V 



60 THE OCEAN FLOOR 



returning sound waves in great detail. WeibuU claims 

 that with the latest modifications in his equipment he 

 is able to distinguish between two reflecting layers 

 situated at levels only about 30 feet apart, a much 

 greater separation than that achieved by other methods. 

 One decided improvement is in the use of depth charges 

 exploding in the sediment itself instead of above the 

 bottom. In this manner the complications due to 

 spurious echoes from hills and hummocks — in which 

 the deep-sea bottom unfortunately abounds — can be 

 minimized or even avoided, giving distinct and clear 

 records where an ordinary depth charge will give a 

 protracted series of echoes overlapping those from 

 deeper layers. 



Returning to the results so far gained, one may say 

 that if we accept Weibull's maximum sediment thickness 

 in the central Atlantic of about 12,000 feet, and if we 

 further assume that the whole of this sediment is of the 

 same type as its surface layer, namely Atlantic red clay, 

 we can make an approximate estimate of the time 

 required for accumulating a layer of this thickness. 

 Taking .3 inches in 1,000 years as a reasonable value 

 for the rate of sedimentation of Atlantic red clay, we 

 arrive at a time span of nearly 500 million years. There 

 are, however, two circumstances which tend to make 

 this estimate too low. One is the so-called "compaction" 

 of the lower layers of the sediment, due to the fact that 

 the pressure from the superposed layers of sediment 

 tends to squeeze the water out of the deposit and may 



