﻿Vol. 
  53.] 
  ON 
  ROCKS 
  FROM 
  THE 
  BALUCHISTAN 
  BORDER. 
  297 
  

  

  to 
  the 
  smallest 
  microlith. 
  In 
  short, 
  there 
  is 
  a 
  gradation 
  in 
  the 
  size 
  

   of 
  the 
  felspars 
  from 
  the 
  largest 
  down 
  to 
  the 
  most 
  minute 
  : 
  this 
  

   characteristic 
  is 
  found 
  not 
  only 
  in 
  the 
  felspars, 
  but 
  in 
  all 
  the 
  original 
  

   minerals. 
  

  

  Zonal 
  structure 
  is 
  well 
  seen 
  in 
  all 
  the 
  large 
  phenocrysts 
  ; 
  and 
  

   that 
  this 
  is 
  due 
  to 
  gradual 
  growth 
  is 
  apparent 
  from 
  the 
  fact 
  that 
  

   the 
  angles 
  of 
  extinction 
  of 
  the 
  different 
  zones 
  vary, 
  and 
  indicate 
  a 
  

   gradual 
  change 
  in 
  the 
  basicity 
  of 
  the 
  mineral. 
  

  

  The 
  larger 
  felspars 
  are 
  sometimes 
  oligoclase 
  and 
  sometimes 
  

   andesine, 
  though 
  the 
  former 
  would 
  seem 
  to 
  predominate. 
  Their 
  

   species 
  were 
  determined 
  partly 
  by 
  the 
  evidence 
  afforded 
  by 
  the 
  

   angles 
  of 
  extinction 
  measured 
  in 
  suitable 
  cases, 
  and 
  partly 
  by 
  the 
  

   determination 
  of 
  the 
  specific 
  gravity 
  of 
  isolated 
  fragments. 
  

  

  There 
  can 
  be 
  no 
  question 
  about 
  the 
  species 
  of 
  a 
  considerable 
  

   number 
  of 
  the 
  crystals, 
  because 
  the 
  simultaneous 
  extinction 
  of 
  

   twins 
  is 
  not 
  uncommon 
  ; 
  and 
  that 
  those 
  which 
  exhibit 
  binary 
  

   twinning, 
  combined 
  with 
  simultaneous 
  and 
  straight 
  extinction, 
  are 
  

   not 
  orthoclase, 
  is 
  shown 
  by 
  the 
  fact 
  that, 
  whenever 
  they 
  occur 
  

   in 
  contact 
  with 
  the 
  Canada 
  balsam 
  of 
  the 
  slide, 
  their 
  refraction 
  

   is 
  invariably 
  found 
  to 
  be 
  higher 
  than 
  that 
  of 
  the 
  balsam. 
  This 
  

   method 
  of 
  distinguishing 
  between 
  orthoclase 
  and 
  oligoclase, 
  recom- 
  

   mended 
  by 
  M. 
  Michel-Levy, 
  1 
  seems 
  to 
  be 
  a 
  valuable 
  and 
  reliable 
  

   test. 
  

  

  The 
  large 
  felspars 
  are, 
  generally 
  speaking, 
  fairly 
  fresh, 
  but 
  some 
  

   are 
  considerably 
  altered. 
  No. 
  1425 
  contains 
  some 
  calcite, 
  as 
  a 
  

   secondary 
  product 
  of 
  decomposition, 
  in 
  the 
  interior 
  of 
  the 
  felspars 
  ; 
  

   and 
  those 
  of 
  No. 
  1438 
  contain 
  dusty-looking 
  matter, 
  arranged 
  either 
  

   in 
  a 
  central 
  core 
  or 
  as 
  a 
  zonal 
  band 
  inside 
  a 
  rim 
  of 
  water-clear 
  

   felspar. 
  This 
  dusty 
  matter 
  may 
  be 
  microscopic 
  granules 
  of 
  limonite, 
  

   but 
  it 
  is 
  not 
  magnetite. 
  Fresh 
  felspars, 
  notably 
  those 
  of 
  No. 
  1424, 
  

   contain 
  very 
  characteristic 
  glass- 
  and 
  stone-inclusions 
  with 
  fixed 
  

   gas- 
  or 
  air-bubbles, 
  and 
  inclusions 
  of 
  the 
  base. 
  Others 
  possess 
  

   liquid 
  inclusions, 
  a 
  few 
  of 
  which 
  contain 
  extremely 
  minute 
  moving 
  

   bubbles. 
  

  

  Magnetite 
  and 
  apatite 
  appear 
  to 
  be 
  original 
  minerals 
  common 
  to 
  

   all 
  these 
  rocks. 
  Magnetite 
  is 
  present 
  in 
  all 
  the 
  slices 
  and 
  varies 
  in 
  

   size 
  from 
  large 
  grains 
  to 
  minute 
  dots 
  ; 
  sometimes 
  it 
  has 
  been 
  largely 
  

   converted 
  into 
  ferric 
  oxide. 
  It 
  occurs 
  as 
  a 
  secondary 
  as 
  well 
  as 
  an 
  

   original 
  mineral. 
  

  

  Apatite 
  is 
  present 
  in 
  seven 
  out 
  of 
  the 
  nine 
  slides, 
  and 
  it 
  is 
  some- 
  

   times 
  very 
  abundant. 
  The 
  contact-action 
  of 
  the 
  acid 
  base 
  has 
  in 
  

   some 
  few 
  cases 
  corroded 
  the 
  crystals, 
  and 
  in 
  some 
  others 
  has 
  pro- 
  

   duced 
  a 
  dark 
  ' 
  resorption 
  '-ring. 
  The 
  crystals 
  that 
  exhibit 
  this 
  

   unusual 
  peculiarity 
  are 
  probably 
  mangan-apatites. 
  The 
  hand-speci- 
  

   mens 
  treated 
  with 
  nitric 
  acid 
  reacted 
  strongly 
  for 
  phosphoric 
  acid. 
  

   Apatite 
  appears 
  to 
  have 
  been 
  one 
  of 
  the 
  first 
  minerals 
  to 
  crystallize 
  

   out 
  of 
  the 
  magma. 
  

  

  The 
  three 
  specimens 
  which 
  stand 
  at 
  the 
  top 
  of 
  my 
  list 
  (p. 
  295) 
  

  

  1 
  ' 
  Etude 
  sur 
  la 
  Determination 
  des 
  Feldspaths,' 
  1894, 
  p. 
  62. 
  

  

  