Darwin1845 - structure and distribution of coral reefs.pdf

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doi:10.1144/GSL.JGS.1845.001.01.82 1845; v. 1; p. 381-389Quarterly Journal of the Geological Society Charles Darwin and D. T. A.

Expedition. London, pp. 214. 1842.R.N. during the Years 1832 to 1836: Naturalist to thethe Beagle under the Command of Capt. Fitzroy,being the first Part of the Geology of the Voyage ofThe Structure and Distribution of Coral Reefs ; Quarterly Journal of the Geological Society

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381

V N O T I C E S O F N E W B O O KS

L T ~ E STRUCTURE AND DISTRIBUTION OF COR AL RE EF S eingthe f i r s t P ar t o f the Geology o f the Voyage o f the B eagle underthe Command o f Cap t. _FitzroyR . N . during t~e Years1832 to

1836. B y CHARLES DARWIN, M .A. , F.R .S . , N atu ra l i s t to theEx ped i t ion . London , pp . 214 . 1842.

M R . D A RW ~ , i n a d d i t i o n to h i s j o u r n a l o f t h e v o y a g e o f th eB e a g le , h a s s i n c e p u b l i sh e d , w i t h t h e a p p r o v a l o f t h e L o r d s C o m -m i s s i o n e rs o f t h e Tr e a s u r y, t w o v o l u m e s h a v i n g r e fe r e n c e tog e o lo g ic a l s u b j e c t s - - t h e r e su l ts o f i n v e s t ig a t i o n s m a d e d u r i n g h i sexped i t ion .

I t i s t o be r eg re t t ed , t ha t t h e pub l i ca t ion o f t hese vo lume s wasd e l a y e d in c o n s eq u e n c e o f t h e i l l h e a r t h o f t h e a u t ho r, w h i c hlong p rev en te d the c lose a t t en t ion nece ssa ry to p rep a re h i s ob -se rva t ions fo r t he p res s . W e hav e r eason to r e jo i ce tha t t hew o r k s t h e m s e l v e s r e q u i r e n o a p o lo g y f o r t h e s ta t e i n w h i c h t h e yappear.

The ob jec t o f t he vo lume a t p re sen t be fo re us , i s t o desc r ibe ind e t a il t h e g e n e r a l s t ru c t u r e a n d t h e c i r c u m s t a n c e s o f f o r m a t i o n o fc o ra l r ee f s, w i t h t h e o b j e ct o f e x p l a i n i n g a n d s u p p o r t in g c e r t a i nv i e w s o f t h e a c t io n o f d i s t u r b i n g f o rc e s, a s s u m e d b y t h e a u t h o r t ohav e p roduce d e l eva t ions and depres s ions o f t he sea bo t tom, andi n t h i s w a y a c c o u n t i n g f o r m a n y g e o l o g i c al p h e n o m e n a i n d i f f e re n tpa r t s o f t he wor ld , an d o f va r ious pe r iods.

Th ere a re two g rea t cl a sses o f co ra l -r ee f s, i n one o r t he o the ro f w h i c h t h e w h o l e n u m b e r m a y b e i n c l u d e d : th os e, n a m e l y,w h i c h e x h i b i t n e a r t h e s u r fa c e n o e v i d e n c e o f a n y s o li d r o c kfro m w h i ch th e y m a y h av e o r i g i n a t e d -the barrier and atoll-formed reefs a n d t h o se w h i c h a r e c a l le df r i n g i n g - r e e f s -where ,o w i n g t o t h e n a t u r e o f t h e s lo p e a n d t h e v i c i n i t y o f l a n d , t h e r ec a n b e n o d o u b t a s t o th e o r i g i n . T h e f o r m e r c l as s ( i n c l u d i n gthe l agoon i s l ands ) have a t tr ac t ed mos t a t t en t ion , and offe r t h em o s t s t r i k i n g p h e n o m e n a , b u t b o th o f t h e m a r e e x t re m e l yin te re s t ing .

T h e t e r m a t o l l i s u s ed b y t h e i n h a b i t a n t s o f t h e i s la n d s i n t h eInd ian ocean to d i s t ingu i sh c i r c u la r g roups o f co ra l i s le t s some-t i m e s c a l le d L a g o o n i s l a n d s , a k i n d o f s a l t - w a t e r l a k e o r l a go o nb e i n g o f t e n f o r m e d b y a n e a r l y c o n t in u o u s c u r v e d l i n e o f s u c hree fs , on ly l eav ing one o r two ou t l e t s . Ba r r i e r- r e e f s a re l i t t l el es s m a r v e l l o u s i n t h e i r s t r u c t u r e t h a n t h e a to ll s, t h e b a n d o fc o r a l - re e f s o m e t im e s f o r m i n g a f r i n g e o f la n d , w h i l e s o m e t i m e s


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~ 8~ NOTICES OF NE W BOOKS.

t h e e x i s t e n c e o f th e r e e f i s o n l y i n d i c a t e d b y a l i n e o f b r e a k e r s ,o r a f e w sm a l l i s l e ts c r o w n e d w i t h c o c o a - n u t t r e e s ; a n d t h e r e e fi n t h i s c a s e is a t a c o n s i d e r a b l e d i s t a n c e f r o m a c e n t r a l is l a n d ,w h i c h i s n o t i t s e lf d u e t o t h e l a b o u r s o f t h e c o r a l p o ly p .

