CORAL FORMATIONS. 623 



The growing reef depends for its upward progress on the growth 

 of the coral, and the waves. The waves act only on the outer 

 margin of a reef, while the dirt and fresh water of the land directly 

 retard the inner part. Hence the outer portion would increase 

 the most rapidly, and would retain itself at the surface during a 

 slow subsidence that would submerge the inner portion. The first 

 step, therefore, in such a subsidence is to change a fringing reef 

 into a barrier-reef (or one with a channel of water separating it from 

 the shore). The continued subsidence would widen and deepen 

 this channel ; then, as the island began to disappear, the channel 

 would become a lake with a few peaks above its surface ; then a 

 single peak of the old land might be all that was left ; and finally 

 this would disappear, and the coral reef come forth an atoll with 

 its lagoon complete. 



Referring again to the figure : if in the subsidence the hori- 

 zontal line 2 become the sea-level, the former fringing reef/ is 

 now at b, a barrier reef, and f' is at b\ and ch, ch', ch // are sec- 

 tions of parts of the broad channel or area of water within ; over 

 one of the peaks, P, of the sinking island, there is an islet of 

 coral i: when the subsidence has made the horizontal line 3 

 the sea-level, the former land has wholly disappeared, leaving 

 the barrier-reef t, V alone at the surface around a lagoon 1 11, 

 with an islet, u, over the peak T, which was the last point to dis- 

 appear. 



These steps are well illustrated at the Feejees. The island Goro 

 (fig. 854) has a fringing reef; Augau (fig. 855), a barrier ; Exploring 

 Isles (fig. 856), a very distant 



barrier, with a few islets ; Nu- Figs. 854-857. 



muku (fig. 857), a lake with a 5 *^n 56 



single rock. The disappear- 

 ance of this last rock would 

 make the island a true atoll. 



Whenever the subsidence 



ceases, the waves build up the 



land above the reach of the 



tides seeds take root and the Islands of the Feejee group: Fig ' 854 ' Goro; 

 tides, seeds tafce root, and tne 855> Augau . 856j Exploring Isles; 857> Nu _ 



reef becomes covered with fo- muku. 



liage. 



The atoll Menchikoff (fig. 850) was evidently formed, as ex- 

 plained by Darwin, about a high island consisting of two distinct 

 ridges or clusters of summits, like Maui and Oahu in the Hawaian 

 group. 



If the subsidence be still continued after the formation of the 



