bain.]  LEAD    AND    ZINC    DEPOSITS    OF    ILLINOIS.  207 
located  well  toward  the  southern  boundary  of  the  district,  has  proved 
that  mineralization,  even  of  the  lower  beds,  is  not  confined  to  the  north, 
and  this  fact  offers  large  hope  for  prospecting-  throughout  a  wide  area. 
In  the  main  the  mines  now  being-  operated  are  clustered  around  Galena 
and  Elizabeth. 
Probably  150  square  miles,  occupying  the  northwestern  portion  of 
Jo  Daviess  County,  may  fairly  be  considered  to  be  within  the  proved 
ore-bearing  district.  With  a  less  degree  of  probability,  the  whole  of 
the  county  may  be  included.  80  far  the  largest  production  has  come 
from  the  areas  from  which  the  Maquoketa  shale  has  been  removed  by 
erosion,  and  no  ore  has,  so  far  as  known,  been  mined  under  the  Niag- 
ara. Within  a  year,  however,  an  important  find  has  been  made  in  the 
Sand  Prairie  mine,  at  the  very  edge  of  the  Niagara  escarpment  and 
where  the  whole  thickness  of  shale  is  present.  At  Dubuque  an  amount 
of  ore,  in  the  aggregate  very  large,  has  been  taken  from  under  an 
important  thickness  of  the  shale.  For  the  present,  therefore,  the 
only  known  areal  limitation  to  be  regarded  in  prospecting-  is  the 
boundary  line  of  Niagara  bluffs.  In  depth  the  St.  Peter  sandstone 
marks  the  horizon  below  which  there  is  no  reason  to  expect  ore,  and 
very  little  ore  has  heretofore  been  found  beneath  the  well-known  oil 
rock. 
Grade  of  the  ore. — The  grade  of  the  crude  ore  so  far  mined  has 
probably  been  notably  above  that  of  the  Joplin  district,  though  it  is 
difficult  to  get  reliable  figures.  It  is  to  be  remembered,  however, 
that  the  concentrates,  since  they  carry  a  high  percentage  of  iron, 
must  either  be  sold  at  low  price  or  be  subjected  to  a  secondary 
treatment  which  increases  the  cost  of  production.  It  is  also  true  here 
as  elsewhere  that  the  grade  of  the  ore  mined  depends  on  cost  of  pro- 
duction and  on  the  state  of  the  market.  There  are  considerable  quan- 
tities of  rock  containing  a  percentage  of  the  sulphides  sufficient  to 
make  the  rock  an  ore  whenever  mining  costs  become  as  low  as  in  the 
Joplin  district. 
In  conclusion,  it  may  be  stated  that  the  upper  Mississippi  district 
may  be  expected  in  the  future  to  produce  large  quantities  of  zinc  ore, 
and  that  the  portions  lying  in  Illinois  give  every  promise  of  develop- 
ing with  the  rest  of  the  district. 
