496 



CHARLES DARWIN 



great distances from the shores of the included islands? It 

 cannot be that the corals will not grow close to the land; 

 for the shores within the lagoon-channel, when not sur- 

 rounded by alluvial soil, are often fringed by living reefs; 

 and we shall presently see that there is a whole class, which 

 I have called Fringing Reefs from their close attachment 

 to the shores both of continents and of islands. Again, on 

 what have the reef-building corals, which cannot live at 

 great depths, based their encircling structures? This is a 

 great apparent difficulty, analogous to that in the case of 

 atolls, which has generally been overlooked. It will be per- 

 ceived 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, Gambier, 

 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 



3032 ft. 



i. Vanikoro. z. Gambier Islands. 3. Maurua. 



The horizontal ^hading 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. 



many other encircled islands, and the general features would 

 have been the same. Now, bearing in mind that reef-build- 

 ing 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 



