336  PRE-CAMBEIAN    GEOLOGY    OF    NORTH    AMERICA. 
but  the  discovery  of  a  great  unconformity  within  the  old  "  Lower 
Huroniarij"  by  A.  E.  Seaman,  showed  that  it  really  consists  of  two 
groups.  The  lower  Huronian  consists  of  three  formations — in 
ascending  succession  the  Mesnard  quartzite,  the  Kona  dolomite,  and 
the  Wewe  slate — together  ranging  from  1,200  to  3,000  feet  in  thick- 
ness. The  middle  Huronian  includes,  from  the  base  up,  the  Ajibik 
quartzite,  the  Siamo  slate,  and  the  iron-bearing  Negaunee  formation, 
together  ranging  from  1,000  to  3,000  feet  in  thickness.  The  iron- 
bearing  Negaunee  formation  carries  the  important  iron-ore  deposits 
of  the  district.  The  upper  Huronian  includes,  in  ascending  succes- 
sion, the  Goodrich  quartzite,  the  iron-bearing  Bijiki  schist,  and  the 
Michigamme  slate.  The  Bijiki  schist,  appearing  principally  in  the 
western  part  of  the  district,  and  the  Goodrich  quartzite,  appearing 
principally  in  the  central  part  of  the  district,  are  taken  to  be  partly 
equivalent  in  age.  The  Bijiki  schist  contains  iron  ores  at  one  or 
more  horizons.  Extrusion  of  volcanic  rocks,  the  Clarksburg  forma- 
tion, began  in  middle  Huronian  time  and  extended  into  upper 
Huronian  time.  Dikes  and  bosses  of  basic  igneous  rocks  are  abun- 
dant in  both  the  lower  and  middle  Huronian. 
The  lower  Huronian  is  the  most  folded  and  metamorphosed,  the 
upper  Huronian  the  least ;  the  lower  Huronian  is  the  thinnest  of  the 
series,  the  upper  Huronian  the  thickest;  the  lower  Huronian  occu- 
pies the  least  area,  the  upper  Huronian  the  greatest.  The  uncon- 
formities between  the  middle  and  lower  Huronian  and  between  the 
upper  and  middle  Huronian  are  indicated  by  basal  conglomerates  and 
truncation  of  the  underlying  rocks.  It  can  not  be  said  at  the  present 
time  which  of  the  two  unconformities  is  the  more  important. 
Crystal  Falls  district. — In  Clements's  monograph  on  the  Crystal 
Falls  district a  the  Huronian  rocks  were  divided,  on  the  map  and  in 
the  text,  into  the  upper  Huronian  and  lower  Huronian,  correspond- 
ing to  the  old  division  of  "  Upper  Marquette  "  and  "  Lower  Mar- 
quette "  for  the  Marquette  district.  The  lower  Huronian  consists, 
from  the  base  upward,  of  the  Sturgeon  quartzite  and  the  Randville 
dolomite,  varying  from  600  to  1,000  feet  in  thickness.  Cutting  and 
resting  on  both  formations  is  the  Hemlock  formation,  consisting 
largely  of  basic  volcanics.  Interbedded  in  this  formation  is  the 
"  Mansfield  "  slate.  The  upper  Huronian  comprises  the  Michigamme 
slate,  containing  an  iron-bearing  horizon  not  separated  in  mapping 
for  much  of  the  district. 
There  is  practically  no  direct  field  evidence  of  unconformity  be- 
tween the  Hemlock  formation  and  the  upper  Huronian  slates,  the 
existence  of  the  brepk  having  been  inferred  from  the  difference 
in  lithological  character  and  from  supposed  correlation  with  the 
Marquette  district. 
«  Mon.  U.  S.  Geol.  Survey,  vol.  36.  1899. 
