DROWNED ANCIENT ISLANDS — HESS 



299 



relationships which should apply to the depths of guyot surfaces with 

 respect to the depths of atoll cores. According to the writer's hypoth- 

 esis, the atoll cores should be j'ounger than the youngest guyot surfaces 

 and should have once projected sufficiently above these surfaces (closer 

 to sea level) so that lime-secreting organisms would grow upon them. 

 Considering the Bikini results, it is evident that an old atoll will have 

 many thousands of feet of limestone deposited on its core. The mass 

 represented by this material is in excess of that which the earth's 

 crust could bear without yielding. Isostatic adjustment will, there- 

 fore, take place. As a result of this adjustment to loading, the peaks 

 of the cores of old atolls must necessarily be considerably deeper than 



FiGUBE 15. — Sketch showing the effects both of loading by limestone and of 

 asymmetrical growth of atoll — more rapid growth to windward toward the 

 right side of the diagram. Note that the high point of the volcanic foundation 

 now lies beneath the lee side of the atoll and that the fomidation has been tilted 

 somewhat by the load. Overlying old atoll surfaces are also tilted but progres- 

 sively less as the surface is approached. 



the upper surfaces of the youngest guyots, as illustrated in the accom- 

 panying diagrams (figs. 13, 14, 15). The settling in the case of 

 Bikini from this change might be estimated roughly as perhaps 3,000 

 feet. 



There was implicit in the original paper a general theory of atoll 

 development in oceanic areas. Island arc areas were specifically ruled 

 out since they present a very different problem. Much confusion has 

 resulted in the past from lumping the two. The theoiy was close to 

 Darwin's original concept, but substitutes slow rise of sea level by 

 partial filling of the ocean basins with sediment for subsidence of 

 undetermined cause. True, there are many reasons why a young 

 oceanic volcano should subside, such as isostatic settling of the load 

 represented by the volcano, squeezing out of weak oceanic clays from 

 beneath the volcano, consolidation by crystallization of the magma 

 in the chamber beneath the volcano with consequent decrease in 

 volume, etc. But all these are comparatively short-lived and could 



