78 JOHN BIRKINBINE 



avenues of approach as a safeguard against accident. From 

 these main arteries drifts are opened either parallel or at dif- 

 ferent angles in wide or shallow deposits, or at determined 

 depths in narrow and deep deposits, and from these in turn 

 supplementary drifts are run and rooms or stopes excavated. 



In deposits dipping considerably from the horizon one 

 prevailing method of exploitation is to open a series of drifts 

 at different levels, from which mining is carried on simultane- 

 ously; the upper levels are farther advanced than those below, 

 the ore being taken out in horizontal sections or slices, known 

 in mming parlance as stopes. When the ore is worked above 

 a given level and allowed to fall by gravity, through chutes or 

 otherwise, to vehicles which deliver it to the main arteries, 

 the method is called overhand stoping. Where the material 

 is attacked below a level and the ore raised to this level, the 

 method is recognized as underhand stoping. The overhand 

 system is, under most circumstances, the cheaper and more 

 advantageous, but the underhand stoping is necessary in tak- 

 ing up floors, removing pillars, and in some open cut work. 



In underground operations the space made void as the 

 ore is removed must be protected, at least in part, either by 

 timbering or by filling in rock or other waste material. In 

 some instances the ore, if hard and if left in pillars alternating 

 with rooms or stopes, will safely support the roof, but often 

 the proportion of ore sacrificed is too great to make this method 

 desirable. 



After shafts or adits have been sunk and main drifts run 

 the ore is taken out by various systems, which may be briefly 

 designated as follows: 



Mining, in which the surface earth is removed and the 

 ore drawn through raises into drifts located some distance 

 below the top of the ore, thus making large sinks or craters. 

 This system requires that the cover be stripped, and is espe- 

 cially adapted to moderately shallow deposits of soft ore 

 covering a large area. 



In caving, a series of levels connecting with the main shaft 

 or with several shafts are simultaneously worked, the ore being 

 taken out from the upper levels and delivered through winzes 

 to lower levels which are protected by the ore in situ. As the 



