autobiography and selected essays-第17节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！



Ehrenberg　had　done　before　him；　that　much　of　its　granular　basis

possesses　a　definite　form。　　Comparing　these　formed　particles　with

those　in　the　Atlantic　soundings；　he　found　the　two　to　be　identical；

and　thus　proved　that　the　chalk；　like　the　soundings；　contains　these

mysterious　coccoliths　and　coccospheres。　　Here　was　a　further　and　a

most　interesting　confirmation；　from　internal　evidence；　of　the

essential　identity　of　the　chalk　with　modern　deep…sea　mud。

Globigerinae；　coccoliths；　and　coccospheres　are　round　as　the　chief

constituents　of　both；　and　testify　to　the　general　similarity　of　the

conditions　under　which　both　have　been　formed。



The　evidence　furnished　by　the　hewing；　facing；　and　superposition　of

the　stones　of　the　Pyramids；　that　these　structures　were　built　by

men；　has　no　greater　weight　than　the　evidence　that　the　chalk　was

built　by　Globigerinae；　and　the　belief　that　those　ancient　pyramid…

builders　were　terrestrial　and　air…breathing　creatures　like

ourselves；　is　it　not　better　based　than　the　conviction　that　the

chalk…makers　lived　in　the　sea？



But　as　our　belief　in　the　building　of　the　Pyramids　by　men　is　not

only　grounded　on　the　internal　evidences　afforded　by　these

structures；　but　gathers　strength　from　multitudinous　collateral

proofs；　and　is　clinched　by　the　total　absence　of　any　reason　for　a

contrary　belief；　so　the　evidence　drawn　from　the　Globigerinae　that

the　chalk　is　an　ancient　sea…bottom；　is　fortified　by　innumerable

independent　lines　of　evidence；　and　our　belief　in　the　truth　of　the

conclusion　to　which　all　positive　testimony　tends；　receives　the　like

negative　justification　from　the　fact　that　no　other　hypothesis　has　a

shadow　of　foundation。



It　may　be　worth　while　briefly　to　consider　a　few　of　these　collateral

proofs　that　the　chalk　was　deposited　at　the　bottom　of　the　sea。



The　great　mass　of　the　chalk　is　composed；　as　we　have　seen；　of　the

skeletons　of　Globigerinae；　and　other　simple　organisms；　imbedded　in

granular　matter。　　Here　and　there；　however；　this　hardened　mud　of　the

ancient　sea　reveals　the　remains　of　higher　animals　which　have　lived

and　died；　and　left　their　hard　parts　in　the　mud；　just　as　the　oysters

die　and　leave　their　shells　behind　them；　in　the　mud　of　the　present

seas。



There　are；　at　the　present　day；　certain　groups　of　animals　which　are

never　found　in　fresh　waters；　being　unable　to　live　anywhere　but　in

the　sea。　　Such　are　the　corals；　those　corallines　which　are　called

Polycoa；　those　creatures　which　fabricate　the　lamp…shells；　and　are

called　Brachiopoda；　the　pearly　Nautilus；　and　all　animals　allied　to

it；　and　all　the　forms　of　sea…urchins　and　star…fishes。



Not　only　are　all　these　creatures　confined　to　salt　water　at　the

present　day；　but；　so　far　as　our　records　of　the　past　go；　the

conditions　of　their　existence　have　been　the　same：　hence；　their

occurrence　in　any　deposit　is　as　strong　evidence　as　can　be　obtained；

that　that　deposit　was　formed　in　the　sea。　　Now　the　remains　of

animals　of　all　the　kinds　which　have　been　enumerated；　occur　in　the

chalk；　in　greater　or　less　abundance；　while　not　one　of　those　forms

of　shell…fish　which　are　characteristic　of　fresh　water　has　yet　been

observed　in　it。



When　we　consider　that　the　remains　of　more　than　three　thousand

distinct　species　of　aquatic　animals　have　been　discovered　among　the

fossils　of　the　chalk；　that　the　great　majority　of　them　are　of　such

forms　as　are　now　met　with　only　in　the　sea；　and　that　there　is　no

reason　to　believe　that　any　one　of　them　inhabited　fresh　waterthe

collateral　evidence　that　the　chalk　represents　an　ancient　sea…bottom

acquires　as　great　force　as　the　proof　derived　from　the　nature　of　the

chalk　itself。　　I　think　you　will　now　allow　that　I　did　not　overstate

my　case　when　I　asserted　that　we　have　as　strong　grounds　for

believing　that　all　the　vast　area　of　dry　land；　at　present　occupied

by　the　chalk；　was　once　at　the　bottom　of　the　sea；　as　we　have　for　any

matter　of　history　whatever；　while　there　is　no　justification　for　any

other　belief。



No　less　certain　it　is　that　the　time　during　which　the　countries　we

now　call　south…east　England；　France；　Germany；　Poland；　Russia；

Egypt；　Arabia；　Syria；　were　more　or　less　completely　covered　by　a

deep　sea；　was　of　considerable　duration。



We　have　already　seen　that　the　chalk　is；　in　places；　more　than　a

thousand　feet　thick。　　I　think　you　will　agree　with　me；　that　it　must

have　taken　some　time　for　the　skeletons　of　animalcules　of　a

hundredth　of　an　inch　in　diameter　to　heap　up　such　a　mass　as　that。　　I

