autobiography and selected essays-第17章

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anything　chemistry　tells　us　to　the　contrary；　the　chalk　might　be　a

kind　of　gigantic　fur　upon　the　bottom　of　the　earth…kettle；　which　is

kept　pretty　hot　below。



Let　us　try　another　method　of　making　the　chalk　tell　us　its　own

history。　　To　the　unassisted　eye　chalk　looks　simply　like　a　very

loose　and　open　kind　of　stone。　　But　it　is　possible　to　grind　a　slice

of　chalk　down　so　thin　that　you　can　see　through　ituntil　it　is　thin

enough；　in　fact；　to　be　examined　with　any　magnifying　power　that　may

be　thought　desirable。　　A　thin　slice　of　the　fur　of　a　kettle　might　be

made　in　the　same　way。　　If　it　were　examined　microscopically；　it

would　show　itself　to　be　a　more　or　less　distinctly　laminated　mineral

substance　and　nothing　more。



But　the　slice　of　chalk　presents　a　totally　different　appearance　when

placed　under　the　microscope。　　The　general　mass　of　it　is　made　up　of

very　minute　granules；　but；　imbedded　in　this　matrix；　are　innumerable

bodies；　some　smaller　and　some　larger；　but；　on　a　rough　average；　not

more　than　a　hundredth　of　an　inch　in　diameter；　having　a　well…defined

shape　and　structure。　　A　cubic　inch　of　some　specimens　of　chalk　may

contain　hundreds　of　thousands　of　these　bodies；　compacted　together

with　incalculable　millions　of　the　granules。



The　examination　of　a　transparent　slice　gives　a　good　notion　of　the

manner　in　which　the　components　of　the　chalk　are　arranged；　and　of

their　relative　proportions。　　But；　by　rubbing　up　some　chalk　with　a

brush　in　water　and　then　pouring　off　the　milky　fluid；　so　as　to

obtain　sediments　of　different　degrees　of　fineness；　the　granules　and

the　minute　rounded　bodies　may　be　pretty　well　separated　from　one

another；　and　submitted　to　microscopic　examination；　either　as　opaque

or　as　transparent　objects。　　By　combining　the　views　obtained　in

these　various　methods；　each　of　the　rounded　bodies　may　be　proved　to

be　a　beautifully　constructed　calcareous　fabric；　made　up　of　a　number

of　chambers；　communicating　freely　with　one　another。　　The　chambered

bodies　are　of　various　forms。　　One　of　the　commonest　is　something

like　a　badly　grown　raspberry；　being　formed　of　a　number　of　nearly

globular　chambers　of　different　sizes　congregated　together。　　It　is

called　Globigerina；　and　some　specimens　of　chalk　consist　of　little

else　than　Globigerina　and　granules。



Let　us　fix　our　attention　upon　the　Globigerina。　　It　is　the　spoor　of

the　game　we　are　tracking。　　If　we　can　learn　what　it　is　and　what　are

the　conditions　of　its　existence；　we　shall　see　our　way　to　the　origin

and　past　history　of　the　chalk。



A　suggestion　which　may　naturally　enough　present　itself　is；　that

these　curious　bodies　are　the　result　of　some　process　of　aggregation

which　has　taken　place　in　the　carbonate　of　lime；　that；　just　as　in

winter；　the　rime　on　our　windows　simulates　the　most　delicate　and

elegantly　arborescent　foliageproving　that　the　mere　mineral　water

may；　under　certain　conditions；　assume　the　outward　form　of　organic

bodiesso　this　mineral　substance；　carbonate　of　lime；　hidden　away

in　the　bowels　of　the　earth；　has　taken　the　shape　of　these　chambered

bodies。　　I　am　not　raising　a　merely　fanciful　and　unreal　objection。

Very　learned　men；　in　former　days；　have　even　entertained　the　notion

that　all　the　formed　things　found　in　rocks　are　of　this　nature；　and

if　no　such　conception　is　at　present　held　to　be　admissible；　it　is

because　long　and　varied　experience　has　now　shown　that　mineral

matter　never　does　assume　the　form　and　structure　we　find　in　fossils。

If　any　one　were　to　try　to　persuade　you　that　an　oyster…shell　（which

is　also　chiefly　composed　of　carbonate　of　lime）　had　crystallized　out

of　sea…water；　I　suppose　you　would　laugh　at　the　absurdity。　　Your

laughter　would　be　justified　by　the　fact　that　all　experience　tends

to　show　that　oyster…shells　are　formed　by　the　agency　of　oysters；　and

in　no　other　way。　　And　if　there　were　no　better　reasons；　we　should　be

justified；　on　like　grounds；　in　believing　that　Globigerina　is　not

the　product　of　anything　but　vital　activity。



