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CHAPTER X. 



WHAT BECOMES OF THE " DUST " FLINT, SALT, ETC. 



What becomes of Nature's " Dust" Difference between Atlantic Ooze and 

 Chalk Origin of Flints Sponges An Extensive Pumping Apparatus 

 Microscopic Population of the Ocean ; Rapid Multiplication Micro- 

 scopic Plants, Seaweeds, Kelp-making Where the Salt of the Sea 

 comes from Animals killed by drinking River-water The Great 

 Salt Lake What would happen if the Straits of Gibraltar were closed 

 The Dead Sea Circulation of the Ocean. 



WE mentioned in the last chapter the strong resem- 

 blance existing between the Atlantic ooze and 

 the chalk, which, wherever found, has evidently been formed 

 in a similar way. But there is one great difference between 

 them. 



Chalk, especially that of the English cliffs, consists 

 almost entirely of carbonate of lime, and such small quan- 

 tities of other minerals as it contains are as equally mixed 

 with it as if the whole had been well stirred. Flints are 

 very commonly associated with chalk, and sometimes occur 

 in layers or sheets at almost regular intervals, but not a 

 particle of flint is scattered about in the chalk itself. 



The Atlantic mud, on the other hand, contains no 

 flints, but does contain a large proportion of silica (twenty 

 to thirty per cent), in the shape of cases, sheaths, and 

 skeletons of minute animals and vegetables, and the bed 

 of the Pacific is to a still larger extent covered with these 

 silicious remains. 



