382  ,  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
tion  of  volcanics  extremely  difficult.  There  are  several  areas  of  vol- 
canics  concerning  whose  age  little  more  than  a  guess  can  be  made.  A 
close  study  may  demonstrate  that  some  of  the  fresher  eruptives  in 
both  the  upper  and  the  lower  and  middle  Huronian,  including  the 
great  intrusive  beds  of  the  Animikie,  are  really  Keweenawan  in  age. 
Also  the  numerous  very  fresh  dikes  of  like  character  cutting  the 
Archean  doubtless  represent  the  same  manifestation  of  igneous  ac- 
tivity. Strong  and  Rominger  long  ago  suggested  that  the  fresh  dikes 
in  the  St.  Louis  River  district  were  the  filling  of  the  pipes  through 
which  passed  the  outflows  of  KewTeenawan  time.  This  idea  is  not  only 
plausible  for  this  district,  but  is  probably  true  for  the  entire  Lake 
Superior  region. 
UNCONFORMITY    BENEATH    THE    ANIMIKIE,    OR    UPPER    HURONIAN. 
A  great  unconformity  at  the  base  of  the  Animikie  group  was  rec- 
ognized by  the  early  Canadian  geologists.  They  first  placed  the 
Animikie  with  the  Huronian,  but  the  fact  that  unconformably  below 
it  was  another  group  which  also  resembled  the  Huronian  led  the  later 
Canadian  geologists  to  exclude  the  entire  Animikie  from  the  Hu- 
ronian, and  they  have  thus  restricted  the  term  in  this  district  to  the 
pre-Animikie  Huronian  rocks.  Lawson  expressed  his  view  of  the 
importance  of  the  unconformity  when  he  proposed  to  call  it  the 
"  Eparchean  interval." 
The  unconformity  at  the  base  of  the  Animikie  was  also  recognized 
by  the  Winchells  near  Gimflint  Lake  and  westward  along  the  Mesabi 
range,  although  they  suggested  the  possibility  of  a  gradation  between 
the  Animikie  and  the  underlying  "  Keewatin  "  and  "  Vermilion  " 
rocks.  Van  Hise,  prior  to  the  detailed  work  of  the  United  States 
Geological  Survey  in  Minnesota,  believed  the  Animikie  to  grade  into 
the  steeply  inclined  and  metamorphosed  sediments  of  the  Vermilion 
district — the  lower-middle  Huronian  of  the  present  classification,  then 
called  the  "  Upper  Vermilion  series."  He  classed  the  Vermilion 
sediments  together  with  the  Animikie  as  upper  Huronian,  placing  in 
the  lower  Huronian  the  iron  formation  and  green  schists  of  the 
Vermilion  iron-bearing  district.  The  detailed  mapping  by  Van 
Hise,  Clements,  Bayley,  Leith,  and  Merriam  of  the  Vermilion  and 
Mesabi  districts  in  the  later  part  of  the  last  century  brought  out 
conclusive  evidence  of  the  unconformity  of  the  Animikie  group  upon 
the  underlying  Vermilion  group  and  its  equivalents.  The  latter 
rocks  were  therefore  called  lower  Huronian  (now  lower-middle)  and 
the  unconformity  between  them  and  the  Animikie  became  known  as 
an  inter-Huronian  unconformity.  The  Michigan  and  Wisconsin 
geologists  include  in  the  Huronian  the  equivalents  of  both  the  Anim- 
ikie and  the  pre-Animikie  Huronian,  and  while  their  reports  con- 
tained facts  which  clearly  pointed  to  a  discordance  within  the  rocks 
