Sect. III. REEF-BUILDING: CORALS LIVE. 113 



considering the manner in which the beds of clean coral 

 off Mauritius, Keeling Island, the Maldiva and Chagos 

 atolls, graduated into a sandy slope, it appears very 

 probable that the depth at which reef-building poly- 

 pifers can exist, is partly determined by the extent of 

 inclined surface which the currents of the sea and the 

 recoiling waves have the power to keep free from 

 sediment. 



MM. Quoy and Gaimard 1 believe that the growth 

 of coral is confined within very limited depths ; and 

 they state that they never found any fragment of an 

 Astrsea (the genus they consider most efficient in form- 

 ing reefs) at a depth above 25 or 30 feet. But we 

 have seen that in several places the bottom of the sea 

 is paved with massive corals at more than twice this 

 depth ; and at 15 fathoms (or thrice this depth) off the 

 reefs of Mauritius, the arming was marked with the 

 distinct impression of a living Astraea. Millepora 

 alcicomis lives in from to 1 2 fathoms, and the genera 

 Madrepora and Seriatopora from to 20 fathoms. 

 Captain Moresby has given me a specimen of Sideropora 

 scabra (Porites of Lamarck) brought up alive from 17 

 fathoms. Mr. Couthouy 2 states that on the Bahama 

 banks he dredged up considerable masses of Mean- 

 drina from 16 fathoms, and has seen this coral growing 

 in 20 fathoms. 



Captain Beechey informs me that branches of pink 



1 Annales des Sci. Nat. torn. vi. 



2 Eemarks on Coral Formations, p. 12. 



I 



