378 



MENARD AND LADD 



[chap. 15 



however, for Funafuti in the EUice Islands some 1700 mi to the southeast. 

 The known Quaternary section Hrilled there is almost twice as thick as that 

 found in the Marshall Islands. 



Practically nothing is known about the foundations of atolls lying outside 

 the Pacific Basin. Dee]) drilling on Kita Daito Jima penetrated the Tertiary 

 hut failed to reach the bottom of the sedimentary section. No other atoll out- 

 side the basin has been drilled. Many of them, as, for example, the Argo Reefs 



"■^o^ 



ELUGELAB 

 (HOLE F-l) 



PARRY ISLAND 

 IHOLE E-l) 



LIMESTONE CAP 



dASALTIC FOUNDATION 



10 



CONTOURS IN FATHOMS 



SEA LEVEL 

 3000' 

 6000' 

 9000' 

 12000' 

 ■I5p00' 



Fig. 6. Plan and structure section through Eniwetok Atoll in the Marshall Islands showing 

 relations of limestone cap to volcanic foundation, as determined by drilling. (After 

 Ladd and Schlanger, U.S. Geological Survey, 1955.) 



and others in Fiji, lie in areas where there has been considerable elevation in 

 post-Tertiary times. Some of the atolls are large and irregular. Even less is 

 known about the foundations of atolls in the Indian Oceans. 



E. Distribution 



As shown in Figs. 2, 7 and 8, atolls are most abundant in the western Pacific, 

 numerous in parts of the Indian Ocean and exceedingly rare elsewhere. 



Structures comparable at least in general form to existing atolls have been 

 described from rocks as old as Paleozoic (Stafford, 1959; Burnside, 1959). 

 Many of these older structures are reservoirs of petroleum and are being 

 intensively investigated. 



F. Development and Age 



Development of existing atolls began at least as far back as Eocene time. 

 Most of those that rise from the deep ocean appear to be caps on ancient 

 volcanoes that have subsided to permit the accumulation of shallow-Avater 



