250  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
6.  The  separation  of  the  Archean  of  Minnesota  into  two  non- 
conformable  parts,  viz,  the  Upper  and  the  Lower  Keewatin,  with  a 
great  basal  conglomerate  between  them. 
7.  The  determination  of  the  oldest  known  rock  of  the  Lake  Su- 
perior region,  a  greenstone  called  Kawishiwin,  the  bottom  rock  of 
the  Keewatin,  the  supposed  earliest  crust  of  the  globe. 
8.  The  great  quartzite  formation  which  is  so  prominent  in  the 
geology  of  Wisconsin  and  Minnesota  is  nonconformable  upon  the 
Animikie,  and  is  a  member  of  the  brecciated  beds  of  the  Keweenawan. 
This  has  been  named  Sioux  quartzite,  Baraboo  quartzite,  and  New 
Ulm  quartzite.  It  is  that  which  contains  the  red  pipestone  (catlinite) 
in  southwestern  Minnesota.  It  is  the  western  representative  of  the 
Potsdam  sandstone,  of  Potsdam,  N.  Y.  This  quartzite  seems  to  be 
the  representative  of  the  middle  Cambrian,  as  the  Beekmantown  is  of 
the  upper  Cambrian. 
9.  The  origin  of  the  Mesabi  iron  ores  is  referred  to  a  greensand, 
which  has  been  altered,  affording  iron  ore  by  concentration  of  the 
iron  in  certain  favorable  positions.  Contemporary  witli  this  altera- 
tion was  a  concentration  of  silica,  and  this  was  increased  by  oceanic 
precipitation.  The  original  greensand  was  found  to  become  pebbles, 
and  to  increase  into  angular  masses  that  were  neither  sand  nor  pebbles, 
but  rather  breccia.  These  breccia  masses  have  at  first  an  amorphous 
crystalline  texture  and  grade  into  a  form  of  the  iron-bearing  rock 
which  was  named  taconite,  and  the  whole  was  referred  to  volcanic 
action,  being  different  forms  of  suddenly  cooled  volcanic  glass  and 
rhyolite,  broken  and  distributed  by  beach  action.  While  this  volcanic 
debris  was  undergoing  this  transformation  great  quantities  of  silica 
were  set  free  from  the  glass,  but  this  silica  immediately  saturated 
the  debris,  producing  spotted  jasperoid,  taconite,  and  sedimentary 
jaspilite. 
This  result,  reached  on  the  Mesabi  range,  opened  the  door  to  the 
understanding  of  the  iron  ores  of  the  Vermilion  range.  At  once  the 
rhyolitic  forms  and  all  the  igneous  associations  of  those  ores  with 
basic  igneous  rocks  were  elucidated,  thus  confirming  Wadsworth's 
idea  of  the  igneous  origin  of  the  jaspilites  of  the  Marquette  region — 
rather  the  igneous  origin  of  the  rock  which  later  was  changed  into 
jaspilite. 
10  and  11.  After  prolonged  field  examinations  the  Minnesota  Sur- 
vey reached  the  conclusion  that  the  granites  of  the  Archean  grade 
into  gneiss,  the  gneiss  into  micaceous  gneiss  and  mica  schist,  and 
finally  into  less  and  less  metamorphic  rocks  that  show  a  plain  frag- 
mental  structure  and  sedimentary  origin.  There  was  found  no  ex- 
ception among  the  Archean  granites.  The  granites  are  of  two  dates 
of  formation,  one  at  the  close  of  the  Lower  Keewatin  and  one  at  the 
close,  or  after  the  close,  of  the  Upper  Keewatin.     A  later  granite, 
