A. N. Winchell— Great Dust fall of 1920. 353 



From the foregoing facts, we infer that the dust was 

 mainly derived from the region of intensest wind, in 

 northeastern Colorado and southeastern Wyoming, and 

 that the supply of soil material in the air was augmented 

 by smaller contributions from most of the southwestern 

 states. 



Distribution of the Dustfall. — The samples of dust that 

 have been sent us, and the reports of observation of dust, 

 are from far too few places to enable us to attempt to 

 map the distribution of dust, as was done by Hellmann 

 and Meinardus 3 and by Mill and Lempf ert 4 for the Euro- 

 pean dustfalls of 1901 and 1903. 



The rain area of the storm of March 15-21, 1920, shown 

 in fig. 1, indicates that most of the eastern half of the 

 United States was a region of possible precipitation of 

 dust by washing down with rain or snow. It is interest- 

 ing to note that the region of intensest blowing between 

 the Rocky Mountains and the Missouri valley remained 

 unwetted during the storm. 



Chemical Composition. — In the chemical study of the 

 samples of dust we have had the help of Dr. E. J. Graul, 

 of the Department of Soils, University of Wisconsin, who 

 determined the nitrogen, phosphorus pentoxide, the alka- 

 lies, and the water lost at 105°. 



A grant of funds for additional chemical work was 

 made from the research fund by the Regents of the 

 University of Wisconsin. The analyses under the grant 

 were made by Mr. Martin Tosterud, working under the 

 direction of Professor A. R. Whitson, in the laboratories 

 of the Department of Soils of the University of Wis- 

 consin. Mr. Tosterud determined silica, alumina, iron, 

 magnesia, lime, titanic acid, and water lost on ignition 

 in three samples. By the additional determination of 

 the total loss on ignition we are enabled to present the 

 following relatively complete analyses of three samples. 



In these analyses it was not practicable to determine 

 the state of oxidation of the iron; total iron was deter- 

 mined and calculated to be ferrous oxide since the dust 

 in mass is gray and not colored red or yellow by ferric 

 iron. But it is recognized that some ferric iron is 

 present, since its colors are distinct under the microscope. 



3 Der grosse Staubfall, Abh. k. Preuss. Meteorl. Inst., II, 1901. 



4 The great dustfall of February, 1903, Quart. Jour. Koy. Met. Soc, 30, 

 p. 57, 1904. 



