Ch. II. BAEEIER REEFS. 67 



the land, leaving a deep and broad moat within, 

 remains altogether unexplained. A supposition of the 

 same nature and appearing at first more probable, is, 

 that the reefs have risen from banks of sediment, 

 which had accumulated round the shore previously to 

 the growth of the coral; but the extension of a bank 

 to the same distance round an unbroken coast, and 

 in front of deep arms of the sea (as in Kaiatea, see 

 Plate II. fig. 3), which penetrate nearly to the heart of 

 some encircled islands, is exceedingly improbable. And 

 why, again, should the reef, in some cases steep on both 

 sides like a wall, spring up at a distance of two, three, 

 or more miles from the shore, leaving a channel often 

 between 200 and 300 feet deep — a depth which we have 

 good reason to believe, is too great for the growth of 

 coral ? The existence, also, of this same channel pre- 

 cludes the idea of the reef having grown outwards, on a 

 foundation slowly formed by the accumulation of its own 

 detritus and sediment. Nor, again, can it be asserted 

 that the reef-building corals will not grow, excepting at 

 a great distance from the land ; for, as we shall soon see, 

 there is a whole class of reefs which take their name 

 from growing (especially where the sea is deep) closely 

 attached to the shore. At New Caledonia (see Plate 

 II. fig. 5), the reefs which run in front of the west coast, 

 are prolonged in the same line for 150 miles beyond the 

 northern extremity of the island, and this shows that 

 some explanation, quite different from any one of those 

 just suggested is requisite. If the island had been 

 originally prolonged to this distance, and if the northern 



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