other questions equally broad are also involved; for instance, 

 have the ocean beds tended to sink under their or/n ^7eight r;ith the 

 lighter niargins of the continents tending to buchle -ip in conpensa- 

 tion? in areas, on the contrari'-, Y;r.exe the sea botton is rising, is 

 its lightness coinpared to the surrounding lands responsible? How is 

 all this related to the weight of the sediuents that accumulate 

 on the sea floor, and this, in turn, to the new hypothesis (raising 

 one of the aost vital problems in modern science) that the huge 

 blocks of the earth's crust that fori:i the existing continents have 

 aoved horisontally? 



For these reasons a net of gravity measureuents is needed over 

 the oceans. A beginning has already been nade in this direction by 

 determinations carried out by the Dutch Geodetic Comais si on, frori 

 submarines, on one voyage from Europe to the East Indies via the 

 Mediterranean and. Suez Canal, on another across the Atla/ntic and. 

 Pacific; ;uore recently by the U. S. llavy in the ^est Indian- 

 Ca.ribbean region Glsev/here . And a gravimetric marine survey :j1 

 the East Indies is now in progress. These observations have o;T3ned 

 interesting problems, for while the flatter parts of the sea floor 

 (along the lines so far run) have given values roughly in accord 

 with the isostatic principle, decided differences have been foTnd. 

 between the observed and the theoretic values of gravity over and 

 near some of the deeper submarine troughs, near oceanic islands and 

 close to the margins of the continental shelves, LIuch more must be 

 done before it will be safe to conclude whether these abnormal! jies 

 of gravity, plus the fact that the grouping of submarine earth- 

 quakes is similar (the deep troughs seem, in particular, to be the 

 seats of the strongest earthquakes), really reflects a lack of 

 isostatic equilibriijim or stability in these parts of the earth's 

 crust, for other explanations are possible. For instance, the sub- 

 marine earthquakes may be caused by e:rpansion or contraction of the 

 crustal material. In fact, the recent discovery Coy sonic methods 

 of sound.ing) of great submarine escarpments with steep slopes points 

 in that direction. Or the abnormal values of gravity so far re- 

 corded at sea may reflect the presence of heavy or of light masses 

 of material close under the surface of the crust in the imi^iediate 

 vicinity, rather than liie mean density of the v/hole underlying thick- 

 ness of the latter, 1 In this case there might be very little 



1, In accordance with the gravitationa.l law that masses attract 

 each other inversely as one square of the distance, a mass of extra 

 heavy material close to the observing station will cs.use an ab- 

 normally high value gravity, while a mass of light unconsolidated 

 material nearby will cause a low value. 



horizontal strain in the material underlying the stations in question, 



The solution of dynamic questions s'ach as these calls for in- 

 tensive studies of the ocean deeps, of the regions around the 

 oceanic islands, and of the m.argins of the continental shelves by 

 means of gravity determinations in combination with detailed 

 topographic surveys of the bottom. A combination of the data 

 secured from the sediments, the configuration of the bottom and the 

 values of gravity should lead to great advances in this general phase 



