LAKE    SUPERIOR   REGION.  225 
Spurr,188  in  1894,  discusses  the  stratigraphic  position  of  the  Thomp- 
son slates  of  northeastern  Minnesota.  These  have  heretofore  been 
correlated  with  the  Animikie  slates.  However,  almost  every  phase  of 
slate  of  the  Thompson  series  can  be  duplicated  by  the  less-altered 
phases  of  the  Keewatin  schists  of  the  Mesabi  range.  In  the  vicinity 
of  Mississippi  River  the  Thompson  series  becomes  partly  crystalline, 
being  changed  into  sericitic,  micaceous,  hornblenclic,  staurolitic,  or 
garnetiferous  schists  which  correspond  exactly  with  the  green  schists 
of  the  Keewatin.  The  cleavage  of  the  Thompson  series  marks  a  dis- 
tinctively pre-Animikie  disturbance.  The  trend  of  the  cleavage  cor- 
responds with  that  of  the  schistosity  of  the  Keewatin  of  the  Mesabi 
range,  and  it  is  thought  that  the  two  were  developed  at  the  same  time. 
The  Thompson  series  has  undergone  considerable  folding,  and  in  this 
respect  also  more  resembles  the  Keewatin  than  the  Animikie  slates, 
which  are  in  an  undisturbed  condition.  The  Thompson  series  is 
therefore  regarded  as  unconformably  below  the  Animikie,  and  is  pro- 
visionally correlated  with  the  Keewatin. 
Spurr,189  in  1894,  gives  an  account  of  the  rocks  of  the  Mesabi  dis- 
trict, and  particularly  of  the  iron-bearing  rocks. 
The  oldest  formation  of  the  district  is  the  Keewatin,  the  most  com- 
mon rock  of  which  is  green  schist,  but  associated  with  this,  especially 
near  the  granites,  are  hornblende  schists  and  mica  schists.  The 
schists  have  a  regional  cleavage,  which  is  nearly  uniform  in  trend, 
about  N.  70°  E.,  and  nearly  vertical.  Next  in  age  to  the  Keewatin 
schists  is  the  hornblende  granite  of  the  Giants  range.  This  range 
has  an  average  width  of  about  10  miles,  and  its  direction  is  the  same 
as  that  of  the  schistosity  of  the  Keewatin  rocks.  The  granite  is  in- 
trusive in  the  schists,  as  shown  by  numerous  fragments  embedded  in 
it,  by  stringers  of  the  granite  in  the  schists,  and  by  the  metamorphism 
of  the  schists  adjacent  to  the  granite. 
Unconformably  upon  the  former  is  the  Animikie  series.  The  Anim- 
ikie series  has  no  marked  folding,  slaty  cleavage,  or  schistose  struc- 
ture. The  rocks  of  the  series  are  in  a  gentle  southern  monocline,  in 
a  direction  perhaps  10°  or  15°  east  of  south.  This  monocline  has 
gentle  undulations,  with  axes  parallel  to  its  dip,  and  in  the  Virginia 
area  has  been  faulted.  The  amount  of  disturbance  is  greater  adjacent 
to  the  central  part  of  the  district,  where  are  found  the  Keweenawan 
rocks.  It  is  probable  that  the  weight  of  the  Keweenawan  rocks  has 
produced  a  sinking  in  the  area  south  of  the  Animikie,  and  that  this 
has  produced  the  tilting.  The  Animikie  series  may  be  divided  into 
three  chief  members — the  Pewabic  quartzite,  the  iron-bearing  mem- 
ber, and  the  upper  slates.  The  Pewabic  quartzite  is  a  fragmental 
rock  indurated  by  the  enlargement  of  quartz  grains.  It  occasionally 
passes  into  a  fine-grained  conglomerate.     The  iron-bearing  member  is 
55721— Bull.  360—09 15 
