IOWA ACADEMY OP SCIENCES. 47 



b. There is no trace of beach deposits, either of ridges or 

 shelves. 



c. There is serious difficulty in accounting for the observed 

 distribution of material, so uniform in thickness and character, 

 in a currentless body of water. And if currents are postu- 

 lated they would have tended to follow deeper valleys and 

 would have differentiated the loess more than we find to be the 

 case 



2. The ^olian hypothesis, first suggested by Richthofen 

 for the loess deposits of China 20 years ago, has had some dis- 

 tinguished advocates who have accepted it for similar forma- 

 tions elsewhere. 



More commonly, however, it has been held as a supplemental 

 theory. 



The following objections lie against its general application: 



a. It does not accord with the distribution of the loess in 

 general, while it may explain its occurence at diiferent levels, 

 it does not explain its common occurrence on windward slopes, 

 nor its greater thickness near streams on both sides alike. 



b. It can not be harmonized with the frequent occurrence of 

 coarse material in the loess, in some localities. 



c. It does not explain the horizontal banding, and the flat 

 areas on the same level which are frequently exhibited by the 

 loess. 



3. We may notice also an Aqueo-asolian theory which sup- 

 poses, that streams were flowing at lower levels, sluggish and 

 varying much in volume at different seasons, so that broad bars 

 of fine material contributed dust and fine sand to the winds 

 when the water was low, which was borne up and spread over 

 the adjacent high lands. 



There is little doubt that such a condition has existed often 

 ^nd has been somewhat efficient in many localities, but it is cer- 

 tainly inadequate for our main purpose. The objections against 

 the preceding theory would be ai^plicable here also, especially 

 the extensive flat areas lying at nearly the same altitudes men- 

 tioned under c. 



Before presenting our remaining theory we will call attention 

 to certain facts connected with loess and loams generally. 

 These facts relate to the properties of loess itself, and to its 

 ■erosion as sometimes displayed. We must also consider under- 

 lying formations so far as they affect the problem. 



1. The rigidity of loess. It consists chiefly of grains 



