BARRIER-KEEF3. 265 



(lark, hea\ang waters of the ocean from the light- 

 gi-een expanse of the lagoon-channel. And the 

 quiet waters of this channel generally bathe a fringe 

 of low alluvial soil, loaded with the most beautiful 

 productions of the tropics, and lying at the foot of 

 the wild, abrupt, central mountains. 



Encircling barrier-reefs are of all sizes, from 

 three miles to no less than forty-four miles in di- 

 ameter ; and that which fronts one side, and encir- 

 cles both ends of New Caledonia, is 400 miles long. 

 Each reef includes one, two, or several rocky isl- 

 ands of various heights; and, in one instance, even as 

 many as twelve separate islands. The reef runs at 

 a gi'eater or less distance from the included land ; 

 in the Society Archipelago, generally from one to 

 three or four miles ; but at Hogoleu the reef is 

 20 miles on the southern side, and 14 miles on 

 the opposite or northern side, from the included 

 islands. The depth within the lagoon-channel also 

 varies much ; from 10 to 30 fathoms may be taken 

 as an average ; but at Vanikoro there are spaces 

 no less than 56 fathoms, or 336 feet deep. Inter- 

 nally the reef either slopes gently into the lagoon- 

 channel, or ends in a perpendicular wall sometimes 

 between two and three hundred feet under water 

 in height : externally, the reef rises, like an atoll, 

 with extreme abruptness out of the profound depths 

 of the ocean. What can be more singular than 

 these structures'? We see an island, which may 

 be compared to a castle situated on the summit of 

 a lofty submarine mountain, protected by a great 

 wall of coral-rock, always steep externally and 

 sometimes internally, with a broad level summit, 

 here and there breached by narrow gateways, 

 through which the largest ships can enter the vvid(j 

 and deep encircling moat. 



As far as the actual reef of coral is concerned, 

 II.— Z 



