*4 CORAL REEFS. 



face of the water. Our island is now surrounded by 

 a circular wall, rising to the level of the sea (No. 19). 



Above that no 

 Corals can live ; 

 and therefore as 

 soon as the wall 



No rises above high- 



water mark, the 



work of the little builders is done, they can bring it 

 up no higher, and they die for want of the constant 

 action of the sea-water. But now other influences 

 come in to complete the structure. The waves beating 

 against the coral wall wear away its surface, break off 

 large pieces from it and wear them into sand by constant 

 rolling and grinding ; in storms these broken masses 

 of coral rock, and quantities of coral sand, are thrown 

 up on the top of the wall. Gradually all the scattered 

 materials floating in the sea around settle upon it, and 

 the summit becomes covered with a soil composed of 

 broken coral masses, sand, mud, parts of shells, drifted 

 sea- weed, &c. And now perhaps birds drop there the 

 seeds of some plant, or such seeds are floated from some 

 neighboring shore : trees spring up there, flowers and 

 grass grow upon it, men come and settle there, they 

 build their houses and plant their gardens on this cir 

 cular island, which lies like a green ring on the sea, and 

 incloses within it a calm ocean lake. And so you see 

 these tiny creatures, many of them no larger than a 

 pin's head, build up from the ocean depths .lands that 

 may grow green and luxuriant with the beautiful vege- 

 tation of the tropics, and in which men may find a 

 pleasant home. 



