»83J.J BJREACHES IN BARRIER-REEFS. 47? 



possible that the very small quantity of water or sediment bronglit 

 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 tlie 

 year, for the mud, sand, or gravel, occasionally washed down, 

 kills the corals on which it is deposited. Consequently, wlien 

 an island thus fringed subsides, though most of the narrow gate- 

 ways w^ill probably become closed by the outward and upward 

 growth of the corals, yet any that are not closed (and some must 

 always be kept open by the sediment and impure water flowing 

 out of the lagoon-channel) will still continue 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 becoming 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 dimensions 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 con- 

 tinued subsidence, would ever be able again to unite the rim . 

 if they did not, as the whole sank downwards, one atoll would be 

 di\ ided 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 be- 

 tween Ross and Ari atolls is 150 fathoms, and that between the 

 north and south Nillandoo atolls is 200 fathoms in depth), thai 