F r i n g i n g , o r sh o r e - r e e fs , d i ff e r f r o m b a r r i e r - r e e f s i n n o t h a v i n gw i t h i n t h e m a b r o a d c h a n n e l o f d e e p w a t e r , a n d i n n o t l y i n g sof a r f r o m th e s h o r e. T h e r e a r e o t h e r k i n d s o f r e e f s a l li e d m o r e o rl es s n e a r l y t o o n e o r t h e o t h e r o f t h e s e ; b u t t h e y p o s se s s c o m -p a r a t i v e l y l i t tl e i n t e r e s t . W i t h r e s p e c t t o t h e o r ig i n o f a l l t h e s er e e fs , th e g r e a t d if f ic u l ty l ie s in e x p l a i n i n g t h e f o u n d a t i o n o f t h eb a r r i e r a n d a t o l l -f o r m e d c l as s, t h e f r i n g i n g - r e e f s r e q u i r i n g l i t t l ec o n s i d e r a t i o n .

K Z l ~ L I N O AT O L L

Chart and Vert ical Sect ion) .

. , $ . ' :' . . . . . . . . . '

b c d e f

a. Sea level at low w ater ; depth 25 fathoms at 150 yards from the edge ofthe reef.

b. O ut er edge o the f iat part of the reef wh ich dries at low water ; this edgeis ei ther a m ou nd o r consists of jagg ed rugg ed points .

c . A f iat of corm rock covered at high w ater.

d. A projec t ing ledge of brecciated coral rock washed by the waves at highwater.e . Loo se fragmemts of coral only reached ~by the sea durin g gales , and i ts

up per pa rt 6 to 12 feet hig h) clothed with vegetation.f . Lo w water leve l of the lagoon.N o t e . Th e dot ted l ine in the char t marks the l imi t o f sha llow water.

:M r . D a r w i n h a s d e s c r i b e d e v e r y e x i s ti n g c o r a l - r e e f i n g e o g r a -p l a ie a l o r d e r so f a r as h e p o ss e s se d i n f o r m a t i o n w i t h t h e e x c e p t i o n


4/10

DARW1N ON CORAL REEFS 3 8 3

o f s om e o n t h e c o as t o f B r a z i l ), a n d h a s m a r k e d t h e m o n a m a pw i t h c o lo u r s d i s t in g u i s h i n g t h e d i f fe r e n t k in d s . H e h a s th u si l l u s t r a t ed h i s ow n hypo thes i s on t he sub jec t, na me ly, t ha t i n bo tha t ol ls a n d b a r r i e r - r e e f s , t h e f o u n d a t io n o n w h i c h t h e c o r a l w a sp r i m a r i l y a t t a c h e d h a s s u b si de d , a n d t h a t d u r i n g t h is d o w n w a r dm o v e m e n t t h e r e e f s h a v e g r o w n u p w a r d s .

In o rde r t o g ive a s d i s t inc t an i dea a s pos s ib l e o f t he n a tu re o fthe var io us l~inds of cora l reefs , a de ta i led acco unt i s g iv en of oneexam ple cha ra c t e r i s t i c o f each c l a s s : t he one se l ec ted a s i l lu s -t r a t i n g t he sub jec t o f l agoon i s lands o r a to ll s , i s Ke e l ing Ato l l,o f w h i c h a r e d u c e d c h a r t a n d v e r t i c a l s e ct io n a r e h e r e a p p e n d e d .

T h e f i r st t h i n g t o b e o b s e rv e d w i t h r e g a r d t o t h e f o r m a t i o n o fthese r ee f s, i s, t ha t t he l i v ing co ra l an ima l does no t en dure ex -p o s u r e to t h e r a y s o f t h e s u n e v e n f o r a v e r y s h o r t t i m e ; a n d t h a tconseq uen t ly i t i s on ly a t t he ou t e r edge , be low the l eve l o f l ow-wa te r, t ha t t he r ee f i nc reases , o r a t l e a s t t ha t t hose spec ie s t ow h i c h t h e g r e a t m a s s o f t h e r e e f is d u e c a n e x i st . T h e m o s ta b u n d a n t c o r al o n t h is o u t e r e d g e w a s f o u n d t o b e t h ePorites,w h i c h fo r m s g r e a t i r r e g u l a r l y ro u n d e d m a s se ~ f r o m f o u r t o e i g h tf ee t b road , and l i tt l e le s s i n t h i ck ne s s ; bu t a spec ie s o fMitlepora(M. complanata)i s a lso ab l e t o r e s i s t t he fu ry o f t he wa ves , andgrow s in t h i ck ve r t i ca l p l a t e s , i n t e r sec t ing each a t he r a t va r iou sa n g le s , f o r m i n g a n e x t r e m e l y s tr o n g h o n e y - c o m b e d m a s s , w h i c hgen e ra l l y a f fec ts a c i r cu l a r fo rm, t he m arg ina l p l a t e s on ly be inga l ive . Th e co ra ls i nhab i t i ng t he p ro t ec t ed c r ev i ce s on the ou t e rr ee f , and those i n t he i n t e r io r o f t he l agoon, a r e qu i t e d i s ti nc t , andbe long to much more de l i ca t e va r i e t i e s .

F r o m s o u n d i n g s t a k e n o u t si d e t h e r e e f a t K e e l i n g A t o l l, t h ew a t e r w a s f o u n d to d e e p e n g r a d u a l l y f o r a sp a c e b e t w e e n 1 0 0 a n d2 0 0 y a r d s w i d e , b u t b e y o n d t h is , t h e s id e s p lu n g e i n to th eu n f a t h o m a b l e o c e a n a t a n a n g l e o f 4 5 . To t h e d e p t h ~ o f 1 0o r 12 f a thoms , t he bo t tom seems to be , i nva r i ab ly, l i v ing co r a l ;

b e t w e e n t h a t d e p t h a n d 2 0 f a th o m s , a b o u t a n e q u a l n u m b e r o ft im e s s a n d a n d c o r al ; b u t o f t w e n t y - f i v e s o u n d i n g s a t a g r e a t e rd e p t h t h a n 2 5 f a t h o m s , e v e r y o n e s h o w e d t h e b o t t o m c o v e r e d w i t hs a n d, t h e s a n d c o n s i s ti n g o f f in e ly p o u n d e d f r a g m e n t s o f s t o n yc o ra ls . A t a d i s ta n c e o f a m i l e a n d a q u a r t e r f ro m t h e b r e a k e r sn o b o t t o m w a s f o u n d w i t h a l i n e 7 2 0 0 f e e t i n l e n g t h , p r o v i n g t h a tt h e s u b m a r i n e s lo p e w a s g r e a t e r t h a n t h a t o f a n y v o l c a n i c co n e.

