148  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
values  vary  in  different  parts  of  the  camp,  so  the  degree  of  concentra- 
tion required  in  milling  ores  varies.  Some  ores  are  concentrated  four 
times,  others  five.  It  may  be  stated  that  in  general  the  degree  of 
concentration  ranges  from  four  to  six  into  one.  A  good  concentrate 
of  sulphide  ore  carries  approximately  55  ounces  silver,  30  to  40  per 
cent  lead,  0.05  ounce  gold,  and  15  per  cent  zinc.  The  above  figures 
are  based  on  published  and  reported  average  values  of  ores  extracted 
at  the  present  time.  Bonanzas  have  been  and  are  encountered,  and  I 
excessively  high  assays  of  silver,  lead,  and  gold  are  occasionally 
reported. 
Occurrence  of  ores. — The  principal  ore  bodies  which  have  been 
opened  in  this  district  occur  either  (1)  in  lodes  or  fissures  or  (2)  in 
limestone  beds.  Although  in  early  days  the  high-grade  silver  ore  was 
from  fissure  veins,  at  present  the  bulk  of  the  shipments  of  crude  ore 
is  from  bedded  ore  bodies  in  limestone.  Fissures  occur  abundantly 
throughout  the  district.  Those  fissures  which  have  been  found  to 
carr}7  the  largest  and  richest  ore  bodies  lie  in  a  northeast-southwest 
zone  in  the  center  of  the  camp  and  dip  (with  rare  exceptions)  steeply 
toward  the  northwest.  They  are  branching  systems,  in  which  indi- 
vidual members  may  be  single,  narrow  fissures,  or  broad,  breccia 
zones.  Thus  ore  is  found  in  narrow  pay  streaks  frozen  to  the  walls  and 
in  groups  of  pa}r  streaks  included  in  lodes,  which  are  2  to  35  feet  wide. 
The  ore  bodies  in  limestone  have  replaced  portions  of  certain  mem- 
bers in  both  the  upper  Carboniferous  and  "Permo-Carboniferous" 
limestone  formations,  and  they  lie  between  siliceous  members  as  walls. 
Those  occurring  in  the  upper  Carboniferous  formation  which  have 
been  studied  thus  far  have  replaced  a  limestone  bed  that  overlies  a 
brown  sandy  or  quartzitic  member  at  a  distance  varying  from  50  to 
100  feet  above  the  Ontario  quartzite.  Occasional^  the  calcareous  bed 
underlying  this  sandy  member  is  found  replaced  by  ore,  and  the  sand- 
stone, normally  the  foot  wall,  is  left  barren  between  these  two  pro- 
ductive horizons.  Ore  in  the  Carboniferous  limestones  has  been 
observed  to  have  replaced  certain  metamorphosed  calcareous  mem- 
bers, and  also  to  have  been  deposited  in  a  quartzose  breccia  adjacent 
to  a  dike. 
Both  the  lodes  and  the  replacement  bodies  occur  in  frequent  and 
intimate  association  with  porphyry.  Fissures  occupied  by  ore  bodies 
cut  indiscriminately  across  porphyry  and  all  other  formations.  Among 
the  numerous  cases  observed  in  which  ore-bearing  limestone  has  suf- 
fered intrusion  by  porphyry,  several  have  been  noted  in  which  the 
ore  is  in  immediate  contact  with  porphyry.  The  evidence  now  at  hand 
tends  to  show  that  a  period  of  igneous  activity  preceded  the  period 
of  ore  deposition. 
Exploitation. — In  underground  exploration,  in  mining,  in  milling, 
and  in  shipping  ore,  the  economies  afforded  by  perfected  mine  and  mill 
equipment  arc  practiced.     Air  drills  are  used  in  development  work, 
