1836.] FRINGING REEFS. 463 



protected by the reefs, thus having left a shallow ledge 

 round them under water, the present shores would have 

 been invariably bounded by great precipices ; but this is 

 most rarely the case. Moreover, on this notion, it is not 

 possible to explain 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 im- 

 probable, 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 one hundred and 

 fifty 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 stralghlly 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 thirty 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. Where 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 further, sometimes even as much as a mile 

 from the land ; but in such cases the soundings outside the 

 reef always 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 twenty to thirty fathoms, is found. 

 As far as the actual reef is concerned, thee 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 



