﻿442 
  E. 
  A. 
  Paly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  actually 
  present 
  in 
  a 
  natural 
  magma 
  and 
  as 
  to 
  their 
  capacity 
  

   for 
  diffusion 
  is 
  so 
  profound 
  that 
  this 
  third 
  speculation 
  cannot 
  

   be 
  profitably 
  discussed. 
  

  

  Difficulty 
  of 
  explanation 
  cannot 
  alter 
  the 
  fact 
  that 
  the 
  red- 
  

   rock 
  shells 
  appear 
  to 
  be 
  due 
  iu 
  part 
  to 
  separation 
  of 
  material 
  

   from 
  the 
  general 
  magma. 
  These 
  enyelopes 
  are, 
  therefore, 
  not 
  

   direct 
  proofs 
  of 
  an 
  origin 
  of 
  the 
  main 
  red-rock 
  mass 
  in 
  mere 
  

   contact 
  fusion 
  of 
  the 
  Aoimikie 
  sediments. 
  Nevertheless, 
  the 
  

   discovery 
  of 
  indigenous 
  micropegmatite 
  in 
  the 
  contact- 
  

   metamorphosed 
  sediments 
  shows 
  that 
  some 
  red-rock 
  material 
  

   has 
  been 
  generated 
  by 
  contact 
  fusion. 
  

  

  Magmatic 
  Stopin'g. 
  — 
  As 
  Bayley 
  noted, 
  the 
  roof 
  rock 
  of 
  the 
  

   sill 
  is 
  locally 
  much 
  shattered. 
  Besides 
  the 
  isolated 
  xenoliths, 
  

   evidence 
  of 
  the 
  disruptive 
  action 
  is 
  given 
  in 
  comparatively 
  

   thick 
  breccias 
  of 
  roof 
  quartzite, 
  now 
  cemented 
  by 
  subordinate 
  

   amounts 
  of 
  red 
  rock. 
  Xenolith 
  and 
  breccia 
  block 
  still 
  remain 
  

   near 
  or 
  at 
  the 
  roof, 
  evidently 
  because 
  the 
  magma 
  was 
  nearly 
  

   frozen 
  when 
  the 
  enclosure 
  of 
  roof 
  fragments 
  took 
  place. 
  Dur- 
  

   ing 
  the 
  long 
  preceding 
  magmatic 
  period, 
  other 
  shattering 
  

   must 
  have 
  occurred, 
  and, 
  because 
  of 
  the 
  smaller 
  magmatic 
  vis- 
  

   cosity 
  then 
  ruling, 
  the 
  roof 
  blocks 
  sank 
  into 
  the 
  heart 
  of 
  the 
  

   sill.* 
  As 
  in 
  the 
  case 
  of 
  the 
  normal 
  batholith, 
  the 
  efficiency 
  

   of 
  fusion 
  and 
  syntexis 
  can 
  here 
  be 
  estimated 
  only 
  after 
  the 
  fate 
  

   of 
  the 
  sunken 
  blocks 
  is 
  considered. 
  In 
  all 
  batholiths 
  the 
  deeper 
  

   levels 
  are 
  never 
  exposed, 
  so 
  that 
  the 
  subsequent 
  history 
  of 
  

   downstoped 
  blocks 
  can 
  only 
  be 
  inferred. 
  The 
  outcrops 
  of 
  the 
  

   Pigeon 
  Point 
  intrusive 
  are 
  sufficiently 
  continuous 
  to 
  warrant 
  

   the 
  statement 
  that 
  xenoliths 
  are 
  very 
  rare 
  at 
  depths 
  greater 
  

   than 
  20 
  meters 
  below 
  the 
  roof. 
  If 
  the 
  initial, 
  hot 
  magma 
  did 
  

   actually 
  shatter 
  and 
  stope 
  more 
  effectively 
  than 
  the 
  nearly 
  

   frozen 
  magma, 
  one 
  is 
  compelled 
  to 
  believe 
  the 
  older 
  xenoliths 
  

   to 
  have 
  been 
  melted 
  and 
  more 
  or 
  less 
  completely 
  dissolved 
  in 
  

   the 
  primary 
  gabbro 
  magma, 
  If 
  this 
  " 
  abvssal 
  " 
  assimilation 
  

   strongly 
  supplemented 
  any 
  assimilation 
  at 
  the 
  roof, 
  some 
  dif- 
  

   ferentiation 
  must 
  be 
  postulated, 
  for 
  the 
  gabbro 
  has 
  been 
  

   cleansed 
  from 
  nearly 
  all 
  the 
  material 
  which 
  is 
  not 
  present 
  in 
  

   normal 
  gabbro. 
  The 
  actual 
  stratiform 
  structure 
  of 
  the 
  sill 
  

   implies, 
  as 
  already 
  noted, 
  gravitative 
  control 
  in 
  the 
  separation, 
  

   which 
  would 
  presumably 
  progress 
  simultaneously 
  with 
  the 
  

   stoping 
  and 
  internal 
  solution 
  of 
  blocks.f 
  

  

  *The 
  lighter 
  shales 
  would 
  float 
  in 
  the 
  initial 
  gabbro. 
  Practically 
  all 
  

   xenoliths 
  would 
  sink 
  in 
  the 
  acidified 
  magma. 
  Blocks 
  of 
  the 
  dominant 
  

   quartzites 
  would 
  probably 
  not 
  sink 
  all 
  the 
  way 
  to 
  the 
  sill 
  floor, 
  but 
  would 
  

   come 
  to 
  rest 
  at 
  intermediate 
  levels. 
  There 
  high 
  temperature 
  was 
  long 
  main- 
  

   tained, 
  with 
  the 
  result 
  that 
  the 
  blocks 
  would 
  tend 
  to 
  be 
  completely 
  dissolved 
  

   or 
  fused. 
  

  

  f 
  On 
  the 
  shore, 
  east 
  of 
  the 
  bay 
  marked 
  at 
  " 
  C" 
  in 
  fig. 
  1, 
  the 
  red 
  rock 
  cuts 
  

   an 
  older, 
  nearly 
  vertical 
  dike 
  of 
  coarse 
  anorthositic 
  gabbro. 
  For 
  a 
  distance 
  

   of 
  about 
  30 
  centimeters 
  from 
  their 
  contact, 
  the 
  red 
  rock 
  is 
  charged 
  with 
  

  

  