bain.]  LEAD    AND    ZINC    DEPOSITS    OF    ILLINOIS.  203 
NORTHWESTERN    DISTRICT. 
The  northwestern  Illinois  area  forms  a  portion  of  the  upper  Missis- 
sippi Valley  district,  in  which  mining  has  been  carried  on  since  1788 
and  which  up  to  the  middle  of  the  nineteenth  century  was  the  principal 
source  of  American  lead.  With  the  opening-  of  the  silver-lead  mines 
of  the  Western  States  attention  was  diverted  from  the  district,  and  at 
about  the  same  time  the  mines  reached  a  depth  at  which  pumping 
became  a  serious  burden  and  at  which  the  lead  ores  gave  place  com- 
monly to  zinc  ore.  There  was  at  that  time  no  considerable  market 
for  such  ore.  Indeed,  up  to  the  period  of  high  prices  in  1899  no 
serious  effort  was  made  to  develop  the  sulphide  ores,  the  carbonates  of 
zinc  alone  being  salable.  The  principal  production  now  is  of  zinc  sul- 
phide, though  some  galena  and  zinc  carbonate  still  find  their  way  to 
the  market.  The  producers  of  zinc  blende  in  this  district  areata 
disadvantage  in  competition  Avith  those  of  the  Joplin  district,  since  the 
Wisconsin-Illinois  ores  carry  a  higher  percentage  of  iron.  Up  to 
recent  years  there  has  been  no  convenient  method  of  separating  the 
iron  from  the  zinc,  and  the  northern  ores  have  accordingly  lacked  a 
market. 
The  Mineral  Point  Zinc  Company,  the  principal  buyer  of  the  ores 
of  the  district,  has  equipped  its  plant  for  the  manufacture  of  sulphuric 
acid,  and  hence  is  now  able  to  economically  handle  the  sulphide  ores. 
The  development  of  the  method  of  roasting  and  magnetic  separation 
has  made  it  possible  to  build  plants  in  small  units  adapted  to  the  wants 
of  individual  mines,  and  the  ore  when  cleaned  by  this  process  has  a 
wide  market.  Experiments  with  static  electricity  operating  on  un- 
roasted  ores  are  now  being  made,  with  most  encouraging  results,  and 
it  seems  probable  that  the  district  as  a  whole  will  gradually  become  an 
important  producer  of  zinc  ores. 
Topography  of  the  district. — The  lead  and  zinc  deposits  of  the 
upper  Mississippi  district  lie  within  the  driftless  area.  The  region 
is  one  in  which  the  topography  seems  exceedingly  rugged  in  contrast 
with  the  smooth  drift  plains  surrounding  it.  The  principal  topo- 
graphic feature  of  the  region  is  an  elevated  peneplain,  well  seen  from 
the  top  of  the  bluffs  of  the  Mississippi  River.  Above  it  rise  certain 
detached  hills,  spoken  of  locally  as  mounds.  Below  it  the  streams  of 
the  region  have  cut  their  channels,  in  the  case  of  the  Mississippi  to  a 
depth  of  more  than  200  feet. 
Stratigraphy  of  northwestern  Illinois. — The  stratigraphy  of  the 
area  within  which  the  mines  are  located  is  simple.  There  are  no 
igneous  rocks  within  the  area  or  in  its  vicinity,  and  the  strata  have 
suffered  very  little  deformation.  There  is  a  rather  uniform  dip  to 
the  southwest  of  about  10  feet  to  the  mile.  The  outcropping  forma- 
tions include  the  Niagara  limestone,  the  Maquoketa  shale,  the  Galena 
