(ii:XEI!AL CONCLUSIONS. 07 



coai'seuess, within tlie limits of the tran.sportiiig powei- of 

 the air, and the proportion of the different ingredients 

 will be determined by the position of the place of its 

 accumulation with regard to different areas of erosion. 

 Small areas of erosion are found almost everyAvhere, and 

 local material will therefore seldom be absent from any 

 wind deposit. Should places of erosion be numerous in 

 any particular region this may itself be regarded as the 

 windward angle in a great triangle with a great area of 

 deposition to the leeward. 



THE PROBLEM OF THE LOESS. 



It seems probable that the Western plains and the 

 Mississippi A^alley maintain the windward-leeward rela- 

 tion to each other. Dust which is stirred up over the 

 plains must be carried east by the prevailing winds, and 

 a part of it no doubt settles over the great central valley. 

 The loess and surface silts, which are spread over most of 

 the territory' in this valley, resemble atmospheric sed- 

 iments considerably in their mechanical composition. *) 



It is generally finer in the east and coarser in the west, 

 and it decreases in thickness from west to east. The ques- 

 tion whether it is, in the main, aqueous or eolian, cannot 

 be considered as yet settled. It seems doubtful if the 

 deposition at present exceeds erosion over all of this 

 area, but a very slight change in elevation or in climate 

 may lately have reversed the condition in this respect. 

 The question of changed conditions is a very complex one. 



*) See Report on the Examination of Some Soils from Illinois, by Milton Whitney 

 in the Report of the Illinois Board of World's Fair Commissioners; edso Prelim- 

 inarij Report of the Driftless Area of the Upper Mississippi Valley, by Chaniberlin 

 and Salisbury. 



