220 CORAL ISLANDS. 



The manner in which these islands are made is, 

 to some extent, a matter of uncertainty. The most 

 generally received opinion is, that the insects fasten 

 round the summit of a submarine mountain, and 

 build upwards until they reach the surface of the 

 sea, where they die, and their labours cease. As, 

 however, the sea is sometimes unfathomable close 

 to those islands, it has been supposed that the sub- 

 marine islands on which the corallines began to 

 build have gradually subsided, and that, as they 

 did so, the insects always built a little more, so as 

 to keep the top of their structures on a level with 

 the sea. Above the sea they cannot build. To be 

 washed by the waves is essential to their existence. 



We do not think this a very satisfactory theory, 

 because it supposes a prolonged subsiding of these 

 islands, and then an unaccountably sudden stop- 

 page. For although the corallines might continue 

 to build during the whole time of subsidence, it 

 were utterly impossible that the coral island, with 

 its luxuriant herbage, could be formed until that 

 subsidence should have ceased. The manner in 

 which the islands are formed makes this obvious. 



"When the coral reef, as it is called, reaches the 

 surface, it advances no further. Soon the action of 

 the waves breaks off the branches of the upper por- 

 tions of coral, which are tossed upon the reef, and 

 pulverized into fine sand. This goes on increasing 

 until the island rises a little above the waves. 

 When this happens, birds alight there ; sea-drift 



