458 BKEACHES IN BARRIEE-BEEFS. [CHAP. xs. 



quantity of water or sediment brought down could injure the 

 corals on the reef. Now, every reef of the fringing class is 

 breached by a narrow gateway in front of the smallest rivulet, 

 even if dry during the greater part of the year, for the mud, sand, 

 or gravel, occasionally washed down kills the corals on which it is 

 deposited. Consequently, when an island thus fringed subsides, 

 though most of the narrow gateways will probably become closed 

 by the outward and upward growth of the corals, yet any that arc 

 not closed (and some must always be kept open by the sediment 

 and impure water flowing out of the lagoon-channel) will still con- 

 tinue to front exactly the upper parts of those valleys, at the mouths 

 of which the original basal fringing-reef was breached. 



We can easily see how an island fronted only on one side, or on 

 one side with one end or both ends encircled by barrier-reefs, might 

 after long- continued subsidence be converted either into a single 

 wall-like reef, or into an atoll with a great straight spur projecting 

 from it, or into two or three atolls tied together by straight reefs 

 all of which exceptional cases actually occur. As the reef -building- 

 corals require food, are preyed upon by other animals, are killed 

 by sediment, cannot adhere to a loose bottom, and may be easily 

 carried down to a depth whence they cannot spring up again, we 

 need feel no surprise at the reefs both of atolls and barriers becom- 

 ing in parts imperfect. The great barrier of New Caledonia is thus 

 imperfect and broken in many parts; hence, after long subsidence, 

 this great reef would not produce one great atoll 400 miles in length, 

 but a chain or archipelago of atolls, of very nearly the same dimen- 

 sions with those in the Maldiva archipelago. Moreover, in an atoll 

 once breached on opposite sides, from the likelihood of the oceanic 

 and tidal currents passing straight through the breaches, it is 

 extremely improbable that the corals, especially during contimtecl 

 subsidence, would ever be able again to unite the rim ; if they did 

 not, as the whole sank downwards, one atoll would be divided into 

 two or more. In the Maldiva archipelago there are distinct atolls 

 so related to each other in position, and separated by channels 

 either unfathomable or very deep (the channel between Boss and 

 Ari atolls is 150 fathoms, and that between the north and south 

 Nillaudoo atolls is 200 fathoms in depth), that it is impossible to 

 look at a map of them without believing that they were once more 

 intimately related. And in this same archipelago, Mahlos-Mahdoo 

 atoll is divided by a bifurcating channel from 100 to 132 fathoms 