T h e c h a r t a n d s e c ti o n r e p r e s e n t a r i n g p a r t l y fo r m e d o f s m a l li s le t s on a co ra l r ee f , bu t i f t he pa r t above the l eve l (c ) , w h ich i sj u s t c o v e r e d a t h i g h w a t e r, w e r e r e m o v e d , t h e r e w o u l d b e a s i m p l er e e f w h i c h f o r m s w h a t i s e s s e n t ia l ly t h eatoll, and i t enc losest h e lagoono n a l l s id e s, e x c e p t a t t h e n o r t h e r n e n d , w h e r e t h e r ea r e t w o o p e n s p a c es . T h e r e e f v a r i e s i n w i d t h f r o m250 t o 500ya r ds ; i ts su r f ace i s leve l , o r ve ry s l i gh t ly i nc l i ned Cowards t hel ag o on , a n d t h e i s l et s a r e f ir s t f o r m e d b e t w e e n 2 0 0 a n d 3 0 0 y a r d sf r o m i ts o u t e r e d g e , t h r o u g h t h e a c c u m u l a t i o n o f a p i l e o f f r a g -m e n t s t h r o w n t o g e t h e r b y so m e u n u s u a l l y s t r o n g g a le . T h e y a r ea f t e r w a r d s i n c r e a s e d b y t h e a d d i ti o n o f f r a g m e n t s o n t h e i r o u t e r


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~ 4 N~TI CES OF NEW BOOKS ~

s ide and the f r ag m en t s become in t ime cemen ted in to a so lid masse i t h e r m a d e u p o f p a r t i a l l y r o u n d e d f r a g m e n t s o f c o r a ls o f v a r io u ss ize s o r o f ca l ca reous s ands tone fo rm ed o f a cong lom era t e in wh icht h e s t r u c t u r e o f t h e c o r a l h a s o f t e n be c o m e m u c h o b s c u re d o ren t i r e ly l o s t .

T h e i n t e r io r o f t h e la g o o n is t h e n e x t s u b j e c t t h a t r e q u i r e sno t i ce . I n t he ca se be fo re u s abo u t ha l f i t s a r e a cons is t s o fs e d i m e n t a n d h a l f o f c o r a l re e f s ; b u t t h e c o ra ls a r e v e r y d i f f e re n tf rom those on the ou t side and a r e m uc h mo re de l ica t e . Th e r ee f sa l so a r e so f t er and inc rease ve ry r ap id ly.

Th e sed imen t f rom the deepes t pa r t s i n t he l agoon cons i s ts o fca l ca reous s and and l a rg e so ft banks o s imi l a r mud hav e been ob -.s e rv ed in o the r a tol ls . W he n d ry t h i s mu d so c lo sely r e sembles

cha lk t ha t i t c an s ca rce ly be d i s t ingu i shed from tha t subs t ance .L a rg e s h o al s o f f is he s a n d m a n y o t h e r a n i m a ls a r e f o u n d t of e e d u p o n t h e b r a n c h e s o f th e l iv i n g co r a l a n d t h e e x c r e m e n t o fthese m us t cons t an t ly p roduce vas t quan t i t i e s o f f ine ca lca reousm u d .

J u d g i n g f r o m o ld c h a r t s o f K e e l i n g A t o ll t h e c o r a l p o ly p h a sa d d e d m u c h t o t h e s o li d m a t t e r o f t h e r e e f w i t h i n a v e r y m o d e r a t epe r iod an d t lm is l e ts have a l so g rea t ly inc reased ; bu t so f a r i s t h i sf r o m b e i n g t h e c o n se q u e n c e o f a n y g e n e r a l e l e v a ti o n o f t h e l an dtha t t he re is good ev idence in p roo f o f a sma l l subs idence . A t anyra t e t he r e i~ ev idence he re o f a s eve re s t rugg le i n p rog re s s be -t w e e n t h e n i ce ly b a l a n c e d p o w e r s o f l a n d a n d w a t e r a n d i f l e ftund i s tu rbed i t i s man i f e s t t ha t t hough the is l et s m ay increasean d the l agoon becom e pa r t l y f il led up the gen e ra l i nc rease s ea -wa rds and even the f i nal conve r s ion o f t he l agoon in to land m us tneces sa r i l y be exceed ing ly s low.

K e e l i n g A t o l l m a y b e t a k e n a s a f a i r e x a m p l e o f t h e v e r ynum erou s r i ngs o f co ra l r ee f s and co ra l i s lands i n t he Pac i f i c andI n d i a n O c e a n s ; b u t m a n y o f t h e m a r e m u c h l a rg e r a n d so m e o nt h e o t h e r h a n d e x c e e d i n g l y s m a ll a n d t h e n a t u r e o f t h e s ub -m ar in e s lope i s i n t he g rea t m a jo r i t y o f ca se s s imi l a r. Bu t a I -

t ho ug h such i s the ca se gen e ra l l y t he re a r e two excep t ions sorem ark ab le t ha t some spec ia l no t i ce i s due to t hem . These ex -c e p t io n s c o n s i st o f t h e r e e f s w h i c h f o r m t h e M a l d i v e A r c h i p e la g o

