70 CORAL-REEFS. 



coral are not of great thickness, but that before their first 

 growth, the coasts of these encircled islands were deeply 

 eaten into, and a broad but shallow submarine ledge thus 

 left, on the edge of which the coral grew ; but if this had 

 been the case, the shore would have been invariably 

 bounded by lofty cliffs, and not have sloped down to the 

 lagoon-channel, as it does in many instances. On this 

 view, 1 moreover, the cause of the reef springing up at such 

 a great distance from 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 sprung up 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 those deep arms of the sea (as in 

 Raiatea, see Plate II., Fig. 4) which penetrate nearly to the 

 heart of some encircled islands, is exceedingly improbable. 

 And why, again, should the reef spring up, in some cases 

 steep on both sides like a wall, at a distance of two, three 

 or more miles from the shore, leaving a channel often 

 between 200 and 300 feet deep, and rising from a depth 

 which we have reason to believe is destructive to the growth 

 of coral ? An admission of this nature cannot possibly be 

 made. The existence, also, of the deep channel, utterly 

 precludes the idea of the reef having grown outwards, on 

 a foundation slowly formed on its outside, by the accumula- 

 tion of sediment and coral detritus. 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 



1 The Rev. D. Tyerman and Mr. Bennett (Jonrn. of Voyage and 

 Travels, vol. i. p. 215) have briefly suggested this explanation of the 

 origin of the encircling reefs of the Society Islands. 



