14 



Transactions. 



/ 



above sea-level. In the case of many volcanic islands it is not 

 unreasonable to conclude that a barrier reef a mile from the island-shore 

 has a vertical thickness of 1,000 ft. This conclusion evidently rejects 

 the idea that a barrier reef is built upon a shallow platform of non-reef 

 origin, which appears to me as improbable as that a volcanic island rests 

 upon a shallow foundation of non-volcanic origin. 



Mature Reef Plains. — Although coasts of recent submergence usually 

 present favourable conditions for the growth of fringing or of barrier reefs, 

 these conditions may not persist indefinitely on coasts that long remain 

 stationary after a less recent submergence ; for, if the land drained by 

 the coastal rivers is of large-enough area, deltas, E (fig. 3), will in time not 



Fig. 3. 



only fill the drowned-valley embayments of a still-standing island, but 

 will unite around the spur-ends and form a confluent alluvial lagoon 

 plain, F, with a shore-line of comparatively simple pattern. As the 

 advance or the progradation of such a plain continues, the fringing reefs 

 on the spur-ends will be smothered with detritus. Such appears to have 

 been the fate of many fringing reefs on the island o^ Tahiti, where an 

 alluvial plain extends along much of the island-border. As the plain is 

 still farther prograded, and as overwash of debris from the. outer face to 

 the inner slope of the outgrowing reef continues, the lagoon will be filled and 

 converted into a mature reef plain, MP. 



If the outwash of alluvium still goes on, the barrier reef, R, iu 

 spite of the width it may have then attained by outward growth, will be 

 smothered, and its corals will be killed. Thereupon the sea will attack 

 the reef and cut it away ; and if this process be once begun there appears 

 to be no reason for it to stop. The alluvial reef plain must in time be 

 consumed, and then the central island will be attacked and clified ; for 

 as long as the island stands stiU, and as long as outwashed alluvium 

 supplies material for a beach, coral growth cannot be re-established 

 (Davis, 1917a). This sequence of events is evidently hypothetical in a 

 high degree : nevertheless, the successive stages of such a sequence, and 

 of all other reasonable sequences, should be carefully conceived by an 

 observer of coral reefs while he is still on his voyage of investigation, m 

 order that he may be able to confront the successive stages of the various 

 sequences with the reefs that he sees, and thus discover which sequence 

 gives the best history of their origin. 



The danger of being buried and smothered in alluvium appears to 

 threaten the barrier reef along the south coast of Viti Levu, Fiji, where 

 the delta of the Rewa River has almost filled the lagoon ; and a long 

 stretch of the barrier reef on the south side of New Guinea appears to be 



