﻿60 
  E. 
  J. 
  DUNN 
  ON 
  SOUTH-AFRICAN 
  DIAMONDS. 
  

  

  pentinized. 
  The 
  diallage 
  was 
  the 
  true 
  diallage, 
  viz. 
  a 
  mineral 
  of 
  

   augitic 
  type. 
  The 
  mineral 
  resembling 
  elinochlore 
  was, 
  he 
  had 
  

   proved, 
  vermiculite 
  ; 
  and 
  the 
  phlogopite 
  was 
  not 
  improbably 
  Jeffrey- 
  

   site. 
  The 
  calcite 
  had 
  been 
  thought 
  to 
  have 
  been 
  derived 
  from 
  

   the 
  decomposition 
  of 
  augitic 
  silicates; 
  but 
  this 
  he 
  doubted. 
  One 
  

   specimen, 
  from 
  a 
  depth 
  of 
  130 
  feet, 
  exhibited 
  a 
  completely 
  serpen- 
  

   tinous 
  metamorphosis, 
  which 
  had 
  involved 
  one 
  mineral 
  as 
  yet 
  

   undetermined, 
  but 
  probably 
  enstatite. 
  The 
  absence 
  of 
  felspars 
  

   in 
  all 
  the 
  rocks 
  but 
  one 
  was 
  singular 
  ; 
  in 
  one 
  specimen 
  from 
  Bult- 
  

   fontein, 
  however, 
  there 
  was 
  an 
  abundance 
  of 
  kaolinite, 
  proving 
  the 
  

   original 
  presence 
  of 
  a 
  felspar. 
  He 
  thought 
  that 
  the 
  fracture 
  of 
  the 
  

   diamonds 
  might 
  be 
  due 
  to 
  the 
  breaking 
  up 
  of 
  the 
  rocks 
  in 
  which 
  

   they 
  are 
  now 
  found. 
  A 
  fact 
  pointed 
  out 
  by 
  the 
  late 
  Prof. 
  Rose, 
  

   viz. 
  that 
  the 
  octahedral 
  faces 
  of 
  diamonds 
  became 
  grooved 
  after 
  

   burning 
  by 
  triangular 
  striations, 
  was 
  of 
  great 
  importance 
  in 
  con- 
  

   sidering 
  the 
  changes 
  these 
  rock-fragments 
  had 
  undergone, 
  as 
  

   many 
  of 
  the 
  diamonds 
  from 
  the 
  upper 
  beds 
  had 
  their 
  surface 
  

   striated 
  in 
  this 
  manner. 
  It 
  would 
  be 
  a 
  question 
  of 
  interest 
  to 
  

   observe 
  whether 
  those 
  from 
  the 
  lower 
  depths 
  were 
  similarly 
  marked 
  

   and 
  equally 
  shattered. 
  

  

  Mr. 
  Forbes 
  observed 
  that, 
  owing 
  to 
  his 
  not 
  having 
  had 
  the 
  op- 
  

   portunity 
  of 
  examining 
  the 
  rocks 
  in 
  question, 
  he 
  could 
  not 
  speak 
  

   as 
  to 
  their 
  nature 
  ; 
  but 
  as 
  he 
  looked 
  upon 
  the 
  rock 
  gabbro 
  as 
  being 
  

   in 
  the 
  main 
  composed 
  of 
  triclinic 
  felspar 
  with 
  diallage, 
  he 
  could 
  not 
  

   regard 
  any 
  of 
  the 
  rock-specimens 
  shown 
  as 
  entitled 
  to 
  this 
  name, 
  

   especially 
  as 
  they, 
  as 
  a 
  rule, 
  were 
  even 
  devoid 
  of 
  felspar. 
  The 
  black 
  

   rock, 
  found 
  deepest, 
  appeared 
  to 
  him 
  to 
  resemble 
  one 
  of 
  the 
  altered 
  

   basalts, 
  in 
  which 
  Zirkel 
  had 
  shown 
  that 
  the 
  olivine 
  was 
  changed 
  into 
  

   a 
  peculiar 
  greenish 
  serpentine-like 
  mineral. 
  

  

  Mr. 
  Dunn, 
  in 
  reply, 
  stated 
  that 
  he 
  had 
  merely 
  made 
  use 
  of 
  the 
  

   term 
  gabbro 
  as 
  a 
  provisional 
  name. 
  There 
  could, 
  he 
  thought, 
  be 
  

   no 
  doubt 
  of 
  the 
  volcanic 
  origin 
  of 
  the 
  rock 
  ; 
  and, 
  moreover, 
  near 
  

   Bultfontein 
  and 
  elsewhere, 
  there 
  were 
  sheets 
  and 
  dykes 
  of 
  dolerite, 
  

   but 
  quite 
  different 
  from 
  the 
  rocks 
  in 
  the 
  pipes. 
  Some 
  of 
  the 
  fragments 
  

   in 
  the 
  matrix 
  were 
  fragments 
  of 
  shale, 
  mica-schist, 
  and 
  hornblende, 
  

   which 
  had 
  probably 
  been 
  brought 
  up 
  into 
  the 
  pipes 
  from 
  below. 
  

  

  Prof. 
  Ramsay 
  remarked 
  on 
  the 
  wide 
  application 
  of 
  the 
  term 
  

   gabbro, 
  which 
  could 
  hardly 
  be 
  said 
  to 
  imply 
  any 
  distinct 
  mine- 
  

   ralogical 
  character. 
  Looking 
  at 
  the 
  question 
  broadly, 
  the 
  facts 
  

   seemed 
  to 
  be, 
  that 
  among 
  nearly 
  horizontal 
  shales 
  there 
  were 
  

   patches 
  of 
  rock 
  of 
  approximately 
  circular 
  form, 
  with 
  the 
  shale 
  turned 
  

   up 
  around 
  their 
  edges, 
  and 
  altered 
  for 
  a 
  certain 
  distance. 
  The 
  

   appearances 
  were 
  therefore 
  those 
  of 
  the 
  extrusion 
  of 
  a 
  heated 
  or 
  

   molten 
  body 
  by 
  expansion 
  from 
  below 
  ; 
  and 
  possibly, 
  as 
  the 
  author 
  

   suggested, 
  other 
  traces 
  of 
  eruption 
  had 
  been 
  removed 
  by 
  subse- 
  

   quent 
  denudation. 
  He 
  could 
  not 
  regard 
  the 
  rocks 
  in 
  the 
  pipes 
  as 
  

   in 
  any 
  way 
  metamorphic 
  in 
  the 
  usual 
  acceptation 
  of 
  the 
  term. 
  

   Though 
  the 
  volcanic 
  origin 
  of 
  those 
  rocks 
  might 
  be 
  accepted, 
  he 
  

   thought 
  it 
  more 
  probable 
  that 
  the 
  diamonds 
  were 
  brought 
  up 
  from 
  

   some 
  subterranean 
  metamorphosed 
  rock, 
  than 
  generated 
  in 
  the 
  lava. 
  

  

  