﻿438 
  B. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  of 
  the 
  latter 
  are 
  usually 
  evident. 
  The 
  originally 
  gray 
  or 
  

   greenish 
  quartzites 
  become 
  of 
  a 
  deeper 
  and 
  deeper 
  red 
  tint, 
  

   according 
  to 
  the 
  degree 
  of 
  their 
  metamorphism. 
  When 
  most 
  

   changed 
  the 
  quartzites 
  have 
  a 
  color 
  nearly 
  identical 
  with 
  that 
  

   of 
  the 
  red 
  rock. 
  

  

  More 
  locally, 
  drusy 
  cavities, 
  lined 
  with 
  well-terminated 
  

   quartz 
  and 
  feldspar 
  crystals, 
  are 
  developed 
  in 
  the 
  roof 
  quartz- 
  

   ites. 
  These 
  are 
  of 
  habit 
  similar 
  to 
  that 
  of 
  the 
  yet 
  more 
  abun- 
  

   dant 
  druses 
  in 
  the 
  red 
  rock. 
  

  

  Both 
  the 
  reddening 
  and 
  the 
  formation 
  of 
  drusy 
  cavities 
  in 
  

   the 
  roof 
  rocks 
  might 
  be 
  regarded 
  as 
  effects 
  of 
  water-gas 
  and 
  

   other 
  emanations 
  from 
  the 
  red 
  rock, 
  in 
  which 
  the 
  same 
  features 
  

   were 
  developed 
  because 
  of 
  its 
  own 
  content 
  of 
  these 
  gases. 
  By 
  

   this 
  hypothesis 
  the 
  color 
  change 
  is 
  attributed 
  to 
  the 
  chemical 
  

   influence 
  of 
  the 
  gas 
  ; 
  the 
  existence 
  of 
  the 
  cavities, 
  to 
  its 
  high 
  

   tension. 
  Thus, 
  the 
  two 
  characteristics 
  mentioned 
  would 
  be 
  

   considered 
  to 
  have 
  no 
  significance 
  in 
  the 
  problem 
  of 
  the 
  red 
  

   rock. 
  

  

  Yet 
  some 
  mode 
  of 
  origin 
  for 
  the 
  emanating 
  gas 
  itself 
  is 
  

   demanded. 
  A 
  critic 
  of 
  the 
  foregoing 
  hypothesis 
  may 
  reply 
  

   that 
  it 
  puts 
  the 
  cart 
  before 
  the 
  horse, 
  holding 
  that 
  the 
  mag- 
  

   matic 
  gas 
  involved 
  was 
  largely 
  water-gas 
  derived 
  from 
  the 
  sedi- 
  

   ments, 
  as 
  really 
  implied 
  in 
  Bay 
  ley's 
  theory 
  of 
  the 
  red 
  rock. 
  

  

  The 
  consanguinity 
  of 
  red 
  rock 
  and 
  sedimentary 
  rock 
  is 
  more 
  

   clearly 
  suggested 
  from 
  the 
  fact 
  that 
  each 
  bears 
  indigenous 
  

   micrographic 
  intergrowths, 
  composed 
  of 
  quartz 
  and 
  feldspar. 
  

   Some 
  of 
  this 
  material 
  has 
  doubtless 
  been 
  introduced 
  into 
  the 
  

   roof 
  sediments, 
  through 
  impregnation, 
  from 
  the 
  red-rock 
  

   magma, 
  but 
  careful 
  microscopic 
  study 
  shows 
  this 
  explanation 
  

   to 
  be 
  unacceptable 
  for 
  much 
  of 
  the 
  micropegmatite 
  developed 
  

   in 
  the 
  sediments. 
  

  

  The 
  original 
  quartzites 
  and 
  metargillites 
  contain 
  quartz, 
  

   feldspar, 
  sericite 
  (also 
  paragonite 
  ?), 
  chlorite, 
  biotite, 
  and 
  iron 
  

   oxides. 
  In 
  an 
  early 
  stage 
  of 
  the 
  contact 
  metamorphism 
  the 
  

   quartz 
  grains 
  are 
  seen 
  to 
  have 
  been 
  embayed 
  by 
  minute 
  tongues 
  

   of 
  their 
  alkaline 
  and 
  aluminous 
  cement. 
  These 
  minute, 
  but 
  

   relatively 
  long 
  and 
  narrow, 
  tongues 
  can 
  only 
  be 
  due 
  to 
  mutual 
  

   solution 
  of 
  quartz 
  and 
  feldspar, 
  or 
  quartz 
  and 
  general 
  cement, 
  

   aided 
  by 
  connate 
  fluids. 
  Where 
  the 
  mutual 
  solution 
  has 
  not 
  

   gone 
  beyond 
  the 
  incipient 
  stage, 
  the 
  product 
  is 
  generally 
  

   obscure. 
  In 
  a 
  slightly 
  more 
  advanced 
  stage, 
  most 
  or 
  all 
  of 
  the 
  

   tongues 
  in 
  an 
  attacked 
  quartz 
  grain 
  are 
  filled 
  with 
  alkaline 
  

   feldspar, 
  mixed 
  with 
  accessory 
  material. 
  The 
  feldspathic 
  parts 
  

   of 
  the 
  tongues 
  commonly 
  extinguish 
  simultaneously 
  under 
  the 
  

   microscope. 
  With 
  further 
  metamorphism 
  this 
  new 
  feldspar 
  is 
  

   found 
  to 
  be 
  intergrown 
  micrographically 
  with 
  quartz, 
  indicat- 
  

   ing 
  a 
  quite 
  local, 
  but 
  complete, 
  mutual 
  solution. 
  The 
  micro- 
  

  

  