Chap. 5] GEOPHYSICAL METHODS IN MINING 55 



are more magnetic than are sedimentary rocks, and basic (and more mag- 

 netic) igneous rocks are preferred to acidic rocks in regard to toughness 

 and abrasive resistance. Seismic methods have good possibilities for the 

 location of road materials as well as for determining their composition, 

 alteration, and general nature. 



F. Abrasives 



Where deposits are of sufficient size, as in the cases of diatomaceous 

 earth, quartz sand and sandstone, the application of seismic refraction and 

 electrical resistivity is indicated. 



G. Materials for Various Industrial Uses 



1. Fluorspar. As this mineral is usually found in fissure and fracture 

 zones, indirect structural prospecting, or locating of such zones by resis- 

 tivity or electromagnetic methods is suitable. 



2. Talc and soapstone. One instance is known in the literature where 

 talc formations could be located by an electromagnetic method, since they 

 reacted as poor conductors in contrast with adjacent graphite deposits. 



3. Lithographic stone. See road and building materials. 



4. Sand (used as construction material, abrasive, for glass manufac- 

 ture, molding, filtering, furnace lining, and so on). Provided an applica- 

 tion of geophysics is at all economical compared with surface-geological 

 methods, electrical-resistivity method will be most suitable for the delinea- 

 tion of sand lenses. 



5. Monacite sands. In addition to resistivity and refraction surveys 

 usable for the location of channels (see Chapter 6), radioactivity methods 

 may be "'^^ lied to sands of high thorium content. 



6. Serpentine. Magnetic methods are best suited for reconnaissance, 

 to be supplemented by electrical and possibly seismic measurements for 

 detail. 



7. Barite. Its high density makes the torsion balance or gravimeter 

 applicable, provided that topography is suitable. Barite, being a poor 

 conductor, may be located in more conductive rocks by surface-potential 

 methods. When barite occurs with conductive minerals such as pyrite, 

 almost any electrical method could be suited. 



8. Graphite. This mineral may be readily located by most electrical 

 methods. It produces strong self-potentials, usually of positive sign. 



9. Magnesite, feldspar, asbestos, mica. The geologic occurrence of these 

 minerals suggests that any application of geophysical methods (probably 

 magnetic or electrical) would have to be of an indirect nature. 



