﻿R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  433 
  

  

  at 
  master 
  joints 
  in 
  the 
  country 
  rock, 
  with 
  a 
  square 
  termination, 
  

   like 
  that 
  of 
  a 
  commercial 
  lath 
  (fig. 
  5, 
  e). 
  

  

  The 
  injection 
  mechanism 
  of 
  the 
  largest 
  ribbon 
  is, 
  in 
  appear- 
  

   ance, 
  much 
  like 
  that 
  of 
  a 
  laccolith 
  with 
  a 
  lateral 
  conduit. 
  The 
  

   thinner 
  ribbons 
  are 
  not 
  so 
  obviously 
  explained. 
  They 
  seem 
  to 
  

   follow 
  joints 
  in 
  the 
  argillite, 
  and 
  to 
  have 
  made 
  room 
  for 
  

   themselves 
  by 
  crowding 
  aside 
  and 
  compacting 
  the 
  argillaceous 
  

   material 
  on 
  each 
  side. 
  No 
  other 
  feasible 
  interpretation 
  of 
  the 
  

   rectangular 
  cross-section 
  has 
  yet 
  been 
  imagined. 
  Where 
  

   erosion 
  has 
  exposed 
  the 
  top 
  of 
  a 
  ribbon 
  as 
  well 
  as 
  the 
  argil- 
  

   laceous 
  layer 
  on 
  each 
  side, 
  the 
  ribbon 
  looks 
  like 
  the 
  filling 
  of 
  a 
  

   sun-crack 
  in 
  the 
  sediment. 
  However, 
  the 
  cross-section 
  of 
  the 
  

   same 
  ribbon 
  always 
  shows 
  normally 
  bedded 
  sediment 
  im- 
  

   mediately 
  below 
  the 
  red 
  rock 
  ; 
  so 
  that 
  any 
  genetic 
  connection 
  

   with 
  sun-cracks 
  is 
  improbable. 
  The 
  explanation 
  by 
  lateral 
  

   crowding 
  presents 
  patent 
  difficulty. 
  Each 
  shale 
  lamina 
  con- 
  

   cerned 
  may 
  have 
  been 
  prepared 
  for 
  the 
  required 
  sudden 
  con- 
  

   densation 
  through 
  preliminary 
  contact 
  metamorphism 
  ; 
  the 
  

   main 
  contact 
  of 
  the 
  great 
  sill 
  is 
  not 
  more 
  than 
  about 
  10 
  meters 
  

   below 
  these 
  gently 
  dipping 
  metargillites. 
  Perhaps 
  the 
  heating 
  

   of 
  water, 
  specially 
  abundant 
  in 
  the 
  shale, 
  prepared 
  the 
  material 
  

   for 
  sudden 
  condensation 
  under 
  great 
  stress. 
  

  

  Some 
  of 
  the 
  probable 
  feeders 
  of 
  the 
  ribbon 
  injections 
  are 
  

   visible 
  in 
  low 
  cliffs 
  along 
  the 
  shore. 
  The 
  feeders 
  are 
  true 
  

   dikes 
  or 
  veins, 
  always 
  very 
  thin 
  (one 
  millimeter 
  or 
  less 
  to 
  three 
  

   millimeters 
  in 
  maximum 
  thickness) 
  and 
  often 
  of 
  capillary 
  

   dimensions. 
  They 
  are 
  interrupted 
  and 
  some 
  seem 
  to 
  peter 
  

   out, 
  both 
  above 
  and 
  below, 
  in 
  the 
  cliff 
  sections. 
  None 
  of 
  these 
  

   dikelets 
  could 
  be 
  traced 
  with 
  certainty 
  into 
  a 
  ribbon. 
  The 
  

   ribbons 
  regularly 
  cross 
  the 
  planes 
  of 
  the 
  dikelets 
  at 
  high 
  angles. 
  

   In 
  none 
  of 
  the 
  observed 
  ribbons 
  is 
  the 
  immediate 
  roof 
  or 
  floor 
  

   diked 
  by 
  red 
  rock. 
  Hence 
  the 
  ribbons 
  cannot 
  be 
  parts 
  of 
  co- 
  

   terminous 
  dikelets 
  abruptly 
  widened 
  at 
  shaly 
  horizons. 
  Each 
  

   ribbon 
  chamber 
  appears 
  to 
  have 
  been 
  forced 
  open 
  by 
  magma 
  

   which 
  entered 
  from 
  one 
  end. 
  The 
  forcing 
  of 
  a 
  nail 
  into 
  wood 
  

   by 
  very 
  strong, 
  steady 
  pressure 
  is 
  an 
  analogy, 
  though 
  the 
  

   injected 
  red 
  rock 
  was, 
  of 
  course, 
  not 
  rigid. 
  The 
  viscosity 
  of 
  

   the 
  ribbon 
  magma 
  must 
  rather 
  have 
  been 
  of 
  an 
  extremely 
  low 
  

   order. 
  

  

  Time 
  Relations 
  of 
  the 
  Magmatic 
  Phases.— 
  Because 
  the 
  

   Pigeon 
  Point 
  gabbro 
  is 
  cut 
  by 
  dikes 
  of 
  the 
  red 
  rock, 
  a 
  few 
  

   observers 
  have 
  argued 
  that 
  all 
  of 
  the 
  red 
  rock 
  here 
  belongs 
  to 
  

   a 
  magma 
  distinctly 
  younger 
  than 
  the 
  gabbro 
  magma. 
  Accord- 
  

   ingly, 
  the 
  intermediate 
  rock 
  has 
  been 
  explained 
  as 
  the 
  product 
  

   of 
  the 
  fusion 
  of 
  the 
  older 
  gabbro 
  and 
  its 
  peripheral 
  inter- 
  

   mingling 
  with 
  the 
  red-rock 
  magma. 
  This 
  view 
  has 
  little 
  to 
  

   commend 
  it. 
  The 
  irregular 
  main 
  layer 
  of 
  red 
  rock 
  averages 
  

  

  