XVI 



PREFA TOR Y NOTE. 



us believe that coral-reefs are built. 'Whether built up 

 sufficiently high to rise above the surface of the sea and thus 

 form islands, or brought up only to varying heights below the 

 sea-level, these volcanic eminences tend to become platforms on 

 which coral-reefs may be formed.' Thus he conceives that if 

 the volcanic peak be above the surface, it will be brought down 

 to the lower limit of breaker action by the force of the waves, — as 



Fig. 6.— Section of Red Coral showing the Polypes. 



was the case, for example, with Graham's Island in the Mediter- 

 ranean 1 which arose on July 18th, 1831, as an active volcanic 

 crater, about thirty miles off the south-western shores of Sicily, 

 but which was soon demolished by the waves, until a shoal of 



1 Phil. Trans., 1832; Prevost, Mem. Soc. Geol. France, ii. p. 91 ; and 

 Ann. des Sci. Nat. , vol. xxiv. 



