ISOLATED   AREAS    TN    MISSISSIPPI   VALLEY.  719 
cal  and  microscopical  study  shows  this  rock  to  be  a  quartz  kerato- 
phyre.  It  is  shown  to  be  a  volcanic  rock  by  its  flowage  structure, 
broken  crystals,  and  by  volcanic  breccias.  The  rock  has  a  schistosity 
parallel  to  the  bedding  of  the  quartzite.  The  quartz  keratophyre 
rests  upon  the  topmost  layer  of  quartzite,  with  a  possible  erosion  in- 
terval. It  has  been  folded  with  the  quartzite,  and,  like  that  rock, 
rests  unconformably  below  the  undisturbed  Cambrian. 
Weidman,13  in  1904,  describes  and  maps  the  Baraboo  quartzite 
region  of  south-central  Wisconsin. 
A  pre-Cambrian  quartzite  formation,  having  an  estimated  thick- 
ness of  3,000  to  5,000  feet,  forms  an  east-west  synclinorium  about  20 
miles  long  and  ranging  in  width  from  2  miles  on  the  east  to  10  or  12 
miles  on  the  west,  resting  on  a  basement  of  igneous  rock  consisting 
of  granite,  rhyolite,  and  diorite,  in  isolated  and  widely  separated 
areas  both  north  and  south  of  the  quartzite  synclinorium.  The 
largest  area  is  one  of  rhyolite  near  the  lower  narrows  of  the  Baraboo. 
The  upturned  north  and  south  edges  of  the  quartzite  form,  respec- 
tively, the  North  and  South  ranges  of  the  Baraboo  Bluffs,  standing 
700  to  800  feet  above  the  surrounding  country  and  above  the  inter- 
vening valley.  In  the  valley  are  pre-Cambrian  formations  younger 
than  and  conformable  with  the  quartzite.  These  are  the  Seeley  slate, 
having  an  estimated  thickness  of  500  to  800  feet,  and,  above  this,  the 
Freedom  formation,  mainly  dolomite,  having  a  thickness  estimated 
to  be  at  least  800  feet,  bearing  iron-ore  deposits  in  its  lower  horizon. 
Flat-lying  Paleozoic  sediments,  unconformably  overlying  the  pre- 
Cambrian  rocks,  occupy  the  surrounding  area  and  partly  fill  the  \  al- 
ley. The  Paleozoic  rocks  range  from  Upper  Cambrian,  Potsdam,  in 
the  valley  bottom,  to  Lower  Silurian,  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  val- 
ley. Glacial  drift  is  abundant  over  the  quartzite  ranges  and  in  the 
valley  in  the  eastern  half  of  the  district,  but  occurs  only  in  the 
valleys  in  the  western  half. 
The  iron  ore  is  mainly  a  Bessemer  hematite,  with  soft  and  earthy, 
hard  and  black,  and  banded  siliceous  phases.  A  very  small  amount 
of  hydrated  hematite  or  limonite  is  also  present.  The  rocks  imme- 
diately associated  with  the  ore,  and  into  which  the  ore  grades,  are 
dolomite,  cherty  ferruginous  dolomite,  ferruginous  chert,  ferruginous 
slate,  and  ferruginous  dolomite  slate — in  fact,  all  possible  grada- 
tions and  mixtures  of  the  minerals  dolomite,  hematite,  quartz,  and 
such  argillaceous  minerals  as  kaolin  and  chlorite.  In  the  ferruginous 
rocks  associated  with  the  iron  ore  the  iron  occurs  as  hematite  and  also 
in  the  form  of  carbonate,  isomorphous  with  carbonates  of  calcium, 
magnesium,  and  manganese,  in  the  proportions  to  constitute  ferro- 
dolomite  and  manganic    Perrodolomite ;   a-  silicates  combined   with 
