60 
BARRIER-REEFS. 
Ch. n. 
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 Gambier 
Island as an instance of this structure. Submerged 
reefs, dead, covered with sand, and with a less defined 
outline, have been observed (see Appendix I.) 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 
accounted for without much difficulty. In cases like 
