LAKE   SUPERIOR   REGION.  349 
rocks  than  into  the  higher  series,  the  lower  series  shows  in  general 
more  profound  metamorphism  than  the  upper  series. 
FOSSILS     IN     LAKE     SUPERIOR     PRE-CAMBRIAN. 
The  discovery  of  fossils  has  been  reported  from  time  to  time  in  the 
pre-Cambrian  formations  in  various  parts  of  the  Lake  Superior  re- 
gion, but  in  no  case  has  the  discovery  been  verified  by  subsequent 
study.  Presumptive  evidence  of  life  is  afforded  by  the  presence  of 
carbonaceous  slates  and  of  limestone,  presumably  developed  at  least 
in  part  through  organic  agencies.  Also  the  existence  of  cleanly 
sorted  sediments  in  the  Algonkian  points  to  complete  decomposition 
in  preexisting  land  areas,  a  process  usually  requiring  organic  agencies. 
UNCONFORMITY  BENEATH  THE  CAMBRIAN. 
In  discussing  the  relation  between  the  Cambrian  and  Keweenawan, 
distinction  must  be  made  between  the  lower-middle  Keweenawan 
and  the  upper  Keweenawan.  It  is  agreed  by  all  that  there  is  a  pro- 
found unconformity  between  the  lower-middle  Keweenawan  and  the 
Cambrian.  The  former  beds  dip  steeply;  the  latter  are  flat-lying. 
Actual  unconformable  contacts  may  be  observed  in  the  St.  Croix  area 
and  in  the  area  southeast  of  Keweenaw  Point.  But  the  dips  of  the 
Keweenawan  become  progressively  lower  in  the  upper  part  of  the 
series,  and  the  dip  of  the  upper  beds  is  so  low  as  to  suggest  possible 
gradation  into  the  Cambrian  sandstone.  It  is  the  contention  of 
Seaman,  Lane,  and  others  that  such  gradation  exists.  The  evidence 
favoring  it  is  as  follows: 
(1)  At  the  mouth  of  Montreal  River  the  dip  of  the  Keweejiawan 
beds  is  80°  toward  the  lake;  half  a  mile  to  the  west,  at  the  mouth  of 
Oronto  River,  it  is  52°  in  the  same  direction;  and  the  next  exposures 
5  miles  to  the  west,  mapped  as  Cambrian  by  Irving,  dip  as  high  as 
15°  in  the  same  direction.  Farther  to  the  west,  toward  Ashland,  are 
the  flat-lying  Cambrian  sandstones.  Fossils  have  not  been  found  in 
the  Cambrian  near  the  contact. 
(2)  Lithologically  the  upper  Keweenawan  and  the  Cambrian  are 
similar  in  their  feldspathic  content,  in  their  reddish  color,  in  contain- 
ing shale  bands,  and  in  their  degree  of  induration,  though  the  Cam- 
brian as  a  whole  may  be  somewhat  more  quartzose  and  less  shaly  and 
feldspathic  than  the  Keweenawan. 
(3)  No  contacts  between  the  Cambrian  and  upper  Keweenawan 
have  been  observed  where  not  complicated  by  fault  relations. 
(4)  The  Cambrian  sandstone  and  the  upper  Keweenawan  were 
naturally  regarded  as  the  same  by  some  of  the  earlier  writers  and  the 
burden  of  proof  rests  on  one  who  would  separate  them.  In  fact  there 
has  always  been  doubt  as  to  the  assignment  of  certain  exposures  to 
one  or  the  other  series. 
