2G4 CARBIER-UEEF3. 



ner how it is possible that numerous islands should 

 be distributed throughout vast areas — all the isl- 

 ands being low — all being built of corals, absolutely 

 requiring a foundation within a limited depth from 

 the surface. 



Before explaining how atoll-formed reefs ac- 

 quire 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 chaimel 

 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 won- 

 derful 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 Ime of leef 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 



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, namelv, the growth of coral and volcanic action. 



