232  PRE-CAMBEIAN   GEOLOGY    OF   NORTH    AMERICA. 
igneous  fusion  of  the  fragmentals  of  the  Archean  itself;  that  when 
deeply  buried,  under  heat  and  pressure,  the  Archean  elastics  Avere 
rendered  plastic,  penetrating  openings  in  the  adjacent  and  super- 
jacent strata;  and  that  when  the  plastic  mass  was  not  moved  from  its 
place  it  was  simply  recrystallized  in  situ. 
The  clastic  rocks  must  have  been  derived  from  the  basal  greenstone, 
which  is  considered  representative  of  the  original  crust  of  the  earth. 
The  presence  in  such  elastics  of  sufficient  potassa  and  silica  to  yield 
upon  fusion  the  granitic  magmas  is  explained  on  the  hypothesis  that 
they  must  have  come  from  the  waters  depositing  the  fragmentals,  and 
primarily  from  the  atmosphere,  in  its  condition  normal  to  the  Arch- 
ean age,  just  following  the  congealing  of  the  first  crust. 
While  numerous  instances  of  such  transition  from  clastic  to  igneous 
rock  have  been  noted  in  Minnesota,  there  has  been  a  careful  study  of 
but  one.  That  was  the  case  of  the  granite  and  porphyry  which  in- 
trude the  elastics  at  Kekequabic  Lake. 
Winchell  (N.  H.),198  in  1898,  discusses  some  resemblances  between 
the  Archean  of  Minnesota  and  of  Finland.  The  succession  in  north- 
eastern Minnesota,  as  made  out  largely  from  field  work  done  in  1897, 
is  as  follows,  in  descending  order: 
1.  Granitic  intrusion,  cutting  and  metamorphosing  the  earlier 
schists  and  fragmentals.  This  rock  is  seen  about  Snowbank  Lake 
and  Moose  Lake,  about  the  western  confine  of  Disappointment  Lake, 
and  at  Kekequabic  Lake. 
2.  Upper  Keewatin,  consisting  of  conglomerates  (at  Stuntz  Island 
and  at  Saganaga  and  Ogishkie  Muncie  lakes),  sericitic  schists,  quartz- 
ose  and  micaceous  schists,  graywackes,  clay  slates,  chloritic  schists, 
and  porphyroids.  The  mica  schists,  embracing  many  conspicuous 
bowlder-like  forms  on  the  weathered  surfaces,  are  to  be  seen  about 
Moose  Lake  and  southeast  of  Snowbank  Lake,  about  Disappointment 
Lake,  Kekequabic  Lake,  and  eastward  to  Zeta  Lake. 
3.  Granitic  intrusion,  chiefly  represented  by  the  granite  of  Sag- 
anaga Lake,  where  the  Upper  Keewatin  lies  unconformably  upon  it. 
It  is  also  seen  a  little  west  of  Ely  and  on  Kawishiwi  River.  At  West 
Seagull  Lake  this  granite  cuts  older  greenstones  and  green  schists. 
4.  Lower  Keewatin  or  Kawishiwin,  mainly  a  greenstone  formation, 
both  massive  and  fragmental,  constituting  the  oldest  formation  in  the 
State.  When  stratified  it  consists  of  basic  tuffs,  agglomerates,  and 
green  stratified  schists  and  "  greenwackes."  It  contains  the  banded 
jaspilites  and  iron  ores  at  Vermilion  Lake.  Where  cut  by  granite 
and  porphyry  (1)  these  rocks  are  converted  to  mica  schist  and 
banded  gneiss. 
Unconformably  above  all  these  is  the  Animikie  formation,  of  Ta- 
conic  age,  the  base  of  the  Paleozoic. 
