218  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
MESABI   DISTRICT. 
During  the  year  exploration  has  been  vigorous,  though  with  less 
success  than  in  preceding  years.  Some  of  the  results  of  the  work 
have  been  to  show  the  iron  formation  in  the  vicinity  of  Biwabik  to  be 
much  wider  than  indicated  on  the  recent  Survey  map  of  this  range. 
The  slate  layers  there  found,  which  have  been  previously  taken  as  the 
northern  margin  of  the  overlying  black  slate  (Virginia  slate),  are  now 
shown  to  be  but  interstratifieations  within  the  iron  formation.  Explo- 
ration has  also  seemed  to  confirm  conclusions  as  to  the  essential  barren- 
ness of  the  extreme  eastern  and  western  portions  of  the  Mesabi  district. 
BARABOO   DISTRICT. 
During  the  present  year  the  Wisconsin  geological  and  natural 
history  survey  made  an  investigation  of  the  newly  found  occurrence 
of  iron  ore  in  the  Baraboo  ranges  in  southern  Wisconsin.  This  inves- 
tigation was  carried  out  by  S.  Weidman,  and  his  report  is  in  the  form 
of  a  bulletin  of  the  Wisconsin  survey  (Bull.  13),  containing  a  general 
map  of  the  district  on  a  scale  of  1  inch  to  the  mile. 
A  pre-Cambrian  quartzite  formation,  having  an  estimated  thickness 
of  3,000  to  5,000  feet,  forms  an  east- west  S3Tnclinorium  about  20  miles 
long  and  ranging  in  wTidth  from  2  miles  on  the  east  to  10  or  12  miles 
on  the  west,  resting  on  a  basement  of  igneous  rock.  The  upturned 
north  and  south  edges  of  the  quartzite  forms  respectively  the  north 
and  south  ranges  of  the  Baraboo  Bluffs,  standing  TOO  to  800  feet 
above  the  surrounding  country  and  above  the  intervening  valley. 
This  valley  is  occupied  by  formations  }rounger  than  and  conformable 
with  the  quartzite.  Mr.  Weidman  has  named  these  formations  the 
Seeley  slate  and  Freedom  formation.  The  Seeley  slate  has  an  esti- 
mated thickness  of  500  to  800  feet,  and  above  this  is  the  Freedom 
formation,  mainly  dolomite,  which  has  a  thickness  estimated  to  be  at 
least  800  feet,  and  which  bears  the  iron-ore  deposits  in  its  lower 
horizon. 
Flat-lying  Paleozoic  sediments,  unconformably  overlying  the  pre- 
Cambrian  rocks,  occupy  the  surrounding  area  and  partly  till  the  val- 
ley. The  Paleozoic  rocks  range  from  upper  Cambrian  (Potsdam)  in 
the  valley  bottom  to  Ordovician  (Trenton)  on  the  upper  portions  of  the 
quartzite  ranges.  The  Potsdam  sandstone  has  a  thickness  ranging 
from  a  few  feet  to  a  maximum  of  about  570  feet  in  the  valley.  Glacial 
drift  is  abundant  over  the  quartzite  ranges  and  in  the  valleys  in 
the  eastern  half  of  the  district,  but  occurs  only  in  the  valleys  in  the 
western  half. 
1  he  iron  ore  occurs  in  the  lower  horizons  of  the  Freedom  formation 
and  is  mainly  a  Bessemer  hematite  with  soft  and  earthy,  hard  and  black, 
