VOLCANIC DUST ON OCEAN-FLOORS 



107 



very gradually, it rises and takes the place of the part of the 

 sea-water which has been driven away by the currents moving 

 toward the arctic and antarctic regions. The prime movers 

 of the superficial streams, like that which flows from the Gulf 

 of Mexico to the North Atlantic, are the trade-winds; and 

 thus it comes about that the really arctic climate of the deep 

 sea-floors of all the open oceans is caused by these wonderful 

 permanent winds. 



On the bottom of all the seas there is constantly gathering 

 a coating of materials derived 

 from the bodies of animals and 

 plants which perish there, or 

 which fall down upon the floor 

 from the higher parts of the 

 water ; with this is mingled, 

 more or less finely divided 

 rocky matter. If the portion 

 of the submarine surface where 

 the accumulation is making is 

 near the coast line, these sev- Tomopteris 



eral mineral substances may a curious wormUkeanimal inhabiting the surface 



of tropical seas. 



be derived from the land and 



brought into the sea by the rivers or dragged away from 

 the beaches by the reflux of the tides. In high latitudes 

 the icebergs raft off from the land quantities of stony frag- 

 ments which, when the ice is melted, fall swiftly to the bottom. 

 But this importation of detritus from the continents can aftect 

 but a small portion of the ocean-floors : it is probable that the 

 greater part of the sediment, other than that derived from 

 orcranic remains which come to rest on these surfaces, is 

 thrown out from volcanoes. The amount of these ejections 



