234 DISINTEGEATION Chap. V. 



drainage must be lowered :rrrff ^^ ^ ^°°* 

 annually, or 1 foot in 4566 years. Con- 

 sequently, taking the best estimate of the 

 mean heigbt of tbe Nortb American continent, 

 viz. 748 feet, and looking to the future, the 

 whole of the great Mississippi basin will be 

 washed away, and " brought down to the sea- 

 " level in less than 4,500,000 years, if no 

 " elevation of the land takes place." Some 

 rivers carry down much more sediment re- 

 latively to their size, and some much less than 

 the Mississippi. 



Disintegrated matter is carried away by 

 the wind as well as by running water 

 During volcanic outbursts much rock is 

 triturated and is thus widely dispersed; and 

 in all arid countries the wind plays an im- 

 portant part in the removal of such matter. 

 Wind-driven sand also wears down the 

 hardest rocks. I have shown* that during 

 four months of the year a large quantity of 

 dust is blown from the north-western shores 

 of Africa, and falls on the Atlantic over a 



* " An account of the fine dust which often falls on Vessels in 

 the Atlantic Ocean," Froc. Geclog. Soc. of London, J\me 4tli, 

 1846. 



