Ch. 27] DEVELOPMENT AND MINING 499 



facts themselves can be extended considerably. • When evidence to 

 the contrary is lacking, the nature of the deposit in the area explored 

 may be taken as a random sample of the situation that prevails at 

 depth. If the bed in outcrops is of the same quality and thickness 

 for miles around and, as sampled underground, shows no effects of 

 leaching or secondary enrichment, it is reasonable to suppose that it 

 is of the same quality and thickness wherever it is present at depth. 

 The chief question then becomes one of structure, and the answer may 

 require mapping of a more extended area than that in which the de- 

 posit crops out. If, on the other hand, the deposit is discontinuous 

 where explored, it is sometimes justifiable to assume that the same 

 quantity and quality of material is present per unit of area at depth 

 as is present near the surface. 



DEVELOPMENT AND MINING 



If, on careful appraisal, the deposit appears to contain the required 

 tonnage of minable material, steps are taken to prepare it for mining. 

 Depending on the type of deposit, this development work may con- 

 sist in detailed drilling and mapping to establish more precisely the 

 position and attitude of that part of the deposit that is to be extracted, 

 or it may consist only in driving haulage ways, removing overburden, 

 and otherwise providing access to the deposit and means of removing 

 the material from the mine once it has been loosened at the face. 

 Mining usually begins on the first minable unit developed, and there- 

 after development work is carried on concurrently with mining. 



Only rudimentary principles of geology were applied to the prob- 

 lems of mining most sedimentary deposits in the past, and these were 

 applied by miners, shovel operators, and mining engineers who ac- 

 quired their knowledge of the principles involved through actual ex- 

 perience with the deposit being mined. Many of the larger companies 

 mining epigenetic deposits now employ geologists; there is a growing 

 tendency to do the same in the mining of many syngenetic sedimentary 

 deposits. 



If a mining geologist is employed, his chief responsibility is to trace 

 the ore, both in the development work and in mining. In many areas, 

 exploration sufficiently detailed to form the basis for a sound appraisal 

 of the property is by no means detailed enough to disclose all the 

 faults and other irregularities or discontinuities in the bed to be 

 mined ; nor will all of them be found in the course of the development 

 work. The principal means used in keeping track of the bed to be 

 mined is detailed geologic mapping — mapping not only of the surface 



