FORMATION OF COBAL BEEFS. 89 



ramparts, perhaps a hundred miles or more in circuit." 

 Darwin has estimated that some reefs in the Pacific Ocean 

 are at least 3000 feet in thickness. 



Thus far we have spoken of reefs surrounding mountainous 

 Islands ; coral islands or atolls (Fig. 58) resemble such reefs, 

 except that they surround a lake or lagoon instead of a high 

 island, the coral island itself being seldom more than ten or 

 twelve feet above the sea, and usually siipporting a growth of 

 cocoanut trees, while the sea maybe of great depth very near 

 the outer edge of the atoll, which " usually seems to stand as 

 if stilted up in a fathomless sea " (Dana). These reefs and 

 atolls are formed and raised above the sea by the action of 

 the winds and waves, in breaking up the living corals, 

 comminuting it and forming with the debris of shells and 

 other limestone-secreting animals and plants, banks or de- 

 posits of coral mixed witli a chalky limestone, as the base of 

 the reef. When it rises above the waves, cocoanuts and other 

 seeds are caught and washed up on the top, and gradually 

 the island becomes large enough to support a few human 

 beings. The Bermudas are the remnants of a single atoll, 

 and are situated farther from the equator than any other 

 reefs. Most barrier reefs and coral islands or atolls are 

 formed in an area of subsidence, where the bottom of the 

 ocean is gradually sinking ; this accounts for the peculiar 

 form and great thickness of many reefs. On the other 

 hand, the coral reefs of the West Indies are, generally 

 speaking, in an area of elevation. 



A section of a coral reef is shown by Fig. 59 : n is the jooint 

 where the shore slopes rapidly down within the lagoon 

 (which lies to the right), and m is where the reef suddenly 

 descends toward the open ocean. Between I c and d e lies 

 the higher part of the reef. The shore toward the lagoon 

 slopes away regularly from d to n ; while toward the open 

 ocean there is a broad horizontal terrace (« to l c) which 

 becomes uncovered at low water. 



The theory of the formation of barrier reefs is shown by 

 the diagram. Fig. 60. The island, for example, the volcanic 

 island Core, which is slowly sinking, at tlie ancient sea-level 

 I is surrounded by a fringing leef //, a small rock-terrace 



