bllis.]  ZINC    AND    LEAD    NEAR    DODGEVILLE,    WIS.  315 
from  1  to  5  feet  and  the  proportion  of  ore  varies  greatly  from  place 
to  place  in  the  same  mine. 
Genesis  of  the  ores. — The  ore  bodies  of  the  glass-rock  opening  are 
probably  all  of  a  secondary  nature,  as  all  the  evidence  points  toward 
he  Galena  limestone  as  the  original  source  of  the  lead  and  zinc 
Dres  of  the  Wisconsin  district.  These  flats  are  all  in  a  deeply  eroded 
irea  in  which  there  remains  a  comparatively  small  thickness  of 
'Jalena  limestone,  and  are  close  to  or  below  the  level  of  ground 
vater.  In  the  deposits  above  ground-water  level  the  ores  are  galena 
uid  smithsonite.  The  predominant  ore  below  water  level  is  blende, 
vhile  crevices  leading  from  above  show  the  transition  from  the  oxid- 
zed  ores  to  the  unaltered  sulphides.  The  Galena  limestone  carried 
considerable  ore,  as  is  shown  by  the  old  lead  and  dry-bone  diggings 
n  this  formation,  but  as  erosion  progressed  and  lowered  the  level  of 
•  round  water  the  solution  and  downward  transportation  of  the  less 
table  sulphides — blende  and  marcasite — progressed  equally,  leaving 
he  less  soluble  sulphide  of  lead  and  some  of  the  zinc,  which  was 
eprecipitated  as  the  carbonate.  The  brittle  glass  rock  was  easily 
ractured  under  the  stresses  which  the  strata  have  undergone,  and 
mall,  irregular  cracks  have  been  formed.  These,  enlarged  by  solu- 
ion  and  filled  with  ore,  give  the  rock  a  brecciated  appearance.  The 
>re-bearing  waters  coming  from  above  followed  these  comparatively 
pen  channels  and,  taking  calcium  carbonate  into  solution,  deposited 
he  sulphides,  the  precipitation  being  assisted  by  the  carbonaceous 
hale  at  the  base  of  the  glass  rock.  The  shale  in  this  area  has  per- 
ormed  the  function  exercised  by  the  "  oil  rock  '•'  in  a  considerable 
>ortion  of  the  Wisconsin  lead  and  zinc  district,  and  has  acted  as  an 
mpervious  layer,  stopping  the  dowivward  circulation  of  ore-bearing 
waters.  Nowhere  in  this  vicinity  has  ore  been  found  below  this 
hale  bed,  while  in  all  the  mines  a  sheet  of  ore  occurs  directly  above. 
The  ores  of  the  glass-rock  opening  constitute  the  most  important 
odies  yet  developed  in  the  vicinity  of  Dodgeville,  but  in  the  greater 
)art  of  the  Wisconsin  lead  and  zinc  district  the  more  valuable  lead 
nd  zinc  deposits  occur  above  this  horizon,  and  there  is  no  reason 
o  suppose  that  such  deposits  may  not  exist  in  the  less  eroded  por- 
ions  of  the  Dodgeville  area.  Considerable  work  has  been  done 
q  the  past  on  lead  crevices  at  these  higher  elevations,  and  one  com- 
»any  has  recently  started  development  work  on  one  of  these  old  lead 
nges. 
Mention  should  also  be  made  of  the  fact  that  some  prospecting  has 
hown  the  presence  of  galena  in  the  upper  portion  of  the  "  Lower 
lagnesian "  limestone  near  Dodgeville,  although  no  development 
rork  has  been  done  at  that  place  and  no  definite  idea  can  be  formed 
f  the  possible  extent  of  such  bodies. 
