2o6 SEA AND LAND 



reef various accumulations of or^^anic waste shoal the water or 

 construct spits across it so that the lagoon, much after the 

 manner of those enclosed by sand reefs, as above described, 

 becomes divided into sections of no very great extent. It 

 occasionally happens, as in the case of a great reef off the 

 eastern coast of Australia, that the wall is very far from the 

 land, so that the lagoon area, being very wide, has little value 

 as a harbor. 



A large part of the coral matter which is developed on the 

 front of the reef is broken off in times of heavy storms and 

 ground to powder in the surf. A portion of this waste is tossed 

 over the reef or drifts in through the inlets, and is thus added 

 to the sediments accumulatinof on the bottom of the la<roon. 

 Another portion of the debris is dragged seaward by the under- 

 tow and distributed in front of the reef. In this way the sea- 

 floor next the barrier is gradually shallowed. When in the 

 course of this process it is elevated to the level where the water 

 is only a little more than a hundred feet deep, a new coral reef 

 may begin to form which will in time rise to such a height as 

 to deprive the older barrier of its due share of food-giving 

 water. When this condition comes about the old reef dies, 

 and the frail materials of wliich it is composed may to a cer- 

 tain extent, at least in the upper part of the structure, become 

 broken up by the waves or dissolved by their waters so that the 

 once marked ridge may lose its distinct character. In certain 

 cases, where the sea waters are not saturated with limy matter, 

 they may take into solution and bear away to other regions a 

 part of the material which is afforded by the dead coral and 

 other organic remains in the section within the growing reef. 

 It is perhaps to this solvent action that we owe, in part at 

 least, the sometimes remarkably deep channels which penetrate 



