190 CORALS AND CORAL ISLANDS. 



This feature is well exhibited in the Mahlos Mahdoo atoll, 

 an enlarged map of which, from Darwin's work, is here in- 

 serted. 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 five 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 their 

 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 illus- 

 trate 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 

 that the ring-formed reefs are based on a shallow foundation 

 instead of on the floor of the open sea, and that instead of being 



