﻿Vol. 
  53.] 
  MICROPEGMATITE 
  IN 
  SOUTHERN 
  INDIA. 
  417 
  

  

  EXPLANATION 
  OF 
  PLATE 
  XXIX. 
  

  

  Fig. 
  1. 
  Augite-diorite 
  with 
  rnicropegmatite. 
  Large 
  dyke 
  crossing 
  the 
  gneiss. 
  

   Seven 
  Pagodas, 
  Cbingelput 
  district. 
  X^y. 
  The 
  augite 
  and 
  plagio- 
  

   clase 
  make 
  up 
  the 
  main 
  mass 
  of 
  the 
  minerals 
  included 
  within 
  the 
  

   field. 
  Where 
  the 
  augite 
  abuts 
  directly 
  against 
  the 
  water-clear 
  

   rnicropegmatite, 
  as 
  in 
  the 
  centre 
  and 
  lower 
  parts 
  of 
  the 
  field, 
  green 
  

   hornblende, 
  brown 
  biotite, 
  and 
  opaque 
  magnetite 
  are 
  formed, 
  while 
  

   the 
  edges 
  of 
  the 
  augite 
  meeting 
  the 
  plagioclase 
  are 
  generally 
  quite 
  

   unaltered. 
  

  

  Fig. 
  2. 
  A 
  finer- 
  grained 
  variety 
  of 
  the 
  same 
  rock 
  occurring 
  as 
  a 
  narrow 
  dyke 
  

   near 
  Nemeli, 
  South 
  Arcot 
  district. 
  The 
  rnicropegmatite, 
  as 
  well 
  as 
  

   the 
  augite 
  and 
  plagioclase, 
  is 
  finer 
  in 
  grain 
  and 
  sometimes 
  almost 
  

   cryptographic. 
  This 
  rock 
  shows 
  the 
  intergrowths 
  of 
  augite 
  and 
  

   plagioclase 
  to 
  indicate 
  an 
  average 
  .simultaneous 
  crystallization 
  of 
  the 
  

   two 
  minerals. 
  X^. 
  

  

  Fig. 
  3. 
  Augite-andesite. 
  the 
  hemicrystalline 
  selvage 
  of 
  an 
  augite-diorite 
  dyke 
  

   near 
  Perumbakam, 
  South 
  Arcot 
  district. 
  The 
  glomero-porphyritic 
  

   groups 
  of 
  augite 
  and 
  plagioclase 
  show 
  these 
  two 
  minerals 
  crystallizing 
  

   at 
  about 
  the 
  same 
  time, 
  being 
  intimately 
  intergrown 
  although 
  they 
  are 
  

   in 
  a 
  free 
  matrix. 
  The 
  presence 
  of 
  vitreous 
  material 
  in 
  this 
  matrix 
  

   could 
  not 
  be 
  demonstrated. 
  X^. 
  

  

  Fig. 
  4. 
  Augite-diorite 
  with 
  rnicropegmatite. 
  A 
  dyke 
  in 
  biotite-gneiss. 
  Jauli- 
  

   kerai, 
  Hosurtaluk, 
  Salem 
  district. 
  The 
  hydrous 
  decomposition 
  which 
  

   has 
  commenced 
  in 
  this 
  rock 
  has 
  manifested 
  itself 
  most 
  markedly 
  in, 
  

   and 
  immediately 
  around, 
  the 
  rnicropegmatite, 
  the 
  felspars 
  of 
  which 
  

   have 
  been 
  converted 
  into 
  an 
  aggregation 
  of 
  minute 
  chloritic 
  flakes, 
  

   while 
  the 
  plagioclase 
  in 
  the 
  immediate 
  neighbourhood 
  has 
  been 
  

   kaolinized, 
  and 
  secondary 
  quartz 
  has 
  been 
  deposited 
  in 
  crystallographic 
  

   continuity 
  with 
  that 
  forming 
  part 
  of 
  the 
  rnicropegmatite. 
  This 
  rock 
  

   is 
  an 
  example 
  also 
  of 
  the 
  types 
  in 
  which 
  en 
  statite 
  forms 
  a 
  considerable 
  

   portion 
  of 
  the 
  pyroxenic 
  constituent, 
  and 
  which 
  form 
  a 
  link 
  with 
  

   certain 
  norites 
  described 
  in 
  a 
  paper 
  in 
  the 
  Eecords 
  of 
  the 
  Geological 
  

   Survey 
  of 
  India 
  (vol. 
  xxx). 
  X^r. 
  

  

  Fig. 
  5. 
  Portion 
  of 
  rock 
  shown 
  in 
  fig. 
  4, 
  seen 
  in 
  polarized 
  light, 
  so 
  as 
  to 
  show 
  

  

  the 
  interstitial 
  rnicropegmatite. 
  X^r* 
  

   Fig. 
  6. 
  Portion 
  of 
  rock 
  shown 
  in 
  fig. 
  1; 
  seen 
  in 
  polarized 
  light. 
  X-^-. 
  

  

  Discussion. 
  

  

  Gen. 
  McMahon 
  said 
  that 
  he 
  had 
  listened 
  to 
  the 
  paper 
  with 
  great 
  

   interest, 
  and 
  looked 
  forward 
  to 
  studying 
  it 
  in 
  print. 
  He 
  thought 
  

   that 
  the 
  micropegmatitic 
  structure 
  arose 
  in 
  different 
  ways 
  in 
  different 
  

   rocks, 
  and 
  that 
  one 
  explanation 
  was 
  not 
  true 
  for 
  all 
  cases. 
  A 
  

   difficulty 
  in 
  holding 
  that 
  the 
  structure 
  was 
  formed 
  during 
  the 
  

   primary 
  consolidation 
  of 
  the 
  rock 
  arose 
  from 
  the 
  fact 
  mentioned 
  by 
  

   the 
  Author 
  that 
  the 
  micropegmatitic 
  portion 
  of 
  his 
  rock 
  altered 
  the 
  

   pyroxenic 
  minerals 
  in 
  contact 
  with 
  it. 
  The 
  Author 
  spoke 
  of 
  heat 
  

   being 
  generated 
  in 
  igneous 
  masses 
  by 
  mechanical 
  motion. 
  Might 
  

   not 
  this 
  superinduced 
  heat 
  account 
  for 
  the 
  formation 
  of 
  a 
  micro- 
  

   pegmatitic 
  structure 
  ? 
  His 
  observations 
  had 
  led 
  him 
  to 
  believe 
  that 
  

   the 
  initial 
  stage 
  of 
  remelting 
  resulted, 
  in 
  some 
  cases, 
  in 
  the 
  

   separation 
  of 
  free 
  silica 
  from 
  felspars 
  in 
  the 
  way 
  seen 
  in 
  micro- 
  

   pegmatite. 
  

  

  Q.J. 
  G. 
  S. 
  No. 
  211. 
  2k 
  

  

  