﻿OF 
  THE 
  DIAMANTIFEROTTS 
  ROCK 
  OF 
  SOUTH 
  AFRICA. 
  

  

  409 
  

  

  II. 
  Specimens 
  from 
  Du 
  Toit's 
  Pan. 
  

  

  The 
  drab-coloured 
  mineral, 
  or 
  hydrated 
  bronzite, 
  again 
  forms 
  the 
  

   ground-mass 
  of 
  the 
  rock. 
  Through 
  it 
  is 
  disseminated 
  a 
  considerable 
  

   amount 
  of 
  a 
  vermiculite 
  mineral, 
  which 
  often 
  takes 
  upon 
  the 
  surface- 
  

   layers 
  a 
  fine 
  bluish 
  green-colour 
  like 
  that 
  of 
  elinochlore, 
  the 
  whole 
  

   mass 
  in 
  some 
  cases 
  having 
  this 
  colour. 
  Besides 
  these 
  there 
  occur 
  

   small 
  imperfect 
  crystals 
  of 
  the 
  bright 
  green 
  bronzite, 
  garnet, 
  and 
  

   ilmenite, 
  while 
  fragments 
  of 
  brownish-white 
  zircon 
  and 
  crystals 
  of 
  

   a 
  hornblendic 
  mineral, 
  with 
  much 
  the 
  aspect 
  of 
  smaragdite, 
  are 
  

   authentically 
  stated 
  to 
  come 
  from 
  this 
  locality, 
  with 
  grossular 
  

   garnet 
  and 
  brilliant 
  little 
  black 
  tourmalines. 
  One 
  diamond 
  crystal 
  

   in 
  the 
  British 
  Museum 
  is 
  in 
  situ 
  in 
  a 
  mass 
  of 
  this 
  rock, 
  singularly 
  

   gneiss-like 
  in 
  appearance, 
  the 
  mica-like 
  mineral 
  really 
  consisting 
  

   of 
  the 
  mineral 
  resembling 
  vermiculite, 
  which 
  we 
  now 
  proceed 
  to 
  

   describe. 
  

  

  1. 
  Vaalite. 
  

  

  The 
  mineral 
  resembling 
  vermiculite 
  occurs 
  in 
  hexagonal 
  prisms, 
  

   the 
  angles 
  of 
  which 
  are 
  nearly 
  60° 
  and 
  120°. 
  Several 
  measurements 
  

   gave 
  a 
  mean 
  augle 
  of 
  59° 
  50'. 
  The 
  crystals 
  are 
  ciinorhombic, 
  the 
  

  

  Fig. 
  I. 
  — 
  Projection 
  of 
  Vaalite 
  on 
  Plane 
  of 
  Symmetry. 
  

  

  angle 
  (y) 
  of 
  the 
  oblique 
  axes 
  being 
  very 
  near 
  103° 
  30'. 
  The 
  faces 
  

   measured 
  were 
  those 
  of 
  the 
  forms 
  c 
  (001), 
  a 
  (100), 
  m 
  (110), 
  b 
  (010). 
  

  

  100, 
  

  

  001 
  

  

  = 
  

  

  76 
  30 
  

  

  

  

  110, 
  

  

  110 
  

  

  = 
  

  

  59 
  50 
  

  

  

  

  110, 
  

  

  001 
  

  

  = 
  

  

  83 
  20, 
  

  

  calc. 
  

  

  83 
  

  

  001, 
  

  

  010 
  

  

  = 
  

  

  90 
  

  

  

  

  19' 
  

  

  The 
  plane 
  c 
  offers 
  a 
  very 
  facile 
  cleavage, 
  far 
  less 
  ready 
  cleavages 
  

   following 
  the 
  directions 
  of 
  the 
  faces 
  m. 
  

  

  The 
  normal 
  of 
  the 
  face 
  001 
  is 
  actually, 
  or 
  very 
  nearly, 
  coincident 
  

   with 
  the 
  mean 
  line 
  of 
  the 
  optic 
  axes, 
  which 
  are 
  inclined 
  at 
  so 
  very 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  120. 
  2 
  g 
  

  

  