﻿Vol. 
  53.] 
  MICROPEGMATITE 
  IN 
  SOUTHERN 
  INDIA. 
  411 
  

  

  and 
  plagioclase 
  would 
  be 
  sufficiently 
  strong 
  to 
  prevent 
  the 
  collapse 
  

   of 
  the 
  rock 
  under 
  any 
  but 
  extreme 
  pressure, 
  the 
  micropegmatitic 
  

   patches 
  would 
  be 
  less 
  compact 
  than 
  the 
  rest 
  of 
  the 
  rock; 
  they 
  

   should 
  in 
  fact 
  be 
  miarolitic 
  on 
  a 
  very 
  small 
  scale 
  — 
  to 
  save 
  words, 
  

   micromiarolitic. 
  That 
  this 
  actually 
  is 
  the 
  case 
  is 
  confirmed 
  

   by 
  a 
  very 
  interesting 
  feature 
  displayed 
  by 
  all 
  the 
  rocks 
  in 
  which 
  

   hydrous 
  decomposition 
  has 
  commenced. 
  In 
  these 
  it 
  is 
  seen 
  that 
  

   the 
  decomposition 
  has 
  invariably 
  developed 
  around 
  the 
  micropegma- 
  

   titic 
  patches, 
  the 
  felspars 
  in 
  the 
  immediate 
  neighbourhood 
  of 
  the 
  

   patches 
  being 
  kaolinized, 
  the 
  secondary 
  biotite 
  and 
  hornblende 
  

   changed 
  to 
  chloritic 
  products, 
  and 
  the 
  iron-ores 
  rusted. 
  The 
  greatest 
  

   change 
  of 
  all 
  has 
  been 
  effected 
  in 
  the 
  felspars 
  which 
  originally 
  

   formed 
  part 
  of 
  the 
  micro-pegmatite 
  ; 
  these 
  have, 
  as 
  a 
  rule, 
  completely 
  

   lost 
  their 
  original 
  characters, 
  and 
  their 
  places 
  are 
  now 
  occupied 
  by 
  

   chloritic 
  products, 
  which 
  have 
  been 
  manufactured 
  from 
  their 
  

   remains 
  with 
  the 
  aid 
  of 
  iron, 
  magnesia, 
  and 
  other 
  compounds 
  

   derived 
  from 
  the 
  adjoining 
  biotite 
  and 
  iron-ores. 
  Not 
  the 
  least 
  

   interesting 
  among 
  the 
  changes 
  brought 
  about 
  by 
  the 
  introduction 
  

   of 
  water 
  is 
  the 
  formation, 
  in 
  the 
  adjoining 
  kaolinized 
  felspars, 
  of 
  

   secondary 
  quartz 
  in 
  crystallographic 
  continuity 
  with 
  that 
  forming 
  

   an 
  original 
  constituent 
  of 
  the 
  micropegmatite, 
  and 
  thus 
  becoming 
  

   a 
  secondary 
  extension 
  of 
  the 
  structure. 
  These 
  unmistakable 
  signs 
  

   of 
  hydrous 
  decomposition 
  limited 
  to 
  the 
  neighbourhood 
  of 
  the 
  micro- 
  

   pegmatite 
  show 
  that 
  the 
  water 
  is 
  distributed 
  through 
  the 
  rock 
  by 
  

   this 
  means. 
  The 
  intercommunicating 
  lacunae 
  once 
  occupied 
  by 
  the 
  

   residual 
  mother-liquor, 
  and 
  subsequently 
  filled 
  loosely 
  by 
  the 
  quartz 
  

   and 
  felspar 
  into 
  which 
  it 
  crystallized, 
  now 
  evidently 
  form 
  a 
  most 
  

   intricate 
  arterial 
  system 
  for 
  water-communication. 
  

  

  Y. 
  Comparison 
  with 
  so-called 
  Granophtric 
  Gabbros. 
  

  

  In 
  his 
  interesting 
  memoir 
  ' 
  On 
  the 
  Relation 
  of 
  the 
  Granite 
  to 
  

   the 
  Gabbro 
  of 
  Barnavave, 
  Carlingford,' 
  Prof. 
  Sollas 
  has 
  brought 
  

   together 
  a 
  great 
  assemblage 
  of 
  facts 
  to 
  show 
  that 
  the 
  micropeg- 
  

   matite 
  in 
  the 
  gabbro 
  had 
  been 
  produced 
  by 
  minute 
  intrusions 
  of 
  

   ' 
  granophyric 
  ' 
  material 
  in 
  a 
  state 
  of 
  great 
  fluidity, 
  subsequent 
  to 
  

   the 
  consolidation 
  and 
  even 
  jointing 
  of 
  the 
  older 
  gabbro. 
  In 
  com- 
  

   paring 
  the 
  Carlingford 
  case 
  with 
  the 
  similar 
  instances 
  of 
  micro- 
  

   pegmatite 
  occurring 
  in 
  basic 
  rocks 
  in 
  Great 
  Britain, 
  Prof. 
  Sollas 
  

   pointed 
  out 
  the 
  association 
  of 
  granophyric 
  massive 
  rocks 
  with 
  the 
  

   well-known 
  enstatite-diorite 
  of 
  Penmaenmawr 
  and 
  with 
  the 
  gabbro 
  

   (augite-diorite) 
  of 
  Carrock 
  Fell. 
  For 
  want 
  of 
  evidence, 
  however, 
  

   concerning 
  the 
  presence 
  of 
  granophyric 
  intrusions 
  in 
  connexion 
  with 
  

   the 
  Whin 
  Sill, 
  Prof. 
  Sollas 
  was 
  unable 
  to 
  extend 
  his 
  explanation 
  to 
  

   that 
  well-known 
  instance 
  ; 
  at 
  the 
  same 
  time 
  he 
  remarked 
  that 
  

   ' 
  till 
  minute 
  veins 
  or 
  dykes 
  of 
  granophyre 
  have 
  been 
  specially 
  

   searched 
  for 
  in 
  this 
  dyke 
  without 
  success, 
  it 
  will 
  be 
  impossible, 
  in 
  

   the 
  light 
  of 
  our 
  recent 
  observations, 
  not 
  to 
  feel 
  some 
  suspicion 
  as 
  

   to 
  the 
  real 
  nature 
  of 
  the 
  granophyric 
  inclusions.' 
  1 
  

  

  1 
  Trans. 
  Roy. 
  Irish 
  Acad. 
  vol. 
  xxx. 
  (1894) 
  p. 
  477. 
  

  

  