LIFE : ITS MECHANICAL WORK AND ROCK CONTRIBUTIONS. 149 



3. Origin of the Forms of Beefs, — tJie Atoll and the Distant Barrier. 



The origin of the atoll form of reefs was first explained by Darwin. 

 According to his theory, each atoll began as a fringing reef, around an 

 ordinary island ; and the slow sinking of the island till it disappeared, while 

 the reef continued to grow upward, left the reef at the surface, a ring of 

 coral around a lake. 



As reef-forming corals grow only within depths not greater than 150 

 feet, the bottom on which they began must have been no deeper than this ; 

 and as such a shallow depth is to be found, with rare exceptions, only 

 along the shores of lands or islands, the reef formed would be at first 

 nothing but a fringing reef. 



A fringing reef, the first step in coral formations, being begun, slow 

 subsidence would make it a barrier reef. 



In the lower part of Fig. 148, a section of a high island, ATPB, is repre- 

 sented. The horizontal line 1 is the level of the sea, / a section of the 

 fringing reef on the left, and /' of that on the right. The reef depends for 

 its upward progress on the growth of the coral, and on the waves. The 

 waves act only on the outer margin of a reef, while the dirt and fresh water 



148. 



Section of an island bordered by a coral reef, to illustrate -the effects of a subsidence. 



of the land directly retard the inner part. Hence the outer portion increases 

 most rapidly, and retains itself at the surface, during a slow subsidence that 

 would submerge the inner portion. The first step, therefore, in such a sub- 

 sidence, is to change a fringing reef into a barrier reef (or one with a channel 

 of water separating it from the shore). Continued subsidence widens and 

 deepens this channel. Then, as the island begins to disappear, the channel 

 becomes a lake, with a few peaks above its surface ; and later, a single peak 

 of the old land is all that is left. Finally this peak disappears, and the coral 

 reef comes forth an atoll, with its lagoon complete. 



Eeferring again to the figure : if, in the subsidence, the horizontal line 2 

 becomes the sea level, the former fringing reef / is then at b, a barrier reef, 

 and /' is at b', and ch, ch', ch" are sections 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 



