﻿488 
  DE. 
  C. 
  CALLAWAY 
  ON 
  SECONDARY 
  MINERALS 
  LN 
  THE 
  

  

  from 
  Bischof 
  downwards. 
  In 
  ordinary 
  decomposition, 
  the 
  biotite 
  

   usually 
  passes 
  into 
  chlorite, 
  but 
  such 
  an 
  alteration 
  would 
  be 
  obviously 
  

   impossible 
  in 
  the 
  environment 
  in 
  which 
  the 
  biotite 
  has 
  been 
  produced, 
  

   and 
  any 
  mineral 
  change 
  must 
  be 
  in 
  another 
  direction. 
  There 
  are 
  

   frequent 
  indications 
  in 
  the 
  Malvern 
  Hills 
  of 
  the 
  conversion 
  of 
  biotite 
  

   into 
  white 
  mica. 
  The 
  evidence 
  is 
  of 
  two 
  kinds. 
  In 
  some 
  slides, 
  in 
  

   which 
  seams 
  of 
  chlorite 
  alternate 
  with 
  other 
  minerals, 
  chiefly 
  felspar 
  

   and 
  quartz, 
  and 
  where 
  the 
  shearing 
  increases 
  in 
  intensity 
  towards 
  

   one 
  side, 
  the 
  chlorite 
  is 
  represented 
  in 
  the 
  intermediate 
  zone 
  by 
  

   biotite, 
  and 
  in 
  the 
  most 
  schistose 
  seams 
  by 
  white 
  mica. 
  These 
  slides 
  

   will 
  be 
  more 
  fully 
  noticed 
  in 
  the 
  next 
  section. 
  Again, 
  black 
  mica 
  

   appears 
  to 
  pass 
  into 
  white 
  mica 
  in 
  the 
  same 
  crystalline 
  form. 
  The 
  

   crystals 
  observed 
  were 
  cut 
  normal 
  to 
  the 
  basal 
  plane 
  and 
  were 
  lath- 
  

   shaped. 
  One 
  half 
  or 
  so 
  of 
  the 
  length 
  of 
  the 
  lath 
  was 
  brown, 
  with 
  

   strong 
  dichroisni 
  ; 
  the 
  other 
  transparent, 
  without 
  pleochroism, 
  and 
  

   polariziug 
  in 
  bright 
  colours 
  of 
  the 
  first 
  and 
  second 
  orders. 
  Tet 
  the 
  

   margin 
  of 
  the 
  lath-section 
  was 
  bounded 
  by 
  parallel 
  straight 
  lines, 
  

   the 
  parallel 
  cleavages 
  passed 
  uninterruptedly 
  from 
  end 
  to 
  end 
  of 
  

   the 
  section, 
  and 
  both 
  micas 
  extinguished 
  simultaneously. 
  It 
  would 
  

   appear 
  more 
  probable 
  that 
  we 
  have 
  here 
  a 
  case 
  of 
  alteration 
  than 
  of 
  

   simultaneous 
  crystallization. 
  The 
  change 
  from 
  biotite 
  to 
  white 
  mica 
  

   is 
  associated 
  with 
  the 
  intrusion 
  of 
  granite-veins, 
  as 
  noticed 
  by 
  

   30L 
  Fouque 
  and 
  Levy 
  *. 
  

  

  White 
  Mica 
  from 
  Chlorite. 
  — 
  This 
  transformation 
  was 
  an 
  unex- 
  

   pected 
  one 
  ; 
  but 
  the 
  changes 
  from 
  chlorite 
  to 
  biotite, 
  and 
  from 
  biotite 
  

   to 
  white 
  mica, 
  remove 
  a 
  great 
  part 
  of 
  the 
  antecedent 
  improbability. 
  

   The 
  facts 
  here 
  described 
  were 
  observed 
  at 
  the 
  Ragged 
  Stone 
  Hill. 
  

   In 
  a 
  certain 
  stage 
  of 
  metamorphism 
  we 
  get 
  a 
  schisty 
  rock, 
  mainly 
  

   consisting 
  of 
  chlorite 
  alternating 
  with 
  fragments 
  of 
  broken 
  crystals 
  

   of 
  triclinic 
  felspar. 
  This 
  schist 
  is 
  penetrated 
  by 
  granite-veins. 
  How 
  

   this 
  rock 
  was 
  produced 
  does 
  not 
  concern 
  ns 
  in 
  the 
  present 
  paper. 
  

   Following 
  the 
  field-section 
  in 
  one 
  direction, 
  a 
  progressive 
  change 
  is 
  

   seen 
  ; 
  the 
  rock 
  becomes 
  more 
  and 
  more 
  schistose, 
  and 
  in 
  the 
  advanced 
  

   stage 
  the 
  foliation 
  surfaces 
  glisten 
  with 
  films 
  of 
  mica, 
  and 
  sometimes 
  

   there 
  is 
  great 
  contortion. 
  The 
  felspar 
  fragments 
  in 
  this 
  stage 
  are 
  

   sometimes 
  intact, 
  but 
  they 
  are 
  often 
  replaced 
  by 
  a 
  colourless, 
  trans- 
  

   parent, 
  rather 
  highly 
  refractive 
  mineral, 
  which 
  I 
  have 
  not 
  yet 
  fully 
  

   studied, 
  and 
  by 
  minute 
  water-clear 
  granules 
  with 
  the 
  polarization 
  of 
  

   quartz, 
  and 
  some 
  of 
  them 
  at 
  least 
  are 
  uniaxial. 
  The 
  felspar 
  is 
  thus 
  

   mostly 
  accounted 
  for. 
  The 
  chlorite 
  sometimes 
  passes 
  into 
  black 
  mica, 
  

   as 
  stated 
  in 
  the 
  preceding 
  section 
  ; 
  but 
  where 
  the 
  shearing, 
  as 
  evi- 
  

   denced 
  by 
  increased 
  schistosity 
  and 
  by 
  the 
  distortion 
  of 
  calcite-veins, 
  

   is 
  very 
  great, 
  the 
  change 
  is 
  directly 
  into 
  white 
  mica. 
  In 
  one 
  slide 
  

   (303) 
  the 
  chlorite 
  is 
  largely 
  represented 
  by 
  biotite, 
  which 
  towards 
  

   one 
  side, 
  where 
  the 
  schistosity 
  is 
  greater, 
  passes 
  into 
  bundles 
  of 
  a 
  white 
  

   mica 
  (sericite 
  ?), 
  with 
  included 
  opaque 
  lines 
  (iron-oxide 
  ?) 
  running 
  in 
  

   the 
  same 
  direction 
  as 
  the 
  films. 
  Within 
  a 
  few 
  inches, 
  the 
  rock 
  is 
  

   wholly 
  schistose, 
  with 
  glistening 
  mica-surfaces. 
  Very 
  little 
  biotite 
  is 
  

   present, 
  but 
  there 
  is 
  an 
  abundance 
  of 
  white 
  mica 
  in 
  patches 
  of 
  undu- 
  

   * 
  Mineralogie 
  Iticrographique. 
  p. 
  339. 
  

  

  