DROWNED ANCIENT ISLANDS — HESS 



295 



amount of water in pore space of the sediment) . Even though the fig- 

 ure cited for the rate of sedimentation may be inaccurate it neverthe- 

 less follows that oceanic islands are and have always been slowly sink- 

 ing relative to sea level. 



It stands to reason that once lime-secreting organisms appeared in 

 the oceans, presumably in Cambrian time they would grow upon any 

 available shallow, wave-cut platform and both tend to protect it from 

 further wave action and build it up to sea level. These reef- forming 

 organisms need not have been very efficient reef builders to keep pace 

 with a settling rate of 1 cubic centimeter in perhaps 5,000 years. So 



Figure 13. — Sequence depicting diagrammatically guyots and atoUs in steps of 

 decreasing age. C R stands for Cenozoic and Recent ; M, Mesozoic ; P, Paleozoic. 

 In upper half of diagram the effect of the limestone load on atolls is neglected 

 to illustrate the original depth relations to the volcanic foundations. In the 

 lower diagram the effect of load has been added. 



that beginning in Cambrian time every island in warm seas which at 

 that time had not been submerged below the level at which these or- 

 ganisms could live, would be built up to sea level or nearly to sea level 

 and could henceforth maintain its growth. In other words all Pale- 

 ozoic, Mesozoic, and Tertiary islands which were eroded to low relief 

 and submerged in warm seas must inevitably become banks or atolls 

 and be maintained as such throughout the remainder of geologic time 

 except for the interference of some rare diastrophic accident. Epochs 

 of glaciation might inhibit growth of reef-forming organisms tem- 

 porarily. But these epochs are too short to permit the islands to sink 

 to such a level that erowth would not recommence with the return of 

 warmer water. 



We may now turn to the ultimate objective of this long series of 

 digressions, the guyots. It is proposed that they represent the relics 



777488 — 48- 



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