The Mighty Deep 



depth in the sea than about twenty fathoms, 

 and the limits of temperature which the polyps 

 can endure are narrow. Their growth is quickly 

 checked by water a little too deep or a little too 

 cold, and by water not very pure. Abundance of 

 lime also is needful. 



An " atoll " is a coral-reef standing alone, 

 generally of a round or oval shape, with a lake 

 of calm sea-water in its centre, while ocean- 

 billows thunder on the outer margin. The 

 polyps love those waves ; but they fade and die 

 in the inclosed lagoon, where the water is not 

 often enough renewed to supply them with food. 



Much the same in make are the Fringing 

 Reefs and Barrier Reefs, except that the former 

 skirt the shore more closely, while the latter 

 may lie at a distance of many miles from island 

 or mainland. 



It is impossible for the polyps to raise their 

 stony structure above the sea. Some kinds 

 can endure exposure to the air for a short 

 time without dying, but only for a short time. 

 A coral island, when first built, is merely level 

 with the ocean-surface at low tide. 



But old Ocean carries on the work which the 

 polyps have begun. Heavy rollers break inces- 

 santly against the reef, loosening blocks of coral, 



170 



