﻿Yol. 
  53.] 
  ORIGIN 
  OF 
  THE 
  RA.UENTHAL 
  SERPENTINE. 
  253 
  

  

  similar 
  characters. 
  In 
  the 
  enstatite 
  and 
  in 
  some 
  of 
  the 
  hornblende, 
  

   gummy-brown 
  granular 
  aggregates 
  are 
  clustered 
  along 
  the 
  cleavage- 
  

   planes, 
  and 
  perhaps 
  cause 
  a 
  filmy 
  appearance 
  over 
  intervening 
  parts. 
  

   This 
  also 
  is 
  not 
  unlike 
  perofskite, 
  possibly 
  a 
  second 
  form 
  of 
  that 
  

   mineral 
  or 
  a 
  stage 
  in 
  its 
  development, 
  but 
  it 
  is 
  in 
  grains 
  too 
  minute 
  

   to 
  be 
  tested. 
  The 
  serpentinized 
  hornblende 
  in 
  one 
  slide 
  encloses 
  

   small 
  rhomboidal 
  nets, 
  which 
  might 
  be 
  clusters 
  of 
  a 
  similar 
  mineral, 
  

   but 
  have 
  rather 
  the 
  appearance 
  of 
  skeleton 
  crystals. 
  

  

  (7) 
  Rutile* 
  

  

  In 
  one 
  slide 
  of 
  serpentine 
  (from 
  a 
  loose 
  specimen) 
  many 
  of 
  the 
  

   rounded 
  kernels 
  within 
  the 
  network 
  exhibit 
  acicular 
  brownish 
  

   crystals. 
  They 
  are 
  long, 
  but 
  too 
  slender 
  to 
  be 
  examined 
  in 
  detail. 
  

   Their 
  colour 
  and 
  form 
  suggest 
  a 
  dark 
  impure 
  rutile, 
  and 
  geniculate 
  

   twins, 
  although 
  not 
  common, 
  occur. 
  The 
  needles 
  are 
  developed 
  

   usually 
  towards 
  the 
  border 
  of 
  the 
  kernel 
  and 
  point 
  in 
  a 
  more 
  or 
  less 
  

   radial 
  direction. 
  The 
  late 
  Prof. 
  Carvell 
  Lewis, 
  as 
  will 
  be 
  seen 
  from 
  

   his 
  forthcoming 
  work 
  on 
  the 
  Genesis 
  and 
  Matrix 
  of 
  the 
  Diamond, 
  

   identified 
  rutile 
  in 
  serpentinized 
  olivine. 
  

  

  (8) 
  Serpentine. 
  

  

  The 
  mineral 
  serpentine 
  in 
  these 
  slides 
  shows 
  varietal 
  forms. 
  It 
  

   may 
  consist 
  of 
  tubular 
  or 
  fibrous 
  elements, 
  sometimes 
  parallel 
  in 
  

   bundles, 
  sometimes 
  in 
  a 
  matted 
  mass. 
  This 
  serpentine 
  is 
  greenish, 
  

   gives 
  yellowish 
  polarization-colours, 
  and 
  extinguishes 
  parallel 
  to 
  

   the 
  direction 
  of 
  the 
  fibres, 
  which 
  thus 
  are 
  probably 
  chrysotile. 
  

   Secondly, 
  the 
  serpentine 
  may 
  show 
  the 
  lattice- 
  work 
  or 
  the 
  somewhat 
  

   rectangular 
  network 
  surrounding 
  angular 
  kernels 
  as 
  described 
  by 
  

   Weigand 
  and 
  others. 
  1 
  Thirdly, 
  it 
  exhibits 
  kernels 
  of 
  less 
  regular 
  form 
  

   which, 
  although 
  they 
  sometimes 
  have 
  rectilinear 
  boundaries, 
  are 
  

   generally 
  rather 
  more 
  rounded. 
  Several 
  differences 
  between 
  the 
  last 
  

   two 
  cases 
  are 
  often 
  recognizable. 
  These 
  are 
  for 
  the 
  most 
  part 
  best 
  

   seen 
  with 
  high 
  powers 
  in 
  ordinary 
  light 
  by 
  the 
  use 
  of 
  a 
  diaphragm. 
  

   The 
  parts 
  with 
  angular 
  network 
  are 
  paler 
  in 
  colour 
  ; 
  the 
  net 
  gives 
  

   an 
  effect 
  of 
  higher 
  refractive 
  power 
  than 
  the 
  mesh, 
  appearing 
  like 
  a 
  

   green 
  rim 
  bordering 
  a 
  colourless 
  area. 
  With 
  crossed 
  nicols, 
  the 
  

   rim 
  gives 
  low 
  colours 
  and 
  extinguishes 
  nearly 
  or 
  quite 
  parallel, 
  

   while 
  the 
  enclosed 
  parallelogram 
  is 
  faintly 
  or 
  not 
  at 
  all 
  doubly 
  

   refracting. 
  The 
  part 
  with 
  rounded 
  grains 
  is 
  greener, 
  the 
  kernels 
  

   are 
  paler, 
  more 
  highly 
  refracting 
  and 
  prominent 
  than 
  the 
  inter- 
  

   vening 
  bands 
  ; 
  where 
  they 
  have 
  a 
  greener 
  tint 
  we 
  may 
  sometimes 
  be 
  

   looking 
  at 
  a 
  band 
  cut 
  parallel 
  to 
  its 
  plane. 
  In 
  either 
  case 
  the 
  

   nucleus 
  has 
  no 
  effect 
  on 
  polarized 
  light, 
  but 
  the 
  borders 
  give 
  the 
  

   usual 
  low 
  colours 
  of 
  serpentine. 
  

  

  Thus, 
  as 
  was 
  pointed 
  out 
  by 
  Prof. 
  Bonney, 
  there 
  are 
  two 
  type- 
  

  

  1 
  This 
  form 
  of 
  serpentine 
  results 
  from 
  the 
  alteration 
  of 
  hornblende, 
  trans- 
  

   verse 
  sections 
  of 
  that 
  mineral 
  showing 
  a 
  lattice-structure, 
  and 
  longitudinal 
  

   sections 
  exhibiting 
  the 
  prism-cleavages 
  crossed 
  roughly 
  at 
  right 
  angles 
  by 
  less 
  

   regular 
  lines. 
  

  

  