160 THE ILLINOIS GLACIAL LOBE. 



C( mtains a greater percentage of fine particles than the soil. An interesting 

 contrast between loess and residuary earths is thus indicated. As noted 

 above, the surface portion of the loess probably owes its finer constitution 

 in part to atmospheric action, through a breaking "up of the particles. It 

 is probable also that dust transported by the wind, and very fine material, 

 such as is constantly floating in the atmosphere, tend to increase the 

 percentag-e of fine material at the surface. There may also have been an 

 original difference in the coarseness, for there is generally found to be ar. 

 increase in porosity toward the base of the loess. 



In the table of wind-deposited soils two analyses of fine sand are 

 included, one sample obtained at Mason City on the east border of a sand- 

 covered basin along the Illinois, the other from a belt of sand which fringes 

 the Sangamon River bluffs in northern Christian County. The entire 

 absence of coarse sand in these deposits distinguishes them from water- 

 bedded sand on the terraces of neighboring streams. Analysis No. 1338, 

 from an Illinois River terrace where stream action was operative, shows 

 nearly 5 per cent of coarse sand. 



The final tabic presents analyses of a variety of soils. Two of these 

 (the gumbo from Mississippi bottoms near East St. Louis and the soil from 

 the old lake bottom near Chicago) contain a greater percentage of clay 

 than is found either in the compact loess or the bowldery clay. All other 

 analyses of water-deposited silts and sands from Illinois indicate a partial 

 removal of the clay or fine material, but in these samples there appears to 

 have been an accession or concentration of this material. The sample from 

 Rockford represents a thin deposit of silt capping the Iowan till sheet. It 

 contains coarser particles than are found in the loess outside the border of 

 the Iowan drift. 



In comparing the measurements given in these tables with those made 

 by Salisbury it should be borne in mind that Salisbury's measurements 

 refer to number of grains, while Whitney's refer to the bulk of the material. 

 The percentage in number of grains in Whitney's tables may be readily 

 estimated from the data presented in the column showing the number of 

 grains per gram. Thus at Virginia City the subsoil analysis (No. 1318) 

 shows about 96 per cent, and that at Dubuque (No. 1347) shows 97.7 per 

 cent below 0.005 millimeter in diameter. As the average of Salisbury's 

 analvses show that about 97^ per cent fall below 0.005 millimeter in 

 diameter, the results obtained are very similar to those obtained bv Whitney. 



