498 BARRIER-REEFS 



of the corals. It is impossible here to enter into all the 

 necessary details, but I venture to defy ^ 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, 

 analoTOus to the lagoon within an atoll. It is remarkable how 



^sg 



BARRIER-REEF, BOLABOLA. 



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

 they are truly wonderful structures. The accompanying sketch 

 represents part of the barrier encircling the island of Bolabola 

 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, central mountains. 



1 It is remarkable that Mr. Lyell, even in the first Edition of his Principles 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. 



