Subsurface Methods as Applied in Mining Geology 977 



course, differs. Depending on the complexity of the deposit, the scales 

 most commonly used are 20, 40, or 50 feet to the inch. An underground 

 geologic map also differs from a surface map in that either a waist-high or 

 breast-high horizontal mapping plane is assumed, and all data are pro- 

 jected to that plane. The mapping height varies with the practice of dif- 

 ferent geologists, but, if the plane is consistent and noted, either is accept- 

 able. Under certain conditions, the mapping level may be the back or top 

 of the mine working. Underground mapping has an advantage over sur- 

 face work in tliat the vertical dimension normally is better revealed by 

 underground workings, but the geologist, because of restricted mine open- 

 ings, is hampered when attempting fully to develop the plan maps. As in 

 surface mapping, conventional color and pattern symbols are used 

 (pl. 13). 



Geophysics 



For a number of years geophysics was considered, without much 

 success, as a possible aid in finding new ore bodies. One of the reasons 

 for failure was the lack of understanding between the mining geologist 

 and the geophysicist. The mining geologist knew little of geophysics and 

 the geophysicist knew even less of geologic problems encountered in min- 

 ing. In recent years, however, the picture has been changing. Mining 

 companies are realizing that geophysics can be an important tool of the 

 mining geologist. Research has been started by various companies and 

 government agencies, so that at the present time geophysical staffs have 

 been established who are devoting all of their time and effort to the appli- 

 cation of geophysics to mining. Such organizations certainly are useful 

 now and will be more important in the future in aiding the geologist to 

 find new ore. The use of geophysical methods is described in chapter 14 

 of this volume. Numerous papers and summaries ^^ ^^ discuss the use, ap- 

 plications, and limitations of mining geophysics. 



Laboratory Methods ^^ 



The laboratory investigation of ore deposits involves the study of two 

 types of minerals; those tliat are nonopaque in thin section and those that 

 are opaque. Nonopaque minerals are studied in transmitted light using a 

 petrographic microscope, and opaque minerals are studied in reflected 

 light using a reflecting or ore microscope. Many problems, such as min- 

 eral content and size of mineral grains, are readily solved in the labora- 

 tory, while other questions, particularly those relating to origin of the 

 specimen, may be more difficult to answer. 



Petrographic examination of fragments crushed to about 125-150 



^^ Heiland, C. A., Geophysical Exploration, New York, Prentice-Hall, Inc., 1940. 

 ^' McKinstry, H. E., op. cit., pp. 115-132, 1948. 

 '* McKinstry, H. E., op. cit., pp. 113-161, 1948. 



