74 THE OCEAN FLOOR 



of lime is probably an important factor in deep-sea 

 geochemistry. 



Below the volcanic layer, where it exists, there are, 

 according to Ewing, crystalline rocks four to five kilo- 

 meters thick, which are probably basaltic. At the base 

 of this basaltic layer there is the so-called Mohorovicic 

 Discontinuity, which under the deep ocean lies at a 

 depth of only ten to twelve kilometers, whereas under 

 the continents this discontinuity lies at a depth of 

 between 30 and 40 kilometers. Below the discontinuity 

 the earth's mantle extends to great depths and is prob- 

 ably of the type of ultramafic rock called peridotite. 

 It seems probable that the peridotites found on St. Paul's 

 Rocks in the equatorial Atlantic Ocean are represen- 

 tative of the layers below the Mohorovicic Discontinuity. 

 Our knowledge of these deeper parts of the earth's 

 mantle is still very imperfect, and the velocity of the 

 compressional earthquake waves passing through them 

 has not been determined with any high degree of ac- 

 curacy, the highest figure so far reported being about 

 7% km/sec. 



The Transportation of Sediments along 

 the Ocean Floor 



Kuenen has devoted extensive studies on the labora- 

 tory scale to the so-called turbidity currents. He ascribes 

 to them a great importance not only over the continental 

 slopes but also on the deep ocean floor, where they have 



