﻿490 
  DK. 
  C. 
  CALLAWAY 
  ON 
  SECONDARY 
  MINERALS 
  IN 
  THE 
  

  

  slide 
  has 
  a 
  structure 
  resembling 
  modified 
  kersantite 
  ; 
  but 
  both 
  biotite 
  

   and 
  felspar 
  have 
  nearly 
  disappeared, 
  and 
  the 
  rock 
  is 
  mainly 
  composed 
  

   of 
  granular 
  quartz, 
  with 
  scattered 
  zoisite, 
  replacing 
  felspar, 
  and 
  

   garnet, 
  possibly 
  representing 
  the 
  mica. 
  The 
  mineral 
  which 
  I 
  take 
  

   to 
  be 
  zoisite 
  is 
  in 
  irregular 
  translucent 
  granules, 
  and 
  its 
  refraction 
  

   being 
  high, 
  it 
  resembles 
  a 
  garnet 
  in 
  plain 
  light 
  ; 
  but 
  under 
  crossed 
  

   nicols 
  it 
  polarizes 
  rather 
  faintly 
  in 
  shades 
  of 
  indigo, 
  passing 
  through 
  

   grey 
  and 
  white 
  to 
  yellow. 
  

  

  No. 
  417, 
  from 
  the 
  Wych, 
  is 
  crushed 
  granite 
  with 
  infiltrated 
  

   chlorite. 
  There 
  are 
  numerous 
  garnets, 
  some 
  of 
  which 
  are 
  traversed 
  

   by 
  irregular 
  fissures 
  filled 
  with 
  chlorite. 
  Most 
  of 
  them 
  are 
  immersed 
  

   in 
  chlorite 
  ; 
  but 
  a 
  small 
  proportion 
  of 
  them 
  are 
  enclosed 
  in 
  felspar. 
  

  

  In 
  the 
  above 
  slides, 
  garnets, 
  as 
  well 
  as 
  the 
  zoisite, 
  appear 
  to 
  be 
  

   the 
  result 
  of 
  secondary 
  action 
  ; 
  for 
  in 
  the 
  earlier 
  slides 
  they 
  are 
  seen 
  

   only 
  in 
  the 
  most 
  highly 
  altered 
  diorite, 
  and 
  in 
  the 
  last 
  they 
  would 
  

   appear 
  either 
  to 
  have 
  been 
  introduced 
  with 
  the 
  infiltrated 
  chlorite 
  or 
  

   subsequently 
  formed 
  out 
  of 
  it. 
  There 
  has 
  evidently 
  been 
  some 
  recon- 
  

   struction 
  of 
  the 
  felspar 
  of 
  the 
  granite 
  after 
  the 
  garnets 
  were 
  formed. 
  

  

  Ilmenite 
  and 
  Sphene. 
  — 
  Mr. 
  Teall 
  called 
  my 
  attention 
  to 
  the 
  

   association 
  of 
  sphene 
  with 
  some 
  of 
  the 
  iron-ore 
  of 
  Malvern. 
  I 
  

   followed 
  out 
  this 
  hint 
  through 
  50 
  or 
  60 
  slides, 
  with 
  some 
  interesting 
  

   results. 
  The 
  ore 
  (ilmenite) 
  sometimes 
  shows 
  rhombohedral 
  forms, 
  

   simple 
  or 
  twinned, 
  but 
  is 
  more 
  frequently 
  in 
  irregular 
  granules. 
  

   It 
  is 
  very 
  often 
  incrusted 
  with 
  a 
  whitish, 
  finely 
  granular 
  substance 
  

   (leucoxene) 
  ; 
  but 
  as 
  the 
  rock 
  (diorite) 
  undergoes 
  progressive 
  alter- 
  

   ation, 
  the 
  leucoxene 
  becomes 
  more 
  coarsely 
  crystalline, 
  minute 
  

   sphenoid 
  forms 
  appearing 
  in 
  the 
  granular 
  crust 
  and 
  increasing 
  in 
  

   size, 
  till 
  we 
  are 
  able 
  to 
  distinguish 
  the 
  high 
  refraction 
  and 
  double 
  

   refraction 
  of 
  sphene. 
  These 
  observations 
  thus 
  confirm 
  the 
  opinion 
  

   of 
  MM. 
  Fouque 
  and 
  Levy 
  * 
  that 
  the 
  incrusting 
  leucoxene 
  is 
  a 
  form 
  

   of 
  sphene. 
  As 
  the 
  sphene 
  increases 
  in 
  quantity 
  in 
  the 
  altered 
  

   diorite, 
  the 
  ilmenite 
  diminishes, 
  until 
  in 
  some 
  cases 
  it 
  is 
  represented 
  

   by 
  a 
  cluster 
  of 
  granules 
  or 
  crystals 
  of 
  sphene, 
  with 
  a 
  little 
  opacite. 
  

   It 
  is 
  not 
  asserted 
  that 
  all 
  the 
  free 
  sphene 
  is 
  formed 
  from 
  ilmenite. 
  

   Very 
  commonly, 
  slender 
  sphene-crystals, 
  clearly 
  derived 
  from 
  asso- 
  

   ciated 
  iron-ore 
  (335), 
  lie 
  in 
  the 
  cleavage-planes 
  of 
  chlorite. 
  In 
  

   an 
  advanced 
  stage 
  of 
  the 
  metamorphism 
  of 
  diorite, 
  when 
  the 
  rock 
  

   has 
  become 
  micaceous 
  (333), 
  the 
  cleavages 
  of 
  the 
  mica, 
  and 
  the 
  

   planes 
  of 
  junction 
  between 
  separate 
  crystals 
  of 
  mica, 
  are 
  often 
  occu- 
  

   pied 
  by 
  dark 
  lines, 
  which, 
  on 
  examination 
  with 
  a 
  high 
  power, 
  are 
  

   seen 
  to 
  be 
  acicular 
  or 
  vermicular 
  microliths 
  of 
  a 
  colourless 
  aniso- 
  

   tropic 
  mineral 
  with 
  black 
  margins, 
  as 
  if 
  highly 
  refracting, 
  and 
  some- 
  

   times 
  crossed 
  by 
  dark 
  lines, 
  like 
  jointing. 
  It 
  can 
  hardly 
  be 
  doubted 
  

   that 
  this 
  mineral 
  is 
  sphene. 
  

  

  The 
  history 
  of 
  this 
  sphene 
  tends 
  to 
  confirm 
  the 
  evidence 
  for 
  the 
  

   secondary 
  origin 
  of 
  the 
  black 
  mica. 
  Much 
  of 
  the 
  sphene 
  is 
  certainly 
  

   formed 
  from 
  ilmenite 
  t, 
  and 
  the 
  position 
  it 
  occupies 
  in 
  the 
  cleavage- 
  

  

  * 
  Mineralogie 
  Micrographique, 
  1879, 
  p. 
  42(>. 
  

  

  t 
  It. 
  is 
  not 
  affirmed 
  that 
  the 
  ore 
  associated 
  with 
  sphene 
  may 
  not 
  sometimes 
  

   be 
  titaniferous 
  magnetite. 
  

  

  