356 



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



square mile. T. E. Thorpe, Nature, 68, p. 222, 1903. 9.89% of 

 organic carbon and 0.32% of CoO included in total. 



The dustfall at Madison is the most siliceous of these 

 analyses, but an incomplete analysis of a dust which fell 

 in Tunis 6 in 1901 revealed 70.95%? of silica, and the tenor 

 of silica in siroccan dust from the Sahara reaches a 

 maximum of 73.45 %, 7 calculated on a water-free basis. 



The dust from the Sahara is always red and produces 

 the "red rain" or "rain of blood" sometimes noted in 

 Europe. This is due to the thoroughly oxidized state of 

 the iron which is unlike the condition found in the Madi- 

 son dust. In the percentage of alkalies and alumina the 

 latter does not differ markedly from the samples from 

 the Sahara, but it contains far less lime and very little 

 carbonic acid as carbonate, in contrast with the Saharan 

 dust. In these particulars the Madison dust closely 

 resembles that fallen at Otakaia, New Zealand, which 

 was derived from the continent of Australia. 



In some respects the Madison dustfall resembles, in 

 chemical composition, the loess of the Mississippi valley 

 more closely than it does the dust from the African 

 desert. This is shown in the following table. 



Table III. Chemical composition of Mississippi valley loess 



compared with the average composition of the 



Madison dustfall. 





1 



2 



3 



4 



5 



6 



Si0 2 ... 



64.61 



67.20 



70.11 



70.86 



72.68 



74.46 



A1 2 3 .. 



10.64 



13.71 



14.25 



8.91 



12.03 



12.26 



Fe 2 3 .. 



2.61 





4.02 



2.97 



3.53 



3.25 



FeO ... 



.51 



2.17 





.10 



.96 



.12 



MgO ... 



3.69 



1.76 



1.32 



3.12 



1.11 



1.12 



CaO .... 



5.41 



1.74 



1.53 



4.13 



1.59 



1.69 



Na 2 . . . 



1.35 



2.11 



1.09 



1.69 



1.68 



1.43 



K o .... 



2.06 



2.30 



2.03 



1.18 



2.13 



1.83 



H,0"+ . 



2.05 



3.22 



2.48 



1.10 



2.50 



2.70 



C0 9 .... 



6.31 







4.70 



.39 



.49 



Ti0 2 ... 



.40 



.52 





.59 



.72 



.14 



P 2 5 ... 



.06 



.15 





.40 



.23 



.09 



MnO ... 



.05 



.38 





.28 



.06 



.02 



Total 



100.06 101.28 100.33 



99.98 100.22 



99.83 



6 E. Bertainchand, Compt. Send., 132, p. 1153, 1901. According to 

 L. Cayeux this dust contained quartz, staurolite, tourmaline, rutile, zircon, 

 magnetite, yellow phosphate, and diatoms. 



7 E. E. Free, 77. S. Bur. Soils, Bull. 68, p. 95, 1911. 



