REEF-MAKING CORALS. 133 



long, and seventy miles wide near the southern end, while 

 that on the west of New Caledonia is 400 miles long. 



The Bell Rock lighthouse, in the German Ocean, though 

 112 feet high, is often completely buried in foam and spray 

 during a ground-swell, though there be no wind to lash the 

 waves. But the great rollers of the Pacific are far heavier 

 than any waves in the German Ocean, and during a storm 

 they do as much damage in the coral groves as a gale does 

 among the trees on shore. Sometimes, especially where the 

 reef has been weakened by boring mollusks, masses twenty 

 and thirty feet long are torn off, many a coral tree is pros- 

 trated, boughs and twigs are torn off, and the polyps over a 

 large surface destroyed. The dead parts are, however, soon 

 overgrown and protected by smaller encrusting corals and 

 other zoophytes, as well as by serpulae, mollusks, and lichen- 

 like nullipores, just as the dead trunk of a tree is overgrown 

 with mosses, &c. 



There are deep-sea corals, but the reef-making kinds do 

 not flourish at a greater depth than from 120 to 150 feet, and 

 prefer being within reach of the light, but where they may 

 be either quite covered or constantly washed by the waves, 

 since a very short exposure to the sun kills them. Only two 

 species, however, are able to stand the full violence of the 

 breakers on the upper and outer edge of the reef, and of 

 these one grows in thick vertical plates, while the other grows 

 in masses from four to eight feet broad and nearly as thick. 

 These massive kinds actually thrive best where they are 

 most exposed, and are less perfect in sheltered spots, where 

 more delicate kinds flourish. 



The largest number of species, however, are found where 



