1836.] BREAKS IN CORAL REEFS. 469 



ajl 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 archi- 

 pelago 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 Archipelago 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 explana- 

 tion in the upward and outward growth of the corals, 

 originally based both on small detached reefs in their 

 lagoons, such as occur in conmion 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 great sandy and generally concave disc rises 

 abruptly from the unfathomable ocean, with Its central 

 expanse studded, and its edge symmetrically bordered with 



