LAKE    SUPERIOR   REGION.  353 
the  Animikie  and  the  usually  small  sizes  of  the  pebbles  also  are  in 
accord  with  the  supposition  of  base-leveling  in  the  preexisting  land 
areas. 
In  the  past  the  Canadian  geologists,  observing  the  unconformity  at 
the  base  of  the  upper  Huronian,  principally  on  the  north  shore,  have 
been  so  impressed  with  its  magnitude  and  the  striking  differences  in 
deformation  and  metamorphism  of  the  rocks  above  and  below  that 
they  have  refused  to  regard  it  as  an  inter-Huronian  unconformity, 
and  have  maintained  that  it  separates  the  Cambrian  above  (in  which 
they  included  Keweenawan  and  Animikie  rocks)  from  the  Huronian 
below.  Their  emphasis  on  this  unconformity  is  not  surprising  when 
we  remember  that  on  the  Canadian  shore  the  Animikie  group  rests 
on  steeply  tilted  and  highly  metamorphosed  rocks  of  Archean  age 
and  only  locally  on  rocks  of  lower-middle  Huronian  age.  On  the 
south  shore  the  unconformity  is  still  a  great  one,  but  not  so  striking 
as  that  on  the  north  shore,  for  the  Animikie  largely  rests  upon  middle 
and  lower  Huronian  rocks  not  contrasting  so  markedly  in  lithology 
and  structure  with  the  Animikie  as  do  the  Archean  rocks. 
Farther  south,  in  the  Crystal  Falls,  Felch,  Iron  River,  and  Menomi- 
nee districts,  the  upper  Huronian  appears  to  be  nearly  conformable, 
structurally,  with  underlying  sediments  which  are  similar  lithologi- 
cally  to  and  have  been  correlated  with  the  lower  Huronian  of  the 
Marquette  district.  The  significance  of  this  relation  is  discussed 
on  page  362. 
UNCONFORMITY    BENEATH    THE    AJIBIK    QUARTZITE     (MIDDLE 
HURONIAN). 
Seaman  has  shown  the  existence  of  an  unconformity  at  the  base  of 
the  Ajibik  quartzite  in  the  Marquette  district,  evinced  by  basal  con- 
glomerates and  truncation  of  the  underlying  series.  The  Ajibik 
quartzite  is  found  in  contact  with  each  of  the  three  members  of  the 
sedimentary  series  below  and  with  the  Keewatin,  indicating  that  the 
erosion  plane  has  cut  completely  across  the  underlying  series.  The 
Ajibik  quartzite  and  younger  rocks  are  also  slightly  less  metamor- 
phosed than  those  beneath  the  unconformity. 
UNCONFORMITY   BENEATH   THE  HURONIAN   SERIES. 
The  unconformity  at  the  base  of  the  Huronian  series  is  more 
obscure  than  higher  ones,  and  has  been  less  widely  recognized.  Fail- 
ure to  recognize  this  break  has  been  due  to  the  high  degree  of 
metamorphism,  which  has  often  obliterated  evidence  of  unconform- 
ity and  given  rocks  parallel  secondary  structures  on  both  sides  of  it ; 
to  the  similar  lithological  character  shown  by  the  metamorphosed 
sediments  and  the  underlying  phases  of  the  Archean,  particularly 
55721— Bull.  360—09 23 
