LAKE    SUPERIOR   REGION.  215 
constitute  the  fundamental  complex  or  Archean,  which  is  a  unit  in  its 
grander  features. 
The  structure  and  origin  of  the  foregoing  series  are  considered  in 
some  detail.  It  is  concluded  that  stratification  can  always  be  discrim- 
inated from  schistosity  or  slaty  cleavage  by.  the  varying  shades  of 
color  bands  which  sweep  across  the  surface  of  the  rocks  and  by  grada- 
tions of  the  kind  and  size  of  grains  across  the  bands.  These  layers 
may  vary  from  one-sixteenth  of  an  inch  to  several  inches  or  several 
feet  across. 
Lawson,178  in  1893,  gives  a  petrographical  and  structural  account 
of  the  anorthosites  of  the  northwest  shore  of  Lake  Superior.  The 
anorthosite  is  wholly  massive,-  completely  granitic  in  structure,  and 
is  composed  almost  entirely  of  basic  feldspar,  varying  in  composition 
from  labradorite  to  anorthite.  The  rock  occurs  near  Encampment 
Island,  in  the  vicinity  of  Split  Rock  Point,  at  Beaver  Bay  and  vicin- 
ity, at  Baptism  River,  on  the  slopes  of  Sawteeth  Mountain,  and  at 
Carlton  Peak.  In  nearly  all  of  these  localities  the  rock  is  found  in 
rounded,  dome-shaped  masses  below  the  other  eruptives  of  the  coast. 
It  is  cut  by  these  different  eruptives,  and  in  the  lava  flows  are  found 
very  numerous  blocks  and  bowlders  of  anorthosite,  which  were  caught 
at  the  time  of  their  extrusion.  These  facts  show  that  the  anorthosite 
is  of  pre-Keweenian  age,  and  as  the  anorthosite  is  a  plutonic  rock  it 
must  have  suffered  profound  erosion  prior  to  the  extravasation  of  the 
Keweenian  eruptives.  Norwood,  Irving,  and  Winchell  have  described 
the  blocks  of  anorthosite  in  the  lavas  at  some  of  the  points.  Win- 
chell regarded  the  anorthosite  at  Split  Rock  as  older  than  the  erup- 
tives, containing  masses  of  them,  and  Irving  reached  the  same  con- 
clusion in  reference  to  the  anorthosite  at  Carlton  Peak.  However, 
none  of  them  differentiated  the  anorthosite  mass  from  the  general 
aggregation  of  volcanic  flows  constituting  the  Keweenian  series  of  the 
Minnesota  coast.  The  surface  of  the  pre-Keweenian  anorthosite  is 
domed  and  hummocky  like  that  of  the  other  Archean  terranes  of 
Canada,  and  it  is  thought  to  have  been  only  modified  by  Pleistocene 
erosion.  The  interval  between  the  anorthosite  and  the  Keweenian  is 
probably  the  same  as  the  pre-Paleozoic  interval  which  effected  the 
reduction  of  the  Archean  to  the  great  hummocky  plain  to  which  it 
was  reduced  before  the  Animikie  was  deposited  upon  it.  As  the  Ke- 
weenian rests  directly  upon  the  anorthosite,  the  Animikie  is  absent 
for  the  middle  third  of  the  Minnesota  coast.  Irving  places  the  thick- 
ness of  the  Keweenian  of  the  area  at  20,000  feet,  stating  that  it  may 
reach  22,000  or  24,000  feet.  The  maximum  thickness  of  the  Kewee- 
nian is  not  more  than  one-tenth  of  this  thickness.  Irving's  subdivision 
of  the  Keweenian  into  groups  and  his  estimates  of  the  thickness  of 
various  portions  of  the  series  are  of  little  value — a  statement  which  it 
