LAKE    SUPERIOR    REGION.  27*7 
most  part  of  plutonic,  volcanic,  and  pyroclastic  rocks,  although  in 
sonic  of  the  upper  members  there  arc  more  or  less  aqueous  sediments. 
The  Coutchiching  and  Keewatin  arc  everywhere  in  strict  conformity, 
although  at  tin4  base  of  the  Keewatin  in  certain  localities  there  are 
conglomerates  regarded  as  local  and  volcanic. 
The  Laurentian  granites  and  gneisses  are  intrusive  in  the  Ontarian, 
and  are  therefore  younger,  the  relations  between  the  two  being  the 
same  as  described  by  Lawson  in  the  Rainy  Lake  district. 
Resting  discordantly  upon  the  Laurentian  and  Ontarian  rocks  is 
the  Steep  Rock  series,  presumably  of  Archean  age.  This  series  is  be- 
lieved to  be  a  folded  syncline,  rather  than  a  monocline,  as  described 
by  Smyth.  As  the  Animikie  series  exhibits  no  such  folding,  the  in- 
ference is  strong  that  the  Steep  Rock  series  is  older  than  the  Animikie. 
While  the  unconformity  between  the  Steep  Rock  series  and  the 
Laurentian  is  undoubted,  the  unconformity  between  the  Keew-atin  of 
Seine  River  and  the  Steep  Rock  Lake  series  is  not  at  all  obvious. 
Lithologically  the  two  series  are  strikingly  similar,  and  could  not  be 
separated  by  the  most  care  fid  study.  It  wTould  seem  that  to  the  west 
of  Steep  Rock  Lake  this  series  has  been  faulted  up  and  swrept  away, 
so  that  it  is  really  unconformably  above  the  KeewTatin.  The  Atic 
Oban  series  is  an  eruptive  one,  probably  belonging  to  the  Keewatin. 
Lawson,-1''  in  1893,  describes  the  laccolithic  sills  of  the  northwest 
coast  of  Lake  Superior.  The  trap  sills  are  mainly  diabases,  but  they 
occasionally  pass  into  gabbros.  It  is  held  that  there  are  no  con- 
temporaneous volcanic  rocks  in  the  Animikie  group,  and  that  the  trap 
sheets  are  intrusive  in  origin,  rather  than  subsequent  volcanic  flows, 
for  the  following  reasons:  They  are  simple  geological  units,  one  not 
overlapping  another;  they  have  a  uniform  thickness  over  areas  more 
than  100  square  miles  in  extent;  where  inclined,  the  dip  is  due  to 
faulting  and  tilting;  they  have  no  pyroclastic  rocks  associated  with 
them;  they  are  not  glassy  nor  amygdaloidal ;  they  show  no  flow 
structure  or  other  distinct  properties  of  effusive  rocks;  their  contacts 
with  the  slates  are  sharp:  they  never  repose  upon  a  surface  which  has 
been  exposed  to  weathering  or  erosion;  they  are  analogous  to  the  great 
dikes  of  the  region  in  all  their  relations;  they  may  be  observed  in 
direct  continuity  with  dikes;  they  pass  from  one  horizon  to  another; 
they  have  a  columnar  structure  extending  throughout  their  thickness; 
apophyses  pass  from  the  main  sheets  into  cracks  of  (he  slate  above 
and  below;  they  locally  alter  the  slates  above  and  below  them. 
The  Animikie  strata  have  been  dislocated  by  a  great  system  of 
faults,  the  orographic  blocks  having  been  frequently  tilted.  The  non- 
recognition  <>f  this  prevalent  tilted  structure  has  led  to  very  excessive 
estimates  of  the  thickness  of  the  series  by  Irving  and  Ingall.  In  the 
vicinity  of  Black  Sturgeon  River  and  on  the  isles  of  Nipigon  Bay 
