252  PRE-CA  AT  BRIAN    GEOLOGY    OF    NORTH   AMERICA. 
From  the  information  so  far  available,  consisting  largely  of  drill 
samples,  the  succession  of  rocks  for  the  Cuyuna  district,  from  the 
base  upward,  is  as  follows: 
Quartzite  and  its  altered  equivalents,  quartzose,  micaceous,  and  hornblende 
schists. 
Iron  formation,  consisting  principally  of  iron  carbonate  where  unaltered,  but 
largely  altered  to  amphibole-magnetite  rock,  ferruginous  slates,  and  iron  ore. 
The  ores  thus  far  found  are  soft,  reddish,  slightly 'hydra ted  hematite,  reddish 
slaty  hematite,  and  hard,  blue,  banded,  siliceous  magnetite  and  hematite. 
Chloride  and  carbonaceous  slate,  interbedded  in  its  lower  part  with  iron 
formation. 
Intrusive  granite  and  diorite,  principally  the  latter. 
Cretaceous  sediments. 
Glacial  drift,  SO  to  350  feet. 
It  is  possible  that  some  of  the  igneous  rock  is  really  older  than 
the  sedimentary  series  and  lies  unconformably  beneath  it,  but  no 
evidence  of  this  has  yet  been  found,  and  the  structural  relations  do 
not  favor  this  view.  All  the  sediments  are  conformable  with  the 
exception  of  the  Cretaceous  and  the  drift. 
The  sediments  are  bent  in  upright  symmetrical  folds  and  the  slaty 
members  are  cut  by  schistosity  dipping  steeply  away  from  the  axial 
planes  of  the  folds.  For  the  most  part  erosion  has  not  gotten  down 
through  the  slate.  Ore  is  found  only  where  erosion  has  cut  through 
the  slate  to  the  iron  formation.  The  magnetic  belts  are  carried  by 
the  lower  members  of  the  iron  formation  and  therefore  mark  the  foot 
walls  near  the  axes  of  the  folds. 
The  pre-Cretaceous  rocks  are  pre-Cambrian  and  the  sediments  are 
correlated  with  the  Upper  *r!uronian  series. 
Zapffe,214  in  1907,  gives  a  full  account  of  the  geology  of  the  Cuy- 
una iron  district  of  Minnesota  as  known  to  date,  with  details  as  to 
exploration.  The  general  succession  and  structure  are  essentially 
those  described  by  Leith.  Much  of  the  fieldwork  was  done  in  com- 
mon by  the  two  men. 
Abbott,215  in  1907,  describes  the  Ely  iron-bearing  trough  of  the 
Vermilion  district  of  Minnesota  and  shows  in  detail  the  relations  of 
the  iron  formation  to  the  associated  green  schists  of  the  Keewatin  and 
the  sediments  of  the  Lower  Huronian.  It  is  concluded  that  while 
the  iron  formation  is  in  general  above  the  greenstone  and  infolded 
with  it,  it  is  in  part  interbedded  with  the  greenstone  flows  of  the 
Keewatin. 
Sardeson,21521  in  1908,  discusses  the  geological  history  of  the  Red- 
stone quartzite  and  its  associated  rocks  between  New  Ulm  and  Court- 
land  in  the  valley  of  Minnesota  River,  and  principally  the  post-Cam- 
brian history  as  shown  by  relations  with  later  rocks. 
For  fuller  summaries  of  articles  referring  to  the  Minnesota  ore- 
bearing  districts  see  Monographs  XLIII  and  XLV  of  the  United 
States  Geological  Survey. 
