350  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
(5)  The  lack  of  igneous  rocks  in  the  Cambrian  is  not  regarded  as 
good  evidence  for  unconformity,  because  igneous  rocks  are  also  lack- 
ing from  the  upper  part  of  the  Keweenawan. 
(6)  The  lack  of  fossils  in  the  lower  series  is  to  be  explained  because 
of  the  character  of  the  sediments. 
(7)  It  is  improbable  that  a  very  considerable  interval  should 
elapse  with  no  deposition  in  the  Lake  Superior  basin,  and  that  depo- 
sition should  begin  again  with  so  little  evidence  of  erosion  or  valley 
trenching  of  the  upper  Keweenawan. 
(8)  If  the  Animikie  group  is  ever  found  to  be  the  equivalent  of 
the  Belt  series  of  British  Columbia,  as  is  thought  possible,  this  would 
make  more  probable  the  Cambrian  age  of  the  Keweenawan  lying 
above  the  Animikie,  in  view  of  the  fact  that  in  British  Columbia  the 
Belt  series  apparently  grades  up  into  the  Cambrian. 
Against  this  evidence  of  possible  conformity  and  favoring  the 
view  here  held  that  the  Cambrian  lies  unconf  ormably  above  the  upper 
Keweenawan  are  the  following  facts : 
(1)  The  Cambrian  is  fossiliferous,  but  the  Keweenawan  is  not. 
(2)  The  Cambrian  contrasts  with  the  Keweenawan  in  lacking 
volcanism  and  deformation.  So  far  as  they  share  in  deformation,  it 
is  through  faulting  which  may  have  taken  place  as  late  as  the 
Cretaceous. 
(3)  The  Cambrian  rests  upon  a  peneplain  of  continental  extent, 
over  which  the  Paleozoic  sea  swept  and  deposited  Paleozoic  sediments, 
with  overlap  relations  to  the  pre-Cambrian  rocks.  This  sea  did  not 
reach  the  Lake  Superior  country  until  upper  Cambrian  time,  and 
parts  of  Canada  were  not  reached  until  Ordovician  time.  If  the 
Keweenawan  is  Cambrian  it  constitutes  a  local  marked  variation  from 
the  general  uniform  conditions  of  overlap.  The  upper  Keweenawan 
sediments  rest  on  a  plane  which  cuts  the  pre-Cambrian  peneplain  at 
a  considerable  angle,  as  is  well  shown  on  Keweenaw  Point. 
(4)  The  similarity  of  lithology  and  accordance  of  structure  be- 
tween the  Keweenawan  and  Cambrian  are  the  natural  sequence  of 
transgression  of  a  sea  over  flat-lying  sediments.  The  conditions  are 
not  different  from  those  that  would  prevail  if  the  ocean  were  to 
transgress  to-day  from  the  Gulf  of  Mexico  across  the  flat-lying  and 
little-consolidated  Paleozoic  sediments  of  the  upper  Mississippi 
Valley.  It  would  be  extremely  difficult  to  prove  the  unconformity  in 
any  limited  area,  especially  where  exposures  are  not  numerous.  In 
this  connection  it  should  be  remarked  that  the  low  dips  of  the  Cam- 
brian strata  up  to  15°,  as  at  Ashland,  are  no  evidence  against  this 
hypothesis,  for  the  highest  dips  which  the  Cambrian  has  are  not 
greater  than  may  occur  with  similar  sediments  along  a  steep  shore. 
Therefore  the  dips  of  the  Cambrian  sandstone  may  be  initial  and  con- 
sequently may  afford  no  evidence  that  deformation  has  taken  place 
since  their  deposition. 
