Ch. 1] CHEMICAL ACTION 15 



ment or mass of earth material is likely to be weaker than it was be- 

 fore the slide took place. Landslides commonly leave characteristic 

 scars upon the hills, showing the places from which they slide, and the 

 surface of the landslide deposits is hummocky. 



Factors that affect landslides are the slope of the land, the strength 

 of the earth materials, and the load that exists or is imposed. Loads 

 can be imposed by adding material to the upper parts or by under- 

 cutting the lower parts. The type of clay minerals and the amount 

 of water in the soil materially affect the strength of the soil. 



Volcanism 



Volcanic sediments consist mainly of layers of wind-blown ash. As 

 eruptions of ash come at irregular intervals and as the distance to 

 which the ash is transported varies with the wind, successive layers 

 of ash vary in grain size. They also vary in chemical or mineral 

 composition if the source of the volcanic material changes. Ash de- 

 posits commonly are interspersed with lava flows. Most deposits of 

 volcanic ash contain bombs or fragments of rock and lava blown from 

 the volcano. When these bombs land on newly deposited ash they 

 bury themself in the ground, in many places more than one or two feet, 

 and thus deform the layers upon which they land. The presence of 

 these buried bombs leads to irregular consolidation when the overlying 

 soil is loaded. Furthermore, volcanic ash commonly gives rise to 

 montmorillonite clay, which has a great affinity for water and pro- 

 duces relatively weak deposits when wet. 



Organisms 



Sediments of biologic origin are essentially a product of environ- 

 ment, because living organisms are dependent upon environment for 

 existence. Furthermore, since deposits consisting chiefly of organic 

 material can form only when the rate of production of organic material 

 is significantly greater than the influx of inorganic material, deposits 

 of organic origin, such as coal, diatomite, oil shale, bog iron ore, or 

 algal limestone, require special conditions of formation (Bradley, 1931 ; 

 Bramlette, 1946; Cady, 1942; Harder, 1919). Deposits rich in or- 

 ganic matter commonly are relatively rich in radioactive material 

 (Beers and Goodman, 1944). 



CHEMICAL ACTION 



Chemical action is essentially a question of degree of saturation. If 

 water is undersaturated it can dissolve material; if it is oversaturated 



