108 Kansas Academy of Science. 



obscure or inconspicuous. (5) It commonly contains unoxi- 

 dized carbonate of lime in such quantities as to effervesce 

 freely under acids, (c) It frequently contains nodules and 

 minute ramifying tubules of carbonate of lime, {d) In many 

 regions it contains abundant shells of land and fresh-water 

 mollusca. (e) It is commonly so friable that it may be re- 

 moved with a spade or impressed with the fingers; yet it re- 

 sists weathering and erosion in a remarkable manner, second 

 only to the more obdurate clastic or crystalline rocks. 



In the north of Iowa the loess grades either into water-laid 

 gravel-beds or into stratified sand, and in the middle latitudes 

 it commonly passes into stratified and evidently water-laid 

 sand. At the south it grades either into a peculiarly assorted 

 but variable glacial deposit evidently modified by contempora- 

 neous aqueous action, or into water-laid sand. So the strat- 

 igraphic relations, apart from the structural features, ally 

 the formation with water-laid deposits and indicate a certain 

 community of origin between its finely comminuted materials 

 and the coarser aqueo-glacial materials of its base. It is the 

 finer grist of the ice-mill laid down in ice-bound lakes and 

 gorges as the Pleistocene glacier shrunk by surface melting 

 and retreated northward. 



Remarks on the Origin of the Loess. — It seems evident that 

 a large part of the loess is seolian in formation ; but further in- 

 vestigations and the discovery of criteria by which it can be 

 distinguished from the aqueous type are yet needed to de- 

 termine how much. It is quite probable that the seolian and 

 aqueous agencies worked simultaneously in the same region, 

 and this accounts for the geological tangle. As the glacier de- 

 parted the plains of America it must have left a barren strip 

 in its rear many miles in width, an area at its final retreat 

 probably half as large as the United States. In this barren 

 waste the agencies of water and wind vied with each other 

 in moving the lighter material left by the ice-sheet. The 

 streams deposited their sediments in the slackened-water re- 

 gions, while the wind drifted its deposits everywhere, mixing 

 with the aqueous formation wherever it came in contact with 

 it, otherwise depositing it on the leeward side of obstructions 

 and in the grassy regions to the south, which were gradually 

 advancing northward as the glacier retreated. 



