306  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
Valley  Cambrian  sandstone.  It  has,  however,  already  been  shown 
that  the  Keweenawan  rocks  lie  unconformably  under  the  Cambrian 
sandstone  of  the  Mississippi  Valley  in  western  Wisconsin. 
The  Animikie  series  in  the  Thunder  Bay  district  is  of  great  thick- 
ness, probably  upward  of  10,000  feet,  comprising  quartzites,  quartz 
slates,  clay  slates,  magnetitic  quartzites,  sandstones,  thin  limestone 
beds,  and  beds  of  cherty  and  jaspery  material.  With  these  are  asso- 
ciated in  great  volume,  both  in  interbedded  and  in  intersecting 
masses,  coarse  gabbro  and  fine-grained  diabase  like  those  well  known 
in  the  Keweenaw  series.  A  broad  examination  of  the  region  shows 
that  there  is  little  ground  for  the  belief  in  one  crowning  overflow. 
The  Animikie  series  is  lithologically  like  the  Penokee  range  in  Wis- 
consin ;  both  series  bear  the  same  relations  to  the  newer  Keweenawan 
rocks  and  the  older  gneisses,  and  the  two  groups  are  regarded  as  the 
same. 
The  Animikie  rocks  are  also  the  equivalent  of,  if  not  actually  con- 
tinuous with,  the  Mesabi  iron  range  running  to  Pokegama  Falls  and 
the  slates  of  St.  Louis  River,  although  these  latter  are  affected  by 
slaty  cleavage. 
The  Original  Huronian  of  Logan  is  compared  with  the  Animikie 
slates  of  Thunder  Bay  and  the  two  are  regarded  as  equivalent.  The 
Marquette  and  the  Menominee  Huronian,  with  minor  exceptions  due 
perhaps  to  metasomatic  changes,  are  lithologically  like  the  Animikie 
and  Penokee  series,  and  are  also  regarded  as  belonging  to  the  same 
horizon.  The  Original  Huronian,  the  Animikie  slates,  the  Penokee 
iron  rocks,  and  the  iron-bearing  rocks  of  Marquette  and  Menominee 
appear,  then,  to  belong  together  and  hence  may  properly  be  called 
Huronian.  The  Huronian  schists  in  each  of  these  areas  are  limited 
by  granite  and  gneiss.  There  are,  however,  considerable  areas  of 
crystalline  schists  the  relations  of  which  are  doubtful,  and  it  is  sus- 
pected that  in  several  of  the  iron  regions  there  are  two  distinct  kinds 
of  schists — those  belonging  to  the  Huronian,  and  a  schistose  greenish 
phase  belonging  with  an  older  series.  It  is  also  possible  that  a  por- 
tion of  the  granites  are  eruptive  and  relatively  new,  while  others, 
and  especially  those  connected  with  the  gneisses,  may  be  of  some  sort 
of  metamorphic  origin  not  understood.  The  iron-bearing  schists  of 
Vermilion  Lake  are,  however,  so  like  the  Huronian  that  they  are  re- 
garded as  a  folded  continuation  of  the  Animikie  beds. 
That  the  Animikie  Huronian  is  beneath  the  Keweenawan  rocks  is 
shown  by  the  fact  that  the  Keweenawan  beds  along  the  Minnesota 
coast  are  passed  in  descending  order  until  the  Animikie  slates  are 
reached  at  Grand  Portage  Bay,  but  there  is  not  a  direct  downward 
continuation  of  the  Keweenawan  into  the  Animikie,  for  between  the 
two  there  has  been  an  intervening  period  of  erosion.  This  is  shown 
by  the  fact  that  at  Grand  Portage  Bay,  where  the  two  formations 
come  together,  the  underlying  slates  suddenly  rise  entirely  across  the 
