﻿P. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  429 
  

  

  shore 
  of 
  the 
  peninsula. 
  At 
  many 
  places 
  their 
  contact 
  with 
  the 
  

   red 
  rock, 
  for 
  distances 
  of 
  from 
  one 
  to 
  ten 
  or 
  more 
  meters, 
  meas- 
  

   ured 
  down 
  the 
  dip, 
  is 
  seen 
  to 
  be 
  clearly 
  accordant 
  (fig. 
  3). 
  In 
  

   general, 
  the 
  surface 
  of 
  contact 
  dips 
  southward, 
  under 
  the 
  

   quartzites, 
  at 
  angles 
  of 
  from 
  5° 
  to 
  20°, 
  averaging 
  about 
  15°. 
  

   Parallel 
  to 
  that 
  general 
  direction 
  is 
  an 
  unusually 
  well 
  devel- 
  

   oped 
  system 
  of 
  rift 
  joints 
  in 
  the 
  red 
  rock 
  (see 
  fig. 
  8 
  in 
  Bay 
  ley's 
  

   memoir). 
  These 
  are 
  also 
  parallel 
  to 
  a 
  distinct, 
  though 
  less 
  con- 
  

   spicuous, 
  rift 
  system 
  in 
  the 
  adjacent 
  intermediate 
  rock 
  ; 
  and 
  

   roughly 
  parallel 
  to 
  the 
  main 
  rift 
  system 
  in 
  the 
  gabbro, 
  as 
  

   already 
  implied. 
  Assuming 
  that 
  these 
  rifts 
  are 
  parallel 
  to 
  the 
  

  

  Fig. 
  4. 
  

  

  

  IOO 
  200 
  METERS 
  

  

  Fig. 
  4. 
  Dip-section 
  of 
  the 
  Pigeon 
  Point 
  sill 
  through 
  "A," 
  fig. 
  1, 
  where 
  

   the 
  sill 
  is 
  relatively 
  thin. 
  Q, 
  Animikie 
  quartzite 
  with 
  metargillitic 
  inter- 
  

   beds 
  ; 
  D, 
  diabase 
  and 
  gabbro 
  dikes 
  ; 
  1, 
  gabbro 
  ; 
  2, 
  intermediate 
  rock 
  ; 
  3, 
  red 
  

   rock. 
  

  

  original 
  cooling 
  surface 
  of 
  the 
  complex 
  intrusive 
  — 
  a 
  probable 
  

   interpretation 
  — 
  the 
  conclusion 
  that 
  the 
  intrusive 
  is 
  a 
  sill 
  is 
  

   again 
  indicated.* 
  

  

  Nevertheless, 
  the 
  upper 
  contact 
  of 
  the 
  intrusive 
  is 
  by 
  no 
  

   means 
  always 
  concordant 
  with 
  the 
  bedding 
  of 
  the 
  roof 
  quartz- 
  

   ites. 
  In 
  the 
  low 
  cliffs 
  of 
  the 
  south 
  shore 
  the 
  red 
  rock 
  may 
  be 
  

   seen 
  at 
  some 
  points 
  to 
  cross-cut 
  the 
  sedimentaries 
  at 
  high 
  angles. 
  

   There 
  the 
  red 
  rock 
  fills 
  blocky 
  re-entrants 
  in 
  the 
  roof. 
  The 
  

   maximum 
  observed 
  degree 
  of 
  cross-cutting 
  is 
  a 
  couple 
  of 
  

   meters, 
  measured 
  at 
  right 
  angles 
  to 
  the 
  bedding 
  of 
  the 
  sedi- 
  

   ments, 
  but 
  it 
  may 
  be 
  locally 
  several 
  times 
  that 
  amount. 
  Thus, 
  

   in 
  spite 
  of 
  the 
  fact 
  that 
  bedding 
  of 
  the 
  roof 
  rock 
  and 
  the 
  con- 
  

   tact 
  surface 
  are 
  on 
  the 
  whole 
  roughly 
  parallel, 
  the 
  roof 
  exhibits 
  

   re-entrants 
  which 
  seem 
  to 
  be 
  small-scale 
  analogues 
  of 
  those 
  

   depicted 
  in 
  BarrelPs 
  study 
  of 
  the 
  intrusive 
  stock 
  at 
  Marysville, 
  

   Montana.f 
  As 
  at 
  Marysville 
  such 
  sharp-angled 
  embayments 
  

  

  * 
  The 
  wonderfully 
  perfect 
  rift 
  in 
  the 
  red 
  rock 
  is 
  clearly 
  a 
  primary 
  struc- 
  

   ture 
  and 
  in 
  no 
  way 
  related 
  to 
  relief 
  of 
  load 
  by 
  erosion 
  or 
  to 
  the 
  present 
  

   topography. 
  This 
  Pigeon 
  Point 
  case 
  strongly 
  suggests 
  that 
  the 
  rift 
  in 
  batho- 
  

   lithic 
  granite 
  is 
  similarly 
  related, 
  in 
  a 
  genetic 
  way, 
  to 
  the 
  forms 
  of 
  the 
  cor- 
  

   responding 
  batholithic 
  roofs. 
  For 
  these 
  larger 
  granitic 
  bodies 
  also 
  the 
  

   explanation 
  of 
  the 
  nearly 
  flat 
  rifting 
  through 
  cooling 
  contraction 
  still 
  seems 
  

   to 
  be 
  the 
  best. 
  

  

  f 
  J. 
  Barrell, 
  Professional 
  Paper 
  57, 
  U. 
  S. 
  Geol. 
  Survey, 
  1907, 
  page 
  72 
  and 
  

   plate 
  IT. 
  

  

  