478  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
although  most  widespread  and  abundant  in  Canada.  That  such  a  basal 
system  exists  is  no  new  idea;  but  it  has  not  generally  been  recognized 
that  between  it  and  the  Cambrian  there  elapsed  an  era  in  which  were 
alternating  cycles  of  the  deposition  of  systems  of  rocks  and  of  vast  ero- 
sion intervals. 
As  here  used  the  term  Archean  is  restricted  to  this  fundamental  com- 
plex. It  is  no  longer  possible  to  regard  as  a  unit  or  treat  together  all 
the  pre-Cambrian  rocks.  The  rocks  included  in  the  fundamental  com 
plex  are  everywhere  called  Azoic  or  Archean.  The  crystallines  and 
semicrystallines  above  this  complex,  often  called  Archean,  must  be  dis- 
tributed from  the  Devonian  or  later  to  the  pre-Cambrian.  It  is  clear 
that  if  Archean  is  to  remain  a  serviceable  term  it  must  be  restricted  to 
some  unit.  Such  a  unit  is  the  fundamental  complex,  and  to  it  this  term 
is  most  appropriate. 
ORIGIN   OP   THE   ARCHEAN. 
As  has  been  shown,  whatever  the  origin  of  the  Archean,  it  is  of  vast 
age.  It  will  be  remembered  that  south  of  the  lake  Superior  region, 
where  is  known  the  greatest  volume  of  pre-Cambrian  elastics,  there  is 
abundant  evidence  showing  that  the  lowest  of  the  clastic  series  has 
derived  its  debris  from  and  rests  upon  the  foliated  edges  of  the  Archean. 
In  the  Arizona  region,  in  which  the  time  of  the  pre-Cambrian  elastics 
is  only  inferior  to  that  of  the  lake  Superior  region,  the  evidence  of  a 
great  hiatus  below  these  elastics  is  of  the  most  decisive  character. 
The  same  may  be  said  of  several  of  the  other  areas  of  Archean.  Conse- 
quently in  many  regions  the  Archean  complex,  in  essentially  its  present 
condition,  maybe  asserted  upon  definite  structural  evidence,  to  be  vastly 
older  than  the  Cambrian. 
As  to  the  origin  of  the  Archean  rocks,  three  different  views  are 
prominent:  (1)  The  Archean  has  been  considered  as  metamorphosed 
detrital  rocks;  (2)  it  has  been  considered  as  igneous,  but  later  in  origin 
than  certain  of  the  x>re- Cambrian  elastics  with  which  it  is  in  contact: 
(3)  it  has  been  considered  as  igneous  and  representing  a  part  of  the 
original  crust  of  the  earth,  and  therefore  earlier  than  any  sedimentarie^ 
A  modification  of  this  theory  is  suggested  under  the  topic  Delimitation 
of  the  Archean. 
(1)  Those  who  believe  in  the  detrital  origin  of  the  Archean,  as  above ( 
defined,  will  not  question  the  conclusion  reached  as  to  the  age  of  the 
fundamental  complex ;  for  to  produce  results  so  different  from  any 
known  metaniorphic  clastic  series  must  not  only  imply  great  age,  but 
probably  sediments  which  were  originally  deposited  under  different  con- 
ditions from  those  of  later  times.  This  school,  while  believing  in  the 
detrital  origin  of  the  Archean  as  a  whole,  is  conscious  that  it  has 
been  cut  again  and  again  by  eruptives  of  all  kinds;  that  the  supposed 
elastics  have  thereby  been  profoundly  metamorphosed  by  contact  and 
dynamic  action,  and  often  have  been  so  changed  that  the  place  can  not 
bej)ointed  out  where  the  intrusives  end  and  the  elastics  begin, 
