﻿Vol. 
  53.] 
  MICROPEGMATITE 
  IN 
  SOUTHERN 
  INDIA.. 
  419 
  

  

  ' 
  As 
  regards 
  the 
  primary 
  origin 
  of 
  micropegmatite 
  in 
  basic 
  rocks, 
  

   I 
  would 
  prefer 
  to 
  wait 
  for 
  further 
  information 
  before 
  committing 
  

   myself. 
  As 
  against 
  it, 
  I 
  note 
  the 
  alteration 
  in 
  adjoining 
  minerals 
  

   which 
  seems 
  to 
  be 
  usually 
  associated 
  with 
  the 
  presence 
  of 
  micro- 
  

   pegmatite. 
  This 
  alteration 
  is 
  very 
  marked 
  in 
  the 
  augite 
  of 
  the 
  

   slide 
  sent 
  to 
  me. 
  The 
  crystallographic 
  continuity 
  of 
  the 
  orthoclase 
  

   with 
  the 
  plagioclase 
  proves 
  nothing, 
  except 
  that 
  these 
  two 
  kinds 
  of 
  

   felspars 
  are 
  sufficiently 
  isomorphous 
  for 
  the 
  growth 
  of 
  the 
  one 
  

   to 
  follow 
  on 
  the 
  lines 
  of 
  the 
  other 
  : 
  as 
  well 
  might 
  one 
  argue 
  

   that 
  the 
  quartz 
  enlargements 
  of 
  quartz-grains 
  in 
  a 
  sandstone-rock 
  

   were 
  primary, 
  because 
  they 
  are 
  in 
  crystallographic 
  continuity. 
  

  

  ' 
  The 
  co-variation 
  in 
  coarseness 
  of 
  grain 
  simply 
  indicates 
  to 
  my 
  

   mind 
  that 
  where 
  druses 
  exist 
  there 
  is 
  room 
  for 
  the 
  growth 
  of 
  large 
  

   crystals, 
  and 
  that 
  the 
  size 
  of 
  the 
  crystals 
  will 
  bear 
  some 
  proportion 
  

   to 
  the 
  size 
  of 
  the 
  druses. 
  We 
  see 
  this 
  in 
  the 
  druses 
  of 
  the 
  Mourne 
  

   Mountains 
  granite, 
  and 
  I 
  have 
  frequently 
  observed 
  in 
  basalts 
  and 
  

   dolerites 
  structures 
  in 
  the 
  immediate 
  neighbourhood 
  of 
  cavities 
  

   which 
  are 
  now 
  filled 
  with 
  large 
  zeolitic 
  crystals. 
  The 
  interstitial 
  

   character 
  of 
  the 
  micropegmatite 
  does 
  not 
  seem 
  to 
  me 
  an 
  argument 
  

   of 
  much 
  force. 
  

  

  * 
  The 
  strongest 
  argument 
  to 
  my 
  mind 
  against 
  the 
  secondary 
  

   origin 
  of 
  the 
  micropegmatite 
  is 
  the 
  existence 
  of 
  druses 
  which 
  it 
  

   postulates; 
  and 
  it 
  is 
  this 
  which 
  leads 
  me 
  to 
  a 
  suspense 
  of 
  judgment. 
  

  

  ' 
  As 
  regards 
  Barnavave, 
  I 
  take 
  up 
  a 
  decided 
  position, 
  at 
  least 
  as 
  

   regards 
  the 
  so-called 
  contemporaneous 
  veins. 
  They 
  are 
  not 
  only 
  

   easy 
  to 
  trace 
  into 
  connexion 
  with 
  the 
  great 
  granitic 
  masses 
  of 
  the 
  

   district, 
  but 
  they 
  include 
  clastic 
  fragments 
  of 
  the 
  gabbro 
  and 
  of 
  its 
  

   constituent 
  minerals. 
  If 
  a 
  rock 
  injected 
  in 
  a 
  fluid 
  state 
  into 
  

   another 
  previously 
  consolidated 
  can 
  be 
  called 
  contemporaneous 
  

   with 
  the 
  rock 
  it 
  penetrates, 
  the 
  Author's 
  argument 
  may 
  possibly 
  

   hold, 
  but 
  I 
  maintain 
  that 
  to 
  apply 
  the 
  word 
  contemporaneous 
  

   in 
  this 
  case 
  is 
  a 
  misuse 
  of 
  terms. 
  

  

  ' 
  As 
  regards 
  the 
  Madras 
  dykes, 
  the 
  absence 
  of 
  exposed 
  acid 
  rocks 
  

   is 
  negative 
  evidence 
  solely; 
  there 
  is 
  probably 
  acid 
  rock 
  in 
  every 
  

   igneous 
  district 
  ; 
  but 
  it 
  does 
  not 
  always 
  come 
  to 
  the 
  surface: 
  I 
  

   quite 
  agree 
  with 
  the 
  Author 
  that 
  his 
  supposed 
  case 
  (6) 
  will 
  not 
  

   hold.' 
  

  

  Prof. 
  Judd 
  added 
  that 
  he 
  felt 
  sure 
  that 
  the 
  Author 
  would 
  be 
  

   greatly 
  gratified 
  to 
  find 
  that 
  his 
  views 
  had 
  met 
  with 
  the 
  support 
  of 
  

   Mr. 
  Teall, 
  Mr. 
  Rutley, 
  and 
  other 
  speakers 
  that 
  evening. 
  In 
  adopting 
  

   a 
  name 
  for 
  the 
  rock, 
  the 
  Author 
  stated 
  in 
  the 
  paper 
  that 
  he 
  followed 
  

   Prof. 
  Cole. 
  It 
  would 
  be 
  seen 
  that 
  from 
  some 
  of 
  the 
  admitted 
  

   facts 
  the 
  Author 
  of 
  the 
  paper 
  and 
  Prof. 
  Sollas 
  drew 
  deductions 
  of 
  

   an 
  exactly 
  opposite 
  character. 
  With 
  respect 
  to 
  the 
  use 
  of 
  the 
  term 
  

   < 
  contemporaneous 
  veins,' 
  Mr. 
  Holland 
  maintained 
  that 
  the 
  old 
  

   geologists 
  who 
  first 
  employed 
  the 
  term 
  were 
  justified 
  in 
  using 
  it 
  in 
  

   cases 
  where, 
  though 
  there 
  must 
  evidently 
  have 
  been 
  a 
  succession 
  in 
  

   time 
  between 
  the 
  formation 
  of 
  two 
  kinds 
  of 
  igneous 
  rock, 
  yet 
  both 
  

   were 
  comprised 
  within 
  the 
  same 
  general 
  period 
  of 
  eruption. 
  

  

  2f 
  2 
  

  

  