have　said　that　throughout　the　thickness　of　the　chalk　the　remains　of

other　animals　are　scattered。　　These　remains　are　often　in　the　most

exquisite　state　of　preservation。　　The　valves　of　the　shell…fishes

are　commonly　adherent；　the　long　spines　of　some　of　the　sea…urchins；

which　would　be　detached　by　the　smallest　jar；　often　remain　in　their

places。　　In　a　word；　it　is　certain　that　these　animals　have　lived　and

died　when　the　place　which　they　now　occupy　was　the　surface　of　as

much　of　the　chalk　as　had　then　been　deposited；　and　that　each　has

been　covered　up　by　the　layer　of　Globigerina　mud；　upon　which　the

creatures　imbedded　a　little　higher　up　have；　in　like　manner；　lived

and　died。　　But　some　of　these　remains　prove　the　existence　of

reptiles　of　vast　size　in　the　chalk　sea。　　These　lived　their　time；

and　had　their　ancestors　and　descendants；　which　assuredly　implies

time；　reptiles　being　of　slow　growth。



There　is　more　curious　evidence；　again；　that　the　process　of　covering

up；　or；　in　other　words；　the　deposit　of　Globigerina　skeletons；　did

not　go　on　very　fast。　　It　is　demonstrable　that　an　animal　of　the

cretaceous　sea　might　die；　that　its　skeleton　might　lie　uncovered

upon　the　sea…bottom　long　enough　to　lose　all　its　outward　coverings

and　appendages　by　putrefaction；　and　that；　after　this　had　happened；

another　animal　might　attach　itself　to　the　dead　and　naked　skeleton；

might　grow　to　maturity；　and　might　itself　die　before　the　calcareous

mud　had　buried　the　whole。



Cases　of　this　kind　are　admirably　described　by　Sir　Charles　Lyell。＇67＇

He　speaks　of　the　frequency　with　which　geologists　find　in　the　chalk

a　fossilized　sea…urchin；　to　which　is　attached　the　lower　valve　of　a

Crania。　　This　is　a　kind　of　shell…fish；　with　a　shell　composed　of　two

pieces；　of　which；　as　in　the　oyster；　one　is　fixed　and　the　other

free。



〃The　upper　valve　is　almost　invariably　wanting；　though　occasionally

found　in　a　perfect　state　of　preservation　in　the　white　chalk　at　some

distance。　　In　this　case；　we　see　clearly　that　the　sea…urchin　first

lived　from　youth　to　age；　then　died　and　lost　its　spines；　which　were

carried　away。　　Then　the　young　Crania　adhered　to　the　bared　shell；

grew　and　perished　in　its　turn；　after　which；　the　upper　valve　was

separated　from　the　lower；　before　the　Echinus　＇68＇　became　enveloped

in　chalky　mud。〃



A　specimen　in　the　Museum　of　Practical　Geology；　in　London；　still

further　prolongs　the　period　which　must　have　elapsed　between　the

death　of　the　sea…urchin；　and　its　burial　by　the　Globigerinae。　　For

the　outward　face　of　the　valve　of　a　Crania；　which　is　attached　to　a

sea…urchin　（Micraster）；　is　itself　overrun　by　an　incrusting

coralline；　which　spreads　thence　over　more　or　less　of　the　surface　of

the　sea…urchin。　　It　follows　that；　after　the　upper　valve　of　the

Crania　fell　off；　the　surface　of　the　attached　valve　must　have

remained　exposed　long　enough　to　allow　of　the　growth　of　the　whole

corraline；　since　corallines　do　not　live　imbedded　in　mud。



The　progress　of　knowledge　may；　one　day；　enable　us　to　deduce　from

such　facts　as　these　the　maximum　rate　at　which　the　chalk　can　have

accumulated；　and　thus　to　arrive　at　the　minimum　duration　of　the

chalk　period。　　Suppose　that　the　valve　of　the　Crania　upon　which　a

coralline　has　fixed　itself　in　the　way　just　described；　is　so

attached　to　the　sea…urchin　that　no　part　of　it　is　more　than　an　inch

above　the　face　upon　which　the　sea…urchin　rests。　　Then；　as　the

coralline　could　not　have　fixed　itself；　if　the　Crania　had　been

covered　up　with　chalk　mud；　and　could　not　have　lived　had　itself　been

so　covered　it　follows；　that　an　inch　of　chalk　mud　could　not　have

accumulated　within　the　time　between　the　death　and　decay　of　the　soft

parts　of　the　sea…urchin　and　the　growth　of　the　coralline　to　the　full

size　which　it　has　attained。　　If　the　decay　of　the　soft　parts　of　the

sea…urchin；　the　attachment；　growth　to　maturity；　and　decay　of　the

Crania；　and　the　subsequent　attachment　and　growth　of　the　coralline；

took　a　year　（which　is　a　low　estimate　enough）；　the　accumulation　of

the　inch　of　chalk　must　have　taken　more　than　a　year：　and　the　deposit

of　a　thousand　feet　of　chalk　must；　consequently；　have　taken　more

than　twelve　thousand　years。



The　foundation　of　all　this　calculation　is；　of　course；　a　knowledge

of　the　length　of　time　the　Crania　and　the　coralline　needed　to　attain

their　full　size；　and；　on　this　head；　precise　knowledge　is　at　present

wanting。　　But　there　are　circumstances　which　tend　to　show；　that

nothing　like　an　inch　of　chalk　has　accumulated　during　the　life　of　a

Crania；　an