78 THE OCEAN FLOOR 



thousands of square miles, ultimately reaching the ocean 

 bed and sometimes in quantities sufficient to form layers 

 of volcanic ash. Submarine volcanism is also a con- 

 tributor, on a scale probably much vaster than we have 

 hitherto been inclined to assume. 



The relative amounts of these different components 

 vary all over the ocean according to latitude, distance 



Fig, 27. Ash rain over the Tyrrhenian Sea 



from continents and islands, water depth, water move- 

 ments, etc. The quantity of dust and clay particles from 

 the continents varies also according to the great cycles 

 of mountain building (orogenesis) and erosion (vas- 

 tage), processes which are themselves dependent on 

 climatic variations. These climatic changes, moreover, 

 directly affect the quantity of planktonic and benthonic 

 remains which, to a considerable extent, depend on the 

 temperature of the surrounding sea water. It is only 



