THE EIGHT-RAYED POLYPS 



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surrounded by the reef (b'" &'"), with two small reef islands (i'" z"') developed on 

 mountain peaks which have disappeared below the surface. The coral rock has 

 greatly increased in thickness, and almost entirely covers the former island. 



Such an ideal section corresponds exactly with the reality. In the following 

 illustration the outline of the island of Aiva, one of the Fiji group, is given. In the 

 lagoon there are two islands resembling the summits of mountains, just as in 



DIAGRAM EXPLAINING THEORY OF SUBSIDENCE. 



the diagram. The exact altitudes and depths are unfortunately not known, 

 but it will be seen how well the theory of subsidence appears to explain the 

 conditions. 



The chief objections which have been made to this theory of subsidence are 

 the following: The simultaneous occurrence of atolls, barrier reefs, and fringing 

 reefs in neighboring regions does not coincide with the theory, nor does the appear- 



OUTUNE OF THE ISLAND OF AIVA, WITH PROJECTED SECTION. 



ance of atolls and barrier reefs in regions in which recent elevation of the land has 

 t>een proved. The discovery of extensive submarine banks of sediment formed of 

 the calcareous portions of foraminifera, deep-sea corals, mollusks, etc., makes it 

 possible to explain the formation of atolls and barrier reefs without the help of sub- 

 sidence, this explanation being more probable than that involving the sinking of 

 extensive areas of land. The formation of atolls can be explained by the better 



