568 W. M. DAVIS SUBSIDENCE OF KEEF-EXCIRCLED ISLANDS 



in both these doctrines. Indeed^ such instability is involved in the sug- 

 gestion above quoted^ that the level of the sea "changes within limits 

 from time to time by warpings in different places of the sea-floor/' al- 

 though it is easily shown that a change of sealevel by 200 feet as a result 

 of such warpings demands a vertical movement as many times greater 

 than 200 feet as the upwarped area is smaller than the total area of the 

 ocean, and is therefore a very extravagant method of submerging an 

 island as compared with the local subsidence of the island itself. 



The 30-foot bed of beach sand at from 910 to 940 feet can be accounted 

 for as having been water-worn at sealevel in situ during a lull in volcanic 

 activity, then buried under lava flows when eruptions began again, and 

 lowered by subsidence to its present depth; this is quite as reasonable 

 as to suppose that the sand was washed from the beach on which it was 

 worn 700 feet or more down a submarine volcanic slope, during a lull 

 in volcanic activity, and then buried by new lava flows. If it be ac- 

 cepted that barrier and atoll reefs rest on platforms of independent origin, 

 then the truncation of still-standing volcanic islands — if such islands ex- 

 ist — would produce platforms of the desired shape; but it has not yet 

 been shown that barrier and atoll reefs do rest on such platforms. None 

 of the elevated reefs in the Pacific have been found to lie on flat volcanic 

 foundations; wherever the foundation has been examined, it has a slope 

 such as would be produced by subaerial erosion and not by marine abra- 

 sion. If v^e are to learn about one island by studying others, it seems 

 more reasonable to regard the submerged volcanic foundation of the Ber- 

 muda limestones as an eroded volcanic mountain, like the visible volcanic 

 foundations of many uplifted reefs in the Pacific, rather than to regard 

 the submerged foundations of the many Pacific atolls as truncated vol- 

 canic platforms, because that has been inferred, on the evidence of a single 

 boring, to be the form of the submerged foundation beneath the Bermuda 

 limestones. 



DEPTHS OF ATOLL LAGOONS 



Two other considerations regarding the atolls of the Pacific deserve 

 mention. It is, in the first place, desirable to point out that no very well 

 assured conclusions can now be drawn from the average depth of atoll 

 lagoons, because the greater number of atolls in the Pacific are at present 

 so incompletely surveyed that the depths of their lagoons are not well 

 known. The international collection of charts in the Hydrographic Office 

 at Washington, in which the latest issues from all countries are accessible, 

 does not afford full details as to depth of lagoons in the atolls of the 

 eastern Carolines, the Marshall, or the Paumotu group. It is therefore 