n d t h o s e w h i c h c o m p o s e t h e g r e a t C h a g o s B a n k .T h e M a l d i v e A r c h i p e l a g o i s 4 7 0 m i l es in l e n g t h w i t h a n

a v e r a g e b r e a d t h o f a b o u t 5 0 m i le s t h e di m e n s io n s o f t h e l a rg e s ta to l l a r e 88 mi l e s by 20 and l i ke t he o the r l a rg e r ones i t i sb r e a c h e d b y n u m e r o u s d e e p - w a t e r c h a n n e l s l e a d i n g in t o t h e l a -g o o n . T h e s e p a r a t e p o r t io n s o f t h e r e e f b e t w e e n t h e s e c h a n n el sa re i n m an y cases e spec i a ll y i n t he more no r the rn a to ll s t r uer ing - sha pe d a to ll s d i f f e r ing on ly fr com those i n t he open sea bybe ing b ased on a sha l low founda t ion and g roup ed c lo se ly t oge the ro n o n e la rg e p l a tf o r m w i t h t h e m a rg i n a l r i n g s f o r m i n g t h e e x -t e rna l a tol l a r r an ged in a rude ly fo rm ed c i rc l e. Th e l agoons i nthe a to l ls o f th i s la rge g ro up a r e f rom 10 to 20 f a thom s deepe r inthe sou the rn t h an in t he no r the rn p a r t . I t i s obse rved in t h i s


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D A RW I N O N C O RA L R E E F S . 385

group, as in atolls generally, that t he channel is genera lly situated

on the side least exposed to t he pr evail ing winds and t he heav yseas.It appears, with regard to the islets formed in this group, that

some of them h ave come into existence as land on ly withi n a fewyears, while others are fast wearing away.

The great Chagos Ban k is described by Capt. Moresby, as ahalf-d rowned atoll. Its longer axis is about 90 miles, and it is70 miles in width across the broadest park The central partconsists of a level muddy flat between 40 and 50 fathoms deep,surrounded on all sides, with the exception of some breaches, bythe steep edges of a set of sand-banks, v ary ing in breadth from 5to 12 miles, and rudely arr an ge di n a circle. These banks are about16 fathoms ben eath the surface, and are bordered by a rim about amile wide, consist ing of smooth hard rock, and o nly about 5 or l 0fathoms beneat h the surface. At the distance of half a mile fromone part of this rim, no bottom was found with 190 fathoms, andoff anoth er point, none with 210 fathoms. Th e circumstancesand cause of this submerged condition will require subsequentconsideration.

a r r i e r r ee f s resemble, in general form of structure, the atollreef~ already described, bu t they inclose l~:nd and a lagoon channel

instead of a lagoon or mere sheet of water. Th ey are, however,sometimes of vast dimensions, one on the west coast of :NewCaledonia being 400 miles in length and u pwards of eight leaguesfrom the shore,, and the Austral ian barri er extendi ng along theeastern and north- eastern coast of tha t vast island for nearly athousand miles, its average distance from the coast being between20 and 30 miles. It is observed in many of these reefs tha tthe coral banks rise from far greater depths than the animal canexist in, and the living corals form a comp aratively small proportionof the whole amount.

Fr ing ing o r shore r ee f sdiffer from barrier reefs, as has beenalread y stated, by th e absence of a lagoon channel and b y t he closerelation in their horizontal extension with the probable slope be-neath the sea of the adjoinin g land. Th e reefs round the islandof Mauriti us offer a good example of thi s-c las s ; and here, as inbarrier reefs, the reef is breached by a straight passage in front ofevery river and streamlet. ~

Many islands are fringed by reefs of this kind, but where thesea deepens rapidly, the reef~ are narrow, often not more than50 or 100 yards wide, and they have a nearly smooth hard surfacescarcely uncovered at low water, and without an interior shoal

channel. The ir dimensions and structure, in fact, depend enti rely* T h i s p e c u l i a r i t y is e x p l a i n e d a ft e r w a r d s b y M r. D a r w i n , a n d a n a c c o u n t . i s

g i v e n o f o t h e r c a u s e s u n f a v o u r a b l e t o t h e a d v a n c e o f t h e c o r a l a n i m a l , b u t i t iss t t e d t h a t o n e o f t h e s e f r i n g i n g re e fs , i f e l e v a t e d i n a p e r f e c t c o n d i t i o n a b o v et h e le v e l o f t h e s ea , o u g h t t o p r e s e n t t h e si n g u l a r a p p e a r a n c e o f a b r o a d d r ym o a t w i t h i n a l o w m o u n d . T h i s , h o w e v e r , w o u l d b e g r e a t l y m o d i f i e d b yt h ea c t i o n o f t h e s e d u r i n g t h e e l e v at i o n. S e e p . . 5 4 . )

VOL . I . C C


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8 6 N O T I C E S O F N E W B O O K S

on the deg ree o f i nc l ina t i on o f t he subm ar ine s lope , con jo inedw i th t he f ac t , t ha t r ee f -bu i ld ing po lyps c an on ly ex i s t a t l im i ted

dep ths ; so t ha t , wh e re t he s ea i s sha llow, t hey lose t he i r f r i ng ingcha rac t e r, and ap pea r a s s epa ra t e and i r r eg u la r ly s ca t t e r ed pa t ches,o f t en o f cons ide rab l e a r ea . Th ese r ee f s a lso a r e gene ra l l y h ighe rand mo re pe r f ec t on t he ou t e r s ide , a s suming the appea ran ce o fa tol ls , owing to the m ore v igo rous g row th o f t he co ra l whe re mos texpo sed . T he y d i ffe r, how ever , f rom a to l ls in be in g less deep , inhav ing the i r fo rm l es s de f ined , a nd in be ing based on a sha l lowfounda t ion .

