SIR JOHN MURRAY 175 



it is also confoundedly cold never more than a 

 degree or two above zero and it is still. 



The pressure under which animals live at great 

 depths is enormous. At the surface of the ocean 

 the pressure per square inch is fifteen pounds, 

 at 3,000 fathoms it is three and a half tons. The 

 change of pressure when deep-sea animals are 

 hauled to the surface often distorts their shapes. 

 Only occasionally the erosion of a marine cable 

 demonstrates the presence of slow but persistent 

 deep-sea currents ; but for the most part all is 

 still, motionless. The bottom of the sea, like the 

 soil of the land, grows. It is added to by all 

 sorts of mud carried from the land by rivers and 

 winds ; broken-off ends of glaciers icebergs, 

 carrying with them chunks of rocks contribute 

 larger particles. All these at any rate are terri- 

 genous, and so are, as a rule, the products of 

 volcanoes, which are constantly dispersing volcanic 

 dust over large areas of the ocean. Pumice-stone 

 of various kinds plays, at any rate in certain parts 

 of the ocean, a conspicuous rule in the bottom 

 deposits. A third source is extra-terrestrial, and 

 meteorites of the interstellar space reach in very 

 perceptible numbers the bottom of the sea. 



But far the most bulky and widely distributed 



