M\, 



f 



H 





■»* •. 



ct 



u^ the "^^ 



Ho* ' . 





I 



•«llv derV 



■1 



: fi,^ 



m\ 



lienti 



e 



^ , 





e 





wlcareous 



I mav lie 



,xl in a soli'l 



♦ 



il fonning the 



linnok^ 



• .1:3 of the 



hundred 



[»T * oT 



BBOIt 



P 



i 



teristie of 



of r 



eef'^ 



d 



iJJ 



tie 



' J ti^-' ^ 



doe ' jt 



P^ 



]pv' • 



-«H 



i.r- 



i^ 



^ 





Ch. XLIX.l 



CIRCULAR FORM OF CORAL ISLANDS. 



589 



of a series of circular assemblaofes 



is composed tliroiigliout 

 of islets all formed of coral, the larger groups being from 

 to 90 miles in their longest diameter. Captain Hors- 



40 



burcli whose chart of these islands is 



subjoined, states, that outside of each 



circle or atoll, as it is termed, there are 



coral reefs sometimes extending to the 



distance of two or three miles, beyond 



r 



which there are no 



7 



soundings at 



im- 



mense depths. But in the centre of 

 each atoll there is 



G 



a lagoon from 



15 



to 49 fathoms deep. In the chan- ' 

 nels between the atolls no sound- 

 ings can usually be obtained at the *~ 

 depth of 150 or even 250 fathoms, but 



Captain Moresby's survey, - 

 soundings were struck at 150 and 200 

 fathoms, the only instances as yet >- 



during 



known of the bottom 



having 



been 



reached, either in the Indian or Pacific 

 oceans, in a space intervening between 

 two separate and w^ell characterised .; 

 atolls. 



The singularity in the form of the 

 atolls of this archipelago consists in 

 their being made up, not of one contin- 

 uous circular reef, but of a ring of small 

 coral islets sometimes more than a 

 hundred in number, each of which is a 

 miniature atoll in itself; in other words, 

 a ring-shaped strip of coral surround- - 

 ing a lagoon of salt water. To account 

 for the origin of these, Mr. Darwin 



supposes the larger annular reef to have 

 been broken up into a number of frag- o 

 ments, each of which acquired its pecu- 

 liar configurations under the influence . 

 of causes similar to those to which the 



Fig. 155. 



.^:* 



■-*■ 





J 



4+: 



J- i:** *■"*'■• 



4« 



IT , 





^1- 











.J 



^ v^ *»■ 



'%. ;*'-?^%^' 



/ 





m* mm 



"5- 









".+ 



> + 



■it' >■ 







/y/z/^ 



",+ V 



'4- 



'+-■ 



.++; 



'+; 





■;> 



S^ a half detjree 





Ecjuatorial i'hanm: 



/ 



;5? 





structure of the parent atoll has been due. Many of the 



