the past condition of organic nature-第2章

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source　will；　of　course；　be　carried　to　the　same　place；　and；　as　it　is
quite　impossible　for　it　to　get　beneath　the　layer　already　there；　it
deposits　itself　above　it；　and　forms　another　layer；　and　in　that　way　you
gradually　have　layers　of　mud　constantly　forming　and　hardening　one　above
the　other；　and　conveying　a　record　of　time。

It　is　a　necessary　result　of　the　operation　of　the　law　of　gravitation　that
the　uppermost　layer　shall　be　the　youngest　and　the　lowest　the　oldest；
and　that　the　different　beds　shall　be　older　at　any　particular　point　or
spot　in　exactly　the　ratio　of　their　depth　from　the　surface。　　So　that　if
they　were　upheaved　afterwards；　and　you　had　a　series　of　these　different
layers　of　mud；　converted　into　sandstone；　or　limestone；　as　the　case
might　be；　you　might　be　sure　that　the　bottom　layer　was　deposited　first；
and　that　the　upper　layers　were　formed　afterwards。　　Here；　you　see；　is　the
first　step　in　the　historythese　layers　of　mud　give　us　an　idea　of　time。

The　whole　surface　of　the　earth；I　speak　broadly；　and　leave　out　minor
qualifications；is　made　up　of　such　layers　of　mud；　so　hard；　the
majority　of　them；　that　we　call　them　rock　whether　limestone　or
sandstone；　or　other　varieties　of　rock。　　And；　seeing　that　every　part　of
the　crust　of　the　earth　is　made　up　in　this　way；　you　might　think　that　the
determination　of　the　chronology；　the　fixing　of　the　time　which　it　has
taken　to　form　this　crust　is　a　comparatively　simple　matter。　　Take　a
broad　average；　ascertain　how　fast　the　mud　is　deposited　upon　the　bottom
of　the　sea；　or　in　the　estuary　of　rivers；　take　it　to　be　an　inch；　or　two；
or　three　inches　a　year；　or　whatever　you　may　roughly　estimate　it　at；
then　take　the　total　thickness　of　the　whole　series　of　stratified　rocks；
which　geologists　estimate　at　twelve　or　thirteen　miles；　or　about　seventy
thousand　feet；　make　a　sum　in　short　division；　divide　the　total　thickness
by　that　of　the　quantity　deposited　in　one　year；　and　the　result　will；　of
course；　give　you　the　number　of　years　which　the　crust　has　taken　to　form。

Truly；　that　looks　a　very　simple　process！　　It　would　be　so　except　for
certain　difficulties；　the　very　first　of　which　is　that　of　finding　how
rapidly　sediments　are　deposited；　but　the　main　difficultya　difficulty
which　renders　any　certain　calculations　of　such　a　matter　out　of　the
questionis　this；　the　sea…bottom　on　which　the　deposit　takes　place　is
continually　shifting。

Instead　of　the　surface　of　the　earth　being　that　stable；　fixed　thing　that
it　is　popularly　believed　to　be；　being；　in　common　parlance；　the　very
emblem　of　fixity　itself；　it　is　incessantly　moving；　and　is；　in　fact；　as
unstable　as　the　surface　of　the　sea；　except　that　its　undulations　are
infinitely　slower　and　enormously　higher　and　deeper。

Now；　what　is　the　effect　of　this　oscillation？　　Take　the　case　to　which　I
have　previously　referred。　　The　finer　or　coarser　sediments　that　are
carried　down　by　the　current　of　the　river；　will　only　be　carried　out　a
certain　distance；　and　eventually；　as　we　have　already　seen；　on　reaching
the　stiller　part　of　the　ocean；　will　be　deposited　at　the　bottom。

Let　C　y　（Fig。　4）　be　the　sea…bottom；　y　D　the　shore；　x　y　the　sea…level；
then　the　coarser　deposit　will　subside　over　the　region　B；　the　finer　over
A；　while　beyond　A　there　will　be　no　deposit　at　all；　and；　consequently；
no　record　will　be　kept；　simply　because　no　deposit　is　going　on。　　Now；
suppose　that　the　whole　land；　C；　D；　which　we　have　regarded　as　stationary；
goes　down；　as　it　does　so；　both　A　and　B　go　further　out　from　the　shore；
which　will　be　at　yl；　x1；　y1；　being　the　new　sea…level。　　The　consequence
will　be　that　the　layer　of　mud　（A）；　being　now；　for　the　most　part；
further　than　the　force　of　the　current　is　strong　enough　to　convey　even
the　finest　'debris'；　will；　of　course；　receive　no　more　deposits；　and
having　attained　a　certain　thickness　will　now　grow　no　thicker。

We　should　be　misled　in　taking　the　thickness　of　that　layer；　whenever　it
may　be　exposed　to　our　view；　as　a　record　of　time　in　the　manner　in　which
we　are　now　regarding　this　subject；　as　it　would　give　us　only　an
imperfect　and　partial　record：　it　would　seem　to　represent　too　short　a
period　of　time。

