34? BREACHES IN BARRIER-REEFS. [CHAP. XX. 



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 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 four hundred 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 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 be called three separate atolls, or one great atoll not yet finally 

 divided. 



I will not enter on many more details ; but I must remark that the 

 curious structure of the northern Maldiva atolls receives (taking into 

 consideration the free entrance of the sea through their broken margins) 

 a simple explanation in the upward and outward growth of the corals, 

 originally based both on small detached reefs in their lagoons, such 

 as occur in common atolls, and on broken portions of the linear 

 marginal reef, such as bounds every atoll of the ordinary form. I 

 cannot refrain from once again remarking on the singularity of these 

 complex structures a jgreat sandy and generally concave disc rises 



