﻿OF 
  THE 
  DIAHANTIFEROTIS 
  ROCK 
  OF 
  SOUTH 
  AFRICA. 
  413 
  

  

  Among 
  these 
  a 
  small 
  specimen 
  offered 
  special 
  advantages 
  for 
  the 
  

   investigation. 
  Its 
  first 
  aspect 
  is 
  that 
  of 
  a 
  piece 
  of 
  serpentine 
  of 
  dark 
  

   greenish 
  hue 
  pervaded 
  by 
  porphyritic-looking 
  spots, 
  many 
  of 
  which 
  

   possess 
  crystalline 
  outline, 
  and, 
  at 
  first 
  sight, 
  somewhat 
  resemble 
  

   the 
  crystals 
  of 
  olivine 
  in 
  an 
  augitic 
  lava 
  rich 
  in 
  that 
  mineral. 
  The 
  

   minerals 
  that 
  make 
  up 
  this 
  rock 
  are, 
  first, 
  the 
  dark 
  greenish 
  base, 
  

   secondly 
  the 
  once 
  crystalline 
  greenish-grey 
  substance 
  scattered 
  

   through 
  it, 
  crystals 
  of 
  the 
  mineral 
  we 
  have 
  called 
  vaalite, 
  rarely 
  

   garnet, 
  and 
  interspersed 
  calcite, 
  which 
  pervades 
  the 
  whole 
  mass. 
  

  

  A 
  thin 
  section 
  of 
  this 
  rock, 
  seen 
  in 
  the 
  microscope, 
  exhibits, 
  be- 
  

   sides 
  the 
  very 
  metamorphosed 
  crystals 
  before 
  alluded 
  to, 
  a 
  ground- 
  

   mass 
  containing 
  crystals 
  of 
  a 
  similar 
  kind, 
  only 
  much 
  smaller, 
  an 
  ir- 
  

   regular 
  granular 
  mixture 
  of 
  what 
  appears 
  to 
  be 
  the 
  same 
  mineral, 
  

   mingled 
  throughout 
  with 
  nearly 
  opaque 
  minute 
  irregularly 
  formed 
  

   particles 
  that 
  sometimes 
  seem 
  to 
  present 
  crystalline 
  outline. 
  The 
  

   last-mentioned 
  microscopic 
  grains 
  are 
  of 
  a 
  deep 
  olive-yellow 
  to 
  

   olive-green 
  ; 
  and 
  of 
  one 
  the 
  crystalline 
  outline 
  gave 
  normal 
  angles 
  

   of 
  57° 
  to 
  59° 
  30' 
  for 
  the 
  measurements 
  of 
  two 
  prism-planes 
  upon 
  a 
  

   third 
  plane 
  that 
  appeared 
  to 
  be 
  a 
  pinacoid, 
  which 
  might 
  be 
  taken 
  to 
  

   represent 
  the 
  inclination 
  of 
  the 
  plane 
  101 
  on 
  the 
  cleavage-plane 
  100 
  

   of 
  olivine. 
  It 
  would 
  be 
  the 
  angle 
  g'm 
  of 
  Descloizeaux, 
  of 
  which 
  the 
  

   value 
  of 
  the 
  normal 
  angle 
  would 
  be 
  59° 
  37'. 
  The 
  mineral 
  is 
  too 
  

   much 
  metamorphosed 
  to 
  present 
  any 
  optical 
  characters, 
  so 
  that 
  it 
  is 
  

   impossible 
  to 
  confirm 
  this 
  conclusion 
  by 
  their 
  aid. 
  If, 
  however, 
  

   this 
  olive-green 
  ingredient 
  be 
  olivine, 
  it 
  is 
  present 
  only 
  in 
  absolutely 
  

   microscopic 
  particles, 
  the 
  bulk 
  of 
  the 
  rock 
  being 
  the 
  mineral 
  which 
  

   fills 
  the 
  place 
  once 
  occupied 
  by 
  crystallized 
  bronzite, 
  while 
  throughout 
  

   the 
  whole 
  the 
  interstices 
  are 
  filled 
  with 
  infiltrated 
  calcite. 
  

  

  The 
  measurement 
  of 
  the 
  angles 
  afforded 
  by 
  the 
  outlines 
  of 
  what 
  

   were 
  once 
  bronzite 
  crystals 
  offers 
  the 
  usual 
  difficulties 
  where 
  the 
  

   observations 
  are 
  not 
  controlled 
  by 
  optical 
  characters 
  ; 
  a 
  distinct 
  

   prism-angle 
  of 
  87° 
  to 
  87° 
  30' 
  in 
  one 
  very 
  symmetrical 
  section 
  would 
  

   correspond 
  fairly 
  well 
  with 
  the 
  angle 
  of 
  the 
  Breitenbach 
  bronzite, 
  

   viz. 
  110, 
  110 
  = 
  88° 
  16'. 
  

  

  4. 
  Altered 
  Bronzite. 
  

  

  The 
  metamorphosed 
  bronzite-like 
  mineral 
  was 
  carefully 
  picked 
  

   and 
  examined. 
  It 
  contained 
  no 
  carbonic 
  acid, 
  and 
  when 
  boiled 
  with 
  

   potash 
  gave 
  up 
  a 
  very 
  small 
  amount 
  of 
  silicic 
  acid. 
  The 
  composition 
  

   of 
  the 
  mineral 
  is 
  as 
  follows 
  : 
  — 
  

  

  Oxygen 
  Ratios. 
  

  

  Silicic 
  acid 
  53-462 
  28-513 
  4 
  

  

  Alumina 
  0-948 
  

  

  Iron 
  protoxide 
  .... 
  8-768 
  1-947 
  1 
  

  

  Magnesia 
  25-925 
  10-369 
  I 
  13-039 
  2 
  

  

  Lime 
  2-534 
  0-723 
  J 
  

  

  Water 
  8-363 
  7-434 
  1 
  

  

  100-000 
  

  

  