510 



W. M. DAVIS— SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



Lakemba, scmewhat farther south, has been lately interpreted by Foye as 

 having been first maturely eroded, then submerged 320 feet or more and 

 bordeied by an unconformable cover of coral limestone, and finally ele- 

 vated with a tilt to the east (1917, h, 348). And yet, simply because 

 uplifted limestones occur on various members of the Fiji group, these 

 islands have been said to occupy a "region of elevation"; and, no atten- 

 tion being paid to the unconformable contact of the elevated limestones 

 with their volcanic foundations or to the embayed shorelines of the vol- 

 canic and limestone islands, subsidence has been excluded from the con- 

 ditions of reef-making around them. 



ELEVATED REEFS MAT HAVE BEEN FORMED DURING SUBSIDENCE 



Elevated reefs have often been treated by observers who took no account 

 of unconformable contacts, as if the reef had been formed during pauses 



Figure 7. — Inferred Structure of Reefs formed during Submergence and Emergence 



in the elevation of their foundation, and as therefore" invalidating Dar- 

 win's theory. Yet it should be manifest, as soon as the unconformity 

 with their foundation is recognized, that submergence must have taken 

 place before emergence, and hence that such reefs are reasonably explained 

 either as formed during pauses in submergence, followed by an emergence 

 too rapid for reef growth, or as formed during pauses in emergence pre- 

 ceded by a submergence too rapid for reef growth. Safe choice between 

 these evident alternatives can be made only by detailed observational 

 studies of reef structures, which, as is always the case in such problems, 

 are much facilitated if the theoretical possibilities are carefully deduced 

 while or before observation is in progress. 



For example, section K, figure 7, illustrates the expectable relations of 

 a series of unconformable reefs formed in ascending order during inter- 

 mittent subsidence; section M shows the expectable relations of a series 

 of reefs formed in descending order during intermittent upheaval; sec- 



