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



the microscope. Most of the material is cloudy and 

 isotropic. 



It is possible to calculate the approximate mineral com- 

 position from the gross chemical composition by making 

 certain assumptions similar to those made in calculating 

 the mineral composition of igneous rocks. Such calcula- 

 tions are greatly facilitated by the use of the circular slide 

 rule for minerals devised by W. J. Mead. 8 In this way 

 we have computed the mineral compositions of the Mad- 

 ison dustfall as well as that of European dustfalls and 

 of Mississippi Valley loess. In these calculations, it is 

 assumed that all the soda is in albite feldspar, all the 

 potassium is in orthoclase feldspar, all the phosphoric 

 acid is in apatite, all the titanic acid is in ilmenite, all the 

 magnesia is in chlorite (if there is sufficient alumina — 

 otherwise in enstatite), all the lime remaining after form- 

 ing apatite and calcite is in anorthite, all the alumina 

 remaining after forming feldspars and chlorite- is in 

 kaolinite and all the silica remaining after forming these 

 silicates is in quartz. It must, of course, be admitted that 

 these assumptions are not all true, but they are approxi- 

 mations and represent possible, even if not actual, com- 

 binations of the oxides into minerals. They furnish a 

 useful means of comparing analyses, especially when the 

 underlying assumptions are controlled as far as possible 

 by microscopic study of the material. The results of 

 these computations are given in the following table so far 

 as they relate to dustfalls. 



Table IV. Calculated mineral composition of Madison and 

 European dustfalls. 

 1 234 5 6 78 9 



Quartz.... 39.91 32.82 36.87 36.47 18.36 13.34 5.75 17.32 



Albite 13.87 25.80 13.87 17.89 14.21 9.82 22.14 14.04 21.98 



Orthoclase. 13.16 12.52 15.21 13.69 15.32 13.64 12.28 9.38 16.15 

 Anorthite . 8.11 8.06 6.86 7.64 4.73 4.38 4.44 16.17 7.75 



Kaolinite . 10.09 3.59 10.09 7.94 26.20 28.85 '25.22 13.01 



Chlorite . . 8.92 9.04 9.83 9.26 4.20 5.13 8.66 6.11 



Enstatite 2.30 



Calcite 13.86 7.85 19.17 8.36 



Apatite... .33 .37 .37 .35 46 



Anhydrite 2.33 



Hematite 6.54 6.57 7.03 9.19 6.40 



Ilmenite . . 1.01 1.01 .99 1.01 



Water, etc. 6.00 7.99 7.13 7.03 10.42 4.41 4.41 11.43 20.89 



Total 101.40 101.20 101.22 101.28 99.98 100.00 100.57 99.00 100.65 



6 Economic Geology, 7, p. 136, 1912. 



