15 



in bowls of this sort off He"»v England. 



The degree of alteration undergone by the particles that -aake 

 up the abyssal red clay in its different layers T;ould also shon seine- 

 thing of the age of tiiis material relative to other geologic pro- 

 cesses, even if it cannot be measured in years. 



Gores are like-rise needed to tell us the relative abundance of 

 organic natter in the mud, from its upper surface do'.vnrard, a 

 question that bears on many of the cheiuical reactions that tend tc 

 alter the rau material sifting dov:n on the bottom, 



3. Submarine Dynamics 



The study of dynarnc and structural geology has been great- 

 ly handicap-oed in the past by the fact that t:7o thirds or more of^ the 

 earth' s surface v/as put out of reach by its covering of water, irnile 

 it y/as possible to survey the topography of the bottom, and to gather 

 samples of the sediments, these are only t-;o of the fo.ctors in the 

 problem of the cause of basins and continents, or of the existence 

 of troughs, submarine ridges and oceanic islandSo 



'7e still lack any means of obtaining samples of the rocks that 

 underly the oceanic sediments. But studies of earthquakes and of 

 the volcanic rocks of oceanic islands suggest that the regional 

 grouping of these may throv; light on the constitution of the crustal 

 material below the oceans. And the recent development of a mxeaxis 

 for m.easuring the strength of the force of gravity- at sea {e.s 

 geophysicists have for many years been able to do on land) opens a 

 wholly -:-e\u field of oceanic research, for previously there had heen 

 no way of determining whether the high values of gravity that pre- 

 vail on oceanic islands did or did not indicate an excess of 

 material in the crust under the oceans as a whole. And an ans^rer 

 to this question is prerequisite for any general conclusion as to 

 Tjhether the state of hydrostatic equilibriujn or "Isostasy" that has 

 been proved to be the normal condition of the emergent portion of the 

 earth's surface is equally characteristic of the ocean beds; in other 

 words, whether these depressions represent uhe heavy sectors of 'Lhe 

 crust, just as the masses above sea level are compensated for by a 

 deficiency of material (i,e, likeness of the crust) beneath the 

 continsnts. 



This is a major problem of geophysics because our interpreta- 

 tion of irre.gu.larities of the earth's surface must depend largely 

 on determining the relative densities of the crust under oceans of 

 different depths, compared with lands elevated to different heighths 

 above the mean crust level. Gravity measurements at sea supple- 

 mented by analyses of the igneous rocks found on oceanic islands 

 may, therefore, be expected to throw much light on the causes of 

 ocean basins and continents, of the sinkings and risings of oceanic 

 islands, and of the volcanic activity occurring o". the latter 'ao\i 

 or in the past. Such measurements may also 'oe expected to show 

 whether the processes that caused broad up-lifts in the past are now 

 at work under the oceans, to m.ake up-lifts that v;ill appear abO'/e 

 the sea in geologic ages to come. 



