﻿430 
  R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  in 
  the 
  country 
  rock 
  are 
  best 
  explained 
  as 
  due 
  to 
  its 
  dislocation 
  

   along 
  fractures, 
  accompanied 
  by 
  downstoping 
  of 
  the 
  blocks 
  

   immersed 
  in 
  the 
  magma. 
  At 
  other 
  points 
  along 
  the 
  contact, 
  

   much 
  more 
  intense 
  shattering 
  of 
  the 
  sedimentary 
  rocks 
  is 
  in 
  

   evidence, 
  and 
  swarms 
  of 
  quartzitic 
  xenoliths 
  show 
  that 
  mag- 
  

   matic 
  stoping 
  has, 
  once 
  again, 
  been 
  " 
  caught 
  in 
  the 
  act." 
  

  

  In 
  summary, 
  the 
  field 
  facts 
  seem 
  to 
  show 
  that 
  the 
  Pigeon 
  

   Point 
  body 
  is 
  a 
  sill 
  (fig. 
  4), 
  modified, 
  as 
  to 
  the 
  detailed 
  form 
  of 
  

   its 
  roof, 
  by 
  heat 
  shattering 
  and 
  moderate 
  overhead 
  stoping. 
  

   So 
  far 
  as 
  could 
  be 
  observed, 
  the 
  lower 
  contact 
  is 
  more 
  per- 
  

   fectly 
  concordant 
  with 
  the 
  Animikie 
  beds, 
  the 
  cross-cutting 
  

   there 
  proved 
  being 
  restricted 
  to 
  rare 
  local 
  offsets 
  of 
  a 
  few 
  

   centimeters.* 
  

  

  Tilting 
  after 
  Injection 
  of 
  the 
  Sill. 
  — 
  Almost 
  all 
  of 
  the 
  many 
  

   diabase 
  and 
  gabbro 
  dikes 
  cutting 
  the 
  sediments 
  follow 
  master 
  

   joints, 
  developed 
  at 
  right 
  angles 
  to 
  bedding. 
  The 
  east-west 
  

   dikes 
  therefore 
  now 
  dip 
  about 
  75° 
  northward 
  (see 
  iig. 
  4), 
  since 
  

   the 
  quartzites 
  dip 
  about 
  15° 
  in 
  a 
  southerly 
  direction. 
  It 
  is 
  

   simplest 
  to 
  assume 
  that 
  the 
  joints 
  were 
  formed 
  when 
  the 
  strata 
  

   lay 
  flat; 
  and 
  that 
  the 
  dikes, 
  some 
  of 
  which 
  merge 
  into 
  the 
  gab- 
  

   bro 
  at 
  its 
  lower 
  contact 
  and 
  thus 
  seem 
  to 
  represent 
  feeders 
  for 
  

   the 
  sill, 
  were 
  injected 
  before 
  the 
  Animikie 
  beds 
  were 
  tilted. 
  

   If 
  so, 
  the 
  great 
  sill 
  was 
  injected 
  before 
  being 
  turned 
  up 
  into 
  

   its 
  present 
  position. 
  Yet 
  absolute 
  proof 
  of 
  this 
  hypothesis 
  has 
  

   not 
  been 
  found. 
  (See 
  page 
  446.) 
  

  

  During 
  or 
  after 
  the 
  tilting 
  the 
  sill 
  and 
  sedimentaries 
  have 
  

   been 
  affected 
  by 
  a 
  few 
  dip-faults 
  of 
  small 
  displacement. 
  One 
  

   fault 
  has 
  offset 
  a 
  trap 
  dike 
  cutting 
  the 
  main 
  gabbro 
  at 
  the 
  

   extreme 
  end 
  of 
  the 
  point. 
  Some 
  of 
  the 
  irregularities 
  in 
  the 
  

   ground-plan 
  of 
  the 
  complex 
  eruptive 
  may, 
  perhaps, 
  be 
  explained 
  

   by 
  similar 
  faults. 
  

  

  Stratiform 
  Structure 
  of 
  the 
  Sill. 
  — 
  Bayley's 
  view 
  that 
  the 
  

   principal 
  intrusive 
  of 
  Pigeon 
  Point 
  is 
  a 
  practically 
  vertical 
  

   dike, 
  does 
  not 
  agree 
  well 
  with 
  his 
  explanation 
  of 
  the 
  red 
  rock 
  

   as 
  the 
  product 
  of 
  the 
  contact 
  fusion 
  of 
  the 
  sediments 
  by 
  the 
  

   primary 
  gabbro. 
  Except 
  for 
  a 
  few 
  dikelets, 
  the 
  red 
  rock 
  is 
  

   restricted 
  to 
  the 
  southern 
  side 
  of 
  the 
  main 
  gabbro. 
  If 
  the 
  

   gabbro 
  were 
  injected 
  as 
  a 
  dike, 
  there 
  is 
  no 
  apparent 
  reason 
  

   why 
  the 
  sediments 
  along 
  the 
  northern 
  contact 
  should 
  not 
  be 
  

   fused 
  in 
  the 
  same 
  way. 
  

  

  * 
  The 
  time 
  spent 
  in 
  the 
  field 
  did 
  not 
  permit 
  of 
  full 
  study 
  of 
  the 
  relations 
  

   between 
  the 
  small 
  patches 
  of 
  gabbro 
  mapped 
  by 
  Bayley 
  on 
  Plate 
  XV, 
  to 
  the 
  

   southward 
  of 
  the 
  red 
  rock. 
  The 
  patch 
  on 
  Fisherman's 
  Point 
  is 
  clearly 
  a 
  thick 
  

   dike 
  with 
  normal 
  chilled 
  contacts. 
  The 
  other 
  three 
  patches 
  may 
  represent 
  

   one 
  or 
  more 
  other 
  dikes 
  older 
  than 
  the 
  sill, 
  or 
  they 
  may 
  be 
  residual 
  chill 
  

   phases 
  of 
  the 
  sill 
  magma, 
  analogous 
  to 
  those 
  observed 
  locally 
  at 
  the 
  roof 
  of 
  

   the 
  Duluth 
  laccolith. 
  A 
  dip 
  fault 
  may 
  explain 
  the 
  offset 
  of 
  the 
  red-rock 
  

   band 
  in 
  the 
  area 
  mapped 
  on 
  Plate 
  XIV 
  of 
  Bayley's 
  memoir. 
  These 
  uncer- 
  

   tainties 
  do 
  not 
  seriously 
  affect 
  the 
  conclusion 
  that 
  the 
  main 
  eruptive 
  is 
  a 
  

   sill. 
  

  

  