CORAL-REEFS. 91 



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 encircling 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 described 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 air; whereas, three species of 

 Nullipora also live amidst the breakers, but are able to 

 survive uncovered for a part of each tide; at greater 

 depths, a strong Madrepora and Millepora alcicornis are the 

 commonest kinds ; the former appearing to be confined to 

 this part : beneath the zone of massive corals, minute 

 encrusting corallines and other organic bodies live. If we 

 compare the external margin of the reef at Keeling atoll 

 with that on the leeward side of Mauritius, which are very 

 differently circumstanced, we shall find a corresponding 

 difference in the appearance of the corals. At the latter 

 place, the genus Madrepora is preponderant over every 

 other kind, and beneath the zone of massive corals there 

 are large beds of Seriatopora. There is also a marked 



