1836.] BARRIER-REEFS. 451 



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 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 Yanikoro there are spaces no less than 56 fathoms or 363 

 feet deep. Internally 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 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, 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 structures? 

 This is a great apparent difficulty, analogous to that in the case 

 of atolls, which has generally been overlooked. It will be per- 

 ceived more clearly by inspecting the following sections, which 

 arc real ones, token in north and south lines, through the islands 

 with their barrier-reefs, of Vanikoro, Gambier, and Maurua ; and 



