VANHisEl  LAKE    SUPERIOR    REGION.  149 
original  clierty  carbonate  of  iron  which  is  yet  abundantly  present  in 
the  upper  horizons  of  the  ore-bearing  formation. 
Hall203  (C.  W.),  in  1889,  describes  the  distribution  of  the  granites  of 
the  Northwestern  states,  and  particularly  those  of  Minnesota.  They 
are  found  to  be  either  intrusive  or  granitic  veinstones,  the  latter  being 
insignificant  in  quantity.  The  granites  of  Minnesota  as  to  age  are 
probably  later  than  the  Laurentian  floor  of  the  continent  but  earlier 
than  the  close  of  the  Agnotozoic  era.  There  are  three  or  four  grand 
periods  of  eruptive  activity. 
Williams,203  in  1890,  as  a  result  of  an  extended  examination  of  the 
field  relations  and  microscopical  characters  of  the  widespread  green- 
stones, greenstone  schists,  and  agglomerates  of  the  Marquette  and  Me- 
nominee districts,  concludes  that  they  are  all  of  eruptive  origin.  This 
conclusion  is  reached  from  a  consideration  of  the  field  evidence,  the 
schistose  phases  being  frequently  traced  by  continuous  gradations  into 
massive  forms;  and  from  the  microscopical  evidence,  these  unaltered 
forms  having  all  the  characteristics  of  eruptive  rocks.  The  original 
rock  types  were  rather  numerous,  including  gabbro,  diabase,  diabase- 
porphyry,  melaphyre,  diorite,  diorite-porphyry,  and  tuffs.  These  rocks 
have  been  compressed,  faulted,  and  crushed,  as  a  result  of  which,  com- 
bined with  metasomatic  changes,  their  present  condition  is  produced. 
Irving,204  in  1890,  discusses  the  field  relations  of  the  greenstones  and 
greenstone  schists  of  the  Marquette  and  Menominee  districts.  Afield 
study  of  these  rocks,  heretofore  generally  considered  sedimentary,  led 
to  the  conclusion  that  they  are  largely  of  eruptive  origin,  and  the  de- 
tailed study  of  Williams  has  shown  this  conclusively.  In  the  Marquette 
district  the  line  of  demarcation  between  the  schists  and  granites  is  not 
a  sharp  one,  the  granites  intricately  intruding  the  schists,  often  in  such 
a  manner  as  to  render  it  certain  that  the  granite  is  the  later  rock.  Also 
the  basic  dikes  which  cut  the  greenstone  schists  are  of  wholly  subse- 
quent date  to  the  schists  themselves,  and  are  equivalent  in  age  to  those 
which  have  intruded  the  overlying  detrital  iron  bearing  series.  On  the 
other  hand,  it  is  concluded  that  the  greenstone  schists  themselves  do  not 
belong  within  the  same  geological  period  as  that  which  holds  the  remain- 
der of  the  stratiform  rocks  of  the  region;  that  is,  the  greenstone  schists 
are  placed  along  with  the  granites  and  gneisses  to  form  the  basement 
upon  which  the  overlying  detrital  iron-bearing  series  was  horizontally 
and  unconformably  spread.  This  is  shown  by  the  fact  that  at  a  number 
of  points  the  detrital  beds  which  form  the  basement  member  of  the  iron- 
bearing  series  proper  bear  numerous  water  worn  fragments  of  the  gran- 
ite when  in  contact  with  that  rock,  and,  when  in  contact  witli  the  green- 
stone schists,  fragments  of  those  rocks.  In  some  cases  the  basal  quartz- 
ite  appears  to  grade  into  the  granite,  but  a  study  of  this  quartzite  in 
the  thin  section  shows  its  completely  fragmental  character.  These  con- 
tacts or  basal  conglomerates  are  described  in  Sees.  1,  2,  3,  4,  5,  T.  47  N"., 
E.  25  W.,  Mich.;  in  Sees.  21  and  22,  T.  47  K,  B.  20  W.,  Mich.;  in  Sec. 
