314  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
small  amount  of  smithsonite  is  mined  in  connection  with  them.  Cal- 
cite  is  frequently  associated  with  these  ores,  while  marcasite  almost 
invariably  accompanies  the  blende  in  varying  amounts. 
Very  little  mining  in  this  area  has  been  done  in  the  Galena  lime- 
stone, the  important  ore  bodies  occurring  in  the  glass  rock.  The  ores 
obtained  from  the  Galena  limestone  have  been  galena  and  smithsonite, 
which  occur  mainly  in  vertical  crevices,  though  with  a  small  develop- 
ment in  flats  and  pitches. 
The  ore  bodies  now  worked  are  in  the  so-called  "  glass-rocl 
opening  ?1  of  the  Platteville  limestone,  and  consist  of  irregular  flats 
with  occasional  small  vertical  crevices.  The  beds  forming  the  flats 
of  the  glass-rock  opening  range  in  vertical  height  from  1  to  7  feet, 
average  GO  feet  in  width,  and  frequently  have  very  considerable 
longitudinal  extent.  In  another  portion  of  the  Wisconsin  district 
deposits  of  the  same  class  have  been  worked  in  individual  mines  foi 
three- fourths  of  a  mile  in  one  direction.  In  the  Dodgeville  are? 
these  fiats  show  no  parallel  arrangement,  and  even  the  individual 
ore  bodies  do  not  keep  a  single  direction,  but  vary  from  place  t( 
place.  This  variation  in  direction  is  probably  due  to  the  positioi 
of  the  feeding  crevices  above. 
Where  the  ore  bodies  are  below  the  level   of  ground  water  th( 
predominant  ore  is  zinc  blende,  while  above  or  near  water  level  the 
important   ore   is  galena   with   associated   smithsonite.     In   the   on 
bodies  which  have  been  formed  above  ground  water  and  which  wer( 
consequently  subjected  to  the  oxidizing  influences  of  surface  waters, 
the  limestone  is  generally  soft  and  disintegrated  and  the  ore  occurs 
in  a  flat  at  the  base  of  the  glass  rock,  directly  above  a  bed  of  hard, 
impervious,  black  shale  2  to  5  inches  in  thickness.     These  flats  oi 
galena   and   smithsonite   are  very   irregular,   varying   from   a   soli< 
sheet  4  inches  in  thickness  to  small,  lenticular  masses  only  a  fea 
inches  in  longest  diameter  and  separated  by  barren  rook.     In  th( 
ore  bodies  which  have  escaped  the  oxidizing  surface  waters,  eithei 
because  of  an   impervious  bed  of  oil  rock  above  or  because  the] 
were  sufficiently  below  ground- water  level,  the  predominant  ore  is 
zinc  blende  with  a  small  proportion  of  intimately  associated  galena 
and  with  considerable  iron  sulphide,  generally  in  the  form  of  mar- 
casite.    In  ore  bodies  of  this  latter  type  ore  occurs  in  a  flat  at  the! 
base  of  the  glass  rock  and  above  the  thin  shale  bed,  but  the  main 
body  of  ore  is  in  irregular  seams  traversing  the  rocks  in  diverse 
directions,  but  with  the  majority  parallel  to  the  bedding  planes  of 
the  limestone.     The  irregular  seams  represent  fractures  in  the  glass 
rock  which  have  been  enlarged  by  solution  and  filled  with  ore,  and 
give  a  marked  brecciated  appearance  to  the  limestone.     The  stratum: 
occupied  by  these  bedding  and  cross  seams  has  a  vertical  height  oft 
