VARIETIES OF CORAL REEFS. 143 



Each wrought alone, yet all together wrought 



Unconscious, not unworthy instruments, 



By which a hand invisible was rearing 



A new creation in the secret deep. 



Omnipotence wrought in them, with them, by them ; 



Hence, what Omnipotence alone could do, 



Worms did. I saw the living pile ascend, 



The mausoleum of its architects, 



Still dying upwards as their labours closed : 



Slime the material, but the slime was turn'd 



To adamant by their petrific touch : 



Frail were their frames, ephemeral their lives, 



Their masonry imperishable." 



MONTGOMERY, Pelican Islands. 



These zoophytic productions present themselves 

 under three principal forms, ee atolls, 3 ' <( barrier -reefs" 

 and "fringing reefs*" 



The term " atoll " is the name given to the coral 

 islands, or lagoon islands, in the Indian Ocean, by their 

 inhabitants. An atoll consists of a wall or mound of 

 coral-rock rising in the ocean from a considerable depth, 

 and forming a ring, or circular island, with a lagoon or 

 sheet of water in its interior. The outer wall is gene- 

 rally breached in one or more places, and, when the 

 breach is deep enough to admit a ship, the atoll affords 

 a convenient and safe harbour. The outer side of the 

 atoll usually sinks to a depth of from two to three 

 hundred fathoms, shelving at an angle of forty-five 

 degrees or more; the inner side slopes gradually 

 towards the centre of the lagoon, forming a saucer- 

 shaped cavity, the depth of which varies from one 



* Darwin, Structure and Distribution of Coral Reefs. 8vo, 

 1842. 



