206  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903. 
[bull.  225. 
expected  when  they  roach  somewhat  deeper  horizons  than  those  now 
penetrated. 
I  \  rtical  distribution  of  the  ores. — The  characteristic  ore  of  the  upper 
openings  is  galena.  This  occurs  in  well-developed  crystals,  and  is 
frequently  embedded  in  red,  residual  clay.  It  was  this  ore  which  was 
first  mined  and  which  led  to  the  development  of  the  region.  Below 
it  is  the  zinc  carbonate  which  next  attracted  attention  and  which  for 
two  decades  has  formed  the  main  output  of  the  region.  At  still 
lower  horizons  is  zinc  sulphide  with  minor  amounts  of  lead  and  iron 
sulphide.  It  is  the  sulphide  ore  which  is  now  attracting  attention. 
The  lines  between  the  three  classes  of  ore  are  not  perfectly  sharp,  and 
to  some  extent  the  ores  mingle,  but  in  any  general  survey  the  three 
classes  become  quite  distinct.  In  general  the  galena  and  the  zinc- 
carbonate  ores  are  most  abundant  in  the  crevices  and  openings,  while 
the  zinc-sulphide  ores  characterize  the  pitches  and  fiats. 
Origin  of  the  ores. — The  order  of  superposition  of  the  ores  is  readily 
explained  as  a  result  of  secondaiy  concentration  by  descending  waters. 
By  their  action  the  zinc  and  iron  sulphides  originally  present  with  the 
galena  in  the  upper  levels  have  been  oxidized  and  carried  below  to 
be  reprecipitated.  The  galena,  being  less  readily  soluble,  is  left 
behind.  The  belt  of  carbonates  marks  the  zone  in  which  this  leaching 
is  }^et  in  process.  The  great  pitches  and  flats  filled  with  mixed 
sulphides,  of  which  blende  is  dominant,  mark  the  zone  of  secondary 
enrichment.  Whether  this  second  concentration  marks  a  reversal  in 
direction  of  flow  of  the  local  ground  water  or  merely  the  final  stages 
of  a  general  downward  movement  is  yet  in  question,  and  is,  in  this 
instance,  mainly  of  academic  interest.  The  ore  bodies  of  the  region  are 
confined  to  the  Galena-Trenton  rocks,  and  neither  hypothesis  offers 
encouragement  for  the  prospecting  of  lower-lying  beds.  The  second 
concentration  is  the  dominant  one  and  accounts  for  the  fact  that  the 
richest  ores  occur  in  the  lower  portion  of  the  section  and  good  bodies 
of  zinc  ore  are  below  the  old  lead  workings. 
Present  development. — In  northwestern  Illinois  there  is  at  present 
no  important  producing  mine.  A  considerable  amount  of  develop- 
ment is  under  way  and  an  increasing  production  may  confidently  be 
expected.  There  are  thirteen  properties  upon  which  work  is  now 
being  done  with  a  view  to  zinc  production.  A  few  of  these  are  hoist- 
ing ore,  but  none  are  as  yet  down  to  the  horizons  at  which  large  bodies 
may  be  expected.  The  Oldenburg  mine  was  for  many  years  a  pro- 
ducer of  lead  ore,  and  later  of  zinc  carbonate.  The  bottom  of  the  old 
workings,  as  reported,  is  at  least  100  feet  above  the  oil  rock.  The 
mine  has  hitely  resumed  operation.  The  Black  Jack,  or  Peru  mine, 
south  of  Galena,  has  produced  an  important  amount  of  zinc  ore.  The 
workings  extend  well  down  toward  the  bottom  of  the  Galena,  and  drill 
boles  have  been  carried  down  to  the  St.  Peter.     This  mine,  since  it  is 
