BARRIER-REEFS. 267 



which are real ones, taken in north and south lines 

 through the islands, with their barrier-reefs, of 

 Vanikoro, Gambler, and Maurua ; and they are 

 laid down, both vertically and horizontally, on the 

 scale of three sixteenths of an inch to a mile. 



It should be observed that the sections might 

 have been taken in any direction through these 

 islands, or through many other encircled islands, 

 and the general features would have been the 

 same. Now, bearing in mind that reef-building 

 coral cannot live at a greater depth than from 20 

 to 30 fathoms, and that the scale is so small that 

 the plummets on the right hand show a depth of 

 200 fathoms, on what are 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 1 If the sea had foiTnerly eaten deeply 

 into the islands, 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 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 gi'owth of 

 corals. The accumulation of a wide bank of sedi- 

 ment 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, 

 vv^hich 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 



