﻿TOURMALINE, 
  &c, 
  WITHIN 
  MICA 
  AND 
  OTHER 
  MINERALS. 
  551 
  

  

  in 
  a 
  very 
  irregular 
  manner 
  from 
  a 
  nucleus, 
  sometimes 
  having 
  the 
  form 
  of 
  a 
  thin 
  

   film 
  ; 
  sometimes 
  of 
  oriental 
  characters 
  ; 
  and 
  sometimes 
  it 
  is 
  disseminated 
  in 
  grains 
  

   so 
  extremely 
  minute, 
  that 
  the 
  flame 
  of 
  a 
  candle 
  seen 
  through 
  it 
  is 
  surrounded 
  with 
  

   a 
  halo 
  of 
  five 
  or 
  six 
  perfectly-formed 
  coloured 
  rings. 
  

  

  3. 
  Distribution 
  of 
  Quartz 
  in 
  Mica. 
  

  

  In 
  mica 
  from 
  various 
  localities, 
  I 
  have 
  found 
  large 
  crystallisations 
  of 
  quartz, 
  the 
  

   quartz 
  replacing 
  the 
  mica. 
  I 
  have 
  never 
  even 
  once 
  met 
  with 
  a 
  regular 
  crystal 
  of 
  

   quartz 
  ; 
  and 
  what 
  is 
  curious, 
  all 
  the 
  crystalline 
  masses 
  of 
  it 
  which 
  I 
  have 
  exa- 
  

   mined 
  have 
  their 
  axis 
  of 
  double 
  refraction 
  in 
  the 
  plane 
  of 
  the 
  laminae 
  of 
  mica. 
  

   In 
  some 
  very 
  large 
  specimens 
  of 
  Bengal 
  mica 
  given 
  to 
  me 
  by 
  Mr 
  Swinton, 
  I 
  have 
  

   found 
  layers 
  of 
  quartz, 
  several 
  inches 
  in 
  area, 
  and 
  about 
  the 
  200th 
  of 
  an 
  inch 
  

   thick. 
  The 
  two 
  surfaces 
  of 
  the 
  plates 
  are 
  exceedingly 
  inequal 
  and 
  corrugated, 
  

   owing 
  to 
  the 
  circumstances 
  under 
  which 
  they 
  were 
  formed, 
  but 
  they 
  possessed 
  

   regular 
  double 
  refraction, 
  and 
  gave 
  the 
  colours 
  of 
  polarised 
  light. 
  

  

  4. 
  Distribution 
  of 
  Titanium 
  in 
  Amethyst. 
  

  

  While 
  examining, 
  many 
  years 
  ago, 
  along 
  with 
  the 
  late 
  Marquis 
  of 
  North- 
  

   ampton, 
  several 
  bags 
  of 
  amethyst 
  which 
  had 
  been 
  imported 
  into 
  Scotland 
  from 
  

   the 
  Brazils, 
  we 
  were 
  surprised 
  to 
  observe 
  a 
  number 
  of 
  fine 
  pyramidal 
  crystals, 
  

   which 
  seemed 
  to 
  have 
  a 
  powdery 
  matter 
  distributed 
  through 
  their 
  mass. 
  Upon 
  

   more 
  narrowly 
  examining 
  these 
  crystals, 
  I 
  found 
  that 
  this 
  dust 
  formed 
  an 
  inner 
  

   pyramid, 
  all 
  the 
  faces 
  of 
  which 
  were 
  parallel 
  to 
  the 
  faces 
  of 
  the 
  pyramid 
  of 
  ame- 
  

   thyst. 
  When 
  two 
  parallel 
  faces 
  were 
  ground 
  upon 
  the 
  pyramid, 
  and 
  perpendi- 
  

   cular 
  to 
  its 
  axis, 
  the 
  particles 
  of 
  dust 
  were 
  seen 
  by 
  the 
  microscope 
  to 
  consist 
  each 
  

   of 
  several 
  spicular 
  crystals 
  of 
  titanium, 
  crossing 
  one 
  another 
  at 
  angles 
  of 
  60° 
  and 
  

   30°, 
  and 
  forming 
  distinct 
  groups. 
  In 
  one 
  crystal 
  there 
  were 
  two 
  interior 
  pyra- 
  

   mids 
  composed 
  of 
  these 
  groups 
  ; 
  and 
  it 
  will 
  be 
  seen, 
  from 
  the 
  explanation 
  which 
  

   I 
  shall 
  presently 
  give 
  of 
  this 
  phenomenon, 
  that 
  there 
  may 
  be 
  any 
  number 
  of 
  such 
  

   pyramids. 
  

  

  As 
  the 
  crystals 
  of 
  amethyst 
  are 
  supposed 
  to 
  have 
  been 
  produced 
  by 
  the 
  gradual 
  

   enlargement 
  of 
  a 
  small 
  crystal 
  placed 
  in 
  an 
  amethystine 
  solution, 
  we 
  have 
  only 
  to 
  

   assume 
  that 
  a 
  solution 
  containing 
  titanium 
  has 
  been 
  introduced 
  into 
  the 
  ame- 
  

   thystine 
  solution 
  at 
  different 
  times 
  during 
  the 
  growth 
  of 
  the 
  crystal. 
  The 
  small 
  

   crystals 
  of 
  titanium 
  will 
  deposit 
  themselves 
  on 
  each 
  of 
  the 
  surfaces 
  of 
  the 
  pyra- 
  

   mid 
  ; 
  and 
  when 
  the 
  whole 
  of 
  the 
  introduced 
  titanium 
  has 
  been 
  thus 
  deposited, 
  

   the 
  enlargement 
  of 
  the 
  amethyst 
  will 
  go 
  on, 
  leaving 
  a 
  pyramid 
  of 
  titanium 
  crys- 
  

   tals 
  in 
  its 
  interior. 
  If 
  a 
  second 
  solution 
  of 
  titanium 
  is 
  introduced, 
  a 
  second 
  pyra- 
  

   mid 
  of 
  its 
  particles 
  will 
  be 
  formed 
  in 
  the 
  same 
  manner 
  ; 
  and 
  this 
  process 
  may 
  

   be 
  repeated 
  any 
  number 
  of 
  times. 
  

  

  If 
  we 
  now 
  suppose 
  that 
  the 
  amethystine 
  solution 
  is 
  exhausted, 
  just 
  when 
  the 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  7 
  K 
  

  

  