BREACHES IN BARRIER-REEFS. 275 



tinue to front exactly the upper part of those val- 

 leys, 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 bairier-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 car- 

 ried down to a depth whence they cannot spring 

 up again, we need feel no surpi'ise at the reefs 

 both of atolls and barriers becoming in parts im- 

 perfect. 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 Archi- 

 pelago. Moreover, in an atoll once breached on 

 opposite sides, from the likelihood of the Oceanic 

 and tidal currents passing straight through the 

 bi^eaches, it is extremely miprobable that the corals, 

 especially during continued 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 Archi- 

 pelago there are distinct atolls so related to each 

 other in position, and separated by channels either 

 unfathomable or very deep (the channel between 

 Ross and Ari atolls is 150 fathoms, and that be- 

 tween the north and south Nillandoo atolls is 200 

 fathoms in depth), that it is impossible to look at 



