976 Subsurface Geologic Methods 



of vegetation, ash from plants, twigs, and leaves has been analyzed for 

 metal content. Some exacting work has been done along these lines, but, 

 because of the lack of sufficient data, the results are still inconclusive. Like 

 geochemical sampling, biogeochemical sampling shows great promise, and 

 more work is being carried out at the present time.^^ 



Stratigraphic Control — As a guide in layered rocks, stratigraphy 

 is important in that certain beds are more easily replaced by mineralizing 

 solutions and are better hosts than others. No clear and definite reason 

 can be given for the preference of epigenetic ores for one particular type 

 of rock or one particular formation, but certainly permeability, as re- 

 flected by relative brittleness, and composition, as reflected by chemical 

 reactiveness, are important. On a purely statistical basis, it can be demon- 

 strated that certain formations, as the Leadville in Colorado, the Home- 

 stake in South Dakota, and the Boone of the Tri-State region, are host 

 rocks for the great bulk of the ore in those areas. In other areas as Bisbee, 

 Arizona, much of the ore occurs in one formation but is not restricted to 

 that formation. In either case, if such facts are known, initial prospecting 

 certainly will be aimed toward the better host rocks, but until the district 

 is well explored it cannot be assumed that the ore is restricted to those cer- 

 tain horizons. Many exceptions to the foregoing statements show that all 

 favorable guides, of which stratigraphy is but one, must be examined by 

 the mining geologist in his search for ore. 



It is obvious that in sedimentary ore deposits, stratigraphy is of prime 

 importance, and any prospecting must be based on sedimentation studies. 

 The Tertiary gold-bearing channels of California, the potash deposits of 

 New Mexico, and the phosphate deposits of Idaho all are examples of 

 deposits of which the sedimentary features must be known before there 

 can be efficient prospecting. 



Geologic Mapping 



Surface geologic mapping, as carried out by the mining geologist, 

 diff'ers very little from surface mapping performed by other geologists. 

 Such necessary equipment as a Brunton compass, plane table, aneroid 

 barometer, and airplane photographs are employed by all geologists. The 

 principal difference between a mining problem and most other types of 

 geologic mapping is in the choice of scale. It is not at all uncommon in 

 mining geology to map the surface using a scale of one inch equals 100 

 feet. Detail is important, and it is only on such maps that detailed repre- 

 sentation is possible. When mapping the general relations of the district, 

 the geologist may use a scale as small as one inch equals 1,000 feet. 



Underground geologic mapping (see chap. 13) has been well des- 

 scribed,^^ ^^ and in principle is similar to surface mapping. The scale, of 



^^ Warren, H. V., and Delavault, R. E., Further Studies in Biogeochemistry: Geol. Soc. America, 

 vol. 60, no. 3, pp. 531-560, 1949. 



^' Forrester, J. D., Principles of Field and Mining Geology: pp. 331-348, New York, John Wiley and 

 Sons, 1948. 



^McKinstry, H. E., op. cit.. pp. 1-34, 1948. 



