Sect. III. MALDIVA ATOLLS. 47 



The lagoons of all the atolls in the southern part of 

 the Archipelago are from 10 to 20 fathoms deeper than 

 those in the northern part* This is well exemplified in 

 the case of Addoo, the southernmost atoll in the group, 

 for although only 9 miles in its longest diameter, it has 

 a depth of 39 fathoms, whereas all the other small atolls 

 have comparatively shallow lagoons ; I can assign no 

 adequate cause for this difference in depth, excepting 

 that the southern part of the Archipelago has subsided 

 to a greater degree or at a quicker rate than the 

 northern part; and this conclusion agrees well with the 

 fact that, in the Chagos group, lying 280 miles still 

 further southwards, most of the atolls are sunken and 

 half destroyed with the dead corals. In the central and 

 deepest part of the Maldiva lagoons, the bottom consists, 

 as I am informed by Captain Moresby, of stiff clay (pro- 

 bably a calcareous mud) ; nearer the border it consists 

 of sand, and in the channels through the reef, of hard 

 sand-banks, sandstone, conglomerate rubble, and a little 

 live coral. Close outside the reef the bottom is 

 sandy, and slopes abruptly into unfathomable depths. 

 In most lagoons the depth is considerably greater 

 in the centre than in the channels ; but in Tilla- 

 dou-Matte, where the marginal ring-formed reefs 

 stand far apart, the same 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 singular structure of these atolls, 

 — a great sandy and generally concave disk rises 



