4S IOWA ACADEMY OF SCIENCES. 



of quartz. These are cemented by carbonates of lime and iron 

 and chinked more or less with clay. It stands like a rock if it 

 is kept dry throughout. 



2. It is on the other hand very plastic when wet to a cer- 

 tain degree. Water, particularly if charged with a little car- 

 bonic acid, dissolves the cement, and the clay serves as a lubri- 

 cant to the rounded quartz grains. We have only to notice the 

 behavior of it when thrown from a well, or to mix a little of it 

 with water to be impressed with this fact. 



This property is further exhibited in roads passing through 

 cuts in loess, and in the rapid wash from hills and hillsides 

 after a continued rainy season. Two and three feet of sediment 

 have been deposited on the flood plains of adjacent streams 

 after a single flood. 



3. The porosity of loess and loams generally is marked. 

 Water is quickly absorbed in any direction, by capillary action. 

 This has been often noted in it as a subsoil. It affords admi- 

 rable under drainage and on the other hand furnishes moisture 

 from below in time of drought. 



This character tends to promote plasticity and to render that 

 character more general. By promoting absorption it decreases 

 much the surface erosion. 



4. The easy and perfect recementation or "setting" of 

 loess after being wet, or the sudden change from plasticity to 

 rigidity. 



When water mixes with loess as sometimes on a side hill 

 after soaking rains, or in sudden rainfall, it flows down, cover- 

 ing the surface below, and accumulating as a talus, and as soon 

 as the water has soaked out of it, it is as firm and solid as the 

 original loess. It may be almost impossible to show that it is 

 a secondary formation except by inference from its relations, 

 unless there be some fragment of plant, or shell, or position of 

 concretions, or distribution of color to reveal the fact. 



5. The vertical cleavage or column structure of the loess is 

 a well recognized featui'e which has an important bearing on 

 our subject Several things, probably aid in producing this. 

 The lateral shrinking in drying, the prevalent vertical direction 

 of the roots of plants, and the formation of light faults by the 

 unequal settling of different portions because of the plasticity 

 of its lower portions, or of underlying clays or sands, are some 

 of the more important. 



As illustrations of this property we may refer to the way in 



