i34 THE WORLD'S LUMBER ROOM. 



the heat is intense and the water calm, as in the Red Sea, 

 which has some 120 varieties. No coral can live except in 

 quite clear water ; and as all rivers bring down more or less 

 sediment, gaps usually occur in the reef opposite their 

 mouths. 



Since reef-corals cannot live below a certain depth, it is 

 quite evident that they have not worked their way up from 

 the bed of the ocean, as used to be believed, and the method 

 in which the numerous coral islands, or atolls, which stud 

 the Pacific have been formed, seems to be this : 



In the first place the coral has grown upon the submerged 

 rocky bed immediately surrounding some island, and has 

 gradually formed a fringing reef. Here it grows upwards 

 until it gets too near the surface, when it will cease growing 

 in that direction and grow outwards instead, the inner parts 

 dying, being broken into fragments, ground up into sand, 

 and consolidated into a compact rock, and fresh shoots 

 constantly growing in their place. 



As long as the island remains stationary, the reef can 

 do nothing but grow outwards, but if the island should sink 

 a little, the sea will flow in and a channel will be formed 

 between the island and the reef, which is now called a 

 barrier reef. This apparently is what has taken place on 

 the coast of Australia, where the channel between it and the 

 reef is generally from five to fifteen, but, in one part, nearly 

 a hundred miles wide.* 



But supposing that the land should continue to sink until 



* Mr. Wallace tells us that the outer side of the Great Barrier reef 

 sinks 2, ooo feet, which shows how much the rock upon \\hich the corals 

 first grew must have subsided. 



