﻿P. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  439 
  

  

  pegmatite 
  so 
  formed 
  is 
  indistinguishable 
  from 
  that 
  in 
  typical 
  

   red 
  rock, 
  of 
  which 
  micropegmatite 
  forms 
  an 
  essential 
  part. 
  

   The 
  thin 
  sections 
  in 
  which 
  this 
  series 
  of 
  changes 
  can 
  be 
  traced 
  

   show 
  no 
  sign 
  of 
  fracturing 
  and 
  no 
  indication 
  of 
  any 
  channel 
  

   through 
  which 
  magmatic 
  material 
  has 
  been 
  introduced. 
  In 
  

   other 
  words, 
  the 
  micropegmatite 
  has 
  apparently 
  been 
  formed 
  

   of 
  material 
  original 
  in 
  the 
  sediment, 
  without 
  importation. 
  

  

  Professor 
  Grout, 
  in 
  a 
  personal 
  letter, 
  has 
  raised 
  the 
  question 
  

   whether 
  the 
  intergrowths 
  in 
  the 
  metamorphosed 
  sediment 
  have 
  

   been 
  produced 
  through 
  the 
  agency 
  of 
  juvenile 
  water-gas 
  

   emanating 
  from 
  the 
  magma. 
  Of 
  this 
  there 
  is 
  no 
  evidence 
  in 
  

   the 
  thin 
  section. 
  On 
  the 
  other 
  hand, 
  chemical 
  analysis 
  of 
  the 
  

   sediments 
  collected 
  where 
  they 
  have 
  not 
  undergone 
  any 
  con- 
  

   tact 
  metamorphism 
  proves 
  that 
  they 
  now 
  bear 
  a 
  relatively 
  high 
  

   percentage 
  of 
  connate 
  water. 
  In 
  Keweenawan 
  times 
  these 
  

   beds 
  were 
  less 
  affected 
  by 
  regional 
  (static) 
  metamorphism 
  and 
  

   were, 
  as 
  above 
  noted, 
  probably 
  even 
  more 
  hydrous. 
  That 
  the 
  

   connate 
  water 
  must 
  have 
  acted 
  as 
  a 
  solvent 
  in 
  the 
  highly 
  heated 
  

   sediments 
  is 
  an 
  obvious 
  fact 
  ; 
  to 
  attribute 
  the 
  visible 
  solution 
  

   to 
  the 
  influence 
  of 
  juvenile 
  gas 
  is 
  a 
  speculative 
  assumption, 
  

   unsupported 
  by 
  facts 
  won 
  from 
  a 
  study 
  of 
  the 
  thin 
  sections. 
  

   The 
  matter 
  is 
  admittedly 
  delicate, 
  but 
  the 
  present 
  writer 
  pre- 
  

   fers 
  to 
  dwell 
  on 
  the 
  simpler 
  inference 
  from 
  observed 
  facts. 
  

  

  The 
  indigenous 
  character 
  of 
  the 
  micropegmatite 
  observed 
  in 
  

   the 
  contact 
  aureole 
  is 
  further 
  suggested 
  by 
  its 
  occurrence 
  in 
  

   the 
  floor 
  quartzites, 
  next 
  the 
  gabbro, 
  in 
  which 
  the 
  granophyric 
  

   material 
  is 
  almost 
  or 
  quite 
  absent, 
  and 
  where, 
  to 
  judge 
  from 
  

   the 
  actual 
  mineralogy 
  of 
  the 
  rocks, 
  magmatic 
  gases 
  were 
  never 
  

   greatly 
  concentrated. 
  

  

  The 
  problem 
  is 
  identical 
  with 
  that 
  studied 
  in 
  connection 
  

   with 
  the 
  micropegmatite 
  of 
  the 
  Moyie 
  sills 
  of 
  British 
  Colum- 
  

   bia. 
  There 
  the 
  roof 
  aureoles 
  are 
  sometimes 
  richly 
  charged 
  

   with 
  typical 
  micropegmatite, 
  distributed 
  through 
  the 
  bedded 
  

   quartzites. 
  Chemical 
  analysis 
  shows 
  that 
  little 
  or 
  none 
  of 
  this 
  

   quartz-feldspar 
  intergrowth 
  is 
  to 
  be 
  explained 
  by 
  emanation 
  of 
  

   feldspathic 
  material 
  from 
  the 
  magma.* 
  According 
  to 
  Pro- 
  

   fessor 
  Grout 
  the 
  same 
  chemical 
  relations 
  hold 
  at 
  Pigeon 
  Point. 
  

  

  While, 
  then, 
  feldspathization 
  in 
  many 
  other 
  contact 
  aureoles 
  

   is 
  not 
  to 
  be 
  doubted, 
  that 
  principle 
  seems 
  not 
  to 
  account 
  for 
  

   much 
  of 
  the 
  micropegmatite 
  in 
  the 
  country 
  rocks 
  of 
  the 
  

   Pigeon 
  Point 
  sill. 
  

  

  Nor 
  is 
  it 
  likely 
  that 
  the 
  close 
  field 
  association 
  of 
  micropeg- 
  

   matite 
  in 
  the 
  red 
  rock 
  with 
  micropegmatite 
  in 
  the 
  metamor- 
  

   phosed 
  sediments 
  is 
  a 
  pure 
  accident. 
  The 
  material 
  is 
  of 
  a 
  

   nature 
  too 
  specialized 
  for 
  that. 
  The 
  preferable 
  explanation 
  is 
  

   probably 
  to 
  be 
  found 
  in 
  the 
  principle 
  of 
  " 
  ultra-metamorphism," 
  

  

  * 
  Cf 
  . 
  Memoir 
  38, 
  Geological 
  Survey 
  of 
  Canada, 
  1912, 
  page 
  243. 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Vol. 
  XLIII, 
  No. 
  258. 
  — 
  June, 
  1917. 
  

   30 
  

  

  