LAKE    SUPERIOR   REGION.  239 
This  volcanic  epoch  may  have  a  deep-seated  connection  with  the 
Cabotian  or  lower  division  of  the  Keweenawan  (described  later). 
Above  the  iron-bearing  member  is  an  impure  dark-colored  lime- 
stone a  few  feet  in  thickness,  not  exceeding  20.  It  extends  appar- 
ently the  whole  length  of  the  Mesabi  range,  but  has  been  identified 
in  two  places  only — sec.  7,  T.  58  N.,  R.  IT  W.,  and,  doubtfully,  on  the 
shores  of  Gunflint  Lake.  This  limestone  may  be  regarded  as  the  basal 
horizon  of  the  next  overlying  rock. 
The  black  slate  is  probably  several  thousand  feet  in  thickness  and 
constitutes  the  bulk  of  the  Animikie.  In  the  neighborhood  of  Gun- 
flint  Lake  it  has  been  divided  by  Grant  into  a  lower  black  slate  divi- 
sion and  an  upper  graywacke  slate  division,  both  of  which  members 
are  interleaved  with  diabase  sills. 
In  the  Indian  reservation  at  Grand  Portage  and  at  various  places 
along  the  Grand  Portage  trail  is  a  graywacke  which  is  supposed  to 
overlie  the  black  slate  member,  but  its  extent  and  stratigraphic  posi- 
tion have  not  been  satisfactorily  established. 
The  top  of  the  Animikie  has  not  been  identified.  The  first  recog- 
nizable datum  plane  after  the  close  of  the  Animikie  is  the  Puckwunge 
conglomerate,  supposed  to  be  the  fragmental  base  of  the  Keweenawan. 
At  one  or  two  places  southwestward  from  Birch  Lake,  at  Little 
Falls  on  Mississippi  River,  and  in  Morrison  County  the  Animikie 
has  been  converted  into  a  mica  schist. 
The  age  of  the  Animikie  is  believed  to  be  Lower  Cambrian  for  the 
following  reasons:  It  graduates  upward  into  Upper  Cambrian  rocks, 
as  seen  on  the  south  side  of  Lake  Superior.  The  derivation  of  the 
iron  ores  from  a  glauconitic  greensand  indicates  that  large  quantities 
of  foraminiferal  organisms  once  lived  in  the  Animikie  ocean,  and 
Matthew  has  shown  the  existence  of  foraminiferal  organisms  associ- 
ated with  the  iron  ore  in  the  St.  Johns  group  of  NeAV  Brunswick. 
Further,  the  Animikie  has  a  uniformly  low  dip,  while  the  lower 
strata  are  all  highly  tilted.  There  must  therefore  have  been  a  great 
lapse  of  time  between  the  deposition  of  the  two  series. 
The  Puckwunge  conglomerate  is  taken  to  be  the  fragmental  base  of 
the  Keweenawan,  although  certain  igneous  rocks  which  antedate  it 
and  which  perhaps  are  contemporaneous  with  the  upper  portions  of 
the  Animikie  are  also  called  Keweenawan.  The  conglomerate  is 
found  at  Grand  Portage  Island,  at  Isle  Royal,  on  Baptism  River, 
at  Little  Marais,  on  Manitou  River,  at  the  deep  well  at  Short  Line 
Park  near  Duluth,  and  at  New  Ulm. 
Above  this  conglomerate  are  conglomerates  and  sandstones  of 
Keweenawan  age  which  are  stratified  with  lavas  of  diabasic  nature. 
Still  higher  up  the  eruptive  rocks  become  less  in  quantity  and  the  frag- 
mental rock  is  a  sandstone,  known  as  the  Hinckley  sandstone,  quar- 
ried in  the  gorge  of  the  Kettle  River  in  Pine  County.     This  in  turn 
