56 



ATMOSPHERIC DUST. 



finest of all the samples of the shower dust examined, 

 having a larger percentage than the rest of all the grades 

 containing particles less than one thirty-second of a 

 millimeter in diameter. 



The common belief that this shower dust is brought 

 from distant places receives some support from the wide 



Table XXXVII. Mechanical 



Composition of Storm Dust : |an 



average for eigliteen samples). 



Tahh XXXV III. Average 



IVIectianical Composition of 57 



samples of Atmosplieric Dust. 



Length o! 



diameter in 



mm. 



Average. 



16-8 





8 4 





4-2 





2-1 





1-i 



tr. . 



i-i 



.5 



i-i 



2.1 



1 1 



8 16 



8.6 1 



tV-A 



40.3 ■ 



1 1 



32 64 



39.5 ■ 



i'i 1 a 8 



7.8 \ 



TITS ^66 



.6 1 



Length, of 



diameter in 



mm. 



Average. 



16-8 





8-4 





4-2 





2-1 





1-i 



.1 > 



i-i 



2 



i-i 



1.1 



1 1 



8 16 



7.9 



TB— jV 



33.1 ■ 



1V-6V 



40.4 p 



"B"4 128 



14.9 r 



12s 2S6 



1.6 



areal extent of the storms which bring it. The prevailing 

 westerly' direction of the winds in these latitudes, taken 

 in connection with the gradual change exhibited by these 

 samples (see Tab. XXXVI) from coarse in Missouri to fine 

 in New York, maybelooked upon as supporting the same 

 view. This change in the deposits may be the result of a 

 slow sifting out of the coarser particles during transit 



