312  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  200. 
000,000  pounds  would  probably  be  a  fair  estimate  of  the  annual  pro- 
duction before  1900.  During  the  last  five  years  about  150  tons  of 
galena  and  300  tons  of  zinc  ore  a  year  have  been  shipped  from 
Dodge  ville. 
Dodgeville  is  particularly  well  situated  in  reference  to  markets 
for  its  ores.  It  is  on  the  Illinois  Central  Railroad,  and  therefore  ha^ 
a  direct  line  to  the  zinc  works  at  Peru  and  Lasalle,  111.,  which  con- 
sume the  higher-grade  ores.  The  low-grade  zinc  ores  may  be  shipped 
on  the  Chicago  and  Northwestern  to  the  works  at  Waukeaan,  111., 
but  generally  find  a  ready  market  at  the  Mineral  Point  Zinc  Works. 
These  are  9  miles  away  by  wagon  road,  and  are  also  reached  by  raill 
roads  which  gave  a  good  rate.  A  smelter  recently  reestablished  all 
Dodgeville  gives  a  home  market  for  all  the  galena  produced. 
The  large  horizontal  extent  of  the  flats  and  the  small  amount  of 
pumping  necessary  make  the  working  of  these  mines  relatively 
cheap,  and  permits  the  development  of  leaner  bodies  of  ore  than 
would  generally  be  profitable  under  the  mining  methods  followed.] 
The  galena  and  smithsonite  above  the  level  of  ground  water  occur! 
in  soft  rock,  as  a  rule,  and  are  readily  separated  by  hand  sorting. 
They  are  relatively  free  from  iron,  and  consequently  are  of  good  grade. 
The  ores  below  wrater  level  are  intimately  associated  with  marcasitej 
and  are  separated  from  it  with  difficulty,  the  grade  of  the  blende 
concentrates  in  consequence  being  considerably  lowered,  although 
averaging  well  up  with  those  of  the  Wisconsin  district  in  general. 
Physiography  of  the  area. — Dodgeville  is  situated  on  the  divide 
between  the  drainage  of  the  Pecatonica  and  Wisconsin  rivers,  while 
the  center  of  the  mining  district  is  among  the  small  headwater 
streams  of  Dodges  Branch,  which  flows  into  the  Pecatonica.  The 
country  is  perfectly  drained,  being  part  of  the  driftless  area  of 
Wisconsin ;  and  as  the  streams  all  flow  to  the  southeast  and  are  nearly 
parallel,  a  series  of  ridges  is  left,  with  long,  gentle  slopes  to  the  south- 
east. In  the  least  eroded  portion  of  the  area  the  hills  rise  to  an 
average  height  of  1,250  feet  above  tide,  which  marks  the  level  of  an 
old  peneplain.  The  small  streams  have  an  average  fall  of  60  feet  to> 
the  mile  near  their  sources  and  have  flat  bottoms,  while  the  valleys 
have  gently  rounded  side  slopes. 
Stratigraphy. — The  formations  represented  are  all  sedimentary 
and  of  early  Paleozoic  (Ordovician)  age.  Those  exposed  within  the 
area  are  the  Galena  limestone,  the  Platteville  limestone,  the  St. 
Peters  sandstone,  and  the  "  Lower  Magnesian  "  limestone. 
The  full  thickness  of  the  Galena  limestone  does  not  occur  within 
the  area,  although  at  places  200  feet  of  this  dolomite  remain.     In 
the  territory  where  the  larger  proportion  of  the  mining  is  carried  I 
on  erosion  has  left  not  more  than  100  feet  of  the  limestone.     This 
includes  the  lower  50  feet  of  nonflinty  dolomite,  above  which  occur 
