of　rocks　which　must　have　covered　up　the　Weald　within　so　limited　a　period　as　since　the　latter　part　of　the　Chalk　formation。　The　distance　from　the　northern　to　the　southern　Downs　is　about　22　miles；　and　the　thickness　of　the　several　formations　is　on　an　average　about　1100　feet；　as　I　am　informed　by　Prof。　Ramsay。　But　if；　as　some　geologists　suppose；　a　range　of　older　rocks　underlies　the　Weald；　on　the　flanks　of　which　the　overlying　sedimentary　deposits　might　have　accumulated　in　thinner　masses　than　elsewhere；　the　above　estimate　would　be　erroneous；　but　this　source　of　doubt　probably　would　not　greatly　affect　the　estimate　as　applied　to　the　western　extremity　of　the　district。　If；　then；　we　knew　the　rate　at　which　the　sea　commonly　wears　away　a　line　of　cliff　of　any　given　height；　we　could　measure　the　time　requisite　to　have　denuded　the　Weald。　This；　of　course；　cannot　be　done；　but　we　may；　in　order　to　form　some　crude　notion　on　the　subject；　assume　that　the　sea　would　eat　into　cliffs　500　feet　in　height　at　the　rate　of　one　inch　in　a　century。　This　will　at　first　appear　much　too　small　an　allowance；　but　it　is　the　same　as　if　we　were　to　assume　a　cliff　one　yard　in　height　to　be　eaten　back　along　a　whole　line　of　coast　at　the　rate　of　one　yard　in　nearly　every　twenty…two　years。　I　doubt　whether　any　rock；　even　as　soft　as　chalk；　would　yield　at　this　rate　excepting　on　the　most　exposed　coasts；　though　no　doubt　the　degradation　of　a　lofty　cliff　would　be　more　rapid　from　the　breakage　of　the　fallen　fragments。　On　the　other　hand；　I　do　not　believe　that　any　line　of　coast；　ten　or　twenty　miles　in　length；　ever　suffers　degradation　at　the　same　time　along　its　whole　indented　length；　and　we　must　remember　that　almost　all　strata　contain　harder　layers　or　nodules；　which　from　long　resisting　attrition　form　a　breakwater　at　the　base。　Hence；　under　ordinary　circumstanc