﻿Vol. 
  53.] 
  MICROPEGMATITE 
  IN 
  SOUTHERN 
  ESDIA. 
  413- 
  

  

  acid 
  magma 
  being 
  intruded 
  among 
  rocks 
  already 
  hot. 
  1 
  Such 
  a 
  

   conclusion 
  accounts 
  for 
  the 
  coarseness 
  of 
  grain 
  which 
  has 
  been 
  so 
  

   frequently, 
  almost 
  invariably, 
  observed 
  in 
  the 
  granophyric 
  rocks 
  at 
  

   the 
  junctions 
  with 
  their 
  basic 
  associates. 
  2 
  

  

  But 
  still 
  more 
  interesting 
  are 
  the 
  secondary 
  changes 
  induced 
  in 
  

   the 
  gabbro 
  at 
  its 
  junction 
  with 
  the 
  granophyre. 
  Harker 
  has 
  noticed 
  

   that 
  near 
  the 
  granophyre 
  the 
  augites 
  of* 
  the 
  gabbros 
  (augite-diorite) 
  

   are 
  largely 
  replaced 
  by 
  green 
  hornblende, 
  and 
  biotite 
  sometimes 
  

   occurs. 
  3 
  - 
  I 
  have 
  already 
  referred 
  to 
  precisely 
  similar 
  changes 
  in 
  the 
  

   augites 
  of 
  the 
  Madras 
  dykes 
  where 
  they 
  come 
  into 
  contact 
  with 
  the 
  

   micropegmatite, 
  while 
  the 
  same 
  augite-crystal 
  where 
  it 
  abuts 
  cleanly 
  

   againsta 
  plagioclase 
  is 
  often 
  quite 
  fresh 
  and 
  unchanged 
  (supra, 
  p. 
  408). 
  

   The 
  explanation 
  of 
  these 
  phenomena 
  which 
  agrees 
  most 
  nearly 
  

   with 
  the 
  facts 
  seems 
  to 
  me 
  to 
  be 
  somewhat 
  as 
  follows: 
  — 
  The 
  

   water 
  originally 
  contained 
  in 
  the 
  molten 
  magma 
  would 
  become, 
  

   by 
  the 
  separation 
  of 
  the 
  anhydrous 
  minerals, 
  augite 
  and 
  plagioclase, 
  

   excluded 
  to 
  the 
  final 
  stages 
  of 
  consolidation, 
  before 
  the 
  completion 
  

   of 
  which 
  there 
  must 
  have 
  been 
  a 
  more 
  perfect 
  condition 
  of 
  what 
  

   has 
  been 
  frequently 
  described 
  as 
  aquo-igneous 
  fusion. 
  The 
  separa- 
  

   tion 
  of 
  the 
  silica 
  and 
  alumino-alkaline 
  silicate, 
  as 
  quartz 
  and 
  

   felspar 
  respectively, 
  from 
  this 
  aquo-igneous 
  melt 
  would 
  leave 
  the 
  

   miarolitic 
  channels 
  filled 
  with 
  heated 
  vapours, 
  which 
  would 
  be 
  

   free 
  to 
  attack 
  the 
  susceptible 
  ferromagnesian 
  silicate, 
  and 
  bring 
  

   about 
  the 
  excretion 
  of 
  magnetite 
  and 
  formation 
  of 
  biotite 
  by 
  altera- 
  

   tion 
  of 
  the 
  highly 
  ferriferous, 
  and 
  almost 
  non- 
  aluminous, 
  augite. 
  4 
  

  

  Although 
  more 
  limited 
  in 
  extent, 
  the 
  secondary 
  changes 
  shown 
  by 
  

   the 
  augites 
  where 
  they 
  come 
  into 
  contact 
  with 
  the 
  micropegmatite 
  

   are 
  precisely 
  similar 
  in 
  kind 
  to 
  those 
  described 
  by 
  Prof. 
  Sollas 
  5 
  as 
  

   brought 
  about 
  by 
  the 
  action 
  of 
  the 
  granophyre 
  on 
  the 
  pyroxenic 
  

   xenocrysts 
  obtained 
  from 
  the 
  gabbro 
  of 
  Barnavave 
  — 
  the 
  assumption 
  

   of 
  a 
  green 
  colour, 
  the 
  excretion 
  of 
  magnetite, 
  and 
  the 
  formation 
  of 
  

   biotite 
  and 
  hornblende 
  referred 
  to 
  by 
  Prof. 
  Sollas 
  as 
  results 
  of 
  

   the 
  transformation 
  of 
  diallage-xenocrysts, 
  being 
  phenomena- 
  con- 
  

   stantly 
  observed 
  wherever 
  the 
  augite-crystals 
  come 
  into 
  contact 
  

   with 
  micropegmatite 
  in 
  the 
  Madras 
  dykes. 
  Although 
  these 
  facts 
  

   suggest 
  the 
  likelihood 
  of 
  a 
  similarity 
  of 
  physical 
  conditions 
  

   during 
  the 
  consolidation 
  of 
  the 
  Barnavave 
  granophyre 
  and 
  the 
  

   micropegmatite 
  in 
  the 
  Madras 
  basic 
  dykes, 
  there 
  is 
  no 
  escaping 
  the 
  

   conclusion 
  that 
  in 
  the 
  latter 
  instance 
  the 
  acid 
  material 
  was 
  formed 
  

   after, 
  and 
  in 
  direct 
  succession 
  to, 
  the 
  consolidation 
  of 
  the 
  augite 
  and 
  

   felspar, 
  and 
  was 
  thus 
  formed 
  at 
  probably 
  a 
  lower 
  temperature. 
  

   If 
  this 
  conclusion 
  be 
  the 
  correct 
  one, 
  it 
  follows 
  that 
  the 
  change 
  of 
  

  

  1 
  Quart. 
  Journ 
  Geol. 
  Soc. 
  vol. 
  li. 
  (1895) 
  p. 
  133. 
  

  

  2 
  Ibid. 
  p. 
  148. 
  

  

  3 
  Ibid. 
  pp. 
  133, 
  134, 
  & 
  135. 
  

  

  4 
  That 
  water 
  existed 
  in 
  the 
  original 
  molten 
  material 
  is 
  more 
  than 
  likely, 
  and 
  

   that 
  its 
  presence 
  would 
  result 
  in 
  a 
  suspension 
  of 
  consolidation 
  will, 
  in 
  view 
  of 
  

   the 
  researches 
  which 
  all 
  point 
  to 
  the 
  conclusion 
  first 
  indicated 
  by 
  Scheerer 
  in 
  

   1846, 
  be 
  now 
  generally 
  conceded. 
  

  

  5 
  Trans. 
  Roy. 
  Irish 
  Acad. 
  vql. 
  xxx. 
  (1894) 
  pp. 
  493, 
  494. 
  

  

  