16 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



it can precipitate material. The principal factors are ionic concentra- 

 tion, temperature, oxidation-reduction potential, amount of water, mi- 

 crobial action, types of earth materials in contact with the water, and 

 rate at which conditions change. Pressure usually is not a critical 

 factor. Climate is a dominant influence because of its effect upon 

 temperature, rainfall, and microbial action. Variations in moisture 

 and temperature throughout the year or from one year to another 

 strongly influence chemical reactions because of their effects upon 

 equilibrium conditions and the degree of saturation of dissolved sub- 

 stances. 



Evaporation changes the concentration of dissolved salts and thus 

 changes equilibrium relationships. The distribution of water in the 

 ground affects the oxidation-reduction potential. Earth materials 

 lying above the water table tend to be more oxidized than correspond- 

 ing materials lying below the water table. Changes in water level thus 

 can significantly affect chemical reactions in the earth. Rate of flow 

 influences the degree of saturation. Slow movement of water through 

 pore spaces favors the development of a relatively high degree of 

 saturation of dissolved substances, but, if the rate is so slow that the 

 water becomes saturated, no further material can be dissolved. This 

 feature has been brought out by Davis (1930) and others in respect to 

 the development of cavities in limestone. 



The concentration of the different ions, particularly the hydrogen- 

 ion concentration, is an important factor. The presence of even a 

 small amount of some minor element may significantly affect the solu- 

 bility relations of other substances. Boron, for example, materially 

 affects solubility of calcium carbonate in sea water. The solution of 

 many substances is aided by the activity of microorganisms or soil- 

 forming processes (Waksman, 1936; Jenny, 1941; Zobell, 1946). Mi- 

 crobial action is favored by warm and humid climate. 



Parent earth materials vary greatly in their susceptibility to solu- 

 tion. Limestone is readily dissolved by acid solutions and volcanic 

 ash by basic or alkaline solutions. Salt, gypsum, nitrate, and other 

 products of chemical evaporation are dissolved by moving water of 

 almost any character. 



Chemical deposition is essentially a question of environment. The 

 chief requirement is the development of a condition of oversaturation. 

 The temperature, pressure, or concentration of dissolved substances 

 must change before material can be precipitated. If the solutions are 

 supersaturated, foreign material sometimes needs to be added to start 

 the precipitating action. Evaporation is the chief cause of changes 



