THE VOYAGE OF THE BEAGLE 497 



these barrier-reefs based? Are we to suppose that each 

 island is surrounded by a collar-like submarine ledge of rock, 

 or by a great bank of sediment, ending abruptly where the 

 reef ends? 



If the sea had formerly eaten deeply into the isl- 

 ands, before they were protected by the reefs, thus having 

 left a shallow ledge round them under water, the present 

 shores would have been invariably bounded by great preci- 

 pices; 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 150 miles beyond 

 the northern point of the islands, 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 ab- 

 ruptly under it. On what then, I repeat, are these barrier- 

 reefs based? Why, with their wide and deep moat-like chan- 

 nels, 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 ab- 

 ruptly 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 



