Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

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 aU 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 hj 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.