LAKE   SUPERIOR  REGION.  209 
and  numerous  fragments  are  found  contained  in  the  schists.  The 
jaspilite  is  regarded  as  a  sedimentary  formation  which  was  broken 
up  and  involved  in  the  eruptions  of  Keewatin  age.  The  Huronian 
quartzite  associated  with  the  magnetite,  lying  unconformably  upon 
the  syenite,  is  believed  to  lie  conformably  upon  the  Animikie  slates. 
Grant,103  in  1880,  gives  geological  observations  made  in  northeast- 
ern Minnesota.  North  of  Gunflint  Lake  the  vertical  Keewatin  and 
crystalline  schists,  with  an  east-west  strike,  strike  directly  across  a 
range  of  immediately  adjacent  gneisses,  the  schists  showing  no  evi- 
dence of  being  twisted  or  bent  within  200  feet  of  the  gneiss.  In  the 
syenites  of  Gunflint  Lake  are  found  fragments  of  schist,  which  in- 
dicates that  the  syenite  is  eruptive  later  than  the  schists.  At  Winchell 
Lake  the  syenite  upon  the  top  grades  down  by  an  apparent  transition 
into  gabbro.  The  gabbro  is  sometimes  cut  by  veins  or  dikes  of  syenite, 
which  indicates  that  the  latter  is  of  later  age,  although  the  syenite 
is  generally  below  the  gabbro. 
Winchell  (Alexander),104  in  1889,  maintains  that  the  Saganaga 
and  West  Sea  Gull  granite  conglomerate  before  described  is  produced 
from  a  fragmental  rock  by  selective  metamorphism,  the  completely 
crystalline  gneissoid  rocks  retaining  rounded  fragments  which  are 
residual  clastic  material.  The  conglomerate  of  Wonder  Island  is  not 
one  consisting  originally  of  a  mass  of  pebbles  over  which  a.  fluid 
magma  has  been  poured,  for  the  pebbles  are  not  in  contact;  they 
could  not  have  lain  where  they  are  before  the  gneissic  magma  existed. 
The  gneissic  magma  was  present,  and  it  was  this  which  supported  the 
pebbles  and  prevented  their  contact.  It  is,  then,  contemporaneous 
with  the  pebbles.  The  magma  must  have  been  plastic,  but  it  was  low 
temperature  igneo-aqueous  plasticity. 
Winchell  (N.  H.),105  in  1889,  in  a  general  discussion  of  the  origin 
of  the  eruptive  rocks,  maintains  that  there  are  four  epochs  of  basic 
eruption  in  Minnesota,  the  first  represented  by  the  Vermilion  group; 
the  second,  by  the  rocks  succeeding  the  graywackes;  the  third,  by 
those  succeeding  the  Animikie,  and  the  fourth,  by  those  of  the  Cuprif- 
erous formation. 
Meads,166  in  1889,  describes  the  Stillwater,  Minn.,  deep  well.  The 
well,  after  passing  through  about  700  feet  of  St.  Croix  and  Potsdam 
sandstone,  enters  rocks  which  are  in  every  respect  identical  with  those 
of  Keweenaw  Point;  hence  the  Keweenaw  rocks  are  below  the  light- 
colored  sandstones  of  the  Northwest.  For  the  first  1,500  feet  these 
are  brown  shales  and  brown  feldspathic  sandstones;  these  gradually 
assume  the  characters  of  a  volcanic  detrital  tuff — amygdaloid;  and 
finally,  at  a  depth  of  3,800  feet,  unmistakable  beds  of  trap  were  en- 
countered alternating  with  sandstone  beds.  At  this  depth  grains  of 
native  copper  were  seen  in  the  drillings. 
55721— Bull.  3G0— 00 14 
