﻿P. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  441 
  

  

  the 
  quartzite 
  of 
  the 
  xenolith. 
  The 
  writer 
  has 
  confirmed 
  his 
  

   view 
  by 
  microscopic 
  study 
  of 
  the 
  same 
  and 
  of 
  other 
  rimmed 
  

   inclusions 
  in 
  the 
  intermediate 
  rock. 
  As 
  shown 
  by 
  the 
  thin 
  

   sections, 
  each 
  shell 
  contrasts 
  with 
  its 
  xenolith 
  in 
  carrying 
  at 
  

   least 
  twice 
  as 
  much 
  potash 
  and 
  also 
  more 
  soda. 
  Moreover, 
  the 
  

   shells 
  examined 
  are 
  nearly 
  uniform 
  in 
  composition 
  and 
  therein 
  

   are 
  equivalent 
  to 
  the 
  main 
  mass 
  of 
  typical 
  red 
  rock 
  of 
  the 
  sill 
  ; 
  

   while 
  the 
  xenoliths 
  vary 
  considerably 
  in 
  their 
  original 
  content 
  

   of 
  alkalies. 
  As 
  Professor 
  Grout 
  has 
  pointed 
  out, 
  the 
  differ- 
  

   ences 
  of 
  alkaline 
  content 
  between 
  the 
  shells 
  and 
  xenoliths 
  are 
  

   of 
  the 
  same 
  order 
  as 
  those 
  between 
  the 
  alkali 
  content 
  in 
  the 
  

   average 
  analysis 
  of 
  the 
  main 
  red 
  rock 
  and 
  the 
  alkali 
  content 
  of 
  

   the 
  average 
  Animikie 
  sediment 
  of 
  the 
  region. 
  (See 
  Table 
  I.) 
  

  

  (1) 
  Assuming 
  the 
  red-rock 
  shell 
  to 
  have 
  resulted 
  from 
  the 
  

   fusion 
  of 
  the 
  xenolith, 
  without 
  addition 
  of 
  material 
  from 
  the 
  

   general 
  magma, 
  a 
  large 
  amount 
  of 
  silica, 
  iron, 
  and 
  water 
  must 
  

   have 
  been 
  expelled 
  from 
  the 
  secondary 
  melt. 
  It 
  is 
  conceivable 
  

   that 
  one 
  or 
  more 
  compounds 
  of 
  iron 
  and 
  silica, 
  aided 
  by 
  the 
  

   water, 
  might 
  rapidly 
  diffuse 
  into 
  the 
  general 
  magma, 
  leaving 
  

   behind 
  the 
  less 
  volatile 
  feldspar 
  and 
  some 
  free 
  silica. 
  By 
  such 
  

   differential 
  diffusion 
  feldspar 
  would 
  be 
  more 
  concentrated 
  in 
  

   the 
  shell 
  than 
  in 
  the 
  original 
  quartzite. 
  As 
  yet 
  no 
  compelling 
  

   evidence 
  either 
  for 
  or 
  against 
  this 
  hypothesis 
  has 
  been 
  found 
  ; 
  

   however, 
  one 
  may 
  well 
  doubt 
  the 
  quantitative 
  value 
  of 
  the 
  

   imagined 
  process. 
  

  

  (2) 
  Is 
  the 
  red-rock 
  shell 
  rich 
  in 
  alkalies 
  because 
  these 
  have 
  

   been, 
  as 
  it 
  were, 
  sweated-out 
  from 
  the 
  hot 
  interior 
  of 
  the 
  

   xenolith 
  ? 
  The 
  connate 
  water 
  of 
  the 
  inclusion 
  must 
  have 
  at- 
  

   tained 
  high 
  gas-tension, 
  which 
  might 
  be 
  conceived 
  to 
  have 
  led 
  

   to 
  the 
  expulsion 
  of 
  a 
  water-feldspar-quartz 
  solution, 
  analogous 
  

   to 
  an 
  ordinary, 
  low-temperature 
  pegmatitic 
  magma. 
  This 
  

   second 
  guess 
  as 
  to 
  the 
  cause 
  of 
  the 
  concentration 
  of 
  feldspar 
  

   in 
  the 
  shell 
  at 
  once 
  meets 
  the 
  difficulty 
  that 
  certain 
  shells 
  have 
  

   volumes 
  greater 
  than 
  their 
  respective 
  enclosures. 
  

  

  (3) 
  On 
  the 
  whole, 
  it 
  seems 
  more 
  probable 
  that, 
  while 
  some 
  

   fusion 
  of 
  each 
  xenolith 
  has 
  taken 
  place, 
  some 
  of 
  the 
  soda 
  and 
  

   potash, 
  and 
  perhaps 
  alumina, 
  concentrated 
  in 
  the 
  red-rock 
  

   shells 
  have 
  been 
  derived 
  from 
  the 
  general 
  magma. 
  How 
  this 
  

   was 
  accomplished 
  is 
  a 
  residual 
  question, 
  as 
  yet 
  without 
  adequate 
  

   answer. 
  At 
  least 
  half 
  of 
  the 
  soda 
  and 
  potash 
  in 
  the 
  feldspar 
  

   of 
  the 
  average 
  shell 
  must, 
  apparently, 
  have 
  been 
  imported, 
  the 
  

   remainder 
  being 
  the 
  alkalies 
  of 
  the 
  xenolith 
  fused 
  in 
  situ. 
  If 
  

   the 
  oxides 
  of 
  sodium, 
  potassium, 
  and 
  aluminum, 
  in 
  the 
  presence 
  

   of 
  water, 
  were 
  highly 
  diffusible 
  in 
  the 
  roof 
  phase 
  of 
  the 
  general 
  

   magma, 
  those 
  oxides 
  might 
  conceivably 
  have 
  been 
  fixed, 
  both 
  

   in 
  chemical 
  combination 
  and 
  in 
  position, 
  by 
  the 
  free 
  silica 
  of 
  

   the 
  partly 
  melted 
  xenolith. 
  The 
  mystery 
  as 
  to 
  the 
  phases 
  

  

  