﻿446 
  P. 
  A. 
  Paly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  with 
  the 
  final 
  development 
  of 
  steam 
  pressure 
  which 
  tended 
  to 
  

   expel 
  the 
  water 
  from 
  the 
  wall-rock 
  into 
  the 
  conduit 
  magma. 
  

   How 
  great 
  this 
  effect 
  was 
  cannot 
  be 
  estimated 
  without 
  a 
  

   knowledge 
  of 
  the 
  geothermal 
  gradient 
  in 
  the 
  Keweenawan 
  

   period. 
  That 
  it 
  was 
  steeper 
  than 
  now 
  is 
  indicated 
  by 
  the 
  

   efficiency 
  of 
  static 
  metamorphism 
  in 
  pre-Cambrian 
  time. 
  Pos- 
  

   sibly 
  the 
  conduit 
  walls 
  of 
  the 
  Pigeon 
  Point 
  eruptive, 
  where 
  

   passing 
  through 
  the 
  wet 
  sediments, 
  had 
  a 
  general 
  temperature 
  

   of 
  200° 
  C. 
  or 
  higher. 
  Experiments 
  have 
  shown 
  the 
  ample 
  

   power 
  of 
  water, 
  at 
  temperatures 
  little 
  above 
  200°, 
  to 
  promote 
  

   the 
  solution 
  of 
  siliceous 
  materials. 
  

  

  Herein, 
  perhaps, 
  is 
  an 
  explanation 
  of 
  the 
  fact 
  that 
  very 
  

   many 
  of 
  the 
  known 
  gabbroid 
  sills 
  and 
  laccoliths 
  bearing 
  acid, 
  

   granophyric 
  differentiates, 
  have 
  pre-Ordovician, 
  if 
  not 
  pre- 
  

   Cambrian, 
  dates 
  of 
  intrusion.* 
  

  

  Before 
  leaving 
  this 
  subject, 
  one 
  other 
  speculative 
  point 
  may 
  

   be 
  noticed. 
  The 
  Pigeon 
  Point 
  eruptive 
  is 
  precisely 
  en 
  axe 
  

   with 
  the 
  D 
  ul 
  u 
  th 
  laccolith, 
  which 
  in 
  its 
  essential 
  petrography^ 
  in 
  

   its 
  gravitative 
  differentiation, 
  and 
  in 
  its 
  general 
  structural 
  rela- 
  

   tions 
  is 
  a 
  colossal 
  replica 
  of 
  the 
  sill 
  under 
  discussion. 
  The 
  

   known 
  outcrops 
  of 
  the 
  two 
  bodies 
  are 
  separated 
  by 
  little 
  more 
  

   than 
  30 
  kilometers. 
  Is 
  the 
  sill 
  an 
  apophysis 
  from 
  the 
  200-kil- 
  

   ometer 
  laccolith 
  ? 
  Was 
  some 
  syntexis 
  accomplished 
  in 
  the 
  

   vaster 
  chamber 
  before 
  the 
  offshooting 
  sill 
  forced 
  a 
  way 
  east- 
  

   ward 
  as 
  far 
  as 
  Pigeon 
  Point 
  ? 
  In 
  the 
  face 
  of 
  this 
  possibility, 
  

   is 
  it 
  safe 
  to 
  deny 
  the 
  efficiency 
  of 
  magmatic 
  assimilation 
  simply 
  

   because 
  of 
  facts 
  observed 
  at 
  the 
  visible 
  contacts 
  of 
  the 
  sill 
  '(\ 
  

  

  Summary 
  on 
  the 
  Origin 
  of 
  the 
  Med 
  Pock. 
  — 
  Assimilation 
  of 
  

   the 
  Animikie 
  or 
  underlying 
  rocks 
  had 
  three 
  possible 
  loci 
  : 
  in 
  

   the 
  sill 
  conduit, 
  including 
  one 
  or 
  more 
  dikes 
  and 
  perhaps 
  lacco- 
  

   lithic 
  or 
  sill 
  enlargements 
  ; 
  at 
  the 
  main 
  contacts 
  of 
  the 
  visible 
  

   sill 
  chamber; 
  and 
  in 
  the 
  heart 
  of 
  the 
  sill, 
  where 
  downstoped 
  

   blocks 
  are 
  concerned. 
  

  

  Differentiation 
  of 
  the 
  syntectic 
  material 
  had 
  two 
  possible 
  

   loci 
  : 
  in 
  the 
  conduit, 
  and 
  in 
  the 
  visible 
  sill 
  chamber. 
  The 
  

   actual 
  differentiation, 
  implied 
  by 
  the 
  existing 
  rock 
  types 
  in 
  the 
  

   sill, 
  had 
  two 
  phases 
  : 
  that 
  controlled 
  directly 
  by 
  gravity, 
  and 
  

   that 
  controlled 
  by 
  gas 
  tension. 
  

  

  * 
  Compare 
  list 
  in 
  the 
  writer's 
  " 
  Igneous 
  Eocks 
  and 
  Their 
  Origin," 
  New 
  

   York, 
  1914, 
  pages 
  230 
  and 
  344 
  ft. 
  

  

  f 
  Elsewhere 
  (Memoir 
  38, 
  Geological 
  Survey 
  of 
  Canada, 
  1912, 
  page 
  250) 
  the 
  

   writer 
  has 
  described 
  a 
  mechanism 
  by 
  which 
  syntectic 
  magma, 
  formed 
  in 
  one 
  

   sill 
  chamber, 
  may 
  be 
  injected 
  into 
  a 
  new, 
  higher 
  sill 
  or 
  dike 
  chamber, 
  where 
  

   differentiation 
  unaccompanied 
  by 
  further 
  assimilation 
  of 
  country 
  rock 
  is 
  

   possible. 
  The 
  country 
  rocks 
  of 
  the 
  second 
  chamber 
  may 
  be 
  chemically 
  little 
  

   related 
  to 
  those 
  of 
  the 
  chamber 
  where 
  the 
  assimilation 
  occurred, 
  or 
  to 
  any 
  

   differentiate 
  in 
  the 
  second 
  chamber. 
  The 
  origin 
  of 
  the 
  red 
  rock 
  in 
  the 
  

   Duluth 
  laccolith 
  is 
  not 
  here 
  specially 
  considered, 
  but 
  the 
  size 
  of 
  that 
  body 
  

   warrants 
  a 
  sympathetic 
  reception 
  for 
  the 
  syntectic 
  theory 
  when 
  applied 
  to 
  

   its 
  red 
  rock. 
  

  

  