98  CONTKIBUTTONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260 
The  occurrence  of  these  exhalations  over  a  large  part  of  the  ore 
bearing  area  is  of  much  interest.  They  certainly  increase  in  quaii 
tity  with  depth,  and  it  is  to  be  feared  that  in  some  cases  they  mai 
seriously  affect  mining  operations.  The  evil  has  proved  very  diffi- 
cult to  cope  with.  Ventilation  alone  has  rarely  proved  efficient,  an< 
the  only  practicable  remedial  measures  appear  to  be  cementation  o 
drifts  at  particularly  bad  places  and  working  the  mine  under  ai 
lock  at  a  pressure  slightly  exceeding  the  normal. 
The  origin  of  these  gases  can  not  reasonably  be  sought  in  any  sue 
explanation  as  the  oxidizing  of  sulphides  and  accompanying  absorp 
tion  of  oxygen.  They  probably  represent  the  last  exhalations  fro! 
the  throat  of  the  extinct  Cripple  Creek  volcano. 
