LAKE    SUPERIOR  REGION.  315 
rentian,  separated  from  it  by  a  great  unconformity.  This  is  a  series 
of  granites,  gneisses,  hornblende  schists,  mica  schists,  and  other  green 
schists. 
These  correlations  are  held  to  be  warranted  both  by  the  lithological 
likenesses  of  the  rocks  in  the  different  districts  referred  to  the  same 
grand  division  and  by  the  lithological  contrasts  between  the  divisions, 
as  well  as  by  the  fact  that  such  unconformities  as  exist  between  the 
series  must  necessarily  have  a  very  wide  extent.  That  one  or  two 
organic  forms  have  been  found  in  the  rocks  referred  to  the  Huronian 
is  not  sufficient  evidence  for  extending  the  term  Cambrian  down  to 
cover  this  and  the  Keweenawan  groups.  In  the  Huronian  are  shales 
and  slates  which  have  abundant  organic  matter,  and  important  beds 
of  ferruginous  strata  which  were  probably  accumulated  because  of 
the  existence  of  organic  matter.  The  fossils  discovered  are  of  types 
which  have  a  great  vertical  range  above  the  Cambrian  and  may  have 
as  great  a  vertical  range,  below  it.  That  a  pre-Cambrian  fauna 
existed  is  evident,  while  it  is  probable  that  this  fauna  had  affinities 
with  the  Cambrian  itself.  Such  weak  paleontological  evidence  is 
not  sufficient  reason  to  disregard  the  enormous  thickness  of  the  forma- 
tions to  be  included  in  the  Cambrian  in  case  the  Keweenawan  and 
Huronian  are  here  placed,  as  well  as  the  two  great  unconformities 
below  the  Potsdam,  which  must  also  be  covered  by  this  term.  Archean 
is  restricted  to  the  pre-Huronian  rocks.  The  volume  of  the  clastic 
series  between  the  Cambrian  and  the  Archean  is  such  as  to  demand 
a  term  of  value  equivalent  with  Paleozoic,  and  Agnotozoic  or  Epar- 
chean  is  proposed  as  this  term. 
Van  Hise,325  in  1889,  finds  the  iron  ores  of  the  Penokee-Gogebic 
series  to  be  of  sedimentary  origin  and  to  have  been  derived  from  an 
original  cherty  carbonate  of  iron  which  is  yet  abundantly  present  in 
the  upper  horizons  of  the  ore-bearing  formation. 
Van  Hise,326  in  1891,  describes  the  physical  break  between  a  Lower 
and  an  Upper  Huronian  series.  In  the  Marquette  district  the  Lower 
series  includes  the  lower  quartzite  and  novaculite  of  Brooks,  the  lime- 
stone formations  as  well  as  the  chief  iron-bearing  formation  contain- 
ing the  hard  ore,  which  is  composed  chiefly  of  jasper  and  actinolitic 
and  magnetitic  slates.  The  Upper  series  has  at  its  base  a  vitreous 
quartzite,  but  is  chiefly  composed  of  black  slates,  sometimes  carbo- 
naceous, graywackes,  and  mica  schists,  together  of  great  thickness, 
and  locally  contains  belts  of  ferruginous  cherts  and  slates,  including 
ore  bodies,  which  are,  however,  of  a  different  character  from  the  ores 
of  the  Lower  series.  The  area  occupied  by  the  Upper  series  is  equal 
to  or  greater  than  that  of  the  Lower  series.  That  the  two  series  are 
separated  by  a  great  unconformity  is  shown  by  numerous  contacts. 
