Nos. 620-621] THE HAWAIIAN CORAL BEEFS 



415 



is cut by more or less numerous channels, which mark 

 those places where streams flow down from the land. 

 There is usually a lagoon— of sufficient depth to be navi- 

 gable by canoes or small boats— between the reef rim and 

 the shore. The outer wall of the fringing reef is steep, 

 and in the Hawaiian Islands descends abruptly into deep 

 water. The reef rim is the region of most active coral 

 growth, the shoreward coral being gradually killed by 

 fresh water and the deposition of mud and sand. 







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Barrier reefs may be considered as fringing reefs upon 

 a large scale. Although rare in the North Pacific Ocean, 

 there are many fine examples in the South Pacific The 

 grandest in the world is the Great Australian barrier reef, 

 which is 1,250 miles long, and supports a wonderfully 

 rich marine life. 



An atoll is an annular or ring-shaped reef, either awash 

 at low tide or surmounted by several islets, or less fre- 

 quently by a complete circle of dry land surrounding a 

 central lagoon. The outer wall of the atoll generally de- 

 scends with a very steep but irregular slope to a depth of 



