WOEK PEEFOEMED BY THE WINDS. 223 



between the productions and the climates, no less than in the general 

 aspect of nature. 



The winds are also powerful geological agents. Thus the aerial 

 currents of certain latitudes transport clouds of dust, which may at 

 length render vast countries sterile or fertile, either by covering the 

 natural soil with an unfruitful layer, or by effecting a happy mixture 

 with it. On the banks of the Nile, the sand of the desert which the 

 wind mingles with the thick mud of the river, contributes to develop 

 the marvellous productive force of the land, while in the neighbour- 

 ing plains, which are destitute of moisture, it buries the plants and 

 renders the soil wholly unfit for vegetation. Elsewhere, and princi- 

 pally on the low coasts of the sea, the wind drives hills of sand 

 across the plains, barring the outlets of the streams, and gradually 

 driving the water up the slope of the continent.* 



In certain places the aerial current even goes so far as to tempora- 

 rily change the level of the sea ; it sometimes arrests the waves, or 

 hurls them against the shores, and alternately dries up the bed, and 

 causes disastrous inundations. Sometimes the wind, which descends 

 with violence from the polar regions of North America to the Gulf of 

 Mexico, keeps back three or even four successive tides. Then these, 

 returning altogether in one foaming mass, sweep over whole Islands 

 off the low coasts of Louisiana and Texas. In the same way when the 

 pampero or south-west wind blows over the great estuary of La Plata, 

 its waters are sometimes lowered by 12 or even 18 feet in less than 

 half a day, and the vessels that were floating in the road remain 

 stranded in the mud.f 



This is not all. The wind can also modlfj^ the configuration of the 

 shores, since the waves of the sea, which contribute so largely 

 towards the sculpturing of them, receive from it their impulsive 

 force. Thus the large arm of the Ehone perhaps owes its south-east- 

 erly direction to the mistral which descends from the Oevennes.J 



As to the delta of the Mississippi, its exterior contours are probably 

 modelled by the south-east monsoon which prevails in that country ; 

 the southern passage, which opens exactly in the direction of the 

 prevailing wind, is almost entirely obstructed by the dike of mud 

 that the surf has raised across its current. The two arms of the 

 Mississippi which carry the greatest quantity of water are directed, 



* See above, p. 211. 

 t Fitzroy, Adventure and Beagle, vol. ii. Appendix, p. 89. 

 X See The Earth, the section entitled, Mivers. 



