494 CHARLES DARWIN 



they were not lifted up to the required level, they must of 

 necessity have subsided into it; and this at once solves the 

 difficulty. For as mountain after mountain, and island after 

 island, slowly sank beneath the water, fresh bases would be 

 successively afforded for the growth of the corals. It is 

 impossible here to enter into all the necessary details, but I 

 venture to defy 12 any one to explain in any other manner 

 how it is possible that numerous islands should be distributed 

 throughout vast areas all the islands being low all being 

 built of corals, absolutely requiring a foundation within a 

 limited depth from the surface. 



Before explaining how atoll-formed reefs acquire their 

 peculiar structure, we must turn to the second great class, 

 namely, Barrier-reefs. These either extend in straight lines 

 in front of the shores of a continent or of a large island, or 

 they encircle smaller islands; in both cases, being separated 

 from the land by a broad and rather deep channel of water, 

 analogous to the lagoon within an atoll. It is remarkable 

 how little attention has been paid to encircling barrier-reefs ; 

 yet they are truly wonderful structures. The following sketch 

 represents part of the barrier encircling the island of Bola- 

 bola in the Pacific, as seen from one of the central peaks. 

 In this instance the whole line of reef has been converted 

 into land; but usually a snow-white line of great breakers, 

 with only here and there a single low islet crowned with 

 cocoa-nut trees, divides the dark heaving waters of the ocean 

 from the light-green expanse of the lagoon-channel. And 

 the quiet waters of this channel generally bathe a fringe of 

 low alluvial soil, loaded with the most beautiful productions 

 of the tropics, and lying at the foot of the wild, abrupt, cen- 

 tral mountains. 



Encircling barrier-reefs are of all sizes, from three miles 

 to no less than forty-four miles in diameter ; and that which 

 fronts one side, and encircles both ends, of New Caledonia, 

 is 400 miles long. Each reef includes one, two, or several 

 rocky islands of various heights; and in one instance, even 



13 It is remarkable that Mr. Lyell, even in the first edition of his " Prin- 

 ciples of Geology," inferred that the amount of subsidence in the Pacific 

 must have exceeded that of elevation, from the area of land being very 

 small relatively to the agents there tending to form it, namely, the growth 

 of coral and volcanic action. 



