FRINGING- REEFS 501 



why the corals should have sprung up, like a wall, from the 

 extreme outer margin of the ledge, often leaving a broad space 

 of water within, too deep for the growth of corals. The 

 accumulation of a wide bank of sediment all round these islands, 

 and generally widest where the included islands are smallest, is 

 highly improbable, considering their exposed positions in the 

 central and deepest parts of the ocean. In the case of the 

 barrier-reef of New Caledonia, which extends for 150 miles 

 beyond the northern point of the island, in the same straight 

 line with which it fronts the west coast, it is hardly possible to 

 believe that a bank of sediment could thus have been straightly 

 deposited in front of a lofty island, and so far beyond its 

 termination in the open sea. Finally, if we look to other 

 oceanic islands of about the same height and of similar geological 

 constitution, but not encircled by coral-reefs, we may in vain 

 search for so trifling a circumambient depth as 30 fathoms, 

 except quite near to their shores ; for usually land that rises 

 abruptly out of water, as do most of the encircled and non- 

 encircled oceanic islands, plunges abruptly under it. On what 

 then, I repeat, are these barrier-reefs based ? Why, with their 

 wide and deep moat-like channels, do they stand so far from the 

 included land ? We shall soon see how easily these difficulties 

 disappear. 



We come now to our third class of Fringing-reefs, which 

 will require a very short notice. W^here the land slopes 

 abruptly under water, these reefs are only a few yards in width, 

 forming a mere ribbon or fringe round the shores : where the 

 land slopes gently under the water the reef extends farther, 

 sometimes even as much as a mile from the land ; but in such 

 cases the soundings outside the reef alwa}-s show that the 

 submarine prolongation of the land is gently inclined. In 

 fact the reefs extend only to that distance from the shore at 

 which a foundation within the requisite depth from 20 to 30 

 fathoms is found. As far as the actual reef is concerned, there 

 is no essential difference between it and that forming a barrier 

 or an atoll : it is, however, generally of less width, and 

 consequently few islets have been formed on it From the 

 corals growing more vigorously on the outside, and from the 

 noxious effect of the sediment washed inwards, the outer edge 

 of the reef is the highest part, and between it and the land 



