﻿IGNEOUS 
  ROCKS 
  AFTER 
  THEIR 
  CONSOLIDATION. 
  181 
  

  

  that, 
  after 
  an 
  interval, 
  growth 
  had 
  been 
  resumed 
  under 
  somewhat 
  

   different 
  conditions. 
  In 
  the 
  felspars 
  I 
  am 
  now 
  describing, 
  however, 
  

   the 
  zone 
  of 
  extinction 
  passes 
  gradually, 
  and 
  ivithout 
  any 
  Jcind 
  of 
  

   break 
  whatever, 
  from 
  the 
  original 
  central 
  core 
  through 
  the 
  secondary 
  

   peripheral 
  fringe. 
  Taking 
  note 
  of 
  the 
  amount 
  of 
  rotation 
  required 
  

   to 
  produce 
  extinction 
  in 
  different 
  zones 
  in 
  the 
  cases 
  of 
  crystals 
  

   exhibiting 
  the 
  largest 
  periphery, 
  I 
  have 
  found 
  it 
  to 
  be 
  such 
  as 
  to 
  

   lead 
  me 
  to 
  conclude 
  that 
  some 
  crystals 
  with 
  a 
  composition 
  between 
  

   labradorite 
  and 
  anorthite 
  have 
  an 
  irregular 
  fringe 
  which, 
  as 
  we 
  pass 
  

   outwards, 
  corresponds 
  to 
  every 
  intermediate 
  stage 
  through 
  the 
  

   Andesineand 
  Oligoclase 
  series, 
  and 
  sometimes 
  approaches, 
  if 
  it 
  does 
  

   not 
  actually 
  reach, 
  the 
  Albite-limit 
  *. 
  

  

  JSowhere, 
  I 
  think, 
  could 
  we 
  have 
  a 
  more 
  convincing 
  illustration 
  

   of 
  the 
  truth 
  of 
  Tschermak's 
  beautiful 
  theory 
  of 
  the 
  felspars. 
  We 
  

   find 
  absolute 
  evidence 
  of 
  the 
  'perfectly 
  gradual 
  change 
  of 
  the 
  optical 
  

   characters 
  of 
  these 
  crystals 
  as 
  we 
  follow 
  them 
  from 
  the 
  original 
  

   central 
  mass 
  through 
  the 
  portions 
  representing 
  consecutive 
  additions 
  

   to 
  it. 
  

  

  Nor 
  is 
  the 
  explanation 
  of 
  the 
  facts 
  observed 
  difficult. 
  When 
  the 
  

   rock, 
  by 
  cooling, 
  assumed 
  the 
  solid 
  state, 
  the 
  stable 
  well-formed 
  por- 
  

   phyritic 
  crystals 
  were 
  separated 
  by 
  portions 
  of 
  vitreous 
  material, 
  

   this 
  vitreous 
  material 
  consisting 
  of 
  unstable 
  mixtures 
  of 
  various 
  

   silicates 
  of 
  lime 
  and 
  the 
  alkalies. 
  

  

  We 
  must 
  never 
  forget 
  that 
  in 
  the 
  deep-seated 
  rocks 
  — 
  and 
  it 
  is 
  in 
  

   such 
  only 
  that 
  these 
  and 
  similar 
  changes 
  seem 
  to 
  occur 
  — 
  the 
  whole 
  

   mass, 
  crystals 
  and 
  base 
  alike, 
  must 
  be 
  permeated 
  by 
  liquids 
  and 
  gases 
  ; 
  

   and 
  chemical 
  reactions 
  (like 
  those 
  involved 
  in 
  the 
  process 
  of 
  

   schillerization) 
  can 
  readily 
  take 
  place. 
  

  

  The 
  tendency 
  of 
  the 
  more 
  basic 
  minerals 
  to 
  separate 
  from 
  a 
  magma 
  

   before 
  the 
  more 
  acid 
  ones, 
  which 
  has 
  been 
  so 
  clearly 
  pointed 
  out 
  by 
  

   Eosenbusch 
  and 
  other 
  authors, 
  leads 
  to 
  the 
  gradual 
  exhaustion 
  of 
  

   the 
  lime 
  ; 
  and 
  as 
  the 
  proportion 
  of 
  this 
  ingredient 
  in 
  the 
  limited 
  

   portion 
  of 
  magma 
  that 
  can 
  be 
  drawn 
  upon 
  is 
  diminished, 
  more 
  and 
  

   more 
  of 
  the 
  alkaline 
  constituent 
  must 
  be 
  taken 
  up 
  by 
  the 
  growing 
  

   felspar, 
  which 
  thus 
  gradually 
  passes 
  from 
  a 
  basic 
  lime-felspar 
  to- 
  

   wards 
  an 
  acid 
  alkali-felspar. 
  At 
  last 
  little 
  else 
  than 
  the 
  silica 
  may 
  

   be 
  left, 
  and, 
  as 
  we 
  shall 
  see 
  hereafter, 
  this 
  tends 
  to 
  crystallize 
  as 
  

   quartz, 
  sometimes 
  simultaneously 
  with, 
  and 
  sometimes 
  after 
  the 
  

   whole 
  of 
  the 
  felspar 
  has 
  crystallized. 
  

  

  Prof. 
  Lagorio, 
  in 
  a 
  very 
  useful 
  and 
  painstaking 
  investigation, 
  has 
  

   endeavoured 
  to 
  apply 
  the 
  important 
  results 
  obtained 
  by 
  the 
  late 
  

   Dr. 
  Guthrie 
  on 
  " 
  Eutectic 
  Compounds 
  " 
  to 
  the 
  explanation 
  of 
  the 
  

  

  * 
  M. 
  Michel-Levy 
  is 
  of 
  opinion 
  that 
  the 
  same 
  phenomenon 
  would 
  result 
  if 
  

   the 
  different 
  zones 
  of 
  the 
  felspars, 
  instead 
  of 
  consisting 
  of 
  homogeneous 
  materials 
  

   of 
  different 
  chemical 
  compositions, 
  were 
  variously 
  built 
  up 
  of 
  " 
  submicroscopi- 
  

   cal" 
  («*. 
  e. 
  ultramicroscopical) 
  twin-lamellse 
  arranged 
  on 
  the 
  pericline 
  and 
  

   albite 
  types. 
  For 
  the 
  purpose 
  of 
  my 
  argument 
  it 
  is 
  not 
  of 
  great 
  importance 
  

   which 
  theoretical 
  view 
  of 
  the 
  molecular 
  structure 
  of 
  the 
  felspars 
  is 
  accepted. 
  

   It 
  is 
  sufficient 
  to 
  have 
  demonstrated 
  that 
  in 
  these 
  cases 
  the 
  angle 
  of 
  extinction 
  

   varies 
  gradually 
  and 
  progressively 
  as 
  the 
  crystal 
  grows 
  at 
  the 
  expense 
  of 
  the 
  

   residual 
  glass. 
  

  

  