720  PRE-CAMBETAN    GEOLOGY    OF    NORTH   AMERICA. 
various  proportions  of  alumina,  lime,  magnesia,  and  manganese;  as 
chlorite  and  mica ;  and  also  very  probably  to  a  small  extent  as  iron 
phosphate. 
It  is  believed  that  the  iron  ore  of  the  Baraboo  district  was  origi- 
nally a  deposit  of  ferric  hydrate,  or  limonite,  formed  in  comparatively 
stagnant,  shallow  water,  under  conditions  similar  to  those  conditions 
existing  where  bog  or  lake  ores  are  being  formed  to-day,  and  that 
through  subsequent  changes,  long  after  the  iron  was  deposited  as 
limonite,  while  the  formation  was  deeply  buried  below  the  surface 
and  subjected  to  heat  and  pressure,  the  original  limonite  became  to  a 
large  extent  dehydrated  and  changed  to  hematite. 
For  literature  on  correlation  of  this  district  with  the  rocks  of  Lake 
Superior,  see  summaries  of  articles  by  Irving,  Van  Hise,  and  others 
in  Chapter  III,  Lake  Superior  region,  pages  108-252. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
The  Baraboo  ranges  constitute  an  outlier  of  pre-Cambrian  rocks 
in  the  Paleozoic  area  of  south-central  Wisconsin.  The  crystalline 
rocks  are  mainly  quartzite,  and  until  recently  it  was  not  known  that 
other  rocks  were  present  in  abundance.  It  was  formerly  supposed 
also  that  the  quartzite  constituted  the  remnfnt  of  the  north  limb  of  a 
great  anticline.  It  is  now  known,  from  the  work  of  Weidman,  that 
the  quartzite  is  in  the  form  of  a  syncline,  the  north  and  south  edges 
of  which  form  respectively  the  North  and  South  Baraboo  ranges, 
and  that  within  the  intervening  low  synclinal  area,  beneath  a  con- 
siderable thickness  of  Cambrian  sediments,  there  are  other  sedimen- 
tary rocks  conformably  above  the  quartzite.  These  are  the  Seeley 
slate,  500  to  800  feet  thick,  and  above  this  the  Freedom  dolomite, 
800  feet  thick,  the  lower  portion  of  which  is  ferruginous  and  shows 
concentrations  of  iron  ore  in  commercial  quantities  and  grades.  The 
quartzite  is  found  to  rest  unconformably  upon  an  igneous  basement 
which  appears  both  north  and  south  of  the  ranges.  This  basement 
consists  of  granites  and  of  acidic  volcanics,  principally  metarhyolite, 
showing  brecciated,  spherulitic,  perlitic,  and  fluxion  structures.  It 
was  formerly  supposed  that  these  rocks  overlay  the  quartzite. 
The  crystalline  series  lie  unconformably  below  the  upper  Cam- 
brian of  this  area,  but  their  precise  ages  are  not  known.  Hall  and 
Irving  referred  it  to  the  Huronian  at  an  early  date,  and  Irving,  fol- 
lowed by  Van  Hise,  later  concluded  it  to  be  upper  Huronian  on  the 
basis  of  its  general  lithological  character.  With  the  recent  discovery 
of  new  formations  associated  with  the  quartzite,  the  sedimentary 
series  shows  closer  resemblance  to  the  middle  Huronian  of  the  Mar- 
quette district  of  Michigan  than  to  any  other  group,  the  Baraboo 
quartzite  corresponding  to  the  Ajibik  quartzite,  the  Seeley  slate  to 
