LAKE    SUPERTOR   REGION.  241 
Beaver  Bay  diabase,  are  believed  to  represent  segregation  phases  in 
the  main  gabbro  flow,  and  to  be  the  same  as  anorthosite  masses  in  the 
gabbro  proper  to  the  west. 
The  Beaver  Bay  diabase  is  believed  to  represent  the  upper  portion 
of  the  great  gabbro  flow,  and  to  be  due  to  the  first  and  greatest  move- 
ment of  the  gabbro  toward  Lake  Superior.  The  Logan  sills  belong 
to  this  part  of  the  gabbro  flow. 
The  Manitou  division  of  the  Keweenawan  includes  the  surface 
flows,  sills,  and  dikes  which  accompanied  and  followed  the  Puck- 
wunge  conglomerate.  These  eruptives,  with  the  elastics  associated 
with  them,  do  not  have  a  thickness  in  Minnesota  of  more  than  1,000 
feet.  These  lava  sheets  extend  along  the  shore  of  Lake  Superior 
from  near  Baptism  River  to  near  Grand  Marais,  except  where  re- 
placed at  intervals  by  the  Beaver  Bay  diabase  or  some  of  the  inter- 
sheeted  fragmentals.  They  occur  also  in  the  neighborhood  of  Grand 
Portage  Bay,  but  their  extent  here  is  not  definitely  known. 
The  most  important  petrological  conclusions  determined  from  the 
examination  of  the  Minnesota  crystalline  rocks  are  three  in  number: 
1.  All  the  granites  of  the  Archean  can  be  explained  on  the  assump- 
tion that  they  are  intrusives  representing  the  metamorphosed  condi- 
tions of  clastic  rocks  adjacent  to  the  observed  intrusions,  rendered 
plastic  by  the  force  of  dynamic  metamorphism  accompanied  by 
moisture. 
2.  The  basic  Keweenawan  gabbro  and  its  derivatives  are  derived 
from  the  metamorphism  and  complete  refusion  of  the  Archean  green- 
stones and  their  attendants. 
3.  The  greensand  of  the  Mesabi  iron-bearing  formation  appears 
to  have  resulted  from  a  volcanic  sand,  and  the  taconite  itself  from 
igneous  forces. 
Hall,20:?  in  1899,  describes  the  pre-Cambrian  crystalline  rocks  of 
the  Minnesota  River  valley  of  soutliAvestern  Minnesota.  These 
rocks  appear  in  numerous  exposures  along  the  river,  protruding  from 
the  drift,  from  southeast  of  New  Ulm  to  Ortonville  on  the  northwest. 
The  great  bulk  of  the  crystalline  rocks  are  granites  and  gneisses. 
These  appear  for  the  most  part  in  the  river  bottoms,  but  stand  also  in 
a  few  isolated  knobs  on  the  higher  ground  south  and  west  of  the  river. 
There  arc1  many  varieties  of  granites  and  gneisses,  and  all  gradations 
between  them.  They  are  taken  as  a  whole  to  represent  the  Archean 
or  Basement  Complex. 
Associated  with  the  granites  and  gneisses  are  a  much  smaller  num- 
ber of  exposures  of  gabbros  and  gabbro  schists.  These  present  many 
varieties,  all  of  which  are  believed  to  have  resulted  from  the  alteration 
of  two  original  forms  and  their  intergradations — a  hypersthene-bear- 
ing  gabbro  and  a  hvpersthene-free  gabbro. 
55721— Bull.  3G0— 09 16 
