54 THEOCEANFLOOR 



clay of roughly one millimeter, or not quite .04 inches, 

 in 1 ,000 years. For the Globigerina ooze the correspond- 

 ing figure is .4 inches in 1,000 years. Obviously, these 

 calculations require a huge extrapolation from known 

 figures. The rate of denudation of the continental rocks 

 and the rate of productivity of plankton organisms in 

 the sea are bound to have varied greatly with both 

 climatic changes and alterations in the orography of 

 continents. Furthermore, it is evident that the actual 

 thickness of the sediment carpet in a given locality is 

 likely to show considerable deviation from the average 

 for the whole ocean floor. In general the very lowest 

 rates of sedimentation are found in the central parts of 

 the oceans, especially in that of the Pacific Ocean, where 

 the distance from the surrounding continents reduces 

 contributions from terrigenous components and where 

 the accumulation of biogenetic matter has been largely 

 counterbalanced by chemical action from the bottom 

 water, acting especially on calcareous components. 



Direct measurements of the thickness of the sediment 

 carpet in great depths down to the underlying bedrock 

 or, to be more exact, to transition surfaces within the 

 sediment which are capable of reflecting acoustic 

 signals have become possible only in the last decade. 

 The method used may be called "seismic," since it is 

 analogous to that used by seismologists for studying 

 the internal structure of the earth and its crust by ob- 

 serving the elastic vibrations emanating from earth- 

 quakes. However, when studying the sediment layers in 



