524 



CCELENTERA TES. 



the channel (e"). On the one side (/"), the f ringing-reef is retained, while on the other 

 it has disappeared, a fact due to currents and other such agencies. Finally, when 

 the water is at the level (IV), two rocky islands are visible in a large lagoon 

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



DIAGRAM EXPLAINING THEORY OP SUBSIDENCE. 



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 

 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 



OUTLINE OF THE ISLAND OF AIVA, WITH PROJECTED SECTION. 



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

 reefs in neighbouring regions does not coincide with the theory, nor does the 

 appearance of atolls and barrier-reefs in regions in which recent elevation of the 

 land has been proved. The discovery of extensive submarine banks of sediment 

 formed of the calcareous portions of foraminifera, deep-sea corals, molluscs, etc., 

 makes it possible to explain the formation of atolls and barrier-reefs without the 

 help of subsidence, this explanation being more probable than that involving the 

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



