60 BAREIER UEEFS. Ch. II. 



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 be- 

 neath the surface ; I have already mentioned Grambier 

 Island as an instance of this structure. Submerged 

 reefs, dead, covered with sand, and with a less denned 

 outline, have been observed (see Appendix) off some 

 parts of Huaheine and Tahiti. The reef is more fre- 

 quently breached to leeward than to windward, although 

 this is not so frequent as in the case of atolls. Thus I 

 find in Krusenstern's Memoir on the Pacific, that there 

 are passages through the encircling reef on the lee- 

 ward side of the seven Society Islands, which possess 

 ship-harbours ; but that there are openings to wind- 

 ward through only three of them. The breaches 

 in the reef are seldom as deep as the interior 

 lagoon-like channel ; they generally occur in front of 

 the main valleys, a circumstance which can . be ac- 

 counted for, as will be seen in the fourth chapter, 

 without much difficulty. The breaches being generally 

 situated in front of the valleys which descend on 

 all sides, explains their more frequent occurrence 

 through the windward side of barrier-reefs than 

 through the windward side of atolls, — for in atolls 

 there is no included land to influence the position of 

 the breaches. 



It is remarkable that the lagoon-channels round 

 mountainous islands have not in every instance been 

 long ago filled up with coral and sediment ; but it is 



