I40 THE OCEAN 



force of the waves they are constantly being 

 broken off at the surface and the bits of coral, 

 lodging among the living growths and piling 

 up on the bottom, gradually form a solid 

 mass. About the sides of this the living corals 

 continue to grow and spread and in time a 

 little islet appears above the breaking waves. 

 Slowly but surely the little spot of coral in- 

 creases in size, bits of floating weed and wood 

 are added to it, seeds and nuts find lodgment 

 in the crevices, vegetable life appears and 

 eventually the once-submerged reef becomes 

 a true coral-island with nodding palms and 

 rich green vegetation covering its surface. 

 Sometimes the corals may grow in more or 

 less circular form with deep water in the cen- 

 tre and the islets formed retain this circular 

 shape and are known as "atolls." 



But the circular form of most atolls is due 

 to the corals growing about the rims of sub- 

 merged volcanic craters. Other atolls, known 

 as "Serpuline Atolls," are produced in quite 

 a different manner. These seldom are of 

 large size and are composed mainly of the 

 hard, calcareous tubes of a group of marine 



