54 CORAL-REEFS. 



depth is carried across the entire atoll, from the deep-water 

 line on one side to that on the other. I cannot refrain from 

 once again remarking on the singularity of these atolls, — a 

 great sandy and generally concave disc rises abruptly from 

 the unfathomable ocean, with its central expanse studded 

 and its border symmetrically fringed with oval basins of 

 coral-rock, just lipping the surface of the sea, sometimes 

 clothed with vegetation, and each containing a little lake of 

 clear water ! 



In the southern Maldiva atolls, of which there are nine 

 large ones, all the small reefs within the lagoons come to 

 the surface, and are dry at low water spring-tides ; hence in 

 navigating them, there is no danger from submarine banks. 

 This circumstance is very remarkable, as within some atolls, 

 for instance those of the neighbouring Chagos group, not a 

 single reef comes to the surface, and in most other cases a 

 few only do, and the rest lie at all intermediate depths from 

 the bottom upwards. When treating of the growth of coral 

 I shall again refer to this subject. 



Although in the neighbourhood of the Maldiva Archi- 

 pelago the winds, during the monsoons, blow during nearly 

 an equal time from opposite quarters, and although, as I 

 am informed by Capt. Moresby, the westerly winds are the 

 strongest, yet the islets are almost all placed on the eastern 

 side of the northern atolls, and on the south-eastern side of 

 the southern atolls. That the formation of the islets is due 

 to detritus thrown up from the outside, as in the ordinary 

 manner, and not from the interior of the lagoons, may, I 

 think, be safely inferred from several considerations, which 

 it is hardly worth while to detail. As the easterly winds 

 are not the strongest, their action probably is aided by 

 some prevailing swell or current. 



In groups of atolls, exposed to a trade-wind, the ship 



