34  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
As  a  general  rule  it  may  be  said  that  it  seems  inadvisable  to  drive  long  tunnels  or  sink 
shafts  in  barren  rock,  as  has  been  done  in  some  cases,  in  the  remote  hope  of  striking  an 
ore  body.  Only  actual  indications  of  ore  should  be  followed  by  such  explorations  until 
the  existence  of  workable  bodies  has  been  proved  by  workings  sufficiently  extensive  to 
permit  the  determination  of  the  geologic  character  of  the  deposits  that  may  exist  here. 
For  the  deposits  in  the  Archean,  where  the  probable  values  lie  in  the  copper  ore,  it  must 
be  borne  in  mind  that  these  ores  make  a  disproportionately  large  showing  at  the  surface. 
Until  the  existence  of  strong  vein  deposits  has  been  actually  proved  by  sinking  on  the 
outcrop,  it  is  inadvisable  to  drive  crosscut  tunnels  to  cut  a  lode  in  depth  which  shows, 
as  do  most  of  the  prospects,  only  little  stringers  and  impregnations  of  the  country  rock  at 
the  surface. 
