CORALS. 133 



How the Coral Islands are produced must now be seen. 



The bed of the ocean is not one uniform plain, but, 

 like the surface of the earth, has its deep valleys and 

 lofty mountains. Sometimes, the tops of the mountains 

 are not covered by water, and then we call them islands; 

 but they are not the Coral Islands of which we are now 

 treating. 



These appear mysteriously, and give no premonitions 

 of their appearance. A ship, perhaps, passes over the 

 track which has been traced by hundreds of vessels pre- 

 viously, strikes upon a rock that is not in the charts, and 

 sinks. She has come upon a coral island that has not 

 yet reached the surface, but which in a few years will be 

 known, and its place noted in the charts. 



Below the surface is the apex of a mountain-peak, 

 submerged so deeply that a ship cannot touch it with 

 her keel. But upon that peak the Coral colonies have 

 settled, and have continued their hidden work until their 

 sharp, stony ridges have gradually approached the surface, 

 and become a danger to the next vessel that sails in that 

 direction. 



By degrees, the Coral reaches the limits of high water, 

 and the polypes which make it not being able to exist 

 without water, can rise no higher, but spread laterally in 

 all directions, until, according to Captain Basil Hall's 

 graphic simile, it looks like a huge cauliflower on its 

 stem. Consequently, there is deep water within a foot 

 of its edge, the lead gives no warning, and so a vessel is 

 wrecked without any fault of those in charge of her. 



