﻿R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  437 
  

  

  and 
  relative 
  volume 
  to 
  the 
  red 
  rock 
  at 
  Pigeon 
  Point. 
  One 
  or 
  

   more 
  special 
  conditions 
  must, 
  therefore, 
  be 
  assumed 
  in 
  order 
  

   to 
  explain 
  these 
  remarkable 
  Minnesota 
  bodies, 
  if 
  due 
  to 
  pure 
  

   differentiation. 
  N~o 
  such 
  condition 
  has 
  yet 
  been 
  deduced 
  from 
  

   the 
  field 
  facts 
  ; 
  until 
  some 
  are 
  found, 
  the 
  pure-differentiation 
  

   idea 
  must 
  remain 
  in 
  the 
  realm 
  of 
  speculation. 
  

  

  On 
  the 
  other 
  hand, 
  specially 
  rapid 
  and 
  thorough 
  differentia- 
  

   tion 
  of 
  an 
  originally 
  basic 
  magma 
  might 
  be 
  thought 
  to 
  have 
  

   been 
  induced 
  by 
  its 
  absorption 
  of 
  much 
  water 
  from 
  the 
  walls 
  

   of 
  its 
  conduit. 
  The 
  Pigeon 
  Point 
  magma 
  rose 
  through 
  a 
  great 
  

   thickness 
  of 
  Animikie 
  strata 
  before 
  reaching 
  the 
  level 
  of 
  the 
  

   sill 
  chamber. 
  The 
  sediments 
  were 
  then 
  hydrous, 
  probably 
  

   more 
  so 
  than 
  now. 
  If 
  the 
  original 
  gabbroid 
  magma 
  absorbed 
  

   some 
  water 
  from 
  them, 
  the 
  temperature 
  of 
  consolidation 
  was 
  

   lowered 
  and 
  the 
  magmatic 
  life 
  thereby 
  lengthened. 
  Any 
  ten- 
  

   dency 
  to 
  spontaneous 
  differentiation 
  would 
  become 
  more 
  effec- 
  

   tive 
  merely 
  because 
  of 
  the 
  longer 
  time 
  available, 
  and 
  the 
  resur- 
  

   gent 
  water 
  itself 
  might 
  be 
  an 
  independent 
  cause 
  of 
  magmatic 
  

   differentiation. 
  

  

  If 
  this 
  were 
  true 
  for 
  the 
  sill, 
  similar 
  differentiation 
  might 
  be 
  

   regularly 
  expected 
  in 
  the 
  thick 
  Keweenawan 
  dikes 
  cutting 
  the 
  

   upper 
  Animikie 
  sediments 
  ; 
  since 
  dikes, 
  crossing 
  the 
  bedding, 
  

   are 
  favorably 
  situated 
  for 
  the 
  absorption 
  of 
  connate 
  water. 
  As 
  

   far 
  as 
  known, 
  the 
  thicker 
  Minnesota 
  dikes 
  show 
  no 
  special 
  ten- 
  

   dency 
  to 
  extreme 
  differentiation.* 
  

  

  Evidences 
  of 
  Assimilation. 
  — 
  The 
  second 
  objection 
  to 
  the 
  

   pure-differentiation 
  hypothesis 
  is 
  more 
  telling. 
  There 
  is 
  an 
  

   unmistakable 
  consanp-uinitv 
  between 
  the 
  red 
  rock 
  and 
  the 
  Ani- 
  

   mikie 
  siliceous 
  sediments, 
  just 
  as 
  there 
  is 
  between 
  the 
  ucticro- 
  

   pegmatitic 
  phases 
  of 
  the 
  Movie 
  sills 
  of 
  British 
  Columbia 
  and 
  

   the 
  quartzites 
  invaded 
  by 
  them. 
  The 
  consanguinity 
  is 
  both 
  

   mineralogical 
  and 
  chemical. 
  In 
  neither 
  respect 
  are 
  sediment 
  

   and 
  red 
  rock 
  identical 
  ; 
  yet 
  qualitatively 
  the 
  two 
  formations 
  

   are 
  closely 
  parallel, 
  and 
  one 
  cannot 
  rest 
  in 
  the 
  belief 
  that 
  their 
  

   similarity 
  is 
  accidental. 
  On 
  the 
  contrary, 
  their 
  qualitative 
  

   likeness 
  suggests 
  that 
  the 
  red 
  rock 
  is 
  the 
  result 
  of 
  syntexis 
  plus 
  

   differentiation. 
  

  

  For 
  a 
  distance 
  of 
  from 
  5 
  to 
  20 
  meters, 
  measured 
  at 
  right 
  

   angles 
  to 
  the 
  roof 
  contact 
  of 
  the 
  sill, 
  the 
  quartzites 
  and 
  metar- 
  

   gillites 
  have 
  been 
  strongly 
  metamorphosed 
  by 
  the 
  magma. 
  

   Their 
  maximum 
  alteration 
  is 
  like 
  that 
  in 
  the 
  many 
  xenoliths 
  

   close 
  to 
  the 
  shattered 
  roof. 
  Nearly 
  always 
  the 
  contact 
  between 
  

   red 
  rock 
  and 
  sedimentary 
  rock 
  is 
  sharp 
  and 
  the 
  bedding 
  planes 
  

  

  * 
  Other 
  things 
  being 
  equal, 
  the 
  volume 
  of 
  water 
  absorbed 
  by 
  an 
  ordinary 
  

   dike 
  would 
  be 
  less 
  than 
  that 
  entering 
  the 
  magma 
  of 
  a 
  conduit 
  to 
  a 
  great 
  sill, 
  

   so 
  that 
  the 
  postulated 
  differentiating 
  influence 
  of 
  water 
  might 
  be 
  more 
  con- 
  

   spicuous 
  in 
  the 
  sill 
  than 
  in 
  the 
  dike 
  ; 
  yet 
  some 
  effect 
  should, 
  by 
  the 
  hypo- 
  

   thesis, 
  be 
  illustrated 
  in 
  visible 
  thick 
  dikes. 
  

  

  