PAPERS ON BOTANY . 101 



level bordering the streams, the latter being designated 

 "yellow-grey silt loam." Aside from differences of color 

 and water content it contains less of the finer wind blown 

 material and has a lower humus content. The excellent 

 maps accompanying the report of the Soil Survey, pub- 

 lished in 1913, show the larger streams bordered by a nar- 

 rower or broader band of this yellow-grey silt loam soil. 

 Two or three of the streams flowing through the townships 

 covered during the autumn of 1918 in our Forest Survey 

 may be taken as examples of this soil distribution. Along 

 the Little Vermilion in Troy Grove and Dimmick Town- 

 ships the bordering strip of yellow-grey silt loam varies in 

 width up to a maximum of 1500 yards. 



Along Big Indian Creek in Earle Township it reaches a 

 width of over a mile, while along Little Indian Creek in 

 Adams and Serena Townships at its widest part the 

 stream valley as indicated by the soil difference is two 

 miles across. 



For some reason not as yet clearly understood the strip 

 of yellow-grey silt loam is decidedly wider upon the east 

 side of these streams than upon the west. At times this 

 difference is not great but it frequently happens that three- 

 fourths of the entire strip is upon the left bank of the 

 creek. An examination of other north and south streams 

 shows that they possess the same soil fringe with a similar 

 unequal lateral distribution. 



It has been suggested that since the prevailing winds 

 are from the west the inequality of the soil strip must be 

 connected with the action of such winds in causing more 

 deposition of wind-carried soil on the west bank of the 

 stream or more wind erosion upon the east side. Another 

 attempted explanation is connected with the movement of 

 prairie fires from west to east but the causes of the distri- 

 bution of this soil in this peculiar way is not of immediate 

 importance in the present discussion. It is sufficient to 

 point out that the soil has been recognized as essentially 

 different, in some particulars, from the upland brown silt 

 loam and its extent has been mapped in La Salle and 

 other counties in the published reports of soil surveys. 



In making our forest survey of portions of La Salle 

 County it was soon noted that with the exception of a very 

 narrow fringe of such trees as black willow along the small 



