Sect. I. THE GROWTH OF CORAL-REEFS. 89 



with the salt, which prevented the growth of coral, the 

 reef certainly would not terminate abruptly ; but as 

 the polypifers nearest the impure stream would grow 

 less vigorously than those farther off, so would the 

 reef gradually thin away. On the other hand, the 

 sediment brought down from the land would only 

 prevent the growth of the coral in the line of its 

 deposition, but would not check it on the side, so that 

 the reefs might increase till they overhung the bed of 

 the channel. The breaches are much fewer in number, 

 and front only the larger valleys in reefs of the en- 

 circling barrier class. They probably are kept open 

 in the same manner as those into the lagoon of an 

 atoll, namely, by the force of the currents and the 

 drifting outwards of fine sediment. Their position in 

 front of valleys, although often separated from the 

 land by deep-water lagoon-channels, which it might 

 be thought would entirely remove the injurious effects 

 both of the fresh water and the sediment, will receive 

 a simple explanation when we discuss the origin of 

 barrier-reefs. 



In the vegetable kingdom every different station 

 has its peculiar group of plants, and similar relations 

 appear to prevail with corals. We have already de- 

 scribed the great difference between the corals within 

 the lagoon of an atoll and those on its outer margin, 

 The corals, also, on the margin of Keeling Island occurred 

 in zones : thus the Porites and Millepora complanata 

 grow to a large size, only where they are washed by a 

 heavy sea, and are killed by a short exposure to the 



