﻿II. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  447 
  

  

  The 
  observed 
  characters 
  and 
  geological 
  relations 
  of 
  the 
  sill 
  

   were 
  established 
  after 
  both 
  assimilation 
  and 
  differentiation 
  had 
  

   been 
  completed. 
  

  

  In 
  view 
  of 
  so 
  many 
  complexities, 
  it 
  is 
  not 
  astonishing 
  that 
  

   the 
  problem 
  of 
  the 
  red 
  rock 
  still 
  awaits 
  definitive 
  solution. 
  

   The 
  solution 
  must, 
  of 
  course, 
  depend 
  on 
  a 
  patient 
  application 
  

   of 
  the 
  principle 
  of 
  inference 
  from 
  known 
  facts. 
  Of 
  these 
  the 
  

   most 
  significant, 
  in 
  the 
  writer's 
  opinion, 
  is 
  the 
  development 
  of 
  

   indigenous 
  granophyric 
  intergrowths 
  of 
  quartz 
  and 
  feldspar 
  in 
  

   the 
  contact-metamorphosed 
  sediments. 
  The 
  typical 
  intergrowth 
  

   has 
  properties 
  identical 
  with 
  those 
  of 
  the 
  abundant 
  micropeg- 
  

   matite 
  of 
  the 
  red 
  rock. 
  That 
  the 
  red-rock 
  material 
  was 
  derived 
  

   from 
  dissolved 
  or 
  fused 
  sediments 
  is 
  further 
  suggested 
  : 
  by 
  

   the 
  thorough 
  reddening 
  of 
  the 
  Animikie 
  strata 
  where 
  strongly 
  

   metamorphosed 
  ; 
  by 
  the 
  discovery 
  of 
  isolated 
  druses, 
  like 
  the 
  

   druses 
  of 
  the 
  red 
  rock, 
  in 
  the 
  same 
  strata 
  ; 
  and 
  by 
  the 
  unusual 
  

   abundance 
  of 
  gas, 
  presumably 
  water-gas, 
  in 
  the 
  red 
  rock. 
  

   Some 
  genetic, 
  though 
  not 
  easily 
  deduced, 
  relation 
  between 
  red 
  

   rock 
  and 
  quartzite 
  is 
  implied 
  by 
  the 
  presence 
  of 
  red-rock 
  shells 
  

   around 
  quartzite 
  xenoliths 
  and 
  their 
  absence 
  at 
  xenoliths 
  of 
  the 
  

   older 
  gabbro. 
  Supplemented 
  by 
  the 
  principle 
  of 
  differentia- 
  

   tion, 
  Bayley's 
  idea 
  of 
  contact 
  fusion 
  seems 
  to 
  give 
  one 
  essential 
  

   element 
  in 
  a 
  valid 
  explanation 
  of 
  the 
  progressive, 
  and 
  ulti- 
  

   mately 
  complete, 
  replacement 
  of 
  quartz 
  xenoliths 
  by 
  red 
  rock. 
  

  

  The 
  principal 
  difficulty 
  with 
  the 
  assimilation-fusion 
  hypothesis 
  

   is 
  the 
  higher 
  alkali, 
  especially 
  potash, 
  content 
  of 
  the 
  red 
  rock, 
  

   as 
  compared 
  with 
  the 
  Animikie 
  sediments 
  already 
  analyzed. 
  

   Bay 
  ley 
  states 
  that 
  individual 
  strata 
  carry 
  75 
  per 
  cent 
  of 
  feld- 
  

   spar, 
  but 
  such 
  beds 
  must 
  be 
  rare, 
  and 
  it 
  is 
  unsafe 
  to 
  assume 
  for 
  

   the 
  average 
  sediment 
  a 
  percentage 
  of 
  alkalies 
  as 
  large 
  as 
  that 
  

   characterizing 
  the 
  red 
  rock. 
  The 
  alkalies 
  concentrated 
  in 
  the 
  

   red 
  rock 
  may, 
  in 
  part, 
  have 
  been 
  derived 
  from 
  the 
  gabbroid 
  

   magma. 
  Combining 
  with 
  the 
  silica 
  and 
  alumina 
  of 
  the 
  Ani- 
  

   mikie 
  shales, 
  these 
  juvenile 
  alkalies 
  may 
  have 
  risen 
  to 
  the 
  sill 
  

   roof 
  dtfring 
  the 
  general 
  differentiation. 
  The 
  analyzed 
  gabbro 
  

   has, 
  in 
  fact, 
  proportions 
  of 
  soda 
  and 
  potash 
  which 
  are 
  abnor- 
  

   mally 
  low 
  for 
  gabbro. 
  The 
  same 
  feature 
  appears 
  in 
  other 
  

   instances 
  where 
  granophyric 
  and 
  gabbroid 
  rock 
  have 
  differen- 
  

   tiated 
  from 
  each 
  other.* 
  

  

  As 
  the 
  proximate 
  mode 
  of 
  origin 
  for 
  the 
  red 
  rock, 
  differentia- 
  

   tion 
  must 
  be 
  assumed. 
  Both 
  syntexis 
  and 
  mere 
  fusion 
  in 
  situ, 
  

   though 
  probably 
  important, 
  have 
  been 
  masked 
  by 
  differentia- 
  

   tion. 
  The 
  mechanism 
  of 
  magmatic 
  separation 
  represents 
  an 
  

   unsolved 
  problem. 
  Specifically, 
  future 
  investigation 
  may 
  well 
  

   be 
  devoted 
  to 
  the 
  question 
  as 
  to 
  how 
  the 
  feldspar 
  molecules 
  

  

  *E. 
  A. 
  Daly, 
  Igneous 
  Kocks 
  and 
  Their 
  Origin," 
  New 
  York, 
  1914, 
  page 
  320. 
  

  

  