Co ra l reefs a re , fo r the m os t pa r t conf ined to the t rop ica l seas,bu t ex t end a s f a r a s l a t i t ude 32 15 1~. ( a t t he B e rm ud a i s l ands ) ,and on the w es t e rn sho re s o f A us t r a l i a t o 29 S .

T he y do no t s eem to be i n f luenced by the p re sence o r absenceo f vo l can i c cen t r e s , bu t a r e t o t a l l y absen t i n ce r t a in l a rge d i s t r i c t swi th in t h e t rop i ca l s eas , and th i s app a ren t ly cap r ic ious d i s t r i bu t ioncanno t be exp la ined by an y obv ious causes . I t s eems beyond adoub t t h a t t he s t ronges t a nd m os t mass ive co ra l s fl ou ri sh bes tw he re mos t exposed , wh i l e t he p re sence o f mu d and sand , and o fb ra ck i s h o r f r e sh wa te r, i s h igh ly i n ju r iou s D i ff e r en t speci esabou nd in each o f t he d i f f e r en t zones o f dep th , and in t he ca se o fsome r ee f s t he re i s no new ma te r i a l added above the lowes t l eve lwh ich the w a te r a t t a in s ; a l t hough , i n o the r s , t he g ro w th con t inuest o h i g h w a t e r m a r k . T h e r e i s a ls o a f a c t o f s i n g u l a r i n t e r e s t a n dimp or t ance w i th r eg a rd to t h i s po in t i n t he occas iona l subm ergedree f s in t he Chag os Bank and e l s ewhere , wh ich , i n some in s tances ,app ea r t o have r em a ined s t a t i ona ry fo r a ve ry l ong pe riod , andh a v e n o t e n d e n c y t o g r o w u p w a r d s . I t is s u p p o se d b y t h e a u t h o rtha t , i n t he i r c a se t he who le g rou p o f r ee fs has subs ided someseven o r e igh t f a thom s , so t ha t t he spec ie s bu i ld ing nea r t he su r-f ace w ere des t royed , and o the r spec i es p re f e r r i n g th i s dep th h av ingsucceeddd to t hem, t he re i s no w l i tt l e t endency to g row upw ards .Th e re i s, a t an y r a t e , no e qua l t endenc y to g ro w upw ards i n a l l

co ra l r ee fs , bu t t he i r g ro w th is unequ a l an d depe nden t on c ir cum -s tances ,Th e re i s no doub t t ha t co ra l, unde r f avoura b l e cond it ions o f

g r o w t h , i n c re a s e s t o a n e n o r m o u s e x t e n t, a n d v e r y r a p i d l y ; a n da l t h o u g h t h e r e a r e m a n y i n s ta n c e s o n r e c o r d o f r e e f s w h i ch h a v en o t in c r e a s e d f o r m a n y y e a r s , t h e r e a r e o t h e r s t e ll in g a v e r y d if -f e r en t t a le . Th e ca se o f M a t i l da A to l l , de sc r ibed b y Cap ta inBeec hey, i s quo ted a s an ex am ple o f t h i s l a t t e r k ind , t h i s a to l lh a v i n g b e e n c o n v e r t e d i n 3 4 y e a r s f r o m b e i n g a r e e f o f r o c k s i n t oa lagoon i sland , 14 mi les in leng th , wi th one of i t s s ides coveredn e a r l y a l l t h e w a y w i t h h i g h t r e e s. S o m e e x p e r i m e n t s a r e a ls om e n t i o n e d i n w h i c h it h a s b e e n a t t e m p t e d t o m e a s u r e t h e r a t e o finc rease o f d i f fe r en t k inds o f co ra ls , an d as one o f t he r e su l t s o fthese i s an i n s t ance o f a g ro w th two f ee t th i ck o f co ral accum u-l a t ed on the coppe r bo t tom o f a ves se l i n t he cou r se o f twe n tymon ths .

C a r e f u l e x a m i n a t i o n b y s o u n d i n g s , t o g e t h e r w i t h s u c h o t h e robse rva t ions a s can be mad e on the sub jec t , a l l un i t e i n p rov ing


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D A R ~ V I N O N C O R A L R E E F S 8 7

t ha t t he r ee f -bu i ld ing co ra l an ima l does no t gen e ra l l y l i ve a t a

g r e a t e r d e p t h t h a n a b o u t t h i r t y f a th o m s a n d t h is i s n o t in v a l i d a t e dby the f ac t t ha t li v ing spec imens ha ve been found occas iona lly a tm u c h g r e a t e r d e p t h . T h e g r e a t e s t d e p t h s a t w h i c h t h e o r d i n a r ys p ec ie s g r o w a p p e a r to b e t h o se o f t h e m o r e n o r t h e r n p a r t s o f t h eR e d S e a ; b u t i t i s w e l l k n o w n a n d h a s b e e n a l r e a d y m e n t i o n edtha t co ra l ree f s occas iona lly exh ib i t a mu ch g r ea t e r t h i cknes s t ha nth i s dep th a l t houg h the l ower po r ti on i s i n such ca se s dead .

H a v i n g t h u s d e sc r ib e d th e c h ie f p h e n o m e n a o f c o r a l re e ih a n dt h e c i r c u m s t a n c e s o f t h e i r g r o w t h t h e a u t h o r e n t e rs o n t h e s u b j ec to f t he t he o ry o f t he fo rm a t ion o f t he d i f f e r en t c la s se s o f them ands h o w s t h e u n s o u n d n a t u r e o f t h e t h e o r y m o s t g e n e r a l l y r e c e iv e dn a m e l y t h a t t h e y a r e b a s e d o n s u b m a r i n e c r a te r s a n d a ls o o f t h eb e t t e r th e o r y o f C h a m i s so t h a t t h e o u t e r p o r t io n s o f a r e e f b e i n go f t h e m o s t v i g o ro u s g r o w t h w o u l d f i rs t r e a c h t h e s u r fa c e . ~ e i t h e ro f t he se t heo r i e s how eve r accoun t fo r ba r r i e r r ee f s .

