222  PEE-CAMBPJAN    GEOLOGY    OF    NORTH    AMERICA. 
Mesabi  range.  No  true  quartzite  is  found  at  the  base  of  the  Animikie 
in  the  Akeley  Lake  area,  but  the  iron-bearing  rocks  at  Gunflint  Lake 
rest  directly  upon  the  Keewatin. 
The  quartzites  of  Pigeon  Point  are  lithologically  similar  to  the 
quartzite  at  the  top  of  the  graywacke  slate  member  and  are  supposed 
to  be  equivalent  to  it.  The  igneous  rocks  are  all  intrusive.  The 
diabase  sills  in  places  have  a  thickness  of  100  feet.  They  have  not 
been  found  in  contact  with  nor  to  extend  into  the  gabbro  below.  The 
great  Keweenawan  gabbro  of  the  district  has  a  varying  mineralogical 
composition,  in  places  being  composed  almost  entirely  of  feldspar, 
thus  forming  anorthosite,  and  elsewhere  being  exceedingly  rich  in 
olivine.  This  gabbro  includes  fragments  of  the  Animikie  slates,  and 
was  found  directly  overlying  and  in  contact  with  the  uppermost 
member  of  the  Animikie,  thus  showing  it  to  be  of  post- Animikie  age. 
Associated  with  the  coarse-grained  gabbros  are  finer  grained  rocks, 
including  gabbros,  olivine  gabbros,  norites,  and  olivine  norites,  which 
have  been  called  muscovado.  These  are  slightly  older  than  the  main 
mass  of  gabbro,  which  is  seen  cutting  and  including  fragments  of 
them. 
The  acidic  eruptive  rocks,  called  augite  syenite  by  Irving,  includ- 
ing reddish,  hornblendic,  granitic  rocks,  are  found  cutting  the  gabbro. 
In  passing  toward  the  granitic  rocks,  at  first  a  few  small  acidic  dikes 
are  seen.  These  increase  in  frequency  in  approaching  the  central  mass 
of  the  granite,  and  at  the  edge  of  the  mass  apophyses  can  be  traced 
directly  from  the  granite  into  the  gabbro.  The  dikes  are  not  finer 
grained,  as  a  whole  or  at  their  edges,  than  the  main  mass,  thus  indi- 
cating the  heated  condition  of  the  gabbro  when  the  dikes  were  in- 
truded. It  is  concluded  that  while  the  granite  is  of  later  date  than 
the  gabbro,  it  is  not  much  later,  and  was  perhaps  intruded  before  the 
complete  solidification  of  the  basic  rock. 
Culver,186  in  1894,  describes  the  rocks  of  Itasca  County,  Minn.  The 
Pokegama  quartzite  was  found  to  extend  from  the  north  end  of 
Pokegama  Lake  northeasterly  to  the  rapids  of  Prairie  River.  This 
rock  is  flat  lying,  with  low  southerly  or  southeasterly  dip,  and  seems 
to  have  been  bowed  into  a  series  of  low,  flat  arches.  The  lower  beds 
are  fine  grained,  hard,  and  massive,  although  broken  into  cubical 
blocks.  In  the  upper  portions  of  the  quartzite  in  many  places  is 
found  a  considerable  quantity  of  iron  ore.  In  cross  section  there  are 
alternately  sheets  of  ore  and  sheets  of  quartz.  In  the  hand  specimen 
these  quartz  layers  show  no  grains.  The  structure  is  porous,  and  the 
quartz  is  usually  stained  red.  Both  the  ore  and  the  quartz  layers  are 
exceedingly  irregular,  and  are  often  interrupted  or  cut  by  each  other. 
The  Prairie  River  granite  lies  in  a  belt  parallel  to  the  Pokegama 
quartzite.    It  contains  some  bodies  of  schist,  which  are  taken  to  indi- 
