﻿428 
  R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  south 
  of 
  the 
  most 
  southerly 
  outcrop 
  of 
  quartzite, 
  the 
  maiu 
  gab- 
  

   bro 
  appears 
  in 
  a 
  low 
  cliff. 
  Here 
  the 
  gabbro 
  bears 
  feldspar 
  

   phenocrysts 
  ranging 
  from 
  2 
  to 
  5 
  centimeters 
  in 
  length. 
  These 
  

   are 
  commonly 
  parallel, 
  as 
  if 
  arranged 
  by 
  flow 
  in 
  the 
  magma, 
  

   and 
  lie 
  parallel 
  to 
  the 
  bedding 
  planes 
  of 
  the 
  adjacent 
  strata. 
  

   The 
  same 
  parallelism 
  of 
  orientated 
  feldspars 
  to 
  the 
  regional 
  

   dip-plane 
  of 
  the 
  Animikie 
  sediments 
  was 
  observed 
  in 
  the 
  gab- 
  

   bro 
  at 
  several 
  other 
  localities. 
  Such 
  repetition 
  in 
  the 
  arrange- 
  

   ment 
  of 
  the 
  gabbro 
  phenocrysts, 
  where 
  that 
  rock 
  is 
  distinctly 
  

   porphyritic, 
  is 
  inexplicable 
  on 
  the 
  dike 
  hypothesis. 
  As 
  Bay- 
  

   ley 
  hints 
  (page 
  23), 
  the 
  phenomenon 
  rather 
  suggests 
  that 
  the 
  

   gabbro 
  was 
  injected 
  after 
  the 
  manner 
  of 
  a 
  typical 
  sill. 
  

  

  Fig. 
  3. 
  

  

  Fig. 
  3. 
  Section 
  of 
  roof 
  contact 
  of 
  sill, 
  made 
  east 
  of 
  "C," 
  fig. 
  1, 
  and 
  

   similar 
  to 
  other 
  roof 
  sections 
  to 
  the 
  west. 
  Q, 
  contact-metamorphosed 
  

   Animikie 
  quartzite 
  with 
  metargillitic 
  interbeds 
  ; 
  2, 
  intermediate 
  rock; 
  3, 
  red 
  

   rock. 
  Blocky 
  character 
  of 
  upper 
  contact 
  illustrated 
  diagrammatically. 
  

  

  Again, 
  the 
  gabbro, 
  especially 
  near 
  its 
  northern 
  contact, 
  has 
  

   a 
  principal 
  system 
  of 
  joints 
  or 
  rift, 
  dipping 
  5° 
  to 
  20° 
  south- 
  

   ward, 
  the 
  other 
  joint 
  systems 
  being 
  roughly 
  perpendicular 
  to 
  

   that 
  rift. 
  These 
  structural 
  details 
  are 
  also 
  consonant 
  with 
  the 
  

   sill 
  hypothesis. 
  

  

  The 
  contact 
  of 
  the 
  red 
  rock 
  with 
  the 
  sedimentary 
  rocks 
  is 
  

   much 
  better 
  exposed. 
  With 
  a 
  few 
  short 
  interruptions, 
  quartz- 
  

   ites 
  and 
  interbedded 
  metargillites 
  extend 
  all 
  along 
  the 
  southern 
  

  

  younger 
  component. 
  After 
  the 
  consolidation 
  of 
  the 
  porphyrite, 
  this 
  older 
  

   dike 
  was 
  split, 
  on 
  a 
  nearly- 
  central 
  plane, 
  and 
  a 
  red 
  granite 
  dike 
  was 
  injected 
  

   along 
  that 
  plane. 
  The 
  granite 
  dike 
  is 
  a 
  little 
  over 
  one 
  meter 
  in 
  width 
  ; 
  so 
  

   that 
  the 
  whole 
  composite, 
  as 
  far 
  as 
  visible, 
  is 
  composed 
  of 
  porphyrite 
  and 
  

   granite 
  in 
  nearly 
  equal 
  proportions. 
  The 
  porphyrite 
  shows 
  chilled 
  edges. 
  

   The 
  granite 
  does 
  not, 
  but 
  is 
  coarse-grained 
  throughout 
  and 
  locally 
  even 
  peg- 
  

   matitic. 
  The 
  granite, 
  almost 
  wholly 
  made 
  up 
  of 
  quartz 
  and 
  alkaline 
  feldspar, 
  

   has 
  been 
  much 
  kaolinized 
  and 
  otherwise 
  altered, 
  as 
  if 
  hydrothermally. 
  Its 
  

   original 
  structure 
  was 
  the 
  hypidiomorphic-granular 
  ; 
  no 
  granophyric 
  inter- 
  

   growth 
  is 
  found 
  in 
  the 
  thin 
  section. 
  

  

  The 
  geological 
  relation 
  of 
  this 
  good 
  example 
  of 
  composite 
  dikes 
  to 
  the 
  sill, 
  

   only 
  a 
  few 
  meters 
  distant, 
  could 
  not 
  be 
  determined. 
  

  

  