﻿440 
  R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  the 
  red-rock 
  material 
  having 
  been 
  chiefly 
  derived 
  from 
  dis- 
  

   solved 
  quartzite 
  aud 
  metargillite. 
  

  

  Red-rock 
  Shells 
  Surrounding 
  Xenoliths. 
  — 
  One 
  set 
  of 
  field 
  

   observations 
  made 
  by 
  Bayley 
  and 
  repeated 
  by 
  the 
  present 
  

   writer 
  at 
  first 
  sight 
  appear 
  to 
  corroborate 
  that 
  theory 
  most 
  

   emphatically. 
  In 
  gabbro, 
  in 
  intermediate 
  rock, 
  and 
  in 
  red 
  

   rock 
  alike, 
  are 
  xenoliths 
  of 
  quartzite 
  which 
  are 
  completely 
  

   enclosed 
  in 
  shells 
  of 
  typical 
  red 
  rock. 
  These 
  small 
  masses 
  of 
  

   red 
  rock 
  thus, 
  in 
  the 
  field, 
  look 
  like 
  direct 
  contact 
  fusions 
  of 
  

   the 
  sediment. 
  The 
  shells 
  vary 
  from 
  one 
  centimeter 
  to 
  50 
  

   centimeters 
  or 
  more 
  in 
  thickness. 
  In 
  general 
  their 
  contacts 
  

   with 
  the 
  enclosed 
  quartzite 
  and 
  with 
  the 
  enclosing 
  igneous 
  

   rock 
  are 
  fairly 
  sharp. 
  The 
  quartzite 
  of 
  the 
  xenolith 
  often 
  

   shows 
  clear 
  bedding 
  and 
  always 
  a 
  texture 
  different 
  from 
  that 
  

   of 
  the 
  surrounding 
  red 
  rock, 
  so 
  that 
  there 
  is 
  seldom 
  much 
  

   doubt 
  as 
  to 
  the 
  limiting 
  surface 
  of 
  the 
  non-magmatic 
  material. 
  

   Just 
  east 
  of 
  the 
  little 
  bay 
  marked 
  " 
  C 
  v 
  in 
  fig. 
  1, 
  a 
  large 
  group 
  

   of 
  these 
  shelled 
  xenoliths 
  is 
  exposed 
  along 
  the 
  shore. 
  In 
  the 
  

   group 
  a 
  series 
  of 
  xenoliths 
  can 
  be 
  found, 
  wherein 
  the 
  relative 
  

   thickness 
  (and 
  volume) 
  of 
  the 
  red-rock 
  shell 
  increases, 
  the 
  vol- 
  

   ume 
  of 
  the 
  respective 
  xenoliths 
  decreasing, 
  until 
  thick 
  shells 
  of 
  

   red 
  rock 
  are 
  seen 
  to 
  enclose 
  mere 
  shreds 
  of 
  quartzite. 
  Finally, 
  

   at 
  the 
  end 
  of 
  the 
  series, 
  no 
  quartzite 
  is 
  seen, 
  the 
  whole, 
  roughly 
  

   globular 
  mass 
  being 
  composed 
  of 
  red 
  rock. 
  All 
  of 
  these 
  bodies 
  

   lie 
  in 
  a 
  general 
  matrix 
  of 
  intermediate 
  rock. 
  The 
  masses 
  of 
  

   pure 
  red 
  rock, 
  one 
  or 
  two 
  decimeters 
  in 
  diameter, 
  have 
  the 
  

   appearance 
  of 
  being 
  "ghosts" 
  of 
  completely 
  fused 
  xenoliths. 
  

   That 
  explanation 
  would 
  seem 
  the 
  more 
  probable 
  since 
  xeno- 
  

   liths 
  of 
  feidspathic 
  gabbro 
  (torn 
  from 
  a 
  dike 
  older 
  than 
  the 
  

   sill) 
  have 
  no 
  such 
  envelopes 
  of 
  red 
  rock 
  ; 
  thus 
  a 
  genetic 
  con- 
  

   nection 
  between 
  sedimentary 
  rock 
  and 
  red 
  rock 
  is 
  the 
  more 
  

   readily 
  credited. 
  

  

  Lawson 
  found 
  quartzite 
  xenoliths 
  in 
  the 
  diabase 
  of 
  the 
  

   Logan 
  sills, 
  north 
  of 
  Pigeon 
  Point, 
  and 
  noted 
  the 
  reddening 
  of 
  

   the 
  diabase 
  in 
  their 
  vicinity.* 
  

  

  The 
  importance 
  of 
  the 
  red-rock 
  shells 
  first 
  came 
  to 
  the 
  

   writer's 
  attention 
  in 
  the 
  course 
  of 
  a 
  personal 
  discussion 
  with 
  

   Professor 
  Grout. 
  He 
  seems 
  to 
  have 
  been 
  the 
  first 
  to 
  see 
  clearly 
  

   that 
  mere 
  fusion 
  of 
  the 
  xenoliths 
  in 
  situ' 
  is 
  not 
  the 
  whole 
  

   explanation 
  of 
  the 
  red-rock 
  shells. 
  He 
  has 
  made 
  a 
  special 
  

   study 
  of 
  the 
  large 
  rimmed 
  xenolith 
  in 
  the 
  intermediate 
  rock 
  

   at 
  the 
  locality 
  mentioned 
  on 
  page 
  110 
  of 
  Bay 
  ley's 
  memoir. 
  

   After 
  a 
  detailed 
  microscopic 
  and 
  chemical 
  investigation 
  of 
  this 
  

   xenolith 
  and 
  its 
  red-rock 
  shell, 
  Professor 
  Grout 
  has 
  concluded 
  

   that 
  the 
  shell 
  is 
  much 
  richer 
  in 
  alkalies, 
  especially 
  potash, 
  than 
  

  

  * 
  A. 
  C. 
  Lawson, 
  Bulletin 
  8, 
  Geological 
  and 
  Natural 
  History 
  Survey 
  of* 
  

   Minnesota, 
  1893, 
  page 
  30. 
  

  

  