88 IOWA ACADEMY OF SCIENCES. 



only of loess-like soil is fouad/'^ Quite important too is the argu- 

 ment furnished by the physical properties of the loess mate- 

 rial. This in eastern Iowa is always very easily eroded, so 

 much so that upon cleared hillsides it is often impossible even 

 for bluegrass to gain a foothold, and failure has been the uni- 

 versal result of all attempts to cultivate such slopes. This 

 being the case it seems hardly probable that trees, which 

 require more time to become established than do smaller 

 plants, could have gained a foothold upon these unstable hill- 

 tops had they been formed. The organic matter which 

 undoubtedly accumulated in these forests gradually decayed, 

 mingled with the alluvium brought by the winds, and was 

 finally consumed in leaching iron oxides from the lower strata 

 of deposit. 



Other, smaller, vegetation no doubt effected the deposition 

 of fine alluvium in the same manner, but to a lesser degree, and 

 by the aid of this probably were formed the thin layers of loess 

 which sometimes occur in prairie country. 



The element of time still remains to be considered. Without 

 an attempt at exact computations, attention is simply called to 

 the fact that in eastern Iowa the loess in no place exceeds fifty 

 feet in thickness, the average being probably about ten or twelve 

 feet, and that if we assume, for example, the deposition of a 

 minimum of one mm. a year, the time required for the forma- 

 tion of the entire deposit would not be unreasonably great. 



The deposition of loess material is no doubt going on in this 

 manner to-day, and the investigation of this phase of the sub- 

 ject is worthy the attention of the most careful observers. The 

 foregoing statements apply particularly to the loess of east- 

 ern Iowa. In the western part of the state and in eastern 

 Nebraska much thicker deposits occur, which differ in many 

 respects from the loess of eastern Iowa. 



The western loess is thicker, coarser, with more siliceous 

 material, and the writer has found it more frequently inter- 

 laminated with sand. Tiiat it is much less easily eroded because 

 of this difference in composition is a well known fact. 



From the general topographical and climatic relations which 

 exist between the eastern and western regions to day, it is prob- 

 able that during the loess period, as now, the western region 

 was drier (a fact also attested by the rather greater abundance 

 of dry- region molluscs in its loess), and that strong winds were 



16A further investigation of the soils in prairie groves of tliis kind is contemplated 

 during the coming summer. 



