﻿Yol. 
  53.] 
  MICROPEGMATITE 
  IN 
  SOUTHERN 
  INDIA. 
  415 
  

  

  constituting 
  the 
  first 
  phase 
  in 
  the 
  consolidation 
  of 
  the 
  magma, 
  

   while 
  micropegmatite 
  — 
  subsequently 
  formed 
  and 
  representing 
  the 
  

   final 
  stage 
  in 
  the 
  consolidation 
  of 
  the 
  magma 
  — 
  fills 
  in 
  the 
  angles 
  

   between 
  the 
  augite 
  and 
  plagioclase, 
  and 
  so 
  plays 
  the 
  part 
  of 
  ground- 
  

  

  The 
  augite 
  and 
  plagioclase 
  being 
  in 
  excess, 
  their 
  simultaneous 
  

   separation 
  from 
  the 
  magma 
  resulted 
  in 
  the 
  production 
  of 
  a 
  strong 
  

   solid 
  framework, 
  the 
  angles 
  and 
  interstices 
  of 
  which 
  were 
  filled 
  in 
  

   with 
  more 
  acid 
  mother-liquor, 
  which 
  ultimately 
  gave 
  rise 
  to 
  the 
  

   micropegmatite. 
  As 
  the 
  crystallization 
  of 
  the 
  mother-liquor 
  filling 
  

   these 
  intercrystal 
  lacunae 
  and 
  their 
  ramifying 
  connexions 
  would 
  be 
  

   attended 
  with 
  the 
  usual 
  reduction 
  in 
  volume 
  due 
  to 
  crystallization, 
  

   and 
  as 
  the 
  framework 
  of 
  augite 
  and 
  plagioclase 
  previously 
  formed 
  

   would 
  be 
  strong 
  enough 
  to 
  resist 
  any 
  but 
  extreme 
  pressure, 
  the 
  

   micropegmatite 
  would 
  be 
  less 
  compact 
  than 
  the 
  rest 
  of 
  the 
  rock, 
  and 
  

   would 
  be 
  miarolitic 
  on 
  a 
  small 
  scale 
  (micromiarolitic). 
  Hence 
  

   the 
  lacunae 
  and 
  channels, 
  loosely 
  filled 
  with 
  micropegmatite, 
  would 
  

   form 
  an 
  intricate 
  arterial 
  system 
  throughout 
  the 
  rock 
  for, 
  first 
  of 
  all, 
  

   the 
  passage 
  of 
  the 
  liberated 
  hot 
  vapours, 
  and 
  finally 
  for 
  water. 
  As 
  

   a 
  consequence 
  of 
  this 
  fact, 
  we 
  find 
  that 
  the 
  portions 
  of 
  the 
  augites 
  

   abutting 
  directly 
  against 
  the 
  micropegmatite 
  are 
  almost 
  invariably 
  

   attacked 
  in 
  the 
  freshest 
  of 
  the 
  rocks, 
  while 
  in 
  those 
  wherein 
  hydrous 
  

   decomposition 
  has 
  appreciably 
  commenced 
  the 
  micropegmatitic 
  

   patches 
  are 
  always 
  the 
  centres 
  of 
  very 
  marked 
  changes, 
  the 
  felspars 
  

   in 
  their 
  immediate 
  neighbourhood* 
  being 
  kaolinized, 
  the 
  biotites 
  

   converted 
  into 
  chloritic 
  products, 
  and 
  the 
  iron-ores 
  rusted. 
  

  

  In 
  these 
  augite-diorite 
  dykes 
  the 
  coarseness 
  of 
  the 
  micropegmatite 
  

   varies 
  with 
  that 
  of 
  the 
  other 
  constituents 
  ; 
  in 
  the 
  coarser-grained 
  

   varieties 
  the 
  quartz 
  and 
  felspars 
  are 
  easily 
  distinguished 
  between 
  

   crossed 
  nicols, 
  while 
  in 
  the 
  finer-grained 
  varieties, 
  forming 
  the 
  

   margins 
  of 
  large 
  masses 
  or 
  constituting 
  smaller 
  dykes, 
  the 
  inter- 
  

   growth 
  of 
  the 
  constituents 
  of 
  the 
  micropegmatite 
  is 
  as 
  minute 
  as 
  

   that 
  in 
  the 
  structure 
  to 
  which 
  Harker 
  has 
  given 
  the 
  name 
  ' 
  crypto- 
  

   graphic' 
  The 
  felspar 
  of 
  the 
  micropegmatite 
  is 
  sometimes 
  micrc- 
  

   cline, 
  but 
  more 
  often 
  plagioclase 
  : 
  when 
  the 
  latter, 
  it 
  is 
  generally 
  in 
  

   crystallographic 
  continuity 
  with 
  the 
  outer 
  zone 
  of 
  an 
  adjoining 
  and 
  

   unquestionably 
  original 
  plagioclase. 
  For 
  these 
  reasons, 
  principally, 
  

   the 
  micropegmatite 
  is 
  regarded 
  as 
  primary 
  in 
  origin. 
  The 
  rock^ 
  

   being, 
  as 
  a 
  rule, 
  remarkably 
  fresh 
  and 
  unaltered, 
  there 
  is 
  no 
  reason 
  

   for 
  considering 
  the 
  micropegmatite 
  to 
  be 
  secondary 
  in 
  origin. 
  As 
  

   the 
  numerous 
  augite-diorite 
  dykes 
  are 
  never 
  found 
  to 
  be 
  crossed 
  

   by 
  veins 
  of 
  granite 
  or 
  by 
  subsequent 
  intrusions 
  of 
  any 
  sort, 
  the 
  

   explanation 
  applied 
  by 
  Prof. 
  Sollas 
  to 
  the 
  ' 
  granophyric 
  gabbro 
  ' 
  of 
  

   Carlingford, 
  and 
  suggested 
  by 
  the 
  same 
  author 
  for 
  similar 
  cases 
  of 
  

   augite-diorites 
  with 
  micropegmatite 
  in 
  Great 
  Britain, 
  is 
  not 
  applic- 
  

   able 
  to 
  any 
  of 
  the 
  numerous 
  dykes 
  which 
  have 
  been 
  very 
  carefully 
  

   examined 
  in 
  Southern 
  India. 
  

  

  Where 
  the 
  augite 
  and 
  plagioclase 
  far 
  exceed 
  the 
  micropegmatite 
  

   in 
  quantity, 
  and 
  where 
  the 
  pressure 
  is 
  not 
  too 
  great, 
  the 
  formation 
  

   of 
  a 
  strong 
  solid 
  framework 
  by 
  the 
  simultaneous 
  crystallization 
  of 
  

  

  