412 THE ICE AGE IN NORTH AMERICA. 



It is to President Chamberlin, again,* that we are in- 

 debted for the most careful study of this problem in the Mis- 

 sissippi Yalley. According to him, the loess is limited pre- 

 dominantly to the river valleys, and the belt is ordinarily not 

 over forty miles in width. As a rule, also, it is thicker and 

 slightly coarser in character near the banks of the great riv- 

 ers, and there shows some signs of stratification. 



A comparison of loess with sand and clay reveals some 

 interesting facts. The loess is intermediate in size and fine- 

 ness. When the particles are suspended in water, the loess 

 settles much more rapidly than clay — as much of the loess 

 settling in four hours as of the clay in thirty-six hours. Out 

 of 150,000 particles of loess examined under the microscope, 

 146,000, or about ninety- seven per cent, were less than -005 

 of a millimetre in diameter. A grain of sand one millimetre 

 in diameter is considered fine. But this would make 200,000 

 particles of loess of the size mentioned, and 100,000 particles 

 of the fineness of a large part of true clay. The size of the 

 largest particles of loess noted by President Chamberlin was 

 about one tenth of a millimetre in size, and consisted of scales 

 of mica. The loess of Yicksburg is a little finer than that of 

 Kansas City, and both a little finer than that from the Rhine. 

 The particles of loess were found to be angular and irregular. 

 "Sharp corners and rough surfaces are the rule, and any ap- 

 proach to regularity or smoothness is the exception." In 

 the vicinity of the great rivers the grains are coarser than at 

 points removed from them. In chemical composition the 

 difference between loess and true residual clays is not very 

 striking ; but, as compared with glacial clays, occurring in the 

 till, there is a marked difference in several respects. The gla- 

 cial clays have far less amount of silica and alumina, and a far 

 larger amount of calcic and magnesian oxides and of carbonic 

 dioxides. The glacial clays are evidently the result largely 

 of mechanical abrasion ; but the loess would seem to consist 

 in larger proportion of material resulting from chemical dis- 



* " Driftless Area of the Upper Mississippi Valley," pp. 278-30?. 



