314  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
schists,  gneisses,  and  granites,  above  which,  separated  by  a  great 
structural  hiatus,  is  the  Huronian  group,  mainly  of  detrital  rocks, 
which  is  followed  in  turn,  after  a  severe  structural  break,  by  the 
Keweenaw  group,  upon  the  eroded  edges  of  which  rests  the  Potsdam 
or  Upper  Cambrian  sandstone.  For  the  combination  of  clastic  series 
above  the  basement  complex  and  below  the  Potsdam  sandstone  the 
system  name  Agnotozoic  is  proposed. 
Winchell  (N.  H.),323  in  1888,  maintains  that  there  is  a  great  Pri- 
mordial quartzite  extending  from  New  England  through  Canada, 
Wisconsin,  and  Minnesota  to  the  Black  Hills  of  Dakota.  It  includes 
the  Taconic  quartzite  of  Emmons,  that  of  Sauk  and  Barron  counties 
in  Wisconsin,  the  Sioux  quartzite  of  Dakota,  the  quartzites  of  Min- 
nesota, and  those  of  the  Black  Hills  of  Dakota.  At  the  exhibition  in 
New  Orleans  in  1884  was  seen  a  block  of  the  Potsdam  sandstone  of  the 
State  of  New  York  exactly  similar  to  the  Pipestone  quartzite  of  Min- 
nesota, and  as  the  latter  bears  Primordial  fossils  there  is  no  lack  of 
evidence  to  parallelize  these  outcrops.  An  examination  of  the  quartz- 
ites of  the  Original  Huronian  convinced  the  author  of  the  parallel- 
ism of  the  great  quartzite  there  displayed  with  those  of  Wisconsin 
and  Minnesota.  But  things  that  are  equal  to  the  same  thing  are  equal 
to  each  other,  hence  the  Huronian  quartzite  is  no  other  than  the  Pots- 
dam sandstone  of  New  York,  the  Red  sand  rock  of  Vermont,  and  the 
granular  quartz  of  the  Taconic. 
Irving,324  in  1888,  discusses  the  classification  of  the  early  Cambrian 
and  pre-Cambrian  formations,  and  particularly  those  of  the  North- 
western States.  The  relations  of  the  Baraboo  quartzites  to  the  Pots- 
dam sandstone,  of  the  Potsdam  to  the  Keweenawan  series,  and  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.  Evidence  before 
given  is  repeated,  and  important  additional  evidence  of  the  break 
is  found  in  northeastern  Minnesota.  At  the  base  of  the  Keweenawan 
is  a  great  mass  of  gabbro,  which  extends  from  Duluth  northeast  to 
the  national  boundary,  more  than  100  miles,  and  at  its  maximum  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  granite. 
In  the  Huronian  are  placed  the  Original  Huronian,  the  iron-bearing 
series  of  Michigan  and  Wisconsin,  the  Black  River  Falls  iron-bearing 
series,  the  Animikie  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  Lau- 
