XX 
BARRIER-REEFS 
499 
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 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 1 o to 
30 fathoms may be taken as an average ; but at Vanikoro 
there are spaces no less than 56 fathoms or 336 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 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- 
