174  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
called  Mareniscan  by  the  United  States  Geological  Survey,  but  now 
called  Keewatin,  intruded  by  a  granitic  complex  called  Laurentian. 
Seaman,89  in  1904,  revised  the  geological  maps  of  the  Marquette, 
Menominee,  and  Gogebic  districts  for  the  Michigan  exhibit  at  the 
St.  Louis  world's  fair. 
Bayley  and  Van  Hise,90  in  1904,  describe  and  map  the  geology  of 
the  Menominee  iron-bearing  district  of  Michigan.  The  essential 
facts  are  covered  in  a  preliminary  report.0  An  additional  feature  of 
interest  is  the  discovery  of  minute  granules  in  the  Menominee  iron 
formation  similar  to  the  greenalite  granules  from  which  the  Mesabi 
ores  are  largely  derived. 
Rose,91  in  1904,  discusses  the  geology  of  the  Humboldt,  Champion, 
Marquette,  Michigamme,  L'Anse,  Republic,  Crystal  Falls,  and  Amasa 
districts.  A  number  of  interesting  details,  particularly  concerning 
mines  and  magnetic  lines,  are  added  to  the  description  of  these  local- 
ities, but  the  description  of  the  general  geology  follows  in  general 
that  of  the  United  States  Geological  Survey  monographs. 
Corey  and  Bowen,92  in  1905,  find  evidence  along  Pine  River  that 
the  Keewatin  schists,  probably  the  continuation  of  the  Quinnesec 
schists,  are  unconformably  beneath  Upper  Huronian  sediments  of 
this  area.  They  also  find  evidence  that  the  greenstones  mapped  in 
the  Menominee  monograph  as  equivalent  to  the  Quinnesec  schists 
(Keewatin)  on  the  north  side  of  the  Menominee  syncline  are  really  in- 
trusive into  the  Upper  Huronian.  The  same  is  true  of  greenstones 
to  the  west  along  Brule  River.  The  evidence  is  the  increased  meta- 
morphism  of  the  slates  near  the  contact  and  the  existence  of  occa- 
sional dikes  and  sharp  contacts  without  coarse  fragmental  material. 
They  conclude  further  that  the  greenstones  of  this  area  are  in  more 
limited  areas  than  now  mapped;  that  low-lying  covered  areas  with- 
out exposure  between  greenstone  knobs  are  probably  underlain  by 
slate  rather  than  by  greenstone. 
Lane,93  in  1905,  discusses  the  Black  River  section  of  the  Gogebic 
range  across  the  Keweenawan  series.  Observations  on  the  grain  of 
the  gabbro  show  it  to  be  a  deep-seated  intrusive.  Irving's  estimate 
of  the  great  thickness  of  the  Keweenawan  is  confirmed.  Evidence  of 
unconformity  is  found  between  the  upper  Keweenawan  and  the 
lower  Keweenawan.  Pebbles  in  the  conglomerate  in  the  upper 
Keweenawan  are  derived  from  the  lower  Keweenawan.  Some  of 
these  are  deep-seated  intrusives,  and  there  must  accordingly  have 
been  a  considerable  amount  of  erosion  preceding  the  formation  of 
the  upper  Keweenawan  conglomerate.  Abundant  evidence  of  fault- 
ing across  the  series  is  observed. 
a  Van  Hise,  Charles  R.,  and  Bayley,  W.  S.,  The  Menominee  special  folio  :  Geologic 
Atlas  U.   S.,  folio  62,  U.    S.  Geol.   Survey,  1900. 
