A. N. Wincliell— Great Dustfall of 1920. 361 



Physical Composition. — Dustfalls vary considerably in 

 the average size of the component particles as well as in 

 the range of size and the abundance of particles of various 

 sizes. The dustfall at Madison in 1920 may well be 

 compared in this respect with the dustfall of 1918 as 

 shown in the following table. 



Table VI. — Size of constituents of dustfalls. 



12 3 4 



Clay, less than .005 mm 11.15 27.49 25.57 23.32 



Fine silt, .005 to .010 22.01] 11.911 



Medium silt, .010 to .025. . . . 56.17 V 66.86 44.09 f 70.11 



Coarse silt, .025 to .050 5.99J 11.35J 



Very fine sand, .05 to .10. . . . 1.22 4.78 5.04 5.05 



Fine sand, .10 to .25 1.04 .81 .87 .77 



Medium sand, .25 to .50. . . . 0.58 .03 .05 .04 



Coarse sand, .50 to 1.00 0.29 .01— .03 .03 



Fine gravel, 1.00 to 2.00. . . . 1.08 .00 .00 .00 



99.53 99.98 98.91 99.31 



1. Dustfall at Madison, Wis., 9 March, 1918, this Journal, 46, 

 p. 602, 1918. 



2. Sample 1 of dustfall at Madison, "Wis., 19 March, 1920. 

 Mechanical analysis of this and two next were made by Hazel 

 Hankinson in Dept. Soils, Univ. Wis. 



3. Sample 2 of dustfall at Madison, Wis., 19 March, 1920. 



4. Sample 3 of dustfall at Madison, Wis., 19 March, 1920. 



It is evident that the dustfall of 1920 is composed of 

 even finer particles than that of 1918. This is probably 

 due to the fact that the velocity of the wind east of the 

 Missouri River was less in the later storm, though the 

 recorded velocity west of that river was greater. We 

 have no explanation of the fact that the 1920 dustfall 

 contains about four times as great a tenor of very fine 

 sand as that of 1918; the "fine gravel" of the latter 

 consists largely of fragments of vegetation. In all these 

 samples nearly 95 per cent of the material is finer than 

 0.05 millimeter. Unfortunately we know of no mechan- 

 ical analyses of European dustfalls with which these may 

 be compared. It is reported merely that the commonest 

 size of the particles in the great dustfall 1 of March, 1901 



1 Hellmann and Meinardus, loc. cit. p. 63-65. 



