320  PKE-CAMBKIAN    GEOLOGY    OF    NORTH    AMERICA. 
The  Taconic  age  is  represented  in  the  Lake  Superior  basin,  as  in 
New  England  and  Newfoundland,  by  a  great  series  of  quartzites  and 
slates  and  a  few  limestones. 
Those  rocks  which  have  been  described  and  mapped  as  Keweena- 
wan  embrace  three  eruptive  systems,  separable  by  two  erosion  inter- 
vals marked  by  basal  conglomerates  and  by  faunal  differences,  viz,  the 
eruptives  of  the  Animikie  revolution,  those  of  the  Keweenawan 
proper,  and  those  of  the  regions  of  Thunder  Bay  and  Black  Bay. 
It  is  added  as  a  corollary  to  the  foregoing  that  the  ocean  which  cov- 
ered the  spot  where  North  America  was  to  exist  was  subject  to  forces 
which  acted  simultaneously  over  a  very  wide  area,  producing  oceanic 
deposits  of  like  nature  and  of  like  succession  in  widely  separated 
regions;  and,  again,  that  some  other  widely  operating  forces  caused 
the  simultaneous  elevation,  depression,  and  finally  the  breaking  of 
the  earth's  crust  and  the  escape  of  vast  quantities  of  basic  rock  at 
various  points  far  distant  from  one  another. 
Walcott,331  in  1899,  discusses  markings  which  have  been  reported 
as  found  in  the  Huronian  iron  formation  of  the  Menominee  iron  dis- 
trict of  Michigan.  An  examination  of  the  specimens  indicates  that 
they  probably  are  from  the  basal  detrital  material  of  the  Cambrian 
which  rests  upon  the  Huronian  iron  formation.  In  the  Animikie 
rocks  of  the  Lake  Superior  region  the  evidence  of  life  consists  of  the 
presence  of  graphitic  material  in  the  slates  and  of  a  supposed  fossil 
mentioned  by  G.  F.  Matthew.  In  the  Minnesota  quartzite  of  the 
Upper  Huronian  series  lingula-like  forms  and  an  obscure  trilobitic- 
looking  impression  are  described  by  Winchell.  The  latter  has  been 
examined  and  the  conclusion  is  reached  that  it  is  of  inorganic  origin. 
As  to  the  lingula  forms,  the  weight  of  evidence  is  in  favor  of  their 
being  small  flattened  concretions. 
Van  Hise,332  in  1901,  describes  the  geology  of  the  Lake  Superior 
iron-ore  deposits  and  gives  the  general  succession  of  formations  in  the 
iron-bearing  districts.     (See  table,  pp.  328-329.) 
Van  Hise  and  others"  have  discussed  the  geology  of  the  Lake 
Superior  region  in  previous  reports,  both  general  and  detailed.  The 
present  report  is  a  summary  of  the  earlier  reports,  but  it  contains  in 
addition  many  new  features  of  interest.  Attention  will  be  directed 
only  to  such  conclusions  as  are  new  or  vary  from  those  given  in  the 
preceding  reports. 
In  the  Vermilion  district  the  great  Stuntz  conglomerate  and 
equivalent  rocks  have  been  found  to  lie  unconformable  under  the 
Animikie  series,  which  has  been  referred  to  the  Upper  Huronian 
a  See  especially  Principles  of  pre-Cambrian  geology  :  Sixteenth  Ann.  Rept.  U.  S.  Geol. 
Survey,  pt.  1,  1896,  pp.  571-874  ;  Bull.  U.  S.  Geol.  Survey  No.  86,  1892  ;  Mon.  IT.  S.  Geol. 
Survey,  vols.  19,  1892  ;  28,  1895,  and  36,  1899  ;  and  Folio  62,  Geologic  Atlas  U.  S.,  U.  S. 
Geol.  Survey,  1890. 
