EOLIAN TRANSPORTATION 6i 



the exceptional ones to average this amount for every period of five 

 years, we would get a deposit 4.78 mm. thick in a century, while dur- 

 ing the three thousand odd years during which such storms have 

 been recorded, the total amount would be 143.4 mm. or over" 5^/2 

 inches of material brought from the desert. Dust fogs are charac- 

 teristic of the Red Sea at certain seasons and are known on parts 

 of the Atlantic reached by the winds from the desert. If dust-laden 

 winds reach a district of more rainfall, or if their velocity is dimin- 

 ished, they will be relieved of their burden, and the dust will settle. 

 This occurs in the steppe surrounding the desert, where the further 

 transport of the material is in large measure prevented by the grassy 

 vegetation. Thus the surface of the steppe is gradually built up by 

 deposits of fine material derived often from great distances. Skele- 

 tons of terrestrial animals may readily be buried where accumulation 

 is rapid, while along the water courses leaves and shells of fresh 

 water or land molluscs may become entombed. The loess, discussed 

 more at length below, is believed to be an accumulation of this sort, 

 and older deposits of this type are coming to be more generally 

 recognized. 



Sorting and Rounding of Sand Grains by JVind. The sands 

 picked up by the winds on the shores ordinarily travel only at a 

 slow and probably constantly diminishing rate inland. But the 

 finer dust particles are blown away rapidly. If the sand consists of 

 or contains minerals which are readily ground into dust, this dust 

 will be blown away and the destructible minerals are thus removed 

 from the dune sand. It is due to this destruction of other minerals 

 that the dune sand of some coasts consists almost entirely of pure 

 quartz grains. 



During this movement, these dry sand particles are not only ac- 

 tively engaged in abrading whatever rocks or other objects are in 

 their path, but they themselves are well rounded through mutual 

 attrition. So perfect is this rounding of the grains in some wind- 

 blown sands that even the lens will reveal no irregularities. (Wal- 

 ther-103 : /'93-) Since particles o.i mm. or less in diameter are not 

 reduced by mutual attrition in water, it follows that rounded grains 

 below this size must in general be regarded as wind-worn grains. 

 (The relative efficacy of wind and water as agents of rounding will 

 be discussed in Chapter V.) The glassy character is, moreover, 

 dimmed by this attrition, and the surface of the grains takes on the 

 character of ground glass. Such sands nxiy later be worked over 

 by the waves of a transgressing sea or lake, and so become well- 

 stratified waterlaid deposits with remains of aquatic organisms. Not 

 infrequently shells are driven far up on the shore by -the winds, 



