Ch. 30] SAMPLING FOR CHEMICAL ANALYSIS 543 



"indicator" plants (Beath et at., 1939) grow only on soil containing 

 large concentrations of a particular element. In Germany it is re- 

 ported that a variety of violet grows only on zinc-rich soil (Jensch, 

 1894) and has been useful in locating zinc ore in the underlying bed- 

 rock. Indicator plants have been mentioned for many other metals 

 and minerals (Dorn, 1937; Lidgey, 1897; Thyssen, 1942; Rickard, 

 1926; White, 1929; Monigatti et at., 1947), though it is difficult to 

 confirm some of the reported observations. 



More frequently, the effect of high concentrations of certain metals 

 in the soil is to modify the entire plant assemblage (Vogt, 1942a, b) . 

 For example, the plant ecology over the outcrops of the San Manuel 

 copper deposit in Arizona, where the copper content of the soil is very 

 high, is quite different from that on the adjoining normal soil (Lover- 

 ing, Huff, and Almond, in preparation). A more commonly observed 

 phenomenon is the relative infertility of soils either because of their 

 acidity or their metal content (Robinson, Edgington, and Byers, 1935 ; 

 Bateman, 1930; Guillemain, 1913; Vogt and Braadlie, 1942; Bell, 

 1931). 



Another botanical indication of metals is the development of char- 

 acteristic plant symptoms due to deficiencies or excesses of metals in 

 the soil. Agricultural scientists have been particularly concerned with 

 mineral deficiencies not only of major mineral constituents but also 

 of many elements normally present in the soil only in very small 

 quantities. Insufficient copper, zinc, molybdenum, boron, and cobalt 

 in the soil may cause serious deficiency diseases either in the vegeta- 

 tion or in the animals feeding on that vegetation (Brenchley, 1947; see 

 also references listed in Willis, 1948). Toxicity symptoms in plants 

 and animals due to excesses of certain elements such as zinc, copper, 

 cobalt, and nickel have also been reported (Brenchley, 1927; Robinson 

 and Edgington, 1948 ; Jensch, 1894 ; Vogt and Bugge, 1943 ; Piper, 1942 ; 

 Staker, 1942; Bergh, 1948; Hewitt, 1948; Millikan, 1948). Under 

 favorable conditions, systematic searching for such diagnostic plant 

 symptoms may be useful in prospecting. 



Such direct botanical indications of ore are especially worth in- 

 vestigating as they offer a reconnaissance method of scouting without 

 the immediate necessity of careful sampling and chemical analysis. 



SAMPLING FOR CHEMICAL ANALYSIS 



The delineation of a geochemical indication of ore in any type of 

 surface material involves a contrast between the anomalous metal con- 

 centrations on one hand and the normal or background concentra- 



