WIND DEI'OSITS 689 



the greater proportions of the extreme grades, which may be accounted 

 for by the diminished proportionate chances of the grains of the maxi- 

 mum ingredient to find and maintain a secure lodgment, all of these 

 samples resemble those collected on surfaces rendered adhesive by the 

 application of glycerine. 



List of Samples in TaMe 28 



306. Dust shaken from the trunk of an oak, Rock Island, Illinois. 



307. Dust in rain-water from the roof of a house, Rock Island, Illinois. 



308. Dust from rain-water coming down on the trunk of an oak during a 



shower. Rock Island, Illinois. 



309. Dust from the trunk of an oak. Rock Island, Illinois. 



310. Dust in rain-water from the trunk of an oak, Rock Island, Illinois. 



311. Dust washed from the trunk of an oak. Rock Island, Illinois. 



312. Dust washed from some poplar leaves, Rock Island, Illinois. 



313. Dust washed from the leaves of a hickory tree. Rock Island, Illinois. 



314. Dust washed from the leaves of a linden tree, Rock Island, Illinois. 



315. Dust washed from the leaves of an oak tree. Rock Island, Illinois, June, 



1895. 



316. Dust washed from the leaves of an oak tree. May, 1895, Rock Island, 



Illinois. 



317. Dust washed from foliage of trees. La Salle, Illinois. 



318. Dust washed from foliage of trees. New Bedford, Illinois. 



319. Dust washed from dry leaves of oak trees, Rock Island, Illinois, Febru- 



ary, 1895. 



Shower dust. — Deposits of an impalpable dust are sometimes observed 

 in the central States, especially during winter, when it is apt to fall on 

 the snow. It generally appears after strong westerly winds, which have 

 been called- dust storms. Eighteen samples of such dust have been ex- 

 amined, and these represent six different storms. The coarsest fell in 

 Kansas City in the summer of 1890. Nearly 60 per cent of its weight 

 consists of coarse dust, and less than 30 per cent is medium dust. Two 

 samples taken near Alta, in Iowa, come next to this in coarseness. An 

 average of the two analyses has 52 per cent of coarse dust and 35 of the 

 medium. This was collected during and after a heavy wind in the early 

 part of June, in 1895. 



Thirteen samples were gathered from the surface of ice and from 

 snow at Eock Island, in Illinois, and these represent three different 

 showers. One such shower occurred in the latter part of November, in 

 1894; one in the latter part of January, in 1895, and one in February, 

 in 1896. Dust gathered on the ice of the i iasissippi, close to land, con- 

 tains comparatively large quantities of coarse admixtures, evidently de- 

 rived from the ground close by, and the same is the case with some 



