Plotting the number of known seamounts by classifying them 

 according to the depth in fathoms of the top (from 500 to 1,000 fathoms) 

 in a graphic representation (see figure) it becomes obvious that certain 

 levels are clearly preferred. The result is practically the same if we use 

 only the data given by Hess (1946) or the general sounding list of the 

 Hydrographic Office at Washington (the two lower curves) for the Pacific 

 north of the Equator (Pilot Chart 2603). The visible maxima seem to be 

 more or less evenly spaced (about 80 fathoms) especially towards depth. 



If the seamounts are drowned islands and are the older the deeper 

 below the actual surface they are, we can hardly escape the conclusion 

 that all islands in the open oceans are subjected to a slow secular drowning. 

 One probable cause of this slow drowning may be the secular sedimenta- 

 tion in the open ocean, as Hess has suggested. If for rather obvious 

 reasons the sedimentation rate on the flat-topped submarine cones is 

 less than on the surrounding ocean bottom, then a slow sedimentary 

 burying of these cones must take place with the ages, calling for a slow 

 drowning of the tops due to isostatic adjustment. Recent probmg of 

 bottom sediments, made on a flat-topped seamount in the Atlantic, north- 

 east of the Bermudas seems to confirm this explanation. The top plateau 

 stands at 841 fathoms and the sounding core of the sediments showed 

 Eocone chalk below 8 in. of globigerina ooze (Ewing, 1948). On the 

 surrounding deeper ocean bottom the sedimentation is far more intense, 

 and only far younger sediments can be found there in the cores. There 

 may be other causes for the secular drowning such as a tectonic deepening 

 of the oceans with time, an increase of the total water content. I have 

 dealt with these possibilities in other papers (1942,. 1948). 



Assuming that these flat-topped seamounts are old volcanoes which 

 preferentially rose at times of pitched tectonic diastrophism within the 

 earth's crust, one is tempted to parallelize the preferred levels of the 

 graph with the times of pitched tectonic volcanic activity, clearly visible 

 in statistics of radioactive age determinations of minerals (Holmes, Wahl). 

 Such an attempt has been made in the graph. If this interpretation 

 should be correct, then the secular drowning rate of the seamounts would 

 be about 110 m. in a hundred million years. 



How does such an estimate match with known borings on coral 

 islands, which may be seamounts with an age younger than the first 

 appearance of corals in the seas ? To the International Geological 

 Congress some new data have been presented in this respect. According 

 to Wells and others (1948), borings down to about 760 m. (2,5-56 ft.) 

 on Bikini were still in the Lower Tertiary. Also on the other atolls, 

 where borings are known — for instance, Funafuti — the results were 

 similar. Notwithstanding that these results suggest at first sight a far 

 higher sinking rate of these islands than the present estimate, it is not 

 certain that this must be so. 



The borings made up to date are marginal borings and do not give 

 the conditions at the centre of the atoll, where we have to expect probably 

 the original volcanic cone. Coral islands do not only grow upwards, but 

 also sidewards. Coral material broken loose by wave action may roll 

 down the submarine slopes, with the result that in marginal borings 

 beds of certain age are found at far lower levels than in more central 

 borings. As we have to consider also the possibility of special tectonic 

 level displacements which may have affected certain islands or of especially 

 strong displacements of sea-level during the more recent history of the 



333 



13 — Pac. Congress 



