38  PRE-CAMBRIAN    GEOLOGY    OF. NORTH    AMERICA. 
of  amount  of  volcanism,  by  differences  in  metamorphism,  and  by  the 
field  distribution.  The  evidence  is  such  that  their  importance  can 
not  be  doubted. 
While  these  unconformities  doubtless  do  mark  unequal  intervals 
of  time,  no  evidence  is  available  to  justify  the  emphasis  of  one  of  them 
more  than  another.  However,  Lawson  has  selected  the  unconformity 
below  the  Animikie  as  of  greatest  importance  and  has  named  it  the 
"  Eparchean  interval."  Where  the  Animikie  rests  directly  upon  the 
lower  Huronian  or  the  Archean  this  unconformity  is  naturally 
greater  than  where  it  rests  upon  the  middle  Huronian,  for  in  such 
cases  the  unconformity  at  the  base  of  the  Animikie  marks  not  merely 
one  unconformity  but  is  equivalent  to  two  or  three  elsewhere  with  the 
intervening  groups.  It  is  plain  that  the  relative  importance  of  the 
various  unconformities  can  be  determined  only  where  the  succession 
is  full  and  where  therefore  the  Animikie  is  in  contact  with  the  under- 
lying middle  Huronian.  Where  this  is  the  case,  as  in  the  Marquette 
district,  there  is  no  evidence  whatever  that  the  unconformity  at 
the  base  of  the  Animikie  is  more  important  than  the  one  separating 
the  middle  and  the  lower  Huronian,  or  the  one  separating  the 
Animikie  and  the  Keweenawan,  and  in  the  Crystal  Falls  and  Me- 
nominee districts  this  unconformity  is  very  slight  or  lacking. 
In  no  other  part  of  North  America  have  so  many  unconformities 
within  the  Algonkian  been  determined  as  in  the  Lake  Superior  re- 
gion. Over  the  great  northern  area  of  Canada  it  is  probable  that  all 
three  of  the  unconformities  are  represented,  but  there  have  been  de- 
tailed mapping  and  correlation  in  but  limited  areas. 
In  the  Grand  Canyon  district  is  exposed  an  unconformity  between 
the  two  groups  of  the  Algonkian. 
In  Montana  an  unconformity  is  supposed  to  exist  between  the  Belt 
series  and  the  Cherry  Creek  group,  believed  to  belong  to  the  Algon- 
kian, although  such  unconformity  has  not  been  proved. 
In  general,  outside  of  the  Lake  Superior  and  Lake  Huron  regions 
unconformities  within  the  Algonkian  have  been  inferred  at  a  number 
of  places,  but  the  only  definitely  proved  unconformity  is  in  the  Grand 
Cairyon  region. 
Each  of  the  Algonkian  unconformities  may  be  represented  else- 
where in  North  America  by  a  continuous  section  of  sediments.  To 
what  extent  this  is  true  is  not  yet  known.  The  discussion  of  the  cor- 
relation shows  how  fragmentary  is  the  knowledge  on  which  we  must 
rely  to  decide  this  matter. 
UNCONFORMITY  BETWEEN  ALGONKIAN  AND  ARCHEAN. 
In  large  areas  of  the  Lake  Superior  region  the  Archean  was  nearly 
base-leveled  before  the  deposition  of  the  Algonkian.  The  contrast 
of  the  two  systems  in  volcanism,  in  metamorphism,  and  in  relative 
