500 



BARRIER-REEFS 



building corals, which cannot h've at great depths, based their 

 encirchng structures ? This is a great apparent difficulty, 

 analogous to that in the case of atolls, which has generally been 

 overlooked. It will be perceived more clearly by inspecting 

 the following sections, 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 same scale of a quarter 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 



I Vanikoro. 2. Gambier Islands. 3. Maurua. 



The horizontal shading shows the barrier-reefs and lagoon-channels. The inclined shading above 

 the level of the sea (AA) shows the actual form of the land ; the inclined shading below this line 

 shows its probable prolongation under water. 



SECTIONS OF BARRIER-REEFS. 



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 ? If the sea had 

 formerly 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 



