204  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull.  225. 
limestone,  and  the  Trenton  limestone.  The  St.  Peter  sandstone 
occurs  a  short  distance  below  the  surface  and  outcrops  only  a  few 
miles  north  of  the  State  line.  The  Niagara  limestone  is  a  massive 
dolomite  with  certain  cherty  layers.  It  forms  the  tops  of  the  mounds 
and  of  the  escarpment  which  encircles  the  whole  district  on  the  south 
and  west.  It  is  not  ore  bearing.  The  Maquoketa  shale  is  a  soft,  blue 
to  drab,  argillaceous  shale,  about  175  feet  thick.  It  underlies  the 
gentler  lower  slopes  of  the  mounds  and  the  escarpment,  and  spreads 
out  in  thin  disconnected  patches  over  the  higher  portions  of  the  flat 
upland.     It,  like  the  Niagara,  is  not  ore-bearing. 
The  Galena  limestone  is  a  very  massive  dolomite,  weathering  with 
a  rough,  carious  surface,  and  with  heavy  chert  beds  occupying  the 
middle  of  the  section,  non-cherty  members  occurring  above  and  below. 
It  is  the  main  ore-bearing  horizon  and  is  about  240  feet  thick  in  Jo 
Daviess  County.  Beneath  it  is  what  has  been  called  the  Trenton  lime- 
stone, which,  in  contrast  with  the  Galena,  is  not  a  dolomite,  though 
usually  not  entirely  free  from  magnesia.  Near  the  top  of  the  Trenton 
are  certain  thin  shale  beds,  the  most  important  being  locally  known 
as  the  "  oil  rock."  The  Trenton  in  this  area  is  about  40  feet  thick. 
To  the  north  a  greater  thickness  has  been  assigned  to  it,  but  this  is 
due,  not  to  a  thickening  of  the  strata,  but  to  the  reference  of  a  portion 
of  the  overlying  beds  to  this  formation.  The  Trenton  does  not  out- 
crop in  northwestern  Illinois  except  in  a  small  area  near  Millbrig. 
Below  the  Trenton  is  the  St.  Peter  sandstone,  which  is  a  common 
source  of  artesian  water  to  the  south  and  west.  Below  it  are  sand- 
stones and  dolomites  representing  Ordovician  and  Cambrian  sediments, 
none  of  which  outcrops  within  this  particular  area. 
Ore  bodies. — The  ores  now  being  mined  in  northwestern  Illinois  are 
found  in  the  Galena  limestone.  In  Wisconsin  the  Trenton  also  yields 
ore,  but  the  mines  in  the  vicinity  of  Galena  have  not  as  }^et  been 
worked  at  these  lower  horizons.  The  Illinois  mines  resemble  more 
closel}-  those  at  Dubuque,  Iowa,a  than  those  near  Platteville  and  the 
other  Wisconsin  shipping  points.6  Indeed,  almost  every  feature  of 
the  Dubuque  mines  can  be  duplicated  in  Illinois,  and  vice  versa, 
though  the  Black  Jack,  or  Peru  mine,  not  now  open,  has  been  worked 
to  a  horizon  below  that  of  any  of  the  mines  opened  at  Dubuque.  A 
somewhat  generalized  section  of  the  Galena  limestone  is  given  below. 
Generalized  section  of  the  Galena  limestone. 
Feet. 
4.  Dolomite,  earthy,  thin  bedded 30 
3.  Dolomite,  coarsely  crystalline,  massive  to  thick  bedded 60 
2.  Dolomite,  thick  to  thin  bedded,  coarsely  crystalline,  cherty 90 
1.  Dolomite,  thick  bedded,  coarsely  crystalline 60 
«Geology  of  Dubuque  County:  low?  Geol.  Surv.,  vol.  10,  1900,  pp.  385-657. 
b  Grant,  U.  S.,  Preliminary  report  on  the  lead  and  zinc  deposits  of  southwestern  Wisconsin:  Wis- 
consin Geol.  Nat.  Hist.  Surv.,  bull.  9,  1903,  103  pp. 
