bain]  FLUORSPAR    DEPOSITS    OF    SOUTHERN    ILLINOIS.  509 
correct,  the  intrusion  is  older  than  some  of  the  faulting,  and  it  seems 
not  unlikely  that  the  two  were  approximately  contemporaneous,  and 
occurred  somewhere  after  the  close  of  the  Carboniferous  and  before  the 
formation  of  the  presumably  Tertiary  peneplain. 
THE    ORE    DEPOSITS. 
Character  of  the  ores. — The  ores  are  essentially  made  up  of  fluor- 
spar, with  calcite  as  the  most  abundant  gangue  mineral.  Associated 
with  these  are  subordinate  amounts  of  lead  and  zinc  sulphides,  of  stib- 
nite,  and  possibly  other  sulphides,  barium  sulphate,  and  occasional 
traces  of  copper.  In  the  same  area  are  iron  deposits  (limonite)  of  some 
size,  but  not  at  present  of  commercial  importance.  Fluorspar  is  the 
most  abundant  mineral,  and  is  the  only  ore  now  shipped  in  quantity. 
The  lead  occurs  as  galena  intimately  associated  with  the  fluorspar,  and 
its  production  is  incidental  to  the  preparation  of  the  spar  for  the  mar- 
ket. In  contrast  with  the  galena  found  elsewhere  in  the  Mississippi 
Valley,  this  is  argentiferous,  seemingly  reliable  assays  indicating  the 
constant  presence  of  silver  running  up  to  9  ounces  to  the  ton.  Gold 
has  also  been  reported  to  have  been  found,  though  not  with  the  same 
degree  of  reliability.  These  facts,  with  the  occasional  occurrence  of 
stibnite,  are  of  considerable  importance  in  indicating  for  the  ores  of 
the  district  a  different  genesis  from  that  of  the  other  lead  and  zinc 
ores  of  the  Mississippi  Valley. 
Form  of  the  ore  bodies. — The  ore  bodies  are  of  the  t}^pe  known  as 
fissure  veins,  tabular  deposits  filling  fissures.  Together  with  the 
process  of  fissure  filling  there  has  been  considerable  replacement  of 
the  country  rock,  and  this  in  places  gives  irregular  form  to  the  ore 
bodies.  As  a  general  thing,  however,  the  veins  are  tabular  bodies  of 
ore  having  marked  longitudinal  direction,  occurring  in  vertical  posi- 
tion or  cutting  across  the  strata  at  high  angles.  They  usually  occupy 
fault  planes,  though  the  faulting  is  not  in  every  case  of  great  amount. 
The  largest  ore  bodies  so  far  developed  have  been  found  along  or 
near  fault  planes  which  have  brought  sandstone  and  limestone  into 
contact.  In  these  cases  the  limestone  near  the  ore  body  is  usually 
recrystallized  as  calcite  and  the  sandstone  has  been  indurated  to  a 
quartzite.  A  very  common  phenomenon  is  Assuring  parallel  to  the 
vein,  particularly  in  the  sandstone.  The  result  of  the  processes  of 
Assuring  and  induration  is  the  production  of  hard  quartzite  ridges, 
dike  like  in  form,  traversing  the  country  parallel  to  the  vein.  They 
are  the  most  common  and  most  reliable  of  the  indications  followed  by 
the  prospector. 
Size  of  the  ore  bodies. — The.  ore  bodies  are  of  rather  unusual  size. 
In  the  Fairview  and  Rosiclare  mines  a  continuous  body  of  ore  has 
been  worked  for  a  distance  of  nearly  a  mile.  The  workings  now 
extend  to  a  depth  of  300  feet  and  stopes  12  to  20  feet  wide  have  been 
