ATOLL FORM OF CORAL ISLANDS. 



95 



with a few island patches of reef over the peaks of the moun- 

 tains.* At a still greater subsidence (to the line VI) the islets, 

 excepting one, have disappeared, owing to their increasing less 

 rapidly than the barrier. The lagoon is in exact correspondence, 

 in all its characters, with those of atoll reefs. Should subsidence 

 now cease, the reefs, no longer increasing in \.. v 

 height, would go on to widen, and the accu- 

 mulations produced by the sea would com- 

 mence the formation of dry land, as exhibited 

 in figure 6, (p. 91.) Verdure may soon after 

 appear, and the coral island is finally com- 

 pleted. It is not impossible that the land 

 should form in certain favorable spots, while 

 the subsidence is in progress if it be not be- 

 yond a certain rate. 



The annexed figure represents the effect of 

 a cessation or diminution of subsidence on the 

 barrier reef, about a high island. The bar- 

 rier reef has become a finished island, and 

 forms a green belt to the land. The figure 

 shows a section of such a belt. 



All the features of atolls harmonize com- 

 pletely with this view of their origin. In 

 form they are as various and irregular as the 

 outlines of barrier reefs. Compare Angau of 

 the Feejees, with Tari-tari of the Tarawan 

 Group; Nairai or Moala with Tarawa: Na- 

 nuku with Maiana or Apamama. The resem- 

 blance is close ; and in the same manner we 

 might find all the forms of lagoon reefs repre- 

 sented among barrier reefs. We observe all 

 those configurations which would be derived 

 from land of various shapes of outline, whether 

 the narrow mountain ridge, (as at Taputeouea, 

 one of the Tarawan Islands,) or wide areas of 

 irregular slopes and mountain ranges. Among 

 the groups of high islands, we observe that 

 abrupt shores may occasion the absence of a 

 reef on one side, as on Moala ; and a like 

 interruption is found among coral islands. Many of the passages 

 through the reefs may be thus accounted for. 



The fact that the submerged reef is often much prolonged 

 from the capes or points of a coral island, a-coiUs wetl with 



* As the lagoon islets cover the summits of the subsided mountains, they afford 

 the most favorable spots for reaching the rocks below by boring. In fig. 6. p. 91 

 the depth required for this purpose on an islet in the lagoon would be hardly a 

 fourth what would be necessary on the enclosing reef. 



