348  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
region,  the  pre-Cambrian  sedimentary  series  with  interbedded  vol- 
canics.  These  are  grouped  about  the  western  half  of  Lake  Superior. 
The  west  end  of  Lake  Superior  consists  of  an  eastward-pitching 
synclinorium  of  Keweenawan  rocks  with  gentle  southerly  dips  on  its 
north  limb  and  fairly  steep  northerly  dips  on  its  south  limb,  the 
Keweenawan  series  there  fringing  the  western  half  of  the  lake.  The 
underlying  rocks,  the  upper  Huronian,  may  be  considered  also  as 
taking  part  in  this  synclinorial  structure  and  as  having  a  distribu- 
tion consonant  with  such  structure — that  is,  bordering  the  outer  edge 
of  the  Keewenawan  areas.  In  detail  the  structure  of  the  upper 
Huronian  is  far  more  complex  than  that  of  the  Keweenawan  series, 
and  in  Michigan,  in  the  triangular  area  extending  eastward  from 
the  Penokee-Gogebic  district  and  fingering  out  in  the  Marquette, 
Crystal  Falls,  and  Menominee  districts,  the  upper  Huronian  appears 
in  a  series  of  rolls  which  apparently  have  no  counterparts  in  the 
Keweenawan  series.  The  lower  and  middle  Huronian  sediments, 
occupy  but  limited  areas  as  compared  with  the  other  Algonkian  rocks. 
For  the  most  part  they  are  closely  folded  and  have  steep  dips.  Their 
strikes  and  dips  give  evidence  that  they  were  complexly  folded  before 
the  beginning  of  the  earth  movements  which  resulted  in  the  Lake 
Superior  synclinorium.  They  accord  in  structure  only  in  that  the 
prevailing  strike  of  bedding,  folds,  and  cleavage  is  east-northeast, 
parallel  to  the  axis  of  the  Lake  Superior  syncline.  The  same  is  of 
course  true  of  the  Archean. 
It  has  been  possible  to  explain  the  major  structure  and  distribution 
of  the  Algonkian  rocks  on  the  basis  of  folding.  Faulting  has  been 
recognized  in  many  places,  and  locally  has  been  important.  Recent 
study  has  tended  to  show  that  faulting  has  played  a  greater  part  than 
had  been  supposed,  although  still  a  subordinate  part.  A  system  of 
definitely  recognizable  strike  and  dip  faults  has  been  worked  out  in 
the  Marquette  district  by  Seaman,  and  the  effect  of  these  faults  on 
the  distribution  and  structure  of  the  ore  deposits  has  been  noted  in  at 
least  one  of  the  mines.  Presumably  further  work  will  show  more  of 
such  features.  In  the  Gogebic  district  important  faults,  both  across 
and  parallel  to  the  iron-bearing  formation,  have  been  found  to  have 
an  important  bearing  on  the  distribution  and  structure  of  the  ore 
deposits.  In  the  Mesabi  range  normal  faulting,  inferred  but  not 
observed  during  the  preparation  of  the  Geological  Survey  report  on 
the  Mesabi  range,0  has  been  found  by  further  mining  operations  in 
the  Biwabik  mine.  Strike  faults  of  considerable  magnitude  also 
occur  in  the  Animikie  iron  range  east  of  Thunder  Bay. 
As  a  result  of  the  closer  folding  of  the  lower  series  in  the  region  as 
a  whole,  as  compared  with  the  higher  ones,  and  as  a  result  also  of  the 
intrusion  into  the  lower  series  of  larger  masses  of  deep-seated,  igneous 
°Mon.  U.  S.  Geol.  Survey,  vol.  43,  1903. 
