﻿54 
  E. 
  J. 
  DUNN 
  ON 
  THE 
  MODE 
  OF 
  OCCURRENCE 
  

  

  11. 
  On 
  the 
  Mode 
  of 
  Occurrence 
  of 
  Diamonds 
  in 
  South 
  Africa. 
  

   By 
  E. 
  J. 
  Dunn, 
  Esq. 
  (Read 
  December 
  17, 
  1873.) 
  

  

  [Communicated 
  by 
  Professor 
  Ramsay, 
  E.E.S., 
  V.P.G.S.] 
  

  

  The 
  conditions 
  under 
  which 
  diamonds 
  occur 
  in 
  South 
  Africa 
  are 
  

   quite 
  different 
  from 
  those 
  of 
  every 
  other 
  known 
  locality, 
  and 
  are 
  so 
  

   unusual 
  as 
  to 
  deserve 
  the 
  earnest 
  attention 
  of 
  all 
  geologists. 
  

  

  It 
  is 
  quite 
  certain 
  that 
  the 
  present 
  " 
  dry 
  diggings," 
  such 
  as 
  

   " 
  Colesberg 
  Kopje," 
  « 
  Du 
  Toit's 
  Pan," 
  " 
  De 
  Beer's," 
  " 
  Bultfontein," 
  

   and 
  " 
  Jagersfontein," 
  are 
  being 
  worked 
  in 
  the 
  rock 
  which 
  has 
  brought 
  

   the 
  diamonds 
  to 
  the 
  present 
  surface 
  ; 
  they 
  all 
  possess 
  certain 
  com- 
  

   mon 
  features 
  of 
  very 
  marked 
  character. 
  Each 
  area 
  is 
  more 
  or 
  less 
  

   circular 
  in 
  form 
  (fig. 
  1, 
  a) 
  ; 
  the 
  boundary 
  of 
  each 
  is 
  usually 
  horizontal, 
  

   or 
  nearly 
  horizontal, 
  shale. 
  At 
  the 
  junction, 
  and 
  back 
  for 
  a 
  distance 
  

   of 
  from 
  one 
  to 
  several 
  feet, 
  the 
  edges 
  of 
  the 
  shale 
  are 
  bent 
  sharply 
  

   upwards 
  (fig. 
  1, 
  b). 
  

  

  The 
  contents 
  of 
  these 
  " 
  pipes 
  " 
  in 
  the 
  shale 
  are 
  the 
  same 
  in 
  all 
  

   eases, 
  and 
  show 
  distinctly 
  that 
  they 
  are 
  of 
  igneous 
  origin. 
  The 
  base 
  

   is 
  more 
  or 
  less 
  decomposed 
  gabbro 
  (?) 
  or 
  euphotide, 
  (?), 
  through 
  which 
  

   are 
  scattered 
  particles, 
  fragments, 
  and 
  huge 
  masses 
  of 
  shale, 
  nodides 
  

   of 
  dolerite, 
  occasional 
  fragments 
  of 
  chloritic 
  schist, 
  micaceous 
  schist, 
  

   and 
  gneiss. 
  The 
  principal 
  foreign 
  ingredient 
  is 
  the 
  shale, 
  which 
  in 
  

   many 
  places, 
  particularly 
  at 
  Colesberg 
  Kopje, 
  is 
  thoroughly 
  commi- 
  

   nuted, 
  forming 
  a 
  breccia, 
  with 
  euphotide 
  (?) 
  as 
  a 
  base. 
  Where 
  large 
  

   masses 
  of 
  shale 
  occur, 
  the 
  lines 
  of 
  bedding, 
  as 
  might 
  be 
  expected, 
  are 
  

   not 
  horizontal, 
  but 
  lie 
  in 
  all 
  directions. 
  

  

  Erom 
  the 
  forms, 
  contents, 
  and 
  general 
  features 
  of 
  the 
  pipes, 
  it 
  is 
  

   reasonable 
  to 
  infer 
  that 
  they 
  are 
  merely 
  the 
  channels 
  that 
  con- 
  

   nected 
  ancient 
  volcanic 
  craters 
  with 
  deep-seated 
  reservoirs 
  of 
  molten 
  

   rock. 
  

  

  On 
  sinking 
  into 
  these 
  pipes, 
  the 
  following 
  sequence, 
  with 
  slight 
  

   variations, 
  occurs 
  : 
  — 
  Eirst, 
  from 
  a 
  few 
  inches 
  to 
  several 
  feet 
  of 
  red 
  

   sand, 
  or 
  sandy 
  soil, 
  brought 
  into 
  its 
  present 
  position 
  by 
  the 
  agency 
  

   of 
  wind. 
  Even 
  within 
  the 
  last 
  20 
  years 
  portions 
  of 
  the 
  neigh- 
  

   bouring 
  country 
  have 
  been 
  stripped 
  of 
  soil 
  down 
  to 
  the 
  bed-rock, 
  

   and 
  other 
  portions 
  covered 
  by 
  such 
  means. 
  

  

  Then 
  a 
  layer 
  of 
  tufaceous 
  lime, 
  from 
  a 
  few 
  inches 
  to 
  8 
  or 
  10 
  feet 
  

   thick, 
  generally 
  much 
  harder 
  and 
  purer 
  on 
  the 
  surface, 
  shading 
  

   gradually 
  underneath 
  into 
  the 
  decomposed 
  rock 
  from 
  the 
  altered 
  

   pyroxenic 
  minerals 
  of 
  which 
  it 
  has 
  doubtless 
  been 
  formed. 
  The 
  

   segregation 
  of 
  the 
  lime 
  to 
  the 
  surface 
  would 
  be 
  materially 
  assisted 
  

   by 
  climatic 
  conditions 
  ; 
  for 
  in 
  this 
  subtropical 
  region 
  rain 
  usually 
  

   falls 
  in 
  heavy 
  showers 
  during 
  the 
  summer. 
  No 
  sooner 
  is 
  the 
  rain 
  

   over 
  than 
  the 
  intense 
  heat 
  causes 
  rapid 
  evaporation. 
  

  

  Lower 
  still 
  is 
  the 
  very 
  friable 
  decomposed 
  euphotide 
  (?), 
  so 
  soft 
  and 
  

   crumbling 
  that, 
  for 
  a 
  few 
  feet, 
  it 
  may 
  be 
  almost 
  dug 
  out 
  with 
  a 
  spade. 
  

  

  