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



which the interior highlands gained most of their altitude, mnst also have 

 had a reef -free shoreline, as in block B, figure 14, for previous to the sub- 

 recent emergence of the coast the seaward slope of the uplifted highlands 

 had been cut back many miles by the waves, thus producing a broad and 

 low rock-platform backed by high cliffs, block C, v^hich are now, since the 

 later emergence, seen inland from the low coastal plain, as in block D, 

 figure 14, or as in figure 13. 



This region may therefore be taken as exemplifying the principles 

 above stated regarding coasts of ancient as well as of modern emergence, 

 and as warranting another of the principles expressed above in the section 



FiGDRK 14. — Evolution of the reefless Coast of Madi-as 



on clift islands in the coral seas, to the effect that as long as a coast of 

 emergence stands still enough not to interrupt the action of abrasion in 

 wearing it back, no reefs can be formed along its shore. If, then, reefs 

 can not be developed on coasts of emergence, either at the time of emer- 

 gence or during a succeeding stationary period, they must be developed 

 on coasts of submergence. 



REEFS ON COASTS OF SUBMERGENCE 



The bearing of the reef-free condition of coasts of emergence on the 

 coral-reef problem will be clearer if the favorable conditions for reef 

 growth on coasts of submergence are considered. Unlike the simple 

 shoreline of a coast of emergence, the shorelhie of a coast of submergence 

 is of irregularly varied pattern : it usually presents a succession of ad- 



