958 EXPLORATION GEOPHYSICS 



outlining the dispersion halos as determinations of the metals themselves. 

 In dealing with ores containing several metals, the choice of the indicator 

 metal is determined entirely by environment and the judgment of the 

 prospector. 



The sampling procedure foUov^^ed also depends upon the particular 

 problem and environment. The Russian geochemists usually have sampled 

 the soil on a close grid pattern and at fixed intervals in depth. 



A large number of testing procedures adapted for field use are known. 

 They include color reactions between traces of metals and organic dyes, 

 spectrographic analyses, electric potential measurements, flame tests, and 

 others. In general, the color tests appear to be the most rapid and satis- 

 factory. Since prospecting in the field requires a very large number of 

 tests, rapidity and qualitative sensitivity of the test are preferred to quanti- 

 tative accuracy. In outlining the dispersion halos, determination of the 

 relative amounts of the indicator metal in the samples is sufficient, and 

 the results may be expressed as "high," "medium," and "low" rather than 

 by actual numbers. By following this procedure, from 40 to 200 tests per 

 day can be made by a single chemist, whereas a quantitative analysis of the 

 same number of samples in the laboratory would require weeks or months. 



A map showing chemical anomalies, plotted with reasonable accuracy, 

 is the final result of any geochemical prospecting job. Interpretation of 

 such a map is rarely possible without correlation with the geologic features. 

 However, it is conceivable that in areas of thick residual soils, geochemical 

 prospecting alone may locate ores at much lower cost than other prospecting 

 methods. 



Plants. — The interest in plants, in connection with ore prospecting, 

 has been motivated chiefly by three observations. (1) Certain species of 

 plants have a specific requirement for certain metals and may not develop 

 in the absence of these metals. Such species are regarded as indicator plants. 

 The so-called "zinc flora," for example, is a group of species, chiefly grasses 

 and sedges that develop best on soils high in zinc. (2) By means of their 

 roots, plants sample a large mass of soil, and some of them concentrate 

 certain metals, chiefly in their tissues. Thus unusually high concentrations 

 of metals in the soil, water, and rock in which the plants grow may be 

 expressed by a high concentration of those metals in the plant material. 

 Zinc, nickel, copper, and cobalt are stored chiefly in the plant leaves. Lead, 

 arsenic, and some others, on the other hand, are believed to be concentrated 

 chiefly in the roots. Once such accumulator plants have been recognized, 

 the presence and even the boundaries of a geochemical anomaly may be 

 established by analysis under favorable conditions. (3) The presence of 

 unusually high amounts of certain metals in the soil, water, or rock may 

 cause injuries or abnormalities in the plants, although the plants them- 

 selves may be neither indicators nor accumulators of the element in ques- 



