208  PEE-CAMBH1AN    GEOLOGY    OF    NORTH   AMERICA. 
ing  schists  in  Wisconsin,  and  of  the  quartzites  of  the  Black  Hills. 
All  are  of 'Taconic  age,  for  the  Lower  Cambrian  is  equal  to  the 
Taconic,  the  Huronian  is  equal  to  the  Taconic;  therefore  the  Lower 
Cambrian  is  equal  to  the  Huronian. 
In  the  Potsdam  sandstone,  which  is  unconformably  on  the  Taconic, 
is  included  the  upper  quartzites  of  the  Original  Huronian,  certain  of 
the  quartzites  of  Marquette,  the  Sioux  quartzites  of  Dakota,  and  the 
quartzites  of  Minnesota  and  Wisconsin.  This  is  also  the  age  of  the 
Copper-bearing  rocks,  which  are  an  alternation  of  basic  and  acidic 
eruptions  with  interbedded  sandstones  and  conglomerates.  The  great 
gabbro  eruption  is  later  than  the  beginning  of  the  Potsdam  age. 
Unconformably  above  the  Potsdam  is  the  St.  Croix  sandstone. 
The  general  succession  in  descending  order  is,  then,  as  follows : 
Calciferous Magnesian  limestones  and  sandstones 
St.  Croix Sandstones  and  shales \Dikelocephalus  horizon. 
Overlap  unconformity. 
Potsdam Quarzite,  gabbro,  red  granite,  and  Kewee- 
nawan Paradoxides  horizon. 
Overlap  unconformity. 
Taconic Black  and  gray  slates  and  quartzites,  iron 
ore  (Huronian,  Animikie) Olenellus  horizon. 
Overlap  unconformity. 
Keewatin (Including  the  Kawishiwin  or  greenstone 
belt,  with  its  jaspilite)  sericitic  schists 
and  gray wackes 
Vermilion (Coutchiching)  crystalline  schists 
Eruptive  unconformity. 
Laurentian Gneiss 
"Archean. 
Winchell  (H.  V.),102  in  1889,  gives  further  observations  on  the 
iron  regions  of  Minnesota.  On  the  Giants  range  the  Animikie  is 
found  to  rest  upon  the  syenite.  Here  is  a  semicrystalline  rock  be- 
tween the  two,  which  grades  into  the  syenite.  The  character  of  the 
transition  is  not  metamorphic,  but  rather  fragmental,  there  appear- 
ing to  be  a  certain  amount  of  loose  crystalline  material  which  has 
resulted  from  the  decay  and  erosion  of  the  syenite  lying  on  top  of 
this  rock  in  the  bed  of  the  sea  upon  and  around  which  the  Animikie 
sediments  were  deposited.  The  coarse  detritus  grades  up  into  the  fine 
detritus  of  the  Animikie.  The  Animikie  beds  are  found  also  to  rest 
unconformably  upon  the  upturned  edges  of  the  Keewatin  schists. 
The  same  relations  are  found  to  prevail  in  the  Birch  Lake  region. 
The  gabbro  containing  ores  in  the  vicinity  of  Kawishiwi  River  is 
found  to  contain  fragments  of  the  Animikie  slates  and  quartzites, 
and  is,  therefore,  of  later  origin.  At  Gunflint.  Lake  the  Animikie 
rests  unconformably  upon  the  Keewatin,  and  is  found  upon  green- 
stone. The  Keewatin  schists  are  largely  of  eruptive  origin.  The 
contacts  of  the  jaspilite  with  the  basic  schists  are  abrupt  and  angular, 
