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



before, and the continents would be characterized rather by the predomi- 

 nance of coasts of submergence, as they really are, than of coasts of emer- 

 gence. 



For if many lofty volcanic cones are successively built up on the ocean 

 floor, the ocean surface must be thereby somewhat raised; and if, as the 

 cones successively subside, their place is largely taken by an atoll crown, 

 the ocean surface will retain its rise instead of sinking again, as I have 

 recently shown elsewhere (1917, h). [Here be it noted that the with- 

 drawal of limestone from solution in water diminishes the water volume 

 by only a small fraction of the volume of the withdrawn limestone.] 

 Hence, as far as the prevalence of coasts of submergence around conti- 

 nental borders is concerned, they testify in favor of the local subsidence 

 of volcanic islands which are locally reconstituted by reef growth as they 

 subside, and against a broad subsidence of the ocean bottom, the effects of 

 vv^hich could not be compensated by reef growth. It is true that geolog- 

 ical opinion has usually been in favor of regarding volcanoes as situated 

 in areas of elevation, and that active volcanoes in particular have been 

 usually regarded as rising, not sinking ; but the opposite opinion deserves 

 consideration. 



The well supported interpretation given by Branner (1903) to the great 

 canyons in the northeastern part of the island of Hawaii carries with it 

 the implication that that great volcanic mass has been subsiding during 

 the later stages of its eruptive activity at least. Similarly, the relation 

 of the partly submerged valleys in the older (western) and younger (east- 

 ern) parts of the Oahu volcanic doublet gives strong indication that the 

 subsidence of that island, which now amounts to 1,000 feet at least, had 

 begun before its volcanic activity ceased. Taviuni, in northeastern Fiji, 

 bears several young volcanic cones, yet near by are several deeply dissected 

 volcanic islands, the embayed shorelines of which indicate a recent sub- 

 sidence of several hundred feet. Kanda^vu, a larger island in southwest- 

 ern Fiji, has a young volcanic cone, little dissected, near its western end ; 

 the rest of the island is maturely dissected and elaborately embayed, as if 

 it had recently suffered strong subsidence. 



Besides these islands which bear recently active volcanoes there are 

 numerous other volcanic islands so old that they have been deeply dis- 

 sected; but these also have been shown, by various lines of evidende al- 

 ready set forth, to have subsided, namely, by the rock-bottom depth of 

 their embayed valleys, which is frequently much greater than the Glacial 

 lowering of sealevel; by the thickness of their encircling reefs as meas- 

 ured up from the submarine prolongation of the island slopes; by the 

 unconformable contacts of reef limestones on the eroded spurs; and by 



