THE VOYAGE OF THE BEAGLE 



495 



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 Vanikoro 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 sometimes 

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

 by a narrow gateway, 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 



