﻿"Vol. 
  53.] 
  MICROPEGMATITE 
  IN 
  SOUTHERN 
  INDIA. 
  40 
  

  

  The 
  micropegmatite, 
  however, 
  is 
  distinctly 
  later 
  in 
  origin, 
  being 
  

   never 
  older 
  than 
  the 
  'water-clear' 
  outer 
  margins 
  of 
  the 
  felspar- 
  

   crystals. 
  But 
  although 
  distinctly 
  the 
  last-formed 
  original 
  con- 
  

   stituent, 
  there 
  is 
  no 
  reason 
  for 
  regarding 
  it 
  as 
  other 
  than 
  the 
  

   result 
  of 
  one 
  continuous 
  process, 
  and 
  the 
  final 
  stage 
  in 
  the 
  consolida- 
  

   tion 
  of 
  the 
  magma 
  which 
  gave 
  rise 
  to 
  the 
  rock. 
  1 
  

  

  The 
  order 
  of 
  the 
  crystallization 
  of 
  the 
  constituents 
  is, 
  then, 
  the 
  

   following: 
  — 
  

  

  (1) 
  Augite 
  and 
  plagioclase 
  together 
  forming 
  the 
  main 
  mass 
  

  

  of 
  the 
  rock, 
  and 
  

  

  (2) 
  Micropegmatite, 
  playing 
  the 
  part 
  of 
  groundmass. 
  

  

  The 
  augite 
  is 
  pale 
  brown 
  in 
  colour, 
  sometimes 
  distinctly 
  pleo- 
  

   chroic, 
  frequently 
  twinned 
  according 
  to 
  the 
  common 
  law, 
  and, 
  so 
  

   far 
  as 
  its 
  original 
  microscopic 
  characters 
  are 
  concerned, 
  calls 
  for 
  

   no 
  further 
  remark. 
  An 
  analysis 
  of 
  material 
  carefully 
  separated 
  

   from 
  the 
  dyke 
  at 
  the 
  Seven 
  Pagodas, 
  Chingelput 
  district, 
  whose 
  

   bulk-analysis 
  is 
  given 
  below, 
  shows 
  the 
  close 
  resemblance 
  between 
  

   the 
  monoclinic 
  pyroxene 
  of 
  this 
  rock 
  and 
  that 
  of 
  the 
  petrol 
  ogic 
  ally 
  

   similar 
  Whin 
  Sill 
  analysed 
  by 
  Mr. 
  Teall. 
  

  

  I. 
  II. 
  

  

  SiO., 
  5002 
  4903 
  

  

  Al.,63 
  5-61 
  5*46 
  

  

  Fe"^ 
  15-61 
  15-57 
  

  

  MnO 
  trace 
  022 
  

  

  CaO 
  1484 
  15-34 
  

  

  MgO 
  1201 
  11-66 
  

  

  Is 
  T 
  a 
  2 
  0&K 
  2 
  096 
  124 
  

  

  Loss 
  on 
  ignition 
  0'76 
  0*81 
  

  

  99-81 
  99-33 
  

  

  I. 
  Monoclinic 
  pyroxene 
  in 
  augite-diorite 
  dyke, 
  Seven 
  Pagodas, 
  Uhmgelput 
  

  

  district, 
  Madras 
  Presidency. 
  

   II. 
  Monoclinic 
  pyroxene 
  from 
  the 
  Whin 
  Sill, 
  Teall, 
  Quart. 
  Journ. 
  Greol. 
  

   Soc. 
  vol. 
  xl. 
  (1884) 
  p. 
  648. 
  

  

  The 
  central 
  portions 
  of 
  the 
  plagioclase-crystals 
  give 
  the 
  extinc- 
  

   tion-angles 
  of 
  varieties 
  approaching 
  labradorite 
  in 
  composition. 
  But 
  

   as 
  the 
  margins 
  of 
  the 
  crystals 
  are 
  approached 
  the 
  well-known 
  zoning 
  

   by 
  change 
  of 
  chemical 
  composition 
  is 
  well 
  displayed. 
  The 
  central 
  

   portions 
  of 
  the 
  crystals 
  are 
  pale-brown 
  in 
  colour 
  through 
  innumerable 
  

   original 
  inclusions, 
  a 
  feature 
  which 
  is 
  also 
  well 
  shown 
  by 
  the 
  

   phenocrysts 
  of 
  the 
  hemicrystalline 
  varieties. 
  But 
  in 
  the 
  holo- 
  

   crystalline, 
  coarser-grained 
  tj'pes, 
  the 
  plagioclase-crystals 
  become 
  

   paler 
  in 
  colour 
  near 
  the 
  margins 
  and 
  finally 
  ' 
  water-clear 
  ' 
  in 
  their 
  

   outermost 
  zones. 
  

  

  The 
  micropegmatite 
  generally 
  fills 
  in 
  the 
  angles 
  between 
  the 
  

   felspars 
  and 
  pyroxenes, 
  and 
  is 
  composed 
  of 
  the 
  usual 
  intergrowth 
  of 
  

   quartz 
  with 
  felspar, 
  which 
  is 
  sometimes 
  microcline, 
  but 
  generally 
  

   plagioclase. 
  When 
  the 
  intergrown 
  felspar 
  is 
  plagioclase 
  its 
  

  

  1 
  The 
  evidence, 
  upon 
  which 
  the 
  conclusion 
  as 
  to 
  the 
  primary 
  origin 
  of 
  the 
  

   micropegmatite 
  is 
  based, 
  is 
  detailed 
  below 
  and 
  on 
  p. 
  409. 
  

  

  