214  PRE-CAM  BRIAN    GEOLOGY    OF    NORTH   AMERICA. 
4.  The  Animikie.  This  series  is  characterized  by  a  great  quartzite 
associated  with  the  iron  ores  and  cherts.  The  quartzite  (Pewabic) 
lies  unconformably  on  all  the  older  rocks.  In  many  places  it  is  con- 
glomeratic, bearing  debris  of  the  underlying  formations.  Within 
it  are  mingled  volcanic  tuffs  from  contemporaneous  eruptions.  The 
Pewabic  quartzite  includes  that  of  Pokegama  Falls,  on  the  Mississippi 
River,  and  of  Pipestone  County.  In  the  vicinity  of  contemporaneous 
volcanic  disturbances  its  grain  is  fine,  like  jaspilite,  and  sometimes  it 
has  acquired  a  dense  crystalline  structure  from  contact  with  the 
gabbro. 
5.  The  Keewatin.  This  is  a  volcanic  series  of  great  thickness, 
being  composed  mainly  of  volcanic  tuffs,  presenting  more  or  less 
evidence  of  aqueous  sedimentation,  but  conglomerates,  graywackes, 
quartzitic  schists,  and  glossy  serpentinous  schists  are  present.  The 
Kawishiwin  formation,  apparently  the  upper  member  of  the  series, 
embraces  the  great  bulk  of  the  greenstones,  chloritic  schists,  jaspers, 
and  hematites.  The  iron  ores  are  in  lenticular  lodes,  and  stand  up- 
right, conformable  with  the  general  position  of  the  rocks. 
G.  The  Keewatin  series  becomes  more  crystalline  toward  the  bottom, 
and  passes  conformably  into  completely  crystalline  mica  schists  and 
hornblende  schists,  which  are  named  the  Vermilion  series.  The  rocks 
are  usually  stratiform,  contain  magnetic  iron  ore,  and  embrace  some 
dark  massive  greenstone  belts,  in  which  no  stratification  bands  are 
visible. 
7.  The  Laurentian.  When  not  disturbed  by  upheaval  the  Ver- 
milion schists  pass  into  Laurentian  gneiss,  there  being  a  gradual  in- 
crease in  the  feldspathic  and  siliceous  ingredients.  Even  after  the 
Laurentian  characters  are  apparently  fully  established  conformable 
bands  of  Vermilion  schists  reappear;  from  which  it  is  plain  that  the 
base  of  the  Vermilion  is  an  uncertain  plane,  which  can  not  be  located 
exactly.  This  normal  passage  from  the  Vermilion  to  the  Laurentian 
is  frequently  disturbed  by  the  intrusion  of  numerous  dikes  of  light- 
colored  granitic  and  basic  rocks.  These  were  both  in  a  fluid  state, 
the  only  nonfluid  rocks  being  the  schists  which  are  embraced  within 
them  in  isolated  pieces.  In  a  similar  manner  small  areas  of  Lauren- 
tian granite,  sometimes  directly  in  contact  with  the  schists,  have  the 
imperfectly  crystalline  condition  of  the  Keewatin. 
Nos.  3  and  4  are  separable  from  No.  2  by  divergence  in  dip  and 
strike,  as  well  as  by  a  marked  difference  of  lithology.  There  is  conse- 
quently some  evidence  of  unconformity  between  them.  Below  No.  4 
is  a  great  physical  break,  which  separates  Nos.  1,  2,  3,  and  4  from  5,  6, 
and  7  throughout  the  Lake  Superior  region.  This  break  is  the  great- 
est erosion  interval  which  has  been  discovered  in  Paleozoic  geology. 
Nos.  1,  2,  3,  and  4  together  constitute  the  Taconic;  Nos.  5,  0,  and  7 
