60 GENERAL COXCLUSIOXS. 



the dust in suspension, while sand is dropped, and in the 

 same way lesser vertical components higiier up in the air 

 mosth^ retain particles less than one sixty-fourth of a 

 millimeter in diameter, while particles larger than this 

 are slowly settling'. Where no exceptional conditions 

 prevail, the two slopes should therefore be symmetrical, 

 since both are determined by the velocity of the atmos- 

 pheric currents. 



GENERAL CONCLUSIONS. 



While the wind-borne materials which were collected for 

 these analj^ses may not represent the greatest extremes 

 of wind work, such extremes were sought in their selec- 

 tion. Even if more extended observation should show, 

 as it hardly can fail to do, that pebbles considerably 

 larger than any seen in these samples, may be moved by 

 the wind, it is evident that atmospheric transportation is 

 confined to rock fragments of comparatively limited 

 range of sizes. The largest pebble found in any of these 

 anah'ses, measured less than eight millimeters in diam- 

 eter. In the opposite direction infinity is of course the 

 extreme limit, but in the dust collected for this study the 

 quantit}" of particles measuring less than one two 

 hundred and fifty-sixth of a millimeter in diameter prob- 

 abh' in no case amounted to as much as one per cent of 

 the whole, and generally it constituted merely a trace, 

 when at all present. It was therefore neglected in the 

 anal3'Ses. 



The limited range of coarseness of wind-borne mate- 

 rials is, of course, due to the lightness of the air. Within 



