THE VOYAGE OF THE BEAGLE 503 



sides, though most of the narrow gateways will 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 excep- 

 tional 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 bar- 

 riers 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 pro- 

 duce one great atoll 400 miles in length, but a chain or 

 archipelago of atolls, of very nearly the same dimension 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 

 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 between the north and south Nillandoo 

 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 in depth, in such a manner, that 

 it is scarcely possible to say whether it ought strictly to 



