ATMOSPHERIC DUST. 



51 



SHOWER DUST. 



Deposits of an impalpable dust are sometimes observed 

 over this region, especially during winter, when it is apt 

 to fall on the snow and discolor its surface. It generally' 

 appears after strong westerly winds, which have been 

 called dust storms. Eighteen samples of such dust have 



Table XXXII. Mechanical Composition of Sliower Dusf fallen west of the 

 Mississippi River. 



Leogth or 

 diameter in 



Dust from 

 Kansas City, Mo. 



16—8 

 8-4 

 4-2 

 2-1 



1— i 



s 



_ 1 

 TB" 



_ I 

 3^ 



_1 



16 



_I 1 _ 



04, TSS 



1 1 



TITS TST 



Dust from 

 Alta, Iowa, 

 June 7, 1895. 



Dust from 

 Alta, Iowa, 

 Junes, 1895. 



1.7 



4.0 

 46.0 

 42.0 



7.6 



.7 



F 



been examined, and these represent six different storms. 

 The coarsest fell in Kansas City in the summer of 1890. 

 Nearly sixty per cent of its Aveight consists of coarse dust, 

 .and less than thirty per cent is medium dust (Tab. 

 XXXII). Two samples taken near Alta in Iowa come 

 next to this in coarseness. An average of the two ana- 

 lyses has fifty-two per cent of coarse dust and thirty-five 



