﻿56 
  E. 
  J. 
  DTJN5T 
  ON 
  THE 
  MODE 
  OF 
  OCCURRENCE 
  

  

  surprising 
  that 
  not 
  a 
  vestige 
  of 
  a 
  crater, 
  or 
  of 
  any 
  of 
  the 
  usual 
  

   subaerial 
  products 
  of 
  volcanoes, 
  has 
  been 
  found 
  in 
  S. 
  Africa. 
  

  

  A 
  brief 
  sketch 
  of 
  the 
  various 
  intrusive 
  rocks 
  of 
  S. 
  Africa 
  will 
  help 
  

   to 
  explain 
  their 
  relation 
  to 
  the 
  one 
  under 
  notice. 
  

  

  The 
  oldest 
  are 
  the 
  diorite 
  dykes 
  of 
  Namaqualand, 
  running 
  about 
  

   east 
  and 
  west, 
  extremely 
  rich 
  in 
  copper 
  ores, 
  frequently 
  very 
  mica- 
  

   ceous. 
  They 
  were 
  formed 
  subsequently 
  to 
  the 
  "Namaqualand 
  Schists" 
  

   (pre-Silurian), 
  but 
  do 
  not 
  penetrate 
  the 
  " 
  Table 
  Mountain 
  Sandstone 
  " 
  

   (Devonian 
  ?). 
  Penetrating 
  the 
  same 
  rocks, 
  gneiss 
  and 
  metamorphic, 
  

   are 
  dykes 
  of 
  granite, 
  syenite, 
  and 
  porphyry, 
  probably 
  of 
  the 
  same 
  

   age 
  as 
  the 
  diorites. 
  

  

  The 
  extensive 
  intrusive 
  action 
  by 
  which 
  thousands 
  of 
  square 
  miles 
  

   of 
  Dicynodon-shales 
  were 
  penetrated 
  by 
  enormous 
  dykes 
  and 
  lateral 
  

   sheets 
  of 
  dolerite, 
  took 
  place 
  at 
  a 
  much 
  more 
  recent 
  period, 
  as 
  the 
  

   " 
  Stormberg 
  beds" 
  (containing 
  coal 
  and 
  abundant 
  fossil 
  plant-remains, 
  

   such 
  as 
  Sphenopteris 
  elongata, 
  Pecopteris 
  odontopteroides, 
  Cyclopteris 
  

   cuneata, 
  &c.) 
  are 
  penetrated 
  by 
  doleritic 
  amygdaloids 
  &c, 
  while 
  the 
  

   "Sundays-River 
  beds" 
  (Middle 
  or 
  Upper 
  Oolite) 
  are 
  not 
  intersected 
  

   by 
  any 
  intrusive 
  rock. 
  

  

  The 
  most 
  recent 
  and, 
  at 
  the 
  same 
  time, 
  most 
  interesting 
  eruptive 
  

   rock 
  in 
  S. 
  Africa 
  is 
  the 
  " 
  Trap 
  Conglomerate," 
  or 
  breccia, 
  apparently 
  

   a 
  doleritic 
  rock, 
  which 
  in 
  places 
  becomes 
  a 
  conglomerate 
  on 
  account 
  

   of 
  the 
  intermixture 
  of 
  pebbles 
  and 
  boulders 
  of 
  other 
  rocks, 
  while 
  in 
  

   other 
  places 
  it 
  is 
  filled 
  with 
  angular 
  fragments 
  forming 
  a 
  friction- 
  

   breccia. 
  It 
  extends 
  continuously 
  in 
  a 
  horse-shoe 
  form 
  for 
  several 
  

   hundred 
  miles 
  in 
  length, 
  varying 
  from 
  1 
  to 
  3 
  miles 
  in 
  breadth, 
  and 
  

   throwing 
  off 
  many 
  bifurcations. 
  Detached 
  outliers 
  are 
  not 
  uncom- 
  

   mon. 
  It 
  penetrates 
  the 
  diamond-field 
  rocks 
  at 
  Pniel, 
  and 
  contains 
  

   fragments 
  of 
  amygdaloid 
  entangled 
  in 
  its 
  mass. 
  

  

  It 
  is 
  possible 
  that 
  the 
  diamond-bearing 
  rock 
  filling 
  the 
  pipes 
  

   belongs 
  to 
  one 
  or 
  the 
  other 
  of 
  the 
  last-mentioned 
  intrusive 
  out- 
  

   flows. 
  

  

  Areas 
  similar 
  in 
  every 
  respect 
  to 
  those 
  worked 
  at 
  the 
  diamond- 
  

   fields 
  occur 
  near 
  Schietfontein 
  (see 
  fig. 
  1), 
  about 
  200 
  miles 
  from 
  the 
  

   present 
  workings, 
  while 
  Jagersfontein 
  is 
  70 
  miles 
  distant 
  in 
  another 
  

   direction. 
  Those 
  near 
  Schietfontein 
  have 
  not 
  yet 
  been 
  proved 
  ; 
  but 
  

   as 
  all 
  similar 
  places 
  at 
  the 
  fields 
  have 
  yielded 
  diamonds, 
  it 
  is 
  probable 
  

   that 
  they 
  will 
  also 
  when 
  properly 
  opened. 
  

  

  Near 
  the 
  diggings 
  it 
  is 
  by 
  no 
  means 
  an 
  easy 
  task 
  to 
  find 
  the 
  pipes, 
  

   through 
  the 
  accumulation 
  of 
  surface- 
  soil 
  ; 
  but 
  at 
  Schietfontein 
  the 
  

   boundary 
  can 
  be 
  traced 
  to 
  a 
  foot 
  by 
  means 
  of 
  the 
  upturned 
  edges 
  of 
  

   the 
  shale. 
  

  

  The 
  variety 
  of 
  rocks 
  occurring 
  within 
  a 
  very 
  limited 
  area 
  sur- 
  

   rounding 
  Pniel, 
  on 
  the 
  Vaal 
  river, 
  is 
  very 
  noticeable, 
  and 
  would 
  

   require 
  considerable 
  attention 
  to 
  the 
  details 
  of 
  their 
  occurrence 
  

   before 
  satisfactory 
  conclusions 
  could 
  be 
  arrived 
  at. 
  There 
  are 
  

   dolerites, 
  diorites, 
  amygdaloids, 
  porphyries, 
  trap-breccia, 
  <fcc. 
  Wear 
  

   Du 
  Toit's 
  Pan 
  the 
  rock, 
  which 
  is 
  of 
  doleritic 
  character, 
  presents 
  very 
  

   different 
  texture 
  and 
  colour 
  within 
  a 
  few 
  yards. 
  Along 
  the 
  banks 
  

   of 
  the 
  Vaal 
  " 
  almonds 
  " 
  of 
  quartz 
  that 
  have 
  weathered 
  out 
  of 
  amyg- 
  

  

  