498 



BARRIER-REEFS 



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

 necessary details, but I \ enture to def)- ' an)- one to explain in 

 an)- other manner, how it is possible that numerous islands 

 should be distributed throughout \-ast areas — all the islands 

 being low — all being built of corals, absolutel)- requiring a 

 foundation within a limited depth from the surface. 



Before explaining how atoll -formed reefs acquire their 

 peculiar structure, ne 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 lac;"oon within an atoll. It is remarkable how 



ARKIEK-REEF. BiJL.\l-OLA. 



little attention has been paid to encircling barrier-reefs ; }'et 

 they are trul\- wonderful structures. The accompanying sketch 

 represents part of the barrier encircling the island of Bolabola 

 in the Pacific, as seen froni one of the central peaks. In this 

 instance the whole line of reef has been con\'erted into land ; 

 but usually a snow-white line of great breakers, with onh' here 

 and there a single low islet crowned with cocoa-nut trees, 

 di\-ides 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 mcist beautiful productions of the tropics, and 

 h'ing at the foot of the wild, abrupt, central mountains. 



1 It is icm.irk,Tlilc Uial ^Tr. Lyell, ^vcn in llie fiisl Ediiiun uf his /';/«(V//t,r o/" 

 Geology, inferred thai the amuunt of siiiisiilcnce in the Pacific must have e\ceetle"d 

 tliat of clevatidn, i^mm tlie area etf land heini^' very small relatively to tile assents 

 there tendinL; tu luiin it, namely, tlie Ljiowlli of coral and volcanic action. 



