450 AREAS OF StJi^IDENCE. tcnAr. xx. 



any oilier 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 Bolabola in the Pacific, as seen from one of the central 

 peaks. In this instance the whole line of reef has been converted 



into laud ; 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. 



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. 



Pacific must have exceeded that of elevation, from the area of laud bein 



l " crc tCDdins lo fo " 