Happily；　however；　better　evidence　in　proof　of　the　organic　nature　of

the　Globigerinae　than　that　of　analogy　is　forthcoming。　　It　so

happens　that　calcareous　skeletons；　exactly　similar　to　the

Globigerinae　of　the　chalk；　are　being　formed；　at　the　present　moment；

by　minute　living　creatures；　which　flourish　in　multitudes；　literally

more　numerous　than　the　sands　of　the　sea…shore；　over　a　large　extent

of　that　part　of　the　earth's　surface　which　is　covered　by　the　ocean。



The　history　of　the　discovery　of　these　living　Globigerinae；　and　of

the　part　which　they　play　in　rock　building；　is　singular　enough。　　It

is　a　discovery　which；　like　others　of　no　less　scientific　importance；

has　arisen；　incidentally；　out　of　work　devoted　to　very　different　and

exceedingly　practical　interests。



When　men　first　took　to　the　sea；　they　speedily　learned　to　look　out

for　shoals　and　rocks；　and　the　more　the　burthen　of　their　ships

increased；　the　more　imperatively　necessary　it　became　for　sailors　to

ascertain　with　precision　the　depths　of　the　waters　they　traversed。

Out　of　this　necessity　grew　the　use　of　the　lead　and　sounding　line；

and；　ultimately；　marine…surveying；　which　is　the　recording　of　the

form　of　coasts　and　of　the　depth　of　the　sea；　as　ascertained　by　the

sounding…lead；　upon　charts。



At　the　same　time；　it　became　desirable　to　ascertain　and　to　indicate

the　nature　of　the　sea…bottom；　since　this　circumstance　greatly

affects　its　goodness　as　holding　ground　for　anchors。　　Some　ingenious

tar；　whose　name　deserves　a　better　fate　than　the　oblivion　into　which

it　has　fallen；　attained　this　object　by　〃arming〃　the　bottom　of　the

lead　with　a　lump　of　grease；　to　which　more　or　less　of　the　sand　or

mud；　or　broken　shells；　as　the　case　might　be；　adhered；　and　was

brought　to　the　surface。　　But；　however　well　adapted　such　an

apparatus　might　be　for　rough　nautical　purposes；　scientific　accuracy

could　not　be　expected　from　the　armed　lead；　and　to　remedy　its

defects　（especially　when　applied　to　sounding　in　great　depths）

Lieut。　Brooke；＇61＇　of　the　American　Navy；　some　years　ago　invented

a　most　ingenious　machine；　by　which　a　considerable　portion　of　the

superficial　layer　of　the　sea…bottom　can　be　scooped　out　and　brought

up　from　any　depth　to　which　the　lead　descends。



In　1853；　Lieut。　Brooke　obtained　mud　from　the　bottom　of　the　North

Atlantic；　between　Newfoundland　and　the　Azores；　at　a　depth　of　more

than　ten　thousand　feet；　or　two　miles；　by　the　help　of　this　sounding

apparatus。　　The　specimens　were　sent　for　examination　to　Ehrenberg　＇62＇

of　Berlin；　and　to　Bailey　of　West　Point；＇63＇　and　those　able

microscopists　found　that　this　deep…sea　mud　was　almost　entirely

composed　of　the　skeletons　of　living　organismsthe　greater　proportion

of　these　being　just　like　the　Globigerinae　already　known　to　occur

in　the　chalk。



Thus　far；　the　work　had　been　carried　on　simply　in　the　interests　of

science；　but　Lieut。　Brooke's　method　of　sounding　acquired　a　high

commercial　value；　when　the　enterprise　of　laying　down　the　telegraph…

cable　＇64＇　between　this　country　and　the　United　States　was　undertaken。

For　it　became　a　matter　of　immense　importance　to　know；　not　only　the

depth　of　the　sea　over　the　whole　line　along　which　the　cable　was　to

be　laid；　but　the　exact　nature　of　the　bottom；　so　as　to　guard　against

chances　of　cutting　or　fraying　the　strands　of　that　costly　rope。　　The

Admiralty　consequently　ordered　Captain　Dayman；　an　old　friend　and

shipmate　of　mine；　to　ascertain　the　depth　over　the　whole　line　of　the

cable；　and　to　bring　back　specimens　of　the　bottom。　　In　former　days；

such　a　command　as　this　might　have　sounded　very　much　like　one　of　the

impossible　things　which　the　young　prince　in　the　Fairy　Tales　is

ordered　to　do　before　he　can　obtain　the　hand　of　the　Princess。

However；　in　the　months　of　June　and　July；　1857；　my　friend　performed

the　task　assigned　to　him　with　great　expedition　and　precision

without；　so　far　as　I　know；　having　met　with　any　reward　of　that　kind。

The　specimens　of　Atlantic　mud　which　he　procured　were　sent　to　me　to

be　examined　and　reported　upon。*





*　S