The Dustfall of March 9, 1918. 603 



much, if not all of the four largest sizes consist of organic 

 material, and the very fine sand includes both organic 

 and inorganic matter. 



The organic constituents were so obviously plant tis- 

 sue that they were submitted to Professor R. H. Dennis- 

 ton of the Department of Botany, who reports that they 

 include fragments of blades of grass, of leaves of clover 

 or some similar legume, fibers of cotton, and fragments 

 of coniferous wood, all more or less decayed, as shown by 

 the presence of saprophytic fungi and their spores. The 

 only inorganic material in the so-called "gravel" con- 

 sists of white particles which effervesce with acetic acid ; 

 it is therefore a carbonate. 



An attempt was made to separate the constituents of 

 the dust by means of a heavy solution of potassium mer- 

 curic iodide. Most of the material sank in a liquid of 

 specific gravity of 2-3, but the material still floating con- 

 tained the same materials as the part which sank. A 

 portion of the dust separated by mechanical analysis to 

 the size 0-010 to 0-025 mm. was tested in the same way. 

 Practically all of it floated at 2-7, less than one quarter 

 of it sank at 2-6 ; again the two parts contained the same 

 materials; the heavier seemed to contain less limonite 

 stain than the other. It seems probable that submicro- 

 scopic porosity modifies considerably the apparent speci- 

 fic gravity of much of the dust, 



It will be useful to compare the results of the mechan- 

 ical analysis of this dust with similar analyses of soils, 

 volcanic dust, atmospheric dust, as shown in the following 

 table (p. 604). 



This comparison shows that the Madison dust has two 

 peculiarities, namely, it is finer than the other dusts and 

 it contains a large percentage within a small range of 

 sizes. Some soils contain much larger amounts of clayey 

 material (smaller than -005 mm.) than the Madison 

 material, but a hasty search of the literature makes it 

 clear that few, if any, soils contain as much silt; on the 

 other hand shower and volcanic dust contains much less 

 clay " than the Madison dust. This may be explained as 

 due to the fact that shower and volcanic dusts fall wholly 

 through the action of gravity, while the Madison dust 

 was brought down not by its own weight, but by the 

 weight of the snow or rain condensed upon it. 



