180  PRK-OAMBRIAN    ROCKS    OF.  NORTH    AMERICA.  [bull.86. 
grained  ferruginous  quartzite,  the  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  water- worn  beaches- 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.  Recently  this 
author  has  changed  his  opinion  in  this  particular.  Irving  recognized 
the  break,  and  the  fragments  included  in  the  conglomerate  overlying 
the  iron  belt  are  said  to  prove  the  existence  of  the  jaspery  and  chalce- 
donic  material  in  its  present  condition  before  the  formation  of  the  upper 
quartzite.  Lately  the  break  Avas  noticed  by  the  Profs.  Winch  ell,  and 
N.  H.  Winchell  regarded  it  as  so  great  that  the  rocks  above  the  break 
were  provisionally  referred  to  the  Potsdam.  The  writer  has  described 
the  break  as  of  universal  extent  and  as  representative  of  a  great  uncon- 
formity, for  the  banded  and  contorted  jasper  and  ore  are  found  to  abut 
perpendicularly  against  a  quartzite  bearing  abundant  fragments  of  the 
underlying  formation,  which  are  in  exactly  the  condition  there  found. 
The  lower  series  is  a  semicrystalliue,  much  folded  one,  while  the  upper 
series  has  usually  not  become  crystalline  nor  closely  folded.  Before  the 
upper  series  was  deposited  the  lower  series  was  folded  and  truncated. 
It  is,  then,  plain  that  in  the  Marquette  district,  within  the  rocks  whicH 
have  heretofore  been  referred  to  the  Huronian,  are  two  series.  Below 
the  break  which  separates  them  are  the  lower  quartzite  of  Brooks,  the 
associated  novaculite  and  limestone,  and  the  lower  ore-bearing  forma- 
tion, including  the  hematitic,  magnetitic,  and  actinolitic  schists  and  jas- 
pers, which  contain  the  larger  number  of  great  mines.  Above  the  phys- 
ical break  are  Brooks's  upper  quartzite,  the  base  of  which  is  generally 
the  conglomerate  already  described.  Over  the  upper  quartzite  follow 
the  black  slates,  sometimes  carbonaceous,  gray wackes,  and  mica  schists, 
together  of  great  thickness,  and  occupying  an  area  as  large  as  or  larger 
than  the  Lower  Marquette  series.  In  these  upper  slates,  apparently  at 
rather  persistent  horizons,  are  locally  belts  of  chert  and  iron  carbonate 
associated  with  ore  bodies  of  considerable  size.  These  ores  are,  how- 
ever, of  a  very  different  character  from  those  which  occur  in  the  lower  ore 
formation. 
In  the  Menominee  district  as  evidence  in  favor  of  a  physical  break 
within  the  clastic  series  are  the  conglomerates  described  by  Brooks  at 
the  Pine  and  Poplar  rivers  district,  and  in  the  Commonwealth  section. 
At  the  first  is  found  conglomeratic  quartz-schists,  containing  micaceous 
iron  and  magnetite;  in  the  second  are  included  conglomeratic  quarts 
schists,  containing  pebbles  of  white  quartz  (chert)  and  jasper.  Simi- 
lar jasper  conglomerates  have  been  found  by  Pumpelly  and  the  writer 
over  the  ore  at  certain  of  the  mines.  The  relations  here  are,  then,  ex- 
actly like  those  in  the  Marquette  district,     Also,  the  structural  break 
