CORAL REEFS. 



141 



Fig. 220. Caryophy'llia fastigiata. 



Fig. 223. Madrepo'ra murica ta 



Fig. 221. Meandri'na labyri'nthica. 



Fig. 222. Astrea viridis. 



between the tropics. They crown most submarine mountains, and 

 cover thousands of square leagues, distributed among thousands of 

 islands and reefs. 



40. These sa'xigenous polypa'ria, attached to every kind of 

 rock, surround most large islands with their products, forming 

 around them a kind of rampart, separated frequently by deep 

 water. In other instances they form islets, detached or grouped in 

 different ways, and they are, when there are breakers, the more 

 dangerous, because they are not seen before being cast upon them, 

 and because the depth of water is so great as not to afford anchorage. 

 It is these deposits which render navigation so difficult in certain 

 parts of the South Sea, and cause so many deplorable losse's by 

 shipwreck. Some of the forms assumed by these deposits at the 

 surface of the sea are particularly remarkable, and are not ye. 

 entirely explained ; sometimes these reefs are completely annular 



40. What is the form of coral islands ? 



