542 hawkes. GE0CHEM1CAL PROSPECTING FOR ORES [Ch. 30 



TABLE 1 



pH of Precipitation of Hydroxide or Basic Salt from Dilute Solutions 



(Britton, 1942, p. 79) 



gardless of the richness of the source of copper (Leach, 1947; Lover- 

 ing, Huff, and Almond, in preparation). A similar reaction takes 

 place in soils, where a high pH tends to reduce the mobility of zinc, 

 copper, and some other ore metals (Peech, 1941). 



Soluble material present in solution under some conditions has an 

 important effect on the mobility of a given element. Some anions form 

 slightly soluble compounds, such as silver chloride or lead sulphate, and 

 their presence reduces the mobility of those metals. Some anions form 

 soluble complex ions with certain metals and consequently tend to in- 

 crease the mobility of those metals. Furthermore, precipitation of 

 minerals such as manganese and iron hydrates tends to "scavenge" 

 many metallic ions and remove them from solution (Goldschmidt and 

 Peters, 1934; Goldschmidt and Hefter, 1934; Forrester, 1942; Vogt, 

 1942c; Landergren, 1948, p. 135). 



Cation exchange with organic or inorganic colloidal matter tends 

 to modify the mobility (Burd, 1947; Antipov-Karataev, 1947; Noll, 

 1931) . Organic matter tends to remove certain metallic ions from solu- 

 tion, although the exact mechanism is not well understood (Hibbard, 

 1940; Lucas, 1948; Bremner et al., 1946; Hasler, 1943; Fischer, 1937). 

 There is also a selective action by growing organisms, some of which 

 can take up certain elements and reject others, depending on their 

 metabolic requirements (Goldschmidt, 1937; Robinson and Edgington, 

 1945; Thyssen, 1942; Lovering, 1927; Riley, 1939). The oxidation 

 potential of the environment may influence the mobility of elements 

 having two or more common valence states (Chapman and Schweitzer, 

 1947; Murata, 1939). 



Mobility is thus not a simple thing. However, if we are to deal 

 intelligently with problems involving dispersion in the cycle of weath- 

 ering, it is a subject that we must explore further and understand more 

 fully. 



DIRECT BOTANICAL INDICATIONS 



Under certain conditions, plants respond in a specific and diagnostic 

 manner to variations in soil composition. Certain species known as 



