THE WORKMEN OF THE SEA. 



471 



of the great coral banks and coral islands of the tropical 

 seas. Their growth, if we may use the expression, is, 

 under favourable circumstances, very rapid ; so that a 

 channel cut in the reef surrounding a coral island to per- 

 mit the passage of a vessel, has been found blocked up 

 with coral in ten years. The animals begin their work 

 in water a few fathoms deep, the basement being laid by 



BilAlNSTONE CORAL. 



the Astrceans, generally upon some volcanic mountain, 

 which past convulsions have upheaved beneath the surface 

 of the sea ; as soon as the edifice reaches the level of six 

 fathoms, the task of building is undertaken by the Mean- 

 drinas and Porites; and these, in their turn, are succeeded 

 by the Madreporidce, Milleporidce, and Gorgonidce, which 

 carry the wonderful structure to the surface of the sea. 



