326  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
The  following  succession  and  nomenclature  are  recognized  and 
adopted : 
Cambrian — Upper  sandstones,  etc.,  of  Lake  Superior. 
Unconformity. 
Pre-Cambrian  : 
Keweena wan    ( Nipigon )  .a 
Unconformity. 
Upper   ( Animikie ) . 
Unconformity. 
Huronian<j  Middle. 
Unconformity. 
Lower. 
Unconformity. 
Keewatin. 
Eruptive  contact. 
Laurentian. 
Lane,339  in  1905,  discusses  the  observations  and  report  of  the  inter- 
national geological  committee.  He  disagrees  with  the  committee  in 
believing  that  the  Keweenawan  is  Cambrian  rather  than  pre-Cam- 
brian,  for  the  following  reasons: 
1.  The  Keweenawan  is  very  thick,  but  this  thickness  is  composed  of  rocks 
which  may  accumulate  with  extreme  rapidity — sandstones,  conglomerates,  and 
sheets  of  trap,  single  flows  of  which  are  hundreds  of  feet  thick— so  that  geo- 
logically it  may  represent  no  greater  time  than  the  Iceland  deposits. 
2.  Its  lithological  character  is  such  that  fossils  would  hardly  be  expected. 
3.  Intense  volcanic  activity  characterizes  its  middle  and  lower  parts.  Intru- 
sions occur  in  the  lower  part.  Conglomerates  and  sandstones  are  often  derived 
from  material  of  the  formation  itself.  We  even  find  pebbles  of  Lower  Keweena- 
wan agates  and  intrusives  in  the  Upper  Keweenawan. 
4.  We  are  therefore  prepared  to  find,  as  we  do,  that  the  lower  part  has  steeper 
dips,  more  faults  with  greater  throws,  and  was  evidently  much  disturbed  before 
the  Upper  Keweenawan  was  laid  down. 
5.  The  Lower  Keweenawan,  which  is  overlain  very  unconformably  by  the 
horizontal  Lake  Superior  (Potsdam)  sandstone,  also  stands  in  the  same  relation 
to  the  Upper  Keweenawan. 
6.  There  is  a  fairly  steady  approximation  in  dip,  lithological  character,  geo- 
graphical distribution,  and  degree  of  disturbance,  from  the  base  of  the  Keweena- 
wan, through  its  upper  members,  to  the  Lake  Superior  sandstone. 
7.  This  points  to  one  great  volcanic  epoch,  gradually  dying  out,  and  one  great, 
generally  continuous  movement  of  depression.  I  do  not  know  of  any  evidence 
of  uplift  and  erosion  of  the  Upper  Keweenawan  before  the  laying  down  of  the 
Lake  Superior  sandstone,  which  may  be  conceived  as  a  normally  following  envel- 
oping mantle,  and  generally  contains  practically  no  material  which  must  and 
but  little  that  may  be  derived  from  this  Keweenawan. 
8.  But  along  the  southern  contact  of  the  Copper  (Keweenawan)  range  motion 
along  a  great  fault  line  took  place,  disturbing  the  unconformable  contact  between 
the  Lake  Superior  sandstone  and  the  base  of  the  Keweenawan,  probably  down 
to  the  time  of  the  Niagara,  which  is  caught  in  the  Limestone  Mountain  syn- 
a  Lane  dissents  as  to  the  position  of  the  Keweenawan  as  follows  :  "  The  use  of  the 
pre-Cambrian  above  does  not  imply  unanimity  in  the  committee  with  regard  to  the  pre- 
Cambrian  correlation  of  the  Keweenawan — a  topic  the  committee  as  such  did  not  investi- 
gate." 
