224  PKE-CAMBKTAN    GEOLOGY    OF    NORTH    AMERICA. 
proximity  of  the  gabbro.  These  slates  disappear  before  Dunke  River 
is  reached,  having  been  removed  at  the  time  of  the  gabbro  intrusion. 
The  Pewabic  quartzite  at  the  bottom  of  the  Animikie  decreases  in 
thickness  as  Birch  Lake  is  approached  from  the  west,  and  in  the 
vicinity  of  Iron  Lake  disappears  entirely.  From  this  locality  east- 
ward the  iron-bearing  rock  rests  upon  the  granite.  It  is  concluded 
that  the  Pewabic  quartzite  between  Birch  Lake  and  Gunflint  Lake 
belongs  to  the  middle  iron-bearing  member  of  the  Animikie. 
In  Tps.  61  and  G2  N.,  Rs.  10  and  11  W.,  occurs  a  heavily  bedded 
olivine  gabbro.  In  going  from  the  northern  and  southern  limits  of 
the  gabbro  toward  the  center  of  the  area  it  is  noticeable  that  the  ferro- 
magnesian  minerals  decrease  and  the  feldspar  increases  in  proportion, 
until  in  the  center  of  the  mass  occur  numerous  knobs  and  areas,  a  mile 
or  more  in  extent,  composed  of  plagioclase  rock  or  anorthosite,  which 
are  regarded  as  segregations.  In  the  center  of  the  mass  the  rock  has 
greater  coarseness  of  texture,  and  also  more  of  a  stratified  appearance, 
arising  from  the  arrangement  of  the  constituent  minerals  in  bands. 
The  mineral  and  chemical  compositions  of  the  various  parts  of  the 
formation  correspond  to  the  known  rules  which  govern  the  cooling  of 
liquid  magmas,  and  the  whole  is  regarded  as  a  batholithic  intrusion 
rather  than  a  surface  flow. 
Red  rocks,  comprising  augite  syenite,  quartz  porphyry,  felsite,  etc., 
occur  in  the  vicinity  of  Greenwood  Lake,  and  were  followed  to  the 
shores  of  Lake  Superior,  making  together  one  prominent  group  of 
rocks. 
The  dark  gabbros  of  Irving,  the  diabases,  and  the  amygdaloids  are 
placed  in  another  group,  called  the  diabase  group.  The  anorthosites  of 
the  coast  of  Lake  Superior,  described  by  Lawson  as  pre-Keeweenawan, 
and  newly  discovered  masses  back  from  the  coast,  are  found  to  be 
detached  blocks  from  the  great  gabbro  mass  inclosed  in  and  under- 
lain by  the  black  gabbro,  as  previously  held  by  Irving  and  Winchell. 
The  latter  rock  is  considered  as  the  effusive  equivalent  of  the  great 
basal  gabbro.  After  the  aggregations  of  feldspar  had  separated  from 
the  gabbro  magma,  and  were  floating  around  in  it,  they  were  ejected 
in  portions  of  the  unsolidified  magma,  and,  being  lighter  than  it, 
floated  near  the  surface,  and  are  found  only  near  the  top  of  the  first 
outburst  of  lava  or  black  gabbro.  Later,  when  the  rock  was  some- 
what eroded,  the  feldspar  knobs  projected  above  the  surrounding 
rocks,  and  later  were  covered  by  the  flows  of  the  red  rock  group. 
Therefore  the  conclusion  of  Lawson  that  the  anorthosite  forms  a  pre- 
Keweenawan  terrane  is  rendered  valueless. 
In  chronological  order  the  Keweenawan  of  the  north  shore  of  Lake 
Superior  can  be  divided  into  gabbro,  diabase,  red  rocks,  and  later 
dikes. 