T h e c a u s e t h a t h a s g i v e n t o a t o ll s a n d b a r r i e r r e e f s t h e i r c h a -r a c t e r i st i c f o rm s i s s u p p o se d b y M r. D a r w i n t o h a v e b e e n t h eg r a d u a l su b s id e n c e o f p o r t io n s o f t h e b e d o f t h e o c e an o v e r l a rg ea reas and i s pa r t l y deduced f rom the cons ide ra t i on o f t he se twoc i r cums tances v i z. : - - f ir st t ha t r ee f -bu i ld ing co ra l s f l ou ri sh on ly a tl im i ted dep ths and second ly t ha t va s t a r ea s a r e i n te r s lbe r sed wi th

co ra l r ee f s and co ra l i sl e ts none o f w h ich r i s e t o a g r e a t e r he ig h ta b o v e t h e l ev e l o f t h e s e a t h a n t h a t a t t a i n e d b y m a t t e r t h r o w nu p b y t h e w a v e s a n d w i n d s . T h e f o u n d a t io n o f e a c h r e e f i s a s -s u m e d to h a v e b e e n r o c k y b u t i t c a n n o t b e t h o u g h t p r o b a b l e th a tt h e b ro a d s u m m i t o f a m o u n t a i n l ie s b u r i e d a t t h e d e p t h o f a f e wfa thoms ben ea th eve ry a to ll w i th no one po in t o f rock p ro j ec t i nga b o v e t h e s u r f a c e o v e r s u c h a w i d e e x t e n t a n d i t is k n o w n t h a tn e i t h e r t h e f la t d i s k o f a n a b r a d e d p o i nt n o r a n y o t h e r r o c k yfounda t ion i s ev e r a t t he t op o f t he r ee f . Bes ides t he ev idencei n f a v o u r o f a s u b s id e n c e h a v i n g t a k e n p la c e d e r i v e d f r o m t h e s e

a n d o t h e r c o n si d e ra t io n s c o n n e c t e d w i t h t h e g e n e r a l a p p e a r a n c e o fthe rocks o the r ev idence i s addu ced show ing the p robab i l i tyo f th i s v i ew wh ich is t hen p rove d to exp la in i n a v e ry s imp lem a n n e r t h e o r d i n a r y p h e n o m e n a o f f r i n g i n g r e ef s b a r r i e r s a n da to l ls and some o f t he l es s r eg u la r fo rms o f t he two g rea t c l a sse so f r ee f s and i t on ly r em a ins t o cons ide r w he the r t he ac tua l d i s-t r i bu t ion o f. co ra l r ee f s and the i r r e l a t i ve cond i ti on ag rees w i thsuch a t heo ry o f t he i r f o rma t ion . A chap te r is added on th i s sub -j ec t and i s gccompan ied by a co lou red ch a r t o f a l l kno wn co ra lr ee f s and i s lands one co lou r be ing used to m ark b a r r i e r r ee f s anda to ll s and ano the r d i s ti ngu i sh ing the f r i ng ing ree f s wh ich fo rm ad i s t inc t t yp e o f s t ruc tu re . I n t h i s cha r t t he d i f fe r ence be tw eenthe a to ll s and ba r r i e r r ee f s i s i nd i ca t ed b y the t i n t ; so t ha t t heone co lou r m ark s those d i s t r i c ts in wh ic h i t i s supposed tha t su b -s idence has t aken p l ace a t a s low bu t s t eady r a t e a l l owing o f t heg r a d u a l a c c u m u l a ti o n o f e x t e n s i v e c o r a l r e e fs w h i l e t h e o t h e rdeno te s t h a t no such chang e o f l eve l t o an y sens ib l e ex t en t haStal~en place.

c c ~


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3 8 8 N O T I C E S O F N E W B O O K S .

O n e x a m i n i n g t h e m a p w i t h a v i e w t o d e t e r m i n e t h e v a lu e o f

t h e t h e o r y o f s u b s i d e n c e , i t i s a t o n c e s e e n , t h a t w h i l e a t o ll s a n db a r r i e r r e e f s a r e g e n e r a l l y n e a r t o g e t h e r , t h e f r i n g i n g r e e f s a r ee i t h e r d i s t a n t f r o m t h e s e , o r i n t h e e x c e p t i o n a l c a se s , t h e r e i s i n -d e p e n d e n t e v i d e n c e o f o s c il l a ti o n s o f l e v e l h a v e t a k e n p la c e, s u b -s u b s i d e n c e h a v i n g p r e c e d e d t h e e l e v a t i o n o f th e l a tt e r , an d e l e v a -t i o n h a v i n g p r e c e d e d t h e s u b s i d e n c e o f t h e f o r m e r c la ss .

