136  CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,  1903.  [bull. '225. 
beneath  the  Carboniferous  limestones  that  cover  the  country  to  the 
north.  The  horizon  is  just  beneath  the  scolithus  or  "worm-eaten" 
sandstone.  Ore  bodies  of  considerable  size  have  recently  been  opened 
here  and  the  district  has  become  a  productive  one. 
Value  of  the  ores.—  The  gold  contents  of  the  ores  in  the  Bald  Moun- 
tain area  run  from  $3  or  $1  per  ton  to,  in  rare  instances,  $100.  The 
general  average  for  the  ores  in  this  district  is  about  $17,  those 
containing  from  $10  to  $20  being  of  the  most  common  occurrence. 
Ore  carrying  $35  per  ton  is  considered  high  grade.  Some  of  the  ore 
from  the  Ben  Hur  mine  has  yielded  upward  of  $60  per  ton  in  gold. 
As  compared  with  the  ores  of  the  lower  beds,  those  from  the  upper 
contact  are  of  slightly  poorer  grade,  so  that  much  of  the  ore  is  often 
left  in  the  mines.  The  upper  ores  have  also  been  reported  to  carry  a 
higher  relative  proportion  of  silver,  but  although  this  is  true  in  indi- 
vidual instances,  in  general,  silver  ores  are  as  frequent  in  the  lower 
as  in  the  upper  beds. 
The  three  smaller  areas  of  siliceous  ore,  Yellow  Creek,  Lead,  and 
Garden,  lying  over  or  to  the  west  of  the  Homestake  ore  body  or  its 
continuation,  produce  ores  of  uniform^  higher  grade  than  those  from 
the  Bald  Mountain  country.  The  mineralization  is  probabty  later  than 
the  igneous  activity,  for  the  verticals  which  supplied  the  ores  often 
cut  all  varieties  of  eruptive  rocks. 
As  igneous  rocks  cut  strata  of  the  Fort  Benton  Cretaceous  and 
pebbles  of  the  same  rock  have  been  found  in  the  basal  conglomerates 
of  the  Neocene,  it  would  seem  then  that  the  mineralization  occurred 
somewhere  between  Fort  Benton  and  Neocene  times,  and  it  probably 
represents  the  final  phase  of  volcanism  that  was  concomitant  with  the 
elevation  of  the  Black  Hills.  This  occurred  while  the  Cambrian  was 
still  deeply  buried  beneath  its  covering  of  later  formations. 
Lead-silver  ores  of  Galena  and  vicinity.- — -The  ores  belonging  to  the 
fourth  division  of  Cambrian  ore  deposits  are  similar  in  form  and  mode 
of  occurrence  to  the  refractory  siliceous  ores.  They  occur  in  the 
vicinit}^  of  the  town  of  Galena. 
At  one  time  these  ores  filled  an  important  place  in  the  mineral  pro- 
duction of  the  Black  Hills.  About  twenty  years  ago  a  smelter  was  in 
operation  and  several  mines  were  producing  heavily,  the  Richmond  or 
Sitting  Bull  mine  especially  having  figured  prominently  in  the  silver 
production.  After  a  brief  period  of  activity,  however,  operations 
were  rather  abruptly  discontinued  and  the  district  was  idle  until  the 
year  1886,  when  operations  were  resumed,  although  upon  a  somewhat 
smaller  scale.  Work  is  now  being  conducted  in  a  rather  desultory 
manner.  Mines  that  produced  lead-silver  ores  are  situated  in  and  about 
the  town  of  Galena.     Most,  if  not  all,  of  the  ore  bodies  are  in  strata  of 
imbrian  age.    Some  of  the  principal  producing  mines  were  the  Rich- 
