368  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
tural  basis,  if  the  Animikie  group  be  included.  The  successions  de- 
duced by  the  geologists  who  have  made  the  most  extended  study  of  the 
Lake  Superior  region  are  as  follows : 
Logan  makes  the  Keweenawan  a  downward  extension  of  the  Lake 
Superior  sandstone.  Below  the  Keweenawan  series,  as  before  defined, 
is  a  set  of  slates  (the  Animikie)  of  very  considerable  thickness,  which 
are,  however,  a  part  of  the  "  Upper  Copper-bearing  group,"  and 
therefore  superior  to  the  original  Huronian  or  "  Lower  Copper-bear- 
ing group."  The  Animikie  rests  unconformably  upon  the  Huronian. 
As  to  the  relations  of  the  Huronian  and  "  Laurentian  "  about  Lake 
Superior  little  is  said,  except  that  at  one  place  they  appear  to  be 
conformable  and  grade  into  each  other.  We  thus  have  Logan's  suc- 
cession :  Lake  Superior  sandstone ;  Keweenawan ;  Animikie ;  uncon- 
formity; Huronian;  Laurentian.  The  Animikie,  as  well  as  the 
Keweenawan,  is  regarded  as  a  part  of  the  Cambrian  system. 
Selwyn's  succession  differs  from  Logan's  only  in  that  he  maintains 
that  all  of  the  rocks  underlying  the  Animikie  in  Canada  constitute 
one  general  conformable  succession,  but  divisible  into  tAVo  systems  on 
lithological  grounds.  These  are  the  Laurentian  and  the  Huronian. 
This  order  is  also  that  of  Bell.  With  these  authors  the  "  Laurentian  " 
is  granitoid  and  gneissic,  while  the  Huronian  is  quartzose,  horn- 
blendic,  schistose,  and  slaty. 
Foster  and  Whitney's  succession  is:  Keweenawan,  which  includes 
the  Lake  Superior  sandstone ;  unconformity ;  Azoic — the  latter  said  to 
be  indivisible  except  on  the  north  shore,  and  the  granites  are  intrusive 
rocks  later  than  the  "Azoic  "  slates.  On  the  north  shore  the  Animikie 
reposes  upon  the  granite.  Until  recently  Wadsworth  has  held  to  the 
same  succession  as  Foster  and  Whitney.  In  his  last  paper,  however, 
he  states  that  it  is  probable  that  in  the  Marquette  "Azoic  "  there  are 
three  distinct  geological  formations  or  ages,  to  which  he  applies, 
beginning  at  the  base,  the  terms  Cascade,  Republic,  and  Holyoke. 
The  last  two  are  unconformable  with  each  other. 
Macfarlane  recognizes  a  "  Huronian  "  and  a  "  Laurentian,"  but 
regards  both  series  as  wholly  of  igneous  origin  and  the  distinction 
between  the  two  a  lithological  one,  the  basic  green  schists  referred  to 
the  Huronian  being  newer  than  the  granite  and  gneiss,  and  the 
pseudo-conglomerates  found  in  the  "  Huronian  "  a  consequence  of  the 
intrusion  of  the  latter,  in  which  process  fragments  of  granite  and 
gneiss  were  caught.  The  Keweenawan  is  full  of  debris  from  the 
Huronian. 
Brooks,  Pumpelly,  Irving,  Chamberlin,  Sweet,  and  Wright  recog- 
nize about  the  same  general  succession:  Lake  Superior  sandstone; 
unconformity;  Keweenawan;  unconformity;  then  a  great  system  of 
rocks  included  in  the  Huronian;  another  unconformity;  and  then  a 
complex  of  granites,  gneisses,  and  schists.    Irving  in  his  later  work 
