198 



MINERALS AND GEOLOGY 



Veins often cut or cross each 

 other or are cut by eruptive dykes. 

 In this case the intersected vein 

 is very generally faulted or dis- 

 placed. In mining language, a 

 break of this kind in the con- 

 tinuity of the vein is commonly 

 termed a "trouble," "heave," 

 or " thrust," or an " upthrow " 

 or "downthrow" as the case 

 may be. The displacement may 

 be very slight, or it may exceed Fia. 107. 



many fathoms ; and great expense is often incurred in seeking for 

 the displaced portion of a vein thus affected. As a general rule, if 

 the intersecting vein or dyke be entered at its hanging- wall, as in 

 working from A to A', Fig. 107, the continuation of the broken vein 

 may be looked for " up-hill ;" whereas, if the intersecting vein or 

 dyke be entered at its foot-wall, or at J3', the search for the displaced 

 vein should be made "down-hill." This rule is not without its 

 exceptions, but the exceptions are comparatively rare. 



In order to ascertain the depth at which an inclined vein or bed of 

 any kind may be reached by vertical sinking at a given depth from its 

 outcrop, as at S, for example, 

 in Fig. 108, we have the for- 

 mula : s = tan i x d \ or, Log 

 s log tan i + log d ; in which ^ 

 s the depth of the shaft ; 

 i = the dip or inclination of the 

 vein in degrees or minutes ; and d the distance between the out- 

 crop and the mouth of the shaft. If the ground at the proposed site 

 of the shaft be higher or lower than at the outcrop of the vein or 

 bed, the difference of level must of course be added to or deducted 

 from s, as the case may require. 



^^S^^^^SS^;^ 

 FIG. 



