388 PRINCIPLES OF STRATIGRAPHY 



Where the barrier reef is not far from land, gaps always occur in it 

 opposite the mouths of rivers. 



Other examples of barrier reefs occur in the Pacific, where 

 they encircle some of the Society Islands, including Tahiti, the 

 Fiji Islands, and New Caledonia. The last is 400 miles long and 

 about ten miles distant from shore. The Pelew Islands and the 

 Comoro Isles in the Mozambique Channel are surrounded by bar- 

 rier reefs and other examples occur in the middle of the Red Sea. 



The Atoll is an elliptical, oval or roundish ring of coral, its 

 continuity broken here and there by channels which lead into the 

 central lake-like expanse of water, the lagoon. The outside water 

 is commonly very deep, while the water of the atoll is shallow. 

 Thus, ofif the Cocos-Keeling atoll, the depth sounded at a distance 



^<!-^9tieK^>!itry^^'^^&Zii^. 



Fig. 77. The atoll of Whitsunday Island. (From Le Conte.) 



of 2,200 yards from the reef's edge was 1,200 fathoms, while the 

 depth of the lagoon is only from two to seven fathoms (3.5 to 13 

 meters). In the atolls of the Low Archipelago the depth varies 

 from 35 to 70 meters (20 to 38 fathoms), in the Marshall group 

 from 50 to 60 meters (30-35 fathoms), and in the Maldive group 

 up to 90 meters (45 to 49 fathoms). (Figs. 75-77.) 



The openings in the reefs are always on the leeward side, and so 

 the water within the lagoon is seldom much disturbed. In it live 

 swarms of organisms of all kinds, including fishes and, preeminently, 

 sharks. The calcareous alga Halimeda grows abundantly over the 

 floor of the lagoon of Funafuti in the Ellis group, and of many 

 other atolls in the Pacific, and it has been collected in quantity in 

 the lagoon of Diego Garcia in tl^? Chagos group in the Indian 

 Ocean and elsewhere. 



Atolls are especially abundant in the Pacific and the Indian 



