CORAL REEFS, AND ISLANDS. 267 



There are knolls in the Southern Maldive atolls, 

 some of which, according to Captain Moresby, are 

 less than a hundred yards in diameter, and rise to the 

 surface from a depth of between 250 feet and 300 feet. 

 " Considering their number, form, and position," says 

 Darwin, " it would be preposterous to suppose that 

 they are based on pinnacles of any rock, not of coral 

 formation ; or that sediment could have been heaped 

 up into such small and steep isolated cones. As no 

 kind of living coral grows above the height of a few 

 feet, we are compelled to suppose that these knolls 

 have been formed by the successive growth and 

 death of many individuals, first one being broken 

 off or killed by some accident, and then another, 

 and one set of species being replaced by another set 

 with different habits, as the reef rose nearer the 

 surface, or as other changes supervened. In reefs of 

 the barrier class we may feel sure that masses of 

 great thickness have been formed by the growth of 

 coral ; in the case of Vanikoro, judging only from 

 the depth of the moat between the land and the reef 

 the wall of coral rock must be at least 300 feet in 

 vertical thickness." l There can be little doubt that 

 Matilda atoll, in the Low Archipelago, has been con- 

 verted, in the space of thirty-four years, from a " reef 

 of rocks" into a lagoon island, fourteen miles in 



1 Darwin, "Structure of Coral Reefs" (1851), p. 72 