T h e r e i s h o w e v e r s o m e d i r e c t e v id e n c e i n t h e e x t e r n a l a p p e a r -a n ce , o f m a n y o f t h e a t o ll s t h a t s u b s i d e n c e h a s t a k e n p l a c e ;f is s u r es f o r m e d b y e a r t h q u a k e s a n d o t h e r r e c o r d e d p h e n o m e ns u f f i ci e n t ly p r o v i n g t h a t s u b t e r r a n e o u s m o v e m e n t o f t h i s k i n d i si n p ro g r e s s , w h i l e, o n t h e o t h e r h a n d , w i t h r e g a r d t o t h e f r i n g i n gr e e f s, t h e y a re a l l s i t u a t e d i n d i s tr i c t s w h e r e e l e v a t i o n o f t h e s u r -f a c e w o u l d s e e m to b e g o i n g o n . T h e S o c i e t y I sl e s , a n d th en u m e r o u s i s la n d s a n d r e ef s o f th e L o w A r c h i p e l a g o , a r e r e m a r ka b l e i n s t a n c e s o f s u b s id e n c e , w h i l e t h e S a n d w i c h Is l a n d s a n d t h eP h i l i p p i n e s a r e e q u a l l y s t r i k i n g e x a m p l e s o f r e c e n t e l ev a t io n . I ti s w o r t h y o f n o t i c e al so , t h a t i n t h e v i c i n i t y o f t h o s e a r e a s o f s u b -s i d e n ce w h i c h a r e m a r k e d b y t h e p r e s e n c e o f c o r al r e e f s o f t h eb a r r i e r a n d a t o l l k i n d , t h e r e i s a t o t a l a b s e n c e o f v o l c a n o e s o f r e c e n ta c t io n , w h i l e t h e y a b o u n d i n t h e d i s t r i c t s w h e r e r i n g i n g r e e fs o c c u r.

I n c o n c lu s io n , M r. D a r w i n d i r e ct s a t t e n t i o n t o t h e v a s t e x t e n t

o f t h e a r e a s o f s u b s i d e n c e a n d e l e v a t i o n o n t h e e a r t h s s u r fa c e ,w h e t h e r a s in d i c a t e d b y t h e e x i s t e n c e o f c o r a l r e ef s , o r b y t h e p l a i ne v i d e n c e o f o b s e r v e d a p p e a r a n c e s . I t s e e m s t h a t i n t h e s p a c e o fo c e an e x t e n d i n g f r o m t h e s o u t h e r n e n d o f t h e L o w A r c h i p e l a g o t ot h e n o r t h e r n e n d o f M a r s h a l l A r c h i p e l a g o , a l e n g t h o f 4 5 0 0 m i le s ,e v e r y i s l a n d w i t h o n e e x c e p t i o n i s a t o l l- f o r m e d . ~ Vi t h r e g a r d t ot h e d i s t r i c t w e a r e t o l d : - -

T h e e a s t e r n a n d w e s t e r n b o u n d a r i e s ( t h e c o n t i n e n t s o f A f r i c a a n d S .A m e r i c a ) a r e r i s in g a r e a s, t h e c e n t r a l s p a ce s o f t h e g r e a t I n d i a n a n d P a c i f ico c e a n s a r e m o s t l y s u b s i d i n g ; b e t w e e n t h e m , n o r t h o f A u s t r a l i a , l ie s t h e m o s tb r o k e n l a n d o n t h e g lo b e , a n d t h e r e t h e r i s i n g p a r t s a r e s u r r o u n d e d a n d p e n e -t r a t e d b y a r e a s o f su b s i d en c e , s o t h a t t h e p r e v a i l i n g m o v e m e n t s n o w in p r o -g r e s s s e e m t o a c c o rd w i t h t h e a c t u a l s ta t e s o f s u r f a c e o f t h e g r e a t d i vi si o ns o ft h e w o r l d P a g e 1 43 .

S i n c e , t h e r e f o r e , i t a p p e a r s , t h a t w h i l e t h e s u r f a c e o f d i f f e r e n tk i n d s o f c o r a l r e e f s d i f fe r s b u t l i tt l e, t h e b a r r i e r r e e f o r r i n g o fc o r al s u r r o u n d i n g a n i s l a n d b e c o m i n g a n a to l l w h e n t h e l a n d d i s-a p p e a r s, a n d b e i n g d i s t i n g u i s h e d f r o m t h e f r i n g i n g r e e f b y t h e p r e -s e n c e o f a d e e p w a t e r l a g o o n o r m o a t b e t w e e n t h e r e e f a n d t h ec e n t r a l e x p a n s e ; a n d s i n c e a ls o t h e c o r a l a n i m a l i s f o u n d n o t t oe x i s t e x c e p t a t v e r y l im i t e d d e p t h s c o m p a r e d w i t h t h e a c t u a l d e p t ho f c o r a l i n t h e r e e f s , i t r e s u l t s t h a t N

,, The re is no dif~culty respecting the foundations on which frin g|n g reefsare based, while with barrier reef and atolls there is a great apparent difllcultyon th is head ; - - in barrier reefs, from th e imp robability of the rock of the coastor of banks of sediment extending in every instance so far seaward within therequired depth ; and in atolls, f ro m the imm ensity o f the spaces ov er whichthe y are interspersed, and the a ppa rent necessity for believing tha t they are allsupported on mou ntain summ its, which, although rising very near to the sur-


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D X R W ~ O N C OR AL R E EF S 3 8 9

f a c e l e v e l o f t h e se a , i n n o o n e i n s t a n c e e m e rg e a b o v e i t. T o e s c a p e t h i s l a t t e rm o s t i m p r o b a b l e a d m i s s io n , w h i c h i m p l i e s t h e e x i s t e n c e o f s u b m a r i n e c h a i n so fm o u n t a i n s o f a l m o s t t h e s a m e h e i g h t e x t e n d i n g o v e r a re a s o f m a n y th o u s a n d

s q u a r e m i le s , t h e r e is b u t o n e a l t e r n a t i v e , n a m e l y, t h e p r o l o n g e d s u b s i d e n c eo f

t h e f o u n d a t i o n s o n w h i c h t h e a t o ll s w e r e p r i m a r i l y b a s ed , to g e t h e r w i t h t h eu p w a r d g r o w t h o f t h e r e e f - c o n s t r u c t i n g c o ra l s. O n th i s v i e w e v e r y d i f fi c u l tyv a n i s h e s ; f r i n g i n g r ee f s a r e t h u s c o n v e r t e d i n b a r r i e r r e e fs , a n d b a r r i e r r e e f s,w h e n e n c i r c l i n g i sl a n ds , a r e t h u s c o n v e r t e d i n t o a t o l l s t h e i n s t a n t t h e l a s t p i n -n a c l e o f l a n d s i n k s b e n e a t h t h e s u r f a c e o f t h e o c e a n .

