ellis.]  ZINC    AND    LEAD    NEAR    DODGEVILLE,    WIS.  313 
the  intercalated  beds  of  flint  and  dolomite,  which,  when  complete, 
attain  a  thickness  of  100  feet.  The  base  of  the  Galena  is  marked 
by  a  constant  shale  bed  varying  from  1  to  3  feet  in  thickness.  This 
shale  consists  of  bine  clay  and  of  brown  carbonaceous  material,  which 
gives  it  the  name  of  "  oil  rock."  It  is  a  very  important  horizon, 
and  throughout  the  greater  portion  of  the  Wisconsin  lead  and  zinc 
district  marks  the  lower  limit  of  mining  operations. 
The  Platteville  limestone  has  here  a  rather  uniform  thickness  of 
(35  feet  and  is  much  less  magnesian  than  the  Galena.  The  upper  beds, 
called  the  "  glass  rock,"  constitute  the  important  ore  horizon.  The 
glass  rock  is  a  fine-grained,  hard  limestone  from  4  to  12  feet  in  thick- 
ness. It  shows  considerable  variation  in  coarseness  of  grain  within 
short  distances  and  carries  varying  amounts  of  magnesium  carbonate, 
although  not  approaching  a  dolomite  in  composition.  Near  the  base 
of  the  Platteville  is  a  generally  recognized  series  of  beds  of  magnesian 
limestone,  known  as  the  "  quarry  rock." 
Generalized  section  of  the  Platteville  limestone  near  Dorff/erillc. 
Ft.  in. 
7.  Glass  rock 10  0 
6.  Dark-brown  to  black,  bard  carbonaceous  shale 0  3 
5.  Hard,  gray  limestone 10  0 
4.  Fossilii'erous  limestone,  fine  grained  and  separating  into 
thin,  irregular  beds 18  0 
3.  Clay   bed 0  2 
2.  Dolomite  limestone  in  heavy  beds  (quarry  rock) 25  0 
1.  Sandy  shale 0  8 
The  St.  Peter  sandstone,  occurring  below  the  Platteville,  varies  in 
thickness  from  50  to  150  feet  and  consists  of  pure  quartz  sandstone, 
frequently  iron  stained  and  with  considerable  variation  in  the  degree 
of  its  cementation. 
The  "  Lower  Magnesian  "  limestone  is  a  cherty  dolomite,  varying 
greatly  in  thickness  and  character  and  frequently  containing  sand- 
stone layers  interbedded  with  the  limestone.  Only  the  upper  portion 
of  this  limestone  occurs  within  the  limits  of  the  Dodgeville  area  and 
there  is  considerable  evidence  of  an  erosion  interval  between  it  and 
the  St.  Peter  sandstone. 
With  the  exception  of  this  small  erosion  interval,  the  relations  of 
the  formations  are  those  of  normal  sedimentation  with  changing  lit- 
toral conditions.  The  strata,  while  in  general  nearly  horizontal,  have 
been  subjected  to  stresses  which  locally  have  produced  gentle  t hough 
well-defined  folds. 
Ore  deposits. — The  ore  bodies  of  this  district  occur  in  vertical  crev- 
ices, in  pitching  crevices,  and  in  fiats,  occupying  a  largo  horizontal 
space  as  compared  with  their  vertical  extent.  The  quantities  of 
sphalerite  and  galena  produced  are  at  present  nearly  equal,  while  a 
