﻿336 
  

  

  W. 
  H. 
  HTTDLESTON 
  ON 
  SOME 
  

  

  The 
  veinstone 
  being 
  found 
  more 
  or 
  less 
  mixed 
  up 
  with 
  the 
  decom- 
  

   posed 
  rock, 
  appears 
  to 
  indicate 
  that 
  their 
  ages 
  may 
  be 
  identical. 
  

  

  4th. 
  The 
  quartz-trachytes 
  and 
  sandstone 
  are 
  recent, 
  and 
  most 
  

   certainly 
  Post-tertiary. 
  

  

  APPENDIX. 
  

  

  Report 
  on 
  some 
  Rock 
  specimens 
  collected 
  by 
  Me. 
  George 
  ATrwood. 
  

   By 
  W. 
  H.Hudleston, 
  Esq., 
  F.G.S., 
  F.C.S., 
  &c. 
  

  

  Six 
  specimens 
  were 
  carefully 
  examined. 
  Five 
  of 
  these 
  are 
  igneous 
  

   rocks, 
  having 
  all 
  the 
  appearance 
  of 
  lavas 
  ; 
  and 
  although 
  considerable 
  

   differences 
  exist 
  between 
  some 
  of 
  them, 
  yet 
  four 
  out 
  of 
  the 
  five, 
  

   and 
  perhaps 
  all 
  five, 
  must 
  come 
  within 
  the 
  category 
  of 
  augite-ande- 
  

   sites. 
  The 
  hand-specimens 
  show 
  a 
  dark 
  compact 
  base 
  with 
  numerous 
  

   crystals 
  of 
  felspars, 
  usually 
  small. 
  Thus 
  the 
  general 
  structure, 
  

   viewed 
  macroscopically, 
  is 
  micro-porphyritic. 
  In 
  most 
  cases 
  the 
  

   felspars 
  are 
  inclined 
  to 
  be 
  vitreous 
  ; 
  but 
  in 
  one 
  instance, 
  where 
  the 
  

   crystals 
  are 
  rather 
  larger, 
  they 
  are 
  of 
  a 
  dull 
  white. 
  

  

  The 
  character 
  of 
  the 
  ground-mass 
  in 
  thin 
  section 
  is 
  seen 
  to 
  range 
  

   from 
  a 
  trachydolerite, 
  where 
  there 
  is 
  a 
  considerable 
  development 
  

   of 
  microcrystalline 
  matter, 
  consisting 
  for 
  the 
  most 
  part 
  of 
  acicular 
  

   prisms 
  of 
  triclinic 
  felspar, 
  to 
  a 
  rock 
  in 
  which 
  the 
  felsitic 
  texture 
  is 
  

   in 
  the 
  ascendant 
  and 
  the 
  tendency 
  to 
  doleritic 
  texture 
  is 
  reduced 
  

   to 
  a 
  minimum. 
  

  

  Generally 
  speaking, 
  the 
  felspars 
  are 
  fresh, 
  and 
  polarize 
  with 
  con- 
  

   siderable 
  brilliancy. 
  They 
  are 
  mostly 
  triclinic, 
  though 
  not 
  without 
  

   orthoclase 
  (sanidine) 
  in 
  some 
  cases. 
  Without 
  analysis 
  it 
  is 
  impos- 
  

   sible 
  to 
  state 
  positively 
  what 
  the 
  triclinic 
  felspars 
  may 
  be, 
  though 
  

   probably 
  oligoclase 
  is 
  the 
  most 
  abundant. 
  Judging 
  from 
  the 
  analogy 
  

   of 
  similar 
  rocks, 
  andesine 
  is 
  also 
  present 
  ; 
  but 
  the 
  more 
  basic 
  vari- 
  

   eties 
  of 
  felspar 
  are 
  probably 
  rare 
  or 
  absent. 
  

  

  There 
  is 
  a 
  fair 
  amount 
  of 
  very 
  fresh 
  augite, 
  in 
  cr} 
  T 
  stals 
  which 
  

   are 
  smaller 
  than 
  those 
  of 
  the 
  felspars 
  and 
  less 
  numerous. 
  Asso- 
  

   ciated 
  with 
  these 
  augite 
  crystals, 
  and 
  sometimes 
  independant 
  of 
  

   them, 
  occurs 
  a 
  fair 
  amount 
  of 
  magnetite 
  or 
  pyrite, 
  which 
  in 
  some 
  

   cases 
  has 
  undergone 
  partial 
  oxidation. 
  The 
  accessory 
  minerals 
  are 
  

   by 
  no 
  means 
  plentiful, 
  though 
  one 
  may 
  note 
  a 
  stray 
  needle 
  of 
  apatite 
  

   here 
  and 
  there. 
  If 
  any 
  olivine 
  exists, 
  it 
  must 
  be 
  in 
  very 
  small 
  

   quantity 
  ; 
  but 
  some 
  of 
  the 
  augite 
  crystals 
  are 
  so 
  rounded 
  at 
  the 
  

   edges 
  that 
  they 
  might 
  perhaps 
  be 
  taken 
  for 
  grains 
  of 
  olivine. 
  

  

  The 
  specific 
  gravity 
  ranges 
  from 
  about 
  2-55 
  to 
  2*80, 
  being 
  on 
  the 
  

   whole 
  rather 
  low 
  for 
  rocks 
  with 
  some 
  augite 
  and 
  so 
  much 
  triclinic 
  

   felspar 
  in 
  addition 
  to 
  a 
  fair 
  proportion 
  of 
  iron 
  minerals. 
  This 
  cir- 
  

   cumstance 
  is 
  an 
  additional 
  point 
  in 
  favour 
  of 
  the 
  notion 
  that 
  in 
  

   most 
  cases 
  the 
  bulk 
  of 
  the 
  felspar 
  is 
  oligoclase. 
  Furthermore 
  the 
  

   microscopic 
  examination 
  would 
  lead 
  one 
  to 
  believe 
  that 
  the 
  specimens 
  

   for 
  the 
  most 
  part 
  belong 
  to 
  a 
  class 
  of 
  rocks 
  intermediate 
  between 
  

   the 
  dolerites 
  and 
  the 
  acidic 
  trachytes. 
  

  

  No. 
  1. 
  " 
  In 
  situ, 
  Paradise 
  Yalley." 
  Specific 
  gravity 
  2- 
  76. 
  Black 
  

  

  