CORAL-REEFS. 25 



the lowest classes of the vegetable kingdom, should be 

 limited to a zone so peculiarly circumstanced. Hence the 

 layer composed by their growth merely fringes the reef for 

 a space of about 20 yards in width, either under the form of 

 separate mammillated 1 projections, where the outer masses 

 of coral are separate, or, more commonly, where the corals 

 are united into a solid margin, as a continuous smooth 

 convex mound (B in woodcut), like an artificial breakwater. 

 Both the mound and mammillated projections stand about 

 three feet higher than any other part of the reef, by which 

 term I do not include the islets, formed by the accumula- 

 tion of rolled fragments. We shall hereafter see that other 

 coral-reefs are protected by a similar thick growth of 

 Nulliporse on the outer margin, the part most exposed to the 

 breakers, and this must effectually aid in preserving it from 

 being worn down. 



The woodcut represents a section across one of the islets 

 on the reef, but if all that part which is above the level of C 

 were removed, the section would be that of the simple reef, 

 as it occurs where no islet has been formed. It is this reef 

 which essentially forms the atoll. It is a ring, enclos- 

 ing the lagoon on all sides except at the northern end, 

 where there are two open spaces, through one of which 

 ships can enter. The reef varies in width from 250 to 

 300 yards; its surface is level, or very slightly inclined 

 towards the lagoon, and at high tide the sea breaks 

 entirely over it : the water at low tide thrown by the breakers 

 on the reef, is carried by the many narrow and shoal gullies 

 or channels on its surface, into the lagoon : a return stream 

 sets out of the lagoon through the main entrance. The 

 most frequent coral in the hollows on the reef is Pocillopora 

 verrucosa^ which grows in short sinuous plates, or branches, 

 1 Nipple-shaped. — Ed. 



