﻿R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  443 
  

  

  The 
  probable 
  importance 
  of 
  stoping 
  is 
  indicated 
  by 
  the 
  pro- 
  

   nounced 
  irregularity 
  of 
  the 
  sill 
  roof. 
  The 
  reader 
  will 
  recall 
  

   that, 
  at 
  many 
  places, 
  the 
  red 
  rock 
  sharply 
  cross-cuts 
  the 
  roof 
  

   strata 
  for 
  several 
  meters, 
  the 
  eruptive 
  occupying 
  blocky 
  re- 
  

   entrants 
  in 
  the 
  roof. 
  One 
  result 
  is 
  the 
  development 
  of 
  local, 
  

   dike-like 
  contacts, 
  some 
  of 
  which 
  were 
  emphasized 
  by 
  Bayley.* 
  

   (See 
  fig. 
  3.) 
  On 
  the 
  other 
  hand, 
  the 
  floor 
  contact, 
  where 
  seen, 
  

   is 
  that 
  characteristic 
  of 
  a 
  typical 
  sill, 
  with 
  observed 
  cross-cut- 
  

   ting 
  to 
  the 
  extent 
  of 
  only, 
  at 
  most, 
  a 
  few 
  centimeters, 
  measured 
  

   at 
  right 
  angles 
  to 
  bedding. 
  Correspondingly, 
  the 
  eruptive 
  at 
  

   the 
  floor, 
  nearly 
  the 
  normal 
  gabbro 
  in 
  composition, 
  has 
  a 
  con- 
  

   tinuous 
  chilled 
  contact 
  and 
  shows 
  little 
  or 
  no 
  evidence 
  of 
  hav- 
  

   ing 
  there 
  shattered 
  or 
  assimilated 
  the 
  sedimentary 
  rock. 
  Since 
  

   roof 
  and 
  floor 
  originally 
  matched, 
  the 
  roof 
  must 
  have 
  been 
  

   roughened 
  by 
  some 
  kind 
  of 
  magmatic 
  activity 
  ; 
  that 
  roughen- 
  

   ing 
  is 
  most 
  readily 
  ascribable 
  to 
  stoping. 
  

  

  The 
  reason 
  for 
  the 
  practical 
  restriction 
  of 
  stoping 
  to 
  the 
  

   upper 
  contact 
  is 
  discussed 
  in 
  a 
  following 
  paragraph. 
  

  

  Capacity 
  of 
  the 
  Original 
  Magma 
  for 
  Assimilation 
  and 
  

   Contact 
  Fusion. 
  — 
  The 
  syntectic 
  theory 
  is 
  very 
  commonly 
  dis- 
  

   missed 
  from 
  sympathetic 
  consideration 
  by 
  those 
  who 
  emphasize 
  

   the 
  " 
  enormous 
  " 
  amount 
  of 
  latent 
  and 
  other 
  heat 
  required 
  to 
  

   fuse 
  or 
  dissolve 
  rock. 
  For 
  the 
  present 
  case, 
  as 
  for 
  many 
  other 
  

   debated 
  cases, 
  the 
  heat 
  demanded 
  may 
  be 
  regarded 
  as 
  " 
  enor- 
  

   mous" 
  in 
  absolute 
  measure 
  but 
  as 
  small 
  when 
  compared 
  with 
  

   the 
  initial 
  heat 
  supply. 
  

  

  In 
  the 
  Pigeon 
  Point 
  area, 
  trap 
  dikes 
  and 
  sills 
  only 
  a 
  few 
  

   centimeters 
  thick 
  were 
  observed. 
  Such 
  thinness, 
  combined 
  

   with 
  great 
  length, 
  demonstrates 
  some 
  superheat 
  in 
  their 
  

   diabasic 
  or 
  gabbroid 
  magma. 
  The 
  main 
  sill 
  itself, 
  less 
  than 
  

   200 
  meters 
  thick 
  and 
  probably 
  more 
  than 
  10 
  kilometers 
  long, 
  

   could 
  not 
  have 
  been 
  emplaced 
  unless 
  it 
  were 
  somewhat 
  super- 
  

   heated. 
  The 
  temperature 
  of 
  its 
  magma 
  would 
  be 
  raised 
  by 
  

  

  large 
  plagioclase 
  individuals 
  which, 
  seem 
  to 
  have 
  crystallized 
  from 
  a 
  hybrid 
  

   magma, 
  formed 
  by 
  the 
  solution 
  of 
  the 
  gabbro 
  in 
  the 
  invading 
  red-rock 
  

   magma. 
  This 
  contact 
  phase 
  of 
  the 
  red 
  rock 
  is 
  specially 
  dark 
  in 
  color 
  

   because 
  of 
  the 
  abundance 
  of 
  a 
  femic 
  mineral, 
  now 
  completely 
  chloritized. 
  

   Thus 
  the 
  red-rock 
  magma 
  was 
  capable 
  of 
  performing 
  some 
  assimilation, 
  

   even 
  at 
  a 
  late 
  stage 
  in 
  its 
  history. 
  If 
  that 
  deduction 
  is 
  correct, 
  it 
  tends 
  to 
  

   fortify 
  the 
  assumption 
  that 
  hydrous, 
  feldspathic 
  quartzite, 
  related 
  to 
  red 
  

   rock 
  in 
  composition 
  could 
  be 
  wholly 
  dissolved 
  by 
  the 
  initial, 
  hot, 
  gabbroid 
  

   magma. 
  

  

  *The 
  ''gabbro" 
  (really 
  a 
  coarse 
  porphyrite) 
  shown 
  in 
  fig. 
  3 
  of 
  Bayley's 
  

   paper 
  belongs 
  to 
  a 
  thick 
  dike, 
  which 
  is 
  certainly 
  older 
  than 
  the 
  red 
  rock 
  

   and 
  probably 
  older 
  than 
  the 
  main 
  gabbro. 
  The 
  cross-cutting 
  quality 
  of 
  the 
  

   red 
  rock 
  at 
  this 
  point 
  may 
  be 
  explained 
  either 
  by 
  stoping 
  or 
  by 
  local 
  up- 
  

   faulting 
  of 
  the 
  sill 
  roof 
  along 
  the 
  contact 
  of 
  the 
  massive, 
  strong 
  dike. 
  In 
  

   any 
  case 
  the 
  section 
  does 
  not 
  invalidate 
  the 
  conclusion 
  that 
  the 
  main 
  erup- 
  

   tive 
  is 
  a 
  sill. 
  

  

  