RECORDS 'LEFT BY THE SEA 



blocks as they roll down form a new foundation on which 

 new generations of polyps can grow and feed. Moreover, 

 it is better for the polyp to take the risks of these evictions 

 than to vegetate inside the reef, for there in the calmer 

 water he will not have enough to eat, and will dwindle 

 and die. Thus the tendency of all reefs must be to grow 

 seawards, and to increase in breadth. Perhaps their 

 breadth may tell us roughly how old they are. But 

 there is another possibility to be taken into considera- 

 tion, which is that while the polyps are building the sea 

 bottom or island foundation may be slowly sinking. In that 

 case it is quite likely that the coral builders might just 

 keep pace with the subsiding foundations of their home, 

 and build up a great thickness of coral rock during the 

 countless years of change. 



Sir Archibald Geikie has called attention to the swift- 

 ness with which the structure of the coral polypes skeleton 

 is effaced from the foundation and a compact mass of 

 rock put iii its place. The sea -water's chemical and 

 dissolving action, and the vast amount of mud and sand 

 produced by the breakers are chiefly responsible for this. 

 As the rock is being formed it is always being cemented. 

 On the portion of a reef laid dry at low water, the coral 

 rock looks in many places as solid and old as some of 

 the ancient white limestones and marbles of the land. 

 In pools where a current of water keeps the grains of 

 coral sand in motion, each grain may be seen to be 

 rounded. This is because on each particle of coral the 

 dissolved carbonate of lime in the water is always being 

 deposited (like the sediment in the bottom of a kettle). 

 E 65 



