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SCIENCE PROGRESS 



Wharton on the Truncation of Atoll Foundations. — Wharton's 

 hypothesis that the flat floors of atoll lagoons represent volcanic 

 islands truncated by marine abrasion x is, like various other 

 theories, easily conceivable, but it involves contradictory pro- 

 cesses, and it has no independent support. It does not explain 

 why reef-forming corals should fail to form reefs while the 

 truncation is in progress, and yet succeed in forming reefs 

 after the truncation is completed. If atoll foundations have 

 been truncated, then uplifted and dissected atolls, such as occur 



Fig. 3. — Sector diagram, illustrating successive stages in the truncation of a still-standing 

 volcanic island by ocean waves, and the subsequent growth of a reef on the resulting 

 submarine platform. 



Before the reef is formed, the cliffs at the back of the platform will be free from deltas and talus, as in 

 sectors H, j, and K : after the reef is formed, deltas and talus will advance from the cliff base into the 

 enclosed lagoon, as in sectors h' and j'. The reefs must be of small thickness and must rest uncon- 

 formably on a flat rock floor. Tahiti is one of the very few reef-encircled islands that has such 

 features ; it resembles sector f, except that its delta plain extends inland into the valleys, thus showing 

 that submergence has taken place since its cliffs were cut. Most reef-encircled islands resemble 

 sector o. 



in Fiji in moderate number, should show a flat platform of 

 volcanic rocks beneath their limestones ; but no such platforms 

 are known. If volcanic islands were truncated before atoll 

 reefs grew up around the margin of the truncated surface, the 

 central volcanic islands of barrier reefs should be at least cut 

 back in cliffs, as shown in Fig. 3, but that is very rarely the 

 case. As the central islands of barrier and of fringing reefs 

 give good evidence by a slanting unconformity that strong 

 subsidence occurred before or during reef formation, it is 



1 W. J. L. Wharton, [Address], Report Br. Assoc. Adv. Set. 1894, 699-710; 

 " Foundations of Coral Atolls," Nature, lv. 1897, 39Q-393- 



