240  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
grades  up  into  typical  Upper  Cambrian  sandstones  of  the  St.  Croix 
Valley.  The  term  Potsdam  is  restricted  to  the  Puckwunge  conglom- 
erate and  the  hardened  quartzites  immediately  overlying  it,  repre- 
sented by  the  Sioux  quartzite,  the  Baraboo  and  Barron  County 
quartzites  of  Wisconsin,  the  quartzite  at  Grand  Portage  Island  and 
west  of  Grand  Portage  village,  the  New  Ulm  quartzite  in  Cotton- 
wood County,  and  the  quartzite  in  Pipestone  County. 
The  igneous  rocks  of  the  Keweenawan  vary  in  age  from  late 
Animikie  time  to  the  close  of  the  Keweenawan.  They  are  divided 
into  two  groups,  the  Cabotian  or  Lower  Keweenawan,  and  the  Man- 
itou  or  Upper  Keweenawan. 
The  Cabotian  division  includes  gabbro  and  contemporaneous  red 
rock  and  their  surface  lavas,  and  all  other  dikes  and  sills  which  are 
associated  with,  but  are  younger  than,  the  Animikie  clastic  rocks  and 
which  are  older  than  the  Puckwunge  conglomerate.  The  lower  mem- 
ber of  the  Cabotian  is  the  gabbro,  Avhich  covers  an  enormous  area.  It 
extends  on  the  east  to  East  Greenwood  Lake,  in  T.  64  N.,  R.  2  E. 
On  the  north  it  is  bounded  by  the  Animikie  strata  of  the  Mesabi  iron 
range.  Its  westernmost  exposure  is  in  the  vicinity  of  Short  Line 
Park,  Duluth.  The  southern  limit  is  irregular,  swinging  from  East 
Greenwood  Lake  in  a  zigzag  manner  through  T.  63  N.,  R.  1  W. ;  T. 
62  N.,  R.  2  W. ;  T.  62  N.,  R.  4  W. ;  T.  60  N.,  R.  6  W. ;  T.  60  N.,  R.  7 
W. ;  T.  58  N.,  R.  10  W.,  and  T.  55  N.,  R.  11  W.,  to  Duluth. 
Along  the  northern  and  northwestern  sides  of  the  great  gabbro 
mass  the  gabbro  is  plainly  intrusive  on  the  older  formations,  Anim- 
ikie and  Keewatin. 
From  the  northern  border  of  the  gabbro  many  sills  oifshoot  and 
penetrate  the  Animikie  strata  parallel  to  the  bedding.  These,  are 
known  as  the  Logan  sills. 
Near  its  contact  with  the  underlying  rocks,  both  the  Animikie  and 
the  Keewatin  series,  there  are  various  altered  rocks  which  can  be  con- 
nected in  places  with  the  gabbro  and  in  places  with  the  underlying 
rocks.  To  these  altered  rocks  the  term  muscovadite  has  been  applied. 
It  includes  the  various  so-called  peripheral  phases  of  the  gabbro. 
On  the  southern  and  eastern  border  the  gabbro  is  penetrated  by  and 
penetrates  in  a  confused  manner  the  red  rock,  with  which  it  alternates 
both  structurally  and  areally.  It  is  believed  to  have  resulted  from 
the  metamorphism  by  the  gabbro  of  the  Animikie  and  perhaps  earlier 
fragmentals. 
As  the  granites  of  the  Archean  are  believed  to  have  resulted  from 
the  softening  of  acidic  fragmentals,  so  the  gabbro  may  probably  have 
been  the  result  of  the  metamorphism  or  refusion  of  the  Keewatin 
greenstones. 
The  anorthosite  masses  of  the  Beaver  Bay  diabase,  supposed  by 
Lawson  to  be  of  Archean  age  and  to  underlie  unconformably  the 
