LEK SAND. :51 



distance exceeds that over which fine sand is generally 

 lifted in single leaps. And this quite likely also applies to 

 the transportation of the next finer grade. It should 

 also be noticed here that only a very subordinate percen- 

 tage of pai'ticles smaller than one thirty-second of a milli- 

 meter in diameter settles within the distance observed. 



We ma,y infer that the grades of rock fragments which 

 range in diameter from one eigth to one sixteenth of a 

 millimeter in diameter are mainly deposited, together 

 with some coarser and some finer ones, in front of drift- 

 ing tracts as a thin apron, which becomes finer in com- 

 position with increasing distance to the leeward. There 

 is little doubt that the change in the texture of this apron 

 deposit is most rapid at first and more slow farther out. 

 Its deposition results from temporary lulls in the wind, 

 which allow the coarser grains to fall to the ground. Go- 

 ing down the scale of diminishing particles a. size will at 

 last be found, which is capable of almost indefinite sus- 

 pension in the changeable currents of the atmosphere. 

 Material of this kind is scattered over wide distances and 

 the change in the texture of the deposits formed from this 

 dust progresses with extreme slowness from one place to 

 another. 



ATMOSPHERIC DUST. 



To determine the size of the particles that may readily 

 be transported such long distances by ordinar^^ winds, it 

 is only needed to examine the nature of the loads which 

 these winds generally carry. I have collected a number of 

 samples of such dust by different methods, under dif- 



