Volcanic 

 products. 



152 



DEPTHS OF THE OCEAN 



distributed over the ocean-floor, the latter being derived from 

 the disintegration of continental land through atmospheric and 

 physical agencies and distributed in comparatively close proximity 

 to that land. Of volcanic products the most characteristic is 

 pumice, which may float for a long time in the surface waters of 



Fig. 120. 

 Hastlgerina pelagica, d'Orbigny. From the surface (\°). 



the ocean and may be carried far from its original source before 

 finally becoming water-logged and sinking to the bottom. 

 While floating on the surface these stones are knocked against 

 one another by the waves, and the broken-off fragments fall to 

 the bottom. Three varieties of pumice have been recognised 

 among the fragments from the sea-bottom : liparitic, basaltic 



