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Lagoon channels of Barrier reefs represent in every respect 

 the trae lagoons of AtoUs ; they have a shaUow level bottom of 

 fine sand, and are fiUed with a fauna analogous to those Hvmg 

 in lagoons; they have a depth of water of from 30 to 50 

 fathoms. The lagoon channel round the Society Islands vanes 

 from 3 to 80 fathoms; at Vanikoro, from 30 to 561; at Gamhier 

 Islands, from 23 to 29. Some Barrier reefs have only a few 

 islets on them, whilst that which encircles Bolabola is converted 

 into a strip of land. 



The heights of the islands encircled by Barrier reefs vary : 

 Tahiti, according to Beechey, is 7000 feet; Bolabola, (Lesson,) 

 4026 feet; Maurua, (Bennett,) 800 feet; Aitutaki, (Fitzrot,) 

 360 feet; and Manouai, (Williams,) 50 feet. Some of these 

 islands, like New Caledonia, are formed of primary strata; others, 

 like Tahiti, of madreporic Limestone; and many are composed 

 of volcanic rocks. The central land consists either of one 

 island, or of several, encircled by a Barrier reef. There is no 

 essential difference between Barrier r^efs and AtoUs: the latter 

 . enclose a lagoon in a circle of Coral rock, the former surround 

 an island, or group of islands, at some distance from the shore, 

 with a reef having a lagoon channel separating it from the 

 island. Were the central land being removed by subsidence, as 

 in the case of Bolabola, there would remain a circular Atoll, 

 formed of numerous Coral islets, clothed with cocoa-nut trees, 

 having in the centre a deep lagoon. 



The Barrier reefs of AustraHa and of New Caledonia, from 

 their immense extent, are marvellous structures. The reef on 

 the West Coast of New Caledonia is 400 miles in length, and 

 for many leagues is distant eight miles from the shore, and 

 near the southern end of the island it is about sixteen mUes in 

 width. The great Australian Barrier reef extends, with few 

 interruptions, for nearly one thousand miles, and its average 

 distance from the land is from twenty to thirty miles, and in 

 some parts from fifty to seventy. The lagoon channel in this 

 case becomes a great arm of the sea, having an average 

 depth of from ten to twenty-five fathoms where the reef is near 

 the island, but where it is most distant the depth is from fifty 

 J 



