LAKE    SUPERIOR   REGION.  139 
which,  combined  with  metasomatic  changes,  their  present  condition 
is  produced. 
Wadsworth,53  in  1890,  gives  a  general  account  of  the  geology  of 
the  Marquette  and  Keweenaw  districts  based  upon  his  own  and  other 
works.  The  Azoic  system  includes  fragmental  and  eruptive  rocks. 
Among  the  former  are  various  argillites  and  schists  of  the  Marquette 
district.  Among  the  eruptive  rocks  are  placed  the  jaspilites  and  their 
associated  ores,  with  the  exception  of  certain  soft  iron  ores  of  chemical 
origin.  The  Keweenawan  is  again  placed  as  a  part  of  the  Potsdam, 
since  the  first  lava  flow  found  on  Keweenaw  Point  flowed  over  the 
Eastern  sandstone.  Subsequently  there  has  been  a  fault  line  or  fis- 
sure running  near  the  contact  of  the  sandstones  and  lavas.  This 
fault  is  regarded  as  normal,  and  it  accounts  for  the  fact  that  some- 
times the  lavas  and  sometimes  the  associated  conglomerates  are 
brought  in  contact  with  the  Eastern  sandstone  along  the  fault  line. 
Irving,54  in  1890,  discusses  the  field  relations  of  the  greenstones  and 
greenstone  schists  of  the  Marquette  and  Menominee  districts.  A  field 
study  of  these  rocks,  heretofore  generally  considered  sedimentary,  led 
to  the  conclusion  that  they  are  largely  of  eruptive  origin,  and  the 
detailed  study  of  Williams  has  shown  this  conclusively.  In  the  Mar- 
quette district  the  line  of  demarcation  between  the  schists  and  the 
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  subsequent  date  to  the  schists  themselves,  and  are  equiv- 
alent 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  remainder  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  unconformable 
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  waterworn  fragments  of  the  granite 
when  in  contact  with  that  rock,  and  fragments  of  the  greenstone 
schists  when  in  contact  with  those  rocks.  In  some  cases  the  basal 
quartzite  appears  to  grade  into  the  granite,  but  a  study  of  this quartzite 
in  the  thin  section  shows  its  completely  fragmental  character.  These 
contacts  or  basal  conglomerates  are  described  in  sees.  1.  2.  3,  1.  5,  T. 
47  N.,  R.  25  TV.;  in  sees.  21  and  22,  T.  47  X.,  R.  26  AY.;  in  sec.  2D.  T. 
48-N.,  Pv.  25  W.;  in  sec.  20,  T.  48  N.,  R.  27  W. ;  in  sec.  17.  T.  48  N., 
R.  2G  W.;  in  sec.  21,  T.  48  N.,  R.  27  W.;  and  at  various  points  in 
T.  49  N.,  R.  28  W.,  all  in  Michigan. 
