688 J. A. UDDEN COMPOSITIOK OF CLASTIC SEDIMENTS 



298. Like the preceding ; maximum wind velocity, 18 miles per hour. 



299. Like the preceding ; maximum wind velocity, 18 miles per hour. 



300. Like the preceding ; maximum wind velocity, 19 miles per hour. 



301. Like the preceding ; maximum wind velocity, 20 miles per hour. 



302. Like the preceding ; maximum wind velocity, 21 miles per hour. 



303. Like the preceding ; maximum wind velocity, 23 miles per hour. 



304. Like the preceding ; maximum wind velocity, 24 miles per hour. 



305. Dust collected in a hollow cylinder 10 feet above the ground at Rock 



Island, Illinois, February 19, 1896. 



Dust taken on surfaces above the ground. — Several analyses have been 

 made of dust found adhering to surfaces of objects more or less elevated 

 above the ground. Eight such samples were washed from the foliage of 

 trees, on which appreciable dej^osits of dust may always be observed. 

 The maximum grade in this material is medium dust, but the lesser 

 weight and the smaller size of the particles of less than this size renders 

 them less subject to dislodgment by the wind and by occasional shaking 

 and rubbing of the leaves against each other. When collected in the 

 early part of the summer, this dust is therefore found to be coarser than 

 it is later on, owing to the more frequent removal of the coarser parti- 

 cles and the more persistent adhering of the finer particles. In some 

 dust which was washed from the leaves of oak trees in the months of 

 May and June, in 1895, there was about 20 per cent of fine dust, while 

 in two samples taken in August and September there was a little over 

 30 per cent of the same ingredient, and in another sample, which was 

 washed from leaves remaining on some trees in February, the fine dust 

 was the maximum ingredient, making nearly 40 per cent of the whole 

 sample. Four analyses are of dust taken on the bark of some trees, and 

 two are of dust coming with rain-water from a house-roof. Such rough 

 surfaces as these give a secure lodgment to grains of sand as well as 

 dust. From the analyses it is quite evident that some coarse material is 

 moved even by the gentle winds of the Mississippi Valley. It may be 

 that many of these grains are raised by the aid of lighter objects to 

 which they adhere, such as bits of straw and leaves; but their abun- 

 dance in these last samples is best accounted for by the action of occa- 

 sional strong convection currents and by the increased chances for larger 

 grains to find lodgment on rough surfaces. This may be inferred from 

 two analyses, one of which gives the composition of some dust collected 

 from the trunk of a small tree by striking it repeatedly with a hammer, 

 while the other shows the ingredients in the material which remained 

 on the bark after this procedure and which was secured afterward by 

 washing. The former has a small and the latter a large proportion of 

 the admixtures on either side of the maximum ingredient. Aside from 



