1836.] ENCIRCLING BARRIER-REEFS. 461 



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 diameter ; and that which 

 fronts one side, and encircles both ends, of New Caledonia, 

 is 400 miles long. Each reef includes one, two, or several 

 rocky islands of various heights ; and in one instance, even 

 as many as twelve separate islands. The reef runs at a 

 greater 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 twenty miles on the 

 southern side, and fourteen miles on the opposite or 

 northern side, from the included islands. The depth 

 within the lagoon-channel also varies much ; from ten to 

 thirty fathoms may be taken as an average ; but at 

 Vanikoro there are spaces no less than fifty-six fathoms or 

 336 feet deep. Internally the reef either slopes gently into 

 the lagoon-channel, or ends in a perpendicular wall some- 

 times 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 some- 

 times internally, with a broad level summit, here and there 

 breached by narrow gateways, through which the largest 

 ships can enter the wide and deep encircling moat. 



As far as the actual reef of coral is concerned, there is 

 not the smallest difference, in general size, outline, group- 

 ing, and even in quite trifling details of structure, between 

 a barrier and an atoll. The geograpiier Balbi has well 

 remarked, that an encircled island is an atoll with high 

 land rising out of its lagoon ; remove the land from within, 

 and a perfect atoll is left. 



But what has caused these reefs to spring up at such 

 'feat distances from the shores of the included islands? 

 1 1 cannot be that the corals will not grow close to the land ; 

 for the shores within the lagoon-channel, when not sur- 

 rounded by alluvial soil, are often fringed by living reefs; 

 and we shall presently see that there is a whole class, 

 wliich I have called Etinging Reefs, from their close 



