260 THE OCEAN. 



Tlie principal and most important place is filled by 

 the genus Porites, wliich occupies the outside of tlie 

 reef, at the exposed edge, constructing large rounded 

 masses. The next in importance is the Millepora 

 comjplanata, which forms thick vertical plates, unit- 

 ing at different angles by their edges, so as to pre- 

 sent the appearance of a honeycomb : the marginal 

 plates only being alive. These two kinds alone 

 are able to endure the intermitting exposure to 

 which the upper edge is subject, in being conti- 

 nually washed over by the surf; other species are 

 found a few fathoms down. Inside the lagoon, 

 there are quite distinct sorts, generally brittle, and 

 thinly branched ; while great round Brainstones 

 [Meandrina), and ilower-like Caryophyllice occupy 

 the bottom. In the shallow hollows of the reef, 

 Pocilhpora verrucosa, a species having short waved 

 plates or branches, is found : when alive it is a beau- 

 tiful object, being of a delicate pale crimson hue. 



Conflicting statements have been made respect- 

 ing the activity of the building processes going on 

 in the present age ; some affirming that the reefs 

 have acquired no perceptible addition, either to their 

 height or extent, since they have been known ; others 

 anticipating a speedy filling up of the Pacific from 

 their rapid growth. The truth seems to be, that 

 while in some localities no change in extent can be 

 traced through many years, in others very rapid 

 enlargements are made. As showing the rate at 

 which coral grows under favourable circumstances, 

 Mr. Darwin mentions two or three interesting cases. 

 In the lagoon of Keeling AtoU, a channel was dug 



