A. N. WincheU— Great Dustfall of 1920. 357 



1. Loess from a stratum overlying residual clay, 350 feet 

 above the Mississippi River, near Galena, Illinois. Chamberlin 

 and Salisbury, 6th Ann. Rep. U. S. Geol. Survey, 1885, p. 282. 

 Dried at 100° C. 0.13% of organic carbon, 0.11% S0 3 and 

 0.07% CI included in total. 



2. Average composition of Madison dustfall, March 19, 1920. 

 5.62% ignition loss and 0.39% N. included in total. 



3. Loess from Kansas City, Mo. W. E. McCourt, Missouri 

 Bureau Geol. & Mines, vol. 14,' p. 91, 1917. 3.50% ignition loss 

 included in total. 



4. Loess from depth of 8 feet at brickyard at Mt. Vernon, la. 

 N. Knight, Am. Geol. 29, p. 189, 1902, 



5. Loess from 300 feet above the Mississippi River, 3!/2 miles 

 north of Dubuque, la. Chamberlin and Salisbury. 6th Ann. 

 Rep. U. S. Geol. Survey, 1885, p. 282. Dried at 100° C. 0.51% 

 S0 3 , 0.09% organic carbon and 0.01% CI included in total. 



6. Loess from Kansas Citv, Mo. Chamberlin and Salisbury : 

 6th Ann. Rep. U. S. Geol. Survey, 1885, p. 282. Dried at 100° C. 

 0.12% organic carbon, 0.06% S0 3 and 0.05% CI included in 

 total. 



Except as to the state of oxidation of the iron the 

 Madison dust is closely similar to the loess from Kansas 

 City studied by McCourt; it is also much like the loess 

 from Dubuque and that from Kansas City examined by 

 Chamberlin and Salisbury; it differs from the loess of 

 Galena, Illinois, and that from Mt. Vernon, Iowa, in the 

 scarcity of carbonates of calcium and magnesium. 



Mineral Composition. — The components of the dust are 

 so extremely fine grained that it is quite difficult to make 

 satisfactory determinations of the mineral constituents 

 with the microscope and it seems to be wholly imprac- 

 ticable to attempt any quantitative determinations micro- 

 scopically because so large a proportion of the material 

 is too small for identification. However, samples from 

 Madison contain abundant quartz grains in tiny angular 

 forms and no other recognizable mineral in any important 

 amount. Feldspar may be present, but if so twinning 

 crossing the tiny particles is rare ; biotite flakes are very 

 uncommon; one shows an optic angle of about 40° (2E) 

 and pleochroic colors with Z = brown and Y = yellow. 

 Very rare fragments of calcite are also present. Still 

 more rarely a fragment of microcline, a light-colored 

 amphibole and a mineral resembling staurolite may be 

 detected. In spite of the gray color in mass, the red and 

 yellow colors of hematite and limonite are common under 



