﻿OP 
  THE 
  DIAMANTIFEROTTS 
  ROCK 
  OF 
  SOUTH 
  AFRICA. 
  407 
  

  

  these 
  rock-masses 
  ; 
  and 
  calcite 
  has 
  penetrated 
  them 
  everywhere, 
  in 
  

   some 
  cases 
  even 
  surrounding 
  and, 
  as 
  it 
  were, 
  cementing 
  the 
  diamonds 
  

   in 
  their 
  rocky 
  setting. 
  

  

  The 
  analyses 
  of 
  the 
  several 
  minerals 
  composing 
  the 
  rocks 
  are 
  

   given 
  below, 
  with 
  the 
  localities 
  from 
  which 
  they 
  were 
  taken. 
  They 
  

   will 
  be 
  seen 
  to 
  exhibit 
  this 
  undoubtedly 
  once 
  igneous 
  rock 
  in 
  the 
  

   light 
  of 
  a 
  bronzite 
  rock 
  converted 
  (except 
  where 
  the 
  remains 
  of 
  

   crystals 
  have 
  still 
  survived 
  the 
  process 
  of 
  metamorphism) 
  into 
  a 
  

   hydrated 
  magnesium 
  silicate, 
  which 
  has 
  the 
  chemical 
  character 
  of 
  a 
  

   hydrated 
  bronzite. 
  It 
  is 
  possible 
  that 
  the 
  steatite-like 
  magma 
  in 
  

   which 
  the 
  other 
  minerals 
  and 
  shale 
  fragments 
  are 
  contained 
  may 
  

   have, 
  in 
  fact, 
  originated 
  in 
  an 
  augitic 
  mineral 
  once 
  more 
  plentiful, 
  

   but 
  of 
  which 
  part 
  of 
  the 
  calcium 
  and 
  the 
  silica 
  have 
  been 
  separated 
  

   as 
  opaline 
  silica 
  and 
  calcite. 
  

  

  It 
  is, 
  however, 
  not 
  very 
  probable 
  that 
  a 
  separation 
  of 
  ingredients 
  

   in 
  this 
  way 
  from 
  a 
  decomposed 
  augite 
  would 
  leave 
  a 
  hydrated 
  

   bronzite 
  with 
  so 
  definite 
  a 
  character 
  as 
  is 
  the 
  base 
  of 
  this 
  rock. 
  

   It 
  is 
  far 
  more 
  probable 
  that 
  the 
  calcite 
  is 
  an 
  infiltered 
  ingredient, 
  

   and 
  that 
  the 
  silica 
  has 
  been 
  imported 
  by 
  the 
  agency 
  of 
  water, 
  

   probably 
  thermal, 
  and 
  possibly 
  associated 
  with 
  the 
  secondary 
  results 
  

   of 
  the 
  volcanic 
  outburst 
  to 
  which 
  the 
  columnar 
  pipes 
  filled 
  with 
  these 
  

   rocks, 
  and 
  of 
  which 
  they 
  were 
  first 
  shown 
  by 
  Dr. 
  Cohen 
  to 
  have 
  

   been 
  probably 
  the 
  volcanic 
  throats, 
  were 
  due. 
  

  

  It 
  would 
  seem 
  that 
  the 
  diamonds 
  are 
  more 
  plentifully, 
  if 
  not 
  

   almost 
  exclusively, 
  found 
  in 
  the 
  neighbourhood 
  of 
  dykes 
  of 
  diorite 
  

   that 
  intersect 
  the 
  hydrated 
  rock 
  or 
  occur 
  near 
  it 
  in 
  the 
  horizontal 
  

   strata 
  through 
  which 
  the 
  igneous 
  rocks 
  have 
  been 
  projected. 
  This 
  

   is 
  the 
  view 
  taken 
  by 
  Mr. 
  "William 
  Nevill, 
  whose 
  experience 
  on 
  the 
  

   diamond-fields 
  is 
  the 
  more 
  valuable 
  from 
  his 
  having 
  carried 
  with 
  

   him 
  to 
  South 
  Africa 
  some 
  mineralogical 
  experience. 
  At 
  Bultfontein, 
  

   where 
  the 
  diamantiferous 
  rock 
  is 
  in 
  contact 
  with 
  a 
  narrow 
  dyke 
  of 
  

   diorite, 
  small 
  diamonds, 
  numerous 
  and 
  remarkable 
  for 
  the 
  absence 
  of 
  

   colour 
  in 
  them, 
  occur 
  in 
  the 
  immediate 
  neighbourhood 
  of 
  the 
  diorite. 
  

   At 
  the 
  " 
  diggings 
  " 
  of 
  Du 
  Toit's 
  Pan, 
  on 
  the 
  other 
  hand, 
  where 
  the 
  

   diamond-bearing 
  rock 
  intervenes 
  between 
  the 
  diorite 
  and 
  the 
  horizon- 
  

   tal 
  strata 
  of 
  shale, 
  large 
  yellow 
  diamonds, 
  accompanied 
  by 
  a 
  few 
  white 
  

   ones, 
  were 
  found. 
  In 
  the 
  old 
  De 
  Beer, 
  again, 
  diamonds 
  were 
  found 
  

   in 
  a 
  rock 
  that 
  abutted 
  on 
  a 
  large 
  boss-like 
  mass 
  of 
  a 
  much 
  more 
  

   coarse-grained 
  diorite. 
  The 
  same 
  thing 
  was 
  true 
  of 
  the 
  rich 
  deposits 
  

   of 
  diamonds 
  found 
  at 
  the 
  so-called 
  New 
  Rush 
  or 
  Colesberg 
  Kopje. 
  

  

  The 
  distinctive 
  character 
  of 
  the 
  diamonds 
  met 
  with 
  in 
  these 
  

   different 
  localities 
  would 
  seem 
  to 
  point 
  to 
  a 
  source 
  in 
  each 
  case 
  not 
  

   very 
  remote 
  from 
  the 
  place 
  where 
  they 
  are 
  found, 
  a 
  confirmation 
  of 
  

   which 
  is 
  further 
  to 
  be 
  seen 
  in 
  the 
  sharp 
  and 
  unabraded 
  character 
  of 
  

   the 
  edges 
  of 
  the 
  crystals, 
  while 
  the 
  fact 
  that 
  so 
  many 
  diamonds 
  

   have 
  undergone 
  fracture 
  and 
  in 
  fact 
  are 
  found 
  as 
  fragments, 
  would 
  

   not 
  be 
  inconsistent 
  with 
  such 
  a 
  view. 
  

  

  The 
  presence, 
  in 
  intruded 
  masses, 
  of 
  diorite, 
  and 
  the 
  crushed 
  and 
  

   broken 
  appearance 
  so 
  often 
  presented 
  by 
  the 
  metamorphosed 
  en- 
  

   statite 
  rock 
  containing 
  the 
  diamonds 
  — 
  the 
  fragmentary 
  nature 
  of 
  so 
  

  

  