176 STUDIES IN INSECT LIFE, ETC. 

 material which drops down from above are the 

 skeletons or shells of innumerable organisms 

 which live on the surface or a little below the 

 surface of the ocean or on its margins : 



" The wrecks dissolve above us ; their dust drops down 



from afar 



Down to the dark, to the utter dark, where the blind white 

 sea-snakes are." 



Calcareous algae, corals, molluscs, Crustacea, skele- 

 tons of fishes and of seals and whales are per- 

 petually falling on to the ocean bed like the 

 " gentle rain from heaven/' and nearer inland 

 the shells of many of the shore-dwelling animals 

 are ever being washed down to the depths. 



Besides these, the ocean-bed consists of many 

 substances which are regarded as chemical or 

 secondary products. Certain kinds of clay arise 

 from the decomposition of minerals on the sea 

 floor, manganese nodules and greensand are 

 examples. Further examples are the boulders 

 and rock fragments which have been carried by 

 icebergs far from the land where they originated, 

 and these lie scattered over the sea bottom. All 

 these varied materials are unequally scattered 

 like patchwork over our ocean floor, and mixed 

 with nearly all of them are sharks' teeth and 



