248 TOILERS IN THE SEA. 



he defined them in his " Journal " : " Barrier reefs 

 either extend in straight lines in front of the shores 

 of a continent, or of a large island, or they encircle 

 smaller islands ; in both cases being separated from 

 the land by a broad, and rather deep, channel of 

 water." Fringing reefs, where the land slopes 

 abruptly under water, are only a few yards in width, 

 forming a mere ribbon or fringe round the shores ; 

 where the land slopes gently under water the reef 

 extends further, sometimes even as much as a mile 

 from the land ; but in such cases the soundings out- 

 side the reef always show that the submarine pro- 

 longation of the land is gently inclined. 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 reef and an atoll, or lagoon island (fig. 52). 

 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." 



These distinctions between coral reefs and coral 

 islands (lagoon islands, or atolls, whichever name be 

 employed) are recognised also by Dana, although he 

 combines the two kinds of reefs, and generally treats 

 of coral reefs (barrier reefs, and fringing reefs) and 

 coral islands. " Coral reefs and coral islands," he 

 says, "are structures of the same kind under some- 



