LAKE    SUPERIOR   REGION.  155 
Van  Hise,  Bayley,  and  Smyth,7"  in  189G,  map  and  describe  a  the 
geology  of  the  Marquette  iron-bearing  district  of  Michigan.  The 
pre-Cambrian  rocks  of  the  district  comprise  three  series,  separated  by 
unconformities.  These  are  the  Basement  Complex,  or  Archean;  the 
Lower  Marquette,  and  the  Upper  Marquette,  the  two  latter  constitut- 
ing the  Algonkian  for  this  district.  All  of  these  are  cut  by  basic  in- 
trusives.  The  pre-Cambrian  rocks  are  unconformably  overlain  by 
Cambrian  sandstone. 
The  Basement  Complex  occurs  in  two  main  areas,  one  north  of  the 
Marquette  series,  called  the  Northern  Complex,  and  one  south  of  the 
Marquette  scries,  called  the  Southern  Complex.  There  are  also  iso- 
lated areas  within  the  Algonkian.  The  oldest  rocks  of  the  Basement 
Complex  are  thoroughly  crystalline,  foliated  schists  and  gneisses.  A 
close  field  and  laboratory  study  has  failed  to  detect  in  them  any  evi- 
dence of  sedimentary  origin.  These  gneisses  and  schists  have  been 
cut  by  various  igneous  rocks  at  different  epochs.  The  latter  occur 
both  in  the  form  of  great  bosses  and  in  dikes,  sometimes  cutting, 
sometimes  parallel  to,  the  foliation  of  the  rocks.  In  the  area  of  the 
Northern  Complex  there  have  been  volcanic  outbursts,  and  a  vast 
series  of  lavas,  agglomerates,  greenstone  conglomerates,  and  tuffs  have 
been  piled  up.  By  far  the  greater  part  of  the  volcanic  material  is  of 
an  intermediate  or  basic  character. 
The  Northern  Complex  is  treated  under  the  divisions  of  Mona 
schists,  Kitchi  schists,  gneissoid  granites,  hornblende  syenites,  basic 
dikes,  acid  dikes,  peridotite,  and  ferruginous  veins.  The  Mona  and 
Kitchi  schists  are  greenstone  schists,  which  are  believed  to  be  largely 
recrystallized  volcanic  materials.  Their  original  forms  included  both 
tuffs  and  lavas.  The  gneissoid  granites  and  syenites  are  plutonic 
intrusive  rocks  within  the  greenstone  schists.  The  basic  dikes  are 
mainly  diabase.  The  peridotite  is  older  than  the  Cambrian  sandstone 
and  younger  than  the  greenstone  schists  of  the  Basement  Complex. 
The  ferruginous  veins  are  believed  to  be  water  deposited,  and  were 
formed  previous  to  the  deposition  of  the  Lower  Marquette  series. 
The  Southern  Complex  differs  from  the  Northern  Complex  in  the 
smaller  quantity  of  greenstone  schists  in  the  former  and  in  the  pres- 
ence in  it  of  the  micaceous  and  hornblendic  schists  and  the  Palmer 
gneiss.  It  is  treated  under  the  divisions  micaceous  schists,  amphi- 
bole  schists,  gneissoid  granites.  Palmer  gneiss,  and  intrusives.  The 
micaceous  schists  include  muscovite  schists,  biotite  schists,  feldspathj  : 
biotite  schists,  and  hornblendic  biotite  schists.  They  are  thought  to 
be  mashed  acid  eruptives.  The  amphibole  schists  include  greenstone 
schists,  hornblende  schists,  and  micaceous  hornblende  schists.     They 
"The  Algonkian  rocks  arc  described  by  Van  Hise;  the  Basemenl  Complex  and  later 
igneous  rocks  are  described  by  Bayley;  the  Republic  trough  is  described  by  Smyth. 
