154 



CORALS AND CORAL LSLANDS. 



most, is forty-four miles long from north to south, and thirty- 

 four miles across. 



The point of special interest in their structure is the occur- 

 rence of atolls or annular reefs within the larger atolls. The 

 islets of the lagoon and those of the incircling reef are in 

 many instances annular reefs, each with its own little lake. 

 Gems within gems are here clustered together. 



This feature is well exhibited in the Mahlos Mahdoo atoll, 

 an enlarged map of which, from Darwin's work, is inserted on 

 the preceding page. The atoll consists of three main atoll- 

 shaped portions ; but in each of these, the border is made up 

 in part of atolls. Many of the subordinate atolls of the border 

 are " three, and some even hve miles in diameter, while those 

 within the lagoon are usually smaller, few being more than two 

 miles across, and the greater number less than one. The depth 

 of the little lagoons within these small annular reefs is generally 

 from five to seven fathoms, but occasionally more ; and in Ari 

 atoll, many of the central ones are twelve, and some even 

 more than twelve fathoms deep. These subordinate atolls rise 

 abruptly from the platform or bank on which they stand, with 

 iheir outer margin bordered by living corals." " The small atolls 

 of the border, even where most perfect and standing farthest 

 apart, generally have their longest axis directed in the line 

 which the reef would have held if the atoll had been bounded 

 by an ordinary wall." (Darwin on Coral Reefs, pp. 33, 34.) 



The Maldives are among the largest atoll-reefs known ; 

 and they are intersected by many large open channels ; and 

 Mr. Darwin observes, that the interior atolls occur only near 

 these channels, where the sea has free access. We may view 

 each large island in the archipelago as a sub-archipelago of 

 itself. Although thus singular in their features, they illustrate 

 no new principles with regard to reef-formations. 



Mr. Darwin thus remarks (op. cit. pp. 33, 34), — "I can 

 in fact point out no essential difference between these little 

 ring-formed reefs (which, however, are larger, and contain 

 deeper lagoons than many true atolls that stand in the open 

 sea), and the most perfectly characterized atolls, excepting 



