April, 1836. 



ENCIRCLING REEFS. 



555 



both ingenious and very plausible. Yet the sinuous margin 

 of some^ as in the Radack Islands of Kotzebue^ one of which 

 is fifty-two miles long^ by twenty broad^ and the narrowness 

 of others^ as in Bow Island (of which there is a chart on a 

 large scale^ forming part of the admirable labours of Captain 

 Beechey)^ must have startled every one who considered this 

 subject. 



The very general surprise of all those who have beheld 

 lagoon islands^ has perhaps been one chief cause why other 

 reefs^ of an equally curious structure have been almost over- 

 looked :* I allude to the encircling reefs. We will take, as an 

 instance, Vanikoro, celebrated on account of the shipwreck 

 of La Peyrouse. The reef there runs at the distance of 

 nearly two, and in some parts three miles from the shore, 

 and is separated from it by a channel having a general depth 

 between thirty and forty fathoms, and, in one part, no less 

 than fifty, or three hundred feet. Externally, the reef rises 

 from an ocean profoundly deep. Can any thing be more sin- 

 gular than this structure ? It is analogous to that of a lagoon, 

 but with an island standing, like a picture in its frame, in the 

 middle. A fringe of low alluvial land in these cases generally 

 surrounds the base of the mountains ; this, covered by the 

 most beautiful productions of a tropical land, backed by the 

 abrupt mountains and fronted by a lake of smooth water, only 

 separated from the dark waves of the ocean by a line of 

 breakers, form the elements of the beautiful scenery of Ta- 

 hiti — so well called the Queen of Islands. We cannot 

 suppose these encircling reefs are based on an external 

 crater, for the central mass sometimes consists of primary 

 rock, or on any accumulation of sedimentary deposits, for the 

 reefs follow indifferently the island itself, or its submarine 

 prolongation. Of this latter case there is a grand instance 



* Mr. De la Beche, however, seems to have been fully aware of the 

 difficulty. He says, " there are certain situations, where coral reefs run, 

 as it were, in a line with the coast, but separated from it by deep water, 

 which would seem to require a different explanation." — Geological 

 Manual, p. 142. 



