﻿R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  445 
  

  

  dental 
  to 
  differentiation 
  must 
  have 
  stirred 
  the 
  upper 
  layer 
  of 
  

   magma 
  and 
  prevented 
  rapid 
  chilling 
  at 
  the 
  roof. 
  

  

  To 
  this 
  rather 
  bold 
  sketch 
  of 
  the 
  probable 
  events, 
  objection 
  

   may 
  be 
  urged 
  that 
  the 
  magmatic 
  gas, 
  so 
  clearly 
  forming 
  an 
  

   original 
  part 
  of 
  the 
  magma 
  of 
  the 
  drusy 
  red 
  rock, 
  may 
  have 
  

   been 
  purely 
  juvenile 
  and 
  in 
  no 
  degree 
  resurgent. 
  Such 
  a 
  

   speculation 
  is 
  founded 
  on 
  a 
  difficult 
  thesis, 
  for 
  it 
  denies 
  the 
  

   efficiency 
  of 
  gas 
  tension 
  to 
  drive 
  connate 
  water 
  into 
  the 
  initial 
  

   magma 
  from 
  the 
  hot 
  walls 
  of 
  the 
  conduit. 
  Some 
  penologists 
  

   will 
  have 
  it 
  that 
  gas 
  can 
  move 
  centrifugally 
  from 
  an 
  intrusive 
  

   body, 
  but 
  never 
  in 
  the 
  reverse 
  direction, 
  into 
  the 
  intrusive 
  

   magma. 
  They 
  think 
  of 
  the 
  contact 
  surface 
  of 
  the 
  magma 
  as 
  a 
  

   one-way 
  gate. 
  This 
  remarkable 
  conception 
  surely 
  needs 
  criti- 
  

   cal 
  examination. 
  The 
  gas 
  moves 
  because 
  of 
  differential 
  

   pressure. 
  In 
  contact 
  aureoles 
  developed 
  in 
  relatively 
  imper- 
  

   vious, 
  hydrous 
  sediments, 
  the 
  steam 
  pressure 
  must, 
  in 
  very 
  

   many 
  cases, 
  rise 
  enormously. 
  If 
  the 
  pores 
  of 
  the 
  sediments 
  

   were 
  originally 
  full 
  of 
  water, 
  such 
  pressure 
  might 
  be 
  very 
  much 
  

   greater 
  than 
  that 
  of 
  the 
  initial 
  magma 
  at 
  the 
  level 
  concerned. 
  

   To 
  explain 
  dehydration 
  in 
  such 
  contact 
  aureoles, 
  it 
  is 
  quite 
  

   gratuitous 
  to 
  assume 
  that 
  all 
  the 
  water 
  moves 
  centrifugally. 
  

   The 
  certainty 
  of 
  pegmatitic 
  emanation 
  in 
  the 
  late 
  magmatic 
  

   stage 
  proves 
  nothing 
  at 
  all 
  regarding 
  the 
  direction 
  of 
  migration 
  

   for 
  gases 
  during 
  the 
  long 
  preceding 
  stage 
  of 
  higher 
  tempera- 
  

   tures 
  and 
  gas 
  pressures. 
  In 
  the 
  present 
  case 
  the 
  conditions, 
  

   including 
  the 
  existence 
  of 
  many 
  horizons 
  of 
  impervious 
  shales 
  

   in 
  the 
  Animikie 
  series, 
  seem 
  to 
  imply 
  the 
  necessity 
  of 
  expulsion 
  

   of 
  much 
  water-gas 
  from 
  the 
  wall-rocks 
  into 
  the 
  gabbroid 
  

   magma. 
  

  

  Further, 
  no 
  good 
  reason 
  is 
  in 
  sight 
  for 
  crediting 
  this 
  particu- 
  

   lar 
  body 
  of 
  magma 
  with 
  a 
  much 
  higher 
  proportion 
  of 
  juvenile 
  

   gas 
  than 
  that 
  fairly 
  attributable 
  to 
  many 
  Keweenawan 
  injec- 
  

   tions 
  in 
  Minnesota. 
  On 
  the 
  other 
  side, 
  no 
  one 
  can 
  doubt 
  that 
  

   the 
  Animikie 
  sediments, 
  especially 
  the 
  shaly 
  beds, 
  were 
  during 
  

   Keweenawan 
  time 
  rich 
  in 
  water. 
  So 
  true 
  is 
  this 
  that 
  it 
  would 
  

   not 
  be 
  utterly 
  absurd 
  to 
  regard 
  the 
  abundance 
  of 
  gas, 
  probably 
  

   water-gas, 
  in 
  the 
  red 
  rock 
  as 
  another 
  suggestion 
  of 
  consanguin- 
  

   ity 
  between 
  that 
  formation 
  and 
  the 
  Animikie 
  sediments. 
  

  

  Again, 
  the 
  heat 
  producing 
  gas 
  tension 
  in 
  the 
  wall-rocks 
  of 
  

   the 
  conduit, 
  and 
  also 
  aiding 
  in 
  the 
  preparation 
  of 
  the 
  tempera- 
  

   ture 
  suitable 
  for 
  solution 
  of 
  their 
  solid 
  material 
  in 
  the 
  magma, 
  

   was 
  not 
  merely 
  magmatic 
  heat. 
  Before 
  the 
  gabbro 
  eruption, 
  

   the 
  lower 
  beds 
  of 
  the 
  Animikie 
  series 
  and 
  the 
  unconformable 
  

   underlying 
  rocks 
  had 
  been 
  heated 
  because 
  of 
  their 
  burial 
  under 
  

   the 
  thick, 
  upper 
  part 
  of 
  the 
  Animikie 
  series, 
  perhaps 
  already 
  

   covered 
  by 
  early-Keweenawan 
  lava 
  flows. 
  General 
  earth 
  heat 
  

   had 
  thus 
  begun 
  the 
  process 
  of 
  superheating 
  the 
  connate 
  water, 
  

  

  