Fig。4。

Suppose；　on　the　other　hand；　that　the　land　（C　D）　had　gone　on　rising
slowly　and　graduallysay　an　inch　or　two　inches　in　the　course　of　a
century；what　would　be　the　practical　effect　of　that　movement？　　Why；
that　the　sediment　A　and　B　which　has　been　already　deposited；　would
eventually　be　brought　nearer　to　the　shore…level；　and　again　subjected　to
the　wear　and　tear　of　the　sea；　and　directly　the　sea　begins　to　act　upon
it；　it　would　of　course　soon　cut　up　and　carry　it　away；　to　a　greater　or
less　extent；　to　be　re…deposited　further　out。

Well；　as　there　is；　in　all　probability；　not　one　single　spot　on　the　whole
surface　of　the　earth；　which　has　not　been　up　and　down　in　this　way　a
great　many　times；　it　follows　that　the　thickness　of　the　deposits　formed
at　any　particular　spot　cannot　be　taken　（even　supposing　we　had　at　first
obtained　correct　data　as　to　the　rate　at　which　they　took　place）　as
affording　reliable　information　as　to　the　period　of　time　occupied　in　its
deposit。　So　that　you　see　it　is　absolutely　necessary　from　these　facts；
seeing　that　our　record　entirely　consists　of　accumulations　of　mud；
superimposed　one　on　the　other；　seeing　in　the　next　place　that　any
particular　spots　on　which　accumulations　have　occurred；　have　been
constantly　moving　up　and　down；　and　sometimes　out　of　the　reach　of　a
deposit；　and　at　other　times　its　own　deposit　broken　up　and　carried　away；
it　follows　that　our　record　must　be　in　the　highest　degree　imperfect；　and
we　have　hardly　a　trace　left　of　thick　deposits；　or　any　definite
knowledge　of　the　area　that　they　occupied；　in　a　great　many　cases。　　And
mark　this！　　That　supposing　even　that　the　whole　surface　of　the　earth　had
been　accessible　to　the　geologist；that　man　had　had　access　to　every
part　of　the　earth；　and　had　made　sections　of　the　whole；　and　put　them　all
together；even　then　his　record　must　of　necessity　be　imperfect。

But　to　how　much　has　man　really　access？　　If　you　will　look　at　this　Map　you
will　see　that　it　represents　the　proportion　of　the　sea　to　the　earth：
this　coloured　part　indicates　all　the　dry　land；　and　this　other　portion
is　the　water。　　You　will　notice　at　once　that　the　water　covers
three…fifths　of　the　whole　surface　of　the　globe；　and　has　covered　it　in
the　same　manner　ever　since　man　has　kept　any　record　of　his　own
observations；　to　say　nothing　of　the　minute　period　during　which　he　has
cultivated　geological　inquiry。　　So　that　three…fifths　of　the　surface　of
the　earth　is　shut　out　from　us　because　it　is　under　the　sea。　　Let　us　look
at　the　other　two…fifths；　and　see　what　are　the　countries　in　which
anything　that　may　be　termed　searching　geological　inquiry　has　been
carried　out：　a　good　deal　of　France；　Germany；　and　Great　Britain　and
Ireland；　bits　of　Spain；　of　Italy；　and　of　Russia；　have　been　examined；
but　of　the　whole　great　mass　of　Africa；　except　parts　of　the　southern
extremity；　we　know　next　to　nothing；　little　bits　of　India；　but　of　the
greater　part　of　the　Asiatic　continent　nothing；　bits　of　the　Northern
American　States　and　of　Canada；　but　of　the　greater　part　of　the　continent
of　North　America；　and　in　still　larger　proportion；　of　South　America；
nothing！

Under　these　circumstances；　it　follows　that　even　with　reference　to　that
kind　of　imperfect　information　which　we　can　possess；　it　is　only　of　about
the　ten…thousandth　part　of　the　accessible　parts　of　the　earth　that　has
been　examined　properly。　Therefore；　it　is　with　justice　that　the　most
thoughtful　of　those　who　are　concerned　in　these　inquiries　insist
continually　upon　the　imperfection　of　the　geological　record；　for；　I
repeat；　it　is　absolutely　necessary；　from　the　nature　of　things；　that
that　record　should　be　of　the　most　fragmentary　and　imperfect　character。
Unfortunately　this　circumstance　has　been　constantly　forgotten。　　Men　of
science；　like　young　colts　in　a　fresh　pasture；　are　apt　to　be　exhilarated
on　being　turned　into　a　new　field　of　inquiry；　to　go　off　at　a
hand…gallop；　in　total　disregard　of　hedges　and　ditches；　losing　sight　of
the　real　limitation　of　their　inquiries；　and　to　forget　the　extreme
imperfection　of　what　is　really　known。　Geologists　have　imagined　that
they　could　tell　us　what　was　going　on