114 DEPTH AT WHICH Ch. IV. 



and yellow coral were frequently brought up from be- 

 tween 20 and 25 fathoms off the Low atolls; and Lieut. 

 Stokes, writing to me from the N.W. coast of Australia, 

 says that a strongly branched coral was procured there 

 from 30 fathoms : unfortunately it is not known to what 

 genera these corals belong. 



Although the limit of depth, at which each particular 

 kind of coral ceases to exist, is thus far from being 

 accurately known ; yet when we bear in mind the 

 manner in which the clumps of coral gradually became 

 infrequent at about the same depth, and wholly dis- 

 appeared at a greater depth than 20 fathoms 'on the 

 slope round Keeling atoll, off the reefs in the Pacific 

 (according to Dana), on the leeward side of the Mauri- 

 tius, and at rather less depth both within and without 

 the atolls of the Maldiva and Chagos Archipelagoes ; 

 and when we know that the reefs round these islands do 

 rot differ from other coral formations in their form and 

 structure, we may, I think, conclude that in ordinary 

 cases, reef-building polypifers do not flourish at greater 

 depths than between 20 and 30 fathoms, and rarely at 

 above 15 fathoms. 



It has been argued, 1 that reefs may possibly rise 

 from very great depths through the means of small 

 corals first making a platform for the growth of the 

 stronger kinds. This, however, is an arbitrary supposi- 

 tion : it is not always remembered, that in such cases 

 there is an antagonistic power at work, namely, the decay 

 of organic bodies when not protected by a covering of 

 1 Journal of the Royal Geograph. Soc. 1831, p. 218. 



