LAKE    SUPERIOR   REGION.  383 
referred  to  the  Huronian,  no  attempt  was  made  to  carry  these  facts 
to  their  conclusion  and  to  subdivide  the  series.  Alexander  Winchell, 
in  his  last  paper,  which  appeared  almost  simultaneously  with  his 
death,  announced  definitely  that  two  series  had  been  confounded  in 
the  Huronian. 
Evidence  of  this  break  in  the  Marquette  district  was  first  noticed 
by  Foster  and  by  Foster  and  Whitney,  who  found  over  the  ore  hori- 
zon at  what  has  since  become  the  Republic  mine,  and  at  one  or  two 
other  localities,  a  conglomerate  bearing  fragments  of  the  ore,  jasper, 
and  other  rocks  associated  with  the  iron  ore.  It  was  next  noted  by 
Kimball,  who  mentions  beds  of  specular  conglomerate.  Credner  de- 
scribes a  conglomerate  over  the  iron  formation  at  Michigamme  mine, 
the  fragments  of  jasper  and  quartz  being  in  an  iron  and  quartz  base. 
Brooks  describes  the  upper  quartzite  of  Republic  Mountain  and 
that  at  the  New  England  mine  as  a  conglomerate  containing  frag- 
ments of  ore.  Rominger  noticed  the  break  at  so  many  places  that  he 
remarked  that  above  the  iron-bearing  rock  is  generally  a  very  coarse 
quartzite  conglomerate  which  often  has  the  character  of  a  coarse- 
grained ferruginous  quartzite,  the4  fragments  of  which  are  chiefly 
ore,  jasper,  and  quartz.  This  occurrence  is  so  general  as  to  suggest 
to  this  author  that  great  disturbances  not  of  a  local  extent  must  have 
occurred  at  the  end  of  the  era  of  iron  sediments.  Wadsworth  says 
that  these  conglomerates  mark  old  waterworn  beaches  existing  after 
the  jasper  and  ore  were  in  situ  in  nearly  their  present  condition. 
Believing  in  the  eruptive  origin  of  these  rocks,  Wadsworth  did  not 
regard  the  conglomerates  as  evidence  of  the  existence  of  more  than  a 
single  series;  but  that  author  later  changed  his  opinion  in  this  par- 
ticular. Irving  recognized  the  break,  and  the  fragments  included  in 
the  conglomerate  overlying  the  iron  belt  are  said  to  prove  the  exist- 
ence of  the  jaspery  and  chalcedonic  material  in  its  present  condition 
before  the  formation  of  the  upper  quartzite.  Later  the  break  was 
noticed  by  the'Winchells,  and  N.  H.  Winchell  regarded  it  as  so  great 
that  the  rocks  above  the  break  were  provisionally  referred  to  the 
Potsdam.  On  the  basis  of  the  two  unconformities  the  Huronian  of 
the  Marquette  district  is  now  divided  into  three  groups,  upper, 
middle,  and  lower. 
In  the  Menominee  district  the  observations  of  Brooks  seemed  to 
indicate  the  presence  of  an  unconformity  within  the  Huronian  series. 
It  was  first  thought  by  Pumpelly,  Irving,  Van  Hise,  and  others  that 
the  break  should  come  above  the  iron-bearing  formation,  but  the 
detailed  mapping  by  Bayley,  Clements,  and  Van  Hise  suggested  one 
below  the  iron-bearing  horizon  and  above  the  quartzite  and  marble 
of  the  lower  Huronian.  The  evidence  of  considerable  unconformity 
is  slight. 
