724  PEE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
Hall,23  in  1862,  concludes  that  the  quartzite  ranges  of  Baraboo  and 
Necedah  hold  the  same  position  relative  to  the  Potsdam  sandstone  as 
the  Huronian  system  of  the  Canadian  Survey. 
Murrish,24  in  1873,  describes  granitic  and  hornblendic  Azoic  and 
plutonic  rocks  on  the  Black  and  Yellow  rivers.  At  Black  River  Falls 
are  knobs  of  magnetic  ore  in  a  series  of  elongated  knobs  or  mounds, 
associated  with  quartz  and  micaceous  slate.  At  Grand  Rapids,  on 
Wisconsin  River,  are  found  Azoic  rocks  similar  to  those  on  Black 
River.  A  quartzite  mound  at  Necedah  occupies  a  geological  position 
similar  to  the  iron  ores  at  Black  River  Falls. 
Irving,25  in  1877,  describes  the  quartzite  at  Necedah.  The  struc- 
ture is  not  apparent.     It  is  mapped  as  Archean. 
Hancock,26  in  1901,  maps  and  describes  the  iron  formation  and  ad- 
jacent rocks  along  the  Black  River  valley  northward  from  Black 
River  Falls.  They  are  observed  in  unconformable  contact  with 
overlying  Paleozoic  rocks. 
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. 
At  Necedah,  in  Juneau  County,  and  at  North  Bluff,  in  Wood 
County,  are  quartzite  exposures  projecting  through  the  Cambrian. 
Drilling  at  Necedah  has  disclosed  the  presence  of  granite,  probably 
intrusive  into  quartzite.  The  quartzite  is  highly  metamorphosed. 
The  age  of  this  quartzite  is  not  known,  but  its  lithological  similarity 
is  with  the  lower  Huronian. 
In  the  Black  River  valley  northward  from  Black  River  Falls  are 
exposures  of  gneiss,  granite,  hornblende  schist,  magnesian  schist,  and 
ferruginous  quartz  schist,  mapped  by  Irving  in  1873,  and  subse- 
quently by  Hancock  in  1901.  The  relation  of  these  rocks  to  one  an- 
other is  not  definitely  known.     All  are  pre-Cambrian. 
SECTION  2.    SIOUX   QUARTZITE   OF   CONTIGUOUS   POR- 
TIONS OF  MINNESOTA,  SOUTH  DAKOTA,  AND  IOWA. 
SUMMARY   OF  LITERATURE. 
Hayden,27  in  1867,  in  his  sketch  of  the  geology  of  northeastern 
Dakota,  describes  quartzites  along  James  River,  Vermilion  River,  and 
at  Sioux  Falls.  These  quartzites  are  sometimes  conglomeratic.  On 
James  River  the  lines  of  stratification  are  nearly  obliterated,  but  the 
rock  appears  to  be  metamorphic.  The  pipestone  bed  on  Pipestone 
Creek  is  associated  with  the  quartzites  already  mentioned,  and  this 
rock  is  undoubtedly  of  the  same  age.     At  Sioux  Falls,  while  no  well- 
