52 HISTORY OF THE OCEANS 



the deep-sea floor, the work is barely begun. Only in the past 

 three decades have large-scale efforts on a continuing basis fur- 

 nished the descriptive material for stratigraphic interpretations. 

 The first valid rough draft of the geologic history of the world 

 awaits the conclusions of the marine geologists and other oceanog- 

 raphers. The sediments and rocks of the deep-sea floor will, in the 

 end, yield the raw material for the unraveling of the earth's history. 

 Of prime importance will be the thickness of sediments and rocks 

 in the deep-sea floor, their total volume, their ages, and geo- 

 chemistry. The geochemical balance of the world cannot be ascer- 

 tained until the true thickness, amounts, and kinds of mineral 

 solids in the sea floor are determined; this infonnation will also 

 bear on, if not solve, the problem of the ages of ocean basins. 



Here will be mentioned some of the presently available data 

 concerning the gross stratigraphy of the deep-sea floor, namely 

 the large-scale layering in the ocean floor. After two decades of 

 seismic research at sea (by far the greater part concentrated in 

 the late 1940's and the 1950's) it has been shown that in many 

 areas a topmost layer of unlithified sediments overlies layers of 

 higher seismic velocity. The material of the layers immediately 

 under the unlithified sediment has been thought to be granite, 

 lava, pyroclastics, metamorphics, sedimentary rocks, and mix- 

 tures of these rocks and others of appropriate velocity. 



In 1956-1959 evidence increased to indicate that the higher 

 velocity layers can be sedimentary rocks. In 1957, Laughton. and 

 also Nafe and Drake pointed out that the known velocity gradient 

 in the upper sediments is too great to be explained by compac- 

 tion alone and must reflect the existence of cementation and re- 

 crystallization. Hamilton (1957) enumerated physical factors 

 which make it probable that these higher velocity layers are 

 sedimentary rock. Hill (1957) and Gaskell el al. (1958) stated 

 that around volcanic islands the second layers may be \olcanic, 

 but in the deep-ocean basins it is much more plausible to ascribe 

 them to a sedimentary origin. Ewing and Ewing (1959) suggested 

 that part of the geologic column computed to have a sedimentary 

 velocity may be high-velocity rocks masked by the geometry of 

 the water-sediment column. Their sections are labeled "uncon- 