F i n a ll y , w h e n t h e t w o g r e a t t y p e s o f s t r u c t u r e - - t h e b a r r i e r r ee fs a n da t o ll s o n t h e o n e ha n d , a n d f r i n g i n g - re e f s o n ti le o t h e r - - a r e l a ld d o w n inc o l o u r s o n t h e m a p , a m a g r , i fi c en t a n d h a r m o n i o u s p i c t u r e o f t h e m o v e m e n t sw h i c h t h e c r u s t o f t h e e a r t h h a s w i t h i n a la t e p e r i o d u n d e r g o n e , i s p r e s e n t e dt ou s . W e t h e r e s ee v a s t a r ea s r is i ng , w i t h v o lc a n i c m a t t e r e v e r y n o w a n d t h e nb u r s t i n g f o r t h t h r o u g h t h e v e n t s o r fi ss ur e s w i t h w h i c h t h e y a r e t r av e r se d . W es e e o t h e r w i d e s p a c es s l o w l y s i n k i n g w i t h o u t a n y v o l ca n i c o u t b u r s t s , a n d w em a y f e e l s u r e t h a t t h i s s i n k i n g m u s t h a v e b e e n i m m e n s e in a m o u n t a s w e l l a si n a r e a , t h u s t o h a v e b u r i e d , o v e r t h e b r o a d f a c e o f th e o c e a n , e v e r y o n e o f t h o s em o m ~ t a in s a b o v e w h i c h a t o ll s P o w st a nd , l i k e m o n u m e n t s , m a r k i n g t h e p l a c eo ft h e i r fo r m e r e x i st e n c e. R e f l e c t i n g h o w p o w e r f u l a ll a g e n t w i t h r e s p e c t t o d e -n u d a t i o n , a n d c o n s e q u e n t l y t o t h e n a t u r e a n d t h i c k n e s s o f t h e d e p o s it s i na c c u m u l a t i o n , t h e s e a m u s t e v e r b e w h e n a c t i n g f o r p r o l o n g e d p e r i o d s o n t h el a n d d u r i n g e i t h e r i ts s l o w e m e r g e n c e o r s u b s i d e n c e : r e f le c t i n g a l so o nt hf in a l e f fe c ts o f t h e s e m o v e m e n t s i n t h e i n t e r c h a n g e o f l a n d a n d o c e a n - w a t e r o nt h e c l i m a t e o f t h e ea r t h , a n d o n t h e d i s t r ib u t i o n o f o r g a n i c b e i ng s , I m a y b ep e r m i t t e d t o h o p e , t h a t t h e c o n c l u s io n s d e r i v e d f r o m t h e s t. ud y o f c o r a l f o r m a -t io n s , o r i g i n a l l y a t t e m p t e d m e r e l y to e x p l a i n t h e i r p e c u l i a r f o r m s , m a y b et h o u g h t w o r t h y o f t h e a t t e n t i o n o f g e o l o g i st s . P a g e s 1 4 6 - - 1 't 8.

D . T. A .

I I . TRAVELS in NORTH AMERmA,wit h eological Observations outhe UN ITED STATES, CANADA, an d NO VA SCOTIA. B y CHARLESLYELL, Esq . F. R .S . 2 Vo l s . 12mo . pp . 588 . M ap and P l a t e s .

S o f a r a s r e g a r d s g e o lo g y, M r. Ly e l l s w o r k i s e m i n e n t l y v a l u -ab l e, si nce i t p r e sen t s a c onnec t ed v i ew o f t he r e su l t s o f a l a rgen u m b e r o f c a r e f u l s u r v e y s o f d if f er e n t p a r t s o f t h e C o n t i n e n t o fN o r t h A m e r i c a , b y m e a n s o f a c o lo u r e d g e o lo g ic a l m a p , i n w h i c ht h e w h o l e a m o u n t o f i n f o r m a t i o n a t p r e s e n t k n o w n o n t h e s u b j e c to f N o r t h A m e r i c a n g e o l o g y is i n c o r p o ra t e d . T h e i m p o r t a n c e o fth i s a s a means o f s imp l i fy ing and gene ra l ] s ing the no t ions o fEn g l i sh geo log i s ts w i th r ega r d t o t he succes s ion o f s t r a t a onthe o the r s ide o f t he A t l an t i c , i t wou ld be d i f f i cu l t t o e s t ima te t ooh igh ly.

M r. Ly e l l h a s a l so a d d e d m u c h to t h e k n o w l e d g e h i t h e r t opossessed on the sub jec t o f A m er i c an ge o logy by h i s own inves t i -ga t ions i n t he f i e ld , and in t he p re sen t no t i ce we p ropose t o po in tou t i n o rde r t he va r ious geo log ica l m a t t e r s t ouched on in t he

wo rk be fo re u s , com men c ing wi th t he o lde r rocks, and so ap -p roach ing l a s t o f a l l t o t hose o f new er da t e . Th e l a t t e r i ndeed ,a l t h o u g h p e r h a p s t h e m o s t i m p o r t a n t , w e s h a ll h e r e s c a r c e l ya l lude t o, because , hav ing fo rme d the sub jec t o f com mu nica t ions t othe Geo log ica l Soc ie ty, t hey e i t he r hav e b een a l r ead y o r w i l l beh e r e a f t e r d e s c r ib e d i n g r e a t e r d e t a il t h a n e v e n i n t h e b o o k i t s e lf

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