354  PKE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
where  these  are  greenstones  and  green  schists  of  the  Keewatin,  and, 
finally,  to  the  fact  that  the  banded  and  contorted  gneiss,  which  is 
prominent  in  the  basal  complex,  does  not  differ  greatly  in  lithological 
character  from  some  areas  of  later  granite  gneiss  which  have  in- 
truded the  elastics.  This  later  granite  gneiss  is,  however,  usually 
somewhat  nearer  the  normal  form  of  an  eruptive  rock,  not  having 
suffered  so  many  vicissitudes  in  its  briefer  history.  Those  whose 
attention  has  been  mainly  directed  to  the  contact  phenomena  of  the 
intrusive  granite  gneisses  have  generally  refused  to  believe  in  an 
.earlier  granite  gneiss,  although  recognizing,  at  least  in  some  cases, 
that  the  lowest  detrital  rocks  bear  numerous  fragments  of  a  granite 
gneiss.  On  the  other  hand,  those  whose  attention  has  been  directed 
to  the  unconformities,  as  indicated  by  basal  contacts  and  other  phe- 
nomena, between  the  basal  complex  and  the  clastic  series  have  some- 
times been  disinclined  to  believe  in  the  existence  of  important  areas 
of  granite  gneiss  which  are  intrusives  later  than  the  clastic  series. 
Generally,  in  the  districts  which  have  been  studied  by  individual 
writers,  the  phenomena  of  the  one  class  are  conspicuous  while  those 
of  the  other  class  are  unimportant  or  perhaps  lacking  altogether. 
Naturally  this  has  engendered  an  inclination  in  each  observer  to  con- 
clude that  the  relations  which  have  strongly  impressed  him  are  true 
of  the  entire  Lake  Superior  region. 
Notwithstanding  the  above  considerations,  conclusive  evidence  of 
the  existence  of  a  profound  unconformity  between  the  lower  Huro- 
nian  and  the  underlying  rocks  is  now  known  in  many  parts  of  the 
Lake  Superior  region.  Some  of  the  best  evidence,  and  the  first  to 
be  worked  out,  appears  in  the  Marquette  district.  Here  basal  con- 
glomerates of  the  lower  Huronian  are  found  in  contact  with  all  the 
members  of  the  Keewatin  and  Laurentian  divisions  of  the  Archean. 
Commonly  parallel  schistosity  has  been  developed  along  the  contact, 
in  places  entirely  destroying  the  conglomerates  and  giving  the  ap- 
pearance of  a  gradational  zone.  However,  there  is  usually  a  con- 
siderable contrast  between  the  massive  and  schistose  igneous  rocks 
on  one  side  of  the  contact  and  the  clearly  recognizable  sedimentaries 
on  the  other.  In  places  also,  as  near  the  Carp  River  and  south  of 
Marquette,  the  cleavage  of  the  underlying  Keewatin  stands  at  a  high 
angle  to  the  surface  of  contact  with  the  Huronian.  Dikes  are  found 
penetrating  the  Archean  rocks  which  do  not  enter  the  Huronian 
rocks  above.  Met  amor  phism  and  deformation  in  general  are  much 
more  pronounce'd  in  the  Archean  rocks  than  in  the  Huronian.  In 
short,  there  appear  all  of  the  evidences  usually  cited  to  prove  the 
existence  of  unconformity. 
In  the  Crystal  Falls  and  Menominee  districts  the  basal  conglom- 
erates of  the  Sturgeon  quartzite  at  the  base  of  the  Huronian  series, 
resting  against  members  of  both  the  Keewatin  and  the  Laurentian 
