ATMO.SJMIKKIC IJTST. 



55 



dium and the fine dust are present in nearly equal 

 proportions and constitute from seventy-four to eighty- 

 seA^en per cent of the whole. In the samples collected on 

 the ice, there is nearly thi'ee times as much of the two 

 grades of sand as in those taken on snow, owing, it seems, 

 to the greater quantity of local drift raised by the wind 

 from bare ground along the banks of the river. 



Tnhle XXXVI. Mechanical Composition of Shower Dust, Averaged for five Localities. 



Length of 



diameter in 



mm. 



Kansas City, Mo. 



Alta, Iowa. 

 (Average). 



Hock Island, III. 

 (Average). 



Chicago, 111. 



Maysvllle, N. Y. 



16-S 

 8-4 

 4—2 

 2-1 

 1-1 



1 j_ 



■S ID 



_1_ 1 



1 li 32" 



(if 

 1 



1 1 



TS'S STS 



ttV 



3.0 



5.5 



7.8 



3L.5 



3(5.2 



14.1 



1.5 



} 



A sample of dust was taken just west of Chicago, soon 

 after the shower which occurred in tlie latter part of Feb- 

 ruary in 1896 (Tab. XXXVI). It contains a considerable 

 admixture of local coarse fragments, but aside from this 

 it is slightly finer than the average deposit from the same 

 storm at Eock Island. Still another sample was collected 

 at Maysville in New York, after this storm. This also 

 contains a small quantity of sand, but it is otherwise the 



