﻿408 
  ME. 
  T. 
  H. 
  HOLLAND 
  OBT 
  ATTGITE-DIOBITES 
  WITH 
  [Aug. 
  1897, 
  

  

  crystallographic 
  continuity 
  with 
  an 
  adjacent 
  normal, 
  and 
  unques- 
  

   tionably 
  original, 
  plagioclase 
  can 
  generally 
  be 
  demonstrated 
  between 
  

   crossed 
  nicols. 
  The 
  quartz 
  and 
  felspar 
  of 
  the 
  micropegmatite 
  both 
  

   contain 
  numerous 
  colourless, 
  or 
  very 
  pale-green, 
  acicular 
  crystals, 
  

   which, 
  being 
  thinner 
  than 
  the 
  sections 
  of 
  doubly-refracting 
  minerals 
  

   in 
  which 
  they 
  lie, 
  cannot 
  be 
  determined 
  with 
  certainty. 
  On 
  account 
  

   of 
  the 
  almost 
  constant 
  presence 
  of 
  these 
  acicular 
  crystals 
  and 
  on 
  

   account 
  of 
  the 
  constantly 
  ' 
  water-clear/ 
  limpid 
  appearance 
  of 
  the 
  

   quartz 
  and 
  felspar 
  in 
  which 
  they 
  lie, 
  the 
  patches 
  of 
  micropegmatite 
  

   can 
  easily 
  be 
  detected 
  in 
  ordinary 
  light. 
  

  

  As 
  noticed 
  by 
  Teall 
  in 
  connexion 
  with 
  the 
  Whin 
  Sill, 
  1 
  the 
  

   coarseness 
  of 
  the 
  micropegmatitic 
  patches 
  varies 
  with 
  that 
  of 
  the 
  

   remaining 
  constituents 
  of 
  the 
  rock, 
  and 
  in 
  the 
  finer-grained 
  

   varieties 
  the 
  separation 
  of 
  the 
  constituents 
  becomes 
  less 
  and 
  less 
  

   pronounced, 
  until 
  the 
  quartz- 
  and 
  felspar-individuals, 
  becoming 
  

   narrower 
  than 
  the 
  thickness 
  of 
  the 
  section, 
  are 
  indistinguishable 
  

   one 
  from 
  another 
  in 
  polarized 
  light, 
  thus 
  passing 
  into 
  the 
  structure 
  

   to 
  which 
  Harker 
  has 
  given 
  the 
  name 
  cryptographic. 
  2 
  This 
  

   interesting 
  character 
  strongly 
  supports 
  the 
  conclusion 
  that 
  the 
  

   micropegmatite 
  is 
  primary 
  in 
  origin. 
  

  

  The 
  minerals 
  which 
  are 
  wholly, 
  or 
  in 
  part, 
  secondary 
  in 
  origin 
  

   are 
  opaque 
  iron-ores, 
  hornblende, 
  and 
  biotite. 
  In 
  the 
  larger 
  number 
  

   of 
  dykes 
  these 
  minerals 
  are 
  comparatively 
  small 
  in 
  quantity, 
  and 
  are 
  

   almost 
  always 
  situated 
  on 
  the 
  margins 
  of 
  the 
  augite 
  which 
  abut 
  

   directly 
  against 
  the 
  micropegmatite, 
  while 
  the 
  faces 
  of 
  the 
  same 
  

   mineral 
  abutting 
  directly 
  against 
  the 
  ordinary 
  plagioclase-crystals 
  

   are 
  more 
  often 
  quite 
  free 
  of 
  any 
  such 
  signs 
  of 
  secondary 
  change. 
  

   The 
  secondary 
  changes 
  in 
  the 
  augites 
  are 
  thus 
  facilitated 
  by 
  proxi- 
  

   mity 
  to 
  the 
  micropegmatite, 
  and 
  in 
  the 
  few 
  dykes 
  in 
  which 
  hydrous 
  

   decomposition 
  has 
  appreciably 
  advanced 
  these 
  peculiar 
  circumstances 
  

   are 
  still 
  more 
  marked. 
  In 
  such 
  cases 
  the 
  felspars 
  of 
  the 
  micropeg- 
  

   matite 
  itself 
  have 
  been 
  completely 
  decomposed 
  and 
  converted 
  into 
  

   an 
  aggregate 
  of 
  minute 
  chlorite-tiakes, 
  sometimes 
  clear 
  green 
  and 
  

   sometimes 
  coloured 
  by 
  rusty 
  stains. 
  The 
  felspars 
  around 
  the 
  micro- 
  

   pegmatitic 
  patches 
  are 
  at 
  the 
  same 
  time 
  kaolinized 
  to 
  varying 
  

   extents, 
  accompanied 
  by 
  the 
  deposition 
  of 
  secondary 
  quartz 
  in 
  crys- 
  

   tallographic 
  continuity 
  with 
  that 
  forming 
  part 
  of 
  the 
  micropegmatite 
  ; 
  

   while 
  the 
  augites 
  are, 
  on 
  the 
  faces 
  exposed 
  to 
  the 
  micropegmatite, 
  

   corroded 
  by 
  the 
  formation 
  of 
  hornblende, 
  biotite, 
  and 
  iron-ores 
  (see 
  

   PI. 
  XXIX. 
  fig. 
  1). 
  The 
  significance 
  of 
  these 
  phenomena 
  is 
  indicated 
  

   farther 
  on 
  (p. 
  413). 
  

  

  III. 
  Chemical 
  Composition 
  of 
  the 
  Bocks. 
  

   For 
  the 
  purposes 
  of 
  chemical 
  analysis, 
  I 
  have 
  selected 
  the 
  dyke 
  

   exposed 
  near 
  the 
  rock-hewn 
  Seven 
  Pagodas, 
  in 
  the 
  Chingelput 
  

   district, 
  as 
  an 
  example 
  of 
  the 
  well-crystallized 
  types 
  comparatively 
  

   free 
  from 
  enstatite; 
  and 
  the 
  selvage 
  of 
  a 
  dyke 
  near 
  Perumbakam, 
  in 
  

   the 
  South 
  Arcot 
  district, 
  as 
  the 
  most 
  typical 
  of 
  the 
  fine-grained 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xl. 
  (1884) 
  r 
  . 
  644. 
  

  

  2 
  * 
  Petrology 
  for 
  Students' 
  (1895;, 
  p. 
  92. 
  

  

  