132  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  8 
as  follows:  (1)  The  hematites  and  limonites  of  the  Mesabi  range,  tt 
equivalents  of  the,hematites  of  the  Penokee- Gogebic  range  in  Wiseoi 
sin ;  (2)  the  gabbro  titaniferous  magnetites  near  the  bottom  of  the  rock 
of  the  Mesabi  range ;  (3)  Olivinitic  magnetites,  just  below  the  gabbr 
in  the  basal  portion  of  the  Mesabi  rocks;  (4)  the  hematites  and  ma^ 
netites  of  the  Yermilion range  in  the  Keewatin  formation;  (5)  the  ma£ 
netites  of  the  crystalline  schists  of  the  Yermilion  formation.  It  is  mail 
tained  that  the  upper  iron  deposits  of  the  Mesabi  and  those  of  th  I 
Penokee- Gogebic  are  the  equivalents  of  theTaconic  ores  of  western  Ne^ 
England. 
Winchell  (N.  H.)187,  In  1891,  gives  numerous  additional  field  obsei 
vations.  The  relations  of  the  jaspilite,  argillite,  and  green  sdhist  ar 
considered,  and  the  argillite  at  least  is  regarded  as  a  sedimentar 
rock.  The  position  of  the  Pewabic  quartzite  is  left  uncertain.  It  i 
considered,  however,  to  overlie  the  Animikie  black  slate,  unless  then 
are  two  great  quartzites.  This  quartzite  has  heretofore  been  made  th< 
parallel  of  the  great  quartzite  that  overlies  the  Animikie  unconform 
ably,  but  it  is  possible  that  it  runs  below  it  conformably.  In  the  Stunts 
conglomerate  is  found  a  large  bowlder  which  contains  pebbles  of  chal 
cedonic  quartz  and  quartzose  felsite  and  the  porphyrel  at  Kekekabit 
lake.  A  study  of  the  ore  formation  leads  to  the  conclusion  that  a] 
three  of  the  known  agencies  for  rock-forming  were  intermittently  ai 
work  and  concerned  in  the  formation  of  the  iron  ore,  viz :  Eruption,  tc 
afford  the  basic  eruptive  material;  sedimentation,  to  arrange  it  (in  the 
main),  and  chemical  precipitation  in  the  same  water,  to  give  the  pure 
hematite  and  the  chalcedonic  silica.  The  great  gabbro  of  the  Cuprifer- 
ous formation  is  regarded  as  lying  below  the  Animikie,  among  othei 
reasons,  because  it  lies  next  to  and  immediately  south  of  the  gneiss  oi 
the  Giant  Eange  without  the  appearance  of  any  black  slate  between 
them ;  and  because  bowlders  of  characteristic  gabbro,  red  syenite,  and 
quartz-porphyry  occur  abundantly  in  the  later  traps  of  the  Cupriferous. 
Winchell  (N.  H  and  H.  V),188  in  1891,  give  an  extended  treatment 
of  the  iron  ores  of  northeastern  Minnesota  and  the  rocks  in  which  they 
are  contained.  Excluding  the  Cretaceous,  the  rocks  here  found  are 
divided  in  descending  order,  as  follows :  Keweenawan — trap  rocks,  tuffs, 
red  sandstones,  and  conglomerates  (Potsdam?);  Animikie — black  slates, 
gray,  feldspathic  sandstones  and  limestones;  Norian — gabbro  of  the 
Mesabi  hills,  red  granite,  quartz-porphyry,  red  felsite ;  Pewabic  quartz- 
ite (Granular  quartz,  Potsdam?);  Keewatin — sericitic  schists,  gray- 
wackes,  greenstones,  agglomerates,  jaspilite;  Yermilion — mica- schists 
and  hornblende-schists  (Coutchiching?),  Laurentian — sedimentary, 
gneissic  and  eruptive,  massive  or  porphyritic.  The  Keweenawan  to  the 
Pewabic  inclusive  are  placed  in  the  Taconic,  and  the  Keewatin  to  the 
Laurentian  inclusive  in  the  Archean  or  Azoic. 
The  jaspilite  and  schist  of  the  Keewatin  are  found  to  occur  some- 
times minutely  interlaminated;  at  other  times  the  jasper  is  in  irregular 
layers,  which  never  have  any  great  extent  and  always  finally  pinch 
