1 4 ATOLLS. Ch. I. 



(B in wood-cut) 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 

 accumulation 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 wood-cut (at p. 8) represents a section across 

 one of the islets on the reef, but if all that part which 

 is above the level of G were removed, the section 

 would be that of the reef, as it occurs where islets 

 have not been formed. It is this reef which essen- 

 tially forms the atoll. In Keeling atoll the ring 

 encloses 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 500 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 sur- 

 face, 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 ver- 

 rucosa, which grows in short sinuous plates, or branches, 

 and when alive is of a beautiful pale lake-red : a 

 Madrepora, closely allied or identical with ll.pocilliferas 

 is also common. As soon as an islet is formed, and the 

 waves are prevented from breaking entirely over the 



