94 THE OCEAN FLOOR 



a kind of manure on the surface layers, giving rise to 

 an abundance of phytoplankton, and hence of zoo- 

 plankton. 



Of great interest also is the mechanical structure of 

 the sediment, whether it consists mainly of clay with 

 very minute inorganic particles or larger mineral frag- 

 ments bordering on sand, of biogenetic remains like 

 calcareous tests and siliceous skeletons, or of volcanic 

 debris. An exhaustive mineralogical examination of 

 representative samples from all our cores would have 

 been a very heavy task, and the time and expense in- 

 volved prohibitive. A granulometric study, in which 

 components of different grain size are separated 

 through a fractionated sedimentation process, was 

 therefore carried out only with certain samples of 

 special importance. 



Of very great interest is the deep-sea sand, that is, 

 coarse particles occurring as separate layers in cores 

 from great depths. For instance, a core nearly fifty feet 

 long was raised from that most interesting pit in the 

 equatorial Atlantic Ocean, the Romanche Deep, 7,500 

 meters at that point. Although the sediment in the upper 

 levels was largely fine grained and intercalated with thin 

 dark streaks rich in organic remains, in the lower 

 levels calcareous ooze predominated; but there were in 

 certain levels very peculiar layers of sand. They con- 

 sisted of angular fragments of mafic rocks which must 

 have come from the substratum, that is, from the nearby 

 Mid-Atlantic Ridge. Judging from the echograms 



