6 4 CORAL-REEFS, 



with true lagoons. In some cases they are open, with a 

 level bottom of fine sand ; in others they are choked up 

 with reefs of delicately branched corals, which have the 

 same general character as those within the Keeling atoll. 

 These internal reefs either stand separately, or more com- 

 monly skirt the shores of the included high islands. The 

 depth of the lagoon-channel round the Society Islands 

 varies from two or three to thirty fathoms; in Cook's 1 chart 

 of Ulietea, however, there is one sounding laid down of 

 48 fathoms ; at Vanikoro there are several of 54 and one of 

 56-I fathoms (English), a depth which even exceeds by a 

 little that of the interior of the great Maldiva atolls. Some 

 barrier-reefs have very few islets on them ; whilst others are 

 surmounted by numerous ones ; and those round part of 

 Bolabola (Plate II., Fig. 1) form a single linear strip. The 

 islets first appear either on the angles of the reef, or on the 

 sides of the breaches through it, and are generally most 

 numerous on the windward side. The reef to leeward 

 retaining its usual width, sometimes lies submerged several 

 fathoms beneath the surface ; I have already mentioned 

 Gambier Island as an instance of this structure. Sub- 

 merged reefs, having a less defined outline, dead, and 

 covered with sand, have been observed (see Appendix) off 

 some parts of Huaheine and Tahiti. The reef is more 

 frequently breached to leeward than to windward; thus I 

 find in Krusenstern's Memoir on the Pacific that there 

 are passages through the encircling reef on the leeward 

 side of each of the seven Society Islands, which possess 

 ship-harbours ; but that there are openings to windward 

 through the reef of only three of them. The breaches in 

 the reef are seldom as deep as the interior lagoon-like 



1 See the chart in vol. i. of Hawkesworlh's 410 ed. of Cook's First 

 Voyage. 



