bain.]  LEAD    AND    ZINC    DEPOSITS    OF    ILLINOIS.  205 
The  thickness  of  these  divisions  varies  slightly,  the  difference  being 
due  to  variations  in  the  levels  at  which  chert  begins  to  appear  and  to 
disappear.  There  are  certain  horizons  characterized  b}T  fossils  which 
maintain  constant  relations  to  the  top  and  bottom  of  the  formation  and 
which  make  it  possible  to  recognize  stratigraphic  horizons.  Notable 
among  these  are  two  horizons  marked  by  Reeeptaculites  oweni,  known 
locally  as  the  "sunflower  fossil"  and  found  constantly  70  feet  below 
the  top  and  40  feet  above  the  bottom  of  the  formation. 
The  mines  now  open  are  working  in  the  flint  beds  and  the  dolomite 
above.  In  these  beds  the  characteristic  mode  of  occurrence  of  the 
ore  is  in  crevices  or  open  fissures  which  are  at  certain  horizons  locally 
constant.  These  horizons  are  marked  by  greater  weathering  of  the 
rock  and  the  consequent  production  of  openings — a  local  term  for 
either  actual  open  spaces  or  horizontal  bands  of  soft  decayed  rock 
occurring  along  a  crevice.  It  is  characteristic  of  the  area  that  crevices 
which  are  tight  or  are  marked  by  cracks  of  the  thickness  of  a  knife  edge 
open  out  into  caves  from  2  to  12  feet  wide  at  these  definite  horizons. 
By  the  action  of  solutions  and  the  dropping  of  unsupported  rock  from 
above,  the  openings  are  extended  upward  irregularly,  even  to  the  extent 
of  connecting  one  horizontal  opening  with  another  through  chimneys. 
Both  the  openings  and  the  chimneys  are  often  loosely  filled  with  tumbled 
rock  and  with  dolomite  sands  residual  from  the  breaking  down  of  the 
wall  rock.  The  ore  occurs  in  part  as  crystals  and  clusters  of  crystals 
attached  to  the  sides  of  these  open  caves,  and  as  broken  masses  scat- 
tered with  the  tumbled  rock  through  the  loose  sand.  To  a  limited 
extent  there  is  a  tendency  for  the  ore  to  penetrate  the  wall  rock  and 
to  metasomatically  replace  it.  There  are  some  crevices,  usually  veiy 
small  ones,  which  are  entirely  filled  from  wall  to  wall  with  ore.  The 
two  last-named  modes  of  occurrence  are  not  common  in  the  beds  so  far 
worked  in  Illinois.  They  are,  on  the  other  hand,  characteristic  of  the 
occurrence  of  ore  in  the  lower  beds  worked  in  Wisconsin.  In  these 
lower  beds  the  simple  vertical  crevice  with  horizontal  openings  gives 
way  to  flats  and  pitches.  The  fiats  are  horizontal  sheets  of  ore  in 
openings  along  bedding  planes  or  partially  replacing  the  beds.  The 
pitches  are  similar  bodies  occurring  in  crevices  parallel  to  the  main 
crevice  but  cutting  across  the  beds  and  pitching  away  from  the  crevice 
proper.  The  pitches  connect  the  flats  with  one  another  and  with  the 
vertical  crevice. 
Flats  and  pitches  are  found  in  the  rock  from  the  top  of  the  flint 
beds  downward,  but  reach  their  greatest  development  in  the  lower 
portion  of  the  Galena,  ending  in  most  cases  with  a  rather  extensive 
flat  at  the  horizon  of  the  oil  rock,  a  shale  and  shaly  limestone  bed 
found  within  a  few  feet  of  the  top  of  the  Trenton.  They  afford  the 
largest  and  richest  bodies  of  ore,  and  accordingly  the  maximum  devel- 
opment of  the  mines  now  working  in  and  around  Galena,  111.,  is  to  be 
