54* BARRIER-REEFS. [CHAP. xx. 



one of the central peaks. In this instance the whole line of reef has 

 been converted into land; but usually a snow-white line of great 

 breakers, with only here and there a single low islet crowned with 

 cocoa-nut trees, divides the dark heaving waters of the ocean from the 

 light-green 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 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 

 *ne 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 sometimes 

 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, grouping, and even in quite trifling 

 details of structure, between a barrier and an atoll. The geographer 

 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 great distances 

 from the shores of the included islands ? It cannot be that the corals 

 will not grow close to the land ; for the shores within the lagoon- 

 channel, when not surrounded by alluvial soil, are often fringed by 

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

 which I have called Fringing Reefs from their close attachment to the 

 shores both of continents and of islands. Again, on what have the 

 reef-building corals, which cannot live at great depths, based their 

 encircling structure ? This is a great apparent difficulty, analogous to 

 that in the case of atolls, which has generally been overlooked. It will 

 be perceived more clearly by inspecting the following sections, which 

 are real ones, taken in north and south lines, through the islands with 

 their barrier-reefs, of Vanikoro, Gambler, and Maurua; and they are 



