544 hawkes. GEOCHEMICAL PROSPECTING FOR ORES [Ch. 30 



tions on the other. The background varies widely from one locality 

 to another with local geology, climate, drainage conditions, and many 

 other factors and can best be determined separately for each project. 

 For general reference, however, the literature contains useful com- 

 pilations of the average metal content of rocks (Goldschmidt, 1937), 

 living organisms (Vinogradov, 1935) , and surface water (Braidech and 

 Emery, 1935). No comprehensive estimate of the average composi- 

 tion of soils has been made, although abundant data have been gath- 

 ered on individual elements (Holmes, 1943; Mitchell, 1944; Siniakova, 

 1945; Jacks and Scherbatoff, 1940; Maliuga, 1944; Robinson, 1914; 

 Slater et al, 1937). 



Gossans. Information on the mineralogy and grade of underlying 

 sulphide ore can under some conditions be gained from mineralogical 

 studies of surface material (Locke, 1926; Blanchard, 1930, 1931, 1942; 

 Blanchard and Boswell, 1925, 1934, 1935; Boswell and Blanchard, 1927, 

 1929) . Chemical analysis for traces of ore metals has apparently not 

 been applied to gossan study, and it is impossible to discuss sampling 

 procedures at present. 



Soils. The term "soil" as used by agriculturists refers to surface 

 material that has been modified chemically and physically by bacterio- 

 logical and other organic activity, and that can, as a result, support 

 growing vegetation. It is commonly layered, with an upper "horizon" 

 of leaching (the A horizon or topsoil), an intermediate zone of con- 

 centration (the B horizon or subsoil), and the parent material (the C 

 horizon, usually weathered rock, alluvium, or glacial material). De- 

 pending on local conditions any one of the three soil horizons may give 

 optimum results in sampling for metals. Some metals are taken up in 

 solution through the circulatory system of trees, where they are con- 

 centrated in the leaves ; when the leaves fall to the ground and rot, the 

 metals remain near the surface in the organic part of the A horizon 

 (Goldschmidt, 1937; Vogt and Bergh, 1946, 1947; Sokoloff, 1948). In 

 the absence of forest trees, the metals may be leached from the topsoil, 

 and it becomes necessary to collect deeper samples from the B or C 

 horizon in the soil (Hawkes and Lakin, 1949; Sergeev, 1941, Fig. 8). 

 Each problem should be treated as a special case, with a preliminary 

 investigation of the optimum depth of sampling before the routine work 

 is started. The optimum size of sample and the mechanical fraction 

 containing the most diagnostic variation in metal content should also 

 be determined by preliminary experiments for each field problem. 



Alkaline soils tend to be richer in copper and zinc than acid soils 

 (Holmes, 1943) , whereas the reverse is apparently true for molybdenum 

 (Lewis, 1933). Thus in areas where the soil acidity varies from place 



