148  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
I r vino,200  in  1888,  discusses  tlie  classification  of  the  early  Cambrian 
and  pie-Cambrian  formations,  and  particularly  those  of  the  north- 
western states.  The  relations  of  the  Baraboo  quartzites  to  the  Pots- 
dam sandstone,  the  relations  of  the  Potsdam  to  the  Keweenaw  series, 
the  relations  of  the  Animikie,  Penokee,  Marquette,  Menominee,  and 
Vermilion  lake  iron-bearing  series  to  the  underlying  and  overlying 
series  are  again  fully  discussed.  The  Keweenawan  is  held  to  overlie 
the  Huronian  everywhere  by  a  very  considerable  unconformity.  Evi- 
dence before  given  is  repeated,  and  important  additional  evidence  of 
the  break  is  found  in  nortb eastern  Minnesota.  At  the  base  of  the  Ke- 
weenawan is  a  great  mass  of  gabbro,  which  extends  from  Duluth  north- 
east to  the  national  boundary,  more  than  100  miles,  and  at  its  maxi- 
mum is  more  than  20  miles  wide.  This  basal  gabbro  is  now  in  contact 
with  one  member  of  the  Animikie,  and  now  with  another,  while  in 
other  places  it  is  in  contact  with  the  lower  crystalline  schists  or  gran- 
ite. In  the  Huronian  are  placed  the  Original  Huronian,  the  iron-bear- 
ing series  of  Michigan  and  Wisconsin,  the  Black  river  falls  iron-bearing 
series,  theAnimikie  series,  the  St.  Louis  and  Mississippi  slate  series,  the 
Vermilion  lake  iron-bearing  series,  the  Baraboo  quartzite  series,  and 
the  Sioux  quartzite  series.  Under  the  Huronian  is  the  Laurentian, 
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 
ikenesses  of  the  rocks  in  the  different  districts  referred  to  the  same 
grand  division,  and  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  suf- 
ficient 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 "i 
weak  paleontological  evidence  is  not  sufficient  reason  to  disregard  the 
enormous  thickness  of  the  formations  to  be  included  in  the  Cambrian  in:i 
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  vol- 
ume of  the  clastic  series  between  the  Cambrian  and  the  Archean  is  such 
as  to  demand  a  term  of  value  equivalent  with  Paleozoic,  and  Agnoto- 
zoic  or  Eparchean  is  proposed  as  this  term. 
Van  Hise,201  in  1889,  finds  the  iron  ores  of  the  Penokee-Gogebil 
series  to  be  of  sedimentary  origin  and  to  have  been  derived  from  an 
