﻿R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  431 
  

  

  On 
  the 
  other 
  hand, 
  the 
  asymmetry 
  of 
  the 
  complex 
  presents 
  

   no 
  mystery 
  if 
  the 
  whole 
  igneous 
  mass 
  is, 
  in 
  essence, 
  a 
  sill 
  dip- 
  

   ping 
  gently 
  southward. 
  (See 
  figs. 
  1 
  and 
  4.) 
  Many 
  direct 
  field 
  

   observations, 
  as 
  well 
  as 
  numerous 
  homologies 
  among 
  the 
  sills 
  

   of 
  British 
  Columbia, 
  Ontario, 
  South 
  Africa, 
  etc., 
  indicate 
  that 
  

   the 
  gabbro 
  of 
  the 
  Pigeon 
  Point 
  body 
  forms 
  a 
  thick 
  layer 
  at 
  

   the 
  sill 
  floor; 
  and 
  that 
  this 
  is, 
  in 
  succession, 
  overlain 
  by 
  much 
  

   thinner 
  layers 
  of 
  intermediate 
  rock 
  and 
  red 
  rock. 
  The 
  prin- 
  

   ciple 
  of 
  gravitative 
  differentiation 
  is 
  obviously 
  suggested. 
  

  

  Segregation 
  of 
  Red 
  Rock 
  through 
  Gas 
  Tension. 
  — 
  Gravity 
  

   has 
  not 
  been 
  the 
  sole 
  control 
  in 
  the 
  separation 
  of 
  the 
  red 
  rock 
  

   magma. 
  Narrow, 
  short 
  dikes 
  of 
  red 
  rock 
  cut 
  the 
  intermediate 
  

   rock, 
  the 
  gabbro, 
  and 
  the 
  quartzites 
  underlying 
  the 
  sill 
  gabbro. 
  

   The 
  dikes 
  seen 
  in 
  those 
  quartzites 
  appeared 
  in 
  all 
  cases 
  to 
  peter 
  

   out 
  within 
  a 
  distance 
  of 
  about 
  10 
  meters 
  below 
  the 
  lower 
  con- 
  

   tact 
  of 
  the 
  sill, 
  and 
  to 
  be, 
  in 
  a 
  sense, 
  apophyses 
  from 
  the 
  sill. 
  

   The 
  existence 
  and 
  location 
  of 
  these 
  dikes 
  imply 
  that 
  the 
  red 
  

   rock 
  differentiate 
  was 
  fluid 
  longer 
  than 
  the 
  gabbroid 
  differen- 
  

   tiate 
  ; 
  and 
  that 
  the 
  red 
  rock 
  magma 
  was 
  injected 
  into 
  the 
  sill 
  

   floor, 
  into 
  the 
  roof, 
  or 
  into 
  the 
  already 
  solidified 
  gabbroid 
  

   phase, 
  by 
  virtue 
  of 
  strong 
  gaseous 
  tension 
  in 
  the 
  red-rock 
  

   magma. 
  

  

  That 
  this 
  magma 
  was 
  charged 
  with 
  gas 
  in 
  large 
  amount 
  is 
  

   shown 
  by 
  the 
  great 
  development 
  of 
  miaroles 
  or 
  drusy 
  cavities 
  

   in 
  the 
  red 
  rock 
  of 
  sill 
  and 
  dike. 
  Similar 
  miaroles 
  were 
  

   observed 
  in 
  the 
  intermediate 
  rock, 
  but 
  never 
  in 
  the 
  gabbro. 
  

   The 
  unusual 
  driving 
  force 
  which 
  must 
  have 
  been 
  resident 
  in 
  

   the 
  red-rock 
  magma 
  is 
  suggested 
  also 
  by 
  the 
  lengths 
  of 
  numer- 
  

   ous, 
  exceedingly 
  thin 
  veins 
  of 
  red 
  rock 
  in 
  the 
  sediments 
  of 
  the 
  

   roof. 
  

  

  u 
  Ribbon 
  Injections." 
  — 
  A 
  very 
  striking 
  proof 
  of 
  enormous 
  

   tension 
  in 
  the 
  red-rock 
  magma 
  was 
  found 
  in 
  other 
  minute, 
  

   though 
  locally 
  numerous 
  bodies, 
  which 
  maybe 
  called 
  "linear 
  

   injections" 
  or, 
  perhaps 
  better, 
  "ribbon 
  injections." 
  On 
  the 
  

   south 
  shore, 
  immediately 
  west 
  of 
  the 
  little 
  bay 
  marked 
  " 
  C" 
  in 
  

   fig. 
  1, 
  the 
  roof 
  of 
  the 
  sill 
  consists 
  of 
  micaceous 
  and 
  feldspathic 
  

   quartzites 
  enclosing 
  thin, 
  originally 
  argillaceous 
  beds. 
  Into 
  

   two 
  different 
  layers 
  of 
  the 
  argillite, 
  separated 
  by 
  about 
  12 
  deci- 
  

   meters 
  of 
  quartzite, 
  the 
  red 
  rock 
  has 
  been 
  injected 
  in 
  the 
  

   form 
  of 
  nearly 
  straight, 
  flattened 
  needles, 
  or 
  thin, 
  narrow 
  rib- 
  

   bons 
  of 
  relatively 
  great 
  lengths. 
  The 
  exposures 
  in 
  three 
  di- 
  

   mensions 
  here 
  happen 
  to 
  be 
  almost 
  perfect, 
  so 
  that 
  one 
  can 
  make 
  

   out 
  the 
  form 
  and 
  relations 
  characterizing 
  this 
  novel 
  kind 
  of 
  

   intrusive 
  body. 
  

  

  The 
  smaller 
  ribbons 
  are 
  one 
  millimeter 
  or 
  less 
  in 
  thickness, 
  

   5 
  to 
  10 
  millimeters 
  in 
  width, 
  and 
  of 
  various 
  exposed 
  lengths 
  

   of 
  10 
  centimeters 
  to 
  one 
  meter. 
  The 
  largest 
  ribbon 
  seen 
  is 
  

  

  