﻿448 
  P. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  have 
  been 
  concentrated 
  in 
  the 
  red-rock 
  envelopes 
  surrounding 
  

   xenoliths 
  and 
  in 
  the 
  red-rock 
  layer, 
  thin 
  or 
  thick, 
  between 
  the 
  

   main 
  gabbro 
  and 
  the 
  sill 
  roof. 
  Until 
  that 
  mechanism 
  is 
  under- 
  

   stood, 
  a 
  final 
  decision 
  concerning 
  the 
  origin 
  of 
  the 
  red 
  rock 
  

   must 
  be 
  delayed.* 
  Nevertheless, 
  the 
  facts 
  now 
  in 
  hand 
  seem 
  

   to 
  show 
  the 
  origin 
  of 
  the 
  red 
  rock 
  to 
  lie 
  in 
  both 
  assimilation 
  

   and 
  differentiation, 
  rather 
  than 
  in 
  the 
  differentiation 
  of 
  a 
  wholly 
  

   primary 
  magma. 
  In 
  any 
  case, 
  be 
  the 
  fate 
  of 
  past 
  and 
  present 
  

   speculation 
  what 
  it 
  may, 
  Pigeon 
  Point 
  will 
  long 
  engage 
  the 
  

   profound 
  interest 
  of 
  penologists, 
  for 
  its 
  mysteries 
  are 
  the 
  mys- 
  

   teries 
  of 
  igneous 
  rocks 
  in 
  general 
  and 
  at 
  but 
  few 
  other 
  locali- 
  

   ties 
  are 
  the 
  rocks 
  representing 
  the 
  two 
  commonest 
  magmas 
  — 
  

   the 
  basaltic 
  and 
  the 
  granitic 
  — 
  better 
  or 
  more 
  suggestively 
  

   exposed. 
  

  

  Harvard 
  University, 
  

   Cambridge, 
  Mass. 
  

  

  * 
  While 
  fully 
  believing 
  in 
  fractional 
  crystallization 
  as 
  a 
  cause 
  for 
  the 
  

   diversity 
  of 
  igneous 
  rocks 
  (see 
  Journal 
  of 
  Geology, 
  vol. 
  xvi, 
  1908, 
  pp. 
  401- 
  

   420), 
  the 
  writer 
  finds 
  much 
  difficulty 
  in 
  applying 
  that 
  principle 
  as 
  the 
  sole 
  

   explanation 
  of 
  the 
  Pigeon 
  Point 
  differentiation. 
  The 
  matter 
  is 
  not 
  here 
  dis- 
  

   cussed. 
  

  

  