174  PRE-OAMBEIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
cutting  the  fundamental  complex  doubtless  represent  in  other  series 
the  same  manifestation  of  igneous  activity. 
UNCONFORMITY   AT  BASE   OF   CLASTIC   SERIES. 
In  attempting  to  determine  how  far  the  different  views  held  as  to  lake 
Superior  stratigraphy  are  really  in  harmony,  it  is  desirable  to  have,  if 
possible,  as  starting  planes  upper  and  lower  horizons.  From  the  pre- 
ceding pages  it  is  evident  that  for  the  first  of  these  we  have  the  base 
of  the  Keweenawan.  Nearly  all  are  agreed  that  below  this  series  is  a 
break  more  or  less  considerable,  and  all  are  agreed  that  it  is  a  recog- 
nizable plane.  It  appears  to  the  writer  that  the  evidence  shows  the 
existence  of  another  general  plane  for  the  lake  Superior  region  at 
the  horizon  elsewhere  defined  as  separating  the  Archean  from  the 
Algonkian — that  is,  the  plane  between  the  basement  granite-gneiss- 
schist  complex  and  the  overlying  clastic  series  with  their  equivalent 
crystallines.  The  failure  generally  to  recognize  this  plane  is  due  to  the 
fact  that  the  banded  and  contorted  gneiss,  which  is  the  most  prominent 
rock  of  the  basal  complex,  does  not  differ  greatly  in  lithological  char- 
acter from  areas  of  later  granite-gneiss  which  have  intruded  the  elas- 
tics. This  later  granite-gneiss  is,  however,  usually  somewhat  nearer 
the  normal  form  of  an  eruptive  rock,  not  having  suffered  so  many  vicis- 
situdes 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  phenomena  between  the  basal  complex  and 
the  clastic  series,  have  sometimes  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  con- 
spicuous while  those  of  the  other  class  are  unimportant  or  perhaps 
lacking  altogether.  Naturally  this  has  engendered  an  inclination  in 
each  observer  to  conclude  that  the  relations  which  have  strongly  im- 
pressed him  are  true  of  the  entire  lake  Superior  region. 
Bell  and  Selwyn  find  no  evidence  of  discordance  between  their  Lau- 
rentian and  Huronian.  Bell,  in  1873,  says  the  distinction  between  the 
Laurentian  and  Huronian  is  chiefly  of  a  lithological  character,  while 
Selwyn,  in  1883,  states  that  he  can  give  no  better  reason  for  supposing 
that  certain  sets  of  beds  belong  to  the  Laurentian  and.  others  to  the 
Huronian  than  a  considerable  difference  in  lithological  character,  the 
former  being  essentially  granitoid,  gneissic,  and  feldspathic,  while  the 
latter  is  quartzose,  hornblendie,  schistose,  and  slaty. 
There  is,  however,  much  evidence  that  the  plane  between  the  Algom 
kian  and  Archean  is  definitely  fixed  over  much  of  the  lake  Superior 
region  by  a  great  unconformity. 
