﻿416 
  CHARACTER 
  OE 
  THE 
  DIAMANTIEEROTJS 
  ROCK 
  OE 
  SOUTH 
  AFRICA. 
  

  

  When 
  treated 
  with 
  potash, 
  55-345 
  per 
  cent, 
  were 
  dissolved. 
  The 
  

   residue 
  was 
  somewhat 
  richer 
  in 
  silicic 
  acid 
  than 
  was 
  that 
  of 
  the 
  en- 
  

   closed 
  mineral 
  ; 
  and 
  this 
  may 
  have 
  been 
  due 
  to 
  insufficient 
  digestion 
  

   with 
  alkali. 
  The 
  silicate 
  appears 
  to 
  contain 
  more 
  iron 
  oxide 
  in 
  this 
  

   ease, 
  the 
  oxygen 
  ratios 
  of 
  iron 
  protoxide 
  to 
  magnesia 
  being 
  1 
  : 
  2 
  

   here, 
  and 
  1 
  : 
  4 
  in 
  the 
  former 
  instance. 
  

  

  The 
  portion 
  insoluble 
  in 
  potash, 
  amounting 
  to 
  44*655 
  per 
  cent., 
  

   was 
  of 
  a 
  dull 
  olive-oolour 
  and 
  exceedingly 
  hard. 
  Moistened 
  with 
  a 
  

   few 
  drops 
  of 
  cold 
  water, 
  it 
  immediately 
  swelled 
  up 
  to 
  twice 
  or 
  three 
  

   times 
  its 
  bulk 
  and 
  crumbled 
  to 
  powder, 
  the 
  action 
  in 
  every 
  way 
  re- 
  

   sembling 
  that 
  observed 
  in 
  the 
  slaking 
  of 
  lime. 
  Treated 
  with 
  a 
  mix- 
  

   ture 
  of 
  one 
  part 
  of 
  strong 
  hydrochloric 
  acid 
  (spec. 
  grav. 
  = 
  l*16) 
  and 
  

   two 
  parts 
  of 
  water 
  in 
  the 
  cold 
  during 
  about 
  two 
  hours, 
  14*267 
  per 
  

   cent, 
  (or 
  about 
  one 
  third 
  of 
  the 
  material 
  employed) 
  were 
  dissolved. 
  

   A 
  comparison 
  of 
  the 
  percentages 
  of 
  iron 
  protoxide 
  and 
  magnesia 
  

   dissolved 
  by 
  the 
  acid 
  (I.), 
  with 
  those 
  of 
  the 
  same 
  bases 
  in 
  the 
  residue 
  

   from 
  the 
  potash 
  treatment 
  (II.), 
  shows 
  the 
  action 
  of 
  the 
  acid 
  to 
  have 
  

   been 
  that 
  of 
  a 
  solvent. 
  

  

  I. 
  II. 
  

  

  Iron 
  protoxide 
  4*955 
  6*964 
  

  

  Magnesia 
  4*356 
  7*225 
  

  

  If 
  we 
  regard 
  the 
  matrix 
  as 
  made 
  up, 
  in 
  addition 
  to 
  calcite, 
  of 
  a 
  

   hydrated 
  bronzite 
  of 
  the 
  form 
  already 
  described, 
  and 
  an 
  opal 
  having 
  

   the 
  composition 
  

  

  Silicic 
  acid 
  95*766 
  

  

  Water 
  4*234 
  

  

  100*000 
  

  

  and 
  regard 
  the 
  free 
  silicic 
  acid 
  occurring 
  in 
  the 
  striated 
  mineral 
  as 
  

   an 
  opal 
  of 
  the 
  same 
  composition, 
  this 
  striated 
  mineral 
  may 
  be 
  re- 
  

   garded 
  as 
  made 
  up 
  of 
  

  

  Bronzite 
  43*850 
  

  

  Hydrated 
  bronzite 
  24*017 
  

  

  Opaline 
  silica 
  30*895 
  

  

  Alumina 
  0*970 
  

  

  Chromium 
  oxide 
  0*251 
  

  

  99*983 
  

  

  The 
  lithological 
  character 
  of 
  the 
  rock, 
  even 
  down 
  to 
  a 
  depth 
  of 
  

   180 
  feet 
  from 
  the 
  Dempster 
  Claim, 
  is 
  precisely 
  the 
  same 
  as 
  that 
  be- 
  

   fore 
  described. 
  The 
  base 
  of 
  the 
  rock 
  consists 
  of 
  the 
  same 
  ingredients 
  

   as 
  that 
  described 
  above, 
  the 
  mass 
  of 
  it 
  being 
  the 
  hydrated 
  bronzite. 
  

   The 
  rock 
  is 
  further 
  very 
  full 
  of 
  fragments 
  of 
  the 
  shale, 
  which 
  has 
  

   been 
  altered 
  but 
  still 
  contains 
  carbon 
  • 
  indeed 
  the 
  character 
  of 
  the 
  

   rock 
  is 
  almost 
  that 
  of 
  a 
  breccia, 
  in 
  which 
  these 
  masses 
  of 
  shale 
  are 
  

   cemented 
  by 
  hydrated 
  bronzite 
  containing 
  the 
  vaalite 
  and 
  the 
  

   bright 
  green 
  bronzite, 
  with 
  ilmenite 
  and 
  the 
  other 
  minerals 
  associ- 
  

   ated 
  with 
  it. 
  

  

  