178  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
the  basal  complex  and  the  elastics.  When  a  clastic  series  grades  down 
into  a  crystalline  one,  this  niay "be  caused  by  progressive  metamorphism5 
by  contact  action  of  a  subsequent  intrusive,  or  by  later  folding,  which 
destroys  the  original  bedding  of  both  scries  and  produces  a  common 
secondary  structure,  at  the  same  time  causing  the  newer  scries  to  assume 
a  crystalline  character.  Such  induced  conform  it  y  and  transit  i<  >n  are  the 
more  likely  to  occur  when  the  materials  of  the  newer  series  are  chiefly 
derived  from  the  older,  and,  as  shown  by  Pumpelly,  will  be  particularly 
likely  to  be  perfect  when  the  earlier  series  has  suffered  atmospheric  dis- 1 
integration  before  the  deposition  of  the  later.  Future  study  may  show 
still  other  causes  of  gradations  between  unconformable  series. 
Evidently  north  and  east  of  lake  Superior  we  are  not  without  posi- 
tive evidence  that  a  great  physical  break  occurs  at  certain  points 
between  an  ancient  granite-gneiss-schist  complex  and  the  clastic  series, 
which,  however,  are  often  cut  by  recent  intrusive  granite-gneisses.  Also 
at  many  localities  in  which  an  unconformity  has  not  been  positively 
shown  the  evidence  at  hand  points  in  this  direction.  When  the  condrl 
tions  are  considered  which  are  necessary  to  produce  an  unconformity 
it  is  difficult  to  see  how  one  can  really  be  of  a  local  nature.  Thus  while 
it  can  not  be  asserted  that  a  universal  structural  break  north  and  east 
of  lake  Superior  exists  between  the  Algonkian  and  Arckean,  there  is: 
a  probability  that  such  is  the  case. 
The  foregoing  evidence  combined  gives  a  strong  case  of  probability 
for  a  general  structural  break  in  the  lake  Superior  region  between  the 
lowest  clastic  series  and  a  basement  crystalline  complex.  However,  it 
can  not  be  denied  that  certain  of  the  contacts,  cited  as  showing  this  un- 
conformity, although  unquestionably  beneath  the  lowest  clastic  series 
for  particular  districts,  may  be  at  higher  strati  graphical  positions  than 
the  base  of  the  lowest  clastic  series  of  the  whole  lake  Superior  region. 
Before  the  question  of  a  break  at  this  position  can  be  considered  as  set- 
tled beyond  all  question  for  the  entire  lake  Superior  region  much 
more  detailed  mapping  must  be  done.  However,  the  existence  of  this 
break  for  many  districts  is  so  strongly  supported,  that  it  gives  for  the 
present  the  best  available  guidance  as  to  one  fixed  horizon  for  Compaq 
isons  of  the  rock  series  of  the  different  districts.  The  recognition  of  this  | 
break  does  not  imply  that  the  lowest  clastic  series  at  certain  localities 
are  not  penetrated  by,  and  now  rest  upon,  intrusive  granite-gneiss,  but 
in  such  cases  the  evidence  of  this  break,  if  it  once  existed,  has  been  de- 
stroyed. 
We  have,  then,  for  structural  work  two  starting  planes,  the  base  of  the 
Keweenawan  and  the  base  of  the  elastics,  included  between  which  are 
the  larger  part  of  the  series  of  rocks  placed  in  the  Huronian  by  the 
earlier  <  Janadian  geologists,  the  Animikie,  Lawson/s  Keewatin  the  Profs. 
Winchell's  Keewatin  and  Animikie,  Irving's  Huronian,  and  perhaps  in 
part  the  Profs.  Winchells'  Vermilion  and  Lawson's  Ooutchiching. 
